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We report a new class of stable mesoionic N‐heterocyclic olefins, featuring a highly polarized (strongly ylidic) double bond. The ground‐state structure cannot be described through an uncharged mesomeric Lewis‐structure, thereby structurally distinguishing them from traditional N‐heterocyclic olefins (NHOs). mNHOs can easily be obtained through deprotonation of the corresponding methylated N,N′‐diaryl‐1,2,3‐triazolium and N,N′‐diaryl‐imidazolium salts, respectively. In their reactivity, they represent strong σ‐donor ligands as shown by their coordination complexes of rhodium and boron. Their calculated proton affinities, their experimentally derived basicities (competition experiments), as well as donor abilities (Tolman electronic parameter; TEP) exceed the so far reported class of NHOs., Highly charged: A new class of mesoionic N‐heterocyclic olefins is described featuring a highly ylidic double bond. The compounds can be utilized as ligands in main‐group and transition metal chemistry. Their donor properties exceed previously reported traditional N‐heterocyclic olefins.

The synthesis and study of a library of cyclic (aryl)(amido)carbenes (CArAmCs), which represent a class of electrophilic NHCs that feature low calculated singlet-triplet gaps (ΔEST =19.9 kcal mol-1 ; B3LYP/def2-TZVP) and exhibit reactivity profiles expected from triplet carbenes, are described. The electrophilic properties of the CArAmCs were quantified by analyzing their respective selenium adducts, which exhibited the largest downfield 77 Se NMR chemical shifts (up to 1645 ppm) measured for any NHC derivative known to date, as well as their Ir carbonyl complexes, from which large Tolman electronic parameter (TEP) values (up to 2064 cm-1 ) were ascertained. The CArAmCs were found to engage in reactions that are typically observed with triplet carbenes, including C-H insertions, [2+1] cycloadditions with alkenes as well as alkynes, and spontaneous oxidation upon exposure to oxygen.

Stimuli-responsive ancillary ligands are valuable tools to control the activity and selectivity of transition-metal catalysts. The synthesis and characterization of a series of metal complexes containing phosphines with proton-responsive imidazolin-2-ylidenamino substituents are reported. Determination of the ligand-donor properties revealed that protonation of each substituent increases the Tolman electronic parameter (TEP) of the phosphine by 22 cm-1 , hence allowing for switching of the electron-donor power of phosphine 2 within an unprecedented range (ΔTEP=43.4 cm-1 ).

Atomically precise gold superatoms offer useful templates to evaluate tunable properties via ligand engineering. Herein, the role of different linked N-heterocyclic carbene (NHC) protecting ligands ranging from strong to weak σ-donors was evaluated according to the Tolman electronic parameter (TEP), in species related to the classical [Au13Cl2(dppe)5]3+ nanocluster (1). Our results show a strong dependency on the nature of NHC, providing a useful design principle for the efficient tuning of the structural, optical, chiroptical and emission properties of the Au13Cl2 core. A sizable decrease is observed in the HOMO–LUMO gap for weaker σ-donor ligand cases, with a change in the LUMO nature from core-based orbitals in 1, to a π*-ligand nature. Furthermore, a shorter bridge results in interesting structural changes between the eclipsed ↔ staggered Au13Cl2 core unraveling the potential to convert light energy into mechanical work. Thus, the noticeable modulation of [Au13Cl2(NHC)5]3+ properties by different ligands underlies design rules for tunable clusters towards nanostructured materials, by taking advantage of the recent introduction of NHC-protected gold clusters.

The synthesis and coordination chemistry of a saturated analogue of a 'bulky-yet-flexible' N-heterocyclic carbene (NHC) ligand are described. "SIPaul" is a 4,5-dihydroimidazol-2-ylidene ligand with unsymmetrical aryl N-substituents, and is one of the growing class of 'bulky-yet-flexible' NHCs that are sufficiently bulky to stabilise catalytic intermediates, but sufficiently flexible that they do not inhibit productive chemistry at the metal centre. Here, the synthesis of SIPaul.HCl and its complexes with copper, silver, iridium, palladium, and nickel, and its selenourea, are reported. The steric impact of the ligand is quantified using percent buried volume (%Vbur), while the electronic properties are probed and quantified using the Tolman Electronic Parameter (TEP) and δSe of the corresponding selenourea. This work shows that despite the often very different performance of saturated versus unsaturated carbenes in catalysis, the effect of backbone saturation on measurable properties is very small.

Palladium terminal imido complexes hold promise as catalysts for the amination of CH bonds. This computational study elucidates the electronic structure of palladium(II) and palladium(IV) terminal imido complexes by CASSCF as well as DFT calculations. In search of stable palladium terminal imido complexes, a series of synthetically relevant ancillary ligands are evaluated computationally. The thermodynamic stability of the palladium(II) complexes is correlated with the singlet/triplet gap of the compounds, whereas the palladium(IV) complexes are stabilized by strong electron donor ligands as indicated by the Tolman electronic parameter.

