Zerovalent

L. Malatesta and S. Cenimi, Zerovalent Compounds of Metals, Academic Press, London, 1974. 

Dehalogenation of monochlorotoluenes can be readily effected with hydrogen and noble metal catalysts (34). Conversion of -chlorotoluene to Ncyanotoluene is accompHshed by reaction with tetraethyl ammonium cyanide and zero-valent Group (VIII) metal complexes, such as those of nickel or palladium (35). The reaction proceeds by initial oxidative addition of the aryl haHde to the zerovalent metal complex, followed by attack of cyanide ion on the metal and reductive elimination of the aryl cyanide. Methylstyrene is prepared from -chlorotoluene by a vinylation reaction using ethylene as the reagent and a catalyst derived from zinc, a triarylphosphine, and a nickel salt (36). 

J. Chatt and H. R. Watson, Complexe.s of zerovalent transition metals with the ditertiary phosphine, Me2PCH2CH2PMe2, J. Chem. Soc. 2545-9 (1962). 

The versatility of cyanide and bipyridyl ligands has been used to stabilize low oxidation states. By using potassium in liquid ammonia, K3Ti (CN)6 is reduced to K2Ti (CN)4 and TiBt3 - - KCN to K4Ti°(CN)4. With ZrBt3 and M CN (M = K, Rb) in liquid ammonia, ammonolysis occurs and zerovalent Zr is produced ... 

The tertiary phosphine complexes are the most important zerovalent compounds. They are frequently prepared by reductive methods, often using the phosphine as the reducing agent [43], e.g. 

Zerovalent complexes are also obtained with polydentate phosphines [53] Pd2Cl2(dppm)2 NaBH > Pd2(dppm)3... 

A few isocyanides of palladium and platinum are known in the zerovalent oxidation state. The best characterized compounds involve triangular M3 clusters with M-M bonds. 

The coordination sphere of transition-metal complexes can furthermore be utilized for the fixation of silicon ligands in their lowest oxidation states. Even examples of compounds containing a formally zerovalent silicon (E) are now known [41]. 

Molecular compounds of silicon in a formally zerovalent oxidation state can be stabilized by appropriate transition-metal fragments. An entry to such polyme-tallated complexes of silicon is given by the chlorosilylene compounds 7, 11 as a starting-point. 

Reactions of zerovalent platinum and palladium complexes with organometallic compounds of nontransition metals. V. I. Sokolov and O. A. Reutov, Coord. Chem. Rev., 1978, 27, 89-107 (80). 

The transition-metal catalyzed decomposition of thiirene dioxides has been also investigated primarily via kinetic studies103. Zerovalent platinum and palladium complexes and monovalent iridium and rhodium complexes were found to affect this process, whereas divalent platinum and palladium had no effect. The kinetic data suggested the mechanism in equation 7. 

Scheme 6 Synthesis of platinum disulfido complexes from zerovalent platinum complexes...

Hidai et al. have reported the reaction of a diruthenium complex, (RuCp )2(yU-Si-Pr)2(//-S2), with zerovalent complexes of group 10 metals, [M(PPh3)4] (M=Pd, Pt) (Scheme 65) [146]. The reaction with the Pt complex... 

Scheme 65 Reaction of (RuCp )2( t-Si-Pr)2( U-S2) with zerovalent platinum and palladium complexes...

The first ruthenium porphyrin alkyls to be reported were prepared from the zerovalent dianion, [Ru(Por)] with iodomethane or iodocthane, giving the ruthe-nium(lV) dialkyl complexes Ru(Por)Me2 or Ru(Por)Et2 (Por = OEP, TTP). Alternatively, the Ru(lV) precursors Ru(Por)X2 react with MeLi or ArLi to produce Ru(Por)Mc2 or Ru(Por)Ar2 (Ar = / -C(,H4X where X = H, Me, OMe, F or Cl) 147-149 The osmium analogues can be prepared by both methods, and Os(Por)R2 where R = Me, Ph and CH2SiMe2 have been reported.Some representative structures are shown in Fig. 5, and the preparation and interconversion of ruthenium porphyrin alkyl and aryl complexes are shown in Scheme 10. 

Thiourea was used as stabilising agent for zerovalent Pd species [117]. The Pd-thiourea (H2NCSNH2) catalysed carbonylation of terminal alkynes and allylic alcohols has been described by Chiusoh [118]. More recently, Pd-thiourea-catalysed carbonylative annulation was studied. The reaction proceeds between alkynes, iodophenol acetates and carbon monoxide, in the presence of dppp, thiourea (H2NCSNH2) and base at 40 °C. Flavones have been obtained in good yields (Scheme 30) [119]. 

Tindall D,PawlowJH,Wagener KB (1998) RecentAdvancesin ADMET Chemistry. 1 183-198 Tobisch S (2005) Co-Oligomerization of 1,3-Butadiene and Ethylene Promoted by Zerovalent Bare Nickel Complexes. 12 187-218 Tomioka K, see Iguchi M (2003) 5 37-60 Tomooka K, see Hodgson DM (2003) 5 217-250... 

Scheme 4 Oxidative-addition of NCAs to zerovalent cobalt and nickel complexes...

Totten LA, U Jans, AL Roberts (2001) Alkyl bromides as mechanistic probes of reductive dehalogenation reactions of vicinal stereoisomers with zerovalent metals. Environ Sci Technol 35 2268-2274. 

Yak HK, BW Wenclawiak, IE Cheng, JG Doyle, CM Wai (1999) Rednetive dechlorination of polychlorinated biphenyls by zerovalent iron in snbcritical water. Environ Sci Technol 33 1307-1310. 

In addition, if we accept the conventional mechanism of Brpnsted acid site generation by hydrogen spillover from Pt sites, as proposed for Pt-S04=/Zr02, we must recognize that this would be in fact a reduction process (zerovalent hydrogen yielding -OH groups). 

This study could be extended to the synthesis of iron nanoparticles. Using Fe[N(SiMe3)2]2 as precursor and a mixture of HDA and oleic acid, spherical nanoparticles are initially formed as in the case of cobalt. However, a thermal treatment at 150 °C in the presence of H2 leads to coalescence of the particles into cubic particles of 7 nm side length. Furthermore, these particles self-organize into cubic super-structures (cubes of cubes Fig. ) [79]. The nanoparticles are very air-sensitive but consist of zerovalent iron as evidenced by Mossbauer spectroscopy. The fact that the spherical particles present at the early stage of the reaction coalesce into rods in the case of cobalt and cubes in the case of iron is attributed to the crystal structure of the metal particles hep for cobalt, bcc for iron. 

In 1977, Fahey and Mahan described the first C-F oxidative addition of hexafluo-robenzene (C ) to the zerovalent nickel complex [Ni(cod)(PEt3)j] [11], The reaction was very slow and led, after several days at 30-35°C, to the formation of trani-[Ni(CgF3)(F)(PEt3)2] in very poor yield (7%). The use of [NiCPEtj) ] for the same reaction by Perutz and co-workers 20 years later allowed for a better conversion but the reaction rate remained slow (4 weeks were necessary to obtain 48% of the desired product) [12],... 

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