  Methods are reported for the preparation of mixed-carboxylate versions of the
[Mn12O12(O2CR)16(H2O)4] family of single-molecule magnets (SMMs).
[Mn12O12(O2CCHCl2)8(O2CCH2But)8(H2O)3] (5) and
[Mn12O12(O2CHCl2)8(O2CEt)8(H2O)3] (6) have been obtained from the 1:1 reaction
of the corresponding homocarboxylate species. Complexes 5 and 6 both contain a
[Mn12O12] core with the CHCl2CO2- ligands ordered in the axial positions and
the RCO2- ligands (R = CH2But (5) or Et (6)) in equatorial positions. There is,
thus, a preference for the CHCl2CO2- to occupy the sites lying on the MnIII
Jahn-Teller axes, and this is rationalized on the basis of the relative
basicities of the carboxylate groups. Direct current magnetic susceptibility
studies in a 10.0 kG field in the 2.00-300 K range indicate a large
ground-state spin, and fitting of magnetization data collected in the 10.0-70.0
kG field and 1.80-4.00 K temperature range gave S = 10, g = 1.89, and D = -0.65
K for 5, and S = 10, g = 1.83, and D = -0.60 K for 6. These values are typical
of [Mn12O12(O2CR)16(H2O)4] complexes. Alternating current susceptibility
studies show the out-of-phase susceptibility (chi'') signals characteristic of
the slow relaxation in the millisecond time scale of single-molecule magnets.
Arrhenius plots obtained from chi'' versus T data gave effective barriers to
relaxation (Ueff) of 71 and 72 K for 5 and 6, respectively. 1H NMR spectra in
CD2Cl2 show that 5 and 6 are the main species present on dissolution, but there
is evidence for some ligand distribution between axial and equatorial sites, by
intra- and/or intermolecular exchange processes.
