Anthraquinone- 2-sulphonate sensitized reactions of pentacyanonitrosylferrate(II), [Fe(CN)₅NO]^2-

Anthraquinone- 2-sulphonate sensitized reactions of pentacyanonitrosylferrate(II),

[Fe(CN)5N0 ]2-

K S Sidhu*, W R Bansal &Sumanjit Department of Chemistry, Punjabi University,

Patiala 147 001, India

Received 22 October 1993; revised and accepted 21 July 1994

Evidence is presented for energy transfer from trip- let anthraquinone- 2-sulphonate,

e

AQS), to pentacya- nonitrosylferrate(II), (NP), in aqueous medium at dif- fusion rate to yield [Fe(CN)5H20j2- (LSP-ligand sub- stituted product). In the presence of hydrogen donor, 2-propanol, [Fe(CN)4N0j2- (TNF-tetracyanonitrosyl- ferrate(II)) is formed in proportion to'[IPA] at the cost of LSP. Ground state AQS' does not interfere with the various processes for the quenching of 3AQS and dis- solved oxygen does not compete with NP for the quenching of 3AQS, but removes the intermediate rad- ical AQSI-i responsible for the formation of TNF. Ex- perimental results for the competitive quenching of

3AQS by NP and IPA are shown to fit into the de- rived rate expression based on the proposed mechan- ism.

Photochemistry of the title ion has been investi- gated in aqueous medium f-or many years'r' whereas the investigation of the reaction in non- aqueous medium is of recent originv'. In all these studies pentacyanonitrosylferrate (II), is both the absorber and the disappearing species and as a result it has not been possible to quantitatively ex- amine the various pathways of the reaction. We have reported two studies+"! where we have used photosensitizers and elucidated the transition state which leads to ligand substitution reaction and the mechanism for the total reaction. It has also been brought out that the solvent plays an important role. In these photosensitized studies, alcohols constituted the medium and the results could not be compared with the direct photolysis studies in aqueous medium. Herein we report the results of sensitization in water with anthraquinone-2-sul- phonate (AQS) whose triplet state is at 257.7 kJ mol-^{l, ~ISC}

>

0.9 (ref. 7) and which is known to transfer energy in many systems"^-12 and undergo reaction with high quantum yields.

Notes

Experimental

Sodium pentacyanonitrosylferrate(II) dihydrate, (BDH, India) and sodium anthraquinone-z-sul- phonate, (AQS), (K&K, New York) were repea- tedly crystallized from water and aqueous ethanol respectively. All other materials were purified by known methods. AQS absorbs 366 nm line of mercury 30 times strongly as compared to NP (nitroprusside ion) and therefore, sensitization through AQS was carried out under conditions si- milar to that of benzophenone (BP) sensitization.

Photolysis were carried in a pyrex cell (1 x 1 x 6 em"), The other experimental and analytical and monitoring procedures have been described else- where''.

For deoxygenation, system was flushed with pu- rified nitrogen and potassium ferrioxalate acti- nometer was employed for measuring light inten- sity.

Results and discussion

Spectral changes were observed on the photoly- sis at 366 om of aqueous solution of NP (1 x 10- 2 mol dm - 3) without and with AQS (2 x 10- 3 mol dm - 3). It has been established that the product formed in the direct photolysis of NP with ^Amax at 395 om is [Fe(CN)sH²

0F-

(LSP- ligand substituted productj-'. There is a slight shift in the absorption spectra from 395 to 400 om in the presence of AQS. In the absence of any other competing process water has been shown to quench triplet AQS (k= 1 x 10^{7 S-I)} to give hy- droxylated products with quantum yields

=

0.02 (ref. 14). However, the products of photolysis have been measured only in basic medium. To ex- amine any interference of the hydroxylated pro- ducts in monitoring the products in the present system, AQS was photo lysed in water in a blank run. The linear increase in optical density at 400 om with the time of photolysis in Fig. (1) indi- cates that the quenching of triplet AQS by water does not interfere with the measurements of the product formed by the quenching of triplet AQS by NP. It is to be further noted that the extent of reaction with water has to considerably decrease in the presence of NP. Results in Fig. (1), where the ratio of total light absorbed in curve (II) and that of in curve (I)

=

2.6 and in curve (II) more than 90% of the total light is absorbed by AQS

t

^0.10

0.30 ₀

0

I 11

0.06 0 5 10 15

-minut.s~

0 0 0.04

0 ci

0.02 m

<.

