The role of temperature in the solvothermal synthesis of hybrid vanadium oxyfluorides

Abstract

The solvothermal synthesis of a new series of organically templated vanadium oxyfluorides is presented. The materials were synthesised from identical starting mixtures using temperature as the only independent variable. This shows a trend towards increasing dimensionality and decreasing vanadium oxidation state as a function of temperature. The materials were synthesised from a system containing V 2 O 5 -HF-H 2 O-ethylene glycol-piperazine which resulted in four new structure types, together with a previously known phase. 1) consists of V (V) -centred monomeric units and V (IV) edge-sharing dimers, which are held together by hydrogen bonding to form infinite chains. 2) features a Y-shaped tetramer containing both V (III) and V (IV) -centred octahedra which are held together via hydrogen bonding from the water molecules to form infinite sheets. 3) is a complex chain containing five different V-centred octahedra, which shares structural similarities with compound (2). The inter-relations between these three structure types as a function of temperature, together with two further compounds isolated from this system, are discussed.

A hybrid vanadium fluoride with structurally isolated S = 1 kagome layers

“…The kagome layer consists of three crystallographicallydistinct VF 6 octahedra, sharing in-plane corners. As discussed in our previous studies 8,20 , increasing the reaction temperature in these systems favours reduction of vanadium to the trivalent state; bond-valence sum (BVS) analysis (see ESI) and magnetic data † show that in this case all three distinct V sites correspond to V 3+ , rather than the more oxidised state (6V 4+ /1V 3+ ) occurring in DQ-VOF 8 . In each of the three octahedra, the geometry is tetragonally shortened, such that the apical V-F distances are in the range 1.86 -1.89 Å and the in-plane distances are 1.94 -1.99 Å; the underbonding at the apical sites is partially alleviated by acceptance of a network of hydrogen bonds from the interlayer cations.…”

“…This study forms part of our ongoing efforts to explore and understand the complex solvothermal/ionothermal chemistry of vanadium fluoride-based systems 8,[19][20][21] . Specifically, compound 1 was isolated by using identical chemistry ¶ to that in the preparation of the QSL compound DQ-VOF, except that the temperature of the reaction was increased from 170 to 210 C and the time was increased from 1 day to 5 days.…”

New structural units in molybdenum oxyfluoride chemistry

“…In particular, for the same reaction mixture an increase in reaction temperature typically results in both a reduction in the average vanadium oxidation state and an increase in the dimensionality of the vanadium oxyfluoride sublattice 4 . Both of these features can be seen in our preliminary exploration of the molybdenum oxyfluoride systems presented here.…”

“…All non-H atoms were refined with anisotropic thermal parameters; H atoms in 1, 2 and 5 were treated as 'riding'. Hydrogen atoms on the NH 4 + ions in 3 were not located.Correct assignments of O/F atoms were confirmed by bond-valence sum (BVS) analysis 24,25 . Crystallographic details are provided in Table 3, with selected bond distances in Table 4.…”

“…Recently we have greatly expanded the chemistry of vanadium oxyfluorides, in particular using solvothermal methods, and this has led to a reasonably well-developed understanding of the factors that dictate their compositional and structural features [2][3][4][5][6][7] . In the case of molybdenum oxyfluorides (MoOFs) both solid-state and solution-based methods have been used in the past to prepare a select number of examples of MoOFs containing a variety of structural building units.…”