Metal Bonding with 3d and 6d Orbitals: An EPR and ENDOR Spectroscopic Investigation of Ti³⁺–Al and Th³⁺–Al Heterobimetallic Complexes

Abstract

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 Cptt 2ThH3AlCTMS3 [TMS = Si­(CH3)3; Cptt = 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 (a iso = −9.7 and −10.7 MHz, respectively), the spin density on the Al ion in the Th3+ complex is ∼5-fold larger than that in the titanium­(3+) (Ti3+) analogue. This suggests a direct orbital overlap between Th and Al, leading to a covalent interaction between Th and Al. Our quantitative investigation by a pulse EPR technique deepens our understanding of actinide bonding to main-group elements.

Structure and dynamics of catalytically competent but labile paramagnetic metal-hydrides: the Ti(iii)-H in homogeneous olefin polymerization

“…In the case of Ti(III)-Al linked by two m-H bridging hydrides with a Ti(III)-H distance of 1.892Å, 1 H couplings with a iso ¼ 9.7 MHz were observed accompanied by a 27 Al isotropic hyperne coupling a iso ¼ 9.4 MHz. 42 Comparison to our values shows that our system is characterized by a larger 1 H and a smaller 27 Al hyperne coupling, implying a reduced spin density on the Al and a larger spin density at the proton. This fact suggests a stronger Ti-H interaction with respect to bridging H. 42 Even though we cannot denitely exclude an H-Al interaction, this would be rather loose, and all evidence point toward a terminal character of the Ti-H hydride.…”

“…1 H Davies ENDOR experiments allow the obtainment of compelling evidence for a 1 H hyperne coupling characterized by a iso ¼ À18.16 MHz and a dipolar tensor T ¼ [À10.16, À0.67, 10.82] MHz. The experimental 1 H dipolar tensor can be satisfactorily reproduced adopting a three-point dipole model 42,44 with a Ti-H distance of the order of r ¼ 1.8 AE 0.1Å and a tilting angle of $70 with respect to the cylindrical axis of the d z 2 orbital. M-H distances in early transition metals of the rst row range typically in the interval 1.55-1.77Å, 59 while longer distances are usually observed for bridging hydride species.…”

“…Both a iso and the dipolar matrix correlate with values observed in the case of VO 2+ -O-Al linkages in V doped aluminumphosphate 47 and Ti 3+ -O-Al linkages in Ti doped aluminosilicate materials. 48 Somewhat larger values have been reported for Ti 3+ ions coordinated to an anionic aluminum hydride (alanate) ligand 42 and for Ti 3+ ions on the surface of chlorinated aluminum oxide. 49 Finally, we note that no evidence of Cl interactions could be detected.…”

“…The model, derived as an extension of the case of p p C-H fragment, 44 has been validated by successfully reproducing the anisotropies of the Ni-C state of the [NiFe] regulatory hydrogenase of Ralstonia eutropha 1 and to the Ti(III) complex Cp 2 TiH 3 AlCTMS 3 (TMS ¼ Si(CH 3 ) 3 ; Cp ¼ m 5 -C 5 H 5 ). 42 By applying the same model to Ti-H (Scheme 1) and by setting the angle between g z and the Ti/H vector equal to the Euler angle b, considering a spin density on the Ti 3d z 2 orbital of 0.95, the displacement of the 3d z 2 lobes equal to 1.40Å and a Ti/H bond length of r ¼ 1.8 AE 0.1Å, the experimental rhombicity derived by ENDOR spectroscopy can be reasonably reproduced T calc ¼ [À10.4, À5.7, 16.1] MHz, providing an independent validation of the hyper-ne components obtained by tting.…”

“…4) indicates that the experimental pattern can be satisfactorily reproduced with a slightly rhombic hyperne tensor The latter value is in line with values reported for Ti(III)-Al heterobimetallic complexes. 42 From the isotropic part of the hyperne coupling a iso Al ¼ 1/3(A x + A y + A z ) ¼ 3.6 AE 1.0 MHz and considering the value of a 0 ¼ 3367.76 MHz for unit spin density in the 27 Al 3s orbital, one can calculate the electron spin population in the Al 3s orbital to be 0.1%. From the anisotropic hyperne tensor, a lower limit for the Ti-Al distance of about 2Å can be estimated (Scheme 1a).…”