3d Transition Metal Complexes Research Articles

Systematic Variation of 3d Metal Centers in a Redox-Innocent Ligand Environment: Structures, Electrochemical Properties, and Carbon Dioxide Activation.

Coordination compounds of earth-abundant 3d transition metals are among the most effective catalysts for the electrochemical reduction of carbon dioxide (CO2). While the properties of the metal center are crucial for the ability of the complexes to electrochemically activate CO2, systematic variations of the metal within an identical, redox-innocent ligand backbone remain insufficiently investigated. Here, we report on the synthesis, structural and spectroscopic characterization, and electrochemical investigation of a series of 3d transition-metal complexes [M = Mn(I), Fe(II), Co(II), Ni(II), Cu(I), and Zn(II)] coordinated by a new redox-innocent PNP pincer ligand system. Only the Fe, Co, and Ni complexes reveal distinct metal-centered electrochemical reductions from M(II) down to M(0) and show indications for interaction with CO2 in their reduced states. The Ni(0) d10 species associates with CO2 to form a putative Aresta-type Ni-η2-CO2 complex, where electron transfer to CO2 through back-bonding is insufficient to enable electrocatalytic activity. By contrast, the Co(0) d9 intermediate binding CO2 can undergo additional electron uptake into a formal cobalt(I) metallacarboxylate complex able to promote turnover. Our data, together with the few literature precedents, single out that an unsaturated coordination sphere (coordination number = 4 or 5) and a d7-to-d9 configuration in the reduced low oxidation state (+I or 0) are characteristics that foster electrochemical CO2 activation for complexes based on redox-innocent ligands.

Crystal growth and physical properties of an antiferromagnetic molecule: trans-dibromidotetrakis(acetonitrile)chromium(III) tribromide, [CrBr2(NCCH3)4](Br3)

The synthesis, crystal structure determination, magnetic properties and bonding interaction analysis of a novel 3d transition-metal complex, [CrBr2(NCCH3)4](Br3), are reported. Single-crystal X-ray diffraction results show that [CrBr2(NCCH3)4](Br3) crystallizes in space group C2/m (No. 12) with a symmetric tribromide anion and the powder X-ray diffraction results show the high purity of the material specimen. X-ray photoelectron studies with a combination of magnetic measurements demonstrate that Cr adopts the 3+ oxidation state. Based on the Curie–Weiss analysis of magnetic susceptibility data, the Néel temperature is found to be around 2.2 K and the effective moment (μeff) of Cr3+ in [CrBr2(NCCH3)4](Br3) is ∼3.8 µB, which agrees with the theoretical value for Cr3+. The direct current magnetic susceptibility of the molecule shows a broad maximum at ∼2.3 K, which is consistent with the theoretical Néel temperature. The maximum temperature, however, shows no clear frequency dependence. Combined with the observed upturn in heat capacity below 2.3 K and the corresponding field dependence, it is speculated that the low-temperature magnetic feature of a broad transition in [CrBr2(NCCH3)4](Br3) could originate from a crossover from high spin to low spin for the split d orbital level low-lying states rather than a short-range ordering solely; this is also supported by the molecular orbital diagram obtained from theoretical calculations.

Hydricity of 3d Transition Metal Complexes from Density Functional Theory: A Benchmarking Study.

A range of modern density functional theory (DFT) functionals have been benchmarked against experimentally determined metal hydride bond strengths for three first-row TM hydride complexes. Geometries were found to be produced sufficiently accurately with RI-BP86-D3(PCM)/def2-SVP and further single-point calculations with PBE0-D3(PCM)/def2-TZVP were found to reproduce the experimental hydricity accurately, with a mean absolute deviation of 1.4 kcal/mol for the complexes studied.

Complexation of free-base and 3d transition metal(II) phthalocyanines with endohedral fullerenes H@C60, H2@C60 and He@C60: The effect of encapsulated species

We studied theoretically the bonding of unsubstituted free-base phthalocyanine H2Pc and its 3d transition metal complexes MPc (MMn, Fe, Co, Ni, Cu, Zn) with endohedral non-metalofullerenes H@C60, H2@C60 and He@C60, by using PBE functional with Grimme's dispersion correction and DNP basis set. The calculated formation energies, geometries and electronic parameters (energies and distribution of HOMO and LUMO orbitals, electrostatic potential and spin density plots) were analyzed and compared to those of the dyads formed between the same series of unsubstituted Pcs and empty-cage C60. Particular differences were found in MnPc coordination pattern and the resulting frontier orbital distribution.

