Samarium diodide

Samarium(II)diiodide-induced radical cyclizations
in the synthesis of biologically important molecules




Samarium diiodide ( Sm(II)I2 is probably the most versatile lanthanide compound . Discovered in 1906 by Matignon and Caze [1] , it is easily prepared [2] and , since the most stable oxidation state is +3 , readily donates one electron making it a suitable promoter of organic transformations that need one-electron transfer or consecutive one-electron transfers .

The first use of SmI2 in organic synthesis was reported by Kagan et al. in 1980[2] . The team performed deoxygenations , reductions of conjugated double bonds , carbonyl compounds and organic halides , alkylations of ketones with organic halides , etc. some of them in high yields and showing some chemoselectivity .

At the time the mechanisms of the reactions or the scope of the reactivity of SmI2 weren’t fully understood . In the next few years Kagan conducted some mechanistic studies on reactions involving organic halides[3] , synthesized homoallylic and homobenzilic alcohols from aldehydes and halides coupling [4] , pinacols from aldehydes and ketones [5] and reductions of nitro compounds , imines and oximes[6] , using SmI2 as a single electron transfer reagent .


In 1984 the first cyclic compounds obtained by SmI2 promoted radical reactions were reported by Molander and Etter [7] . By treating 2-(n-iodoalkyl)cycloalkanones with SmI2 in tetrahydrofuran in the presence of catalytic amounts of iron tris(dibenzoylmethane) [Fe(DBM)3],
the team afforded bicyclic alcohols in good yields .



Highly stereoselective products were obtain in the case of 2-(3-iodopropyl)cyclopentanone (>99.5% cis product) and 2-(4-iodobutyl)cyclopentanone ( ~ 95% cis product) and , in the case of trans-2-(3-iodopropyl)-4-t-butylcyclohexanone only the cis ring-fused isomer was afforded , showing that , with the right substituents and reaction conditions , SmI2 can be used as a powerful stereoselective reagent .

Two years later Fukuzawa et. al[8] found that SmI2 was an efficient reagent for the reductive coupling of carbonyl compounds with electro-deficient alkenes to afford γ-lactones . By adding a mixture of ethyl acrylate or ethyl crotonate , aliphatic or aromatic carbonyl compound , and t-
butyl alcohol as a proton donor , to a solution of SmI2 in THF , the team obtained α- and β- substituted γ-lactones as a mixture of cis and trans isomers , in good yields .



In the same year Inanaga et. al [9] published the preparation of medium and large-ring lactones by SmI2-induced cyclization of ω-(α-bromoacyloxy)aldehydes giving 8- to 14-membered ring β-hydroxy lactones , structures frequently encountered in macrolide antibiotics . Using a method used by Kagan in 1980 [2] to afford a Reformatski product, the team noted that the optical purity of the starting α-bromo ester was completely lost in the product, concluding that the reaction proceeded via a radical process .



While the product was obtained in excellent yields (92%) , no satisfactory stereoselectivity was achieved .

At the same time Molander and Etter[10] , following their earlier success in synthesizing bicyclic alcohols[7] by using samarium diiodide, developed an improved method for the cyclization of
2-(ω-iodoalkyl)cycloalkanones.
Recognizing the importance of ring annulation in organic synthesis , the team published mechanistic studies regarding the stereoselectivity of the reactions . They used the data from their initial method to design a stereoselective intramolecular coupling by incorporating substituents adjacent to the carbonyl at position 2 . The substituents forced the attack to take place from the opposite face ensuring stereoselectivity .















The method proved to be a general procedure for 5- and 6-membered ring annulation due to the mild nature of samarium diiodide that permitted the incorporation of various functional groups into the substrate and , as a result , the control of the stereochemistry of the products .

The method developed by Molander and Etter was used in the following year by Suginome and Yamada [11] to synthesize exaltone and (±)-muscone , two naturally occurring macrocyclic ketones .
Based on a three-carbon annelation of cyclic ketones followed by the regioselective cleavage of the fused bond , at the time it was the simplest method to synthesize muscone and it offered the possibility to be used repeatedly for the ring expansion of cyclic ketones .


In 1988 Kagan with Sasaki and Collin[12] achieved an unusual double cyclization of allyloxybenzoic acid chloride mediated by samarium diiodide to afford cyclopropanols . While the team attempted to discuss the mechanism , they couldn’t fully exclude the possibility of an intramolecular addition of a carbene to a double bond .



