A Joint Experimental and Computational Study on the Electronic Communication in Diethynylaryl‐Bridged (η 5 ‐C 5 H 5 )Fe(η 2 ‐dppe) and (η 5 ‐C 5 H 5 )Fe(CO) 2 Units

A family of bimetallic complexes [Cp(CO) 2 Fe–C≡C–Ar–C≡C–Fe(CO) 2 Cp] {Cp = C 5 H 5 ; 6a – g : Ar = C 4 H 2 S ( a ), 3‐(C 4 H 9 )‐C 4 HS ( b ), 3‐(C 16 H 33 )‐C 4 HS ( c ), C 6 H 4 ( d ), 2,5‐bis(OC 4 H 9 )‐C 6 H 2 ( e ), 2,5‐bis(OC 8 H 17 )‐C 6 H 2 ( f ), (C 6 H 4 ) 2 ( g )} was prepared by the three‐step Pd‐catalysed extended one‐pot (EOP) synthetic protocol from Bu 3 Sn–C≡CH, X–Ar–X (X = I, 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 voltammetry (CV). The most significant electrochemical information comes from the oxidation of the dppe derivatives. The Δ E ° 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 [BF 4 ] {Cp* = C 5 (CH 3 ) 5 } and [Fe(Cp)] 2 [BF 4 ] 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 C 4 H 2 S 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 (PH 3 ) 2 Fe] and [Cp(CO) 2 Fe] binuclear model complexes are reported. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)