4463
Kinetics and Mechanism of the Formation of Organochromium( 111) Complexes in the Reactions of Organic Peroxides with Chromium(I1)' Michael R. Hyde and James H. Espenson* Contribution from the Ames Laboratory and Department of Chemistry, Iowa S t a t e University, Ames, Iowa 5001 I . Received October 30, 1975
Abstract: Bimolecular rate constants for reduction of a series of 17 organic peroxides, hydroperoxides, peresters, and peroxyacids by Cr2+vary within the range 7.7 X to 3 X lo6 M-' s-l (25O in 1:l v / v CH3OH/H20). Steric factors provide the dominant influence upon reactivity, whereas electronic effects are relatively unimportant. Reaction products in both CH3OH/ H20 and in aqueous solution are consistent with the formation of alkoxy-radical intermediates. Organochromium(II1) species, (H20)5CrR2+,are detected in all but the slowest reactions. The observation of a transient intermediate in reaction of perbenzoic acid with Cr(I1) is attributed to formation and decomposition of phenylchromium(II1) ion.
Reduction of hydrogen peroxide by a variety of metal ions including Fe2+,2 Cr2+,3and V 0 2 + indicates that reaction occurs at least in part by a successive one-electrontransfer mechanism.
M"+
H+
+ HOOH
M"+
+ *OH
M("+')+ H+
M("+')+
+ *OH
+ H20
(1)
(2)
Addition of organic substrates R H to these "Fenton-like" reagents4 diverts hydroxyl radical in reaction 2 to produce organic radicals via reaction 3,
RH
+ .OH
+
Re
+ H20
(3)
providing an excellent route into the study of these latter species5 More recently, chemical and radiolytic generation of carbon-centered radicals have been adapted to prepare organochromium(II1) species, via radical capture by Cr2+.6,7Similar complexes of the formula (H20)5CrR2+ have been proposeds and sometimes detected9 in the reactions of Cr2+ with a number of organic peroxides. There is presently available a good deal of information concerning reaction products in these systems.1° However, with few exceptionsI0 kinetic data have not been reported concerning the rates and mechanism of interaction between Cr2+ and organic peroxides. Our own interest in the preparation and chemistry of organochromium(II1) complexes' prompted an investigation into the factors affecting the reactions between various organic peroxides (a term we use to include peroxides, hydroperoxides, peroxyacids, and peresters), particularly the generation of CrR2+ complexes thereby. Details are reported herein. Formation of the strongly absorbing organochromium(II1) species has proven to be an extremely useful kinetic probe, facilitating study of peroxide reactions with chromium(I1) where previously the necessarily poorer quality of data has been shown to be r e ~ t r i c t i v e . ~
Experimental Section Preparation of Organic Peroxides and Other Reagents. A number of the peroxides used in this study were commercially available: tert- butyl hydroperoxide, di-tert-butyl peroxide, and tert- butyl perbenzoate (Matheson, Coleman and Bell) and benzoyl peroxide (Aldrich Chemical Co.). Other samples including some of the above, were prepared by methods available in the literature: ethyl tert-butyl peroxide and isopropyl tert- butyl peroxide,'* cumene hydr~peroxide,'~ a,a-dimethyl 6-phenethyl hydroperoxide? n-butyl hydroperoxide, l4 sec- butyl hydropero~ide,'~ tert-amyl hydroperoxide,I6 substituted
tert-butyl perbenzoates,17and perbenzoic acid.18 The characterization and purity of samples were checked by NMR spectroscopy, boiling points of samples (as cited in the literature), elemental analysis where possible, and always by peroxide assay, using iodometric titration.I9 In all cases the compounds used satisfied the known data, and were sufficiently pure, after vacuum distillation, for kinetic runs. Samples of the neat compounds were stored at -10' when not in use, and solutions were always freshly made prior to reaction. Solutions of Cr(C104)2 were prepared by reduction of chromium(II1) perchlorate with amalgamated zinc under a nitrogen atmosphere in dilute aqueous perchloric acid. Kinetic Experiments. Preliminary runs in a variety of mixed solvents, for example, THF/H20, C ~ H ~ O H / H Zand O , CH3OH/H20, provided evidence for hydrogen atom abstraction from the solvent by radicals produced during the course of reaction. A strictly aqueous solution would eliminate this problem, but this was unfortunately not feasible owing to the very