Organic Electron Delocalization Modulated by Ligand Charge States in [L₂M]^n–Complexes of Group 13 Ions

Abstract

Water-stable organic mixed valence (MV) compounds have been prepared by the reaction of reduced bis­(imino)­pyridine ligands (I2P) with the trichloride salts of Al, Ga, and In. The coordination of two tridentate ligands to each ion affords octahedral complexes that are accessible with five ligand charge states: [(I2P0)­(I2P–)­M]2+, [(I2P–)2M]+, (I2P–)­(I2P2–)­M, [(I2P2–)2M]−, and [(I2P2–)­(I2P3–)­M]2–, and for M = Al only, [(I2P3–)2M]3–. In solid-state structures, the anionic members of the redox series are stabilized by the intercalation of Na+ cations within the ligands. The MV members of the redox series, (I2P–)­(I2P2–)­M and [(I2P2–)­(I2P3–)­M]2–, show characteristic intervalence transitions, in the near-infrared regions between 6800–7400 and 7800–9000 cm–1, respectively. Cyclic voltammetry (CV), NIR spectroscopic, and X-ray structural studies support the assignment of class II for compounds [(I2P2–)­(I2P3–)­M]2– and class III for M = Al and Ga in (I2P–)­(I2P2–)­M. All compounds containing a singly reduced I2P– ligand exhibit a sharp, low-energy transition in the 5100–5600 cm–1 region that corresponds to a π*−π* transition. CV studies demonstrate that the electron-transfer events in each of the redox series, Al, Ga, and In, span 2.2, 1.4, and 1.2 V, respectively.

Delocalization tunable by ligand substitution in [L2Al]n−complexes highlights a mechanism for strong electronic coupling

“…We have recently been studying ligand-based MV compounds with Group 13 metals as the bridging ion. 26 Our initial studies provided experimental evidence that bis(imino)pyridine (I 2 P) redox-active ligands bridged by Group 13 metals display Class III behavior and are delocalized. We also noted in that work that the octahedral coordination geometry enables ligand donor atoms to achieve good orbital overlap with metal p-valence orbitals, and we reasoned that this may facilitate the strong coupling between the two ligand end-groups.…”

Harnessing Electrostatic Interactions for Enhanced Conductivity in Metal-Organic Frameworks

“…However, these peaks disappeared in the CV scan of Zn-S@PFC-8, which is a strong indication of the change in electron delocalization. This phenomenon is usually observed in some metal complexes where the variation of the coordination environment can greatly change the electron delocalization of metal clusters and result in different electrochemical properties [41,42]. Therefore, by loading the same guest molecules in a similar structure but different charge nature, we can infer that the electrostatic interaction between guest molecules and the host framework (including both ligands and metal centers) plays a vital role in altering the electrochemical properties of materials.…”

A delocalized cobaltoviologen with seven reversibly accessible redox states and highly tunable electrochromic behaviour

“…The distinct properties of 1 4+ appear to arise from significant electronic delocalization in its reduced states, with spectroscopic and structural analyses confirming 1 3+ features strong ligand-ligand electronic coupling mediated by Co II . Metal-mediated coupling between organic redox centres is uncommon, 24 and it is remarkable that this property has long been overlooked for simple homoleptic cobaltoviologens like 1 4+ , especially since other efforts to achieve strong electronic coupling between viologens have employed elaborate schemes involving macrocycles and mechanical bonds. 25 In contrast, 1 4+ is easily prepared and similar complexes are commonly employed in (nano)materials, 26 suggesting that our results are promising for the development of new functional electronic materials.…”