A Comparison of 4f vs5fMetal−Metal Bonds in (CpSiMe₃)₃M−ECp* (M = Nd, U; E = Al, Ga; Cp* = C₅Me₅): Synthesis, Thermodynamics, Magnetism, and Electronic Structure

Abstract

Reaction of (CpSiMe(3))(3)U or (CpSiMe(3))(3)Nd with (Cp*Al)(4) or Cp*Ga (Cp* = C(5)Me(5)) afforded the isostructural complexes (CpSiMe(3))(3)M-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 (1)H 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 (1)H NMR spectroscopy but were not isolated as pure materials. Comparison of the equilibrium constants and thermodynamic parameters DeltaH and DeltaS obtained by (1)H NMR titration methods revealed a much stronger U-Ga interaction in 2 than the Nd-Ga interaction in 4. Competition reactions between (CpSiMe(3))(3)U and (CpSiMe(3))(3)Nd indicate that Cp*Ga selectively binds U over Nd in a 93:7 ratio at 19 degrees C and 96:4 at -33 degrees C. For 1 and 3, comparison of (1)H NMR peak intensities suggests that Cp*Al also achieves excellent U(III)/Nd(III) selectivity at 21 degrees 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 Cp(3)M-ECp (M = Nd, U; E = Al, Ga), and empirical fitting of the values for Cp(3)M-ECp allowed the prediction of binding energy estimates for Cp*Al compounds 1 and 3. NBO/NLMO bonding analyses on Cp(3)U-ECp indicate that the bonding consists predominantly of a E-->U sigma-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 pi-donation. The overall experimental and computational bonding analysis suggests that Cp*Al and Cp*Ga behave as good sigma-donors in these systems.

Evaluating f‐Element Bonding from Structure and Thermodynamics

“…[27] Computations on [b] [25] 3.246 [b] [25] 2 Nd Cp*Al [a] C 5 H 4 SiMe 3 7.4 [b] [26] 3.220 [b] [ www.chemeurj.org a simple H 3 U À AlH model indicated that the potential energy (PE) wells for the U À Al bond are quite broad, and that each U À Al bond may vary by AE 0.2 with as little as 1 kcal mol À1 of added energy. [26] If this predicted behavior extends to the real Cp-substituted system then, taken together with the structural results, these findings suggest that the U À Al bond is easily perturbed, or "elastic".…”

“…[26] As discussed earlier, within 1.0 kcal mol À1 there could be AE 0.2 of variation in the UÀAl bond lengths in [(CpSiMe 3 ) 3 UA C H T U N G T R E N N U N G (AlCp*)]. This prediction suggests that making judgments regarding electronic structure or bond strength in the three related systems with M À Ga bonds would be a risky endeavor.…”

“…[25][26][27] Both uranium complexes could be prepared on Figure 8. This series of isostructural and isoelectronic molecules provided a unique opportunity to compare trends in chemical bonding for f-elements with weakly coordinated small molecules.…”

“…Additionally, because these solid-state experimental bond lengths are expected to be easily influenced by crystal packing forces and minor intramolecular steric clashes, assignment of merit for theoretical methods based on agreement with the geometric data would be, at best, a dubious process. Along these lines, DFT methodology which had been benchmarked to experimental BDEs, [26] not the bond lengths, for U À Ga and Nd À Ga was found to have predictive power, in that the simulated CeÀGa BDE (4.1 kcal mol À1 ) was statistically identical to the experimental value (4.2(1) kcal mol À1 ). [25] …”

Rare Earth Arene-Bridged Complexes Obtained by Reduction of Organometallic Precursors

“…Although the first structurally characterized lanthanide-transition metal bonds were reported in the early 1990s, ([(MeCN)3YbFe(CO)4]2·MeCN)∞ (Deng and Shore, 1991) and Cp2Lu(THF)-RuCp(CO)2 (Beletskaya et al, 1993), the field started developing at a fast pace only in the last decade Butovskii et al, 2010;Liddle and Mills, 2009;Minasian et al, 2009;Oelkers et al, 2012), spurred by Kempe's report of [O(SiMe2N-2-C5H3N-4-Me)2]2NdRh( 2 -C2H4) and…”