Complexes of alkenes, alkynes, and dienes

Created by Margaret L. Scheuermann, Princeton University; Abby R. O’Connor, The College of New Jersey, oconnora@tcnj.edu. Copyright Scheuermann and O’Connor, 2014. This work is licensed under the Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License. To view a copy of this license visit http://creativecommons.org/about/license/

2

History & Utility

ClPt

ClClK+

W. C. Zeise, Annalen der Physik, 1831, 21, 497.Review: Hunt. Platin. Met. Rev. 1984, 28, 76.

First example Popular starting materials and precatalysts:

Rh

+

IrCl

ClIr

PtCl

Cl

Fe

OC CO CO

Catalytic intermediates:Alkene hydrogenation^*Alkene dimerization/ oligomerization/ polymerization^*Olefin metathesis^*Mizoroki-Heck^*Wacker Oxidation^Hydrofunctionalization- Hydroboration/ hydosilylation^/ hydrocyanation^/ hydroamination^/ hydroformylation^/ etc.

^ significant industrial use; * Nobel Prize

Orbital Overlap- Octahedral Example

3

C-C π

C-C π*

C-C σ

C-C σ*

M-olefin σ

M-olefin π

M-olefin σ*

M-olefin π* M-olefin π*

M-olefin σ*

M-olefin π

M-olefin σ

dz2

dxz

(or dyz)

dx2-y2

Dxy (or dyz or dxz)

OlefinOlefin along x (or y) axisOlefin along z axisx

y

z

Types of π-bound ligands

4

M

M

M

M

M M

MM

MM

Extremes

M M

Less backbonding into π*… electron poor metal center

More backbonding into π*… electron rich metal center

M

Properties

5

Rotation around metal-olefin bond

Orientation determined by steric and electronic factors

Bond lengths and angles

Electron rich metal centers tend to form more stable olefin complexes

NMR

M M M

• As backbonding increases so do geometry differences

• C-C bonds get longer

• sp2 becomes more sp3-like

• sp becomes more sp2-like• can help relieve ring strain

ZrPMe3

Buchwald. JACS 1986, 165, 7441.

• late metals• Low oxidation states• high d-electron counts• low overall charge

M

M

• When backbonding is minimal (electron poor metal center) 1H and 13C shifts resemble free olefin

• When backbonding is significant (electron rich metal center) 1H and 13C resonances shift upfield- smaller δ

M

M

Synthesis & Reactivity

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Ligand substitution*

Reduction

β-hydride elimination*

Displacement by ligand substitution*

Nucleophilic attack* (when metal center is a poor π donor)

M L + M + L M L' +M + L'

MM + nuc.nuc.M

Insertion*

M R MR

R = H, alkyl, aryl

α

β

M+2 + + reductant

M + reductant+2

* Often present in catalytic cycles

M HMH