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Issue 25, 2008
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Magnetic circular dichroism spectroscopy of antiferromagnetically coupled hetero-metallic rings [H2NR2][Cr7MF8(O2CCMe3)16]

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Abstract

The optical and magnetic properties of the multi-metal rings [NH2R2][Cr7MF8(O2CCMe3)16], where M = Cd(II), Mn(II) or Ni(II), have been studied using variable-field and variable-temperature magnetic circular dichroism (MCD) in the UV-visible spectra. Spectra of samples were recorded in a frozen organic matrix or cast in a polymethacrylate (PMMA) polymer film between 1.7 and 75 K. The spectra are characteristic of the Cr(III) ion (d3) in a rhombic field when M = Cd(II). In the case that M = Ni(II) additional optical transitions arise from the d8 ion whereas for M = Mn(II) no additional transitions are observed. The influence of magnetic exchange is apparent from a change in the sign of the MCD signal between complexes in which the hetero-atom has a local spin moment greater, or less, than that of Cr(III), S = 3/2, namely, Mn(II), S = 5/2, and Ni(II), S = 1. The exchange coupling generates a manifold of thermally accessible electronic states that give rise to variations in MCD intensity as well as additional spectral features as the temperature is raised. Equations have been derived to relate the splittings observed in the optical spectrum to the single-ion ground state zero-field splittings of chromium(III). There is reasonable agreement between the sign and magnitude of the contribution to the cluster anisotropy from that of the single ion with values estimated from other techniques.

Graphical abstract: Magnetic circular dichroism spectroscopy of antiferromagnetically coupled hetero-metallic rings [H2NR2][Cr7MF8(O2CCMe3)16]

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Supplementary files

Article information


Submitted
26 Feb 2008
Accepted
07 Apr 2008
First published
13 May 2008

Dalton Trans., 2008, 3311-3319
Article type
Paper

Magnetic circular dichroism spectroscopy of antiferromagnetically coupled hetero-metallic rings [H2NR2][Cr7MF8(O2CCMe3)16]

J. M. Bradley, A. J. Thomson, E. J. L. McInnes, R. E. P. Winpenny and G. Timco, Dalton Trans., 2008, 3311
DOI: 10.1039/B803269F

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