Stephen P. Watton

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.

Methane Monooxygenase: Models and Mechanism

There is much current interest in non-heme diiron carboxylate proteins. Included are the hydroxylase enzyme of methane monooxygenase (MMO), hemerythrin, ribonucleotide reductase, and purple acid phosphatase, all of which contain a dinuclear iron center at their active site. We ultimately desire an understanding of how these units are tuned in each protein to exhibit diverse functions ranging from the reversible binding of dioxygen in hemerythrin to activation of dioxygen for converting methane to methanol in MMO. In pursuit of this objective, we are investigating the proteins of the MMO system and exploring the fundamental chemistry of the hydroxylase diiron center. In the present article we review some of our recent progress in this area.