Bis(pyridyl)siloxane Oligomeric Ligands for Palladium(II) Acetate

Organometallics 2010, 29, 3769–3779 DOI: 10.1021/om100316n

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Bis(pyridyl)siloxane Oligomeric Ligands for Palladium(II) Acetate: Synthesis and Binding Properties Michael N. Missaghi, John M. Galloway, and Harold H. Kung* Department of Chemical and Biological Engineering, Northwestern University, Evanston, Illinois 60208 Received April 19, 2010

The complexation of a series of new bis(meta-pyridyl)methylsiloxane ligands with palladium(II) acetate in dilute toluene-d8 solution was studied by 1H NMR at 233-363 K, measuring the binding affinity and ring-chain distribution as a function of ligand structure, temperature, and concentration. Significant differences in the ring distribution and Pd binding affinity were observed as a function of siloxane chain length, with hexa- and heptasiloxane spacers binding Pd most effectively. The 1H NMR resonance of the singlet ortho aromatic proton was shifted unusually far downfield for the bidentate complex; the effect was strongest for the shortest chains and disappeared for chains with 10 or more siloxane units. Fitting the experimental data to a modified Jacobson-Stockmayer model demonstrated that the bidentate bis(pyridyl)siloxane complexes function as chelates in the concentration range 1-10 mM. Variable-temperature studies showed that all of the observed cyclic coordination oligomers are strainless. Therefore, entropic effects are implicated as the main cause of the observed trends in binding affinity, particularly the loss of conformational freedom of the siloxane chain upon binding.

1. Introduction Bidentate ligands play a variety of important roles in coordination chemistry. They affect the stability and selectivity of coordination complex catalysts1 and are essential structural elements in the formation of multinuclear complexes and extended three-dimensional networks, such as metal-organic frameworks.2 They have found applications as components in nonlinear optical materials,3 sorbents, and heavy metal chelators.4 The rigidity of a bidentate ligand influences the ability of a ligand-metal system to form cyclic structures and to interconvert between rings of different sizes. It also affects the ability of the functional groups to act cooperatively at a single metal center, whether for purposes of tighter binding (the thermodynamic chelate effect) or for enhancement of catalytic activity. In general, rigid and divergent ligands have a lower probability of forming cyclic coordination monomers (bidentate complexes). Often, the ring-chain distributions of these coordination systems are driven by entropic effects, since redistribution without a net change in the number of coordination bonds is usually possible. Most bidentate ligands are derived from hydrocarbon skeletons. Despite the unique structural possibilities offered

by an inorganic backbone, there are few reports of bidentate ligands based on inorganic frameworks, such as organosiloxane. A recent example is the Ag-chelating bis(2-pyridylethynyl)disiloxane reported by Son et al., which takes advantage of the unique conformational properties of the Si-O bond.5 In our recent study of aerobic benzyl alcohol oxidation catalyzed by Pd(OAc)2/bis(meta-pyridyl)siloxane,6 we found that some of the bidentate ligands can stabilize Pd0 intermediates more effectively against deactivation by metal aggregation, relative to unsubstituted pyridine. In some situations, this catalyst is a useful extension of the original Pd(OAc)2/pyridine system,7,8 which generally relies on a stoichiometric excess of monofunctional pyridine. The stabilization effect depends on the separation between the pyridyl groups, which also affects the dependence of catalytic activity on the pyridine-to-metal ratio. These catalysts are efficient at low metal concentrations (1-5 mM), where cyclic coordination oligomers are expected to predominate. In order to better understand the Pd(OAc)2/bis(meta-pyridyl)siloxane coordination system, we set out to investigate the ligand binding affinity and distribution of coordination states as a function of temperature, concentration, and ligand chain length. For this purpose 1H NMR is a convenient and highly informative technique to study the ringchain distribution of these diamagnetic complexes. In this paper, we report on the synthesis of the bis(metapyridyl)siloxane ligands of various chain lengths and the

*To whom correspondence should be addressed. E-mail: hkung@ northwestern.edu. (1) Xing, B. G.; Choi, M. F.; Xu, B. Chem.;Eur. J. 2002, 8 (21), 5028–5032. (2) Kuppler, R. J.; Timmons, D. J.; Fang, Q. R.; Li, J. R.; Makal, T. A.; Young, M. D.; Yuan, D. Q.; Zhao, D.; Zhuang, W. J.; Zhou, H. C. Coord. Chem. Rev. 2009, 253 (23-24), 3042–3066. (3) Janiak, C. Dalton Trans. 2003, No. 14, 2781–2804. (4) Logar, N. Z.; Kaucic, V. Acta Chim. Slov. 2006, 53 (2), 117–135.

(5) Sengupta, P.; Zhang, H. M.; Son, D. Y. Inorg. Chem. 2004, 43 (6), 1828–1830. (6) Missaghi, M. N., Galloway, J. M, Kung, H. H., submitted. (7) Nishimura, T.; Onoue, T.; Ohe, K.; Uemura, S. Tetrahedron Lett. 1998, 39 (33), 6011–6014. (8) Nishimura, T.; Uemura, S. Synlett 2004, No. 2, 201–216.

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Scheme 1. Synthesis of Monopyridylsiloxane 2c and Bis(pyridyl)siloxanes 4a-ga

a Reaction conditions: (i) 1 equiv iPrMgCl, THF, rt, 1 h; 1.2 equiv (CH3)2SiHCl, 15 min; (ii) H2O/Et2O or THF, Pd(OH)2/C, rt, 1-4 h; (iii) 1.2 equiv (CH3)2SiHCl, rt, 15 min; (iv) 0.5 equiv ClSi(CH3)2(Si(CH3)2O)nCl, n = 0-3, 1.1 equiv TEA, CHCl3, rt, 15-30 min; (v) 60 C, vacuum, 1 h; (vi) 1.2 equiv (CH3)3SiCl, rt, 15 min.

binding of these ligands to Pd(OAc)2 over a range of temperatures. The results of competitive binding studies with pyridine are presented, along with measurements of the concentration dependence of the ring interconversion equilibrium. In general, higher dilution favors smaller rings and shorter chains. For tight-binding ligands that do not exhibit significant autodissociation, linear coordination oligomers are observed only when the ligand-to-metal ratio deviates from the stoichiometric value, creating chain-terminating groups. The ability to form a cyclic coordination monomer (bidentate complex) also depends on the chain length of the ligand: a minimum ligand length is necessary to avoid excessive ring strain. In this study, we employed a modified ring-chain equilibrium model of Jacobson and Stockmayer9 to describe the ring-chain distribution and to interpret the data with respect to the enthalpic and entropic penalties of cyclization. A discussion of trends and exceptions in the fitted model parameters and the relationship between these trends and the coordination geometries of the complexes provides a comprehensive view of the Pd(OAc)2-binding properties of these siloxane-modified organic ligands.

