Synthesis, characterization and electrochemical behavior of unsymmetric transition metal-terminated biphenyl ethynyl thiols

Abstract

The synthesis of the biphenyl alkynyl thiols and thioesters R′–C C–C 6H 4–C 6H 4–SR ( 3: R′ = SiMe 3, R = C(O)Me; 4: R′ = SiMe 3, R = H; 5: R′ = H, R = C(O)Me) from I–C 6H 4–C 6H 4–SC(O)Me ( 1) is described. Molecules 1 and 5 have been used as starting materials in the synthesis of mono- and heterobimetallic transition metal complexes of type L n M′–C C–C 6H 4–C 6H 4–SR ( 7: L n M′ = Fc, R = C(O)Me; 8: L n M′ = Fc, R = H; 10: L n M′ = (Ph 3P)Au, R = C(O)Me; 14: L n M′ = FcPPh 2-Au, R = C(O)Me; Fc = (η 5-C 5H 5)(η 5-C 5H 4)Fe; FcPPh 2 = (η 5-C 5H 5)(η 5-C 5H 4PPh 2)Fe). While complex 7is accessible by the Sonogashira cross-coupling of Fc–C CH ( 6) with 1, molecules 10 and 14 can be prepared by treatment of the thioester 5 with (Ph 3P)AuCl ( 9) and FcPPh 2-AuCl ( 13), respectively. The molecular solid state structures of 3, 7, 10 and 13– 15 have been determined by single crystal X-ray crystallographic analysis. Typical features of these species are their linear M-C C–C 6H 4–C 6H 4–SR structure and the lack of coplanarity of the biphenyl arene rings. The overall length of these complexes are 13.345(2) Å for 3 (molecule A), 15.146(3) Å for 7, 15.705(2) Å for 10 (molecule A) and 15.649(4) Å for 14. The thioester groups are pointing away from the ferrocene building block. In 7 a linear 1D chain is set-up by π-interactions between two independent molecules of 7. Characteristic for 15 is the formation of a Au 2I 2 ring, while 13 is monomeric. All compounds were studied with cyclic voltammetry. Characteristic are the reversible ferrocene Fe(II)/Fe(III) redox wave, the irreversible reduction of Au(I) to Au(0), the oxidative cleavage of the S–C(O)Me sulfur–carbon ( 3, 5, 7, 10 and 14) and of the sulfur–hydrogen bond ( 4 and 8), respectively. Electronic effects extending from the –SH-end group to the ferrocene unit resulting in considerable shifts of the redox potential of the latter entity are found. Coordination of Au(I) at the FcPPh 2 moiety also results in a shift of the redox potential of the ferrocene group indicative of an electron withdrawing effect of the Au(I) species.