CH3CN Ligands Research Articles

Syntheses of Pyrazolato‐ and Formamidinato‐lanthanoid(III) diiodides from in situ generated lanthanoid(III) pseudo‐Grignard reagents

Trivalent lanthanoid pseudo‐Grignard reagents PhpLnIIII(3‐p) (Ln = La, Ce, Nd, Er and Lu) are readily prepared by the oxidative addition of iodobenzene to the lanthanoid metal at 25°C, following pre‐activation of the metal with 1 drop Hg in tetrahydrofuran (thf) or acetonitrile (CH3CN) and brief sonication. The protolysis of PhpLnIIII(3‐p) with 3,5‐diphenylpyrazole (Ph2pzH) yields pyrazolato complexes [Ln(Ph2pz)I2(thf)n] (n = 4: Ln = La 1, Ce 2, Nd 3; n = 3: Ln = Er 4, Lu 5). Interestingly, for Er, a competing complex [ErPh(Ph2pz)I(thf)3]⋅thf (4a) was obtained along with the expected [Er(Ph2pz)I2(thf)3] (4). In contrast, in CH3CN, [Ln(Ph2pz)3(CH3CN)3]⋅2CH3CN (Ln = Nd 6, Gd 7) complexes were obtained through Schlenk equilibrium redistribution. Reactions with bulky N,N′‐bis(2,6‐di‐isopropylphenyl)formamidine (DippFormH) and N,N’ –bis(2,6‐dimethylphenyl)formamidine (XylFormH) produced the Ln(III) complexes, [Ln(DippForm)I2(thf)3]⋅nthf (n = 1, Ln = Nd 8; n = 0.5, Ln = Er 9) and [Ln(XylForm)I2(thf)3]⋅thf (Ln = Nd 10, Er 11). All the complexes isolated in this study are monomeric; complexes 4‐5 and 8‐11 are seven coordinate, complexes 1‐3 are eight coordinate with one η2‐N‐donor ligand and two trans iodine ligands, and complexes 6‐7 are nine coordinate, featuring three η2‐Ph2pz ligands, and three CH3CN ligands.

Kinetic study of the azo – Hydrazone photoinduced mechanism in complexes with a -Re(CO)3L0/+ core by flash photolysis

The photoprocesses of three rhenium(I) tricarbonyl complexes, fac-Re(NN)(CO)3L0/+, bearing an azo/hydrazone ligand, NN, and L = Cl, Br or CH3CN as axial ligand, were studied by time-resolved spectroscopic techniques. Flash irradiation of deaerated solutions of the complexes in acetonitrile photogenerated intermediates with intense optical absorptions in the UV–vis spectral region of the hydrazone ligand. The decay of the intermediates photogenerated with the Cl, Br and CH3CN ligands in the axial position proceeds by three different mechanisms. For the fac-Re(NN)(CO)3Cl, a three-step mechanism involving isomerization, tautomerization and rotation accounts for the spectroscopic transformations. In contrast to the flash photolysis result, the continuous photolysis of the complex in acetonitrile does not show spectroscopic changes associated with the generation of a stable product, i.e., the complex exhibits a photoreversible behavior. The first step observed in the flash irradiation of fac-Re(NN)(CO)3Cl, is missing in the decay of the photogenerated intermediates with fac-Re(NN)(CO)3Br and fac-[Re(NN)(CO)3CH3CN]+. A possible reason for this absence is that the process is too fast for the time-response of the flash photolysis set-up. Time-resolved spectra and associated reaction rate data for the Re(I) complexes are communicated and discussed. DFT calculations support a plausible E → Z mechanism with thermal back-isomerization mediated by tautomerization.

Exploring Rhenium Arene Piano-Stool Chemistry with [Re(η6-C6H6)(NCCH3)3]+: A Powerful Semi-Solvated Precursor.

