The chemistry of 1,4-diazepane-based ligands is attracting increasing attention since these ligands have strong binding capabilities with different metal ions, including main group metals, transition metals and lanthanides (Comba et al.
; Ge et al.
; Peralta et al.
; Rey et al.
). In particular, it has been proposed that gadolinium(III) complexes of 6-amino-6-methylperhydro-1,4-diazepinetetraacetic acid (AAZTA) ligands are good candidates as MRI (magnetic resonance imaging) contrast agents due to their good thermodynamic stability, kinetic inertness and high relaxivity at neutral pH (Aime et al.
Recently, we reported the syntheses and characterization of new bifunctional AAZTA ligands with hydroxy side chains, and the crystal structures of their gadolinium and europium complexes (Sengar et al.
). The title compound, (I)
, was used as a precursor for one of the bifunctional ligands with a hydroxy arm and the complete synthesis and spectroscopic characterization of (I)
was provided (Sengar et al.
). We report herein the molecular structure of compound (I)
determined by X-ray crystallography (Fig. 1).
Figure 1 The molecular structure of (I), showing one of the molecules in the unit cell, with displacement ellipsoids drawn at the 50% probability level. H atoms have been omitted for clarity.
crystallizes with two independent molecules in the asymmetric unit and these exhibit similar conformations. The r.m.s. deviation of the two molecules based on a fit of all non-H atoms is 0.353 Å, calculated by PLATON
). The maximum deviations were observed for the tert
-butyl ester groups. As expected, compound (I)
contains two rings, viz.
a seven-membered 1,4-diazepane ring with a twisted-chair conformation and a six-membered morpholine ring which adopts a boat conformation (Fig. 1). For the seven-membered ring, the approximate plane can be defined by N4/C29/N5/C27, with atoms C30, C43 and C28 deviating above and below this plane. For the six-membered ring, atoms C44 and C46 occupy the prow and stern positions, respectively. The two rings are connected to each other via
spiro atom C43 in a near orthogonal fashion; the dihedral angles for the N5/C30/C43/N6 and N4/C27/C43/C44 planes are –87.0 (2) and −71.9 (2)°, respectively [−85.6 (2) and –72.8 (2)°, respectively, for the other molecule in the unit cell].
The diazepane and morpholine rings in the two molecules have slightly different puckering parameters (Cremer & Pople, 1975
), as given in Table 1. The puckering parameters for the seven-membered ring in (I)
are different from the values for the AAZTA chelate (Table 1) or the protonated diazepane (daza-3HCl-3H2
O) chelate (Romba et al.
). This shows that a large degree of flexibility is associated with the diazepane ring, which gives rise to the different puckering parameters for noncoordinated ligands. However, upon metal coordination, the diazepane ring adopts a pseudo-chair conformation and all three N atoms coordinate to the metal ion in a facial mode (Aime et al.
Puckering parameters for (I) and the AAZTA chelate