Phosphine complexes - PowerPoint Presentation

Phosphine complexes Lecture 12a

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 spectroscopyThe polarity of the solvent determines which product precipitates i.e., SnCl4(THT)2: dichloromethane (trans), pentane (cis) Introduction I

L as p -complex only (C2H4, alkenes)Ligands like ethylene form strong p-complexes with low-valent metalsThe HOMO is the C=C p-bond, which is used to form the M-L s-bondOften 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)]), Introduction II

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-onExample: [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 groupsIntroduction III

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 stableMetal acts as a hard acid and the ligand as a hard baseExamples: [M(NH3)4]2+ (M=Cu, Zn), [M(NH3) 6 ]2+ (M=Co, Ni)Introduction IV

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-Introduction V

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 stableThe 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 groupThe ligand and the metal act as baseExamples: Mo(CO)6 , [Fe(CN)6] 4-, [Co(NO)4] Introduction VI

L as s -donor and s*-acceptor (PR3)In the older literature, phosphine ligands are often referred to as p-acceptorsIn 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)Phosphines I

The degree of p -acidity largely depends on the substituents on the phosphorus atomWhile alkyl phosphines are weak p-acids, the acidity increases for aryl, dialkylamino and alkoxy phosphinesThe 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*-orbital increases and the size of the orbital pointing towards the metal as well allowing for a better overlapBased on this argument, the order of p-acidity of phosphines is PMe3 < PAr3 < P(OMe ) 3 < P( OAr ) 3 < PCl 3 < PF 3 ≈ CO Phosphines II

Aside of the p -acidity, the steric impact of the phosphine ligand has to be considered as wellC.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. Tolman observed 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)). Phosphines III

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 centerThus , the bulkiness of the phosphine ligand can be quantified by its cone angle (Q )The observed cone angles for phosphines range from Q=87o (PH 3 ) to Q =212 o (P( mes ) 3 ) (neither one is shown in the diagram below). The cone angles for PMe 3 , PPh 3 and PCy 3 are Q =118 o , Q =145 o and Q=170o, respectively, consistent with the observations above. Generally, phosphines with aryl groups or highly branched alkyl chains exhibit large cone angles while phosphite have much smaller cone angles Phosphines IV

Phosphines V

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. Phosphines VI

These complexes can easily be prepared from Mo(CO)6 by the reaction with one equivalent of LThe resulting compounds exhibit colors ranging from white to red depending in the ligand L95Mo-NMR and infrared spectroscopy can be used to assess the effect of the ligand L on the metal and the remaining CO ligandsMo(CO)5L complexes I

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-nucleusLigands that are s-donors and good p-acceptor i.e., PF3 and P(OR)3 are comparable to the CO ligand itself Mo(CO)5L complexes IILigandd(ppm)Piperidine-1433CH3CN-1440PCl3-1523PCl2Ph-1615 PClPh2-1702 PPh3-1743 PBr 3 -1396 PF 3 -1860 P( OPh ) 3 -1819 Mo(CO) 6 -1857

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 longIf 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 Mo(CO)5L complexes III Mo-PMo-C (trans)C-O (trans)PPh3256.0199.6114.1P(2-MeOPh) 3 258.8198.1 114.3 P(2,4,6-MeOPh) 3 263.6 197.4 114.5 P(NC 5 H 10 ) 3 260.5 198.5 114.0 PCy 3 259.4 197.2 115.4 PMe 3 250.8 198.4 115.1 PCl 3 237.9 203.5 113.0

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

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 ci s-configuration while the trisubstituted compounds are in fac-configuration.Mo(CO)4L2 and Mo(CO)3L3 complexesLigandMo(CO)4L2 Mo(CO) 3L3Piperidine -1093 CH 3 CN -1307 -1112 PCl 3 -1206 -910 PCl 2 Ph -1369 -1124 PClPh 2 -1522 -1320 PPh 3 -1556 -1265 PBr 3 -977 PF 3 -1860 -1860 P( OPh ) 3 -1819 Mo(CO) 6 -1857

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 alkenesThe complex itself is the 16 VE systemCatalysis I

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

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