Mesoionic Carbene Complexes Research Articles

Scrutinizing the redox chemistry of group 10 complexes supported by a redox-active bis-phenolate mesoionic carbene

We present the synthesis of an isostructural series of group 10 mesoionic carbene complexes with the general formula [M(MIC)Py] (with M = Ni (1), Pd (2) and Pt (3), Py = pyridine and MIC = bis-phenolate triazolylidene). Their electrochemical properties are investigated, revealing two quasi-reversible oxidations for all compounds, while no reductions are observed. Isolation of the oxidized species has been achieved using either silver triflate or nitrosonium tetrafluoroborate for the nickel and the platinum complex yielding the corresponding mono-cationic complexes [1]X and [3]X (X = OTf or BF4). For the palladium complex, oxidation reactions yielded different products 4 and 5 in which the pyridine and/or the phenol(oxy) donors were found to be labile compared to Ni and Pt. Additionally, for Pt the electron transfer reaction was found to be quite slow and short reaction times led to the isolation of an unusual heterobimetallic silver-platinum complex 6. The location of the redox event has been investigated using EPR spectroscopy and computational methods, which suggest, that the oxidation is mostly centered on one of the phenolate anchors.

A Series of Rare-Earth Mesoionic Carbene Complexes.

We report the synthesis and characterisation of a series of rare-earth mesoionic carbene complexes, [RE{N(SiMe3 )2 }3 {CN(Me)C(Me)N(Me)CH}] (3RE, RE=Sc, Ce, Pr, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu), greatly expanding the limited library of f-block mesoionic carbene complexes. These complexes were prepared by treatment of the parent RE-triamides with an N-heterocyclic olefin (NHO), where an NHO backbone proton undergoes a formal 1,4-proton migration to the NHO-methylene group. For all RE(III) metals, as expected, quantum chemical calculations suggest only a σ-component to the metal-carbene bonding, in contrast to a previously reported uranium(III) congener where the 5f3 metal engages in a weak π-back-bond to the MIC. All complexes were characterised by static variable-temperature magnetic measurements, and dynamic magnetic measurements reveal that 3Dy and 3Er are field-induced single-molecule magnets (SMMs), with Ueff energy barriers of 35 and 128 K, respectively. Complex 3Dy is, as expected, a poorly performing SMM, but conversely 3Er performs unexpectedly well.

Mesoionic Carbene Complexes of Uranium(IV) and Thorium(IV).

We report the synthesis and characterization of uranium(IV) and thorium(IV) mesoionic carbene complexes [An{N(SiMe3)2}2(CH2SiMe2NSiMe3){MIC}] (An = U, 4U and Th, 4Th; MIC = {CN(Me)C(Me)N(Me)CH}), which represent rare examples of actinide mesoionic carbene linkages and the first example of a thorium mesoionic carbene complex. Complexes 4U and 4Th were prepared via a C–H activation intramolecular cyclometallation reaction of actinide halides, with concomitant formal 1,4-proton migration of an N-heterocyclic olefin (NHO). Quantum chemical calculations suggest that the An–carbene bond comprises only a σ-component, in contrast to the uranium(III) analogue [U{N(SiMe3)2}3(MIC)] (1) where computational studies suggested that the 5f3 uranium(III) ion engages in a weak one-electron π-backbond to the MIC. This highlights the varying nature of actinide-MIC bonding as a function of actinide oxidation state. In solution, 4Th exists in equilibrium with the Th(IV) metallacycle [Th{N(SiMe3)2}2(CH2SiMe2NSiMe3)] (6Th) and free NHO (3). The thermodynamic parameters of this equilibrium were probed using variable-temperature NMR spectroscopy yielding an entropically favored but enthalpically endothermic process with an overall reaction free energy of ΔG298.15K = 0.89 kcal mol–1. Energy decomposition analysis (EDA-NOCV) of the actinide–carbon bonds in 4U and 4Th reveals that the former is enthalpically stronger and more covalent than the latter, which accounts for the respective stabilities of these two complexes.

