Initial members of the family of molecular mid-valent high-nuclearity iron nitrides: [Fe4N2X10]4- and [Fe10N8X12]5- (X = Cl-, Br-)

Current theoretical and experimental evidence points toward X = N as the identity of the interstitial atom in the [MoFe7S9X] core of the iron-molybdenum cofactor cluster of nitrogenase. This atom functions with mu6 bridging multiplicity to six iron atoms and, if it is nitrogen as nitride, raises a question as to the existence of a family of molecular iron nitrides of higher nuclearity than known dinuclear Fe(III,IV) species with linear [Fe-N-Fe]5+,4+ bridges. This matter has been initially examined by variation of reactant stoichiometry in the self-assembly systems [FeX4]1-/(Me3Sn)3N (X = Cl-, Br-) in acetonitrile. A 2:1 mol ratio affords [Fe4N2Cl10]4- (1), isolated as the Et4N+ salt (72%). This cluster has idealized C2h symmetry with a planar antiferromagnetically coupled [Fe(III)4(mu3-N)2]6+ core containing an Fe2N2 rhombus to which are attached two FeCl3 units. DFT calculations have been performed to determine the dominant magnetic exchange pathway. An 11:8 mol ratio leads to [Fe10N8Cl12]5- (3) as the Et4N+ salt (37%). The cluster possesses idealized D2h symmetry and is built of 15 edge- and vertex-shared rhomboids involving two mu3-N and six mu4-N bridging atoms, and incorporates two of the core units of 1. Four FeN2Cl2 and four FeN3Cl sites are tetrahedral and two FeN5 sites are trigonal pyramidal. The cluster is mixed-valence (9Fe(III) + Fe(IV)); a discrete Fe(IV) site was not detected by crystallography or M?ssbauer spectroscopy. The corresponding clusters [Fe4N2Br10]4- and [Fe10N8Br12]5- are isostructural with 1 and 3, respectively. Future research is directed toward defining the scope of the family of molecular iron nitrides.     

Iron-catalyzed olefin epoxidation in the presence of acetic acid: insights into the nature of the metal-based oxidant

The iron complexes [(BPMEN)Fe(OTf)2] (1) and [(TPA)Fe(OTf)2] (2) [BPMEN = N,N'-bis-(2-pyridylmethyl)-N,N'-dimethyl-1,2-ethylenediamine; TPA = tris-(2-pyridylmethyl)amine] catalyze the oxidation of olefins by H2O2 to yield epoxides and cis-diols. The addition of acetic acid inhibits olefin cis-dihydroxylation and enhances epoxidation for both 1 and 2. Reactions carried out at 0 degrees C with 0.5 mol % catalyst and a 1:1.5 olefin/H2O2 ratio in a 1:2 CH3CN/CH3COOH solvent mixture result in nearly quantitative conversions of cyclooctene to epoxide within 1 min. The nature of the active species formed in the presence of acetic acid has been probed at low temperature. For 2, in the absence of substrate, [(TPA)FeIII(OOH)(CH3COOH)]2+ and [(TPA)FeIVO(NCCH3)]2+ intermediates can be observed. However, neither is the active epoxidizing species. In fact, [(TPA)FeIVO(NCCH3)]2+ is shown to form in competition with substrate oxidation. Consequently, it is proposed that epoxidation is mediated by [(TPA)FeV(O)(OOCCH3)]2+, generated from O-O bond heterolysis of the [(TPA)FeIII(OOH)(CH3COOH)]2+ intermediate, which is promoted by the protonation of the terminal oxygen atom of the hydroperoxide by the coordinated carboxylic acid.     

Octahedral non-heme non-oxo Fe(IV) species stabilized by a redox-innocent N-methylated cyclam-acetate ligand

The ligand 1,4,8-tri-N-methyl-1,4,8,11-tetraazacyclotetradecane-11-acetic acid (Me3cyclam-acetic acid) has been synthesized by Eschweiler-Clarke methylation of cyclam-acetic acid, and the iron(III) complex [(Me3cyclam-acetate)FeN3]PF6, 1, has been synthesized, which has been found to have significantly different properties than its unmethylated analogue, [(cyclam-acetate)FeN3]PF6, 2. Whereas the iron ion in 2 is low spin with S = 1/2, 1 is found to be high spin at temperatures above 100 K, though low-spin species are observed at lower temperatures, indicating a spin crossover phenomenon. The iron(II) species 1red is electrochemically more accessible than 2red since the Fe2+/3+ redox wave in 1 appears approximately 350 mV more positive than the corresponding wave in 2. Also, 1 displays a reversible Fe3+/4+ redox wave, which is irreversible in 2, denoting that the Fe(IV) species 1ox is kinetically stable. 1red and 1ox have been generated electrochemically in solution and studied spectroscopically. M?ssbauer spectroscopy has confirmed that, in both reduction and oxidation, iron is the redox center, that 1red is high spin (S = 2), and that 1ox is low spin (S = 1), in contrast to 2red which is low spin and 2ox which could not be isolated.     

