Redution of LNiF eCl

The distint isolated redution event of LNiFeCl reommends itself for its redution

to ferrous LNiFe. Besides eletrohemial redution, a variety of dierent typial

redutants for ferri iron porphyrins were used, for example sodium hydrogensulde

(NaHS),ethanethiol (EtSH),sodiumdithionite(Na

2 S

2 O

4

),potassiumsuperoxide(KO

2 )

andobaltoene(CoCp

2 ).

[8,147149℄

To get rst insights into the redution of LNiFeCl , eletrohemial redution was

performed at a potential of

−

^{1.2 V vs. F/F}

⁺

^{under inert onditions, sine yli}

voltammetry has shown a suitable redution wave at

−

^{1.12 V vs. F/F}

⁺

^{(Fig. 5.8)}

whose irreversibilty wasexplained with a oordination hange upon reoxidation (Chap.

5.1). A potential slightly lower wasapplied to assureomplete redution. The reation

wasfollowed withsimultaneousUV-visspetrosopy (Fig. 5.11).

Figure5.11:UV-visspetrumoftheeletrohemialredutionof LNiFeClin CH

2 Cl

2 at

am-bient temperature with [Bu

4 N℄PF

6

aseletrolyte at a potential

−

^{1.2 V vs. the}

redox oupleF/F

+

. Data was olletedevery5seonds. Isosbesti points are

markedwithblakirles

LNiFeCl getsrapidly redued to LNiFe and theredution isompleted within 90

se-onds. The redution resulted in a olor hange from dark brown to green. The rst

Q-band at 535 nm vanishes, indiating this band to have some iron(III) ontribution.

Thus,this delineinabsorptionanbetaken asa goodindiator fortheoxidationstate

of the iron nikel omplex. Furthermore the isosbesti point (Fig. 5.11) indiates a

leanonversionfromone speiesintoanother. LNiFe showedrapidreoxidation in

non-oordinatingsolventslikeCH

2 Cl

2

,whenthe negativepotentialwasnotappliedanymore.

Theiron(II)ioniswithinarhombioordinationenvironmentafterredution,duetothe

abseneofasuitable neutralaxialligand. Suhaoordinationenvironmentisunfavored

for iron(II),leading to aninstable omplex. A stabilization waspossible withthe

addi-tion of a oordinating solvent like THF, but reoxidation still ourred rapidly,as ould

be observed inUV-visspetrosopy. Möÿbauer spetrosopy of theorresponding

solu-tion, obtained from eletrohemial redution, wasnot suitable, due to the tremendous

exess of eletrolyte [Bu

4 N℄PF

6

. Halides areknownto absorb

γ

^{-radiation inMöÿbauer}

spetrosopy, resulting in an enormous drop of the intensity. A leanup of the solution

of LNiFe was not possible due to immediate reoxidation, even under inert onditions

as ould be observed byUV-vis spetrosopy. To obtain a LNiFe solution suitable for

MB spetrosopy,the former mentioned redutants(NaHS, EtSH, Na

2

used on labeled ferriLNi 57

FeCl, to enhane the signal to noise ratio. The benet of

these redutants inomparison withfor example CoCp

2

is their demonstrated

seletiv-ity. Independent of the amount used, these redutants normaly redue ferriporphyrin

omplexes only one.

[8,147149℄

Obviously NaHSand EtSH arestrong ligands, whih

re-sult in deoordination of theiron ion inthe ase of LNiFeCl, asould be observed by

HRMS and UV-vis spetrosopy and exludes these as suitable redutants. However,

withNa

suitableredutantsfor LNiFeClwerefound,asouldbe

distin-guished byUV-vis spetrosopy,whih areeasy to seperate from thesolutionbysimple

ltration. Howeverwiththeaddition ofNa

2 S

2 O

4

,deomposition was observed after

l-tration, whih doesnot ourwith KO

2

. Interestingly, potassium superoxideis suitable

to redueferri porphyrinseventhoughdioxygen isreleased, aslongasan exessof the

reduingagentispresentorthereationisperformedinoordinatingsolvents,stabilizing

the iron(II) speies due to oordination.

[148,150℄

Neverthelessno MB spetrum from the

resulting solution of LNi 57

Fe ouldbe obtainedsofar.

With the stoihiometri use of CoCp

2

the redution of LNi 57

FeCl was suessful and

the resulting solution in THFould diretly be used for MB spetrosopy (Fig. 5.12).

CoCp

2

hasa potential of

−

^{1.33 V vs. F/F}

⁺

^{,[141℄whih is intheperfet range for the}

redution of the iron ionin LNiFeCl (E

1/2

=

−

^{1.12V) and isonly slightly lower than}

thepotential of

−

^{1.2V applied in}eletrohemial redution(Fig. 5.11).

CoCp 2

Sheme5.3: RedutionoftheferrinikelironomplexLNiFeClwithobaltoeneat

−

⁴⁰

^◦

^C.

