Transition metal carbonyls irradiation

Photochemical activation of transition metal carbonyls has been used as a preparative tool for substitution of carbonyl ligands by donor molecules or unsaturated hydrocarbons for many years (7-6). The advantage of photochemical activation in comparison with thermal activation is the possibility of conducting reactions at fairly low temperatures. Hence even thermolabile products can be prepared and isolated by appropriate treatment of the reaction mixtures. However, due to the various activation modes of transition metal carbonyls by UV light, often more than one product is obtained, and chromatographic separation is necessary. Limitations are set primarily by the amount of substance which can be irradiated in solution at one time. 

Transition metal carbonyl complexes can be suitable precursors, for example, treatment of hydrido complexes or their alkali salts with PF3 under UV irradiation followed if necessary by acidification of the reaction mixture (method E). Interestingly, vanadium forms the hydrido trifluorophosphine complex [VH(PF3)6], whereas with CO, the paramagnetic [V(CO)6] is formed. 

A rather different route to short-lived intermediates is to irradiate a suitable precursor with radiation of an appropriate energy. Most common is UV-visible photolysis. Section 5 discusses some of the rich photochemistry displayed by transition metal carbonyls, these being very amenable to UV-visible photolysis. Thus unsaturated carbonyl fragments may be produced ... 

Table 6. Polymerization of HCsCPh by UV irradiation of group 6 and 7 transition metal carbonyls ...

Most transition metal complexes are sensitive towards oxygen and/or water, especially under irradiation. This fact sometimes impedes photochemical work in this field. Detailed information about the properties and thermal chemistry of transition metal carbonyl compounds is found in 519>2 and there are excellent reviews on arene complexes 177 387,525,532) and olefin complexes i78,22i) ... 

Substitution of CO. The substitution of a CO by a PH3 ligand In transition metal carbonyl compounds can be Initiated thermally or by UV Irradiation. In some cases, when such reactions are carried out In donor solvents, like THF or acetonitrile, complexes In which one CO ligand Is replaced by a solvent molecule are assumed to be Intermediates. These intermediates, however, generally can not be Isolated or have not been Isolated In practice Instead the resulting solution Is reacted In situ with PH3. Therefore, these solvent stabilized 16-electron Intermediates were not Included In the subsequent description of the various reactions. Attempts to replace more than one CO ligand in a certain complex directly with the same number of PH3 molecules led mostly to the formation of a mixture of complexes containing different numbers of PH3 ligands. 

The role of transition-metal carbonyls and particularly those of the Group 6 metals in homogeneous photocatalytic and catalytic processes is a matter of considerable interest [1]. UV irradiation especially provides a simple and convenient method for generation of thermally active co-ordinately unsaturated catalyst for alkenes or alkynes transformation. By using tungsten and molybdenum carbonyl compounds as catalysts, alkenes and alkynes can be metathesized, isomerised and polymerized. Photocatalytic isomerization of alkenes in the presence of molybdenum hexacarbonyl was observed by Wringhton thirty years ago [2]. Carbonyl complexes of molybdenum catalyze not only... 

Liicke et al. have prepared other phosphinated POSS compounds Tg[(CH2)2-PMe2]8 and Tg[(CH2)3-PMe2]8 by treating T8[CH = CH2]8 or T8[CH2-CH = CH2]8 with H-PMe2 under UV irradiation. The former compound has shown to have good coordination properties to carbonyl transition metal complexes such as CpMn(CO)3 (Table 15). 

Yeom and Frei [96] showed that irradiation at 266 nm of TS-1 loaded with CO and CH3OH gas at 173 K gave methyl formate as the main product. The photoreaction was monitored in situ by FT-IR spectroscopy and was attributed to reduction of CO at LMCT-excited framework Ti centers (see Sect. 3.2) under concurrent oxidation of methanol. Infrared product analysis based on experiments with isotopically labeled molecules revealed that carbon monoxide is incorporated into the ester as a carbonyl moiety. The authors proposed that CO is photoreduced by transient Ti + to HCO radical in the primary redox step. This finding opens up the possibility for synthetic chemistry of carbon monoxide in transition metal materials by photoactivation of framework metal centers. 

The possible mechanisms which one might invoke for the activation of these transition metal slurries include (1) creation of extremely reactive dispersions, (2) improved mass transport between solution and surface, (3) generation of surface hot-spots due to cavitational micro-jets, and (4) direct trapping with CO of reactive metallic species formed during the reduction of the metal halide. The first three mechanisms can be eliminated, since complete reduction of transition metal halides by Na with ultrasonic irradiation under Ar, followed by exposure to CO in the absence or presence of ultrasound, yielded no metal carbonyl. In the case of the reduction of WClfc, sonication under CO showed the initial formation of tungsten carbonyl halides, followed by conversion of W(C0) , and finally its further reduction to W2(CO)io Thus, the reduction process appears to be sequential reactive species formed upon partial reduction are trapped by CO. 

Raman and UV-visible spectroscopy, but no precise characterization was made. A report was made in 1981 where the IR spectrum of Cu atoms deposited with C02 at 80 K was interpreted in terms of the formation of a -coordinated complex between C02 and zerovalent copper [32]. Almond et al. [33] prepared a (C02) M(CO)5 molecule (M = Cr, W), that led to the formation of CO and oxometal carbonyl under UV irradiation. The first complete study of the reactivity of C02 with the first row of transition metals was made by Mascetti et al. [34, 35]. Here, it was shown that the late transition metal atoms (Fe, Co, Ni, and Cu) formed one-to-one M(C02) complexes, where C02 was bonded in a side-on (Ni), end-on (Cu), or C-coordinated (Fe, Co) manner, while the earlier metal atoms (Ti, V, and Cr) spontaneously inserted into a CO bond to yield oxocarbonyl species OM(CO) or 0M(C0)(C02). Normal coordinate analysis showed that the force constants of CO bonds were significantly decreased by 50%, compared to free C02, and that the OCO angle was bent between 120 and 150°. 

The zinc-mediated Reformatsky reaction is one of the classical methods for carbon-carbon bond formation. To date, various main group metals and transition metals have been used for this reaction. Rieke s activated indium powder mediates readily the coupling of ethyl a-bromoacetate and a variety of carbonyl compounds yielding /3-hydroxy esters in good yields (Scheme 87).3 Later, commercially available indium powder has been found to be equally effective for the indium-based Reformatsky reaction in THF.28 This indium Reformatsky reaction is accelerated by ultrasound irradiation (Scheme 88).322,323 Indium(i) iodide also mediates the Reformatsky reaction of aldehydes and ketones to give /3-hydroxy esters, presumably via organoindium(m) diiodide (Scheme 89).27... 

The first reported arene-transition metal trifluorophosphine complex [Cr(i/6-C6H6)(PF3)3] was obtained by UV irradiation of [Cr(i/6-C6H6)(C0)3] with PF3 (method A), whereas the corresponding thermal reaction afforded instead the mixed carbonyl trifluorophos-... 

Carbonyl precursors of the type [R3MM (CO)5] (M = group IV metal M = transition metal) react readily with PF3 either thermally or under the influence of UV irradiation (method D), and PF5 has also been utilized in the case of silyl-substituted metal carbonyl complexes (method E). 

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