Effects of ligand topology and iron coordination number on the electronic structure of FeIII-μ-oxo-CrIII complexes supported by tetramethylcyclam

⁵⁷Fe Mössbauer, electron paramagnetic resonance, and ⁵⁷Fe nuclear resonance vibrational spectroscopies are applied to characterize three different Fe^III-O-Cr^III complexes derived from the tetramethylcyclam (TMC, 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane) ligand. [(CH₃CN)(TMC)Fe^III-O_anti-Cr^III(OTf)₄(NCCH₃)] (1) features an anti configuration where the bridging O-atom is located on the opposite face of all four methyl groups of TMC, which leads to a six-coordinate iron center. Similarly, due to the presence of a pyridine appended to one of the methyl groups of TMC, [(TMC-Py)Fe^III-O_anti-Cr^III(OTf)₄(NCCH₃)] (3) also exhibits an anti configuration with a six-coordinate iron center. But for [(TMC)Fe^III-O_syn-Cr^III(OTf)₄(NCCH₃)] (2), the iron center is five-coordinate due to a syn configuration where the bridging O atom is on the same face of all four methyl groups of TMC. The detailed spectroscopic characterizations reported here reveal that all three complexes exhibit S = 1 spin ground states, which result from antiferromagnetic coupling between an S = 5/2 Fe^III center and an S = 3/2 Cr^III center. However, the detailed magnetic properties derived from Mössbauer and EPR measurements and vibrational properties of these complexes, particularly the symmetric and asymmetric Fe-O-Cr stretching modes, closely reflect the ligand topology difference (anti vs. syn) and the coordination number of the iron center. These spectroscopic properties are used to guide the density functional theory calculations to show how the ligand topology affects the electronic structures of these complexes. Thus, the current study provides a comprehensive geometric and electronic structure correlation of these unique hetero-dinuclear complexes.