A hydrogen bond scaffold supported synthetic heme FeIII–O2− adduct

Abstract

A hydrogen bonded heme-Fe(III)-O(2)(-) adduct is stabilized and characterized using resonance Raman and EPR spectroscopy. The low O-O vibrations of this complex are quite different from those reported for other heme-Fe(III)-O(2)(-) adducts.

Direct observation of intermediates formed during steady-state electrocatalytic O 2 reduction by iron porphyrins

“…2). Both are known to electrocatalytically reduce O 2 , and the latter recently was shown to have a hydrogen-bonding distal superstructure that can stabilize an Fe-O 2 adduct (28).…”

“…This is in sharp contrast to the investigation of O 2 activation by iron porphyrin catalysts and heme enzymes in homogeneous solutions. In these cases, a broad spectrum of reactive intermediate species, formed during singleturnover reactions, is reported (25)(26)(27)(28)(29). The problem lies mainly in investigating a thin layer of catalyst (a monolayer in many cases) on an electrode and getting a good signal-to-noise ratio.…”

Heme: From quantum spin crossover to oxygen manager of life

“…to increase the O2 binding affinity by hydrogen-bond stabilization of the adduct [146]. These effects appear in synthetic models that stabilize the "Weiss" resonance form by explicit hydrogen bonding [177] and are summarized in Figure 4 as "XDISTAL". The main structure-spectroscopy correlations that document this importance of back-bonding are shown in Figure 5, which summarizes the effect of changing the distal amino acid residue of myoglobin models [146].…”

Significantly improved electrocatalytic oxygen reduction by an asymmetrical Pacman dinuclear cobalt(ii) porphyrin–porphyrin dyad

“…16 As inspired by nature, metal porphyrins have been extensively studied as ORR catalysts. 4, [19][20][21][22][23][24][25][26][27][28][29][30][31][32][33] In addition to biologically relevant Fe porphyrins, Co porphyrins have also received great attention because of their high activity and stability. [19][20][21][22][23][24][25][26][32][33][34] However, mononuclear Co porphyrins are much less selective than Fe analogues to catalyze the 4e reduction of O 2 to water.…”

“…Unlike Fe complexes, Co complexes in general catalyze the 2e reduction of O 2 because the peroxo intermediates of Co are challenging to undergo heterolytic O-O bond cleavage to generate terminal Co-oxo species that have large d electron counts and thus are high in energy. [35][36][37] Several molecular design strategies have been reported to improve the selectivity for the 4e ORR, including (1) formation of intramolecular hydrogen bonding interactions to stabilize O 2 adducts, 27,30,38,39 (2) appending acid groups to assist proton delivery, 19,28,29,40 and (3) introduction of functional groups to ensure rapid electron transfer between catalyst molecules and electrodes. 34,[41][42][43][44] Additionally, for complexes of late-transition metals, such as Co, it is found that dinuclear complexes are typically more efficient than mononuclear analogues to catalyze the 4e reduction of O 2 .…”