Cadmium(II), Nickel(II), and Zinc(II) Complexes of Vacataporphyrin: A Variable Annulene Conformation inside a Standard Porphyrin Frame

5,10,15,20-Tetraaryl-21-vacataporphyrin, 1 (butadieneporphyrin, annulene-porphyrin hybrid), which contains a vacant space instead of heteroatomic bridge, gives diamagnetic zinc(ll) 1-ZnCl and cadmium(ll) 1-CdCl and paramagnetic nickel(ll) 1-NiCI complexes. A metal ion is bound in the macrocyclic cavity by three pyrrolic nitrogens. Coordination imposes a steric constraint on the geometry of the ligand and leads to two stereoisomers with a butadiene fragment oriented toward 1-MCI-i or outward 1-MCI-o of the macrocyclic center. 1-CdCl-o, 1-ZnCl-o, and the free base share a common 1 H NMR spectral pattern as the basic structural features of 1 are preserved after metal ion insertion. The 1 H NMR spectra of 1-CdCl-i and 1-ZnCl-i reflect a decrease of aromaticity accounted for by the inverted butadiene geometry. The proximity of the butadiene fragment to the metal ion induces direct couplings between the spin-active nucleus of the metal ( 1 1 1 / 1 1 3 Cd) and the adjacent 1 H nuclei of butadiene. The pattern of chemical shifts detected for isomeric 1-NiCl-i and 1-NiCl-o is typical for high-spin nickel(ll) complexes of porphyrin analogues. Resonances 2,3-H of 1-NiCl-o or 1-NiCl-i present the chemical shift typical for the β-H pyrrolic position despite the vacancy in the location of nitrogen-21. Coordination of imidazole, methanol-d 4 , acetonitrile-d 3 , or chloride converts 1-NiCl-i and 1-NiCl-o into distinct species which contain two axial ligands: 1-Ni(Im) 2 + ; 1-Ni(CD 3 OD) 2 + ; 1-Ni(CD 3 -CN) 2 + ; 1-Ni(Cl) 2 - . The density functional theory (DFT) has been applied to model the molecular and electronic structure of feasible 1-CdCl stereoisomers. The total energies calculated using the B3LYP/LANL2DZ approach demonstrate a very small energy difference (2.3 kcal/mol) between 1-CdCl-o and 1-CdCl-i stereoisomers consistent with their concurrent formation.