Flashcards in Stasch Deck (99):
general properties of s-block metals in compounds
1. typically ionic bonding (non-directional)
2. flexible metal ligand interaction
3. various coord. modes and metal coord. nos
4. redox inert
what is organometallic chemistry?
study of compounds that have at least one interaction/bond between a carbon and metal centre
compounds with M-C bonds
compounds that contain metal and organic groups but don't have direct M-C bond
properties of M-C bonds depend on
1. bond polarity (electronegativity differences)
3.bond strengths (atomic/ionic radii, orb size and overlaps)
covalent contribution in bonds
orb overlap (wavefunctions)
ionic contribution in bonds
attraction from opposite charges
what type of bonds make up main group organometallics?
largely M-C σ bonds of various strengths and polarity
electronegativity of element increases with
higher oxidation state
eg Ti(IV) > Ti(I)
is electronegativity of element such as carbon constant?
no- depends on the amount of s-contribution to hybrid orbs (s-orb more strongly affected by effective nuclear charge than other orbs)
the more s character, the more electronegatiive and the stronger the M-C bond strength
polarity trends in main group organometallics
1. M-C bond polarity decreases from left to right in table
2. M-C bond polarity increases down a group
bond strength trends in main group organometallics
decreases down a group
what does mean bond energy do going a group and why?
energy decreases due to different radical expansion and poorer orb overlap for heavier metals and period 2 elements eg carbon
are organometallic compounds stable/unstable with respect to oxidation to MOn, CO2, H2O
organometallic compounds are thermodynamically unstable
generally what types of organometallics are reactive towards air and moisture?
organometallics with e-poor metal centres and vacant coord. sites (low-lying vacant orbs, free e- pairs and highly polar M-C bonds
`is M-C interaction hard or soft?
why are ionic R- groups preferred to covalent ones?
prefer good distribution of anionic charge on R
what do the aggregation states of lithium alkyls in solution depend on?
solvent (hydrocarbon vs donor solvent) and temperature
why can RLi compounds be more reactive at low temperatures compared with at RT in donor solvent?
due to a lower aggregation state at low temperatures. driving force is (-TΔS) towards more free donors and higher RLi aggregation at higher temp.
why are deprotonations of toluene/benzene very slow?
how can the kinetic basicity of lithium alkyls be increased?
use donor molecules eg THF, chelating amines or MOtBu
how can the speed of deprotonation of benzene be improved
addition of TMEDA- induces break up of (nBuLi)6 cluster, breaking Li-C interactions to form a lower coordinated species. Polarity of Li- bond is increased and reactivity of nBuLi is increased
mixtures of strong bases (different alkali metals or alkaline earth metals) can react as strong bases/nucleophiles
mixtures of nBuLi and KOtBu
what can schlosser bases be used for?
deprotonate quite inert CH bonds eg synthesis of benzylpotassium from toluene
why are sterically demanding alkali amides strong bases but poorer nuleophiles
due to donor atom with e- lone pair in the amide, kinetic barriers are less of an issue
synthesis of ketone from aldehyde- convert aldehyde to a 1,3-dithiane to generate a CH-acidic compound that is readily deprotonated with nBuLi to form an acyl anion equivalent
how can you estimate the concentration of RLi solutions?
titrations- need to be air and moisture free
eg use diphenylacetic acid and N-pivaloyl-o-toluidine
what does the reaction of lithium metal and naphthalene give?
lithium naphthalenide (radical anion) and lithium naphthalenediide (dianion) species
what "holds together" s-block metals coordinating to neutral/anionic π-systems?
electrostatic (ionic) interactions
common donor solvents for Schlenk equillibrium, grignard reagents
diethyl ether, tetrahydrofuran (THF), dimethoxyethane (DME), 1,4-dioxane
ways to synthesise MgR2 compounds
1. grignard formation followed by formation of salt
synthesis of MgR2 compounds : direct synthesis
grignard formation : Mg + nBuCl → nBuMgCl
salt formation: nBuMgCl + nBuLi → MgnBu2 + LiCl
synthesis of MgR2 compounds : metallation
MgnBu2 + 2RH → MgR2 +2nBuH
synthesis of MgR2 compounds : redox- transmetallation
Mg + HgR2 → MgR2 + Hg
2RMgX ↔ MgR2 + MgX2
addtion of dioxane to grignard solutions gives?
solutions of MgR2
2RMgX + 2 dioxane → MgR2 + MgX2(dioxane)2
aggreation state of RMgX in THF
monomeric eg 1 Mg (Mg has coordination number of ~5/6
aggreation state of RMgX in Et2O
dimeric eg 2 Mg - bridging groups
Mg coordination number ~4
what side of schlenk equil. does it lie on in Et2O?
reagents side- RMgX
what side of schlenk equil. does it lie on in THF?
products - MgR2 + MgX2
grignard formation mechanism
1. reaction initiated by single e- transfer from Mg to σ* orb on R-X
2. Get ransfer of X to Mg leaving radical R°, which combines with MgX to give RMgX
grignard formation mechanism - order of reactivity of RX
I> Br> Cl> > F (RF is typically unreactive)
Problem associated with using RI as RX in grignard formation?
it is fastest however gives most Wurtz coupling
2RX + 2Na → R-R + 2NaX
in the reaction of RMgX with ketones what type of transition states are favoured?
