Unique solubility of polyoxoniobate salts in methanol: coordination to cations and POM methylation

Abstract

Sodium salts of hybrid organometallic POM complexes consisting of [M₆O₁₉]⁸⁻ (M = Nb, Ta) and half-sandwich fragments {Cp*M}²⁺ (Cp* = η⁵-C₅(CH₃)₅; M = Rh, Ir) and {(C₆H₆)Ru}²⁺ dissolve only in one organic solvent, that is, in methanol. From methanolic solutions, crystals of Na₄[trans-{(C₆H₆)Ru}₂Nb₆O₁₉]·14.125MeOH·2H₂O (1), K₄[trans-{Cp*Rh}₂Nb₆O₁₉]·4MeOH·10H₂O (2) and K₄[trans-{Cp*Ir}₂Nb₆O₁₉]·10MeOH·4H₂O (3) were isolated and characterized by X-ray analysis. Methoxo species [{LM′}₂M₆O_19−n(OCH₃)_n] (n = 1–3) were detected in solution by ESI-MS and NMR, and they account only for about half of the total speciation in the solution. DFT calculations were used to calculate ¹³C NMR chemical shifts in the methoxo complexes and to assess their relative stability. Reasons for the preferred solubility in methanol are discussed.