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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...

Four new dimeric metalloporphyrins have been prepared in which the two porphyrin rings are linked by an anthracene bridge attached to meso positions. The electrocatalytic behavior of the diporphyrins toward the reduction of 02 at graphite electrodes has been examined for the following combination of metal centers: Co-Cu, Co-Fe, Fe-Fe, Fe-H2. The Co-Cu diporphyrin catalyzes the reduction of 02 to H202 but no further. The other three catalysts all exhibit mixed reduction pathways leading to both H202 and H20. However, the pathways that lead to H20 do not involve H202 as an intermediate. A possible mechanistic scheme is offered to account for the observed behavior.

Cofacial diporphyrins and their metal complexes, first synthesized in 1977, have found interesting applications in mediating the 4e path of O2 reduction on graphite electrodes['.'] as well as in modeling light-induced charge separation in photosynthetic reaction centerd3I. To date, most cofacial dimers have been prepared by linking two porphyrin rings via an amide (or ester) bondc4]. While this method of covalent linkage is synthetically straightforward, the yields from the lengthy sequence have been low. More serious is the fact that the long-term stability of the amide bond is low in the acid environments present in many potentially useful electrochemical 02-reduction systems, e.g., fuel cells. It is therefore worthwhile to explore other approaches to synthesize dimeric porphyrins. We report here the synthesis of a novel biphenylene cofacially arranged diporphyrin 7.For diporphyrins used in electrocatalytic 0,-reductions, the interporphyrin distance has been shown to be critical. The proven optimum length consists of four C atoms [',21. Bearing this in mind, a biphenylene having two porphyrins attached at the 1 and 8 positions should be very attractive. 1,8-Biphenylenedicarboxaldehyde, prepared from the dimethyl precursor, could serve as a convenient starting material. 3-Methylbenzyne 1[41 was condensed in situ to yield a mixture of 1,8-2 and 1,s-dimethylbiphenylene 3. The separation of the two isomers was found to be much simpler if the mixture was converted first to the dialdehydes and then chromatographed. Attempts to condense 1,8-biphenylenedicarboxaldehyde with a mixture of pyrrole and benzaldehyde invariably produced only tetraphenylporphyrin and untractable materials. The 1,8-dialdehyde 4, however, reacted cleanly with ethyl 3-ethyl-4-methyl-2-pyrrolecarboxylate 8l6I to give almost quantitatively the bis(dipyrrylmethane) 5 . The tetraester 5 was saponified and the acid 6 decarboxylated. Unlike common a-unsubstituted dipyrrylmethanes, 6 did not yield any porphyrin when allowed to react with dipyrrylmethane-5,S'dialdehydes such [**I Financial support from the National Science Foundation is gratefully acknowledged. We thank M. Shih for preparing the precursors of 1.as the (5,5'-diformyl-4,4'-dimethyl-3,3'-diethyl-2,2'-dipyr-ry1)methane 9 in 0.4% HI/HOAc (MacDonald method). However, if the same reaction was carried out in 0.4% HClO,/methanol, the desired diporphyrin 7 was obtained, surprisingly in nearly 20% yield. Mw t" 0 0 < 0 0 Scheme 1. A ) See [4b]. B) See [5]. C ) 4 + 8 (molar ratio 1 :4), HCI, in EtOH, 1 h reflux. D ) 5+NaOH; H"; 2-amino ethanol, 180°C. E ) 6 + 9 in 0.4% HCIO,/MeOH, 5 h, RT; o-chloranil.The 'H-NMR spectrum of 7 shows two sharp singlets for the ring methyl groups since the four methyl groups flanking the biphenylene are shielded by the six-membered rings and shifted ca. 0.3 ppm ~pfield [~]. The magnitude of this shift is notably smaller compared to other meso-phenyletioporphyrins (> 1 ppm)['I. We consider this as unequivocal proof of the theoretical prediction that the sixmembered ri...

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The structure of the primary electron-donor-acceptor complex in reaction centers as well sis the molecular events involved in the electron-transport processes remain to be Scheme I

Et,N)2Fe&16. Die pli%ttchenfUrmigen, schwarzen Kristalle sind an der Luft mehrere Tage haltbar. [4] Eine ahnliche Struktur wie die FesSs-Einheit haben die Ag.,M2S6-G~ppen (M=Mo. W) in den Verbindungen (R3P)&,M2S8: A. MUller, E. Diemann, R Jostes, H. BUgge, Angew. Chem. 93 (1981) 957; Angew. Chem. fnt. Ed. Engf. 20 (1981) 934 (zit. Lit.): J. K. Stalick, A. R. Siedle, A. D. Mighell, C. R Hubbard, J. Am. Chem. SOC. IOI (1979) 2908. [5] Vgl. East Lansing, MI 48824 (USA) Wir danken M . Shih fur die Herstellung der Vorlaufer von 1. [**I Dicse Arbeit w r d e von der National Science Foundation unterstiitzt. Abb. 1. X-Band-ESR-Spektrum des Dikupfer(r1)-Komplexes von 7 in Toluol bei -150°C. c n 1 mmol/L. Amplitudenmodulation 10 GauB; v-9.2 GHz. D ist der halbe Abstand zwischen den Zentren der parallelen Absorptionen. = 2.18, I -2.05. 154 0 Verlag Chemie GmbH. 0-6940 Weinheim, 1984 0044-8249/84/0202-0154 $02.SO/O Angew. Chem. 96 (1984) Nr. 2