L18

Front 1

Describe what species that vanadium can occur as?

Back 1

Vanadium has rich oxygen chemistry associated with oxyanionand cation species, VO4 3- (vanadate), VO2+ (vanadyl) and VO2 +. These species are oxidising agents, which canreact by one electron transfer steps and V can act as an O-atom donor. In aqueous solutions in the presence of oxygen,vanadium is in the +5, d0 oxidation state as the vanadate species. Vanadate has long been recognised as astructural and electronic analogue of phosphate with similar protonationreactions. The analogy is most evident in the tetrahedral trianionic forms VO43- and PO4 3.

Front 2

What happen to vanadate species H2VO4 - and HV04 2- atneutral pH and draw a picture?

Back 2

They oligermise to form dimeric, trimeric and tetrameric structuresthat are similar to analogous phosphates. (See L18 pg.2 notes to confirmpicture).

Front 3

Describe how hypohalous acids are formed and what their roleis ?

Back 3

VCPO and VCBO enzymes are involved, in the 2e oxidation of thehalide ion by H2O2 to give the intermediate hypohalous acids, which acts as asource X + (X - = chloride or bromide). H2O2 + X - + H + ® HOX + H2O. If anucleophilic acceptor (RH) is present, a reaction will occur in which HOXtransfers X+ to the substrate (AH) and halogenated compounds are produced alongwith water. HOX + RH ®RX + H2O. HOX will react with a broad range of acceptors (RH) to form a diversenumber of halogenated reaction products (RX). These vanadium-containing enzymesare probably responsible for the formation of huge amounts of bromoformproduced by seaweeds and plankton. Some oceans are even saturated with thiscompound and other halogenated methane’s; with the result, that ventilation tothe atmosphere occurs. There they play a role in the regulation of ozone in thestratosphere and have an effect on atmospheric processes, but why algae producethese compounds is not clear.f8"

Front 4

What is the difference between VCPO and VCBO ?

Back 4

The general reactions performed by the chloroperoxidases(VCPO) and bromoperoxidases (VBPO) are shown below: (L18. Pg.3) The VCPOenzymes can also perform the reactions of VBPO since chloride is the moreelectronegative element and thus has a higher ionisation energy than bromide,so therefore chloride is harder to oxidise than bromide. Hence, VCPO has thecapacity to oxidise both chloride and bromide.i

Front 5

Describe the first stage of the reaction mechanism for VCPO ?

Back 5

The catalytic cycle involves protonation ofthe hydroxyl ligand in the apical position of the active site by hydrogenperoxide to generate a coordinate aqua ligand and the HOOanion.

Front 6

Describe the structure of VCPO ?

Back 6

X-ray structural analysis of vanadium chloroperoxidase hasshown that vanadium(V) is bound by oxygen and nitrogen containing ligands. Threeoxygen’s bind in a trigonal equatorial plane and the fourth oxygen is presentas a hydroxyl (OH- ) ligand at one apical position. The N atom from a histidinebonds directly to the metal in the other apical position to complete thetrigonal bipyrimidal structure, see below (H atoms have been omitted). 5 Thecrystal structure has revealed an intricate hydrogen bonding network around thesite which helps to stabilise the 5-coordinate trigonal bipyramidal geometry atthe metal centre

Front 7

Describe the second stage of the reaction mechanism for VCPO ?

Back 7

The mechanism the vanadium centre experiences nucleophilic attack from the HOOanion and generates the 5-coordinate square pyramidal peroxovanadium intermediate with the elimination of 2 molecules of water, one of which is due to the protonation of the hydroxyl ligand in the equatorial plane of the complex. HOO- ® O2 2- + H+ HO- + H+ ® H2O

Front 8

Describe the third stage of the reaction mechanism for VCPO ?

Back 8

This step involves nucleophilic attack of the coordinatedperoxide ligand by the chloride ion to give the oxo and oxychloride ligandwhich both rapidly become protonated by water and an acid. O2 2- + Cl-® ClO- + O2- ClO- + O2- +H+ + H2O ® ClOH+ 2OH- · Thehydroxyl group, generated by lose of H+ from the water, binds to the V centrein a 6-coordinate octahedral species.mrWG3Wf(\3ba

Front 9

Describe the fourth stage of the reaction mechanism for VCPO ?

