Heme/Cu/O2 reactivity: change in FeIII-(O2 2-)-CuII unit peroxo binding geometry effected by tridentate copper chelation

A new heme-peroxo-copper complex structural type with mu-eta2:eta2 peroxo ligation has been generated utilizing a heterobinucleating ligand with bis(2-(2-pyridyl)ethyl)amine tridentate chelate for copper. Oxygenation of [(2L)FeIICuI]+ (1) at -80 degrees C in CH2Cl2/6%EtCN, 1 (lambdamax, 426, 530 nm) produces [(2L)FeIII-(O22-)-CuII)]+ (3) (lambdamax, 419, 488, 544, 575 nm). Stopped-flow kinetic/spectroscopic probing reveals that a superoxo complex, [(2L)FeIII-(O2-)...CuI(NCEt)]+ (2) (lambdamax = 544 nm), initially forms, k1 = 5.23 +/- 0.09 x 104 M-1 s-1 (-105 degrees C). Subsequent intramolecular reaction of the copper(I) ion in 2 occurs with k2 = 2.74 +/- 0.04 x 101 s-1 (-105 degrees C), producing 3. Resonance Raman spectroscopy (rR) confirms the peroxo assignment for 3; nu(O-O) = 747 cm-1 (Delta(18O2) = -40 cm-1). In an 16O-18O mixed isotope experiment a single band is observed at 730 cm-1. The low nu(O-O) value and the absence of a splitting of the 730 cm-1 band are indicative of a symmetrical binding of the peroxide group in a side-on mu-eta2:eta2 geometry. This conclusion is supported by X-ray absorption spectroscopy on 3. Copper K-edge EXAFS indicates a five-coordinate metal center: 2 N, 2.028(7) A; 2 O, 1.898(7) A; 1 N, 2.171(12) A. An outer-sphere Fe scatterer is found at 3.62(1) A. The iron center K-edge EXAFS fits to either a five- or six-coordinate metal center: 4 N(pyrrole), approximately 2.1 A; 1,2 O, approximately 1.9 A. A preedge feature (Fe(1s) --> Fe(3d) transition) at 7113.2(2) eV resembles that obtained for a eta2-peroxo ferric heme complex, being weaker and at approximately 1.5 eV lower energy than those found in five-coordinate (P)FeIII-X (in C4v symmetry) complexes. Arguments based on rR properties of relevant peroxo compounds also effectively point to the copper(II) ion in 3 as being side-on bound, leading to the very low O-O stretching frequency observed in comparison to those of heme-peroxo species or heme-peroxo-copper complexes with a tetradentate copper chelate. These investigations derive from interest in establishing relevant and/or fundamental O2 chemistry at heme-copper centers, in relation to heme-copper oxidase active-site chemistry.     

Tridentate copper ligand influences on heme-peroxo-copper formation and properties: reduced, superoxo, and mu-peroxo iron/copper complexes

In cytochrome c oxidase synthetic modeling studies, we recently reported a new mu-eta2:eta2-peroxo binding mode in the heteronuclear heme/copper complex [(2L)Fe(III)-(O2(2-))-CuII]+ (6) which is effected by tridentate copper chelation (J. Am. Chem. Soc. 2004, 126, 12716). To establish fundamental coordination and O2-reactivity chemistry, we have studied and describe here (i) the structure and dioxygen reactivity of the copper-free compound (2L)FeII (1), (ii) detailed spectroscopic properties of 6 in comparisons with those of known mu-eta2:eta1 heme-peroxo-copper complexes, (iii) formation of 6 from the reactions of [(2L)FeIICuI]+ (3) and dioxygen by stopped-flow kinetics, and (iv) reactivities of 6 with CO and PPh3. In the absence of copper, 1 serves as a myoglobin model compound possessing a pyridine-bound five-coordinate iron(II)-porphyrinate which undergoes reversible dioxygen binding. Oxygenation of 3 below -60 degrees C generates the heme-peroxo-copper complex 6 with strong antiferromagnetic coupling between high-spin iron(III) and copper(II) to yield an S = 2 spin system. Stopped-flow kinetics in CH2Cl2/6% EtCN show that dioxygen reacts with iron(II) first to form a heme-superoxide moiety, [(EtCN)(2L)FeIII-(O2-)...CuI(EtCN)]+ (5), which further reacts with Cu(I) to generate 6. Compared to those properties of a known mu-eta2:eta1-heme-peroxo-copper complex, 6 has a significantly diminished resonance Raman nu(O-O) stretching frequency at 747 cm(-1) and distinctive visible absorptions at 485, 541, and 572 nm, all of which seem to be characteristics of a mu-eta2:eta2-heme-peroxo-copper system. Addition of CO or PPh3 to 6 yields a bis-CO adduct of 3 or a PPh(3) adduct of 5, the latter with a remaining FeIII-(O2-) moiety.     

