Substitution photochemical

Photochemical reactions can occur when light is absorbed by a compound. In this process, an electron is promoted and the ground-state electronic configuration is changed to that of one of the excited states. Even the longer-lived of these states only survive 10 to 10 sec, and so if any photochemistry is to occur, the excited state must react very quickly. If a molecule of product is formed for every photon absorbed, the quantum yield, 4 , is said to be unity. Otherwise the electron falls back to the ground state and the compound either emits light (luminescence) or is heated up thermally in this case, chemistry does not occur and 4 ifor product formation will normally be less than unity. 

The photolysis of W(CO)5L can lead either to loss of L or of a CO group cis to L, according to the wavelength used. This result can be understood in terms of the crystal field diagram for the complex, shown in Fig. 4.5. Since the symmetry is lower than octahedral because of the presence of L, both the da and the d levels split up in a characteristic pattern. The L ligand, conventionally placed on the z axis, is usually a lower-field ligand than CO and so the d 2 orbital is stabilized with respect to the As we saw in 

Section 1.5, these are really M—L ct orbitals, d -y playing this role 

FIGURE 4.5 The crystal field basis for the selectivity observed in the photolysis of M(C0)5L complexes. Irradiation at a frequency v, raises an electron from the filled d), level to the empty ct (z), where it helps to labilize ligands along the z axis of the molecule. Irradiation at vi labilizes ligands in the xy plane. 

L ligand because it lies on the z axis. Irradiation at V2 will tend to populate (j y, and so one of the cis COs will be labilized, because they lie in the xy plane, cis to L. Where L is pyridine, the appropriate wavelengths are 400 nm (vi) and 250 nm (V2), respectively. The method has often been used to synthesize cw-Mo(CO)4L2 complexes. 

Photochemical reactions can occur when light is absorbed by a compound. In this process, an electron is promoted and the ground-state electronic configuration is 

Increased pressure accelerates an associative process because the volume of the transition state L M L is smaller than that of the separated L M and V molecules the reverse is true for a dissociative process because is 

The second most common photosubstitution is the extrusion of H2 from a di- or polyhydride discovered in the case of the yellow crystalline complex, CP2WH2 (Eq. 4.54). This is most probably the result of the promotion of an electron into the M-L cr orbital corresponding to the MH2 system. Sometimes the reductive elimination product is stable, or as in Eq. 4.54, it can also be very unstable and 

Another important photochemical process is the homolysis of M—M bonds. The fragments produced are likely to be odd-electron and therefore substitutionally labile. For example, the photosubstitution of CO in Mn2COio by PPhs proceeds via the 17e intermediates Mn(CO)5. Equation 4.57 is an interesting example because the replacement of three COs by the non-w-acceptor NH3 leads to a buildup of electron density on the metal. This is relieved by an electron transfer from a 19e Mn(CO)3(NH3)3 intermediate to a 17e Mn(CO)s fragment to give the disproportionation product 4.20 in a chain mechanism.-  

Increased pressure accelerates an associative process because the volume of the transition state L M—L is smaller than that of the separated L M and L the reverse is true for a dissociative process because L iM -L has a larger volume than L M. Several hundred atmospheres are required to see substantial effects, however. Van Eldik has shown that pressure accelerates the MLCT photosubstitution of W(CO)4(phen) but decelerates the LF photosubstitution. As the MLCT excited state is effectively a 17e species, an A mechanism is reasonable for this process the LF process is evidently a D mechanism, probably as a result of populating the M-L t levels. 

Thermal substitution in (ri -C7H8)Cr(CO)3 goes by loss of C/Hg because the triene binds much more weakly than CO. In contrast. 

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Photochemical substitution reactions of this type which involve selective hydrogen abstractions from intramolecular sites by the m.tt ketone oxygen, are reviewed in chapter 12. ... 

Photochemical substitutions on metal carbonyls and their derivatives. W. Strohmeier, Angew. Chem., Int. Ed. Engl., 1964, 3, 730-737 (68). 

Mechanistic information from the effect of pressure on thermal and photochemical substitution reactions. R. van Eldik, Comments Inorg. Chem., 1986, 5,135 (33). 

It is generally believed (1) that the photochemical substitution reactions of Cr(C0)5 can be summarised ... 

Photochemical Substitution Reactions in Synthetic Organic Chemistry... 

Photoexcited aromatic compounds undergo substitution reactions with (non-excited) nucleophiles. The rules governing these reactions are characteristically different and often opposite to those prevailing in aromatic ground state chemistry 501a,b>, in contrast to the well known ortho/para activation in thermal aromatic substitutions, nitro groups activate the meta position in the photochemical substitution, as shown in (5.1) 502). 

The photochemistry of octacyanometallates, and of mixed cyano-dii-mine complexes of the type [W(CN)6(diimine)]2 and [MO(CN)3(bpy)] M = Mo, W, has been reviewed (183). The authors pay particular attention to the role of the counterion in this type of reaction they also call attention to questions which were, at the time of writing, unresolved. A mainly structural and redox review of octacyano-, nitridotetracyano-, and oxotetracyano-metallates (Nb, Ta Mo, W Tc, Re) contains some kinetic and mechanistic information on thermal and photochemical substitution in these complexes, with the main conclusion being that much more needs to be done on such reactions (184). 

