J . Chern. SOC., Faraday Trans. 1, 1982, 78,1525-1531 Kinetics of the Oxidation of Benzyl Alcohol by Tris(2,2’-bipyridine)nickel(111) Ions in Aqueous Perchlorate Media B Y DAVID F O X AND CECIL F. WELLS* Department of Chemistry, University of Birmingham, Edgbaston, P.O. Box 363, Birmingham B15 2TT Received 10th June, 1981 The oxidation of benzyl alcohol by [Ni(bipy),I3+ is first-order in both Ni”I and the alcohol. The observed independence of the second-order rate constant with acidity conforms with the following previous findings: (a) [Ni(bipy),13+ oxidizes substrates in the outer sphere without removal of a 2,2’-bipyridine from Ni”’; (b) equilibrium studies on solvent-sorting in the solvation of the proton predict that the tendency for benzyl alcohol (BzOH) to form an oxidatively inactive {H+(H,O),-, ROH) complex from ROH and (H+(H,O),} should be low.The transition-state parameters are compared for several substrates oxjdized by [Ni(bipy),13+. Following the kinetic investigation of the oxidation of various inorganic substrate ligands by [Ni(bipy)3]3+,1-3 an account is now given of the kinetic investigation of benzyl alcohol by this complex in aqueous perchlorate media. This complex participates in genuine cation + ligand redox reactions rather than in free radical oxidation, as the e.s.r. spectrum of the complex shows3 that it is a complex of the highly oxidizing Ni3+ cation and not a cation-radical complex involving NiII and a radical site on a bipyridine ligand. It was hoped that the oxidation of propan-2-01 by [Ni(bipy),13+ could be investigated kinetically to provide a comparison with the kinetic studies involving this alcohol and oxidizing aqua-~ations.~ However, although the rate of disappearance of [Ni(bipy),13+ in an excess of propan-2-01 in aqueous perchlorate media is first-order in the complex, the reaction is very slow and the observed rates at 20 O C are too close to that found for the oxidation of water by [Ni(bipy),13+ under the same conditions to provide accurate rate mea~urernents.~ At the other end of the scale, the rate of oxidation of p-benzohydroquinone by [Ni(bipy),13+ was found5 to be too fast to follow using the stopped-flow technique at 15 OC.However, the stoichiometry of this reaction is reported here, as it has a general significance for the mechanism of the oxidation of organic hydroxy compounds by this complex.Benzyl alcohol is oxidized by [Ni(bipy),I3+ at a convenient rate and, despite the restrictions imposed by the low solubility of the alcohol, a complete kinetic investigation involving the variation of [alcohol] and of [H+] was possible. EXPERIMENTAL MATERIALS [Ni(bipy),13+ was prepared as described A 5 x lo-* mol dm-, solution of [Fe(bipy),I2+ was prepared by dissolving the calculated weight of AnalaR FeSO, 7H,O and AnalaR 2,2’-bipyridine in water. Solutions of sodium perchlorate were prepared by the accurate neutralization of a solution of AnalaR HClO, with solid AnalaR sodium carbonate with boiling to expel CO, and subsequent filtration through Whatman no. 42 paper. Water was distilled 15251526 OXIDATION OF BENZYL ALCOHOL BY [Ni(bipy),13+ once in an all-glass still and AnalaR HC10, was used in reaction mixtures.Peroxy compounds were removed from the benzyl alcohol by shaking with a solution of Fe", followed by washing of the alcohol layer four times with distilled water and fractional