Cyclic (amino)[bis(ylide)]carbene as an anionic bidentate ligand for transition-metal complexes

Single-Site and Cooperative Bond Activation Reactions with Ylide-Functionalized Tetrylenes: A Computational Study

Due to their transition metal-like behavior divalent group 14 compounds bear huge potential for their application in bond activation reactions and catalysis. Here we report on detailed computational studies on the use of ylide-substituted tetrylenes in the activation of dihydrogen and phenol. A series of acyclic and cyclic ylidyltetrylenes featuring various α-substituents with different σ- and π-donating capabilities have been investigated which demonstrate that particularly π-accepting boryl groups lead to beneficial properties and low barriers for single-site activation reactions, above all in the case of silylenes. In contrast, for the thermodynamically more stable germylenes and stannylenes an alternative mechanism involving the active participation of the ylide ligand in the E-H bond (E=H or PhO) activation process by addition across the element carbon linkage was found to be energetically favored. Furthermore, the boryl substituted tetrylenes allowed for a further activation pathway involving the active participation of the boron element bond. These cooperative mechanisms are especially attractive for the heavier cyclic ylidyltetrylenes in which the loss of the protonated ylide group is prevented due to the cyclic framework. Overall, the present studies suggest that cyclic ylide-substituted germylenes and stannylenes bear huge potential for cooperative bond activations at mild conditions which should be experimentally addressed in the future.

Carbenes and phosphonium ylides: a fruitful association in coordination chemistry

Among a plethora of σ-donor ligands available, carbon-centered ones have become essential, in particular with the emergence of N-heterocyclic carbenes (NHCs), positioning themselves as credible alternatives to traditional nitrogen- and phosphorus-based systems. Phosphonium ylides representing another class of neutral η¹-bonded carbon ligands have also been shown to act as effective Lewis bases. Considering the intrinsic features of the carbene and phosphonium ylide ligands, similar in terms of electronic properties, but different in terms of bonding mode, the design of hybrid systems combining these two types of carbon functionalities appeared to be a natural and exciting challenge. This Perspective comprehensively covers the chemistry of such ligand architectures from synthesis and fundamental aspects to catalytic applications.

Nickel Complexes in C‒P Bond Formation

This review is a comprehensive account of reactions with the participation of nickel complexes that result in the formation of carbon-phosphorus (C‒P) bonds. The catalytic and non-catalytic reactions with the participation of nickel complexes as the catalysts and the reagents are described. The various classes of starting compounds and the products formed are discussed individually. The several putative mechanisms of the nickel catalysed reactions are also included, thereby providing insights into both the synthetic and the mechanistic aspects of this phosphorus chemistry.

A 2,2'-diphosphinotolane as a versatile precursor for the synthesis of P-ylidic mesoionic carbenes via reversible C-P bond formation

A metal-templated synthetic route to cyclic (aryl)(ylidic) mesoionic carbenes (CArY-MICs) featuring an endocyclic P-ylide is presented. This approach, which requires metal templates with two cis-positioned open coordination sites, is based on the controlled cyclisation of a P,P'-diisopropyl-substituted 2,2'-diphosphinotolane (1) and leads to chelate complexes coordinated by a phosphine donor and the CArY-MIC carbon atom. The C-P bond formation involved in the former partial cyclisation of 1 proceeds under mild conditions and was shown to be applicable all over the d-block. In the presence of a third fac-positioned open coordination site, the P-C bond formation was found to be reversible, as shown for a series of molybdenum complexes. DFT modelling studies are in line with an interpretation of the target compounds as CArY-MICs.

Mercury(II) Complexes of Anionic N-Heterocyclic Carbene Ligands: Steric Effects of the Backbone Substituent

Mercury(II) complexes (Me-maloNHC_Dipp)HgCl (1b), (t-Bu-maloNHC_Dipp)HgCl (2b) and (t-Bu-maloNHC_Dipp)HgMe (2c) supported by anionic N-heterocyclic carbenes have been obtained in good yields from the reaction of the potassium salt of N-heterocyclic carbene ligand precursors and mercury(II) salts, HgCl₂ and MeHgI. These molecules have been characterized by ¹H-NMR, ¹³C-NMR and IR spectroscopy and elemental analysis. X-ray crystal structures of 1b and 2b are also presented. Interestingly, complex 1b is polymeric {(Me-maloNHC_Dipp)HgCl}_n in the solid state, as a result of inter-molecular Hg-O contacts, and features rare three coordinate mercury sites with a T-shaped arrangement, whereas the (t-Bu-maloNHC_Dipp)HgCl (2b) is monomeric and has a linear, two-coordinate mercury center. The formation of T-shaped structure and the aggregation of complex 1b is attributable to the reduced steric demand of the N-heterocyclic carbene ligand backbone substituent.

