Structure Transformation of Methylammonium Polyoxomolybdates via In-Solution Acidification and Solid-State Heating from Methylammonium Monomolybdate and Application as Negative Staining Reagents for Coronavirus Observation.

We prepared polyoxomolybdates with methylammonium countercations from methylammonium monomolybdate, (CH 3 NH 3 ) 2 [MoO 4 ], through two dehydrative condensation methods, acidifying in the aqueous solution and solid-state heating. Discrete (CH 3 NH 3 ) 10 [Mo 36 O 112 (OH) 2 (H 2 O) 14 ], polymeric ((CH 3 NH 3 ) 8 [Mo 36 O 112 (H 2 O) 14 ]) n , and polymeric ((CH 3 NH 3 ) 4 [γ-Mo 8 O 26 ]) n were selectively isolated via pH control of the aqueous (CH 3 NH 3 ) 2 [MoO 4 ] solution. The H 2 SO 4 -acidified solution of pH < 1 produced "sulfonated α-MoO 3 ", polymeric ((CH 3 NH 3 ) 2 [(MoO 3 ) 3 (SO 4 )]) n . The solid-state heating of (CH 3 NH 3 ) 2 [MoO 4 ] in air released methylamine and water to produce several methylammonium polyoxomolybdates in the sequence of discrete (CH 3 NH 3 ) 8 [Mo 7 O 24 -MoO 4 ], discrete (CH 3 NH 3 ) 6 [Mo 7 O 24 ], discrete (CH 3 NH 3 ) 8 [Mo 10 O 34 ], and polymeric ((CH 3 NH 3 ) 4 [γ-Mo 8 O 26 ]) n , before their transformation into molybdenum oxides such as hexagonal-MoO 3 and α-MoO 3 . Notably, some of their polyoxomolybdate structures were different from polyoxomolybdates produced from ammonium molybdates, such as (NH 4 ) 2 [MoO 4 ] or (NH 4 ) 6 [Mo 7 O 24 ], indicating that countercation affected the polyoxomolybdate structure. Moreover, among the tested polyoxomolybdates, (CH 3 NH 3 ) 6 [Mo 7 O 24 ] was the best negative staining reagent for the observation of the SARS-CoV-2 virus using transmission electron microscopy.