Palladium Goes First: A Neutral Asymmetric Heteroditopic N,P Ligand Forming Pd-3d Heterobimetallic Complexes.

A facile two-step synthesis of bis(1-methylhydrazinyl)pyrimidine from pyridine-2-carbaldehyde and 2-diphenylphosphanylbenzaldehyde gave access to the new asymmetric ligand 1. The phosphane selectively guides PdII into the softer tridentate N,N,P pocket, yielding monometallic complex 2. A second reaction with a 3d transition metal complex precursor (groups 7 to 12) fills the vacant N,N,N pocket and thus provides a variety of heterobimetallic complexes of the type PdII/MII (M = Mn (3), Fe (4), Co (5), Ni (6), Cu (7), Zn (8)). Single-crystal X-ray diffraction studies were performed for all complexes. The assembly of μ2-chlorido-bridged dimers was observed for complexes 5-7 in the solid state, while DOSY NMR experiments have shown that 5-7 are unbridged monomers in solution. As an exception, FeII prefers to form the homoleptic meridional complex [Fe{PdCl(1)}2](OTf)4 (4). The electrochemical behavior and the effective magnetic moment in solution were investigated for all complexes by cyclic voltammetry and Evans method, respectively. Experimental UV/vis results were interpreted by performing TD-DFT calculations on 1, 2, and 3.

BONDING FEATURES BETWEEN 2-PHENYLPYRIDINE AND 3d METAL IONS IN POLYNUCLEAR PIVALATES

The effect of a bulky phenyl substituent on the formation and stability of pivalate complexes of 3d transition metals is studied on the example of 2-phenylpyridine (phpy). Quantum chemical calculations in the B3LYP/Def2-TZVP approximation are used to optimize binuclear molecules of [M2(μ-piv)4L2] complexes, where M = Co, Ni, Cu, $$\text{piv} = \text{Me}_3\text{CCO}^-_2$$ , L = 2,3-lutidine (lut) for which the analyzed compounds are known, phpy, and 7,8-benzoquinoline (bquin). The calculated energy characteristics indicate a significant stability of lut complexes and suggest that similar compounds with phpy can be prepared. The accuracy of theoretical calculations is confirmed by the successful synthesis of complex [Cu2(piv)4(phpy)2] (3). The computational simulation of binuclear coordination compounds with bquin suggests that there is a theoretical possibility of their formation; however, the fact that such structures have lower stabilization energies than those of reference complexes ([M2(piv)4(DMF)2]) indicates that their preparative isolation is hindered. Molecular structures of novel molecular complexes [Ni9(OH)6(piv)12(Hpiv)2(phpy)]·C6H6 (2) and [Li2Co2(piv)6(phpy)2] (4) are isolated and determined.

Theranostics Through the Synergistic Cooperation of Heterometallic Complexes.

Heterometallic drugs are emerging as a great alternative to conventional metallodrugs. Careful selection of different metallic fragments makes possible to enhance not only the therapeutic potential by a synergistic effect, but also to incorpore key features like traceability. Drugs that integrate traceability and therapy in one system are known as theranostic agents. In cancer research, theranostic agents are becoming increasingly important. They deliver crucial information regarding their biological interplay that can ultimately be used for optimization. The well-established therapeutic potential of PtII -, RuII - and AuI -based drugs combined with the outstanding optical properties of d6 transition metal complexes grant the delivery of traceable metallodrugs. These species can be easily fine-tuned through modification of their respective ligands to provide a new generation of drugs.

Tailoring the properties of 3d transition metal complexes with different N-cycloalkyl-substituted tetrazoles

N-Cycloalkyl-substituted tetrazoles were coordinated with different late 3d transition metal(ii) salts to give moderately sensitive energetic coordination compounds.

Predicted structure and selectivity of 3d transition metal complexes with glutamicN,N-bis(carboxymethyl) acid

Structure and selectivity of 3d transition metal complexes with glutamicN,N-bis(carboxymethyl) acid are analyzed and predicted from DFT calculations.

Capturing photochemical and photophysical transformations in iron complexes with ultrafast X-ray spectroscopy and scattering.