Thermal treatment of the ReIII hydride complex [ReH(η5-C6H7)(η6-C6H6)]+ in CH3CN results in the formation of [Re(η6-C6H6)(NCCH3)3]+. This semi-solvated complex is remarkably stable under an ambient atmosphere and exhibits a fast CH3CN self-exchange, which facilitates substitution reactions. The CH3CN ligands are replaced by σ-donating phosphines such as trimethyl phosphine (PMe3), triphenyl phosphine (PPh3), or the bidentate 1,2-bis(diphenylphosphino)ethane (dppe) to afford [Re(η6-C6H6)(NCCH3)3-x(PR3)x]+ (if R = Me, then x = 2; if R = Ph, then x = 1 or 2) or [Re(η6-C6H6)(dppe)(NCCH3)]+, respectively. [Re(η6-C6H6)(NCCH3)3]+ also reacts with π-acceptors such as 2,2'-bipyridine (bipy), 1,10-phenanthroline (phen), or CO (1 atm) to give [Re(η6-C6H6)(L)(NCCH3)]+ (L = bipy or phen) and [Re(η6-C6H6)(CO)(NCCH3)2]+, respectively. The latter does not show any signs of decomposition after being exposed to an ambient atmosphere for multiple days. Additionally, [Re(η6-C6H6)(NCCH3)3]+ reacts with π-donors such as the dienes 2,3-dimethyl-1,3-butadiene (DMBD), norbornadiene (NBD), or 1,5-cyclooctadiene (COD) to give [Re(η6-C6H6)(η4-diene)(NCCH3)]+ (diene = DMBD, NBD, and COD). All three complexes are extremely stable and do not decompose during purification by preparative high-performance liquid chromatography (aqueous acidic gradient). In the presence of 18-crown-6, [Re(η6-C6H6)(NCCH3)3]+ reacts with lithium cyclopentadienyl to give the sandwich complex [Re(η5-C5H5)(η6-C6H6)]. Loss of the coordinated benzene was observed when treating [Re(η6-C6H6)(NCCH3)3]+ with diphenylacetylene (PhC≡CPh), yielding the tetra-coordinated [Re(NCCH3)(η2-PhC≡CPh)3]+.

Synthesis, characterization and anticancer activities of cationic η6-p-cymene ruthenium(II) complexes containing phosphine and nitrogenous ligands

Ruthenium-based anticancer agents have created a center of attention in the field of inorganic medicinal chemistry. The first fully characterized cationic ruthenium(II)-arene complexes [Ru(η6-p-cymene) (PAr3)LNCl]+ with highly lipophilic PAr3 ligands where Ar = 3,5-((CH3)3C)2C6H3– (L1), 3,5-(CH3)2C6H3– (L2), 4-CH3O-3,5-(CH3)2C6H2– (L3) and 4-CH3O-C6H4– (L4) with N = 3-methylpyridine (1–4, respectively), or L4 and 4-methylpyridine (5), or L4 and CH3CN (6) were obtained (yields 67–91%) as solids stable to light and air. Electrical conductance indicates that all the complexes are 1:1 electrolytes in solution. Their composition and purity have been unambiguously established by single-crystal X-ray diffraction, NMR spectroscopy and elemental analysis. The coordination geometries are uniform for all six complexes and each structure consist of a unipositive complex cation bearing the phosphine ligands L1-L4 and LN = 3-methylpyridine, 4-methylpyridine or CH3CN attached to the organometallic fragment. The equivalent unit cell volumes per formula unit decrease with 1 > 3 > 2 > 4 > 5 > 6, accurately reflecting the decreasing sizes of the phosphines L1-L4, and a greater occupied volume for 3-methyl- vs. 4-methylpyridine, and the smallest volume contribution from CH3CN. Electrochemical studies showed mixed electrochemical mechanisms (EC/ECE) from partial substitution of p-cymene by CH3CN ligands from the solvent. A large electrochemical stability window (>2.2 V) for Ru(II) was observed extending beyond the physiological E° range. The complexes were cytotoxic against human cancer cell lines in vitro, and some complexes altered cell morphology.

Molecular and Electronic Structures of a Series of Dinuclear CoII Complexes Varied by Exogeneous Ligands: Influence of π‐Bonding on Redox Potentials

AbstractFour dinuclear CoII complexes have been synthesized and structurally characterized with the dinucleating ligand susan (susan=4,7‐dimethyl‐1,1,10,10‐tetra(2‐pyridylmethyl)‐1,4,7,10‐tetraazadecane) varying in the exogeneous ligands: [(susan){CoII(CH3CN)2}2](PF6)4, [(susan){CoIICl}2](ClO4)2, [(susan){CoIIBr}2](ClO4)2, and [(susan){CoII(μ‐OH)CoII}](ClO4)3. The CoII ions are six‐coordinate with CH3CN ligands and five‐coordinate with anionic ligands. The electronic absorption spectra reflect the differences in the electronic structures, not only between the six‐ and five‐coordinate complexes, but also between the five‐coordinate complexes. These variations are also reflected in the magnetic properties with the highest orbital angular momentum contribution in six‐coordinate [(susan){CoII(CH3CN)2}2](PF6)4. The lower symmetry in the five‐coordinate complexes reduces the orbital angular momentum contribution. The hydroxo‐bridged complex [(susan){CoII(μ‐OH)CoII}](ClO4)3 exhibits an additional antiferromagnetic interaction. The electrochemical characterization reveals that the π‐acceptor ligand CH3CN facilitates not only reduction from CoII to CoI but also oxidation to CoIII, while the π‐donor ligands Br−, Cl−, and μ‐OH− impede both reduction to CoI and oxidation to CoIII. [(susan){CoII(CH3CN)2}2](PF6)4 and [(susan){CoIICl}2](ClO4)2 show electrocatalytic proton reduction at a potential of ≈−1.9 V vs Fc+/Fc that is associated with a ligand‐centered reduction.

