William H. Armstrong

Sterically crowded manganese-oxo complexes of N,N-bis(2-pyridylmethyl)-tert-butylamine with bridged binuclear core types {MnIII2(μ-O)(μ-OAc)2} and {MnIV2(μ-O)2(μ-OAc)}

Abstract Reactions of Mn(O2CCH3)3 · 2H2O with N,N-bis(2-pyridylmethyl-tert-butylamine (bpta) in ethanol followed by the addition of excess NaClO4 or 70% aqueous HClO4 afforded complexes [Mn2O(O2CCH3)2(bpta)2](ClO4)2 (1) or [Mn2O2(O2CCH3)(bpta)2](ClO4)3 (2), respectively. Both 1 and 2 were characterized by X-ray crystallographic studies. Compound 1 crystallizes with two acetonitriles of solvation in the space group C2/c with a = 37.5526(6) A , b = 10.6000(2) A , c = 24.1931(4) A , β = 91.5190(10)°, V = 9626.9(3) A 3 , and Z = 8 . The two manganese atoms in 1 are bridged by an O2− ligand and two CH3CO2− groups. One facially coordinated tridentate bpta ligand, with the aliphatic nitrogen positioned trans to an acetate oxygen atom, completes a distorted octahedral geometry at each manganese center. Structural parameters for 1 are consistent with both manganese ions being in the + 3 formal oxidation state. The MnMn separation in 1 is 3.1719(5) A. The cyclic voltammogram of a pure sample of 1 in CH3CN shows III,III/III,IV oxidation ( i p a i p c = 1.6 ) and III,III/III,II reduction (irreversible) waves at + 0.88 and −0.28 V, respectively, versus the ferrocene/ferrocenium (Fc/Fc+) couple. UV-Vis and IR spectroscopic properties of 1 are consistent with the presence of a {MnIII2(μ-O)(μ-OAc)2}2+ core. Compound 2 crystallizes as an acetonitrile solvate in the space group P21/n with a = 11.536 80(10) A , b = 10.2865(2) A , c = 36.9831 (4) A , β = 98.835(12)°, V = 4336.96(10) A 3 , and Z = 4. Bond distances and angles for 2 are generally similar to those of other complexes containing a {MnIV2(μ-O)2(μ-OAc)}3+ and polypyridyl ligands. UV-Vis and IR properties of 2 are also reported. Cyclic voltammetry waves for 2 in MeCN, assigned as IV,IV/III,IV and III,IV/III,III reductions, were observed at +0.60( i p a i p c = 1) and −0.55 V (i p c = 1) versus Fc/Fc, respectively. The effects of the steric size of the t-butyl group are manifested in longer Mn-Nalkyl distances for both 1 and 2. This elongation results in markedly more positive first reduction potentials for both 1 (III,III/III,II) and 2 (IV,IV/IV,III) relative to their N-ethyl analogs. Finally, the reactions of 1 and 2 in water are dramatically different as compared to those of the same analogs.

A novel triply bridged dinuclear manganese (III) complex containing the [Mn2O(OAc)2]2+ core: synthesis, crystal structure and properties of [Mn2(μ-O)(μ-OAc)2(bpea)2](ClO4)2☆

