The Core Difference between a Mesoionic and a Normal N-Heterocyclic Carbene.

The properties of the abnormal N-heterocyclic carbene (NHC) 1,4-dimesityl-3-methyl-1,2,3-triazolin-5-ylidene were comprehensively compared to those of the related normal carbene 1,3-dimesitylimidazolin-2-ylidene using a range of steric and electronic probe techniques (% V bur, steric maps, Tolman electronic parameter, alane, Huynh electronic parameter, selone, and pK a values). The two NHCs were determined to be sterically equivalent (isostructural), while the triazolin-5-ylidene was found to be a stronger σ-electron donor and a much weaker π-electron acceptor. These results were used to demonstrate that the electronic properties of these NHCs could affect the stereochemical outcome of an NHC-catalyzed reaction.

Bulky bis(aryl)triazenides: just aspiring amidinates? A structural and spectroscopic study.

The synthesis and molecular structures (by single crystal X-ray diffraction) of s-, p- and d-metal complexes of the sterically demanding N,N'-bis(2,6-diisopropylphenyl)triazenide are reported and the spectroscopic (NMR spectroscopy and infrared spectroscopy) and physical properties of these complexes compared to related formamidinate complexes. Through the use of infrared spectroscopy the σ-donor capacity of this ligand is demonstrated to be reduced relative to the structurally isomorphous formamidinate congener, which supports previously advanced theoretical calcluations and DFT results reported herein. These electronic differences are highlighted by the stark contrast in reaction outcomes at rhodium; where [(Dipp2N3)Rh(CO)2] (1) is an isolable, stable complex and the formamidinate complex is not. The coordination chemistry of the triazenide ligand for the s-block metal complexes (M = Li, Na, K) has been shown to give structurally isomorphous complexes to the formamidinate analogue. In contrast to the amidinate complexes, these complexes show extreme lability of coordinated, volatile Lewis-bases, which in turn-yields the highly insoluble base-free triazenide complexes. These complexes are also synthesized directly in the absence of donor solvents. This triazenide ligand has proven to be a suitable ligand for stabilising reactive main group hydrides of Group 13 (M = Ga, In) and attempts at the analogous thallium hydride complex by halide-hydride exchange are reported. Finally attempts at the synthesis of low valent main group complexes are reported ([MIL], M = In, Ga) are also reported, which yield instead disproportionation products ([MIIIXL2], M = Ga, In; [{MIIXL}2], M = Ga).

The pKa values of N-aryl imidazolinium salts, their higher homologues, and formamidinium salts in dimethyl sulfoxide.

A series of imidazolinium salts, their six-, seven- and eight-membered homologues, and the related formamidinium salts were prepared, and their pKa values were determined in DMSO at 25 °C using the bracketing indicator method. The effect of each type of structural variation on the acidity of each salt was considered, particularly noting the importance of ring size and the effect of the steric and electronic nature of the N-aryl substituents. The effect of a cyclic structure was also probed through comparing the cyclic systems with the corresponding formamidinium salts, noting the importance of conformational flexibility in the latter cases. Along with allowing choice of appropriate bases for deprotonation of these species, it is anticipated that the data presented will aid in the understanding of the nucleophilicity, and potentially catalytic efficacy, of the corresponding carbenes.

The impact of cation structure upon the acidity of triazolium salts in dimethyl sulfoxide.

A series of triazolium salts, selected for their varying electronic and steric properties, were prepared and their pKa values were determined in DMSO at 25 °C using the bracketing indicator method. The effect of each systematic structural variation upon the acidity of the triazolium cation has been considered, in particular examining the effects of systematically altering electronic properties, quantified through the use of Hammett σ parameters. The first pKa value for an azolium salt that generates a mesionic carbene is also reported. These new data allow for the selection of appropriate bases for the deprotonation of such triazolium salts and the potential to correlate the pKa values determined herein with the nucleophilicity of the corresponding carbenes.

Kinetic stabilization of low-oxidation state and terminal hydrido main group metal complexes by a sterically demanding N,N'-bis(2,6-terphenyl)triazenide.