limited solubility of many of these peroxides. Hence it was necessary to employ a mixed-solvent system, although as reported herein, some important product distribution data were obtained in a strictly aqueous medium. A 1:l v/v CH30H/H20 solvent was chosen as the standard kinetic solvent, with runs being conducted under an atmosphere of Cr2+scrubbed nitrogen with ionic strength maintained a t 1.0 M with LiC104 and HC104. Runs were carried out at 25' unless stated otherwise. A Durrum stopped-flow spectrophotometer was employed for many kinetic runs, and in the case of the slower reactions a Cary 14 spectrophotometer was used. For the slower reactions, particularly those of ethyl tert-butyl peroxide and isopropyl tert-butyl peroxide, conditions had to be carefully chosen to avoid interference from the slow acidolysis of the organochromium products. In such cases the growth of Cr3+ was monitored through the small absorbance increases at X 400-410 nm, although in most instances the reaction was followed by monitoring large absorbance increases at appropriate wavelengths corresponding to absorption maxima for the organochromium(II1) species, generally between 260 and 390 nm. With the exception of the reaction of Cr2+ with perbenzoic acid, whose rate was so high that second-order conditions were required, the reactant concentrations were selected with a 10- to 50-fold excess of Cr2+over peroxide. The concentration of Cr2+ in run solutions was determined by reaction with Co(NH3)5Cl2+, determining Co2+ as the thiocyanate complex in 50% acetone-water at X 623 nm (t 1.84 X IO3 M-' cm-' 1. Kinetic Data. Conventional pseudo-first-order rate plots were constructed to permit computation of a rate constant designated kobsd. An exception to this was made in a few cases where the product decomposition rate was not negligibly slow compared to its formation, in which instance a consecutive reaction scheme or Swinbourne's methodZowas used to obtain a self-consistent set of kinetic data. We here assume, and will later verify, that in every instance the rate-limiting step is the simple bimolecular process of eq 4,followed by numerous and varied rapid reactions of the radical so generated.
Hyde, Espenson
Cr2+
/
+ R O O R ' k C [CrOR']2+ 3. RO.
(4)
Reactions of Organic Peroxides with Chromium(Ii)
4464 Table I. Kinetic Data for the Reduction oforganic Peroxides ROOR’ by Chromium(II), 25.0°, p = 1.00 M (LiC104) in 1:l v/v CH30H/H20 Range of initial concentration ~
ROOR
[ROOR’] / M
~ _ _ _ _ _ _ _
[Cr2+]/ M
k1lM-l s-’ (no.).
h/nm ~
t-BuOO-t-Bu 1.7 x t -BuOOCH (CH3) 2 (1.2-6.0) X t-BuOOC2Hs (0.50-1.00) X t-BuOOH (1.50-3.00) X C ~ H S CCH3)200H ( (1.0-2.0) x 10-4 PhC(CH3)200H (0.50-3.00) X PhCHzC(CH3)200H (1.4-2.8) X n-BuOOH (0.50-1.00) X sec-BuOOH (0.50-1.00) X PhC(0)OOH (2-4) x 10-5 PhC(O)OOC(O)Ph (1.0-3.0) X t-Bu-OOC(0)Ph (0.50-1.50) X r-BU-OOC(O)-p-CIC6H4 (2.98-5.58) X t-Bu-00C(O)-m-CiC6H4 (1.01-7.9) X ~ - B u - O O C ( O ) - P - C H ~ O C ~ H ~(1.02-10.2) x 10-3 ~ - B u - O O C ( O ) - ~ - C H ~ C ~ H (1.0-8.7) ~ X t-BU-OOC(O)-p-NCC6H4 (1.02-8.55) X
3.3 x 10-1 (2.20-10.0) x 10-2 (0.71-4.50) X (0.61-7.51) X 10’: (0.81-7.30) X (0.55-6.13) X (0.48-7.53) X (0.40-5.05) X (0.59-5.03) X lob3 (0.89-1.90) X (0.53-6.03) X (0.73-7.58) X (3.6-7.0) X (3.4-10.1) X (3.6-10.1) X (3.5-10.0) X (3.4-10.1) X
408 408 258,390 258,392 275, 394 258 297,358 260,280 275, 330 260,280 392,580 288,392 300 300 300 300 300
~~~~
G7.7 x 10-6 ( 1 ) b . C (4.19 f 0.40) X (7) 2.35 f 0.25 (16) (1.65 f 0.11) x 104 (20) (1.60 f 0.11) X IO4 (14) (0.99 f 0.06) X lo4 (19) (2.13 f 0.18) X lo4 (22) (5.77 f 0.43) X lo4 (16) (3.63 f 0.61) X IO4 (16) (3.3 f 0.5) X IO6 (8) (5.2 f 0.2) X lo1 (16)c (1.18 f 0.09) X lo4 (14) (8.9 1.1) x 103 (4)d (1.02 f 0.13) x 104 (4)d (1.50 f 0.12) x 104 (4)d (1.23 f 0.04) X IO4 (4)d (8.2 0.1) x 103 (4)d
The mean value of kl, the standard deviation, and the number of determinations in parentheses. One run at 23O, assuming a first-order dependence upon [Cr2+]. Experimental kl divided by a statistical factor of 2 for this symmetrical compound. Reference 45. Some but by no means all of the subsequent reactions produce an organochromium(II1) species. Provided the overall stoichiometry 1 peroxide:2 Cr2+is maintained, the yield of organochromium complex does not affect the determination of kl?which is given by kobsd/[Cr2+] regardless of the fraction of the intermediate one of whose rapid reactions yields that particular product whose absorbance is being used to monitor the reaction rate. That is to say, the concentration of a particular organochromium product [Pi] is given by the relation [Pi] / [peroxidelo = fi[ 1 - exp(-k I [Cr2+]t ]
