Alkylation of dinitrogen in [(HIPTNCH2CH2) 3N]Mo complexes (HIPT = 3,5-(2,4,6-i-Pr3C 6H2)2C6H3

Article abstract of DOI:10.1021/ja904535f

In this paper we explore the ethylation of dinitrogen (employing [Et ₃O][BAr^f₄]; Ar^f = 3,5-(CF ₃)₂C₆H₃) in [HIPTN₃N]Mo (Mo) complexes ([HIPTN₃N]<

Full text of DOI:10.1021/ja904535f

Published on Web 08/12/2009
Alkylation of Dinitrogen in [(HIPTNCH2CH2)3N]Mo Complexes
(HIPT ) 3,5-(2,4,6-i-Pr3C6H2)2C6H3)
Thomas Kupfer† and Richard R. Schrock*
Department of Chemistry, Massachusetts Institute of Technology,
Cambridge, Massachusetts 02139
Received June 4, 2009; E-mail: rrs@mit.edu
Abstract: In this paper we explore the ethylation of dinitrogen (employing [Et3O][BArf4]; Arf ) 3,5-(CF3)2C6H3)
in [HIPTN3N]Mo (Mo) complexes ([HIPTN3N]3- ) [N(CH2CH2NHIPT)3]3-; HIPT ) 3,5-(2,4,6-i-Pr3C6H2)2C6H3)
with the objective of developing a catalytic cycle for the conversion of dinitrogen into triethylamine. A number
of possible intermediates in a hypothetical catalytic cycle have been isolated and characterized: MoNdNEt,
[ModNNEt2][BArf4], ModNNEt2, [ModNEt][BArf4], ModNEt, MoNEt2, and [Mo(NEt3)][BArf4]. Except for
MoNEt2, all compounds were synthesized from other proposed intermediates in a hypothetical catalytic
reaction. All alkylated species are significantly more stable than their protonated counterparts, especially
the Mo(V) species, ModNNEt2 and ModNEt. The tendency for both ModNNEt2 and ModNEt to be readily
oxidized by [Et3O][BArf4] (as well as by [H(Et2O)2][BArf4], [ModNNH2][BArf4], and [ModNH][BArf4]) suggests
that their alkylation is unlikely to be part of a catalytic cycle. All efforts to generate NEt3 in several
stoichiometric or catalytic runs employing MoN2 and MotN as starting materials were unsuccessful, in
part because of the slow speed of most alkylations relative to protonations. In related chemistry that employs
a ligand containing 3,5-(4-t-BuC6H4)2C6H3 amido substituents alkylations were much faster, but a preliminary
exploration revealed no evidence of catalytic formation of triethylamine.
Introduction
Ever since the discovery of the first transition metal dinitrogen
catalytic with respect to it. Hydrazine is the primary product,
with ammonia being formed through a metal-catalyzed dispro-
portionation of hydrazine to dinitrogen and ammonia. Shilov
proposed that dinitrogen is bound and reduced to hydrazine
between two metal centers.
1
complex, [Ru(NH3)5(N2)]2+
, scientists have been trying to
reduce dinitrogen to ammonia or to prepare organic compounds
from dinitrogen under mild conditions. Although the groups of
Chatt and Hidai made significant advances toward the catalytic
reduction of dinitrogen to ammonia,2-9 no catalytic reduction
under mild conditions was achieved. Chatt proposed that
coordination of dinitrogen to the appropriate single metal could
activate it toward addition of the first proton or electron and
that subsequent alternating addition of a proton and an electron
would result in formation of 2 equiv of ammonia.
Two systems are known in which dinitrogen is reduced under
mild conditions. The first is one in which dinitrogen is reduced
catalytically to a ∼10:1 mixture of hydrazine and ammonia in
methanol employing a relatively strong reducing agent such as
sodium amalgam.10 The reaction requires molybdenum and is
The second system catalytically reduces dinitrogen to am-
monia as the primary product at a molybdenum center in a
complex that contains the [HIPTN3N]3- ligand, where HIPT )
3,5-(2,4,6-i-Pr3C6H2)2C6H3 (HexaIsoPropylTerphenyl; see Figure
1).11-18 The [HIPTN3N]3- ligand discourages bimetallic chem-
istry by sterically protecting the metal coordination site and also
provides increased solubility of Mo complexes in nonpolar
solvents. The proposed catalytic reduction of dinitrogen to
ammonia that involves [HIPTN3N]Mo complexes is shown in
Figure 2 (R ) H; [HIPTN3N]Mo ) Mo). The oxidation states
of molybdenum in the system range from Mo(III) to Mo(VI).
Eight of these intermediates have been prepared and crystallo-
graphically characterized: MoN2, [MoN2]-, Mo-NdNH, [Mo-
NdNH2]+, MotN, [MotNH]+, [Mo(NH3)]+, and Mo(NH3).
All hypothetical compounds that could not be observed and/or
isolated were Mo(V) species, e.g., ModN-NH2 (not observed)
† Present address: Institut fu¨r Anorganische Chemie, Universita¨t Wu¨rzburg,
Germany.
(1) Allen, A. D.; Senoff, C. V. J. Chem. Soc., Chem. Commun. 1965,
621.
(2) Chatt, J.; Dilworth, J. R.; Richards, R. L. Chem. ReV. 1978, 78, 589.
(3) Fryzuk, M. D.; Johnson, S. A. Coord. Chem. ReV. 2000, 200-202,
379.
(11) Schrock, R. R. Angew. Chem., Int. Ed. 2008, 5512.
(12) Weare, W. W.; Dai, X.; Byrnes, M. J.; Chin, J. M.; Schrock, R. R.;
Mueller, P. Proc. Nat. Acad. Sci. U.S.A. 2006, 103, 17099.
(13) Weare, W. W.; Schrock, R. R.; Hock, A. S.; Mu¨ller, P. Inorg. Chem.
2006, 45, 9185.
(4) Hidai, M.; Mizobe, Y. Chem. ReV. 1995, 95, 1115.
(5) Hidai, M. Coord. Chem. ReV. 1999, 185-186, 99.
(6) Barriere, F. Coord. Chem. ReV. 2003, 236, 71.
(7) Henderson, R. A.; Leigh, G. J.; Pickett, C. J. AdV. Inorg. Chem.
Radiochem. 1983, 27, 197.
(14) Ritleng, V.; Yandulov, D. V.; Weare, W. W.; Schrock, R. R.; Hock,
A. R.; Davis, W. M. J. Am. Chem. Soc. 2004, 126, 6150.
(15) Yandulov, D. V.; Schrock, R. R. J. Am. Chem. Soc. 2002, 124, 6252.
(16) Yandulov, D. V.; Schrock, R. R.; Rheingold, A. L.; Ceccarelli, C.;
Davis, W. M. Inorg. Chem. 2003, 42, 796.
(8) Allen, A. D.; Harris, R. O.; Loescher, B. R.; Stevens, J. R.; Whiteley,
R. N. Chem. ReV. 1973, 73, 11.
(9) Richards, R. L. Coord. Chem. ReV. 1996, 154, 83.
(10) Shilov, A. E. Russ. Chem. Bull. Int. Ed. 2003, 52, 2555.
(17) Yandulov, D.; Schrock, R. R. Inorg. Chem. 2005, 44, 1103.
(18) Yandulov, D. V.; Schrock, R. R. Science 2003, 301, 76.
9
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J. AM. CHEM. SOC. 2009, 131, 12829–12837 12829

A R T I C L E S
Kupfer and Schrock
in the chemistry of relatively low oxidation state molybdenum
and tungsten dinitrogen complexes,2,5 similar alkylations of
relatively high oxidation state molybdenum-based triamidoamine
complexes has been investigated only sporadically.24-26 In this
paper we explore the possibility that triethylamine might be
prepared catalytically in the [HIPTN3N]Mo system.
Results and Discussion
Synthesis of [Et3O][BArf4]. The proton source that was chosen
for the catalytic reduction of dinitrogen by [HIPTN3N]Mo
catalysts was [2,6-Lut][BArf4] (2,6-Lut ) 2,6-dimethylpyri-
dinium). Since [2,6-Lut][BArf4] is relatively insoluble in heptane
(the reaction solvent), the direct reduction of protons to H2 by
CrCp*2, the reducing agent that is slowly added to the mixture,
is minimized. Alkylating agents that contain the [BArf4]- anion
would be most desirable as they would perturb the system
minimally. Therefore we turned to the possibility of employing
[R3O][BArf4] reagents. All attempts to prepare [Me3O][BArf4]
by treating commercially available [Me3O][BF4] with Na[BArf4]
under various conditions yielded only complex mixtures.
