J. Am. Chem. SOC.1992, 114, 758-759
758
Communications to the Editor Catalytic Disproportionation and Reduction of Hydrazine by Sulfur-Ligated Molybdenum(IV) Complexes. Characterization of the Catalytic Precursor
[(2-SC5H3N-3-SiMe3)C12Mo(p-S2)(p-2-SC5H3NH-3-
c12
SiMe3)MoCI2(2-SC5H3N-3-SiMe3) fthf Eric Block* and Gabriel Ofori-Okai Department of Chemistry State University of New York at Albany Albany, New York 12222
Hyunkyu Kang and Jon Zubieta* CJB
Department of Chemistry, Syracuse University Syracuse, New York 13244 Received July 3, 1991
While the role played by molybdenum in the reduction of dinitrogen by nitrogenases remains enigmatic,lS2 the coordination chemistry of molybdenum with sulfur-containing ligands may be relevant to this process since it is possible that at least one end of dinitrogen binds to a molybdenum center in an intermediate oxidation state.3 Synthetic and mechanistic studies of model complexes, which focus on low-oxidation-state, mononuclear molybdo-phosphine complexes of the type [M(NZ),(PR3),],4 have established that a single metal center is sufficient for stoichiometric reduction of N2 to ammonia and that hydrazido(2-) complexes, M=NNH2, are key intermediates. In contrast, relatively few studies of N-N bond activation by molybdenum species in high oxidation states: or with sulfur ligands, and/or of higher nuclearity have been described. Although thiolate ligands may be most suitable for modeling the sulfido environment of the molybdenum center in nitrogenase, few examples of high-oxidation-state Mo-thiolate complexes capable of further substitution chemistry are known,6V8with the notable exceptions of [ M O ( S C M ~ ~and ) ~ ][Mo(SC6H2-2,4,6~ Pri3),].’ As part of our continuing investigations of the chemistry of metal-thiolate c o m p l e ~ e s we , ~ have described the chemistry of singly or multiply silylated 2-pyridinethiols. The silylated 2-pyridinethiol ligands 2-HSC5H3N-3-SiR3(R = Me, 1; R = Pr’, 2) and 2-HSCSH3N-6-SiMe2Bu‘(3) react with Mo(1V) precursors such as [MoC14(CH3CN),] to give mononuclear and binuclear complexes of the types [MoCl,(SR),] (SR = 2-SC5H3NH-3SiMe3, 4 2-SCSH3NH-3-SiPri3,5 and 2-SC5H3NH-6-SiMe2Bu‘, (1) Burgess, B. K. Chem. Rev. 1990, 90, 1377. (2) Leigh, G. J.; Jimenez-Tenorio, M. J . Am. Chem. SOC.1991,113,5862 and references therein. (3) Caughlin, M., Ed. Molybdenum and Molybdenum-Containing Enzymes; Pergamon: New York, 1980. Stephens, P. J. In Molybdenum Enzymes; Spiro, T.. Ed.; Wiley-Interscience: New York, 1985; p 117. (4) Chatt, J.; Pearman, A. J.; Richards, R. L. J. Chem. Soc.,Dalton Tram. 1978, 1766. Anderson, S. N.; Fakley, M. E.; Richards, R. L.; Chatt, J. J . Chem. SOC.,Dalton Tram. 1981, 1973. ( 5 ) Schrock, R. R.; Glassman, T. E.; Vale, M. G. J . Am. Chem. SOC.1991, 113, 715. (6) Otsuka, S.;Kamata, M.; Hirotsu, K.; Higuchi, T. J . Am. Chem. SOC. 1981,103, 301 1. Kamata, M.; Yoshida, T.; Otsuka, S.; Hirotsu, K.; Higuchi, T. J. Am. Chem. SOC.1981, 103, 3572. (7) Roland, E.; Walborsky, E. C.; Dewan, J. C.; Schrock, R. R. J . Am. Chem. SOC.1985, 107, 5795. (8) Walborsky, E. C.; Wigley, D. E.; Roland, E.; Dewan, J. C.; Schrock, R. R. Inorg. Chem. 1987, 26, 1615. (9) Block, E.; Gernon, M.; Kang, H.; Ofori-Okai, G.; Zubieta, J. Inorg. Chem. 1991, 30, 1736 and references therein.
