The Structure of Molecular and Surface Platinum Sites Determined by DNP-SENS and Fast MAS¹⁹⁵Pt Solid-State NMR Spectroscopy

Abstract

The molecular level characterization of heterogeneous catalysts is challenging due to the low concentration of surface sites and the lack of techniques that can selectively probe the surface of a heterogeneous material. Here, we report the joint application of room temperature proton-detected NMR spectroscopy under fast magic angle spinning (MAS) and dynamic nuclear polarization surface enhanced NMR spectroscopy (DNP-SENS), to obtain the 195 Pt solid-state NMR spectra of a prototypical example of highly dispersed Pt sites (single site or single atom), here prepared via surface organometallic chemistry, by grafting [(COD)Pt(OSi(OtBu) 3 ) 2 ] (1, COD = 1,5-cyclooctadiene) on partially dehydroxylated silica (1@SiO 2 ). Compound 1@SiO 2 has a Pt loading of 3.7 wt %, a surface area of 200 m 2 /g, and a surface Pt density of around 0.6 Pt site/nm 2 . Fast MAS 1 H{ 195 Pt} dipolar-HMQC and S-REDOR experiments were implemented on both the molecular precursor 1 and on the surface complex 1@SiO 2 , providing access to 195 Pt isotropic shifts and Pt−H distances, respectively. For 1@SiO 2 , the measured isotropic shift and width of the shift distribution constrain fits of the static wide-line DNPenhanced 195 Pt spectrum, allowing the 195 Pt chemical shift tensor parameters to be determined. Overall the NMR data provide evidence for a well-defined, single-site structure of the isolated Pt sites.

Proton-detected solid-state NMR spectroscopy of spin-1/2 nuclei with large chemical shift anisotropy

“…59 But, inversion can double the dephasing as compared to a saturation pulse for spin-1/2 nuclei. We have previously incorporated SHAPs to invert 195 Pt patterns in aMAT D-HMQC 40 and TONE D-HMQC schemes, 44 however, the inversion efficiency was not systematically investigated.…”

“…We have previously observed this effect in a molecular Pt-based organometallic compound. 44 Furthermore, the dynamics of the NH3 groups in cisplatin partially averages the Pt-H dipolar couplings which leads to a slower build-up of the S-REDOR curves.…”

“…[41][42] We recently reported modified pulse sequences to reduce t1-noise in 2D 1 H detected D-HMQC spectra with half-integer quadrupolar nuclei and spin-1/2 nuclei, dubbed t1-noise eliminated (TONE) D-HMQC. [43][44] The combination of fast MAS 1 H{ 195 Pt} TONE D-HMQC and static DNP enhanced 1 H→ 195 Pt BRAIN-CP-WCPMG was then used to characterize a Pt-based organometallic and single-site silica-supported compound. 44 Short high-power adiabatic pulses (SHAPs) 39,45 have been previously incorporated into aMAT D-HMQC, 40 TONE D-HMQC and S-REDOR (symmetry-based resonance echo double resonance) experiments 44 on samples with 195 Pt CSA (span) of greater than 8000 ppm, however the efficiency of inversion was not systematically investigated.…”

“…[43][44] The combination of fast MAS 1 H{ 195 Pt} TONE D-HMQC and static DNP enhanced 1 H→ 195 Pt BRAIN-CP-WCPMG was then used to characterize a Pt-based organometallic and single-site silica-supported compound. 44 Short high-power adiabatic pulses (SHAPs) 39,45 have been previously incorporated into aMAT D-HMQC, 40 TONE D-HMQC and S-REDOR (symmetry-based resonance echo double resonance) experiments 44 on samples with 195 Pt CSA (span) of greater than 8000 ppm, however the efficiency of inversion was not systematically investigated. We note that Carvalho and Pell have recently provided an in-depth investigation of the inversion of large CSA sideband manifolds with SHAPs and sideband selective WURST pulses.…”

Proton-Detected Solid-State NMR Spectroscopy of Spin-1/2 Nuclei with Large Chemical Shift Anisotropy

“…Therefore, the pulse sequence for AID D-HMQC can also be used for rotorsynchronized experiments. 40 and TONE D-HMQC schemes, 44 however, the inversion efficiency was not systematically investigated.…”