Spin crossover in iron(II) complexes of 3,4,5-tri-substituted-1,2,4-triazole (Rdpt), 3,5-di-substituted-1,2,4-triazolate (dpt−), and related ligands

Abstract

A general introduction to spin crossover involving iron(II) is provided. Then the 26 examples of 3,4,5-tri-substituted-1,2,4-triazole (mostly Rdpt and Rppt) ligands and the 11 examples of 3,5-di-substituted-1,2,4-triazole (including Hdpt) ligands that are of interest to this review are introduced. The 26 examples of 3,4,5-tri-substituted-1,2,4-triazole ligands fall into main two categories: (a) the 13 examples of symmetric 4-substituted 3,5-di(2-pyridyl)triazole (Rdpt) ligands and (b) the 13 ligands closely related to the Rdpt ligands but featuring phenyl (known as Rppt ligands), pyrazine or pyrimidine rings in place of one of more of the pyridine rings. In contrast, of the 11 examples of 3,5-di-substituted-1,2,4-triazole (HL, including Hdpt) ligands, only 2 are symmetric (3,5-di(2-pyridyl)triazole and 3,5-di(2-pyrazyl)triazole ligands); the other 9 are non-symmetric as two different rings are attached to the 3 and 5 positions of the triazole ring. In all but one example, in iron complexes of the 3,5-di-substituted-1,2,4-triazole ligands (HL, including Hdpt) the ligand deprotonates and coordinates as an anionic 3,5-di-substituted-1,2,4-triazolate (L−) ligand.This field was reviewed in 2008, and considered the 17 such complexes published up until 2007. Hence the present review provides a comprehensive update on the substantial progress that has been made in this field since then. Specifically it covers the 92 new iron(II) complexes of these 37 ligands, published since then and before 1 December 2015. Of the 92 of new iron(II) complexes reported, 72 have been structurally characterized, and almost half of them, 40, are SCO-active. A wide range of nuclearities, from mono- to di- to tri- to penta- and poly-nuclear, as well as a range of binding modes, are observed, so this review is organised into sections accordingly. In each section, after briefly summarising the findings of the 2008 review, the new developments are detailed. Finally, the findings of this rich avenue of investigation into such ligands are summarised, indicating a bright future ahead.