Exclusive Stabilization of cis-cis Conformation in a Flexible Urea-Bridged Diiron(III) Porphyrin Dimers upon Axial Coordination.

Herein, we report an explicit stabilization of the cis-cis conformer, for the first time, in a series of penta-coordinated diiron(III) porphyrin dimer linked by a conformationally flexible urea bridge. The synthesis, structure, and characterization of a new diiron(III)-μ-oxo porphyrin dimer have also been reported here, which serves as a precursor for the corresponding five-coordinate complexes. Treatment with phenol and thiophenol axial ligands affords a series of penta-coordinated diiron(III) complexes that explicitly adopt the cis-cis conformation despite the flexibility of the bridging framework. Detailed structural and spectroscopic investigations using single-crystal X-ray diffraction, EPR, 1H NMR, and Mössbauer spectroscopy demonstrated the high-spin nature of the reported complexes. Comparative studies reveal that the nature of the axial ligand, i.e., phenolato versus thiophenolato, exerts a significant influence on paramagnetic spin delocalization, electronic structure, and redox properties. DFT calculations further support the experimental findings and indicate that favorable interporphyrin interactions contribute substantially to the stabilization of the cis-cis arrangement relative to the other conformations within the urea-bridged scaffold.