Four-coordinate, low-spin (S=0) and six-coordinate, high-spin (S=1) nickel(II) complexes of tetraphenylporphyrins with beta-pyrrole electron-withdrawing substituents: Porphyrin-core expansion and conformation

To examine the porphyrin-core expansion and the conformational variations i nduced by a change in the coordination sphere of nickel(II) from four-coord inate, low-spin (S = 0) to six-coordinate, high-spin (S = 1), several nicke l(II) derivatives of tetraphenylporphyrins, substituted in their beta-pyrro le positions with electron-withdrawing groups, were isolated and studied by X-ray crystallography. The four-coordinate nickel(II) complex studied, Ni( TPP(Br)(4)(CN)(4)) (1), is a derivative of the antipodal beta-pyrrole bromi nated and cyanated macrocycle, 2,3,12,13-tetrabromo-7,8,17,1 8 tetracyano-5 ,10,15,20-tetraphenylporphyrin. One of the six-coordinate species is the bi s-pyridine nickel(II) adduct, Ni(TPP(Br)(4)(CN)(4))(py)(2) (2), of this ant ipodal beta-pyrrole brominated and cyanated ring, whereas the second six-co ordinate complex is the bis-1-methylimidazole nickel(II) adduct, Ni(TPP(CN) (4))(1-MeIm)(2) (3), of the antipodal tetracyanated macrocycle, 7,8,17,18-t etracyano-5,10,15,20-tetraphenylporphyrin. This study confirms that, upon c onversion of a four-coordinate, low-spin nickel(II) derivative of a beta-py rrole-substituted tetraphenylporphyrin into a six-coordinate, high-spin com plex, a radial expansion of the porphyrin core also takes place. This radia l expansion flattens the porphyrin core, inducing more planarity in the con formations of the six-coordinate nickel(II) porphyrin species. The X-ray st ructure of 1 reveals an average Ni-N-p bond distance of 1.920(2) Angstrom, similar to that present in tetragonal Ni(OEP) [(Ni-N-p)(av) = 1.924(3) Angs trom], the ruffled low-spin nickel(II) (S = 0) porphyrin derivative, and ma ny other low-spin nickel(II) complexes of distorted porphyrins. The conform ation of the macrocycle present in 1 is severely saddle-shaped with a small ruffling and a minor doming. The structures of 2 and 3 indicate a clear ex pansion of the porphyrin cores with two sets of (Ni-N-p)(av) bond lengths o f 2.040(2) and 2.073(2) Angstrom in 2 and 2.037(2) and 2.068(2) Angstrom in 3, which differ significantly from the average Ni-N-p distance occurring i n 1. Relative to the conformation of the porphyrin ring present in 1, the h igh-spin complex 2 is less saddle-shaped, but more ruffled. In 3, in which only two opposite pyrrole rings of the porphyrin are substituted by cyano g roups, the macrocycle adopts a slightly waved conformation. Crystal data: N i(TPP(Br)(4)(CN)(4)). 2.5(1,2-Cl2C2H4), [1.2.5(1,2-Cl2C2H4)], triclinic, sp ace group P (1) over bar, a = 13.741(1) Angstrom, b = 14.029(2) Angstrom, c = 15.201(2) Angstrom, alpha = 89.037(9)degrees, beta = 70.829(8)degrees, g amma = 67.607(8)degrees, V = 2539.4(5) Angstrom(3), T = 20 degrees C, Z = 2 , R-F = 0.047 and R-wF = 0.067 based on 5341 reflections with I > 3 sigma(I ); Ni(TPP(Br)(4)(CN)(4))(py)(2). 2.5(1,2-Cl2C2H4), triclinic, space group P (1) over bar, a = 11.9472(9) Angstrom, b = 14.3651(7) Angstrom, c = 19.556 (1) Angstrom, alpha = 87.049(5)degrees, beta = 73.181(6)degrees, gamma = 70 .613(5)degrees, V = 3026.8(4) Angstrom(3), T = 173 K, Z = 2, R-F = 0.036, R wF = 0.041 based on 6846 reflections with I > 3 sigma(I); Ni(TPP(CN)(4))(1- MeIm)2 . 2CH(2)Cl(2), [3 2(CH2Cl2)], monoclinic, space group C2/c, a = 26.8 04(8) Angstrom, b = 10.948(3) Angstrom, c = 19.453(5) Angstrom, beta = 113. 13(2)degrees, V = 5251. 5 Angstrom(3), T = 20 degrees C, Z = 4, R-F = 0.047, R-wF = 0.067 based on 4384 reflections with I > 3 sigma(I).