Structural and Photochemical Properties of Zn(II) Phenanthroline-Derived Complexes: From Mononuclear to Bimetallic and Circular-Trimetallic Helicates.

In the design of self-assembled compounds, small variations in the linkers connecting the coordinating moieties can produce large differences in the obtained structures. Here, we report three novel zinc(II) complexes with phenanthroline-derived ligands as building blocks (L1-L3): A mononuclear complex, a bimetallic helicate, and a trimetallic circular helicate. The even-number spacer in L2 promotes the formation of a bimetallic helicate stabilized by π-π interactions of adjacent phenanthrolines. The addition of an extra methylene in L3 increases the distance between where the phenanthrolines can stack, and CH-π noncovalent interactions give stability to the circular helicate. When irradiated at 308 nm in acetonitrile, long-lived excited states are formed with all three complexes, which are able to participate in oxidation of 2-propanol and in reduction of methylviologen, MV2+. While the overall behavior of the three complexes is similar, the bimetallic helicate is able to form a ground-state adduct with MV2+, while the trimer reaches the excited state to form an exciplex with MV2+.

Excited States versus Reaction Intermediates as Active Species in Photoinduced Redox Reactions of Cyrhetrenyl and Ferrocenyl Chalcones: A 351 nm Flash Photolysis Study.

The photoinduced redox reactions of two organometallic chalcones: trans, E, (η5-C5H4C(O)CH═CH-4-benzo-15-crown-5)Re(CO)3, 1, and trans, E, (η5-C5H4C(O)CH═CH-4-benzo-15-crown-5)Fe(η5-C5H4C(O)CH═CH-4-benzo-15-crown-5), 2, were investigated in fluid solution using the flash photolysis technique. For a better understanding of the photoinduced redox processes of these organometallic chalcones, an electron donor, triethylamine (TEA), and an electron acceptor, methylviologen dichloride (MVCl2), were chosen. Two parallel reaction paths for the decay of the intermediate 1-I, that is, the anion radical of 1, were observed in the presence of TEA. One generates a radical anion, while the other reaction path produces the Z isomer. Instead, the photoinduced reaction of 2 with TEA in MeOH generates an intense absorption band at λmax = 660 nm, which is attributed to a 2-I·MeOH adduct. The oxidative process between 1-I and MV2+ in CH3CN generates transient spectra consistent with the formation of the radical cation MV•+. In contrast, the photoinduced reaction between 2 and MV2+ showed that the generation of MV•+ occurs through a complex mechanism. MV•+ is formed in two steps where the first one is the formation of an adduct between the long-lived metal-to-ligand charge transfer (MLCT)Fe→Chalcone excited state and MV2+. These results have shown that intermediates 1-I and 2-I can function as photo-oxidants and photoreductants better than the chalcone short-lived excited states.

Steric and Electronic Factors Affecting the Conformation of Bimetallic CuI Complexes: Effect of the Aliphatic Spacer of Tetracoordinating Schiff-Base Ligands.

A family of six homoleptic [CuI (Ln )]2 (ClO4 )2 and six heteroleptic [CuI (Ln )(PPh3 )2 ]2 (ClO4 )2 bimetallic complexes, in which Ln are bis-Schiff base ligands with alkyl spacers of variable length (n=2-7 -CH2 -), were prepared to evaluate the role of the spacer on the formation of helicates or mesocates. In the homoleptic series, spectroscopic and theoretical studies indicate that preferences for a conformation are based on energetic parameters, mainly, the establishment of noncovalent interactions. The odd-even nature of the spacers preconditions the superposition of the aromatic rings to allow the juxtaposition necessary for noncovalent interactions, whereas the increase of the length reduces the strength of such interactions. Consequently, complexes with even-spacers of short length were identified as helicates in solution, [CuI (Ln )]22+ (n=2, 4). Complexes [CuI (Ln )]22+ (n=3-7) dissociate in solution to produce the monometallic complexes in equilibrium, [CuI (Ln )]+ . The stability of the bimetallic species is discussed in terms of their conformations. The set of heteroleptic complexes was prepared to evaluate the reach of the "odd-even rule" in the solid, which is based on the "zig-zag" arrangements of the spacers. Based on crystallographic information, "S-" and "C"-type conformations of Ln are related to even and odd spacers, respectively. This trend is considered in addition to other factors to explain preferences for either a mesocate or helicate conformation in the homoleptic series.

