The guanine cation radical: investigation of deprotonation states by ESR and DFT

This work reports ESR studies that identify the favored site of deprotonation of the guanine cation radical (G*+) in an aqueous medium at 77 K. Using ESR and UV-visible spectroscopy, one-electron oxidized guanine is investigated in frozen aqueous D2O solutions of 2'-deoxyguanosine (dGuo) at low temperatures at various pHs at which the guanine cation radical, G*+ (pH 3-5), singly deprotonated species, G(-H)* (pH 7-9), and doubly deprotonated species, G(-2H)*- (pH > 11), are found. C-8-deuteration of dGuo to give 8-D-dGuo removes the major proton hyperfine coupling at C-8. This isolates the anisotropic nitrogen couplings for each of the three species and aids our analyses. These anisotropic nitrogen couplings were assigned to specific nitrogen sites by use of 15N-substituted derivatives at N1, N2, and N3 atoms in dGuo. Both ESR and UV-visible spectra are reported for each of the species: G*+, G(-H)*, and G(-2H)*-. The experimental anisotropic ESR hyperfine couplings are compared to those obtained from DFT calculations for the various tautomers of G(-H)*. Using the B3LYP/6-31G(d) method, the geometries and energies of G*+ and its singly deprotonated state in its two tautomeric forms, G(N1-H)* and G(N2-H)*, were investigated. In a nonhydrated state, G(N2-H)* is found to be more stable than G(N1-H)*, but on hydration with seven water molecules G(N1-H)* is found to be more stable than G(N2-H)*. The theoretically calculated hyperfine coupling constants (HFCCs) of G*+, G(N1-H)*, and G(-2H)*- match the experimentally observed HFCCs best on hydration with seven or more waters. For G(-2H)*-, the hyperfine coupling constant (HFCC) at the exocyclic nitrogen atom (N2) is especially sensitive to the number of hydrating water molecules; good agreement with experiment is not obtained until nine or 10 waters of hydration are included.     

On the evolution of one-electron-oxidized deoxyguanosine in damaged DNA under physiological conditions: a DFT and ONIOM study on proton transfer and equilibrium

Different deprotonation paths of the radical cation formed by one-electron oxidation of 2'-deoxyguanosine (2dG) sites in DNA have been studied using Density Functional Theory (M05-2X/6-31+G(d,p)) and ONIOM methodology (M05-2X/6-31+G(d,p):PM6) in conjunction with the SMD model to include the solvent effects. Models of increased complexity have been used ranging from the isolated nucleoside to a three unit double-stranded oligomer including the sugar units, the base pairing with cytidine, and the phosphate linkage. The reported results correspond to aqueous solution, at room temperature, and pH = 7.4. Under such conditions it was found that the proton transfer (PT) within the base pair is a minor path compared to the PT between the base pair and the surrounding water. It was also found that the deprotonation of ground-state 2dG˙(+) sites mainly yields C centered radicals in the sugar unit, with the largest populations corresponding to C4'˙ and C5'˙, followed by C3'˙. The different aspects of the presented theoretical study have been validated with experimental results.     

Proton-coupled hole hopping in nucleosomal and free DNA initiated by site-specific hole injection

Nucleosomes were reconstituted from recombinant histones and a 147-mer DNA sequence containing the damage reporter sequence 5'-…d([2AP]T[GGG](1)TT[GGG](2)TTT[GGG](3)TAT)… with 2-aminopurine (2AP) at position 27 from the dyad axis. Footprinting studies with ˙OH radicals reflect the usual effects of "in" and "out" rotational settings, while, interestingly, the guanine oxidizing one-electron oxidant CO(3)(˙-) radical does not. Site-specific hole injection was achieved by 308 nm excimer laser pulses to produce 2AP(˙+) cations, and superoxide via the trapping of hydrated electrons. Rapid deprotonation (~100 ns) and proton coupled electron transfer generates neutral guanine radicals, G(-H)˙ and hole hopping between the three groups of [GGG] on micro- to millisecond time scales. Hole transfer competes with hole trapping that involves the combination of O(2)(˙-) with G(-H)˙ radicals to yield predominantly 2,5-diamino-4H-imidazolone (Iz) and minor 8-oxo-7,8-dihydroguanine (8-oxoG) end-products in free DNA (Misiaszek et al., J. Biol. Chem. 2004, 279, 32106). Hole migration is less efficient in nucleosomal than in the identical protein-free DNA by a factor of 1.2-1.5. The Fpg/piperidine strand cleavage ratio is ~1.0 in free DNA at all three GGG sequences and at the "in" rotational settings [GGG](1,3) facing the histone core, and ~2.3 at the "out" setting at [GGG](2) facing away from the histone core. These results are interpreted in terms of competitive reaction pathways of O(2)(˙-) with G(-H)˙ radicals at the C5 (yielding Iz) and C8 (yielding 8-oxoG) positions. These differences in product distributions are attributed to variations in the local nucleosomal B-DNA base pair structural parameters that are a function of surrounding sequence context and rotational setting.     

