Versatile Silylphosphine Ligands for Transition Metal Complexation

Abstract

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.

1. 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 ³¹P and ²⁹Si nuclei.

2. Silylphosphine ligands: definition, general structure and bonding

Silylphosphines can be described as bi- or polydentate ligands bearing at least one basic phosphorous (III) atom, usually a phosphine PR₃ or phosphite P(OR)₃, and at least one silicon-substituted 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 H₂, 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.

Scheme 1.

General structure of silylphosphine ligands and examples of main coordination modes.

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.

Figure 1.

Molecular orbital diagrams of the phosphine and non-classical Si─H sigma moieties.

3. 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 characterisation of an extensive family of silanes modified with a phosphorous fragment connected to the silicon atom by a polymethylene chain, of general formula (XYZ)Si(CH₂)_nPR₂ (where X, Y, Z = Me, Ph, Cl or H; n = 1–3; R = Me or Ph;) (Figure 2, compounds 1–33) [1]. The reactivity of Vaska’s complex trans-[Ir(PPh₃)₂(CO)(Cl)] towards Ph₂P(CH₂)₂SiRR’H (compounds: 12, 14, 16, 18, 20) was also investigated. The reaction results in the formation of air stable six-coordinated Ir(III) compounds, resulting from coordination of the ligands through the P atom and of the oxidative addition of the Si─H bond (compounds: 34–38) (Figure 3). Furthermore, the reactions of Ph₂P(CH₂)₂SiMe₂H towards the dimers [M(μ-Cl)(COD)]₂ (M = Rh, Ir; COD = 1, 5-cyclooctadiene), also afford the M(III) complexes [MCl{Ph₂P(CH₂)₂SiMe₂}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).

Figure 2.

The silylphosphine ligands bearing alkyl bridges reported in Ref. [1].

Figure 3.

The reactivity of group 9 metals as reported in Ref. [2].

Figure 4.

Design of poly(phosphino)-silane ligands reported in Ref. [12].

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).

4. Silylphosphines complexation in tetra-coordinated systems

4.1. Square-planar geometry

Turculet and co-workers have further made significant contributions in the field of silylphosphine chemistry. They introduced a PSiP-type ligand [(ο-C₆H₄-PCy₂)₂SiMe₂] (53) and explored its reactivity with transition metal precursors. The complexes [MCl{(ο-C₆H₄-PCy₂)₂SiMe}] (M = Ni or Pd) (64, 65) were treated with alkyl lithium or Grignard reagents. In the case of 65 treatment with stoichiometric amounts of MeLi led to the formation of [Pd(Me){(ο-C₆H₄-PCy₂)₂SiMe}] (61) which regenerates complex 65, upon reaction with Ph₂SiClH while renders [Pd{(ο-C₆H₄-PCy₂)₂SiMe}(SiHPh₂)] (66) from reaction with Ph₂SiH₂. The direct reaction of 53 and [Pd₂(dba)₃] provides complex [Pd{(ο-C₆H₄-PCy₂)SiMe₂}(ο-C₆H₄-PCy₂)] (63) derived from Si-C(sp²) bond activation. Treatment of complex 64 with MeMgBr led to [Ni(Me){(ο-C₆H₄-PCy₂)₂SiMe}] (60) and complex [Ni{(ο-C₆H₄-PCy₂)SiMe₂}(ο-C₆H₄-PCy₂)] (62) resulting from ligand rearrangement. Complexes 62 (Ni) and 63 (Pd) constitute rare examples of reversible Si-C(sp²) and Si-C(sp³) bond cleavage (Figure 5) [13, 14].

Figure 5.

[PSiP] ligands and their square planar group 10 metal complexes [13-19, 26, 59].

Moreover, the reactivity of 54 towards [PtCl₂(SEt₂)₂] leads to the generation of [PtCl{(ο-C₆H₄-PPh₂)₂SiMe}] (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].