distillation. The procedure used previously4 for the removal of carbonyl compounds from other alcohols proved to be impracticable with benzyl alcohol owing to elevation of the boiling point by the added 2,4-dini trophenylhydrazine. PROCEDURE The rates of disappearance of [Ni(bipy)J3+ were followed at 360 nm in the thermostatted cell compartment of a Unicam SP500 series 2 spectrophotometer. Water was circulated from a thermostat for the higher temperatures and a water + alcohol mixture was circulated from a cryostat for the lower temperatures.The initial [NiIIIJ was ca. 5 x lo-, mol dm-3. +p-benzoquinone reaction was measured using an excess of Ni"' and quenching with a solution containing [Fe(bipy),12+. The decrease in [Fe(bipy)32+] was determined spectrophotometrically at 522 nm and comparison with a blank solution with p-benzohydroquinone absent. The consumption ratio of the RESULTS AND DISCUSSION STOICHIOMETRY The application of the procedure for the estimation of carbonyl compounds6 used for a wide range of alcoholic materials with aqua-cations [e.g. ref. (4)] was not possible here owing to the uncertainty which exists for the extinction coefficient for benzaldehyde 2,4-dinitrophenylhydrazone in alkaline conditions and because the rate of oxidation of a dilute solution of benzyl alcohol by excess [Ni(bipy),13+ is comparable with the rate of oxidation of water by this complex.By comparison with the oxidation of simple alcohols by aqua-cations [e.g. ref. (4)], and as benzaldehyde is the principal product of the oxidation of benzyl alcohol by the permanganate a consumption ratio of IAINirrl]I/JAIBzOH]J = 2.0 would be expected. p-Benzohydroquinone also requires the removal of two electrons to produce benzoquinone, as found in its oxidation by aqua-catiom8 We found that IA[Nirrr]l/lA[QH,]1 = 2.0 0.1 (QH, = p-benzohydro- quinone) for [HClO,] = 1-5 mol dm-3, initial [NiII'] = 7.5 x mol dm-3 and initial [QH,] = 1.56 x mol dm-3; this supports the above assumption for the oxidation of benzyl alcohol in two one-electron steps by [Ni(bipy),13+. ORDERS OF REACTION Plots of log (optical density) against time for [C,H,CH,OH] + [NiIII] were always linear, showing that the reaction is first-order in [NilI1].Fig. 1 shows that the observed first-order rate contrast k , taken from the slopes of such first-order plots gives a straight line passing through the origin when plotted against the concentration of benzyl alcohol, showing that the reaction is also first-order in benzyl alcohol in 2.00 mol dm-3 HClO, at 3.00 OC. Values for k, are given in table 1, which shows that the rate is unaffected when the reaction is carried out under nitrogen. With the high yield of benzaldehyde from the oxidation by the permanganate ion7 it is unlikely that there will be attack of Nirrl on benzaldehyde when benzyl alcohol is in high excess over the Ni'II.VARIATION OF RATE WITH ACIDITY A N D TEMPERATURE Linear pseudo-first-order plots were observed for other acidities at 3.00 "C and values of k, are given in table 1 . Ionic strength was adjusted to 2.00 mol dm-3 by the addition of sodium perchlorate. Calculated values for the second-order rate constant k , given in table 1 show that k , is independent of acidity in the range 0.4- 2.0 mol dm-3 HClO, at a constant ionic strength of 2.00 rnol dmP3.D. FOX AND C. F. WELLS 1527 Linear pseudo-first-order plots were found also for 12.7, 25.3, 32.8 "C with acidity varying in the range 0.40-2.00 mol dm-, HCIO, and ionic strength maintained at 2.00 mol dm-3. Values for k , and k , are collected in table 1.The constancy of k , at constant acidity and temperature confirms that the reaction is first-order in [NiIII] and in [benzyl alcohol] and the invariance of k , at constant temperature with varying FIG. 1 5 10 15 20 1 0 [ benzyl alcohol] /mol dm-:% .