Thermal reactions of metal complexes under quasi-isothermal and -isobaric conditions. 3. Solid-phase thermal reactions of the doubly complexed salts of the forms [(Cr or Co)(NH3)6][(Cr or Co)(CN)6], [(Cr or Co)(H2O)(NH3)5][(Cr or Co)(CN)6], and cis-[(Co(H2O)2(NH3)4][Co(CN)6]
Abstract:Durch Reaktion der [Hexammin‐ bzw. Aquopentammin‐metall(III)]‐nitrate oder ‐chloride mit K‐hexacyanocobaltat(III) bzw. ‐chromat(III) wurden die neun Titelkomplexe dargestellt.
“…This is not an unlikely event, because CN ligand flipping upon heating has been previously reported in Prussian blue, 14,25 its analogues, 26,39 and other CNcontaining transition-metal complexes. 27 In the present work, we have not evaluated whether the cyanide ligand inversion is caused by a flip or an alternative mechanism.…”
Section: Discussionmentioning
confidence: 94%
“…It has been previously reported that an increased temperature can lead to “cyanide flipping” in the bridged complexes, ,– and this may play a role in the reduction processes. It has also been observed that the temperature of reduction is affected by the chemical environment of the cyanide; bridging cyanide ligands reduce metal centers at lower temperatures than the corresponding terminal cyanides. , The above-mentioned features of the cyanometalate chemistry imply that the coordination chemistry of cyanide-containing compounds at elevated temperature is quite distinctive from the relatively well-understood chemistry of these compounds at room temperature.…”
Section: Introductionmentioning
confidence: 98%
“…It has been previously reported that an increased temperature can lead to "cyanide flipping" in the bridged complexes, 14, [25][26][27] and this may play a role in the reduction processes. It has also been observed that the temperature of reduction is affected by the chemical environment of the cyanide; bridging cyanide ligands reduce metal centers at lower temperatures than the corresponding terminal cyanides.…”
Cyanogels are coordination polymers made from the reaction of a chlorometalate and a cyanometalate in aqueous solution, which undergo a sol-gel transition to form stable gels. At temperatures above 240 degrees C, the cyanide ligand acts as a reducing agent and reduces the metal centers to lower oxidation states. To understand the mechanism of the autoreduction, the thermal reduction of the Pd-Co cyanogel system formed by the reaction of PdCl4(2-) and Co(CN)6(3-) was studied in an inert atmosphere. It was found that the reduction proceeds through two polymeric cyanide-containing intermediates, CoPd(CN)4 and Pd(CN)2, that form upon reduction of Co(3+) to Co(2+) and involves a significant rearrangement of the coordination structure. The two intermediates upon further heating reduce to metallic products, which by solid-state diffusion form a single Pd/Co alloy product. CoPd(CN)4 was found to have a hydrated form Co(H2O)2Pd(CN)4 x 4 H2O with a layered structure crystallizing in an orthorhombic Pnma space group. The Pt-Co cyanogel was found to autoreduce via a similar route. CoPt(CN)4 was confirmed as an intermediate. Understanding of the mechanism of the cyanogel autoreduction is an important step toward better understanding of opportunities that cyanogels offer in materials chemistry, as well as an expansion of the knowledge of coordination chemistry at elevated temperatures in general.
