Structural diversity in the first metal complexes of 2,5-dicarboxamidopyrroles and 2,5-dicarbothioamidopyrroles

Metal complexes of 2,5-dicarboxamidopyrroles and 2,5-dicarbothioamidopyrroles have been structurally characterised for the first time, complementing the significant amount of work that has been reported for the analogous pyridine ligands. N,N'-Bis(3,5-dinitrophenyl)-3,4-diphenyl-1H-pyrrole-2,5-dicarboxamide forms octahedral bis(tridentate) complexes with cobalt(iii) and nickel(ii), where the ligands are bound to the metal centres through deprotonated pyrrole and amide N atoms. N,N'-Dibutyl-3,4-diphenyl-1H-pyrrole-2,5-dicarboxthioamide and N,N'-diphenyl-3,4-diphenyl-1H-pyrrole-2,5-dicarboxthioamide also form bis(tridentate) cobalt complexes but are only deprotonated at the pyrrole N atom, the remainder of the coordination sphere comprising the thioamide S atoms. The dibutyl derivative was isolated as a Co(ii) complex, whereas the diphenyl system deposited a Co(iii) complex. In contrast, N,N'-dibutyl-3,4-dichloro-1H-pyrrole-2,5-dicarboxamide was found to act as a bidentate ligand, in an octahedral cobalt(ii) complex comprising of two bidentate pyrrole ligands, and two aqua ligands. Synthesis of N,N-bis(pyridin-2-ylmethyl)-3,4-diphenyl-1H-pyrrole-2,5-carboxamide gave a pyrrole ligand with increased denticity. Reaction with cobalt(ii) chloride resulted in the isolation of a dinuclear helicate complex. The ligand was found to have undergone addition of a methoxy group to one of the linking methylene carbons, presumably as a result of the oxidative addition of solvent methanol.     

Further insight into aryl nitration of tetraphenylporphyrin

We report an in-depth study on meso-aryl nitration of tetraphenylporphyrin. In contrast to previous studies, new evidence reveals that tetrakis(p-nitrophenyl) derivative can be obtained as a major product. Successful isolation of the barely soluble product toward a remarkable yield of nearly 90% has been reached by means of a solid phase extraction technique. Distribution of different nitro-porphyrin components is reassessed with respect to varying acid content in the reactions. An ortho-effect model is proposed to describe the formation mechanism.     

A combined computational and experimental study of polynuclear Ru-TPPZ complexes: insight into the electronic and optical properties of coordination polymers

We report a combined experimental and computational study of polynuclear [Ru(n)(TPPZ)(n)(+1)](2)(n)(+) complexes, of interest in the field of photoactive polymers. The complexes with n = 1, 2, 3 and n > 5 have been synthesized and spectroscopically characterized. A red-shift of the visible band maximum from 2.59 to 2.03 eV is observed going from the monomer to the longer oligomeric species (n > 5). To characterize the geometries, electronic structure, and excited states of these complexes, density functional theory (DFT) and time-dependent DFT calculations on the [Ru(n)(TPPZ)(n)(+1)](2)(n)(+) series with n = 1-4 in solution have been performed. The agreement between experimental and calculated spectra is good, both in terms of absorption maximum energies and relative intensities for different values of n. For all the investigated complexes, we assign the main band in the visible region as a metal-to-metal plus ligand charge transfer (MMLCT) transition. The resulting excited states are delocalized throughout the entire complexes, as they originate from a superposition of pi(TPPZ)-t(2g)(Ru) states. The low-energy shoulder of the main visible absorption band, present in the experimental spectra for n > 1, is proposed to arise from spin-forbidden singlet-triplet transitions of similar MMLCT character, consistent with the observed enhancement of this feature in the spectra of the corresponding Os oligomers.     

