Rare Example of Stereoisomeric 2 + 2 Metallacycles of Porphyrins Featuring Chiral-at-Metal Octahedral Ruthenium Corners

In this paper, we describe three new stereoisomers of the already known 2 + 2 metallacycle of porphyrins [trans,cis,cis-RuCl2(CO)2(4′cisDPyP)]2 (2, 4′cisDPyP = 5,10-bis(4′-pyridyl)-15,20-diphenylporphyrin), namely [{trans,cis,cis-RuCl2(CO)2}(4′cisDPyP)2{cis,cis,cis-RuCl2(CO)2}] (14) and [cis,cis,cis-RuCl2(CO)2(4′cisDPyP)]2 (15), in which the chiral {cis,cis,cis-RuCl2(CO)2} fragment has either a C or A handedness. The least abundant 15 exists as a mixture of two stereoisomers defined as alternate (15alt, both porphyrins are trans to a Cl and a CO) and pairwise (15pw, one porphyrin is trans to two chlorides and the other to two carbonyls), each one as a statistical mixture of meso (AC) and racemic (AA and CC) diastereomers. Remarkably, both 14 and 15 are—to the best of our knowledge—unprecedented examples of 2D metallacycles with octahedral chiral-at-metal connectors, and 14 is the first example of a 2 + 2 molecular square with stereoisomeric Ru(II) corners. Whereas 2 is selectively obtained by treatment of trans,cis,cis-RuCl2(CO)2(dmso-O)2 (1) with 4′cisDPyP, 14 and 15 were obtained, together with 2 (major product), using stereoisomers of 1, either cis,cis,trans-RuCl2(CO)2(dmso-S)2 (5) or cis,cis,cis-RuCl2(CO)2(dmso)2 (6), as precursors. From a general point of view, this work demonstrates that—even for the smallest 2 + 2 metallacycle and using a symmetric organic linker—several stereoisomers can be generated when using octahedral metal connectors of the type {MA2B2} that are not stereochemically rigid. As a proof-of-concept, it also opens the way to new—even though challenging—opportunities: unprecedented and yet unexplored chiral metallosupramolecular assemblies can be obtained and eventually exploited (e.g., for supramolecular catalysis) by using stereogenic octahedral metal connectors amenable to become chiral centers.