First direct evidence for stereospecific olefin epoxidation and alkane hydroxylation by an oxoiron(IV) porphyrin complex

We report in this study that an oxoiron(IV) porphyrin complex bearing electron-deficient porphyrin ligand, (TPFPP)FeIV=O (TPFPP = meso-tetrakis(pentafluorophenyl)porphinato dianion), shows reactivities similar to those found in oxoiron(IV) porphyrin pi-cation radicals. In the epoxidation of olefins by the (TPFPP)FeIV=O complex, epoxides were yielded as major products; cyclohexene oxide was the sole product formed in the epoxidation of cyclohexene, and stilbenes were stereospecifically oxidized to the corresponding epoxide products. More striking results were obtained in alkane hydroxylation reactions; the hydroxylation of adamantane afforded a high degree of selectivity for tertiary C-H bonds over secondary C-H bonds, and the hydroxylation of cis-1,2-dimethylcyclohexane yielded a tertiary alcohol product with >99% retention of stereochemistry. The latter result demonstrates that an oxoiron(IV) porphyrin complex hydroxylates alkanes with a high stereospecificity. Isotope labeling studies performed with H218O and 18O2 in the olefin epoxidation and alkane hydroxylation reactions demonstrated that oxygen atoms in oxygenated products derived from the oxoiron(IV) porphyrin complex.     

Rapid freeze-quench ENDOR study of chloroperoxidase compound I: the site of the radical

The classical heme-monooxygenase active intermediate, compound I (Cpd-I), incorporates a heme which is oxidized by two equivalents above the resting ferric state, one equivalent associated with a ferryl center, [Fe=O]2+ (FeS = 1), and the other with an active-site radical (RS = 1/2). Theoretical calculations on models of a Cpd-I with a thiolato axial ligand have presented divergent views about its electronic structure. In one picture, the radical is on the porphyrin; in the other, it is on the sulfur. In this report, ENDOR spectroscopy answers the question, does Cpd-I of the enzyme chloroperoxidase contain a porphyrin pi-cation radical or an iron-bound cysteinyl radical: the radical is predominantly on the porphyrin, with spin density on sulfur having an upper bound, rhoS 相似文献   

Redox potentials of oxoiron(IV) porphyrin π-cation radical complexes: participation of electron transfer process in oxygenation reactions

The oxoiron(IV) porphyrin π-cation radical complex (compound I) has been identified as the key reactive intermediate of several heme enzymes and synthetic heme complexes. The redox properties of this reactive species are not yet well understood. Here, we report the results of a systematic study of the electrochemistry of oxoiron(IV) porphyrin π-cation radical complexes with various porphyrin structures and axial ligands in organic solvents at low temperatures. The cyclic voltammogram of (TMP)Fe(IV)O, (TMP = 5,10,15,20-tetramesitylporphyrinate), exhibits two quasi-reversible redox waves at E(1/2) = 0.88 and 1.18 V vs SCE in dichloromethane at -60 °C. Absorption spectral measurements for electrochemical oxidation at controlled potential clearly indicated that the first redox wave results from the (TMP)Fe(IV)O/[(TMP(+?))Fe(IV)O](+) couple. The redox potential for the (TMP)Fe(IV)O/[(TMP(+?))Fe(IV)O](+) couple undergoes a positive shift upon coordination of an anionic axial ligand but a negative shift upon coordination of a neutral axial ligand (imidazole). The negative shifts of the redox potential for the imidazole complexes are contrary to their high oxygenation activity. On the other hand, the electron-withdrawing effect of the meso-substituent shifts the redox potential in a positive direction. Comparison of the measured redox potentials and reaction rate constants for epoxidation of cyclooctene and demethylation of N,N-dimethylanilines enable us to discuss the details of the electron transfer process from substrates to the oxoiron(IV) porphyrin π-cation radical complex in the oxygenation mechanisms.     

