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C34H57BBr6N6O4V2, orthorhombic, Pna21 (no. 33), a = 17.198(2) Å, b = 12.9497(11) Å, c = 20.496(2) Å, V = 4564.8(9) Å3, Z = 4, Rgt(F) = 0.0441, wRref(F2) = 0.1093, T = 183(2) K.

The IR spectra and normal-mode analysis of the adamantane-like compound [Mn4O6(bpea)4]n+ in two oxidation states, MnIV4 and MnIIIMnIV3, that are relevant to the oxygen-evolving complex of photosystem II are presented. Mn-O vibrational modes are identified with isotopic exchange, 16O->18O, of the mono-(mu)-oxo bridging atoms in the complex. IR spectra of the MnIIIMnIV3 species are obtained by electrochemical reduction of the MnIV4 species using a spectroelectrochemical cell, based on attenuated total reflection [Visser 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 MnIV4 species at 745 and 707 cm-1 and a weaker band at 510 cm-1. Upon reduction, the MnIIIMnIV3 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 MnIV4 and reduced MnIIIMnIV3 species. The calculated force constants for the MnIV4 species are = 3.15 mdynAngstrom, = 0.55 mdyn/Angstrom, and = 0.20 mdyn/Angstrom. The force constants for the MnIIIMnIV3 species are = 3.10 mdyn/Angstrom, = 2.45 mdyn/Angstrom, = 0.40, and = 0.15 mdyn/Angstrom. 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.

The synthesis and characterization of a series of adamantane-shaped [Mn4O6](super 4+) complexes is reported. A comparison of spectroscopic and redox properties is presented as well as X-ray structure determinations for two members of the series.

A study is conducted towards the synthesis and characterization of Mn-oxo complexes to understand the structural and physicochemical properties of the WO site in PSII. A major emphasis is put towards complexes that had direct or indirect relevance to the PSII Mn (sub 4) site.

The IR spectra and normal-mode analysis of the adamantine-like compound [Mn4O6(bpea)(sub 4)](super n+)(bpea=N,N-bis(2-pyridylmethyl)ethylamine) in two oxidation states, Mn(super IV)(sub 4) and Mn(super III)Mn(super IV)(sub 3), that are relevant to the oxygen-evolving complex of photosystem II are presented. A novel method of subtraction is used to reduce background contributions from solvent and ligand modes.

He came to Dick's lab as a postdoc, having received a Ph.D. with Animesh Chakravorty, who himself had been a postdoc with Holm in the 1960's. Dick had moved his lab relatively recently from M.I.T., but he was well settled into lab space that would later be taken over by Ed Solomon when Dick moved once again in 1980 back to Harvard University. B My Memories of Dick Holm: a Wizard of Inorganic Synthesis, a Pioneer in Bioinorganic Chemistry, and a Connoisseur of the Written Word b by Stas Groysman I worked in the Holm laboratory at Harvard as a postdoc during the concluding part of his career. My outstanding undergraduate student at Berkeley, Mike Scott, went on to do his Ph.D. with Dick and one of my graduate students - Joel Gohdes - carried out postdoctoral research in Dick's lab. Dick's Ph.D. work with Al primarily concerned the magnetic properties of tetrahedral Co(II) complexes[[3], [5]] and the structures and properties of acetylacetonate coordination complexes.[[7], [9]] Upon finishing work in Cotton's lab in 1961, Dick took a position as instructor of inorganic chemistry at Harvard University. [Extracted from the article]

Research was conducted to investigate the proton NMR spectra of a series of manganese-oxo aggregates yielding a characteristic signature for each complex. Data for the derivatives support a coordination geometry-dependent pathway for spin delocalization. Data derived from proton NMR spectra also leads to the conclusion that the solution-state structures of all members of the series are the same as the reported solid-state structures.

The structure and mechanism of oxygen-evolving complex (OEC) were studied through the synthesis and properties of the adamantane-like cluster [Mn4O6-(bpea)4]4+ (1) and its analog, [Mn4O6-(bpea)4]3+ (2). OEC have distinct manganese-oxo aggregated involved in the catalysis of water oxidation to dioxygen. It was determined that complex 1 presented various reactivities affecting substrate oxidation at the active site. It was also capable of subsite differentiation of the oxo-bridges attributed from the steric and electronic characteristics of the ligand.

A study of 55Mn electron spin echo-electron nuclear double resonance (ESE-ENDOR) for (di-mu-oxotetrakis(2,2'-bipyridine)dimanganese(III,IV)) indicates that large hyperfine interactions are predominant for Mn(III) and Mn(IV) ENDOR spectra. The ENDOR spectra at the outermost pairs of lines in EPR spectrum can be easily resolved, whereas the spectra in the central region contains multiple 55Mn nuclear spin transitions. The ENDOR data on the g = 2 Mn multiline EPR signal of the S2 state indicates that the EPR signal does not arise from Mn(III)-Mn(IV) center.

