A set of four [Mn(II)(L(7)py(2)(R))](2+) complexes, supported by the tetradentate 1,4-bis(2-pyridylmethyl)-1,4-diazepane ligand and derivatives with pyridine substituents in the 5 (R = Br) and 6 positions (R = Me and MeO), are reported. X-ray crystal structures of these complexes all show the L(7)py(2)(R) ligands bound to give a trans complex. Treatment of these Mn(II) precursors with either H(2)O(2)/Et(3)N or KO(2) in MeCN at -40 degrees C results in the formation of peroxomanganese complexes [Mn(III)(O(2))(L(7)py(2)(R))](+) differing only in the identity of the pyridine ring substituent. The electronic structures of two of these complexes, [Mn(III)(O(2))(L(7)py(2)(H))](+) and [Mn(III)(O(2))(L(7)py(2)(Me))](+), were examined in detail using electronic absorption, low-temperature magnetic circular dichroism (MCD) and variable-temperature variable-field (VTVH) MCD spectroscopies to determine ground-state zero-field splitting (ZFS) parameters and electronic transition energies, intensities, and polarizations. DFT and TD-DFT computations were used to validate the structures of [Mn(III)(O(2))(L(7)py(2)(H))](+) and [Mn(III)(O(2))(L(7)py(2)(Me))](+), further corroborating their assignment as peroxomanganese(III) species. While these complexes exhibit similar ZFS parameters, their low-temperature MCD spectra reveal significant shifts in electronic transition energies that are correlated to differences in Mn-O(2) interactions among these complexes. Taken together, these results indicate that, while the [Mn(III)(O(2))(L(7)py(2)(H))](+) complex exhibits symmetric Mn-O(peroxo) bond lengths, consistent with a side-on bound peroxo ligand, the peroxo ligand of the [Mn(III)(O(2))(L(7)py(2)(Me))](+) complex is bound in a more end-on fashion, with asymmetric Mn-O(peroxo) distances. This difference in binding mode is rationalized in terms of the greater electron-donating abilities of the methyl-appended pyridines and suggests a simple way to modulate Mn(III)-O(2) bonding through ligand perturbations.

Three peroxomanganese(III) complexes [Mn(III)(O(2))(mL(5)(2))](+), [Mn(III)(O(2))(imL(5)(2))](+), and [Mn(III)(O(2))(N4py)](+) supported by pentadentate ligands (mL(5)(2) = N-methyl-N,N',N'-tris(2-pyridylmethyl)ethane-1,2-diamine, imL(5)(2) = N-methyl-N,N',N'-tris((1-methyl-4-imidazolyl)methyl)ethane-1,2-diamine, and N4py = N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine) were generated by treating Mn(II) precursors with H(2)O(2) or KO(2). Electronic absorption, magnetic circular dichroism (MCD), and variable-temperature, variable-field MCD data demonstrate that these complexes have very similar electronic transition energies and ground-state zero-field splitting parameters, indicative of nearly identical coordination geometries. Because of uncertainty in peroxo (side-on η(2) versus end-on η(1)) and ligand (pentadentate versus tetradentate) binding modes, density functional theory (DFT) computations were used to distinguish between three possible structures: pentadentate ligand binding with (i) a side-on peroxo and (ii) an end-on peroxo, and (iii) tetradentate ligand binding with a side-on peroxo. Regardless of the supporting ligand, isomers with a side-on peroxo and the supporting ligand bound in a tetradentate fashion were identified as most stable by >20 kcal/mol. Spectroscopic parameters computed by time-dependent (TD) DFT and multireference SORCI methods provided validation of these isomers on the basis of experimental data. Hexacoordination is thus strongly preferred for peroxomanganese(III) adducts, and dissociation of a pyridine (mL(5)(2) and N4py) or imidazole (imL(5)(2)) arm is thermodynamically favored. In contrast, DFT computations for models of [Fe(III)(O(2))(mL(5)(2))](+) demonstrate that pyridine dissociation is not favorable; instead a seven-coordinate ferric center is preferred. These different results are attributed to the electronic configurations of the metal centers (high spin d(5) and d(4) for Fe(III) and Mn(III), respectively), which results in population of a metal-peroxo σ-antibonding molecular orbital and, consequently, longer M-O(peroxo) bonds for peroxoiron(III) species.

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Deposition of CH3ReO3 onto the surface of dehydrated, amorphous silica-alumina generates a highly active, supported catalyst for the metathesis of olefins. However, silica-alumina with a high (10 wt %) Re loading is no more active than silica-alumina with low (1 wt %) loading, while CH3ReO3 on silica is completely inactive. Catalysts prepared by grafting CH3ReO3 on silica-alumina contain two types of spectroscopically distinct sites. The more strongly bound sites are responsible for olefin metathesis activity and are formed preferentially at low Re loadings (< or =1 wt %). They are created by two Lewis acid/base interactions: (1) the coordination of an oxo ligand to an Al center of the support and (2) interaction of one of the adjacent bridging oxygens (AlOSi) with the Re center. At higher Re loadings (1-10 wt %), CH3ReO3 also interacts with surface silanols by H-bonding. This gives rise to highly mobile sites, most of which can be observed by 13C solid-state NMR even without magic-angle spinning. Their formation can be prevented by capping the surface hydroxyl groups with hexamethyldisilazane prior to grafting CH3ReO3, resulting in a metathesis catalyst that is more selective, more robust, and more efficient in terms of Re use.