(5)
wherefi represents the fraction of the radical intermediate yielding the product Pi; clearly, only the yield of Pi, and not the specific reaction rate, depends uponfi. Product Identification. Three different approaches were adopted, depending on the information required: (1) Ion exchange separation was used in a few reactions to identify the Cr(II1) complexes, and proved particularly useful for the products Cr02CPh2+, C ~ ( C ~ H S O H )and ~ +C, ~ ( ~ - B U O Has) ~well + as some organometallic products. The separations were effected by transferring reacted SOlutions onto a column of Dowex 50W-X8 cation exchange resin in the Li+ form, followed by elution with appropriate combinations of LiC104 and HC104, up to 4 M HC104, and ultimately using 3 M H2SO4 to remove the tightly bound tripsitive Cr(II1) complexes with coordinated alcohols. (2) Measurement of the acidolysis rate of the organochromium product produced from reactions in 50% CH30H/ H2O provides in most cases an unambiguous identification, largely because the products of all 17 reactions produced one of five possible alkyl derivatives, each of which is known from independent unambiguous reactions.7.9-1 Comparison of the products formed in strictly aqueous solvent, whenever solubility considerations permitted, afforded additional information on the reactions of radical intermediates. (3) Mass spectrometric investigations of volatile products of reaction of perbenzoic acid were also carried out.
Results Kinetics of Cr2+ Reductions. Reduction of each of the organic peroxides by Cr2+ conforms strictly to a second-order rate expression, given by eq 6:21 -d[ROOR‘]/dt = kl[ROOR’][Cr2+]
as k&sd vs. [Cr2+].These plots are linear within experimental error and pass through the origin, in conformity with eq 6. The slopes of these lines afford the values k l , which are summarized along with the experimental conditions in Table I for all 17 peroxides studied. On the basis of the absorbance changes measured in the kinetic runs,22the yields of the (H20)5CrR2+ products based on the limiting initial quantity of peroxide were typically 10-20% of theoretical for the situation in which 100% of the alkoxy radical RO- led to the formation of an organometallic product. Kinetics of Organochromium(II1) Decomposition. Each of the organochromium(II1) products undergoes slow decomposition corresponding to an acidolysis process resulting in Cr-C bond cleavage as shown in eq 7. CrR2+
(7)
Preliminary experiments showed that the rate constant for a given decomposition reaction was the same whether or not the complex was first isolated from the reaction solution by ionexchange chromatography. These determinations were made using the CrR2+ complexes derived from the reaction of tertbutyl hydroperoxide in each of two solvents, H2O (producing CrCH32+) and C H 3 O H / H 2 0 (producing CrCH20H2+).23 Consequently in subsequent reactions the rate of decomposition of the CrR2+ product was determined directly in the solution immediately after completion of the peroxide reaction, by simply air-oxidizing the residual Cr2+. Conditions for these experiments were chosen such that substantial absorbance changes could be monitored, principally around A,, 380-390 nm,24with [ROOR’] z 2-5 X M, [Cr2+]o z 1-3 X M, at p = 1.00 M (LiC104) and 25.0’. Pseudo-first-order plots of In ( D t - D m )vs. time were linear to at least 80% completion. A general expression for the rate of reaction 7 is
(6)
Plots of In (D,- D J vs. time were linear for at least three half-lives throughout, and kl proved independent of [H+] in the range 0.05 M < [H+] < 1 .O M. Typical data for the Cr2+ reduction of tert- b a y 1 hydroperoxide, sec-butyl hydroperoxide, and n-butyl hydroperoxide are shown in Figure 1 plotted
+ H 3 0 + = Cr3+,q + R H
-d[CrR2+]/dt = ( k 2
+ k3[H+])[CrR2+]
(8)
although for R = CH3 no k 2 term was observed. Plots of k o b d vs. [H+] are shown in Figure 2 for the three major organochromium products, R = CH3, CH2OH, and C2H5. Each point in the figure is keyed to the organochromium(II1) cations listed in Table 11, the major identification of the CrR2+ product
