J . Am. Chem. SOC.1981, 103, 3559-3564
3559
Reactivity of Coordinated Nitriles I. I. Creaser, J. MacB. Harrowfield, F. R. Keene, and A. M. Sargeson* Contribution from the Research School of Chemistry, Australian National University, Canberra ACT 2600, Australia. Received July 14, 1980
Abstract: The base hydrolysis of coordinated acrylonitrile in [ (NH3)5CoN=CCH=CH2]3+ to the acrylamide complex in carbonate buffers obeys the rate law kOw = koHIOH-] k,[C032-] (koH = 35 M-' s-l; k, = 1 M-' s-'; 25 "C, p = l.O), and l*O tracer studies indicate the mechanism of hydrolsis by carbonate ion to involve direct nucleophilic attack at the nitrile group by CO,*- with subsequent elimination of C02. Coordination of malononitrile, cyanoacetic acid, and ethyl cyanoacetate to the (NH3)5C~r'1 moiety leads to greatly increased acidity of the methylene protons (pK, of [ (NH3)sCoN=CCH2CzN]3+ is 5.7 ie 0.1 ( p = 1.0,25 "C) compared with 11.3 for uncoordinated malononitrile). In basic aqueous solutions, these complexes equilibrate rapidly between the original complexes and the complexes containing the deprotonated activated methylene group. The protonated complexes then hydrolyze to the coordinated amides with both hydroxide ion and water acting as nucleophiles while the coordinated carbanions undergo an intramolecular electron transfer of a single electron to yield Co(I1) and the ligand radical. The C-deprotonated complexes also function as nucleophiles for appropriate substrates (such as methyl iodide, methyl pyruvate, or bromine) to form the corresponding substituted species. In nonaqueous media, generation of the ligand radical initiates polymerizations of monomers methacrylate, styrene, acrylonitrile, and methacrylonitrile by a radical (rather than an ionic) mechanism.
+
gous way to that of [CO(NH~)~(OP(OC~H~)~](CIO~)~ by using the corresponding BF, salts. Nitrosyl triflate was prepared by passing N20, gas (from H2S04/NaN02)into purified CF3S03Huntil crystallization was complete. [ C O ( N H , ) S ( N ~ - C H ~ ~ N ) I ( C I O ?[C~(NHS),(~P(OCH~)~)I(B),. F4)3(4.0 g) was dissolved in freshly distilled malononitrile (50 mL) and concentrated H2S04(0.5 mL) added. The mixture was maintained at ca. 50 "C until it turned yellow (ca. 3 h). Ethanol (50 mL) and ether (300 mL) were added, the solution was filtered and the product was washed with ethanol and acetone and air dried. The resultant complex (3.0 g, 80%) was purified by dissolution in 0.1 M HBr (50 mL) and adding excess LiBr (ca. 5 g) to remove any amide complex. The solution was cooled quickly and filtered, and excess solid NaC10, was added to the filtrate whereupon [CO(NH,)~(N=CH,C%N)] (CIO,), crystallized out. This material was further recrystallized by precipitation using NaC104 from a 0.1 M HCIO, solution and was collected as indicated above. Yield: 1.9 g, 51%. A solution in 0.01 M HClO,, measured rapidly after dissolution, gave two maxima in the visible spectrum (e,, = 72, t341= 82). The infrared spectrum showed two absorptions at 2263 and 2330 cm-l, attributed to the -C=N stretching modes of the terminal and bound -CN groups, respectively. Anal. Calcd for [CO(NH~)~NCCH~CN](CIO,),: Co, 11.6; C, 7.09; H, 3.37;, N, 19.3. Found: Co, 11.7; C, 7.3; H, 3.6; N, 19.4. [CO(NHS)~(N~CH~C~~C~HS)I(~~,),. [ C o W H d d ~(BF4)z l (2.0 g), ethyl cyanoacetate (30 mL), and trimethyl phosphate (10 mL, to Experimental Section affect dissolution) were stirred, and excess NOBF, was added slowly. Electronic spectra and molar absorptivities e (M-l cm-I) were recorded The mixture was stirred for a further 2 h, and the complex (as the B F i by using a Cary 118C spectrophotometer, 'H NMR spectra by using salt) was precipitated by using ether. The product was recrystallized either a Varian HA-100 or a JEOL MH-100 spectrometer, and IR twice by precipitation using NaC10, from solutions in slightly acidified spectra by using a Perkin-Elmer PE 225 spectrometer with KBr disks (ca. water. It was washed with ethanol and ether and