Synthesis of (η 3 ‐Allyl)bromodicarbonylbis(pyrazole)molybdenum( II ) and Reactivity towards [Au(acac)PPh 3 ]: Structure and Dynamic Behavior of the Monometallic Pyrazole and Heterometallic Pyrazolate Complexes

The reactions of [Mo(η 3 ‐allyl)Br(CO) 2 (NCMe) 2 ] and two equivalents of Hpz (pyrazole) or Hdmpz (3,5‐dimethylpyrazole) lead to [Mo(η 3 ‐allyl)Br(CO) 2 (Hpz) 2 ] or [Mo(η 3 ‐allyl)Br(CO) 2 (Hdmpz) 2 ]. The Hpz and Hdmpz ligands are coordinated cis and trans with respect to the allyl group in the solid state, whereas in their solutions a slow trigonal twist process is detected. These complexes react with Tl(acac) giving [Mo(acac)(η 3 ‐allyl)(CO) 2 (Hpz)] and [Mo(acac)(η 3 ‐allyl)(CO) 2 (Hdmpz)], which also undergo a slow trigonal twist process in solution. The latter complex may also be obtained by the reaction of equimolar amounts of [Mo(η 3 ‐allyl)Br(CO) 2 (Hdmpz) 2 ] and [Au(acac)PPh 3 ], giving [AuBrPPh 3 ] as a by product. The reaction of these molybdenum acac complexes with [Au(acac)PPh 3 ] give rise to the heterobimetallic bridging pyrazolate complexes [Mo(acac)(η 3 ‐allyl)(CO) 2 (μ‐pz)AuPPh 3 ] and [Mo(acac)(η 3 ‐allyl)(CO) 2 (μ‐dmpz)AuPPh 3 ]. The solid state structure of the latter, which is also obtained from [Mo(η 3 ‐allyl)Br(CO) 2 (Hdmpz) 2 ] and two equivalents of [Au(acac)PPh 3 ], with [AuBrPPh 3 ] as a by product, reveals the coordination of the bridging dimethylpyrazolate ligand trans to the allyl group. Two different dynamic processes which are slowed down at low temperatures are detected in solution for each heterobimetallic complex: trigonal twist for the pyrazolate bridging complex, and hindered rotation of the Mo−N bond for the more hindered dimethylpyrazolate complex. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)