Bitupon Borthakur

Tuning the electronic and ligand properties of remote carbenes: a theoretical study.

The effect of annulation and carbonylation on the electronic and ligating properties of remote N-heterocyclic carbenes (rNHCs) has been studied quantum-chemically. The thermodynamic stability of these complexes has been assessed on the basis of their hydrogenation and stabilization energies, while HOMO-LUMO gaps are used to measure the kinetic stabilities. Annulated/carbonylated rNHCs are found to be weaker σ donors but better π acceptors compared with the parent rNHCs. The reactivity of these rNHCs has been studied by evaluating their nucleophilicity and electrophilicity indices. The nucleophilicity values are in good agreement with the σ basicities of all of the rNHCs. The (31)P NMR chemical shifts of the corresponding rNHC-phosphinidene adducts have been calculated and found to correlate well with the π acidities of these rNHCs.

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.

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)2Cl] fragment. NICS and QTAIM based bond magnetizability calculations indicate the presence of cyclic electron delocalization in majority of the molecules.

Structure and Reactivity of Carbones and Ylide Stabilized Carbenes: Contributions from Theory

This chapter presents the results of theoretical studies on divalent C(0) bases as well as on ylide stabilized carbenes. The primary difference between C(0) and C(II) compounds is the presence of two lone pairs at the central carbon atom of the former, unlike one in the latter. Theoretical studies reveal that the first and especially second proton affinity values can be used to distinguish carbenes from carbones. Further, the difference in reactivity of C(0) and C(II) compounds towards electrophiles such as H+, BH3 and AuCl is also found to be effective in distinguishing these two classes of compounds. Moreover, the 13C NMR chemical shifts of the central carbon atom as well as reactivity towards nucleophiles such as CNMe provide another criteria for distinguishing not only C(0) and C(II) compounds but also those having hidden C(0) characters. In addition, this chapter also deals with the recent studies carried out on ylide stabilized carbenes which possess excellent electron donation abilities. However, the ylide substituted carbenes are found to be ineffective towards activation of small molecules like H2 and NH3 due to the absence of a suitable low-lying π-acceptor orbital at the carbene centre.

Release of Isonitrile- and NHC-Stabilized Borylenes from Group VI Terminal Borylene Complexes

A family of doubly isonitrile-stabilized terphenyl borylenes could be obtained by addition of three equivalents of isonitrile to the corresponding Cr and W terminal terphenyl-borylene complexes. The mechanism of isonitrile- and carbon-monoxide-induced borylene liberation was investigated computationally and found to be significantly exergonic in both cases. Furthermore, addition of a small N-heterocyclic carbene (NHC) to a terminal Cr borylene complex results in release of an NHC-stabilized borylene carbonyl species, whereas the analogous reaction with bulkier phosphines results in metal-centered substitution.