air-dried. Yield: 0.8 5 mg of complex/200 mg of KBr). g, 26%. Synthesis and Reagents. Cyanoacetic acid (Unilab), ethyl cyanoThe visible spectrum (0.01 M HCIO,) showed two maxima (ey5 = 64, acetate (Fluka, puriss), trimethyl phosphate (Pflaum), nitrosyl tetraeZg5 = 89), and the infrared spectrum (KBr disk) indicated vCN = 2325 fluoroborate (Ozark-Mahoning), and acrylonitrile (Aldrich) were used C ~ H ~ ) ]Co, cm-I. Anal. Calcd for [ C O ( N H ~ ) ~ ( N C C H ~ C O ~(CIO,),: without further purification. Malononitrile (Pfaltz and Bauer) was 10.6; C, 10.8; H, 3.99; N, 15.1. Found: Co, 10.8; C, 10.7; H, 4.1; N, freshly distilled before use (76-77 "C, (1 mmHg)). [ C O ( N H ~ ) ~ N ~ ] C I ~15.1. was converted to the tetrafluoroborate salt by using AgOOCCH3/ A much higher yield was obtained by adding NO03SCF, .(8 g) to a NaBF,, and [CO(NH~)~(OP(OCH,),)](BF,), was isolated in an analovigorously stirred solution of [(NH,),CoN,] (CFSS03)2(15 g) in acetone (20 mL) and NCCH2COOC2H5(100 mL). Only a little complex remained undissolved in this solvent mixture. The solution rapidly turned (1) Breslow, R.; Fairweather, R.; Keana, J. J . Am. Chem. SOC.1967,89, yellow-brown as N2 and N20 evolution occurred, and after 15 min a 2135. Sakai, K.; Ito, T.; Watanabe, K. Bull. Chem. SOC.Jpn. 1967,40,1660. copious yellow precipitate had appeared. The mixture was cooled at 0 Watanabe, K.;Komiya, S.; Suzuki, S. Ibid. 1973, 46, 2792. Barnard, P. F. OC for 30 min and then mixed with ether (100 mL) and the product B. J . Chem. Soc. A 1969, 2140. collected. The triflate salt was recrystallized from CHIOH by addition (2) Diamond, S.E.; Grant, B.; Tom, G. M.; Taube, H. Tetrahedron Lett. 1974, 46, 4025. of ether to give fine needles (17 g): (3) Balahura, R. J.; Purcell, W. L. Inorg. Chem. 1979, 18, 937. [C~(NHS)S(N~C(CH,)~C~~C,H,)I(N [CO(NHS)~(NCCH~C~,),. (4) Pinnell, D.; Wright, G. B.; Jordan, R. B. J. Am. Chem. SOC.1972,94, OzC2H5)](C104)S (0.36 g) was dissolved in dry Me2S0 (10 mL), and 6104. Me1 (ca. 1 mLI was added. followed immediatelv bv imidazole (0.10 a). (5) Rouschais, G.; Wilkinson, G. J . Chem. SOC.A 1968,489. The mixture was shaken, stoppered, and left to itand for 10 min du&g (6) Buckingham, D. A.; Keene, F. R.; Sargeson, A. M. J. Am. Chem. SOC. which time the bright red color obtained on the addition of imidazole 1973, 95, 5649. faded to orange. Ether (500 mL) precipitated the complex as an oil, (7) Buckingham, D. A.; Morris, P.; Sargeson, A. M.; Zanella, A. Inorg. which after two triturations with ether was solidified by the addition of Chem. 1977, 16, 1910. (8) Jordan, R. B.; Sargeson, A. M.; Taube, H. Inorg. Chem. 1966,5, 1091. isopropyl alcohol. The product was filtered and dried in vacuo. h r i -
Increased reactivity of coordinated nitriles has been observed on a number of Marked acceleration in the rates of hydrolysis to the amide by OH- compared with the uncoordinated species has been recorded both for inter- and intramolecular hydrolysis processes. The acceleratory effect for coordinated acetonitrile was also accompanied by an enhancement in the rate of proton exchange for the methyl group: in fact, the latter process was slightly faster than OH- attack a t the nitrile carbon.6 This property is in accord with some earlier observations on the treatment of [(NH3)&o(NCCH2CN)l3+ with base.' A solution of this latter ion in water gave an intense red color when OH- was added. The color faded gradually to give a yellow solution similar in appearance to the original solution, although the visible spectra were not identical. The initial observation was attributed to deprotonation of the acidic methylene group, but the second reaction was not investigated further.' If the interpretation of the initial reaction is correct, the acidity of coordinated malononitrile is considerably greater than that of the uncoordinated species, and this paper reports on the hydrolysis and reactivity of such nitrile complexes. Studies on several related nitriles have also been made.
