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⧧(Tc = 148 K) = 7.1 kcal mol–1. Both −SiMe3 groups in Lu­[CH­(SiMe3)2]3 have 1JCH = 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 1JCH = 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.