Cobalt(II) carbon paste electroactive electrodes (Co-CPEEs) comprising of a series of Co(II) pincer complexes bearing pyridyl benzothiazole/carbo(thio)amide moieties and carbon (graphite) powder were fabricated by direct mixing method. The electrochemical behavior and electrocatalysis towards proton reduction were studied and the Co-CPEEs were found to be efficient hydrogen evolving electrocatalysts in acetate buffers (pH = 3.6). Faradaic efficiency exceeding 90% were obtained at pH 3.6 with the Co-CPEE containing κ3-SNN Co(II) modification achieving up to 99% faradaic efficiency. No significant changes in the FTIR spectral features of the Co(II)-CPEEs were observed after 60 min of controlled potential electrolysis (CPE), suggesting that the immobilized Co(II) complexes remained structurally intact and exhibited good short-term operational stability under the electrocatalytic conditions.

The synthesis, X-ray crystallographic, and spectroscopic studies of 2-benzoylthiophene ketone thiophene-2-carboxylic acid hydrazone (bttcah) is reported. The compound crystallized by slow evaporation from ethanol in a monoclinic space group P 21/c with a = 9.2783(3) Å, b = 16.4236(6) Å, c = 9.8605(3) Å, β = 106.0017(12)° and V = 1444.35(8) Å3 with Z = 4. The thienyl rings of bttcah are pseudo-planar with the hydrazone backbone, however, the phenyl ring is out of the plane of the hydrazone backbone by 108.96o. The packing of the molecules showed classical intermolecular hydrogen bonding between NH and C=O groups. In the 1H NMR the NH was observed at 9.96 ppm, and in the FTIR spectrum, the νNH and the νC=O were observed at 3167 cm–1 and 1621 cm–1, respectively.

Three Co(III) pincer complexes of the general formula [Co(L)I2(solv)]& sdot;(solv)n (solv = CH3CH2OH (n = 0) or H2O (n = 1) and L = bis-N-(2,5-dimethoxyphenyl)pyridine-2,6-dicarbothioamide (L1), N-(2,5-dimethoxyphenyl)-6[(2,5-dimethoxyphenyl)carbamothioyl]pyridine-2-carboxamide (L2), and 6-(4,7-dimethoxy-2-benzothiazolyl)-N(2,5-dimethoxyphenyl)-2-pyridinecarbothioamide (L3); were prepared by oxidation of the corresponding Co(II) species using iodine in ethanol to give the Co(III) pincer complexes, 1-3. The Co(III) complexes were shown to be active electrocatalysts for hydrogen evolution reaction at an unmodified glassy carbon electrode in acetonitrile, with Faradaic efficiencies in the range 80-88%, and overpotentials between 490 and 670 mV with acetic acid as the proton source. Faradaic efficiencies ranging from 35 to 65% and overpotentials between 660 and 700 mV were obtained with p-toluene sulfonic acid monohydrate as the proton source. The potential for light-driven hydrogen production of the catalysts were evaluated under blue light (445 nm, 44 mW) under photocatalytic conditions using acetonitrile as solvent, triethanolamine as a sacrificial electron donor, tetrafluoroboric acid (HBF4, aqueous solution 48%) as a proton donor and ruthenium complex [RuII(bpy)3](PF6)2 as a photosensitizer (PS). Complex 1, which had a kappa 3-SNS coordination of the ligand, showed the best results, achieving a similar efficiency when compared to the [Co(dmgH)(dmgH2)Cl2] reference (within ca 90% of the TON and TOF) with a TON of up to 232 molH2.molcat-1 and a TOF of up to 13,880 mmolH2.molcat-1.min-1. Complexes 2 and 3 having kappa 3ONS and kappa 3-SNN gave similar results, achieving TON ca. 130 molH2.molcat-1 and TOF ca. 11,000 mmolH2.molcat min-1, respectively. Complex 1 was able to produce hydrogen over a longer period, suggesting that the kappa 3-SNS coordination mode improved the lifetime of the Co(III) and the transient species generated in the catalytic cycle.

