Synthesis and characterization of tris(bipyridyl)ruthenium(II)-modified mono-, di-, and trinuclear manganese complexes as electron-transfer models for photosystem II

With the aim of modeling the arrangement of redox-active and photoactive components along the electron-transfer pathway of photosystem II, tetra- to nonanuclear transition metal complexes have been synthesized, comprising one, two, or three manganese ions, oxidizable phenolates, and tris(2,2'-bipyridyl)ruthenium(II)-type units as photosensitizers. These model complexes are considered to be mononuclear ([LnMn](PF6)m), dinuclear ([L1aMnIV2(mu-O)2](PF6)6), or trinuclear ([LnMnIIMnIIMnIILn](PF6)12) with respect to the number of manganese centers present. Electronic coupling between the manganese ions is strongly antiferromagnetic in the case of the di(mu-oxo)-dimanganese compound [L1aMnIV2(mu-O)2](PF6)6, where the "ligand" [H2L1a]4+ consists of two tris(bipyridyl)ruthenium(II)-type units covalentely bound to a bismacrocyclic Me2dtne backbone to which the manganese ions are coordinated via an additional phenolate oxygen (Me2dtne = 1,2-bis(4-methyl-1,4,7-triazacyclononyl)ethane). Weak antiferromagnetic coupling is observed in compounds [LnMnIIMnIIMnIILn](PF6)12, where the three metals are in a linear arrangement (face-sharing octahedral). They are bridged by three phenolate oxygens of each of the deprotonated "ligands" [H3Ln]6+, respectively. Each ligand [H3Ln]6+ (n = 1, 2) consists of a tacn ring with three pendent arm phenols which are each bound to a tris(bipyridyl)ruthenium(II)-type unit (tacn = 1,4,7-triazacyclononane). In these compounds several electron-transfer steps were detected by electrochemical methods which are assigned to different redox processes located at individual electrochemically active components (Mn, Ru, bipyridyl, phenolate). For example, in the "mononuclear" compounds [LnMn](PF6)m (n = 1 or 2) Mn(II), Mn(III), and Mn(IV) are accessible and three Ru(II) centers are reversibly oxidized to Ru(III), and in addition, the coordinated phenolate can be oxidized to a highly reactive, coordinated phenoxyl radical. In several cases very slow heterogeneous electron-transfer rates were observed for redox processes involving the manganese centers.     

The Radical Anions of [2.2.2.2]Paracyclophane-1,9,17,25-tetraene and Some of its Heteroaromatic Analogues

The radical anions of [2.2.2.2]paracyclophane- 1,9,17,25-tetraene (I), [2] (2, 5)-furano [2]paracyclo [2] (2,5)furano [2]paracyclophane-1,8, 16,23-tetraene (II), [2]-(2,5)thiopheno [2]paracyclo [2] (2,5)thiopheno [2]paracyclophane-1,8,16,23-tetraene (III) and [2.2.2.2](2,5)thiophenophane-1,8,15,22-tetraene (IV) have been studied by ESR. and ENDOR. spectroscopy. The assignment of the proton coupling constants, a_Hμ is to a large extent based on investigations of deuteriated derivatives. These investigations impressively demonstrate the potential of ENDOR. spectroscopy as an analytical tool. The Arrhenius activation energies, E_a, for the rotation of phenylene fragments about the bonds linking them with the ethylenic parts in I ? and II ? are 36±6 and 28±4 kJ/mol, respectively. The value a_Hμ of the olefinic protons in I? appears substantially smaller than expected for the corresponding planar radical anion. The hyperfine data for II ?, III ? and IV ? are consistent with the conformations which should minimize the deviations of the macrocyclic π-systems from planarity. In the case of II ?, tight ion pairs are formed by the radical anion and its counter-ion, K ⊕, in DME , owing to the strong association of the alkali metal cation with one of the furan moieties. An analogous interaction of K ⊕ with a thiophene moiety in III ? must be weaker, since no effects of ion pairing on the ESR. and ENDOR. spectra have been observed for this radical anion.     