Photocytotoxicity and photoinduced phosphine ligand exchange in a Ru(ii) polypyridyl complex.

Two new tris-heteroleptic Ru(ii) complexes with triphenylphosphine (PPh3) coordination, cis-[Ru(phen)2(PPh3)(CH3CN)]2+ (1a, phen = 1,10-phenanthroline) and cis-[Ru(biq)(phen)(PPh3)(CH3CN)]2+ (2a, biq = 2,2′-biquinoline), were synthesized and characterized for photochemotherapeutic applications. Upon absorption of visible light, 1a exchanges a CH3CN ligand for a solvent water molecule. Surprisingly, the steady-state irradiation of 2a followed by electronic absorption and NMR spectroscopies reveals the photosubstitution of the PPh3 ligand. Phosphine photoinduced ligand exchange with visible light from a Ru(ii) polypyridyl complex has not previously been reported, and calculations reveal that it results from a trans-type influence in the excited state. Complexes 1a and 2a are not toxic against the triple negative breast cancer cell line MDA-MB-231 in the dark, but upon irradiation with blue light, the activity of both complexes increases by factors of >4.2 and 5.8, respectively. Experiments with PPh3 alone show that the phototoxicity observed for 2a does not arise from the released phosphine ligand, indicating the role of the photochemically generated ruthenium aqua complex on the biological activity. These complexes represent a new design motif for the selective release of PPh3 and CH3CN for use in photochemotherapy.

Effect of Lewis Basic Amine Site on Proton Reduction Activity of NNN‐Co Pincer Complex

Electrochemical proton reduction is a promising energy storage method because H2 molecule has a simple structure with a relatively low potential energy. Current interest in hydrogen catalysts has increased research efforts on synthetic analogs of hydrogenase active sites. In this study, we demonstrated the electrochemical H2 evolution reactivity of [NNNR‐Co(CH3CN)3]2+ (R  CH2 (1b), NCH3 (2b)) complexes and examined a proton‐relay process in the H2 evolution reaction (HER). Upon one‐electron reduction, the Co(II) ion center in a high‐spin state dissociated a CH3CN ligand, while opening a reaction site. Cyclic voltammograms of the Co complexes indicated quasi‐reversible Co(II/I) redox behaviors, and both complexes 1b and 2b showed catalytic H2 evolution activity. Interestingly, 2b, assisted by a proton‐relaying NCH3 group, exhibited more efficient catalytic activity than 1b.

Photosubstitution Reaction of cis-[Ru(bpy)2(CH3CN)2]2+ and cis-[Ru(bpy)2(NH3)2]2+ in Aqueous Solution via Monoaqua Intermediate.

The photoinduced ligand exchange reaction of Ru(II) complexes in aqueous solution was studied using density functional theory (DFT). The optimized structures of the lowest triplet state of cis-[Ru(bpy)2(CH3CN)2]2+ (bpy = bipyridine), cis-[Ru(bpy)2(NH3)2]2+, and their monoaqua complexes were analyzed. The metal-centered (3MC) structure was lower than the metal-to-ligand charge transfer (3MLCT) structure for cis-[Ru(bpy)2(CH3CN)2]2+, whereas the 3MLCT structure was lower than the 3MC structure for cis-[Ru(bpy)2(NH3)2]2+. Such a difference would correlate with the higher quantum yield of the former complex. For the monoaqua complexes, the most stable local minimum structure was the 3MC structure, in which the Ru-OH2O and Ru-Nbpy ( trans to the oxygen) bonds were elongated. Therefore, the dissociation of the H2O ligand would be preferred to that of the CH3CN (or NH3) ligand from the monoaqua intermediate, which might result in the reformation of the monoaqua intermediate, and thus, the formation of the bis-aqua product would take a longer time than that of the monoaqua intermediate.

Monocycloplatinated Solvento Complex Displays Turn-on Ratiometric Phosphorescence Responses to Histamine.