Effect of the N-terminal basic residue on facile Cα-C bond cleavages of aromatic-containing peptide radical cations

Fragmentation of radical cationic peptides [R(G)(n-2)X(G)(7-n)]˙(+) and [R(G)(m-2)XG]˙(+) (X = Phe or Tyr; m = 2-5; n = 2-7) leads selectively to a(n)(+) product ions through in situ C(α)-C peptide backbone cleavage at the aromatic amino acid residues. In contrast, substituting the arginine residue with a less-basic lysine residue, forming [K(G)(n-2)X(G)(7-n)]˙(+) (X = Phe or Tyr; n = 2-7) analogs, generates abundant b-y product ions; no site-selective C(α)-C peptide bond cleavage was observed. Studying the prototypical radical cationic tripeptides [RFG]˙(+) and [KFG]˙(+) using low-energy collision-induced dissociation and density functional theory, we have examined the influence of the basicity of the N-terminal amino acid residue on the competition between the isomerization and dissociation channels, particularly the selective C(α)-C bond cleavage viaβ-hydrogen atom migration. The dissociation barriers for the formation of a(2)(+) ions from [RFG]˙(+) and [KFG]˙(+)via their β-radical isomers are comparable (33.1 and 35.0 kcal mol(-1), respectively); the dissociation barrier for the charge-induced formation of the [b(2)- H]˙(+) radical cation from [RFG]˙(+)via its α-radical isomer (39.8 kcal mol(-1)) was considerably higher than that from [KFG]˙(+) (27.2 kcal mol(-1)). Thus, the basic arginine residue sequesters the mobile proton to promote the charge-remote selective C(α)-C bond cleavage by energetically hindering the competing charge-induced pathways.     

Deprotonation/protonation of coordinated secondary thioamide units of pincer ruthenium complexes: modulation of voltammetric and spectroscopic characterization of the pincer complexes

New pincer ruthenium complexes, [Ru(SCS)(tpy)]PF(6) (1) (SCS = 2,6-bis(benzylaminothiocarbonyl)phenyl), tpy = 2,2':6',2'-terpyridyl) and [Ru(SNS)(tpy)]PF(6) (2) (SNS = 2,5-bis(benzylaminothiocarbonyl)pyrrolyl), having κ(3)SCS and κ(3)SNS pincer ligands with two secondary thioamide units were synthesized by the reactions of [RuCl(3)(tpy)] with N,N'-dibenzyl-1,3-benzenedicarbothioamide (L1) and N,N'-dibenzyl-2,5-1H-pyrroledicarbothioamide (L2), respectively, and their chemical and electrochemical properties were elucidated. The structure of 1 was determined by X-ray crystallography. The complexes 1 and 2 showed a two-step deprotonation reaction by treatment with 1,8-diazabicyclo[5,4,0]undec-7-ene (DBU), and the addition of DBU led to a shift of the metal-centered redox couples to a lower potential by 720 and 550 mV, respectively. The di-deprotonated complexes were also studied by (1)H-NMR and UV-vis spectroscopy. The addition of methanesulfonic acid (MSA) to the di-deprotonated complexes enabled the recovery of 1 and 2, indicating that the thioamide moiety underwent a reversible deprotonation-protonation process, which resulted in regulating the redox potentials of the metal center. The Pourbaix diagram of 1 revealed that 1 underwent a one-proton/one-electron transfer process in the pH range of 5.83-10.35, and a two-proton/one-electron process at a pH of over 10.35, indicating that the deprotonation/protonation process of the complexes is related to proton-coupled electron transfer (PCET).     

