- Email: [email protected]
Kinetics and mechanism of the oxidation of D-galactono-1,4-lactone by CrVI and CrVfn2
\
Pergamon PII ] S9166Ð4276"87#99951ÐX
Polyhedron Vol[ 06\ No[ 05\ pp[ 1628Ð1638\ 0887 Þ 0887 Elsevier Science Ltd All rights reserved[ Printed in Great Britain 9166Ð4276:87 ,08[99¦9[99
Kinetics and mechanism of the oxidation of VI V D!galactono!0\3!lactone by Cr and Cr $ S[ Signorella\a M[ Santoro\a C[ Palopoli\a C[ Brondino\b J[ M[ Salas!Peregrin\c M[ Quirozc and L[ F[ Salaa a
b
Departamento de Qu(mica\ Facultad de Ciencias Bioqu(micas\ y Farmaceuticas\ UNR\ Suipacha 420\ 1999 Rosario\ Argentina\
Instituto de Desarrolo Tecnologico para la Industria Qu(mica\ Guemes 2349\ 2999 Santa Fe\ Argentina\ c
Departamento de Qu(mica Inorganica\ Facultad de Ciencias\ Universidad de Granada\ Fuentenueva s:n\ 07960 Granada\ Espan½a "Received 10 January 0887 ^ accepted in revised form 15 January 0887#
Abstract*The oxidation of D!galactono!0\3!lactone by CrVI yields D!lyxonic acid\ carbon dioxide and Cr2¦ as _nal products when an excess of sugar acid over CrVI is used[ The redox reaction occurs through CrVI : CrIII and CrVI : CrV : CrIII paths[ The complete rate law for the CrVI oxidation reaction is expressed by −dðCrVIŁ:dt "k9¦kH ðH¦Ł# ðgalŁðCrVIŁ\ where k9 "2022#×09−3 M−0 s−0 and kH "8824#×09−3 M−1 s−0\ at 39>C[ CrV is formed in a rapid step by reaction of the CO1=− radical with CrVI[ CrV reacts with the substrate faster than does CrVI[ The CrV oxidation follows the rate law ] −dðCrVŁ:dt "k?9 ¦k?H ðH¦Ł# ðgalŁ\ where k?9 "0421#×09−2 M−0 s−0 and k?H "2323#×09−2 M−1 s−0\ at 39>C[ The EPR spectra show that several intermediate ðCr"O#"gala# 1Ł − linkage isomers are formed in rapid pre!equilibria before the redox steps[ Þ 0887 Elsevier Science Ltd[ All rights reserved Keywords] galactonolactone ^ chromium ^ oxidation ^ kinetics ^ mechanism ^ CrV ^ CrVI[ ———————————————————————————————————————————————
Hexavalent chromium\ in many di}erent compounds\ is a well established carcinogen and mutagen ð0Ð4Ł[ The observation of CrV and CrIV intermediates in the selective oxidation of organic substrates by CrVI and their implication in the mechanism of Cr!induced can! cers ð5Ð7Ł has generated a considerable amount of interest in their chemistry and biochemistry ð8Ð01Ł[ The biological reduction of CrVI to lower states has been observed with a wide variety of naturally occur! ring cellular reductants ð02Ð04Ł[ Ligands that possess two oxygen atoms able to form _ve!membered rings about the metal ion\ such as 0\1!diols and a!hyd! roxyacids\ are e}ective as non enzymatic reductants and complexation agents towards hypervalent chro! mium and can stabilise the labile oxidation states of chromium ð01\ 05Ð08Ł[ For this reason sugars\ or their
Author to whom correspondence should be addressed[ e!mail ] inquibirÝsatlink[com $ The present work was presented at the X Meeting on Physical!Chemistry\ Tucuman\ Argentina\ April 10\ 0886[
derivatives\ may play an important role in the chem! istry of CrVI[ Aldonic acids "or their lactones# have been found as metabolic intermediates in animals and plants\ as products of the action of micro!organisms on carbohydrates and as constituents of acidic poly! saccharides ð19Ł[ CrVI oxidation of a!hydroxyacids has been extensively studied ð10Ð18Ł[ These oxidations may yield products coming from either C0C ð10Ð 11\14Ð16\18Ł or C0H ð12Ð13\17\18Ł cleavage\ depend! ing on the particular molecular characteristics of the organic substrate\ through mechanisms involving either two!electron or three!electron initial oxidation of substrate by CrVI[ Polyhydroxyacids o}er additional binding sites to form the CrVI "CrV# ester precursor of slow electron!transfer steps and provide helpful information about stereoelectronic e}ects a}ecting the reaction course ð17Ł[ In previous work we have found that the number and arrangement of hydroxyl groups of aldoses a}ect the CrVI and CrV oxidation rates and mechanism ð29Ð23Ł[ At present\ we are interested in correlating the stereochemistry of aldonic acids with their ability for reducing and co!
1628
1639
S[ Signorella et al[
ordinating CrVI and CrV\ as well as determining the preferred donor sites of these polyhydroxyacids[ In this paper we report our results on the chromic oxi! dation of D!galactono!0\3!lactone "Scheme 0# and we present structural evidences for the intermediate CrV complexes formed in acid and neutral aqueous solu! tions[
Table 0[ Proportion of D!galactonolactone "galL# and D! galactonic acid "gala# in the starting solutiona ðHClO3Ł:M
091 CTb:M
Rc
9[14 9[27 9[49 9[52 9[64 9[89 0[99
5[59 4[99 4[34 4[03 4[95 5[18 4[47
−9[659 −9[428 −9[473 −9[467 −9[462 −9[559 −9[571
EXPERIMENTAL Materials
galL ")# gala ")# 67[8 61[1 60[5 65[3 66[0 71[9 74[2
10[0 16[7 17[3 12[5 11[8 07[9 03[6
a
D!Galactono!0\3!lactone "Sigma grade#\ D!gluconic acid "Sigma grade#\ potassium dichromate "BDH#\ sodium perchlorate "Fluka grade#\ acrylonitrile "Ald! rich grade# and perchloric acid "P[ A[ Merck# were used without further puri_cation[ Water was puri_ed by deionisation\ followed by double distillation from a potassium permanganate solution[ NaðC! r"O#"ehba# 1Ł = H1O was synthesised from 1!ethyl!1! hydroxybutanoic acid "Aldrich grade# and sodium dichromate "Merck# in acetone "Merck\ A[ R[ grade# according to the literature method ð24Ł[ The stability of the organic substrate under con! ditions used in the kinetic studies was tested[ The acid "gala# ] lactone "galL# proportion of the reductant in the starting solution was calculated by optical rotation measurements at given hydrogen ion concentrations ð25Ł and the composition of the substrate solution at di}erent perchloric acid concentrations is shown in Table 0[ The equilibrated solutions are stable over the time required for the kinetic measurements[ In the text that follow gal refers to the total reductant con! centration "ðgalŁ ðgalLŁ¦ðgalaŁ#[ For experiments performed in the 1Ð7 pH range\ the pH of the solutions was adjusted by addition of 9[4 M NaOH and 9[4 M HClO3[ Spectrophotometric measurements Most kinetics measurements were made at 249 nm\ by monitoring the absorbance changes on a Guilford Response II spectrophotometer with fully ther! mostated cell compartments[ Mixtures of sodium per!
