Anionic and cationic Bi- and tri-nuclear tert-butylimido complexes of manganese-(V) and -(VI)

Abstract

Dimeric anionic complexes have been obtained by reduction of MnCl(NBu^t)₃ or [Mn(NBu^t)₂(µ-NBu^t)]₂1 with lithium powder in 1,2-dimethoxyethane (dme) to give [Li(dme)]₂[{Mn(NBu^t)₂(µ-NBu^t)}₂]2, or with sodium amalgam in tetrahydrofuran (thf) to give [Na(thf)][{Mn(NBu^t)₂(µ-NBu^t)}₂]3 which have oxidation states V,V and V,VI respectively. Interaction of 1 with iodine gave a mixed-valence (VI,V,VI) compound, [{Mn(NBu^t)₂(µ-NBu^t)₂}₂Mn]I₃4. Interaction of 3 with HgCl₂ gave [{Mn(NBu^t)₂(µ-NBu^t)₂}₂Mn]₂[Hg₂Cl₆]·2CHCl₃5a and [(Bu^tN)₂Mn(µ-NBu^t)₂Mn(NHBu^t)(NBu^t)]₂[Hg₃Cl₈]5b. Oxidation of 1 by silver trifluoromethanesulfonate resulted in an unisolable cation in CH₂Cl₂, presumably [{Mn(NBu^t)₂(µ-NBu^t)}₂]⁺, the EPR spectrum of which suggests that the single electron is located on one N atom of the terminal imide groups. The interaction of this cation with thf resulted in abstraction of H to give the manganese(VI) amido species [(Bu^tN)(Bu^tNH)Mn(µ-NBu^t)₂Mn(NBu^t)₂]⁺ which is also formed on protonation of 1 with stoichiometric amounts of CF₃SO₃H. The amido cation has been isolated with the anions CF₃SO₃^–6 and [B(C₆F₅)₄]^–7, but use of [B{C₆H₃(CF₃)₂-3,5}₄]^– produces, after crystallisation from toluene, the salts of the amido cation 8 and of an oxo species [Mn₃(µ-O)(µ-NBu^t)₄]⁺9. Interaction of [{Mn(NBu^t)₂(µ-NBu^t)}₂]⁺in situ with dry O₂ gave a mixture 10 of [{(Bu^tN)₂Mn(µ-NBu^t)₂}₂Mn]⁺ and [{(Bu^tN)₂Mn(µ-O)(µ-NBu^t)}₂Mn]⁺ which cocrystallise and cannot be separated. Thermal decomposition of the alkoxide Mn(NBu^t)₃[OCH(CF₃)₂] gave the manganese(V) dimer [Mn(NBu^t)(NHBu^t)(µ-NBu^t)]₂. The structures of compounds 3, 4, 5a, 5b, 7, 9 and the mixture 10 have been confirmed by single-crystal X-ray studies. In all cases the Mn atoms have distorted-tetrahedral geometry. The Mn–N bonds to terminal imide ligands lie in the range 1.554–1.674 Å, whilst bridging distances vary considerably from 1.724 to 1.872 Å, reflecting differences for Mn atoms which are co-ordinated only by bridging imides and those which are also bonded to terminal ligands. The Mn–O bonds in the oxo-bridged species are slightly longer than the analogous Mn–N (bridging) distances, indicating less multiple bonding. The structure of the [Hg₃Cl₈]^2– ion in 5b shows it to conform quite closely to the ‘composite’ species [(Cl₂Hg)(Cl₂HgCl₂)(HgCl₂)]^2–