Carbanionic Ligands Research Articles

The coordination of alkali-metal nickelates to organic π-systems: synthetic, structural and spectroscopic insights.

Low-valent nickelates have recently been shown to be key intermediates in challenging cross-coupling reactions using aryl ethers as electrophiles. Key for the success of these transformations is the activation of the substrate through π-coordination to the nickelate intermediate, however there is still limited knowledge about the fundamental structure and coordination chemistry of these heterobimetallic complexes. Herein, we report the synthesis, structures, and spectroscopic analysis of a diverse family of alkali-metal nickelates derived from phenyl-alkali-metal reagents and Ni(ttt-CDT), where ttt-CDT = trans,trans,trans-1,5,9-cyclododecatriene. The co-complexation of PhLi with Ni(ttt-CDT) was found to yield 1 : 1, 2 : 1 or 4 : 2 lithium nickelates depending on the stoichiometry and reaction conditions employed. The high lability of the ttt-CDT ligand enables facile ligand exchange with an assorted series of organic π-acceptors, ranging from polyaromatic hydrocarbons to ketones, imines and nitriles. For anthracene and phenanthrene, a homologous series of Li, Na and K nickelates could be obtained, which lead to different structural motifs or degrees of aggregation in the solid-state spanning solvated monomers to complex polymeric arrangements. For π-extended systems such as perylene or coronene, competing single-electron-transfer to give the corresponding radical anions was observed, illustrating the highly reducing nature of the alkali-metal nickelates. X-ray crystallographic analysis and NMR spectroscopy of the phenyl-alkali-metal nickelates reveal extreme back-bonding from Ni(0) to the organic π-acceptors due to strong σ-donation from the carbanionic ligands.

Bioinspired iron(II)-β-diketonate and iron(II)-α-hydroxy ketone complexes of a carbanionic N3C ligand: Oxidation of metal center vs C[sbnd]C bond cleavage of co-ligand with dioxygen

Two bioinspired low-spin iron(II)-β-diketonate complexes, [(L1)FeII(BA)] (2) and [(L1)FeII(DBM)] (3), and an iron(II)-α-hydroxy ketone complex [(L1)FeII(HAP)] (4) (L1 = tris(2-pyridylthio)methanido anion, BA = monoanionic benzoylacetone, DBM = monoanionic dibenzoylmethane and HAP = monoanionic 2-hydroxyacetophenone) of a tripodal carbanionic ligand were prepared from an iron(II) precursor complex [(L1)FeII(CH3CN)2](ClO4) (1). The dioxygen reactivity of the complexes (2, 3 and 4) was investigated to evaluate the effect of ligand geometry and of the spin state of iron on the CC bond cleavage of the co-ligands. Complexes 2 and 3 react with dioxygen to yield benzoic acid as a minor product. The major pathway involves the formation of the corresponding iron(III)-β-diketonate complexes, [(L1)FeIII(BA)](ClO4) (2ox-ClO4) and [(L1)FeIII(DBM)](ClO4) (3ox-ClO4). Complex 4, however, undergoes the CC bond cleavage of the iron-coodinated HAP to form benzoic acid as the major product. In the reaction, phenylglyoxal is formed as the minor product with concomitant generation of an iron-oxygen oxidant. The oxidant is able to transfer an oxygen atom to thioanisole and can exchange its oxygen atom with water. The reactivity patterns of the low-spin iron(II) complexes reported here are distinctly different from that of the corresponding high-spin complexes supported by a monoanionic facial N3 ligand. The results emphasize the role of spin-state of iron, the denticity of the supporting ligand, and the nature of co-ligand in affecting the CC bond cleavage reaction.

Unprecedented Formation of a Binuclear Au(II)-Au(II) Complex through Redox State Cycling: Electrochemical Interconversion of Au(I)-Au(I), Au(II)-Au(II), and Au(I)-Au(III) in Binuclear Complexes Containing the Carbanionic Ligand C6F4PPh2.