Direct detection of a hydrogen ligand in the [NiFe] center of the regulatory H2-sensing hydrogenase from Ralstonia eutropha in its reduced state by HYSCORE and ENDOR spectroscopy

The regulatory H2-sensing [NiFe] hydrogenase of the beta-proteobacterium Ralstonia eutropha displays an Ni-C "active" state after reduction with H2 that is very similar to the reduced Ni-C state of standard [NiFe] hydrogenases. Pulse electron nuclear double resonance (ENDOR) and four-pulse ESEEM (hyperfine sublevel correlation, HYSCORE) spectroscopy are applied to obtain structural information on this state via detection of the electron-nuclear hyperfine coupling constants. Two proton hyperfine couplings are determined by analysis of ENDOR spectra recorded over the full magnetic field range of the EPR spectrum. These are associated with nonexchangeable protons and belong to the beta-CH(2) protons of a bridging cysteine of the NiFe center. The signals of a third proton exhibit a large anisotropic coupling (Ax = 18.4 MHz, Ay = -10.8 MHz, Az = -18 MHz). They disappear from the 1H region of the ENDOR spectra after exchange of H2O with 2H2O and activation with 2H2 instead of H2 gas. They reappear in the 2H region of the ENDOR and HYSCORE spectra. Based on a comparison with the spectroscopically similar [NiFe] hydrogenase of Desulfovibrio vulgaris Miyazaki F, for which the g-tensor orientation of the Ni-C state with respect to the crystal structure is known (Foerster et al. J. Am. Chem. Soc. 2003, 125, 83-93), an assignment of the 1H hyperfine couplings is proposed. The exchangeable proton resides in a bridging position between the Ni and Fe and is assigned to a formal hydride ion. After illumination at low temperature (T = 10 K), the Ni-L state is formed. For the Ni-L state, the strong hyperfine coupling observed for the exchangeable hydrogen in Ni-C is lost, indicating a cleavage of the metal-hydride bond(s). These experiments give first direct information on the position of hydrogen binding in the active NiFe center of the regulatory hydrogenase. It is proposed that such a binding situation is also present in the active Ni-C state of standard hydrogenases.     

A study of metal chelation of dinucleotide analogs in the gas phase by fast-atom bombardment mass spectrometry

Fast-atom bombardment (FAB) mass spectrometry was used to investigate the interaction of proton and alkali metal ions with dinucleotide analogs such as T-n-T (T = thymine moiety, n = polyether chain, e.g., triethylene, tetraethylene, pentaethylene, and hexaethylene ether 1–4), A-n-T (A = adenine unit 5–8), and T-n-OMe (9–12) in 3-nitrobenzyl alcohol matrix. The [M + H]⁺ ion is the most abundant ion for the A-n-T series, whereas in 1–4 and 9–12 the (TC₂H₄)⁺ ion is the most abundant. Formation of [M + H -C₂H₄O]⁺ ions, a characteristic fragmentation of crown ethers under electron ionization, is observed for compounds 1–12 and is more pronounced in 6 and 7. An abundant [M ? H]^? ion is observed for all the compounds studied under negative ion FAB due to the presence of the (-CO-NH-CO-) group of thymine, an indication of existence of intramolecular H bonding. The FAB mass spectra of 1–12 with alkali metal ions (Li⁺, Na⁺, K⁺, Rb⁺, and Cs⁺) showed formation of abundant metal-coordinated ions ([M + Met]⁺ and [TC₂H₄ + Met]⁺). Compounds 3, 4, 6, 7, and 10–12 showed ions due to the substitution of the thymine moiety by a hydroxyl group ([M + Met ? 108]⁺, Met = metal ion). For compound 3 alone, substitution of two thymine groups ([M + Met - 216]⁺) was observed. Metastable ion studies were used to elucidate the structures of these potentially significant ions, and the ion formule were confirmed with high resolution measurements. Selectivity toward metal complexation with ligand size was seen in the T-n-T and A-n-T series and was even more pronounced in A-n-T series. These dinucleotide analogs fall in the following order of chelation of alkali metal ions, acyclic glymes < dinucleotide analogs (acyclic glymes substituted with nitrogen bases) < crown ethers, which places them in perspective as receptor models.     

Excellent chiralselectivity in sulfur-bridged CoIIIMCoIII (M = NiII and PdII) trinuclear complexes containing 1,2-cyclohexanediamine

The reaction of [Ni(aet)2] with [CoCl2(R,R-chxn)2]+ (aet = 2-aminoethanethiolate, R,R-chxn = 1R,2R-cyclohexanediamine) in water gave a CoIIINiIICoIII trinuclear complex, DeltaRRDeltaRR-[Ni(Co(aet)(2-)(R,R-chxn))2]4+ ([1a]4+), in which two cis(S)-[Co(aet)2(R,R-chxn)]+ units are linked by a central NiII ion through sulfur bridges. The two CoIII units in [1a]4+ uniformly adopt the Delta configuration, which is induced by the chirality of the terminal R,R-chxn ligands. The central NiII ion in [1a]4+ was replaced by a PdII ion to produce an analogous CoIIIPdIICoIII trinuclear complex, DeltaRRDeltaRR-[Pd(Co(aet)2(R,R-chxn))2]4+ ([2a]4+), with retention of the Delta configuration. When racemic R,R/S,S-chxn was employed instead of R,R-chxn, not only the chirality about two CoIII centers but also the chirality about two chxn ligands was unified in the S-bridged trinuclear structure, leading to the selective formation of a pair of enantiomers, DeltaRRDeltaRR/LambdaSSLambdaSS-[M(Co(aet)2(chxn))2]4+ (M = NiII ([1b]4+) and PdII ([2b]4+)). The stereochemical and spectroscopic features of these complexes are discussed on the basis of the electronic absorption, CD, and NMR spectroscopies, along with the crystal structures of [1a]4+ and [2a]4+.     

