1532
Langmuir 1998, 14, 1532-1534
The Reaction between Tetra-tert-butoxytin and Al(110)-OH in Ultrahigh Vacuum: Contrasting Behavior vs Its Zirconium Analogue Gang Lu, Jeffrey Schwartz,* and Steven L. Bernasek Department of Chemistry, Princeton University, Princeton, New Jersey 08544-1009 Received October 15, 1997 The organometallic chemical vapor deposition reaction between tetra-tert-butoxytin and hydroxylated Al(110) proceeds in ultrahigh vacuum to generate a surface-bound tin alkoxide product. The stoichiometry of the product, [Al]-[O]3-Sn(OBut), is different from that observed for zirconium alkoxide analogues. Facile ligand exchange of tert-butoxy for phenoxy can be accomplished for the surface-bound tin complex. Heating either surface tert-butoxytin or phenoxytin species ultimately gives the zerovalent metal. Deposition and thermal evolution reactions were followed by both reflection-absorption infrared and X-ray photoelectron spectroscopies.
Organometallic chemical vapor deposition (OMCVD) processes involving protolytic ligand loss1 for ostensibly similar systems might suggest similar reactivity profiles, but mechanistic considerations for the deposition step could engender significantly different outcomes for these analogues. We have reported2 that tetra-tert-butoxyzirconium (1) reacts with the heavily hydroxylated Al(110) surface3 in ultrahigh vacuum (UHV) to generate a surface complex with the stoichiometry [Al]-[O]4-n-Zr(OBut)n (n ) 1 or 2), and we have noted2 that thermolytic decomposition of these materials ultimately led to formation of ZrO2. Because heterogenized tin complexes have use in catalyst formulations4 or as precursors of tin oxide surface coatings,5 we studied the reaction between heavily hydroxylated Al(110) and the Sn analogue, tetra-tert-butoxytin (3), in UHV. We were surprised to discover that, despite the apparent similarity of the tin and zirconium compounds, rather different surface deposition and reaction chemistries derive from these two species. Hydroxylation of Al(110) was carried out as previously described.2 A multiple water dosing and desorption protocol was effected to ensure saturation surface hydroxyl group content. Deposition of tetra-tert-butoxytin was accomplished from the vapor phase onto the hydroxylated surface at 170 K. X-ray photoelectron spectroscopic (XPS) analysis of the resulting multilayer (see Table 1) showed C/Sn ) 16 (for intact 3, expected: C/Sn ) 16). Desorption of the multilayer was monitored by temperature-programmed desorption spectrometry (TDS) and reflectanceabsorbance infrared spectroscopy (RAIRS).2 As for 1, desorption of intact 3 occurred between 220 and 270 K. However, in contrast to 1, which gives [Al]-[O]1-Zr(OBut)3 (2; XPS measured ratios:2 C/Zr ) 12.6 ( 0.5), the surface tin compound formed under identical conditions was the (1) For a general discussion, see: Iwasawa, Y. Tailored Metal Catalysts; D. Reidel: Boston, MA, 1986. (2) Lu, G.; Purvis, K. L.; Schwartz, J.; Bernasek, S. L. Langmuir 1997, 13, 5791. (3) Miller, J. B.; Bernasek, S. L.; Schwartz, J. Langmuir 1994, 10, 2629. (4) For recent examples, see: (a) Merlen, E.; Beccat, P.; Bertolini, J. C.; Delichere, P.; Zanier, N.; Didillon, B. J. Catal. 1996, 159, 178. (b) Lee, A. F.; Baddeley, C. J.; Hardacre, C.; Moggridge, G. D.; Ormerod, R. M.; Lambert, R. M. J. Phys. Chem. B 1997, 101, 2797. (5) For recent examples, see: (a) Simianu, V. C.; Hossenlopp, J. M. Appl. Organomet. Chem. 1997, 11, 147. (b) Jime´nez, V. M.; Mejı´as, J. A.; Espino´s, J. P.; Gonza´lez-Elipe, A. R. Surf. Sci. 1996, 366, 545. (c) Jime´nez, V. M.; Espino´s, J. P.; Gonza´lez-Elipe, A. R. Surf. Sci. 1996, 366, 556.