Journal of the American Chemical Society / 98:15 / July 21, 1976
4465 Table 11.
I
m-
Identity" of Organochromium(II1) Products Product
7
250 -
Oxidant source
ROOR'
In aq H3O+
In 1:l CH30H/H20
200-
H202 t-BuOOH PhC(CH3)2OOH C2HsC(CH3)200H PhCH2(CH3)200H t-BuOOC2Hs n-BuOOH sec-BuOOH t-BuOOC(0)Ph PhC(O)OOC(O)Ph PhC(0)OOH
cr3faq (1) CrCH32+ ( 3 ) CrCH32+ (5) CrC2Hs2+ (7) CrCH2Ph2+ CrCH32+ (10) Cr(CH2)40H2+ (1 2) CrC2Hs2+ (1 3) CrCH32+ (15)
CrCH20H2+ (2) CrCH20H2+ (4) CrCH20H2+ (6) CrC2Hs2+ (8) CrCH2Ph2+ CrCH20H2+ ( 1 1 )
150~
1
/ I
I
1
2
3 103
I
4
1-BuOOH
I
5
I
I
I
6
[cr2+i/ M
Figure 1. Plots of pseudo-first-orderrate constant vs. [Cr*+]for reduction of three typical hydroperoxides.
i - - ' - i -
--CrC6Hs2+ e
_-_
CrC2Hs2+ (14) CrCH20H2+ (16) CrCH20H2+ (17) CrCH20H2+ (1 8)
+
" The organochromium product from the particular ROOR' Cr2+ reaction in the respective solvent. Each product is keyed by the parenthetical number to the experirnent(s) on acidolysis illustrated 355 nm. The in Figure 2. From its uv-visible spectrum with A, product in CH30H/H20 was not determined but earlier work9 identified this product in C2HsOH/H20 solutions. e The presumed product from perbenzoic acid Cr2+in 100% aqueous solution (see text).
+
Table 111. Values of Rate Constants k2 and k3 for Acidolysis of Organochromium(II1) Cations, eq 7 and 8"
CrR2+ CrCH32+ CrC2Hs2+ C ~ - ~ Z - C - , He ~ ~ + CrCH20H2+ Cr(CH2)40H2+ C r C H 5 2+ g
k2/~-'
___
2.2 x 10-4 3.0 x 10-4 6.6 X 10-4f 6.12 X 1.3
k3lM-l
s-I
NO.'
4.94 x 10-3 c i . i 5 x 10-4 6.5 X 4.65 X 10-4f
4 4 0 6
1.64 x 103
1
___
1
" At fi = 1.0 M (LiC104, HC104) in mixed aqueous methanol containing from 0 to 50 vol % CH30H. The number of different peroxide systems which produce the particular O R 2 + product. Reference 6 gives k3 = 5.0 X M-' S - ' . ~ Determined on authentic and purified CrC2Hs2+ by Mr. T. J. McNeese. Determined on authentic Cr-r~-CjH7~+ by Mr. M. J. Haasl. f Reference 7b gives k2 = 7 X s-' and k3 = 3.2 X M-1 s-I. g The presumed product from benzoic acid Cr2+ in 100% aqueous solution (see text).
+
T
I3
1
\3 P I 4 he12
02
04 06 : H ~ / M
08
I0
1
Figure 2. Plots of rate constants for acidolysis of the four organochromium(II1) reaction products against [H+]. The number designations refer to the reactions as given in Table 111.
