Organometallics 1996, 14, 542-546
542
Concerted Multiple Dehydrogenation of Gas-Phase Saturated Cyclic c4-cfjHydrocarbons by Os+ Xinzhen Xiang and Fernando R. Tollens Department of Chemistry, The Ohio State University, Columbus, Ohio 43210
Alan G. Marshall* National High Magnetic Field Laboratory and Department of Chemistry, Florida State University, Tallahassee, Florida 32306 Received October 4, 1994@ The gas-phase reactions of laser-generated Os+ with various cycloalkanes, C5H10, C6H12, C7H14, C8H16, C6H10, CsHs, cis-l,2-dimethylhexane, and cis-l,3-dimethylhexane, have been examined by Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. In contrast to many transition-metal ions, whose reactions with alkanes show extensive cleavage of C-C bonds, dehydrogenation is the major process in all reactions. No evidence of C-C insertion is found, and the carbon ring is left intact. Saturated cyclic hydrocarbons exposed to Os+ undergo repeated dehydrogenation to form the complex ions (CnHn)Os+,which then react with another corresponding neutral hydrocarbon molecule to form [CnHn-Os-CnHn]+. The Os-ligand complex ions may be identified unambiguously from isotopic distributions. Reaction mechanisms for the various reaction pathways are proposed.
Introduction The gas-phase reactions of transition-metal ions have been studied intensively for the past two decades;l-14 the chemistry of naked transition-metal ions or metal cluster ions with various hydrocarbons provides valuable insight into the mechanisms of condensed-phase reactions as well as the efficiencies of stoichiometric or catalytic processes in general.2 Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR/MS) has the capability of ultrahigh resolution, wide mass range, multistage MS/MS capability with a single mass spectrometer, simultaneous detection of all ions, long ion storage period, and relatively easy and accurate adjustment of ion kinetic energy.2-5,11,12,15-31 These
* To whom correspondence should be addressed.
Abstract published in Advance ACS Abstracts, December 1,1994. (1)d e n t r o u t , P. B.; Beauchamp, J. L. Acc. Chem. Res. 1989,22, 315-321. (2) Eller, K.; Schwarz, H. Chem. Rev. 1991, 91, 1121-1177. (3) Freiser, B. S. Talunta 1985, 32, 697-708. (4) Freiser, B. S. Chemtrmts: Anal. Phys. Chem. 1989, I, 65-109. ( 5 ) Freiser, B. S. In Bonding Energetics in Organometallic Compounds; Marks, T. J., Ed.; ACS Symposium Series 428: American Chemical Society: Washington, DC, 1990; pp 55-69, (6)Hettich, R. L.; Freiser, B. S. J . Am. Chem. SOC.1987,109,35373542. (7) Huang, Y.; Wise, M. B.; Jacobson, D. B.; Freiser, B. S. Organometallics 1987, 6, 346-354. (8) Irikura, K. K.; Beauchamp, J. L. J . Am. Chem. SOC.1989,111, @
75-85. ..
(9) Jacobson, D. B.; Freiser, B. S. J . A m . Chem. SOC.1983, 105, 5197-5206. (10)Jacobson, D. B.; Freiser, B. S. J . Am. Chem. SOC.1983, 105, 7492-9500. (11) Nibbering, N. M. M. Acc. Chem. Res. 1990, 23, 279-285. (12) Sharpe, P.; Richardson, D. E. Coord. Chem. Rev. 1989,93,59-
--. 85
(13) SimGes, J. A. M.; Beauchamp, J. L. Chem. Rev. 1990,90,629688. (14) Tolbert, M. A.; Beauchamp, J. L. J . Am. Chem. SOC.1984,106, 8117-8122. (15)Ghaderi, S. Ceram. Trans. 1989,5, 73-86. (16) Gord, J. R.; Freiser, B. S. Anal. Chim. Acta 1989,225, 11-24. (17) Wanczek, K-P. Int. J . Muss Spectrom. Ion Processes 1989,95, 1-38.
