Understanding the Solution and Solid-State Structures of Pd and Pt PSiP Pincer-Supported Hydrides

The PSiP pincer-supported complex (CyPSiP)­PdH [CyPSiP = Si­(Me)­(2-PCy2-C6H4)2] has been implicated as a crucial intermediate in carboxylation of both allenes and boranes. At this stage, however, there is uncertainty regarding the exact structure of (CyPSiP)­PdH, especially in solution. Previously, both a Pd­(II) structure with a terminal Pd hydride and a Pd(0) structure featuring an η2-silane have been proposed. In this contribution, a range of techniques were used to establish that (CyPSiP)­PdH and the related Pt species, (CyPSiP)­PtH, are true M­(II) hydrides in both the solid state and solution. The single-crystal X-ray structures of (CyPSiP)­MH (M = Pd and Pt) and the related species (iPrPSiP)­PdH [iPrPSiP = Si­(Me)­(2-PiPr2-C6H4)2] are in agreement with the presence of a terminal metal hydride, and the exact geometry of (CyPSiP)­PtH was confirmed using neutron diffraction. The 1H and 29Si­{1H}­NMR chemical shifts of (CyPSiP)­MH (M = Pd and Pt) are consistent with a structure containing a terminal hydride, especially when compared to the chemical shifts of related pincer-supported complexes. In fact, in this work, two general trends relating to the 1H NMR chemical shifts of group 10 pincer-supported terminal hydrides were elucidated: (i) the hydride shift moves downfield from Ni to Pd to Pt and (ii) the hydride shift moves downfield with more trans-influencing pincer central donors. DFT calculations indicate that structures containing a M­(II) hydride are lower in energy than the corresponding η2-silane isomers. Furthermore, the calculated NMR chemical shifts of the M­(II) hydrides using a relativistic four-component methodology incorporating all significant scalar and spin–orbit corrections are consistent with those observed experimentally. Finally, in situ X-ray absorption spectroscopy (XAS) was used to provide further support that (CyPSiP)­MH exist as M­(II) hydrides in solution.