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 1-5 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 1-5 are singly or doubly charged dinuclear Cu(I) species with an eight-membered Cu₂C₂P₄ ring of {Cu(μ-dppm)₂Cu} unit, in which 3 and 4 adopt the boat-boat conformation, while 1, 2, and 5 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 1-5 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 (ortho-, meta-, and para-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.