Ruthenium complexes containing 2-(2-nitrosoaryl)pyridine: structural, spectroscopic, and theoretical studies

Ruthenium complexes containing 2-(2-nitrosoaryl)pyridine (ON(^)N) and tetradentate thioether 1,4,8,11-tetrathiacyclotetradecane ([14]aneS4), [Ru(ON(^)N)([14]aneS4)](2+) [ON(^)N = 2-(2-nitrosophenyl)pyridine (2a), 10-nitrosobenzo[h]quinoline (2b), 2-(2-nitroso-4-methylphenyl)pyridine, (2c), 2-(2-nitrosophenyl)-5-(trifluoromethyl)pyridine (2d)] and analogues with the 1,4,7-trithiacyclononane ([9]aneS3)/tert-butylisocyanide ligand set, [Ru(ON(^)N)([9]aneS3)(C≡N(t)Bu)](2+) (4a and 4b), have been prepared by insertion of a nitrosonium ion (NO(+)) into the Ru-aryl bond of cyclometalated ruthenium(II) complexes. The molecular structures of the ON(^)N-ligated complexes 2a and 2b reveal that (i) the ON(^)N ligands behave as bidentate chelates via the two N atoms and the bite angles are 86.84(18)-87.83(16)° and (ii) the Ru-N(NO) and N-O distances are 1.942(5)-1.948(4) and 1.235(6)-1.244(5) ?, respectively. The Ru-N(NO) and N-O distances, together with ν(N═O), suggest that the coordinated ON(^)N ligands in this work are neutral moiety (ArNO)(0) rather than monoanionic radical (ArNO)(?-) or dianion (ArNO)(2-) species. The nitrosated complexes 2a-2d show moderately intense absorptions centered at 463-484 nm [ε(max) = (5-6) × 10(3) dm(3) mol(-1) cm(-1)] and a clearly discriminable absorption shoulder around 620 nm (ε(max) = (6-9) × 10(2) dm(3) mol(-1) cm(-1)), which tails up to 800 nm. These visible absorptions are assigned as a mixing of d(Ru) → ON(^)N metal-to-ligand charge-transfer and ON(^)N intraligand transitions on the basis of time-dependent density functional theory (TD-DFT) calculations. The first reduction couples of the nitrosated complexes range from -0.53 to -0.62 V vs Cp(2)Fe(+/0), which are 1.1-1.2 V less negative than that for [Ru(bpy)([14]aneS4)](2+) (bpy = 2,2'-bipyridine). Both electrochemical data and DFT calculations suggest that the lowest unoccupied molecular orbitals of the nitrosated complexes are ON(^)N-centered. Natural population analysis shows that the amount of positive charge on the Ru centers and the [Ru([14]aneS4)] moieties in 2a and 2b is larger than that in [Ru(bpy)([14]aneS4)](2+). According to the results of the structural, spectroscopic, electrochemical, and theoretical investigations, the ON(^)N ligands in this work have considerable π-acidic character and behave as better electron acceptors than bpy.     

2,4,6-Tris(2-pyridyl)-1,3,5-triazine (tptz)-derived [Ru(II)(tptz)(acac)(CH3CN)]+ and mixed-valent [(acac)2Ru(III){(mu-tptz-H+)-}Ru(II)(acac)(CH3CN)]+

