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.