Stabilisation of Li(0)-Li(0) bond by normal and mesoionic carbenes and electride characteristics of the complexes

Density functional theory (DFT) has been used to study the structure, stability, and bonding in the Li2L2 complexes containing Li(0)-Li(0) bond using both normal and mesoionic carbenes (MICs) as binding ligands (L) with Li2 molecule. The geometries and the nature of bonding of Li2(SNHCMe)2 and Li2(cAACMe)2 normal carbene complexes and Li2(MIC1)2 and Li2(MIC2)2 mesoionic carbene complexes have been compared with the previously reported Li2(NHC)2 normal carbene complex. The stability of the designed complexes is determined by positive Gibbs’ free energy changes (ΔG) towards the dissociation of the complexes into different fragments. Both normal and mesoionic carbenes stabilise the Li2 moiety and both the types of complexes are almost equally stable. From the natural bond orbital (NBO) analysis and the atoms in molecule (AIM) analysis, it is shown that two Li atoms are bonded by a single covalent bond with the oxidation state of zero on each Li. Again, the electron density analysis confirmed that each Li-Li bond has a non-nuclear attractor (NNA) at the middle of the bonds and the values of Laplacian of electron density (∇2 ρ(r)) are negative therein. Both the types of complexes show electride characteristics. The reactivity descriptors of all the studied complexes have been analyzed.

Excitation Energy-Dependent Branching Dynamics Determines Photostability of Iron(II)-Mesoionic Carbene Complexes.

Photoactive metal complexes containing earth-abundant transition metals recently gained interest as photosensitizers in light-driven chemistry. In contrast to the traditionally employed ruthenium or iridium complexes, iron complexes developed to be promising candidates despite the fact that using iron complexes as photosensitizers poses an inherent challenge associated with the low-lying metal-centered states, which are responsible for ultrafast deactivation of the charge-transfer states. Nonetheless, recent developments of strongly σ-donating carbene ligands yielded highly promising systems, in which destabilized metal-centered states resulted in prolonged lifetimes of charge-transfer excited states. In this context, we introduce a series of novel homoleptic Fe-triazolylidene mesoionic carbene complexes. The excited-state properties of the complexes were investigated by time-resolved femtosecond transient absorption spectroscopy and quantum chemical calculations. Pump wavelength-dependent transient absorption reveals the presence of distinct excited-state relaxation pathways. We relate the excitation-wavelength-dependent branching of the excited-state dynamics into various reaction channels to solvent-dependent photodissociation following the population of dissociative metal centered states upon excitation at 400 nm.

Anomalous magnetism of uranium(IV)-oxo and -imido complexes reveals unusual doubly degenerate electronic ground states

Anomalous magnetism of uranium(IV)-oxo and -imido complexes reveals unusual doubly degenerate electronic ground states

A 2,2'-diphosphinotolane as a versatile precursor for the synthesis of P-ylidic mesoionic carbenes via reversible C-P bond formation.

A metal-templated synthetic route to cyclic (aryl)(ylidic) mesoionic carbenes (CArY-MICs) featuring an endocyclic P-ylide is presented. This approach, which requires metal templates with two cis-positioned open coordination sites, is based on the controlled cyclisation of a P,P′-diisopropyl-substituted 2,2′-diphosphinotolane (1) and leads to chelate complexes coordinated by a phosphine donor and the CArY-MIC carbon atom. The C–P bond formation involved in the former partial cyclisation of 1 proceeds under mild conditions and was shown to be applicable all over the d-block. In the presence of a third fac-positioned open coordination site, the P–C bond formation was found to be reversible, as shown for a series of molybdenum complexes. DFT modelling studies are in line with an interpretation of the target compounds as CArY-MICs.

Coupling of CO2 and epoxides catalysed by novel N-fused mesoionic carbene complexes of nickel(II).

We report the syntheses of two rigid mesoionic carbene (MIC) ligands with a carbazole backbone via an intramolecular Finkelstein-cyclisation cascade and investigate their coordination behavior towards nickel(II) acetate. Despite the nickel(II) carbene complexes 4a,b showing only minor differences in their chemical composition, they display curious differences in their chemical properties, e.g. solubility. Furthermore, the potential of these novel MIC complexes in the coupling of carbon dioxide and epoxides as well as the differences in reactivity compared to classical NHC-derived complexes are evaluated.

Tuning electronic structure through halide modulation of mesoionic carbene cobalt complexes.

The first examples of Co(ii) mesoionic carbene complexes (CoX2DippMIC2; X = Cl-, Br-, I-) demonstrate a new electronic perturbation on tetrahedral Co(ii) complexes. Using absorption spectroscopy and magnetometry, the consequences of the MIC's strong σ-donating/minimal π-accepting nature are analyzed and shown to be further tunable by the nature of the coordinated halide.