0276-733319512314-0542$09.0OlO
features make FT-ICR/MS ideally suited for the study of gas-phase ion-molecule reaction chemistry.2~4.5~7-14~32-37 Among the transition metals, osmium holds special interest because it has catalytic value for dehydrogenation, hydr~genation,~~ and dehydroxylationreactions. Irikura et a1.8 have investigated the ion-molecule reactivities of OsOn+( n = 0-4) with a number of hydrocarbons and small molecules. They observed cycloaddition with Hz,bond metathesis, oxo transfer, and hydrogen atom abstraction. Although much investigation has been focused on catalytic surfaces,38there has been virtually no examination of the fundamental reactions in the gas phase between Os+ and organic species, because Os+ presents several difficulties for mass spectrometric analysis: (a) high mass resolving power is needed to distinguish the large number of isotopes of osmium; (b) Os+ oxidizes readily, so that experiments are best conducted at very low pressure (5 Torr); (c) osmium does not form a simple stable volatile metal carbonyl (for ready gas-phase electron (18) Wilkins, C. L.; Chowdhury, A. K.; Nuwaysir, L. M.; Coates, M. L. Mass Spectrom. Rev. 1989, 8, 67-92. (19) Laude, D. A., Jr.; Hogan, J. D. TM, Tech. Mess. 1990,57,155159. (2O)Lasers in Muss Spectrometry; Lubman, D. M., Ed.; Oxford University Press: New York, 1990. (21) Campana, J. E. In Proceedings of SPIE-Applied Spectroscopy in Material Science; International Society for Optical Engineering: Bellingham, WA, 1991; pp 138-149. (22) Marshall, A. G.; Grosshans, P. B. Anal. Chem. l991,63,215A229A. (23) Nuwaysir, L. M.; Wilkins, C. L. In Proceedings ofSPIE-Applied Spectroscopy in Material Science; International Society for Optical Engineering: Bellingham, WA, 1991; pp 112-123. (24) Dunbar, R. C. Mass Spectrom. Rev. 1992, 11, 309-339. (25) Jacoby, C. B.; Holliman, C. L.; Gross, M. L. In Mass Spectrometry in the Biological Sciences: A Tutorial; Gross, M. L., Ed.; Kluwer Academic: Dordrecht, The Netherlands, 1992; pp 93-116. (26)Koster, C.; Kahr, M. S.; Castoro, J. A.; Wilkins, C. L. Mass Spectrom. Reu. 1992, 11, 495-512. (27) Marshall, A. G.; Schweikhard, L. Int. J . Mass Spectrom. Ion Processes 1992, 1181119, 37-70. (28) Schweikhard, L.; Alber, G. M.; Marshall, A. G. Phys. Scr. 1992, 46, 598-602.
0 1995 American Chemical Society
Dehydrogenation of Gas-Phase Hydrocarbons by Os+
ionization), as do many of the more accessible transition metals (e.g., Fe(C0)d. In this paper, we show that all of these difficulties may be overcome by use of Fourier transform ion cyclotron resonance mass spectrometry to examine the primary and secondary ion-molecule reaction products of laser-desorbed Os+ with various cyclic (c5-c8) hydrocarbons in the gas phase, and we propose mechanisms for the major reactions.
Organometallics, Vol. 14,No.1, 1995 543
secondary reactions: (5)
Experimental Section All hydrocarbons were obtained from Aldrich Chemical Co. (Milwaukee, WI).Each was introduced at a pressure of 5 x Torr through a leak valve from a sample reservoir after degassing by several freeze-pump-thaw cycles on the foreline vacuum chamber of the instrument. Osmium ions were generated by laser desorption from osmium metal sponge with a Continuum Model YG-660A Nd:YAG laser operated at 1.064 pm, at a power density of -500 MW/cm2 (-50 m J in -10 ns over a spot size -1 mm in diameter). A subsequent variable delay period allowed for thermal ion-molecule reactions to occur. Mass spectra were obtained on a standard Extrell Millipore FTMS-2000 FT-ICR mass spectrometer (Extrel FTMS, Madison, WI) with dual 1.875-in. cubic ion traps centered in the bore of a superconducting magnet (3.0 T). The instrument was modified with CTI Corp. Helix CryoTorr 8 Cryopumps (1250 L s-l pumping speed for Nz) rather than the usual diffision pumps on both the source and analyzer sides of the dual trap, and both sides of the dual vacuum chamber were backed by a single 50 L s-l Leybold-Hereaus Model TMP50 turbopump. The trapping voltage was typically 2 V. Broadband frequency-sweep excitation at a sweep rate of 1000 Hz ps-l was followed by acquisition of a 16K time-domain transient signal which was digitized in direct mode at a Nyquist bandwidth of 2 MHz and padded with an additional 16K of zeroes before discrete Fourier transformation (without prior apodization).
