Intimate Binding Mechanism and Structure of Trigonal Nickel(I) Monocarbonyl Adducts in ZSM-5 Zeolite—Spectroscopic Continuous Wave EPR, HYSCORE, and IR Studies Refined with DFT Quantification of Disentangled Electron and Spin Density Redistributions along σ and π Channels

Abstract

Interaction of tetracoordinated nickel(I) centers generated inside the channels of ZSM-5 zeolite with carbon monoxide ((12,13)CO, pCO < 1 Torr) led to the formation of T-shaped, top-on monocarbonyl adducts with a unique trigonal nickel core, supported by two oxygen donor ligands. The mechanism of the formation of the {Ni(I)-CO}ZSM-5 species was accounted for by a quantitative molecular orbital correlation diagram of CO ligation. Detailed electronic and magnetic structure of this adduct was obtained from comprehensive DFT calculations, validated by quantitative reproduction of its continuous wave electron paramagnetic resonance (CW-EPR), hyperfine sublevel correlation (HYSCORE), and IR fingerprints, using relativistic Pauli and ZORA-SOMF/B3LYP methods. Molecular analysis of the stretching frequency, νCO = 2109 cm(-1), g and A((13)C) tensors (g(xx) = 2.018, g(yy) = 2.380, g(zz) = 2.436, A(xx) = +1.0 ± 0.3 MHz, A(yy) = -3.6 ± 0.9 MHz, A(zz) = -1.6 ± 0.3 MHz) and Q((27)Al) parameters (e(2)Qq/h = -13 MHz and η = 0.8) supported by quantum chemical modeling revealed that the Ni-CO bond results from the π overlap between the low-laying π(2p) CO states with the 3d(xz) and 3d(yz) orbitals, with a small σ contribution due to the overlap of σ(2p+2s) orbital and a protruding lobe of the in-plane 3d(xz) orbital. Two types of orbital channels (associated with the σ and π overlap) of the electron and spin density flows within the {Ni(I)-CO} unit were identified. A bathochromic shift of the νCO stretching vibration was accounted for by resolving quantitatively the separate contributions due to the σ donation and π back-donation, whereas the (13)C hyperfine coupling was rationalized by incongruent α and β spin flows via the σ and π channels. As a result the very nature of the carbon-metal bond in the Ni(I)-CO adduct and the molecular backbone of the corresponding spectroscopic parameters were revealed with unprecedented accuracy.

Interfacial coordination chemistry for catalyst preparation

“…This preliminary model was then refined and extended to dicarbonyl species, by embedding the fragment in an environment comprising a large silica ring and a small, strained, neighboring 2T ring, as can exist on the surface of silica [62]. The reactivity of NO on these tripodal Ni 2+ complexes was ultimately studied [66], concluding a large and coherent body of work that spanned over a period of almost 30 years and that would further expand, for example through the works of the Krakow and Torino groups [67]. At each stage, this work had included new sophisticated methodologies of analysis, and from the investigation of the puzzling migration of Ni 2+ ions in zeolites, led to the description at the atomic level of isolated supported complexes and of their first-and second-sphere environment (Fig.…”

Nature and Topology of Metal–Oxygen Binding Sites in Zeolite Materials:17O High‐Resolution EPR Spectroscopy of Metal‐Loaded ZSM‐5

“…The technique can selectively probe metal ions with open‐shell electronic configurations, which are often directly involved in catalytic cycles acting—from the EPR perspective—as probes directly reporting on their own activity. As in the case of NMR, 27 Al, 29 Si, and 1 H, two‐dimensional EPR (HYSCORE), and ENDOR spectroscopies have been used to obtain exquisite details on the local coordination of catalytically relevant metal ions in zeotype materials, but no information relative to the metal–oxygen bond and electron delocalization over the zeolite framework is directly available. This very information can be gathered through the detection of hyperfine interactions ( hfi ) of 17 O‐labeled lattice sites.…”

Diagnostic Features of EPR Spectra of Superoxide Intermediates on Catalytic Surfaces and Molecular Interpretation of Their g and A Tensors

“…Metallozeolites that accommodate 3d ions act as efficient cationic redox centers for O 2 activation, and their three-dimensional channel structure makes the cage metal centers easily accessible for gaseous dioxygen [29,30,50]. In particular, intrazeolite nickel(I) sites generated in situ via reduction of oxo-nickel species with CO (Ni 2?…”