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 (Ksp), implying predictable syntheses by controlling the crystallization conditions.