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Pentamethylcyclopentadienyl-rhodium and -iridium complexes. Part 27. Catalysed oxygenation reactions; tetrahydrofuran to γ-butyrolactone
JoountaZof Molecrclar Catalys.& 10 (1981) 203 - 211 @ Ekesier Sequoia S.A., Iausenne - Printed in the Netheriands
203
R3NT-LRHODIUM AND -ERlIDIUM COMPLEXES. PART 27*. CATALYSED OXYGENATION REACHONS; TETRAHYDROFURAN TO r-BUTYROEACIYINE
PENFAMETHYLCXCLOPE
KOICHE HERA& ANDREW Depement
of Chemistry,
NUTTON
and PETER M. MAl???LIS*f
!Fke Lhiuersity.
Skeffield
S3 7HF
(Gt_ Britain)
(Received June 3.1980)
The tri-N-hydroxocomplex [Rh2(C,MeS),(OH),] Cl- 4Ii,O (1) catiyses the oxygenation of tetrahydrofuran to y-butyrolactone in the presence of small amounts of water. Peroxides cause a similar reaction. Complex (1) also catalyses the c ygenation of Ph3P to Ph,PO; however, (EtO),P is not oxidised but is sted into [(RhCs~~eS){(EtQ),P),]*‘. Cornprep (1) is itself oxidised by Lrd2 to give initially a pammagnetic species with an ESR spectrum similar to that of I&(H)-0, complexes, which is tentatively identified as IRb”(C5Me5)(02)X], and finally to acetic acid, acetate, and COz. Solids containing acetate can be isolated Tom this reaction that are similar to materials obtained by the air-oxidation of (1) in isopropanol and base. The mechanisms of these oxidation reactions are discud.
Oxidation of triphenyiphosphine We have previously reported on the use of peukethylcyclopenta-
dienyl-rhodium and -iridium complexes as hydrogenation catalysts 121. During some investigations into the use of the rhodium(III) tri-.Khydroxy complex (1) [3] as a hydrogenation catalyst [4] we observed that at the end of a reaction the c2talyst vms able to convert triphenylphosphine to the oxide. We then found that (1) itself was also an efkient catiyst for this
*For Eart 26, se ref. 1. **Author to whom correspondence
should he addressed.
20-I
oxygenation reaction under a variety of conditions in different solvents. This reaction was, however, rather limited in scope; (EtO),P was not oxidised but formed the salt [Rh(CsMe,) {P(OEt)s }s] ‘* in 85% yield. No complex could be isolated from the reaction with Ph,P. It also proved impossible to co-oxygenate olefins with PPhs in the manner that Read and others have shown for Rh(PPh,),Cl-catalysed oxygenation reactions [ 51. However, w-hen tetrahydrofuran (THF) was used as solvent for the triphenylphosphine oxygenations, we observed a reaction with the solvent as indicated by the appearance of a band at 1780 cm‘-’ in the IR spectrum. This was shown to be due to the formation of 7-butyrolactone by comparison with an authentic sample. Oxidation of teimhydrofrcmn to y-butyrolactone The oxidation of THF also proceeded in the absence of FFh,. Since commercial THF contains traces of y-butyrolactone, in addition to peroxides, a sample of carefully purified THF was prepared. The catalyst (1) was only partly soluble in the THF and no reaction took place on stirring the suspension in air (20 “C/18 h). However, on addition of 1% of water a slow reaction occurred and turnover numbers of up to 34” (over 3 d were obtained. Addition of more water (10%) stopped the reaction entirely. Blank experiments showed negligible formation of butyrolactone in the absence of catalyst; further, [Rh(PPh,),Cl] was inactive both in the presence and absence of water. Although (1 j was p&y insoluble in the reaction medium, it slowly dissolved to give a deep red solution. Attempts to isolate and characterise a complex from the reaction were frustrated by polymerisation of the butyrolactone. Both [(RhC+¶e,),Cl, ] and [ (IrCsMes)2Cl-I] reacted similarly to (1) but the related ruthenium complexes [(RuCsMe,)&ll] and [RuCsMe,), (OH),] PFs were much less active under the same conditions (Table 1). The conversion of THF to butyrolactone by (1) was also induced by peroxides. Thus, addition of 0.1 mmol cf t-BuOOH to a solution of (1) (0.04 mmol) in THF (2.5 ml) gave 0.63 mmol butyrolactone after 18 h at 20 “C. In the absence of (1) only traces were ob’r;ained. [ Rh(PPh,),ClJ and [ (RhCSMeS)&lI] also catalysed the reaction in the presence of t-BuOOH (0.2 mmol), but yields of butyrolactone were much poorer (0.2 and 0.15 mmol, respectively, after 84 h/20 “2). Although crystals of (1) were.unsuitable for an X-ray crystal structure determination, the tetraphenylborate salt, [(RhCSMe5)2(OH),] BFh,, derived fro-m (1) has been found to have the expected structure, with three p-hydroxy bridges linking the two rho&urns IS]. Further, the c!osely related complex [(LrCSMe&(OH),]OAc-14Hr0 [3] has recently been shown also to possess three p-OH bridges [7] . There is therefore no likelihood that com*Yields and turnover numbers not optimised since they also depend of the oxygen concentrations; the reactions were run in air under reflux condensers with stirring.