A convenient access to a triad of triazoles with ferrocenyl and cobaltoceniumyl substituents is reported. N‐Alkylation, deprotonation and metalation with CuI/AgI/AuI synthons affords the heteroleptic triazolylidene complexes. Due to the combination of neutral, electron‐donating ferrocenyl substituents and cationic, strongly electron‐withdrawing cobaltocenium substituents, the mesoionic carbene (MIC) ligands of these complexes are electronically interesting “push–pull”, “pull–push” and “pull–pull” metalloligands with further switchable redox states based on their fully reversible FeII/FeIII, (ferrocene/ferrocenium) and CoIII/CoII, (cobaltocenium/cobaltocene) redox couples. These are the first examples of metal complexes of (di)cationic NHC ligands based on cobaltoceniumyl substituents. DFT calculated Tolman electronic parameter (TEP) of the new MIC ligands, show these metalloligands to be extremely electron‐poor NHCs with properties unmatched in other carbene chemistry. Utilization of these multimetallic electronically tunable compounds in catalytic oxazoline synthesis and in antitumor studies are presented. Remarkably, 1 mol % of the AuI complex with the dicationic MIC ligand displays full catalytic conversion, without the need for any other additives, in less than 2 hours at ambient temperatures. These results thus firmly establish these new classes of cobaltoceniumyl based (di)cationic MIC ligands as prominent players in several branches of chemistry.

The fundamental reactivity leading to N-heterocyclic fluoroalkene adducts is explored in detail, featuring a total of 15 N-heterocyclic carbenes (NHCs) with various electronic and steric environments. The activity of these carbenes towards tetrafluoroethylene (TFE), hexafluoropropene (HFP), trifluoroethylene (HTFE) and vinylidene fluoride (VDF) is assessed in THF and toluene. Attempts were made to correlate the observed reactivity with electronic (Tolman Electronic Parameters) and steric (% buried volume) parameters unique to each NHC, but a trend has yet to be fully determined. However, the unique steric constraints of a cyclic (alkyl)(amino)carbene (CAAC) were shown to modify the initial point of nucleophilic attack on HTFE, providing selective transformation to a different adduct than has been observed to date with all reactions involving this fluoroalkene.

Cyclic (alkyl)(amino)carbenes cAACcy (1b) and cAACmenthyl (1c) react with [Ni(CO)4] to give the 18 VE complexes [Ni(CO)3(cAACcy)] (2b) and [Ni(CO)3(cAACmenthyl)] (2c). With these in hand, the donor-strength and the steric profile of the respective cAAC ligands were evaluated. CAACcy and cAACmenthyl possess similar overall-donating properties (Tolman electronic parameter (TEP) = 2046 (1b) and 2042 (1c)) as common NHCs, though they are also known to be better π-acceptors. 3,3-Diamino-2-aryloxyacrylimidamide 3b, arising from the reaction of cAACcy (1b) with released CO molecules, was obtained as side-product of CO substitution reactions at nickel carbonyls. In contrast to cAACmenthyl (%Vbur = 42), the sterically less encumbered cAACcy (%Vbur = 38) undergoes a subsequent CO substitution at [Ni(CO)3(cAACCy)] (2b) to afford the 16 VE complex [Ni(CO)(cAACcy)2] (4b). Treatment of both [Ni(CO)3(cAACmethyl)] (2a) and [Ni(CO)(cAACmethyl)2] (4a) with allyl bromides led to the formation of cAAC-stabilized allyl nickel...

A new rhenium complex, cis-[ReO(btmp)PPh3Cl2] (2a) with 2-benzothiazolyl-5-methoxyphenol (Hbtmp) has been synthesized and characterized by means of FT-IR, UV–Vis, 1H NMR and elemental analyses. The structural parameters of 2a as obtained from XRD studies have been compared with the calculated [PBEPBE/STMIDI] values. The observed results are in good agreement. The vibrational frequencies, electronic transitions of 2a have been calculated and compared with the experimental results. The global chemical reactivity descriptors based on conceptual density functional theory have also been calculated for a set of analogous systems derived from 2a varying the central metal center (Re or Tc) and substituted phosphine moieties. A good relationship between the calculated hardness (η) and Tolman’s cone angle (θ) of phosphines is observed both for Re and Tc complexes. Similar harmony between electronic chemical potential (μ) and Tolman’s electronic parameter (ν) of phosphines is also noted.

Achieving chemo- and regioselectivity simultaneously is challenging in organic synthesis. Transition metal-catalyzed reactions are effective in addressing this problem by the diverse ligand effect on the catalyst center. Ligand-controlled regioselective Pd-catalyzed carbonylation of styrenes with aminophenols was realized, chemoselectively affording amides. Using a combination of boronic acid and 5-chlorosalicylic acid as the additives, linear amides were obtained in high yields and selectivity using tris(4-methoxyphenyl)phosphine (L3) in acetonitrile, while branched amides were obtained in high yields and selectivity in butanone by changing the ligand to 1,3,5,7-tetramethyl-2,4,8-trioxa-6-phenyl-6-phosphaadamantane (L5). Further studies show that the nature of the ligand is key to the regioselectivity. Cone angle and Tolman electronic parameter (TEP) have been correlated to the reactivity and regioselectivity. Studies on the acid additives show that different acids act as the proton source and the corresponding counterion can help enhance the reactivity and selectivity.

4-Phenylsemicarbazide and 1,5-diphenylcarbazide are suitable starting materials for the syntheses of N-heterocyclic carbene (NHC) compounds with new backbone structures. In the first case, cyclisation and subsequent methylation leads to a cationic precursor whose deprotonation affords the triazolon-ylidene 2, which was converted to the corresponding sulfur and selenium adducts and a range of metal complexes. In contrast, cyclisation of diphenylcarbazide affords a neutral betain-type NHC-precursor 7, which is not in equilibrium with its carbene tautomer 7a. Precursor 7 can either be deprotonated to give the anionic NHC 8 or methylated at the N or O atom of the backbone resulting in two isomeric cationic species 16 and 20. Deprotonation of the latter two provides neutral NHC compounds with a carboxamide or carboximidate backbone, respectively. The ligand properties of the new NHC compounds were evaluated by IR and (77) Se NMR spectroscopy. Tolman electronic parameter (TEP) values range from 2050 to 2063 cm(-1) with the anionic NHC 8 being the best overall donor.

