Synthesis and Characterization of the Bis-Cyclometalating Ligand 3,3‘,5,5‘-Tetrakis[(dimethylamino)methyl]biphenyl and Its Use in the Preparation of Bimetallic M(II), M(IV) (M = Pt, Pd), and Mixed-Valence Pt(II)−Pt(IV) Complexesviaa Dilithio-Derivative. Crystal Structure of the Pd Dimer [ClPd{2,6-(Me2NCH2)2C6H2}]2

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The potentially bis-cyclometalating ligand 3,3‘,5,5‘-tetrakis[(dimethylamino)methyl]biphenyl, was prepared from 3,5-(Me2NCH2)2C6H3Br in a one-pot synthesis involving lithium−halogen exchange, transmetalation, and reductive-elimination reactions. This ligand undergoes selective dilithiation in the 4 and 4‘ positions of the biphenyl group to form [Li2{2,6-(Me2NCH2)2C6H2}2]n, which is a versatile precursor to bimetallic complexes via a double transmetalation reaction. The syntheses and characterization of the new homobinuclear Pt(II) and Pd(II) compounds, [(MeCN)Pt{2,6-(Me2NCH2)2C6H2}−]2(CF3SO3)2 and [ClPd{2,6-(Me2NCH2)2C6H2}−]2, is reported in addition to a novel synthetic route to the complex [ClPt{2,6-(Me2NCH2)2C6H2}−]2. The solid-state structure of Pd dimer was determined by single-crystal X-ray diffraction. Oxidative addition of Cl2 to the latter Pt compound produced the bis-platinum(IV) complex [Cl3Pt{2,6-(Me2NCH2)2C6H2}−]2, which reacts with 2 equiv of Ag(O3SCF3) in MeCN solution to form [(MeCN)Cl2Pt{2,6-(Me2NCH2)2C6H2}−]2(CF3SO3)2. This complex could also be obtained directly by reaction of the former PtII dimer with 2 equiv of Cl2IPh. 1H NMR spectroscopic studies show that mixtures of the PtII and PtIV dimers in acetonitrile solution are in equilibrium with a mixed-valence (PtII,PtIV) species [(MeCN)PtII{2,6-(Me2NCH2)2C6H2−C6H2(CH2NMe2)-2,6}PtIVCl2(NCMe)](CF3SO3)2, which, however, could not be isolated in pure form. The reaction of the PtII dimer with ≤1 equiv of Cl2IPh also gave a PtIV dimer, along with unreacted starting material and the mixed-valence complex. A reversible double C−C bond formation has been achieved by reaction of the PtII dimer with excess of methyl iodide. This procedure yields the bis(arenonium) species [IPtII{2,6-(Me2NCH2)2-1-MeC6H2}−]2(CF3SO3)2 in which both the Me groups derived from I−Me bond cleavage are bound to the Cipso of the aryl rings. The mono(arenonium) compound [IPtII{2,6-(Me2NCH2)2-1-MeC6H2-C6H2(CH2NMe2)2-2‘,6‘}PtII](CF3SO3)2 has been identified as an intermediate in this reaction by 1H NMR spectroscopy.