PINCER COMPLEXES

A SPECIAL TOPIC

Ir

PtBu2

PtBu2

Cl

H

Pd

P(CMe3)2

P(CMe3)2

Cl

Pincer Complexes

Complexes of mixed donor polydentate ligands pioneered by Shaw in mid-70s

M

DR

DR

XnLm

R'

*cavity for metal bonding with tunable accessibility*sites for counterions or ancillary ligand

*hardness/softness*metal-binding rigidity*steric constraints of substituents*coordinating 2e- donor or free Lewis base

*anchoring site*remote electronic modulations

*chiral pocket*steric constraints

Generic Structure of Pincer Complex

Dehydrogenation of Alkanes

I + COOMeCOOMe

140oC, N-methyl pyrrolidone, Na2CO3

Pd

PiPr2

PiPr2

TFA

Palladium Pincer Complexes as Catalysts for the Heck Reaction

+

O

Pd

O

PiPr2

PiPr2

Cl

Cl

180oC, Dioxane, CsOAc

Palladium Phosphinito PCP Pincer Complex in Heck Reactions Involving Aryl Chlorides

CsOAC, 180oC, Dioxane

The Classical Mechanism PdL4 precatalyst

Pd(0)L2

R'-Pd(II)L2-X

R'X

Pd(II)L2X

R''

R'

H

H

R''

H

Pd(II)L2X

H

R'

H

R''

H

H-Pd(II)L2-X

R'R''

HX-BaseBase

oxidative addition

coordination

internal rotation

elimination

reductive elimination

ligand dissociation

R'-Pd(II)L2-X

R''

alkyl migration

The Pd(II)/Pd(IV) Mechanism

O

Pd

O

PPri2

PPri2

Cl

O

Pd

O

PPri2

PPri2

O

Pd

O

PPri2

PPri2

Ar

O

Pd

O

PPri2

PPri2

H

Cl

Ph

Ph

Ph

Cl

HCl

Ar

Ph

Ph

ArCl

II

IV

II

IV

Platinum NCN and PCP pincer complexes as catalysts for alkane CH bond activation

and functionalization

OH OH+HO OH

OH

+ CH3CH2CO2H

Pt

NEt2

NEt2

ClPt

PPri2O

O PPri2

Cl

PtII

+ RHa)

PtII + H+

BA

PtIV

PtII

PtIV

b)

C

R

R

H2O

ROH + H+

c)

Cl-

RCl

Mechanism for the Shilov system

OH

OHHO

POP-Pd-Cl (1 mol %)

CuCl2.2H2O (2 mol %), HCl (2 mol %)

Air, H2O

OHCl

+

OH OH

Pd

PiPr2O

O PiPr2

Cl

TFA, H2O, Oxidant, 95oC

Pd

PiPr2

PiPr2

Cl Pd

NEt2

NEt2

Br Pd

StBu

StBu

Cl

HO1 or 2 or 3 or 4

1 2 3 4

OH

OHHO

POP-Pd-Cl (1 mol %)

CuCl2.2H2O (2 mol %), HCl (2 mol %)

Air, H2O

OHCl

+

OH

OHHO

POP-Pd-Cl (1 mol %)

CuCl2.2H2O (2 mol %), HCl (2 mol %)

Air, H2O

OHCl

+

Pd

PiPr2O

O PiPr2

Cl Pd

PiPr2

PiPr2

Cl Pd

NEt2

NEt2

Br Pd

StBu

StBu

Cl

1 2 3 4

Preparation of Pincer Complex

synthesis of pincer ligand

synthesis of metal precursor

Metalation of pincer ligand

A solution of resorcinol (0.36 g, 3.28 mmol) and DMAP (0.81 g, 6.60 mmol) in THF (30 ml) was added a solution of ClPPri

2 (1.0 g, 6.55 mmol) in THF (20 ml), while stirring at 0°C. The resulting mixture was allowed to reach room temperature (r.t.) and stirred for an additional 24 h. Following removal of the solvent in vacuo, the solid residue was extracted with toluene (220 ml). The combined extracts were filtered through a short plug of Celite and the toluene removed in vacuo to yield the product (1.1 g, 95%) as a colorless oil. NMR spectroscopy showed the product to be greater than 98% pure and it was used in further steps without further purification. 1H NMR (400.03 MHz, CDCl3): d 7.10–6.80 (m, 4H, arom.); 2.00–1.80 (m, 4H, CH(CH3)2); 1.15–1.00 (m, 24H, CH(CH3)2). 13C NMR (100.59 MHz, CDCl3): d 160.22 (s, ArC), 129.38 (s, ArC), 111.77 (s, ArC), 109.25 (s, ArC), 28.26 (d, 1JPC 17.9 Hz, PCH(CH3)2), 17.71 (s, PCH(CH3)2), 16.98 (s, PCH(CH3)2). 31P{1H} NMR (161.93 MHz, CDCl3): d 149.0. Anal. Calc. for C18H32O2P2 (342.40.): C, 63.14; H, 9.42. Found: C, 63.42; H, 9.74%.

Preparation of C6H4 -2,6 -(OPPri2)2

DMAP = 4 dimethylaminopyridine

A toluene (50 ml) solution of C6H4-2,6-(OPPri2)2 (500 mg,

1.46 mmol), and PdCl2(COD) (417 mg, 1.46 mmol) was refluxed for 5 h. The solvent was evaporated under vacuum and the crude product was extracted with pentane. Following recrystalization from diethyl ether, a purified product was obtained (Yield 630 mg, 90 %). 1H NMR (400.03 MHz, CDCl3): d 1.20–1.40 (m, 24H, CH(CH3)2), 2.40–2.52 (sep m, 3JHH7.2 Hz, 4H, CH(CH3)2), 6.54 (d, 3JHH8.0 Hz, 2H, arom), 6.96 (t, 3JHH8.0 Hz, 1H, arom); 13C NMR (100.59 MHz, CDCl3) d 166.29 (bs, ArC), 149.84 (s, ArC), 128.02 (s, ArC), 105.94 (vt, JPC14.48 Hz, ArC), 28.78 (vt, JPC23.14 Hz, PCH(CH3)2), 17.26 (s, PCH(CH3)2), 16.69 (s, PCH(CH3)2); 31P NMR (161.93 MHz, CDCl3): d 187.68 (s, 1P). Anal. Calc. for C18H31Cl1O2P2Pd (483.24): C, 44.74; H, 6.47. Found: C, 44.49; H, 6.15%.

Preparation PdCl{C6H3 -2,6 -(OPPri2)}

Single-crystal X-ray diffraction study