-Variation of the pseudo-first-order rate constant k , with [BzOH] in 2.00 3.0 O C . mol dmP3 HCIO, at acidity confirms that the rate is independent of acidity. The mean values for k , at each temperature are also given in table 1. Fig. 2 shows that a plot of log k , against reciprocal of absolute temperature is linear and the application of the least-squares procedure gives the enthalpy of activation AH* = 77& 1 kJ mol-1 and the entropy of activation AS* = 2.0+ 1.3 J K-l mol-l. MECHANISM OF THE OXIDATION The consumption ratio found above for the oxidation of p-benzohydroquinone by [Ni(bipy),I3+, together with the evidence from the oxidation of other s~bstratesl-~ by [Ni(bipy),13+, and the consumption ratios [A[cation]l/IA[substrate]l = 2 usually found for the oxidation of alcohols by aqua-cations all suggest a two-step mechanism as in reactions (1) and (2): k2 Nil" + C,H,CH,OH -+ NilI1 + C,H,cHOH + H+ (1) with k , 9 k,.No evidence is found above for the existence of intermediate complex formation between [Ni(bipy),13+ and the alcohol and it is therefore concluded that the oxidation is an outer-sphere reaction without removal of a bipyridine from the NiIII, as found for the oxidation of H,O,,l Br-, and HN,., The absence of any kinetic effect of the hydrogen ion also supports this view, as a removal of a bipyridine from NilI11528 OXIDATION OF BENZYL ALCOHOL BY [Ni(bipy)J3+ TABLE 1 .-VALUES FOR THE PSEUDO-FIRST-ORDER RATE CONSTANT AND THE SECOND-ORDER RATE CONSTANT FOR THE OXIDATION OF BENZYL ALCOHOL BY [Ni(bipy),13+ AT IONIC STRENGTH 2.00 mol dm-l ~~ temp.[HCIO,I 1 03[BzOH] 10 k , /"C /mol dm-3 /mol dm-3 lo3 k,/ S-' /dm3 mol-' s-' 3.0 3 .O 3.0 3.0 3.0 3.0 3.0 3 .O 3 .O 3.0 3.0 3.0 3.0 12.7 12.7 12.7 12.7 12.7 12.7 12.7 12.7 12.7 12.7 12.7 12.7 12.7 12.7 25.3 25.3 25.3 25.3 25.3 25.3 25.3 25.3 25.3 25.3 25.3 25.3 25.3 32.8 32.8 32.8 32.8 32.8 32.8 32.8 32.8 32.8 32.8 2.00 15.4 0.69 2.00 30.9 1.32 2.00 62 2.76 2.00" 62 2.60 2.00 124 5.4 2.00 185 8.8 1.60 19.3 0.84 I .60 93 4.14 1 .oo 62 2.78 1 .oo 93 4.20 1 .oo 124 5.8 0.40 49.4 2.40 0.40 62 2.70 0.09 2.00 9.3 1.34 2.00 19.3 2.70 2.00 38.6 5.2 2.00 62 8.4 2.00 74 10.2 1.60 9.3 1.48 1.60 74 11.0 1 .oo 19.3 2.84 1 .oo 38.6 5.6 1 .oo 62 8.7 1 .oo 74 10.6 0.40 38.6 5.4 0.40 62 8.4 0.40 74 11.0 mean lo2 k,/dm3 mol-l s-' = 7.2k0.3 2.00 3.86 2.50 2.00 7.7 4.72 2.00 15.4 8.7 2.00 19.3 10.5 1.60 3.86 2.60 1.60 7.7 5.0 1.60 15.4 9.2 1.60 19.3 11.0 1 .oo 7.7 5.1 1 .oo 15.4 8.8 1 .oo 19.3 11.0 0.40 15.4 9.2 0.40 19.3 11.6 mean k,/dm3 mol-' s-' = 0.303 k0.020 2.00 2.35 1.51 2.00 3.86 2.49 2.00 6.2 3.80 2.00 9.3 5.7 1.60 3.86 2.53 1.60 6.2 3.90 1.60 9.3 5.7 1 .oo 6.2 4.08 1 .oo 9.3 6.0 0.40 12.3 7.7 mean k2/dm3 mol-' s-' = 0.64k0.02 mean lo2 k,/dm3 mol-l s-' = 2.24 0.224 0.214 0.223 0.210 0.218 0.238 0.218 0.223 0.224 0.226 0.232 0.243 0.218 0.72 0.70 0.68 0.68 0.69 0.80 0.74 0.74 0.72 0.7 1 0.72 0.70 0.68 0.74 3.24 3.07 2.84 2.75 3.37 3.25 2.99 2.85 3.29 2.86 2.85 2.99 3.01 6.4 6.5 6.1 6.1 6.6 6.3 6.1 6.6 6.5 6.3 a Under nitrogen.D.FOX AND C . F. WELLS 1529 3.2 3.3 3.4 3.5 3.6 3.7 lo3 KIT FIG. 2.