Base hydrolysis of acetonitrile coordinated to a ruthenium(II)_polypyridine complex

~

Pergamon PII : S0277-5387(96)00447--0

Polyhedron Vol. 16, No. 1 I, pp. 1921-1923, 1997 t~ 1997 Elsevier Science Ltd All rights reserved. Printed in Great Britain 0277 5387/97 $17.00+0.00

Base hydrolysis of acetonitrile coordinated to a ruthenium(II) polypyridine complex Florencia Fagalde, Noemi D. Lis de Katz and N6stor E. Katz* Instituto de Quimica Fisica, Facultad de Bioquimica, Quimica y Farmacia, Universidad Nacional de Tucumfin, Ayacucho 491, (4000) San Miguel de Tucumfin, Argentina

(Received 25 June 1996; accepted 3 September 1996)

A b s t r a c t - - O n coordination to Ru(terpy)(bipy) 2+ (terpy = 2,2':6',2"-terpyridine, bipy = 2,2'-bipyridine), acetonitrile is converted to acetamide in aqueous basic solution, through hydroxide attack with a catalytic factor o f ca 3 x 103 (koH = 4.6 x 10 .3 M t s '). This is a remarkable effect for a d ° transition metal in the (I1) oxidation state, and can be ascribed to 7r backbonding from the metal to the polypyridyl ligands, which makes the ruthenium(II) centre more electropositive than ruthenium(II) in the Ru(NH3) 2+ moiety. The final hydrolysis product is the [Ru(terpy)(bipy)(OH)] + complex, since amides, being poor ~-acceptor ligands, are rapidly released from the coordination sphere of ruthenium(If). @, 1997 Elsevier Science Ltd. All rights reserved. K e y w o r d s : base hydrolysis ; acetonitrile complexes ; ruthenium(II) ; polypyridine ; catalysis ; ~-backbonding.

The hydrolysis of nitrile groups catalysed by transition metal complexes has been extensively studied in aqueous solutions, in part because of the connection to enzymatic proteolytic reactions [1]. Large metalpromoted acceleration of the rate of basic nitrile hydrolysis has been observed, especially when the metal is trivalent. Thus, ConI(NH3)5 enhances the rate of acetonitrile conversion into acetamide [2] by a factor of c a 106, while RuHI(NH3)5 has a catalytic factor of ca 108, over the free ligand value [3]. In contrast, RuH(NH3)5 has little or almost no effect on this rate [3], a result attributed to ~ backbonding from ruthenium(II) into the nitrile group, which increases the electron density at the nitrile C atom and thus makes it less susceptible to nucleophilic attack by hydroxide ion. We have been interested in the general properties of mono- and di-nuclear complexes derived from the Ru"(terpy)(bipy) group (terpy = 2,2' : 6',2"terpyridine; bipy = 2,2'-bipyridine) [4]. In a recent paper [5], we reported a nitrile hydrolysis reaction induced by oxidation of the dinuclear complex [(terpy)(bipy)Ru"(4-CNpy)Ru"(NH3)5)] 4+ (4-CNpy = 4-cyanopyridine). There, we attributed the observed rate enhancement to an inductive effect of the Ru" (terpy)(bipy) moiety acting as a remote substituent. Now, we wish to report on a rate increase owing to

* A u t h o r to w h o m c o r r e s p o n d e n c e s h o u l d be a d d r e s s e d .

an electronic effect caused by direct coordination of a nitrile group to Rull(terpy)(bipy), the catalytic factor being remarkable for a divalent transition metal complex.

EXPERIMENTAL Syntheses

The previously reported PF6 salt of the [Ru (terpy)(bipy)(NCCH3)] 2+ ion was prepared by a modified technique [6]. A solution of 83 mg of [Ru (terpy)(bipy)(C1)] (PF6)], prepared as described in ref. [7], dissolved in 30 cm 3 of MeCN/ethylene glycol (2 : 1 v/v) was refluxed for 2 h. Then, the solution was rotoevaporated to 8 cm 3, cooled and sorbed on to a neutral alumina column, previously equilibrated with M e C N / t o l u e n e ( 1 : 2 v/v). M e C N / t o l u e n e was used as an eluent, followed by pure M e C N . The last fraction was collected and the solvent was removed by evaporation. The solid residue was dissolved in 10 cm ~ of water and then 1 g of NH4PF 6 was added. The redorange precipitate was filtered off, washed with cold water and diethyl ether and dried in vacuo over P40~0. It was recrystallized from acetone/ether. Yield : 75 mg (74%). The B r - salt of the ion [Ru(terpy)(bipy) (NCCH3)] 2+ was obtained by dissolving 50 mg of the PF6 salt in 5 cm 3 of acetone and adding 1 g of Bu]

1921

F. Fagalde et al.

1922

1.00

N B r in 3 cm 3 of Me2CO. The precipitate was collected by filtration, washed with cold acetone a n d dried in vacuo over P40~o. Yield: 34 m g (81%).

