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)2FeCCArCCFe(CO)2Cp] {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=I,Br) and Cp(CO)2FeI. Complexes 6a,d,g were then exposed to ultraviolet irradiation in presence of an equivalent amount of 1,2-bis(diphenylphosphyne)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 Ciclic Voltammetry (CV). The most significant electrochemical information comes from the oxidation of the dppe derivatives. The E° separations between the subsequent reversible waves suggest that the efficiency of the metal-metal electronic coupling decreases in the order 6a > 6d > 6g. Complexes 7a and 7g were also chemically oxidised with [Fe(Cp*)]2[BF4] {Cp*= C5(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 Charge Transfer band (ICT) was observed only for the radical cation 7a+. While this finding confirms the existence of 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)2Fe and [Cp(CO)2Fe binuclear model complexes are reported.