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Inorganic Chemistry > Main group organometallic chemistry [Mike Hill] > Flashcards

Main group organometallic chemistry [Mike Hill] Flashcards

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1
Q

organometallics

A

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

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2
Q

electron precise

A

octet configuration at metal centre

leads to molecular compounds with reduced lewis acidity/electrophilicity

saturated centre -> inert

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3
Q

electron rich

A

leads to molecular compounds with reduced lewis acidity/electrophilicity

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4
Q

electron poor

A

too few valence e-

provide pair for every bonded atom in a Lewis acidic/highly electrophilic molecule

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5
Q

how do organometallic compounds in groups 1,2 and 13 use its valence orbitals in bonding?

A

addition donors

formation of multi centre bonds + aggregation

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6
Q

what happens to the polarity as the difference in electronegativity (effective nuclear charge) between metal and carbon increases

A

increases (+ reactivity of M-C bond increases)

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7
Q

effect of metal size on M-C bonding

A

larger metal = weaker M-C bond

due to less efficient orbital overlap

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8
Q

which group are π-bonds restricted to?

A

1

heavier elements = σ-bonds only

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9
Q

kinetically labile

A

low energy decomposition energy

highly reactive

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10
Q

how can kinetic lability be suppressed?

A

[increased thermal stability]

via formation of Lewis base adduct or by use of ligands without hydrogens in the position (no β-hydrogen)

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11
Q

3 factors that govern the kinetic reactivity of main group organometallic with water and air

A
  1. high polarity of M-C bond (kinetic)
  2. availability of low-lying empty orbitals at metal (kinetic)
  3. availability of free electron pairs at metal (kinetic)
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12
Q

group 13 + air/water

A

vacant p orbital

water/air sensitive

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13
Q

group 14 + air/water

A

electron precise

water/air stable

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14
Q

group 15 + air/water

A

lone pair

water/air sensitive

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15
Q

M-C bond formation - direct synthesis from metal and organic halide

A

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

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16
Q

M-C bond formation - redox transmetallation

A

M + RM’ -> RM + M’

metal of lower electronegativity (M) replaces metal of higher electronegativity (M’)

M = electropositive group 1,2,13 metal
M’ = Hg

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17
Q

M-C bond formation - metallisation (metal-hydrogen exchange)

A

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

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18
Q

M-C bond formation - salt metathesis

A

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

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19
Q

M-C bond formation - addition reactions

A

[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

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20
Q

lithium + carbon - electronegativity

A

large difference

BUT Li-C bonds = somewhat covalent due to polarising ability of Li+

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21
Q

LiR formation - direct synthesis

A

2Li + RX -> LiR + LiX

high enthalpy of formation of LiX salt = exothermic reaction

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22
Q

LiR formation - transmetallation

A

2Li + HgR2 -> 2LiR + Hg

based upon relative ease of reduction of Hg and Li

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23
Q

LiR formation - metal-halogen exchange

A

[at low temp.]

Li-R + R’-X -> Li-R’ + R-X

if carbanion formed = more stable, equilibrium will be driven to the right

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24
Q

Wurtz coupling process

A

R’Li + R’‘X -> R’R’’ + LiX

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25
Q

LiR formation - metallation of acidic hydrocarbons with organolithiums

A

[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

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26
Q

organolithium structures

A

2 valence electrons - electron-deficient

compensated by formation of multi centre bonds (aggregation)

tend to form oligomeric aggregates in solution + solids

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27
Q

what does the degree of association of organolithium compounds in solution depend on?

A

polarity

coordinating ability of solvent

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28
Q

Li-CH3 in THF or Et2O

A

tetramer

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29
Q

Li-CH3 in TMEDA

A

dimer

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30
Q

Li-n-C4H9 in hexane

A

hexamer

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31
Q

Li-n-C4H9 in Et2O

A

tetramer

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32
Q

Li-t-Bu in hexane

A

tetramer

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33
Q

Li-C6H5 in THF or Et2O

A

dimer

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34
Q

Li-CH2C6H5 in THF or Et2O

A

monomer

35
Q

what happens to the reactivity as the polar nature of Li-C increases?

A

reactivity increases

36
Q

reactivity of organolithiums - metallation of CH, NH, OH

A

[i.e. as a base]

EH + RLi -> ELi + RH

EH = stronger acid than RH

37
Q

reactivity of organolithiums - addition to multiple bonds

A

[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

38
Q

reactivity of organolithiums - reaction with transition metal/p-block halides

A

[produces salt]

e.g. 4PhLi + SiCl4 -> SiPh4 + 4LiCl

salt metathesis = thermodynamic driving force

39
Q

heavier alkali metal organometallics

A

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

40
Q

polyhapto n^n interactions

A

organic anion with metal via >1 interaction

41
Q

group 2 oxn states and general properties

A

[alkaline earth metals]

+2

ionic radii smaller than group 1 in same period

all elements (M0) highly electropositive/reducing

42
Q

what happens to the reactivity of the M-C bond with increasing electropositivity of the metal?