Abstract The reaction of Mn(O 2 CCH 3 ) 3 ·2H 2 O with the ligand N , N -bis(2-pyridylmethyl)ethylamine (bpea) in a 1:1 ratio in methanol followed by the addition of NaClO 4 afforded the title compound [Mn 2 O(O 2 CCH 3 ) 2 (bpea) 2 ](ClO 4 ) 2 ( 1 ) in 75% yield. Compound 1 was characterized by using several physical methods including single-crystal X-ray diffractometry. Compound 1 ·0.5H 2 O crystallizes in the orthorhombic space group Pbca (No. 61) with a = 38.780(2), b = 22.058(4), c = 19.719(5) A , V = 16 868(8) A 3 and Z = 16. There are two independent equivalents of compound 1 in the crystallographic asymmetric unit and there is a single unique water molecule in the crystal lattice. Both manganese atoms in each of the independent dinuclear units are in the +3 oxidation state and they are bridged by one oxo ligand (O 2− ) and two acetate groups. Each manganese atom is also bound to one facially-coordinating tridentate bpea ligand, with the aliphatic nitrogen donor situated trans to an acetate oxygen atom. Deviations from idealized octahedral symmetry at each manganese center are consistent with expectations for a d 4 electronic configuration. Compound 1 has an average Mn…Mn separation of 3.11 A and an average Mn(μ-O)Mn angle of 122.7°. The electronic absorption spectrum of 1 in CH 3 CN is very similar to other compounds that contain the {Mn 2 (μ-O)(μ-O 2 CCH 3 ) 2 } 2+ core. A cyclic voltammogram of 1 in CH 3 CN exhibits an oxidation wave (III,III to III,IV) at +0.66 V and a reduction wave (III,III to III,II) at −0.39 V versus the ferrocene/ferrocenium (Fc/Fc + ) couple. The IR spectrum of 1 reveals two strong bands at 1566 and 1432 cm −1 corresponding to the bridging acetate groups present in the compound. The X-band electron paramagnetic resonance (EPR) spectrum of 1 in dry CH 3 CN at 77 and 4 K is featureless. Compound 1 disproportionates in an H 2 O/CH 3 CN solvent mixture to give the (III,IV) binuclear species [Mn 2 O 2 (O 2 CCH 3 )(bpea) 2 ] 2+ , which displays a characteristic 16-line g = 2 EPR signal.

Pulsed 1H and 55Mn ENDOR studies of dinuclear Mn(III)Mn(IV) model complexes

The high resolution pulsed EPR technique of ESE-ENDOR is applied to bis-μ-oxo dinuclear Mn(III)Mn(IV) model complexes ligated by either 2,2′-bipyridine (bipy) or 1,10-phenanthro-line (phen). Such complexes, which model a building block of which the Mn4 cluster of the oxygen evolving complex of photosystem II is thought to be contracted, are strongly anti-ferromagnetically coupled, class II mixed valence compounds. The 1H hyperfine couplings between ligand protons and the dinuclear spin centre reflect significant couplings to each Mn ion: a situation which requires a more complex analysis than that used in the case of a simple single point-dipole. 55Mn ENDOR spectra of phen recorded at field positions across the EPR envelope reveal its Mn hyperfine and quadrupole coupling constants. These ENDOR spectra are discussed in relation to a general analysis of 55Mn ENDOR spectra, including the importance of angle and m I selection, in addition to obtaining ENDOR spectra across the EPR envelope in cases where 55Mn ...

A novel vanadium(V) homocitrate complex: synthesis, structure, and biological relevance of [K2(H2O)5][(VO2)2(R,S-homocitrate)2]·H2O

Abstract Initial investigations into the possible roles of homocitric acid in the biosynthesis and function of the active site cofactor of nitrogenase resulted in the isolation and characterization of the dinuclear vanadium(V) species [K2(H2O)5][(VO2)2(R,S-C7H8O7)2]·H2O ( 1). Complex 1 represents the first synthetic structurally characterized transition metal homocitrate complex and may represent an early mobilized precursor in the biosynthesis of VFeco. Compound 1 was characterized by a variety of physical methods, including X-ray crystallography. Crystal data: space group P * (#2), with a = 10.292 (3) Å, b = 16.663 (3) Å, c = 8.343 (1) Å, α = 95.93 (1)°, β = 105.74 (2)°, γ = 90.86 (2)°, V = 1386 (1) Å3, and Z = 2. The homocitrate ligand is coordinated to the vanadium(V) atoms in a bidentate fashion via the deprotonated bridging hydroxyl group and a carboxylate donor. This unique coordination mode accurately mimics the coordination of homocitrate to the cofactor of nitrogenase.

