Synthesis and Characterization OF Wnuclear Tungsten Carbonyl Anions Tim~thyJ. ~ c ~ e e s e ' Loyc'a College. Baltimore, MD 21210 T h purpose of the synthesis and characterization of the co~r:.ounds discussed here is to introduce upper level inorga chemistry students t o certain fundamental principles slI $oratorytechniques of organometallic chemistry. ~g the most general reaction sequences of the group 6 t: ion metal hexacarhonyls are those in which CO g! are rlisplaced by other ligands, resultingin complexes th; nay then undergo further reactions. For example, an impurtant intermediate in the synthesis of tungsten carbonyl derivatives is W(C0)3(CH3CN)3. This complex, prepared from W(C0)6 by a CO-substitution reaction, contains labile acetonitrile ligands (I). Tungsten carbonyl aryloxide complexes are valuahle model syztems for the study of low-valent group 6 transitionmet:)' dkoxides. The im~ortanceof these alkoxides as interrn. es in catalytic processes such as methanol carbonylatn.. and the hvdroeenation of aldehvdes and ketones (3) hzi .en established: Unfortunately, -the alkoxides ha"; bet lifficult to characterize because of their instability rela! ive to formation of tungsten hydride analogs via a 8hydride elimination process. The metal carbonyl aryloxide derivatives, however, can be readily synthesized, and an interesting series of interconversions between mono- and polynucle& tungsten complexes has been reported that also illu3trate;. a number of the various bonding modes of the phenoxo !igand (4). In this experiment a dinuclear tungsten carbonyl anion c o o h m i n g bridging phenoxo ligands, [ W 2 ( C 0 ) 6 ( ~ OC.iJ;)3]3-, is prepared by a substitution reaction from W(C':I 13(CH3CN)3and phenoxide ion, OCsH5- (4,5). Reac-
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Journal of Chemical Education
tion of this trianion with allyl bromide yields another dinuclear anion, [WZ(CO)~(?~-C~H~)~(P-OC~H~)~]-, by an oxidative addition process. Both complex ions are isolated as their EtaN+salts. The structure of [Wz(CO)s(p-OCsHs)3I3- has recently been confirmed by X-ray diffraction (4), while that of [Wz(C0)4(q3-C3Hs)z(r-OCsH5)3]is proposed on spectroscopic evidence. Each tungsten atom of these dimers possesses a closed-shell electron configuration and neither complex contains a W-W bond. The number of CO ligands and the structural arrangement of the metal carbonyl fragment within each complex ion are deduced from infrared spectral data (6). The chemical formula of both compounds is consistent with the IR spectral data, proton NMR integration ratios, electron counting, and reactant stoichiometry. A variety of inert atm o s ~ h e r emethods. such as Schlenk technioues. dove box opeiations, and drisolvent distillations are requkid for the ~ r e ~ a r a t i oand n handline of these air-sensitive com~ounds i 7 L ' ~ h eentire experiment can be completed by students in three 4-h laboratow periods. Two additional 30-min time periods a r e required if t h e reaction mixture of t h e W(C0)3(CH&N)3 preparation is periodically monitored by infrared spectroscopy over a two-day period. Alternatively, the instructor can record the IRspectra over this time period and present the data to the students for subsequent analysis. Experimental
Caution: All solvents and reagents should be handled with care in an efficientfume hwd, and gloves should he worn during all trans. fers. General Methods
Manipulations involving air-sensitive compounds and solvents were performed with Schlenk techniques under an argon atmo-
sphere in an efficient hood or in a Vacuum Atmospheres inert atmosphere chamber under prepurified argon. Acetonitrile was dried over calcium hydride, methanol over molecular sieves, and tetrshydrofuran over lithium aluminum hydride hefare distillation under prepurified nitrogen. Solvents were further deoxygensted with argon prior to use. IR samples were prepared in acetonitrile solution and loaded in a Barnes Analytical cell containing NaCl windows and a 0.1 mm-path length spacer. IR spectra were obtained with a Perkin-Elmer 283 spectrometer. 'H NMR spectra were recorded with Varian HFT-80 or EM-360 spectrometers. NMR samples were prepared in CDsCN solution and data are listed in parts per million relative to TMS. Tungsten hexacarhonyl was purchased from Pressure Chemical Co.; acetonitrile, ally1 bromide, methanol, phenol, and tetraethylammonium hydroxide (25% (wlw) in methanol) were purchased from Aldrich Chemical Co. Phenol was purified by vaeuum sublimation prior to use. Neutral alumina, Brockman activity I, 8&2W mesh was purchased from Fiseher Scientific and dried in an oven at 120 "C before use. Tetraethylammonium phenoxide (81, W(C0)dCHsCN)s ( I ) , and IEt~Nla[W~(C0)fi(fi-OCsH)sl (4,5)were synthesized by modifications of published methods. Elemental analyses were performed hy Schwarzkopf Microanalytical Laboratories.
Results and Discussion T h e tungsten carbonyl complexes were prepared according to t h e reaction sequence shown in Figure 1.IR spectroscopy was tested to monitor t h e extent of stepwise CO substitution during t h e synthesis of W(C0)3(CH3CN)3 from W(C0)6 b y comparing t h e carbonyl band pattern of t h e reaction mixture t o those of a series of W(CO)s.,(CH3CN), ( x = 1-3) derivatives (9). T h e spectrum of the product, W(C0)3(CH3CN)3,contains two strong absorption bands in the carbonyl region of relative intensity -1:2 a t 1917 and 1794 cm-', respectively. This pattern represents fac configuration of CO ligands bound t o the metal with C3" symmetry of the W(C0)3 fragment.
[ WC0)3(cH~CN)31 Approximately 50 ml. of aretonitrile was added via cannula u, s 100-mL Schlenk tube containing W(COji (1.0 g, 2.8 mmol! and a stirbar. The Schlenk rube wan then fitted with a reflur condenser and stopcock, and the reaction mixture was refluxed under argon with stirring for 2 days. During this time, the solution developed a bright vellow color. Tbeeatenr of subsritution. rndicated by the C 0 baid nittern of the reaction mixture..was . neriodicallv monitored bv 1R an;ctrosconv. samnles were orenared hvdilutian of 0.i~ ~ Suitable -~~~~~~~~ ~ = , ~~. m~ali~uutaufreaction mirrurp with CH .c