多硫二硫醇烯配合物[Ni(C5S9)2]电磁学性能的研究

多硫 1 ,2 二硫醇烯能与过渡金属离子形成具有特殊平面共轭结构的稳定配合物 ,是构成有机导体或超导体的基本“砖块”[1,2 ] 。配合物的晶体结构研究表明 ,分子之间通过Ｓ…Ｓ原子ｐ轨道的重叠而存在的超分子相互作用是配合物具有高导电性的一个重要特征[3 ] 。增加分子间超分子相互作用的一个有效方法是增加配体上硫原子的个数。我们以Ｃ6Ｓ10 (图 1 )为中间体得到Ｃ5Ｓ2 -9(图2 ) ,与Ｎｉ2 + 离子配位后在四种不同阳离子作用下生成 (Ｃａｔ)2 [Ｎｉ(Ｃ5Ｓ9)2 ]型二价阴离子配合物。当这些配合物用Ｉ2 氧化时都生成了中性配位化合物 [Ｎｉ…     

Generation of 8‐oxo‐7,8‐dihydroguanine in G‐Quadruplexes Models of Human Telomere Sequences by One‐electron Oxidation

The mechanistic aspects of one‐electron oxidation of G‐quadruplexes in the basket (Na⁺ ions) and hybrid (K⁺ ions) conformations were investigated by transient absorption laser kinetic spectroscopy and HPLC detection of the 8‐oxo‐7,8‐dihydroguanine (8‐oxoG) oxidation product. The photo‐induced one‐electron abstraction from G‐quadruplexes was initiated by sulfate radical anions (SO₄˙^?) derived from the photolysis of persulfate ions by 308 nm excimer laser pulses. In neutral aqueous solutions (pH 7.0), the transient absorbance of neutral guanine radicals, G(‐H)˙, is observed following the complete decay of SO₄˙^? radicals (~10 μs after the actinic laser flash). In both basket and hybrid conformations, the G(‐H)˙ decay is biphasic with one component decaying with a lifetime of ~0.1 ms, and the other with a lifetime of 20–30 ms. The fast decay component (~0.1 ms) in G‐quadruplexes is correlated with the formation of 8‐oxoG lesions. We propose that in G‐quadruplexes, G(‐H)˙ radicals retain radical cation character by sharing the N1‐proton with the O⁶‐atom of G in the [G˙⁺: G] Hoogsteen base pair; this [G(‐H)˙: H⁺G G˙⁺: G] leads to the hydration of G˙⁺ radical cation within the millisecond time domain, and is followed by the formation of the 8‐oxoG lesions.     

One- and two-electron reduced 1,2-diketone ligands in [CrIII(L*)3] (S = 0) and Na2(Et2O)2[VIV(LRed)3] (S = 1/2)

The electronic structures of chromium and vanadium centers coordinated by three reduced 1,2-diketones have been elucidated by using density functional theory (DFT) calculations and a host of physical methods: X-ray crystallography; cyclic voltammetry; ultraviolet-visible (UV-vis), nuclear magnetic resonance (NMR), and electron paramagnetic resonance (EPR) spectroscopy; and magnetic susceptibility measurements. The metal center in octahedral [CrIII(L*)3]0 (1), a CrIII (d3) ion is coupled antiferromagnetically to three monoanionic ligand pi-radicals affording an S ) 0 ground state. In contrast, Na2(Et2O)2[VIV(LRed)3] (2) (S ) 1/2), possesses a central VIV (d1) ion O,OE-coordinated to three closed-shell, doubly reduced ligands which in turn are coordinated by two Na cations enforcing a trigonal prismatic geometry at the vanadium center. 2 can be oxidized electrochemically by one and two electrons generating a monoanion, [V(L)3]1-, and a neutral species, [V(L)3]0, respectively. DFT calculations atthe B3LYP level show that the one-electron oxidized product contains an octahedral VIV ion coupled antiferromagnetically to one monoanionic ligand pi-radical [VIV(L*)(LRed)2]1- (S ) 0). In contrast, the two-electron oxidized product contains a VIII ion coupled antiferromagnetically to three ligand pi-radicals in an octahedral field[VIII(L*)3]0 (S ) 1/2).     

