Transcript Group 2 and Group 12 - University of Ottawa

Group 2 and Group 12
We will discuss groups 2 an 12 together due to their similar reactivity.
Group 12 has completely filled and low-lying d-orbitals, mimicking
group 2.
Magnesium compounds are the most important with respect to reactivity.
Like the heavier alkali metals, the heavier alkali earth metals (Ca, Sr, Ba)
are not as useful magnesium compounds, due to their tendency to be
ionic rather than covalent compounds.
Beryllium
Beryllium’s extremely high charge density gives some structural
anomalies.
No compelling reason to use due to toxicity.
Metal
Charge Density
Metal
3
Charge Density
3
Li
(C/mm )
52
Na+
24
Mg2+
120
K+
11
Ca2+
52
+
8
Sr
2+
33
+
6
Ba
2+
23
+
Rb
Cs
2+
Be
(C/mm )
1108
Organoberyllium Synthesis
More electronegative than Li and Mg so metathesis reaction with
organolithium or Grignard works.
BeCl2 + 2 LiPh  2 LiCl + BePh2
Transmetallation with HgR2 also works.
HgR2 + Be  R2Be + Hg
Organomagnesium Compounds
The most important of these, RMgX, are the Grignard reagents. They
were first isolated by Victor Grignard.
Grignard reagents are mainly made by direct
reaction of haloalkane with Mg in ether
solvents (trace of I2)
CH3Br + Mg  CH3MgBr (ether, 20°C)
Can also be made by transmetallation with
organomercury:
MgCl2 + (CH3CH2)2Hg  2 CH3CH2MgCl + HgCl2
Organoberyllium Structures
Compounds are electron-deficient with a high charge density on Be2+
and thus display some uncommon bonding arrangements.
BeMe2 is a polymeric material, with the 3-center, 2-electron (3c,2e) bond
Me
Me
Me
Be
Me
Be
Me
Me
Be
Me
Be
Be-C 1.92A
Cl
Cl
Be
Cl
Be
Cl
BeCl2
Me
Me
BeMe2
Cl
Me
Cl
Cl
Be
Be
Cl
Cl
Cl
Structures
Be, Mg covalent : tendency to adopt a four coordinate tetrahedral
structure via bridging atoms.
However, steric effects can lead to decreased association.
Monomeric Be(tBu)2 and Mg{C(SiMe3)3}2
Coordinating solvents break the polymeric structure down to dimers
L
Me
Me
Be
Be
Me
Me
L
BeMe2 Dimer with Coordinating Solvent (L)
Beryllocene
An unusual case of bonding exists in beryllocene (Cp2Be), due to
the very small size of the beryllium center. (h1-Cp(h5-Cp)Be in the
solid but electron diffraction and spectroscopy suggest a different
structure.
1.47A
1.53A
Be
1.83A
Solid State
Be
1.90A
Gas Phase
This structure is fluxional in the NMR, with all protons equivalent
down to 163K
Schlenk Equilibrium
In solution, Grignard reagents undergo a complex equilibrium called the
Schlenk equilibrium. This equilibrium is important in the reactivity of
Grignard reagents:
X
R
Mg
Mg
2 L + 2 RMgX
L
X
L
L
Mg
Mg
R +
R
X
R
-
L
X
R-+ RMgX + L2MgX+
MgCl2 + MgR2
X
R
Mg
Mg
R
L
X
R2MgX- + L2MgX+
L
Schlenk Equilibrium
This equilibrium is more pronounced, and thus a Grignard is more
reactive, as:
•the halide becomes less basic
more electronegative (up the group)
•the R group becomes a more stable carbanion
greater branching
Grignard reagents exist as a mixture of
species due to equillibria in solution
– 2RMgX == MgR2 + MgX2 and more complex
processes
Magnesium Dialkyls
Magnesium dialkyls can be synthesized by transmetallation.
Mg + Et2Hg  MgEt2 + Hg (THF)
Taking advantage of the Schlenk equilibrium.
