Oxomolybdenum Chemistry An Experiment Charles G. Young University of Melbourne, Parkville 3052, Australia

Metal oxides are important industrial catalysts (1) and certain biological systems, notably those containing Mn, Fe and Mo, exploit 0x0-metal active sites in a wide variety of catalytic reactions (2). Indeed, the high-valent chemis- try of the early and middle transition metals is dominated by 0x0 complexes (3). Diverse and important, oxomolybde- num chemistry is particularly instructive and relevant in undergraduate inorganic chemistry (4). I t forms the basis of this undergraduate experiment.

The 0x0 lieand. formallv 02-. stabilizes high-valent met- - , -

al centers by both o- and ;-bask interactions and is suscep- tible to both nucleo~hilic and electro~hilic attack. Nncleo- philic attack by phkphine on a d i o x o - ~ o ( ~ ~ ) center leads to oxygen atom transfer (51, two-electron reduction of mo- lybdenum and concomitant oxidation of phosphorus (eq 1).

[ ~ 0 ~ ' 0 ~ 1 ~ + + :PR, + [ M O ~ O ] ~ ~ + OPR, (1)

In a complementary reaction, oxygen atomdonors such as S- andN-oxides, peroxides and dioxygen are capable of oxi- dizing 0x0-Mo(1V) complexes, thus generating dioxo- Mo(V1) species (e.g., eq 2). Under suitable conditions, coupling of reactions 1 and 2 permits catalytic oxygen atom transfer processes to be realized (5). Electrophilic attack on an 0x0 group is exemplified by simple protonation, as shown in eq 3; these reactions do not change the oxidation state of the molybdenum. In the absence of other ligands, protonation is followed by condensation and polyoxo- molyhdate formation (4). In the presence of other ligands, selective replacement of the 0x0 ligands may he promoted by protonation. Another prevalent reaction in oxomolybde- num chemistry is the comproportionation reaction shown in eq 4.

Reactions 14 are exploited in the synthesis of innumer- able Mo complexes (41, account for much of the chemistry of such comalexes (4). and are im~or tan t in catalvtic redox reactions s;ch as those mediated by industrial'oxidation catalysts (1) and the molybdoenzvmes sulfite oxidase, xan- thine oxidase and nitrate reduccase (6). Comproportiona- tion reaction 4 must he sterically prevented in realistic models of mononuclear oxomolybdenum enzyme centers (6).

This experiment involves the synthesis and charac- terization of several oxomolybdenum complexes contain- ing the N,N- diethyldithiocarhamate ligand and the explo- ration of an historically important system for catalytic oxveen atom transfer. (See firmre.) Firstlv. acidification of a kTxture of ~ 0 0 4 " and N ~ & C N E ~ ~ (prepared in situ) is em~loved to svnthesize the octahedral cis-dioxo-Mo(V1) c0&~1ex e i s - ~ o ~ z ( ~ z ~ ~ ~ t z ) z (1) (7). This is converted to t he square pyramidal 0x0-Mo(1V) complex

(YI (81 The chemical reactions involved in this experiment.

MoO(S2CNEtz)z [Redl and the dinuclear poxo-MOW) com- plex syn-MozOz(p-O)(SzCNEtz)4 [Purple], by reactions with PPhn (c.f.. ea 1) (8). These three com~lexes are compo-

- . .

nmts of the now classlc catalytic oxygen :Itom transfer iys- :em first dcwrihd b\. B x r a l and co-workers (9.101. 'l'heir structures, or thosewof closely related derivatives, have been determined by X-ray diffraction (11). Reaction of 1 with hydrochloric acid results in the formation of seven-co- ordinate , pentagonal hipyramidal cis-mer- MoOCIz(SzCNEtz)z Eellow] (12). If time permits, the structurally related chiral seven-coordinate complex MoO(S2)(SzCNEtz)z [Blue] may be prepared and sulfur atom transfer reactions contrasted with their oxygen atom counterparts (13). The literature syntheses of all com- plexes have been simply and successfully modified for the undergraduate laboratory. Moreover, the experiment may be tailored to suit the level of student and time available. Less advanced students may simply be required to per- form the syntheses and observe and explain the oxygen atom transfer chemistry. For more advanced students, the identity of the compounds can be withheld and, upon pro- vision or collection of analytical and spectroscopic data, the students may be required to determine the formulae and structures of the compounds and explain the chemistry. The experiment is typically performed over a four-hour pe- riod with the simultaneous synthesis of P, R and Y. An

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additional period is generally required if B is to be pre- pared along with the aforementioned.

