A cyclometalated resting state for a reactive molybdenum amide: Favorable consequences of β-hydrogen elimination including reductive cleavage, coupling, and complexation [6]

Full text of DOI:10.1021/ja9917464

10426
J. Am. Chem. Soc. 1999, 121, 10426-10427
Initially it was presumed that the cyclometalated nature of 1
would inhibit the type of reaction chemistry observed for related
three-coordinate derivatives.2,3 This proved not to be the case.
For example, the reaction of 1-d18 with dinitrogen was found to
give, in high yield, the nitrido-bridged dimolybdenum complex
(µ-N)[Mo(N[iPr-d6]Ar)3]2 (2). Here, as in the other reactions of
1 to be described, the hydrido ligand is observed to have migrated
back to carbon, rendering all three -N[iPr]Ar ligands per Mo
center equivalent.
A Cyclometalated Resting State for a Reactive
Molybdenum Amide: Favorable Consequences of
â-Hydrogen Elimination Including Reductive
Cleavage, Coupling, and Complexation
Yi-Chou Tsai, Marc J. A. Johnson, Daniel J. Mindiola, and
Christopher C. Cummins*
Department of Chemistry, Room 2-227, Massachusetts Institute
of Technology, Cambridge, Massachusetts 02139-4307
The X-ray structure of 2 shows that it possesses a linear Mod
NdMo core, as found previously for the dimethylamide ana-
logue.11 Additionally, an inversion center at the bridging nitrogen
atom is indicated by the crystallography. According to magnetic
susceptibility studies, 2 is a ground-state doublet.
Wim T. Klooster and Thomas F. Koetzle
Chemistry Department, BrookhaVen National Laboratory
Upton, New York 11973-5000
ReceiVed May 26, 1999
The ultimate step in the formation of 2 is thought to be the
combination of the terminal nitrido complex NtMo(N[iPr-d6]Ar)3
(3) with 1-d18. Formation of 3 presumably occurrs via dinuclear
N2 reductive cleavage,10,12 a process evidently slow relative to
consumption of 1-d18. Terminal nitride 3 was prepared indepen-
dently by reaction of 1-d18 with NtCr(OtBu)3,13 and in control
experiments was found to combine rapidly with 1-d18 to produce
purple 2. These results are in stark contrast with those obtained
for Mo(N[R]Ar)3 (R ) tBu), inasmuch as a µ-nitrido linkage was
obviated sterically in that system.
Whereas dimethylamide ligation of molybdenum(III) supports
MotMo triple bond formation,1 and N-tert-butylanilide ligation
engenders reactive yet isolable monomeric three-coordinate
derivatives,2,3 it is now shown that ligation of Mo(III) by
N-isopropylanilide gives rise to the cyclometalated4 (â-H elimi-
nated) species Mo(H)(η2-Me2CdNAr)(N[iPr]Ar)2 (1, Ar ) 3,5-
C6H3Me2, Figure 1), which can be thought of alternatively as an
imine complex or as a metallaziridine derivative. â-H elimination
for complexes of organoamide ligands is a rarely documented
phenomenon.5-9
Divergent pathways were observed for the reaction of 1 with
benzonitrile (PhCN). Under conditions of rapid mixing of
solutions of 1 and PhCN in a 1:1 ratio, nitrile coupling
chemistry14-18 was the result, leading to (µ-N[Ph]C-C[Ph]N)-
[Mo(N[iPr]Ar)3]2 (4). Compound 4 is thought to arise from
dimerization of putative intermediate adduct (PhCN)Mo(N[iPr]-
Ar)3 (5). An analogue 16 of diamagnetic 4, prepared from
Compound 1, which is freely soluble in hydrocarbon and
ethereal solvents, was obtained as a brown solid in 70% yield
from the reaction of Li(N[iPr]Ar)(OEt2) with MoCl3(THF)3.
Characterization of 1 includes Evans method magnetic suscep-
tibility (µeff ) 2.1 µB) and 2H NMR spectra for the d18 isotopomer
Mo(H)(η2-(D3C)2CdNAr)(N[iPr-d6]Ar)2 (1-d18). Three 2H NMR
signals in a 1:1:1 ratio are observed, consistent with a pseudo-Cs
structure, as sketched in Figure 1. The three 2H NMR signals for
1-d18 were observed to coalesce under observation at ∼70 °C
(benzene, 500 MHz), consistent with the â-H elimination process
being reversible. Infrared spectroscopy substantiates the presence
of the terminal hydride ligand (νΜoΗ ) 1876 cm-1).
t
acetonitrile and Mo(N[R]Ar)3 (R ) Bu), was characterized by
X-ray diffraction and found to have a C-C bond distance of 1.43-
(2) Å. All indications are that 4 is a 1e (per Mo) coupling product.