Addition of a very strong donor ligand (e.g. dioxane) will
precipitate the dihalomagnesium adduct, leaving the dialkyl in
solution:
O
2 RMgX + O
O
Mg Cl + R Mg
2
Cl
O
O
O
Structure of the Grignards
The structures of magnesium mono- and di-alkyl compounds are similar
to that of beryllium. They range in oligomerization from polymers to
monomers depending on the R group and presence of base.
Halide bridges with 2c,2e bonds preferred over alkyl bridges.
CH2CH3
Et2O
Et2O
Br
Et2O
Mg
But
Br
Mg
Mg
tBu
OEt2
Br
Polymeric species are common in weak donor solvents (Et2O) and/or
with high Grignard concentration.
Crystal Structures
Crystallization commonly carries coordinated solvents because Grignard
reagents are usually not soluble in hydrocarbons.
Oligomerization of both RMgX and R2Mg compounds is a function of
steric bulk and coordinative saturation.
•The larger the group, the less oligomerization
•The stronger the base, the less oligomerization
Et
Et
Mg
Mg
Et
Me3Si
Et
Et
Mg
Mg
Et
Et
Me3Si
Mg
Et
Et
A polymer
Et
SiMe3
Me3Si
SiMe3
SiMe3
A monomer,
due to steric bulk
Crystal Structures
Structure of Cp2Mg contrast with Be
An Unusual Grignard
This Grignard has no M-C bond:
Me
Me
Me
N
Si
Me2 Me2
Si
N
C
Me
Me
Mg
Si
Me
N
I
Me
Me
Organometallics, 1997, 16(4), 503.
Grignard Reagents
Particularly useful carbanion reagents in organic chemistry
Presences of ether can some limite to uses – can form unwanted
complexes with Lewis acidic reagents (e.g. BX3) (can be avoided by
using lithium reagents that are hydrocarbon soluble)
Group 12 Synthesis and Structure
Zinc organometallics are most commonly prepared by metathesis with
alkyllithium or alkylaluminum.
ZnCl2 + 2 RLi  R2Hg + 2 LiCl
3 Zn(OAc)2 + 2 AlR3  3 ZnR2 + 2 Al(OAc)3
Note that in the case of Al reagents that this does not conform to the
electronegativity (1.61 Al, 1.65 Zn) but does correlate with hardness (Zn2+
softer with CH3- and Al3+ with Cl-)
Transmetallation:
Zn + HgR2  Hg + ZnR2
Notably, group 12 compounds are not a very electropositive metal centres.
They are much less reactive than the groups 1 and 2, and thus have seen less
utility in synthesis.
Organozinc Compounds
Reactivity – pyrophoric and readily protonated/hydrolyzed
Mild Lewis acidity – will coordinate amines esp. if chelating
Do not behave as good Lewis acids. Lewis acidity decreases on going down the group
Carbanionic character exhibited by reactivity with ketones to yield alkoxide – same as
Li, Al, Mg (note this does not go with R Hg or Cd)
2
Decreasing bond polarity and carbanion like behavior on going down the group
– e.g. Dialkylmercury compounds do not add across ketones
On going down the group the softness of the metal center increases
– Organomercury compounds show a strong
tendency to bind to sulfur leading to high toxicity
Structures of Organozinc
Compounds
Zinc shows the kind of diversity that is common among the less
electropositive metals, although it favours the linear arrangement.
Structures of Organozinc
Compounds
Generally formation of linear species that are not associated in solid,
liquid, gas or haydrocarbon solution
Suggests that these species possess 2c, 2e bonds
Do not complete octect through association via alkyl bridges like Mg and
Be
Structure and Zinc
Cyclopentadienyl compounds of Zn – monomeric pentahapto in the gas phase
zig zag pentahapto to two Zn polymer in the solid state
CH3
Zn
Zn
Zn
CH3
CH3
Zn
CH3