Synthesis of tripod Ligands Containing Mixed Donor Sets with Phosphane and Pyrazole Donors − Bidentate Chelate Binding Modes and Dynamic Behaviour of tripod Ligands in d<SUP>8</SUP>-Metal Complexes (Ni<SUP>II</SUP>, Pd<SUP>II</SUP>, Pt<SUP>II</SUP>)

The synthesis of a series of tripod ligands MeOCH<INF>2</INF>C(CH<INF>2</INF>X)(CH<INF>2</INF>Y)(CH<INF>2</INF>Z) (X = PR<INF>2</INF>; Y, Z = PR'<INF>2</INF> or pz; pz = 1-pyrazolyl) based on the 3,3-difunctionalized oxetane O^a[CH<INF>2</INF>]<INF>2</INF>C^a(CH<INF>2</INF>OMs)(CH<INF>2</INF>Br)(O^a−C^a) as the starting material is described. With their mixed donor sets these ligands coordinate to d⁸-metal ions in a bidentate binding mode. One of the three donors remains uncoordinated in each case. Coordination of phosphane donors is generally preferred over coordination of pyrazole donors. With bulky phosphanes such as −CH<INF>2</INF>PMes<INF>2</INF>, however, pyrazole may compete with PR<INF>2</INF>, depending on the kind of the d⁸ fragment. Dynamic exchange between coordinated and noncoordinated pyrazole donors is observed. Tetracoordinate complexes of the general type L<INF>2</INF>MR<INF>2</INF> are formed (L<INF>2</INF> symbolizes the tripod ligand in its bidentate chelate binding mode with two donors functions coordinating and the third one serving as a dangling arm; M = Ni^II, Pd^II, Pt^II). With nickel(<SC>II</SC>) as the d⁸ species, tetrahedral or square-planar coordination geometries ensue, depending on the kind of donor functions and on the kind of co-ligands R. With palladium(<SC>II</SC>) and platinum(<SC>II</SC>) as the d⁸ centres, square-planar coordination is observed throughout. The size of the chelate cycles varies from six to eight depending on the number of pyrazole entities within the cycle. The conformations and the conformational flexibility characterizing the chelate cycles have been analysed by X-ray analyses and by variable-temperature NMR. The classes of conformations observed may be formally reduced to chair, half-chair and twist-boat conformations. Dynamic exchange between these conformations is observed. In one case, the same compound forms two different types of crystals that differ by the conformation adopted by their chelate cycles. All compounds have been fully characterized by standard analytical techniques including X-ray structure analysis of 14 chelate compounds. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)