Calcium Hydride Reactivity: Formation of an Anionic N‐Heterocyclic Olefin Ligand

An anionic N‐heterocyclic olefin ligand was serendipitously obtained by reaction of an amidinate calcium hydride complex with 1,3‐dimethyl‐2‐methyleneimidazole (NHO). Instead of anticipated addition to the polarized C=CH₂ bond to form an unstabilized alkylcalcium complex, deprotonation of the NHO ligand in the backbone was observed. Preference for deprotonation versus addition is explained by loss of aromaticity in the latter conversion. Theoretical calculations demonstrate the substantially increased ylidic character of this anionic NHO ligand which, like N‐heterocyclic dicarbenes, shows strong bifunctional coordination.     

N-Heterocyclic Olefin-Ligated Palladium(II) Complexes as Pre-Catalysts for Buchwald–Hartwig Aminations

New N-heterocyclic olefins (NHOs) are described with functionalization on the ligand heterocyclic backbone and terminal alkylidene positions. Various Pd^II–NHO complexes have been formed and their use as pre-catalysts in Buchwald–Hartwig aminations was explored. The most active system for catalytic C−N bond formation between hindered arylamine and arylhalide substrates was accessed by combining a backbone methylated NHO with [Pd(cinnamyl)Cl]₂ in the presence of NaOtBu as a base. In these active systems evidence suggests that catalysis is mediated by colloidal palladium metal, highlighting a different coordination ability of NHOs in comparison with commonly used N-heterocyclic carbene co-ligands.     

Oxazolines as chiral building blocks for imidazolium salts and N-heterocyclic carbene ligands

Enantiomerically pure imidazolium triflates can be readily prepared from bioxazolines and oxazolineimines; deprotonation of imidazolium triflate 2 gives a chiral N-heterocyclic carbene that can act as a ligand in a catalytically active palladium complex.     

Sydnone Methides: Intermediates between Mesoionic Compounds and Mesoionic N-Heterocyclic Olefins

Sydnone methides represent an almost unknown class of mesoionic compounds which possess exocyclic carbon substituents instead of oxygen (sydnones) or nitrogen (sydnone imines) in the 5-position of a 1,2,3-oxadiazolium ring. Unsubstituted 4-positions give rise to the generation of anionic N-heterocyclic carbenes by deprotonation. Preparations of new sydnone methides are described here. They can be represented by mesomeric structures with either exocyclic carbanionic groups like −C(CN)₂⁻, −C(CN)(COOMe)⁻, −C(CN)(CONH₂)⁻, and −C(CN)(SO₂Me)⁻, or with the corresponding exocyclic C=C double bonds as a common feature with mesoionic N-heterocyclic olefins. An X-ray single structure analysis revealed a length of 140.7(3) pm of the exocyclic bond in the solid state. From the coalescence temperature (55 °C) determined by a series of ¹³C NMR experiments (150 MHz) at various temperatures, an energy of rotation of 18.5 Kcal/mol was calculated for this bond. The ¹³C NMR signals of the anionic N-heterocyclic carbenes, from which the 2-mesityl-substituted anionic NHC proved to be stable up to 10 °C, are highly shifted upfield (δ_carbene=157.9 ppm−160.5 ppm). The carbenes can be reacted in situ with elemental selenium and chlorophosphanes to yield sydnone methide selenoethers after methylation and 4-phosphanylsydnone methides in good to excellent yields, respectively.     

Synergic Deprotonation Generates Alkali-Metal Salts of Tethered Fluorenide-NHC Ligands Co-Complexed to Alkali-Metal Amides

Synergic combinations of alkali-metal hydrocarbyl/amide reagents were used to synthesise saturated N-heterocyclic carbene (NHC) ligands tethered to a fluorenide anion through deprotonation of a spirocyclic precursor, whereas conventional bases were not successful. The Li₂ derivatives displayed a bridging amide between two Li atoms within the fluorenide-NHC pocket, whereas the Na₂ and K₂ analogues displayed extended solid-state structures with the fluorenide-NHC ligand chelating one alkali metal centre.     

