The co-ordination of small molecules by manganese(II) phosphine complexes. Part 1. The reversible co-ordination of dioxygen by [MnX2(PR3)](X = Cl, Br, or I; R3= Bun 3 or PhBun 2) complexes in solution and an infrared criterion for predicting dioxygen binding activity in the di-isothiocyanato(phosphine)manganese(II) complexes

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Abstract
The reversible binding of dioxygen by the novel manganese(II) phosphine complexes [MnX2(PR3)](X = Cl, Br, or I; R3= Bun 3 or PhBun 2) in solution in tetrahydrofuran (thf), toluene, and 1,2-dichloroethane has been studied. Quantitative absorption of dioxygen to form [MnX2(PR3)(O2)] complexes is observed, and gas chromatographic methods have been used to measure desorption. Repeated cycling of dioxygen absorption has been shown to be possible, e.g. [MnI2(PBun 3)] absorbed and desorbed dioxygen in thf at –20 °C more than 400 times without any complex deterioration. Colourless solutions of [MnX2(PR3)] become intensely coloured as [MnX2(PR3)(O2)] forms, and changes in εmax. With p(O2) employed allow construction of dioxygen binding curves. Using the Hill equation, equilibrium data have been deduced. Tentative conclusions have been drawn relating the nature of the phosphine, the halide, and the solvent to the ability of the [MnX2(PR3)] complexes to bind dioxygen. The solid-state dioxygen binding by bis(isothiocyanato)(phosphine)manganese(II) complexes [Mn(NCS)2(PPh3–nRn)](n= 0–3; R = alkyl) have been studied and only the [Mn(NCS)2(PR3)] compounds are found reversibly to bind dioxygen. The compounds are characterised by their different i.r. spectra [ν(CN)] and magnetic moments to those of the inactive compounds. The phosphine oxide complexes [Mn(NCS)2(OPR3)](OPR3= OPPhMe2, OPPrn 3, or OPBun 3) have also been synthesised. Their i.r. and magnetic properties are similar to those of the inactive phosphine complexes and they do not bind dioxygen.
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