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

The redox chemistry of cube-type titanium(iv) nitrido complexes [{Ti-4(eta(5)-C5Me5)(3)(R)}(mu(3)-N)(4)] (R = eta(5)-C5Me5 (1), N(SiMe3)(2) (2), eta(5)-C5H4SiMe3 (3), and eta(5)-C5H5 (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(eta(5)-C5Me5)}(4)(mu(3)-N)(4)] (5). The treatment of complexes 1-4 in toluene with one equivalent of [K(C5Me5)] in the presence of macrocycles (L) leads to C10Me10 and the formation of more stable derivatives [K(L)][{Ti-4(eta(5)-C5Me5)(3)(R)}(mu(3)-N)(4)] (R = eta(5)-C5Me5, L = 18-crown-6 (6), crypt-222 (7); R = N(SiMe3)(2), L = 18-crown-6 (8), crypt-222 (9); R = eta(5)-C5H4SiMe3, L = 18-crown-6 (10), crypt-222 (11); R = eta(5)-C5H5, L = crypt-222 (12)). However, the analogous reaction of 4 with [K(C5Me5)] and 18-crown-6 affords [{(18-crown-6)K}(2)(mu-eta(5):eta(5)-C5H5)][{Ti-4(eta(5)-C5Me5)(3)(eta(5)-C5H5)}(mu(3)-N)(4)] (13) via abstraction of one cyclopentadienide group from a putative intermediate [(18-crown-6)K(mu-eta(5):eta(5)-C5H5)Ti-4(eta(5)-C5Me5)(3)(mu(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(eta(5)-C5Me5)(mu-NH)}(3)(mu(3)-N)] (14) does not reveal any reversible redox event and 14 readily reacts with [K(C5Me5)] to afford C5Me5H and the diamagnetic derivative [{K(mu(4)-N)(mu(3)-NH)(2)Ti-3(eta(5)-C5Me5)(3)(mu(3)-N)}(2)] (15). The treatment of 15 with two equiv. of 18-crown-6 polyethers produces the molecular species [(L)K{(mu(3)-N)(mu(3)-NH)(2)Ti-3(eta(5)-C5Me5)(3)(mu(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(eta(5)-C5Me5)(3)(mu(3)-N)(mu-N)(mu-NH)(2)] (18) similar to the ion pair [K(crypt-222)][Ti-3(eta(5)-C5Me5)(3)(mu(3)-N)(mu-N)(mu-NH)(2)] (19) obtained in the treatment of 15 with cryptand-222.