Organometallic Chalcones Functioning as Radical Traps: Observations on the Solvated Electron Reduction and the C·H2OH and C·H2Cl Addition Processes. A Time-Resolved, Mechanistic Study.

The pulse radiolysis method was used to investigate reactions between organometallic chalcones derived from ferrocene, cyrhetrene, and cymantrene substituted with a phenyl (R1) or a 4'-benzo-15-crown-5 (B1) moiety and the solvated electron e-sol and two radicals. The principal spectroscopic transformations resolved with the e-sol reactions reveal that the chalcone compounds react under a diffusion controlled condition. The chromophore in the reaction product is the radical anion chalcone, -C(═O)CH═CH-. The kinetics of the redox processes are consistent with the formation of the radical anion in a first step, followed by an E/Z isomerization reaction in the second step. The rate of the E/Z isomerization reaction is dependent on the chalcone substituent. Reactions with C·H2OH and C·H2Cl radicals are strongly affected by the nature of the organometallic fragment. The transient spectra for the reactions between C·H2OH and C·H2Cl radicals with the phenyl-substituted chalcone show weak, broad absorption bands at λob < 400 nm. Only the ferrocenyl derivatives showed an intense absorption band with λmax = 350 nm. The C-centered radicals react with organometallic chalcones with lifetimes less than or equal to a few microseconds, and the products are assigned as adducts of the C-centered radicals to the chalcone.

Double-salt formation in crystal structures containing [Co(en)3]Cl3.

The structures of three racemic double salts of [Co(en)3]Cl3 (en is ethane-1,2-diamine, C2H8N2), namely, bis[tris(ethane-1,2-diamine-κ2N,N')cobalt(III)] hexaaquasodium(I) heptachloride, [Co(en)3]2[Na(H2O)6]Cl7, bis[tris(ethane-1,2-diamine-κ2N,N')cobalt(III)] hexaaquapotassium(I) heptachloride, [Co(en)3]2[K(H2O)6]Cl7, and ammonium bis[tris(ethane-1,2-diamine-κ2N,N')cobalt(III)] heptachloride hexahydrate, (NH4)[Co(en)3]2Cl7·6H2O, have been determined, and the structural similarities with the parent compound, tris(ethane-1,2-diamine-κ2N,N')cobalt(III) trichloride tetrahydrate, [Co(en)3]Cl3·4H2O, are highlighted. All four compounds crystallize in the trigonal space group P-3c1. When compared with the parent compound, the double salts show a modest increase in the unit-cell volume. The structure of the chiral derivative [Λ-Co(en)3]2[Na(H2O)6]Cl7 has also been redetermined at cryogenic temperatures (120 K) and the disorder noted in a previous report has been accounted for.

An X-ray investigation of stereoselectivity in the interactions of [Co(en)3]3+ with [Co(ox)3]3- (en is ethane-1,2-diamine and ox is oxalate).

The X-ray structure of racemic tris(ethane-1,2-diamine-κ2N,N')cobalt(III) tris(oxalato-κ2O1,O2)cobaltate(III) pentahydrate, [Co(C2H8N2)3][Co(C2O4)3]·5H2O or [Co(en)3][Co(ox)3]·5H2O, has been determined. Hydrogen-bonding interactions along the C3-axis of the [Co(en)3]3+ cation with the [Co(ox)3]3- anion are heterochiral, while those perpendicular to this axis are homochiral. Implications for the interpretation of chiral discriminations and induction in electron-transfer reactions in solution are discussed.

Structure of Λ(δλλ)-[Co(en)3]I3(I)2.