Five new pseudotetrahedral nickel(II) arylthiolate complexes Tp (R,Me)Ni-SR' [(Tp (R,Me)) (-) = 2,2,2-kappa (3)-hydridotris(3-R,5-methylpyrazolyl)borate; R = Me, R' = C 6H 5 (Ph), 2,4,6-C 6H 2(CH 3) 3 (Mes); R = Ph, R' = C 6H 5 (Ph), 2,4,6-C 6H 2(CH 3) 3 (Mes), and 2,6-C 6H 3(CH 3) 2 (Xyl)] were prepared by metathesis reactions of known chloride complexes with sodium arylthiolate salts in THF. The new products were fully characterized. The effect of increasing bulk of substituents at the proximal 3-pyrazolyl and ortho-thiolate positions represented in this series was evident in spectroscopic studies (UV-vis-NIR, (1)H NMR) of the product complexes. Increased steric contact induced red-shifting of nickel-thiolate ligand to metal charge transfer (LMCT) bands and enhanced contact shifts of arylthiolate protons with the paramagnetic ( S = 1) nickel(II) ion. These spectroscopic effects arise from structural distortion of the nickel(II)-thiolate bond revealed by X-ray crystal structure determinations of the structural extremes of the series, Tp (Me,Me)Ni-SPh and Tp (Ph,Me)Ni-SXyl. The distortion consists of a significantly increased tilting of the Ni-S bond from an ideal trigonal axis and increased linearity of the Ni-S-R angle that alters covalency of the Ni-S coordinate bond. Reactivity of the nickel-thiolate linkage toward electrophilic alkylation with MeI is also significantly affected, showing enhanced rates according to two distinct competing mechanisms, direct bimolecular alkylation of intact complex and rate-limiting unimolecular dissociation of free thiolate. Possible biochemical relevance of these observations to tetrahedral nickel(II) centers in metalloenzymes is considered.

The electronic structures of the bis(hydroxo)manganese(IV) and oxohydroxomanganese(IV) complexes [Mn(IV)(OH)(2)(Me(2)EBC)](2+) and [Mn(IV)(O)(OH)(Me(2)EBC)](+) were probed using electronic absorption, magnetic circular dichroism (MCD), and variable-temperature, variable-field MCD spectroscopies. The d-d transitions of [Mn(IV)(OH)(2)(Me(2)EBC)](2+) were assigned using a group theory analysis coupled with the results of time-dependent density functional theory computations. These assignments permit the development of an experimentally validated description for the pi and sigma interactions in this complex. A similar analysis performed for [Mn(IV)(O)(OH)(Me(2)EBC)](+) reveals that there is a significant increase in the ligand character in the Mn pi* orbitals for the Mn(IV)=O complex relative to the bis(hydroxo)manganese(IV) complex, whereas the compositions of the Mn sigma* orbitals are less affected. Because of the steric features of the Me(2)EBC ligand, we propose that H-atom transfer by these reagents proceeds via the sigma* orbitals, which, because of their similar compositions among these two compounds, leads to modest rate enhancements for the Mn(IV)=O versus Mn(IV)OH species.

Herein we describe the chemical reactivity of the mononuclear [MnII(N4py)(OTf)](OTf) (1) complex with hydrogen peroxide and superoxide. Treatment of 1 with one equivalent superoxide at −40 °C in MeCN formed the peroxomanganese(III) adduct, [MnIII(O2)(N4py)]+ (2) in ~30% yield. Complex 2 decayed over time and the formation of the bis(μ-oxo)dimanganese(III,IV) complex, [MnIIIMnIV(μ-O)2(N4py)2]3+ (3) was observed. When 2 was formed in higher yields (~60%) using excess superoxide, the [MnIII(O2)(N4py)]+ species thermally decayed to MnII species and 3 was formed in no greater than 10% yield. Treatment of [MnIII(O2)(N4py)]+ with 1 resulted in the formation of 3 in ~90% yield, relative to the concentration of [MnIII(O2)(N4py)]+. This reaction mimics the observed chemistry of Mn-ribonucleotide reductase, as it features the conversion of two MnII species to an oxo-bridged MnIIIMnIV compound using O2− as oxidant. Complex 3 was independently prepared through treatment of 1 with H2O2 and base at −40 °C. The geometric and electronic structures of 3 were probed using electronic absorption, electron paramagnetic resonance (EPR), magnetic circular dichroism (MCD), variable-temperature, variable-field MCD (VTVH-MCD), and X-ray absorption (XAS) spectroscopies. Complex 3 was structurally characterized by X-ray diffraction (XRD), which revealed the N4py ligand bound in an unusual tetradentate fashion.

The room-temperature, gas−solid reaction of volatile GaMe3 with a nonporous silica was studied by elemental and gas-phase analysis, in situ IR and 1H, 13C, and 29Si solid-state NMR, and extended X-ray absorption fine structure (EXAFS) spectroscopy. Most of the grafting (∼85%) occurred on Q3 sites, O3SiOH, but a small amount (∼15%) of siloxane (O3SiO−SiO3) bond cleavage was also observed. The major, if not the only, gallium product has the empirical formula ⋮SiOGaMe2, but it is not an “isolated” site. The Ga K-edge EXAFS of GaMe3-modified silica, recorded at 10 K, reveals that each Ga has a Ga neighbor at 2.97−2.99 Å. The sites are best described as [GaMe2(μ-OSi⋮)]2. To strengthen this assignment, a molecular analogue, [GaMe2(μ-OSiPh3)]2, was characterized by both single-crystal X-ray diffraction and EXAFS. The Ga2O2 rings in the molecular complex and the silica-supported gallium dimer have very similar dimensions. The gallium dimer is formed on the silica surface regardless of the extent of partial dehydroxylation (varied by pretreatment in vacuo at 100 and 500 °C). This result is interpreted in terms of a vicinal disposition for the majority of Q3 grafting sites.