Light-driven chemical transformations provide a compelling approach to understanding chemical reactivity with the potential to use this understanding to advance solar energy and catalysis applications. Capturing the non-equilibrium trajectories of electronic excited states with precision, particularly for transition metal complexes, would provide a foundation for advancing both of these objectives. Of particular importance for 3d metal compounds is characterizing the population dynamics of charge-transfer (CT) and metal-centered (MC) electronic excited states and understanding how the inner coordination sphere structural dynamics mediate the interaction between these states. Recent advances in ultrafast X-ray laser science has enabled the electronic excited state dynamics in 3d metal complexes to be followed with unprecedented detail. This review will focus on simultaneous X-ray emission spectroscopy (XES) and X-ray solution scattering (XSS) studies of iron coordination and organometallic complexes. These simultaneous XES-XSS studies have provided detailed insight into the mechanism of light-induced spin crossover in iron coordination compounds, the interaction of CT and MC excited states in iron carbene photosensitizers, and the mechanism of Fe–S bond dissociation in cytochrome c.

Why Conventional Design Rules for C–H Activation Fail for Open-Shell Transition-Metal Catalysts

The design of selective and active C–H activation catalysts for direct methane-to-methanol conversion is challenging. Bioinspired complexes that form high-valent metal–oxo intermediates capable of hydrogen abstraction and rebound hydroxylation are promising candidates. This promise has made them a target for computational high-throughput screening, typically simplified through the use of linear free energy relationships (LFERs). However, their mid-row transition-metal centers have numerous accessible spin and oxidation states that increase the combinatorial scale of design efforts. Here, we carry out a computational design screen of over 2500 mid-row 3d transition-metal complexes with four metals in numerous spin and oxidation states. We demonstrate the importance of spin/oxidation state in dictating design principles, limiting the generalization of strategies derived for widely studied high-spin Fe(II) catalysts to other metals or spin/oxidation states. Combined assessment of the effect of ligand-field tuning on reaction step energetics and on the identity of the ground state allows us to propose refined design strategies for spin-allowed methane-to-methanol catalysis. We observe weak coupling of energetics and design principles between reaction steps (e.g., oxo formation vs methanol release), meaning that LFERs do not generalize across our larger catalyst set. To rationalize relative reactivity in known catalysts, we instead compute independent reaction energies and propose strategies for further improvements in catalyst design.

Synthesis and structure of mono and bis {1,3-bis(2- pyridylimino)isoindoline} supported 3d transition metal complexes

Series of homoleptic and heteroleptic 3d transition metal complexes were prepared in analytically pure form and high yield, by reacting appropriate metal precursors with 1,3-bis(2-pyridylimino)isoindolate (BPIH) ligand under suitable reaction conditions. The complexes were characterized by various spectro-analytical techniques, including IR, UV, NMR, ESI Mass and X-ray crystal analysis. A distorted octahedral structure containing two meridionally coordinating BPI ligands was assigned to all the homoleptic complexes. While, for the heteroleptic complexes, a highly distorted square pyramidal geometry, with a planar tridentate BPI ligand coordinating to the metal center along with chloro and aqua ligands was assigned. Considering the unique structural features of the complexes, potential catalytic applications are envisaged.

Quantitative and Chemically Intuitive Evaluation of the Nature of M-L Bonds in Paramagnetic Compounds: Application of EDA-NOCV Theory to Spin Crossover Complexes.

To improve understanding of M−L bonds in 3d transition metal complexes, analysis by energy decomposition analysis and natural orbital for chemical valence model (EDA‐NOCV) is desirable as it provides a full, quantitative and chemically intuitive ab initio description of the M−L interactions. In this study, a generally applicable fragmentation and computational protocol was established and validated by using octahedral spin crossover (SCO) complexes, as the transition temperature (T 1/2) is sensitive to subtle changes in M−L bonding. Specifically, EDA‐NOCV analysis of Fe−N bonds in five [FeII(L azine)2(NCBH3)2], in both low‐spin (LS) and paramagnetic high‐spin (HS) states led to: 1) development of a general, widely applicable, corrected M+L6 fragmentation, tested against a family of five LS [FeII(L azine)3](BF4)2 complexes; this confirmed that three L azine are stronger ligands (ΔE orb,σ+π=−370 kcal mol−1) than 2 L azine +2 NCBH3 (=−335 kcal mol−1), as observed. 2) Analysis of Fe−L bonding on LS→HS, reveals more ionic (ΔE elstat) and less covalent (ΔE orb) character (ΔE elstat:ΔE orb 55:45 LS→64:36 HS), mostly due to a big drop in σ (ΔE orb,σ ↓50 %; −310→−145 kcal mol−1), and a drop in π contributions (ΔE orb,π ↓90 %; −30→−3 kcal mol−1). 3) Strong correlation of observed T 1/2 and ΔE orb,σ+π, for both LS and HS families (R 2=0.99 LS, R 2=0.95 HS), but no correlation of T 1/2 and ΔΔE orb,σ+π(LS‐HS) (R 2=0.11). Overall, this study has established and validated an EDA‐NOCV protocol for M−L bonding analysis of any diamagnetic or paramagnetic, homoleptic or heteroleptic, octahedral transition metal complex. This new and widely applicable EDA‐NOCV protocol holds great promise as a predictive tool.

