A Rhenium Trioxide–Copper Oxide Layer Compound

 

This brief note supplements a previous Chemexplore web page , “New Layered Compounds for High Temperature Superconductivity”. In that study various multilayered compounds (heterostructures) were proposed as possible HTS (high temperature superconductor) materials . Most notable among them was the composite of rhenium trioxide and nickel oxide :

 

Copper(II) oxide should be an equally effective AFM induction agent with rhenium trioxide . CuO is antiferromagnetic , with TN = 230 K . However , this is an anomalously low value compared to related Transition metal binary oxides :

If the graph's trend was extrapolated upward for CuO , its Nel temperature would actually be ~ 600 K !

Why is copper(II) oxide's TN relatively low ? MnO , FeO , CoO , and NiO all have a cubic rocksalt crystal structure (or a slightly distorted version of it) ; for example , that of nickel(II) oxide :

In contrast copper(II) oxide is composed of layers of infinite CuO2 ribbons whose directions alternate from plane to plane , and which are joined at the hip by their oxide anions :

The electronic structure of CuO is readily visualized in a Picture VB (Valence Bond) sketch :

VB predicts the Cu(II) 3d9 singlet electron will be relocated to the 4pz orbital . Since the energy levels of the 4s–3d–4p orbitals are fairly close together in the Transition metal elements , its promotion , which is required because the Cu–O covalent bonds take its 3d orbital , should be energetically facile . It's this relocated 3d9 singlet electron that causes the antiferromagnetic behaviour in copper(II) oxide . Some sort of electron interactions [the references are below , at the end of this web page] within the CuO lattice are thought to reduce the strength of its antiferromagnetism . Such interactions might arise because of local spin pairing between the 3d9 electrons in the CuO2 ribbons :

The copper 4pz orbital may be hybridizing with the energetically accessible 5s orbital , forming a larger linear spz hybrid orbital with a greater volume of positive symmetry electron probability density :

The sp hybrid orbital is quite prevalent in the chemistry of copper(I) compounds , many of which have a linear coordination geometry .

The CuO2 ribbons are magnetically isolated from one another , and the 4pz electron spins alternate from copper to copper along the ribbons . A weak p bonding between the antiparallel singlet electrons would explain the anomalously low TN in copper(II) oxide . Such a feeble magnetic coupling has been experimentally observed in the copper(II) acetate dimer , [Cu(OAc)2]2 , with its unusual molecular structure :

In this latter case the Cu–Cu bond would be formed by an end-to-end sigma type of overlap between the 4pz orbitals , which are stereochemically prominent above and below the x-y square planar Cu–O coordinate covalent bonds .

In the sketch of copper(II) oxide above it should be noted that the CuO2 ribbons are actually tilted slightly away from 90 with respect to the neighbouring planes of ribbons . Also , the O–Cu–O angles aren't exactly at 90 , either (refer to Wells's sketch of tenorite for the correct bond angles) . It's interesting to see in this regard that my sketch is reasonably accurate for a crystallographically related compound , palladium(II) oxide , PdO , whose ribbon angles are at 90 . Pd(II) is 4d8 electronically , and PdO would have the same electronic structure as CuO per picture VB , but without the 4pz singlet electron . It's therefore likely that the angular distortions in CuO are caused by the 4pz singlet electron , all the more so if it's located in the voluminous spz hybrid orbital .

The stacks of CuO2 ribbons in CuO can be straightened out into flat sheets , as in the ternary copper oxide SrCuO2 :

It might be possible to intercalate such flat CuO2 sheets between layers of rhenium trioxide . A [ReO3]2–CuO layered compound could have the following crystal structure :

The singlet electrons in the Cu(II) 4pz orbitals can superexchange through the oxide 2pz orbitals with the free electrons in the rhenium 6py,z orbitals , in which the Re–O metallic bond free electrons are located . A picture VB sketch of the electronic structure of ReO3 is presented in the ReO3-NiO web page . The CuO2 sheets should be an effective AFM induction agent in [ReO3]2–CuO , thereby enabling it to become superconducting at an elevated temperature .