205
TABLE
1
Olrygenation
of tetiydrofuran8
Cafdystb
(1) (1) (1) (1) none (1) L(~C5Me5hCbl IUK5Me5i2Ql C(RuC6Me&Chl [(RuC6nie,),(GH),,1PF6 c=wh3)3(=ll CRMPPh3)&11
at 50 ‘C
Water (ml)
0.025 0.025 0.025 0.025 0.025 0.25 0.025 0.025 0.025 0.025 0 0.025
Time
(h)
18 44 72 72 72 18 72 72 72 72 72 72
-y-butyro[aelcne
(mmol)=
T/Ad
0.22 0.34 1.17 l-37 0.09 0 1.11 1.11 0.17 O.iN3 0.01 0
6 9 29 34 @ 28 28 4 2 0 0
‘Tetrahydrofwan (2-5 ml) was carefuily puriried and dried by distilling under nitrogen from molecular sieve. GC and IR spectroscopy shoved that no butyrolactone was present. b In each case 0.04 mmol of catalyst was used_ cAnalysis by GC (1 m Cubowzx/lSO “C); no other products detected. dTurnover number; number of moles of product per mole of cataIyst_
plex (1) has peroxidic of (1) were negative.
ligands. Starch-iodide
tests for peroxide
on solutions
Oxidation of complex (I) Complex (1) itself also reacted with air under ‘reducing conditions’, for example in isopropanol/triethylamine. It was possible to isolate a brown solid km such a reaction sequence, which was a mixture, by spectioscopy (e.g. several peaks in the IFI XMR). Elemental analysis indicated the presence of variable amounts of Et,NIICl and (probably) some silica, but they were consistent in showing a ratio of C5Me5 to Rh of significantly less than one. Another interesting feature of these solids was the presence of a strong broad band in the IR at 1550 cm-l _ This is consistent with the presence of a coordinated acetate ligand (see below). A very similar material could be obtained by the action of hydrogen peroxide on (1). Wh*n an aa_ueous solution of (1) was treated with 14 - 16 molar equivalents of 30% hydrogen peroxide the colour became dark and a paramagnetic solution was obtained (ESR parameters, <9> = 2.024 at 20 “C; g, = 2.071, g2 = 2.013, g3 = 1.988 at - 70 “C). When such a solution was aIlowed to stand for 11 d and then e-vaporated to dryness, it gave an oily solid that showed a skong broad band in in the ‘II the IR at 1550 cm-’ and two broad resonances (6 1.63,1.73)
206
NMR
spectrum.
The higher field resonance was probably
due to unreacted
(1).
Smaller amounts (6 equivalents) of hydrogen peroside gave a product indistinguishable from (1). When an aqueous solution of (1) was reacted with a very large excess (123 molar equivalents) of HZ02, the resultant almost black solution showed no ESR signal. Analysis OF the gas evolved during this reaction showed the presence of carbon dioxide (0.3 mmol from 1.6 mmol of (1)) and NMR and GC analysis of the solution showed the presence of acetic acid (- 1 mmol). When the solution was evaporated to dryness, it gave a dark solid; when hydrogen was bubbled through an aqueous solution of this solid, rhodium metal precipitated and much acetic acid was produced. Although these results are complicated and at best only semiquantitative, the following points are &ar. (i) Complex (1) reacts with H202 (TO - 20 molar equivalents) to give a pammagnetic species, together with complexed acetate (IR and NMR). The ESR parameters for the pammagnetic species (which appears, horn the skength of the signal, to be present in large amount) are very similar to those found for Rh(II)-O2 complexes [ 8] , which all show similar (g), g,, g,. and g, and appear to be essentially ligand independent. These ESR specWa are quite different from those of Rh(II) or Rh(IV) species, and could well be due to species such as [C5hle,Rh”(02)X] (see below). (ii) A diamagnetic solid can be obtained from this soIution, the spectroscopic properties of which are consistent with the presence of coordinated acetate. When larger amounts of peroxide are used, clear evidence is obtained for the formation of CO2 and acetic acid as well as of a rhodium acetate complex ?vhich gives metal and acetic acid on exposure to hydrogen_ (iii) Since the only source of carbon in these experiments is the CsMes ligand this indicates that oxidation of the ligand in (I) (to CO2 and acetic acid or acetate) Eas occumed. (iv) Since oxygenation of (1) can yield apparently similar materials, here too the hgand is at least partially oxidis4. We conclude that in (l), as in many other homogeneous catalysts that are active for oxidation and oxygenation reactions, there is a balance between oxidation of the ligand and of the substrate, in this case C&es and THF, respectively_ This is not unexpected since both are presum ably able to interact with the oxidising reactent. We also note that the chloride dimer ~(RhC,Me,),Cl,j is much more resistant and is not appreciably oxidised by peroxide_
Discussion There are two mechanisms for oxygenation reactions which are relevant to the systems discussed here and that should be considered. One is a variant of the Haber-Weiss mechanism which applies to the decomposition of peroxides by mekzls in higher oxidation states:
207
W%~~M
fl
=
_mm(OH)X
f Hz02
+
mm(OHa)(OsH)X
m”‘(OH)X
f HO*.
