The structure and magnetic properties of the bimetallic
borohydride-bridged
dysprosocenium compound [{(η5-Cpttt)(η5-CpMe4t)Dy}2(μ:κ2:κ2-BH4)]+[B(C6F5)4]−([3Dy][B(C6F5)4])
are reported along with the solution-phase dynamics of the isostructural
yttrium and lutetium analogues (Cptttis 1,2,4-tri(tert-butyl)cyclopentadienyl, CpMe4tis tetramethyl(tert-butyl)cyclopentadienyl). The synthesis of [3M][B(C6F5)4] was accomplished in the 2:1 stoichiometric reactions of [(η5-Cpttt)(η5-CpMe4t)Dy(BH4)] (2M) with [CPh3][B(C6F5)4], with the metallocenes2Mobtained from reactions of the
half-sandwich complexes [(η5-Cpttt)M(BH4)2(THF)] (1M) (M = Y, Dy, Lu) with NaCpMe4t. Crystallographic studies
show significant lengthening of the M···B distance
on moving through the series1M,2M, and3M, with essentially linear {M···B···M}
bridges in3M. Multinuclear NMR
spectroscopy indicates restricted rotation of the Cptttligands in3Yand3Luin solution. The single-molecule magnet
(SMM) properties of [3M][B(C6F5)4] are characterized by Raman and
Orbach processes, with an effective barrier of 533(18) cm–1and relaxation via the second-excited Kramers doublet. Although
quantum tunneling of the magnetization (QTM) was not observed for
[3M][B(C6F5)4], it was, surprisingly, found in its magnetically dilute
version, which has a very similar barrier ofUeff= 499(21) cm–1. Consistent with this
observation, slightly wider openings of the magnetic hysteresis loop
at 2 K are found for [3M][B(C6F5)4] but not for the diluted analogue.
The dynamic magnetic properties of the dysprosium SMMs and the role
of exchange interactions in3Dyare interpreted with the aid of multireference ab initio calculations.