[Et3O][B(C6F5)4] has been mentioned briefly in the literature,
although experimental details of its synthesis were not pro-
vided.27 The reaction between [Et3O][BF4] and 1.05 equiv of
Na[BArf4] proceeded smoothly in diethyl ether over a period
of 3 days to give [Et3O][BArf4] in yields of 90-95% as a white
solid. [Et3O][BArf4] became the reagent of choice for exploring
dinitrogen alkylation reactions.
Figure 1. Drawing of MoN2.
Synthesis of MoNdNEt. The first plausible intermediate in a
hypothetical catalytic alkylation that begins with MoN2 is Mo-
NdNEt (3 in Figure 2; R ) Et). Diamagnetic MoNdNEt is
accessible through reduction of MoCl with excess Na sand under
1 atm of dry N2 in THF to give [MoN2]- in situ, followed by
addition of [Et3O][BArf4] in diethyl ether; MoNdNEt prepared
in this manner can be isolated in a moderate yield (42%). The
yield of MoNdNEt is higher (69%) when [Bu4N][MoN2] is
first prepared and purified and then treated with [Et3O][BArf4]
in ether (eq 1); this is the preferred method. MoNdNEt also is
obtained in 54% yield from MoN2, [Et3O][BArf4], and CrCp*2
in benzene at room temperature after 2 days. After recrystalli-
zation from heptane, MoNdNEt was isolated as a pale yellow,
crystalline solid.
Figure 2. Possible intermediates in the reduction of dinitrogen to ammonia
or a trialkylamine at a Mo center through the stepwise addition of R+ (R
) H or alkyl, respectively) and electrons.
and ModNH (observed, but not isolable). Calculations by
Reiher and his group on complexes with the full [HIPTN3N]3-
ligand set are consistent with the proposed catalytic scheme.19-21
Turnover employing CrCp*2 as the reducing agent and [2,6-
Lut][BArf4] (Arf ) 3,5-(CF3)2C6H3) as the proton source in
heptane is limited to approximately 4 equiv of dinitrogen out
of a possible six, as a consequence, it is proposed, of the
[HIPTN3N]3- ligand being removed from the metal under
catalytic conditions. The only other product of the catalytic
reduction is dihydrogen.
We became interested in alkylation of dinitrogen and its
derivatives in the [HIPTN3N]Mo system for two reasons. First,
alkylated dinitrogen and related ligands are likely to be much
more robust than their protonated counterparts, a result that
might lead to a more detailed understanding of the catalytic
reduction of dinitrogen to ammonia. Second, trialkylamines
might be synthesized directly from dinitrogen via a set of
intermediates analogous to that proposed for reduction to
ammonia (Figure 2; R ) alkyl). Transition-metal-mediated
reduction of dinitrogen to form amines under homogeneous
catalysis conditions has been reported by Hidai22 and by Mori.23
Both processes require alkali metals as reducing agents, and
mechanistic details are unknown. Although alkylations of
dinitrogen and reduced dinitrogen fragments have been explored
[Bu4N][MoN2] + [Et3O][BArf4] f MoNdNEt +
[Bu4N][BArf4] + Et2O (1)
1
The H NMR spectrum of MoNdNEt in C6D6 displays a
triplet at 0.87 ppm and a quartet at 3.31 ppm for the ethyl group,
as well as the expected resonances for the [HIPTN3N]Mo core.
MoNdNEt can be heated to 80 °C for several days in solution
with no sign of decomposition. (MoNdNH is known to
decompose to MoH under similar conditions.16) The oxidation
of MoNdNEt is fully reversible in PhF (0.1 M [Bu4N][BArf4];
E1/2 ) -0.18 V vs Fc/Fc+) at scan rates between 50 and 700
mV/s (Table 1; Figure 3); the oxidized species is assigned as
[MoNdNEt]+. In contrast, oxidation of MoNdNH begins near
0 V but is irreversible.17 Attempts to prepare [MoNd
(19) Reiher, M.; Le Guennic, B.; Kirchner, B. Inorg. Chem. 2005, 44, 9640.
(20) Schenk, S.; Le Guennic, B.; Kirchner, B.; Reiher, M. Inorg. Chem.
2008, 47, 3634; erratum 7934.
(24) Mo¨sch-Zanetti, N. C.; Schrock, R. R.; Davis, W. M.; Wanninger, K.;
Seidel, S. W.; O’Donoghue, M. B. J. Am. Chem. Soc. 1997, 119,
11037.
(21) Schenk, S.; Kirchner, B.; Reiher, M. Eur. J. Chem. 2009, 15, 5073.
(22) Komori, K.; Oshita, H.; Mizobe, Y.; Hidai, M. J. Am. Chem. Soc.
1989, 111, 1939.
(25) O’Donoghue, M. B.; Davis, W. M.; Schrock, R. R. Inorg. Chem. 1998,
37, 5149.
(26) Greco, G. E.; Schrock, R. R. Inorg. Chem. 2001, 40, 3861.
(27) Huang, W.; Berke, H. Chimia 2005, 59, 113.
(23) Hori, M.; Mori, M. J. Org. Chem. 1995, 60, 1480.
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A R T I C L E S
Table 1. Electrochemical Properties of Selected Protonated and
Alkylated Mo Speciesa
of any significant side products. The reaction is complete within
10-15 min at initial concentrations of 0.047 M in MoNdNEt
and [Et3O][BArf4] and within 1 h at initial concentrations of
0.024 M of each. Disappearance of the proton resonances for
[Et3O][BArf4] were monitored by 1H NMR in order to obtain a
second-order rate constant of k2 ) 0.23 L mol-1 s-1. The cyclic
voltammogram of [ModNNEt2][BArf4] in PhF (0.1 M
[Bu4N][BArf4]) at a scan rate of 50 mV/s reveals an irreversible
reduction at -1.87 V (Ipc; E1/2 ) -1.76 V; vs Fc/Fc+), which
we assign to the [ModNNEt2]+/0 couple. This couple is shifted
by about ∼200-300 mV compared to the [ModNNH2]+/0
couple (Table 1).17 Like most of the other Mo compounds
described here, [ModNNEt2][BArf4] is highly soluble even in
pentane, and X-ray quality crystals could not be obtained.
couple
E1/2 [V]
couple
E1/2 [V]
[MoNdNEt]+/0
[ModNNEt2]+/0
[ModNEt]+/0
[MoNEt2]+/0
[Mo(EtNH2)]+/0
CrCp*2
–0.18
–1.76b
–1.64c
–0.98
–1.83
–1.63
[MoNdNH]+/0
[ModNNH2]+/0
[ModNH]+/0
∼0d
–1.56e
–1.38e
[Mo(NH3)]+/0
CoCp*2
–1.63
–2.01
a Potentials measured by cyclic voltammetry in 0.1 M [Bu4N][BArf4]
in PhF at a 3.0 mm glassy carbon disk under Ar and referenced to Fc/
Fc+. b Irreversible process (Ipc ) -1.87 V). c Irreversible process for
[ModNEt][BArf4] (Ipc ) -1.82 V); reversible process for ModNEt.
d Onset of multiple irreversible oxidation waves. e Quasi-reversible
process.
MoNdNEt + [Et3O][BArf4]. Et2O 8 [ModNNEt2][BArf4]
(2)
Reduction of [ModNNEt2][BArf4] with 1 equiv of CrCp*2
in benzene yielded [CrCp*2][BArf4] and a red, paramagnetic
1
species in ∼90% yield according to H NMR spectroscopy of
the reaction mixture (eq 3). This species is tentatively assigned
as the neutral Mo(V) hydrazido complex, ModNNEt2;
ModNNEt2 is much more stable than ModNNH2, which cannot
be observed upon attempts to generate it in situ.17 The 1H NMR
spectrum of ModNNEt2 displays only broad peaks in the
diamagnetic region and no characteristic paramagnetically
shifted resonances. Attempts to obtain analytically pure material
have failed so far, in part because traces of CrCp*2 and
[CrCp*2][BArf4] are present, but also because ModNNEt2
decomposes slowly in solution over a period of days. Attempts
to alkylate ModNNEt2 with [Et3O][BArf4] (5 f 6 in Figure 2)
resulted only in one-electron oxidation of ModNNEt2 to reform
[ModNNEt2][BArf4] in 90-95% yield and butane, which was
observed by 1H NMR spectroscopy of the volatile products. As
the electrochemical potentials suggest, [ModNNH2][BArf4]
should oxidize ModNNEt2. It does so readily (eq 4).
ModNNEt2 is also oxidized by [H(OEt2)2][BArf4] in C6D6 and
CD2Cl2 in virtually 100% yield.
Figure 3. Cyclic voltammograms of MoNdNEt in PhF (0.1
M
[Bu4N][BArf4] vs FeCp2/[FeCp2]+).
NEt][BArf4] through oxidation of MoNdNEt with [Fc][BArf4]
(Fc ) FeCp2) or silver salts such as Ag[BArf4] in benzene,
diethyl ether, or methylene chloride have been unsuccessful;
only immediate decomposition to unidentified species was
observed.
