From Silyl- to Cyanophosphanides: Novel Routes to PCN Building Blocks

Anionic phosphorus and arsenic compounds have found numerous uses as building blocks in synthetic main group chemistry. In particular, phosphanides and arsanides ([RER’]−, E = P, As) with ample structural diversity have been documented. Likewise, because of the lability and polarity of pnictogen-silicon bonds, silylated phosphorus and arsenic species are highly sought-after nucleophilic precursors for the synthesis of challenging low-coordinate heavier pnictogen species or crystalline semiconductor materials. In contrast to pnictogen compounds bearing silyl substituents, cyano-substituted phosphorus and arsenic species are more scarce in literature, owing to the lack of efficient general methods for their synthesis. Preparation of [PCN] and [AsCN] nucleophilic building blocks remains a challenge for main-group chemists. Despite this, cyano-substituted phosphorus compounds would present a significant interest as precursors for phosphorus-doped graphitic carbon nitride photocatalysts. Chapter 2 presents the synthesis of silylated phosphorus and arsenic species from the corresponding sodium pnictogenides, focusing on compounds featuring underexplored substitution patterns at the silyl group. Chemistry of pnictogenide anions featuring unusual silyl substituents is explored in the third chapter, showing the applications of the triphenylsilyl derivatives to synthesis of low-coordinate pnictogen compounds and transition metal complexes. Chapter 4 describes two independent preparations of the [Ph3SiPCN]− anion using sodium phosphide as a source of phosphorus. The structure and electronic properties of [Ph3SiPCN]−, a hybrid between a bis(cyano)phosphanide and a bis(silyl)phosphanide, were investigated. The cyano(triphenylsilyl)phosphanide anion is primarily described as a phosphanide species but significant delocalisation towards the cyano moiety is evidenced in DFT computations. 2 Chapter 5 describes the reactivity of [Ph3SiPCN]−, highlighting the nucleophilic character of both the phosphorus and nitrogen atoms of the [PCN] unit and the lability of the phosphorus-silicon bond. Cyano(triphenylsilyl)phosphanide can therefore be considered as a [PCN]2− synthon. Transition metal complexes of the cyano(silyl)phosphanide show significant differences with complexes of bis(silyl)phosphanides. Cyano(trialkylsilyl)phosphanides would present an interest compared to [Ph3SiPCN]− because of reduced steric bulk and enhanced atom economy. Chapter 6 documents the attempts at preparing such species using the methods described in Chapters 3 and 4 for the corresponding triphenylsilyl derivative. Chapter 7 focuses on methods to prepare N-heterocyclic carbene-cyanophosphinidene (NHC-PCN) adducts from cyanophosphanides. NHC-PCN adducts with substituents of minimal size on the NHC moiety would present an interest for the synthesis of graphitic carbon nitride by thermolysis. In summary, this work shows the use of sodium pnictogenides as inexpensive and convenient sources of pnictogens for the preparation of reactive phosphorus and arsenic species, and in particular for the synthesis of the [Ph3SiPCN]− anion. This anion shows potential as a [PCN]2− synthon and exhibits peculiar coordination chemistry compared to parent phosphanides