Cooperativity-Driven Reactivity of a Dinuclear Copper Dimethylglyoxime Complex.

In this report, we present the dinuclear copper(II) dimethylglyoxime (H2 dmg) complex [Cu2 (H2 dmg)(Hdmg)(dmg)]+ (1), which, in contrast to its mononuclear analogue [Cu(Hdmg)2 ] (2), is subject to a cooperativity-driven hydrolysis. The combined Lewis acidity of both copper centers increases the electrophilicity of the carbon atom in the bridging µ2 -O-N=C-group of H2 dmg and thus, facilitates the nucleophilic attack of H2 O. This hydrolysis yields butane-2,3-dione monoxime (3) and NH2 OH that, depending on the solvent, is then either oxidized or reduced. In ethanol, NH2 OH is reduced to NH4 + , yielding acetaldehyde as the oxidation product. In contrast, in CH3 CN, NH2 OH is oxidized by CuII to form N2 O and [Cu(CH3 CN)4 ]+ . Herein are presented the combined synthetic, theoretical, spectroscopic and spectrometric methods that indicate and establish the reaction pathway of this solvent-dependent reaction.

How Solid Surfaces Control Stability and Interactions of Supported Cationic CuI(dppf) Complexes─A Solid-State NMR Study.

Organometallic complexes are frequently deposited on solid surfaces, but little is known about how the resulting complex-solid interactions alter their properties. Here, a series of complexes of the type Cu(dppf)(Lx)+ (dppf = 1,1'-bis(diphenylphosphino)ferrocene, Lx = mono- and bidentate ligands) were synthesized, physisorbed, ion-exchanged, or covalently immobilized on solid surfaces and investigated by 31P MAS NMR spectroscopy. Complexes adsorbed on silica interacted weakly and were stable, while adsorption on acidic γ-Al2O3 resulted in slow complex decomposition. Ion exchange into mesoporous Na-[Al]SBA-15 resulted in magnetic inequivalence of 31P nuclei verified by 31P-31P RFDR and 1H-31P FSLG HETCOR. DFT calculations verified that a MeCN ligand dissociates upon ion exchange. Covalent immobilization via organic linkers as well as ion exchange with bidentate ligands both lead to rigidly bound complexes that cause broad 31P CSA tensors. We thus demonstrate how the interactions between complexes and functional surfaces determine and alter the stability of complexes. The applied Cu(dppf)(Lx)+ complex family members are identified as suitable solid-state NMR probes for investigating the influence of support surfaces on deposited inorganic complexes.

Mesoionic Imines (MIIs): Strong Donors and Versatile Ligands for Transition Metals and Main Group Substrates.

We report the synthesis and the reactivity of 1,2,3-triazolin-5-imine type mesoionic imines (MIIs). The MIIs are accessible by a base-mediated cycloaddition between a substituted acetonitrile and an aromatic azide, methylation by established routes and subsequent deprotonation. C=O-stretching frequencies in MII-CO2 and -Rh(CO)2 Cl complexes were used to determine the overall donor strength. The MIIs are stronger donors than the N-heterocyclic imines (NHIs). MIIs are excellent ligands for main group elements and transition metals in which they display substituent-induced fluorine-specific interactions and undergo C-H activation. DFT calculations gave insights into the frontier orbitals of the MIIs. The calculations predict a relatively small HOMO-LUMO gap compared to other related ligands. MIIs are potentially able to act as both π-donor and π-acceptor ligands. This report highlights the potential of MIIs to display exciting properties with a huge potential for future development.

[(η6-p-Cymene)[3-(pyrid-2-yl)-1,2,4,5-tetrazine]chlororuthenium(II)]+, Redox Noninnocence and Dienophile Addition to Coordinated Tetrazine.