[ModNNEt2][BArf4] + CrCp* f ModNNEt2 +
2
[CrCp* ][BArf4] (3)
2
Two plausible products in a reaction between MoN2 and
[Et3O][BArf4] are [MoNdNEt][BArf4] (2 in Figure 2; R ) Et)
or a species in which an amido nitrogen is alkylated. The latter
would be analogous to an unstable and potentially important
intermediate observed when MoN2 is treated with [LutH][BArf4]
in fluorobenzene,17 in which an amido nitrogen has been
protonated. However, under a variety of conditions (solvent,
temperature, stoichiometry) no significant amounts (<10%) of
any identifiable alkylated species were formed. Since alkylation
of an amido nitrogen is not likely to be reversible, the synthesis
of MoNdNEt from MoN2, [Et3O][BArf4], and CrCp*2 in
benzene at room temperature noted above must proceed either
through formation first of a small amount of [MoNdNEt]+
(followed by reduction) or a small amount of [MoN2]- (followed
by alkylation).
Mo ) NNEt2 + [ModNNH2][BArf4] f
[ModNNEt2][BArf4] + ModNNH2 (4)
Synthesis of [ModNEt][BArf4] and ModNEt. Treatment of
MotN with 1 equiv of [Et3O][BArf4] in CH2Cl2 cleanly afforded
diamagnetic [ModNEt][BArf4] in high yields (up to 93%) as
the only product (eq 5). The reaction required about 13 h to go
to completion at initial concentrations of 0.047 M.
MotN + [Et3O][BArf4]. Et2O 8 [ModNEt][BArf4] (5)
[ModNEt][BArf4] was isolated as an orange solid. The high
solubility of [ModNEt][BArf4] in all common solvents and
therefore failure to obtain single crystals suitable for an X-ray
study again prevented structural elucidation. The electrochem-
istry of [ModNEt][BArf4] is comparable to that observed for
[ModNNEt2][BArf4]. An irreversible reduction is observed at
-1.82 V at a scan rate of 50 mV/s (Ipc; E1/2 ) -1.64 V vs
Synthesis and Reduction of [ModNNEt2][BArf4]. Alkylation
of MoNdNEt with 1 equiv of [Et3O][BArf4] in CH2Cl2 afforded
diamagnetic [ModNNEt2][BArf4] as an orange solid in 89%
1
isolated yield (eq 2). According to H NMR spectroscopy of
the reaction mixture, the conversion proceeds without formation
9
J. AM. CHEM. SOC. VOL. 131, NO. 35, 2009 12831

A R T I C L E S
Kupfer and Schrock
Fc/Fc+; recorded in PhF and 0.1 M [Bu4N][BArf4]); we assign
of the reaction mixtures by methods analogous to that shown
in eq 7.
this reduction to the [ModNEt]+/0 couple.
Despite the irreversible [ModNEt]+/0 couple observed in the
CV of [ModNEt][BArf4], [ModNEt][BArf4] can be reduced
with 1 equiv of CrCp*2 in benzene to give analytically pure
ModNEt in virtually 100% yield (eq 6). ModNEt is stable
both in the solid state and in solution over a period of several
weeks at room temperature. The stability of ModNEt contrasts
with the instability of ModNH,17 which is prone to decomposi-
tion in the absence of excess reducing agent to form MotN
and [Mo(NH3)]+. As expected for a paramagnetic Mo(V) d1
species, the 1H NMR Spectrum of ModNEt shows only broad
resonances in the diamagnetic region. No characteristic, para-
magnetically shifted resonances for either the ethylene backbone
or the ethyl protons could be observed readily at room
temperature. The electrochemical oxidation of ModNEt to
[ModNEt]+ is fully reversible in PhF (E1/2 ) -1.64 V vs Fc/
Fc+; 0.1 M [Bu4N][BArf4], Table 1). We cannot explain why
the reduction of [ModNEt][BArf4] is irreversible (vide supra),
while the oxidation of ModNEt is reversible.
+ Na[BArf4] + amine
MoCl
8 [Mo(amine)][BArf4]
(7)
- NaCl
Reactions similar to that shown in eq 7 that involve Et2NH,
Et3N, and Me3N did not yield any [Mo(amine)][BArf4] over a
period of several days, even at elevated temperatures. We ascribe
the failure to produce these [Mo(amine)][BArf4] species by
methods analogous to that shown in eq 7 to a failure to form
what is proposed to be the required Mo(Cl)(amine) intermediate.
We therefore turned to alternative approaches.
A successful synthesis of [Mo(Et2NH)][BArf4] is analogous
to the reaction between MoH and [2,6-LutH][B(C6F5)4] to yield
[Mo(2,6-LutH)][B(C6F5)4].17 [Et2NH2][BArf4] and [Et3NH]-
[BArf4] were prepared by salt metathesis reactions of Na[BArf4]
with either [NEt2H2]Cl or [Et3NH]Cl, respectively, and isolated
as white solids in high yields. Treatment of a benzene solution
of MoH with 1.1 equiv of [Et2NH2][BArf4] at room temperature
smoothly afforded [Mo(Et2NH)][BArf4] within 2 h (eq 8).
[Mo(Et2NH)][BArf4] is the sole Mo species present at any
significant concentration in the crude reaction mixture.
[Mo(Et2NH)][BArf4] was isolated as an analytically pure red,
crystalline solid in yields up to 84% after recrystallization from
pentane. The 1H NMR spectrum of [Mo(Et2NH)][BArf4] in C6D6
features two paramagnetically shifted resonances at -9.8 and
-88.3 ppm, similar to those observed for the ethylene backbone
protons in [Mo(EtNH2)][BArf4] (vide supra). The reduction of
[Mo(Et2NH)][BArf4] with 1 equiv of CrCp*2 in C6D6 under 1
atm of N2 produced [CrCp*2][BArf4], MoN2, and free Et2NH
+ CrCp*
2
[ModNEt][BArf4]
8 Mo ) NEt
(6)
- [CrCp*2][BArf4
]
The reactivity of ModNEt toward protonation and alkylation
is comparable to that observed for ModNNEt2 described above.
The reaction between ModNEt and 1 equiv of [Et3O][BArf4]
in various solvents at various temperatures only led to clean
oxidation of ModNEt to [ModNEt][BArf4] and formation of
butane (by proton NMR; not quantitated). No evidence for
formation of [MoNEt2][BArf4] was obtained. Addition of
[H(OEt2)2][BArf4] to ModNEt in C6D6 and CD2Cl2 also led
simply to oxidation of ModNEt to [ModNEt][BArf4]. As
expected on the basis of electrochemical potentials (Table 1),
ModNEt readily reduces [ModNH][BArf4] in benzene.
1
within 15 min, as judged by H NMR spectroscopy of the
reaction mixture. The presumed intermediate (Mo(Et2NH)) was
not observed.
Synthesis of Cationic [Mo(amine)]+ Complexes. We turned
to reactions between MoCl and Na[BAr4] (Ar ) Ph, Arf) in the
presence of NR3, R2NH, or RNH2 (R ) Me, Et) in order to
determine whether the crowded HIPT system would allow alkyl
amines to coordinate to the metal, just as ammonia does.
+ [Et2NH2][BArf4
MoH.
]8 [Mo(Et2NH)][BArf4]
(8)
- H2
All efforts to prepare [Mo(NEt3)][BArf4] in a reaction between
MoH and [Et3NH][BArf4] in several solvents (C6H6, CH2Cl2,
PhF, NEt3) were not successful, even at temperatures up to 100
°C.
[Mo(EtNH2)][BArf4] was isolated as a red-brown, crystalline
solid in good yield (75%) as shown in eq 7 (amine ) EtNH2).
1
Its H NMR spectrum displays paramagentically shifted reso-
Synthesis and Characterization of MoNEt2. Because we could
not prepare [MoNEt2][BArf4] through the reaction of ModNEt
with [Et3O][BArf4] (vide supra), we did not have the option to
prepare MoNEt2 through reduction of [MoNEt2]+. Therefore
we had to devise an alternative synthesis of MoNEt2.
nances for the ethylene backbone at -16.6 and - 98.9 ppm.
Reduction of [Mo(EtNH2)][BArf4] with 1 equiv of CrCp*2 in
C6D6 under 1 atm of N2 afforded MoN2, [CrCp*2][BArf4], and
free EtNH2 in less than 15 min. The complete exchange of the
ethylamine ligand in presumed intermediate Mo(EtNH2) by
dinitrogen contrasts with what is found for the reaction between
Mo(NH3) and N2, a reaction for which Keq ) 1.16(6) in benzene
at 22 °C.17 Whether the reaction between Mo(EtNH2) and
dinitrogen is zero order or first order in dinitrogen (as it is for
the reaction between Mo(NH3) and N2) is not known. The CV
of [Mo(EtNH2)][BArf4] under argon shows a quasireversible
reduction couple in PhF (E1/2 ) -1.83 V; 0.1 M [Bu4N][BArf4];
Table 1) that we assign as [Mo(EtNH2)]+/0. Interestingly, even
though CrCp*2 is technically not a good enough reducing agent
in PhF to accomplish the reduction of [Mo(EtNH2)][BArf4] (E1/2
) -1.63 V; Table 1), the reduction proceeds quantitatively.
