An iron(III)-porphyrin complex with a vinylidene group inserted into an iron-nitrogen bond: relevance to the structure of the active oxygen complex of catalase

Iron(iv) alkyl complexes: electronic structure contributions to Fe-C bond homolysis and migration reactions that form N-C bonds from N2

High-valent iron alkyl complexes are rare, as they are prone to Fe-C bond homolysis. Here, we describe an unusual way to access formally iron(iv) alkyl complexes through double silylation of iron(i) alkyl dinitrogen complexes to form an NNSi2 group. Spectroscopically validated computations show that the disilylehydrazido(2-) ligand stabilizes the formal iron(iv) oxidation state through a strongly covalent Fe-N π-interaction, in which one π-bond fits an "inverted field" description. This means that the two bonding electrons are localized more on the metal than the ligand, and thus an iron(ii) resonance structure is a significant contributor, similar to the previously-reported phenyl analogue. However, in contrast to the phenyl complex which has an S = 1 ground state, the ground state of the alkyl complex is S = 2, which places one electron in the π* orbital, leading to longer and weaker Fe-N bonds. The reactivity of these hydrazido(2-) complexes is dependent on the steric and electronic properties of the specific alkyl group. When the alkyl group is the bulky trimethylsilylmethyl, the formally iron(iv) species is stable at room temperature and no migration of the alkyl ligand is observed. However, the analogous complex with the smaller methyl ligand does indeed undergo migration of the carbon-based ligand to the NNSi2 group to form a new N-C bond. This migration is followed by isomerization of the hydrazido ligand, and the product exists as two isomers that have distinct η1 and η2 binding of the hydrazido group. Lastly, when the alkyl group is benzyl, the Fe-C bond homolyzes to give a three-coordinate hydrazido(2-) complex which is likely due to the greater stability of a benzyl radical compared to that for methyl or trimethylsilylmethyl. These studies demonstrate the availability of a hydrocarbyl migration pathway at formally iron(iv) centers to form new N-C bonds directly to N2, though product selectivity is highly dependent on the identity of the migrating group.

Phenanthrene cyclocarbonylation – Core post-synthetic modification of phenanthriporphyrin

The unique [Fe(CO)5]-induced cyclocarbonylation of the phenanthriporphyrin's core is an intriguing example of a post-synthetic core modification of the macrocycle. The reaction involves the activation of C(22)−H and C(25)−H bonds,...

Bridging and axial carbene binding modes in cobalt corrole complexes: effect on carbene transfer

Catalytically relevant intermediates in carbene transfer reactions from a diazo precursor were investigated using cobalt corrole complexes. Two divergent mechanisms are proposed depending on the oxidation state of the cobalt center. Mechanistically driven factors for the usage of cobalt corroles in carbene transfer reactions are discussed.

Spin modification of iron(ii) complexes via covalent (dative) and dispersion guided non-covalent bonding with N-heterocyclic carbenes: DFT, DLPNO-CCSD(T) and MCSCF studies

Spin crossover from high spin Fe(ii)-phthalocyanine to low or intermediate spin via either dative covalent or non-covalent interaction by just varying the substituent using the same core ligand.

Iron- and cobalt-catalyzed C(sp3)–H bond functionalization reactions and their application in organic synthesis

This review highlights the developments in iron and cobalt catalyzed C(sp³)–H bond functionalization reactions with emphasis on their applications in organic synthesis, i.e. natural products and pharmaceuticals synthesis and/or modification.

Catalytic iron-carbene intermediate revealed in a cytochrome c carbene transferase

Recently, heme proteins have been discovered and engineered by directed evolution to catalyze chemical transformations that are biochemically unprecedented. Many of these nonnatural enzyme-catalyzed reactions are assumed to proceed through a catalytic iron porphyrin carbene (IPC) intermediate, although this intermediate has never been observed in a protein. Using crystallographic, spectroscopic, and computational methods, we have captured and studied a catalytic IPC intermediate in the active site of an enzyme derived from thermostable Rhodothermus marinus (Rma) cytochrome c High-resolution crystal structures and computational methods reveal how directed evolution created an active site for carbene transfer in an electron transfer protein and how the laboratory-evolved enzyme achieves perfect carbene transfer stereoselectivity by holding the catalytic IPC in a single orientation. We also discovered that the IPC in Rma cytochrome c has a singlet ground electronic state and that the protein environment uses geometrical constraints and noncovalent interactions to influence different IPC electronic states. This information helps us to understand the impressive reactivity and selectivity of carbene transfer enzymes and offers insights that will guide and inspire future engineering efforts.

