COMPRESSIBILITY AND HIGH-PRESSURE PHASE-TRANSITION OF A METALLOPORPHYRIN - (5,10,15,20-TETRAPHENYL-21H,23H-PORPHINATO)COBALT(II)

Unit-cell parameters of the tetragonal metalloporphyrin, meso-(tetraphenylporphinato)cobalt(II), were determined at several pressures between 1 bar and 9 kbar, by using single-crystal X-ray diffraction techniques. Compression is uniform to about 5 kbar, with the c axis 3.4 times more compressible than a; a and c axis compressibilities are 1.47 (4) and 5.0 (2) (both x 10-3 kbar-1), respectively. Bulk modulus K0 of this low-pressure phase is 108 (7) kbar, and K′ = 7 (4). At 4.9 (2) kbar the crystal transforms reversibly to a triclinic (pseudotetragonal) phase in which a = b and γ = 90°, but cell angles α and β deviate significantly from 90°. The axial compression ratio is 3.6, and a and c axis compressibilities are 2.0 (1) and 7.2 (2) (both x10-3 kbar-1), respectively. Zero-pressure bulk modulus of the high-pressure phase is 85 (12) kbar, and K′ = 1 (1). Thus, the high-pressure phase is significantly more compressible than the low-pressure phase. Infrared spectra were measured at several pressures to 141 kbar. Absorption bands broadened, decreased in intensity, and shifted at rates varying from -0.25 to +1.3 cm-1 kbar-1. All but two of fifteen identifiable bands had positive shifts. No major discontinuities in the intensities or band positions occurred during compression. The observed crystallographic transformation is due predominantly to changes in intermolecular distances and configuration, with relatively little distortion of the intramolecular structure. © 1987 American Chemical Society.