Patent Application
20090250661
The present invention relates to trimetallic nitride clusters encased within heterofullerene CnN cages and to the methods of making them with oxidizing gases (e.g., O2 and NOx) and combustion.
Fullerenes are a family of closed-caged molecules made up of carbon atoms. The closed-caged molecules consist of a series of five and six member carbon rings. The fullerene molecules can contain 60 or more carbon atoms. The most common fullerene is the spherical C60 molecule taking on the familiar shape of a soccer ball.
Fullerenes are typically produced by an are discharge method using a carbon rod as one or both of the electrodes in a Krätschmer-Huffman generator. Krätschmer, W. et al., Chem. Phys. Lett., 170, 167-170 (1990). Typically the generator has a reaction chamber and two electrodes. The reaction chamber is evacuated and an inert gas is introduced in the reaction chamber at a controlled pressure. A potential is applied between the electrodes in the chamber to produce an are discharge. The are discharge forms a carbon plasma in which fullerenes of various sizes are produced.
Many derivatives of fullerenes have been prepared including encapsulating metals inside the fullerene cage. Metal encapsulated fullerenes are typically prepared by packing a cored graphite rod with the metal oxide of the metal to be encapsulated in the fullerene cage. The packed graphite rod is placed in, the generator and are discharged to produce fullerene products. The formation of metal encapsulated fullerenes is a complicated process and typically yields only very small amounts of the metal fullerenes.
U.S. Pat. No. 6,303,760, herein incorporated by reference in its entirety, describes a family of endohedral metallofullerenes where a trimetallic nitride is encapsulated in an all-carbon fullerene cage. The endohedral metallofullerenes have the general formula A3-nXnN@Cm (n=0-3) where A is a metal, X is a second trivalent metal, n is an integer from 0 to 3, and m is an even integer from about 60 to about 200. The metals A and X may be an element selected from the group consisting of a rare (earth element and a group IIIB element and may be the same or different. In some embodiments, A and X may be selected from the group consisting of scandium, yttrium, lanthanum, gadolinium, holmium, erbium, thulium, and ytterbium, where A and X may be the same or different. These novel trimetallic nitride endohedral metallofullerenes are produced by introducing nitrogen gas into the Kratschmer-Huffman generator during vaporization of packed graphite rods containing corresponding metal oxides, known as the trimetallic nitride template (TNT) process.
The present invention seeks to provide trimetallic nitride endohedral metalloheterofullerenes. Heterofullerene cages (e.g., C59N, C69N, C79N have one carbon on the fullerene cage surface substituted with a different type of atom (e.g., N). Having just one atom of difference has the following effects: (1) changing the reactivity of the entire molecule relative to all carbon cages C60, C70, C80 (2) the ability of the heteroatom to serve as a unique linking site to subsequent chemistry (i.e., functionalization). These phenomena serve as the motivation to pursue heterofullerene cages. With metallic nitride clusters entrapped within the heteroatom cage, we can now also take advantage of the metals' utility in application areas such as Gd3N@C79N for MRI contrast agents, Ho3N@C79N for radiopharmaceuticals, Er3N@C79N for optical and photovoltaic applications, and Lu3N@C79N for X-Ray contrast agents.
The present invention is directed to a family of endohedral trimetallic nitride metalloheterofullerenes having the formula A3N@CnN or AxX3-xN@CnN, (x=0, 1, 2 or 3) (n=number of carbon atoms in the cage, typically between 59 and 199), wherein A and X are metal atoms encased in the cage, C is carbon and N is nitrogen. A and X are preferably selected from rare earth elements or group IIIB elements. A or X metal atoms can be scandium, yttrium, lanthanum, neodymium, cerium, terbium, thulium, gadolinium, holmium, erbium, thulium, dysprosium, praseodymium and ytterbium. Representative embodiments include Sc3N@C79N, Y3N@C79N, La3N@C79N, Gd3N@C79N, Tb3N(C79N, Ho3N@C79N, and mixed-metal trimetallic nitride metalloheterofullerenes such as LaSc2N@C79N, PrSc2N@C7-9N, GdSc2N@C79N, and Gd2ScN@C79N. Additionally, the present invention provides an endohedral metalloheterofullerene having the formula: AXZN@CnN, (n=an odd integer between about 59 and about 199), wherein A, X, and Z are any combination of all dissimilar transition metal or rare-earth metal atoms, such as GdScHoN@CnN or GdHoErN@CnN.
