Reactivity among first and second coordination spheres using a multiprotonated ligand and Cu(ii) in the solid-state

Abstract

Second sphere interactions by means of charge-assisted hydrogen bonds have been used to study the solid-state reactivity of two new Cu(II) super-complexes. A tetradentate flexible ligand (1R,2R)-N,N′-bis(pyridine-3-ylmethyl)cyclohexane-1,2-diamine (L) with two pyridine and two amine groups self-assembled with [CuCl₄]²⁻ and Cl⁻ anions gave the second sphere complex [H₄L]⁴⁺·[CuCl₄]²⁻·2Cl⁻ (2). In air, 2 transforms into a new hydrated outer sphere structure [H₄L]⁴⁺·[CuCl₃(H₂O)]⁻·3Cl⁻ (3) differing in the chemistry and geometry of the Cu(II) metal center. Upon heating, 3 reversibly transforms into 2 in the solid-state. The 3 → 2 reaction occurs via ligand exchange with the transfer of a Cl⁻ from the outer sphere to the Cu(II) first sphere replacing the coordinated H₂O. Adduct 3 reacted using mechanochemistry with KOH to form a neutral first sphere coordination complex [LCuCl₂] (4). The transformation from first-to-second coordination spheres 3 → 4 with the formation of a five-membered chelated ring and its reversible process (second-to-first coordination sphere) 4 → 3 can be carried out upon chemisorption of HCl. The gas–solid chemisorption process occurs in a dynamic non-porous material able to adapt to the incoming HCl and H₂O molecules. The amines in the internal part of L appear to be more reactive towards the formation/destruction of coordination of bonds than the external pyridine groups. Insights into the relative stabilities of key crystalline phases have been obtained by means of quantum-mechanical calculations. Considering that the reaction [CuCl₄]²⁻ + H₂O → [CuCl₃H₂O]⁻ + Cl⁻ is exothermic, a simple rationalisation of the experimental results has been provided.