Biomimetic Chemistry - American Chemical Society

22 Molybdothiol and Molybdoselenol Complex Catalysts

Downloaded by SUFFOLK UNIV on January 19, 2018 | http://pubs.acs.org Publication Date: December 10, 1980 | doi: 10.1021/ba-1980-0191.ch022

Acetylene Reduction and Electron Spin Resonance Characteristics YUKIO SUGIURA, TAKANOBU KIKUCHI, and HISASHI TANAKA Faculty of Pharmaceutical Sciences, Kyoto University, Kyoto 606, Japan In the reduction of acetylene with molybdothiol and molybdoselenol complex catalysts, the effects of structural variation in ligands, variety of coordination-donor atom, kind of transition-metal ion, and other factors have been surveyed systematically. These factors have profound effects on the catalytic activity. The Mo complexes of cysteamine (or selenocysteamine), its N , N dimethyl derivative, and its ß,ß-dimethyl derivative give ethylene, ethane, and 1,3-butadiene, respectively, as the major product. The Co(II) complexes of cysteine and cysteamine show higher catalytic activity than do the corresponding Mo complexes, and the order of the activity in the donor atom, namely S > Se >> O in the Co(II) complexes is consistent with that in the Mo complex systems. On the basis of electron spin resonance (ESR) features of these Mo complex catalysts, a relationship between their ESR characteristics and catalytic activities is discussed.

O

ne of the more remarkable properties of nitrogenase is its ability to catalyze the reduction of diverse small, unsaturated molecules besides molecular nitrogen. Prominent among these substrates is acetylene, which the enzyme readily reduces to ethylene. Recently, Schrauzer and his collaborators discovered that the Mo complexes of cysteine and glutathione with N a S 0 or N a B H mimic the enzyme in this respect, catalyzing the formation of ethylene (I, 2). The catalytic systems were based initially on the premise that nitrogenase contained Mo and sulfhydryl-containing amino acids. In contrast with the native 2

2

4

4

0-8412-0514-0/80/33-191-393$05.00/0 © 1980 American Chemical Society

Dolphin et al.; Biomimetic Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1980.


394

BIOMIMETIC CHEMISTRY

enzyme, the molybdothiol model system produces 1,3-butadiene from acetylene. Therefore, the difference of the selectivity between the enzyme and the molybdothiol complex system is of special interest. In this chapter we survey the effects of various factors that affect the catalytic activity (i.e., total yield, product distribution, and reaction rate) in the acetylene reduction with the molybdothiol and molybdoselenol complex catalysts. Variations in ligand structure, the nature of the coordination-donor atom and the transition-metal ion, and other factors have been investigated. In addition, the catalytic activity of the molybdothiol and molybdoselenol complexes has been probed through the electron spin resonance (ESR) features, which reflect the structural characteristics of the complex species. The information presented here presumably opens the way for a superior catalytic system and also will define the essential factors that determine the reduction of acetylene in aqueous solution, though the molybdothiol model complex systems may not mimic the native enzyme in an important aspect. Experimental N- and /*-Substituted derivatives of cysteamine and selenocysteamine were synthesized according to Klayman's method (3), with some modifications. Cysteamine, L-cysteine, selenocysteamine, and L-selenocystine were purchased from Sigma Company; L-selenocysteine was prepared by the reduction of L-selenocystine with sodium borohydride. Various sulfhydrylcontaining peptides were synthesized according to our previous method (4, 5). The selenohydryl-containing ligands were used freshly, as they readily oxidize to diselenides. All other reagents used were of commercial reagent grade. A typical catalytic system consisted of a 20-mL glass containerfittedwith a rubber serum cap containing borate buffer (pH 9.2; 3.5 mL), Na Mo0 or CoCl (0.5 mL; O.lmM aqueous solution), and the ligand (0.5 mL of 0.2mM solution; borate buffer). Water-washed acetylene (1 atm) wasflushedinto the solution and the reaction was initiated by the injection of 0.5 mL of NaBH (0.5 mL of 2mM solution; borate buffer). The reaction mixture was shaken at 20°C and the gas phase analyzed by gas chromatography using a Shimadzu gas chromatograph, Model GC-5A, equipped with a 0.3-cm x 2-m column of activated aluminum oxide containing 1% squalene and aflame-ionizationdetector. Two-component systems consisting of solutions of the metal ion and NaBH alone exhibited no significant catalytic activity. X-Band ESR spectra were obtained at 77 and 293 K with a JES-FE-3X spectrometer. The g-values were determined relative to Li-TCNQ(g = 2.0026) and the magnetic fields were calibrated by the splitting of Mn(II) in MgO(AH _ = 86.9 G). Magnetic circular dichroism (MCD) measurements, using a 11.7-kG magnet, were carried out on a Jasco J-20 spectropolarimeter and are expressed in terms of molecular ellipticity, [0] = 2.303 (4500/7r) (^-€ ), with units of (deg cm )/d mol. 2

4

2

4

4

3

4

R

2

Results and Discussion Ligand Effect. In M o complexes containing cysteine-related ligands as potential catalysts, structural variations in the ligand have



22.

