Phosphines are good donor ligands, which are readily employed in organometallic chemistry to fine-tune the reactivity of late transition metal complexes. The bonding of phosphines to metal centres is similar to that of carbonyl and cyanide ligands, in that they are π acids; σ donors and π acceptors (Figure 1.30 describes the bonding of phosphines to metal centres). However, alkyl phosphines generally rank lower in the spectrochemical ligand series than CO or -CN and are therefore not expected to bind as strongly to metal centres. 55, 57 This allows for their remarkable versatility and role as spectator ligands in many chemical reactions.
Figure 1.30 Metal-phosphorus bonding for phosphine bound complexes
M P
R
R R a
b a) Donation of the phosphorus lone pair from the sp hybridised PR3 orbital (HOMO) to the dx2-y2 orbital
of the metal.
b) Back-donation from the dxy metal orbital to the
(LUMO) σ*
orbitals of the P-R bond. This also lengthens the P-R bonds, relative to those in free phosphine.
The greater the electronegativity of the R groups of the phosphine, the greater the stability of the σ* orbital of the alkyl/aryl carbon (R) which bonds to the phosphorus atom. Consequentially, this also stabilises the σ*
orbital of the P-R bond, 134, 135 by becoming lower in energy. This makes the empty σ*
orbital more accessible for back- donation, increasing the overall π acidity of the ligand (i.e. PF3, has a slightly greater π acidity than CO). Figure 1.31 provides an overview of the order of π acidity for phosphine ligands.
Figure 1.31 General order of π acidity for phosphine ligands (highest to lowest)
PF3 ≈ (CO) > PCl3 > P(OAr)3 > P(OR)3 > P(Ar)3 > P(NR2)3 ≈ PR3
The electronic properties of phosphines have long been known to be partly responsible for their strong binding character to metal centres. In 1977 Tolman compiled IR v(CO) data for phosphine complexes of the type Ni(CO)3(PR3), in order to quantify the relative
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binding strength of phosphines to metal centres. 136 The stronger donor phosphine ligands would increase the electron density on the Ni centre. Some of this increased density would be expected to flow to the bound carbonyl ligands, lowering the v(CO) frequency. The IR data demonstrated that phosphine ligand coordination at the Ni(0) centre was decreased, and could not be explained fully in terms of electronic effects. Tolman deduced that steric hindrance of the moieties bound to the phosphorus atom were responsible.
The steric properties of phosphines (and other ligands) may be determined by measuring the cone angle. Tolman described this concept by using crystallography data to calculate the cone angle (θ) for a variety of different phosphine ligands, with a broad range of steric properties. The cone angle data is standardised by taking measurements from a point 2.28 Å from the phosphorus atom, towards the metal centre, along the M-P bond. The cone extends to encompass the van der Waals radii of the outermost atoms of the phosphine ligand (Figure 1.32).
Figure 1.32 Depiction of the Tolman cone angle (θ) for sterically hindered phosphines
P
[M]
Å 2.28
Tolman compiled this steric and electronic data into what has since become known as the Tolman map (Figure 1.33), which relates the electronic properties (v(CO)) to the steric properties, to demonstrate relative binding strength of phosphine ligands.
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Figure 1.33 Tolman map correlating the electronic and steric factors of phosphine ligands 100 110 120 130 140 150 160 170 180 190 2050 2060 2070 2080 2090 2100 2110 v( C O ) / c m -1
cone angle / degrees
PF3 P(OCH2CCl3)3 PCl3 P(OCH3)2OHMe P(OCH2)CH2Cl)3 P(OMe)3 P(OEt)3 PH(OEt)2Ph P(Me)3 PH(Me)2Ph P(Bu)3 P(Et)3 P(Ph)3 P(Et)2Ph PMe(Ph)2 P(OiPr)3 P(OPhMe)3 P(OPh)3 P(OPhCl)3 PH(OCN)3 P(PhClH2)3 P(PhMe)3 P(iPr3)3 P(NMe2)3 PH(PhCH4)3 PCy3 P( iBu) 3 P(C6F5)3 PH(C6F5)Ph2 P(OC6H4Me)3 P(C6H4Me)3 P(OC6H4PhN)3 P(OC6H4iPr)3 P(OC6H4MeCN)3 P(OC6H4iBu)3 P(C6H4Me2)3
The effect of the sterically hindered phosphine ligands on the formation of predominant isomeric forms of metal complexes, has been known since 1968. 137 The formation of the trans configuration of Mn(CO4)(COMe)(PR3) was reported to be favoured over the
cis-isomer, when the steric bulk of the coordinated phosphine ligand was increased.
Concerning catalytic activity, Grubbs et al. observed that when sterically hindered phosphine ligands were incorporated into the structure of the complexes Ru(X)2(C=CR’H)(PR3)2 and Ru(X)2(C=CR’H)(NHC)PR3, catalytic activity for olefin metathesis was increased. 138 This was deduced to be caused by the stabilising effect of the phosphine ligand on the Ru(IV) cyclometallated butyl intermediate, owing to the electron donating character of the phosphine. For the case of Ru(X)2(C=CR’H)(PR3)2, the increased steric bulk of the phosphine facilitated the dissociation of one of the phosphine ligands, which was an initial step in the catalytic cycle. The complex Ru(X)2(C=CR’H)(NHC)PR3, was observed to possess even greater activity owing to the presence of the N-heterocyclic carbene (NHC) ligand, which was more sterically encumbered and electron donating than the trialkylphosphine ligands. More recent investigations into phosphine design by Pringle et al. have led to the effective design of
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ligands which enable the catalytic hydromethoxycarbonylation of ethane, 139 and tri / tetra-merisations of ethene, 140 when incorporated into metal complexes.
For complexes of the type CpRu(PR3)X, the steric hinderance around the ruthenium centre is relieved by favouring ligand loss, therefore the M-P bond is increasingly sensitive to sterically hindered R groups. This was explained by Bruce et al. 141 for the complex CpRu(PPh3)2Cl, which was observed to readily undergo thermal dissociation of one of the triphenylphosphine ligands, to relieve steric hindrance about the metal centre. However, the relief of the steric encumbrance led to strengthening of the M-P bond for the remaining phosphine ligand, as a result of the binding now relying more on electronic effects. This was demonstrated by the inability of CO (which processes greater π acidity) to displace the remaining triphenylphosphine ligand, for the mono- substituted complex CpRu(CO)(PPh3)Cl. 120
For CpRu(PMe3)2X The barrier to the loss of PMe3 is approximately lowered by the extent to which X acts as a π donor (relative to the case where X is a non-pi donor, e.g. Me). This electronic effect coupled with the steric effect of phosphine ligands, means that no one set of bond dissociation energies will apply over the full spectrum of organometallic complexes. This is evidenced by the varying stability of M-CO bonds, where a range of bond dissociation energies have been found (between 22 and 84 kcal mol-1), for a variety of metal carbonyl complexes.