Factors affecting the electronic ground state of low-spin iron(III) porphyrin complexes

To determine the factors affecting the ground-state electron configuration of low-spin Fe(III) porphyrin complexes, we have examined the H-1 NMR, C-13 NMR, and EPR spectra of a series of low-spin bis-ligated Fe(III) porphyrin complexes [Fe(Por)L-2](+/-), in which the positions of porphyrin substitue nts and the coordination ability of axial ligands are different. The seven porphyrins used in this study are meso-tetraalkylporphyrins (TRP: R is prop yl, cyclopropyl, or isopropyl), meso-tetraphenylporphyrin (TPP), meso-tetra kis(2,3,4,5,6-pentafluorophenyl)porphyrin, and 5,10,15,20-tetraphenyl-2,3,7 ,8,12,13,17,18-octaalkylporphyrins (ORTPP: R is methyl or ethyl). The porph yrin cores of TRP are more or less S-4-ruffled depending on the bulkiness o f the alkyl substituents, while those of ORTPP are highly S-4-saddled. Thre e types of axial ligands are examined which have the following characterist ics in ligand field theory: they are (i) strong sigma -donating imidazole ( HIm), (ii) strong sigma -donating and weak pi -accepting cyanide (CN-), and (iii) weak sigma -donating and strong pi -accepting tert-butyl isocyanide ((BuNC)-Bu-t). In the case of the bis(HIm) complexes, only the isopropyl co mplex, [Fe((TPrP)-Pr-i)(HIm)(2)](+), has shown the less common (d(xz), d(yz ))(4)(d(xy))(1) ground state; the other six complexes have exhibited the co mmon (d(xy))(2)(d(xz), d(yz))(3) ground state. When the axial imidazole is replaced by cyanide, even the propyl and cyclopropyl complexes have shown t he (d(xz), d(yz))(4)(d(xy))(1) ground state; the TPP and ORTPP complexes ha ve still maintained the common (d(xy))(2)(d(xz), d(yz))(3) ground state. In the case of the bis((BuNC)-Bu-t) complexes, all the complexes have shown t he (d(xz), d(xy))(4)(d(xy))(1) ground state. However, the contribution of t he (d(xz), d(yz))(4)(d(xy))(1) state to the electronic ground state differs from complex to complex; the (d(xz), d(yz))(4)(d(xy))(1) contribution is t he largest in [Fe((TPrP)-Pr-i)((BuNC)-Bu-t)(2)](+) and the smallest in [Fe( OETPPP)((BuNC)-Bu-t)(2)](+). We have then examined the electronic ground st ate of low-spin [Fe(OEP)((BuNC)-Bu-t)(2)](+) and [Fe(ProtoIXMe(2))((BuNC)-B u-t)(2)](+); OEP and ProtoIXMe(2) represent 2,3,7,8,12,13,17,18-octaethylpo rphyrin and protoporphyrin-IX dimethyl ester, respectively. These porphyrin s have al, HOMO in contrast to the other seven porphyrins that have at, HOM O. The C-13 NMR and EPR studies have revealed that the contribution of the (d(xz), d(yz))(4)(d(xy))(1) state in these complexes is as small as that in [Fe(OETPP)((BuNC)-Bu-t)(2)](+). On the basis of these results, we have con cluded that the low-spin iron(m) porphyrins that have (i) strong axial liga nds, (ii) highly saddle shaped porphyrin rings, (iii) porphyrins with a(1u) HOMO, and (iv) electron withdrawing substituents at the meso positions ten d to maintain the common (d(xy))(2)(d(xz), d(yz))(3) ground state.