Oxidizing intermediates from the sterically hindered iron salen complexes related to the oxygen activation by nonheme iron enzymes

Oxidizing intermediates are generated from nonheme iron(III) complexes to investigate the electronic structure and the reactivity, in comparison with the oxoiron(IV) porphyrin pi-cation radical (compound I) as a heme enzyme model. Sterically hindered iron salen complexes, bearing a fifth ligand Cl (1), OH(2) (2), OEt (3), and OH (4), are oxidized both electrochemically and chemically. Stepwise one-electron oxidation of 1 and 2 generates iron(III)-mono- and diphenoxyl radicals, as revealed by detailed spectroscopic investigations, including UV-vis, EPR, M?ssbauer, resonance Raman, and ESIMS spectroscopies. In contrast to the oxoiron(IV) formation from the hydroxoiron(III) porphyrin upon one-electron oxidation, the hydroxo complex 4 does not generate oxoiron(IV) species. Reaction of 2 with mCPBA also results in the formation of the iron(III)-phenoxyl radical. One-electron oxidation of 3 leads to oxidative degradation of the fifth EtO ligand to liberate acetaldehyde even at 203 K. The iron(III)-phenoxyl radical shows high reactivity for alcoxide on iron(III) but exhibits virtually no reactivity for alcohols including even benzyl alcohol without a base to remove an alcohol proton. This study explains unique properties of mononuclear nonheme enzymes with Tyr residues and also the poor epoxidation activity of Fe salen compared to Mn and Cr salen compounds.     

Influence of Meso Substituents on Electronic States of (Oxoferryl)porphyrin pi-Cation Radicals

A series of (oxoferryl)porphyrin pi-cation radicals generated from porphyrins substituted at the meso positions with highly electron-withdrawing aryl groups has been characterized: tetrakis-5,10,15,20-(2,6-dichlorophenyl)-, 5-(2-chloro-6-nitrophenyl)-10,15,20-tris(2,6-dichlorophenyl)-, and 5-(2,6-dinitrophenyl)-10,15,20-tris(2,6-dichlorophenyl)porphyrins (porphyrins 1-3, respectively). The physical-chemical properties of the oxidized complexes of 1-3 are compared to those of two (oxoferryl)porphyrin pi-cation radical complexes substituted with electron-releasing aryl groups: tetramesitylporphyrin (TMP) and 2-iodotetramesitylporphyrin (2-iodoTMP). While all of the complexes examined show close correspondance in a number of spectroscopic parameters, some significant differences were observed. In contrast to observations for the oxidized complexes of TMP and 2-iodoTMP, the resonance Raman marker bands nu(2) and nu(11), which are indicators of symmetry state of porphyrin pi-cation radicals of 1-3, do not show the expected downfrequency shifts for oxidation to compound I analogs in a(2u) symmetry states. The upfield hyperfine NMR shifts of the pyrrole beta-proton signals of the compound I analogs of 1-3 are much larger than those for TMP and 2-iodoTMP. These data may be explained by admixture of some a(1u) character into the ground state of radical cations of 1-3, consistent with the hypothesis that electron-withdrawing meso substituents lower the energy of the a(2u) molecular orbital, favoring an a(1u) admixture.     

Observation and analysis of the 2g(1D) ion-pair state of I2: the g/u mixing between the 1u(1D) and 2g(1D) states

We report the analysis of the 2g(1D) ion-pair state of I2 by perturbation-facilitated optical-optical double resonance. The present study began with the observation of the 2g(1D)-A' 3Pi(2u) emission at around 230 nm during the analysis of the ultraviolet emissions originating form the 1u(1D) ion-pair state. The identification of this new transition helped us to specify the wavelengths for detecting the 2g(1D) state by emission, and also to estimate its absolute position. The intermediate states used to observe the 2g(1D) state were the B 3Pi(0u(+))-b' 2u mixed states by the hyperfine interaction, which allowed us to combine the X 1Sigmag(+) ground state with the 2g(1D) state in the (1+1) photon excitation following the optical selection rules for one-photon transitions: 2g(1D)<--b' 2u-B 3Pi(0u(+))<--X 1Sigmag(+). Our analysis covered the 2g(1D) state in the 0< or =v< or =12 and 9< or =J< or =40 ranges. The molecular constants and Rydberg-Klein-Rees (RKR) potential of the 2g(1D) state were reported. We discussed the occurrence of the 2g(1D)-A' 3Pi(2u) emission, when exciting to the 1u(1D) v=0 state, and attributed it to the g/u mixing between the 2g(1D) and 1u(1D) states by the hyperfine interaction. The effect of the perturbation on measured line intensities and lifetimes was evident.     

