A New Family of Sandwich-Type Polytungstophosphates Containing Two Types of Metals in the Central Belt: M′2M2(PW9O34)212−(M′ = Na or Li, M = Mn2+, Co2+, Ni2+, and Zn2+)

A new family of sandwich-type polytungstophosphates containing two different types of metals in the central belt, M′2M2(PW9O34)212− (M′ = Na or Li, M = Mn2+, Co2+, Ni2+, and Zn2+), have been synthesized and characterized by infrared spectroscopy, 31P solution NMR spectroscopy, and elemental analysis. Compounds Na2Co2(PW9O34)212− (Na2Co2), Na2Ni2(PW9O34)212− (Na2Ni2), Li2Ni2(PW9O34)212− (Li2Ni2), Na2Mn2(PW9O34)212− (Na2Mn2), and Li2Zn2(PW9O34)212− (Li2Zn2) were characterized by X-ray crystallography. All these compounds have similar structures, in which two transition-metal ions and two alkali metal ions (sodium or lithium) are sandwiched between two [B-α-PW9O34]9− units; the transition and alkali metals reside in the internal and external (solvent exposed) positions of the central belt, respectively. By adding LiCl to aqueous solutions of compounds Na2M2, lithium−sodium exchanges in the external belt positions are observed by 31P solution NMR spectroscopy and X-ray crystallography. Magnetic measurements indicate ferromagnetic exchange interactions between the two Ni2+ ions in Na2Ni2 at 10−300 K and the two Co2+ ions in Na2Co2 at 6−30 K. In contrast, Na2Mn2 exhibits an antiferromagnetic interaction between the Mn2+ ions at 2−50 K.