Tuning tetranuclear manganese-oxo core electronic properties: adamantane-shaped complexes synthesized by ligand exchange.

A series of adamantane-shaped [Mn4O6]4+ aggregates has been prepared. Ligand substitution reactions of [Mn4O6(bpea)4](ClO4)4 (1) with tridentate amine and iminodicarboxylate ligands in acetonitrile affords derivative clusters [Mn4O6(tacn)4](ClO4)4 (4), [Mn4O6(bpea)2(dien)2](ClO4)4)(5), [Mn4O6(Medien)4](ClO4)4 (6), [Mn4O6(tach)4](ClO4)4 (7), [Mn4O6(bpea)2(me-ida)2] (8), [Mn4O6(bpea)2(bz-ida)2] (9), [Mn4O6(bpea)2((t)bu-ida)2] (10), and [Mn4O6(bpea)2((c)pent-ida)2] (11) generally on the order of 10 min with retention of core nuclearity and oxidation state. Of these complexes, only 4 had been synthesized previously. Characterization of two members of this series by X-ray crystallography reveals that compound 7 crystallizes as [Mn4O6(tach)4](ClO4)4 x 3CH3CN x 4.5H2O in the cubic space group Fmm and compound 11 crystallizes as [Mn4O6(bpea)2((c)pent-ida)2].7MeOH in the monoclinic space group C2/c. The unique substitution chemistry of 1 with iminodicarboxylate ligands afforded asymmetrically ligated complexes 8-11, the mixed ligand nature of which is most likely unachievable using self-assembly synthetic methods. A special feature of the iminodicarboxylate ligand complexes 8-11 is the substantial site differentiation of the oxo bridges of the [Mn4O6]4+ cores. While there are four site-differentiated oxo bridges in 8, the solution structural symmetry of 8H+ reveals essentially a single protonation isomer, in contrast to the observation of two protonation isomers for 1H+, one for each of the site-differentiated oxo bridges in 1. Magnetic susceptibility measurements on 4, 7, 8, and 9 indicate that each complex is overall ferromagnetically coupled, and variable-field magnetization data for 7 and 9 are consistent with an S = 6 ground state. Electrochemical analysis demonstrates that ligand substitution of bpea affords accessibility to the Mn(V)(Mn(IV))3 oxidation state.

FTIR spectra and normal-mode analysis of a tetranuclear manganese adamantane-like complex in two electrochemically prepared oxidation states: relevance to the oxygen-evolving complex of photosystem II.

The IR spectra and normal-mode analysis of the adamantane-like compound [Mn(4)O(6)(bpea)(4)](n+) (bpea = N,N-bis(2-pyridylmethyl)ethylamine) in two oxidation states, Mn(IV)(4) and Mn(III)Mn(IV)(3), that are relevant to the oxygen-evolving complex of photosystem II are presented. Mn-O vibrational modes are identified with isotopic exchange, (16)O-->(18)O, of the mono-micro-oxo bridging atoms in the complex. IR spectra of the Mn(III)Mn(IV)(3) species are obtained by electrochemical reduction of the Mn(IV)(4) species using a spectroelectrochemical cell, based on attenuated total reflection [Visser, H.; et al. Anal. Chem. 2001, 73, 4374-4378]. A novel method of subtraction is used to reduce background contributions from solvent and ligand modes, and the difference and double-difference spectra are used in identifying Mn-O bridging modes that are sensitive to oxidation state change. Two strong IR bands are observed for the Mn(IV)(4) species at 745 and 707 cm(-1), and a weaker band is observed at 510 cm(-1). Upon reduction, the Mn(III)Mn(IV)(3) species exhibits two strong IR bands at 745 and 680 cm(-1), and several weaker bands are observed in the 510-425 cm(-1) range. A normal-mode analysis is performed to assign all the relevant bridging modes in the oxidized Mn(IV)(4) and reduced Mn(III)Mn(IV)(3) species. The calculated force constants for the Mn(IV)(4) species are f(r)(IV)= 3.15 mdyn/A, f(rOr) = 0.55 mdyn/A, and f(rMnr) = 0.20 mdyn/A. The force constants for the Mn(III)Mn(IV)(3) species are f(r)(IV)= 3.10 mdyn/A, f(r)(III)= 2.45 mdyn/A, f(rOr) = 0.40 mdyn/A, and f(rMnr) = 0.15 mdyn/A. This study provides insights for the identification of Mn-O modes in the IR spectra of the photosynthetic oxygen-evolving complex during its catalytic cycle.

Proton NMR Study of Oxo-Bridged Dimanganese(III) Complexes: Solution State Structures and an Isotropic Shift/Magnetic Exchange Correlation.

The (1)H NMR spectra of a series of manganese-oxo aggregates have been examined, and a characteristic signature was found for each complex. For the dimanganese(III,III) complexes [Mn(2)O(OAc)(2)(HB(pz)(3))(2)], [Mn(2)O(OAc)(2)(tacn)(2)](2+), [Mn(2)O(OAc)(2)(H(2)O)(2)(bpy)(2)](2+), and [Mn(2)O(OAc)(2)(bpta)(2)](2+) (HB(pz)(3) = hydrotris(pyrazol-1-yl)borate; tacn = 1,4,7-triazacyclononane; bpy = 2,2'-bipyridine, and bpta = N,N-bis(2-pyridylmethyl)-tert-butylamine), the (1)H NMR spectra reveal a resonance associated with acetate, found downfield between 58 and 80 ppm, and a generally well resolved set of terminal ligand resonances which can be divided into two classes: those resonances associated with pyridyl or pyrazolyl ring protons and those of methylene groups. A number of the pyridine ring resonances have been unambiguously assigned by the examination of methyl-substituted derivatives. Data for these derivatives also support a coordination geometry-dependent pathway for spin delocalization. Moreover, interpretation of the (1)H NMR spectra leads to the conclusion that the solution-state structures of all members of the series are the same as the reported solid-state structures. A strong linear correlation between the magnetic coupling constant (J) and the isotropic shift of the acetate resonance was observed within this series of {Mn(2)O(OAc)(2)}(2+) core complexes. Furthermore, comparisons of the acetate proton isotropic shift ratio (DeltaH(Mn)/DeltaH(Fe)) to the ratio of the squared effective magnetic moments &mgr;(eff)(2)(Mn)/ &mgr;(eff)(2)(Fe) for complexes with the {M(2)O(OAc)(2)}(2+) core (where M = Mn(3+) or Fe(3+)) revealed excellent agreement (within 10%) between these two quantities.