By employing boronato instead of carboxylato bridges in the dication [Mn 2IV(μ‐O)‐(μ‐PhBO2)2L2]2+ (L = 1,4,7‐trimethyl‐1,4,7‐triazacyclononane), a Mn 2IV unit has been stabilized for the first time. Together with the di‐μ‐carboxylato‐μ‐oxodimanganese complexes of analogous structure, there is now an “isostructural series” available containing Mn 2II, MnIIMnIII, Mn 2III, MnIIIMnIV, Mn 2IV units. The Mn K‐edge energy of the corresponding complexes increases linearly with increasing oxidation level. This is of importance for EXAFS investigations of metalloproteins containing manganese.

The mixed‐valent FeIIFeIII complex 1 (structure shown on the right; the dashed line indicates a weak hydrogen bond to one ClO 4− counterion) with the ground state St = 1/2 is the first functional model for the mixed‐valent form of the metalloprotein hemerythrin. Like the protein, 1 disproportionates autocatalytically in CH3CN to the FeIIFeII and FeIIIFeIII forms. L = 1,4,7‐trimethyl‐1,4,7‐triazacyclonane.

Hexadentate macrocyclic ligands containing a 1,4,7-triazacyclononane backbone and three N-bound pendent-arm phenolates form extremely stable neutral complexes with Fe III Cl 3 . The octahedral complexes [Fe III L] undergo three reversible one-electron oxidation processes to yield the mono-and dications, [FeL] and [FeL] 2 , which are stable in solution for hours, whereas the trications, [FeL] 3 , are only stable in solution on the time scale of a cyclic voltammetric experiment. These oxidations are shown to be ligand-rather than metal-centered. Three coordinated phenoxyl radicals are formed successively as shown conclusively by Mössbauer spectroscopy. The neutral, mono-, di-, and tricationic species each contain an octahedral, high-spin ferric ion (S Fe 5 2), which is intramolecularly, antiferromagnetically coupled to the spin (S 1 2) of the bound phenoxyl ligands to yield an S t 2 ground state for the monocation, and an S t 3 2 ground state for the dications as shown by EPR spectroscopy. The vibrations of the coordinated phenolate are observed by resonance Raman (RR) spectroscopy by excitation in resonance with the phenolate-to-iron charge-transfer (CT) transition above 500 nm or, alternatively, of the coordinated phenoxyl by excitation in resonance with the intraligand p 3p* transition at about 410 nm. Use of 18 O isotopomers selectively labeled at the phenolic oxygen allowed the identification of the C À O stretching and Fe À O stretching and bending modes. It is shown that the substitution pattern of phenolates and phenoxyls in their respective ortho and para positions and the charge of the complexes have a pronounced influence on the vibrational modes observed.

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