This paper focuses on the stable, ferrocene-based N-heterocyclic carbene (NHC) rac-[Fe{(η(5)-t-BuC5H3)NpN}2C:] (A'-Np, Np = neopentyl), which is planar-chiral due to the two tert-butyl substituents in 3,3'-positions. A'-Np was synthesized in nine steps starting from 1,1'-di-tert-butylferrocene (1), the first step being its 3,3'-dilithiation to afford rac-[Fe(η(5)-t-BuC5H3Li)2] (rac-fc'Li2, 2). The structures of rac-fc'(SiMe3)2 (3), rac-fc'Br2 (4), rac-fc'(N3)2 (5), and the immediate carbene precursor [A'-NpH]BF4 were determined by single-crystal X-ray diffraction (XRD). The chemical properties of A'-Np were found to be very similar to those of its tert-butyl-free congener A-Np, both being ambiphilic NHCs with rather high calculated HOMO energies (ca. -4.0 eV) and low singlet-triplet gaps (ca. 35 kcal/mol). A Tolman electronic parameter value of 2050 cm(-1) was derived from IR data of cis-[RhCl(A'-Np)(CO)2], indicating the high donicity of A'-Np as a ligand. Consistent with its ambiphilic nature, A'-Np was found to react readily with carbon monoxide, affording the betainic enolate (A'-Np)2CO as four stereoisomers, viz. (RpRp-A'-Np)═C(O(-))(RpRp-A'-Np(+)), (SpSp-A'-Np)═C(O(-))(SpSp-A'-Np(+)), (RpRp-A'-Np)═C(O(-))(SpSp-A'-Np(+)), and (SpSp-A'-Np)═C(O(-))(RpRp-A'-Np(+)). The former two isomers were structurally characterized as a racemic compound by single-crystal XRD. A'-Np was found to react swiftly with dichloromethane, affording the addition product A'-NpH-CHCl2 in a reaction that is unprecedented for diaminocarbenes. A-NpH-CHCl2 was obtained analogously. Both compounds were structurally characterized by single-crystal XRD. An electrochemical investigation of A'-Np by cyclic and square wave voltammetry revealed a reversible oxidation of the carbene at a half-wave potential of -0.310 vs ferrocene/ferrocenium (THF/NBu4PF6). The electrochemical data previously published for A-Np were identified to be incorrect, since unnoticed hydrolysis of the NHC had taken place, affording A-Np(H2O). The hydrolysis products of A-Np and A'-Np were found to be reversibly oxidized at half-wave potentials of -0.418 and -0.437 V, respectively.

A series of iridium complexes containing phenanthro[4,5-abc]phenazino[11,12-d]imidazol-2-ylidene and acetonaphtho[1,2-b]quinoxaline[11,12-d]imidazol-2-ylidene ligands have been obtained and fully characterized. These complexes display highly extended polyaromatic systems attached to the backbone of the N-heterocyclic carbene. The presence of this extended polyaromatic system makes the electron-donating character of these ligands sensitive to the presence of π-stacking additives, such as pyrene and hexafluorobenzene. The computational studies predict that the addition of pyrene affords an increase of the electron-donating character of the polyaromatic ligand (TEP decreases), while the addition of hexafluorobenzene has the opposite effect (TEP increases). This prediction is experimentally corroborated by IR spectroscopy, by measuring the shift of the CO stretching bands of a series of IrCl(NHC)(CO)2 complexes, where NHC is the N-heterocyclic carbene ligand with the polyaromatic system. Finally, the energy of the π-stacking interaction of one of the key Ir(I) complexes with pyrene and hexafluorobenzene has been estimated by using the Benesi-Hildebrand treat-ment, based on the δ-shifts observed by 1H NMR spectroscopy. MEC of Spain (CTQ2011-24055/BQU)

The catalytic activity of transition metal complexes (R)nM–L can be predicted utilizing the metal–ligand electronic parameter (MLEP) that is based on the local stretching force constant of the M-L bond. Vibrational spectroscopy is an excellent tool to accurately determine vibrational mode properties such as stretching frequencies. These correspond, because of mode–mode coupling, to delocalized vibrational modes, which have to be first converted into local vibrational modes, and their properties. Each bond of a molecule can be uniquely characterized by the local stretching force constant and frequency. The former is ideally suited to set up a scale of bond strength orders, which identifies weak M–L bonds with promising catalytic activity. It is shown how the MLEP replaces the TEP (Tolman Electronic Parameter), which is based on the CO stretching frequencies of a (CO)nM–L complex and which is now exclusively used in hundreds of investigations. However, the TEP is at best a qualitative parameter that suffers from relatively large mode–mode coupling errors and the basic deficiency of most indirect descriptors: They cannot correctly describe the intrinsic M–L bond strength via the CO stretching frequencies.

