Preparation and Reaction An Experiment in Organometallic Photochemistry David M. Manuta and Alistair J. Lees' State University of New York at Binghamton, Binghamton, NY 13901 At the present time there are relatively few experiments in the undergraduate curriculum t h a t expose students t o projects in organometallic chemistry or photochemistry. T h e experiment described herein meets both these goals and is suitable for third and fourth year chemistry majors. This project involves (1) synthesis of Mo(C0)4(2,2'-bpy) by photolysis of a solution containing the parent hexacarhnnyl and 22-bipyridine (2,2'-hpy), (2) spectroscopic identification of the product, and (3) a preliminary investigation of the phot o c h e m i s t r y of t h i s m o l e c u l e . T h e s t r u c t u r e of M0(C0)~(2,2'-hpy)is shown in Fig. 1.
purging (gentle Np stream) or constant stirring. Upon irradiation the solution turns violet (indicative of product) before clearing as precipitate forms. It is not necessary to use quartz vessels for this experiment. It is important to use a nonpolar solvent medium to aid precipitation of Mo(CO)4(2,2'-hpy)and therefore to prevent further photoreaction from taking place. Coaling in an ice bath for 15 min after photolysis helps to precipitate the product. The orange solid is collected hy suction filtration and purified by repeated washings with hexane. Typical yields are 80%;lower values indicate insufficient photolysis. The filtrate can, however, be returned to the reaction vessel and the procedure repeated. Safety Notes:The irradiation portion of the procedure should be performed in the hood. Throughout photolysis the Hg lamp must be kept cool (either by air or water circulation) to avoid contact of solvent vapors with a hat lamp housing. In this respect the reaction vesselshould be keptatleast 6 in. from the lamp. It isstressed that it is harmful to look at Hg radiation, and the lamp should be screened from direct view. Infrared Spectroscopy Infrared spectroscopy is an extremely useful method to ohtain structural information of metal earbonyl complexes ( 6 ) .The speetrum obtained from a saturated solution of Mo(CO)&,2'-hpy) in chloroform is shown in Figure 2. This spectrum was recorded on a Perkin-Elmer 283B spectrometer usingliquid cells with 1-mm spac-
Figure 1. The structure of M0(CO)~(2,2'-bpy).
The following reactions illustrate t h e photochemical route to Mo(CO)&,2'-bpy):
Ir has been shown pre\.iously that the quantum efficiency of phorodissoriarion of ('0 troni Mu(COl; in solurion (resrriun 11i i unir) 11.21and that with suffirient r o d i n a t i n n linnnd present, t h e unsaturated Mo(C0)s intermediate is &venged (reaction 2) a t approximately diffusion-controlled rates ( 3 , 4 ) .Recent studies have shown t h a t the lifetime of the monodentate M0(CO)~(2,2'-hpy) intermediate is also short and t h a t the complex chelates rapidly losing a further CO molecule (reaction 3) (5). Experimental Procedure
Preparation of Mo(CO)&,Z'-bpy) A hexane solution (30 mL) containing Mo(COIs (0.017 mmol) and 2,2'-bpy (0.017 mmol) is de-oxygenated (gentle NI-purging for 10 min) in a 100-mL round-bottom flask. The solution is irradiated with a 200 W Hg lamp for approximately 30 min with continued
' Author to whom correspondence should be addressed.
-
02200
2000
1900
1800
Frequency, cm-'
Figure 2. Infrared spectrum of the C - 0 stretching region of Ma(CO),(2,P-bpy) in chloroform.
Volume 64 Number 7
July 1967
637
-
........
-
-
-
0.1 -
-
4 0.2
Wavelength, nm Figure 3. C-0 sfretching mcdes of Mo(CO)d2,2'-bpyI.
...
