Phosphandiides

It is a challenge to study dimetalated derivatives of primary phos-phanes that possess more covalent metal-phosphorus bonds. This can be achieved through replacement of the alkali metal ions in bimetallic phosphandiides by monovalent coin metal centers. 

The employment of CuO Bu as copper-transfer reagent in this Bronsted acid-base reaction is remarkable. There is no hint of the 

The cluster 15, which crystallizes in the rhombohedral space group R3, has a remarkably different structure with respect to the structural features of 3a, 5a, and 10 (see Section II,D). Although a dode-cameric aggregate is present, as is the case for 5a (21), the cluster framework in 15 does not adopt a globular shape (39). The structure is built up of 24 Cu atoms surrounded by 12 triorganosilylphos-phanediyl moieties. The 24-metal-atom cluster consists of three planar Cu6 rings and two peripheral Cu3 cycles that all lie parallel to one another (Fig. 17). 

The 12 RP fragments cap alternately the Cu4 faces of the Cu24 polyhedron, resulting in fivefold-coordinated phosphorus atoms. This structure resembles that of the recently described [Cu24(NPh)i4]4 anionic cluster (40). The Cu-P and Si-P distances are unremarkable. The construction principle of parallel Cu layers to form a metal-like package has also been observed for other Cu clusters (41). The main reason for the different structures of Cu2PR and Li2PR clusters is the covalent character of the Cu-P bond, with the additional involvement of favorable Cu-Cu interactions. The latter are probably due to relativistic d10-d10 interactions (dispersion-type of interaction) (42, 43). 

A series of metal-rich, coin-metal containing phosphanediyl clusters (phosphandiides) with additional terminal donor ligands (i.e., triorga-nophosphanes) have been prepared only one example of a donor-free dicopper phosphandiide has been reported up to now (37, 38). Thus, the neutral cluster 16 has been prepared by the reaction of the primary phosphane la with CuOfBu in toluene at 60°C and has been isolated in 81% yield in the form of dark red crystals that are only sparingly soluble (39)  

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Figure 26 Structure of the dilithiated phosphandiide 338. Hydrogen atoms have been omitted for clarity.

The dilithium phosphandiide dimer 9 is complexed by two molecules of a fluorosilane. The complex crystallizes in the monoclinic space group P2i/n. The framework of the aggregate consists of a cen-... 

Since the Mg-P bond is probably as ionic as the Li-P bond, it is expected that the structural features of their aggregates are very similar. However, structurally characterized magnesium phosphanides are scarce (44-46). Only recently have the first magnesium phosphandiides been prepared and structurally characterized (47, 48). 

Reaction of the primary phosphane Bu3SiPH2 If with MgBu2 furnishes the solvent-free hexameric cluster 17 (Eq. 10) (47). Yellow crystals, have been isolated in 39% yield, which are thermochromic. The NMR spectrum, especially the 31P NMR signal at S = -263.8, suggested that the molecule prefers a high symmetry or dissociates rapidly on the NMR time scale. Since 15 is highly soluble in aromatic hydrocarbons even at low temperature and free of metal oxide, it can thus be regarded as a valuable source of phosphandiide, that is, for nucleophilic RP2 transfer reactions. 

In contrast to magnesium phosphandiides, analogous tin(II) derivatives possess more covalent metal-phosphorus bonds. This basic difference is also apparent for dilithium versus dicopper(I) phosphandiides (see Section II and III). It is, therefore, interesting to assess the structural and electronic features of such species in a similar way. To date, only three tin(II) phosphandiide derivatives have been prepared... 

They are promising precursors for phosphandiide transmet-alation reactions (tin-metal exchange), that is, for the synthesis of other types of bimetallic phosphandiides. 

The Sn6P6 cages 19c and 19d are accessible by two different Bronsted acid-base reaction pathways Reaction of lc and Id, respectively, with two different stannanediyl derivatives furnished in 80-89% yield red-black crystals of the aggregates (Eq. 12) (39). The tin(II) phosphandiides are somewhat related to the previously described oligomeric bis (phosphaneyl) stannanediyls of the type PkSn, which easily form intermolecular aggregates (50, 51) or remain monomeric, if the phosphorus atoms bear very crowded organosilyl substituents (52). 

Related gallium phosphandiides and some sterically congested monomeric digallyl phosphandiides have also been synthesized, and these results have been summarized in a detailed review (63). Molec-... 

An important result of the multinuclear NMR investigations of 23— 25, 27, 28, and 31-35 is that the structures, in contrast to aggregates of monometalated secondary phosphanes and arsanes, are retained in solution. Thus, the phosphandiide derivatives here discussed show resonance signals in their 31P NMR spectra that are independent of concentration and temperature (see Table III). The 31P and 27A1 NMR chemical shifts of 23-25, 27, and 31-35 differ with respect to ring size of the clusters and electronic influences by the substituents at phosphorus and aluminum. 

Replacement of the NMe2 groups in Sb(NMe2)3 by phosphandiide ligands has been achieved by the reaction shown in Eq. (25) (70). Interestingly, all of the HNMe2 produced by this reaction coordinates to the Li ions in 42, which leads to a rhombododecahedral Li6Sb2P6 skeleton. 

The first Ca/Sn-mixed phosphandiide cluster 43 has been prepared by reaction of the calcium phosphanide 44 with Sn[N(SiMe3)2]2 in THF... 

The Ca/Sn-mixed metalated phosphanide-phosphandiide cluster 43 crystallizes in the triclinic space group PI (71). It consists of a trigonal Ca2SnP2 bipyramid, where the phosphandiide P centers serve as ju -bridging centers to the metals. The two phosphanide ligands, however, are. -bridging between two Ca and Ca/Sn centers (Fig. 32). Both Ca ions are octahedrally coordinated. 

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