Redox-catalyzed binding of dinitrogen by molybdenum N-tert- hydrocarbylanilide complexes: Implications for dinitrogen functionalization and reductive cleavage

Article abstract of DOI:10.1021/ja991435t

The splitting of dinitrogen (1 atm, THF, 25 °C) by Mo(N[R]Ar)₃ (R = C(CD₃)₂CH₃, Ar = 3,5-C₆H₃Me₂) giving 2 equiv of nitride N≡Mo(N[R]Ar)₃ is found to be accelerated in the presence of sodium amalgam. Careful control of the Mo(N[R]Ar)₃ concentration led to the isolation and characterization of the anionic dinitrogen complex, [(THF)(xNa)][(N₂)Mo(N[R]Ar)₃], where x is from 0 to 3. Via electrochemical experiments and synthetic studies, [(THF)(x)Na][(N₂)Mo(N[R]Ar)₃] is found to be a key intermediate in the acceleration of N₂ splitting by Mo(N[R]Ar)₃ in the presence of sodium amalgam. Accordingly, in the presence of an electron acceptor, [(THF)(x)Na][(N₂)Mo(N[R]Ar)₃] reacts with Mo(N[R]Ar)₃ to give the neutral N₂-bridged complex (μ-N₂){Mo(N[R]Ar)₃}₂, which in turn splits to 2 equiv of nitride N≡Mo(N[R]Ar)₃. It is seen that the function of sodium amalgam in this system is as a redox catalyst, accelerating the conversion of Mo(N[R]Ar)₃ to (μ-N₂){Mo(N[R]Ar)₃}₂, a dinuclear dinitrogen complex that does not lose N₂ readily. Electrochemical or chemical outer-sphere oxidation of [(THF)(x)Na] [(N₂)Mo(N[R]Ar)₃] leads to rapid N₂ evolution with regeneration of Mo(N[R]Ar)₃, presumably via the neutral mononuclear dinitrogen complex (N₂)Mo(N[R]Ar)₃. In situ generated [(THF)(x)Na][(N₂)Mo(N[R]Ar)₃] was efficiently trapped by ClSiMe₃ to give (Me₃SiNN)Mo(N[R]Ar)₃. This complex underwent reaction with methyl triflate to give the dimethyl hydrazido cationic species, [(Me₂NN)Mo(N[R]Ar)₃][OTf]. The synthesis of the monomethyl complex (MeNN)Mo(N[R]Ar)₃ also was achieved. Experiments designed to trap the neutral mononuclear dinitrogen complex (N₂)Mo(N[R]Ar)₃ gave rise to efficient syntheses of heterodinuclear dinitrogen complexes including (ph[¹Bu]N)₃Ti(μ-N₂)Mo(N[R]Ar)₃, which also was synthesized in its ¹⁵N₂-labeled form. Synthesis and characterization data for the new N-adamantyl-substituted three-coordinate molybdenum(III) complex Mo(N[Ad]Ar)₃ (Ad = 1-adamantyl, Ar = 3,5-C₆H₃Me₂) are presented. The complex is found to react with dinitrogen (1 atm, THF, 25 °C) in the presence of sodium amalgam to give the dinitrogen anion complex [(THF)(x)Na] [(N₂)Mo(N[Ad]Ar)₃]; the synthesis does not require careful regulation of the Mo(N[Ad]Ar)₃ concentration. Indeed, under no conditions has Mo(N[Ad]Ar)₃ been observed to split dinitrogen or to give rise to a dinuclear μ-N₂ complex; this striking contrast with the reactivity of Mo(N[R]Ar)₃ (R = C(CD₃)₂CH₃) is attributed to the enhanced steric protection at Mo afforded by the 1-adamantyl substituents.

Full text of DOI:10.1021/ja991435t

J. Am. Chem. Soc. 1999, 121, 10053-10067
10053
Redox-Catalyzed Binding of Dinitrogen by Molybdenum
N-tert-Hydrocarbylanilide Complexes: Implications for Dinitrogen
Functionalization and Reductive Cleavage
Jonas C. Peters,† John-Paul F. Cherry, J. Christopher Thomas, Luis Baraldo,
Daniel J. Mindiola, William M. Davis, and Christopher C. Cummins*
Contribution from the Department of Chemistry, Room 2-227, Massachusetts Institute of Technology,
Cambridge, Massachusetts 02139-4307
ReceiVed May 3, 1999
Abstract: The splitting of dinitrogen (1 atm, THF, 25 °C) by Mo(N[R]Ar)3 (R ) C(CD3)2CH3, Ar ) 3,5-
C6H3Me2) giving 2 equiv of nitride NtMo(N[R]Ar)3 is found to be accelerated in the presence of sodium
amalgam. Careful control of the Mo(N[R]Ar)3 concentration led to the isolation and characterization of the
anionic dinitrogen complex, [(THF)xNa][(N2)Mo(N[R]Ar)3], where x is from 0 to 3. Via electrochemical
experiments and synthetic studies, [(THF)xNa][(N2)Mo(N[R]Ar)3] is found to be a key intermediate in the
acceleration of N2 splitting by Mo(N[R]Ar)3 in the presence of sodium amalgam. Accordingly, in the presence
of an electron acceptor, [(THF)xNa][(N2)Mo(N[R]Ar)3] reacts with Mo(N[R]Ar)3 to give the neutral N2-bridged
complex (µ-N2){Mo(N[R]Ar)3}2, which in turn splits to 2 equiv of nitride NtMo(N[R]Ar)3. It is seen that the
function of sodium amalgam in this system is as a redox catalyst, accelerating the conversion of Mo(N[R]Ar)3
to (µ-N2){Mo(N[R]Ar)3}2, a dinuclear dinitrogen complex that does not lose N2 readily. Electrochemical or
chemical outer-sphere oxidation of [(THF)xNa][(N2)Mo(N[R]Ar)3] leads to rapid N2 evolution with regeneration
of Mo(N[R]Ar)3, presumably via the neutral mononuclear dinitrogen complex (N2)Mo(N[R]Ar)3. In situ
generated [(THF)xNa][(N2)Mo(N[R]Ar)3] was efficiently trapped by ClSiMe3 to give (Me3SiNN)Mo(N[R]-
Ar)3. This complex underwent reaction with methyl triflate to give the dimethyl hydrazido cationic species,
[(Me2NN)Mo(N[R]Ar)3][OTf]. The synthesis of the monomethyl complex (MeNN)Mo(N[R]Ar)3 also was
achieved. Experiments designed to trap the neutral mononuclear dinitrogen complex (N2)Mo(N[R]Ar)3 gave
rise to efficient syntheses of heterodinuclear dinitrogen complexes including (Ph[tBu]N)3Ti(µ-N2)Mo(N[R]-
Ar)3, which also was synthesized in its 15N2-labeled form. Synthesis and characterization data for the new
N-adamantyl-substituted three-coordinate molybdenum(III) complex Mo(N[Ad]Ar)3 (Ad ) 1-adamantyl, Ar
) 3,5-C6H3Me2) are presented. The complex is found to react with dinitrogen (1 atm, THF, 25 °C) in the
presence of sodium amalgam to give the dinitrogen anion complex [(THF)xNa][(N2)Mo(N[Ad]Ar)3]; the synthesis
does not require careful regulation of the Mo(N[Ad]Ar)3 concentration. Indeed, under no conditions has Mo-
(N[Ad]Ar)3 been observed to split dinitrogen or to give rise to a dinuclear µ-N2 complex; this striking contrast
with the reactivity of Mo(N[R]Ar)3 (R ) C(CD3)2CH3) is attributed to the enhanced steric protection at Mo
afforded by the 1-adamantyl substituents.
Introduction
A key intermediate in the binding and splitting of dinitrogen
by 1 is the dinuclear complex (µ-N2){Mo(N[R]Ar)3}2 (3), which
is formed during a slow (∼76 h) incubation period at -35 °C
(1 atm N2, OEt2 or toluene).2 The dimolybdenum dinitrogen
complex 3 then converts to 2 equiv of the terminal nitrido
derivative NtMo(N[R]Ar)3 (4) with a first-order kinetic profile,
a fragmentation reaction constituting the final step in the splitting
of dinitrogen by 1. Although the fragmentation of 3 to 2 equiv
of 4 is facile at 28 °C and above, the reaction is slow at -35
°C. The kinetic profile of the steps in Figure 1 has facilitated
the study of the properties of dinuclear 3 and its conversion to
1. These reactions also have been the subject of theoretical
scrutiny.9-11
It has been reported that the three-coordinate molybdenum-
(III) complex Mo(N[R]Ar)3 (1, R ) C(CD3)2CH3, Ar ) 3,5-
C6H3Me2) effects the six-electron reductive cleavage of dini-
trogen (Figure 1),1-3 thus providing a homogeneous-solution
analogue to the proposed rate-determining step in the Haber-
Bosch ammonia synthesis.4-6 The splitting of N2 by niobium
calixarene complexes also has been documented,7 and atomic
scandium has been shown to split N2 to give scandium(III)
nitride via a putative dinuclear intermediate.8
† Present address: Division of Chemistry and Chemical Engineering,
California Institute of Technology, Pasadena, CA 91125.
(1) Laplaza, C. E.; Cummins, C. C. Science 1995, 268, 861.
(2) Laplaza, C. E.; Johnson, M. J. A.; Peters, J. C.; Odom, A. L.; Kim,
E.; Cummins, C. C.; George, G. N.; Pickering, I. J. J. Am. Chem. Soc.
1996, 118, 8623.
(7) Zanotti-Gerosa, A.; Solari, E.; Giannini, L.; Floriani, C.; Chiesi-Villa,
A.; Rizzoli, C. J. Am. Chem. Soc. 1998, 120, 437.
(8) Chertihin, G.; Andrews, L.; Bauschlicher, C. J. Am. Chem. Soc. 1998,
120, 3205.
(3) Cummins, C. C. Chem. Commun. 1998, 1777.
(4) Jennings, J., Ed. Catalytic Ammonia Synthesis; Plenum: New York,
1991.
(5) Nielsen, A., Ed. Ammonia: Catalysis and Manufacture; Springer-
Verlag: New York, 1995.
(9) Cui, Q.; Musaev, D. G.; Svensson, M.; Sieber, S.; Morokuma, K. J.
Am. Chem. Soc. 1995, 117, 12366.
(10) Neyman, K. M.; Nasluzov, V. A.; Hahn, J.; Landis, C. R.; Ro¨sch,
N. Organometallics 1997, 16, 995-1000.
(11) Hahn, J.; Landis, C. R.; Nasluzov, V. A.; Neyman, K. M.; Ro¨sch,
N. Inorg. Chem. 1997, 36, 3947.
(6) Travis, T. Chem. Ind. 1993, 581.
10.1021/ja991435t CCC: $18.00 © 1999 American Chemical Society
Published on Web 11/03/1999

10054 J. Am. Chem. Soc., Vol. 121, No. 43, 1999
Peters et al.
others who study dinitrogen chemistry involving metal com-
plexes supported by hard π-donor ligands.35 Alternative syn-
thetic approaches to nitrogen fixation, including those involving
organophosphine-stabilized systems, have been reviewed.36-39
Results and Discussion
(1) Reaction of Mo(N[R]Ar)3 with Dinitrogen in the
Presence of Sodium Amalgam. Stirring THF solutions of 1
over stoichiometric Na/Hg under a dinitrogen atmosphere
resulted in a rapid transformation at 25 °C (Figure 2). Initially
displaying the bright orange color of 1, such solutions acquired
the intense purple of intermediate 3 within 1 h. 2H NMR
spectroscopy revealed that a steady consumption of orange 1
coincided with the formation of the neutral dinuclear complex
3. A new, sharper peak at 1.5 ppm, indicative of a diamagnetic
species, also was observed.
An FTIR spectrum of this solution featured an intense νNN
vibration at 1761 cm-l. Other stretches appearing in this
spectrum are typical of complexes bearing the N(R)Ar ligand.2,40,41
The most prominent are those at 1596 and 1583 cm-1 arising
from aryl νCC vibrations.
As the reaction mixture was stirred over a period of several
hours, the intense purple color gradually diminished, and the
final solution acquired a red-brown color. 2H NMR spectroscopy
of the crude product mixture revealed complete consumption
Figure 1. Reaction of Mo(N[R]Ar)3 (1) with dinitrogen in the absence
of added reagents. Reaction conditions for complete conversion of 1
to 3: 1 atm of N2, Et2O or toluene, -35 °C, ∼72 h. At 25 °C under
otherwise identical conditions, consumption of 1 is negligible over ∼24
h. Conversion of 3 to 4 at 25 °C occurrs with t1/2 ≈ 30 min.
1
of both the starting material 1 and intermediate 3. A H NMR
spectrum of said mixture indicated that nitride 4 had formed in
85% yield. Approximately 10% of a new diamagnetic product
also was present, displaying a single set of anilido ligand
resonances along with resonances for solvated THF. A solution
FTIR spectrum of this mixture still showed the presence of νNN
at 1761 cm-1, its intensity diminished relative to data acquired
at earlier reaction times. In part on the basis of the literature
precedent provided by Schrock and co-workers,27,42,43 we assign
the new diamagnetic product as the sodium salt of the anionic
dinitrogen complex [(THF)xNa][(N2)Mo(N[R]Ar)3] (5). These
observations are depicted in Figure 2.
Since the slow step in the binding and splitting of dinitrogen
by 1 appears to be the formation of dinuclear 3,2 it was of
interest to see whether the binding of N2 could be accelerated
by some means. In this regard, the reaction of 1 with N2 in the
presence of reducing equivalents has been examined both
chemically and electrochemically.
A further point of interest is whether the formation of a
dinuclear dinitrogen complex such as 3 could be avoided
entirely, to give rise to a manifold of chemistry in which the
N2 to Mo ratio would be 1:1. Such a manifold of chemistry
might be fruitful in terms of obtaining complexes of function-
alized N2-derived ligands. This particular goal has been achieved
straightforwardly through the use of more sterically demanding
(adamantyl-substituted12,13) ligands, and less-straightforwardly
via careful control of reaction conditions.
The product ratio of neutral 4 to salt 5 produced in the
reaction system of Figure 2 was found critically to be dependent
(22) Berno, P.; Gambarotta, S. Organometallics 1995, 14, 2159.
(23) Berno, P.; Hao, S.; Minhas, R.; Gambarotta, S. J. Am. Chem. Soc.
1994, 116, 7417.
The results described in this paper bear on both of the
preceding points, namely (i) the acceleration of N2 splitting by
1 as a consequence of accelerated N2 uptake in the presence of
reducing agents and (ii) diversion of the N2 chemistry of 1 into
a manifold where N2 functionalization is possible and where
the N2 to Mo ratio is 1:1.
Of course, the new N2 coordination chemistry described here
is complementary to work from the groups of Floriani,7,14-16
Fryzuk,17-20 Gambarotta,21-25 Schrock,26-29 Scott,30-34 and
(24) Song, J.-I.; Berno, P.; Gambarotta, S. J. Am. Chem. Soc. 1994, 116,
6927.
(25) Beydoun, N.; Duchteau, R.; Gambarotta, S. Chem. Commun. 1992,
244.
(26) O’Donoghue, M.; Davis, W.; Schrock, R. Inorg. Chem. 1998, 37,
5149.
(27) O’Donoghue, M. B.; Zanetti, N. C.; Davis, W. M.; Schrock, R. R.
J. Am. Chem. Soc. 1997, 119, 2753.
(28) Schrock, R. R. Acc. Chem. Res. 1997, 30, 9.
(29) Neuner, B.; Schrock, R. R. Organometallics 1996, 15, 5-6.
(30) Roussel, P.; Scott, P. J. Am. Chem. Soc. 1998, 120, 1070.
(31) Roussel, P.; Tinker, N.; Scott, P. J. Alloy. Compd. 1998, 271, 150.
(32) Kaltsoyannis, N.; Scott, P. Chem. Commun. 1998, 1665.
(33) Roussel, P.; Hitchcock, P.; Tinker, N.; Scott, P. Inorg. Chem. 1997,
36, 5716.
(12) Johnson, A. R.; Cummins, C. C. Inorg. Synth. 1998, 32, 123.
(13) Ruppa, K. B. P.; Desmangles, N.; Gambarotta, S.; Yap, G.;
Rheingold, A. L. Inorg. Chem. 1997, 36, 1194.
(14) Campazzi, E.; Solari, E.; Floriani, C.; Scopelliti, R. Chem. Commun.
1998, 2603.
(15) Ferguson, R.; Solari, E.; Floriani, C.; Osella, D.; Ravera, M.; Re,
N.; Chiesi-Villa, A.; Rizzoli, C. J. Am. Chem. Soc. 1997, 119, 10104.
(16) Ferguson, R.; Solari, E.; Floriani, C.; Chiesi-Villa, A.; Rizzoli, C.
Angew. Chem., Int. Ed. Engl. 1993, 32, 396.
(17) Fryzuk, M.; Johnson, S.; Rettig, S. J. Am. Chem. Soc. 1998, 120,
11024.
(18) Cohen, J.; Fryzuk, M.; Loehr, T.; Mylvaganam, M.; Rettig, S. Inorg.
Chem. 1998, 37, 112.
(19) Fryzuk, M. D.; Love, J. B.; Rettig, S. J.; Young, V. G. Science
1997, 275, 1445.
(20) Fryzuk, M. D.; Haddad, T. S.; Mylvaganam, M.; McConville, D.
H.; Rettig, S. J. J. Am. Chem. Soc. 1993, 115, 2782.
(21) Song, J.-I.; Gambarotta, S. Chem. Eur. J. 1996, 2, 1258.
(34) Roussel, P.; Hitchcock, P.; Tinker, N.; Scott, P. Chem. Commun.
1996, 2053.
(35) Cummins, C. C. Prog. Inorg. Chem. 1998, 47, 685.
(36) Hidai, M.; Mizobe, Y. Chem. ReV. 1995, 95, 1115.
(37) Bazhenova, T.; Shilov, A. Coord. Chem. ReV. 1995, 144, 69.
(38) Leigh, G. Acc. Chem. Res. 1992, 25, 177.
(39) Sellmann, D.; Sutter, J. Acc. Chem. Res. 1997, 30, 460.
(40) Peters, J. C.; Johnson, A. R.; Odom, A. L.; Wanandi, P. W.; Davis,
W. M.; Cummins, C. C. J. Am. Chem. Soc. 1996, 118, 10175.
(41) Peters, J. C.; Odom, A. L.; Cummins, C. C. Chem. Commun. 1997,
1995.
(42) Shih, K.-Y.; Schrock, R. R.; Kempe, R. J. Am. Chem. Soc. 1994,
116, 8804.
(43) Kol, M.; Schrock, R. R.; Kempe, R.; Davis, W. M. J. Am. Chem.
Soc. 1994, 116, 4382.