Electronic Configuration and Ligand Nature of Five-Coordinate Iron Porphyrin Carbene Complexes: An Experimental Study

The five-coordinate iron porphyrin carbene complexes [Fe(TPP) (CCl₂)] (TPP = tetraphenylporphyrin), [Fe(TTP) (CCl₂)] (TTP = tetratolylporphyrin) and [Fe(TFPP) (CPh₂)] (TFPP = tetra(pentafluorophenyl)porphyrin), utilizing two types of carbene ligands (CCl₂ and CPh₂), have been investigated by single crystal X-ray, XANES (X-ray absorption near edge spectroscopy), Mössbauer, NMR and UV-vis spectroscopies. The XANES suggested the iron(II) oxidation state of the complexes. The multitemperature and high magnetic field Mössbauer experiments, which show very large quadrupole splittings (QS, ΔE_Q), determined the S = 0 electronic configuration. More importantly, combined structural and Mössbauer studies, especially the comparison with the low spin iron(II) porphyrin complexes with strong diatomic ligands (CS, CO and CN^-) revealed the covalent bond nature of the carbene ligands. A correlation between the iron isomer shifts (IS, δ) and the axial bond distances is established for the first time for these donor carbon ligands (:C-R).

'Carbene radicals' in Co(II)(por)-catalyzed olefin cyclopropanation

The mechanism of cobalt(II)-porphyrin-mediated cyclopropanation of olefins with diazoesters was studied. The first step--reaction of cobalt(II)-porphyrin with ethyl diazoacetate (EDA)--was examined using EPR and ESI-MS techniques. EDA reacts with cobalt(II)-porphyrin to form a 1:1 Co(por)(CHCOOEt) adduct that exists as two isomers: the 'bridging carbene' C' in which the 'carbene' is bound to the metal and the pyrrolic nitrogen of the porphyrin that has a d(7) configuration on the metal, and the 'terminal carbene' C in which the 'carbene' behaves as a redox noninnocent ligand having a d(6) cobalt center and the unpaired electron residing on the 'carbene' carbon atom. The subsequent reactivities of the thus formed 'cobalt carbene radical' with propene, styrene, and methyl acrylate were studied using DFT calculations. The calculations suggest that the formation of the carbene is the rate-limiting step for the unfunctionalized Co(II)(por) and that the cyclopropane ring formation proceeds via a stepwise radical process: Radical addition of the 'carbene radical' C to the C=C double bonds of the olefins results in formation of the gamma-alkyl radical intermediates D. Species D then easily collapse in almost barrierless ring-closure reactions (TS3) to form the cyclopropanes. This radical mechanism readily explains the high activity of Co(II)(por) species in the cyclopropanation of electron-deficient olefins such as methyl acrylate.

Regioselective pyridination of m-benziporphyrin

[reaction: see text] Reaction of tetraaryl-m-benziporphyrin with pyridine and silver tetrafluoroborate yields 22-pyridiniumyl-m-benziporphyrin as the only substitution product. This compound is further rearranged to a fused m-benziphlorin containing a 4a-azafluorene fragment. A mechanism involving a high-valent silver complex is proposed for the pyridination reaction.