Stoichiometric and catalytic secondary O-atom transfer by Fe(III)-NO2 complexes derived from a planar tetradentate non-heme ligand: reminiscence of heme chemistry

An Fe(III) nitro complex [(bpb)Fe(NO2)(py)] (2) of the tetradentate ligand 1,2-bis(pyridine-2-carboxamido)benzene (H2bpb, H is the dissociable amide proton) has been synthesized via addition of NaNO2 to [(bpb)Fe(py)2](ClO4) (1) in MeCN or DMF. This structurally characterized Fe(III) nitro complex exhibits its nuNO2 at 1384 cm(-1). The reaction of 1 with 2 equiv of Et4NX (X = Cl-, Br-) affords the high-spin complexes (Et4N)[(bpb)Fe(Cl2)] (3) and (Et4N)[(bpb)Fe(Br)2] (4), respectively. The structure of 4 has been determined. The addition of an equimolar amount of Et4NCl, Et4NBr, or Et4NCN to a solution of 2 affords the mixed-ligand complexes (Et4N)[(bpb)Fe(NO2)(Cl)] (5), (Et4N)[(bpb)Fe(NO2)(Br)] (6), and (Et4N)[(bpb)Fe(NO2)(CN)] (7), respectively. These complexes are all low spin with isotropic g values of 2.15. Under anaerobic conditions, the reactions of 5-7 with Ph3P in MeCN afford the five-coordinate {Fe-NO}7 nitrosyl [(bpb)Fe(NO)] (and Ph3PO) via secondary oxygen-atom (O-atom) transfer. The O-atom transfer to Ph3P by 5-7 becomes catalytic in the presence of dioxygen with transfer rates in the range of 1.70-13.59 x 10-3 min(-1). The O-atom transfer rates and turnover numbers (5 > 6 > 7) are reflective of the strength of the axial donors (Cl- > Br- > CN-). The catalytic efficiencies of complexes 5-7 are limited due to formation of the thermodynamic end products [(bpb)Fe(X)2]- (where X = Cl- for 5, Br- for 6, and CN- for 7).     

Single- and double-cubane clusters in the multiple oxidation states [VFe(3)S(4)](3+,2+,1+)

A new series of cubane-type [VFe(3)S(4)](z)() clusters (z = 1+, 2+, 3+) has been prepared as possible precursor species for clusters related to those present in vanadium-containing nitrogenase. Treatment of [(HBpz(3))VFe(3)S(4)Cl(3)](2)(-) (2, z = 2+), protected from further reaction at the vanadium site by the tris(pyrazolyl)hydroborate ligand, with ferrocenium ion affords the oxidized cluster [(HBpz(3))VFe(3)S(4)Cl(3)](1)(-) (3, z = 3+). Reaction of 2 with Et(3)P results in chloride substitution to give [(HBpz(3))VFe(3)S(4)(PEt(3))(3)](1+) (4, z = 2+). Reaction of 4 with cobaltocene reduced the cluster with formation of the edge-bridged double-cubane [(HBpz(3))(2)V(2)Fe(6)S(8)(PEt(3))(4)] (5, z = 1+, 1+), which with excess chloride underwent ligand substitution to afford [(HBpz(3))(2)V(2)Fe(6)S(8)Cl(4)](4)(-) (6, z = 1+, 1+). X-ray structures of (Me(4)N)[3], [4](PF(6)), 5, and (Et(4)N)(4)[6] x 2MeCN are described. Cluster 5 is isostructural with previously reported [(Cl(4)cat)(2)(Et(3)P)(2)Mo(2)Fe(6)S(8)(PEt(3))(4)] and contains two VFe(3)S(4) cubanes connected across edges by a Fe(2)S(2) rhomb in which the bridging Fe-S distances are shorter than intracubane Fe-S distances. M?ssbauer (2-5), magnetic (2-5), and EPR (2, 4) data are reported and demonstrate an S = 3/2 ground state for 2 and 4 and a diamagnetic ground state for 3. Analysis of (57)Fe isomer shifts based on an empirical correlation between shift and oxidation state and appropriate reference shifts results in two conclusions. (i) The oxidation 2 --> 3 + e(-) results in a change in electron density localized largely or completely on the Fe(3) subcluster and associated sulfur atoms. (ii) The most appropriate charge distributions are [V(3+)Fe(3+)Fe(2+)(2)S(4)](2+) (Fe(2.33+)) for 1, 2, and 4 and [V(3+)Fe(3+)(2)Fe(2+)S(4)](3+) (Fe(2.67+)) for 3 and [V(2)Fe(6)S(8)(SEt)(9)](3+). Conclusion i applies to every MFe(3)S(4) cubane-type cluster thus far examined in different redox states at parity of cluster ligation. The formalistic charge distributions are regarded as the best current approximations to electron distributions in these delocalized species. The isomer shifts require that iron atoms are mixed-valence in each cluster.     

Diboryl and Diboranyl Porphyrin Complexes: Synthesis,Structural Motifs,and Redox Chemistry: Diborenyl Porphyrin or Diboranyl Isophlorin?

