Oxo Ligand Reactivity in the [ReO₂L₄]⁺ Complex of 1-Methylimidazole. Preparation and Crystal Structures of Salts Containing the ReOL₄³⁺ Core and Apical CH₃O^-, BF₃O^2-, and (CH₃O)₂PO₂^- Groups

The oxo group of [ReO2(1-MeIm)4]+ can be methylated with excess methyl trifluoromethanesulfonate in CH2Cl2 under mild conditions, leading to [ReO(OCH3)(1-MeIm)4](CF3SO3)2, which is converted to the B(C6H5)4- and PF6- salts. When the reaction is carried out in methanol in the presence of excess PF6-, the phosphate ester complex [ReO{OP(O)(OCH3)2}(1-MeIm)4](PF6)2 is isolated. [ReO2(1-MeIm)4]+ in the presence of acid, 2,2-dimethoxypropane, and excess BF4- transforms to the oxo−boron adduct [ReO(OBF3)(1-MeIm)4]+. The presence of the trans-ORe−OR core was demonstrated by crystallographic studies on one compound of each type: [ReO(OCH3)(1-MeIm)4](PF6)2, tetragonal, P4/ncc, Z = 4, a = 13.022(3) Å, c = 17.218(5) Å, R = 0.0271; [ReO{OP(O)(OCH3)2}(1-MeIm)4](PF6)2·toluene, monoclinic, P21/c, Z = 4, a = 14.165(4) Å, b = 13.052(6) Å, c = 20.931(6) Å, β = 95.83(2)°, R = 0.0373; [ReO(OBF3)(1-MeIm)4](I3), monoclinic, P21/c, Z = 4, a = 13.373(4) Å, b = 13.237(3) Å, c = 16.689(6) Å, β = 103.67(3)°, R = 0.0445. The IR spectra show ν(ReO) vibrations near 960 cm-1 in all cases. The solution UV−vis spectra of the CH3O- and BF3O2- compounds are similar to that of the parent oxo−hydroxo [ReO(OH)(1-MeIm)4]2+ species, whereas the visible spectrum of the blue phosphate ester compound resembles that of the oxo−aquo [ReO(OH2)(1-MeIm)4]3+ ion. The compounds were also characterized by solution 1H, 13C, and 31P NMR spectroscopy. This work confirms earlier conclusions that ReO2L4+ units with good σ-donor imidazole ligands are more resistant to ligand loss than the pyridine analogues.