PYRIDINE AND RELATED ADDUCTS, (SILOX)3ML (M = SC, TI, V, TA) - ETA-1-PYRIDINE-N VS ETA-2-PYRIDINE-N,C LIGATION

Adducts of (silox)3M (M = Ta (1), Ti (2), Sc (3), V (4)) have been prepared in order to assess the various electronic factors responsible for eta-1-pyridine-N vs. eta-2-pyridine-N,C ligation. Treatment of ScCl3(THF)3 or VCl3 with 3 equiv of Na(silox) in THF yielded (silox)3M(THF) (M = Sc (3-THF), V (4-THF)), and exposure of [(Me3Si)2N]3Sc to 5 equiv of (silox)H provided (silox)3ScNH3 (3-NH3), but the bases could not be removed. Addition of C2H4, C2H3Me, 1-butene, and cis-2-butene to 1 afforded (silox)3Ta(olefin) (olefin = C2H4 (5a), C2H3Me (5b), C2H3Et (5c), cis-MeHC=CHMe (5d)), although cyclometalation to give (silox)2H activated TaOSi(t)Bu2CMe2CH2 (6) competed with the latter two. In concentrated benzene solution (approximately 0.10 M), 1 trapped C6H6 to yield [(silox)3Ta]2{mu-eta-2(1,2):eta-2(4,5)-C6H6} (7; approximately 7%) along with 6. Acetylene, 2-butyne, and F3CC=CCF3 reacted with 1 to give (silox)3Ta(alkyne) (alkyne = C2H2 (8a), C2Me2 (8b), C2(CF3)2 (8c)). Ethylene did not displace the THF and NH3 from 3-THF, 3-NH3, and 4-THF, but reacted with 2 to provide [(silox)3Ti]2(mu-C2H4) (9). Treatment of 1 with pyridine, 2-picoline, 2,6-lutidine, pyridazine (1,2-N2C4H4) and pyrimidine (1,3-N2C4H4) provided (silox)3Ta(eta-2-NC5H5-N,C) (10a), (silox)3Ta(eta-2-6-NC5H4Me-N,C) (10b), (silox)3Ta(eta-2-2,6-NC5H3Me2-N,C) (11b), (silox)3Ta(eta-2-N2C4H4-N,N') (12), and (silox)3Ta(eta-2-1,3-N2C4H4-N1,C6) (13), respectively. Formation of these eta-2-heterocyclic adducts is proposed to occur via nucleophilic attack by 1 at the LUMO (predominantly C=N pi*) of the substrate, a process consistent with the generation of the pyridyl hydride (silox)3Ta(H)(C5H2Me2N) (11a) from 2,6-lutidine prior to equilibration with 11b. Similar treatments of 2 yielded (silox)3Ti(eta-1-py) (2-py) and related eta-1-py derivatives of 3,5-lutidine (2-3,5-NC5H3Me2), 4-picoline (2-4-NC5H4Me), and 4-NC5H4(t)Bu (2-4-NC5H4(t)Bu). Upon application of the McConnell equation to a(H) values obtained from the H-1 contact shifts of 2-py, the following spin density probabilities were obtained: rho-2(2,6) = 0.13%; rho-2(3,5) = 0.01%; rho-2(4) = 0.16%. eta-1-Pyridine adducts (silox)3M(py) (M = Sc (3-py), V (4-py)) were produced upon exposure of (silox)3M(THF) to py. Interpretation of the UV-vis spectrum of 1 and EHMO calculations of eta-1 and eta-2 forms of (silox)3Ta(py) provide a rationale for the variation in pyridine ligation. Of critical importance are the four-electron repulsion between the filled d(z)2 orbital of 1 and the py N-donor orbital and the capability of pyridine to function as a good pi-acceptor in the eta-2-mode.