2. Results 2.1. Preparation of Ligands and PdII Complexes. Scheme 1 shows the approach to the synthesis of the bis(meta-pyridyl)siloxane ligands by coupling of two mono(metapyridyl)sil(ox)anes. Starting from 3-bromopyridine, (m-py)(CH3)2SiH (1a) was prepared by metal-halogen exchange10 and nucleophilic displacement of (CH3)2SiHCl. Oxidative hydrolysis11 of the Si-H bond over Pd(OH2)/C resulted in quantitative conversion of 1a to the pyridylsilanol 1b, which was found to be stable as a 10 wt % solution in diethyl ether (9) De Greef, T. F. A.; Smulders, M. M. J.; Wolffs, M.; Schenning, A. P. H. J.; Sijbesma, R. P.; Meijer, E. W. Chem. Rev. 2009, 109 (11), 5687– 5754. (10) Trecourt, F.; Breton, G.; Bonnet, V.; Mongin, F.; Marsais, F.; Queguiner, G. Tetrahedron 2000, 56 (10), 1349–1360. (11) Jens, B.; Dainis, D.; Andrew, D.; Richard, C. F. Silicon Chem. 2003, 2, 27–36.

or THF. Repeating the steps of heterofunctional condensation12 of the silanol group in 1b and 2b with chlorodimethylsilane and oxidative hydrolysis of the resulting silane compounds 2a and 3a produced pyridylsiloxanols 2b and 3b, respectively. These compounds were condensed with dichloromethylsil(ox)ane reagents to form the corresponding symmetric bis(pyridyl)methylsiloxane oligomers. Flash chromatography on silica gel afforded 4(a-g) as clear colorless oils in fair to good yields. Alternative preparations of 3- and 4-pyridylsil(ox)anes based on organolithium chemistry have been reported in the literature,13-16 but the approach used here offers several advantages. These include the introduction of sensitive Si-O bonds after the harsh Si-C bond-forming process and the convergent synthesis step, which offers numerous opportunities for varying the ligand size, shape, and connectivity. Combining toluene-d8 solutions of Pd(OAc)2 and ligand 4(a-g) (2:1 py:Pd unless otherwise noted) resulted in spontaneous formation of the corresponding Pd-pyridylsiloxane complexes, designated Pd-4(a-g). The characteristic yellow color of Pd(OAc)2 disappeared within 5-20 s of addition of stoichiometric quantities of these ligands at room temperature. While short-chain complex Pd-4a was slightly soluble in toluene (ca. 10 mM), complexes Pd-4(b-g) were soluble throughout the temperature range studied. No attempt was made to crystallize these complexes, since most complexes of this type crystallize in an infinite chain configuration quite different from the anticipated solution-phase structures, as was the case for a reported Ag complex of trisiloxane 4b.13 Complexes Pd-4 were found to be stable toward Si-C and (12) Uchida, H.; Kabe, Y.; Yoshino, K.; Kawamata, A.; Tsumuraya, T.; Masamune, S. J. Am. Chem. Soc. 1990, 112 (19), 7077–7079. (13) Jung, O. S.; Lee, Y. A.; Kim, Y. J. Chem. Lett. 2002, No. 11, 1096–1097. (14) Schmitz, M.; Leininger, S.; Fan, J.; Arif, A. M.; Stang, P. J. Organometallics 1999, 18 (23), 4817–4824. (15) Zeldin, M.; Tian, C. X.; Xu, J. M., Abstr. Pap.-Am. Chem. Soc. 1987, 193, 51. (16) Zeldin, M.; Xu, J. M.; Tian, C. X. J. Organomet. Chem. 1987, 326 (3), 341–346.

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Figure 1. Effect of concentration on the ortho aromatic region of the 1H NMR spectra for Pd-4(a-f) (toluene-d8, [Pd] = 1.5-23.8 mM, 2:1 pySi:Pd, 353 K). R1, R2, and R3 are peaks assigned to monomeric, dimeric, and trimeric cyclic coordination complexes, respectively. Riþ refers to the i-mer and larger cyclic coordination oligomers. Scheme 2. Structures of Smaller Cyclic Coordination Oligomers R1, R2, and R3, with the Siloxane Chains Represented by Dashed Lines for Clarity

Si-O bond cleavage over the temperature range studied, with no decomposition detected by 1H NMR after several days of storage at room temperature. 2.2. Spectral Features and Concentration Dependence of Pd-4(a-g). The 1H NMR spectra of Pd-4(a-g) (Figures 1 and S5-S11) differ from those of 4(a-g) in several important ways. One or more singlets appeared near 1.9 ppm, due to the acetate CH3 group of Pd(OAc)2(py)2. In addition, the chemical shifts of the aromatic (6.4-9.6 ppm) and Si-CH3 (0.0-0.6 ppm) resonances changed significantly upon binding of Pd. These shifts are expected for the aromatic peaks, due to their close proximity to the pyridinic nitrogen. The changes in the peak shifts for Si-CH3 are more surprising, since these protons are located many bonds away from the coordination site; most likely it is due to close proximity to the cationic metal center in a low dielectric constant medium.

For the monofunctional ligand 2c, only one set of peaks was observed in the 1H NMR spectrum of its Pd complex Pd2c, indicating that only one coordination species was formed. On the other hand, bifunctional ligand complexes Pd-4(a-g) all exhibited multiple sets of 1H NMR peaks. While the chemical shifts of these resolved peaks were not sensitive toward dilution, their peak integrals were highly sensitive (Figure 1). This behavior strongly implied that the multiple sets of peaks corresponded to various cyclic oligomers. The structures of the smaller ones are illustrated in Scheme 2. We assume here that the pyridylsiloxanes coordinate in the trans configuration, since the complex Pd(OAc)2py2 appears in the crystallography literature only as the trans isomer.17 (17) Kravtsova, S. V.; Romm, I. P.; Stash, A. I.; Belsky, V. K. Acta Crystallogr. Sect. C 1996, 52 (9), 2201–2204.

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Table 1. Best-Fit Values of Kinter and EMi for Pd-4(a-g) -1

b

ligand

B (mol L )

4a 4b 4c 4d 4e 4f 4g

0.201 0.130 0.0987 0.0710 0.0584 0.0487 0.0303

EM1a (mol L-1)

EM2a (mol L-1)

EM3a (mol L-1)

EM4a (mol L-1)

Kinter

0.0041 0.0129 0.0162 0.0142

0.0027 0.0075 0.0091 (0.0103) (0.0086) (0.0054)

0.0030 0.0032 (0.0063) (0.0046) (0.0037) (0.0031) (0.0019)

(0.0063) (0.0041) (0.0031) (0.0022) (0.0018) (0.0015) (0.0009)

2.27 1.54 1.59 0.48 1.94 1.03 0.76

a Values in bold represent optimized model parameters, while values in parentheses represent values of EMi calculated from B by the formula EMi = Bi-5/2. b Estimated from the empirical correlation in Mandolini, Advances in Physical Organic Chemistry, Table 19. See the Supporting Information for method of estimation.