0

400 500 600 700

10 20

Tim •. (minuttsl

30

Fig. I-Plots of 0.0. versus time of photolysis. at 400 nm un- der different conditions. (Solvent-water, [NPIo-l x 10-² mol dm-3, 10 = 26.1 X10-6 Ein 1-1 min-I. Curves I to III are for NP only, containing both NP and AQS, and AQS only re-

spectively).

only, clearly show that AQS is sensitizing the reaction of NP.

In the presence of hydrogen donors, triplet AQS produces AQSfi like BP by C

=

0 - C - OH mechanism 7. Wells has deduced the reactivity of various alcohols with triplet AQS relative to the deactivation of triplet AQS by other paths includ- ing that of by water". The ratio of these two rates for IPA has been given to be nearly unity. Figure 2 shows the spectral changes in deoxygenated so- lution of NP (1 x 10 -² mol dm - 3) containing AQS (2 x 10-3 mol dm-3) and IPA (1.31 mol dm= ') in water. The maximum corresponding to tetracya- nonitrosylferrate(D), (TNF), grows with time and the increase in O.D. at 610 nm is linear as shown in the inset plot of Fig. (2). The variations in quantum yields of LSP and TNF at different con- centrations of IPA are given in Table 1. trNF in- creases from zero to one and ^~LSP decreases from 0.12 to zero when [IPA] changes from zero to 1.31 mol dm - 3in water.

Data in Table 2 show the effect of changes in concentration of NP. For low concentration of NP

«2.5 ^X 10-³ mol dm="), quantum yield of TNF formation (tn.F) remains nearly constant and +Up is zero. But for higher concentration of NP

(>

2.5 x 10 - 3 mol dm - 3), ~ decreases from 1.02 to 0.44 and ^~LSP increases from 0 to 0.08.

A( nm)

Fig. 2-Spectral changes on photolysis of NP and AQS in water containing IPA. ([NPlu= I x 10-, mol dm-.1.

[AQSj=2 X 10-3 mol dm-3, [IPAj= 1.31 mol dm-J Curves I to III are after O.8 and 24 minutes of photolysis respectively.

Inset figure shows the plots of 0.0..at 610 nm versus time of photolysis).

Table I-Effect of [IPA! on the quantum yields of LSP and TNF

Solvent = water, [NPIo= 1 x 10-3 mol, dm - -', IAQS1= 2 x 10-.1

mol dm-.l lo=26.1xlO-6 Ein.-I min-I. !,;'P=0.22 x 10-6 Eln I-I min-I. r.os-25.9X 10-1> Einl-I min-I. yield (di- rect)=0.07 x 10-6 Mmin- I.

[IPA! Yield x 106(Mmin-I) Quantum yield (f)

(mol dm" '] _

Total Sensitized" TNF LSP TNF Sensitized

0.0 3.13 3.06 0.12

0.131 2.66 2.59 5.84 0.10 0.23

0.393 1.36 1.29 13.21 0.05 0.51

0.655 1.11 1.04 18.65 0.04 0.72

1.048 24)8 0.94

1.310 26.42 1.02

Sensitized: The yields arising from the interaction with.lAQS.

The observed spectral changes are in accordance with the predominance of energy transfer route at higher NP concentrations.

