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

The reaction of the unsaturated anion [Re3(μ-H)4(CO)10]- with pyrazole (used as solvent at 80°C) proceeds in a stepwise manner, at first giving the [Re3(μ-H)3(μ-η2-pz)(CO) 10]- anion (2, pz = pyrazolate), with H2 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 [Re3(μ-H)3(μ-η2-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 2. 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 π-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 π-electron cloud; (ii) a 2D NOESY experiment has shown a dipolar interaction between Hα of the pyrazolate and the N-H protonic resonance (and, to a lesser extent, Hα 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 (ID and 2D) experiments. Further reacting 1 in molten pyrazole causes fragmentation of the triangular cluster moiety with formation of the novel dinuclear anion [Re2(μ-H)(μ-η2-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 hydrido and two pyrazolato bridging ligands, with the two μ-η2-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 [Re3(μ-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 [Re3(μ-H)3(μ-η2-pz)(CO) 9-(NCMe)]-, characterized spectroscopically. Also, the unsaturated anion [Re3(μ-H)4(CO)9(PMe2-Ph)] - reacts with Hpz, giving two isomers of the [Re3(μ-H)3(μ-η2-pz)(CO) 9(PMe2Ph)]- anion (with the phosphine in an axial and equatorial position, respectively). The reactions rates of the 46 valence electron complexes [Re3(μ-H)4(CO)9L]- 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.