26 research outputs found
Are N-Heterocyclic Carbenes āBetterā Ligands than Phosphines in Main Group Chemistry? A Theoretical Case Study of Ligand-Stabilized E<sub>2</sub> 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 E<sub>2</sub> 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 Sn<sub>2</sub> and Pb<sub>2</sub> 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 Ļ 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 ārenaissanceā
of low-valent main group compounds supported by ligands
Ring Opening of Epoxides Induced by Pentaphenylborole
The unsaturated antiaromatic BC<sub>4</sub> 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 Ī²-hydrogen
atoms, 1,1-diphenylethylene oxide inserts the C<sub>2</sub>O unit
to furnish a BOC<sub>6</sub> heterocycle, and cyclohexene oxide inserts
two epoxides to form an unusual BC<sub>8</sub>O<sub>2</sub> 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)<sub>2</sub> Reagents
We report the outcomes
of the reactions of aromatic group 16 thiophene, selenophene, and
tellurophene rings with the IĀ(III) oxidants PhIĀ(OAc)Ā(OTf) and [PhIĀ(Pyr)<sub>2</sub>]Ā[OTf]<sub>2</sub> (Pyr = pyridine). In all reactions, oxidative
processes take place, with generation of PhI as the reduction product.
However, with the exception of tellurophene with PhIĀ(OAc)Ā(OTf), +4
oxidation state complexes are not observed, but rather a variety of
other processes occur. In general, where a CāH unit is available
on the 5-membered ring, an electrophilic aromatic substitution reaction
of either āIPh or pyridine onto the ring occurs. When all positions
are blocked, reactions with PhIĀ(OAc)Ā(OTf) give acetic and triflic
anhydride as the identifiable oxidative byproducts, while [PhIĀ(Pyr)<sub>2</sub>]Ā[OTf]<sub>2</sub> 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<sup>+</sup>, 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 HOMOāLUMO and singletātriplet 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 BeāC
bond
Ring Opening of Epoxides Induced by Pentaphenylborole
The unsaturated antiaromatic BC<sub>4</sub> 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 Ī²-hydrogen
atoms, 1,1-diphenylethylene oxide inserts the C<sub>2</sub>O unit
to furnish a BOC<sub>6</sub> heterocycle, and cyclohexene oxide inserts
two epoxides to form an unusual BC<sub>8</sub>O<sub>2</sub> 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 NāHeterocyclic Carbene CāN 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 (H<sub>2</sub>BNHR, BH<sub>3</sub>; R = Me, Ph) and beryllium hydrides (BeH<sub>2</sub>) in
comparison with silanes (SiH<sub>2</sub>R<sub>2</sub>; 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) CāN 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 NHCĀ·BH<sub>3</sub> 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 BeH<sub>2</sub> insertion product provides a rare example of a BeāN
Ļ interaction
Reactions of [PhI(pyridine)<sub>2</sub>]<sup>2+</sup> with Model Pd and Pt II/IV Redox Couples
The results of the
reactions of the dicationic iodineĀ(III) family of oxidants [PhIĀ(pyridine)<sub>2</sub>]<sup>2+</sup> with model PdĀ(II) and PtĀ(II) complexes are
described. Depending on the specific reaction pairs, a variety of
outcomes are observed. For palladium, PdĀ(IV) complexes cannot be observed
but are implicated in CāC and CāN bond formation for
PdĀ(II) starting materials based on phenylpyridine and 2,2-bipyridine,
respectively. Theoretical comparisons with similar processes for āCl
and āOAc 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 PtĀ(IV) complexes that may be
isolated and structurally characterized
Te(II)/Te(IV) Mediated CāN Bond Formation on 2,5-Diphenyltellurophene and a Reassignment of the Product from the Reaction of PhI(OAc)<sub>2</sub> with 2 TMS-OTf
We report a novel
CāH to CāN bond metathesis at the 3-position of 1,2-diphenyltellurophene
via oxidation of the TeĀ(II) center to TeĀ(IV) using the IĀ(III) oxidant
[PhIĀ(4-DMAP)<sub>2</sub>]<sup>2+</sup>. Spontaneous reduction of a
transient TeĀ(IV) coordination compound to TeĀ(II) generates an electrophilic
equivalent of 4-DMAP that substitutes at a CāH bond at the
3-position of the tellurophene. Theoretical and synthetic reaction
pathway studies confirm that a TeĀ(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 IĀ(III) oxidant generated from
PhIĀ(OAc)<sub>2</sub> and 2 TMS-OTf is PhIĀ(OAc)Ā(OTf) and not PhIĀ(OTf)<sub>2</sub>, as had been previously thought
Homoleptic PnictogenāChalcogen 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, [ChĀ(dppe)]Ā[OTf]<sub>2</sub> [Ch = Se, Te; dppe = 1,2-bisĀ(diphenylphosphino)Āethane; OTf
= trifluoromethanesulfonate], are formed using [Ch]<sup>2+</sup> reagents
via a ligand-exchange protocol and represent extremely rare examples
of homoleptic pnictogen ā chalcogen coordination complexes.
The corresponding arsenic compounds were also prepared, [ChĀ(dpAse)]Ā[OTf]<sub>2</sub> [Ch = Se, Te; dpAse = 1,2-bisĀ(diphenylarsino)Āethane], exhibiting
the first instance of an arsenic ā chalcogen dative bond. The
electronic structures of these unique compounds were determined and
compared to previously reported chalcogen dications
Heterobimetallic <i>N</i>āHeterocyclic 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 āpincerā
ligand. The synthetic route involved the initial preparation of a
mononuclear [AuĀ(NHC)<sub>2</sub>]<sup>+</sup> complex with pendent
imidazole moieties on the NHC ligands. Subsequent alkylation of the
imidazole groups with Et<sub>3</sub>OBF<sub>4</sub> and metalation
with a second metal ion (AgĀ(I) or HgĀ(II)) provided two heterobimetallic
complexes. Four homobimetallic (CuĀ(I)<sub>2</sub>, AgĀ(I)<sub>2</sub>, AuĀ(I)<sub>2</sub>, and HgĀ(II)<sub>2</sub>) complexes of the same <i>bis</i>-NHC āpincerā ligand were also prepared.
The homobimetallic CuĀ(I)<sub>2</sub>, AuĀ(I)<sub>2</sub>, and HgĀ(II)<sub>2</sub> complexes and heterobimetallic AuĀ(I)āAgĀ(I) and AuĀ(I)āHgĀ(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 ātwistedā
conformations in the solid state, supporting short MĀ·Ā·Ā·M
interactions. Crystalline samples of the homobimetallic AgĀ(I)<sub>2</sub> and AuĀ(I)<sub>2</sub> and heterobimetallic AuĀ(I)āAgĀ(I)
and AuĀ(I)āHgĀ(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 MĀ·Ā·Ā·M interactions for all synthesized complexes
was investigated using natural bond orbital (NBO) calculations