Molecular and Electronic Structures of the Members of the Electron Transfer Series [Mn(bpy)3]n (n = 2+, 1+, 0, 1−) and [Mn(tpy)2]m (m = 4+, 3+, 2+, 1+, 0). An Experimental and Density Functional Theory Study

Influence of Donor-Acceptor Interactions on MLCT Hole Reconfiguration in {Ru(bpy)} Chromophores

In MLCT chromophores, internal conversion (IC) in the form of hole reconfiguration pathways (HR) is a major source of dissipation of the absorbed photon energy. Therefore, it is desirable to minimize their impact in energy conversion schemes by slowing them down. According to previous findings on {Ru(bpy)} chromophores, donor-acceptor interactions between the Ru ion and the ligand scaffold might allow to control HR/IC rates. Here, a series of [Ru(tpm)(bpy)(R-py)]2+ chromophores, where tpm is tris(1-pyrazolyl)methane, bpy is 2,2'-bipyridine and R-py is a 4-substituted pyridine, were prepared and fully characterized employing electrochemistry, spectroelectrochemistry, steady-state absorption/emission spectroscopy and electronic structure computations based on DFT/TD-DFT. Their excited-state decay was monitored using nanosecond and femtosecond transient absorption spectroscopy. HR/IC lifetimes as slow as 568 ps were obtained in DMSO at room temperature, twice as slow as in the reference species [Ru(tpm)(bpy)(NCS)]+.

Effect of density functional approximations on the calculated Jahn-Teller distortion in bis(terpyridine)manganese(III) and related compounds

CONTEXT

Bis(terpyridine)manganese(III) exhibits Jahn-Teller distortion due to the inequivalent occupation of the degenerate eg orbitals of this high-spin d4 pseudo octahedral complex. Due to the spatially constrained nature of the terpyridine ligand, the central Mn-N bonds will always be shorter than the Mn-N terminal bonds, making it more difficult to distinguish between compression and elongation Jahn-Teller structures for bis(terpyridine)manganese(III). Density functional theory (DFT) calculations were utilized as a tool to evaluate the type of Jahn-Teller distortion in the high-spin d4 bis(terpyridine)manganese(III). The nature of the Jahn-Teller distortion calculated does depend upon the choice of density functional approximation (DFA) with the B3LYP, M06, and OLYP-D3 DFAs giving compression and the PW6B95D3, MN15, and MN15-D3 DFAs giving elongation in gas-phase calculations. All solvent-phase calculations yield an elongated structure for the bis(terpyridine)manganese(III) compound, which is yet to be structurally characterized experimentally. However, both gas and solvent OLYP-D3 calculations result in a compressed structure for the only experimentally isolated and characterized bis(terpyridine)manganese(III) complex, specifically the complex with terpyridine = 4'-(4-methylphenyl)-2,2':6',2''-terpyridine. This alignment with the experimentally observed compression Jahn-Teller structure enhances the credibility of OLYP-D3 calculations in reproducing the observed geometries. The compressed Jahn-Teller geometries were near D2d symmetry with the z-axis for compression defined along the Mn-N central bonds. Elongation Jahn-Teller distortion is not possible along the Mn-N central bonds, due to their spatially constrained nature. Thus, elongation occur along one pair of opposite Mn-N terminal bonds that are longer than the other pair of opposite terminal bonds, with shorter central bonds. The highest symmetry of the elongation Jahn-Teller distortion geometry of bis(terpyridine)manganese(III) is C2v. Criteria to distinguish between a compression and elongation Jahn-Teller geometry for bis(terpyridine)manganese(III) are identified. The nature of the singly occupied eg molecular orbital, exhibiting anti-bonding interaction with the nitrogen-p MOs involved, dictates the type of Jahn-Teller distortion that occurs. The low-energy occupied bonding t2g molecular orbitals establish bonds with and undergo mixing with the ligand molecular orbitals. The OLYP-D3 functional is recommended for calculating bis(terpyridine)manganese(III) and related compounds due to its consistent generation of metal-ligand bonds slightly longer than observed in experiments, in line with the required behavior. Additionally, OLYP-D3 offers a realistic electronic structure for Jahn-Teller distorted bis(terpyridine)manganese(III), correctly identifying alpha eg molecular orbitals as the highest occupied molecular orbital and lowest unoccupied molecular orbital in agreement with experimental electrochemical studies. Furthermore, OLYP-D3 accurately reproduces the experimental compression geometry for the only structurally known bis(terpyridine)manganese(III) compound, instilling confidence in its reliability for such calculations.

