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Nitriles and isonitriles are important σ-donor ligands in coordination chemistry. Isonitriles also function in low-valent complexes as π-acceptor ligands similar to CO. Herein we present the unusual behavior of the highly reducing, high-spin iron(I) complex [Fe(hmds)2]- toward these compound classes. Rare examples of side-on coordination of nitriles to the metal center are observed. Insights gained by 57Fe Mössbauer spectroscopy as well as DFT and CASSCF calculations give an interplay between the resonance structures of not only an iron(I) π-complex and an iron(III) metallacycle but also point to the importance of an iron(II) nitrile radical anion. For an aromatic isonitrile end-on coordination is observed, which is best described as an iron(I) complex with only minor unpaired spin transfer onto the isonitrile. For aliphatic isonitriles, the selective R-CN bond cleavage occurs and yields stoichiometric mixtures of alkyl iron(II) and cyanido iron(II) complexes. Attempts to isolate presumed (iso)nitrile radical anions void of 3d-metal coordination give for the reaction of an aromatic isonitrile with KC8 facile reductive coupling to the corresponding diamido acetylene.
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Aiming at intramolecular frustrated Lewis pairs (FLPs) based on soft Lewis acidic bismuth centers, a phosphine function was combined with a dichloridobismuthane unit on a phenylene backbone utilizing a scrambling approach. The reaction between two equivalents of BiCl3 and (o-(Ph2P)C6H4)3Bi yielded (o-(Ph2P)C6H4)BiCl2(THF), the structure of which indicated Bi P interactions and thus a pronounced Lewis acidity at the bismuth center that was confirmed by the Gutmann-Beckett method. However, the system turned out to be insufficient to be utilized for FLP reactivity. Hence, the chloride ligands were exchanged by iodide and C2F5 substituents, respectively. Despite a lower electronegativity the iodide compound exhibits a shorter Bi P contact, while the C2F5 substituents led to a further decrease of the Lewis acidity, despite their high group electronegativity. DFT calculations rationalized this by a quenching of the Lewis acidity inherent to the σ*(Bi-C) orbital by negative hyperconjugation from occupied p-orbitals at the F atoms. Furthermore, it turned out that the strength of the covalent Bi-X σ-bond is a more important factor than the charge at Bi in determining the energetic accessibility and thus Lewis acidity of the antibonding σ*(Bi-C) orbital.
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Nickel carbonite complexes supported by alkaline earth metals have been accessed via salt-metathesis of the corresponding alkali metal precursors. The new complexes undergo Schlenk-like exchange reactions in solution which have been investigated by NMR spectroscopy. Also their reactivity towards epoxides and carbon monoxide was studied.
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Compounds containing Mn-O bonds are of utmost importance in biological systems and catalytic processes. Nevertheless, mononuclear manganese complexes containing all O-donor ligands are still rare. Taking advantage of the low tendency of the pentafluoroorthotellurate ligand (teflate, OTeF5) to bridge metal centers, we have synthesized two homoleptic manganese complexes with monomeric structures and an all O-donor coordination sphere. The tetrahedrally distorted MnII anion, [Mn(OTeF5)4]2-, can be described as a high spin d5 complex (S = 5/2), as found experimentally (magnetic susceptibility measurements and EPR spectroscopy) and using theoretical calculations (DFT and CASSCF/NEVPT2). The high spin d4 electronic configuration (S = 2) of the MnIII anion, [Mn(OTeF5)5]2-, was also determined experimentally and theoretically, and a square pyramidal geometry was found to be the most stable one for this complex. Finally, the bonding situation in both complexes was investigated by means of the Interacting Quantum Atoms (IQA) methodology and compared to that of hypothetical mononuclear fluoromanganates. Within each pair of [MnXn]2- (n = 4, 5) species (X = OTeF5, F), the Mn-X interaction is found to be comparable, therefore proving that the similar electronic properties of the teflate and the fluoride are also responsible for the stabilization of these unique species.
