Modeling the syn disposition of nitrogen donors at the active sites of carboxylate-bridged diiron enzymes. Enforcing dinuclearity and kinetic stability with a 1,2-diethynylbenzene-based ligand

The syn coordination of histidine residues at the active sites of several carboxylate-rich non-heme diiron enzymes has been difficult to reproduce with small molecule model compounds. In this study, ligands derived from 1,8-naphthyridine, phthalazine, and 1,2-diethynylbenzene were employed to mimic this geometric feature. The preassembled diiron(II) complex [Fe(2)(micro-O(2)CAr(Tol))(2)(O(2)CAr(Tol))(2)(THF)(2)] (1), where Ar(Tol)CO(2)(-) is the sterically hindered carboxylate 2,6-di(p-tolyl)benzoate, served as a convenient starting material for the preparation of iron(II) complexes, all of which were crystallographically characterized. Use of the ligand 2,7-dimethyl-1,8-naphthyridine (Me(2)-napy) afforded the mononuclear complex [Fe(O(2)CAr(Tol))(2)(Me(2)-napy)] (2), whereas dinuclear [Fe(2)(micro-DMP)(micro-O(2)CAr(Tol))(2)(O(2)CAr(Tol))(2)(THF)] (3) resulted when 1,4-dimethylphthalazine (DMP) was employed. The dinuclear core of compound 3 is kinetically labile, as evidenced by the formation of [Fe(O(2)CAr(Tol))(2)(vpy)(2)] (4) upon addition of 2-vinylpyridine (vpy). The diiron analogue of 4, [Fe(2)(micro-O(2)CAr(Tol))(2)(O(2)CAr(Tol))(2)(vpy)(2)] (5), was prepared directly from 1. When the sterically more demanding ligand 2,6-di(4-tert-butylphenyl)benzoate (Ar(4-tBuPh)CO(2)(-)) was used, mononuclear [Fe(O(2)CAr(4)(-)(tBuPh))(2)(THF)(2)] (6) and [Fe(O(2)CAr(4)(-)(tBuPh))(2)(DMP)(2)] (7) formed. The difficulty in stabilizing a dinuclear core with these simple (N)(2)-donor ligands was circumvented by preparing a family of 1,2-diethynylbenzene-based ligands, from which were readily assembled the complexes [Fe(2)(Et(2)BCQEB(Et))(micro-O(2)CAr(Tol))(3)](OTf) (15) and [Cu(2)(Et(2)BCQEB(Et))(micro-I)(2)] (16), where Et(2)BCQEB(Et) is 1,2-bis(3-ethynyl-8-carboxylatequinoline)benzene ethyl ester. The Et(2)BCQEB(Et) framework provides both structural flexibility and the desired syn nitrogen donor geometry, thus serving as a good first-generation ligand in this class.     

Modeling dioxygen-activating centers in non-heme diiron enzymes: carboxylate shifts in diiron(II) complexes supported by sterically hindered carboxylate ligands

General synthetic routes are described for a series of diiron(II) complexes supported by sterically demanding carboxylate ligands 2,6-di(p-tolyl)benzoate (Ar(Tol)CO(2)(-)) and 2,6-di(4-fluorophenyl)benzoate (Ar(4-FPh)CO(2)(-)). The interlocking nature of the m-terphenyl units in self-assembled [Fe(2)(mu-O(2)CAr(Tol))(2)(O(2)CAr(Tol))(2)L(2)] (L = C(5)H(5)N (4); 1-MeIm (5)) promotes the formation of coordination geometries analogous to those of the non-heme diiron cores in the enzymes RNR-R2 and Delta 9D. Magnetic susceptibility and M?ssbauer studies of 4 and 5 revealed properties consistent with weak antiferromagnetic coupling between the high-spin iron(II) centers. Structural studies of several derivatives obtained by ligand substitution reactions demonstrated that the [Fe(2)(O(2)CAr')(4)L(2)] (Ar' = Ar(Tol); Ar(4-FPh)) module is geometrically flexible. Details of ligand migration within the tetracarboxylate diiron core, facilitated by carboxylate shifts, were probed by solution variable-temperature (19)F NMR spectroscopic studies of [Fe(2)(mu-O(2)CAr(4-FPh))(2)-(O(2)CAr(4-FPh))(2)(THF)(2)] (8) and [Fe(2)(mu-O(2)CAr(4-FPh))(4)(4-(t)BuC(5)H(4)N)(2)] (12). Dynamic motion in the primary coordination sphere controls the positioning of open sites and regulates the access of exogenous ligands, processes that also occur in non-heme diiron enzymes during catalysis.     

