Diphosphene

On the other hand the alkyl- or amino-substituted congeners should adopt a more classical structure, in which the twofold coordinated phosphorus atom is bent, 8b. A first hint of the cation was reported by mass-spectroscopic investigations [67]. The synthetic verification of this prediction starts from amino-substituted diphosphene 22 via protonation with CF3SO3H [68] (Scheme 15). 

In the structure 24a, the triphenylphosphine is strongly bound to the electrophilic phosphorus centre (PP=2.206A) which indicates a strong covalent character of this bond. Upon warming the solution to 20 °C decomposition takes place and a mixture of bicyclotetraphosphanes is formed. Interestingly, some structural trends towards the formation of ion pairs between a donor and an acceptor were also reported in the push-puU diphosphene structures 25-27 [69] (Fig. 4). 

With increasing electronegativity of one ligand in the diphosphene the ZPPR angle shrunk and decreased finally below 90° for the diphosphene 27. The experimental findings were accompanied by the quantum chemical calculations. There are no other reports regarding further experimental investigations or possible dicoordination in these compounds. 

The investigations indicate that, in agreement with the calculations on the cations, the push-pull substituted diphosphenes tend to form a bridged structure of one ligand the other substituent can easily depart under formation of an ion pair structure. 

The group of Protasiewicz, who has reported Cp2Zr complexed phosphinidenes with very bulky substituents and phosphine ligands, has explored the interchange of ligands and the formation of diphosphenes [113]. 

As a final note, during the final stages of preparing this review the first example of a diphosphene-PPV was reported [111]. This exciting new polymer contains P=P bonds spaced by p-phenylenevinylene units in the main chain, has a degree of polymerization of approximately 6, and shows emissive properties. 

Diphosphenes stabilised by bulky groups, e.g. (157, R = 2,4,6-tri-t-butylphenyl), can be isolated in high yield from the reactions of alkyl- or aryl-(trichlorogermyl)phosphines with an excess of the base DBU. The formation of less sterically crowded systems, e.g. (157, R = t-butyl), is also possible by this route,... 

A novel mode of coordination of diphosphenes is seen in the ligand... 

The highly reactive diphosphene (157, R = trifluoromethyl), accessible from the reaction of trifluoromethyldiiodophosphine... 

An unusual complex is (228), which forms upon reaction of [NiCl2(PMe3)2] with K2(BulP)2.659 Different mechanisms have been proposed for its formation, generally involving diphosphene Ni° intermediates.660... 

Complexes with diphosphenes, phosphabenzene, biphosphinine, and other unsaturated phosphorus donor ligands... 

Solventless arylations of norbornene have been reported recently with low ee s (4.5%), in the presence of a chiral diphosphene ligand (Equation (150)).127... 

Anion-radicals of compounds with the azo and diphosphene central bonds (R N=NR and Rip=PR ) have three n electrons between two nitrogen or phosphorus atoms (Stagko et al. 1998, Binder et al. 1996, Shah et al. 1997). 

Closely related with the synthesis of ylides from carbenes is the use of ylides as carbene transfer reagents (CTR), that is processes in which the ylide is cleaved homolytically, liberating the nucleophile and the carbene, which could remain both coordinated to the metal or not (Scheme 10). Diphosphirane (34) can be obtained from the diphosphene by reaction with sulfur ylide Me2S(0)=CH2, which behave as a carrier of the CH2 unit [95]. Recent work of Milstein et al. shows that sulfur ylides decompose in the presence of Rh derivatives with vacant coordination sites affording Rh(l)-carbene complexes [96, 97]. Complexes (35-37) can be obtained from... 

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