J . Am. Chem. SOC.1991, 113, 9155-9161
9155
Photochemical Reaction of Dinuclear Manganese Carbonyl Compounds with Tributyltin Hydride and with Silanes Richard J. Sullivan and Theodore L. Brown* Contribution from the School of Chemical Sciences, University of Illinois, Urbana-Champaign, Illinois 61801. Received February 27, 1991. Revised Manuscript Received July 19, 1991
Abstract: The photochemical reactions of Mn2(CO)8L2(L = CO, PMe,, P(n-Bu),, P(i-Pr)d with HSnBu, or HSiEt3 in hexane solutions have been studied, using 366- or 313-nm irradiation, and under CO or Ar atmospheres. Under CO, 1.1-3.7 atm, the products of the reaction of Mn2(CO)lowith HSnBu3 are HMn(CO)Sand Bu3SnMn(C0),. Under Ar or low CO pressures, a third product, assigned as HMn(C0)4(SnBu3)2,is formed at the expense of B U , S ~ M ~ ( C OFor ) ~ .a given photon flux, the reaction rate is inversely related to [CO]. The behavior of the system is consistent with a reaction pathway that involves oxidative addition of the hydride to the coordinatively unsaturated metal center formed upon CO loss. Analogous results are observed for the phosphine-substitutedmanganese carbonyl dimers. Reaction with HSiEt, proceeds much more slowly under equivalent conditions of irradiation. In the reaction with Mn2(CO)lo,only HMn(CO)Sis seen as a significant product, with trace amounts of Et3SiMn(C0)s also observed. These results are also consistent with oxidative addition to the Co-loss product as the only pathway for the photochemical reaction. None of the manganese dimers undergo photochemical reaction with either fluorene or triphenylmethane, in spite of the comparatively low C-H bond energy in each case.
through formation of bridging (Cp2Fe2(p2-C0)3)2-11 or semiThe photochemical properties of dinuclear metal carbonyl bridging (Mn2(C0)9)12CO linkages. Alternatively, or in addition compounds have been the subject of intense study during the past ~ -rearrangements mentioned, the dinuclear CO-loss inter20 years. For compounds such as Mn2(CO)lo,( v ~ - C ~ H ~ ) ~ Fto~the mediate may subsequently react with other substrates. (CO), and their variously substituted analogues, there are two In this and the following paper in this issue, we report on the primary photochemical processes, homolysis of the metal-metal results of studies of the photochemical reactions of dinuclear bond and CO loss.’v2 The former process gives rise to 17-electron manganese carbonyl compounds Mn2(C0)*L2(L = C O or Pradicals which may undergo a variety of reactions, including atom (alkyl),) with tributyltin hydride, triethylsilane, and a few other transfer, recombination, electron transfer, and Under appropriate conditions, these 17-electron species may add selected molecules in which atom transfer or oxidative addition reactions to the metal center are possibilities. The present cona 2-electron donor to become, at least formally, 19-electron in tribution concerns the results of continuous photolysis studies to ~haracter.