Comparison of structural, spectroscopic, theoretical and thermal properties of metal complexes (Zn(II), Mn(II), Cu(II), Ni(II) and Co(II)) of pyridazine-3-carboxylic acid and pyridazine-4-carboxylic acids

In our present work, we studied complexes of 3d-transition metals with pyridazine-3-carboxylic and pyridazine-4-carboxylic acids. The synthesis of manganese(II), cobalt(II), nickel(II), copper(II) and zinc(II) complexes with pyridazinecarboxylic acids was performed. The effect of selected 3d-transition metal ions on the electronic system of ligands and the thermal properties of the complexes by the use of TG-FTIR were studied. The products of dehydration and decomposition processes of studied compounds were determined from the TG (thermogravimetric) and DSC (differential scanning calorimetry) curves. The crystal structures of manganese(II) pyridazine-3-carboxylate acid and pyridazine-4-carboxylate were analysed by the use of single-crystal X-ray analysis. The geometries, NBO atomic charges, energy of HOMO and LUMO orbitals were calculated in the B3LYP/6-311++G(d,p) level. The aromaticity of ligands and complexes were discussed on the basis of HOMA and Bird’s indices. The influence of the type of metal ion coordination and the location of the carboxylate anion in the pyridazine ring on the aromaticity of complexes were discussed. The comparison of the effect of metal ions on the electronic charge distribution and aromaticity of pyridazynecarboxylic and pyridinecarboxylic acids (i.e. nicotinic and picolinic acids) was done.

Quantitative evaluation of transient valence orbital occupations in a 3d transition metal complex as seen from the metal and ligand perspective

It is demonstrated for the case of photo-excited ferrocyanide how time-resolved soft X-ray absorption spectroscopy in transmission geometry at the ligand K-edge and metal L3-edge provides quantitatively equivalent valence electronic structure information, where signatures of photo-oxidation are assessed locally at the metal as well as the ligand. This allows for a direct and independent quantification of the number of photo-oxidized molecules at two soft X-ray absorption edges highlighting the sensitivity of X-ray absorption spectroscopy to the valence orbital occupation of 3d transition metal complexes throughout the soft X-ray range.

Spin Crossover in 3D Metal Centers Binding Halide-Containing Ligands: Magnetism, Structure and Computational Studies

The capability of a given substance to change its spin state by the action of a stimulus, such as a change in temperature, is by itself a very challenging property. Its interest is increased by the potential applications and the need to find sustainable functional materials. 3D transition metal complexes, mainly with octahedral geometry, display this property when coordinated to particular sets of ligands. The prediction of this behavior has been attempted by many authors. It is, however, made very difficult because spin crossover (SCO), as it is called, occurs most often in the solid state, where besides complexes, counter ions, and solvents are also present in many cases. Intermolecular interactions definitely play a major role in SCO. In this review, we decided to analyze SCO in mono- and binuclear transition metal complexes containing halogens as ligands or as substituents of the ligands. The aim was to try and find trends in the properties which might be correlated to halogen substitution patterns. Besides a revision of the properties, we analyzed structures and other information. We also tried to build a simple model to run Density Functional Theory (DFT) calculations and calculate several parameters hoping to find correlations between calculated indices and SCO data. Although there are many experimental studies and single-crystal X-ray diffraction structures, there are only few examples with the F, Cl, Br and series. When their intermolecular interactions were not very different, T1/2 (temperature with 50% high spin and 50% low spin states) usually increased with the calculated ligand field parameter (Δoct) within a given family. A way to predict SCO remains elusive.

Large-scale comparison of 3d and 4d transition metal complexes illuminates the reduced effect of exchange on second-row spin-state energetics

