Redox Non-innocence of Thioether Macrocycles: Elucidation of the Electronic Structures of Mononuclear Complexes of Gold(II) and Silver(II)

Stabilizing Au2+ in a mixed-valence 3D halide perovskite

Although Cu2+ is ubiquitous, the relativistic destabilization of the 5d orbitals makes the isoelectronic Au2+ exceedingly rare, typically stabilized only through Au-Au bonding or by using redox non-innocent ligands. Here we report the perovskite Cs4AuIIAuIII2Cl12, an extended solid with mononuclear Au2+ sites, which is stable to ambient conditions and characterized by single-crystal X-ray diffraction. The 2+ oxidation state of Au was assigned using 197Au Mössbauer spectroscopy, electron paramagnetic resonance, and magnetic susceptibility measurements, with comparison to paramagnetic and diamagnetic analogues with Cu2+ and Pd2+, respectively, as well as to density functional theory calculations. This gold perovskite offers an opportunity to study the optical and electronic transport of the uncommon Au2+/3+ mixed-valence state and the characteristics of the elusive Au2+ ion coordinated to simple ligands. Compared with the perovskite Cs2AuIAuIIICl6, which has been studied since the 1920s, Cs4AuIIAuIII2Cl12 exhibits a 0.7 eV reduction in optical absorption onset and a 103-fold increase in electronic conductivity.

Novel silver-phosphine coordination polymers incorporating a Wurster's blue - like radical cation with enhanced photoelectric properties

Tetraphosphine ligand dpppda reacts with AgNO3 to generate two 1D coordination polymers containing cubic and chair-like [Ag4(NO3)4] clusters. ESR spectroscopy supported by DFT calculations indicated the presence of dpppda˙+, a Wurster's blue - like radical cation formed by intramolecular ligand-to-metal charge transfer (LMCT). This LMCT results in higher AgI → Ag0 potentials and good photocurrent responses.

Porphyrin-based mixed-valent Ag(i)/Ag(ii) and Cu(i)/Cu(ii) networks as efficient heterogeneous catalysts for the azide-alkyne "click" reaction and promising oxidation of ethylbenzene

By using a new porphyrin-based linker, two unusual mixed-valent Ag(i,ii)- and Cu(i,ii)-organic networks were synthesized. Most strikingly, 1 and 2 exhibit highly efficient catalytic activities for the azide-alkyne "click" reaction and oxidation of ethylbenzene.

CW-EPR Spectral Simulations: Solid State

This chapter summarizes the core concepts underlying the simulation of EPR spectra from biological samples in the solid state, from a user perspective. The key choices and decisions that have to be made by a user when simulating an experimental EPR spectrum are outlined. These include: the choice of the simulation model (the network of spins and the associated spin Hamiltonian), the dynamic regime (solid, liquid, slow motion), the level of theory used in the simulation (matrix diagonalization, perturbation theory, etc.), the treatment of orientational order and disorder (powder, crystal, partial ordering), the inclusion of the effects of structural disorder (strains), the effects of other line broadening mechanisms (unresolved hyperfine couplings, relaxation), and the inclusion of experimental distortions (field modulation, power saturation, filtering). Additionally, the salient aspects of utilizing least-squares fitting algorithms to aid the analysis of experimental spectra with the help of simulations are outlined. Although drawing from the experience gained from implementing EasySpin and from interacting with EasySpin's user base, this chapter applies to any EPR simulation software.

Crystal structure of μ-cyanido-1:2κ(2) N:C-dicyanido-1κC,2κC-bis-(quinolin-8-amine-1κ(2) N,N')-2-silver(I)-1-silver(II): rare occurrence of a mixed-valence Ag(I,II) compound

The title dinuclear complex, [Ag2(CN)3(C9H8N2)2], may be considered as an Ag(II) compound with the corresponding metal site coordinated by two bidentate quinolin-8-amine mol-ecules, one cyanide group and one dicyanidoargentate(I) anion, [Ag(CN)2](-). Since this latter ligand contains an Ag(I) atom, the complex should be a class 1 or class 2 mixed-valence compound, according to the Robin-Day classification. The Ag(II) atom is six-coordinated in a highly distorted octa-hedral geometry, while the Ag(I) atom displays the expected linear geometry. In the crystal, the amino groups of the quinolin-8-amine ligands form N-H⋯N hydrogen bonds with the N atoms of the non-bridging cyanide ligands, forming a two-dimensional network parallel to (102). The terminal cyanide ligands are not engaged in polymeric bonds and the title compound is an authentic mol-ecular complex. The title mol-ecule is thus a rare example of a stable Ag(I,II) complex, and the first mixed-valence Ag(I,II) mol-ecular complex characterized by X-ray diffraction.

