Dinuclear Cobalt Complexes Research Articles

Modulating the Microenvironments of Robust Metal Hydrogen-Bonded Organic Frameworks for Boosting Photocatalytic Hydrogen Evolution.

Hydrogen-bonded organic frameworks (HOFs) are outstanding candidates for photocatalytic hydrogen evolution. However, most of reported HOFs suffer from poor stability and photocatalytic activity in the absence of Pt cocatalyst. Herein, a series of metal HOFs (Co2-HOF-X, X=COOMe, Br, tBu and OMe) have been rationally constructed based on dinuclear cobalt complexes, which exhibit exceptional stability in the presence of strong acid (12 M HCl) and strong base (5 M NaOH) for at least 10 days. More impressively, by varying the -X groups of the dinuclear cobalt complexes, the microenvironment of Co2-HOF-X can be modulated, giving rise to obviously different photocatalytic H2 production rates, following the -X group sequence of -COOMe>-Br>-tBu>-OMe. The optimized Co2-HOF-COOMe shows H2 generation rate up to 12.8 mmol g-1 h-1 in the absence of any additional noble-metal photosensitizers and cocatalysts, which is superior to most reported Pt-assisted photocatalytic systems. Experiments and theoretical calculations reveal that the -X groups grafted on Co2-HOF-X possess different electron-withdrawing ability, thus regulating the electronic structures of Co catalytic centres and proton activation barrier for H2 production, and leading to the distinctly different photocatalytic activity.

Tuning Valence Tautomerism in Dinuclear Cobalt Complexes by Modulating Communication in the Bridging Ligand

AbstractThe capability of bis(dioxolene) ligands to access multiple redox states makes them ideal candidates to tune the electronic properties of metal complexes, for example to achieve valence tautomerism (VT). In this study, a family of dinuclear cobalt complexes have been isolated with the bridging bis(dioxolene) thean− ligand in the cat2−‐cat2−, cat2−‐SQ⋅− and SQ⋅−‐SQ⋅− states (cat2−=catecholate, SQ⋅−=semiquinonate): [{CoIII(tpa)}2(theacat−cat)](PF6)2 (1), [{CoIII(tpa)}2(theacat−SQ)](PF6)3 (2), and [{CoIII(tpa)}2(theaSQ−SQ)](PF6)4 (3) (theaH4=2,3,6,7‐tetrahydroxy‐9,10‐dimethyl‐9,10‐dihydro‐9,10‐ethanoanthracene, tpa=tris(2‐pyridylmethyl)amine). Multi‐technique analysis confirms that 1, 2 and 3 adopt low spin‐CoIII containing {CoIII‐cat‐cat‐CoIII}, {CoIII‐cat‐SQ‐CoIII} and {CoIII‐SQ‐SQ‐CoIII} states, respectively. Compound 1 undergoes thermally‐induced {CoIII‐cat‐cat‐CoIII} ⇌ {CoIII‐cat‐SQ‐CoII} VT in the solid‐ and solution‐states above 300 K, involving high spin‐CoII. This interconversion is contrary to expectations, as tpa typically stabilizes low spin‐CoIII‐catecholate. Compound 2 is mixed‐valence class II/II–III, indicating a localized electronic structure, with electron transfer faster than the EPR and solvent rearrangement timescales, but slower than the infrared timescale. Compound 3 exhibits strong antiferromagnetic exchange. The overlap between the dioxolene π‐orbitals in thean− increases the accessibility of the cat2−‐SQ⋅− state, resulting in VT for 1. This study demonstrates that thean− can be isolated in multiple oxidation states in metal complexes, which is promising for applications in magnetic materials.

