Dinitrogen cleavage stemming from a heterodinuclear niobium/molybdenum N2 complex: New nitridoniobium systems including a niobazene cyclic trimer

Article abstract of DOI:10.1021/om000159k

Dimolybdenum systems have been found to effect N₂ splitting in the absence of added reagents, while certain diniobium and divanadium systems also split dinitrogen when used in conjunction with alkali metals. While dinuclear dinitrogen complexes

Full text of DOI:10.1021/om000159k

1622
Organometallics 2000, 19, 1622-1624
Din itr ogen Clea va ge Stem m in g fr om a Heter od in u clea r
Niobiu m /Molybd en u m N2 Com p lex: New Nitr id on iobiu m
System s In clu d in g a Nioba zen e Cyclic Tr im er
Daniel J . Mindiola, Karsten Meyer, J ohn-Paul F. Cherry, Thomas A. Baker, and
Christopher C. Cummins*,†
Massachusetts Institute of Technology, Department of Chemistry, Room 2-227,
Cambridge, Massachusetts 02139-4307
Received February 17, 2000
Summary: Preparation of the terminal nitrido anion
-[NNb(N[iPr]Ar)3] and the niobazene trimer {(µ-N)Nb-
(N[iPr]Ar)2}3 (iPr ) CH(CH3)2 or CH(CD3)2, Ar ) 3,5-
C6H3Me2) entails cooperative splitting of dinitrogen upon
reduction of the heterobimetallic paramagnetic dinitro-
Recent work with anionic molybdenum dinitrogen
complexes has shown them to be good nucleophiles, in
that they are smoothly alkylated or silylated at the â
(terminal) nitrogen atom.16-18 For example the anion
[(N2)Mo(N[R]Ar)3]- is converted to Me3SiNNMo(N[R]-
Ar)3 and MeNNMo(N[R]Ar)3 upon reaction with chlo-
rotrimethylsilane and methyl tosylate, respectively.
Furthermore, it has been possible to assemble a variety
of heterodinuclear dinitrogen complexes by reaction of
dinitrogen molybdenum complex anions with various
metal halide (M ) Zr, V, Fe, and U) electrophiles.16,17,19,20
t
gen complex (Ar[R]N)3Mo(µ-N2)Nb(N[iPr]Ar)3 (R ) Bu
or C(CD3)2 CH3).
Dinitrogen gas rarely is employed as the nitrogen
source in the synthesis of molecular nitridometal com-
pounds,1 but recent advances involving well-defined N2-
splitting reactions are improving the outlook for the
direct utilization of molecular nitrogen. Dimolybdenum
systems2-4 have been found to effect N2 splitting in the
absence of added reagents, while certain diniobium and
divanadium systems5-7 also split dinitrogen when used
in conjunction with alkali metals. While dinuclear
dinitrogen complexes represent an emerging theme for
N2-splitting systems,2-6,8-12 thus far the systems in
question have been homobimetallic in nature. The
present work shows that molybdenum and niobium can
be used together to effect the splitting of molecular
nitrogen in a cooperative fashion. Nitridoniobium com-
pounds stemming from this intriguing heterodinuclear
N2-scission process include a niobazene cyclic trimer,
the structure of which provides insight into the bonding
in M3N3 rings (M ) transition metal).13-15
Accordingly, we find that the reaction of the purple
chloroniobium(IV) complex NbCl(N[iPr]Ar)3
with
21
[Na(THF)x][(N2)Mo(N[R]Ar)3]18 or [Mg(THF)2][(N2)Mo-
(N[R]Ar)3]2 provides in high yield (74-78%) the neu-
21
tral, paramagnetic, brown-green N2-bridged complex
(Ar[tBu]N)3Mo(µ-N2)Nb(N[iPr]Ar)3 (2,21 Figure 2).