Metal coordination study at Ag and Cd sites in crown thioether complexes through DFT calculations and hyperfine parameters

Structural and electronic properties of [C12H24S6X], [C13H26S6OX], and [C14H28S6OX] (X: Ag(+), Cd(2+)) crown thioether complexes were investigated within the framework of the density functional theory (DFT) using the projector augmented wave (PAW) method. The theoretical results were compared with time-differential perturbed γ-γ angular correlations (TDPAC) experiments reported in the literature using the (111)Ag→(111)Cd probe. In the case of X=Ag(+), a refinement of the structure was performed and the predicted equilibrium structures compared with available X-ray diffraction experimental data. Structural distortions induced by replacing Ag(+) with Cd(2+) were investigated as well as the electric-field gradient (EFG) tensor at the Cd(2+) sites. Our results suggest that the EFG at Cd(2+) sites corresponds to the Ag(+) coordination sphere structure, i.e., before the structural relaxations of the molecule with X=Cd(2+) are completed. The results are discussed in terms of the characteristics of the TDPAC (111)Ag→(111)Cd probe and the time window of the measurement, and provide an interesting tool with which to probe molecular relaxations.

Epitaxial Retrieval of a Disappearing Polymorph

Recrystallization of [PdCl2([9]aneS2O)] ([9]aneS2O = 1-oxa-4,7-dithiacyclononane), 1, and [PtCl2([9]aneS2O)], 2, by diffusion of Et2O vapor into solutions of the complexes in MeNO2 yielded three phases of 1 and two phases of 2. The known phase of 1, herein designated α-1, was obtained under ambient conditions. A second phase, designated β-1, was initially also obtained by this method; however, following the advent of a third phase, γ-1, all subsequent efforts over a period of a year to crystallize β-1 yielded either γ-1, obtained by carrying out the recrystallization at elevated temperature, or α-1, commonly found throughout the study. This persistent absence of a phase which could initially be crystallized with ease led us to the conclusion that β-1 was an example of a "disappearing polymorph". The first phase obtained of 2, designated α-2, was obtained by recrystallization under ambient conditions and is isomorphous and isostructural with α-1. The second phase β-2 was obtained by slight elevation of the recrystallization temperature and was found to be isomorphous and isostructural with β-1. Subsequently, β-2 was used to seed the growth of the disappearing polymorph β-1. No third phase of 2 (γ-2) has been isolated thus far.

Synthesis and coordination chemistry of 1,1,1-tris-(pyrid-2-yl)ethane

The complexes [ML₂]ⁿ⁺(Mⁿ⁺= Fe²⁺, Co²⁺, Co³⁺, Cu²⁺and Ag⁺), [PdCl₂L] and [CuI(L)] are described. [AgL₂]⁺is an unusual square planar silver(i) centre (left). Exposure of [CuI(L)] to air affords mono- or dinuclear copper(ii)/carbonato products. Two of the copper complexes form crystalline hydrates with complicated hydrogen bond networks (right).

Gold(iii) assisted C–H activation of 1,4,7-trithiacyclononane: synthesis and spontaneous resolution of a bicyclic chiral sulfonium salt

A trithiamacrocyclic ligand complex of Au(iii) undergoes a redox-mediated thermal reaction to form a chiral bicyclic sulfonium salt.