METHODS

DFT geometry optimization and frequency calculations were done on the two different modes of Jahn-Teller distortion of bis(terpyridine)manganese(III), using the OLYP, B3LYP, M06, PW6B95D3, and MN15 functionals, with and without the Grimme's D3 dispersion correction, and the 6-311G(d,p) or def2TZVPP basis set, as implemented in Gaussian 16. All optimizations were in the gas phase and also in the solvent phase with CH3CN as implicit solvent using IEFPCM. DFT calculations were utilized to determine the Jahn-Teller effect on the geometry of high-spin d4 bis(terpyridine)manganese(III) complex containing two structurally constrained tridentate ligands.

Spin State in Homoleptic Iron(II) Terpyridine Complexes Influences Mixed Valency and Electrocatalytic CO2 Reduction

Two homoleptic Fe(II) complexes in different spin states bearing superbasic terpyridine derivatives as ligands are investigated to determine the relationship between spin state and electrochemical/spectroscopic behavior. Antiferromagnetic coupling between a ligand-centered radical and the high-spin metal center leads to an anodic shift of the first reduction potential and results in a species that shows mixed valency with a moderately intense intervalence-charge-transfer band. The differences afforded by the different spin states extend to the electrochemical reactivity of the complexes: while the low-spin species is a precatalyst for electrocatalytic CO₂ reduction and leads to the preferential formation of CO with a Faradaic efficiency of 37%, the high-spin species only catalyzes proton reduction at a modest Faradaic efficiency of approximately 20%.

Redox Data of Tris(polypyridine)manganese(II) Complexes

Very little cyclic voltammetry data for tris(polypyridine)manganese(II) complexes, [MnII(N^N)3]2+, where N^N is bipyridine (bpy), phenanthroline (phen) or substituted bpy or phen ligands, respectively; are available in the literature. Cyclic voltammograms were found for tris(4,7-diphenyl-1,10-phenanthroline)manganese(II) perchlorate only. In addition to our recently published related research article, the data presented here provides cyclic voltammograms and corresponding voltage-current data obtained during electrochemical oxidation and the reduction of four [MnII(N^N)3]2+ complexes, using different scan rates and analyte concentrations. The results show increased concentration and scan rates resulting in higher Mn(II/III) peak oxidation potentials and increased peak current-voltage separations of the irreversible Mn(II/III) redox event. The average peak oxidation and peak reduction potentials of the Mn(II/III) redox events stayed constant within 0.01 V. Similarly, the average of the peak oxidation and reduction potentials of the ligand-based reduction events of [MnII(N^N)3]2+ were constant within 0.01 V.

A photoinduced mixed valence photoswitch

The ground state and photoinduced mixed valence states (GSMV and PIMV, respectively) of a dinuclear (Dp⁴⁺) ruthenium(II) complex bearing 2,2'-bipyridine ancillary ligands and a 2,2':4',4'':2'',2'''-quaterpyridine (Lp) bridging ligand were investigated using femtosecond and nanosecond transient absorption spectroscopy, electrochemistry and density functional theory. It was shown that the electronic coupling between the transiently light-generated Ru(II) and Ru(III) centers is H_DA ∼ 450 cm^-1 in the PIMV state, whereas the electrochemically generated GSMV state showed H_DA ∼ 0 cm^-1, despite virtually identical Ru-Ru distances. This stemmed from the changes in dihedral angles between the two bpy moieties of Lp, estimated at 30° and 4° for the GSMV and PIMV states, respectively, consistent with a through-bond rather than a through-space mechanism. Electronic coupling can be turned on by using visible light excitation, making Dp⁴⁺ a competitive candidate for photoswitching applications. A novel strategy to design photoinduced charge transfer molecular switches is proposed.