Density functional theory (DFT) is widely used in transition-metal chemistry, yet essential properties such as spin-state energetics in transition-metal complexes (TMCs) are well known to be sensitive to the choice of the exchange-correlation functional. Increasing the amount of exchange in a functional typically shifts the preferred ground state in first-row TMCs from low-spin to high-spin by penalizing delocalization error, but the effect on properties of second-row complexes is less well known. We compare the exchange sensitivity of adiabatic spin-splitting energies in pairs of mononuclear 3d and 4d mid-row octahedral transition-metal complexes. We analyze hundreds of complexes assembled from four metals in two oxidation states with ten small monodentate ligands that span a wide range of field strengths expected to favor a variety of ground states. We observe consistently lower but proportional sensitivity to exchange fraction among 4d TMCs with respect to their isovalent 3d TMC counterparts, leading to the largest difference in sensitivities for the strongest field ligands. The combined effect of reduced exchange sensitivities and the greater low-spin bias of most 4d TMCs means that while over one-third of 3d TMCs change ground states over a modest variation (ca. 0.0-0.3) in exchange fraction, almost no 4d TMCs do. Differences in delocalization, as judged through changes in the metal-ligand bond lengths between spin states, do not explain the distinct behavior of 4d TMCs. Instead, evaluation of potential energy curves in 3d and 4d TMCs reveals that higher exchange sensitivities in 3d TMCs are likely due to the opposing effect of exchange on the low-spin and high-spin states, whereas the effect on both spin states is more comparable in 4d TMCs.

Synthesis and Serviceability of New Symmetric Bis-pyrazolone Metal Complex Acid Dyes

Synthesis of new symmetric bis-pyrazolone acid dye and its 3d transition metal complexes is studied. Bis-pyrazolone is synthesized from (E)-6,6’-(ethene-1,2-diyl)bis(3-hydrazinylbenzenesulfonic acid) and ethyl ace-toacetate. The synthesized bis-pyrazolone is coupled with diazonium salt of 4-sulpho-2-aminophenol in alkaline medium targeting bis-pyrazolone ligand acid dye. Various metal complexes are obtained by treating a methanolic solution of metal salts with ligand acid dye in basic media. Structures of the synthesized ligand dye and its metal complexes are confirmed by UV-Vis, FTIR, and NMR spectra. The ligand acid dye and metal complex dyes demonstrate absorption in UV-Vis in the range of 400–600 nm. Ligand acid dye and its metal complexes demonstrate good to high value of light speed, washing fastness and rubbing fastness on leather.

Domain-based local pair natural orbital CCSD(T) calculations of strongly correlated electron systems: Examination of dynamic equilibrium models based on multiple intermediates in S1 state of photosystem II

ABSTRACT Domain-based local pair natural orbital (DLPNO) coupled cluster single and double (CCSD) methods with perturbative triples (T) correction with NormalPNO were used to compute energies for twelve different S1 structures of the CaMn4O5 cluster in the oxygen evolving complex (OEC) of photosystem II (PSII). The DLPNO-CCSD(T0) calculations with TightPNO for the important six structures among them revealed that the right (R)-opened S1XYZW structures were more stable than the corresponding left (L)-opened structures (X = O(5), Y = W2, Z = W1, and W = O(4)) of CaMn4O5. The three different S1 structures belonging to the R-opened type (S1acca, S1bbca, and S1abcb, where O2- = a, OH- = b and H2O = c) were found nearly degenerated in energy, indicating the possibility of the coexistence of different structures in the S1 state. The DLPNO-CCSD(T0) calculations with TightPNO supported the proposal of a dynamic equilibrium model based on the multi-intermediate structures for the S1 state, which is also in agreement with EPR and other experimental and hybrid DFT computational results. Implications of the computational results are discussed in relation to scope and applicability of NormalPNO and TightPNO for the CCSD(T0) calculations of strongly correlated electron systems such as 3d transition-metal complexes.

Correlation between the M←OH2 Bond Force Constants, Energies, and Lengths and Magnetic Moments of the Metal Atoms in Isostructural Nitrilotris(methylenephosphonic) Complexes [MII(H2O)3μ-NH(CH2PO3H)3] (MII = Cr-Zn)

The isostructural 3d transition metal complexes [MII(H2O)3μ-NH(CH2PO3H)3] (MII = Cr-Zn) were studied by X-ray diffraction, Raman spectroscopy, thermogravimetric analysis, and X-ray photoelectron spectroscopy. In order to gain more in-depth knowledge about the chemical bonds in coordination compounds, correlation analysis was carried out between the force constants and energies of the M←OH2 coordination bonds and the magnetic moments of transition metal atoms. The correlation of the force constants of the M←OH2 bonds with the M-O(w) interatomic distances was in line with the known empirical Badger’s rule. The double linear regression between the bond energy and reciprocal of M-O(w) interatomic distances and atomic magnetic moments shows a close relationship between the M←OH2 bond energy and the spin magnetic moment of 3d-metal atoms. The results account for the stability of Fe-containing heterometallic polymeric NTP complexes and high efficiency of NTP complexes as corrosion inhibitors for iron alloys in aqueous media.