Generation, Characterization, and Electrochemical Behavior of the Palladium–Hydride Cluster [Pd 3 (dppm) 3 (μ 3 ‐CO)(μ 3 ‐H)] + (dppm=Bis(diphenylphosphinomethane)

Addition of formate on the dicationic cluster [Pd 3 (dppm) 3 (μ 3 ‐CO)] 2+ (dppm=bis(diphenylphosphinomethane) affords quantitatively the hydride cluster [Pd 3 (dppm) 3 (μ 3 ‐CO)(μ 3 ‐H)] + . This new palladium–hydride cluster has been characterised by 1 H NMR, 31 P NMR and UV/Vis spectroscopy and MALDI‐TOF mass spectrometry. The unambiguous identification of the capping hydride was made from 2 H NMR spectroscopy by using DCO 2 − as starting material. The mechanism of the hydride complex formation was investigated by UV/Vis stopped‐flow methods. The kinetic data are consistent with a two‐step process involving: 1) host–guest interactions between HCO 2 − and [Pd 3 (dppm) 3 (μ 3 ‐CO)] 2+ and 2) a reductive elimination of CO 2 . Two alternatives routes to the hydride complex were also examined : 1) hydride transfer from NaBH 4 to [Pd 3 (dppm) 3 (μ 3 ‐CO)] 2+ and 2) electrochemical reduction of [Pd 3 (dppm) 3 (μ 3 ‐CO)] 2+ to [Pd 3 (dppm) 3 (μ 3 ‐CO)] 0 followed by an addition of one equivalent of H + . Based on cyclic voltammetry, evidence for a dual mechanism (ECE and EEC; E=electrochemical (one‐electron transfer), C=chemical (hydride dissociation)) for the two‐electron reduction of [Pd 3 (dppm) 3 (μ 3 ‐CO)(μ 3 ‐H)] + to [Pd 3 (dppm) 3 (μ 3 ‐CO)] 0 is provided, corroborated by digital simulation of the experimental results. Geometry optimisations of the [Pd 3 (H 2 PCH 2 PH 2 ) 3 (μ 3 ‐CO)(μ 3 ‐H)] n model clusters were performed by using DFT at the B3 LYP level. Upon one‐electron reductions, the PdPd distance increases from a formal single bond ( n =+1), to partially bonding ( n =0), to weak metal–metal interactions ( n =−1), while the PdH bond length remains relatively the same.