Host-Guest Feature of DPPP Bridged Arene-Ruthenium Clip Derived Molecular Rectangle

The development of DPPP^2- (H₂DPPP = 2,5-dihydro-3,6-di-2-pyridylpyrrolo(3,4-c)pyrrole-1,4-dione) bridged (NN∩NN) diruthenium complexes [(Cym)(X)Ru^II(μ-dppp)Ru^II(X)(Cym)] (Cym = para-cymene and X = OTf^- (1), SCN^- (2), N₃^- (3), NO₂^-(4)) are considered as the probable molecular clips for the construction of metallarectangle. Crystal structures of 2-4 established anticonfiguration with respect to monodentate SCN^-, N₃^- and NO₂^- groups, respectively. Though molecular clips 2-4 failed to provide the desired metallarectangle in combination with the 4,4'-bipyridine spacer, 1 with the labile OTf groups facilitated to achieve the metallarectangle 5. The crystal structure of 5 revealed that two twisted 4,4'-bipyridine spacers bridged between the two units of dimeric 1 in symmetric fashion, which in effect led to the newer class of molecular rectangle 5 with a hydrophobic cavity size of the cationic host of 8.32 × 11.11 Ų. Furthermore, the host-guest interaction potential of 5 with special reference to the guest molecule, pyrene, was explored. The crystal structure of the resultant molecule 6 ascertained the partial encapsulation of two pyrene molecules inside the hydrophobic cavity of 5, due to the twisted 4,4'-bipyridine spacer units between the two ruthenium clips. It also attributed a noncovalent CH-π interaction involving protons of pyrene and the π-electron cloud of 4,4'-bipyridine as well as a weak interaction between pyrene protons and the pendant C═O group of DPPP. Encapsulation of the guest molecule (pyrene) inside the cavity of the metallarectangle was also monitored by following the quenching of florescent intensity of pyrene on addition of 5.