Electronic and structural data of 4’-substituted bis(2,2’;6’2’’-terpyridine)manganese in mono-, bis-, tris- and tetra-cationic states from DFT calculations

This data article provides density functional theory calculated structural (bond lengths and angles, coordinates of optimized geometries) and electronic (Mulliken spin population and character of frontier molecular orbitals) data of a series of 4’-substituted bis(2,2’;6’2’’-terpyridine)manganese complexes in four different oxidation states. The bis-cationic (n = 2) [Mn(tpy)₂]²⁺ complexes are experimentally well known (Sjödin et al., 2008), while little or none experimental structural data of the tetra-cationic (n = 4, Romain et al., 2009, 2009), tris-cationic (n = 3, Romain et al., 2009) and mono-cationic (n = 1, Wang et al., 2014) [Mn(tpy)₂]ⁿ⁺ complexes are available. For more insight into the provided data, see related research article “Redox chemistry of bis(terpyridine)manganese(II) complexes – a molecular view” (Conradie, 2022).

Extensive Redox Non-Innocence in Iron Bipyridine-Diimine Complexes: a Combined Spectroscopic and Computational Study

Metal-ligand cooperation is an important aspect in earth-abundant metal catalysis. Utilizing ligands as electron reservoirs to supplement the redox chemistry of the metal has resulted in many new exciting discoveries. Here, we demonstrate that iron bipyridine-diimine (BDI) complexes exhibit an extensive electron-transfer series that spans a total of five oxidation states, ranging from the trication [Fe(BDI)]3+ to the monoanion [Fe(BDI]-1. Structural characterization by X-ray crystallography revealed the multifaceted redox noninnocence of the BDI ligand, while spectroscopic (e.g., 57Fe Mössbauer and EPR spectroscopy) and computational studies were employed to elucidate the electronic structure of the isolated complexes, which are further discussed in this report.

Redox Activity of Noninnocent 2,2'-Bipyridine in Zinc Complexes: An Experimental and Theoretical Study

We report on a systematical reactivity study of β-diketiminate zinc complexes with redox-active 2,2'-bipyridine (bpy). The reaction of LZnI (L = HC[C(Me)N(2,6-iPr₂C₆H₃)]₂) with NaB(C₆F₅)₄ in the presence of bpy yielded [LZn(bpy)][B(C₆F₅)₄] (1), with bpy serving as a neutral ligand, whereas reduction reactions of LZnI with 1 or 2 equiv of KC₈ in the presence of bpy gave the radical complex LZn(bpy) (2) and [2.2.2-Cryptand-K][LZn(bpy)] (3), in which bpy either acts as a π-radical anion or a diamagnetic dianion, respectively. The paramagnetic nature of 2 was confirmed via solution magnetic susceptibility measurements, and UV-vis spectroscopy shows that 2 exhibits absorption bands typical for bpy radical species. The EPR spectra of 2 and its deuterated analog 2-d ₈ demonstrate that the spin density is localized to the bpy ligand. Density functional theoretical calculations and natural bond orbital analysis were employed to elucidate the electronic structure of complexes 1-3 and accurately reproduced the structural experimental data. It is shown that reduction of the bpy moiety results in a decrease in the β-diketiminate co-ligand bite angle and elongation of the Zn-N(β-diketiminate) bonds, which act cooperatively and in synergy with the bpy ligand by decreasing Zn-N(bpy) bond lengths to stabilize the energy of the LUMO.

Homoleptic tris(6,6′-dimethyl-2,2′-bipyridine) rare earth metal complexes

Homoleptic tris(bipy) rare earth metal complexes were synthesized and structurally characterized. While two parallel bipy radical anions were strongly antiferromagnetically coupled, the remaining bipy radical anion hosted most spin densities.

Effect of Metal-Ligand Coordination Complexes on Molecular Dynamics and Structure of Cross-Linked Poly(dimethylosiloxane)

Poly(dimethylosiloxane) (PDMS) cross-linked by metal-ligand coordination has a potential functionality for electronic devices applications. In this work, the molecular dynamics of bipyridine (bpy)–PDMS-MeCl₂ (Me: Mn²⁺, Fe²⁺, Ni²⁺, and Zn²⁺) are investigated by means of broadband dielectric spectroscopy and supported by differential scanning calorimetry and density functional theory calculations. The study of molecular motions covered a broad range of temperatures and frequencies and was performed for the first time for metal-ligand cross-linked PDMS. It was found that the incorporation of bpy moieties into PDMS chain prevents its crystallization. The dielectric permittivity of studied organometallic systems was elevated and almost two times higher (ε′ ~4 at 1 MHz) than in neat PDMS. BpyPDMS-MeCl₂ complexes exhibit slightly higher glass transition temperature and fragility as compared to a neat PDMS. Two segmental type relaxations (α and α_ac) were observed in dielectric studies, and their origin was discussed in relation to the molecular structure of investigated complexes. The α_ac relaxation was observed for the first time in amorphous metal-ligand complexes. It originates from the lower mobility of PDMS polymer chains, which are immobilized by metal-ligand coordination centers via bipyridine moieties.

