Hydrogen bonding in substituted-ammonium salts of the tetracarbonylcobaltate(−I) anion: some insights into potential roles for transition metals in organometallic crystal engineering

Abstract

This review examines the supramolecular structures of substituted-ammonium salts of the tetracarbonylcobaltate(−I) anion, Co(CO) 4 −. The salts discussed contain cations of increasing complexity, ranging from simple tertiary ammonium cations, through cations consisting of dimeric or trimeric units, to a protonated tetraamine. Across the series distinct changes in the supramolecular structure are apparent. For the simplest cations the principal cation–anion interaction is a short (strong) direct N–H⋯Co hydrogen bond, which is augmented by longer (weaker) C–H⋯O hydrogen bonds. The availability of amine acceptor sites results in competition with the metal center for the available N–H donors and leads to the formation of dimeric, trimeric, and even polymeric N–H +⋯N hydrogen-bonded ammonium/amine cations. Such cations typically utilize all N–H donors, leaving cation–anion links to be formed solely through C–H⋯O hydrogen bonds. In the extreme case of the protonated tetraamine, a strong N–H +⋯N hydrogen bonded cation network is formed, the anions reside in channels and few cation–anion links are present. The Co(CO) 4 − anion is revealed as a rich source of hydrogen bond acceptor sites. Three types are identified and discussed in detail, namely the metal center (Co), the carbonyl oxygens, and the carbonyl π-system. Further, in determining the overall supramolecular arrangement, it is clear that a balance exists between the effect of hydrogen bonds of different types (strengths) depending upon their relative populations. The observations presented in the review are discussed in the context of organometallic crystal engineering, and the potential roles for metal-mediated hydrogen bonding are illustrated.