Iminopyridine Complexes of Manganese, Rhenium, and Molybdenum Derived from Amino Ester Methylserine and Peptides Gly-Gly, Gly-Val, and Gly-Gly-Gly: Self-Assembly of the Peptide Chains

Complexes containing pyridine-2-carboxaldehyde (pyca)
ligand acting
as κ2-(N,O) chelates in [MX­(CO)3(pyca)]
(M = Mn, Re; X = Cl, Br), or [MoX­(methallyl)­(CO)2(pyca)]
(X = Cl, Br), are good precursors for iminopyridine complexes derived
from amino esters and peptides of formula [MX­(CO)3(py-2-C­(H)NCHX-COOY)]
or [MoX­(methallyl)­(CO)2(py-2-C­(H)NCHX-COOY)], via
Schiff condensation of the aldehyde function of pyca with the terminal
NH2 group of the amino ester or peptide. X-ray determinations
confirm the structures and show that in solid phase the peptide chains
assemble through H-bonds adopting different patterns which depend
on the geometry of the metal–ligand fragments. The H-bonding
patterns have been analyzed in detail and described by using graph
set methods. In most cases, Mo complexes show intramolecular arrangement
involving the halogen (Cl or Br) and an NH group of the side chain.
For the Mn and Re complexes, the peptide side arms form infinite chains,
helices, and rings. In many cases, the terminal carboxylic O–H
function is engaged in a “terminal” H-bond with a polar
molecule of solvent (THF or acetone), instead of forming the usual
head-to-head arrangement found in simple carboxylic acids. For the
longer tripeptide Gly-Gly-Gly, a discrete, dimeric association is
observed, in which the peptide chains show antiparallel arrangement
with a complementary disposition of the internal NH and CO
functions. DOSY experiments in solution show significant changes in
the diffusion rates upon addition of OPBu3, which indicate
H-bonding interaction of OPBu3 with the peptide hydrogens.