Supercritical noble gases

Liquid and supercritical noble gases are ideal media for studying H2 species since H2 is completely miscible under these conditions. The group VI metal carbonyl hydrides, M(CO)5(H2), and cis-Cr(CO)4(H2)2 were formed photochemically in liquid Xe/If2 mixtures. H2 and N2 complexes of a number of half-sandwich compounds were formed in supercritical Xe. These included (C Rn)M(CO)2Y, where R = H, Me, M = Mn, Re (n = 5), Cr (n = 6), and (C4H4)Fe(CO)2Y, where Y = H2 or N2. In all cases except Re, nonclassical H2 complexes were formed. ... 

IR bands due to vCO for [Rh(CO)2(L)]+, where L = bis[2-(3,5-dimethyl-1-pyrazolyl)ethyl]-ether, showed that two isomers were present in solution (four bands seen).140 Fast TRIR data were used to follow the reactions of (r 5-CsR5)Rh(CO)2 (R = H or Me) in supercritical noble gases (Xe, Kr). Evidence was found for the formation of CpRh(CO)(L), where Cp = C5H5 or C5Me5, L = Xe or Kr, at room temperature.141... 

Crystallization occurs for many common fluids, such as carbon tetrachloride, benzene, and cyclohexane, at pressures less than 200 MPa, thus their entire fluid range is rather limited. The supercritical noble gases significantly extend this range, achieving a maximum crystallization pressure for helium of P = 11.6 GPa. The viscosity increase prior to all such transitions is modest. The viscosity for a typical dense fluid is 1-100 mPa and this will increase by, at most, about three orders of magnitude. Experimentally, this viscosity and pressure regime is covered by many of the viscometers discussed below, and hard-sphere theory can explain most of the viscosity increase. 

Of all the compounds capable of becoming supercritical fluids under relatively moderate temperatures and pressures, supercritical carbon dioxide (CO2) is unique in that, unlike supercritical alkanes (ethane, propane, and n-butane), it is nonflammable and environmentally friendly. Supercritical noble gases, such as krypton and xenon, are benign but very expensive. Although water is environmentally friendly, the critical temperature of 374" C and pressure of 212 atm are much higher that those for CO2. 

These complexes are transient species generated from photolysis of M(CO)6 in supercritical noble gas solution at room temperature. Their octahedral structure (symmetry C4V) has been characterized by IR spectroscopy and solution NMR. The noble gas atom E can be formally considered as a neutral two-electron donor ligand, and the bonding involves interactions between the p orbitals of E and orbitals on the equatorial CO groups. The stabilities of the complexes decrease in the order W > Mo Cr and Xe > Kr > Ar. Organometallic noble gas complexes... 

O. S. Jina, X. Z. Sun and M. W. George, Do early and late transition metal noble gas complexes react by different mechanisms A room temperature time-resolved infrared study of (z)5-C5H5)Rh(CO)2 (R = H or Me) in supercritical noble gas solution at room temperature. Dalton Trans., 1773-8 (2003). 

Fig. 14. Values of In (A2) ( 2 = bimolecular rate constant) for the reaction of a series of group VI noble gas complexes with CO in supercritical noble gas solution at 298 K, representing their relative stabilities. The data were taken from Ref. (64).

The Bimoleculae Rate Constants for the Reaction of a Series OF Transition Metal-Noble Gas Complexes with CO in Supercritical Noble Gas Solution at 298 K... 

In the absence of a reactant gas (e.g., CO), all the noble gas complexes we have studied in supercritical noble gas solution at room temperature have been kinetically unstable with respect to decomposition pathways... 

This has allowed us to identify, for the first time in solution at room temperature, organometallic noble gas complexes which are formed following irradiation of metal carbonyls in supercritical noble gas solution. We have found that these complexes are surprisingly stable and have reactivity comparable to organometallic alkane complexes. In addition, we have studied the co-ordination of COj to metal centres in supercritical CO2 (scCOj) and shown that v(C-O) bands provide a very sensitive probe for the oxidation state of the metal centre. We found evidence, albeit circumstantial, for the formation and reactivity of ri -O bound metal COj complexes in solution at or above room temperature and found these highly reactive COj complexes have similar reactivity to the analogous Xe complexes [11-12]. We have also used TRIR to examine the reactivity of CpMofCO), radicals in scCOj and found evidence for an interaction, possibly Lewis Acid/Base, between CpMo(CO), and scCO [13]. 

X-Z Sun, MW George, SG Kazarian, SM Nikiforov, M Poliakoff. Can organometal-lic noble gas compounds be observed in solution at room temperature A time-resolved infrared (TRIR) and UV spectroscopic study of the photochemistry of M(C0)6 (M = Cr, Mo, and W) in supercritical noble gas and CO2 solution. J Am Chem Soc 118 10525-10532, 1996. 

Recently the MEM was applied to the calculation of optical absorption spectra and coirelation functions for an excess electron in water and supercritical noble gas fluids. Semi-quantitative agreement with experimental spectra was found in the case of water, where the electron is confined in cavities. In addition, the electron mobility was successfully modeled in the supercritical helium environment where the electronic states are extended. 

A special type of TM ligands are the noble gas atoms argon, krypton, and xenon [61]. Although they are weak Lewis bases, TM complexes M(CO)sNg with M = Cr, Mo, W and Ng = Ar, Kr and Xe have been experimentally investigated in the gas phase as well as in the liquid phase and in supercritical C02 [62, 63], The M-Ng BDEs were estimated with... 

As described earlier, high pressure cells have been developed for the use of noble gases as solvents for IR spectroscopic studies, either at low temperature, or at ambient temperature where the supercritical phase exists. A particular focus of this work was the study of reactive complexes containing coordinated noble gas atoms or molecular H2, the latter being particularly relevant to hydrogenation reactions. 

These Rh complexes have been the subject of intense interest due to their propensity for C-H activation of alkanes (Section 3.3.2.7). The noble gas complexes [CpRh(CO)L] and [Cp Rh(CO)L] (L = Kr, Xe) have also been studied in supercritical fluid solution at room temperature [120]. For both Kr and Xe, the Cp complex is ca. 20-30 times more reactive towards CO than the Cp analogue. Kinetic data and activation parameters indicated an associative mechanism for substitution of Xe by CO, in contrast to Group 7 complexes, [CpM(CO)2Xe] for which evidence supports a dissociative mechanism. 

Matrix isolation spectroscopy has proved an invaluable technique for the isolation and characterization of transition metal—noble gas complexes (see Table III). However, this technique has obvious limitations. Although photoproducts in low-temperature matrices can be made to react with added dopants, it is impossible to accurately predict their reactivity and mechanisms in solution at room temperature. Therefore, in the years following the original discovery of transition metal-noble gas interactions in matrices, new techniques have been used to probe these species in solution, gas phase, and supercritical fluids. 

Gas-phase studies have not been restricted to the group VI hexacar-bonyls. Fu and co-workers (54) have used TRIR to study the coordina-tively unsaturated species CpMn(CO) (x = 1 and 2) generated by 266-and 355-nm laser photolysis of CpMn(CO)3 in the gas phase. In the presence of noble gas L (L = He, Ar, or Xe), they were able to measure the rate constant for reaction of the noble gas complex CpMn(CO)2L with CO. Interestingly, they foimd that only Ar significantly perturbed the rate fi om that observed in the absence of noble gas. This was thought to be because He has too high an ionization potential and Xe is too bulky to interact with the Mn center. In light of recent TRIR experiments conducted in supercritical fluid solution, the conclusion that Xe is unable to coordinate is incorrect. 

---