Six Co(II) pincer complexes of the general formula [Co(Ln)(OAc)x(H2O)]& sdot;yH2O (x = 1 or 2, y = 0-2) and L = bis-N-(4-chlorophenyl)pyridine-2,6-dicarbothioamide (L1), bis-N-(2,5-dimethoxyphenyl)pyridine-2,6-dicarbothioa-mide (L2), N-(2,5-dimethoxyphenyl)-6-[(2,5-dimethoxyphenyl)carbamothioyl]pyridine-2-carboxamide (L3), 6-(6-chloro-1,3-benzothiazol-2-yl)-N-(4-chlorophenyl)pyridine-2-carbothioamide (L4), 6-(4,7-dimethoxy-2-benzo-thiazoyl)-N-(2,5-dimethoxyphenyl)-pyridinecarboxamide (L5) and 6-(4,7-dimethoxy-2-benzothiazolyl)-N-(2,5-dimethoxyphenyl)-2-pyridinecarbothioamide (L6) were prepared by refluxing Co(OAc)2 & sdot;4H2O with Ln in refluxing ethanol. Their ability as electrocatalyst for the hydrogen evolution reaction (HER) from acidified (acetic or trifluoroacetic acid) CH3CN solutions were investigated at an unmodified glassy carbon electrode. The Co(II) complexes of L1-L6 displayed Faradaic efficiencies in the range 67-92 % and overpotentials between 510-760 mV with acetic acid as the proton source, and 43-95 % Faradaic efficiencies and overpotentials 490 to 760 mV with trifluoroacetic acid as the proton source. Foot of the wave analysis (FOWA) suggests that the mechanism for the HER followed an ECEC mechanism where the CoII-H is the active species.

Six copper(II) pincer complexes of kappa 3-SNS, ONS, SNN or ONN coordination modes were investigated as electrocatalysts for the hydrogen evolution reaction (HER) in dimethylformamide using acetic acid and trifluoroacetic acid as proton sources. The copper(II) complexes of general formula [Cu(Ln)(OAc)]center dot zH2O (z = 0-3) and Ln = bis-N-(4-chlorophenyl)pyridine-2,6-dicarbothioamide (L1), bis-N-(2,5-dimethoxyphenyl)pyridine-2,6-dicar-bothioamide (L2), N-(2,5-dimethoxyphenyl)-6-[(2,5-dimethoxyphenyl)carbamothioyl]pyridine-2-carboxamide (L3), 6-(6-chloro-1,3-benzothiazol-2-yl)-N-(4-chlorophenyl)pyridine-2-carbothioamide (L4), 6-(4,7-dimethoxy-2benzothiazoyl)-N-(2,5-dimethoxyphenyl)-pyridinecarboxamide (L5) and 6-(4,7-dimethoxy-2-benzothiazolyl)-N-(2,5-dimethoxyphenyl)-2-pyridinecarbothioamide (L6), were prepared by refluxing Cu(OAc)2 center dot H2O with Ln in ethanol. The copper complexes with the kappa 3-SNS coordination mode showed the highest catalytic enhancement and Faradaic efficiencies. The results also suggested that the methoxy substituents were slightly advantageous to the chloro substituent. Moderate overpotentials between 0.69 and 0.83 V at Faradaic yields between 77 and 97 % were obtained for the Cu(II) complexes in acetic acid. Lower overpotentials within the range of 0.57 and 0.73 V were obtained at Faradaic yields between 86 and 98 % when trifluoroacetic acid was used as the proton source. Rate constants were extracted from foot-of-the-wave analysis (FOWA) plots where an EECC mechanism is proposed for the catalytic formation of H2.

The synthesis, X-ray crystallographic, and spectroscopic studies of N-(4-chlorophenyl)-6-[(4-chlorophenyl)carbamothioyl]pyridine-2-carboxamide (Cl-pcta) grown from EtOAc/ hexane (1:8) is reported. The compound crystallized in a monoclinic space group P 21/c with a = 9.8905(4) Å, b = 21.5491(9) Å, c = 8.9818(4) Å, β = 111.1277(13)° and V = 1785.62(13) Å3 with Z = 4. The structure of Cl-pcta is pseudo-planar with the Cl-aryl rings twisted in opposite directions out of the plane of the pyridyl ring. The packing of the molecules were stabilized by classical hydrogen bonding between NH and C = O/C = S groups. In the 1H NMR the NH(C = O) and the NH(C = S) where observed at 9.44 and 11.19 ppm, respectively. In the IR spectrum, the NH(C = O) and the NH(C = S) were also resolved and observed at 3216 and 3302 cm–1, respectively; coherent with the asymmetrical nature of the molecule.

