Regioselective Coordination toward Silver(I) Ions by Bis- and Tris(Tetra-Armed Cyclen)s via Differential Electron Density of Aromatic Side Arms

Abstract

Bis- and tris(tetra-armed cyclen)s have been synthesizedby connecting two or three cyclens bearing electron-rich or electron-deficientaromatic side arms, enabling regioselective coordination toward Ag+ by leveraging the influence of the electron density variationin the aromatic side arms of each cyclen. These compounds can be readilydistinguished in terms of which cyclen forms a complex with Ag+ through the proton signals of the aromatic side arms. Four types of bis- and tris(tetra-armed cyclen)s wereprepared. 2 ( MF ) is a bis(tetra-armedcyclen) withelectron-rich side arms (three 4-methoxybenzyl groups) and electron-deficientside arms (three 3,5-difluorobenzyl groups), connected by a 4,4 & PRIME;-dimethyl-1,1 & PRIME;-biphenylmoiety. 3 ( MFM ) is a tris(tetra-armedcyclen) with 4-methoxybenzyl and 3,5-difluorobenzyl groups introducedon both ends and the central cyclen, respectively. 4 ( MFM ) is a V-shaped analogue of 3 ( MFM ). 3 ( FMF ) is a tris(tetra-armed cyclen) where the aromatic side arms at bothends and the central cyclen of 3 ( MFM ) are exchanged. The regioselective coordination of silver(I)ions by these ligands is reported. Initially, we added Ag+ ions to a compound (2 ( MF )) thatconsists of tetra-armed cyclen with 4-methoxybenzyl groups as an electron-donatingsubstituent on the aromatic rings and 3,5-difluorobenzyl groups asan electron-withdrawing substituent. H-1 NMR and F-19{H-1} NMR spectroscopy exhibited that the cyclen withthe 4-methoxybenzyl groups formed a complex with Ag+ ionsfirst, followed by the cyclen with the 3,5-difluorobenzyl groups.Next, we employed a compound (3 ( MFM )) with three cyclen units, where the cyclen at each end was functionalizedwith three 4-methoxybenzyl groups, and the central cyclen was functionalizedwith two 3,5-difluorobenzyl groups. Upon adding Ag+ ions,we observed that the cyclen units at both ends formed complexes withAg(+) ions initially, followed by the central cyclen forminga complex with Ag+ ions. When we used 4 ( MFM ), which is a V-shaped compound, the Ag+-induced-H-1 NMR chemical shift changes are almostthe same results as the 3 ( MFM ).Furthermore, we synthesized a compound (3 ( FMF )) by interchanging the substituents on the cyclen units atthe ends and the center. Interestingly, the order of the complex formationwas reversed in this compound. These results demonstrated that tuningthe electron density on the aromatic side arms through substituentsmakes it possible to achieve regioselective coordination with Ag+ ions in bis- or tris(tetra-armed cyclen)s.