A Comparison of 4f vs 5f Metal−Metal Bonds in (CpSiMe3)3M−ECp (M = Nd, U; E = Al, Ga; Cp = C5Me5): Synthesis, Thermodynamics, Magnetism, and Electronic Structure

Reaction of (CpSiMe3)3U or (CpSiMe3)3Nd with (Cp*Al)4 or Cp*Ga (Cp* = C5Me5) afforded the isostructural complexes (CpSiMe3)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 Cp3M−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.