Synthesis, structural properties and reactivity of ruthenocene-based pincer Pd(ii) tetrahydroborate

Two novel ruthenocene-based pincer palladium tetrahydroborates were characterized by XRD, NMR and FTIR. The alcoholysis of Pd(ii) tetrahydroborate LPd(BH₄) (L = κ³-[{2,5-(^tBu₂PCH₂)₂C₅H₂}Ru(C₅H₅)]) yields the dinuclear cationic Pd(ii) tetrahydroborate with the bridging BH₄^- ligand [(LPd)₂(μ,η^1,2:η^1,2-BH₄)]⁺. The bifurcate dihydrogen-bonded complexes are the active intermediates of the first proton transfer in the step-wise alcoholysis of LPd(BH₄), yielding eventually [(LPd)₂(μ,η^1,2:η^1,2-BH₄)]⁺. According to the X-ray and DFT/M06 geometry analysis, the BH₄^- ligand in both palladium tetrahydroborates has a mixed coordination mode η^1,2. The possibility of BH₃-group abstraction from LPd(BH₄) by an excess of organic base (THF, Py) with the formation of hydride LPd^IIH is shown. This Pd(ii) hydride is a very reactive compound able to rapidly capture CO₂ (ca. 15 min) converting into the formate complex LPd^II(η¹-OC(O)H). The hydrolysis of LPdH with subsequent CO₂ insertion yields a hydrocarbonate complex LPd^II(η¹-OC(O)OH). The hydrocarbonate complex forms hydrogen-bonded dimers in the crystal due to hydrogen bonds between the OC(O)OH fragments.