The bidentate ligand 3-(pyrid-2-yl)-1,2,4,5-tetrazine (TzPy) coordinated in the complex [CyRuCl(TzPy)]PF6 ([1]+; Cy = η6-p-cymene) shows noninnocent behavior and can be modified through the addition of dienophiles, vinylferrocene (ViFc) or ethynylferrocene (EthFc). The kinetics and transition-state thermodynamic analysis of the reaction of [1]+ + ViFc found ΔG⧧(298 K) = 67 kJ mol-1, while that of [1]+ + EthFc was ΔG⧧(298 K) = 83 kJ mol-1. The room temperature second-order rate of [1]+ + EthFc, k2 = 1.51(4) × 10-2 M-1 s-1, was 3 orders of magnitude faster than that of EthFc + TzPy, k2 = 1.05(15) × 10-4 M-1 s-1. The [1H2Fc]+ complex was converted to [1Fc]+ by oxidation with oxygen and 3,5-di-tert-butyl-o-quinone, and the molecular structure of [1Fc]+ was determined by single-crystal X-ray diffraction. The title complex [1]+ showed a quasi-reversible reduction in the cyclic voltammogram, and the electrochemical reduction mechanism was determined by UV-vis spectroelectrochemistry (SEC) experiments, as well as supported by density functional theory (DFT) calculations. The dihydropyridazine [1H2Fc]+ and pyridazine [1Fc]+ states of the ligand showed ligand noninnocence similar to that of the parent tetrazine but at a cathodically shifted potential. The dihydropyridazine [1H2Fc]+ showed a mixture of several products; however, upon oxidation, only a single product, [1Fc]+, was formed from the endo addition of the dienophile to [1]+. The electrochemical mechanism of [1Fc]+ was also studied by cyclic voltammetry and UV-vis SEC experiments, as well as supported by DFT calculations.

Electronic Structure of a Diiron Complex: A Multitechnique Experimental Study of [(dppf)Fe(CO) 3]+/0.

Here we explore the electronic structure of the diiron complex [(dppf)Fe(CO)3]0/+ [10/+; dppf = 1,1'-bis(diphenylphosphino)ferrocene] in two oxidation states by an advanced multitechnique experimental approach. A combination of magnetic circular dichroism, X-ray absorption and emission, high-frequency electron paramagnetic resonance (EPR), and Mössbauer spectroscopies is used to establish that oxidation of 10 occurs on the carbonyl iron ion, resulting in a low-spin iron(I) ion. It is shown that an unequivocal result is obtained by combining several methods. Compound 1+ displays slow spin dynamics, which is used here to study its geometric structure by means of pulsed EPR methods. Surprisingly, these data show an association of the tetrakis[3,5-bis(trifluoromethylphenyl)]borate counterion with 1+.

Tricoordinate Coinage Metal Complexes with a Redox-Active Tris-(Ferrocenyl)triazine Backbone Feature Triazine-Metal Interactions.

Invited for the cover of this issue is the group of Evamarie Hey-Hawkins at the University of Leipzig and colleagues at the University of Stuttgart and University of Regensburg. The image depicts the reported complexes as three similar flowers growing from one common branch representing the ligand. Read the full text of the article at 10.1002/chem.202000226.

Tricoordinate Coinage Metal Complexes with a Redox-Active Tris-(Ferrocenyl)triazine Backbone Feature Triazine-Metal Interactions.

2,4,6-Tris(1-diphenylphosphanyl-1'-ferrocenylene)-1,3,5-triazine (1) coordinates all three coinage metal(I) ions in a 1:1 tridentate coordination mode. The C3 -symmetric coordination in both solid state and solution is stabilised by an uncommon cation-π interaction between the triazine core and the metal cation. Intramolecular dynamic behaviour was observed by variable-temperature NMR spectroscopy. The borane adduct of 1, 1BH3 , displays four accessible oxidation states, suggesting complexes of 1 to be intriguing candidates for redox-switchable catalysis. Complexes 1Cu, 1Ag, and 1Au display a more complicated electrochemical behaviour, and the electrochemical mechanism was studied by temperature-resolved UV/Vis spectroelectrochemistry and chemical oxidation.

