Indian Journal of ChemistryVol. 28A, November 1989, pp. 958-960

Hydrolysis of diisopropyl phosphorofluoridate catalysed by copper (II)-diamine complexes

D N Marjit & U S Sharma"Defence Research and Development Establishment, Tansen Road, Gwalior 474 002

Received 5 August 1988; revised 19 December 1988; accepted 2 February 1989

The hydrolysis of diisopropylphosphorofluoridate (DFP) has been studied using various diaminecomplexes of Cu salts. The complexes of Cu(II) perchlorate salt have been found to be the most ef-fective catalysts. Lewis acid character of the central Cu metal atom is enhanced by strong anionwhich in turn increases the catalytic activity.

Most of the phosphono and phosphoro esterswhich are very useful insecticides are highly toxicand are also reported as chemical warfare agents.Rapid hydrolysis of such compounds is consi-dered as one of the methods for their effective de-toxification. Wagner-Jauregg et al) studied Cufll}bipyridyl sulphate catalysed hydrolysis of diisop-ropylphosphorofluoridate (DFP) under unbufferedcondition at a constant pH. Augustinsson andHeimburger- described the effectiveness of Cu(II),Ni(II), Ag(I), Au(III), and Pd(lI) salts in accelerat-ing the hydrolysis of N,N-dimethylethylphosphor-ocyanidate (Tabun). Copper (II) nitrate complexof N,N,N',N'-tetramethylethylenediarnine (TMEN)(1:1 mixture) was reported:' to bring down thehalf-lives of DFP and isopropylmethyl phosphon-ofluoridate (Sarin) to 15 min and 1.5 min respect-ively at pH 7.

Menger et al.4 studied the hydrolysis of p-nitrophenyldiphenylphosphate (PNPDPP) and pin-acolylmethylphosphonofluoridate (Soman) cata-lysed by micellar Cu(II) diarnine complex. Theyfound the kobs for the hydrolysis of PNPDPP veryhigh and the half-life of Soman at pH 7.0 was0.85 min. But work so far done in this area suf-fers from certain disadvantages i.e. in some casesthe hydrolysis takes longer time while in othersthe solubility of the catalyst in water is low or thecatalysts are found effective at high pH only.

Both Courtney et al. and Menger et al haveconfined their work to complexes of Cu(II) nitratewith different diarnines or of Cu(lI) chloride withdiarnines having long alkyl chain. Considering thetwo influencing factors, viz. solubility of the com-plexes and electrophilicity of the central metal at-om, presently' we have investigated the hydrolysisof DFP using diamine complexes of Cu(lI) salts

958

with different anions. Diamines of varied chainlengths were also studied.

Materials and MethodsThe complexes CUX2:~ (where X = NO;,

CIO", CCl]COO- and CH3COO-, L=diamine)were prepared by mixing 1:2 molar quan-tities of Cu (II) salt and diamine[N,N,N',N' -tetramethylethylenediarnine (TMEN),1,3-propanediamine (DAP) and N,N,N',N'-te-tramethyl-l,2-phenylenediamine (TMPD)] in 70%aqueous ethanol, recrystallised from aq. ethanoland characterised by elemental analyses. Stabilityof the complexes was studied spectrophotometri-cally by monitoring the UV/visible absorptionpeak of the aqueous solution of the complexes at35°C. The conductivity measurements were carri-ed out using digital conductivity meter Apx-185and the cell was kept in a Julabo F-50 Ultratemp2000 thermostat.

Diisopropylphosphorofluoridate (DFP) was syn-thesised by the reported procedure and its puritywas checked by GLC. The hydrolysis of DFP wasfollowed by measuring the change in conductivityof liberated HF in unbuffered aqueous solution.Generally ionic strength was not maintained andthe reaction kinetics were followed under pseudo-first order condition viz. [metal chelate] > > [DFP].Linear first order plots were always obtained foratleast upto three half-lives. The rate constantswere calculated from the linear plot of In (a/(a-x)versus time. Kinetic runs carried out always in du-plicate gave rate constants with an uncertainity ofless than ± 5%.

