Mesoionic Janus-Type Dicarbene: Complexes, Adducts, and Catalytic Studies.

The preparations of homo- and hetero-bimetallic complexes as well as thiourea and selenourea derivatives of a mesoionic Janus-type N-heterocyclic dicarbene (diNHC) are reported. Analogues of its monocationic intermediate NHC have also been obtained for comparison. Using the main group adducts, the π-acceptor properties of both NHCs were determined using low temperature 77Se NMR spectroscopy completing their stereoelectronic profiling. Moreover, catalytic investigations reveal that the mesoionic dipalladium Janus-diNHC complex can be used in the sequential C2- and C5-arylation of 1-methylpyrrole for the preparation of non-symmetrical 2,5-diarylpyrroles.

Design of a Mesoionic Janus-type Dicarbene.

A versatile synthetic strategy for the preparation of homo- and heterobimetallic complexes bearing an unprecedented mesoionic Janus-type diNHC is presented. Moreover, its electronic property is evaluated, and a preliminary catalytic application in the direct diarylation of 1-methylpyrrole is demonstrated.

Electronic Profiling of N-Phosphine Oxide-Substituted Imidazolin-2-ylidenes (PoxIms) and Imidazolidin-2-ylidenes (SPoxIms).

A detailed electronic study of the N-phosphine oxide functionalized imidazolin-2-ylidenes (PoxIms) and imidazolidin-2-ylidenes (SPoxIms) has been performed experimentally using IR, 13C, and 77Se NMR spectroscopies. While the net donor/acceptor properties of the (S)PoxIms could not be differentiated via IR spectroscopy (TEP), NMR spectroscopic methods (HEP, Se) reveal that the (S)PoxIms are slightly weaker σ-donors but stronger π-acceptors compared to common NHCs. Moreover, backbone and substituent-effects could also be resolved by the latter, allowing for a ranking of their electronic properties. Finally, the donicities of these well-designed NHC ligands in their bidentate κ2-C,O modes were evaluated using HEP2 and compared to those of classical chelators.

Diverse Post-Metathesis Reactivities of Mixed Amido-Isothiocyanato Molybdenum Complexes.

Attempts to prepare mixed isothiocyanato-bis(imido) MoVI complexes led to the discovery of post-metathesis rearrangements toward three distinct products (1-3), which feature the NCS-derived chelators [N(NMe2)CS]2- (L1 in dinuclear 1 and 2) and [N(SiMe3)(NMe2)CS]- (L2 in mononuclear 3). Notably, the preparation of bidentate ligand L1 and its coordination chemistry are unprecedented. Together with computational studies, it is proposed that the putative "mono-substituted" intermediate [Mo(NtBu)2(NMe2)(NCS)] serves as the common starting point for the observed molecular transformations. Construction of the [Mo(NtBu)2(NCS)2] core was ultimately possible in the presence of additional stabilizing donors (THF or PMe3), which yielded the complexes [Mo(NtBu)2(NCS)2(THF)2] (4) and [Mo(NtBu)2(NCS)2(PMe3)2] (5).

Donor Strength Determination of Anionic Ligands.

14 new gold(I) NHC complexes of the type [AuX(iPr2-bimy)] (iPr2-bimy = 1,3-diisopropylbenzimidazolin-2-ylidene) have been prepared and fully characterized. These complexes and their reported analogues were used to systematically compare and rank the donating abilities of overall 34 anionic X-type donors by 13C NMR spectroscopy. Specifically, the carbene chemical shift of the iPr2-bimy ligand was found to be responsive to the ligand X spanning an overall range Δδ > 37 ppm between the strongest and weakest donor in this study.

Stereoelectronic Profiling of Neutral and Monoanionic Biimidazoles and Mixed Diimines.

14 mono-, di-, and tetranuclear palladium complexes were prepared to study the coordination chemistry of symmetrical and unsymmetrical azole-derived diimines and their anions. The diverse range of complexes obtained highlights the structural and electronic diversities imposed by these ligands. Using the monopalladium species, the electronic properties of selected bidentate ligands were determined, ranked, and compared by 13C NMR spectroscopy, extending the scope of the HEP2 (Huynh electronic parameter 2) scale, which can detect even subtle differences. Moreover, the %Vbur (percentage volume buried) values as an estimate for the steric bulk of some ligands were determined using the solid-state molecular structures of their complexes, and a preliminary stereoelectronic map was established.