Organic-Inorganic Hybrid Nanomaterials for Electrocatalytic CO2 Reduction

Electrochemical CO₂ reduction (ECR) to value-added chemicals and fuels is regarded as an effective strategy to mitigate climate change caused by CO₂ from excess consumption of fossil fuels. To achieve CO₂ conversion with high faradaic efficiency, low overpotential, and excellent product selectivity, rational design and synthesis of efficient electrocatalysts is of significant importance, which dominates the development of ECR field. Individual organic molecules or inorganic catalysts have encountered a bottleneck in performance improvement owing to their intrinsic shortcomings. Very recently, organic-inorganic hybrid nanomaterials as electrocatalysts have exhibited high performance and interesting reaction processes for ECR due to the integration of the advantages of both heterogeneous and homogeneous catalytic processes, attracting widespread interest. In this work, the recent advances in designing various organic-inorganic hybrid nanomaterials at the atomic and molecular level for ECR are systematically summarized. Particularly, the reaction mechanism and structure-performance relationship of organic-inorganic hybrid nanomaterials toward ECR are discussed in detail. Finally, the challenges and opportunities toward controlled synthesis of advanced electrocatalysts are proposed for paving the development of the ECR field.

Structure of Electronically Reduced N-Donor Bidentate Ligands and Their Heteroleptic Four-Coordinate Zinc Complexes: A Survey of Density Functional Theory Results

The role of Hartree-Fock exchange in describing the structural changes occurring upon reduction of bipyridine-based ligands and their complexes is investigated within the framework of density functional theory (DFT) calculations. A set of four free ligands in their neutral and radical anionic forms, and two of their zinc complexes in their dicationic and monocationic radical forms, is used to compare a large panel of pure, conventional, and long-range corrected hybrid DFT functionals; coupled cluster single and double calculations are used alongside experimental results as benchmarks. Particular attention has been devoted to the magnitude of the change, upon reduction, of the Δ-parameter, which measures the difference between the C_py-C_py and the C-N bond lengths in bipyridine ligand and is known to experimentally correlate with the charge of the ligands. Our results indicate that the structural changes significantly depend on the amount of exact exchange included in the functional. A progressive evolution is observed for the free ligands, whereas two distinct sets of results are obtained for the complexes. Functionals with a small degree of HF exchange, e.g., B3LYP, do not adequately describe geometric changes for the considered species, and, quite surprisingly, the same holds for the CC2 method. The best agreement to experimental and CCSD values is obtained with functionals that include a significant but not excessive part of exact exchange, e.g., CAM-B3LYP, M06-2X, and ωB97X-D. The calculated localization of the added electron after reduction, which depends on the self-interaction error, is used to rationalize these outcomes. Static correlation is also shown to play a role in the accurate description of the electronic structure.

A 2,2'-bipyridine-containing covalent organic framework bearing rhenium(i) tricarbonyl moieties for CO2 reduction

The reduction of CO2 into higher energy products such as carbon-based fuels and feedstocks is an attractive strategy for mitigating the continuous rise in CO2 emissions associated with the growing global energy demand. Rhenium tricarbonyl complexes bearing 2,2'-bipyridine (2,2'-bpy) ligands are well-established molecular electrocatalysts for the selective reduction of CO2 to CO. Construction of efficient devices for this electrochemical process requires the immobilization of electrocatalysts to electrode surfaces. To integrate Re(2,2'-bpy)(CO)3 fragments into a covalent organic framework (COF), Re(5,5'-diamine-2,2'-bpy)(CO)3Cl (1) was synthesized and electrochemically investigated. Complex 1 is an active and selective electrocatalyst for the reduction of CO2 to CO with excellent faradaic efficiency (99%). The presence of the amine substituents leads to a destabilization of the π* orbital of the 5,5'-diamine-2,2'-bpy ligand with respect to the metal center. Therefore, 1 requires more negative potentials (-2.47 V vs. Fc+/0) to reach the doubly reduced catalytically active species. DFT studies were conducted to understand the electronic structure of 1, and support the destabilizing effect of the amine substituents. The Re-2,2'-bpy fragments were successfully integrated into a COF containing 2,2'-bpy moieties (COF-2,2'-bpy) via a post-metallation synthetic route to generate COF-2,2'-bpy-Re. A composite of COF-2,2'-bpy-Re, carbon black, and polyvinylidene fluoride (PVDF) was readily immobilized onto glassy carbon electrodes and electrocatalytic CO2 reduction to CO was observed at -2.8 V vs. Fc0/+, with a faradaic efficiency of 81% for CO production.