Mechanistic insight into fragmentation reactions of titanapinacolate complexes

Reactions between terminal alkynes or aromatic ketones and titanapinacolate complexes (DMSC)Ti(OCAr(2)CAr(2)O) (2, Ar = Ph, and 3, Ar = p-MeC(6)H(4); DMSC = 1,2-alternate dimethylsilyl-bridged p-tert-butylcalix[4]arene dianion) occur via rupture of the C-C bond of the titanacycle. Thus, reactions of 2 and 3 with terminal alkynes produce 2-oxatitanacyclopent-4-ene or 2-oxatitanacycloheptadiene complexes along with free Ar(2)CO. These compounds have been characterized spectroscopically and by X-ray crystallography. Because metallapinacolate intermediates have been implicated in important C-C bond-forming reactions, such as pinacol coupling and McMurry chemistry, the mechanism of the fragmentation reactions was studied. Analysis of the kinetics of the reaction of (DMSC)Ti[OC(p-MeC(6)H(4))(2)C(p-MeC(6)H(4))(2)O] (3) with Bu(t)Ctbd1;CH revealed that the fragmentation reactions proceed via a preequilibrium mechanism, involving reversible dissociation of titanapinacolate complexes into (DMSC)Ti(eta(2)-OCAr(2)) species with release of a ketone molecule, followed by rate-limiting reaction of (DMSC)Ti(eta(2)-OCAr(2)) species with an alkyne or ketone molecule.     

Further insight into the mass-spectral fragmentation of tetracyclone. Tetraphenyltetrahedrane ion

The p-fluoro labeling technique is used to demonstrate that the ions formed by the loss of CO from tetracyclones pass through a tetrahedral transition state or intermediate before they fragment.     

Heteroscorpionate-based Co2+, Zn2+, and Cu2+ complexes: coordination behavior, aerobic oxidation, and hydrogen sulfide detection

The coordination behavior and reactivity of the phenol-substituted bis(pyrazolyl)methane ligands, (3,5-(t)Bu(2)-2-phenol)bis(3,5-Me(2)-pyrazol-1-yl)methane (L1-H) and 2-phenol-bis(3,5-Me(2)-pyrazol-1-yl)methane (L2-H) have been investigated in the metal complexes (L1-H)CoCl(2) (1), (L1-H)ZnCl(2) (2), (L3)CuCl(2) (3), (L2)(2)Co(2)Cl(2) (4) (L2-H)ZnCl(2) (5), and (L2-H)CuCl(2)·H(2)O (6). The mononuclear tetrahedral cobalt complex 1 was isolated and fully characterized by X-ray single crystal diffraction and (1)H NMR spectroscopy and relaxometry. The neutral L1-H is κ(2)-coordinated to the metal center whereas the not coordinated hydroxy-phenyl group is involved in extended intermolecular hydrogen bonds. Aerobic oxidation of L1-H was observed in the reaction of this ligand with CuCl(2) to yield the para-quinone derivative L3 (L3 = 2-(t)Bu-6-(bis(3,5-Me(2)-pyrazol-1-yl)methyl)cyclohexa-2,5-diene-1,4-dione). Upon oxidation L3 resulted κ(2)-coordinated to the tetrahedral Cu(II) metal center, affording 3. The reaction of L2-H with CoCl(2)·6H(2)O produced the elimination of 1 equiv of hydrochloric acid and the formation of the binuclear complex 4 in which one cobalt is in an octahedral environment featuring two κ(3)-coordinated deprotonated ligands whereas the second cobalt center is detected in tetrahedral coordination geometry, bound to the octahedral cobalt via two phenoxo bridging moieties. Interestingly L2-H, (3-(t)Bu-2-phenol)bis(3,5-Me(2)-pyrazol-1-yl)methane (L4-H), or (5-(t)Bu-2-phenol)bis(3,5-Me(2)-pyrazol-1-yl)methane (L5-H) were not oxidized in the reaction with CuCl(2). The reaction of the ligand L2-H with ZnCl(2) and CuCl(2)·2H(2)O yielded the κ(2)-coordinated tetrahedral complex 5 and the square planar complex 6, respectively. The application of the cobalt complex 1 as molecular dosimeter for H(2)S was explored and compared to that of the zinc analogue 2. Density functional theory (DFT) calculations and NMR experiments to assess the possible mechanisms of H(2)S detection by both 1 and 2 are also described.     