The syntheses of diboryl porphyrin complexes [(BX2)2(ttp)] (ttp: dianion of tetra-p-tolylporphyrin) and the B-B single-bond diboranyl complexes [(BX)2(ttp)] (X=F, Cl, Br, I) are given. The former are prepared from the reactions of BX3 (X=F, Cl) with [Li2(ttp)] and the latter from B(2)Cl(4) (X=Cl), the reaction of SbF3 with [(BCl)2(ttp)] (for X=F), and, in the cases of X=Br or I, in a remarkable reductive coupling reaction resulting directly from the reaction of BBr3 or BI3 with [Li2(ttp)]. Density functional theory (DFT) calculations on the thermochemical parameters for the reductive coupling reactions (and those calculated for related dipyrromethene complexes) indicate that a combination of the reducing ability of bromide and iodide ions combined with the constrained environment of the porphyrin ligand contribute to the driving force. The reductive coupling is also observed in the reaction of [(BCl2)2(ttp)] with nBuLi to give [(BnBu)2(ttp)], which was characterised crystallographically. The reaction of [(BCl)2(ttp)] with catechol gives a boron catecholato porphyrin complex, [B2(O(2)C(6)H(4))(ttp)]. Chloride abstraction from [(BCl)2(ttp)] gives the planar dication [B2(ttp)]2+, whereas chemical reduction of [(BCl)2(ttp)] by using magnesium anthracenide gives a neutral complex, [B(2)(ttp)], in which the TTP ligand has been reduced by two electrons to give an unusual example of an isophlorin complex. The cationic and neutral complexes [B2(ttp)]2+ and [B2(ttp)] were characterised through a combination of spectroscopic data that is supported by DFT calculations on the porphine analogues.     

2-rhodaoxetanes: their formation of oxidation of [RhI(ethene)]+ and their reactivity upon protonation

New cationic, pentacoordinate complexes [(TPA)Rh1(ethene)]+, [1a]+, and [(MeTPA)Rh1(ethene)]+, [1b]+, have been prepared (TPA = N,N,N-tri(2-pyridylmethyl)amine, MeTPA = N-[(6-methyl-2-pyridyl)-methyl]-N,N-di(2-pyridylmethyl)amine). Complex [1a]+ is selectively converted by aqueous HCl to [(TPA)RhIII-(ethyl)Cl]+, [2a]+. The same reaction with [1b]+ results in the [(MeTPA)RhIII-(ethyl)Cl]+ isomers [2b]+ and [2c]+. Treatment of [1a]+ and [1b]+ with aqueous H2O2 results in a selective oxygenation to the unsubstituted 2-rho-da(III)oxetanes (1-oxa-2-rhoda(III)cyclo-butanes) [(TPA)RhIII(kappa2-C,O-2-oxyethyl)]+, [3a]+, and [(MeTPA)RhIII(kappa2-C,O-2-oxyethyl)]+, [3b]+. The reactivity of 2-rhodaoxetanes [3a]+ and [3b]+ is dominated by the nucleophilic character of their 2-oxyethyl oxygen. Reaction of [3a]+ and [3b]+ with the non-coordinating acid HBAr(f)4 results in the dicationic protonated 2-rhodaoxetanes [(TPA)RhIII(kappa2-2-hydroxyethyl)]2+, [4a]2+, and [(MeTPA)RhIII(kappa2-2-hydroxyethyl)]2+, [4b]2+. These eliminate acetaldehyde at room temperature, probably via a coordinatively unsaturated kappa1-2-hydroxyethyl complex. In acetonitrile, complex [4a]2+ is stabilised as [(TPA)-RhIII(kappa1-2-hydroxyethyl)(MeCN)]2+, [5a]2+, whereas the MeTPA analogue [4b]2+ continues to eliminate acetaldehyde. Reaction of [3a]+ with NH4Cl and Mel results in the coordinatively saturated complexes [(TPA)RhIII(kappa1-2-hydroxyethyl)(Cl)]+, [6a]+, and [(TPA)-RhIII(kappa1-2-methoxyethyl)(I)+, [7a]+, respectively. Reaction of [3a]+ with NH4+ in MeCN results in formation of the dicationic metallacyclic amide [(TPA)-RhIII [kappa2-O,C-2-(acetylamino)ethyl]]2+, [9]2+, via the intermediates [4a]2+, [5a]2+ and the metallacyclic iminoester [(TPA)RhIII[kappa2-N,C-2-(acetimidoyloxy)ethyl]]2+, [8]2+. The observed overall conversion of the [Rh(I)(ethene)] complex [1a]+ to the metallacyclic amide [9]2+ via 2-rhodaoxetane [3a]+, provides a new route for the amidation of a [RhI(ethene)] fragment.     

基于非血红素铁模型配合物[FeⅣ(O)(N4Py)]2+的新型N杂环卡宾配合物的理论设计

基于四价非血红素铁模型配合物[FeⅣ(O)(N4Py)]2+, 通过理论计算设计出一种新型N杂环卡宾配合物[FeⅣ(O)(N4Py)]2+. 采用密度泛函理论B3LYP方法, 计算了[FeⅣ(O)(N4Py)]2+的几何结构和电子结构, 并研究了[FeⅣ(O)(N4Py)]2+使环己烷C-H键羟基化的反应机理. 结果表明, [FeⅣ(O)(N4Py)]2+的五重态能量比基态三重态能量高约5.7 kJ/mol, 故五重态几乎不能参与反应. 赤道方向的配位基N杂环卡宾(NHC)对FeO单元的σ-贡献要大于N4Py的贡献, 而它的空间位阻效应也大于N4Py, 因此2+的稳定性强于[FeⅣ(O)(N4Py)]2+. [FeⅣ(O)(N4Py)]2+的三重态的反应能垒比[FeⅣ(O)(N4Py)]2+的三重态反应能垒高2.0 kJ/mol, 且为单态反应, 所以[FeⅣ(O)(N4Py)]2+的反应活性要高于[FeⅣ(O)(N4Py)]2+.     