+
m”(H,O)X
2mm(OH)X
(1) +
mLL(OHs)X f HOs-
+ 0s
(2) (3)
where m = C&e&h and X = OH, Cl or acetate. The dissociation of dinuclear (1) into active mononuclear species (eqn. The (1)) with vacant sites is suggested by analogy to other reactions ]2,9] rhodium(U) species formed in reactions (2) or (3) could then react with oxygen (that possibly formed in step (3)) to give [C,Me&h”(X)(O,)] the characteristics of which are consistent with the ESR spectrum of the paramagnetic species deribed above. Such species could then transfer their coordinated O2 intramolecularly to attack the &Me5 ligand in a series of steps such as:
c+5d1~o*11
-
-
4O
Rh’r’(Ei!,Xb!!Ol,
+
Rhm(OH),X(H,O),
Kzc0+4
+ MeCO,H
+
co2
+
Presumably intramolecular attack on the ring occurs very much more readily than intermolecular attack at another unsaturated carbon and hence olefins (1-hexene, cyclohexene) are not significantly oxidised under these conditions. It also appears that, once the fti step has occurred, subs=quent o_xidation steps of this organic product are very much faster. This accounts for the presence of some (I) even at higher peroxide ratios. The peroxidation and oxygenation reactions may be linked by a path (4): [m,(OH)x]
Cl + O2 + 2m(02H)(X)
(4)
Alternatively, as the oxidative degradation is particularly efficient under ‘reducing conditions’ this suggests the potibiliw of a path where the starting Rh(ILI) complex is fmt reduced to Rh(II) which then picks up 0,. The second tVpe of oxidation mechanism relevant to the experiments discussed here is the one which has recently been she-wn by Sen and Halpem [lo] to accounf, for the [Pt(R3P)202]catalysed oxygenation of R3P to R3PO: Pt(PR&02
f PR3 +
Pt(PR&02
f R’OH
Pt(PR&02 f 2PRa
Pt(R3P)-L’+ t 2RaP f HO, Pt(PR&
+ 02
f R1O-
+
Ft(R,P)4z’
+ HO;
i- RIO-
*
Pt(RsP)40
c 2RsPO
+
Pt(PR&Ox
f 2R3P
+ R’OH
Such a mechanism could also easily account for the catalysed oxygenation of triphenylphosphine that we observe by the steps in eqns. (5) - (7): m”‘(O,H)X
02H OH
+ Ph,P
+ P&P + [m(PPh,)(X)]’
*
[m”‘(PPh,)X]’
c O&-i-
+
Ph3P0 f OH
(6)
+
m(OH)X
(7)
+ PPh3
(5)
Support for this proposal comes from our observations [ll] that, in with phosphines and phosphites, the reacticns of [Rh(C,Me,)(solvent)3]2’ small ligands (L = MetPhP or (EtO)sP) give tris-ligand complexes [Rh(C,Me,)L,] ** for solvent = acetone. However, no products could be isolated from larger phosphines. The more inert this-acetonitrile complex 2* reacted with triphenylphosphine to give the isolable iRh(C,Me,)(MeCN)3] [ Rh(C&Ie,)(MeCN),(PPh,)l ** ; with excess PPh, spectroscopic evidence was obtained for the formation of [Rh(C,Me,)(MeCN)(PPh,)2] **, but no trisPPh, complex could be detected. This suggests that the formation of such trk-ligand complexes is dominated by steric effects. We therefon- propose that when a small ligand (eg. (EtO),P) is reacted with (1) all available coordination sites are rapidly blocked and the siower formation of a peroxy species (eqn. (4)) cannot occur. With the bigger Ph,P ligand, where the coordinatively saturated t&-complex i ; not stable, a peroxy species can form at one of the vacant sites, and even t le equilibrium described by eqn. (7) may lie over to ‘the right. Although the oxidation of T3F to its hydroperoxide and the conversitin of this (for example in the stoichiometric reaction, with Fe’*) to ybutyrolzctone is well-known [ 121, a catalysed oxygenation of the type we have observed does not appear to have been described_ -0 paths may be considered for this reaction_ One is based on a HaberWeiss mechanism, in which the role cf the metal is chiefly to decompose the THF-hydroperoxide which is formed directly : RH+Y-
+
R- +02
-, R02-
R02-
+RH+
R-+YH
RO,H
+ R-
R- -
However, this scheme does not explain the need for small quantities of water, and we prefer the alternative mm(02H)X
[plll(x)(~‘)]+
+ THF
=
[mm(TIiF)(X)]+
+ THF + I@+- -
+ HO?
(8)
OOiH
(9)
0.
+&dF), +HX
209
Reoxidation can then oatus: m’(TIIF),
of the b-ansient solvated Rh(I) f 5 Oz f Hz0
+
mn’(OH)z
species, [RhC&e5(TIIF),],
+ nTIIF
The small amounts of water may be need& to promote the ionisation in eqn. (S), and attack of the hydroperoxy anion may occur preferentially at the coordtiakd THF; but this latter process would be inhibited by the presence of larger amounts of water. The abilify of [IMz(CSMeS)zCIC] (M = Rh or Ir) to catalyse this reaction to the same degree suggests that a similar path is available_ Finally we drawv attention to the fact that the peroxide reactions of (1) are amongst the fern undergone by &Me,-Rh or -Ir complexes in which there is evidence for loss of the &Me5 ligand [2] _
Experimental Complex (1) was prepared by the literature method [3] _ TIIF was refluxed Oirer sodium and distilled under nitrogen; it was then fractionaM under nitrogen from molecular sieve. The resultant THF showed no v(C0) in the IR and no trace of butyrolactone in the GC. Triphenylphosphine was crystallised from petroleum ether and showed no triphenylphosphine oxide by IR or 31P NMK (Jeol PET-IOO). Oxidation of friphenyiphosphine A solution of (1) (25 mg, 0.04 mmol) in THF (5 ml) containing triphenylphosphine (0.46 g, 1.7 mmol) was stirred in air (I9 h/20 “C). Analysis ofthesolutionby 31P NMR spectroscopy showed the presence of 79% PhaPO, corresponding to a turnover number of 28 with respect to (1) as catalyst. Similar reactions occurred in isopropanol, isopropanol + triethylamine, cr w2ter + benzene; because of solubiliw problems water by itself was not suitable.