Phosphorus-ylides: powerful substituents for the stabilization of reactive main group compounds

Phosphorus ylides are 1,2-dipolar compounds with a negative charge on the carbon atom. This charge is stabilized by the neighbouring onium moiety, but can also be shifted towards other substituents thus making ylides strong π donor ligands and hence ideal substituents to stabilize reactive compounds such as cations and low-valent main group species. Furthermore, the donor strength and the steric properties can easily be tuned to meet different requirements for stabilizing reactive compounds and for tailoring the properties and reactivities of the main group element. Although the use of ylide substituents in main group chemistry is still in its infancy, the first examples of isolated compounds impressively demonstrate the potential of these ligands. This review summarizes the most important discoveries also in comparison to other substituents, thus outlining avenues for future research directions.

Towards the Preparation of Stable Cyclic Amino(ylide)Carbenes

Cyclic amino(ylide)carbenes (CAYCs) are the ylide-substituted analogues of N-heterocyclic Carbenes (NHCs). Due to the stronger π donation of the ylide compared to an amino moiety they are stronger donors and thus are desirable ligands for catalysis. However, no stable CAYC has been reported until today. Here, we describe experimental and computational studies on the synthesis and stability of CAYCs based on pyrroles with trialkyl onium groups. Attempts to isolate two CAYCs with trialkyl phosphonium and sulfonium ylides resulted in the deprotonation of the alkyl groups instead of the formation of the desired CAYCs. In case of the PCy₃-substituted system, the corresponding ylide was isolated, while deprotonation of the SMe₂-functionalized compound led to the formation of ethene and the thioether. Detailed computational studies on various trialkyl onium groups showed that both the α- and β-deprotonated compounds were energetically favored over the free carbene. The most stable candidates were revealed to be α-hydrogen-free adamantyl-substituted onium groups, for which β-deprotonation is less favorable at the bridgehead position. Overall, the calculations showed that the isolation of CAYCs should be possible, but careful design is required to exclude decomposition pathways such as deprotonations at the onium group.

A strong organic electron donor incorporating highly π-donating triphenylphosphonium ylidyl substituents

The π-electron donor strength of a triphenylphosphonium ylidyl group (Ph₃P[double bond, length as m-dash]CH-) was explored through its substitution onto a bispyridinylidene (BPY) scaffold. Electrochemical studies revealed that the new triphenylphosphonium ylidyl-substituted BPY is the most reducing di-substituted derivative reported to date (E_1/2 = -1.55 V vs. SCE). By using a previously established correlation between the redox potential of the substituted BPYs and the corresponding substituent, a Hammett constant for the Ph₃P[double bond, length as m-dash]CH- group was determined (σ_p⁺ = -2.33), establishing it as the most donating neutral substituent currently quantified. The BPY is readily oxidized by hexachloroethane to produce the corresponding dicationic bipyridinium salt as a mixture of isomers owing to hindered C_ylidyl-C_pyridyl bond rotation. In preliminary tests of the BPY as a reductant, dichlorotricyclohexylphosphorane and chlorodiphenylphosphine were reduced to the corresponding phosphine and diphosphine, respectively.

Carbeniophosphines versus Phosphoniocarbenes: The Role of the Positive Charge

The chemistry of carbeniophosphines and phosphoniocarbenes, which have general structures derived formally from the three-component "carbene/phosphine/positive charge" association, is presented. These two complementary classes of carbon-phosphorus-based ligands, defined by the presence of an inverted cationic coordinating structure (C+ ∼P: vs. P+ ∼C:) have the common purpose of positioning a positive charge in the vicinity of the metal center. Through selected examples, the synthetic methods, coordination properties, and general reactivity of both cationic species is described. Particular emphasis is placed on the influence of the positive charge on the respective chemical behavior of the two classes of compound.

