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

Abstract

The field strength of the axial ligands determines the spin state of saddled iron(III) porphyrin complexes (see picture). 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.

Recent developments in the 13 C NMR spectroscopic analysis of paramagnetic hemes and heme proteins

“…Ferrihemes possessing the S=/2, (d xy ) 2 (d xz , d yz ) 2 (d z 2 ) 1 spin state have been shown to exhibit complicated distortions from planarity [85,98,99], which suggests they might exist in solution as a complex mixture of interconverting conformers with similar energies. Nonplanar hexacoordinated ferrihemes possessing a pure S=3/2 spin state also exhibit a unique pattern of 13 C NMR shifts [86] with very large downfield β shifts (≈1,000 ppm), large downfield C α shifts (≈600 ppm), and large upfield C m shifts (≈-300 ppm) (Fig.…”

“…However, as will be discussed below, there is a straightforward correlation between the chemical shifts of these core carbons and the coordination state and electronic structure of the heme, which warrants the effort needed to observe and assign these resonances. Recent studies conducted with low-spin ferriheme complexes have contributed to solidifying the idea that chemical shifts originating from porphyrin core carbons, C α , C β , and C meso (C m ) greatly facilitate the assessment of electronic structure [83,84,85,86,87]. Furthermore, calculations utilizing density functional theory methods have recently been used to successfully predict the chemical shift of core carbon resonances from hemes exhibiting different oxidation and spin states [88].…”

“…11 Correlation of the 13 C chemical shifts of C α , C β , and C m carbons against the 1 H chemical shifts of the pyrrole hydrogens of several mono-imidazole ligated complexes of (meso-tetramesitylporphyrinato)Fe(III) exhibiting different contributions of the S=3/2 state to the quantum mechanically admixed S=5/2, S=3/2 spin state. Adapted from reference [87] served in a model ferriheme complex [85]. The presence of these unusual spin states, which are typically accompanied by large nonplanar deformations of the macrocycle, were interpreted to suggest that the water molecules in the distal pocket of HO lower the ligand field strength of the coordinated hydroxide by virtue of hydrogen bonding.…”

Reversible mechanism for spin crossover in transition-metal cyanides

“…Such spin transitions are observed in molecular-based magnets, [1][2][3][4][5] oxides, 6-8 supra-molecular systems, 9 and metal-organic complexes, 11 which show promising application capabilities in sensors, switches, and recording devices.…”

Manipulation of the heme electronic structure by external stimuli and ligand field

“…This type-1 spin crossover between the S = 5/2 and S = 1/2 has been most extensively studied in six-coordinated complexes such as [Fe(OEP)(3-ClPy [4,5], while the type-2 spin-crossover was only recently found by ourselves in 2001 [6,7]. As for the type-3 spin-crossover, no example has ever been reported until recently.…”