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The formation of isolable monatomic BiI complexes and BiII radical species is challenging due to the pronounced reducing nature of metallic bismuth. Here, we report a convenient strategy to tame BiI and BiII atoms by taking advantage of the redox noninnocent character of a new chelating bis(germylene) ligand. The remarkably stable novel BiI cation complex 4, supported by the new bis(iminophosphonamido-germylene)xanthene ligand [(P)GeII(Xant)GeII(P)] 1, [(P)GeII(Xant)GeII(P) = Ph2P(NtBu)2GeII(Xant)GeII(NtBu)2PPh2, Xant = 9,9-dimethyl-xanthene-4,5-diyl], was synthesized by a two-electron reduction of the cationic BiIIII2 precursor complex 3 with cobaltocene (Cp2Co) in a molar ratio of 1:2. Notably, owing to the redox noninnocent character of the germylene moieties, the positive charge of BiI cation 4 migrates to one of the Ge atoms in the bis(germylene) ligand, giving rise to a germylium(germylene) BiI complex as suggested by DFT calculations and X-ray photoelectron spectroscopy (XPS). Likewise, migration of the positive charge of the BiIIII2 cation of 3 results in a bis(germylium)BiIIII2 complex. The delocalization of the positive charge in the ligand engenders a much higher stability of the BiI cation 4 in comparison to an isoelectronic two-coordinate Pb0 analogue (plumbylone; decomposition below -30 °C). Interestingly, 4[BArF] undergoes a reversible single-electron transfer (SET) reaction (oxidation) to afford the isolable BiII radical complex 5 in 5[BArF]2. According to electron paramagnetic resonance (EPR) spectroscopy, the unpaired electron predominantly resides at the BiII atom. Extending the redox reactivity of 4[OTf] employing AgOTf and MeOTf affords BiIII(OTf)2 complex 7 and BiIIIMe complex 8, respectively, demonstrating the high nucleophilic character of BiI cation 4.
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Nickel complexes with a two-electron reduced CO2 ligand (CO2 2-, "carbonite") are investigated with regard to the influence alkali metal (AM) ions have as Lewis acids on the activation of the CO2 entity. For this purpose complexes with NiII(CO2)AM (AM=Li, Na, K) moieties were accessed via deprotonation of nickel-formate compounds with (AM)N(iPr)2. It was found that not only the nature of the AM ions in vicinity to CO2 affect the activation, but also the number and the ligation of a given AM. To this end the effects of added (AM)N(R)2, THF, open and closed polyethers as well as cryptands were systematically studied. In 14 cases the products were characterized by X-ray diffraction and correlations with the situation in solution were made. The more the AM ions get detached from the carbonite ligand, the lower is the degree of aggregation. At the same time the extent of CO2 activation is decreased as indicated by the structural and spectroscopic analysis and reactivity studies. Accompanying DFT studies showed that the coordinating AM Lewis acidic fragment withdraws only a small amount of charge from the carbonite moiety, but it also affects the internal charge equilibration between the LtBuNi and carbonite moieties.
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Deprotonation of O(iPr2 SiOH)2 (iPr LH2 ) with LiOtBu followed by reaction with FeCl2 in THF led to the complex [iPr L2 Fe][Li(THF)2 ]2 , 2, which represents a structural and spectroscopic model of the α-Fe sites of Fe/ZSM-5. Reaction with O2 in THF solution proceeds rather fast and is complete within 200â ms; an intermediate O2 adduct could not be identified by stopped-flow methods. Cooling blue solutions of 2 to -80 °C led to the growth of blue crystals of 2â THF, the analysis of which by XRD revealed a FeO4 core that is somewhat distorted from planarity towards a tetrahedral structure. By contrast, cooling such solutions to -30 °C led to pink crystals of an allogon featuring a perfectly square planar FeO4 entity. Hence, 2 represents a unique case where two different structural isomers (allogons) can be crystallized from the same solvent selectively, controlled by the temperature. DFT calculations were performed to understand this finding.
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The pentafluoroorthotellurate group (teflate, OTeF5) is considered as a bulky analogue of fluoride, yet its coordination behavior in transition metal complexes is not fully understood. By reaction of [CoCl4]2- and neat ClOTeF5, we synthesized the first cobalt teflate complex, [Co(OTeF5)4]2-, which exhibits moisture-resistant Co-OTeF5 bonds. Through a combined experimental and theoretical (DFT and NEVPT2) study, the properties and electronic structure of this species have been investigated. It exhibits a distorted tetrahedral structure around the cobalt center and can be described as a d7 system with a quartet (S = 3/2) ground state. A comparative bonding analysis of the (pseudo)tetrahedral [CoX4]2- anions (X = OTeF5, F, Cl) revealed that the strength of the Co-X interaction is similar in the three cases, being the strongest in [Co(OTeF5)4]2-. In addition, an analysis of the charge of the Co center reinforced the similar electron-withdrawing properties of the teflate and fluoride ligands. Therefore, the [Co(OTeF5)4]2- anion constitutes an analogue of the polymeric [CoF4]2- in terms of electronic properties, but with a monomeric structure.
