Structural, dynamic, and theoretical studies of [AunPt2(PPh3)(4)(mu-S)(2-n)(mu(3)-S)(n)L][PF6]n [n=1, L = PPh3; n=2, L = Ph2PCH2PPh2, (C5H4PPh2)(2)Fe]

Three heterometallic Au-Pt complexes [Pt-2(PPh3)(4)(mu -S)(mu (3)-S)AU(PPh3 )][PF6] (2), [Pt-2(PPh3)(4)(mu (3)-S)(2)Au-2(mu -dppm)]-[PF6](2) (3), and [ Pt-2(PPh3)(4)(mu (3)-S)(2)Au-2(mu -dppf)][PF6](2) (4) have been synthesized from Pt-2(PPh3)(4)(mu -S)(2) (1) [dppm = Ph2PCH2PPh2; dppf = (C5H4PPh2)(2) Fe] and characterized by single-crystal X-ray crystallography. In 2, the Au (I) atom is anchored on only one of the sulfur centers. In 3 and 4, both su lfur atoms are aurated, showing the ability of 1 to support an overhead bri dge structure, viz. [Au-2(P-P)], with or without the presence of Au-Au bond . The change of dppf to dppm facilitates such active interactions. Two ster eoisomers of complex 3 (3a,b) have been obtained and characterized by singl e-crystal X-ray crystallography. NLDFT calculations on 2 show that the line ar coordination mode is stabilized with respect to the trigonal planar mode by 14.0-kJ/mol. All complexes (2-4) are fluxional in solution with differe nt mechanisms. In 2, the [Au(PPh3)] fragment switches rapidly between the t wo sulfur sites. Our hybrid MM-NLDFT calculations found a transition state in which the Au(I) bears an irregular trigonal planar geometry (DeltaG(doub le dagger) = 19.9 kJ/mol), as well as an intermediate in which Au(I) adopts a regular trigonal planar geometry. Complexes 3a,b are roughly diastereois omeric and related by sigma (minor plane) conversion. This symmetry operati on can be broken, down to two mutually dependent fluxional processes: (i) r apid flipping of the dppm methylene group across the molecular plane define d by the overhead bridge; (ii) rocking motion of the two Au atoms across th e S . . .S axis of the {Pt2S2} core. Modeling of the former by molecular me chanics yields a steric barrier of 29.0 kJ/mol, close to that obtained from variable-temperature P-31{H-1} NMR study (33.7 kJ/mol). In 4, the twisting of the ferrocenyl moiety across the S S axis is in concert with a rocking motion of the two gold atoms. The movement of dppf is sterically most deman ding, and hence, 4 is the only complex that shows astatic structure at lowe r temperatures. Pertinent crystallographic data: (2) space group P1, a = 15 .0340(5) Angstrom, b = 15.5009(5) Angstrom, c = 21.9604(7), Angstrom, alpha = 74.805(1)degrees, beta = 85.733(1)degrees, gamma = 78.553(1)degrees, R = 0.0500; (3a) space group Pna2(1), a = 32.0538(4) Angstrom, b = 16.0822(3) Angstrom, c = 18.9388(3) Angstrom, R = 0.0347; (3b) space group Pna2(1),a = 31.950(2) Angstrom, b, 16.0157(8) Angstrom, c = 18.8460(9) Angstrom, R = 0 .0478; (4) space group P2(1)/cl a = 13.8668(2) Angstrom, b = 51.7754(4) Ang strom, c = 15.9660(2) Angstrom, beta 113.786(1)degrees, R = 0.0649.