Effect of Ligand Coordination on the Structures and Visible-Light Photocatalytic Activity of Manganese Vanadate Hybrids

A new manganese–vanadate hybrid
structure, Mn­(H2O)­(bpy)­V2O6 (I; bpy = 2,2′-bipyridine),
has been synthesized via hydrothermal methods and characterized by
single crystal X-ray diffraction [P21/n, Z = 4, a = 6.8557(4)
Å, b = 10.4900(6) Å, c = 19.7921(13) Å, β = 96.419(4)°], infrared spectroscopy,
thermogravimetric analysis, magnetic susceptibility measurements,
and UV–vis diffuse reflectance. The structure is comprised
of manganese vanadate layers with 2,2’-bipyridine ligands coordinated
to the Mn­(II) cations. The water molecules coordinated to the manganese
sites can be reversibly desorbed at ∼190 °C with the formation
of a new hybrid structure before then further decomposing to MnV2O6 upon heating to 300 °C. Notably, I undergoes a reversible structural transformation to Mn­(bpy)­V4O11(bpy) (II) under hydrothermal conditions.
This structural transformation results from additional bpy-ligand
coordination to 1/4 of the vanadium sites. Magnetic data indicate
Mn­(II) cations in both I and II are high
spin (S = 5/2). The optical bandgap sizes of I and II were measured to be ∼2.2 eV and
∼1.6 eV, respectively, and that are calculated by DFT methods
to arise primarily from Mn-to-bpy and Mn-to-V electronic transitions
between the valence and conduction bands. Visible-light irradiation
of II in aqueous solutions leads to photocatalytic activities
for total water splitting at rates of ∼92 μmol H2/1/2O2 g–1 h–1 and ∼21 μmol H2/1/2O2 g–1 h–1 for II, with and without a 1 wt % Pt surface cocatalyst,
respectively, but no measurable activity for I. Rates
for only H2 production using aqueous methanol solutions
were significantly lower. Results from electronic structure calculations
show that the change in ligand coordination from I to II causes the excited electrons to populate slightly lower-energy
bpy ligand π* orbitals that are coordinated to V­(V), and thus
this structural change in II leads to a better excited-state
charge separation within its hybrid structure.