Luminescent Three- and Four-Coordinate Dinuclear Copper(I) Complexes Triply Bridged by Bis(diphenylphosphino)methane and Functionalized 3‑(2′-Pyridyl)-1,2,4-triazole Ligands

A new series of bimetallic Cu­(I) complexes <b>1</b>–<b>5</b> triply bridged by a monoanionic or charge-neutral functionalized 3-(2′-pyridyl)-1,2,4-triazole in a μ–η¹(N),η²(N,N) tridentate binding mode and two bis­(diphenylphosphino)­methane (dppm) ligands have been synthesized. Complexes <b>1</b>–<b>5</b> are singly or doubly charged dinuclear Cu­(I) species with an eight-membered Cu₂C₂P₄ ring of {Cu­(μ-dppm)₂Cu} unit, in which <b>3</b> and <b>4</b> adopt the boat–boat conformation, while <b>1</b>, <b>2</b>, and <b>5</b> display the chair–boat form. In these dimeric copper­(I) complex cations, one of the two Cu­(I) ions is four-coordinated, in a highly distorted N₂P₂ tetrahedral environment and the other is three-coordinated, in a distorted NP₂ trigonal planar arrangement. All these Cu­(I) complexes exhibit a comparatively weak low-energy absorption in CH₂Cl₂ solution, ascribed to the charge-transfer transitions with appreciable ¹MLCT contribution, as suggested by time-dependent density functional theory (TDDFT) analyses. Complexes <b>1</b>–<b>5</b> display good emission properties in both solution and solid states at ambient temperature, which are well-modulated via structural modification of 3-(2′-pyridyl)-1,2,4-triazole, including the alteration of the substituent type (−CF₃, −H, −CH₃, and −C­(CH₃)₃) and position (<i>ortho</i>-, <i>meta</i>-, and <i>para</i>-position). Furthermore, the variation of the substituent (−CF₃ and −C­(CH₃)₃) on the 5-site of the 1,2,4-triazolyl ring markedly influences the proton activity of the 1,2,4-triazolyl-NH, thus leading to the formation of both singly and doubly charged bimetallic Cu­(I) species regulated by the NH ↔ N^– conversion, resulting from NH deprotonation of the 1,2,4-triazolyl ring