Half‐Sandwich Iridium Complexes Bearing a Diprotic Glyoxime Ligand: Structural Diversity Induced by Reversible Deprotonation

Synthesis and deprotonation reactions of half‐sandwich iridium complexes bearing a vicinal dioxime ligand were studied. Treatment of [{Cp*IrCl(μ‐Cl)}₂] (Cp*=η⁵‐C₅Me₅) with dimethylglyoxime (LH₂) at an Ir:LH₂ ratio of 1:1 afforded the cationic dioxime iridium complex [Cp*IrCl(LH₂)]Cl ( 1 ). The chlorido complex 1 undergoes stepwise and reversible deprotonation with potassium carbonate to give the oxime–oximato complex [Cp*IrCl(LH)] ( 2 ) and the anionic dioximato(2?) complex K[Cp*IrCl(L)] ( 3 ) sequentially. Meanwhile, twofold deprotonation of the sulfato complex [Cp*Ir(SO₄)(LH₂)] ( 4 ) resulted in the formation of the oximato‐bridged dinuclear complex [{Cp*Ir(μ‐L)}₂] ( 5 ). X‐ray analyses disclosed their supramolecular structures with one‐dimensional infinite chain ( 1 and 2 ), hexagonal open channels ( 3 ), and a tetrameric rhomboid ( 4 ) featuring multiple intermolecular hydrogen bonds and electrostatic interactions.     

Exploring the Coordination Properties of Phosphonium-Functionalized N-Heterocyclic Carbenes Towards Gold

Gold(I) complexes with N-heterocyclic carbene ligands functionalized with a pendant phosphonium moiety were synthesized by simple procedures. In particular, the simple addition of LiBr salt in the reaction media allows the formation of the NHC gold(I) mononuclear complexes, whilst in the absence of excess bromide ions the deprotonation of the methylene group in alpha position to the phosphonium group occurs, allowing the isolation of the dinuclear complexes with two C,C-bridging NHC-phosphonium ylide ligands. The complexes were characterized in solution with NMR spectroscopy and ESI-MS spectrometry, as well as in the solid state by means of single crystal X-ray diffraction analysis. Mononuclear gold(III) species were also isolated by Br₂ oxidative addition to the mononuclear gold(I) species and fully characterized. Preliminary results of the biological effects on MCF7 cancer cells are also reported.     

ProPhenol Derived Ligands to Simultaneously Coordinate a Main-Group Metal and a Transition Metal: Application to a Zn−Cu Catalyzed Reaction

A new bifunctional ligand bearing chiral N-heterocyclic carbene (NHC) and prolinol moieties is presented. Utilizing the designed ligand, an in situ formed Cu/Zn hetero-bimetallic complex unlocks the asymmetric allylic alkylation reactions of allyl phosphates with zinc keto-homoenolates, leading to the formation of various γ-vinyl ketones with good regio- and enantio-selectivity. DF sT calculation supports that the chelation of allyl phosphates with catalyst promotes the S_N2’ addition and the ligand-substrate steric interactions account for the stereoselective outcome.     

The synthesis and catalytic activity of the iridium(I) hydroquinone complex [(H2Q)Ir(COD)]

The η⁶-hydroquinone iridium complex [(η⁶-H₂Q)Ir(COD)]BF₄ undergoes facile double deprotonation to the η⁴-quinone anionic analogue. The latter has been shown to be the first example of an iridium complex that catalyzes 1,4 conjugate addition reactions of aryl boronic acids to electron deficient olefins.     

Tin(IV) complexes of 2‐benzoylpyridine N(4)‐phenyl‐thiosemicarbazone: spectral characterization,structural studies and antifungal activity

Three tin(IV) complexes of 2‐benzoylpyridine N(4)‐phenylthiosemicarbazone (H2Bz4Ph) were prepared: [Sn(L)Cl₃] (1), [BuSn(L)Cl₂] (2) and [(Bu)₂Sn(L)Cl] (3), in which L stands for the anionic ligand formed upon complexation with deprotonation and release of HCl. The complexes were characterized by a number of spectroscopic techniques. The crystal structures of H2Bz4Ph and complex 3 were determined. The antifungal activity of the ligand and its tin(IV) complexes was tested against Candida albicans. The thiosemicarbazone proved to be more active than the tin(IV) complexes. Copyright © 2003 John Wiley & Sons, Ltd.     

