Star Porphyrazines: Peripheral Chelation of Porphyrazineoctathiolate by Diphosphinonickel Ions

We have synthesized new pentametallic macrocyclic complexes based on the polynucleating ligand, porphyrazine-2,3,7,8,12,13,17,18-octathiolate, (pzot)⁸⁻. This ligand can be thought of as a porphyrazine (tetraazaporphyrin) bearing four dithiolene moieties peripherally at the β-pyrrole positions, and we show that a transition-metal ion can be chelated to each of the four peripheral dithiolene moieties. The complexes [(P-P)Ni]₄[Ni(pzot)] (2a, -P = dppe; 2b, P-P = dppy; 2c, P-P = dppb; 2d, P-P = dcpe) have been synthesized and characterized. Complex 2a crystallizes in the orthorhombic space group Cmc2₁ (No. 36) with 4 macrocycles and 16 solvent molecules (N,N-dimethylformamide) in a unit cell of dimensions a = 24.630(5) Å, b = 20.607(4) Å, c = 30.675(6) Å. The [Ni(pzot)]⁸⁻ ligand in 2a coordinates four Ni(P-P) moieties around its periphery with the two thiolate sulfur atoms of each pyrrole binding a nickel ion in a bidentate (S-S) coordination mode. This is in contrast to the reaction of [Ni(pzot)]⁸⁻ with SnR₂X₂, which yielded Ni(pzot)(SnR₂)₄ (1), where each tin is coordinated in the tridentate (S-N-S) mode, with the thiolate sulfur atoms from two pyrroles and one m-nitrogen atom as ligands from the macrocycle (Velázquez, C. S.; Fox, G. A.; Broderick, W. E.; Andersen, K; Anderson, O. P.; Barrett, A. G. M.; Hoffman, B. M. J. Am. Chem. Soc. 1992, 114, 7416–7424). Comparison of the crystal structures of 1, 2a, and aquo(octakis(methylthio)porphyrazinato)magnesium-(II) (Mg(omtp)), a structurally unconstrained reference compound with eight peripheral methylthio groups, shows that [Ni(pzot)]⁸⁻ adjusts to accommodate either the (S-S) or (S-N-S) modes of coordinating the metal ions at the periphery by a swing of the C_β-S bond. ¹H and ³¹P NMR studies have been used to determine the nature of the coordination of the Ni(P-P) units to the [Ni(pzot)]⁸⁻ macrocycle in solution. Model compounds were synthesized as structural references to mimic both possible coordination sites of the [Ni(pzot)]⁸⁻ ligand. The ¹H and ³¹P NMR chemical shifts of macrocycles 2a-d and their respective bidentate model complexes clearly indicate that the macrocycle in solution also binds the Ni(P-P) units in a bidentate fashion. The electronic absorption spectra of complexes 2a-d show a red-shift of the absorbance in the Q-band region due to interaction of the porphyrazine π-system with the four peripheral metal ions.