Crystal and molecular structure of anthracene and biphenylene pillared cofacial diporphyrins

Abstract

The structures of two cofacial metalloporphyrins anchored by rigid pillaring spacer groups of anthracene and biphenylene have been determined by using X-ray crystallographic methods (DP-A and DP-B, respectively). The cobalt complexes of these two porphyrin hosts have demonstrated electrocatalytic activity of mediating the four-electron reduction of dioxygen to water. The structures reported here are as the dinickel (DP-A) and the dicopper (DP-B) complexes. The porphyrins of both molecules slip with respect to each other: 2.40 A in DP-A giving a Ni-Ni distance of 4.566 and 1.60 A in DP-B giving a Cu-Cu distance of 3.807 A. The slip of the former leads to an average porphyrin plane-to-plane separation of 3.88 A while that of the latter corresponds to about 3.45 A. The larger distance of the DP-A is a consequence of the greater lack of planarity of the Ni porphyrin structure. The slip of the porphyrin rings appears to be an optimizational positioning of the rings with respect to van der Waals interactions and its extent is limited by the following: ( I ) repulsive interactions between pyrrole substituent methyl groups and atoms of the pillaring connector group and (2) the ruffling of the porphyrin rings by the metal. The structures of the pyrroles adjacent to the aromatic connector groups are different from the outer pyrroles while the structures of the connectors compare quantitatively with those of the isolated molecules. Interestingly, although both crystal structures are triclinic, the crystal packing of the two is very different. The results of these studies suggest indirectly that an exact metal-metal distance is not absolutely crucial for four-electron dioxygen reduction.In recent years great interest has been shown in the use of metalloporphyrins for catalyzing dioxygen activation and reduction.',2 The ability of certain cofacial dicobalt porphyrins to catalyze the electroreduction of dioxygen a t unusually positive potentials has made such complexes, supported on inexpensive graphite, promising alternatives to platinum as electrode material in air battery and fuel cell application^.^-^ I t has been shown, however, that the performance of these macrocycle catalysts is extremely sensitive to the porphyrin structure. To date, only three diporphyrins are capable of achieving the four-electron electroreduction of dioxygen to water without accumulating a substantial amount of hydrogen peroxide. Among these, the first compound consists of two stacked porphyrin rings doubly linked via short alkyl amide straps3v4 while the other two are based on a design in which two porphyrins are anchored cofacially onto a rigid pillaring spacer group (e.g., anthracene or biphenylene).s The strapped system is, due to the synthetic approach employed, always composed of more than one stereoisomer6 which undoubtedly has contributed to the difficulty of obtaining good quality crystals for X-ray diffraction studies. There are two published crystallographic diporphyrin ~t r u c t u r e s~-~ but neither is for the active (4-e process) ca...

Triplet–Triplet Energy Transfer Controlled by the Donor–Acceptor Distance in Rigidly Held Palladium‐Containing Cofacial Bisporphyrins

“…Such large deviations have not been reported for other spacers such as anthracenyl, biphenylenyl, and dibenzofuran. [17,38] The C meso -C meso (4.32 ) and Ct 1 -Ct 2 (4.01 ) distances determined in this work compare favorably with those of other DPX systems (4.27 < C meso -C meso < 4.47 ; 3.87 < Ct 1 -Ct 2 < 4.70 ; see the Supporting Information). [17,39,40] Absorption spectra: The UV/Vis absorption data are reported in Table 3 and the spectra for (Pd)H 2 DPS, (Pd)H 2 DPX, (Pd)H 2 DPB, (PdZn)DPS, (PdZn)DPX, (Pt)H 2 DPX, and (Pt)H 2 DPB are shown in Figure 3.…”

Synthesis, Spectroscopy, and Electrochemistry of Tetra‐tert‐butylated Tetraazaporphyrins, Phthalocyanines, Naphthalocyanines, and Anthracocyanines, together with Molecular Orbital Calculations

“…R.5.1 program,43a and by the PPP Hamiltonian that we have adopted several times previously 43b. For the PPP calculations, the structures of Pc analogues were constructed from X‐ray structural data for standard Pc,48 naphthalene,49a and anthracene49b and by making the rings perfectly planar and adopting either the D 4 h (metallocomplexes) or D 2 h symmetry (metal‐free species), and were therefore essentially the same as those we had used in our previous work 20. For the ZINDO/S calculations the central metal was assumed to be dihydrogen, zinc, and Co I .…”

Role of Proton‐Coupled Electron Transfer in O—O Bond Activation

“…8,10,11 The unique reactivity of the singly pillared DPA and DPB bisporphyrins as compared to that of doubly strapped bisporphyrin counterparts has been ascribed to their ability to maintain the face-to-face arrangement of porphyrin subunits while maintaining vertical flexibility via the Pacman effect. 8 Nevertheless, the DPA and DPB systems of Chart 1 differ in vertical pocket size by only ∼1 Å, 12 offering a limited range of conformational flexibility for examination of structure-reactivity relationships.…”