Main group organometallic chemistry [Mike Hill] Flashcards
organometallics
synthesis, structure and reactivity (i.e. s/p block) of compounds containing a metal to carbon bond
metal (M) includes metalloids such as boron, silicon
ligand (L) includes alkoxides (oxygen), thiolates (sulphur), enolates
electron precise
octet configuration at metal centre
leads to molecular compounds with reduced lewis acidity/electrophilicity
saturated centre -> inert
electron rich
leads to molecular compounds with reduced lewis acidity/electrophilicity
electron poor
too few valence e-
provide pair for every bonded atom in a Lewis acidic/highly electrophilic molecule
how do organometallic compounds in groups 1,2 and 13 use its valence orbitals in bonding?
addition donors
formation of multi centre bonds + aggregation
what happens to the polarity as the difference in electronegativity (effective nuclear charge) between metal and carbon increases
increases (+ reactivity of M-C bond increases)
effect of metal size on M-C bonding
larger metal = weaker M-C bond
due to less efficient orbital overlap
which group are π-bonds restricted to?
1
heavier elements = σ-bonds only
kinetically labile
low energy decomposition energy
highly reactive
how can kinetic lability be suppressed?
[increased thermal stability]
via formation of Lewis base adduct or by use of ligands without hydrogens in the position (no β-hydrogen)
3 factors that govern the kinetic reactivity of main group organometallic with water and air
- high polarity of M-C bond (kinetic)
- availability of low-lying empty orbitals at metal (kinetic)
- availability of free electron pairs at metal (kinetic)
group 13 + air/water
vacant p orbital
water/air sensitive
group 14 + air/water
electron precise
water/air stable
group 15 + air/water
lone pair
water/air sensitive
M-C bond formation - direct synthesis from metal and organic halide
2M + nR-X -> RnM + MXn or RnMXn
useful for electropositive metas and alkyl magnesium
= OXIDATIVE ADDITION
driven by enthalpy of formation of M-X bond or MXn salt
M-C bond formation - redox transmetallation
M + RM’ -> RM + M’
metal of lower electronegativity (M) replaces metal of higher electronegativity (M’)
M = electropositive group 1,2,13 metal
M’ = Hg
M-C bond formation - metallisation (metal-hydrogen exchange)
R-M + R’-H -> R’-M + R-H
driving force = relative acidities of the 2 C-H acids involved (RH and R’H)
forward reaction will proceed if pKa of R’H is lower than RH
M-C bond formation - salt metathesis
R-M + M’-X -> R-M’ + MX
important for generation of p-block M-C bonds
driving force = formation of salt with high lattice enthalpy
predicted by electronegativity of 2 metals involved
- if M’ = more electronegative than M; reaction = feasible
M-C bond formation - addition reactions
[hydrometallation]
common for B, Al, Si
driving force = formation of C-H bond from weaker M-H bond
[carbometallation]
only occurs with v. electropositive metal
lithium + carbon - electronegativity
large difference
BUT Li-C bonds = somewhat covalent due to polarising ability of Li+
LiR formation - direct synthesis
2Li + RX -> LiR + LiX
high enthalpy of formation of LiX salt = exothermic reaction
LiR formation - transmetallation
2Li + HgR2 -> 2LiR + Hg
based upon relative ease of reduction of Hg and Li
LiR formation - metal-halogen exchange
[at low temp.]
Li-R + R’-X -> Li-R’ + R-X
if carbanion formed = more stable, equilibrium will be driven to the right
Wurtz coupling process
R’Li + R’‘X -> R’R’’ + LiX
LiR formation - metallation of acidic hydrocarbons with organolithiums
[equilibrium reaction] - position depends on relative pKa of the 2 C-H involved -> stronger C-H acid will promote higher yield of metallation by lithium salt of weaker acid
R’-Li + R’‘H -> R’-H + R’‘-Li
organolithium structures
2 valence electrons - electron-deficient
compensated by formation of multi centre bonds (aggregation)
tend to form oligomeric aggregates in solution + solids
what does the degree of association of organolithium compounds in solution depend on?
polarity
coordinating ability of solvent
Li-CH3 in THF or Et2O
tetramer
Li-CH3 in TMEDA
dimer
Li-n-C4H9 in hexane
hexamer
Li-n-C4H9 in Et2O
tetramer
Li-t-Bu in hexane
tetramer
Li-C6H5 in THF or Et2O
dimer
Li-CH2C6H5 in THF or Et2O
monomer
what happens to the reactivity as the polar nature of Li-C increases?