Evidently precipitation of [CrCp*2][BArf4] provides the ad-
ditional driving force to complete the reduction in PhF.12,13,15,17,18
[Mo(MeNH2)][BArf4] and [Mo(Me2NH)][BArf4] could be gen-
erated and characterized in situ through 1H NMR spectroscopy
MoNEt2 was isolated as a green solid in a yield of 54% upon
treatment of [Mo(NH3)][BPh4] with LiNEt2 in diethyl ether (eq
9). The reaction had to be conducted in the absence of N2,
otherwise significant amounts of MoN2 were formed as a side
product. In the absence of N2, the conversion proceeded cleanly
in 2 h and the 1H NMR spectrum of the crude reaction mixture
indicated that MoNEt2 was present in a yield of 90-95%.
MoNEt2 is noticeably more stable than MoNH2 (attempted
isolation of which led to the formation of MotN) and could
be stored in the solid state for several weeks at -30 °C without
decomposition. We could not prepare MoNEt2 through substitu-
tion of the chloride ligand in MoCl by LiNEt2 or through
deprotonation of [Mo(Et2NH)][BArf4] with LiNEt2, Li[N-
(SiMe3)2], or KO-t-Bu, an approach that was successful for
preparing MoNH2.17
9
12832 J. AM. CHEM. SOC. VOL. 131, NO. 35, 2009

A R T I C L E S
+ LiNEt2
[Mo(NH3)][BPh4]
8 MoNEt2
(9)
- Li[BPh4] - NH3
1
The H NMR spectrum of MoNEt2 at room temperature
features paramagnetically shifted resonances for the ethylene
backbone at -6.5 and -39.5 ppm, which is suggestive of a
ground-state high-spin d2 Mo configuration or a high-spin/low-
spin equilibrium, as found in the related compounds
24
[(RNCH2CH2)3N]MoNMe2 (R ) SiMe3 or C6F524,28). The
resonances of the ethyl groups are also significantly shifted to
-1.0 ppm (NCH2CH3) and -66.6 ppm (NCH2CH3). The
temperature dependence of the four resonances observed in the
1H NMR spectrum of MoNEt2 in the range -70 to +80 °C
allowed us to determine the spin multiplicity of MoNEt2 in the
ground state. A plot of the chemical shift of each resonance
versus 1/T revealed a linear relationship for all, as expected for
a Curie-Weiss paramagnet in solution (Figure 4). The methyl
resonances in the diethylamide group were almost independent
of T as a consequence of their being further away from Mo
than the C2H4 protons in the ligand and the methylene protons
in the diethylamide group. Thus, in contrast to the low-spin
ground-state d2 configurations found for [(RNCH2CH2)3N]-
MoNMe2 (R ) SiMe324 or C6F524,28), MoNEt2 has a high-spin
ground-state d2 configuration.
Figure 4. Plot of the chemical shift of selected resonances in MoNEt2
versus 1/T.
The second temperature-dependent process that is observed
in NMR spectra of MoNEt2 is loss of C3 symmetry at low
temperature (Figure 5). As the temperature is lowered, the ligand
is locked into a C3 configuration and the dimethylamido group
no longer freely rotates. At 200 K at least nine resonances are
observed for the two former ethylene backbone protons.
The cyclic voltammogram of MoNEt2 in PhF (0.1 M
[Bu4N][BArf4]) reveals a quasireversible oxidation process at
E1/2 ) -0.98 V (vs Fc/Fc+), as well as an irreversible process
at -1.74 V (Ipc vs Fc/Fc+; scan rate 50 mV/s; Table 1). We
assign the former to the one-electron oxidation of MoNEt2 to
[MoNEt2]+, while the nature of the latter is not known. Chemical
oxidation of MoNEt2 with [Fc][BArf4] in toluene over a period
of 3 days is accompanied by a gradual color change of the deep
blue solution to deep red. Ferrocene is formed along with a
new species with paramagnetically shifted resonances at -8.2,
-21.3, -35.7, and -104.3 ppm. Unfortunately, side products
are also formed in minor amounts upon oxidation of MoNEt2,
and the identity of the major product, which we propose is
[MoNEt2]+, therefore cannot be confirmed.
To be able to compare compounds that contain the same
dialkylamido ligand, MoNMe2 was prepared through reaction
of [Mo(NH3)][BPh4] with LiNMe2 in diethyl ether in the absence
of dinitrogen. MoNMe2 was obtained as a dark green solid in
a moderate yield (39%). MoNMe2 is significantly less stable
than MoNEt2 both in the solid state and in solution. Decomposi-
tion of MoNMe2 to undefined products was observed within
24 h in C6D6 solution and within 5 d in the solid state. The 1H
NMR spectrum of MoNMe2 in C6D6 displays paramagnetically
shifted resonances for the ethylene backbone at -3.0 and -40.4
ppm at room temperature, as well as for the amine methyl groups
at -28.3 ppm. Variable-temperature proton NMR spectra of
MoNMe2 in toluene-d8 in a range of -20 to +80 °C (Figure 6)
are analogous to those obtained for MoNEt2. Therefore, we
conclude that MoNMe2 also has a high-spin ground state d2
configuration. A low-spin d2 configuration (e.g., for [(RNCH2-
Figure 5. Part of the temperature-dependent 1H NMR spectrum of MoNEt2.
24
CH2)3N]MoNMe2 (R ) SiMe3 or C6F524,28)) would suggest
that the amido ligand is planar, while a high-spin d2 configu-
ration for MoNEt2 and MoNMe2 would suggest that the amido
ligand is not planar but rather pyramidal. We propose that a
planar geometry for a diethylamino or dimethylamido group in
MoNEt2 and MoNMe2 must not be possible because of steric
interaction with the isopropyl HIPT substituents. It should be
noted that the energy difference between high-spin and low-
24
spin forms of [(RNCH2CH2)3N)MoNMe2] (R ) SiMe3 or
C6F524,28) is small (∼2 kcal/mol), so the steric interaction that
is required for the lowest energy state to be high-spin instead
of low-spin need not be dramatic.
Alkylation and Reduction of MoNEt2. Alkylation of MoNEt2
with 1 equiv of [Et3O][BArf4] in diethyl ether in the absence of
dinitrogen cleanly afforded [Mo(NEt3)][BArf4] as a deep red
(28) Kol, M.; Schrock, R. R.; Kempe, R.; Davis, W. M. J. Am. Chem.
Soc. 1994, 116, 4382.
9
J. AM. CHEM. SOC. VOL. 131, NO. 35, 2009 12833

A R T I C L E S
Kupfer and Schrock
[DTBTN3N]MoCl + NaN3 f [DTBTN3N]MotN +
NaCl + N2 (11)
Alkylation of [DTBTN3N]MotN with one equiv of
[Et3O][BArf4] in CD2Cl2 was complete within 10 min, which is
much faster than alkylation of [HIPTN3N]MotN. Even in
benzene, the formation of [(DTBTN3N)ModNEt][BArf4] was
complete within 15 min. [(DTBTN3N)ModNEt][BArf4] was
obtained in a 34% yield through crystallization from heptane.
Higher yields were thwarted by the poor crystallization behavior
of [(DTBTN3N)ModNEt][BArf4], which tended to form a deep
red oil when concentrated solutions of it in pentane or heptane
were cooled. These results suggest that the Mo center in
[DTBTN3N]Mo species is much more accessible than in the
[HIPTN3N]Mo system, as expected.
Attempts To Generate Triethylamine. We first attempted to
generate triethylamine in the [HIPTN3N]Mo system. MoN2 and
MotN were chosen as starting compounds in experiments that
employed [Et3O][BArf4] and CrCp*2. Addition of 7 equiv of
[Et3O][BArf4] and 8 equiv of CrCp*2 to a benzene solution of
MoN2 or 3.5 equiv of [Et3O][BArf4] and 4.2 equiv of CrCp*2
to a benzene solution of MotN led to no detectible NEt3 or
[Et4N][BArf4] by 1H NMR spectroscopy or gas chromatography.
[Et3O][BArf4] is almost insoluble in benzene but is consumed,
and diethyl ether is formed. The solid residues contained
[CrCp*2][BArf4], and either a mixture of ModNNEt and
[ModNNEt2][BArf4] or unreacted MotN.
