Reversible binding of nitric oxide to an Fe(iii) complex of a tetra-amido macrocycle

Abstract

Nitric oxide binds reversibly to the Fe(III) complex of a well-developed tetra-amido macrocyclic ligand. Reaction with NO results in formation of a species consistent with an S=1 {Fe-NO}6 ground state as characterized by UV-vis, IR, EPR, and Mössbauer spectroscopy. The resultant nitrosyl is labile and dissociates readily upon purging with N2, thus providing a rare example of reversible NO binding to non-heme iron.

A mononuclear nonheme {FeNO}6 complex: synthesis and structural and spectroscopic characterization

“…2. Fitting of these data shows a single species (100%) with δ = –0.19 mm s –1 and |Δ E Q | = 3.29 mm s –1 , in agreement with previously reported parameters for 1 ( δ = –0.20 mm s –1 and |Δ E Q | = 3.29 mm s –1 ) 15. It should be noted that, in the previous study, the nitrosyl species only accounted for 36% of the Mössbauer spectrum.…”

“…Fitting of these data shows a single species (100%) with δ = –0.19 mm s –1 and |Δ E Q | = 3.29 mm s –1 , in agreement with previously reported parameters for 1 ( δ = –0.20 mm s –1 and |Δ E Q | = 3.29 mm s –1 ). 15 It should be noted that, in the previous study, the nitrosyl species only accounted for 36% of the Mössbauer spectrum. These parameters are similar to a low-spin, six-coordinate {FeNO} 6 compound with a macrocyclic tetracarbene ligand, [( NHC L)FeNO(ONO)](OTf) 2 , which has δ = –0.16 mm s –1 and |Δ E Q | = 3.12 mm s –1 (Table S4, ESI † ).…”

“…Complex 1 was generated as described by Pluth and Lippard 15 (see the Experimental section and Fig. S1–S4, ESI † ); 1 was prepared in situ at –40 °C and excess NO (g) was removed by vacuum/argon-purge cycles.…”

“…The starting complex, [Fe III (TAML)] – ( 1 ) was bubbled with Ar for 20 min in CH 3 CN at –40 °C. 15 To this solution, excess NO (g) (∼10 cm 3 ) was injected into the head-space and stirred for 10 min as the color changed from orange to deep purple. 15 N-incorporated {FeNO} 6 complex was generated using 15 NO (g) under the identical reaction conditions.…”

“… 13 Mascharak has synthesized and characterized non-heme complexes with a photolabile Fe–NO bond, including [(PaPy 3 )Fe III (NO)] 2+ . 14 Recently, Pluth and Lippard reported that an Fe( iii ) complex with a tetraamido macrocycle TAML ligand, [Fe III (TAML)] – , 15 where the TAML scaffold is known to provide remarkable stability to higher oxidation states of iron, 16 promotes NO binding and release, stated to occur without formal oxidation state changes at iron.…”