Molybdenum N-tert-Hydrocarbylanilide Complexes
J. Am. Chem. Soc., Vol. 121, No. 43, 1999 10055
when stirred over 0.4% Na/Hg. This amount of decay was
indistinguishable from that observed in the absence of Na/Hg
under otherwise identical conditions.
The lack of reactivity between 1 and sodium amalgam in the
absence of N2, as well as the electrochemical stability of 1 at
potentials as low as -2.8 V in THF (vide infra), precludes direct
reduction of 1 to its corresponding anion under the conditions
of Figure 2. Rather, the unobserved neutral dinitrogen complex
2 is implicated as the electron-accepting species.
In a full paper, which included electrochemical data, Floriani
and co-workers reported recently that the anionic d3 species
[(THF)V(Mes)3]- (Mes ) 2,4,6-Me3C6H2) binds N2 readily.15
(3) Synthesis and Characterization of [(THF)xNa][(N2)-
Mo(N[R]Ar)3] (5). The clean isolation of 5 relied upon
circumventing a subsequent bimolecular reaction between 1 and
5. In a typical synthetic run, a dilute solution of 1 was added
dropwise, very slowly, to a vigorously stirred mixture of Na/
Hg in THF under a dinitrogen atmosphere. The rate of addition
was determined empirically by an array of attempts until
conditions were found in which the intense purple color of the
undesired bridged species 3 was no longer noticeable throughout
the addition. A 10-h dropwise addition of 1 to a stirring amalgam
under N2 reproducibly converted ∼0.01 mmol of 1 to 5 in good
isolated yield (e.g., 69%). Syringe pump addition of a THF
solution of 1 to the Na/Hg mixture at 25 °C under 1 atm N2
was found to be optimal.
1
A H NMR spectrum of the crude product mixture showed
that 5 formed in ∼96% yield under the above conditions with
only trace impurities of nitride 4 and the free ligand HN(R)Ar.
Consistent with earlier observations, pure 5 showed an intense
ν
NN at 1761 cm-1 in THF. Dinitrogen complex [(THF)xNa][(N2)-
Mo(N[R]Ar)3] was scarlet in the presence of donor solvents
such as THF and OEt2. Upon trituration, pentane solutions of 5
acquired an orange color and the THF was effectively removed.
Recrystallization from pentane afforded the stable, solvent-free
derivative [(THF)xNa][(N2)Mo(N[R]Ar)3] (5; x ) 0) in 69%
yield. The latter species is likely to be dimeric in nature.41,44
(4) Synthesis and Structure of [Na(12-crown-4)2][(N2)Mo-
(N[R]Ar)3] (7). Solvent-free [(THF)xNa][(N2)Mo(N[R]Ar)3] (5;
x ) 0) was moderately soluble in pentane but precipitated upon
addition of 2 equiv of 12-crown-4. An intense violet solid was
easily isolated, which was insoluble in pentane, benzene, and
OEt2, reflecting its distinctly ionic nature, formulated as [Na-
(12-crown-4)2][(N2)Mo(N[R]Ar)3] (7). Complex 7 was solubi-
lized by tetrahydrofuran, and a single crystal suitable for an
X-ray diffraction study was obtained by storing a THF/pentane
solution at -35 °C. The solid-state structure confirmed assign-
ment of 7 as the salt [Na(12-crown-4)2][(N2)Mo(N[R]Ar)3].
A thermal ellipsoid representation of 7 is given in Figure 3.
The complex, displaying a terminally bound dinitrogen ligand,
adopts the expected geometry in which the dinitrogen ligand
resides on the pseudo-C3 axis of the Mo(N[R]Ar)3 template and
is centered within the C3 propeller of the tBu substituents. The
N(4)-N(5) bond distance of 1.171(14) Å seems to be shorter
than that found in (Ph[tBu]N)3Ti(µ-N2)Mo(N[R]Ar)3 (14, dNN
) 1.229(4) Å, vide infra) but the difference is not statistically
significant. This may reflect a greater degree of N2 reduction
in 14, further manifested by the respective νNN stretching
frequencies for the two complexes: 1761 cm-1 for 7 vs 1575
cm-1 for 14.
Figure 2. Reaction of 1 with dinitrogen (1 atm, 28 °C, THF solvent)
in the presence of sodium amalgam. (i) Na/Hg or 6 generated in situ;
(ii) compound 2 or possibly Hg. Note that intermediates 6 and 2 are
not observed. Conditions can be optimized for isolation of either 5 or
4 (see text).
upon factors including (i) the rate of stirring, (ii) the concentra-
tion of 1, and (iii) the quantity of Na/Hg employed.
Experiments resulting in virtually quantitative formation of
nitride 4 were those in which compound 1 was added all at
once to 1 equiv of Na/Hg in THF with moderate stirring. On
the other hand, slow addition of 1 to an excess of Na/Hg in
THF with rapid stirring gave rise to good yields of the anionic
N2 complex 5.
The reaction scheme of Figure 2 accounts for the observation
that high concentrations of 1 lead via reaction of 1 with 5 (in
the presence of an oxidant) to good yields of nitride 4. On the
other hand, slow addition of 1 in the presence of an excess of
Na/Hg minimizes the pathway for conversion of 5 to 3, thereby
affording a route to good yields of dinitrogen complex 5. Rapid
stirring presumably increases the rate at which N2 is taken up.
Thus, the scheme of Figure 2 provides a tunable product
distribution via careful control of the reaction conditions.
To this point our discussion of the reactions in Figure 2 has
focused on experiments carried out at ∼28 °C under 1 atm of
dinitrogen. Our attention now turns to control experiments
carried out under argon.
(5) Ferrocenium Oxidation of [(THF)xNa][(N2)Mo(N[R]-
Ar)3] (5). With the key compound [(THF)xNa][(N2)Mo(N[R]-
(2) Treatment of Mo(N[R]Ar)3 with Sodium Amalgam
under Argon. Compound 1 was found to be stable to excess
sodium amalgam under an argon atmosphere. Less than 5%
decay of 1 was observed in THF over a period of 24 h at 25 °C
(44) Fickes, M. G.; Odom, A. L.; Cummins, C. C. Chem. Commun. 1997,
1993.

10056 J. Am. Chem. Soc., Vol. 121, No. 43, 1999
Peters et al.
Figure 4. Cyclic voltammogram of 5 in THF with [N(n-Bu)4][PF6]
electrolyte. Potentials indicated are vs ferrocene/ferrocenium. See text
for assignments.
SiNCH2CH2)3N]MotN was measured to be -2.9 V (THF,
[N(n-Bu)4][PF6], vs ferrocene/ferrocenium).
(6) Electrochemistry of Mo(N[R]Ar)3 (1) and [(THF)xNa]-
[(N2)Mo(N[R]Ar)3] (5). The inherent instability of neutral (N2)-
Mo(N[R]Ar)3 (2) prompted us to utilize another technique for
its study. Due to the inherent redox nature of the Na/Hg-
facilitated N2 cleavage reaction, an electrochemical study was
deemed appropriate.
Cyclic voltammetry of a THF solution of [(THF)xNa][(N2)-
Mo(N[R]Ar)3] (5; see Figure 4) revealed a distinct oxidation
wave at -1.7 V. No return wave was observable at scan rates
ranging from 0.1 to 10 V/s, indicating that the oxidation product
was unstable and had decayed completely before the scan was
reversed. This irreversible process is assigned to the oxidation
of the anionic component of [(THF)xNa][(N2)Mo(N[R]Ar)3] (5)
to generate neutral (N2)Mo(N[R]Ar)3 (2). The latter quickly
decays through loss of N2 to afford Mo(N[R]Ar)3 (1). On the
basis of this result, the t1/2 for neutral 2 appears to be less than
1 s.
A reduction wave was observed at -2.9 V. Through
successive scans it was found that the wave at -1.7 V gradually
decayed while the wave at -2.9 V increased, confirming that
the latter belongs to the decomposition product of 2, namely 1.
Independent electrochemical study of a solution of 1 cor-
roborated the assignment of the -2.9-V wave. A further
noteworthy feature of the -2.9-V wave is that it was found to
be irreversible. The latter observation is suggestive of fast uptake
of N2 (or, indeed, some other chemical process) by a highly
reactive reduced molybdenum(II) species “[Mo(N[R]Ar)3]-”.
Exhaustive electrolysis of a solution of 1 confirmed that the
final product, under a dinitrogen atmosphere, contained the
anionic component of 5. When a potential of -3.0 V was
maintained for extensive periods, the solution color changed
from the bright orange of 1 to the scarlet of 5. At this point, the
current had reached a plateau and 1 equiv of electrons (based
on 1) had passed through the cell. The electrochemical response
confirmed the presence of the anionic component of 5. As
expected, returning this solution to a potential of -1.3 V
reverted the process, oxidizing 5 and regenerating the orange
solution of 1. Thus, the electrochemistry of 1 and 5 dramatically
illustrates the redox chemistry outlined in Figure 2.
Figure 3. Structural drawing of 7 with ellipsoids at the 35% probability
level. Selected bond distances (Å) and angles (deg): Mo(1)-N(1),
1.994(12); Mo(1)-N(2), 2.016(11); Mo(1)-N(3), 1.999(11); Mo(1)-
N(4), 1.840(11); N(4)-N(5), 1.171(14); N(4)-Mo(1)-N(3), 98.8(4);
N(4)-Mo(1)-N(1), 97.1(5); N(4)-Mo-N(2), 96.7(4); N(3)-Mo(1)-
N(2), 118.8(5); N(3)-Mo(1)-N(1), 116.5(5); N(1)-Mo(1)-N(2),
119.6(5).
Ar)3] (5) in hand, it was possible to access the neutral N2
complex 2 from the other side of the proposed equilibrium for
N2 binding (Figure 1). Ferrocenium triflate45 ([Cp2Fe][OTf])
was selected as an appropriate one-electron oxidant. Fast
addition of a scarlet solution of 5 to an intense blue THF solution
of [Cp2Fe][OTf] effected the rapid oxidation of 5.
Effervescence was observed upon mixing, indicating dis-
sociation of the coordinated dinitrogen ligand. FTIR spectra of
the crude mixture acquired just after mixing showed that the
intense νNN band of 5 at 1761 cm-1 had completely decayed to
baseline. No stretches were observed that might have cor-
responded to the neutral dinitrogen complex 2 in question. A
2H NMR spectrum of the crude mixture corroborated this by
showing that Mo(N[R]Ar)3 had been generated cleanly and
quantitatively by the chemical oxidation. The oxidation pro-
ceeded analogously when carried out at -35 °C. It is apparent
that 5 spontaneously and rapidly loses dinitrogen upon chemical
oxidation and that neutral 2 is not a chemically isolable species.
Interestingly, O’Donoghue and Schrock reported that oxida-
tion of the related complex {[(Me3SiNCH2CH2)3N]Mo(N2)}2-
Mg(THF)2 gave a moderately stable dinitrogen adduct [(Me3-
SiNCH2CH2)3N]Mo(N2).26,27 This complex was structurally
characterized and formulated as a low-spin neutral dinitrogen
complex of molybdenum(III). Apparently, the apical donor atom,
provided by the triamidoamine ligand and absent in our
trisamido system, provides just enough electron density at the
metal center to make the N2 adduct of the [(Me3SiNCH2CH2)3N]-
Mo core stable.9 Electrochemical experiments comparing the
two systems suggest that the trianionic (Me3SiNCH2CH2)3N
ancillary ligand set is appreciably more reducing than the
trisanilido ligand set employed here. For example, the reduction
potential of NtMo(N[R]Ar)3 (4) was found to be -2.7 V,
whereas the reduction potential for the related complex [(Me3-
Fruitless attempts have been made to prepare synthetically a
reduced trisanilido molybdenum species “[Mo(N[R]Ar)3]-” in
the absence of N2 using solid sodium as the reductant. An
important subtlety of the chemistry described here is that 1 is
not itself reduced by 0.5% Na/Hg. Electrochemically, 1 is
reduced at -2.9 V, at which point it picks up dinitrogen. When
subjected to less reducing conditions (0.5% Na/Hg can be
estimated at ∼-2.4 V vs ferrocene/ferrocenium in THF), the
data indicate that 5 is formed by reduction of 2, the potential
for which is a mere -1.7 V.
(45) Sharp, P. R.; Schrock, R. R. J. Am. Chem. Soc. 1980, 102, 1430.