The study of biological histamine (HA) requires probes capable of ratiometric photoluminescence detection of HA. We discovered that a monocycloplatinated complex having two solvento ligands ([Pt(2-(2-naphthyl)quinolinate)(NCCH3)2]ClO4) could produce ratiometric phosphorescence responses to HA in aerated aqueous solutions buffered to pH 7.4. The HA response was characterized with a hypsochromic shift of an emission peak wavelength from 635 to 567 nm. The corresponding phosphorescence intensity ratio (i.e., I567 nm/ I635 nm) increased from 0.26 to 1.90. Spectroscopic and spectrometric investigations indicated an occurrence of spontaneous displacement of the labile CH3CN ligands with HA. An independently prepared HA adduct supported this notion. The ratiometric phosphorescence responses to HA were highly tolerant to other biological stimuli, including changes in pH and the presence of biometals and biological Lewis bases such as amino acids, nucleosides, biothiols, neurotransmitters, and small molecular metabolites. Of note was the high selectivity toward HA over common biological ligands, including histidine, cysteine, and homocysteine, which was ascribed to tighter HA binding. Our phosphorescence measurements employing Boc-protected derivatives of HA suggested that the bis-chelate motif involving imidazolyl and terminal amino groups was crucial for eliciting the ratiometric phosphorescence signaling. Finally, the bioimaging utility of the HA probe was validated using RAW 264.7 macrophages that were exogenously supplemented with HA or stimulated with thapsigargin to enrich intracellular HA. Ratiometric phosphorescence imaging microscopy experiments demonstrated the ability of the probe for monitoring intracellular HA uptake. In addition, photoluminescence lifetime imaging microscopy techniques could be applied for visualization of HA within the RAW 264.7 cells, because the HA binding elongated the photoluminescence lifetime. Our study demonstrated the promising utility of inner-sphere interactions of phosphorescent Pt(II) complexes for detection of biological HA.

Crystal structure of a mononuclear copper(II) complex with 2-methoxy-N,N-bis-(quinolin-2-yl-meth-yl)-ethyl-amine (DQMEA).

Structural characterization of the compound [Cu(C2H3N)(C23H23N3O)](ClO4)2] or [Cu(C2H3N)(DQMEA)](ClO4)2] [DQMEA = 2-methoxy-N,N-bis-(quinolin-2-ylmeth-yl)ethyl-amine] {systematic name: (aceto-nitrile)[2-methoxy-N,N-bis(quinolin-2-ylmeth-yl)ethyl-amine]-copper(II) diperchlorate} by single-crystal X-ray diffraction reveals a complex cation with a tetra-dentate coordination of the DQMEA ligand along with monodentate coordination of a CH3CN ligand to a single CuII center, with two perchlorate anions providing charge balance. The CuII center has a distorted square-pyramidal geometry in which the nitro-gen atoms of the DQMEA and CH3CN ligands occupy the equatorial positions, while the oxygen atom of the DQMEA ligand resides in the axial position with an elongated Cu-O bond. The quinoline ring systems are nearly co-planar in the structure, while the linear CH3CN ligand is tilted significantly below this plane, and the central nitro-gen of DQMEA is above it. Within the complex, weak C-H⋯N hydrogen bonding takes place between the nitro-gen of CH3CN and a neighboring quinolyl group. The perchlorate ions are disordered within the structure, but undergo a number of weak inter-molecular C-H⋯O hydrogen-bonding inter-actions. Additional weak π-stacking inter-actions between the quinolyl groups of neighboring complexes further stabilize the crystal packing.

Atomically Precise Site-Specific Tailoring and Directional Assembly of Superatomic Silver Nanoclusters.

Convenient generation of stable superatomic silver clusters and their systematic site-specific tailoring and directional assembly present an enduring and significant challenge. In this work, we prepared a face-centered cubic (fcc) array of Ag14 superatoms protected by face-capping 1,2-dithiolate-o-carborane (C2B10H10S2) ligands, each produced from 1-thiol-o-carborane in crystallization with simultaneous reduction of Ag+ to Ag0. We find that the corner N-donor ligands contribute predominately to the stability and luminescence of the Ag14 superatom. As the first-formed nanocluster [Ag14(C2B10H10S2)6(CH3CN)8]·4CH3CN (NC-1) with labile vertex-coordinated CH3CN ligands is highly unstable, monodendate pyridine ligands were used to replace these CH3CN species site-specifically, giving [Ag14(C2B10H10S2)6(pyridine/p-methylpyridine)8] (NCs-2,3) in gram scale with its core structure intact, which features ultrastability up to 150 °C in air. Moreover, using bidentate N-containing ligands to bridge the superatomic Ag14 building blocks, we constructed an unprecedented hierarchical series of 1D-to-3D superatomic silver cluster-assembled materials (SCAM-1,2,3,4), and SCAM-4 is air-stable up to 220 °C. Furthermore, this series of stable solid-state superatomic-nanocluster materials exhibit tunable dual emission with wide-range thermochromism. The present study constitutes a major step toward the development of ligand-modulation of the structure, stability, assembly, and functionality of superatomic silver nanoclusters.