Mononuclear [Ru(II)(tptz)(acac)(CH3CN)]ClO4 ([1]ClO4) and mixed-valent dinuclear [(acac)2Ru(III){(mu-tptz-Eta+)-}Ru(II)(acac)(CH3CN)]ClO4 ([5]ClO4; acac = acetylacetonate) complexes have been synthesized via the reactions of Ru(II)(acac)2(CH3CN)2 and 2,4,6-tris(2-pyridyl)-1,3,5-triazine (tptz), in 1:1 and 2:1 molar ratios, respectively. In [1]ClO4, tptz binds with the Ru(II) ion in a tridentate N,N,N mode (motif A), whereas in [5]ClO4, tptz bridges the metal ions unsymmetrically via the tridentate neutral N,N,N mode with the Ru(II) center and cyclometalated N,C- state with the Ru(III) site (motif F). The activation of the coordinated nitrile function in [1]ClO4 and [5]ClO4 in the presence of ethanol and alkylamine leads to the formation of iminoester ([2]ClO4 and [7]ClO4) and amidine ([4]ClO4) derivatives, respectively. Crystal structure analysis of [2]ClO4 reveals the formation of a beautiful eight-membered water cluster having a chair conformation. The cluster is H-bonded to the pendant pyridyl ring N of tptz and also with the O atom of the perchlorate ion, which, in turn, makes short (C-H- - - - -O) contacts with the neighboring molecule, leading to a H-bonding network. The redox potentials corresponding to the Ru(II) state in both the mononuclear {[(acac)(tptz)Ru(II)-NC-CH3]ClO4 ([1]ClO4) > [(acac)(tptz)Ru(II)-NH=C(CH3)-OC2H5]ClO4 ([2]ClO4) > [(acac)(tptz)Ru(II)-NH2-C6H4(CH3)]ClO4 ([3]ClO4) > [(acac)(tptz)Ru(II)-NH=C(CH3)-NHC2H5]ClO4 ([4]ClO4)} and dinuclear {[(acac)2Ru(III){(mu-tptz-H+)-}Ru(II)(acac)(NC-CH3)]ClO4 ([5]ClO4), [(acac)2Ru(III){(mu-tptz-H+(N+-O-)2)-}Ru(II)(acac)(NC-CH3)]ClO4 ([6]ClO4), [(acac)2Ru(III){(mu-tptz-H+)-}Ru(II)(acac)(NH=C(CH3)-OC2H5)]ClO4 ([7]ClO4), and [(acac)2Ru(III){(mu-tptz-Eta+)-}Ru(II)(acac)(NC4H4N)]ClO4 ([8]ClO(4))} complexes vary systematically depending on the electronic nature of the coordinated sixth ligands. However, potentials involving the Ru(III) center in the dinuclear complexes remain more or less invariant. The mixed-valent Ru(II)Ru(III) species ([5]ClO4-[8]ClO4) exhibits high comproportionation constant (Kc) values of 1.1 x 10(12)-2 x 10(9), with substantial contribution from the donor center asymmetry at the two metal sites. Complexes display Ru(II)- and Ru(III)-based metal-to-ligand and ligand-to-metal charge-transfer transitions, respectively, in the visible region and ligand-based transitions in the UV region. In spite of reasonably high K(c) values for [5]ClO4-[8]ClO4, the expected intervalence charge-transfer transitions did not resolve in the typical near-IR region up to 2000 nm. The paramagnetic Ru(II)Ru(III) species ([5]ClO4-[8]ClO4) displays rhombic electron paramagnetic resonance (EPR) spectra at 77 K (g approximately 2.15 and Deltag approximately 0.5), typical of a low-spin Ru(III) ion in a distorted octahedral environment. The one-electron-reduced tptz complexes [Ru(II)(tptz.-)(acac)(CEta3CN)] (1) and [(acac)2Ru(III){(mu-tptz-Eta+).2-}Ru(II)(acac)(CH3CN)] (5), however, show a free-radical-type EPR signal near g = 2.0 with partial metal contribution.     

Synthesis of nitrosylruthenium complexes containing 2,2':6',2"-terpyridine by reactions of alkoxo complexes with acids

Nitrosylruthenium complexes containing 2,2':6',2"-terpyridine (terpy) have been synthesized and characterized. The three alkoxo complexes trans-(NO, OCH3), cis-(Cl, OCH3)-[RuCl(OCH3)(NO)(terpy)]PF6 ([2]PF6), trans-(NO, OC2H5), cis-(Cl, OC2H5)-[RuCl(OC2H5)(NO)(terpy)]PF6 ([3]PF6), and [RuCl(OC3H7)(NO)(terpy)]PF6 ([4]PF6) were synthesized by reactions of trans-(Cl, Cl), cis-(NO, Cl)-[RuCl2(NO)(terpy)]PF6 ([1]PF6) with NaOCH3 in CH3OH, C2H5OH, and C3H7OH, respectively. Reactions of [3]PF6 with an acid such as hydrochloric acid and trifluoromethansulforic acid afford nitrosyl complexes in which the alkoxo ligand is substituted. The geometrical isomer of [1]PF6, trans-(NO, Cl), cis-(Cl, Cl)-[RuCl2(NO)(terpy)]PF6 ([5]PF6), was obtained by the reaction of [3]PF6 in a hydrochloric acid solution. Reaction of [3]PF6 with trifluoromethansulforic acid in CH3CN gave trans-(NO, Cl), cis-(CH3CN, Cl)-[RuCl(CH3CN)(NO)(terpy)]2+ ([6]2+) under refluxing conditions. The structures of [3]PF6, [4]PF6.CH3CN, [5]CF3SO3, and [6](PF6)2 were determined by X-ray crystallograpy.     

