FORMATION OF ETA-2-IMINOACYL COMPOUNDS BY PROTONATION OF A KETENIMINE LIGAND IN BIS((TRIMETHYLSILYL)CYCLOPENTADIENYL) COMPOUNDS OF NIOBIUM - MOLECULAR-STRUCTURES OF [NB(ETA-5-C5H4SIME3)2CL(ETA-2(C,N)-ETPHHCCNPH)]+BF4- AND [NB(ETA-5-C5H4SIME3)2F(ETA-2(C,N)-PH2HCCNPH)]+BF4-

Nb(eta5-C5H4SiMe3)2X(eta2(C,N)-R1R2CCNR3) (X = Cl, Br) reacts with 1 equiv of HBF4.OEt2 to give in one step the eta2-iminoacyl complexes [Nb(eta5-C5H4SiMe3)2X(eta2(C,N)-R1R2HCCNR3]+BF4-(1, X = Cl, R1 = R2 = R3 = C6H5 (Ph); 2, X = Br, R1 = R2 = R3 = Ph; 3, X = F, R1 = R2 = R3 = Ph; 4, X = Cl, R1 = R3 = Ph, R2 = C2H5 (Et); 5, X = Br, R1 = R3 = Ph, R2 = Et; 6, X = Cl, R1 = R3 = Ph, R2 = CH3 (Me); 7, X = Br, R1 = R3 = Ph, R2 = Me; 8, X = Cl, R1 = R2 = Ph, R3 = p-Br-C6H4; 9, X = Br, R1 = R2 = Ph, R3 = p-Br-C6H4) through protonation at the free terminus of the complexed ketenimine ligands. Complex 3 has also been isolated as both the N-outside and N-inside conformers in the three-step reaction (oxidation, F- abstraction, and protonation processes) of Nb(eta5-C5H4SiMe3)2(eta2(CN)-Ph2CCNPh) with 2 equiv of HBF4.OEt2. The chemical (Na/Hg) and electrochemical one-electron reductions of complexes 1-9 yield the starting ketenimine complexes with the evolution of H2. The structures of 3 and 4 were determined by single-crystal diffractometry. Compound 3 crystallizes in the monoclinic space group P2(1)/n with a = 11.004 (4) angstrom, b = 17.702 (5) angstrom, c = 18.679 (4) angstrom, beta = 91.60 (2)-degrees, Z = 4, V = 3637.3 angstrom3, rho(calcd = 1.358 g/mL, R = 0.051, and R(w) = 0.058 based on 6832 reflections. Compound 4 crystallizes in the monoclinic space group C2/c with a = 23.962 (8) angstrom, b = 21.408 (5) angstrom, c = 16.456 (3) angstrom, beta = 123.30 (2)-degrees, Z = 8, V = 7055.5 angstrom3, rho(calcd) = 1.368 g/mL, R = 0.046, and R(w) = 0.048 based on 5856 reflections. The molecular structures show a typical bent-metallocene geometry around the niobium atom with an eta2(C,N)-bonded iminoacyl ligand.