Manganese(I) selenoether chemistry: synthesis, multinuclear NMR studies and the structures of [MnCl(CO)3(MeSeCH2CH2SeMe)], [MnCl(CO)3(MeSeCH2CH2CH2SeMe)] and [MnBr(CO)3{C6H4(SeMe)2-o}]

Abstract

Reaction of [Mn(CO)₅X] (X = Cl, Br or I) with RSe(CH₂)_nSeR (R = Me or Ph, n = 2; R = Me, n = 3) or C₆H₄(SeMe)₂-o in refluxing CHCl₃ yielded the neutral manganese(II) complexes [MnX(CO)₃{RSe(CH₂)_nSeR}] or [MnX(CO)₃{C₆H₄(SeMe)₂-o}] as yellow or orange solids. Infrared spectroscopic studies confirmed the fac-tricarbonyl arrangement. Proton, ¹³C-{¹H}, ⁷⁷Se-{¹H} and ⁵⁵Mn NMR spectroscopy has been used to probe the solution behaviour, and show that only those compounds involving PhSeCH₂CH₂SePh are undergoing rapid pyramidal inversion on the NMR timescale, with the individual NMR distinguishable invertomers observed for the other complexes (meso-1, meso-2 and DL) in varying ratios. X-Ray crystallographic analyses on three examples confirmed a fac-tricarbonyl arrangement, with the diselenoether ligand chelating: [MnCl(CO)₃(MeSeCH₂CH₂SeMe)] and [MnCl(CO)₃(MeSeCH₂CH₂CH₂SeMe)] adopt the DL arrangement, while [MnBr(CO)₃{C₆H₄(SeMe)₂-o}] is in the meso-2 form in the solid state. These are the first structure determinations on selenoether complexes of manganese(I) carbonyl halides. Most importantly, ⁵⁵Mn NMR spectroscopic studies show that δ(⁵⁵Mn) is to low frequency of those of the corresponding thioether compounds, lying in the range δ –175 to –702, the lowest frequencies occurring for the iodo derivatives. The CO stretching frequencies and ⁵⁵Mn NMR shifts show that, for a given X, the manganese(I) centre in [MnX(CO)₃(diselenoether)] is more shielded than in [MnX(CO)₃(dithioether)], possibly indicating increased σ donation in the former.