Figure 6. Part of the temperature-dependent 1H NMR spectrum of
MoNMe2.
solid in 61% yield after 20 h (eq 10). This result demonstrates
that triethylamine can coordinate to the metal in a cationic Mo
core, a fact that would be necessary for a viable catalytic cycle
(11 and 12 in Figure 2). The 1H NMR spectrum of paramagnetic
[Mo(NEt3)][BArf4] features characteristic resonances for the
ethylene backbone protons at -16.3 and -99.1 ppm, as well
as resonances for the amine protons at -2.5 and -10.8 ppm.
As expected, reduction of [Mo(NEt3)][BArf4] with CrCp*2 in
C6D6 under 1 atm of N2 was accompanied by precipitation of
yellow [CrCp*2][BArf4] and formation of MoN2, as judged by
1H NMR spectroscopy of the reaction mixture. Therefore, if
[Mo(NEt3)][BArf4] could be formed under catalytic conditions,
it would be reduced under dinitrogen to yield triethylamine and
MoN2, thus completing the catalytic cycle.
Several experiments were conducted in a manner analogous
to that employed for the catalytic reduction of dinitrogen to
ammonia except the CrCp*2 was added over a period of 10-16
h. In all cases, no NEt3 was formed, as judged by gas
chromatography, and the residues contained [CrCp*2][BArf4],
MotN, and undefined decomposition products; no [Et4N]-
1
[BArf4] was observed by H NMR spectroscopy.
Similar experiments were carried out employing
[(DTBTN3N)ModNEt][BArf4], [Et3O][BArf4], and CrCp*2. Ad-
dition of 2.2 equiv of [Et3O][BArf4] and 3.2 equiv of CrCp*2 to
a benzene solution of [(DTBTN3N)ModNEt][BArf4] was fol-
lowed by stirring the mixture over a period of 20 h.
[(DTBTN3N)ModNEt][BArf4], [Et3O][BArf4], and CrCp*2 were
all consumed, but no evidence for the formation of NEt3 or
+ [Et3O][BArf4
MoNEt2
]8 [Mo(NEt3)][BArf4]
(10)
- Et2O
1
[Et4N][BArf4] could be obtained from H NMR spectroscopic
[DTBTN3N]Mo System. The [HIPTN3N]Mo system appears
to be well-suited for the addition of protons and electrons to
dinitrogen and NxHy ligands,29 but for steric reasons not for
addition of ethyl groups and electrons. Therefore, smaller
terphenyl substituents might lead to faster alkylation rates and
still relatively stable intermediates. For example, the
[DTBTN3N]3- ligand (DTBT ) 3,5-(4-t-BuC6H4)2C6H3), which
is a sterically less demanding terphenyl analogue of the HIPT
system, is known.30 Its complexes are also known to form
diazenidoderivatives,e.g.,[(DTBTN3N)MoN2]- and[DTBTN3N]-
MoNdNSiMe3.26 Therefore we reasoned that the [DTBTN3N]3-
ligand might solve some of the problems associated with the
alkylation chemistry of [HIPTN3N]Mo species.
[DTBTN3N]MotN was synthesized through a reaction
between [DTBTN3N]MoCl and 2 equiv of NaN3 in THF at room
temperature (eq 11). It was isolated as a yellow powder in a
yield of 74%. We could not obtain [DTBTN3N]MotN through
addition of several equivalents of Me3SiN3 to [DTBTN3N]MoCl
in toluene at ambient and elevated temperatures.
data, and no (DTBTN3N)Mo derivatives could be identified.
Similar results were obtained in attempted catalytic runs using
36 equiv of [Et3O][BArf4] and 48 equiv of CrCp*2 in octane or
toluene. Proton NMR analysis of the nonvolatile components
revealed no [Et4N][BArf4]; only [CrCp*2][BArf4] could be
identified. Analysis of the volatiles by gas chromatography
revealed a product with a retention time identical to that of NEt3,
but the amount was e0.2 equiv.
Summary and Conclusions
We have prepared several possible intermediates in a hy-
pothetical catalytic reduction of dinitrogen to triethylamine with
(HIPTN3N)Mo species, among them MoNdNEt, [Mod
NNEt2][BArf4], ModNNEt2, [ModNEt][BArf4], ModNEt, Mo-
NEt2, and [MoNEt3][BArf4]. ModNNEt2 and ModNEt are
especially interesting because ModNNH2 has not been ob-
served, and ModNH loses dihydrogen to give MotN upon
attempted isolation. The protio analogue of MoNEt2 (MoNH2)
is also unstable with respect to loss of dihydrogen to give
MotN. We were surprised to find that both ModNNEt2 and
ModNEt are oxidized by [Et3O][BArf4] to reform the cationic
(29) Schrock, R. R. Acc. Chem. Res. 2005, 38, 955.
(30) Greco, G. E.; Schrock, R. R. Inorg. Chem. 2001, 40, 3850.
9
12834 J. AM. CHEM. SOC. VOL. 131, NO. 35, 2009

A R T I C L E S
and used without further purification. NMR spectra were recorded
on a Bruker Avance 400 spectrometer (1H, 400 MHz; 11B, 128
MHz; 13C, 100 MHz), and VT NMR spectra on Varian Inova 500
spectrometers (1H, 500 MHz). Chemical shifts for 1H NMR spectra
were referenced to the residual 1H NMR resonance of the deuterated
solvent, those of the 13C NMR spectra to the 13C NMR resonances
of the solvent itself, and are reported as parts per million relative
to tetramethylsilane. 11B NMR spectra were referenced externally
to a solution of B(OH)3 in D2O. Electrochemical measurements
were carried out in an argon-filled glovebox using a CHI 620C
potentiate, 0.1 M [Bu4N][BArf4]/PhF electrolytes, and a standard
three-electrode cell assembly with a glassy carbon (3.0 mm
diameter) disk working electrode, a platinum wire auxiliary
electrode, and a reference electrode consisting of a AgCl-coated
silver wire submerged in 0.1 M [Bu4N][BArf4]/PhF electrolyte. All
measurements were referenced externally and/or internally with
FeCp2. Elemental analyses were performed by Midwest Microlabs,
Indianapolis, IN.
Mo(VI) species instead of being alkylated. ModNNEt2 is also
oxidized by [ModNNH2]+ or [H(Et2O)2][BArf4]. Therefore it
seems unlikely that it will be possible to complete a catalytic
cycle in which ModNNEt2 must be alkylated (5 f 6; Figure
2) and ModNEt must be alkylated (9 f 10; Figure 2).
We know that reduction of [ModNNH2]+ with CrCp*2 gives
Mo-NdNH, MotN, [Mo(NH3)]+, Mo(NH3), and ammonia.17
ModNNH2 is proposed to be the first product of reduction
of [ModNNH2]+, but ModNNH2 is then protonated, possibly
by [ModNNH2]+ in a base-catalyzed proton transfer, to give
[ModNNH3]+ (and ModNNH), and [ModNNH3]+ is then
reduced by CrCp*2 or some other species in solution (including
ModNNH2) to give MotN and ammonia. Therefore Mod
NNH2 and [ModNNH3]+ are most likely intermediates, but how
they are formed and consumed is unclear.
All efforts to generate NEt3 employing (HIPTN3N)Mo species
failed, a result that is consistent with slow alkylations. However,
efforts to generate NEt3 in the (DTBTN3N)Mo also have failed
so far, or at least NEt3 was not formed catalytically. Neverthe-
less, sterically less demanding triamidoamine ligands would
seem to be more likely to allow formation of NEt3 catalytically
in a manner analogous to that proposed for catalytic formation
of ammonia.
[Et3O][BArf4]. A mixture of [Et3O][BF4] (0.15 g, 0.79 mmol)
and Na[BArf4] (0.74 g, 0.83 mmol, 1.05 equiv) was stirred in Et2O
(10 mL) at room temperature for 3 d. The solvent removed in vacuo.
The residue was extracted with CH2Cl2, and the extract was filtered
through Celite. Removal of all volatiles afforded [Et3O][BArf4] (0.72
g, 0.74 mmol, 94%) as a white solid, which was washed with
pentane and dried in vacuo. A concentrated CH2Cl2 solution was
layered with pentane and cooled to -30 °C to afford analytically
Unlike protonation of an amido nitrogen in the ligand,
alkylation of an amido nitrogen in the ligand would likely be
irreversible and probably would lead to dissociation of the amine
donor from the metal. Some evidence to support this proposal
exists in triamidoamine chemistry of molybdenum.24-26 Alky-
lation of an amido nitrogen in the [HIPTN3N]3- ligand could
be regarded as another potential pitfall in the catalytic formation
of triethylamine by a sequence of reactions analogous to those
proposed for catalytic formation of ammonia.
1
pure [Et3O][BArf4]: H NMR (400 MHz, CD2Cl2, 297 K) δ 1.61
(t, 9H, CH3), 4.61 (q, 6H, CH2), 7.74 (s, 4H, BArf), 7.60 (s, 8H,
BArf); 11B{1H} NMR (128 MHz, CD2Cl2, 297 K) δ -26.0. Anal.