Dioxygen reactivity of copper and heme-copper complexes possessing an imidazole-phenol cross-link

Recent spectroscopic, kinetics, and structural studies on cytochrome c oxidases (CcOs) suggest that the histidine-tyrosine cross-link at the heme a3-CuB binuclear active site plays a key role in the reductive O2-cleavage process. In this report, we describe dioxygen reactivity of copper and heme/Cu assemblies in which the imidazole-phenol moieties are employed as a part of copper ligand LN4OH (2-{4-[2-(bis-pyridin-2-ylmethyl-amino)-ethyl]-imidazol-1-yl}-4,6-di -tert-butyl-phenol). Stopped-flow kinetic studies reveal that low-temperature oxygenation of [CuI(LN4OH)]+ (1) leads to rapid formation of a copper-superoxo species [CuII(LN4OH)(O2-)]+ (1a), which further reacts with 1 to form the 2:1 Cu:O2 adduct, peroxo complex [{CuII(LN4OH)}2(O2(2-))]2+ (1b). Complex 1b is also short-lived, and a dimer Cu(II)-phenolate complex [CuII(LN4O-)]2(2+) (1c) eventually forms as a final product in the later stage of the oxygenation reaction. Dioxygen reactivities of 1 and its anisole analogue [CuI(LN4OMe)]+ (2) in the presence of a heme complex (F8)FeII (3) (F8 = tetrakis(2,6,-difluorotetraphenyl)-porphyrinate) are also described. Spectroscopic investigations including UV-vis, 1H and 2H NMR, EPR, and resonance Raman spectroscopies along with spectrophotometric titration reveal that low-temperature oxygenation of 1/3 leads to formation of a heme-peroxo-copper species [(F8)FeIII-(O2(2-))-CuII(LN4OH)]+ (4), nu(O-O) = 813 cm(-1). Complex 4 is an S = 2 spin system with strong antiferromagnetic coupling between high-spin iron(III) and copper(II) through a bridging peroxide ligand. A very similar complex [(F8)FeIII-(O2(2-))-CuII(LN4OMe)]+ (5) (nu(O-O) = 815 cm(-1)) can be generated by utilizing the anisole compound 2, which indicates that the cross-linked phenol moiety in 4 does not interact with the bridging peroxo group between heme and copper. This investigation thus reveals that a stable heme-peroxo-copper species can be generated even in the presence of an imidazole-phenol group (i.e., possible electron/proton donor source) in close proximity. Future studies are needed to probe key factors that can trigger the reductive O-O cleavage in CcO model compounds.     

Further insights into the spectroscopic properties, electronic structure, and kinetics of formation of the heme-peroxo-copper complex [(F8TPP)FeIII-(O2(2-)-CuII(TMPA)]+

In the further development and understanding of heme-copper O2-reduction chemistry inspired by the active-site chemistry in cytochrome c oxidase, we describe a dioxygen adduct, [(F8TPP)FeIII-(O22-)-CuII(TMPA)](ClO4) (3), formed by addition of O2 to a 1:1 mixture of the porphyrinate-iron(II) complex (F8TPP)FeII (1a) {F8TPP = tetrakis(2,6-difluorophenyl)porphyrinate dianion} and the copper(I) complex [(TMPA)CuI(MeCN)](ClO4) (1b) {TMPA = tris(2-pyridylmethyl)amine}. Complex 3 forms in preference to heme-only or copper-only binuclear products, is remarkably stable {t1/2 (RT; MeCN) approximately 20 min; lambda max = 412 (Soret), 558 nm; EPR silent}, and is formulated as a peroxo complex on the basis of manometry {1a/1b/O2 = 1:1:1}, MALDI-TOF mass spectrometry {16O2, m/z 1239 [(3 + MeCN)+]; 18O2, m/z 1243}, and resonance Raman spectroscopy {nu(O-O) = 808 cm-1; Delta16O2/18O2 = 46 cm-1; Delta16O2/16/18O2 = 23 cm-1}. Consistent with a mu-eta2:eta1 bridging peroxide ligand, two metal-O stretching frequencies are observed {nu(Fe-O) = 533 cm-1, nu(Fe-O-Cu) = 511 cm-1}, and supporting normal coordinate analysis is presented. 2H and 19F NMR spectroscopies reveal that 3 is high-spin {also muB = 5.1 +/- 0.2, Evans method} with downfield-shifted pyrrole and upfield-shifted TMPA resonances, similar to the pattern observed for the structurally characterized mu-oxo complex [(F8TPP)FeIII-O-CuII(TMPA)]+ (4) (known S = 2 system, antiferromagnetically coupled high-spin FeIII and CuII). M?ssbauer spectroscopy exhibits a sharp quadrupole doublet (zero field; delta = 0.57 mm/s, |DeltaEQ| = 1.14 mm/s) for 3, with isomer shift and magnetic field dependence data indicative of a peroxide ligand and S = 2 formulation. Both UV-visible-monitored stopped-flow kinetics and M?ssbauer spectroscopic studies reveal the formation of heme-only superoxide complex (S)(F8TPP)FeIII-(O2-) (2a) (S = solvent molecule) prior to 3. Thermal decomposition of mu-peroxo complex 3 yields mu-oxo complex 4 with concomitant release of approximately 0.5 mol O2 per mol 3. Characterization of the reaction 1a/1b + O2 --> 2 --> 3 --> 4, presented here, advances our understanding and provides new insights to heme/Cu dioxygen-binding and reduction.     

