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Cobalt complexes of the chelating dicarboxylate ligand “esp”: a paddlewheel-type dimer and a heptanuclear coordination cluster
Reaction of CoCO 3 with α,α,α′,α′-tetramethyl-1,3-benzenedipropionic acid (H 2 esp) in EtOH forms the paddlewheel-type dimer Co 2 (esp) 2 (EtOH) 2 , which hydrolyzes in MeCN to give a bowl-shaped heptacobalt coordination cluster complex; both compounds display antiferromagnetic interactions between high-spin Co( ii ) ions.
Sterically attenuated electronic communication in cobalt complexes of meridional isoquinoline-derived ligands for applications in electrocatalysis
The ability to synthetically tune the ligand frameworks of redox-active molecules is of critical importance to the economy of solar fuels because manipulating their redox properties can afford control over the operating potentials of sustained electrocatalytic or photoelectrocatalytic processes. The electronic and steric properties of 2,2′:6′,2″-terpyridine (Terpy) ligand frameworks can be tuned by functional group substitution on ligand backbones, and these correlate strongly to their Hammett parameters. The synthesis of a new series of tridentate meridional ligands of 2,4,6-trisubstituted pyridines that engineers the ability to finely tune the redox potentials of cobalt complexes to more positive potentials than that of their Terpy analogs is achieved by aryl-functionalizing at the four-position and by including isoquinoline at the two- and six-positions of pyridine (Aryl-DiQ). Their cobalt complex syntheses, their electronic properties, and their catalytic activity for carbon dioxide (CO2) reduction are reported and compared to their Terpy analogs. The cobalt derivatives generally experience a positive shift in their redox features relative to the Terpy-based analogs, covering a complementary potential range. Although those evaluated fail to produce any quantifiable products for the reduction of CO2 and suffer from long-term instability, these results suggest possible alternate strategies for stabilizing these compounds during catalysis. We speculate that lower equilibrium association constants to the cobalt center are intrinsic to these ligands, which originate from a steric interaction between protons on the pyridine and isoquinoline moieties. Nevertheless, the new Aryl-DiQ ligand framework has been engineered to selectively tune homoleptic cobalt complexes’ redox potentials.
Computational modeling of mixed-ligand cobalt complexes with o-quinone derivative of corannulene
The structure and energy characteristics of isomers of electrically neutral adducts of tetracoordinate cobalt complexes with o-quinone derivative of corannulene were studied quantum chemically within the density functional theory (DFT) in the UB3LYP*/6-311++G(d,p) approximation. It was shown that the framework rigidity of the redox-active ligand favors convergence of the energy levels of the high-spin and low-spin isomers of the complexes under study as compared to the o-benzoquinone-containing analogues. At the same time, steric strain in the corannulene moiety causes the stability of the adducts to reduce. A compound characterized by high stabilization energy and the ability to switch its spin states via valence tautomeric rearrangements was found by systematic variation of structural parameters of cobalt bis(chelate).
Electronically-coupled redox centers in trimetallic cobalt complexes
Synthesis and isolation of molecular building blocks of metal–organic frameworks (MOFs) can provide unique opportunities for characterization that would otherwise be inaccessible due to the heterogeneous nature of MOFs. Herein, we report a series of trinuclear cobalt complexes incorporating dithiolene ligands, triphenylene-2,3,6,7,10,11-hexathiolate (THT) (1 3+ ), and benzene hexathiolate (BHT) (2 3+ ), with 1,1,1,-tris(diphenylphosphinomethyl)ethane (triphos) employed as the capping ligand. Single crystal X-ray analyses of 1 3+ and 2 3+ display three five-coordinate cobalt centers bound to the triphos and dithiolene ligands in a distorted square pyramidal geometry. Cyclic voltammetry studies of 1 3+ and 2 3+ reveal three redox features associated with the formation of mixed valence states due to the sequential reduction of the redox-active metal centers (Co III/II ). Using this electrochemical data, the comproportionality values were determined for 1 and 2 (log K c = 1.4 and 1.5 for 1, and 4.7 and 5.8 for 2), suggesting strong resonance-stabilized coupling of the metal centers, with stronger electronic coupling observed for complex 2 compared to that for complex 1. Cyclic voltammetry studies were also performed in solvents of varying polarity, whereupon the difference in the standard potentials (ΔE 1/2 ) for 1 and 2 was found to shift as a function of the polarity of the solvent, indicating a negative correlation between the dielectric constant of the electrochemical medium and the stability of the mixed valence species. Spectroelectrochemical studies of in situ generated multi-valent (MV) states of complexes 1 and 2 display characteristic NIR intervalence charge transfer (IVCT) bands, and analysis of the IVCT transitions for complex 2 suggests a weakly coupled class II multi-valent species and relatively large electronic coupling factors (1700 cm –1 for the first multi-valent state of 2 2+ , and 1400 and 4000 cm –1 for the second multi-valent state of 2 + ). Here, density functional theory (DFT) calculations indicate a significant deviation in relative energies of the frontier orbitals of complexes 1 3+ , 2 3+ , and 3 + that contrasts those calculated for the analogous trinuclear cobalt dithiolene complexes employing pentamethylcyclopentadienyl (Cp*) as the capping ligand (Co 3 Cp* 3 THT and Co 3 Cp* 3 BHT, respectively), and may be a result of the cationic nature of complexes 1 3+ , 2 3+ , and 3 + .