Results and Discussion Reactions of Os+ with Cyclic Alkanes C,H2* (n The primary and secondary ion-molecule reactions of Os+ with cyclic alkanes CnHzn( n = 5-8) are shown in eqs 1-7. Minor products appearing only after long reaction periods have been ignored. FT-ICR mass spectra at various reaction periods are shown in Figure 1. = 5-81.
primary reactions:
L
--
la
0 0 7 bS+
2
02
(3)
3
O+(==J 02 4
n
n (7)
7
The reactivities of three of the four cyclic hydrocarbons are strikingly similar. Initially, the only process observed for C5H10, C6H12, and C7H14 is multiple dehydrogenation, leading to formation of complexes which are well-described by ligands of stable aromatic (4n 2 electrons) structure: (pCsHs)OsH+,( ~ - C ~ H ~ ) O and S+, (~pC7HaOs+ (see reactions 1-3). We see no evidence for ring-opening reactions by Os+, in contrast to Fe+, which has been observed to react with alkanes by means of both C-H and C-C bond insertions.1° (Fe+ does not cleave bonds in C g and c6 ringslo but does so in C7 and c8 ringsS2)M e r longer reaction periods, the primary reaction products react further with another saturated cyclic hydrocarbon molecule t o generate the adducts CloHloOs+,C12H120sf, and C14H16Os+,which have the presumed structures (~-CsH5)20s+,(?pC6H&OS+, and (q-C7H&os+ shown in reactions 5-7. These Os+ reactions parallel those of La+, which forms CsHsLa+ and CloHloLa+with cyclopentane and C6H6Laf and C12H12La+ with cy~lohexane.~ After an extended reaction period, products from C-C bond cleavage begin to appear. In contrast, the larger ring compound C8Hl6 exhibits noticeable ring cleavage to yield products such as C6H6Os+ and C7H&s+, although the most abundant product is still the dehydrogenated species: C8H@s+. The difference between C8H16 on the one hand and C5H10, C6H12, and C7H14 on the other may be attributed to the
+
.I
Ib
Os+
O+O
6
(4)
(29)Speir, J.P.;Gorman, G. S.; Amster, I. J. In Mass Spectrometry in the Biological Sciences: A Tutorial; Gross, M. L., Ed.; Kluwer Academic: Dordrecht, The Netherlands, 1992;pp 199-212. (30)Buchanan, M.V.;Hettich, R. L. Anal. Chem. 1993,65,245A259A. (31)Schweikhard, L.;Marshall, A. G. J. Am. SOC.Mass Spectrom. 1993,4,433-452. (32)Kiplinger, J. P.;Tollens, F. R.; Marshall, A. G.; Kobayashi, T.; Lagerwall, D. R.; Paquette, L. A.; Bartmess, J. E. J. Am. Chem. Soc. 1989,111,6914-6919. (33)Irikura, K.K.;Beauchamp, J. L. J.Am. Chem. SOC.1991,113, 2767-2768. (34)Larson, B. S.;Ridge, D. P. J . Am. Chem. SOC.1984,106,1912. (35)Sunderlin, L. S.;Armentrout, P. B. J . Am. Chem. SOC.1989, 111,3845-3855. (36)Dunbar, R. C.;Solooki, D.; Tessier, C. A.; Youngs, W. J.; Asamoto, B. Organometallics 1991,10, 52-54. (37)Stirk, K. M.; Kiminkinen, M.; Kenttamaa, H. I. Chem. Rev. 1992,92,1649-1665. (38)Dil'magambetov, S. N.; Dzhardamalieva, K. K.; Sokol'skii, D. V. Khim. Khim. Tekhnol. (Alma-Ata)1978,199-206.
544 Organometallics, Vol. 14,No. 1, 1995
Xiang et al.
OS+ + cyclopentane
os+ r*lr
180
mlz
200
, 250 OS+ + cyclooctane
os+
C~H~OS'
T
*
-.--
os
ri ---A L - I I -ALA-, 200
250
300 mlz
350
260
270
280
290
300
mlz Figure 2. Experimental FT-ICR mass spectral isotopic abundance patterns of the products of the reaction of Os+ with cyclic alkanes CnHgn(n = 5-8).