Molybdenum N-tert-Hydrocarbylanilide Complexes
J. Am. Chem. Soc., Vol. 121, No. 43, 1999 10057
(7) Reaction of Mo(N[R]Ar)3 (1) with [(THF)xNa][(N2)-
Mo(N[R]Ar)3] (5). The foregoing results establish that dini-
trogen cleavage is dramatically accelerated at ambient temper-
atures by the presence of 0.5% Na/Hg. It is presumed (Figure
2) that salt 5 reacts directly with 1 in solution to generate
dinuclear 3, with the aid of an oxidant. With the key compound
5 having been isolated, this model became subject to direct
testing.
Mixing stoichiometic amounts of 1 and 5 in THF under a
dinitrogen atmosphere at 25 °C resulted in a steady buildup of
intense purple 3 within 1 h (2H NMR). IR spectroscopy
confirmed that a significant amount of 5 was present throughout
the course of the reaction.
Figure 5. Cyclic voltammogram of 3 in THF with [N(n-Bu)4][PF6]
electrolyte. Potentials indicated are vs ferrocene/ferrocenium. See text
for assignments.
Analysis of the final reaction products by 1H NMR spectros-
copy after 14 h showed a mixture of NtMo(N[R]Ar)3 and 5 in
an approximate 2:1 ratio. Inclusion of an internal integration
standard indicated that ∼90% of the starting materials were
accounted for by 1H NMR spectroscopy. Additionally, a small
amount of free ligand, HN(R)Ar, was observed. Notably, the
reaction proceeded in similar fashion even when carried out in
the presence of added mercury.
The described reactions demonstrate clearly that 5 reacts
readily with Mo(N[R]Ar)3, (1), to generate (µ-N2){Mo(N[R]-
Ar)3}2 (3), en route to the irreversible formation of NtMo-
(N[R]Ar)3 (4). This is as depicted in Figure 2. Hence, reactions
in which the redox catalyst [(THF)xNa][(N2)Mo(N[R]Ar)3] (5)
is formed in the presence of 1 are expected to generate the nitride
product 4.
(8) Effect of Other Reducing Agents. In our efforts to
explore the diverse reaction chemistry of Mo(N[R]Ar)3, several
one-electron reductants, in addition to Na/Hg, have been
observed to facilitate the dinitrogen cleavage reaction. A
particularly clean instance of this occurred upon interaction of
sodium triethylborohydride (superhydride) with 1 under N2.
When a homogeneous THF solution of Mo(N[R]Ar)3 and an
excess of superhydride were stirred under an atmosphere of
dinitrogen at 25 °C, nitride 4 was formed quantitatively over a
period of 24 h. 1H NMR and IR spectroscopies of the final crude
mixture showed neither [(THF)xNa][(N2)Mo(N[R]Ar)3] (5) nor
any free HN(R)Ar ligand. As was the case for the Na/Hg-
facilitated cleavage reaction, the intense purple of the N2-bridged
intermediate 3 was observed during the course of the reaction.
The purple color dissipated gradually over 24 h, consistent with
its decay to NtMo(N[R]Ar)3.
Undoubtedly, a plethora of reaction pathways may be
postulated to account for formation of the observed nitride
product in the superhydride-mediated reaction. To date, no
intermediates have been observed.
(9) Attempted Synthesis of [(THF)xNa][(µ-N2){Mo(N[R]-
Ar)3}2] (6). A salt such as [(THF)xNa][(µ-N2){Mo(N[R]Ar)3}2]
(6) as invoked in Figure 2, seemed a likely intermediate in the
generation of (µ-N2){Mo(N[R]Ar)3}2 from the bimolecular
reaction between 1 and 5. It was therefore of interest to discover
whether a dinitrogen-free THF mixture of 5 and Mo(N[R]Ar)3
would contain detectable concentrations of 6.
To address this question, rigorously degassed THF was
vacuum transferred into an NMR tube containing stoichiometric
amounts (as solids) of 5 and 1. 2H NMR spectra acquired after
mixing showed no evidence for the presence of 6. Both starting
materials, 5 and 1, were observed, in addition to a small amount
(<10%) of the neutral bridged complex (µ-N2){Mo(N[R]Ar)3}2.
A similar experiment carried out with the crown-containing salt
7 and Mo(N[R]Ar)3 likewise did not show signals indicative of
formation of an N2-bridged dimolybdenum anion.
Though its intermediacy remains likely, [(THF)xNa][(µ-N2)-
{Mo(N[R]Ar)3}2] (6) does not appear to be an isolable or
observable complex, at least under the reaction conditions of
the redox-facilitated cleavage reaction. Thus, it may be con-
cluded that 6 dissociates readily to 1 and 5 in the absence of a
(N2)Mo(N[R]Ar)3 oxidant (Figure 2). As described below, the
electrochemical data support this notion.
(10) Electrochemistry of (µ-N2){Mo(N[R]Ar)3}2 (3). In the
cyclic voltammogram of a freshly prepared THF solution of
(µ-N2){Mo(N[R]Ar)3}2 (3) (see Figure 5), a cathodic wave at
-2.4 V is clearly in evidence. It was not possible to detect the
corresponding anodic wave at scan rates between 0.1 and 10
V/s. However, a new anodic peak was detected at -1.7 V
concomitant with the irreversible reduction process at -2.4 V.
The peak at -1.7 V also displayed a lack of reversibility and is
readily assigned to oxidation of the dinitrogen complex anion
5, an oxidation that we have already analyzed chemically
(section 5). Repetitive reductive scans effected a decrease in
the magnitude of the wave at -2.4 V and a corresponding
increase in the wave at -1.7 V.
Thus may be assigned the cathodic wave at -2.4 V to the
reductive conversion of 3 to its corresponding unstable anion,
the same N2-bridged dimolybdenum anion considered above (6,
section 9). Consistent with the chemical instability of dinuclear
anion 6, the species was unstable on the electrochemical time
scale, splitting apart into the mononuclear anionic component
of 5 along with neutral Mo(N[R]Ar)3 (1).
Dinitrogen anion 5 was indeed detected at -1.7 V subsequent
to a sequence of reductive scans. On the basis of its independent
electrochemical investigation described above (section 6), we
know that 1 itself is not electroactive through the potential
window under consideration here. When more positive anodic
potentials were investigated, a reversible wave at -1.2 V was
detected. The intensity of the latter wave decreased along with
consumption of 3, as judged by decay of the peak at -2.4 V.
Thus we assign the -1.2-V anodic wave to the electrochemically
reversible conversion of 3 to its corresponding monocation, an
entity that has not yet been subjected to attempted synthesis
and characterization.
(11) Observation of Intermetal N2- Transfer. Having been
unable directly to observe [(THF)xNa][(µ-N2){Mo(N[R]Ar)3}2]
(6; sections 9 and 10) it was desirable to secure further indirect
evidence for its proposed intermediacy in the reaction scheme
of Figure 2.
Because the deuterated anilide ligand N(R)Ar (R ) C(CD3)2-
CH3, Ar ) 3,5-C6H3Me2) can be prepared readily in its natural-
isotopic-abundance form, e.g., N(tBu)Ar, it was possible to do
a crossover experiment.
2H NMR spectroscopy showed clearly that Mo(N[R]Ar)3 (1)
was generated upon mixing a THF solution of stoichiomotetric
Mo(N[tBu]Ar)3 and 5 under vacuum (Figure 6). Integration of

10058 J. Am. Chem. Soc., Vol. 121, No. 43, 1999
Peters et al.
Figure 7. Reaction of 1 with Ti(N[tBu]Ph)3 (13) to give the
molybdenum/titanium µ-dinitrogen complex 14. The high-yield forma-
tion of 14 (25 °C, 1 atm N2, 4 h) suggests that 13 is a far more efficient
trap of putative 2 than is 1 itself. The data do not, however, rule out
initial binding of N2 by 13.
Figure 6. Reaction in THF and in the absence of N2 of Mo(N[tBu]-
Ar)3 with [(THF)xNa][(N2)Mo(N[R]Ar)3]. The reaction constitutes an
example of degenerate [N2]- transfer. Detection of progress in the
2
forward direction is by H NMR spectroscopy.
the 2H NMR signals for the respective N(R)Ar ligands revealed
that ∼38.5% of the deuterium-labeled 5 was converted to
deuterium-labeled 1.
the consequences of the interception of 2 by the reducing agent,
sodium amalgam. Now, it seemed likely that reducing agents
other than sodium amalgam might also be capable of intercept-
ing 2. A sterically demanding titanium(III)-based reducing agent
we have studied in the past is the complex Ti(N[tBu]Ph)3 (13).56
In this section is described the cooperative binding of N2 by 1
and 13.
In accord with the above observation, 1H NMR spectroscopy
confirmed that [(THF)xNa][(N2)Mo(N[tBu]Ar)3] (9) also was
generated. Control experiments have shown that 5 is stable for
days in THF at 25 °C with no appreciable decomposition to
three-coordinate 1. Hence, transfer of the dinitrogen ligand from
5 to Mo(N[tBu]Ar)3 (8) is best explained by the intermediate
formation of an anionic N2-bridged species, [(Ar[t-Bu]N)3Mo-
(µ-N2)Mo(N[R]Ar)3]-, identical to 6 but for the consideration
of deuterium substitution.
A 1:1 mixture of 1 and 1356 were stirred in OEt2 under a
dinitrogen atmosphere at 25 °C, giving initially an olive-green
solution which gradually lightened in color. Over the course of
a 15-h period, a deep-orange solid determined to be (Ph[t-
Bu]N)3Ti(µ-N2)Mo(N[R]Ar)3 (14) precipitated and was isolated
in 81% yield (Figure 7).
Despite repeated attempts to generate an ideal 1:1 mixture
2
of 5 and 9, the H NMR spectra of such solutions typically
Complex 14 was found to be robust both in solution and in
the solid state, as expected given the π8 electronic configuration
for this system. The designation π8 refers to the fact that in C3V
symmetry the π molecular orbitals for the linear Mo-N-N-
Ti moiety occur as four increasingly energetic doubly degenerate
levels, which in the Ti/Mo case are populated with eight
electrons. The corresponding dimolybdenum complex 3 carries
a corresponding π10 designation, accounting for its paramagnet-
ism and (in part) its propensity to split to 2 equiv of 4. Other
examples of stable π8 systems are the linear nitrosyl complex
(ON)Mo(N[R]Ar)3,57 the isoelectronic Mo-N2 anion in 5, and
the terminal phosphorus monoxide complex (OP)Mo(N[R]-
Ar)3.58 Note that 2 equiv of 4 together constitutes a π8 system.
A further interesting way to think about these systems is to note
that, while d3 1 can be regarded as isolobal with a quartet-state
nitrogen atom, the π8 systems are isolobal with N2O, azide ion,
and CO2. The very stable terminal nitride 4 is isolobal with
dinitrogen itself.
showed that a small amount of dinuclear 3 (∼7%) had been
generated. As mentioned above (section 7), this was also
observed when 5 was mixed with 1.
Note that the result summarized by Figure 6 constitutes an
example of intermetal dinitrogen monoanion (N2-) transfer. A
conceivable extension of this type of reaction is to systems for
the cooperative reduction of N2 by two different metal com-
-
plexes or to catalytic systems in which the transfer of N2
provides a means for regenerating 1. The transfer of N2- from
one metal complex to another is reminiscent of intermetal
nitrogen atom-transfer reactions, a class of reactions that have
undergone intense scrutiny in recent years.46-55
(12) A Titanium/Molybdenum Heterodinuclear Dinitrogen
Complex. In effect, the new chemistry of Figure 2 represents
(46) Woo, L.; Goll, J. J. Am. Chem. Soc. 1989, 111, 3755.
(47) Woo, L.; Goll, J.; Czapla, D.; Hays, J. J. Am. Chem. Soc. 1991,
113, 8478.
(48) Woo, L. Chem. ReV. 1993, 93, 1125.
1H NMR spectroscopy supports our formulation for 14, it
being the case that two inequivalent ligand environments are
(49) Bottomley, L.; Neely, F. J. Am. Chem. Soc. 1989, 111, 5955.
(50) Neely, F.; Bottomley, L. Inorg. Chim. Acta 1992, 192, 147.
(51) Tong, C.; Bottomley, L. Inorg. Chem. 1996, 35, 5108.
(52) Bottomley, L.; Neely, F. Inorg. Chem. 1997, 36, 5435.
(53) Neely, F.; Bottomley, L. Inorg. Chem. 1997, 36, 5432.
(54) Laplaza, C. E.; Johnson, A. R.; Cummins, C. C. J. Am. Chem. Soc.
1996, 118, 709.
(56) Kim, E.; Odom, A. L.; Cummins, C. C. Inorg. Chim. Acta 1998,
278, 103.
(57) Laplaza, C. E.; Odom, A. L.; Davis, W. M.; Cummins, C. C.;
Protasiewicz, J. D. J. Am. Chem. Soc. 1995, 117, 4999.
(58) Johnson, M. J. A.; Odom, A. L.; Cummins, C. C. Chem. Commun.
1997, 1523.
(55) Johnson, M. J. A.; Lee, P. M.; Odom, A. L.; Davis, W. M.;
Cummins, C. C. Angew. Chem., Int. Ed. Engl. 1997, 36, 87.

