Publications (5)14.51 Total impact
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Article: Tuning tetranuclear manganese-oxo core electronic properties: adamantane-shaped complexes synthesized by ligand exchange.
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ABSTRACT: 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.Inorganic Chemistry 08/2005; 44(14):5161-75. · 4.60 Impact Factor -
Article: 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.
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ABSTRACT: 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.Journal of the American Chemical Society 09/2002; 124(37):11008-17. · 9.91 Impact Factor -
Article: A Spin Topological Model for the g = 4.1 S2 State Photosystem II Water Oxidase Manganese Aggregate
03/1999; -
Article: Tetranuclear Manganese-Oxo Aggregates Relevant to the Photosynthetic Water Oxidation Center. Crystal Structure, Spectroscopic Properties and Reactivity of Adamantane-Shaped [Mn4O6(bpea)4]4+ and the Reduced Mixed-Valence Analog [Mn4O6(bpea)4]3+
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ABSTRACT: The tetranuclear MnIV adamantane-like complex [Mn4O6(bpea)4](ClO4)4(1(ClO4)4) was isolated from a comproportionation reaction of Mn(ClO4)2·6H2O and [n-Bu4N][MnO4] with the ligand N,N-bis(2-pyridylmethyl)ethylamine (bpea) in acetonitrile. Characterization by X-ray crystallography reveals that the [Mn4O6(bpea)4]4+ cation approaches S4 point symmetry. There are three distinct types of Mn−N bonds and two types of bridging oxo ligands in 1. 1H NMR protonation studies of 1 in acetonitrile reveal that each type of oxo bridge renders a different protonation isomer and that the isomers are readily distinguished by their differences in solution structural symmetry and oxo bridge acidity. In addition, pH-dependent aqueous electrochemical studies show that the proton-coupled electron-transfer behavior of 1 is significant because it is the first example of a Mn-oxo aggregate exhibiting a e-/2H+ stoichiometry and because it displays pH-dependent e-/H+ stoichiometry. Remarkably, reaction of 1(BF4)4 with the tridentate nitrogen donor ligand 1,4,7-trimethyl-1,4,7-triazacyclononane (Me3tacn) in acetonitrile affords the one-electron reduced complex [Mn4O6(bpea)4](BF4)3 (2(BF4)3), rather than the anticipated ligand substituted product. Like 1, the [Mn4O6(bpea)4]3+ cation also contains the adamantane skeleton. The (MnIII)(MnIV)3 cluster of 2 contains a crystallographically distinguishable MnIII ion, as noted by elongation along the Nalkyl-Mn−Ooxo axis. The 1H NMR solution spectrum of 2 is consistent with a valence-delocalized (MnIII)(MnIV)3 cluster, indicating fast intramolecular electron transfer on the NMR time scale, and redox titration of 1 to 2 indicates slow intermolecular electron transfer on the same time scale. Solution magnetic susceptibility measurements in acetonitrile show that conversion of 1(ClO4)4 to 2(ClO4)3 is attendant with a change from net ferromagnetic coupling to overall moderate antiferromagnetic coupling within the manganese-oxo core. Isolation of the {Mn4O6} core in the (IV)4 and (III)(IV)3 oxidation states is facilitated by the relatively weak donor nature of the bpea ligand, in marked contrast to the strongly basic donor ligands 1,4,7-triazacyclononane (tacn) and 1,1,1-tris(aminomethyl)ethane (tame), which stabilize the higher oxidation state of the {Mn4O6} cluster, making the one-electron reduced form less accessible. The novel protonation and electrochemical properties of 1 are discussed in the context of the Kok cycle of photosynthetic water oxidation.04/1998; -
Article: Multiple Reversible Protonations of the Adamantane-Shaped {Mn4O6}4+ Core: Detection of Protonation Stereoisomers at the {Mn4O4(OH)2}6+ Level
11/1996;