Influence of Multisite Metal-Ligand Cooperativity on the Redox Activity of Noninnocent N2S2 Ligands.

Metal-ligand cooperativity (MLC) relies on chemically reactive ligands to assist metals with small-molecule binding and activation, and it has facilitated unprecedented examples of catalysis with metal complexes. Despite growing interest in combining ligand-centered chemical and redox reactions for chemical transformations, there are few studies demonstrating how chemically engaging redox active ligands in MLC affects their electrochemical properties when bound to metals. Here we report stepwise changes in the redox activity of model Ru complexes as zero, one, and two BH3 molecules undergo MLC binding with a triaryl noninnocent N2S2 ligand derived from o-phenylenediamine (L1). A similar series of Ru complexes with a diaryl N2S2 ligand with ethylene substituted in place of phenylene (L2) is also described to evaluate the influence of the o-phenylenediamine subunit on redox activity and MLC. Cyclic voltammetry (CV) studies and density functional theory (DFT) calculations show that MLC attenuates ligand-centered redox activity in both series of complexes, but electron transfer is still achieved when only one of the two redox-active sites on the ligands is chemically engaged. The results demonstrate how incorporating more than one multifunctional reactive site could be an effective strategy for maintaining redox noninnocence in ligands that are also chemically reactive and competent for MLC.

Polyfunctional Imidazolium Aryloxide Betaine/Lewis Acid Catalysts as Tools for the Asymmetric Synthesis of Disfavored Diastereomers.

Enzymes are Nature's polyfunctional catalysts tailor-made for specific biochemical synthetic transformations, which often proceed with almost perfect stereocontrol. From a synthetic point of view, artificial catalysts usually offer the advantage of much broader substrate scopes, but stereocontrol is often inferior to that possible with natural enzymes. A particularly difficult synthetic task in asymmetric catalysis is to overwrite a pronounced preference for the formation of an inherently favored diastereomer; this requires a high level of stereocontrol. In this Article, the development of a novel artificial polyfunctional catalyst type is described, in which an imidazolium-aryloxide betaine moiety cooperates with a Lewis acidic metal center (here Cu(II)) within a chiral catalyst framework. This strategy permits for the first time a general, highly enantioselective access to the otherwise rare diastereomer in the direct 1,4-addition of various 1,3-dicarbonyl substrates to ß-substituted nitroolefins. The unique stereocontrol by the polyfunctional catalyst system is also demonstrated with the highly stereoselective formation of a third contiguous stereocenter making use of a diastereoselective nitronate protonation employing α,ß-disubstituted nitroolefin substrates. Asymmetric 1,4-additions of ß-ketoesters to α,ß-disubstituted nitroolefins have never been reported before in literature. Combined mechanistic investigations including detailed spectroscopic and density functional theory (DFT) studies suggest that the aryloxide acts as a base to form a Cu(II)-bound enolate, whereas the nitroolefin is activated by H-bonds to the imidazolium unit and the phenolic OH generated during the proton transfer. Detailed kinetic analyses (RPKA, VTNA) suggest that (a) the catalyst is stable during the catalytic reaction, (b) not inhibited by product and (c) the rate-limiting step is most likely the C-C bond formation in agreement with the DFT calculations of the catalytic cycle. The robust catalyst is readily synthesized and recyclable and could also be applied to a cascade cyclization.

Beyond Common Metal-Metal Bonds, κ3 -Bis(donor)ferrocenyl→Transition-Metal Interactions.

Ligands with 1,1'-bis(donor)ferrocene motif are capable of a wide range of binding modes, including the trans chelation mode in which there is a Fe-M interaction (κ3 -D,Fe,D), in the form of a dative Fe→TM bond (TM=transition metal). This Minireview will explore the nature of this Fe-TM interaction thorough select examples as well as how to characterize a Fe→TM dative bond using physical, computational, and spectroscopic techniques.