0002-7863/81/1503-3559$01.25/0
0 1981 American Chemical Society
3560 J. Am. Chem. SOC.,Vol. 103, No. 12, 1981
Creaser et al.
fication was achieved by anion exchange using a column of DEAENaOH solution (4 mL) added. After being left to stand for 90 min, the Sephadex A-25 in the NO3- form (to remove CIO, and I,- ions), evap solution was filtered to remove precipitated cobalt oxides, and the product orating the eluant to dryness and recrystallizing the residue from hot was crystallized from the filtrate by the addition of solid NaCIO,. The water (5 mL). The orange product was filtered, washed with ethanol and pink-yellow solid was filtered, washed with ethanol and ether, and airether, and air-dried. Yield: 0.13 g, 42%. dried. Yield 0.1 g. The complex showed uPN = 2260 cm-' and tlpn= The IR spectrum (YCN = 2330 cm-') was consistent with coordinated 74 and e345 = 95 for a solution in 0.01 M NaOH. Anal. Calczlfor nitrile. Anal. Calcd for [CO(NH,)~(NCC(CH~)~CO~C~H,)](NO,),: C, [Co(NH,),(NHCOCH~CN)](ClO,)~: C, 8.46;H, 4.26;N, 23.0. Found 17.8;H, 5.56;N, 26.8. Found: C, 17.0;H, 5.6;N,26.8. C, 8.5; H, 4.5;H, 22.8. [(NH3)sCoNHCOCBr2C02Cc,HsIo,. Bromine (3 mL) was added A yellow protonated form was also isolated by addition of 12 M dropwise to a stirred ice-cold solution of [ (NH3)5CoNCCHzC0zCzHClO, to the solution obtained from the reaction of [ C O ( N H ~ ) ~ H,](CF,SO3)3 (17g) in HzO (100mL). With continued cooling of the (NCCH2CN)13+with OH-, but as with protonated forms of the commixture, a slurry of NaHCO, (4 g) in H 2 0 (20 mL) was added in plexes [CO(NH~)~(NHCOC,H~)]~+' and [ C O ( N H ~ ) ~ ( N H C O C H ~ ) ] ~ + , ~ analytical results indicated incomplete protonation or possibly one proton portions over 5 min. The solution was then stirred for 20 min at 20 OC shared between two of the cations via hydrogen bonding. before HClO, (20 mL, 70%) was added. On cooling of the mixture, a mass of flesh pink crystals formed which were collected and washed with Base Hydrolysis of [CO(NH,)~(N=CCH=CH~)~)+ and [Coethanol and ether (yield 11 g). The product was recrystallized from (NHJ5N4CH313+. A solution of the complex perchlorate in water water by cooling and adding HC10,. Anal. Calcd for ((0.5-3) X l(r M) was deaerated with nitrogen and mixed with solutions [(NH3)5CoNHCOCBrzC02C2H5](C104)2: C, 9.52;H, 3.35; N, 13.32. of deaerated sodium hydroxide solution by using a hand-operated stop Found: C, 9.3;H, 3.8;N, 13.2. ped-flow device fitted to a Cary 16K spectrophotometer. The reaction was followed at 260 and 250 nm, respectively, and constant ionic strength [(NH3)5CoNHCOCHBr#304)2. NaOH (2 M, 5 mL) was added to of 1 .O (NaClO,). The amide product [(NH3),CoNHCOCH=CH2]12 (Clod), (1.6g) in H 2 0 (1 5 a slurry of [(NH3)5CoNHCOCBr2C02C2H5] was isolated and characterized by microanalysis and 'H NMR specmL). The solid rapidly dissolved to give an orange solution, and after troscopy (see paragraph in this section of tracer studies). 15 s the reaction was quenched with HClO, (12 M, 5 mL). Cooling on Catalysis of Base Hydrolysis of [CO(NH,),(N=CCH=CH~)]~+by ice produced a minor but voluminous amount of orange precipitate which Carbonate Ion. In a similar method to the base hydrolysis studies above, was collected and discarded. It appeared to be a mixture of [(NH3)5CoNHCOCHBr2]2+and [(NH3)5CoNHCOCBr2COOH]2+. solutions of the complex were mixed with carbonate/bicarbonate buffers of varying total carbonate concentrations (pH maintained at ca. 9.6)by After 12 h at 20 'C clusters of orange needles had deposited which were using the hand-operated stopped-flow device. Ionic strength was maincollected, washed with ethanol, and dried in vacuo. Anal. Calcd for tained at 1.0 (NaClO.,), T = 25.0 O C , and all runs were followed at X [(NH3)5CoNHCOCHBr2](C104)2: C, 4.30;H, 3.07;N, 15.04. Found: = 260 nm by using a Cary 16K spectrophotometer. C, 4.7:H. 3.4: N. 15.2. Tracer Studies of COj- Reaction with [ C O ( N H J ~ ( N W C H ~ ~ ) ~ + . [ C O ( N H , ) , ( N ~ ~ ~ C O ~ H ) ] ( C[CO(NH~),OH~](C~O~)~ ~O~)~. (1 1.6 A sample of anhydrous NazC03(2g) was enriched by allowing a solution g) and cyanoaceticacid (8.5 g, 100 "01) were slumed in water (10mL) in H2I80(ca. 1.5 atom %) to stand for 14 days at 25 OC: the H2I80was in an evaporating basin, and