Pincer complexes have aided in the understanding of a number of processes mediated by transition metal compounds. In homogenous catalytic transformations, pincers as ligands provide enhanced chemical and thermal stability which serve to minimize the leaching of the metal during the catalytic cycle. The electronic and the steric properties of pincer ligands can be independently fine-tuned, thereby increasing the scope of their applications. Pincer complexes are being explored as tools in the quest for solving the global energy crisis and methods of slowing climate change viz sequestering CO2. In this review, the complexes are organized according to the coordinating atoms from the pincer ligand. We will explore some of the progress in made in recent years towards photo- and electro-catalytic hydrogen evolution reaction (HER), and the carbon dioxide reduction reaction (CO2RR) by pincer complexes. The review focuses on reports within the last 12 years, and, where possible, emphasises how modifications to the ligand framework (or reaction conditions) impacts the efficacy of the resulting pincer complexes.

Hydrazones have been studied for a myriad of chemical and physiochemical properties, such as sensors, chelators and numerous biological activities. Experimental data indicates that hydrazones are unstable under cathodic potentials irrespective of the solvent. The single electron reduction of hydrazones to produce radical anions result in unstable species that cleaves at the N–N bond in a heterolytic manner. The literature has proposed a mechanism favouring the radical on the imine moiety, however in this study DFT calculations suggest the radical on the amine product is more likely upon bond cleavage. This has implications on electrochemical mechanisms, and the active molecule in biological studies viz the method of delivery to target areas. Density functional theory calculations were carried out using the GAMESS software package. The structures were optimized in the gas phase (B3LYP/6-31G(d,p)) as indicated by the absence of imaginary frequencies in the Hessian, and in CH3CN (B3LYP/6-31G(d,p)/SMD) with the Pople polarization functions. As a comparison, selected pathways were fully optimized using PBE0/6-31G(d,p) and PBE0/6-31G(d,p)/SMD for gas phase and CH3CN, respectively with the Pople polarization functions. The values were not significantly different (< 5

The ability of six pincer type molecules (di-2-pyridyl ketone acetic acid hydrazone (1), N-(2,5-dimethoxyphenyl)-6-[(2,5-dimethoxyphenyl)carbamothioyl]pyridine-2-carboxamide (2), 6-(4,7-dimethoxy-2-benzothiazolyl)-N-(2,5-dimethoxyphenyl)-2-pyridinecarboxamide (3), di-2-pyridyl ketone thiosemicarbazone (4), bis-N-(2,5-dimethoxyphenyl)pyridine-2,6-dicarbothioamide (5), and 6-(4,7-dimethoxy-2-benzothiazolyl)-N-(2,5-dimethoxyphenyl)-2-pyridinecarbothioamide (6)), to detect MBr2 (where M=Co, Ni, and Cu) in DMSO was quantified via formation constants determined from spectrophotometric and voltammetric titrations. The limit of detection (from electronic spectroscopy) of 1-6 ranged from 0.9-9.5 ppm. The values of beta (=log K) for the six species were found to be in the ranges of 4.3-4.9 and 4.0-6.3 for Co(II), 3.9-5.5 and 3.5-4.3 for Ni(II), and 4.0-4.3 and 3.7-5.0 for Cu(II) by UV-visible and voltammetric methods, respectively. The results suggested that kappa(3)-NNS (6) and the kappa(3)-NNO (3) had good affinity for Ni(II) and Cu(II), whereas the kappa(3)-SNS (5) had greater affinities for Co(II) and Ni(II). The hydrazones (kappa(3)-NNO, 1 and kappa(3)-NNS, 4) were generally in the middle of the pack, displaying good affinity for the three metal ions. In a contrasting vein, the kappa(3)-ONS species (2) was generally the lowest across the pincers. DFT calculations (PBE0/6-31G(d,p)/SMD) suggested that stable complexes of coordination numbers=4 to 6 were plausible in solution.