Iwasawa and collaborators reported an interesting system for the catalytic hydrocarboxylation of allenes using the Pd(II) hydride complex [PdH{(ο-C₆H₄-PPh₂)₂SiMe}] (68) as the active catalyst. Their methodology also served for the synthesis of β,γ-unsaturated carboxylic acids. In general, complex [Pd(OTf){(ο-C₆H₄-PPh₂)₂SiMe}] (69) in catalytic ratios 1.0 mol% or 2.5 mol % was used in soft conditions of CO₂ pressure (1 atm) with 150 mol% of AlEt₃ or ZnEt₂ for carboxylation of 1,1-disubstituted, monosubstituted or disubstituted allenes to the respective carboxylic acid or ester [15]. In addition, the chemical properties of complex 69 were described as well. To mention some, 69 undergoes transmetalation with AlEt₃ followed by β-hydride elimination to generate the proposed complex 68. Complex 69 reacted with an excess of B₂pin₂ at room temperature leading to HBpin and the monoborylpalladium complex [Pd(Bpin){(ο-C₆H₄-PPh₂)₂SiMe}] (70) which promotes the product of borylation of styrene as well as other alkenes (Figure 5) [16, 17, 18].

Milstein and co-workers described the design and synthesis of the first pincer-type silanol-Pt(II) compound by using a PSiP ligand. The ligand {(ο-C₆H₄-PiPr₂)₂SiH₂} (58) was obtained in moderate yields from the o-bromophosphine. It readily reacts with [(Me₂S)₂Pt(Me)Cl] at room temperature to give the bicyclic complex [PtCl{(ο-C₆H₄-PiPr₂)₂SiH}] (72), which then undergoes autoxidation yielding the silanol complex [PtCl{(ο-C₆H₄-PiPr₂)₂Si(OH)}] (73) in moderate yields (65%) (Figure 5) [19].

Interestingly, changes on the identity of the substituents on the P atoms in the PSiP ligand backbone bring about a great strategy for the coordination of ligand 55 towards group 9 metals, in particularly rendering an Ir system able to activate intermolecular arene C─H bonds. Indeed, the reaction of 55 and [MCl(coe)₂]₂ (M = Rh, Ir; coe = 1-cyclooctadiene) or [RhCl(PPh₃)₃] produced the monomeric complexes [MH(Cl){(ο-C₆H₄-PCy₂)₂SiMe}] (M = Rh, 74; Ir, 75), which react with organolithium compound [Me₃SiCH₂Li] forming neutral three-coordinate intermediate species able to subsequently coordinate neutral ligands, thus generating [M(L){(ο-C₆H₄-PCy₂)₂SiMe}] (M = Rh, L = H₂NPh (76); M = Rh, L = NH₃ (77); M = Ir, L = C₂H₄ (78); M = Ir, L = PMe₃ (79); M = Rh, L = PMe₃ (80)) (Figure 7) [20].

4.2. Tetrahedral and trigonal pyramidal geometries

Ligand 54 (see Figure 5) reacted with [Pd(PPh₃)₄] or [CpPd(C₃H₅)] yielding complex [Pd{η²-(ο-C₆H₄-PPh₂)₂SiHMe}(PPh₃)] (82) instead of the hypothesised hydride complex 68 which was proposed as the intermediate in the reduction of compound [PdCl{(ο-C₆H₄-PPh₂)₂SiMe}] (71, see Figure 5) with LiHBEt₃ in presence of PPh₃ to afford also 82 [21]. Likewise, the reaction of ligand 54 with [Ni(PPh₃)₄] led to the formation of the Ni(0) complex [Ni{η²-(ο-C₆H₄-PPh₂)₂SiHMe}(PPh₃)] (81). The Si─H, Ni─H and Ni─Si distances are 1.62(3), 1.44(2) and 2.2782(4) Å respectively, suggesting that the Si─H bond was preserved. This non-classical complexation mode is kept in solution because the observed NMR parameters such as coupling constants J_SiH = 89 Hz at 300 K and 77 Hz at 193 K are large (²J_SiH ≤ 20 Hz for a complete Si─H bond cleavage) and in line with the conservation of the η²-Ni(0) structure seen in solid state (Figure 6). On the other hand, the mixture of 54 with [Pt(PPh₃)₄] is discussed in Section 5.2 [22].

Figure 6.

Reactivity of 54. Tetra-coordination is highlighted in blue and penta-coordinated in purple [21-23, 26-29].

Bourissou and co-workers reported the reactivity of the PSiP ligand 54 towards CuCl and AuCl(SMe₂), which was subsequently followed by a stoichiometric addition of GaCl₃ (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 back-donation 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].

