Catenation of heavier p-block elements Flashcards
Catenation
The bonding of atoms of the same element into a series/chain
“Propagation of a linear chain through homonuclear bonding interactions”
Why should it become easier to make ‘carbene’ analogues as you go down group 14?
As you go down the group, the energy separation between the valence s and p orbitals increases
Therefore the compounds are less likely to ‘use’ their much lower energy electrons in their ns orbitals for bonding
More likely to just use the 2 electrons in the valence p orbital - i.e. just form two bonds
Can see this with PbCl2 - bottom of group 14, divalent state is very common
Propensity to form these divalent compounds then decreases as you go back up the group
‘Inert pair effect’
Inert pair effect
The increasing stability of oxidation states that are 2 less than the group oxidation state for the heavier elements of groups 13, 14, 15 and 16
The outermost s electrons are more tightly held to the nucleus as you go down the group due to ineffective shielding by d- and f-block electrons
Thermodynamic/kinetic stabilisation of carbon chains
Catenated carbon chains are not thermodynamically stable with respect to their own oxidation
However, they are kinetically stable - the C-C/C-H bonds are sufficiently compact and strong that they cannot be oxidised unless energy is provided - i.e. they do not ignite spontaneously
Thermodynamic/kinetic stabilisation of chains of heavier group 14 elements
As you go down group 14, PQN increases so the orbital overlap becomes less efficient and the bond strengths decrease
These compounds are more thermodynamically unstable
The longer bonds means the compounds are also more readily oxidised - less kinetically stable
i.e. the degree of kinetic stability seen for catenated carbon chains is not as pronounced as you go down the group
Why is an N-N bond weaker than a P-P or As-As bond?
Because N is a lot more electron rich
How can we increase the stability of catenated group 14 elements?
By adding kinetic ‘protection’ through introducing bulky substituents
Why is SiH4 more pyrophoric than CH4?
Si-H bonds are longer and weaker than C-H bonds
Polysilanes are known up to…
Si8H18 (but really unstable, even in the absence of oxygen - just falls apart into smaller oligomers)
Poly-diorganosilanes
i.e. polymers of SiR2 rather than SiH2
Presence of large R groups introduces kinetic stability and protects the Si-Si bonds
(still prone to oxidation - would produce SiO2 which is really stable)
Synthesis of poly-diorganosilanes
Most commonly synthesised by Wurtz coupling
nR2SiCl2 + 2n Na —> (R-Si-R)n + 2n NaCl
See flashcard
Disadvantage of Wurtz couplings
Yields a bimodal distribution of molecular weights, as well as small cyclic materials with n = 4, 5, 6
It is difficult to control the MW of the products made
Affected by lots of factors e.g. temp of rxn, volume of solvent, concentration of rxn etc
How can the yield of the Wurtz coupling be improved?
By adding crown ethers e.g. 15-crown-5 to solubilise Na
Using ultrasound activation to produce monomodal molecular weight distribution - e.g. adding an ultrasound horn directly into the reaction bath yields only one product
Gel permeation chromatography
A chromatographic method for determining ranges of MWs of polymeric compounds
Alternative synthetic route for producing polysilanes
Silane dehydrocoupling: using a different type of monomer e.g. RSiH3 and then applying a catalytic method to effect a dehydrocoupling reaction via a sigma-bond metathesis mechanism
See flashcard
Types of catalysts used for polysilane dehydrocouplings
A variety of mid-late TM catalysts have been reported
Most commonly these are d0 species e.g. Ti(IV), Zr(IV), Hf(IV), Ln(III) complexes
UV absorption spectra of high MW polysilane derivatives
In contrast to saturated C-C-based polymers (that have no electronic absorption above 160 nm so are colourless), all soluble high MW polysilane derivatives absorb strongly in the UV region of the spectrum (>250 nm) irrespective of chain length
This highlights that the electronics of these are different
Red shift
= moving to longer wavelengths
Changing the identity of the substituents leads to an electronic modification
Polysilanes start to absorb at longer wavelengths with aromatic substituents and at lower temperatures
Also, as the value of n increases (i.e. longer chains), the compounds become weakly coloured, as lambda(max) moves from the UV into the visible region of the spectrum - up to a saturation point
This red shift overall suggests there must be a change in the HOMO-LUMO gap
Smaller HOMO-LUMO gap =
= stronger absorption (i.e. more coloured), absorbing at longer wavelength
Electronic properties of polysilanes
Can think of this electronic behaviour of polysilanes as directly analogous to increasing double bond conjugation in polyalkenes
e.g. ethene lamba(max) = 180 nm, buta-1,3-diene lambda(max) = 217 nm and typical polyacetylenes lambda(max) = 500-600 nm
The pi-pi* gap narrows with increasing conjugation and the electronic transition moves to a lower energy (longer wavelength)
Why does the pi-pi* gap get narrower with increasing conjugation?
