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

The synthesis of the biphenyl alkynyl thiols and thioesters R'-C=C-C6H4-C6H4-SR (3: R' = SiMe3, R = C(O)Me; 4: R' = SiMe3, R = H; 5: R' = H, R = C(O)Me) from I-C6H4-C6H4-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 LnM'-C=C-C6H4-C6H4-SR (7: LnM' = Fc, R = C(O)Me; 8: LnM' = Fc, R = H; 10: LnM' = (Ph3P)Au, R = C(O)Me; 14: LnM' = FcPPh(2)-Au, R = C(O)Me; Fc = (eta(5)-C5H5)(eta(5)-C5H4)Fe; FcPPh(2) = (eta(5)-C5H5)(eta(5)-C5H4PPh2)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 (Ph3P)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-C6H4-C6H4-SR structure and the lack of coplanarity of the biphenyl arene rings. The overall length of these complexes are 13.345(2) angstrom for 3 (molecule A), 15.146(3) angstrom for 7, 15.705(2) angstrom for 10 (molecule A) and 15.649(4) angstrom for 14. The thioester groups are pointing away from the ferrocene building block. In 7 a linear 1D chain is set-up by pi-interactions between two independent molecules of 7. Characteristic for 15 is the formation of a Au2I2 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. (c) 2006 Elsevier B.V. All rights reserved.