Defect models of the three trigonal Ti3+ centers in LiF:Ti3+ and LiF:Ti3+:Mg2+ crystals

The EPR g factors of the trigonal Ti3+ center A in LiF:Ti3+ and two additional trigonal Ti3+ centers B and C in LiF:Ti3+:Mg2+ crystals are calculated from the third-order perturbation formulas based on the cluster approach. From the calculations and by considering the Ti3+ displacement along 111 axis obtained by ENDOR experiment, the defect models for the three Ti3+ centers are suggested. For center A, there are two possible models: (i) [Ti3+F3-O3(2-)] cluster and (ii) [Ti3+F6-] cluster with the Ti3+ off-center caused by a neighboring Li+ vacancy (VLi+) at <111> axis. The latter seems the more likely. The defect models of centers B and C are the [Ti3+F3-O(3)2-] clusters associated with a neighboring: Mg2+ ion at the Li+ site along 111 axis in the vicinity of three F- ions and three O2- ions, respectively. The reasonableness of these models is discussed.     

An Anionic Amido-Substituted Seven-Vertex Siliconoid Cluster

The silanide [Si₄{N(SiMe₃)Dipp}₃]⁻ ( 1 ) transforms into the anionic siliconoid cluster [Si₇{N(SiMe₃)Dipp}₃]⁻ ( 2 ) with four unsubstituted silicon atoms as a contact ion pair with [K([18]crown-6)] in C₆D₆ at room temperature within five weeks. Anion 2 was investigated by natural population analysis and visualization of intrinsic atomic orbitals. Magnetically induced current-density calculations of 2 revealed two distinct strong diatropic vortices that sum up in one direction and create a strongly shielded apical silicon atom in 2 .     

Organometallic[bond]polyoxometalate hybrid compounds: metallosalen compounds modified by Keggin type polyoxometalates

Hybrid compounds with two functional centers consisting of a metallosalen moiety (M[bond]salen; M = Mn, Co, Ni, and Pd) connected by an alkylene bridging group to a lacunary Keggin type polyoxometalate were synthesized and characterized. In these metallosalen-polyoxometalate compounds (M[bond]salen[bond]POM) it was shown by the use of a combination of UV[bond]vis, (1)H NMR, EPR, XPS, and cyclic voltammetry measurements that the polyoxometalate exerts a significant intramolecular electronic effect on the metallosalen moiety leading to formation of an oxidized metallosalen moiety. For the Mn[bond]salen[bond]POM, the metallosalen center is best described as a metal[bond]salen cation radical species; that is, a localized "hole" is formed on the salen ligand. For the other M[bond]salen[bond]POM compounds, the metallosalen moiety can be described as a hybrid of a metal[bond]salen cation radical species and an oxidized metal[bond]salen species, that is, a delocalized "hole" is formed at the metallosalen center. It is proposed that these oxidized metallosalen centers are best characterized as stabilized charge transfer (metallosalen donor[bond]polyoxometalate acceptor) complexes despite the relatively large distance between the two functional centers.     

ENDOR investigation of the liganding environment of mixed-spin ferric cytochrome c'

The electronic structure of the 5-coordinate quantum-mechanically mixed-spin (sextet-quartet) heme center in cytochrome c' was investigated by electron nuclear double resonance (ENDOR), a technique not previously applied to this mixed-spin system. Cytochrome c' was obtained from overexpressing variants of Rhodobacter sphaeroides 2.4.3. ENDOR for this study was done at the g(//) = 2.00 extremum where single-crystal-like, well-resolved spectra prevail. The heme meso protons of cytochrome c' showed a contact interaction that implied spin delocalization arising from the heme (d(z)(2)) orbital enhanced by iron out-of-planarity. An exchangeable proton ENDOR feature appeared from the proximal His123 Ndelta hydrogen. This Ndelta hydrogen, which crystallographically has no hydrogen-bonding partner and thus belongs to a neutral imidazole, showed a larger hyperfine coupling than the corresponding hydrogen-bonded Ndelta proton from metmyoglobin. The unique residue Phe14 occludes binding of a sixth ligand in cytochrome c', and ENDOR from a proton of the functionally important Phe14 ring, approximately 3.3 A away from the heme iron, was detected. ENDOR of the nitrogen ligand hyperfine structure is a direct probe into the sigma-antibonding (d(z)(2)) and (d(x)(2)-d(y)(2)) orbitals whose energies alter the relative stability and admixture of sextet and quartet states and whose electronic details were thus elucidated. ENDOR frequencies showed for cytochrome c' larger hyperfine couplings to the histidine nitrogen and smaller hyperfine couplings to the heme nitrogens than for high-spin ferric hemes. Both of these findings followed from the mixed-spin ground state, which has less (d(x)(2)-d(y)(2)) character than have fully high-spin ferric heme systems.     