0.80

0.60

Materials, instrumentation and techniques

A 0.40

All chemicals were reagent grade a n d used as received. Triply distilled water was used for all kinetic determinations. p H m e a s u r e m e n t s were carried out with a precision o f _+0.05 p H units, using a L u f t m a n - R e l i a n c e | I p H meter. U V - v i s i b l e spectra were r u n with a S h i m a d z u U V - 1 6 0 A s p e c t r o p h o t o m e t e r , equipped with a t h e r m o s t a t t e d cell c o m p a r t m e n t . Stock solutions o f N a O H were prepared at the desired total concentrations. A fresh solution o f [Ru (terpy)(bipy)(NCCH3)] 2+ in water was used for each set o f experiments. A b s o r b a n c e vs time data were recorded at a value of 2 = 453 n m ( m a x i m u m o f the initial complex). Duplicate or triplicate runs were m a d e at each value o f p H a n d temperature. Rate c o n s t a n t s were o b t a i n e d by least-squares fits of I n ( A , - - A ~ ) vs time t, which were linear up to three half-lives. The estimated error in the base hydrolysis rate c o n s t a n t koH is + 5%. RESULTS AND DISCUSSION Figure 1 shows consecutive spectra o b t a i n e d at p H = 12.5 a n d T = 25.0'~C o f an aqueous solution of [Ru(terpy)(bipy)(NCCH3)] 2+ (C = 4.4 > 10 5 M). The metal-to-ligand charge transfer ( M L C T ) absorption b a n d at 2max = 454 n m (same value in M e C N ) [6] is shifted to higher wavelengths, reaching a value of 2m,x = 500 n m after 1 day. Isosbestic points are observed at 470 a n d 390 nm. These changes are indicative of a n hydrolysis process followed by a q u a t i o n ; i.e. a reaction o f the c o o r d i n a t e d ligand followed by a substitution process. The observed final peak value is coincident with t h a t of the [ R u ( t e r p y ) ( b i p y ) ( O H ) ] +

0.80 0.60

l

l

0.20

0.00

~

400

500

600

"-~--

700

800

WAVELENGTH [nm] Fig. 2. Absorption spectra of the hydrolysed products of the complex [Ru(terpy)(bipy)(NCCH3)] 2+ at pH = 11.9 () and a t p H = 0 ( ) , C = 7 . 0 x 1 0 5M.

ion [8]. To confirm the identity of the product, one d r o p of 6 N HC1 was added to this last solution. As s h o w n in Fig. 2, a value of 2max = 475 n m obtains, which agrees very well with the already reported value for the [Ru(terpy)(bipy)(OH2)] 2+ ion [8]. The value o b t a i n e d for k o b s = 4.7 x 10 4 s ~ at p H = 13.0 (# = 0.1 M) is several orders o f m a g n i t u d e higher t h a n t h a t o f the expected rate c o n s t a n t for a ligand substitution process [9]. Thus, after adding excess pyrazine (pz) to the [Ru(terpy)(bipy) (NCCH3)] 2+ ion at p H = 5.0, n o evidence of form a t i o n of [Ru(terpy)(bipy)(pz)] 2+ (2 .... = 428 rim) [4] was o b t a i n e d after several days. O n the o t h e r h a n d , the acetamide ligand was a q u a t e d very rapidly, since, as usually f o u n d in R u u complexes [10], amides are p o o r g-acceptor ligands a n d are thus rapidly released from the c o o r d i n a t i o n sphere of R u n . As s h o w n in Table 1, the observed first-order rate c o n s t a n t s k o b s for the hydrolysis process vary linearly with [ O H - ] . F r o m the rate law kobs = koH[OH ], a value of kon = 4.6 x 10 3 M - t s-~ was calculated, which can be c o m p a r e d with o t h e r metal-catalysed hydrolysis second-order rate c o n s t a n t s for acetonitrile, as s h o w n in Table 2. The value o b t a i n e d for the complex studied in this work is intermediate between those of the acetonitrile complexes of RuH(NH3)5 a n d Com(NH3)5. The catalytic factor is ca 3 x 103 over the free ligand value. This effect is quite r e m a r k a b l e for a d 6 transition metal in the (II) oxidation state a n d can

/1 2

A 0.40

Table 1. kob s for the hydrolysis of coordinated CH3CN in [Ru(terpy)(bipy)(NCCH3)] 2+ at different hydroxide concentrations (25.0°C)

0.20 0.00 350

[OH ], M 400

450

500

550

600

650

WAVELENGTH [nm] Fig. 1. Spectra obtained after adding 0.05 M NaOH to an aqueous solution of [Ru(terpy) (bipy) (NCC H3)]2+, C = 4.4x 10 5 M, at T = 25.0"C. Approximate times of reaction are: 1)0 h; 2) 1 h, 3) 2 h ; 4 ) 6 h ; 5) 8 h; 6) 24 h.