The structure of tris-(ethane-1,2-di-amine-κ2 N,N')cobalt(III) bis-(iodide) triiodide, [Co(C2H8N2)3]I3(I)2, at 120 K has ortho-rhom-bic (P212121) symmetry. The di-amine nitro-gen atoms form N-H⋯I hydrogen bonds throughout the lattice, resulting in a three-dimensional network, which involves the iodide and all atoms in the triiodide anions.

Oxygenation of cobalt porphyrinates: coordination or oxidation?

The X-ray characterization of the five-coordinate picket-fence porphyrin complex, [Co(TpivPP)(2-MeHIm)], is reported. The complex has the displacement of cobalt from the porphyrin plane = 0.15 A, and Co-N(Im) = 2.145(3) and (Co-N(p))(av) = 1.979(3) A. This five-coordinate complex, in the presence of dioxygen and excess 2-methylimidazole, undergoes an unanticipated, photoinitiated atropisomerization of the porphyrin ligand, oxidation of cobalt(II), and the formation of the neutral cobalt(III) complex [Co(alpha,alpha,beta,beta-TpivPP)(2-MeHIm)(2-MeIm(-)]. Two distinct examples of this complex have been structurally characterized, and both have structural parameters consistent with cobalt(III). The two new Co(III) porphyrin complexes have axial Co-N(Im) distances ranging from 1.952 to 1.972 A, but which allow for the distinction between imidazole and imidazolate. An interesting intermolecular hydrogen bonding network is observed that leads to infinite helical chains. UV-vis spectroscopic study suggests that [Co(TpivPP)(2-MeHIm)(O(2))] is an intermediate state for the oxidation reaction and that the atropisomerization process is photocatalyzed. A reaction route is proposed based on the spectroscopic studies.

Unfolding of the [Cu2(1,3-bis(9-methyl-1,10-phenanthrolin-2-yl)propane)2]2+ helicate. Coupling of the chlorocarbon dehalogenation to the unfolding process.

A new helical dimeric copper(I) complex [Cu(2)(mphenpr)(2)](ClO(4))(2) where mphenpr is 1,3-bis(9-methyl-1,10-phenanthrolin-2-yl)propane has been prepared and characterized by X-ray crystallography and NMR. In the solid state, the metal centers are 6.42 A apart, and the electronic structure has been investigated with use of density functional theory (DFT) calculations. In solution the dimer equilibrates with a monomeric form [Cu(mphenpr)](ClO(4)), and the mechanism of unfolding of the dimer into monomer has been studied. In the presence of CCl(4), formation of the monomer is coupled to the reductive dehalogenation of the halocarbon. The mechanism of this process has been probed by the study of short-lived potential reaction intermediates using fast kinetic pulse radiolysis techniques and comparisons with DFT calculations. The copper(II) product [Cu(mphenpr)Cl](ClO(4)) and an analogue [Cu(mphenpr)](ClO(4))(2) have been isolated and characterized by X-ray crystallography.

Mechanistic and energetic aspects of the thermal and photochemical redox chemistry of the octanuclear cubane complexes, Fe(III)(8)(mu(4)-O(4))(mu-pyrazolate)(12)X(4) (X = Cl or Br).

The mechanisms of the thermal and photochemical redox reactions of clusters [Fe(III)(8)(mu(4)-O(4))(mu-Pz)(12)X(4)] (Pz = pyrazolate anion, X = Cl or Br) were investigated in this work. Reactions of the complexes with e(-)(sol), C(*)H(2)OH, and several powerful reducing transition metal complexes were investigated using the pulse radiolysis technique. Reaction rates of the outer-sphere electron transfer reactions with transition metal complexes had to be rationalized by invoking the formation of a [Fe(III)(7)Fe(II) '(mu(4)-O(4))(mu-Pz)(12)X(4)](-) intermediate or excited state. A transient species observed in the reaction of the e(-)(sol) with the cubanes can be either an excited state or a reaction intermediate mediating the formation of the stable product, [Fe(III)(7)Fe(II)(mu(4)-O(4))(mu-Pz)(12)X(4)](-). Photoredox reactions, characteristic of the ligand X(-) to Fe(III) charge transfer excited sates, were observed in the 350 nm steady state and 351 nm laser flash irradiations of the cubanes. Quantum yields are limited by the rapid recombination of the photofragments. The charge transfer spectroscopy of the products was rationalized on the basis of parameters derived from the thermal electron transfer reactions.