Hydride-containing models for the active site of the nickel-iron hydrogenases

The [NiFe]-hydrogenase model complex NiFe(pdt)(dppe)(CO)(3) (1) (pdt = 1,3-propanedithiolate) has been efficiently synthesized and found to be robust. This neutral complex sustains protonation to give the first nickel-iron hydride [1H]BF(4). One CO ligand in [1H]BF(4) is readily substituted by organophosphorus ligands to afford the substituted derivatives [HNiFe(pdt)(dppe)(PR(3))(CO)(2)]BF(4), where PR(3) = P(OPh)(3) ([2H]BF(4)); PPh(3) ([3H]BF(4)); PPh(2)Py ([4H]BF(4), where Py = 2-pyridyl). Variable temperature NMR measurements show that the neutral and protonated derivatives are dynamic on the NMR time scale, which partially symmetrizes the phosphine complex. The proposed stereodynamics involve twisting of the Ni(dppe) center, not rotation at the Fe(CO)(2)(PR(3)) center. In MeCN solution, 3, which can be prepared by deprotonation of [3H]BF(4) with NaOMe, is about 10(4) stronger base than is 1. X-ray crystallographic analysis of [3H]BF(4) revealed a highly unsymmetrical bridging hydride, the Fe-H bond being 0.40 ? shorter than the Ni-H distance. Complexes [2H]BF(4), [3H]BF(4), and [4H]BF(4) undergo reductions near -1.46 V vs Fc(0/+). For [2H]BF(4), this reduction process is reversible, and we assign it as a one-electron process. In the presence of trifluoroacetic acid, proton reduction catalysis coincides with this reductive event. The dependence of i(c)/i(p) on the concentration of the acid indicates that H(2) evolution entails protonation of a reduced hydride. For [2H](+), [3H](+), and [4H](+), the acid-independent rate constants are 50-75 s(-1). For [2H](+) and [3H](+), the overpotentials for H(2) evolution are estimated to be 430 mV, whereas the overpotential for the N-protonated pyridinium complex [4H(2)](2+) is estimated to be 260 mV. The mechanism of H(2) evolution is proposed to follow an ECEC sequence, where E and C correspond to one-electron reductions and protonations, respectively. On the basis of their values for its pK(a) and redox potentials, the room temperature values of ΔG(H?) and ΔG(H-) are estimated as respectively as 57 and 79 kcal/mol for [1H](+).     

Gold(III) diazonium complexes for electrochemical reductive grafting

Gold(III) diazonium complexes were synthesized for the first time and studied for electrochemical reductive grafting. The diazonium complex [CN-4-C(6)H(4)N≡N]AuCl(4) was synthesized by protonating CN-4-C(6)H(4)NH(2) with chloroauric acid H[AuCl(4)]·3H(2)O to form the ammonium salt [CN-4-C(6)H(4)NH(3)]AuCl(4), which was then oxidized by the one-electron oxidizing agent [NO]PF(6) in CH(3)CN. The highly irreversible reduction potential of 0.1 mM [CN-4-C(6)H(4)N≡N]AuCl(4) observed at -0.06 V versus Ag/AgCl in CH(3)CN/0.1 M [Bu(4)N]PF(6) encompasses both gold(0) deposition and diazonium reduction. Repeated scans showed the absence of the reduction peak on the second run, which indicates that surface modification with a blocking gold aryl film has occurred and is largely complete.     

