Inorg. Chem. 1988, 27, 2050-2054
2050
Contribution from Department of Chemistry 6-331, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
Tbiolato Dinitrogen (or Hydrazido(4-)) Complexes, [Ta(SAr),(THF)12(p-N2)(Ar = 2,6-C6H3-i-Pr2,2,4,6-C6Hz-i-P1’3),and Phenoxide Analogues. Structural Comparison of [Ta(S-2,6-C6H3-i-Pr2)3( THF)],( p N 2 ) and [Ta(0-2,6-C6H3-i-Pr2)3(THF)],( p-N2) R. R. Schrock,* M. Wesolek, A. H. Liu, K. C. Wallace, and J. C. Dewan Received September 22, I987 [TaC13(THF)2]2(p-N2) reacts with 6 equiv of LiDIPP (DIPP = 0-2,6-C6H3-i-Pr2)to give [Ta(DIPP)3(THF)]2(p-N,)(1) in 60% isolated yield. Crystals of [Ta(DIPP),(THF)]2(p-N2)belong to the space group C 2 / c with a = 20.276 (8) A, b = 16.737 (7) A, c = 28.227 (19) A, @ = 105.55 (S)”, V = 9228 A’, and 2 = 4. The structure contains a hydrazido(4-) p-N, ligand with N-N = 1.32 (1) A, Ta-N = 1.796 (5) A, and Ta-N-N = 176.6 ( 6 ) O . The p-N, ligand is bound to the metal in an equatorial position of a trigonal bipyramid. [Ta(TIPT),(THF)],(p-N,) (3; TIPT = S-2,4,6-C6H2-i-Pr3) and [Ta(DIPT),(THF)]2(p-N2)(4; DIPT = S-2,6-C6H3-i-Pr2) can be prepared analogously in high yield. Crystals of [Ta(DIPT),(THF)],(p-N,) (4) belong to the space group PcZln with a = 13.606 (6) A, b = 23.096 (9) A, c = 28.845 (10) A, V = 9064.5 A3, and 2 = 4. In this relatively poor quality structure N-N = 1.29 (6) A, Ta-N = 1.90 (6) and 1.72 (6) A, and Ta-N-N = 166 (5)’. In this case the p-N2ligand is bound in an axial position trans to THF in a trigonal-bipyramidalligand arrangement about each metal. Neither 1 nor 3 can be prepared by treating Ta(CHCMe3)(DIPP),(THF) or Ta(CHCMe,)(TIPT),(THF) with benzaldehyde azine. 1 reacts readily with benzaldehyde to give benzaldehyde azine in high yield, while 3 gives only -25% benzaldehyde azine.
Introduction In spite of the fact that hundreds of dinitrogen complexes are now known,’ complexes that contain an anionic sulfur ligand are rare. To our knowledge the only reported examples are diethyldithiocarbamate complexes of the type [M(SzCNEtz)3]z(p Nz) ( M = Nb, Ta)za and complexes of the type mer-[Re(L)(N2)(PMe2Ph)3]where L = SzCOEt, SZPPh2, or S2CNR2.2b This is ironic since the molybdenum in nitrogenase, to which dinitrogen possibly binds in Qrderto be reduced, is likely to have sulfur in its coordination spheree3 Therefore, it is important to demonstrate that dinitrogen can be found in complexes that contain other types of sulfur ligands (e.g., thiolates) and to structurally characterize such species. A logical starting point for preparing pdinitrogen complexes that contain anionic ligands are complexes of the type [MC13(THF),]2(p-NZ)( M = Nb, Ta).2a34 The reason is that dithiocarbamate derivatives have been prepared from themza (as noted above), the chlorides in [TaC13(THF)z]z(p-N2)have been substituted by tert-butoxide ligands to give [Ta(OCMe,),(THF)],has been prepared (p-Nz),4 and [Nb(CHzCMe3)3(THF)]2(p-N2) by adding Mg(CH2CMe3)2(dioxane)to [NbC13(THF)2]z(p-N2).4 We chose tantalum over niobium in the belief that in circumstances where complexes might be only marginally stable toward loss of dinitrogen, what can be called a ditantalum(5+) hydrazido(4-) complex would be more stable than an analogous niobium complex to “reduction” of the metal to the 3+ oxidation state. We report here the preparation and structural characterization of complexes of the general type [Ta(XR),(THF)lz(p-Nz) where XR is a bulky benzenethiolate or an analogous bulky phenoxide ligand. Henderson, R. A,; Leigh, G. J.; Pickett, C. J. Adv. Inorg. Chem. Radiochem. 1983, 27, 197. (b) Dilworth, J. R.; Richards, R. L. In Comprehensive Organometallic Chemistry; Wdkinson, G., Stone, F.G. A., Abel, E. W., Eds.; Pergamon: Elmford, NY, 1982; Vol. 8, Chapter 60. (2) (a) Dilworth, J. R.; Harrison, S. J.; Henderson, R. A.; Walton, D. R. M. J. Chem. Soc., Chem. Comrnun. 