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 chemistry of the early and middle transition metals is dominated by 0x0 complexes (3).Diverse and important, oxomolybdenum chemistry is particularly instructive and relevant in undergraduate inorganic chemistry (4). I t forms the basis of this undergraduate experiment. stabilizes high-valent metThe 0x0 lieand. , formallv 02-. al centers by both o-and ;-bask interactions and is susceptible to both nucleo~hilicand electro~hilicattack. Nncleophilic attack by phkphine on a d i o x o - ~ o (center ~ ~ ) leads to oxygen atom transfer (51, two-electron reduction of molybdenum and concomitant oxidation of phosphorus (eq 1).
-
[ ~ 0 ~ ' 0+~:PR, 1 ~+ + [ M O ~ O+ ]OPR, ~~
(1)
In a complementary reaction, oxygen atomdonors such a s S- andN-oxides, peroxides and dioxygen are capable of oxidizing 0x0-Mo(1V) complexes, thus generating dioxoMo(V1) species (e.g., eq 2). Under suitable conditions, coupling of reactions 1and 2 permits catalytic oxygen atom transfer processes to be realized (5).Electrophilic attack on a n 0x0 group is exemplified by simple protonation, as shown in eq 3; these reactions do not change the oxidation state of the molybdenum. I n the absence of other ligands, protonation is followed by condensation and polyoxomolyhdate formation (4). In the presence of other ligands, selective replacement of the 0x0 ligands may he promoted by protonation. Another prevalent reaction in oxomolybdenum chemistry is the comproportionation reaction shown in eq 4. Reactions 1 4 are exploited in the synthesis of innumerable Mo complexes (41, account for much of the chemistry t catalvtic redox of such comalexes (4). and are i m ~ o r t a nin reactions s;ch a s those mediated by industrial'oxidation catalysts (1)and the molybdoenzvmes sulfite oxidase, xanthine oxidase and nitrate reduccase (6). Comproportionation reaction 4 must he sterically prevented in realistic models of mononuclear oxomolybdenum enzyme centers
(6). This experiment involves the synthesis and characterization of several oxomolybdenum complexes containing the N,N- diethyldithiocarhamate ligand and the exploration of a n 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 e m ~ l o v e dto svnthesize the octahedral cis-dioxo-Mo(V1) c0&~1exe i s - ~ o ~ z ( ~ z ~(1) ~ (7). ~ t This z ) z is converted to t h e s q u a r e p y r a m i d a l 0x0-Mo(1V) complex
(YI
(81
The chemical reactions involved in this experiment. MoO(S2CNEtz)z [Redl and the dinuclear poxo-MOW)complex syn-MozOz(p-O)(SzCNEtz)4 [Purple], by reactions with PPhn- (c.f... ea. 1)(8).These three com~lexesare compon m t s of the now classlc catalytic oxygen :Itom transfer iys:em first d c w r i h d b\. B x r a l and co-workers (9.101. 'l'heir structures, or thosewofclosely related derivatives, have been determined by X-ray diffraction (11).Reaction of 1 with hydrochloric acid results in the formation of seven-coordinate, pentagonal hipyramidal cis-merMoOCIz(SzCNEtz)z Eellow] (12). If time permits, t h e 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 complexes 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 perform 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 provision 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 period with the simultaneous synthesis of P, R and Y. An Volume 72 Number 8 August 1995
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additional period i s generally required if B i s to be prepared 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.
C ~ S - ~ ~ ~ - M O ~ % C Z (Y) ~ ((121 S~CNE~~)~
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 solvents and propylene sulfide should be carried out i n a fume hood.
+
NaOH + NaS2CNEt2 + H20
(5)
Success depends on vigorous agitation or magnetic stirring during the addition of the hydrochloric acid. Diethylamine (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 period) with a solution of 6 mL of concentrated hvdrochloric acidin water (100 mL). Vigorous stirring is required during the dropwise addition; the dense yellow-brown product precipitates. The solid i s 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 remainder of the sample can be recrystallized by dissolving it in dichloromethane (15 m u g ) , 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(R) E ~(81 ~)~ This compound is moderately air-sensitive and all work should be nerformed anicklv and efficientlv. I n a small round-bottomed flask connected with a water or air condenser. a mixture of 1 (1.0 e, 2.3 mmol) and triphenvl. . phosphine I 1.0g , 3.8 mniol: thc excess alloursthe synthrsii to be ~ e r t o r m e din air. in I Z d i r h l o n ~ e t h e n~~l )~X" v ' C , 10 mL) is refluxed for 10-15 min. Ensure that reflux (in a preheated bath) is commenced immediatelv after adding the solvent to the starting materials. upon-completion of the reflux pour the reaction mixture, with swirling, into icecold ethanol (50 mL) contained i n 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,3J7.5 Hz, 4 x CH3),3.87 and 3.93(m, 8H, '5 15 Hz, 3~ 7.5 Hz, 4 x diastereotapie CH2).
752
Journal of Chemical Education
A solution of 1 (0.5 g, 1.2 mmol) i n dichloromethane (5 mL) is filtered through a fluted filter paper into a 25-mL Erlenmeyer flask, then the filtrate is treated with a solution 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 vacuum filtered, washed with methanol a n d dried a t t h e 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-'.
Experimental Procedure Preparations
HNEt, + CS,
S ~ ~ - M O ~ ~ O ~ ~ ~ - O )(P) ( S(81 ZCNE~Z)~
Asolution of crude 1(0.5 g, 1.2 mmol) in acetone (35 mL) i s filtered through a fluted filter paper, then the filtrate is treated with concentrated hydrochloric acid (2.5 mL, excess) and the mixture stirred for 20 min. The product is isolated bv filtration. washed with 10 mL of acetone and 0.39 g, 70%. If time permits, large dried a t t
