η5-Pentamethylcyclopentadienyliridium(III) and -rhodium(III) Labeling of Amino Acids with Aromatic Side Chains – The Importance of Relativistic Effects for the Stability of Cp*Irııı Sandwich Complexes

Abstract

η5-Pentamethylcyclopentadienyliridium(III) and -rhodium(III) sandwich complexes of the type [(η5-Cp*)M(η6-aa)](CF3SO3)2 (M = Ir, Rh; 3–14) containing L-tyrosine, L-tryptophan and L-phenylalanine derivatives (aa) can be prepared by treatment of [(η5-Cp*)ML3] (CF3SO3)2 [L = thf, (CH3)2CO, CH3CN] with the appropriate bioligand in thf for N-protected compounds and in CF3COOH for α-amino acids with unprotected amino groups. Coordination to the Cp*MIII fragments stabilizes the ketonic form of the tyrosine aromatic side chains, leading to a marked enhancement in the acidity 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 η5-oxohexadienyl coordination mode as may be gauged from the tilting of the p-OH plane C13/C14/C15 by no less than θ = 12.9° from that of the remaining ring atoms. Facial isomers are present in an effective 1:1 ratio for all tryptophan derivatives. Whereas the Cp*IrIII sandwich complexes of aromatic α-amino acids are stable in polar solvents, rapid decay is observed for analogous Cp*RhIII complexes of N-unprotected derivatives in polar solvents. Comparative nonrelativistic and relativistic all-electron density functional calculations on the cationic sandwich complexes [Cp*M(η6-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 (k1 > k2). A much larger relativistic enhancement of k2 for M = Ir (279 vs 207 Nm−1) could be responsible for the greater stability of Cp*IrIII complexes in solution.