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 co ntaining a bridging orthometalated pyridine moiety reacts with strong acids, in the presence of acetonitrile, to give the cisoid and transoi d diaxial isomers of [Re-3(mu-H)(3)(CO)(10)(NCMe)(Py)] (4a and 4b) con taining a terminally bound pyridine. The major isomer 4a, identified a s the transoid species, is thermodynamically favoured in solvents of l ow 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 vari es 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-positi on of the terminally bound pyridine. The protonation has also been per formed in the absence of added ligands, with acids with weakly coordin ating anions (CF3SO3H or HBF4/Et(2)O): the NMR data suggest the format ion of derivatives analogous to the isomers 4, in which the nitrile li gand is replaced by very poor donors, such as CF3SO3-, BF4- or Et(2)O. Successive substitution reactions indicate the following thermodynami c 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 les s than MeCN [ Cl-, Py. Under CO atmosphere, both the isomers 4 give th e 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 h as 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 f aster than with CO, and in a few minutes both the isomers give the pre viously known trans-diaxial [Re-3(mu-H)(3)(CO)(10)(Py)(2)] (7a) comple x. 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 concentr ation. A kinetic cis/trans ratio higher than the thermodynamic one has been found both for 4 and 7, in all the reactions studied.