In the same year Curran published his first paper on radical cyclizations promoted by samarium diiodide with Fevig and Elliot [13] . At the time the team was focusing on the synthesis of triquinane sesquiterpenes due to their unique structural features and biological activities . Employing samarium diiodide promoted tandem radical cyclizations the team achieved the total synthesis of (±)-hypnophilin and the formal synthesis of (±)-coriolin , two highly oxygenated linear triquinanes .



A year later Molander and Kenny [14] reported the first stereocontrolled intramolecular Barbier reaction mediated by samarium diiodide . Using coupling of unsaturated β-ketoesters and Β-ketoamides , they afforded highly substituted cis-2-hydroxycycloalkanecarboxylates and
cis-2- hydroxycycloalkanecarboamides in high yields . On the basis that ketyls are the initial intermediates formed by electron transfer from samarium diiodide to the ketone , Lewis basic functional groups , such as esters and amides , were incorporated into substrate to serve as stereochemical control elements for the cyclization .





Thus stereochemical control was achieved by virtue of kinetically favoured ring closure from the most accessible face of the chelated ketyl intermediate . This was an unique route to highly substituted , stereodefined carbocyclic ring systems , difficult to access by traditional synthetic strategies .
The team also achieved an excellent diastereoselectivity at three stereocenters by using substituted allylic halides to afford stereodefined vinyl-substituted carbocycles in good yields .







By the end of the 1980s SmI2 was established as a versatile , mild , stereoselective single electron transfer reagent that can reduce chemoselectively a wide variety of functional groups , with radical cyclizations dominating its promoted processes .

In 1991-1992 De Pouilly et al. published the first example of SmI2 use in the carbohydrates chemistry [15] . The team investigated the reactivity of thioglycosides and sulfones towards SmI2 in order to synthesize substituted pyranoid glycals , which are versatile intermediates in the carbohydrates chemistry . While the thioglycosides gave no reactions , the glycosyl phenyl sulfones that were O-acetylated at C-2 were successfully converted to glycals .




Two years later Guidot et. al [16] prepared D- 3,4,5,6-tetra-O- benzyl-myo-inositol, a valuable intermediate for the synthesis of various optically active inositol derivatives . Using a method of reductive coupling of dialdehydes by SmI2 , proposed earlier by Hanessian et al [17] , the group conducted a Swern oxidation of a derivative diol of L-iditol into the dialdehyde , followed by a pinacol closure with SmI2 , accomplishing a 5-step synthesis of a carbocycle from a sugar .




A year later Chiara and Valle[18] synthesized L-chiro-inositol and (-)-conduritol F starting from readily available D-sorbitol . They used the same highly steroselective intramolecular pinacol coupling promoted by SmI2 in an efficient one-pot sequence .






The first use of SmI2 in the synthesis of taxol derivatives was reported by Arseniyades et al. in 1993 . The team synthesised an A-ring building unit by a SmI2 mediated reductive pinacol coupling achieving the stereocenter at C1 in a highly stereospecific manner [19] .




Two years later , using the same methodology , the team reported an efficient formal total synthesis of an A-seco taxane framework .[20] .

In 1996 Swindell and Fan[21] reported the synthesis of a taxane intermediate with a stereoselective intramolecular pinacol coupling that joins C1 and C2 within the eight-membered B-ring as a key step .



In the light of previous failed efforts to cause the cyclization of other substrates , the team concluded that the key structural feature that facilitates the closure of the B-ring in trans-fused fashion relative to the C-ring , is the Δ olefinic bond .


The first use of SmI2 in a successful total synthesis of taxol was achieved by Mukayama et.al in 1997 by constructing the 8-membered ring B in an intramolecular samarium pseudo-Reformatski reaction [22] .




In 1994 Molander and Mckie reported an efficient synthesis of substituted cyclooctanols by a samarium diiodide promoted 8-endo radical cyclization process . Starting with various substituted unsaturated olefinic ketones the team obtained substituted monocyclic , fused bicyclic, and bridged bicyclic cyclooctanols [23] .




Three years later , Molander with McWilliams and Noll [24] published the first example of the use of a chelating samarium metal to affect high levels of remote asymmetric induction in a radical cyclization . They achieved a high diastereoselectivity in a samarium(II) iodide –promoted ketyl-olefin radical cyclization using tartramide-derived keto allylic acetals as chiral auxiliaries , high levels of stereochemical induction being observed at the newly created stereocenters .



In 1999 Nakata et al. reported a highly efficient and facile synthesis of a trans-fused polytetrahydropyran ring system using a cascade of SmI2-induced reductive intramolecular cyclizations . [25]


The team concluded that the mechanism could be explained by a transition state initiated by single-electron reduction of the aldehyde by SmI2 and chelation of Sm(III) to the ester which contributes to control the configuration of the product with high stereoselectivity .







