Acc. Chem. Res. 2000, 33, 207-214

of radicals with different substrates and the study of the High-Pressure Pulse Radiolysis properties of transients formed in these reactions, as well as a Tool in the Study of as for the study of the chemical properties of transition metal complexes with uncommon oxidation states, e.g., Transition Metal Reaction Ni(I),2,3 Co(I),4 Zn(I),2 Ni(III),5 Cu(III),6 and Fe(V).7 Thus, pulse radiolysis enables the study of the proper- Mechanisms ties of key intermediates in a large variety of catalytic RUDI VAN ELDIK† AND DAN MEYERSTEIN*,‡ processes. This technique is also used for the study of fast Institute for Inorganic Chemistry, University of reactions between relatively stable compounds, e.g., III - 8 + 9 2+ 10 Erlangen-Nuernberg, Egerlandstrasse 1, reactions of Ni edta , Cu(phen)2 , Cr(H2O)6 , and 91058 Erlangen, Germany, Chemistry Department, nickel (1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradec- Ben-Gurion University of the Negev, Beer-Sheva, Israel, and ane)+ 11 with dioxygen; ligand exchange reactions on Cu- The College of Judea and Samaria, Ariel, Israel (I),12 Co(II),13 Cr(II),13 and Ru(II);13 and redox processes, Received April 21, 1999 + - 14 - e.g., the reactions of Cu aq with Cl3CCOO and ONOO with I-.15 ABSTRACT The measurement of the volume of activation, ∆V#, and The unique combination of high-pressure and pulse radiolysis the overall reaction volume, ∆V°, has been shown to be a kinetic techniques enables detailed mechanistic insight for a large variety of chemical processes to be gained. Typical examples for powerful technique to obtain detailed information on the transition metal reactions are presented. These include ligand nature of the transition state for a large variety of inorganic substitution, electron transfer, reactions of complexes with uncom- reactions.16 The volume of activation is determined from mon oxidation states, and reactions with radicals, including the - rate constant (k) measurements performed as a function formation and decomposition reactions of complexes with metal - carbon σ bonds. The volumes of activation and volume profiles of pressure (usually in the range 0.1 200 MPa) and from # obtained form the basis of a critical analysis of different plausible application of the relationship (σ ln k/σP)T )-∆V /RT. reaction mechanisms. Special instrumentation is required to perform kinetic experiments at elevated pressure. ∆V# can be determined Pulse radiolysis is a powerful technique for the elucidation rather accurately from the slope of ln k vs P. The overall of a variety of inorganic reaction mechanisms, especially reaction volume can be determined from the difference in aqueous solutions.1 This technique is based on the in the partial molar volumes of the reactant and product - • • formation of the radicals e aq,H, and OH by the radiolysis species (usually obtained from density measurements), or of water.1 These radicals can be transformed into a single from the pressure dependence of the equilibrium constant reducing or oxidizing radical, or a mixture thereof, via the (similar to that given for a rate constant above) in the case use of suitable scavengers.1 In pulse radiolysis experi- of a reversible reaction. From this information, a volume ments, observable concentrations of the desired radical profile for a particular reaction can be constructed, which are formed within a short period, e.g., <2 µs. Therefore, represents the chemical process in terms of volume this technique is the optimal one for the study of the changes along the reaction coordinate.16 Such volume kinetics and mechanisms of the reactions of a large variety changes can be analyzed in terms of intrinsic (due to changes in bond lengths and bond angles) and solvational (due to changes in electrostriction as a result of changes Rudi van Eldik was born in Amsterdam, Holland, in 1945 and grew up in in charge and/or dipole moment) volume contributions. Johannesburg, South Africa. He received his Ph.D. degree (1971) from the Potchefstroom University. He spent two years (1972 and 1978) with Gordon M. There are numerous examples now available from the Harris at SUNY at Buffalo, NY, and one year as an Alexander von Humboldt literature of cases in which this technique has contributed Fellow with Hartwig Kelm at the University of Frankfurt, Germany. He was group significantly to the elucidation of the intimate mechanisms leader at the Institute for Physical Chemistry, University of Frankfurt (1980-1987) and Professor of Inorganic Chemistry at the University of Witten/Herdecke (1987- of thermal and photoinduced reactions in inorganic, 1994). He is presently Professor of Inorganic and Analytical Chemistry at the bioinorganic, and organometallic chemistry.16 University of Erlangen-Nurnberg. In 1997 he received an honorary doctorate from It seemed therefore logical to combine these unique the Potchefstroom University. His research interests are in mechanistic studies techniques and to measure ∆V# and ∆V° for reactions of inorganic, organometallic, and bioinorganic reactions, with special emphasis on the application of fast kinetic and high-pressure techniques. studied by pulse radiolysis with the aim of obtaining a more detailed insight into the mechanisms of such Dan Meyerstein was born in Jerusalem, Israel, in 1938. He received his M.Sc. processes. The main difficulty in developing the combined (1961) and Ph.D. (1965) degrees from the Hebrew University of Jerusalem. He spent three years (1966-67 and 1978) with Dr. Max S. Matheson at Argonne technique was that, in many pulse radiolysis setups, National Laboratory, Argonne, IL. Since 1968 he has been at the Ben-Gurion electrons with an energy e5 MeV are used. The penetra- University of the Negev, Beer-Sheva, Israel, where he has been the Irene Evens tion of these electrons through windows which are Professor of Inorganic Chemistry since 1987. He was president of the Israel Chemical Society (1988-1991). Since 1995 he has been president of the College resistant to high pressure, e.g. sapphire, is limited. A of Judea and Samaria, Ariel, Israel. He received the Meitner-Humboldt Research stainless steel window with a special geometric design Prize (1997) and the Kolthoff Prize (1998). His main research interests include (Figures 1 and 2) was therefore required. This window radical reactions with transition metal complexes, transition metal complexes with uncommon oxidation states, ligand design, complexes with metal-carbon † University of Erlangen-Nuernberg. σ bonds in aqueous solution, catalysis using pulse radiolysis, stopped-flow, ‡ Ben-Gurion University of the Negev and The College of Judea and electrochemistry, etc. Samaria.