Figure 1. ORTEP view of the structure [Mo~CI~(~-SC~H,N-~-S~M~,) (p-S2)(p-2-SC5H3NH-3-SiMe3)]. 6 ) and [ M o ~ ( S R ) ~ C(SR ~ ~ ]= 2-SC5H3NH-3-SiMe3,7; 2SCSH3NH-6-SiMe2But,8): where the ligand is present in the
neutral pyridine-2(lfll-thione form with the nitrogen protonated.I0 We now report that reactions of [MoCIS]with excess 1 result in reduction of the molybdenum to give the unusual binuclear complex [Mo2Cl4(p-S2)(p-2-SC5H3NH-3-SiMe3)(2-SCsH3N-3SiMe3),]-thf (9), which serves as a precursor for both catalytic disproportionation and reduction of hydrazine. Complex 9 is produced in the reaction of [MoC15]with 4 equiv of 1 in CH,C12, requiring that 1 act as a reductant and source of S22-,to give the bispyridyl sulfide [(3-Me3Si-2-NCSH3)2S]” as the major organic product. As shown in Figure 1, the structure of 9 consists of discrete binuclear molcules with the molybdenum centers bridged by a disulfide and a thione sulfur donor,ll an unusual geometry for Mo(1V)-Mo(IV) binuclear complexesI2but somewhat reminiscent of those of the tetrahydrothiophenecomplex [MO~C~~(C,H~S)~(~-S,)(~L-C~H~S)] (lo)’3 and Of (NEt4)2[MozC16(r-sz)(pc-C1)21(1 In the absence of external reductant and proton source, complex 9 effects the catalytic disproportionationof hydrazine to ammonia and molecular nitrogen,I5 as illustrated by the data summarized in Table I. At room temperature, a total of 1200 equiv of NH3/(equiv of dimer) was obtained over a 17-day period. The rate of ammonia evolution increased monotonically from 0.3 mol of NH3/(mol of 9)h at -5 OC to 2.4 mol of NH3/(mol of 9)h at +35 OC. Dinitrogen was identified as the other nitrogenous product of this reaction, and in one example 95% of the nitrogen
w4
(10) Deeming, A. J.; Hardcastle, K. I.; Meak, M. N.; Bates, P. A.; Dawes, H. M.; Hursthouse, M. B. J . Chem. SOC.,Dalton Trans. 1988, 227. (1 1) Experimental details are available as supplementary material. (12) Gamer, C. D. In Comprehemiue Coordination Chemistry; Wilkinson, G., Ed.; Pergamon: New York, 1987; Vol. 3, p 1301. (13) Hughes, D. L.; Richards, R. L.; Shortman, C. J. Chem. SOC.,Chem. Commun. 1986, 1731. (14) Muller, U.; Klingelhofer, P.; Friebel, C.; Pebler, J. Angew. Chem., In?. Ed. Engl. 1985, 24, 689. (1 5 ) Reactions were carried out in CH2C12at room temperature under argon. The ammonia was quantified by the indophenol method16 and hydrazine using p-(dimethylamino)benzaldehyde.” In one set of experiments (see Table 1 and supplementary material), NZevolution was measured by use of a Toepler pump and confirmed by mass spectroscopy/gas chromatography. (16) Weatherburn, M. W. Anal. Chem. 1967, 39, 971. (17) Chaney, A. L.; Marback, E. P. Clin. Chem. (Winsfon-Salem,N.C.) 1962, 8, 130.
0002-7863/92/1514-758%03.00/0 0 1992 American Chemical Society
Communications to the Editor
J . Am. Chem. SOC.,Vol. 114, No. 2, 1992 159
Table I. Catalytic Ammonia Production under Conditions of Disproportionation (A) and Reduction (B)" A: no external reductanth B: Zn amalgam/lutidine.HCI added' complex
added N2H4,bequiv
N H 3 yield,' equiv
conversion,' %
added N2H4, equiv
N H , yield!
