Phosphanides

In order to overcome the difficulties of thermal instability, the phosphasilene derivatives 15, which bear a silyl or germyl group attached to phosphorus, were synthesized. Indeed, they proved to have stronger Si=P bonds (stable up to 100°C), thus allowing studies of their structures and reactivity.l0b 14 Phosphasilenes 15a-15i were synthesized from the corresponding Iithium(fluorosilyl)phosphanides 16a-16i by the thermally induced elimination of LiF (see Scheme 4).10b It has been shown that excellent steric protection of the highly reactive Si=P bond in 15 is provided by the 2,4,6-triisopropylphenyl (Is = isityl) substituent attached to the low-coordinate silicon center. The appropriate precursors 16a-16i were synthesized in a multiple-step procedure, starting from 17 (Scheme 4).10b U... 

Hitherto no monometalated molecular pnictide exists without solvation of the main group metal atom. Therefore, the monomeric species L (Fig. 2) can only be stabilized if the Li ion has its coordination sphere enlarged through donor solvation. More importantly, the lithium phosphanides of the type K undergo oligomerization processes to form dimer, tetramer, hexamer, or polymeric assemblies M—Q (Fig. 2), which dissociate in solution more easily than related amides (2, 11, 12). 

Compound 14 is diamagnetic and represents the first tetrasodium-dication cluster that is stabilized by two sterically congested silyKflu-orosilyl)phosphanide counterions (see Section II,D). It has been also independently synthesized through sodium consumption of 13 in the presence of styrene as catalyst in 24% yield. The electron reservoir of the Na) cluster can serve for reduction processes, that is, it reduces Me3SiCl to hexamethyldisilane (see Section II,F). The fact that 14 is intensely yellow, and not red or blue as observed for Na-loaded zeolites (28), suggests that the residual metal electrons are probably much less delocalized. 

The Na-Na distances of 3.076(3) (Nal-Na2) and 3.202(3) A (Nal-Nal ) reflect Na-Na bonds (3.82 A in elemental Na), whereas the Nal-Na2 distance of 3.530(3) A suggested less attractive interactions. The Na4-dication cluster is embedded between two silyl(fluoro-silyl)phosphanide counterions. The relatively low-coordinated Na centers are remarkably stabilized by the -fluorine atoms and by... 

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). 

The average value of the Mg-P distance of 2.53 A resembles that of 17 and of other related magnesium phosphanides (47). However, the endocyclic angles at phosphorus (average 81.3°) are much smaller than that values in 17. 

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. 

Scheme 3.6-1. Structural motifs of main group metal amides, phosphanides and arsanides (E = N, P, As).

Considering the importance of alkali metal phosphanides it is not surprising that numerous review articles have dealt with this subject [34-36]. The solid state and solution structures vary from dimers with central M2 P2 cycles to larger rings and from chain to ladder structures as described for the lithium amides (see Sections 3.6.1 and 3.6.2). Cage compounds in the field of lithium phosphanides are unusual... 

Oxygen and moisture has to be excluded carefully during the preparation procedures to avoid oxygen-centered cages. Then unusual metal deficient phosphanediides of lithium of the type [(Li2PR) (PR)m] with Li2 P( +m) cages are isolated. Investigations of phosphanides of the heavier alkali metals are far less common [37]. 

The interconnection of two phosphanide substituents by sterically demanding RR Si groups allows the preparation of cage compounds [38], The two-fold lithia-tion of bis(phosphanyl)(alkyl)arylsilane and the dimerization of this bis(phos-phanido)silane leads to the formation of 19 with a strongly distorted Li4P4 hetero-cubane structure according to Eq. (5). 

Fig. 3.6-6. Molecular structures of the mixed lithium phosphanide-LiCI cluster 20 and the anionic cluster core in 21.

Scheme 3.6-7. Synthesis of the sodium (fluorosilyl)phosphanide clusters 22 and 23.

Fig. 3.6-7. Core structure of the lithium phosphanide cluster 28. Small dark circles lithium atoms, big dark circles phosphorus atoms.

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. 

In contrast to the lithium amides and phosphanides, dimeric alkaline-earth metal bis(phosphanides) of the heavier group 2 metals show bicyclic structures of the... 

In Table 3.6-4 the relative energies of coligand-free dimeric alkaline earth metal bis(phosphanides) are summarized [56, 57]. Whereas for Ca and Sr the bicyclic... 

Tab. 3.6-4. Relative energies obtained by ab initio SCF calculations of dimeric alkaline-earth metal dihydrides and bis(phosphanides) (kj mol-1).

The metalation of trialkylsilylphosphane and -arsane with the alkaline earth metal bis[bis(trimethylsilyl)amides] of calcium, strontium, and barium yields the mixed phosphanides and phosphanediides as well as arsanides and arsanediides depending on the stoichiometry and the demand of the trialkylsily] substituents according to Scheme 3.6-11. The main feature is the M2E3 bipyramid with the metal atoms in apical positions. These cages are often interconnected via common faces (61, 63, 64, 65, 67, and 69). A substitution of the phosphanide substituents by other Lewis bases such as THF or benzonitrile is not possible for these compounds and, consequently, homoleptic phosphanediides and arsanediides with inner M4E4 heterocubane moieties are so far unknown for M = Ca, Sr, and Ba. In all these cases a further metalation to obtain homoleptic phosphanediides failed. 

Oxygen-centered phosphanides are accessible by hydrolysis of the phospha-nides as shown for the oxygen-centered phosphanide 73 of the formula [Sr40- P(SiMe2Prl)2 6] with an Sr4P6 adamantane-like structure as shown in Eq. (7) [74], The strontium atoms are coordinatively saturated by agostic interactions to the silicon-bonded alkyl substituents. 

The synthesis of heterobimetallic cages which contain alkaline-earth metals and tin(+2) atoms succeeds by the metalation of trialkylsilyl substituted phosphanes with the bis(trimethylsilyl)amides of tin(+2) and of calcium, strontium, or barium according to Scheme 3.6-13. Heterobimetallic cages of tin and magnesium are unknown, instead their formation mixtures of the homometallic phosphanides are observed [75],... 

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