Heteroleptic complexes via solubility control: examples of Cu(ii), Co(ii), Ni(ii) and Mn(ii) complexes based on the derivatives of terpyridine and hydroxyquinoline

Abstract

We describe the construction of synthetically challenging heteroleptic complexes by capitalizing on the solubility properties of their corresponding favored homoleptic complexes. We demonstrate that the formation of a heteroleptic Cu²⁺ complex based on 2,2′:6′,2′′-terpyridine (Terpy) and 8-hydroxyquinoline (HQ) is not possible due to the insolubility of (HQ)₂Cu²⁺. Replacing HQ with 8-hydroxy-2-quinolinecarbonitrile (HQCN) enabled the solubility of (HQCN)₂Cu²⁺ in acetonitrile, leading to the formation of the heteroleptic complex Terpy(HQCN)Cu²⁺, TQCu. Applying these conditions to the synthesis of the corresponding heteroleptic Co²⁺ complex resulted in TerpyCo²⁺(acetate)₂, which is insoluble in acetonitrile. Upon changing the solvent to methanol, the carbonitrile group of HQCN was converted to carboxyimidate HQOMe yielding a heteroleptic complex Terpy(HQOMe)Co²⁺, TQ′Co. Using this method, we also generated the heteroleptic complex TQ′Ni and the polynuclear heteroleptic complex Q′₄Q′′₂Mn₄ (Q′′ = HQO₂Me). Detailed analysis of the complexes included characterization by X-ray diffraction, EPR, UV-Vis, high resolution ESI MS, DFT calculations and electrochemistry. X-ray analysis of TQCu revealed distorted square pyramidal geometry, while TQ′Co and TQ′Ni exhibit distorted octahedral geometry, which includes metal coordination via the carboxyimidate nitrogen site. Interestingly, Q′₄Q′′₂Mn₄ was found to contain a [Mn^II₄(μ₃-O)₂(μ₂-O)₄N₁₀]²⁺ core, which adopts a distorted octahedral geometry, and two types of HQ chelators. Thus, Q′₄Q′′₂Mn₄ is also heteroleptic even though it does not contain a Terpy ligand. Solution studies revealed that while TQCu is stable in solution, TQ′Co and TQ′Ni go through ligand exchange and are partially converted to their corresponding homoleptic complexes. Based on these data we could propose a mechanism for the formation of TQ′Co and TQ′Ni and show that TQ′Co can be prepared directly from Terpy and HQOMe.