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Lecture 12a

Lecture 12a. Phosphine complexes. Introduction I. In order to understand what a ligand does, one has to look at its electronic and its steric properties

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Lecture 12a

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  1. Lecture 12a Phosphine complexes

  2. Introduction I • In order to understand what a ligand does, one has to look at its electronic and its steric properties • The reaction conditions (kinetic and thermodynamic control) during the reaction determine the configuration is observed in the product (cis-trans, fac-mer) • In many cases, there is an equilibrium in solution, which can be detected by NMR or infrared spectroscopy • The polarity of the solvent determines which product precipitates i.e., SnCl4(THT)2: dichloromethane (trans), pentane (cis)

  3. Introduction II • L as p-complex only (C2H4, alkenes) • Ligands like ethylene form strong p-complexes with low-valent metals • The HOMO is the C=C p-bond, which is used to form the M-L s-bond • Often times, there is also a back-bonding into the p*-orbital of the C=C bond (free C2H4: 134 pm, complex: 137 pm) • Example: Zeise’s salt (K[PtCl3(h2-C2H4)]),

  4. Introduction III • L as s-complex only (H2) • Molecular hydrogen does not exhibit a lone pair or a p-bond, yet it binds to some metal centers as intact molecule (meaning it does not perform an oxidative addition!) • The s-bond of the H2 molecule is the electron donor in this bond (red bond), while the s*-orbital acts as an acceptor for the back-bonding (blue bond) • In order to maximize the overlap, the H2-molecule binds side-on • Example: [W(h2-H2)(CO)3(PR3)2] (free H2: 74 pm, complex: 75.5 pm) • Sigma complexes are also found for C-H, Si-H, B-H and M-H groups

  5. Introduction IV • L as s-donor only (NH3, NR3) • The metal has to exhibit a medium or high oxidation state in order for these complexes to be stable • Metal acts as a hard acid and the ligand as a hard base • Examples: [M(NH3)4]2+ (M=Cu, Zn), [M(NH3)6]2+ (M=Co, Ni)

  6. Introduction V • L as s-and p-donor (H2O, OH-, OR-, NR2-, F-) • The metal has to exhibit a medium or high oxidation state in order for these complexes to be stable • The ligand acts as very hard base and the metal as hard acid • Examples: [Ni(H2O)6]2+, [CoF6]3-,[Sn(OH)6]2-

  7. Introduction VI • L as s-donor and p-acceptor (CO, CN-, NO) • The metal has to exhibit a low oxidation state in order for these complexes to be stable • The s-bond is formed from the sp-orbital of the carbon atom with a suitable empty d-orbital of the metal while the p-back bond is formed by the interaction of a filled d-orbital of the metal with the p*-orbital of the carbonyl group • The ligand and the metal act as base • Examples: Mo(CO)6, [Fe(CN)6]4-,[Co(NO)4]

  8. Phosphines I • L as s-donor and s*-acceptor (PR3) • In the older literature, phosphine ligands are often referred to as p-acceptors • In the more recent literature (after 1980), they are usually referred to as s*-acceptor • As electron-withdrawing groups (i.e., halogen atoms) are placed on the phosphorous atom, the s-donating capacity of the phosphine ligand tends to decrease • At the same time, the energy of the s*-orbital on phosphorous is lowered in energy, providing an increase in backbonding ability (p-acid)

  9. Phosphines II • The degree of p-acidity largely depends on the substituents on the phosphorus atom • While alkyl phosphines are weak p-acids, the acidity increases for aryl, dialkylamino and alkoxyphosphines • The extreme cases are PCl3 and PF3, which is equivalent to CO in its p-acidity because more electronegative elements on the phosphorous atom stabilize the s-bond and lower the energy of the s*-orbital(see diagram) • The contribution of the phosphorus atom to the s*-orbitalincreases and the size of the orbital pointing towards the metal as well allowing for a better overlap • Based on this argument, the order ofp-acidity of phosphines is • PMe3 < PAr3 < P(OMe)3 < P(OAr)3 < PCl3 < PF3 ≈ CO

  10. Phosphines III • Aside of the p-acidity, the steric impact of the phosphine ligand has to be considered as well • C.A. Tolman(Chem. Rev.1977, 77, 313) summarizes the electronic parameters and cone angles of phosphine ligands: • The electronic parameter can be adjusted by changing the R-group(see above). Stronger donor groups increase the electron density on the metal atom, which is capable of more backbonding to ligands like CO, CN-, etc. • Tolmanobserved for Ni(CO)3L that the carbonyl stretching frequency decreases as the donor ability of the R-group increases (i.e., PCy3 (2056 cm-1) vs. P(OMe)3 (2070 cm-1) vs. PF3 (2111 cm-1)).