AsolutionofCoCp

2

indryTHFwasadded toasolutionof LNiFeClindryTHFunder

inert onditions at low temperature (Fig. 5.3). The solution was stirred for 5 minutes

and diretly used for the haraterization and further reations. The resulting UV-vis

spetrumofthesolutionmathedtheoneoftheeletrohemialredution(seeAppendix

Fig. A33). ForMBspetrosopytheorrespondinglabeledomplexLNi 57

FeClwasused

and the reation mixture was transferred into a Möÿbauer sample holder and diretly

frozen.

Figure5.12:Möÿbauerspetrumof labeledferrous highspin LNi 57

Fein THFat 80K with

δ F e

^=1.07and

∆E Q

^=3.81.

The Möÿbauer spetrum learly shows a high spin iron(II) (Fig. 5.12), proving a

su-essfulredutionof LNiFeCl. A highspinstate iron(II)ionan beassigned, due to an

isomere shift of more than 0.9mms

−1

. [82℄

Insolution thehs-iron(II) ion in LNiFe an

eitherbe in otahedral or a rhombi pyramidal geometry. Even thoughboth

oordina-tiongeometries areknown fromferrous highspin ironporphyrins (Tab. 5.3), arhombi

pyramidal geometry isnormallypreferred.

[32,147,150153℄

Table 5.3:Möÿbauer parameter of a seletion of [meso-tetraphenyl-porphyrinato℄iron(II)

(TPPFe) omplexeswithdierentspinstatesat80K.

Complex S

δ F e

^{/ mms}

− 1

∆E Q

^/mms

− 1

Ref.

TPPFe 1 0.50 1.51

[154℄

TPPFe(2-MeHIm) [a℄

2 0.92 2.26

[154℄

TPPFe(py)

2

^{0 0.40 1.15}

[155℄

TPPFe(THF)

2

^{2 0.95 2.64}

[150℄

[a℄2-MeHIm: 2-methyl-3-H-imidazole.

Interestingly,theinnerMöÿbauerdoublet,whihwasalwaysobservedforferriLNiFeCl

(Fig. 5.5),vanished. Thisdisappearane furtherindiates theinner signal(f. Fig. 5.5)

not to be an impurity, beause an impurity would still give a Möÿbauer signal after

redution, likelydierent to theoneof LNiFe.

Intriguingly,the innerdoublereappearswhenLNiFeisreoxidizedbytheadditionofthe

oxidant m-CPBA(Fig. 5.13).

Asolution ofm-CPBAinTHFwasaddedto the solutionof reduedLNiFe, transfered

into a MB sample holder and diretly frozen like before. When a substoihiometri

amount ofthe oxidant wasadded, amixture of LNiFe (green) andLNiFeCl (blue and

orange)wasobserved (Fig. 5.13(a)), thatturnedintothetypialtwodoubletsregularly

observed for LNiFeCl when theamount was inreased up to one equivalent (Fig. 5.13

(b)). Thepreviously observed ratio of20:80 isnearly reovered(30:70).

(a) (b)

Figure5.13: (a)Möÿbauer spetrum of isotopially labeled ferrous high spin LNi 57

Fe after

reoxidationwithsubstoihiometri(

δ F e

^{=0.53,0.64and1.07;}

∆E Q

^=1.33,2.82

and3.76)and(b)withstoihiometriamountofm-CPBA(

δ F e

^{=0.53and0.64;}

∆E Q

^{=1.07and2.85.) inTHFat80K.}

Table5.4:Möÿbauerparameterof LNiFe, LNiFeClandLNiFeafter reoxidationwith

sub-(a) and stoihiometri (b) amount of m-CPBA (f. Fig. 5.15) in frozen THF

solutionat80K.

Complex

δ _{F e} ∆E _Q

^{rel. Int. / %}

LNiFe 1.07 3.81 100

LNiFeCl

0.46 1.01 20

0.54 2.79 80

Reoxidation(a)

1.07 3.76 18

0.53 1.33 18

0.64 2.82 64

Reoxidation(b)

0.53 1.07 30

0.64 2.85 70

The slight dierenes in ratio, isomer shift and quadrupole splitting an be explained

with possible side reations and/or deomposition, supported by the loss in intensity

from the partly reoxidized (Fig. 5.13 (a)) to the fully reoxidized spetrum (Fig. 5.13

(b)). Ifthereationmixturewasstirredforalongertimeaftertheadditionofm-CPBA,

deomposition ould be observed in UV-vis spetrosopy. The reovery of the two

doubletsindiates onemore theinner doubletto bepart of LNiFeCl.

With ferrous LNiFe in hand, rst preliminary reativity studies were performed

(Chap. 7) and the diret synthesis from the free base Siamese-twin porphyrin was

performed (Chap. 5.1.5), due to the now known spetrosopi details and behavior of

LNiFe.

Im Dokument New Derivatives and Iron Complexes of the Siamese-Twin Porphyrin (Seite 56-60)