Molecular modelling for coordination compounds: Cu(II)-amine complexes

The Ligand Field Molecular Mechanics (LFMM) method has been applied to 85 Cu(II)-amine complexes, eighteen of which were selected to form a training set. A single set of parameters yields Cu-N bond lengths for four-, five- and six-coordinate systems generally within 0.04 A of their X-ray crystallographic values. Larger deviations are indicative of counterion effects and/or crystallographic artefacts arising from Jahn-Teller averaging. The LFMM treatment is flexible and unbiased and for simple ligands, automatically gives planar CuN(4) and tetragonally elongated CuN(6) complexes. In agreement with experiment, square-pyramidal coordination is marginally favoured over trigonal bipyramidal coordination for CuN(5) species. However, if the ligand requirements dictate, the LFMM accommodates trigonal bipyramidal coordination for CuN(5) species, tetrahedral distortions of CuN(4) species and cis-elongated CuN(6) species.     

Preparation of stable and metastable coordination compounds: insight into the structural, thermodynamic, and kinetic aspects of the formation of coordination polymers

The reaction of ZnI2 and pyrimidine in acetonitrile results in the formation of the 1:2 compound ZnI2(pyrimidine)2 (1), which consists of discrete tetrahedral building blocks. Slow heating of 1 at 1 degrees C/min leads to its transformation into the ligand-deficient intermediate 1:1 compound ZnI2(pyrimidine) (3), which upon further heating decomposes into the most ligand-deficient 2:1 compound (ZnI2)2(pyrimidine) (4). In contrast, the 2:3 compound (ZnI2)2(pyrimidine)3 (2) is formed as an intermediate by decomposing 1 using a faster heating rate of 8 degrees C/min. Compound 2 consists of oligomeric units in which each ZnI2 unit is coordinated by two iodine atoms and one bridging and one terminal pyrimidine ligand. The crystal structure of compound 3 is built up of ZnI2 units, which are connected by the ligands into chains. For the thermal transformation of 1 into 3 via 2 as the intermediate, a smooth reaction pathway is found in the crystal structure, for which only small translational and rotational changes are needed. The metastable solvated compound (ZnI2)(pyrimidine)(acetonitrile)0.25 (5) consisting of (ZnI2)4(pyrimidine)4 rings is obtained by quenching the reaction of ZnI2 and pyrimidine in acetonitrile using an antisolvent. On heating, 5 decomposes into a new polymorphic 1:1 compound 6, which consists of (ZnI2)(pyrimidine) chains. On further heating, 6 transforms into a third polymorphic 1:1 compound 7, which consists of (ZnI2)3(pyrimidine)3 rings, and finally into the 1:1 compound 3. Solvent-mediated conversion experiments reveal that compounds 1-4 are thermodynamically stable, whereas compounds 5-7 are metastable. Time-dependent crystallization experiments unambiguously show that compound 7 is formed by kinetic control and transforms within minutes into compound 6, which finally transforms into 3. Compound 3 represents the thermodynamically most stable 1:1 modification, whereas compounds 6 and 7 are metastable. The different compounds obtained by thermal decomposition and by crystallization from solution represent a snapshot of the species in solution and thus provide insight into the formation of coordination compounds.     

