Halogen bonding in cobaloxime systems

In the past twenty years the halogen bonding has investigated as a useful tool for synthesis of the new materials in solid state chemistry and crystal engineering. The most common building blocks in halogen bonded systems are perfluorinated halogenated hydrocarbons as halogen bond donors (for example 1, 4- and 1, 2-diiodotetrafluorobenzene) and Lewis bases as halogen bond acceptors (pyridines, amines and carbonyl compounds). On the other side, the potential of halogen atom coordinated to metal centre to act as halogen bond donor have not been extensively studied to date. For this purpose, we have prepared ten cobaloximes ([CoX(Hdmg)2L], where X = Cl, Br, I ; Hdmg = hydrogendimethylglyoximate anion and L is a Lewis base). In spite of the fact that all synthesized compounds have atoms which may act as a halogen bond acceptor (O atoms at Hdmg and in some cases additional O or N atoms on L), no halogen bonding between these atoms and halogen coordinated to metal centre has been observed in any of the investigated compounds. Quite the opposite, in all crystal structures the halogen atoms are at considerable distances from all the potential acceptor atoms. Such behaviour is explained by the ab-initio calculation which have shown that halogen atom coordinated at the metal centre does not have a positive σ-hole and consequently, cannot act as halogen bond donor. This result, as well as several similar cases reported in the literature, indicated that the cobaloxime halogen atoms may be good halogen bond acceptors. This prompted us to attempt cocrystallization of the prepared cobaloximes with halogen bond donors (1, 2- and 1, 4-diiodotetrafluorobenzene) rather than halogen bond acceptors. These experiments have resulted with corresponding halogen bonded co-crystals. All obtained compounds exhibit I∙∙∙X contacts (X = Br, Cl), while in some of them I∙∙∙O and I∙∙∙pi halogen bonds have found.