Organometallics 1983,2, 486-489
486
Studies of Molybdenum Compounds. 3. Vinyl Molybdate(V1): Synthesis and Detection in the Molybdothiol-Catalyzed Reduction of Acetylene Laura A. Hughes, Liu Nan Hui,' and Gerhard N. Schrauzer" Department of Chemistry, University of California at San Diego, Revelle College, La Jolla, California 92093 Received October 12, 1982
Vinyl molybdate(VI), CH2=CHMo03-, generated by alkaline hydrolysis of CH2=CHMo(0),Br(bpy) (bpy = 2,2'-bipyridyl), is a colorless anion undergoing Mo-C bond hydrolysis to molybdate and ethylene with a half-life of 3 h at 23 O C in aqueous solution. In aqueous NaOH, the rate of Mo-C bond hydrolysis exhibits a linear dependence on [OH-]. M d : bond hydrolysis is also accelerated in strongly acidic solutions; the half-life of vinyl molybdate at pH 1.3 (0.1 M HC1) is 1.3 h. The apparent Arrhenius energies of activation for the hydrolysis of the M d : bond of vinyl molybdate are 7 and 3 kcal/mol at pH 11and 1.3, respectively. Vinyl molybdate species are reactive intermediates in the molybdothiol-catalyzed reduction of acetylene to ethylene and were detected in quenched reaction solutions, employing a chromatographic separation technique. The previously derived mechanism of the molybdothiol-catalyzed reduction of acetylene is extended on the basis of these observations.
Introduction Previous work2-' has suggested that the reduction of acetylene to ethylene in molybdothiol and related model systems of nitrogenase occurs by way of a symmetrically side-on bonded, mononuclear organomolybdenum(V1) intermediate (A), from which ethylene is generated by way of Mc-C bond hydrolysis. Since the hydrolysis of the two M o C bonds in A is likely to occur sequentially rather than simultaneously, a vinylmolybdenum(V1) species B should be formed before ethylene is released.
A
B
In this paper we report the results of experiments which led us to detect vinyl- and other organomolybdenum species in quenched molybdothiol-acetylene reaction solutions. As intermediate A is expected to be short-lived, we concentrated on detecting vinyl molybdate species related to B, first by studying the properties of the vinyl molybdate anion CH2=CHMo03-, synthesized by the controlled hydrolysis of the complex CH,=CHMo(0)2Br(bpy) (bpy = 2,2'-bipyridyl) in analogy to the corresponding alkyl derivative^.^^^ On the basis of these experiments, it was found that the room-temperature half-life of vinyl molybdate is significantly shorter than that of the known alkyl molybdate^.^^^ However, chromatographic separation of this species from other products in quenched molybdothiol-acetylene reaction solutions was still possible and was employed for its detection. (1) On leave of absence from Lanzhou University, Lanzhou, Gansu, China. (2) Schrauzer, G. N.; Doemeny, P. A. J. Chem. SOC.1971, 93, 1608. (3) Schrauzer, G. N. Angew. Chem. 1975,87,579; Angew. Chem., Znt. Ed. Engl. 1975, 14, 514 and references cited therein. (4) Weathers, B. J.; Grate, J. H.; Schrauzer, G. N. J.Am. Chem. SOC. 1979, 101,917. (5) Schrauzer, G. N.; Moorehead, E. L.; Grate, J. H.; Hughes, L. J.Am. Chem. SOC.1978, 100,4760. (6) (a) Schrauzer, G. N.;Hughes, L. A.; Strampach, N. Z. Naturjorsch., B: Anorg. Chem., Org. Chem. 1982, 37B, 380. (b) Schrauzer, G. N.; Hughes, L. A.; Strampach, N. 'Proceedings of the 4th International Conference on Chemistry and Uses of Molybdenum", Climax Molybdenum Co.: Greenwich, CT, 1982, in press.