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The open-shell cationic stannylene-iron(0) complex 4 (4=[PhiP DippSnâ Feâ IPr]+ ; PhiP Dipp={[Ph2 PCH2 Si(i Pr)2 ](Dipp)N}; Dipp=2,6-i Pr2 C6 H3 ; IPr=[(Dipp)NC(H)]2 C:) cooperatively and reversibly cleaves dihydrogen at the Sn-Fe interface under mild conditions (1.5â bar, 298â K), in forming bridging hydrido-complex 6. The One-electron oreduction of the related GeII -Fe0 complex 3 leads to oxidative addition of one C-P linkage of the PhiP Dipp ligand in an intermediary Fe-I complex, leading to FeI phosphide species 7. One-electron reduction reaction of 4 gives access to the iron(-I) ferrato-stannylene, 8, giving evidence for the transient formation of such a species in the reduction of 3. The covalently bound tin(II)-iron(-I) compound 8 has been characterised through EPR spectroscopy, SQUID magnetometry, and supporting computational analysis, which strongly indicate a high localization of electron spin density at Fe-I in this unique d9 -iron complex.
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[NiFe]-hydrogenases are biotechnologically relevant enzymes catalyzing the reversible splitting of H2 into 2e- and 2H+ under ambient conditions. Catalysis takes place at the heterobimetallic NiFe(CN)2(CO) center, whose multistep biosynthesis involves careful handling of two transition metals as well as potentially harmful CO and CN- molecules. Here, we investigated the sequential assembly of the [NiFe] cofactor, previously based on primarily indirect evidence, using four different purified maturation intermediates of the catalytic subunit, HoxG, of the O2-tolerant membrane-bound hydrogenase from Cupriavidus necator. These included the cofactor-free apo-HoxG, a nickel-free version carrying only the Fe(CN)2(CO) fragment, a precursor that contained all cofactor components but remained redox inactive and the fully mature HoxG. Through biochemical analyses combined with comprehensive spectroscopic investigation using infrared, electronic paramagnetic resonance, Mössbauer, X-ray absorption and nuclear resonance vibrational spectroscopies, we obtained detailed insight into the sophisticated maturation process of [NiFe]-hydrogenase.
Asunto(s)
Cupriavidus necator , Hidrogenasas , Dominio Catalítico , Hidrogenasas/química , Hidrogenasas/metabolismo , Cupriavidus necator/química , Cupriavidus necator/metabolismo , Oxidación-Reducción , NíquelRESUMEN
Diiron cofactors in enzymes perform diverse challenging transformations. The structures of high valent intermediates (Q in methane monooxygenase and X in ribonucleotide reductase) are debated since Fe-Fe distances of 2.5-3.4â Å were attributed to "open" or "closed" cores with bridging or terminal oxido groups. We report the crystallographic and spectroscopic characterization of a FeIII 2 (µ-O)2 complex (2) with tetrahedral (4C) centres and short Fe-Fe distance (2.52â Å), persisting in organic solutions. 2 shows a large Fe K-pre-edge intensity, which is caused by the pronounced asymmetry at the TD FeIII centres due to the short Fe-µ-O bonds. A ≈2.5â Å Fe-Fe distance is unlikely for six-coordinate sites in Q or X, but for a Fe2 (µ-O)2 core containing four-coordinate (or by possible extension five-coordinate) iron centres there may be enough flexibility to accommodate a particularly short Fe-Fe separation with intense pre-edge transition. This finding may broaden the scope of models considered for the structure of high-valent diiron intermediates formed upon O2 activation in biology.
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Hierro , Oxígeno , Hierro/química , Análisis Espectral , Cristalografía por Rayos X , Oxígeno/química , Oxidación-ReducciónRESUMEN
FLPs featuring aluminum-phosphane interactions, spring-loaded by a rigid biphenylene linker, have been accessed through a route where trimethyltin units at phosphane-functionalized organic backbones are exchanged by an AlCl2 moiety. Upon contact with substrates like CO2 these are readily bound by the Al/P site with release of strain. The system could also be utilized for a unique reactivity, namely the activation of allene.
