Structural characterization of surface immobilized platinum hydrides by sensitivity-enhanced 195Pt solid state NMR spectroscopy and DFT calculations

Abstract

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-tert-butylphosphino)platinum, Pt(P^tBu₃)₂, is supported on dehydroxylated SiO₂ or SiO₂–Al₂O₃. First, we obtain magic angle spinning (MAS) ¹H, ³¹P and ¹⁹⁵Pt 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 ¹⁹⁵Pt ssNMR spectra were obtained using ³¹P{¹⁹⁵Pt} sideband selective J-resolved and J-HMQC experiments. The measured ¹H and ³¹P chemical shifts, ³¹P–¹⁹⁵Pt J-couplings and ¹⁹⁵Pt chemical shift (CS) tensors are shown to be diagnostic of oxidation state and coordination number. Room temperature ¹H ssNMR spectra of Pt(P^tBu₃)₂ supported on SiO₂ or SiO₂–Al₂O₃ show diagnostic hydride NMR signals, suggesting that Pt(P^tBu₃)₂ undergoes oxidative addition, resulting in surface hydrides and Pt–oxygen bonds to the support surface. MAS dynamic nuclear polarization (DNP) enables ³¹P{¹⁹⁵Pt} correlation NMR experiments on the supported compounds. These experiments enable the measurement of the ³¹P–¹⁹⁵Pt J-coupling constants and ¹⁹⁵Pt CS tensors. Combined NMR and DFT analyses suggest that the primary surface platinum species are [HPt(P^tBu₃)₂OSi] on SiO₂ and [HPt(P^tBu₃)₂]⁺[Si–O⁻–Al] on SiO₂–Al₂O₃. 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.