The understanding and control of non-bonding intra- and intermolecular forces and of the supramolecular self-assembly process is a prerequisite for the progress of crystal engineering [1]. In recent years, crystal engineers have begun to explore the potential of weak interactions other than classical hydrogen bonds as directors of molecular association such as C-H···Y (Y = N, O, halogen), π-π, halogen-halogen, and C-H···π [2]. Our initial results in this area include the first example of self-assembly of a mercaptaruthenacarborane complex by an unconventional cooperative effect, including a new dihydrogen bond (S-H···H-B) and its combination with a C-H···S hydrogen bond in crystalline closo-[3-Ru(η⁶-C₆H₆)-8-HS-1,2-C₂B₉H₁₀] [3]. The latter cooperative chain can be explained by the enhanced (hydrogen)acceptor properties of the B-SH group due to its low acidity (compared with the acidity of a related C-SH group). Thus, we expect that other classical hydrogen bond acceptors will also become better acceptors when attached to boron. The primary aim of this work is to explore further this idea in combination with established ideas in transition metal complexes and in hydrogen bonding as a strategy for metallacarborane crystal engineering. We will show how supramolecular structure of a given cobaltacarborane complex can be controlled by selecting the appropriate substituents. In the absence of substituents, polymorphism is also found in these compounds.

[1] D. Braga et al., Cryst. Eng. Comm 2005, 7, 1.

[2] G. R. Desiraju, T. Steiner, The Weak Hydrogen Bond in Structural Chemistry and Biology, Oxford, 2001.

[3] a) J. G. Planas, C. Viñas, F. Teixidor et al., J. Am. Chem. Soc. 2005, 7, 15976. b) Dalton Trans. 2004, 2059. c) C. Viñas, F. Teixidor et al., Dalton Trans. 2002, 3647.

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