Regioselective phosphine attack on the coordinated alkyne in Co2( μ-alkyne) complexes reactivity studies and X-ray diffraction structures of Co2(CO)4(bma)(μ-HC≡CtBu) and the zwitterionic hydrocarbyl complexesn

Article abstract of DOI:10.1016/0022-328X(95)06090-J

The alkyne-bridged compounds Co₂(CO)₆( μ-R′C≡CR) (where R′ = H, R = Ph, ¹Bu; R′ = Me, R = Ph) have been examined for their reactivity with the redox-active diphosphine ligand 2,3-bis(diphenylphosphino)maleic anhydride (bma). Thermal and Me₃NO-induced activation of each Co₂(CO)₆( μ-R′C≡CR) compound initially furnishes the binuclear compounds Co₂(CO)₄(bma)( μ-R′C≡CR) (1c, R′ = H, R = Ph; 2c, R′ = H, R = ¹Bu; 3c, R′ = Me, R = Ph), all of which are shown to possess a chelating bma ligand. These compounds exhibit a reversible chelate-to-bridge bma ligand isomerization that proceeds by a pathway involving dissociative CO loss. The stability of the chelating isomers is dependent on the steric bulk of the alkyne substituents, with the bridging isomer (1b, 2b, 3b) being favored at ambient temperature only in the case of Co₂(CO)₄(bma)( μ-HC≡C¹Bu) (2b). Regioselective phosphine attack at the least-substituted alkyne carbon is observed at temperatures above 60 °C to afford the corresponding zwitterionic hydrocarbyl compounds Co₂(CO)₄[ μ-η²:η²:η¹:η¹-RC=C(R′)PPh₂C=C(PPh₂)C(O)OC(O)] (1z, 2z, 3z). Depending on the ease of separation, the new compounds have been characterized by IR and NMR spectroscopy, and the molecular structure of the bridged compound 2b and all three zwitterionic hydrocarbyl compounds have been determined by X-ray crystallography, 1z crystallizes in the triclinic space group P1: a = 9.8093(7) A, b = 12.215(1) A, c = 15.744(2) A, α = 98.251(8)°, β = 95.060(8)°, γ = 90.640(7)°, V = 1859.1(3) A³, Z = 2, d_calc = 1.426 g cm ^-3; R = 0.0365, R_w = 0.0469 for 2749 observed reflections. 2b crystallizes in the triclinic space group P1: a = 11.1272(7) A, b = 12.142(1) A, c = 13.565(1) A, α = 97.123(7)°, β = 94.951(6)°, γ = 100.197(6)°, V = 1778.8(2) A³, Z = 2, d_calc = 1.453 g cm^-3; R = 0.0469, R_w = 0.0484 for 3357 observed reflections. 2z crystallizes in the monoclinic space group P2₁/n: a = 9.465(7) A, b = 20.071(8) A, c = 18.389(6) A, β = 98.88(5)°, V = 3452(3) A³, Z = 4, d_calc = 1.498 g cm^-3; R = 0.0544, R_w = 0.0589 for 3114 observed reflections. 3z, as the CH₂Cl₂ solvate, crystallizes in the triclinic space group P1: a = 9.732(3) A, b = 12.286(2) A, c = 16.715(3) A, α = 98.39(1)°, β = 102.91(2)°, γ = 91.93(2)°, V = 1922.6(8) A³, Z = 2, d_calc = 1.550 g cm^-3; R = 0.0730, R_w = 0.0825 for 2896 observed reflections. The X-ray data on 1z, 2z, and 3z confirm the site of phosphine attack on the coordinated alkyne ligand and the presence of the eight-electron hydrocarbyl ligand that accompanies this reaction. The redox properties of the zwitterionic compounds Co₂(CO)₄[ μ-η²:η²:η¹:η¹-RC=C(R′)PPh₂C=C(PPh₂)C(O)OC(O)] have been examined by cyclic voltammetry, and the data discussed relative to the coordination mode adopted by the bma ligand and the redox properties exhibited by the related alkyne-bridged compound Co₂(CO)₄(bma)( μ-PhC≡CPh) (chelating and bridging isomers). Extended Hueckel calculations on the model compound Co₂(CO)₄[ μ-η²:η²:η¹:η¹-HC=C(H)PH₂C=C(PH₂)C(O)OC(O)] have been carried out and are used in a discussion concerning the site of electron reduction and oxidation in 1z, 2z, and 3z.