Substituent Effect on Formation of Heterometallic Molecular Wheels: Synthesis, Crystal Structure, and Magnetic Properties

The reaction of manganese(III) Schiff bases of the type salen2- (N,N‘-ethylenebis(salicylideneaminato)) with
X-substituted (X = CH3, Cl) pyridinecarboxamide dicyanoferrite(III) [Fe(X-bpb)(CN)2]- gave rise to a series of
cyanide-bridged Mn6Fe6 molecular wheels, [MnIII(salen)]6[FeIII(bpmb)(CN)2]6·7H2O (1), [Mn(salen)]6[Fe(bpClb)(CN)2]6·4H2O·2CH3OH (2), [Mn(salen)]6[Fe(bpdmb)(CN)2]6·10H2O·5CH3OH (3), [Mn(5-Br(salpn))]6[Fe(bpmb)(CN)2]6·24H2O·8CH3CN (4), and [Mn(5-Cl(salpn))]6[Fe(bpmb)(CN)2]6·25H2O·5CH3CN (5). Compared with [Fe(bpb)(CN)2]-,
which always gives rise to 1D or polynuclear species when reacting with Mn(III) Schiff bases, the introduction of
substituents (X) to the bpb2- ligand has a driving force in formation of the novel wheel structure. Magnetic studies
reveal that high-spin ground state S = 15 is present in the wheel compounds originated from the ferromagnetic
Mn(III)−Fe(III) coupling. For the first time, the quantum Monte Carlo study has been used to modulate the magnetic
susceptibility of the huge Mn6Fe6 metallomacrocycles, showing that the magnetic coupling constants J range from
3.0 to 8.0 K on the basis of the spin Hamiltonian H = −J(∑i,jsFeiSMnj + sFe1SMn12). Hysteresis loops for 1 have been
observed below 0.8 K, indicative of a single-molecule magnet with a blocking temperature (TB) of 0.8 K. Molecular
wheels 2−5 exhibit frequency dependence of alternating-current magnetic susceptibility under zero direct-current
magnetic field, signifying the slow magnetization relaxation similar to that of 1. Significantly, an unprecedented
archlike Mn2Fe2 cluster, [Mn(5-Cl(salpn))]2[Fe(bpmb)(CN)2]2·3H2O·CH3CN (6), has been isolated as an intermediate
of the Mn6Fe6 wheel 5. Ferromagnetic Mn(III)−Fe(III) coupling results in a high-spin S = 5 ground state. Combination
of the high-spin state and a negative magnetic anisotropy (D) results in the observation of slow magnetization
relaxation in 6.