10.1021/ar990067v CCC: $19.00  2000 American Chemical Society VOL. 33, NO. 4, 2000 / ACCOUNTS OF CHEMICAL RESEARCH 207 Published on Web 01/28/2000 High-Pressure Pulse Radiolysis van Eldik and Meyerstein

# # measured ∆V equals ∆V1° + ∆V2° + ∆V3 and usually cannot be separated for each step in the process. In the following sections the application of high- pressure pulse radiolysis in different types of reactions is discussed.

Oxidations by •OH Radicals Most of the reactions of •OH radicals with low-valent transition metal complexes have specific rates approach- ing the diffusion-controlled limit. There is no point in studying their pressure dependence, since pressure will FIGURE 1. Schematic diagram of the electron beam window for high-pressure pulse radiolysis. The electron beam enters from the only affect the viscosity of the medium, which in turn will right side. control the diffusion process. One of the exceptions is the 2+ 6- oxidation of [Cu(H2O)n] and [Cu(P2O7)2(H2O)2] by hydroxyl radicals. The volumes of activation for these reactions are close to zero, i.e., + 0.718 and -1.8 cm3 mol-1,19 respectively. These results rule out the possibility that electron transfer via either the outer-sphere or the inner-sphere mechanisms is the rate-determining step:18 Electron-transfer steps which involve the creation of

k n+ + • w\Kx { n+ • } 9et8 [M(H2O)m] OH M(H2O)m , OH { (n+1)+ + - M(H2O)m] OH aq (4)

k FIGURE 2. Schematic diagram of the pillbox sample cell showing n+ + • w\K x n+ 9et8 [M(H2O)m] OH - [(H2O)m-1M(OH)] the electron beam and light beam paths and the irradiated volume. H2O + n+1 - n+ 9H 8 (n+1)+ [(H2O)m-1M (OH )] [M(H2O)m] (5) enables the study of a large variety of processes using the pulse radiolysis technique at pressures up to 200 MPa.17 charges are expected to have significant negative volumes It should be kept in mind that measurement of volumes of activation due to changes in electrostriction.16 Also, of activation is meaningful only for processes which meet hydrogen abstraction from one of the water molecule the following criteria:16 ligands according to (1) The process is not diffusion controlled, or ap- n+ + • f n+ + proaching this limit, since such studies measure the effect [M(H2O)m] O*H [(H2O)m-1M(OH)] H2O* of pressure on the diffusion rate. + n+ f n+1 - n+ 9H 8 (2) The process is not pH dependent in the pH range [(H2O)m-1M(OH)] [(H2O)m-1M (OH )] e e + + 5 pH 9, as the dissociation of water depends on pH [M(H O) ](n 1) (6) and is affected by pressure. 2 m (3) The process is not general base or acid catalyzed, (where the asterisk is introduced to distinguish this as the dissociation of any acid or base strongly depends reaction from reaction 7 below) can be ruled out, as the on the pressure. Therefore, for a system of the type hydrogen abstraction can occur only if the OH in the product strongly interacts with the central cation, i.e., if partial oxidation, and therefore an increase in electro- striction, occurs in the abstraction step.18 It was therefore concluded18 that the almost zero volume of activation favors the following mechanism:

. < n+ + • f n+ + measurements if k1 k2 are meaningful only at pH [M(H2O)m] O*H [(H2O)m-1M(O*H)] H2O - . > + pKa 1, and if k2 k1 only at pH pKa 1. + n+ f n+1 - n+ 9H 8 (4) The process is not complex, i.e., [(H2O)m-1M(O*H)] [(H2O)m-1M (O*H )] + + [M(H O) ](n 1) (7) + H + 2 m A B C D K1 (1) C H C′ K (2) where the rate-determining step involves ligand exchange. 2 This ligand exchange reaction then has to proceed via an ′ f C P k3 (3) interchange mechanism, I, and in the case of [Cu(P2O7)2- 6- (H2O)2] probably via a slightly associative interchange 19 where the equilibria 1 and 2 are fast, K1 , 1, K2 , 1, and mechanism, Ia, in order to rationalize the small volumes k3 is the rate-determining step. In such systems, the of activation observed.

