Yoshiki Ohgo

Saddle-Shaped Six-Coordinate Iron(III) Porphyrin Complexes Showing a Novel Spin Crossover between S=1/2 and S=3/2 Spin States

The field strength of the axial ligands determines the spin state of saddled iron(III) porphyrin complexes. Strong axial ligands (L), such as imidazole and 4-dimethylaminopyridine, lead to the formation of complexes with a pure S=1/2 state, while weak ligands, such as THF, give complexes with a pure S=3/2 state. Intermediate strength ligands, such as pyridine and 4-cyanopyridine, give complexes that show a novel spin crossover between the S=1/2 and S=3/2 states.

Electronic Structure of Six-Coordinate Iron(III) Monoazaporphyrins

The electronic structures of six-coordinate iron(III) octaethylmonoazaporphyrins, [Fe(MAzP)L 2] (+/-) ( 1), have been examined by means of (1)H NMR and EPR spectroscopy to reveal the effect of meso-nitrogen in the porphyrin ring. The complexes carrying axial ligands with strong field strengths such as 1-MeIm, DMAP, CN (-), and (t)BuNC adopt the low-spin state with the (d xy ) (2)(d xz , d yz ) (3) ground state in a wide temperature range where the (1)H NMR and EPR spectra are taken. In contrast, the complexes with much weaker axial ligands, such as 4-CNPy and 3,5-Cl 2Py, exhibit the spin transition from the mainly S = 3/2 at 298 K to the S = 1/2 with the (d xy ) (2)(d xz , d yz ) (3) ground state at 4 K. Only the THF complex has maintained the S = 3/2 throughout the temperature range examined. Thus, the electronic structures of 1 resemble those of the corresponding iron(III) octaethylporphyrins, [Fe(OEP)L 2] (+/-) ( 2). A couple of differences have been observed, however, in the electronic structures of 1 and 2. One of the differences is the electronic ground state in low-spin bis( (t)BuNC) complexes. While [Fe(OEP)( (t)BuNC) 2] (+) adopts the (d xz , d yz ) (4)(d xy ) (1) ground state, like most of the bis( (t)BuNC) complexes reported previously, [Fe(MAzP)( (t)BuNC) 2] (+) has shown the (d xy ) (2)(d xz , d yz ) (3) ground state. Another difference is the spin state of the bis(3,5-Cl 2Py) complexes. While [Fe(OEP)(3,5-Cl 2Py) 2] (+) has maintained the mixed S = 3/2 and 5/2 spin state from 298 to 4 K, [Fe(MAzP)(3,5-Cl 2Py) 2] (+) has shown the spin transition mentioned above. These differences have been ascribed to the narrower N4 cavity and the presence of lower-lying pi* orbital in MAzP as compared with OEP.

Electronic ground states of low-spin iron(III) porphyrinoids

Six-coordinate low-spin iron(III) porphyrinates adopt either common (d(xy))(2)(d(xz),d(yz))(3) or less common (d(xz),d(yz))(4)(d(xy))(1) ground state. In this review article, three major factors that affect the electronic ground state have been examined. They are (i) nature of the axial ligand, (ii) electronic effect of peripheral substituents, and (iii) deformation of porphyrin ring. On the basis of the (1)H NMR, (13)C NMR, and EPR data, it is now clear that (i) the axial ligands with low-lying pi* orbitals such as tert-butylisocyanide and 4-cyanopyridine, (ii) the electron donating groups at the meso-carbon atoms, and (iii) the ruffled deformation of porphyrin ring stabilize the (d(xz),d(yz))(4)(d(xy))(1) ground state. By manipulating these factors, we are able to prepare various low-spin iron(III) porphyrinates with unusual electronic structures such as bis(imidazole) complexes with the (d(xz),d(yz))(4)(d(xy))(1) ground state or bis(tert-butylisocyanide) complexes with the (d(xy))(2)(d(xz),d(yz))(3) ground state; bis(imidazole) and bis(tert-butylisocyanide) complexes usually adopt the (d(xy))(2)(d(xz),d(yz))(3) and (d(xz),d(yz))(4)(d(xy))(1) ground state, respectively.

[(R)-1,2-Bis(allyloxycarbonyl)ethyl]bis(dimethylglyoximato-N,N')[(R)-1-phenylethylamine]cobalt(III)

A new bis(dimethylglyoximato)cobalt(III) complex with a bulky chiral alkyl group bonded to the Co atom has been prepared, [Co(C4H7N2O2)2(C10H13O4)(C8H11N)]. The crystal was irradiated with a xenon lamp. Racemization of the crystalline state was not observed for the crystal since the reaction cavity for the chiral alkyl group appears to be too small.

(12,17-Diethoxy­carbonyl-2,3,6,­7,11,­18-hexa­methyl­corrphy­cenato-­κ4N)iodoiron(III) chloro­form solvate

The title complex, (diethyl 3,4,8,15,19,20-hexamethyl-21,22,23,24-tetraazopentacyclo[16.2.1.1(2,5).1(7,11).1(14,17)]tetracosa-1(20),2(22),3,5,7,9,11,13(24),14,16,18-undecaene-9,14-dicarboxylate-kappa4N)iodoiron(III) chloroform solvate, [Fe(C32H32N4O4)I].CHCl3, shows an almost planar arrangement of the corrphycene moiety with a slightly distorted trapezoid pyramidal core; the Fe(III) atom is 0.416 (1) A from the plane of the C20N4 system. The Fe-N distances are 2.049 (3), 2.044 (3), 2.079 (3) and 2.075 (3) A. The solvated chloroform forms a C-H...O hydrogen bond [C...O 3.107 (10) A] to an adjacent carbonyl O atom. This is the first X-ray structure analysis of a corrphycenatoiron(III) derivative.