A

becomes more reactive

43
Q

Be

A

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)

44
Q

Grignard reagents

A

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

45
Q

Rieke method

A

= extremely reactive form of Mg

MgCl2 + 2K —-> Mg + 2KCl -> (RX) RMgX

finely divided (high SA) black powder

46
Q

Grignard reagent structures - solid state

A

solvated by ether (4 coordinate Mg centres)

less heavily solvated oligomers feature halide bridges

47
Q

dialkylmagnesium compounds - synthesis

A

[redox transmetallation of Mg metal with diorganomercury compounds]

Mg + R2Hg -> R2Mg + Hg

48
Q

dialkylmagnesium - bonding

A

sp3 hybridised bridging methyl group

3-centre-2-electron bonds to sp3 metal

49
Q

R2Mg + water/protic reagents

A

[eliminates alkanes]

R2Mg + R’OH -> RMgOR’ + RH

50
Q

reactivity of organomagnesium compounds - alkylating/arylating for main group and transition metal halides

A

source of R- (from grignard)

formation of magnesium dihalides = thermodynamic force

e.g. SbCl3 + 3CH3MgX -> (CH3)3Sb + 3MgXCl

Mg-C = less polar + reducing

51
Q

organometallic compounds of heavier alkaline earth metals - Ca, Sr, Ba

A

far more reactive

catalysts - sustainable, common, cheap (compared to Rhodium, Palladium, Platinum)

52
Q

organometallic compounds of heavier alkaline earth metals - Ca, Sr, Ba - structural studies

A

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

53
Q

group 13 organometallics

A

+3 oxn state

stabilisation of lower oxn state (+1) as you go down group -> inert pair effect

54
Q

R3B synthesis

A

[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

55
Q

R3B properties

A

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)

56
Q

R3Al lab prep

A

[metathesis with RLi or RMgX]

AlCl3 + 3t-BuLi -> t-Bu3Al + 3LiCl

[transmetallation]

2Al + 3HgPh2 -> 2AlPh3 + 3Hg

** 2:3 stoichiometry **

57
Q

structures of R3Al

A

> 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)

58
Q

R2AlX and RAlX2 prep.

A

[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

59
Q

reactivity of R3Al

A

organoaluminium compounds = hard acids (6 valence e-)

readily form adducts with bases

hard donors

60
Q

role of methylaluminoxane (MAO)

A

acts as a co-catalyst for alkene polymerisation

61
Q

RnMX3n-1

M = Ga, In, Tl

SUMMARY

A

prepared the same as R3Al

halides RnMX3-n most readily prepared by redistribution reactions

reactivity of R3Ga and R3In similar to R3Al

62
Q

RnMX3n-1

M = Ga, In, Tl

STRUCTURAL CHEMISTRY

A

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

63
Q

preparation of thallium (I) cyclopentadienyls

A

[v. toxic]

Tl2SO4 + 2C5H6 + 2NaOH -> 2(C5H5)Tl + Na2SO4 + 2H2O

64
Q

what type of ligands is best for lighter group members?

A

bulky ligands

helps prevent disproportionation of monovalent metal (M0 -> M3+)

65
Q

group 14 organometallics

A

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

66
Q

what are tetravalent group 14 organometallics used for?

A

[technological applications]

silicones

organotin compounds

lead alkyls

67
Q

Rochow process

A

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

68
Q

organosilanes

A

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

69
Q

polyorganosiloxanes (silicones)

A

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

70
Q

Si-O bond enthalpy

A

466 kJ mol-1

71
Q

flexibility of Si-O-Si link

A

delocalisation of lone pairs on oxygen into available σ* orbitals on Si

reduces directionality of Si-O bond + makes structure more flexible

72
Q

heavier group 14 (Ge, Sn, Pb) organometallics

A

+4

stabilisation of lower oxn state as you go down group

inert pair effect -> tendency of 6s2 electrons remained paired

73
Q

organogermanium compounds

A

similar to silicon compounds - due to shielding effect of filled 3d shell

much less common -> low terrestrial abundance of the element

74
Q

R4Sn structure

A

tetrahedral - sp3

air/moisture stable

75
Q

R3SnX

A

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

76
Q

organolead compounds

A

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

77
Q

group 15 (P, As, Sb, Bi) organometallics

A

+3 and +5

stabilisation of +3 increases as you go down group -> inert pair effect (tendency of the ns2 electrons remain paired)

78
Q

group 15 R3E compound shape

A

pyramidal with stereochemically active lone pair

79
Q

σ-donor capacity of phosphine ligands

A

PH3 < PRH2 < PR2H < PR3

due to inductive effect of alkyl sub.

80
Q

π-donor capacity of phosphine ligands

A

[depends on availability of low energy σ* orbitals]

PF3 > PCl3 > P(OAr)3 > P(OR)3 > PAr3 > PAr2R > PR3

81
Q

R5E shape

A

trigonal bipyramid in solid state

EXCEPTION = pentaphenylstiborane, Ph5Sb = pyramidal

82
Q

phosphazenes

A

family of compounds constructed from P-N linkages

83
Q

phosphonitrilic dichloride (PNCl2)3

A

earliest reported inorganic heterocycle

nPCl5 + nNH4Cl -> (NPCl2)3-10 + 4n HCl

130°C

84
Q

phosphonitrilic dichloride (PNCl2)3 - structure

A

[cyclic trimer]

planar six-membered ring

all P-N bonds = identical (shorter than normal => suggested multiple/delocalisation)

Inorganic Chemistry (15 decks)

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