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II • Oxidation by Other Inorganic Radicals Scheme 1. Mechanism of Oxidation of Ni LbyHO2 The volumes of activation for the oxidation of a variety of •- Mn(II), Fe(II), Co(II), and Ni(II) complexes by Br2 , •- • • 20,21 (SCN)2 , O2CH3, and HO2 were measured. The results clearly indicate that the rate-determining step in all these reactions, which proceed via the inner-sphere mechanism, III forming M X (where X ) Br, SCN, O2CH3,orO2H), is the ligand interchange step. The volumes of activation are small, slightly positive (<6cm3 mol-1) for the Ni(II) and

Co(II) complexes, i.e., indicating an Id mechanism, and around zero or slightly negative (-1to-4cm3 mol-1) for the Fe(II) and Mn(II) complexes, i.e., indicating a pure I or an Ia mechanism for these complexes, in agreement with expectations.20 The displacement of coordinated water by a nucleophile in these reactions closely resembles the Z axis and have to be displaced in the process. This the water exchange process on such metal complexes, for conclusion is in accord with earlier observations.22 The - which typical volumes of activation of around +6 and -6 formation of the contact ion pairs, {NiIIL;X }, accelerates 3 -1 cm mol for Id and Ia mechanisms, respectively, have the rate of the reaction; i.e., k2 is ca. 10 times larger than • been reported.16 In the case of limiting D and A mecha- k3, and under these conditions the reaction with the HO2 nisms, significantly larger values of around +13 and -13 radicals becomes an associative process, as expected for 8 21 cm3 mol-1, respectively, are expected. a low-spin planar d nickel complex. •- For the oxidation of a series of Fe(II) complexes by Br2 an interesting trend is observed.20 The faster the reaction, Reactions of Transition Metal Complexes with i.e., the faster the ligand exchange process, the more Alkyl Radicals: Formation of Metal-Carbon σ negative the volume of activation. In other words, the Bonds more labile complexes tend to react via an Ia mechanism, In principle, the reactions whereas the less labile complexes react via an Id or a pure I mechanism.20 Alternatively, it could be argued that the n + • f n+1- + M Lm R Lm-1M R L (8) faster reactions involve an “early” transition state, i.e., mainly an associative process, and the slower reactions are inner-sphere oxidation processes. Indeed, the mea- involve a later transition state, i.e., at a stage where partial surement of the volumes of activation for the reactions - • 2+ 3 -1 23 Br Br bond breakage already occurs. of CH3 radicals with [Cr(H2O)6] (+6.3 cm mol ), - - - II - II - 3. 1 20 II For the oxidation of [Mn (N(CH2CH2CO2 )3)aq]by [Fe (nta)(H2O)2] ( 0.3 cm mol ), [Co (nta)(H2O)2] - •- # 3 -1 + 3 1 24 II (SCN)2 , ∆V ) 2.8 cm mol ; i.e., surprisingly a small ( 6.0 cm mol ), and [Ni (trans-III-1,4,8,11-tetraazacy- + + - positive volume of activation is observed.20 This observa- clotetradecane)]2 (NiL′ 2 )(+4.0 cm3.mol 1)22 points out tion is probably related to the fact that this reaction is that the rate-determining step of all these reactions is the II - considerably less exothermic than the others studied and ligand interchange step, pure I for the [Fe (nta)(H2O)2] 20 therefore has a relatively late transition state. system and Id for the other systems. Of special interest is the oxidation of [NiII(trans-III-C- Of special interest is the study of the reaction of [Cr- 2+ 23 meso-5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotet- (H2O)6] with a variety of alkyl radicals. The specific 2+ II • 21 rates of these reactions for a large variety of radicals, with radecane)] ,Ni L, by HO2 . This reaction is endothermic 2- a varying degree of steric hindrance, are in the range 2 × in the absence of a stabilizing axial ligand, e.g., SO4 . The 7 e e × 8 -1 -1 reaction occurs, therefore, only in the presence of such a 10 k 3 10 M s . Furthermore, the volumes of ligand. The experimental data are in accord with the activation of these reactions are all small positive, with ( 3 -1 23 mechanism presented in Scheme 1. A detailed study an average value of 4.3 1.0 cm mol . These results enabled the determination of the following values: indicate that the rate-determining step in these reactions # 3 -1 # is the ligand exchange step and that this step occurs via ∆V (k3) ) 1.4 ( 1.0 cm mol ; ∆V (k2) )-7.2 ( 1.0 and - - - the I mechanism. This observation suggests that ligand -7.7 ( 1.0 cm3 mol 1 for X ) SO 2 and H PO , respec- d 4 2 4 2+ - and solvent exchange on Cr(H O) occurs via the I tively;21 ∆V°(K) ) 7 ( 2 and -5 ( 2cm3 mol 1 for X ) 2 6 d 2- - 21 mechanism, and not via the Ia mechanism which occurs SO4 and H2PO4 , respectively. The values of ∆V°(K) are for the analogous reactions on V(H O) 2+ and Mn(H O) 2+. in accord with expectations for the electrostatic interaction 2 6 2 6 The deviation for Cr(H O) 2+ is attributed to the Jahn- ofa2+ cation with 2- and 1- charged nucleophiles.21 2 6 # Teller distortion, which facilitates a dissociative mecha- The value of ∆V (k3) is in accord with the suggestion that nism.23 As the volume of activation of the homolysis the rate-determining step of this reaction is a ligand 21 reaction exchange process occurring via the Id mechanism. This is a surprising result as the NiIIL complex has a low-spin + [(H O) CrIII-C(CH ) OH]2 + H O f d8 planar configuration. It was therefore concluded that 2 5 3 2 2 2+ + • water molecules must be present at some distance along [Cr(H2O)6] C(CH3)2OH (9)