Electronic structure of highly ruffled low-spin iron(III) porphyrinates with electron withdrawing heptafluoropropyl groups at the meso positions.

Bis(pyridine)[meso-tetrakis(heptafluoropropyl)porphyrinato]iron(III), [Fe(THFPrP)Py(2)](+), was reported to be the low-spin complex that adopts the purest (d(xz), d(yz))(4)(d(xy))(1) ground state where the energy gap between the iron d(xy) and d(π)(d(xz), d(yz)) orbitals is larger than the corresponding energy gaps of any other complexes reported previously (Moore, K. T.; Fletcher, J. T.; Therien, M. J. J. Am. Chem. Soc. 1999, 121, 5196-5209). Although the highly ruffled porphyrin core expected for this complex contributes to the stabilization of the (d(xz), d(yz))(4)(d(xy))(1) ground state, the strongly electron withdrawing C(3)F(7) groups at the meso positions should stabilize the (d(xy))(2)(d(xz), d(yz))(3) ground state. Thus, we have reexamined the electronic structure of [Fe(THFPrP)Py(2)](+) by means of (1)H NMR, (19)F NMR, and electron paramagnetic resonance (EPR) spectroscopy. The CD(2)Cl(2) solution of [Fe(THFPrP)Py(2)](+) shows the pyrrole-H signal at -10.25 ppm (298 K) in (1)H NMR, the CF(2)(α) signal at -74.6 ppm (298 K) in (19)F NMR, and the large g(max) type signal at g = 3.16 (4.2 K) in the EPR. Thus, contrary to the previous report, the complex is unambiguously shown to adopt the (d(xy))(2)(d(xz), d(yz))(3) ground state. Comparison of the spectroscopic data of a series of [Fe(THFPrP)L(2)](+) with those of the corresponding meso-tetrapropylporphyrin complexes [Fe(TPrP)L(2)](+) with various axial ligands (L) has shown that the meso-C(3)F(7) groups stabilize the (d(xy))(2)(d(xz), d(yz))(3) ground state. Therefore, it is clear that the less common (d(xz), d(yz))(4)(d(xy))(1) ground state can be stabilized by the three major factors: (i) axial ligand with low-lying π* orbitals, (ii) ruffled porphyrin ring, and (iii) electron donating substituent at the meso position.

Chloro­(3,6,13,16-tetraethyl-2,7,12,17-tetra­methyl­porphycenato-κ4N)iron(III) chloro­form solvate

The X-ray crystallographic analysis of the title complex, chloro[3,10,13,20-tetraethyl-4,9,14,19-tetramethylpentacyclo[16.2.1.1(2,5).1(8,11).1(12,15)]tetracosa-2,4,6,8(23),9,12,14,16,18(21),19-decaene]iron(III) chloroform solvate, [Fe(C(33)H(37)N(4))Cl].CHCl(3), reveals a twisted macrocyclic framework with a slightly distorted rectangular pyramidal core, where the deviation of the central Fe(III) atom from the least-squares plane of the C(20)N(4) core is 0.594 (1) A. Some important bond distances are as follows: Fe-N 2.019 (3), 2.026 (3), 2.028 (3) and 2.034 (3) A; Fe-Cl 2.232 (1) A.

Barriers to rotation of axially coordinated imidazole ligands in nonplanar meso-tetraalkylporphyrinato-cobalt(III) complexes

Dynamic NMR study of a series of meso-[Co(TRP)(L)2]Cl, where R is an alkyl group and L is a substituted imidazole, has been carried out. While the complexes with unhindered imidazole show no splitting of the signals, those with bulky imidazoles exhibit change in line shape, indicating the hindered imidazole rotation. The activation free energy increases as R and/or L become bulkier. Based on the red-shifted Soret and Q bands in the UV-Visible and the downfield shifted imidazole protons in the 1H NMR spectra of these complexes as compared with the meso unsubstituted complexes, it is concluded that the deformation of the porphyrin ring slows down the rate of rotation of the coordinated ligand.

[(R)-1,2-Diethoxycarbonylethyl]bis(dimethylglyoximato)(methyldiphenylphosphine)cobalt(III)

The crystal structure of the title compound, [Co(C 4 H 7 N 2 O 2 ) 2 (C 8 H 13 O 4 )(C 13 H 13 P)], has been determined in order to examine whether or not the chiral 1,2-diethoxycarbonylethyl group is racemized on exposure to either visible light or X-rays with retention of the single-crystal form. Although the reaction cavity of the chiral group is considerably large, the chiral group is isolated from the other chiral groups in the structure. The cooperative motion between the chiral groups necessary for the racemization of bulky groups may be impossible in such an isolated environment

Unusual electronic structure of bis-isocyanide complexes of iron(III) porphyrinoids.

A series of isocyanide complexes, [Fe(Porphyrinoid)((t)BuNC)(2)](+), were synthesized and examined for their physicochemical properties. The molecular structure of the bis((t)BuNC) adduct of the iron(III) porphycene (1) and corrphycene (2) adopting the (d(xy))(2)(d(xz), d(yz))(3) ground state were determined for the first time. Furthermore, 1 and 2 showed unusual crossover phenomena between different electron configurations, (d(xy))(2)(d(xz), d(yz))(3) ground state and (d(xz), d(yz))(4)(d(xy))(1) ground state, by the addition of the external stimuli.