The Mo(η-allyl)(CO)2 moiety as a robust marker group in bioorganometallic chemistry.: Unusual crystal structure of the phenylalanine derivative Mo(C5H4-CO-Phe-OMe)(η-allyl)(CO)2

The MoCp(eta-C3H5)(CO)(2) (Cp = eta-cyclopentadienyl moiety is introduced as a new labeling group in bioorganometallic chemistry. The acid Mo(C5H4-CO2H)(eta-C3H5)(CO)(2) (2) was obtained from the reaction of MoCp(eta-C3H5)(CO)(2) (1) with BuLi and solid CO2 followed by aqueous workup. Coupling of 2 to amino acids with various complexity and C-terminal functionality by standard peptide chemistry methods yielded the amino acid derivatives Mo(C5H4-CO-AA-R)(eta-C3H5)(CO)(2), 3 (3a, AA = Phe, R = OCH3; 3b, AA= Leu, R = NH2; 3c, AA = Gly, R = OCH3). In addition, the dipeptide derivative Mo(C5H4-CO-Leu-Phe-OCH3)-(eta-C3H5)(CO)(2) (4) was synthesized by reacting 2 with H-Leu-Phe-OCH3. All new compounds are characterized by elemental analysis, IR, MS, and NMR spectroscopy. X-ray analysis on 3a shows the unit cell to contain two independent molecules, A and B, which differ mainly lay the orientation of the allyl and carbonyl groups with respect to the amino acid substituent on the Cp ring. Furthermore, an allyl-endo conformation for both A and B is observed. This is the first example of such a conformation in the crystal structure of a MoCp(C3H5)(CO)(2) derivative. In solution, both the exo and endo isomer are present, as concluded from H-1 NMR spectroscopy approximately in a 4:1 ratio. The activation barriers of interconversion were determined to be 62.7 +/- 0.5 kJ mol(-1) (3a) and 60.5 +/- 0.5 kJ mol(-1) (3c).