The interaction between two Ag porphyrins, connected covalently through a highly flexible ethane bridge, in a metalloporphyrin dimer has been investigated upon stepwise oxidation. X-ray structure determination of one and two-electron oxidized complexes has clearly revealed only metal-centered oxidation that results in short Ag-N (porphyrin) distance with large distortion in the porphyrin macrocycle. The 2e-oxidized complex exhibits significant metallophilic interaction in the form of a close Ag ⋅⋅⋅Ag contact that brings two porphyrin rings more cofacial with syn-conformation, which would otherwise stabilize in an anti-form. The interaction also leads to an intense emission peak at 546 nm at 77 K in the photoluminescence study.

The synthesis, structure, and properties of bischloro, μ-oxo, and a family of μ-hydroxo complexes (with BF4 (-) , SbF6 (-) , and PF6 (-) counteranions) of diethylpyrrole-bridged diiron(III) bisporphyrins are reported. Spectroscopic characterization has revealed that the iron centers of the bischloro and μ-oxo complexes are in the high-spin state (S=(5) /2 ). However, the two iron centers in the diiron(III) μ-hydroxo complexes are equivalent with high spin (S=(5) /2 ) in the solid state and an intermediate-spin state (S=(3) /2 ) in solution. The molecules have been compared with previously known diiron(III) μ-hydroxo complexes of ethane-bridged bisporphyrin, in which two different spin states of iron were stabilized under the influence of counteranions. The dimanganese(III) analogues were also synthesized and spectroscopically characterized. A comparison of the X-ray structural parameters between diethylpyrrole and ethane-bridged μ-hydroxo bisporphyrins suggest an increased separation, and hence, less interactions between the two heme units of the former. As a result, unlike the ethane-bridged μ-hydroxo complex, both iron centers become equivalent in the diethylpyrrole-bridged complex and their spin state remains unresponsive to the change in counteranion. The iron(III) centers of the diethylpyrrole-bridged diiron(III) μ-oxo bisporphyrin undergo very strong antiferromagnetic interactions (J=-137.7 cm(-1) ), although the coupling constant is reduced to only a weak value in the μ-hydroxo complexes (J=-42.2, -44.1, and -42.4 cm(-1) for the BF4 , SbF6 , and PF6 complexes, respectively).

The binding of a series of substituted thiophenols as axial ligands on a highly flexible ethane-bridged diiron(III)bisporphyrin framework has been investigated as a model of diheme proteins. Spectroscopic characterization reveals a high-spin (S = 5/2) state of iron for all of the pentacoordinate thiophenolato complexes. In the UV-visible spectra of the complexes, the positions of the Soret and band I have been found to be dependent on the pKa of thiophenols. The alternating shift pattern, which has opposite sign of the chemical shifts for meta- vs. ortho- and para-protons in the (1)H NMR spectra, is attributed to negative and positive spin densities, respectively, on thiophenolate carbon atoms and is indicative of π-spin delocalization to the bound thiophenolate ligand. The Fe(III)/Fe(II) redox couple of the complexes bears a linear relationship with the pKa of thiophenol and is found to be positively shifted with decreasing pKa. The effect of the electronic nature of the substituent on the thiophenolate ring has also been demonstrated in which a large potential range of 540 mV was observed (in contrast to the value of only 270 mV in case of monoheme analogues) for the Fe(III)/Fe(II) redox couple on going from monoheme to diheme and is attributed to the interheme interaction. Also, the Fe(III)/Fe(II) redox potential of the thiophenolato complexes has been found to be more positively shifted compared to their phenolato analogues, which was further supported by DFT calculation. The addition of another thiophenol at the sixth axial position of the five-coordinate thiophenolato complex causes a change in iron spin from high (S = 5/2) to low (S = 1/2) along with a large positive shift of 490 mV for the Fe(III)/Fe(II) redox couple.

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The family of multiheme proteins constitutes one of the fascinating molecular machineries designed by Nature to execute a large variety of functions. A high level of conservation among the structural arrangement of heme units is evident among various multiheme cytochromes. The relative arrangement of the heme centers and the intermacrocyclic interactions therein have been found to exhibit a major role in functional properties of such a widely distributed family. The existence of more than one heme center provides an effective and efficient tool to modulate various structures and properties that are needed for its function. This Frontier overviews a brief account of our on-going efforts to examine some of the design principles in which the inter-heme distance and their relative orientations are appropriately chosen to elucidate, at the molecular level, the effects of heme-heme interactions and electronic communication in the synthetic dihemes.