VOL. 33, NO. 4, 2000 / ACCOUNTS OF CHEMICAL RESEARCH 209 High-Pressure Pulse Radiolysis van Eldik and Meyerstein

Scheme 2. Volume Profile for the Formation and Homolysis of Scheme 3. Volume Profile for the Reverse Reaction 10, III 2+ • II II 3 -1 [(H2O)5Cr -C(CH3)2OH] (R ) C(CH3)2OH; Volumes in Units of M L(H2O)2 ) Co (nta)(H2O)2- (Volumes in Units of cm mol ) cm3 mol-1)

(2) The volume of reaction 8 is always significantly 25 was known, the full volume profile for this equilibrium negative, as expected for an associative process which process could be constructed (Scheme 2). This scheme furthermore involves partial oxidation of the central clearly demonstrates that the formation and homolysis of cation. However, the absolute values of the reaction - the Cr C bond have a dissociative character, as expected. volumes, ∆V°, are more complex to interpret: (a) The The partial molar volume of the transition state of this value observed in the chromium system23 (Scheme 2) is reaction is significantly larger than those of the reactant in accord with expectations based on the known partial and the product species. molar volumes of the exchanging radical and water ligand. A special case is the effect of acetate on the reaction (b) The considerably more negative ∆V° in the cobalt of •C(CH ) OH radicals with Cr(II).26 The addition of 3 2 system24 (Scheme 3) is somewhat surprising and is at- acetate slows the rate of the reaction. An analysis of the tributed to the large difference in the radii of high-spin data suggests that the complexes [Cr(H O) ]2+, [Cr(H O) - 2 6 2 5 CoII and low-spin CoIII complexes. (c) The even larger (CH CO -)]+, and [Cr(H O) (CH CO -) ] have a similar 3 2 2 4 3 2 2 negative ∆V° observed in the nickel system22 (Scheme 4) reactivity toward the radical, whereas the dimer, [Cr(H O)- 2 is attributed to the fact that the NiIIL′ complex is a low- (CH CO -) ] , is practically unreactive.26 This observation 3 2 2 2 spin d8 planar complex. Thus, the two water molecules clearly demonstrates that the quadruple Cr-Cr bond located near the nickel divalent cation in Scheme 4 are considerably affects the rate of the ligand exchange of the not bound to it but are rather located in cavities formed axial water ligand. Also, the volume of activation for the by the macrocyclic ligand.22 The oxidation of the complex reaction decreases and becomes more negative on in- forms an octahedral low-spin d7 complex. Thus, a water creasing the acetate concentration. This effect is attributed molecule is ligated as an additional ligand during the to the large negative reaction volume for the dissociation oxidation reaction. This change in the coordination sphere of the dimer, and not to an intrinsic property of the accounts for the large negative ∆V° observed. Finally, in reaction with the radical.26 theoretical simulations of complexes with NiIII-C bonds,27 octahedral and not pentacoordinated complexes, as re- Homolysis of Metal-Carbon σ Bonds cently suggested, have to be considered. The volume of activation for the homolysis of the metal- III- - 24 ′ carbon σ bond in [(nta)(H2O)Co CH3] and in [L - Heterolysis of Metal-Carbon σ Bonds (H O)NiIII-CH ]2+ 22 was determined by following the 2 3 Heterolysis of the metal-carbon bond, homolytic insertion of dioxygen into the metal-carbon σ bond, which occurs via the reactions f n+1 + + - M Lm RH OH (13) III • II L(H O)M -CH + H O H CH + M L(H O) + 2 3 2 3 2 2 L Mn 1-R + H O s II + m 2 (or M L 2H2O ) (10) f n-1 + - + + M Lm ROH/R( H) H3O • + f • CH3 O2 CH3O2 (11) (14) II II + + • f M L(H2O)2 (or M L 2H2O) CH3O2 is the major decomposition mechanism of complexes with III- + metal-carbon σ bonds in aqueous solutions. The activa- L(H2O)M OOCH3 H2O (12) tion volumes for the heterolysis of several, relatively stable III 2+ III The results are summarized in Schemes 3 and 4. A complexes, e.g., [(H2O)5Cr -CH2OH] ,withaCr -C comparison of the results presented in Schemes 2-4 bond which proceed via reaction 13 were measured.25 reveals several interesting conclusions:22,24 These reactions are acid catalyzed; i.e., their rate of H + (1) All the reactions proceed via an Id mechanism. This reaction obeys the equation k13 ) k° + k [H3O ]. It was means that measuring ∆H# of the homolysis reaction does found that ∆V# for both the acid-independent and acid- not yield the M-C bond strength as proposed in the catalyzed pathways are approximately zero.25 This result literature.24 was interpreted as suggesting that reaction 13 can be

210 ACCOUNTS OF CHEMICAL RESEARCH / VOL. 33, NO. 4, 2000 High-Pressure Pulse Radiolysis van Eldik and Meyerstein

II 2+ 3 -1 Scheme 4. Volume Profile for the Reverse Reaction 10, M L(H2O)2 ) Ni(L′)(H2O)2 (Volumes in Units of cm mol )