Functional mimic of dioxygen-activating centers in non-heme diiron enzymes: mechanistic implications of paramagnetic intermediates in the reactions between diiron(II) complexes and dioxygen

Two tetracarboxylate diiron(II) complexes, [Fe(2)(mu-O(2)CAr(Tol))(2)(O(2)CAr(Tol))(2)(C(5)H(5)N)(2)] (1a) and [Fe(2)(mu-O(2)CAr(Tol))(4)(4-(t)BuC(5)H(4)N)(2)] (2a), where Ar(Tol)CO(2)(-) = 2,6-di(p-tolyl)benzoate, react with O(2) in CH(2)Cl(2) at -78 degrees C to afford dark green intermediates 1b (lambda(max) congruent with 660 nm; epsilon = 1600 M(-1) cm(-1)) and 2b (lambda(max) congruent with 670 nm; epsilon = 1700 M(-1) cm(-1)), respectively. Upon warming to room temperature, the solutions turn yellow, ultimately converting to isolable diiron(III) compounds [Fe(2)(mu-OH)(2)(mu-O(2)CAr(Tol))(2)(O(2)CAr(Tol))(2)L(2)] (L = C(5)H(5)N (1c), 4-(t)BuC(5)H(4)N (2c)). EPR and M?ssbauer spectroscopic studies revealed the presence of equimolar amounts of valence-delocalized Fe(II)Fe(III) and valence-trapped Fe(III)Fe(IV) species as major components of solution 2b. The spectroscopic and reactivity properties of the Fe(III)Fe(IV) species are similar to those of the intermediate X in the RNR-R2 catalytic cycle. EPR kinetic studies revealed that the processes leading to the formation of these two distinctive paramagnetic components are coupled to one another. A mechanism for this reaction is proposed and compared with those of other synthetic and biological systems, in which electron transfer occurs from a low-valent starting material to putative high-valent dioxygen adduct(s).     

Synthetic analogue of the [Fe(2)(mu-OH)(2)(mu-O(2)CR)](3+) core of soluble methane monooxygenase hydroxylase via synthesis and dioxygen reactivity of carboxylate-bridged diiron(II) complexes

We describe the synthesis and dioxygen reactivity of diiron(II) tetracarboxylate complexes [Fe(2)(mu-O(2)CAr(Tol))(2)(O(2)CAr(Tol))(2)(N,N-Me(2)en)(2)] (2) and [Fe(2)(mu-O(2)CAr(Tol))(2)(O(2)CAr(Tol))(2)(N,N-Bn(2)en)(2)] (6), where Ar(Tol)CO(2)(-) = 2,6-di(p-tolyl)benzoate. These complexes were prepared as models for the diiron(II) center in the hydroxylase component of soluble methane monooxygenase (MMOH). Compound 6 reacts with dioxygen to afford PhCHO in approximately 60(5)% yield, following oxidative N-dealkylation of the pendant benzyl group on the diamine ligand. The diiron(III) complex [Fe(2)(mu-OH)(2)(mu-O(2)CAr(Tol))(O(2)CAr(Tol))(3)(N-Bnen)(N,N-Bn(2)en)] (8) was isolated from the reaction mixture. The 4.2 K M?ssbauer spectrum of 8 displays a single quadrupole doublet with parameters delta = 0.48(2) mm s(-1) and Delta E(Q) = 0.61(2) mm s(-1). The [Fe(2)(mu-OH)(2)(mu-O(2)CR)](3+) core structure in 8 matches that of the fully oxidized form of MMOH. The conversion of 6 to 8 closely parallels the chemistry of MMOH in which an O(2)-derived oxygen atom is inserted into the C-H bond of methane. Several reaction pathways are considered to account for this novel chemical transformation, and these are compared with mechanistic frameworks previously developed for related cytochrome P450 and copper(I) dioxygen chemistry.     

Synthesis, characterization, and preliminary oxygenation studies of benzyl- and ethyl-substituted pyridine ligands of carboxylate-rich diiron(II) complexes

In this study benzyl and ethyl groups were appended to pyridine and aniline ancillary ligands in diiron(II) complexes of the type [Fe(2)(mu-O(2)CAr(R))(2)(O(2)CAr(R))(2)(L)(2)], where (-)O(2)CAr(R) is a sterically hindered terphenyl carboxylate, 2,6-di(p-tolyl)- or 2,6-di(p-fluorophenyl)benzoate (R = Tol or 4-FPh, respectively). These crystallographically characterized compounds were prepared as analogues of the diiron(II) center in the hydroxylase component of soluble methane monooxygenase (MMOH). The use of 2-benzylpyridine (2-Bnpy) yielded doubly bridged [Fe(2)(mu-O(2)CAr(Tol))(2)(O(2)CAr(Tol))(2)(2-Bnpy)(2)] (1) and [Fe(2)(mu-O(2)CAr(4)(-)(FPh))(2)(O(2)CAr(4)(-)(FPh))(2)(2-Bnpy)(2)] (4), whereas tetra-bridged [Fe(2)(mu-O(2)CAr(Tol))(4)(4-Bnpy)(2)] (3) resulted when 4-benzylpyridine (4-Bnpy) was employed. Similarly, 2-(4-chlorobenzyl)pyridine (2-(4-ClBn)py) and 2-benzylaniline (2-Bnan) were employed as N-donor ligands to prepare [Fe(2)(mu-O(2)CAr(Tol))(2)(O(2)CAr(Tol))(2)(2-(4-ClBn)py)(2)] (2) and [Fe(2)(mu-O(2)CAr(Tol))(2)(O(2)CAr(Tol))(2)(2-Bnan)(2)] (5). The placement of the substituent on the pyridine ring had no effect on the geometry of the diiron(II) compounds isolated when 2-, 3-, or 4-ethylpyridine (2-, 3-, or 4-Etpy) was introduced as the ancillary nitrogen ligand. The isolated [Fe(2)(mu-O(2)CAr(Tol))(2)(O(2)CAr(Tol))(2)(2-Etpy)] (6), [Fe(2)(mu-O(2)CAr(Tol))(2)(O(2)CAr(Tol))(2)(3-Etpy)] (7), [Fe(2)(mu-O(2)CAr(Tol))(2)(O(2)CAr(Tol))(2)(4-Etpy)] (8), and [Fe(2)(mu-O(2)CAr(4)(-FPh))(2)(O(2)CAr(4)(-)(FPh))(2)(2-Etpy)(2)] (9) complexes all contain doubly bridged metal centers. The oxygenation of compounds 1-9 was studied by GC-MS and NMR analysis of the organic fragments following decomposition of the product complexes. Hydrocarbon fragment oxidation occurred for compounds in which the substrate moiety was in close proximity to the diiron center. The extent of oxidation depended upon the exact makeup of the ligand set.     

