SYNTHESIS, STRUCTURE, AND REACTIVITY OF PT3AU AND PT3AU2 CLUSTER COMPLEXES

The complex [Pt3(mu-3-H)(mu-dppm)3]+ (dppm = Ph2PCH2PPh2) reacts with LAu+ (L = PPh3) to give [Pt3(mu-3-H)(mu-3-AuL)(mu-dppm)3]2+ and with excess LAu+ to give [Pt3(mu-3-AuL)2(mu-dppm)3]2+. The complex [Pt3(mu-3-H)(mu-3-AuL)(mu-dppm)3]2+ can be reversibly deprotonated to give [Pt3(mu-3-AuL)(mu-dppm)3]+; this reacts with LAu+ to give [Pt3(mu-3-AuL)2(mu-dppm)3]2+, from which one LAu+ unit can be removed as L2Au+ by reaction with free L to regenerate [Pt3(mu-3-AuL)(mu-dppm)3]+. These reactions indicate clearly how electrophilic substitution at a triplatinum center can occur by a stepwise mechanism (S(E)2 stepwise). The complexes [Pt3(mu-3-AuL)(mu-dppm)3]+ (L = PPh3) and [PT3(mu-3-AuL)2(mu-dppm)3]2+ (L = PMe3) can be prepared most conveniently by reaction of [Pt3(mu-3-CO)(mu-dppm)3]2+ with LAuCl (or LAuBr) and NaBH4. The silver complex [Pt3(mu-3-H)(mu-3-AgPPh3)(mu-dppm)3][PF6]2 is much less thermally stable than the gold analogue. The complex [Pt3(mu-3-AuPPh3)(mu-dppm)3]+ reversibly adds CO to give [Pt3(CO)(mu-3-AuPPh3)(mu-dppm)3]+; the carbonyl ligand is terminal with nu(CO) = 1995 cm-1, but it migrates rapidly around the Pt3 triangle even at -90-degrees-C so that, by NMR spectroscopy, the molecule appears to have 3-fold symmetry. The complexes were characterized by multinuclear NMR methods and [Pt3(mu-3-AuPMe3)2(mu-dppm)3][PF6]2 was also characterized crystallographically [space group P2(1)/c, Z = 4, a = 14.442 (2) angstrom, b = 26.676 (2) angstrom, c = 25.749 (2) angstrom, beta = 104.117 (8)-degrees, R = 0.049]. It is the first example of a Pt3Au2 cluster with this electron count, and the Pt3Au2 core is trigonal bipyramidal with Pt-Pt distances ranging from 2.635 (1) to 2.642 (1) angstrom and Pt-Au distances ranging much more widely from 2.678 (1) to 2.843 (1) angstrom.