Versatile Silylphosphine Ligands for Transition Metal Complexation Versatile Silylphosphine Ligands for Transition Metal Complexation

In this chapter, a review throughout the literature on the chemistry of multidentate silylphosphines is presented. The incorporation of P and Si functionalities in cooperation in a single ligand backbone is exceptionally versatile, and examples of this rich chemistry stemming from the works of many research groups around the world are herein provided. The ligand systems can be flexible or rigid and incorporate varying numbers of P, Si and even other atoms. Exceptional ligand-metal systems are discussed in terms of their structure, reactivity and, in some cases, catalytic activity.


Introduction
In modern Coordination and Organometallic Chemistry, ligand design is recognised as crucial for the development of efficient and selective complexes for important transformations including medicinal chemistry, material science and catalysis. Polydentate-rigid or semi-rigid ligands constrain the geometry at the metal centre providing inherently well-defined coordination geometries for potential incoming substrates. Indeed, a good number of these metal-ligand systems are capable of performing selectively difficult activations and many research groups around the world have directed their endeavours to the study of their chemical properties.
The incorporation of dual functionalities in a single ligand backbone has also been shown to modify the properties of the compounds making them especially prone to undergo selective transformations resulting from differing reactivity of the coordinating atoms in the ligand. A wide variety of combinations of donor atoms have been employed to date, including for example, soft and hard donor atoms in what is known as hemilabile ligands.
In this chapter, the chemistry related to silylphosphine ligands which include in their structure both a basic P as well as a Si is reviewed. Si derivatives are exceptionally good sigma donors and exert a considerably high trans-influence/effect, thus their coordination generates electron rich metal centres in turn capable of activating otherwise inert substrates. Phosphines have long been preferred ligands due to their ability to tune their steric and electronic properties depending on the substituents on P. The incorporation of P and Si in a ligand framework also allows for the employment of NMR spectroscopic tools deriving from 31 P and 29 Si nuclei. moiety. The P (III) group is able to form a coordination bond to the transition metal, while the silyl moiety is potentially prone to bind by means of loss of H 2 , alkane or arene molecules. Between the P and Si atoms, there are generally a number of carbon atoms in the form of an alkyl or aryl bridges (Scheme 1). Else a direct P─Si bond can be established. Silylphosphines are potentially bi-, tri-or polydentate ligands, the coordination number depending on the number of P or Si moieties present in the ligand backbone.
Therefore, the molecular orbitals can be described as those of the phosphine and silicon donor moieties. For example, for the non-classical bidentate coordination mode, the frontier orbitals are shown schematically in Figure 1. A bidentate P, Si ligand can readily coordinate to the metal centre both through the phosphorous atom via the donation of the electron lone pair on P to an empty d-orbital on the metal and through the σ-Si─H electron pair donated to a suitable empty d-orbital on the metal generating a 3c-2e non-classical bond. In both bonds, the stabilisation is given by the retro-donation of electron density of a filled d-orbital to an anti-bonding orbital. In the full oxidative addition process of the Si─H bond to the metal, due to the strong retro-donation of the d-orbital → σ*(Si─H), the final product results in the formation of two 2c-2e bonds: M─H, M─Si. As expected, depending on the substituents on both the P and Si atoms, the molecular orbital diagrams and the energy of the HOMO and LUMO will vary. In general, it could be said according to Figure 1, the HOMO generally possess a higher ligand character, while the LUMO is more metal centred.

Silylphosphine ligands throughout the chemical literature: a review
Stobart and co-workers pioneered the systematic study of transition metals bound to silylphosphine ligands. As early as in 1983, they reported the synthesis and full characteri-   (Figure 3). Furthermore, the reactions of Ph 2 P(CH 2 ) 2 SiMe 2 H towards the dimers [M(μ-Cl) (COD)] 2 (M = Rh, Ir; COD = 1, 5-cyclooctadiene), also afford the M(III) complexes [MCl{Ph 2 P(CH 2 ) 2 SiMe 2 } 2 ] (M = Rh 39; Ir 40) which are quiral with the two phosphorous atoms in trans disposition while the two Si dispose in a cis fashion (Figure 3) [2]. The fact that the reactivity of complexes 39 and 40 was remarkably constrained due to the trans-labilising effect of the silyl groups, was exploited in their use as catalysts for transformations of organic substrates [3]. Several works reported in the literature have argued on the high extent of trans-influence silyl groups exercise on a transition metal centre. There are various reasons for this behaviour including an excellent sigma orbital overlap as well as a favourable electronic release of the Si [4,5]. This is in agreement with only a few compounds exhibiting a trans coordination of the Si atoms in many cases as kinetic products in equilibria with their cis isomers [6,7] even when employing chelating silylphosphines (vide supra) [8][9][10][11] (Section 8).
It was found that the ligands with two or three phosphorous atoms and a Si-H bond (compounds 41-52, Figure 4) coordinate via oxidative addition to the metal centre (i.e. rhodium, iridium, ruthenium and platinum) and impose steric constraints on the coordination sphere in turn restraining substrate entry to sites which could suffer the strongly labilising trans effect of the silyl group, increasing the complexes' capabilities as catalysts [12] ( Figure 4).

