Four-Coordinate Mo(II) as (silox)2Mo(PMe3)2 and Its W(IV) Congener (silox)2HW(η2-CH2PMe2)(PMe3) (silox = t Bu3SiO)

The reduction of [( t Bu3SiO)2MoCl]2 (2 2) provided the cyclometalated derivative, (silox)2HMoMo(κ-O,C-OSi t Bu2CMe2CH2)(silox) (3), and alkylation of 2 2 with MeMgBr afforded [( t Bu3SiO)2MoCH3]2 (4 2). The hydrogenation of 4 2 was ineffective, but the reduction of 2 2 under H2 generated [( t Bu3SiO)2MoH]2 (5 2), and the addition of 2-butyne to 3 gave [(silox)2Mo]2(μ:η2η2-C2Me2) (6), thereby implicating the existence of [(silox)2Mo]2 (1 2). The addition of (silox)H to Mo(NMe2)4 led to (silox)2Mo(NMe2)2 (7), but further elaboration of the core proved ineffective. The silanolysis of MoCl5 afforded (silox)2MoCl4 (8) and (silox)3MoCl3 (9) as a mixture from which pure 8 could be isolated, and the addition of THF or PMe3 resulted in derivatives of 9 as (silox)2Cl3MoL (L = THF, 10; PMe3, 11). Reductions of 11 and (silox)2WCl4 (15) in the presence of excess PMe3 provided (silox)2Cl2MPMe3 (M = Mo, 12; W, 16) or (silox)2HW(η2-CH2PMe2)PMe3 (14). While “(silox)2W(PMe3)2” was unstable with respect to W(IV) as 14, a reduction of 12 led to the stable Mo(II) diphosphine, (silox)2Mo(PMe3)2 (17). X-ray crystal structures of 10 (pseudo-O h ), 12 (square pyramidal), and 14 and 17 (distorted T d ) are reported. Calculations address the diamagnetism of 12 and 16, and the distortion of 17 and its stability to cyclometalation in contrast to 14.