Anionic N-heterocyclic carbenes with N,N'-bis(fluoroaryl) and N,N'-bis(perfluoroaryl) substituents

A series of rhodium complexes, [Rh(cod)(NHC-F(x))(OH(2))] (cod = 1,5-cyclooctadiene; NHC = N-heterocyclic carbene), incorporating anionic N-heterocyclic carbenes with 2-tert-butylmalonyl backbones and 2,6-dimethylphenyl (x = 0), 2,6-difluorophenyl (x = 4), 2,4,6-trifluorophenyl (x = 6), and pentafluorophenyl (x = 10) N,N'-substituents, respectively, has been prepared by deprotonation of the corresponding zwitterionic precursors with potassium hexamethyldisilazide, followed by immediate reaction of the resulting potassium salts with [{RhCl(cod)}(2)]. These complexes could be converted to the related carbonyl derivatives [Rh(CO)(2)(NHC-F(x))(OH(2))] by displacement of the COD ligand with CO. IR and NMR spectroscopy demonstrated that the degree of fluorination of the N-aryl substituents has a considerable influence on the σ-donating and π-accepting properties of the carbene ligands and could be effectively used to tune the electronic properties of the metal center. The carbonyl groups on the carbene ligand backbone provided a particularly sensitive probe for the assessment of the metal-to-ligand π donation. The ortho-fluorine substituents on the N-aryl groups in the carbene ligands interacted with the other ligands on rhodium, determining the conformation of the complexes and creating a pocket suitable for the coordination of water to the metal center. Computational studies were used to explain the influence of the fluorinated N-substituents on the electronic properties of the ligand and evaluate the relative contribution of the σ- and π-interactions to the ligand-metal interaction.     

Self-Assembled Monolayers of N-Heterocyclic Olefins on Au(111)

Self-assembled monolayers (SAMs) of N-heterocyclic olefins (NHOs) have been prepared on Au(111) and their thermal stability, adsorption geometry, and molecular order were characterized by X-ray photoelectron spectroscopy, polarized X-ray absorption spectroscopy, scanning tunneling microscopy (STM), and density functional theory (DFT) calculations. The strong σ-bond character of NHO anchoring to Au induced high geometrical flexibility that enabled a flat-lying adsorption geometry via coordination to a gold adatom. The flat-lying adsorption geometry was utilized to further increase the surface interaction of the NHO monolayer by backbone functionalization with methyl groups that induced high thermal stability and a large impact on work-function values, which outperformed that of N-heterocyclic carbenes. STM measurements, supported by DFT modeling, identified that the NHOs were self-assembled in dimers, trimers, and tetramers constructed of two, three, and four complexes of NHO−Au-adatom. This self-assembly pattern was correlated to strong NHO−Au interactions and steric hindrance between adsorbates, demonstrating the crucial influence of the carbon-metal σ-bond on monolayer properties.     

Chelating alkoxy-N-heterocyclic carbene complexes of silver and copper

Incorporation of an alkoxide functional group into an N-heterocyclic carbene (NHC) ligand allows the synthesis of the first anionic NHC chelating ligands, which react to give the first neutral, molecular silver(I) alkoxide carbene complex, and a copper(I) derivative containing the first nonmacrocyclic, square planar Cu(I) centres.     