Silylphosphines complexation in tetra-coordinated systems
Moreover, the reactivity of 54 towards [PtCl 2 (SEt 2 ) 2 ] leads to the generation of [PtCl{(ο-C 6 H 4 -PPh 2 ) 2 SiMe}] (67) where the ligand coordination results in adoption of a distorted square planar geometry around Pt with a persistent Cl atom bonded trans to the silyl group ( Figure 5) [9].
Milstein and co-workers described the design and synthesis of the first pincer-type silanol-Pt(II) compound by using a PSiP ligand.  (Figure 5) [19].

Tetrahedral and trigonal pyramidal geometries
Ligand 54 (see   Bourissou and co-workers reported the reactivity of the PSiP ligand 54 towards CuCl and AuCl(SMe 2 ), which was subsequently followed by a stoichiometric addition of GaCl 3 (complexes 84, 85). The addition of the gallium halide was envisioned to increase the electrophilicity of the central metal and thus to escalate the strength of non-classical σ-SiH bond interaction at the metal. In complex 84, the coordination of the ligand occurs through the two phosphorous atoms and a weak sigma interaction Si─H⋯Cu. The spectroscopic evidence as well as computational analyses (geometry optimisations and NBO analyses) are in agreement with weak donation σ-SiH → Cu in combination with a negligible Cu → σ*SiH backdonation in 84. Meanwhile in the cationic gold complex 85, the coordination of 54 took place only through the two phosphorous atoms as any non-classical Si─H bond interaction to the metal was strongly disfavoured as it was found to be by computational means 15.9 kcal/mol ( Figure 6) [23].

Square pyramidal geometry
The reactivity of 55 with [RuCl 2 (p-cymene)] 2 was carried out in the presence of PCy 3 and Et 3 N rendering a binuclear complex that preserves bridging chloride ligands [Ru(μ-Cl){(ο-C 6 H 4 -PCy 2 ) 2 SiMe}] 2 (86), which was exhaustively characterised spectroscopically (Figure 7). The ligand coordinates each Ru atom through two P, one Si and two Cl atoms in a distorted square pyramidal geometry, in which the silyl group occupies the apical coordination site.

Trigonal bipyramidal geometry
The   (137)). For the last two complexes, NMR spectroscopy reveals the presence of the fac/mer isomers; the meridional and facial disposition of the PSiP ligand was supported by single crystal X-ray diffraction (Figure 9) [35].