Activation of alkenes and H2 by [Fe]-H2ase model complexes

The established ability of the Fe(II) bridging hydride species (micro-H)(micro-pdt)[Fe(CO)2(PMe3)]2+, 1-H+, to take-up and heterolytically activate dihydrogen, resulting in H/D scrambling of H2/D2 and H2/D2O mixtures (Zhao et al. Inorg. Chem. 2002, 41, 3917) has prompted a study of simultaneous alkene/H2 activation by such [Fe]H2ase model complexes. That the required photolysis produced an open site was substantiated by substitution of CO in 1-H+ by CH3CN with formation of structurally characterized [(micro-H)(micro-pdt)[Fe(CO)2(PMe3)][Fe(CO)(CH3CN)(PMe3)]]+[PF6]-. Under similar photolytic conditions, H/D exchange reactions between D2 and terminal alkenes (ethylene, propene and 1-butene), but not bulkier alkenes such as 2-butene or cyclohexene, were catalyzed by 1-H+ and the edt (SCH2CH2S) analogue, 2-H+. Substantial regioselectivity for H/D exchange at the internal vinylic hydrogen was observed. The extent to which the olefins were deuterium enriched vs deuterated was catalyst dependent. The stabilizing effect of the binuclear chelating ligands, SCH2CH2CH2S, pdt, and SCH2CH2S, edt, is required for the activity of binuclear catalysts, as the mono-dentate micro-SEt analogue decomposed to inactive products under the photolytic conditions of the catalysis. Reactions of 1 and 2 with EtOSO2CF3 yielded the S-alkylated products, [(micro-SCH2CH2CH2SEt)[Fe(CO)2(PMe3)]2]+[SO3CF3]- (1-Et+), and 2-Et+, rather than micro-C2H5 analogues to the micro-H of 1-H+. The stability and lack of reactivity toward H2 of 1-Et+ and 2-Et+, indicates they are not on the reaction path of the olefin/D2 H/D exchange process. A mechanism with olefin binding to an open site created by CO loss and formation of an Fe-(CH2CHDR) intermediate is indicated. A likely role of a binuclear chelate effect is implicated for the unique S-XXX-S cofactor in the active site of [Fe]H2ase.     

XAS characterization of a nitridoiron(IV) complex with a very short Fe-N bond

X-ray absorption spectroscopy has been used to characterize the novel nitridoiron(IV) units in two [PhBPR3]Fe(N) complexes (R=iPr and CyCH2) and obtain direct spectroscopic evidence for a very short Fe-N distance. The distance of 1.51-1.55 A reflects the presence of an FeN triple bond in accord with the observed FeN vibration observed for one of these species (nuFeN=1034 cm(-1)). This highly covalent bonding interaction results in the appearance of an unusually intense pre-edge peak, whose estimated area of 100(20) units is much larger than those of the related tetrahedral complexes with FeI-N2-FeI, FeII-NPh2, and FeIIINAd motifs, and those of recently described six-coordinate FeVN and FeVIN complexes. The observation that the FeIV-N distances of two [PhBPR3]Fe(N) complexes are shorter than the FeIV-O bond lengths of oxoiron(IV) complexes may be rationalized on the basis of the greater pi basicity of the nitrido ligand than the oxo ligand and a lower metal coordination number for the Fe(N) complex.     

Mono- vs. bi-metallic assembly on a bulky bis(imino)terpyridine framework: a combined experimental and theoretical study

The bis(imino)terpyridine ligands, 6,6'-{(2,6-i-Pr2C6H3)N=CR}2-2,2':6',2'-C15H9N3 (R = H L1, Me L2), have been prepared in high yield from the condensation reaction of the corresponding carbonyl compound with two equivalents of 2,6-diisopropylaniline. The molecular structure of L2 reveals a transoid relationship between the imino and pyridyl nitrogen groups throughout the ligand framework. Treatment of aldimine-containing L1 with one equivalent or an excess of MX2 in n-BuOH at 110 degrees C gives the mononuclear five-coordinate complexes, [(L1)MX2] (M = Fe, X = Cl 1a; M = Ni, X = Br 1b; M = Zn, X = Cl 1c), in which the metal centre occupies the terpyridine cavity and the imino groups pendant. Conversely, reaction of ketimine-containing L2 with excess MX2 in n-BuOH at 110 degrees C affords the binuclear complexes, [(L2)M2X4] (M = Fe, X = Cl 3a; M = Ni, X = Br 3b; M = Zn, X = Cl 3c), in which one metal centre occupies a bidentate pyridylimine cavity while the other a tridentate bipyridylimine cavity. 1H NMR studies on diamagnetic 3c suggests a fluxional process is operational at ambient temperature in which the central pyridine ring undergoes an exchange between metal coordination. Under less forcing conditions (room temperature in dichloromethane), the monometallic counterpart of 1b [(L2)NiBr2] (2b) has been isolated which can be converted to 3b by addition of one equivalent of (DME)NiBr2 (DME = 1,2-dimethoxyethane) in n-BuOH at 110 degrees C. Quantum mechanical calculations (DFT) have been performed on [(L1)ZnCl2] and [(L2)ZnCl2] for different monometallic conformations and show that 1a is the energetically preferred structure for L1 while there is evidence for dynamic behaviour in L2-containing species leading to bimetallic formation. Single-crystal X-ray diffraction studies have been performed on 1a, 1b, 1c, 2b, 3a, 3b(H2O) and 3c.     