Based on systematic trends in chemical shift as a function of ligand length and the corresponding ring size, as well as the observed response of the peaks to changes in concentration, assignments of the ortho aromatic peaks to cyclic coordination oligomers Ri were made, where i refers to the number of Pd atoms in the ring (assignments shown in Figure 1). For the shorter ligands 4(a-c), this proton produced multiple singlets at 9.1-9.2 ppm, while for 4d, peaks at 9.49, 9.18, and 9.17 ppm were observed. The peak at 9.49 ppm decreased monotonically with increasing concentration, while the peak at 9.17 ppm increased monotonically, and the peak at 9.18 ppm passed through a maximum at intermediate concentration. A similar trend was observed for the remaining Pd-4, in that the relative intensities of the peaks farthest downfield increased upon dilution. According to ring distribution statistics, smaller cyclic species are favored at lower concentrations. Also, a simple molecular model of Pd-4(a-g) (MM2 potential) indicated that R1 is highly distorted from the square-planar geometry for ligands smaller than 4d. Therefore, we assigned the resonance at 9.49 ppm for Pd-4d to the R1 complex, 9.18 ppm to R2, and 9.17 ppm to R3þ (i.e., R3 and larger). Similarly, for Pd-4e, the peaks at 9.29 and 9.18 ppm were assigned to R1 and R2þ (i.e., R2 and larger), respectively; and for Pd-4f, the peaks at 9.23 and 9.19 ppm were assigned to R1 and R2þ, respectively. For decasiloxane Pd-4g no distinction in the ortho singlet was observed for the various cyclic oligomers, so the acetate CH3 resonance was used instead (see Figure S11, middle region), with the singlets at 1.87 and 1.89 assigned to R1 and R2þ, respectively. A similar reasoning was used for the shorter ligands where R1 cannot be formed. Molecular modeling of the higher macrocycles excluded R2 for short-chain complex Pd-4a. For tetrasiloxane complex Pd-4c, the ortho singlets at 9.19 and 9.16 ppm were assigned to R2 and R3þ. For trisiloxane complex Pd-4b, the peaks at 9.16, 9.14, and 9.12 ppm were assigned to R2, R3, and R4þ. Finally, for disiloxane complex Pd-4a, the peaks at 9.10 and 9.09 ppm were assigned to R3 and R4þ, respectively. Supporting characterization for these peak assignments was obtained using the 2D DOSY technique, which is sensitive to the diffusion coefficient (and, consequently, molecular weight) of the molecular species present. A set of peaks is observed in the 2D DOSY map of Pd-4d near log10 D = -9.08, with D given in m2/s, while the nearest peaks in the map of Pd-4c lie at log10 D = -9.26 (for comparison, the toluene CHD2 peak appears at log10 D = -8.6). This transition correponds with our prediction of the appearance of the cyclic coordination monomer R1 for pentasiloxane ligands and longer. Interestingly, the peaks assigned to R1 sometimes had chemical shifts quite different from the corresponding

Table 2. Difference in Chemical Shifts between Complexes R1 and R¥ for 4d-g, Where R¥ Refers to the Larger Cyclic Oligomers Ri.1 (whose chemical shifts rapidly converge to a single observed peak; see Figure 1) C(Si)-CH-N

C(dO)CH3

Si-CH3

ligand

δR1

δR¥

δR1

δR¥

Si positiona

δR1

δR¥

4d 4e 4f 4g

9.492 9.290 9.230 9.189

9.168 9.186 9.187 9.189

1.814 1.830 1.840 1.877

1.897 1.896 1.897 1.896

5 5, 7 5, 9 9, 11

0.543 0.445 0.350 0.264b

0.121 0.140 0.038 0.039

a Refers to the position of the Si(CH3)2 unit along the siloxane chain, according to the standard convention where oxygens are counted in the numbering. The positions reported here correspond to the strongest downfield shifts. Doubled entries refer to symmetric chain positions. b Assignment is tentative, due to ambiguity in peak structure.

peaks for Ri>1 (Figure 1, 4d-f; see also Table 2). This was first observed with the pentasiloxane complex Pd-4d, the shortest siloxane chain for which the formation of R1 is feasible. The chemical shift anomaly became less pronounced for complexes of longer siloxanes. For example, the pyridyl ortho singlet for R1 was found at 9.49 ppm for Pd-4d, 9.29 ppm for Pd-4e, 9.23 ppm for Pd-4f, and 9.19 ppm for Pd-4g. Similar trends were observed for the acetate and methylsiloxane resonances. The effect is most likely due to the unique chemical shielding environment and molecular conformation of the chelate R1, compared to larger macrocycles. 2.3. Competitive Coordination of Bis(pyridyl)siloxane and Pyridine. Displacement of bifunctional ligands 4(a-g) from complexes Pd-4(a-g) was investigated by adding successive aliquots of pyridine-d5 (0.2 N in toluene-d8) at 353 K. Since pyridine-d5 is invisible by 1H NMR, the displacement process could be monitored by quantifying the decrease in peak intensities associated with bound pyridylsiloxane and the appearance and increase in intensity of the unbound pyridylsiloxane. For the case of equal binding affinity of pyridine-d5 and pyridylsiloxane, half of the pyridylsiloxane would be displaced if equimolar pyridine-d5 were added. This was observed for monopyridylsiloxane 2c, for which equimolar pyridine-d5 displaced half of the ligand from Pd (Figure 2, 2c). On the other hand, for most of the bis(pyridyl)siloxanes, a greater-than-equimolar concentration of pyridine-d5 was needed to displace half of the ligand, showing that the displacement was more difficult than predicted by simple exchange (Figure 2, 4(a-g)). Performing the displacement in reverse (displacing pyridine-d5 coordinated to Pd(OAc)2 with 4) yielded very similar results, confirming that the effect was thermodynamic rather than kinetic in nature. Surprisingly, the observed binding affinity among the bis(pyridyl)siloxanes was found to pass through a minimum

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Figure 2. Effect of pyridine-d5 on the aromatic region of 1the H NMR spectra for Pd-2c and Pd-4(a-g) (353 K, 5.9 mM Pd(OAc)2). Spectra are labeled by μmol of pyridine-d5 added (equivalence point at 8.9 μmol).

Figure 3. Effect of dilution on competitive binding of pyridined5 and Pd-4(a-g) (353 K, pySi:Pd = 1-1.2:1, py-d5:pySi = 0.9-1.1:1).

as a function of chain length, with 4c and 4d showing the weakest binding. In order to evaluate the concentration dependence of the competitive binding equilibrium, a roughly equimolar mixture of Pd-4 and pyridine-d5 was successively diluted from 5.7 to 1.4 mM. The results are plotted in Figure 3 (based on spectral data in Figures S20-S26). For the shortchain ligands 4(a-c), the equilibrium shifted very slightly toward bound pyridylsiloxane upon dilution, while for 4(d-f) the equilibrium shift upon dilution was more significant. This behavior suggests that the intramolecular (bidentate) coordination with pyridylsiloxane is favored

over intermolecular coordination with pyridine-d5 at the lower concentrations. 2.4. Effect of Temperature on 1H NMR Spectra and Binding Equilibrium. The temperature dependence of the 1H NMR spectra of the complexes Pd-4(a-g) was studied in the range 233-363 K. Experiments were performed in the absence and presence of pyridine-d5, with the former probing the temperature dependence of ring interconversion and the latter probing the competitive binding equilibrium. Figure 4 shows representative results for 4f. At around 293 K and in the absence of pyridine-d5, a dramatic change in the spectrum of Pd-4f was observed, with crossover in chemical shift of the singlet (position 2) and doublet (position 6) ortho aromatic peaks, suggesting a transition in the orientation of pyridine at the coordination site. No such transition was observed for the monodentate ligand 2c, indicating that the transition for Pd-4f was due to ring conformation. It is likely that the effect is due to a change in the preferred conformation of the siloxane chain upon cooling. The other complexes studied showed more or less similar behavior (results for 2c and 4(a-g) are provided as Supporting Information). While the 1H NMR chemical shifts of Pd-4(a-g) were highly sensitive to temperature, the peak integrals were found to be nearly independent of temperature, in both the presence and absence of pyridine-d5. Figure 5 illustrates this behavior for the pentasiloxane complex Pd-4d. The weight fractions of cyclic oligomers R1, R2, and R3þ did not show a statistically significant trend over the temperature range 273-353 K (Figure 5a), suggesting that the enthalpic

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Figure 4. Effect of temperature on the 1H NMR spectrum of heptasiloxane complex Pd-4f (toluene-d8, 233-353 K, [Pd] = 5.9 mM, pySi:Pd = 2:1). Peak assignments are for R1, which is the predominant species under these conditions. Si1, Si3, Si5, and Si7 are for different siloxane positions along the chain, starting from the pyridylsilane end group and counting oxygen atoms in the numbering.