400 ms L r* .-800 ms
Scheme 1
1.2s
400
Figure 1. FT-ICR mass spectra of the products of the reaction of Os+ with cyclic alkanes CnHgn(n = 5-8), after each of several stated reaction periods.
4n-electron antiaromatic character (and thus lower stability) of CsHsOs+. For example, CsHsOs+ may undergo carbon-carbon cleavage followed by elimination of C2H2 to form CsHsOs+. Alternatively (see Figure 21, CsHsOs+ may react with another CsH8 neutral to form ClsHlsOs+, which presumably has the sandwich structure (pCsH&Os+. In some cases, the mass spectra are complicated by the isotopic distribution of osmium, leading to dificulty in distinguishing (e.g.1 CnHm+~1900~+ from CnHmig20s+, whose masses differ by only 0.0126 amu. Nevertheless, the appropriate osmium-ligand complex ion may be identified unambiguously in such cases from the isotopic abundance pattern. For example, Figure 2 shows experimental isotopic abundance multiplets for C,H,Os+. In principle, it is possible t o tell whether Os+ ions are in their ground state or excited states before they react with neutrals. For ground-state Os+, the Os+ concentration should decrease exponentially with reaction period, whereas for excited-state ions, Osf concentration will vary nonexponentially with reaction period. Unfortunately, we were unable to make quantitative comparisons of Os+ concentration after different reaction periods, because the signal-to-noise ratio was limited (due in part to the large number of isotopes of
la
lb
- 2
U
5
osmium) and the laser power was not sufficiently reproducible from one shot to the next. The various reactions may be rationalized by C-H insertion followed by dehydrogenation, on the basis of the reaction between Os+ and C5H10 as an odd-number carbon example (Scheme 1) and the reaction of Os+ and C6H12 as an even-number carbon example (Scheme 2). (A similar mechanism has been proposed by Land et al.39 for the surface reaction between platinum and cyclohexane, for which cyclohexene intermediates were detected by mass spectrometry.) Os+ first inserts into a C-H bond; the resulting Os-ligand complex then easily loses H2. This process is repeated until a stable aromatic or maximally conjugated species is formed. (39)Land,D. P.; Pettiette-Hall, C. L.; McIver, R. T. J.; Hemminger, J. C. J.Am. Chem. SOC.1989,111, 5970-5972.
Dehydrogenation of Gas-PhaseHydrocarbons by Os+
Organometallics, Vol. 14,No.1, 1995 545
Reactions of Os+ with Unsaturated Cyclic Hydrocarbons. If our analysis of the mechanism of reaction of Os+ with saturated cyclic hydrocarbons is correct, then we should expect to see similar products from the reaction of Os+ with unsaturated cyclic hydrocarbons. FT-ICR mass spectra for the reaction of Os+ with cyclohexene and benzene, C6HlO and C6H6, following each of several reaction periods are shown in Figure 3 (top). The reactivity of Os+ toward cyclohexene is clearly similar to that with cyclohexane: we observe dehydrogenation exclusively, to give stable CsH6Os+ (reaction 81,
Scheme 2
-0 - H2
8a
8b
9a
9b
os+ 2
0 0 OS'
6
Os+ chemistry, we reacted Os+ with cis-l,a-dimethylhexane and cis-1,3-dimethylcyclohexane,to yield the FTICR mass spectra shown in Figure 4. The products of the primary and secondary reactions are shown in reactions 13 and 14. The chemical formula of both 1,29b
lla
llb
12a
12b
15s
1
13a
13b
14a
14b