formed in each instance being just this agreement between the rate constant a t a particular [H+] compared to that for the authentic compound. The products so identified are given in Table 11, and in Table I11 are listed values of k2 and k3 for these and related complexes. Ion-Exchange Separations. Further separations and product identifications were carried out in three systems, t-BuOOH, t-BuOOCzHs, and t-BuOOC(0)Ph. For the first of these, reaction with Cr2+was carried out in both 1:l CH30H/H20 and in aqueous solution. The former, after ion-exchange separation, yielded a dispositive organochromium(II1) cation with,,,A 390 nm; the complex underwent acidolysis a t the same rate as authentic CrCH20H2+. No CrCH32+ was detected in this experiment, and its concentration can conservatively be estimated as IO9 in this series. This regular and large effect is consistent with direct attack of Cr2+ a t -01 based on steric crowding a t the -CI position. When -C1 = CI’ (i.e., for di-tert-butyl peroxide), it is no longer possible to conclude that Cr2+ attack occurs a t oxygen, and this reaction may possibly occur by an outer-sphere mechanism. In the series of tert-butyl p e r b e n ~ o a t e sstudied, ~~ there is a mild but systematic variation of 10-4kl (0.82-1.50 M-I s - I ) with the Hammett substituent parameter u (+0.66 to -0.27), The overall resulting in a reaction constant p = -0.28 f 0. reaction may be considered in two steps, a preequilibrium between perester and Cr2+ (forming a “precursor complex”) followed by intramolecular electron transfer to the peroxide. The two steps would appear to have opposing electronic requirements, which nearly cancel, although the former effect may be slightly dominant in view of the small negative value of p.47 The product of the reaction of tert-butyl perbenzoate with Cr2+ is the benzoato complex of Cr(III), but the question of the molecular configuration of its transition state remains open. Kochi8Jo originally pointed to the analogy between the reactions
+ Cr2+ t-BuOX + Cr2+
(NH3)5CoX2+
-
-
+ (NH3)5Co1l. (20) CrX2+ + t-BuO. (21) CrX2+
a n analogy which we believe demonstrates considerable merit. The question of which oxygen atom is attacked in the former
Hyde, Espenson
/ Reactions of Organic Peroxides with Chromium(II)
4468 reaction for X = carboxylate has never been settled by direct experiment, although indirect but seemingly convincing arguments have been advanced for attack at the carbonyl oxygen.49Similarly, we are unable to say whether Cr2+ attack on tert- butyl perbenzoate, which is definitely an inner-sphere process in view of the formation of Cr02CC6Hs2+,occurs via carbonyl or peroxidic oxygen.50 ValuesS' of kl for Cr2+ benzoyl peroxide (50 M-' s - l ) and valeryl peroxide (1 .3)1° are both considerably smaller than the specific rates for tert-butyl perbenzoate (1.2 X lo4) and perbenzoic acid (3 X lo6). In the latter case attack occurs a t a completely different site, the OH oxygen, and the rates are not directly comparable. For the other three, both steric and electronic effects complicate the reactivity pattern; ignoring the former, the effect of symmetrical electron-withdrawing groups at both oxygen atoms can but induce positive charge in each oxygen,52and once again the preequilibrium (precursor complex) term appears dominant. Perbenzoic Acid and Cr(I1) and the Search for CrCgHs2+. Reaction of Cr2+ with perbenzoic acid occurs rapidly, 10-6k 1 = 3 f. 1 M-' s-l, some 102-fold faster than the H202 react i o n . ' ~The ~ ~ two reactions presumably afford similar steric constraints in that attack of Cr2+ occurs a t OH in both cases. The increased reactivity of the former is presumably due to its greater oxidizing strength. The product of reaction in CH30H/H20 solvent is C r C H 2 0 H 2 + ,the origin of which appears to us to arise from reaction of the benzoyloxy radical with methanol (eq 22), which under these conditions would be expected to compete favorably with decarboxylation (eq 2 3 ) . 5 3
+
C6H5C02.
+ HCH2OH
-+
C6H5COy
C6H5C02H
+
C6H5.
+ aCH20H
+ CO2
(22) (23)
The observation of a transient intermediate (Table 111, Figures 3 and 4) in the strictly aqueous solvent suggested to us the occurrence of decarboxylation (eq 23) followed by recombination of phenyl radical and Cr2+ (eq 24), a process well known to be very rapid for other radical^.^ C6Hj.
+ Cr2+
-
CrC6Hj2+
(24)
The mass spectra of product solutions give evidence for only benzene and carbon dioxide, which is consistent with our identification of the transient as C T C ~ H ~presuming ~+, the latter reacts with HzO and H3O+ as the other compounds d o (eq 7 ) . The available data do not constitute definitive evidence that the observed transient is CrC6Hs2+, but this formulation is consistent with known reactions of the radical intermediate^.^^ The phenylchromium cation represents a new and interesting addition to the series of known (Hz0)5CrR2+ complexes'~" being the first and only aryl. Its acidolysis is orders of magnitude faster than the other derivatives (ref 7b and Table 111), perhaps reflecting the bond strength of Cr-C compared to that of H-C in benzene. Acknowledgments. W e are grateful to the Undergraduate Summer Trainees in the Ames Laboratory who contributed data useful here, Messrs. M . J. Haasl. T. J. McNeese, and J. B. Raff, to Mr. G. D. Flesch of the Ames Laboratory for mass spectral determinations, and to Dr. K. U. Ingold for a helpful discussion of the chemistry of organic free radicals. One of us (M.R.H.) wishes to acknowledge a Senior Fulbright-Hays travel award. Helpful collaborative discussion with Dr. M. D. Johnson under support of a NATO Research Grant is gratefully acknowledged. References and Notes (1) Work supported by the U. S. Energy Research Development Administration. Contract No. W-7405-Eng-82.