Modulating the Microenvironments of Robust Metal Hydrogen‐Bonded Organic Frameworks for Boosting Photocatalytic Hydrogen Evolution

AbstractHydrogen‐bonded organic frameworks (HOFs) are outstanding candidates for photocatalytic hydrogen evolution. However, most of reported HOFs suffer from poor stability and photocatalytic activity in the absence of Pt cocatalyst. Herein, a series of metal HOFs (Co2‐HOF‐X, X=COOMe, Br, tBu and OMe) have been rationally constructed based on dinuclear cobalt complexes, which exhibit exceptional stability in the presence of strong acid (12 M HCl) and strong base (5 M NaOH) for at least 10 days. More impressively, by varying the ‐X groups of the dinuclear cobalt complexes, the microenvironment of Co2‐HOF‐X can be modulated, giving rise to obviously different photocatalytic H2 production rates, following the −X group sequence of −COOMe>−Br>−tBu>−OMe. The optimized Co2‐HOF‐COOMe shows H2 generation rate up to 12.8 mmol g−1 h−1 in the absence of any additional noble‐metal photosensitizers and cocatalysts, which is superior to most reported Pt‐assisted photocatalytic systems. Experiments and theoretical calculations reveal that the −X groups grafted on Co2‐HOF‐X possess different electron‐withdrawing ability, thus regulating the electronic structures of Co catalytic centres and proton activation barrier for H2 production, and leading to the distinctly different photocatalytic activity.

A Planar-Structured Dinuclear Cobalt(II) Complex with Indirect Synergy for Photocatalytic CO2-to-CO Conversion.

Dinuclear metal synergistic catalysis (DMSC) has been proved an effective approach to enhance catalytic efficiency in photocatalytic CO2 reduction reaction, while it remains challenge to design dinuclear metal complexes that can show DMSC effect. The main reason is that the influence of the microenvironment around dinuclear metal centres on catalytic activity has not been well recognized and revealed. Herein, we report a dinuclear cobalt complex featuring a planar structure, which displays outstanding catalytic efficiency for photochemical CO2-to-CO conversion. The turnover number (TON) and turnover frequency (TOF) values reach as high as 14457 and 0.40 s-1 respectively, 8.6 times higher than those of the corresponding mononuclear cobalt complex. Control experiments and theoretical calculations revealed that the enhanced catalytic efficiency of the dinuclear cobalt complex is due to the indirect DMSC effect between two CoII ions, energetically feasible one step two-electron transfer process by Co2 I,I intermediate to afford Co2 II,II(CO2 2-) intermediate and fast mass transfer closely related with the planar structure.

A Planar‐Structured Dinuclear Cobalt(II) Complex with Indirect Synergy for Photocatalytic CO2‐to‐CO Conversion

Dinuclear metal synergistic catalysis (DMSC) has been proved an effective approach to enhance catalytic efficiency in photocatalytic CO2 reduction reaction, while it remains challenge to design dinuclear metal complexes that can show DMSC effect. The main reason is that the influence of the microenvironment around dinuclear metal centres on catalytic activity has not been well recognized and revealed. Herein, we report a dinuclear cobalt complex featuring a planar structure, which displays outstanding catalytic efficiency for photochemical CO2‐to‐CO conversion. The turnover number (TON) and turnover frequency (TOF) values reach as high as 14457 and 0.40 s‐1 respectively, 8.6 times higher than those of the corresponding mononuclear cobalt complex. Control experiments and DFT calculations revealed that the enhanced catalytic efficiency of the dinuclear cobalt complex is due to the indirect DMSC effect between two CoII ions, energetically feasible one step two‐electron transfer process by Co2I,I intermediate to afford Co2II,II(CO22‐) intermediate and fast mass transfer closely related with the planar structure.

Tuning valence tautomerism in a family of dinuclear cobalt complexes incorporating a conjugated bridging bis(dioxolene) ligand with weak communication.

