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

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+)center dot[CuCl4](2-)center dot 2Cl(-) (2). In air, 2 transforms into a new hydrated outer sphere structure [H4L](4+)center dot[CuCl3(H2O)](-)center dot 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.