Direct evidence for an iron(IV)-oxo porphyrin pi-cation radical as an active oxidant in catalytic oxygenation reactions

A high-valent iron(IV)-oxo porphyrin pi-cation radical is an active oxidant in the catalytic oxygenation of organic substrates by an iron(III) porphyrin complex and peracids, whereas an iron(III)-oxidant porphyrin adduct is a sluggish oxidant in iron porphyrin model reactions.     

Structural, NMR, and EPR studies of S = (1)/(2) and S = (3)/(2) Fe(III) bis(4-cyanopyridine) complexes of dodecasubstituted porphyrins

The NMR and EPR spectra for three complexes, iron(III) octamethyltetraphenylporphyrin bis(4-cyanopyridine) perchlorate, [FeOMTPP(4-CNPy)(2)]ClO(4), and its octaethyl- and tetra-beta,beta'-tetramethylenetetraphenylporphyrin analogues, [FeOETPP(4-CNPy)(2)]ClO(4) and [FeTC(6)TPP(4-CNPy)(2)]ClO(4), are presented. The crystal structures of two different forms of [FeOETPP(4-CNPy)(2)]ClO(4) and one form of [FeOMTPP(4-CNPy)(2)]ClO(4) are also reported. Attempts to crystallize [FeTC(6)TPP(4-CNPy)(2)]ClO(4) were not successful. The crystal structure of [FeOMTPP(4-CNPy)(2)]ClO(4) reveals a saddled porphyrin core, a small dihedral angle between the axial ligand planes, 64.3 degrees, and an unusually large tilt angle (24.4 degrees ) of one of the axial 4-cyanopyridine ligands with respect to the normal to the porphyrin mean plane. There are 4 and 2 independent molecules in the asymmetric units of [FeOETPP(4-CNPy)(2)]ClO(4) crystallized from CD(2)Cl(2)/dodecane (1-4) and CDCl(3)/cyclohexane (5-6), respectively. The geometries of the porphyrin cores in 1-6 vary from purely saddled to saddled with 15% ruffling admixture. In all structures, the Fe-N(p) distances (1.958-1.976 A) are very short due to strong nonplanar distortion of the porphyrin cores, while the Fe-N(ax) distances are relatively long ( approximately 2.2 A) compared to the same distances in S = (1)/(2) bis(pyridine)iron(III) porphyrin complexes. An axial EPR signal is observed (g( perpendicular ) = 2.49, g( parallel ) = 1.6) in frozen solutions of both [FeOMTPP(4-CNPy)(2)]ClO(4) and [FeTC(6)TPP(4-CNPy)(2)]ClO(4) at 4.2 K, indicative of the low spin (LS, S = (1)/(2)), (d(yz)d(xz))(4)(d(xy))(1) electronic ground state for these two complexes. In agreement with a recent publication (Ikeue, T.; Ohgo, Y.; Ongayi, O.; Vicente, M. G. H.; Nakamura, M. Inorg. Chem. 2003, 42, 5560-5571), the EPR spectra of [FeOETPP(4-CNPy)(2)]ClO(4) are typical of the S = (3)/(2) state, with g values of 5.21, 4.25, and 2.07. A small amount of LS species with g = 3.03 is also present. However, distinct from previous conclusions, large negative phenyl-H shift differences delta(m) - delta(o) and delta(m) - delta(p) in the (1)H NMR spectra indicate significant negative spin density at the meso-carbons, and the larger than expected positive average CH(2) shifts are also consistent with a significant population of the S = 2 Fe(II), S = (1)/(2) porphyrin pi-cation radical state, with antiferromagnetic coupling between the metal and porphyrin unpaired electrons. This is the first example of this type of porphyrin-to-metal electron transfer to produce a partial or complete porphyrinate radical state, with antiferromagnetic coupling between metal and macrocycle unpaired electrons in an iron porphyrinate. The kinetics of ring inversion were studied for the [FeOETPP(4-CNPy)(2)]ClO(4) complex using NOESY/EXSY techniques and for the [FeTC(6)TPP(4-CNPy)(2)]ClO(4) complex using DNMR techniques. For the former, the free energy of activation, deltaG, and rate of ring inversion in CD(2)Cl(2) extrapolated to 298 K are 63(2) kJ mol(-)(1) and 59 s(-)(1), respectively, while for the latter the rate of ring inversion at 298 K is at least 4.4 x 10(7) s(-)(1), which attests to the much greater flexibility of the TC(6)TPP ring. The NMR and EPR data are consistent with solution magnetic susceptibility measurements that show S = (3)/(2) in the temperature range from 320 to 180 K for [FeOETPP(4-CNPy)(2)](+), while both [FeOMTPP(4-CNPy)(2)](+) and [FeTC(6)TPP(4-CNPy)(2)](+) change their spin state from S = (3)/(2) at room temperature to mainly LS (S = (1)/(2)) upon cooling to 180 K.     