Molybdenum N-tert-Hydrocarbylanilide Complexes
J. Am. Chem. Soc., Vol. 121, No. 43, 1999 10059
are directed proximal to the N2 molecule, a typical conforma-
tional motif for X-M(N[R]Ar)3 complexes.3 The ligands adopt
a moderately staggered disposition, with an average N-Ti-
Mo-N dihedral angle of ∼18°. The Mo-N(7) bond length of
1.787(3) Å is indicative of substantial π bonding to the
dinitrogen ligand by the Mo(N[R]Ar)3 portion of the complex.
This distance is significantly shorter than the Mo-N bonding
distances to the anilido ligands (average 1.968 Å). The Ti-
N(8) distance of 1.880(3) Å suggests a lesser degree of π
bonding between the Ti(N[tBu]Ph)3 fragment and the N2 ligand,
but this distance nonetheless is appreciably shorter than the
average Ti-Nanilide distance of 1.954 Å. The N(8)-N(7) bond
length of 1.229(4) Å reflects an appreciable degree of coopera-
tive dinitrogen reduction by the respective trisanilido molyb-
denum and titanium fragments. By way of comparison, the N-N
distance in 3 is ∼1.20 Å, a distance identified with an N-N
bond order of 2.
In an important control experiment, the trisanilido complexes
13 and 1 were found not to react with each other when stirred
in OEt2 at 25 °C in the absence of dinitrogen (1 atm of argon).
While it is known that low-valent N-donor-stabilized complexes
of titanium can exhibit dinitrogen chemistry,60,61 studies of the
Figure 8. A 35% thermal ellipsoid representation of (Ph[tBu]N)3Ti-
(µ-N2)Mo(N[R]Ar)3, 14, from X-ray coordinates. Selected distances
(Å) and angles (deg): Mo-N(7), 1.787(3); Ti-N(8), 1.880(3); N(7)-
N(8), 1.229(4); Mo-N(1), 1.969(3); Mo-N(3), 1.971(3); Mo-N(5),
1.963(3); Ti-N(2), 1.956(3); Ti-N(4), 1.951(3); Ti-N(6), 1.954(3);
N(7)-Mo-N(1), 102.59(13); N(7)-Mo-N(3), 103.17(13); N(7)-Mo-
N(5), 104.03(13); N(5)-Mo-N(1), 113.23(13); N(5)-Mo-N(3),
115.22(13); N(1)-Mo-N(3), 116.25(13). N(8)-Ti-N(6), 109.87(87);
N(8)-Ti-N(2), 111.11(13); N(8)-Ti-N(4), 109.98(13); N(4)-Ti-
N(6), 109.09(13); N(4)-Ti-N(2), 108.80(13); N(6)-Ti-N(2), 107.93-
(13).
solution chemistry of 1356 and related Ti(N[R]Ar)3 have not
40
revealed any independent propensity for reactivity with dini-
trogen. Similarly, dinitrogen chemistry was not reported for
Wolczanski’s siloxide Ti(OSitBu3)3.62-64 Taken collectively,
these results indicate that the formation of 14 (Figure 7) results
from the capture of (N2)Mo(N[R]Ar)3 (2) by reactive Ti(N[t-
Bu]Ph)3 (13). Like the reductant sodium amalgam, 13 efficiently
intercepts 2 at 25 °C on a time scale much faster than formation
(Figure 1) of the dimolybdenum N2 complex 3.
(13) Synthesis and Structure of (Me3SiNN)Mo(N[R]Ar)3
(12). The synthesis of 12 was accomplished by trapping the in
situ generated anionic dinitrogen complex with the electrophile
chlorotrimethylsilane. The optimized procedure involved addi-
tion of complex 1 to a THF solution of ClSiMe3 while stirring
over sodium amalgam under dinitrogen (1 atm; see Figure 9).
Allowing the reaction to proceed for 2 h prior to filtration led
to the best results. Silyldiazenido complex 12 is readily obtained
in pure form upon crystallization from pentane solution. An
alternative approach to the synthesis of (Me3SiNN)Mo(N[R]-
Ar)3 was found to involve the addition of equimolar amounts
of 5 and ClSiMe3 to a vessel containing ether.
in evidence in a 1:1 ratio. Broad resonances observed at room
temperature for the ortho protons of the 3,5-C6H3Me2 residues
of 14 are taken to be a reflection of the gearing effect involving
t
the six Bu groups inhibiting rapid interconversion of two
degenerate C3V enantiomers.55 Along these lines, note that low-
temperature 1H NMR spectra of X-M(N[R]Ar)3 complexes are
consistent with C3 symmetry,55 with three aryl resonances
observed in a 1:1:1 ratio for the Ar substituents. The two
resonances for the ortho protons coalesce to a single signal upon
warming. Indeed, in the case of 14, the broad ortho signals
sharpened upon warming.
An FTIR spectrum of 14 confirmed the presence of the
bridging dinitrogen ligand with a νNN at 1575 cm-1, to be
compared with the 1630-cm-1 νNN observed by Raman spec-
troscopy for 3.2 Because this region of the spectrum is
complicated by the presence of rather intense anilido νCC
vibrations, a 15N2-labeled derivative 15, (Ph[tBu]N)3Ti(µ-15N2)-
Mo(N[R]Ar)3, was prepared. A shift in νNN from 1575 to 1524
cm-1 was consistent with the simple harmonic oscillator model
(calculated 1522 cm-1). The 15N NMR spectrum of 15 showed
doublets at 437.15 and 433.09 ppm relative to liquid ammonia
(0 ppm) with a 1JNN of 12.6 Hz. These shifts can be compared
with the large downfield 15N NMR shifts found for terminal
nitrido derivatives of chromium59 and molybdenum.54
Heterodinuclear 14 was crystallized from pentane, and single
crystals suitable for an X-ray diffraction study were obtained
from a pentane solution stored at -35 °C. A thermal ellipsoid
representation of 14 is presented in Figure 8. The complex
displays a dinitrogen ligand suspended linearly between the Mo
and Ti centers. Both metal centers adopt a pseudotetrahedral
geometry, and the N2 ligand rests in the aliphatic pocket
provided by the six tert-butyl groups. The tert-butyl groups all
Infrared spectra (THF, KBr) of 12 showed a signature band
at 1650 cm-1 for the νNN vibration of the N2 ligand.
X-ray diffraction studies were carried out using a single
crystal of silyldiazenido 12 grown from a chilled pentane
solution. A thermal ellipsoid representation of 12 is presented
in Figure 10. The complex displays a dinitrogen ligand
suspended between the Mo and Si centers. The Mo-N(4) bond
length of 1.753(2) Å is indicative of substantial π bonding to
the dinitrogen ligand by the Mo(N[R]Ar)3 fragment. This
distance is significantly shorter than the Mo-N bonding
distances to the anilido ligands (average 1.970 Å). The N(4)-
N(5) bond length of 1.221(3) Å reflects an appreciable degree
of dinitrogen reduction relative to free dinitrogen and taken
together with the observed νNN frequency indicates an N-N
(60) Duchateau, R.; Gambarotta, S.; Beydoun, N.; Bensimon, C. J. Am.
Chem. Soc. 1991, 113, 8986.
(61) Hagadorn, J.; Arnold, J. Inorg. Chem. 1997, 36, 2928.
(62) Covert, K. J.; Neithamer, D. R.; Zonnevylle, M. C.; LaPointe, R.
E.; Schaller, C. P.; Wolczanski, P. T. Inorg. Chem. 1991, 30, 2494.
(63) Covert, K. J.; Wolczanski, P. T.; Hill, S. A.; Krusic, P. J. Inorg.
Chem. 1992, 31, 66.
(59) Odom, A. L.; Cummins, C. C.; Protasiewicz, J. D. J. Am. Chem.
Soc. 1995, 117, 6613.
(64) Covert, K. J.; Mayol, A.-R.; Wolczanski, P. T. Inorg. Chim. Acta
1997, 263, 263.

10060 J. Am. Chem. Soc., Vol. 121, No. 43, 1999
Peters et al.
Figure 11. Skeletal drawing of 1 and 16. Whereas 1 forms the
µ-dinitrogen complex 3, an analogous dinuclear N2-bridged species
derived from 16 does not form; this result is attributable to the increased
steric requirements of the 1-adamantyl groups in 16. Ar ) 3,5-C6H3-
Me2.
marginally shorter (by ∼0.05 Å) than that found for [N3N]MoN2-
SiMe3.
(14) Methylation of the Dinitrogen Ligand. The cationic
dimethylhydrazido complex [(Me2NN)Mo(N[R]Ar)3][OTf] (21)
was synthesized by adding the silyldiazenido precursor 12 to a
pentane solution containing 7 equiv of methyl triflate. Reaction
occurs with precipitation of the red-orange product 21 over a
period of 6 h. A 79% yield of pure diamagnetic 21 was obtained
in one crop.
Figure 9. One-pot syntheses of 12 and 22 by silylation or methylation,
respectively, of 5 generated by in situ reduction of the dinitrogen adduct
of 1. Note that the success of this procedure relies on the fact that the
electrophile (EX ) ClSiMe3 or MeOTs) reacts at a negligible rate with
the reductant (0.5% Na/Hg) under the reaction conditions (1 atm of
N2, 28 °C, THF solvent).
A corresponding monomethylated complex, namely, the
neutral species (MeNN)Mo(N[R]Ar)3 (22), also has been
synthesized and characterized (see Figure 9). The first prepara-
tion of methyldiazenido 22 comprised a one-pot synthesis in
which Mo(N[R]Ar)3 was added slowly to a rapidly stirred
solution of excess methyl tosylate in THF over sodium amalgam.
This procedure provides the desired monomethylated product;
however, its separation from impurities proved somewhat
troublesome using this regimen. Therefore a second, preferred
procedure was developed, involving the reaction of preisolated
9 with 1 equiv of methyl tosylate in OEt2. The latter reaction
was run for 2 h, and it produced the desired compound in
spectroscopically and analytically pure form. The IR spectrum
of 22 revealed a νNN at the relatively low frequency of 1538
cm-1. It should be noted that the one-pot syntheses of the silyl-
and methyldiazenido derivatives 12 and 22 are interesting in
that the reactions involve both N2 uptake and N-E (E ) SiMe3
or Me) bond formation. Additionally, both of the successful
one-pot procedures employ electrophiles that fail to react with
complex 1 and also fail to react with sodium amalgam under
the conditions of the experiment, while the in situ generated
anionic N2 complex is readily derivatized by these electrophiles.
(15) An N-Adamantyl-Substituted Three-Coordinate Mo-
lybdenum(III) Complex. The successful trapping (section 12)
of transient (N2)Mo(N[R]Ar)3 (2) by the titanium(III) reductant
13 was an advance consistent with the interpretation given above
(section 3) for the formation of 5. The chemistry of Mo(N[R]-
Ar)3 (1) in the presence of N2 and sodium amalgam is complex
and quite dependent upon the reaction conditions, primarily
Figure 10. Structural drawing of (Me3SiNN)Mo(N[R]Ar)3 (12) with
ellipsoids at the 35% probability level. Selected distances (Å) and angles
(deg): Mo-N(1), 1.977(2); Mo-N(2), 1.970(2); Mo-N(3), 1.970(2);
Mo-N(4), 1.753(2); N(4)-N(5), 1.221(3); N(5)-Si, 1.678(3); N(4)-
Mo-N(3), 100.68(9); N(4)-Mo-N(2), 99.21(9); N(3)-Mo-N(4),
115.98(8); N(4)-Mo-N(1), 99.85(9); N(5)-N(4)-Mo, 173.7(2);
N(4)-N(5)-Si 157.0(3).
t
because the size requirements of the Bu substituents are not
sufficient to rule out dinuclear binding of N2 (Figure 2).
Providing the anilido ligands with an even greater degree of
steric bulk was to allow the clean generation of a molybdenum
dinitrogen anion species without the complication of bimetallic
reaction pathways (see Figure 11 for a skeletal structural
comparison between the two systems).
A preparation of the new three-coordinate molybdenum
complex Mo(N[Ad]Ar)3 (16) (Ad ) 1-adamantyl,12,13 Ar ) 3,5-
C6H3Me2) was developed as a modification of the synthetic
protocol2 for 1.
bond order of ∼2. Recently, Schrock et al.26 synthesized and
structurally characterized an analogous silyldiazenido compound,
[N3N]MoN2SiMe3, where [N3N] ) [(Me3SiNCH2CH2)3N]3-
.
The reported core bond distances for [N3N]MoN2SiMe3 were
as follows: Mo-NR ) 1.803(7) Å; N-N ) 1.206(9) Å; and
Nâ-Si ) 1.670(9) Å. Although the N-N and Nâ-Si bond
distances are essentially identical (within experimental error)
for the two systems, the Mo-NR bond distance for 12 is

Molybdenum N-tert-Hydrocarbylanilide Complexes
J. Am. Chem. Soc., Vol. 121, No. 43, 1999 10061
Figure 12. Structural drawing of Mo(N[Ad]Ar)3 (16) with ellipsoids
at the 35% probability level. Selected distances (Å) and angles (deg):
Mo-N(1), 1.989(3); Mo-(N2), 1.974(3); Mo-N(3), 1.958(3); N(1)-
C(11), 1.429(5); N(1)-C(17), 1.485(5); N(2)-C(21), 1.435(5); N(2)-
C(27), 1.481(5); N(3)-C(31), 1.450(5); N(3)-C(37), 1.484(5); N(3)-
Mo-N(2), 121.55(13); N(3)-Mo-N(1), 118.01(14); N(2)-Mo-N(1),
120.17(13); C(11)-N(1)-C(17), 122.3(3); C(11)-N(1)-Mo, 109.5-
(2); C(17)-N(1)-Mo, 128.1(2); C(21)-N(2)-C(27), 117.9(3); C(21)-
N(2)-Mo, 109.9(2); C(27)-N(2)-Mo, 132.2(2); C(31)-N(3)-C(37),
118.9(3); C(31)-N(3)-Mo, 110.0(2); C(37)-N(3)-Mo, 131.0(3).
Figure 13. Synthesis of 18 proceeding in a fashion similar to that for
5 (1 atm of N2, 28 °C), except that the adamantyl-substituted
molybdenum complex 16 need not be added slowly to the Na/Hg/THF
mixture. Putative 17 is not observed. The synthesis of 20 by addition
of benzoyl chloride to an isolated sample of 18 went in 45% isolated
yield (OEt2, -35 f 25 °C).
1). Inspection of Figure 12 indicates that steric factors are
prohibitive with regard to assembling two molecules of 16 about
a single dinitrogen molecule.
(16) Synthesis and Properties of Mo(N[Ad]Ar)3 (16; Ad
Accordingly, no special precautions are required with respect
to the handling of 16 under dinitrogen, and under no circum-
stances has 16 been observed to convert to its corresponding
molybdenum(VI) nitride as a consequence of simple exposure
to dinitrogen.
(17) Binding of N2 by Mo(N[Ad]Ar)3 (16) upon Na/Hg
Reduction. When a THF solution of 16 was stirred over 0.4%
Na/Hg under a dinitrogen atmosphere at 25 °C, the anionic
dinitrogen complex [(THF)xNa][(N2)Mo(N[Ad]Ar)3] (18) was
the major product (Figure 13). While the lack of deuterium
incorporation in the N(Ad)Ar ligands obviated 2H NMR
spectroscopy as a viable spectroscopic probe, it was found that
the formation of 18 was easily monitored by IR spectroscopy.
Dinitrogen complex 18 evinces two prominent νNN bands at
1783 and 1757 cm-1 in THF solution, and its formation was
determined to be complete within several hours.
The observation of two νNN bands for 18 was unexpected,
since only one band at 1761 cm-1 was observed under similar
conditions for 5. It is tempting to suggest that this phenomenon
arises as a consequence of the presence of two conformational
isomers being present in solution, isomers that are easily
discriminated on the fast time scale of IR spectroscopy. For
example, isomers of both C3 and Cs symmetries may be present
in significant concentrations in solution, despite the fact that
pseudo-C3 symmetry is often manifested in the solid state (see,
e.g., Figure 12).
Dinitrogen complex 18 is scarlet in the presence of donor
solvents such as THF or OEt2 but precipitates as an orange solid
when added to hydrocarbon solvents such as pentane or benzene.
1H NMR spectroscopy showed that this color change coincided
with complete or near-complete loss of the donor solvent
molecules which solvate the sodium cation. The orange, THF-
free solid was virtually insoluble in pentane, and its isolation
and purification as the solvent-free complex 18 (x ) 0) in ∼64%
yield was straightforward. Orange 18 (x ) 0) is stable in the
solid state, both under a dinitrogen atmosphere and under
) 1-Adamantyl). The synthesis of 16 entails addition of 2 equiv
of the lithium salt Li(N[Ad]Ar)12,13 to MoCl3(THF)3 in OEt2
65
to effect the desired salt-elimination reaction. After removal of
LiCl and excess molybdenum(III) chloride, the crude product
is extracted into a pentane/benzene mixture and filtered through
Celite to remove residual impurities including the free ligand
HN(Ad)Ar. The use of benzene serves selectively to solubilize
16, helping to separate it from the impurities. The purification
can be somewhat problematic because both 16 and HN(Ad)Ar
have limited solubility in pentane and in OEt2. However,
multigram quantities of 16 have been obtained reproducibly in
yields typically ranging from 35 to 50% based on the starting
lithium salt. Analytically pure samples of solid 16 are bright
yellow-orange, appreciably lighter in hue than 1.
A solution magnetic susceptibility measurement obtained by
the method of Evans66,67 for 16 was consistent with a quartet
ground state (µeff ) 3.96 µB), as expected.
Infrared spectra (CaF2, THF) of 16 showed two signature
bands at 1597 and 1583 cm-1 for the aryl νCC vibrations of the
N(Ad)Ar ligands. Compound 16 was stable in the solid state
for long periods at 25 °C and, once properly purified, provided
a sturdy manifold for the molybdenum(III) dinitrogen chemistry
of interest (vide infra).
Single crystals of 16 suitable for an X-ray diffraction study
were deposited from an OEt2 solution on standing at 25 °C. A
thermal ellipsoid representation of 16 is shown in Figure 12.
As anticipated, the molybdenum center adopts a trigonal planar
geometry and the small-molecule binding pocket is in a cage
consisting of the highly encumbering and preorganized ada-
mantyl substituents.
In stark contrast to observations involving 1, three-coordinate
16 proved to be stable indefinitely under the conditions for
conversion of 1 to 3 (1 atm of N2, toluene, -35 °C; see Figure
(65) Dilworth, J. R.; Zubieta, J. Inorg. Synth. 1986, 24, 193.
(66) Evans, D. F. J. Chem. Soc. 1959, 2003.
(67) Sur, S. K. J. Magn. Reson. 1989, 82, 169.