Trans-fused polycyclic ether ring systems are the characteristic structural feature of the marine polycyclic ethers such as brevetoxins , cigautoxins , gambierol , etc. Due to their synthetically challenging unique structures combined with their potent biological activities , these compounds have been extensively studied since brevetoxin B was first isolated in 1981 [26] .


Soon after their first success , the team reported the stereoselective synthesis of trans-fused 6,6- and 6,7-memebered ether ring systems , this time with an angular methyl group , another structural characteristic of marine polycyclic ethers . Four types of cyclic ethers were synthesized using two routes based on samarium diiodide-induced intramolecular cyclization of an aldehyde or a ketone and a β-alkoxy acrylate [27] .

The trans-tetrahydropyran 1 , obtained exclusively in 90% yield , corresponds to the BC-ring system of brevetoxin B .



The trans-oxepane 2 , obtained with complete stereoselectivity , corresponds to the enantiomer of the BC-ring system of hemibrevetoxin B .



The trans-fused bicyclic tetrahydropyran 3 , corresponding to the D-ring system of maitotoxin , was synthesized efficiently using a methyl ketone instead of an aldehyde .

The trans-fused 6,7-membered ether 4 corresponds to the enantiomer of the DE-ring of yessotoxin .

Therefore , the team developed an efficient strategy for the synthesis of cyclic ethers with an angular methyl group , that could be widely applied to the total synthesis of natural polycyclic ethers .

Two years later the method was applied by another team to synthesize ABC-ring fragments of ciguatoxin 3C , a congener of ciguatoxin , the principal causative toxins of ciguatera , a disease that affect thousands of people in tropical and subtropical regions [28] .


Starting from the readily available tri-O-acetyl-D-glucal , the team used a samarium diiodide radical cyclization to stereoselectively afford a trans-fused tricyclic ether , which was further conjugated with bovine serum albumine (BSA) in order to study its interactions with monoclonal antibodies .


In 2002 Fuwa et. al published the first total synthesis of (-)-gambierol , another marine polycyclic ether toxin [29] .




The team used the samarium diiodide-induced reductive cyclization method to stereoselectively construct the F and H rings with remarkable overall efficiency .



In 2002 Nakata et.al [30] achieved the total synthesis of Brevetoxin B using samarium diiodide-induced intramolecular cyclization to stereoselectively and efficiently construct each ether ring .
Again , starting from the commercially available tri-O-acetyl-D-glucal , the team conducted a radical-induced reductive cyclization with concomitent lactonization to give an oxepane with complete stereochemistry , followed by a double cyclization to afford a trans-fused ether-lactone corresponding to the CDE-ring system . Same protocol was used for I-ring and a samarium diiodide-induced intramolecular Reformatski-type reaction was used for the construction of
K-ring .

In 2006 Fuwa with Ebine and Sasaki [31] published the total synthesis of brevenal , a pentacyclic polyether natural product .


The team conducted a samarium siiodide-induced radical cyclization to afford a tricyclic lactone as a single stereoisomer after acidic treatment .



At the same time Kimura and Nakata [31] published a samarium diiodide-induced cyclization of (E)- and (Z)-β-alkoxyvinyl sulfones with aldehydes to afford 2,6-syn-2,3-trans- and 2,6-syn-2,3-cis-tetrahydropyrans .
The mechanism would be a first single electron reduction of the aldehyde to give the ketyl radical , followed by a C-C bond formation in a chelate intermediate with complete stereoselectivity , then a second reduction to produce an anion that is immediately protonated by methanol .





(E)- β-alkoxyvinyl sulfone






(Z)- β-alkoxyvinyl sulfone

In 2002 Nakata et. al [32] achieved the total synthesis of mucocin , an antitumor agent , using a samarium diiodide-induced reductive cyclization as a key step to afford high stereo- and chemoselectivity .


The team explained the high stereoselectivity by a transition state involving a cyclic chelate , noting that the formyl group was retained during the reaction .




A year later , in 2003 , Curran with Rivkin and Nagashima published a paper on samarium(II) diiodide mediated radical/polar crossover reactions of cyclobutenes to construct the BCD ring systems of penitrems [33] . Penitrems are important biologically active indole alkaloids characterized by at least nine interlocking rings with an unusual juxtaposition .

At the time this was the first example of intramolecular cyclization of radicals with cyclobutenes.
The team adopted a Barbier procedure for the cascade reactions , using various substituents at the key carbon atom that bears an exo methylene group to probe generality .