Ferromagnetic spin alignment in head-to-tail coupled oligo(1, 4-phenyleneethynylene)s and Oligo(1,4-phenylenevinylene)s bearing pendant p-phenylenediamine radical cations

The intramolecular and long-range ferromagnetic coupling between p-phenylenediamine radical cations in head-to-tail coupled oligo(1, 4-phenyleneethynylene)s and oligo(1,4-phenylenvinylene)s between neighbors and next-nearest neighbors is described. UV/vis/near-IR experiments show that the radical cations are localized in the pendant p-phenylenediamine units of the conjugated oligomers. The ESR spectra of these oligo(1,4-phenyleneethynylene) and oligo(1, 4-phenylenvinylene) di(radical cation)s are consistent with those of a triplet state. A linear behavior is observed for the doubly integrated ESR intensity of the DeltaM(s) = +/-1 and DeltaM(s) = +/-2 signals with the inverse temperature (I approximately 1/T), consistent with Curie's law. This behavior indicates a triplet ground-state diradical with a large triplet-singlet energy gap or possibly a degeneracy of singlet and triplet states.     

Characterization of a shallow-bound 0g+ valence state of I2 using emission from the D 0u+(3P2) and F' 0u+(1D2) ion-pair states populated by amplified spontaneous emission

A range of vibrational levels of the D 0(u)(+)((3)P(2)) and F' 0(u)(+)((1)D(2)) ion-pair states of I(2) is shown to be easily generated by amplified spontaneous emission (ASE) from their more accessible partners, E 0(g)(+)((3)P(2)) and f' 0(g)(+)((1)D(2)), in sufficient concentration for dispersed fluorescence studies of the D 0(u)(+)((3)P(2)) --> 0(g)(+)(bb) and F' 0(u)(+)((1)D(2)) --> 0(g)(+)(bb) transitions to be carried out. T(0) (J = 49) of this shallow-bound 0(g)(+)(bb) valence state is unambiguously determined and an improved R(e) value of 3.952 +/- 0.005 A is obtained from optimizing the fit of the intensities of the vibrational progressions in the 0(g)(+)(bb) state, and T(e) is found to be 27311.3 +/- 2 cm(-1), leading to D(e) = 442.0 +/- 2 cm(-1).     

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.     

Femtosecond Time-Resolved Stimulated Raman Spectroscopy of the S(2) (1B(u)) Excited State of beta-Carotene

The electronic and vibrational structure of beta-carotene's early excited states are examined using femtosecond time-resolved stimulated Raman spectroscopy. The vibrational spectrum of the short-lived ( approximately 160 fs) second excited singlet state (S(2),1B(u) (+))of beta-carotene is obtained. Broad, resonantly enhanced vibrational features are observed at approximately 1100, 1300, and 1650 cm(-1) that decay with a time constant corresponding to the electronic lifetime of S(2). The temporal evolution of the vibrational spectra are consistent with significant population of only two low-lying excited electronic states (1B(u) (+) and 2A(g) (-)) in the ultrafast relaxation pathway of beta-carotene.     

Formation of iron(III) meso-chloro-isoporphyrin as a reactive chlorinating agent from oxoiron(IV) porphyrin π-cation radical

Iron(III) isoporphyrin, a tautomer of porphyrin with a saturated meso carbon, is one of the isoelectronic forms of oxoiron(IV) porphyrin π-cation radical, which is known as an important reactive intermediate of various heme enzymes. The isoporphyrin has been believed to be incapable of catalyzing oxygenation and oxidation reactions. Here, we report that an oxoiron(IV) porphyrin π-cation radical can be converted to iron(III) meso-chloro-isoporphyrin in the presence of trifluoroacetic acid and chloride ion. More importantly, this study shows the first evidence that iron(III) meso-chloro-isoporphyrin is an excellent reactive agent for chlorinating aromatic compounds and olefins. The results of this study suggest that the mechanism involves electrophilic chlorination of substrate with iron(III) meso-chloro-isoporphyrin.     