In addition to the coupling of PhCN, 1 was found to effect the
cleaVage of this substrate. This reaction was optimized with a
3:1 ratio of 1:PhCN; the products are benzylidyne PhCt
Mo(N[iPr]Ar)3 (7) (the 13C NMR spectrum shows a resonance
for the R-C at 294.9 ppm) and 2. Slow addition of PhCN to a
solution of 1 produced the best yields of 7. Splitting of the nitrile
functionality to alkylidyne and nitride components is known for
ditungsten compounds.19 A possible mechanism for the new PhCN
cleavage reaction involves attack on putative intermediate adduct
5 by 1, giving a dinuclear intermediate which can fragment to 3
and 7. Finally, capture of 3 by the third equivalent of 1 produces
2. The low solubility of 2 enables its facile separation from
benzylidyne 7.
X-ray crystallography was used to interrogate the structure of
1-d18. The structural parameters thereby determined are consistent
2
with the H NMR data. The effect of â-H elimination from the
methine position is to give a short Mo-C(27) distance of 2.166-
(7) Å. A conformational feature of interest is that the aryl ring
component of the cyclometalated amide is favorably oriented for
π-interaction with the contiguous nitrogen atom, the opposite of
what is observed normally for unperturbed ligands of the
-N[tBu]Ar variety.10
Single-crystal neutron diffraction also was used to characterize
1, due to interest in the exact position of the presumed hydride
ligand (Figure 2). Suffice it to say that the neutron structure of 1
is directly in accord with the X-ray structure, giving a Mo-H
distance of 1.69(5) Å. No bonding interaction between the hydride
ligand and C(27) is indicated by the relevant internuclear C‚‚‚H
distance of 2.36(6) Å. Density functional theory calculations were
found to corroborate the structure of 1 as determined by the X-ray
and neutron diffraction studies.
Treatment of 1 with benzophenone (1 equiv) produced not the
pinacol-coupling product, as might have been imagined,20,21 but
rather an adduct 8 with the carbonyl moiety complexed in an η2
(11) Johnson, M. J. A.; Lee, P. M.; Odom, A. L.; Davis, W. M.; Cummins,
C. C. Angew. Chem., Int. Ed. Engl. 1997, 36, 87.
(12) Zanotti-Gerosa, A.; Solari, E.; Giannini, L.; Floriani, C.; Chiesi-Villa,
A.; Rizzoli, C. J. Am. Chem. Soc. 1998, 120, 437.
(13) Chiu, H.-T.; Chen, Y.-P.; Chuang, S.-H.; Jen, J.-S.; Lee, G.-H.; Peng,
S.-M. Chem. Commun. 1996, 139.
(14) Blight, D. G.; Deutscher, R. L.; Kepert, D. L. J. Chem. Soc., Dalton
Trans. 1972, 87.
(15) Finn, P. A.; Schaefer King, M.; Kilty, P. A.; McCarley, R. E. J. Am.
Chem. Soc. 1975, 97, 220.
(16) Cotton, F. A.; Hall, W. Inorg. Chem. 1978, 17, 3525.
(17) Roskamp, E. J.; Pedersen, S. F. J. Am. Chem. Soc. 1987, 109, 3152.
(18) Duchateau, R.; Williams, A. J.; Gambarotta, S.; Chiang, M. Y. Inorg.
Chem. 1991, 30, 4863.
(19) Schrock, R. R.; Listemann, M. L.; Sturgeoff, L. G. J. Am. Chem. Soc.
1982, 104, 4291. Chisholm, M. H.; Hoffman, D. M.; Huffman, J. C. Inorg.
Chem. 1983, 22, 2903.
(20) Covert, K. J.; Mayol, A.R.; Wolczanski, P. T. Inorg. Chim. Acta 1997,
263, 263.
(1) Chisholm, M. H.; Cotton, F. A.; Frenz, B. A.; Reichert, W. W.; Shive,
L. W.; Stults, B. R. J. Am. Chem. Soc. 1976, 98, 4469.
(2) Cummins, C. C. Chem. Commun. 1998, 1777.
(3) Cummins, C. C. Prog. Inorg. Chem. 1998, 47, 685.
(4) Berno, P.; Gambarotta, S. Organometallics 1995, 14, 2159.
(5) Mayer, J. M.; Curtis, C. J.; Bercaw, J. E. J. Am. Chem. Soc. 1983, 105,
2651.
(6) Bryndza, H. E.; Tam, W. Chem. ReV. 1988, 88, 1163.
(7) Fryzuk, M. D.; Montgomery, C. D. Coord. Chem. ReV. 1989, 95, 1.
(8) Driver, M. S.; Hartwig, J. F. J. Am. Chem. Soc. 1996, 118, 4206.
(9) Hartwig, J. F. J. Am. Chem. Soc. 1996, 118, 7010.