the mixture was taken to dryness on a steam evaporated and the Na2C1'0, dried for 2 h at 260 'C and then kept in bath. Water (5 mL) was added to the resultant orange solid, and the an evacuated desiccator. mixture was again taken to dryness. The solid residue was cooled, disNazC1803(0.50 g) was dissolved in H2160 (COz free; 3 mL): imsolved in water (300mL), and sorbed on Dowex 50W-X2resin (2OC-400 (0.30.g) susmediately, yellow [CO(NH,),(N=CCH=CH~)](C~O~)~ mesh, H+ form). Elution with 1 M NaC104 (pH 3) gave two bands, the pended in H2I60(2 mL) was added, whereupon the complex dissolved first red and the second yellow. The separate eluates were reduced in and the solution became pink. Excess NaI was added after a fixed period volume on a rotary evaporator until crystallization occurred. After ad(1, 5, and 7 min) to crystallize anhydrous [Co(NH,),(NHCOCH=Cdition of methanol and cooling of the mixture, the product was collected, H2)]12. The complex was washed thrice with cold 1 M NaI solution and washed with ethanol and ether, and air-dried. The first band gave rise then several times with methanol and dried overnight in vacuo over P205 to pink-orange needles (0.8g), having uCN = 2260 cm-' in the infrared Half of the solid was recrystallized from water (ca. 2 mL) containing 1 spectrum, indicating free C S N , whence this compound was formulated drop of glacial acetic acid. 'H NMR (Dz0, ABX spectrum): 8 5.2-6.6 Anal. Calcd for as [CO(NH~)~(O~CCH~CN)](C~O,)~. relative to DSS internal reference. Anal. Calcd for [(Co(NH& C0CjH17N6010C12: C, 8.44;H, 4.01;N, 19.68;CO, 13.80;C1, 16.61. (NHCOCH=CH2)]12: C, 7.70;H, 4.09;N, 18.0;CO, 12.6; I, 54.2. Found: C, 8.5; H, 3.9;N, 19.6;Co, 13.9;C1, 16.5. Found: C, 7.9;H, 4.0;N, 17.9;Co, 12.7;I, 54.1. The second band resulted in bright yellow flakes (1.5 g, 11%) with uCN The analysis of the enrichment in the acrylamide product was achieved = 2325 cm-l, indicating a coordinated nitrile group; this complex had a by using the Anbar and Guttman method.g [Co(NH&(NHCOCH= visible spectrum €470 = 68 and E,% = 66 in 0.01 M HClO, solution. Anal. CH2)]Iz(0.1g) was mixed with HgC1z.Hg(CN)zand heated at 400 OC Calcd for [CO(NH~)~(NCCH~CO~H)](C~O~),: Co, 11.2;C, 6.83;H, for 15 h in a break-seal tube. The C02, separated from the other gases 3.44;N, 15.9. Found: Co, 11.3;C, 6.9;H, 3.7;N, 15.7. in a gas chromatograph, was collected by using He as the carrier gas and [CO(NH~),(N~CH=CIIZ)I(C~O [CO(NHS)~N,I ~)P. (Clod2 (109) the I8Ocontent analyzed by using an Atlas M-86mass spectrometer, as was dissolved in trimethyl phosphate (30 mL) and then diluted with outlined previously.* The enrichment in COj- was determined by adding acrylonitrile (-75 mL). Solid NO(CF,SO,) was added slowly until all acid to the solid sample in a stream of He and trapping the C 0 2 procomplex had reacted and the resulting mixture left overnight at 40-50 duced. In one experiment, the reaction mixture was also treated in this "C. After the mixture was cooled, ether was added to precipitate the way to sample the enrichment of reacted C03z-. complex. It was recrystallized by dissolution in hot water at pH -3, and pK, Determination. The pK, values for [Co(NH3),(N=CCH2C= the complex was precipitated by the addition of solid NaC104. The yield were determined by the N)]'+ and [CO(NH,),(N=CCH~CO~C~H,)]~+ was >90%. IH NMR (D20, ABX spectrum): 8 6.0-7.0relative to DSS following method. A solution (ca. 8 X lo4 M) of the complex as the internal reference. Anal. Calcd for [Co(NH3)~(N=CCH=CH2)]perchloratesalt in 1 M NaC10, (pH ca. 3) was mixed with equal voluma (C104)3: Co, 11.9; C, 7.27;H, 3.66;N, 17.0. Found: Co, 11.8; C, 7.4; of various buffer solutions ([buffer] = 0.2M; p = 1.0,NaC10,) by using H, 3.8;N, 16.7. Kinetics of Base Hydrolysis of [CO(NH,)~(N~CH,GN)](C~O,)~.a stopped-flow apparatus. The subsequent reaction was followed for ca. 3 min at a fixed wavelength ( A = 397 nm for [Co(NH,),A solution of the complex in 1 M NaClO, (ca. 8 X lo-' M) was mainby (NCCH2CN)]", X = 527 nm for [CO(NH~)~(NCCH~CO~C~H~)]~+) tained at the desired pH with a pH-stat apparatus and the reaction using a Cary 16K spectrophotometer at a cell temperature of 25.0OC. followed at 397 nm by using a Cary 16 K spectrophotometer at a cell The pH of each solution