mono-tert-butoxytin species, [Al]-[O]3-Sn(OBut) (4; XPS measured ratio6,7 C/Sn ) 3.7 (0.1; expected C/Sn ) 4) (Figure 1 and Scheme 1). XPS analysis also showed the presence of a second, minor tin species (≈5% of the total signal) corresponding to Sn(0).5,8,9 In contrast to thermolysis of [Al]-[O]-Zr(OBut)3, heating 4 in the range 300-400 K resulted in complete decomposition (as determined by Fourier transform RAIRS (FT-RAIRS)10). Surface graphitic carbon (for C1s, binding energy (BE) ) 284.6 eV9) and both oxidized Sn (measured Sn3d5/2 BE ) 486.5 eV; for SnO2, BE ) 486.5-486.9 eV6) and Sn(0) (BE ) 484.2 eV8,9) were formed (see Table 1). Continued heating above 400 K showed conversion of all the tin to Sn(0), with concomitant loss of surface carbon (by XPS) and evolution of CO (by TDS, Figure 2): Reduction of Sn(IV) to Sn(0) by carbon in this temperature range is possible.11,12 Surface complex 4 undergoes ligand metathesis with phenol in UHV similar to that of 2.13 Following four cycles of dosing (170 K) and desorption (240 K), FT-RAIRS analysis showed complete loss of the tert-butoxy group. Although phenoxy for tert-butoxy ligand exchange in [Al][O]4-n-Zr(OBut)n (n ) 1, 2) can be accomplished with (6) Willeman, H.; Van De Vondel, D. F.; Van Der Kelen, G. P. Inorg. Chim. Acta 1979, 34, 175. (7) (a) Clark, D. T.; Kilcast, D.; Musgrave, W. K. R. J. Chem. Soc., Chem. Commun. 1971, 517. (b) Gelius, W.; Heden, P. F.; Hedman, J.; Lindberg, B. J.; Manne, R. R.; Nordberg, R.; Nordling, C.; Siegbahn, K. Phys. Scr. 1970, 2, 70. (8) The measured binding energy (Sn 3d5/2 BE ) 484.0 eV) is slightly lower than that reported (Sn 3d5/2 BE ) 484.5-485.0 eV),9 but it is in the range recorded in our control experiments for analysis of Sn deposited in UHV onto clean (Sn 3d5/2 BE ) 483.8 eV) and oxidized Al(110) (Sn 3d5/2 BE ) 484.3 eV). In all cases, binding energies were compared with those measured for Al(2p) (73.41-73.50 eV) and O(1s) (534.8-535.1 eV) in order to minimize effects of possible surface charging. (9) (a) Moulder, J. F.; Stickle, W. F.; Sobol, P. E.; Bomben, K. D. Handbook of X-ray Photoelectron Spectroscopy; Chastain, J., Ed.; PerkinElmer Co., Physical Electronics Division: Eden Prairie, MN, 1992. (b) Ko¨ve´r, L.; Barna, P. B.; Sanjine´s, R.; Kova´cs, Zs.; Margaritondo, G.; Adamik, M.; Radi, Zs. Thin Solid Films 1996, 281-282, 90. (10) Characteristic IR peaks were recorded at 2973 cm-1 (νC-H) and 1207 cm-1 (νC-O). (11) Tin metal is prepared by reduction of the oxide with carbon. See, Kirk-Othmer Encyclopedia of Chemical Technology, 3rd ed.; Grayson, M., Ed.; John Wiley & Sons: New York, 1983; Vol 23, pp 21-22. (12) Reduction of zirconium oxide can be accomplished under forcing conditions using Mg or Ca. See, Kirk-Othmer Encyclopedia of Chemical Technology, 3rd ed.; Grayson, M., Ed.;John Wiley & Sons: New York, 1983; Vol 24, pp 872-873. (13) VanderKam, S. K.; Lu, G.; Bernasek, S. L.; Schwartz, J. J. Am. Chem. Soc. 1997, 119, 11639.
S0743-7463(97)01125-6 CCC: $15.00 © 1998 American Chemical Society Published on Web 03/14/1998
Letters
Langmuir, Vol. 14, No. 7, 1998 1533
Figure 1. FT-RAIRS spectrum of [Al]-[O]3-Sn(OBut) (4).
Figure 2. TDS analysis of the preparation and thermolysis of [Al]-[O]3-Sn(OBut) (4).
Figure 3. FT-RAIRS spectrum of [Al]-[O]3-Sn(OC6H5) (5).
