Trigonal-planar-coordinated organogold(I) complexes stabilized by organometallic 1,4-diynes: Reaction behavior, structure, and bonding

The reaction of the bis(alkynyl) titanocenes [Ti](C=CR1)(C=CR2) ([Ti] = (eta(6)-C5H4SiMe3)(2)Ti; la R-1 = R-2 = SiMe3; Ib R-1 = R-2 = Bu-t; 1c R-1 = SiMe3, R-2 = Bu-t) With (C5H5N)AuCl3 (2), LAuCl (5a, L = PPh3; 5b, L = SMe2) as well as (Me2S)AuR3 (6b, R-3 = C=CSiMe3; 6c, R-3 = C=(CBu)-Bu-t; 6d, R-3 = C6H2(CF3)(3)-2,4,6; 6e, R-3 = Me) is described. Treatment of [Ti](C=CSiMe3)(2) (la) with (C5H5N)AuCl3 (2) produces [Ti]Cl-2 (3) and Me3SiC=C-C=CSiMe3 (4a) together with Au(O). However, the linear two-coordinated gold(I) chlorides LAuCl (5a,b) react with la to afford different products, depending on the Lewis bases applied. While in the reaction of the Ph3P donor-stabilized gold(I) chloride 5a, the titanocene dichloride (3) along with (Ph3P)AuC=CSiMe3 (6a) is obtained, with the appropriate Me2S donor-stabilized molecule 5b, the titanocene dichloride (3) along with the heterobimetallic tweezer molecule {[Ti](C=CSiMe3)(2)}AuC=CSiMe3 (7a) is formed. A possible mechanism for the different chemical behavior is discussed. Likewise, molecules of the latter type (compounds 7a-e) can be synthesized in much better yields by the reaction of [Ti](C=CR1)(C=CR2) (la-c) with (Me2S)AuR3 (6b-e). In the heterobimetallic titanium-gold complexes {[Ti](C=CR1)(C=CR2)}AuR3 (7a, R-1 = R-2 = SiMe3, R-3 = C=CSiMe3; Tb, R-1 = R-2 = Bu-t, R-3 = C=(CBu)-Bu-t; 7c, R-1 = SiMe3, R-2 = Bu-t, R-3 = C=CSiMe3; 7d, R-1 = R-2 = SiMe2, R-3 = C6H2(CF3)(3)-2,4,6; 7e, R-1 = R-2 = SiMe3, R-3 = Me), a low-valent monomeric organogold(I) moiety in a trigonal-planar environment is present, which is stabilized by the chelating effect of the organometallic pi-tweezer bis(alkynyl) titanocene. The thermolysis of selected organogold(I) complexes affords, on elimination of the bis(alkynyl) titanocene fragment, the coupling products R-3-R-3 (4) and gold films. The X-ray structure analyses of compounds 7a, 7b, and 7d are reported. It is found that in all compounds short titanium-gold bond lengths are present (7a, 3.007(2) Angstrom; 7b, 2.975(1) Angstrom; 7d, 2.995(1) Angstrom). Calculations show that the complexation of the Au-R-3 monomers with the organometallic pi-tweezer bis(alkynyl) titanocene is described by a four-center two-electron bond. The monomeric organogold(I) moieties are stabilized by a synergetic in-plane donation and back-donation of electron density between the bis(alkynyl) titanocene and the Au-R-3 species. A direct donor-acceptor Au-Ti interaction contributes to this stabilization.