A Triarylated 1,2,3‐Triazol‐5‐ylidene Ligand with a Redox‐Active Ferrocenyl Substituent for Rhodium(I)‐Catalyzed Hydroformylation of 1‐Octene

A series of rhodium(I)–1,2,3‐triazol‐5‐ylidene (trz) complexes are described, containing either a novel triarylated trz ligand A′ (N1, N3‐arylated, C4‐ferrocenyl) {complexes 1, [Rh(A′)Cl(cod)]; 4, [Rh(A′)Cl(CO)2]}, or N3‐alkylated triazolylidenes with a C4‐ferrocenyl {2, [Rh(B′)Cl(cod)]; 5, [Rh(B′)Cl(CO)2]} or C4‐phenyl substituent {3, [Rh(C′)Cl(cod)]; 6, [Rh(C′)Cl(CO)2]}. The free mesoionic carbene (MIC) A′ is structurally characterized and its electronic properties evaluated by employing the complex [Pd(Br)2(iPr2‐bimy)(A′)] (7) in an NMR spectroscopic analysis method. The redox activity of A′ is exploited, and the chemically oxidized precursor Aox and complex 4ox are isolated. The mesoionic carbene complexes 1–3, as well as in situ oxidized 1ox, are used as homogeneous catalysts for the hydroformylation of 1‐octene for the first time, and the influence of chemical oxidation of the catalyst on the activity and chemo‐ and regioselectivity of the catalyst precursor 1 is evaluated.

Steroid Derived Mesoionic Gold and Silver Mono- and Polymetallic Carbenes.

A two-step synthesis of gold mesoionic carbene complexes containing estrone moieties has been developed. The method uses the methylation of the triazole nucleus, followed by the treatment of the triazolium salt with Ag2O and transmetalation with [AuCl(SMe2)]. Mono-, bi-, tri-, and tetrametallic gold complexes can be obtained depending on the structure of the starting triazolium salts. Tetrametallic gold carbene embedded in a macrocylic stereoidal cavity containing four estrone nuclei has been also prepared. Additionally, the mono- and bimetallic silver carbene complexes containing triazole-steroid ligands have been isolated and characterized. These complexes resulted to be stable and have been characterized by spectroscopic and HRMS techniques. The gold and silver complexes having triazole-steroid ligands are unprecedented in the literature and the method reported here to access to these compounds is easy and efficient. Preliminary results regarding the catalytic activity of some of the gold-carbenes prepared in the insertion of diazoalkanes into alcohols are presented.

Synthesis of Imidazole-Based Functionalized Mesoionic Carbene Complexes of Palladium: Comparison of Donor Properties and Catalytic Activity toward Suzuki–Miyaura Coupling

Three different backbone-monofunctionalized imidazolium salts have been synthesized using the metal–halogen exchange procedure, and their corresponding mesoionic carbene complexes with palladium were prepared via oxidative addition without protection of the C2 position. The donor properties were evaluated with 31P NMR spectroscopy of the respective palladium complexes. The catalytic activity of these complexes toward Suzuki–Miyaura coupling of aryl bromides was also explored. Also, in one case, a comparison of donor properties was made with those of a “normal” carbene with similar steric bulk.

Iridium, ruthenium, and palladium complexes containing a mesoionic fused imidazolylidene ligand

Imidazo[1,2-a]pyridine consisting of a pyridine fused to an imidazolium salt at the imidazolium N1–C2 bond and hence protected from forming normal imidazole-2-ylidene complexes undergoes selective activation of the C5–H bond with Ag2O, i.e. at the imidazolium carbon that is proximal to the pyridine nitrogen. While the silver carbene complex is unstable, transmetallation with [IrCp*Cl2]2, [RuCl2(cym)]2, and [PdCl(allyl)]2 afforded stable mesoionic carbene complexes. Two iridium(III) complexes containing one fused carbene ligand and one palladium(II) complex containing two carbene ligands at the metal centre were structurally characterized. The absence of substituents adjacent to the carbene carbon prevents wingtip group activation, and it imparts a reduced stability of the complexes in particular under (mildly) acidic conditions.

Carbene iridium complexes for efficient water oxidation: scope and mechanistic insights

Combined kinetic & in operando spectroscopic studies, factor analysis, and DFT calculations provide insight into the water oxidation reaction catalyzed by operationally homogeneous iridium mesoionic carbene complexes.

Ynamides: stable ligand equivalents of unstable oxazol-4-ylidenes (novel mesoionic carbenes)

Deprotonation of an oxazolium salt induces a ring opening process leading to the corresponding ynamide. Although the expected mesoionic carbene is not obtained, the acyclic ynamide readily reacts with various transition metals yielding robust mesoionic carbene complexes.