Magnetic circular dichroism spectroscopy of weakly exchange coupled transition metal dimers: A model study

A detailed study of the magnetic circular dichroism (MCD) spectra of weakly exchange coupled transition metal heterodimers is reported. The systems consist of three isostructural complexes of the type [LM(III)(PyA)3M(II)](ClO4)2 where L represents 1,4,7-trimethyl-1,4,7-triazacyclonanane and PyA− is the monoanion of pyridine-2-aldoxime. The trivalent metal ion M(III) is either diamagnetic Ga(III) or paramagnetic Cr(III) (SCr = 3/2). The divalent metal ion M(II) is either diamagnetic Zn(II) or paramagnetic Ni(II) (SNi = 1). The three systems 1 (CrZn), 2 (GaNi) and 3 (CrNi) have been structurally and magnetically characterized through magnetic susceptibility measurements. For 1 the zero-field splitting is D = 0.6 cm−1 while for 2 the value D = 3.5 cm−1 was found. These values have been fixed in analyzing 3 which was found to be characterized by an antiferromagnetic interaction of JCrNi = −8.4 cm−1 (H = −2JSASB). These values served as benchmarks in the MCD analysis. The zero-field splitting of 1 and 2 was qualitatively recovered using a multi-wavelength analysis of the variable-temperature variable-field (VTVH) MCD data. The observed ligand field bands were assigned to individual d3 and d8 multiplets. Using an extension of an earlier developed theory of the nonlinear MCD response, the stunningly complex multiwavelength VTVH-MCD curves of 3 could be quantitatively reproduced with only the relative transition polarizations as input into the fitting procedure. Interestingly, the MCD bands of the minority spin Ni(II) ligand field bands were observed to change sign relative to the parent complex 2. This behavior has been analyzed. The present work hence provides a benchmark study for the application of MCD spectroscopy to weakly interacting transition metal dimers.