IrPd, IrHg, IrCu, and IrTl Binuclear Complexes Bridged by the Short-Bite Ligand 2-(Diphenylphosphino)pyridine. Catalytic Effect in the Hydroformylation of Styrene Due to the Monodentate P-Bonded 2-(Diphenylphosphino)pyridine Ligands oftrans-[Ir(CO)(Ph2PPy)2Cl]

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The complex trans -[Ir(CO)Cl(Ph 2 PPy) 2 ] ( 1; Ph 2 PPy = 2-(diphenylphosphino)pyridine) was obtained in good yield from the reaction of [Ir( p -toluidine)(CO) 2 Cl] with Ph 2 PPy. It has a square-planar geometry, as does the analogous Vaska complex, with two monodentate Ph 2 PPy ligands P-bonded to the iridium(I) center. Compound 1 exhibits a reactivity reminiscent of Vaska's complex; in fact, it adds SO 2, halogens, HCl, and CH 3 I. The addition of HCl to 1 initially occurs at the pyridine nitrogen atoms of the pendant Ph 2 PPy and subsequently at the iridium(I) center, affording [Ir(CO)H(Cl) 2 (Ph 2 PPyH) 2 ][Cl] 2 ( 6 ). The addition of a benzene solution of [Pd(PhCN) 2 Cl 2 ] to a solution of 1 in the same solvent yielded, nearly quantitatively, the Ir II Pd I complex [IrPd(CO)Cl 3 (μ-Ph 2 PPy) 2 ] ( 8 ). The reaction of 1 with HgCl 2, in benzene, in a 1:1 molar ratio afforded the binuclear complex [Ir(CO)Cl 2 (μ-Ph 2 PPy) 2 HgCl] ( 9 ). Using a 1:2 molar ratio the product [Ir(CO)Cl 2 (HgCl 2 )(μ-Ph 2 PPy) 2 HgCl]·2CH 2 Cl 2 ( 10 ) was obtained. Compounds 8 and 10 were also characterized by a single-crystal X-ray diffraction analysis. The reaction of 1 with [Cu(NCCH 3 ) 4 ]BF 4 afforded, almost quantitatively, the ionic compound [Ir(CO)Cl(μ-Ph 2 PPy) 2 Cu]BF 4 ( 11 ) as an orange solid. The compound [Ir(CO)Cl(μ-Ph 2 PPy) 2 Tl]PF 6 ( 12 ) obtained from the reaction of 1 with TlPF 6, shows the Ir−Tl bonding and is luminescent in frozen CH 2 Cl 2 solution at 77 K with a lifetime of 5 ns (±10%). As opposed to analogous compounds, in the bimetallic complexes 8 − 12 the bridging Ph 2 PPy ligands assume a head-to-head structure. The aim of the work was to obtain some insight into the beneficial effect of catalytic precursors containing the pendant Ph 2 PPy ligand in the hydroformylation catalytic cycle. Using complex 1 as a precatalyst in the hydroformylation of styrene, at 80 °C and under 80 atm of CO/H 2 (1:1) pressure, the rate of the catalytic process increases significantly with respect to that observed with Vaska's complex, making clear a favorable effect of the pendant Ph 2 PPy ligand. However, the chemoselectivity of the process was low. A scheme of the catalytic cycle is proposed. The new aspects of the catalytic cycle are: (i) the protonation under equilibrium conditions of one of the two pyridine nitrogen atoms of the pendant Ph 2 PPy ligands in the reductive elimination of HCl from the dihydride−iridium(III) species formed by oxidative addition of H 2 to 1 and (ii) protonolysis, by the proton coordinated to the pyridine nitrogen atom, of the acyl or σ-alkyl groups present in the intermediates formed in the reaction pathway. Unexpectedly, 11 and 12 exhibit catalytic activity comparable with that of 1 . This occurs because under the experimental conditions used the Cu−N or Tl−N bonds are broken, giving 1 .