Role of Organic Components in Electrocatalysis for Renewable Energy Storage

Carbon dioxide electroreduction and water splitting are known as two promising strategies to convert renewable intermittent electrical energy into chemical energy. Thus, the three half-reactions, namely, CO₂ reduction reaction, hydrogen evolution reaction, and the oxygen evolution counter reaction, in these two electrolytic processes have attracted wide research interest. Organic polymer electrocatalysts or electrocatalysts containing organic components play important roles in these catalytic processes. It has been shown that the organic molecules can efficiently catalyze the reactions themselves, and modulate the active sites towards high selectivity and efficiency. The roles of the organic molecules in conducting polymers, the metal complexes, and the framework materials are extracted for the three half-reactions mentioned above, and this comprehensive review will serve as a guide for future research and aid in the design of electrocatalysts related to organic molecules.

Molecular polypyridine-based metal complexes as catalysts for the reduction of CO2

Polypyridyl transition metal complexes represent one of the more thoroughly studied classes of molecular catalysts towards CO₂ reduction to date. Initial reports in the 1980s began with an emphasis on 2nd and 3rd row late transition metals, but more recently the focus has shifted towards earlier metals and base metals. Polypyridyl platforms have proven quite versatile and amenable to studying various parameters that govern product distribution for CO₂ reduction. However, open questions remain regarding the key mechanistic steps that govern product selectivity and efficiency. Polypyridyl complexes have also been immobilized through a variety of methods to afford active catalytic materials for CO₂ reductions. While still an emerging field, materials incorporating molecular catalysts represent a promising strategy for electrochemical and photoelectrochemical devices capable of CO₂ reduction. In general, this class of compounds remains the most promising for the continued development of molecular systems for CO₂ reduction and an inspiration for the design of related non-polypyridyl catalysts.

Radical non-radical states of the [Ru(PIQ)] core in complexes (PIQ = 9,10-phenanthreneiminoquinone)

9,10-Phenanthreneiminosemiquinonate anion radical (PIQ˙(-)) complexes of ruthenium of types trans-[Ru(II)(PIQ˙(-))(PPh3)2(CO)Cl] () and trans-[Ru(III)(PIQ˙(-))(PPh3)2Cl2] () are reported. Reactions of and with I2 afford trans-[Ru(III)(PIQ˙(-))(PPh3)2(CO)Cl](+)I3(-)·½CH2Cl2 ((+)I3(-)·½CH2Cl2) and trans-[Ru(PIQ˙(-))2(PPh3)2(μ-Cl)3](+)I3(-)·¼I2·¼toluene) ((+)I3(-)·¼I2·¼toluene), while the reaction of with Br2 yields a 9,10-phenanthreneiminoquinone (PIQ) complex of the type mer-[Ru(III)(PIQ)(PPh3)Br3]·½CH2Cl2 (·½CH2Cl2). In comparison, the reaction of trans-[Ru(III)(PQ˙(-))(PPh3)2Cl2] (PQ), a 9,10-phenanthrenequinone (PQ) analogue of affords only trans-[Ru(III)(PQ)(PPh3)2Cl2](+)Br3(-) ((+)Br3(-)). Considering the X-ray bond parameters, EPR spectra and the atomic spin densities obtained from the density functional theory (DFT) calculations, is defined as a PIQ˙(-) (average C-O/N and C-C lengths, 1.280(2) and 1.453(3) Å) complex of ruthenium(ii) while is a neutral PIQ (average C-O, C-N, C-C and C-O/N lengths, 1.248(7), 1.284(7), 1.485(8) and 1.266(7) Å) complex of the ruthenium(iii) ion. The single crystal X-ray bond parameters proposed that (+)I3(-) (average C-O/N and C-C lengths, 1.294(8) and 1.449(9) Å) and (average C-O/N and C-C lengths, 1.289(2) and 1.447(4) Å) are PIQ˙(-) complexes of ruthenium(iii), while the (+) ion (average C-O/N and C-C lengths, 1.288 ± 0.004 and 1.450 ± 0.017 Å) is a co-facial bi-octahedral complex of ruthenium(iii). In contrast, the (+) ion is a PQ complex of the ruthenium(iii) ion. EPR spectra and the calculated atomic spin densities authenticated that the (+) ion obtained after constant potential coulometric oxidation of is a PIQ complex of ruthenium(iii), while the (-) ion is a hybrid state of [Ru(II)(PIQ˙(-))] and [Ru(III)(PIQ(2-))] states. It is observed that the PIQ˙(-) state in which spin is more localized on the nitrogen (∼38% in and ∼35% in (-) ion) is stable and the coordination of the PIQ(2-) state is not observed in this study. Redox activities, UV-vis/NIR absorption spectra and their origins and the spectroelectrochemical measurements for → (+), → (-) and (+) → (2+) conversions are analyzed.