Resonance raman investigation of equatorial ligand donor effects on the Cu(2)O(2)(2+) core in end-on and side-on mu-peroxo-dicopper(II) and bis-mu-oxo-dicopper(III) complexes

The effect of endogenous donor strength on Cu(2)O(2) bonds was studied by electronically perturbing [[(R-TMPA)Cu(II)]](2)(O(2))](2+) and [[(R-MePY2)Cu](2)(O(2))](2+) (R = H, MeO, Me(2)N), which form the end-on mu-1,2 bound peroxide and an equilibrium mixture of side-on peroxo-dicopper(II) and bis-mu-oxo-dicopper(III) isomers, respectively. For [[(R-TMPA)Cu(II)](2)(O(2))](2+), nu(O-O) shifts from 827 to 822 to 812 cm(-1) and nu(Cu)(-)(O(sym)) shifts from 561 to 557 to 551 cm(-1), respectively, as R- varies from H to MeO to Me(2)N. Thus, increasing the N-donor strength to the copper decreases peroxide pi(sigma) donation to the copper, weakening the Cu-O and O-O bonds. A decrease in nu(Cu-O) of the bis-mu-oxo-dicopper(III) complex was also observed with increasing N-donor strength for the R-MePY2 ligand system. However, no change was observed for nu(O-O) of the side-on peroxo. This is attributed to a reduced charge donation from the peroxide pi(sigma) orbital with increased N-donor strength, which increases the negative charge on the peroxide and adversely affects the back-bonding from the Cu to the peroxide sigma orbital. However, an increase in the bis-mu-oxo-dicopper(III) isomer relative to side-on peroxo-dicopper(II) species is observed for R-MePY2 with R = H < MeO < Me(2)N. This effect is attributed to the thermodynamic stabilization of the bis-mu-oxo-dicopper(III) isomer relative to the side-on peroxo-dicopper(II) isomer by strong donor ligands. Thus, the side-on peroxo-dicopper(II)/bis-mu-oxo-dicopper(III) equilibrium can be controlled by electronic as well as steric effects.     

An iron-peroxo porphyrin complex: new synthesis and reactivity toward a Cu(II) complex giving a heme-peroxo-copper adduct

Side-on eta2-peroxo-iron porphyrins are strong nucleophiles. In cytochrome P450-like aromatase and other enzymes, such species are postulated as the active oxidants. In cytochrome c oxidase, hemea3-peroxo, hemea3-hydroperoxo, or hemea3-(mu-peroxo)-copper species are proposed as transient intermediates forming prior to O-O bond cleavage. In this report, we describe (1) a facile method for reduction of a heme-O2 species [(F8TPP)FeIII(O2-)(S)] (2), generating the ferric peroxo porphyrin complex [(F8TPP)FeIII(O22-)]- (3) (UV-vis, THF: lambdamax = 435 (Soret), 540(sh), 561; EPR: g = 8.7, 4.2), and (2) that this can be subsequently reacted with a ligand-copper(II) complex, [CuII(TMPA)-(CH3CN)](ClO4)2 (4), affording a heme-peroxo-copper heterobinuclear compound, [(F8TPP)FeII(O22-)-CuII(TMPA)](ClO4) (5). Generation of [(F8TPP)FeIII(O22-)]- (3) using cobaltocene as a one-electron reductant was monitored by UV-vis, EPR, and 1H NMR spectroscopies. Reaction between 3 and 4 was followed by UV-vis spectroscopy, and the product 5 could be precipitated and characterized. Coordination by copper(II) in 5 makes possible further reduction of the mu-peroxo complex by cobaltocene yielding the mu-oxo analogue, [(F8TPP)FeIII(O2-)-CuII(TMPA)](ClO4) (6).     

PPh4]3[W(CN)7(O2)].4H2O as the representative of the [M(L)7(LL)] class for nine-coordinate complexes

The photoinduced dissociation of a W-CN bond in [W(CN)8]4- in an aqueous solution under ambient conditions, in conjunction with the uptake of molecular oxygen, affords the W(VI) mixed-ligand complex anion [W(CN)(7)(eta2-O2)]3-, conveniently isolable as its [PPh4+] salt. Although research into the chemistry of cyanomolybdates and cyanotungstates has been pursued with great interest and vigor over several decades, there is a paucity of structurally characterized cyano-peroxo complexes of Mo and W. The side-on coordination mode of the peroxo moiety in [W(CN)7(eta2-O2)]3- has been ascertained with X-ray crystal structure determination [d(O-O) = 1.41 A; peroxo bite angle: 41.0 degrees ] and corroborated with vibrational spectroscopy [nu(O-O) = 915 cm(-1)]. The complex ion exhibits trapezoidal tridecahedral geometry and represents the new class of nine-coordinate complexes with one bidentate and seven monodentate ligands. Cyclic voltammetry shows a reversible redox behavior of [W(CN)7(eta2-O2)]3- in CH3CN with its standard reduction potential equal to 1.130 V. Generally, interest in atmospheric oxygen derives from the versatility of this molecule as a ligand and oxidant and extends to the physicochemical features it imparts to transition metals such as copper and iron in biological oxygen carriers.     

