A comparison of 4f vs 5f metal-metal bonds in (CpSiMe₃) ₃M-ECp* (M = Nd, U; E = Al, Ga; Cp* = C ₅Me₅): Synthesis, thermodynamics, magnetism, and electronic structure

Reaction of (CpSiMe₃)₃U or (CpSiMe₃) ₃Nd with (Cp*Al)₄ or Cp*Ga (Cp* = C ₅Me₅) afforded the isostructural complexes (CpSiMe ₃)₃M-ECp* (M = U, E = Al (1); M = U, E = Ga (2); M = Nd, E = Al (3); M = Nd, E = Ga (4)). In the case of 1 and 2 the complexes were isolated in 39 and 90% yields, respectively, as crystalline solids and were characterized by single-crystal X-ray diffraction, variable-temperature ¹H NMR spectroscopy, elemental analysis, variable-temperature magnetic susceptibility, and UV-visible-NIR spectroscopy. In the case of 3 and 4, the complexes were observed by variable-temperature ¹H NMR spectroscopy but were not isolated as pure materials. Comparison of the equilibrium constants and thermodynamic parameters ΔH and ΔS obtained by ¹H NMR titration methods revealed a much stronger U-Ga interaction in 2 than the Nd-Ga interaction in 4. Competition reactions between (CpSiMe₃)₃U and (CpSiMe₃)₃Nd indicate that Cp*Ga selectively binds U over Nd in a 93:7 ratio at 19°C and 96:4 at -33°C. For 1 and 3, comparison of ¹H NMR peak intensities suggests that Cp*Al also achieves excellent U(III)/Nd(III) selectivity at 21°C. The solution electronic spectra and solid-state temperature-dependent magnetic susceptibilities of 1 and 2, in addition to X-ray absorption near-edge structure (XANES) measurements from scanning transmission X-ray microscopy (STXM) of 1, are consistent with those observed for other U(III) coordination complexes. DFT calculations using five different functionals were performed on the model complexes Cp₃M-ECp (M = Nd, U; E = Al, Ga), and empirical fitting of the values for Cp ₃M-ECp allowed the prediction of binding energy estimates for Cp*Al compounds 1 and 3. NBO/NLMO bonding analyses on Cp₃U-ECp indicate that the bonding consists predominantly of a E→U σ-interaction arising from favorable overlap between the diffuse ligand lone pair and the primarily 7s/6d acceptor orbitals on U(III), with negligible U→E π-donation. The overall experimental and computational bonding analysis suggests that Cp*Al and Cp*Ga behave as good σ-donors in these systems.