Electrochemistry and Stability of 1,1′-Ferrocene-Bisphosphonates

Here, we investigate the electrochemical properties and stability of 1,1′-ferrocene-bisphosphonates in aqueous solutions. 31P NMR spectroscopy enables to track decomposition at extreme pH conditions revealing partial disintegration of the ferrocene core in air and under an argon atmosphere. ESI-MS indicates the decomposition pathways to be different in aqueous H3PO4, phosphate buffer, or NaOH solutions. Cyclovoltammetry exhibits completely reversible redox chemistry of the evaluated bis­phosphonates, sodium 1,1′-ferrocene-bis­(phosphonate) (3) and sodium 1,1′-ferrocene-bis­(methylphosphonate) (8), from pH 1.2 to pH 13. Both the compounds feature freely diffusing species as determined using the Randles-Sevcik analysis. The activation barriers determined by rotating disk electrode measurements revealed asymmetry for oxidation and reduction. The compounds are tested in a hybrid flow battery using anthraquinone-2-sulfonate as the counterside, yielding only moderate performance

Synthesis and Structure of Six-Coordinate Iron Borohydride Complexes Supported by PNP Ligands

The preparation of a number of iron complexes supported by ligands of the type HN­{CH₂CH₂(PR₂)}₂ [R = isopropyl (^ⁱPrPNP) or cyclohexyl (^CyPNP)] is reported. This is the first time this important bifunctional ligand has been coordinated to iron. The iron­(II) complexes (^ⁱPrPNP)­FeCl₂(CO) (<b>1a</b>) and (^CyPNP)­FeCl₂(CO) (<b>1b</b>) were synthesized through the reaction of the appropriate free ligand and FeCl₂ in the presence of CO. The iron(0) complex (^ⁱPrPNP)­Fe­(CO)₂ (<b>2a</b>) was prepared through the reaction of Fe­(CO)₅ with ^ⁱPrPNP, while irradiating with UV light. Compound <b>2a</b> is unstable in CH₂Cl₂ and is oxidized to <b>1a</b> via the intermediate iron­(II) complex [(^ⁱPrPNP)­FeCl­(CO)₂]Cl (<b>3a</b>). The reaction of <b>2a</b> with HCl generated the related complex [(^ⁱPrPNP)­FeH­(CO)₂]Cl (<b>4a</b>), while the neutral iron hydrides (^ⁱPrPNP)­FeHCl­(CO) (<b>5a</b>) and (^CyPNP)­FeHCl­(CO) (<b>5b</b>) were synthesized through the reaction of <b>1a</b> or <b>1b</b> with 1 equiv of ⁿBu₄NBH₄. The related reaction between <b>1a</b> and excess NaBH₄ generated the unusual η¹-HBH₃ complex (^ⁱPrPNP)­FeH­(η¹-HBH₃)­(CO) (<b>6a</b>). This complex features a bifurcated intramolecular dihydrogen bond between two of the hydrogen atoms associated with the η¹-HBH₃ ligand and the N–H proton of the pincer ligand, as well as intermolecular dihydrogen bonding. The protonation of <b>6a</b> with 2,6-lutidinium tetraphenylborate resulted in the formation of the dimeric complex [{(^ⁱPrPNP)­FeH­(CO)}₂(μ₂,η¹:η¹-H₂BH₂)]­[BPh₄] (<b>7a</b>), which features a rare example of a μ₂,η¹:η¹-H₂BH₂ ligand. Unlike all previous examples of complexes with a μ₂,η¹:η¹-H₂BH₂ ligand, there is no metal–metal bond and additional bridging ligand supporting the borohydride ligand in <b>7a</b>; however, it is proposed that two dihydrogen-bonding interactions stabilize the complex. Complexes <b>1a</b>, <b>2a</b>, <b>3a</b>, <b>4a</b>, <b>5a</b>, <b>6a</b>, and <b>7a</b> were characterized by X-ray crystallography

Synthesis and Structure of Six-Coordinate Iron Borohydride Complexes Supported by PNP Ligands

The preparation of a number of iron complexes supported by ligands of the type HN­{CH₂CH₂(PR₂)}₂ [R = isopropyl (^ⁱPrPNP) or cyclohexyl (^CyPNP)] is reported. This is the first time this important bifunctional ligand has been coordinated to iron. The iron­(II) complexes (^ⁱPrPNP)­FeCl₂(CO) (<b>1a</b>) and (^CyPNP)­FeCl₂(CO) (<b>1b</b>) were synthesized through the reaction of the appropriate free ligand and FeCl₂ in the presence of CO. The iron(0) complex (^ⁱPrPNP)­Fe­(CO)₂ (<b>2a</b>) was prepared through the reaction of Fe­(CO)₅ with ^ⁱPrPNP, while irradiating with UV light. Compound <b>2a</b> is unstable in CH₂Cl₂ and is oxidized to <b>1a</b> via the intermediate iron­(II) complex [(^ⁱPrPNP)­FeCl­(CO)₂]Cl (<b>3a</b>). The reaction of <b>2a</b> with HCl generated the related complex [(^ⁱPrPNP)­FeH­(CO)₂]Cl (<b>4a</b>), while the neutral iron hydrides (^ⁱPrPNP)­FeHCl­(CO) (<b>5a</b>) and (^CyPNP)­FeHCl­(CO) (<b>5b</b>) were synthesized through the reaction of <b>1a</b> or <b>1b</b> with 1 equiv of ⁿBu₄NBH₄. The related reaction between <b>1a</b> and excess NaBH₄ generated the unusual η¹-HBH₃ complex (^ⁱPrPNP)­FeH­(η¹-HBH₃)­(CO) (<b>6a</b>). This complex features a bifurcated intramolecular dihydrogen bond between two of the hydrogen atoms associated with the η¹-HBH₃ ligand and the N–H proton of the pincer ligand, as well as intermolecular dihydrogen bonding. The protonation of <b>6a</b> with 2,6-lutidinium tetraphenylborate resulted in the formation of the dimeric complex [{(^ⁱPrPNP)­FeH­(CO)}₂(μ₂,η¹:η¹-H₂BH₂)]­[BPh₄] (<b>7a</b>), which features a rare example of a μ₂,η¹:η¹-H₂BH₂ ligand. Unlike all previous examples of complexes with a μ₂,η¹:η¹-H₂BH₂ ligand, there is no metal–metal bond and additional bridging ligand supporting the borohydride ligand in <b>7a</b>; however, it is proposed that two dihydrogen-bonding interactions stabilize the complex. Complexes <b>1a</b>, <b>2a</b>, <b>3a</b>, <b>4a</b>, <b>5a</b>, <b>6a</b>, and <b>7a</b> were characterized by X-ray crystallography