Chemistry of a family of osmium(II) metallacycles incorporating isonitrile coordination

Abstract

The reaction of Os(RL)(PPh₃)₂(CO)Br (3) with CNBu^t in dichloromethane solution has furnished the cationic complex [Os(RL)(PPh₃)₂(CO)(CNBu^t)]⁺ isolated as PF₆-salt, 4 which behaves as a 1:1 electrolyte in acetone solution (RL is C₆H₂O-2-CHNHC₆H₄R(p)-3-Me-5 and R is Me, OMe and Cl). The presence of significant Os-CNBu^t back-bonding in 4 is reflected in the blue shift of both the MLCT transition maximum (~500 nm) and CºO stretching frequency (~1930 cm^-1) compared to those in 3 (~550 nm and ~1870 cm^-1, respectively). For the same reason, the Os^III/Os^II reduction potential (~0.8 V vs. SCE) in 4 is ~100 mV higher than those in 3. The complex 4·2CH₂Cl₂ (R = Me) represents the first example of a structurally characterized cyclometallated osmium(II) isonitrile system. In the distorted octahedral OsC₃OP₂ coordination sphere of 4·2CH₂Cl₂ (R = Me), the Os-P, Os-CO and Os-O bond lengths are closely similar to those in a analogue of 3. Due to the trans influence of the CNBut ligand, the Os-C(aryl) bond is however lengthened by ~0.15 Å in 4·2CH₂Cl₂ (R = Me). The type 3 complexes stand in strong contrast to the corresponding ruthenium system, which undergoes halide substitution as well as insertion into the Ru-O bond upon treatment with CNBu^t. This happens because the Os-O bond (2.166(6) Å) is stronger than the Ru-O bond (2.235(4) Å).