Isomers of Re(CO)3(CNtBu)LX: Synthetic strategies starting from MnRe(CO)8(CNtBu)L and Re(CO)4LX (X  halogen; L Group 15 donor ligand)

Reaction of Re(CO)4LX (L  PMePh2, PMe2Ph, PPh3, P(OMe)3, P(OiPr)3, P(O-o-tol)3; X  Br, I) with tBuNC in the presence of PdO catalyst gave the new complexes Re(CO)3(CNtBu)LX in high yield (> 60%). The new complexes were shown by spectroscopic techniques (IR, 1H and 31P NMR) to comprise a mixture of mer and fac isomers. The mer/fac ratio decreased with reaction temperatures (e.g. L  P(OMe)3; 10°C, ratio = 3; 45°C, ratio = 0.25). At high temperatures (90°C) isomerization of the mer to the fac isomers (L  PMe2Ph, P(OMe)3) occurred, suggesting that the mer isomer was the kinetic product of the catalysed reaction. Reactions induced by Me3NO gave similar effects. Halogen cleavage of MnRe (CO)8(CNtBu)L, prepared from MnRe(CO)9(CNtBu) and L in the presence of Me3NO, yielded either one or two isomers of Re(CO)3(CNtBu)(L)X (X  I, Br; 40% yield). For large L (e.g. PPh3) a new mer isomer with L trans to X was synthesized, and characterized by IR and NMR spectroscopy. For small L (e.g. P(OMe)3) a mixture of the two different mer products was obtained. The product isomer ratio was determined predominantly by the position of L in the starting dimer.