Synthesis, Characterization, and Spectroelectrochemistry of Cobalt Porphyrins Containing Axially Bound Nitric Oxide

Several cobalt nitrosyl porphyrins of the form (T(p/m-X)PP)Co(NO) (p/m-X = p-OCH(3) (1), p-CH(3) (2), m-CH(3) (3), p-H (4), m-OCH(3) (5), p-OCF(3) (6), p-CF(3) (7), p-CN (8)) have been synthesized in 30-85% yields by reaction of the precursor cobalt porphyrin with nitric oxide. Compounds 1-7 were also prepared by reaction of the precursor cobalt porphyrin with nitrosonium tetrafluoroborate followed by reduction with cobaltocene. Compounds 1-8 have been characterized by elemental analysis, IR and (1)H NMR spectroscopy, mass spectrometry, and UV-vis spectrophotometry. They are diamagnetic and display nu(NO) bands in CH(2)Cl(2) between 1681 and 1695 cm(-)(1). The molecular structure of 1, determined by a single-crystal X-ray crystallographic analysis, reveals a Co-N-O angle of 119.6(4) degrees. Crystals of 1 are monoclinic, P2/c, with a = 15.052(1) ?, b = 9.390(1) ?, c = 16.274(2) ?, beta = 111.04(1) degrees, V = 2146.8(4) ?(3), Z = 2, T = 228(2) K, D(calcd) = 1.271 g cm(-)(3), and final R1 = 0.0599 (wR2 = 0.1567, GOF = 1.054) for 3330 "observed" reflections with I >/= 2sigma(I). Cyclic voltammetry studies in CH(2)Cl(2) reveal that compounds 1-7 undergo two reversible oxidations and two reversible reductions at low temperature. This is not the case for compound 8, which undergoes two reversible reductions but an irreversible oxidation due to adsorption of the oxidized product onto the electrode surface. Combined electrochemistry-infrared studies demonstrate that each of the compounds 1-7 undergoes a first oxidation at the porphyrin pi ring system and a first reduction at either the metal center or the nitrosyl axial ligand. The formulation for the singly oxidized products of compounds 1-7 as porphyrin pi-cation radicals was confirmed by the presence of bands in the 1289-1294 cm(-)(1) region (for compounds 1-5), which are diagnostic IR bands for generation of tetraarylporphyrin pi-cation radicals.     

Crystal Structure and Spectroscopic Characterization of K(6)(VO)(4)(SO(4))(8) Containing Mixed-Valent Vanadium(IV)-Vanadium(V)

Pleochroistic crystals (dark green to colorless) of a mixed-valence V(IV)-V(V) compound, K(6)(VO)(4)(SO(4))(8), suitable for X-ray determination have been obtained from the catalytically important K(2)S(2)O(7)-V(2)O(5)/SO(2)-O(2)-SO(3)-N(2) molten salt-gas system, at approximately 400 degrees C. The compound crystallizes in the monoclinic space group P2(1) (No. 4) with a = 8.931(2) ?, b = 18.303 (3) ?, c = 9.971(2) ?, beta = 90.11(2) degrees, and Z = 2. It contains two rather similar V(IV)-V(V) pairs of VO(6) octahedra distorted as usual having a short V-O bond of around 1.57 ?, a long bond of around 2.40 ? trans to this, and four equatorial bonds around 2.00 ?. The bond lengths of the V(V)O(6) octahedra are significantly shorter than those found for the V(IV)O(6) octahedra. The eight different SO(4)(2)(-) groups are all bridging bidentate between the V(IV) and V(V) atoms; a third oxygen is coordinated to a vanadium atom of a neighboring chain trans to the short V=O bond, and the fourth oxygen remains uncoordinated. The measured bond distances and angles show a considerable distortion of the SO(4) tetrahedra. This is confirmed by the IR spectra of the compound, where large shift and splitting of the sulfate nu(3) bands up to wave numbers of around 1300 cm(-)(1) is observed. The ESR spectra of the compound exhibit weak anisotropy with g(iso) = 1.972 +/- 0.002 and DeltaB(pp) = 65 +/- 2 G. The compound may cause the deactivation for industrial sulfuric acid catalysts observed around 400 degrees C in highly converted SO(2)-O(2)-N(2) gas mixtures.     

