Transcript Group 14 - University of Ottawa

Cyclopentadienyl Complexes
Revisited
General MO diagram for CpEY
Y = coordinate bond for groups 1 and 2, a nonbonding pair in CpAl or CpSn+
Most effective interaction is
between the s/p on E and
occupied a1
The interaction with p orbitals
on E (e1) is secondary
The C5v structure represents
a minimum for most
compounds including:
CpLi, CpBe+, CpE (E = B, Al,
Ga, In, Tl) and CpE+ (E = Ge,
Sn)
Cyclopentadienyl Compounds
Cp2Si – two structures in the solid state
Divalent state is accessible for Ge and Sn and Pb – so directly
form Cp2E
GeX2 + 2NaC5R5
(5-C5R5)2Ge + 2NaX (X = Cl, Br)
in the gas phase discrete monomers with the bent structure
Solid state structure of Cp2Pb - polymeric
generally air sensitive but bulky groups offer kinetic stability
bulkier R groups will give monomeric solid state structures
Cp2Pb compounds can react with Lewis bases such as bipyridine
or TMEDA to make adducts – structure.
Reactivity of Group 14 Alkyl Halides
Me3SiCl + NaCp give the Cp-SiMe3
Similarly, Cp*2SiBr2 can be prepared
reduction with K/anthracene leads to Cp*2Si
Structure: two isomers in the solid state
one with parallel rings Si-C range from 2.41-2.44Å
one with centroid-Si-centroid angle of 167.4 degrees
Cyclopentadienyl Complexes
Revisited
General MO diagram for Cp2E
Two possible structures presented – D5d (parallel rings) and C2v (bent)
For a 14 electron system all of these orbitals are filled.
In both D5d and C2v the
primary interaction is
stabilization of three pairs of
electrons.
Note that the difference is
2a1g and 2a1 is an important
difference between the two
structures (due to the
interaction of the element p
orbital)
Note that the 2a1 is the lone
pair in a valence model
Cyclopentadienyl Complexes
Revisited
General MO diagram for Cp2E
Two possible structures presented – D5d (parallel rings) and C2v (bent)
For a 14 electron system all of these orbitals are filled.
The energy of the 2a1g (D5d)
is sensitive to the E-Cp
distances – it becomes more
antibonding as the Cp’s get
closer to E
The bent structure represents
a minimum but < 10 kJ mole-1
Heavier Cp Compounds
With the exception of (Ph5C5)2M all are bent compounds (30-50 degrees)
in gas phase and solution
Stereochemically active lone pair
Stannocene
Stannocene, Cp2Sn prepared from LiCp and SnCl2 – air sensitive, white
solid m.p. 105C
Structure - 46 degree angle between the centroids
Cp*2Sn from Cp*Br and Sn or LiCp* and SnCl2 (36 degree angle)
(Ph5C5)2Sn has co-planar rings, bright yellow, low basicity suggests
delocalized electrons
Reactivity
- Cp2Sn + MgCp2 → Sn(3-Cp)3(Mg(THF)6)
- protic acids cleave the Sn-Cp bond to give CpSnX
- non-coordinating borate or triflate – Cp*Sn+ cation (Cp*Sn+B(C6F5)4- has
been used as co-catalyst)
- these cations can coordinate nitrogen bases like pyridine
- Cp2Sn Lewis base adducts with BBr3, AlCl3 and AlBr3
Catenation in Heavier Group 14
Compounds
Organocompounds with (R3E)2 are quite common. In fact they are
known for essentially all combinations of C, Si, Ge, Sn, and Pb.
Some longer chain and cluster compounds are known.
Polymers:
R
*
R
E
R
*
n
*
E
*
*
n
Polymers show extensive s-delocalization
Catenation in Heavier Group 14
Compounds
Polymers show characteristics of sigma delocalization
Near UV absorption σ --> σ* moves to longer wavelengths
as polymer increases in length
Can form delocalized anions
Catenation in Heavier Group 14
Compounds
Rings and Cages:
Cyclotrisilanes, germanes, stannanes and mixed species
By reduction of R2EX2
R
R
Si
3 R2SiCl2 + 6 Li(C10H8)
6 LiCl + 6C10H8
R
Si
Si
R
R
R
Catenation in Heavier Group 14
Compounds
Reactivity –
R
R
Si
R
Si
Si
R
R
R
source of silylene – “R2Si”- and disilene – R2Si=SiR2
The ratio of products depends on the identity of R
R
h
R
Si
R
Si
Si
R
R
R2Si: + R2Si=SiR2
Li
R
Si
Li
Si
Si
Li
Catenation in Heavier Group 14
Compounds
Polycyclic and cluster compounds
R
R
E
R
E
R
R
R
E
E
E
E
R
R
E
E
E
E
R
R
E
E
E
E
R
R
R
R
Sn
RSn RSn
RSn
RSn
RSn
SnR
SnR
SnR
SnR
R
Catenation in Heavier Group 14
Compounds
“Catorcanes”
Organometallics 1996, 15, 429.
Catenation in Heavier Group 14
Compounds
J.Organomet. Chem. 2003, 686 379.
Carbenes and Analogues
Carbenes are 6-electron carbon centers, typically s-bonded to two other
groups, and with two lone electrons.
These can have ground states that are singlet or triplet.
R
R
C
R
triplet
C
R
singlet
Heavier Group 14 Analogues
In the singlet state these compounds can be Lewis bases or Lewis acids.
E
R
R
When R is a “small” group these species oligomerize.
If R is large or is there is a site for intermolecular coordination to the E
center (increases the coordination number up to three or four) then these
species can be stabilized.