Neutral trans-Dioxorhenium(V) Complexes with the Anionic Tetrakis(pyrazolyl)borate Ligand

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Abstract
Reduction of [ReO3{η3-B(pz)4}] (1) with PPh3 in the presence of mono- or bidentate σ-donor ligands (pyridines, imidazoles, or diphosphines) is a very convenient method for the synthesis of the neutral dioxorhenium(V) complexes: trans-[ReO2{η2-B(pz)4}(L)2] (L = py (3), 4-Mepy (4), 4-NMe2py (5), 1-MeImz (6)) and trans-[ReO2{η2-B(pz)4}(P⌒P)] (P⌒P = dmpe (7), dppe (8)). In the presence of pyridine or dimethylphosphinoethane, the analogous [ReO3{η3-HB(pz)3}] is also reduced by PPh3 to trans-[ReO2{η2-HB(pz)3}(py)2] (9) and trans-[ReO2{η2-HB(pz)3}(dmpe)] (10), respectively. In contrast, the reaction of [ReO2(py)4]Cl with K[B(pz)4] leads to a mixture of species from which were identified the neutral mono-oxo complexes [ReO(η2-N,O)(μ-O)B(pz)3}(pz)(pzH)2] (11) and [ReO{(η2-N,O)(μ-O)B(pz)3}Cl(py)2] (12). Complexes 3−12 were characterized by different techniques, namely, IR, 1H/31P{1H} NMR spectroscopies and X-ray crystallographic analysis (5, 10, and 11). Compound 5 crystallizes from dichloromethane/n-hexane as orange crystals containing 3 molecules of solvated CH2Cl2 (crystal data: triclinic space group P1̄, with cell parameters a = 10.907(2) Å, b = 11.113(1) Å, c = 16.922(2) Å, α = 97.91(1)°, β = 102.37(1)°, γ = 94.21(1)°, V = 1973(1) Å3, Z = 2). Compound 10 crystallizes from dichloromethane/n-hexane as yellowish crystals containing one molecule of pzH (crystal data: orthorhombic space group Pnma, with cell parameters a = 18.422(2) Å, b = 11.850(1) Å, c = 11.434(1) Å, α = β = γ = 90°, V = 2496.1(4) Å3, Z = 4). Compound 11 crystallizes from dichloromethane/n-hexane in the monoclinic space group P21/n, with cell parameters a = 10.890(1) Å, b = 15.162(1) Å, c = 14.137(2) Å, β = 102.07(1)°, V = 2282.6(4) Å3, Z = 4.