Pyridylporphyrin Metallacycles with a Slipped Cofacial Geometry:  Spectroscopic, X-ray, and Photophysical Characterization

Abstract

Treatment of the octahedral Ru(II) complex [trans,cis,cis-RuCl(2)(DMSO-O)(2)(CO)(2)] with an equimolar amount of 5,10-bis(3'-pyridyl)-15,20-diphenylporphyrin (3'-cis-DPyP) yielded, upon selective replacement of the DMSO ligands, the neutral 2 + 2 metallacycle 2. NMR spectroscopy provided unambiguous evidence that only one highly symmetrical species, in which the two chromophores are held in a slipped cofacial arrangement by the external Ru(II) metal fragments, exists in solution. The unprecedented geometry of 2, and of the fully zincated analogue 2a, were confirmed in the solid state by X-ray structural investigations. The spatial arrangement of the two parallel chromophores in 2, with an interplanar distance of 4.18 A and a lateral offset (center-to-center distance) of 9.82 A, is reminiscent of those of the special pair of bacteriophylls in the reaction centers and of adjacent B850 units in the LH2 light-harvesting antenna systems of photosynthetic bacteria. For comparison, the X-ray structure of the corresponding metallacycle with 4'-cis-DPyP, 1a, is also reported. In 1a, the two porphyrins have an almost perfect coplanar arrangement. The semi-zincated metallacycles 1b and 2b, in which only one of the two chromophores bears an inner zinc atom, were prepared from 1 and 2, respectively, and isolated in pure form. Detailed photophysical investigations of the above porphyrin assemblies were performed. In particular, very fast photoinduced intercomponent energy transfer processes from the zinc porphyrin to the free-base unit were detected in the semi-metalated derivatives 1b and 2b (time constants: 14 and 12 ps, respectively).

Metal-Mediated Multi-Porphyrin Discrete Assembliesand Their Photoinduced Properties

“…17), the metric parameters of 12 are very similar to those found in the corresponding homometallic ruthenium metallacycle 9Zn (see above) [71].…”

“…14). The Ru · · · Ru and the Zn · · · Zn distances are [71]). Top: front view; bottom: side view along the Ru· · ·Ru axis, evidentiating the almost perfect coplanar geometry 14.009(3) and 14.028(3)Å, respectively; metal-to-porphyrin center distances (sides of the metallacycle) are about 9.9Å [71].…”

“…Similarly, the reaction of precursor 2 with an equimolar amount of 3 -cisDPyP, rather than 4 -cisDPyP, yielded the corresponding neutral 2 + 2 metallacycle [trans,cis,cis-RuCl 2 (CO) 2 (3 -cisDPyP)] 2 (10) [71]. NMR spectroscopy provided unambiguous evidence that only one highly symmetrical metallacycle, in which the two chromophores are held in a slipped cofacial arrangement by the external Ru(II) metal fragments, exists in solution (Fig.…”

“…2), the change in the position of the N atom in the peripheral pyridyl rings of cisDPyP from 4 to 3 led indeed, upon coordination to the same cis bifunctional ruthenium fragment trans,cis-[RuCl 2 (CO) 2 ], to porphyrin cyclic assemblies with the same nuclearity but with very different geometries: from a flat two-dimensional assembly (9) to a staggered three-dimensional structure with a rigid spatial arrangement of the two chromophores (10) [71]. Compared to the other neutral 2 + 2 metallacycles of porphyrins that feature octahedral fac-[ReX(CO) 3 ] corner units, compounds 8-10 have the advantage of being more symmetrical: the two axial ligands of the Re(I) corners are different (one halide and one CO), thus generating mixtures of isomers when more units are present in the same assembly.…”

“…Treatment of the metallacycle 9 with zinc acetate in chloroform/methanol mixtures led, depending on the Zn/Ru ratio, to the isolation in pure form of the corresponding fully zincated adduct 9Zn or of the semi-zincated metallacycle 9Zn 1/2 , in which only one of the two chromophores has an inner zinc ion (see below) [71].…”