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Alcadienos , AluminioRESUMEN
The pentafluoroorthotellurate group (teflate, OTeF5 ) is able to form species, for which only the fluoride analogues are known. Despite nickel fluorides being widely investigated, nickel teflates have remained elusive for decades. By reaction of [NiCl4 ]2- and neat ClOTeF5 , we have synthesized the homoleptic [Ni(OTeF5 )4 ]2- anion, which presents a distorted tetrahedral structure, unlike the polymeric [NiF4 ]2- . This high-spin complex has allowed the study of the electronic properties of the teflate group, which can be classified as a weak/medium-field ligand, and therefore behaves as the fluoride analogue also in ligand-field terms. The teflate ligands in [NEt4 ]2 [Ni(OTeF5 )4 ] are easily substituted, as shown by the formation of [Ni(NCMe)6 ][OTeF5 ]2 by dissolving it in acetonitrile. Nevertheless, careful reactions with other conventional ligands have enabled the crystallization of nickel teflate complexes with different coordination geometries, i.e. [NEt4 ]2 [trans-Ni(OEt2 )2 (OTeF5 )4 ] or [NEt4 ][Ni(bpyMe2 )(OTeF5 )3 ].
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Frustrated Lewis pairs (FLPs) composed of acidic alane and basic phosphane functions, separated by a xanthene linker, can be prepared through the corresponding Me3 Sn derivative and methyl aluminum compounds with elimination of Me4 Sn. This way MeClAl-, Cl2 Al- and (C6 F5 )2 Al- moieties could be introduced and the resulting FLPs are stabilized by a further equivalent of the alane precursors. In contact with the FLPs CO2 is bound via the C atom at the phosphane functions and the two O atoms at the Al centers. The residues at the latter determine the binding strength. Hence, in case of MeClAl CO2 capture occurs at higher pressure and under ambient conditions CO2 is released again, while for Cl2 Al and (C6 F5 )2 Al CO2 binding becomes irreversible. The results of DFT calculations rationalize these findings by the high thermodynamic stabilization in case of more electronegative residues, which concomitantly lead to higher barriers, and in case of (C6 F5 )2 Al further stabilization is achieved through a low reorganization energy.
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Carbonyl and iminyl based radical anions are reactive intermediates in a variety of transformations in organic synthesis. Herein, the isolation of ketyl, and more importantly unprecedented ketiminyl and aldiminyl radical anions coordinated to cobalt and iron complexes is presented. Insights into the electronic structure of these unusual metal bound radical anions is provided by X-Ray diffraction analysis, NMR, IR, UV/Vis and Mössbauer spectroscopy, solid and solution state magnetometry, as well as a by a detailed computational analysis. The metal bound radical anions are very reactive and facilitate the activation of intra- and intermolecular C-H bonds.
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Light-driven N2 cleavage into molecular nitrides is an attractive strategy for synthetic nitrogen fixation. However, suitable platforms are rare. Furthermore, the development of catalytic protocols via this elementary step suffers from poor understanding of N-N photosplitting within dinitrogen complexes, as well as of the thermochemical and kinetic framework for coupled follow-up chemistry. We here present a tungsten pincer platform, which undergoes fully reversible, thermal N2 splitting and reverse nitride coupling, allowing for experimental derivation of thermodynamic and kinetic parameters of the N-N cleavage step. Selective N-N splitting was also obtained photolytically. DFT computations allocate the productive excitations within the {WNNW} core. Transient absorption spectroscopy shows ultrafast repopulation of the electronic ground state. Comparison with ground-state kinetics and resonance Raman data support a pathway for N-N photosplitting via a nonstatistically vibrationally excited ground state that benefits from vibronically coupled structural distortion of the core. Nitride carbonylation and release are demonstrated within a full synthetic cycle for trimethylsilylcyanate formation directly from N2 and CO.
