Carbonyl oxides matrix isolation

The carboxylic acid derivatives li-lm can only be matrix-isolated if the corresponding quinone diazides 2i-2m are irradiated with monochromatic blue light (k = 436 nm).81 91 92 UV or broad-band visible irradiation rapidly results in the decarboxylation of the carbenes. As expected, the IR and UV/vis spectra of the carbenes are very similar to that of la. Oxygen trapping results in the formation of the photolabile carbonyl oxides 7. Thus, the carbenes li-lm were identified both spectroscopically and by their characteristic reaction with molecular oxygen. 

The matrix photochemistry of 2v proved to be fairly complicated.108 The primary product of the photolysis of 2v is carbene lv, which was identified by ESR spectroscopy. Under the conditions of matrix isolation the carbene showed the expected reactivity towards molecular oxygen (formation of carbonyl oxide 7v) and carbon monoxide (formation of ketene lOv) (Scheme 22). In contrast to the oxocyclohexadienylidenes (la and derivatives) carbene lv slowly reacted with CO2 to give an a-lactone with the characteristic C=0 stretching vibration at 1896 cm-1. The latter reaction indicates that lv is — as expected — more nucleophilic than la. 

Infrared spectroscopy has also been used to characterize carbonyl oxides in matrix isolation. The carbonyl oxides are identified by their intense 0—0 stretching... 

Carbonyl oxides are usually highly unstable and can be detected only by matrix isolation spectroscopy at low temperature or by FFP techniques. However, they can be stabilized through steric protection. Dimesityl ketone oxide (80c, ),niax = 398 nm) generated by the reaction of triplet dimesitylcarbene (19c) with O2 at... 

Photolysis of matrix-isolated (trimethoxysilyl)diazomethane at X > 305 nm produced carbene 3e (equation 1), which was characterized by IR and UV-Vis spectroscopy23. Under these conditions, no other species besides the carbene could be detected spectroscopically. In an 02-doped (1%) argon matrix, the carbene rapidly reacted with oxygen to give carbonyl oxide 4 which was further photoisomerized to formylsilane 5. Again, the fast reaction of 3e with 302 points to a triplet ground state of the carbene. 

The organometallic chemistry of aluminum is dominated by the chemistry of aluminum(lll), but lower oxidation state compounds are now accessible. The first examples of this class of compounds are carbonyl complexes such as Al(CO), A1(C0)2, and Al3(CO), which were generated upon exposure of aluminum atoms to CO in matrix-isolation experiments near 20 K. The number, relative intensities, and frequency of the carbonyl stretches in the IR spectra, along with isotopic labeling and EPR studies were used to verify these compositions. These complexes exhibit vco values of 1868, 1985 and 1904, and 1715 cm , respectively, indicative of Al- CO 7t backbonding. The carbonyl species are unstable at higher temperatures and no stable carbonyl complex of aluminum, in any oxidation state, has been reported. The monomeric aluminum-alkene adducts A1( -C2H4) and k rf-CeHe) were similarly identified in inert matrices at low temperature. No room-stable alkene complexes of aluminum have been reported. 

The pale yellow [Ni(PEt3)4] is also tetrahedral but with some distortion. In sharp contrast to nickel, palladium forms no simple carbonyl, Pt(CO)4 is prepared only by matrix isolation at very low temperatures and reports of K4[M(CN)4] (M = Pd, Pt) may well refer to hydrido complexes in any event they are very unstable. The chemistry of these two metals in the zero oxidation state is in fact essentially that of their phosphine and arsine complexes and was initiated by L. Malatesta and his school in the 1950s. Compounds of the type [M(PR3)4], of which [Pt(PPh3)4] has been most thoroughly studied, are in general yellow, air-stable solids or liquids obtained by reducing complexes in H2O or H20/EtOH solutions with hydrazine or sodium borohydride. They are tetrahedral molecules whose most important property is their readiness to dissociate in solution to form... 

Nearly all of the compounds were synthesized photochemically, which imposes a nuihber of quite strict limitations on larger-scale preparations of these compounds. The primary, or at least the initial, identification of these compounds has been via IR spectroscopy. (The sole exception has been Cp Re(CO)(C2H4)2 which was tentatively identified by NMR [18]). In most cases, characterization of the metal carbonyl moiety via v(C-O) IR bands is definitive, particularly if library spectra are available from liquid noble gas [23] or matrix isolation experiments [4]. This is because these bands are sharp and intense, and the C-O stretching vibrations are largely uncoupled from other vibrations of the molecule. Furthermore, the precise wavenumber of the bands are extremely sensitive to the oxidation state of the metal center as illustrated, for example, by Kazarian et al. in their study of hydrogen bonding to metal centers [25]. 