(Spectro)electrochemical and Electrocatalytic Investigation of 1,1'-Dithiolatoferrocene-Hexacarbonyldiiron.

Hexacarbonyldiiron bridged by a 1,1'-dithiolatoferrocene, [Fe(C5H4S)2{Fe(CO)3}2] (1), was synthesized, and the electrochemistry showed reversible oxidation at the Fe(C5H4S)2 site and quasi-reversible reduction at the hexacarbonyldiiron site. Spectroelectrochemical techniques showed reduction-induced ligand isomerization, where the thiolate ligand went from bridging to terminal and one carbon monoxide ligand moved to a quasi-bridging position; this mechanism was further supported by cyclic voltammetry simulation and density functional theory calculations. Complex 1 showed electrocatalytic activity toward hydrogen-evolving reaction.

(Benz-)imidazolin-2-ylidene-benzimidazolatonickel(II) Chelates: Syntheses, Structures, and Tuning of Noninnocent Chelate Ligand Behavior.

The deprotonation of 1-(1H-benzimidazol-2-yl)-3-methylbenzimidazolium hexafluorophosphate (2MeH2[PF6]) and 1-(1H-benzimidazol-2-yl)-3-isopropylbenzimidazolium hexafluorophosphate (2iPrH2[PF6]) with potassium tert-butoxide in THF afforded the benzimidazolium-benzimidazolates 2MeH and 2iPrH. The "instant carbene" behavior of these conjugated mesomeric betaines was demonstrated by trapping their carbenic tautomers 2'MeH and 2'iPrH with elemental sulfur and selenium, which afforded the corresponding thio- and selenourea derivatives 2'MeHE and 2'iPrHE (E = S, Se). The treatment of 2MeH and 2iPrH with nickelocene furnished the nickel(II) complexes [NiCp(2'Me)] and [NiCp(2'iPr)], which contain an anionic C,Namido-chelating NHC ligand. The electronic structure and redox behavior of the nickel(II) chelates were investigated, as well as those of the closely related chelates [NiCp(1'Me)] and [NiCp(1'iPr)] derived from the corresponding imidazolium-benzimidazolates 1MeH and 1iPrH. According to DFT calculations, the highest occupied molecular orbital (HOMO) is located over the NiCp moiety and the π system of the chelate ligand with a large contribution from the (benz-)imidazolate moiety. Cyclic voltammetry revealed a reversible oxidation to the monocation [NiCp(L)]+ (E1/2 = 0.315, 0.222, 0.396, 0.265 V vs ferrocene/ferrocenium for L = 1'Me, 1'iPr, 2'Me, 2'iPr, respectively) in CH2Cl2/0.1 M n-Bu4N[B(ArF)4] (B(ArF)4- = tetrakis(3,5-bis(trifluoromethyl)phenyl)borate), and isosbestic behavior was found in UV-vis-NIR spectroelectrochemical experiments. The different redox potentials reflect the different donor/acceptor properties of the NHC part of the chelate ligands, with 1'iPr being the strongest and 2'Me the weakest net electron donor. The EPR spectroscopic signature of [NiCp(2'Me)]+ in CH2Cl2/0.1 M n-Bu4N[B(ArF)4] at 100 K is consistent with a chelate-ligand-based radical with strong spin-orbit coupling to the Ni center. In contrast, the EPR spectra of [NiCp(1'Me)]+, [NiCp(1'iPr)]+, and [NiCp(2'iPr)]+ indicate that these monocations are best described as NiIII complexes, the comparatively higher contribution of the NiIII(L) vs the NiII(L•+) valence tautomer being supported by the results of open-shell DFT calculations.

Organometallic Fe-Fe Interactions: Beyond Common Metal-Metal Bonds and Inverse Mixed-Valent Charge Transfer.

