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Langmuir 1998, 14, 3720-3722
Ligand Metathesis in Surface-Bound Alkoxyzirconium Complexes. 2. Preparation of Alkanecarboxylate Complexes in Ultrahigh Vacuum Kathleen L. Purvis, Gang Lu, Jeffrey Schwartz,* and Steven L. Bernasek* Department of Chemistry, Princeton University, Princeton, New Jersey 08544-1009 Received February 17, 1998
Introduction Carboxylic acids are appealing candidates for use in adsorption of organics to metal surface native oxide coatings1 because of the relative ease of their synthesis and the variety of chain lengths, steric factors, and functionalities readily available to them. Indeed, the reaction between carboxylic acids and metallic oxide surfaces, particularly of aluminum,2 has been studied as a means to attach organic films to oxides. Using quartz crystal microbalance (QCM) measurements, we found that, although simple alkanoic acids readily adsorb onto the oxidized aluminum surface, such adsorption was reversible;3 we also found that treating the oxide surface first with a zirconium alkoxide enables irreversible subsequent adsorption of the alkanoic acid through formation of a surface-bound zirconium carboxylate, and average stoichiometries of zirconium alkoxide complex deposition and metathesis were deduced. We have recently demonstrated4 a simple methodology to accomplish ligand metathesis in ultrahigh vacuum (UHV); this technique enables preparation of surface complex species not accessible by direct chemical vapor deposition techniques. We now report the metathetical synthesis of zirconium carboxylates in UHV; using a combination of infrared, X-ray photoelectron spectroscopic (XPS), and thermal desorption (TDS) techniques, the stoichiometries, structural features, and thermal stabilities of these surface complexes were directly measured. These studies help elucidate structural features of Zr-complex-mediated bonding of carboxylic acid derivatives, including polymeric ones,5 to a variety of oxide-coated substrates. Experimental Section General. Tetra(tert-butoxy)zirconium (Aldrich) was distilled in vacuo and stored under dry N2 prior to transfer to the UHV apparatus. Degassed butanoic and octanoic acids (Alfa) were used without further purification. The UHV apparatus has been described elsewhere;6 briefly, it is equipped with a Mattson Research Series FT-IR spectrometer, a quadrupole mass spec(1) For example, see: (a) Laibinis, P. E.; Hickman, J. J.; Wrighton, M. S.; Whitesides, G. M. Science 1989, 245, 845. (b) Kumar, A.; Biebuyck, H.; Whitesides, G. M. Langmuir 1994, 10, 1498. (2) For example, see: (a) Allara, D. L.; Nuzzo, R. G. Langmuir 1985, 1, 45. (b) Allara, D.; Nuzzo, R. G. Langmuir 1985, 1, 52. (c) Golden, W. G.; Snyder, C. D.; Smith, B. J. Phys. Chem. 1982, 86, 4675. (c) Tao, Y. T. J. Am. Chem. Soc. 1993, 115, 4350. (3) Aronoff, Y. G.; Chen, B.; Lu, G.; Seto, C.; Schwartz, J.; Bernasek, S. L. J. Am. Chem. Soc. 1997, 119, 259. (4) VanderKam, S. K.; Lu, G.; Bernasek, S. L.; Schwartz, J. J. Am. Chem. Soc. 1997, 119, 11639. (5) VanderKam, S. K.; Bocarsly, A. B.; Schwartz, J. Chem. Mater. 1998, 10, 685. (6) Miller, J. B.; Bernasek, S. L.; Schwartz, J. J. Am. Chem. Soc. 1995, 117, 4037. Miller, J. B.; Bernasek, S. L.; Schwartz, J. Langmuir 1994, 10, 2629.