Valence tautomerism (VT) involves the stimulated reversible intramolecular electron transfer between a redox-active metal and ligand. Dinuclear cobalt complexes bridged by bis(dioxolene) ligands can undergo thermally-induced VT with access to {CoIII-cat-cat-CoIII}, {CoIII-cat-SQ-CoII} and {CoII-SQ-SQ-CoII} states (cat2- = catecholate, SQ˙- = semiquinonate, CoIII refers to low spin CoIII, CoII refers to high spin CoII). The resulting potential for two-step VT interconversions offers increased functionality over mononuclear examples. In this study, the bis(dioxolene) ligand 3,3',4,4'-tetrahydroxy-5,5'-dimethoxy-benzaldazine (thMH4) was paired with Mentpa (tpa = tris(2-pyridylmethyl)amine, n = 0-3 corresponds to methylation at 6-position of the pyridine rings) to afford [{Co(Mentpa)}2(thM)](PF6)2 (1a, n = 0; 2a, n = 2; 3a, n = 3). Structural, magnetic susceptibility and spectroscopic data show that 1a and 3a remain in the temperature invariant {CoIII-cat-cat-CoIII} and {CoII-SQ-SQ-CoII} forms in the solid state, respectively. In contrast, 2a exhibits incomplete thermally-induced VT between these two tautomeric forms via the mixed {CoIII-cat-SQ-CoII} tautomer. In solution, room temperature electronic absorption spectra are consistent with the assignments from the solid-state, with VT observed only for 2a. From electrochemistry, the proximity of the two 1e--processes for the thMn- ligand indicates weak electronic communication between the two dioxolene units, supporting the potential for a two-step VT interconversion in thMn- containing complexes. Comparison of the redox potentials of the Co and thMn- processes suggests that only 2a has these processes in sufficient proximity to afford the thermally-induced VT observed experimentally. Density functional theory calculations are consistent with the prerequisite energy ordering for a two-step transition for 2a, and temperature invariant {CoIII-cat-cat-CoIII} and {CoII-SQ-SQ-CoII} states for 1a and 3a, respectively. This work presents the third example, and the first formally conjugated example, of a bridging bis(dioxolene) ligand that can afford two-step VT in a Co complex, suggesting new possibilities towards applications based on multistep switching.

106-fold faster C-H bond hydroxylation by a CoIII,IV2(µ-O)2 complex [via a CoIII2(µ-O)(µ-OH) intermediate] versus its FeIIIFeIV analog.

The hydroxylation of C-H bonds can be carried out by the high-valent CoIII,IV2(µ-O)2 complex 2a supported by the tetradentate tris(2-pyridylmethyl)amine ligand via a CoIII2(µ-O)(µ-OH) intermediate (3a). Complex 3a can be independently generated either by H-atom transfer (HAT) in the reaction of 2a with phenols as the H-atom donor or protonation of its conjugate base, the CoIII2(µ-O)2 complex 1a. Resonance Raman spectra of these three complexes reveal oxygen-isotope-sensitive vibrations at 560 to 590 cm-1 associated with the symmetric Co-O-Co stretching mode of the Co2O2 diamond core. Together with a Co•••Co distance of 2.78(2) Å previously identified for 1a and 2a by Extended X-ray Absorption Fine Structure (EXAFS) analysis, these results provide solid evidence for their "diamond core" structural assignments. The independent generation of 3a allows us to investigate HAT reactions of 2a with phenols in detail, measure the redox potential and pKa of the system, and calculate the O-H bond strength (DO-H) of 3a to shed light on the C-H bond activation reactivity of 2a. Complex 3a is found to be able to transfer its hydroxyl ligand onto the trityl radical to form the hydroxylated product, representing a direct experimental observation of such a reaction by a dinuclear cobalt complex. Surprisingly, reactivity comparisons reveal 2a to be 106-fold more reactive in oxidizing hydrocarbon C-H bonds than corresponding FeIII,IV2(µ-O)2 and MnIII,IV2(µ-O)2 analogs, an unexpected outcome that raises the prospects for using CoIII,IV2(µ-O)2 species to oxidize alkane C-H bonds.

Synthesis and Characterization of Cobalt(II/II) and Cobalt(II/III) Macrocyclic Complexes and Their Application in the Copolymerization of Epoxides and CO2

The synthesis and characterization of six dinuclear cobalt complexes bearing a conjugated macrocyclic ligand are reported. The condensation of 2,6-diformylphenol and o-phenylenediamine led to semi-hydrogenated macrocycles A–C. The ligand precursor A reacted with Co(OAc)2·4H2O and generated the Co(II/II) complex 1, and a conjugated macrocyclic ligand was formed during the complexation. The partial oxidation of 1 with air resulted in mixed-valence Co(II/III) complexes 2 and 3 bearing different anionic ligands. Analogous Co(II/III) complexes 4–6 of Cl- and F-substituted ligand backbones were also synthesized. Structural characterization and charge determination of all complexes were achieved through X-ray diffraction analysis and X-ray photoelectron spectroscopy. These complexes were applied in the copolymerization of CO2 and cyclohexene oxide (CHO), and the mixed valence Co(II/III) complex 2 showed good reactivity and selectivity (>99% polycarbonate).