Dynamic Ligand Reactivity in a Rhodium Pincer Complex

Ligand cooperativity provides (transition) metal complexes with new reactivities in substrate activation and catalytic reactions, but usually the ligand acts as an internal (Brønsted) base, while the metal acts as a (Lewis) acid. We describe the synthesis and stepwise activation of a new phosphane‐pyridine‐amide ligand PNN^H2 in combination with Rh^I. The ligand is susceptible to stepwise proton and hydride loss from the nitrogen arm (imine formation) and deprotonation at the pyridylphosphine arm (dearomatization), giving rise to amine complex 1 , amido species 2 , imine complex 3 and dearomatized compound 4 . Complex 4 bears a dual‐mode cooperative PNN′ ligand containing both a (nucleophilic) basic methine fragment and a reactive (electrophilic) imine moiety. The basic ligand arm enables substrate deprotonation while the imine ligand arm enables reversible “storage” of the activated (nucleophilic) form of a sulfonamide substrate at the ligand. In combination with metal‐based reactivity, this allows for the mono‐alkylation of o‐toluenesulfonamide with iodomethane. Compounds 1 , 3 and 4 are structurally characterized. We also report the first structurally characterized example of an aminal in the coordination sphere of rhodium, complex 5 , [Rh(CO)( PNN′′ )], formed by sequential N?H activation of sulfonamide by the dearomatized ligand PNN′ and follow‐up nucleophilic attack of anionic sulfonamide onto the imine fragment.     

Ligand‐Centred Reactivity of Bis(picolyl)amine Iridium: Sequential Deprotonation,Oxidation and Oxygenation of a “Non‐Innocent” Ligand

Treatment of [Ir(bpa)(cod)]⁺ complex [ 1 ]⁺ with a strong base (e.g., tBuO^?) led to unexpected double deprotonation to form the anionic [Ir(bpa?2H)(cod)]^? species [ 3 ]^?, via the mono‐deprotonated neutral amido complex [Ir(bpa?H)(cod)] as an isolable intermediate. A certain degree of aromaticity of the obtained metal–chelate ring may explain the favourable double deprotonation. The rhodium analogue [ 4 ]^? was prepared in situ. The new species [M(bpa?2H)(cod)]^? (M=Rh, Ir) are best described as two‐electron reduced analogues of the cationic imine complexes [M^I(cod)(Py‐CH₂‐N?CH‐Py)]⁺. One‐electron oxidation of [ 3 ]^? and [ 4 ]^? produced the ligand radical complexes [ 3 ]^. and [ 4 ]^.. Oxygenation of [ 3 ]^? with O₂ gave the neutral carboxamido complex [Ir(cod)(py‐CH₂‐N‐CO‐py)] via the ligand radical complex [ 3 ]^. as a detectable intermediate.     

Molybdenum(VI) complexes with N-substituted hydroxylamines

New molybdenum(VI) complexes with N-monoalkylsubstituted hydroxylamines have been synthesized and studied. The structure of [MoO₂(C₂H₅NHO)₂] and [MoO₂(i-C₃H₇NHO)₂], whose distinguishing feature is the bidentate chelate coordination of the ligand to molybdenum with the formation of the three-membered NMoO chelate ring, has been determined by X-ray diffraction.     

Synthesis of Platinum(II) N-Heterocyclic Carbenes Based on Adenosine

Organometallic derivatization of nucleosides is a highly promising strategy for the improvement of the therapeutic profile of nucleosides. Herein, a methodology for the synthesis of metalated adenosine with a deprotected ribose moiety is described. Platinum(II) N-heterocyclic carbene complexes based on adenosine were synthesized, namely N-heterocyclic carbenes bearing a protected and unprotected ribose ring. Reaction of the 8-bromo-2′,3′,5′-tri-O-acetyladenosine with Pt(PPh₃)₄ by C8−Br oxidative addition yielded complex 1, with a Pt^II centre bonded to C-8 and an unprotonated N7. Complex 1 reacted at N7 with HBF₄ or methyl iodide, yielding protic carbene 2 or methyl carbene 3, respectively. Deprotection of 1 to yield 4 was achieved with NH₄OH. Deprotected compound 4 reacted at N7 with HCl solutions to yield protic NHC 5 or with methyl iodide yielding methyl carbene 6. Protic N-heterocyclic carbene 5 is not stable in DMSO solutions leading to the formation of compound 7, in which a bromide was replaced by chloride. The cis-influence of complexes 1–7 was examined by ³¹P{¹H} and ¹⁹⁵Pt NMR. Complexes 2, 3, 5, 6 and 7 induce a decrease of ¹J_Pt,P of more than 300 Hz, as result of the higher cis-influence of the N-heterocyclic carbene when compared to the azolato ligand in 1 and 4.     