  11. Phosphines IV • The second important parameter is the steric demand, which can also be controlled by changing the R-group. • Very bulky phosphines often favor low-coordinate compounds, which can coordinate additional small ligand as observed in catalytic cycles • Metals like Mo and W can coordinate up to six PMe3 ligands (i.e., M(PMe3)6)), while a maximum of four PPh3 ligands (i.e., M(PPh3)4, M=Pd, Cu+, Ag+, Au+) or two PCy3 ligands (i.e., Cu+, Ag+, Au+, Ni2+, Pd2+, Pt2+) can be coordinated to a metal center • Thus, the bulkiness of the phosphine ligand can be quantified by its cone angle (Q) • The observed cone angles for phosphines range from Q=87o (PH3) to Q=212o (P(mes)3) (neither one is shown in the diagram below). • The cone angles for PMe3, PPh3 and PCy3 are Q=118o, Q=145o andQ=170o, respectively, consistent with the observations above. • Generally, phosphines with aryl groups or highly branched alkyl chains exhibit large cone angleswhile phosphite have much smaller cone angles

  12. Phosphines V

  13. Phosphines VI • The ability of a metal to perform backbonding can easily be tuned by manipulating the electronic effect of the phosphine ligand. • For instance, a change of the ligand from PBu3 to P(OiPr)3, which possess virtually identical cone angles, decreases the ability of the metal for backbonding as can be seen from the higher carbonyl stretching frequency in Ni(CO)3L. • If the same electronic effect is desired but a larger cone angle to lower the number of coordinated ligands, one could move from PBu3 to P(iPr)3, which exhibits a 30o larger cone angle, but is electronically speaking identical.

  14. Mo(CO)5L complexes I • These complexes can easily be prepared from Mo(CO)6by the reaction with one equivalent of L • The resulting compounds exhibit colors ranging from white to red depending in the ligand L • 95Mo-NMR and infrared spectroscopy can be used to assess the effect of the ligand L on the metal and the remaining CO ligands

  15. Mo(CO)5L complexes II • 95Mo-NMR studies have shown that the chemical shift varies significantly with the ligand • Ligands that are good s-donors, but poor orno p-acceptor causing a significant decreasein the HOMO-LUMO gap, which results in a deshielding of the Mo-nucleus • Ligands that are s-donors and good p-acceptor i.e., PF3 and P(OR)3 are comparable to the CO ligand itself

  16. Mo(CO)5L complexes III • If the phosphine ligand is a good p-acid, the Mo-P bond is very short (i.e. PCl3) and the Mo-C bond is fairly long • If donor groups are attached to the phenyl group, the Mo-P bond length increases while the Mo-C bond length increases because the phosphorus atom becomes a weaker p-acid

  17. Cone Angle • For the sequence, the chemical shift in the 95Mo-NMR spectrum follows a straight trend • The chemical shift depends linearly with the cone angle of the phosphine • The weaker of a p-acid the phosphine is, the more negative the chemical shift is because the Mo-atom is more shielded • The comparison of the first and the second diagram shows that a larger number of phosphine groups increases the effect (~200-300 ppm/group)

  18. Mo(CO)4L2 and Mo(CO)3L3 complexes • The di- and trisubstituted compounds exhibits the same trends like the monosubstituted compounds, just to a much larger degree i.e., L=PPh3(d=-1743ppm, -1556 ppm, -1265 ppm) • Note that all disubstituted compounds are in cis-configuration while the trisubstituted compounds are in fac-configuration.

  19. Catalysis I • Wilkinson’s catalyst (RhCl(PPh3)3) • It is obtained by the reaction of RhCl3 with four equivalents of triphenylphosphine as a red-violet solid (note that the phosphine acts as ligand and as reducing reagent here) • It exhibits a square-planar coordination of around the Rh(I)-ion (d8) • It catalyzes the hydrogenation of alkenes • The complex itself is the 16 VE system

  20. Catalysis II • Step 1:The dissociation of one triphenylphosphine ligands to give 14 VE complexes • Step 2: Oxidation addition of H2 to the metal (cis) • Step 3: The π-complexationof alkene to the metal • Step 4: Intramolecular hydride transfer (olefin insertion) • Step 5: Reductive elimination results in extrusion of the alkane product

  21. Catalysis III • When the triphenylphosphine ligands are replaced by chiral phosphines (i.e., DIPAMP), the catalyst becomes chiral and converts prochiralalkenes into enantiomerically enriched alkanes via the process called asymmetric hydrogenation (i.e., L-DOPA process, Monsanto)

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