thomas - coordination chem Flashcards
Why are crowns soluble in lyophilllic and hydrophillic solvents?
They can rotate oxygen lp
what does it mean by “crowns can lipophilise alkali metal ions”?
Salts such as KCl and KOH and MnO4 become soluble in solvents like toluene by addition of crown ethers cordinating.
why do the first 5 crown ethers show a strong affinity for potassium ions?
Increase in ring size means there are more donor groups therefore more coordination interactions with metal ion which increases enthalpy.
how does the fact that the free energy of solvation for potassium is less than it is for other metals such as Na, Ca, and Li explain with crown ethers have a stronger affinity for K.
Coordination sphere around K has to be reomoved to allow access to metal ion
Large sphere and a +1 interaction with the solvent means that the coordination sphere is weak and is therefore easier to remove.
How does the size of the ion explain why crown ethers have a stronger affinity for K ions.
K is right size to form chelating rings and crown ether is less constrained when coordination to larger K ion
why do cryptands show selectivity towards metal ions?
The rigidity of cryptands means that they cant flex around metal ions that are smaller than their cavity or accommodate larger ions.They show a cavity diameter effect.
Why do spherands show high binding affinities in comparison to their acylic analogous?
most rigid. The donors group in the correct geometry for metal binding. Highly selective and undergo no conformational change.
how do crowns and cryptands bind to complex cation eg NaH4+?
by H bonding
How do crowns and crows bind to NH4+?
Via O—-H-N interaction to give a perching complex with the cation sitting above the crown
Why do crpytands bind NH4+ over K?
Can form 4 tetraherdally arranged N—–H-N interaction. K is too small to fit
what is the template effect?
a metal ion or another centre with definite sterochem and electronic properties that acts as a mould for forming a product that is difficult or impossible to make
describe the kinetic template effect
Competition between intra mol and inter mol reaction.. The metal ion facilitates the intra mol reactions, and provides a site for reactants to assemble themselves in the correct geometry to produce the target product structure.
Describe the thermodynamic template effect
coordination to the metal pushes equilibrium to the desired product. The reaction then proceeds via the kinetic template effect.
Why is synthesis of cryptands synthetically demanding?
they are made in several steps and unsymmetrical cryptands are accessible.
Describe the process of high dilution
A few g of each reactant is dissolved in 100 mls of solvent slowly and mixed in a flask with 5 ml of solvent.
Intra mol - 1st order with low conc (high dilution)
inter mol - 2nd order with high conc (low dilutions)
Is the oxidation of phenol in toluene with KMNO4- possible?
Yes but only with [18]- C- O6
why is CO3- a strong base in organic solvents?
Crowns and cryptands are phase transfer catalyst - they are able to transport species from one phase into another from aq solvents into non aq solvents.
true or false: A crown can not facilitate the reaction of hydrophilic inorganic salts with lipophilic organic compounds?
false - it can by phase transfer catalysis
why do solvated anions have a high activation barrier?
In aq solvents ions are solvated by water molecules and these have to be removed
In organic solvents anions and cations pair via electrostatic interactions.
Why is an anion activated in the presence of crowns and cryptands?
They are large lipophilic cations so do not pair up. In non polar organic solvents anions are weakly solvated. This bare anion is activated for any reaction involving base and nucleophiles.
what is monesin and what are the two forms of it?
Monesin is a pH switchable binder and is an antibiotic.
There is a linear non bonding and a cyclic Na+ binding from upon deprotonation of terminal acid groups.
how can crown ether derivatives behave as ionphores?
clue monesin mimics
Podands with low affinity for alkyl metal ion eg Na+ and K+. The deprotonated H bonded macrocycle binds to these metal ions with high affinity and can carry metal ions from ag sols across membranes into acid solutions.
how does sulphur based redox switches work?
switched using ox and red agents. presence of metal get macrocyle without get podand produces polymeric material
how do ferrocene based redox switches work?