Cobalt(II)/nickel(II)-centered Keggin-type heteropolymolybdates: syntheses, crystal structures, magnetic and electrochemical properties

New Keggin-type cobalt(II)/nickel(II)-centered heteropolymolybdates, (C3H5N2)6[Co(II)Mo12O40]10H2O (1) and (NH4)3(C4H5N2O2)3[Ni(II)Mo12O40] (2), were isolated and characterized by IR, UV-vis, single-crystal X-ray diffraction, thermogravimetric, magnetic, as well as electrochemical analyses. The polyanion in the two compounds displays the well-known alpha-Keggin structure, which is composed of four Mo3O13 units formed by edge-sharing octahedra. Four Mo3O13 units connect each other by vertices, and the Co(2+) or Ni(2+) is located in the center. Magnetic measurements show that the central Co(2+) and Ni(2+) are in high spin states (with S = 3/2 and S = 1, respectively) exhibiting paramagnetic behaviors. Cyclic voltammetric experiments for 1 represent a quasi-reversible one-electron redox Co(3+)/Co(2+) couple and two four-electron reversible redox processes ascribed to Mo centers, while 2 only shows two four-electron redox processes attributed to Mo centers in pH = 0.5 H2SO4 solution.     

Synthesis, characterization, and DFT/TD-DFT calculations of highly phosphorescent blue light-emitting anionic iridium complexes

Highly phosphorescent blue-light-emitting anionic iridium complexes (C4H9)4N[Ir(2-phenylpyridine)2(CN)2] (1), (C4H9)4N[Ir(2-phenyl-4-dimethylaminopyridine)2(CN)2] (2), (C4H9)4N[Ir(2-(2,4-difluorophenyl)-pyridine)2(CN)2] (3), (C4H9)4N[Ir(2-(2,4-difluorophenyl)-4-dimethylaminopyridine)2(CN)2] (4), and (C4H9)4N[Ir(2-(3,5-difluorophenyl)-4-dimethylaminopyridine)2(CN)2] (5) were synthesized and characterized using NMR, UV-vis absorption, and emission spectroscopy and electrochemical methods. In these complexes color and quantum yield tuning aspects are demonstrated by modulating the ligands with substituting donor and acceptor groups on both the pyridine and phenyl moieties of 2-phenylpyridine. Complexes 1-5 display intense photoluminescence maxima in the blue region of the visible spectrum and exhibit very high phosphorescence quantum yields, in the range of 50-80%, with excited-state lifetimes of 1-4 micros in acetonitrile solution at 298 K. DFT and time dependent-DFT calculations were performed on the ground and excited states of the investigated complexes to provide insight into the structural, electronic, and optical properties of these systems.     

Synthesis of paramagnetic tetranuclear rhodium and iridium complexes with the 2,6-pyridinedithiolate ligand. Redox-induced degradation to diamagnetic triiridium compounds

The tetranuclear complexes [M4(mu-PyS2)2(diolefin)4] [PyS2 = 2,6-pyridinedithiolate; M = Rh, diolefin = cod (1,5-cyclooctadiene) (1), tfbb (tetrafluorobenzo[5,6]bicyclo[2.2.2]octa-2,5,7-triene) (2); M = Ir, diolefin = cod (3), tfbb (4)] exhibit two one-electron oxidations at a platinum disk electrode in dichloromethane at potentials accessible by chemical reagents. The rhodium tetranuclear complexes were selectively oxidized to the monocationic complexes [Rh4(mu-PyS2)2(diolefin)4](+) (1(+), 2(+)) by mild one-electron oxidants such as [Cp2Fe](+) or [N(C6H4Br-4)3](+) and isolated as the PF6(-), BF4(-), and ClO4(-) salts. Silver salts behave as noninnocent one-electron oxidants for the reactions with the rhodium complexes 1 and 2 since they give sparingly soluble coordination polymers. The complex [Ir4(mu-PyS2)2(cod)4](+) (3(+)) was obtained as the tetrafluoroborate salt by reaction of 3 with 1 molar equiv of AgBF4, but the related complex 4(+) could not be isolated from the chemical oxidation of [Ir4(mu-PyS2)2(tfbb)4] (4) with AgBF4. Oxidation of 3 and 4 with 2 molar equiv of common silver salts resulted in the fragmentation of the complexes to give the diamagnetic triiridium cations [Ir3(mu-PyS2)2(diolefin)3](+). The molecular structure of [Ir3(mu-PyS2)2(cod)3]BF4, determined by X-ray diffraction methods, showed the three metal atoms within an angular arrangement. Both 2,6-pyridinedithiolate tridentate ligands bridge two metal-metal bonded d(7) centers in pseudo octahedral environments and one d(8) square-planar iridium center. An interpretation of the EPR spectra of the 63-electron mixed-valence paramagnetic tetranuclear complexes suggests that the unpaired electron is delocalized over two of the metal atoms in the complexes 1(+)-3(+).     