equiv
conversion! %
9
95 12 (2) 20.6 (24)d 86 12 (5) 15.20 (16) 90 36 (1) 61.2 (72) 83 36 (2) 43.20 (48) 216 (2) 184.32 (288) 64 280 (2) 476.0 (560) 85 1440 (1) 1232.00 (1920) 64 1200 (1) 2045.0 (2400) 85 4 12 (2) 14.80 (16) 93 180 (2) 315.2 (360) 88 216 (1) 179.25 (288) 62 360 (2) 583.2 (720) 81 N2H4 g g 2 g g 3 " A complete table of all experiments is provided in the supplementary material. *Number of experiments is in parentheses. CMaximumyield possible by disproportionation is in parentheses. dMaximum yield possible by reduction is in parentheses. eIn one set of experiments, the yields of all nitrogen-containing products were quantified: for 122 equiv of N2H4 added, 15.20 equiv of NH,, 3.82 equiv of N2, and a trace amount of N2H4 were observed; these results give a N balance of 95%. Using 95% I5N labeled hydrazine, a nearly quantitative amount of I5NH3and 15N2(91%) was recovered as determined by ISN N M R and by mass spectroscopy/gas chromatography. 'For 12 equiv of N2H4, 20.6 equiv of N H , and 1.3 equiv of N 2 were obtained, for a 97% nitrogen balance. ZWith no complex present, traces of ammonia could be detected upon stirring CH2C12 solutions containing 0.05, 0.10, 0.20, and 0.50 mol of NzH4. A maximum conversion of 2% was observed after 7 days at room temperature. Addition of Zn amalgam and lutidine hydrochloride in the absence of complex did not significantly enhance ammonia concentrations. hThe rate of ammonia evolution at room temperature for 9 is 1.8 mol of NH,/(mol of 9)h, averaged over a 5-day period. 'The rate of ammonia evolution upon addition of lutidine hydrochloride is 6.6 mol of NH3/mol of 9)h, averaged over a 5-day period.
hydrazido species, as in the reaction of [ M ~ ~ C I ~ ( m b d(mbdsHz s)~] was accounted for. Furthermore, in the presence of zinc amalgam = PhCH(SH)SiMe,CH(SH)Ph) with H2NNPhMe to give and 2,6-lutidine hydrochloride, enhanced levels of ammonia as [MO~C~(~~~S)~(NNR~)~]+.'~ well as increased rates of evolution are observed, features consistent The evidence suggests that the catalytic production of ammonia with catalytic reduction." from hydrazine by 9 and 4 involves a common bimetallic interThe structurally related Mo(IV)-disulfido bridged binuclear mediate of composition [ M O ~ C I ~ ( ~ - S C ~ H ~ N (12), -~-S~M~~ complexes 10 and 11 were treated with hydrazine under conditions whose formation requires deprotonation of a ligand thione moiety identical to those employed for 9 and were found to yield levels with concomitant structural rearrangement involving loss of of ammonia slightly above background. On the other hand, the chloride groups and, in the case of 9, the disulfido group. While mononuclear complex 4 is effective in both the disproportionation a single metal center has been shown to be sufficient for the and the reduction of hydrazine. However, 4 has been shown to s t o i c h i o m e t r i ~and ~ ~ *catalytic ~~ reduction of hydrazine to amyield binuclear species under reaction with h y d r a ~ i n ea, ~reaction bonds constitutes a persistent, monia: bimetallic activation of N-N which may be monitored by changes in the electronic spectrum albeit minor, theme in the literature of nitrogen f i x a t i ~ n *and ~-~~ of 4 upon addition of various amounts of hydrazine. Addition has received added impetus from the recently described bridged of ether to solutions of 4 which have been treated with more than double cubane structure^.