Scheme 5. General Base Catalysis of the Heterolysis of CrIII-C Decomposition of Complexes with III- 2+ Bonds in [(H2O)5Cr R] Metal-Carbon σ Bonds via â-Elimination Reactions n+1 1 2 3 4 Many complexes of the type LmM -CR R CR R X, where X is a good leaving group, decompose via n+1- 1 2 3 4 f LmM CR R CR R X n+1 + 1 2 d 3 4 + - considered as a substitution reaction of the Cr(III) central M Lm R R C CR R X (16) cation which occurs via an interchange, I, mechanism.25 Reaction 13 for Mn+1 ) CrIII was shown to be also The volumes of activation of such reactions for a variety III- 1 ′ 2+ general base catalyzed; i.e., it proceeds via the mechanism of complexes of the type [(H2O)5Cr CHR CH2OR ] , ′ ) 29 described in Scheme 5.28 The volumes of activation for where R H or alkyl, were measured. It is known that several of these reactions were measured.28 It was found all these reactions have an acid-catalyzed and an acid- - # 3 independent pathway and that the first product formed that for a large variety of anions, An , ∆V (k2) ≈10 cm mol-1. These results were interpreted as indicating that is a π-complex between the produced alkene and the - < the An- ligand, usually in the trans position to R, weakens central metal cation. The results indicate that 7.0 # <- 3 -1 the CrIII-C bond, thus shifting the mechanism from a pure ∆V 4.0 cm mol for all these reactions and that 28 these values are independent of the pH; i.e., the volumes ItoanId mechanism. To check whether the positive volume of activation is, of activation for the acid-independent and the acid- 29 indeed, due to an intrinsic effect induced by the An- catalyzed pathways are identical. These results suggest ligand, and not just due to the higher rate of reaction, ∆V# that the mechanism of these reactions can be formulated III as in Scheme 6. It is proposed that the protonation for the heterolytic decomposition of [(H2O)5Cr -CH2CH- 2+ 29 equilibrium does not involve a large volume change since (OH)2] was measured. The rate of heterolysis of the CrIII-C bond in this complex is considerably faster than it does not involve a change in the net charge. For both # pathways it is proposed that the formation of the transi- that of k2 for the anion-catalyzed reactions. However, ∆V is only 3.3 and 1.9 cm3 mol-1 for the spontaneous and tion state involves a coherent partial ring-closure process, acid-catalyzed pathways of this reaction.29 This result, which is associated with a volume collapse, and a stretch- - therefore, confirms the earlier conclusions. ing of the C O bond, which is associated with a volume 29 ∆V# )-8.7 cm3 mol-1 was determined29 for the increase. The results clearly demonstrate that the former reaction process has the dominant effect. The volumes of activation for the â-elimination of III- 2+ + + f 3+ + ammonia from [(H O) CrIII-CH C(CH ) NH +]3+ and from [(H2O)5Cr H] H3O [Cr(H2O)6] H2 (15) 2 5 2 3 2 3 II + 2+ [(H2O)mCu -CH2C(CH3)2NH3 ] were also measured and which is the fastest of these reactions. This result is in found to be + 3.1 and + 3.6 cm3 mol-1 respectively.29 The agreement with the small size of the leaving group, which ca. 10 cm3 mol-1 difference between the volumes of - is overruled by the volume decrease due to the coordina- activation for the â-elimination of OR and NH3 is tion of the larger water molecule and the net concentra- attributed to the fact that the C-N bond is stronger than tion of charge. the C-O bond, which makes the â-elimination of am-

VOL. 33, NO. 4, 2000 / ACCOUNTS OF CHEMICAL RESEARCH 211 High-Pressure Pulse Radiolysis van Eldik and Meyerstein

Scheme 6. Proposed Mechanism for â-Elimination Reactions from Scheme 7. Volume Profiles for Reaction 18, L ) 4-Ethylpyridine III 1 2+ Complexes of the Type [(H2O)5Cr -CHR CH2OR′]

major volume change on going from the reactants to the transition state is due to the increased electrostriction at the ruthenium center. Data in the literature suggest that 2+ 3+ the oxidation of [Ru(NH3)6] to [Ru(NH3)6] is ac- companied by a volume decrease of ca. 30 cm3 mol-1;33 - monia considerably slower than that of OR .30 Therefore, thus, the activation volumes quoted above could princi- - the transition state in the elimination of OR is probably pally be a consequence of the volume changes associated an early one, i.e., at the stage of ring closure, and in the with the oxidation of the redox partner. This conclusion ammonia system a late one, i.e., at the stage of the suggests that the enzyme does not change its volume cleavage of the C-N bond. during the redox process and thus probably requires only minor rearrangement prior to the electron-transfer step. Intramolecular Electron-Transfer Processes This was confirmed later through application of high- pressure electrochemical measurements on cytochrome Due to the importance of understanding the factors c and ruthenated cytochrome c.34 More recently,35 pulse affecting the rate of intramolecular electron-transfer radiolysis techniques were used to study the effect of processes in inorganic and bioinorganic systems, the pressure on a series of intramolecular electron-transfer volume of activation for a number of such reactions was reactions of the type studied. III- II f II- III (a) The volume of activation for the reaction (NH3)4(L)Ru Cyt c (NH3)4(L)Ru Cyt c (18)

