Synthesis, electrochemistry and electronic spectra of tetranuclear bis(η2-alkynyl) transition-metal complexes. The molecular structure of [(η5-C5H4SiMe3)2Ti(CCFc)2]CuBr

Tetrametallic complexes of type {[Ti](CCFc)2}MX (M=Cu: 2a, X=Cl; 2b, X=Br; 2c: X=OTf; M=Ag: 3a, X=Cl; 3b, X=BF4; [Ti]=(η5-C5H4SiMe3)2Ti, Fc=(η5-C5H4)Fe(η5-C5H5), OTf=OSO2CF3) were synthesised by the reaction of trinuclear [Ti](CCFc)2 (1) with one equivalent of [MX] (M=Cu, Ag; X=Cl, Br, OTf, BF4). The solid-state structure of {[Ti](CCFc)2}CuBr (2b) displays a three-coordinate Cu(I) centre with two η2-coordinated CCFc units and a η1-bound Br ligand. The Fc groups are both positioned on one side of the Ti(CC)2Cu plane, while the Br ligand points towards the opposite side and is located 0.2566(57) Å out of this array. The electronic spectra of complexes 2b (M=Cu) and 3a (M=Ag) show a bathochromic shift of λmax upon η2-coordination of the FcCC units in 1 to the M(I) centre in 2b or 3a. Cyclic voltammetric studies reveal a behaviour dependent on M(I). In the case of Cu(I), the Ti σ-acetylide bonds are retained upon oxidation of the remote Fc units, while with Ag(I) immediate TiCCC σ-bond cleavage is observed.