Structure, reactivity and solution behaviour of [Re(ser)(7-MeG)(CO)3] and [Re(ser)(3-pic)(CO)3]: “nucleoside-mimicking” complexes based on the fac-[Re(CO)3]+ moiety

Abstract

The fac-[Re(CO)₃]⁺ moiety was reacted with the amino acid serine (D- and L-ser) and with 7-methylguanine (7-MeG), 3-methylpyridine (3-pic) or adenine (ade) to yield novel complexes intended as nucleoside-mimicking compounds. Reaction of [Re(H₂O)₃(CO)₃]⁺ (1) with L-ser yields the complex [Re(L-ser)₂(CO)₃] (L-2). X-Ray structure analysis of L-2 reveals that one of the two amino acids is bound to the metal centre in a bidentate fashion while the other amino acid is bound as a zwitterion via the carboxylate oxygen only. Reaction of L-2 and of [Re(D-ser)₂(CO)₃] (D-2) with 7-MeG yields complexes [Re(L-ser)(7-MeG)(CO)₃] (L-3) and [Re(D-ser)(7-MeG)(CO)₃] (D-3) respectively. Complexes L-3 and D-3 are received as a mixture of diastereomers. If 3-pic is used instead of 7-MeG complex [Re(L-ser)(3-pic)(CO)₃] (L-4) is obtained in good yield, while interaction of L-2 with ade gives a mixture of five distinct species. Crystallization gave one single diastereomer for L-3 and D-3 and the two forms for 4 respectively. X-Ray structure analyses reveal that in all cases the amino acid is bound in a chelate fashion with the base occupying the sixth co-ordination site. When crystals of either 2 or 3 are dissolved in a CD₃OD/D₂O mixture (1 : 1, 293 K) rapid transformation to the diastereomeric mixture is observed. While for L-2 this reorganisation is fast on the NMR time scale even at 193 K, the rate constant for the rearrangement of L-3 and D-3 is 1.36 ± 0.24 × 10⁻² s⁻¹ at 293 K.