Journal of the American Chemical Society
/
98:15
(2) W. G. Barb, J. H. Baxendale, P. George, and K. R. Hargrave, Trans. Faraday SOC..47. 462 (1951). (3) A. Samuni, D. Meisel; and G. Czapski, J. Chem. SOC.,Dalton Trans., 1273 (1972). (4) H.J. H. Fenton, J. Chem. Soc., 65, 899 (1894). (5) (a) W. T. Dixon and R. 0. C. Norman, J. Chem. Soc., 31 19 (1973): (b) G. Czapski, A. Samuni, and D. Meisel, J. Phys. Chem., 75,3271 (1971). (6) W. Schmidt, J. H. Swinehart, and H. Taube, J. Am. Chem. SOC.,93, 1117 (1971). (7) (a) H. Cohen and D. Meyerstein, J. Chem. SOC.,Chem. Commun., 320 (1972); (b) lnorg. Chem., 13, 2434 (1974). (8)J. K. Kochi, Rec. Chem. Prog., 27, 207 (1966). (9) J. K. Kochi and D. D. Davies, J. Am. Chem. SOC.,86, 5264 (1964). (10) J. K. Kochi and P. E. Mocadlo, J. Org. Chem., 30, 1133(1965). (11) See, for example, (a) J. H. Espenson and D. A. Williams. J. Am. Chem. Soc., 96, 1008 (1974); (b) J. S. Shveima and J. H. Espenson. ibid., 95, 4368 (1973); (c) R. S. Nohr and J. H. Espenson, ibid., 97, 3392 (1975). (12) F. F. Rust, R. H. Seubold, and W. E. Vaughan, J. Am. Chem. SOC.,72, 338 (1950). (13) G. P. Armstrong, R. H. Hall, andD. C. Quin, J. Chem. Soc., 666 (1950). (14) H. R. Williams and H. S. Mosher, J. Am. Chem. SOC.,76, 2984 (1954). (15) H. R. Williams and H. S. Mosher, J. Am. Chem. Soc., 76, 2987 (1954). (16) N. A. Milas and D. M. Surgenor, J. Am. Chem. Soc.. 68, 643 (1946). (17) (a) A. T. Blomquist and I. A. Berstein, J. Am. Chem. SOC.,73,5546 (1951); (b) N. A. Milas and D. M. Surgenor, ibid., 68, 642 (1946). (18) G. Braun, "Organic Syntheses", Collect. Voi. 1. Wiley, New York, N.Y., 1941, p 431. (19) (a) R. M. Johnson and I. W. Siddigi, "The Determination of Organic Peroxides", Pergamon Press, Oxford, 1966; (b) R. D. Mair and R. T. Han, "Organic Peroxides", Vol (I, D. Swern, Ed., Wiley-lnterscience, New Yo&, N.Y., 1971, Chapter VI. (20) E. S. Swinbourne, J. Chem. SOC.,2371 (1960). (21) in the case of the series of tert-butyl perbenzoatescontaining substituents in the phenyl ring, reaction conditions were such that the perester concentration was in excess of [Cr2+]. Consequently rate constants k, were obtained accordin to the expression -d[Cr2+] /dt = 2 kl [Cr2+] [ROOR'], with ki = k,b~d/*QRooR']. (22) Molar absorptivities of aik ichromium(lll) complexes are typically -200 M-' cm-' at 390 nm.6,7,' Other reactions of the alkoxy radical such as eq 12b and 12c are responsible for the yields of CrR2+ being lower than theoretical, the Cr(lli) products so formed being a tert-butyl alcohol complex (12b) or the hexaaquochromium(l1l) ion (12c). The relative proportion of the latter products was not determined. (23) The decomposition rates were determined in solutions containing 0-50% CH30H, and the rate is largely independent of the methanol content. The implications are that the rate constant is nearly solvent independent in this range and that mixed alcohol-water complexes ( H 2 0 ) 5 - ~ C H 3 0 HCrR2+ )~ undergo acidolysis nearly independent of X . Some of the scatter in the data in Figure 2 may reflect the approximate nature of this treatment. (24) Since the spectra of most aikylchromium species are rather similar, particularly around X 390 nm, the spectrum itself is insufficient to distinguish between (say) and CrCH20H2+. At lower wavelengths X 250-290 nm the presence of other absorbing species often masks the more definitive absorbance maxima of RCrZC,thus necessitating ionexchange separation of products in these cases. In certain instances. for example, C&!