In view of the known mechanistic features of AQS and the observations with respect to BP sen- sitizations. The results can be understood in terms of the following mechanism:

AQS_3AQS (la)

3AOS kl (water~AOS (1)

k,

3AQS

+

AQS .; 2AQS . .. (2)

Table 2-Effect of[NPJ on the quantum yields ofLSP and TNF

solvent = water" [AQS] = 2 xlO-3 mol dm -\ 10= 26.1 X lO-6 Ein I-I min-I, [IPAj= 1.31 mol dm --', 4jld"ecl = 0.32 lax 10". (Ein I-I min-I) Yield xlO",(Mmin-l) Quantum yield (4jI)

LSP TNF

Sensitized

[NP]X IOJ NP AQS ^Tot(l_~ Direct Sensitized TNF

moldm-J

0.5 0.18 25.92 26.27

1.0 0.22 25.88 26.43

2.5 0.61 25.49 0.45 0.20 0.25 23.96

5.0 1.08 25.02 1.10 0.35 0.75 19.76

10.0 2.07 24.03 1.86 0.66 1.20 15.07

20.0 3.84 22.26 3.01 1.23 1.78 9.79

·Direct: The yields arising from the direct absorption of incident radiations by NP.

·"Sensitized: The yields arising from the interaction with3AQS.

1.01 1.02

0.01 0.94

0.D3 0.79

0.05 0.62

0.08 0.44

k3

3AQS + NP ^-+ NP*

k4 NP" ^-+ LSP

ks

NP" ^-+ NP

... (3) ... (4)

· .. (5)

· .. (6)

· .. (7) ... (8) ... (9) k6

3AQS + AH2 ^-+ AQSH + AI-i k7

AH + AQS ^-+ AQSI-i + A k8

NP+AQSI-iJAQS- ^-+ TNF+AQS k9

AQSI-i + AQSH ^-+ AQSH2 + AQS

Step (1) is deactivation of triplet AQS7.J1. Rate constant for step (1) has been reported to be 7x 10^{6 S - I} by Hulme et al:¹⁶ and later on has been modified to 1.0x 107^{S - 1}by Loeff et all,It has been shown that decay rate of triplet AQS is not affected by the AQS concentration up to 10 - 2 mol dm -3 and hence the upper limit for the reac- tion between triplet AQS and ground state AQS in both water •and^l acetonitrile has been put at

<lOs M-^{1 S-I} (ref. 7). The maximum concentra- tion of AQS in the present work is 2 x 10 -³ mol dm -³ and thus step (2) is unimportant. Step (3) involves exothermic energy transfer and is expect- ed to be diffusion controlled. Steps (4), (5), (7) and (8) are supported by BP sensitisation'. Ratio of the constants k6 (different hydrogen donorj/x, (H²0) is in the range 0 to 4.85 (ref. 15)..The sec- ond order decay rate constant of AQSH (Step 9) has been approximately reported to be 5.5 x lOs M-¹ S-I (ref. 12).

The scheme gives the quantum yield of TNF formation:

trnF(kl +k2[AQS]+k3[NP] +k6[AH2]Xk8[NP] +2k9[AQSHj) ... (i)

which yields,

1 1 k,

+

k²[AQS]

+

k³[NP]

_=_+ ...

(ii)

+-rnF 2 2k6[AH2]

as 2k^9[AQSI-i]

< <

ks[NP].

A plot of this expression is given in Fig. (3). The intercept

=

0.55, which is in good agreement with (ii),The slope is

k,

+

k²[AQS]

+

k³[NP]

0.55 ... (ill)

2k6

By substituting the values of rate constants as discussed above and the value of [NP]=1x 10 -³ mol dm -³ and [AQS]=2x 10 -³ mol dm -^3, gives k3

=

1.1^X 10⁹ M -1^{S - I.} It is to be noted that the corresponding value of rate constant for BP sensi- tisation is 4.9

±

0.5 x 10⁹M-^{1 S-I.}

For a Stem Volmer plot:

1 k³[NP] kl

+

k²[AQS] 1

--=

+ +-.

