Tripodal N-heterocyclic Carbene Research Articles

Closed Synthetic Cycle for Nickel-Based Dihydrogen Formation.

Dihydrogen evolution was observed in a two-step protonation reaction starting from a Ni0 precursor with a tripodal N-heterocyclic carbene (NHC) ligand. Upon the first protonation, a NiII monohydride complex was formed, which was isolated and fully characterized. Subsequent protonation yields H2 via a transient intermediate (INT) and an isolable NiII acetonitrile complex. The latter can be reduced to regenerate its Ni0 precursor. The mechanism of H2 formation was investigated by using a deuterated acid and scrutinized by 1 H NMR spectroscopy and gas chromatography. Remarkably, the second protonation forms a rare nickel dihydrogen complex, which was detected and identified in solution and characterized by 1 H NMR spectroscopy. DFT-based computational analyses were employed to propose a reaction profile and a molecular structure of the Ni-H2 complex. Thus, a dihydrogen-evolving, closed-synthetic cycle is reported with a rare Ni-H2 species as a key intermediate.

Dioxygen Activation by Internally Aromatic Metallacycle: Crystallographic Structure and Mechanistic Investigations.

SummaryMononuclear metal-peroxo species are invoked as the key intermediates in metalloenzymatic or synthetic catalysis. However, either transience or sluggishness reactivity of synthetic analogs of metal-peroxo species impedes our understanding of oxygen activation mechanism. Herein, we designed and characterized a dioxygen-derived mononuclear osmium-peroxo complex, in which the peroxo ligand is stabilized by internally aromatic metallacycle. We demonstrate that the osmium-peroxo species shows catalytic activity toward promoterless alcohol dehydrogenations. Furthermore, computational studies provide a new mechanism for the osmium-peroxo-mediated alcohol oxidation, starting with the concerted double-hydrogen transfer and followed by the generation of osmium-oxo species. Interestingly, the internally aromatic metallacycle also plays a vital role in catalysis, which mediates the hydrogen transfer from osmium center to the distal oxygen atom of Os–OOH moiety, thus facilitating the Os–OOH→Os=O conversion. We expect that these insights will advance the development of aromatic metallacycle toward aerobic oxidation catalysis.

Tripodal N-Heterocyclic Carbene Complexes of Palladium and Copper: Syntheses, Characterization, and Catalytic Activity

The tris-palladium tripodal N-heterocyclic carbene (NHC) complexes (timtebtBu){Pd(ICy)I2}3 (4a), (timtebtBu){Pd(PPh3)I2}3 (5a), and (timtebdipp){Pd(PPh3)I2}3 (5b) (timtebtBu = 1,3,5-{tris(tert-butylimidazol-2-ylidene)methyl}-2,4,6-triethylbenzene, 3a; ICy = 1,3-dicyclohexylimidazol-2-ylidene; timtebdipp = 1,3,5-{tris({2,6-diisopropylphenyl}imidazol-2-ylidene)methyl}-2,4,6-triethylbenzene, 3b) were prepared by reaction of Pd(II) precursors with either the free carbenes or imidazolium salts. Treatment of [Cu(NCMe)4]X (X = PF6, BF4) with 3a or 3b produced the tris-copper(I) bridged complexes [(timtebR)Cu3(μ3-O)]X (R = tBu, X = PF6, 6a; R = dipp, X = BF4, 6b). Complexes 4a, 5a, and 6a were structurally characterized. The palladium complexes were tested as catalysts for Suzuki−Miyaura and Sonogashira coupling reactions and the copper species also employed for the Sonogashira reaction, as well as for the Ullmann-type arylation of imidazoles and phenols.

Unexpected reactivity resulting from modifications of the ligand periphery: Synthesis, structure, and spectroscopic properties of iron complexes of new tripodal N-heterocyclic carbene (NHC) ligands

The chelating ligand tris-[2-(3-aryl- imidazol-2-ylidene) ethyl]ami ne (TIMEN R, R = aryl = 2,6-xylyl (xyl), mesityl (mes)) has provided access to reactive transition metal complexes. Here, two new tripodal N-heterocyclic carbene ligands of the TIMEN R system (R = aryl = tolyl (tol), 3,5-xylyl (3,5xyl)), featuring sterically less demanding aryl substituents were synthesized. With these ligands, Fe(II) precursor complexes could be obtained, namely [(TIMEN tol)Fe](BF 4) 2 ( 3) and [(TIMEN 3,5xyl)Fe(CH 3CN)](PF 6) 2 ( 7), which showed unexpected reactivity upon reduction. Treatment of the compounds with sodium amalgam yield the tris- and bis-metallated products, [(TIMEN tol∗∗∗)Fe] ( 4) and [(TIMEN 3,5xyl∗∗)Fe] ( 8), respectively. While the Fe(III) complex 4 is relatively inert towards oxygen, the Fe(II) complex 8 is prone to oxidation. This oxidation of 8 can readily be observed in chlorinated solvents, producing the Fe(III) complex [(TIMEN 3,5xyl∗∗)Fe](PF 6) ( 9). All new ligand imidazolium precursors and metal complexes were characterized by single crystal X-ray structure determination.

