Studies on the chemical reduction of polynuclear titanium(IV) nitrido complexes.

The redox chemistry of cube-type titanium(IV) nitrido complexes [{Ti 4 (η 5 -C 5 Me 5 ) 3 (R)}(μ 3 -N) 4 ] (R = η 5 -C 5 Me 5 (1), N(SiMe 3 ) 2 (2), η 5 -C 5 H 4 SiMe 3 (3), and η 5 -C 5 H 5 (4)) was investigated by electrochemical methods and chemical reactions. Cyclic voltammetry studies indicate that 1-4 undergo a reversible one-electron reduction at ca. -1.8 V vs . ferrocenium/ferrocene. Thus, complex 1 reacts with sodium sand in tetrahydrofuran to produce the highly reactive ionic compound [Na(thf) 6 ][{Ti(η 5 -C 5 Me 5 )} 4 (μ 3 -N) 4 ] (5). The treatment of complexes 1-4 in toluene with one equivalent of [K(C 5 Me 5 )] in the presence of macrocycles (L) leads to C 10 Me 10 and the formation of more stable derivatives [K(L)][{Ti 4 (η 5 -C 5 Me 5 ) 3 (R)}(μ 3 -N) 4 ] (R = η 5 -C 5 Me 5 , L = 18-crown-6 (6), crypt-222 (7); R = N(SiMe 3 ) 2 , L = 18-crown-6 (8), crypt-222 (9); R = η 5 -C 5 H 4 SiMe 3 , L = 18-crown-6 (10), crypt-222 (11); R = η 5 -C 5 H 5 , L = crypt-222 (12)). However, the analogous reaction of 4 with [K(C 5 Me 5 )] and 18-crown-6 affords [{(18-crown-6)K} 2 (μ-η 5 :η 5 -C 5 H 5 )][{Ti 4 (η 5 -C 5 Me 5 ) 3 (η 5 -C 5 H 5 )}(μ 3 -N) 4 ] (13) via abstraction of one cyclopentadienide group from a putative intermediate [(18-crown-6)K(μ-η 5 :η 5 -C 5 H 5 )Ti 4 (η 5 -C 5 Me 5 ) 3 (μ 3 -N) 4 ]. In contrast to the cube-type nitrido systems 1-4, the cyclic voltammogram of the trinuclear imido-nitrido titanium(IV) complex [{Ti(η 5 -C 5 Me 5 )(μ-NH)} 3 (μ 3 -N)] (14) does not reveal any reversible redox event and 14 readily reacts with [K(C 5 Me 5 )] to afford C 5 Me 5 H and the diamagnetic derivative [{K(μ 4 -N)(μ 3 -NH) 2 Ti 3 (η 5 -C 5 Me 5 ) 3 (μ 3 -N)} 2 ] (15). The treatment of 15 with two equiv. of 18-crown-6 polyethers produces the molecular species [(L)K{(μ 3 -N)(μ 3 -NH) 2 Ti 3 (η 5 -C 5 Me 5 ) 3 (μ 3 -N)}] (L = 18-crown-6 (16), dibenzo-18-crown-6 (17)). Complex 17 further reacts with one equiv. of dibenzo-18-crown-6 to yield the ion-separated compound [K(dibenzo-18-crown-6) 2 ][Ti 3 (η 5 -C 5 Me 5 ) 3 (μ 3 -N)(μ-N)(μ-NH) 2 ] (18) similar to the ion pair [K(crypt-222)][Ti 3 (η 5 -C 5 Me 5 ) 3 (μ 3 -N)(μ-N)(μ-NH) 2 ] (19) obtained in the treatment of 15 with cryptand-222.