DIOXYGEN AND CARBON-MONOXIDE UPTAKE BY IRIDIUM(I) COMPLEXES STABILIZED BY MIXED N,P-DONOR LIGANDS

The preparation of a new family of iridium(I) complexes with the mixed -donor bidentate ligands o-Ph2PC6H4CH=NR (PNalkyl) (R = Et, Pr(i), Bu( t)) is described. In these complexes, the iridium center is coordinate d by two PNalkyl ligands in a square-planar environment. All[(PNalkyl) 2Ir]+ compounds undergo reversible electron transfer to the Ir(0) oxid ation state as well as irreversible oxidation to Ir(III). Only for R = Bu(t) is the lr(II) oxidation state accessible through a reversible o xidation process. For comparative purposes, the electrochemical behavi or of the Rh(I) congeners [(PNalkyl)2Rh]PF6 has been investigated. Ill ustrative examples of the paramagnetic Ir(0),Rh(0), Ir(II), and Rh(II) derivatives have been studied by ESR spectroscopy, which confirms tha t the unpaired electron is mainly localized on the metal center. The I r(I) complexes [(PNalkyl)2Ir]Y (Y = BPh4-, PF6-, Cl-; alkyl = Et, Pr(i )) react with O2, inorganic solvent solutions at room temperature, to form peroxo complexes of formula [(PNalkyl)2Ir(O2)]Y. The molecular st ructure of the PNEt derivative [(PNEt)2(O2)]PF6 has been determined by X-ray diffraction methods: monoclinic P2(1)/a, a = 17.426(4) angstrom , b = 14.685(3) angstrom, c = 16.131(4) angstrom, alpha = 90.00(0)degr ees, beta = 98.07(2)degrees, gamma = 90.00(0)degrees, Z = 4, R = 0.053 . The O-O bond distance is 1.47(1) angstrom. The coordination of the m etal may alternatively be described as distorted trigonal bipyramidal or distorted octahedral, according to whether the dioxygen molecule is treated as occupying one or two equatorial sites, respectively. The s teric hindrance of the alkyl substituent on the imino donors appears a s a limiting factor for the dioxygen uptake, as shown by the fact that [(PNBu(t))2Ir]+ does not react with O2. Unlike the rhodium congeners, the Ir(I) complexes do not behave as oxygen carriers, the O2 molecule being irreversibly coordinated to the metal center even at high tempe rature. In contrast, reversible square-pyramidal adducts form upon rea ction of the PNEt and PNPr(i) Ir(I) complexes with CO. No reaction occ urs with the PNBu(t) derivative, confirming that the size of the imino substituent controls the access of incoming reagents at the metal cen ter.