(10) 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.
10.1021/ja9917464 CCC: $18.00 © 1999 American Chemical Society
Published on Web 10/21/1999

Communications to the Editor
J. Am. Chem. Soc., Vol. 121, No. 44, 1999 10427
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Figure 1. Reaction pinwheel highlighting the chemistry of imine-hydride 1 (Ar ) 3,5-C6H3Me2).
Figure 2. Ball-and-stick drawing of Mo(H)(η2-Me2CdNAr)(N[iPr]Ar)2
(1) from a single-crystal neutron diffraction study. Selected distances (Å)
and angles (deg): Mo-H, 1.69(5); Mo-N(1), 1.93(2); Mo-N(2), 1.95-
(2); Mo-N(3), 2.02(2); Mo-C(27), 2.18(2); N(1)-C(17), 1.56(2); N(2)-
C(27), 1.44(2), N(3)-C(37), 1.47(2); N(1)-Mo-N(2), 117(1); N(1)-
Mo-N(3), 117(1); N(2)-Mo-N(3), 122(1); N(1)-Mo-C(27), 116(1);
Figure 3. 35% thermal ellipsoid plot of (η2-OdCPh2)Mo(N[iPr]Ar)3 (8).
N(2)-Mo-C(27), 40.3(7); N(3)-Mo-C(27), 122(1).
Selected distances (Å) and angles (deg): Mo-N(1), 1.954(4); Mo-N(2),
1.980(4); Mo-N(3), 1.950(5); Mo-C(1), 2.149(5); Mo-O, 2.049(3);
O-C(1), 1.377(6); N(3)-Mo-N(1), 114.8(2); N(3)-Mo-N(2), 106.3-
(2); N(1)-Mo-N(2), 108.1(2); N(3)-Mo-O, 88.3(2); N(1)-Mo-O,
93.4(2); N(2)-Mo-O, 144.9(2); N(3)-Mo-C(1), 115.6(2); N(1)-Mo-
C(1), 103.7(2); N(2)-Mo-C(1), 108.1(2); O-Mo-C(1), 38.2(2); C(1)-
Mo-O, 74.8(3); Mo-C(1)-O, 67.0(2).
fashion to Mo (Figure 3). Note that the tert-butyl-substituted
analogue of 1, namely Mo(N[R]Ar)3 (R ) tBu), does not exhibit
any reaction with benzophenone. The observed η2-complexation
of benzophenone by 1 raises the question of whether nitriles, as
in putative 5, also would prefer to engage in η2-binding.22
As for alkynes, it was hoped that 1 would be competent for
their cleavage to corresponding alkylidyne species.19 However,
treatment of 1 with tolane produced only the green adduct (η2-
PhCCPh)Mo(N[iPr]Ar)3 (9), even under conditions of excess 1.
In conclusion, an attempt to prepare the putative iPr-substituted
three-coordinate2,3 molybdenum(III) complex Mo(N[iPr]Ar)3 (10)
led instead to the isolation and characterization of a new type of
reactive hydride-imine complex, 1. In all reactions studied thus
far, 1 behaves as a source of 10, serving to transfer the
tricoordinate Mo fragment. Hydride 1 is electrochemically
versatile, in that it serves as a 3e reductant in its dinitrogen and
PhCtN splitting reactions and as a 1e reductant in its coupling
of PhCN. The PhCN-coupling reaction, producing 4, was found
to be unaffected by the presence of 1 equiv of ethyl acetate, an
observation hinting at the functional-group-selective nature of 1.
Acknowledgment. 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
Foundation are gratefully acknowledged for financial support. Oak Ridge
National Laboratory is managed by Lockheed Martin Energy Research
Corp. for the U.S. Department of Energy under Contract No. DE-AC05-
96OR22464. Work at BNL was carried out under Contract No. DE-AC02-
98CH10886 with the U.S. Department of Energy, Office of Basic Energy
Sciences.
Supporting Information Available: Synthetic, spectroscopic and
analytical data for complexes 1, 1-d18, 2-4, 6-9; tables of crystal data,
atomic coordinates, structure solution and refinement, bond lengths and
angles, and anistropic thermal parameters for compounds 1 (neutron
study), 1-d18 (X-ray study), 2, 3, 6, and 8; and information pertaining to
the density functional theory study of hypothetical Mo(H)(η2-H2 CdNH)-
(NH2)2 (PDF). This material is available free of charge via the Internet
at http://pubs.acs.org.
(21) Covert, K. J.; Wolczanski, P. T.; Hill, S. A.; Krusic, P. J. Inorg. Chem.
1992, 31, 66.
(22) Wright, T. C.; Wilkinson, G.; Montevalli, M.; Hursthouse, M. B. J.
Chem. Soc., Dalton Trans. 1986, 2017.
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