Nitrogen atom transfer coupled with dinitrogen cleavage and Mo-Mo triple bond formation

Full text of DOI:10.1021/ja953573y

J. Am. Chem. Soc. 1996, 118, 709-710
Nitrogen Atom Transfer Coupled with Dinitrogen
709
Cleavage and Mo-Mo Triple Bond Formation
Catalina E. Laplaza, Adam R. Johnson, and
Christopher C. Cummins*
Department of Chemistry, Massachusetts Institute of
Technology, 77 Massachusetts AVenue
Cambridge, Massachusetts 02139-4307
ReceiVed October 24, 1995
In principle, catalytic dinitrogen fixation1 could be achieved
by combining known dinitrogen cleavage2 and intermetal
nitrogen atom transfer3 reactions in a single system. This would
solve “the problem of regenerating the molybdenum(III) starting
material to make a cyclic system”.4 In this report, we describe
the first prototype of such a system. We show that the recently-
reported2 cleavage of dinitrogen by three-coordinate Mo(NRAr)3
[R ) C(CD3)2CH3, Ar ) 3,5-C6H3Me2, Figure 1, giving the
terminal nitrido complex NMo(NRAr)3,5 is accelerated in the
presence of Chisholm’s6 nitrido complex NMo(OR)3 [R )
C(CH3)3].
Figure 1. Reprinted with permission from ref 2. Copyright 1995
American Association for the Advancement of Science.
Under 1 atm of dinitrogen, the reaction of Mo(NRAr)3 with
NMo(OR)3 (benzene, 28 °C, 3.5 mM in both metal complexes)
again led to quantitative formation of nitrido NMo(NRAr)3, a
process requiring ∼6 h to reach completion. 1H NMR analysis
showed that under these conditions, only a small amount (∼7%)
of dimer Mo2(OR)6 was produced. Pure NMo(OR)3 was
recovered via fractional crystallization in 74% yield, while pure
NMo(NRAr)3 was isolated in 83% yield. Control experiments
show that the reaction of Mo(NRAr)3 (benzene, 28 °C, 3.5 mM)
with dinitrogen (1 atm) to give NMo(NRAr)3 is quite slow,
proceeding only to e5% in 12 h.8 Thus, NMo(OR)3 accelerates
the reaction of Mo(NRAr)3 with dinitrogen.
In the absence of dinitrogen (eq 1), Mo(NRAr)3 reacts with
NMo(OR)3, giving NMo(NRAr)3 along with 0.5 equiv of the
known7 dimer Mo2(OR)6. This reaction (benzene, 28 °C) is
The reaction of Mo(NRAr)3 with NMo(OR)3 (benzene, 28
°C, 3.5 mM in both metal complexes) under an atmosphere of
99% 15N2 was carried out next to substantiate dinitrogen
cleavage (eq 2). The nitrido products 14/15NMo(NRAr)3 and
(1)
essentially quantitative (as determined by 1H and 2H NMR) and
is complete in under 12 h. It was possible to separate Mo2-
(OR)6 and NMo(NRAr)3 via fractional crystallization at -35
°C. Obtained as red-orange needles, a sample of dimer Mo2-
(OR)6 was characterized by elemental analysis, by EIMS, and
by comparison of its 1H and 13C NMR spectra with those extant
in the literature.7 These results establish facile N atom transfer
from NMo(OR)3 to Mo(NRAr)3 and demonstrate that the
reaction can be coupled with Mo-Mo triple bond formation.
(2)
14/15NMo(OR)3 (respectively 43% and 42% 15N by EIMS) were
separated and purified via fractional crystallization. IR9 and
15N NMR10 spectroscopic data obtained for 14/15NMo(NRAr)3
and 14/15NMo(OR)3 were consistent with the level of 15N-
enrichment indicated by EIMS. Formation of a small amount
(∼7% by 1H NMR) of dimer Mo2(OR)6 in this reaction accounts
for the less-than-quantitative (∼85%) incorporation of 15N into
the nitrido functions of 14/15NMo(NRAr)3 and 14/15NMo(OR)3.
To test for reversibility of N atom transfer, we investigated
the reaction of Mo(NRAr)3 with NMo(OR)3 (benzene, 28 °C,
3.5 mM in both metal complexes) in the presence of 15NMo-
(1) Catalytic Ammonia Synthesis; Jennings, J. R., Ed.; Plenum: New
York, 1991.
(2) Laplaza, C. E.; Cummins, C. C. Science 1995, 268, 861.
(3) (a) Woo, L. K. Chem. ReV. 1993, 93, 1125. (b) Woo, L. K.; Goll,
J. G.; Czapla, D. J.; Hays, J. A. J. Am. Chem. Soc. 1991, 113, 8478. (c)
Woo, L. K.; Goll, J. G. J. Am. Chem. Soc. 1989, 111, 3755. (d) Neely, F.