~,’ determine the reaction products and to provide evidence regarding The focus for some years has been on the odd-electron products a plausible mechanism based on product distribution. The folof photolysis of the dinuclear species. However, the chemical lowing paper deals with flash photolysis studies of intermediates. behavior of the dinuclear C O loss intermediates has taken on The flash photolysis results support the general mechanism proincreased importance as interest has increased in the reactions posed on the basis of the continuous photolysis results. They add of mononuclear coordinatively unsaturated species, such as to our understanding of the detailed course of the reaction, and (qS-CSHS)Rh(C0),8( V ~ - C ~ H ~ ) M ~ (and C OCr(CO)s.lo )~,~ In the dinuclear compounds the coordinatively unsaturated molecule shed important light on the lifetimes and relative stabilities of resulting from CO loss can engage in a form of “self-repair”, e.g., intermediates. The interactions of main group element hydrides with unsatA urated metal carbonyl fragments are ~ell-established?*~~~~~~ variety of mononuclear and dinuclear metal carbonyls have been (1) Geoffroy, G. L.; Wrighton, M. S. Organometallic Photochemistry; Academic Press: New York, 1979. employed as catalysts in hydr~silation.”*~~ For metal carbonyl (2) Meyer, T. L.; Caspar, J. V. Chem. Reu. 1985, 85, 187. dimers, both 17-electron metal carbonyl radicals and coordinatively (3) (a) Brown, T. L. Ann. N . Y. Acad. Sci. 1980,333,SO. (b) Brown, T. unsaturated 16-electron species have been invoked as the active L. In Organometallic Radical Processes; Trogler, W. C., Ed.; Elsevier: Amintermediates. The most widely accepted mechanism for the metal sterdam, 1990; p 67. (c) Trogler, W. C. Inr. J . Chem. Kinet. 1987, 19, 1025. (4) Baird, M. C. In Organometallic Radical Processes; Trogler, W. C., carbonyl catalyzed hydrosilation of olefins is that proposed by Ed.; Elsevier: Amsterdam, 1990; p 49. Chalk and Harrod for C O ~ ( C O ) ~Because . ~ ~ of its relatively high (5) (a) Kochi, J. K. In Organometallic Radical Processes; Trogler, W. C., lability toward CO loss, C O ~ ( C Ois) ~effective as a catalyst under Ed.; Elsevier: Amsterdam, 1990; p 201. (b) Kochi, J. K. J . Organomet. thermal reaction conditions. By contrast, Mn,(CO) is effective Chem. 1986, 300, 139. (6) (a) Stiegman, A. E.; Tyler, D. R. Comments Inorg. Chem. 1986, 5, only under photochemical conditions. It has been proposed that 215. (b) Tyler, D. R. In Organometallic Radical Processes; Troger, W. C . , the photochemical catalytic cycle is initiated through a hydrogen Ed.; Elsevier: Amsterdam, 1990; p 338. (7) Astruc, D. Chem. Rev. 1988, 88, 1189. (8) (a) Hoyano, J . K.; Graham, W. A. G. J . Am. Chem. SOC.1982,104, 3723. (b) Bergman, R. G.; Janowicz, A. H. J . Am. Chem. Soc. 1982, 104, 352. (c) Weiller, B. H.; Wasserman, E. P.; Bergman, R. G.; Moore, C. B.; Pimentel, G. C. J . Am. Chem. SOC.1989, 1 1 1 , 8288. (9) (a) Schubert, U. Ado. Organometal. Chem. 1990, 30, 151. (b) Hill, R. H.; Wrighton, M. S. Organometallics 1987,6, 632. (c) Lichtenberger, D. L.;Rai-Choudhari, A. J . Am. Chem. Soc. 1989, 1 1 1 , 3583. (IO) (a) Wrighton, M.; Schroeder, M. A. J . Am. Chem. SOC.1973, 95, 5764. (b) Platbrood, G.; Wilprette-Steinert, L. J. Organomet. Chem. 1974, 70, 407. (c) Wuce, Y.-M.; Bentsen, J . G.; Brinckley, C. G.; Wrighton, M. S. Inorg. Chem. 1987,26,530. (d) Lee,M.; Harris, C. B. J . Am. Chem. Soc. 1989, 1 1 1 , 8963. (e) ODriscoll, E.; Simon, J. D. J . Am. Chem. SOC.1990, 112,6580. (f) Perutz, R. N.;Turner, J. J. J . Am. Chem.Soc. 1975,97,4791. (g) Burdett, J . K.; Gryzbowski, J. M.; Perutz, R. N.; Poliakoff, M.; Turner, J. J.; Turner, R. F. Inorg. Chem. 1978.17, 147. (h) Dobson, G. R.; Zhang, S. J . Coord. Chem. 1990, 21, 155. (i) Bickelhaupt, F. M.; Baerends, E. J.; Ravenek, W. Inorg. Chem. 1990, 29, 350.