Design and synthesis of a novel triptycene-based ligand for modeling carboxylate-bridged diiron enzyme active sites

A novel triptycene-based ligand with a preorganized framework was designed to model carboxylate-bridged diiron active sites in bacterial multicomponent monooxygenase (BMM) hydroxylase enzymes. The synthesis of the bis(benzoxazole)-appended ligand L1 depicted was accomplished in 11 steps. Reaction of L1 with iron(II) triflate and a carboxylate source afforded the desired diiron(II) complex [Fe(2)L1(μ-OH)(μ-O(2)CAr(Tol))(OTf)(2)].     

Water affects the stereochemistry and dioxygen reactivity of carboxylate-rich diiron(II) models for the diiron centers in dioxygen-dependent non-heme enzymes

Carboxylate-bridged high-spin diiron(II) complexes with distinctive electronic transitions were prepared by using 4-cyanopyridine (4-NCC(5)H(4)N) ligands to shift the charge-transfer bands to the visible region of the absorption spectrum. This property facilitated quantitation of water-dependent equilibria in the carboxylate-rich diiron(II) complex, [Fe(2)(mu-O(2)CAr(Tol))(4)(4-NCC(5)H(4)N)(2)] (1), where (-)O(2)CAr(Tol) is 2,6-di-(p-tolyl)benzoate. Addition of water to 1 reversibly shifts two of the bridging carboxylate ligands to chelating terminal coordination positions, converting the structure from a paddlewheel to a windmill geometry and generating [Fe(2)(mu-O(2)CAr(Tol))(2)(O(2)CAr(Tol))(2)(4-NCC(5)H(4)N)(2)(H(2)O)(2)] (3). This process is temperature dependent in solution, rendering the system thermochromic. Quantitative treatment of the temperature-dependent spectroscopic changes over the temperature range from 188 to 298 K in CH(2)Cl(2) afforded thermodynamic parameters for the interconversion of 1 and 3. Stopped flow kinetic studies revealed that water reacts with the diiron(II) center ca. 1000 time faster than dioxygen and that the water-containing diiron(II) complex reacts with dioxygen ca. 10 times faster than anhydrous analogue 1. Addition of {H(OEt(2))(2)}{B}, where B(-) is tetrakis(3,5-di(trifluoromethyl)phenyl)borate, to 1 converts it to [Fe(2)(mu-O(2)CAr(Tol))(3)(4-NCC(5)H(4)N)(2)](B) (5), which was also structurally characterized. Mossbauer spectroscopic investigations of solid samples of 1, 3, and 5, in conjunction with several literature values for high-spin iron(II) complexes in an oxygen-rich coordination environment, establish a correlation between isomer shift, coordination number, and N/O composition. The products of oxygenating 1 in CH(2)Cl(2) were identified crystallographically to be [Fe(2)(mu-OH)(2)(mu-O(2)CAr(Tol))(2)(O(2)CAr(Tol))(2)(4-NCC(5)H(4)N)(2)].2(HO(2)CAr(Tol)) (6) and [Fe(6)(mu-O)(2)(mu-OH)(4)(mu-O(2)CAr(Tol))(6)(4-NCC(5)H(4)N)(4)Cl(2)] (7).     

Diiron species containing a cyclic P(Ph)2N(Ph)2 diphosphine related to the [FeFe]H2ases active site

A new dissymmetrically disubstituted diiron dithiolate species, [Fe(2)(CO)(4)(κ(2)-P(Ph)(2)N(Ph)(2))(μ-pdt)] (pdt = S(CH(2))(3)S), was prepared by using a flexible cyclic base-containing diphosphine, 1,3,5,7-tetraphenyl 1,5-diaza-3,7-diphosphacyclooctane (P(Ph)(2)N(Ph)(2) = {PhPCH(2)NPh}(2)). Preliminary investigations of proton and electron transfers on the diiron system have been done.     

