Effects of Solvents on the Electron Configurations of the Low-Spin Dicyano[meso-tetrakis(2,4,6-triethylphenyl)porphyrinato]iron(III) Complex:  Importance of the C−H···N Weak Hydrogen Bonding

Abstract

There are two types of electron configurations, (d(xy))(2)(d(xz), d(yz))(3) and (d(xz), d(yz))(4)(d(xy))(1), in low-spin iron(III) porphyrin complexes. To reveal the solvent effects on the ground-state electron configurations, we have examined the (13)C- and (1)H-NMR spectra of low-spin dicyano[meso-tetrakis(2,4,6-triethylphenyl)porphyrinato]ferrate(III) in a variety of solvents, including protic, dipolar aprotic, and nonpolar solvents. On the basis of the NMR study, we have reached the following conclusions: (i) the complex adopts the ground state with the (d(xz), d(yz))(4)(d(xy))(1) electron configuration, the (d(xz), d(yz))(4)(d(xy)())(1) ground state, in methanol, because the d(pi) orbitals are stabilized due to the O-H...N hydrogen bonding between the coordinated cyanide and methanol; (ii) the complex also exhibits the (d(xz), d(yz))(4)(d(xy))(1) ground state in nonpolar solvents, such as chloroform and dichloromethane, which is ascribed to the stabilization of the d(pi) orbitals due to the C-H...N weak hydrogen bonding between the coordinated cyanide and the solvent molecules; (iii) the complex favors the (d(xz), d(yz))(4)(d(xy))(1) ground state in dipolar aprotic solvents, such as DMF, DMSO, and acetone, though the (d(xz), d(yz))(4)(d(xy))(1) character is less than that in chloroform and dichloromethane; (iv) the complex adopts the (d(xy))(2)(d(xz), d(yz))(3) ground state in nonpolar solvents, such as toluene, benzene, and tetrachloromethane, because of the lack of hydrogen bonding in these solvents; (v) acetonitrile behaves like nonpolar solvents, such as toluene, benzene, and tetrachloromethane, though it is classified as a dipolar aprotic solvent. Although the NMR results have been interpreted in terms of the solvent effects on the ordering of the d(xy) and d(pi) orbitals, they could also be interpreted in terms of the solvent effects on the population ratios of two isomers with different electron configurations. In fact, we have observed the unprecedented EPR spectra at 4.2 K which contain both the axial- and large g(max)-type signals in some solvents such as benzene, toluene, and acetonitrile. The observation of the two types of signals has been ascribed to the slow interconversion on the EPR time scale at 4.2 K between the ruffled complex with the (d(xz), d(yz))(4)(d(xy))(1) ground state and, possibly, the planar (or nearly planar) complex with the (d(xy))(2)(d(xz), d(yz))(3) ground state.

Proton NMR Study of Low‐Spin meso‐Unsubstituted β‐Substituted Alkyl Iron Porphyrins: Remarkable Influence of Peripheral Substitution on Spin Density

“…For example, the 13 C signal at δ = 76.18 ppm (a β-pyrrole carbon) that showed correlation peaks with the proton signal at +1.29 ppm (meso β,δ-H) was assigned to C-12 and C-18. Consequently, the proton resonance at δ = -11.48 ppm was assigned to the β-pyrrole protons 12,18-H by using the HMQC spectrum.…”

“…13 C NMR chemical shifts can also be a good probe to determine the electronic ground state, because they give the information on the spin distribution of any porphyrin carbons including meso and β-pyrrole carbons. [12,13] 2 ] at 298 K. Signals were obtained and assigned on the basis of HMQC spectra. Examination of the data in Table 3 reveals that the meso carbons appear in a range of 35-97 ppm, and the β-pyrrole carbons appear in a range of 70-142 ppm.…”

“…[9,24,25] This remarkable resolution was also detected with iron(III) geoporphyrins. [26] It should be emphasized that the meso 13 C chemical shift is also particularly useful to determine the electronic configuration. [27,28] Thus, the meso carbon signal in complexes with the (d xy ) 2 (d xz ,d yz ) 3 configuration appears more upfield than the corresponding signal in diamagnetic complexes because the 3e g orbital, which interacts with the half-occupied d π orbital has a node at the meso carbon atom.…”

Conformações distorcida e planar do anel porfirínico em complexos e hemoproteínas: propriedades físico-químicas e implicações espectroscópicas

“…Esta estabilidade, juntamente com a conhecida versatilidade dos sistemas macrocíclicos, além da evidente relevância biológica, tem motivado várias pesquisas desses compostos como protótipos em modificação de superfícies (filmes finos) 9,10 . No entanto, há de se acrescentar entre esses fatores a estereoquímica, que também desempenha papel determinante.…”

“…9 , partindo-se complexos de Fe(III) de porfirinas, clorinas e tetrafenilporfirinas com ligantes axiais diversificados, entre eles piridinas, pirazóis e imidazóis, pode-se identificar três tipos de centros ferro-porfirínicos: a) tipo I, que apresenta o estado fundamental eletrônico ( . Em princípio, pode-se dizer que são duas as variáveis que determinam qual configuração eletrônica (Figura 1) a porfirina vai apresentar: os ligantes axiais e o solvente 9,10 . No entanto, há de se acrescentar entre esses fatores a estereoquímica, que também desempenha papel determinante.…”