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

Abstract

AbstractA 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 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 voltammetry (CV). The most significant electrochemical information comes from the oxidation of the dppe derivatives. The ΔE° 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* = 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 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)2Fe] and [Cp(CO)2Fe] binuclear model complexes are reported. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)