-Variation of log (second-order rate constant) with reciprocal of absolute temperature at ionic strength = 2.00 mol dm-3. will be a~id-dependent.~'~ With its acid-independence the reaction resembles the oxidation of B r 2 and H202,1 the acid-dependence of the oxidation of HN, being ascribed to N; being the oxizable form of hydrazoic acid., The oxidation of secondary alcohols by Mn&1,47 CoIII aq, Vv aq and Ag& are all believed to be outer-sphere reactions without the involvement of intermediate cation + substrate complexes, whereas such complexes were found for the oxidation of secondary alcohols by Ceiz.4+ lo Intermediate cation + substrate complexes are believed to be involved in the oxidation of methanol by Mn:i1.l1 Following the observation12 of kinetic effects on redox reactions in aqueous solu- tions of solvent-sorting in the environment of protons, it was found that this solvent- sorting could be characterised using equilibrium measurements by a concentration quotient13 which increases markedly as the co-solvent concentration increases14 and changes the environment.It was then found4q that the relative insensitivity to changes in acidity of the rate of oxidation of secondary alcohols by Mnikl via an outer-sphere mechanism arises from a balance between the two equilibria (3) and (4) K h Mnii MnOHii + H,+, (3) Kh and K, being the concentration quotients for reactions (3) and (4), respectively, and the species (H+(H20)x-1 ROH) being unreactive towards oxidation.12 This assignment of mechanism for the outer-sphere oxidation by aqua-cations was supported by the kinetic investigation4 of the outer-sphere oxidation of propan-2-01 by Coit where Kh % Kh for Mnky: the above balance achieved with Mniy is destroyed and an acid-variation of the rate is ob~erved.~ Alternatively, this balance found with secondary alcohols and Mn;" will be destroyed producing an acid-variation in the rate if an alcohol is used which has K , @ K , for propan-2-01.As [Ni(bipy),13+ 50 FAR 11530 OXIDATION OF BENZYL ALCOHOL BY [Ni(bipy),I3t has no hydrolytic equilibrium analogous to reaction (3), the absence of an acid-variation on the rate of oxidation of benzyl alcohol is explainable if K , for benzyl alcohol is very low.Unfortunately, the solubility of benzyl alcohol in water is too low for the equilibrium methodl39 l4 involving spectrophotometric measurements on added p- nitroaniline to be used. However, the variation of K, with structure for a wide range of alcohols and carbonyl compounds shows that K, is decreased by the presence of an electron-withdrawing substituent and enhanced by the presence of an electron- releasing substituent. Although the phenyl group in phenol enhances K, by the electrons supplied to the basic oxygen atom by the n-bonding mechanism overriding the electron-withdrawing inductive effect of C6H5- as repre~entedl~ by the Taft o* function, the former will not operate at all in benzyl alcohol, allowing the latter effect to operate fully in producing this very low value for K,.TABLE 2.-cOMPARISON OF ENTHALPIES AND ENTROPIES OF ACTIVATION FOR THE OXIDATION OF SUBSTRATES BY [Ni(bipy),13+ IN AQUEOUS PERCHLORATE MEDIA A€€* AS* substrate /kJ mo1-l /J K-' mol-l H202 38+2 - 126f7 Br- 60f4 -3+11 N3 36+3 -3+10 C,H,CH,OH 77f 1 2 . 