J . Am. Chem. SOC.1981, 103, 357-363
351
The hydrogen atoms were located from a difference Fourier. Anisoinery parts).22 Final positional and thermal parameters are presented in Table I. A tropic refinement of all nonhydrogen atoms and hydrogen positional table of observed and calculated structure factors is available as supparameters (isotropic E value at one more than average value of ring carbons and boron atoms) yielded a final R1 = 0.0428 and R2 = 0.0408. plementary material (Table V). Inclusion of the unobserved reflections (Convergence was considered complete when all the shifts were less than had little or no effect on the bond distances and angles. The final dif, one-tenth their standard deviation.) Here Rl = x/IFol - ~ F c ~ / ~R2~ F o ~ference map was relatively smooth with maxima of *0.345 e/A3 in the region of the P-N bonds. = [Xw(lF0I- IFc1)2j,X~Fo211’2, w = 1 / d F o ) 2 ,dF0) = 4F,2)/2F0, F : = p / L p , and a(Fo) = [u(Z,,,)~ + (0.25P)2]1/2/Lp. The function minimized was xwlF,I - IFc1)2. Atomic scattering facAcknowledgment. This work was supported by N.A.S.A., Ames tors fnr all atoms were taken from the compilation of Cromer and WaResearch Center. her', and were corrected for anomalous dispersion (both real and imag(22) Cromer, D. T.;Waber, J. T. “International Tables for X-Ray Crystallography”; The Kynoch Press: Birmingham, England, 1974; Vol. IV, Tables 2.2 B and 2.31.
Supplementary Material Available: A listing of structure factor amplitudes for IV (17 pages). Ordering information is given on a n y current masthead page.
Structures of [ (CH3),( t-BuN)W],(p-t-BuN), and [ ((CH3)2N)2TiI2(p-t-BuN),. A Molecular Orbital Explanation of the Existence of Unsymmetrically or Symmetrically Bridging Organoimido Ligands David. L. Thorn,* William A. Nugent,* and Richard L. Harlow Contribution No. 2766 from the Central Research and Development Department, E . I . du Pont de Nemours and Company, Experimental Station, Wilmington, Delaware 19898. Received April 1 I , 1980
Abstract: The crystal structures of the unsymmetrically bridging imido complex [(CH3)2(t-BuN)W]2(p-t-BuN)2 (Ib) and the symmetrically bridging imido complex [((CH3)2N)2Ti]2(pt-BuN)2 (IIb) have been determined. Both compounds I b and IIb crystallize in the space group P 2 n with two molecules in the unit cell. Compound l b has the cell dimensions (at -100 “ C ) a = 12.987 (2) A, b = 9.477 (2) c = 11.252 (2) A, @ = 103.85 (l)’, V = 1345 A3, and pcalcd= 1.759 g ~ m - compound ~; IIb has the cell dimensions (at -20 “ C ) a = 9.434 (2) A, b = 15.984 (4) A, c = 8.795 (2) A, @ = 115.55 (2)O, V = 1197 A3, and pcalcd= 1.150 g cm-3. Final conventional and weighted agreement indices on F,, for F: > 3u(F?) are 0.033 and 0.031 for I b and 0.040 and 0.037 for IIb. Molecular orbital calculations suggest that the unsymmetrical bridging mode in Ib, and in the previously studied Mo analogue Ia, is a manifestation of an “antiaromatic” electronic structure and that the symmetrical bridging mode in IIb and in the previously examined Zr analogue IIa is consistent with a n “aromatic” electronic structure.
td,
Transition-metal imido (NH a n d NR) species a r e ubiquitous intermediates in industrial1 a n d laboratory2 organic synthesis. Nevertheless, o u r systematic understanding of t h e chemistry of imido compounds is limited, particularly with regard t o t h e relationship between structure a n d r e a ~ t i v i t y . ~Structural studies in these laboratories4 a n d elsewhere3 have established t h a t five different bonding modes (1-5) can be present in these complexes. A n understanding of the electronic differences reflected by modes
M=N-R M=“R
(1) terminal linear
( 2 ) terminal bent
R
R
I
I
R
(1) Examples of such reactions and relevant literature include the following. (a) Ammoxidation of propylene to acrylonitrile: Burrington, J. D.; Grasselli, R. K. J . Cutal. 1979, 59, 79-99. (b) Hydrogenation of nitriles: Andrews, J . A.; Kaesz, H. D.1.Am. Chem. Sot. 1977, 99,6763-6765. (c) Haber ammonia synthesis: Jones, A,; McNicol, B. D.J . Cutal. 1977, 47, 384-388; Irgranova, E. G.; Ostrovskii, V . E.; Temkin, M . I. Kiner. Kutal. 1976, 17, 1257-1262. (2) Examples include the following. (a) Osmium-catalyzed oxyamination of olefins: Sharpless, K. B.; Chong, A. 0.;Oshima, K. J . Org. Chem. 1976, 41, 177-179; Herranz, E.; Sharpless, K. B. Ibid. 1978, 43, 2544-2548; Herranz, E.; Biller, s. A.; Sharpless, K. B. J . Am. Chem. Sot. 1978, 100, 3596-3598. (b) Electrochemical tosylamidation at a vanadium anode: Breslow, R.; Kluttz, R. Q.; Khanna, P. L. Tetrahedron Lett. 1979, 3273-3274. (c) Reduction of organic azides: Ho, T.-L.; Henninger, M.; Olah, G. A. Synthesis 1976,815-816; Kwart, H.; Kahn, A. A. J . Am. Chem. Sot. 1967, 89, 1950-1950. (3) For a review on organoimido and related complexes of transition metals see: Nugent, W. A.; Haymore, B. L. Coord. Chem. Reu. 1980, 31, 123-175. (4) (a) Nugent, W. A.; Harlow, R. L. J . Chem. SOC.,Chem. Commun. 1978, 579-580. (b) Ibid. 1979, 342-343. (c) Ibid. 1979, 1105-1 106. (d) Inorg. Chem. 1979. 18,2030. (e) Ibid. 1980, 19,777-779. (f) J . Am. Chem. Sot. 1980, 102, 1759-1760. (9) Nugent, W. A,; Harlow, R. L.; McKinney, R. J. Ibid. 1979, 101, 7265-7268.
