Metal Bonding with 3d and 6d Orbitals: An EPR andENDOR Spectroscopic Investigation of Ti3+–Al andTh3+–Al Heterobimetallic Complexes

摘要

Accessing covalent bonding interactions between actinides and ligating atoms remains a central problem in the field. Our current understanding of actinide bonding is limited because of a paucity of diverse classes of compounds and the lack of established models. We recently synthesized a thorium (Th)–aluminum (Al) heterobimetallic molecule that represents a new class of low-valent Th-containing compounds. To gain further insight into this system and actinide–metal bonding more generally, it is useful to study their underlying electronic structures. Here, we report characterization by electron paramagnetic resonance (EPR) and electron–nuclear double resonance (ENDOR) spectroscopy of two heterobimetallic compounds: (i) a Cp^tt2ThH3AlCTMS3 [TMS = Si(CH3)3; Cp^tt = 1,3-di-tert-butylcyclopentadienyl] complex with bridging hydrides and (ii) an actinide-free Cp2TiH3AlCTMS3 (Cp = cyclopentadienyl) analogue. Analyses of the hyperfine interactions between the paramagnetic trivalent metal centers and the surrounding magnetic nuclei, ¹H and ²⁷Al, yield spin distributions over both complexes. These results show that while the bridging hydrides in the two complexes have similar hyperfine couplings (aiso = −9.7and −10.7 MHz, respectively), the spin density on the Al ionin the Th³⁺ complex is ∼5-fold larger than thatin the titanium(3+) (Ti³⁺) analogue. This suggests a directorbital overlap between Th and Al, leading to a covalent interactionbetween Th and Al. Our quantitative investigation by a pulse EPR techniquedeepens our understanding of actinide bonding to main-group elements.