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Keywords:

  • Iridium;
  • Phosphanes;
  • Carbonyl ligands;
  • Oxygen;
  • Oxidative addition;
  • ­Vaska's complex

Abstract

The in-situ-generated dimeric precursor [Ir(CO)2Cl]2 reacts with four molar equivalents of the ligands 2-Ph2PC6H4COOMe (a) and 2-Ph2PC6H4OMe (b) to afford tetracoordinated complexes of the type trans-[Ir(CO)ClL2] (1a, 1b), where L = a and b. The IR spectra of 1a and 1b in CHCl3 solution show the terminal υ(CO) bands at around 1957 and 1959 cm–1, respectively, which are significantly lower in frequency compared to Vaska's complex, trans-[Ir(CO)Cl(PPh3)2] (1965 cm–1) and substantiate the enhanced electron density at the metal centre. The single-crystal X-ray structure of 1a indicates iridium–oxygen (ester group) distances [Ir···O(2) 3.24 Å, Ir···O(5) 3.29 Å] and angle [O(5)···Ir···O(2) 157.25°] suggesting a long-range intramolecular “secondary” Ir···O interaction resulting in a pseudo-hexacoordinated complex. Complex 1b reacts with O2 to generate [Ir(O2)(CO)Cl(2-Ph2PC6H4OMe)2] (2b), while 1a remains unreactive. Complex 2b shows a distorted octahedral structure with peroxo O–O linkage (O2–O3 1.47 Å). The kinetic study of the reaction of 1b and Vaska's complex towards dioxygen addition reveals that the rate of dioxygen addition to 1b is about three times faster than Vaska's complex. Complexes 1a and 1b undergo oxidative addition with small molecules like CH3I and I2 to produce IrIII carbonyl species of the type [Ir(CO)Cl(CH3)IL2] (3a, 3b) and [Ir(CO)ClI2L2] (4a, 4b), where L = a, b.