Exploring Differences between Bis(aldimino)- and amino-aldimino- N, C, N-Pincer-Stabilized Pnictinidenes: Limits of Synthesis, Structure, and Reversible Tautomerization-Controlled Oxidation

Abstract

Two types of N,C,N-chelated pnictinidenes, i.e., [2-(RNHCH2)-6-(RN = CH)C6H3]E (1-E-R) and [2,6-(RN=CH)2C6H3]E (2-E-R, where E = As, Sb, Bi), are targeted in this study. 2-E-R could be obtained by simple reduction of respective precursors [2,6-(RN=CH)2C6H3]ECl2(i.e., (2-E-R)Cl2) with KC8for E = As, Sb, or Bi with R being tBu, Ad, or Dipp. By contrast, the utilization of a bulky hydride, i.e., K-selectride, allowed the isolation of 1-E-R for E = As or Sb and R = Dipp, while the utilization of other pincer ligands, where R = tBu or Ad, led only to mixtures of both 1-E-R and 2-E-R. For bismuth, even the Dipp-substituted pincer ligand provided a mixture of 1-Bi-Dipp and 2-Bi-Dipp only. The structures and chemical properties of these sets of compounds, i.e., 1-E-Dipp, 2-E-tBu, and 2-E-Dipp, were compared using X-ray diffraction analysis, UV-vis spectroscopy including an extensive theoretical study, and CV measurements. The oxidation of 2-E-Dipp (E = As, Sb, Bi) by air resulted only in the formation of complicated mixtures, while oxidation of 1-E-Dipp (E = As or Sb) cleanly produces (1′-E-Dipp)OH as a result of a tautomeric NH → OH shift at elusive oxides (1-E-Dipp)O under formation of a new E-N covalent bond. To examine the potential of such tautomerization-driven stabilization of oxidation products, the reactivity of 1-E-Dipp (E = As or Sb) toward other chalcogens is also reported, including a theoretical study.