Synthesis, structures and properties of platinum(II) complexes of oligothiophene-functionalized ferrocenylacetylene

A series of rigid-rod alkynylferrocenyl precursors with oligothiophene (from thiophene to terthiophene) linkage units in the backbone,[(η⁵-C₅H₅)Fe(η ⁵-C₅H₄)C≡C(C₄H ₂S)_mBr] 2a-2c, [(η⁵-C₅H₅)Fe(η⁵-C ₅H₄)C≡C(C₄H₂S) _mC≡CSiMe₃] 3a-3c and [(η⁵-C₅H₅)Fe(η⁵-C ₅H₄)C≡C(C₄H₂S) _mC≡CH] 4a-4c (m = 1-3), have been prepared in moderate to good yields. The ferrocenylacetylene complexes 4a-4c can be used to form a range of stable platinum(II) alkynyl and bis(alkynyl) compounds trans-[(η⁵-C₅H₅)Fe(η⁵-C ₅H₄)C≡C(C₄H₂S) _mC≡CPt(PEt₃)₂Ph] 5a-5c and trans-[(η⁵-C₅H₅)-Fe(η⁵-C ₅H₄)C≡C(C₄H₂S) _mC≡CPt(PBu₃)₂C≡C(C₄H ₂S)_mC≡C(η⁵-C₅H ₄)Fe(η⁵-C₅H₅)] 6a-6c (m = 1-3). All these new compounds have been fully characterized by analytical and spectroscopic methods and the molecular structures of the bithienyl-linked complexes [(η⁵-C₅H₅)Fe(η⁵-C ₅H₄)C≡C(C₄H₂S) ₂C≡C(η⁵-C₅H₄)Fe(η ⁵-C₅H₅)], 2b, 4b, 5b and 6b have been established by X-ray crystallography. Structural analysis of 6b confirms its rigid-rod structural motif, featuring coplanar bithienyl rings and a trans arrangement of the two bithiophene groups. An iron-iron through-space distance of ca. 32 Å is observed in 6b. Although there is no significant metallocene-metallocene interaction through the alkynyl-platinum-oligothiophene bridge, a slight negative shift of the ferrocene-ferrocenium redox potential in the platinum-containing species indicates some degree of electron delocalization into the platinum segment, in line with the results from theoretical studies. Oxidation of the thiophene units is facilitated by the presence of the platinum centre and increased conjugation in the chain.