L.; Bottomley, L. A. Inorg. Chim. Acta 1992, 192, 147. (e) Bottomley, L.
A.; Neely, F. L. J. Am. Chem. Soc. 1989, 111, 5955. (f) Mayer, J. M.
Chemtracts: Inorg. Chem. 1992, 4, 380. (g) Bakir, M.; White, P. S.;
Dovletoglou, A.; Meyer, T. J. Inorg. Chem. 1991, 30, 2835. (h) Evans, D.
A.; Bilodeau, M. T.; Faul, M. M. J. Am. Chem. Soc. 1994, 116, 2742. (i)
Jitsukawa, K.; Hata, T.; Yamamoto, T.; Kano, K.; Masuda, H.; Einaga, H.
Chem. Lett. 1994, 7, 1169. (j) Svastits, E. W.; Dawson, J. H.; Breslow,
R.; Gellman, S. H. J. Am. Chem. Soc. 1985, 107, 6427.
5
(NRAr)3 (3.5 mM, ∼99% 15N) under dinitrogen (natural
1
abundance N2, eq 3). H and 2H NMR monitoring showed that
(4) Leigh, G. J. Science 1995, 268, 827.
(5) Laplaza, C. E.; Odom, A. L.; Davis, W. M.; Cummins, C. C.;
Protasiewicz, J. D. J. Am. Chem. Soc. 1995, 117, 4999. NMo(NRAr)3 has
been structurally characterized via an EXAFS study: d(MoNnitrido) ) 1.66
Å, d(MoNamido) ) 1.98 Å. Results to be published in an upcoming full
paper (Laplaza, C. E.; Cummins, C. C.; Pickering, I.; George, G., manuscript
in preparation). In addition, we have determined via single-crystal X-ray
diffraction that NMo[N(tBu)Ph]3 is a discrete, three-fold-symmetric mono-
mer in the solid state: Johnson, M. J. A.; Cummins, C. C., unpublished
results. The structure of NMo[N(tBu)Ph]3 is very similar to that of the
recently-reported terminal phosphido complex PMo(NRAr)3: Laplaza, C.
E.; Davis, W. M.; Cummins, C. C. Angew. Chem., Int. Ed. Engl. 1995, 34,
2042. A single-crystal X-ray diffraction study has been carried out for
NMo(NPh2)3: Gebeyehu, Z.; Weller, F.; Neumu¨ller, B.; Dehnicke, K. Z.
Anorg. Allg. Chem. 1991, 593, 99.
(3)
the reaction proceeded with quantitative conversion of Mo-
(NRAr)3 to its nitrido counterpart 14/15NMo(NRAr)3. Dimer
(8) Note that our original report on dinitrogen cleavage by Mo(NRAr)3
(ref 2 above) involved its conversion to purple (µ-N2)[Mo(NRAr)3]2 at low
temperature (-35 °C, 1 atm of N2) for ∼76 h. Subsequent warming to 30
°C gave NMo(NRAr)3 (t1/2 ) 35 min).
(6) Chan, D. M.-T.; Chisholm, M. H.; Folting, K.; Huffman, J. C.;
Marchant, N. S. Inorg. Chem. 1986, 25, 4170.
(9) We assign the MoN stretching frequency for NMo(OR)3 (pentane
solution/KBr plates) as follows: νMo N ) 1052 cm-1; νMo N ) 1024 cm-1
.
14
15
(7) (a) Chisholm, M. H.; Cotton, F. A.; Murillo, C. A.; Reichert, W. W.
Inorg. Chem. 1977, 16, 1801. (b) Cotton, F. A.; Walton, R. A. Multiple
Bonds Between Metal Atoms, 2nd ed.; Oxford University Press: New York,
1993.
(10) We find the 15N NMR shift for 15NMo(OR)3 to be +811 ppm relative
to liquid ammonia (0 ppm). The corresponding shift for 15NMo(NRAr)3 is
+840 ppm; see ref 2 above.
0002-7863/96/1518-0709$12.00/0 © 1996 American Chemical Society

710 J. Am. Chem. Soc., Vol. 118, No. 3, 1996
Communications to the Editor
Mo2(OR)6 formed only to a small extent (∼7%). EIMS analysis
of the NMo(OR)3 recovered from this reaction mixture indicated
negligible 15N-enrichment. We conclude that N atom transfer
from NMo(OR)3 to Mo(NRAr)3 is irreversible on the time scale
and under the conditions of the reactions studied here. This
means that the 15N label in 14/15NMo(OR)3 (eq 2) cannot have
come by way of 14/15NMo(NRAr)3. Thus, it appears that the
Mo(OR)3 fragment plays an active dinitrogen-splitting role in
the present reaction system.
observed) is responsible for dinitrogen cleavage in the reaction
system of eq 2.
