Sunlight Photochemistry The Preparation of Dicarbonyl(y5-methylcyclopentadienyl) triphenylphosphinemanganese David C. Calabro and Dennis L. Lichtenbergerl University of Arizona, Tuscon, AZ 85721 The recent growth of interest in solar energy lends increasing significance to solar-activated chemistry. One example is photoelectrochemistry, which is attracting much attention ( I ) . Another example is photoactivated chemical synthesis, where solar radiation may be employed to assist and carry out chemical reactions. Although many photochemical reactions are known, few are carried out using sunlight, partly because of the low intensity of atmospherically filtered radiation in the useful wavelength ranges. We have ~erformed a previously reported pholor'hrmical ligand substitution reaction using sunlight activation and have found the reaction t o be very well s&ed for an undergraduate laboratory experiment. The reaction involves the photosubstitution of PPh3 for CO in CH3C6H4Mn(C0)3. I t provides a novel, straightforward demonstration of photochemistry and some basic mechanisms of transition metal reactions. The reaction can be done with ordinary glassware, uses inexpensive starting materials, and can be conveniently followed by infrared spectroscopy. The reaction time can be varied from 1-2 hr to several days with workable yields a t both extremes. The product is a stable, crystalline solid which precipitates out of the reaction mixture and can he recrystallized to high purity. Background Transition metal carbonyl complexes serve as common starting materials for many inorianic and org~nomrt~llic reactions. The substitution of ;I coordinated carhonyl ligand can he induced hy the photochemit:i~lclvavage of an bl -CO bond (1).
Irradiation of M(CO), in a non-coordinating solvent in the nresence of the incomine lieand .. L oroceeds hv an assumed courdinatively unqarurated, tnlniition state. IM~COI.-II~, which readily reacts with I. to give rhe substitution prodnct (2).
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' Author to whom correspondence should be addressed.
686
Journal of Chemical Education
oil bubbler
I Erlenmeyer flask 7
To increase yields and decrease photodecomposition, the reaction is frequently carried out in a weakly-coordinating solvent such as T H F or diethyl ether. In these solvents the transition state is stabilized to give an observable M(CO),-l (solvent) intermediate. The addition of L then gives the product by the facile replacement of the coordinated solvent molecule (3).
The compound tricarbonylmetbylcyclopentadienylmanganese, which has been widely used as an octane-booster in unleaded gasoline, undergoes extensive carbonyl photosubstitution reactions to give CH3C5HaMn(C0)2Lwhere L =
Reaction Conditions, Products, and Yields Irradiation Time
Sunlight Exposure Procedure A (direct;under Np) Procedure B (window:under air) Procedure B (window;under N2) Procedure B (window; under N2)
Product
Color
"CO
2 hr
CW.H4Mn(COj2PPh3
Yellow
1930, 1861
4 days
CH~C~~M~(CO)ZPP~~
Yellow
1930.1861
4 days
CH&~,M~(CO)~PP~~
Yellow
1930, 1861
55 %
4 days
CHsCdbMn(COXPPh&
Red
1836
19%
olefins, amines, phosphines, S02, etc., by the two routes indicated in reaction (4) (2,3).
When L equals triphenylphosphine, a convenient introduction to photochemical synthetic techniques results. Two optional approaches are described below. Experimental
Although this reaction was first carried out in a Hanoviatype reactor (4),we have obtained improved yields with ordinary Pyrexmglassware. The reaction can be carried out in several ways depending on the desired reaction time. The shortest irradiation time (2.2% hr; Procedure A) was obtained by setting the reaction in direct sunlight. The reaction may be carried out for days, or as long as a week by simply placing the reaction mixture by a window (Procedure B).Satisfactory yields can be obtained with or without inert atmosphere techniques. In air considerably more decomposition occurs, while a greater tendency to form the disuhstituted product, CH3CsHaMn(CO)(PPh3)2was observed in the absence of air (5).Tricarbonylmethylcyclopentadienylmanganese was obtained from Ethvl Cor~orationand was distilled prior to use. Reagent grade hexan; was used without any fuither purification. Required Chemicals and Equipment: methylcyclopentadienyl manganese tricarbonyl triphenylphosphine nitrogen bubblers hexane scblenk vessel (optional) 500 ml Erlenmeyer flask filtration flask (and water aspirator) Procedure A A 500-mlErlcnmeypr flask was charged with 200 ml reagent-grade hexane, 1.5 ml (9.59 mmoles, of (:H:CsH4Mn(CO)xand 3.00 g ( I 1.32 mmdesi ~riohenvlohosohin~. The vrllow solution was shaken until the ohosohke di&olved After Nn was streamed throueh the flask for 2-3 min, it was placed in direct sunlight. 'The incrmbingly yell~,wre orange solution became cluudy and contained a fluffy p r ~ i p ~ r aafter 35 min. Periodically monitoring the 111
