CRYSTALLOGRAPHIC AND EXAFS STUDIES OF CONFORMATIONALLY DESIGNED NONPLANAR NICKEL(II) PORPHYRINS

A series of Ni(II) tetraphenylporphyrins with varying beta substituent s 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 13,17,18-octaethyl-5,10,15,20- tetraphenylporphyrin (1), 3,17-18-octapropyl-5,10,15,20-tetrapbenylpor phyrin (2), -tetracyclohexenyl-5,10,15,20-tetraphenylporphyrin (3), 3, 5,7,8,10,12,13,15,17,18,20-dodecaphenylporphyrin (4), tetracyclopenten yl-5,10,15,20-tetraphenylprophyrin (5), and 10,15,20-tetrakis(3,4,5-tr imethoxyphenyl)porphyrin (6). X-ray structures of 1, 2, and 3 reveal t hat the molecules are severely nonplanar and assume saddle shapes in w hich the pyrrole rings lie alternately above and below the porphyrin p lanes with beta carbon displacements of more than 1 angstrom 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 i s taken of the fact that short and long Ni-N distances are diagnostic of ruffled and planar Ni macrocycles, respectively, to establish the c onformations of the molecules in solution and in the amorphous state b y EXAFS. Within the precision of the EXAFS data (0.02 angstrom), the N i-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. EX AFS data for 4, whose structure is unknown, indicate an equally distor ted 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. Furth er evidence that 6 is not sterically constrained derives from the obse rvation that it can be converted to a high-spin hexacoordinated Ni(II) complex in pyridine (5 is insoluble). The conformations and Ni-N dist ances obtained crystallographically or by EXAFS for 1-6 agree well wit h previous molecular mechanics calculations. The macrocycle distortion s induce optical red shifts attributed to a smaller gap between the HO MOs and LUMOs of the porphyrins. In particular, the first optical tran sition, which is principally a HOMO to LUMO excitation, is correctly p redicted by INDO/s calculations based on the crystal coordinates for 1 , 2, and 3 reported here. An additional assessment of the effects of t he substituents and macrocycle conformations on the frontier orbitals of the molecules is obtained from cyclic voltammetry measurements of o xidation and reduction potentials which provide an experimental probe of the migration of the HOMOs and LUMOs; the electrochemically determi ned differences in redox potentials mirror the first optical transitio ns. Crystallographic data: NiN4C60H60.3CH3OH (1): triclinic space grou p P1BAR, a = 13.739(1) angstrom, b = 17.055(4) angstrom, c = 12.938(2) angstrom, alpha = 96.89(1)degrees, beta = 107.66(1)degrees, gamma = 1 04.58(2)degrees, V = 2731.7 angstrom3, Z = 2, R(F) =0.066 and R(wF) = 0.091 based on 8077 reflections with F(o) > 6sigmaF(o). NiN4C68H76 (2) : monoclinic space group P2(1)/n, a = 15.195(8) angstrom, b = 19.577(9 ) angstrom, c = 19.137(5) angstrom, beta = 98. 77(3)degrees, V = 5626. 2 angstrom3, Z = 4, R(F) = 0.054 and R(wF) = 0.060 based on 5716 refle ctions with F(o) > 3sigmaF(o). NiN4C60H52.CH2Cl2 (3): tetragonal space group I4BAR, a = b = 32.111(11) angstrom, c = 9.876(8) angstrom, V = 10183 angstrom3, Z = 8, R(F) = 0.081 and R(wF) = 0.108 based on 3161 r eflections with F(o) > 2sigmaF(o). T = 200 K.