Breathing Effect in a Cobalt Phosphonate upon Dehydration/Rehydration: A Single‐Crystal‐to‐Single‐Crystal Study

<jats:title>Abstract</jats:title><jats:p>Two cobalt phosphonates, [Co<jats:sub>2</jats:sub>(2,2′‐bpy)<jats:sub>2</jats:sub>(H<jats:sub>2</jats:sub>O)(pbtcH)] (<jats:bold>1</jats:bold>) and [Co<jats:sub>2</jats:sub>(H<jats:sub>2</jats:sub>O)(pbtcH)(phen)<jats:sub>2</jats:sub>] (<jats:bold>2</jats:bold>; pbtcH<jats:sub>5</jats:sub>=5‐phosphonatophenyl‐1,2,4‐tricarboxylic acid, 2,2′‐bpy=2,2′‐bipyridine, phen=1,10‐phenanthroline), with layer structures are reported. Compound <jats:bold>1</jats:bold> contains O‐C‐O and O‐P‐O bridged tetramers of Co<jats:sub>4</jats:sub>, which are further connected by pbtcH<jats:sup>4−</jats:sup> units to form a layer. In compound <jats:bold>2</jats:bold>, the cobalt tetramers made up of water‐bridged Co<jats:sub>2</jats:sub> dimers and O‐P‐O linkages are connected into a layer by pbtcH<jats:sup>4−</jats:sup> units. Upon dehydration, compounds <jats:bold>1</jats:bold> and <jats:bold>2</jats:bold> experience single‐crystal‐to‐single‐crystal (SC–SC) structural transformations to form [Co<jats:sub>2</jats:sub>(2,2′‐bpy)<jats:sub>2</jats:sub>(pbtcH)] (<jats:bold>1 a</jats:bold>) and [Co<jats:sub>2</jats:sub>(pbtcH)(phen)<jats:sub>2</jats:sub>] (<jats:bold>2 a</jats:bold>), respectively. The process is reversible in each case. Notably, a breathing effect is observed for <jats:bold>1</jats:bold>, accompanied by pore opening and closing due to the reorientation of the coordinated 2,2′‐bpy molecules. The transformation was also monitored by in situ IR measurements. Magnetic studies reveal that antiferromagnetic interactions are mediated between the magnetic centers in compounds <jats:bold>1</jats:bold> and <jats:bold>1 a</jats:bold>, whereas ferromagnetic interactions are dominant in compound <jats:bold>2</jats:bold>.</jats:p>