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Effect of Heme–Heme Interactions and Modulation of Metal Spins by Counter Anions in a Series of Diiron(III)‐μ‐hydroxo Bisporphyrins: Unusual Stabilization of Two Different Spins in a Single Molecular Framework
Author(s) -
Ghosh Sudip Kumar,
Bhowmik Susovan,
Sil Debangsu,
Rath Sankar Prasad
Publication year - 2013
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/chem.201301318
Subject(s) - protonation , spins , crystallography , chemistry , dimer , metal , stereochemistry , ion , physics , organic chemistry , condensed matter physics
A new family of five ethene‐bridged diiron(III)‐μ‐hydroxo bisporphyrins with the same core structure but different counter anions, represented by the general formula [Fe 2 (bisporphyrin)]OH ⋅ X (X=counter anion), is reported herein. In these complexes, two different spin states of Fe are stabilized in a single molecular framework. Protonation of the oxo‐bridged dimer 1 by strong Brønsted acids such as HI, HBF 4 , HPF 6 , HSbF 6 , and HClO 4 produces the μ‐hydroxo complexes with I 5 − ( 2 ), BF 4 − ( 3 ), PF 6 − ( 4 ), SbF 6 − ( 5 ), and ClO 4 − ( 6 ) as counter anions, respectively. The X‐ray structures of 2 and 6 have been determined, which provide a rare opportunity to investigate structural changes upon protonation. Spectroscopic characterization has revealed that the two iron(III) centers in 2 are nonequivalent with nearly high and admixed‐intermediate spins in both the solid state and solution. Moreover, the two different Fe III centers of 3 – 5 are best described as having admixed‐high and admixed‐intermediate spins with variable contributions of S =5/2 and 3/2 for each state in the solid, but two different admixed‐intermediate spins in solution. In contrast, the two Fe III centers in 6 are equivalent and are assigned as having high and intermediate spin states in the solid and solution, respectively. The X‐ray structures reveal that the FeO bond length increases on going from the μ‐oxo to the μ‐hydroxo complexes, and the Fe‐O(H)‐Fe unit becomes more bent, with the dihedral angle decreasing from 150.9(2)° in 1 to 142.3(3)° and 143.85(2)° in 2 and 6 , respectively. Variable‐temperature magnetic data have been subjected to a least‐squares fitting using the expressions derived from the spin Hamiltonians H =−2 JS 1 ⋅ S 2 − μ ⋅ B + D [ ${S{{2\hfill \atop z\hfill}}}$ −1/3 S ( S +1)] (for 2 , 3 , 4 , and 5 ) and H =−2 JS 1 ⋅ S 2 (for 6 ). The results show that strong antiferromagnetic coupling between the two Fe III centers in 1 is attenuated to nearly zero (−2.4 cm −1 ) in 2 , whereas the values are −46, −32.6, −33.5, and −34 cm −1 for 3 , 4 , 5 , and 6 , respectively.