Figure 5. Effect of temperature on (a) ring distribution ([Pd] = 5.9 mM, pySi:Pd = 2:1) and (b) competitive binding (py-d5:pySi = 1.1:1) of pentasiloxane complex Pd-4d. Similar lack of temperature dependence was obtained for Pd-2c and the other Pd-4.

contributions to the ring interconversion equilibria (iþ1)Ri / iRiþ1 are minimal. The competitive binding equilibrium with pyridine-d5 was also nearly independent of temperature (Figure 5b), demonstrating that the N-Pd bond energy is unaffected by silylation at the pyridine ring. Similar results were obtained for the other pyridylsiloxane ligands. 2.5. Fitting of Experimental Data to Ring-Chain Equilibrium Model. The experimental data were analyzed using the ring-chain equilibrium theory of Jacobson and Stockmayer (JS),9,18 as modified by Mandolini et al.19 (A detailed description of the model, derivation of the equations, and method of solution are provided in the Supporting Information.) The JS theory models the ring-chain equilibrium for a linear coordination system lacking branching units, assuming Gaussian chain dynamics and equal chain end reactivity. The modification by Mandolini et al. (hereafter referred to as JSM) accounts for small coordination rings that suffer from ring strain or non-Gaussian chain dynamics and expresses the concentrations in terms of effective molarities, which is particularly useful for the characterization of macrocycles and small coordination oligomers. The model consists of mole balances for the participants in the binding (18) Jacobson, H.; Stockmayer, W. H. J. Chem. Phys. 1950, 18 (12), 1600–1606. (19) Ercolani, G.; Mandolini, L.; Mencarelli, P.; Roelens, S. J. Am. Chem. Soc. 1993, 115 (10), 3901–3908.

equilibrium, with the functional forms of the ring and chain distributions being taken from the JS model. Simultaneous solution of the mole balances gives a unique value of the polymerization parameter x, which, in the JSM model, is the fraction of reacted monomer groups in the chain fraction. In our system, the monomeric unit for ring-chain equilibrium can be written as -py(SiMe2O)npyPd(OAc)2-. Therefore x is proportional to the fraction of the pyridyl groups in the chain species that is coordinated to Pd. (For cyclic species this fraction would be fixed at 1, since all pyridyl groups must be coordinated to Pd to close the ring.) The value of x and the total concentrations of ligand and metal determine the weight fraction of cyclic complexes in the system and the distribution of linear and cyclic species. In applying the JSM model to our experiments, the set of parameters must satisfy all observations, including the competitive binding experiments, where the coordinated bis(pyridyl)siloxane ligands are displaced by the spectroscopically invisible probe molecule pyridine-d5. In order to account for the competitive binding of pyridine-d5, the equilibrium constant for the intermolecular reaction, Kinter, is defined using three intermolecular reactions, corresponding to the three types of possible chain species:

CðiÞpp þ CðiÞll / Cð2iÞpl þ P, 4Kinter ¼

2CðiÞll / Cð2iÞll þ L, Kinter ¼

½Cð2iÞpl ½P ð1Þ ½CðiÞpp ½CðiÞll

½Cð2iÞll ½L

2CðiÞpp þ L / Cð2iÞpp þ 2P, 4Kint er ¼

½CðiÞll 2 ½Cð2iÞpp ½P2 ½CðiÞpp 2 ½L

ð2Þ

ð3Þ

In these equations, [P] refers to the concentration of probe pyridine-d5 in solution, and [L] refers to the concentration of free bis(pyridyl)siloxane ligand. Thus, C(i)pp is the class of coordination chains (py)Pd(OAc)2[(L)Pd(OAc)2]i-1(py), C(i)ll is the class (L)Pd(OAc)2[(L)Pd(OAc)2]i-1(L), and C(i)pl is the class (py)Pd(OAc)2[(L)Pd(OAc)2]i-1(L). For C(i)ll and C(i)pl, the L at the chain ends possesses one uncoordinated pyridyl group. An implicit assumption here is that the bond

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energies of P or L to Pd in these species are identical and independent of chain length; this assumption is validated by experiment (see Discussion). In these equations, the prefactors of Kinter account for changes in symmetry number going from reactants to products. Formally, these reactions can be classified as condensations, since they eliminate probe molecule or ligand with an increase in chain length. As a result, Kinter is a relative intermolecular equilibrium constant, with pyridine-d5 serving as the reference. It should be noted that autodissociation of Pd-N is neglected in this treatment, due to the strong binding of these ligands to Pd. In general, cyclic (ring) complexes Ri may exist alongside the chain complexes Ci in solution. The smallest ring, LPd(OAc)2 or R1, consists of a bis(pyridyl)siloxane ligand with both pyridyl groups coordinated to the same Pd. Likewise, R2 consists of two ligands and two Pd with each ligand bridging two Pd, R3 consists of three bridging ligands and three Pd, and so on. Because autodissociation of Pd-N is negligible, only Ri complexes were observed in experiments without added pyridine-d5, due to the lack of available chain terminations. The ring-chain equilibrium system is governed by mole balances for the metal, ligand, and probe. Substituting in the equilibrium relationships 1-3 and the distributions from JSM theory and replacing the infinite sums in x with analytical expressions (see Supporting Information for full derivation), the following mole balance equations were obtained:

the EMi for the smallest cyclic oligomers are best fitted to experimental data, since they deviate significantly from their ideal values given by the formula EMi = Bi-5/2. Since the system concentration in this work lies in the range 1-25 mM, only cyclic oligomers with effective molarities of this magnitude make a significant contribution to the mole balances. The fitting process for EMi and Kinter started with the experiments satisfying [Ltot] = [Pdtot] and [Ptot] = 0, for which eqs 4-6 can be simplified due to the lack of chain species. The NMR peak integrals were assigned to their corresponding cyclic oligomers, and the results were normalized to give weight fractions WRi. If the chemical shifts were found to overlap (as is the case for the higher oligomers), the weight fractions were lumped in the fitting procedure (denoted as WRiþ). Equations 4-6 were then used to predict the observed weight fractions, using the formulas