A solution of (1) (25 mg, 0.04 mmol) in THF (5 ml) was treated with triethyl phosphite (0.98 g, 5.9 mmol) and stirred in air (19 h/20 “C). Analysis of the solution by 31P NMR spectroscopy showed the presence of cornplexed (EtO)3P, as well as free (Et0)3P (3.9 mmol), (EtO)3P0 (1.1 mmol), and (EtO),P(O)H (0:9 mmol). A sample of 5.9 mm01 of (EtO)3P tre2ted in the same way without catalyst showed the presence of (EtO)3P (3.2 mmol), (EtO)sPO (2.6 mmol) and (EtO)zP(O)S (0.2 mmol). Thus the effect of complex (1) appears to be to repress (somewhat) the normal autoxidation but to accelerate a hydrolysis to (EtO),P(O)H, presumably caused by the waters of crystallisation of (1). The complex obsenwd in ‘Je jlI? NMR spectrum was also formed (in 85% yield) on ad&g RPF, to an aqueous solution of (1) and (EtO)3P and was identified as the known [Rh(C,Mes){(EtO),P),] (PF6)z [lI] _
210
Oxidution of tetruhydrqkwn Complex (1) (0.025 g, 0.04 mmol) was partly dissolved in putied THF (2.5 ml) containing water (0.025 ml) and stirred in air under a short reflex condenser at 50 “C. The complex dissolved after a few minutes to give a red solution. GC analysis (Carbowax, i50 “C) showed the _Dple to contain 0.22 mmol y-butyrolactone after 18 h =d 0.34 mm01 after 44 h. No other products were detected. Details of further experiments are given in Table 1. Oxidation of complex (1) (0 By H, O2 It was observed that the colour of an aqueous solution of (1) deepened on addition of H,O,; this was found to be due to the appearance of a band at 57C nm in the visible spectrum. Attempts to use this to characterise a product were, however, unsuccessful since the intensity of this new band increased continuously with adaition of more peroxide without any maximum being reached; clearly this arose from total decomposition_ Addition of hydrogen peroxide (0.05 g of 30%, 0.5 mmol) in water (11 ml) to a solution cf (1) (0.025 g, 0.04 mmol) in water (7 ml) gave ti dark solution_ This was allowed to stznd (11 d/ 20 “C) and the solvent was then removed in oacuo to leave a brown solid (5.7 mg), the IR of which showed a broad strong band at 1550 cm-’ and the ‘H NMR (in D20) of which showed a broad band containing peaks at 6 1.73 and 1.63, the latter probably due to some unreacted (1). The solid was analysed and contained C, 31.3; H, 4.7 and Cl, 4.6%. Calculated for complex (1); C, 37.8; H, 6.5; and Cl, 5.6%. In a further experiment 1.01 g (1.6 mmol) of (1) in 150 ml water was reacted with 25 ml of 30% hydrogen peroxide under a stream of nitrogen. The nitrogen was bubbled through standard po’kssium hydroxide solution to trap any CO2 evolved; about 0.8 mmol of CO2 was produced. In addition, GC, ‘H and “C NMR spectroscopy showed the presence of about 1 mmol of acetic acid in the solution_ The solution was taken to dryness, the solid redissolved in water and thi - solution, was exposed to hydrogen (20 min/ 20 “C). Rhodium metal was precipitated ar-d the solution was shown by GC to contain acetic acid. The solid analysed for C, 24.8; H, 3.7 and Cl, 10.1%. (ii) By air Similar materials with similar spectroscopic properties were obtained by the following procedure_ A slow stream of air was bubbled for 20 min through a solution (at 80 “C) of complex (1) (0.65 g) in isopropanol(200 ml) containing triethylamine (911 g). The solution was then cooled and filtered to leave a dark solid, which aftir reprecipitation from acetone and diethyl ether weighed 94 mg and analysed for: C, 32.1; H, 4.7; N, 0.5; Cl, 5.1; Rt, 31.7 and 0,18_05L The fdtrate was taken to dryness, dissolved in acetone and ether added; the residue was discarded and the soluble &tion was evaForated down to leave a solid that was agti taken up in isopropanol containing tiethylamine =d exposed to a slow stream of hydrogen (20 min/ 20 “C). Some black solid (metal) precipitated and was filtered off to :eave
211
a solution tiich was evaporated to dryness and fnctionally precipitated from acetone and ether to give 0.68 g of a brown solid analysing for C, 38.3; H, 5.0; N, 0.7; Cl, 12.7; Rh, 28.3 and 0,15.1%.
Acknowkdgements We thank the Science Research Council, Johnson Matthey Co. Ltd., and Ube Chemical Industries for supporting this work, and Professor N. M. Athertan For measuring ESR spectra. We also thank Johnson Matthey Re search Centre for Rh and 0 analyses and for the loan of some rhodium salts.
Referents 1 Part 26. P. &pin&, P. M Bailey, R. Downey and P. M. Maitlis. J. Chem. Sot. Dalton Trans.. (19&n) 1046. 2 D. S. Gill, C. White and P. 51. Maitlis, J. Chem. 5x. Daiton Trarrr.. (1978) 617; see also P. M. Maitlis. Act. Chem. Res.. I1 (1978) 301. 3 J. W. Kang and P. M. MeitLs, L OrgznometaI. Chem.. 30 (1971) 393. 4 J_ E. Han&n, V. C. Gibson, K. Hirai, P. Piraino and P. M. Maitlis, unpublished results. 5 G. Read and P. J. C. Walker, J_ Chem. Sot. Daltorr T’mrrs., (1977) 883; G. Read, J. Mol. Catal., 4 (1975) 83; F. Igenheim and H. Mimoun. J. Chem. Sot. Chem. Commun.. (1978) 559; R. Tang, F. Mares; N. Neary and D. E. Smith, J_ Chem. Sot. Ch!m. Commun.. (1979) 27% 6 ,M. R. Churchill. unpublished results. 7 P. M. Bailey, A Nutton and P. XI. Maitlis, unpublished cults. 8 B. R. Jzrnq F. T. T. Ng and E. Ochiai. Canad. J_ Chem.. 50 (1972) 590; B. H. var. Vugt and W. Drenth, Rec. Z’kau. Claim.. 96 (1977) 225; B. B. Wayland and -4. R. Newman, d_ Am. Chem. Sot.. 101 (1979) 6473. 9 J. Cook, J. E. Hamlin, A. Nutton and P. M. Maitiis, J. Chem. Sot. Chem. Commun., 1980.144. 10 A Sen and J. Halpem, J. Am. Chem. Sot.. 99 (1977) 8337. 11 S. J. Thompson, C. White 2nd P. M. Mtitlis, J. Oganometal. Chem.. 136 (1977) 37. 12 G. Sosnovsky and D. J_ Rawlinson, in D. Swem (ed.), Organic Peroxides. Wiley-Laterscience, New York, lYi1, Vol. H, p. 179.