The role of chalcogen-chalcogen interactions in the intrinsic basicity acidity of beta-chalcogenovinyl(thio)aldehydes HC([double bond]X)[bond]CH[double bond]CH[bond]CYH

The intrinsic acidity and basicity of a series of beta-chalcogenovinyl(thio)aldehydes HC([double bond]X)[bond]CH[double bond]CH[bond]CYH (X=O, S; Y=Se, Te) were investigated by B3LYP/6-311+G(3df,2p) density functional and G2(MP2) calculations on geometries optimized at the B3LYP/6-31G(d) level for neutral molecules and at the B3LYP/6-31+G(d) level for anions. The results showed that selenovinylaldehyde and selenovinylthioaldehyde should behave as Se bases in the gas phase, because the most stable neutral conformer is stabilized by an X[bond]H...Se (X=O, S) intramolecular hydrogen bond (IHB). In contrast the Te-containing analogues behave as oxygen or sulfur bases, because the most stable conformer is stabilized by typical X...Y[bond]H chalcogen-chalcogen interactions. These compounds have a lower basicity than expected because either chalcogen-chalcogen interactions or IHBs become weaker upon protonation. Similarly, they are also weaker acids than expected because deprotonation results in a significantly destabilized anion. Loss of the proton from the X[bond]H or Y[bond]H groups is a much more favorable than from the C[bond]H groups. Therefore, for Se compounds the deprotonation process results in loss of the X[bond]H...Se (X=O, S) IHBs present in the most stable neutral conformer, while for Te-containing compounds the stabilizing X...Y[bond]H chalcogen-chalcogen interaction present in the most stable neutral conformer becomes repulsive in the corresponding anion.     

Theoretical study on structures and stabilities of [H,Ge,C,N

Theoretical investigations are performed for the first time on the simplest hydrogenated germanium cyanide [H,Ge,C,N], whose analogs [H,C(2),N] and [H,Si,C,N] have been detected in space and laboratory, respectively. The detailed potential energy surfaces in both singlet and triplet states are constructed at the CCSD(T)/6-311+G(3df,2p)//B3LYP/6-31G(d)+ZPVE level, including 18 minimum isomers and 26 interconversion transition states. The former three low-lying and kinetically stabilized isomers are HGeCN (1)1 (0.0 kcal/mol), HGeNC (1)2 (5.1 kcal/mol), and cyclic cCHNGe(1)7 (11.1 kcal/mol). In addition, five isomers HCNGe (1)3 (33.8), HNCGe (1)5 (29.8), cNHCGe (1)8 (37.9), HGeCN (3)1 (30.1), and HNCGe (3)5 (26.5) each have considerable barriers, despite their high energies. Future laboratory characterization and astrophysical detection of the eight [H,Ge,C,N] isomers, especially the former three low-lying species (1)1, (1)2, and (1)7, are highly recommended. The accurate spectroscopic data at the QCISD/6-311G(d,p) level are provided. For some species, the CBS-QB3 calculations are also performed. Wherever possible, comparisons with the analogous [H,C(2),N] and [H,Si,C,N] are made on the structural, energetic, and bonding properties.     

