Laura E. Pence

Alkali metal-responsive geometric and spectral changes in a cobalt(II) complex of a constrained dicarboxylate: a carboxylate shift-mediated metallochromoionophore☆

Abstract A mononuclear Co(II) complex of the convergent dicarboxylate ligand xylenediamine bis-(Kemps triacid imide) (XDK) and neocuproine (neo, 2,9-dimethyl-1,10-phenanthroline) has been prepared both by demetallation of a dinuclear precursor with neocuproine and by direct synthesis. The X-ray structure of this complex, [Co(XDK)(neo)]·3CH3OH (1·dCH3OH) (monoclinic P2 1 /c, a = 13.352(2), b = 16.026(3), c = 23.145(7) A , β = 96.64(2)°, V = 4919(2) A 3 , Z = 4, T = 173 K ) shows the cobalt ion to be in a highly distorted trigonal bipyramidal environment, with asymmetrical binding by the carboxylates of XDK. Complex 1 reacts readily with alkali metal salts to afford heterodimetallic complexes, such as [KCo(XDK)(neo)(PF6)] (2) and [Rb2Co2(XDK)2-(neo)2(H2O)(BPh4)2] (3). The X-ray structures of 2·2CH3CN and 3·2CHCl3 reveal that XDK undergoes a carboxylate shift upon formation of the alkali metal adducts, resulting in a highly unusual distorted trigonal pyramidal geometry at the cobalt ion (2·2CH3CH: monoclinic P2 1 /c, a = 14.261(2), b = 14.275(2), c = 26.113(4) A , β = 100.95(1)°, V = 5219(1) A 3 , Z = 4, T = 188 K . 3·2CHCl3: triclinic, P 1 , a = 12.130(2), b = 16.409(3), c = 17.315(3) A , α = 100.09(2), β = 103.83(1), γ = 96.35(2)°, V = 3252(1) A 3 , Z = 1, T = 213 K ). The alkali metal ions bind to the carboxylate and amide oxygen atoms as well as to the PF6− or BPh4− anion. The alteration in Co(II) geometry upon binding of the alkali metal ions to 1 is reflected by significant changes in its visible spectrum. These changes permit the reactions to be monitored spectroscopically, from which their 1:1 stoichiometries were clearly evident.