203
R3NT-LRHODIUM AND -ERlIDIUM COMPLEXES. PART 27*. CATALYSED OXYGENATION REACHONS; TETRAHYDROFURAN TO r-BUTYROEACIYINE
PENFAMETHYLCXCLOPE
KOICHE HERA& ANDREW Depement
of Chemistry,
NUTTON
and PETER M. MAl???LIS*f
!Fke Lhiuersity.
Skeffield
S3 7HF
(Gt_ Britain)
(Received June 3.1980)
The tri-N-hydroxocomplex [Rh2(C,MeS),(OH),] Cl- 4Ii,O (1) catiyses the oxygenation of tetrahydrofuran to y-butyrolactone in the presence of small amounts of water. Peroxides cause a similar reaction. Complex (1) also catalyses the c ygenation of Ph3P to Ph,PO; however, (EtO),P is not oxidised but is sted into [(RhCs~~eS){(EtQ),P),]*‘. Cornprep (1) is itself oxidised by Lrd2 to give initially a pammagnetic species with an ESR spectrum similar to that of I&(H)-0, complexes, which is tentatively identified as IRb”(C5Me5)(02)X], and finally to acetic acid, acetate, and COz. Solids containing acetate can be isolated Tom this reaction that are similar to materials obtained by the air-oxidation of (1) in isopropanol and base. The mechanisms of these oxidation reactions are discud.
Oxidation of triphenyiphosphine We have previously reported on the use of peukethylcyclopenta-
dienyl-rhodium and -iridium complexes as hydrogenation catalysts 121. During some investigations into the use of the rhodium(III) tri-.Khydroxy complex (1) [3] as a hydrogenation catalyst [4] we observed that at the end of a reaction the c2talyst vms able to convert triphenylphosphine to the oxide. We then found that (1) itself was also an efkient catiyst for this
*For Eart 26, se ref. 1. **Author to whom correspondence
should he addressed.
20-I
oxygenation reaction under a variety of conditions in different solvents. This reaction was, however, rather limited in scope; (EtO),P was not oxidised but formed the salt [Rh(CsMe,) {P(OEt)s }s] ‘* in 85% yield. No complex could be isolated from the reaction with Ph,P. It also proved impossible to co-oxygenate olefins with PPhs in the manner that Read and others have shown for Rh(PPh,),Cl-catalysed oxygenation reactions [ 51. However, w-hen tetrahydrofuran (THF) was used as solvent for the triphenylphosphine oxygenations, we observed a reaction with the solvent as indicated by the appearance of a band at 1780 cm‘-’ in the IR spectrum. This was shown to be due to the formation of 7-butyrolactone by comparison with an authentic sample. Oxidation of teimhydrofrcmn to y-butyrolactone The oxidation of THF also proceeded in the absence of FFh,. Since commercial THF contains traces of y-butyrolactone, in addition to peroxides, a sample of carefully purified THF was prepared. The catalyst (1) was only partly soluble in the THF and no reaction took place on stirring the suspension in air (20 “C/18 h). However, on addition of 1% of water a slow reaction occurred and turnover numbers of up to 34” (over 3 d were obtained. Addition of more water (10%) stopped the reaction entirely. Blank experiments showed negligible formation of butyrolactone in the absence of catalyst; further, [Rh(PPh,),Cl] was inactive both in the presence and absence of water. Although (1 j was p&y insoluble in the reaction medium, it slowly dissolved to give a deep red solution. Attempts to isolate and characterise a complex from the reaction were frustrated by polymerisation of the butyrolactone. Both [(RhC+¶e,),Cl, ] and [ (IrCsMes)2Cl-I] reacted similarly to (1) but the related ruthenium complexes [(RuCsMe,)&ll] and [RuCsMe,), (OH),] PFs were much less active under the same conditions (Table 1). The conversion of THF to butyrolactone by (1) was also induced by peroxides. Thus, addition of 0.1 mmol cf t-BuOOH to a solution of (1) (0.04 mmol) in THF (2.5 ml) gave 0.63 mmol butyrolactone after 18 h at 20 “C. In the absence of (1) only traces were ob’r;ained. [ Rh(PPh,),ClJ and [ (RhCSMeS)&lI] also catalysed the reaction in the presence of t-BuOOH (0.2 mmol), but yields of butyrolactone were much poorer (0.2 and 0.15 mmol, respectively, after 84 h/20 “2). Although crystals of (1) were.unsuitable for an X-ray crystal structure determination, the tetraphenylborate salt, [(RhCSMe5)2(OH),] BFh,, derived fro-m (1) has been found to have the expected structure, with three p-hydroxy bridges linking the two rho&urns IS]. Further, the c!osely related complex [(LrCSMe&(OH),]OAc-14Hr0 [3] has recently been shown also to possess three p-OH bridges [7] . There is therefore no likelihood that com*Yields and turnover numbers not optimised since they also depend of the oxygen concentrations; the reactions were run in air under reflux condensers with stirring.