0276-7333/83/2302-0486$01.50/0
Table I. Pseudo-First-Order Rates of Mo-C Bond Cleavage in Vinyl Molybdate conditions 103k, no. buffer pH T,"C min-' 1 0.1 M phosphate 14.0 70 21.4 2 0.1 M phosphate 13.0 18.0 11.0 17.5 3 0.1 M phosphate 4 0.1 M phosphate 8.8 16.7 6.2 16.1 5 0.1 M phosphate 3.3 14.4 6 0.1 M phosphate 7 0.1 M HCl 1.0 35.7 11.0 23 3.5 8 0.1 M phosphate 9 0.1 M phosphate 7.0 3.3 10 0.1 M HC1 1.0 21.1 13.9 0 6.1 11 0.8MNaOH 12 0.1MNaOH 13.0 1.8 13 0.1 M phosphate 11.0 1.1 7.0 1.3 14 0.1 M phosphate 1.0 11.9 15 0.1 MHCl
Experimental Section Preparation of Vinyl Molybdate in Solution. Vinyl molybdate was prepared by the alkaline hydrolysis of the complex CH2=CHMo(bpy)(0)2Bras described in ref 6. For the preparation of CH2=CHMo(bpy)(0)2Br,25 mL of a 1 M solution of vinylmagnesium bromide in THF was added dropwise to a stirred in 50 mL of dry suspension of 4.5 g (10 mmol) of M~(O)~(bpy)Br~ THF at 0 "C, while a slow stream of dry argon was passed through the system. The reaction mixture turned slowly dark purple and was allowed to react for an additional 60 min. The purple crystals were collected by filtration through a Schlenk tube. The product was washed with anhydrous diethyl ether and dried at 1 mmHg for 24 h. For analysis, weighed aliquots of the complex were hydrolized in hot 1 M KOH, and the ethylene released was determined by GLPC. On this basis, the crude products isolated were 60-78% pure. For purification, samples were recrystallized from hot THF or CH2C12 Anal. Calcd for Cl2Hl1N2O2BrMo:Mo, 24.5;CH2=CH,6.9. Found: Mo,23.7;CH=CH2(as determined from the amount of C2H4released on alkaline hydrolysis), 6.0. For the preparation of vinyl molybdate solutions, fresh CH2=CHMo(0)2(bpy)Br was always used since the complex undergoes slow irreversible changes on prolonged storage at room temperature. The nature of these changes will be reported elsewhere. Into a beaker of 250-mL capacity was suspended 1 g of CH2=CHMo(0)p(bpy)Brin 100 mL of deionized water. To the slurry was added 1mL of 6 N NHIOH solution, and the mixture was magnetically stirred at room temperature for 3 min. The reaction solution was subsequentlycooled with ice. Bipyridyl wm removed by suction filtration and extraction with CH2C12.De0 1983 American Chemical Society
Organometallics, Vol. 2, No. 4, 1983 487
Synthesis of Vinyl Molybdate( V I )
Table 111. Relative Rates (%) of Reductive Cleavage of Methyl, Ethyl, and Vinyl Molybdates Using Ferredoxins at 21 "C and 1 h reactants" methyl ethyl vinyl no. 1 R-MO 2.9 17.8 22.6 5.0 27.1 57.9 2 R-MO t S,O,'2 t cysteine 2.0 53.0 67.6 58.3 73.7 3 t Fe4S,(S-n-C,H,)4]2- 3.2 3 t Fe,S,(S-n-C3H,),l3- 71.3 72.4 78.6 3 t Fe4S,(S-n-C,H,),I4' 83.8 80.2 87.3 6 t ATP 100.0b 100.0b 100.0b \70°'C
0
40
80
120 160 minutes
200
240
-
Figure 1. Rate plot of the decomposition of vinyl molybdate according to CH2=CHMo03-+ OH- CH&H2 + MOO:-, in pH 11,O.l M phosphate buffer at temperatures of 0,23,52, and 70 "C. Table 11. Half-Lives and Product Distribution of Aqueous Alkyl (R)Molybdates at 7 2 "C pH 11" pH 1.3b alkane/ t,,, , alkane/ t,,?, R min alkene h alkene methyl 60 methane 2.7 methane ethyl 29.3 66.5 0.3 0.08 n-propyl 34.3 24.7 1.9 0.88 13.2 isopropyl 40.8 4.6 0.07 tert-butyl 96.0 14.0 is0bu tene 1.0 neopentyl 282.0 neopen tane 16.2 neopentane crotyl 16.0 3.4 0.4 1.2c vinyl 39.6 ethylene 0.3 ethylene a In pH 11, 0.1 M phosphate buffer. 1-Buteneltrans-2-butene.
