Organometallics 1986, 5, 387-388
387
Formation of Adducts of Molecular Hydrogen and Dicarbonyl(~5-cyclopentadlenyl)hydridomolybdenum In Low-Temperature Matrices Ray L. Sweany Department of Chemistry, University of New Orleans New Orleans, Louisiana 70 148 Received June 7, 1985
Summary: Dihydrogen reacts with photolytically generated HMoCp(CO), to form cis and trans dihydrogen adducts in argon matrices. In addition to adduct formation, hydrogen exchanges for deuterium during photolysis in the parent molecule, presumably via the intermediency of the D, adduct.
Hydrogen has shown a remarkable reactivity toward coordinatively unsaturated transition-metal complexes in inert-gas matrices. Several complexes have oxidatively added hydrogen1v2 whereas others give complexes of dihydrogen with the H-H bond intacte3 This novel mode of attachment is fairly resilient; pentacarbonyl(dihydrogen)chromium, is stable at temperatures as high as 238 K in liquid Xe4 and the hydrogen adduct of bis(ph0sphine)tricarbonyltungsten is stable at room temperature in hydrogen atmospheres and has been characterized by X-ray and neutron diffraction method^.^ Recently, even a stable iridium(II1) complex of dihydrogen has been detected by NMR.6 Presuming that a more electron-rich group 6 metal center might oxidatively add hydrogen, H M o C ~ ( C O )was ~ photolyzed in hydrogencontaining matrices. Here, I report the formation of cis and trans dihydrogen adducts of HMoCp(CO),, formed by the photolysis of H M o C ~ ( C O in ) ~ hydrogen-containing matrices. Prolonged photolysis of these complexes gives evidence for exchange of the hydride ligand with one of the hydrogen atoms of the Hz moiety. The photochemistry of HMoCp(CO), has been ably described by Rest and c o - ~ o r k e r s . ~In matrices which have been doped with up to 20 mol% hydrogen, new bands appear upon ultraviolet irradiation along with the bands due to HMoCp(CO),. (See Figure 1.) When the matrix is exposed to the visible light of the glower the new bands gain additional intensity as the bands due to HMoCp(CO), become attenuated. The new bands belong to two sets of two. The first set includes bands at 2000 and 1930 cm-'. The second set include bands at 1987 and 1920 cm-l. The position of the lower energy band of either set is shifted 2 cm-' to longer wavelengths when D2 is substituted for H, in the matrix. The bands of the two sets can be differentiated by their behavior in response to irradiation by a medium-pressure mercury lamp filtered by cobalt glass. The bands of set I1 become attenuated whereas the bands of set I are barely affected. The bands of set I are assigned (1) Sweany, R. L. J. Am. Chem. Soc. 1981,103, 2410. (2) Sweany, R. L. J. Am. Chem. Soc. 1982,104,3739. Hydrogen has also oxidatively added to naked metal atoms: Ozin, G. A.; Mitchell, S. A.; Prieto-Garcia, J. Angew. Chem., Int. Ed. Engl. 1982,21, 380. Ozin, G. A.; Gracie, C. J . Phys. Chem. 1984,88, 643. Ozin, G. A,; McCaffrey, M. G. Ibid. 1984, 88, 645. (3) Sweany, R. L. J . Am. Chem. Soc. 1985, 107, 2374. (4) Upmacis, R. K.; Gadd, G. E.; Poliakoff, M.; Simpson, M. B.; Turner, J. J.; Whyman, R.; Simpson, A. F. J. Chem. Soc., Chem. Commun. 1985, 27. (5) Kubas, G. J.; Ryan, R. R.; Swanson, B. I.; Vergamini, P. J.; Wasserman, H. J. J. Am. Chem. SOC.1984,106,451. (6) Crabtree, R. H.; Lavin, M. J.Chem. Soc., Chem. Commun. 1985, 794. . ._.
(7) Mahmoud, K.A.; Rest, A. J.; Alt, H. G.J. Chem. Soc., Dalton Trans. 1984, 187.