An Iron Nitride Complex

Coordination compounds of iron in high oxidation states have been invoked as reactive intermediates in biocatalyses. Iron(IV) ferryl species are examples of such highly reactive species that have long been known to be at the catalytic centers of oxygenases. Supported by X-ray diffraction studies on nitrogenase, the iron nitride moiety has recently been suggested to be present at the site of biological nitrogen reduction. As a result, well-characterized high-valent iron complexes have been sought as biomimetic models for transformations mediated by iron-containing enzymes. To gain understanding of iron nitride reactivity and the possible role of such species in biocatalysis, insight into the molecular structure of complexes stabilizing the [FeN] synthon is highly desirable. Whereas significant progress has been made in the synthesis and spectroscopic elucidation of Fe=NR and Fe N species, X-ray crystallographic characterization of a complex with a terminal Fe N functionality has not been accomplished. The first mononuclear Fe=O entity crystallographically characterized was stabilized in an octahedral environment provided by a macrocyclic tetra-N-methylated cyclam ligand. Similar cyclam derivatives also allow the stabilization and detailed spectroscopic characterization of octahedral Fe and Fe nitride complexes in unusually high oxidation states. 10] Recently, Peters and Betley developed a stunningly redox-rich iron system employing the tripodal tris(phosphino)borate ligand system (PhBP3 ), which stabilizes tetrahedral L3Fe=Nx species in oxidation states ranging from + I to + IV. Remarkably, this ligand system enabled the first room-temperature spectroscopic characterization of a terminal Fe nitride species. Concentration-dependent coupling to the Fe-N2-Fe I dinuclear product, however, prevents crystallization of this nitride species. We herein present the synthesis, spectroscopy, and most significantly, the X-ray diffraction analysis of a discrete iron nitride complex stabilized by the sterically encumbering Nanchored tris(carbene) ligand, tris[2-(3-aryl-imidazol-2-ylidene)ethyl]amine (TIMEN, R= xylyl (xyl), mesityl (mes)). Structurally and electronically related to the tetrahedral phosphinoborate ligand system by Peters and Betley, this tripodal N-heterocyclic carbene (NHC) system coordinates a high-spin Fe center in a trigonal-planar fashion, thus forming four-coordinate complexes with the metal ion in trigonalpyramidal environments. Under inert atmosphere, treatment of TIMEN with one equivalent of anhydrous ferrous chloride in pyridine at room temperature yields the four-coordinate Fe complexes [(TIMEN)Fe(Cl)]Cl (1, 1) as analytically pure, white powders in 80% yield (Scheme 1).

Dioxygen activation by a low-valent cobalt complex employing a flexible tripodal N-heterocyclic carbene ligand.

The novel versatile cobalt(I) tris-carbene complex [(TIMEN(xyl))Co]Cl (1) (where TIMEN = (tris[2-(3-arylimidazol-2-ylidene)ethyl]amine) reacts with CO, one-electron oxidizers such as CH(2)Cl(2), and O(2) to yield the cobalt complexes [(TIMEN(xyl))Co(CO)]Cl (2), [(TIMEN(xyl))Co(Cl)]Cl (3), and peroxo species [(TIMEN(xyl))Co(O(2))](BPh(4)) (5). All new complexes were fully characterized by (1)H NMR, UV/vis, and IR spectroscopy as well as superconducting quantum interference device (SQUID) magnetization measurements and single-crystal X-ray crystallography. The nucleophilic character of the eta(2)-bound dioxygen ligand in 5 was confirmed by density functional theory (DFT) studies and allows for oxygen-transfer reactions with electron-deficient organic substrates, such as benzoyl chloride.

Carbene complexes of rhodium and iridium from tripodal N-heterocyclic carbene ligands: synthesis and catalytic properties.

Two tripodal trisimidazolium ligand precursors have been tested in the synthesis of new N-heterocyclic carbene rhodium and iridium complexes. [Tris(3-methylbenzimidazolium-1-yl)]methane sulfate gave products with coordination of the decomposed precursor. [1,1,1-Tris(3-butylimidazolium-1-yl)methyl]ethane trichloride (TIMEH(3)(Bu)) coordinated to the metal in a chelate and bridged-chelate form, depending on the reaction conditions. The crystal structures of two of the products are described. The compounds resulting from the coordination with TIME(Bu) were tested in the catalytic hydrosilylation of terminal alkynes.

Intramolecular C-H...Ccarbene hydrogen bonds and competing interactions in monoprotonated tripodal carbenes.