205
TABLE
1
Olrygenation
of tetiydrofuran8
Cafdystb
(1) (1) (1) (1) none (1) L(~C5Me5hCbl IUK5Me5i2Ql C(RuC6Me&Chl [(RuC6nie,),(GH),,1PF6 c=wh3)3(=ll CRMPPh3)&11
at 50 ‘C
Water (ml)
0.025 0.025 0.025 0.025 0.025 0.25 0.025 0.025 0.025 0.025 0 0.025
Time
(h)
18 44 72 72 72 18 72 72 72 72 72 72
-y-butyro[aelcne
(mmol)=
T/Ad
0.22 0.34 1.17 l-37 0.09 0 1.11 1.11 0.17 O.iN3 0.01 0
6 9 29 34 @ 28 28 4 2 0 0
‘Tetrahydrofwan (2-5 ml) was carefuily puriried and dried by distilling under nitrogen from molecular sieve. GC and IR spectroscopy shoved that no butyrolactone was present. b In each case 0.04 mmol of catalyst was used_ cAnalysis by GC (1 m Cubowzx/lSO “C); no other products detected. dTurnover number; number of moles of product per mole of cataIyst_
plex (1) has peroxidic of (1) were negative.
ligands. Starch-iodide
tests for peroxide
on solutions
Oxidation of complex (I) Complex (1) itself also reacted with air under ‘reducing conditions’, for example in isopropanol/triethylamine. It was possible to isolate a brown solid km such a reaction sequence, which was a mixture, by spectioscopy (e.g. several peaks in the IFI XMR). Elemental analysis indicated the presence of variable amounts of Et,NIICl and (probably) some silica, but they were consistent in showing a ratio of C5Me5 to Rh of significantly less than one. Another interesting feature of these solids was the presence of a strong broad band in the IR at 1550 cm-l _ This is consistent with the presence of a coordinated acetate ligand (see below). A very similar material could be obtained by the action of hydrogen peroxide on (1). Wh*n an aa_ueous solution of (1) was treated with 14 - 16 molar equivalents of 30% hydrogen peroxide the colour became dark and a paramagnetic solution was obtained (ESR parameters, <9> = 2.024 at 20 “C; g, = 2.071, g2 = 2.013, g3 = 1.988 at - 70 “C). When such a solution was aIlowed to stand for 11 d and then e-vaporated to dryness, it gave an oily solid that showed a skong broad band in in the ‘II the IR at 1550 cm-’ and two broad resonances (6 1.63,1.73)
206
NMR
spectrum.
The higher field resonance was probably
due to unreacted
(1).
Smaller amounts (6 equivalents) of hydrogen peroside gave a product indistinguishable from (1). When an aqueous solution of (1) was reacted with a very large excess (123 molar equivalents) of HZ02, the resultant almost black solution showed no ESR signal. Analysis OF the gas evolved during this reaction showed the presence of carbon dioxide (0.3 mmol from 1.6 mmol of (1)) and NMR and GC analysis of the solution showed the presence of acetic acid (- 1 mmol). When the solution was evaporated to dryness, it gave a dark solid; when hydrogen was bubbled through an aqueous solution of this solid, rhodium metal precipitated and much acetic acid was produced. Although these results are complicated and at best only semiquantitative, the following points are &ar. (i) Complex (1) reacts with H202 (TO - 20 molar equivalents) to give a pammagnetic species, together with complexed acetate (IR and NMR). The ESR parameters for the pammagnetic species (which appears, horn the skength of the signal, to be present in large amount) are very similar to those found for Rh(II)-O2 complexes [ 8] , which all show similar (g), g,, g,. and g, and appear to be essentially ligand independent. These ESR specWa are quite different from those of Rh(II) or Rh(IV) species, and could well be due to species such as [C5hle,Rh”(02)X] (see below). (ii) A diamagnetic solid can be obtained from this soIution, the spectroscopic properties of which are consistent with the presence of coordinated acetate. When larger amounts of peroxide are used, clear evidence is obtained for the formation of CO2 and acetic acid as well as of a rhodium acetate complex ?vhich gives metal and acetic acid on exposure to hydrogen_ (iii) Since the only source of carbon in these experiments is the CsMes ligand this indicates that oxidation of the ligand in (I) (to CO2 and acetic acid or acetate) Eas occumed. (iv) Since oxygenation of (1) can yield apparently similar materials, here too the hgand is at least partially oxidis4. We conclude that in (l), as in many other homogeneous catalysts that are active for oxidation and oxygenation reactions, there is a balance between oxidation of the ligand and of the substrate, in this case C&es and THF, respectively_ This is not unexpected since both are presum ably able to interact with the oxidising reactent. We also note that the chloride dimer ~(RhC,Me,),Cl,j is much more resistant and is not appreciably oxidised by peroxide_
Discussion There are two mechanisms for oxygenation reactions which are relevant to the systems discussed here and that should be considered. One is a variant of the Haber-Weiss mechanism which applies to the decomposition of peroxides by mekzls in higher oxidation states:
207
W%~~M
fl
=
_mm(OH)X
f Hz02
+
mm(OHa)(OsH)X
m”‘(OH)X
f HO*.
