Manganese–Vanadate Hybrids: Impact of Organic Ligands on Their Structures, Thermal Stabilities, Optical Properties, and Photocatalytic Activities

Manganese­(II)–vanadate­(V)/organic
hybrids were prepared in high purity using four different N-donor
organic ligands (2,6:2′,2″-terpyridine = terpy, 2,2′-bipyrimidine
= bpym, o-phenanthroline = o-phen,
and 4,4′-bipyridine = 4,4′-bpy), and their crystalline
structures, thermal stabilities, optical properties, photocatalytic
activities and electronic structures were investigated as a function
of the organic ligand. Hydrothermal reactions were employed that targeted
a 1:2 molar ratio of Mn­(II)/V­(V), yielding four hybrid solids with
the compositions of Mn­(terpy)­V2O6·H2O (I), Mn2(bpym)­V4O12·0.6H2O (II), Mn­(H2O)­(o-phen)­V2O6 (III), and Mn­(4,4′-bpy)­V2O6·1.16H2O (IV). The inorganic component within these
hybrid compounds, that is, [MnV2O6], forms infinite
chains in I and layers in II, III, and IV. In each case, the organic ligand preferentially
coordinates to the Mn­(II) cations within their respective structures,
either as chelating and three-coordinate (mer isomer in I) or two-coordinate (cis isomers in II and III), or as bridging and two coordinate (trans isomer in IV). The terminating ligands in I (terpy) and III (o-phen) yield nonbridged “MnV2O6” chains and layers, respectively, while the
bridging ligands in II (bpym) and IV (4,4′-bpy)
result in three-dimensional, pillared hybrid networks. The coordination
number of the ligand, that is, two- or three-coordinate, has the predominant
effect on the dimensionality of the inorganic component, while the
connectivity of the combined metal-oxide/organic network is determined
by the chelating versus bridging ligand coordination modes. Each hybrid
compound decomposes into crystalline MnV2O6 upon
heating in air with specific surface areas from ∼7 m2/g for III to ∼41 m2/g for IV, depending on the extent of structural collapse as the lattice water
is removed. All hybrid compounds exhibit visible-light bandgap sizes
from ∼1.7 to ∼2.0 eV, decreasing with the increased
dimensionality of the [MnV2O6] network in the
order of I > II ≈ III > IV. These bandgap sizes are smaller by ∼0.1–0.4
eV in comparison to related vanadate hybrids, owing to the addition
of the higher-energy 3d orbital contributions from the Mn­(II) cations.
Each compound also exhibits temperature-dependent photocatalytic activities
for hydrogen production under visible-light irradiation in 20% methanol
solutions, with threshold temperatures of ∼30 °C for III, ∼36 °C for I, and ∼40
°C for II, IV, and V4O10(o-phen)2. Hydrogen production
rates are ∼142 μmol H2 g–1·h–1, ∼673 μmol H2 g–1·h–1, ∼91 μmol
H2 g–1·h–1, and
∼218 μmol H2 g–1·h–1 at 40 °C, for I, II, III, and IV, respectively, increasing
with the oxide/organic network connectivity. In contrast, the related
V4O10(o-phen)2 exhibits
a much lower photocatalytic rate of ∼36 H2 g–1·h–1. Electronic structure
calculations based on density-functional theory methods show that
the valence band edges are primarily derived from the half-filled
Mn 3d5 orbitals in each, while the conduction band edges
are primarily comprised of contributions from the empty V 3d0 orbitals in I and II and from ligand π*
orbitals in III. Thus, the coordinating
organic ligands are shown to significantly affect the local and extended
structural features, which has elucidated the underlying relationships
to their photocatalytic activities, visible-light bandgap sizes, electronic
structures, and thermal stabilities.