0002-7863/81/1503-357$01.00/0
I
M
(5) triply bridging
1-5 should provide insight into t h e chemistry of t h e respective complexes. Moreover such a n investigation should also shed light o n structure-reactivity trends in t h e related alkylidene5 a n d oxo6 transition-metal complexes. Recently w e have isolated a n d structurally characterized t h e imido-bridged dimeric compound [(CH3)2(t-BuN)Mo]~(p-t-BuN), (Ia),4f d r a w n below. T h e distinctly asymmetric structure in t h e bridging region of Ia4‘ is remarkably different f r o m t h e symmetrical structure of t h e imido-bridged dimer [((CH,),N),Zr],(pt-BUN), (IIa).4d T h i s difference h a s prompted us t o determine t h e structures a n d establish t h e mode of bridge bonding in t h e (5) Schrock, R. R. Arc. Chem. Res. 1979, 12, 98-104. (6) Griffith, W. P. Coord. Chem. Rev. 1970, 5 , 459-517.
0 1981 American Chemical Society
358 J . Am. Chem. SOC.,Vol~103, No. 2, 1981 Chart I t-Ru
I
t-Bu
I t. BU
Ja, M = M o Ib,M=W
IIa, M = Zr IIb, M = Ti
related compounds [(CH3)2(t-BuN)W]2(p-t-BuN)2 (Ib) and [((CH3)2N)2Ti]2(p-BuN)2 (IIb).7 We find that the titanium complex IIb is isostructural with its Zr analogue IIa and has a symmetrically bridged structure and that the tungsten complex Ib is isostructural with the Mo complex Ia and possesses the unsymmetrical bridging structure.* In this paper we report the X-ray crystal structures of Ib and IIb. The persistence of the different bridging modes in compounds I and I1 has encouraged us to probe the electronic causes of the asymmetry in the compounds Ia,b. Our ultimate conclusion, upon which we elaborate in the remainder of this paper, is conveniently summarized in the representations of I and I1 illustrated in Chart I. The bridging region of I1 is, electronically, a delocalized, “aromatic” system and has the requisite equal bond lengths, while the bridging region of I is “antiaromatic” and prefers a localized bonding system of alternating long--short bond length^.^ It is not at all obvious how this difference can arise between two complexes which are, at first glance, electronically very similar; both are formally do metal compounds and are nominally isoelectronic in the bridging region. However, the different natures and locations of the terminal ligands result in significantly different bonding in the bridging regions in the two complexes, and it is these effects which we wish to examine. The factors responsible for establishing the geometries of ligand-bridged metal dimers have been discussed by several workers,1° notably Dahl and colleagues” and Hoffmann and co-workers.I2 However, the specific geometrical issue which we are addressing, the question of why a dimer will have a distinctly unsymmetrical bridging structure when closely related structures of high symmetry are intuitively and experimentally realizable, has to our knowledge not been previously studied.I3 In this paper we will confine our attention to the imido-bridged compounds I and I1 and close structural analogues thereof. (7) This complex was originally reported by: Bradley, D. C.; Torrible, E. G. Can. J . Chem. 1963, 41, 134-138. (8) All the compounds (Ia. Ib, Ha, IIb) have crystallographically imposed inversion centers and no other exact symmetry. There remain two crystallographically independent metal-N(bridge) separations. By “unsymmetrical” we mean the significant nonequivalence of these two independent separations in 1 (a and b), and by “symmetrical” we mean the equivalence of these independent separations in I1 (a