Ambidentate and redox-properties of 4,7-phenanthroline-5,6-dione in cobalt complexes: a quantum chemical study
Mono- and dinuclear adducts of cobalt diketonates with tetradentate 4,7-phenanthroline- 5,6-dione were modeled within the framework of DFT UB3LYP*/6-311++G(d,p) approximation. A competitive coordination of the metal ion to different donor centers of the redox-active ligand was studied. Variation of substituents in the diketone moieties allowed one to reveal compounds than can undergo thermally initiated one- and two-step valence tautomeric rearrangements. The calculated energy and magnetic characteristics of the dinuclear complexes give reasons to consider them as potential basis of molecular electronics and spintronics devices.
Hydride and Seek: Comparing Crystallographic Hydride Placement Techniques with an Open-Shell Cobalt Complex
Locating hydrides is crucial in organometallic chemistry but difficult to do accurately using X-ray diffraction. Electron diffraction has been proposed as a way to overcome this problem but has not been systematically compared to neutron diffraction and to quantum crystallography (Hirshfeld atom refinement, HAR) to test this hypothesis. Here, we present a comparative analysis of methods for a terminal cobalt hydride complex by comparing a single-crystal neutron diffraction reference structure to results from single-crystal X-ray diffraction with and without Hirshfeld atom refinement (HAR, NoSpherA2), density functional theory (DFT), and electron diffraction (3D-ED/MicroED) refined under kinematical and dynamical formalisms. Conventional X-ray diffraction gives lower precision than neutron diffraction as expected. Despite expected improvements, HAR gives systematic deviation from the neutron benchmark. Interestingly, optimized DFT equilibrium geometries are closer to the neutron value than the value from HAR. On the other hand, electron diffraction with a high-quality data set coupled with dynamical refinement localizes the hydride in difference maps and gives excellent agreement with the neutron data. Dynamical refinement is crucial, as kinematical refinement does not allow assignment of a hydride peak. This cross-modal comparison defines the conditions under which 3D-ED/MicroED delivers high-precision metal–hydride distances for this open-shell cobalt hydride.
Synthesis and Crystal Structure of a Four-coordinated Cobalt Complex with 1-Methylbenzimidazole
A four-coordinated complex, L{sub 2}Co(NCO){sub 2}, (L = 1-methylbenzimidazole) has been synthesized and characterized by IR spectroscopy, elemental analyses, and single crystal X-ray diffraction. The crystal is monoclinic, a = 14.522(2) Å, b = 14.610(2) Å, c = 8.6850(13) Å, β = 92.037(3)°, Z = 4, sp. gr. P2{sub 1}/c. Bond lengths Co–N(L) are equal to 2.0165(16) and 2.0174(15) Å, Co–O(NCO) are 1.944(2) and 1.945(2) Å. Intermolecular C–H···O, C–H···N, and C–H···π interactions link the molecules into three-dimensional networks in the crystal.