0002-7863/91/1513-9155%02.50/0
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T. J. J . Am. Chem. SOC.1980, 102,7795. (b) Hepp, A. F.; Blaha, J. P.; Lewis, C.; Wrighton, M. S. Organometallics 1984, 3, 174. (c) Moore, B. 0.;Simpson, M. B.; Poliakoff, M.; Turner, J. J . J . Chem. SOC.,Chem. Commun. 1984, 972. (d) Dixon, A. J.; Healy, M. A.; Poliakoff, M.; Turner, J. J . Chem. SOC.,Chem. Commun. 1986, 944. (12) (a) Fox, A.; PO%A. J . Am. Chem. SOC.1980,102,2497. (b) Hepp, A. F.; Wrighton, M. S . J . Am. Chem. SOC.1983, 105, 5934. (c) Dunkin, I. A,; HBrter, P.; Shields, C. J. J . Am. Chem. Soc. 1984, 106, 7248. (d) Church, S. P.; Hermann, H.; Grevels, F.-G.; Shaffner, K. J . Chem. Soc., Chem. Commun. 1984, 785. ( e ) Seder, T. A,; Church, S. P.; Weitz, E. J . Am. Chem. SOC.1986, 108, 7518. (1 3) Ford, P. C.; Friedman, A. F. In Photocata1ysis; Serpone, N., Pelizzetti, E., Eds.; J. Wiley & Sons, Inc.: New York, 1989. (14) Harrod, J. F.; Chalk, A. J. Organic Syntheses Via Metal Carbonyls; Wiley: New York, 1977; Vol. 2, pp 687-690 and references therein. (15) (a) Harrod, J. F.; Chalk, A. J. J . Am. Chem. Soc. 1965, 87, 1133. (b) Chalk, A. J.; Harrod, J . F. J . Am. Chem. SOC.1967, 89, 1640. (11) (a) Caspar, J. V.; Meyer,
0 1991 American Chemical Society
9156
Sullivan and Brown
J. Am. Chem. SOC.,Vol. 113, No. 24, 1991
atom transfer to the Mn(CO)5' radical.I6 Given the relative bond energies, about 60-65 kcal mo1-I for H M ~ I ( C O ) and ~ I ~ 90 kcal mol-l for HSi(C2Hs)3,18such a pathway seems unlikely. Others have invoked a catalytic cycle similar to the Chalk-Harrod process.I9 In such a scheme, oxidative addition of the silane would occur to the coordinatively unsaturated metal center following CO loss, followed by reductive elimination of H M n ( C O ) 5 , and forming R , S i M n ( C 0 ) 5 after CO uptake. Three pathways suggest themselves as possibilities for reaction of a main group element hydride with Mn2(C0)*L2(where L may be CO a s well a s a phosphine) under photochemical conditions. They involve reaction with either the radical formed via metalmetal bond homolysis or with the CO loss product. 1. Atom transfer t o Mn(C0)4L*: Mn(C0)4L'
+ HMR,
-
HMn(CO),L
+ MR,'
(1)
If atom transfer were the primary pathway for reaction, the products should consist of the metal carbonyl hydride and M2R6. Only small amounts, if any, of R 3 M - M n ( C 0 ) 4 L should be observed. This reaction must compete with other processes involving Mn(C0)4L', notably recombination to form t h e dimer. Atom transfer from HSnBu, to R e ( C 0 ) 5 ' has been shown to compete effectively with recombination of the radicals.20 Persistent manganese carbonyl radicals Mn(CO),L2', in which there is no competing radical recombination, undergo H atom transfer from HSnBu,.2' In the present case, given the comparatively low bond dissociation energy for the Mn-H bond, H atom abstraction by Mn(C0)4L' radicals may be endergonic to an extent that renders the reaction too slow t o compete effectively with M n ( C 0 ) 4 L * recombination. 2. Oxidative addition to Mn(C0)4L':
+ HMR,
MII(CO)~L'
-
+ CO
Mn(H)(MR3)(CO),L'
(2)
When the H atom transfer is energetically unfavorable, the high substitutional lability of t h e 17-electron radical may enable t h e hydride t o react with t h e metal center via an oxidative addition reaction. T h e resulting 17-electron radical, being more electron rich, can then possibly undergo H atom transfer with a second molecule of hydride, followed by reductive elimination of H 2 : Mn(H)(MR,)(CO),L*
+ HMR,
-
-
+ MR3' (3)
Mn(H)dMR,)(CO)&
+
M I I ( H ) ~ ( M R , ) ( C O ) ~ L CO
H2 + R,MMn(CO),L
(4)
A mechanism of this type has been proposed for both thermal and photochemical reactions of some dinuclear carbonyl compounds with hydrides.22 T h e products of the overall process are H2 and t h e metal-metal bonded species R3M-Mn(C0)4L. 3. Oxidative addition t o M n 2 ( C 0 ) 7 L 2 :
