Methylene complexe

Table 9-11. Experimental binding energies D0 [kcal/mol] for the cationic methylene complexes of the 3d elements and deviations from these data obtained at...

Holthausen, M. C., Mohr, M., Koch, W., 1995, The Performance of Density Functional/Hartree-Fock Hybrid Methods The Bonding in Cationic First-Row Transition Metal Methylene Complexes , Chem. Phys. Lett., 240, 245. 

The tantalum center in the Schrock methylene complex 2 is electron rich by virtue of the electron-releasing ligands coordinated to it, while the osmium and ruthenium center in the methylene 3 and difluorocarbene 4 species are electron-rich because they have d8 electron configurations in neutral complexes. 

The intermediacy of cationic d6 osmium methylene complex 6 is implicated in the reactions of OsI(CH2I)(CO)2(PPh3)2 with nucleophiles (23), e.g.,... 

The nucleophilic reactivity of the d8 methylene complex Os(=CH2) Cl(NO)(PPh3)2 (3) with HC1 was noted above. 

While the tantalum methylene complex 2 reacts with Mel (15), the osmium complex 3 is quite unreactive, suggesting greater nucleo-philicity of the former compound. 

The cationic methylene complexes [(t71 5-C5H5)(CO)3M=CH2]+ (M = Mo, W) react rapidly with nucleophiles (e.g., olefins) but are unreactive toward electrophiles (26). 

Terminal methylene complexes are relatively rare—less than 10 such compounds have been isolated and about as many again have been characterized by spectroscopic techniques only. The methylene complexes previously reported fall into two groups, (i) neutral complexes of the early transition metals (e.g., Ti, Ta) and (ii) cationic complexes of the later transition metals (e.g., Re, Fe). The osmium complex 47 is important, then, as it is a new example extending the neutral group to the later transition metals. Compound 47 is the prototype for the series Os(=CHR)Cl(NO)(PPh3)2 and is one of only three terminal methylene complexes to be structurally characterized by X-ray crystallography (see Section IV,B). 

The reaction of Vaska s compound (46) with diazomethane was reported in 1966 (81). A diethyl ether suspension of 46 afforded chloromethyl complex 52 on treatment with CH2N2 at -30°C, and a d6 iridium methylene complex 51 was proposed as the likely intermediate ... 

A recent reinvestigation of this reaction has substantiated the intermediacy of the 5-coordinate methylene complex 51. Thermally unstable 51 is formed when a THF solution of 46 is treated with CH2N2 at -60°C (56). The iodo analog Ir(=CH2)I(CO)(PPh3)2 has been similarly prepared and is somewhat more stable than 51, with iodide migrating less readily to the carbene carbon than chloride (56). 

The dihaptoformaldehyde complex 0s(Tj2-CH20)(C0)2(PPh3)2 reacts with hydrogen halides, affording hydroxymethyl species. Further reaction leads to the formation of halomethyl complexes, probably via the intermediacy of methylene complexes (60) (73) ... 

The spectroscopically characterized iron methylene complex 63 is formed in a closely related reaction of an alkoxymethyl compound (92) ... 

X-Ray crystallographic studies of the methylene complex Os(=CH2)Cl(NO)(PPh3)2 (47) and the phenylcarbene complex OsCl2(CO)(=CHPh)(PPh3)2 (59) have been undertaken to fully characterize these compounds. The only other terminal methylene complexes to be crystallographically characterized are (77S-C5H5 )2MeTa=CH2 (94) and... 

The effect of metal basicity on the mode of reactivity of the metal-carbon bond in carbene complexes toward electrophilic and nucleophilic reagents was emphasized in Section II above. Reactivity studies of alkylidene ligands in d8 and d6 Ru, Os, and Ir complexes reinforce the notion that electrophilic additions to electron-rich compounds and nucleophilic additions to electron-deficient compounds are the expected patterns. Notable exceptions include addition of CO and CNR to the osmium methylene complex 47. These latter reactions can be interpreted in terms of non-innocent participation of the nitrosyl ligand. 

The ruthenium and iridium methylene complexes and the osmium alkylidenes 48-50 also react with S02 to afford sulfene complexes (104). 

Thus methyl and chalcoformaldehyde complexes of osmium are accessible by both electrophilic addition to a neutral d8 methylene complex and nucleophilic addition to a cationic d6 methylene complex. 

The insertion reactivity of the electrophilic iridium methylene species 57 was noted above. A cationic iridium methylene complex is also the likely intermediate in the thermal rearrangement of Ir(=CH2)I(CO)(PPh3)2 to the ortho-metallated ylide complex 75 ([Pg.167]

Unlike the analogous S02 adducts of the d8 Ru, Os, and Ir methylene complexes, 108 is rather unstable, decomposing to OsClPh(CO)2(PPh3)2 on exposure to moisture. 

Aside from the Ziegler-Natta polymerization, alkene and alkyne metathesis, and other reactions of Ti-methylene complexes, carbometallation reactions induced by alkyltitanium compounds have been dominated by those involving... 

Scheme 108 Cathodic cyclopropanation with an iron-methylene complex.

Recent mechanistic work has shown that 16 e Ru methylene complexes (such as bisphosphine 11) are slow to re-enter the catalytic cycle. Their reluctance to initiate can result in competitive decomposition see Mechanism and Activity of Ruthenium Olefin Metathesis Catalysts, M.S. Sanford, J.A. Love, R.H. Grubbs,/. 

Ethylidenation. Flclquist et al. have extended the cyclopropanation of olefins with an iron methylene complex (9, 143) to ethylidenation of olefins with the iron cthylidcnc complex 1. Since the sulfide precursor (2) is more stable, the reagent is generated in the presence of the olefin. The reagent gives methyl-substituted cyclo-... 

In the area of Os organometallic chemistry, a comprehensive review of the literature up until 1982 has been produced (4). More recent reviews include the annual surveys on the organometallic chemistry of Ru and Os (8-10), organometallic arene chemistry of Ru and Os (11), and terminal methylene complexes (12). 

A thioformaldehyde complex of titanium (55) was similarly obtained from the titanocene methylene complex 54 and either elemental sulfur or sulfur sources such as propene sulfide, cyclohexene sulfide, styrene sulfide, or Ph3P = S. Best yields were achieved with propene sulfide [Eq. (II)].67... 

Such higher order prerequisites could be fulfilled by ensemble operation of several sites. For example, a dimeric cluster of cuprous ions on silica gel is very active for the oxidation of CO with NzO at room temperature, but isolated cuprous ions are entirely inactive for this reaction 60). More interesting selectivity may be found in the reaction of olefins with methylene complexes the reaction of olefins with mononuclear methylene undergoes an olefin metathesis reaction, but the reaction of ethylene with bridging methylene in /i-CH2Co2(CO)2(Cp)2 61), /<-CH2Fe2(CO)8 (62), and /<-CH2-/i-ClTi(Cp)2Al(Me)2 (65) (Cp = cyclopentadiene) leads to propene formation (homologation reaction). 

Most of the sustained work on the different aspects of /x-methylene complexes has been performed in our own laboratories at Regensburg (59-61, 296 -299), and by the innovative groups of Knox (Bristol) (62-67), Levisalles and Rudler (Paris) (68-71), Pettit (Austin) (72-74), Shapley (Urbana, Illinois) (75-83), and Stone (Bristol) (84-94). Other significant contributions originate from the research of Wilkinson (London) (95-97), Bergman (Berkeley) (98), Tebbe (Du Pont) (99-101), Puddephatt (Liverpool) (102. 103), and Ziegler (Heidelberg) (104). 

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