A Biphenylenediporphyrin: Two Cofacially Arranged Porphyrins with a Biphenylene Bridge

Abstract

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...

Oxygen Activation Chemistry of Pacman and Hangman Porphyrin Architectures Based on Xanthene and Dibenzofuran Spacers

“…The xanthene-bridged cofacial bisporphyrin H 4 (DPX), as well as the dibenzofuran-bridged homologue H 4 (DPD) have been prepared as outlined in Schemes 1 and 2. This three-branch approach (Scheme 1) borrows from methods originally developed by Chang et al (44,46) and Collman et al (49) for the preparation of pillared bisporphyrins bridged by anthracene and biphenylene. This convergent route involves the coupling of rigid dicarboxaldehyde bridges with the appropriate a-free pyrrole ethyl esters and dipyrrylmethane dialdehydes to form the cofacial bisporphyrin (Scheme 2).…”

“…The substantial body of work on cofacial bisporphyrins as O 2 -reduction catalysts has clarified the structural attributes of an effective architecture, including the restriction of macrocyclic subunits to a face-to-face arrangement with minimal lateral displacements, while at the same time allowing sufficient vertical flexibility to bind and activate the O 2 substrate. The dicobalt(II) derivatives of both DPA and DPB (45,46,49) have been shown to efficiently electrocatalyze the complete four-electron reduction of oxygen to water (as opposed to the two-electron pathway involving peroxide). Given that the intermetallic distances between the DPA and DPB derivatives generally only differ by $1 Å (see below) it was unclear as to the effect that Pacman cleft size and oxygen orientation has on the efficiency of the complete 4e À þ 4H þ oxygen reduction process.…”

“…1(b) were targeted. The requisite dialdehyde pillars needed for cofacial assembly, obtained through arduous multistep routes for DPB and DPA (44,46), are much more easily prepared for the DPX and DPD spacers. The DPX and DPD series of cofacial bisporphyrins display an active multielectron catalysis that is predicated on a proton-coupled electron-transfer (PCET) reactivity (69)(70)(71)(72)(73).…”

Electrocatalytic four-electron reduction of oxygen to water by a highly flexible cofacial cobalt bisporphyrin

“…1,2 Nevertheless, the pursuit of structural and functional models for O 2 activation have emphasized, for the most part, bimetallic reaction centers poised within well-defined, rigid pockets. [3][4][5][6][7] For example, pillared cofacial dicobalt bisporphyrins bridged by anthracene (DPA) and biphenylene (DPB) [8][9][10][11] impair ring slippage, and as a result, these complexes efficiently electrocatalyze the direct fourelectron reduction of oxygen to water (as opposed to the twoelectron pathway involving peroxide) with little structural reorganization of juxtaposed subunits. Can efficient oxygenactivation chemistry be preserved when this cofacial structural motif exhibits a large range of motion?…”

“…The more compressed structure of 2 engenders mixed-valence behavior, as two reversible electrochemical oxidations are observed at +0.28 and +0.17 V; the Co(II)/Co(III) DPX complex reacts with dioxygen in the presence of 1,5-dicyclohexylimidazole to give a 15-line EPR spectrum typical of a symmetrical biscobalt(III) superoxo complex (g = 2.02, A Co = 11.14 G). [8][9][10] Nevertheless, 2 efficiently catalyzes the reduction of oxygen at a potential (0.38 V vs. AgCl/Ag) and with a selectivity for the four-electron pathway to produce water (72%) commensurate to 1 [ Fig. 2(b)].…”