+ HMR,
-
(CO)3L(H)(MR3)Mn-Mn(CO)4L (5) Following t h e oxidative addition step shown in eq 5, reductive elimination and uptake of CO would lead t o H M n ( C 0 ) 4 L and Mn2(C0I7L2
R,MMn( C 0 ) 4 L . ~~
~
~~~
(16) (a) Kuz'mina, N. A.; Il'inskaya, L.V.;Gasanov, R. G.; Chukovskaya, E.TS.; Freidlina, R. Kh. Izv. Akad. NaukSSSR, Ser. Khim. 1986, 212. (b) Terent'ev, A. B.; Moskalenko, M. A.; Freidlina, R. Kh. Izu. Akad. Nauk SSSR,Ser. Khim. 1984, 2825. ( I 7) (a) Pilcher, G. In Thermochemistry and Its Applications to Chemical and Biological Systems; Riberio de Silva, M. A. V.,Ed.; NATO AS1 Series 119; D. Reidel Publishing Co.: Dordrecht, Holland, 1984; p 353. (b) Connor, J. A.; Zafarini-Moattar, M. T.;Bickerton, J.; El Saie, N. I.; Suradi, S.; Carson, R.; AI Takhin, G.; Shinnes, H. Organometallics 1982, I , 1166. (c) Eisenberg, D.; Norton, J. R. Isr. J . Chem., in press. (18) Kanabus-Kaminski, J. M.;Hawari, J.; Griller, D. J . Am. Chem. Sot. 1987, 109, 5261. (19) Hilal, H. S.; Maher, A.-E.; AI-Subu, M.; Khalaf, S. J. Mol. Catal. 1987, 39, I . (20) Hanckel, J. M.; Lee, K.-W.; Rushman, P.;Brown, T.L.Inorg. Chem. 1986, 25, 1852. (21) McCullen, S.B.; Brown, T.L. J . Am. Chem. SOC.1982, 104, 7496. (22) (a) Wegman, R. W.; Brown, T. L. J . Am. Chem. Soc. 1980, 102, 2494. (b) Wegman, R. W.; Brown, T. L. Organometallics 1982, I, 41.
Our results, as described below, support oxidative addition to the CO-loss product as the only significant reaction pathway for reactions of the variously substituted dinuclear manganese compounds with HSnBu, and related compounds. Experimental Section Solvents. Hexane obtained from Burdich and Jackson Laboratories, Inc., was purified by stirring over concentrated H2S04 for at least 1 week, followed by washing with aqueous NaHCO, and then water, and predried with CaCI,. It was then passed over freshly activated silica gel and refluxed over CaH2 under N2 for a minimum of 12 h prior to distillation. The distilled hexane was collected under an N, atmosphere, purged with Ar, freezed-pumped-thawed with Ar three times, and stored in an oven-dried amber bottle in the glovebox. Tetrahydrofuran, THF, obtained from Fischer Scientific was purified by predrying a freshly opened bottle with KOH, followed by refluxing over LiAIH, under N2 for at least 12 h prior to distillation. The distilled T H F was collected under an N2 atmosphere and freezed-pumpedthawed three times with Ar prior to use. Benzene, obtained from Fischer Scientific, was purified by predrying with CaCI,, followed by refluxing over CaH, under N2 for at least 12 h prior to distillation. The distilled benzene was collected under N, and freezed-pumped-thawed three times with Ar prior to use. Toluene was obtained from Fischer Scientific and was treated in the same manner as benzene. Reagents. Manganese carbonyl Mn2(CO)Io, obtained from Pressure Chemical Co., was sublimed (50 OC, 0.3 mmHg) prior to use and stored in the refrigerator. Phosphines were obtained from Strem Chemical Co. They were used as received and stored in the glovebox. Tri-n-butyltin hydride, HSnBu,, was obtained from Aldrich Chemical Co. It contained trace amounts of CISnBu,, which was removed by distillation over LiAlH4 (65 "C, 0.3 mmHg). The purified HSnBu, was stored in a glass vial in the freezer under Ar. Triethylsine, HSiEt,, obtained from Aldrich, was distilled from P205 under Ar prior to use and stored in the glovebox. Fluorene was obtained from Lancaster Synthesis and was used as received. Triphenylmethane, Ph,CH, was obtained from Aldrich. It was recrystallized from ethanol prior to use (mp 93.4 "C). Carbon tetrachloride, CCI,, was obtained from Mallinckrodt. It was purified by washing with hot ethanolic KOH and then with H20, predried with CaCI,, and then distilled from P205 under Ar. Argon (research grade, minimum purity 99.9995%, < I ppm 02,
