Special effects of ortho-isopropylphenyl groups.: Diastereoisomerism in platinum(II) and palladium(II) complexes of helically chiral PAr3 ligands

The coordination chemistry of the four phosphines, P {C6H3(o-CH3) (p-Z)}(3) where Z = H( 1a) or OMe (1b) and P {C6H3(o-CHMe2) (p-Z)}(3) Z = H(1c) or OMe (1d) with platinum( II) and palladium( II) is reported. Mononuclear complexes trans - [PdCl2L2] (L = 1a, b) and trans - [PtCl2L2] (L = 1a-c) have been prepared and the crystal structures of trans - [PdCl2(1b)(2)] and trans - [PtCl2(1c)(2)] as their dichloromethane solvates have been determined. The structures show that in these complexes, the ligands adopt g(+)g(+) a conformations. Examination of the Cambridge Structural Database confirms this to be one of only two conformer types that tri-o-tolylphosphines adopt and the only viable conformer in 4 and 6 coordinate complexes. The binuclear complexes trans - [Pd2Cl4L2] (L = 1c, d) are formed even when an excess of the bulky 1c, d is used in the synthesis and the crystal structure of trans - [Pd2Cl4(1c)(2)] as its chloroform solvate is reported. Reaction of [PtCl2(NCBut)(2)] with 1b-d in refluxing toluene gave the cycloplatinated species [Pt2Cl2(L-H)(2)] where L-H is phosphine 1b-d deprotonated at one of the ortho - methyl carbon atoms. Variable temperature P-31 and H-1 NMR spectroscopy reveals that all the complexes reported are fluxional. The processes are analysed in terms of restricted P-C and P-M rotations that give rise to diastereoisomeric rotamers because of the helically chiral orientations of the aryl substituents. For the complexes of the bulky ligands 1c, d, rotation about the P-C bond is slow on the NMR timescale even up to 75degreesC. The crystal structure of the cyclometallated complex [Pt2Cl2(1d-H)(2)] has been determined.