Values used in calculations ] molar rotations ] −02[73 "galL# ^ −1[73 "gala#\ ¦9[794 "gal−# ^ K ðH¦Łðgal−Ł "ðgalaŁ¦ðgalLŁ# 1[14×09−4[ b CT ðgalaŁ¦ðgalLŁ¦ðgal−Ł\ where gal− refers to gal! actonate and represents ³9[91) of CT[ c Optical rotation[
chlorate and perchloric acid were used to maintain a constant ionic strength "I# of 0[9 M[ Reactant solu! tions were previously thermostated and transferred into a cell of 0 cm path length immediately after mix! ing and disappearance of CrVI was followed until at least 79) conversion[ Experiments were performed at 39>C unless otherwise mentioned[ In most exper! iments the concentration of CrVI was kept constant at 2[5×09−3 M while the gal concentration was varied from 9[907 to 9[011 M[ The _rst!order dependence of the rate upon ðCrVIŁ was veri_ed[ The pseudo!_rst! order rate constants were calculated at various ðCrVIŁ 9\ but at constant temperature\ ðgalŁ 9\ ðH¦Ł and I[ As expected on the basis of a −d"lnðCrVIŁ#dt kobs rate law where kobs f"ðgalŁðH¦#\ kobs was found to be essentially constant with increasing ðCrVIŁ 9[ CrV was monitored by following the absorbance growth and decay at 649 nm of mixtures of CrVI "2[5×09−2 M# and excess gal "9[07Ð9[46 M# at _xed perchloric acid concentrations[ The formation of CrIII was monitored at 469 nm "ðCrVIŁ 9 0[7×09−1 M and ratios of gal to CrVI from 09 ] 0 to 19 ] 0\ at ðHClO3Ł in the 9[14Ð9[64 M range#[ At the end of the reaction the two dÐd bands ascribed to CrIII were observed at
Scheme 0[
The oxidation of D!galactono!0\3!lactone by CrVI and CrV lmax 309 "o 22# and 455 nm "o 16[4 dm2 mol−0 cm−0# "ðHClO3Ł 9[14 M#[ Both\ visible and UV spectral maxima and intensities\ are in close agreement "vide infra# with those observed for the acid solutions of the KðCr"OH#"gala#"lyxa#Ł "lyxa D!lyxonic acid# ternary complex\ independently synthesised ð26Ł according to a literature method ð27Ł[ These dÐd bands changed with time toward bands of lower intensity at lmax 397 "o 08[6# and 462 nm "o 04 dm2 mol−0 cm−0#\ characteristic of the ðCr"H1O# 5Ł 2¦ ion[
Products analysis Under the conditions used in the kinetic measure! ments "ratios of gal to CrVI from 49 ] 0 to 239 ] 0#\ qualitative identi_cation of D!lyxonic acid as the reac! tion product was carried out by paper chro! matography[ D!lyxonic acid was identi_ed against an authentic sample using n!butanol:acetic acid:water "3 ] 0 ] 4# as eluent[ Paper chromatograms were visu! alised by two kinds of development reagents ] a three! stage dip of silver nitrate\ sodium hydroxide and sodium thiosulphate ð28Ł and p!anisidine reagent ð39Ł[ D!lyxonic acid identi_cation as the only reaction prod! uct was also made by 02C NMR[ A mixture of gal "6[53×09−2 mmol# and potassium dichromate "0[55×09−2 mmol# in 0 M HClO3 "09 cm2# was allowed to react at 39>C[ After the reaction had pro! ceeded to completion the mixture was stirred with a Dowex 49W!X7 cation exchange resin until all the CrIII was removed from the solution and a con! centrated sample was analysed by 02C NMR[ Lyxonic acid and unreacted galL and gala were identi_ed by comparing against authentic samples[ Under all these experimental conditions "large to moderate excess of organic substrate# a quantitative yield of carbon dioxide was observed[ The presence of free radicals was tested for in the reaction of gal with CrVI[ A solution of potassium dichromate "3×09−3 mmol# in 1 cm2 of 9[4 M HClO3 was added to a mixture of 3×09−1 mmol gal and acrylonitrile "9[14 cm2# in 9[4 M HClO3 "9[14 cm2# at 39>C[ After 0 h a white precipitate appeared[ Control experiments "without potassium dichromate or reductant present# did not show the formation of a precipitate[ Besides\ the addition of an excess of acry! lonitrile on the reaction mixture results in the complete suppression of the carbon dioxide evolution[
EPR spectroscopy The EPR spectra were obtained on a Bruker ESP 299 spectrometer[ The microwave frequency was gen! erated with a Bruker 93 ER "8Ð09 GHz# and measured with a Racal!Dana frequency meter[ The magnetic _eld was measured with a Bruker NMR!probe gaussmeter[ A least!squares EPR spectrum!_tting program\ e12new\ was used to estimate the spectral
1630
parameters and relative concentrations of the CrV complexes\ using Lorentzian line shapes ð30\31Ł[ RESULTS AND DISCUSSION Rate studies The absorbance vs time curves at 249 nm exhibit a monotonic decrease of absorbance which cannot be described by a single exponential decay[ At this wave! length\ kinetic traces show an initial deviation from _rst!order decay over short time periods[ These kinetic pro_les could be adequately described by the set of consecutive _rst!order reactions of Scheme 1 ] It is known that CrV species absorb strongly at 249 nm and may superimpose CrVI absorbance yielding the wrong interpretation of spectrophotometric absorbance decay values ð18Ł[ Thus\ considering the CrV absorption superimposition\ the absorbance at 249 nm\ at any time during the reaction\ is given by Abs249 oVI ðCrVI Ł¦oV ðCrV Ł
"0#
Combining the eqn "0# with rate expressions ð32Ł derived from scheme 1\ yields ] Abs249 Abs9 e−k5t ¦k5 oV ðCrVI Ł 9 "e−k4t −e−1k5t #: "1k5 −k4 #
"1#
In this equation\ oV refers to the molar absorptivity of CrV at 249 nm and was assumed to be the same as for the complex ðCrV "O#"ehba# 1Ł − ð24Ł[ Parameters k5 and k4 refer to the rate of disappearance of CrVI and CrV\ respectively\ and were evaluated from a non!linear iterative computer _t of eqn "1#[ It must be noted that in eqn "1#\ k5 appears in the numerator of the pre! exponential term and 1k5 appears in the denominator and in the exponential terms because\ according to the proposed reaction scheme\ only half of the CrVI reaches CrIII through a CrV intermediate[ Figure 0"a# shows a typical curve for one run at 249 nm and the curve _t according to eqn "1#[ The calculated kinetic parameters\ k5 and k4\ for various concentrations of gal at _xed concentrations of HClO3\ are summarised in Table 1[ In multiple measurements\ the reproducibility of the two rate con! stants is better than 09)[ Values of k4 and k5 may also be obtained from absorbance data at 649 nm[ At this wave length\ only CrV absorbs and the experimentally observed inter! mediate CrV growth and decay curve could be _tted by eqn "2# Abs649 k5 ðCrVI Ł 9 oV "e−k4t −e−1k5t #:"1k5 −k4 # V
V
"2#
where o refers here to Cr molar absorptivity at 649 nm[ A good _t of data is obtained with oV 27 M−0 cm−0 ð15Ł\ as shown in Fig[ 0"b#[ Since at least tenfold higher initial CrVI and gal concentrations were necess! ary to follow absorbance changes at 649 nm\ k4:09 and 1k5:09 values calculated by eqn "2# are coincident
1631
S[ Signorella et al[
Scheme 1[ S organic substrate ^ I> intermediate radical
Fig[ 0[ Absorbance vs time curves for the chromic oxidation of gal\ I 0M\ T 39>C at "a# 249 nm\ ðCrVIŁ 2[5×09−3 M\ ðgalŁ 2[5×09−1 M\ ðH¦Ł 9[27 M ^ "b# 649 nm\ ðCrVIŁ 2[5×09−1 M\ ðgalŁ 2[5×09−0 M\ ðH¦Ł 9[27 M ^ "c# 469 nm\ ðCrVIŁ 0[7×09−1 M\ ðgalŁ 0[7×09−0 M\ ðH¦Ł 9[64 M[
Table 1[ Observed pseudo!_rst!order rate constants "k5 and k4# for di}erent concentrations of HClO3 and organic substrate[ T 39>C ^ ðCrVIŁ 9 2[5×09−3 M ^ I 0 M
ðgalŁ:M
9[907 9[925 9[949 9[946 9[961 9[975 9[982 9[097 9[011
1k5 "s−0# and k4 "s−0# for ðHClO3Ł:M of 9[27 9[49 9[64
9[14 0931k5 9[89 0[77 1[49 * 2[69 * * 4[28 5[50
093k4 3[99 5[18 09[4 * 04[5 * * 10[9 15[0
0931k5 0[28 1[67 2[71 3[28 4[99 * * 6[60 7[67
093k4 4[29 09[6 04[1 06[5 19[6 * * 21[0 25[1
0931k5 0[50 2[01 * 3[50 5[11 * * 7[60 8[88
093k4 5[0 01[5 06[9 07[8 13[0 * * 25[7 30[9
0931k5 0[87 3[09 4[07 * 7[90 8[29 * 09[8 01[5
093k4 6[19 03[3 19[5 * 18[5 22[6 * 31[0 35[5
0[99 0931k5 1[30 3[69 5[37 * 8[31 * 01[1 * 04[5
093k4 7[80 06[4 13[7 * 22[8 * 34[7 * 47[1
The oxidation of D!galactono!0\3!lactone by CrVI and CrV with those obtained from eqn "1#\ when the same gal:CrVI ratio and ðHClO3Ł were employed[ For gal and CrVI concentrations used in the kinetic measure! ments\ either at 249 or 649 nm\ the calculated maximum CrV concentrations "ðCrVŁ max# represent up to 7) of the total chromium concentration[ The CrIII formation was followed at 469 nm[ At this wave length\ changes in CrIII concentration can be described by the pair of consecutive reactions shown in the Scheme 2 and the experimental absorbance can be _tted by eqn "3# Abs469 Abs"ðCrIII Ł#¦Abs"Cr2¦ # ðCrVI Ł 9 k0 "e−k0t −e−k2t #"o0III −o1III #: "k2 −k0 #¦Abs "0−e−k0t #
"3#