Complex 2 is characterized by an intense νNN at 1583
cm-1, a value to be compared with that (1548 cm-1) for
the isotopomer prepared from 15N2. While EPR spec-
troscopy (25 °C, toluene) revealed a classic 10-line pat-
tern resulting from coupling to the I ) 9/2 niobium
nucleus 93Nb (100%), no coupling to the spin 5/2
molybdenum nuclei 95Mo and 97Mo (15.92% and 9.55%,
respectively) was resolved.21 The magnitude of the
93Nb coupling was found by simulation to be 99.3 G, a
value typical for niobium(IV) systems of the type
Nb(X)(N[iPr]Ar)3 (X- ) I, Cl).22 Inclusion in the simula-
tion of a 32.0 G coupling to 95/97Mo (this value being
typical for S ) 1/2 molybdenum systems of the type
MoCl2(N[iPr]Ar)3)23 did not lead to resolution of the
molybdenum hyperfine. Thus one cannot conclude that
* To whom correspondence should be addressed. E-mail:
ccummins@mit.edu.
† Alfred P. Sloan Fellow, 1997-2000.
(1) Dehnicke, K.; Stra¨hle, J . Angew. Chem., Int. Ed. Engl. 1992, 31,
955.
(2) Laplaza, C. E.; Cummins, C. C. Science 1995, 268, 861-863.
(3) Laplaza, C. E.; J ohnson, 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, 8623-8638.
(4) Cummins, C. C. Chem. Commun. 1998, 1777-1786.
(5) Zanotti-Gerosa, A.; Solari, E.; Giannini, L.; Floriani, C.; Chiesi-
Villa, A.; Rizzoli, C. J . Am. Chem. Soc. 1998, 120, 437.
(6) Ferguson, R.; Solari, E.; Floriani, C.; Osella, D.; Ravera, M.; Re,
N.; Chiesi-Villa, A.; Rizzoli, C. J . Am. Chem. Soc. 1997, 119, 10104-
10115.
(13) Plenio, H.; Roesky, H. W.; Noltemeyer, M.; Sheldrick, G. M.
Angew. Chem., Int. Ed. Engl. 1988, 27, 1330.
(14) Holl, M. M. B.; Kersting, M.; Pendley, B. D.; Wolczanski, P. T.
Inorg. Chem. 1990, 29, 1518.
(15) Wheeler, R. A.; Hoffmann, R.; Stra¨hle, J . J . Am. Chem. Soc.
1986, 108, 5381.
(16) O’Donoghue, M. B.; Zanetti, N. C.; Davis, W. M.; Schrock, R.
R. J . Am. Chem. Soc. 1999, 119, 2753.
(7) Clentsmith, G. K. B.; Bates, V. M. E.; Hitchcock, P. B.; Cloke, F.
G. N. J . Am. Chem. Soc. 1999, 121, 10444.
(8) Campazzi, E.; Solari, E.; Floriani, C.; Scopelliti, R. Chem.
Commun. 1998, 2603.
(9) Ferguson, R.; Solari, E.; Floriani, C.; Chiesi-Villa, A.; Rizzoli, C.
Angew. Chem., Int. Ed. Engl. 1993, 32, 396.
(10) Berno, P.; Gambarotta, S. Angew. Chem., Int. Ed. Engl. 1995,
34, 822.
(11) Fryzuk, M. D.; Love, J . B.; Rettig, S. J .; Young, V. G. Science
1997, 275, 1445.
(12) Basch, H.; Musaev, D. G.; Morokuma, K.; Fryzuk, M. D.; Love,
J . B.; Seidel, W. W.; Albainati, A.; Koetzle, T. F.; Klooster, W. T.;
Mason, S. A.; Eckert, J . J . Am. Chem. Soc. 1999, 121, 523.
(17) O’Donoghue, M. B.; Davis, W. M.; Schrock, R. R. Inorg. Chem.
1998, 37, 5149.
(18) Peters, J . C.; Cherry, J .-P.; Thomas, J . C.; Baraldo, L.; Mindiola,
D. J .; Davis, W. M.; Cummins, C. C. J . Am. Chem. Soc. 1999, 121,
10053-10067.
