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Synthesis of Dinitrogen and Dihydrogen Complexes of Molybdenum Leonard J. Archer,' T. Adrian G e ~ r g eand , ~ Mark E. Noble University of Nebraska-Lincoln, Lincoln, NE 68588 Since the first reoort of a stahle transition metal comolex containing dinitrogen (N2) as a ligand in 1965 (1), many such complexes have been prepared of almost all the transition metals (2). Of all these complexes, those of molyhdenum and tungsten are among the most stahle and easiest to work with and have shown the most diverse chemistry (2). This is particularly interesting since molybdenum is found in the enzyme nitmgenase, and its function in the enzyme remains a mystery. However, the preparation of these complexes was often lengthy and not suitable as an experiment for a standard undergraduate laboratory class. Recently, a one-step method has been developed for the preparation of a number of dinitrogen complexes of molybdenum and tungsten that we have used in our advanced inorganic laboratory class (3).The preparation that we report here is for trons-Mo(Nz)z(dppe)z,I (dppe = Ph2PCH2CHzPPhz) and is shown in eqn. (1).A mixture of molybdenum(V) chloride, dppe, and sodium amalgam in THI
MoCIs + 2 dppe + 5 NaIHg +Mo(N&(dppe)z Nn
+ 5NaCI
the two u(C0) hands a t 1853 and 1785 cm-' due to the thermodynamically more stable cis isomer (6). Comolex I1 mav he oreoared in hieh vield hv suhstitutine
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yellow complex displays hroad V ~ M ~ stretches H) a t 11830 and 1750 cm-'. The proton NMR mectrum in chlorobenzene four equivalent hidridic hydrogen atoms coupled to the four apparently equivalent phosphorus atoms (JPH= 30 HZ)
(7). Experimental SAFETY NOTE: CAUTION: Molybdenum(V) chloride reacts vigorouslv with water (and most oreanic solvents a t room temperature) forming corrosive a n d noxious hsdrozen chloride. Weiehine out of samoles can he done in
(1)
dry tetrahydrofuran (THF) is vigorously stirred under an atmosphere of dinitrogen. The better the quality of reagents, the drier the solvents and dinitrogen and the more anaerohic the reaction conditions, the easier is the work up and the higher the yield of product. Dinitrogen uptake is rapid and the extent of reaction can he monitored hy following the growth of the very strone u(NN) stretchine-freauencv . . a t 1980 cm-' in the i n f k e d sfictrum. This is most easily done by removing a few drous of solution from the reaction mixture with a svringe and-placing them on the surface of asalt plate. ~ o l v e n t is allowed to evaporate under dinitrogen and the resulting film is pressed against a similar salt plate. The infrared spectrum is then recorded between 2100 and 1800 cm-I. The solid state structure of I has heen determined by X-ray diffraction methods (4). The two Nz ligands are mutually trans and the two bidentate ditertiary organophosphine ligands occupy the remaining four coordination sites. The overall symmetry of the molecule is low hut can be considered to have D4h or D2h local symmetry. The antisymmetric iA2= in D4h) w(NN) stretch aooears a t e l 9 8 0 cm-I and the svmmetrv forbidden
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solid state but does decompose insolution in the presence of oxygen. Thoroughly deoxygenated solutions (e.g. benzene or THF) of compound 1are stable for up to 6 hr under dinitrogen. Compound 1is a useful starting material for the formation of MoH4(dppeI2, I1 and cis-Mo(CO)z(dppe)z,111; eqns. (2) and (3). Merely bubbling dihydrogen or carhon monoxide through a benzene solution of I will produce
the corresponding eight coordinate tetrahydride 11, and 111, respectively. Initially, carbon monoxide replaces dinitrogen to form trans-Mo(CO)z(dppe)z which slowly isomerizes to 111. The reaction can he monitored by infrared spectroscopy showing the decrease of w(NN) as u(C0) a t 1820 cm-', due to the trans isomer, increases. With time, this band gives way to
in a dessicator. Preparation of M ~ ( N ~ ) ~ ( d p p e ) ~ . ~ A dry two- or three-necked, 500-mL round-bottomed flask with a magnetic stirring bar was flushed withdinitrogen. Whilemaintaining a flow of dinitrogen, 17osodium amalgam (425g;185mmolof Na) was added followed by 250 ml, of dry, freshly distilled THF4 and dppe (8.81 g, 22.1 mmole). MolybdenumiV) chloride (2.52 g, 9.22 mmol) was then added all at one time5 to produce a brown suspension. A dinitrogen atmosphere was maintained during the reaction. After stirring 16 hr6 the THF solution was decanted,filtered aver Celite,7 and washed through with 200 mL of dry benzene.'Solvent was then removed using a rotary evaporatorHThe orange-brown residue was
' Present Address: Missouri Western State College. St. Joseph, MO 64507. ~
~~
