Crystal structure of abelsonite, the only known crystalline geoporphyrin

The crystal structure of the unique nickel porphyrin mineral abelsonite, NiC31H32N4, has been solved using direct methods with 2195 independent reflections to a final R-1 = 0.0406. Abelsonite crystallizes in the triclinic space group P (1) over bar, with Z = 1 and unit-cell parameters a = 8.4416(5) angstrom, b = 10.8919(7) angstrom, c = 7.2749(4) angstrom, alpha = 90.465(2)degrees, beta = 113.158(2)degrees, and gamma = 78.080(2)degrees at the measurement condition of 100 K, in very good agreement with previous unit-cell parameters reported from powder diffraction. The structure consists of nearly planar, covalently bonded porphyrin molecules stacked approximately parallel to (1 (1) over bar1), and held together by weak intermolecular Van der Waals forces. The molecules within a layer are slightly tilted such that molecular planes do not overlap, and an up-turned ethyl group on one molecule sits adjacent to a down-turned ethyl group on a neighboring molecule of the same layer. Layers are stacked along a vector normal to (1 (1) over bar1) such that an aromatic ring at one corner of the molecule lies directly above the opposite aromatic ring of the molecule below. Although a single molecule does not quite possess (1) over bar symmetry, matching ethyl groups at roughly opposite ends of the molecule enable orientational disorder, in which molecules can randomly adopt one of two different orientations while still stacking in the same manner. The aggregate of these two random orientations produces an overall symmetry of P (1) over bar.