The Nature of Secondary Interactions at Electrophilic Metal Sites of Molecular and Silica-Supported Organolutetium Complexes from Solid-State NMR Spectroscopy

Lu­[CH­(SiMe3)2]3 reacts with [SiO2–700] to give [(SiO)­Lu­[CH­(SiMe3)2]2] and CH2(SiMe3)2. [(SiO)­Lu­[CH­(SiMe3)2]2] is characterized by solid-state NMR and EXAFS spectroscopy, which show that secondary Lu···C and Lu···O interactions, involving a γ-CH3 and a siloxane bridge, are present. From X-ray crystallographic analysis, the molecular analogues Lu­[CH­(SiMe3)2]3–x [O-2,6-tBu-C6H3] x (x = 0–2) also have secondary Lu···C interactions. The 1H NMR spectrum of Lu­[CH­(SiMe3)2]3 shows that the −SiMe3 groups are equivalent to −125 °C and inequivalent below that temperature, ΔG ⧧ (T c = 148 K) = 7.1 kcal mol–1. Both −SiMe3 groups in Lu­[CH­(SiMe3)2]3 have 1 J CH = 117 ± 1 Hz at −140 °C. The solid-state 13C CPMAS NMR spectrum at 20 °C shows three chemically inequivalent resonances in the area ratio of 4:1:1 (12:3:3); the J-resolved spectra for each resonance give 1 J CH = 117 ± 2 Hz. The 29Si CPMAS NMR spectrum shows two chemically inequivalent resonances with different values of chemical shift anisotropy. Similar observations are obtained for Lu­[CH­(SiMe3)2]3–x [O-2,6-tBu-C6H3] x (x = 1 and 2). The spectroscopic data point to short Lu···Cγ contacts corresponding to 3c-2e Lu···Cγ–Siβ interactions, which are supported by DFT calculations. Calculated natural bond orbital (NBO) charges show that Cγ carries a negative charge, while Lu, Hγ, and Siβ carry positive charges; as the number of O-based ligands increases so does the positive charge at Lu, which in turns shortens the Lu···Cγ distance. The change in NBO charges and the resulting changes in the spectroscopic and crystallographic properties show how ligands and surface-support sites rearrange to accommodate these changes, consistent with Pauling’s electroneutrality concept.