Download Phosphines - U of L Class Index

Phosphines
:PR3
•
- very important ligands
- σ-donors
- π-acceptors
For years, it was assumed that π-backdonation occurred from the metal into empty d-orbitals
on phosphorus.
•
Actually: π-backdonation occurs into MOs formed by combination of two d-orbitals on
phosphorus and the σ* orbitals involved in P-R bonding.
P
+
•
P
σ* orbitals
d-orbitals
P
P
+
acceptor MOs
P
P
Each acceptor MO has 2 lobes (similar to a d-orbital) but is antibonding with respect to the
P-R bond
•
As the amount of π-backdonation increases, the length of P-R increases. This can be
observed in the crystal structures of phosphine complexes
- eCoI
Et3P
CoII
PEt3
Et3P
Average Co-P
221.8 pm
223.0 pm
Average P-C
184.6 pm
182.9 pm
•
PEt3
A huge variety of phosphines have been prepared, many of which are commercially
available. One can just choose a phosphine with the desired steric (size) and electronic
properties (σ-donation/π-acceptance).
•
The size of ligands (not just phosphines) can be measured using the concept of a cone
angle (Chadwick A. Tolman, Chem. Rev., 1977, 313).
-
TM-P distance fixed at 228 pm to standardize the cone
angle (θ).
M
θ
P
-
The cone encompases the van der Waals radii of the
outermost atoms of the ligand.
228 pm
•
Cone angles measured by crystallography (although TM-P bond distance fixed at 228 pm)
•
Electronic properties measured by IR and electrochemistry – ν(CO) and E1/2 for oxidation or
reduction measured for a large series of carbonyl phosphine complexes with different PR3
(e.g. [Ni(CO)3(PR3)] or [CpFeMe(CO)(PR3)])
x-axis = cone angle (θ)
y-axis = ν(CO) for [Ni(CO)3L]
[φ = old fashioned abbreviation for phenyl]
Typical Ligands – 2 (Phosphines) - Bite angles for
bidentate ligands
Xantphos
DPPM
t-Bu
Ph2P
P
βn
PPh2
P
OC
βn = bite angle =
ligand-metalligand angle for
bidentate ligands
O
H
Rh
M
t-Bu
PPh3
β = 72 o
small bite angle
Ph2P
H
Rh
PPh2
OC
PPh3
β = 111 o = large bite angle
Bite angle can have a profound effect on the
rate and selectivity of metal catalyzed reactions
Reviews: (1) P. C. J. Kamer, P. W. N. M. van Leeuwen, Wide Bite Angle Diphosphines:
Xantphos Ligands in Transition Metal Complexes and Catalysis, Acc. Chem. Res., 2001,
895-904; (2) Z. Freixa, P. W. N. M. van Leeuwen, Bite angle effects in diphosphine metal
catalysis: steric or electronic?, Dalton Trans., 2003, 1890-1901; (3) P. W. N. M. van
Leeuwen, Ligand Bite Angle Effects in Metal-catalyzed C-C bond formation, Chem. Rev.,
2000, 2741-2769.
Phosphines/Arsines in early TM and lanthanide chemistry
• PR3 excellent ligands for soft late transition metals
• PR3 typically NOT v. suitable to form complexes with hard early TMs (PR3
> PAr3)
• AsR3 even worse, and early TM SbR3 or BiR3 complexes unknown
• NPh3 useless as a ligand for early or late TMs (terrible donor), NMe3 OK
for early TMs
Ph2P
N
Me3Si
AsPh2
Ph2P
[CpTiCl3]
N
AsPh2
Ti
Cl
Cl
[PdCl2(NCPh)2]
•Route to early/late
heterobimetallics
Ph2P
N
R. G. Cavell (U of A),
OM, 1991, 539
Ti
Pd
Cl
Cl
AsPh2
Cl
Cl
2. xs TMS-Cl
•Hard/Soft donor combination
M. D. Fryzuk (UBC), OM, 2005, 1112
•Rare early TM (Ti, Zr, Hf)
arsine complexes
G. Reid (U Southampton),
DT, 2004, 3005 & EJIC,
2001, 2927