So far samarium diiodide proved to be an outstanding versatile reagent in stereoselective reactions to synthesize highly functionalised , structurally complex organic compounds inaccessible by established conventional methods .
The publishing of many research papers this year alone [34-40] proves that the applicability of samarium diiodide as a reagent in organic synthesis is far from being exhausted .








REFERENCES


1. Matignon, C.A.; Caze, E. Ann. Chim. Phys. 1906 , 8 , 417

2. Girard, P.; Nany, J.-L.; Kagan, H.B. J. Am. Chem. Soc. , 1980, 102 , 2693

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Journal of Organometallic Chemistry 1983, 250 , 227

7. Molander , G.A.; Etter , J.B. Tetrahedron Letters 1984 , 31 , 3281

8. Fukuzawa , S.; Iida , M.; Nakanishi , A.; Fujinami , T.; Sakai , S.
J. Chem. Soc.,Chem. Commun. 1987 , 920

9. Inanaga, J.; Ujikawa, O.; Yamaguchi, M. Chem.. Lett. 1987 , 1485

10. Molander , G.A.; Etter , J.B. J. Org. Chem. 1986 , 51 , 1778

11. Suginome , H.; Yamada , S. Tetrahedron Lett. 1987 , 28 , 3963

12. Sasaki , M.; Collin J.; Kagan , H.B. Tetrahedron Letters , 1988 , 29 , 6105

13. Ferrig , T.L.; Elliot , R.I.; Curran , P. J. Am. Chem. Soc. 1988 , 110 , 5064

14. Molander, G. A.; Kenny, C. J. Am. Chem. Soc. 1989 , 111 , 8236

15. De Pouilly, P.; Chenede, A.; Mallet, J.-M.; Sinai, P. Tetrahedron Lett. 1992 , 33 , 8065

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Brenda Vinter 11/01/2007














Noble gases
Compounds

ReviewDiscovered at the end of 1800s noble gases were believed to be unable to form compounds . However , in 1962 when Bartlett isolated the first xenon compound , a new era of noble gases chemistry began .


Introduction

The development of quantum mechanics in the 1920s explained , in theory , the inactivity of noble gases . But even from those early days scientist speculated that inert gases might be able to form compounds with other chemical elements .The first compound reported by Bartlett was XePtF6 [1] , now known to be the ionic salt [XeF]+[PtF6]- .

Compounds with Fluorine and Oxygen

Compounds of xenon chemically bonded to fluoride and oxygen were the first to be synthesised and characterised .
Xenon fluoride (XeF2) was the first to be synthesised shorly after XePtF6 and it is surprising that it remained elusive for a long time since it is fairly simple to be prepared from a gaseous mixture of xenon and fluorine using different types of energy[2] .

F2(g) + Xe(g) XeF2

Other Xe fluorides that have been prepared are XeF4 and XeF6 .

Fig.1 XeF2 and XeF4
http://web1.caryacademy.org/chemistry/rushin/StudentProjects/ElementWebSites/xenon/compounds.htm

Fig.2 XeF6
http://www.3dchem.com/inorganics/XeF6.jpg


XeF2 is one of the most stable and therefore easy to handle noble gas compound and as a result its chemistry is the most extensive . It is a strong Lewis base , a fluorinating and oxidising agent and its cations (XeF+ and Xe2F3+ ) , obtained in anhydrous HF and Lewis acids , are even stronger such agents [3] .

XeF2 XeF+ + F- [4]
2XeF2 Xe2F3+ + F- [5]


Fig.3 V-shaped structure of Xe2F3+ cation in Xe2F3AsF6 [3]



XeF2 readily reacts with metals pentafluorides forming complexes which can be mainly ionic in the case of strong Lewis acids such as SbF5 and AsF5 , or mainly covalent for milder Lewis acids , when XeF2 is coordinated to MF5 via a fluorine bridge .



Fig.4 Structure of XeFAsF6 [6]



XeF2 reacts as well with the weaker Lewis acids tetrafluorides , includind the xenon tetrafluoride ,
XeF4 [7] .

Fig.5 XeF2.CrF4 [8]

Due to its semi ionic character and its small size XeF2 is a very good ligand and a variety of metal complexes have been synthesised , were XeF2 acts as a ligand , either coordinated to one metal centre or as a bridging ligand between two metal centres [3] .