The formation constant for interaction of six pincer type molecules (di-2-pyridyl ketone acetic acid hydrazone (dpkaah, 1 ), N -(2,5-dimethoxyphenyl)-6-[(2,5-dimethoxyphenyl)carbamothioyl]pyridine-2-carboxamide (pcta, 2 ), 6-(4,7-dimethoxy-2-benzothiazolyl)- N -(2,5-dimethoxyphenyl)-2-pyridinecarboxamide (pbca, 3 ), di-2-pyridyl ketone thiosemicarbazone (dpktsc, 4 ), bis- N -(2,5-dimethoxyphenyl)pyridine-2,6-dicarbothioamide (pdcta, 5 ), and 6-(4,7-dimethoxy-2-benzothiazolyl)- N -(2,5-dimethoxyphenyl)-2-pyridinecarbothioamide (pbcta, 6 )), with MBr 2 (where M = Co, Ni, and Cu) were studied in DMSO via spectrophotometric and voltammetric titrations, and DFT calculations. The limit of detection (from electronic spectroscopy) was found to range between 1.2-6.5 ppm for the hydrazonic species 1 and 4 , and between 0.9-9.5 ppm for the other species. The formation constants ( β = log K ) for all the pincers are generally comparable, and ranged from 4.3-4.9 and 4.0-6.3 for Co(II), 3.9-5.5 and 3.5-4.3 for Ni(II), and 4.0-4.3 and 3.7-5.0 for Cu(II) by UV-visible and voltammetric methods, respectively. The measurements and calculation suggests that the thiazole containing species 6 (κ 3 - NNS) and 3 (κ 3 - NNO) have good affinity for Ni(II) and Cu(II), whereas the dithioamide 5 (κ 3 - SNS) has greater affinities for Co(II) and Ni(II). The hydrazones (κ 3 -NNO, 1 and κ 3 -NNS, 4 ) were generally in the middle of the pack, possessing good affinity for the three metal ions. In a contrasting vein, the κ 3 - ONS species ( 2 ) was generally the lowest across the pincers. DFT calculations (PBE0/6-31G(d)/SMD) suggest that stable complexes of coordination numbers (C.N.=) 4-6 are plausible in solution for the complexes formed in situ between 1 - 6 and Co(II), Ni(II), and Cu(II).

fac-[Re(CO)(3)(kappa(2)-N-im,S-bptsc)Cl] (2), isolated from the reaction between Re(CO) 5 Cl and benzophenone thiosemicarbazone, bptsc, (1) in refluxing toluene in air, is the first Re compound of 1 and is the second Re(CO)(3)Cl compound of kappa(2)-N-im,S-coordinated ligand (im = imine). The authenticity of 2 was established from the results of its elemental composition, spectroscopic measurements, and X-ray crystallographic analysis. Single crystals of 2 grown from DMF are in the monoclinic space group P2(1)/c. The asymmetric unit of 2 revealed pseudo-octahedral coordination about Re. Two carbonyl C atoms, an imine N atom and a thione S atom occupy the equatorial sites and the axial sites are occupied by a carbonyl C atom and a Cl atom. The kappa(2)-N,S-coordination of 1 to Re forms a semi-planar five-membered [Re-N-N-C-S] metallocyclic ring. The extended structure disclosed stacks of molecules interlocked via a web of N-H center dot center dot center dot X (X = Cl or O), S center dot center dot center dot S and C-H center dot center dot center dot pi interactions. DFT calculations divulged facile delocalization of electron density in the molecule and interatomic distances and angles in good agreement with the solid state structure. Electrochemical measurements on 2 in CH3CN and DMF revealed sequential irreversible electron transfers pointing to structural changes due to electrochemically induced thione-thiol tautomerization of the thioamide moiety. Plausible mechanisms for the oxidative and reductive electrochemical decomposition of 1 and 2 are reported. The proposed mechanisms are in good agreement with those reported in the literature for closely related compounds. (c) 2021 Elsevier B.V. All rights reserved.

