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η 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 ııı Sandwich Complexes
Author(s) -
Agaid Herebian Diran,
Schmidt Claudia S.,
Sheldrick William S.,
van Wüllen Christoph
Publication year - 1998
Publication title -
european journal of inorganic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.667
H-Index - 136
eISSN - 1099-0682
pISSN - 1434-1948
DOI - 10.1002/(sici)1099-0682(199812)1998:12<1991::aid-ejic1991>3.0.co;2-3
Subject(s) - chemistry , rhodium , medicinal chemistry , aromatic amino acids , cationic polymerization , stereochemistry , chirality (physics) , crystal structure , ligand (biochemistry) , aromaticity , toluene , crystallography , side chain , amino acid , catalysis , molecule , organic chemistry , biochemistry , chiral symmetry breaking , physics , receptor , quantum mechanics , nambu–jona lasinio model , quark , polymer
η 5 ‐Pentamethylcyclopentadienyliridium(III) and ‐rhodium(III) sandwich complexes of the type [(η 5 ‐Cp*)M(η 6 ‐aa)](CF 3 SO 3 ) 2 (M = Ir, Rh; 3–14 ) containing L ‐tyrosine, L ‐tryptophan and L ‐phenylalanine derivatives (aa) can be prepared by treatment of [(η 5 ‐Cp*)ML 3 ] (CF 3 SO 3 ) 2 [L = thf, (CH 3 ) 2 CO, CH 3 CN] with the appropriate bioligand in thf for N ‐protected compounds and in CF 3 COOH for α‐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 acidity of the p ‐hydroxy function. The crystal structure of [Cp*Ir(ActyrOMe)] (CF 3 SO 3 ) 2 ( 3b , ActyrOMe = N ‐acetyltyrosine methyl ester) confirms a marked distortion towards an η 5 ‐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*Ir III sandwich complexes of aromatic α‐amino acids are stable in polar solvents, rapid decay is observed for analogous Cp*Rh III complexes of N ‐unprotected derivatives in polar solvents. Comparative nonrelativistic and relativistic all‐electron density functional calculations on the cationic sandwich complexes [Cp*M(η 6 ‐C 6 H 5 Me)] 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 relativistic enhancement of k 2 for M = Ir (279 vs 207 Nm −1 ) could be responsible for the greater stability of Cp*Ir III complexes in solution.