A sterically demanding N,N'-bis(2,6-terphenyl)triazenide was employed to stabilize a number of low-coordinate main group metal terminal hydride complexes. These complexes display enhanced thermal stabilities relative to analogous complexes ligated by ß-diketiminates, which we attribute to the steric shielding of the metal hydride moiety by the ligand. We also show this triazenide ligand can stabilize low-oxidation state Group 13 metals. DFT calculations conducted on these triazenide ligated Group 13 metal(i) complexes revealed they possess a narrower HOMO-LUMO gap relative to analogous complexes ligated by other monoanionic N,N'-ligands. In addition, several heteroleptic Group 13 metal(iii) and Group 14 metal(ii) complexes featuring this triazenide are reported.

Structural diversity in a homologous series of donor free alkali metal complexes bearing a sterically demanding triazenide.

The reactivity of the sterically demanding triazene Dmp-N[double bond, length as m-dash]N-N(H)-Dmp (Dmp2N3H, Dmp = 2,6-bis(2,4,6-trimethylphenyl)phenyl) with alkali metal bases and elemental heavy alkali metals has been examined. All reactions afforded the respective alkali metal triazenide complex. The solid-state molecular structures of these complexes display κ2-N,N'-chelation of the triazenide by the metal. The coordination sphere of each metal center is further furnished by Mπ-arene interactions to the pendant mesityl groups of the triazenide, which enables each complex to exist as a mononuclear species with complexes of the heavier alkali metals display weak aggregation in solid-state. A heteroleptic dilithium complex, featuring µ-κ1-N,N'-bridging of the triazenide by the two lithium centers, was also characterized.

An exceptionally stable NHC complex of indane (InH3).

A spectroscopic study of the decomposition of a Lewis base adduct of indane is reported. The outcomes of this study have prompted the synthesis of the exceptionally stable indane complex [InH3(IPr*)]. The ease at which [InH3(IPr*)] can be handled shows promise for the exploration of indium trihydride reactivity.

Targeted and Systematic Approach to the Study of pKa Values of Imidazolium Salts in Dimethyl Sulfoxide.

A range of more than 25 imidazolium salts, chosen for their differing steric and electronic features, were prepared, and their pKa values were determined using the bracketing indicator method. Through the systematic change in the structure of the imidazolium cation, the effect of varying substituents at each position on the heterocyclic ring was determined; particularly, the transmission of electronic effects was quantified using Hammett parameters. These new data give an indication of the strength of base required for deprotonation and the potential to correlate these data with the nucleophilicity of the corresponding carbenes.

The stabilization of gallane and indane by a ring expanded carbene.

7Dipp Complexes of the group 13 binary hydrides GaH3 and InH3 have been prepared. Both adducts possess excellent thermal stability in the solid state but decompose readily in solution at ambient temperature. The presence of short M-H···H-CAlkyl distances in the structures of both [MH3(7Dipp)] complexes is discussed.

Air stable NHCs: a study of stereoelectronics and metallorganic catalytic activity.

The air stable NHC IPrBr is reported. A stereoelectronic study of IPrBr and its similarly stable relative IMesBr demonstrates metal complex specific changes in NHC donicity versus the ubiquitous IPr and IMes. Application to a Suzuki coupling and an iridium transfer hydrogenation gives superior outcomes using IPrBr and IMesBr.

A 3-D diamondoid MOF catalyst based on in situ generated [Cu(L)2] N-heterocyclic carbene (NHC) linkers: hydroboration of CO2.

A new MOF, [Zn4O{Cu(L)2}2] (1), with a 4-fold interpenetrated 3D diamondoid structure was synthesised from in situ generated [Cu(L)2] NHC linkers. MOF 1 possesses tetrahedral Zn4O nodes, which are unusually coordinated by four pairs of carboxylates from four [Cu(L)2] linkers, and 14 Å 1-D pore channels lined with [Cu(L)2] moieties that catalyse the hydroboration of CO2.

