Stereoselectivity in the Formation of a Cyclic Trinuclearcis-Dioxomolybdenum(VI) Complex of a Chiral Siderophore Analogue

Abstract

The stereoselective synthesis of coordination compounds can be achieved by using enantiomerically pure ligands, which, on metal binding, transfer their chiral information to the metal center. [1] Siderophores, high-affinity chelators synthesized by microorganisms to solubilize iron(iii) for uptake into the cell, represent a biologically important example of this ligand type. [2] Most siderophores are enantiomerically pure amino acid derivatives, and the chirality of the molecules can play an important role in their biological activity. This is well documented for enterobactin, the native siderophore of Escherichia coli: Only the naturally occurring complex, having an l-seryl ligand backbone and a D configuration at the metal center, is able to supply iron to the cells. [3,4] Interestingly, the N 2 -fixing cells of Azotobacter vinelandii secrete azotochelin (Scheme 1), a tetradentate iron Scheme 1. siderophore which also binds molybdenum. [5, 6] The structures of relevant model complexes have been reported. [7,8] Molybdenum, as well as iron, is a component of the conventional nitrogenase, and in A. vinelandii the siderophore production depends not only on the amount of iron in the environment but also on the concentration of molybdenum. [5b, 6b] We report here that the l-lysine derivative azotochelin shows a high diastereoselectivity in forming the two possible D and L cisdioxomolybdenum complexes. Use of the azotochelin analogue H 4 -(1R,2R)-dachcam (Scheme 1) allowed us to investigate the chiral induction on molybdenum binding in the solid state and to compare the CD spectra of the model complex Further study of the magnetic properties of these irregular alternating systems are planned but a general law may be easily envisaged for homometallic systems with sequences of more than one F/AF interaction. Ferromagnetism emerges, as observed for 2, if 2n1 AF interactions are followed by one or more F interactions; the ground state here is always S 0. An alternative possibility consists of an even number of AF interactions (2n) followed by one or more (m) F interactions, as observed for 1, which induces net ferrimagnetic behavior with ground state S T m S for each (2nm) local S spin carrier. 3 ], Z 6 [10], 1 calc 1.451 g cm À3 [1.593 g cm À3 ], crystal size 0.15 Â 0.28 Â 0.50 mm [0.30 Â 0.60 Â 1.0 mm], m 0.893 mm À1 [0.807 mm À1 ]. Of 4887 [17 574] reflections collected, 3164 [6593] were observed with I > 2s(I); 289 [612] parameters. R(F) 0.0363 [0.0641], wR(F 2 ) 0.0943 [0.1861], min./max. residual electon density 1 À 0.22/0.21 e À3 [À 1.94/ 1.80 e À3 ]. The data were collected on a modified STOE [IPDS] diffractometer with Mo Ka irradiation 0.71069 [0.71073 ] at room temperature. Lorentz polarization effects, intensity decay, and absorption (DIFABS) [Lorentz polarization effects and numerical absorption (X-SHAPE, min./max. transmission 0.604/0.803)] corrections were made. Both structures were solved by direct methods and refined by full-matrix least-squares method on F 2 values. Refinement used the SHELXTL/PC packag...

Metal complexes with the quinolone antibacterial agent N-propyl-norfloxacin: Synthesis, structure and bioactivity

“…For all the metal ions we used a default parameter set with R * = 2.20 Å and e * = 0.02 kcal mol À1 , since their van der Waals interactions are not expected to play a major role in such bonded representations as in octahedral complexes. Metal-ligand bond lengths were corrected in the final models using harmonic restraints with equilibrium values that were extracted from crystallographic data of similar compounds [34][35][36][37][38][39][40][41][42][43][44][45][46] (Tables S1, S3, S5). Nonbonded interactions were calculated in vacuo with no cutoff and the electrostatic energy term was treated using bond dipoles.…”

Diastereoselective Synthesis and Catalytic Activity of Two Chiral cis‐Dioxidomolybdenum(VI) Complexes

“…This kind of chelation leads to an overall Δ‐wrapping of the salan ligand around the molybdenum center, consistent with chiral induction from ligand to metal (Scheme ). Such high diastereoselectivities were also observed for other ( R , R )‐enantiopure salan ligands like H 4 L 2 , leading to the Δ‐configuration at the molybdenum atom . Kol and co‐workers reported similar behavior for titanium(IV) complexes, and could also show that the ( S , S )‐configured cyclohexyldiamine ligand gives the opposite Λ‐configuration at the metal atom , , …”

“…Both C 2‐symmetric cis ‐α and C 1‐symmetric cis ‐β configurations for dioxidomolybdenum(VI) complexes have been previously reported, dependent on the employed ligand , , . The 1 H and 13 C NMR spectra of complex 1 showed distinct signals for both ligand half‐units, in accordance with the asymmetric cis ‐β configuration.…”

From Siderophores and Self‐Assembly to Luminescent Sensors: The Binding of Molybdenum by Catecholamides

“…The even shorter and conformationally restricted cyclohexyl linker in H 2 ‐ L 4 (Figure 9) leads to the self‐assembly of a molecular triangle (Figure 13). 64 In this structure, Mo2 lies on a pseudo‐twofold axis of symmetry, with the catecholamide units coordinated in cis,trans,cis orientation. In contrast, Mo1 and Mo3 adopt a cis,cis,cis arrangement in order to enable the formation of a cyclic structure.…”

“…Interestingly, the visible region of the CD spectrum of the cis ‐MoO 2 complex of azotochelin mirrors that of the Λ‐configured diastereomer of [{MoO 2 ( L 4 )} 3 ] 6– . Consequently, the predominant configuration of the molybdenum centre in the siderophore complex can be assigned as Δ,64 the configuration that is also adopted by the iron(III) centre in the biologically active enterobactin complex, which is recognised stereoselectively by its outer membrane receptor protein FepA. The identical charge and similar shape of Δ‐[MoO 2 (azotochelin)] 3– and Δ‐[Fe(enterobacin)] 3– suggest that the molybdenum–azotochelin complex may be recognised by iron–siderophore receptors, although experimental evidence for this hypothesis has not yet been obtained.…”