where k0 and k2 refer to the rates of CrVI and ðCrIIIŁ disappearance\ and o0III \ o1III are the molar absorptivities of the CrIII complex formed at the end of the redox reaction and ðCr"H1O# 5Ł 2¦\ respectively[ A typical _t of experimental data at 469 nm by using eqn "3# is shown in Fig[ 0"c#[ Here\ we have considered that the rate of CrIII complex formation equals the rate of CrVI decay\ without taking into account CrV[ This is consistent with the observation of an isosbestic point at 414 nm under conditions employed to follow CrIII at 469 nm "Fig[ 1# and the low ðCrVŁ max "up to 7)# observed at 649 nm "Fig[ 0#\ for the same oxidant and reductant concentration range[ Values of k0 calculated by eqn "3# are coincident with 1k5 values obtained by using either eqn "1# or "2#[ On the other hand\ k2 depends strongly on ðH¦Ł and a more detailed study is required for the complete description of the hydrolysis mechanism[
CrVI oxidation of `al In the proton concentration range studied\ plots of 1k5 vs ðgalŁ gave good straight lines from which values of k were determined "Table 2#[ A plot of k vs ðH¦Ł gave a straight line with a positive intercept\ showing that the second!order rate constant may be expressed as consisting of an acidity independent and an acidity dependent term ] k k9 ¦kH ðH¦ Ł M−0 s−0 where k9 "2022#×09−3 −3 −1 −0 kH "8824#×09 M s The complete rate law is then given by
"4# and
1632
−d ðCrVI Ł:dt 1k5 ðCrVI Ł T "k9 ¦kH ðH¦ Ł#ðgalŁðCrVI Ł T VI
"5#
VI
where ðCr Ł T represents the total Cr concentration[ The two terms in the rate law "eqn "5## results from the fact that there are at least two transition states\ di}ering in composition but similar in energy\ through which the reduction of CrVI can proceed[ Thus\ the oxidation of gal by CrVI should occur through two parallel slow steps leading to the redox products[ Besides\ in the range of substrate concentrations employed\ only substrate _rst!order dependence was observed and D!lyxonic acid and carbon dioxide were the only detected reaction products\ clearly showing that the C0C scission is preferred to the C0H cleav! age at the a!carbon[ A mechanism taking account of these facts is proposed in Scheme 3[ In this mechanism we have assumed that the open carboxylic acid is the reactive form of the organic substrate[ This assumption is based on previous _n! dings showing the enhanced ability of hydroxyacids to bind CrVI and CrV ð02\14Ð17Ł[ In any case\ a possible mechanism involving the lactone may not be discarded[ However\ if the lactone was the reactive form of the organic reductant\ co!ordination of CrVI should be favoured at a pair of adjacent hydroxyl groups of the molecule ð33Ł[ The intermediate complex formed in this way might be present in the equilibrium mixture but should not be a direct precursor of the slow redox steps[ Opening of the lactone ring and re! arrangement should be required for the slow electron transfer steps to take place within a complex with the carboxylate bound to CrVI[ On the other hand\ in the ðH¦Ł range studied\ CrVI exists mainly as Cr1 O 6 ð34\35Ł[ Thus\ the _rst step of the proposed mechanism may be interpreted as the formation of a monochelate with gala acting as a bidentate ligand bound to CrVI at Cl0O and C10O to yield the anionic species A1−[ This intermediate\ or its monoprotonated form "AH−#\ may decompose to the products[ The rate law corresponding to this mechanism may be expressed as in eqn "6# ] −dðCrVI Ł:dt "k0 K0 ¦k1 K0 K1 ðH¦ Ł#ðgalaŁðCrVI Ł "6# Equation "6# may be expressed as a function of the total organic substrate concentration as in eqn "7# ] −dðCrVI Ł:dt "k0 K0 ¦k1 K0 K1 ðH¦ Ł#ðgalŁKh ðCrVI Ł: "0¦Kh #
"7# 1−
Scheme 2[ ðCrIIIŁ CrIII!gala!lyxa ternary complex
We propose that redox paths\ involving either A or AH− intermediate complexes\ are three electron steps\ just as observed for the chromic oxidation of mandelic ð14Ł\ 1!hydroxy!2!methylbutanoic ð10Ł and D!ribonic ð11Ł acids\ and CrIII is formed with the oxidation prod! ucts[ The alternative path corresponding to the sequence CrVI:CrIV:CrIII was discarded on the basis of the complete CO1 suppression after addition of excess
1633
S[ Signorella et al[
Fig[ 1[ Time evolution of the UVÐvis spectrum of a mixture of gal "9[64 M# and CrVI "0[7×09−1 M#[ ðHClO3Ł 9[4 M ^ T 14>C\ over a period of ca 34 min[
Table 2[ Values of k and k? for di}erent ðHClO3Ł ðHClO3:M
9[14
9[27
9[4
9[64
092 k:M−0 s−0 092 k?:M−0 s−0
4[19 6[13 7[11 09[4 19[5 18[6 22[7 28[1
0[9 01[8 37[1
acrylonitrile on the reaction mixture and the fact that the 1!ketoacid was not observed over the whole set of conditions used here[ Besides\ CrII formation was not proposed since CrO11¦ \ which is taken as evidence that CrII is an intermediate product of the reaction\ was not observed as we did for the oxidation of homo! serine by CrVI ð13Ł[ CO1=− radicals formed in the slow steps may react with CrVI in a subsequent fast step to yield CrV and carbon dioxide[ Polymerisation after addition of acrylonitrile support the formation of these radicals[ CrV can further oxidise gala as will now be discussed[
spectrum dominated by a single detectable signal at `iso 0[8670 "Fig[ 2 Inset# with the four weak 42Cr "8[44) abundance\ I 2:1# hyper_ne peaks at 07 G spacing\ typical of _ve!co!ordinate oxochromate"V# complexes ð36\37Ł[ The rate of formation and disappearance of the intermediate CrV calculated from the peak!to!peak heights "and:or areas# of the EPR signals as a function of time "Fig[ 2# are consistent with values obtained from the spectrophotometric measurements[ The delayed tmax observed by this technique\ account for the lower temperature "14>C\ instead of 39>C# used in the EPR measurements[ In the 9[14Ð0 M proton concentration range\ plots of k4 vs ðgalŁ gave good straight lines from which values of k? were determined "Table 2#[ A plot of k? vs ðH¦Ł gave a straight line with a positive intercept\ showing that the second!order rate constant may be expressed as consisting of an acidity independent and an acidity dependent term[ The rate constant for the disappearance of CrV then may be expressed as ] k4 "k?9 ¦k?H ðH¦ Ł#ðgalŁ
"8#
CrV oxidation of `al At the ðHClO3Ł used in the kinetics measurements\ the reaction of CrVI with excess gal generates an EPR
where k?9 "0421#×09−2 M−0 s−0 and k?H "2323#×09−2 M−1 s−0
"09#
The oxidation of D!galactono!0\3!lactone by CrVI and CrV
1634
Scheme 3[ A1− ð"galaH−1#"O# 1 "OH1#CrOCrO2Ł 1−
Fig[ 2[ Peak!to!peak heights of CrV EPR signals vs time[ ðgalŁ 9[033 M ^ ðCrVIŁ 2[5×09−2 M ^ ðH¦Ł 9[4 M ^ I 0 M ^ T 14>C[ Tmax 497 s[ Inset ] EPR signal recorded at t 04 min[
This means that the CrV intermediate decays to the product directly or through an acid!catalysed step[ Since the latter is of the same order as the non!cat! alysed step\ at low acid concentration "i[e[ neutral pH# the oxidation occurs mainly through the non!acid!
catalysed path[ In this way\ the intermediate CrV is consumed faster than observed for the reaction of CrVI with sugars for which the non!acid catalysed path is extremely slow and the CrV intermediates remain in solution after several weeks ð29Ð21Ł[
1635
S[ Signorella et al[
A mechanism taking into account the structural evidence for the CrV intermediate complex and the kinetics results is given in Scheme 4[ In this mechanism\ only one of the two gala mol! ecules involved in the CrV complex formation is oxi! dised to D!lyxonic acid through the two!electron intramolecular redox path\ in accordance with the observed _rst!order rate on ðgalaŁ[ Possibly\ several linkage isomers might be formed by co!ordination of gala with CrVI via any pair of properly disposed hydroxyl group "see discussion below#[ However\ since D!lyxonic acid is the only reaction product observed in the solution\ the CrV complex\ precursor of the redox steps\ must be one with at least one gala molecule bound to CrV at the carboxylate group[ Besides\ the `iso and Aiso values observed for the CrV intermediate are close to those of ðCr"O#"ehba# 1Ł −[ Since the `iso and\ specially\ Aiso values are very sen! sitive to the CrV co!ordination environment\ our assumption seems to be a reasonable one[ It must be noted that\ in Scheme 4\ the two!electron substrate C0C cleavage a}ords formic acid[ However\ under our experimental conditions\ we have never detected formic acid[ This fact may be explained if\ under conditions used here\ formic acid is oxidised faster than gal[ E}ectively\ CrVI oxidation of formic acid to carbon dioxide is faster than gal oxidation ð38Ł\ a}ording the observed quantitative yield of carbon dioxide[ The proton superhyper_ne "shf# coupling has been shown to be useful in determining the binding of 0\1! diolate moieties of sugars to the CrV centre ð29\33Ł[ We found that performing the reaction at pH × 1\ the single broad "linewidth ½ 1 G# signal observed at higher ðH¦Ł sharpens and the shf pattern resolves\ thus providing new insights about the gala co!ordi! nation sites[ The reaction of CrVI with an excess of 4Ð19 times of gal\ in the 1Ð5 pH range\ at 14>C\ results in the formation of a CrV EPR spectrum which is a com!
