## 1 Introduction  
Electochromic materials are currently attracting a great deal of research interest due to their ability to controllably change optical absorption when an electrical voltage is applied [1]. This property makes them prospective functional components for a number of applications. For example, smart windows based on these materials can dramatically improve the energy efficiency of buildings by regulating the exchange of thermal energy between the room and the environment [2, 3]. Reflective and transparent displays and e-paper based on electrochromic materials are also promising in terms of energy savings and are perfectly readable in outdoor lighting and safer for the eye than light-emitting displays [4, 5, 6]. The possibility of using such materials in devices for adaptive camouflage is also of great interest [7]. Furthermore, electrochromic materials can be used as auxiliary elements of non-optical devices--for example, as charge indicators in batteries--or they can perform electrochromic and energy functions simultaneously [8, 9]. The most popular electrochromic materials are various conducting polymers (such as polyaniline) or transition metal oxides, such as NiO or WO\({}_{3}\)[1; 2; 10]. The latter have a number of advantages, such as high stability, spectral purity, and coloration efficiency. Among transition metal oxides, tungsten oxide WO\({}_{3}\) stands out due to its characteristic large optical contrast in the visible and near-infrared range, high ionic conductivity, and high coloration efficiency [11; 12; 13; 14]. In addition to its use as an electrochromic material, tungsten oxide is also a promising photocatalyst for water splitting [15], photochromic material [16; 17; 18], photoreceptor [19], material for resistive memory with random access [19], gas sensors [16; 20], lithium-ion batteries [21], etc. Since electrochromic processes occur on the surface of a material, its dispersity and microstructure have a great influence on their course, so the use of nanomaterials with a developed surface, such as hierarchic materials, is promising in this field. Hierarchic materials may also have specific active centers, which may further improve the functional properties of such materials [22; 23; 24; 25; 26; 27]. Hydrothermal synthesis is known as the most convenient approach to the preparation of hierarchic oxide nanomaterials [23; 27; 28; 29; 30]. Fine control over product morphology is possible by varying the duration and temperature of the synthesis and other parameters. Hydrothermal synthesis can also be used to grow oxide films, avoiding film deposition from powders [18; 31; 32; 33; 34]. The choice of precursor plays a key role in the production of material using the hydrothermal method. In the case of the hydrothermal synthesis of tungsten(VI) oxide, the following precursors are mostly used: peroxotungstic acid [29; 35; 36], ammonium or sodium tungstate [37; 38; 39], and freshly precipitated tungsten oxide [27; 40; 41; 42]. In our opinion, expanding this list with complex compounds will allow a finer control over the product structure and properties by choosing a precursor that has the necessary geometry at the molecular level. In particular, obtaining metal oxides with the hydrothermal method from the corresponding acetylacetonates may be of interest. Tungsten acetylacetonate [WO\({}_{2}\)(C\({}_{5}\)H\({}_{7}\)O\({}_{2}\))\({}_{2}\)] use can result in special oxide structure due to presence of the WO\({}_{2}\)\({}^{+}\) cation in its structure. At the same time, the [WO\({}_{2}\)(C\({}_{5}\)H\({}_{7}\)O)\({}_{2}\)] complex is little described in the literature, and it was scarcely used for WO\({}_{3}\) preparation. As an example, we were able to find only work [43], in which tungsten(VI) oxide films were obtained by the decomposition of the complex under the influence of irradiation. This is due to the relatively complex method of synthesizing it compared to other precursors of tungsten oxide and acetylacetonates of other metals.