Special effects of ortho-isopropylphenyl groups.Diastereoisomerism in platinum(II) and palladium(II) complexes of helically chiral PAr_3 ligands

摘要

The coordination chemistry of the four phosphines,P{C_6H_3(o-CH_3)(p-Z)}_3 where Z = H (la) or OMe (1b) and P{C_6H_3(o-CHMe_2)(p-Z)}_3 Z = H (1c) or OMe (1d) with platinum(II) and palladium(II) is reported.Mononuclear complexes trans-[PdCl_2L_2] (L = la,b) and trans-[PtCl_2L_2] (L = la-c) have been prepared and the crystal structures of trans-[PdCl_2(1b)_2] and trans-[PtCl_2(lc)_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-[Pd_2Cl_4L_2] (L = lc,d) are formed even when an excess of the bulky lc,d is used in the synthesis and the crystal structure of ;trans-[Pd_2Cl_4(lc)_2] as its chloroform solvate is reported.Reaction of [PtCl_2(NCBu~t)_2] with 1b-d in refluxing toluene gave the cycloplatinated species [Pt_2Cl_2(L - H)_2] where L - H is phosphine 1b-d deprotonated at one of the ortho-methyl carbon atoms.Variable temperature ~(31)P and ~1H 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 lc,d,rotation about the P-C bond is slow on the NMR timescale even up to 75 deg C.The crystal structure of the cyclometallated complex [Pt_2Cl_2(ld - H)_2] has been determined.