Reversible Single-Crystal-to-Single-Crystal Cross-Linking of a Ribbon of Cobalt Citrate Cubanes To Form a 2D Net

An unprecedented 1-D polymer of cobalt citrate cubanes, 1, has been prepared by wet chemistry techniques and isolated as single crystals with two units of formula Cs₂Co₇(citr)₄(H₂O)₂₁ in the unit cell [H₄citr = C₆H₈O₇, citric acid], as characterized by X-ray diffraction at T = 278 K. In addition to the four Co atoms in the cubane unit, there are four independent peripheral citrate-bound Co sites. Upon warming at T = 303 K, the polymer undergoes cross-linking through a complex substitution/addition reaction at one of the peripheral Co centers to produce a new 2-D polymer based on the same Co citrate cubane building block. In the transition a second peripheral Co atom undergoes a substitution reaction and separates from the polymer. The new phase, 2, analyzed by X-ray diffraction from the same crystalline sample as 1, has two units of formula Cs₂Co₇(citr)₄(H₂O)_13.5 in the unit cell. Water egress in the transition amounts to 35.7 mole percent. The volume of the unit cell diminishes by 12.4%. The Co center at the cross-link in the 2-D polymer is coordinated by seven oxygen atoms. Upon sitting at ambient conditions for an extended period of time, the same crystal reverts to its original phase, 1, as characterized by unit cell determination. When heated again at T = 303 K, it again acquires the unit cell of 2.