Non-oxo chemistry of manganese in high oxidation states. Part 1. Mononuclear tert-butylimido compounds of manganese-(VII) and -(VI)

Abstract

The interaction of manganese(III) chloride in acetonitrile with NHR(SiMe₃)(R = Bu^t or CMe₂CH₂Me) gave the manganese(VII) species Mn(NR)₃Cl as thermally and air-stable green crystals. The chlorine atom in Mn(NBu^t)₃Cl 1 has been substituted by Br, OC(O)R (R = Me or CF₃). OC₆X₅(X = F or Cl), OCH(CF₃)_2′ SC₆F_5′ C₆F₅ or NHBu^t by use of SiMe₃Br and the silver, thallium or lithium salts of other reagents. Interaction of compound 1 with Ag(O₃SCF₃) or AgPF₆ and NH₂Bu^t gave the salts of the cation [Mn(NBu^t)₃(NH₂Bu^t)]⁺, with Li(NHBu^t) under selected conditions gave the paramagnetic manganese(VI) analogues of the manganate(VI) anion, [LiL]₂[Mn(NBu^t)₄][L = 1,2-dimethoxyethane (dme) or N,N,N′,N′-tetramethylethylenediamine (tmen)], and with HCl gave Mn(NBu^t)₂(NHBu^t)Cl₂, the reaction being reversed by addition of pyridine. Unstable compounds, e.g Mn(NBu^t)₃(C₆F₅) and Mn(NBu^t)₃(NHBu^t), were characterised spectroscopically. The mechanism of the formation of 1 is discussed together with a mechanism accounting for the instability of compounds where the X-group has β-H atoms as in Mn(NBu^t)₃[OCH(CF₃)₂]. β-Hydrogen transfer to a NBu^t group to give a manganese(V) dimer [Mn(NBu^t)₂(NHBu^t)]₂ occurs with the formation of ketones from alkoxides; for alkyls decomposition appears to involve homolysis of the Mn–C bond and a radical pathway. The crystal structures of the compounds Mn(NBu^t)₃X, X = OC(O)Me, OC₆F₅, SC₆F₅, [Mn(NBu^t)₃(NH₂Bu^t)]Y, Y = CF₃SO₃ or PF₆ and [Li(dme)]₂[Mn(NBu^t)₄] have been determined. In all the compounds the manganese atom has distorted tetrahedral geometry. In the first three the Mn–N(imido) bond lengths are 1.664(3)–1.673(9)Å and the Mn–N–C angles 139.9(2) to 144.6(3)°, indicating partial multiple-bond character. The Mn–X distances, 1.91(3), 1 896(2) and 2.289(1)Å respectively indicate single bonds. In the cationic compounds the Mn–N(imido) distances are 1.531 (8)–1.655(8)Å, with Mn–N–C angles of 1 35.8(6)–170.2(9)°, suggesting more uneven distribution of the Mn–N multiple bonding, although these structures were less precisely determined due to disorder. In the manganate(VI) anion the Mn–N(imido) lengths are longer, 1.746(3)–1.749(3)Å, as expected.