Figure 1. FT-RAIRS monitoring of the reaction between 2 and octanoic acid; (a) 2; (b) following exposure to octanoic acid, before desorption of the acid multilayer (νCO ) 1708 cm-1); (c) following two cycles of octanoic acid exposure-desorption. trometer (QMS), and X-ray photoelectron spectroscopy (XPS) capabilities. The aluminum (110) single crystal was cleaned and characterized in situ utilizing the above spectroscopies. Preparation of Surface [Al(110)]-[O]-Zr(OBut)3 (1) and [Al(110)]-[O]2-Zr(OBut)2 (2) in UHV. An Al(110) singlecrystal surface was cleaned in UHV by Ar+ ion sputtering, followed by annealing at 575K for 30 min. XPS spectral analysis of this surface and water dosing to achieve saturation hydroxylation have been previously described.7 The crystal was cooled to 170 K and exposed to ∼15 L of tetra(tert-butoxy)zirconium to give a thick multilayer of the intact tetraalkoxide on the hydroxylated aluminum surface;4 the C/Zr atomic ratio was measured to be 16:1. Fourier transform reflectance absorbance infrared spectroscopic (FT-RAIRS) analysis also showed the intact tetraalkoxide. A TDS profile of the overlayer (monitoring m/z ) 57) showed three desorption regions: a first peak at 240 K, for desorption of physisorbed multilayers; a second at 340 K, for conversion of [Al]-[O]-Zr(OBut)3 (1); and a final one at ∼500 K, for decomposition of [Al]-[O]2-Zr(OBut)2 (2). By heating the sample to either 300 or 400 K, 1 or 2 can be produced, respectively. Characterization of these two surface species has been described elsewhere.7 Ligand Metathesis for [Al]-[O]-Zr(OBut)3 (1) and Butanoic Acid in UHV. Ligand metathesis was accomplished by repetitive dosing of the substrate with butanoic acid at 170 K followed by heating to 300 K. The course of ligand metathesis was followed by RAIRS, XPS, and TDS. After one cycle of treatment of 1 with 0.6L (based on ion gauge sample pressure) of butanoic acid, FT-RAIRS analysis showed the presence of both alkoxide and exchanged carboxylate ligands. Even after eleven more dose-desorb cycles, FT-RAIRS showed only two tert-butoxy ligands had been exchanged for butanoate ones. Ligand Metathesis for [Al]-[O]2-Zr(OBut)2 (2) and Butanoic or Octanoic Acids in UHV. Ligand metathesis was accomplished as for 1 by repetitive dosing of the substrate with butanoic acid at 170 K followed by warming to 300 K. After one cycle of treatment of 2 with 0.6 L (based on ion gauge sample (7) Lu, G.; Purvis, K. L.; Schwartz, J.; Bernasek, S. L. Langmuir 1997, 13, 5791.
S0743-7463(98)00190-5 CCC: $15.00 © 1998 American Chemical Society Published on Web 05/30/1998
Notes
Langmuir, Vol. 14, No. 13, 1998 3721 Scheme 1. Synthesis of 3 and 4 by Replacement of Alkoxide Ligation by Carboxylates
Table 1. Ligand Metathesis with Butanoic Acid in UHV XPS species [Al]s[O]sZr(OC[CH3]3)3 (1) [Al]s[O]2sZr(OC[CH3]3)2 (2) [Al]s[O]2sZr(O2CC3H7)2 (3a) [Al]s[O]2sZr(O2CC7H15)2 (3b) [Al]s[O]sZr(O2CC3H7)2(OC[CH3]3)1 (4)
C1s binding energy (eV)
rel peak area
RAIRS (cm-1)
287.3 (CsO) 285.7 (CsCH3) 287.1 (CsO) 285.5 (CsCH3) 290.2 (C[dO]O) 286.1 (C3H7) 290.1 (C[dO]O) 285.8 (C7H15) 289.4 (C[dO]O) 286.7 (CsO) 285.5 (C3H7) 285.1 (CsCH3)
1 2.9 1 2.8 2 2.9 1 7.7 2.1 1 6.3 3.0
2973, 2913, 1361, 1230, 1200 2973, 2913, 1361, 1230, 1200 2965, 2877, 1600, 1477 2962, 2927, 2854, 1589, 1477 2965, 2877, 1600, 1477, 1195
Scheme 2. Ligand Metathesis Involving a Sterically Congested Intermediate (5)
pressure) of butanoic acid, FT-RAIRS analysis showed the presence of both alkoxide and exchanged carboxylate ligands. After seven more dose-desorb cycles, FT-RAIRS showed complete ligand exchange. A similar sequence (of 2-3 cycles) was used for reaction of 2 with octanoic acid.