Synthesis and structural characterization of novel Rh(I) complexes bearing 1,2,4-triazol-3-ylidene or imidazol-2-ylidene substituted with an N-2-nitrobenzenesulfonyl (N-nosyl) group are described. The complexes were characterized by infrared (IR), NMR, and single-crystal X-ray diffraction analyses. The Tolman electronic parameter (TEP) values of both of the 1,2,4-triazol-3-ylidene (2064 cm−1) and imidazol-2-ylidene (2059 cm−1), as determined from the average carbonyl (CO) stretching frequencies for the corresponding RhCl(NHC)(CO)2 (NHC = N-heterocyclic carbene) complexes, were higher than that of RhCl(IMes)(CO)2 (IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene) (2050 cm−1). As the TEP values of the NHCs increased, the 13C NMR signal of the carbenic carbon in the corresponding RhCl(NHC)(cod) (cod = 1,5-cyclooctadiene) and RhCl(NHC)(CO)2 complexes shifted downfield. Moreover, the crystallographic analysis of the RhCl(NHC)(cod) complexes revealed that one of the S O bonds in the sulfonyl group could be conjugated with the NHC frameworks, while the remaining S O bond conjugated with the 2-nitrophenyl groups of the N-nosyl substituent.

This paper focuses on ferrocene-based expanded-ring N-heterocyclic carbenes (NHCs) of the type [Fe(C5H4–NR–C–NR′–C5H4)] (1-R/R′), which contain two different N-substituents. Three combinations were addressed, with R = neopentyl (Np) in each case and R′ being either 2-adamantyl (Ad), phenyl (Ph) or 9-anthracenylmethyl (Acm). The NHCs were generated by reaction of the corresponding formamidinium tetrafluoroborates [H–1-R/R′][BF4] with lithium diisopropylamide (LDA). While only 1-Np/Ad was sufficiently stable for isolation, 1-Np/Ph and 1-Np/Acm could be efficiently trapped in situ by complexation reactions. Two series of RhI complexes were prepared, viz. [RhCl(cod)(1-R/R′)] (cod = 1,5-cyclooctadiene) by reacting [{Rh(μ-Cl)(cod)}2] with 1-R/R′ and cis-[RhCl(CO)2(1-R/R′)] by reacting [RhCl(cod)(1-R/R′)] with CO. All complexes exhibit pronounced anagostic α-CH⋯Rh interactions, both in solution and in the solid state, in accord with a strong influence of the N-substituents on the steric ligand properties, as is chemically illustrated by the huge reactivity difference of [RhCl(cod)(1-Ad)] (R = R′ = Ad) and [RhCl(cod)(1-Np/Ad)] towards CO, the former complex being inert. Tolman electronic parameter (TEP) values are 2050 ± 1 cm−1 for the unsymmetrical NHCs studied, indicating only a weak influence of the N-substituents on the electronic ligand properties.

The new N-heterocyclic carbene (NHC) precursors 4, -dicyano-1, -dimesityl- (9) and 4, 5-dicyano-1, 3-dineopentyl-2-(pentafluorophenyl)imidazoline (14) were synthesized. The structure of 9 could be determined by X-ray crystallography. With the 2-pentafluorophenyl-substituted imidazolines 9 and 14, the [AgCl(NHC)], [RhCl(COD)(NHC)], and [RhCl(CO)2(NHC)] complexes [NHC = 4, 5-dicyano-1, 3-dimesitylimidazol-2-ylidene (3) and 4, 5-dicyano-1, 3-dineopentylimidazol-2-ylidene (4)] were obtained. Crystal structures of [AgCl(3)] (15), [RhCl(COD)(3)] (17), [RhCl(COD)(4)] (18), and [RhCl(CO)2(3)] (19) were solved and with the crystal data of 19, the percent buried volume ( %Vbur) of 31.8(±0.1) % was determined for NHC 3. Infrared spectra of the imidazolines 9 and 14 and of the complexes 15–20 were recorded and the CO stretching frequencies of complexes 19 and 20 were used to determine the Tolman electronic parameters of the newly obtained NHCs 3 (TEP: 2060 cm–1) and 4 (TEP: 2061 cm–1), thus proving that 1, 3-substitution of maleonitrile-NHCs does not have a significant effect for the high π-acceptor strength of these carbenes.

Attempts to evaluate experimentally the donor characteristics of the N,N'-bis(di-tert-butylphosphanyl)-imidazole-2-ylidene (PC(NHC)P) hybrid ligand are described. Thus, reactions of PC(NHC)P with [Rh(μ-Cl)(COD)]2 and [Rh(μ-Cl)(CO)2]2 led to the formation of the mononuclear and dinuclear complexes, [Rh(PC(NHC)P,κP,κC(NHC))2]Cl (PC(NHC)P-RhCl) and [Rh(CO)(PC(NHC),κP,κC(NHC),κN)]2 (PC(NHC)-RhCO), respectively, the latter resulting after in situ cleavage of one (t-Bu)2P-Nimid bond of PC(NHC)P. With ligands acting as a P,C-chelate, a straightforward evaluation of the Tolman electronic parameter (TEP) of the C(NHC) donor is problematical; the viability of dangling P- and bound C(NHC)-donors (i.e.κC(NHC)) has been observed in the trinuclear Fe(ii) chain complex [Fe3Cl2(μ-Cl)4(THF)2(PC(NHC)P,κC(NHC))2] (PC(NHC)P-Fe), obtained by the reaction of PC(NHC)P with [Fe4Cl8(THF)6] and, recently, established on Cr(II), Co(II) and Au(I) centres. Evaluation of the π-accepting properties of the PC(NHC)P (and the related Dipp-PC(NHC)) was based on the (77)Se NMR chemical shifts of the corresponding NHC-Se adducts, PC(NHC)P-Se (and Dipp-PC(NHC)-Se), which were prepared from the free PC(NHC)P (and Dipp-PC(NHC)) and Se. The π-acidity of PC(NHC)P is found to be higher than that of Dipp-PC(NHC) but lower than that of SIPr. The donor ability of the C(NHC) in PC(NHC)P was explored by its reaction with the Lewis acids tris(pentafluorophenyl)borane (B(C6F5)3) and tris(pentafluorophenyl)boroxine ([(C6F5)BO]3), which resulted in stable donor-acceptor adducts with no FLP reactivity. The steric properties of PC(NHC)P and Dipp-PC(NHC) are conformation dependent, with the percent buried volume (%Vbur) of PC(NHC)P in the structurally characterised conformer calculated at 67.6, the largest value currently reported for NHC ligands.