The present invention is also directed to a method of making the inventive endohedral metalloheterofullerenes having the formula AxX3-xN@CnN. These methods involve use of oxidizing gases (e.g. O2 and NOx) coupled with combustion as a means for making trimetallic nitride clusters encapsulated in heteroatom CnN cages made of both carbon and nitrogen. NOx is a generic term for NO and NO2 and further including N2O, N2O5, N2O3, N2O4. Briefly, the method includes charging a reactor with a first metal, carbon, O2 and NOx; and reacting by combusting the O2 and NOx, the first metal, and the carbon in the reactor to form an endohedral metalloheterofullerene. The first metal and carbon are introduced in the reactor in the form of a rod filled with a mixture of a first metal oxide (with or without graphite), wherein the first metal oxide is an oxide of the first metal.
The first metal is selected from the group consisting of a rare earth element and a group IIIB element. Typically, the first metal is selected from the group consisting of group IIIB or rare-earth elements, e.g., scandium, yttrium, lanthanum, gadolinium, holmium, erbium, thulium, and ytterbium. The first metal may have an ionic radius below about 0.095 nm. Further, the first metal may be a trivalent metal. The mixture comprises from about 1% to about 5% first metal oxide by weight. Typically the mixture comprises about 3% first metal oxide by weight. The method includes a mixture having from about 1% to about 5% first metal oxide by weight and from about 1% to about 5% second metal oxide by weight. Typically, the mixture has about 3% first metal oxide and about 2% second metal oxide by weight. Alternatively the mixture of the first metal or metal oxide plus the second metal or metal oxide can sum to 99>% with as little as <1% NOx. Addition of graphite powder to the metal mixture is optional.
The present invention is directed to trimetallic nitride endohedral metalloheterofullerenes. In accordance with an embodiment of the present invention, trimetallic nitride endohedral metalloheterofullerenes are synthesized by use of oxidizing gases (e.g. O2 and NOx) coupled with combustion.
As used herein, “endohedral” refers to the encapsulation of atoms inside the fullerene cage network. Accepted symbols for elements and subscripts to denote numbers of elements are used herein. Further, all elements to the right of an @ symbol are part of the fullerene cage network, while all elements listed to the left are contained within the fullerene cage network. Under this notation, Sc3N@C79N indicates that the Sc3N trimetallic nitride is situated within a C79N heterofullerene cage.
The present invention is directed to a family of endohedral metalloheterofullerenes having the formula A3N@CnN or AxX3-xN@CnN, (x=0, 1, 2 or 3) (n=number of carbon atoms in the cage), wherein A and X are metal atoms encased in the cage, C is carbon and N is nitrogen. A or X metal atoms are transition and/or rare-earth elements such as scandium, yttrium, lanthanum, cerium, lutetium, gadolinium, holmium, erbium, thulium, dysprosium, praseodymium and ytterbium. Representative embodiments include Sc3N@C79N, Y3N@C79N, La3N@C79N, Tb3N@C79N, Ho3N@C79N, LaSc2N@C79N, PrSc2N@C79N, GdSc2N@C79N, and Gd2ScN@C79N. Representative examples cover both generic molecular formulas (i.e., homometallic, Type I (A3N@CnN) or mixed-metal nitride clusters, Type II (AxX3-xN@CnN)) described above. Examples of Type I homometallic class of trimetallic nitride metallofullerenes include, but are not limited to, species such as Sc3N@C79N, Y3N@C79N, Tb3N@C79N, Ho3N@C79N, La3N@C79N, Gd3N@C79N etc). Type II compositions of matter include, but are not limited to, mixed-metal, rare-earth containing, trimetallic nitride clusters in CnN heteroatomic fullerene cages, such as LaSc2N@C79N, PrSc2N@C79N, GdSc2N@C79N, Gd2ScN@C79N, etc.
In accordance with the present invention, the fullerene cage, Cn, can range from about 67 carbon atoms to about 199 carbon atoms. In preferred embodiments, n can be about 67, about 77, 79 or about 87. The hetero N making up the cage is generally limited to a single N atom. In one embodiment, the fullerene cage has a portion of the cage that corresponds to a corranulene-type unit. The corranulene-type unit consists of a five-member ring surrounded by five, six-member rings forming a five-member ring and six-member ring juncture, also called a [5,6] ring juncture. The C79N cages are the highest yielding of the process, but other cage sizes include, but are not limited to C87N, C95N. These larger cages are created in the plasma along with the trimetallic nitride metalloheterofullerene C79N cage.