Effects of non-covalent interactions on the electronic and electrochemical properties of Cu(i) biquinoline complexes.

A Cu(i) complex {[CuI(biq)2]ClO4-biq} with biq = 2,2'-biquinoline was prepared, fully characterized and its properties compared with those of the well-known [CuI(biq)2]ClO4 complex. The crystal structures were obtained for both complexes (crystal structure for [CuI(biq)2]ClO4 has not been previously reported). Complex [CuI(biq)2]ClO4 crystallizes as a racemate where each enantiomer has a different τ4 value while compound {[CuI(biq)2]ClO4-biq} crystallizes as a non-chiral supramolecular aggregate with an uncoordinated biq molecule forming a π-π stacking interaction with a coordinated biq. 1H-NMR spectroscopy in non-coordinating solvents reveals that structures in solution are similar to those in the solid phase, confirming the presence of a supramolecular arrangement for compound {[CuI(biq)2]ClO4-biq}. The stability of the non-covalent aggregate in solution of {[CuI(biq)2]ClO4-biq} causes significant differences between the spectroscopic and electrochemical properties of {[CuI(biq)2]ClO4-biq} and [CuI(biq)2]ClO4.

Crystal structure of tris-(trans-1,2-di-amino-cyclo-hexane-κ(2) N,N')cobalt(III) trichloride monohydrate.

The synthesis of the title hydrated complex salt, [Co(C6H14N2)3]Cl3·H2O, from racemic trans-1,2-di-amino-cyclo-hexane and [CoCl(NH3)5]Cl2 and its structural characterization are presented in this paper. The product was synthesized in the inter-est of understanding the hydrogen-bonding patterns of coordination complexes. Previous characterizations of the product in the I-42d space group have not yielded coordinates; thus, this paper provides the first coordinates for this complex in this space group. The octahedrally coordinated cation adopts twofold rotation symmetry, with outer-sphere chloride counter-ions and solvent water mol-ecules forming a hydrogen-bonded network with amine H atoms.

Convenient Synthesis and Characterization of the Chiral Complexes cis- and trans-[Ru((SS)-Pr(i)pybox)(py)(2)Cl]PF(6) and [Ru((SS)-Pr(i)pybox)(bpy)Cl]PF(6).

The chiral oxazoline complexes cis- and trans-[Ru((SS)-Pr(i)pybox)(py)(2)Cl]PF(6) were prepared from the common precursor trans-[Ru((SS)-Pr(i)pybox)(py)Cl(2)], and all three complexes were spectroscopically and structurally characterized. The complex cis-[Ru((SS)-Pr(i)pybox)(py)(2)Cl]PF(6) crystallized in P2(1)2(1)2(1) with Z = 4, a = 8.396(2) Å, b = 18.396(3) Å, c = 20.954(3) Å, and R = 0.039 for 4131 reflections. The complex trans-[Ru((SS)-Pr(i)pybox)(py)(2)Cl]PF(6) crystallized in P2(1) with Z = 2, a = 10.110(2) Å, b = 13.566(2) Å, c = 12.043(2) Å, beta = 106.10(2) degrees, and R = 0.039 for 3869 reflections. The precursor trans-[Ru((SS)-Pr(i)pybox)(py)Cl(2)] crystallized in P2(1)2(1)2(1) with Z = 8, a = 15.856(2) Å, b = 17.216(1) Å, c = 17.768(1) Å, and R = 0.030 for 5826 reflections. A related reagent, [Ru((SS)-Pr(i)pybox)(bpy)Cl]PF(6), was prepared by a more direct route and was also characterized. Aqua complexes were formed by hydrolysis of the chloride ligand in the bis(pyridine) complexes with retention of the stereochemistry. Electrochemical oxidation of the aqua complexes yielded oxo species which are formally ruthenium(IV). Reactions of the oxo complexes with the prochiral reductant, methyl p-tolyl sulfide led to stereoselective formation of the sulfoxide with enantiomeric excess of the R isomer ranging from 7% to 13%.

