Combining hydrogen bonds with coordination chemistry or organometallic π-arene chemistry:: strategies for inorganic crystal engineering

Two strategies are described for the construction of extended arrays of transition metal-containing molecules linked via hydrogen bonds. The first approach connects the transition metal to the ligand bearing the hydrogen-bonding group(s) via a coordinative bond. Thus, coordination complexes with peripheral hydrogen-bonding functional groups constitute the (molecular) building blocks of the assembly. The ligands, substituted pyridines, have been selected to provide rigidity and thus permit the metal coordination geometry to guide the direction of propagation of the hydrogen-bonded links between building blocks. The second approach uses arenes bearing hydrogen-bonding functional groups, the arenes being pi -bound to transition metals as is typical of sandwich and half-sandwich organometallic compounds. In the first approach the metal can be viewed as serving a directing role in propagation of the hydrogen bonded assembly. In the second, the metal can be thought of as a (potentially functional) appendage to an organic hydrogen-bonded array, where substituent position at the arene ring serves the directing role for propagation. Examples presented for discussion of the coordination chemistry approach are compounds of the form [PtL4]X-2 (L=nicotinamide or isonicotinamide; X-=Cl- or PF6-). Examples presented in the context of the organometallic approach are members of the series [Cr{eta (6)-C6H6 epsilon-epsilonn(CO2H)(n)}(CO)(3)], where n=1-3.