Dinuclear palladium complexes with two ligand-centered radicals and a single bridging ligand: Subtle tuning of magnetic properties

The facile and tunable preparation of unique dinuclear [(L)Pd-X-Pd(L)] complexes (X=Cl or N₃), bearing a ligand radical on each Pd, is disclosed, as well as their magnetochemistry in solution and solid state is reported. Chloride abstraction from [PdCl(NNO^ISQ)] (NNO^ISQ=iminosemiquinonato) with TlPF₆ results in an unusual monochlorido-bridged dinuclear open-shell diradical species, [{Pd(NNO^ISQ)}₂(μ-Cl)]⁺, with an unusually small Pd-Cl-Pd angle (ca. 93°, determined by X-ray). This suggests an intramolecular d⁸-d⁸ interaction, which is supported by DFT calculations. SQUID measurements indicate moderate antiferromagnetic spin exchange between the two ligand radicals and an overall singlet ground state in the solid state. VT EPR spectroscopy shows a transient signal corresponding to a triplet state between 20 and 60 K. Complex 2 reacts with PPh₃ to generate [Pd(NNO^ISQ)(PPh₃)]⁺ and one equivalent of [PdCl(NNO^ISQ)]. Reacting an 1:1 mixture of [PdCl(NNO^ISQ)] and [Pd(N₃)(NNO^{I SQ})] furnishes the 1,1-azido-bridged dinuclear diradical [{Pd(NNO^ISQ)}₂(κ¹-N;μ-N₃]⁺, with a Pd-N-Pd angle close to 127° (X-ray). Magnetic and EPR measurements indicate two independent S=1/2 spin carriers and no magnetic interaction in the solid state. The two diradical species both show no spin exchange in solution, likely because of unhindered rotation around the Pd-X-Pd core. This work demonstrates that a single bridging atom can induce subtle and tunable changes in structural and magnetic properties of novel dinuclear Pd complexes featuring two ligand-based radicals. Double up! The synthesis, X-ray structural, spectroscopic, and magnetic properties of unusual mono(pseudo)-halide bridged open-shell dipalladium species are reported. Each Pd^II center harbors one redox-active ligand radical. Combined magnetic, spectroscopic, and computational studies show that the bridging group has a significant effect on the spin-exchange interaction and the intramolecular d⁸-d⁸ interaction in solution vs. solid state.