Electron transfer within β-diketiminato nickel bromide and cobaltocene redox couples activating CO2

Reduction of β-diketiminato nickel(II) complexes (LtBuNiII) to the corresponding nickel(I) compounds does not require alkali metal compounds but can also be performed with the milder cobaltocenes. LtBuNiBr and Cp2Co have rather similar redox potentials, so that the equilibrium with the corresponding electron transfer compound [LtBuNiIBr][Cp2CoIII] (ETC) clearly lies on the side of the starting materials. Still, the ETC portion can be used to activate CO2 yielding a mononuclear nickel(II) carbonate complex and ETC can be isolated almost quantitatively from the solutions through crystallisation. The more negative reduction potential of Cp*2Co shifts the equilibrium formed with LtBuNiBr strongly towards the ETC and accordingly the reaction of such solutions with CO2 is much faster.Peer Reviewe

Influencing the coordination mode of tbta (tbta = tris[(1-benzyl- 1H-1,2,3-triazol-4-yl)methyl]amine) in dicobalt complexes through changes in metal oxidation states

The complexes [(tbta)Co(μ-CA-2H)Co(tbta)(CH3CN)](BF4)21 and [(tbta)Co(μ-OH)2Co(tbta)](BF4)42 (tbta = tris[(1-benzyl- 1H-1,2,3-triazol-4-yl)methyl]amine and CA = chloranilic acid) were synthesized and characterized by X-ray crystallography, SQUID magnetometry and NMR spectroscopy. The reactions to form these complexes deliver 1 as a paramagnetic species containing two high spin Co(II) centers, and 2 as a diamagnetic compound with two low spin Co(III) centers. Structural analysis shows that in 1 the capped-octahedral environment around the Co(II) centers is highly distorted with rather long bonds between the metal and donor atoms. The tbta ligand binds to the Co(II) centers through the three triazole nitrogen donor atoms in a facial form, with the Co–N(amine) distance of 2.494(2) Å acting as a capping bond to the octahedron. In the crystal an unusual observation of one acetonitrile molecule statistically occupying the coordination sites at both Co(II) centers is made. 1 displays a series of intermolecular C–HCl and π–π interactions leading to extended three- dimensional structures in the solid state. These interactions lead to the formation of voids and explain why only one acetonitrile molecule can be bound to the dinuclear complexes. In contrast to 1, the cobalt centers in 2 display a more regular octahedral environment with shorter cobalt–donor atom distances, as would be expected for a low spin Co(III) situation. The tbta ligand acts as a perfect tetradentate ligand in this case with the cobalt–N(amine) distance of 2.012(3) Å falling in the range of a normal bond. Thus, we present the rare instances where the ligand tbta has been observed to bind in a perfectly tetradentate fashion in its metal complexes. The room temperature magnetic moment of 6.30 μB for 1 shows values typical of two high spin Co(II) centers, and this value decreases at temperatures lower than 30 K indicating a weak antiferromagnetic coupling and zero field splitting. Mass spectrometric analysis of 2 provided evidence for the formation of an oxo- bridged dicobalt complex in the gas phase

Influencing the coordination mode of tbta (tbta = tris[(1-benzyl- 1H-1,2,3-triazol-4-yl)methyl]amine) in dicobalt complexes through changes in metal oxidation states

The complexes [(tbta)Co(μ-CA-2H)Co(tbta)(CH3CN)](BF4)21 and [(tbta)Co(μ-OH)2Co(tbta)](BF4)42 (tbta = tris[(1-benzyl- 1H-1,2,3-triazol-4-yl)methyl]amine and CA = chloranilic acid) were synthesized and characterized by X-ray crystallography, SQUID magnetometry and NMR spectroscopy. The reactions to form these complexes deliver 1 as a paramagnetic species containing two high spin Co(II) centers, and 2 as a diamagnetic compound with two low spin Co(III) centers. Structural analysis shows that in 1 the capped-octahedral environment around the Co(II) centers is highly distorted with rather long bonds between the metal and donor atoms. The tbta ligand binds to the Co(II) centers through the three triazole nitrogen donor atoms in a facial form, with the Co–N(amine) distance of 2.494(2) Å acting as a capping bond to the octahedron. In the crystal an unusual observation of one acetonitrile molecule statistically occupying the coordination sites at both Co(II) centers is made. 1 displays a series of intermolecular C–HCl and π–π interactions leading to extended three- dimensional structures in the solid state. These interactions lead to the formation of voids and explain why only one acetonitrile molecule can be bound to the dinuclear complexes. In contrast to 1, the cobalt centers in 2 display a more regular octahedral environment with shorter cobalt–donor atom distances, as would be expected for a low spin Co(III) situation. The tbta ligand acts as a perfect tetradentate ligand in this case with the cobalt–N(amine) distance of 2.012(3) Å falling in the range of a normal bond. Thus, we present the rare instances where the ligand tbta has been observed to bind in a perfectly tetradentate fashion in its metal complexes. The room temperature magnetic moment of 6.30 μB for 1 shows values typical of two high spin Co(II) centers, and this value decreases at temperatures lower than 30 K indicating a weak antiferromagnetic coupling and zero field splitting. Mass spectrometric analysis of 2 provided evidence for the formation of an oxo- bridged dicobalt complex in the gas phase