Hydroxide-promoted core conversions of molybdenum-iron-sulfur edge-bridged double cubanes: oxygen-ligated topological PN clusters

The occurrence of a heteroatom X (C, N, or O) in the MoFe7S9X core of the iron-molybdenum cofactor of nitrogenase has encouraged synthetic attempts to prepare high-nuclearity M-Fe-S-X clusters containing such atoms. We have previously shown that reaction of the edge-bridged double cubane [(Tp)2Mo2Fe6S8(PEt3)4] (1) with nucleophiles HQ- affords the clusters [(Tp)2Mo2Fe6S8Q(QH)2](3-) (Q = S, Se) in which HQ- is a terminal ligand and Q(2-) is a mu2-bridging atom in the core. Reactions with OH- used as such or oxygen nucleophiles generated in acetonitrile from (Bu3Sn)2O or Me3SnOH and fluoride were examined. Reaction of 1 with Et4NOH in acetonitrile/water generates [(Tp)2Mo2Fe6S9(OH)2]3- (3), isolated as [(Tp)2Mo2Fe6S9(OH)(OC(=NH)Me)(H2O)](3-) and shown to have the [Mo2Fe6(mu2-S)2(mu3-S)6(mu6-S)] core topology very similar to the P(N) cluster of nitrogenase. The reaction system 1/Et4NOH in acetonitrile/methanol yields the P(N)-type cluster [(Tp)2Mo2Fe6S9(OMe)2(H2O)](3-) (5). The system 1/Me3SnOH/F- affords the oxo-bridged double P(N)-type cluster {[(Tp)2Mo2Fe6S9(mu2-O)]2}5- (7), convertible to the oxidized cluster {[(Tp)2Mo2Fe6S9(mu2-O)]2}4- (6), which is prepared independently from [(Tp)2Mo2Fe6S9F2(H2O)](3-)/(Bu3Sn)2O. In the preparations of 3-5 and 7, hydroxide liberates sulfide from 1 leading to the formation of P(N)-type clusters. Unlike reactions with HQ-, no oxygen atoms are integrated into the core structures of the products. However, the half-dimer composition [Mo2Fe6S9O] relates to the MoFe7S9 constitution of the putative native cluster with X = O. (Tp = hydrotris(pyrazolyl) borate(1-)).     

Mononuclear (nitrido)iron(V) and (oxo)iron(IV) complexes via photolysis of [(cyclam-acetato)FeIII(N3)]+ and ozonolysis of [(cyclam-acetato)FeIII(O3SCF3)]+ in water/acetone mixtures

Reaction of the monoanionic, pentacoordinate ligand lithium 1,4,8,11-tetraazacyclotetradecane-1-acetate, Li(cyclam-acetate), with FeCl3 yields, upon addition of KPF6, [(cyclam-acetato)FeCl]PF6 (1) as a red microcrystalline solid. Addition of excess NaN3 prior to addition of KPF6 yields the azide derivative [(cyclam-acetato)FeN3]PF6 (2a) as orange microcrystals. The X-ray crystal structure of the azide derivative has been determined as the tetraphenylborate salt (2b). Reaction of 1 with silver triflate yields [(cyclam-acetato)Fe(O3SCF3)]PF6 (3), which partially dissociates triflate in nondried solvents to yield a mixture of triflate and aqua bound species. Each of the iron(III) derivatives is low-spin (d5, S = 1/2) as determined by variable-temperature magnetic susceptibility measurements, M?ssbauer and EPR spectroscopy. The low-spin iron(II) (d6, S = 0) complexes 1red and 2ared have been prepared by electrochemical and chemical methods and have been characterized by M?ssbauer spectroscopy. Photolysis of 2a at 419 nm in frozen acetonitrile yields a nearly colorless species in approximately 80% conversion with an isomer shift delta = -0.04 mm/s and a quadrupole splitting delta EQ = -1.67 mm/s. A spin-Hamiltonian analysis of the magnetic M?ssbauer spectra is consistent with an FeV ion (d3, S = 3/2). The proposed [(cyclam-acetato)FeV=N]+ results from the photooxidation of 2a via heterolytic N-N cleavage of coordinated azide. Photolysis of 2a in acetonitrile solution at -35 degrees C (300 nm) or 20 degrees C (Hg immersion lamp) results primarily in photoreduction via homolytic Fe-Nazide cleavage yielding FeII (d,6 S = 0) with an isomer shift delta = 0.56 mm/s and quadrupole splitting delta EQ = 0.54 mm/s. A minor product containing high-valent iron is suggested by M?ssbauer spectroscopy and is proposed to originate from [((cyclam-acetato)Fe)2(mu-N)]2+ with a mixed-valent (FeIV(mu-N)FeIII))4+S = 1/2 core. Exposure of 3 to a stream of oxygen/ozone at low temperatures (-80 degrees C) in acetone/water results in a single oxidized product with an isomer shift delta = 0.01 mm/s and quadrupole splitting delta EQ = 1.37 mm/s. A spin-Hamiltonian analysis of the magnetic M?ssbauer yields parameters similar to those of compound II of horseradish peroxidase which are consistent with an FeIV=O monomeric complex (S = 1).     