It is apparent that the chemistry of such systems is rich, but the preparation by either thermal or photochemical substitution normally leads to complex mixtures of compounds. Recently, substituted products, which can be prepared in high yield, have been utilized as precursors. Two classes of reactions (Table IX) may be employed for the preparation of cluster derivatives those involving displacement in systems typified by complexes (a), (b), (c), and (d), or addition reactions to the nominally "unsaturated species H2Os3(CO)10 (see also Section 11,1,2). 

The photochemistry of [Cr(CO)e] has been investigated in several studies. Flash photolysis of cyclohexane solutions of [CrfCO) ] affords two species one has a of 470 nm and a lifetime of 5 ms and the other, = 440 nm, has a lifetime > 1 s. The relationship between photolysed species of [CrfCO) ] and photochemical substitution reactions described in Scheme 4 has been suggested from i.r. and u.v. spectroscopic studies of matrix-isolated species. ... 

Reviews " of pentacyanoferrate substitution kinetics have included a detailed consideration of high-pressure studies of thermal and photochemical substitution and electron transfer reactions of pentacyanoferrates-(II) and -(III). Photochemical activation can result in the loss of L or of CN . The best way to study the latter is through photochemical chelate ring closure in a pentacyanoferrate complex of a potentially bidentate ligand LL [Fe(CN)5(TL)]" rFe(CI 4(LL)] " +... 

A photochemical substitution reaction that does involve radical intermediates, but in a different way, is the photo-Fries reaction... 

Photochemical substitution reactions can however follow other pathways than the concerted one which is the rule in the ground state processes. The orientation effects of electron donor and electron acceptor substituents are based on the model of a transition state of a complex which implies a concerted reaction (Figure 4.65). 

Photochemical substitution reactions can proceed through high-energy products such as radical ions, the primary process being a dissociation or an ionization of the excited molecule. Such processes do not have to follow the orientation rules dictated by the charge distribution of the excited molecule, and in many instances the product distribution is still little understood. 

Photochemical substitutions of carbohydrates can be differentiated by describing the carbon atom which is transformed in the reaction. This deals with reactions where the photochemical event is the breaking and the creation of a bond in a carbohydrate moiety ... 

The results clearly show that the same kinetic anomeric effects governs these intramolecular cyclization reactions as well as the preceding anomeric hydrogen abstractions described in intermolecular photochemical substitutions. 

Photochemical substitution of some iodo-substituted pyrroles 597 in the presence of aromatic compounds depends on the structure of the pyrrole and on reaction conditions and gives the corresponding aryl derivatives 598 and/or dehalogenated product 599 (Scheme 120) <1997J(P1)2369>. Use of 4,5-diiodopyrrole-2-carbaldehyde 597 (R = I, = H, R = CHO) as substrate and irradiation in benzene, w-xylene, thiophene and 2-chlorothiophene as solvents gives solely the corresponding 5-aryl derivatives 598 in good yields (57-100%). There is no competition between 4-and 5-substitution. 

A series of stable organometallic SO2 complexes of Cr, W, and Mn, including CpMn(C0)2(S02), has been synthesized by photochemical substitution of carbonyl ligands. Unstable pentacarbonyls, M(C0)5(S02) (M = Cr, W), were claimed but not isolated The complex CpMn(C0)2(S02) was found to contain j -planar SO2 (M-S = 2.037(5) A) and is one of the few sublimable metal-S02 complexes The SO2 ligand lies approximately in the plane which also contains the Mn atom and one atom of the Cp ring (see Fig. 5). The bonding has been discussed by Hofhnaim and coworkers who concluded that the observed orientation allows interaction between the best 71 acceptor orbital of SO2 and the best jt donor orbital of the CpMn(CO)2 fragment ... 

Chromium hexacarbonyl is extremely photolabile (equation 6) therefore photochemical substitution is an efficient means of preparing derivatives. Oxidation of the Cr center requires nitric or sulfuric acid, or chlorine. Alternatively, some hgands induce complete carbonyl dissociation with concomitant oxidation, for example, acetylacetonate. Chemical reduction with alkali or alkaline-earth metals or electrochemical reduction proceeds in two-electron steps with loss of two CO molecules to first give [Cr2(CO)io]" and then [Cr(CO)s]. Nucleophilic attack at CO generates a number of stable (Nu = R) and unstable (Nu = N3, OH, H, NEt2) products. The stable [(OC)5CrCOR] ion is a carbene precursor. 

The 17-electron species see Seventeen Electron Configuration) formed can undergo rapid substitution since associative pathways of low activation energy exist for them. Recombination of the substituted flagments can yield the monosubsti-tuted, disubstituted, or more fully substituted complexes. The rate-determining step would be cleavage of the metal-metal bond since all other steps are relatively fast. This pathway may be preferred for photochemical substitution. Clearly, this pathway is not open to mononuclear analogs. The reactivity of low valent metal radical complexes has been reviewed. ... 

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