Iron–molybdenum-oxo complexes as initiators for olefin autoxidation with O2

The reaction between [(TPA)Fe(MeCN)2](OTf)2 and [nBu4N](Cp*MoO3) yields the novel tetranuclear complex [(TPA)Fe(μ-Cp*MoO3)]2(OTf)2, 1, with a rectangular [Mo–O–Fe–O–]2 core containing high-spin iron(II) centres. 1 proved to be an efficient initiator/(pre)catalyst for the autoxidation of cis-cyclooctene with O2 to give cyclooctene epoxide. To test, which features of 1 are essential in this regard, analogues with zinc(II) and cobalt(II) central atoms, namely [(TPA)Zn(Cp*MoO3)](OTf), 3, and [(TPA)Co(Cp*MoO3)](OTf), 4, were prepared, which proved to be inactive. The precursor compounds of 1, [(TPA)Fe(MeCN)2](OTf)2 and [nBu4N](Cp*MoO3) as well as Cp2*Mo2O5, were found to be inactive, too. Reactivity studies in the absence of cyclooctene revealed that 1 reacts both with O2 and PhIO via loss of the Cp* ligands to give the triflate salt 2 of the known cation [((TPA)Fe)2(μ-O)(μ-MoO4)]2+. The cobalt analogue 4 reacts with O2 in a different way yielding [((TPA)Co)2(μ-Mo2O8)](OTf)2, 5, featuring a Mo2O84− structural unit which is novel in coordination chemistry. The compound [(TPA)Fe(μ-MoO4)]2, 6, being related to 1, but lacking Cp* ligands failed to trigger autoxidation of cyclooctene. However, initiation of autoxidation by Cp* radicals was excluded via experiments including thermal dissociation of Cp2*

A di-iron(III) mu-oxido complex as catalyst precursor in the oxidation of alkanes and alkenes

The oxido-bridged diiron(III) complex [Fe-2(mu-O)(mu-OAc)(DPEAMP)(2)](OCH3) (1), based on a new unsymmetrical ligand with an N4O donor set, viz. [2-((bis(pyridin-2-ylmethyl)amino)methyl)-6-((ethylamino)methyl)-4-meth-ylphenol (HDPEAMP)], has been prepared and characterized by spectroscopic methods and X-ray crystallog-raphy. The crystal structure of the complex reveals that each Fe(III) ion is coordinated by three nitrogen and three oxygen donors, two of which are the bridging oxido and acetate ligands. Employing H2O2 as a terminal oxidant, 1 is capable of oxidizing a number of alkanes and alkenes with high activity. The catalytic oxidation of 1,2-dimethylcyclohexane results in excellent retention of configuration. Monitoring of the reaction of 1 with H2O2 and acetic acid in the absence of substrate, using low-temperature UV-Vis spectroscopy, suggests the in situ formation of a transient Fe(III)(2)-peroxido species. While the selectivity and nature of oxidation products implicate a high-valent iron-oxido complex as a key intermediate, the low alcohol/ketone ratios suggest a simultaneous radical-based process.Peer reviewe

Widening the Window of Spin-Crossover Temperatures in Bis(formazanate)iron(II) Complexes via Steric and Noncovalent Interactions

Bis(formazanate)iron(II) complexes undergo a thermally induced S = 0 to S = 2 spin transition in solution. Here we present a study of how steric effects and π-stacking interactions between the triarylformazanate ligands affect the spin-crossover behavior, in addition to electronic substituent effects. Moreover, the effect of increasing the denticity of the formazanate ligands is explored by including additional OMe donors in the ligand (7). In total, six new compounds (2-7) have been synthesized and characterized, both in solution and in the solid state, via spectroscopic, magnetic, and structural analyses. The series spans a broad range of spin-crossover temperatures (T1/2) for the LS ⇌ HS equilibrium in solution, with the exception of compound 6 which remains high-spin (S = 2) down to 210 K. In the solid state, 6 was shown to exist in two distinct forms: a tetrahedral high-spin complex (6a, S = 2) and a rare square-planar structure with an intermediate-spin state (6b, S = 1). SQUID measurements, 57Fe Mössbauer spectroscopy, and differential scanning calorimetry indicate that in the solid state the square-planar form 6b undergoes an incomplete spin-change-coupled isomerization to tetrahedral 6a. The complex that contains additional OMe donors (7) results in a six-coordinate (NNO)2Fe coordination geometry, which shifts the spin-crossover to significantly higher temperatures (T1/2 = 444 K). The available experimental and computational data for 7 suggest that the Fe···OMe interaction is retained upon spin-crossover. Despite the difference in coordination environment, the weak OMe donors do not significantly alter the electronic structure or ligand-field splitting, and the occurrence of spin-crossover (similar to the compounds lacking the OMe groups) originates from a large degree of metal-ligand π-covalency