Aqueous solution speciation of Fe(III) complexes with dihydroxamate siderophores alcaligin and rhodotorulic acid and synthetic analogues using electrospray ionization mass spectrometry

Aqueous solutions of Fe3+ complexes of cyclic (alcaligin) and linear (rhodotorulic acid) dihydroxamate siderophores and synthetic linear eight-carbon-chain and two-carbon-chain dihydroxamic acids ([CH3N(OH)C=O)]2(CH2)n; H2Ln; n = 2 and 8) were investigated by electrospray ionization mass spectrometry (ESI-MS). Information was obtained relevant to the structure and the speciation of various Fe(III)-dihydroxamate complexes present in aqueous solution by (1) comparing different ionization techniques (ESI and FAB), (2) altering the experimental parameters (Fe3+/ligand ratio, pH, cone voltage), (3) using high-stability hexacoordinated Fe(III) siderophore complex mixtures (ferrioxamine B/ferrioxamine E) as a calibrant to quantify intrinsically neutral (H+ clustered or protonated) and intrinsically charged complexes, and (4) using mixed-metal complexes containing Fe3+, Ga3+, and Al3+. These results illustrate that for all dihydroxamic acid ligands investigated multiple tris- and bis-chelated mono- and di-Fe(III) species are present in relative concentrations that depend on the pH and Fe/L ratio.     

Nitrogen-ligated iron complexes: photolytic approach to the FeN+ moiety

The synthesis of (PNP)FeCl, (PNP)Fe[NH(xylyl)], and (PNP)FeN3 are reported(PNP = (tBu2PCH2SiMe2)2N-). While the azide is thermally stable, it is photosensitive to lose N2 and form [(PNPN)Fe]2,in which the nitride ligand has formed a double bond to one phosphorus, and this N bridges to a second iron to form a 2-fold symmetric dimer. The reaction energy to form the (undetected) monomeric [eta3- tBu2PCH2SiMe2NSiMe2CH2PtBu2N]Fe is -15.9 kcal/mol, so this PIII --> PV oxidation is favorable. The eta2 version of this same species is less stable by 23.7 kcal/mol, which shows that the loss of one P--> Fe bond is caused by dimerization, and therefore, it does not precede and cause dimerization. A comparison is made to Ru analogs.     

Novel iron(III) complexes of tripodal and linear tetradentate bis(phenolate) ligands: close relevance to intradiol-cleaving catechol dioxygenases

Four new iron(III) complexes of the bis(phenolate) ligands N,N-dimethyl-N',N'-bis(2-hydroxy-3,5-dimethylbenzyl)ethylenediamine [H2(L1)], N,N-dimethyl-N',N'-bis(2-hydroxy-4-nitrobenzyl)ethylenediamine [H2(L2)], N,N'-dimethyl-N,N'-bis(2-hydroxy-3,5-dimethylbenzyl)ethylenediamine [H2(L3)], and N,N'-dimethyl-N,N'-bis(2-hydroxy-4-nitrobenzyl)ethylenediamine [H2(L4)] have been isolated and studied as structural and functional models for the intradiol-cleaving catechol 1,2-dioxygenases (CTD). The complexes [Fe(L1)Cl] (1), [Fe(L2)(H2O)Cl] (2), [Fe(L3)Cl] (3), and [Fe(L4)(H2O)Cl] (4) have been characterized using absorption spectral and electrochemical techniques. The single-crystal X-ray structures of the ligand H2(L1) and the complexes 1 and 2 have been successfully determined. The tripodal ligand H2(L1) containing a N2O2 donor set represents the metal-binding region of the iron proteins. Complex 1 contains an FeN2O2Cl chromophore with a novel trigonal bipyramidal coordination geometry. While two phenolate oxygens and an amine nitrogen constitute the trigonal plane, the other amine nitrogen and chloride ion are located in the axial positions. In contrast, 2 exhibits a rhombically distorted octahedral coordination geometry for the FeN2O3Cl chromophore. Two phenolate oxygen atoms, an amine nitrogen atom, and a water molecule are located on the corners of a square plane with the axial positions being occupied by the other nitrogen atom and chloride ion. The interaction of the complexes with a few monodentate bases and phenolates and differently substituted catechols have been investigated using absorption spectral and electrochemical methods. The effect of substituents on the phenolate rings on the electronic spectral features and FeIII/FeII redox potentials of the complexes are discussed. The interaction of the complexes with catecholate anions reveals changes in the phenolate to iron(III) charge-transfer band and also the appearance of a low-energy catecholate to iron(III) charge-transfer band similar to catechol dioxygenase-substrate complexes. The redox behavior of the 1:1 adducts of the complexes with 3,5-di-tert-butylcatechol (H2DBC) has been also studied. The reactivities of the present complexes with H2DBC have been studied and illustrated. Interestingly, only 2 and 4 catalyze the intradiol-cleavage of H2DBC, the rate of oxygenation being much faster for 4. Also 2, but not 4, yields an extradiol cleavage product. The reactivity of the complexes could be illustrated not on the basis of the Lewis acidity of the complexes alone but by assuming that the product release is the rate-determining phase of the catalytic reaction.     

