Metallacycles of Porphyrins as Building Blocks in the Construction of Higher Order Assemblies through Axial Coordination of Bridging Ligands: Solution- and Solid-State Characterization of Molecular Sandwiches and Molecular Wires

Treatment of the octahedral Ru(II)−dimethyl sulfoxide complexes trans-RuCl2(dmso-S)4 (1), trans-RuCl2(dmso-O)2(CO)2 (2), and trans-RuCl2(dmso)3(CO) (3) with a stoichiometric amount of 5,10-bis(4‘-pyridyl)-15,20-diphenylporphyrin (4‘-cis-DPyP) yields, after chromatographic purification, the novel 2+2
molecular squares of formula [trans,cis,cis-RuCl2(dmso-S)2(4‘-cis-DPyP)]2 (4), [trans,cis,cis-RuCl2(CO)2(4‘-cis-DPyP)]2 (5), and [trans,cis,cis-RuCl2(dmso-S)(CO)(4‘-cis-DPyP)]2 (6), respectively. Compound 6 exists
as an equimolar mixture of the isomeric metallacycles 6a and 6b, depending on whether the 4‘-N(py) rings
of 4‘-cis-DPyP's are trans to CO or to dmso-S. Compounds 4−6 were fully characterized by NMR and IR
spectroscopy and by FAB mass spectrometry. Treatment of 5 with excess zinc acetate in chloroform/methanol mixtures led to the isolation of the corresponding zinc adduct [trans,cis,cis-RuCl2(CO)2(Zn·4‘-cis-DPyP)]2 (5Zn). Treatment of 5Zn with 1 equiv of a trans ditopic N-donor ligand L (L = 4,4‘-bipy, 5,15-bis(4‘-pyridyl)-2,8,12,18-tetra-n-propyl-3,7,13,17-tetramethylporphyrin (4‘-trans-DPyP-npm), or 5,15-bis(4‘-pyridyl)-10,20-diphenylporphyrin (4‘-trans-DPyP)) leads readily and selectively, according to 1H NMR
spectroscopy, to the quantitative assemblying of 2:2 supramolecular adducts of stacked metallacycles of
formula [(5Zn)2(μ-L)2] (7−9), which were thoroughly characterized in solution by NMR spectroscopy. NMR
features indicate that, at ambient temperature, the equilibrium between 5Zn and L to yield [(5Zn)2(μ-L)2]
has an intermediate to slow rate on the NMR time scale (relatively broad signals for L) and is totally shifted
toward the 2:2 product (all or nothing process). Single-crystal X-ray investigations showed that, depending
on the nature of the bridging ligand, in the solid state these sandwich structures can either be maintained
or originate polymeric chains formulated as [(5Zn)(μ-L)]∞. When L = 4‘-trans-DPyP, both solution- and
solid-state data indicate that [(5Zn)2(μ-4‘-trans-DPyP)2] (9) is a discrete supramolecular assembly of two
molecular squares of metalloporphyrins axially connected through other porphyrins. In this molecular box,
the two bridging porphyrins are coplanar at a distance of about 11.4 Å. When L = 4,4‘-bipy, the
corresponding adduct 7 has the anticipated sandwich-like discrete architecture [(5Zn)2(μ-4,4‘-bipy)2] in
solution, but it assumes a stairlike polymeric wire structure in the solid state. The polymer [(5Zn)(μ-4,4‘-bipy)]∞ is made by 5Zn squares bridged by 4,4‘-bipy ligands which are axially coordinated alternatively on
the two opposite sides of each square. Our work clearly established that relatively simple supramolecular
adducts of porphyrins, such as molecular squares, are suitable building blocks for the construction of more
elaborate assemblies of higher order by axial coordination of bridging ligands.