A joint experimental and computational study on the electronic communication in diethynylaryl-bridged (η5-C5H5)Fe(η2-dppe) and (η5-C5H5)Fe(CO)2 units

A family of bimetallic complexes [CP(CO)(2)Fe-C C-Ar-C C-Fe(CO)(2)CP] {CP = C5H5; 6a-g: Ar = C4H2S (a), 3-(C4H9)-C4HS (b), 3-(C16H33)-C4HS (c), C6H4 (d), 2,5-bis(OC4H9)-C6H2 (e), 2,5-bis(OC8H17)-C6H2 (f), (C6H4)(2) (g)} was prepared by the three-step Pd-catalysed extended one-pot (EOP) synthetic protocol from Bu3Sn-C CH, X-Ar-X (X = 1, Br) and CP(CO)(2)FeI. Complexes 6a,d,g were then exposed to ultraviolet irradiation in the presence of an equivalent amount of 1, 2-bis(diphenylphosphanyl) ethane (dppe) to form the corresponding bimetallic complexes [Cp(dppe)Fe-C C-Ar-C C-Fe(dppe)Cp] (7a,d,g). Compounds 6a-g and 7a,d,g were characterised by cyclic voltarnmetry (CV). The most significant electrochemical information comes from the oxidation of the dppe derivatives. The Delta E degrees separations between the subsequent reversible waves suggest that the efficiency of the metal-metal electronic coupling decreases in the order 7a > 7d > 7g. Complexes 7a and 7g were also chemically oxidised with [Fe(Cp*)](2)[BF4] {CP* = C-5(CH3)(5)} and [Fe(Cp)](2)[BF4] respectively, and the near infrared (NIR) spectra of the mixed-valence species 7a(+) and 7g(+) were recorded. A strong intervalence transition (IT) band was observed only for the radical cation 7a(+). While this finding confirms the existence of an electronic interaction between the two termini when a 2,5-thiophene group is present in the spacer, the NIR spectrum of 7g(+) reveals a reduced efficiency in conveying electrons when the C4H2S moiety is replaced by a 4,4'-biphenyl. In order to rationalise and quantify the extent of electronic communication, ruled by geometrical and electronic factors, density functional computational results on selected [Cp (PH3)(2)Fe] and [CP(CO)(2)Fe] binuclear model complexes are reported.