A series of Ru-based olefin metathesis catalysts containing N,N′-diamidocarbenes (DACs) were synthesized and studied. X-ray crystallographic analysis revealed that the Ru−Ccarbene distances (1.938(5)−1.984(4) A) measured in the DAC-supported complexes were relatively short, particularly in comparison to the range of Ru−Ccarbene distances typically observed in analogous N-heterocyclic carbene (NHC) supported complexes (1.96−2.03 A). While the Tolman electronic parameters (TEP) of various DACs (2056−2057 cm−1) were calculated to be similar to that of PCy3 (2056 cm−1), the ring-closing metathesis (RCM) of diethyl diallylmalonate facilitated by DAC-supported Ru complexes proceeded at a relatively slow rate. However, unlike the phosphine-containing complexes, the DAC analogues catalyzed the RCM of diethyl dimethallylmalonate to its respective tetrasubstituted olefin. A series of electrochemical experiments revealed that the Ru complexes bearing a DAC ligand underwent oxidation at significantly higher potential...

Condensation of N,N′-dimesitylformamidine with phthaloyl chloride afforded 1·HCl, which, upon treatment with base, afforded 2,4-dimesitylbenzo[e][1,3]diazepin-1,5-dione-2-ylidene (1), a seven-membered N,N′-diamidocarbene (DAC), in high yield (85%). The free DAC was used to synthesize four new, late transition metal complexes: [Rh(cod)(1)Cl] (2a) (cod = 1,5-cyclooctadiene), [Ir(cod)(1)Cl] (2b), [Rh(CO)2(1)Cl] (3a), and [1−AuCl] (5). The Tolman electronic parameter (TEP) of 1 was calculated to be 2047 cm−1 from the IR spectrum of 3a. This TEP value is approximately 10 cm−1 lower than known DACs and 5 cm−1 lower than known imidazol-2-ylidenes, indicating that DAC 1 is a relatively strong electron donor. Additionally, electrochemical analyses of 2a and 2b corroborated the IR data obtained on 3a and revealed E1/2 values that were shifted cathodically by ca. 0.16 V when compared to analogous complexes supported by N-heterocyclic carbenes. The gold complex 5 was found to catalyze the hydration of phenylacetylene...

Complexes of the type [(L)Ir(COD)Cl] and [(L)Ir(CO)2Cl] (L = N,N′-dimesityl-N,N′-dimethylformamidin-2-ylidene (3) and N,N′-bis(2,6-di-isopropylphenyl)-N,N′-dimethylformamidin-2-ylidene (4); COD = cis,cis-1,5-cyclooctadiene) were synthesized and studied in solution as well as in the solid state. While the acyclic diaminocarbene (ADC) ligand in [(3)Ir(COD)Cl] adopted a conformation in which the N-aryl substituents were anti with respect to the coordinated metal, the respective Ir carbonyl complex was prepared as separable isomers ([(anti-3)Ir(CO)2Cl] and [(amphi-3)Ir(CO)2Cl]). The ADC ligands in [(4)Ir(COD)Cl] and [(4)Ir(CO)2Cl] adopted exclusively amphi conformations, where one N-aryl substituent was oriented toward the coordinated metal and the other was oriented away. The Tolman electronic parameter (TEP) for anti-3 (2047.8 cm−1) was derived from the carbonyl stretching energy (νCO) of the aforementioned Ir(CO)2Cl complex and was found to be larger than the TEPs calculated for amphi-3 (2044.4 cm−1) and 4...

A large group of two-electron donor ligands were compared and ranked with the help of DFT calculations. The following characteristics were considered: (a) C≡O stretching frequencies and distances of Ni(CO)3L, IrCl(CO)2L, and IrCp(CO)L, (b) Ir−(η2-C2H4) bond enthalpies, stretching frequencies, and distances of IrCp(η2-H2C═CH2)L, (c) enthalpies of the ligand exchange reactions IrCp(CO)L + IrCpXe(PF3) → IrCp(CO)(PF3) + IrCpXeL and Ir(III)H2CpL + Ir(I)CpXe(PF3) → Ir(III)H2Cp(PF3) + Ir(I)CpXeL, (d) Ir−(η2-H2) bond enthalpies and H−H distances of Os(η2-H2)Cl2(CO)L2. Useful trends and correlations have been established for several common ligand types.

The effect of tuning the electronic properties of N-heterocyclic carbene (NHC) ligands was evaluated in multiple, mechanistically distinct, metal-mediated reactions. Hydroboration and Heck reactions, catalyzed by Rh–NHC and Pd–NHC complexes, respectively, were found to result in yields that were up to ten times lower when π-withdrawing substituents were incorporated into the NHC backbone relative to analogues bearing σ-withdrawing groups.

