The final fate of NHC stabilized dicarbon

In this paper we report the outcome of the reduction of NHC stabilized acetylenic dications, [NHC-Cn-NHC]2+ for n=2 and 4. The target compounds were NHC stabilized di- and tetracarbon in the form of NHC-Cn-NHC. However, upon chemical reduction, decomposition ensues with release of the free NHC. This effect is also observed in electrochemical studies. This lends credence to Bestman’s hypothesis that two donor ligands cannot stabilize Cn for n=even numbers.

NHC-Stabilised Acetylene-How Far Can the Analogy Be Pushed?

Experimental studies suggest that the compound (NHC^bz )₂ C₂ H₂ can be considered as a complex of a distorted acetylene fragment which is stabilised by benzoannelated N-heterocyclic carbene ligands (NHC^bz )→(C₂ H₂ )←(NHC^bz ). A quantum chemical analysis of the electronic structures shows that the description with dative bonds is more favourable than with electron-sharing double bonds (NHC^bz )=(C₂ H₂ )=(NHC^bz ).

The fate of NHC-stabilized dicarbon

The attempted synthesis of NHC-stabilized dicarbon (NHC=C=C=NHC) through deprotonation of a doubly protonated precursor ([NHC-CH=CH-NHC](2+) ) is reported. Rather than deprotonation, a clean reduction to NHC=CH-CH=NHC is observed with a variety of bases. The apparent resistance towards deprotonation to the target compound led to a reinvestigation of the electronic structure of NHC→CC←NHC, which showed that the highest occupied molecular orbital/lowest unoccupied molecular orbital (HOMO/LUMO) gap is likely too small to allow for isolation of this species. This is in contrast to the recent isolation of the cyclic alkylaminocarbene analogue (cAAC=C=C=cAAC), which has a large HOMO-LUMO gap. A detailed theoretical study illuminates the differences in electronic structures between these molecules, highlighting another case of the potential advantages of using cAAC rather than NHC as a ligand. The bonding analysis suggests that the dicarbon compounds are well represented in terms of donor-acceptor interactions L→C2 ←L (L=NHC, cAAC).

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.

On the Bonding in Bis-pyridine Iodonium Cations

A study on the bonding in bis-pyridine halonium cations has been carried out using both theoretical and synthetic techniques. The primary thrust for the study is to highlight the opportunities potentially afforded by considering iodine as a Lewis acid in a classic coordination sense. Our results suggest that the iodine dipyridine complex ([pyr-I-pyr]+) can be considered as a coordination complex of [I]+. The lighter bromine and chlorine analogues are more towards the covalent rather than the dative side of bonding, while [pyr-F-pyr]+ is best described as an ion-molecule complex with one strong covalent F-pyr bond and one weak F-pyr dispersion interaction. Finally, theoretical and synthetic studies suggest that the commercially available [pyr-F]+ cation cannot be considered as a coordination complex of ‘F+’, despite its use as a source of electrophilic fluorine.