Multiple silicon-hydrogen bond activations at adjacent rhodium and iridium centers

The reaction of 1 equiv of primary silanes, SiH(3)R (R = Ph, Mes), with [RhIr(CO)(3)(dppm)(2)] yields mono(silylene)-bridged complexes of the type [RhIr(H)(2)(CO)(2)(μ-SiHR)(dppm)(2)] (R = Ph or Mes), while for R = Ph the addition of 2 equiv yields the bis(silylene)-bridged complexes, [RhIr(CO)(2)(μ-SiHPh)(2)(dppm)(2)]. The kinetic isomer of this bis(silylene)-bridged product has the phenyl substituent axial on one silylene unit and equatorial on the other, and in the presence of excess silane this rearranges to the thermodynamically preferred "axial-axial" isomer, in which the phenyl substituents on each bridging silylene unit are axial and parallel to one another. The reaction of 1 equiv of diphenylsilane with [RhIr(CO)(3)(dppm)(2)] produces the mono(silylene)-bridged product, [RhIr(H)(2)(CO)(2)(μ-SiPh(2))(dppm)(2)], and the subsequent addition of silane in the presence of CO yields the silyl/silylene product [RhIr(H)(SiPh(2)H)(CO)(3)(κ(1)-dppm)(μ-SiPh(2))(dppm)]. The reaction of [RhIr(CO)(3)(dppm)(2)] with 2 equiv of SiH(2)Me(2) yields the analogous product [RhIr(H)(SiMe(2)H)(CO)(3)(κ(1)-dppm)(μ-SiMe(2))(dppm)]. Low-temperature NMR spectroscopic observation of some key intermediates, such as [RhIr(H)(SiH(2)Ph)(CO)(2)(μ-CO)(dppm)(2)], formed during the formation of the mono(silylene)-bridged species provides evidence for a mechanism involving initial Si-H bond activation at Rh, followed by the subsequent Si-H bond activation at Ir. The Si-H bond activation of a second equivalent of silane seems to be initiated by dissociation of the Rh-bound end of one diphosphine. The reaction of diphenylsilane with the cationic complex [RhIr(CH(3))(CO)(2)(dppm)(2)][CF(3)SO(3)] gives rise to a different reactivity pattern in which Si-H bond activation is initiated at Ir. In this case, the cationic silyl-bridged species, [RhIr(CH(3))(CO)(2)(κ(1):η(2)-SiHPh(2))(dppm)(2)][CF(3)SO(3)], contains an agostic Si-H interaction with Rh. In solution, at ambient temperature, this complex converts to two species, [RhIr(H)(COCH(3))(CO)(μ-H)(μ-SiPh(2))(dppm)(2)][CF(3)SO(3)] and [RhIr(CO)(2)(μ-H)(μ-SiPh(2))(dppm)(2)] [CF(3)SO(3)], formed by the competing methyl migration to CO and reductive elimination of methane, respectively. In the diphenylsilylene dihydride product, a weak interaction between the bridging silicon and the terminal Ir-bound hydride is proposed on the basis of NMR evidence.     

Modeling features of the non-heme diiron cores in O2-activating enzymes through the synthesis, characterization, and oxidation of 1,8-naphthyridine-based complexes

Multidentate naphthyridine-based ligands were used to prepare a series of diiron(II) complexes. The compound [Fe(2)(BPMAN)(mu-O(2)CPh)(2)](OTf)(2) (1), where BPMAN = 2,7-bis[bis(2-pyridylmethyl)aminomethyl]-1,8-naphthyridine, exhibits two reversible oxidation waves with E(1/2) values at +310 and +733 mV vs Cp(2)Fe(+)/Cp(2)Fe, as revealed by cyclic voltammetry. Reaction with O(2) or H(2)O(2) affords a product with optical and M?ssbauer properties that are characteristic of a (mu-oxo)diiron(III) species. The complexes [Fe(2)(BPMAN)(mu-OH)(mu-O(2)CAr(Tol))](OTf)(2) (2) and [Fe(2)(BPMAN)(mu-OMe)(mu-O(2)CAr(Tol))](OTf)(2) (3) were synthesized, where Ar(Tol)CO(2)(-) is the sterically hindered ligand 2,6-di(p-tolyl)benzoate. Compound 2 has a reversible redox wave at +11 mV, and both 2 and 3 react with O(2), via a mixed-valent Fe(II)Fe(III) intermediate, to give final products that are also consistent with (mu-oxo)diiron(III) species. The paddle-wheel compound [Fe(2)(BBAN)(mu-O(2)CAr(Tol))(3)](OTf) (4), where BBAN = 2,7-bis(N,N-dibenzylaminomethyl)-1,8-naphthyridine, reacts with dioxygen to yield benzaldehyde via oxidative N-dealkylation of a benzyl group on BBAN, an internal substrate. In the presence of bis(4-methylbenzyl)amine, the reaction also produces p-tolualdehyde, revealing oxidation of an external substrate. A structurally related compound, [Fe(2)(BEAN)(mu-O(2)CAr(Tol))(3)](OTf) (5), where BEAN = 2,7-bis(N,N-diethylaminomethyl)-1,8-naphthyridine, does not undergo N-dealkylation, nor does it facilitate the oxidation of bis(4-methylbenzyl)amine. The contrast in reactivity of 4 and 5 is attributed to a difference in accessibility of the substrate to the diiron centers of the two compounds. The M?ssbauer spectroscopic properties of the diiron(II) complexes were also investigated.     