was measured with a Radiometer 26 pH meter. temperature of 25.0 OC. The following Radiometer apparatus was used In each case, the curve was extrapolated to give the optical density at the in the pH-stat assembly: pH meter 26, ABUl autoburet and TTTl instant of mixing and the pK, evaluated.IO titrator with a G202B glass electrode and a saturated calomel electrode Polymerization Studies. Acrylonitrile, methacrylonitrile, and dicoupled by a salt bridge containing 1.6 M NH4N03and 0.2 M NaNO,. methylformamide (DMF) were purified and distilled before use.l' Product Isolation. The resultant solutions from the above kinetic Styrene, methyl methacrylate, and pyridine were dried and distilled bedeterminations were adjusted to pH 7,diluted, and sorbed on a cationexchange resin (Dowex 50W-X2,20CF400 mesh, Na+ form) and the products eluted with 2 M NaC104. The products of the reaction were (9) Anbar, M.; Guttman, S.J. Appl. Radiation Isotopes 1959,5,223.See: Co2+,[CO(NH,)~OH~]~+, and [CO(NH~)(NHCOCH,CN)]~+ measured Buckingham, D. A.; Foster, D. M.; Sargeson, A. M.J. Am. Chem. Soc. 1968, by atomic absorption spectroscopy. 90, 6032 and references therein. The amide complex was isolated by evaporation of the above eluant (10) Albert, A.; Serjeant, E. P."Ionization Constants of Acids and Basea"; solution but was obtained more simply by the following method. [CoMethuen: London, 1962. (NH3),(NCCHzCN)](ClO,), (0.2 g) was dissolved in the minimum (11) Colebourne, N.; Collinson, E.; Dainton, F. S.Tram. Faraday SOC. quantity of warm water (10 mL, acidified with CH,COOH) and 1.0 M 1963,59, 886.
J. Am. Chem. SOC.,Vol. 103, No. 12, 1981 3561
Reactivity of Coordinated Nitriles
Table I. Base Hydrolysis of [Co(NH,), (N=CCH=CH,)]3+ a
0.0010 0.0025 0.0050 0.010 0.010
3.49 x 8.49 x 1.67 X 10-l 3.67 X 10" 3.70 X lo-'
34.9 34.0 33.3 36.7 37.0 knHav=
35.2
a [Co] = 6.70 X M; p = 1.0 (NaC10,); T = 25.0'C; A = 270 nm; all solutions deaerated.
ordinated nitrile group for [ C O ( N H ~ ) ~ ( N C C H ~ C N( )k] ~o+=~ 9.6 X s-I; pH 3.0, p = 1.0, 25 "C), followed by a slower substitution of the coordinated amide by water. C The addition of base to solutions of the three yellow nitrile complexes resulted in the immediate formation of deep red solutions. These changed slowly to yellow solutions which differed _JL spectrally from the initial nitrile species. The initial rapid reaction is reversible on adding acid and is attributed to the deprotonation of the acidic methylene protons. For uncoordinated malononitrile 6 4 2 0 the pK, of the methylene protons is 1 1.3.16 On coordination, this P PM value falls to 5.7 f 0.1 (0.1 M pyridine/HC104 buffer, p = 1.0 Figure 1. 100-MHz IH NMR spectra of [Co(NH,),(NaC104), 25.0 "C). Similarly for ethyl cyanoacetate, the pK,I6 (NCCH2C02C2H5)13+in Me2SO-d6solution (A) and of [Co(NH,),is reduced on coordination from >9 to 7.58 (0.1 M "Tris"/HC104 (NCC(CH3)2C02C2H5)]3+ in Me2SO-d6solution (B) and in acidified buffer, p = 1.O (NaC104), 25.0 "C). The deprotonated complexes D 2 0 solution (C) (sodium 4,4-dimethyl-4-silapentanesulfonate, internal are presumably stabilized by extensive delocalization of the reference, DSS). negative charge throughout the coordinated ligand. A consequence fore use. All polymerizations were carried out in an all-glass vacuum line of this is the intense absorption which develops in the first ligand by using the following procedure. Dried and deaerated DMF (ca. I O field band on deprotonation: the maximum molar absorptivity mL) was distilled into a 50" flask containing a magnetic stirrer bar €532 = 450 M-I cm-' for [ C O ( N H ~ ) ~ ( N C C H C N ) is ] ~to + be and the desired complex (ca. 0.02 9). The monomer (ca. 2 mL) was then compared with t474= 72 M-' cm-' for the protonated form. The distilled into the flask, followed by pyridine (ca. 0.1 mL) if required. The former intensity is more consistent with a substantial contribution flask was sealed off from the line; the contents were melted, mixed, and from ligand-to-metal charge transfer whereas the latter intensity left for 48 h protected from the light. Subsequently, the solvent was is consistent with ligand field absorption and is typical in complexes removed and the resultant polymer recrystallized (twice) by dissolution of this