Extraordinarily, 86 (described in more detail in Section 5.2) demonstrated to be a suitable precursor for the synthesis of stable 14-electron [Ru(X){(ο-C₆H₄-Cy₂)₂SiMe}] (X = O^tBu (87), N(SiMe₃)₂ (88); NHPh (89); NH(2,6-Me₂C₆H₃) (90)) complexes, donning unusual trigonal pyramidal coordination geometries explained once again by the presence of the strongly σ-donating silyl group in the apical site with the contribution of steric effects of the phosphino substituents in the equatorial plane (Figure 7) [24].

Figure 7.

Reactivity of ligand 55. Tetra-coordination is highlighted in blue and penta-coordination in purple [20, 24-25, 36].

5. Silylphosphines complexation in penta-coordinated systems

5.1. Square pyramidal geometry

The reactivity of 55 with [RuCl₂(p-cymene)]₂ was carried out in the presence of PCy₃ and Et₃N rendering a binuclear complex that preserves bridging chloride ligands [Ru(μ-Cl){(ο-C₆H₄-PCy₂)₂SiMe}]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. This Ru(II)-ligand system brings about stability and selectivity in catalytic transformations including the transfer hydrogenation of ketones. In this case, Li et al observed high conversion of the corresponding secondary alcohols for numerous dialkyl, diaryl, and alkyl/aryl ketones, employing 0.2 mol% of 86 with 5 mol% of KO^tBu at 80°C [25].

The activation of Si-H bonds in ligands of general formula (ο-C₆H₄-PR₂)₂SiHMe (R = Ph or Cy; 54 and 55) was also observed in monomeric Ru systems. Compound 54 reacted with [RuCl₂(PPh₃)₃] in the presence of triethylamine as a base affording complex [RuCl{(ο-C₆H₄-PPh₂)₂SiMe}(PPh₃)] (91). The X-ray diffraction study confirms the coordination of the ligand through the two phosphorous and the silicon atom adopting a facial arrangement with the silyl group occupying the basal position of a distorted square pyramidal geometry around Ru. The Cl atom disposes trans to the silyl group, and the remaining site was occupied by a PPh₃. Complex 91 reacted with LiEt₃BH to form octahedral-Ru hydride complex 92 in moderate yield (Figures 6) [26].

5.2. Trigonal bipyramidal geometry

The mixture of 54 with [Pt(PPh₃)₄] at room temperature led to the generation of five-coordinated Pt(II) complex [PtH{(ο-C₆H₄-PPh₂)₂SiMe}(PPh₃)] (93). In contrast with the derivatives of Ni(0) 81 and Pd(0) 82 (Figure 6), where the Si─H bond is only slightly activated, the Pt(II)-hydride complex 93 derives from the complete oxidative addition of the Si─H bond. The crystalline structure displays a trigonal bipyramidal geometry with the silyl group in the apical position in the metal centre. The opposed apical site was taken by the hydride ligand which in the ¹H NMR spectrum revealed a quartet at δ −7.92 ppm (²J_PH = 18.9 Hz) with ¹⁹⁵Pt satellites exhibiting a measured coupling constant ¹⁹⁵Pt-¹H of 650 Hz, which is considerably small compared with some cis-H-Pt(II)-Si species previously reported (¹J_PtH = 890–1010 Hz); supporting the proposal that the hydride is located trans to Si atom (Figure 6) [22].

A study of the reaction of complex 82 and its related analogue [Pd{η²-(ο-C₆H₄-PPh₂)₂SiHMe}(PMe₃)] (83) towards B₂pin₂ was made, since it could provide a means of accessing Pd(II) hydrides via oxidative addition of the Si─H bond. Two isomers: cis and trans were proposed. Depending on the phosphine choice, the isolation of one isomer was possible through a reversible σ-bond metathesis pathway. In the case of the PMe₃ ligated complex, the kinetic product cis-[Pd(Bpin){(ο-C₆H₄-PPh₂)₂SiMe}(PMe₃)] (94) showed a slow reverse reaction and was obtained predominantly. In contrast, for the PPh₃ derivative, the equilibrium favoured the thermodynamic isomer trans-[Pd(Bpin){(ο-C₆H₄-PPh₂)₂SiMe}(PPh₃)] (95) as a major product (Figure 6) [27, 28]. In relation to unusual bonding modes, Sun and collaborators reported the systematic reactivity of the tridentate ligand 54 towards the low-valent nickel compound [Ni(PMe₃)₄] which induced the formation of Ni(0) complexes [Ni{η²-(ο-C₆H₄-PPh₂)₂MeSi-H}(PMe₃)] (96, Figure 6). Complex 96 did not undergo the oxidative addition process of a Si─H bond even in the presence of independent silanes (Et₃SiH, Ph₂MeSiH). However, the reactivity with chlorosilanes Me₃SiCl or MeHSiCl₂ led to the formation of Ni(II) complex [NiCl{(ο-C₆H₄-PPh₂)₂MeSi}] (97). The halogenated products [NiX{(ο-C₆H₄-PPh₂)₂SiMe}(PMe₃)] (X = Br (98); I (99)) were easily obtained from reaction with EtBr or MeI of complex 96 (Figure 6) [29].