III- - • - f which resulted in the construction of the first complete [(NH3)5Co p OC(O)C6H4NO2 ] + - volume profiles for such intramolecular electron-transfer Co2 + 5NH + p- O CC H NO (17) aq 3 2 6 4 2 processes. The volume profiles demonstrated a significant volume increase associated with the reduction of the 31 was measured. This reaction was chosen as a model ruthenium center. In contrast to earlier results on a series system as it is a simple one and, although the electron is intermolecular reactions involving cytochrome c and the transferred only over a short distance, it is a relatively slow corresponding series of pentaamine complexes, for which ) × 3 -1 ° 31 # ) process, k 2.6 10 s at 25 C. The result, ∆V the volume profiles were reported to be completely + 3 -1 7.5 cm mol , suggests an early transition state with an symmetrical,36 the studied intramolecular reactions exhibit intrinsic volume increase due to partial reduction of the asymmetric volume profiles (a typical example is shown Co(III) central cation and desolvation of the anion radical in Scheme 7). The overall volume change can be ac- 31 ligand. counted for in terms of electrostriction effects centered (b) Of special interest are the intramolecular electron- around the amine ligands on the ruthenium center. transfer reactions in metal complex-modified proteins. A challenging question in such studies is whether “long- Properties of Complexes with Uncommon distance” electron-transfer reactions exhibit characteristic Oxidation States pressure dependencies that could reveal further informa- tion on the electron-transfer mechanism itself.32 Pulse The pulse radiolysis technique enables the study of the radiolysis techniques were used to measure the volumes properties of various complexes with uncommon, and of activation for the intramolecular electron transfer in usually unstable, oxidation states. The volumes of activa- II tion for the reactions of several such complexes were (NH3)5Ru -His33 horse heart ferricytochrome c and in II studied. (NH3)5Ru -His39 Candida krusei ferricytochrome c. For both systems, ∆V# ≈ - 18 cm3 mol-1 was observed.32 For (a) Cu(III) Complexes. The volume of activation for the ligand exchange reaction comparison purposes, the volume of activation of the intermolecular reduction of horse heart ferricytochrome III - 6- f 2+ # 3 -1 [Cu (P2O7)2(H2O)(OH )] c by [Ru(NH3)6] was measured, ∆V )-15.6 cm mol . III - 2- + 4- These results were interpreted as pointing out that the [Cu (P2O7)(H2O)(OH )] P2O7 (19)

212 ACCOUNTS OF CHEMICAL RESEARCH / VOL. 33, NO. 4, 2000 High-Pressure Pulse Radiolysis van Eldik and Meyerstein

Scheme 8. Mechanism of Decomposition of the CuIII(GlyGlyHis) Complex

I Scheme 9. Mechanism of Reaction of Cu (L) with Dioxygen k3 have the same values for L ) (H2O)n and (phen)2, 3 -1 38 ∆V°(1) )-8.9 cm mol is observed for L ) (H2O)n. Several plausible sources for the difference in ∆V°(1) for the two systems were discussed.38 Concluding Remarks was determined, ∆V# )-2.4 cm3 mol-1.19 It should be It was the aim of this Account to point out the advantage noted that Cu(III) has a d8 electronic configuration, and of combining pulse radiolysis with high-pressure tech- its complexes are therefore expected be planar. Thus, the niques for the elucidation of the detailed mechanisms of coordination geometry might differ from that proposed. a large variety of inorganic reactions. Volumes of activa- The slight negative volume of activation probably indicates tion, along with the construction of volume profiles as that the ligand exchange process occurs either via an outlined in this presentation, can discriminate between associative mechanism, as expected, with some compen- various mechanistic possibilities and support the assign- sation due to charge dilution, or via an I mechanism.19 a ment of a specific mechanism. In general, activation The mechanism of decomposition of the CuIII(GlyGly- volumes can be determined more accurately than activa- His) complex was proposed to proceed via the mechanism tion entropies due to the inherent extrapolation involved outlined in Scheme 8.6 However, only two consecutive in the determination of the latter parameter. A correlation reactions were observed experimentally, and the question between these parameters should, in principle, exist since was, which ones are they? The first of these reactions has they both reveal information on order in the transition ∆V# )+14 cm3 mol-1,6 which is difficult to reconcile with state. However, such correlations usually exist only in a closure of the free carboxylate to form a chelate ring. cases where the activation entropy has a large absolute This value is, however, in good agreement with expecta- value. In the case of smaller absolute values, the uncer- tions for a decarboxylation process. It was therefore tainty in the activation entropy is too large for this concluded that decarboxylation is the rate-determining parameter to be considered as a reliable mechanistic step observed experimentally.6 The ∆V# of the second indicator. Therefore, the more accurately determinable experimentally observed reaction is +8cm3 mol-1,in activation volume is, indeed, a powerful mechanistic agreement with expectations for the heterolytic cleavage discrimination parameter. It is hoped that this review will of the CuIII-C bond, as proposed.6 also convince other research groups to apply this tech- (b) Cu(I) Complexes. The kinetics of reaction of CuI- 37 9 nique to other mechanistic studies. (L), where L ) (H2O)n or (phen)2, with dioxygen were studied. The measured reaction volume for equilibrium The authors are indebted to their co-workers whose studies are 1 in Scheme 9, ∆V°(1) )-22 cm3 mol-1 in the phenan- cited in this review, since it is their work that has enabled the throline system,9 clearly demonstrates that the reaction rather unique combination of high-pressure and pulse radiolysis has to proceed via the formation of an intermediate in kinetic techniques. They gratefully acknowledge financial support which a bond is formed, since an outer-sphere redox from the Deutsche Forschungsgemeinschaft, Volkswagen Founda- tion (R.v.E.), The German-Israeli Foundation for Scientific Re- process, in which the charge of the complex is increased + + search and Development (R.v.E., D.M.), the Budgeting and Plan- from 1 to 2 and a monovalent anion is formed, is ning Committee of the Council of Higher Education, the Israel expected to have a considerably smaller negative volume Atomic Energy Commission, and the Alexander von Humboldt # 3 -1 of activation. The results suggest that ∆V (2) ≈ 0cm mol Foundation (D.M.). R.v.E. appreciates the stimulating collabora- in the phenanthroline system and that k3 approaches the tion with Jim Wishart (Brookhaven National Laboratory) on the 9 diffusion-controlled limit. Surprisingly, though K1 and development and application of high-pressure pulse radiolysis.