1.80x10 1.05x10 2.09x10 3.98x10 1.00×10

kob s S

~ 2 "~ 2 L

L

2.64x10 -~ 4.64x10 5 1.42x10 4 2.77x10 4 4.68x10 4

Base hydrolysis of acetonitrile coordinated to a ruthenium(II) polypyridine complex Table 2. Base-catalysed hydrolysis of acetonitrile complexes at 25.0c'C Nitrile CH3CN [(NH3)sRu(NCCH3)] 2+ [(terpy)(bipy)Ru(NCCH3)] 2+ [(NH3)sCo(NCCH3)]

3+

[(NH3)sRu(NCCH3)] 3+

koH, M J s -~

Reference

1.60 × 10 - 6 < 6 × 10 5 4.6 x 10 3 3.40 2.2 x 102

3 3 This work 3 3

be ascribed to 7z backbonding from the metal to the polypyridyl ligands, which makes the Ru n centre more electropositive than the Ru" centre in the Run(NH3)5 moiety. The ability of Ru" to promote the hydrolysis of a nitrile group when coordinated to ~z-acceptor ligands was previously detected in a work [11] on the stability of aqueous solutions of [Ru(bipy)2(4CNpy)2] 2+. The values of koH as a function of temperature are shown in Table 3. The variation of ln(koH/T) with ( l / T ) is displayed in Fig. 3. F r o m Eyring's rate equation [12], values of A H ~ = ( 7 4 + 4 ) kJ mol -~ and AS # = ( - 4 2 _ _ 12) J mol ~K -L are determined, which

Table 3. Temperature dependence of the second-order rate constant koH for the hydrolysis of coordinated CH3CN in[Ru(terpy) (bipy) (NCCH3)] 2+

T, 'C

koH, M ~s t

17.0 20.0 25.0 30.0 35.0 40.0

1.95 × 10 -3 2.30 X 10 - 3 4.68 × 10 3 7.54× 10 3 1.23× 10 2 1.84× 10 z

-9.0

-10.0

©

-11.0

.E -12.0

-13.0 3.10

I

I

I

3.20

3.00

3.40

3.50

( I / T ) x 10 -3 ( K -1)

Fig.

Eyring plots for base hydrolysis of [Ru(terpy) (bipy)(NCCH3)] 2+ in aqueous solutions.

1923

compare reasonably well with other nitrile hydrolysis activation parameters. For example, values of AH" = 63.2 kJ m o l i and AS # = - 8 4 J m o l i K - I have been obtained for the base-catalysed reaction of 2-cyano-l,10-phenthroline [13]. These values indicate the operation of an associative process. By following the arguments already pointed out by Deutsch et al. [14], we can anticipate that the orbitals of Ru(terpy)(bipy) 2+ are not as basic as those of Ru (NH3)~ +, because of the presence of the ~t electronaccepting polypyridyl ligands. Therefore, the Ru" centre in Ru(terpy)(bipy) 2+ is capable of stabilizing a bimolecular reaction mechanism, either in the H202 oxidation of a chalcogenoether [14], or in the hydroxide attack on a nitrile group, as reported here. Work in progress in our laboratory will address the extent of the catalytic action of the Ru(terpy) (bipy)2 + moiety on the hydrolysis rate of other nitrile derivatives.

Acknowled#ement~We are grateful to Consejo Nacional de Investigaciones Cientificas y T6cnicas (CONICET, Argentina), Consejo de Investigaciones de la Universidad Nacional de Tucumf.n (CIUNT, Argentina) and Consejo de Ciencia y T6cnica de la Provincia de Tucum~in (COCYTUC, Argentina) for financial help. F.F. thanks CONICET for a postgraduate fellowship. N.E.K. is a Member of the Research Career (CONICET). REFERENCES

1. Belokon, Y. N., Tararov, V. Y., Savel'eva, T. E., Vitt, S. V., Paskonova, E. A., Dotdayev, S. C., Borisov, Y. A., Struchkov, Y. T., Batasanov, A. S. and Belikov, V. M., Inor9. Chem., 1988, 27, 4046. 2. Buckingham, D. A., Keene, F. R. and Sargeson, A. M., J. Am. Chem. Soc., 1973, 95, 5649. 3. Zanella, A. W. and Ford, P. C., Inor9. Chem., 1975, 14, 42. 4. Fagalde, F. and Katz, N. E., Polyhedron, 1995, 14, 1213. 5. Lis de Katz, N. D., Fagalde, F. and Katz, N. E., Polyhedron, 1995, 14, 3111. 6. Hecker, C. R., Fanwick, P. E. and McMillin, D. R., Inorg. Chem., 1991, 30, 659. 7. Ware, D. C., Lay, P. A., Taube, H., Inor9. Synth., 1986, 24, 300. 8. Davies, N. R. and Mullins, T. L., Aust. J. Chem., 1967, 20, 657. 9. Davies, N. R. and Mullins, T. L., Aust. J. Chem., 1968, 21,915. 10. Naal, Z., Tfouni, E. and Benedetti, A. V., Polyhedron, 1994, 13, 133. 11. Katz, N. E., Creutz, C. and Sutin, N., Inor9. Chem., 1988, 27, 1687. 12. Wilkins, R. G., Kinetic and Mechanism of Reactions o f Transition Metal Complexes, 2nd edn. VCH, Weinheim, 1991, p. 88. 13. Breslow, R., Fairweather, R. and Keana, J., J. Am. Chem. Soc., 1967, 89, 2135. 14. Root, M. J. and Deutsch, E., Inor9. Chem., 1985, 24, 1464.