Structural Characterization of Surface Immobilized Platinum Hydrides by Sensitivity-Enhanced 195Pt Solid State NMR Spectroscopy and DFT Calculations

Supported single-site platinum hydride compounds are promising heterogeneous catalysts catalysts for organic transformations. Unfortunately, few methods exist to describe the structures of single-site Pt catalysts with atomic resolution because of their ill-defined structures and low Pt loadings. 195Pt solid-state NMR spectroscopy (ssNMR) is potentially useful for characterizing the structures of low loading Pt catalysts because the 195Pt chemical shift (CS) tensor is sensitive to the coordination environment. Here, we study the compounds formed when Pt(PtBu3)2 is supported on dehydroxylated SiO2 or SiO2-Al2O3. First, we use MAS 1H{195Pt} and 1H-31P{195Pt} J-resolved and J-HMQC sideband selective experiments to obtain 195Pt ssNMR spectra of four model Pt phosphine compounds with various oxidation states and coordination environments. These compounds are analogs of potential structures present in the supported compounds. We then apply MAS dynamic nuclear polarization (DNP) surface-enhanced ssNMR to enable 31P{195Pt} correlation NMR experiments on the characterize the Pt hydrides present on SiO2 and SiO2-Al2O3. These experiments enable the measurement of the 31P-195Pt J-coupling constants and 195Pt CS tensors. Combined NMR/DFT analysis suggest that the primary surface platinum species is [HPt(PtBu3)2]+. The Pt-oxygen bond length is dependent on the support and estimated as 2.1-2.3 Å and 2.7-3.0 Å for SiO2 and SiO2-Al2O3, respectively.