DFT STUDY OF NEW P -HETEROCYCLIC SILYLENES
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DFT STUDY OF NEW P-HETEROCYCLIC SILYLENES M. Ayoubi-Chianeh1 and M. Z. Kassaee1*
Following our quest for new divalents, we report the synthesis of P-heterocyclic silylenes (PHSis) by adding from one to four phosphorous atoms to saturated cyclopentasilylene (1) and unsaturated cyclopentasilylene-3-ene (1′) resulting in respective PHSis (2-10 and 2′-10′, respectively) and their study at the B3LYP/6-311++G** level of theory. Stabilities of 1-10 and 1′-10′ assumed as the energy gaps between the singlet (s) and triplet (t) states (ΔEs-t) are compared and contrasted. With increasing number of phosphorous atoms, singlet-triplet energy gap (ΔEs-t), nucleophilicity (N), electrophilicity (ω), chemical potential (μ), band gap (ΔEH-L), positive natural bond orbital (NBO) charge on the Si atom, and reactivity decrease. Changes in the hydrogenation enthalpy (ΔHHyd) of 1-10 and 1′-10′ show negative values indicating that the formation of hydrogenated divalent silylenes is exothermic (1H-10H and 1H′-10H′, respectively). The positive overall change in the Gibbs free energy of reaction (ΔGover for the conversion of 1′-10′ to 1-10, respectively) confirms that every 1′-10′ is more stable than its corresponding 1-10. DOI: 10.1134/S0022476620070033 Keywords: computational chemistry, quantum chemical calculations, density functional theory, silicon compounds.
INTRODUCTION Silylenes are divalent dicoordinate silicon species and heavier analogs of carbenes [1]. Because of their high reactivity, they are mostly observable at low temperatures and have been studied as transient species in the gas phase [2] or a liquid solution [3], or by matrix isolation [4]. They usually exist in a singlet state. Their chemistry has been rapidly developed since the synthesis of first isolable silylene [5]. The reactivity of singlet silylenes is due to the presence of both lone pair and vacant p-orbital on the silicon atom [6]. Studies of stable silylenes as electron donors to transition metals and lanthanides show that silylenes are mostly unique ligands. Silicon analogs of aromatic and heteroaromatic compounds are less studied [7]. However, the advent of isolable silylenes [8] has opened up a rich field of synthetic organosilicon chemistry [9] and enabled their use as ligands in transition metal complexes with valuable catalytic properties [10]. A key factor in the stability of isolable silylenes is the presence of a singlet electronic ground state with a lone pair on an orbital of the mostly s-character. The vacant silicon p-orbital has a high reactivity and must be protected in some fashion to achieve isolable silylene. The stability of N-heterocyclic silylenes (NHSis) and carbenes (NHCs) is widely understood in terms of the electron-withdrawing ability of α-nitrogen atoms in the σ-system and donation through the π-system. The incorporation of the latter feature in an extended π-system is often described as aromaticity but it is clearly absent for saturated silylenes,
1
Department of Chemistry, Tarbiat Modares University, Tehran, Iran; *kassaeem@modares
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