New dinuclear cobalt(II) complexes as “self‐activating” chemical nuclease and inhibitor of tumor cell invasion: High selective cytotoxicity against NCI‐H460 cells

Three new dinuclear Co (II) complexes [Co2(L1)2Cl2](PF6)2 (1), [Co2L2(OAc)2(CH3OH)4](ClO4)2 (2) and [Co2L3(OAc)2(CH3OH)4](ClO4)2 (3) were synthesized and structurally determined by X-ray single crystal diffraction analysis as well as IR, elemental analysis and mass spectrometry. All complexes exhibit efficiently self-activated cleavage activity with hydrolytic mechanism, which was accurately proved by addition of regular radical scavengers, anaerobic cleavage experiment and 2-thiobarbituric acid reaction. Complex 1 exhibited higher selective cytotoxicity against tumor cells than normal cells, which is significantly better than that of cisplatin. Hoechst 33342 staining, intracellular ROS generation and mitochondrial membrane potential were conducted to assess the apoptosis-inducing activity of 1. The effect of complex 1 on cell invasion was observed by Trans-Well assay and Angiogenesis experiment, and the results demonstrated that 1 could inhibit the cell invasion and angiogenesis.

Electrocatalytic CO2 Reduction with a Binuclear Bis-Terpyridine Pyrazole-Bridged Cobalt Complex.

A pyrazole-based ligand substituted with terpyridine groups at the 3 and 5 positions has been synthesized to form the dinuclear cobalt complex 1, that electrocatalytically reduces carbon dioxide (CO2 ) to carbon monoxide (CO) in the presence of Brønsted acids in DMF. Chemical, electrochemical and UV-vis spectro-electrochemical studies under inert atmosphere indicate pairwise reduction processes of complex 1. Infrared spectro-electrochemical studies under CO2 and CO atmosphere are consistent with a reduced CO-containing dicobalt complex which results from the electroreduction of CO2 . In the presence of trifluoroethanol (TFE), electrocatalytic studies revealed single-site mechanism with up to 94 % selectivity towards CO formation when 1.47 M TFE were present, at -1.35 V vs. Saturated Calomel Electrode in DMF (0.39 V overpotential). The low faradaic efficiencies obtained (<50 %) are attributed to the generation of CO-containing species formed during the electrocatalytic process, which inhibit the reduction of CO2 .

An insight into the non-covalent interactions in the solid state structures of dinuclear cobalt(ii) complexes with N,O-donor ligands: application of the complexes in the fabrication of Schottky devices

Two new dinuclear cobalt(ii) complexes were synthesized using compartmental N,O-donor ligands. The optical properties of the complexes were studied with focus on the band gaps.

Electrochemical hydrogen evolution reaction catalysed by a dinuclear cobalt complex with doubly N-confused hexaphyrin

A dinuclear cobalt complex with a doubly N-confused hexaphyrin, Co2DNCH, showed high catalytic activity for electrochemical hydrogen reduction due to the wide π-conjugated system of DNCH.

Design and synthesis of dinuclear cobalt(ii) complexes derived from strong π-acidic ligands: crystal structure and studies on the oxidation of sp3 C–H bonds

Cobalt complexes (1 and 2) supported by strong π-acidic ligands were synthesized and characterized. These complexes were employed as catalysts for the oxidation of sp3 C–H bonds. Mechanistic pathways are proposed on the basis of experimental evidence.

Structural comparison of geometrical isomers of N'-(1H-imidazol-4-ylmethylene)picolinohydrazide and their mononuclear and dinuclear cobalt(III) complexes

The crystal structures and spectroscopic properties of both E- and Z-isomers of N'-(1H-imidazol-4-ylmethylene)-picolinohydrazide (H2Lim) were investigated. In addition, an aerobic reaction of H2Lim with Co(BF4)2 in methanol afforded a mononuclear cobalt(III) complex, [Co(Z-HLim)2]BF4 (1) bearing tridentate hydrazonate ligands, and a dinuclear complex of [Co2(μ-E-HLim)3](BF4)3 (2), where the hydrazonate bridged two CoIII centers in the μ-κ2N,N': κ2N'',N''' mode to form an C3-symmetry triple helicate complex.

