Reactions of pyrazole with unsaturated triangular clusters of rhenium. Solid-state and solution characterization of an intramolecular N-H -pi hydrogen bond

The reaction of the unsaturated anion [Re-3(mu-H)(4)(CO)(10)](-) with pyrazole: (used as solvent at 80 degrees C) proceeds in a stepwise manner, at first giving the [Re-3(mu-H)(3)(mu-eta(2)-pz)(CO)(10)](-) anion (2, pz pyrazolate), with Hz evolution. Spectroscopic characterization has shown that 2 contains a pz ligand bridging through the two N atoms, one edge of the triangular cluster. At longer times, CO is evolved and 2 transforms into the novel species [Re-3(mu-H)(3)(mu-eta(2)-pz)(CO)(9)(Hpz)](-)(3), containing one pz ligand bridging a cluster edge and a second pyrazole molecule terminally bonded on the third vertex of the triangle, replacing an axial carbonyl of. The treatment with CO (atmospheric pressure, room temperature) slowly restores 2. A single-crystal X-ray analysis of 3 has revealed the syn coordination of the two heterocycles with respect to the metal triangle plane and an intramolecular hydrogen bond between the N-H proton of the pyrazole ligand and the aromatic pi-electrons of the bridging pyrazolate. There is NMR evidence that this intramolecular hydrogen bond is maintained in CD2Cl2 solution: (i) the N-H protonic resonance is significantly shifted upfield, with respect to the values typical of complexes containing Hpz, due to the screening effect Of the pi-electron cloud; (ii) a 2D NOESY experiment has shown a dipolar interaction between H-alpha of the pyrazolate and the N-H protonic resonance (and, to a lesser extent, H-alpha of the pyrazole). In acetone, intermolecular N-H ... solvent interactions replace, in part, the intramolecular H bond and different conformers are populated in different ratios on varying the temperature; as shown by variable-temperature and nOe (1D and 2D) experiments. Further reacting 1 in molten pyrazole causes fragmentation of the triangular cluster moiety with formation of the novel dinuclear anion [Re-2(mu-H)(mu-eta(2)-pz)(2)(CO)(6)](-) (4) and of the neutral complex [Re(pz)(CO)(3)(Hpz)(2]). A single-crystal X-ray analysis of the [NEt4](+) Salt of 4 showed that the dinuclear anion contains two fac-Re(CO)(3) units linked by one hydride and two pyrazolato bridging Ligands, with the two mu-eta(2)-pz units almost orthogonal to each other. A more selective route to anion 3 is provided by the reaction of the 46 valence electron complex [Re-3(mu-H)4(CO)(9)(Hpz)](-) (6) with Hpz. On using pyrazole deuterated in the nitrogen site. HD evolution has been demonstrated. The treatment of 6 with MeCN affords, even if very slowly, another derivative containing a bridging pyrazolate, namely the anion [Re-3(mu-H)(3)(mu-eta(2)-pz)(CO)(9)(NCMe)](-), characterized spectroscopically. Also, the unsaturated anion [Re-3(mu-H)(4)(CO)(9)(PMe2Ph)](-) reacts with Hpz, giving two isomers of the [Re-3(mu-H)(3)(mu-eta(2)-pz)(CO)(9)(PMe2Ph)](-) anion (with the phosphine in an axial and equatorial position, respectively). The reactions rates of the 46 valence electron complexes [Re-3(mu-H)(4)(CO)(9)L](-) with pyrazole increase on increasing the electron density on the cluster (in the order L = Hpz > PMe2Ph > CO), suggesting that the rate is controlled by the interaction of a "hydridic" H ligand with the acidic N-H bond of pyrazole. In no case has activation of the ortho C-H bond of pyrazole been observed.