+
m”(H,O)X
2mm(OH)X
(1) +
mLL(OHs)X f HOs-
+ 0s
(2) (3)
where m = C&e&h and X = OH, Cl or acetate. The dissociation of dinuclear (1) into active mononuclear species (eqn. The (1)) with vacant sites is suggested by analogy to other reactions ]2,9] rhodium(U) species formed in reactions (2) or (3) could then react with oxygen (that possibly formed in step (3)) to give [C,Me&h”(X)(O,)] the characteristics of which are consistent with the ESR spectrum of the paramagnetic species deribed above. Such species could then transfer their coordinated O2 intramolecularly to attack the &Me5 ligand in a series of steps such as:
c+5d1~o*11
-
-
4O
Rh’r’(Ei!,Xb!!Ol,
+
Rhm(OH),X(H,O),
Kzc0+4
+ MeCO,H
+
co2
+
Presumably intramolecular attack on the ring occurs very much more readily than intermolecular attack at another unsaturated carbon and hence olefins (1-hexene, cyclohexene) are not significantly oxidised under these conditions. It also appears that, once the fti step has occurred, subs=quent o_xidation steps of this organic product are very much faster. This accounts for the presence of some (I) even at higher peroxide ratios. The peroxidation and oxygenation reactions may be linked by a path (4): [m,(OH)x]
Cl + O2 + 2m(02H)(X)
(4)
Alternatively, as the oxidative degradation is particularly efficient under ‘reducing conditions’ this suggests the potibiliw of a path where the starting Rh(ILI) complex is fmt reduced to Rh(II) which then picks up 0,. The second tVpe of oxidation mechanism relevant to the experiments discussed here is the one which has recently been she-wn by Sen and Halpem [lo] to accounf, for the [Pt(R3P)202]catalysed oxygenation of R3P to R3PO: Pt(PR&02
f PR3 +
Pt(PR&02
f R’OH
Pt(PR&02 f 2PRa
Pt(R3P)-L’+ t 2RaP f HO, Pt(PR&
+ 02
f R1O-
+
Ft(R,P)4z’
+ HO;
i- RIO-
*
Pt(RsP)40
c 2RsPO
+
Pt(PR&Ox
f 2R3P
+ R’OH
Such a mechanism could also easily account for the catalysed oxygenation of triphenylphosphine that we observe by the steps in eqns. (5) - (7): m”‘(O,H)X
02H OH
+ Ph,P
+ P&P + [m(PPh,)(X)]’
*
[m”‘(PPh,)X]’
c O&-i-
+
Ph3P0 f OH
(6)
+
m(OH)X
(7)
+ PPh3
(5)
Support for this proposal comes from our observations [ll] that, in with phosphines and phosphites, the reacticns of [Rh(C,Me,)(solvent)3]2’ small ligands (L = MetPhP or (EtO)sP) give tris-ligand complexes [Rh(C,Me,)L,] ** for solvent = acetone. However, no products could be isolated from larger phosphines. The more inert this-acetonitrile complex 2* reacted with triphenylphosphine to give the isolable iRh(C,Me,)(MeCN)3] [ Rh(C&Ie,)(MeCN),(PPh,)l ** ; with excess PPh, spectroscopic evidence was obtained for the formation of [Rh(C,Me,)(MeCN)(PPh,)2] **, but no trisPPh, complex could be detected. This suggests that the formation of such trk-ligand complexes is dominated by steric effects. We therefon- propose that when a small ligand (eg. (EtO),P) is reacted with (1) all available coordination sites are rapidly blocked and the siower formation of a peroxy species (eqn. (4)) cannot occur. With the bigger Ph,P ligand, where the coordinatively saturated t&-complex i ; not stable, a peroxy species can form at one of the vacant sites, and even t le equilibrium described by eqn. (7) may lie over to ‘the right. Although the oxidation of T3F to its hydroperoxide and the conversitin of this (for example in the stoichiometric reaction, with Fe’*) to ybutyrolzctone is well-known [ 121, a catalysed oxygenation of the type we have observed does not appear to have been described_ -0 paths may be considered for this reaction_ One is based on a HaberWeiss mechanism, in which the role cf the metal is chiefly to decompose the THF-hydroperoxide which is formed directly : RH+Y-
+
R- +02
-, R02-
R02-
+RH+
R-+YH
RO,H
+ R-
R- -
However, this scheme does not explain the need for small quantities of water, and we prefer the alternative mm(02H)X
[plll(x)(~‘)]+
+ THF
=
[mm(TIiF)(X)]+
+ THF + I@+- -
+ HO?
(8)
OOiH
(9)
0.
+&dF), +HX
209
Reoxidation can then oatus: m’(TIIF),
of the b-ansient solvated Rh(I) f 5 Oz f Hz0
+
mn’(OH)z
species, [RhC&e5(TIIF),],
+ nTIIF
The small amounts of water may be need& to promote the ionisation in eqn. (S), and attack of the hydroperoxy anion may occur preferentially at the coordtiakd THF; but this latter process would be inhibited by the presence of larger amounts of water. The abilify of [IMz(CSMeS)zCIC] (M = Rh or Ir) to catalyse this reaction to the same degree suggests that a similar path is available_ Finally we drawv attention to the fact that the peroxide reactions of (1) are amongst the fern undergone by &Me,-Rh or -Ir complexes in which there is evidence for loss of the &Me5 ligand [2] _
Experimental Complex (1) was prepared by the literature method [3] _ TIIF was refluxed Oirer sodium and distilled under nitrogen; it was then fractionaM under nitrogen from molecular sieve. The resultant THF showed no v(C0) in the IR and no trace of butyrolactone in the GC. Triphenylphosphine was crystallised from petroleum ether and showed no triphenylphosphine oxide by IR or 31P NMK (Jeol PET-IOO). Oxidation of friphenyiphosphine A solution of (1) (25 mg, 0.04 mmol) in THF (5 ml) containing triphenylphosphine (0.46 g, 1.7 mmol) was stirred in air (I9 h/20 “C). Analysis ofthesolutionby 31P NMR spectroscopy showed the presence of 79% PhaPO, corresponding to a turnover number of 28 with respect to (1) as catalyst. Similar reactions occurred in isopropanol, isopropanol + triethylamine, cr w2ter + benzene; because of solubiliw problems water by itself was not suitable.