In 0.1 N HCl.
pending on speed of workup, solutions containing 0.1-0.5 pmol/mL vinyl molybdate were obtained as determined from the amount of C2H4generated on alkaline hydrolysis. For ethylene detection, a Hewlett-Packard Model 700 gas chromatograph was used, fitted with an FID detector and an 8 ft X 1/8 in. column packed with phenyl isocyanate on Porasil C, operating at 23 OC. Hydrolysis Experiments. Hydrolysis of the Mo-C bond of vinyl molybdate was studied in silicone-rubber serum capped bottles of 38-cm3capacity by measuring the amounts of ethylene released into the gas phase under the conditions indicated in the legends of Table I and Figure 1. Chromatography of Vinyl Molybdate Solutions. Vinyl molybdate was separated from molybdate by using a 15 X 1 cm silica gel column and 0.6 M HC1/CH30H as the eluent. This method was chosen because it allows separation' of Reo4-from MOO?-. Because of the instability of vinyl molybdate, the chromatography and all subsequent operations must be performed as rapidly as possible. The separated fractions were assayed for vinyl molybdate by measuring, with GLPC, the ethylene released into the gas phase on alkaline hydrolysis. This method allows detection of small traces of vinyl molybdate species. Typical elution profiles are shown in Figure 2. Vinyl molybdate appears in fractions immediately preceding molybdate. Molybdate was assayed colorimetrically by using the method described by Sandall? Detection of Vinyl Molybdate Species in MolybdothiolAcetylene Reaction Solutions. The conditions used for acetylene reduction with molybdothiol catalysts were modified to allow the detection of organomolybdenumintermediates. The binuclear p-oxo-bridged L(+)-cysteine complex of Mo(V) (7) Phipps, A. M. Anal. Chem. 1971, 43, 467. (8)Sandall,E. B. "Colorimetric Determination of Traces of Metals", 2nd ed.; Interscience: New York, 1950.
a Where indicated, reaction solutions contained 0.24 mol of Lis-n-C,H,, 0.06 mol of Li'S, 0.06 mol of FeCl, and/or FeCl,, and 0.10 mol of cysteine, all with 5 mmol of alkyl molybdate in a total reaction volume of 1 0 cm3; solvent, CH,OH/O.2 N, pH 9.6, borate buffer (3:7 by Yields measured volume). R indicates alkyl group. correspond to 15%methyl, 25% ethyl, and 40% vinyl,
Table IV. Percentage of Buta-1,3-diene Relative to Ethylene in Catalytic Acetylene Reduction Experiments with Complex I/NaBH4 as a Function of pH, after 60 min of Reaction at 23 O c a DH buffer 3'% 1.3-CAH. 11 phosphate 108 9.6 borate 55.6 7.6 borate 7.0 6.5 phosphate 4.5 5.1 phosphate 2.9 0 3.5 acetate 2.0 phosphate 0 a Reaction conditions: reaction bottles of 38-mL capacity contained in a total solution volume of 5 mL; complex I, 25 pmol; NaBH, (at t = 0), 0.133 mmol, in the buffers (0.2 M) as indicated. The acetylene pressure was 1atm.
("complexI") was employed as the catalyst precumor,with NaBH, as the reducing agent. Typically, 0.1 g (159 pmol) of complex I was dissolved in 2 cm3of H20,and the solution was placed into a serum-capped bottle. The solution was purged with argon and allowed to incubate for 2 h. Thereafter, 10 cm3of acetylene gas were injected, followed by 0.5 cm3 of a freshly prepared 0.3 M NaBH4solution. After 30 min, the reaction was quenched by the addition of 50 cm3 of acetone. The acetone served the dual purpose of destroying the excessive NaBH4and of precipitating all solutes. These were collected by filtration, redissolved in 0.6 M HC1/CH30Hand subjected to column chromatographyon silica gel as outlined above. As in the experiments with authentic vinyl molybdate, the fist fractions that produced ethylene on alkaline hydrolysis appeared immediately prior to the fractions that contained moat of the total molybdenum. In addition to ethylene, alkaline hydrolysis of the collected fractions produced variable traces of ethane and buta-1,3-diene, suggesting the presence of other organomolybdatespecies. The yields of C2H,were invariably very low and are not shown in Figure 2. The later fractions contained unreacted complex I and other reduced molybdenum species. However, they released hydrocarbons on alkaline hydrolysis and thus presumably contained complexes of L(+)-cysteine of vinyl and other organomolybdates whose separation was not attempted. These fractions also produced varying amounts of acetylene on alkaline hydrolysis, suggesting the existence of molybdenum(V)-acetylene complexes. For elimination of the possibility that the hydrocarbons generated on hydrolysis in any of the fractions were due to the secondary reduction of residual acetylene by disproportionating Mo(V) species: the hydrolysis was run at pH 11,where these secondary disproportionation and reduction reactions do not occur to an appreciable extent. This was confirmed through control experiments with added excess acetylene and complex I. Reductive Cleavage of Vinyl Molybdate with Ferredoxin Model Compounds. The M A bond cleavage experimenb were carried out in serum-capped bottles of 38-cm3capacity. Details of the experimental conditions are given in the legend of Table
488 Organometallics, Vol. 2, No. 4, 1983
Hughes, Hui,and Schrauzer
n
80
60
i
/
V o
*
I
02
04
0.6
08
IO
[NoOH, N]
60 t
1
Figure 3. Linear dependence of the rates of Mc-C bond cleavage of vinyl (A),methyl (bsd),and ethyl molybdates ( O ) ,on [OH-], at 0 "C. Data for methyl and ethyl molybdates from ref 6a, loc. cit.