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Figure 1. Spectrum of HMoC~(CO)~ which has been photolyzed for 2.5 h with a low-pressuremercury lamp in the presence of 15 mol % D,. Bands which are marked by Arabic numbers refer to the number of carbonyls on a complex which does not contain D2.Thus, 3 refers to HMoCp(CO),. Bands marked by I are assigned to trans-HMoCp(CO)2(D2)while bands marked by I1 are due to the cis isomer. The spectrum which has been traced by a dashed l i e is that of photolyzed HMoCp(CO), in argon. The abscissa scale of the original spectra changes at 1990 cm-' from 20 cm-'/in to 25 cm-l/in.
to trun~-HMoCp(C0)~(H~) and the bands of set I1 are assigned to the cis isomer. There are several reasons for claiming that both sets of bands are due to species that have reacted with one molecule of hydrogen. The new bands grow in as the bands due to HMoCp(CO), become attenuated, yet they do not appear at all in the absence of dihydrogen. In the presence of 7 mol % CO, both sets of bands appear in the same ratio of intensity as they are found in the absence of extra CO. This is not the expected behavior if the loss of more than one CO was required to form one of the species. In the presence of 13 mol 90CO, no significant intensity is observed for either set. The simplest explanation is to assume that HMoCp(CO), reacts with H2 or CO, the choice of which is determined by the availability of the incoming ligand to the vacant coordination site, as well as by the stability of the resulting adduct under photolytic conditions. If the hydrogen simply substitutes for CO in a complex that retains the gross structure of the parent, two isomers are expected, cis and trans. The relative intensity of the two bands within the set will be most nearly equal for the cis isomer.' It is unlikely that the hydrogen has
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reacted with the cyclopentadienyl ligand. If hydrogen were being oxidatively added to the metal and then transferred to the ring, or if the ring were being reduced directly, the presence of CO in the matrix would produce a variety of new compounds with absorptions at higher energy than what are observed because four or possibly more carbonyls would be competing for the same x-electron density. The mode of the interaction of hydrogen with the metal can be deduced from the comparison of the frequency of 0 1986 American Chemical Society
Organometallics 1986, 5, 388-390
388
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of the molecules have not been detected.'l The regions of the spectrum between 700 and 400 cm-l of the parent molecule, HMoCp(CO),, contain many of the M-CO stretching and deformation modes, and they are quite sensitive to the isotope of hydrogen. When HMoCp(CO), is photolyzed in D2-containingmatrices, the spectrum gives evidence of slow isotope exchange in the parent molecule as shown in Figure 2. The bands of the deuterium isotopomer that are marked by D are, in two instances, quite well resolved from the correlated bands of HMoCp(CO),, and over periods of several hours of photolysis there is clear evidence of the formation of DMoCp(CO),, especially in the presence of 7 mol % CO. The extra CO increases the probability that H M O C ~ ( C Owill ) ~ react with CO to give back starting material but is not so concentrated that no adduct forms. In the presence of 13 mol % CO no significant quantities of adduct forms and no H-D exchange is noted. This precludes the possibility that the exchange is due to reactions of hydrogen atoms.'J2 Registry No. HMoC~(CO)~, 85150-17-0; HMoCp(CO),, 12176-06-6;DMoCp(CO),, 79359-05-0.
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Figure 2. Spectra obtained during the photolysis of HMoCp(CO), in a matrix composed of 20 mol % D2 and 7 mol % CO in argon. Tracing a is of the unphotolyzed matrix. Tracing b was of a spectrum taken after 60 of irridation by a low-pressure mercury lamp. Tracing c was made of a spectrum taken after 75 additional minutes of photolysis, using a medium pressure mercury lamp. The spectrum was recorded with a 2X ordinate expansion. Bands marked by D show the positions of absorptions which are assignable to D M o C ~ ( C O The ) ~ shoulder on the high energy side of the band at 655 cm-' is due to COz. The species to which the band at 630 cm-' belongs is unknown.
