Synthesis, Spectroelectrochemical, and EPR Spectroscopic Studies of Mixed Bis(alkynyl)biferrocenes of the Type (L_nMC≡C)(L_nM′C≡C)bfc

Abstract

The synthesis and properties of a series of complexes containing bis(ethynyl)biferrocene as a bridge between different redoxactive group 8 metal fragments is described. These metal acetylide compounds of type (L n MCC)(L n M′CC)bfc (5a, L n M = Fe(Cp*)-(η 2 -dppe), L n M′ = Ru(Cp)(Ph 3 P) 2 ; 5b, L n M = Fe(Cp*)(η 2 -dppe), L n M′ = Os(Cp)(Ph 3 P) 2 ; 5c, L n M = Ru(Cp)(Ph 3 P) 2 , L n M′ = Os(Cp)(Ph 3 P) 2 ; bfc = biferrocene-1′,1‴-diyl, ((η 5 -C 5 H 4 ) 2 Fe) 2 ; dppe = 1,2-bis(diphenylphosphino)ethane, C 2 H 4 (PPh 2 ) 2 ; Cp = η 5 -C 5 H 5 , Cp* = η 5 -C 5 Me 5 ) were prepared either by treatment of (HCC)(L n M′CC)bfc (L n M′ = Ru(Cp)(Ph 3 P) 2 (4b), Os(Cp)(Ph 3 P) 2 (4c)) with Fe(Cp*)(η 2 -dppe)Cl (2a) or by the reaction of 4c with Ru(Cp)(PPh 3 ) 2 Cl (2b) in the presence of [H 4 N][PF 6 ] and KO t Bu, respectively. Compounds 5a−c show wellseparated reversible one-electron redox events in their cyclic voltammograms using [ n Bu 4 N][PF 6 ] as supporting electrolyte in dichloromethane solutions. Absorption and vibrational spectroscopic studies were achieved for mixed-valence 5a−c[PF 6 ] and 5a−c[PF 6 ] 2 by spectroelectrochemical methods (OTTLE), and in the case of the more robust Fe/Os system the higher oxidation states 5b[PF 6 ] 3 and 5b[PF 6 ] 4 were also characterized. Taken as a whole, our data indicate that direct electron transfer between the redox termini does not take place. Electron exchange results from dominant interactions between the redox termini and the proximal fc units (fc = Fe(η 5 -C 5 H 4 ) 2 ) of the bfc moiety and a weak but sizable interaction between the fc units. Furthermore, EPR spectroscopy of 5a−c[PF 6 ] allowed the simultaneous observation of the EPR signatures of half-sandwich metal-centered radicals and biferrocenium-centered radicals. This feature strongly supports that a multistep electron exchange mechanism takes place between the ML n /M′L n redox termini of this molecular array, with bridge-centered low-lying mediating states thermally populated even at 66 K. The g tensors of anisotropy (Δg = g ∥ − g ⊥ ) of the bis(ethynyl)biferrocenyl moiety ranging between 2.26 and 2.42 for 5a−c[PF 6 ] are consistent with a slow electron exchange rate between the fc units and confirmed that these mixed-valence complexes belong to class II compounds as defined by Robin and Day.

Why is Ferrocene so Exceptional?

“…For instance, studies conducted by Lang et al in particular have scrutinized electronically intercommunicating iron centers in di‐ and tetraferrocenylpyrroles and other heterocycles. Diethynylbiferrocene has been used by the Lapinte and Lang groups as a bridge between several redox‐active group 8 metal fragments in order to examine the nature of the electronic communication between these groups with absorption and vibrational spectroscopy of the mixed valences, and the interaction was shown to be weak but measurable …”

“…Diethynylbiferrocene has been used by the Lapinte and Lang groups as a bridge between several redox-active group 8 metal fragments in order to examine the nature of the electronic communication between these groups with absorption and vibrational spectroscopy of the mixed valences, and the interaction was shown to be weak but measurable. [111] The Nishihara group synthesized azo-bridged ferrocene oligomers and a polymer and conducted electrochemical and optical analyses of internuclear electronic interactions in the mixed-valence states. The redox potentials and intervalence band characteristics were shown to depend significantly on the solvent.…”

Electrochemical Properties and Structure of Multi-Ferrocenyl Phosphorus Thioesters

“…Currently, a number of new practical applications of multi-ferrocenyl compounds are being actively developed. [1][2][3][4][5][6][7][8][9][10][11] The multi-ferrocenes are widely used as model systems for understanding intramolecular charge transfer processes. Consequently, more and more multi-ferrocenes are synthesized to study their charge transfer processes and to match intramolecular charge transfer interactions numbers.…”

Twists and turns: Studies of the complexes and properties of bimetallic complexes featuring phenylene ethynylene and related bridging ligands

“…ligand redox non-innocence) [47]. Further complications arise in the middle ground of electronic structures where bridge and metal states mix, leading to significant deviations from the well-known Hush two-state model of electronic coupling in mixed valence systems, whilst retaining some spectroscopic characteristics associated with mixed valency [48][49][50][51]. Identifying the electronic characteristics of the redox products derived from [{L x M n }(-bridge){M n L x }] systems therefore requires application of a range of physical and spectroscopic methods which provide information across a range of timescales, and is often supported by computational investigations and which precede discussions of the electron transfer characteristics of these species.…”