Rhodium(I) complexes of acceptor substituted N-heterocyclic carbenes were obtained either by transmetalation from the corresponding Ag(I) complexes or by thermal decomposition of corresponding pentafluorobenzene carbene adducts. All complexes were fully characterized by means of NMR- and mass spectroscopy. Compounds 5, 6, 7 and 11 were although characterized by single-crystal X-ray analysis. The relative σ-donor/π-acceptor strength of the NHC ligands was determined by means of IR spectroscopy. Dimerisation behaviour of Rh carbonyl complexes was studied.

Complexes of iridium bearing NHC (NHC = N-heterocyclic carbene) ligands were synthesized and fully characterized. The series [(NHC)Ir(cod)Cl] were obtained by simple cleavage of [Ir(cod)Cl]2. The [(NHC)Ir(cod)Cl] complexes were reacted with excess carbon monoxide, leading to [(NHC)Ir(CO)2Cl]. The infrared carbonyl stretching frequencies of these were recorded to quantify the electronic parameter of NHC ligands. X-ray diffraction study results allow for determination of NHC steric parameters within this series. These data allow for comparison with other ligand families.

The ability of N-heterocyclic carbenes (NHCs) to participate in π-back-bonding interactions was evaluated in a range of transition metal complexes. Rh chloride complexes containing a systematic ser...

The synthesis of the novel seven-membered N-heterocyclic carbene (NHC) 1,3-dicyclohexyl-1,3-diazepan-2-ylidene (3) and its 5,6-dioxolane derivative 4 is reported and their coordination chemistry with Rh(I), Ir(I), and Pt(0) discussed. The M(cod)(3)Cl, where M = Rh and Ir, complexes display a high rotation barrier at room temperature about the M−CNHC bond, whereas for the M(CO)2(3)Cl and Pt(nbe)2(3) complexes rapid rotation of the carbene ligand is observed at ambient temperature. The infrared ν(CO) values of the Rh(I) and Ir(I) derivatives M(CO)2(3)Cl give a measure of the donor ability of the new carbene ligands. The crystal structures of the amidinium salts 3·HPF6 and 4·HPF6 together with those of M(cod)(3)Cl [M = Rh, Ir], Ir(cod)(4)Br, Ir(CO)2(3)Cl, and Pt(nbe)2(3) are reported. Both the salts and the coordinated carbene ligands exhibit extremely large NCN angles; for the complexes the angles are in the range 115.5(3)° [Pt(3)] to 122(11)° [Ir(4)].

Palladium-catalyzed C-C and C-N bond-forming reactions are among the most versatile and powerful synthetic methods. For the last 15 years, N-heterocyclic carbenes (NHCs) have enjoyed increasing popularity as ligands in Pd-mediated cross-coupling and related transformations because of their superior performance compared to the more traditional tertiary phosphanes. The strong sigma-electron-donating ability of NHCs renders oxidative insertion even in challenging substrates facile, while their steric bulk and particular topology is responsible for fast reductive elimination. The strong Pd-NHC bonds contribute to the high stability of the active species, even at low ligand/Pd ratios and high temperatures. With a number of commercially available, stable, user-friendly, and powerful NHC-Pd precatalysts, the goal of a universal cross-coupling catalyst is within reach. This Review discusses the basics of Pd-NHC chemistry to understand the peculiarities of these catalysts and then gives a critical discussion on their application in C-C and C-N cross-coupling as well as carbopalladation reactions.

The latest advances in N-heterocyclic carbene-based organometallic chemistry have drawn increasing attention to this class of ancillary ligands as an attractive alternative to tertiary phosphines. Studies focusing on the fundamental steric and electronic factors characterizing this family of compounds are therefore essential for the rationalization of the activity observed for such organometallic complexes in metal-mediated organic transformations. This knowledge is also of fundamental importance in catalyst design efforts. This review intends to provide a comprehensive overview of the progress in this area.

Deprotonation of 1,3-di(2-tolyl)benzimidazolium tetrafluoroborate with a strong base afforded 1,3-di(2-tolyl)benzimidazol-2-ylidene (BTol), which dimerized progressively into the corresponding dibenzotetraazafulvalene. The complexes [RhCl(COD)(BTol)] (COD is 1,5-cyclooctadiene) and cis-[RhCl(CO)2(BTol)] were synthesized to probe the steric and electronic parameters of BTol. Comparison of the percentage of buried volume (%VBur) and of the Tolman electronic parameter (TEP) of BTol with those determined previously for 1,3-dimesitylbenzimidazol-2-ylidene (BMes) revealed that the two N-heterocyclic carbenes displayed similar electron donicities, yet the 2-tolyl substituents took a slightly greater share of the rhodium coordination sphere than the mesityl groups, due to a more pronounced tilt. The anti,anti conformation adopted by BTol in the molecular structure of [RhCl(COD)(BTol)] ensured nonetheless a remarkably unhindered access to the metal center, as evidenced by steric maps. Second-generation ruthenium-benzylidene and isopropoxybenzylidene complexes featuring the BTol ligand were obtained via phosphine exchange from the first generation Grubbs and Hoveyda-Grubbs catalysts, respectively. The atropisomerism of the 2-tolyl substituents within [RuCl2(=CHPh)(PCy3)(BTol)] was investigated by using variable temperature NMR spectroscopy, and the molecular structures of all four possible rotamers of [RuCl2(=CH-o-O(i)PrC6H4)(BTol)] were determined by X-ray crystallography. Both complexes were highly active at promoting the ring-closing metathesis (RCM) of model α,ω-dienes. The replacement of BMes with BTol was particularly beneficial to achieve the ring-closure of tetrasubstituted cycloalkenes. More specifically, the stable isopropoxybenzylidene chelate enabled an almost quantitative RCM of two challenging substrates, viz., diethyl 2,2-bis(2-methylallyl)malonate and N,N-bis(2-methylallyl)tosylamide, within a few hours at 60 °C.