0 + 1.3 The values of AH* and AS* found for the oxidation of various sub~tratesl-~ by [Ni(bipy),13+ are compared with those for the oxidation of benzyl alcohol in table 2. The major contributions to AS* are1-, AS,*, arising from the release of oriented water following the lowering of the charge on [Ni(bipy),ln+ in the transition state and the loss of any charge on the substrate, and AS:, arising from the restriction imposed on the solvent in the transition state through any loss of a proton.Thus values of AS* for the similar reactions NiIII + Br- and NilI1 + N;, where only AS,* contributes, agree very well ; whereas AS* for NiIII + H202 has a large negative value through AS: counterbalancing AS,*. However, NilI1 + C,H5CH20H, which is very similar to NiI" + H202 (both have a radical and H+ in addition to NiII as products), has a small positive AS*. As the contributions arising from AS,* for these last two reactions should be broadly similar, the difference found suggests that there might be a different mechanism for the proton release in the two cases.It is possible that, whereas electron release and proton release are coincident with H202, they may be sequential with C6H5CH20H. In the latter case an electron may be lost first from the n-system of the phenyl group and the proton lost in a later rearrangement of charge and electron density. The restriction imposed on the solvent molecules by the diffuse charge on the phenyl group will be much less than that imposed by a proton, resulting in a higher entropy in the transition state and a smaller negative or a small positive AS* value, as observed. C. F. Wells and D. Fox, J. Chem. SOC., Dalton Trans., 1977, 1498. C. F. Wells and D. Fox, J. Chem. SOC., Dalton Trans., 1977, 1502. J. K. Brown, D. Fox, M. P. Heyward and C. F. Wells, J. Chem. SOC., Dalton Trans., 1979, 735.C. F. Wells and G. Davies, Trans. Faraday Soc., 1967, 63, 2737; C. F. Wells and M. Husain, Trans. Faraday Soc., 1970,66,679; R. Varadarajan and C. F. Wells, J. Chem. Soc., Faraday Trans. I , 1973, 69,521 ; C. F. Wells and A. F. M. Nazer, J. Chem. SOC., Faraday Trans. I , 1976,72,910; C. F. Wells and D. Fox, J. Inorg. Nucl. Chem., 1976, 38, 287.D. FOX AND C . F. WELLS 1531 D. Fox and C. F. Wells, unpublished results. ti C. F. Wells, Tetrahedron, 1966, 22, 2685. ’ K. K. Banerji and P. Nath, Bull. Chem. SOC. Jpn, 1969, 42, 2038. * C. F. Wells and L. V. Kuritsyn, J. Chem. SOC. A , 1969, 2575, 2931; 1970, 676, 1372. l o C. F. Wells and M. Husain, Trans. Furuduy Sac., 1970, 66, 2855. l 2 C. F. Wells, Discuss. Faraday SOC., 1960, 29, 219; Trans. Faraday SOC., 1961, 57, 1703, 1719. l3 C. F. Wells, Trans. Furaday SOC., 1965, 61, 2194; 1966, 62, 2815; 1967, 63, 147; Hydrogen-bonded Solvent Systems, ed. A. K. Covington and P. Jones (Taylor and Francis, London, 1968), pp. 323-324; J , Phys. Chem., 1973, 77, 1994, 1997; J . Chem. SOC., Faruduy Trans. I , 1972, 68, 993. C. F. Wells, J . Chem. SOC., Faruduy Trans. I , 1973, 69,984; 1974,70, 694; 1975,71, 1868; 1976,72, 601; 1978, 74, 636, 1569; 1981, 77, 1515; Adv. Chem. Ser., 1~79, 177, 53; G. S. Groves and C. F. Wells, unpublished results. C. F. Wells, C. Barnes and G. Davies, Trans. Faraday SOC., 1968, 64, 3069. C. F. Wells and C . Barnes, J. Chem. SOC. A , 1968, 1626; 1971, 430. l5 K . B. Wiberg, Ph-ysical Organic Chemistry (Wiley, New York, 1964), p. 415. (PAPER 1/933) 50-2