Versatile halogenation via a CNHC

Stable cyclopalladated complexes containing an (sp3)C-Pd bond were synthesized via α-CH2 deprotonation and palladation of N-alkyl groups of carbene ligands bearing electron-withdrawing substituents. The strong electron donating strengths of the resulting CNHC^Csp3 chelators were experimentally identified, and the palladacycle underwent template-directed, versatile C-halogenation with X2.

Stereoelectronic Mapping of Dithiocarbamates and Xanthates.

A library of 12 palladium(II) complexes of the type [PdBr(iPr2-bimy)(L∧X)] comprising 10 dithiocarbamato (R2NCS2-) and two xanthato (ROCS2-) ligands have been prepared and fully characterized. With these complexes in hand, the electronic and steric properties of the bidentate, monoanionic ligands were evaluated using the HEP2 and %Vbur methodologies. Moreover, the construction of the first stereoelectronic map for dithiocarbamates enabled the in-principle identification of optimal ligand parameters for enhanced cytotoxic activities of their gold(III) complexes. This application of the stereoelectronic map showcases its viability as a useful tool to establish structure-activity relationships for rational ligand design.

Controlled Access to Four- and Six-Membered Palladacycles via Modifying Donor Abilities of ß-Ketiminato Ligands ("NacAcs").

The synthesis of Pd complexes of the type [PdBr(iPr2-bimy)(NacAc)] (NacAc = ß-ketiminate, iPr2-bimy = 1,3-diisopropylbenzimidazolin-2-ylidene) was attempted, in a continuing effort to quantify donor abilities of chelating ß-ketiminate ligands using the Huynh electronic parameter for bidentate donors (HEP2). Subtle variation of N-substituents on the NacAc backbone was discovered to induce a drastic change in the preferred chelating mode, in that the commonly encountered κ2-N,O-six-membered palladacycles were observed with R = Me and Et, while the unusual κ2-C,N-four-membered palladacycles were isolated with R = iPr, Cy, and tBu. Computational studies subsequently corroborated these findings, in the form of an overall exergonic six-to-four-membered ring contraction process and a lower associated activation energy for the three more electron-donating alkyl moieties. This trend in the established energy profiles can be attributed to a reduced HOMO-LUMO gap in the corresponding optimized structures of the six-membered ring complexes.

Gauging the donor strength of iron(0) complexes via their N-heterocyclic carbene gold(I) adducts.

We isolate and characterize the gold(I)-iron(0) adducts [(iPr2-bimy)Au-Fe(CO)3(PMe3)2][BArF4] and [Au-{Fe(CO)3(PMe3)2}2][BArF4] (iPr2-bimy = 1,3-diisopropylbenzimidazolin-2-ylidene, BArF4 = tetrakis(pentafluorophenyl)borate). DFT analysis reveals that the gold-iron interaction in [(iPr2-bimy)Au-Fe(CO)3(PMe3)2][BArF4] is predominantly a σ-donation from iron to gold. We further extend this class of compounds to include [(iPr2-bimy)Au-Fe(CO)3(PR3)2][BArF4] (PR3 = PPh3, PCy3, PCyPh2, PMePh2, PMe2Ph, P(4-C6H4F)3) and [(iPr2-bimy)Au-Fe(CO)4(PPh3)][BArF4] and correlate the iPr2-bimy carbenic 13C NMR signal with the relative donor strength of the iron(0) ligand. This approach allows for a fast and simple approach to gauge relative donor strength of Fe(0) donors.

Mixed NHC-thiolato complexes of palladium: understanding the formation of di-versus mononuclear complexes.

The preference for the formation of mono- versus dinuclear mixed carbene/thiolato complexes of PdII has been studied with three types of N-heterocyclic carbenes derived from benzimidazole, imidazole and 1,2,4-triazole. The complexes were prepared by treatment of halido/NHC precursors with sodium isopropylthiolate in a salt metathesis reaction. Mononuclear complexes are formed when the sulfur and carbon donors are exclusively cis to each other, while their trans arrangement preferably leads to dinuclear complexes with µ2-bridging thiolato ligands. The increased electron density in the latter case cannot be sufficiently compensated by one PdII center alone and leads to the formation dinuclear species with bridging thiolato ligands.