Formation of ternary complexes by coordination of (diethylenetriamine)-platinum(II) to N1 or N7 of the adenine moiety of the antiviral nucleotide analogue 9

The synthesis of (Dien)Pt(PMEA-N1), where Dien = diethylenetriamine and PMEA2- = dianion of 9-[2-(phosphonomethoxy)ethyl]adenine, is described. The acidity constants of the threefold protonated H3[(Dien)Pt(PMEA-N1)]3+ complex were determined and in part estimated (UV spectrophotometry and potentiometric pH titration): The release of the proton from the (N7)H+ site in H4[(Dien)Pt(PMEA-N1)]3+ occurs with a rather low pKa (= 0.52+/-0.10). The release of the proton from the -P(O)2(OH) group (pKa = 6.69+/-0.03) in H[(Dien)Pt(PMEA-N1)]+ is only slightly affected by the N1-coordinated (Dien)Pt2+ unit. Comparison with the acidic properties of the H[(Dien)Pt(PMEA-N7)]+ species provides evidence that in the (Dien)Pt(PMEA-N7) complex in aqueous solution an intramolecular, outer-sphere macrochelate is formed through hydrogen bonds between the -PO3(2-) residue of PMEA2- and a PtII-coordinated (Dien)NH2 group; its formation degree amounts to about 40%. The stability constants of the M[(Dien)Pt(PMEA-N1)]2+ complexes with M2+ = Mg2+, Ca2+, Ni2+, Cu2+ and Zn2+ were measured by potentiometric pH titrations in aqueous solution at 25 degrees C and I = 0.1 M (NaNO3). Application of previously determined straight-line plots of log K(M(R-PO3))M versus pK(H(R-PO3)H for simple phosph(on)ate ligands. R-PO3(2-), where R represents a non-inhibiting residue without an affinity for metal ions, proves that the primary binding site of (Dien)Pt(PMEA-N1) is the phosphonate group with all metal ions studied; in fact, Mg2+, Ca2+ and Ni2+ coordinate (within the error limits) only to this site. For the Cu[(Dien)Pt(PMEA-N1)]2+ and Zn[(Dien)Pt(PMEA-N1)]2- systems also the formation of five-membered chelates involving the ether oxygen of the -CH2-O-CH2-PO3(2-) residue could be detected; the formation degrees are about 60% and 30%, respectively. The metal-ion-binding properties of the isomeric (Dien)Pt(PMEA-N7) species studied previously differ in so far that the resulting M[(Dien)Pt(PMEA-N7)]2+ complexes are somewhat less stable, but again Cu2+ and Zn2+ also form with this ligand comparable amounts of the mentioned five-membered chelates. In contrast, both M[(Dien)Pt(PMEA-N1/N7)]2+ complexes differ from the parent M(PMEA) complexes considerably; in the latter instance the formation of the five-membered chelates is of significance for all divalent metal ions studied. The observation that divalent metal-ion binding to the phosphonate group of (Dien)Pt(PMEA-N1) and (Dien)Pt(PMEA-N7) is only moderately inhibited (about 0.2-0.4 log units) by the twofold positively charged (Dien)Pt2+ unit at the adenine residue allows the general conclusion, considering that PMEA is a nucleotide analogue, that this is also true for nucleotides and that consequently participation of, for example, two metal ions in an enzymatic process involving nucleotides is not seriously hampered by charge repulsion.     

Biomimetic iron(III) complexes of N3O and N3O2 donor ligands: protonation of coordinated ethanolate donor enhances dioxygenase activity

A series of iron(III) complexes 1-4 of the tripodal tetradentate ligands N,N-bis(pyrid-2-ylmethyl)-N-(2-hydroxyethyl)amine H(L1), N,N-bis(pyrid-2-ylmethyl)-N-(2-hydroxy- propyl)amine H(L2), N,N-bis(pyrid-2-ylmethyl)-N-ethoxyethanolamine H(L3), and N-((pyrid-2-ylmethyl)(1-methylimidazol-2-ylmethyl))-N-(2-hydroxyethyl)amine H(L4), have been isolated, characterized and studied as functional models for intradiol-cleaving catechol dioxygenases. In the X-ray crystal structure of [Fe(L1)Cl(2)] 1, the tertiary amine nitrogen and two pyridine nitrogen atoms of H(L1) are coordinated meridionally to iron(III) and the deprotonated ethanolate oxygen is coordinated axially. In contrast, [Fe(HL3)Cl(3)] 3 contains the tertiary amine nitrogen and two pyridine nitrogen atoms coordinated facially to iron(III) with the ligand ethoxyethanol moiety remaining uncoordinated. The X-ray structure of the bis(μ-alkoxo) dimer [{Fe(L5)Cl}(2)](ClO(4))(2)5, where HL is the tetradentate N(3)O donor ligand N,N-bis(1-methylimidazol-2-ylmethyl)-N-(2-hydroxyethyl)amine H(L5), contains the ethanolate oxygen donors coordinated to iron(III). Interestingly, the [Fe(HL)(DBC)](+) and [Fe(HL3)(HDBC)X] adducts, generated by adding ～1 equivalent of piperidine to solutions containing equimolar quantities of iron(III) complexes 1-5 and H(2)DBC (3,5-di-tert-butylcatechol), display two DBC(2-)→ iron(III) LMCT bands (λ(max): 1, 577, 905; 2, 575,915; 3, 586, 920; 4, 563, 870; 5, 557, 856 nm; Δλ(max), 299-340 nm); however, the bands are blue-shifted (λ(max): 1, 443, 700; 2, 425, 702; 3, 424, 684; 4, 431, 687; 5, 434, 685 nm; Δλ(max), 251-277 nm) on adding 1 more equivalent of piperidine to form the adducts [Fe(L)(DBC)] and [Fe(HL3)(HDBC)X]. Electronic spectral and pH-metric titration studies in methanol disclose that the ligand in [Fe(HL)(DBC)](+) is protonated. The [Fe(L)(DBC)] adducts of iron(III) complexes of bis(pyridyl)-based ligands (1,2) afford higher amounts of intradiol-cleavage products, whereas those of mono/bis(imidazole)-based ligands (4,5) yield mainly the auto-oxidation product benzoquinone. It is remarkable that the adducts [Fe(HL)(DBC)](+)/[Fe(HL3)(DBC)X] exhibit higher rates of oxygenation affording larger amounts of intradiol-cleavage products and lower amounts of benzoquinone.     