(19) O’Donoghue, M. B.; Davis, W. M.; Schrock, R. R.; Reiff, W. M.
Inorg. Chem. 1999, 38, 243.
(20) Odom, A. L.; Arnold, P. L.; Cummins, C. C. J . Am. Chem. Soc.
1998, 120, 5836-5837.
(21) Experimental details are included in the Supporting Informa-
tion.
(22) Mindiola, D. J .; Meyer, K.; Cummins, C. C. Unpublished results.
(23) Tsai, Y.-C.; Meyer, K.; Cummins, C. C. Unpublished results.
10.1021/om000159k CCC: $19.00 © 2000 American Chemical Society
Publication on Web 03/31/2000

Communications
Organometallics, Vol. 19, No. 9, 2000 1623
F igu r e 3. Structural diagram of {(µ-N)Nb(N[iPr]Ar)2}3 (5)
with ellipsoids at the 35% probability level. Selected
distances (Å) and angles (deg): Nb(1)-N(7), 1.792(6);
Nb(1)-N(8), 2.004(6); Nb(2)-N(8), 1.795(7); Nb(2)-N(9),
2.003(6); Nb(3)-N(9), 1.785(6); Nb(3)-N(7), 2.004(6);
Nb(1)-N(7)-Nb(3), 129.4(3); N(8)-Nb(1)-N(7), 110.6(3);
N(7)-Nb(3)-N(9), 110.2(3); Nb(3)-N(9)-Nb(2), 130.2(3);
N(9)-Nb(2)-N(8), 110.0(3). Peripheral substituents are
omitted for clarity.
F igu r e 1. Preparation of niobazene trimer 5. Monochlo-
ride 1 was obtained by reduction of the corresponding
niobium(V) dichloride NbCl2(N[iPr]Ar)3 (see Supporting
Information.
of complex 2 is due to a π f σ* transition, where the σ*
orbital in question is localized primarily on Mo and
oriented largely distal to the bridging N2 ligand.24-28
Structural characterization of complex 2 was achieved
by single-crystal X-ray diffraction (Figure 2). The topo-
logically linear Nb-N-N-Mo core exhibits respective
distances of 1.969(9), 1.235(10), and 1.783(9) Å, high-
lighting the asymmetry in the bonding of N2 to the two
metals. In addition, the solid-state structure of complex
2 reveals distortion with a descent in symmetry from
an ideal C3 to the observed C1 resulting from an amide
rotation and bending about the Nb-N2-Mo frame.
Electrochemically it was found that 2 undergoes a
reversible oxidation at -1.4 V (vs Fc/Fc+), in accord
with which chemical oxidation provided the cation [(Ar-
[tBu]N)3Mo(µ-N2)Nb(N[iPr]Ar)3]+, as its scarlet-colored
[B(3,5-C6H3(CF3)2)4]- salt, in 77% isolated yield. When
prepared using 15N2, the latter cation exhibited two 15
N
NMR signals, located at 429.8 and 415.9 ppm and
mutually related by a coupling of 15 Hz. Possibly due
to coupling to 93Nb, the 415.9 ppm signal was severely
broadened at 25 °C but relatively sharp at -40 °C.29
The electrochemistry21 of 2 revealed an irreversible
reduction at ca. -2.6 V, successive scans resulting in
the production of a new quasi-reversible wave at
-2.8 V, a value characteristic of the terminal molybde-
num(VI) nitrido derivative NtMo(N[R]Ar)3.18 This re-
duction was investigated chemically by reaction of
F igu r e 2. Structural diagram of (Ar[tBu]N)3Mo(µ-N2)Nb-
(N[iPr]Ar)3 (2) with ellipsoids at the 35% probability
level. Selected distances (Å) and angles (deg): Nb-N(7),
1.969(9); Nb-N(6), 1.991(8); Nb-N(5), 1985(8); Nb-N(4),
2054(8); Mo-N(8), 1.783(9); Mo-N(1), 2.009(8); Mo-N(2),
1.971(8); Mo-N(3), 1.974(8); N(8)-N(7), 1.235(10); Mo-
N(8)-N(7), 1.78(10); N(8)-N(7)-Nb, 167.0(8).
