Reactions of fac-[Re(CO)3(H2O)3]+ with nucleoside diphosphates and thiamine diphosphate in aqueous solution investigated by multinuclear NMR spectroscopy

Products formed between monoester diphosphates (MDPs) and fac-Re(CO)3(H2O)3]OTf at pH 3.6 were examined. Such adducts of the fac-Re(CO)3]+ moiety have an uncommon combination of properties for an "inert" metal center in that sharp NMR signals can be observed, yet the products are equilibrating at rates allowing NMR EXSY cross-peaks to be observed. Thiamine diphosphate (TDP) and uridine 5'-diphosphate (5'-UDP) form 1:1 bidentate {Palpha,Pbeta} chelates, in which the MDP binds Re(I) via Palpha and Pbeta phosphate groups. Asymmetric centers are created at Re(I) (RRe/SRe) and Palpha (Delta/Lambda), leading to four diastereomers. The two mirror pairs of diastereomers (RReDelta/SReLambda) and (RReLambda/SReDelta) for TDP (no ribose) and for all four diastereomers (RReDelta, RReLambda, SReDelta, SReLambda) for 5'-UDP (asymmetric ribose) gave two and four sets of NMR signals for the bound MDP, respectively. 31Palpha-31Palpha EXSY cross-peaks indicate that the fac-Re(CO)3(H2O)({Palpha,Pbeta}MDP)]- isomers interchange slowly on the NMR time scale, with an average k approximately equal to 0.8 s(-1) at 32 degrees C; the EXSY cross-peaks could arise from chirality changes at only Re(I) or at only Palpha. Guanosine 5'-diphosphate (5'-GDP), with a ribose moiety and a Re(I)-binding base, formed both possible diastereomers (RRe and SRe) of the fac-Re(CO)3(H2O)({N7,Pbeta}GDP)]- macrochelate, with one slightly more abundant diastereomer suggested to be RRe by Mn2+ ion 1H NMR signal line-broadening combined with distances from molecular models. Interchange of the diastereomers requires that the coordination site of either N7 or Pbeta move to the H2O site. 31Palpha-31Palpha EXSY cross-peaks indicate a k approximately equal to 0.5 s(-1) at 32 degrees C for RRe-to-SRe interchange. The similarity of the rate constants for interchange of fac-Re(CO)3(H2O)({Palpha,Pbeta}MDP)]- and fac-Re(CO)3(H2O)({N7,Pbeta}GDP)]- adducts suggest strongly that interchange of Pbeta and H2O coordination positions accounts for the EXSY cross-peaks present in the spectra of all adducts.