10062 J. Am. Chem. Soc., Vol. 121, No. 43, 1999
Peters et al.
SiNCH2CH2)3N]Mo(N2) core.26,27 The chemistry of such com-
plexes, which employ triamidoamine supporting ligands, was
reviewed recently.28
(19) Reaction of [(THF)xNa][(N2)Mo(N[Ad]Ar)3] (18) with
Electrophiles. Addition of ClSiMe3 to an ethereal solution of
18 unveiled the nucleophilic nature of this anionic dinitrogen
complex. A change in color from the scarlet of ethereal 18 to
bright yellow occurred upon mixing at 25 °C. Following salt
1
removal a H NMR spectrum of the crude product mixture
showed the expected resonance for a SiMe3 group and confirmed
a nearly quantitative conversion to the anticipated silyldiazenido
complex (Me3SiNN)Mo(N[Ad]Ar)3 (19).
An IR spectrum of 19 evinced a dramatic shift from the
intense νNN vibration at 1761 cm-1 for 18 to a very broad
absorbance centered at 1646 cm-1
.
The addition of benzoyl chloride to a cold ethereal solution
of 18 effected N-C bond formation with concomitant salt
elimination. The reaction was not quantitative, and the orange,
crystalline benzoyldiazenido product (PhC(O)NN)Mo(N[Ad]-
Ar)3 (20) was obtained in 45% yield from the green-tinted
reaction mixture (Figure 13). By analogy with the Curtius
rearrangement, it was thought that 20 might exhibit a tendency
to rearrange thermally to phenylisocyanate and NtMo(N[Ad]-
Ar)3. However, NtMo(N[Ad]Ar)3 was not observed and 20
was not consumed when a benzene solution of the latter was
maintained at 75 °C for 2 h.
The reactions of ClSiMe3 and benzoyl chloride with 18
represent examples of N-Si and N-C bond formation stem-
ming from a nucleophilic coordinated dinitrogen ligand.68,69
The use of transition metals to coordinate and activate
molecular nitrogen toward functionalization remains an intense
and challenging area of study. The construction of N-C and
N-Si bonds from dinitrogen bound to zero-valent d6 Mo and
W complexes has been reported for metal centers bearing
phosphine and macrocyclic thioether coligands. Chatt and co-
workers reported the first examples of N-C bond formation
from metal-coordinated dinitrogen in 1972.70 Hidai69 reported
the molecular structure of a group 6 benzoyldiazenido complex
obtained by functionalization of coordinated N2. Noteworthy
for comparison, the complexes trans-[M(N2)2(diphos)2] (M )
Mo, W) undergo reaction with iodotrimethylsilane at 50 °C in
benzene to afford silyldiazenido complexes, but chlorotrimeth-
ylsilane under otherwise identical conditions was unable to effect
the silylation.68
(20) Chemical Oxidation of [(THF)xNa][(N2)Mo(N[Ad]-
Ar)3] (18). As was the case for 5 (section 5), 18 was readily
oxidized by ferrocenium triflate. Such a proclivity towards redox
chemistry was also manifested in a brief study of the chemistry
of 18 as follows.
We were interested in the possibility of using simple salt-
elimination reactions to furnish connections between the
(Ar[Ad]N)3ModNdN- moiety and transition metal centers such
as Mo, Re, or W. Though this concept remains to be explored
in detail, initial probe reactions indicate that redox-susceptible
metal halides such as MoCl3(THF)3 can oxidize 18 to liberate
N2.
Figure 14. Structure of 18 with ellipsoids at the 35% probability level.
Selected bond distances (Å) and angles (deg): Mo-N(1), 2.003(5);
Mo-N(2), 2.002(5); Mo-N(3), 2.038(6); Mo-N(4), 1.830(8); N(4)-
N(5), 1.152(8); Na-O(1), 2.193(10); Na-O(2), 2.254(10); Na-O(3),
2.221(10); Na-N(5), 2.227(9); N(1)-Mo-N(3), 115.8(2); N(1)-Mo-
N(2), 119.1(2); N(2)-Mo-N(3), 119.9(2); N(4)-Mo-N(1), 98.3(2);
N(4)-Mo-N(2), 96.8(2); N(4)-Mo-N(3), 97.7(2); N(4)-N(5)-Na,
177.6(6); N(5)-N(4)-Mo, 179.2(6); O(1)-Na-N(5), 110.0(4); O(2)-
Na-N(5), 110.4(3); O(3)-Na-N(5), 113.(4).
vacuum. Its scarlet color is regenerated upon dissolution in OEt2
or THF. As was found to be the case for 1 (section 2), three-
coordinate 16 exhibited no reaction with 0.4% Na/Hg under an
argon atmosphere, suggesting that the N2-binding step precedes
reduction in the generation of 18 (as indicated for 1 in Figure
2).
(18) X-ray Structure of [(THF)xNa][(N2)Mo(N[Ad]Ar)3]
(18). An X-ray diffraction study on a single crystal of 18 (x )
3), grown from heptane/THF, revealed an ion-paired dinitrogen
complex in which a sodium cation is ligated by three THF
molecules in addition to the â-nitrogen atom of the terminally
coordinated N2 ligand, with a Na‚‚‚N(5) distance of 2.227(9)
Å (Figure 14). The dinitrogen ligand itself resides along the
pseudo-C3 axis of the Mo(N[Ad]Ar)3 fragment and does not
protrude from the completely adamantyl-caged binding site. The
structure observed for 18 (Figure 14) offers a forceful graphic
illustration of how the steric constraints of the Mo(N[Ad]Ar)3
system render the synthesis of 18 straightforward relative to
that of 5 (compare section 3), by preventing the assembly of
dimolybdenum N2-bridged systems.
While bimetallic complexes featuring a bridging dinitrogen
ligand are ubiquitous, structurally characterized examples where
one of the metal centers is an alkali metal are rare. Schrock et
al. reported in 1994 that, under appropriate conditions, Na/Hg
effected the transformation of a molybdenum(IV) triflato
complex to a corresponding anionic dinitrogen complex,
formulated with a ModNdN‚‚‚Na core.43 Although structural
data were not reported for the latter complex, the reaction
chemistry served to demonstrate nucleophilic character for the
â-nitrogen atom of the coordinated dinitrogen ligand. Silylation
and stannylation were achieved at the â-N atom upon reaction
with ClSi(iPr)3 and ClSn(nBu)3, respectively.
A few attempts were made to protonate 18. The hope was to
generate a parent diazenido species, (HNN)Mo(N[Ad]Ar)3, but
the attempts evidently resulted in undesired redox chemistry.
(68) Hidai, M.; Komori, K.; Kodama, T.; Jin, D. M.; Takahashi, T.;
Sugiura, S.; Uchida, Y.; Mizobe, Y. J. Organomet. Chem. 1984, 272, 155.
(69) Hidai, M.; Sato, M.; Kodama, T.; Uchida, Y. J. J. Organomet. Chem.
1978, 152, 239.
(70) Chatt, J.; Heath, G. A.; Leigh, G. J. J. Chem. Soc., Chem. Commun.
1972, 444.
Recently, O’Donoghue and Schrock have extended this work
to a host of related dinitrogen complexes bearing a [(Me3-

Molybdenum N-tert-Hydrocarbylanilide Complexes
J. Am. Chem. Soc., Vol. 121, No. 43, 1999 10063
Addition of sterically hindered 2,6-di-tert-butylphenol to an
ethereal solution of 18 resulted in a gradual color change from
scarlet to green. IR spectra showed complete consumption of
starting material 18 and no new νNN vibrations, consistent with
redox-triggered dissociation of N2. tert-Butyl iodide also was
investigated as a potential proton source. It was found, however,
that addition of this reagent to 18 also effected the ejection of
N2.
Figure 15. Numbering scheme for H and 13C NMR assignments.
1
Concluding Remarks
activated in vacuo overnight at a temperature above 180 °C. Compounds
1,2 13,40 ferrocenium triflate,45 and LiN(Ad)Ar12 were prepared ac-
cording to published procedures. 15N-Labeled N2 gas was purchased
form Cambridge Isotope Laboratory (CIL). ClSiMe3 and benzoyl
chloride were degassed and dried over molecular sieves (4 Å) prior to
use. Other chemicals were purified and dried by standard procedures
or were used as received. The syringe pump used was a KD Scientific
model 200 Jen Houser Series infusion pump. Infrared spectra were
recorded on a Bio-Rad 135 Series FTIR spectrometer. UV-visible
spectra were recorded on a Hewlett-Packard 8453 diode-array spec-
The chemistry of Mo(N[R]Ar)3 (1) and related complexes
of three-coordinate molybdenum(III) represents a unique chapter
in coordination chemistry and in the chemistry of dinitrogen.
Several reports have documented the binding and splitting of
dinitrogen by 1 in the absence of added reagents.1-3,35,54 Such
reactivity represents a solution-chemistry analogue of the rate-
determining step in Haber-Bosch nitrogen fixation,4-6 namely,
the dissociative adsorbtion of N2.
With the present work we have shown that the binding of N2
by 1 and its analogues is accelerated dramatically by the
judicious utilization of appropriate reducing agents. The elec-
trochemistry of several of the relevant dinitrogen complexes,
mononuclear and dinuclear, has been investigated thoroughly,
such that a detailed understanding is now available of the redox-
catalyzed splitting of N2 by 1. Furthermore, in the case of the
adamantyl-substituted system (section 16) it has been demon-
strated that steric control afforded by synthesis can be exerted
to shut down the dinitrogen-splitting pathway, which requires
dinuclear N2 complex intermediates.
1
trophotometer. H and 13C NMR spectra were recorded on a Varian
VXR-500, Varian XL-300, or Varian Unity-300 spectrometer. 1H and
13C NMR chemical shifts are reported with reference to solvent
resonances (residual C6D5H in C6D6, 7.15 ppm; C6D6, 128.0 ppm;
CHCl3 in CDCl3, 7.24 ppm; CDCl3, 77.0 ppm). 15N NMR spectra are
referenced as reported previously.59 19F NMR referenced against CFCl3,
0.0 ppm. 2H NMR chemical shifts are reported with respect to external
C6D6 (7.15 ppm). Solution magnetic susceptibilities were determined
by 1H NMR at 300 MHz using the method of Evans.66,67 Routine
coupling constants are not reported. Combustion analyses (C, H, N)
were performed by Microlytics (South Deerfield, MA) and Mikroana-
lytisches Labortorium (Mu¨lheim, Germany). X-ray diffraction data were
collected on a Siemens Platform goniometer with a charge coupled
device (CCD) detector. Structures were typically solved by direct
methods (SHELXTL V5.0, G. M. Sheldrick and Siemens Industrial
In the course of investigating the intricacies of the redox
chemistry of 1 in the presence of dinitrogen, syntheses of
formally molybdenum(II) anionic dinitrogen complexes have
been developed, and structural characterization by X-ray dif-
fraction has been achieved.
1
Automation, Inc., 1995) unless otherwise noted. Peaks in the H and
13C NMR spectra are denoted according to Figure 15.
A related advance is the development of rational high-yield
routes to heterodinuclear Mo/M (M ) Ti, U71) dinitrogen
complexes, arising as a consequence of experiments designed
to intercept the elusive neutral dinitrogen complex 2.
The chemistry of Figure 2 constitutes a succinct summary of
the basis for the wealth of fundamentally new dinitrogen
chemistry included in this paper. Among other things, the first
clear instance of intermetal dinitrogen-monoanion transfer has
been documented, a new principle for the high-yield assembly
of heterodinuclear N2 complexes has been espoused, and the
critical role of the state of charge with respect to N2 binding
has been made abundantly clear.
Electrochemical Measurements. Electrochemical measurements
were performed in THF solution containing the desired compounds
and 0.5 M [N(n-Bu)4][PF6]. In a typical procedure, 5 mg of the complex
was dissolved in 0.75 mL of THF. To the solution was added 0.75 mL
of a saturated THF solution of [N(n-Bu)4][PF6]. A platinum disk (1.6
mm diameter, Bioanalytical Systems), a platinum wire, and a silver
wire were employed as the working electrode, the auxiliary, and the
reference, respectively. The electrochemical response was collected with
the assistance of an Eco-Chemie Autolab potentiostat (pgstat20) and
the GPES 4.3 software. An IR correction drop was always employed
due to the high resistance of the solutions. A typical resistance value
measured with the positive feedback technique for these solutions was
975 Ω. All of the potentials are reported against the ferrocenium/
ferrocene couple measured in the same solution.
Reaction of Mo(N[R]Ar)3 (1) with Na/Hg (1 Equiv) under 1 atm
of N2. A 0.40% Na/Hg was prepared by dissolving Na metal (5.7 mg,
0.2479 mmol) in Hg (1.41 g, 7.03 mmol) in a 20-mL scintillation vial.
THF was added (5 mL) and the amalgam was stirred for 5 min, followed
by addition of solid orange 1 (173.4 mg, 0.2697 mmol) at 22 °C.
Vigorous stirring was continued and within 1 h the solution had acquired
an intense purple color, indicative of formation of 3, an interpretation
Experimental Section
General Considerations. Unless stated otherwise, all operations
were performed in a Vacuum Atmospheres drybox under an atmosphere
of purified nitrogen or using standard Schlenk techniques under an argon
or dinitrogen atmosphere. Anhydrous OEt2 and toluene were purchased
from Mallinckrodt; n-pentane and n-hexane were purchased from EM
Science. Ether was purified according to the procedure of Grubbs et
al.72 Aliphatic hydrocarbon solvents were distilled under a nitrogen
atmosphere from sodium/benzophenone ketyl solubilized with a small
quantity of tetraglyme. Distilled solvents were transferred under vacuum
into Teflon-stopcocked glass vessels and stored, prior to use, in a
Vacuum Atmospheres drybox. C6D6 was degassed and dried over
activated molecular sieves (4 Å) and transferred under vacuum into a
storage vessel. Molecular sieves (4 Å), Celite, and alumina were
2
confirmed by a H NMR spectrum acquired at this time. 1 had been
consumed after 1 h (∼3% remaining). An FTIR spectrum showed a
ν
NN band at 1761 cm-1 consistent with the presence of 5. Stirring was
continued for 7 h, by which time the purple color had faded and the
solution had turned orange-red. The solution was filtered through Celite,
and solvent was removed in vacuo from the filtrate, yielding an orange-
red solid. A 1H NMR spectrum of the orange solid showed it to consist
of 85% nitride 4 and 10% dinitrogen anion 5, the independent
characterization of which is described below.
Reaction of Mo(N[R]Ar)3 (1) with [Na][Et3BH] under 1 atm of
N2. Compound 1 (1, 61.8 mg, 0.0961 mmol) was dissolved in THF (2
mL) in a 20-mL scintillation vial equipped with a magnetic stir bar.
(71) Odom, A. L.; Arnold, P. L.; Cummins, C. C. J. Am. Chem. Soc.
1998, 120, 5836.
(72) Pangborn, A.; Giardello, M.; Grubbs, R.; Rosen, R.; Timmers, F.
Organometallics 1996, 15, 1518.

