Trinuclear {M¹}CN{M²}₂Complexes (M¹= Cr^III, Fe^III, Co^III; M²= Cu^II, Ni^II, Mn^II). Are Single Molecule Magnets Predictable?

Abstract

The reaction of the hexacyanometalates K3[M(1)(CN)6] (M(1) = Cr(III), Fe(III), Co(III)) with the bispidine complexes [M(2)(L(1))(X)](n+) and [M(2)(L(2))(X)](n+) (M(2) = Mn(II), Ni(II), Cu(II); L(1) = 3-methyl-9-oxo-2,4-di-(2-pyridyl)-7-(2-pyridylmethyl)-3,7-diazabicyclo[3.3.1]nonane-1,5-dicarboxylic acid dimethyl ester; L(2) = 3-methyl-9-oxo-7-(2-pyridylmethyl)-2,4-di-(2-quinolyl)-3,7-diazabicyclo[3.3.1]nonane-1,5-dicarboxylic acid dimethyl ester; X = anion or solvent) in water-methanol mixtures affords trinuclear complexes with cis- or trans-arrangement of the bispidine-capped divalent metal centers around the hexacyanometalate. X-ray structural analyses of five members of this family of complexes (cis-Fe[CuL(2)]2, trans-Fe[CuL(1)]2, cis-Co[CuL(2)]2, trans-Cr[MnL(1)]2, trans-Fe[MnL(1)]2) and the magnetic data of the entire series are reported. The magnetic data of the cyanide bridged, ferromagnetically coupled cis- and trans-Fe[ML]2 compounds (M = Ni(II), Cu(II)) with S = 3/2 (Cu(II)) and S = 5/2 (Ni(II)) ground states are analyzed with an extended Heisenberg Hamiltonian which accounts for anisotropy and zero-field splitting, and the data of the Cu(II) systems, for which structures are available, are thoroughly analyzed in terms of an orbital-dependent Heisenberg Hamiltonian, in which both spin-orbit coupling and low-symmetry ligand fields are taken into account. It is shown that the absence of single-molecule magnetic behavior in all spin clusters reported here is due to a large angular distortion of the [Fe(CN)6](3-) center and the concomitant quenching of orbital angular momentum of the Fe(III) ((2)T2g) ground state.

Magnetic properties and spin dynamics in the single-molecule paramagnetsCu6FeandCu6Co

“…These molecules are thus a prototype of single-molecule paramagnets. As it will be shown by the experimental results, the Cu 6 ions via the bond to the central Co 3+ ion ͑Fig. 1͒.…”

“…In the case of high spin ground state and high magnetic anisotropy, quantum tunneling of magnetization and quantum coherence at low temperature have been observed, making these nanomagnets promising candidates for magnetic storage and quantum computing among other applications. 2,3 In this paper we present the magnetic properties of heptanuclear molecular magnets Cu 6 Fe and Cu 6 Co with very small intramolecular magnetic coupling. These molecules are thus a prototype of single-molecule paramagnets.…”

“…The results are presented in separate subsections for the magnetization, the electron paramagnetic resonance ͑EPR͒, the static and the dynamic NMR. Section IV contains a comparison between Cu 6 Fe and Cu 6 Co and a discussion of the results obtained with the different techniques and the relevant conclusions. 6 ͔ ͑H saldmen is the Schiff base resulted by reacting salicylaldehyde with N,N-dimethylethylenediamine as will be described elsewhere 4 ͒.…”

“…It is found that in Cu 6 Fe, whose magnetic core is constituted by six Cu 2+ ions and one Fe 3+ ion all with s =1/ 2, a weak ferromagnetic interaction between Cu 2+ moments through the central Fe 3+ ion with J = 0.14 K is present, while in Cu 6 Co the Co 3+ ion is diamagnetic and the weak interaction is antiferromagnetic with J = −1.12 K. The NMR spectra show the presence of nonequivalent groups of protons with a measurable contact hyperfine interaction consistent with a small admixture of s wave function with the d function of the magnetic ion. The NMR relaxation results are explained in terms of a single-ion ͑Cu 2+ , Fe 3+ , Co 3+ ͒ uncorrelated spin dynamics with an almost temperature-independent correlation time due to the weak magnetic exchange interaction.…”

“…The magnetic properties and the spin dynamics of two molecular magnets have been investigated by magnetization and dc susceptibility measurements, electron paramagnetic resonance and proton nuclear magnetic resonance ͑NMR͒ over a wide range of temperatures ͑1.6-300K͒ at applied magnetic fields H = 0.5 and 1.5 T. The two molecular magnets consist of ͓ ͕Cu II ͑saldmen͒͑H 2 O͖͒ 6 ͕Fe III ͑CN͒ 6 ͖͔͑ClO 4 ͒ 3 ·8H 2 O in short Cu 6 Fe ͑here H saldmen is the Schiff base resulted by reacting salicylaldehyde with N,N-dimethylethylenediamine͒ and the analog compound with cobalt, Cu 6 Co. It is found that in Cu 6 Fe, whose magnetic core is constituted by six Cu 2+ ions and one Fe 3+ ion all with s =1/ 2, a weak ferromagnetic interaction between Cu 2+ moments through the central Fe 3+ ion with J = 0.14 K is present, while in Cu 6 Co the Co 3+ ion is diamagnetic and the weak interaction is antiferromagnetic with J = −1.12 K. The NMR spectra show the presence of nonequivalent groups of protons with a measurable contact hyperfine interaction consistent with a small admixture of s wave function with the d function of the magnetic ion.…”