Calcd (%) for C38H27BF24O (966.39): C, 47.23; H, 2.82. Found: C,
47.30; H, 2.80.
(HIPTN3N)MoNdNEt. Method A. (HIPTN3N)MoCl (0.15 g,
87.37 µmol) was reduced with sodium sand (380.1 mg, 0.87 mmol,
10 equiv) in THF (4 mL) under 1 atm of dinitrogen to form
[(HIPTN3N)MoN2]-, as reported in the literature.16 The deep green
reaction mixture was filtered through Celite, and the solvents were
removed from the filtrate in vacuo. The residue was dissolved in
ether (2 mL). The mixture was cooled to -30 °C and treated
dropwise with a solution of [Et3O][BArf4] (84.4 mg, 87.37 µmol,
1 equiv) in ether (2 mL). The reaction was allowed to stir at ambient
temperatures over a period of 24 h, and the solvent was subsequently
removed under vacuum. The residue was extracted into pentane
and filtered through Celite. After removal of all volatiles, the crude
product was recrystallized from heptane at -30 °C to afford
(HIPTN3N)MoNdNEt (63.8 mg, 36.69 µmol, 42%) as a pale
yellow, crystalline solid in several crops: 1H NMR (400 MHz, C6D6,
297 K) δ 0.87 (t, 3H, -NCH2CH3), 1.14 (d, 36H, -CH(CH3)2), 1.22
(d, 36H, -CH(CH3)2), 1.38 (d, 36H, -CH(CH3)2), 2.08 (br t, 6H,
-NCH2CH2N-), 2.94 (m, 6H, -CH(CH3)2), 3.32 (m, 12H, -CH(CH3)2),
3.31 (q, 2H, -NCH2CH3), 3.72 (br t, 6H, -NCH2CH2N-), 6.68 (s, 3H,
4,4′,4′′-TerH),7.20(s,12H,3,5,3′,5′,3′′,5′′-TipH),7.29(s,6H,2,6,2′,6′,2′′,6′′-
TerH). Anal. Calcd (%) for C116H164MoN6 (1738.52): C, 80.14; H,
9.51; N, 4.83. Found: C, 79.84; H, 9.54; N, 4.85.
Method B (Preferred). An ether solution (8 mL) of [Et3O]-
[BArf4] (0.52 g, 0.54 mmol, 1.05 equiv) was cooled to -30 °C
and treated with solid [Bu4N][(HIPTN3N)MoN2] (1.00 g, 0.51
mmol). A red solution formed within 30 min and was stirred for
20 h. The volatile components were removed in vacuo, and the
residue was dried at 50 °C for 3 h. The residue was extracted into
pentane, and the mixture was filtered through Celite. All volatiles
were removed and the crude product was recrystallized from
heptane at -30 °C to afford (HIPTN3N)MoNdNEt (0.61 g total,
0.35 mmol, 69%) in several crops.
Experimental Section
General. Air- and moisture-sensitive compounds were manipu-
lated under 1 atm of dry N2 or argon by standard Schlenk techniques
or in a glovebox using flame- and oven-dried glassware. HPLC
grade pentane, benzene, toluene, ether, THF, and CH2Cl2 were
sparged with dinitrogen, passed through activated alumina, and
stored over 4 Å Linde-type molecular sieves prior to use. Heptane,
octane, NEt2H, and NEt3 were dried by refluxing over molten
potassium, vacuum transferred, degassed, and stored over molecular
sieves. Gaseous H2NMe, HNMe2, NMe3, and NEt2H were con-
densed onto sodium sand, stirred over a period of 3 h at -40 °C,
and vacuum transferred into the reaction vessel. PhF was dried over
CaH2, degassed, and vacuum distilled prior to use. Benzene-d6,
toluene-d8, and methylene chloride-d2 were freeze-pump-thaw
degassed and stored over activated molecular sieves for at least 2
days prior to use. MoCl,16 MoN2,16 [Bu4N][MoN2],16 MotN,16
[Mo(NH3)][BPh4],16 MoH,17 [ModNNH2][BArf4],16 [ModNH]-
[BArf4], (DTBTN3N)MoCl,26 [H(OEt2)2][BArf4],31 Na[BArf4], Ag-
[BArf4],32 CrCp*2,33 and [Fc][BArf4]34 were prepared according to
known methods. [Bu4N][BArf4] was prepared by salt metathesis of
[Bu4N]Cl with Na[BArf4] in diethyl ether and recrystallized from
CH2Cl2/pentane. [NEt2H2][BArf4] and [NEt3H][BArf4] were syn-
thesized by reaction of NEt2H·HCl and NEt3 ·HCl with Na[BArf4]
in diethyl ether. All other chemicals were obtained commercially
(31) Brookhart, M.; Grant, B.; Volpe, A. F., Jr. Organometallics. 1992,
11, 3920.
(32) Hayashi, Y.; Rohde, J. J.; Corey, E. J. J. Am. Chem. Soc. 1996, 118,
5502.
Method C. A solid mixture of (HIPTN3N)MoN2 (25.0 mg, 14.62
µmol), [Et3O][BArf4] (14.2 mg, 14.62 µmol, 1 equiv), and CrCp*2
(6.2 mg, 14.62 µmol, 1 equiv) was stirred in benzene (7 mL) at
room temperature over a period of 2 days. All volatile components
were removed in vacuo, and the residue was extracted into pentane.
The extract was filtered through Celite, and the volatiles were
(33) Robbins, J. L.; Edelstein, N.; Spencer, B.; Smart, J. C. J. Am. Chem.
Soc. 1982, 104, 1882.
(34) Chavez, I.; Alvarez-Carena, A.; Molins, E.; Roig, A.; Maniukiewicz,
W.; Arancibia, A.; Arancibia, V.; Brand, H.; Manriquez, J. M. J.
Organomet. Chem. 2000, 601, 126.
9
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A R T I C L E S
Kupfer and Schrock
removed from the filtrate in vacuo again. The crude product was
recrystallized from heptane at -30 °C to afford (HIPTN3N)-
MoNdNEt (13.7 mg, 7.90 µmol, 54%).
The mixture was filtered through Celite and all volatiles were
removed in vacuo to yield (HIPTN3N)ModNEt (0.15 g, 85.98
µmol, 89%) as a red, crystalline solid: 1H NMR (400 MHz, C6D6,
297 K) δ 0.91 (br t, 3H, -NCH2CH3), 1.31 (br s, 72H, -CH(CH3)2),
1.40 (br s, 36H, -CH(CH3)2), 2.95 (br m, 12H, -CH(CH3)2), 3.16
(br m, 6H, -CH(CH3)2), 7.06 (br s, 12H, 3,5,3′,5′,3′′,5′′-TipH); the
other signals could not be observed at room temperature. Anal.
Calcd (%) for C116H164MoN5 (1724.52): C, 80.79; H, 9.59; N, 4.06.
Found: C, 80.52; H, 9.23; N, 4.29.
[(HIPTN3N)ModNNEt2][BArf4]. An CH2Cl2 solution (3 mL)
of (HIPTN3N)MoNdNEt (0.15 g, 86.28 µmol) and [Et3O][BArf4]
(83.4 mg, 86.28 µmol, 1 equiv) was stirred at room temperature
for 1 h to give a dark orange solution. Volatiles were removed in
vacuo, and the solid residue was extracted into pentane. The extracts
were filtered through Celite, and the solvents were removed from
the filtrate in vacuo. The remaining material was dried at 60 °C
under high vacuum to yield [(HIPTN3N)ModNNEt2][BArf4] (0.20
g, 76.79 µmol, 89%) as an orange, amorphous solid: 1H NMR (400
MHz, C6D6, 297 K) δ 0.66 (t, 6H, -NCH2CH3), 1.08 (d, 36H,
-CH(CH3)2), 1.14 (d, 36H, -CH(CH3)2), 1.42 (d, 36H, -CH(CH3)2),
2.52 (br t, 6H, -NCH2CH2N-), 2.81 (m, 12H, -CH(CH3)2), 2.98 (m,
12H, -CH(CH3)2, -NCH2CH3), 3.77 (br t, 6H, -NCH2CH2N-), 6.89
(s, 9H, 2,4,6,2′,4′,6,′2′′,4′′,6′′-TerH), 7.19 (s, 12H, 3,5,3′,5′,3′′,5′′-
TipH), 7.75 (s, 4H, BArf), 8.45 (s, 8H, BArf). Anal. Calcd (%) for
C150H181BF24MoN6 (2630.8): C, 68.48; H, 6.93; N, 3.19. Found:
C, 68.73; H, 7.00; N, 3.10.
