Anomalous stereochemistry of pyrazolato-3,5-dicarboxylato-bridged dinuclear chromate(III) complexes containing ethylenediamine-N,N '-dicarboxylates with entrapped unstable conformations: X-ray structure of Na[Cr-2(eddp)(mu-pzdc)]center dot 6H(2)O

Several new pyrazolato-3,5-dicarboxylato (pzdc) bridged dinuclear chromate( III) complexes containing linear tetradentate O-N-N-O type ligands were syn thesized and structurally characterized. Among them, the X-ray structure of the eddp complex Na[Cr-2(eddp)(mu -pzdc)]. 6H(2)O (eddp = ethylenediamine- N,N'-dipropionate) was determined to have a (sym-cis)-(unsym-cis) geometric al configuration with intramolecular three-center hydrogen bonds, entrappin g the unfavored sym-cis configuration for the Cr(eddp) moiety as well as th e favored unsym-cis one. As a pair of positional disorders, there were also found to be two conformational isomers with respect to the absolute config urations of the coordinated asymmetric nitrogen atom at the G (in-plane) ri ng for the unsym-cis moiety. Moreover, chiral pzdc-bridged dinuclear comple xes with another type of O-N-N-O ligand, 1,2-cyclohexanediamine-N,N'-diacet ate (cdda), were successfully synthesized, isolated, and characterized by c olumn chromatographic behavior, elemental analysis, and chiroptical spectra . There were two diastereomers for Na[(R,R-cdda)Cr(mu -pzdc)Cr(S, S-cdda)] and only one isomer for Na[(R,R-cdda)Cr(mu -pzdc)Cr(R,S-cdda)] and Na[(R,R- cdda)Cr(mu -pzdc)Cr(edda)] (R,R- or S,S- and R,S-cdda = R,R-trans- or S,S-t rans- and R,S-cis-1,2-cyclohexanediamine-N,N'-diacetate, and edda = ethylen ediamine-N,K-diacetate). From their circular dichroism (CD) spectra, these complexes could exhibit the Delta-Delta absolute configuration with ((sym-c is-R,R-cdda)-(unspm-cis-edda or S,S- or R,S-cdda)) geometrical configuratio n, indicating the abnormal eq-eq (N-C-eq) configuration for the R,R-cdda. T he comparison among the CD spectra of the ((cdda)-(cdda)) complexes reveale d that two diastereomers of the ((R, R-cdda)- (S, S-cdda)) complex correspo nd to the conformational isomers resulting from the difference in geometric al orientations of the secondary amine protons on two coordinated asymmetri c nitrogen atoms with the opposite absolute configuration in the unsym-cis- S,S-cdda moiety. Tn a series of the pzdc-bridged cr(III) complexes the anom alous conformations in two different geometrical configurations could be en trapped probably owing to stereognostic coordination through the intramolec ular N-H . . .O hydrogen bond interaction.