Metal–carbon bonding in early lanthanide substituted cyclopentadienyl complexes probed by pulsed EPR spectroscopy

Abstract

We examine lanthanide (Ln)–ligand bonding in a family of early Ln³⁺ complexes [Ln(Cp^tt)₃] (1-Ln, Ln = La, Ce, Nd, Sm; Cp^tt = C₅H₃^tBu₂-1,3) by pulsed electron paramagnetic resonance (EPR) methods, and provide the first characterization of 1-La and 1-Nd by single crystal XRD, multinuclear NMR, IR and UV/Vis/NIR spectroscopy. We measure electron spin T₁ and T_m relaxation times of 12 and 0.2 μs (1-Nd), 89 and 1 μs (1-Ce) and 150 and 1.7 μs (1-Sm), respectively, at 5 K: the T₁ relaxation of 1-Nd is more than 10² times faster than its valence isoelectronic uranium analogue. ¹³C and ¹H hyperfine sublevel correlation (HYSCORE) spectroscopy reveals that the extent of covalency is negligible in these Ln compounds, with much smaller hyperfine interactions than observed for equivalent actinide (Th and U) complexes. This is corroborated by ab initio calculations, confirming the predominant electrostatic nature of the metal–ligand bonding in these complexes.