Synthesis, crystal structure, aqueous speciation, and kinetics of substitution reactions in a water-soluble Mo3S4 cluster bearing hydroxymethyl diphosphine ligands

The [Mo3S4Cl3(dhmpe)3]Cl ([1]Cl) cluster has been prepared from [Mo3S7Cl6]2- and the water-soluble 1,2-bis(bis(hydroxymethyl)-phosphino)ethane (dhmpe, L) ligand. The crystal structure has been determined by X-ray diffraction methods and shows the incomplete cuboidal structure typical of the M3Q4 clusters (M=Mo, W; Q=S, Se), with a capping sulfide ligand to the three metal centers and the other three sulfides acting as bridges between two Mo atoms. The octahedral coordination around each metal center is completed with a chlorine and two phosphorus atoms of one L ligand. The chemistry of aqueous solutions of [1]Cl is dominated by the formation of the [Mo3S4L(L-H)2(H2O)]2+ complex ([2]2+), where the three chlorides have been replaced by one water molecule and two alkoxo groups of two different dhmpe ligands, thus leading to a solution structure where the three metal centers are not equivalent. A detailed study based on stopped-flow, 31P{1H} NMR, and electrospray ionization mass spectrometry techniques has been carried out to understand the behavior of [2]2+ in aqueous solution. In this way, it has been established that the addition of an excess of X- (Cl-, SCN-) leads to [Mo3S4X3(dhmpe)3]+ complexes in three resolved kinetic steps that correspond to the sequential coordination of X- at the three metal centers. However, whereas the first two steps involve the opening of the chelate rings formed with the alkoxo groups of the dhmpe ligands, the third one corresponds to the substitution of the coordinated water molecule. These results demonstrate that the asymmetry introduced by the closure of chelate rings at only two of the three Mo centers makes the kinetics of the reaction deviate significantly from the statistical behavior typically associated with M3Q4 clusters. The results obtained for the reaction of [2]2+ with acid and base are also described, and they complete the picture of the aqueous speciation of this cluster.     

1,4,7,10-tetraazacyclododecane metal complexes as potent promoters of carboxyester hydrolysis under physiological conditions

New 1,4,7,10-tetrazacyclododecane ([12]aneN4 or cyclen) ligands with different heterocyclic spacers (triazine and pyridine) of various lengths (bi- and tripyridine) or an azacrown pendant and their mono- and dinuclear Zn(II), Cu(II), and Ni(II) complexes have been synthesized and characterized. The pKa values of water molecules coordinated to the complexed metal ions were determined by potentiometric pH titrations and vary from 7.7 to 11.2, depending on the metal-ion and ligand properties. The X-ray structure of [Zn2L2]mu-OH(ClO4)3.CH3CN.H2O shows each Zn(II) ion in a tetrahedral geometry, binding to three N atoms of cyclen (the average distance of Zn-N = 2.1 A) and having a mu-OH bridge at the apical site linking the two metal ions (the average distance of Zn-O- = 1.9 A). The distance between the Zn(II) ion and the fourth N atom is 2.6 A. All Zn(II) complexes promote the hydrolysis of 4-nitrophenyl acetate (NA) under physiological conditions, while those of Cu(II) and Ni(II) do not have a significant effect on the hydrolysis reaction. The kinetic studies in buffered solutions (0.05 M Tris, HEPES, or CHES, I = 0.1 M, NaCl) at 25 degrees C in the pH range of 6-11 under pseudo-first-order reaction conditions (excess of the metal complex) were analyzed by applying the method of initial rates. Comparison of the second-order pH-independent rate constants (kNA, M-1 s-1) for the mononuclear complexes ZnL1, ZnL3, and ZnL8, which are 0.39, 0.27, and 0.38, respectively, indicates that the heterocyclic moiety improves the rate of hydrolysis up to 4 times over the parent Zn([12]aneN4) complex (kNA = 0.09 M-1 s-1). The reactive species is the Zn(II)-OH- complex, in which the Zn(II)-bound OH- acts as a nucleophile, which attacks intermolecularly the carbonyl group of the acetate ester. For dinuclear complexes Zn2L2, Zn2L4, Zn2L5, Zn2L6, and Zn2L7, the mechanism of the reaction is defined by the degree of cooperation between the metal centers, determined by the spacer length. For Zn2L7, having the longest triaryl spacer, the two metal centers act independently in the hydrolysis; therefore, the reaction rate is twice as high as the rate of the mononuclear analogue (kNA = 0.78 M-1 s-1). The complexes with a monoaryl spacer show saturation kinetics with the formation of a Michaelis-Menten adduct. Their hydrolysis rates are 40 times higher than that of the Zn[12]aneN4 system (kNA approximately 4 M-1 s-1). Zn2L6 is a hybrid between these two mechanisms; a clear saturation curve is not visible nor are the metal cores completely independent from one another. Some of the Zn(II) complexes show a higher hydrolytic activity under physiological conditions compared to other previously reported complexes of this type.     