含类立方烷簇芯[MoFe_3S_4]~(n+)(n=4,5)钼铁硫原子簇化合物的电子顺磁共振谱

本文报道簇合物(Et_4N)(MoFe_3S_4(Et_2dtc)_5)CH_3CN(1)和(MoFe_3S_4(C_5H_10NCSS)_5)CH_2Cl_2(2)在室温和液氮温度(77K)测试的ESR谱。在77K簇合物(1)和(2)的ESR谱分别解释为来自具有双量子跃迁(△W=2hv_0)的S=1和菱形对称的S=1/2的自旋体系,基态自旋S值与根据反铁磁性自旋耦合模型和9N—L规则计算结果一致。 簇合物(1)出现gx=gy=gz=(15/7)g_0≈4.3(E》g_0βH,D≈0)的ESR谱可以确认为Fe~(2+)碎片信号,而具有~(95,97)Mo特征超精细结构(a_0(1)≈38G)的g_0(1)=1.9735±0.0002信号来自Mo~(5+)碎片。簇合物(2)亦出现Mo~(5+)碎片信号(g_(2)=1.9748±0.0002,a_0(2)≈38G)。上述事实说明在分子内发生Fe~(2+)→Mo~(6+)净电子密度转移自氧化还原作用,选种电子非定域化有利于簇骼的形成与稳定。     

Sequential electron-transfer and proton-transfer pathways in hydride-transfer reactions from dihydronicotinamide adenine dinucleotide analogues to non-heme oxoiron(IV) complexes and p-chloranil. Detection of radical cations of NADH analogues in acid-promoted hydride-transfer reactions

Hydride transfer from dihydronicotinamide adenine dinucleotide (NADH) analogues, such as 10-methyl-9,10-dihydroacridine (AcrH 2) and its derivatives, 1-benzyl-1,4-dihydronicotinamide (BNAH), and their deuterated compounds, to non-heme oxoiron(IV) complexes such as [(L)Fe (IV)(O)] (2+) (L = N4Py, Bn-TPEN, and TMC) occurs to yield the corresponding NAD (+) analogues and non-heme iron(II) complexes in acetonitrile. Hydride transfer from the NADH analogues to p-chloranil (Cl 4Q) also occurs to produce the corresponding NAD (+) analogues and the hydroquinone anion (Cl 4QH (-)). The logarithms of the observed second-order rate constants (log k H) of hydride transfer from NADH analogues to non-heme oxoiron(IV) complexes are linearly correlated with those of hydride transfer from the same series of NADH analogues to Cl 4Q, including similar kinetic deuterium isotope effects. The log k H values of hydride transfer from NADH analogues to non-heme oxoiron(IV) complexes are also linearly correlated with those of deprotonation of the radical cations of NADH analogues. Such linear correlations indicate that overall hydride-transfer reactions of NADH analogues to both non-heme oxoiron(IV) complexes and Cl 4Q occur via electron transfer from NADH analogues to the oxoiron(IV) complexes, followed by rate-limiting deprotonation from the radical cations of NADH analogues and subsequent rapid electron transfer from the deprotonated radicals to the Fe(III) complexes to yield the corresponding NAD (+) analogues and the Fe(II) complexes. The electron-transfer pathway was accelerated by the presence of perchloric acid, and the resulting radical cations of NADH analogues were detected by electron spin resonance spectroscopy and UV-vis spectrophotometry in the acid-promoted hydride-transfer reactions from NADH analogues to non-heme oxoiron(IV) complexes. This result provides the first direct evidence that a hydride transfer from NADH analogues to non-heme oxoiron(IV) complexes proceeds via an electron-transfer pathway.     

