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The Binding Ability of Iron Bonded to Porphodimethene: Structural, Magnetic, and Electronic Relationship to Iron Porphyrin Complexes
Author(s) -
Da Silva Carlos,
Bonomo Lucia,
Solari Euro,
Scopelliti Rosario,
Floriani Carlo,
Re Nazzareno
Publication year - 2000
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/1521-3765(20001215)6:24<4518::aid-chem4518>3.0.co;2-e
Subject(s) - hexacoordinate , porphyrin , chemistry , crystallography , diamagnetism , metal , ground state , spectrochemical series , ligand (biochemistry) , aromaticity , electron configuration , electronic structure , stereochemistry , ligand field theory , photochemistry , computational chemistry , molecule , atomic physics , magnetic field , physics , organic chemistry , ion , biochemistry , receptor , quantum mechanics , silicon
The availability of the parent compound, meso ‐hexaethylporphodimetheneiron( II ), [(Et 6 N 4 )Fe] ( 2 ), of this report results from a novel synthetic methodology that makes [Et 6 N 4 Li 2 ] ( 1 ) easily available. The major focus is on how the axial positions, which are the key reactive sites in metalloporphyrins, and the electronic configuration of the metal can be affected by the breakdown of the aromaticity of the porphyrin skeleton and by the nonplanar conformation of the ligand. DFT calculations indicate a 3 B 1 (d z 2) 1 (d yz ) 1 ground state for 2 versus the 3 A 2 (d xz ) 1 (d yz ) 1 ground state in the porphyrin analogue. The intermediate‐spin state ( S =1) of 2 changed drastically upon addition of one or two axial ligands, as hexacoordination is preferred by iron( II ). The hexacoordinate complexes [(Et 6 N 4 )Fe(L)(L′)] (L=L′=THF, 3 ; L=L′=Py, 4 ; L=PhNO, L′=Py, 14 ) have been isolated and structurally characterized. Strong‐field ligands lead to a low‐spin diamagnetic state for iron( II ), namely for complexes 4 – 7, 9 , and 14 , whereas 3 is a typical d 6 high‐spin complex, as is the pentacoordinate [(Et 6 N 4 )Fe(CN)]Bu 4 N ( 8 ). The structural analysis showed common features for 6, 7, 9 , and 14 : i) a small displacement of the metal from the N 4 plane, and ii) an N 4 cavity, larger than that in the corresponding porphyrins, affecting the Fe−N bond lengths. The 1 H NMR spectrum is quite diagnostic of the two‐fold symmetry in the diamagnetic hexacoordinate complexes, revealing either a D 2h or a C 2v symmetry. The CO stretching frequency (1951 cm −1 ) in complex 6 probes the good electron density at the metal. The one‐electron oxidation of 2 led to pentacoordinate iron( III ) derivatives [(Et 6 N 4 )Fe(Cl)] ( 10 ), [(Et 6 N 4 ) 2 Fe 2 (μ‐O)] ( 11 ), and [(Et 6 N 4 ) 2 Fe 2 (μ‐ p ‐OC 6 H 4 ‐O)] ( 12 ). Complex 10 is a typical high‐spin iron( III ) (5.85 μ B at 298 K), while 11 and 12 behave as antiferromagnetic coupled iron( III ) ( J =−9.4 cm −1 , 12 , and J =−115 cm −1 , 11 ). In complexes 10, 11 , and 12 iron is sitting in a quite distorted square pyramidal geometry, in which the ligand displays a very distorted roof conformation with different degrees of ruffling. Distinctive structural and magnetic features have been found for the nitrosyl derivative [(Et 6 N 4 )Fe−NO], which has a low‐spin state ( S =1/2) and the following structural parameters: Fe−N−O, 147.3(2)°; Fe−N, 1.708(2) Å; N−O, 1.172(3) Å. A comparative structural, magnetic, and theoretical analysis of the compounds listed above has been made with the analogous porphyrin derivatives. The detailed structural investigation has been mapped through the X‐ray analysis of 2, 7, 8, 9, 11, 13 , and 14 .