Spectroscopic approaches to elucidating novel iron-sulfur chemistry in the "radical-Sam" protein superfamily

Electron paramagnetic resonance (EPR), electron-nuclear double resonance (ENDOR), and M?ssbauer spectroscopies and other physical methods have provided important new insights into the radical-SAM superfamily of proteins, which use iron-sulfur clusters and S-adenosylmethionine to initiate H atom abstraction reactions. This remarkable chemistry involves the generation of the extremely reactive 5'-deoxyadenosyl radical, the same radical intermediate utilized in B12-dependent reactions. Although early speculation focused on the possibility of an organometallic intermediate in radical-SAM reactions, current evidence points to novel chemistry involving a site-differentiated [4Fe-4S] cluster. The focus of this forum article is on one member of the radical-SAM superfamily, pyruvate formate-lyase activating enzyme, and how physical methods, primarily EPR and ENDOR spectroscopies, are contributing to our understanding of its structure and mechanism. New ENDOR data supporting coordination of the methionine moiety of SAM to the unique site of the [4Fe-4S]2+/+ cluster are presented.     

ENDOR characterization of a synthetic diiron hydrazido complex as a model for nitrogenase intermediates

Molybdenum-dependent nitrogenase binds and reduces N2 at the [Fe7, Mo, S9, X, homocitrate] iron-molybdenum cofactor (FeMo-co). Kinetic and spectroscopic studies of nitrogenase variants indicate that a single Fe-S face is the most likely binding site. Recently, substantial progress has been made in determining the structures of nitrogenase intermediates formed during alkyne and N2 reduction through use of ENDOR spectroscopy. However, constraints derived from ENDOR studies of biomimetic complexes with known structure would powerfully contribute in turning experimentally derived ENDOR parameters into structures for species bound to FeMo-co during N2 reduction. The first report of a paramagnetic Fe-S compound that binds reduced forms of N2 involved Fe complexes stabilized by a bulky beta-diketiminate ligand (Vela, J.; Stoian, S.; Flaschenriem, C. J.; Münck, E.; Holland, P. L. J. Am. Chem. Soc. 2004, 126, 4522-4523). Treatment of a sulfidodiiron(II) complex with phenylhydrazine gave an isolable mixed-valence FeII-Fe(III) complex with a bridging phenylhydrazido (PhNNH2) ligand, and this species now has been characterized by ENDOR spectroscopy. Using both 15N, 2H labeled and unlabeled forms of the hydrazido ligand, the hyperfine and quadrupole parameters of the -N-NH2 moiety have been derived by a procedure that incorporates the (near-) mirror symmetry of the complex and involves a strategy which combines experiment with semiempirical and DFT computations. The results support the use of DFT computations in identifying nitrogenous species bound to FeMo-co of nitrogenase turnover intermediates and indicate that 14N quadrupole parameters from nitrogenase intermediates will provide a strong indication of the nature of the bound nitrogenous species. Comparison of the large 14N hyperfine couplings measured here with that of a hydrazine-derived species bound to FeMo-co of a trapped nitrogenase intermediate suggests that the ion(s) are not high spin and/or that the spin coupling coefficients of the coordinating cofactor iron ion(s) in the intermediate are exceptionally small.     

