phases are prepared by a new organometallic chimie douce concept employing the organometallic precursor methyltrioxorhenium.Crystalline WO 3 has found widespread interest as electrochromic material, applied e.g. as thin film for "smart windows", 1 and as gas sensor.2 At room temperature monoclinic -WO 3 (P2 1 /n) represents the thermodynamically most stable phase and consists of tilted WO 6 octahedra. Its structure is therefore related via groupsubgroup relationships with cubic ReO 3 (Pm-3m), which represents the aristotype of the BO 3 perovskite family. Accordingly, it should be possible to form solid solutions of mixed Re x W 1-x O 3 phases due to the structural resemblance of both parent oxides and the similar ionic radii of Re VI and W VI . 3 While WO 3 represents an insulator, ReO 3 displays metallic behaviour with a specific conductivity in the same range as crystalline copper. 4 Hence, solid solutions of both oxides might lead to new phases with interesting electronic properties like the related Na x WO 3 phases, which are benchmark systems to study chemically-induced metal-to-insulator transitions. However, up to now mixed Re x W 1-x O 3 phases were only accessible in small quantities under extreme conditions at high pressure and high temperatures (65 kbar, 1200°C). 6 Here, we propose a new organometallic chimie douce method which allows to synthesise Re x W 1-x O 3 phases in large quantities by a low temperature process at ambient pressure.A convenient aqueous synthesis of WO 3 ·yH 2 O (y = 1, 2, 1/3) from tungstates via tungstic acid has been reported and discussed earlier.7 11 We note that only pure WO 3 ·H 2 O is precipitated when other inorganic Re VII species like perrhenate salts are added to the reaction mixture instead of MTO. Hence, the salient capability of our organometallic precursor to form Re VI oxide species by methane elimination in water appears to be crucial for the successful tungsten/rhenium substitution. 12 The composition and homogeneity of the samples was ascertained by ICP, SEM-EDX, X-ray diffraction and microanalysis up to a Re content of 12%. For x larger than 0.12 phase separation via ReO 3 formation is observed.IR spectroscopic studies (KBr pellets; Fig. 1) show a characteristic band at 940 cm -1 in the parent tungstite,WO 3 ·H 2 O, which shifts to lower frequency in our new Re x W 1-x O 3 ·H 2 O samples with increasing rhenium content. This mode can be assigned by the stretching mode of the W=O bond (see structural motif in Fig. 5) which appears to become softened by stepwise substitution of W A further indication of electronic changes induced by the rhenium content of the mixed phases can be found in the dehydration behaviour as studied by TGA using a temperature ramp from 25 to 230°C during 45 minutes. As shown in Fig. 2, all phases loose a total of one molecule of water per formula unit (7%), but the ease of dehydration depends on the rhenium content. While significant water loss (5% of the total loss) of WO 3 ·H 2 O starts only above 180°C, 9 the mixed phases with x = 0...
The metal oxide polymeric methyltrioxorhenium [(CH3)xReO3]∞ is an unique representative of a layered inherent conducting organometallic polymer which adopts the structural motifs of classical perovskites in two dimensions (2D) in form of methyl-deficient, corner-sharing ReO 5 (CH 3 ) octahedra. In order to improve the characteristics of polymeric methyltrioxorhenium with respect to its physical properties and potential usage as an inherentconducting polymer we tried to optimise the synthetic routes of polymeric modifications of 1 to obtain a sintered ceramic material, denoted ceramic MTO. Ceramic MTO formed in a solvent-free synthesis via auto-polymerisation and subsequent sintering processing displays clearly different mechanical and physical properties from polymeric MTO synthesised in aqueous solution. Ceramic MTO is shown to display activated Re-C and Re=O bonds relative to MTO. These electronic and structural characteristics of ceramic MTO are also reflected by a different chemical reactivity compared with its monomeric parent compound. First examples of the unprecedented reactivity of ceramic MTO in the field of amine oxidations are shown -results which warrant further exploitation.
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