Synthesis, characterization, and solution redox properties of (trimpsi)M(CO)2(NO) complexes [M = V, Nb, Ta; trimpsi = tBuSi(CH2PMe2)3]

Treatment of [Et4N][M(CO)(6)] (M = Nb, Ta) with I-2 in DME at -78 degreesC produces solutions of the bimetallic anions [M-2(mu-l)(3)(CO)(8)](-). Addition of the tripodal phosphine (BuSi)-Bu-t(CH2PMe2)(3) (trimpsi) followed by refluxing affords (trimpsi)M(CO)(3)I [M = Nb (1), Ta (2)], which are isolable in good yields as air-stable, orange-red microcrystalline solids. Reduction of these complexes with 2 equiv of Na/Hg, followed by treatment with Diazald in THF, results in the formation of (trimpsi)M(CO)(2)(NO) [M = Nb (3), Ta (4)] in high isolated yields. The congeneric vanadium complex, (trimpsi)V(CO)(2)(NO) (5), can be prepared by reacting [Et4N][V(CO)(6)] with [NO][BF4] in CH2Cl2 to form V(CO)(5)(NO). These solutions are treated with 1 equiv of trimpsi to obtain (eta(2)-trimpsi)V(CO)(3)(NO). Refluxing orange THF solutions of this material affords 5 in moderate yields. Reaction of (trimpsi)VCl3(THF) (6) with 4 equiv of sodium naphthalenide in THF in the presence of excess CO provides [Et4N][(trimpsi)V(CO)(3)] (7), (trimpsi)V(CO)(3)H, and [(trimpsi)V(mu-Cl)(3)V(trimpsi)][(eta(2)-trimpsi)V(CO)(4)].3THF ([8][9].3THF). All new complexes have been characterized by conventional spectroscopic methods, and the solid-state molecular structures of 2.(1)/2THF, 3-5, and [8][9].3THF have been established by X-ray diffraction analyses. The solution redox properties of 3-5 have also been investigated by cyclic voltammetry. Cyclic voltammograms of 3 and 4 both exhibit an irreversible oxidation feature in CH2Cl2 (E-p,E-a = -0.71 V at 0.5 V/s for 3, while E-p,E-a = -0.55 V at 0.5 V/s for 4), while cyclic voltammograms of 5 in CH2Cl2 show a reversible oxidation feature (E-1/2 = -0.74 V) followed by an irreversible feature (0.61 V at 0.5 V/s). The reversible feature corresponds to the formation of the 17e cation [(trimpsi)V(CO)(2)(NO)](+) ([5](+)), and the irreversible feature likely involves the oxidation of [5](+) to an unstable 16e dication. Treatment of 5 with [Cp2Fe][BF4] in CH2Cl2 generates [5][BF4], which slowly decomposes once formed. Nevertheless, [5][BF4] has been characterized by IR and ESR spectroscopies.