The rational design of binuclear Au(I)-Au(I), Au(II)-Au(II), and Au(I)-Au(III) complexes requires an understanding of how the redox states interconvert. Herein, the electrochemical interconversion of the three oxidation states I, II, and III is reported on the voltammetric (cyclic and rotating disk electrode) time scales for binuclear gold complexes containing C6F4PPh2 as a ligand, to demonstrate for the first time formation of a binuclear Au(II)-Au(II) from a Au(I)-Au(III) complex. Results are supported by bulk electrolysis and coulometry with reaction products being identified by 31P NMR and UV-vis spectroscopy. All electrochemical processes involve an overall two-electron charge-transfer process with no one-electron intermediate being detected. Importantly, the kinetically rather than thermodynamically favored isomer [Au2IIX2(μ-2-C6F4PPh2)2] is formed on redox cycling of [XAuI(μ-2-C6F4PPh2)(κ2-2-C6F4PPh2)AuIIIX] (X = Cl, ONO2). Finally, a mechanism is proposed to explain the simultaneous change of coordination of the chelating carbanionic ligand to bridging mode and interconversion of oxidation states in binuclear gold complexes.

Asymmetric Cyclometalated RuII Polypyridyl-Type Complexes with π-Extended Carbanionic Donor Sets.

A series of novel cyclometalated RuII complexes were investigated featuring the tridentate dqp ligand platform (dqp is 2,6-di(quinolin-8-yl)pyridine), in order to utilize the octahedral coordination mode around the Ru center to modulate the electrochemical and photophysical properties. The heteroleptic complexes feature C1 symmetry due to symmetry breaking by the peripheral five- or six-membered carbanionic chelate (phenyl, naphthyl, or anthracenyl units). The chelation mode is controlled by the steric effects and C-H activation selectivity of the ligand, which prompted the development of a general synthesis protocol. The optimized conditions to achieve high overall yields (55-75%) involve NaHCO3 as the base and an simplified purification protocol: i.e., facile chromatographic separation using commercially available amino-functionalized silica applying nonaqueous salt-free conditions to omit the necessity of counterion exchange. The structural, photophysical, and electrochemical properties were studied in depth, and the results were corroborated by density functional theory (DFT) calculations. Steady state and time-resolved spectroscopy revealed red-shifted absorption (up to 750 nm) and weak IR emission (800-1000 nm) combined with prolonged emission lifetimes (up to 20 ns) in comparison to classical tpy-based (tpy is 2,2':6',2″-terpyridine) complexes. An enhanced stability was observed by blocking the reactive positions of the carbanionic ligand framework, while the reactive positions may be exploited for further functionalization.

P,C-bond cleavage in the ligand sphere of a nickel(II) complex.

Reacting nickel(ii)perchlorate with a bidentate P,N-ligand in methanol leads to P,C-bond cleavage and gives a five-coordinate nickel complex wherein the nickel(ii) site is coordinated by a tridentate P,N,P-ligand and a bidentate N,C-ligand. The carbanion of the latter is the result of the P,C-bond cleaving process. The diamagnetic nickel(ii) complex was characterized by means of elemental analysis, NMR spectroscopy, cyclic voltammetry and X-ray structure analysis.

Synthesis, Characterization and Antitumor Activity of 4-Ferrocenylpyridine-3, 5-Dicarbonitrile Derivatives and Sodium Polymeric Complexes Containing Carbanionic Ligands

ABSTRACTThe reactions of 2-cyano-3-ferrocenylacrylonitrile with malononitrile in a ROH/H2O medium in the presence of Na2CO3 afforded 6-alkoxy-2-amino-4-ferrocenylpyridine-3,5-dicarbonitriles, 6-alkoxy-2-amino-4-ferrocenyl-3-ferrocenyl-methyl-3,4-dihydropyridine-3,5-dicarbonitriles and Na+ polymeric complexes: {[Na+(2-ferrocenyl(tetracyano)propenyl)−L]∞ and [Na+(2-amino-3,5-dicyano-4-ferrocenyl-6-pyridyl-dicyanomethyl)−L]∞ where L = ethanol, methanol. Complexes with L = acetonitrile (c), dimethylformamide (d), acetone (e), ethyl acetate (f) were prepared by recrystallization. The structures of the compounds 4b and Na+ polymeric complexes 5c and 6d, e were established by the spectroscopic data and X-ray diffraction analysis. Two compounds 3a and 4a were tested in vitro against six human tumor cell lines U-251, PC-3, K-562, HCT-15, MCF-7 and SKLU-1 to assess their in vitro antitumor activity. The results suggest biological specificity towards PC-3, K-562 and HCT-15 cells for compound 3a, and towards PC-3 cell for compound 4a at doses 50 μM, which are lower than Cisplatin IC50′s in the three cell lines.