More and more molecular orbitals gives rise to a band
The pi-electrons in the pi-orbitals all interact which gives rise to a band
What must happen in polysilanes for there to be conjugation?
For polysilanes, conjugation cannot occur through pi-bonds
Therefore, there must be delocalisation of the sigma-electrons
This does not occur in hydrocarbons, which means a significant change in the electronic structure must take place between carbon and silicon
Sigma-delocalisation in polysilanes
The catenated polysilane chain is propagated by the overlap of approximately sp3-hybridised Si valence orbitals
Two of the hybrid orbitals are used to form bonds to the R groups on Si
The other two are responsible for the vicinal bonding interactions to an adjacent sp3 hybrid orbital of another Si centre i.e. the bonding interaction that propagates the Si chain overall
In order for delocalisation of the sigma electrons to occur, the geminal overlap of the sp3 orbitals must also be significant
This can occur because Si 3s and 3p orbitals are more diffuse than the 2s and 2p orbitals on C
This leads to a splitting of each pair of sp3 orbitals into a strongly bonding sigma(SiSi) orbital and a strongly anti-bonding (sigma*SiSi) orbital delocalised over the silicon backbone (= “sigma-delocalisation”)
i.e. each orbital interaction is either stabilised or destabilised
Overall leading to a narrowing of the HOMO LUMO gap and giving rise to the pronounced electronic transition in the visible region
What affects the degree of delocalisation in polysilanes?
The degree of delocalisation depends on the efficiency of the geminal interaction (at the same Si) in comparison to the vicinal interaction
i.e. delocalisation is a function of the ratio gem/vic - the closer the value is to one, the closer to perfection the electronic delocalisation
Size of geminal interaction in comparison to vicinal
Geminal interaction is always less/smaller than the vicinal interaction
Conformational effects on sigma-delocalisation in polysilanes
All anti (/trans) configuration maximises the geminal overlap/interaction, destabilising the HOMO and stabilising the LUMO, resulting in a red shift
A cis-kink would disrupt the sigma-delocalisation along the chain
e.g. poly(di-n-hexyl)silane absorbs at 317 nm in solution but at 371 nm in the solid state
e.g. octasilane (see flashcard)
What do polysilanes behave as?
Semiconductors
Can increase their conductivity by doping and introduce metallic behaviour
What does the colour of polysilanes arise from?
A sigma to sigma* transition of about 300 nm (4 eV)
Catenated compounds down the rest of group 14
As n increases, the gem/vic ratio increases due to the increased diffusivity of the orbitals
This leads to a narrowing of the HOMO/LUMO gap and a red-shifted lambda(max)
Electronic properties of polystannanes
Also show a red shift in the lambda(max) with increasing chain length, reflecting more geminal overlap and a narrower HOMO/LUMO gap
Absorption maxima plateaus at approximately 20 Sn units
Synthesis of polystannanes
Same as polysilanes - Wurtz coupling
Can also do catalytic dehydrocoupling - using the same Hf catalyst as for polysilanes, but there are mechanistic differences
Involves alpha-hydride elimination of the stannylene and chain growth by insertion into Sn-Sn bonded species
This reflects the increased stability of the Sn(II) oxidation state down the group
Properties of polystannanes c.f. polysilanes
Polystannanes are even more air/light sensitive (but can be isolated)
Isoelectronic species in groups 13 and 15
Using different electron donors
e.g. beta-diketiminate ligands for group 13
Draw
Catenation in other groups of p-block elements
Not commonly recognised, but has been shown for In in group 13 and As in group 15
Draw