which further reacts with another molecule of cyclohexene (reaction 10) to form ClzH120s+(with the presumed structure ( ~ ~ - C ~ H ~ ) ZThe O S +sole ) . product of the reaction of Os+ with benzene (Figure 3, bottom) is C6H40s+ (reaction 9) rather than the direct adduct CsHsOs+, further corroborating that reaction of Os+ with saturated cyclic hydrocarbons indeed proceeds by dehydrogenation rather than by direct attachment. In contrast, Hettich et a1.6 found that the major product for reaction of iron cluster ions (M = CuFe+, ScFe+,TiFe+, and Fez+) with benzene was CsH&I+,whereas NbFe+ did not react with benzene. Finally, CsH40s+ reacts further (reactions 11, 12) with benzene to form ClzHaOs+ (presumably (C6H4)20se) and ClzHloOs+ and subsequently ClaHlzOs+ and ClaH140s+. Reactions of Os+ with Substituted Cyclohexanes. To demonstrate how ring substituents affect the
15b
15c
I
I
16a
16b
,
dimethylcyclohexane and 1,3-dimethylcyclohexane is C8H16, the same as that of cyclooctane. Interestingly, the reactions of Os+ with cis-1,2-dimethylcyclohexane, cis-1,3-dimethylcyclohexane,and cyclooctane are similar in that each loses 4 H2 to yield CsHsOs+, which is relatively unstable and either dissociates to form noticeable amounts of C6H60s+ and C7HaOs+ or reacts with another molecule of C8H16 to form complex ions with two ligands. In summary, saturated cyclic hydrocarbons exposed t o Os+ undergo exclusively C-H insertion to form the complex ions C,H,-Os+, which then react with another corresponding neutral hydrocarbon molecule t o form [C,H,-Os-C,H,I+ ions. Dehydrogenation of Benzene. Perhaps the most novel feature of the present results is the observation of the dehydrogenation of benzene. Benzene itself is unreactive with Fe+ (the group 8 congener of Os+) as well as with Ti+,V+, Cr+,Mn+, Fe+, Co+,Ni+, Cu+, Mo+, Ag+, and W+.2 However, early-transition-metal ions (S C + Nb+,40v41 ,~ and Ta+42,43) can dehydrogenate benzene. (40)Buckner, S. W.;MacMahon, T. J.; By-rd, G. D.; Freiser, B. S. Inorg. Chem. 1989,28, 351. (41)Higashide, H.; Oka, T.; Kasatani, K.; Shinohara, H.; Sato, H. Chem. Phys. Lett. 1989,171, 297. (42)Wise, M. B.;Jacobson, D. B.; Freiser, B. S. J.Am. Chem. SOC. 1985,107, 1590.
---
Xiang et al.
546 Organometallics, Vol. 14, No. 1, 1995
+ cyclohexene
0s'
os+
Os' + ckl,2-dlmethyl cyclohexane
os+
A
*
A
11
QH~OS'
Ci2Hi20s'
'%H60s+
0
0
0.2 s
L A
- * .
-
1.0s
0.5 s
I
il,
I ,
,
+ 3.0 s
dl
1.0 s
200
250
300 mlz
Os'
+
350
benzene
, -+-A-. .L
0s' + cis-1 ,3-dimethyl cyclohexane
0.2 s
-
I L
400
-
,
A
Jl
'
250
350
300
400
A
.
1.0 s
0.5 s
-_ + - - - + A L A
---I-.---
200
p 1.0s 450
mlz
Figure 3. FT-ICR mass spectra of the products of the reaction of Os+ with cyclohexene and benzene after each of several stated reaction periods. Moreover, neutral niobium clusters dehydrogenate ben~ e n e Finally, . ~ ~ Fe+ reacts with alkyl halides to form of~the polyphenylene c o m ~ l e x e s ~ - ~ ~type postulated here as products of the Os+ reaction with benzene. Acknowledgment. This work was supported by the National Science Foundation (Grant No. CHE-90(43)Wise, M.B.; Jacobson, D. B.; Freiser, B. S. J . Am. Chem. SOC. 1985,107,6744. (44)St. Pierre, R.J.;Chronister, E. L.; El-Sayed, M. J . Phys. Chem. 1987,91,5228. (45)Bjarnason, A.;Taylor, J. W. Organometallics 1989,8, 2020.
L 200
250
L ' 5.0 s 300 mk
350
400
Figure 4. FT-ICR mass spectra of the products of the reaction of Os+ with cis-1,2-dimethylcyclohexaneand cis1,3-dimethylcyclohexaneafter each of several stated reaction periods. 210581,The Ohio State University, and the National High Magnetic Field Laboratory at Florida State University. OM940770H (46)Bjamason, A.;Taylor, J. W. Organometallics 1990,9,1493. (47)Huang, Y.;Freiser, B. S. J . Am. Chem. SOC.1989,111, 2387.