Reaction of (HIPTN3N)ModNEt with [Et3O][BArf4]. De-
gassed CD2Cl2 (0.6 mL) was added to a solid mixture of
(HIPTN3N)ModNEt (20.3 mg, 11.77 µmol) and [Et3O][BArf4]
(11.4 mg, 11.77 µmol, 1 equiv). The mixture was thawed to 22
°C, and allowed to react for 1 h, during which time an orange
solution formed. The gaseous components of the reaction mixture
were vacuum transferred onto frozen and degassed CD2Cl2 (0.6
mL). The 1H NMR spectrum of the volatiles indicated the presence
1
of butane: H NMR (400 MHz, CD2Cl2, 297 K) δ 0.88 (t, 6H,
CH2CH3), 1.28 (m, 4H, CH2CH3).
[(HIPTN3N)Mo(H2NEt)][BArf4]. Gaseous H2NEt was con-
densed onto Na sand, and the mixture was stirred over a period of
4 h at -30 °C. Two equivalents (57 mL, 0.29 mmol, 95 Torr) were
transferred onto a frozen solution of (HIPTN3N)MoCl (0.25 g, 0.15
mmol) and Na[BArf4] (0.15 g, 0.15 mmol, 1.05 equiv) in CH2Cl2
(10 mL). The mixture was allowed to thaw to room temperature
and was stirred for another 2 h. During this time, the reaction
mixture turned red brown in color. The volatiles were removed in
vacuo, and the residue was extracted into pentane. The extract was
filtered through Celite. The filtrate was reduced in volume and
stored at -30 °C to afford [(HIPTN3N)Mo(H2NEt)][BArf4] (0.28
g, 0.11 mmol, 75%) as a red brown, microcrystalline solid. The
product was washed with cold pentane and dried in vacuo: 1H NMR
(400 MHz, C6D6, 297 K) δ -98.9 (br s, 6H, -NCH2CH2N-), -16.6
(br s, 6H, -NCH2CH2N-), -2.63 (br s, 2H, -H2NCH2CH3), 0.56
(br s, 3H, -NCH2CH3), 1.09 (br s, 36H, -CH(CH3)2), 1.17 (br s,
36H, -CH(CH3)2), 1.40 (br s, 36H, -CH(CH3)2), 2.77 (br m, 12H,
-CH(CH3)2), 2.99 (br m, 8H, -CH(CH3)2, -NCH2CH3), 7.25 (br s,
12H, 3,5,3′,5′,3′′,5′′-TipH), 7.64 (s, 4H, BArf), 8.28 (s, 8H, BArf),
9.73 (br s, TerH); the other signals could not be observed at room
temperature. Anal. Calcd (%) for C148H178BF24MoN5 (2589.74): C,
68.64; H, 6.93; N, 2.70. Found: C, 68.43; H, 6.94; N, 2.80.
[(HIPTN3N)Mo(NEt2H)][BArf4]. A solid mixture of (HIPTN3N)-
MoH (0.25 g, 0.15 mmol) and [H2NEt2][BArf4] (0.15 g, 0.16 mmol,
1.1 equiv) was treated with benzene (3 mL), and stirred over a
period of 2 h at room temperature. All volatiles were removed in
vacuo, and the residue extracted into pentane and filtered through
Celite. Removal of all volatile components under high vacuum at
60 °C for 3 h yielded [(HIPTN3N)Mo(NEt2H)][BArf4] (0.33 g, 0.13
mmol, 84%) as a red brown solid: 1H NMR (400 MHz, C6D6, 297
K): δ -88.3 (br s, 6H, -NCH2CH2N-), -9.8 (br s, 6H, -NCH2CH2N-
), 0.89 (t, 6H, -NCH2CH3), 1.17 (br s, 72H, -CH(CH3)2), 1.40 (d,
36H, -CH(CH3)2), 2.31 (br s, 4H, -NCH2CH3), 2.90 (br m, 6H,
-CH(CH3)2), 3.00 (br m, 12H, -CH(CH3)2, -NCH2CH3), 7.23 (s,
12H, 3,5,3′,5′,3′′,5′′-TipH), 7.64 (s, 4H, BArf), 8.26 (s, 8H, BArf),
10.6 (br s, TerH); the other signals could not be observed at room
temperature. Anal. Calcd (%) for C150H182BF24MoN5 (2617.8): C,
68.82; H, 7.01; N, 2.68. Found: C, 68.92; H, 7.09; N, 2.72.
(HIPTN3N)MoNEt2. Diethyl ether (10 mL) was freeze-pump-
thaw degassed thoroughly and condensed onto a solid mixture of
[(HIPTN3N)Mo(NH3)][BPh4] (0.45 g, 0.22 mmol) and LiNEt2 (1.8
mg, 0.22 mmol, 1 equiv). The reaction mixture was allowed to
warm to ambient temperature and was stirred for 2 h. All volatiles
were removed in vacuo, and the residue was extracted into pentane
under 1 atm of argon. The mixture was filtered through Celite, and
the filtrate was brought to dryness in vacuo. The crude reaction
product was recrystallized from heptane at -30 °C. (HIPTN3N)-
MoNEt2 (0.21 g, 0.12 mmol, 54%) was isolated as a green,
(HIPTN3N)ModNNEt2. A solution of [(HIPTN3N)ModNNEt2]-
[BArf4] (0.10 g, 39.53 µmol) in benzene (6 mL) was treated with
solid CrCp*2 (12.7 mg, 39.53 µmol, 1 equiv), and stirred for 18 h
at room temperature, during which time a yellow precipitate and a
dark red solution formed. All volatiles were removed in vacuo,
and the residue was extracted with pentane. The extracts were
filtered through a medium porosity frit to yield a deep red filtrate
and yellow [CrCp*2][BArf4] (45.9 mg, 38.74 µmol, 98%). The
1
filtrate was brought to dryness to afford a deep red solid. The H
NMR spectroscopy of the crude reaction mixture indicated the
formation of (HIPTN3N)ModNNEt2 with a purity of 90%. All
attempts to further purify the crude product have been unsuccessful:
1H NMR (400 MHz, C6D6, 297 K) δ 1.22 (br m, 72H, -CH(CH3)2),
1.35 (br s, 36H, -CH(CH3)2), 2.91 (br m, 12H, -CH(CH3)2), 3.18
(br m, 6H, -CH(CH3)2), 7.08 (br s, 12H, 3,5,3′,5′,3′′,5′′-TipH), all
other signals were not observed at room temperature.
[(HIPTN3N)ModNEt][BArf4]. A CH2Cl2 solution (5 mL) of
(HIPTN3N)MotN (0.40 g, 0.24 mmol) and [Et3O][BArf4] (0.23 g,
0.24 mmol, 1 equiv) was stirred at room temperature for 18 h to
give a dark orange solution. Volatiles were removed in vacuo, and
the solid residue was extracted with pentane. The extracts were
filtered through Celite, and the filtrate was brought to dryness. The
remaining material was dried at 60 °C under high vacuum to yield
[(HIPTN3N)ModNEt][BArf4] (0.57 g, 0.22 mmol, 93%) as an
1
orange, amorphous solid: H NMR (400 MHz, CD2Cl2, 297 K) δ
0.62 (t, 3H, -NCH2CH3), 0.85 (d, 36H, -CH(CH3)2), 0.96 (d, 36H,
-CH(CH3)2), 1.28 (d, 36H, -CH(CH3)2), 2.51 (m, 12H, -CH(CH3)2),
2.91 (m, 6H, -CH(CH3)2), 3.27 (br t, 6H, -NCH2CH2N-), 3.59 (q,
2H, -NCH2CH3), 4.28 (br t, 6H, -NCH2CH2N-), 6.80 (s, 3H, 4,4′,4′′-
TerH), 6.84 (s, 6H, 2,6,2′,6,′2′′,6′′-TerH), 6.98 (s, 12H, 3,5,3′,5′,3′′,5′′-
TipH), 7.56 (s, 4H, BArf), 7.72 (s, 8H, BArf); 1H NMR (400 MHz,
C6D6, 297 K): δ ) 0.68 (t, 3H, -NCH2CH3), 1.00 (d, 36H,
-CH(CH3)2), 1.10 (d, 36H, -CH(CH3)2), 1.34 (d, 36H, -CH(CH3)2),
2.30 (br t, 6H, -NCH2CH2N-), 2.69 (m, 12H, -CH(CH3)2), 2.91 (m,
6H, -CH(CH3)2), 3.64 (m, 8H, -NCH2CH2N-, -NCH2CH3), 6.74 (s,
3H, 4,4′,4′′-TerH), 6.82 (s, 6H, 2,6,2′,6,′2′′,6′′-TerH), 7.14 (s, 12H,
3,5,3′,5′,3′′,5′′-TipH), 7.69 (s, 4H, BArf), 8.34 (s, 8H, BArf). Anal.
