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Iron Carbonyl Clusters as Proton Reduction Catalysts

Rahaman, Ahibur LU (2016)
Abstract
Abstract – The mixed-valence triiron complexes [Fe3(CO)7-x(PPh3)x(µ-edt)2] (x = 0, 1, 2; edt = SCH2CH2S) and [Fe3(CO)5(κ2-diphosphine)(µ-edt)2] (diphosphine = dppv, dppe, dppb, dppn) have been prepared and structurally characterized. In comparison to the diiron complex [Fe2(CO)6(µ-edt)], [Fe3(CO)7(µ-edt)2] catalyzes proton reduction at 0.36 V less negative potentials, which is a significant energetic gain. In all complexes the HOMO comprises an iron-iron bonding orbital localized between the two iron atoms not ligated by the semi-bridging carbonyl, while the LUMO is... (More)
Abstract – The mixed-valence triiron complexes [Fe3(CO)7-x(PPh3)x(µ-edt)2] (x = 0, 1, 2; edt = SCH2CH2S) and [Fe3(CO)5(κ2-diphosphine)(µ-edt)2] (diphosphine = dppv, dppe, dppb, dppn) have been prepared and structurally characterized. In comparison to the diiron complex [Fe2(CO)6(µ-edt)], [Fe3(CO)7(µ-edt)2] catalyzes proton reduction at 0.36 V less negative potentials, which is a significant energetic gain. In all complexes the HOMO comprises an iron-iron bonding orbital localized between the two iron atoms not ligated by the semi-bridging carbonyl, while the LUMO is highly delocalised in nature and is anti-bonding between both pairs of iron atoms but also contains an anti-bonding dithiolate interaction. The clusters [Fe3(CO)9(μ3-E)2] (E = S, Se, Te), [Fe3(CO)7(μ3-E)2(μ- κ2-diphosphine)] (E = S, Se, Te), [Fe3(CO)7(μ3-CO)(μ3-E)(μ-dppm)] (E = S, Se) and [Fe3(CO)8(µ3-Te)2(κ2-diphosphine)] have been prepared and examined as proton reduction catalysts. The reduction potentials for the tellurium-capped clusters occur at lower potentials than for their sulfur and selenium analogues, and the redox processes also show better reversibility than for the S/Se analogues. The 52-electron clusters [Fe3(CO)8(µ3-Te)2(κ2-diphosphine)] consist of Fe2(CO)6(µ-Te)2 “butterfly” units that are capped by a Fe(CO)2(κ2-diphosphine) moiety. Cyclic voltammetry studies reveal that their redox behaviour and properties as proton reduction catalysts largely stem from the Fe2(CO)6(µ-Te)2 entities, although computational modelling indicates that their LUMOs are centered on the briding tellurium ions and the unique “capping” iron ion. The influence of the substitution, orientation and structure of the phosphido bridges on the electrochemical and electrocatalytic properties of [Fe2(CO)6(µ-phosphido)2] clusters and bis(phosphinidene)-capped triiron carbonyl clusters, including electron-rich derivatives formed by substitution with chelating diphosphines, have been studied. The electrochemistry and electrocatalyses of the [Fe2(CO)6(µ-PR2)2] dimers show subtle variations with the nature of the bridging phosphido group(s), including the orientation of bridgehead hydrogen atoms. The reduction potential of the phosphinidene- capped clusters shift negative way to increase the electron density on the iron centers. (Less)
Abstract (Swedish)
En utveckling av den s. k. väte-ekonomin, där vätgas (H2) används som energibärare, skulle – rent teoretiskt – kunna lösa människans energiproblem då vatten skulle kunna användas som bränsle. För att nå detta mål krävs dock väsentlig teknologisk och vetenskaplig utveckling. Ur ett rent vetenskapligt perspektiv, måste bl a (energisnåla) kemiska metoder att skapa stora mängder vätgas utvecklas. Många forskare söker att efterhärma de fundamentala biologiska reaktioner som sker i fotosyntesen, bl a oxidation av vatten till syrgas, protoner (vätejoner, H+) och elektroner. Det är lika viktigt att kunna omvandla protoner och elektroner till vätgas på ett effektivt sätt, och intensivt forskningsarbete bedrivs för närvarande inom detta område.... (More)
En utveckling av den s. k. väte-ekonomin, där vätgas (H2) används som energibärare, skulle – rent teoretiskt – kunna lösa människans energiproblem då vatten skulle kunna användas som bränsle. För att nå detta mål krävs dock väsentlig teknologisk och vetenskaplig utveckling. Ur ett rent vetenskapligt perspektiv, måste bl a (energisnåla) kemiska metoder att skapa stora mängder vätgas utvecklas. Många forskare söker att efterhärma de fundamentala biologiska reaktioner som sker i fotosyntesen, bl a oxidation av vatten till syrgas, protoner (vätejoner, H+) och elektroner. Det är lika viktigt att kunna omvandla protoner och elektroner till vätgas på ett effektivt sätt, och intensivt forskningsarbete bedrivs för närvarande inom detta område. Denna forskning har hittills fokuserat på att efterhärma de aktiva säten (bestående av järn-nickel eller järnkluster) i vissa enzymer – hydrogenaser – som kan katalysera jämvikten 2 H+ + 2 e-  H2 (i båda riktningar). Denna forskning har lett till en djupare förståelse av de fundamentala kemiska principerna för hydrogenasreaktionerna, men har inte lett till goda katalysatorer. I denna avhandling beskrivs framställandet av ett antal järnkomplex som innehåller två, tre eller fyra järnatomer, och som besitter liknande kemiska egenskaper som hydrogenas-enzymer. Likt vissa sådana enzymer, kan komplexen katalysera elektrokemisk reduktion av protoner som skapar vätgas, d v s driva ovanstående jämvikt åt höger. Med hjälp av elektrokemiska metoder, spektroskopiska studier och datorberäkningar har mekanismen för protonreduktion studerats och för ett flertal katalysatorer har det mest sannolika första steget i reaktionen kunnat fastställas, nämligen om komplexet först binder en proton eller erhåller en elektron (reduceras). (Less)
Please use this url to cite or link to this publication:
author
supervisor
opponent
  • Professor Schollhammer, Philippe, Université de Bretagne Occidentale, Brest, France
organization
publishing date
type
Thesis
publication status
published
subject
keywords
iron-only hydrogenase, phosphine-substitution, electrocatalysis, proton reduction
pages
256 pages
publisher
Lund University, Faculty of Science, Department of Chemistry
defense location
Center for chemistry and chemical engineering, lecture hall B, Naturvetarvägen 14, Lund
defense date
2016-06-03 9:30:00
ISBN
978-91-7422-457-3
978-91-7422-460-3
language
English
LU publication?