Kinetic and Mechanistic Characterization of Low-Overpotential, H2O2-Selective Reduction of O2 Catalyzed by N2O2-Ligated Cobalt Complexes
A soluble, bis-ketiminate-ligated Co complex [Co(N2O2)] was recently shown to catalyze selective reduction of O2 to H2O2 with an overpotential as low as 90 mV. Here we report experimental and computational mechanistic studies of the Co(N2O2)-catalyzed O2 reduction reaction (ORR) with deca-methylferrocene (Fc*) as the reductant in the presence of AcOH in MeOH. Analysis of the Co/O2 binding stoichiometry and kinetic studies support an O2 reduction pathway involving a mononuclear cobalt species. The catalytic rate exhibits a first-order kinetic dependence on [Co(N2O2)] and [AcOH], but no dependence on [Fc*] or [O2]. Differential pulse voltammetry and computational studies support CoIII-hydroperoxide as the catalyst resting state and protonation of this species as the rate-limiting step of the catalytic reaction. These results contrast previous mechanisms proposed for other Co-catalyzed ORR systems, which commonly feature rate-limiting protonation of a CoIII-superoxide adduct earlier in the catalytic cycle. Computational studies show that protonation is strongly favored at the proximal oxygen of the CoIII(OOH) species, accounting for the high selectivity for formation of hydrogen peroxide. Further analysis shows that a weak dependence of the ORR rate on the pKa of the protonated CoIII(OOH) species across a series of Co(N2O2) catalysts provides a rationale for the unusually low overpotential observed for O2 reduction.
Linkage Effect in the Heterogenization of Cobalt Complexes by Doped Graphene for Electrocatalytic CO2 Reduction
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Redox Pairs of Diiron and Iron-Cobalt Complexes with High-Spin Ground States
Abstract not provided.
Graphite Conjugation of a Macrocyclic Cobalt Complex Enhances Nitrite Electroreduction to Ammonia
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Synthesis of four-, five-, and six-coordinate cobalt(III) bis-o-iminobenzosemiquinone complexes
Cobalt(III) complexes with different 4,6-di-tert-butyl-N-(aryl)-o-iminobenzosemiquinone ligands were synthesized. Four-, five-, or six-coordinate cobalt(III) derivatives can be prepared using various starting metal compounds (CoCl{sub 2}•6H{sub 2}O, (acac){sub 2}Co•2H{sub 2}O) and varying substituents in the N-aryl moiety of the organic ligand. The structures of five synthesized compounds were determined by X-ray diffraction. The electronic and spin states of the five-coordinate cobalt(III) complexes were studied in detail by DFT quantum chemical calculations.
Fischer-Tropsch Cobalt Catalyst Improvements with the Presence of TiO2, La2O3, and ZrO2 on an Alumina Support
The objective of this study was to evaluate the effect of titanium oxide, lanthanum oxide, and zirconium oxide on alumina supported cobalt catalysts. The hypothesis was that the presence of lanthanum oxide, titanium oxide, and zirconium oxide would reduce the interaction between cobalt and the alumina support. This was of interest because an optimized weakened interaction could lead to the most advantageous cobalt dispersion, particle size, and reducibility. The presence of these oxides on the support were investigated using a wide range of characterization techniques such as SEM, nitrogen adsorption, x-ray diffraction (XRD), temperature programmed reduction (TPR), temperature programmed reduction after reduction (TPR-AR), and hydrogen chemisorptions/pulse reoxidation. Results indicated that both La2O3 and TiO2 doped supports facilitated the reduction of cobalt oxide species in reference to pure alumina supported cobalt catalysts, however further investigation is needed to determine the effect of ZrO2 on the reduction profile. Results showed an increased corrected cluster size for all three doped supported catalysts in comparison to their reference catalysts. The increase in reduction and an increase in the cluster size led to the conclusion that the support-metal interaction weakened by the addition of TiO2 and La2O3. It is also likely that the interaction decreased upon presence of ZrO2 on the alumina, but further research is necessary. Preliminary results have indicated that the alumina-supported catalysts with titanium oxide and lanthanum oxide present are of interest because of the weakened cobalt support interaction. These catalysts showed an increased extent of reduction, therefore more metallic cobalt is present on the support. However, whether or not there is more cobalt available to participate in the Fischer-Tropsch synthesis reaction (cobalt surface atoms) depends also on the cluster size. On one hand, increasing cluster size alone tends to decrease the active site density; on the other hand, by increasing the size of the cobalt clusters, there is less likelihood of forming oxidized cobalt complexes (cobalt aluminate) during Fischer-Tropsch synthesis. Thus, from the standpoint of stability, improving the extent of reduction while increasing the particle size slightly may be beneficial for maintaining the sites, even if there is a slight decrease in overall initial active site density.