Characterization of a 1:1 Cu-O2 adduct supported by an anilido imine ligand

Copper(I) complexes of sterically hindered anilido imine ligands o-C6H4{N(C6H3(i)Pr2)}{C(R)=NC6H3(i)Pr2}- (L(1), R = H; L(2), R = CH3) have been prepared and characterized by spectroscopic and X-ray crystallographic methods. These complexes are highly reactive with O2, and in the case of L2 the product of low-temperature oxygenation was fully characterized by spectroscopic, X-ray crystallographic, and computational methods. The resonance Raman spectrum features an isotope-sensitive vibration at 974 cm(-1) (Delta(18O) = 66 cm(-1)), consistent with assignment as an O-O stretch. Despite the asymmetric coordination environment provided by the supporting anilido imine ligand, the X-ray crystal structure confirms rather symmetric side-on binding of the O2 moiety to the copper center, and the O-O bond length of 1.392(2) Angstroms indicates that this intermediate has significant Cu(III)-peroxo character. Theoretical calculations support this interpretation and predict that while a fully optimized end-on singlet geometry can be obtained, it is higher in energy than the side-on isomer by 3.5 kcal mol(-1) at the CASPT2/TZP level.     

Copper-dioxygen adducts and the side-on peroxo dicopper(II)/bis(mu-oxo) dicopper(III) equilibrium: Significant ligand electronic effects

The variation of ligand para substituents on pyridyl donor groups of tridentate amine copper(I) complexes was carried out in order to probe electronic effects on the equilibrium between mu-eta2:eta2-(side-on)-peroxo [Cu(II)2(O2(2-))]2+ and bis(mu-oxo) [Cu(III)2(O(2-))2] species formed upon reaction with O2. [Cu(I)(R-PYAN)(MeCN)n]B(C6F5)4 (R-PYAN = N-[2-(4-R-pyridin-2-yl)-ethyl]-N,N',N'-trimethyl-propane-1,3-diamine, R = NMe2, OMe, H, and Cl) (1R) vary over a narrow range in their Cu(II)/Cu(I) redox potentials (E(1/2) vs Fe(cp)2(+/0) = -0.40 V for 1(NMe2), -0.38 V for 1(OMe), -0.33 V for 1H, and -0.32 V for 1Cl) and in C-O stretching frequencies of their carbonyl adducts, 1R-CO: nu(C-O) = 2080, 2086, 2088, and 2090 cm(-1) for R = NMe2, OMe, H, and Cl, respectively. However, within this range of electronic properties for 1R, dioxygen reactivity is significantly affected. The reaction of 1Cl or 1H with O2 at -78 degrees C in CH2Cl2 gives UV-vis and resonance Raman spectra indicative of a mu-eta2:eta2-(side-on)-peroxo dicopper(II) adduct (2R). Compound 1(OMe) reacts with O2, yielding equilibrium mixtures of side-on peroxo (2(OMe)) and bis(mu-oxo) (3(OMe)) species. Oxygenation of 1(NMe2) leads to the sole generation of the bis(mu-oxo) dicopper(III) complex (3(NMe2)). A solvent effect was also observed; in acetone or THF, increased ratios of bis(mu-oxo) relative to side-on peroxo complex are observed. Thus, the equilibrium between a dicopper side-on peroxo and bis(mu-oxo) species can be tuned by ligand design-specifically, more electron donating ligands favor the formation of the latter isomer, and the peroxo/bis(mu-oxo) equilibrium can be shifted from one extreme to the other within the same ligand system. Observations concerning the reactivity of the dioxygen adducts 2H and 3(NMe2) toward external substrates are also presented.     

Contrasting copper-dioxygen chemistry arising from alike tridentate alkyltriamine copper(I) complexes

Copper(I)-dioxygen interactions are of great interest due to their role in biological O2-processing as well as their importance in industrial oxidation processes. We describe here the study of systems which lead to new insights concerning the factors which govern Cu(II)-mu-eta2:eta2 (side-on) peroxo versus Cu(III)-bis-mu-oxo species formation. Drastic differences in O2-reactivity of Cu(I) complexes which differ only by a single -CH3 versus -H substituent on the central amine of the tridentate ligands employed are observed. [Cu(MeAN)]B(C6F5)4 (1) (MeAN = N,N,N',N',N'-pentamethyl-dipropylenetriamine) reacts with O2 at -80 degrees C to form almost exclusively the side-on peroxo complex [{CuII(MeAN)}2(O2)]2+ (3) in CH2Cl2, tetrahydrofuran, acetone, and diethyl ether solvents, as characterized by UV-vis and resonance Raman spectroscopies. In sharp contrast, [Cu(AN)]B(C6F5)4 (2) (AN = 3, 3'-iminobis(N,N-dimethyl-propylamine) can support either Cu2O2 structures in a strongly solvent-dependent manner. Extreme behavior is observed in CH2Cl2 solvent, where 1 reacts with O2 giving 3, while 2 forms exclusively the bis-mu-oxo species [{CuIII(AN)}2(O)2]2+ (4Oxo). Stopped-flow kinetics measurements also reveal significant variations in the oxygenation reactions of 1 versus 2, including the observations that 4Oxo forms much faster than does 3; the former decomposes quickly, while the latter is quite stable at 193 K. The solvent-dependence of the bis-mu-oxo versus side-on peroxo preference observed for 2 is opposite to that reported for other known copper(I) complexes; the factors which may be responsible for the unusual behavior of 1/O2 versus 2/O2 (possibly N-H hydrogen bonding in the AN chemistry) are suggested. The factors which affect bis-mu-oxo versus side-on peroxo formation continue to be of interest.     