The versatility of ligand 55 was also probed in the coordination towards Ru. With the aim of preparing highly valuable 16-electron complexes, complex 86 was reacted with monodentate phosphines. The reaction with PPh₃ results in small conversion to the five-coordinated compound [Ru(Cl){(ο-C₆H₄-PCy₂)₂SiMe}(PPh₃)] (100) in equilibrium with 86. Interestingly, this latter compounds are also formed from the reaction of ligand 55 and [RuCl₂(PPh₃)₃] in the presence of NEt₃ albeit in low yields. However, the production of the isolable penta-coordinate complex [Ru(Cl){(ο-C₆H₄-PCy₂)₂SiMe}(PMe₃)] (101) was possible in quantitative yields when employing 86 in solution and the smaller, more σ-electron-donating PMe₃ (Figure 7) [30].

In complexes 74 and 75 (Figure 7), the ligand coordinates in a tridentate fashion through the phosphorous atoms which dispose in trans and the Si which sits in the equatorial plane of a trigonal bipyramidal geometry. The remaining sites were taken by the hydride derived from the ligand and a Cl atom. Remarkably, besides the intermolecular C─H activation ability imposed by the coordination of ligand 55 to Ir, complex 75 also exhibits facile N─H bond activation of ammonia and anilines while its Rh analogues undergo mainly adduct formation. Likewise, the starting complexes 74 and 75 react with lithium anilides [Li(NHR)] generating isolable anilido hydride complexes [MH(NHR){(ο-C₆H₄-PCy₂)₂SiMe}] (M = Rh, R = Ph (102); M = Rh, R = {2, 6-Me₂C₆H₃} (103); M = Ir, R = Ph (104); M = Ir, R = {2, 6-Me₂C₆H₃} (105); M = Ir, R = H (106)) upon mixing. The new compounds were described as being very resistant to N─H bond reductive elimination even in the presence of alkyl or aryl substrates (Figure 7) [20].

The ligand (ο-C₆H₄-PPh₂)₃Si-H (107) reacts with [Ni(PPh₃)₄] to yield the complex [Ni{η₂-(ο-C₆H₄-PPh₂)₂Si-H(ο-C₆H₄-PPh₂)}(PMe₃)] (109) bearing non-classical σ-Si─H bonds. On the other hand, complex 109 undergoes thermal oxidative addition at the Ni centre and loss of PMe₃ to allow the coordination of the previously uncoordinated phosphorous, thus rendering a compound of formula [NiH{(ο-C₆H₄-PPh₂)₃Si)}] (110). In a subsequent step, HCl was added to afford the formation of [NiCl{(ο-C₆H₄-PPh₂)₃Si)}] (111), which was also obtained when compound 109 was combined with one equivalent of MeHSiCl₂. Compounds [NiX{(ο-C₆H₄-PPh₂)₃Si}] (X = Br, 112; I, 113) were obtained from the reaction of 110 with either EtBr or MeI (Figure 8).

Figure 8.

The chemistry of P₃Si systems [31-34].

