Semiempirical method for examining asynchronicity in metal-oxido-mediated C-H bond activation.

The oxidation of substrates via the cleavage of thermodynamically strong C-H bonds is an essential part of mammalian metabolism. These reactions are predominantly carried out by enzymes that produce high-valent metal-oxido species, which are directly responsible for cleaving the C-H bonds. While much is known about the identity of these transient intermediates, the mechanistic factors that enable metal-oxido species to accomplish such difficult reactions are still incomplete. For synthetic metal-oxido species, C-H bond cleavage is often mechanistically described as synchronous, proton-coupled electron transfer (PCET). However, data have emerged that suggest that the basicity of the M-oxido unit is the key determinant in achieving enzymatic function, thus requiring alternative mechanisms whereby proton transfer (PT) has a more dominant role than electron transfer (ET). To bridge this knowledge gap, the reactivity of a monomeric MnIV-oxido complex with a series of external substrates was studied, resulting in a spread of over 104 in their second-order rate constants that tracked with the acidity of the C-H bonds. Mechanisms that included either synchronous PCET or rate-limiting PT, followed by ET, did not explain our results, which led to a proposed PCET mechanism with asynchronous transition states that are dominated by PT. To support this premise, we report a semiempirical free energy analysis that can predict the relative contributions of PT and ET for a given set of substrates. These findings underscore why the basicity of M-oxido units needs to be considered in C-H functionalization.

Crystal structures of two cross-bridged chromium(III) tetra-aza-macrocycles.

The crystal structure of di-chlorido-(4,10-dimethyl-1,4,7,10-tetra-aza-bicyclo-[5.5.2]tetra-deca-ne)chromium(III) hexa-fluorido-phosphate, [CrCl2(C12H26N4)]PF6, (I), has monoclinic symmetry (space group P21/n) at 150 K. The structure of the related di-chlorido-(4,11-dimethyl-1,4,8,11-tetra-aza-bicyclo-[6.6.2]hexa-deca-ne)chromium(III) hexa-fluorido-phosphate, [CrCl2(C14H30N4)]PF6, (II), also displays monoclinic symmetry (space group P21/c) at 150 K. In each case, the Cr(III) ion is hexa-coordinate with two cis chloride ions and two non-adjacent N atoms bound cis equatorially and the other two non-adjacent N atoms bound trans axially in a cis-V conformation of the macrocycle. The extent of the distortion from the preferred octa-hedral coordination geometry of the Cr(III) ion is determined by the parent macrocycle ring size, with the larger cross-bridged cyclam ring in (II) better able to accommodate this preference and the smaller cross-bridged cyclen ring in (I) requiring more distortion away from octa-hedral geometry.

High-performance liquid chromatography/high-resolution multiple stage tandem mass spectrometry using negative-ion-mode hydroxide-doped electrospray ionization for the characterization of lignin degradation products.

In the search for a replacement for fossil fuel and the valuable chemicals currently obtained from crude oil, lignocellulosic biomass has become a promising candidate as an alternative biorenewable source for crude oil. Hence, many research efforts focus on the extraction, degradation, and catalytic transformation of lignin, hemicellulose, and cellulose. Unfortunately, these processes result in the production of very complex mixtures. Further, while methods have been developed for the analysis of mixtures of oligosaccharides, this is not true for the complex mixtures generated upon degradation of lignin. For example, high-performance liquid chromatography/multiple stage tandem mass spectrometry (HPLC/MS(n)), a tool proven to be invaluable in the analysis of complex mixtures derived from many other biopolymers, such as proteins and DNA, has not been implemented for lignin degradation products. In this study, we have developed an HPLC separation method for lignin degradation products that is amenable to negative-ion-mode electrospray ionization (ESI doped with NaOH), the best method identified thus far for ionization of lignin-related model compounds without fragmentation. The separated and ionized compounds are then analyzed by MS(3) experiments to obtain detailed structural information while simultaneously performing high-resolution measurements to determine their elemental compositions in the two parts of a commercial linear quadrupole ion trap/Fourier-transform ion cyclotron resonance mass spectrometer. A lignin degradation product mixture was analyzed using this method, and molecular structures were proposed for some components. This methodology significantly improves the ability to analyze complex product mixtures that result from degraded lignin.

C-H bond cleavage with reductants: re-investigating the reactivity of monomeric Mn(III/IV)-oxo complexes and the role of oxo ligand basicity.