Donor Strength Determination of Pyridinylidene-amide Ligands using Their Palladium-NHC Complexes.

Pyridinylidene-amides (PYAs) are a relatively new type of N-donor ligands that can exist in three isomeric forms and adopt various resonance structures. This makes them electronically flexible, and in order to evaluate their electronic profile using the Huynh electronic parameter (HEP), seven structurally diverse mixed N-heterocyclic carbenes (NHCs)/PYA palladium complexes of the type trans-[PdBr2(iPr2-bimy)(PYA)] were prepared and fully characterized by various spectroscopic and spectrometric methods. This study shows that PYAs are among the strongest, formally neutral N-donors, but they are still weaker than phosphines and organometallic ligands such as NHCs. Notably, the donating abilities of isomeric PYAs are distinct and can be further fine-tuned by the choice of two substituents making them structurally and electronically versatile. These characteristics and the ease of their preparation hold promise for a wide applicability in coordination chemistry.

Stereoelectronic Profiling of Acyclic Diamino Carbenes (ADCs).

A library of 14 heterobis(carbene) complexes of the general formula [Au(iPr2-bimy)(ADC)]BF4 (7-20) containing the N-heterocyclic carbene reporter iPr2-bimy and various protic acyclic diaminocarbenes (ADCs) have been prepared to estimate their stereoelectronic properties by 13C NMR spectroscopy and percentage buried volume (%Vbur) determinations. Their preparation was achieved by nucleophilic attack of five secondary amines on six mixed NHC/isocyanide complexes of the type [Au(iPr2-bimy)(CN-R)]BF4 (1-6). Analyses of the iPr2-bimy carbene signals reveal that protic ADCs are stronger donors than classical and expanded-ring NHCs. On the other hand, they are weaker donating compared to NHCs with reduced-heteroatom stabilization. Moreover, stereoelectronic fine-tuning of these ligands is possible by a diverse range of substituents originating from the employed isocyanides and amines.

Fused azole-thiazolines via one-pot cyclization of functionalized N-heterocyclic carbene precursors.

A one-pot two-step methodology was exploited to synthesize fused thiazoline-azolium salts via reactions of bromoalkyl-azolium salts with KSCN and NaOH. The synthetic feasibility and versatility was demonstrated by the high yield (>80%) preparation of 13 salts with different backbones, linkers and substituents. Using methylpropionato as an N-protecting group, the resulting salts could be further derivatized to their neutral azole-thiazolines. The reaction sequence proceeds via (i) Br → SCN substitution, (ii) N-heterocyclic carbene formation, (iii) carbene attack of the S atom and CN- displacement in the alkyl-S-C[triple bond, length as m-dash]N unit, and (iv) methyl acrylate elimination.

Donor Strengths Determination of Pnictogen and Chalcogen Ligands by the Huynh Electronic Parameter and Its Correlation to Sigma Hammett Constants.

The suitability and accuracy of the Huynh electronic parameter (HEP) was further tested to reveal remote substituent effects in pyridines, which are located five or six bonds away from the reporter probe. These values show an excellent correlation to Hammett σ-constants of the respective substituents with coefficients of R2 =0.9856 (σm ) and R2 =0.9857 (σp ). Based on this observation, a methodology for the re-evaluation of certain Hammett constants with larger uncertainties has been proposed and demonstrated. Moreover, the scope of HEP was extended to various neutral pnictogen and chalcogen donors during which "transphobia effects" were revealed for mixed NHC complexes containing phosphites, arsine and stibine for the first time.

Stereoelectronic Profiling of Expanded-Ring N-Heterocyclic Carbenes.

Heterobis(carbene) complexes of palladium(II) and gold(I) containing expanded-ring N-heterocyclic carbenes (erNHCs) have been prepared in order to study their electronic properties. erNHCs with mesityl substituents were found to exhibit anisotropic interferences, which hampered ranking of their donicities by 13C NMR spectroscopy. The anisotropy effects were found to be stronger in the linear gold complexes, where a smaller coordination number allows the wingtips to spread out more. erNHCs with flexible N-benzyl groups are more suitable, and their donor strengths were found to gradually increase from five- to seven-membered heterocycles. The same trend can also be obtained by comparing the 1 J(C-H) coupling constants of the respective salts, although significant differences between seven- and eight-membered erNHCs could not be detected. The % Vbur values of erNHCs obtained from structures of their palladium and gold complexes revealed that the anisotropic interferences increase with overall steric bulk.