Decorating poly(alkyl aryl-ether) dendrimers with metallacarboranes

A new family of polyanionic poly(alkyl aryl-ether) metallodendrimers decorated with four and eight cobaltabisdicarbollide units have been obtained in high yield by the ring-opening reaction of cyclic oxonium [3,3'-Co(8-(C(2)H(4)O)(2)-1,2-C(2)B(9)H(10))(1',2'-C(2)B(9)H(11))] with alkoxides formed by deprotonation of terminal alcohols in the α,α'-bis[3,5-bis(hydroxymehyl)phenoxy]-p-xylene, α,α'-bis[3,5-bis(hydroxymehyl)phenoxy]-m-xylene, α,α'-bis[3,5-bis-[3,5-bis(hydroxymethyl)phenoxy]methylen]phenoxy]-p-xylene, and α,α,'-bis[3,5-bis-[3,5-bis(hydroxymethyl)phenoxy]methylen]phenoxy]-m-xylene dendrimers. The crystal structure of the precursor α,α'-bis[3,5-bis(chloromethyl)phenoxy]-p-xylene is also described. Final products are fully characterized by FTIR, NMR, UV-vis spectroscopies and elemental analysis. For metallodendrimers, the UV-vis absorptions have been a good tool for estimating the experimental number of cobaltabisdicarbollide units peripherally attached to the dendrimeric structure and consequently to corroborate the complete functionalization of the dendrimers.     

Synthesis of two pentaarylated [60]fullerene derivatives Ar_5C_(60)-H (Ar = p-MeC_6H_4, m-MeC_6H_4) and their novel electrochemical properties

Through one pot reaction of C60 with organocopper/magne-sium reagent ( p - MeQ H4 )2 CuMgBr or ( m - MeC6 H )2 -CuMgBr prepared from CuBr-Me2S and p-MeC6H4MgBr or m-MeC6H4MgBr and subsequent quenching with aqueous NH4Cl, two pentaarylated [60] fullerene derivatives (p-MeC6H4)5C60H (1) and (m-MeC6H4)5C60H (2) have been synthesized in 94% and 96% yields, respectively. While known compound 1 prepared via this improved method is unambiguously identified, new compound 2 is fully characterized by elemental analysis, IR, UV-vis, 1H NMR and 13C NMR spectroscopies. Additionally, electrochemical study shows that the two [60] fullerene derivatives 1 and 2 in dichloromethane solution display two sequential one-electron reductions which are shifted by about 0.4V towards more negative potential values with respect to free C60. Such remarkable cathodic shift is attributed to the multiple breakage of the double-bond conjugation within the fullerene core.     

Binding of nitrate to a CuII-cyclen complex bearing a ferrocenyl pendant: synthesis, solid-state X-ray structure, and solution-phase electrochemical and spectrophotometric studies

The reaction of Cu(NO3)2.3H2O with the ligand 1-(ferrocenemethyl)-1,4,7,10-tetraazacyclododecane (L) in acetonitrile leads to the formation of a blue complex, [Cu(L)(NO3)][NO3] (C1). The X-ray structure determination shows an unexpected binding of a nitrate anion in that the CuII center is surrounded by four N atoms of the 1,4,7,10-tetraazacyclododecane (cyclen) macrocycle and two O atoms from a chelating nitrate anion, both Cu-O distances being below the sums of the van de Waals radii. Hydrogen-bonding interactions in the crystal lattice and a weak interaction between a second nitrate O and the CuII center in C1 give rise to a highly distorted CuII geometry relative to that found in the known structure of [Cu(cyclen)(NO3)][NO3] (C5). Electrochemical studies in acetonitrile containing 0.1 M [Bu4N][NO3] as the supporting electrolyte showed that oxidation of C1 in this medium exhibits a single reversible one-electron step with a formal potential E degrees f of +85 mV vs Fc0/+ (Fc = ferrocene). This process is associated with oxidation of the ferrocenyl pendant group. Additionally, a reversible one-electron reduction reaction with an E degrees f value of -932 mV vs Fc0/+, attributed to the CuII/I redox couple, is detected. Gradual change of the supporting electrolyte from 0.1 M [Bu4N][NO3] to the poorly coordinating [Bu4N][PF6] electrolyte, at constant ionic strength, led to a positive potential shift in E degrees f values by +107 and +39 mV for the CuII/I(C1) and Fc0/+(C1) redox couples, respectively. Analysis of these electrochemical data and UV-vis spectra is consistent with the probable presence of the complexes C1, [Cu(L)(CH3CN)2]2+ (C2), [Cu(L)(CH3CN)(NO3)]+ (C3), and [Cu(L)(NO3)2] (C4) as the major species in nitrate-containing acetonitrile solutions. In weakly solvating nitromethane, the extent of nitrate complexation remains significant even at low nitrate concentrations, due to the lack of solvent competition.     