type. A similar result was observed for the ethyl cyanoin DMF and precipitation by the slow addition of water. The product acetate complex where the deprotonated complex (t527mx= 550 was dried in vacuo for 3 days. The 'H NMR spectra of samples of polymethacrylonitrile(PMAN) were measured by using a Varian HAM-l cm-I) absorbs much more strongly than the parent ion 100 spectrometer and a 7% solution in CD3N02at 90 OC with hexa= 64 M-' cm-'). methyldisiloxane as an internal reference,I2and trimethyl phosphate as The subsequent decay of the deprotonated complex was unan internal lock. The polystyrene-polymethyl methacrylate copolymer usually rapid for coordinated cyanoacetic acid and produced was not soluble in CD3N02,and its 'H NMR spectrum was measured predominantly Co(I1). For the other two complexes, the decay in C6D6solution (7%) by using a JEOL MH-100 spectrometer with of the red solution was slower and gave both Co(I1) and hydrolysis tetramethylsilane as internal reference at 34 "C. of the coordinated nitrile. Three interesting aspects arise from Results and Discussion these properties: the mechanism of the base-hydrolysis process, The pentaamminecobalt(II1) complexes of three nitriles, the reactions of the coordinated carbanion, and the electronmalononitrile (NC-CH2-CN), ethyl cyanoacetate (NC-CH2transfer process from the deprotonated ligand resulting in the C02C2H5),and cyanoacetic acid (NC-CH2-C02H), have been reduction of Co(II1) to Co(I1) and a ligand radical. isolated and characterized analytically and spectrally. The I R Base hydrolysis of coordinated nitrile in [CO(NH,),(NCR)]~+ spectra of the complexes all showed an absorption in the range ions has been attributed to direct attack of OH- a t the nitrile carbon atom.496For [(NH3)5CoN=cC6H5]3+,this seems the only 231 5-2330 cm-' attributed to the -C=N stretching mode for the feasible explanation4 for the first-order dependence on [OH-]. coordinated nitrile g r o ~ p . ~ , ' ~In, 'addition, ~ coordinated maloAlthough C-proton exchange occurs with coordinated CH3CN nonitrile gave an absorption at 2260 cm-', appropriate to vCN for faster than amide formation,6 no exchange was observed on the uncoordinated nitriles (ca. 2275 cm-')15 a s did the aromatic ring of coordinated NCC6H5. Also, it does not seem [(NH3)2CoOOCCH2CN]*+ion. The IH N M R spectrum of [Co(NHJ5(NCCH2CO2C2H5)I3+ likely that deprotonation of the ammine centers would lead to amide formation. The results for hydrolysis of [Coin Me2SO-d6is shown in Figure 1A. The complexes of malononitrile and cyanoacetic acid were found to have limited stability (NH&NCCH=CH2]3+ also support this proposal (Table I). The linear pseudo-first-order plots of log ( A , - A , ) vs. time obtained in Me2S0, with the solvent replacing the nitrile in the coordination sphere over a period of hours a t 20 "C. In all three cases, the over at least 3tl/2demonstrate a first-order dependence of the rate 'H N M R spectra measured in D+/D20 mixture indicated a rapid on [OH-] for the production of coordinated acrylamide. Clearly exchange of the methylene protons. OH- adds at the nitrile carbon atom and not at the olefin @carbon Acidified aqueous solutions of these complexes varied considatom nor is there any proton exchange a t the olefin moiety. erably in stability, the ethyl cyanoacetate complex being the most Similar results are obtained in the presence of the C0:- ion which stable. Both the cyanoacetic acid and malononitrile complexes also generates the acrylamide complex. underwent an initial reaction, identified as hydrolysis of the coOne striking feature of this study was the remarkable dependence of the base-hydrolysis rate on C032-concentration. The hydrolysis reaction was therefore studied at 25 "C by using (12) Yamada, A.; Yanagita, M. J . Polymer Sci., Part B 1971, 9, 103. (13) Foust, R. D.;Ford, P. C. Inorg. Chem. 1972, 11, 899. carbonate/bicarbonate buffer mixtures, and the results are sum(14)Clarke, R. E.;Ford, P. C. Inorg. Chem. 1970, 9, 227.
I
,.'I.,,k-
(15) Nakamoto, K. "Infrared Spectra of Inorganic and Coordination Compounds", 2nd ed.; Wiley-Interscience: New York, 1970; p 186.
(16) Pearson, R. G.; Dillon,
R. L. J. Am. Chem. SOC.1953, 75, 2439.
3562 J. Am. Chem. Soc., Vol. 103, No. 12, 1981
Creaser et al.