Results and Discussion Tetra(tert-butoxy)zirconium was adsorbed on surface hydroxylated Al(110)7 to give a thick multilayer of the intact tetraalkoxide. Desorption of physisorbed material was accomplished at 240 K to give [Al]s[O]sZr(OBut)3 (1); heating 1 at 340 K afforded [Al]s[O]2sZr(OBut)2 (2).7
Complete exchange4 of tert-butoxy ligands for carboxylates occurred for 2 after several cycles of dosing (170 K) and warming (300 K), depending on the acid (Scheme 1). Ligand metathesis was monitored by RAIRS by noting the disappearance of the band at 1200 cm-1, characteristic of νCsO for the tert-butoxy ligands, and the increase in intensity of bands attributable to carboxylate νsym and νantisym at 1477 and 1600 cm-1, respectively (Figure 1).2,8 XPS analysis of the product of metathesis (3) established (8) Compare with zirconium tetraacetate. For the η2-carboxylate: 1540 (νantisym); 1455 (νsym) cm-1. Tackett, J. E. Appl. Spectrosc. 1989, 43, 483.
3722 Langmuir, Vol. 14, No. 13, 1998
Notes
Figure 2. Comparative TDS analysis of the decomposition of 3a versus 4.
the stoichiometry of the overlayer to be [Al]s[O]2sZr(O2CR)2 (see Table 1). It is interesting to note that although the same ratio, C/Zr ) 8:1, is expected for both 2 and 3a (R ) C3H7), resolution of the C1s spectrum, focusing on differences between the alkoxide carbon (CsO) of 2 and the carboxylate carbon (C(dO)O) of 3a (or 3b),9 easily distinguishes between these ligand types. TDS analysis for the final surface species showed essentially no signal at m/z ) 57 below 450 K and confirms the absence of tert-butoxy groups (Figure 2). No carboxylate ligand desorption was noted for 3a or 3b which were stable to >500 K.10 Only incomplete metathesis was observed for reaction between butanoic acid and 1 (Scheme 1). After even 12 cycles of dose (170 K) and warm (300 K), bands characteristic of mixed alkoxide (1200 cm-1) and carboxylate (1477; 1600 cm-1) ligation were observed by RAIRS (Figure 3). Furthermore, TDS analysis for the resulting material (4) showed a significant signal at m/z ) 57 in the temperature range 300-400 K, characteristic of thermal decomposition of the tert-butoxy ligand (Figure 2).7,10 As for 2 and 3, the expected C/Zr ratios for 1 and 4 are the same (12:1). Here, analysis of the deconvoluted C1s XPS signal as a function of butanoic acid dose cycle showed asymptotic replacement of alkoxide ligands by butanoate (Figure 4), and the final stoichiometry [Al]-[O]Zr(O2CC3H7)2(OC[CH3]3)1 was established. Incomplete ligand metathesis for 1, even after exhaustive cycles of dose and heat, has been observed for its reaction with phenols.4 We attribute this change in metathesis stoichiometry to steric factors in which the (9) Gelius, U.; Heden, P. F.; Hedman, J.; Lindberg, B. J.; Manne, R.; Nordberg, R.; Nordling, C.; Siegbahn, K. Phys. Scr. 1970, 2, 70. (10) These observations are consistent with estimates of bond energies: (alkoxide) C-O, 147 kJ/mol; (carboxylate) C-C, 310 kJ/mol; and O-Zr, 776 kJ/mol. See: CRC Handbook of Chemistry and Physics; Lide, D. R., Ed.; CRC Press: Boca Raton, FL, 1990-1991; Vol. 71, pp 9-89-9-98.
Figure 3. FT-RAIRS monitoring of the reaction between 1 and butanoic acid: (a) 1; (b) following 3 cycles, (c) 7 cycles, and (d) 12 cycles of butanoic acid exposure-desorption.
Figure 4. C1s XPS determined ratios for alkoxide (CsO) and carboxylate (C[dO]O) ligation versus butanoic acid dose cycle.
tert-butoxy ligands of 1 are in a more congested environment than are those of 2. Replacement of an alkoxide ligand likely involves Zr complexation of the attacking acid (to give 5) followed by proton transfer; therefore, alkoxide ligands in a spatially exposed environment would be expected to exchange fastest with carboxylate. In this model, replacement of two of the three alkoxide ligands of 1 leaves the third alkoxide group sufficiently sterically inaccessible that further metathesis is unlikely (Scheme 2). We are attempting to directly observe putative metathesis intermediate 5 to probe this mechanistic hypothesis. Acknowledgment. The authors acknowledge support for this work given by the National Science Foundation and the New Jersey Council on Science and Technology. Note Added in Proof. Further treatment of 3a with butanoic acid rapidly gave [Al]-[O]-Zr(O2CC3H7)3 by cleavage of a surface Zr-O bond. LA980190F
