Axially asymmetric metal alkyls. Part 3. Chemical, electrochemical, and structural studies of Group 5A d0,1,2 metallepines [M{(2-CH2C6H4)2}(η-C5H5)2]z(M = Nb or Ta; z=1–, 0, or 1+); synthesis of [Nb{2-CH2(C6H4)2Me-2′}X(η-C5H5)2](X = Cl or CO) and [{(η-C5H5)2ClV}2{(2-CH2C6H4)2}], and electrochemistry of [M(CH2Ph)2 –nCln(η-C5H5)2](n= 0, M = Nb, or Ta; n= 1, M = V)

Abstract

Reaction of [MCl₂(η-C₅H₅)₂] with the di-Grignard reagent {o-[(thf)_nClMgCH₂]C₆H₄}₂ in tetrahydrofuran (thf) yields either the bimetallic complex [{(η-C₅H₅)₂ClV}₂{(2-CH₂C₆H₄)₂}](2) or the thermally stable metallepines, [M{(2-CH₂C₆H₄)₂}(η-C₅H₅)₂][M = Nb (3a) or Ta (3b)]. E.s.r. data on these and the compounds [Nb{2-CH₂(C₆H₄)₂Me-2′}Cl(η-C₅H₅)₂](7) and [M(CH₂Ph)_{2 –n}Cl_n(η-C₅H₅)₂][n= 1, M = V (8); n= 0, M = Nb (9a) or Ta (9b)], prepared by similar methods, are consistent with each being a d¹ complex. Electrochemical reduction and oxidation of (3a) is reversible; the cationic d⁰ and anionic d² species have been generated chemically (using AgBF₄ and Na[C₁₀H₈]) in thf and isolated as salts of [BF₄]^–, (4a), and [Na(18-crown-6)(thf)₂]⁺, (5), respectively (18-crown-6 = 1,4,7,10,13,16-hexaoxacyclo-octadecane). Electrochemical studies on the other d¹ compounds are also reported. Reductive carbonylation of (7) or treating (5) with CF₃CH₂OH under CO yields [Nb{2-CH₂(C₆H₄)₂Me-2′}(CO)(η-C₅H₅)₂](6). X-Ray structures of the d^0,1,2 niobepines (4a), (3a), and (5) show a large change in CH₂–Nb–CH₂ angle [106.3(1), 83.₀, 80(1)°] in accordance with molecular orbital predictions, and in the Nb–CH₂–C angles [102.₉, 118.₇, and 123(2)°] and the torsion along the biphenyl axis (78.₄, 62.₄, and 59.₆°).