Magnetic, electronic and electrochemical studies of mono and binuclear Cu(II) complexes using novel macrocyclic ligands

A series of new mono and binuclear copper (II) complexes [Cul]X(2)and [Cu(2)lX(2)] where 1 = L(1), L(2) and L(3) are the macrocyclic ligands. In mononuclear complexes the geometry of Cu(II) ion is distorted squareplanar and in binuclear complexes the geometry of Cu(II) is tetragonal. The synthesized complexes were characterized by spectroscopic (IR,UV-vis and ESR) techniques. Electrochemical studies of the complexes reveals that all the mononuclear Cu(II) complexes show a single quasireversible one-electron transfer reduction wave (E(pc) = -0.76 to -0.84V) and the binuclear complexes show two quasireversible one electron transfer reduction waves (E(pc)(1) = -0.86 to -1.01V, E(pc)(2) = -1.11 to -1.43V) in cathodic region. The ESR spectra of mononuclear complexes show four lines with nuclear hyperfine splittings with the observed g(11) values in the ranges 2.20-2.28, g( perpendicular) = 2.01-2.06 and A(11) = 125-273. The binuclear complexes show a broad ESR spectra with g = 2.10-2.11. The room temperature magnetic moment values for the mononuclear complexes are in the range [mu(eff) = 1.70-1.72BM] and for the binuclear complexes the range is [mu(eff) = 1.46-1.59BM].     

Mechanistic insight into the aromatic hydroxylation by high-valent iron(IV)-oxo porphyrin pi-cation radical complexes

Mechanistic studies of the aromatic hydroxylation by high-valent iron(IV)-oxo porphyrin pi-cation radicals revealed that the aromatic oxidation involves an initial electrophilic attack on the pi-system of the aromatic ring to produce a tetrahedral radical or cationic sigma-complex. The mechanism was proposed on the basis of experimental results such as a large negative Hammett rho value and an inverse kinetic isotope effect. By carrying out isotope labeling studies, the oxygen in oxygenated products was found to derive from the iron-oxo porphyrin intermediates.     

Reactivity of the indole ring in palladium(II) complexes of 2N1O-donor ligands: cyclopalladation and pi-cation radical formation

The Pd(II) complexes of new 2N1O-donor ligands containing a pendent indole, 3-[N-2-pyridylmethyl-N-2-hydroxy-3,5-di(tert-butyl)benzylamino]ethylindole (Htbu-iepp), 1-methyl-3-[N-2-pyridylmethyl-N-2-hydroxy-3,5-di(tert-butyl)benzylamino]ethylindole (Htbu-miepp), 3-[N-2-pyridylmethyl-N-2-hydroxy-3,5-di(tert-butyl)benzylamino]methylindole (Htbu-impp), and 3-(N-2-pyridylmethyl-N-4-hydroxybenzylamino)ethylindole (Hp-iepp) (H denotes a dissociable proton), were synthesized, and the structures of [Pd(tbu-iepp)Cl] (1a), [Pd(tbu-iepp-c)Cl] (1b), [Pd(tbu-miepp)Cl] (3), and [Pd(p-iepp-c)Cl] (4) (tbu-iepp-c and p-iepp-c denote tbu-iepp and p-iepp bound to Pd(II) through a carbon atom, respectively) were determined by X-ray analysis. Complexes 1a prepared in CH(2)Cl(2)/CH(3)CN and 3 prepared in CH(3)CN have a pyridine nitrogen, an amine nitrogen, a phenolate oxygen, and a chloride ion in the coordination plane. Complex 1b prepared in CH(3)CN has the same composition as 1a and was revealed to have the C2 atom of the indole ring bound to Pd(II) with the Pd(II)-C2 distance of 1.973(2) A. The same Pd(II)-indole C2 bonding was revealed for 4. Interconversion between 1a and 1b was observed for their solutions, the equilibrium being dependent on the solvent used. Reaction of 1b and 4 with 1 equiv of Ce(IV) in DMF gave the corresponding one-electron-oxidized species, which exhibited an ESR signal at g = 2.004 and an absorption peak at approximately 550 nm, indicating the formation of the Pd(II)-indole pi-cation radical species. The half-life, t(1/2), of the indole radical species at room temperature was calculated to be 20 s (k(obs) = 3.5 x 10(-)(2) s(-)(1)) for 1b. The cyclic voltammogram for 1b in DMF gave two irreversible oxidation peaks at E(pa) = 0.68 and 0.80 V (vs Ag/AgCl), which were ascribed to the oxidation processes of the coordinated indole and phenolate moieties, respectively.