[Co(tpy)(phen)Cl](PF6)2?0.25CH3CN (where tpy = 2,2?;6?,2?-terpyridine and phen = 1,10-phenanthroline) was prepared from a one pot mixture involving stoichiometric quantities of tpy and phen. The structure of [Co(tpy) (phen)Cl](PF6)2?0.25CH3CN was confirmed by elemental analysis, high resolution mass spectroscopy (HRMS), various spectroscopic analyses, and X-ray crystallography. Density functional theory calculations were also carried out. The crystal structure of [Co(tpy)(phen)Cl](PF6)2?0.25CH3CN, which was grown from acetonitrile, revealed a monoclinic crystal system with a C2/c space group. The cyclic voltammogram which was acquired in acetonitrile revealed reversible CoIII/II, CoII/I, and CoI/0 mixed with ligand-based redox couples at E1/2 = +0.35, ?0.81, and ?1.37 V (vs Ag/AgCl), respectively. In the presence of p-cyanoanillinium tetrafluoroborate with acetonitrile as the solvent, [Co(tpy)(phen)Cl](PF6)2?0.25CH3CN displayed electrocatalytic hydrogen evolution activity at a 830 mV overpotential, as evidenced by a catalytic wave which was observed in the voltammogram, and by the detection of hydrogen in the headspace of the reaction vessel of a controlled potential electrolysis experiment. Photocatalytic hydrogen evolution studies with [Co(tpy)(phen)Cl](PF6)2?0.25CH3CN produced a turnover frequency (TOF) of 3300 mmol H2 mol- 1 CAT min-1 when compared to [Co(dmgH)2(py)Cl] (where dmgH = dimethylglyoximato), which had a TOF of 4500 mmol H2.mol- 1 CAT min-1 under the same conditions. [Co(tpy)(phen)Cl](PF6)2?0.25CH3CN produced a turnover number (TON) of 79 when compared to 141 for [Co (dmgH)2Cl(py)] in DMF in ca 3 h.

Hypertension has been for decades a major contributor to overall mortality and morbidity worldwide. This study was aimed towards evaluating changes in blood pressure in normotensive and L-NAME hypertensive induced rats subjected to treatment with a synthesized semicarbazone copper complex (CSCZ)**. CSCZ was synthesized from the reaction between di-2-pyridylketone semicarbazone (dpksc) and CuCl2.2H2O in refluxing ethanol and characterized from its spectroscopic and elemental analyses. The infrared and electronic absorption spectra are in accord with the proposed formulation of the complex. Animals were rendered hypertensive by oral administrations of Nω-nitro-L-arginine methyl ester (L-NAME) daily (dose 40 mg/kg body weight) for 5 weeks. Oral daily dosages (20 mg/kg body weight) of CSCZ was administered to the animals. A significant decrease in systolic blood pressure (SBP) at 132.91 ± 7.62 was observed in the L-NAME/CSCZ treated group (LCZ) when compared to the L-NAME group at 161.00 ± 5.34 while no significant changes were observed in heart rate (HR) and pulse pressure (PP). Treatment with CSCZ also seemingly normalised and prevented the development of L-NAME induced hypertension in the rats. A significant decrease in SBP and PP was also seen in the normotensive group treated with CSCZ at 91 ± 2.11 and 21.67 ± 1.43 respectively when compared to the control group at 130.63 ± 2.98 and 44.10 ± 5.15 respectively. These findings reveal the blood pressure lowering effects of CSCZ in normotensive rats and its ability to prevent L-NAME induced hypertension.

Bis-N-(2,5-dimethoxyphenyl)pyridine-2,6-dicarbothioamide (pdcta) and N-(2,5- dimethoxypheny1)-6-1(2,5-dimethoxyphenyecarbamothioyllpyridine-2-carboxamide (pcta) were synthesized from a one pot mixture as pincer ligands. Crystals of pcta, grown from CDCl3 were structurally characterized, and found to crystallize in the space group P2(1)/c. The Co(II) complexes [Co(II)((kappa(3)-SNS-pdcta)(CH3CO2 )(H2O)] (1) and [Co(II)(kappa(3)-ONS-pcta) (CH3CO2 )(H2O)]center dot H2O (2) were prepared from the reaction of pdcta and pcta and Co(CH3CO2)(2)center dot H2O in ethanol at reflux under argon atmosphere. The identities of 1 and 2 were confirmed from their elemental analyses, ESI MS and a series of spectroscopic measurements. Both complexes displayed electrocatalytic hydrogen evolution with p-toluene sulfonic acid monohydrate in the presence and absence of bpy or PPh3 co-ligands. The hydrogen evolution occurred at moderate overpotentials (605-780) mV, with 1 giving better Faradaic efficiencies than 2 under all the conditions explored herein. Thermodynamic estimates, based on the reduction potentials, suggest that the homolytic pathway for the production of hydrogen from the Co-III-hydride species generated in the presence of the proton source, is more favourable than the heterolytic pathway in CH3CN.

