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Reaction of [Fe(N-Et-HPTB)(CHCOS)](BF) (1) with (NO)(BF) produces a nonheme mononitrosyl diiron(II) complex, [Fe(N-Et-HPTB)(NO)(DMF)](BF) (2). Complex 2 is the first example of a [Fe{Fe(NO)}] species and is also the first example of a mononitrosyl diiron(II) complex that mediates the reduction of NO to NO. This work describes the selective synthesis, detailed characterization and NO reduction activity of 2 and thus provides new insights regarding the mechanism of flavodiiron nitric oxide reductases.

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Desulfurization of organosulfur compounds is a highly important reaction because of its relevance to the hydrodesulfurization (HDS) process of fossil fuels. A reaction system involving Co(BF)·6HO and the dinucleating ligands HBPMP or HPhBIMP has been developed that could desulfurize a large number of thiophenes, sulfides, and thiols to generate the complexes [Co(BPMP)(μ-SH)(MeCN)](BF) (1a), [Co(BPMP)(SH)](BF) (1b), and [Co(PhBIMP)(μ-SH)(X)](BF) [X = DMF (2a), MeCN (2c)], while the substrates are mostly converted to the corresponding alcohols/phenols. This convenient desulfurization process has been demonstrated for 25 substrates in 6 different solvents at room temperature.

A reaction system involving Fe­(BF4)2·6H2O and two dinucleating ligands, HBPMP and HPhBIMP, mediates the desulfurization of aliphatic and aromatic thiols at room temperature. This rare C–S bond cleavage reaction produces two nonheme diiron­(II) complexes, [Fe2(BPMP)­(SH)2­(MeOH)2]1+ (1a) and [Fe2(PhBIMP)­(μ-SH)­(DMF)]2+ (2a), possibly via an active species similar to [Fe2(PhBIMP)­(H2O)2­(DMF)2]3+ (2c), while the thiols are converted to the corresponding alcohols/phenols. In the case of thioacetic acid, a bidentate chelating organosulfur substrate, the use of HBPMP produces the corresponding bis-thiocarboxylato bridged complex, [Fe2(BPMP)­(CH3­COS)2]1+ (1b), instead of 1a. However, the use of HPhBIMP allows the Fe­(II) mediated desulfurization of thioacetic acid as well to yield 2a, along with the formation of [Fe2(PhBIMP)­(CH3COS)­(MeCN)]2+ (2b). This convenient desulfurization reaction has been demonstrated for different substrates in different solvents along with the structural and spectroscopic characterizations of the diiron­(II)-hydrosulfide complexes in comparison with two isostructural chloride complexes, [Fe2(BPMP)­(Cl)2­(MeOH)2]1+ (1c) and [Fe2(PhBIMP)­(μ-Cl)­(DMF)]2+ (2d). The role of the individual reactants in the desulfurization process has been thoroughly investigated using control reactions, and on the basis of these results and the identification of intermediate species, such as [Fe2(PhBIMP)­(S t Bu)­(DMF)3]2+ and [Fe2(PhBIMP)­(S t Bu)­(H2O)­(DMF)2]2+, in solution by mass spectrometry, a possible mechanism has been proposed.

Synthesis and reactivity of a series of thiolate/thiocarboxylate bridged dicobalt(II) complexes were investigated in comparison with their carboxylate bridged analogues bearing free thiol/hydroxyl groups. Upon one-electron oxidation, complexes [Co(N-Et-HPTB)(μ-SR)](BF) (R = Ph, 1a; Et, 1b; Py, 1c) and [Co(N-Et-HPTB)(μ-SCOR)](BF) (R = Ph, 2a; Me, 2b) yielded [Co(N-Et-HPTB)(DMF)](BF) (6) (DMF = dimethylformamide) along with the corresponding disulfides (where N-Et-HPTB is the anion of N,N,N',N'-tetrakis[2-(1-ethylbenzimidazolyl)]-2-hydroxy-1,3-diaminopropane). Unlike the inertness of carboxylate bridged complexes [Co(N-Et-HPTB)(μ-OC-R-SH)](BF) (R = Ph, 3a; CHCH, 3b) and [Co(N-Et-HPTB)(μ-OCR)](BF) (R = Ph, 4a; Me, 4b; CHCHCHOH, 5) toward O, the bridging ethanethiolate in 1b was oxidized to yield a sulfinate bridged complex, [Co(N-Et-HPTB)(μ-OSEt)](BF) (10). Detailed investigation of the synthetic aspects of 1a-1c led to the discovery of a C-S bond cleavage reaction and yielded the dicobalt(II) complexes [Co(N-Et-HPTB)(SH)(HO)](BF) (8a), [Co(N-CHPy-HPTB)(SH)(HO)](BF) (8b) (where N-CHPy-HPTB is the anion of N,N,N',N'-tetrakis[2-(1-picolylbenzimidazolyl)]-2-hydroxy-1,3-diaminopropane)), and [Co(N-Et-HPTB)(μ-S)](BF) (9). Both 8a and 8b feature nonheme dinuclear Co(II) units containing a terminal hydrosulfide. The present study thus reports comparative redox reactions for a rare class of 16 dicobalt(II) complexes and introduces a selective synthetic strategy for the synthesis of unprecedented dicobalt(II) complexes featuring only one terminal hydrosulfide.

The reduction of NO to N2O by flavodiiron nitric oxide reductases (FNORs) is related to the disruption of the defense mechanism in mammals against invading pathogens. The proposed mechanism for this catalytic reaction involves both nonheme mono- and dinitrosyl diiron­(II) species as the key intermediates. Recently, we reported an initial account for NO reduction activity of an unprecedented mononitrosyl diiron­(II) complex, [Fe2(N-Et-HPTB)­(NO)­(DMF)3]­(BF4)3 (1) (N-Et-HPTB is the anion of N,N,N′,N′-tetrakis­(2-(l-ethylbenzimidazolyl))-2-hydroxy-1,3-diaminopropane; DMF = dimethylformamide) with [FeII{FeNO}7] formulation [J. Am. Chem. Soc.201713914380]. Here we report the full account for the selective synthesis, characterization, and reactivity of FNOR model complexes, which include a dinitrosyl diiron­(II) complex, [Fe2(N-Et-HPTB)­(NO)2(DMF)2]­(BF4)3 (2) with [{FeNO}7]2 formulation and a related, mixed-valent diiron­(II, III) complex, [Fe2(N-Et-HPTB)­(OH)­(DMF)3]­(BF4)3 (3). Importantly, whereas complex 2 is able to produce 89% of N2O via a semireduced mechanism (1 equiv of CoCp2 per dimer = 50% of NO reduced), complex 1, under the same conditions (0.5 equiv of CoCp2 per dimer = 50% of NO reduced), generates only ∼50% of N2O. The mononitrosyl complex therefore requires superreduction for quantitative N2O generation, which constitutes an interesting dichotomy between 1 and 2. Reaction products obtained after N2O generation by 2 using 1 and 2 equiv of reductant were characterized by molecular structure determination and electron paramagnetic resonance spectroscopy. Despite several available literature reports on N2O generation by diiron complexes, this is the first case where the end products from these reactions could be characterized unambiguously, which clarifies a number of tantalizing observations about the nature of these products in the literature.