This work unites well-defined dinitrogen cleavage and
nitrogen atom-transfer reactions. Preliminary experiments
indicate that the system reported here (eq 2), although cyclic,
is not efficient enough to become truly catalytic.14 The finding
that alkoxide-ligated molybdenum(III) may participate in dini-
trogen cleavage chemistry is in line with reports on catalytic
dinitrogen fixation by molybdenum(III) and vanadium(III) in
protic media.15 In future reports, we will seek to more brightly
illuminate mechanistic aspects of the chemistry described here.16
We sought independent access to “Mo(OR)3”, in order to
determine whether N2 cleavage by Mo(OR)3 alone [i.e., in the
absence of Mo(NRAr)3] is competitive with formation of dimer
11
Acknowledgment. For funding, C.C.C. is grateful to the National
Science Foundation (CAREER Award CHE-9501992), to DuPont
(Young Professor Award), to the Packard Foundation, and to the MIT
Department of Chemistry. C.E.L. thanks MIT’s UROP (Undergraduate
Research Opportunities Program) for funding.
Mo2(OR)6. Treatment of MoCl3(THF)3 (benzene, 3.5 mM,
28 °C, 1 atm of N2) with 3 equiv of LiOR led only to clean
formation of dimer Mo2(OR)6 (70% isolated yield, eq 4).12
Supporting Information Available: Experimental details for all
procedures described in the text (5 pages). This material is contained
in many libraries on microfiche, immediately follows this article in
the microfilm version of the journal, can be ordered from the ACS,
and can be downloaded from the Internet; see any current masthead
page for ordering information and Internet access instructions.
(4)
NMo(OR)3 was not detected (1H NMR) in the crude reaction
mixture. This result suggests that both Mo(NRAr)3 and Mo-
(OR)3 are required for dinitrogen cleavage in the reaction system
of eq 2. Since the dinitrogen-splitting reaction described
previously for Mo(NRAr)3 is known to proceed via the
bimetallic intermediate (µ-N2)[Mo(NRAr)3]2,13 we propose that
an analogous intermediate (RO)3Mo(µ-N2)Mo(NRAr)3 (not
JA953573Y
(13) A recent EXAFS investigation confirmed the proposed linear
structure for the MoNNMo core of (µ-N2)[Mo(NRAr)3]2; these results will
be published in an upcoming full paper (Laplaza, C. E.; Cummins, C. C.;
Pickering, I.; George, G., manuscript in preparation) along with activation
parameters for the first-order conversion of (µ-N2)[Mo(NRAr)3]2 to NMo-
(NRAr)3. For the single-crystal X-ray structure of a complex related to
(µ-N2)[Mo(NRAr)3]2, see: Shih, K.-Y.; Schrock, R. R.; Kempe, R. J. Am.
Chem. Soc. 1994, 116, 8804.
(11) We prepare orange MoCl3(THF)3 according to: Dilworth, J. R.;
Zubieta, J. Inorg. Synth. 1986, 24, 193. For a paramagnetic 1H NMR study
of the solution stability of monomeric mer-MoCl3(THF)3, see: Poli, R.;
Mui, H. D. J. Am. Chem. Soc. 1990, 112, 2446. For the single-crystal X-ray
structure of mer-MoCl3(THF)3, see: Hofacker, P.; Friebel, C.; Dehnicke,
K.; Ba¨uml, P.; Hiller, W.; Stra¨hle, J. Z. Naturforsch. 1989, 44b, 1161.
(12) This route to Mo2(OR)6 appears to be simpler and higher-yielding
than the original literature route involving treatment of Mo2(NMe2)6 with
ROH (ref 7a); another improved route to Mo2(OR)6 compounds has recently
appeared: Gilbert, T. M.; Landes, A. M.; Rogers, R. D. Inorg. Chem. 1992,
31, 3438.
(14) For example, treatment of Mo(NRAr)3 (benzene, 0.05 M, 28 °C, 1
atm of N2) with 10, 2, or 1 mol % NMo(OR)3 led to e20% conversion to
1
NMo(NRAr)3 over a 12 h period as determined by H NMR.
(15) (a) Antipin, M.; Struchkov, Y.; Shilov, A.; Shilova, A. Gazz. Chim.
Ital. 1993, 123, 265. (b) Luneva, N. P.; Mironova, S. A.; Shilov, A. E.;
Antipin, M. Y.; Struchkov, Y. T. Angew. Chem., Int. Ed. Engl. 1993, 32,
1178. (c) Shilov, A. E. Pure Appl. Chem. 1992, 64, 1409.
(16) For example, we are currently attempting to characterize an
observable blue intermediate, presumably (RO)3Mo(µ-N)Mo(NRAr)3, in the
N atom abstraction from NMo(OR)3 by Mo(NRAr)3.