The experiment highlights:

1. Avariety of synthetic strategies based on eqs 1 and 2. 2. The study of complexes with a variety of coordination

numbers, geometries, and nuclearities. 3. Analysis of the infrared, 1H NMR and mass spectra of the

complexes. 4. An examination of stoichiametrie and catalytic oxygen

atom transfer reactions. 5. A comparison of simple oxygen atom transfer and sulfur

atom transfer reactions.

Experimental Procedure Preparations

While most syntheses can be performed on a n open bench, the dispensing of malodorous or toxic substances such as HNEtz, CSz, concentrated HC1, chlorinated sol- vents and propylene sulfide should be carried out in a fume hood.

HNEt, + CS, + NaOH + NaS2CNEt2 + H20 (5)

Success depends on vigorous agitation or magnetic stir- ring during the addition of the hydrochloric acid. Diethy- lamine (2.4 mL, 23 mmol) and sodium hydroxide (0.9 g, 23 mmol) are added to water (50 mL) in a 250-mL Erlenmeyer flask. After stirring for 5 min, the mixture is treated with carbon disulfide (1.4 mL, 23 mmol), a watchglass is placed over the top of the flask and the solution is stirred for a further 10 min. Sodium molybdate(V1) dihydrate (3.5 g, 14.5 mmol) is added to the mixture. which is then treated dropwise (from a dropping funnel, over about a 10 min pe- riod) with a solution of 6 mL of concentrated hvdrochloric acidin water (100 mL). Vigorous stirring is required dur- ing the dropwise addition; the dense yellow-brown product precipitates. The solid is isolated by vacuum filtration, washed well with water (60 mL), ethanol (60 mL), then ether (60 mL) and dried a t the pump. The crude material may be employed in the syntheses that follow. The remain- der of the sample can be recrystallized by dissolving it in dichloromethane (15 mug) , filtering, and adding ether (20 mL/g) to the clear filtrate. Yield 4.0 g, 85 %.

IR (KBr): MCN) 1510s; v(Ma=Ol 920, 880s cm-'. 'H NMR (CDCl,, 400 MHz): 6 1.32 (t, 12H, 3~ 7.5 Hz, 4 x CH31, 3.80 (q, 8H, 4 x CH,) (fluxional on NMR timescale). M O ~ O ( S ~ C N E ~ ~ ) ~ (R) (81

This compound is moderately air-sensitive and all work should be nerformed anicklv and efficientlv. In a small round-bottomed flask connected with a water or air con- denser. a mixture of 1 (1.0 e, 2.3 mmol) and triphenvl-

. .

phosphine I 1.0 g , 3.8 mniol: thc excess allours the synthrsii to be ~er tormed in air. in I Z d i r h l o n ~ e t h e n ~ ~ ~ l ) v X" ' C , 10 mL) is refluxed for 10-15 min. Ensure that reflux (in a pre- heated bath) is commenced immediatelv after adding the solvent to the starting materials. upon-completion of the reflux pour the reaction mixture, with swirling, into ice- cold ethanol (50 mL) contained in a 100-mL Erlenmeyer flask. Filter the crystals, wash with ethanol, then ether, and vacuum dry. Yield 0.77 g, 80%.

IR (KBr): v(CN) 1520s; v(Mo=Ol 960s em-'. 'H NMR (CDCI3, 400 MHz): 6 1.35 (t, 12H, 3J 7.5 Hz, 4 x CH3), 3.87 and 3.93 (m, 8H, '5 15 Hz, 3~ 7.5 Hz, 4 x diastereotapie CH2).

S ~ ~ - M O ~ ~ O ~ ~ ~ - O ) ( S Z C N E ~ Z ) ~ (P) (81 A solution of 1 (0.5 g, 1.2 mmol) in dichloromethane (5

mL) is filtered through a fluted filter paper into a 25-mL Erlenmeyer flask, then the filtrate is treated with a solu- tion of triphenylphosphine (0.16 g, 0.6 mmol) in methanol (10 mL). The mixture is swirled for a few seconds then left to stand for 15 min (longer times may be employed if the flask is tightly stoppered). The purple solid formed is vac- uum filtered, washed with methanol and dried a t the pump. Yield 0.42 g, 85 %. The true color of the compound is revealed only when a sample is crushed on a white surface (e.g., tile or paper).

IR (KBr): v(CN) 1500s; v(Mo=O) 940s, 920sh; v(MaOMal750w em-'.

