Olefin complexes photochemical

Although complexes with C—H—metal three-center, two-electron bonds were first observed several years ago (40-42), they have received increasing attention recently as model systems for C—H activation by transition metal complexes (43). A general route to such compounds involves the protonation of diene (35,44-51) or olefin complexes (52-56). The resulting 16-electron species are stabilized by the formation of C—H—metal bridges. Irradiation of the complexes [Cr(CO)s L] [L = CO, P(CH3)3, P(OCH 3)3 jin presence of conjugated dienes having certain substituents provides a photochemical route to electron-deficient >/4 CH-diene complexes. 

The catalytic activity of zeolites in alkane to olefin reactions, photochemical conversion reactions, Fischer-Tropsch hydrogenation, isocyana-tion, carbonylation, and related chemistry make up the last theme. An important focus of this is to explore the utility of zeolites as selective heterogeneous catalysts for reactions that involve Group VIII metals. The mechanistic nature of some of this chemistry is presented, along with the characterization of supported organometallic transition metal complexes. 

Qualitative photochemical studies concentrate on diene complexes because of their greater thermal inertness and ease of characterization. Formation of olefin complexes is induced in situ either by photochemical or thermal means and their presence determined by spectroscopy. The photocatalyzed hydrogenation and hydrosilation of 1,3-di-enes the photocatalyzed valence isomerization of norbomadiene to quadricyc-lane and the cis trans photoisomerization of coordinated olefins are potentially usefulHowever, these transformations are not photosubstitution reactions and are not discussed here the reader should consult ref. 1 and references cited therein. Photolysis of olefin complexes leads to olefin loss with high quantum efficiency unless the olefin is a chelating di- or polyene where, as with most chelating ligands, other reactions occur. 

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) ... 

Photochemistry of Metal Carbonyls, Metallocenes, and Olefin Complexes Table 4. Photochemical substitution of metal carbonyl compounds with n-donors... 

Iron-olefin complexes have been prepared photochemically from Fe(CO)5, Fe CCO), and FesCCO) ... 

A review of the photochemical properties of copper complexes includes a survey of the photocatalysed reactions of copper-olefin complexes. The addition of acetonitrile to norbornene may be induced by irradiation in the presence of silver ions. The reaction appears to involve excitation of a LMCT excited state of the norbomene-silver complexes and the formation of norbornene radical cations. 

Thus, it can be concluded that reactions, in which there is a possibility of the formation of platinum(IV) derivatives with alkyl fragments containing a hydrogen bond in the p-position, lead to olefin ff-cotnp exes of platinum(II). These reactions are the interactions of platinum chlorides with alkyl iodides and alkyl derivatives of tin as well as the thermal and photochemical dehydrogenation of n-hexane by PtCU (Scheme VI.5). One possible mechanism for the dehydrogenation of n-hexane is the formation of a cr-hcxyl complex of plati-num(IV) and its subsequent transformation into a n-olefin complex of plati-num(II) via p-elimination of hydrogen [16] ... 

Olefin complexes have most often been prepared by displacement of some other ligand by the olefin. Both thermal and photochemical reactions have been observed. Olefins can form complexes with platinum and palladium by displacing solvent or halide molecules from the coordination sphere of the metal ... 

Photochemical studies of olefin complexes are more concentrated on diene compounds because they are more stable. Photochemical reactions of hydrogenation, isomerization, and hydrosilation of olefins were carried out. ... 

The olefin complex (22 X,Y = F P,Q = Br) has also been prepared in low yield by a photochemical reaction of CFi CBra with pentacarbonyliron, together with a small amount of material formulated from its mass spectrum and an i.r. band at 1675 cm" as the fenacyclopentene (23). A Russian patent claims that bis(halogenoperfluoroalkene)iron derivatives can be prepared by heating Fe(CO)s with CF2 CFX (X = Br, CF3, or CFiCFj) in an autoclave at 125—280° for 23—38 h. ... 

