Four-Coordinate, Low-Spin (S = 0) and Six-Coordinate, High-Spin (S = 1) Nickel(II) Complexes of Tetraphenylporphyrins with β-Pyrrole Electron-Withdrawing Substituents: Porphyrin-Core Expansion and Conformation

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Abstract
To examine the porphyrin-core expansion and the conformational variations induced by a change in the coordination sphere of nickel(II) from four-coordinate, low-spin (S = 0) to six-coordinate, high-spin (S = 1), several nickel(II) derivatives of tetraphenylporphyrins, substituted in their beta-pyrrole 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 brominated and cyanated macrocycle, 2,3,12,13-tetrabromo-7,8,17,18-tetracyano-5,10,15,20-tetraphenylporphyrin. One of the six-coordinate species is the bis-pyridine nickel(II) adduct, Ni(TPP(Br)(4)(CN)(4))(py)(2) (2), of this antipodal beta-pyrrole brominated and cyanated ring, whereas the second six-coordinate 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-tetracyano-5,10,15,20-tetraphenylporphyrin. This study confirms that, upon conversion of a four-coordinate, low-spin nickel(II) derivative of a beta-pyrrole-substituted tetraphenylporphyrin into a six-coordinate, high-spin complex, a radial expansion of the porphyrin core also takes place. This radial expansion flattens the porphyrin core, inducing more planarity in the conformations of the six-coordinate nickel(II) porphyrin species. The X-ray structure of 1 reveals an average Ni-N(p) bond distance of 1.920(2) Å, similar to that present in tetragonal Ni(OEP) [(Ni-N(p))(av) = 1.929(3) Å], the ruffled low-spin nickel(II) (S = 0) porphyrin derivative, and many other low-spin nickel(II) complexes of distorted porphyrins. The conformation 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 expansion of the porphyrin cores with two sets of (Ni-N(p))(av) bond lengths of 2.040(2) and 2.073(2) Å in 2 and 2.037(2) and 2.068(2) Å in 3, which differ significantly from the average Ni-N(p) distance occurring in 1. Relative to the conformation of the porphyrin ring present in 1, the high-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 groups, the macrocycle adopts a slightly waved conformation. Crystal data: Ni(TPP(Br)(4)(CN)(4)).2.5(1,2-Cl(2)C(2)H(4)), [1.2.5(1,2-Cl(2)C(2)H(4))], triclinic, space group P&onemacr;, a = 13.741(1) Å, b = 14.029(2) Å, c = 15.201(2) Å, alpha = 89.037(9) degrees, beta = 70.829(8) degrees, gamma = 67.607(8) degrees, V = 2539.4(5) Å(3), T = 20 degrees C, Z = 2, R(F)() = 0.047 and R(wF) = 0.067 based on 5341 reflections with I > 3sigma(I); Ni(TPP(Br)(4)(CN)(4))(py)(2).2.5(1,2-Cl(2)C(2)H(4)), [2.2.5(1,2-Cl(2)C(2)H(4))], triclinic, space group P&onemacr;, a = 11.9472(9) Å, b = 14.3651(7) Å, c = 19.556(1) Å, alpha = 87.049(5) degrees, beta = 73.181(6) degrees, gamma = 70.613(5) degrees, V = 3026.8(4) Å(3), T = 173 K, Z = 2, R(F)() = 0.036, R(wF) = 0.041 based on 6846 reflections with I > 3sigma(I); Ni(TPP(CN)(4))(1-MeIm)(2).2CH(2)Cl(2), [3.2(CH(2)Cl(2))], monoclinic, space group C2/c, a = 26.804(8) Å, b = 10.948(3) Å, c = 19.453(5) Å, beta = 113.13(2) degrees, V = 5251.5 Å(3), T = 20 degrees C, Z = 4, R(F)() = 0.047, R(wF) = 0.067 based on 4384 reflections with I > 3sigma(I).

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