The synthesis and characterization of multiply bonded dirhenium(III) alkoxide complexes containing two or three alkoxide ligands, including further studies on complexes of the type Re2Cl4(OMe)2(PAr3)2 (Ar=aryl)

The dirhenium(III) complex cis-Re2(μ-O2CCH3)2Cl4(H2O)2 reacts in refluxing methanol with triarylphosphines (PAr3) with relatively low basicities (pKa values in the range 1.0–4.6) and moderately large cone angles (145–165°) to afford unsymmetrical quadruply bonded dirhenium(III) alkoxides of the type Cl2(MeO)2ReReCl2(PAr3)2 (PAr3=P(p-tolyl)3 (1), P(m-tolyl)3 (2), P(p-ClPh)3 (3), P(p-MeOPh)3 (4), PPh3). The more basic phosphines PCyPh2 and PBz3 (Bz=benzyl), with pKa values of 5.05 and 6.50 and cone angles of 153 and 165°, respectively, afford the complexes Re2(μ-O2CCH3)Cl3(OMe)(PCyPh2)3 (5), with a Re–Re bond order of 4, and Re2(μ-O2CCH3)Cl4(PBz3)2 (6), with a Re–Re bond order of 3.5, under the same conditions. Attempts to incorporate more than two alkoxide ligands about a multiply bonded dirhenium core were successfully achieved in the synthesis of Re2Cl3(OEt)3(PPh3)2 (7) from the reactions of Re2Cl6(PPh3)2 and Re2Cl4(OEt)2(PPh3)2 with NaOEt in ethanol. Single-crystal X-ray structure determinations have been carried out on compounds 4, 5 and 7 which have Re–Re bond distances of 2.2476(4), 2.2872(15) and 2.2399(12) Å, respectively.