Porphodimethene–porphyrinogen relationship: the generation of unprecedented forms of porphyrinogen† Lucia Bonomo, a Euro Solari, a Rosario Scopelliti, a Mario Latronico b and Carlo Floriani* a a Institut de Chimie Min´ erale et Analytique, BCH, Universit´ e de Lausanne, CH-1015 Lausanne, Switzerland. E-mail: carlo.floriani@icma.unil.ch b Dipartimento di Ingegneria e Fisica dell’Ambiente, Universit` a della Basilicata, I-85100 Potenza, Italy Received (in Cambridge, UK) 23rd August 1999, Accepted 17th September 1999 The electrophilic reactivity of the porphodimethene skeleton towards nucleophiles led the establishment of a synthetic methodology to unprecedented forms of porphyrinogen containing the vinylidene substituents as well as other functionalities in the meso-positions The intermediacy of porphodimethenes 1,2 in the oxidation of porphyrinogen to porphyrin is well accepted. Work on the chemistry of the porphodimethene skeleton 3 has suffered, however, for a long time from the absence of a real synthetic methodology, which is now available 4 for exploring its reactivity. Porphodimethenes are the target molecules of this report, since they allow one to enter the field of unprecedented forms of porphyrinogen. The latter compounds have been obtained studying the reactivity of lithium and nickel hexa- ethylporphodimethenes (5,15-diethyl-meso-tetraethylpor- phyrin) towards nucleophiles. The synthetic sequences are displayed in Schemes 1 and 2. Complex 1 4 underwent a stepwise deprotonation by LiNMe 2 to the monovinylidene porphomethene complex 2, 5 containing a trianionic tetrapyrrolic derivative, which undergoes further deprotonation to the bisvinylidene porphyrinogen Ni derivative 3. Both complexes 2 and 3 can be protonated back to the starting material by employing PyHCl. The stepwise deprotonation of 1 to 3 requires the preliminary attack of a nucleophile, i.e. [NMe 2 ] 2 , at the mono-substituted meso-position of 1. Such a pathway is supported by the reactivity of 1 with nucleophiles other than [NMe 2 ] 2 , namely LiBu, LiHBEt 3 and LiCH 2 CN, and by the reactions in Scheme 2, leading to meso-functionalized octaalkylporphyrinogens. 6 Compounds 2 and 3 have been fully characterized including the X-ray analysis of 3. The 1 H NMR of 3 revealed the presence of an equimolar mixture of the two possible isomers with two quartets and two doublets of equal intensity for the vinylidene groups and eight doublets for the b-protons to the pyrroles. This spectroscopic analysis has been confirmed with an X-ray structure revealing a statistical distribution of the Me and H groups around the vinylidene carbon, although not in the same ratio observed in solution. We should draw attention to the structural similarity of 3 with the 5,15-dioxoporphyrinogen. 7 The structure of the anionic moiety of 3 is displayed in Fig. 1, with a selection of the structural parameters which support the proposed bonding scheme.‡ The presence of two meso-sp 3 carbons gives rise to a saddle shape conformation of the porphyrinogen with nickel 0.011(3) Å above the N 4 plane. A relevant structural feature is the two short Ni–H contacts [Ni1···H15A and Ni1···H15AA, 2.92 Å (prime denotes a symmetry operation 2x, y, 2z + 1/2)] with two meso-ethyl groups from the same face of the N 4 coordination plane. Complex 3 contains a novel form of porphyrinogen, which may have considerable synthetic potential and may be available either from the demetalation or the transmetalation of 3. We † Syntheses of complexes 2, 3, 5, 6, 7 and 8, and complete ORTEP drawings of 3 and 7, are available from the RSC web site, see http://www.rsc.org/ suppdata/cc/1999/2227/ Scheme 1 Scheme 2 This journal is © The Royal Society of Chemistry 1999 Chem. Commun., 1999, 2227–2228 2227