title: Studies of a molecular hourglass: synthesis and magnetic characterisation of a cyclic dodecanuclear {Cr10Cu2} complex.
abstract: The synthesis, structure, EPR, and magnetic studies of two dodecanuclear heterometallic cyclic clusters are reported. The compounds have the general formula [R(2)NH(2)](2)[Cr(10)Cu(2)F(14)(O(2)CCMe(3))(22)] (R=Me, 1 or iPr, 2). Both structures contain an array of metal centers which describe an approximate "hourglass", with an ammonium cation in the center of each half of the figure. The chromium sites are all six-coordinate, with the two copper sites five-coordinate. The majority of metal-metal edges are bridged by a single fluoride and two pivalate ligands, while two Cr--Cu edges are bridged by a single fluoride and a single pivalate. Magnetic studies show that 1 and 2 exhibit similar (but not identical) behavior, which can be attributed to ten antiferromagnetic and two ferromagnetic exchange interactions around the ring which gives an S=0 ground state. Quantum Monte Carlo calculations have been used to quantify the exchange interactions by successfully simulating the susceptibility for the full temperature range and thus clarifying the distinction between 1 and 2. EPR spectroscopy shows signals due to excited states, and a variable-temperature study has provided an estimate of the energy gap between the first excited state (S=1) and second excited state (S=2) for 1 that is consistent with the value obtained using the QMC method.
authors.affiliation: School of Chemistry, The University of Manchester, Oxford Road, Manchester M13 9PL, UK.
authors.forename: Muralidharan
authors.initials: M
authors.lastname: Shanmugam
authors.forename: Larry P
authors.initials: LP
authors.lastname: Engelhardt
authors.forename: Finn K
authors.initials: FK
authors.lastname: Larsen
authors.forename: Marshall
authors.initials: M
authors.lastname: Luban
authors.forename: Eric J L
authors.initials: EJ
authors.lastname: McInnes
authors.forename: Christopher A
authors.initials: CA
authors.lastname: Muryn
authors.forename: Jacob
authors.initials: J
authors.lastname: Overgaard
authors.forename: Eva
authors.initials: E
authors.lastname: Rentschler
authors.forename: Grigore A
authors.initials: GA
authors.lastname: Timco
authors.forename: Richard E P
authors.initials: RE
authors.lastname: Winpenny