~cii2Cr~+ , , ,A 355 nm, ion-exchange is not necessary. For CrCH20H2+, c is given as 570 M-' cm-I at 390 nm.7b /+ ,m, (e/M-' cm-') are: 392 (240) and 258 (2520).'lb For C T C H ~ ~Xn In this situation the simultaneous formation of a tert-butyl aicohoi complex undoubtedly occurs (from the reaction of t-BuO. with Cr2+, however, not from an alternative mode of attack of Cr2+ on the peroxide, a point readily seen from the kinetic data of Table I). The tertiary alcohol will not, of course, respond to the oxidation procedure used for analysis of coordinated C2H50H. D. W. Kemp and E. L. King, J. Am. Chem. SOC.,89,3433 (1967). Methanol must, of course, be avoided in experiments designed to detect coordinated ethanol. The sample was obtained by reaction of benzoatopentaamminecobalqlll) ion with Cr2+, according to E. S.Gould and H. Taube, J. Am. Chem. SOC., 86, 1318 (1964). Elution with 0.5 M K l O 4gave a green complex with peaks at X 415 and 585 nm. We note without explanation that the peroxide reaction product appears to decompose rather more rapidly than the authentic sample. The solubility of the perbenzoate in aqueous solution is much lower than the concentration cited, however, and reaction with Cr2+ was not homogeneous. The approximate absorption spectrum was obtained by monitorin the absorbance decrease accom anyin acidoiysis. With [PhC(O)OOHQ = 5 X M, [Cr2+] = 1 X 10-'M, [H?] = 2 X M, and = 1.0 M, the initial Cr2+ reduction is complete within the 4 ms mixing time, whereas acidolysis of CrCgH5'+ requires ca. 1 sat this [H+]. Extrapolation to zero time and subtraction of the final absorbance gives the desired spectrum. We are grateful to Mr. G. D. Flesch for the mass spectra determinations. A Considerably smaller value, 2.1 X lo3 M-' s-', from G. Davies and N. Sutin (unpublished results) is cited by G. Davies, N. Sutin. and K. 0. Watkins, J. Am. Chem. SOC.,92, 1892 (1970). H. Taube and H. Meyers, J. Am. Chem. SOC.,76, 2103 (1954). (a) M. Ardon and R. A. Plane, J. Am. Chem. Soc., 81, 3197 (1959); (b) L. B. Anderson and R. A. Plane, Inorg. Chem., 3, 1470 (1964). M. R. Hyde and J. H. Espenson, unpublished results. See,for example, M. R. Hyde,R. S.Taylor, and A. G. Sykes. J. Chem. SOC., Dalton Trans., 2730 (1973). There are also strong precedents for tert-alkoxy radical formation in both Fez+ and Ti3+ reductions of related systems: J. K. Kochi, J. Am. Chem. SOC..84, 1193 (1962).
/ July 21, 1976
Y
4469 (a) C. Walling and A. Padwa, J. Am. Chem. Soc., 85, 1593 (1963); (b) ibid., 85, 1587 (1963). C. Walling and J. A. McGuiness, J. Am. Chem. SOC.,91, 2053 (1969). (a) C. Walling and B. B. Jacknow, J. Am. Chem. SOC., 82, 6108, 6113 (1960); (b) C. Walling and W. Thalen, ibid., 83, 3877 (1961). K. U. lngold in "Free Radicals", J. K. Kochi, Ed., Vol. I, Wiley, New York. N.Y., 1973, Chapter 2. J. K. Kochi, ref 43, Chapter 23. We gratefully acknowledge the assistance of Mr. John B. Raff, who determined the rate constants fot the substituted fert-butyl perbenzoates. J. Shorter, "Correlation Analysis in Organic Chemistry". Clarendon Press, Oxford, 1972. . . Similar small effects have been observed in theacickatalvzed esterification of benzoic acids (JI = -0.52)46and in the thermolysis of t&f4utyl peresters (p = -0.56).48 Cleavage of the 0-0 bond in the latter reaction is more facile when the bond is less polar. (48) A. T. Blomquist and I. A. Berstein, J. Am. Chem. Soc., 73, 5546 (1951). (49) K. L. Scott and A. G. Sykes, J. Chem. SOC.,Dalton Trans., 1832 (1972). ( 5 0 ) Unlike the Inorganic situation, in the case of the perbenzoate, I8O tracer
experlments appear feasible in that It is possible to label each oxygen specifically, such as f-Bu'80180C(160)Phand t-Bu160160C('80)Ph.The experiment requires a knowledge of the mode of cleavage during the subsequent acid hydrolysis of the Cr(lll) carboxylate, which for (H20)5Cr0&CH32+ was inferred to occur at the C-0 bond (E. A. Deutsch and H. Taube, Inorg. Chem., 7, 1532 (1968)), a conclusion which would also be verified. (51) Experimental values of kl for the symmetric peroxides have been divided by a statistical factor of two. (52) S. Hasegawa, N. Nishimura, S.Mitsumoto. and K. Yokoyama. Bull. Chem. Soc. Jph., 36, 522 (1963). (53) Another likely source of hydroxymethyl radical is its formation from reaction of CBH5-with methanol solvent (eq 23a).54 CBHS*4- HCH2OH 4 C&
-I-GHzOH
(2%)
(54) J. E. Packer, D. 8. House, and E. J. Rasburn (J. Chem. SOC.B,1574 (1974)) give k2% 1.4 X IO5 M-l s-'. (55) R. Hiatt in "Organic Peroxides", Vol. 11, D. Swern. Ed., Wiley, New York. N.Y., 1971, Chapter VIII.