Synthesis, Structure, and Properties of Al((R)bpy)3 Complexes (R = t-Bu, Me): Homoleptic Main-Group Tris-bipyridyl Compounds

The neutral homoleptic tris-bpy aluminum complexes Al((R)bpy)3, where R = tBu (1) or Me (2), have been synthesized from reactions between AlX precursors (X = Cl, Br) and neutral (R)bpy ligands through an aluminum disproportion process. The crystalline compounds have been characterized by single-crystal X-ray diffraction, electrochemical experiments, EPR, magnetic susceptibility, and density functional theory (DFT) studies. The collective data show that 1 and 2 contain Al(3+) metal centers coordinated by three bipyridine (bpy(•))(1-) monoanion radicals. Electrochemical studies show that six redox states are accessible from the neutral complexes, three oxidative and three reductive, that involve oxidation or reduction of the coordinated bpy ligands to give neutral (R)bpy or (R)bpy(2-) dianions, respectively. Magnetic susceptibility measurements (4-300 K) coupled with DFT studies show strong antiferromagnetic coupling of the three unpaired electrons located on the (R)bpy ligands to give S = (1)/2 ground states with low lying S = (3)/2 excited states that are populated above 110 K (1) and 80 K (2) in the solid-state. Complex 2 shows weak 3D magnetic interactions at 19 K, which is not observed in 1 or the related [Al(bpy)3] complex.

Platinum(ii) complexes of mixed-valent radicals derived from cyclotricatechylene, a macrocyclic tris-dioxolene

Three complexes of cyclotricatechylene (H6ctc), [{PtL}3(μ3-ctc)], have been synthesised: (L = 1,2-bis(diphenylphosphino)benzene {dppb}, 1; L = 1,2-bis(diphenylphosphino)ethane {dppe}, 2; L = 4,4'-bis(tert-butyl)-2,2'-bipyridyl { t Bu2bipy}, 3). The complexes show three low-potential, chemically reversible voltammetric oxidations separated by ca. 180 mV, corresponding to stepwise oxidation of the [ctc]6- catecholato rings to the semiquinonate level. The redox series [1]0/1+/2+/3+ and [3]0/1+/2+/3+ have been characterised by UV/vis/NIR spectroelectrochemistry. The mono- and di-cations have class II mixed valent character, with reduced radical delocalisation compared to an analogous bis-dioxolene system. The SOMO composition of [1˙]+ and [3˙]+ has been delineated by cw EPR, ENDOR and HYSCORE spectroscopies, with the aid of two monometallic model compounds [PtL(DBsq˙)]+ (DBsqH = 3,5-bis(tert-butyl)-1,2-benzosemiquinone; L = dppe or t Bu2bipy). DF and time-dependent DF calculations confirm these interpretations, and demonstrate changes to spin-delocalisation in the ctc macrocycle as it is sequentially oxidised.