yes
id
b7ecd2d1-ca06-4b48-9c5f-5e642bca884d
date added to LUP
2016-05-11 17:36:06
date last changed
2018-11-21 21:23:37
@phdthesis{b7ecd2d1-ca06-4b48-9c5f-5e642bca884d,
  abstract     = {{Abstract – The mixed-valence triiron complexes [Fe<sub>3</sub>(CO)<sub>7-x</sub>(PPh<sub>3</sub>)<sub>x</sub>(µ-edt)<sub>2</sub>] (x = 0, 1, 2; edt = SCH<sub>2</sub>CH<sub>2</sub>S) and [Fe<sub>3</sub>(CO)<sub>5</sub>(κ<sup>2</sup>-diphosphine)(µ-edt)<sub>2</sub>] (diphosphine = dppv, dppe, dppb, dppn) have been prepared and structurally characterized. In comparison to the diiron complex [Fe<sub>2</sub>(CO)<sub>6</sub>(µ-edt)], [Fe<sub>3</sub>(CO)<sub>7</sub>(µ-edt)<sub>2</sub>] catalyzes proton reduction at 0.36 V less negative potentials, which is a significant energetic gain. In all complexes the HOMO comprises an iron-iron bonding orbital localized between the two iron atoms not ligated by the semi-bridging carbonyl, while the LUMO is highly delocalised in nature and is anti-bonding between both pairs of iron atoms but also contains an anti-bonding dithiolate interaction. The clusters [Fe<sub>3</sub>(CO)<sub>9</sub>(μ<sub>3</sub>-E)<sub>2</sub>] (E = S, Se, Te), [Fe<sub>3</sub>(CO)<sub>7</sub>(μ<sub>3</sub>-E)<sub>2</sub>(μ- κ<sup>2</sup>-diphosphine)] (E = S, Se, Te), [Fe<sub>3</sub>(CO)<sub>7</sub>(μ<sub>3</sub>-CO)(μ<sub>3</sub>-E)(μ-dppm)] (E = S, Se) and [Fe<sub>3</sub>(CO)<sub>8</sub>(µ<sub>3</sub>-Te)<sub>2</sub>(κ<sup>2</sup>-diphosphine)] have been prepared and examined as proton reduction catalysts. The reduction potentials for the tellurium-capped clusters occur at lower potentials than for their sulfur and selenium analogues, and the redox processes also show better reversibility than for the S/Se analogues. The 52-electron clusters [Fe<sub>3</sub>(CO)<sub>8</sub>(µ<sub>3</sub>-Te)<sub>2</sub>(κ<sup>2</sup>-diphosphine)] consist of Fe2(CO)6(µ-Te)2 “butterfly” units that are capped by a Fe(CO)<sub>2</sub>(κ<sup>2</sup>-diphosphine) moiety. Cyclic voltammetry studies reveal that their redox behaviour and properties as proton reduction catalysts largely stem from the Fe<sub>2</sub>(CO)<sub>6</sub>(µ-Te)<sub>2</sub> entities, although computational modelling indicates that their LUMOs are centered on the briding tellurium ions and the unique “capping” iron ion.  The influence of the substitution, orientation and structure of the phosphido bridges on the electrochemical and electrocatalytic properties of [Fe<sub>2</sub>(CO)<sub>6</sub>(µ-phosphido)<sub>2</sub>] clusters and bis(phosphinidene)-capped triiron carbonyl clusters, including electron-rich derivatives formed by substitution with chelating diphosphines, have been studied. The electrochemistry and electrocatalyses of the [Fe<sub>2</sub>(CO)<sub>6</sub>(µ-PR<sub>2</sub>)<sub>2</sub>] dimers show subtle variations with the nature of the bridging phosphido group(s), including the orientation of bridgehead hydrogen atoms. The reduction potential of the phosphinidene- capped clusters shift negative way to increase the electron density on the iron centers.}},
  author       = {{Rahaman, Ahibur}},
  isbn         = {{978-91-7422-457-3}},
  keywords     = {{iron-only hydrogenase; phosphine-substitution; electrocatalysis; proton reduction}},
  language     = {{eng}},
  publisher    = {{Lund University, Faculty of Science, Department of Chemistry}},
  school       = {{Lund University}},
  title        = {{Iron Carbonyl Clusters as Proton Reduction Catalysts}},
  url          = {{https://lup.lub.lu.se/search/files/7645274/PhD_Thesis_Ahibur_Rahaman_without_papers.pdf}},
  year         = {{2016}},
}