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(22) van Eldik, R.; Cohen, H.; Meshulam, A.; Meyerstein, D. The Methyl References (cyclam) nickel (III) Dication in Aqueous Solutions: Determination (1) Tabata, Y. Pulse Radiolysis; CRC Press: Boca Raton, FL, 1990. of the Volume of Reaction and Volume of Activation for the (2) Meyerstein, D.; Mulac, W. A. Reduction of Co(III) Complexes by Homolysis of the Nickel-Carbon Bond. A Pulse Radioysis Study. Monovalent Zinc, Cadmium and Nickel Ions in Aqueous Solutions. Inorg. Chem. 1990, 29, 4156. J. Phys. Chem. 1969, 73, 1091. (23) van Eldik, R.; Gaede, W.; Cohen. H.; Meyerstein, D. High-Pressure (3) Zilbermann, I.; Winnik, M.; Sagiv, D.; Rotman, A.; Cohen, H.; Kinetic Evidence for a Dissociative Interchange (Id) Substitution Meyerstein, D. Properties of Monovalent Nickel Complexes with Mechanism for Aquated Chromium(II). Inorg. Chem. 1992, 31, Tetraazamacrocyclic Ligands in Aqueous Solutions. Inorg. Chim. 3695. Acta 1995, 240, 503. (24) van Eldik, R.; Gaede, W.; Cohen. H.; Meyerstein, D. Pressure- (4) Behar, D.; Dhanasekaran, T.; Neta, P.; Hosten, C. M.; Hambright, Assisted Formation of a Cobalt-Carbon σ Bond: A High-Pressure P.; Fujita, E. Cobalt Porphyrin Catalyzed Reduction of CO2. Pulse Radiolysis Study. Angew. Chem. 1991, 30, 1158. Radiation Chemical, Photochemical, and Electrochemical Studies. (25) Sisley, M. J.; Rindermann, W.; van Eldik, R.; Swaddle, T. W. J. Phys. Chem. A 1998, 102, 2870. Pressure and Temperature Effects on the Rates of Concurrent (5) Zilbermann, I.; Golub, G.; Cohen, H.; Meyerstein, D. Kinetic Homolytic and Heterolytic Decomposition Pathways of Aqueous Stabilization of Trivalent Nickel Complexes with Tertiary-Tetra- 2-propyl- and 2-hydroxy-2-propylchromium(III) ions. J. Am. Chem. Aza-Macrocyclic Ligands in Aqueous Solutions. J. Chem. Soc., Dalton Trans. 1997, 141. Soc. 1984, 106, 7432. (6) Goldstein, S.; Czapski, G.; Cohen, H.; Meyerstein, D.; van Eldik, (26) Gaede, W.; van Eldik, R.; Cohen, H.; Meyerstein, D. Influence of R. The mechanism of decomposition of Cu(III)-GlyGlyHis. A pulse Acetate Ion on the Formation Reactions of Organochromium(III) radiolysis study. Inorg. Chem. 1994, 33, 3255. Species. A Rapid-Scan and High-Pressure Pulse-radiolysis Study. (7) Bielski, I. H.; Sharma, V. K.; Czapski, G. Ractivity of Ferrate(V) with J. Chem. Soc., Dalton Trans. 1993, 2065. Carboxylic Acids: A Pre-Mix Pulse Radiolysis Study. Radiat. Phys. (27) De Gioia, L.; Fantucci, P. A Theoretical Study of the Methyl Chem. 1994, 44, 479. Ligation in Tetraaza Macrocyccle Nickel Complexes which Model (8) Zilbermann, I.; Maimon, E.; Cohen, H.; van Eldik, R.; Meyerstein, the Acetyl-CoA Synthase Active Site. Inorg. Chim. Acta 1998, 273, D. Cooperative Oxidation of EDTA by Ni(III) and Dioxygen. A Pulse 379. Radiolysis Study. Inorg. Chem. Commun. 1998, 1, 46. (28) Gaede, W.; van Eldik, R.; Cohen, H.; Meyerstein, D. Anion- (9) Goldstein, S.; Czapski, G.; van Eldik, R.; Cohen, H.; Meyerstein, Catalyzed Heterolysis of Chromium-Carbon σ Bonds. Effect of D. Determination of the Volume of Activation of the Key Reaction Different Anions, Temperature and Pressure. Inorg. Chem. 1993, Steps in the Oxidation of Phenanthroline-Cu(I) by Molecular 32, 1997. Oxygen. J. Phys. Chem. 1990, 95, 1282. (29) Cohen, H.; van Eldik, R.; Gaede, W.; Gerhard, A.; Goldstein, S.; 2+ (10) Ilan, Y. A.; Czapski, G.; Ardon, M. The Formation of CrO2 in the Czapski, G.; Meyerstein, D. â-Elimination and related reactions 2+ Reaction of Cr + O2 in Aqueous Acid Solutions. Isr. J. Chem. of chromium(III)-alkyl complexes in aqueous solutions. Mecha- 1975, 13, 15. nistic information from high pressure kinetic measurements. (11) Jubran, N.; Ginzburg, G.; Cohen, H.; Koresh, Y.; Meyerstein, D. Inorg. Chim. Acta 1994, 227, 57. 