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Abstract
Low-valent group 15 compounds stabilized by pincer ligands have gained particular interest, given their direct access to fine-tune their reactivity by the coordination pattern. Recently, bismuth has been employed in a variety of catalytic transformations by taking advantage of the (+1/+3) redox couple. In this work, we present a detailed quantum–chemical study on the electronic structure of bismuth pincer complexes from two different families, namely, bis(ketimine)phenyl (NCN) and triamide bismuthinidene (NNN). The use of the so-called effective oxidation state analysis allows the unambiguous assignation of the bismuth oxidation state. In contrast to previous studies, our calculations suggest a Bi(+1) assignation for NCN pincer ligands, while Bi(+3) character is found for NNN pincer complexes. Notably, regardless of its oxidation state, the central bismuth atom disposes of up to two lone pairs for coordinating Lewis acids, as indicated by very high first and second proton affinity values. Besides, the Bi–NNN systems can also accommodate two Lewis base ligands, indicating also ambiphilic behavior. The effective fragment orbital analysis of Bi and the ligand allows monitoring of the intricate electron flow of these processes, revealing the noninnocent nature of the NNN ligand, in contrast with the NCN one. By the dissection of the electron density into effective fragment orbitals, we are able to quantify and rationalize the Lewis base/acid character.
Highlights
In recent years, there has been an increasing interest in using heavier main group elements as a potential replacement of transition metals (TMs) in catalytic reactions.[1−3] The work on heavier group 15 elements, “pnictogen(Pn)-based” species P, As, Sb, and Bi, has showcased their capability to participate as catalysts in a number of reaction transformations.[4−8]It has been recognized that the activity sharply depends on the nature of the ligand and the pnictogen center since special combinations allow to fine-tune the geometry and the oxidation state of the central pnictogen atom
Efforts are justified as nontoxic bismuth has potential applications in medicinal chemistry, in contrast to its lighter congeners (P, As, and Sb).[19−21] The utilization of tridentate rigid meridional pincer ligands has been the key to engineering the energetic levels of frontier orbitals, encompassing similar chemical bonding and reactivity patterns to transition metals and, in some cases, exhibiting unprecedented reactivity.[1]
May lead to either oxidation state +1 or +3, which can be reduced to the question: does bismuth possess one or two lone pairs? To gain insight into the electronic structure of these complexes, we examined the oxidation state involving a series of structural variations where the size of the flanking groups R1 is increased, and the electronic nature of the π-conjugated system is tuned by donor or electron-withdrawing groups
Summary
There has been an increasing interest in using heavier main group elements as a potential replacement of transition metals (TMs) in catalytic reactions.[1−3] The work on heavier group 15 elements, “pnictogen(Pn)-based” species P, As, Sb, and Bi, has showcased their capability to participate as catalysts in a number of reaction transformations.[4−8]. Many of these approaches take advantage of the use of localized orbitals.[32−36] We have recently developed an automated method so-called effective oxidation state (EOS) analysis.[36] This method is based on Mayer’s spin-resolved effective fragment orbitals (EFOs)[37,38] and their occupations (λ) to perform the OS assignation. Article while R (%) values of around 65−70 are expected for systems with more intricate electronic structures.[39] The presence of noninnocent or redox-active ligands such as nitrosyl may lead to close-call situations with R (%) < 60 between NO(+)/ NO(−) due to the high covalent character of the sigma metal− nitrosyl bond.[40] Similar high covalent character was observed for the Ru−C bonds along the catalytic cycle of Rubased olefin metathesis.[41]. Base character from ground-state properties, without recurring to intermediate states
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