Pyrophosphate-mediated magnetic interactions in Cu(II) coordination complexes

The reaction in water of Cu(NO(3))(2)·2.5H(2)O with 2,2'-bipyridine (bipy), 1,10-phenanthroline (phen), or 1,10-phenanthroline-5-amine (phenam), and sodium pyrophosphate (Na(4)P(2)O(7)), at various pHs, afforded three new copper(II)-pyrophosphate complexes, namely, {[Cu(bipy)(cis-H(2)P(2)O(7))](2)}·3H(2)O (1a), {[Cu(phen)(H(2)O)](4)(HP(2)O(7))(2)}(ClO(4))(2)·4H(2)O (2), and {[Cu(2)(phenam)(2)(P(2)O(7))](2)·25H(2)O}(n) (3). A solvent free crystalline phase of 1a was also isolated with formula {[Cu(bipy)(trans-H(2)P(2)O(7))](2)} (1b), which can be regarded as a pseudo-polymorph of 1a. Single crystal X-ray analyses revealed these compounds to have uncommon molecular architectures, with 3 being an unprecedented pyrophosphate-containing two-dimensional (2D) polymer. Compounds 1a/1b and 2 are discrete di- and tetra-nuclear complexes, respectively. The cationic {[Cu(phen)(H(2)O)](4)(HP(2)O(7))(2)}(2+) unit in 2 presents a unique quasi-flat structure, held together by solely in-plane pyrophosphate bridging modes (short O(eq)-P-O(eq) and long O(eq)-P-O-P-O(eq) pathways), a coordination arrangement also not previously reported. A different tetranuclear copper(II)-pyrophosphate arrangement is found in 3, with two classically bridged dimers (O(eq)-P-O(eq) pathway) joined together by auxiliary equatorial-axial μ-O pyrophosphate bridges. Here, the bidimensionality is reached through bridging phenam ligands, which provide further inter-"tetramer" metal-metal connections [(N,N')(eq)-(N')(ax) pathway], leading to the formation of an expanded covalent network based on the [Cu(2)(phenam)(2)(P(2)O(7))](2) moiety. Variable-temperature magnetic susceptibility measurements on polycrystalline samples of 2 and 3 revealed net antiferromagnetic coupling between metal centers with J(2a) = -7.9(2) cm(-1), J(2b) = -46.9(3) cm(-1), J(2c) = 0 cm(-1) in 2 (H = -J(2a)[S(Cu(1))·S(Cu(2)) + S(Cu(1a))·S(Cu(2a))] - J(2b)[S(Cu(1))·S(Cu(2a)) + S(Cu(1a))·S(Cu(2))] - J(2c)S(Cu(2))·S(Cu(2a))), and J(3a) = -87.9(2) cm(-1), J(3b) = -5(1) cm(-1) and J(3c) = +5(3) cm(-1) in 3 (H = -J(3a)[S(Cu(1))·S(Cu(2)) + S(Cu(1a))·S(Cu(2a))] - J(3b)[S(Cu(1))·S(Cu(2a)) + S(Cu(1a))·S(Cu(2))] - J(3c)S(Cu(2))·S(Cu(2a))). For 1a, a net ferromagnetic coupling is observed with J(1a) = +0.86(1) cm(-1) (H = -J S(A)·S(B) + S(A)·D· S(B) + βH (g(A)S(A) + g(B)S(B)). This is the first example of ferromagnetic coupling in pyrophosphate-complexes reported to date. A structure-function correlation study focusing on magnetic exchange across the observed diverse pyrophosphate-bridges is described with density functional theory (DFT) calculations included to support the stated observations.     

Hydroquinone-bridged dinuclear Cu(II) complexes and single-crystalline Cu(II) coordination polymers