Sander and co-workers [94ACIE2212] have reported the first preparative-scale synthesis of a dioxirane via carbene oxidation. Thus, the relatively stable dimesityldioxirane 106 was prepared by the low-temperature O2 oxidation of carbene 104 in matrix isolation in CFCI3 an intermediate carbonyl oxide (105) was observed spectroscopically. 

The mechanism of the ozonolysis reaction of alkenes has been investigated in the gas phase and solid state using matrix isolation spectroscopy. While alkene/ozone 7C-complexes and the primary ozonides are readily observed by IR und UV/vis spectroscopy, there is no direct spectroscopic evidence for the Criegee intermediate (carbonyl O oxide) in these reactions. However, these elusive species can be synthesized and characterized via the carbene/oxygen route. Comparison of experimental and calculated spectroscopic data allows for the prediction of the spectroscopic properties of carbonyl oxides which are not accessible by this method. 

The only route to carbonyl O oxides 1 which is suitable for matrix isolation is the oxidation of free carbenes 2 with molecular oxygen 02 [5-7]. The carbenes are generated by photolysis of the corresponding diazo compounds or diazirines. Prerequisite for this method is that the carbenes are stable under the conditions of matrix isolation (solid rare gas at 10 K). This excludes simple alkyl substituents, because these carbenes rapidly undergo [1,2]-H shifts to yield alkenes in essentially quantitative yields. During the last years we were able to study the spectroscopy and chemistry of a number of carbonyl oxides (R and R were H, Ph, HC=C, CF3, cyclic systems etc.) [8-12] by IR and UV/vis spectroscopy. 

However, while RE carbonyl complexes are not bottleable, they have been detected under matrix isolation conditions in frozen argon matrixes or in CO atmosphere high-pressure studies. Binary RE carbonyls, [M(CO) ] (n = 1-6), have been observed with some evidence for the formation of M(CO) (n = 7 or 8) species. These binary RE carbonyl adducts typically exhibit a single carbonyl stretch which is 145 195 cm lower than the stretching frequency of 2134 cm for free CO. Backbonding could be assumed to operate, which is reasonable for RE metals in a formal zero-oxidation state where valence orbitals are more chemically accessible. However, it has been noted that the infrared (IR) spectra of all binary RE carbonyls across the RE series... 

The next higher homolog of this series, C6S2, has been discovered in the gas phase . While carbonyl sulfide is a well known stable molecule, the higher sulfide oxides 0=(C) =S are also of recent vintage, and they were obtained by matrix isolation. For example, C2OS 2 was matrix isolated by generating it in the photochemical reaction of carbon monosulfide with carbon monoxide . 

Before these matrix-isolation studies, it had been usual to denote carbonyl oxides as zwitterions cf. 48a), but the experimental IR spectra of 48 and its analogues did not exhibit absorptions that could be assigned to C=0 bonds. It was therefore concluded that carbonyl oxides are better represented as polar singlet biradicaloids cf. 48b). ... 

Palladium(I) complexes are in general dimeric or oligomeric and consequently, although they have a d9 configuration, they are usually diamagnetic. The chemistry of this oxidation state is discussed in Section 51.3. Unlike most transition metals, the chemistry of low valent palladium is not dominated by carbonyls [Pd(CO)4] is only stable at 80 K in a matrix. As with platinum, the most common complexes are those containing phosphines, where complexes of the type [PdL ] (n = 2, 4) have been isolated. The chemistry of palladium(O) is dealt with in Section 51.2 and elsewhere.2... 

Well-defined arene complexes of Group 4 metals in various oxidation states have been isolated. The air- and moisture-sensitive complexes Ti(r -arene)2 (56) have a sandwich structure similar to that of the related chromium compounds [176-178]. They have been used for deoxygenation of propylene oxide and coupling reaction of organic carbonyl compounds [179]. The first synthesis of 56 was cocondensation of metal vapor with arene matrix [176]. Two more convenient methods are reduction of TiCl4 with K[BEt3H] in arene solvent [180] and reaction of TiCl4(THF)2 with arene anions followed by treatment with iodine [170,176]. The latter method involves the formation of an anionic titanate complex, [Ti(ri -arene)2] (57), which can also be formed from KH and 56 [181]. 

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