SUGIURA ET AL.

395

Molybdothiol &• Molybdoselenol

been pursued systematically. (6) The results are summarized in Table I. The presence of two methyl groups in the /3-position, as in penicillamine (/3,/^dimethylcysteine), clearly has compromised the reactivity of the complex. The complex of the threo form of /3-methylcysteine (R = C H , R = H) behaves quite like that of cysteine in the reduction of acetylene, while that of the erythro form (R = H , R = C H ) resembles penicillamine complex and has little activity. A steric factor must be responsible for the obtained results. The N,N-ethylenedicysteine complex, M o 0 (edcys) , wherein an ethylene bridge links the cysteine units on each Mo atom, also was tested to clarify the requirement of sulfur donor and monomeric Mo species for the activity. Despite the 3

x

2

x

2

3

2

2-

4

2-

Table I.

Acetylene Reduction by Mo-Complex Systems of Cysteine and Its Related Ligands" Gas Phase (fimol) Complex (ligand)

(R = R = H ) ( c y s ) (R R = H)(cys)* (R R = CH )(pen) ( R = C H , R = H)(threo-/3-Me) (R = H , R = CH )(erythro-0-Me) (edta) (edcys) 1

2

1 =

2

1

2

=

x

3

3

x

2

2

a

wise. 6

3

C2H4 52.5 130 4.0 52.9 8.0 4.9 0.6

CH 2

6

0.6 4.8 1.1 1.3 1.3 0.9 0.3

CH 4

e

142 27 3.7 149 10.0 0.6 0.1

CH 2

2

362 450 668 345 672 672 668

25-min reaction times and borate buffer(pH 9.6) were used unless noted other0.2M carbonate buffer(pH 9.6) was used in the place of borate buffer.


396


presence of thiol groups, the bridging ethylene group does not allow dissociation into the discrete monomeric units apparently needed for activity. The limited activity of this complex leads to the formation of a substantial proportion of C H , suggesting the ability of the reduced dinuclear complex to effect the required four-electron reduction. Table II shows the effect of structural variations in the cysteamine-related ligands (7). In contrast with the case in cysteine


2

6

C—R

2

ligand, the /3-substitution of cysteamine has a promotive effect on the catalytic activity. Inspection of molecular models reveals a probable explanation for the difference of activity between penicillamine and /3,/3-dimethylcysteamine. Penicillamine requires dissociation of the carboxyl group to exhibit its activity. However, eclipsing caused by methyl and carboxyl groups is unfavorable for the dissociation of the carboxyl group and the /3,/3-dimethyl groups can block partially the position trans to M o — O . However, /3,/^dimethylcysteamine, which lacks a carboxyl group, has a clearly accessible site for the metal ion and the dimethyl substitution may affect electronically the activity. O n the other hand, the N,N-dimethyl substitution of cysteamine induces a drastic change of the product-distribution pattern. In this case, the major product is not ethylene but ethane. Cysteamine (or selenocysteamine), its 2V,N-dimethyl derivative, and its )3,/3-dimethyl derivative give C H , C He, and C H , respectively, as the major product. O f special interest is that seemingly small changes in ligand have remarkable effects on the product distribution and total yield. t

2

4

2

4

6

Donor Effect. Similar catalytic activity of molybdoselenol complexes is predicted through the close resemblance of selenium to sulfur in biochemical aspects. However, the Se atom(covalent radii = 1.16 and ionic radii of Se ~ = 1.98 A) is larger than the S atom (1.02 and 1.84 A) being less electronegative and possesses somewhat more metallic character. Table II summarizes the catalytic activity in the acetylene reduction by various molybdoselenol complexes, together with those by the corresponding molybdothiol complexes. The catalytic activity of the ethanolamine-Mo complex system was negligible. The effect of coordination donor atoms on the catalytic activity clearly decreases in the order S > Se ^> 0. In general, the replacement of S by Se gave the following effects on the catalytic activity and 2



3

3

3

2

H H CH H H H H CH

R

3

3

H CH H H CH CH H H

3

3

3

H CH H H H CH H H 3

3

4

165.5 15.3 200.0 59.1 34.3 14.8 51.9 80.0

2

CH 6

12.0 183.1 22.3 58.9 154.6 256.2 58.8 71.2

2

CH e

29.8 82.2 262.8 58.0 58.4 30.6 61.5 93.6

4

CH

Total

207.3 280.6 485.1 176.0 247.3 301.6 172.2 244.8

Yields were determined after 30-min reaction and rates for the initial 5 min.