30
complex 2 with KC8 in THF solution. As indicated in
(24) Fonseca-Guerra, C.; Snijders, J . G.; te Velde, G.; Baerends, E.
J . J . Chem. Phys. 1998, 99, 391.
(25) UV-vis spectroscopic data in the ground state for the first and
second transition are provided in the Supporting Information.
(26) Baerends, E. J .; Ellis, D. E.; Ros, P. Chem. Phys. 1973, 2, 41.
(27) Fonseca-Guerra, C.; Visser, O.; Snijders, J . G.; te Velde, G.;
Baerends, E. J . Methods and Techniques in Computational Chemis-
try: METECC-95 1995, 305.
(28) te Velde, G.; Baerends, E. J . J . Comput. Phys. 1992, 99, 84.
(29) Harris, R. K., Mann, B. E., Eds. NMR and the Periodic Table;
Academic Press: London, 1978.
the odd electron in S ) 1/2 2 (µeff ) 2.3 µB) is localized
exclusively on niobium. Indeed, density funtional theory
(DFT) calculations24-26 indicate that the orbital housing
the odd electron possesses both Mo (22%) and Nb (29%)
character. The calculations also indicate that the color

1624 Organometallics, Vol. 19, No. 9, 2000
Communications
Figure 1, treatment of 2 with KC8 followed by the
addition of the cryptand 2,2,2-Kryptofix resulted in the
isolation in 74% yield of the diamagnetic [K(2,2,2-
Kryptofix)]+ salt of the nitrido anion [NNb(N[iPr]Ar)3]-
(4), which was separated easily from the neutral Mo
nitride NtMo(N[R]Ar)3. Colorless nitridoniobium anion
[NNb(N[iPr]Ar)3]- exhibited proton NMR spectra char-
acteristic of a single-N(iPr)Ar ligand environment, and
when prepared from 15N2, evinced a singlet at δ ) 793
ppm in its 15N NMR spectrum. The latter large down-
field shift is typical of terminal, d0 nitrido deriva-
tives.2,31-35 Salt 4 also was characterized by single-
crystal X-ray diffraction methods,36 confirming that
the compound consists of discrete anions and cations
the “long” bond to the bridging nitrido ligands averages
to 2.004(6) Å. Such bond length alternation contrasts
sharply with results reported previously for related
“triazatrimetallabenzenes” of formula {(µ-N)TaCp*(X)}3,
where X ) Cl or Me.13,14 Structures of the latter two
compounds revealed equivalent Ta-N bonds in the
respective Ta3N3 rings, a result interpretable either in
terms of the disparity in electronegativity between Ta
and N or alternatively in terms of delocalized bonding.
Careful inspection of the structure of trimer 5 suggests
a different explanation, an explanation based on the
π-donor properties of the planar amido ligand vis-a`-vis
the cylindrical nature of Cp* and chloride ligands in
the tantalum compounds. Focusing attention on Nb(1)
in Figure 3, the relevant structural parameter is the
C(17)-N(1)-Nb(1)-N(7) dihedral angle of near 0°
(1.09(1.11)°). This dihedral angle positions the lone-pair
p orbital on N(1) exactly orthogonal to the short nitrido
Nb(1)-N(7) bond. Structure optimizations using DFT
calculations on the model system39,40 {(µ-N)Nb(NH2)2}3
showed the rotation about the metal amido bonds to
correlate directly with the µ-nitride bond lengths around
the Nb3N3 ring.21 The six Nb-µ-N distances become
equivalent when the amide rotation angle permits an
increase in symmetry to D3h, but such a structure is
energetically less favorable. The observed structure in
C3h can be thought of as enjoying second-order J ahn-
Teller stabilization,41 not attainable in the previously
reported tantalum systems due to the cylindrical nature
of the π-donor ancillary ligands employed.
and that the terminal nitrido functionality (dNb-N
)
1.696(5) Å) was obtained as a mononuclear, pseudo-3-
fold symmetric anion.