Table 11. Catalysis of Base Hydrolysis of [CO(NH,),(N=CCH=CH,)]~+ by C0,Z-a PH
103k',,,*
9.64 9.67 9.59 9.71
2.59 2.78 2.31 3.05
sd
(kobsd -k'OH)/ kobsd,
2.78 x 6.69 X 7.85 X 1.41 X
lo-, 10" 10" lo-'
[CO,*-], M - l s-I
[ c a ~ b o n a t e ]M~ ,
[ C 0 , 2 - ] , cM
1.o 1.0 0.9
0.0455 0.1138 0.1667 0.2275
0.0245 0.0634 0.0853 0.1319
1.0
[Co] = 6.70 X l W 5 M; T = 25 "C; = 1.0 (NaC10,); h = 270 nm. suming pK, = 9.57 for HCO,Z- ZC0,'- + H' at p = l.0.19
Assuming koH = 35.2 M-l s' (from Table I); pKw = 13.77.19
As-
Scheme 11
Table 111. Hydrolysis of [ (NH,),CoN=CCH=CH,](ClO,), (0.12 M) with Na,CL80, (1 M) in H,160 at 25 "C
% '*O
reaction time, min 1
content in species enrichment amide 1.2071 c0,z1.4087 amideC 1.1073 0.9057 75 5 c0,z1.4086 1.2070 amide 0.7402 0.5386 45 1.4088 1.2072 7 c0:0.5993 0.3977 33 amide CO,Z-d 1.3546 1.1530 95.5 a Obtained from the ion current ratios ( R ) using relationships atom % = R/(2 + R). Atom percent levels minus the normal C"0, content 0.2016 atom %. [(NH,),CoNHCOCH=CH,]I, CO, from CO,z- in the reaction mixture. labela atom %
l80
Table IV. Rates of Base Hydrolysis of [Co(NH,),NCCH,] (ClO,), in 0.01 M NaOHa temp, "C 15.2 24.1 30.8
Scheme I 18- 2-
k o ~M-' , s-'
temp, "C
0.945 35.3 2.36 40.0 4.62 E , = 18.8 t 1.0 kcal mor' AS* = +4.4 cal deg-' mol-'
k o ~ M-' ,
S'
8.15 12.6
[complex] = 2.5 x 1W4 M; A = 250 nm; p = 1.0 (NaC10,).
"
marized in Table 11. . The data obey the rate law koM = koH[OH-] k,[CO,2-] with koH = 35 M-ls-I and k, = 1.0 M-ls-l: the carbonate ion being only a factor of 35 less efficient than OHin effecting hydrolysis. A question immediately arises concerning the mechanism of involvement of CO?-. Is it by a nucleophilic mechanism or by a eneral-base process? The determination can be made by using 0 tracer studies, since in the pH region for which the measurements shown in Table I1 are made, CO2exchanges slowly with the oxygen atoms in solvent water (for example, in 0.02 M Na2C03, tl12 for exchange is ca.28 h)." The tracer experiment was carried out by dissolving the complex in a solution of 1 M Na2CI8O3of known enrichment in normal water. In different experiments, the amide product was precipitated after different intervals (1, 5, and 7 min) as the anhydrous iodide salt. (The rate constant for hydrolysis of [ C O ( N H ~ ) ~ ( N = C C H = C H2)](C104)3in 0.25 M Na2C03 is 0.28 s-l at 25 OC, so hydrolysis was complete before the complex was precipitated.) The results of the tracer experiment are given in Table 111. Extrapolation of enrichment to the time required for completion of the hydrolysis process reveals that the initial product contains ca. 90% enrichment, indicating that C032-is involved in direct nucleophilic attack on the nitrile carbon atom, yielding an oxygen atom to the coordinated amide. The observed discrepancy from 100% enrichment in this experiment is due to concomitant hydrolysis of the nitrile by unlabeled OH-.I8 The tracer experiments
+
0
(17) Mills, G. A., Urey, H. C. J . Am. Chem. SOC.1939, 61, 534.