Core-Modified Porphyrin Incorporating a Phenolate Donor. Characterization of Pd(II), Ni(II), Zn(II), Cd(II), and Fe(III) Complexes

Coordinating properties of acetoxybenziporphyrin, (TPBPOAc)H, have been investigated for a number of metal ions. Insertion of Ni, Pd, and Fe results in the cleavage of the acetoxy group leading to complexes (TPBPO)Ni(II), (TPBPO)Pd(II), and (TPBPO)Fe(III)X containing a M-O bond. No cleavage is observed with Zn(II) and Cd(II), which form complexes (TPBPOAc)M(II)Cl, where M = Zn, Cd. (TPBPO)Ni(II) can also be obtained from the dication of hydroxybenziporphyrin, [(TPBPOH)H(3)]Cl(2), which is prepared by acid hydrolysis of the acetoxy compound. The diamagnetic (TPBPO)Ni(II) can be transformed into the paramagnetic (TPBPOAc)Ni(II)Cl in a reaction with acetyl chloride. X-ray structures have been determined for (TPBPO)Pd(II) and (TPBPOAc)Zn(II)Cl. In the palladium species, the phenolate moiety forms a strong bond to the Pd ion and an unusual interaction geometry is observed, enforced by the macrocyclic environment. Association of a TFA molecule to the phenolic oxygen does not cause significant structural changes in the (TPBPO)Pd(II) molecule. In (TPBPOAc)Zn(II)Cl, the metal ion weakly interacts with the phenolic fragment. The paramagnetic Fe(III) complexes, (TPBPO)Fe(III)X, have been investigated with (1)H NMR spectroscopy. The observed spectral patterns are consistent with the presence of a high-spin Fe(III) center and pi delocalization of spin density onto the phenoxide fragment. Each of the compounds (TPBPO)Fe(III)X exists in solution as a mixture of two isomers, which for X = I are shown to remain in a temperature-dependent equilibrium. The observed isomerism results from two nonequivalent orientations of the axial halide with respect to the puckered macrocyclic ring.

Metal complexes of N-tosylamidoporphyrin: cis-acetato-N-tosylimido-meso-tetraphenylporphyrinatothallium(III) and trans-acetato-N-tosylimido-meso-tetraphenylporphyrinatogallium(III)

The crystal structures of acetato-N-tosylimido-meso-tetraphenylporphyrinatothallium(III), Tl(N-NTs-tpp)(OAc) (1), and acetato-N-tosylimido-meso-tetraphenylporphyrinatogallium(III), Ga(N-NTs-tpp)(OAc) (2), were determined. The coordination sphere around the Tl3+ ion is a distorted square-based pyramid in which the apical site is occupied by a chelating bidentate OAc- group, whereas for the Ga3+ ion, it is a distorted trigonal bipyramid with O(3), N(3), and N(5) lying in the equatorial plane. The porphyrin ring in the two complexes is distorted to a large extent. For the Tl3+ complex, the pyrrole ring bonded to the NTs ligand lies in a plane with a dihedral angle of 50.8 degrees with respect to the 3N plane, which contains the three pyrrole nitrogens bonded to Tl3+, but for the Ga3+ complex, this angle is found to be only 24.5 degrees. In the former complex, Tl3+ and N(5) are located on the same side at 1.18 and 1.29 A from its 3N plane, but in the latter one, Ga3+ and N(5) are located on different sides at -0.15 and 1.31 A from its 3N plane. The free energy of activation at the coalescence temperature Tc for the intermolecular acetate exchange process in 1 in CD2Cl2 solvent is found to be delta G++171 = 36.0 kJ/mol through 1H NMR temperature-dependent measurements. In the slow-exchange region, the methyl and carbonyl (CO) carbons of the OAc- group in 1 are separately located at delta 18.5 [3J(Tl-13C) = 220 Hz] and 176.3 [2J(Tl-13C) = 205 Hz] at -110 degrees C.

Disproportionation of Iron(III) Porphyrin pi-Cation Radicals in the Presence of Sterically Hindered Pyridines. Spectroscopic Detection of Asymmetric Highly Oxidized Intermediates