Dioxygen reactivity of a copper(I) complex with a N3S thioether chelate; peroxo-dicopper(II) formation including sulfur-ligation

Employing a tetradentate N3S(thioether) ligand, LN3S, dioxygen reactivity of a copper(I) complex, [(LN3S)CuI]+ (1) was examined. In CH2Cl2, acetone (at -80 degrees C), or 2-methyltetrahydrofuran (at -128 degrees C), 1 reacts with O2 producing the end-on bound peroxodicopper(II) complex [{(LN3S)CuII}2(mu-1,2-O2(2-))]2+ (2), the first reported copper-dioxygen adduct with sulfur (thioether) ligation. Its absorption spectrum contains an additional low-energy feature (but not a Cu-S CT band) compared to the previously well-characterized N4 ligand complex, [{(TMPA)CuII}2(mu-1,2-O2(2-))]2+ (3) (TMPA = tris(2-pyridylmethyl)amine). Resonance Raman spectroscopy confirms the peroxo formulation {nu(O-O) = 817 cm-1 (16-18O2 Delta = 46 cm-1) and nu(Cu-O) = 545 cm-1 (16-18O2 Delta = 26 cm-1), in close analogy to that known for 3 {nu(O-O) = 827 cm-1 and nu(Cu-O) = 561 cm-1}. Direct evidence for thioether ligation comes from EXAFS spectroscopy {Cu K-edge; Cu-S = 2.4 A}.     

Synthesis, structure, and reactivity of ruthenium carboxylato and 2-oxocarboxylato complexes bearing the bis(3,5-dimethylpyrazol-1-yl)acetato ligand

A series of ruthenium(II) acetonitrile, pyridine (py), carbonyl, SO2, and nitrosyl complexes [Ru(bdmpza)(O2CR)(L)(PPh3)] (L = NCMe, py, CO, SO2) and [Ru(bdmpza)(O2CR)(L)(PPh3)]BF4 (L = NO) containing the bis(3,5-dimethylpyrazol-1-yl)acetato (bdmpza) ligand, a N,N,O heteroscorpionate ligand, have been prepared. Starting from ruthenium chlorido, carboxylato, or 2-oxocarboxylato complexes, a variety of acetonitrile complexes [Ru(bdmpza)Cl(NCMe)(PPh3)] (4) and [Ru(bdmpza)(O2CR)(NCMe)(PPh3)] (R = Me (5a), R = Ph (5b)), as well as the pyridine complexes [Ru(bdmpza)Cl(PPh3)(py)] (6) and [Ru(bdmpza)(O2CR)(PPh3)(py)] (R = Me (7a), R = Ph (7b), R = (CO)Me (8a), R = (CO)Et (8b), R = (CO)Ph) (8c)), have been synthesized. Treatment of various carboxylato complexes [Ru(bdmpza)(O2CR)(PPh3)2] (R = Me (2a), Ph (2b)) with CO afforded carbonyl complexes [Ru(bdmpza)(O2CR)(CO)(PPh3)] (9a, 9b). In the same way, the corresponding sulfur dioxide complexes [Ru(bdmpza)(O2CMe)(PPh3)(SO2)] (10a) and [Ru(bdmpza)(O2CPh)(PPh3)(SO2)] (10b) were formed in a reaction of the carboxylato complexes with gaseous SO2. None of the 2-oxocarboxylato complexes [Ru(bdmpza)(O2C(CO)R)(PPh3)2] (R = Me (3a), Et (3b), Ph (3c)) showed any reactivity toward CO or SO2, whereas the nitrosyl complex cations [Ru(bdmpza)(O2CMe)(NO)(PPh3)](+) (11) and [Ru(bdmpza)(O2C(CO)Ph)(NO)(PPh3)](+) (12) were formed in a reaction of the acetato 2a or the benzoylformato complex 3c with an excess of nitric oxide. Similar cationic carboxylato nitrosyl complexes [Ru(bdmpza)(O2CR)(NO)(PPh3)]BF4 (R = Me (13a), R = Ph (13b)) and 2-oxocarboxylato nitrosyl complexes [Ru(bdmpza)(O2C(CO)R)(NO)(PPh3)]BF4 (R = Me (14a), R = Et (14b), R = Ph (14c)) are also accessible via a reaction with NO[BF4]. X-ray crystal structures of the chlorido acetonitrile complex [Ru(bdmpza)Cl(NCMe)(PPh3)] (4), the pyridine complexes [Ru(bdmpza)(O2CMe)(PPh3)(py)] (7a) and [Ru(bdmpza)(O2CC(O)Et)(PPh3)(py)] (8b), the carbonyl complex [Ru(bdmpza)(O2CPh)(CO)(PPh3)] (9b), the sulfur dioxide complex [Ru(bdmpza)(O2CPh)(PPh3)(SO2)] (10b), as well as the nitrosyl complex [Ru(bdmpza)(O2C(CO)Me)(NO)(PPh3)]BF4 (14a), are reported. The molecular structure of the sulfur dioxide complex [Ru(bdmpza)(O2CPh)(PPh3)(SO2)] (10b) revealed a rather unusual intramolecular SO2-O2CPh Lewis acid-base adduct.     