.) diFigure 4. Electronic absorption spectra of Mo(C0)&.2'-bpy) in (. methylformamide.(-) tetrahydrofuran,and (-- - - -) diethyl ether.
ers. Spectra obtained from Nujol mulls were very similar. The CO groups of M0(C0)~(2,2'-bpy)are arranged in a Cn,,-typesymmetry, and from a simole .. erouo theory analvsis it can be shown that these give rise to four fundamental C-0 vibrations; see Figure 3 (7-9). These A;", BI, Af" and Bz modes correspond to the vibrations observed in the spectrum s t 2009 cm-', 1910 cm-', 1882 cm-', and 1832 em-', respectively. The weak vibration centered a t 1970 cm-' is due to a small amount of unreacted henacarhonyl in the product.
.
.
Electronic Absorption Spectroscopy Figure 4 illustrates absorption spectra of M0(C0)~(2,2'-bpy)in various solvents. These spectra were recorded on a Hewlett-Packard 8450A speetrophatometer. The most intense absorption, centered in the 440-500 nm region, is a metal to ligand charge-transfer transition (10) responsible for the compound's deep color. Higher energy ahsorptions, centered in the 340-400 nm region, are d d transitions. The metal to ligand charge-transfer transition is markedly solvent dependent, its maximum shifting to longer wavelengths in less polar solvents (in hexane the maximum is at 554 nm, giving the violet appearance observed in the synthesis) (11). The red shift of the MLCT transition as the solution becomes less polar implies that the excited state dipole moment is less then that of the ground state.
-
Wavelength, nm Figure 5. Absorption spectral sequence observed upon sunlight exposure of a 8 x 10-5 M solution of M0(CO)~(2,2'-bpy) in toluene containing M PPhs. Initial spectrum recorded before photolysis: successive spectra recorded at 5min intervals.
Summary
Photochemistrv
T h i s experiment is suitable for t h r e e laboratory periods: t h e first involving synthesis a n d purification, t h e second being t h e spectroscopic characterization, a n d t h e t h i r d period for monitoring t h e photochemistry. Acknowledgment
M PPhs in A solution of 8 X 10-5 M Mo(C0)&2'-bpy) and toluene was filtered to remove any particulate matter. This solution was contained in a I-cm glass cuvette and placed in a beaker at a sunlit window. Absorption spectra recorded a t successive 5-min irradiation intervals are shown in Figure 5. The growth of the longwavelength absorption is characteristic of the Mo(C0)3(2,2'bpy)(PPh:ll product. lsosbestic points a t 444 nm and 538 nm are ohserved but are lastat >50% conversions, a t which point the photoproduct itseif begins to undergo photochemistry. Of course, in the nhsmwc of .u+hi t ? rommm occurrenrr in Bun+-hamton), mure mptd phr,tt,l\-i.-tr~nh r n c h w e d uith ihr IIg lamp l . a w phorol?s~a \.. = 1 1 4 nm, or > l ~ ~ t ( ' O r , ~ ? .. :.? ' -has l l n been \ ~ shoan virnlnlly tu elminate secondary photoreaetions, enablmg reaction 4 to be carrled out nearly to completion (12).
."
638
Journal
of Chemical Education
W e t h a n k A. J a m e s M a m a r v for his assistance in t h e synthesis a n d spectroscopic measurements a n d a reviewer for s o m e helpful comments. Literature Cited I . Sfmhmeier. w.:v"nHobe.n. C h e m Re,. ls61,94.2031. 2. Stnlhrneier. W. Anms. C h ~ m 1964,76,873. 1. Lees. A.J.: Adarnsnn. A. W. l n o w Chsm. l981,20,4881. 1. Kelly.,l. M.;l.nnp,C.; Bnnneau. R.J.Phyr Chrm. 19R1.87.3944. 6. Srhadt.M.J.:Gressifi. N. J.;Lees. A.J.lnore. Chem. 1985.242942 -n?bonyi Spectra: ~ c ~ d e r n iNew c : York. 1975. 1962. I . 25. enrhn>wmn . l . l ?hem Edur. 1970.47. 33.
.
,
.
.
11. M . : I . . ~ ~ . A . . I . I 1983.zi.382~. ~~~~c~~~. I?. RaIk. R. W.;Snoeck,T.:Shfkanr. D.J.:01knm.A. 1norp.Chrrn. 19RO.19.3015.