Unusual Role of Excited State Mixing in the Enhancement of Photoinduced Ligand Exchange in Ru(II) Complexes.

Four Ru(II) complexes were prepared bearing two new tetradentate ligands, cyTPA and 1-isocyTPQA, which feature a piperidine ring that provides a structurally rigid backbone and facilitates the installation of other donors as the fourth chelating arm, while avoiding the formation of stereoisomers. The photophysical properties and photochemistry of [Ru(cyTPA)(CH3CN)2]2+ (1), [Ru(1-isocyTPQA)(CH3CN)2]2+ (2), [Ru(cyTPA)(py)2]2+ (3), and [Ru(1-isocyTPQA)(py)2]2+ (4) were compared. The quantum yield for the CH3CN/H2O ligand exchange of 2 was measured to be Φ400 = 0.033(3), 5-fold greater than that of 1, Φ400 = 0.0066(3). The quantum yields for the py/H2O ligand exchange of 3 and 4 were lower, 0.0012(1) and 0.0013(1), respectively. DFT and related calculations show the presence of a highly mixed 3MLCT/3ππ* excited state as the lowest triplet state in 2, whereas the lowest energy triplet states in 1, 3, and 4 were calculated to be 3LF in nature. The mixed 3MLCT/3ππ* excited state places significant spin density on the quinoline moiety of the 1-isocyTPQA ligand positioned trans to the photolabile CH3CN ligand in 2, suggesting the presence of a trans-type influence in the excited state that enhances ligand exchange. Ultrafast spectroscopy was used to probe the excited states of 1-4, which confirmed that the mixed 3MLCT/3ππ* excited state in 2 promotes ligand dissociation, representing a new manner to effect photoinduced ligand exchange. The findings from this work can be used to design improved complexes for applications that require efficient ligand dissociation, as well as for those that require minimal deactivation of the 3MLCT state through low-lying metal-centered states.

Electronic Effects on Room-Temperature, Gas-Phase C-H Bond Activations by Cluster Oxides and Metal Carbides: The Methane Challenge.

This Perspective discusses a story of one molecule (methane), a few metal-oxide cationic clusters (MOCCs), dopants, metal-carbide cations, oriented-electric fields (OEFs), and a dizzying mechanistic landscape of methane activation! One mechanism is hydrogen atom transfer (HAT), which occurs whenever the MOCC possesses a localized oxyl radical (M-O•). Whenever the radical is delocalized, e.g., in [MgO]n•+ the HAT barrier increases due to the penalty of radical localization. Adding a dopant (Ga2O3) to [MgO]2•+ localizes the radical and HAT transpires. Whenever the radical is located on the metal centers as in [Al2O2]•+ the mechanism crosses over to proton-coupled electron transfer (PCET), wherein the positive Al center acts as a Lewis acid that coordinates the methane molecule, while one of the bridging oxygen atoms abstracts a proton, and the negatively charged CH3 moiety relocates to the metal fragment. We provide a diagnostic plot of barriers vs reactants' distortion energies, which allows the chemist to distinguish HAT from PCET. Thus, doping of [MgO]2•+ by Al2O3 enables HAT and PCET to compete. Similarly, [ZnO]•+ activates methane by PCET generating many products. Adding a CH3CN ligand to form [(CH3CN)ZnO]•+ leads to a single HAT product. The CH3CN dipole acts as an OEF that switches off PCET. [MC]+ cations (M = Au, Cu) act by different mechanisms, dictated by the M+-C bond covalence. For example, Cu+, which bonds the carbon atom mostly electrostatically, performs coupling of C to methane to yield ethylene, in a single almost barrier-free step, with an unprecedented atomic choreography catalyzed by the OEF of Cu+.

Control of Product Distribution and Mechanism by Ligation and Electric Field in the Thermal Activation of Methane.

