Characterization of the Titanium(III) Tris(alkyl) Ti{CH(SiMe3)2}3 and its Conversion to a Dimeric Alkyl-bridged Titanium(IV) Species.

The reaction of three equivalents of LiCH(SiMe3)2 with TiCl3(NMe3)2 afforded the rare homoleptic Ti(III) alkyl Ti{CH(SiMe3)2}3 (1) which crystallized as blue needles in 32 % yield. Single crystal X-Ray data for 1 showed a trigonal pyramidal coordination geometry around titanium, which could be ascribed to weak interactions between the C-H bonds and the Ti(III) atom based on computational results. X-band EPR spectroscopy give spectral parameters consistent with the proposed Ti(III) formulation.

Structural basis for the conformational protection of nitrogenase from O.

The low reduction potentials required for the reduction of dinitrogen (N) render metal-based nitrogen-fixation catalysts vulnerable to irreversible damage by dioxygen (O). Such O sensitivity represents a major conundrum for the enzyme nitrogenase, as a large fraction of nitrogen-fixing organisms are either obligate aerobes or closely associated with O-respiring organisms to support the high energy demand of catalytic N reduction. To counter O damage to nitrogenase, diazotrophs use O scavengers, exploit compartmentalization or maintain high respiration rates to minimize intracellular O concentrations.

O Activation and Enzymatic C-H Bond Activation Mediated by a Dimanganese Cofactor.

Dioxygen (O) is a potent oxidant used by aerobic organisms for energy transduction and critical biosynthetic processes. Numerous metalloenzymes harness O to mediate C-H bond hydroxylation reactions, but most commonly feature iron or copper ions in their active site cofactors. In contrast, many manganese-activated enzymes─such as glutamine synthetase and isocitrate lyase─perform redox neutral chemical transformations and very few are known to activate O or C-H bonds.

O Cleavage at a Preorganized Triiron Cluster.

In Nature, the four-electron reduction of O is catalyzed at preorganized multimetallic active sites. These complex active sites often feature low-coordinate, redox-active metal centers precisely positioned to facilitate rapid O activation processes that obviate the generation of toxic, partially reduced oxygen species. Very few biomimetic constructs simultaneously recapitulate the complexity and reactivity of these biological cofactors.