Spin Relaxation and Metal–Carbon Bonding in Early Actinide and Lanthanide Organometallic Complexes Probed by Pulsed EPR

Our comprehension of actinide (An) spin relaxation dynamics and chemical bonds lags behind that of other elements, and will need to improve given the technological as well as fundamental importance of <i>f</i>-block elements [1]. Some key differences in the chemical behaviours of actinides and lanthanides, as well as the differences between different actinides, are ascribed to minor differences in covalency, that is, the degree to which electrons are shared between the <i>f</i>-block element and coordinated ligands [2]. Yet there are almost no direct measures of such covalency for actinides.<br/><br/>This paper will discuss our recent results on studies of early actinide(III) and lanthanide(III) organometallic complexes. Recently we have reported the first pulsed electron paramagnetic resonance (EPR) spectra of actinide (An) complexes with substituted cyclopentadienyl ligands, [An(Cp^<i>tt</i>)₃] (An(III) = Th(III), U(III); Cp<i>^tt</i>= C₅H₃(<i>^t</i>Bu)₂), demonstrating that pulsed EPR methods, such as hyperfine sublevel correlation (HYSCORE) spectroscopy, are ideal tools to probe spin densities at the ligand nuclei [2]. This is of great importance because it relates directly to the M-L chemical bond [2]. For [An(Cp<i>^tt</i>)₃], we found greater spin density on the Cp<i>^tt</i> ligands bound to U(III) than Th(III), indicating enhanced covalency for the U(III)-C than Th(III)-C chemical bond, which can be ascribed to the valence electrons residing in the 5f (U) rather than 6d (Th) orbital.<br/><br/>An expansion of this study to the lanthanide (Ln) analogues, [Ln(Cp<i>^tt</i>)₃] (Ln = Nd, Ce, Sm), has allowed a direct comparison of the valence isoelectronic Nd (III) (4f³) and U(III) (5f³) pair, which is the only comparison available between 4f and 5f M(III) ions that does not require designated radiochemical laboratories. We found the Ln-C bond in [LnCp<i>^tt</i>₃] to be predominantly ionic in the sense of minimal transfer of spin density from the 4f to ligand orbitals [3]. We note that HYSCORE studies on a closely related [Yb(Cp)₃] complex, a late Ln complex (4f¹³), showed far more significant transfer of spin density from 4f to Cp ligand, and can hence be described as being “more covalent” in this sense, despite the smaller ionic radius of the Yb(III) ion [4]. This highlights the importance of ligand substituents and Ln ion charge density in controlling the magnitude of 4f metal–ligand interactions, and shows that EPR spectroscopy is a sensitive technique to study these effects.<br/><br/><b>[1 ] </b>S. McAdams, A. M. Ariciu, A. Kostopulos, J. Walsh, F. Tuna, Molecular single-ion magnets based on lanthanides and actinides: Design considerations and new advances in the context of quantum technologies, <i>Coord. Chem. Rev.</i> <b>2017</b>, <i>346</i>, 216.<br/>[2] A. Formanuik, A.-M. Ariciu, F. Ortu, R. Beekmeyer, A. Kerridge, F. Tuna, E. J. L. McInnes, D. P. Mills. Actinide covalency measured by pulsed electron paramagnetic resonance spectroscopy. <i>Nat. Chem.</i><b> 2017</b>, <i>9</i>, 578.<br/>[3] L. E. Nodaraki, J. Liu, A.-M. Ariciu, F. Ortu, M. S. Oakley, L. Birnoschi, G. K. Gransbury, P. J. Cobb, J. Emerson-King, N. F. Chilton, D. P. Mills, E. J. L. McInnes, F. Tuna. Metal-carbon bonding in early lanthanide substituted cyclopentadienyl complexes probed by pulsed EPR spectroscopy. <i>Chem. Sci. </i><b>2024</b>, <i>15</i>, 3003.<br/>[4] R. G. Denning, J. Harmer, J. C. Green, M. Irwin. Covalency in the 4f shell of <i>tris</i>-cyclopentadienyl ytterbium (YbCp₃)—A spectroscopic evaluation. <i>J. Am.Chem. Soc</i>. <b>2011</b>, <i>133</i>, 20644.