Cyano-bridged structures based on [MnIIN3O2-macrocycle)]2+: a synthetic, structural, and magnetic study

Reactions between the complex [MnII(L)]2+, where L is a N3O2 macrocyclic ligand, and different cyanometalate precursors such as [M(CN)n]m- (M(III) = Cr, Fe; M(II) = Fe, Ni, Pd, Pt) lead to cyano-bridged molecular assemblies exhibiting a variety of structural topologies. The reaction between [MnII(L)]2+ and [FeII(CN)6]4- forms a trinuclear complex with formula [(MnII(L)(H2O))2(FeII(micro-CN)2(CN)4)] x 2MeOH x 10H2O (1) which crystallizes in the triclinic space group P1. The reaction between [MnII(L)]2+ and [M(II)(CN)4]2-, where M(II) = Ni (2), Pd (3), Pt (4), gives rise to three isostructural linear chain compounds with stoichiometry [(MnII(L))(M(II)(micro-CN)2(CN)2)]n and which crystallize in the monoclinic space group C2/c. The self-assembly between [MnII(L)]2+ with [M(III)(CN)6]3-, where M(III) = Cr (5), Fe (6, 7, 8), forms three types of compounds. Compounds 5 and 6 are isostructural (monoclinic, space group P2(1)/n), and the structures comprise anionic linear chains [(MnII(L))(M(III)(micro-CN)2(CN)4)]n(n-) with cationic trinuclear complexes [(MnII(L)(H2O))2(M(III)(micro-CN)2(CN)4)]+ as counterions. Using an excess of K3[FeIII(CN)6], an analogous compound to 6 but with K+ as counterion is obtained (7), which crystallizes in the triclinic space group P1. Compound 8 consists of 2-D layers with formula [(MnII(L))3(FeIII(micro-CN)4(CN)2)(FeIII(micro-CN)2(CN)4)]n x 2nMeOH; it crystallizes in the monoclinic space group P2(1)/n. The magnetic properties were investigated for all samples. In particular, compound 5, which shows antiferromagnetic exchange interactions between Mn(II) and Cr(III) ions through cyanide bridging ligands, has been studied in detail; the magnetic exchange parameter amounts to J = -7.5(7) cm(-1). Compound 8 shows a magnetically ordered phase below 6.4 K which is confirmed by M?ssbauer spectroscopy; two hyperfine split spectra were observed below Tc from which IJI values of 2.1 and 1.6 cm(-1) could be deduced.     

Preparation and characterization of diarylphosphazene and diarylphosphinohydrazide complexes of titanium, tungsten and ruthenium and phosphorylketimido complexes of rhenium

Reaction of the proligand Ph2PN(SiMe3)2 (L1) with WCl6 gives the oligomeric phosphazene complex [WCl4(NPPh2)]n, 1 and subsequent reaction with PMe2Ph or NBu4Cl gives [WCl4(NPPh2)(PMe2Ph)] (2) or [WCl5(NPPh2)][NBu4] (3), respectively. DF calculations on [WCl5(NPPh2)][NBu4] show a W=N double bond (1.756 A) and a P-N bond distance of 1.701 A, which combined with the geometry about the P atom suggests, there is no P-N multiple bonding. Reaction of L1 with [ReOX3(PPh3)2] in MeCN (X = Cl or Br) gives [ReX2(NC(CH3)P(O)Ph2)(MeCN)(PPh3)](X = Cl, 4, X = Br, 5) which contains the new phosphorylketimido ligand. It is bound to the rhenium centre with a virtually linear Re-N-C arrangement (Re-N-C angle = 176.6 degrees, when X = Cl) and there is multiple bonding between Re and N (Re-N = 1.809(7) A when X = Cl). The proligand Ph2PNHNMe2(L2H) reacts with [(C5H5)TiCl3] to give [(C5H5)TiCl2(Me2NNPPh2)] (6). An X-ray crystal structure of the complex shows the ligand (L2) is bound by both nitrogen atoms. Reaction of the proligands Ph2PNHNR2[R2 = Me2 (L2H), -(CH2CH2)2NCH3 (L3H), (CH2CH2)2CH2 (L4H)] with [{RuCl(mu-Cl)(eta6-p-MeC6H4iPr)}2] gave [RuCl2(eta6-p-MeC6H4iPr)L] {L = L2H (7), L3H (8), L4H (9)}. The X-ray crystal structures of 7-9 confirmed that the phosphinohydrazine ligand is neutral and bound via the phosphorus only. Reaction of complexes 7-9 with AgBF4 resulted in chloride ion abstraction and the formation of the cationic species [RuCl(6-p-MeC6H4iPr)(L)]+ BF4- {(L = L2H (10), L3H (11), L4H (12)}. Finally, reaction of complex 6 with [{RuCl(mu-Cl)(eta6-p-MeC6H4iPr)}2] gave the binuclear species [(eta6-p-MeC6H4iPr)Cl2Ru(mu2,eta3-Ph2PNNMe2)TiCl2(C5H5)], 13.