 

Synthesis Routes to [ReO3]2–CuO

 

There are at least three possible synthesis routes to [ReO3]2–CuO . First , it could likely be prepared as an epitaxial thin film by a CVD (chemical vapor deposition) procedure such as MBE (molecular beam epitaxy) or atomic sputtering . I've commented on epitaxial film heterostructures and have included several references on their synthesis in the ReO3-NiO web page .

The second method might be used to prepare bulk samples of [ReO3]2–CuO . A direct approach to the composite from the combination of ReO3 and CuO would likely be unsuccessful . ReO3 is metastable and disproportionates to ReO2 and Re2O7 at ~ 400 C , while copper(II) oxide is a refractory solid that melts without decomposition at 1326 C . Rhenium trioxide probably would have decomposed long before it combined to any significant extent with CuO .

In a more feasible approach to [ReO3]2–CuO , the low-melting covalent rhenium precursor Re2O7 (yellow hygroscopic crystals , m.p. 327 C , b.p. 360 C , and a fairly strong oxidizer) would be gently heated with finely divided copper metal reagent :

Re2O7 + Cu0 -------- [thoroughly mix , gently heat to melt] ---------> [ReO3]2–CuO .

Alfa-Aesar offers a grade of copper powder that might be suitable for this reaction : Copper flake , 1-5 micron , 99.9%. Caution : this material may be pyrophoric , as very finely divided metal powders often are . Handle it in a glove bag under nitrogen or argon . They also supply Re2O7 , which unfortunately is very costly , as are rhenium and all of its compounds .

The rhenium(VII) in Re2O7 is a strong enough oxidizer to overcome the resistance of copper metal to oxidation . Copper is considered to be a base metal in the commercial and industrial senses , but in redox terms it's actually a noble metal like the other Group 1B/11 elements , silver and gold :

2 Re(VII) + 2e- -------------> 2 Re(VI) ; E0red = 0.768 V ;

Cu0 – 2e- -------------> Cu2+ ; E0ox = 0.3419 V ;

Net reaction : 2 Re(VII) + Cu0 -------------> 2 Re(VI) + Cu2+ ; E0T = 0.4261 V ; this reaction is predicted to be thermodynamically spontaneous at STP (and somewhat exoergic , so be careful !) .

While simple and straightforward in theory , this proposed reaction of Re2O7 and Cu0 might be problematic in practice . If the reaction became too hot , the CuO could subsequently react further with the disproportionating ReO3 to produce nonmetallic copper(II) perrhenate :

3 ReO3 + CuO -------------> ReO2 + Cu(ReO4)2 .

To circumvent this metastability problem , a third approach to preparing [ReO3]2–CuO would involve the ambient temperature (or even cold) oxidation of an organocopper reagent by Re2O7 , which is thereby reduced under very mild conditions to ReO3 . In the following organic synthesis route the Re2O7 and copper reagents are present in the reaction at the molecular level , dissolved and diluted in a suitable solvent (I have suggested anhydrous ethanol , but there may be better solvents for the reaction . The solvent should be bone-dry , because Re2O7 is rapidly hydrated to perrhenic acid , HReO4) .