Influence of Copper Oxidation State on the Bonding and Electronic Structure of Cobalt-Copper Complexes
Heterobimetallic complexes that pair cobalt and copper were synthesized and characterized by a suite of physical methods, including X-ray diffraction, X-ray anomalous scattering, cyclic voltammetry, magnetometry, electronic absorption spectroscopy, electron paramagnetic resonance, and quantum chemical methods. Both Cu(II) and Cu(I) reagents were independently added to a Co(II) metalloligand to provide (py 3 tren)CoCuCl (1-Cl) and (py 3 tren)CoCu(CH 3 CN) (2-CH 3 CN), respectively, where py3tren is the triply deprotonated form of N,N,N-tris(2-(2-pyridylamino)ethyl)amine. Complex 2-CH 3 CN can lose the acetonitrile ligand to generate a coordination polymer consistent with the formula “(py3tren)CoCu” (2). One-electron chemical oxidation of 2-CH 3 CN with AgOTf generated (py 3 tren)CoCuOTf (1-OTf). The Cu(II)/Cu(I) redox couple for 1-OTf and 2-CH 3 CN is reversible at -0.56 and -0.33 V vs Fc + /Fc, respectively. The copper oxidation state impacts the electronic structure of the heterobimetallic core, as well as the nature of the Co–Cu interaction. Quantum chemical calculations showed modest electron delocalization in the (CoCu) +4 state via a Co–Cu σ bond that is weakened by partial population of the Co–Cu σ antibonding orbital. By contrast, no covalent Co–Cu bonding is predicted for the (CoCu) +3 analogue, and the d-electrons are fully localized at individual metals.
Heteroleptic Square Planar Cobalt(I/II) Complexes
Reduction of the cobalt(II) chloride complex, Ph 2 B( t BuIm) 2 Co(THF)Cl (1) in the presence of t BuN≡C affords the diamagnetic, square planar cobalt(I) complex Ph 2 B( t BuIm) 2 Co(C≡N t Bu) 2 (2). This is a rare example of a 16-electron cobalt(I) complex that is structurally related to square planar noble metal complexes. Accordingly, the electronic structure of 2, as calculated by DFT, reveals that the HOMO is largely d z 2 in character. Complex 2 is readily oxidized to its cobalt(II) congener [Ph 2 B( t BuIm) 2 Co(C=N t Bu) 2 ]BPh 4 (3-BPh 4 ), whose EPR spectral parameters are characteristic of low-spin d 7 with an unpaired electron in an orbital of d z 2 parentage. Further, this is also consistent with the results of DFT calculations. Despite its 16-electron configuration and the d z 2 parentage of the HOMO, the only tractable reactions of 2 involve one electron oxidation to afford 3.
Synthesis, Electronic Structure, and Reactivity of a Planar Four–Coordinate, Cobalt–Imido Complex
A four-coordinate cobalt imido complex, ( tBu mPNP)Co=NMes ( tBu mPNP = modified PNP pincer ligand) has been synthesized from addition of 2,4,6-trimethylphenylazide (Mes–N 3 ) to the corresponding dinitrogen complex. The solid-state structure determined by X-ray diffraction established a rare, idealized planar geometry with a Co=N bond distance of 1.716(2) Å. Magnetic measurements revealed an S = 1 ground state with CAS-SCF calculations supporting radical character on the imide nitrogen. Here, thermolysis of the cobalt-imido compound induced selective insertion of the imido group into a Co–P bond and yielded a three-coordinate cobalt complex with a distorted T-shaped geometry. Transition state analysis conducted with DFT calculations established the thermodynamic stability of the P–N coupled product and provided insight into the exclusive selectivity.
Synthesis, Electronic Structure, and Reactivity of a Planar Four‐Coordinate, Cobalt–Imido Complex
Abstract A four‐coordinate cobalt–imido complex, ( t Bu mPNP)Co=NMes ( t Bu mPNP=modified PNP pincer ligand) has been synthesized from addition of 2,4,6‐trimethylphenylazide (Mes–N 3 ) to the corresponding dinitrogen complex. The solid‐state structure determined by X‐ray diffraction established a rare, idealized planar geometry with a Co=N bond distance of 1.716(2) Å. Magnetic measurements revealed an S =1 ground state with CAS‐SCF calculations supporting radical character on the imide nitrogen. Thermolysis of the cobalt–imido compound induced selective insertion of the imido group into a Co−P bond and yielded a three‐coordinate cobalt complex with a distorted T‐shaped geometry. Transition state analysis conducted with DFT calculations established the thermodynamic stability of the P–N coupled product and provided insight into the exclusive selectivity.