The chemistry of singlet oxygen with a variety of arylphosphines has been studied. Rates of singlet oxygen removal by para -substituted arylphosphines show good correlation with the Hammett σ parameter ( ρ =−1.53 in CDCl 3 ), and with the Tolman electronic parameter. The only products for the reactions of these phosphines with singlet oxygen are the corresponding phosphine oxides. Conversely, for ortho -substituted phosphines with electron-donating substituents, there are two products, namely a phosphinate formed by intramolecular insertion and phosphine oxide. Kinetic analyses demonstrate that both products are formed from the same intermediate, and this allows determination of the rate ratios for the competing pathways. Increasing the steric bulk of the phosphine leads to an increase in the amount of insertion product. VT NMR experiments show that peroxidic intermediates can only be detected for very hindered and very electron-rich arylphosphines.

Recent work is discussed that throws light on synthetic, steric, and electronic aspects of NHC complexes as well as on outer sphere effects in their reactivity. The chemistry of the NHC ligand is much more complex than the more traditional phosphines and provides much greater possibilities for altering steric and electronic properties for tuning reactivity. In synthetic work the Lin Ag 2 O method is shown to be inapplicable to the synthesis of abnormal NHCs bound via C-4(5) where the C2 position is blocked with CH 3 , because Ag(I) oxidizes the CH 3 group to formate with formation of the normal C-2 bound Ag–NHC. Linker effects on the behavior of chelating NHCs depend on the linker locking the azole rings into a conformation that depends on linker length. This gives rise to different complexes being formed when different linker lengths are employed. The failure of M–NHC bonds to reversibly dissociate can prevent potentially chelating bis and tris NHC precursors from forming the desired products but instead being trapped in a kinetic nonchelate form. Imidazolium carboxylates prove to be synthetically useful in that they can act as excellent NHC transfer agents to a variety of transition metals. The Tolman electronic parameter of NHCs can be determined by a variety of experimental and computational methods. Anion dependent chemistry can give rise to a switching of the product of imidazolium salt metallation from normal (C-2) to abnormal (C-4(5)) forms.

N-heterocyclic (NHC) ligands constitute a new class of ligands that is going to commonly be used in organometallic chemistry. Nevertheless, detailed understanding of the bonding properties of these ligands to transition metals is scarce. In particular, a clear separation between steric and electronic effects is missing. Only in recent years combined experimental and computational studies on this topic have been performed. Here we review some advances in the field. We thus present a quantification of steric effects on the bond dissociation energy of various NHC-ligands from transition metals in complexes as Cp*Ru(NHC)Cl and Ni(CO)3(NHC). We further compare the steric requirements of various NHC ligands with the steric requirements of some phosphines. In the second part, we examine the different bonding modes that can contribute to the NHC–metal bond. We will review examples of metal-to-NHC back-donation (σ → d*) as well as of ligand-to-metal-to-NHC back-donation (π → d).

The use of N-heterocyclic carbenes as ligands for transition metals has increased dramatically in the last few years, spurred on by their remarkable successes in the areas of metathesis chemistry and coupling reactions. The stability of the transition metal complexes of N-heterocyclic carbenes is often cited as one of the key advances of these ligands versus their phosphine counterparts. However, to quote Danopoulos, “recent reports are questioning the belief that the NHC–metal bond is inert” [Chem. Commun. (2003) 756]. This review describes some of the reactions that N-heterocyclic carbenes undergo in the metal co-ordination sphere that lead to decomposition of the NHC–metal complex. Implications for catalysis using these species are described.

The cis effects of phosphine, arsine and stibine ligands have been evaluated by measuring the IR stretching frequency in dichloromethane of the carbonyl ligand in a series of Rh(I) Vaska-type complexes, trans-[RhCl(CO)(L)2]. These data were correlated with those obtained by Tolman for the electronic trans influences in the [Ni(L)(CO)3] complexes. The electronic contribution, χFc, of ferrocenyl was determined as 0.8 from these plots by evaluating PPh2Fc as ligand. In order to accommodate arsine and stibine ligands an additional correction term, to compensate for differences in the donor atom, was added to Tolman’s equation for calculation of the Tolman electronic parameter of phosphine ligands. In the resulting equation: ν(CONi)=2056.1+∑i=13χi+CL values for CL of CP=0, CAs=−1.5 and CSb=−3.1 are suggested for phosphine, arsine and stibine ligands, respectively. The crystal and molecular structures of trans-[RhCl(CO)(PPh2Fc)2] · 2C6H6, trans-[RhCl(CO){P(NMe2)3}2] and trans-[RhCl(CO)(AsPh3)2] are reported. The Tolman cone angles for PPh2Fc and P(NMe2)3 were determined as 169° and 166°, while the effective cone angles for PPh2Fc, P(NMe2)3 and AsPh3 were determined as 171°, 168° and 147°, respectively.