The encapsulated metals A and X may vary widely. Preferably, when the metallofullerene cage size is between about 68 carbon atoms and about 80 carbon atoms, the metal atoms are trivalent and have an ionic radius below about 0.095 nm. When the size of the fullerene cage is about 68, the metal atoms preferably have an ionic radius below about 0.090 nm for the A3N endohedral species. As the size of the cage increases, the ionic radius for the metal may increase. Further, A and X may be a rare earth element, a group IIIB element, or combinations thereof. Preferably, A and X may be scandium, yttrium, lanthanum, gadolinium, holmium, erbium, thulium, ytterbium, or heavy other metals, and combination thereof.
The method for making this family of metalloheterofullerenes includes using a Kratschmer-Huffman generator, well known to one skilled in the art. This type of generator typically has a reaction chamber that can be easily evacuated and charged with a controlled pressure of an inert gas such as helium. The generator holds two electrodes within the reaction chamber and is able to apply a potential across the electrodes to produce an arc discharge.
The present method includes mounting a graphite rod, or other source of carbon, that has been filled with a mixture of a metal oxide and graphite in the reaction chamber. The metal or metal oxide contains the metal to be encapsulated in the fullerene cage. The graphite rods are typically cored and filled with a mixture of metal or metal oxide along with graphite, which can be omitted. The metal oxide may be the oxide of a trivalent metal. Preferably the metal oxide is the oxide of a rare earth metal or a group IIIB metal. Metal oxides may include, but are not limited to, Er2O3, Ho2O3, Y2O3. La2O3, Gd2O3, Tm2O3, or Yb2O3. For making trimetallic nitride metallofullerenes, typically, the mixture of metal oxide and graphite may be from about 1% to about 5% metal oxide to graphite by weight. Typically, a 3% metal oxide to graphite loading will produce the desired trimetallic nitride endohedral metallofullerene.
When the encapsulation of more than one type of metal in the fullerene cage is desired, the cored graphite rod is filled with a mixture of metal oxides and graphite. The mixture of metal oxides should correspond to the desired metals and graphite. The metal oxides may be combination of trivalent metals in the form of oxides. Preferably, the metals are rare earth metal oxides or group IIIB metal oxides as discussed above. For making trimetallic nitride metallofullerenes, typically, the loading of each metal oxide may be from a 1% to about 5% metal oxide to graphite.
Once the mixture is loaded into the cored graphite rod, the rod is place in the generator and the reaction chamber is evacuated. Helium is introduced into the reaction chamber at about 300 torr along with a small amount of O2 and NOx, about 1 to about 3 torr. A dynamic atmosphere ranging from about 300 ml/min to 1250 ml/min helium and about 20 ml/min to about 300 ml/min O2 and NOx gas may also be utilized. The ratio of helium to O2 and NOx is not critical. The trimetallic nitride endohedral metalloheterofullerenes will be produced for a wide range of helium to nitrogen ratios, but the yield of the trimetallic nitride metallofullerenes may tend to decrease as the amount of nitrogen approaches the amount of helium. The rods can be packed with either metals, metal oxides or other forms of the metals. The rods may or may not include carbon (e.g., graphite) powder. Often times we pack rods with metals (0.01 weight percent to 99.9 weight percent) plus NO, vapor, which can be from any compound containing nitrites or nitrates (e.g., copper nitrate hydrate) added to the packed rods. Alternatively NOx vapor can be made in the reactor by combustion reactions with N2 gas reacting with O2 or air 0.05-20 torr/min to produce NOx in the chamber, but the yield of trimetallic nitride metalloheterofullerenes is much lower with this experimental design of adding O2 or air to N2. During the burning (vaporization) production process, air is intentionally introduced into the chamber to assist with combustion and provide an oxidative environment within the reactor. In general the O2 and NOx are combusted at temperatures ranging from about 500° C. to about 4000° C. This oxidizing environment of oxygen and NOx is key to making the trimetallic nitride metalloheterofullerenes. Without the NOx vapor, the formation of trimetallic nitride metalloheterofullerenes is difficult. The uniqueness of our invention is the serendipitous discovery of the use of oxidizing gases such as NOx to permit making new trimetallic nitride metalloheterofullerenes. Furthermore the formation of La3N@C79N, can only be created by this method of adding NOx. For example, La3N@C80 cannot be made with the U.S. Pat. No. 6,303,760 or U.S. patent application #20050232842 (Dunsch). Nor does using the procedure in U.S. Pat. No. 6,303,760 or U.S. patent application #20050232842 (Dunsch) make La3N@C80.