The reactivity of iron(III) tetraphenylporphyrin pi-cation radical (TPP(*))Fe(III)(ClO(4))(2), (1-1) iron(III) tetra-p-tolylporphyrin pi-cation radical (TTP(*))Fe(III)(ClO(4))(2) (1-2) and iron(III) tetramesitylporphyrin pi-cation radical (TMP(*))Fe(III)(ClO(4))(2) (1-3) complexes with 2,4,6-collidine, 2,3,6-collidine, 2-picoline, 2,6-di-tert-butylpyridine, and 2,6-dibromopyridine has been examined by (1)H NMR spectroscopy in dichloromethane-d(2) solution at low temperatures. These complexes undergo hydration processes which are essential in the generation of highly oxidized species via acid base/equilibria of coordinated water followed by disproportionation pathway, giving as sole stable products [(TPP(*))Fe(III)OFe(III)(TPP)](+) (4-1), [(TTP(*))Fe(III)OFe(III)(TTP)](+) (4-2), and (TMP)Fe(III)(OH) (6) respectively. The sterically hindered pyridines act as efficient proton scavengers. Two novel highly oxidized iron complexes have been detected by (1)H NMR spectroscopy after addition of 2,4,6-collidine to (TTP(*))Fe(III)(ClO(4))(2) or (TPP(*))Fe(III)(ClO(4))(2) in dichloromethane-d(2) solution at 202 K. New intermediates have been identified as iron porphyrin N-oxide complexes, i.e., iron(III) porphyrin N-oxide cation radical (2-n) and iron(IV) porphyrin N-oxide radical (3-n). The (1)H NMR results indicate that the D(4)(h)() symmetry of the parent iron(III) pi-cation radical is drastically reduced upon disproportionation in the presence of proton scavengers. Both species are very unstable and were observed from 176 to 232 K. The intermediate 2-2 has a (1)H NMR spectrum which demonstrates large hyperfine shifts (ppm) for the meso p-tolyl substituents (ortho 98.0, 94.8, 92.9, 91.7; meta -34.8, -38.7, -41.5, -42.3; p-CH(3) -86.3, -88.0) which are consistent with presence of an N-substituted iron porphyrin radical in the product mixture. The characteristic (1)H NMR spectrum of 2-2 includes six pyrrole resonances at 149.6, 118.2, 115.4, 88.3, 64.6, and 55.7 ppm at 202 K, i.e., in the positions corresponding to iron(III) high-spin porphyrins. On warming to 222 K, the pyrrole resonances broaden and then coalesce pairwaise. Such dynamic behavior is accounted for by a rearrangement mechanism which involves an inversion of the porphyrin puckering. The pattern of p-tolyl resonances revealed the cation radical electronic structure of 3-2. The p-tolyl resonances are divided in two distinct sets showing opposite direction of the isotropic shift for the same ring positions. The pyrrole resonances of 3-2 also demonstrated downfield and upfield shifts. A disproportionation mechanism of the hydrated iron porphyrin cation radicals to generate 2 and 3 has been proposed. Both intermediates react with triphenylphosphine to produce triphenylphosphine oxide and high-spin iron porphyrins. Addition of 2,4,6-collidine to (TMP(*))Fe(III)(ClO(4))(2) does not produce analogs of 2 and 3 found for sterically unprotected porphyrins. It results instead in the formation of a variety of X(TMP(*))Fe(IV)O (5) complexes also accounted for by the disproportionation process.

Biodehalogenation: reactions of cytochrome P-450 with polyhalomethanes

The products, stoichiometry, and kinetics of the oxidation of the enzyme cytochrome P-450 cam by five polyhalomethanes and chloronitromethane are described. The reactivity of the enzyme is compared with that of deuteroheme and with the enzyme in its native cell, Pseudomonas putida (PpG-786). In all cases, the reaction entails hydrogenolysis of the carbon-halogen bond: 2FeIIP + RCXn----2FeIIIP + RCHXn-1 (P = porphyrin or P-450 cam in vitro and in vivo). Trichloronitromethane was the fastest reacting substrate, and chloroform was the slowest. The results establish that P. putida is a valid whole cell model for the reductase activity of the P-450 complement in these reactions. The reactions of cytochrome P-450 with polyhaloalkanes proceed in a manner quite analogous to other iron(II) proteins in the G conformation. The chemistry observed for the enzyme parallels that of its iron(II) porphyrin active site. Iron-bonded carbenes are not intermediates, and hydrolytically stable iron alkyls are not products of these reactions.