An asymmetric mixed-valence dinuclear manganese complex containing the Mn2(mu-O)(sub 2)(mu-O2CCH3)(super 2+) unit has been synthesized using the ligand N,N-bis(2-pyridylmethyl)ethylamine. The ligands binds facially to both metal centers. Additionally, the tridentate ligands in these dihalodimanganese (IV) complexes display a meridional mode. A strong antiferromagnetic spin coupling is displayed by the mixed-valence (III-IV) and dichloro (IV,IV) complexes.

Structurally-disrupted di-mu-oxo (MnIIIMnIV) dimers and superoxidized L. plantarum manganese catalase and azide-perfused manganese catalase were examined in relation to their electronic absorption and magnetic circular dichroism spectra. 10 shifts in optical spectra are identified on the basis of the study of intensity, bandshape, sign and energy of each spectral characteristic.

Protonation of oxo-bridged diiron(II) analogues in ether solution yields hydroxo-bridged complexes having ester bridges of carboxylate or phosphate. These acetate or phosphate-bridged hydroxo complexes are found in metalloprotein interoferrin and beef spleen purple acid phosphatase. The dimetallic core in the hydroxo-bridged analogues is expanded by the protonation of the bridging oxygen atom.

Evidence supporting the formation of two different products from the reaction of the dimanganese (III,IV) complex (Mn2O2(O2CCH3)(tpen))2+ in aqueous solution depending on the pH of the reaction medium is presented. A part of the complex is converted to the IV,IV product Mn2O2(O2CCH3)(tpen)2+ in acidic solution. The species Mn3O4(OH)(tpen)2(mu-tpen)6+ is formed in basic solution.

The electronic structure description of the tetrameric dimer-of-dimers complex ((Mn2O2)2(tphpn)2)4+ was reported. This complex displays a parallel polarization electron paramagnetic resonance signal that resembles that associated with the S1 state of the photosystem II manganese water oxidation catalyst. Each Mn(III, IV) dimer is strongly antiferromagnetically exchanged coupled with S' = 1/2, and the two dimers are weakly exchange coupled resulting in a low-lying singlet and triplet state.

The reaction of chromium(III) chloride and picolinic acid in aqueous solution results in two products which have been structurally characterized. The red monomeric species is identified as the meridional isomer of Cr(pic)3, not the facial isomer. The purple binuclear species is (Cr(pic)2OH)2. The magnetic exchange interaction of this species is analyzed in terms of the Glerup-Hodgson-Pedersen model which relates structural and magnetic properties for the dihydroxo-bridged Cr(III) dimers.

The methanol reaction of N,N-bis(2-pyridylmethyl)ethylamine (bpea) with Mn(O2CCH3)3.2H2O in an aqueous solution of HClO4 results in the isolation of the binuclear species (Mn2O2 O2CCH3 bpea 2)(ClO4)3 in 41% yield. Subsequent addition of aqueous NaClO4 forms the trinuclear species (Mn3O4 OH bpea 3)(ClO4)3 in 90% yield. The binuclear species has a (Mn super IV 2 u-O 2- O2CCH3)3+ core while the trinuclear species features a (Mn3O4)4+ core. The EPR spectral analysis shows that the trinuclear species exhibits S = 3/2 ground state properties. Both species have been subjected to infrared, voltammetry and magnetic susceptibility measurements.

The reaction of Mn(O2CCH3)3.2H2O with the ethylene analogue of N,N,N',N'-tetrakis(2-pyridylmethyl)-1,2-propanediamine (tpen) results in the formation of Mn2O2(O2CCH3)(tpen)(ClO4)2.3H2O. The resulting compound exhibits a face-capped (Mn2O2)3+ core structural group in its structure. In addition, the presence of the core structure in the resulting compound leads to the production of syn-oriented coordination sites which contain bridging acetate ligands.

Air oxidation of a solution of N,N'-bis(salicylidene)-1,3-diaminopropane (H2salpn), sodium hydroxide and manganese perchlorate hexahydrate, followed by ether treatment, yields the mononuclear Mn(salpn)(EtOH)2ClO4 complex. Slow cooling before ether treatment yields Mn(salpn)(EtOH)2(ClO4)-EtOH-H2O, which crystallizes into a triclinic form with two independent Mn(III) monomers per asymmetric unit. The bis(mu-oxo)dimanganese(IV) form results from further oxidation of the monomer in basic methanol or from air oxidation of Mn(salpn)(H2O) in pyridine. The structure and magnetic/electrochemical properties of these complexes are presented.