posite of at least two EPR signals at `0 0[8670 and `1 0[8677[ The relative proportion of the signals depends on the pH\ but is independent of the gal:CrVI ratio employed and the reaction time "the signals were deconvoluted by _tting the spectra by Lorentzian derivatives#[ Figure 3"a# shows the spectrum of a reac! tion mixture at pH 4 with the simulated spectrum\ for which a `0:`1 ratio 9[30 was obtained[ The best _t of the spectrum yields the spectral parameters sum! marised in Table 3[ The `iso values obtained for the two signals are those expected for _ve!co!ordinate oxochromate"V# complexes ^ while the di}erent shf patterns indicate that they are linkage isomers ð49Ł[ In this way\ the shf splitting of the signal at `0 arises from the coupling of two equivalent protons and may be attributed to a complex with two ligand molecules bound to CrV at Cl0O:C10O[ The shf splitting of the signal at `1 may be assumed to arise from two di}erent binding modes of the two ligand molecules co!ordinated to CrV\ via a 0\1!diolate and the a!hyd! roxocarboxylate donor sites\ respectively[ In the oxi! dation of D!gluconic acid "glca# with CrVI\ in this pH range\ only one species is detected[ Interestingly\ the `iso value "0[867# and the shf pattern "Fig[ 4# reveal the same chelation mode of glca as for gala in the complex giving rise to the signal at `1[ For the same gal:CrVI ratios\ in the 5Ð7 pH range\ the spectra are dominated by a signal at 0[8684 with a shf splitting value of 9[81 G[ The `iso and shf pattern clearly indi! cate that CrO2¦ co!ordinates two gala molecules via a 0\1!diolate moiety and that the shf structure derives from the coupling with the four equivalent protons[ In this pH range\ the minor signal "`iso 0[867# can be attributed to the isomer with the ligand acting through a 0\1!diolate and the a!hydroxocarboxylate donor sites[ Figure 3"b# shows the spectrum of a reac! tion mixture at pH 6[3 with the simulated spectrum\ for which the major CrV species represents the 81) of the CrV in solution[ A last point to consider is that\ at each pH\ the
Scheme 4[ R CH1OH"CHOH# 2
The oxidation of D!galactono!0\3!lactone by CrVI and CrV
1636
Fig[ 3[ X!band EPR signal of a mixture of gal and CrVI "4 ] 0 mole ratio# at "a# pH 4[3\ 1 h after mixing\ "b# pH 6[3\ 0 day after mixing\ and the corresponding simulated spectra "linewidth 9[7 and 9[5 G\ respectively# ^ T 14>C\ modulation amplitude 9[52 G\ centre _eld 2444 G ^ frequency 8[73 GHz[
Table 3[ EPR spectral parameters
ðH¦Ł × 9[0 M pH 1Ð5 pH 5Ð7
`0iso "m#
a0iso:G
`1iso "m#
a1iso:G
0[8670 0[8670 "t# 0[8684 "quint#
* 9[64 9[81
0[8677 "dt# 0[867 "dt#
9[64\ 9[81 9[6\ 9[8
m multiplicity ^ t triplet ^ dt double!triplet ^ quint quintet
1637
S[ Signorella et al[
Fig[ 4[ X!band EPR signal of a mixture of D!gluconic acid and CrVI "09 ] 0 mole ratio# at pH 3\ 0 day after mixing and the corresponding simulated spectrum "linewidth 9[44# ^ T 14>C ^ modulation amplitude 9[404 G ^ centre _eld 2429 G ^ frequency 8[66 GHz[
relative proportion of the intermediate CrV complexes remains constant over the reaction time\ meaning that these CrV intermediates are in rapid equilibria com! pared to the time scale of their subsequent reduction to CrIII[ Acknowled`ements*We thank the National Research Coun! cil of Argentina "CONICET#\ The Third World Academy of Sciences "TWAS#\ the National University of Rosario\ Antorchas Foundation and The International Foundation for Sciences "IFS# for _nancial support[ We thank Prof[ Brumby and Prof[ Lay for providing us with the e12new Program[
REFERENCES 0[ O|Brien\ P[ and Wang\ G[\ Polyhedron\ 0882\ 01\ 0398[ 1[ Cohen\ M[\ Kagacin\ B[\ Klein\ C[ and Costa\ M[\ Crit[ Rev[ Toxicol[\ 0882\ 12\ 144[ 2[ Ramana\ V[ and Sastry\ K[\ J[ Inor`[ Biochem[\ 0883\ 45\ 76[ 3[ Katz\ S[ and Salem\ H[\ The Biolo`ical and Environmental Chemistry of Chromium\ VCH Publishers[ Inc[\ New York\ 0883[ 4[ Casadevall\ M[ and Kortenkamp\ A[\ Car! cino`enesis\ 0883\ 04\ 396[ 5[ Farrell\ R[\ Judd\ R[\ Lay\ P[\ Dixon\ N[ and Baker\ R[\ A[ Bonin[ Chem[ Res[ Toxicol[\ 0878\ 1\ 116[ 6[ Moghaddas\ S[\ Gelerinter\ E[ and Bose\ R[\ J[ Inor`[ Biochem[\ 0884\ 46\ 024[ 7[ Luo\ H[\ Lu\ Y[\ Mao\ Y[\ Shi\ X[ and Dalal\ N[\ J[ Inor`[ Biochem[\ 0885\ 53\ 14[ 8[ Cieslak!Golonka\ M[\ Polyhedron\ 0885\ 04\ 2556[
09[ Mitewa\ M[ and Bontchev\ R[\ Coord[ Chem[ Rev[\ 0874\ 50\ 103[ 00[ Gould\ E[\ Coord[ Chem[ Rev[\ 0883\ 024:025\ 540[ 01[ Zhang\ L[ and Lay\ P[\ J[ Am[ Chem[ Soc[\ 0885\ 007\ 01513[ 02[ Stearns\ D[ and Wetterhahn\ K[\ Chem[ Res[ Tox! icol[\ 0883\ 6\ 108[ 03[ Jannette\ K[\ J[ Am[ Chem[ Soc[\ 0871\ 093\ 763[ 04[ O|Brien\ P[\ Barrett\ J[ and Swanson\ F[\ Inor`[ Chim[ Acta\ 0874\ 097\ L08[ 05[ Krumpolc\ M[\ de Boer\ B[ and Rocek\ J[\ J[ Am[ Chem[ Soc[\ 0867\ 099\ 034[ 06[ Brauer\ S[ and Wetterhahn\ K[\ J[ Am[ Chem[ Soc[\ 0880\ 002\ 2990[ 07[ Goodgame\ D[\ Hayman\ P[ and Hathaway\ D[\ Polyhedron\ 0871\ 0\ 386[ 08[ Goodgame\ D[ and Joy\ M[\ J[ Inor`[ Biochem[\ 0875\ 15\ 108[ 19[ Pigman\ W[ and Horton\ D[\ The Carbohydrates[ Chemistry and Biochemistry\ Vol[ 0B[ Academic Press Inc[\ New York\ 0879[ 10[ Signorella\ S[\ Garc(a\ S[ and Sala\ L[\ Polyhedron\ 0881\ 00\ 0280[ 11[ Signorella\ S[\ Garc(a\ S[ and Sala\ L[\ Polyhedron\ 0886\ 05\ 690[ 12[ Signorella\ S[\ Santoro\ M[\ Mulero\ M[ and Sala\ L[\ Can[ J[ Chem[\ 0883\ 61\ 287[ 13[ Palopoli\ C[\ Signorella\ S[ and Sala\ L[\ New J[ Chem[\ 0886\ 10\ 232[ 14[ Ip\ D[ and Rocek\ J[\ J[ Am[ Chem[ Soc[\ 0868\ 090\ 5200[ 15[ Krumpolc\ M[ and Rocek\ J[\ J[ Am[ Chem[ Soc[\ 0866\ 88\ 026[ 16[ Ramesh\ S[\ Mahapatro\ S[\ Lui\ J[ and Rocek\ J[\ J[ Am[ Chem[ Soc[\ 0870\ 092\ 4061[ 17[ Srinivasan\ K[ and Rocek\ J[\ J[ Am[ Chem[ Soc[\ 0867\ 099\ 1678[
The oxidation of D!galactono!0\3!lactone by CrVI and CrV 18[ Haight\ G[\ Jwisich\ G[\ Kelso\ M[ and Merril\ P[\ Inor`[ Chem[\ 0874\ 13\ 1639[ 29[ Sala\ L[\ Signorella\ S[\ Rizzotto\ M[\ Frascaroli\ M[ and Gandolfo\ F[\ Can[ J[ Chem[\ 0881\ 69\ 1935[ 20[ Signorella\ S[\ Rizzotto\ M[\ Daier\ V[\ Frascaroli\ M[\ Palopoli\ C[\ Martino\ D[\ Boussecksou\ A[ and Sala\ L[\ J[ Chem[ Soc[\ Dalton Trans[\ 0885\ 0596[ 21[ Rizzotto\ M[\ Signorella\ S[\ Frascaroli\ M[\ Daier\ V[ and Sala\ L[\ J[ Carbohyd[ Chem[\ 0884\ 03\ 34[ 22[ Sala\ L[\ Palopoli\ C[ and Signorella\ S[\ Poly! hedron\ 0884\ 03\ 0614[ 23[ Rizzotto\ M[\ Frascaroli\ M[\ Signorella\ S[ and Sala\ L[\ Polyhedron\ 0885\ 04\ 0406[ 24[ Krumpolc\ M[ and Rocek\ J[\ Inor`[ Synth[\ 0879\ 19\ 52[ 25[ Dutta\ S[ K[\ Indian J[ Chem[\ 0864\ 02\ 879[ 26[ Signorella\ S[\ Santoro\ M[\ Moreno\ V[\ Salas! Peregrin\ J[ M[ and Sala\ L[ to be submitted for publication at Polyhedron[ 27[ Kaiwar\ S[ P[\ Raghavan\ M[ S[ and Rao\ P[\ J[ Chem[ Soc[\ Dalton Trans[\ 0884\ 0458[ 28[ Travelyan\ W[\ Proctor\ D[ and Harrison\ J[\ Nature\ 0849\ 055\ 33[
1638
