Platinum complexes bridging ligands

Figure 3.98 A dimeric platinum(III) complex with no bridging ligands.

Platinum(IV) exhibits an exceptionless preference for octahedral coordination in these complexes, using bridging ligands when necessary to achieve this. 

The characterization and crystal structure of the dimer [Pt2( -dppm)3] (dppm = bis(diphenyl-phosphino)methane), first reported as a deep red complex in 1978, was described by Manojlovic-Muir et al. in 1986.11 The structure, the first of its type, is made up of two parallel and almost eclipsed trigonal-planar platinum moieties bridged by three diphosphine ligands. The Pf Pt separation is 3.0225(3) A, too long to be considered a bond.11 [Pt2(//-dppm)3] catalyzes the hydrogenation/reduction of carbon dioxide with dimethylamine to give dimethylformamide12 (Equation (1)) and the reverse reaction.13... 

Lippert and co-workers have reported a similar series of A-,O-bridged platinum(III) dimers in which the bridging ligands are the pyrimidines, 1-methyluracil, 1-methylthymine, or 1-ethylthy-mine. Chemical oxidation of dimeric platinum(II) complexes gave [Pt2XY(L)2(NH3)4]2+, X,Y = N03", N02, H20, Cl", or Br", L=l-MeU, 1-MeT, 1-EtT. 27 t30 They too found that the HT dimers are more stable than the HH.430... 

Chiral bis-(binaphthophosphole) (bis(BNP)) ligands have been used in the asymmetric hydroformylation of styrene. In solution, the free diphospholes display fluxional behavior. Consistent with their structure, the reaction of the bis(BNP) compounds with platinum(II) derivatives gives either cis chelate mononuclear complexes or trans phosphorus-bridged polynuclear derivatives. Coordination to platinum enhances the conformational stability of bis(BNP)s and diastereomeric complexes can be detected in solution. In the presence of SnCl2, the platinum complexes give rise to catalysts that exhibit remarkable activity in the hydroformylation of styrene. Under optimum conditions, reaction takes place with high branched selectivity (80-85%) and moderate enantio-selectivity (up to 45% ee). 

Two isomeric forms of the unique tetrameric platinum complex, Pt4(OAc)4 S2P(OEt)2 4 have been isolated and structurally characterized (Scheme 3) one contains chelating and the other contains bridging dithiophosphato ligands (acetato groups not shown for clarity).19... 

Pyrimidinethione complexes, osmium, 37 296 Pyriminethionate, as bridging ligands in platinum dimers, 40 202-205 Pyrochlores, 17 108 Pyrococcus furiosus, 38 344-383 aldehyde ferredoxin oxidoreductase, 38 374-381... 

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... 

Attempts have been made to mimic proposed steps in catalysis at a platinum metal surface using well-characterized binuclear platinum complexes. A series of such complexes, stabilized by bridging bis(diphenyl-phosphino)methane ligands, has been prepared and structurally characterized. Included are diplati-num(I) complexes with Pt-Pt bonds, complexes with bridging hydride, carbonyl or methylene groups, and binuclear methylplatinum complexes. Reactions of these complexes have been studied and new binuclear oxidative addition and reductive elimination reactions, and a new catalyst for the water gas shift reaction have been discovered. 

We have used the ligand bis(diphenylphosphino)methane (dppm) to stabilize binuclear platinum complexes. When chelating, this ligand forms a strained four-membered ring and it therefore tends to bridge between metal atoms. Our first platinum(I) complex was prepared by the following reaction (Equation 1), PP = dppm (1,2). 

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