A potential rhodium cancer therapy: studies of a cytotoxic organorhodium(I) complex that binds DNA.

Described is a novel organorhodium(I) complex that is cytotoxic to the colon cancer cell line HCT116 and alters cell migration, DNA replication, and DNA condensation. Most importantly, the mechanism observed is not seen for the parent organorhodium dimer complex [{RhCl(COD)}2], RhCl3, or the free ligand/proligands (COD and 1-(n)butyl-3-methylimidazolium chloride). Thus, the activity of this organorhodium complex is attributable to its unique structure.

Synthesis, structures and reactivity of lanthanoid(II) formamidinates of varying steric bulk.

New reactive, divalent lanthanoid formamidinates [Yb(Form)(2)(thf)(2)] (Form=[RNCHNR]; R=o-MeC(6)H(4) (o-TolForm; 1), 2,6-Me(2)C(6)H(3) (XylForm; 2), 2,4,6-Me(3)C(6)H(2) (MesForm; 3), 2,6-Et(2)C(6)H(3) (EtForm; 4), o-PhC(6)H(4) (o-PhPhForm; 5), 2,6-iPr(2)C(6)H(3) (DippForm; 6), o-HC(6)F(4) (TFForm; 7)) and [Eu(DippForm)(2)(thf)(2)] (8) have been prepared by redox transmetallation/protolysis reactions between an excess of a lanthanoid metal, Hg(C(6)F(5))(2) and the corresponding formamidine (HForm). X-ray crystal structures of 2-6 and 8 show them to be monomeric with six-coordinate lanthanoid atoms, chelating N,N'-Form ligands and cis-thf donors. However, [Yb(TFForm)(2)(thf)(2)] (7) crystallizes from THF as [Yb(TFForm)(2)(thf)(3)] (7a), in which ytterbium is seven coordinate and the thf ligands are "pseudo-meridional". Representative complexes undergo C-X (X=F, Cl, Br) activation reactions with perfluorodecalin, hexachloroethane or 1,2-dichloroethane, and 1-bromo-2,3,4,5-tetrafluorobenzene, giving [Yb(EtForm)(2)F](2) (9), [Yb(o-PhPhForm)(2)F](2) (10), [Yb(o-PhPhForm)(2)Cl(thf)(2)] (11), [Yb(DippForm)(2)Cl(thf)] (12) and [Yb(DippForm)(2)Br(thf)] (16). X-ray crystallography has shown 9 to be a six-coordinate, fluoride-bridged dimer, 12 and 16 to be six-coordinate monomers with the halide and thf ligands cis to each other, and 11 to have a seven-coordinate Yb atom with "pseudo-meridional" unidentate ligands and thf donors cis to each other. The analogous terbium compound [Tb(DippForm)(2)Cl(thf)(2)] (13), prepared by metathesis, has a similar structure to 11. C-Br activation also accompanies the redox transmetallation/protolysis reactions between La, Nd or Yb metals, Hg(2-BrC(6)F(4))(2), and HDippForm, yielding [Ln(DippForm)(2)Br(thf)] complexes (Ln=La (14), Nd (15), Yb (16)).

Low valent and hydride complexes of NHC coordinated gallium and indium.

The reactions of the N-heterocyclic carbene 1,3-dimesitylimidazol-2-ylidene (IMes) with Ga[GaCl(4)], "GaI", InCl(2) and GaBr(3) have been examined. All reactions using a low valent gallium or indium starting material led to species of the form [{MX(2)(IMes)}(2)], where M = Ga, X = Cl (1), I (2); M = In, X = Cl (3), with disproportionation and loss of gallium metal in the case of 2. Reaction of IMes with gallium tribromide yields the air and moisture stable complex [GaBr(3)(IMes)] (4), which has been used as a precursor to the mixed bromohydrides [GaBrH(2)(IMes)] (5) and [GaBr(2)H(IMes)] (6) by (i) ligand redistribution with [GaH(3)(IMes)], (ii) hydride-bromide exchange with triethylsilane, and (iii) alkylation with (n)butyllithium followed by ß-hydride elimination (6 only). Attempts to prepare 1, or monovalent analogues such as [{GaCl(IMes)}(n)], by thermally induced reductive elimination of dihydrogen from the chlorohydride congeners of 5 and 6 resulted in isolation of the known compounds [IMesCl][Cl] (IMesCl = 1,3-dimesityl-2-chloroimidazolium), and/or 1,3-dimesityl-2-dihydroimidazole, and gallium metal. Preliminary photochemical NMR spectroscopy and catalytic studies of 5 and 6 aimed at reductive dehydrogenation under milder conditions are reported. Compounds 1 and 4 have been characterised by single crystal X-ray structure determination.

