Cyanide, as an ambidentate ligand, plays a pivotal role in providing a simple diatomic building-block motif for controlled metal aggregation (M–CN–M′). Specifically, the inherent hard–soft nature of the cyanide ligand, i.e. , hard-nitrogen and soft-carbon centers, is due to electronic handles for binding Lewis acids following the hard–soft acid–base principle. Studies by Holm and Karlin showed structural and electronic requirements for cyanide-bridged (por)Fe III –CN–Cu II/I (por = porphyrin) molecular assemblies as biomimetics for cyanide-inhibited terminal quinol oxidases and cytochrome-C oxidase. The dinitrosyliron unit (DNIU) that exists in two redox states, {Fe(NO) 2 } 9 and {Fe(NO) 2 } 10 , draws attention as an electronic analogy of Cu II and Cu I , d 9 and d 10 , respectively. In similar controlled aggregations, L-type [(η 5 -C 5 R 5 )Fe(dppe)(CN)] (dppe = diphenyl phosphinoethane; R = H and Me) have been used as N-donor, μ-cyanoiron metalloligands to stabilize the DNIU in two redox states. Two bimetallic [(η 5 -C 5 R 5 )(dppe)Fe II –CN–{Fe(NO) 2 } 9 (sIMes)][BF 4 ] complexes, Fe-1 (R = H) and Fe*-1 (R = CH 3 ), showed dissimilar Fe II CN–{Fe(NO) 2 } 9 angular bends due to the electronic donor properties of the [(η 5 -C 5 R 5 )Fe(dppe)(CN)] μ-cyanoiron metalloligand. A trimetallic [(η 5 -C 5 Me 5 )(dppe)Fe II –CN] 2 –{Fe(NO) 2 } 10 complex, Fe*-2 , engaged two bridging μ-cyanoiron metalloligands to stabilize the {Fe(NO) 2 } 10 unit. The lability of the Fe II –CN–{Fe(NO) 2 } 9/10 bond was probed by suitable X-type (Na + SPh − ) and L-type (PMe 3 ) ligands. Treatment of Fe-1 and Fe*-1 with PMe 3 accounted for a reduction-induced substitution at the DNIU, releasing [(η 5 -C 5 R 5 )Fe(dppe)(CN)] and N-heterocyclic carbene, and generating (PMe 3 ) 2 Fe(NO) 2 as the reduced {Fe(NO) 2 } 10 product.
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Evaluating Diazene to N 2 Interconversion at Iron‐Sulfur Complexes
Biological N2reduction occurs at sulfur‐rich multiiron sites, and an interesting potential pathway is concerted double reduction/ protonation of bridging N2through PCET. Here, we test the feasibility of using synthetic sulfur‐supported diiron complexes to mimic this pathway. Oxidative proton transfer from μ‐η1 : η1‐diazene (HN=NH) is the microscopic reverse of the proposed N2fixation pathway, revealing the energetics of the process. Previously, Sellmann assigned the purple metastable product from two‐electron oxidation of [{Fe2+(PPr3)L1}2(μ‐η1 : η1‐N2H2)] (L1=tetradentate SSSS ligand) at −78 °C as [{Fe2+(PPr3)L1}2(μ‐η1 : η1‐N2)]2+, which would come from double PCET from diazene to sulfur atoms of the supporting ligands. Using resonance Raman, Mössbauer, NMR, and EPR spectroscopies in conjunction with DFT calculations, we show that the product is not an N2complex. Instead, the data are most consistent with the spectroscopically observed species being the mononuclear iron(III) diazene complex [{Fe(PPr3)L1}(η2‐N2H2)]+. Calculations indicate that the proposed double PCET has a barrier that is too high for proton transfer at the reaction temperature. Also, PCET from the bridging diazene is highly exergonic as a result of the high Fe3+/2+redox potential, indicating that the reverse N2protonation would be too endergonic to proceed. This system establishes the “ground rules” for designing reversible N2/N2H2interconversion through PCET, such as tuning the redox potentials of the metal sites.
more » « less- NSF-PAR ID:
- 10497094
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Chemistry – A European Journal
- ISSN:
- 0947-6539
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract Metalation of the polynucleating ligandF,tbsLH6(1,3,5‐C6H9(NC6H3−4‐F−2‐NSiMe2tBu)3) with two equivalents of Zn(N(SiMe3)2)2affords the dinuclear product (F,tbsLH2)Zn2(
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Abstract With the goal of generating anionic analogues to MN2S2
⋅ Mn(CO)3Br we introduced metallodithiolate ligands, MN2S22−prepared from the Cys‐X‐Cys biomimetic, ema4−ligand (ema=N,N′‐ethylenebis(mercaptoacetamide); M=NiII, [VIV≡O]2+and FeIII) to Mn(CO)5Br. An unexpected, remarkably stable dimanganese product, (H2N2(CH2C=O(μ‐S))2)[Mn(CO)3]2resulted from loss of M originally residing in the N2S24−pocket, replaced by protonation at the amido nitrogens, generating H2ema2−. Accordingly, the ema ligand has switched its coordination mode from an N2S24−cavity holding a single metal, to a binucleating H2ema2−with bridging sulfurs and carboxamide oxygens within Mn‐μ‐S‐CH2‐C‐O, 5‐membered rings. In situ metal‐templating by zinc ions gives quantitative yields of the Mn2product. By computational studies we compared the conformations of “linear” ema4−to ema4−frozen in the “tight‐loop” around single metals, and to the “looser” fold possible for H2ema2−that is the optimal arrangement for binucleation. XRD molecular structures show extensive H‐bonding at the amido‐nitrogen protons in the solid state.