Back 9

The final stage of the cycle simply involves the loss of thereactive hypohalous acid (HOCl) which immediately reacts with organicsubstrates and halogenates (chlorinates) them. In general these enzymes displaylittle substrate specificity and have been shown to generate a wide range ofproducts. Such reactions have generatedinterest from the chemical industry since they have important practicalapplications in the halogenation of organic substrates under mild conditions.nt-famNba

Front 10

Explainhow monoatomic oxo p-donor ligands stabilize metal-ions in high oxidation states?

Back 10

This is done through oxygen-p to metal-d pi bonding by theopposite flow of electron flow in p-bonding ligands of the CO type, it is not surprising that the latter are moststable in low oxidation states, whereas M=O bonds are most likely in highoxidation states. Where the filled ligand-p orbitals overlap with empty metal dorbitals of appropriate p-symmetryand the combined s and p bonding of the oxo groupwith metal dz 2, dxz and dyz orbitals produces a strong axial ligand field, seepage 8. The effect of a short multiple V=O bond and the resulting p- donation by the oxop-orbitals is to raise in energy the dorbitals with a z component. The effectof oxygen-p to metal-d p-bondingis to remove the degeneracy of the t2g (d pset) and make the dxy orbital the single occupied HOMO level for a d1 , O=V2+centre.

Front 11

Draw the reaction mechanism for VCPO ?

Back 11

Check on handout 18

Front 12

How many d-d transitions does the O=V2+ ion have ?

Back 12

The O=V2+ ion has three d-d transitions in the electronicabsorption spectra which correspond to the transitionsbetween:dxy dxz, dyzdxy dx2-y2dxy dz29

The presence of these three d-d bands in the electronicabsorption spectra provides a distinctive spectroscopicfingerprint or signature that confirms the presence of aO=V2+ centre. Since the strong axial ligand field created by the pi-bondingof the oxo ligand means that the HOMO orbital is always thedxy, this orbital is the Redox Active orbital. Oxidation from VIV to VV will require the removal of thesingle unpaired electron from the dxy orbital to form VV- d0. Hence, the electronic structure (d orbital splitting) created bythe oxo ligand impacts on the redox properties and reactivityof the O=V2+ ion

Front 13

Describe the properties of amavadin ?

Back 13

The molecular weight was determined to be 415. Elementalanalysis showed the presence of carbon, hydrogen, nitrogen and vanadium, andagreed with the empirical formula – C12H18N2O10V. Electronic absorptionspectrum showed three peaks in the visible region at 775, 715 and 565 cm-1 ,consistent with the d-d transitions of a d1 , O=V2+ centre. The infra-redspectrum contained strong bands in the region 1600-1650 cm-1 assigned tocarboxylate modes and one band at 985 cm-1 assigned to u(V=O). EPR spectroscopy wasconsistent with the presence of a paramagnetic d1 , V IV centre. Titration withsodium hydroxide indicated the presence of 2 free acid groups per molecule. Ձba

Front 14

Name and draw the ligand present in amavadin ?

Back 14

(H3hidpa)

Front 15

Describe the structure of amavadin ?

Back 15

Each hidpa ligand behaves as a tetradentate ligand via thetwo carboxylate oxygen’s, the nitrogen and the oxygen of the hydroxy-iminogroup. The N-OH group of each is deprotonated and forms a unique h 2 -N,O linkage to thevanadium forming a triangle between the V-N-O atoms. The structure contains twoh 2 -N,O bonds mutuallytrans to one another bound at the metal. The ligand is deprotonated at 3 sitesupon coordination and therefore each hidpa carries a -3 charge (tribasic), withtwo ligands bound to the metal in this fashion the overall charge is 2 xhidpa3- = -6.lXxA6B?b

Front 16

What is the oxidation state of vanadium in amavadin ?

Back 16

The vanadium is in the +4 oxidation state; hence thecomplex has a residual charge of -2. This is balanced by two protons, which act as counter cationsand the formula of the compound is H2[V(S,S-hidpa)2].