C ~ S - ~ ~ ~ - M O ~ % C Z ~ ( S ~ C N E ~ ~ ) ~ (Y) (121 Asolution of crude 1 (0.5 g, 1.2 mmol) in acetone (35 mL)

is filtered through a fluted filter paper, then the filtrate is treated with concentrated hydrochloric acid (2.5 mL, ex- cess) and the mixture stirred for 20 min. The product is isolated bv filtration. washed with 10 mL of acetone and dried a t t

farmed. Repeat. Interconversion of R and 1 is effected he- fore decomposition takes place.

4. React a solution of R with excess PPh3. Another observation students should be made aware of

and think about is: When a solution of 1 and a 50-fold mo- lar excess of PPh3 is monitored in air by 3 1 ~ NMR, the in- itial spectrum revealing only the presence of PPh3 i s slowly replaced by a spectrum consistent with the presence of only OPPh3. A similar observation results when a solu- tion of 1 and a 50-fold molar excess of both PPh3 and di- methylsulfoxide (MezS=O) is monitored in a sealed tube by 3 1 ~ NMR. Blank experiments show that in the absence of 1, PPh3 is not converted to OPPh3 in the presence of oxy- gen or MezSO alone.

Analvtical and Swctroscovic Data [A folder containing analytical and spectral data along

with other material useful in the interpretation of these data may be obtained upon request from the author. Some data are summarized after the synthesis of each com- pound.] From analytical and (simdated) mass spectral data students are able to determine the empirical and mo- lecular formulae of the compounds. Having established the formulas, infrared and nuclear magnetic resonance spec- tra are examined and molecular structures deduced. Infra- red spectra exhibit bands due to the dithiocarbamate (typi- call v(CN) 1550-1500 cm-'1 and 0x0 (v(Mo=O) 1000-850

-3 . cm ) hgands as well as co-ligands such as K-0 and szZ-

(14). 1H NMR spectra a t 400 MHz highlight the fluxional behavior of 1, the impact of molecular svmmetrv on spec- tral patterns' (esp. pkntagonal bipyraiidal complexes Y (Cd and B (Cij) and the diastereotopic nature of the CHAHB methylene protons of R, Y and B (131. Arationali- zation of the chemistry is possible once the compounds have been characterized.

Literature Cited 1. Sheldon. R. A,; Koehi, J. K. MelalKatalped Oddation NOrgnnic Compounds; Aca-

demic: New York. 1981. 2. Frausto da Silva. J. R. R.: Williams. R J. P. The Bioiogiml Chemistry of lhr Ele-

menti;: Clarendon: Oxford. 1991. 3. Wilkinson, 0.; Gillard. R. D.; McCleveny, J. A. Compiehhennua Cooniinnfion Chom-

iriiy; Pergamon: Oxford. 1987. Volume 3. 4. Detailedacmunts of oromolybdenum chemistry may be found inRef. 3:1aiSykes,A.

G.Chapter36.1,pp 1229.IblGamer,C.O.:Chamoek,J.M.Chspter36.4.pp 1329. (el Sfiehl, E. I . Chapter 36.5, pp 1375.

5 . For a review of oxygen atom transfer reactions see: Holm, R. H. Cham. Re". 1987, 87, 1401.

6. (a1 Burpayer, S. J. N.: Stiefel, E. I. J Chem. Edue. 1985.62.943. ibiHolm, R. H. Cooid. Chem Reu. 1990. 100, 183. icl Enemark. J. H.; Young, C. G. Ad". Inorg. rham 1-9 A" 1 . . .. . . . . . . . ., . . , . .

7. Moore. F. W.: Larson. M. L. Inore. Cham 1967.6998 . . 8. C h e n , ~ . J.-J.: M e ~ o ~ a l d , J. W.:~ewton, W E. hrorg Chem 1976,15,2612. 9. (a1 Barral, R.: Bocard, C.: Seree de Rah, I.; SaJus, L. 72tmhsdron Lett 1972, 1693.

(bl Balral, R.: B a d C.; Seree de Roch, I.; Sajus, L. Kine'. Coloi. iEngl. Trans.) 1979 ld i?" . . . . , . . , . . . .

10. Reynolds, M. S.: Berg, J.M.; Holm, R. H. Inorg Chem 1984.23,3057. 11. in) Ricard. L : Estienne, J.; Koraglannids. P: Taledano, E: Fiseher, J ; Mitsch1er.A.;

Weirs. R. J. Cnoid. C h i . 1914.3, 277. tbi Berg, J.; Hodgson, K 0. in or^ Chem. 1980,19,2180.

12. Dirand, J.:Rieard. L.; Weirr,R. J. Chem Soc..DoIton Pans 1976,278. 13. Y m . X F.;Young,C. GAuar. J. Chem 1991,44.361. 14. Newton. W E.: McDonald, J. W J. las+Commort Mstols 1977.54, 51.

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