Hydrogen Bond Optimization via Molecular Design for the Fabrication of Crystalline Organometallic Wheel-and-Axle Compounds Based on Half-Sandwich Ru(II) Units

A new series of half-sandwich ruthenium(II) complexes containing p-aminobenzoic acids (4-aminobenzoic acid and 4-aminocynnarnic acid) and different arene ligands (benzene, p-cymene, and indane) have been synthesized and fully, characterized giving particular attention to the analysis of the generated, crystalline supramoleculai architectures. 4 careful. design of the molecular building blocks allows a perfect match to be, reached between hydrogen bond donors and acceptors thus leading. to the construction of crystalline 'wheel-and-axle metal-organic (WAAMO) systems, where the wheels are the half-sandwich units [(arene)RuCl2] and the axles are based on the cyclic dimerization of the COOH functions of the aminobenzoic ligands. The R-2(2)(8) cyclic pattern is however conserved only in absence of hydrogen bond donor crystallintion solvents, which, when present, tend to insert between the COOH group and a Cl-Ru moiety with formation of hydrates or solvates. Far more robust is the supramolecular synthon based on the coupling involving the Ru-NH2 function as donor and the RuCl2 group as acceptor, which has been found in five out of seven X-ray structures reported in this work. The porosity of the WAAMOs can be modulated acting on the steric requirements of the arene ligand and on the length of the covalent segment of the central supramolecular axle.