Structural and Magnetic Diversity in Alkali-Metal Manganate Chemistry: Evaluating Donor and Alkali-Metal Effects in Co-complexation Processes

By exploring co-complexation reactions between the manganese alkyl Mn(CH₂SiMe₃)₂ and the heavier alkali-metal alkyls M(CH₂SiMe₃) (M=Na, K) in a benzene/hexane solvent mixture and in some cases adding Lewis donors (bidentate TMEDA, 1,4-dioxane, and 1,4-diazabicyclo[2,2,2] octane (DABCO)) has produced a new family of alkali-metal tris(alkyl) manganates. The influences that the alkali metal and the donor solvent impose on the structures and magnetic properties of these ates have been assessed by a combination of X-ray, SQUID magnetization measurements, and EPR spectroscopy. These studies uncover a diverse structural chemistry ranging from discrete monomers [(TMEDA)₂MMn(CH₂SiMe₃)₃] (M=Na, 3; M=K, 4) to dimers [{KMn(CH₂SiMe₃)₃C₆H₆}₂] (2) and [{NaMn(CH₂SiMe₃)₃}₂(dioxane)₇] (5); and to more complex supramolecular networks [{NaMn(CH₂SiMe₃)₃}_∞] (1) and [{Na₂Mn₂(CH₂SiMe₃)₆(DABCO)₂}_∞] (7)). Interestingly, the identity of the alkali metal exerts a significant effect in the reactions of 1 and 2 with 1,4-dioxane, as 1 produces coordination adduct 5, while 2 forms heteroleptic [{(dioxane)₆K₂Mn₂(CH₂SiMe₃)₄(O(CH₂)₂OCH=CH₂)₂}_∞] (6) containing two alkoxide-vinyl anions resulting from α-metalation and ring opening of dioxane. Compounds 6 and 7, containing two spin carriers, exhibit antiferromagnetic coupling of their S=5/2 moments with varying intensity depending on the nature of the exchange pathways.