1 arm of cryptand replaced with redox active ferrocene. reversible oxidation to ferrocenium anion.
describe how the CV of ferrocene based cryptand changes with addition of Na ions?
Remain the same with no metal. 0.5 Na assuming high binding affinity you get a 1:1 mixture of the free cyrptand and Na cryptate complex. Two peaks. 1M get exclusively the cryptate complex.
explain the shift in the CV for ferrocene based cryptands?
The more anodic redox process is oxidation of the Na cryptate complex. This is more difficult for electrostatic reasons: The oxidised product has two cations in close contact.
what are butterfly crowns?
these can switch between two binding modes. Trans mode which binds to alkali metals with the similar preferences to crown ethers.
Cis form which binds to larger alkali metals eg K,Rb and Cs. Best for Rb
describe what photoswitchable binding is?
Molecules that photoisomerise. at high energy of light get trans to cis transition. at low levels of light or heat get cis to trans transition
describe how the cis isomer binds in butterfly crowns?
Binds metal ions in an intramol 1:2 sandwich. this is preorganised to bind larger ions.
why does the azobenzene cis form have an almost zero binding affinity but the trans isomer does?
in the cis isomer the binding sites are blocked by intramolecular complexation. in trans it has identical preferences to [18] C- 6.
describe briefly how electrochemical sensoring works?
Receptor supplied by redox couple
sensor output is changes in redox couple
ferrocene unit is used, by attaching it to different macrocycles it can bind to specific cations allowing them to be sensed?
how are K+ ions sensed?
selective electrochemical sensor for potassium. formation of 1:2 sandwich structure, the macocycle binds to K with higher affinity. in MeCN, FC redox couple for Na and K are seperated by 130mv. CV shows in MeCN on K is bound.
how to Trs sensors work?
In water, ferrocene cryptand binds to Ag+ with high affinity producing large shift in FC couple.
how are alkali metal ions sensed with fluorescence sensing?
luminescent portion to macrocycle. flourescence is quenched until binding to metal ion.
why is emission quenched?
in the unbound sensor the lp of the crown are available to quench the excited state of anthracene via PHOTOINDUCED ELETRON TRANSFER. when metal binds the lone pairs are no longer able to quench the excited state via PET.
[15] -C - 5 has a preference for Na over K why?
caused by pseudocryptate structure of the Na + complex, K is too big to fit
How is a gouy balance used to measure susceptibilities?
Involves an accurate balance and magnet. the sample is measured in a cylindrical tube with a magnetic field outside of it. The weight difference the the force exerted by the field. This is compared to a sample where Xmol is known.
how does temp effect magnetic susceptibility on paramagnetic materials?
Follows curie law
X = 1/T = C/T plot T against X^-1 get slope of C^-1
True or false:
The energy difference between alignment with field and against field is»_space; thermal energy at room temp
False the energy difference is «_space;than the thermal energy
what are magnetically dilute materials?
They act as isolated dipoles. They have magnetic centres that do not interact and display simple paramagnetism
what other properties do magnetic materials that show cooperative behaviour exhibit?
Do not obey curies law
In the right conditions will show spontaneous ordering of their magnetic dipoles
what is antiferromagnetism
When orbitals with unpaired electrons interact and eventually spin pairing occurs
what is ferrimagnetism?
Magnetic centres distributed in a lattice with unequal ratios
why does magnetic ordering result in an effect that is seems like ferrimagnetism?
Interactions between neighbouring metal ions in different lattice is more intense than interaction of metal ions in same site.
how to magnetically dilute species interact?
Through bridging ligands providing a super exchange pathway
how can an interaction be fined tuned
depending on the N donor ligand the angle of Cu-O-Cu can turn interaction from anti to ferromagnetic
how does the N donor ligand affect the magentism of Cu-O-Cu?