Results and DiscussionFrom Table 1 it is seen that kobs values did not

MARJIT et al.: HYDROLYSIS OF DIISOPROPYL PHOSPHOROFLUORIDATE

Table 1 - Pseudo-first order rate constants for the metal-chelate catalysed hydrolysis of DFPComplex 103[ complex] W[DFP] Temp. 103

kOb' Half life Eamol drn "? mol dm-3 °C (1)-1 ) t\(s) (kcal/mol)

Cu(II) perchiorate:TMEN (1:2) 7.5 2.88 35 34.0 20.4 3.05.0 2.88 35 18.0 38.55.0 1.44 35 17.5 39.65.0 0.72 35 17.8 38.95.0 2.88 27 16.0 43.35.0 2.88 17 13.0 53.3

Cu(U) trichioroacetate:TMEN (1:2) 5.0 2.88 43 17.0 40.8 9.05.0 2.88 35 12.0 57.85.0 2.88 27 8.0 86.6

Cu(I1) nitrate:TMEN (1:1) 5.0 2.88 35 3.5 198.0 9.05.0 2.88 27 2.4 288.8

Cu(ll) nitrate:TMEN (1:2) 5.0 2.88 42 14.45 48.0 9.25.0 2.88 35 10.0 69.35.0 2.88 27 6.67 103.9

Cu(II) formate:TMEN (1:2) 5.76 2.88 35 0.75 924.0Cu(ll) acetate:TMEN (1:2) 5.76 2.88 35 0.55 1260.0CU(Il) perchiorate:TMPD (1:2) 5.0 2.88 35 19.0 36.5 3.33

5.0 2.88 27 15.5 44.75.0 2.88 17 12.0 57.8

Cu(ll) perchiorate:DAP (1:2) 5.0 2.88 35 6.0 115.5 9.335.0 2.88 27 4.0. 173.35.0 2.88 17 2.4 288.8

Cu( II) trichioroacetate:DAP (1:2) 5.0 2.88 35 5.0 138.6 8.05.0 2.88 27 3.0 231.0

Cu(II) nitrate:DAP (1:2) 5.0 2.88 35 8.0 86.6 8.25.0 2.88 27 5.0 138.6

change ,with [DFP) (0.72 x 10-3 mol dm "? to2.88 x 10-3 mol dm-3 at 5 x 10-3 mol drn "! ofCu(U) perchlorates; TMEN complex). This alsoshows that the reaction is first order in [substrate].The activation energy values. are also given inTable 1. Half life (t,) of each reaction was calcu-lated from the equation t, = 11k (In a/ta-x),

In phosphate buffer (pH 6 and 7) and borax-boric acid buffer (pH 8) the complexes are un-stable whereas in the TRIS buffer the complexesare very stable but the catalytic activity decreasesatleast 10-15 times. Possioly the amino groups ofTRIS molecules occupy the active sites of the me-tal ion which would normally be accessible to theP= 0 functionality of DFP during the catalyticprocess'.

Effect of ionic strength was studied using differ-ent [KCI] and was found to be negligible on therate of reaction. From Table 1 it is observed thatthe rate of hydrolysis is minimum in the case ofCu(II) acetate-TMEN complex and maximum forCU(Il) perchlorate-TMEN. As we proceed fromcomplex of weak acid salts of copper to those ofstrong acid salts, the rate of hydrolysis increasesmany-fold and is 6700 times faster than that inabsence of any catalyst. In the complexes showinga faster rate of hydrolysis, the Lewis acid charac-

'.o~1-6H

~ '-2",,0 ,,0.~·--L:ur------ ..JL__ ---4~E 0-8 @ t _..;0·6

J·4 -

n[ I

~~76~,~70~t~'~~'~~~2~~2~~6~3~~~3L~~3~~8~'~:2-·~,~1.6~~~~pit"

Fig. 1 - Bronsted type plot of log koo. versus 'pk; (__ trichlor-oacetic acid; D-nitric acid; O-formic acid; .6 -acetic acid)

ter of the central metal atom IS also found to in-crease. This is evident from the Bronsted typeplot (Fig. 1) which shows a linear relationship be-tween pKa of various acids and log of kobs valuesof TMEN complexes of Cu(II) salts of the corre-sponding acids at 35°C.

Earlier reports? showed that only 1:1 com-plexes acted as effective catalysts. It has now beenobserved that 1:1 complexes are not only verydifficult to isolate but also if at all they are isolat-ed, their stability is low in the reaction mediumresulting in high apparent tll2 values. It is clearfrom Table 1 that in the presence of Cu(II) nitr-ate-TMEN (1:1) complex kobs is low relative tothat in presence of Cu(II) nitrate-TMEN (1:2).Further Copper (II) nitrate-TMEN1:1 complex

959

INDIAN J CHEM, SEe. A, NOVEMBER 1989

becomes turbid in aqueous solution. This couldbe due to disproportionation reaction in solutionto form more stable (1:2) complex, which is fur-ther supported by the same values of activationenergies in the reaction catalysed by 1:1 and 1:2complexes. The low kobs values in the case of 1:1complex could be due to the low effective con-centration of 1:2 complex after disproportiona-tion. Presently it has been observed that the aque-ous solutions of 1:2 complexes are very stable at35°C and Amax values do not change appreciablyeven upto 5 days with the exception of Cu(II)perchlorate-TMPD complex which decomposedafter 24 hrs.