Phenylcopper is insoluble in organic solvents – it may be polymeric – and is somewhat unstable . Fortunately its methoxylated analogue 2,4,6-trimethoxyphenylcopper(I) is quite soluble in benzene and maybe ethyl ether ; it's probably soluble in ethanol (a good solvent for Re2O7) . It's slowly hydrolyzed by water , releasing Cu2O . It reportedly decomposes quickly in air , so its C–Cu bond must be very labile toward oxidizers . 2,4,6-Trimethoxyphenylcopper should therefore react rapidly and quantitatively with Re2O7 . Its copper(I) atoms would first be be oxidized to Cu(II) ; then they would sequester the by-product oxide anions from the reduced rhenium(VII) oxide :

[ReO3]2–CuO should precipitate from the ethanol as a dense , black , crystalline solid , which is then filtered , washed with solvent , and dried in a vacuum dessicator . It should not be heated , but used as is” in subsequent analyses and testing . For example , it could be ground to a fine powder , then compressed into a cylindrical pellet for electrical conductivity testing , but not annealed at an elevated temperature , which might induce the disproportionation of its Re(VI) . Examination of the product by high power microscopy and X-ray diffraction should reveal whether it's an intimate mixture of ReO3 and CuO or a true chemical compound as illustrated in its sketch above . In either case [ReO3]2–CuO would be a fascinating material to study . It could prove to be a new HTS material with an exceptionally high transition temperature , making it of great interest to researchers and technologists .

 

References

 

Some sort of electron interactions : B.N. Figgis and J. Lewis , “The Magnetic Properties of Transition Metal Complexes”, Prog. Inorg. Chem. 6 , pp. 37-239 , F.A. Cotton (ed.) , Interscience / John Wiley , New York (1964) ; CuO , CuS , and CuSe have ........“very strong magnetic interactions”..........“the sulfide and selenide are diamagnetic” (p. 212) .

copper(II) acetate dimer : H. Krebs , Fundamentals of Inorganic Crystal Chemistry , transl. by P.H.L. Walter , McGraw-Hill , London (UK) , 1968 ; see Fig. 27.1 , p. 322 .

Wells's sketch of tenorite : A.F. Wells , Structural Inorganic Chemistry , 3rd edition , Clarendon Press , Oxford (UK) , 1962 ; see Fig. 152(a) , p. 463 . A sketch of PdO is presented in Fig. 152(b) .

ternary copper oxide : T. Siegrist , S.M. Zahurak , D.W. Murphy , and R.S. Roth ,“The Parent Structure of the Layered High-Temperature Superconductors”, Nature 334 (6179) , pp. 231-232 (1988) ; see Fig. 1 , p. 232 ; Hk. Mller-Buschbaum , “The Crystal Chemistry of Copper Oxometallates”, Angew. Chem. Internat. Ed. Engl. 30 (7) , pp. 723-744 (1991) ; see Fig. 36 , p. 733 (SrCuO2 is discussed on p. 732) ; idem. , “Oxometallates with Planar Coordination”, Angew. Chem. Internat. Ed. Engl. 16 (10) , pp. 674-687 (1977) ; see Fig. 8 , p. 677 . Alkali metal ternary copper(II) oxides with planar CuO2 ribbons or sheets are also known , such as Li2CuO2 .

very costly : The current cost of rhenium metal , industrial grade , is $4275/Kg [quote per May 10 , 2011 , Minormetals.com] . Compare that to the present gold price , which is oscillating at ~ $1500/Troy ounce ($48,231/Kg) . Rhenium is considered to be the most expensive of the industrial (base) metals . Information about rhenium : Wikipedia , Theodore Gray , Reuters , and the U.S. Geological Survey Minerals Yearbook , 2003 (PDF , 200 KB) .

Phenylcopper : K.D. Berlin and D.E. Gibbs , “Attempted Phenylation of Several Organic Solvents by Phenylmagnesium Bromide in the Presence of Cuprous Salts”, Proc. Okla. Acad. Sci. 46 (Sect. C) , pp. 83-84 (1966) [PDF , 166 KB] .

2,4,6-trimethoxyphenylcopper(I) : G. van Koten , A.J. Leusink , and J.G. Noltes , “Synthesis and Characterization of Arylcopper Compounds Containing the Methoxy or Dimethylamino Groups as a Built-in Ligand”, J. Organometallic Chem. 85 (1) , pp. 105-114 (1975) [PDF , 698 KB] .

 

 

 

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