Dinuclear Cobalt Complexes for Homogeneous Water Oxidation: Tuning Rate and Overpotential through the Non-Innocent Ligand.

In this study, dinuclear cobalt complexes (1 and 2) featuring bis(benzimidazole)pyrazolide-type ligands (H2 L and Me2 L) were prepared and evaluated as molecular electrocatalysts for water oxidation. Notably, 1 bearing a non-innocent ligand (H2 L) displayed faster catalytic turnover than 2 under alkaline conditions, and the base dependence of water oxidation and kinetic isotope effect analysis indicated that the reaction mediated by 1 proceeded by a different mechanism relative to 2. Spectroelectrochemical, cold-spray ionization mass spectrometric and computational studies found that double deprotonation of 1 under alkaline conditions cathodically shifted the catalysis-initiating potential and further altered the turnover-limiting step from nucleophilic water attack on (H2 L)CoIII 2 (superoxo) to deprotonation of (L)CoIII 2 (OH)2 . The rate-overpotential analysis and catalytic Tafel plots showed that 1 exhibited a significantly higher rate than previously reported Ru-based dinuclear electrocatalysts at similar overpotentials. These observations suggest that using non-innocent ligands is a valuable strategy for designing effective metal-based molecular water oxidation catalysts.

Triggering of Valence Tautomeric Transitions in Dioxolene-Based Cobalt Complexes Influenced by Ligand Substituents, Co-ligands, and Anions

We report the multistep synthesis of the 1,1′-(piperazine-1,4-diyl)bis(N,N-bis(pyridin-2-ylmethyl)methanamine)(Ltpbap) octadentate ligand, which, in combination with the known 3,5-di-tert-butylcatechol (3,5-dbcat), allowed the preparation of two di-nuclear cobalt complexes [Co2(Ltpbap)(3,5-dbcat)2](SO4)·5.5MeOH·2H2O (3a) and [Co2(Ltpbap)(3,5-dbcat)2 ](ClO4)2·1.5H2O (3b). We also report the synthesis of two mono-nuclear cobalt complexes [Co(3,5-dbsq)(3,5-dbcat)(4-Mepip)2] (1) with 4-Mepip being 4-methylpiperidine and (Hpip)[Co(tbcat)2(pip)2]·CH3CN (2) where Hpip denotes a piperidinium cation and tbcat is the tetra-bromocatechol ligand. The obtained complexes were characterized by single-crystal X-ray crystallography, SQUID magnetometry, and IR spectroscopy. The structure of the crystalline material in all the cases was determined at 173 K. The magnetic properties of all complexes were measured between 2 and 380 K. The magnetic data clearly show that mono-nuclear complex 1 and di-nuclear complex 3a exhibit valence tautomerism with onset around 300 K and 370 K, respectively, whereas the other two complexes 2 and 3b remain unchanged over the measured temperature range.

Interplay and Competition Between Two Different Types of Redox-Active Ligands in Cobalt Complexes: How to Allocate the Electrons?

The field of molecular transition metal complexes with redox-active ligands is dominated by compounds with one or two units of the same redox-active ligand; complexes in which different redox-active ligands are bound to the same metal are uncommon. This work reports the first molecular coordination compounds in which redox-active bisguanidine or urea azine (biguanidine) ligands as well as oxolene ligands are bound to the same cobalt atom. The combination of two different redox-active ligands leads to mono- as well as unprecedented dinuclear cobalt complexes, being multiple (four or six) center redox systems with intriguing electronic structures, all exhibiting radical ligands. By changing the redox potential of the ligands through derivatisation, the electronic structure of the complexes could be altered in a rational way.