WRi ¼

iEMi xi rP -1 ¥ P iEMi xi þ B i - 3=2 xi i¼r

i¼1

EMi ¼ Bi - 5=2 , i g r

WRi þ ¼

ð7Þ

¥ X

WRi

ð8Þ

ð9Þ

j¼i

½Pdtot

0

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r-1 X

¥ X iEMi x þ B i - 3=2 xi þ

1

i

B C B i¼1 C i¼r B C ¼ B 2 C ½P x ½P x ½L x @ A þ þ 2 2 2 4½LKinter ð1 - xÞ Kinter ð1 - xÞ Kinter ð1 - xÞ

½P2 x ½P x þ þ ½P ½Ptot ¼ 2½LKinter ð1 - xÞ Kinter ð1 - xÞ

ð4Þ ð5Þ

0

r-1 ¥ X X iEMi xi þ B i - 3=2 xi þ B B i¼1 i¼r ½Ltot ¼ B B 2 2 x ½P x @ ½P þ 4½LKinter ð1 - xÞ2 Kinter ð1 - xÞ2 1

C C ½L 2x - x2 þ þ ½LC C Kinter ð1 - xÞ2 A

ð6Þ

In eqs 4-6, the variables to be solved for are x, the polymerization parameter, and [L] and [P], the concentrations of unbound ligand and probe, respectively. The model parameters are the ring distribution prefactor B, Kinter, and the effective molarities EMi for the smallest cyclic oligomers (containing fewer than r Pd centers, where r signifies the transition between non-Gaussian and Gaussian chain dynamics). In this model, the parameters B and EMi, which take units of concentration, reflect the favorability of cyclization of the coordination oligomers. While B can be computed from geometric and statistical considerations,

The model parameters EMi were then fitted to the experimental data for WRi by least-squares regression. For the competitive binding studies, the number of species involved was much higher, so the peak integrals were lumped into bound and unbound states, and a single-parameter fit to Kinter was performed. Table 1 gives the best-fit values of EMi and Kinter for the ligands 4(a-g). For 4b and 4d it was necessary to adjust two values of EMi to obtain qualitative agreement with the available ring distribution data. For the other ligands good agreement was obtained by adjusting a single value of EMi, allowing the other values to be defined implicitly by B. The relative standard deviation for the effective molarities and Kinter was estimated to be (5%, on the basis of uncertainties in the peak deconvolution procedure. A visual comparison between the experimental binding data and the JSM model for Pd-4d is presented in Figure 6. The model accurately reflects the concentration dependence of R1, R2, and R3þ for Pd-4d with two adjustable parameters, EM1 and EM2, with the weight fraction of R2 passing through a maximum near 5 mM. The competitive binding equilibrium with pyridine-d5 is also accurately modeled with a single adjustable parameter, Kinter.

3. Discussion The monopyridylsiloxane 2c was observed to bind to palladium acetate with the same affinity as pyridine, demonstrating that silylation of pyridine at the meta position does not significantly affect the binding thermodynamics or the coordination geometry. This is consistent with the reported Hammett substituent constants for the (trimethylsiloxy)dimethylsilyl group (-0.01 for meta substitution), indicating

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Figure 6. Experimental data (points) and fits with the JSM model (curves) for pentasiloxane complex Pd-4d at 353 K. The ring distributions (left) were fitted to the model with two adjustable parameters (EM1 and EM2), and the competitive binding equilibrium with pyridine-d5 (right) can subsequently be modeled with a single adjustable parameter (Kinter).

that the electronic effect of silylation on the pyridinic nitrogen is negligible.20 In contrast, the tethering of two pyridyl groups with a methylsiloxane chain could, in principle, produce significant changes in the binding affinity of the ligand and the distribution of coordination oligomers, depending on the siloxane chain length. If the siloxane chain is too short, the cyclic coordination monomer would be highly strained and unfavorable, and only the larger coordination oligomers would be capable of cyclization. On the other hand, if the siloxane chain is very long, the end groups would act independently of each other. At some optimal chain length, binding of the first pyridyl group to a Pd center would facilitate the binding of the other pyridyl group to form a strainless bidentate complex. This reasoning extends to the higher cyclic oligomers. In applying the JSM model to our system, effective molarity is used to describe the favorability of forming the smallest cyclic oligomers Ri, i < r. The value of EMi is taken to be zero if the corresponding Ri is not experimentally detected. It is depressed from the theoretical value predicted by the formula EMi = Bi-5/2 if Ri is thermodynamically disfavored, and it is enhanced relative to the theoretical value if Ri is thermodynamically favored (for example, due to synergistic effects or preorganization). Based on these considerations, the best-fit EMi values shown in Table 1 indicate that for disiloxane 4a the smallest ring present in a solution containing 5 mM each of Pd(OAc)2 and 4a was R3. Cyclic coordination dimers R2 were observed for ligands 4b and 4c, and bidentate complexes R1 were detected for 4d-g. Interestingly, the value of EM1 was noticeably smaller for 4d than for the longer ligands, suggesting that a thermodynamic penalty of formation of R1 exists for 4d, relative to 4e-g. Another consequence of this positive intramolecular interaction is that ligands 4e-g bind Pd(OAc)2 more tightly than pyridine (Figure 3). The effect is greater at lower concentrations, because the intermolecular interaction is suppressed by the high dilution, while the intramolecular interaction remains unaffected. Nevertheless, all of the EM1 for 4d-g are depressed from the corresponding values of B, indicating that the disordered siloxane linkers do not exhibit any preorganization effect beyond that predicted by the JSM theory. (20) Plzak, Z.; Mares, F.; Hetflejs, J.; Schraml, J.; Papousko., Z; Bazant, V.; Rochow, E. G.; Chvalovs., V. Collect. Czech. Chem. C 1971, 36 (9), 3115.

Missaghi et al.

The Kinter in this model describes the favorability of intermolecular reaction. In arriving at eqs 1-3, it is assumed implicitly that Kinter is independent of coordination oligomer size, although it may be a function of siloxane oligomer length. Since the change in number of coordination bonds in eqs 1-3 is zero, we expect the ΔH contribution to Kinter to be minimal. Also, the change in mole number for eqs 1-3 is zero, suggesting that the ΔS contribution to Kinter is also small. Taken together, these considerations led to a prediction of ΔGinter ≈ 0 and Kinter ≈ 1 for all ligands 4a-g. The best-fit values of Kinter for Pd-4(a-g) vary over the range 0.48-2.27, with a mean value of 1.37 (σ = 0.64), in reasonable agreement with the expected value. The variation in Kinter among Pd-4(a-g) may be due to solvation effects or differences in the conformational entropy of the chain. Abed et al. performed a similar analysis on hydrogenbonded oligomers of bis(carboxy)siloxanes.21 For siloxane linkers containing 8, 14, and 23 Si atoms, they reported EM1 values of 0.15, 1.2, and 1.4 mM, respectively, and B of 7, 14, and 4 mM, respectively. These EM1 values are about an order of magnitude lower than our best-fit EM1 for Pd4(e-g), and the B values are also depressed from our calculated B parameters. This could be due to the more rigid trans configuration of the Pd complexes, compared to the hydrogen-bonded siloxane oligomers. Also, a convergence of EM1 to B for long siloxane spacers was observed by Abed et al., which corresponds closely to our observations. Additional insights into the binding configuration of 4(a-g) can be gained from the systematic variations in the 1H NMR spectra of their coordination complexes (Figures 1 and S5-S11). In particular, the observed chemical shifts of the ortho aromatic, acetate CH3, and Si-CH3 protons in R1 deviate significantly from those for the larger cyclic oligomers Ri.1 (denoted as R¥). These deviations are given in Table 2 for Pd-4(d-g), with the values for R¥ estimated from the spectral data in Figures S5-11. Interestingly, the deviations in chemical shift for the smallest strainless R1, Pd-4d, were much higher than for longer siloxane spacers Pd4(e-g). These large deviations were observed for the aromatic and acetate protons, as well as Si-CH3 protons located many bonds away from the coordinating groups. This suggests that a through-space deshielding effect by the cationic Pd center is responsible for the large chemical shift deviations. As the chain length increases, the siloxane is able to explore more conformational space and the average distance between the internal Si-CH3 groups and the metal center increases, as does the rotational freedom of the squareplanar complex relative to the chain. A schematic diagram of the conformational degrees of freedom of R1 is given in Figure 7. It is possible that the chemical shift of the pyridyl ortho hydrogen is most sensitive to its position relative to the plane of the square complex (defined by j in Figure 7), whereas the chemical shifts of the acetate and Si-CH3 protons are sensitive to their mutual separation (affected by both j and θ). In this scheme, the deviation in chemical shift of the ortho aromatic proton would vanish when the pyridyl ring is able to freely rotate into and out of the plane of the complex. Similarly, the chemical shift deviations of the acetate and SiCH3 protons would approach their limiting values when their average separation exceeds the threshold for throughspace magnetic shielding interaction. Although related, these (21) Abed, S.; Boileau, S.; Bouteiller, L. Macromolecules 2000, 33 (22), 8479–8487.