Theoretical strategies toward stabilization of singlet remote N-heterocyclic carbenes

Theoretical investigations predict that the singlet states of ylide-substituted remote carbenes are significantly stable and comparable to those of experimentally known NHCs. They are also found to be strongly σ-donating in nature as evident from an evaluation of the carbonyl stretching frequencies (νCO ) of their complexes with the [Rh(CO)2 Cl] fragment. NICS and QTAIM based bond magnetizability calculations indicate the presence of cyclic electron delocalization in majority of the molecules. © 2016 Wiley Periodicals, Inc.

Direct estimate of the internal π-donation to the carbene centre within N-heterocyclic carbenes and related molecules

Fifteen cyclic and acylic carbenes have been calculated with density functional theory at the BP86/def2-TZVPP level. The strength of the internal X→p(π) π-donation of heteroatoms and carbon which are bonded to the C(II) atom is estimated with the help of NBO calculations and with an energy decomposition analysis. The investigated molecules include N-heterocyclic carbenes (NHCs), the cyclic alkyl(amino)carbene (cAAC), mesoionic carbenes and ylide-stabilized carbenes. The bonding analysis suggests that the carbene centre in cAAC and in diamidocarbene have the weakest X→p(π) π-donation while mesoionic carbenes possess the strongest π-donation.

Moving toward Ylide-Stabilized Carbenes

The effect of ylide substitution at the α position to the carbene carbon (Cc ) atom on the stability and σ-donating ability of a number of cyclic carbenes has been studied theoretically. The stabilities of all of the carbenes were investigated from an evaluation of their singlet-triplet energy gaps and stabilization energies. All carbenes were found to have a stable singlet state. The energy of the σ-symmetric lone-pair orbital at the Cc atom increases as a result of the introduction of ylide centers near to the Cc atom. This indicates an enhanced σ-donating ability of the ylide-containing carbenes. The calculated carbonyl-stretching frequencies of the corresponding rhodium complexes, proton affinities, and nucleophilicity index values correlate well with the σ basicity of the carbenes.

Annulated boron substituted N-heterocyclic carbenes: theoretical prediction of highly electrophilic carbenes

Annulated boron substituted N-heterocyclic carbenes are found to have significantly enhanced π-accepting ability which is further corroborated by ³¹P NMR shift calculations of the phosphinidene adducts of these carbenes.

Introduction to N-Heterocyclic Carbenes: Synthesis and Stereoelectronic Parameters

N-Heterocyclic carbenes (NHCs) are cyclic compounds containing a divalent carbon atom bound to at least one nitrogen atom within the heterocycle. Variation of the size of the carbene ring, the substituents on the nitrogen atoms or the additional atoms within the heterocycle lead to an array of different NHCs exhibiting a broad range of electronic properties. Their ability to act as donors and the resulting stable bounds to most transition metals make them excellent ligands in coordination chemistry. In addition, free NHCs have found applications as organocatalysts in metal free chemical transformations. In this Chapter synthetic procedures leading to different NHCs and important structural and electronic features of this class of compounds are discussed.

Stable cyclic carbenes and related species beyond diaminocarbenes

The success of homogeneous catalysis can be attributed largely to the development of a diverse range of ligand frameworks that have been used to tune the behavior of various systems. Spectacular results in this area have been achieved using cyclic diaminocarbenes (NHCs) as a result of their strong σ-donor properties. Although it is possible to cursorily tune the structure of NHCs, any diversity is still far from matching their phosphorus-based counterparts, which is one of the great strengths of the latter. A variety of stable acyclic carbenes are known, but they are either reluctant to bind metals or they give rise to fragile metal complexes. During the last five years, new types of stable cyclic carbenes, as well as related carbon-based ligands (which are not NHCs), and which feature even stronger σ-donor properties have been developed. Their synthesis and characterization as well as the stability, electronic properties, coordination behavior, and catalytic activity of the ensuing complexes are discussed, and comparisons with their NHC cousins are made.

A novel ylide-stabilized carbene; formation and electron donating ability of an amino(sulfur-ylide)carbene

A novel N-heterocyclic carbene, amino(sulfur-ylide)carbene, which is stabilized by electron donation from the carbanion of a sulfur ylide as well as a nitrogen atom, was generated and its Rh complex was synthesized; the electron donating properties of the amino(sulfur-ylide)carbene were investigated using the IR carbonyl stretching frequencies of the Rh complex, indicating a relatively strong electron-donating ability among reported carbenes.