“…This is not an unlikely event, because CN ligand flipping upon heating has been previously reported in Prussian blue, 14,25 its analogues, 26,39 and other CNcontaining transition-metal complexes. 27 In the present work, we have not evaluated whether the cyanide ligand inversion is caused by a flip or an alternative mechanism.…”
Section: Discussionmentioning
confidence: 94%
“…It has been previously reported that an increased temperature can lead to “cyanide flipping” in the bridged complexes, ,– and this may play a role in the reduction processes. It has also been observed that the temperature of reduction is affected by the chemical environment of the cyanide; bridging cyanide ligands reduce metal centers at lower temperatures than the corresponding terminal cyanides. , The above-mentioned features of the cyanometalate chemistry imply that the coordination chemistry of cyanide-containing compounds at elevated temperature is quite distinctive from the relatively well-understood chemistry of these compounds at room temperature.…”
Section: Introductionmentioning
confidence: 98%
“…It has been previously reported that an increased temperature can lead to "cyanide flipping" in the bridged complexes, 14, [25][26][27] and this may play a role in the reduction processes. It has also been observed that the temperature of reduction is affected by the chemical environment of the cyanide; bridging cyanide ligands reduce metal centers at lower temperatures than the corresponding terminal cyanides.…”
Cyanogels are coordination polymers made from the reaction of a chlorometalate and a cyanometalate in aqueous solution, which undergo a sol-gel transition to form stable gels. At temperatures above 240 degrees C, the cyanide ligand acts as a reducing agent and reduces the metal centers to lower oxidation states. To understand the mechanism of the autoreduction, the thermal reduction of the Pd-Co cyanogel system formed by the reaction of PdCl4(2-) and Co(CN)6(3-) was studied in an inert atmosphere. It was found that the reduction proceeds through two polymeric cyanide-containing intermediates, CoPd(CN)4 and Pd(CN)2, that form upon reduction of Co(3+) to Co(2+) and involves a significant rearrangement of the coordination structure. The two intermediates upon further heating reduce to metallic products, which by solid-state diffusion form a single Pd/Co alloy product. CoPd(CN)4 was found to have a hydrated form Co(H2O)2Pd(CN)4 x 4 H2O with a layered structure crystallizing in an orthorhombic Pnma space group. The Pt-Co cyanogel was found to autoreduce via a similar route. CoPt(CN)4 was confirmed as an intermediate. Understanding of the mechanism of the cyanogel autoreduction is an important step toward better understanding of opportunities that cyanogels offer in materials chemistry, as well as an expansion of the knowledge of coordination chemistry at elevated temperatures in general.
“…After the second inflection point (280 °C) a very pronounced peak at 2194 cm -1 is seen in the high-frequency region (Figure c) along with the further shift of two peaks in the low-frequency region to 534 and 451 cm -1 which we tentatively ascribe to the stretching vibration and bending modes of cyanide ligands ,, in Co(CN) n ( n < 6). It is also possible that these transitions are due to the to formation of KPdCo(CN) 6 compounds, or to formation of Pd(CN) 2 formed by the “flipping” of the cyanide bridges − to form Co−NC−Pd bridges. 5 High-frequency transmittance FTIR spectra of 60 mM, Pd/Co = 2/1 xerogel samples, which were taken along the main TG inflection points, run under oxygen, at (a) 150, (b) 240, (c) 280, (d) 420, (e) 650, and (f) 795 °C. 6 Low-frequency transmittance FTIR spectra of 60 mM, Pd/Co = 2/1 xerogel samples, which were taken along the main TG inflection points, run under oxygen, at (a) 150, (b) 240, (c) 280, (d) 420, (e) 650, and (f) 795 °C.…”
Section: Resultsmentioning
confidence: 99%
“…After the second inflection point (280 °C) a very pronounced peak at 2194 cm -1 is seen in the high-frequency region (Figure 5c) along with the further shift of two peaks in the low-frequency region to 534 and 451 cm -1 which we tentatively ascribe to the stretching vibration and bending modes of cyanide ligands 23,28,29 in Co(CN) n (n < 6). It is also possible that these transitions are due to the to formation of KPdCo-(CN) 6 compounds, or to formation of Pd(CN) 2 formed by the "flipping" of the cyanide bridges [30][31][32][33] to form Co-NC-Pd bridges. Passing through the third mass loss under an oxygen atmosphere at 500 °C (and 650 °C under an argon atmosphere) removes all high-wavenumber vibrations (Figures 4b and 5e,f).…”
The reaction of aqueous solutions of
K2PdCl4 and
K3Co(CN)6 results in gellike
polymeric
materials, characterized by bridging cyanides between the central
metals of the adducts.
These materials tend to be rigid in nature but contain in excess
of 95% water by weight.
We refer to these novel transition metal based hydrogels as
cyanogels. In contrast to classic
inorganic hydrogels these materials are not based on an oxide network.
Dehydration of the
cyanogels results in amorphous xerogels which maintain the initial
polymeric structure of
the cyanogel. Thermal processing of gels containing palladium and
cobalt centers under an
inert atmosphere at temperatures between 200 and 500 °C, produces a
new metastable
material. Sintering between 500 and 1000 °C produces
ferromagnetic alloys, while processing
under oxygen in the same temperature region gives the ceramic mixed
oxide material PbCoO2,
having the delafossite structure.
Durch Reaktion der [Hexammin‐ bzw. Aquopentammin‐metall(III)]‐nitrate oder ‐chloride mit K‐hexacyanocobaltat(III) bzw. ‐chromat(III) wurden die neun Titelkomplexe dargestellt.
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