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Figure 7. Rotational degrees of freedom about the Pd centers in Ri, the pyridine-pyridine skew angle θ, and the orientation of the coordination plane j. Both of these rotations are restricted when the bridging siloxane chain is too short: θ is restricted because the chain cannot stretch to accommodate the new orientation; j is restricted because of steric hindrance between the siloxane chain and the acetate groups. The dashed line represents the spanning part of the cyclic complex, either a siloxane chain (for R1) or the remaining ligand-Pd units (for higher Ri).

two effects decay at different rates as a function of siloxane chain length, as reflected in Table 2. In summary, we have measured the ring-chain distributions formed by the interaction of bis(meta-pyridyl)siloxane ligands 4(a-g) with palladium(II) acetate in dilute solutions, by fitting the spectroscopic data to a modified JacobsonStockmayer model and expressing the distributions in terms of effective molarities. Although the bidentate complex R1 is strainless for pentasiloxane 4d, R1 predominates at 1-5 mM only for hexasiloxane and longer oligomers (4(e-g)). Furthermore, the conformational restriction of the pentasiloxane ligand in R1 leads to unusual chemical shifts for the ligand and acetate protons, relative to larger cyclic coordination oligomers. The results show that ligand structure and conformation have a stronger influence on the binding properties of this class of bidentate ligands than the coordination bond strength.

4. Conclusion We have shown that the binding of a bidentate bis(pyridyl)siloxane ligand to a Pd(OAc)2 complex depends significantly on the siloxane chain length. Formation of the bidentate complex R1 is observed only when the ring is strainless. However, torsional constraints on the pyridyl group and siloxane chain also exert a strong influence on the ringchain distribution, as well as the magnetic shielding environment of the ligand and acetate protons. These complexes may be of use in homogeneous catalysis (for example, the aerobic oxidation of alcohols to carbonyl compounds), and the binding properties elucidated here provide a starting point for the prediction and correlation of catalytic performance, especially when the activity is sensitive to the binding affinity of the ligand or the geometry of the coordination complex.

5. Experimental Section General Information. Starting materials were purchased from Aldrich and were used as received, except for 3-bromopyridine, which was diluted in dichloromethane and dried over sodium sulfate, the solvent being subsequently removed by rotary evaporation. Manipulations of air- and moisture-sensitive chemicals were performed on a double manifold under N2. Elemental analyses were performed by Atlantic Microlab, Inc. (Norcross, GA). Spectroscopic and HRMS data were collected in the IMSERC facility at Northwestern University. NMR spectra were collected on a Varian INOVA 400 MHz spectrometer with a broadband probe. Typically, 1H NMR spectra were collected with 16 transients and a recycle delay of 4 s, giving signal-to-noise ratios in excess of 100:1 for accurate peak integration. 3-Pyridyldimethylsilane (1a). A 200 mL Schlenk flask equipped with a magnetic stir bar and rubber septum was

evacuated and refilled with N2 and connected to an oil bubbler. In this flask was placed 80 mL of anhydrous THF and 7.6 mL (12.5 g, 80 mmol) of 3-bromopyridine. While the mixture was stirred at room temperature, 40 mL of a 2.0 M solution of isopropylmagnesium chloride in THF was added by syringe over 1 min. The solution turned dark red and formed a dispersed yellow precipitate within 10 min, which underwent further coagulation over 30 min to form a sticky red mass at the bottom of the flask. The solution was stirred for a total of 1 h, after which it was cooled in an ice bath, 10.2 mL (8.67 g, 92 mmol) of chlorodimethylsilane was added by syringe, and the reaction mixture was allowed to warm to room temperature. Disappearance of the precipitated reagent over a period of 15 min was accompanied by the formation of a voluminous yellow-white salt. The reaction mixture was diluted in 2 parts of diethyl ether, washed with 2 100 mL portions of distilled water and 1 50 mL of brine, dried over Na2SO4, and concentrated on a rotary evaporator to give 10.79 g of a clear yellow-red oil. The product was purified by vacuum distillation (87 C, 30 Torr) to give 9.84 g of 1a as a clear liquid (72 mmol, 90%). 1H NMR (400 MHz, CDCl3): δ 8.68 (bs, 1H), 8.56 (dd, J1 = 4.9 Hz, J2 = 1.8 Hz, 1H), 7.77 (dt, J1 = 7.5 Hz, J2 = 1.8 Hz, 1H), 7.22 (ddd, J1 = 7.5 Hz, J2 = 4.9 Hz, J3 = 0.9 Hz, 1H), 4.42 (hept, J = 3.75 Hz, 1H), 0.34 (d, J = 3.75 Hz, 6H) ppm. 13C NMR (100 MHz, CDCl3): δ 154.5, 150.4, 141.8, 132.4, 123.4, -4.0 ppm. 29Si NMR (80 MHz, CDCl3): δ -17.8 ppm. HRMS (ESI): calcd for C7H12NSi ([M þ H]þ) m/z 138.0734; found 138.0722 (Δm/z = 0.001 Da). Anal. Calcd for C7H11NSi: C, 61.25; H, 8.08; N, 10.20. Found: C, 59.23; H, 7.93; N, 9.98. 3-Pyridyldimethylsilanol (1b). A 200 mL Schlenk flask equipped with a magnetic stir bar and rubber septum was connected to an oil bubbler and purged with a stream of N2, then charged with 40 mg of Pearlman’s catalyst (20 wt % Pd(OH)2/C) and 80 mL of diethyl ether that was previously saturated with H2O. The reaction was stirred while 4.0 g (29 mmol) of 3-pyridyldimethylsilane was added by syringe. Evolution of H2, as monitored with the bubbler, was complete within 2 h, at which time the reaction mixture was filtered to remove the catalyst. Removal of solvents gave 1b as a turbid, viscous oil. The neat liquid was found to condense to the disiloxane at room temperature with t1/2 ≈ 6 h, while ether and chloroform solutions of the silanol show much higher stability against condensation. Yield: 90%, with less than 10% of the disiloxane present as a contaminant, as judged by 1H NMR. 1H NMR (400 MHz, CDCl3): δ 8.63 (bs, 1H), 8.47 (dd, J1 = 5.1 Hz, J2 = 1.8 Hz, 1H), 7.90 (dt, J1 = 7.5 Hz, J2 = 1.8 Hz, 1H), 7.28 (ddd, J1 = 7.5 Hz, J2 = 5.1 Hz, J3 = 0.8 Hz, 1H), 5.92 (bs, 1H), 0.39 (s, 6H) ppm. 13 C NMR (100 MHz, CDCl3): δ 152.9, 149.4, 142.1, 135.8, 123.9, 0.4 ppm. 29Si NMR (80 MHz, CDCl3): δ 4.5 ppm. HRMS (ESI): calcd for C7H12NOSi ([M þ H]þ): m/z 154.0683; found 154.0687. 1-(3-Pyridyl)-1,1,3,3-tetramethyldisiloxane (2a). 2a was prepared in the same manner as 1b, but instead of isolating the silanol, the reaction mixture was treated as follows. Drying of the silanol solution was accomplished by percolation through a narrow column packed with 12 g of CaSO4 (Acros, nonindicating,