Bimetallic Complexes Supported by a Redox-Active Ligand with Fused Pincer-Type Coordination Sites

The remarkable chemistry of mononuclear complexes featuring tridentate, meridionally chelating "pincer" ligands has stimulated the development of ligand frameworks containing multiple pincer sites. Here, the coordination chemistry of a novel pentadentate ligand (L(N3O2)) that provides two closely spaced NNO pincer-type compartments fused together at a central diarylamido unit is described. The trianionic L(N3O2) chelate supports homobimetallic structures in which each M(II) ion (M = Co, Cu, Zn) is bound in a meridional fashion by the bridging diarylamido N atom and O,N-donors of the salicyaldimine arms. The metal centers are also coordinated by a mono- or bidentate auxiliary ligand (L(aux)), resulting in complexes with the general form [M2(L(N3O2))(L(aux))2](+) (where L(aux) = 1-methyl-benzimidazole (1MeBI), 2,2'-bipyridine (bpy), 4,4'-dibromo-2,2'-bipyridine (bpy(Br2)), or (S)-2-(4-isopropyl-4,5-dihydrooxazolyl)pyridine (S-(iPr)OxPy)). The fused nature of the NNO pincer sites results in short metal-metal distances ranging from 2.70 Å for [Co2(L(N3O2)) (bpy)2](+) to 3.28 Å for [Zn2(L(N3O2)) (bpy)2](+), as revealed by X-ray crystallography. The complexes possess C2 symmetry due to the twisting of the aryl rings of the μ-NAr2 core; spectroscopic studies indicate that chiral L(aux) ligands, such as S-(iPr)OxPy, are capable of controlling the helical sense of the L(N3O2) scaffold. Since the four- or five-coordinate M(II) centers are linked solely by the amido moiety, each features an open coordination site in the intermetallic region, allowing for the possibility of metal-metal cooperativity in small-molecule activation. Indeed, the dicobalt(II) complex [Co2(L(N3O2)) (bpy(Br2))2](+) reacts with O2 to yield a dicobalt(III) species with a μ-1,2-peroxo ligand. The bpy-containing complexes exhibit rich electrochemical properties due to multiple metal- and ligand-based redox events across a wide (3.0 V) potential window. Using electron paramagnetic resonance (EPR) spectroscopy and density functional theory (DFT), it was determined that one-electron oxidation of [Co2(L(N3O2)) (bpy)2](+) results in formation of a S = 1/2 species with a L(N3O2)-based radical coupled to low-spin Co(II) centers.

Molecular and Electronic Structures of Mononuclear and Dinuclear Titanium Complexes Containing π-Radical Anions of 2,2'-Bipyridine and 1,10-Phenanthroline: An Experimental and DFT Computational Study

Whereas reaction of [(η(5)-Cp*)Ti(IV)Cl3](0) (1) with 2 equiv of neutral 2,2'-bipyridine (bpy) and 1.5 equiv of magnesium in tetrahydrofuran affords the mononuclear complex [(η(5)-Cp*)Ti(III)(bpy(•))2](0) (2), performing the same reaction with only 1 equiv each of magnesium and bpy provides the dinuclear complex [{(η(5)-Cp*)Ti(μ-Cl)(bpy(•))}2](0) (3). Conducting the latter reaction using 1,10-phenanthroline (phen) in place of bpy resulted in formation of dinuclear [{(η(5)-Cp*)Ti(μ-Cl)(phen(•))}2](0) (4). The structures of 2, 3, and 4 have all been determined by high-resolution X-ray crystallography at 153 K; the Cpy-Cpy distances of 1.420(3) and 1.431(4) Å in the N,N'-coordinated bpy ligands of 2 and 3, respectively, are indicative of the presence of (bpy(•))(1-) ligands, rather than neutral (bpy(0)). The electronic spectra (300-1600 nm) of these two complexes are similar in form, and contain intense π → π* transitions associated with the (bpy(•))(1-) radical anion. Temperature dependent magnetic susceptibility measurements (4-300 K) show that mononuclear 2 possesses a temperature independent magnetic moment of 1.73 μB, which is indicative of an S = (1)/2 ground state. Broken symmetry density functional theory (BS-DFT) calculations yield a picture consistent with the experimental findings, in which the central Ti atom possesses a +3 oxidation state and is coordinated by a η(5)-Cp* ligand and two (bpy(•))(1-). Strong intramolecular antiferromagnetic coupling of these three unpaired spins, one each on the Ti(III) center and on the two (bpy(•))(1-) ligands, affords the experimentally observed doublet ground state. The magnetic susceptibility measurements for dinuclear 3 and 4 display weak but significant ferromagnetic coupling, and indicate that these complexes possess S = 1 ground states. The mechanism of the spin coupling phenomenon that yields the observed behavior was analyzed using BS-DFT calculations, and it was discovered that the tight π-stacking of the N,N'-coordinated (bpy(•))(1-)/(phen(•))(1-) ligands in these two complexes results from direct overlap of their SOMOs and formation of a two-electron multicentered bond. This yields a diamagnetic {(bpy)2}(2-)/{(phen)2}(2-) bridging unit whose doubly occupied HOMO is spread equally over both ligands. The two remaining unpaired electrons, one at each Ti(III) center, couple weakly in a ferromagnetic fashion to yield the experimentally observed S = 1 ground states.