Mechanism of Formation of Peroxocarbonates RhOOC(O)O(Cl)(P)(3) and Their Reactivity as Oxygen Transfer Agents Mimicking Monooxygenases. The First Evidence of CO(2) Insertion into the O-O Bond of Rh(eta(2)-O(2)) Complexes

Extended labeling experiments have shown that formation of rhodium peroxocarbonate from CO(2) and [RhCl(eta(2)-O(2))(P)(3)] (P is PEt(2)Ph or PEtPh(2)) proceeds through O-O bond cleavage and CO(2) insertion. O-transfer to ancillary phosphine ligand to give R(3)P=O selectively (>85%) involves the Rh-linked O atom of the peroxo group of RhCl(CO(4))(P)(3).     

Reactivity study of a hydroperoxodicopper(II) complex: hydroxylation, dehydrogenation, and ligand cross-link reactions

Employing a binucleating phenol-containing ligand PD'OH, a mu-phenoxo-mu-hydroperoxo dicopper(II) complex [Cu(II)2(PD'O-)(-OOH)(RCN)2](ClO4)2 (1, R = CH3, CH3CH2 or C6H5CH2; lambda(max) = 407 nm; nu(O-O) = 870 cm(-1); J. Am. Chem. Soc. 2005, 127, 15360) is generated by reacting a precursor dicopper(I) complex [Cu(I)2(PD'OH)(CH3CN)2](ClO4)2 (2) with O2 in nitrile solvents at -80 degrees C. Species 1 is unable to oxidize externally added substrates, for instance, PPh3, 2,4-tert-butylphenol, or 9,10-dihydroanthracene. However, upon thermal decay, it hydroxylates copper-bound organocyanides (e.g., benzylcyanide), leading to the corresponding aldehyde while releasing cyanide. This chemistry mimics that known for the copper enzyme dopamine-beta-monooxygenase. The thermal decay of 1 also leads to a product [Cu(II)3(L")2(Cl-)2](PF6)2 (6); its X-ray structure reveals that L" is a Schiff base-containing ligand which apparently derives from both oxidative N-dealkylation and then oxidative dehydrogenation of PD'OH; the chloride presumably derives from the CH2Cl2 solvent. With an excess of PPh3 added to 1, a binuclear Cu(I) complex [Cu(I)2(L')(PPh3)2](ClO4)2 (5) with a cross-linked PD'OH ligand L' has also been identified and crystallographically and chemically characterized. The newly formed C-O bond and an apparent k(H)/k(D) = 2.9 +/- 0.2 isotope effect in the benzylcyanide oxidation reaction suggest a common ligand-based radical forms during compound 1 thermal decay reactions. A di-mu-hydroxide-bridged tetranuclear copper(II) cluster compound [{Cu(II)2(PD'O-)(OH-)}2](ClO4)4 (8) has also been isolated following warming of 1. Its formation is consistent with the generation of [Cu(II)2(PD'O-)(OH-)]2+, with dimerization a reflection of the large Cu...Cu distance and thus the preference for not having a second bridging ligand atom (in addition to the phenolate O) for dicopper(II) ligation within the PD'O- ligand framework.     

Geometric and electronic structures of peroxomanganese(III) complexes supported by pentadentate amino-pyridine and -imidazole ligands

Three peroxomanganese(III) complexes [Mn(III)(O(2))(mL(5)(2))](+), [Mn(III)(O(2))(imL(5)(2))](+), and [Mn(III)(O(2))(N4py)](+) supported by pentadentate ligands (mL(5)(2) = N-methyl-N,N',N'-tris(2-pyridylmethyl)ethane-1,2-diamine, imL(5)(2) = N-methyl-N,N',N'-tris((1-methyl-4-imidazolyl)methyl)ethane-1,2-diamine, and N4py = N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine) were generated by treating Mn(II) precursors with H(2)O(2) or KO(2). Electronic absorption, magnetic circular dichroism (MCD), and variable-temperature, variable-field MCD data demonstrate that these complexes have very similar electronic transition energies and ground-state zero-field splitting parameters, indicative of nearly identical coordination geometries. Because of uncertainty in peroxo (side-on η(2) versus end-on η(1)) and ligand (pentadentate versus tetradentate) binding modes, density functional theory (DFT) computations were used to distinguish between three possible structures: pentadentate ligand binding with (i) a side-on peroxo and (ii) an end-on peroxo, and (iii) tetradentate ligand binding with a side-on peroxo. Regardless of the supporting ligand, isomers with a side-on peroxo and the supporting ligand bound in a tetradentate fashion were identified as most stable by >20 kcal/mol. Spectroscopic parameters computed by time-dependent (TD) DFT and multireference SORCI methods provided validation of these isomers on the basis of experimental data. Hexacoordination is thus strongly preferred for peroxomanganese(III) adducts, and dissociation of a pyridine (mL(5)(2) and N4py) or imidazole (imL(5)(2)) arm is thermodynamically favored. In contrast, DFT computations for models of [Fe(III)(O(2))(mL(5)(2))](+) demonstrate that pyridine dissociation is not favorable; instead a seven-coordinate ferric center is preferred. These different results are attributed to the electronic configurations of the metal centers (high spin d(5) and d(4) for Fe(III) and Mn(III), respectively), which results in population of a metal-peroxo σ-antibonding molecular orbital and, consequently, longer M-O(peroxo) bonds for peroxoiron(III) species.     