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20-40 mesh), followed by an additional 30 mL portion of dry ether. The dried silanol solution was collected in a 250 mL threenecked flask, which was previously connected to an oil bubbler and purged with N2, and the contents were cooled in an ice bath. To this flask was added 6.8 mL (49 mmol) of triethylamine and immediately afterward 4.9 mL (44 mmol) of chlorodimethylsilane. A voluminous white precipitate immediately formed, which was accompanied by a significant amount of fuming. The reaction mixture was washed with 2 50 mL portions of water, followed by 1 30 mL of brine, and volatile components were removed by rotary evaporation to give 4.88 g of 2a as a clear liquid (23 mmol, 79%). 1H NMR (400 MHz, CDCl3): δ 8.58 (m, 1H), 8.47 (dd, J1 = 4.9 Hz, J2 = 1.9 Hz, 1H), 7.67 (dt, J1 = 7.5 Hz, J2 = 1.9 Hz, 1H), 7.14 (ddd, J1 = 7.5 Hz, J2 = 4.9 Hz, J3 = 1.0 Hz, 1H), 4.62 (hept, J = 2.7 Hz, 6H), 0.24 (s, 6H), 0.06 (d, J = 2.7 Hz, 6H) ppm. 13C NMR (100 MHz, CDCl3): δ 153.8, 150.6, 140.9, 134.5, 123.4, 0.98, 0.71 ppm. 29Si NMR (80 MHz, CDCl3): δ -0.4, -4.2 ppm. HRMS (ESI): calcd for C9H18NOSi2 ([M þ H]þ) m/z 212.0921; found 212.0912. 1-(3-Pyridyl)-1,1,3,3-tetramethyldisiloxan-3-ol (2b). A 200 mL Schlenk flask equipped with magnetic stir bar and rubber septum was connected to an oil bubbler and purged with a stream of N2, then charged with 80 mg of Pd(OH)2/C and 80 mL of diethyl ether that was previously saturated with H2O. The reaction was stirred while 4.2 g (20 mmol) of 2a was added by syringe. After 4 h the evolution of H2 ceased. The catalyst was removed by filtration, and all volatiles were removed on a vacuum line until a pressure of 10 mTorr was achieved to give 4.48 g of 2b as a clear oil (18.9 mmol, 95%). The product was found to be stable at room temperature overnight and at -20 C for several weeks. Alternatively, it could be stored as a ca. 7 wt % solution in CHCl3 for several months without decomposition. 1H NMR (400 MHz, CDCl3): δ 8.69 (s, 1H), 8.54 (dd, J1 = 4.9 Hz, J2 = 1.8 Hz, 1H), 7.85 (dt, J1 = 7.5 Hz, J2 = 1.8 Hz, 1H), 7.19 (ddd, J1 = 7.5 Hz, J2 = 4.9 Hz, J3 = 1.1 Hz, 1H), 3.66 (vbs, 1H), 0.39 (s, 6H), 0.15 (s, 6H) ppm. 13C NMR (100 MHz, CDCl3): δ 153.1, 149.7, 141.5, 135.1, 123.5, 0.89, 0.75 ppm. 29Si NMR (80 MHz, CDCl3): δ -2.5, -10.6 ppm. HRMS (ESI): calcd for C9H18NO2Si2 ([M þ H]þ) m/z 228.0871; found 228.0863. (3-Pyridyl)pentamethyldisiloxane (2c). 2c was prepared according to the synthesis of 2a on the 0.5 g scale, except that chlorotrimethylsilane is used in the condensation step. The product was purified by column chromatography. Yield: 0.258 g (1.14 mmol, 52%). 1H NMR (400 MHz, CDCl3): δ 8.71 (s, 1H), 8.60 (d, J = 4.6 Hz, 1H), 7.80 (dt, J1 = 7.4 Hz, J2 = 1.5 Hz, 1H), 7.27 (m, 1H), 0.35 (s, 6H), 0.10 (s, 9H) ppm. 13C NMR (100 MHz, CDCl3): δ 153.8, 150.4, 140.9, 135.0, 123.4, 2.13, 1.06 ppm. 29Si NMR (80 MHz, CDCl3): δ 7.7, -5.0 ppm. Anal. Calcd for C10H19NSi2O: C, 53.28; H, 8.50; N, 6.21. Found: C, 52.51; H, 8.39; N, 6.18. 1-(3-Pyridyl)-1,1,3,3,5,5-hexamethyltrisiloxane (3a). The siloxanol 2b (4.48 g, 18.9 mmol) was taken up in dry diethyl ether in a 200 mL Schlenk flask under N2, and 4.7 mL of triethylamine (34 mmol) and 3.3 mL of chlorodimethylsilane (30 mmol) were added in rapid succession. The reaction mixture was washed with 2 50 mL of water and 1 30 mL of brine, and volatiles were removed to give 5.11 g of 3a as a clear oil (17.9 mmol, 95%). 1 H NMR (400 MHz, CDCl3): δ 8.72 (s, 1H), 8.60 (dd, J1 = 4.9 Hz, J2 = 1.8 Hz, 1H), 7.82 (dt, J1 = 7.5 Hz, J2 = 1.8 Hz), 7.27 (ddd, J1 = 6.7 Hz, J2 = 4.9 Hz, J3 = 0.8 Hz, 1H), 4.68 (hept, J = 2.8 Hz, 1H, Si-H), 0.38 (s, 6H), 0.16 (d, J = 0.15 Hz, 6H), 0.07 (s, 6H) ppm. 13C NMR (100 MHz, CDCl3): δ 153.8, 150.5, 140.9, 134.6, 123.4, 1.14, 0.86, 0.84 ppm. 29Si NMR (80 MHz, CDCl3): δ -4.8, -8.6, -20.1 ppm. HRMS (ESI): calcd for C11H24NO2Si3 ([M þ H]þ) m/z 286.1109; found 286.1096. 1-(3-Pyridyl)-1,1,3,3,5,5-hexamethyltrisiloxanol (3b). 3b was prepared according to the synthesis of 2b, starting from 3 g (10.5 mmol) of 3a. Yield: 3.05 g (10.1 mmol, 96%). 1H NMR (400 MHz, CDCl3): δ 8.68 (s, 1H), 8.49 (dd, J1 = 4.9 Hz, J2 = 1.7 Hz, 1H), 7.83 (dt, J1 = 7.5 Hz, J2 = 1.8 Hz, 1H), 7.24 (ddd, J1 = 7.3 Hz,

