1.1. Phosphane ligands

Tertiary phosphanes of form PR₃ or PR₂R’ are usually prepared from phosphorus halides and organometallic reagents. Grignard reagents and organolithium reagents are commonly used as the organometallic reagent, but other metals (Al, Sn, Zn) have also found use [5]. The reactions are generally exothermic and are carried out at or below room temperature.

Phosphanes contain a lone electron pair at the phosphorus atom, which is used for the formation of a σ-bond with metals. π -Back-bonding from the d-orbitals of metals in low oxidation states is important in the case of electron-rich metals. The P–R σ*-orbitals are utilized for π -back-bonding, and empty phosphorus 3d-orbitals also play a role — a role that is larger for ligands like trimethylphosphane than for trifluorophosphane [2], [6], [7], [8], [9], [10], [11], [12]. The nature of the R groups attached to phosphorus thus determines the relative donor/acceptor ability of the ligand, and allows adjustment of the properties of the phosphane ligands [10], [11].

Understanding of the ligand system is the first essential step toward catalyst design, since steric and electronic properties of the ligand can drastically influence the rate and selectivity of catalytic reactions. A large number of methods are available to study the stereoelectronic properties of phosphorus ligands and aid in the development of more efficient catalysts. In 1970 Tolman quantified steric and electronic properties in terms of cone angle (-value) and electronic parameter (-value) [13], [14], [15], [16]. Subsequently other methods were developed for calculating the steric effects of ligands, which also take into account the variation in cone angle with ligand conformation [17], [18]. Casey’s natural biting angle has provided a useful tool for elucidating the steric properties of diphosphanes [19]. The quantification of electronic effects has continued, including investigations of parameters such as half neutralization potential for determination of ligand basicity [20], [21], NMR chemical shifts and coupling constants [22], [23], [24], [25], ionization potential [26], [27], enthalpy of reaction [15], [28], [29], [30], molecular electrostatic potential minimum V_min [31] and the “aryl” effect E_ar [32], [33], [34].

Correlations have been sought among these parameters to evaluate steric and electronic effects. Generally, the correlations are limited to similarly modified ligands [22], [31], [35]. Correlations have been demostrated between basicities and cone angles of the ligand series PMe₃, PMe₂Ph, PMePh₂, PPh₃ and P(p-tolyl)₃, P(m-tolyl)₃, P(o-tolyl)₃, where basicity increased as the cone angle decreased [35]. Correlation between molecular electrostatic potential minimum V_min and the Tolman electronic parameter, and also basicity, suggested that the V_min parameter could be used as the σ-donating power of the phosphane ligand to a metallic moiety. If there was significant π -back-bonding from the metal to the phosphane, however, the correlation failed [31].

Triphenylphosphane (odorless solid) is probably the most widely used tertiary phosphane in homogeneous catalysis in part due to its ready availability and stability in air. Typically, trialkylphosphanes and mixed arylalkylphosphanes are liquids, air-sensitive, expensive, and nasty and noxious to handle — properties that may not recommend them for homogeneous catalysis [36]. Moreover, there are cases where they generate less active or even inactive analogues of arylphosphane-based catalysts [36]. The major difference between complexes containing trialkylphosphanes and triaryl­phosphanes is the greater σ-electron donor ability (basicity) of the trialkyl­phosphanes, which leads to the formation of more stable and less active low-pressure transition metal catalysts [37], [38]. More recently, it has become apparent that rhodium trialkylphosphane complexes possess properties that make them suitable for a wide range of catalytic reactions [39]. Sometimes they are even the only systems available: for example, the much higher electron density on metal-containing trialkylphosphanes facilitates oxidative addition reactions, even of difficult substrates [39]. The branching of alkyldiaryl­phosphanes may improve both the i/n selectivity and the activity [37], [40].

Most commercial homogeneous catalysts are based on phosphane complexes. Processes include hydrogenation, hydroformylation, hydrosilylation, hydrocyanation, and oligomerization [1], [40], [41], [42], [43]. The phosphanes with their ability to stabilize low oxidation states of transition metals, and especially the arylphosphanes with their greater steric bulk and weaker bonding affinity for metals relative to the alkylphosphanes, are ideal for the generatation of an empty or potentially reactive coordination sites in the metal reaction sphere [1]. In addition, the good solubility of phosphane complexes, which can also be modified by changing the length of the alkyl substituent and introducing phenyl substituents, makes them highly attractive ligands for homogeneous catalysis. Lately, the synthesis of new water-soluble phosphane ligands such as sulfonated arylphosphanes, pyridylphosphanes, and phosphanes containing a nitrogen group have found industrially promising or relevant applications [44], [45], [46], [47], [48], [49].

Evidently the first tertiary phosphane complex of a transition metal was described by Hoffmann as long ago as 1857. The triphenylphosphane ligand has a major role as a catalyst modifier for example in the famous Reppe compound Ni(CO)₂(PPh₃)₂ used in alkene and acetylene polymerization, in the Wilkinson catalyst Rh(CO)(Ph₃P)₃ used in homogeneous hydrogenation of alkenes, acetylene, and carbonyl compounds, and in Rh(CO)(PPh₃)_2or3-type catalysts used in the hydroformylation of alkenes with hydrogen and CO [5].