1984,176. (b) Chatt, J., Crabtree, R. H.;Dilworth, J. R.; Richards, R. L. J . Chem. Soc., Dalton Trans. 1974, 2358. (3) Coughlin, M., Ed. Molybdenum and Molybdenum-Containing Enzymes; Pergamon: Elmsford, NY, 1980. (b) Hardy, R. W. F., Bottomley, F., Burns, R. C., Eds. A Treatise on Dinitrogen Fixation; Wiley-Interscience: New York, 1979. (c) Gibson, A. H., Newton, W. F., Eds. Current Perspectives in Nitrogen Fixation; Elsevier: Amsterdam, 1981. (d) For a discussion of the most recent theories, see: Conradson, S. D.; Burgess, B. K.; Newton, W. E.; Mortenson, L. E.; Hodgson, K.0.J. Am. Chem. Soc. 1987,109,7507, (e) Recent genetic evidence implicates FeMo-co as the site of binding and reduction of dinitrogen. See: Hawkes, T. R.; McLean, P. A.; Smith, 8. E.Biochem. J. 1984, 217, 317. (4) Rocklage, S.M.; Schrock, R. R. J . A m . Chem. SOC.1982, 104, 3077 (1) (a)
0020-1669/88/1327-2050$01.50/0
Table I. Relevant Bond Distances (A) and Angles (deg) in
[Ta(O-2,6-C6H,-i-Pr2)3(THF)]2(p-N,) (1) Ta-N 1.796 (5) N-Ta-O( 1) Ta-O(1) 1.911 ( 5 ) N-Ta-O(2) N-Ta-O(3) Ta-0(2) 1.945 (5) Ta-0(3) 1.933 (5) N-Ta-O(4) Ta-0(4) 2.232 ( 5 ) O( l)-Ta-0(3) N-N 1.32 (1) O( 1)-Ta-0(2) Ta-O(l)-C(lOl) 175.2 ( 5 ) O( 1)-Ta-0(4) Ta-O(2)-C(20 1) 147.2 (4) O( 2)-Ta-0( 3) Ta-0(3)-C(301) 160.1 (5) 0(2)-Ta-0(4) Ta-N-N 176.6 (6) O( 3)-Ta-0(4)
108.7 (2) 109.8 (2) 109.4 (2)
91.0 (2) 93.3 (2) 91.6 (2) 160.2 (2) 136.5 (2) 81.5 (2) 79.6 (2)
Results and Discussion Preparation and Crystal Structure of [Ta(0-2,6-C6H3-iPrz)3(THF)]2(p-Nz)(1). Addition of 6 equiv of lithium 2,6-diisopropylphenoxideto [TaC13(THF)2]z(~-Nz) in a mixture of ether (1) in and THF gives [Ta(O-2,6-C6H3-i-Prz)3(THF)]2(p-N2) -60% isolated yield as yellow crystals (eq 1). Coordinated T H F [TaC13(THF)2]2(p-N2) 6LiDIPP
+
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[Ta(DIPP)3(THF)I2(11.-N2) (1) DIPP = 0-2,6-CsH3-i-Pr2 exchanges rapidly on the ‘H N M R time scale with free T H F in C6D6 (according to N M R spectra). Since 1 is so crowded, we assume that THF exchanges in a dissociative manner. All DIPP ligands are equivalent and freely rotating at 25 OC, where intermediate pseudotetrahedral species presumably form. Although 1 did not analyze well (becaqse of THF lability; see Experimental Section), a pyridine adduct could be prepared straightforwardly and did analyze satisfactorily. X-ray data were collected on a crystal of 1 at -140 OC (see Experimental Section). Three drawings of the structure are shown in Figure 1. Relevant bond distances and angles can be found in Table I. The molecule has a crystallographically required 2-fold axis that passes through the N-N bond. The configuration about the metal at each end of the molecule is best viewed as a distorted trigonal bipyramid (according to the interligand angles), with the THF ligand (O(4)) and one of the phenoxide ligands (0(1)) being in axial positions. The trigonal-bipyramidal description is also attractive from the point of view of the large Ta-O( I)-C( 101) angle of 175.2 ( 5 ) O and slightly shorter Ta-O(1) bond length (compared to Ta-O(2) and Ta-0(3)), as if that phenoxide were in a position significantly different (electronically and sterically) compared to the other two. There is no evidence that replacing chlorides by phenoxide ligands significantly alters the Ta=N-N=Ta core; Le., the N-N 0 1988 American Chemical Society
m
W Figure 1. Views of [Ta(0-2,6-C6H3-i-Pr2)3(THF)]2(p-N2) (1): (left) side-on, with spheres of fixed radius in order to allow structural details to be seen; (middle) central portion from the side; (right) central portion from the end.
