Review
The unusual coordination chemistry of phosphorus-rich linear and cyclic oligophosphanide anions

The authors wish to dedicate this work to the memory of Prof. Dr. H.C. Hans-Georg von Schnering, an outstanding scientist and distinguished colleague.
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Abstract

In this review, the synthesis, reactivity and properties of linear and cyclic oligophosphanides are described. Specifically the structures and versatile reactivity of the anionic ligands (P4R4)2− (R = But, Ph, Mes), (P4HR4) (R = Ph and Mes) and cyclo-(P5But4) towards main group and transition metal complexes is elucidated. In addition, potential application of metal oligophosphanides as precursors for the preparation of metal phosphides is also briefly discussed.

Introduction

The reports by Baudler and some other authors on their extensive investigations on cyclooligophosphanes cyclo-(PR)n [1] and catenated polyphosphanes (with hydrogen, organic or even organometallic substituents) [2] have shown that the chemistry of these compounds is analogous to that of related carbon compounds [3]. Among other things, this analogy is due to the existence of constitutional and configurational isomerism and valence tautomerism, as well as the existence of mixed P–C ring systems, and is rationalized by the isolobality of the fragments P/CR, PR/CR2 and PR2/CR3 [1], [2], [3]. Like carbon, phosphorus shows a notable propensity to form a wide variety of Pn frameworks due to the comparatively high bond energy of P–P single bonds (ca. 200 kJ/mol, the highest value within group 15) [4].

While the chemistry of catena and cyclic polyphosphanes has been studied in depth, especially with the help of 31P NMR spectroscopy and X-ray crystallography, less is known about metal complexes of these species. Only a few examples have been described, and all of them have shown that cyclic [5] and catenated oligophosphanide anions [6] exhibit a rich coordination chemistry, because each P atom may be involved in coordination via its free electron pair.

On the other hand, the coordination chemistry of oligophosphanide anions has hardly been explored until recently, as selective and facile syntheses were mostly unknown for the corresponding oligophosphanide anions. Besides the academic challenge, metal complexes with anionic polyphosphorus ligands are of interest as potential precursors for the development of rational syntheses of binary metal phosphides (MxPy), which are compounds with rare structures and interesting properties for materials science [7], such as corrosion resistance [8], catalysts for hydrodesulfurization and hydrodenitrogenation of petroleum fuels [9], oxygen barriers in capacitors [10], and magnetic properties [11]. Even today, the number of accessible metal complexes with linear and cyclic oligophosphanide ligands is still relatively small. Thus, this article will review the synthetic methods and reactivity for the preparation of metal complexes with anionic polyphosphorus rings and chains, as well as their structural and spectroscopic properties.

Section snippets

General synthetic methods for the preparation of metal oligophosphanides

The preparation and purification of metal complexes with oligophosphanide ligands has always been a highly challenging task. Several complexes have been reported in the literature which in many cases have been obtained serendipitously or as inseparable mixtures and were characterized only by 31P NMR spectroscopy or X-ray crystallography.

Additionally, some procedures reported for the rational preparation of this kind of complexes have led to mixtures of compounds from which some target products

Dianionic phosphorus-rich chains

As mentioned above, the selective preparation of compounds of the type [M2(L)x(PnPhn)] (M = Na or K, n = 3 or 4, L = donor molecule) was achieved only during the past decade by judicious choice of the correct stoichiometric ratio of RPCl2 and alkali metal (Scheme 10). In the solid state, these compounds form isolated ion-contact complexes, in which the P4 chain of the (P4R4)2− dianion has a syn arrangement and is coordinated to two alkali metal cations [33], [34], [35].

The coordination spheres of the

Alkali metal cyclo-oligophosphanides

The targeted synthesis of anionic ring systems such as M[cyclo-(P5R4)] (M = Li, Na or K; R = Pri, But, Ph) has been reported [36], [37], [41]. These compounds have been synthesized by the reaction of lithium, sodium or potassium with the corresponding RPCl2 and PCl3 in the ratio 12:4:1 in THF (Scheme 11). A product mixture is obtained usually comprising M[cyclo-(P3R2)], M[cyclo-(P4R3)], cyclo-(P4R4), M2(P4R4) and M[cyclo-(P5R4)]. Systematic variation of the reaction conditions led to optimization

Main group metal complexes with the (P4R4)2− ion

The highly reducing nature of (P4R4)2− dianions limits their use as transmetallation reagents for metal salts in high oxidation states [28]. Thus, reactions of the (P4R4)2− anion with p-block main group metal salts such as SnCl2, AlEt2Cl, AlCyCl2, Ga(DAB)I2, GaI, InCyBr2 or GaCl3 (Cy = cyclohexyl, DAB = N,N′-bis(2,6-diisopropyl)phenyldiazabutadiene) did not lead to the desired transmetallation products; instead, either elemental metal, cyclooligophosphanes or unidentified products where obtained

Reactivity of the cyclo-(P5But4) ion towards main group metals

As was already observed for the (P4R4)2− ions, the highly reducing nature of the cyclo-(P5But4) ion complicates its use as transmetallation reagent for metals in high oxidation states.

In the case of p-block main group metals, such as aluminium, germanium, tin, lead and bismuth, differences in reactivity were observed. While the 1:1 reaction of the sodium salt [Na(thf)4][cyclo-(P5But4)] (39) with AlEt2Cl and GeCl4 gave a mixture of products including the expected complexes [AlEt2{cyclo-(P5But4

Future applications of phosphorus-rich metal oligophosphanides

Phosphorus-rich metal oligophosphanides could be suitable candidates for the development of alternative routes for the preparation of metal phosphides (MxPy, with y > x) under mild conditions. One of the key steps in this process is the removal of the R groups at phosphorus, which should be facilitated by thermal decomposition. Organyl-free MxPy species have already been observed in the mass spectra of several of the studied complexes, e.g., [Rh2P3]+ and [Rh2P2]+ in [Rh(P4HMes4)(cod)] (49), [PtP4]

Conclusions

The targeted synthesis of ions such as (P4R4)2−, (P4HR4) (R = Ph, Mes) and cyclo-(P5But4) and their versatile reactivity towards main group and transition metal complexes allowed the preparation of a large variety of phosphorus-rich metal oligophosphanides. In reactions with the anions (P4R4)2− or (P4HR4) (R = Ph, Mes), complexes with intact (P4R4)2− or (P4HR4) ligands are formed or degradation or oxidative coupling was observed resulting in complexes with (P3R3)2−, (P2R2)2−, and diphosphene (P2

Acknowledgements

E.H.-H. would like to thank her co-workers who have worked or are still working in this research area (P. Lönnecke, S. Blaurock, R. Felsberg, A. Schisler, R. Wolf, C. Limburg, S. Bauer, H. Bittig, B. Gallego, R. Herrero, I. Jevtovikj, and A. Kircali). We are also grateful to BASF SE, Chemetall and Umicore AG for their generous donations of chemicals. Financial support from the Alexander von Humboldt-Stiftung (Humboldt-Fellowship for S.G.-R.), the Deutsche Forschungsgemeinschaft (He 1376/22-1,

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