H H CH H H H CH CH

S S S Se Se Se Se Se

a

R,

Donor

a

Product Yield (iumol) 4

13.8 0.08 9.0 1.0 0.2 0.06 0.9 1.1

2

CH

: : : : : : : :

6

1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0

2

. CH

Ratio 4

6

CH

: 2.5 : 0.4 : 11.8 : 1.0 : 0.4 : 0.1 : 1.0 : 1.3

:

Table II. Catalytic Activity of Various Molybdothiol and Molybdoselenol Complex Catalysts in Acetylene Reduction


398

product distribution: lower ethylene production, increased ethane production, decreased 1,3-butadiene production, and smaller difference in product-distribution ratio caused by ^substitution. The /3-substitution of selenocysteamine gives little change on the catalytic activity, and their product distribution ratio is approximately C H : C H : C H = 1 : 1 : 1 . The N-substitution of selenocysteamine clearly increases the catalytic activity, as is the case for cysteamine. However, the product-distribution pattern changes considerably and in particular, the ethane production increases. The complexes with Se donor atoms show catalytic ability similar to that in complexes with S donor atoms show catalytic ability similar to that in complexes with S donor atoms in the reduction of acetylene. However, S —> Se replacement has a distinguishable effect on the catalytic activity and/or product-distribution pattern. The catalytic change that occurs upon the atomic substitution probably reflects the spatial and electronic differences of the two elements, S and Se. Metal Effect Schrauzer and Schlesinger surveyed the relative activity of various transition-metal ions as catalysts in the reduction of acetylene to ethylene in an aqueous solution containing 1-thioglycerol and excess N a S 0 (8). Except for the remarkably specific activity of Mo, only iridium showed appreciable activity, converting acetylene to ethylene at 15% of the rate of the Mo system. In the catalytic system of cysteine and N a B H , tungsten, rhodium, rhenium, and ruthenium demonstrated the catalytic activity of approximate 7.0, 2.7, 2.0, and 1.5%, respectively, relative to the Mo system (9). We recently studied Co(II) complexes containing cysteine- and cysteamine-related ligands that show potential as catalysts, and obtained results that are somewhat different from those mentioned above. The discrepancy presumably is attributable to the difference in p H of the reaction, concentration of the reagents, and molar ratio of the metal/ligand. No formation of finely divided metal was observed under the conditions used. Figure 1 and Table III show the yield and rate of acetylene reduction with the Co(II)-cysteine and -cysteamine ligand systems in the presence of sodium borohydride (JO). These Co(II)-complex catalysts produce ethylene at a rate superior to that with the corresponding Mo-complex systems. One of the salient features is that the formation of 1,3-butadiene is negligible and the major product is ethylene. O n the other hand, the major product from the acetylene reduction with the Mo(V)-cysteine catalysts in borate buffer is not ethylene but 1,3-butadiene (6). The effect of coordination donor atoms on the catalytic activity is S > Se ^> O, consistent with that in the Mo-ligand systems. However, the Co(II) complexes of selenocysteine and selenocysteamine showed a higher ethylene-ethane product ratio than those of cysteine and cysteamine, though the total yield was lower. The aetivity of the Co(II)-cysteine and -cysteamine complexes 2



4

2

6

4

6

2

2

4

4



22.

SUGIURA ET AL.

399

Molybdothiol i? Molybdoselenol

T I M E ( min.) Figure I. Catalytic activity of Co (I I)-complex catalysts as compared with corresponding Mo catalysts: (A) Co-cysteine; (O) Co -cysteamine; (A) Mo-cysteine; and (#) Mo-cysteamine was maximum in the p H region 8.5-10.0, which is consistent with the optimum p H region (8.0-10.5) for the complexation of Co(II) complexes of these ligands. Effects with p H were parallel substantially, indicating that the complex formation plays a specific role in acetylene reduction with these Co(II)-complex systems. The results of visible and M C D spectroscopy suggest that pink-colored Co(II)-cysteine complex [500 nm(c 76) and 570 nm(0 + 0.33 x 10" deg cm /d mol)] and blue-colored Co(II)-cysteamine complex [670 nm(370) and 700 nm(-13.5 x 10" )] have octahedral and tetrahedral geometries, respectively. The magnitude of the M C D bands associated with the d-d transition of the Co(II) chromophore is 50-100-fold larger when the Co(II) ion is in a tetrahedral ligand field rather than an octahedral one (II). The maximal amount of ethylene is formed at the Co(II)-cysteine ratio of 2 : 1 . Probably, the low activity in the presence of excess cysteine is attributed to lack of efficient residual coordination sites of Co(II) for the substrate. These results suggest that the predominant 3

2

3



2

6

0 92 14 0 40 5

C H (fjbmol) 4 519 234 3 446 159

Total Yield (fimol) 0.8 100 45 0.6 100 36

Relative Yield (%) 6

4

— 4.7:1 15.7:1 — 10.2:1 30.8: 1

2

2

CH CH

4

2

6

0 47 25 0 64 38

0 100 52 0 100 60

Rate Relative (/xmol/min) Rate(%)

2

° Yield of the products was obtained at reaction time of 30 min, and rate is represented in terms of C H + C H /min at initial 5 min.