An unexpected mechanistic switch as well as a change of the product distribution in the thermal gas-phase activation of methane have been identified when diatomic [ZnO].+ is ligated with acetonitrile. Theoretical studies suggest that a strong metal-carbon attraction in the pristine [ZnO].+ species plays an important role in the rebound of the incipient CH3. radical to the metal center, thus permitting the competitive generation of CH3. , OH. , and CH3 OH. This interaction is drastically weakened by a single CH3 CN ligand. As a result, upon ligation the proton-coupled single electron transfer that prevails for [ZnO].+ /CH4 switches to the classical hydrogen-atom-transfer process, thus giving rise to the exclusive expulsion of CH3. . This ligand effect can be modeled quite well by an oriented external electric field of a negative point charge.

Exceptional Structural Compliance of the B12F122- Superweak Anion.

The single-crystal X-ray structures, thermogravimetric analyses, and/or FTIR spectra of a series of salts of the B12F122- anion and homoleptic Ag(L)n+ cations are reported (L = CH2Cl2, n = 2; L = PhCH3, n = 3; L = CH3CN; n = 2-4; L = CO, n = 1, 2). The superweak-anion nature of B12F122- (Y2-) was demonstrated by the rapid reaction of microcrystalline Ag2(Y) with 1 atm of CO to form a nonclassical silver(I) carbonyl compound with an FTIR ν(CO) band at 2198 cm-1 (and with the proposed formula [Ag(CO)n]2[Y]). In contrast, microcrystalline Ag2(B12Cl12) did not exhibit ν(CO) bands and therefore did not form Ag(CO)+ species, even after 32 h under 24 atm of CO. When Ag2(Y) was treated with carbon monoxide pressures higher than 1 atm, a new ν(CO) band at 2190 cm-1 appeared, which is characteristic of a Ag(CO)2+ dicarbonyl cation. Both Ag2(CH3CN)8(Y) and Ag2(CH3CN)5(Y) rapidly lost coordinated CH3CN at 25 °C to form Ag2(CH3CN)4(Y), which formed solvent-free Ag2(Y) only after heating above 100 °C. Similarly, Ag2(PhCH3)6(Y) rapidly lost coordinated PhCH3 at 25 °C to form Ag2(PhCH3)2(Y), which formed Ag2(Y) after heating above 150 °C, and Ag2(CH2Cl2)4(Y) rapidly lost three of the four coordinated CH2Cl2 ligands between 25 and 100 °C and formed Ag2(Y) when it was heated above 200 °C. Solvent-free Ag2(Y) was stable until it was heated above 380 °C. The rapid evaporative loss of coordinated ligands at 25 °C from nonporous crystalline solids requires equally rapid structural reorganization of the lattice and is one of three manifestations of the structural compliance of the Y2- anion reported in this work. The second, more quantitative, manifestation is that Ag+ bond-valence sums for Ag2(CH3CN)n(Y) are virtually constant, 1.20 ± 0.03, for n = 8, 5, 4, because the Y2- anion precisely compensated for the lost CH3CN ligands by readily forming the necessary number of weak Ag-F(B) bonds. The third, and most exceptional, manifestation is that the idealized structural reorganization accompanying the conceptual transformations Ag2(CH3CN)8(Y) → Ag2(CH3CN)5(Y) → Ag2(CH3CN)4(Y) involve close-packed layers of Y2- anions that sandwich the Ag(CH3CN)4+ complexes splitting into staggered flat ribbons of interconnected (Y2-)3 triangles that surround the Ag2(CH3CN)52+ complexes on four sides, conceptually re-forming close-packed layers of anions that sandwich the Ag(CH3CN)2+ complexes. The interconnected (Y2-)3 triangle lattice of anions in Ag2(CH3CN)5(Y) may be the first example of this structure type.

Judicious Ligand Design in Ruthenium Polypyridyl CO2 Reduction Catalysts to Enhance Reactivity by Steric and Electronic Effects

A series of RuII polypyridyl complexes of the structural design [RuII (R-tpy)(NN)(CH3 CN)]2+ (R-tpy=2,2':6',2''-terpyridine (R=H) or 4,4',4''-tri-tert-butyl-2,2':6',2''-terpyridine (R=tBu); NN=2,2'-bipyridine with methyl substituents in various positions) have been synthesized and analyzed for their ability to function as electrocatalysts for the reduction of CO2 to CO. Detailed electrochemical analyses establish how substitutions at different ring positions of the bipyridine and terpyridine ligands can have profound electronic and, even more importantly, steric effects that determine the complexes' reactivities. Whereas electron-donating groups para to the heteroatoms exhibit the expected electronic effect, with an increase in turnover frequencies at increased overpotential, the introduction of a methyl group at the ortho position of NN imposes drastic steric effects. Two complexes, [RuII (tpy)(6-mbpy)(CH3 CN)]2+ (trans-[3]2+ ; 6-mbpy=6-methyl-2,2'-bipyridine) and [RuII (tBu-tpy)(6-mbpy)(CH3 CN)]2+ (trans-[4]2+ ), in which the methyl group of the 6-mbpy ligand is trans to the CH3 CN ligand, show electrocatalytic CO2 reduction at a previously unreactive oxidation state of the complex. This low overpotential pathway follows an ECE mechanism (electron transfer-chemical reaction-electron transfer), and is a direct result of steric interactions that facilitate CH3 CN ligand dissociation, CO2 coordination, and ultimately catalytic turnover at the first reduction potential of the complexes. All experimental observations are rigorously corroborated by DFT calculations.