Cationic dinuclear Cu(II) complexes 3 and 4 have been prepared using the novel hydroquinone-based imine chelators 2,5-((i)Pr(2)NCH(2)CH(2)N[double bond, length as m-dash]CH)(2)-1,4-(OH)(2)-C(6)H(2) (1) and 2,5-(pyCH(2)CH(2)N[double bond, length as m-dash]CH)(2)-1,4-(OH)(2)-C(6)H(2) (2), respectively (py = 2-pyridyl). X-Ray quality crystals of both complexes were grown from their DMF solutions. The sterically more encumbered compound crystallizes in the form of discrete dinuclear entities with Cu(II) centres in a distorted square-planar ligand environment (one coordination site is occupied by a DMF molecule). The pyridyl derivative 4 features dinuclear hydroquinone-bridged subunits similar to 3. However, the Cu(II) ions are now six-coordinate with two DMF molecules at an axial and an equatorial position of a Jahn-Teller-distorted octahedron. Moreover, the dinuclear subunits are no longer isolated but linked with each other via bridging hydroquinone oxygen atoms which occupy the second apical position of each octahedron. The structure suggests that the magnetic properties of the resulting coordination polymer of 4 could be described by a model valid for dimerized spin chains. As a result of this analysis the antiferromagnetic coupling constants J(1)/k(B) = 9.9 K (intradimer) and J(2)/k(B) = 0.9 K (interdimer) are obtained. Both in 3 and in 4, the hydroquinone --> semiquinone transition of the central bridging unit (E degrees ' = + 0.57 V, 3; E degrees ' = + 0.51 V, 4; DMF; vs. SCE) displays features of chemical reversibility. In the case of , reduction of Cu(II) centres requires a peak potential of E(p) = - 0.42 V.     

Photochemical and chemical oxidation of alpha-dimine-dithiolene metal complexes: insight into the role of the metal atom

[Pd(bpy)(bdt)], 2 (bpy = 2,2'-bipyridine, bdt = 1,2-benzenedithiolate), was prepared in good yield by the reaction of bdtNa2 with [(bpy)PdCl2] in DMSO. The analogous nickel complex, 1, was prepared in a similar reaction using MeOH/CH2Cl2 and [(bpy)NiCl2.dmf]2. Both 1 (a = 7.9920(1) A, b = 11.4385(1) A, c = 16.1415(1) A, beta = 103.327(1) degrees, V = 1435.86(2) A3, Z = 4) and 2 (a = 8.1631(5) A, b = 11.4379(7) A, c = 16.2475(10) A, beta = 103.7010(10) degrees, V = 1473.84(12) A3, Z = 4) crystallize in the monoclinic space group P2(1)/c and are isostructural with their previously reported platinum analogue. In accord with the results observed for platinum but not nickel, photochemical oxidation of 2 in DMF provides the monosulfinate complex [Pd(bpy)(bdtO2)], 4, along with a minor amount of the corresponding disulfinate [Pd(bpy)(bdtO4)], 5, while chemical oxidation yields only the latter. 4 cocrystallizes with 5 in the monoclinic space group P2(1)/c (a = 8.026(3) A, b = 14.600(6) A, c = 13.371(3) A, beta = 101.80(3) degrees, V = 1533.8(9) A3, Z = 4) as does pure 5 (a = 8.5611(9) A, b = 14.4586(15) A, c = 13.3677(14) A, beta = 108.122(2) degrees, V = 1572.6(3) A3, Z = 4). Comparison of spectroscopic and electrochemical properties of the three complexes, [M(bpy)(bdt)], yields the following ordering for the energy of the HOMO: Pd < Ni < Pt. The observed reactivity patterns and the electronic data suggest that the "anomalous" reactivity of 1 be attributed to the greater relative flexibility of the coordination geometry for nickel(II) complexes rather than electronic differences such as the energies of the frontier orbitals.     

Interpolyelectrolyte complexes: a single-molecule insight

Polyelectrolyte (PE) complexes (PECs) between long polycation poly(methacryloyloxyethyl dimethylbenzylammonium chloride) and short polyanion polystyrene sulfonic acid adsorbed onto mica were studied by atomic force microscopy. If one component is taken in excess, then a rapid coupling of the oppositely charged polyions first leads to the formation of nonequilibrium structures when collapsed PEC particles coexist with unreacted PEs molecules. The equilibrium PEC particles possess micelle-like core-shell morphology if the short polyion is taken in excess. When long PE is given in excess, equilibrium PECs are stabilized by wrapping the long polyion around hydrophobic segments of the PEC. We propose that transformations of initially formed nonequilibrium aggregates proceed through slow reactions (addition or/and substitution) of primary complexes with unreacted PEs chains, which finally leads to equilibrium PECs with optimized morphology. As expected, the mixing of oppositely charged PEs in a near-stoichiometric ratio leads to highly aggregated water-insoluble PECs.     