Dicopper(I) complexes of unsymmetrical binucleating ligands and their dioxygen reactivities

The design, synthesis, and characterization of binuclear copper(I) complexes and investigations of their dioxygen reactivities are of interest in understanding fundamental aspects of copper/O2 reactivity and in modeling copper enzyme active-site chemistry. In the latter regard, unsymmetrical binuclear systems are of interest. Here, we describe the chemistry of new unsymmetrical binuclear copper complexes, starting with the binucleating ligand UN2-H, possessing a m-xylyl moiety linking a bis[2-(2-pyridyl)ethyl]amine (PY2) tridentate chelator and a 2-[2-(methylamino)ethyl]pyridine bidentate group. Dicopper(I) complexes of UN2-H, [Cu2(UN2-H)]2+ (1), as PF6- and ClO4- salts, are synthesized. These react with O2 (Cu:O2 = 2:1, manometry) resulting in the hydroxylation of the xylyl moiety, producing the phenoxohydroxodicopper(II) complex [Cu2(UN2-O-)(OH-)(CH3CN)]2+ (2). Compound 2(PF6)2 is characterized by X-ray crystallography, which reveals features similar to those of a structure described previously (Karlin, K. D.; et al. J. Am. Chem. Soc. 1984, 106, 2121-2128) for a symmetrical binucleating analogue having two tridentate PY2 moieties; here a CH3CN ligand replaces one pyridylethyl arm. Isotope labeling from a reaction of 1 using 18O2 shows that the ligand UN2-OH, extracted from 2, possesses an 18O-labeled phenol oxygen atom. Thus, the transformation 1 + O2-->2 represents a monooxygenase model system. [CuI2(UN2-OH)(CH3CN)]2+ (3), a new binuclear dicopper(I) complex with an unsymmetrical coordination environment is generated either by reduction of 2 with diphenylhydrazine or in reactions of cuprous salts with UN2-OH. Complex 3 reacts with O2 at -80 degrees C, producing the (mu-1,1-hydroperoxo)dicopper(II) complex [CuII2(UN2-O-)(OOH-)]2+ (4) (lambda max 390 nm (epsilon 4200 M-1 cm-1), formulated on the basis of the stoichiometry of O2 uptake by 3 (Cu:O2 = 2:1, manometry), its reaction with PPh3 giving O=PPh3 (85%), and comparison to previously studied close analogues. Discussions include the relevance and comparison to other copper bioinorganic chemistry.     

Photoexcitation in Cu(I) and Re(I) complexes containing substituted dipyrido[3,2-a:2',3'-c]phenazine: a spectroscopic and density functional theoretical study

Copper(I) and rhenium(I) complexes [Cu(PPh(3))(2)(dppz-11-COOEt)]BF(4), [Cu(PPh(3))(2)(dppz-11-Br)]BF(4), [Re(CO)(3)Cl(dppz-11-COOEt)] and [Re(CO)(3)Cl(dppz-11-Br)] (dppz-11-COOEt = dipyrido-[3,2a:2',3'c]phenazine-11-carboxylic ethyl ester, dppz-11-Br = 11-bromo-dipyrido[3,2a:2',3'c]-phenazine) have been studied using Raman, resonance Raman, and transient resonance Raman (TR(2)) spectroscopy, in conjunction with computational chemistry. DFT (B3LYP) frequency calculations with a 6-31G(d) basis set for the ligands and copper(I) centers and an effective core potential (LANL2DZ) for rhenium in the rhenium(I) complexes show close agreement with the experimental nonresonance Raman spectra. Modes that are phenazine-based, phenanthroline-based, and delocalized across the entire ligand structure were identified. The nature of the absorbing chromophores at 356 nm for ligands and complexes was established using resonance Raman spectroscopy in concert with vibrational assignments from calculations. This analysis reveals that the dominant chromophore for the complexes measured at 356 nm is ligand-centered (LC), except for [Re(CO)(3)Cl(dppz-11-Br)], which appears to have additional chromophores at this wavelength. Calculations on the reduced complexes, undertaken to model the metal-to-ligand charge transfer (MLCT) excited state, show that the reducing electron occupies a ligand MO that is delocalized across the ligand structure. Resonance Raman spectra (lambda(exc) = 514.5 nm) of the reduced rhenium complexes show a similar spectral pattern to that observed in [Re(CO)(3)Cl(dppz)](*-); the measured bands are therefore attributed to ligand radical anion modes. These bands lie at 1583-1593 cm(-1) for [Re(CO)(3)Cl(dppz-11-COOEt)] and 1611 cm(-1) for [Re(CO)(3)Cl(dppz-11-Br)]. The thermally equilibrated excited states are examined using nanosecond-TR(2) spectroscopy (lambda(exc) = 354.7 nm). The TR(2) spectra of the ligands provide spectral signatures for the (3)LC state. A band at 1382 cm(-1) is identified as a marker for the (3)LC states of both ligands. TR(2) spectra of the copper and rhenium complexes of dppz-11-Br show this (3)LC band, but it is not prominent in the spectra of [Cu(PPh(3))(2)(dppz-11-COOEt)](+) and [Re(CO)(3)Cl(dppz-11-COOEt)]. Calculations suggest that the lowest triplet states of both of the rhenium(I) complexes and [Cu(PPh(3))(2)(dppz-11-Br)](+) are metal-to-ligand charge transfer in nature, but the lowest triplet state of [Cu(PPh(3))(2)(dppz-11-COOEt)](+) appears to be LC in character.     