The anionic tripodal N-heterocyclic carbene (C3N2H3)3BH- first prepared by Fehlhammer, together with three neutral variants, (C3N2H3)3CH, (C3N2H3)3P, and (C3N2H3)3SiH, have been studied using quantum chemical methods. Isodesmic reactions are used to deduce that the phosphine-bridgehead species in particular has a large-resonance stabilization energy. All the podands undergo substantial conformational change on excitation to the lowest triplet electronic state, with effective localization of the excitation on one of the heterocyclic rings, dearomatizing it. On monoprotonation of the ground states, three of these species display intramolecular C-H...Ccarbene hydrogen bonding: The nature and strength of these interactions is explored using model (intermolecularly hydrogen-bonded) complexes, isodesmic reactions, and GIAO calculations of chemical shifts. One surprising result is that C-H...Ccarbene hydrogen bonds involving ethenic hydrogens can be almost as strong as those involving the imidazolium proton (first identified by Arduengo). The case of the monoprotonated carbon bridgehead species is in particular intriguing. It is stabilized by a competitive Ccarbene...N interaction of sufficient strength to override the C-H...Ccarbene bonding motif observed in the other structures.

Group 11 Metal Complexes of N-Heterocyclic Carbene Ligands: Nature of the MetalCarbene Bond

The silver complex of the tripodal N-heterocyclic carbene ligand TIMEMe, [(TIMEMe)2Ag3](PF6)3 (3), reacts with copper(I) bromide and (dimethyl sulfide)gold(I) chloride to yield the corresponding D3-symmetrical copper(I) and gold(I) complexes [(TIMEMe)2Cu3](PF6)3 (4) and [(TIMEMe)2Au3](PF6)3 (5). Single-crystal X-ray diffraction, spectroscopic, and computational studies of this series of metal NHC complexes are described. The group 11 metal complexes of the TIMEMe ligand exhibit isostructural geometries, with three metal ions bridging two of the TIMEMe ligands. Each metal ion is linearly coordinated to two carbene centers, with each of the carbenoid carbons stemming from a different ligand. Overall, the molecules possess D3 symmetry. The electronic structure of these newly synthesized compounds was elucidated with the aid of DFT calculations. In contrast to the common assumption that NHCs are pure σ-donor ligands, our calculations reveal the existence of both σ- and π-type interactions between the metal io...

Synthesis and characterization of electron-rich nickel tris-carbene complexes.

The synthesis and characterization of the first nickel(0)/(i) tris-carbene complex with a nitrogen anchored tripodal N-heterocyclic carbene ligand are described.

Copper Complexes of Nitrogen-Anchored Tripodal N-Heterocyclic Carbene Ligands

Incorporation of a nitrogen functionality into a tripodal N-heterocyclic carbene ligand system affords the first N-anchored tetradentate tris-carbene ligands TIMEN(R) (R = Me (5a), t-Bu (5b), Bz (5c)). Treatment of the methyl derivatized [H(3)TIMEN(Me)](PF(6))(3) imidazolium salt (H(3)5a) with silver oxide yields the silver complex [(TIMEN(Me))(2)Ag(3)](PF(6))(3) (9), which, in a ligand transfer reaction, reacts with copper(I) bromide to give the trinuclear copper(I) complex [(TIMEN(Me))(2)Cu(3)](PF(6))(3) (10). Deprotonation of the tert-butyl and benzyl derivatives [H(3)TIMEN(t-Bu)](PF(6))(3) and [H(3)TIMEN(Bz)](PF(6))(3) yields the free tris-carbenes TIMEN(t-Bu) (5b) and TIMEN(Bz) (5c), which react readily with copper(I) salts to give mononuclear complexes [(TIMEN(t-Bu))Cu](PF(6)) (11b) and [(TIMEN(Bz))Cu]Br (11c). The solid-state structures of 10, 11b, and 11c were determined by single-crystal X-ray diffraction. While the TIMEN(Me) ligand yields trinuclear complex 10, with both T-shaped three-coordinate and linear two-coordinate copper(I) centers, the TIMEN(t-Bu) and TIMEN(Bz) ligands induce mononuclear complexes 11b and 11c, rendering the cuprous ion in a trigonal planar ligand environment of three carbenoid carbon centers and an additional, weak axial nitrogen interaction. Complexes 11b and 11c exhibit reversible one-electron redox events at half-wave potentials of 110 and -100 mV vs Fc/Fc(+), respectively, indicating sufficient electronic and structural flexibility of both TIMEN(R) ligands (R = t-Bu, Bz) to stabilize copper(I) and copper(II) oxidation states. Accordingly, a copper(II) NHC complex, [(TIMEN(Bz))Cu](OTf)(2) (12), was synthesized. Paramagnetic complex 12 was characterized by elemental analysis, EPR spectroscopy, and SQUID magnetization measurements.