Di-phosphinines

Fig. 6.8 Trans-coordinating di-phosphinine 16 and molecular structure of the corresponding Rh(I) complex 17 in the crystal...

In cooperation with the group of van Leeuwen, we have started to investigate the access to trans-coordmsitmg di-phosphinines via the classical and modular pyrylium salt route. Compound 16 is based on a terphenyl-backbone and the corresponding Rh(I) complex 17 was characterized crystallographically (Fig. 6.8) [43]. 

Taking the special electronic and steric properties of the phosphorus-heterocycles into account, the design of di-phosphinines and di-phosphabarrelenes with natural bite angles close to 120° are expected to give very active and selective catalysts in... 

Fig. 6.17 Bidentate di-phosphinines with fixed coordination geometry...

V. B.7S The 1.707 A bond length (MP2/6-31G(d)) is somewhat shorter than that for the analogue. However, the calculated structure is not entirely planar the hydrogens on the carbon atoms move somewhat out from the plane. The X-ray structure of the heavily substituted l.l,3,3,5,5-hexakis(di-methylamino)-lA .3A ,5A -triphosphinine was planar. In all cases the PC bond lengths were equal and in the range of the aromatic PC bonds (1.687 A from the X-ray structure). It is worth noting that no NIGS data has been reported for y , -phosphinines as yet. 

Phosphinines are a special group of phosphaaUcenes in which the P=C double bond becomes integrated into the aromatic system of benzene and related arenes. The majority of examples are based on the mono-phospha-benzenes, but di-and triphospha-benzenes in various substitution patterns have also been employed as ligands. The coordination chemistry of this class of ligands has been reviewed. ... 

Phosphinines can act mainly as monohapto two-electron <7-donors through the phosphorus atom(s) or as polyhapto six-electron (TT-donors with metal atoms centered above the ring system (73a,b). The cr-mode has been observed, for example, with units Cr(CO)5, Ni(PR3)2, Ni(C2H4)(PR3), Mn(cp)(CO)2, AuCl, and others. 7r-Complexation is achieved in most cases only after P-cr-complexation, as in (L )[cr-Cr(CO)5][7r-Cr(CO)3] with V = 4-Hex-C5H4P. A di(benzene)chromium analog was obtained with two molecules of 2,4,6-tri( butyl)phosphinine, while a mixed complex (cp)Mn(L") could be obtained for a phosphinine L" with a different set of substituents. ... 

Although the molecular geometries of 2 - and 2 -phosphinins are nearly identical (no significant deviations from planarity or bond alternation are observed with either system), the A -phosphinines as well as the mono-, di-, tri- and tetra-aza-A -phosphinines, the A A -diphosphinines and the A A A -triphosphinines are certainly not Hiickel-aromatic systems. Their description as cyclic phosphorus ylides with ionic resonance structures (26) is probably close to the truth. The H and C NMR evidence is particularly strong on this point. 

In the 1,2,3- -diazaphosphinine (38) an ylide-like structure is consistent with the bond length data, and with the NMR measurements <90AG(E)ll23>. A later paper reports a number of crystal structures of other di- and tri-aza-2 -phosphinines <94JA8087>. 

Noteworthy, 2-bromophosphinines have also found an interesting application in the synthesis of a new class of phosphinine-based bidentate ligands featuring either a phosphine or a phosphorus heterocycle like phosphirene or 1,2-dihydrophosphete. Reaction of phosphorus tribromide with 2-bromophosphinine 25 afforded the 2-dibromophosphinophosphinine 27 which in turn could be converted to functional derivatives either through nucleophilic or electrophilic reaction at the P-Br bonds. The mechanism of this phosphination process was shown to involve the transient formation of the A5-di bromophosphinine 26 (Scheme 7) [26],... 

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