The reaction between di-2-thienyl ketone thiosemicarbazone, dtktsc, (1) and Zn(OAc)2·2H2O in ethanol produced [Zn(κ2-Nim,S-dtktsc-H)2] (2). When PdCl2(CH3CN)2 was used in place Zn(OAc)2·2H2O novel [Pd(κ4-C,Nim,(μ-S),(μ-S)-dtktsc-2H)]4 (3) was isolated. These reactions are in contrast with those reported for the synthesis of [Zn(κ3-Npy,Nim,S-dpktsc-H)2] and [PdCl(κ3-Npy,Nim,S),PdCl2(κ2-Npy,Nam)-dpktsc-H)] • CH3CN when di-2-pyridyl ketone thiosemicarbazone (dpktsc) was used in place of 1. The identities of 2 and 3 were established from results of their elemental compositions, spectroscopic and electrochemical properties. The κ2-Nim,S-coordination of the imide deprotonated dtktsc (dtktsc-H)– to the Zn(II) ion was established from 13C and 1H NMR measurements. X-ray structural analysis on a single crystal of 3 • dmf confirmed the κ4-Nim,C,(μ-S),(μ-S)-coordination of amide deprotonated and thienyl metallated dtktsc (dtktsc-2H)2- to the Pd(II) ion. In acidic or basic media 2 suffers dissociation, while 3 is stable under similar conditions. Electrochemical measurements on dmf solutions of 2 and 3, along with electrochemical reactions of dtktsc with Zn(OAc)2·2H2O and PdCl2(CH3CN)2 revealed facile coordination of dtktsc to the metal ions. The palladacyclic 3 shows good electro-catalytic behavior toward proton reduction and moderate catalytic activity toward CC cross-coupling reactions.

The reaction between di-2-thienyl ketone thiosemicarbazone, dtktsc, (1) and Zn(OAc)(2)center dot 2H(2)O in ethanol produced [Zn(kappa(2)-N-im,S-dtktsc-H)(2)] (2). When PdCl2(CH3CN)(2) was used in place Zn(OAc)(2)center dot 2H(2)O novel [Pd(kappa(4)-C,N-im, (mu-S),(mu-S)-dtktsc-2H)](4) (3) was isolated. These reactions are in contrast with those reported for the synthesis of [Zn(kappa(3)-N-py,N-im,S-dpktsc-H)(2)] and [PdCl(kappa(3)-N-py,N-im,S),PdCl2(kappa(2)-N-py,N-am)-dpktsc-H)] center dot CH3CN when di-2-pyridyl ketone thiosemicarbazone (dpktsc) was used in place of 1. The identities of 2 and 3 were established from results of their elemental compositions, spectroscopic and electrochemical properties. The kappa(2)-N-im,S-coordination of the imide deprotonated dtktsc (dtktsc-H)- to the Zn(II) ion was established from C-13 and H-1 NMR measurements. X-ray structural analysis on a single crystal of 3 center dot dmf confirmed the kappa(4)-N-im,C,(mu-S),(mu-S)-coordination of amide deprotonated and thienyl metallated dtktsc (dtktsc-2H)(2-) to the Pd(II) ion. In acidic or basic media 2 suffers dissociation, while 3 is stable under similar conditions. Electrochemical measurements on dmf solutions of 2 and 3, along with electrochemical reactions of dtktsc with Zn(OAc)(2)center dot 2H(2)O and PdCl2(CH3CN)(2) revealed facile co-ordination of dtktsc to the metal ions. The palladacyclic 3 shows good electro-catalytic behavior toward proton reduction and moderate catalytic activity toward C-C cross-coupling reactions.