39[ Haugh\ L[\ Jones\ J[ and Wadman\ N[\ J[ Chem[ Soc[\ 0849\ 0691[ 30[ Brumby\ S[\ Appl[ Spectrosc[\ 0881\ 35\ 065[ 31[ Beckwith\ A[ L[ and Brumby\ S[\ J[ Ma`n[ Reson[\ 0876\ 62\ 141[ 32[ Wilkins\ R[ G[\ The Study of Kinetics and Mech! anism of Reactions of Transition Metal Complexes\ Allyn and Bacon\ Boston\ 0863\ pp[ 19Ð12[ 33[ Branca\ M[\ Dessi\ A[\ Kozlowski\ H[\ Micera\ G[ and Swiatek\ J[\ J[ Inor`[ Biochem[\ 0889\ 28\ 106[ 34[ Cieslak!Golonka\ M[\ Coord[ Chem[ Rev[\ 0880\ 098\ 112[ 35[ Poulopoulo\ V[\ Vrachnou\ E[\ Koinis\ S[ and Katakis\ D[\ Polyhedron\ 0886\ 05\ 410[ 36[ Barr!David\ G[\ Charara\ M[\ Codd\ R[\ Farrel\ R[\ Irwin\ J[\ Lay\ P[\ Brambley\ R[\ Brumby\ S[\ Ji\ J[ and Hanson\ G[\ J[ Chem[ Soc[\ Faraday Trans[\ 0884\ 80\ 0196[ 37[ Farrel\ R[\ Judd\ R[ and Lay\ P[\ Inor`[ Chem[\ 0878\ 17\ 2390[ 38[ Arias\ C[ and Perez!Benito\ J[\ Collect[ Czech[ Chem[ Commun[\ 0881\ 46\ 0710[ 49[ Brambley\ R[\ Ji\ J[\ Judd\ R[ and Lay\ P[\ Inor`[ Chem[\ 0889\ 18\ 2978[
Pergamon PII ] S9166Ð4276"87#99951ÐX
Polyhedron Vol[ 06\ No[ 05\ pp[ 1628Ð1638\ 0887 Þ 0887 Elsevier Science Ltd All rights reserved[ Printed in Great Britain 9166Ð4276:87 ,08[99¦9[99
Kinetics and mechanism of the oxidation of VI V D!galactono!0\3!lactone by Cr and Cr $ S[ Signorella\a M[ Santoro\a C[ Palopoli\a C[ Brondino\b J[ M[ Salas!Peregrin\c M[ Quirozc and L[ F[ Salaa a
b
Departamento de Qu(mica\ Facultad de Ciencias Bioqu(micas\ y Farmaceuticas\ UNR\ Suipacha 420\ 1999 Rosario\ Argentina\
Instituto de Desarrolo Tecnologico para la Industria Qu(mica\ Guemes 2349\ 2999 Santa Fe\ Argentina\ c
Departamento de Qu(mica Inorganica\ Facultad de Ciencias\ Universidad de Granada\ Fuentenueva s:n\ 07960 Granada\ Espan½a "Received 10 January 0887 ^ accepted in revised form 15 January 0887#
Abstract*The oxidation of D!galactono!0\3!lactone by CrVI yields D!lyxonic acid\ carbon dioxide and Cr2¦ as _nal products when an excess of sugar acid over CrVI is used[ The redox reaction occurs through CrVI : CrIII and CrVI : CrV : CrIII paths[ The complete rate law for the CrVI oxidation reaction is expressed by −dðCrVIŁ:dt "k9¦kH ðH¦Ł# ðgalŁðCrVIŁ\ where k9 "2022#×09−3 M−0 s−0 and kH "8824#×09−3 M−1 s−0\ at 39>C[ CrV is formed in a rapid step by reaction of the CO1=− radical with CrVI[ CrV reacts with the substrate faster than does CrVI[ The CrV oxidation follows the rate law ] −dðCrVŁ:dt "k?9 ¦k?H ðH¦Ł# ðgalŁ\ where k?9 "0421#×09−2 M−0 s−0 and k?H "2323#×09−2 M−1 s−0\ at 39>C[ The EPR spectra show that several intermediate ðCr"O#"gala# 1Ł − linkage isomers are formed in rapid pre!equilibria before the redox steps[ Þ 0887 Elsevier Science Ltd[ All rights reserved Keywords] galactonolactone ^ chromium ^ oxidation ^ kinetics ^ mechanism ^ CrV ^ CrVI[ ———————————————————————————————————————————————
Hexavalent chromium\ in many di}erent compounds\ is a well established carcinogen and mutagen ð0Ð4Ł[ The observation of CrV and CrIV intermediates in the selective oxidation of organic substrates by CrVI and their implication in the mechanism of Cr!induced can! cers ð5Ð7Ł has generated a considerable amount of interest in their chemistry and biochemistry ð8Ð01Ł[ The biological reduction of CrVI to lower states has been observed with a wide variety of naturally occur! ring cellular reductants ð02Ð04Ł[ Ligands that possess two oxygen atoms able to form _ve!membered rings about the metal ion\ such as 0\1!diols and a!hyd! roxyacids\ are e}ective as non enzymatic reductants and complexation agents towards hypervalent chro! mium and can stabilise the labile oxidation states of chromium ð01\ 05Ð08Ł[ For this reason sugars\ or their
Author to whom correspondence should be addressed[ e!mail ] inquibirÝsatlink[com $ The present work was presented at the X Meeting on Physical!Chemistry\ Tucuman\ Argentina\ April 10\ 0886[
derivatives\ may play an important role in the chem! istry of CrVI[ Aldonic acids "or their lactones# have been found as metabolic intermediates in animals and plants\ as products of the action of micro!organisms on carbohydrates and as constituents of acidic poly! saccharides ð19Ł[ CrVI oxidation of a!hydroxyacids has been extensively studied ð10Ð18Ł[ These oxidations may yield products coming from either C0C ð10Ð 11\14Ð16\18Ł or C0H ð12Ð13\17\18Ł cleavage\ depend! ing on the particular molecular characteristics of the organic substrate\ through mechanisms involving either two!electron or three!electron initial oxidation of substrate by CrVI[ Polyhydroxyacids o}er additional binding sites to form the CrVI "CrV# ester precursor of slow electron!transfer steps and provide helpful information about stereoelectronic e}ects a}ecting the reaction course ð17Ł[ In previous work we have found that the number and arrangement of hydroxyl groups of aldoses a}ect the CrVI and CrV oxidation rates and mechanism ð29Ð23Ł[ At present\ we are interested in correlating the stereochemistry of aldonic acids with their ability for reducing and co!
1628
1639
S[ Signorella et al[
ordinating CrVI and CrV\ as well as determining the preferred donor sites of these polyhydroxyacids[ In this paper we report our results on the chromic oxi! dation of D!galactono!0\3!lactone "Scheme 0# and we present structural evidences for the intermediate CrV complexes formed in acid and neutral aqueous solu! tions[
Table 0[ Proportion of D!galactonolactone "galL# and D! galactonic acid "gala# in the starting solutiona ðHClO3Ł:M
091 CTb:M
Rc
9[14 9[27 9[49 9[52 9[64 9[89 0[99
5[59 4[99 4[34 4[03 4[95 5[18 4[47
−9[659 −9[428 −9[473 −9[467 −9[462 −9[559 −9[571
EXPERIMENTAL Materials
galL ")# gala ")# 67[8 61[1 60[5 65[3 66[0 71[9 74[2
10[0 16[7 17[3 12[5 11[8 07[9 03[6
a
D!Galactono!0\3!lactone "Sigma grade#\ D!gluconic acid "Sigma grade#\ potassium dichromate "BDH#\ sodium perchlorate "Fluka grade#\ acrylonitrile "Ald! rich grade# and perchloric acid "P[ A[ Merck# were used without further puri_cation[ Water was puri_ed by deionisation\ followed by double distillation from a potassium permanganate solution[ NaðC! r"O#"ehba# 1Ł = H1O was synthesised from 1!ethyl!1! hydroxybutanoic acid "Aldrich grade# and sodium dichromate "Merck# in acetone "Merck\ A[ R[ grade# according to the literature method ð24Ł[ The stability of the organic substrate under con! ditions used in the kinetic studies was tested[ The acid "gala# ] lactone "galL# proportion of the reductant in the starting solution was calculated by optical rotation measurements at given hydrogen ion concentrations ð25Ł and the composition of the substrate solution at di}erent perchloric acid concentrations is shown in Table 0[ The equilibrated solutions are stable over the time required for the kinetic measurements[ In the text that follow gal refers to the total reductant con! centration "ðgalŁ ðgalLŁ¦ðgalaŁ#[ For experiments performed in the 1Ð7 pH range\ the pH of the solutions was adjusted by addition of 9[4 M NaOH and 9[4 M HClO3[ Spectrophotometric measurements Most kinetics measurements were made at 249 nm\ by monitoring the absorbance changes on a Guilford Response II spectrophotometer with fully ther! mostated cell compartments[ Mixtures of sodium per!
