A Trinuclear Gadolinium Cluster with a Three-Center One-Electron Bond and an S = 11 Ground State

The recent discovery of metal-metal bonding and valence delocalization in the dilanthanide complexes (Cp^iPr5)₂Ln₂I₃ (Cp^iPr5 = pentaisopropylcyclopentadienyl; Ln = Y, Gd, Tb, Dy) opened up the prospect of harnessing the 4f5d_z¹ electron configurations of non-traditional divalent lanthanide ions to access molecules with novel bonding motifs and magnetism. Here, we report the trinuclear mixed-valence clusters (Cp^iPr5)₃Ln₃H₃I₂ (1-Ln, Ln = Y, Gd), which were synthesized via potassium graphite reduction of the trivalent clusters (Cp^iPr5)₃Ln₃H₃I₃. Structural, computational, and spectroscopic analyses support valence delocalization in 1-Ln resulting from a three-center, one-electron σ bond formed from the 4d_z and 5d_z orbitals on Y and Gd, respectively. Dc magnetic susceptibility data obtained for 1-Gd reveal that valence delocalization engenders strong parallel alignment of the σ-bonding electron and the 4f electrons of each gadolinium center to afford a high-spin ground state of S = 11. Notably, this represents the first clear instance of metal-metal bonding in a molecular trilanthanide complex, and the large spin-spin exchange constant of J = 168(1) cm^-1 determined for 1-Gd is only the second largest coupling constant characterized to date for a molecular lanthanide compound.