Peters and co-workers have also reported the synthesis and reactivity of silanes functionalised with phosphines and/or sulphur derivatives. In particular, the ligand 107 reacts with [Fe₂Mes₄] leading to the formation of [Fe{(ο-C₆H₄-PPh₂)₃Si}Mes] (114), which was characterised structurally by single crystal X-ray diffraction. The analysis discloses a distorted octahedral geometry around the Fe atom in which the ligand has taken four out of the six coordination positions, a mesityl group occupies one more and the sixth site (trans to silyl group) is occupied by an agostic interaction (C─H⋯Fe) from a methyl group in ortho position of the mesityl bonded to the metal. The reaction of this agostic complex with HCl leads to the formation of complex [FeCl{(ο-C₆H₄-PPh₂)₃Si}] (115). Subsequent reduction with Na/Hg amalgam under a N₂ atmosphere led to the Fe(I) complex [Fe(N₂){(ο-C₆H₄-PPh₂)₃Si}] (116), which was again subjected to reduction with an additional equivalent of Na/Hg in the presence of [12]crown-4 to render dark purple ionic pair [Na([12]crown-4)₂]⁺[Fe(N₂){(ο-C₆H₄-PPh₂)₃Si}]⁻ (117) in which Fe is in a zero oxidation state and the dinitrogen ligand is less labile than in 116 because of stronger π backdonation from the more reduced metal [32]. Once again the nature of the substituents on the P atom is determinant. Indeed, ligand (ο-C₆H₄-PⁱPr₂)₃SiH (108) reacts with FeCl₂ at room temperature producing a species with one uncoordinated phosphorous atom and the Si─H bond intact [FeCl₂{(ο-C₆H₄-PiPr₂)₂SiH(ο-C₆H₄-PiPr₂)}] (118); however, if the reaction was made in the presence of MeMgCl at −78°C, the desired complex [FeCl{(ο-C₆H₄-PiPr₂)₃Si}] (119) was isolated. The reactivity of 108 with metallic precursors CoCl₂, NiCl₂ and [Ir(μ-Cl)(COD)]₂ was found to yield the tripodal species [CoCl{(ο-C₆H₄-PiPr₂)₃Si}] (120), [NiCl{(ο-C₆H₄-PiPr₂)₃Si}] (121) and [IrHCl{(ο-C₆H₄-PiPr₂)₃Si}] (122), respectively (Figure 8) [33].

Another example of the importance of the trans-influence of the silyl groups on the metal coordination sphere is given by the chemical properties of [RuCl{(ο-C₆H₄-PPh₂)₃Si}] (123). The exchange reaction with LiPPh₂ (also with LiPiPr₂) led to expected phosphide complex [Ru(PPh₂){(ο-C₆H₄-PPh₂)₃Si}] (124), which decays at room temperature to the cyclometalated [Ru(PHPh₂){Si(ο-C₆H₄-PPh₂)₂(ο-C₆H₄)P(ο-C₆H₄)Ph}] (125). The reaction of 123 with stoichiometric amounts of MeLi or PhCH₂MgCl leads to [Ru(Z){(ο-C₆H₄-PPh₂)₃Si}] (Z = Me (126); PhCH₂ (127)). Successive loss of methane or toluene renders the formation of the unsaturated square pyramidal species [Ru{Si(ο-C₆H₄-PPh₂)₂(ο-C₆H₄)P(ο-C₆H₄)Ph}] (128), which in turns affords under H₂ or N₂ atmospheres compounds [Ru(H₂)H{(ο-C₆H₄-PPh₂)₃Si}] (129) and [Ru(N₂)H{(ο-C₆H₄-PPh₂)₃Si}] (130) in subsequent reactions steps (Figure 8) [34].

6. Silylphosphines complexation in hexa-coordinated systems: octahedral geometry

Shimada and collaborators reported on the reactivity of ligands (ο-C₆H₄-PR₂)₂SiHMe (R = Cy (55), iPr (56), tBu (57); Figure 5) towards [Ir(μ-Cl)(COD)]₂. The complexes [IrClH{(ο-C₆H₄-PR₂)₂SiMe}] (R = Cy, 75, iPr, 131, tBu, 132) (Figure 7 and 9) reacted with reducing agent Me₄N·BH₄ under argon to produce the tetrahydride complexes [IrH₄{(ο-C₆H₄-PR₂)₂SiMe}] (R = Cy (133), ⁱPr (134), ^tBu(135)) or under dinitrogen gas to produce rare stable Ir(III) dihydride-dinitrogen complexes of formula [IrH₂(N₂){(ο-C₆H₄-PR₂)₂SiMe}] (R = Cy (136), iPr (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].

Figure 9.

(Left) Chemistry of PSiP complexes of group 9. (Right) Related Fe octahedral complexes [35].