Table 11. Comparison of M-N Bond Distances (A), N-N Bond Distances (A), and M-N-N Angles (deg) in Structurally Characterized
Hydrazido(4-) (M=N-N=M) Complexes compd M-N [Ta(CHCMe3)( C H ~ C M ~ ~ ) ( P M ~ ~ ) ~ I ~ ( I L1.837 - W (8) (TaC13[P(CH2Ph)31(THF)h(P-N2) [W(PhC=CPh)C12(dme)12(p-N2) [W(d-C,Me,)Me,l2(p-N,)
[Ta(0-2,6-C6H3-i-Pr2)3(THF)]2(p-N2) (1) [Ta(S-2,6-C6H3-i-Pr2)3(THF)]2(p-N2) (4)
1.842 (8) 1.796 (5) 1.776 (11) 1.735 (11) 1.763 (18) 1.742 (17) 1.796 (5) 1.90 (6) 1.72 (6)
bond length in 1(1.32 (1) A) is characteristic of what we believe to be hydrazido(4-) complexes, several examples of which have now been characterized structurally (Table 11). The average of the N-N bond lengths listed in Table I1 is 1.30 A, and the range is 1.28-1.33 A; an N-N bond length of 1.30 A is -0.1 8,longer than N-N bond lengths in most simple v1-N2or p-N2 dinitrogen complexes,’ consistent with a highly reduced p N Z . The Ta-N bond length and almost 180” Ta-N-N angle are also consistent with this interpretation. The full view of 1 shown in Figure 1 (left) suggests that it is a relatively crowded molecule in which the six proximal isopropyl groups from the phenoxide’ligands fill up the space around the Ta=N-N=Ta core. (There are six N.-H contacts of 3.5 A or less, two of the order of 2.9 A; they are all noted in the Experimental Section.) The two ends of the molecule are twisted 110” with respect to one another, as found in [Ta(CHCMe3)(CHZCMe3)(PMe3),]z(p-Nz)5and [W(PhmCPh)Clz(dme)]2(p-N2).’ In all three cases, one could ascribe the approximate orthogonality of the two ends to a minimization of steric interactions, rather than any overriding electronic factors. It is interesting to compare the structure of 1 with that of M O ( P ~ - P ~ ) ( D I P P ) ~ . ~ Mo(PhC=CPh)(DIPP), is also a trigonal-bipyramidal complex with the “substrate” (the acetylene) bound in an equatorial position. The axial Mo-0 bond lengths are significantly shorter than equatorial M e 0 bond lengths (1 376 (IO) versus 2.004 (IO) A), and the axial M-0-C bond angles, significantly larger than equatorial Mo-0-C angles (160.5 (1 1) versus 144.4 (IO)”). Calling the configuration about each metal
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( 5 ) Churchill, M. R.;Wasserman, H.J. Inorg. Chem. 1981, 20, 2899. (6) Churchill, M. R.; Wasserman, H. J. Inorg. Chem. 1982, 21, 218. (7) Churchill, M. R.; Li, Y.-J.; Thmpold, K.H.;Schrock, R. R. lnorg. Chem. 1984. 23. 4412. (8) Churchill, R.;Li,Y.-J. J. Orgonomet. Chem. 1986, 301, 49. (9) Walborsky, E.C.; Wigley, D. E.; Roland, E.; Dewan, J. C.; Schrock, R. R. Inorg. Chem. 1987, 26, 1615.