Direct application of Ru3(CO)i2 in photochemical synthesis has been described in detail [120]. Thermal reactions of this cluster in presence of two-electron donors L affords [Ru3(CO)9L3]. The discovery in 1974 that irradiation of the cluster under those conditions produces mononuclear products instead of the substituted clusters initiated a wealth of research in Ru-clusters as precursors in photochemical synthesis [121]. Much research has been devoted to the preparation of mononuclear f/ -olefin complexes, as well as alkyne complexes. For example, [Ru(CO)3(PPh3)2] has been reported as an active catalyst for olefin polymerisation, and as such, many investigations have dealt with the reactivity of this compound. Other directions of research include formation of metallacycles, generation of new cluster species, and mixed transition metal/non-metal clusters. 

The chelate olefin complexes (j -C8H5XCH2CH=CHa)Cr(CO)2 have also been prepared by photochemical displacement of L from the (ij -CgHgXCHaCH ... 

Other Metals.—Photochemical or thermal displacement of CO from (ij -allyl)Mn(CO)4 with PX3 (X=R or OR) yields ( -allyl)Mn(CO)2(PX3)2 and (rj3.allyl)Mn(CO)(PX 3) 3 complexes. N.m.r. studies of the latter at low temperature indicate a pseudo-octahedral geometry with rram-phosphorus atoms. C studies of ( -allyl)Mn(CO)4 have also been reported, but quadrupole relaxation by the Mn prohibits any determination of the mechanism of fluxionality. Reaction of [V(CO)4L2] with allyl halides yields ( -allyl)V(CO)3L2 complexes (La = diars or arphos). Similar ( j -methylallyl)V(CO)3L2 complexes (La=arphos or dppm) may be prepared by reaction of isoprene with HV(CO)4La isomers with methyl groups in different positions are obtained. Reaction of CpaTaCla with RMgCl yields Cp2Ta(H)(olefin) complexes (olefin=propene, but-l-ene, pent-l-ene, or cyclopentene). The olefin is easily displaced by phosphine, and the complexes react with Ha to give CpaTaHs. ... 

This section focuses on olefin complexes that contain no other 7r-acceptor ligand such as a carbon monoxide. As such, the reactions will focus on photochemical reactions that occur at the coordinated alkene, and not on transformations where the photoreaction is limited to loss of a carbonyl ligand. Nevertheless, many of the reaction types are similar, and the reader will discern similarities between the photoreactions discussed in this section and those covered in the section devoted to alkene carbonyl complexes. 

Tricarbonyl(6,6-diphenylfulvene)chromium(0) (52) was obtained by reaction of 6,6-diphenylfulvene with hexacarbonylchromium in 64% yield [56]. Later it was shown that almost quantitative yields can be obtained under photochemical reaction conditions starting from (benzene)Cr(CO)3 or from (mesitylene)Cr(CO)3 as the complexation reagent [63]. The corresponding molybdenum and tungsten complexes were prepared by treatment of the ligands with tris(acetonitrile)M(CO)3 (M = Mo, W) [64]. Olefin complex 53 was obtained by Wilkinson and Altman in 51% yield from 6,6-diphenylfulvene and bis(dicarbonylchlororhodium). Similar complexes were prepared with other rhodium reagents [58]. Hiibel and Weiss [59] prepared the diene tri-carbonyliron complex 54 (11-30%) in addition to the dinuclear complex, in which either one of the endocyclic double bonds is coordinated at Fe(CO)4 (49-66%). 

Geiger, D. and Ferraudi, G., Photochemistry of Cu-olefin complexes a flash photochemical investigation of the reactivity of Cu(ethylene)+ and Cu(ds, s-l,5-cyclooctadiene)2+, Inorg. Chim. Acta., 101, 197, 1985. 

The stereospecificity of these reactions is surprising in light of the large energies absorbpd by these molecules. Indeed, the major photochemical product of these photolyses was the alternate olefin isomer (1-butene was also observed). These results indicate that free rotation about the photo-excited double bond does not occur in those molecules that dimerize. This suggests the participation of ground state complexes or excimers in the photodimerization. This view is supported by the observations that dilution of cw-2-butene with neopentane (1 1) decreased the yield of dimers and a 1 4 dilution almost completely suppressed dimerization. 

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