The complex [FeH{(ο-C₆H₄-PPh₂)₂SiMe}(PMe₃)₂] (138) was synthesised from [Fe(PMe₃)₄], its ν(Fe─H) stretching band was found at 1870 cm⁻¹ in the IR spectrum, while a triplet of doublets (td),signal was found at δ −17.09 (J_PH = 71.4, 18.1 Hz) in the ¹H NMR spectrum. By comparison, complex [FeH{(ο-C₆H₄-PPh₂)₃Si}(PMe₃)] (139) was characterised by a ν(Fe─H) band at 1967 cm⁻¹ and a signal in ¹H NMR spectrum at δ −15.00 of a triplet of doublets of doublets (tdd) multiplicity with J_PH = 79.4, 78.8 and 10.8 Hz (Figure 9) [31].

7. Hybrid silylphosphines complexation

PSiN-ligated complexes have also been attractive synthetic targets due to the hemilability property the presence of soft P, Si and hard N atoms could impose. The synthetic strategy for the preparation of PSiN-type ligands involved the synthesis of {(ο-C₆H₄)-NMe₂}SiHMeCl and its reactivity towards organolithium {(ο-C₆H₄Li)-PtBu₂}. In this manner, the mixed-donor ligand {(ο-C₆H₄)-PtBu₂}{(ο-C₆H₄)-NMe₂}SiHMe (140) was synthesised. This new-fangled ligand reacted with group 10 precursors PdBr₂ or [PtCl₂(cod)] to give the complexes [M(X){(ο-C₆H₄)-P^tBu₂}{(ο-C₆H₄)-NMe₂-SiMe}] (M = Pd, X = Br (144); M = Pt, X = Cl (145)), respectively. Treatment of the latter complexes with OTf- led to the formation of compounds [M(OTf){(ο-C₆H₄)-PtBu₂}{(ο-C₆H₄)-NMe₂-SiMe}] (M = Pd (146); Pt (147)) showing in the X-ray diffraction molecular structure a distance Pd-O of 2.3518(11) Å in 146, which once again highlights the strong trans-influence of the silyl donor. The selective reversible de-coordination of the amine arm in 144 and 145 was observed upon the use of PMe₃ which yields compounds [M(X){(ο-C₆H₄)-PtBu₂-SiMe-(ο-C₆H₄)-NMe₂}(PMe₃)] (M = Pd, X = Br (148); M = Pt, X = Cl (149)). The coordination of the PSiN ligand towards group 9 (Rh, Ir) and 8 (Ru) has also been studied. The complex [Ru{(ο-C₆H₄)-PtBu₂}{(ο-C₆H₄)-CHNMe-SiMe}(η³-cyclooctene)] (150) was achieved upon thermal reaction of the PSiN ligand with one equivalent of [Ru(2-methylallyl)₂(cod)]. The complex resulted from the coordination of the P and Si atoms of the ligand as well as a C─H bond activation of the methyl group (NMe) with a hydrogenated cyclooctene, remaining on the coordination sphere of the metal. Overall, a square planar geometry around Ru centre is structurally proposed (Figure 10) [36].

Figure 10.

The chemistry of mixed PSiN ligands [36-37].

Another example of an elegant catalytic application of systems derived of PSiN pincer-like ligands is that comprising the ligands of general formula {(ο-C₆H₄)-PR₂}{(ο-C₆H₄)-NMe₂}SiHMe (R = tBu (140); Cy (141); iPr (142); Ph (143)) [36]. The complexes [PtCl{(ο-C₆H₄)-PR₂(ο-C₆H₄)-NMe₂-SiMe}] (R = tBu (145); Cy (151); iPr (152); Ph (153)) were synthesised by the reaction with [PtCl₂η²-(C₂H₄)]₂ in the presence of Et₃N. 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].