Hybrid silylphosphines complexation
PSiN-ligated complexes have also been attractive synthetic targets due to the hemilability prop- Overall, a square planar geometry around Ru centre is structurally proposed (Figure 10) [36].
Another example of an elegant catalytic application of systems derived of PSiN pincer-like ligands is that comprising the ligands of general formula {(ο-C 6 H 4 )-PR 2 }{(ο-C 6 H 4 )-NMe 2 }SiHMe   2 in the presence of Et 3 N. In particular, the PSiN-platinum complex 151 successfully catalysed C─H borylation not only of highly electron deficient perfluoroarenes but also of the monofluorinated arenes, chloroarenes and benzoate (Figure 10) [37].
Recently, the syntheses of bulky-cage trigonal bipyramidal iron complexes 174 and 175 with remote tertiary amines were reported. The synthesis of ligands 172 and 173 is shown in Figure 13.
Once again, in this regard, the incorporation of secondary sphere interactions into iron-phosphine scaffolds is relevant to synthetic nitrogen fixation research [54]. 183 with respect to a non-benzilic phenylphosphine analogue, coupled with the presence of the non-classical Si─H bond interactions, which could undergo a low energy dissociation-coordination process of the Si─H bonds, was claimed to induce the gradual loss of H 2 in 185 to the final stable bis(carbometallated) complex 187. Thus, it was reasonable to propose that the agostic interactions preceded and favoured the C─H bonds activation process [58]. Ligand SiPSi phosphinodibenzyl-silane PhP{(ο-C 6 H 4 )CH 2 SiMe 2 H} 2 (188) was synthesised from PhP(ο-tolyl) 2 , it behaved as a pincer-like ligand capable of adopting different coordination modes at ruthenium through different degrees of Si─H bond activation. The reaction of 188 towards complex 184 yielded exclusively the formation of 189, in which a Ru(II) centre is coordinated to one ligand 188, through the P atom and two non-fully activated Si─H bonds preserving one PCy 3 and two hydride ligands of the original Ru complex. The phosphorous atoms arrange in a distorted cis with a P-Ru-P angle 113.32(4)° in 189 which should be compared to 107.1(4)° in bis-cyclometallated 187. This sterically encumbered arrangement of the phosphine ligands around ruthenium has been explained due to the favourable exchange of the two formally terminal and two nonclassical sigma hydrides around the metal. Certainly, the measured value of the J SiH together with theoretical calculations and the observed chemical behaviour of 189 in solution agree with the presence of non-classical η 2 -Si-H character of the silyl moieties. Thus, the complex 189 was formulated as an 18-electron species stabilised by two unusual intramolecular ε-non-classical interactions. Complex 189 undergoes facile and reversible loss of dihydrogen to afford quantitatively 16-electron complex 190, which is thought to preserve a single non-classical hydride as well as a terminal one. Moreover, NMR spectroscopic experiments on complex 189 show it to be very fluxional in the temperature range accessible, while hydride exchange in complex 190 takes place at the high-temperature regime but in the slow exchange indicates only one Figure 15. Chemistry of silyl-benzyl phosphines bi-, tri-and tetradentate [58][59][60].  (194, 195). These two complexes feature ligand (188 and 191) in a close to meridional disposition. Complex 195 results from ligand modification at one of the benzylic positions which undergoes formation of a new C-Si bond. Furthermore, d 8 Pt(II) complex 195 is the first case of a silyl-platinum complex that includes a novel C─H⋯Pt anagostic interaction (Figure 15).

Applications of silylphosphines in the chemical industry
From the examples throughout this chapter, one can safely envisage transition metal complexes of silylphosphines as active catalysts in a variety of industrial processes. The industrial application of this type of ligand systems, nevertheless, is still at its cradle with future applications expected to materialise in the mid-term.
In principle, Si and P are capable of displaying nucleophilic behaviour and both also possess the ability to displace leaving groups such as halogens, neutral/monodentate ligands, and so on, while the factors affecting their stereochemistry may also assist the complex in the attainment of specific geometries [61]. Catalysed transfer hydrogenation has been developed mainly based on complexes derived from the platinum-metals group [62], and it is applied in industrial process and organic synthesis [63]. [PSiP-Ru] species also have shown to play an excellent role in the reduction of ketones employing i PrOH as the hydrogen source. The well-known Kumada's cross-coupling reaction is an actual tool for the low-cost synthesis of styrene derivatives in the industrial scale by using Ni and Pd complexes as catalysts [64]. Some advances revealed the crucial use of phosphorous-containing compounds [65][66][67] and/or the very bulky donor ligands [68,69]. Nevertheless, [PSiP-Co] systems have shown efficient conversions in relative mild reaction conditions of an aryl-Grignard reagent reaction with organic halides at 50°C for 24 h [70].

Conclusion and perspectives
The incorporation of dual functionalities P and Si in single ligand backbones, silylphosphines, notably modifies the properties of the complexes they form, making them especially reactive and able to undergo selective transformations resulting from differing reactivity of the coordinating atoms in the ligand in conjunction with the chelate effect.
Predictably, the observed reactivity stems from the combination of the most important qualities of the Si ligands, specifically their extremely high σ-donating character and thus their capability of forming σ-complexes, coupled to those features of the P moieties, which can be greatly modified by the choice of substituents.
Throughout this chapter, it has been shown the study of transition metal systems bonded to silylphosphine ligands has thrived in the last decades, but the findings in the last years highlight the importance of their study. Numerous extraordinary systems displaying unusual bonding modes, structures or physicochemical properties have been reported to date and many more can be envisioned to be informed in the near future given the relatively accessible synthesis of ligands and the seemingly unlimited structural variations.
However, the catalytic and other applications of these compounds have been sparingly explored; yet the potential of many of the reported systems is foreseen. We thus expect this field of chemistry to continue growing rapidly and encourage other research groups to direct their endeavours to this fascinating area of research.