Photoinduced elimination complexes

The photochemical studies of transition metal hydride complexes that have appeared in the chemical literature are reviewed, with primary emphasis on studies of iridium and ruthenium that were conducted by our research group. The photochemistry of the molybdenum hydride complexes Mo(tj5-C5H5)2M2] and [MoH4(dppe)2] (dppe = Ph2PCH2CH2PPh2), which eliminate H2 upon photolysis, is discussed in detail. The photoinduced elimination of molecular hydrogen from di-and polyhydride complexes of the transition elements is proposed to be a general reaction pathway. 

Mo(r75-C5H5)2H2] and [MoH dppe ]. Our studies of the di- and trihydride complexes of ruthenium and iridium, described above and published previously (27,35), and those of other workers (discussed at the beginning of this chapter), indicate that photoinduced elimination of molecular hydrogen is a common reaction pathway for di- and polyhydride complexes. To demonstrate the photoreaction s generality and its utility for generating otherwise unattainable, extremely reactive metal complexes, we have begun to study the photochemistry of polyhydride complexes of the early transition metals. We focused initially... 

In fact, photoinduced elimination of CO from CpM(CO)2R complexes has been used extensively to synthesize mono- and disubstituted derivatives with a number of different entering ligands [Eq. (90)]. The nature and... 

An example of the photoinduced elimination of hydrogen with subsequent ligand coordination to give a coordinately saturated complex as product is found in die conversion of RuH2(CO)(PPh3>3 into Ru(CO)3(PPh3)3 in the presence of CO (Ref. 168) ... 

Photoinduced reductive elimination of from polyhydride complexes, HxMLn (x > 3) can, in principle, lead to the generation... 

The entire photolysis sequence can be reversed readily by H2, as summarized in Scheme 1 (27). A sealed, degassed benzene solution of [IrClH2(PPh3)3], from which the photoreleased H3 is not allowed to escape, can be cycled repeatedly (>50 cycles) through the photoinduced H2 elimination-thermal H2 addition reactions without any observable loss of complex. Elimination of H2 can be induced by irradiation with X < 400 nm. The quantum yield of elimination, measured at 254 nm by monitoring the growth of the 449-nm band of [IrCl(PPh3)3], is 0.56 0.03. 

As the above results indicate, the photochemistry of these uranium-alkyl complexes is not fully resolved. The olefin formation during Marks s siudy of UCP3R in toluene solution suggests that photoinduced /3-hydride elimination occurs to some extent, presumably via a mechanism similar to that of Scheme 1. However, the other data appear to indicate a predominant radical mechanism via photoinduced M—R bond homolysis. 

The above results suggest that the metallacyclobutane complexes 48-51 have at least two photodecomposition pathways available to them (1) r -r shift of the Cp rings with homolysis of one of the metal-carbon bonds, and (2) possibly a concerted, photoinduced reductive-elimination reaction for the formation of cyclopropylbenzene from 51 [Eq. (63)]. 

Irradiation of the title complex in the presence of excess cyclooctadiene (COD) has been reported to yield Pt(cod)2 but no further details have been given 154). TMs reaction may proceed via photoinduced /3-hydride elimination as in Eq. (122). 

This review illustrates the above delineated characteristics of electron-transfer activated reactions by analyzing some representative thermal and photoinduced organometallic reactions. Kinetic studies of thermal reactions, time-resolved spectroscopic studies of photoinduced reactions, and free-energy correlations are presented to underscore the unifying role of ion-radical intermediates [29] in—at first glance—unrelated reactions such as additions, insertions, eliminations, redox reactions, etc. (Photoinduced electron-transfer reactions of metal porphyrin and polypyridine complexes are not included here since they are reviewed separately in Chapters 2.2.16 and 2.2.17, respectively.)... 

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