Dioxygen-initiated oxidation of heteroatomic substrates incorporated into ancillary pyridine ligands of carboxylate-rich diiron(II) complexes

Progress toward the development of functional models of the carboxylate-bridged diiron active site in soluble methane monooxygenase is described in which potential substrates are introduced as substituents on bound pyridine ligands. Pyridine ligands incorporating a thiol, sulfide, sulfoxide, or phosphine moiety were allowed to react with the preassembled diiron(II) complex [Fe(2)(mu-O(2)CAr(R))(2)(O(2)CAr(R))(2)(THF)(2)], where (-)O(2)CAr(R) is a sterically hindered 2,6-di(p-tolyl)- or 2,6-di(p-fluorophenyl)benzoate (R = Tol or 4-FPh). The resulting diiron(II) complexes were characterized crystallographically. Triply and doubly bridged compounds [Fe(2)(mu-O(2)CAr(Tol))(3)(O(2)CAr(Tol))(2-MeSpy)] (4) and [Fe(2)(mu-O(2)CAr(Tol))(2)(O(2)CAr(Tol))(2)(2-MeS(O)py)(2)] (5) resulted when 2-methylthiopyridine (2-MeSpy) and 2-pyridylmethylsulfoxide (2-MeS(O)py), respectively, were employed. Another triply bridged diiron(II) complex, [Fe(2)(mu-O(2)CAr(4)(-)(FPh))(3)-(O(2)CAr(4)(-)(FPh))(2-Ph(2)Ppy)] (3), was obtained containing 2-diphenylphosphinopyridine (2-Ph(2)Ppy). The use of 2-mercaptopyridine (2-HSpy) produced the mononuclear complex [Fe(O(2)CAr(Tol))(2)(2-HSpy)(2)] (6a). Together with that of previously reported [Fe(2)(mu-O(2)CAr(Tol))(3)(O(2)CAr(Tol))(2-PhSpy)] (2) and [Fe(2)(mu-O(2)CAr(Tol))(3)(O(2)CAr(Tol))(2-Ph(2)Ppy)] (1), the dioxygen reactivity of these iron(II) complexes was investigated. A dioxygen-dependent intermediate (6b) formed upon exposure of 6a to O(2), the electronic structure of which was probed by various spectroscopic methods. Exposure of 4 and 5 to dioxygen revealed both sulfide and sulfoxide oxidation. Oxidation of 3 in CH(2)Cl(2) yields [Fe(2)(mu-OH)(2)(mu-O(2)CAr(4)(-)(FPh))(O(2)CAr(4)(-FPh))(3)(OH(2))(2-Ph(2)P(O)py)] (8), which contains the biologically relevant {Fe(2)(mu-OH)(2)(mu-O(2)CR)}(3+) core. This reaction is sensitive to the choice of carboxylate ligands, however, since the p-tolyl analogue 1 yielded a hexanuclear species, 7, upon oxidation.     

Evaluating the identity and diiron core transformations of a (μ-oxo)diiron(III) complex supported by electron-rich tris(pyridyl-2-methyl)amine ligands

The composition of a (μ-oxo)diiron(III) complex coordinated by tris[(3,5-dimethyl-4-methoxy)pyridyl-2-methyl]amine (R(3)TPA) ligands was investigated. Characterization using a variety of spectroscopic methods and X-ray crystallography indicated that the reaction of iron(III) perchlorate, sodium hydroxide, and R(3)TPA affords [Fe(2)(μ-O)(μ-OH)(R(3)TPA)(2)](ClO(4))(3) (2) rather than the previously reported species [Fe(2)(μ-O)(OH)(H(2)O)(R(3)TPA)(2)](ClO(4))(3) (1). Facile conversion of the (μ-oxo)(μ-hydroxo)diiron(III) core of 2 to the (μ-oxo)(hydroxo)(aqua)diiron(III) core of 1 occurs in the presence of water and at low temperature. When 2 is exposed to wet acetonitrile at room temperature, the CH(3)CN adduct is hydrolyzed to CH(3)COO(-), which forms the compound [Fe(2)(μ-O)(μ-CH(3)COO)(R(3)TPA)(2)](ClO(4))(3) (10). The identity of 10 was confirmed by comparison of its spectroscopic properties with those of an independently prepared sample. To evaluate whether or not 1 and 2 are capable of generating the diiron(IV) species [Fe(2)(μ-O)(OH)(O)(R(3)TPA)(2)](3+) (4), which has previously been generated as a synthetic model for high-valent diiron protein oxygenated intermediates, studies were performed to investigate their reactivity with hydrogen peroxide. Because 2 reacts rapidly with hydrogen peroxide in CH(3)CN but not in CH(3)CN/H(2)O, conditions that favor conversion to 1, complex 1 is not a likely precursor to 4. Compound 4 also forms in the reaction of 2 with H(2)O(2) in solvents lacking a nitrile, suggesting that hydrolysis of CH(3)CN is not involved in the H(2)O(2) activation reaction. These findings shed light on the formation of several diiron complexes of electron-rich R(3)TPA ligands and elaborate on conditions required to generate synthetic models of diiron(IV) protein intermediates with this ligand framework.     