Author to whom correspondence should be addressed. The reaction and work up can be carried out satisfactorilyusing a polyethylene glove bag with a dinitrogen atmosphere.Excellent results have been achieved carrying out the reaction on a smaller scale e.g. 1.5 mmol of MoCI,. It is suggested that sodium amalgam should be added first since occasionailv the sodium amalqam has t r a ~ ~ eMoCIS d on the bottom of the flask where it does not unGergo the desired reaction. ' Solvents snodd oe dnsl 1 eo "rider a 0 n trogen nlmosphere and lullher oeaerateo oy buool ng o8n lrogen Inro-gn mem for abo.1 ', nr before .se THF was dlsl I eo from so0 .m benrophenone kety whole benzene was distilled from CaH2. The vessel containing the weighed MoC15 must not be rinsed out with THF and added to the reaction mixture. Under favorable conditions, the reaction may be complete after 2-3 hr. However, it is advisable to monitor the extent of reaction (IR) before working up the reaction after such a short time period. ' Alternatively, the solution can be decanted from the amalgam and stripped to dryness using a rotary evaporator. The residue is dissolved in benzene and filteredthrough Ceiite and product precipitated by addition of methanol. We thank Professor A. E. Finholt of St. Olaf College for this procedure since lhe initial filtration can take a long time. Toluene would be used in the tetrahydride preparation. Use of a water aspirator is satisfactory.However, the yield of 1 is maximized when oxygen and water are totally excluded during the work-up procedure. Volume 58
Number 9
September 1981
727
dissolved in 150 mL of benzene, followed again by Celite filtration and rinsing with 100 mL of henzene. Product was precipitated by the addition of 400 m L of methanol, collected hy filtration, washed with methanol, and dried in vacuo. The bright orange product weighed 4.60 g (53% yield). A possible impurity is yellow MoH,(dppe)?. If present, addition of too large of avolume of methanol will cause it t o precipitate also. Compound I may be recrystallized from benzene-methanol solution.
Preparation of MoH4(dppe)~ (8) T h e reaction was set up analogous to the preparation of1 using the followingquantities: dppe(1.45 g, 3.64 mmol), T H F (SO mL), MoCI6 (0.41 e. -, 152 m m d ) . 1% sodium amaleam (70 e. ... 30 mmol of Na). and dihydrogen atmosphere. Stirring was maintained for 10 hr" after which time the brawu solution was decanted and filtered through Celite.' The residue was washed with dry henzene (100 mL). Solvent was removed using a rotary evaporator. Toluene (SO mL) was added and the solution filtered. Addition of methanol (120 mL) t o the orange-hmwn filtrate caused precipitation of 0.61 g (45%) of the bright yellow product, 11, which may be recrystallized from 1:l chlorobenzene-ethanol solution. Preparation of ci~-MdC0)2(dppe)~ (9). This compound is prepared by bubbling CO through a stirred henzene, T H F or toluene solution of Ma(Nn)n(dppe)zeither a t room temperature ( > 9 hr) or heated a t reflux (1 hr). T h e yellow product, which is less soluhle than its dioitrogen analog, can be cryslallized
728
Journal of Chemical Education
from a hot benzene solution by slow addition of methanol. TransMo(CO)l(dppe)r which cannot he prepared pure is orange. T h e tetrahydride, 11, can be prepared simiiarly from MoIN&(dppeh by using H z ( 8 ) . Cis-M~(CO)~(dppe):, can be oxidized readily either hy using iodine in dichloromethane to give the red triiodide salt t r o , w / M o (C0)2(dppe)z]I:,(10) or by adding perchlorir acid and air t o a T H F solution o f 3 t o give salmon~pinktrans-[M~(CO)z(dppe)~](:lOd(11); d C O ) 1865 cm-'.
Acknowledgment
The authors are indebted to the National Science Foundation (Grant No. CHE76-80878) for support of this work. Thanks are expressed for assistance from J. A. Baumann, G. 1.:.Bmsard, and C. M. Fendrick of our laboratory. Literature Cited !I! Ailen, A. D.and Sennil, C. Y., Chem Commun.. GZL IlYiibi. 12) Chatf,:.. 1lilworth.J. R.. nnd Richards, H.L., Chrm Ilrr, , i n , 589 (19781 1:31 1:oores. T. A. and Noble. M. F... lnnrE. C h ~ r nl.l . 1 6 7 8 119781. 141 Uchida, Y Uchida.T.. Hidai. M.. and Kdsmr.T.. Acla Crystdlu#r .Sect. 8.31.1197 11975). (51 GeorKo.T. A.andSeiboid. C. D..lnlrip. C h ~ m12,264411973). , iGi Wlmmer, F. L., Soow. M. %and H o d A. M..Innri. Chrm.. IS, 1817 (1878). 171 Meakin P., Guggenkrger. L. J., P e t , W. G.. Muttenics. E. L.. and Jes.on.i P.. J A m i r Chpm S o ? . 9E.1667 (1973). 181 Archer,L. J andGeoige.T.A.,inon. Chem 18,2079 119791. 191 Ge0rpe.T. A. andSeibo1d.C. D . . l n a i ~ .Chom. lZ,254R!197:11. (LO1 Lewis. ,I. and Whyman, R . , J Chnm. S o c . 6486 (19651. ( 1 1 1 C-ing, 8'. F. end Snuw. M R.. J Chrm Six ,A,b10(1971!.
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