The activation energies were calculated andfound to be very low specially in the case of reac-tions catalysed by Cu(II) percWorates-TMEN andCu(II) percWorate-TMPD complexes.

The chain length and substitution on nitrogenof the diamines by methyl groups helped to en-hance the catalytic activity of the complexes(Table 1). Complexes of N,N,N',N'-tetramethyl-1,3-propanediarnine,1 ,4-butanediamine and 1,2-phenylenediamine could not be prepared. In thecase of TMPD only two complexes could be pre-pared. It is difficult to derive a correlation be-tween the chain length and catalytic activity. Thekobs values of TMEN and TMPD complexes arealmost the same; while ethylenediamine or 11.2-phenylenediamine complexes showed hardly anycatalytic reactivity, N,N,N',N'-tetramethyl substi-tuted diarnine complexes are very reactive andeven DAP complexes are quite reactive; thus sub-stitution could playa major role in promoting thecatalytic activity by increasing the basicity re-quired to form a better complex with the coppersalt.

The reaction mechanism proposed by Courtneyet al.' or Menger et al" may be operating in thepresent system also. Menger et al showed thattheir complexes had got catalytic turnover whileCourtney et al. mentioned nothing about the fateof the complexes. UV/visible monitoring of thereaction of Cu(II) trichloroacetate-TMEN com-plex with excess DFP indicates that as hydrolysisproceeds the absorption peak at 570 nm due tothe complex progressively disappears and a newpeak at 680 nm makes its appearance. The 680nm peak is due to Cu(II) trichloroacetate. Similareffect was seen by incremental addition of HF so-lution to a solution of Cu(II) trichloroacetate-TMEN complex. This may be due to the neutrali-sation of amine ligand by liberated HF but thepossibility of decomposition of the complex dur-ing catalytic process, giving CU(Il) salt and dia-

960

l R~OH l Rj

/1'1 I 2 Slow /N ?H2Z Cu(1l)+ OFP ~ Z Cu(U) ••• o OPri

"N I "I '1/R OH2 NO."" .: l'

2 Rz H\ r >; IZ Z H F OPr

TMEN:Z. (CHZ)Z'R= CHl jOAP :Z. (CHZ)].R. H REARIlANGESTMPO: Z. o-C6H4,R.CHJ

Cu (II) Solt-.X Hz" 1-Oi_Ift&hydro_

fluoride + Oilsopropyl phocphoricacid.

SCHEME 1

mine cannot be excluded, In any case the reactionseems to proceed stoichiometrically without givingany ~rnover. ?f the catalyst. The very highcatalytic activity of tetramethyl substitutedethylenediamine and N,N,N',N'-tetramethyl-1,2-phenylenediamine complexes indicate that thesubstitution on the nitrogen atom may help thediamine ligand to come out from the "transitioncomplex" formed between the Cu(II) diarninecomplex and DFP during hydrolysis. Though thecomplex formation between the Cu(II) diarninecomplex and DFP is not evidenced by spectraldata, our study on parathion showed that blueCu(II) diamirie complex forms a green complexwith parathion and the 570 nm peak shifted at-least by 70 nm towards longer wavelength with si-multaneous hydrolysis of parathion; so it wouldnot be unreasonable to think that complex forma-tion occurs prior to hydrolysis of DFP. Fromthese results, the mechanism of Cu(II) diarninecomplex catalysed hydrolysis can be proposed asgiven in Scheme 1.

AcknowledgementThe authors express their gratitude to Dr. P K

Ramachandran for his continuous encouragementand highly valued criticisms incorporated in thismanuscript. Thanks are due to Dr. D K Jaiswalfor providing certain facilities. One of the authors(U.S.) thanks the CSIR, New Delhi for financialhelp.

ReferencesI Wagner-Jauregg T, Hackley B E, Jr, Lies T A, Owens 0

0& Proper R, J Arn Chern Sac, 77 (1955) 922.2 Augustinson K B & Heimburger G, Acta Chern Scand, 9

(1955) 383.3 Courtney R C, Gustaffson R L, Westerback S J, Hyytiain-

en H, Chaberek S C, Jr, & Martell A E, J Arn chernSac, 79 (1957) 3030.

4 Menger F M, Gan L H, Johnson E & Durst D H, J Arnchem Sac, 109 (1987) 2800.

5 Gerald Z, A study of agent-reactive fabrics for use in pro-tective clothing, (Andrulis Research Corporation, Be-thesda, Maryland 20014) Contract Report AR CSL-CR-80002, Dec. 1979, Pag 88.