Ring-Opening Isomerization Based on the 3-Connecting Node: Formation of a 0-D M₂L₃ Cage, 1-D Loop-and-Chain, and 2-D (6, 3) Network

Two semirigid ditopic ligands, 1,4-bis(benzimidazol-1-ylmethyl)-2,3,5,6-tetramethylbenzene (L1) and 1,3-bis(benzimidazol-1-ylmethyl)-2,4,6-trimethylbenzene (L2), reacted with Ag⁺ salts to result in two series of complexes, namely [Ag₂(L1)₂](CF₃SO₃)₂ (1-R), [Ag₂(L1)₃](CF₃SO₃)₂ (1-C), {[Ag₂(L1)₃](CF₃SO₃)₂·CH₃CN}_n (1-P), and [Ag₂(L2)₂](ClO₄)₂·1.5CH₃CN (2-R), {[Ag₃(L2)₂](ClO₄)₃}_n (2-P). All complexes have been structurally characterized by single-crystal X-ray diffraction with the phase purity of bulk samples attested by powder X-ray diffraction (PXRD). Four types of structures are formed: (1) a discrete M₂L₂ ring with two Ag⁺ ions and two cis-L ligands comprising a molecular rectangle (1-R and 2-R), (2) a discrete M₂L₃ cage with two Ag⁺ ions and three cis-L ligands comprising a trigonal cage (1-C), (3) a one-dimensional [M₂L₃]_n loop-and-chain with 3-connecting Ag⁺ ions bridged by both cis- and trans-L ligands (1-P), and (4) a two-dimensional [M₂L₃]_n network of (6,3) topology with 3-connecting Ag⁺ ions bridged by trans-L ligands (2-P). The M₂L₃ cage 2-C was not obtained as a solid-state complex but observable in solution by ESI mass spectrometry. The complexes 1-C, 1-P and 2-C, 2-P contain comparable 3-connecting M₂L₃ building blocks, constituting two pairs of ring-opening isomers corresponding to single ring-opening (1-C to 1-P) and double ring-opening (2-C to 2-P) polymerization processes via cis-L to trans-L ligand conformation change, respectively. Investigations on solution behaviors by ¹H NMR and ESI-MS and structural conversions monitored by PXRD disclose that the thermodynamically favored M₂L₂ ring can be converted to a thermodynamically disfavored M₂L₃ cage in solution through an L addition mechanism, which causes crystallization of isomeric structures of an M₂L₃ cage or [M₂L₃]_n polymer due to ring-opening isomerization. Formation of an M₂L₃ cage or [M₂L₃]_n polymer is influenced by kinetic or thermodynamic effects as well as the solubility-product constant (K_sp), implying predictable syntheses by controlling the crystallization conditions.