A C2-symmetric, basic Fe(III) carboxylate complex derived from a novel triptycene-based chelating carboxylate ligand

A triptycene-based bis(benzoxazole) diacid ligand H(2)L2(Ph4) bearing sterically encumbering groups was synthesized. Treatment of H(2)L2(Ph4) with Fe(OTf)(3) afforded a C(2)-symmetric trinuclear iron(III) complex, [NaFe(3)(L2(Ph4))(2)(μ(3)-O)(μ-O(2)CCPh(3))(2)(H(2)O)(3)](OTf)(2) (8). The triiron core of this complex adopts the well known "basic iron acetate" structure where the heteroleptic carboxylates, comprising two Ph(3)CCO(2)(-) and two (L2(Ph4))(2-) ligands, donate the six carboxylate bridges. The (L2(Ph4))(2-) ligand undergoes only minor conformational changes upon formation of the complex.     

Cationic diiron and diruthenium μ-allenyl complexes: synthesis, X-ray structures and cyclization reactions with ethyldiazoacetate/amine affording unprecedented butenolide- and furaniminium-substituted bridging carbene ligands

The novel cationic diiron μ-allenyl complexes [Fe(2)Cp(2)(CO)(2)(μ-CO){μ-η(1):η(2)(α,β)-C(α)(H)=C(β)=C(γ)(R)(2)}](+) (R = Me, 4a; R = Ph, 4b) have been obtained in good yields by a two-step reaction starting from [Fe(2)Cp(2)(CO)(4)]. The solid state structures of [4a][CF(3)SO(3)] and of the diruthenium analogues [Ru(2)Cp(2)(CO)(2)(μ-CO){μ-η(1):η(2)(α,β)-C(α)(H)=C(β)=C(γ)(R)(2)}][BPh(4)] (R = Me, [2a][BPh(4)]; R = Ph, [2c][BPh(4)]) have been ascertained by X-ray diffraction studies. The reactions of 2c and 4a with Br?nsted bases result in formation of the μ-allenylidene compound [Ru(2)Cp(2)(CO)(2)(μ-CO){μ-η(1):η(1)-C(α)=C(β)=C(γ)(Ph)(2)}] (5) and of the dimetallacyclopentenone [Fe(2)Cp(2)(CO)(μ-CO){μ-η(1):η(3)-C(α)(H)=C(β)(C(γ)(Me)CH(2))C(=O)}] (6), respectively. The nitrile adducts [Ru(2)Cp(2)(CO)(NCMe)(μ-CO){μ-η(1):η(2)-C(α)(H)=C(β)=C(γ)(R)(2)}](+) (R = Me, 7a; R = Ph, 7b), prepared by treatment of 2a,c with MeCN/Me(3)NO, react with N(2)CHCO(2)Et/NEt(3) at room temperature, affording the butenolide-substituted carbene complexes [Ru(2)Cp(2)(CO)(μ-CO){μ-η(1):η(3)-C(α)(H)[upper bond 1 start]C(β)C(γ)(R)(2)OC(=O)C[upper bond 1 end](H)] (R = Me, 10a; R = Ph, 10b). The intermediate cationic compound [Ru(2)Cp(2)(CO)(μ-CO){μ-η(1):η(3)-C(α)(H)[upper bond 1 start]C(β)C(γ)(Me)(2)OC(OEt)C[upper bond 1 end](H)](+) (9) has been detected in the course of the reaction leading to 10a. The addition of N(2)CHCO(2)Et/NHEt(2) to 7a gives the 2-furaniminium-carbene [Ru(2)Cp(2)(CO)(μ-CO){μ-η(1):η(3)-C(α)(H)[upper bond 1 start]C(β)C(γ)(Me)(2)OC(OEt)C[upper bond 1 end](H)](+) (11). The X-ray structures of 10a, 10b and [11][BF(4)] have been determined. The reactions of 4a,b with MeCN/Me(3)NO result in prevalent decomposition to mononuclear iron species.     

Lewis base character of the phosphorus atom in phosphanido-niobocene complexes. Synthesis of new early-early homo- and heterobimetallic entities

The reaction of phosphanido complexes [Nb(η(5)-C(5)H(4)SiMe(3))(2)(L)(PPh(2))] [L = CO (1), CNXylyl (2)] with early transition metal halides in high oxidation states has been carried out. New bimetallic niobocene complexes [{Nb(η(5)-C(5)H(4)SiMe(3))(2)(L)}(μ-PPh(2))(MCl(5))] [M = Nb, L = CO (3), L = CNXylyl (4); M = Ta, L = CO (5), L = CNXylyl (6)] have been successfully synthesized by the reaction with [MCl(5)](2) (M = Nb or Ta). In a similar way [{Nb(η(5)-C(5)H(4)SiMe(3))(2)(L)}(μ-PPh(2))(MCl(4))] [M = Ti, L = CO (13), CNXylyl (14); M = Zr, L = CO (15), CNXylyl (16)] were synthesized using MCl(4) (M = Ti or Zr). Solutions of complexes 4-6 in chloroform produced new ionic derivatives [Nb(η(5)-C(5)H(4)SiMe(3))(2)(P(H)Ph(2))(L)] [MCl(6)] [M = Nb, L = CO (7), L = CNXylyl (8); M = Ta, L = CO (9), L = CNXylyl (10)]. Ionic complexes [Nb(η(5)-C(5)H(4)SiMe(3))(2)(P(Cl)Ph(2))(L)] [NbCl(4)O(thf)] [L = CO (11), CNXylyl (12)] were formed from solutions in thf - rapidly in the case of 3 but more slowly for 4. New heterometallic complexes [Nb(η(5)-C(5)H(4)SiMe(3))(2)(L)(μ-PPh(2)){(Ti(η(5)-C(5)R(5))Cl(3)}] [R = H, L = CO (17), CNXylyl (18); R = CH(3), L = CO (19), CNXylyl (20)] were synthesized by the reaction of 1 or 2 with [Ti(η(5)-C(5)R(5))Cl(3)] (R = H or CH(3)). All of these compounds were characterized by IR and multinuclear NMR spectroscopy, and the molecular structures of 9 and 12 were determined by single-crystal X-ray diffraction.     