The synthesis of rhodium(I) and iridium(I) complexes of the bis(diisopropylamino)carbene is described for the first time. The formamidinium chloride and the free bis(diisopropylamino)carbene (L) were used as consecutive precursor compounds to form the metal complexes. Spectroscopic and, for LRh(cod)Cl, crystallographic data are presented for the complexes LRh(cod)Cl and LIr(cod)Cl (L=bis(diisopropylamino)carbene). The ligand properties of the acyclic bis(diisopropylamino)carbene are compared with imidazolin-2-ylidenes and imidazolidin-2-ylidenes as ligands in related rhodium(I) carbonyl complexes. Bis(diisopropylamino)carbene is the most basic known carbene ligand to date.

Values of the molecular electrostatic potential minimum (V(min)) corresponding to the lone pair region of several substituted phosphine ligands (PR(3)) have been determined at the DFT level. The V(min) value is proposed as a quantitative measure of the electronic effect of the PR(3) ligands. Good linear correlation between V(min) and Tolman electronic parameter of PR(3) has been obtained. V(min) is also proportional to the pK(a) values of the conjugate acids of PR(3), viz., [PR(3)H](+). Further, the DeltaE values of the reaction Ni(CO)(3) + PR(3) --Ni(CO)(3)PR(3) and ScH(3) + PR(3) --ScH(3)PR(3) are also linearly proportional to the V(min) values. However, if there is a strong metal to phosphorus pi-back-bonding, the DeltaE and V(min) do not fit to a line. It is also found that the standard reduction potential as well as the enthalpy change corresponding to the electrochemical couple eta-Cp(CO)(PR(3))(COMe)Fe(+)/eta-Cp(CO)(PR(3))(COMe)Fe(0) is linearly proportional to the V(min) values of PR(3). These correlations suggest that V(min) is a quantitative measure of the sigma-donating ability of the phosphine. It is hoped that, in phosphine-metal coordination chemistry, the V(min) based electronic parameter could be more advantageous than nu-CO and pK(a) based electronic parameters as it solely represents the inherent electronic property of the ligand.

The preparation of the novel N-heterocyclic carbene ligand 3 based on an oxalamide backbone is described. Carbene 3 is not stable as a monomeric species at ambient temperature but dimerizes to the olefin 4 in the absence of trapping reagents. Carbene 3 reacts with elemental sulfur, tert-butylisocyanide, or [M(COD)Cl]2 (M = Rh, Ir) to give thione 5, ketenimine 6, or complexes [(3)M(COD)Cl] (7), respectively. The dicarbonyl complexes [(3)M(CO)2Cl] (8) (M = Rh, Ir) show high-energy CO vibrations. The carbene ligand 3 behaves as a remarkably poor net electron donor compared to other NHC ligands.

The chemistry of N-heterocyclic carbenes has long been limited to metal coordination compounds derived from azolium precursors, a development that was started by Ofele and Wanzlick in 1968. Since free carbenes are now available through the work of Arduengo (1991), a renaissance in this little-recognized area of chemistry has occurred. A leading motive is the advantages of N-heterocyclic carbenes as ligands in organometallic catalysts, where they extend the scope of application reached by phosphanes (functionalized, chiral, water-soluble, and immobilized derivatives). The present review summarizes the state of the art with regard to synthesis, structure, bonding theory, metal coordination chemistry, and catalysis. Chelating, functionalized, chiral, and immobilized ligands can be generated and attached to metal centers in straightforward procedures under mild conditions. A wealth of new chemistry is thus opened. It is also shown how carbenes derived from imidazoles and triazoles behave as ligands in catalysis. It is reasonable to assume that N-heterocyclic carbenes surpass the ubiquitous phosphanes as ligands in a number of organometallic catalytic reactions.

A redox-switchable ligand, N,N'-dimethyldiaminocarbene[3]ferrocenophane (5), was synthesized and incorporated into a series of Ir- and Ru-based complexes. Electrochemical and spectroscopic analyses of (5)Ir(CO)2Cl (15) revealed that 5 displayed a Tolman electronic parameter value of 2050 cm(-1) in the neutral state and 2061 cm(-1) upon oxidation. Moreover, inspection of X-ray crystallography data recorded for (5)Ir(cis,cis-1,5-cyclooctadiene)Cl (13) revealed that 5 was sterically less bulky (%V(Bur) = 28.4) than other known diaminocarbene[3]ferrocenophanes, which facilitated the synthesis of (5)(PPh3)Cl2Ru(3-phenylindenylid-1-ene) (18). Complex 18 exhibited quasi-reversible electrochemical processes at 0.79 and 0.98 V relative to SCE, which were assigned to the Fe and Ru centers in the complex, respectively, based on UV-vis and electron pair resonance spectroscopic measurements. Adding 2,3-dichloro-5,6-dicyanoquinone over the course of a ring-opening metathesis polymerization of cis,cis-1,5-cyclooctadiene catalyzed by 18 ([monomer]0/[18]0 = 2500) reduced the corresponding rate constant of the reaction by over an order of magnitude (pre-oxidation: k(obs) = 0.045 s(-1); post-oxidation: k(obs) = 0.0012 s(-1)). Subsequent reduction of the oxidized species using decamethylferrocene restored catalytic activity (post-reduction: k(obs) = up to 0.016 s(-1), depending on when the reductant was added). The difference in the polymerization rates was attributed to the relative donating ability of the redox-active ligand (i.e., strongly donating 5 versus weakly donating 5(+)) which ultimately governed the activity displayed by the corresponding catalyst.