Unusually stable four-membered chelate rings in nickel(II), palladium(II), and platinum(II) complexes with the ligand 2,2-bis(diphenylphosphino)propane (2,2-dppp).: Crystal and molecular structure of [PdI2(2,2-dppp)]•0.8CH2Cl2

The ligand Ph2PC(CH3)(2)PPh2 (2,2-dppp) reacts with Ni(II) salts to give robust square-planar chelate complexes [NiX2(2,2-dppp-P,P')] (X = Cl, 1; X = Br, 2) and the salt [Ni(2,2-dppp)(2)](ClO4)(2) (3). The Pd(II) analogues [PdCl2(2,2-dppp)] (4), [PdI2(2,2-dppp)] (5), and [Pd(2,2-dppp)(2)](BF4)(2) (6) have also been made. The crystal structure of 5 (obtained as 5.0.8CH(2)Cl(2)) has been determined by X-ray diffraction. The geminal C(CH3)(2) group, when compared with related Ph2PCH2PPh2 (dppm) complexes, causes a small compression of the P-C-P chelate ring angle (to 91.3(3)degrees for 5). With [PtCl2(PhCN)(2)], 2,2-dppp affords [PtCl2-(2,2-dppp)] (7), which undergoes metathesis with NaI to give [PtI2(2,2-dppp)] (8), and [Pt(2,2-dppp)(2)]Cl-2 (9) was obtained by reaction of [PtCl2(PhCN)(2)] with 2 equiv of 2,2-dppp. Reactions were then attempted with 7 and 9, which are known to result in ring-opening reactions with the corresponding complexes of dppm, but in all cases, only 2,2-dppp chelate complexes could be isolated. Thus, treatment of 7 or 9 with excess MeLi gave exclusively [PtMe2(2,2-dppp)] (10) rather than cis,cis-[Pt2Me4(2,2-dppp)(2)]. Attempts to ring-open 10 with excess 2,2-dppp to give cis-[PtMe2(2,2-dppp-P)(2)] were unsuccessful. Treatment of 10 with 1 equiv of HCl or, better, treatment of [PtCl(Me)(1,5-cyclooctadiene)] with 2,2-dppp gave only mononuclear [PtCl(Me)(2,2-dppp)] (11) and no dimer of the type [MePt(mu-2,2-dppp)(2)(mu-Cl)PtMe]Cl. Treatment of 7 with 2 equiv of NaCN in EtOH gave the chelate complex [Pt(CN)2(2,2-dppp)] (12) rather than trans, trans-[Pt-2(CN)(4)(mu-2,2-dppp)(2)], and treatment of 7 with LiC=CPh in thf or (better) H2NNH2 . H2O/HC=CPh in EtOH gave [Pt(C=CPh)(2)(2,2-dppp)] (13), rather than trans,trans-[Pt-2(C=CPh)(4)(mu-2,2-dppp)(2)]. Reaction of 7 with LiC=CBun in thf likewise gave [Pt(C=CBun)(2)(2,2-dppp)] (14) in low yield. The complexes were characterized by microanalyses (C, H, N and, in some cases, halide), infrared spectroscopy, (31)p{H-1} and H-1 NMR spectroscopy, and fast atom bombardment mass spectrometry.