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

Abstract

The reaction of Os(RL)(PPh₃)₂(CO)Br (<b>3</b>) with CNBu^t in dichloromethane solution has furnished the cationic complex [Os(RL)(PPh₃)₂(CO)( CNBu^t)]⁺ isolated as PF₆-salt, <b>4</b> which behaves as a 1:1 electrolyte in acetone solution (RL is C₆H₂O-2-CHNHC₆H₄R(<i>p</i>)-3-Me-5 and R is Me, OMe and Cl). The presence of significant Os-CNBu^t back-bonding in <b>4</b> is reflected in the blue shift of both the MLCT transition maximum (~500 nm) and C≡O 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 <b>4 </b>is ~100 mV higher than those in <b>3</b>. The complex <b>4. </b><span style="mso-bidi-font-weight:bold">2CH₂Cl₂ (<span style="mso-bidi-font-weight:bold">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 <b>3</b>. Due to the <i>trans </i>influence of the CNBu^t ligand, the Os-C(aryl) bond is however lengthened by ~0.15Å 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).

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