Structural and Magnetization Dynamics of Borohydride-Bridged Rare-Earth Metallocenium Cations

[Image: see text] The structure and magnetic properties of the bimetallic borohydride-bridged dysprosocenium compound [{(η(5)-Cp(ttt))(η(5)-Cp(Me4t))Dy}(2)(μ:κ(2):κ(2)-BH(4))](+)[B(C(6)F(5))(4)](−) ([3(Dy)][B(C(6)F(5))(4)]) are reported along with the solution-phase dynamics of the isostructural yttrium and lutetium analogues (Cp(ttt) is 1,2,4-tri(tert-butyl)cyclopentadienyl, Cp(Me4t) is tetramethyl(tert-butyl)cyclopentadienyl). The synthesis of [3(M)][B(C(6)F(5))(4)] was accomplished in the 2:1 stoichiometric reactions of [(η(5)-Cp(ttt))(η(5)-Cp(Me4t))Dy(BH(4))] (2(M)) with [CPh(3)][B(C(6)F(5))(4)], with the metallocenes 2(M) obtained from reactions of the half-sandwich complexes [(η(5)-Cp(ttt))M(BH(4))(2)(THF)] (1(M)) (M = Y, Dy, Lu) with NaCp(Me4t). Crystallographic studies show significant lengthening of the M···B distance on moving through the series 1(M), 2(M), and 3(M), with essentially linear {M···B···M} bridges in 3(M). Multinuclear NMR spectroscopy indicates restricted rotation of the Cp(ttt) ligands in 3(Y) and 3(Lu) in solution. The single-molecule magnet (SMM) properties of [3(M)][B(C(6)F(5))(4)] are characterized by Raman and Orbach processes, with an effective barrier of 533(18) cm(–1) and relaxation via the second-excited Kramers doublet. Although quantum tunneling of the magnetization (QTM) was not observed for [3(M)][B(C(6)F(5))(4)], it was, surprisingly, found in its magnetically dilute version, which has a very similar barrier of U(eff) = 499(21) cm(–1). Consistent with this observation, slightly wider openings of the magnetic hysteresis loop at 2 K are found for [3(M)][B(C(6)F(5))(4)] but not for the diluted analogue. The dynamic magnetic properties of the dysprosium SMMs and the role of exchange interactions in 3(Dy) are interpreted with the aid of multireference ab initio calculations.