Bis(ferrocenyl)porphyrins. Compounds with strong long-range metal–metal coupling† Peter D. W. Boyd, b Anthony K. Burrell,* a Wayne M. Campbell, a Paul A. Cocks, c Keith C. Gordon, c Geoffrey B. Jameson, a David L. Officer* a and Zhongde Zhao b a IFS - Chemistry, Massey University, Private Bag 11222, Palmerston North, New Zealand. E-mail: A.K.Burrell@massey.ac.nz b Chemistry Department, The University of Auckland, Auckland, New Zealand c Chemistry Department, University of Otago, PO Box 56, Dunedin, New Zealand Received (in Cambridge, UK) 26th January 1999, Accepted 4th March 1999 The condensation of a dipyrromethane with ferrocene aldehyde leads to a single atropisomer of a,a-5,15-bis(ferro- cenyl)-2,8,12,18-tetrabutyl-3,7,13,17-tetramethylporphyrin 1; electrochemistry of 1 and Ni-1 reveals two consecutive ferrocene-based one-electron oxidation waves, which are separated by 0.19 and 0.41 V, respectively. Discrete systems in which remote sites are electronically coupled have exciting possibilities for applications in molecular electronic devices. 1 However, despite much effort, particularly with compounds containing two connected ferrocene moieties, 2 useful devices have not yet been forthcoming. This is primarily because communication between the electronic (especially the redox) states at the two sites decreases rapidly with distance. Recent studies have identified a combination of factors that influence communication between connected ferrocene moie- ties, including the type of connection, 2 the length of the connector 3 and the orientation of the two ferrocenes. 4 Here, we report the synthesis, structure and properties of a bis(ferroce- nyl)porphyrin 1 in which these factors are synergistically combined to give unprecedented strong coupling between the ferrocene moieties. Porphyrin 1‡ is formed in a classical condensation reaction between ferrocene aldehyde and a tetraalkyl dipyrromethane. The insertion of nickel gives Ni-1.§ To our surprise, compound 1 is formed as a single isomer in high yield (58%), with both ferrocenyl groups in a syn (or a,a-atropisomer) configuration with respect to the porphyrin macrocycle. The anti product, the a,b-atropisomer, is not observed. CPK-model studies indicate that the porphyrinogen conformation which leads to the a,a- isomer is the least sterically congested and thereby the most accessible to chemical oxidation to form 1. This preference has not been observed previously and is a direct result of the unique steric requirements of the ferrocenyl moiety. Upon oxidation of the porphyrinogen, the methyl groups in the b-pyrrolic positions offer sufficient steric hindrance to prevent any isomerisation. The geometry was confirmed by single-crystal structure determinations of 1¶ [Fig. 1(a)] and its nickel(ii)-substituted derivative Ni-1¶ [Fig. 1(b)]. The large steric bulk of the ferrocenyl moiety at opposite meso positions, clashing with the b-methyl substituents, not only prevents rotation of the ferrocenyl moiety but leads to a strongly ruffled porphyrin core. However, comparison of 1 and Ni-1 reveals that the ferrocenyl moieties are not rigidly locked in a single conformation. Indeed the conformational disorder shown by Ni-1 in the solid state provided further indication of the restricted conformational flexibility of the ferrocene groups, relevant to solution-state conformational flexibility and to the distinctive electrochem- istry shown by these compounds. The electrochemistry of 1 and Ni-1 [Fig. 2(d)] shows two consecutive ferrocene-based one-electron oxidation waves † The authors would like to dedicate this paper to Professor Warren R. Roper on the occasion of his 60th birthday. Fig. 1 Molecular structures of 1 (a) and the major conformation of Ni-1 (b). The top view looking down on the porphyrin plane, showing the twist of the ferrocenyl moieties; the bottom view is side-on to the porphyrin plane, showing the distortions of the porphyrin ring. Fig. 2 Spectro-electrochemical UV–VIS spectra of Na-1 (a), 1 (b) and 3 (c); arrows indicate direction of change in peaks during oxidation. (d) Cyclic voltammogram of Ni-1 in CH 2 Cl 2 at room temperature, E°/ versus Fc/ Fc + . Chem. Commun., 1999, 637–638 637