Fig.6 [Mg(XeF2)2](AsF6)2 , an example of metal coordinated by non-bridging XeF2 [9]

Fig.7 [Ba(XeF2)5](AsF6)2 , example of a metal coordinated by bridging XeF2 [3]

Fig. 8 [Ca2(XeF2)9](AsF6)4 , example of a metal coordinated only by XeF2 [10]






Krypton fluorides compounds that have been reported so far are KrF2 and salts of the cations KrF+ and Kr2F3+ ,all thermodynamically unstable , with KrF2 the only krypton compound that has been structurally characterised [11] .
These compounds have similar properties with the xenon fluorides , forming salts with strong fluoride ion acceptors of the type MF6 , where M= As , Sb , Au , Pt , Bi and Ta .





Fig.9 α-KrF2 packaging diagram[11] Fig.10 [Kr2F3][SbF6].KrF2 [11]





There are two known xenon oxides , XeO3 , a solid at room temperature , and XeO4 , a gas at room temperature , both thermodynamically unstable and decompose explosively [12] .

XeO3 XeO4
Fig.11 http://www.3dchem.com/inorganicmolecule.asp?id=817


More versatile are the xenon oxide fluorides XeOF4 and XeO2F2 , which form complexes with strong Lewis acids AsF6 and SbF6 [13] .









Fig.12 XeOF4 Fig.13 XeO2F2
www.apsu.edu/jonesr/Chapter%2010%20Review.pps

Four such complexes have been characterised , α-[XeO2F][SbF6] , β-[XeO2F][SbF6] which exists as a tetramer , [XeO2F][AsF6] and [FO2XeFXeO2F][AsF6] which has a bent F-Xe-F bond [13] .

Fig.14 α-[XeO2F][SbF6] [13] Fig.15 [FO2XeFXeO2F][AsF6] [13]




Fig.16 Tetrameric structure of β-[XeO2F][SbF6] [13]HNgY compouds


Another big group of noble gases compounds are those of the type HNgY , where Y is a fragment with strong electron affinity . This group brought the first xenon-sulfur bond compound , HXeSH ,
a thermodynamically unstable but thermally stable molecule[14] and , most importantly , the first chemically bonded argon compound , HArF [15] .

Fig. 17 Synthetic route of HXeSH [14]

Predicted computationally in 1995 and isolated in 2000 , HArF is a highly polarised , metastable compound . The molecule was experimentally observed in a solid state argon matrix [15] and it was shortly followed by the discovery of HKrF in a solid state krypton matrix[16] .


Fig.18 HKrF in solid Kr a) double-substitutional site and b) single-substitutional site [16]


Other non-metals – noble gas compounds

The first compound of xenon bonded to nitrogen reported in early 70s was FXeN-(SO2F)2 [17] and since a handful of imidodisulfurylfluoride compounds have been characterised such as Xe[N(SO2F)2]2 , F[XeN(SO2F)2]2+ etc . all containing xenon bonded to a sp2 hybridised nitrogen .
Recently a compound with xenon bonded to a sp3 nitrogen has been synthesised as a [AsF6]- salt [18] .

Fig.19 [F5TeN(H)Xe][AsF6] structure [18]

Compounds with krypton-nitrogen bonds such as HC≡ N- KrF+, F3C≡N-KrF+ have been also characterised as salts of [AsF6]- with an octahedral symmetry around the anion [18] .

HArF only opened the first page of argon chemistry . Twelve kinetically stable compounds of noble gases and cyanoacetylene have been reported , four of them with argon . Two types of these compounds were characterised , HNgCCCN and the less stable HNgCCNC [19] .

Fig. 20 Ng-C and Ng-N compounds [19]

Computational research has been carried out on HCCArNC , possibly the first stable neutral compound with an Ar-N bond [19] .

Further theoretical studies have been done on argon-carbon and argon-silicon bonds and two compounds were predicted : FArCCH and FArSiF3 [20] . Computational work showed that the bonding in these compounds is stronger than in HArF , and therefore they are more stable than the only experimentally characterised argon compound [20] . This suggests that a whole family of this type of compounds of argon should be possible to synthesise and that argon chemistry is not science-fiction .









Fig.21 Geometry and electronic density of FArCCH (bond length in Å) [20]



Fig.22 Geometry and electronic density of FarSiF3 (bond length in Å) [20]




Metal – Noble gas bonds

After the discovery of a Au-Xe bond in the bulk compound AuXe42+[SbF11-] [21] a new chapter in the noble gases chemistry opened : noble gases-noble metals compounds . Compounds with linear structure of type NgMF and MNgF have been computationally characterised , including an argon compound , AgArF . The type MNgF have been found to have the strongest nobel gas – metal interaction , with CuXeF the most stable and probably the easiest one yet to be synthesised .



Conclusions

With the chemistry of heavier noble gases , krypton and xenon , well established , the chemistry of the lighter ones , argon and helium , is still at an early stage , while a neon compound is still yet to be identified .


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