Preparation of carbon dioxide, propylene oxide, and norbornene dianhydride terpolymers catalyzed via dinuclear cobalt complexes: Effective improvement of thermal, mechanical, and degradation properties

Dinuclear cobalt complex salen[Co(III)TFA]2 (salen = 3,5-di-tert-butyl- salicylaldehyde-3,3ˊ-diaminobiphenylamine, TFA = trifluoroacetic acid) was synthesized and used to catalyze the preparation of novel polycarbonate terpolymers (PPCEA) of carbon dioxide, propylene oxide and norbornene dianhydride. The results showed high catalyst reactivity under mild conditions, generating a polymer with maximum Mn of 2.3 × 105 g/mol and a narrow molecular weight distribution PDI of 1.01. The 5% thermal weight loss temperature (Td,-5%) of PPCEA can reach 310 °C, which is 150 °C higher than conventional polypropylene carbonate. In addition, its mechanical properties were also improved. The tensile strength increased from10.8 MPa to 22.8 MPa, and elongation at break decreased from 430% to 106%. The polymer has good degradability as well. Furthermore, based on the polymerization results, the corresponding polymerization mechanism is proposed in this paper.

Dinuclear cobalt complexes with an asymmetric biphenyl bridging ligand, [Co2(LFtBu)(bpqa)2](PF6)2 (H4LFtBu = 5-fluoro-5′-tert-butyl-3,3′,4,4′-tetrahydroxybiphenyl, bpqa = bis(2-pyridylmethyl)(2-quinolylmethyl)amine): Spectroscopic, electrochemical and magnetic characterization

Dinuclear cobalt complexes with an asymmetric biscatechol H4LFtBu (5-fluoro-5′-tert-butyl-3,3′,4,4′-tetrahydroxybiphenyl) ligand that has different substituent groups in both phenyl rings, and some ancillary tripod ligands were synthesized. The spectroscopic, electrochemical and magnetic properties of the complexes were compared to the electronic states of dinuclear cobalt complexes with a different ancillary tripod ligand. The result was that [Co2(LFtBu)(bpqa)2](PF6)2 (bpqa: bis(2-methylpyridyl)(2-quinolylmethyl)amine) was isolated with a mixed valence electronic state of lsCoIII-[Cat-Sq]-hsCoII (Cat: catechol, Sq: semiquinone), confirmed by the appearance of the inter-valence charge transfer (IVCT) band at around 1300 nm in the solid and by X-ray Photoelectron Spectroscopy (XPS). In this report, the effect of the substituent group in the biscatechol on the electronic states of such complexes is clarified from examination of their spectroscopic, electrochemical and magnetic properties.

Proton Relay Effects on Oxygen Reduction Reaction Catalyzed by Dinuclear Cobalt Polypyridyl Complexes with OH Groups on Bipyridine Ligands

Abstract Four-electron oxygen reduction reaction (4e−-ORR) is the foundation of both natural and artificial energy conversion systems. Mechanism studies and catalysis improvements of 4e−-ORR are important research for the actualization of a sustainable society. In this study, we present a dinuclear cobalt complex containing mono-deprotonated forms of 6,6′-dihydroxy-2,2′-bipyridine (6DHBP-H+), [Co2(OH)2(6DHBP–H+)2(btpyxa)](PF6)2 (2) is a highly active 4e−-ORR catalyst in a low acid concentration solution. When ferrocene (Fe(Cp)2) was used as a reductant in PhCN containing a low concentration of perchloric acid (1.0 mmol L−1), 2 showed higher selectivity (99%) and reaction rate (kcat = 6.0 × 103 M−1 s−1) for 4e−-ORR than the bpy analog 1 (kcat = 6.2 × 10 M−1 s−1) and 4DHBP analog 3 (kcat = 1.5 × 102 M−1 s−1). A high catalytic current in the cyclic voltammetry (CV) of 2 indicates a high reaction rate for electrochemical ORR under low acid concentration conditions. Moreover, X-ray crystallography of the corresponding monomeric analog [Co(OH2)(6DHBP–2H+)(trpy)](PF6) (4, 6DHBP–2H+: a doubly deprotonated form of 6DHBP) suggests that OH groups of 2 can form hydrogen bonds with a μ-O2 ligand. Hydroxy groups at the 6,6′-position of bpy would deliver protons to the μ-O2 ligand of the intermediate, thereby promoting O–O bond cleavage in the proton-coupled electron transfer (PCET) process.