reactivity increases
reactivity of organolithiums - metallation of CH, NH, OH
[i.e. as a base]
EH + RLi -> ELi + RH
EH = stronger acid than RH
reactivity of organolithiums - addition to multiple bonds
[i.e. as a nucleophile]
uncompleted R-Li only read with conjugated dienes and styrene derivatives to provide polymers
on addition of TMEDA -> nBuLi can initiate polymerisation of ethene
reactivity of organolithiums - reaction with transition metal/p-block halides
[produces salt]
e.g. 4PhLi + SiCl4 -> SiPh4 + 4LiCl
salt metathesis = thermodynamic driving force
heavier alkali metal organometallics
synthesis similar to alkyl lithiums
e.g. metallation of R-X
2Na/K + RX -> MR + MX
most convenient syntheses are by metathesis reactions of RO-M
polyhapto n^n interactions
organic anion with metal via >1 interaction
group 2 oxn states and general properties
[alkaline earth metals]
+2
ionic radii smaller than group 1 in same period
all elements (M0) highly electropositive/reducing
what happens to the reactivity of the M-C bond with increasing electropositivity of the metal?
becomes more reactive
Be
v. small and polarisable
most toxic non-radioactive element
max. coordination number = 4
organo derivatives tend to form 3-centre-2-electron bonds (divalent compounds)
Grignard reagents
R-X + Mg -> RMgX
usually formed from alkyl/aryl halide and Mg in solvent such as diethyl ether (= exothermic)
= oxidative addition reaction to the metal
slow reaction to initiate - due to passivating oxide layer on Mg surface
Rieke method
= extremely reactive form of Mg
MgCl2 + 2K —-> Mg + 2KCl -> (RX) RMgX
finely divided (high SA) black powder
Grignard reagent structures - solid state
solvated by ether (4 coordinate Mg centres)
less heavily solvated oligomers feature halide bridges
dialkylmagnesium compounds - synthesis
[redox transmetallation of Mg metal with diorganomercury compounds]
Mg + R2Hg -> R2Mg + Hg
dialkylmagnesium - bonding
sp3 hybridised bridging methyl group
3-centre-2-electron bonds to sp3 metal
R2Mg + water/protic reagents
[eliminates alkanes]
R2Mg + R’OH -> RMgOR’ + RH
reactivity of organomagnesium compounds - alkylating/arylating for main group and transition metal halides
source of R- (from grignard)
formation of magnesium dihalides = thermodynamic force
e.g. SbCl3 + 3CH3MgX -> (CH3)3Sb + 3MgXCl
Mg-C = less polar + reducing
organometallic compounds of heavier alkaline earth metals - Ca, Sr, Ba
far more reactive
catalysts - sustainable, common, cheap (compared to Rhodium, Palladium, Platinum)
organometallic compounds of heavier alkaline earth metals - Ca, Sr, Ba - structural studies
less covalent than Mg analogues
isolation of alkyl derivatives requires use of bulky alkyl ligands
bent (rather than linear) - due to increased size and polarisability of metal
bonding increasingly ionic as you descend group 2
group 13 organometallics
+3 oxn state
stabilisation of lower oxn state (+1) as you go down group -> inert pair effect
R3B synthesis
[metathesis of a boron halide or trialkyl borate with polar source of carbanion]
BF3.OEt2 + 3RMgX -> BR3 + 3MgFX/Et2O
[hydroboration]
3R-HC=CH2 + BH3 -> B(CH2CH2R)3
R3B properties
monomeric
organoboron alkoxides/amides display 2p(pi)-2p(pi) bonding
stable to water due to low bond polarity
oxidise readily
burn in air
strongly Lewis acidic (6 electron species) = use in many catalytic processes (presence of vacant p orbital)
R3Al lab prep
[metathesis with RLi or RMgX]
AlCl3 + 3t-BuLi -> t-Bu3Al + 3LiCl
[transmetallation]
2Al + 3HgPh2 -> 2AlPh3 + 3Hg
** 2:3 stoichiometry **
structures of R3Al
> 3 coordinate => due to electron deficiency unless R group = bulky
smaller R groups dimerisation occurs via Al-C-Al 2e-3-centre bonds in condensed phase
Al-C-Al bridging persists in non-donor solvents with fast Al-Me exchange (reaction tends to be labile)
R2AlX and RAlX2 prep.