carbonyl stretching vibrations of the hydrogen-containing moiety with those of H M O C ~ ( C O )If~ .the hydrogen had oxidatively added, it is reasonable to expect a fairly dramatic blue shift of the carbonyl modes. For example, the oxidative addition of Hz to Fe(CO), results in a 42 cm-l shift in the totally symmetric breathing m0de.l This behavior has been rationalized by presuming hydride ligands are negatively charged and processes which involve the formation of the hydride from hydrogen atoms result in electron density being withdrawn from the metaL8l9 The dependability of this group property is the basis for the reliability of the force constant calculations by Timney.lo The symmetric stretching vibrations of the cis isomer is shifted by 13 cm-' to shorter wavelengths than the similar mode of H M O C ~ ( C O )short ~ : of the shift observed for Fe(CO), and considerably larger than the 3 cm-' shift observed for Cr(CO)5.3 Taken at face value, these comparisons suggest an interaction which comes closer to being described as oxidative addition than what is exhibited by H2CrCO),. A full vibrational characterization of these species has not been possible because the new species are themselves photolyzed by the radiation which is used to create them. Thus, adequate infrared intensity has only been achieved for the carbonyl modes; the largest optical density of any of the adduct modes that has been achieved thus far is 1.3 0.d. With so little adduct formed in these experiments, other modes which are important for the characterization (8) Sweany, R. L.; Owens, J. W. J . Organomet. Chem. 1983,255,327.
(9) Photolyses of HWCp(CO), in hydrogen-containing matrices lead to the growth of bands which correlate t o those reported herein. In addition, bands grow in which are a t or to higher energy of 2060 cm-'. Work is proceeding to identify the species which are responsible for these bands. Consistent with this line of reasoning would be a claim that these species had resulted from the oxidative addition of hydrogen. (10) Timney, J. A. Inorg. Chem. 1979, 18,2502.
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(11) Unfortunately, both the M-H stretching and deformation modes of HMoCp(CO), are weak. Although the M-H stretching frequency has been observed, there have been no studies which have assigned the M-H deformation mode for this molecule or the analogous HWCP(CO)~. Davison, A.; McCleverty, J. A.; Wilkinson, G. J. Chem. SOC.1963,1133. Davidson, G.; Duce, D. A. J. Organomet. Chem. 1976, 120, 229. (12) The photolysis of HMoCp(CO), gives evidence of the homolytic cleavage of the Mo-H bond in C07and H2-containingmatrices. In solid deuterium, hot hydrogen atoms have been shown to abstract deuterium, although the reaction gives a 40% yield, a t best. The reaction is the function of the excess energy of the hydrogen atom and, in this study, the arobabilitv of the hvdrwen atom collidine with a deuterium molecule befdre it is thermalize& Miyazaki, T.; Tsu;uta, H.; Fueki, K. J . Phys. Chem. 1983,87, 1611.
Pentacoordlnated Slllcon Hydrides: Very Hlgh Afflnlty of the SI-H Bond for the Equatorial Position Claire Brellire, Francis CarrO, Robert J. P. Corriu, Monique Poirier, and GCard Royo Znstitut de Chimie Fine -Hgtgrochimie et aminoacides -UA 1097 Universitg des Sciences et Techniques du Languedoc 34060 Montpellier Cgdex, France Received July 16, 1985
Summary: An X-ray investigation of two pentacoordinated silicon hydrides shows that the Si-H bonds occupy equatorial positions in the trigonal-bipyramidal structures.
It is well-known that nucleophilic substitution at silicon takes place with either retention or inversion of configuration according to the nature of the leaving group.' In a previous paper, we have reported a scale of apicophilicity deduced from the study of pentacoordinate silicon comligand is pounds such as 1 in which the o-Me2NCH2C6H4 intramolecularly bonded to the silicon atom.2
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(1) Corriu, R. J. P.; Guerin, C.; Moreau, J. J. E. Top. Stereochem. 1984, 15, 43 and references therein.
0 1986 American Chemical Society