Unusual condensation of ethylene glycol with coordinated 1,10-phenanthroline-5,6-dione in [Ru(phen-dione)2Cl2] and its cyclometallated complex, [Ru(phpy-κ2N,C2′)(phen-diox)2](PF6)

A novel cyclometallated Ru(II) derivative of 1,10-phenanthroline-5,6-dione (phen-dione), [Ru(phpy-κ2N,C2′)(phen-diox)2](PF6), where phpy-κ2N,C2′ is 2-phenylpyridine and phen-diox is 2,3-dihydro-4a,12b-(epoxyethanooxy)[1,4]dioxino[2,3-f][1,10]phenanthroline, has been prepared by the metal-assisted effect of [Ru(phen-dione)2Cl2] via an unusual condensation of the coordinated phen-dione ligand with ethylene glycol which led to the formation of [Ru(phen-diox)2Cl2]. The condensation reaction could not proceed in the absence of [Ru(phen-dione)2Cl2]. DFT calculations showed that the energy of the LUMO of [Ru(phen-dione)2Cl2] is about 12.430kcal/mol lower than that of the uncoordinated phen-dione ligand. The cyclometallated complex has been characterized by elemental analysis, spectroscopic and spectroelectrochemical (SEC) techniques, cyclic voltammetry and X-ray crystallography. X-ray analysis of the complex shows an elongation of one of the five Ru–N bonds due to a strong trans effect of the carbanionic ligand. The electrochemical data of the complex demonstrate a strong σ-donating character of the anionic phenylpyridine ligand. UV–vis spectroelectrochemical studies of the complex in acetonitrile show a reversible redox behavior, evaluated by the maintenance of isosbestic points in the UV–vis spectrum for both forward oxidation and reverse reduction processes. The complex is very weakly emissive at room temperature in degassed acetonitrile with λem=546nm and ϕem=0.02%.

Double cyclometallation of bridging 3,6-bis(2-thienyl)-1,2,4,5-tetrazine in a dinuclear mesityl(dimethylsulfoxide)platinum(II) complex: Structure and properties

3,6-Bis(2-thienyl)-1,2,4,5-tetrazine (bttz) reacts with trans-Pt(dmso) 2(mes) 2, mes = mesityl = 2,4,6-trimethylphenyl, under twofold cyclometallation to yield structurally characterized (μ-bttz-2H +)[Pt(dmso)(mes)] 2 with uncoordinated thiophene sulfur atoms and bttz deprotonated in the 3,3′ positions. The structural features include cis-positioned carbanionic ligands, twisted mesityl substituents, S-coordinated dmso ligands with the S O bonds lying in the molecular plane, shortened inter-ring bonds, and rather short Pt–C bonds at 1.998(9)/2.00(1) Å (Pt–C mes) and 1.985(9)/1.99(1) Å (Pt–C bttz-2H+). Reversible reduction to {(μ-bttz-2H +)[Pt(dmso)(mes)] 2} − causes a high-energy shift of the charge transfer bands and the appearance of an unresolved EPR signal at g = 1.9905.

ChemInform Abstract: Synthesis and Alkylation fo Cyclic α‐Sulfonimidoyl Carbanions: Non‐Transferable Chiral Carbanionic Ligands in Copper‐Mediated Enantioselective Conjugate Addition.

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Synthesis and Alkylation of Cyclic α-Sulfonimidoyl Carbanions: Non-transferable Chiral Carbanionic Ligands in Copper-Mediated Enantioselective Conjugate Addition

Synthesis and Alkylation of Cyclic α-Sulfonimidoyl Carbanions: Non-transferable Chiral Carbanionic Ligands in Copper-Mediated Enantioselective Conjugate Addition

New results in the coordination chemistry of σ-bonded carbanionic ligands

From the coordination chemical point of view three interrelated topics in o'—organo transition metal chemistry are considered: Complex formation and stability of the transition metal carbon— o—bond, the donor strength of carbanions and the accessibility of unusual low oxidation states in transition metal phenyl complexes.