The compounds [Fe(CO)3 (dRpf)]n+ , n=0, 1, 2 and dRpf=1,1'-bis(dicyclohexylphosphino)ferrocene ([1]n+ ) or 1,1'-bis(diisopropylphosphino)ferrocene ([2]n+ ), were obtained as two-step reversible redox systems by photolytic and redox reactions. The iron-iron distance decreases from about 4 Što about 3 Šon oxidation, which takes place primarily at the tricarbonyliron moiety. Whereas ferrocene oxidation is calculated to occur only in excited states, the near infrared absorptions of the mixed-valent monocations are due to an unprecedented "inverse" inter-valence charge transfer from the electron-rich iron(II) in the ferrocene backbone to the electron-deficient tricarbonyliron(I). Protonation of complex 1 results in the formation of the structurally characterized hydride [1H]BF4 , which reacts with acetone to form the dication, 12+ , and isopropanol. While the hydride [2H]BF4 was found to be unstable, protonation of 2 in acetone resulted in the clean formation of 22+, formally a hydrogen transfer.

Twisting and Tilting l,l'-Bis(dialkylphosphino)ferrocene Bound to Low Valent Tricarbonylmaganese(I to -I). [corrected].

Recently we had reported the noninnocent behavior of 1,1'-bis(diphenylphosphino)ferrocene (dppf) in Fe(CO)3dppf [Ringenberg et al., Inorg. Chem., 2017, 56, 7501]. Moving to the left in the periodic table, HMn(CO)3(dRpf) where dRpf = dppf (1H) and 1,1'-bis(diisopropylphosphino)ferrocene (dippf) (2H) were synthesized. The hydride ligand was removed by protonation with [(Et2O)2H][B(ArF)4] ([B(ArF)4]- = tetrakis[3,5-bis(trifluoromethyl)phenyl]borate), resulting in the rapid evolution of H2 followed by the formation of an Fe→Mn interaction. The reaction mechanism was determined by in situ IR experiments which show that directly following protonation both [1]+ and [2]+ offer an open manganese coordination site that allows for the formation of an intramolecular Fe→Mn dative bond. This process is significantly faster for [2]+ than for [1]+. The reduction chemistry as studied by cyclic voltammetry (CV) reveals that both complexes change from a distorted octahedral coordination with an Fe→Mn interaction to an open square-pyramidal configuration which is more stable for [1]0 than [2]0. Reoxidation of this square-pyramidal species proceeds more reversibly for 2 versus 1 due to the faster ferrocene ligand reorganization. The electrochemical mechanism was studied by in situ spectroscopic techniques, e.g., IR, UV-vis-NIR (near IR), and EPR spectroelectrochemistry (SEC) as well as by CV simulation. The new complexes described offer an exciting platform for the development of electrocatalysts for the reduction of CO2 to CO, or for proton reduction (2H+ + 2e- → H2).

Redox Induced Configurational Isomerization of Bisphosphine-Tricarbonyliron(I) Complexes and the Difference a Ferrocene Makes.

The tricarbonyliron (TCFe) complexes Fe(CO)3(dppf) and Fe(CO)3(dppp), where dppf = 1,1'-bis(diphenylphosphino)ferrocene and dppp = 1,3-bis(diphenylphosphino)propane, exhibit redox activity that induces configurational isomerization. The presence of the ferrocenyl (Fc) group stabilizes higher oxidized forms of TCFe. Using spectroelectrochemistry (IR, UV-vis, Mössbauer, and EPR) and computational analysis, we can show that the Fc in the backbone of the dppf ligand tends to form a weak dative bond to the electrophilic TCFeI and TCFeII species. The open shell TCFeI intermediate was stabilized by the distribution of spin between the two Fe centers (Fc and TCFe), whereas lacking the Fc moiety resulted in highly reactive TCFeI species. The [Fe(CO)3(dppf)]+ cation adopts two possible configurations, square-pyramidal (without an Fe-Fe interaction) and trigonal-bipyramidal (containing an Fe-Fe interaction). The two configurations are in equilibrium with the trigonal-bipyramidal configuration being enthalpically favored (ΔH = -7 kJ mol-1). There is an entropic penalty (ΔS = -20 J mol-1) due to tilting of the Cp (cyclopentadienide) rings of the dppf moieties by ∼8°. Additionally, the terminal iron hydride [FeH(CO)3(dppf)]BF4 was formed by protonation with a strong acid (HBF4·Et2O).