Oligonuclear copper complexes of a bioinspired pyrazolate-bridging ligand: Synthesis, structures, and equilibria in solution

The synthesis of a new bioinspired dinucleating ligand scaffold based on a bridging pyrazolate with appended bis[2-(1-methylimidazolyl)methyl]aminomethyl chelate arms is reported. This ligand forms very stable copper complexes, and a series of different species is present in solution depending on the pH. Interconversions between these solution species are tracked and characterized spectroscopically, and X-ray crystallographic structures of three distinct complexes that correspond to the species present in solution from acidic to basic pH have been determined. Overall, this provides a comprehensive picture of the copper coordination chemistry of the new ligand system. Alterations in the protonation state are accompanied by changes in nuclearity and pyrazolate binding, which cause pronounced changes in color and magnetic properties. Antiferromagnetic coupling between the copper(II) ions is switched on or off depending on the pyrazole binding mode

Electronic Control of Spin-Crossover Properties in Four-Coordinate Bis(formazanate) Iron(II) Complexes

The transition between spin states in d-block metal complexes has important ramifications for their structure and reactivity, with applications ranging from information storage materials to understanding catalytic activity of metalloenzymes. Tuning the ligand field (Delta(O)) by steric and/or electronic effects has provided spin-crossover compounds for several transition metals in the periodic table, but this has mostly been limited to coordinatively saturated metal centers in octahedral ligand environments. Spin-crossover complexes with low coordination numbers are much rarer. Here we report a series of four-coordinate, (pseudo)tetrahedral Fe(II) complexes with formazanate ligands and demonstrate how electronic substituent effects can be used to modulate the thermally induced transition between S = 0 and S = 2 spin states in solution. All six compounds undergo spin-crossover in solution with T-1/2 above room temperature (300-368 K). While structural analysis by X-ray crystallography shows that the majority of these compounds are low-spin in the solid state (and remain unchanged upon heating), we find that packing effects can override this preference and give rise to either rigorously high-spin (6) or gradual spin-crossover behavior (5) also in the solid state. Density functional theory calculations are used to delineate the empirical trends in solution spin-crossover thermodynamics. In all cases, the stabilization of the low-spin state is due to the pi-acceptor properties of the formazanate ligand, resulting in an "inverted" ligand field, with an approximate "two-over-three" splitting of the d-orbitals and a high degree of metal-ligand covalency due to metal -> ligand pi-backdonation. The computational data indicate that the electronic nature of the para-substituent has a different influence depending on whether it is present at the C-Ar or N-Ar rings, which is ascribed to the opposing effect on metal-ligand sigma- and pi-bonding

Supplementary material for the article: Wang, L.; Zlatar, M.; Vlahović, F.; Demeshko, S.; Philouze, C.; Molton, F.; Gennari, M.; Meyer, F.; Duboc, C.; Gruden, M. Experimental and Theoretical Identification of the Origin of Magnetic Anisotropy in Intermediate Spin Iron(III) Complexes. Chemistry - A European Journal 2018, 24 (20), 5091–5094. https://doi.org/10.1002/chem.201705989

Supplementary material for: [https://doi.org/10.1002/chem.201705989]Related to published version: [http://cherry.chem.bg.ac.rs/handle/123456789/2130

Spin-Crossover in a Pseudo-tetrahedral Bis(formazanate) Iron Complex

Spin-crossover in a pseudo-tetrahedral bis(formazanate) iron(II) complex (1) is described. Structural, magnetic, and spectroscopic analyses indicate that this compound undergoes thermal switching between an S=0 and an S=2 state, which is very rare in four-coordinate complexes. The transition to the high-spin state is accompanied by an increase in Fe-N bond lengths and a concomitant contraction of intraligand N-N bonds. The latter suggests that stabilization of the low-spin state is due to the π-acceptor properties of the ligand. One-electron reduction of 1 leads to the formation of the corresponding anion, which contains a low-spin (S=1/2) Fe(I) center. The findings are rationalized by electronic structure calculations using density functional theory