A Flexible Synthetic Strategy for the Preparation of Heteroleptic Metallacycles of Porphyrins

Abstract

We present a stepwise synthetic strategy for the preparation of the unprecedented heteroleptic 2+2 neutral metallacycle [{ t,c,c -RuCl 2 (CO) 2 } 2 (4′ cis DPyP)(3′ cis DPyP)] ( 5 ), in which two different 5,10- meso -dipyridylporphyrins, 4′ cis DPyP [i.e., 5,10-bis(4′-pyridyl)-15,20-diphenylporphyrin] and 3′ cis DPyP [i.e., 5,10-bis(3′-pyridyl)-15,20-diphenylporphyrin], are joined through equal 90°-angular Ru(II) connectors. The synthesis of 5 was accomplished through the preparation of a reactive ditopic intermediate in which one of the two pyridylporphyrins is linked to two neutral ruthenium fragments, each having one residual readily available coordination site (a dmso-O). Thus, compound 5 was obtained under mild conditions through two complementary routes: either by treatment of [{ t , c , c -RuCl 2 (CO) 2 (dmso-O)} 2 (4′ cis DPyP)] ( 3 ) with 1 equiv of 3′ cis DPyP or, alternatively, by treatment of [{ t,c,c -RuCl 2 (CO) 2 (dmso-O)} 2 (3′ cis DPyP)] ( 4 ) with 1 equiv of 4′ cis DPyP. Heteroleptic metallacycle 5 was isolated in pure form in acceptable yield and fully characterized. Spectroscopic data and a molecular model show that 5 has an L-shaped geometry, with the two porphyrins almost orthogonal to one another. The modular approach that we established is highly flexible and opens the way to several possible exciting developments.

Modulating the Shape of Short Metal‐Mediated Heteroleptic Tapes of Porphyrins

“…As always observed by us in the past for previous porphyrinÀ Ru neutral assemblies (see Refs. [46][47][48]), the ESI-MS spectra of the model complexes and metallacycles of porphyrins only showed peaks deriving from the fragmentation (ESI mass spectra were collected in the positive mode on a Perkin-Elmer APII spectrometer at 5600 eV). No molecular ion peak could be detected despite the many attempts.…”

“…As a proof of concept, the novel 2+2 heteroleptic neutral metallacycle [{ t , c , c ‐RuCl 2 (CO) 2 } 2 (4′ cis DPyP)(3′ cis DPyP)] ( 1 ) was prepared by us in pure form and acceptable isolated yield through two complementary stepwise modular approaches that exploited the ditopic reactive intermediates [{ t , c , c ‐RuCl 2 (CO) 2 (dmso‐O)} 2 (4′ cis DPyP)] or [{ t , c , c ‐RuCl 2 (CO) 2 (dmso‐O)} 2 (3′ cis DPyP)] (Scheme 1). [46] Remarkably, we found that the alternative one‐pot synthetic strategy, i. e. the addition of two equiv. of the ruthenium precursor [ t , c , c ‐RuCl 2 (CO) 2 (dmso‐O) 2 ] to a 1 : 1 mixture of 4′ cis DPyP and 3′ cis DPyP, was substantially ineffective for the preparation of 1 in pure form.…”

“…The increased number of orthogonal macrocycles in D 3 À T 4 À D 3 compared to 1 (3 vs 2) leads to an increased mutual shielding, which is particularly relevant for the central porphyrin: for example, the NH' singlets of 4'TPyP in the two isomers of D 3 À T 4 À D 3 (À 3.64 and À 3.67 ppm, respectively) fall at remarkably lower frequencies compared to that of 4'cisDPyP in 1 (À 3.18 ppm). [46] In each set there are four equallyintense doublets for the protons on the four equivalent 4'pyridyl rings. This typical feature is consistent with the meso sixmembered rings laying orthogonal to the 4'TPyP plane: since this latter is not a plane of symmetry for the molecule, the proton facing the adjacent orthogonal 3'cisDPyMP (H6 in the convention adopted by us) is more shielded and thus resonates at lower frequency than the corresponding proton on the other side of the ring (H2).…”

“…The hydrodynamic radii of the 3 + 4 porphyrin tapes are very similar to one another and, consistent with their size, ca. 1/3 larger than those of the smaller 2 + 2 metallacycles previously determined by us [46,61] and included for comparative purposes in Table 1.…”

“…Stepwise synthetic procedures involving polytopic “acceptor” intermediates with reactive metal centers have been exploited before in the coordination‐driven construction of heteroleptic metallacycles and metallacages [37,50–56] . Nevertheless, the application of this approach to porphyrins – with their peculiar chemical‐physical properties and the possibility of being metallated – brings it a step further, opening the way to robust multi‐chromophore assemblies with unprecedented geometries and vast potentiality [46] …”