Thermochemical properties of three isotypic polyborate cluster compounds: Na5[MB24O34(OH)12]·nH2O (M = Cr3+, Al3+, Ga3+)

• Title three compounds have been synthesized and characterized.
• Enthalpies of solution of Na5[MB24O34(OH)12]·nH2O (M = Cr3+, Al3+) were measured.
• Enthalpies of solution of AlCl3·6H2O (s) and CrCl3·6H2O (s) were also measured.
• ΔfHmo for Na5[MB24O34(OH)12]·nH2O (M = Cr3+, Al3+) were obtained by thermochemical cycles.
• ΔfHmo for title three compounds were also estimated by a group contribution method.

Three isotypic polyborate cluster compounds for Na5[AlB24O34(OH)12]·5.5H2O, Na5[CrB24O34(OH)12]·5H2O and Na5[GaB24O34(OH)12]·3H2O have been synthesized and characterized by EDS, XRD, FT-IR, TG techniques and chemical analysis. The molar enthalpies of solution of Na5[AlB24O34(OH)12]·5.5H2O (s) and Na5[CrB24O34(OH)12]·5H2O (s) in 1 mol · dm−3 HCl (aq) were measured to be (58.27 ± 0.15) kJ · mol−1 and (130.99 ± 0.33) kJ · mol−1, respectively. The molar enthalpies of solution of AlCl3·6H2O (s) and CrCl3·6H2O (s) in (HCl + H3BO3 + NaCl) (aq) were also measured, respectively. With the incorporation of the previously determined enthalpy of solution of H3BO3 (s) in 1 mol · dm−3 HCl (aq), together with the standard molar enthalpies of formation for AlCl3·6H2O (s)/CrCl3·6H2O (s), NaCl(s), H3BO3 (s), HCl (aq) and H2O (l), the standard molar enthalpies of formation of −(21316.0 ± 19.3) kJ · mol−1 for Na5[CrB24O34(OH)12]·5H2O (s) and −(21669.7 ± 19.3) kJ · mol−1 for Na5[AlB24O34(OH)12]·5.5H2O (s) at T = 298.15 K were obtained on the basis of the appropriate thermochemical cycles. The molar enthalpy of formation of Na5[GaB24O34(OH)12)]·3H2O was estimated as −20662.9 kJ · mol−1 by using a group contribution method on the basis of these measured results.