The thermodynamic properties of structurally similar Mn(III) and Mn(IV) complexes have been reinvestigated to understand their reactivity with substrates having C-H bonds. The complexes have the general formula [MnH(3)buea(O)](n-), where [H(3)buea](3-) is the tripodal ligand, tris[(N'-tert-butylureaylato)-N-ethylene]aminato. These complexes are unique because of the intramolecular hydrogen-bonding (H-bond) network surrounding the Mn-oxo units. The redox potentials for the Mn(III/IV)(O) couple was incorrectly assigned in earlier reports: the corrected value is -1.0 V vs Cp(2)Fe(+)/Cp(2)Fe in DMSO, while the Mn(IV/V)(O) process is -0.076 under the same conditions. The oxo ligand in the Mn(III)(O) complexes is basic with a pK(a) of 28.3; the basicity of the terminal oxo ligand in the Mn(IV)(O) complex is estimated to be approximately 15. These values were used to re-evalulate the O-H bond dissociation energy (BDE(OH)) of the corresponding Mn(II/III)-OH complexes: BDE(OH) values of 89 and 77 kcal/mol were determined for [Mn(III)H(3)buea(OH)](-) and [Mn(II)H(3)buea(OH)](2-), respectively. Both Mn(O) complexes react with 9,10-dihydroanthracene (DHA) to produce anthracene in nearly quantitative yields. This is surprising based on the low redox potiental of the complexes, suggesting the basicity of the oxo ligand is a major contributor to the observed reactivity. In contrast to the thermodynamic results, a comparative kinetic investigation found that the Mn(III)(O) complex reacts nearly 20 times faster than the Mn(IV)(O) complex. Activation parameters, determined from an Eyring analysis, found that the entropy of activation is significantly different between the two systems (DeltaDeltaS(++) = -35 eu, where DeltaDeltaS(++) = DeltaS(++)(Mn(IV)(O)) - DeltaS(++)(Mn(III)(O)). This unusual kinetic behavior can be explained in the context of the basicity of the oxo ligands that leads to different mechanisms: for [Mn(III)H(3)buea(O)](2-) a proton transfer-electron transfer mechanism is proposed, whereas for [Mn(IV)H(3)buea(O)](-) a hydrogen-atom transfer pathway is likely.

Synthesis and Characterization of the Chromium(III) complexes of Ethylene Cross-Bridged Cyclam and Cyclen Ligands.

Dichloro(4,10-dimethyl-1,4,7,10-tetraazabicyclo[5.5.2]tetradecane)chromium(III) chloride, Dichloro(4,10-dibenzyl-1,4,7,10-tetraazabicyclo[5.5.2]tetradecane) chromium(III) chloride, and Dichloro(4,11-dimethyl-1,4,8,11-tetraazabicyclo[6.6.2] hexadecane)chromium)(III) chloride have been prepared by the reaction of anhydrous chromium(III) chloride with the appropriate cross-bridged tetraazamacrocycle. Aquation of these complexes proved difficult, but Chlorohydroxo(4,11-dimethyl-1,4,8,11-tetraazabicyclo[6.6.2]hexadecane)chromium)(III) chloride was synthesized directly from chromium(II) chloride complexation followed by exposure or the reaction to air in the presence of water. The four complexes were characterized by X-ray crystal structure determination. All contain the chromium(III) ion in a distorted octahedral geometry and the macrocycle in the cis-V configuration, as dictated by the ethylene cross-bridge. Further characterization of the hydroxo complex reveals a magnetic moment of mu(eff) = 3.95 B.M. and electronic absorbtions in acetonitrile at lambda(max) = 583nm (epsilon = 65.8 L/cm.mol), 431nm (epsilon = 34.8 L/cm.mol) and 369nm (epsilon = 17 L/cm.mol).

Preparation and properties of a monomeric Mn(IV)-oxo complex.

Manganese-oxo complexes have long been investigated because of their proposed roles in biological and chemical catalysis. However, there are few examples of monomeric complexes with terminal oxo ligands, especially those with oxomanganese(IV) units. A oxomanganese(IV) complex has been prepared from [MnIIIH3buea(O)]2- ([H3buea]3-, tris[(N'-tert-butylureaylato)-N-ethylene]aminato), a monomeric MnIII-O complex in which the oxo ligand arises from cleavage of dioxygen. Treating [MnIIIH3buea(O)]2- with [Cp2Fe]BF4 in either DMF at -45 degrees C or DMSO at room temperature produces [MnIVH3buea(O)]-: lambdamax = 635 nm; nu(Mn-16O) = 737 cm-1; nu(Mn-18O) = 709 cm-1; g = 5.15, 2.44, 1.63, D = 3.0 cm-1, E/D = 0.26, aMn = 66 G (A = 190 MHz). These spectroscopic properties support the assignment of a mononuclear MnIV-oxo complex with an S = 3/2 ground state. Density functional theory supports this assignment and the Jahn-Teller distortion around the high-spin MnIV center that would alter the molecular structure of [MnIVH3buea(O)]- from trigonal symmetry (as indicated by the highly rhombic EPR signal). [MnIVH3buea(O)]- is relatively unstable in DMSO, converting to [MnIIIH3buea(OH)]- via a proposed X-H bond cleavage. [MnIVH3buea(O)]- reacts with 1,2-diphenylhydrazine to from azobenzene (95% yield) and [MnIIIH3buea(OH)]-. The MnIV-oxo does not react with triphenyl- or tricyclohexylphosphine. However, O-atom transfer is observed with methyldiphenylphosphine and dimethylphenylphosphine, producing the corresponding phosphine oxides. These results illustrate the diverse reactivity of the MnIV-oxo unit.