A single- and double-flash flash photolysis study of the sequential biphotonic photoprocesses of Cu(I) phenanthrolines. Comparison of the helicate complex, [Cu2(1,3-bis(9-methyl-1,10-phenanthrolin-2-yl)propane)2]2+, and [Cu(2,9-dimethyl-1,10-phenanthroline)2]+ photoprocesses.

Photochemical processes induced when two photons are sequentially absorbed by the helicate complex [Cu2(mphenpr)2](2+), where mphenpr = 1,3-bis(9-methyl-1,10-phenanthrolin-2-yl)propane, and [Cu(dmp)2](+) were investigated in CH2Cl2 containing solvents. A strong resemblance was observed between the fs-ns photophysics of [Cu2(mphenpr)2](2+) and Cu(I) phenanthroline complexes having a large steric hindrance. In the biphotonic regime, single-flash flash-photolyzed solutions were used for the determination of the product concentrations and quantum yields. The concentration of Cl(-) produced by the photoinduced decomposition of CH2Cl2 increases linearly with flash intensity as expected for a monophotonic process. In contrast, the concentration of a decomposed Cu(I) complex exhibits the quadratic dependence on flash intensity of a biphotonic process. Results of a sequential double-flash flash photolysis experiment are consistent with the decomposition of CH2Cl2 ahead of the flattened excited state formation and with the absorption of the second photon by the flattened MLCT excited state.

Resolution and characterization of helicate dimer and trimer complexes of 1,3-bis(9-methyl-1,10-phenanthrolin-2-yl)propane with copper(I).

Complexation of copper(I) with the binucleating ligand, 1,3-bis(9-methyl-1,10-phenanthrolin-2-yl)propane, mphenpr, result in formation of helical dimers, [Cu2(mphenpr)2](2+). The resolution of the enantiomeric forms of the dimers has been carried out with Δ-[As(cat)3](-) as resolving agent and X-ray structures for two compounds, P-[Cu2(mphenpr)2](Δ-[As(cat)3])2 and P-[Cu2(mphenpr)2](Δ-[As(cat)3])2·4(CH3CN), are reported. The rate of racemization in poorly-coordinating solvents has been examined by (1)H NMR, and is slow. At saturating concentrations of [[Cu2(mphenpr)2](2+)] in acetonitrile, crystals of the helical trimeric complex [Cu3(mphenpr)3](ClO4)3 are obtained. The X-ray structure of the trimer is reported. This species has also been resolved. As with the helical dimer, racemization in poorly-coordinating solvents is slow, and circular dichroism and (1)H NMR spectra are reported. The absolute configuration of the resolved complex, P-[Cu3(mphenpr)3](Δ-[As(cat)3])3, has been determined by X-ray crystallography.

Stereoselectivity in electron-transfer reactions in chiral media.

The oxidation of [Co(edta)](2-) by [IrCl(6)](2-) proceeds by both inner-sphere and outer-sphere electron-transfer pathways. In the presence of added [Co(en)(3)](3+), the outer-sphere pathway is enhanced. When optically active [Co(en)(3)](3+) is used, the [Co(edta)](-) formed is optically active, reflecting a 1.5% DeltaLambda selectivity. It is proposed that the selectivity arises from preferential formation and reactivity of the DeltaLambda ion pair, {[Co(edta)](2-),[Co(en)(3)](3+)}. Direct reaction of [Co(edta)](-) with [Co(en)(3)](2+) has also been investigated in the optically active solvent, (S)-(-)-1,2-propylene carbonate. The induction is small, forming 0.75% Delta-[Co(en)(3)](3+), consistent with the important role for hydrogen bonding in determining the precursor stereoselectivity to the exclusion of solvent.