Electron-transfer and acid-base properties of a two-electron oxidized form of quaterpyrrole that acts as both an electron donor and an acceptor

Electron-transfer interconversion between the four-electron oxidized form of a quaterpyrrole (abbreviated as P4 for four pyrroles) and the two-electron oxidized form (P4H2) as well as between P4H2 and its fully reduced form (P4H4) bearing analogous substituents in the alpha- and beta-pyrrolic positions was studied by means of cyclic voltammetry and UV-visible spectroelectrochemistry combined with ESR and laser flash photolysis measurements. The two-electron oxidized form, P4H2, acts as both an electron donor and an electron acceptor. The radical cation (P4H2*+) and radical anion (P4H2*-) are both produced by photoinduced electron transfer from dimeric 1-benzyl-1,4-dihydronicotinamide to P4H2, whereas the cation radical form of the compound is also produced by electron-transfer oxidation of P4H2 with [Ru(bpy)3]3+. The ESR spectra of P4H2*+ and P4H2*- were recorded at low temperature and exhibit spin delocalization over all four pyrrole units. Thus, the two-electron oxidized form of the quaterpyrrole (P4H2) displays redox and electronic features analogous to those seen in the case of porphyrins and may be considered as a simple, open-chain model of this well-studied tetrapyrrolic macrocycle. The dynamics of deprotonation from P4H2*+ and disproportionation of P4H2 were examined by laser flash photolysis measurements of photoinduced electron-transfer oxidation and reduction of P4H2, respectively.     

Metal versus ligand alkylation in the reactivity of the (bis-iminopyridinato)Fe catalyst

The alkylation of the Brookhart-Gibson {2,6-[2,6-(i-Pr)2PhN=C(CH3)]2(C5H3N)} FeCl2 precatalyst with 2 equiv of LiCH2Si(CH3)3 led to the isolation of several catalytically very active products depending on the reaction conditions. The expected dialkylated species {2,6-[2,6-(i-Pr)2PhN=C(CH3)]2}(C5H3N)Fe(CH2SiMe3)2 (2) was indeed the major component of the reaction mixture. However, other species in which alkylation occurred at the pyridine ring ortho position, {2,6-[2,6-(i-Pr)2PhN=C(CH3)]2-2-CH2SiMe3}(C5H3N)Fe(CH2SiMe3) (1), and at the imine C atom, {2-[2,6-(i-Pr)2PhN=C(CH3)]-6-[2,6-(i-Pr)2PhNC(CH3)(CH2 SiMe3)](C5H3N)}Fe(CH2SiMe3) (3), have also been isolated and fully characterized. In addition, deprotonation of the methyl-imino functions and formation of a new divalent Fe catalyst {[2,6-[2,6-(i-Pr)2PhN-C=(CH2)]2(C5H3N)}Fe(mu-Cl)Li(THF)3 (4) also occurred depending on the reaction conditions. In turn, the formation of 4 might trigger the reductive coupling of two units through the methyl-carbon wings. This process resulted in the one-electron reduction of the metal center, affording a dinuclear Fe(I) alkyl catalyst {[{[2,6-(i-Pr)2C6H5]N=C(CH3)}(C5H3N){[2,6-(i-Pr)26H5]N=CCH2}Fe(CH2SiMe3)]}2 (5). Different from other metal derivatives, complex 5 could not be prepared from the monodeprotonated version of the ligand. Its reaction with a mixture of FeCl2 and RLi afforded instead [{2,6-[2,6-(i-Pr)2PhN-C=(CH2)]2(C5H3N)}FeCH2Si(CH3)3][Li(THF)4] (6) which is also catalytically active. All of these high-spin species have been shown to have high catalytic activity for olefin polymerization, producing polymers of two distinct natures, depending on the formal oxidation state of the metal center.