Sterically hindered carboxylate ligands support water-bridged dimetallic centers that model features of metallohydrolase active sites

The synthesis and characterization of carboxylate-bridged dimetallic complexes are described. By using m-terphenyl-derived carboxylate ligands, a series of dicobalt(II), dicobalt(III), dinickel(II), and dizinc(II) complexes were synthesized. The compounds are [Co(2)(mu-O(2)CAr(Tol))(2)(O(2)CAr(Tol))(2)L(2)] (1), [Co(2)(mu-OH(2))(2)(mu-O(2)CAr(Tol))(2)(O(2)CAr(Tol))(2)L(2)] (2a-c), [Co(2)(mu-OH)(2)(mu-O(2)CAr(Tol))(2)(O(2)CAr(Tol))(2)L(2)] (3), [Ni(2)(mu-O(2)CAr(Tol))(4)L(2)] (4), [Ni(2)(mu-HO...H)(2)(mu-O(2)CAr(Tol))(2)(O(2)CAr(Tol))(2)L(2)] (5), and [Zn(2)(mu-O(2)CAr(Tol))(2)(O(2)CAr(Tol))(2)L(2)] (6), where Ar(Tol)CO(2)H = 2,6-di(p-tolyl)benzoic acid and L = pyridine, THF, or N,N-dibenzylethylenediamine. Structural analysis of these complexes revealed that additional bridging ligands can be readily accommodated within the [M(2)(mu-O(2)CAr(Tol))(2)](2+) core, allowing a wide distribution of M...M distances from 2.5745(6) to 4.0169(9) A. Unprecedented bridging units [M(2)(mu-OH(2))(2)(mu-O(2)CR)(2)](n+) and [M(2)(mu-HO...H)(2)(mu-O(2)CR)(2)](n+) were identified in 2a-c and 5, respectively, in which strong hydrogen bonding accommodates shifts of protons from bridging water molecules toward the dangling oxygen atoms of terminal monodentate carboxylate groups. Such a proton shift along the O...H...O coordinate attenuates the donor ability of the anionic carboxylate ligand, which can translate into increased Lewis acidity at the metal centers. Such double activation of bridging water molecules by a Lewis acidic metal center and a metal-bound general base may facilitate the reactivity of metallohydrolases such as methionine aminopeptidase (MAP).     

Synthesis, structure and reactivity of novel pyridazine-coordinated diiron bridging carbene complexes

[{mu-(Pyridazine-N(1):N(2))}Fe(2)(mu-CO)(CO)(6)](1) reacts with aryllithium reagents, ArLi (Ar = C(6)H(5), m-CH(3)C(6)H(4)) followed by treatment with Me(3)SiCl to give the novel pyridazine-coordinated diiron bridging siloxycarbene complexes [(C(4)H(4)N(2))Fe(2){mu-C(OSiMe(3))Ar}(CO)(6)](2, Ar = C(6)H(5); 3, Ar =m-CH(3)C(6)H(4)). Complex 2 reacts with HBF(4).Et(2)O at low temperature to yield a cationic bridging carbyne complex [(C(4)H(4)N(2))Fe(2)(mu-CC(6)H(5))(CO)(6)]BF(4)(4). Cationic 4 reacts with NaBH(4) in THF at low temperature to afford the diiron bridging arylcarbene complex [(C(4)H(4)N(2))Fe(2){mu-C(H)C(6)H(5)}(CO)(6)](5). Unexpectedly, the reaction of 4 with NaSCH(3) under similar conditions gave the bridging arylcarbene complex 5 and a carbonyl-coordinated diiron bridging carbene complex [Fe(2){mu-C(SCH(3))C(6)H(5)}(CO)(7)](6), while the reaction of NaSC(6)H(4)CH(3)-p with 4 affords the expected bridging arylthiocarbene complex [(C(4)H(4)N(2))Fe(2){mu-C(SC(6)H(4)CH(3)-p)C(6)H(5)}(CO)(6)](7), which can be converted into a novel diiron bridging carbyne complex with a thiolato-bridged ligand, [Fe(2)(mu-CC(6)H(5))(mu-SC(6)H(4)CH(3)-p)(CO)(6)](8). Cationic can also react with the carbonylmetal anionic compound Na(2)[Fe(CO)(4)] to yield complex 5, while the reactions of 4 with carbonylmetal anionic compounds Na[M(CO)(5)(CN)](M = Cr, Mo, W) produce the diiron bridging aryl(pentacarbonylcyanometal)carbene complexes [(C(4)H(4)N(2))Fe(2)-{mu-C(C(6)H(5))NCM(CO)(5)}(CO)(6)](9, M = Cr; 10, M = Mo; 11, M = W). The structures of complexes 2, 5, 6, 8, and 9 have been established by X-ray diffraction studies.     