Regarding [M(P₃Si)] tripodal systems, an extended series of organometallic species of general formulae [M(X){(ο-C₆H₄-PR₂)₃Si}] or [M(L){(ο-C₆H₄-PR₂)₃Si}] (R = Ph, ⁱPr; X = Me, Cl; L = H₂, N₂, CO, NH₃, N₃R’, PMe₃) has been disclosed with a variety of metals including, Fe [38, 39, 40, 41, 42, 43, 44], Co [45, 46], Ni [47], Ru [48], Os [48], Rh [45] and Ir [45], with outstanding and specific properties. Perhaps among the most novel systems, one can find the chemistry of novel hybrids (thioether/phosphine)-silane ligands (ο-C₆H₄-PiPr₂)_3-n(ο-C₆H₄-SAd)_nSiH (n = 2 (154); 1 (155)) synthesised from silyl-phosphines such as (ο-C₆H₄-PiPr₂)₂SiHCl, 59 (Figure 5, section 4.1), upon lithiation of Br(ο-C₆H₄)E (E = PiPr₂ or SAd) with varying stoichiometric amounts of Li(ο-C₆H₄)E. The reactivity of these hybrids with FeCl₂ afforded a new class of iron complexes featuring a S─Fe─N₂ linkage (Figure 11) [49]. The bulky hexa-dentate ligand {(ο-C₆H₄-PiPr₂)₂HSi-O-SiH(ο-C₆H₄-PiPr₂)₂} (156) was synthesised by the controlled hydrolysis of 59 (Figure 11). The reactivity of 59 was also reported towards FeBr₂ and NiX₂·DME (X = Cl or Br). The complex [Fe₂(N₂)(μ-H)₂{((ο-C₆H₄-PiPr₂)₂Si)₂O}] (157) was formed in an equilibrium mixture with [Fe₂(N₂)₂(μ-H)₂{((ο-C₆H₄-PiPr₂)₂Si)₂O}] (158), which were observed at low temperature in the IR spectrum (two ν(N─N) bands at 2097 and 2060 cm⁻¹) in accordance with the determined thermodynamic parameters including a large negative entropy (−30(2) cal/mol K), consistent with the coordination of a gas molecule and a rather small enthalpy of binding (−9.0(4) kcal/mol) in line with the observation of both species at low temperature (Figure 11) [50]. This research is particularly relevant for the understanding of nitrogenase mimicking systems. From the reactivity of a binucleating variant of a PSiP ligand with NiX₂.DME (X = Cl, Br) in the presence of triethylamine, dinuclear zerovalent nickel complexes bearing both η²-(Si─H) and η²-H₂ moieties were observed by the group of Peters. Theoretical studies suggest that the Ni centre facilitates H atom exchange between the η²-(Si─H) and η²-H₂ ligands via interconversion with a higher valent Ni(II) isomer (compounds 159–161) (Figure 11). This exchange has been exploited in the selective catalytic deuteration of exogenous silanes [51].

Figure 11.

Chemistry of compound 59 and related reactions [49-52].

Interestingly, the synthesis of the bulkier ligand (ο-C₆H₄-PiPr₂)₂HSi-(C₆H₃)SiPr-SiH(ο-C₆H₄-PiPr₂)₂ (162) was reported very recently. The lithiation of (2,6-dibromophenyl)-isopropyl thioether with nBuLi in one pot followed by the stoichiometric addition of 59 affords the formation of (3-bromo-2-(isopropylthio)phenyl)(bis(2-diisopropylphenylphosphino)silane, which received the same treatment that (2,6-dibromophenyl)-isopropyl, to form the desired compound 162 (Figure 11) [52].

8. Bulky silylphosphines complexation

An example of rare kinetic stabilisation of trans bis(silyl) isomers was provided by the contributions of Kang, Ko and coworkers on the reaction of the bulky carborane silyl-phosphines {(R₂P)C₂B₁₀H₁₀(SiMe₂H)} (R = Me (163), OEt (164), Ph (165)) towards [Pt(η²-C₂H₄)(PPh₃)₃] or [Pt(COD)₂], which afforded extremely uncommon trans-bis(P,Si-chelates) [Pt{(R₂P)C₂B₁₀H₁₀(SiMe₂)}₂] (R = Me (166); OEt (167)) formed by “chelate-assisted” oxidative addition. However, in the presence of dimethyl acetylenedicarboxylate, the complexes endure thermally rearrangements to the thermodynamically favoured cis isomers 166′ and 167′. Besides, the reaction of [Pt(η²-C₂H₄)(PPh₃)₃] towards 165 occurs via oxidative addition resulting in the mono(chelate) [PtH{(Ph₂P)C₂B₁₀H₁₀(SiMe₂)}(PPh₃)] (168). These authors also investigated the reactivity of the same bulky carborane silyl-phosphines with the palladium precursor [Pd₂(dba)₃] and observed that depending of the substituents over the phosphorous atoms, the cis isomer was exclusively formed [Pd{(R₂P)C₂B₁₀H₁₀(SiMe₂)}₂] (R = OEt (169); Ph (170)) or a mixture of the trans/cis isomers was detected when R = Me (171/171′) (Figure 12) [53].

Figure 12.

Bulky carborane silyl-phosphine ligands [53].

