Structural Snapshots of Metastable Intermediates Reveals Sequential Addition of Growth Units in the Formation of an Archetypal Coordination Complex: Anisotropic Layer Migration and Solid-State Thermochromic Transitions

Abstract

Kinetically trapped partially preassembled metastable structures afford vital inputs on the reaction progression and the formation mechanism of technologically promising coordination assemblies. We report the structural and transformational relations in a series of coordination complexes, [Co(ad)2(H2O)4](btc-)2(H2O)2 (1), [Co(ad)2(H2O)4][Co(H2O)6](btc-)2(H2O)10 (2), [Co3(ad)2(H2O)14](btc-)2(H2O)4 (3), and [Co3(ad)2(btc-)2(H2O)8] (4), in which ad = adenine and btc = 1,3,5-benzenetricarboxylic acid, to provide insights on sequential structure evolution in an archetypal coordination assembly. Crystals of 3 undergo solid-state thermochromic transformation to a glassy phase 5 consequent to dehydration and anation reaction. With carefully optimized thermal treatment, we obtained a transient crystalline phase 4, which unambiguously proves a restructuring in the Co(II) coordination geometry from octahedral to trigonal bipyramidal. With the 3 → 4 transformation, the crystal surface undergoes drastic modification. Surface reconstruction events associated with photoreactions in the molecular crystals are noted, but analogous observations for thermally induced events are exceptional and unprecedented for coordination complexes. Correlative atomic force microscopy, nanoindentation, and structural inputs provide insights on the surface reconstruction events brought about by anisotropic long-range layer migration subsequent to 3 → 4 transition. The slip plane (011̅) that crosses the crystal face (010) at an optimal angle offers an energetically viable route for layer reorientation and migration.