Novel complexes constructed by flexible 1,2,3,4,5,6- cyclohexanehexacarboxylate and transition metal ions – From 0D mononuclear to 3D porous coordination polymers

Abstract

Nine novel coordination polymers were prepared from flexible 1,2,3,4,5,6-cyclohexanehexacarboxylate (H₆L) and corresponding metal ions at room temperature and/or hydrothermal conditions, namely from binary {[Zn₃(η₉-L^I)(η₂-H₂O)₁(η₁-H₂O)₇]·(H₂O)₅}³_n (1), {[Co₃(η₉-L^I)(η₂-H₂O)₁(η₁-H₂O)₇]·(H₂O)₅}³_n (2), {[Cu₅(η₈-HL^I)₂·10H₂O]·(H₂O)₄}³_n (3), {[Ni₃(η₁₂-L^II)(η₁-H₂O)₆]·1.5H₂O}³_n (4), to ternary {[Zn(η₃-H₄L^I)(4,4′-bipy)(η₁-H₂O)]·(H₂O)₂}²_n (5), {[Zn₂(η₄-H₃L^I)(1,10-phen)₃·(η₁-NO₃)]·H₂O}¹_n (6), {[Cd₂(η₄-H₄L^I)₂(2,2′-bipy)₂(η₁-H₂O)₂]·(2,2′-bipy)·(H₂O)₃}¹_n (7), {[Co_1.5(η₃-H₃L^I)(η₁-4,4′-bipy)₃(η₁-H₂O)₃]·6H₂O}¹_n (8), [Mn(1,10-phen)₂(H₂O)₂]·(H₄L^I)·(H₂O)₅ (9) (L^I = all-cis (a,e,a,e,a,e) conformation L⁶⁻, L^II = all-trans (e,e,e,e,e,e) conformation L⁶⁻, where a and e represent the carboxylate that is almost perpendicular/parallel to the least square of the cyclohexane moiety. 2,2′-bpy = 2,2′-bipyridine, 4,4′-bpy = 4,4′-bipyridine, 1,10-phen = 1,10-phenanthroline). Complexes 1, 2, 3 and 4 have 3D coordination frameworks, in which H₆L are fully deprotonated or only mono-protonated, their coordination numbers are 8, 9 and 12. Complexes 1 and 2 are isomorphous with each other and exhibit 3,5-connected with {3²;4}{3;6³;8⁶} network in the Schlafli notation. Complex 3 is a 3,6-connected {4³}{4⁵;6⁷;8³} network. Complex 4 is a 3,9-connected 9-noted with {4²;6}₃{4⁶;6²¹;8⁹} network. 5–8 are ternary complexes with secondary building blocks where L binds 2 to 4 protons, respectively. The coordination number of L decreased to 3–4 in complexes 5–8. Complex 9 is a mononuclear complex where H₄L²⁻ acts as a counter ion to balance the charge of the metal ion. The ligand in hydrothermal synthesized 4 adopts the all-trans configuration L^II, while in all the other room temperature complexes, L adopts an L^I configuration. As a role, it is always the e-position carboxylate that prefers to coordinate to the metal ion. The solid state photoluminescence studied indicates that there are ligand-centered emissions in 1, 5, 6, and 7. Complex 2 is a breathable porous coordination polymer, X-ray powder diffraction patterns (PXRD) studies have shown that the dehydration/rehydration of 2 can be fully reversible under 100 °C.