[redistribution of trialkyls/trihalides in correct stoichiometry]
2R3Al + AlCl3 -> 3R2AlCl
R3Al + 2AlCl3 -> 3RAlCl2
-> reaction driven by labile nature of compounds in solution
-> @rtp
-> in non-donor solvents, compounds usually oliogemers formed by X-Al-X bridging
reactivity of R3Al
organoaluminium compounds = hard acids (6 valence e-)
readily form adducts with bases
hard donors
role of methylaluminoxane (MAO)
acts as a co-catalyst for alkene polymerisation
RnMX3n-1
M = Ga, In, Tl
SUMMARY
prepared the same as R3Al
halides RnMX3-n most readily prepared by redistribution reactions
reactivity of R3Ga and R3In similar to R3Al
RnMX3n-1
M = Ga, In, Tl
STRUCTURAL CHEMISTRY
don’t show tendency to form dimers (metals have lower Lewis acidity)
R2MX compounds -> larger ln can adopt coordination numbers >4
-> leads to coordination polymers in solid state
preparation of thallium (I) cyclopentadienyls
[v. toxic]
Tl2SO4 + 2C5H6 + 2NaOH -> 2(C5H5)Tl + Na2SO4 + 2H2O
what type of ligands is best for lighter group members?
bulky ligands
helps prevent disproportionation of monovalent metal (M0 -> M3+)
group 14 organometallics
lower polarity of M-C bond
RnMX4-n
electron precise/rich valence shell
no tendency to associate through multicentre bonding via alkyl/aryl bridges
reduced reactivity towards nucleophiles
water/air stable
what are tetravalent group 14 organometallics used for?
[technological applications]
silicones
organotin compounds
lead alkyls
Rochow process
high temp. direct reaction of RX/ArX with fluidised bed of Si
presence of 10% metallic Cu as catalyst
2RCl + Si/Cu —-> R2SiCl2
300°C
organosilanes
Si-C bonds = very thermally stable (348 kJ mol-1)
don’t decompose before ca. 700°C
R4Si compounds = highly resistant to chemical attack
air/water stable because of low polarity of Si-C bonds
polyorganosiloxanes (silicones)
non-degradable; self-sealing
high Mr polymers (Me2SiO)n
propagated by Si-O-Si linkages
v. thermally and chemically stable - due to strength of Si-C and C-H bonds + Si-O-Si
Si-O bond enthalpy
466 kJ mol-1
flexibility of Si-O-Si link
delocalisation of lone pairs on oxygen into available σ* orbitals on Si
reduces directionality of Si-O bond + makes structure more flexible
heavier group 14 (Ge, Sn, Pb) organometallics
+4
stabilisation of lower oxn state as you go down group
inert pair effect -> tendency of 6s2 electrons remained paired
organogermanium compounds
similar to silicon compounds - due to shielding effect of filled 3d shell
much less common -> low terrestrial abundance of the element
R4Sn structure
tetrahedral - sp3
air/moisture stable
R3SnX
more electronegative groups increases Lewis acidity
higher coordination derivatives result from coordination by bases
in absence of external bases -> polymeric structures are common
= larger
= increased Lewis acidity of metal cation
organolead compounds
Pb(II) = most common
Pb-C = sig. weaker than Sn-C bond
= less thermally stable
main application = Et4Pb (endothermic but kinetically stable)
decomposes on heating by Pb-C homolysis as source of ethyl radicals
petrol additive (“anti-knock agent”) to prevent premature ignition in internal combustion engines
group 15 (P, As, Sb, Bi) organometallics
+3 and +5
stabilisation of +3 increases as you go down group -> inert pair effect (tendency of the ns2 electrons remain paired)
group 15 R3E compound shape
pyramidal with stereochemically active lone pair
σ-donor capacity of phosphine ligands
PH3 < PRH2 < PR2H < PR3
due to inductive effect of alkyl sub.
π-donor capacity of phosphine ligands
[depends on availability of low energy σ* orbitals]
PF3 > PCl3 > P(OAr)3 > P(OR)3 > PAr3 > PAr2R > PR3
R5E shape
trigonal bipyramid in solid state
EXCEPTION = pentaphenylstiborane, Ph5Sb = pyramidal
phosphazenes
family of compounds constructed from P-N linkages
phosphonitrilic dichloride (PNCl2)3
earliest reported inorganic heterocycle
nPCl5 + nNH4Cl -> (NPCl2)3-10 + 4n HCl
130°C
phosphonitrilic dichloride (PNCl2)3 - structure
[cyclic trimer]
planar six-membered ring
all P-N bonds = identical (shorter than normal => suggested multiple/delocalisation)
