eta(5)-Pentamethylcyclopentadienyliridium(III) and -rhodium(III) labeling of amino acids with aromatic side chains - The importance of relativistic effects for the stability of Cp*Ir-III sandwich complexes

eta(5)-Pentamethylcyclopentadienyliridium(III) and -rhodium-(III) sandwich complexes of the type [(eta(5)-Cp*)M(eta(6)-aa)]-(CF3SO3)(2) (M = Ir, Rh; 3 -14] containing L-tyrosine, L-tryptophan and L-phenylalanine derivatives (a a) can be prepared by treatment of [(eta(5)-Cp*)ML3] (CF3SO3)(2) [L = thf, (CH3)(2)CO, CH3CN] with the appropriate bioligand in thf for N-protected co mpounds and in CF3COOH far alpha-amino acids with unprotected amino groups. Coordination to the Cp*M-III fragments stabilizes the ketonic form of the tyrosine aromatic side chains, leading to a marked enhancement in the acidi ty of the p-hydroxy function. The crystal structure of [Cp* Ir(ActyrOMe)] ( CF3SO3)(2) (3b, ActyrOMe = N-acetyltyrosine methyl ester) confirms a marked distortion towards an eta(5)-oxohexadienyl coordination mode as may be gau ged from the tilting of the p-OH plane C13/C14/C15 by no less than theta = 12.9 degrees from that of the remaining ring atoms. Facial isomers are pres ent in an effective 1:1 ratio for all tryptophan derivatives. Whereas the C p*Ir-III sandwich complexes of aromatic cr-amino acids are stable in polar solvents, rapid decay is observed for analogous Cp*Rh-III complexes of N-un protected derivatives in polar solvents. Comparative nonrelativistic and re lativistic all-electron density functional calculations on the cationic san dwich complexes [Cp*M(eta(6)-C6H5Me)](n+) (n = 2, M = Ir, Rh; n = 1, M = Ru ) confirm that all three metals bind more tightly to Cp* than to toluene as gauged by the respective force constants (k(1) > k(2)). A much larger rela tivistic enhancement of k(2) for M = Ir (279 vs 207 Nm(-1)) could be respon sible for the greater stability of Cp*Ir-III complexes in solution.