also show that the carbonate ion in the final reaction mixture is diluted in label by 4.5% compared with the expected dilution of 4.2%. The proposed mechanistic scheme for hydrolysis of coordinated acrylonitrile by C032- is shown (Scheme I). Carbonate ion attqcks the nitrile C atom, and either COz is eliminated or a water molecule takes C 0 2 off the complex as H2CO3. If the latter path obtained, it could be expected that this would also be a route to exchange of C032-oxygen atoms with solvent and that was not observed. A preference therefore exists for the path involving decarboxylation of the carbonate ester. Subsequently, protonation of the amide N generates the acrylamide complex. A complicating feature in the reaction should be ion pairing between the parent 3+ ion and the CO?- anion. Analogous association constants2' for [ C O ( N H ~ ) ~ O Hand ~ ] ~SO +:- are in the vicinity of lo3 M-l. If this value holds for the present case, then all the parent complex should be ion paired and the rate law more properly formulated as - d ( C o N C R ) / d t = k[(NH3)5CoN=CR3+.C032-][ C O ~ ~ -A ] . dependence of the rate of hydrolysis of coordinated nitriles on carbonate ion has been reported previously for Ru(II1) and Rh(II1) complexes,20although no direct tracer evidence was presented to distinguish between the nucleophilic and general-base mechanism. However, the involvement of CO?- in the hydrolysis of uncoordinated nitriles does not appear to have been observed. When these reactions were carried out for longer periods, a subsequent slower exchange process occurred with solyent water resulting in the loss of '*O label from the coordinated amide (Table
-
-
(18) A solution of 1 M Na2C0, has a pH of 11.8. With the assumption of pK, = 14.17 for a solution of p = 3.0 at 25 O C , I 9 [OH-] = 4 X M. Come uently, under the conditions of the experiment, k b =~ koH[OH-],= 9 0.14 s- and k'c = k,[CO,*-] = 1.0 s-' whence -14% hydrolysis of the nitrile by unlabeled OH- would be expected. (19) Spec. Pub/.-Chew. SOC.1964, No. 17. (20) Zanella, A. W.; Ford, P. C. Inorg. Chem. 1975, 14, 42. (21) Posey, F. A,; Taube, H, J . Am. Chem. SOC.1956, 78, 15.
'
J . Am. Chem. SOC.,Vol. 103, No. 12, 1981 3563
Reactivity of Coordinated Nitriles
Scheme 111
H*OlH 2 0
L 3
2
1
P PM Figure 2. 100-MHz ' H NMR spectrum of polymethacrylonitrile (7% in CDjN02, 90 " C , hexamethyldisiloxane, internal standard).
111). In 1 M N a 2 C 0 3solution, the concentrations of bicarbonate ion and carbon dioxide are low ([HCOj-] = 6 X loe3M; [CO,] = 6 X lo-' M)22so that it is unlikely that either of these species is involved in the exchange process. Neither does C03" lead to direct exchange, because its own exchange rate with the solvent" is much slower than the observed rate of amide exchange. Consequently, the most reasonable path for the loss of label from the amide would be by OH- addition or general-base (CO?-) catalyzed water addition, and a possible mechanism is presented in Scheme I1 although this exchange process has not been investigated in detail. The base hydrolysis of [ C O ( N H ~ ) ~ ( ~ I C C Hhas ~ )been ] ~ + studied a t 25 OC and p = 1.0.6It was of interest to compare the activation parameters of the coordinated acetonitrile to those of the free ligand, and the rates were measured at five different temperatures (Table IV). From the data, the following activation parameters were calculated: E, = 18.8 f 1.0kcal/mol and AS* = +4 cal/(deg mol). The corresponding data for free a ~ e t o n i t r i l eare ~ ~E, = 19.1 kcal/mol and AS* = -23 cal deg-' mol-', showing that the difference in rate is reflected in the AS*term while E, is unchanged for the two reactions. Essentially the same results were observed for [ C O ( N H ~ ) ~ ( N C C ~ Hand ~ ) free ] ~ +ben~onitrile:~ here, for the complex E, = 16.5 kcal/mol and AS*= +2.7 cal deg-' mol-' compared with E , = 19.9 kcal/mol and AS*= -15.2 cal deg-' mol-' for benzonitrile (in 50% acetone). The change in AS*for the hydrolysis reactions from highly negative for the free nitriles to slightly positive for the coordinated nitriles may simply reflect the 3+ charge on the latter species.24 However, it appears to be a characteristic of the coordinated ligand reactions, and there is as yet no clear correlation between charge neutralization and rate enhancement for reactions of this type. The hydrolysis process was studied throughout the pH range 1-12 for the malononitrile complex. Continuous scans of the electronic spectra during these reactions indicated that the process was not simple. Some [ C O ( N H ~ ) ~ O Hwas ~ ] ~detected + in the reaction mixtures arising from a subsequent loss of the amide ligand. However, linear plots of log (A, - A,) against time were observed for a t least 3t1/2, indicating pseudo-first-order kinetics for the production of the amide and Co2+. In 1 M HClO, 1% Co2+ was produced while the amount increased to 25% at 7 C pH < 10. A reaction profile of the observed rate with pH for the malononitrile species is shown in Figure 2. The data and observations are consistent with Scheme 111. The derived value for K (implying pK, = 5.3) is consistent with the independently measured pK, value of 5.7 f 0.1 for the malononitrile complex. The three paths are competitive but in the acid region nucleophilic attack of O H 2 at the nitrile C atom dominates the kinetics. Only 1% Co2+ was produced under these conditions. Howevet above the pK, for coordinated malononitrile the reduction path is more dominant (25%). Presumably a unimolecular in-
-
(22) These values are calculated using the following equilibrium constants 3.0,25 OC):I9 H C O 3 - e H+ + C0,2-, PK, = 9.56; CO2 + C0:- + H20 2HCO