1534 Langmuir, Vol. 14, No. 7, 1998
Letters
Table 1. Preparation, Thermal Evolution, and Ligand Metathesis for 4 XPS temp (K) 170
multilayer
Sn(OBut)4
species (3)
[Al]-[O]3-Sn(OBut) (4)
273 g273-700 350-500 240 a
observed
Sn(0) graphitic C [Al]-[O]3-Sn(OPh) (5)
Sn 3d5/2 C 1s Sn 3d5/2 C 1s Sn 3d5/2 C 1s Sn 3d5/2 C 1s
binding energy (eV)
C-O:C-C
487.26 285.4, 286.97 486.86 285.5, 287.37 484.08 284.69 487.06 285.3, 287.07
Sn(IV):Sn(0)
RAIRS (cm-1)
a
2973, 2869, 1180
95:5
2973, 2923, 1207
1:2.9 1:3.4 b 71:29
2962, 1600, 1496, 1261
1:4.8
Only Sn(IV) observed. b Only Sn(0) observed.
Scheme 1. Synthesis and Reactions of 4
Scheme 2. Chemisorption Involves Complex Formation and Rate-Determining Proton Transfer
subsequent desorption of unreacted phenol at 300 K, heating no higher than 240 K was attempted for reactions of 4, given its thermal lability (Figure 3 and Scheme 1). After these four cycles, XPS analysis showed two tin species to be present. The lesser component (30% of total signal) corresponded to Sn(0) (apparently formed even below 300 K) and the major component was Sn(IV). The atomic ratio, C/Sn ) 6.0, is consistent with simple ligand metathesis to give [Al]-[O]3-SnIV(OC6H5) (5,14 expected C/Sn ) 6) (Table 1).15 Thermolysis of 5 up to 700 K showed complete decomposition to Sn(0) by XPS. On the basis of direct kinetic isotope effect measurements, the mechanism of CVD of 1 onto hydroxylated aluminum has been proposed16 to involve reversible coordination of a surface -OH group to the zirconium, followed by rate-determining proton transfer from this -OH group to the alkoxide ligand oxygen. Chemisorption efficiency would depend both on the rate of proton transfer in the surface complex and on the stability of the surface complex to desorption (Scheme 2). Organotin compounds similarly undergo electrophilic cleavage (for example, protonolysis) by a pathway involving initial coordination (14) Characteristic IR peaks were recorded at 1600 and 1496 cm-1 (νCdC) and 1261 cm-1 (νC-O). (15) Some graphitic carbon was also observed (by XPS, intensity of C1s comparable to that of the phenoxy carbon atom). (16) Miller, J. B.; Schwartz, J.; Bernasek, S. L. J. Am. Chem. Soc. 1993, 115, 8239.
of the attacking reagent to the metal, followed by transfer of the electrophile to the ligand-tin bond.17 Since chemisorption of either 1 or 3 requires proton transfer to an alkoxide ligand common to both, structural differences for surface hydroxyl group coordination of 3 versus 1 may explain differences in the stoichiometries measured for 2 and 4 in UHV.18 Mechanistic analysis of the protolytic reaction between 3 and surface hydroxylated aluminum is now underway. Acknowledgment. The authors acknowledge support for this work given by the National Science Foundation and the New Jersey Council on Science and Technology. LA971125T (17) Jensen, F. R. Acc. Chem. Res. 1983, 16, 177. (18) Sn(OPri)4 and Zr(OPri)4 adducts with i-PrOH are structurally similar di-µ-alkoxy bridged dimers in which each metal is 6-coordinate. An intriguing observation is that average Sn-O distances for monodentate axial (2.12 Å) and monodentate equatorial (1.95 Å) alkoxide ligands are different by only ca. 0.17 Å (see: Reuter, H.; Kremser, M. Z. Anorg. Allg. Chem. 1991, 598/599, 259), but for the Zr analogue this difference (axial 2.27 Å vs equatorial 1.93 Å) is ca. 0.34 Å (see, Vaartstra, B. A.; Huffman, J. C.; Gradeff, P. S.; Hubert-Pfalzgraf, L. G.; Daran, J.-C.; Parraud, S.; Yunlu, K.; Caulton, K. G. Inorg. Chem. 1990, 29, 3126). Significantly, coordination of the alcohol in two of the four axial sites of the dimer is indicated; perhaps bonding of the alcohol is stronger to Sn(IV) than to Zr(IV) in these model systems. If so, it may be that 3 binds more strongly than 1 to surface -OH groups of oxidized Al, giving rise to the observed difference in chemisorption stoichiometries.