^^^^^ 400 equiv of N2H4 and which remains active with respect to Although the active binuclear species 12 has not been unamdisproportionation results in the precipitation of black crystals biguously characterized, the results described establish that biof the active species 7,9an observation which supports the binuclear nuclear molybdenum complexes with a sulfur ligand environment nature of the reactive intermediates. However, the electronic and in a relatively high oxidation state are capable of N-N bond spectrum of 711is distinct from that of the reactive intermediate, activation. The chemistry of N2H, and NH, ligands with highwhose identity has remained elusive. oxidation-sate Mo-thiolate complexes may be extensivG6and may Complex 9 is also observed to undergo a dramatic color change define alternative chemical possibilities for abiological nitrogen upon addition of hydrazine, from deep red to dark purple. The fixation. infrared spectrum of this solution is characterized by the disapAcknowledgment. We gratefully acknowledge support from pearance of the 620-cm-I band associated with v ( S 2 ) ,while the the National Institutes of Health (GM22566), the donors of the electronic spectrum is identical to that of 4 in the presence of Petroleum Research Fund, administered by the American hydrazine, indicating a common reaction intermediate with no Chemical Society, and the Herman Frasch Foundation. p-SZ2-linkage. The importance of the 2( 1H)-pyridinethione group in the overall Supplementary Material Available: Experimental procedures chemistry of 9 is confirmed by methylation of the pyridine niand tables of crystallographic parameters for the X-ray study of trogen, which yields [ M O ~ C ~ ~ ( ~ - S ~ ) ( ~ - S C ~ H ~ 9Nand - ~atomic - S ~positional M~~)~ parameters, ( ~ - bond lengths, and bond angles for 9 ( 1 3 pages): listing of observed and calculated structure factors SCSH3NMe-3-SiMe3)](13)," a species which is unreactive toward for 9 ( I 6 pages). Ordering information is given on any current hydrazine. However, since hydrazine may act as a Bronsted base and/or a redox reagent, as well as a ligand, complex 9 was treated masthead page. with triethylamine in order to evaluate the consequences of base addition; the reaction was found to give a mixture of HNEt3Cl, (18) Zubieta, J.; Block, E.; Ofori-Okai, G.; Tang, K. Inorg. Chem. 1990, 29, 4595. unidentified molybdenum-sulfido species, and a species analyzing (19) Cummins, C. C.; Baxter, S. M.; Wolczanski, P. T. J . Am. Chem. Soc. for [ M O , C ~ ~ ( ~ - S C ~ H , N(12)." -~-S Compound ~ M ~ ~ )12 ~] was 1988, 110, 8731. also obtained upon reaction of 9 with 1 equiv of hydrazine, with (20) (a) George, T. A.; Kaul, B. B. Inorg. Chem. 1991, 30, 883. (b) the formation of hydrazinium hydrochloride. Compound 12 is George, T. A.; Tisdale, R. C. Inorg. Chem. 1988, 27, 2909. (c) Leigh, G. J. Transition Met. Chem. (London) 1986, 11, 118. extremely unstable toward oxidation, producing [Mo203(2(21) (a) Hardy, R. W. F.; Burns, R. C.; Parshall, G. W. Ado. Chem. Ser. SCSH3N-3-SiMe3)4]in the presence of trace amounts of O2 or 1971, 100, 219. (b) Likhtenstein, G. I.; Shilov, A. E. Zh. Fir. Khim. 1970, H20. The electronic spectrum of 12 is identical to that of the 44, 849. reactive species produced by hydrazine treatment of 9 and 4. (22) Shilov, A. E. J. Mol. Catal. 1987, 41, 221. (23) Homobinuclear dinitrogen complexes have been prepared for metals Furthermore, 12 catalyzes the disproportionation and reduction of each of groups 4-10 of the periodic table. For references,see: Henderson, of hydrazine with quantitative results nearly identical to those R. A. Transition Met. Chem. (London) 1990, I S , 330. = 1.35pB/Mo), of 9 and 4. Since 12 is paramagnetic (pUeff (24) Stach, T. D. P.;Carney, M. J.; Holm, R. M. J. Am. Chem. Soc. 1989, characterization by N M R spectroscopy was not feasible. Reac111, 1670. (25) (a) Challen, P. R.; Koo, S.-M.; Kim, C. G.; Durham, W. R.; Couctions of 12 with organohydrazines yield mononuclear species of ouvanis, and - S[Mo~M~~)~ ] ~ D. J. Am. Chem. Soc. 1990,112,8606. (b) Coucouvanis, D.; Challen, the types [ M O ( N N R ~ ) ( ~ - S C ~ H , N - ~ P. R.; Koo, S.-M.; Davis, W. M.; Butler, W.; Dunham, W. R. Inorg. Chem. (NNR2)2(2-SCSH3N-3-SiMe3)3].9 However, there is precedent 1989, 28, 4183. for the reaction of unsymmetrically coordinated molybdenum (26) Schrauzer, G. N.; Robinson, P. R.; Mwrehead, E. L.; Vickrey, T. M. J. Am. Chem. SOC.1976, 98, 2815. dimers with organohydrazines to give binuclear molybdenum-