A palladacyclic N-heterocyclic carbene system used to probe the donating abilities of monoanionic chelators.

A series of NHC-containing [C^N]- or [C^C']-type palladacyclic complexes of the general formula [PdBr(iPr2-bimy)(L^X)] (5-8, 11, 12, iPr2-bimy = 1,3-diisopropylbenzimidazolin-2-ylidene) have been synthesized and fully characterized. Using these complexes, the donating abilities of monoanionic chelators were probed for the first time. The [C^N]-type palladacycles 5-8 were prepared from acetato-bridged dipalladium complexes [Pd(µ-CH3COO)(C^N)]2 (1-4) and iPr2-bimy·H+Br-as precursors. In the case of the [C^C']-type NHC palladacycles (11, 12), the hetero-bis(NHC) complexes trans-[PdBr2(iPr2-bimy)(trz)] (8, 9, trz = 1,2,3-triazolin-5-ylidene) containing the iPr2-bimy probe were first prepared followed by acetate-assisted cyclopalladations. The 13Ccarbene NMR signals of the iPr2-bimy ligands in all complexes (i.e. HEP and HEP2 values) are found to rationally reflect the donating abilities of the incorporated trz or [L^X]-type chelators with the exception of the Bzpy ligand (Bzpy = 2-(2-pyridinylmethyl)phenyl-C,N). This has been attributed to its larger bite angle, the resulting varied coordination geometry and the lack of electronic delocalization between the two donor units. The donicities of [L^X]-type chelators studied in this work were found to surpass those of all other bidentate ligands evaluated by HEP2 thus far.

Ring-expanded N-heterocyclic carbenes as ligands in iron-catalysed cross-coupling reactions of arylmagnesium reagents and aryl chlorides.

The structure-activity relationship of expanded-ring N-heterocyclic carbenes (NHCs) in the iron-catalysed Kumada aryl-aryl coupling reaction was explored. This was achieved by comparing the catalytic performance of Fe-NHC catalysts generated in situ containing NHCs that differ in steric bulk. In particular, the influences of ring sizes (5-8) and N-aryl substituents were explored in terms of spectroscopic and structural features, which affect their %Vbur values. The three best performing ligands were found on a diagonal of a 5 × 4 structural matrix revealing an optimal steric bulk and significant influences of subtle steric variations on the catalytic activities.

Electronic Properties of N-Heterocyclic Carbenes and Their Experimental Determination.

N-Heterocyclic carbenes (NHCs) have become without doubt one of the most exciting and popular species in chemical science due to the ease of their preparation and modularity in stereoelectronic properties. Numerous types of NHCs have been prepared, and various experimental methodologies have been proposed for the study of their electronic properties in order to rationalize reactivities observed. The objective of this article is to provide a comprehensive overview of the most common and popular ones among them. In particular, these include the nickel(0)-based TEP, its rhodium(I) and iridium(I) variants, LEP and related electrochemical methods, the palladium(II)-based HEP, phosphinidene- and selenourea-based methods, as well as the use of direct 1 J(C-H) coupling constants of the precarbene carbon in azolium salts. Each individual method and the underlying principle of detection it utilizes will be critically discussed in terms of strength and weakness. In addition, comprehensive amounts of data from various NHCs are compiled for the purpose of comparison. These are also meant to help the scientist in better understanding their own research data and possibly providing directions for their future research, which rely on the unique electronic properties of NHCs.

Anion influences on reactivity and NMR spectroscopic features of NHC precursors.

A series of 16 benzimidazolium salts of the type iPr2-bimyH+X- with various anions X were synthesized and characterized by various spectroscopic and spectrometric methods. Significant anion and solvent effects on the chemical shifts of the C2-H protons were found, which allows for a ranking of the anions in terms of their hydrogen-bond acceptor properties. Stronger acceptors could increase the acidity of their respective salts leading to a faster H/D exchange. Similar but less pronounced anion influences were detected for the 13CC2 NMR resonances, while 1 J C2-H coupling constants appear to be anion and solvent independent.