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S.; Wishart, J. F. Mechanistic Information from the First Cobalt and Ruthenium. Inorg. Chem. 1979, 18, 971. Volume Profile Analysis for a Reversible Intermolecular Electron- (14) Navon, N.; Cohen, H.; Meyerstein, D. The Effect of Stabilising Transfer Reaction Involving Penta-ammine(isonicotinamide)ru- Ligands on the Rate of Reaction of Cu(I)L with CCl CO - in 3 2 thenium and Cytochrome c. Inorg. Chem. 1994, 33, 4744. Aqueous Solutions. I. L ) HOOCCHdCHCOO-. Inorg. Chem. 1997, (33) Sachinidis, J. I.; Shalders, R. D.; Tregolan, P. A. Separation of 36, 3781. Intrinsic and Electrostrictive Volume Effects in Redox Reaction (15) Goldstein, S.; Meyerstein, D.; van Eldik, R.; Czapski, G. Spontane- Volumes of Metal Complexes Measured Using High-Pressure ous Reactions and Reduction by Iodide of Peroxonitrite and Peroxonitrate: Mechanistic Insight from Activation Parameters. Cyclic Staircase Voltammetry. Inorg. Chem. 1996, 35, 2497. J. Phys. Chem. A 1997, 101, 7114. (34) Sun, J.; Wishart, J. F.; van Eldik, R.; Shalders, R. D.; Swaddle, T. (16) Drljaca, A. A.; Hubbard, C. D.; van Eldik, R.; Asano, T.; Basilevsky, W. Pressure Tuning Voltammetry. Reaction Volumes for Electron M. V.; le Noble, W. J. Activation and Reaction Volumes in Transfer in Cytochrome c and Ruthenium Modified Cytochrome Solution. 3. Chem. Rev. 1998, 98, 2167. c. J. Am. Chem. Soc. 1995, 117, 2600. (17) Wishart, J. F.; van Eldik, R. High-Pressure Pulse Radiolysis. (35) Sun, J.; Su, C.; Meier, M.; Wishart, J. F.; van Eldik, R. Mechanistic Modification of an Optical Cell for 2-MeV Electron Pulse Radiolysis Information from the First Volume Profile Analysis for Intramo- at Pressure up to 200 MPa. Rev. Sci. Instrum. 1992, 63, 3224. lecular Electron-Transfer Reactions. Tetraammineruthenium(li- (18) Cohen, H.; van Eldik, R.; Masarwa, M.; Meyerstein, D. Mechanism gand) Complexes of Cytochrome c. Inorg. Chem. 1998, 37, 6129. of Oxidation of Aquated Copper(II) Ions by Hydroxyl Free Radicals. (36) Meier, M.; Sun, J.; Wishart, J. F.; van Eldik, R. Comparative Kinetic A High-Pressure Pulse-Radiolysis Study. Inorg. Chim. Acta 1990, Analysis of Reversible Intermolecular Electron-Transfer Reactions 177, 31. Between a Series of Pentaammineruthenium Complexes and (19) Cabelli, D. E.; Wishart, J. F.; Holcman, J.; Meier, M.; van Eldik, R. Cytochrome c. Inorg. Chem. 1996, 35, 1564. Copper(III) Pyrophosphate Complexes in Aqueous Solution. A (37) Kelly, C. A.; Mulazzani, Q. G.; Blinn, E. L.; Rodgers, M. A. J. Kinetics Pulse Radiolysis Study at Ambient and High Pressure. J. Phys. of CO Addition to Ni(cyclam)+ in Aqueous solution. Inorg. Chem. Chem. 1997, 101, 5131. 1996, 35, 5122. (20) van Eldik, R.; Cohen. H.; Meyerstein, D. Ligand Interchange (38) Navon, N.; Cohen, H.; van Eldik, R.; Meyerstein, D. Effect of + Controls Many Oxidations of Divalent First Row Transition Metal Fumarate on the Kinetics and Reaction Mechanism of Cu aq with Ions by Free Radicals. Inorg. 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