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The Molybdenum-Molybdenum Triple Bond. 1. Hexakis( dimethy1amido)dimolybdenum and Some Homologues: Preparation, Structure, and Properties M. H. Chisholm,* la F. A. Cotton,* l b B. A. Frenz,lbW. W. Reichert,la L. W. Shive,lband B. R. Stultslb Contribution from Departments of Chemistry, Princeton University, Princeton, New Jersey 08540, and Texas A&M University, College Station, T4exas77843. Received November 3. 1975
Abstract: Mo2(NR2)6, where NR2 = NMe2, NMeEt, and NEt2, have been prepared from metathetic reactions involving molybdenum halides and the appropriate lithium dialkylamide, and have been characterized by a number of physical techniques. The molecular structure of Mo2('NMe2)6 has been determined by single-crystal x-ray diffraction. The space group is P21/c with u = 11.461( 3 ) A, b = 12.052 (4)A, c = 15.409 ( 3 ) A, 6 = 101.83 (2)O, V = 2083.3 (10)A3, and Z = 4. The molecules lie on centers of inversion-and form two crystallographically independent pairs. The two molecules are essentially identical, each having virtual s 6 symmetry; the central Mo2N6 skeleton has virtual D3d symmetry, i.e., ethane-like geometry. The Mo-Mo distances are 2.211 (2)and 2.217(2)A. The following mean distances and angles were found: Mo-N, 1.98 A; N-C, 1.47A; Mo-Mo-N, 103.7O.The bonds to N are essentially coplanar. The structure has been refined to R I= 0.057 and R2 = 0.075 using 1897 reflections having I > 3 4 1 ) . The short Mo-Mo distance together with other structural and physicochemical properties establish the presence of a Mo-Mo triple bond. The orientations of the planar MoNC2 units lead to six proximal and six distal methyl groups with respect to the Mo-Mo bond. Variable temperature 'HNMR studies show that proximal and distal methyl groups are frozen out in solution below - 3 O O ; at higher temperatures they interconvert rapidly on the NMR time scale. A large chemical shift separation, ca. 2 ppm, between proximal and distal methyl proton resonances reflects the marked diamagnetic anisotropy of the Mo-Mo triple bond. Moz(NMeEt)6 and M02(NEt2)6 show similar 'H NMR spectra: Mo2(NMeEt)6 exists in an equilibrium of isomers favoring proximal methyl and distal ethyl groups. No single Raman active band can be assigned to a totally symmetric Mo-Mo stretching vibration due apparently to extensive coupling between this and other vibrational modes. The bonding and structural properties of Mo2(NR2)6 are compared with previously reported alkyls, Mo2R6, and with related chromium(II1) alkyls, dialkylamides, and alkoxides.
The element molybdenum has already been found to be a prolific former of homonuclear metal-metal bonds, especially quadruple ones.2 A distinct capacity to form triple bonds has also become evident, somewhat more recently. The first triple bond was found3 in Mo2(CHzSiMe&;, where the bond length was found to be 2.167 8, and the second4 in M02(NMe2)6 where the bond length is 2.214 (2) A. Most recently, the compound [ ( Q ~ - C ~ H S ) M O ( C Ohas ) ~ been ] ~ shown5 to have a (presumably) triple bond with a length of 2.448 (1) A, that is, about 0.25 8, longer than in the other two cases.6 In the meantime, in our laboratories several compounds containing tungsten-tungsten triple bonds have been prepared and ~ h a r a c t e r i z e d In . ~ this and the two following paper^^.^ we shall describe the preparation, structures, and certain other properties of these new molybdenum and tungsten compounds
in detail. Preliminary reports on Moz(NMe& have ap~eared.~.'
Results and Discussion Synthesis. Analysis of the crude brown product obtained from petroleum extraction of the reaction between MoCls and LiNR2 (5 equiv) indicated' molybdenum to nitrogen ratios of ca. 1:3.2 and showed valencies of molybdenum somewhat greater than 3. The monomeric compounds Mo(NR& were then obtained by sublimation at 60-90° (Mo(NMez)4) and 80-1 10' (Mo(NEt&) at cmHg. Analysis of the residue after sublimation gave molybdenum to nitrogen ratios of ca. 1:2.6 and suggested valencies of molybdenum less than 3. This situation was reminiscent of the behavior of many of the dialkylamides of titanium(", vanadium(III), and chromi-
Chisholm, Cotton, Frenz, Reichert, Shiue, Stults
/ Molybdenum-Molybdenum Triple Bond
