Are N-Heterocyclic Carbenes “Better” Ligands than Phosphines in Main Group Chemistry? A Theoretical Case Study of Ligand-Stabilized E₂ Molecules, L-E-E-L (L = NHC, phosphine; E = C, Si, Ge, Sn, Pb, N, P, As, Sb, Bi)

A theoretical examination of the L-E-E-L class of molecules has been carried out (E = group 14, group 15 element; L = N-heterocyclic carbene, phosphine), for which Si, Ge, P, and As-NHC complexes have recently been synthesized. The focus of this study is to predict whether it is possible to stabilize the elusive E₂ molecule via formation of L-E-E-L beyond the few known examples, and if the ligand set for this class of compounds can be extended from the NHC to the phosphine class of ligands. It is predicted that thermodynamically stable L-E-E-L complexes are possible for all group 14 and 15 elements, with the exception of nitrogen. The unknown ligand-stabilized Sn₂ and Pb₂ complexes may be considered attractive synthetic targets. In all cases the NHC complexes are more stable than the phosphines, however several of the phosphine derivatives may be isolable. The root of the extra stability conferred by the NHC ligands over the phosphines is determined to be a combination of the NHCs greater donating ability, and for the group 15 complexes, superior π acceptor capability from the E-E core. This later factor is the opposite as to what is normally observed in transition metal chemistry when comparing NHC and phosphine ligands, and may be an important consideration in the ongoing “renaissance” of low-valent main group compounds supported by ligands

Ring Opening of Epoxides Induced by Pentaphenylborole

The unsaturated antiaromatic BC₄ heterocycle pentaphenylborole has been shown to have diverse reactivity with a variety of substrates, including the insertion of polar functional groups into the ring as a route to conjugated boracycles. This work investigates the reactivity of a selection of epoxides with pentaphenylborole, both computationally and experimentally, revealing that the substitution is highly influential on the reaction outcome. Specifically, isobutylene oxide results in protodeborylation to a borabutadiene chain attributed to the acidic β-hydrogen atoms, 1,1-diphenylethylene oxide inserts the C₂O unit to furnish a BOC₆ heterocycle, and cyclohexene oxide inserts two epoxides to form an unusual BC₈O₂ ring. The last two species represent rare boron-containing rings of eight atoms or greater, with the 11-membered species being only the second reported and the first crystallographically characterized

Diverse Reactions of Thiophenes, Selenophenes, and Tellurophenes with Strongly Oxidizing I(III) PhI(L)₂ Reagents

We report the outcomes of the reactions of aromatic group 16 thiophene, selenophene, and tellurophene rings with the I­(III) oxidants PhI­(OAc)­(OTf) and [PhI­(Pyr)₂]­[OTf]₂ (Pyr = pyridine). In all reactions, oxidative processes take place, with generation of PhI as the reduction product. However, with the exception of tellurophene with PhI­(OAc)­(OTf), +4 oxidation state complexes are not observed, but rather a variety of other processes occur. In general, where a C–H unit is available on the 5-membered ring, an electrophilic aromatic substitution reaction of either −IPh or pyridine onto the ring occurs. When all positions are blocked, reactions with PhI­(OAc)­(OTf) give acetic and triflic anhydride as the identifiable oxidative byproducts, while [PhI­(Pyr)₂]­[OTf]₂ gives pyridine electrophilic aromatic substitution onto the peripheral rings. Qualitative mechanistic studies indicate that the presence of the oxidizable heteroatom is required for pyridine to act as an electrophile in a substantial manner

Computational Predictions of the Beryllium Analogue of Borole, Cp⁺, and the Fluorenyl Cation: Highly Stabilized, non-Lewis Acidic Antiaromatic Ring Systems

A computational study of a set of synthetically unknown beryllium-containing rings, anionic analogues of antiaromatic boroles, has been carried out to investigate their structure, stability, and potential reactivity. The results indicate that these compounds should be electronically viable (as assessed from HOMO–LUMO and singlet–triplet gaps) and therefore potential targets for synthesis. In strong contrast with boroles, these beryllium species are predicted to be not Lewis acidic but rather Lewis basic, with reactivity centered on the endocyclic Be–C bond

Ring Opening of Epoxides Induced by Pentaphenylborole

The unsaturated antiaromatic BC₄ heterocycle pentaphenylborole has been shown to have diverse reactivity with a variety of substrates, including the insertion of polar functional groups into the ring as a route to conjugated boracycles. This work investigates the reactivity of a selection of epoxides with pentaphenylborole, both computationally and experimentally, revealing that the substitution is highly influential on the reaction outcome. Specifically, isobutylene oxide results in protodeborylation to a borabutadiene chain attributed to the acidic β-hydrogen atoms, 1,1-diphenylethylene oxide inserts the C₂O unit to furnish a BOC₆ heterocycle, and cyclohexene oxide inserts two epoxides to form an unusual BC₈O₂ ring. The last two species represent rare boron-containing rings of eight atoms or greater, with the 11-membered species being only the second reported and the first crystallographically characterized

Comparison of the Mechanism of Borane, Silane, and Beryllium Hydride Ring Insertion into N‑Heterocyclic Carbene C–N Bonds: A Computational Study