The reaction between di-2-thienyl ketone thiosemicarbazone, dtktsc, (1) and Zn(OAc)2·2H2O in ethanol produced [Zn(κ2-Nim,S-dtktsc-H)2] (2). When PdCl2(CH3CN)2 was used in place Zn(OAc)2·2H2O novel [Pd(κ4-C,Nim,(μ-S),(μ-S)-dtktsc-2H)]4 (3) was isolated. These reactions are in contrast with those reported for the synthesis of [Zn(κ3-Npy,Nim,S-dpktsc-H)2] and [PdCl(κ3-Npy,Nim,S),PdCl2(κ2-Npy,Nam)-dpktsc-H)] • CH3CN when di-2-pyridyl ketone thiosemicarbazone (dpktsc) was used in place of 1. The identities of 2 and 3 were established from results of their elemental compositions, spectroscopic and electrochemical properties. The κ2-Nim,S-coordination of the imide deprotonated dtktsc (dtktsc-H)– to the Zn(II) ion was established from 13C and 1H NMR measurements. X-ray structural analysis on a single crystal of 3 • dmf confirmed the κ4-Nim,C,(μ-S),(μ-S)-coordination of amide deprotonated and thienyl metallated dtktsc (dtktsc-2H)2- to the Pd(II) ion. In acidic or basic media 2 suffers dissociation, while 3 is stable under similar conditions. Electrochemical measurements on dmf solutions of 2 and 3, along with electrochemical reactions of dtktsc with Zn(OAc)2·2H2O and PdCl2(CH3CN)2 revealed facile coordination of dtktsc to the metal ions. The palladacyclic 3 shows good electro-catalytic behavior toward proton reduction and moderate catalytic activity toward CC cross-coupling reactions.

A ruthenium(II) complex of 6-(4,7-dimethoxy-2-benzothiazolyl)-N-(2,5-dimethoxyphenyl)-2-pyridinecarbothioamide (pbcta), of the formula [Ru(pbcta)Cl-2(dmf)] (1, where DMF = dimethyl form-amide) was prepared from RuCl3 center dot xH(2)O and pbcta in DMF at reflux under argon atmosphere. The identity of 1 was confirmed from its elemental analysis, ESI MS, and a series of spectroscopic measurements. Voltammetric measurements on 1 in DMF and DFT studies on the structure optimized in the gas phase revealed predominantly ligand based electron transfer processes under argon. In the presence of a proton source, proton coupled electron transfer to the ligand occurs. Under a carbon dioxide atmosphere, voltammetric studies revealed that 1 is inactive for CO2 reduction, and the redox responses observed in the presence of the proton source and/or CO2 are ligand based leading to reactions with the coordinated pbcta. Transfer hydrogenation (TH) of aryl ketones was efficiently carried out in 2-propanol using 1 at reflux. TH of the aryl ketone substrates proceeded in air with almost quantitative conversions at 0.2-1.0 mol% catalyst. (C) 2020 Elsevier B.V. All rights reserved.

Two mixed-ligand cobalt(III) complexes containing 2,2′-bipyridine (bpy) or 1,10-phenanthroline (phen), and N-(3,5-bis(trifluoromethyl)phenyl)pyridine-2-carbothioamide (PCA-(CF3)2) as ligands were synthesized, and characterized by a variety of spectroscopic techniques and elemental analyses. Crystals of the PCA-(CF3)2 molecule are of the monoclinic, C2/c crystal system and space group as determined via X-ray crystallography. The voltammetric properties of six cobalt(III) complexes, [Co(bpy)2Cl2]Cl 1, [Co(phen)2Cl2]Cl 2, [Co(bpy)2(PCA-(CF3)2)](PF6)2·H2O 3, [Co(phen)2(PCA-(CF3)2)](PF6)2·1.25H2O 4, [Co(bpy)3](PF6)3 5, and [Co(phen)3](PF6)3 6, were compared to assess the potential influence of the PCA-(CF3)2 ligand on the effectiveness of cobalt in the catalytic hydrogen evolution reaction (HER) under electrochemical and photochemical conditions. The PCA-(CF3)2 moiety caused an anodic shift in the reduction potential of CoII/I redox couple of complexes 3 and 4, in comparison to the other complexes. Complexes 3 and 4 demonstrated catalytic HER in the presence of p-cyanoanilinium tetrafluoroborate in CH3CN, with overpotentials for the HER of 730 and 630 mV for complexes 3 and 4, respectively. The reduction potentials suggest that the HER was most likely facilitated by a homolytic pathway. Complexes 3 and 4 also demonstrated photocatalytic HER in the presence of [Ru(bpy)3](PF6)2 as a photosensitizer and triethanolamine as a sacrificial reductant in DMF. Complexes 3 and 4 attained rates of 2700 mmol H2.mol−1 CAT min−1 and 2600 mmol H2 mol−1 CAT min−1, respectively, compared to a standard complex [Co(dmgH)2(py)Cl] which had a rate of 4500 mmol H2 mol−1 CAT min−1, under similar conditions. Turnover numbers (TON) of 140 were observed for [Co(dmgH)2(py)Cl] and complex 4, compared to 91 for complex 3, over a 3 h period.