Synthesis, Structure, and Reactivity of (Dihydrogen)(hydrido)iron(II) Complexes Bearing Chiral Diphos­phanes

AbstractThe heterolytic cleavage of molecular hydrogen by [Fe(H2O)6][BF4]2 and the chiral phosphane (+)‐1,2‐bis[(2R,5R)‐2,5‐dimethylphospholano]benzene [(R,R)‐Me‐DuPhos] or its enantiomer [(S,S)‐Me‐DuPhos] yields the (dihydrogen)(hydrido)iron complexes [FeH(η2‐H2){(R,R)‐Me‐DuPhos}2]BF4 ([R,R‐1]BF4) and [FeH(η2‐H2){(S,S)‐Me‐DuPhos}2]BF4 ([S,S‐1]BF4), respectively. These complexes are fluxional in solution at room temperature, and the trans isomers are observed at 200 K. The (Λ)‐cis‐[R,R‐1]BF4 complex was identified crystallographically as the only stereoisomer at room temperature. The energetic and structural differences between the cis (Λ and Δ) and trans isomers were analyzed from a computational (DFT) perspective. The coordinated dihydrogen is displaced by CO or CH3CN ligands to form a mixture of cis and trans isomers of FeH(L)[(R,R)‐Me‐DuPhos]2BF4 {L = CO ([R,R‐2]BF4), CH3CN ([R,R‐3]BF4)} or FeH(L)[(S,S)‐Me‐DuPhos]2BF4 {L = CO ([S,S‐2]BF4), CH3CN ([S,S‐3]BF4)} complexes, and the cis isomers are the major ones. The formation of trans‐[FeH(NMe3)(Me‐DuPhos*)2]BF4 is suggested for the reaction of [R,R‐1]BF4 and [S,S‐1]BF4 with NMe3 {Me‐DuPhos* = [(R,R)‐Me‐DuPhos] or [(S,S)‐Me‐Duphos]}.

Redox Properties and Catalytic Ability toward Electrochemical Proton Reduction of Sulfur-Bridged Trinuclear Mo3S4 Complexes Containing Acetate, Trifluoroacetate, and/or Dithiophosphate as Bridging Ligands

Abstract A novel complex with the terminal acetonitrile and bridging trifluoroacetate (tfa) ligands of [Mo3S4(dtp)3(μ-tfa)(NCCH3)] (dtp: diethyl dithiophospate) (1-tfa) was prepared from the substitution reaction of [Mo3S4(dtp)3(μ-dtp)(NCCH3)] (1-dtp) with trifluoroacetate anhydride in acetonitrile. The reaction of 1-dtp with trifluoroacetic acid in dichloromethane also afforded 1-tfa. Complex 1-tfa reacts with pyridine in ethanol resulting in the formation of a complex with the terminal pyridine ligand (py), [Mo3S4(dtp)3(μ-tfa)(py)] (2-tfa). Structures of 1-tfa and 2-tfa were determined by single-crystal X-ray structural analysis showing that one of three Mo–Mo distances depend on a kind of the bridging ligands, dtp, OAc, and tfa, while no trends in the other two Mo–Mo distances in each of the complexes with the different bridging ligands. Each of the cyclic voltammograms of 1-dtp, [Mo3S4(dtp)3(μ-OAc)(NCCH3)] (OAc: acetate) (1-OAc), 1-tfa, [Mo3S4(dtp)3(μ-OAc)(py)] (2-OAc), and 2-tfa showed two consecutive one-electron reduction waves and an oxidation wave, which are formally assigned to the Mo(IV IV IV)/Mo(III IV IV), Mo(III IV IV)/Mo(III III IV), and Mo(V IV IV)/Mo(IV IV IV) couples, respectively. The re-oxidation and re-reduction peaks for the second reduction and the oxidation processes were not observed for all of the complexes. The redox potentials of the first reduction processes were shifted up to ca. 200 mV with the bridging and terminal ligands, for example, the redox potentials appeared at −0.98 and −1.19 V vs. Fc/Fc+2-OAc and 1-tfa, respectively, due to the lower electron-donating ability of CH3CN and tfa ligands compared to that of the py and OAc ligands. The re-oxidation peak for the second reduction process of 2-OAc was observed at −80 °C suggesting that this is an EC process and the chemical reaction following the second electrochemical reduction is probably inhibited at low temperature. On the other hand, no re-reduction peak for the oxidation process appeared even at −80 °C implying that the oxidation process is accompanied by a significant structural change. This argument is supported by the results of the DFT calculation. The HOMO component of the complex contains a bonding character between the two acetate-bridged Mo centers and the other acetate-unbridged one, and the spin densities of the 1-electron oxidized complex mostly located at the one of two acetate-bridged Mo centers and the other acetate-unbridged one. Cyclic voltammograms of these complexes in the presence of trifluoroacetic acid as a proton source exhibited catalytic currents around the first reduction wave for each complex. The DFT calculations for 2-OAc and 2-tfa demonstrated that the HOMO components are also distributed over the doubly bridged sulfur ligands in addition to the Mo centers in the Mo3S4 core. These result suggests that a protonation reaction of 2-OAc and 2-tfa probably occurs on the doubly bridged sulfur ligand in the Mo3S4 core.

