Robust, Alkali-Stable, Triscarbonyl Metal Derivatives of Hexametalate Anions, [M6O19{M‘(CO)3} n ](8- n )- (M = Nb, Ta; M‘ = Mn, Re; n = 1, 2)

Ten 1:1 and 2:1 complexes of [Mn(CO)3]+ and [Re(CO)3]+ with [Nb6O19]8- and [Ta6O19]8- have been isolated as potassium salts in good yields and characterized by elemental analysis, 17O NMR and infrared spectroscopy, and single-crystal X-ray structure determinations. Crystal data for 1 (t-Re2Ta6): empirical formula, K4Na2Re2C6Ta6O35H20, monoclinic, space group, C2/m, a = 17.648(3) Å, b = 10.056(1) Å, c = 13.171(2) Å, β = 112.531(2)°, Z = 2. 2 (t-Re2Nb6): empirical formula, K6Re2C6Nb6O38H26, monoclinic, space group, C2/m, a = 17.724(1) Å, b = 10.0664(6) Å, c = 13.1965(7) Å, β = 112.067(1)°, Z = 2. 3 (t-Mn2Nb6): empirical formula, K6Mn2C6Nb6O37H24, monoclinic, space group, C2/m, a = 17.812(2) Å, b = 10.098(1) Å, c = 13.109(2) Å, β = 112.733(2)°, Z = 2. 4 (c-Mn2Nb6): empirical formula, K6Mn2C6Nb6O50H50, triclinic, space group, P1̄, a = 10.2617(6) Å, b = 13.4198(8) Å, c = 21.411(1) Å, α = 72.738(1)°, β = 112.067(1)°, γ = 83.501(1)°, Z = 2. 5 (c-Re2Nb6): empirical formula, K6Re2C6Nb6O54H58, monoclinic, space group, P21/c, a = 21.687(2) Å, b = 10.3085(9) Å, c = 26.780(2) Å, β = 108.787(1)°, Z = 4. The complexes contain M(CO)3 groups attached to the surface bridging oxygen atoms of the hexametalate anions to yield structures of nominal C 3 v (1:1), D 3 d (trans 2:1), and C 2 v (cis 2:1) symmetry. The syntheses are carried out in aqueous solution or by aqueous hydrothermal methods, and the complexes have remarkably high thermal, redox, and hydrolytic stabilities. The Re-containing compounds are stable to 400−450 °C, at which point CO loss occurs. The Mn compounds lose CO at temperatures above 200 °C. Cyclic voltammetry of all complexes in 0.1 M sodium acetate show no redox behavior, except an irreversible oxidation process at ∼1.0 V vs Ag/AgCl. In contrast to the parent hexametalate anions that are stable only in alkaline (pH >10) solution, the new complexes are stable, at least kinetically, between pH 4 and pΗ ∼12.