Protonation reactions of the triangular cluster anion [Re-3(mu-H)(3)(mu-NC5H4)(CO)10](-) containing an orthometallated pyridine molecule

The [Re-3(mu-H)(3)(mu-NC5H4)(CO)(10)](-) triangular cluster anion 2 containing a bridging orthometalated pyridine moiety reacts with strong acids, in the presence of acetonitrile, to give the cisoid and transoid diaxial isomers of [Re-3(mu-H)(3)(CO)(10)(NCMe)(Py)] (4a and 4b) containing a terminally bound pyridine. The major isomer 4a, identified as the transoid species, is thermodynamically favoured in solvents of low polarity (4a/4b equilibrium ratio, at 295 K, 1.9 in MeCN, 2.5 in Me(2)CO, 4.9 in CH2Cl2). The rate of interconversion of the isomers varies with the solvent, ruling out an intramolecular process. Experiments with deuterated isotopomers of 2 have shown that the added proton is distributed among two of the three hydridic sites and the ortho-position of the terminally bound pyridine. The protonation has also been performed in the absence of added ligands, with acids with weakly coordinating anions (CF3SO3H or HBF4/Et(2)O): the NMR data suggest the formation of derivatives analogous to the isomers 4, in which the nitrile ligand is replaced by very poor donors, such as CF3SO3-, BF4- or Et(2)O. Successive substitution reactions indicate the following thermodynamic scale of nucleophilicity towards the vacant site of the Re-3(mu-H)(3)(CO)(10)(Py) fragment: BF4-, Et(2)O, H2O [ CF3SO3- [ Me(2)CO much less than MeCN [ Cl-, Py. Under CO atmosphere, both the isomers 4 give the novel neutral complex [Re-3(mu-H)(3)(CO)(11)(Py)] (k(obs)(I) = 4.21(2) x 10(-5)s(-1), at 300 K, in CO saturated acetone solution), which has been also obtained by protonation of the previously known [Re-3(mu-H)(2)(CO)(11)(Py)](-) anion. The reaction of 4 with pyridine is much faster than with CO, and in a few minutes both the isomers give the previously known trans-diaxial [Re-3(mu-H)(3)(CO)(10)(Py)(2)] (7a) complex. The same complex is obtained by reacting trans-diaxial [Re-3(mu-H)(3)(CO)(10)(NCMe)(2)] with pyridine, the monosubstitution derivatives 4 being present only as intermediates, with a low steady state concentration. A kinetic cis/trans ratio higher than the thermodynamic one has been found both for 4 and 7, in all the reactions studied.