+-rnF 2k6[AH2] 2k6[AH2] 2

Figure 4 shows the plot at constant AH2 and AQS concentrations. Slope of the plot gives.

... (iv)

k3 66.7 , ." (v)

2k⁶[AH^2]

which gives k3

=

3.5^Xi0⁹ M^{-I S-I} and inter- cept=0.9 in agreement with the predicted value by expression (iv). The difference in the values of 'k³ from expressions (Hi) and (v) can be attributed to medium effect.

Concentration of AQS was varied under the conditions favourable for hydrogen abstraction. It confirms that the product does not change with AQS concentrations. Data in Table 3 show that chNF is equal to nearly unity for all concentrations of AQS, and LSP is not formed. It supports that ground state AQS does not interfere with the var- ious processes for the quenching of triplet AQS at least upto its concentration=4 x 10-³ mol dm-^3.

This is in agreement with the earlier reported re- sults^7.

Figure 5 gives the effect of dissolved oxygen in the system containing no hydrogen donor. LSP is the only product and its yield is not affected by

4.0

3.0

1.0

°0L---L---~2----L---i4--~--~L---L---4

~IPA)(M-l)

Fig. 3-A plot of l/chNF versus I/[IPA]. (Solvent = water, [NP]0=lxlO-3 mol dm-3, [AQS]=2xlO-3 mol dm"? and

1,,=26.1 x 10-6 Ein I-I min-I)

2.0

10 3

INP]X10M

20

Fig. 4-A plot of lIchNF versus [NP] (Solvent = water, [AQS]=2xlO-3 mol dm-3, {IPA]=1.31 mol dm-3, and

1.,=26.1 x 10-6 Ein I-I min-I).

ci.;

---,

I

O'()6

0.04

0.Q2

\.'¥----m II I

OL---~40~O---4~50~---~55-0---~

A(nm)

Fig. 5-Effect of dissolved oxygen in the [AQS] sensitization of NP in water ([NP]"= 1 x 10-2 mol dm-3, [AQS]=2 x 10-3

mol dm-.1 1,,=26.1 x 10-⁶ Ein I-I min-I. Curves I to III are for [02]x 104= 13.25, 0.0 and 2.65 mol dm -.1 respectively and after 20 minutes of photolysis. Inset figure shows the plot

of O.D. at 400 nm vs time of photolysis.)

Table 3-Effect of [AQS] on the quantum yields of LSP and TNF Solvent = water; [NP]o= 1 x 10 -.1mol dm -'; 10= 26.1 x 10 -61Ein 1- I min - I

I.X 106(Ein 1- I min - ') Yield x 10\ (M min - ') Quantum Yield (</»

[AQS]XW NP AQS LSP TNF LSP TNF

mol dm-3

0.0 1.21 0.39 0.32

0.2 0.93 10.95 10.26 0.94

0.6 0.59 20.82 20.26 0.97

1.0 0.42 24.02 23.68 0.99

2.0 0.22 25.88 26.42 1.02

4.0 0.11 25.99 25.63 0.99

oxygen which is contrary to previously reported results where oxygen has been shown to affect the intermediates and products'P'". It is apparent that in the present system oxygen does not compete with NP for the quenching of³AQS.

In the presence of [IPA]= 1.31 mol dm-^3, quenching of the formation of 1NF is observed. It is obvious that oxygen, which does not quench

3AQS, removes the intermediate radical (AQSH) which yields TNF.

AQSH

+

O^2-+ AQS

+

H0²

Oxygen is known to decompose TNF, but the spectrum of the decomposed product previously observed is different from the one observed here", This observation eliminates any other possible role of oxygen.

Acknowledgement

We are thankful to the CSIR, New Delhi for awarding SRF to Ms Sumanjit.

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