Coordination chemistry of N-heterocyclic stannylenes: a combined synthetic and Mössbauer spectroscopy study

The N-heterocyclic stannylenes (NHSns), [(Dipp) N(CH(2))(n)N(Dipp)S n] (Dipp = 2,6- (i)Pr(2)C(6)H(3); n = 2, 1; n = 3, 5) and [((t)Bu) N(CHMe)(2)N((t)Bu)S n] (10) are competent ligands toward a variety of transition metal centers, as seen in the complexes [W(CO)(5)·1] (2), [(OC)(4)Fe(μ-1)(2)Fe(CO)(4)] (3), [(OC)(4)Fe(μ-1)Fe(CO)(4)] (4), [Fe(CO)(4)·5](n) (6, n = 1 or 2), [(OC)(4)Fe(μ-5)Fe(CO)(4)] (7), [Ph(3)PPt(μ-1)(2)PtPPh(3)] (8), [Fe(CO)(4)·10] (11), and [(η(5)-C(5)H(5))(OC)(2)Mn·10] (12). X-ray crystallographic studies show that the NHSns are structurally largely unperturbed binding to the metal, but in contrast to the parent NHCs, NHSns often adopt a bridging position across dinuclear metal units. The balance between terminal and bridging positions for the stannylene is evidently closely balanced as shown by the observation of both monomers and dimers for 6 in the solid state and in solution. (119)Sn and (57)Fe Mo?ssbauer spectroscopy of the complexes shows the tin atoms in such complexes to be consistent with electron deficient Sn(II) centers.     

Co-complexation of Lithium Gallates on the Titanium Molecular Oxide {[Ti(η(5)-C(5)Me(5))(μ-O)}(3)(μ(3)-CH)

Amide and lithium aryloxide gallates [Li(+){RGaPh(3)}(-)] (R = NMe(2), O-2,6-Me(2)C(6)H(3)) react with the μ(3)-alkylidyne oxoderivative ligand [{Ti(η(5)-C(5)Me(5))(μ-O)}(3)(μ(3)-CH)] (1) to afford the gallium-lithium-titanium cubane complexes [{Ph(3)Ga(μ-R)Li}{Ti(η(5)-C(5)Me(5))(μ-O)}(3)(μ(3)-CH)] [R = NMe(2) (3), O-2,6-Me(2)C(6)H(3) (4)]. The same complexes can be obtained by treatment of the [Ph(3)Ga(μ(3)-O)(3){Ti(η(5)-C(5)Me(5))}(3)(μ(3)-CH)] (2) adduct with the corresponding lithium amide or aryloxide, respectively. Complex 3 evolves with formation of 5 as a solvent-separated ion pair constituted by the lithium dicubane cationic species [Li{(μ(3)-O)(3)Ti(3)(η(5)-C(5)Me(5))(3)(μ(3)-CH)}(2)](+) together with the anionic [(GaPh(3))(2)(μ-NMe(2))](-) unit. On the other hand, the reaction of 1 with Li(p-MeC(6)H(4)) and GaPh(3) leads to the complex [Li{(μ(3)-O)(3)Ti(3)(η(5)-C(5)Me(5))(3)(μ(3)-CH)}(2)][GaLi(p-MeC(6)H(4))(2)Ph(3)] (6). X-ray diffraction studies were performed on 1, 2, 4, and 5, while trials to obtain crystals of 6 led to characterization of [Li{(μ(3)-O)(3)Ti(3)(η(5)-C(5)Me(5))(3)(μ(3)-CH)}(2)][PhLi(μ-C(6)H(5))(2)Ga(p-MeC(6)H(4))Ph] 6a.     

C-X bond formation and cleavage in the reactions of the ditungsten hydride complex [W2(η5-C5H5)2(H)(μ-PCy2)(CO)2] with small molecules having multiple C-X bonds (X = C, N, O)

Two molecules of C(2)(CO(2)Me)(2) or isocyanides could be added to the title hydride complex under mild conditions to give dienyl-[W(2)Cp(2){μ-η(1),κ:η(2)-C(CO(2)Me)=C(CO(2)Me)C(CO(2)Me)=CH(CO(2)Me)}(μ-PCy(2))(CO)(2)] (Cp = η(5)-C(5)H(5)), diazadienyl-[W(2)Cp(2){μ-κ,η:κ,η-C{CHN(4-MeO-C(6)H(4))}N(4-MeO-C(6)H(4))}(μ-PCy(2))(CO)(2)] or aminocarbyne-bridged derivatives [W(2)Cp(2){μ-CNH(2,6-Me(2)C(6)H(3))}(μ-PCy(2)){CN(2,6-Me(2)C(6)H(3))}(CO)]. In contrast, its reaction with excess (4-Me-C(6)H(4))C(O)H gave the C-O bond cleavage products [W(2)Cp(2){CH(2)(4-Me-C(6)H(4))}(O)(μ-PCy(2))(CO)(2)] and [W(2)Cp(2){μ-η:η,κ-C(O)CH(2)(4-Me-C(6)H(4))}(O)(μ-PCy(2))(CO)].