6-memembered - 1 ketone: 2 grignard
why is the 1:1 mechanism for the reaction of RMgX with ketones not favoured?
can have more side reactions eg enolisation and reduction
why is the 1:2 mechanism for the reaction of RMgX with ketones favoured?
favours nucleophilic addition
how can the the addition of nucleophiles to ketones be improved?
through the addition of anhydrous salts eg Li+, Zn2+, Ln3+ - can supress side reactions through the lewis-acid activation of the ketone by coordination to the salt
what do you get from the reaction of more hindered/basic grignard reagents with carbonyl compounds with acidic CH in the alpha position?
what do you get from the reaction of grignards with β-hyrogens and hindered ketones?
hydride transfer- reduction of carbonyl to "alcohol"
highly selective, reactive and has high functional group tolerance
can be used in Mg-halogen exchange reactions
R2NMgCl.LiCl - strong base
made from turbo grignard
used to deprotonate functionalisaed aromatic molecules to give grignard reagents
synthesis of ZnR2 compounds
direct synthesis, salt-formation, redox-transmetallation
converts alkenes into cyclopropane via an organozinc carbenoid X-CH2-M from diiodomethane and Zn(Cu)
reaction of zinc with α-bromoester to give zinc enolate. Which can go on to to react with carbonyl group to give β- hydroxyester
RCd and RHg compounds
highly toxic, typically linear geometry
CdR2 + acid chlorides
mechanism of RX substitution by Me2CuLi and product
product = R-Me
occurs via cross-coupling mechanism, involves redox reaction - oxidative adddition and reductive elimination
synthesis of AlR3
3RLi + AlCl3 → AlR3 + 3LiCl
synthesis of AlMe3
4Al + 6MeCl → 2Me3Al2Cl3 → Me4Al2Cl2 + Me2Al2Cl4
Reduction to give AlMe3
3Me4Al2Cl2 + 6Na → 2Me6Al2 + 2Al + 6NaCl occurs via disproportionation reaction
structure of AlMe3 in solid/liquid/gas
solid and soultion: dimeric (3c2e bond for bridging methyl groups)
synthesis of AlEt3: Zielger direct process
Al + 1.5 H2 +3C2H4 → AlEt3
organoaluminium from hydroalumination order of reactivity of alkenes
CH2=CR2 < CH2=CHR < CH2=CH2
reaction for hydroaluminium to organoaluminium
Al + 1.5H2 + 3 C4H8 → AliBu3
reverse reaction of β-hydride elimination
ie gives alkenes from alkynes
hydroalumination reaction + product
alkyne + iBu2AlH → alkene
cis- adition and Al adds on to least substituted carbon
growth of alkyl chains using AlEt3 and then displacement to release AlEt3
Aufbau reaction + 1-alkenes
only get single insertion into Al-C bond
Ziegler-Natta polymerisation of alkenes -reaction steps
-migration of Me onto alkyl chain
-insertion of alkene into alykl chain
Methyl alumoxane (MAO) structure, uses
(MeAlO)n, can come in polymer chains, rings or cages
synthesised from hydrolysis of water
used to activate catalysts eg in Zieglar-Natta polymerisation
Role of MAO during polumerisation
1. alkylating agent (M-Cl to M-Me)
2. lewis acid (abstract CH3- from M)
3. picks up impurities
Cp2Ti- (CH2, Cl) bridge- AlMe2
use of Tebbe reagent
can be used to transfer a methylene to a ketone - replaces double bond O
Group 14 organometallics ER4
1. stable - not e- deficient
2. less reactive/nucleophilic due to low bond polarity of E-C
3. E-C bond strength decreases down the group
E-C bond polarity decreases down the group
4.thermal stability of ER4 decreases down the group
Uses and synthesis of PbEt4
used as anti-knocking agent in petrol
4 NaPb + 4EtCl → PbEt4 + 3Pb + 4NaCl
undergo transmetallation reactions with alkyl/phenyl lithium
C3H6SnPh3 + PhLi → C3H6Li + SnPh4
RLi attacks Sn via 5-coordinate intermediate
addition of Ge-R over alkene double bond
addition of Sn-R over alkene double bond
group 15 ER3
group 15 ER5
+5 oc state is less stable going down group due to inert pair effect
inert pair effect
2 electrons in outer most s orb do not ionise or react
group 15 R2EH compounds
don't undergo hydroelementation reactions due to not having strong polarisation of E-H bond
total number of bonds oe can be ox state
number of simga-bonded substituents
Group 15 preparation of organoelement (III) compounds
EX3 + 3MeBr → ER3 + 3MgX2/LiX
ion size decreases with increasing atomic number
form ionic complexes - hard metal cations
+3 common ox state
formation of cyclopentadienyl- Ln complexes
salt metathesis and redox-transmetallations
Ziegler-Natta polymerisation what metal can be used as precatalyst? what is structure of precatalyst?
uses Cp2MCl2 M= Ti, Zr, Hf as precataylst
formation of Tebbe reagent
Cp2TiCl2 + 2AlMe3 → Cp2Ti - (Cl and Me) bridge - AlMe2 + Me2AlCl + MeH
synthesis of PbEt4
4NaPb + 4EtCl → PbEt4 + 3Pb + 4NaCl
decomposition of PbEt4
decomposes at evelated temperatures
PbEt4 → Et• + •PbEt3
PbEt4 + •Et → H3C-CH3 + Et3PbCh2Ch2•
what is the intermediate when RLi attacks Sn
SnMe4 + MeLi → Li[SnMe5]
synthesis of Cp lanthanoid complexes - salt metathesis
LnCl3 + 3Cp M → Cp3Ln
synthesis of Cp lanthanoid complexes - redox transmetallation
Ln metal + 3CpTi → Cp3Ln