Stepwise Expansion of Pd Chains from Binuclear Palladium(I) Complexes Supported by Tetraphosphine Ligands

Reaction of [Pd2(XylNC)6]­X2 (X = PF6, BF4) with a linear tetraphosphine, meso-bis­[(diphenylphosphinomethyl)­phenylphosphino]­methane (dpmppm), afforded binuclear PdI complexes, [Pd2(μ-dpmppm)2]­X2 ([2]­X2), through an asymmetric dipalladium complex, [Pd2(μ-dpmppm)­(XylNC)3]2+ ([1]2+). Complex [2]2+ readily reacted with [Pd0(dba)2] (2 equiv) and an excess of isocyanide, RNC (R = 2,6-xylyl (Xyl), tert-butyl (tBu)), to generate an equilibrium mixture of [Pd4(μ-dpmppm)2(RNC)2]2+ ([3′]2+) + RNC ⇄ [Pd4(μ-dpmppm)2(RNC)3]2+ ([3]2+), from which [Pd4(μ-dpmppm)2(XylNC)3]2+ ([3a]2+) and [Pd4(μ-dpmppm)2(tBuNC)2]2+ ([3b′]2+) were isolated. Variable-temperature UV–vis and 31P­{1H} and 1H NMR spectroscopic studies on the equilibrium mixtures demonstrated that the tetrapalladium complexes are quite fluxional in the solution state: the symmetric Pd4 complex [3b′]2+ predominantly existed at higher temperatures (>0 °C), and the equilibrium shifted to the asymmetric Pd4 complex [3b]2+ at a low temperature (∼−30 °C). The binding constants were determined by UV–vis titration at 20 °C and revealed that XylNC is of higher affinity to the Pd4 core than tBuNC. In addition, both isocyanides exhibited higher affinity to the electron deficient [Pd4(μ-dpmppmF2)2(RNC)2]2+ ([3F′]2+) than to [Pd4(μ-dpmppm)2(RNC)2]2+ ([3′]2+) (dpmppmF2 = meso-bis­[{di­(3,5-difluorophenyl)­phosphinomethyl}­phenylphosphino]­methane). When [2]­X2 was treated with [Pd0(dba)2] (2 equiv) in the absence of RNC in acetonitrile, linearly ordered octapalladium chains, [Pd8(μ-dpmppm)4(CH3CN)2]­X4 ([4]­X4: X = PF6, BF4), were generated through a coupling of two {Pd4(μ-dpmppm)2}2+ fragments. Complex [2]2+ was also proven to be a good precursor for Pd2M2 mixed-metal complexes, yielding [Pd2Cl­(Cp*MCl) (Cp*MCl2)­(μ-dpmppm)2]2+ (M = Rh ([5]2+), Ir ([6]2+), and [Au2Pd2Cl2(dpmppm–H)2]2+ ([7]2+) by treatment with [Cp*MCl2]2 and [AuCl­(PPh3)], respectively. Complex [7]2+ contains an unprecedented PC­(sp3)P pincer ligand with a PCPCPCP backbone, dpmppm–H of deprotonated dpmppm. The present results demonstrated that the binuclear PdI complex [2]2+ was a quite useful starting material to extend the palladium chains and to construct Pd-involved heteromultinuclear systems.