Redox-responsive bi- and tri-nuclear iron/molybdenum complexes incorporating the ferrocenyl unit as a redox spectator

Abstract

Seven new ferrocene derivatives L¹–L⁷ with pendant arms containing a terminal 4-pyridyl binding site have been prepared and characterized. Six have one pendant arm, whereas the seventh has two pendant arms, one on each cyclopentadienyl ring. Two have been crystallographically characterized: (C₅H₅)Fe(C₅H₄)CHCH(4-C₅H₄N)(L¹) and (C₅H₅)Fe(C₅H₄)(4-C₆H₄)CHCH(4-C₅H₄N)(L⁴). In both cases the conjugated side-arm is approximately planar. Attachment of {MoL*(NO)Cl} fragments [L*= tris(3,5-dimethylpyrazolyl)hydroborate] to the vacant pyridyl sites of L¹–L⁷ afforded bimetallic complexes which contain electron-donating ferrocenyl groups attached via conjugated bridges to electron-deficient {MoL*(NO)Cl} fragments. The new ferrocenes and complexes were thoroughly characterized by electrochemical, UV/VIS and EPR spectroscopic methods, and significant intercomponent interactions are evident: co-ordination of the {MoL*(NO)Cl} fragment results in a shift of the ferrocenyl redox potential to more positive potentials by approximately 40 mV per {MoL*(NO)Cl} substituent; the reductions (from 17 to 18 valence electrons) of the {MoL*(NO)Cl} groups are likewise made less negative by the presence of highly conjugated substituents on their pyridyl ligands. The electronic spectra contain transitions from both the ferrocenyl and the {MoL*(NO)Cl} components which are change transfer in origin and therefore solvatochromic. The EPR spectra of the binuclear complexes containing one {MoL*(NO)Cl} substitutent are entirely normal; however the spectrum of [{MoL*(NO)Cl}₂L⁷], which contains two paramagnetic {MoL*(NO)Cl} groups attached to the two pendant arms of L⁷, shows a weak magnetic exchange interaction between the paramagnetic centres which must be transmitted through the bridging ligand and across the ferrocenyl unit.