Crystallographic and EXAFS studies of conformationally designed nonplanar nickel(II) porphyrins

Article abstract of DOI:10.1021/ja00062a029

A series of Ni(II) tetraphenylporphyrins with varying β substituents was examined by X-ray crystallography and EXAFS to assess peripheral steric effects on the conformations of the macrocycles. The compounds are the low-spin Ni(II) derivatives of 2,3,7,8,12,13,17,18-octaethyl-5,10,15,20-tetraphenylporphyrin (1), 2,3,7,8,12,13,17,-18-octapropyl-5,10,15,20-tetraphenylporphyrin (2), 2,3,7,8,12,13,17,18-tetracyclohexenyl-5,10,15,20-tetraphenylporphyrin (3), 2,3,5,7,8,10,12,13,15,17,18,20-dodecaphenylporphyrin (4), 2,3,7,8,12,13,17,18-tetracyclopentenyl-5,10,15,20-tetraphenylporphyrin (5), and 2,3,7,8,12,13,17,18-tetracyclopentenyl-5,10,15,20-tetrakis(3,4,5-t?? methoxyphenyl)?o?hyrin (6). X-ray structures of 1, 2, and 3 reveal that the molecules are severely nonplanar and assume saddle shapes in which the pyrrole rings lie alternately above and below the porphyrin planes with β carbon displacements of more than 1 A? while the meso carbons remain in plane. 1 crystallizes with three methanols of solvation per porphyrin that form an unusual infinite hydrogen-bonded methanol network that transverses one axis of the crystal. Advantage is taken of the fact that short and long Ni-N distances are diagnostic of ruffled and planar Ni macrocycles, respectively, to establish the conformations of the molecules in solution and in the amorphous state by EXAFS. Within the precision of the EXAFS data (0.02 A?), the Ni-N distances in 1, 2, and 3 are the same in solution and in amorphous powders as in the crystals and establish therefore that the distorted conformations of the compounds are maintained in all three phases. EXAFS data for 4, whose structure is unknown, indicate an equally distorted geometry in solution and in the powder. In contrast to 1-4, EXAFS results for 5 and 6 as powders, and for 6 in solution, clearly signal planar conformations for the two tetracyclopentenyl derivatives. Further evidence that 6 is not sterically constrained derives from the observation that it can be converted to a high-spin hexacoordinated Ni(II) complex in pyridine (5 is insoluble). The conformations and Ni-N distances obtained crystallographically or by EXAFS for 1-6 agree well with previous molecular mechanics calculations. The macrocycle distortions induce optical red shifts attributed to a smaller gap between the HOMOs and LUMOs of the porphyrins. In particular, the first optical transition, which is principally a HOMO to LUMO excitation, is correctly predicted by INDO/s calculations based on the crystal coordinates for 1, 2, and 3 reported here. An additional assessment of the effects of the substituents and macrocycle conformations on the frontier orbitals of the molecules is obtained from cyclic voltammetry measurements of oxidation and reduction potentials which provide an experimental probe of the migration of the HOMOs and LUMOs; the electrochemically determined differences in redox potentials mirror the first optical transitions. Crystallographic data: NiN₄C₆₀H₆₀·3CH₃OH (1): triclinic space group P1, a = 13.739(1) A?, b = 17.055(4) A?, c = 12.938(2) A?, α = 96.89(1)°, β = 107.66(1)°, γ = 104.58(2)°, V = 2731.7 A?³, Z = 2, R_F = 0.066 and R_wF = 0.091 based on 8077 reflections with F_○ > 6σF_○. NiN₄C₆₈H₇₆ (2): monoclinic space group P2₁/n, a = 15.195(8) A?, b = 19.577(9) A?, c = 19.137(5) A?, β = 98.77(3)°, V = 5626.2 A?³, Z = 4, R_F = 0.054 and R_wF = 0.060 based on 5716 reflections with F○ > 3σF○. NiN₄C₆₀H₅₂·CH₂Cl₂ (3): tetragonal space group 14, a = b = 32.111(11) A?, c = 9.876(8) A?, V = 10183 A?³, Z = 8, R_F = 0.081 and R_wF = 0.108 based on 3161 reflections with F_○ > 2σF_○. T = 200 K.