N-N 1.298 (12) 1.282(6) 1.292 (16) 1.334 (26) 1.32 (1) 1.29 (6)
M-N-N 171.43 (65) 172.42 (65) 178.9 (4) 176.4 (10) 175.6 (10) 167.0 (16) 170.2 (16) 176.6 (6) 166 (5) 170 (5)
ref 5 6 7 8 this work this work
in 1 trigonal bipyramidal therefore is consistent with the somewhat shorter Ta-O( 1) length and larger Ta-O( l)-C(lOl) angle compared to other Ta-0 lengths and Ta-0-C angles in 1. It is interesting to note that separate N M R resonances for axial and equatorial DIPP ligands can be observed in Mo(PhC=CPh)(DIPP),? and the axial set cannot rotate freely on the N M R time scale, presumably for steric reasons. In contrast, as we mentioned above, the DIPP ligands in 1 are equivalent on the N M R time scale, most likely as a result of dissociation of THF, although we cannot exclude an intramolecular permutation of the phenoxide ligands in a molecule in which T H F remains bound. We thought it possible that 1 could be prepared by a variation of the reaction between an alkylidene complex and an azine, the type of reaction that led to the discovery of hydrazido(4-) comreacts readily with plexes., Ta(CHCMe3)(DIPP),(THF)Io PhCH=N-N==CHPh, but the reaction proceeds only halfway at 25 “C or even after heating to 50 O C for 6 h (eq 2). Almost Ta(CHCMe,)(DIPP),(THF) + PhCH=N-N=CHPh PhCH=CHCMe, + Ta(N-N=CHPh)(DIPP),(THF) 2
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(2)
certainly, steric factors weigh heavily in these sterically finely balanced systems and in part prevent further reaction between 2 and Ta(CHCMe,)(DIPP),(THF) to give 1. (Unfortunately, complexes analogous to Ta(CHCMe,)(DIPP),(THF) that contain a less hindered alkylidene ligand having a @ proton are not stable;lob a small alkylidene (e.g., propylidene) perhaps would be reactive enough to react further with 2.) But it is also likely that the the metal alters the reactivity of the other end of the NN = C H P h ligand system enough-so that an alkylidene complex of borderline reactivity will not react with the Ta=N-N 4u(F0) being used in the structure refinement, which was by full-matrix least-squares techniques (269 variables) using SHELX-76. Final R, = 0.103 and final R2 = 0.1 17. The quality of the final structure is poor because of the marginal quality of the available crystals. After exhaustive attempts it was not possible to locate one of the isopropyl methyl carbons attached to C(661). Benzene rings were refined as rigid planar groups (C-C = 1.395 A; C-C-C = 120') pivoting on the ipso carbon atom. The isopropyl groups were also refined as rigid groups (C-C = 1.54 A; C-C-C = 109") pivoting on the methyne carbon atom. Only the tantalum atoms were refined anisotropically. The two possible space groups are Pcmn and pc2,n, each a nonstandard setting of Pnu2,. The only possible molecular symmetry is a noncrystallographic 2-fold axis passing between the two nitrogen atoms and approximately parallel to the Ta( l)-S(3) and Ta(2)-S(5) bonds. Therefore the structure has been solved and refined in the noncentrosymmetric space group Pc2,n. The centrosymmetric choice is ruled out since the molecule would be required to lie on a site of either T or m symmetry, which does not appear to be possible. The final differenceFourier map showed no chemically significant features. The alternate hand of the structure gave marginally higher R values. Crystal data: space group = Pc2,n, u = 13.606 (6) A, b = 23.096 (9) A, c = 28.845 (10) A, V = 9064.5 A', Z = 4, M , = 1565.8, p(calcd) = 1.147 g mr3, p = 24.6 cm". A semiempirical absorption correction was applied. There are five N-H contacts of less than or about 3.5 A: C(162)H*-N(l) = 3.25 A; C(162)-H-N(2) = 3.53 A; C(761)-H-*N(2) = 3.23 A;C(763)-HA-N(1) = 3.10 A; C(763)-HA*-N(2) = 3.21 A.
Acknowledgment. R.R.S. thanks the National Institutes of Health for support through Grant GM-31978. We thank the Biomedical Research Support-Shared Instrumentation Grant Program, Division of Research Resources, for funds to purchase the X-ray diffraction equipment (NIH Grant RR02243). M.W. thanks the National Science Foundation for a postdoctoral fellowship, and K.C.W. thanks DOW Chemical Co. for a graduate fellowship.
Registry No. 1, 114220-88-1; 2, 114198-82-2; 3, 114198-89-9; 4, 114198-83-3; 5, 114198-84-4; [TaCI,(THF),],(p-N), 90130-65-7; [Ta(0-2,6-C6H,-i-Pr,),(py)z] (p-N,), l 14198-86-6; Ta(CH-t-Bu)(DIPP)3(THF), 106034-24-6; Ta(NNCHPh)(O-2,6-C6H,-i-Pr2),(py), 11419887-7; Ta(CH-t-Bu)(TIPT),(THF), 1 14198-88-8; NaS-2,6-C6H,-i-Pr2, 114198-85-5; N2, 7727-37-9; 2,6-diisopropylaniline, 24544-04-5; sulfur, 7704-34-9; benzaldehyde azine, 588-68-1; benzaldehyde, 100-52-7. Supplementary Material Available: A fully labeled drawing of [Ta(0-2,6-C6H,-i-Pr2)3(THF)]2(p-N2), tables of final positional parameters, final thermal parameters, intramolecular bond distances, and intramolecular bond angles for [T~(O-~,~-C~HJ-~-P~~)~(THF)]Z(~-N~ and a table of final positional and thermal parameters for [Ta(S-2,6-C6H3-iPr2),(THF)],(p-N2) (14 pages); listings of final observed and calculated structure factors for both compounds (66 pages). Ordering information is given on any current masthead page. (17) Silverman, L. D.; Dewan, J . C.; Giandomenico, C. M.; Lippard, S . J. Inorg. Chem. 1980, 19, 3379.
