2410 Inorganic Chemistry, Vol. 17, No. 9, 1978
Gregory L. Geoffroy and Mark G. Bradley Contribution from the Department of Chemistry, The Pennsylvania State University, University Park, Pennsylvania 16802
Photochemistry of Transition Metal Hydride Complexes. 3. Photoinduced Elimination of Molecular Hydrogen from [ M O ( ~ ~ - C ~ H ~ ) ~ H ~ ] GREGORY L. GEOFFROY* and MARK G. BRADLEY
Received April 18, 1978 Ultraviolet irradiation of [Mo(q5-C5H5),H2]results in elimination of molecular hydrogen and generation of [ M O ( ~ ~ - C , H ~ ) ~ ] with a 366-nm quantum yield of 0.1 & 0.02. The reactive molybdenocene intermediate is readily trapped by CO and C2H2 to yield known adducts and by PEt, and PPh, to give the new tertiary phosphine complexes [Mo(q5-C5HJ2PEt3]and [ M o ( ~ ’ - C ~ H , ) ~ P P In ~ , ]the . absence of substrate an uncharacterized, presumably oligomeric [ M O ( ~ ~ - C ~ Hcomplex ~)~], results. Mechanistic experiments suggest that the photochemical reaction proceeds through concerted elimination of molecular hydrogen but that additional H2 is formed through a subsequent secondary reaction between [Mo($-C,H,),] and [Mo(~~-C,H,)~H,]. The nature of the active excited state which leads to H2 elimination is discussed in view of the electronic absorption spectrum of the complex and published molecular orbital calculations.
Introduction We have previously demonstrated that photoinduced elimination of molecular hydrogen from thermally stable diand trihydride complexes is a common reaction pathway. lM3 For example, [IrC1Hz(PPh3)3]and [RuHZ(CO)(PPh3),]do not lose hydrogen under thermal conditions but undergo efficient elimination of H z upon photolysis, eq 1 and 2.’,’ We H2 + [IrCl(PPh,),]
[IrCIH,(PPh,),]
(1)
[RuHz(CO)(PPh3)3l % Hz + [Ru(CO)(PPh3)31 (2) have now set out to demonstrate the generality of this photoreaction and especially its usefulness for the generation of reactive, coordinatively unsaturated intermediates. We have accordingly undertaken a study of the photochemistry of polyhydride complexes of the early transition metals and herein report the results of our investigation of [Mo(q5-C5H5),HZ] and [W(115-C5H5)zHz1. The latter complex has been previously studied by Green and co-workers who showed that UV irradiation of solutions of [W(q5-C5H5)2HZ] led to the formation of the insertion products [W(qS-C5H5),H(R)]and [W(q5-C5H5)z(R)2],in which R is derived from the solvent or from added substrate.&’ Irradiation of [W(q5-C5H5)2Hz] in benzene, for example, gave [W(q5-C5H5)2H(C6H5)],4 and in methanol [W(qsC5H5),H(OMe)] and [W(q5-C5H5)zMe(OMe)]6 were formed. The mechanisms of these reactions have not been worked out although it was recently noted7 that three possibilities should be considered for the initial photochemical step. Photolysis could lead to direct concerted elimination of H2 and generation of tungstenocene, eq 3, or irradiation could induce hydrogen migration to one of the C5H5rings to give a coordinated diene, eq 4. Alternately, photolysis could induce one of the C5H5 [M(?5-C~H5)2H21
-
H2 -t [M(q5-C5H5)21
(3)
hu
[M(11s-csH5)zHz1 [M(?Is-CsH5>H(CsH~)I (4) rings to slip to give an intermediate with an q3-C5H5,ligand, eq 5. Each of these three possible intermediates is a 16-
hv
[M(V5-C&5)zH21 [M(775-C~H~)(?13-C~H~)Hz1 (5) valence-electron complex that should be reactive enough to undergo the initial insertion reaction. No experiments designed to elucidate the actual photochemical mechanism have been reported, and the quantum efficiencies of the [W(q5-C5Hs)zH~] reactions have not been given. The photochemical properties of the corresponding carbonyl complexes have been studied by Brintzinger et aL8 who showed that irradiation leads to loss of carbon monoxide and transient 0020-1669/78/ 1317-2410$01 .OO/O
generation of molybdenocene and tungstenocene, eq 6.
hv
[ M ( V ~ - G H ~ > Z ( C O ) I [M(q5-c5H5)z1 + CO
(6)
Although the reactive metallocenes could not be isolated, they were readily trapped by irradiation in the presence of added substrate to yield adduct complexes. Photolysis of [MO(qsC5H5)2(CO)] in the absence of substrate led only to a poorly characterized material which was suggested to contain “polymeric” [ M O ( ~ ~ - C ~ H , ) , ] ,This . ~ - ~material ~ is similarly obtained upon reduction of [ M O ( ~ ~ - C ~ H with ~)~C~~] Na(Hg).9Jo Experimental Section The complexes [Mo(v5-C5HJ2H2], [Mo(q5-C&5)2D21, [W(q5C5HJ2H2], and [W(q5-C5H5)2D2]were prepared by published procedures.” Triphenylphosphine was obtained from Aldrich Chemical Co. and was recrystallized from benzene/ethanol before use. Triethylphosphine was obtained from Orgmet, Inc. Other chemicals were reagent grade and were used without further purification. All solvents employed were dried by standard methods and were rigorously degassed prior to use. Manipulations and reactions with air-sensitive compounds were carried out under purified N 2 or argon atmospheres unless otherwise indicated. Acetylene and carbon monoxide gases were obtained from Matheson Gas Products. The carbon monoxide was deoxygenated and dried by passing it through BASF BTS Katalysator and Linde 5-A molecular sieve columns. General Irradiation Procedures. Irradiations were conducted at 366 nm using a 450-W Hanovia medium-pressure Hg lamp equipped with Corning Glass 0-52 and 7-37 filters (I = 10” einstein/min), with a 100-W Blak-Ray BlOOA lamp equipped with a 366-nm narrow band-pass filter, or in a 350-nm Rayonet photoreactor. The complex to be studied was placed in an evacuable quartz UV cell or a Schlenk tube, and after degassing, the appropriate solvent was distilled onto the sample. Solutions for infrared studies were transferred in an inert-atmosphere glovebox to 0.5-mm NaCl solution infrared cells. Solutions were irradiated with the appropriate lamp and electronic and infrared spectra periodically recorded. Samples for ‘H and 31P N M R spectra were similarly prepared and the N M R tubes sealed under vacuum. Lamp intensities were measured by ferrioxalate actinometry.I2 The quantum yields of H2 loss from [Mo(q5-C5H5),H2] and [W(q5-C5H,)2H2]were determined by 366-nm irradiation ( I = 1.42 X 10” einstein/min) of multiple samples of thoroughly degassed hexane solutions placed in sealed UV cells. The reactions were monitored by measuring the decrease in intensity of the complexes’ characteristic 270-nm absorption bands. Since the products do not absorb appreciably at 270 nm and the conversions were limited to
