The nonheme iron(IV)-oxido complex trans-N3-[(L 1 ) Fe IV = O(Cl)] + , where L 1 is a derivative of the tetradentate bispidine 2,4-di(pyridine-2-yl)-3,7-diazabicyclo[3.3.1]nonane-1one, is known to have an S = 1 electronic ground state and to be an extremely reactive oxidant for oxygen atom transfer (OAT) and hydrogen atom abstraction (HAA) processes. Here we show that, in spite of this ferryl oxidant having the "wrong" spin ground state, it is the most reactive nonheme iron model system known so far and of a similar order of reactivity as nonheme iron enzymes (CÀ H abstraction of cyclohexane, À 90°C (propionitrile), t 1/2 = 3.5 sec). Discussed are spectroscopic and kinetic data, supported by a DFT-based theoretical analysis, which indicate that substrate oxidation is significantly faster than self-decay processes due to an intramolecular demethylation pathway and formation of an oxido-bridged diiron(III) intermediate. It is also shown that the iron(III)-chlorido-hydroxido/cyclohexyl radical intermediate, resulting from CÀ H abstraction, selectively produces chlorocyclohexane in a rebound process. However, the lifetime of the intermediate is so long that other reaction channels (known as cage escape) become important, and much of the CÀ H abstraction therefore is unproductive. In bulk reactions at ambient temperature and at longer time scales, there is formation of significant amounts of oxidation product -selectively of chlorocyclohexane -and it is shown that this originates from oxidation of the oxido-bridged diiron (III) resting state.

A facile synthesis of meso-thienyl porphyrins containing a phenyl substituent allows fine tuning of the frontier orbitals to suit applications in DSSC and photomedicine.

The mechanistic landscape of H2 generation from formic acid catalyzed by Cp*M­(III) complexes (M = Co or Rh or Ir) with diamino-/dialkylamino-substituted 2,2′-bipyridine ligand architectures have been unveiled computationally. The calculations indicate that the β-hydride elimination process is the rate-determining step for all the investigated catalysts. The dialkylamino moieties on the 2,2′-bipyridine ligand were found to reduce the activation free energy required for the rate-limiting β-hydride elimination step and increase the hydridic nature of the Ir–hydride bond, which accounts for the experimentally observed enhanced catalytic activity. Furthermore, the protonation by H3O+ ion was found to be the kinetically most favorable route than the conventional protonation by formic acid. The origin for this preference lies in the increased electrophilicity of the proton from hydronium ion which facilitates easy protonation of the metal-hydride with low activation energy barrier. The Co and Rh analogues of the chosen iridium catalyst were computationally designed and were estimated to possess a rate-determining activation barrier of 16.9 and 14.5 kcal/mol, respectively. This illustrates that these catalysts are potential candidates for FAD. The insights derived in this work might serve as a vital knowledge that could be capitalized upon for designing cost-effective catalyst for FAD in future.

The catalytic mechanism of hydrogen production via formic acid decomposition by pentamethylcyclopentadienyl (Cp*) rhodium­(III) and cobalt­(III) catalysts with proton-responsive 4,4′-dihydroxy-2,2′-bipyridine (4L) and 6,6′-dihydroxy-2,2′-bipyridine (6L) ligands ([Cp*M­(4L)­(H 2 O)] 2+ and [Cp*M­(6L)­(H 2 O)] 2+ ; M = Rh and Co) were explored using density functional theory calculations. The effect of pH on the protonation state of M­(4L) and M­(6L) ligands was studied using the speciation approach, and the fully protonated dihydroxy-2,2′-bipyridine ligand was found to be the dominated species throughout the catalytic mechanism of formic acid decomposition at pH 2.5. For both Cp*Rh­(III) and Cp*Co­(III) catalysts with 4L or 6L ligands, the β-hydride elimination step was found to be the rate-determining step irrespective of the position of the hydroxyl group on the bipyridine ligand. In the case of M­(6L), both formic acid- and water-assisted hydrogen evolution transition states were considered, and from the computed free energy profile, the water-assisted H2 generation was found to be the most favorable pathway. The electronic origin of the difference in the catalytic efficiency of the chosen catalysts was traced by performing natural bonding orbital analysis. These analyses reveal that the second-order stabilizing interactions and hydricity in the reaction intermediates and transition states play a significant role in altering the energetics of the formic acid decomposition reaction. Furthermore, the calculated activation free energies for the β-hydride elimination step catalyzed by the chosen catalysts were in the range of 15.8 to 20.3 kcal/mol, signifying that these catalysts are promising candidates for hydrogen generation with catalytic activities comparable to its Ir analogue. Especially, Co­(6L) with a relatively low activation energy barrier of 15.8 kcal/mol can be considered as an efficient low-cost catalyst for achieving fast dehydrogenation of formic acid. Overall, the present study paves the way for designing novel catalysts for hydrogen generation via formic acid dehydrogenation.

The iron(II) complexes [Fe(L)(MeCN) 3 ](SO 3 CF 3 ) 2 (L are two derivativeso ft ris(2-pyridyl)-based ligands) have been synthesized as modelsf or cysteined ioxygenase (CDO). The molecular structure of one of the complexesexhibits octahedral coordination geometry and the FeÀN py bond lengths [1.953(4)-1.972(4) ]a re similar to those in the Cysbound Fe II -CDO;F e ÀN His :1 .893-2.199 .T he iron(II) centers of the model complexes exhibit relatively highF e III/II redox potentials (E 1/2 = 0.988-1.380V vs. ferrocene/ferrocenium electrode, Fc/Fc + ), within the range for O 2 activation and typical for the corresponding nonheme iron enzymes.T he reaction of in situ generated [Fe(L)(MeCN)(SPh)] + with excess O 2 in acetonitrile (MeCN) yields selectively the doubly oxygenated phenylsulfinic acid product. Isotopic labeling studies using 18 O 2 confirm the incorporation of both oxygen atomso fO 2 into the product. Kinetic and preliminary DFT studies revealthe involvement of an Fe III peroxido intermediate with ar hombic S = 1 = 2 Fe III center( 687-696 nm; g % 2.46-2.48, 2.13-2.15, 1.92-1.94), similar to the spectroscopic signature of the low-spin Cys-bound Fe III CDO (650 nm, g % 2.47, 2.29, 1.90). The proposed Fe III peroxido intermediates have been trapped, and the OÀOs tretching frequencies are in the expected range (approximately 920 and 820 cm À1 for the alkyl-and hydroperoxido species, respectively). The model complexesh ave as tructure similart ot hat of the enzyme and structural aspects as well as the reactivity are discussed.Scheme1.Chemical structures of the CDO active site and the CDO-Cys adduct.