STUDIES OF DISTORTIONAL ISOMERS .2. EVIDENCE THAT GREEN [LWOCL(2)]PF6 IS A TERNARY MIXTURE

Studies of the ''green isomer of [LWOCl(2)]PF6'' (L = 1,4,7-trimethyl-1,4,7-triazacyclononane) have revealed that this material is not a distortional isomer of blue [LWOCl(2)]PF6 but rather is a ternary mixture of blue [LWOCl(2)]PF6, a tungsten(IV)-oxo species, and a tungsten(VI)-dioxo species characterized by a cis-WO22+ unit. Tungsten 4f X-ray photoelectron spectroscopy (XPS) has shown the green material prepared according to literature methods to be a variable mixture of these three tungsten oxidation states. Blue [LWOCl(2)]PF6 is implicated as the single paramagnetic tungsten(V) component in the green material. Identical EPR spectra were recorded for frozen solutions of blue [LWOCl(2)]PF6 and of the supposed green isomer, showing a rhombic d(1) system (g(1) = 1.869, g(2) = 1.811, g(3) = 1.761; A(1) = 57.4 X 10(-4) cm(-1), W-183). Broad resonances due to blue [LWOCl(2)]PF6 were observed in the proton NMR spectrum of the green material. Visible spectra of the green material revealed two absorptions at 419 and 695 nm, consistent with those reported previously; however, the relative intensities of these peaks were found to vary with reaction time, indicating the composite nature of the green material. Visible spectroscopy also confirmed the rhombic system for blue [LWOCl(2)]PF6, whose broad peak previously reported at 715 nm was resolved into two overlapping peaks centered at 722 and 6 18 nm, respectively. Infrared analyses of the mixed green material characterized the tungsten(VI) component as a cis-WO22+ species from a pair of distinct peaks at 960 and 923 cm(-1). The relative intensity of these and the nu(W=O) peak of blue [LWOCl(2)]PF6 (981 +/- 2 cm(-1); confirmed with O-18 substitution by IR and Raman) varied with reaction time. Cyclic voltammetry of the green material revealed several solvent-dependent waves consistent with its multicomponent character and solvent-dependent equilibria involving the components in solution. Column chromatography separated some of the blue [LWOCl(2)]PF6 from the green material, but additional chromatography yielded no further separation of the remaining green mixture. Competitive solution equilibria are invoked to explain this unusual chromatographic behavior. These combined results refute the existence of distortional isomerism in [LWOCl(2)]PF6, one of the most widely cited examples of this phenomenon in transition-metal complexes.