Ionic liquid effects on Mizoroki-Heck reactions: more than just carbene complex formation.

Reaction profiles for a Mizoroki-Heck reaction in either an ionic liquid or a molecular solvent with different palladium sources demonstrate that the rate enhancements observed in ionic liquids cannot be solely attributed to Pd-carbene complex formation.

Steric control of the reduction of carbodiimides by samarium(II) and the synthesis of very crowded samarium(III) complexes.

Reduction of N,N'-di(2,6-diisopropylphenyl)carbodiimide (DippNCNDipp) by [SmL(2)(thf)(2)] (1) (L = N,N'-bis(2,6-diisopropylphenyl)formamidinate, DippNC(H)NDipp) in PhMe gave [Sm(L)(3)] (2) in good yield. An analogous reaction of 1 with N,N'-dimesitylcarbodiimide (MesNCNMes) gave [SmL(2)(MesNC(H)NMes)] (3). In contrast, reduction of N,N'-dicyclohexylcarbodiimide (CyNCNCy) by 1 in PhMe gave mixture of products from which [SmL(2)(CyNC(CH(2)Ph)NCy)] (4) and [SmL(2)(CyNC(H)NCy)] (5) were isolated by fractional crystallisation. Using thf as the reaction solvent, solely compound 5 was crystallised. Reactions of [Sm(L)(2)(C[triple bond]CPh)(thf)] (6) with the carbodiimides RNCNR (R = Cy, Mes) gave [Sm(L)(2)(RNC(C[triple bond]CPh)NR)] (R = Cy (7) or Mes (8)) which are analogues of 4. No reaction was observed between 6 and DippNCNDipp.

Synthesis of a zinc(II) imidazolium dicarboxylate ligand metal-organic framework (MOF): a potential precursor to MOF-tethered N-heterocyclic carbene compounds.

Two new isomeric dinitrile ligands containing imidazolium salt cores have been synthesized from cyanoanilines in good yield. These have been converted to the corresponding dicarboxylic acids using hydrobromic acid, or the dicarboxylic acids were synthesized directly from the analogous cyanoaniline starting material in a two-step, one-pot reaction. The crystal structures of three of the four compounds are reported. Two reaction pathways with metals are possible for the dicarboxylic acids, initially giving rise to metal carboxylate based metal-organic frameworks (MOFs) as described in this work or N-heterocyclic carbene (NHC) metal complexes en route to the synthesis of MOFs containing tethered NHC complexes. The reaction of 1,3-bis(4-carboxyphenyl)imidazolium bromide with zinc nitrate in dimethylformamide (DMF) gave a one-dimensional (1D) MOF containing the intact imidazolium salt. The potentially porous structure, formed from close packing of the 1D necklace-type polymers, contains channels occupied by disordered DMF solvate molecules. A formate anion also bridges the zinc secondary building units in the 1D polymer, which has an undulating structure resulting from the U-shaped conformation of the dicarboxylate imidazolium ligand.

Tertiary amine and N-heterocyclic carbene coordinated haloalanes--synthesis, structure, and application.

