Studies of Finite Molecular Chains: Synthesis, Structural, Magnetic and Inelastic Neutron Scattering Studies of Hexa‐ and Heptanuclear Chromium Horseshoes

Abstract

We report the synthesis and structural characterisation of a family of finite molecular chains, specifically [{[R(2)NH(2)](3)[Cr(6)F(11)(O(2)CCMe(3))(10)]}(2)] (in which R=nPr 1, Et 2, nBu 3), [{Et(2)NH}(2){[Et(2)NH(2)](3)[Cr(7)F(12)(O(2)CCMe(3))(12)][HO(2)CCMe(3)](2)}(2)] (4), [{[Me(2)NH(2)](3)[Cr(6)F(11)(O(2)CCMe(3))(10)]2.5 H(2)O}(4)] (5) and [{[iPr(2)NH(2)](3)[Cr(7)F(12)(O(2)CCMe(3))(12)]}(2)] (6). The structures all contain horseshoes of chromium centres, with each Cr...Cr contact within the horseshoe bridged by a fluoride and two pivalates. The horseshoes are linked through hydrogen bonds to the secondary ammonium cations in the structure, leading to di- and tetra-horseshoe structures. Through magnetic measurements and inelastic neutron scattering studies we have determined the exchange coupling constants in 1 and 6. In 1 it is possible to distinguish two exchange interactions, J(A)=-1.1 meV and J(B)=-1.4 meV; J(A) is the exchange interactions at the tips of the horseshoe and J(B) is the exchange within the body of the horseshoe (1 meV=8.066 cm(-1)). For 6 only one interaction was needed to model the data: J=-1.18 meV. The single-ion anisotropy parameters for Cr(III) were also derived for the two compounds as: for 1, D(Cr)=-0.028 meV and |E(Cr)|=0.005 meV; for 6, D(Cr)=-0.031 meV. Magnetic-field-dependent inelastic neutron scattering experiments on 1 allowed the Zeeman splitting of the first two excited states and level crossings to be observed. For the tetramer of horseshoes (5), quantum Monte Carlo calculations were used to fit the magnetic susceptibility behaviour, giving two exchange interactions within the horseshoe (-1.32 and -1.65 meV) and a weak inter-horseshoe coupling of +0.12 meV. Multi-frequency variable-temperature EPR studies on 1, 2 and 6 have also been performed, allowing further characterisation of the spin Hamiltonian parameters of these chains.

Magnetic Cluster Excitations

“…Accordingly, the Bose-scaled high-temperature data can estimate the lattice contribution at lower temperatures. Often this works well, especially in large magnetic clusters with a dense higher-lying energy spectrum, and allows an unambiguous identification of magnetic peaks (Dreiser et al, 2010b;Ochsenbein et al, 2008).…”

“…IV.D.2 also afforded the generation of short antiferromagnetic chains, either by replacing one magnetic metal ion in the ring-like structure by a diamagnetic ion such as Zn 2+ or Cd 2+ , or modifying the synthesis method to yield structures called "horseshoes" (Affronte et al, 2007). The members of this family of clusters were studied, besides the thermodynamic techniques, by EPR (Piligkos et al, 2007), XMCD , NMR (Amiri et al, 2010;Micotti et al, 2006), and INS (Baker et al, 2011a;Bianchi et al, 2009;Caciuffo et al, 2005;Ochsenbein et al, 2008Ochsenbein et al, , 2007. Short antiferromagnetic chains were also obtained through doping the Fe 18 wheel (Sec.…”

“…From the view point of the physics of the magnetic excitations, comparison of short antiferromagnetic chains with even-membered antiferromagnetic wheels, or chains with open and periodic boundary conditions, should prove interesting , as well as comparing antiferromagnetic chains with even and odd number of metal centers (Ochsenbein et al, 2008).…”

“…Various derivatives of Cr 6 have been synthesized and studied by INS (Baker et al, 2011a;Larsen et al, 2003b;Ochsenbein et al, 2008Ochsenbein et al, , 2007. Here the cluster [NH 2 R] 3 [Cr 6 F 11 Piv 10 (H 2 O)] is considered (Larsen et al, 2003b).…”

“…(87). The model was later refined to 2J = −1.4 meV, 2J edge = −1.1 meV, D = −0.028 meV and |E| = 0.005 meV, where J edge refers to the coupling strengths of the outer coupling paths (Ochsenbein et al, 2008).…”