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

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.