40:
j0.
0 FRACTIONS (mls)
Figure 2. Chromatographic elution profiles from a 100-200 mesh, 15 X 1 cm silica gel column with 0.6 M HC1 in CH30H: (1) authentic vinyl molybdate, generated from CH2=CHMo(0)2Br(bpy);(2) organomolybdates from the solutes of acetonequenched molybdothiol-acetylene reaction solutions. 'Mo" refers to total molybdenum. 111. The preparation of the ions [Fe4S4(SR)4]z(z = 2-4) was described in ref 9. Acetylene Reduction Experiments as a Function of pH. The reduction of C2H2with complex I/NaBH4 as a function of pH was studied under the conditions indicated in the legend of Table IV.
Results and Discussion Vinyl molybdate, CH2=CHMo03-,may be generated in aqueous solutions by the alkaline hydrolysis of CH2= CHMo(O),(Br)(bpy) (bpy = 2,2'-bipyridyl) in analogy to the reported6 synthesis of the corresponding alkyl derivatives. The colorless anion undergoes spontaneous Mo-C bond hydrolysis more readily than the previously reported alkyl molybdates(VI), RMo03- (see Tables I and 11)and, depending on pH and reaction temperatures, occurs by several mechanisms. In strongly alkaline solutions, Mo-C bond hydrolysis occurs predominantly by the base-dependent mechanism eq la, and a linear dependence of the 1. MOO: t C2H4 (a) OH-
p o l y m o ~ b d i cacids
(9) Tano,
K.;Schrauzer, G. N. J. Am. Chem. SOC.1975, 97, 5404.
hydrolysis rates on [OH-] is observed (see Figure 3). However, the nonzero intercept indicates that Mo-C bond hydrolysis also occurs by a base-independent mechanism formulated in terms of eq l b , which predominates in neutral solution. Mo-C bond hydrolysis of methyl and ethyl molybdates also exhibits a linear dependence on [OH-], but the slopes and intercepts are smaller, indicating a greater resistance of Mo-alkyl bonds to hydrolysis. The enhanced sensitivity of vinyl molybdate to hydrolysis may be caused by the higher ionic character of the Mo-alkenyl bond. This is presumably also the reason for the lower apparent Arrhenius energies of activation for Mo-C bond hydrolysis of 7 kcal/mol (at pH 11; temperature range, 0-70 "C; experimental data from Figure 1). The Arrhenius energy of activation of Mo-C bond hydrolysis of methyl and ethyl molybdates is 17 kcal/mol under the same conditiom6 In 0.1 M HC1, the Arrhenius energy of activation for Mo-C bond hydrolysis is 3 kcal/mol, and the half-life of vinyl molybdate a t 23 "C is reduced to 1.3 h. M d bond hydrolysis under these conditions presumably occurs via a cationic intermediate as indicated in eq IC. Mo-C bond cleavage of vinyl molybdate is accelerated under reducing conditions. Relative rates of reductive cleavage in various systems are given in Table 111. As was previously observed with methyl and ethyl molybdates (see ref 6 and Table 111),the rates of reductive Mo-C bond cleavage reach a maximum with the "complete" molybdothiol catalyst systems, i.e., in the presence of cysteine, fully reduced ferredoxin model compound ( z = 41, a reducing agent (Na2SZO4), and ATP. Mixtures of I,(+)-cysteine with vinyl molybdate also produce active catalysts for the reduction of acetylene. This shows that intermediate vinyl molybdate species formed during reductions of acetylene with molybdothiol catalysts would be short-lived in the presence of excess reductant. Accordingly, acetone was used to destroy excess NaEiH, and to precipitate the solutes, among which vinyl molybdate species were detected on subsequent column chromatography in 0.6 M HC1 in CH30H. The vinyl molybdate species behaved identically on chromatography to those obtained on similar treatment of authentic vinyl molybdate. These observations permit us to extend the previously derived mechanism of molybdothiol-catalyzed acetylene reductionz4 particularly with respect to details
Synthesis of Vinyl Molybdate(VI) Scheme I