Calcd (%) for C148H176BF24MoN5 (2587.73): C, 68.69; H, 6.86; N,
2.71. Found: C, 68.48; H, 6.83; N, 2.88.
(HIPTN3N)ModNEt. A solution of [(HIPTN3N)ModNEt]-
[BArf4] (0.25 g, 96.61 µmol) in benzene (6 mL) was treated with
solid CrCp*2 (31.2 mg, 96.61 µmol, 1 equiv), and the mixture was
stirred for 18 h at room temperature, during which time a yellow
precipitate and a dark red solution formed. All volatiles were
removed in vacuo at 60 °C, and the residue was extracted with
pentane. The extracts were filtered through a medium porosity frit,
yielding a deep red filtrate and yellow [CrCp*2][BArf4] (0.11 g,
93.71 µmol, 97%). The filtrate was taken to dryness in vacuo to
afford a deep red solid, which was again extracted with pentane.
1
microcrystalline solid: H NMR (500 MHz, toluene-d8, 293 K) δ
-66.6 (br s, 6H, -NCH2CH3), -39.5 (br s, 6H, -NCH2CH2N-), -6.5
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12836 J. AM. CHEM. SOC. VOL. 131, NO. 35, 2009

A R T I C L E S
(br s, 6H, -NCH2CH2N-), -1.97 (br s, TerH), -1.01 (br s, 4H,
-NCH2CH3), 1.01 (br s, 36H, -CH(CH3)2), 1.25 (br s, 36H,
-CH(CH3)2), 1.40 (d, 36H, -CH(CH3)2), 2.97 (br m, 6H, -CH(CH3)2),
3.05 (br s, 12H, -CH(CH3)2), 7.15 (s, 12H, 3,5,3′,5′,3′′,5′′-TipH);
other resonances could not be observed. Anal. Calcd (%) for
(DTBTN3N)MotN. THF (4 mL) was added to a solid mixture
of (DTBTN3N)MoCl (0.35 g, 0.27 mmol) and NaN3 (34.6 mg, 0.53
mmol, 2 equiv), and the reaction mixture was stirred at room
temperature for 3 d. The volatile components were removed in
vacuo, and the solid residue extracted into benzene. The extracts
were filtered through Celite and taken to dryness in vacuo. The
remaining material was recrystallized from Et2O/pentane to afford
(DTBTN3N)MotN (33; 0.25 g, 0.20 mmol, 74%) as an orange
powder. 1H NMR (400 MHz, C6D6, 297 K): δ 1.23 (s, 54H,
-C(CH3)3), 2.25 (br t, 6H, -NCH2CH2N-), 3.61 (br t, 6H, -NCH2CH2-
N-),7.27(d,12H,3,5,3′,5,′3′′,5′′-tBu-C6H4),7.75(d,12H,2,6,2′,6,′2′′,6′′-
tBu-C6H4), 7.76 (s, 3H, 4,4′,4′′-TerH), 8.21 (s, 6H, 2,6,2′,6,′2′′,6′′-
TerH). Anal. Calcd (%) for C84H99MoN5 (1274.66): C, 79.15; H,
7.83; N, 5.49. Found: C, 78.94; H, 7.82; N, 5.67.
C
118H169MoN5 (1753.58): C, 80.82; H, 9.71; N, 3.99. Found: C,
81.04; H, 9.54; N, 2.69.
(HIPTN3N)MoNMe2. Diethyl ether (20 mL) was freeze-pump-
thaw degassed thoroughly and condensed onto a solid mixture of
[(HIPTN3N)Mo(NH3)][BPh4] (0.97 g, 0.48 mmol) and LiNEt2 (24.5
mg, 0.48 mmol, 1 equiv). The reaction mixture was allowed to
warm to ambient temperature and was stirred for 2 h. All volatiles
were removed in vacuo, and the residue extracted into heptane/
pentane under 1 atm of argon. The mixture was filtered through
Celite, and the filtrate was cooled to -30 °C. (HIPTN3N)MoNMe2
[(DTBTN3N)ModNEt][BArf4]. A benzene solution (5 mL) of
(DTBTN3N)MotN (0.15 g, 0.11 mmol) was treated with solid
[Et3O][BArf4] (11.1 mg, 0.11 mmol, 1 equiv), and the mixture was
stirred at room temperature for 1 h to give a dark red solution.
Volatiles were removed in vacuo, and the solid residue was
extracted into pentane. The extracts were filtered through Celite,
and the filtrate was brought to dryness in vacuo. The remaining
material was dried at 60 °C under high vacuum to yield
[(DTBTN3N)ModNEt][BArf4] (0.21 g, 97.36 µmol, 85%) as a red,
amorphous solid. Analytically pure material was obtained by
recrystallization from diluted heptane solutions to yield 63.4 mg
(29.26 µmol, 26%) of product: 1H NMR (400 MHz, C6D6, 297 K)
δ -0.24 (t, 3H, -NCH2CH3), 1.25 (s, 54H, -C(CH3)3), 2.43 (br t,
6H, -NCH2CH2N-), 2.78 (q, 2H, -NCH2CH3), 3.60 (br t, 6H,
-NCH2CH2N-), 7.29 (s, 6H, 2,6,2′,6,′2′′,6′′-TerH), 7.40 (d, 12H,
3,5,3′,5,′3′′,5′′-tBu-C6H4), 7.55 (d, 12H, 2,6,2′,6,′2′′,6′′-tBu-C6H4),
7.66 (s, 4H, BArf), 7.81 (s, 3H, 4,4′,4′′-TerH), 8.40 (s, 8H, BArf).
Anal. Calcd (%) for C118H116BF24MoN5 (2166.93): C, 65.40; H,
5.40; N, 3.23. Found: C, 65.29; H, 5.42; N, 3.33.
1
(0.32 g, 0.19 mmol, 39%) was isolated as a green solid: H NMR
(400 MHz, C6D6, 293 K) δ -40.4 (br s, 6H, -NCH2CH2N-), -28.3
(br s, 6H, -NCH3), -3.0 (br s, 6H, -NCH2CH2N-), -0.21 (br s,
TerH), 1.21 (br s, 36H, -CH(CH3)2), 1.35 (s, 36H, -CH(CH3)2),
1.47 (d, 36H, -CH(CH3)2), 2.99 (br m, 6H, -CH(CH3)2), 3.12 (br s,
12H, -CH(CH3)2), 7.19 (s, 12H, 3,5,3′,5′,3′′,5′′-TipH); 7.19 (br s,
3H, -TerH), 26.9 (br s, -TerH). Anal. Calcd (%) for C118H169MoN5
(1753.58): C, 80.82; H, 9.71; N, 3.99. Found: C, 81.04; H, 9.54;
N, 3.86.
[(HIPTN3N)Mo(NEt3)][BArf4]. Diethyl ether (1 mL) was con-
densed onto a solid mixture of (HIPTN3N)MoNEt2 (15.1 mg, 8.55
µmol) and [Et3O][BArf4] (8.3 mg, 8.55 µmol, 1 equiv). The reaction
mixture was thawed and stirred for further 16 h at ambient
temperature, during which time the solution became deep red. All
volatiles were removed in vacuo, and the residue was extracted
with pentane. The mixture was filtered through Celite, and the
red filtrate was brought to dryness in vacuo to afford
[(HIPTN3N)Mo(NEt3)][BArf4] (13.8 mg, 5.21 µmol, 61%) as a red
solid: 1H NMR (400 MHz, C6D6, 297 K) δ -99.1 (br s, 6H,
-NCH2CH2N-), -16.3 (br s, 6H, -NCH2CH2N-), -10.8 (br s, 6H,
-NCH2CH3), -2.5 (br s, 9H, -NCH2CH3), 1.09 (br s, 36H,
-CH(CH3)2), 1.17 (br s, 36H, -CH(CH3)2), 1.40 (d, 36H, -CH(CH3)2),
2.78 (br m, 12H, -CH(CH3)2), 2.99 (br m, 6H, -CH(CH3)2,
-NCH2CH3), 7.30 (s, 12H, 3,5,3′,5′,3′′,5′′-TipH), 7.64 (s, 4H, BArf),
8.27 (s, 8H, BArf), 9.6 (br s, TerH); other resonances could not be
observed. Anal. Calcd (%) for C152H186BF24MoN5 (2645.85): C,
69.00; H, 7.09; N, 2.65. Found: C, 68.86; H, 6.69; N, 2.69.
Acknowledgment. R.R.S. is grateful to the National Institutes
of Health (GM 31978) for research support. T.K. is grateful to the
German Academic Exchange Service (DAAD) for a fellowship
within the Postdoc-Programme, to Dr. Michael Reithofer for helpful
discussions, and to Keith M. Wampler for advice on practical
aspects of electrochemistry.
JA904535F
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J. AM. CHEM. SOC. VOL. 131, NO. 35, 2009 12837