Download Metal Catalysts - UZH - Department of Chemistry

MS101
Metal Catalysts
C.Nevado
1
MS101
TM‐OC: What is special?
C.Nevado
TM‐OC = Transition Metal Organometallic Complex
‐) A convenient classification of organometallics is based on the bond type:
2
MS101
Ionization Potentials
‐)
‐)
‐)
‐) C.Nevado
More electronegative element means lower energies of the orbitals
Early metals more easily oxidized, more basic, more electron‐rich
Mid to late TM’s: third‐row metals can form high oxidation state compounds more easily
In many cases, 3rd row metals are more basic than 2nd row metals which are more basic than 1st row metals.
more basic than 1
row metals
3
MS101
Trends in size
‐)
‐)
C.Nevado
Second‐row metals larger than first‐row (higher quantum number of the valence orbitals) Third‐row metals comparable in size to second row (“lanthanide contraction”, nuclear charges of third‐row much higher than second row and balance the effect of higher quantum number
4
MS101
Historical Perspective
1760
1827
1849
1852
1881
1890
1909
1917
1931
1938
Cadet [(CH3)2As]2O, first organometallic compound
Zeise’s salt Na[PtCl3(C2H4)], first olefin complex
Frankland, Zn(Et)2. Very air‐sensitive, Hydrogen gas was used as a protective atmosphere!
protective atmosphere!
Löwig and Schweizer make Pb(Et)4, Bi(Et)3 and Sb(Et)3.
Grignard, organomagnesium halides.
Mond, Ni(CO)4. First homoleptic metal carbonyl.
Pope (CH3)3PtI, first sigma
Pope, (CH
PtI first sigma‐organotransition‐metal
organotransition metal compound.
compound
Schlenk, lithium alkyls.
Hieber, Fe(CO)4H2 , first TM‐hydride complex.
Roelen, discovers hydroformylation.
1951
Pauson and Miller discover ferrocene.
1955
1964
1965
1983
Ziegler, Natta, polyolefins from ethylene and propylene.
Fischer (CO)5W=C(OMe)Me, first carbene complex. Fischer, (CO)
W C(OMe)Me first carbene complex
Wilkinson, Rh(PPh3)3Cl, catalyst for hydrogenation.
Bergman, Graham, C‐H activation of alkanes with TM’s.
C.Nevado
5
MS101
Metal‐Ligand complexes
C.Nevado
Some definitions:
Some definitions:
Ligand:
A ligand is a molecule or ion that is bonded covalently to a metal center.
Monodentate:
Monodentate ligands have only one point of attachment to the metal center and occupy only one coordination site
Ambidentate:
Ambidentate ligands can bind to a metal center through more than one kind of atom. Classic examples include NO2‐, SCN‐ or R2S=O.
These ligands are examples of linkage isomerism. Ambidentate can also refer to a polydentate ligand that is not fully
These ligands are examples of linkage isomerism. Ambidentate can also refer to a polydentate ligand that is not fully coordinated to a transition metal.
Bidentate:
Bidentate ligands have two points of attachment to the metal center and occupy 2 coordination sites. Examples: 2,2’‐bipyridine (bipy), ethylenediamine (en), diphenylphosphinoethane (dppe), acetylacetonate (acac), etc.
Polydentate:
Polydentate (or multidentate) ligands have more than one point of attachment to the metal center and occupy more than one coordination site. Examples include edta, PCP or TP ligands.
Chelating: A chelating ligand involves a polydentate ligand that forms a ring that includes the metal. Also called a chelate.
Homoleptic Complexes:
Homoleptic Complexes: contain only one kind of ligand.
Primary Coordination sphere
The primary coordination sphere of a metal involves the set of ligands closest to the metal that are directly attached. Mobile cations or counterions are said to be in the outer or secondary coordination sphere.
6
MS101
Metal‐Ligand complexes
C.Nevado
The coordination number:
For simple monodentate and chelating ligands, the coordination number can be defined as p
g g
,
the number of atoms or ligands directly bonded to the metal atom. For example, [Fe(NH3)6]3+ and [Fe(en)3]3+ are both 6‐coordinate complexes.
Complication:
C
li ti
Cp2Fe is considered to be a 6‐coordinate complex, although it should be called a 10‐
coordinate complex. Alkoxide, imido and oxo ligands can donate one, two or three pairs of electrons to a ,
g
,
p
metal, but only occupy one coordination site.
Cp2TiCl2 is not considered to be an 8‐coordinate complex.
Therefore; the coordination number of Cp lies between 1 and three.
Coordinative unsaturation:
The term coordinative unsaturation is used to describe a complex that has one
or more open coordination sites where another ligand can be accommodated
or more open coordination sites where another ligand can be accommodated.
Typically, most complexes with a CN of less than 6 fall into this category.
Coordinative unsaturation is an important concept in organotransition metal
chemistry. For example, for an olefin to undergo insertion, metathesis or
polymerization, there must be room for the olefin to approach or bond to the
metal.
7
MS101
TM‐OC: Metal‐Ligand complexes
C.Nevado
Electron counting:
The 18 electron rule
Because most metals possess d orbitals, they can accommodate 18 valence electrons. And just as the octet rule is often violated, so is the 18 electron rule. Nonetheless, both serve a useful purpose
the octet rule is often violated, so is the 18 electron rule. Nonetheless, both serve a useful purpose in predicting reactivity. Counting electrons in organometallic complexes
Knowing how many valence electrons belong to a transition metal complex allows us to make predictions about the mechanisms of reactions and the possible modes of reactivity. There are two methods for counting electrons:
M th d 1 Th i i ( h
Method 1: The ionic (charged) model
d)
d l
We ‘remove’ all the ligands from the metal and, if necessary, add the proper number of electrons to each ligand to bring it to a closed shell state, for example NH3, but CH3‐.
Method 2: The covalent (neutral) model
Method 2: The covalent (neutral) model
We ‘remove’ the ligands from the metal, but rather than take them to a closed shell state, we do whatever is necessary to make them neutral, for example NH3, but CH3.. Notice that this method does not give us any immediate information about the formal oxidation state of the metal, but is useful when dealing with larger metallic networks such as the ones found in clusters.
8
MS101
C.Nevado
TM‐OC: Geometries of Metal‐Ligand complexes
Sterically preferred geometries: ‐) trigonal planar ‐) tetrahedral ‐) trigonal bipyramidal
‐) octahedral
Sometimes, electronics override steric preferences:
‐) d8 metals, especially of the second and third row, are almost always square planar (dx2‐y2 orbital high in energy)
9
MS101
TM‐OC: Counting electrons
The two methods compared:
10
MS101
TM‐OC: Ligand electron count
11
MS101
TM‐OC: Ligand electron count II
12