A square-planar oxo-alkyl of manganese(III). Synthesis and X-ray crystal structure of {Li₂[MnOMe₃]·2Li₂(Meacac)·tmed}₂

Alkylation of Mn(acac)₃ (acac = 2,4-pentanedionato) with five equiv. of methyl-lithium in diethyl ether, followed by addition of N,N,N′,N′-tetramethylethylenediamine (tmed), gives the deep red manganese(III) oxo-alkyl [MnOMe₃^2-], which has been isolated as a lithium double salt with the 4-methyl-2-penten-2,4-diolate (Meacac) dianion. Overall, the molecular formula is {Li₂[MnOMe₃]·2Li₂(Meacac)·tmed}₂. Six lithium atoms and six oxygen atoms form a hexagonal prismatic core which is capped on each hexagonal face by Li₃(Meacac)₂·tmed groups. Two of the oxygen atoms of the Li₆O₆ core are part of two square-planar [MnOMe₃^2-] centres that are slightly distorted tetrahedrally; this is a rare coordination geometry since manganese(III) is known to adopt a square-planar configuration in only one other case. The oxo group of the [MnOMe₃^2-] unit is bonded to four lithium atoms, and there are Li...HC interactions with two of the methyl groups on manganese. Important bond distances and angles involving the [MnOMe₃^2-] unit are: MnC = 2.040(8) Å (trans to O), MnC = 2.133(8) Å (trans to C), MnO = 1.913(4) Å, OLi = 1.91(2) Å, CMnC = 88.2(3)° (cis), CMnC = 149.1(3)° (trans), CMnO = 98.6(2)° (cis), CMnO = 152.5(3)° (trans). Crystal data for C₄₂H₉₀N₄O₁₀Li₁₂Mn₂ at -100°C: monoclinic, space group P2₁/n, a = 10.616(3), b = 18.189(4), c = 14.973(4) Å, β = 95.06(2)°, V = 2880(2) ų, Z = 2, R_F = 0.037, R_wF = 0.042 for 2084 data and 317 variables. The present compound is the first oxo-alkyl of manganese and only the third σ-organomanganese(III) complex of any kind.