2,3-Bis(1-methylimidazol-2-yl)quinoxaline (bmiq), a new ligand with decoupled electron transfer and metal coordination sites: the very different redox behaviour of isoelectronic complexes with [PtCl2] and [AuCl2]+

The new, potentially ambidentate heterocyclic ligand 2,3-bis(1-methylimidazol-2-yl)quinoxaline (bmiq) was obtained from 2,3-bis(1-methylimidazol-2-yl)glyoxal and 1,2-diaminobenzene. Its coordination to PtCl(2) and to the isoelectronic [AuCl(2)](+) in [AuCl(2)(bmiq)](AuCl(4)) occurs via the imine N donors of the imidazolyl groups, leading to the formation of seven-membered chelate rings with boat conformation. According to the spectroelectrochemistry (UV-vis-NIR, EPR), the reversible electron addition to the [PtCl(2)(bmiq)] and the free ligand takes place in the (non-coordinated) quinoxaline part of the molecule, similarly as for related complexes of dipyrido[3,2-a:2',3'-c]phenazines (dppz), 2,3-bis(2-pyridyl)quinoxalines (bpq) and 2,3-bis(dialkylphosphino)quinoxalines (QuinoxP). DFT calculations confirm the experimental results (structures, spectroscopy) and also point to the coordination potential of the quinoxaline N atoms. The electron addition to [AuCl(2)(bmiq)](+) takes place not at the ligand but at the metal site, according to experimental and DFT results.

In vitro evaluation of a stable monomeric gold(II) complex with hematoporphyrin IX: cytotoxicity against tumor and kidney cells, cellular accumulation, and induction of apoptosis

The antineoplastic potential of a stable monomeric Au(II) complex with hematoporphyrin IX (Hp), namely [Au(II)Hp(-2H).(H(2)O)(2)], was investigated in a panel of tumor cell lines. The complex exhibits strong cytotoxicity, whereby the leukaemia- and lymphoma-derived cell lines are more sensitive, with IC(50) values comparable to those of the reference anticancer drug cisplatin. In contrast, the solid tumor models are more sensitive to the platinum drug. A comparative assessment of both agents against the human kidney cell line 293T has shown that [Au(II)Hp(-2H).(H(2)O)(2)] is less cytotoxic. The gold complex induces oligonucleosomal DNA fragmentation in tumour cells following 24-hour treatment and hence its cytotoxic effect is at least partly mediated by induction of apoptotic cell death. A prominent intracellular gold accumulation was detected after treating tumor cells with [Au(II)Hp(-2H).(H(2)O)(2)] which shows that its putative pharmacological targets are readily accessible after a short incubation period.

Characterization and electronic structures of five members of the electron transfer series [Re(benzene-1,2-dithiolato)3](z) (z = 1+, 0, 1-, 2-, 3-): a spectroscopic and density functional theoretical study

The reaction of ReCl(5) with 3 equiv of a benzene-1,2-dithiolate derivative in CH(3)CN produced, after the addition of [C(8)H(16)N]Br ([C(8)H(16)N](+) is 5-azonia-spiro[4,4]nonane), brownish-green crystals of [C(8)H(16)N][Re(tms)(3)] (1c) and [C(8)H(16)N][Re(Cl(2)-bdt)(3)] (2c), where (tms)(2-) represents 3,6-bis(trimethylsilyl)benzene-1,2-dithiolate and (Cl(2)-bdt)(2-) is 3,6-dichlorobenzene-1,2-dithiolate. Chemical reduction of [Re(bdt)(3)] (3b) with n-butyllithium in the presence of PPh(4)Br yielded [PPh(4)][Re(bdt)(3)] (3c), where (bdt)(2-) is benzene-1,2-dithiolate. The three monoanionic complexes possess a diamagnetic ground state (Re(V), d(2), S = 0). The crystal structures of 1c x 2 CH(3)CN and 2c x C(3)H(6)O have been determined by X-ray crystallography. The electrochemistry establishes that the complexes are members of electron transfer series involving a monocation [Re(V)(L(*))(2)(L)](+) (S = 0(?)), a neutral [Re(V)(L(*))(L)(2)](0) (S = 1/2), a monoanion [Re(V)(L)(3)](1-) (S = 0), a dianion [Re(IV)(L)(3)](2-) (S = 1/2), and a trianion [Re(III)(L)(3)](3-) (S = 1(?)). The unique X-band EPR spectrum of the neutral species clearly describes a diamagnetic Re(V) d(2) central ion with the unpaired electron located in a purely ligand-centered molecular orbital, whereas it is metal-centered in the dianionic form: a Re(IV) d(3) ion with three dithiolate(2-) ligands. S K-edge and Re L-edge X-ray absorption spectroscopy confirms these assignments and furthermore shows that the monoanion has a Re(V) central ion with three dianionic ligands. The geometrical and electronic structures of all members of the electron transfer series have been calculated by density functional theoretical methods, and the S K-pre-edge spectra have been simulated and assigned using a time-dependent DFT protocol.

