Platinum compounds carbene complexes

Co-condensation reaction of the vapors of l,3-di-rcrt-butylimidazol-2-ylidene and nickel, palladium, or platinum gives the coordinatively unsaturated 14-electron sandwiches [L M] (M=Ni, Pd, Pt) of the carbene type (990M3228). Palladium(O) carbene complexes can also be prepared by the direct interaction of l,3-R2-imidazol-2-ylidenes (R=/-Pr, r-Bu, Cy, Mes) (L) with the palladium(O) compound [Pd(P(o-Tol)3)2] (OOJOM(595)186), and the product at the first stage is [(L)PdP(o-Tol)3l, and then in excess free carbene [PdL ]. 

When coordinated to metal ions in their normal or higher oxidation states, an isocyanide is rendered susceptible to attack at the ligating carbon atom by nucleophilic reagents.1 When alcohols or amines are the nucleophiles, carbene complexes that may be prepared for a variety of metals and substituent groups are obtained. The first fully characterized compounds were of platinum(H),2 and general methods of their preparation, with particular examples, are given below. 

Even multiple exchange in methane could be explained this way. For methane, a carbene complex would be postulated, and such complexes are now known (87). One objection to the involvement of platinum-alkene complexes is that they are very stable and once formed might not be expected to react further, as was found when exchange in chloropro-pane was attempted (Section III,C) (55). Also the compound formed is a platinum(O) complex. A mathematical analysis of multiple exchange similar to that developed by Anderson and Kemball (/) has been carried out (82). 

A logical route to the synthesis of platinum carbene complexes is by using a carbene precursor. This method has been successfully used with electron-rich alkenes. Reacting (30) with ci -PtCl2(PPh3)2 and similar complexes leads to the formation of carbene complexes (equation 125).378-380 Heterocyclic chloro compounds have also been used to prepare carbene complexes (equation 126).381... 

Addition of anionic Fischer carbene complexes, of their CS2 adducts and of an anionic thiocarbyne complex to cationic organometallic Lewis acids and to coordinated, unsaturated hydrocarbons gives novel hydrocarbon bridged heterodimetallic compounds. Oxidative addition of halocarbyne complexes to zerovalent platinum and palladium complexes provides a synthesis of carbido bridged complexes. 

Alkyl and arylacetylenes in presence of alcohols react with platinum compounds to furnish carbene complexes ... 

Platinum compounds yield various kinds of carbene complexes [118-137]. For example, if isocyanate platinum compounds react with alcohols or amines, the carbon of the isocyanate bonds to the hetero atom to afford the reactive CXY (X and Y are hetero atoms) group, and the carbene complex yields as shown in eqs. (21.44) [119,127] and (21.45) [120]. On the other hand, as shown in eq. (21.46) [121], the platinum carbene complexes are also obtained by the reaction of cyclic XYC = CXY with platinum compounds on heating for a long time. In these carbene complexes, the cw-isomer is more stable than the /ra/75-isomer as shown in eq. (21.47), the rra/j5-isomers being isomerized to the m-isomer at around 200 C [125]. The Pt-C bond length of these carbene eomplexes is about 2.00 A slightly shorter than 2.16 A, the sum of the covalent radii of the Pt-C bond (Pt= 1.39 A, C = 0.77A). 

The vast majority of platinum carbene complexes are stabilized by one or more adjacent heteroatoms, which allow TT-delocalization. However, there have been a few reports of compounds where there is no such stabilization, and we consider these first, before moving on to look at those with heteroatoms. 

Another common method of introducing NHG ligands has been to isolate silver carbene complexes first, and then react them with a platinum salt. Normally, silver oxide is reacted with an imidazolium salt, before the introduction of the platinum source at ambient or low temperature insoluble silver byproducts are conveniently filtered off. A conceptually similar approach is the prior formation of a tungsten or molybdenum pentacarbonyl carbene complex, and reaction of this with a platinum precursor. The room-temperature reaction with, for example, PtCl2(NGPh)2 results in the release of carbon monoxide and the precipitation of an undefined tungsten or molybdenum compound, which may easily be removed by filtration. 

Non-metathetical reactions catalysed by ruthenium-carbene complexes are multifaceted and cover a broad range of transformations, thanks in part to the large number of oxidation states and coordination geometries available for the metal centre, in sharp contrast with other elements such as rhodium, palladium and platinum, which reluctantly form compounds with high oxidation states and have a strong preference for the square planar geometry. Several... 

Finally, an interesting reaction induced by carbon monoxide leading to the formation of a platinum fris-carbene pincer complex (37) is produced by the reaction at room temperature in pentane or hexanes of the Pt-olefln-carbene complex (36) with CO (1 atm) [16]. This compound contains a tridentate pincer ligand in which all three donors are carbenes (Scheme 2.19). The structure of complex (37) was fully determined by single-crystal X-ray diffraction analysis. 

More recently, Fiirstner has described the hydrosilylation of internal enyne compound 227 to be catalyzed by platinum carbene complex 228 (Scheme 89) [24]. The corresponding silylated diene 229 is thus efficiently obtained in a regio-and stereocontrolled way, en route to lactimidomycin. 

Dehydrocyclization, 30 35-43, 31 23 see also Cyclization acyclic alkanes, 30 3 7C-adsorbed olefins, 30 35-36, 38-39 of alkylaromatics, see specific compounds alkyl-substituted benzenes, 30 65 carbene-alkyl insertion mechanism, 30 37 carbon complexes, 32 179-182 catalytic, 26 384 C—C bond formation, 30 210 Q mechanism, 29 279-283 comparison of rates, 28 300-306 dehydrogenation, 30 35-36 of hexanes over platintim films, 23 43-46 hydrogenolysis and, 23 103 -hydrogenolysis mechanism, 25 150-158 iridium supported catalyst, 30 42 mechanisms, 30 38-39, 42-43 metal-catalyzed, 28 293-319 n-hexane, 29 284, 286 palladium, 30 36 pathways, 30 40 platinum, 30 40 rate, 30 36-37, 39... 

A number of dppm complexes with hydride and methylene bridges have been referenced already in this chapter under the respective sections on hydrides and carbenes. These compounds will not be included again here. The binuclear platinum(I) chemistry of these complexes has also been developed. The complexes have a metal-metal bond between platinums, although this may be broken by formation of A-frame type molecules. This chemistry has been described in a series of papers, and the reaction types are outlined in Schemes 13 and 14, i28 .1438-1445 a similar chemistry will likely develop with the methylated ligand Me2PCH2PMe2.1450... 

An unusual sulfide complex of platinum(II) is obtained with electron-rich alkenes (equation 517). These alkenes are often carbene precursors (Section 52.4), but the reaction with Pt2Cl2(/i-Cl)2(PEt3)2 gives a sulfide complex (188).1738 With the compound (MeS)2C=C(SMe)2, the chelate complex (189) is formed with platinum(H), but with platinum(IV) the ligand will bridge two platinums (190).1739... 

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