Synthesis and characterisation of trisarylphosphine selenides: Further insight into the effect of fluoroalkylation on the electronic properties of trisarylphosphines

Variations in the magnitude of the ¹J_SeP coupling constants for a range of phosphorus(V) selenides allow the efficiency of different spacer groups at insulating the phosphorus centre in triarylphosphines from highly electron-withdrawing perfluoroalkyl groups to be established.     

Heterobimetallic Mn/Co hybrid complexes composed of proximate organometallic and classical coordination sites

A series of highly unsymmetric heterobinuclear Mn/Co complexes is reported, in which an organometallic CpMn(CO)₂ fragment and a classical Werner-type cobalt(II) subunit are arranged in close proximity by means of a bridging pyrazolate. Two ligand scaffolds are employed that differ by the chelate size of the tripodal tetradentate {N4} binding site for cobalt. Molecular structures of three complexes with either nitrate or acetate coligands have been characterized by X-ray crystallography. IR and UV-Vis-spectroelectrochemistry reveals that oxidation of the heterobimetallic systems is highly localized at the organometallic manganese site, while electrochemical reduction occurs at cobalt. Structural and spectroscopic features as well as trends for the redox potentials of the Mn^I/Mn^II couple suggest that changes at the cobalt(II) Werner-type subunit have only minor effects on the properties of the organometallic site.     

N(II) oxide coordination to Co complexes with water-soluble porphyrin

Novel, water-soluble metalloporphyrin of zwitterionic type (Co complex with 5,10,15,20-tetra(4N-carboxymethylenepyridyl)porphyrin tetrabromide) was synthesized and its interaction with N(II) oxide was studied. IR and electronic absorption spectra were analyzed. Alcoholic solutions of this complex saturated with NO were found to contain all three types of coordinated ligands, i.e., NO, NO⁺, and NO^?.     

An ab initio insight into the Cu(111)-mediated Ullmann reaction

The coupling process of phenyl radicals-the important intermediates in the prototypal Ullmann reaction-on Cu(111) is addressed using density functional theory. Consistent with experiments, we prove that the fragments interact attractively already at relatively large distances. An intermediate state involving a "popping-out" surface atom is reached through a non-trivial surface diffusion path. The overall process of coupling to the final biphenyl state (with a barrier of 0.38 eV) is governed by a subtle electronic mechanism that reminds the concepts postulated by Hoffmann about the alignment of molecular frontier orbitals with the metallic Fermi level. Our results can be applied to more complex surface processes in the field of molecular self-assembly.     

Computational insight into the electronic structure and absorption spectra of lithium complexes of N-confused tetraphenylporphyrin

The present work is a theoretical investigation on lithium complexes of N-confused tetraphenylporphyrins (aka inverted) employing density functional theory (DFT) and time-dependent DFT, using the B3LYP, CAM-B3LYP, and M06-2X functionals in conjunction with the 6-31G(d,p) basis set. The purpose of the present study is to calculate the electronic structure and the bonding of the complexes to explain the unusual coordination environment in which Li is found experimentally and how the Li binding affects the Q and the Soret bands. The calculations show that, unlike a typical tetrahedral Li(+) cation, this Li forms a typical bond with one N and interacts with the remaining two N atoms, and it is located in the right place to form an agostic-like interaction with the internal C atom. The reaction energy, the enthalpy for the formation of the lithium complexes of N-confused porphyrins, and the effect of solvation are also calculated. The insertion of Li into N-confused porphyrin, in the presence of tetrahydrofuran, is exothermic with a reaction energy calculated to be as high as -72.4 kcal/mol using the lithium bis(trimethylsilyl)amide reagent. Finally, there is agreement in the general shape among the vis-UV spectra determined with different functionals and the experimentally available ones. The calculated geometries are in agreement with crystallographic data, where available.