A computational investigation has been carried out on the mechanism and energetics of the experimentally observed insertion/ring expansion of N-heterocyclic carbenes (NHCs) by boranes (H₂BNHR, BH₃; R = Me, Ph) and beryllium hydrides (BeH₂) in comparison with silanes (SiH₂R₂; R = Me, Ph). The results suggest that the ring insertion mechanisms are similar for boranes, beryllium hydrides, and silanes. The principal mechanism components are (1) hydrogen atom migration to the carbene carbon, (2) C–N bond expansion of the NHC with insertion of the main-group hydride into the ring, and (3) migration of a second hydrogen atom to the carbene carbon. The synthetically important NHC·BH₃ adduct is also predicted to be thermodynamically unstable with respect to this transformation but is kinetically stabilized with a high barrier to the first hydrogen atom migration. The BeH₂ insertion product provides a rare example of a Be–N π interaction

Reactions of [PhI(pyridine)₂]²⁺ with Model Pd and Pt II/IV Redox Couples

The results of the reactions of the dicationic iodine­(III) family of oxidants [PhI­(pyridine)₂]²⁺ with model Pd­(II) and Pt­(II) complexes are described. Depending on the specific reaction pairs, a variety of outcomes are observed. For palladium, Pd­(IV) complexes cannot be observed but are implicated in C–C and C–N bond formation for Pd­(II) starting materials based on phenylpyridine and 2,2-bipyridine, respectively. Theoretical comparisons with similar processes for −Cl and −OAc rather than pyridine indicate that these provide greater thermodynamic stability, and our results here show that they also give greater kinetic stability (the failure of MP2 methods for these systems is quite dramatic). In contrast, oxidation and delivery of the pyridine ligands gives dicationic Pt­(IV) complexes that may be isolated and structurally characterized

Te(II)/Te(IV) Mediated C–N Bond Formation on 2,5-Diphenyltellurophene and a Reassignment of the Product from the Reaction of PhI(OAc)₂ with 2 TMS-OTf

We report a novel C–H to C–N bond metathesis at the 3-position of 1,2-diphenyltellurophene via oxidation of the Te­(II) center to Te­(IV) using the I­(III) oxidant [PhI­(4-DMAP)₂]²⁺. Spontaneous reduction of a transient Te­(IV) coordination compound to Te­(II) generates an electrophilic equivalent of 4-DMAP that substitutes at a C–H bond at the 3-position of the tellurophene. Theoretical and synthetic reaction pathway studies confirm that a Te­(IV) coordination complex with 4-DMAP is an intermediate. In the course of these pathway studies, it was also found that the identity of the I­(III) oxidant generated from PhI­(OAc)₂ and 2 TMS-OTf is PhI­(OAc)­(OTf) and not PhI­(OTf)₂, as had been previously thought

Homoleptic Pnictogen–Chalcogen Coordination Complexes

The synthesis and structural characterization of dicationic selenium and tellurium analogues of the carbodiphosphorane and triphosphenium families of compounds are reported. These complexes, [Ch­(dppe)]­[OTf]₂ [Ch = Se, Te; dppe = 1,2-bis­(diphenylphosphino)­ethane; OTf = trifluoromethanesulfonate], are formed using [Ch]²⁺ reagents via a ligand-exchange protocol and represent extremely rare examples of homoleptic pnictogen → chalcogen coordination complexes. The corresponding arsenic compounds were also prepared, [Ch­(dpAse)]­[OTf]₂ [Ch = Se, Te; dpAse = 1,2-bis­(diphenylarsino)­ethane], exhibiting the first instance of an arsenic → chalcogen dative bond. The electronic structures of these unique compounds were determined and compared to previously reported chalcogen dications

Heterobimetallic <i>N</i>‑Heterocyclic Carbene Complexes: A Synthetic, Spectroscopic, and Theoretical Study

A new synthetic methodology has been developed for the preparation of heterobimetallic group 11 and group 12 complexes of a symmetrical <i>bis</i>-NHC “pincer” ligand. The synthetic route involved the initial preparation of a mononuclear [Au­(NHC)₂]⁺ complex with pendent imidazole moieties on the NHC ligands. Subsequent alkylation of the imidazole groups with Et₃OBF₄ and metalation with a second metal ion (Ag­(I) or Hg­(II)) provided two heterobimetallic complexes. Four homobimetallic (Cu­(I)₂, Ag­(I)₂, Au­(I)₂, and Hg­(II)₂) complexes of the same <i>bis</i>-NHC “pincer” ligand were also prepared. The homobimetallic Cu­(I)₂, Au­(I)₂, and Hg­(II)₂ complexes and heterobimetallic Au­(I)–Ag­(I) and Au­(I)–Hg­(II) complexes and the synthetic intermediates for the heterobimetallic complexes were characterized by X-ray crystallography. These X-ray structures show that the bimetallic complexes adopt “twisted” conformations in the solid state, supporting short M···M interactions. Crystalline samples of the homobimetallic Ag­(I)₂ and Au­(I)₂ and heterobimetallic Au­(I)–Ag­(I) and Au­(I)–Hg­(II) complexes were emissive at room temperature and at 77 K. The geometries of the synthesized complexes were optimized at the M06-L/def2-SVP level of theory, and the electronic nature of the M···M interactions for all synthesized complexes was investigated using natural bond orbital (NBO) calculations