10064 J. Am. Chem. Soc., Vol. 121, No. 43, 1999
Peters et al.
Under a dinitrogen atmosphere, 480 mL of NaEt3BH (0.480 mmol, 1
M solution in toluene) was added directly to the stirring orange solution
at 22 °C. A gradual change in color resulting in an intense purple
solution was observed during the first few hours. After 24 h, the purple
color had dissipated and the solution was light-brown in color. Volatile
material was removed in vacuo, and spectroscopic analysis (1H NMR)
indicated quantitative formation of nitride 4 relative to an internal
integration standard of O(SiMe3)2.
of a single diamagnetic product exhibiting two ligand environments.
A small quantity of the free ligands HN(R)Ar and HN(tBu)Ph were
also discernable. The crude material was extracted into pentane (15
mL) and recrystallized (two crops) to afford 267 mg (81%) of deep-
orange, crystalline 14: 1H NMR (300 MHz, C6D6, 45 °C) δ 7.20 (m,
8′ and 5′), 7.09 (m, 8′ and 5′), 7.00 (m, 8′ and 5′), 6.73 (s, 5), 6.55 (br
s, 4), 5.92 (br s, 4′), 2.11 (s, 3), 1.56 (s, 1), 1.49 (s, 1′); 13C NMR
(125.66 MHz, C6D6, 25 °C) δ 154.00 (aryl), 148.78 (aryl), 137.26 (aryl),
132.27 (aryl), 128.97 (aryl), 127.69 (aryl), 124.86 (aryl), 65.24 (6 or
6′), 62.25 (6 or 6′), 32.33 (2), 32.07 (2′), 21.86 (3); UV-vis (OEt2) λ
306 (ꢀ ) 34 800), 449 nm (ꢀ ) 32 000); FTIR (CH2Cl2, CaF2) νNN
1575 cm-1. Anal. Calcd for C66H78D18N8MoTi: C, 68.13; H, 8.32; N,
9.63. Found: C, 68.31; H, 8.40; N, 9.54.
Reaction of Mo(N[R]Ar)3 (1) with Na/Hg under Argon. A 0.4%
Na/Hg was prepared in a 50-mL round-bottom flask equipped with a
side arm and stopcock by dissolving solid sodium (22 mg, 0.957 mmol)
in Hg (5.44 g). THF (8 mL) was added to the flask, followed by solid
1 (1 equiv) under argon counterflow. The vessel was sealed, and stirring
was continued under an Ar atmosphere for 22 h at 25 °C. The reaction
mixture retained an orange color throughout. After 22 h, the solvent
was removed in vacuo and the resulting orange residue was extracted
into pentane and filtered away from the amalgam. A 2H NMR spectrum
of this pentane solution showed unreacted 1 along with a small quantity
Synthesis of (Ph[tBu]N)3Ti(µ-15N2)Mo(N[R]Ar)3 (15). A 100-mL
vessel containing 15N2 gas at 1 atm (∼4.1 mmol) was attached to a
50-mL round-bottom Schlenk flask containing a stir bar. The two
vessels were separated by a break-seal. Ether (8 mL) was added to the
Schlenk flask, which was subsequently degassed. Under a counterflow
of argon, 200 mg of solid 1 (0.311 mmol) and 153 mg of 13 were
added to the Schlenk flask. The break-seal was broken with the stir
bar in order to expose the solution to 15N2. The reaction workup
followed the same protocol as that described above for 14: FTIR (CH2-
1
of a diamagnetic entity (δ ) 1.8 ppm). A H NMR spectrum of the
residue reconstituted with C6D6 showed the diamagnetic product to be
the free ligand HN(R)Ar. The estimated amount of HN(R)Ar was less
than 10%.
Cl2, CaF2) 1524 cm-1 (ν N, calcd 1522 cm-1); 15N NMR (50.65 MHz,
Reaction of Mo(N[R]Ar)3 (1) with Na under Argon. Sodium metal
(111 mg, 4.828 mmol) was weighed into a 50-mL flask equipped with
a side arm and stopcock. The sodium was spread around the base of
the flask into a thin film to expose a high degree of surface area. THF
(10 mL) was then added, and the mixture was stirred vigorously under
Ar for 5 min. Compound 1 was added as a solid all at once under Ar
counterflow, and the initially orange solution turned sienna within the
first 30 min of stirring. After 2.5 h, the mixture was dried in vacuo
and the resulting residue was extracted with pentane, the extract was
filtered through Celite, and volatile matter was removed from the filtrate
in vacuo. The process was repeated, and upon the second filtration,
185 mg of a black, hydrocarbon-insoluble solid was removed, affording
a dark orange filtrate. This filtrate was stored at -35 °C and decanted
twice away from a small amount of precipitate that had settled out.
The filtrate was dried in vacuo and a 1H NMR spectrum of the resulting
orange-brown solid was acquired (C6D6), the spectrum evincing
resonances consistent with one N(R)Ar ligand environment and
15 15
N
1
1
C6D6, 25 °C) δ 437.15 (d, JNN ) 12.6 Hz), 433.09 (d, JNN ) 12.6
Hz). 15NH2Ph was used as reference at 55 ppm relative to liquid
ammonia at 0 ppm.59
X-ray structure of (Ph[tBu]N)3Ti(µ-N2)Mo(N[R]Ar)3 (14). Deep
orange crystals were grown slowly from a pentane solution at -35 °C.
The crystals were quickly moved from a scintillation vial to a
microscope slide containing Paratone N oil (an Exxon product). Under
the microscope, an orange plate was selected and mounted on a glass
fiber using wax. A total of 31 317 reflections were collected (-17 e
h e 13, -21 e k e 21, -28 e l e 27) in the θ range of 1.32-23.26°
of which 11 068 were unique (Rint ) 0.0616). The structure was solved
by direct methods in conjunction with standard difference Fourier
techniques. Hydrogen atoms were placed in calculated (dC-H ) 0.96
Å) positions. The largest peak and hole in the difference map were
0.405 and -0.354 e‚Å-3, respectively. A semiempirical absorption
correction was applied based on pseudo-ψ scans with maximum and
minimum transmission equal to 0.1513 and 0.0954, respectively. A
solvent pentane molecule was located in the difference map of the
asymmetric unit and included during refinement. The least-squares
refinement converged normally with residuals of R (based on F) )
0.0579, wR (based on F2) ) 0.1326, and GOF ) 1.086 (based on I >
2σ(I)). Crystal data for C71H107MoN8Ti: monoclinic, space group )
P21/n, z ) 4, a ) 15.679(2) Å, b ) 19.580(4) Å, c ) 25.319(3) Å, â
) 95.61(2)°, V ) 7736(2) Å3, Fcalc ) 1.045 g‚cm-3, F(000) ) 2612.
2
coordinated THF. A H NMR spectrum showed a single resonance at
1.5 ppm. No νNN stretch was observed by infrared spectroscopy. The
coordinated THF was not removed upon lyophilization (benzene).
Analogous 1H NMR spectra were obtained in several independent
1
experiments. H NMR (300 MHz, C6D6, 25 °C) δ ) 6.24 (s, 4), 5.82
(5), 3.24 (t, THF), 2.25 (3), 1.45 (1), 1.23 (m, THF). No change was
observed in the 1H NMR spectrum of the orange-brown material after
exposure to N2 for a period of 1 h. Addition of 2 equiv of 12-crown-4
to a sample of this solid in THF resulted in a complex mixture of
products. An attempt to regenerate 1 by oxidation of the orange-brown
compound with stoichiometric ferrocenium triflate in OEt2 led to a
complex mixture of products including a substantial quantity of the
free ligand, HN(R)Ar. The orange-brown compound reacted slowly with
carbon monoxide over a period of 30 min but did not afford the expected
anionic CO derivative of 1, which has been characterized indepen-
dently.41 Additionally, the orange-brown compound did not react with
N2 to form 5. X-ray-quality crystals of the orange-brown compound
were never obtained.
Synthesis of Mo(N[Ad]Ar)3 (16). To a 500-mL flask was added
7.44 g of Li(N[Ad]Ar)12 (7.44 g, 28.5 mmol) and 150 mL of OEt2.
65
The mixture was stirred for 5 min at 25 °C, and then MoCl3(THF)3
(5.96 g, 14.2 mmol) was added in one portion. The reaction mixture
darkened and attained a light-brown color after 5 h, at which time it
was filtered through Celite. The filtrate was stored at -35 °C to
precipitate out a small quantity of solid. The mixture was then filtered
through a sintered glass frit, and the filter cake was washed with pentane
until the washings were colorless. The orange washings and filtrate
were combined, and the cooling/filtration/extraction process was
repeated. After solvent removal in vacuo, the residue was extracted
with ether (50 mL). The extract was stored at ∼28 °C until a light
orange solid precipitated. The solid (3.1 g) was collected on a frit and
dried in vacuo. The 1H NMR spectrum (C6D6) of the material revealed
∼2% impurity of the free ligand HN(Ad)Ar. The orange solid was
dissolved in a 50:50 pentane/benzene mixture, and the resulting solution
was filtered through Celite. Solvent was removed from the filtrate in
vacuo, affording 2.85 g of a light orange-yellow powder (35% yield):
µeff (C6D6) 2.96 µB. Anal. Calcd for C54H72MoN3: C, 75.49; H, 8.45;
N, 4.89. Found: C, 75.33; H, 8.45; N, 4.68.
Synthesis of (Ph[tBu]N)3Ti(µ-N2)Mo(N[R]Ar)3 (14). Compound
40
1 (187.5 mg, 0.293 mmol) and green Ti(N[tBu]Ph)3 (13, 144 mg,
0.293 mmol) were weighed into a 20-mL scintillation vial containing
a stir bar. Ether (3.5 mL) was added to the vial and the contents were
stirred at 25 °C, initially giving a dark, green-orange solution. The
reaction mixture was stirred under an atmosphere of dinitrogen for 36
2
h and the reaction progress was monitored by H NMR, such that the
gradual depletion of paramagnetic 1 (δ ) 64.5 ppm) and the formation
of a single new product with a single resonance (δ ) 1.8 ppm) was
observed. After 36 h, 1 had been completely consumed, and a large
amount of an orange precipitate was evident in the vessel. This bright
orange powder was collected on a sintered glass frit by filtration, and
the filtrate was concentrated to its solid form. The solids were combined,
and a 1H NMR spectrum of the crude material evinced clean production
X-ray Structure of Mo(N[Ad]Ar)3 (16). Orange crystalline slabs
were grown slowly from an ethereal solution at 25 °C. The crystals
were quickly moved from a scintillation vial to a microscope slide