Values used in calculations ] molar rotations ] −02[73 "galL# ^ −1[73 "gala#\ ¦9[794 "gal−# ^ K ðH¦Łðgal−Ł "ðgalaŁ¦ðgalLŁ# 1[14×09−4[ b CT ðgalaŁ¦ðgalLŁ¦ðgal−Ł\ where gal− refers to gal! actonate and represents ³9[91) of CT[ c Optical rotation[
chlorate and perchloric acid were used to maintain a constant ionic strength "I# of 0[9 M[ Reactant solu! tions were previously thermostated and transferred into a cell of 0 cm path length immediately after mix! ing and disappearance of CrVI was followed until at least 79) conversion[ Experiments were performed at 39>C unless otherwise mentioned[ In most exper! iments the concentration of CrVI was kept constant at 2[5×09−3 M while the gal concentration was varied from 9[907 to 9[011 M[ The _rst!order dependence of the rate upon ðCrVIŁ was veri_ed[ The pseudo!_rst! order rate constants were calculated at various ðCrVIŁ 9\ but at constant temperature\ ðgalŁ 9\ ðH¦Ł and I[ As expected on the basis of a −d"lnðCrVIŁ#dt kobs rate law where kobs f"ðgalŁðH¦#\ kobs was found to be essentially constant with increasing ðCrVIŁ 9[ CrV was monitored by following the absorbance growth and decay at 649 nm of mixtures of CrVI "2[5×09−2 M# and excess gal "9[07Ð9[46 M# at _xed perchloric acid concentrations[ The formation of CrIII was monitored at 469 nm "ðCrVIŁ 9 0[7×09−1 M and ratios of gal to CrVI from 09 ] 0 to 19 ] 0\ at ðHClO3Ł in the 9[14Ð9[64 M range#[ At the end of the reaction the two dÐd bands ascribed to CrIII were observed at
Scheme 0[
The oxidation of D!galactono!0\3!lactone by CrVI and CrV lmax 309 "o 22# and 455 nm "o 16[4 dm2 mol−0 cm−0# "ðHClO3Ł 9[14 M#[ Both\ visible and UV spectral maxima and intensities\ are in close agreement "vide infra# with those observed for the acid solutions of the KðCr"OH#"gala#"lyxa#Ł "lyxa D!lyxonic acid# ternary complex\ independently synthesised ð26Ł according to a literature method ð27Ł[ These dÐd bands changed with time toward bands of lower intensity at lmax 397 "o 08[6# and 462 nm "o 04 dm2 mol−0 cm−0#\ characteristic of the ðCr"H1O# 5Ł 2¦ ion[
Products analysis Under the conditions used in the kinetic measure! ments "ratios of gal to CrVI from 49 ] 0 to 239 ] 0#\ qualitative identi_cation of D!lyxonic acid as the reac! tion product was carried out by paper chro! matography[ D!lyxonic acid was identi_ed against an authentic sample using n!butanol:acetic acid:water "3 ] 0 ] 4# as eluent[ Paper chromatograms were visu! alised by two kinds of development reagents ] a three! stage dip of silver nitrate\ sodium hydroxide and sodium thiosulphate ð28Ł and p!anisidine reagent ð39Ł[ D!lyxonic acid identi_cation as the only reaction prod! uct was also made by 02C NMR[ A mixture of gal "6[53×09−2 mmol# and potassium dichromate "0[55×09−2 mmol# in 0 M HClO3 "09 cm2# was allowed to react at 39>C[ After the reaction had pro! ceeded to completion the mixture was stirred with a Dowex 49W!X7 cation exchange resin until all the CrIII was removed from the solution and a con! centrated sample was analysed by 02C NMR[ Lyxonic acid and unreacted galL and gala were identi_ed by comparing against authentic samples[ Under all these experimental conditions "large to moderate excess of organic substrate# a quantitative yield of carbon dioxide was observed[ The presence of free radicals was tested for in the reaction of gal with CrVI[ A solution of potassium dichromate "3×09−3 mmol# in 1 cm2 of 9[4 M HClO3 was added to a mixture of 3×09−1 mmol gal and acrylonitrile "9[14 cm2# in 9[4 M HClO3 "9[14 cm2# at 39>C[ After 0 h a white precipitate appeared[ Control experiments "without potassium dichromate or reductant present# did not show the formation of a precipitate[ Besides\ the addition of an excess of acry! lonitrile on the reaction mixture results in the complete suppression of the carbon dioxide evolution[
EPR spectroscopy The EPR spectra were obtained on a Bruker ESP 299 spectrometer[ The microwave frequency was gen! erated with a Bruker 93 ER "8Ð09 GHz# and measured with a Racal!Dana frequency meter[ The magnetic _eld was measured with a Bruker NMR!probe gaussmeter[ A least!squares EPR spectrum!_tting program\ e12new\ was used to estimate the spectral
1630
parameters and relative concentrations of the CrV complexes\ using Lorentzian line shapes ð30\31Ł[ RESULTS AND DISCUSSION Rate studies The absorbance vs time curves at 249 nm exhibit a monotonic decrease of absorbance which cannot be described by a single exponential decay[ At this wave! length\ kinetic traces show an initial deviation from _rst!order decay over short time periods[ These kinetic pro_les could be adequately described by the set of consecutive _rst!order reactions of Scheme 1 ] It is known that CrV species absorb strongly at 249 nm and may superimpose CrVI absorbance yielding the wrong interpretation of spectrophotometric absorbance decay values ð18Ł[ Thus\ considering the CrV absorption superimposition\ the absorbance at 249 nm\ at any time during the reaction\ is given by Abs249 oVI ðCrVI Ł¦oV ðCrV Ł
"0#
Combining the eqn "0# with rate expressions ð32Ł derived from scheme 1\ yields ] Abs249 Abs9 e−k5t ¦k5 oV ðCrVI Ł 9 "e−k4t −e−1k5t #: "1k5 −k4 #
"1#
In this equation\ oV refers to the molar absorptivity of CrV at 249 nm and was assumed to be the same as for the complex ðCrV "O#"ehba# 1Ł − ð24Ł[ Parameters k5 and k4 refer to the rate of disappearance of CrVI and CrV\ respectively\ and were evaluated from a non!linear iterative computer _t of eqn "1#[ It must be noted that in eqn "1#\ k5 appears in the numerator of the pre! exponential term and 1k5 appears in the denominator and in the exponential terms because\ according to the proposed reaction scheme\ only half of the CrVI reaches CrIII through a CrV intermediate[ Figure 0"a# shows a typical curve for one run at 249 nm and the curve _t according to eqn "1#[ The calculated kinetic parameters\ k5 and k4\ for various concentrations of gal at _xed concentrations of HClO3\ are summarised in Table 1[ In multiple measurements\ the reproducibility of the two rate con! stants is better than 09)[ Values of k4 and k5 may also be obtained from absorbance data at 649 nm[ At this wave length\ only CrV absorbs and the experimentally observed inter! mediate CrV growth and decay curve could be _tted by eqn "2# Abs649 k5 ðCrVI Ł 9 oV "e−k4t −e−1k5t #:"1k5 −k4 # V
V
"2#
where o refers here to Cr molar absorptivity at 649 nm[ A good _t of data is obtained with oV 27 M−0 cm−0 ð15Ł\ as shown in Fig[ 0"b#[ Since at least tenfold higher initial CrVI and gal concentrations were necess! ary to follow absorbance changes at 649 nm\ k4:09 and 1k5:09 values calculated by eqn "2# are coincident
1631
S[ Signorella et al[
Scheme 1[ S organic substrate ^ I> intermediate radical
Fig[ 0[ Absorbance vs time curves for the chromic oxidation of gal\ I 0M\ T 39>C at "a# 249 nm\ ðCrVIŁ 2[5×09−3 M\ ðgalŁ 2[5×09−1 M\ ðH¦Ł 9[27 M ^ "b# 649 nm\ ðCrVIŁ 2[5×09−1 M\ ðgalŁ 2[5×09−0 M\ ðH¦Ł 9[27 M ^ "c# 469 nm\ ðCrVIŁ 0[7×09−1 M\ ðgalŁ 0[7×09−0 M\ ðH¦Ł 9[64 M[
Table 1[ Observed pseudo!_rst!order rate constants "k5 and k4# for di}erent concentrations of HClO3 and organic substrate[ T 39>C ^ ðCrVIŁ 9 2[5×09−3 M ^ I 0 M
ðgalŁ:M
9[907 9[925 9[949 9[946 9[961 9[975 9[982 9[097 9[011
1k5 "s−0# and k4 "s−0# for ðHClO3Ł:M of 9[27 9[49 9[64
9[14 0931k5 9[89 0[77 1[49 * 2[69 * * 4[28 5[50
093k4 3[99 5[18 09[4 * 04[5 * * 10[9 15[0
0931k5 0[28 1[67 2[71 3[28 4[99 * * 6[60 7[67
093k4 4[29 09[6 04[1 06[5 19[6 * * 21[0 25[1
0931k5 0[50 2[01 * 3[50 5[11 * * 7[60 8[88
093k4 5[0 01[5 06[9 07[8 13[0 * * 25[7 30[9
0931k5 0[87 3[09 4[07 * 7[90 8[29 * 09[8 01[5
093k4 6[19 03[3 19[5 * 18[5 22[6 * 31[0 35[5
0[99 0931k5 1[30 3[69 5[37 * 8[31 * 01[1 * 04[5
093k4 7[80 06[4 13[7 * 22[8 * 34[7 * 47[1
The oxidation of D!galactono!0\3!lactone by CrVI and CrV with those obtained from eqn "1#\ when the same gal:CrVI ratio and ðHClO3Ł were employed[ For gal and CrVI concentrations used in the kinetic measure! ments\ either at 249 or 649 nm\ the calculated maximum CrV concentrations "ðCrVŁ max# represent up to 7) of the total chromium concentration[ The CrIII formation was followed at 469 nm[ At this wave length\ changes in CrIII concentration can be described by the pair of consecutive reactions shown in the Scheme 2 and the experimental absorbance can be _tted by eqn "3# Abs469 Abs"ðCrIII Ł#¦Abs"Cr2¦ # ðCrVI Ł 9 k0 "e−k0t −e−k2t #"o0III −o1III #: "k2 −k0 #¦Abs "0−e−k0t #
"3#
