Beard, C.D., Carr, L., Davis, M.F., Evans, J., Levason, W., Norman, L.D., Reid, G. and Webster, M. (2006) Studies on chromium(III) and vanadium(III) complexes with crown ether and crown thioether coordination - Synthesis, properties and structural systematics. European Journal of Inorganic Chemistry, (21), 4399-4406. (doi:10.1002/ejic.200600573).
Abstract
A series of six-coordinate chloro Cr-III and V-III complexes involving crown ether, crown thioether and mixed ether/thioether crowns, [MCl3(crown)] (M = Cr or V, crown = 12-crown-4, 15-crown-5,18-crown-6, [12]aneS(4), [15]aneS(5), [9]aneS(2)O, [15]aneS(2)O(3), [18]aneS(3)O(3)) has been prepared by the reaction of [MCl3(thf)(3)] with rigorously dried crown in anhydrous CH2Cl2. In the presence of small amounts of water the monoaquo species [MCl3(H2O)(15-crown-5)] and [MCl3(H2O)(18-crown-6)] are obtained. The products have been characterised by IR and UV/Visible spectroscopy, microanalyses and for [CrCl3(H2O)(18-crown-6)], [VCl3(H2O)(15-crown-5)] and [CrCl3([15]aneS(5))], by X-ray crystallography. The unexpected affinity of specifically the 15-crown-5 and 18-crown-6 complexes to pick up H2O is rationalised in terms of the strain within the two adjacent five-membered chelate rings in the anhydrous (kappa(3)-coordinated) species, and a wider survey of the structures reported for six-coordinate M-III crown complexes (M = Sc - Cr) with RECH2CH2ER (E = 0 or S) reveals that while the S-M-S angles in five-membered chelate rings are typically around 82 degrees, those involving O-M-O are very substantially more acute at around 75 degrees. Thus, the [MCl3 (kappa(3)-crown ether)] complexes are much less stable and can alleviate some of this significant ring strain by switching to kappa(2)-crown coordination with the H2O ligand completing the six coordination. In contrast, the crown thioether complexes have much less tendency to do this and hence appear to be more stable. Furthermore, using the structurally related (dimethylene linkages) mixed thia/oxa crowns to probe the M-O vs. M-S binding competitively, strongly indicates that the Cr-III and V-III preferentially coordinate to the thioether rather than the ether donor atoms, contrary to normal expectation based upon HSAB theory.
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