SYNTHESIS AND REACTIVITY OF TRIS(PYRAZOLYL)BORATE-STABILIZED MOLYBDENUM IMIDO ALKYLIDENE COMPLEXES

The addition of LiCH3 to TpMo(CHR)(NAr)(OTf) (1; Tp = hydridotris(1-pyrazolyl)borate; R = C(CH3)(2)Ph; Ar = 2,6-i-Pr-C6H3; OTf = OSO2(CF3)) gave TpMO(CHR)(NAr)(CH3) (2). Compound 2 was used to prepare several solvent-bound, cationic complexes by abstracting the methyl ligand. The addition of tetrakis(3,5-bis(trifluoromethyl)phenyl)boric acid to 2 in Et(2)O gave [TpMo(CHR)(NAr)(Et(2)O)][BAr'(4)] (3; Ar' = 3,5-C6H3(CF3)(2)) and CH4. The reaction of 2 and trityl tetrakis(pentafluorophenyl)borate in the presence of excess acetonitrile gave [TpMo(CHR)(NAr)(NCCH3)][B(C6F5)(4)] (4) and Ph(3)CCH(3). The Lewis acid B(C6F5)(3) abstracted the methyl ligand of 2 in the presence of acetonitrile or tetrahydrofuran to give the compounds [TpMo(CHR)(NAr)(S)][B(CH3)(C6F5)(3)] (5, S = NCCH3; 6, S = THF). The triflate ligand of 1 was displaced by trimethylphosphine to give [TpMo(CHR)(NAr)(P(CH3)(3))][OTf] (7). Stirring 1 in the presence of methanol and Florisil gave TpMo(CHR)(NAr)(OCH3) (8). The addition of excess potassium methoxide to 1 gave TpMo(CR)(NHAr)(OCH3) (9). Attempts to convert 8 to 9 by heating, photolysis, and the addition of NEt(3) or PMe(3) were unsuccessful. Since only excess methoxide converts 8 to 9, a methoxide-mediated proton transfer mechanism is proposed. X-ray structures of syn-8 (P2(1)2(1)2(1), a = 12.620(2) Angstrom, b = 13.492(2) Angstrom, c = 19.682(2) Angstrom, Z = 4, V = 3351.3(7) Angstrom(3), M(r) = 647.48, d(calc) = 1.283 g/cm(3), R = 6.00%, R(w) = 5.89%) and syn-9 (P ($) over bar 1, a = 10.624(2) Angstrom, b = 11.766(2) Angstrom, c = 13.980(2) Angstrom, alpha = 86.96(2)degrees, beta = 86.54(2)degrees, gamma = 67.77(2)degrees, Z = 2, V = 1635.8(7) Angstrom(3), M(r) = 647.48, d(calc) = 1.283 g/cm(3), R = 4.84%, R(w) = 4.87%) were obtained, and a trans-influence series has been established for several ligands. Rotational isomers due to rotation about the Mo-alkylidene bond were observed for all compounds, and marked differences in rates of rotamer interconversion are observed for 1, 2, and 8. Compounds 1 and 2 in the presence of AlCl3 successfully catalyzed the ring-opening metathesis polymerization of cyclooctene and norbornylene and the oligomerization of 1,9-decadiene via acyclic diene metathesis polymerization.