where k0 and k2 refer to the rates of CrVI and ðCrIIIŁ disappearance\ and o0III \ o1III are the molar absorptivities of the CrIII complex formed at the end of the redox reaction and ðCr"H1O# 5Ł 2¦\ respectively[ A typical _t of experimental data at 469 nm by using eqn "3# is shown in Fig[ 0"c#[ Here\ we have considered that the rate of CrIII complex formation equals the rate of CrVI decay\ without taking into account CrV[ This is consistent with the observation of an isosbestic point at 414 nm under conditions employed to follow CrIII at 469 nm "Fig[ 1# and the low ðCrVŁ max "up to 7)# observed at 649 nm "Fig[ 0#\ for the same oxidant and reductant concentration range[ Values of k0 calculated by eqn "3# are coincident with 1k5 values obtained by using either eqn "1# or "2#[ On the other hand\ k2 depends strongly on ðH¦Ł and a more detailed study is required for the complete description of the hydrolysis mechanism[
CrVI oxidation of `al In the proton concentration range studied\ plots of 1k5 vs ðgalŁ gave good straight lines from which values of k were determined "Table 2#[ A plot of k vs ðH¦Ł gave a straight line with a positive intercept\ showing that the second!order rate constant may be expressed as consisting of an acidity independent and an acidity dependent term ] k k9 ¦kH ðH¦ Ł M−0 s−0 where k9 "2022#×09−3 −3 −1 −0 kH "8824#×09 M s The complete rate law is then given by
"4# and
1632
−d ðCrVI Ł:dt 1k5 ðCrVI Ł T "k9 ¦kH ðH¦ Ł#ðgalŁðCrVI Ł T VI
"5#
VI
where ðCr Ł T represents the total Cr concentration[ The two terms in the rate law "eqn "5## results from the fact that there are at least two transition states\ di}ering in composition but similar in energy\ through which the reduction of CrVI can proceed[ Thus\ the oxidation of gal by CrVI should occur through two parallel slow steps leading to the redox products[ Besides\ in the range of substrate concentrations employed\ only substrate _rst!order dependence was observed and D!lyxonic acid and carbon dioxide were the only detected reaction products\ clearly showing that the C0C scission is preferred to the C0H cleav! age at the a!carbon[ A mechanism taking account of these facts is proposed in Scheme 3[ In this mechanism we have assumed that the open carboxylic acid is the reactive form of the organic substrate[ This assumption is based on previous _n! dings showing the enhanced ability of hydroxyacids to bind CrVI and CrV ð02\14Ð17Ł[ In any case\ a possible mechanism involving the lactone may not be discarded[ However\ if the lactone was the reactive form of the organic reductant\ co!ordination of CrVI should be favoured at a pair of adjacent hydroxyl groups of the molecule ð33Ł[ The intermediate complex formed in this way might be present in the equilibrium mixture but should not be a direct precursor of the slow redox steps[ Opening of the lactone ring and re! arrangement should be required for the slow electron transfer steps to take place within a complex with the carboxylate bound to CrVI[ On the other hand\ in the ðH¦Ł range studied\ CrVI exists mainly as Cr1 O 6 ð34\35Ł[ Thus\ the _rst step of the proposed mechanism may be interpreted as the formation of a monochelate with gala acting as a bidentate ligand bound to CrVI at Cl0O and C10O to yield the anionic species A1−[ This intermediate\ or its monoprotonated form "AH−#\ may decompose to the products[ The rate law corresponding to this mechanism may be expressed as in eqn "6# ] −dðCrVI Ł:dt "k0 K0 ¦k1 K0 K1 ðH¦ Ł#ðgalaŁðCrVI Ł "6# Equation "6# may be expressed as a function of the total organic substrate concentration as in eqn "7# ] −dðCrVI Ł:dt "k0 K0 ¦k1 K0 K1 ðH¦ Ł#ðgalŁKh ðCrVI Ł: "0¦Kh #
"7# 1−
Scheme 2[ ðCrIIIŁ CrIII!gala!lyxa ternary complex
We propose that redox paths\ involving either A or AH− intermediate complexes\ are three electron steps\ just as observed for the chromic oxidation of mandelic ð14Ł\ 1!hydroxy!2!methylbutanoic ð10Ł and D!ribonic ð11Ł acids\ and CrIII is formed with the oxidation prod! ucts[ The alternative path corresponding to the sequence CrVI:CrIV:CrIII was discarded on the basis of the complete CO1 suppression after addition of excess
1633
S[ Signorella et al[
Fig[ 1[ Time evolution of the UVÐvis spectrum of a mixture of gal "9[64 M# and CrVI "0[7×09−1 M#[ ðHClO3Ł 9[4 M ^ T 14>C\ over a period of ca 34 min[
Table 2[ Values of k and k? for di}erent ðHClO3Ł ðHClO3:M
9[14
9[27
9[4
9[64
092 k:M−0 s−0 092 k?:M−0 s−0
4[19 6[13 7[11 09[4 19[5 18[6 22[7 28[1
0[9 01[8 37[1
acrylonitrile on the reaction mixture and the fact that the 1!ketoacid was not observed over the whole set of conditions used here[ Besides\ CrII formation was not proposed since CrO11¦ \ which is taken as evidence that CrII is an intermediate product of the reaction\ was not observed as we did for the oxidation of homo! serine by CrVI ð13Ł[ CO1=− radicals formed in the slow steps may react with CrVI in a subsequent fast step to yield CrV and carbon dioxide[ Polymerisation after addition of acrylonitrile support the formation of these radicals[ CrV can further oxidise gala as will now be discussed[
spectrum dominated by a single detectable signal at `iso 0[8670 "Fig[ 2 Inset# with the four weak 42Cr "8[44) abundance\ I 2:1# hyper_ne peaks at 07 G spacing\ typical of _ve!co!ordinate oxochromate"V# complexes ð36\37Ł[ The rate of formation and disappearance of the intermediate CrV calculated from the peak!to!peak heights "and:or areas# of the EPR signals as a function of time "Fig[ 2# are consistent with values obtained from the spectrophotometric measurements[ The delayed tmax observed by this technique\ account for the lower temperature "14>C\ instead of 39>C# used in the EPR measurements[ In the 9[14Ð0 M proton concentration range\ plots of k4 vs ðgalŁ gave good straight lines from which values of k? were determined "Table 2#[ A plot of k? vs ðH¦Ł gave a straight line with a positive intercept\ showing that the second!order rate constant may be expressed as consisting of an acidity independent and an acidity dependent term[ The rate constant for the disappearance of CrV then may be expressed as ] k4 "k?9 ¦k?H ðH¦ Ł#ðgalŁ
"8#
CrV oxidation of `al At the ðHClO3Ł used in the kinetics measurements\ the reaction of CrVI with excess gal generates an EPR
where k?9 "0421#×09−2 M−0 s−0 and k?H "2323#×09−2 M−1 s−0
"09#
The oxidation of D!galactono!0\3!lactone by CrVI and CrV
1634
Scheme 3[ A1− ð"galaH−1#"O# 1 "OH1#CrOCrO2Ł 1−
Fig[ 2[ Peak!to!peak heights of CrV EPR signals vs time[ ðgalŁ 9[033 M ^ ðCrVIŁ 2[5×09−2 M ^ ðH¦Ł 9[4 M ^ I 0 M ^ T 14>C[ Tmax 497 s[ Inset ] EPR signal recorded at t 04 min[
This means that the CrV intermediate decays to the product directly or through an acid!catalysed step[ Since the latter is of the same order as the non!cat! alysed step\ at low acid concentration "i[e[ neutral pH# the oxidation occurs mainly through the non!acid!
catalysed path[ In this way\ the intermediate CrV is consumed faster than observed for the reaction of CrVI with sugars for which the non!acid catalysed path is extremely slow and the CrV intermediates remain in solution after several weeks ð29Ð21Ł[
1635
S[ Signorella et al[
A mechanism taking into account the structural evidence for the CrV intermediate complex and the kinetics results is given in Scheme 4[ In this mechanism\ only one of the two gala mol! ecules involved in the CrV complex formation is oxi! dised to D!lyxonic acid through the two!electron intramolecular redox path\ in accordance with the observed _rst!order rate on ðgalaŁ[ Possibly\ several linkage isomers might be formed by co!ordination of gala with CrVI via any pair of properly disposed hydroxyl group "see discussion below#[ However\ since D!lyxonic acid is the only reaction product observed in the solution\ the CrV complex\ precursor of the redox steps\ must be one with at least one gala molecule bound to CrV at the carboxylate group[ Besides\ the `iso and Aiso values observed for the CrV intermediate are close to those of ðCr"O#"ehba# 1Ł −[ Since the `iso and\ specially\ Aiso values are very sen! sitive to the CrV co!ordination environment\ our assumption seems to be a reasonable one[ It must be noted that\ in Scheme 4\ the two!electron substrate C0C cleavage a}ords formic acid[ However\ under our experimental conditions\ we have never detected formic acid[ This fact may be explained if\ under conditions used here\ formic acid is oxidised faster than gal[ E}ectively\ CrVI oxidation of formic acid to carbon dioxide is faster than gal oxidation ð38Ł\ a}ording the observed quantitative yield of carbon dioxide[ The proton superhyper_ne "shf# coupling has been shown to be useful in determining the binding of 0\1! diolate moieties of sugars to the CrV centre ð29\33Ł[ We found that performing the reaction at pH × 1\ the single broad "linewidth ½ 1 G# signal observed at higher ðH¦Ł sharpens and the shf pattern resolves\ thus providing new insights about the gala co!ordi! nation sites[ The reaction of CrVI with an excess of 4Ð19 times of gal\ in the 1Ð5 pH range\ at 14>C\ results in the formation of a CrV EPR spectrum which is a com!