and b). (9) Aromaticity and antiaromaticity are used here in the sense of a “delocalized” or “localized” bonding system. See: Thorn, D. L.;Hoffmann, R. Nouu. J . Chim. 1979, 3, 39-45 and references therein. See also: Goldstein, M. J.; Hoffmann, R. J . Am. Chem. SOC.1971,93,6193-6204. We have not yet examined these compounds for other manifestations of their “aromaticity” or “antiaromaticity”, e.g., ring currents. (10) (a) Mason, R.; Mingds, D. M. P. J . Orgunomet. Chem. 1973, 50, 53-61. (b) Burdett, J. K. J . Chem. SOC.,Dalton Trans. 1977, 423-428. (c) Norman, J. G.; Gmur, D. J. J . Am. Chem. SOC.1977, 99, 1446-1450. (1 1 ) (a) Dahl, L. F.; deGil, E. R.; Feltham, R. D. J . Am. Chem. SOC.1969, 91, 1653-1664. (b) Teo, B. K.; Hall, M. B.; Fenske, R. F.; Dahl, L. F. J . Organomet. Chem. 1974, 70, 413-420. (c) Teo, B. K.; Hall, M. B.; Fenske, R. F.; Dahl, L. F. Inorg. Chem. 1975, 14, 3103-3117 and references therein. ( 1 2 ) (a) Hay, P. J.; Thibeault, J. C.; Hoffmann, R. J . Am. Chem. SOC. 1975, 97, 4884-4899. (b) Summerville, R. H.; Hoffmann, R. Ibid. 1976, 98, 7240-7253. (c) Summerville, R. H.; Hoffmann, R. Ibid. 1979, 101, 3821-3831. (d) Pinhas, A. R.; Hoffmann, R. Inorg. Chem. 1979, 18, 654-658. (e) Dedieu, A,; Albright, T. A,; Hoffmann, R. J . Am. Chem. SOC. 1979, 100, 3141-3151. (13) An interesting but unrelated issue is the question of unsymmetrically bridging hydride ligands. See for example: Roziere, J.; Williams, J. M.; Stewart, R. P., Jr.; Peterson, J. L.; Dahl, L. F. J . Am. Chem. SOC.1977, 99, 4497-4499; Peterson, J. L.; Johnson, P. L.; O’Connor, J.; Dahl, L. F.; Williams, J. M. Inorg. Chem. 1978, 17, 3460-3469; Bau, R.; Teller, R. G.; Kirtley, S. W.; Koetzle, T. F. Acc. Chem. Res. 1979, 12, 176-183.
Thorn, Nugent, and Harlow Table I. Summary of the Crystal Data for the Two Crystallographic Studies __Ib IIb C,, II,, N, Ti C,, H,, N,W, 712.33 414.35 0.30 X 0.21 X 0.30 0.30 X 0.19 X 0.40 -100 -20 monoclinic monoclinic P 2 1/12 p2, In
mol formula mol wt cryst dimens, mm cryst temp, “C crystal system space group unit cell a, A b, c, a
9,deg cell vol, A’ Z
calcd density, g cni-’ abs coeff, cm-’
__
12.987 (2) 9.477 (2) 11.252 (2) 103.85 (1) 1345 2 1.759 90.7
9.434 (2) 15.984 (4) 8.795 (2) 115.55 (2) 1197 2 1.150 7.01
Table 11. Summary of the Refinement of the Two Structures __.__I
no. of reflctns with I >2 4 ) no. of variables hydrogen atoms
RW peaks in final difference Fourier
-
IIb
Ib
2298
2043
188 not refined 0.033 0.031 four (0.39-0.69 e A-’) near W
193 refined 0.040 0.037 0.29, 0.35 e A-’ near N(1), Ti
Table 111. Selected Bond Distances arid Angles for Compound Ib -__ Bond Distances (a)with W-W 3.093 (1) W-N(l) 1.736 (5) W-N(2) 1.842 (4) W-N(2)’ 2.288 (5) W-C(l) 2.163 (6) W-C(2) 2.171 (5) N(1 )-C( 1) 1.465 (7)
Estimated Standard Deviations N(2)-C(21) 1.488 (7) C(ll)-C(12) 1.495 (9) C(ll)-C(13) 1.531 (10) C(ll)--C(l4) 1.500 (10) C(21)-C(22) 1.537 (8) C(21)-C(23) 1.525 (8) C(21)-C(24) 1.523 (8)
Bond Angles ( I k g ) with N(l)-W-N(2) 108.3 (2) N(l)-W-N(2)’ 168.1 (2) N(1)-W-C(1) 92.6 (2) N(l)-W-C(2) 92.4 (2) N(2)-W-N(2)‘ 83.6 (2) N(2)-W-C(l) 111.2 (2) N(2)-W-C(2) 109.7 (2) N(2)’-W-C(1) 82.4 (2) N(2)’-W-C(2) 83.7 (2) C(l)-W-C(2) 134.7 (2) W-N(l)-C(ll) 168.3 (4) W- N(2)-W‘ 92.4 (2) W-N(2)-C(21) 134.2 (4) W’-N(2)-C(21) 129.4 (3)
I’stimated Standard Ucviations N(l)-C(ll)-C(l2) 110.0 (5) N(l)-C(ll)-C(l3) 107.4 (6) N(l)-C(ll)-C(l4) 108.9 (6) C(12)-C(lI)-C(13) 110.1 (7) C(l2)-C(ll)-C(14) 110.8 (8) C(l3)-C(ll)-C(14) 109.6 (7) N(2)-C(21)-C(22) 110.6 (5) N(2)-C(21)-C(23) 108.7 (5) N(2)-
