18 Intermediate-Spin States of Iron Porphyrins HIROSHI KOBAYASHI and YOUKOH KAIZU Department of Chemistry, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152, Japan KEN EGUCHI Department of Chemistry, University of California, Davis, CA 95616
The observed magnetic properties of the unligated Fe(II) porphyrins were reproduced by theoretical calculations taking into account all possible configuration interac tions and spin-orbit coupling interactions. The calcu lated results were in good agreement with the observa tions only when it was assumed that the axial ligand field is so weak that the d 2 orbital is close to the d and d orbitals. The low-lying states are A , E, E, B and A . The ground state has two sublevels with eigenvec tors that are quantum mechanical admixtures of triplet states ( A , E) and quintet states ( A ). The Mössbauer quadrupole splitting should be attributed to the less temperature-dependent bonding-orbital contribution, however, the d-orbital contribution is almost constant for the ground sublevels and thus a change in the Boltzmann distribution between the sublevels yields no appreciable change in the quadrupole splitting. z
5
xy
π
3
3
2
5
5
2
1
3
2
3
5
1
T
he stereochemistry and functions of all iron porphyrin-containing proteins can be attributed to the varied electronic structure of iron for the oxidation and spin states that are stable in physiological environments. Theoretical descriptions of the electronic structure of iron should be, in principle, applicable to the understanding of structure-function relationships in hemeproteins. Hydroxides and azides of ferrimyoglobin and ferrihemoglobin exhibit the intermediate values of magnetic moment between S = \ 0-8412-0514-0/80/33-191-327$05.00/0 © 1980 American Chemical Society
328
BIOMIMETIC CHEMISTRY
and S = f. George, Beetlestone, and Griffith interpreted that it is near the crossover point between S = \ and S = f and is a thermal mixture of these two spin states (l). Later, more evidence of the thermal equilibria was accumulated with ferric hemeproteins (2-6). Griffith, however, pointed out the possible existence of an intermediate-spin ground state in some ferric porphyrins (7). Ogoshi et al. (8) found an intermediate value of magnetic moment between those expected for S = \ and S = \ with their prepared octaethylporphinatoiron(III) perchlorate [ O E P F e C l O J and interpreted it in terms of thermal equilibrium between S = \ and S = f. Dolphin et al. (9), however, concluded the S = f ground state of O E P F e C 1 0 and the absence of any thermally induced spin crossover on the basis of the temperature-independent Mossbauer quadrupole splitting being much larger than those found for S = \ and S = f Fe(III) porphyrins. 4
The magnetic susceptibility measurements and Mossbauer spectra of Fe(II) phthalocyanine [PcFe(II)] provided evidence of the ferrous intermediate-spin state (10, 11, 12). The Fe(II) ion is incorporated into the central hole of a virtually pure square-planar structure (13). Stillman and Thomson (14) assigned the ground state of Fe(II) phthalocyanine to a A state among possible intermediate-spin states A , B , E on the basis of group-theoretical arguments based on the sign of the M C D observed in the charge-transfer band lower in energy than the lowest ring (TT, TT*) band. The existence of the intermediatespin state of ferrous porphyrin had been predicted (15). However, the recent isolation (16) and x-ray structural determination (17) of unligated tetraphenylporphinatoiron(II) [TPPFe(II)] are convincing evidence for the triplet ground state, since the complex is in a squareplanar structure with a short F e - N bond distance (1.972 A), which argues strongly against occupation of the cr-antibonding d -y2 orbital. Mossbauer spectra also indicate that unligated Fe(II) porphyrins are in a spin state other than the well characterized low-spin and highspin states (17, 18). Other direct evidence of the intermediate-spin state was given by N M R studies (19). By the dominant dipolar shifts observed for the phenyl protons in TPPFe(II), an axial magnetic anisotropy with fi = 4.9fx and m = 3.2/x was concluded. The roomtemperature susceptibility measurements of PcFe(II), TPPFe(II), and OEPFe(II) exhibit the effective magnetic moments of 3.7)JL (10, 12), 4AfjL (l 7), 4.7/JL (18), respectively, while tetra( a, a, a, a-orthopivalamide) phenylporphinatoiron(II) ["picket-fence" Fe(II) porphin] and octamethyltetrabenzporphinatoiron(II) [OTBFe(II)] show the magnetic moments of 5.0/x (19) and 5.9fi , respectively (20). The ground state of PcFe(II), TPPFe(II), and OEPFe(II) have been assigned to the intermediate-spin ground state, and those of the "picket-fence" Fe(II) porphin and OTBFe(II) to the high spin state. The spin-only values, 3
3
ig
3
2g
3
l g
g
x2
±
B
B
b
B
b
B
B
18.
KOBAYASHI ET AL.
329
Iron Porphyrins
however, are 2.8/x and 4.9p, for S = 1 and S = 2, respectively. Thus the observed values for the effective magnetic moment such as 4.44.7pb cannot be attributed necessarily to the S = 1 ground state. The spin-orbit coupling within iron gives rise to sublevels and admixtures of the ground state and the low-lying excited states with different spin states. The structure of the ground state is not described as simply in terms of a single intermediate-spin state. The observed temperature dependence of magnetic susceptibility is well reproduced only by a Boltzmann distribution between sizable numbers of the low-lying sublevels that are different in the contributions of orbital angular moments and spin angular moments. The intermediate-spin state has been suggested both for ferrous (21) and ferric hemeproteins (22, 23). Direct participation of the intermediate-spin iron porphyrins in biological processes might be less probable; however, any reasonable theoretical account of the varied electronic structure of iron in hemeproteins should explain the fact that the so-called intermediate-spin iron porphyrin can exist only when the axial ligand field is extremely weak. B
B
B
Calculations of Ligand-Field Energy An axially symmetrical o--donor ligand is placed on the z axis directing its lone pair to the origin of the coordinate. The energy of the d orbitals of the iron ion fixed at the coordinate origin is given by the electrostatic energy term E and the antibonding energy term X as follows: Y
e(z ) = X + E 2
0
e(yz) = e(zx) = E,
e(x - y ) = e(xy) = E 2
2
2
Among the d orbitals, the d orbital is the only orbital capable of overlapping with the (7-donor orbital on the z axis and thus is given the character of an antibonding orbital by the derealization of ligand o--donor electrons. Here the electrostatic energy terms correspond to the classical crystal-field energy terms. E ,E and E are not necessarily small but much greater than X. However, the electrostatic energy terms are less angular-dependent and thus are assumed as z2
0
EQ = EI
=
E2
2
U
E.
=
The ligand-field splitting of the d orbitals in an octahedral ligand field is described in terms o f E ' s andX (24, 25). E(x - y ) = E(z ) = 3X + 3 E + 3 E = 3X + 6E 2
2
2
0
?
E(xy) = E(yz) = E(zx) = 4E + 2 E = 6E X
2
A = 3X + [3E - 4Ej + E ] = 3X 0
2
330
BIOMIMETIC CHEMISTRY
This indicates that the ligand-fleld splitting, A, is attributable mainly to the derealization effect of ligand a-donor electrons. Similarly, the energy of the d orbitals in a tetragonal planar ligand field is given by the same parameters. E(x - y ) = 3X + 4E 2
2
E(z ) = X + 4E E(xy) = E(yz) = E(zx) = 4E 2
However, it should be noted that the a linear combination of cr-donor orbitals interacts not only with the d z orbital but also with the 4s orbital. This results in a hybridization of the d and s orbitals and grants an extra stabilization to the lower component orbital. The extra stabilization energy is given approximately by lg
z
z2
{H(d, o-) - S(d,