Optimizing the electronic properties of photoactive anticancer oxypyridine-bridged dirhodium(II,II) complexes.

A series of partial paddlewheel dirhodium compounds of general formula cis-[Rh2(xhp)2(CH3CN)n][BF4]2 (n = 5 or 6) were synthesized {xhp = 6-R-2-oxypyridine ligands, R = -CH3 (mhp), -F (fhp), -Cl (chp)}. X-ray crystallographic studies indicate the aforementioned compounds contain two cis-oriented bridging xhp ligands, with the remaining sites being coordinated by CH3CN ligands. The lability of the equatorial (eq) CH3CN groups in these complexes in solution is in the order -CH3 > -Cl > -F, in accord with the more electron rich bridging ligands exerting a stronger trans effect. In the case of cis-[Rh2(chp)2(CH3CN)6][BF4]2 (5), light irradiation enhances the production of the aqua adducts in which eq CH3CN is replaced by H2O molecules, whereas the formation of the aqua species for cis-[Rh2(fhp)2(CH3CN)6][BF4]2 (7) is only slightly increased by irradiation. The potential of both compounds to act as photochemotherapy agents was evaluated. A 16.4-fold increase in cytotoxicity against the HeLa cell line was observed for 5 upon 30 min irradiation (λ > 400 nm), in contrast to the nontoxic compound 7, which is in accord with the results from the photochemistry. Furthermore, the cell death mechanism induced by 5 was determined to be apoptosis. These results clearly demonstrate the importance of tuning the ligand field around the dimetal center to maximize the photoreactivity and achieve the best photodynamic action.

Marked improvement in photoinduced cell death by a new tris-heteroleptic complex with dual action: singlet oxygen sensitization and ligand dissociation.

The new tris-heteroleptic complex [Ru(bpy)(dppn)(CH3CN)2](2+) (3, bpy = 2,2'-bipyridine, dppn = benzo[i]dipyrido[3,2-a;2',3'-c]phenazine) was synthesized and characterized in an effort to generate a molecule capable of both singlet oxygen ((1)O2) production and ligand exchange upon irradiation. Such dual reactivity has the potential to be useful for increasing the efficacy of photochemotherapy drugs by acting via two different mechanisms simultaneously. The photochemical properties and photoinduced cytotoxicity of 3 were compared to those of [Ru(bpy)2(dppn)](2+) (1) and [Ru(bpy)2(CH3CN)2](2+) (2), since 1 sensitizes the production of (1)O2 and 2 undergoes ligand exchange of the monodentate CH3CN ligands with solvent when irradiated. The quantum yield of (1)O2 production was measured to be 0.72(2) for 3 in methanol, which is slightly lower than that of 1, Φ = 0.88(2), in the same solvent (λirr = 460 nm). Complex 3 also undergoes photoinduced ligand exchange when irradiated in H2O (λirr = 400 nm), but with a low quantum efficiency (<1%). These results are explained by the presence of the low-lying ligand-centered (3)ππ* excited state of 3 localized on the dppn ligand, thus decreasing the relative population of the higher energy (3)dd state; the latter is associated with ligand dissociation. Cytotoxicity data with HeLa cells reveal that complex 3 exhibits a greater photocytotoxicity index, 1110, than does either 1 and 2, indicating that the dual-action complex is more photoactive toward cells in spite of its low ligand exchange quantum yield.