of the release of ethylene from the initially formed side-on bonded organomolybdenum intermediate A. This intermediate is now assumed to undergo M d : bond hydrolysis in two steps via vinyl molybdate(VI), from which ethylene is formed in a second step by way of a reductive Mo-C bond cleavage reaction as shown in Scheme I. This modification takes into account that reductive Mo-C bond cleavage of vinyl molybdate is clearly favored over the simple hydrolysis under simulated catalytic conditions (see Table 111). In addition to vinyl molybdate, several other organomolybdenum species appear to be present as judged from the hydrocarbons released on alkaline hydrolysis of the later chromatographic fractions. One of them produces ethane on alkaline hydrolysis and may be a thiol complex of ethyl molybdate(VI), the terminal intermediate of ethane-producing side reactions; it is always formed in much lower yields than vinyl molybdate (yields are not shown in Figure 2), consistent with the generally low yields of ethane in the molybdothiol-catalyzed reduction of acetylene. A species producing ethylene on hydrolysis is the major product. It could be the cysteine complex of vinyl molybdate but has as such not been characterized. Another species that releases buta-1,3-diene on alkaline hydrolysis is observed if the reactions are run under conditions favoring the formation of buta-l,&diene, i.e., at high concentrations of complex I in the presence of excess a ~ e t y l e n e .The ~ early fractions in Figure 2b could contain butadienyl molybdate(VI), CH2=CHCH=CHMo03-, the later fractions cysteine complexes thereof. Butadienyl molybdate could not be obtained by the reaction of vinyl
Organometallics, Vol. 2, No. 4, 1983 489 molybdate with acetylene under conditions of catalytic acetylene reduction. The formation of butadiene in molybdothiol-catalyzed reductions of acetylene presumably occurs by way of C2Hz insertion into an organomolybdenum species other than vinyl molybdate, as has been deduced elsewheread The fact that molybdothiols catalyze the reduction of acetylene to buta-1,3-dieneunder certain conditions was first emphasized by Corbin et al.l0 and was for some time considered to be a serious shortcoming of these nitrogenase model systems, since nitrogenase reduces acetylene only to ethylene. However, the formation of buta-18-diene can be effectively suppressed if the CzH2 reductions are carried out a t low catalyst concentrations," and we have since found that reaction p H is yet another factor controlling the formation of this product. Table IV shows that even a t high catalyst concentrations buta-1,3-diene is formed in substantial yields only in the alkaline pH range. Since the active site of functional nitrogenase is likely to be acidic as protons are generated during ATP hydrolysis,lZthe selectivity of the enzymatic CzHzreduction is no longer difficult to rationalize. Finally, it should be mentioned that, in addition to the hydrocarbons mentioned above, traces of acetylene are released on reaction of the chromatographic fractions with base. This suggests the existence of molybdenum(V)acetylene complexes whose formation is favored a t high concentrations of complex I. These complexes appear to play no role in the catalytic reductions of acetylene, their nature remains to be elucidated.
Acknowledgment. This work wm supported by Grant CHE79-50003 from the National Science Foundation. Registry No. Complex I, 25604-33-5; CH2=CHMo03-, 84521-09-5; CH2=CHMo(bpy)(0)2Br, 84521-10-8; Mo(O),(bpy)Br2,18057-92-6;vinyl bromide, 593-60-2;acetylene, 74-86-2; molybdate, 14259-85-9;ethylene, 74-85-1;1,3-butadiene,106-99-0. (IO) Corbin, J. L.; Pariyadath, N.; Stiefel, E. I. J. Am. Chem. SOC.1976, 98,7862. (11)Robinson, P. R.; Moorehead, E.L.; Weathers, B. J.; Ufkes, E.A.; Vickrey, T. M.; Schrauzer, G. N. J. Am. Chem. SOC.1977, 99,3657. (12) Schrauzer,G. N.;Hughes, L. A,; Palmer, M. R.; Strampach, N.; Grate, J. W. Z. Naturforach., B: Anorg. Chem., Org. Chem. 1980,35B, 1439.