Lanthanide(III) and actinide(III) complexes [M(BH4)2 (THF)5][BPh4] and [M(BH4)2(18-crown-6)][BPh4] (M = Nd, Ce, U): synthesis, crystal structure, and density functional theory investigation of the covalent contribution to metal-borohydride bonding

Treatment of [M(BH4)3(THF)3] with NEt3HBPh4 in THF afforded the cationic complexes [M(BH4)2(THF)5][BPh4] [M = U (1), Nd (2), Ce (3)] which were transformed into [M(BH4)2(18-crown-6)][BPh4] [M = U (4), Nd (5), Ce (6)] in the presence of 18-crown-6; [U(BH4)2(18-thiacrown-6)][BPh4] (7) was obtained from 1 and 18-thiacrown-6 in tetrahydro-thiophene. Compounds 1, 3.C4H8S, 4.THF, 5, and 6.THF exhibit a penta- or hexagonal bipyramidal crystal structure with the two terdentate borohydride ligands in apical positions; the BH4 groups in the crystals of 7.C4H8S are in relative cis positions, and the thiacrown-ether presents a saddle shape, with two diametrically opposite sulfur atoms bound to uranium in trans positions. The crystal structures of these complexes, as well as those of previously reported [M(BH4)2(THF)5]+ cations, do not reveal any clear-cut lanthanide(III)/actinide(III) differentiation. The structural data obtained for [M(BH4)2(18-crown-6)]+ (M = U, Ce) by relativistic density functional theory (DFT) calculations are indicative of a small shortening of the U...B with respect to the Ce...B distance, which is accompanied by a lengthening of the U-Hb bonds and an opening of the Hb-B-Hb angle (Hb = bridging hydrogen atom of the eta3-BH4 ligand). The Mulliken population analysis and the natural bond orbital analysis indicate that the BH4 -->M(III) donation is greater for M = U than for M = Ce, as well as the overlap population of the M-Hb bond, thus showing a better interaction between the uranium 5f orbitals and the Hb atoms. The more covalent character of the B-H-U three-center two-electron bond was confirmed by the molecular orbital (MO) analysis. Three MOs represent the pi bonding interactions between U(III) and the three Hb atoms with significant 6d and 5f orbital contributions. These MOs in the cerium(III) complex exhibit a much lesser metallic weight with practically no participation of the 4f orbitals.

Ligand field photofragmentation spectroscopy of [Ag(L)N]2+ complexes in the gas phase: Experiment and theory

Experiments have been undertaken to record photofragmentation spectra from a series of [Ag(L)N]2+ complexes in the gas phase. Spectra have been obtained for silver(II) complexed with the ligands (L): acetone, 2-pentanone, methyl-vinyl ketone, pyridine, and 4-methyl pyridine (4-picoline) with N in the range of 4-7. A second series of experiments using 1,1,1,3-fluoroacetone, acetonitrile, and CO2 as ligands failed to show any evidence of photofragmentation. Interpretation of the experimental data has come from time-dependent density functional theory (TDDFT), which very successfully accounts for trends in the spectra in terms of subtle differences in the properties of the ligands. Taking a sample of three ligands, acetone, pyridine, and acetonitrile, the calculations show all the spectral transitions to involve ligand-to-metal charge transfer, and that wavelength differences (or lack of spectra) arise from small changes in the energies of the molecular orbitals concerned. The calculations account for an absence in the spectra of any effects due to Jahn-Teller distortion, and they also reveal structural differences between complexes where the coordinating atom is either oxygen or nitrogen that have implications for the stability of silver(II) compounds. Where possible, comparisons have also been made with the physical properties of condensed phase silver(II) complexes.