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].

Figure 13.

Syntheses of bulky cage trigonal bipyramidal iron complexes [54].

9. Non-rigid and semi-rigid silylphosphines

Sola reported tridentate systems exemplified by [IrHCl{[Ph₂P(CH₂)₃]₂SiMe}] (176) [55] derived from the reaction of the ligand PSiP {[Ph₂P(CH₂)₃]₂SiHMe} (47, Figure 3) with the dimeric compound [Ir(μ-Cl)(cod)]₂. In solution, complex 176 displays an equilibrium between the syn (176) and anti (176′) isomers in a ratio 93:7 in C₆D₆ and C₇D₈, while in CDCl₃ or CD₂Cl₂ solutions, the ratio is ca. 83:17. Complex 176 (and 176′) reacted with NaX (X = Br or I) leading to the corresponding complexes [IrHX{[Ph₂P(CH₂)₃]₂SiMe}] ((X = Br (177); I (178)) also in equilibrium with their respective syn and anti-isomers (177′, 178′) in similar ratios that those of 176 [56]. The mixture of isomers 176, also reacted with Me(O₃SCF₃) to produce the isomers syn179 and anti179′ with general formula [IrH(O₃SCF₃){[Ph₂P(CH₂)₃]₂SiMe}]; likewise the reactivity of 176 with AgX or HX (X = PF₆) in the presence of a neutral ligand afforded the mixture of the respective syn/anti cationic species [IrH(L)₂{[Ph₂P(CH₂)₃]₂SiMe}]⁺[PF₆]⁻ (L = NCMe (180/180′), CO (181/181′), bipy (182/182′)) (Figure 14) [57].

Figure 14.

The chemistry of [IrP₂Si] [56, 57].

Our research group studied the reactivity of PSi ligand phosphino-(benzyl)-silane Ph₂P{(ο-C₆H₄)CH₂SiMe₂H} (183) towards the complexes [RuH₂(η²-H₂)(PCy₃)₂] (184) and [Ru(cod)(cot)]. Complex 185 resulted from the substitution of two molecules of dihydrogen and two of the ligands PCy₃ in 184 by two ligands 183 bonded to the ruthenium atom through the phosphorous atoms and two σ-bonds of the fragments Si─H. Following loss of H₂, complex 183 slowly transformed to the cyclometalated complex 186 and subsequently into the bis(cyclometalated) 187. When 183 was added to [Ru(cod)(cot)], the synthetic precursor of 184, it generated directly complex 187 in very high yield. The increase on the acidity of the methylene groups of ligand 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 H₂ 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-C₆H₄)CH₂SiMe₂H}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 PCy₃ 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 non-classical 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 η²-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 hydride is bound to the two silicon atoms. In the solid-state X-ray diffraction analysis, a P-Ru-P angle of 154.37(3)° was determined, which is significantly more obtuse than in 189 and in 187, as expected due to the diminished hydride exchange in 190. The free energy Δ_rG₂₉₈ of the reaction 189 to 190 + H₂ is +16.9 kJ/mol; in line with the experimentally observed conversion at 308 K [59]. The reactivity of the ligands 183, 188 and new PSi₃ ligand P{(ο-C₆H₄)CH₂SiMe₂H}₃ (191) was also investigated with compounds [M(μ-Cl)(cod)]₂ (M = Rh, Ir) [60] and with [Pt(PPh₃)₃] [8]. Compound 191 coordinates to Rh and Ir centres as a tetradentate ligand through the phosphorus and two silyl groups, while a third Si atom engages in an agostic Si─H interaction mode [60]. Complexes 192 and 193 react with adventitious water to generate dimeric siloxane compounds. Additionally, compounds 188 and 191 react with Pt as tridentate ligands leading exclusively to compounds exhibiting a very rare trans silyl disposition at square planar Pt (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, d⁸ Pt(II) complex 195 is the first case of a silyl-platinum complex that includes a novel C─H⋯Pt anagostic interaction (Figure 15).

Figure 15.

Chemistry of silyl-benzyl phosphines bi-, tri- and tetradentate [58-60].

10. 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 ⁱPrOH 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].

11. 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.

Acknowledgments

We acknowledge financial support from CONACyT-Mexico through project CB 242818 and ANR-CONACyT 274001 and a PhD grant to JZ-M

Conflict of interest

The authors declare no conflict of interest.