Tricyanoborane-Functionalized Anionic N-Heterocyclic Carbenes: Adjustment of Charge and Stereo-Electronic Properties

The 1-methyl-3-(tricyanoborane)imidazolin-2-ylidenate anion ( 2 ) was obtained in high yield by deprotonation of the B(CN)₃-methylimidazole adduct 1 . Regarding charge and stereo-electronic properties, anion 2 closes the gap between well-known neutral NHCs and the ditopic dianionic NHC, the 1,3-bis(tricyanoborane)imidazolin-2-ylidenate dianion ( IIb ). The influence of the number of N-bonded tricyanoborane moieties on the σ-donating and π-accepting properties of NHCs was assessed by quantum chemical calculations and verified by experimental data on 2 , IIb , and 1,3-dimethylimidazolin-2-ylidene (IMe, IIa ). Therefore NHC 2 , which acts as a ditopic ligand via the carbene center and the cyano groups, was reacted with alkyl iodides, selenium, and [Ni(CO)₄] yielding alkylated imidazoles 3 and 4 , the anionic selenium adduct 5 , and the anionic nickel tricarbonyl complex 8 , respectively. The results of this study prove that charge, number of coordination sites, buried volume (%V_bur) and σ-donor and π-acceptor abilities of NHCs can be effectively fine-tuned via the number of tricyanoborane substituents.     

Ylidyl‐substituted Phosphonio‐benzophospholides as Chelating Ligands

Phosphonio‐benzo[c]phospholides with an additional phosphonium ylide substituent in 3‐position were synthesized by deprotonation of appropriately substituted 1, 3‐bis‐phosphonio benzophospholide cations and characterized by spectroscopic and analytical data. The ability of these molecules to act as bidentate P, C‐chelating ligands to transition metal atoms was demonstrated in the reactions with [W(CO)₄(norbornadiene)] and [MCl₂(cyclooctadiene)] (M = Pd, Pt). The Pd^II and Pt^II complexes are distinguished by a strong inclination towards addition of H₂O to the 10π‐electron system of the ligand. The molecular structures of a W⁰ complex with a P, C‐chelating ylidyl‐phosphonio‐benzophospholide ligand and of the product resulting from H₂O‐addition to a corresponding Pt^II complex were determined. The structural parameters of the W⁰ complex provide evidence for the presence of substantial steric strain around the metal atom.     

Imidazole to NHC rearrangements at molybdenum centers: an experimental and theoretical study

Both manganese and rhenium complexes of the type [M(bipy)(CO)(3)(N-RIm)](+) (bipy=2,2'-bipyridine) undergo deprotonation of the central CH group of the N-alkylimidazole (N-RIm) ligand when treated with a strong base. However, the outcome of the reaction is very different for either metal. For Mn, the addition of the equimolar amount of an acid to the product of the deprotonation affords an N-heterocyclic carbene (NHC) complex, whereas for Re, once the deprotonation of the central imidazole CH group has occurred, the bipy ligand undergoes a nucleophilic attack on an ortho carbon, affording the C-C coupling product. The extension of these studies to pseudo-octahedral [Mo(η(3)-allyl)(bipy)(CO)(2)(N-RIm)](+) complexes has allowed us to isolate cationic NHC complexes (Mn(I)-type behavior), as well as their neutral imidazol-2-yl precursors. Theoretical studies of the reaction mechanisms using DFT computations were carried out on the deprotonation of [Mn(bipy)(CO)(3)(N-PhIm)](+), [Re(bipy)(CO)(3) (N-MesIm)](+), and [Mo(η(3)-C(4)H(7))(bipy)(CO)(2) (N-MesIm)](+) complexes (Mes=mesityl) at the B3LYP/6-31G(d) (LANL2DZ for Mn, Re, and Mo) level of theory. Our results explain why different products have been found experimentally for Mn, Mo, and Re complexes. For Re, the process leading to a C-C coupling product is clearly more favored than those forming an imidazol-2-yl product. In contrast, for Mn and Mo complexes, the lower stabilizing interaction between the central imidazole and ortho bipy C atoms, along with the higher lability of the ligands, make the formation of an NHC-type product kinetically more accessible, in good agreement with experimental findings.