Illumination with Ultraviolet or Visible Light Induces Chemical Changes in the Water-soluble Manganese Complex, [Mn4O6(bpea)4]Br4

We measured the photosensitivity of an artificial tetranuclear oxo–Mn(IV) complex, [Mn4O6(bpea)4]Br4, which has an adamantane‐shaped {Mn4O6}4+ core. Illumination caused changes in the absorption spectrum of the compound consistent with a one‐electron reduction in the compound. Bromide appears to be the most probable electron donor in the reaction system. Chemical modification of the cluster appears to destabilize it, predisposing it to reductive degradation. UV light was more efficient than visible light in causing the changes. The data support the suggestion that the natural oxygen‐evolving Mn complex is photosensitive and can oxidize components of the oxygen‐evolving complex in its excited state causing photoinhibition, and that photostability is an important issue in designing Mn complexes for artificial photosynthesis. Furthermore, light‐induced oxidation of bromide by [Mn4O6(bpea)4]4+ may suggest that oxidation of chloride is involved in natural water splitting or has been involved during the evolution of the water‐splitting enzyme.

Theoretical analysis of the [Mn2(μ-oxo)2(μ-carboxylato)2]+ core

The first example of a dinuclear manganese complex containing two oxo and two carboxylate bridges, [Mn2(μ-O)2(μ-O2CArTol)2(bpy)2]+ (where bpy = 2,2′-bipyridine, and ArTolCO2− = 2,6-di(p–tolyl)benzoate), was reported recently (J. Am. Chem. Soc. 2003, 125, 13010). X-ray crystallographic analysis performed on this complex reveals a trapped mix-valence species as evidenced, for example, by very different metal–ligand bond distances at the MnIII and MnIV centers. The fact that there are rather bulky bridging carboxylate ligands present in this recently reported dinuclear species raises the question as to whether they affect the extent of valence trapping and the metrical parameters in general. Specifically, it was thought that intramolecular nonbonded contacts could play an important role. In the work reported here, density functional theory calculations were used to address this issue. Structural parameters obtained from calculations on a model compound bearing sterically small bridging carboxylates, [Mn2(μ-O)2(μ-O2CH)2(bpy)2]+, are in good agreement with the experimentally determined single crystal X-ray structure. Thus, the sterically large carboxylate bridges in [Mn2(μ-O)2(μ-O2CArTol)2(bpy)2]+ appear not to have a significant effect on the metal–ligand bond distances and angles. There is calculated to be minimal Mn⋯Mn bonding despite contraction of the Mn⋯Mn distance relative to related complexes. In addition to calculations on the mixed-valence MnIIIMnIV complex, various electronic configurations of the corresponding MnIIIMnIII and MnIVMnIV complexes are explored. Although our calculations support assignment of [Mn2(μ-O)2(μ-O2CH)2(bpy)2]+ as a valence-trapped MnIIIMnIV configuration involving high-spin MnIII, a delocalized configuration arising from low-spin MnIII is calculated to lie very close in energy. The energetic proximity of the delocalized configuration is attributed to an effective crossed-exchange mechanism, which permits mixing of an eg-based orbital (nominally on high-spin MnIII) with a vacant t2g-based orbital (nominally on MnIV).

Toward synthetic models for high oxidation state forms of the photosystem II active site metal cluster: the first tetranuclear manganese cluster containing a [Mn4(μ-O)5]6+ core

The first tetrameric high valent manganese complex consisting of a MnIV4(mu-O)5 bridged core, [Mn4(mu-O)5(dmb)4(dmbO)2](ClO4)4, [symbol: see text] was isolated via dimanganese (III,IV) and (IV,IV) intermediates in presence of the oxidant tert-butyl hydroperoxide and was characterized by X-ray crystallography, electrochemistry, infrared, UV-visible, 1H NMR, and mass spectroscopy; the structure found differs greatly from a proposal for the putative Mn4O5 aggregate found in Photosystem II.

Aggregation/deaggregation processes in vanadium(II) carboxylate chemistry

Reactions of potassium tris(3,5-dimethylpyrazolyl)borate (KTpbMe2) with the low-valent mixed-metal complex [(thf)3V(µ-Cl)(µ-tfa)2V(thf)3][ZnCl3(thf)] resulted first in ZnCl+ abstraction and tetramer formation, then in asymmetric binuclear product formation through ligand substitution.