A potential is applied across the electrodes resulting in an arc discharge. The arc discharge consumes the graphite rod and generates a wide range of carbon products generally referred to as soot. Within the soot is a wide range of fullerenes including the trimetallic nitride endohedral metalloheterofullerenes.
Isolation of the trimetallic nitride endohedral metallofullerenes involves use of carbon disulfide or toluene to extract the soluble fullerenes from the soot. Isolation of trimetallic nitride endohedral metallofullerenes are done by chromatograpy (HPLC), see Stevenson et al, Nature, (1999) 401: 55-57, or by selective uptake to a solid support all non-trimetallic endohedral metallofullerenes. See Stevenson et al, Journal of the American Chemical Society, (2006), 128, 27, 8829-8835.
We have serendipitously discovered an inverse relationship between the propensity to form typical Cn cages (e.g., C80) versus CnN(C79N) cages, as shown in
Heterofullerenes are useful as superconductor materials, catalysts, and nonlinear optical materials. Heterofullerene compounds can also find utility as molecular carriers for drugs or catalysts. Heterofullerenes containing radioactive metals can be useful in missile therapy for cancer and as a radionuclide tracer. The gadolinium containing C79N (e.g., GdSc2N@C79N, Gd2ScN@C79N, and Gd3N@C19N) are MRI active and provide pharmaceutical companies with alternative MRI contrast agents. Another commercial advantage which distinguishes our new molecules is the safety advantage of the encapsulated Gd atom(s) which can't escape from the cage. The advantage of having a dissimilar N atom within the carbon cage network permits selective functionalization at or near the N cage atom. In contrast, current Gd-containing MRI agents are chelates, instead, and hence the Gd can escape from the ligand and would then be a toxic, heavy metal in the body.
The present invention is illustrated in the following examples. The examples are provided for illustration purposes and should not be construed as limiting the scope of the present invention.
Briefly, a metal-packed rod (anode) and a graphite rod (cathode) are placed inside a typical electric arc fullerene reactor. The reactor chamber is pumped down to remove air and backfilled with an inert gas (e.g., helium, He) to achieve a reduced pressure (typically 300 torr). Under dynamic flow of He gas, oxygen gas (O2), is introduced in air at a range of flow rates (typically 0.05 torr/min to 20 torr/min). A pressure control valve permits us to maintain flow rates of He and other gases (e.g., O2, NOx) and still maintain reduced pressures during the experiment. Other chemicals and reagents can be introduced into the packed rod (anode), which is a cored, graphite rod packed with the desired metal to encapsulate (e.g., transition metals such as Sc, Y, La and rare-earth metals such as Gd, Er, Ho, Th, Lu, Dy, Ce, Pr, Nd, etc.). NO, is a generic label for NO and NO2, and also includes other gases such as N2O, N2O5, N2O3, N2O4. Other solids can be added to the packing mixture. For example, catalyst additives (Cu metal) and/or reagents that decompose to release catalysts (e.g., Cu metal) and/or liberate oxidizing gases (e.g., oxygen gas, O2, NOx, etc.) can also be mixed together (see e.g., Stevenson et al., “Chemically Adjusting Plasma Temperature, Energy and Reactivity (CAPTEAR) Method Using NOx and Combustion for Selective Synthesis of Sc3N@C80 Metallic Nitride Fullerenes,” J. Am. Chem. Soc., 129: 16257-15262 (December 2007) in the packing material along with the transition metals and/or rare-earth metals, which are part of the trimetallic nitride cluster.