Molybdenum N-tert-Hydrocarbylanilide Complexes
J. Am. Chem. Soc., Vol. 121, No. 43, 1999 10065
containing Paratone N oil (an Exxon product). Under the microscope
an orange plate was selected and mounted on a glass fiber using wax.
A total of 19 029 reflections were collected (-10 e h e 20, -14 e k
e 13, -23 e l e 21) in the θ range of 1.14-23.25° of which 6804
were unique (Rint ) 0.0469). The structure was solved by direct methods
in conjunction with standard difference Fourier techniques. Hydrogen
atoms were placed in calculated (dC-H ) 0.96 Å) positions. The largest
through the final refinement. All hydrogen atoms were placed in
calculated (dC-H ) 0.96 Å) positions. The largest peak and hole in the
difference map were 0.486 and -0.346 e‚Å-3, respectively. The least-
squares refinement converged normally with residuals of R (based on
F) ) 0.0799, wR (based on F2) ) 0.1829, and GOF ) 1.048 based
upon I > 2σ(I). Crystal data for C66H95MoN5NaO3: trigonal, space
group ) P3h, z ) 6, a ) 31.1129(8) Å, b ) 31.1129(8) Å, c ) 14.9633-
(6) Å, V ) 12544.1(7) Å3, Fcalc ) 0.894 g‚cm-3, F(000) ) 3618.
Synthesis of (Me3SiNN)Mo(N[Ad]Ar)3 (19). In a 20-mL scintil-
lation vial, solid orange 18 (34.5 mg, 0.0379 mmol) was dissolved in
ether (2.5 mL) to form a scarlet solution. A separate ether solution (2
mL) of chlorotrimethylsilane (6 mg, 0.0552 mmol) was prepared, and
the solutions were then mixed at -35 °C. The stirring mixture attained
a bright yellow within 5 min. After 15 min the mixture was filtered
through Celite, abandoning a yellow filter cake, which was washed
with benzene (2 mL) until the washings were colorless. Volatile material
was removed in vacuo, and the resulting yellow residue was recrystal-
lized from pentane in two crops affording 21 mg (58%) of crystalline
yellow 19: 1H NMR (300 MHz, C6D6, 25 °C) δ 6.89 (s, 5), 6.33 (s,
4), 2.16 (s, 3), 2.12 (s, Ad), 2.06 (s, Ad), 1.68 (dd, Ad), 0.39 (s, SiMe3);
13C NMR (125.66 MHz, C6D6, 25 °C) δ 137.31 (aryl), 131.74 (aryl),
121.78 (aryl), 118.13 (aryl), 66.77 (9), 46.30 (Ad), 44.33 (Ad), 37.39
(Ad), 37.16 (Ad), 31.67 (Ad), 30.54 (Ad), 23.07 (3), 21.95; FTIR (THF,
KBr) νNN 1646 cm-1 (br). Anal. Calcd for C57H81N5MoSi: C, 71.29;
H, 8.50; N, 7.29. Found: C, 71.22; H, 8.38; N, 7.19.
Synthesis of (PhC(O)NN)Mo(N[Ad]Ar)3 (20). In a 20-mL scintil-
lation vial, solid orange 18 (92.3 mg, 0.101 mmol) was dissolved in
ether (4 mL), giving a scarlet solution. A separate ether solution (2
mL) of benzoyl chloride (15.1 mg, 0.107 mmol) was prepared. Both
solutions were chilled to -35 °C, and the solution of benzoyl chloride
was then added quickly to the stirring solution of 18, eliciting a rapid
color change to dark olive-green. The reaction mixture was allowed to
stir for an additional 24 h, and then it was filtered through Celite. A
bright orange solid was obtained by crystallization from pentane in
several crops from the green filtrate (45 mg, 45%). A benzene solution
of this solid was thermally stable at 75 °C for 2 h: 1H NMR (300
MHz, C6D6, 25 °C) δ 8.73 (d, phenyl para), 7.24 (m, phenyl meta),
6.69 (5), 6.15 (4), 2.31 (Ad), 2.17 (Ad), 2.09 (3), 1.80 (Ad), 1.57 (Ad);
13C NMR (125.66 MHz, C6D6, 25 °C) 146.98 (aryl), 137.79 (aryl),
131.58 (aryl), 129.61 (aryl), 129.34 (aryl), 121.77 (aryl), 118.13 (aryl),
65.74 (9), 45.54 (Ad), 44.31 (Ad), 37.19 (Ad), 31.74 (Ad), 30.54 (Ad),
21.84 (3). Anal. Calcd for C61H77N5MoO: C, 73.84; H, 7.82; N, 7.06.
Found: C, 73.22; H, 7.88; N, 6.99.
Synthetic Procedures for [(THF)xNa][(N2)Mo(N[R]Ar)3] (5) and
[Na(12-crown-4)2][(N2)Mo(N[R]Ar)3] (7). The following preparation
was carried out in a drybox under 1 atm of dinitrogen. A 0.44% Na/
Hg solution was prepared by dissolving Na (400 mg, 0.0174 mol) in
Hg (91 g, 0.454 mol) in a 250-mL round-bottom flask. The amalgam
was allowed to cool to ambient temperature, and THF (60 mL) was
then added to the flask. The contents were stirred vigorously for 5 min.
Orange 1 (620 mg, 0.0965 mmol) was dissolved in THF (40 mL, 0.024
M), and this solution was placed in a dropping funnel equipped with
a stopcock. The orange solution of 1 was added dropwise, very slowly,
to the vigorously stirring solution containing the amalgam at 25 °C.
The addition rate was adjusted such that the addition took place over
a period of 10 h. During the addition, the solution acquired a scarlet
color. The reaction mixture was stirred for an additional 12 h at 25 °C,
after which time the solvent was removed in vacuo. The resulting
residue was extracted with pentane (30 mL), and the extract was
decanted away from the amalgam. The orange extract was filtered
through Celite and dried to a fine orange powder. Spectroscopic analysis
(1H NMR) revealed near-quantitative conversion (96% crude) to the
desired product [(THF)xNa][(N2)Mo(N[R]Ar)3] (5), with only 2% free
ligand HN(R)Ar and 2% nitride 4 present as side products. The orange
powder was recrystallized from pentane in 69% yield, affording pure
5 (x ) 0) containing a trace amount of THF: 1H NMR (300 MHz,
C6D6, 25 °C) δ 6.804 (s, 4), 6.63 (s, 5), 2.22 (s, 3), 1.38 (s, 1); 13C
NMR (125.66 MHz, C6D6, 25 °C) δ 156.2 (aryl), 137.0 (aryl), 130.1
(aryl), 124.3 (aryl), 60.9 (6), 34.8 (1), 33.0 (2), 23.1 (3), 21.8 (3); FTIR
(THF, KBr) νNN 1761 cm-1. Addition of 2 equiv of 12-crown-4 afforded
the discrete violet salt [Na(12-crown-4)2][(N2)Mo(N[R]Ar)3] (7). This
peak and hole in the difference map were 0.357 and -0.504 e‚Å-3
,
respectively. A semiempirical absorption correction was applied based
on pseudo-ψ scans with maximum and minimum transmission equal
to 0.6720 and 0.6191, respectively. The least-squares refinement
converged normally with residuals of R (based on F) ) 0.0535, wR
(based on F2) ) 0.1040, and GOF ) 1.274 (based on I > 2σ(I)). Crystal
data for C47H72MoN3: monoclinic, space group ) P21/c, z ) 4, a )
18.874(3) Å, b ) 12.6894(11) Å, c ) 20.947(5) Å, â ) 108.805(13)°,
V ) 4749.0(13) Å3, Fcalc ) 1.202 g‚cm-3, F(000) ) 1836.
Synthesis of [(THF)xNa][(N2)Mo(N[Ad]Ar)3] (18). A 0.4% Na/
Hg was prepared by dissolving 100 mg of Na metal (4.350 mmol) in
25 g of Hg in a 50-mL round-bottom flask charged with a stir bar.
THF (17 mL) was added to the amalgam and the mixture was stirred
for 5 min, after which time solid orange 16 (895 mg, 1.045 mmol)
was added directly to the vigorously stirring mixture. The reaction flask
was plugged with a rubber septum fitted with a needle inlet, providing
constant exposure to a dinitrogen atmosphere. The reaction mixture
acquired a scarlet color within 10 h. The mixture was stirred for 35 h,
after which time the deep scarlet supernatant was decanted away from
the amalgam and dried in vacuo. The residue was subsequently extracted
into ether (40 mL) and filtered through Celite. The filtrate was again
divested of solvent in vacuo. Pentane (10 mL) was added to the resulting
scarlet solid, effecting the precipitation of a bright orange solid from
a solution which was now red-orange. The orange solid was collected
on a sintered frit and washed twice with 10 mL of pentane. Solvent
removal afforded 610 mg (64%) of the THF-free species [(THF)xNa]-
[(N2)Mo(N[Ad]Ar)3] (x ) 0). This solid was virtually insoluble in all
hydrocarbon solvents including benzene and was found (not unexpect-
edly) to be unstable in CH2Cl2 and CHCl3. Spectroscopic characteriza-
tion was carried out on samples crystallized from ether at -35 °C,
which produced a scarlet crystalline material in good yield. Drying
under ambient pressure of such a solid, and subsequent extraction into
C6D6, afforded homogeneous solutions amenable to spectroscopic
1
analysis. A H NMR spectrum of such a sample showed shifted OEt2
resonances (there were ∼9 OEt2 molecules for each Mo atom): 1H
NMR (300 MHz, C6D6, 25 °C) δ ) 6.88 (s, 4), 6.63 (s, 5), 3.25 (q,
OEt2), 2.24 (s, 3), 2.18 (s, Ad), 1.93 (s, Ad), 1.67 (s, Ad), 1.11 (t,
OEt2); 13C NMR (125.66 MHz, C6D6, 25 °C) δ 136.89 (aryl), 131.45
(aryl), 121.76 (aryl), 118.11 (aryl), 68.24 (OEt2), 66.3, 61.7, 45.9, 44.3,
37.80, 37.17, 32.63, 31.73, 30.55, 26.18, 23.46, 23.08, 21.93, 15.96
(OEt2), 14.72 (OEt2); FTIR (THF, KBr) νNN 1783, 1757 cm-1; UV-
vis (OEt2) λmax ) 465 nm (ꢀ ) 4000). A sample appropriate for
microanalysis was obtained as the discrete salt complex [Na(12-crown-
4)2][(N2)Mo(N[Ad]Ar)3] by addition of 2 equiv of a benzene solution
of 12-crown-4 to a scarlet solution of 18 in THF/benzene. The resulting
violet solution was dried in vacuo and washed thoroughly with pentane
to afford a violet powder. Recrystallization from a toluene/THF mixture
at -35 °C afforded [Na(12-crown-4)2][(N2)Mo(N[Ad]Ar)3]. Anal. Calcd
for C70H104N5MoNaO8: C, 66.59; H, 8.30; N, 5.55. Found: C, 65.95;
H, 8.41; N, 5.43.
X-ray Structure of [(THF)xNa][(N2)Mo(N[Ad]Ar)3] (18). Deep
scarlet crystals were grown slowly by slow diffusion of heptane into a
scarlet THF solution at 25 °C. The crystals were quickly moved from
a scintillation vial to a microscope slide containing Paratone N oil.
Under the microscope, a scarlet plate was selected and mounted on a
glass fiber using wax. A total of 26 843 reflections were collected (-34
e h e 33, -31 e k e 34, -14 e l e 16) in the θ range of 1.31-
20.00° of which 7153 were unique (Rint ) 0.0835). The structure was
solved by direct methods in conjunction with standard difference Fourier
techniques. Non-hydrogen atoms were refined anisotropically. Two Na-
bound THF moieties were severely disordered, and attempts to model
the problem were unsuccessful. The model that is reported uses the
geometry of the one good THF as a basis for constraining the disordered
ligands. Thermal motion was also constrained for those THF moieties

10066 J. Am. Chem. Soc., Vol. 121, No. 43, 1999
Peters et al.
salt was highly insoluble in solvents such as pentane, diethyl ether,
and benzene but dissolved readily in THF and was recrystallized nearly
quantitatively from a pentane/THF mixture; FTIR (THF, KBr) νNN 1761
cm-1. Anal. Calcd for C52H68D18N5O8MoNa: C, 59.69; H, 8.28; N,
6.69. Found: C, 59.46; H, 8.26; N, 6.61.
Mo(N[R]Ar)3 (1) had been consumed by this time. The reaction mixture
was dried in vacuo after 7 h, by which time the intense purple color of
(µ-N2){Mo(N[R]Ar)3}2 (3) had faded. A 1H NMR spectrum of the crude
mixture showed NtMo(N[R]Ar)3 (4, 62.4%), [(THF)xNa][(N2)Mo-
(N[R]Ar)3] (5, 35.3%), and HN(R)Ar (2.3%).
Syringe Pump Synthesis of [(THF)xNa][(N2)Mo(N[tBu]Ar)3] (9).
Mo(N[tBu]Ar)3 (8, identical to 1 but for lack of deuterium enrichment
in the tert-butyl group, 1.098 g, 1.76 mmol) was divided into two equal
portions. Each portion was dissolved in THF (40 mL) and loaded into
an air-tight glass syringe (two syringes being used simultaneously).
The syringes were mounted on top of a dual-infusion pump that was
set to add the solution continuously for 10 h. Two 250-mL Erlenmeyer
filter flasks each were charged with 30 equiv 0.5% of Na/Hg prepared
by dissolving Na (606 mg, 52.8 mmol) in Hg (121 g). These flasks
were mounted on individual magnetic stir plates set for rapid stirring.
Teflon tubing is used to connect the syringes to their respective flasks.
The tubing is inserted through the side arm on the flask and allowed
to hang downward toward the amalgam. The top of the flasks are capped
to obviate spillage of the rapidly stirring mixture. After complete
addition, the solutions were combined and worked up as one. The scarlet
THF solution was decanted from the amalgam and filtered through
Celite on a sintered glass frit. Solvent was removed in vacuo from the
filtrate. The resulting solid residue was dissolved in pentane. The
pentane solution was filtered through Celite, and volatile material was
removed in vacuo from the filtrate. Spectroscopic analysis (1H NMR)
revealed quantitative conversion (99+% crude) to the desired product
[(THF)xNa][(N2)Mo(N[tBu]Ar)3] (9, x ) 0), with less than 1% nitride
4 present as a side product. The crude product may be crystallized in
77% yield by dissolving in pentane (16 mL) and storing at -35 °C for
48 h. Spectroscopic properties are essentially the same as for 5 above.
X-ray Structure of [Na(12-crown-4)2][(N2)Mo(N[R]Ar)3] (7). Deep
violet crystals were grown slowly from a pentane/THF solution at -35
°C. The crystals were quickly moved from a scintillation vial to a
microscope slide containing Paratone N. Under the microscope a violet
plate was selected and mounted on a glass fiber using wax. A total of
9132 reflections were collected (-13 e h e 13, -16 e k e 18, -13
e l e 19) in the θ range of 1.49-20.00° of which 5796 were unique
(Rint ) 0.0690). The structure was solved by direct methods in
conjunction with standard difference Fourier techniques. Hydrogen
atoms were placed in calculated (dC-H ) 0.96 Å) positions. The largest
Mo(N[R]Ar)3 (1) + [(THF)xNa][(N2)Mo(N[R]Ar)3] (5) + Hg
under N2. In a 20-mL scintillation vial, orange Mo(N[R]Ar)3 (1) (23.2
mg, 0.0360 mmol) was dissolved in THF (1.5 mL) and 207 mg of
mercury was added to the stirring solution at 25 °C. A solution of
[(THF)xNa][(N2)Mo(N[R]Ar)3] (25 mg, 0.0360 mmol) in THF (2 mL)
was then added quickly via pipet. An intense purple color developed
during the first hour of stirring. After 14 h, the solution was dried in
vacuo. Addition of pentane (4 mL) to the dried residue resulted in an
orange solution which was decanted away from a gray particulate
residue at the base of the vial. The mercury was washed further with
pentane until the washings were colorless. The orange solution was
1
dried and weighed at 44.8 mg. H NMR analysis of the crude orange
solid (in C6D6 with an internal integration standard of hexamethyld-
isiloxane) showed the spectroscopically identifiable products to be Nt
Mo(N[R]Ar)3 (60%), [(THF)xNa][(N2)Mo(N[R]Ar)3] (28%), and HN-
2
(R)Ar (6.5%). A H NMR spectrum of this solid (in THF) revealed
that ∼5% of (µ-N2){Mo(N[R]Ar)3}2 remained. The gray particulate
that remained in the original reaction vial was tested for its activity to
facilitate dinitrogen binding and cleavage by preparing a fresh solution
of Mo(N[R]Ar)3 (23.2 mg, 0.0360 mmol) in THF (2 mL) and adding
it to the original reaction vial containing the gray particulate. The
contents of the vial were stirred for several hours. The solution remained
1
orange throughout, and after 7 h a H NMR spectrum of the crude
mixture revealed only ∼8% NtMo(N[R]Ar)3. Starting Mo(N[R]Ar)3
was the only other species detectable (∼90% of the mixture).
Reaction of [(THF)xNa][(N2)Mo(N[R]Ar)3] (5) with Mo(N[tBu]-
Ar)3 (8) in Vacuo. Solid orange Mo(N[tBu]Ar)3 (8; 14.0 mg, 0.0224
mmol) and solid scarlet 5 (17.1 mg, 0.0246 mmol) were carefully
weighed into a sealable NMR tube. Rigorously degassed THF was
subsequently vacuum distilled into the evacuated tube at -196 °C and
the frozen vessel was flame-sealed. Upon thawing, the mixture went
homogeneous and 2H NMR spectroscopy was used to analyze the
deuterium-labeled species present. The spectra obtained clearly showed
the presence of paramagnetic Mo(N[R]Ar)3 at 65 ppm (∼38.5%) as
well as diamagnetic [(THF)xNa][(N2)Mo(N[tBu]Ar)3] at 1.0 ppm
(∼54.5%). A small amount of bridged (µ-N2){Mo(N[R]Ar)3}2 was
observed at 13 ppm (∼7%). A 1H NMR spectrum of the mixture,
reconstituted in C6D6, confirmed that [(THF)xNa][(N2)Mo(N[tBu]Ar)3]
(9) had formed. This result was reproduced in two subsequent
independent experiments.
Attempted Synthesis of [Na(12-crown-4)2][(µ-N2){Mo(N[R]-
Ar)3}2] (11). Solid violet [Na(12-crown-4)2][(N2)Mo(N[R]Ar)3] (7; 44.0
mg, 0.0421 mmol) and orange Mo(N[R]Ar)3 (1; 27.1 mg, 0.0421 mmol)
were weighed carefully into a sealable NMR tube. The tube was
evacuated and rigorously degassed. THF (1.2 mL) was then vacuum
distilled into the the tube at -196 °C. The tube was flame-sealed and
transferred directly to a -78 °C dry ice/2-propanol bath. The cold
sample was then quickly analyzed by 2H NMR spectroscopy. Both
starting materials were present in near-stoichiometric amounts (assign-
ments at δ ) 65 ppm for Mo(N[R]Ar)3 and δ ) 2.2 ppm for [Na(12-
crown-4)2][(N2)Mo(N[R]Ar)3]). A small amount of the neutral bridged
complex (µ-N2){Mo(N[R]Ar)3)2 was observed (δ ) 13 ppm). No other
resonances were observed. Upon standing at 25 °C, the resonance at
13 ppm decayed to a new peak at 2 ppm, consistent with formation of
NtMo(N[R]Ar)3 (the signals for diamagnetic [Na(12-crown-4)2][(N2)-
Mo(N[R]Ar)3] and NtMo(N[R]Ar)3 are coincident by 2H NMR
spectroscopy). The relative ratios of starting Mo(N[R]Ar)3 and [Na-
(12-crown-4)2][(N2)Mo(N[R]Ar)3] remained constant over a period of
24 h at 25 °C. This experiment was repeated several times and always
showed small but varying amounts of 3. A signal assignable to [Na-
(12-crown-4)2][(µ-N2){Mo(N[R]Ar)3)2] was not spectroscopically ob-
served. A similar experiment performed with the ion-pair species
[(THF)xNa][(N2)Mo(N[R]Ar)3] and Mo(N[R]Ar)3 behaved analogously.
Synthesis of (Me3SiNN)Mo(N[R]Ar)3 (12). Sodium amalgam (1%,
10 equiv) was prepared by dissolving Na metal (107.0 mg, 4.652 mmol)
in Hg (10.1877 g) in a 20-mL scintillation vial. Chlorotrimethylsilane
peak and hole in the difference map were 0.632 and -0.552 e‚Å-3
,
respectively. No absorption correction was applied. The least-squares
refinement converged normally with residuals of R (based on F) )
0.1156, wR (based on F2) ) 0.2431, and GOF ) 1.117 (based on I >
2σ(I)). Crystal data for C52H86MoN5NaO8: triclinic, space group ) P1,
z ) 2, a ) 12.3630(8) Å, b ) 16.2797(11) Å, c ) 17.8547(12) Å, R
) 71.3237(12)°, â ) 87.1904(7)°, γ ) 80.7164(12)°, V ) 3359.7(4)
Å3, Fcalc ) 1.016 g‚cm-3, F(000) ) 1100.
[(THF)xNa][(N2)Mo(N[R]Ar)3] (5) + [Cp2Fe][O3SCF3]. A scarlet
solution of [(THF)xNa][(N2)Mo(N[R]Ar)3] (36.0 mg, 0.0519 mmol) in
1.5 mL of THF was prepared in a small vial equipped with a stir bar.
A separate deep blue solution of ferrocenium triflate, [Cp2Fe][O3SCF3]
(17.4 mg, 0.0519 mmol), in 1.5 mL of THF was similarly prepared.
The scarlet solution was added quickly at 25 °C to the intense blue
solution of [Cp2Fe][O3SCF3]. Effervescence was observed at once, and
the resulting homogeneous solution was orange-red. After filtration,
the volatiles were removed in vacuo, and spectroscopic analysis (FTIR,
1H NMR, 2H NMR) of the crude mixture confirmed complete
consumption of 5 and stoichiometric formation of Mo(N[R]Ar)3 and
Cp2Fe. An analogous experiment carried out at low temperature (-35
°C) proceeded similarly.
Mo(N[R]Ar)3 (1) + [(THF)xNa][(N2)Mo(N[R]Ar)3] (5) under N2.
Scarlet 5 (27 mg, 0.0390 mmol) was dissolved in THF (2 mL) in a
20-mL scinitillation vial equipped with a stir bar. An orange solution
of Mo(N[R]Ar)3 (25 mg, 0.0389 mmol) in THF (1.5 mL) was added
dropwise to the stirring scarlet solution over a period of ∼30 s. No
color change was observed after the first few minutes had passed.
Within 30 min, however, the solution was an intense purple. A 2H NMR
spectrum of the reaction mixture after 1.5 h indicated the presence of
the paramagnetic dinuclear complex (µ-N2){Mo(N[R]Ar)3}2 (∼13 ppm)
and a diamagnetic species (1.7 ppm) in a 1:7 ratio. All of the starting