4 428 220 3 406 154

4

Serine Cysteine Selenocysteine Ethanolamine Cysteamine Selenocysteamine

2

C H (fjLmol)

0

Yield and Rate of Ethylene and Ethane Produced from Acetylene with Co(II) Complexes of Cysteine, Cysteamine, and Their Related Ligands

Ligand

Table III.



22.

401

Molybdothiol ir Molybdoselenol

SUGIURA ET AL.

formation of ethylene by these Co(II)-complex systems is due to the monomeric Co(II) complexes as active species. Noteworthy is that the turnover number of acetylene by these Co(II)-complex catalysts is approximately 15, being higher than that (0.5) of the well known M o cysteine complex catalyst (see Table IV). Table V summarizes the reduction activity of acetylene with the Co(II)-complex catalysts of various sulfhydryl-containing peptides. Of interest is the high activity of the sulfhydryl- and imidazolecontaining peptides such as N-mercaptoacetyl-L-histidine and N~ mercaptoacetyl-DL-histidyl-DL-histidine. In addition, the effect of amino-acid residues on the reduction of acetylene with these Co(II) complexes decreases in the order histidine > glycine > cysteine > tryptophan. Other Effect. The product distribution is affected profoundly by changing the buffer ion from borate to carbonate (6). In borate buffer, the major reduction product of acetylene by the Mo-cysteine complex system is 1,3-butadiene, in contrast with ethylene in carbonate buffer. The product-distribution pattern was as follows: C H = 280.2, C H = 21.0, C H = 372.2 /xmol (in borate buffer); and C H = 264.2, C H = 6.4, C H = 82.8 /Ltmol (in carbonate buffer). These results are in agreement with those by Corbin et al. (see Table I) (6). However, further investigations are necessary to clarify the true nature of this buffer effect. One characteristic feature of the molybdothiol complex catalysts is the stimulating effect of A T P on the reduction of acetylene (9, 12, 13). Recently, the effect of A T P as well as A D P was found to depend upon the p H of the A T P solution added to the reaction mixture. In addition, the effect of added H S 0 is similar to that of A T P and ADP. 2

6

2

6

4

4

6

2

4

2

4

6

2

4

Table IV. Comparison of Some Characteristics of the Co-Cysteine and Co-Cysteamine Catalysts with Mo-Cysteine Catalyst and Nitrogenase Co-Cysteine Co-Cysteamine

Characteristic Acetylene reduction, turnover number N reduction to N H , turnover number Enhancement factor of acetylene reduction by A T P Inhibition by C O Inhibition by H 0

2

3

0

2

Mo-Cysteine

Nitrogenase 150-200

13-16

0.5

Unknown

1.5 x 10"

1000

Weak None

Weak None

Strong Competitive

6

50

Turnover numbers defined as mole substrate reduced per mole metal in complex system per minute. a


402


Table V. Effect of Amino-Acid Residues on Acetylene Reduction Parent Compound

R

CH

3

(CH ) CH 3

R—CHCONHCH—R Downloaded by SUFFOLK UNIV on January 19, 2018 | http://pubs.acs.org Publication Date: December 10, 1980 | doi: 10.1021/ba-1980-0191.ch022

I

'

CH

I

SH

2

COOH

3

H

H

CH

3

(CH ) 3

R—CHCH CONHCH—R

I

SH

H

COOH

R—CHCONHCHCONHCHCH | R'

a

H

'

2

I

2

[=1 N NH

2

| COOH

(

:

Yield of the products was obtained at reaction time of 30 min.

Therefore, this ATP-stimulation seems to be attributable to a nonspecific effect as a protic acid (14). Acid catalysis in various redox reactions is well known. Figure 2 presents the relative effects of various charge carriers on the acetylene reduction by the Mo(V)-cysteine complex (15). The optimum enhancement of ethylene formation is observed for additives having reduction potentials around -0.9 V vs. S H E . The charge carriers such as l,l'-trimethylene-2,2 -bipyridylium bromide and iron phthalocyanine-4,4',4",4" -tetrasulfonate sodium, enhance not only the charge transfer from the strong donors such as N a B H and N a S 0 to /

,

4


2

2

4

22.

SUGIURA E T AL.

Molybdothiol 6 Molybdoselenol

403

with 2 : 1 Sulfhydryl-Containing Peptide-Co(II) Complex Catalysts Yield" C H (/Amo

Biomimetic Chemistry - American Chemical Society

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