A solution of (1) (25 mg, 0.04 mmol) in THF (5 ml) was treated with triethyl phosphite (0.98 g, 5.9 mmol) and stirred in air (19 h/20 “C). Analysis of the solution by 31P NMR spectroscopy showed the presence of cornplexed (EtO)3P, as well as free (Et0)3P (3.9 mmol), (EtO)3P0 (1.1 mmol), and (EtO),P(O)H (0:9 mmol). A sample of 5.9 mm01 of (EtO)3P tre2ted in the same way without catalyst showed the presence of (EtO)3P (3.2 mmol), (EtO)sPO (2.6 mmol) and (EtO)zP(O)S (0.2 mmol). Thus the effect of complex (1) appears to be to repress (somewhat) the normal autoxidation but to accelerate a hydrolysis to (EtO),P(O)H, presumably caused by the waters of crystallisation of (1). The complex obsenwd in ‘Je jlI? NMR spectrum was also formed (in 85% yield) on ad&g RPF, to an aqueous solution of (1) and (EtO)3P and was identified as the known [Rh(C,Mes){(EtO),P),] (PF6)z [lI] _
210
Oxidution of tetruhydrqkwn Complex (1) (0.025 g, 0.04 mmol) was partly dissolved in putied THF (2.5 ml) containing water (0.025 ml) and stirred in air under a short reflex condenser at 50 “C. The complex dissolved after a few minutes to give a red solution. GC analysis (Carbowax, i50 “C) showed the _Dple to contain 0.22 mmol y-butyrolactone after 18 h =d 0.34 mm01 after 44 h. No other products were detected. Details of further experiments are given in Table 1. Oxidation of complex (1) (0 By H, O2 It was observed that the colour of an aqueous solution of (1) deepened on addition of H,O,; this was found to be due to the appearance of a band at 57C nm in the visible spectrum. Attempts to use this to characterise a product were, however, unsuccessful since the intensity of this new band increased continuously with adaition of more peroxide without any maximum being reached; clearly this arose from total decomposition_ Addition of hydrogen peroxide (0.05 g of 30%, 0.5 mmol) in water (11 ml) to a solution cf (1) (0.025 g, 0.04 mmol) in water (7 ml) gave ti dark solution_ This was allowed to stznd (11 d/ 20 “C) and the solvent was then removed in oacuo to leave a brown solid (5.7 mg), the IR of which showed a broad strong band at 1550 cm-’ and the ‘H NMR (in D20) of which showed a broad band containing peaks at 6 1.73 and 1.63, the latter probably due to some unreacted (1). The solid was analysed and contained C, 31.3; H, 4.7 and Cl, 4.6%. Calculated for complex (1); C, 37.8; H, 6.5; and Cl, 5.6%. In a further experiment 1.01 g (1.6 mmol) of (1) in 150 ml water was reacted with 25 ml of 30% hydrogen peroxide under a stream of nitrogen. The nitrogen was bubbled through standard po’kssium hydroxide solution to trap any CO2 evolved; about 0.8 mmol of CO2 was produced. In addition, GC, ‘H and “C NMR spectroscopy showed the presence of about 1 mmol of acetic acid in the solution_ The solution was taken to dryness, the solid redissolved in water and thi - solution, was exposed to hydrogen (20 min/ 20 “C). Rhodium metal was precipitated ar-d the solution was shown by GC to contain acetic acid. The solid analysed for C, 24.8; H, 3.7 and Cl, 10.1%. (ii) By air Similar materials with similar spectroscopic properties were obtained by the following procedure_ A slow stream of air was bubbled for 20 min through a solution (at 80 “C) of complex (1) (0.65 g) in isopropanol(200 ml) containing triethylamine (911 g). The solution was then cooled and filtered to leave a dark solid, which aftir reprecipitation from acetone and diethyl ether weighed 94 mg and analysed for: C, 32.1; H, 4.7; N, 0.5; Cl, 5.1; Rt, 31.7 and 0,18_05L The fdtrate was taken to dryness, dissolved in acetone and ether added; the residue was discarded and the soluble &tion was evaForated down to leave a solid that was agti taken up in isopropanol containing tiethylamine =d exposed to a slow stream of hydrogen (20 min/ 20 “C). Some black solid (metal) precipitated and was filtered off to :eave
211
a solution tiich was evaporated to dryness and fnctionally precipitated from acetone and ether to give 0.68 g of a brown solid analysing for C, 38.3; H, 5.0; N, 0.7; Cl, 12.7; Rh, 28.3 and 0,15.1%.
Acknowkdgements We thank the Science Research Council, Johnson Matthey Co. Ltd., and Ube Chemical Industries for supporting this work, and Professor N. M. Athertan For measuring ESR spectra. We also thank Johnson Matthey Re search Centre for Rh and 0 analyses and for the loan of some rhodium salts.
Referents 1 Part 26. P. &pin&, P. M Bailey, R. Downey and P. M. Maitlis. J. Chem. Sot. Dalton Trans.. (19&n) 1046. 2 D. S. Gill, C. White and P. 51. Maitlis, J. Chem. 5x. Daiton Trarrr.. (1978) 617; see also P. M. Maitlis. Act. Chem. Res.. I1 (1978) 301. 3 J. W. Kang and P. M. MeitLs, L OrgznometaI. Chem.. 30 (1971) 393. 4 J_ E. Han&n, V. C. Gibson, K. Hirai, P. Piraino and P. M. Maitlis, unpublished results. 5 G. Read and P. J. C. Walker, J_ Chem. Sot. Daltorr T’mrrs., (1977) 883; G. Read, J. Mol. Catal., 4 (1975) 83; F. Igenheim and H. Mimoun. J. Chem. Sot. Chem. Commun.. (1978) 559; R. Tang, F. Mares; N. Neary and D. E. Smith, J_ Chem. Sot. Ch!m. Commun.. (1979) 27% 6 ,M. R. Churchill. unpublished results. 7 P. M. Bailey, A Nutton and P. XI. Maitlis, unpublished cults. 8 B. R. Jzrnq F. T. T. Ng and E. Ochiai. Canad. J_ Chem.. 50 (1972) 590; B. H. var. Vugt and W. Drenth, Rec. Z’kau. Claim.. 96 (1977) 225; B. B. Wayland and -4. R. Newman, d_ Am. Chem. Sot.. 101 (1979) 6473. 9 J. Cook, J. E. Hamlin, A. Nutton and P. M. Maitiis, J. Chem. Sot. Chem. Commun., 1980.144. 10 A Sen and J. Halpem, J. Am. Chem. Sot.. 99 (1977) 8337. 11 S. J. Thompson, C. White 2nd P. M. Mtitlis, J. Oganometal. Chem.. 136 (1977) 37. 12 G. Sosnovsky and D. J_ Rawlinson, in D. Swem (ed.), Organic Peroxides. Wiley-Laterscience, New York, lYi1, Vol. H, p. 179.