Spectroscopy and bonding in side-on and end-on Cu2(S2) cores: comparison to peroxide analogues

Spectroscopic methods combined with density functional calculations were used to study the disulfide-Cu(II) bonding interactions in the side-on micro -eta(2):eta(2)-bridged Cu(2)(S(2)) complex, [[Cu(II)[HB(3,5-Pr(i)(2)pz)(3)]](2)(S(2))], and the end-on trans- micro -1,2-bridged Cu(2)(S(2)) complex, [[Cu(II)(TMPA)](2)(S(2))](2+), in correlation to their peroxide structural analogues. Resonance Raman shows weaker S-S bonds and stronger Cu-S bonds in the disulfide complexes relative to the O-O and Cu-O bonds in the peroxide analogues. The weaker S-S bonds come from the more limited interaction between the S 3p orbitals relative to that of the O 2s/p hybrid orbitals. The stronger Cu-S bonds result from the more covalent Cu-disulfide interactions relative to the Cu-peroxide interactions. This is consistent with the higher energy of the disulfide valence level relative to that of the peroxide. The ground states of the side-on Cu(2)(S(2))/Cu(2)(O(2)) complexes are more covalent than those of the end-on Cu(2)(S(2))/Cu(2)(O(2)) complexes. This derives from the larger sigma-donor interactions in the side-on micro -eta(2):eta(2) structure, which has four Cu-disulfide/peroxide bonds, relative to the end-on trans- micro -1,2 structure, which forms two bonds to the Cu. The larger disulfide/peroxide sigma-donor interactions in the side-on complexes are reflected in their more intense higher energy disulfide/peroxide to Cu charge transfer transitions in the absorption spectra. The large ground-state covalencies of the side-on complexes result in significant nuclear distortions in the ligand-to-metal charge transfer excited states, which give rise to the strong resonance Raman enhancements of the metal-ligand and intraligand vibrations. Particularly, the large covalency of the Cu-disulfide interaction in the side-on Cu(2)(S(2)) complex leads to a different rR enhancement profile, relative to the peroxide analogues, reflecting a S-S bond distortion in the opposite directions in the disulfide/peroxide pi(sigma) to Cu charge transfer excited states. A ligand sigma back-bonding interaction exists only in the side-on complexes, and there is more sigma mixing in the side-on Cu(2)(S(2)) complex than in the side-on Cu(2)(O(2)) complex. This sigma back-bonding is shown to significantly weaken the S-S/O-O bond relative to that of the analogous end-on complex, leading to the low nu(S)(-)(S)/nu(O)(-)(O) vibrational frequencies observed in the resonance Raman spectra of the side-on complexes.     

Characterization of the side-on coordinated bissuperoxo complexes of aluminum FAl(O(2))(2), ClAl(O(2))(2), and BrAl(O(2))(2) with triplet electronic ground states: a combined matrix IR and quantum chemical study

Matrix isolation has been used to study the photolytically induced reaction of AlX (X = F, Cl, or Br) with O(2). The peroxo and bisperoxo compounds XAlO(2) and XAl(O(2))(2) are found to be the products of these reactions. While the peroxo species XAlO(2) were already addressed in a separate work, we concentrate herein on the bissuperoxo complexes XAl(O(2))(2), which are to our knowledge the first examples of such complexes with Al centers. Our IR spectroscopic results taking in the effect of isotopic substitution ((16)O/(18)O) allied with quantum chemical calculations show that the O(2) moieties in these complexes are side-on coordinated, leading to an overall C(2)(v)() symmetry of the complexes and a spin multiplicity of 3. The O-O distance of about 1.366 A argues for the presence of superoxide units. The force constants are, however, somewhat smaller than expected for a superoxide anion and indicate that the bonding in the complexes cannot be described simply on the basis of an ionic model. Interestingly a photoinduced intramolecular isotopic scrambling process is observed for the compounds resulting in partial conversion of the XAl((16)O(2))((18)O)(2) isotopomer into XAl((16)O(18)O)((16)O(18)O). The properties of the complexes will be compared to those of complexes to transition metal centers.