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The heteroleptic (formazanato)nickel bromide complex LNi(µ-Br)2NiL [LH = Mes-NH-NâC(p-tol)-NâN-Mes] has been prepared by deprotonation of LH with NaH followed by reaction with NiBr2(dme). Treatment of this complex with KC8 led to transformation of the formazanate into azoiminate ligands via N-N bond cleavage and the simultaneous release of aniline. At the same time, the potentially resulting intermediate complex L'2Ni [L' = HNâC(p-tol)-NâN-Mes] was reduced by one additional electron, which is delocalized across the π system and the metal center. The resulting reduced complex [L'2Ni]K(18-c-6) has a S = 1/2 ground state and a square-planar structure. It reacts with dioxygen via one-electron oxidation to give the complex L'2Ni, and the formation of superoxide was detected spectroscopically. If oxidizable substrates are present during this process, these are oxygenated/oxidized. Triphenylphosphine is converted to phosphine oxide, and hydrogen atoms are abstracted from TEMPO-H and phenols. In the case of cyclohexene, autoxidations are triggered, leading to the typical radical-chain-derived products of cyclohexene.
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PURPOSE: Low molecular weight iron(III) complex-based contrast agents (IBCA) including iron(III) trans-cyclohexane diamine tetraacetic acid [Fe(tCDTA)]- could serve as alternatives to gadolinium-based contrast agents in MRI. In search for IBCA with enhanced properties, we synthesized derivatives of [Fe(tCDTA)]- and compared their contrast effects. METHODS: Trans-cyclohexane diamine tetraacetic acid (tCDTA) was chemically modified in 2 steps: first the monoanhydride of Trans-cyclohexane diamine tetraacetic acid was generated, and then it was coupled to amines in the second step. After purification, the chelators were analyzed by high-performance liquid chromatography, mass spectrometry, and NMR spectrometry. The chelators were complexed with iron(III), and the relaxivities of the complexes were measured at 0.94, 1.5, 3, and 7 Tesla. Kinetic stabilities of the complexes were analyzed spectrophotometrically and the redox properties by cyclic voltammetry. RESULTS: Using ethylenediamine (en) and trans-1,4-diaminocyclohexane, we generated monomers and dimers of tCDTA: en-tCDTA, en-tCDTA-dimer, trans-1,4-diaminocyclohexane-tCDTA, and trans-1,4-diaminocyclohexane-tCDTA-dimer. The iron(III) complexes of these derivatives had similarly high stabilities as [Fe(tCDTA)]- . The iron(III) complexes of the trans-1,4-diaminocyclohexane derivatives had higher T1 relaxivities than [Fe(tCDTA)]- that increased with increasing magnetic field strengths and were highest at 6.8 L·mmol-1 ·s-1 per molecule for the dimer. Remarkably, the relaxivity of [Fe(en-tCDTA)]+ had a threefold increase from neutral pH toward pH6. CONCLUSION: Four iron(III) complexes with similar stability in comparison to [Fe(tCDTA)]- were synthesized. The relaxivities of trans-1,4-diaminocyclohexane-tCDTA and trans-1,4-diaminocyclohexane-tCDTA-dimer complexes were in the same range as gadolinium-based contrast agents at 3 Tesla. The [Fe(en-tCDTA)]+ complex is a pH sensor at weakly acidic pH levels, which are typical for various cancer types.
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Medios de Contraste , Hierro , Concentración de Iones de Hidrógeno , Campos Magnéticos , Imagen por Resonancia MagnéticaRESUMEN
Reduction of ß-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.
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The complexes [LtBu Ni(OCO-κ2 O,C)]M3 [N(SiMe3 )2 ]2 (M=Li, Na, K), synthesized by deprotonation of a nickel formate complex [LtBu NiOOCH] with the corresponding amides M[N(SiMe3 )2 ], feature a NiII -CO2 2- core surrounded by Lewis-acidic cations (M+ ) and the influence of the latter on the behavior and reactivity was studied. The results point to a decrease of CO2 activation within the series Li, Na, and K, which is also reflected in the reactivity with Me3 SiOTf leading to the liberation of CO and formation of a Ni-OSiMe3 complex. Furthermore, in case of K+ , the {[K3 [N(SiMe3 )2 ]2 }+ shell around the Ni-CO2 2- entity was shown to have a large impact on its stabilization and behavior. If the number of K[N(SiMe3 )2 ] equivalents used in the reaction with [LtBu NiOOCH] is decreased from 3 to 0.5, the deprotonated part of the precursor enters a complex reaction sequence with formation of [LtBu NiI (µ-OOCH)NiI LtBu ]K and [LtBu Ni(C2 O4 )NiLtBu ]. The same reaction at higher concentrations additionally led to the formation of a unique hexanuclear NiII complex containing both oxalate and mesoxalate ([O2 C-CO2 -CO2 ]4- ) ligands.