Structural characterization of surface immobilized platinum hydrides by sensitivity-enhanced Pt solid state NMR spectroscopy and DFT calculations.

Supported single-site platinum hydride compounds are promising heterogeneous catalysts for organic transformations. Few methods exist to describe the structures of single-site Pt catalysts with atomic resolution because of their disordered structures and low Pt loadings. Here, we study the compounds formed when bis(tri--butylphosphino)platinum, Pt(P Bu), is supported on dehydroxylated SiO or SiO-AlO. First, we obtain magic angle spinning (MAS) H, P and Pt ssNMR spectra of four model Pt phosphine compounds with oxidation states of 0 or +2 and coordination numbers between 2 and 4. These compounds are analogs of potential structures present in the supported compounds. MAS Pt ssNMR spectra were obtained using P{Pt} sideband selective -resolved and -HMQC experiments. The measured H and P chemical shifts, P-Pt -couplings and Pt chemical shift (CS) tensors are shown to be diagnostic of oxidation state and coordination number. Room temperature H ssNMR spectra of Pt(P Bu) supported on SiO or SiO-AlO show diagnostic hydride NMR signals, suggesting that Pt(P Bu) undergoes oxidative addition, resulting in surface hydrides and Pt-oxygen bonds to the support surface. MAS dynamic nuclear polarization (DNP) enables P{Pt} correlation NMR experiments on the supported compounds. These experiments enable the measurement of the P-Pt -coupling constants and Pt CS tensors. Combined NMR and DFT analyses suggest that the primary surface platinum species are [HPt(P Bu)OSi] on SiO and [HPt(P Bu)][Si-O-Al] on SiO-AlO. 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 SiO and SiO-AlO, respectively.