Tris(2,2'-azobispyridine) complexes of copper(II): X-ray structures, reactivities, and the radical nonradical bis(ligand) analogues

Tris(abpy) complexes of types mer-[Cu(II)(abpy)3][PF6]2 (mer-1(2+)[PF6(–)]2) and ctc-[Cu(II)(abpy)2(bpy)][PF6]2 (ctc-2(2+)[PF6(–)]2) were successfully isolated and characterized by spectra and single-crystal X-ray structure determinations (abpy = 2,2′-azobispyridine; bpy = 2,2′-bipyridine). Reactions of mer-1(2+) and ctc-2(2+) ions with catechol, o-aminophenol, p-phenylenediamine, and diphenylamine (Ph–NH–Ph) in 2:1 molar ratio afford [CuI(abpy)2](+) (3(+)) and corresponding quinone derivatives. The similar reactions of [Cu(II)(bpy)3](2+) and [Cu(II)(phen)3](2+) with these substrates yielding [Cu(I)(bpy)2](+) and [Cu(I)(phen)2](+) imply that these complexes undergo reduction-induced ligand dissociation reactions (phen = 1,10-phenanthroline). The average −N═N– lengths in mer-1(2+)[PF6(–)]2 and ctc-2(2+)[PF6(–)]2 are 1.248(4), while that in 3(+)[PF6(–)]·2CH2Cl2 is relatively longer, 1.275(2) Å, due to dCu → πazo* back bonding. In cyclic voltammetry, mer-1(2+) exhibits one quasi-reversible wave at −0.42 V due to Cu(II)/Cu(I) and abpy/abpy(•–) couples and two reversible waves at −0.90 and −1.28 V due to abpy/abpy(•–) couple, while those of ctc-2(2+) ion appear at −0.44, −0.86, and −1.10 V versus Fc(+)/Fc couple. The anodic 3(2+)/3(+) and the cathodic 3(+)/3 redox waves at +0.33 and −0.40 V are reversible. The electron paramagnetic resonance spectra and density functional theory (DFT) calculations authenticated the existence of abpy anion radical (abpy(•–)) in 3, which is defined as a hybrid state of [Cu(I)(abpy(0.5•–))(abpy(0.5•–))] and [Cu(II)(abpy(•–))(abpy(•–))] states. 3(2+) ion is a neutral abpy complex of copper(II) of type [Cu(II)(abpy)2](2+). 3 exhibits a near-IR absorption band at 2400–3000 nm because of the intervalence ligand-to-ligand charge transfer, elucidated by time-dependent DFT calculations in CH2Cl2.

Spectroelectrochemistry of tris(bipyridyl)silicon(iv): ligand localized reductions with potential electrochromic applications

Tris(bipyridyl)silicon(iv) was electrochemically reduced in acetonitrile to obtain the UV-vis spectra of its reduced species. Three stable, reversible, ligand localized reductions were observed.

Terpyridine complexes of first row transition metals and electrochemical reduction of CO2 to CO

Homoleptic terpyridine complexes of 3d transition metals are found to electrocatalytically reduce CO₂ to either CO or tuneable CO–H₂ mixtures.

Electrocatalytic hydrogen generation by a trithiolato-bridged dimanganese hexacarbonyl anion with a turnover frequency exceeding 40 000 s−1

Electrocatalytic proton reduction based on a dimanganese hexacarbonyl anion.