The N donar ligand produce an open Cu-O-Cu angle where super exchange is mediated by a single oxygen p orbtial. The mixing of two atomic orbitals prodcues three MOs yielding a singlet ground state.
what occurs when the Cu-O-Cu angle is about 90 degrees?
super exchange is mediated by 2 orthogonal ocygen p orbitals. This produces two bonding and antibonding orbitals and leads to a triplet ground state
how can a magnetic interaction be extended?
By selecting suitable bridging ligands and controlling the coordination sphere the interaction can be extended eg to ferromagnetism
describe charge transfer salts
metallocene and (draw structures) react together under right conditions to produce ionic charge transfer salts. E transfer from metallocene d orbital to pi* orbital
what happens to charge transfer salts at a certain temp
the ions in the charge transfer salts are close enough for spin int to occur and at certain temps eg (4.8K) the solid becomes a ferromagnet
true or false: Ionic charge transfer salts are soluble in organic polar solvents. This is the first magnetic material where magnetism is partially due to unpaired p electrons
True
what is hysteresis?
the value of 1 variable depends upon whether the other is increasing or decreasing
what is a soft and hard magnet?
hard magnet - remains magnetised even after field is removed
soft magnet - easily magnetised and demagnetised
when is the first example of a room temp synthetic magnet a hard and soft magnetic?
Hard magnet when the curie temp is less that or at RT. above RT it loses its magnetism
it is a soft magnet when separated from reaction solution.
what is the structure of the first room temp synthetic magnet?
6 anions [TCNE]•– coordinated to V(II) units
how is the first room temp synthetic magnet synthesised?
via CVD to produce thin magnetic layers
V(CO)6 + TCNE —-> V(TCNE)2
what are novel magnetic materials?
Show magnetic properties that are not available to traditional materials
why does PB show magnetism
only Fe(III) has spin active sites but substituting in paramagnetic metal ions into every site
how are CoFe PB analogues magnetic
Fe(III)-CN-Co(III) centre is not magnetic, both metals are low spin. but a charge transfer process occurs within solid when photoexcited which has an effect on magnetism. Fe(III) - CN- Co(II) fe is d5 and ls and Co is d7 and hs
what is the faraday effect
in a magnetic field the material rotates the plane of polarised light by a characteristic angle
when is the faraday rotation observed?
observed at room temp in the ferromagentically ordered phase at wavelengths assoc with the charge transfer band
what does the wavelength of bands depend on (in regards to faraday rotation)?
stoichiometry of the material, which can be varied by rxn conditions.
what types of transition metals show spin crossover?
d4-d7, 1st row metals
how to design SCO systems?
- All SCO systems contain oct Fe(III) metal ions with aromatic N donor ligands
- place subs close to N donors prevents close approach to metal ion. lengths are shorter in LS complexes
- change 6 mem ring to 5 mem ring decreases sigma donation and increases pi acceptor character. In bi and tri complexes this introduces strain into chelate rings
- replace aromatic N donors with aliphatic N donors
what are cooperative effects?
Where you get spin transitions
abrupt spin trs usually are para - dia transitions
true or false: For cooperative and hysteresis effect, there needs to be direct bonding?
False: weak interaction will produce these effects
eg Fe stacking between aromatic groups or H bonding
how do you trap hs states at low temps
Irradiate the sample with light, this traps high spin states at low temp. the states are stable for days
these are called Light induced excited spin state trapping (LIESST)
what is stokes shift?
The difference in wavelength between the absorption and emission
describe the energy transfer in a MLCT
energy used to excite one metal passed to another which emits light
what make the photo excited state of metals different to ground states?
The excited state is simultaneously a reducing agent and oxidising agent. it can perform chemistry the ground state can not do
what does reductive quenching produce?
Ru(bipy)3 in +1 state - powering reducing agent and only generated in ground state
what does oxidative quenching produce?
Ru(bipy)3 in +3 state this is a powerful ox and red agent
how do DSSC work?
Ru chromophor absoribs light and generated excited state. e transfer from excited state to conduction band
advantages of DSSC
relatively High efficiencies
work in low light conditions
can be laid onto plastic conditions
cheap, TiO2 is used
