Phosphorus Cage Compounds

Tetraphosphorus Clusters. As mentioned at the beginning of this Chapter, phosphorus is a unique element displaying a modification formed by discrete molecular units. White phosphorus is a molecular solid made up of P4 tetrahedral clusters which are stable in solution as well as in the gas phase. 

Theoretical investigations of the molecule P4 show that the valence shell electron density in this species should be concentrated inside and on the faces of the tetrahedron taking part in multicenter molecular bonding. Although after such a description little or no nucleophilic properties of P4 can be expected, there are some metallic compounds in which the unit participates as a whole. Examples of some transition metal complexes containing the unit P4 are shown in Table 4.15. 

RhL2X(j -P4) L = PPha X = Cl, Br, I L = P(m-tol)3, P(p-tol)3, AsPh3 X = Cl. Synthesis by reaction of L3RhCl with P4 at low temperature. Products are yellow solids, stable under inert atmosphere. Evidence from coordination of P4 from P and H-NMR spectroscopy and X-ray diffraction analysis. 1,2 

Ir(PPh3),Cl( -P4) Orange solids with a chemistry similar to RhL2X( y2-P4) 1 

Fe3(CO)n(P.) Diamagnetic complex obtained from the reaction of Fc2(CO)9 with P4 and characterized by IR and Mossbauer spectroscopies. P-NMR spectrum exhibits only a simple line at room temperature. That has been interpreted in terms of ligand fluxionality. 3 

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Figure 50 The structure of the magnesium-phosphorus cage compound 101.

Table 7 List of structurally characterized phosphorus cage compounds...

Scheme 6-1 All-carbon and all-phosphorus cage compounds. Cp, pentamethylcyclopentadienyl.

As has been demonstrated for the example of 103b, these cage compounds can be isomerized photolytically to the triphosphasemibullvalene 104 [88]. Valence isomerizations of this type are well known and have been intensively investigated in the chemistry of phosphorus cage compounds [5,6,89]. 

Regitz M, Hoffmann A, BergstraBer U (1995) Phosphaalkynes - Starting Point for the Synthesis of Phosphorus-Cage Compounds. In Stang PJ, Diederich F (eds), Modern Acetylene Chemistry. VCH.Weinheim, New York, Basel, Cambridge, Tokyo... 

Cr, Mo, W) by pip, (EtO)3P, and PhgP have been studied by conventional high pressure techniques the reactions followed the reactivity order Mo > Cr > W.Reaction of [Cr(CO)5(thf)] with the phosphorus cage compound ( BU2P)3P7 yields yellow crystals of [ ( BU2P)3P7>Cr(CO)g] in which the Cr atom is coordinated to basal P atom of the cage s three membered... 

Two approaches have been used in the synthesis of these types of compounds. Small boron-phosphorus ring compounds can serve as building blocks, and addition and elimination reactions with other main group elements can then extend the cage structure (see Schemes 23 and 24, Section 12.12.6.4.5). Alternatively, an unsaturated carbenoid fragment can be added to the bicyclic fragment as illustrated in Scheme 31 <1998IC490>. 

The metal to phosphorus (or arsenic) ratio varies between 1 1 (alkali metal phosphanides and arsanides) and 2 1 for bis(alkali metal) phosphanediides or arsanediides. In general, these compounds form pnictogen polyhedra with the faces being capped by monovalent metal atoms. For the alkaline-earth metal phosphorus cages, metal to phosphorus ratios between 1 2 for alkaline earth metal bis(phosphanides) and 1 1 for the phosphanediides of the divalent cations are possible. 

Instances of an R Ge—P bond are confined to one example, the ada-mantane-like cage compound, [(CH3)3Ge]6P4 (51). The Ge—P distance (2.3217 A) is equal to the sum of covalent radii (2.32 A), although the strained trifurcated bonding of each phosphorus atom (Fig. 10) suggests that this may not be representative of a typical covalent Ge—P bond. Such speculation awaits further study for confirmation. 

Substituted phosphorus heterocycles of varying degrees of unsaturation were easily obtained via Diels-Alder reaction and such adducts may be further aromatized or functionalized to provide phosphorus containing cage compounds < 1996PS(109)425, 1999S644, 1999EJ0363>. 

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