Treatment of nitrido anion 4 with iodine (1.0 equiv)
gave a mixture of products from which the niobazene
cyclic trimer {(µ-N)Nb(N[iPr]Ar)2}3 (5) was obtained in
27% yield after purification by recrystallization. This
result was unexpected in that the related nitridonio-
bium anion -[NNb(N[R]Ar)3]37 is known to react smoothly
with iodine (1.0 equiv) to give the neutral iodoimido
complex INNb(N[R]Ar)3.38 Treatment of 4 with [FeCp2]+
(triflate salt) also was found to produce appreciable
quantites of trimer 5 in better yields (41%). Diamagnetic
1
5 exhibits H and 13C NMR spectra in accord with its
crystallographically determined (Figure 3) pseudo-C3h
-
symmetric molecular structure, and the 15N NMR signal
for the isotopically labeled trimer consists of a singlet
located at δ ) 592 ppm.
Ack n ow led gm en t. For financial support are grate-
fully acknowledged the National Science Foundation
(CAREER Award CHE-9501992), the Alfred P. Sloan
Foundation, the National Science Board (1998 Alan T.
Waterman award to C.C.C.), and the Packard Founda-
tion. K.M. thanks the Deutsche Forschungsgemein-
schaft for financial support. The authors would like to
thank Prof. Luis Baraldo for cyclic voltammetry mea-
surements and Dr. J effrey H. Simpson and Dr. Mark
Wall for 15N NMR experiments.
Bond length alternation around the Nb3N3 ring is a
salient crystallographic finding for trimer 5 (Figure 3),
the average “short” NbdN bond being 1.791(7) Å, while
(30) Schwindt, M.; Lejon, T.; Hegedus, L. Organometallics 1990, 9,
2814.
(31) Laplaza, C. E.; Odom, A. L.; Davis, W. M.; Cummins, C. C.;
Protasiewicz, J . D. J . J . Am. Chem. Soc. 1995, 117, 4999-5000.
(32) Odom, A. L.; Cummins, C. C.; Protasiewicz, J . D. J . Am. Chem.
Soc. 1995, 117, 6613-6614.
(33) von Philipsborn, W.; Mu¨ller, R. Angew. Chem., Int. Ed. Engl.
1986, 25, 383.
(34) Dilworth, J . R.; Donovan-Mtunzi, S.; Kan, C. T.; Richards, R.
L.; Mason, J . Inorg. Chim. Acta 1981, 53, L161.
(35) Donovan-Mtunzi, S.; Richards, R. L.; Mason, J . J . Chem. Soc.,
Dalton Trans. 1984, 469.
(36) Single-crystal X-ray diffraction studies and experimentals for
the salt [K(2,2,2-Kryptofix)][NNb(N[iPr]Ar)3] are included in the
Supporting Information.
Su p p or tin g In for m a tion Ava ila ble: Synthetic, spectro-
scopic, and analytical details for new complexes. This material
is available free of charge via the Internet at http://pubs.acs.org.
OM000159K
(39) When optimizing the model [(µ-N)Nb(NH2)2]3, an analogous
tricyclo phosphazene compound was also used as a model for compari-
son.
(40) Golinski, F.; J acobs, H. Z. Anorg. Allg. Chem. 1994, 620, 965.
(41) Albright, T. A., Burdett, J . K., Wnangbo, M.-H., Eds. Orbital
Interactions in Chemistry; J ohn Wiley and Sons: New York, 1985.
(37) Fickes, M. G.; Odom, A. L.; Cummins, C. C. Chem. Commun.
1997, 1993-1994.
(38) Mindiola, D. J .; Fickes, M. G.; Cummins, C. C. Unpublished
results.