Synthesis, characterization and investigation of molecular motion by variable temperature 1H NMR of the bis(quadridentate)zirconium(IV) complexes bis( N,N′-disalicylidene- cis-1,2-diaminocyclohexanato)zirconium(IV) and bis( N,N′-disalicylidene- trans-1,2-diaminocyclohexanato)zirconium(IV), and comparisons with bis( N,N′-disalicylidene-1,2-phenylenediaminato)zirconium(IV). Crystal and molecular structure of the racemic Zr( trans-dsd) 2 and Zr( S,S-trans-dsd) 2 complexes

Abstract

Variable temperature 1H NMR over the range from 165.9 to 19.9 °C in d 6-dimethyl sulfoxide and 22.1 to −95.4 °C in d 2-methylene chloride is employed to investigate the molecular motions in the eight-coordinate zirconium complexes bis( N,N′-disalicylidene- cis-1,2-diaminocyclohexanato)zirconium(IV), Zr( cis-dsd) 2; bis( N,N′-disalicylidene- trans-1,2-diaminocyclohexanato)zirconium(IV), Zr( trans-dsd) 2; and bis( N,N′-disalicylidene-1,2-phenylenedi-aminato)zirconium(IV), Zr(dsp) 2. For Zr( cis-dsd) 2, the resolution of one methine signal into separate axial (upfield) and equatorial (downfield) signals at low temperature is consistent with the chair-to-chair interconversion of the fused cyclohexane ring becoming slow on the NMR time scale. A coalescence temperature of c. −47 °C and a thermodynamic barrier of Δ G ≠ (298)=46.5± 0.9 kJ mol −1, Δ H ≠=27±3 kJ mol −1 and Δ S ≠=−66±13 J mol −1 K −1 are obtained for this process. The resolution of six imine signals at −95.4 °C from two ambient temperature imine signals is attributed to slow interconversion of isomers which arise from different ligand wrapping geometries, cyclohexane ring conformations, and/or polytopal coordination geometries. For Zr( trans-dsd) 2, the resolution of a second set of salicylidene NMR signals as the temperature is decreased is likewise attributed to the slow interconversion of isomers. However, since only one set of averaged signals is observed at high temperature, these isomers must possess symmetrical salicylidene moieties. For Zr(dsp) 2, comparable changes were not observed over the temperature range studied, despite the fact that both dsp(2−) and dsd(2−) ligands are capable of considerable non-planarity. The two new zirconium complexes, Zr( cis-dsd) 2 and Zr( trans-dsd) 2, are synthesized from Zr(O-n-C 4H 9) 4 butanol solvate and Schiff bases made from cis- and trans-diaminocyclohexane (DACH), and characterized by elemental analysis, IR, UV-Vis, 1H and 13C NMR (the latter able to distinguish Zr( R,R-trans-dsd)( S,S-trans-dsd) from other Zr( trans-dsd) 2 isomers), thermogravimetric analysis, optical rotation for the separate enantiomers Zr( R,R-trans-dsd) 2 and Zr( S,S-trans-dsd) 2, and single crystal X-ray diffraction analysis for Zr( trans-dsd) 2. Hitherto unpublished 13C NMR results for Zr(dsp) 2 are included in order to facilitate making assignments. Details of the crystallography include: formula C 40H 40N 4O 4Zr, space group Fddd with a=15.232(3), b=16.994(4), c=26.711(5) Å, V=6914(2) Å 3, Z=8, and D calc=1.406 g cm −3; refinement of 152 variables using 2283 reflections collected (1820 independent and_observed) converge at R=3.84%. Details of the crystallography for the Zr( S,S-trans-dsd) 2 isomer, space group P 1 , can be obtained. Testing of the new complexes for anticancer activity was performed by Bristol-Myers and was negative.