6,7,8,9-四氢-吩嗪并[2,3]C_(60)衍生物的结构、电子光谱和二阶非线性光学性质的理论研究

利用半经验AM1量子化学方法研究了6, 7, 8, 9-四氢-吩嗪并[2, 3]C60衍生物(1～4)及其异构体5, 6, 7, 8-四氢-9, 10-二氮杂蒽并[2, 3]C60衍生物 (5～8) 的结构。 结果表明,目标分子的前线轨道主要由C60部分决定, C60母体与加成基团之间存在较强的分子内电荷转移, C60部分是电子受体,吩嗪环部分为电子给体。 在AM1优化几何构型的基础上, 用INDO/SCI方法计算了目标分子的电子光谱, 用完全态求和(SOS)公式计算了其二阶非线性光学性质。 计算结果表明, 目标分子在400 nm 以上均存在弱吸收峰, 与实验所得结果一致。 5, 6, 7, 8-四氢-9, 10-二氮杂蒽并[2, 3]C60衍生物(5～8)的二阶非线性光学系数(0)比其异构体6, 7, 8, 9-四氢-吩嗪并[2, 3]C60衍生物(1～4)的大得多。     

A new, easily recyclable arylating agent based on a diphosphino-digold(I) complex

The synthesis of two organogold(I) complexes, [(Au(NCN))2(dppbp)] (6) and [(Au(Phebox))2(dppbp)] (9), and their application in subsequent transmetalating reactions are described. A trinuclear organogold(I) complex, [(AuCl)3(tdpppb)] (4) is also reported, which exhibits a surprisingly high solubility in dichloromethane. It was found that 6 and 9 can cleanly transfer the anionic NCN-([C(6)H(3)(CH(2)NMe(2))2-2,6]-) or Phebox-([2,6-bis(oxazolinyl)phenyl]-) moiety to Ti(IV) and Pd(II) centers, respectively. The coproduct [(AuCl)2(dppbp)] (3, dppbp is [4-Ph(2)PC(6)H(4)]2 (1)) formed during this transmetalation reaction, precipitates almost quantitatively from the reaction mixture (toluene) and can thus be separated by simple filtration. In comparison, [AuCl(PPh3)], formed as the coproduct in the transmetalation reaction of [Au(NCN)(PPh3)] with metal salts, has a higher solubility in apolar solvents and thus is more difficult to separate from the resultant organometallic complex. Digold complex 6 has been characterized by NMR spectroscopy and crystallographic analyses. These analyses show that the two gold units are essentially independent. The formation of a dimetallic transmetalating agent based on gold(I) had no effect on its transmetalating properties.     

Neocuproine-extended porphyrin coordination complexes. 2. Spectroscopic properties of the metalloporphyrin derivatives and investigations into the HOMO ordering

The synthesis of a porphyrin compound, 1, containing a 2,9-dimethyl-1,10-phenanthroline moiety that is fused at the beta-pyrrole positions is reported. The absorption spectra of the free-base, copper(II), and zinc(II) derivatives have been studied. On the basis of absorption band intensities, the HOMO of the free base (H21) and its copper and zinc complexes (Cu1 and Zn1) was determined to be of a1u symmetry. Relative to H21, compounds Cul and Znl show enhanced spectral changes upon external metal ion binding. Although the HOMO is the same in all three compounds, the energy gap between the two highest occupied orbitals is greater for Cu1 and Zn1 than it is for the free-base compound. Several metal ions (Ni2+, Cu+, Cu2+, Zn2+, Li+) were examined in their binding to the phenanthrolinic group by measuring the resulting changes in the absorption spectra. It is shown that the observed changes in the absorption spectra are insensitive to the nature of the metal ion coordinated by the phenanthroline moiety. Significant differences in the absorption and emission spectra between Zn1 and [Zn(Zn1)2]2+ clearly demonstrate that the porphyrin pi-system is strongly affected by the binding of metal ions at the fused phenanthrolinic moiety.     

Stabilized Carbenium Ions as Latent,Z‐type Ligands

Controlling the reactivity of transition metals using secondary, σ‐accepting ligands is an active area of investigation that is impacting molecular catalysis. Herein we describe the phosphine gold complexes [(o‐Ph₂P(C₆H₄)Acr)AuCl]⁺ ([ 3 ]⁺; Acr=9‐N‐methylacridinium) and [(o‐Ph₂P(C₆H₄)Xan)AuCl]⁺ ([ 4 ]⁺; Xan=9‐xanthylium) where the electrophilic carbenium moiety is juxtaposed with the metal atom. While only weak interactions occur between the gold atom and the carbenium moiety of these complexes, the more Lewis acidic complex [ 4 ]⁺ readily reacts with chloride to afford a trivalent phosphine gold dichloride derivative ( 7 ) in which the metal atom is covalently bound to the former carbocationic center. This anion‐induced Au^I/Au^III oxidation is accompanied by a conversion of the Lewis acidic carbocationic center in [ 4 ]⁺ into an X‐type ligand in 7 . We conclude that the carbenium moiety of this complex acts as a latent Z‐type ligand poised to increase the Lewis acidity of the gold center, a notion supported by the carbophilic reactivity of these complexes.