Upon vaporization of the packed rod using the electric-arc process under these oxidizing and combustive conditions, this new class of molecules are formed, along with other common types of empty-cage fullerenes (e.g., C60, C70, C84, etc), classical metallofullerenes without nitrogen (e.g., M@Cn, M2@Cn, M3@Cn, M4@Cn, etc), and predominantly amorphous carbon soot. Solvents such as xylene or carbon disulfide can be added to this asproduced dry soot for extraction of fullerene material. Upon subsequent (1) filtration to remove insolubles (e.g., amorphous carbon, nanotubes, etc.) and (2) solvent evaporation, a dried fullerene-containing extract is obtained. Analysis of fullerene extract by MALDI mass spectrometry reveals the presence of our new composition of matter.
For other transition metals and some rare-earth metals, the detection of trimetallic nitride clusters in CnN heteroatom cares is more readily observed directly from the soot extract. For example Y3N(C79N (1243), Lu3N(C79N (1501), Tb3N@C79N (1453) and Ho3N@C79N are readily distinguishable from their respective C80 fullerene counterparts. Given the difference of only 2 mass units between M3N@Cn versus M3N@CnN molecules, the detection and deconvolution of individual isotopic peaks can be difficult. For example, Er3N@C80 contains a broad range of isotope peaks from mass ranges of 1468 to 1487. A similar broad, distribution of isotope peaks for Er3N@C79N, 1470 to 1489 is also evident from mass spectral isotope peak analysis. Similarly, Gd3N@C79N and Dy3N@C79N also provide a broad range of isotope peaks.
Alternatively, the trimetallic nitride cluster can contain a mixture of different metal types to form our new Type II molecule. Synthetically, this species is made by mixing the desired metals into the packing material (anode) prior to electric-arc vaporization. Mass spectral analysis of soot extracts prepared in such a manner is shown in
The Dorn methods of U.S. Pat. No. 6,303,760 and Dorn 20080279745 use a neutral form of nitrogen, i.e., N2 gas as a source of nitrogen in an electric-arc reactor. Using the Dorn method, U.S. Pat. No. 6,303,760, one produces trimetallic nitride clusters in C80 cages. Implementation of the Dorn method 20080279745 produces a trimetallic nitride cluster in a C80 cage or a M2@C79N species, e.g., La2@C79N, Tb2@C79N, but not with both a trimetallic nitride cluster and a CnN cage. It is only with our NOx and combustion method in our reactor that we can make both the trimetallic nitride cluster AND a C79N heteroatom cage. Our experimental results using the Dorn methods, U.S. Pat. No. 6,303,760 and Dorn 20080279745 with N2 demonstrate failure to produce our invention of trimetallic nitride metalloheterofullerenes. Our comparison data in
The Dunsch method (20050232842) uses a reduced form of nitrogen, e.g., NH3, ammonia as a reactive gas in the electric-arc reactor to produce trimetallic nitride metallofullerenes. Using NH3 as a source of nitrogen, the Dunsch method successfully puts a trimetallic nitride inside the C80 fullerene cage, but yet mass spectral results fail to show trimetallic nitride clusters in C79N fullerene cages to make trimetallic nitride metalloheterofullerenes, i.e., an inability to embed and substitute a N atom within the all-carbon fullerene cage PLUS add a N atom inside the cage, i.e., create a trimetallic nitride cluster.
It will therefore be readily understood by those persons skilled in the art that the present invention is susceptible to broad utility and application. Many embodiments and adaptations of the present invention other than those herein described, as well as many variations, modifications and equivalent arrangement, will be apparent from or reasonably suggested by the present invention and the foregoing description thereof, without departing from the substance or scope of the present invention.
Accordingly, while the present invention has been described herein in detail in relation to its preferred embodiment, it is to be understood that this disclosure is only illustrative and exemplary of the present invention and is made merely for purposes of providing a full and enabling disclosure of the invention. The foregoing disclosure is not intended or to be construed to limit the present invention or otherwise to exclude any such other embodiments, adaptations, variations, modifications and equivalent arrangements, the present invention being limited only by the claims and the equivalents thereof.
This application claims the benefit of U.S. Provisional Application No. 61/021,913 filed Jan. 18, 2008, the contents of which are incorporated herein by reference in their entirety.
This invention was made with government support under NSF grant #0547988, NSF NIRT grant #477370 and NIH grant #415509. The government has certain rights in this invention.
| Number | Date | Country | |
|---|---|---|---|
| 61021913 | Jan 2008 | US |