Molybdenum N-tert-Hydrocarbylanilide Complexes
J. Am. Chem. Soc., Vol. 121, No. 43, 1999 10067
(358.0 mg, 3.295 mmol) was dissolved in THF and added to the
amalgam. While stirring, solid orange Mo(N[R]Ar)3 (302.1 mg, 0.4702
mmol) was added to the reaction mixture at 25 °C under a dinitrogen
atmosphere. The orange solution gradually turned yellow-brown. After
6 h, the reaction mixture was filtered through Celite removing a gray
solid. The filtrate was dried in vacuo to a yellow-brown residue.
18.7 mg. Spectroscopic analysis verified this beige solid to be pure
NtMo(N[R]Ar)3 (4; isolated in ∼88% yield).
One-Pot Synthesis of (MeNN)Mo(N[R]Ar)3 (22). The predried
THF used in this protocol was filtered through a column of activated
alumina just prior to use. A 0.5% Na/Hg was prepared by slowly
dissolving sodium (331 mg, 14.41 mmol) in mercury (64 g) in a 250-
mL Erlenmeyer flask equipped with a stir bar. The amalgam was
allowed to cool and then a solution of methyl tosylate, CH3SO3-p-
C6H4Me (869 mg, 4.67 mmol), dissolved in THF (5 mL) was added to
it. An orange solution of Mo(N[R]Ar)3 (1; 870 mg, 0.467 mmol)
dissolved in THF (25 mL) was added dropwise to the stirring reaction
mixture over a period of 1.5 h. The reaction mixture was allowed to
stir an additional 2.5 h, maintaining a red-yellow color throughout. A
gray residue was then removed by filtration of the reaction mixture
through Celite. The filtrate was dried in vacuo, affording a yellow-red
solid. This solid was redissolved in pentane and once again filtered
through Celite to remove a small amount of additional gray material.
Subsequent drying of the filtrate in vacuo produced 287 mg of a crude
2
Spectroscopic analysis (1H, H NMR) of the crude material showed a
relatively clean transformation to a single diamagnetic product. The
crude product was redissolved in pentane and filtered again through
Celite to ensure complete removal of salts. Removal of solvent and
recrystallization from pentane at -35 °C afforded 275.3 mg (79%) of
an orange, crystalline material: 1H NMR (300 MHz, C6D6, 25 °C) δ
) 6.07 (s, 4), 6.64 (s, 5), 2.07 (s, 3), 1.50 (s, 1), 0.49 (s, 9H, SiMe3);
13C NMR (125.66 MHz, C6D6, 25 °C) δ 150.11 (aryl), 137.60 (aryl),
130.89 (aryl), 124.3 (aryl), 63.5 (6), 33.37 (1), 32.5 (2), 21.84 (3), 3.57
(SiMe3); FTIR (THF, KBr) νNN 1650 cm-1. Anal. Calcd for C39H45D18N5-
MoSi: C, 62.95; H, 8.53; N, 9.41. Found: C, 63.18; H, 8.62; N, 9.49.
X-ray Structure of (Me3SiNN)Mo(N[R]Ar)3 (12). Orange crystals
were grown slowly from a pentane solution at -35 °C. The crystals
were quickly moved from a scintillation vial to a microscope slide
containing Paratone N. Under the microscope, an orange plate was
selected and mounted on a glass fiber using wax. A total of 16 637
reflections were collected (-12 e h e 16, -10 e k e 14, -23 e l e
2
product which was yellow-red. Spectroscopic analysis (1H, H NMR)
of this crude material showed a single ligand environment and some
ligand degradation byproduct, HN(R)Ar. Purification for elemental
analysis was not achieved using this synthetic protocol, but see the
section below on synthesis of 23 for a successful synthesis of the pure
material: 1H NMR (300 MHz, C6D6, 25 °C) δ 6.68 (s, 5), 6.29 (s, 4),
3.75 (s, MoNNMe), 2.11 (s, 3), 1.45 (s, 1); FTIR (THF, KBr) νNN 1539
cm-1 (br).
22) in the θ range of 1.81-23.26° of which 5976 were unique (Rint
)
0.0241). The structure was solved by direct methods in conjunction
with standard difference Fourier techniques. Hydrogen atoms were
placed in calculated (dC-H ) 0.96 Å) positions. The largest peak and
hole in the difference map were 0.411 and -0.275 e‚Å-3, respectively.
No absorption correction was applied. The least-squares refinement
converged normally with residuals of R (based on F) ) 0.0287, wR
(based on F2) ) 0.0798, and GOF ) 1.020 (based upon I > 2σ(I)).
Crystal data for C39H63MoN5Si: monoclinic, space group ) P21/c, z
) 4, a ) 15.03890(10) Å, b ) 13.3531(2) Å, c ) 20.7912(2) Å, R )
Synthesis of (MeNN)Mo(N[tBu]Ar)3 (23). A solution of 9 (1.121
g, 1.66 mmol) in 12 mL of diethyl ether was prepared. To the solution
was added methyl tosylate (309 mg, 1.66 mmol) as a solution in ether
(3 mL) at 25 °C. After 2 h, the mixture was filtered through Celite on
a sintered glass frit. Volatile material was removed from the filtrate,
producing a brown-orange solid residue. The residue was extracted into
pentane, the extract was filtered through Celite, and volatile material
was again removed in vacuo. The crude product thus obtained was
spectroscopically pure (1H NMR) and the yield was 89% (986 mg).
Further purification was achieved by dissolution in pentane (6 mL),
followed by cooling to ∼-70 °C. The resulting analytically pure
precipitate was collected and dried. Crystals of 23 were grown by
storing the mother liquor at -30 °C for 24 h: 1H NMR (300 MHz,
C6D6) δ 6.68 (5), 6.29 (4), 3.74 (N2CH3), 2.11 (3), 1.45 (1). 13C NMR
(125.66 MHz, C6D6, 25 °C): δ 150.3 (aryl ipso), 137.7 (4), 130.7 (5),
128.0 (8), 62.6 (6), 43.8 (N2CH3), 32.9 (1), 21.9 (3); FTIR (KBr
windows, THF solution) νNN 1538 cm-1 also 1585, 1285, 1276, 1146,
701, 638, 572 cm-1. Anal. Calcd for C37H57N5Mo: C, 66.54; H, 8.60;
N, 10.49. Found: C, 66.67; H, 8.68; N, 10.41.
90°, â ) 91.23°, γ ) 90°, V ) 4174.24(8) Å3, Fcalc ) 1.155 g‚cm-3
F(000) ) 1552.
,
Synthesis of [(Me2NN)Mo(N[R]Ar)3][OTf] (21). Methyl triflate
(MeOTf, where OTf ) [O3SCF3]) was weighed as a liquid (347 mg,
2.116 mmol) into a 20-mL scintillation vial and subsequently dissolved
by stirring in pentane (5 mL). Solid orange (Me3SiNN)Mo(N[R]Ar)3
(12; 258 mg, 0.346 mmol) was added, at once, to the stirring solution
of MeOTf at 22 °C. The resulting golden solution gradually deposited
a red-orange precipitate over a period of 6 h. The precipitate was
collected on a sintered glass frit, washed with pentane, and dried in
vacuo. This first crop of 233 mg (79%) was judged to be both
spectroscopically and analytically pure: 1H NMR (300 MHz, C6D6,
25 °C) δ 6.65 (s, 5), 6.25 (s, 4), 3.73 (s, MoNNMe2), 2.11 (s, 3), 1.06
(s, 1); 13C NMR (125.66 MHz, C6D6, 25 °C) δ 147.82 (aryl), 138.52
(aryl), 129.63 (aryl), 65.32 (6), 46.25 (Mo(NNMe2), 30.78 (1), 30.15
(2), 21.28 (3); 19F NMR (300 MHz, C6D6, 20 °C) δ -76.92 (O3SCF3).
Anal. Calcd for C39H42D18F3MoN5O3: C, 55.11; H, 7.11; N, 8.24.
Found: C, 55.32; H, 6.63; N, 8.12.
Na/Hg Reduction of [(Me2NN)Mo(N[R]Ar)3][OTf] (21). In an
isolated reaction vessel, [(Me2NN)Mo(N[R]Ar)3][OTf] (21; 27 mg,
0.032 mmol) was stirred over a 0.5% Na/Hg (1.0 mg, 0.043 mmol of
Na; and 197 mg of Hg) in C6D6. After stirring at 22 °C for 3 h, the
volatiles were vacuum-transferred to a sealable NMR tube. The NMR
tube was flame-sealed, and spectroscopic analysis (1H, 13C NMR) of
the isolated volatiles indicated that dimethylamine (HNMe2) was
produced cleanly. This was verified by comparison to spectra of an
authentic sample of HNMe2. Workup of the nonvolatile products
consisted of redissolving the remaining residue in THF and filtering
through Celite to remove an insoluble gray material. The filtrate was
dried in vacuo, producing a pale beige solid. This solid was extracted
into pentane, filtered through Celite, and dried in vacuo to a weight of
Acknowledgment. For support of this work, the authors are
grateful to the National Science Foundation (CAREER Award
CHE-9501992), the Packard Foundation, and the Alfred P. Sloan
Foundation. Additional funding for which the authors are
profoundly grateful has come from the National Science Board
(Alan T. Waterman Award to C.C.C., 1998).
Supporting Information Available: Tables of bond lengths,
bond angles, atomic coordinates, and anisotropic displacement
parameters for the structures of 7, (Ph[tBu]N)3Ti(µ-N2)Mo(N[R]-
Ar)3 (14), (Me3SiNN)Mo(N[R]Ar)3 (12), Mo(N[Ad]Ar)3 (16),
and [(THF)xNa][(N2)Mo(N[Ad]Ar)3] (18); the details of the
structure of 7 have been deposited with the Cambridge Crystal-
lographic Database. This information is available free of charge
via the Internet at http://pubs.acs.org.
JA991435T