posite of at least two EPR signals at `0 0[8670 and `1 0[8677[ The relative proportion of the signals depends on the pH\ but is independent of the gal:CrVI ratio employed and the reaction time "the signals were deconvoluted by _tting the spectra by Lorentzian derivatives#[ Figure 3"a# shows the spectrum of a reac! tion mixture at pH 4 with the simulated spectrum\ for which a `0:`1 ratio 9[30 was obtained[ The best _t of the spectrum yields the spectral parameters sum! marised in Table 3[ The `iso values obtained for the two signals are those expected for _ve!co!ordinate oxochromate"V# complexes ^ while the di}erent shf patterns indicate that they are linkage isomers ð49Ł[ In this way\ the shf splitting of the signal at `0 arises from the coupling of two equivalent protons and may be attributed to a complex with two ligand molecules bound to CrV at Cl0O:C10O[ The shf splitting of the signal at `1 may be assumed to arise from two di}erent binding modes of the two ligand molecules co!ordinated to CrV\ via a 0\1!diolate and the a!hyd! roxocarboxylate donor sites\ respectively[ In the oxi! dation of D!gluconic acid "glca# with CrVI\ in this pH range\ only one species is detected[ Interestingly\ the `iso value "0[867# and the shf pattern "Fig[ 4# reveal the same chelation mode of glca as for gala in the complex giving rise to the signal at `1[ For the same gal:CrVI ratios\ in the 5Ð7 pH range\ the spectra are dominated by a signal at 0[8684 with a shf splitting value of 9[81 G[ The `iso and shf pattern clearly indi! cate that CrO2¦ co!ordinates two gala molecules via a 0\1!diolate moiety and that the shf structure derives from the coupling with the four equivalent protons[ In this pH range\ the minor signal "`iso 0[867# can be attributed to the isomer with the ligand acting through a 0\1!diolate and the a!hydroxocarboxylate donor sites[ Figure 3"b# shows the spectrum of a reac! tion mixture at pH 6[3 with the simulated spectrum\ for which the major CrV species represents the 81) of the CrV in solution[ A last point to consider is that\ at each pH\ the
Scheme 4[ R CH1OH"CHOH# 2
The oxidation of D!galactono!0\3!lactone by CrVI and CrV
1636
Fig[ 3[ X!band EPR signal of a mixture of gal and CrVI "4 ] 0 mole ratio# at "a# pH 4[3\ 1 h after mixing\ "b# pH 6[3\ 0 day after mixing\ and the corresponding simulated spectra "linewidth 9[7 and 9[5 G\ respectively# ^ T 14>C\ modulation amplitude 9[52 G\ centre _eld 2444 G ^ frequency 8[73 GHz[
Table 3[ EPR spectral parameters
ðH¦Ł × 9[0 M pH 1Ð5 pH 5Ð7
`0iso "m#
a0iso:G
`1iso "m#
a1iso:G
0[8670 0[8670 "t# 0[8684 "quint#
* 9[64 9[81
0[8677 "dt# 0[867 "dt#
9[64\ 9[81 9[6\ 9[8
m multiplicity ^ t triplet ^ dt double!triplet ^ quint quintet
1637
S[ Signorella et al[
Fig[ 4[ X!band EPR signal of a mixture of D!gluconic acid and CrVI "09 ] 0 mole ratio# at pH 3\ 0 day after mixing and the corresponding simulated spectrum "linewidth 9[44# ^ T 14>C ^ modulation amplitude 9[404 G ^ centre _eld 2429 G ^ frequency 8[66 GHz[
relative proportion of the intermediate CrV complexes remains constant over the reaction time\ meaning that these CrV intermediates are in rapid equilibria com! pared to the time scale of their subsequent reduction to CrIII[ Acknowled`ements*We thank the National Research Coun! cil of Argentina "CONICET#\ The Third World Academy of Sciences "TWAS#\ the National University of Rosario\ Antorchas Foundation and The International Foundation for Sciences "IFS# for _nancial support[ We thank Prof[ Brumby and Prof[ Lay for providing us with the e12new Program[
REFERENCES 0[ O|Brien\ P[ and Wang\ G[\ Polyhedron\ 0882\ 01\ 0398[ 1[ Cohen\ M[\ Kagacin\ B[\ Klein\ C[ and Costa\ M[\ Crit[ Rev[ Toxicol[\ 0882\ 12\ 144[ 2[ Ramana\ V[ and Sastry\ K[\ J[ Inor`[ Biochem[\ 0883\ 45\ 76[ 3[ Katz\ S[ and Salem\ H[\ The Biolo`ical and Environmental Chemistry of Chromium\ VCH Publishers[ Inc[\ New York\ 0883[ 4[ Casadevall\ M[ and Kortenkamp\ A[\ Car! cino`enesis\ 0883\ 04\ 396[ 5[ Farrell\ R[\ Judd\ R[\ Lay\ P[\ Dixon\ N[ and Baker\ R[\ A[ Bonin[ Chem[ Res[ Toxicol[\ 0878\ 1\ 116[ 6[ Moghaddas\ S[\ Gelerinter\ E[ and Bose\ R[\ J[ Inor`[ Biochem[\ 0884\ 46\ 024[ 7[ Luo\ H[\ Lu\ Y[\ Mao\ Y[\ Shi\ X[ and Dalal\ N[\ J[ Inor`[ Biochem[\ 0885\ 53\ 14[ 8[ Cieslak!Golonka\ M[\ Polyhedron\ 0885\ 04\ 2556[
09[ Mitewa\ M[ and Bontchev\ R[\ Coord[ Chem[ Rev[\ 0874\ 50\ 103[ 00[ Gould\ E[\ Coord[ Chem[ Rev[\ 0883\ 024:025\ 540[ 01[ Zhang\ L[ and Lay\ P[\ J[ Am[ Chem[ Soc[\ 0885\ 007\ 01513[ 02[ Stearns\ D[ and Wetterhahn\ K[\ Chem[ Res[ Tox! icol[\ 0883\ 6\ 108[ 03[ Jannette\ K[\ J[ Am[ Chem[ Soc[\ 0871\ 093\ 763[ 04[ O|Brien\ P[\ Barrett\ J[ and Swanson\ F[\ Inor`[ Chim[ Acta\ 0874\ 097\ L08[ 05[ Krumpolc\ M[\ de Boer\ B[ and Rocek\ J[\ J[ Am[ Chem[ Soc[\ 0867\ 099\ 034[ 06[ Brauer\ S[ and Wetterhahn\ K[\ J[ Am[ Chem[ Soc[\ 0880\ 002\ 2990[ 07[ Goodgame\ D[\ Hayman\ P[ and Hathaway\ D[\ Polyhedron\ 0871\ 0\ 386[ 08[ Goodgame\ D[ and Joy\ M[\ J[ Inor`[ Biochem[\ 0875\ 15\ 108[ 19[ Pigman\ W[ and Horton\ D[\ The Carbohydrates[ Chemistry and Biochemistry\ Vol[ 0B[ Academic Press Inc[\ New York\ 0879[ 10[ Signorella\ S[\ Garc(a\ S[ and Sala\ L[\ Polyhedron\ 0881\ 00\ 0280[ 11[ Signorella\ S[\ Garc(a\ S[ and Sala\ L[\ Polyhedron\ 0886\ 05\ 690[ 12[ Signorella\ S[\ Santoro\ M[\ Mulero\ M[ and Sala\ L[\ Can[ J[ Chem[\ 0883\ 61\ 287[ 13[ Palopoli\ C[\ Signorella\ S[ and Sala\ L[\ New J[ Chem[\ 0886\ 10\ 232[ 14[ Ip\ D[ and Rocek\ J[\ J[ Am[ Chem[ Soc[\ 0868\ 090\ 5200[ 15[ Krumpolc\ M[ and Rocek\ J[\ J[ Am[ Chem[ Soc[\ 0866\ 88\ 026[ 16[ Ramesh\ S[\ Mahapatro\ S[\ Lui\ J[ and Rocek\ J[\ J[ Am[ Chem[ Soc[\ 0870\ 092\ 4061[ 17[ Srinivasan\ K[ and Rocek\ J[\ J[ Am[ Chem[ Soc[\ 0867\ 099\ 1678[
The oxidation of D!galactono!0\3!lactone by CrVI and CrV 18[ Haight\ G[\ Jwisich\ G[\ Kelso\ M[ and Merril\ P[\ Inor`[ Chem[\ 0874\ 13\ 1639[ 29[ Sala\ L[\ Signorella\ S[\ Rizzotto\ M[\ Frascaroli\ M[ and Gandolfo\ F[\ Can[ J[ Chem[\ 0881\ 69\ 1935[ 20[ Signorella\ S[\ Rizzotto\ M[\ Daier\ V[\ Frascaroli\ M[\ Palopoli\ C[\ Martino\ D[\ Boussecksou\ A[ and Sala\ L[\ J[ Chem[ Soc[\ Dalton Trans[\ 0885\ 0596[ 21[ Rizzotto\ M[\ Signorella\ S[\ Frascaroli\ M[\ Daier\ V[ and Sala\ L[\ J[ Carbohyd[ Chem[\ 0884\ 03\ 34[ 22[ Sala\ L[\ Palopoli\ C[ and Signorella\ S[\ Poly! hedron\ 0884\ 03\ 0614[ 23[ Rizzotto\ M[\ Frascaroli\ M[\ Signorella\ S[ and Sala\ L[\ Polyhedron\ 0885\ 04\ 0406[ 24[ Krumpolc\ M[ and Rocek\ J[\ Inor`[ Synth[\ 0879\ 19\ 52[ 25[ Dutta\ S[ K[\ Indian J[ Chem[\ 0864\ 02\ 879[ 26[ Signorella\ S[\ Santoro\ M[\ Moreno\ V[\ Salas! Peregrin\ J[ M[ and Sala\ L[ to be submitted for publication at Polyhedron[ 27[ Kaiwar\ S[ P[\ Raghavan\ M[ S[ and Rao\ P[\ J[ Chem[ Soc[\ Dalton Trans[\ 0884\ 0458[ 28[ Travelyan\ W[\ Proctor\ D[ and Harrison\ J[\ Nature\ 0849\ 055\ 33[
1638
39[ Haugh\ L[\ Jones\ J[ and Wadman\ N[\ J[ Chem[ Soc[\ 0849\ 0691[ 30[ Brumby\ S[\ Appl[ Spectrosc[\ 0881\ 35\ 065[ 31[ Beckwith\ A[ L[ and Brumby\ S[\ J[ Ma`n[ Reson[\ 0876\ 62\ 141[ 32[ Wilkins\ R[ G[\ The Study of Kinetics and Mech! anism of Reactions of Transition Metal Complexes\ Allyn and Bacon\ Boston\ 0863\ pp[ 19Ð12[ 33[ Branca\ M[\ Dessi\ A[\ Kozlowski\ H[\ Micera\ G[ and Swiatek\ J[\ J[ Inor`[ Biochem[\ 0889\ 28\ 106[ 34[ Cieslak!Golonka\ M[\ Coord[ Chem[ Rev[\ 0880\ 098\ 112[ 35[ Poulopoulo\ V[\ Vrachnou\ E[\ Koinis\ S[ and Katakis\ D[\ Polyhedron\ 0886\ 05\ 410[ 36[ Barr!David\ G[\ Charara\ M[\ Codd\ R[\ Farrel\ R[\ Irwin\ J[\ Lay\ P[\ Brambley\ R[\ Brumby\ S[\ Ji\ J[ and Hanson\ G[\ J[ Chem[ Soc[\ Faraday Trans[\ 0884\ 80\ 0196[ 37[ Farrel\ R[\ Judd\ R[ and Lay\ P[\ Inor`[ Chem[\ 0878\ 17\ 2390[ 38[ Arias\ C[ and Perez!Benito\ J[\ Collect[ Czech[ Chem[ Commun[\ 0881\ 46\ 0710[ 49[ Brambley\ R[\ Ji\ J[\ Judd\ R[ and Lay\ P[\ Inor`[ Chem[\ 0889\ 18\ 2978[