The preparation of several tertiary amine and N-heterocyclic carbene coordinated chloro- and bromoalanes has been studied and routes to their gram-scale synthesis optimized. This provides a catalogue of well-characterized, thermally stable haloalanes for future application. All complexes have been investigated by spectroscopy (IR, NMR) and, where possible, single-crystal X-ray diffraction structure determination. A particular focus of this article is the relative thermal stabilities of the complexes, which provides a useful handle for the aerobic stability of Group 13 hydride complexes. These thermal data have been elucidated in full and rationalized relative to one another on the basis of Lewis base donation, steric shielding, and relative inductive halide strengthening of the aluminum hydride bonds by halides. All of the four-coordinate complexes reported exist as distorted tetrahedra in the solid state with aluminum to N/C-donor bonds that shorten with the increasing Lewis acidity of the aluminum Lewis acid. The five-coordinate complexes [AlBrH(2)(Quin)(2)] and [AlBr(2)H(Quin)(2)] (Quin = quinuclidine) exist in a trigonal-bipyramidal geometry in the solid state with the amine donors situated in the apical positions. Five chloroalanes; [AlClH(2)(Quin)], [AlClH(2)(Quin)(2)], [AlCl(2)H(Quin)(2)], [AlClH(2)(IMes)], and [AlCl(2)H(IMes)] (IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene), the latter two of which are aerobically stable, have been applied to the hydroalumination of carbonyl and heterocycle substrates and their chemo-, regio-, and stereoselectivities compared to those of Group 13 hydride reagents cited in the literature. Overall, the reactivities of these species are comparable to non-halogenated alane complexes with the additional benefit of aerobic stability, non-pyrophoricity, and enhanced regioselectivity borne out of greater Lewis acidity.

Hydride-bromide exchange at an NHC-a new route to brominated alanes and gallanes.

The reaction of [MH(3)(Quinuclidine)] (M = Al or Ga) with an air stable dibrominated N-heterocyclic carbene (NHC) affords the hydride-bromide exchange product [MBr(2)H(NHC)].

Bulky triazenide complexes of alumino- and gallohydrides.

The sterically bulky triazenes DitopN(3)(H)pTol, DitopN(3)(H)Mes, DmpN(3)(H)pTol and DmpN(3)(H)Mes, where Ditop = 2,6-di-p-tolylphenyl, Dmp = 2,6-dimesitylphenyl, pTol = p-MeC(6)H(4) and Mes = 2,4,6-Me(3)C(6)H(2), have been prepared. The reactivity of these triazenide precursors with LiAlH(4) and, in the cases of DmpN(3)(H)pTol and DmpN(3)(H)Mes, LiGaH(4), with diethyl ether as the solvent, has been examined. All reactions were undertaken in a 1:1 ratio giving rise to a variety of aluminium and gallium complexes that either incorporate LiH with a metal to triazenide ratio of 1:1 or generate 'LiH-free' aluminohydrides with aluminium to triazenide ratios of 1:1 or 1:2 dependant on triazenide bulk. Increasing triazenide bulk from DitopN(3)pTol through to DmpN(3)Mes results in a transition from complexes of structure [{Li(OEt(2))(mu-H)(mu-L)AlH (mu-H)}(n)] (L = triazenide ligand; n = 2 DitopN(3)pTol, n = 1 DitopN(3)Mes, DmpN(3)pTol), to bis(triazenide) monohydride complexes [AlH(L)(2)], through to monotriazenide dihydride complexes [AlH(2)(L)]. By contrast, both DmpN(3)(H)Ar (Ar = pTol or Mes) triazenides react with LiGaH(4) to afford the monomeric, lithium hydride containing, complexes [Li(ether)(mu-H)(mu-L)GaH(2)] (L = triazenide, ether = OEt(2) or THF). The molecular structures of [AlH(DitopN(3)Mes)(2)], [AlH(DmpN(3)pTol)(2)], [AlH(2)(DmpN(3)Mes)(THF)] and [Li(THF)(mu-H)(mu-DmpN(3)Mes)GaH(2)] are reported, as well as the structure of the triazene DmpN(3)(H)Mes which exists in the E-syn isomeric form in the solid-state.