Magnetic circular dichroism spectroscopy of antiferromagnetically coupled hetero-metallic rings [H2NR2][Cr7MF8(O2CCMe3)16].
The optical and magnetic properties of the multi-metal rings [NH(2)R(2)][Cr(7)MF(8)(O(2)CCMe(3))(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 (d(3)) in a rhombic field when M = Cd(II). In the case that M = Ni(II) additional optical transitions arise from the d(8) 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.
School of Chemical Sciences and Pharmacy, University of East Anglia, Norwich, NR4 7TJ, UK. justin.bradley@uea.ac.uk
Justin M
JM
Bradley
Andrew J
AJ
Thomson
Eric J L
EJ
McInnes
Richard E P
RE
Winpenny
Grigore
G
Timco
D016880:Anisotropy; D002104:Cadmium / Q000737:chemistry; D002412:Cations / Q000737:chemistry; D002857:Chromium / Q000737:chemistry; D002942:Circular Dichroism / Q000379:methods*; D005296:Ferrous Compounds / Q000737:chemistry*; D008280:Magnetics; D008345:Manganese / Q000737:chemistry; D008670:Metals / Q000737:chemistry*; D015394:Molecular Structure; D009532:Nickel / Q000737:chemistry; D055095:Optics and Photonics; D009942:Organometallic Compounds / Q000737:chemistry*; D011109:Polymethacrylic Acids / Q000737:chemistry; D013455:Sulfur / Q000737:chemistry; D013816:Thermodynamics
D002412:Cations; D005296:Ferrous Compounds; D008670:Metals; D009942:Organometallic Compounds; D011109:Polymethacrylic Acids; D002104:Cadmium; D002857:Chromium; C030613:polymethacrylic acid; D008345:Manganese; D013455:Sulfur; D009532:Nickel