Borane-protected cyanides as surrogates of H-bonded cyanides in [FeFe]-hydrogenase active site models.

Triarylborane Lewis acids bind [Fe2(pdt)(CO)4(CN)2](2-) [1](2-) (pdt(2-) = 1,3-propanedithiolate) and [Fe2(adt)(CO)4(CN)2](2-) [3](2-) (adt(2-) = 1,3-azadithiolate, HN(CH2S(-))2) to give the 2:1 adducts [Fe2(xdt)(CO)4(CNBAr3)2](2-). Attempts to prepare the 1:1 adducts [1(BAr3)](2-) (Ar = Ph, C6F5) were unsuccessful, but related 1:1 adducts were obtained using the bulky borane B(C6F4-o-C6F5)3 (BAr(F)*3). By virtue of the N-protection by the borane, salts of [Fe2(pdt)(CO)4(CNBAr3)2](2-) sustain protonation to give hydrides that are stable (in contrast to [H1](-)). The hydrides [H1(BAr3)2](-) are 2.5-5 pKa units more acidic than the parent [H1](-). The adducts [1(BAr3)2](2-) oxidize quasi-reversibly around -0.3 V versus Fc(0/+) in contrast to ca. -0.8 V observed for the [1](2-/-) couple. A simplified synthesis of [1](2-), [3](2-), and [Fe2(pdt)(CO)5(CN)](-) ([2](-)) was developed, entailing reaction of the diiron hexacarbonyl complexes with KCN in MeCN.

Redox-active ligands in catalysis.

Nature's use of redox-active moieties combined with 3d transition-metal ions is a powerful strategy to promote multi-electron catalytic reactions. The ability of these moieties to store redox equivalents aids metalloenzymes in promoting multi-electron reactions, avoiding high-energy intermediates. In a biomimetic spirit, chemists have recently developed approaches relying on redox-active moieties in the vicinity of metal centers to catalyze challenging transformations. This approach enables chemists to impart noble-metal character to less toxic, and cost effective 3d transitional metals, such as Fe or Cu, in multi-electron catalytic reactions.

Metal-metal communication between 1,1'-bis(diphenylphosphino)cobaltocenium and an organonickel moiety.

1,1'-Bis(diphenylphosphino)cobaltocenium (dppc)+, similar to 1,1'-bis(diphenylphosphino)ferrocene, can be coordinated by dicarbonylnickel(0) or cyclopentadienylnickel(II), respectively. The cyclic voltammogram of [Ni(CO)2(dppc)]+ ([1]+) showed two reversible reductions, at potentails nearly identical to (dppc)+. The ligand-based reductions were confirmed by IR spectroelectrochemistry (SEC), although the ΔνCO was larger than might be expected if the ligand was electronically decoupled to the Ni(CO)2 moiety. The larger ΔνCO was attibuted to changes in the donor/accpetor properties of the -PPh2 moeities based on the occupation of the Co-Cp anti-bonding orbital, where the Cp ligand adopts an ylide-like structure. A similar analysis was performed on [CpNi(dppc)]2+ ([2]2+), where the two reductions were anodically shifted by ΔE = 292 and 520 mV from (dppc)+, indicating an increase in the M-M communication. The EPR SEC spectrum for [2]+ showed that the cobalt was reduced, while the UV-Vis-NIR SEC spectrum for [2]+ showed a NIR absportion at 1200 nm assinged as a MMCT CoII → NiII band. The second reduction formed [2]0 which was EPR silent but the UV-Vis-NIR SEC spectrum showed an increase in the intensity of the MMCT CoI → NiII.