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Synthesis, Characterization, and Solution Redox Properties of (trimpsi)M(CO)2(NO) Complexes [M = V, Nb, Ta; trimpsi = tBuSi(CH2PMe2)3]

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posted on 26.06.2002, 00:00 by Trevor W. Hayton, P. James Daff, Peter Legzdins, Steven J. Rettig, Brian O. Patrick
Treatment of [Et4N][M(CO)6] (M = Nb, Ta) with I2 in DME at −78 °C produces solutions of the bimetallic anions [M2(μ-I)3(CO)8]-. Addition of the tripodal phosphine tBuSi(CH2PMe2)3 (trimpsi) followed by refluxing affords (trimpsi)M(CO)3I [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 (η2-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)3H, and [(trimpsi)V(μ-Cl)3V(trimpsi)][(η2-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, 35, and [8][9]·3THF have been established by X-ray diffraction analyses. The solution redox properties of 35 have also been investigated by cyclic voltammetry. Cyclic voltammograms of 3 and 4 both exhibit an irreversible oxidation feature in CH2Cl2 (Ep,a = −0.71 V at 0.5 V/s for 3, while Ep,a = −0.55 V at 0.5 V/s for 4), while cyclic voltammograms of 5 in CH2Cl2 show a reversible oxidation feature (E1/2 = −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.