Platinum complexes fluxionality

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). 

The two platinum complexes 478 and 479 are less fluxional, the pzTp complex 478 being stereochemically rigid below 110 °C, above which it becomes unstable in solution, while for the Tp analogue 479 equivalence of the pyrazole groups is achieved at 85 °C. This equilibration slows at ambient temperature, but poor solubility below 10 °C precluded acquisition of limiting spectra. 

Finally in this section, the platinum complex BpP rihr -CgH OMe) (186) and related bis(pyrazolyl)methane complex salts [(H2Cpz2)M(r 1 r 2-CgHi2OMe)]PF6 (M = Pd 189.PF6, Pt 190.PF6) have been obtained from [M (ri1 ri2-C8FIi2OMe)Cl]2 and NaBp, or H2Cpz2/NFl4PF6, respectively (Scheme 15, Section III.B.l).73 The palladium analogue of 186 has proven inaccessible. These complexes all exhibit fluxionality, ascribed to inversion of the EN2M boat (E = B, C). 

The platinum complex formed in the reaction between C6(CF3)6 and tris(triethylphosphine)platinum shows only a single band at 30°C [5 -51-2, y(P-CFj) 3-0, /(Pt-CFj) 27-5 Hz], which must result from fluxional behaviour in solution, since X-ray crystallography shows the structure of the solid to be as in [370], with alternate long and short bonds in the non-planar C6 ring. (159, 160) At — 90°C the spectrum... 

A platinum complex in which the ligand contains two independent olefinic bonds, PtCl2-C10H14O3, will be referred to in Sect. E.6. Cyclo-octatetraene (COT) may frequently be regarded as a conjugated ring system (e.g. in fluxional... 

Fluxionality in platinum complexes includes inversion of the face-to-face structure of the Pt(II) complex and a twisting of the bridging 2,3-bis(2-... 

The JHNMR spectrum of the platinum 1,8-naphthyridine complex (124) shows the naphthyridine protons to be equivalent above -30 °C, suggesting fluxional behavior (77IC261). Other examples of this phenomenon have also been observed (73JA4855). 

The structures of jj3-allyl complexes of platinum have been fully summarized by Hartley in Chapter 39 of Comprehensive Organometallic Chemistry . This article also details the NMR methods used to investigate fluxionality of the r/3-allyl ligand. 

Numerous other aromatic nitrogen compounds are known, and a significant number have been used to prepare complexes of platinum(II). Phthalazine (101) is one such compound, and the complex ds-PtCl(phthalazine)(PEt3)2 has been prepared in order to compare its fluxional properties with the phenanthroline and naphthyridine analogues. The monodentate complex is fluxional, but in this phthalazine case the orientation of the lone pairs is unsuitable for the five-coordinate intermediate required by an intramolecular process and the mechanism of exchange becomes dissociative.11 6... 

The types of compounds formed by gold(I) and gold(III) often differ from those of other metals due to the constraints imposed by coordination number and electron count at the metal. Thus, for example, whereas 7r-bonded cyclopentadienyl complexes of palladium and platinum are numerous (336), and a copper(I) species of this type is known (337), cyclopentadienyl complexes of univalent (94, 96, 97) and trivalent (228) gold have invariably been found to be fluxional behavior, similar to that in dicyclopentadienylmercury, was involved (228). 

Unsuccessful attempts to prepare stable complexes of six-membered ring allenes that succeeded for their seven-membered counterparts (Section IV,C) include reaction of 299 with KOt-Bu in the presence of tris(triphenyl-phosphine)platinum(O) [Eq. (50)]118 and abstraction of methoxy from 301 (Scheme 37)119120 (L = CO) with trimethylsilyl triflate. The latter gave products suggestive of an intermediate complex of 1,2-cyclohexadiene, but it could not be detected or trapped. Based on fluxionality studies in the seven-membered counterpart (Section IV,C), equilibration of 302 with a... 

Although 2,2 -bipyridine and 1,10-phenanthroline usually give chelate complexes, unidentate complexes can be formed as in [PtCl(PEt3)2(V phen)]+i in solution they are fluxional and the platinum atom moves from one nitrogen atom to the other. 

These fluxional complexes have been fully characterized in solution by multinuclear NMR techniques. The data are consistent with a c -arrangement of the hydride and the silyl ligand. In the H NMR spectrum of 2a (in CDCI3 at 243 K) the hydride resonance is found at 6 = -2.66 ppm and consists of a doublet of doublets (Vp-h = 19.6, 152.7 Hz), which is flanked by platinum satellites ( pt-H = 972.3 Hz). A single crystal X-ray determination performed on 2a revealed that intramolecular short contacts between a thioether group and the platinum center are absent in the solid state. 

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