Missaghi et al. J2 = 4.9 Hz, J3 = 0.9 Hz, 1H), 5.85 (vbs, 1H), 0.35 (s, 6H), 0.080 (s, 6H), 0.062 (s, 6H) ppm. 13C NMR (100 MHz, CDCl3): δ 153.2, 149.5, 141.4, 135.1, 123.5 ppm. 29Si NMR (80 MHz, CDCl3): δ -4.9, -15.0, -21.9 ppm. HRMS (ESI): calcd for C11H24NO3Si3 ([M þ H]þ) m/z 302.1058; found 302.1047. 1,3-Bis(3-pyridyl)tetramethyldisiloxane (4a). 4a was prepared according to the synthesis of 2a, except that 0.5 g (3.3 mmol) of 3-pyridyldimethylsilanol was isolated and warmed to 60 C under vacuum for 1 h. The product was purified by column chromatography on silica gel. Yield: 0.191 g (0.6 mmol, 36%). 1 H NMR (400 MHz, CDCl3): δ 8.70 (s, 2H), 8.60 (dd, J1 = 4.8 Hz, J2 = 1.6 Hz, 2H), 7.76 (dt, J1 = 7.5 Hz, J2 = 1.8 Hz), 7.25 (ddd, J1 = 7.5 Hz, J2 = 5.0 Hz, J3 = 0.9 Hz, 2H), 0.38 (s, 12H) ppm. 13C NMR (100 MHz, CDCl3): δ 153.7, 150.7, 140.8, 134.2, 123.5, 0.96 ppm. 29Si NMR(80 MHz, CDCl3): -0.1 ppm. HRMS (ESI): calcd for C14H21N2OSi2 ([M þ H]þ) m/z 289.1187; found 289.1194. Anal. Calcd for C14H20N2OSi2: C, 58.29; H, 6.99; N, 9.71. Found: C, 57.27; H, 7.25; N, 9.50. 1,5-Bis(3-pyridyl)hexamethyltrisiloxane (4b). 4b was prepared from 0.5 g (3.3 mmol) of 1b and 0.213 g (1.65 mmol) of dichlorodimethylsilane, in the same manner as 4d. Yield: 0.33 g (0.91 mmol, 55%). 1H NMR (400 MHz, CDCl3): δ 8.69 (s, 2H), 8.59 (dd, J1 = 4.9 Hz, J2 = 1.5 Hz, 2H), 7.77 (dt, J1 = 7.4 Hz, J2 = 1.8 Hz, 2H), 7.24 (ddd, J1 = 7.4 Hz, J2 = 4.9 Hz, J3 = 0.9 Hz), 0.35 (s, 12H), 0.08 (s, 6H) ppm. 13C NMR (100 MHz, CDCl3): δ 153.7, 150.5, 140.9, 134.5, 123.4, 1.46, 0.83 ppm. 29Si NMR (80 MHz, CDCl3): δ -2.1, -18.1 ppm. HRMS (ESI): calcd for C16H27N2O2Si3 ([M þ H]þ) m/z 363.1375; found 363.1379. Anal. Calcd for C16H26N2O2Si3: C, 52.99; H, 7.23; N, 7.72. Found: C, 52.72; H, 7.23; N, 7.76. 1,7-Bis(3-pyridyl)octamethyltetrasiloxane (4c). 4c was prepared from 1.0 g (6.6 mmol) of 1b and 0.67 g (3.3 mmol) of 1,3-dichlorotetramethyldisiloxane, in the same manner as 4d. The product was purified by column chromatography (silica gel, 100% ethyl acetate). Yield: 0.57 g (1.3 mmol, 39%). 1H NMR (400 MHz, CDCl3): δ 8.67 (s, 2H), 8.55 (d, J = 4.5 Hz, 2H), 7.78 (d, J = 7.5 Hz, 2H), 7.21 (dd, J1 = 7.5 Hz, J2 = 4.5 Hz, 2H), 0.33 (s, 12H), 0.02 (s, 12H) ppm. 13C NMR (100 MHz, CDCl3): δ 153.6, 150.3, 141.0, 134.6, 123.4, 1.32, 0.88 ppm. 29Si NMR (80 MHz, CDCl3): δ -2.3, -19.6 ppm. Anal. Calcd for C18H32N2Si4O3: C, 49.49; H, 7.38; N, 6.41. Found: C, 49.60; H, 7.47; N, 6.39. 1,9-Bis(3-pyridyl)decamethylpentasiloxane (4d). In a dry 20 mL septum vial was placed 10 mL of a 0.44 M solution of 3 in CHCl3 (4.4 mmol). Separately, 0.96 mL (8 mmol) of dichlorodimethylsilane, 2.6 mL (19 mmol) of triethylamine, and 8 mL of CHCl3 were mixed to make a solution that was 0.69 M in dichlorodimethylsilane. Of this solution, 3.19 mL (2.2 mmol) was added to the silanol solution by syringe. A mild exotherm was observed, but no precipitate formed. After 15 m, the reaction mixture was washed with 2 5 mL of water and 1 3 mL of brine. After evaporation of volatile components, the crude product was chromatographed on silica gel (1:2 hexane/ethyl acetate) to give 0.63 g of 4d (1.2 mmol, 55%). 1H NMR (400 MHz, CDCl3): δ 8.71 (s, 2H), 8.60 (dd, J1 = 4.9 Hz, J2 = 1.8 Hz, 2H), 7.82 (dt, J1 = 7.5 Hz, J2 = 1.8 Hz, 2H), 7.26 (ddd, J1 = 7.5 Hz, J2 = 4.9 Hz, J3 = 0.8 Hz), 0.37 (s, 12H), 0.070 (s, 12H), 0.035 (s, 6H) ppm. 13C NMR (100 MHz, CDCl3): δ 153.8, 150.5, 140.1, 134.7, 123.4, 1.36, 1.23, 0.93 ppm. 29Si NMR (80 MHz, CDCl3): δ -2.4, -19.7, -21.3 ppm. HRMS (ESI): calcd for C20H39N2O4Si5 (Mþ) m/z 511.1751; found 511.1756. Anal. Calcd for C20H38N2O4Si5: C, 47.01; H, 7.50; N, 5.48. Found: C, 46.77; H, 7.72; N, 5.38. 1,11-Bis(3-pyridyl)dodecamethylhexasiloxane (4e). 4e was prepared from 1 g (4.4 mmol) of 2b and 0.45 g (2.2 mmol) of 1,3-dichlorotetramethyldisiloxane, in the same manner as 4d. The product was purified by chromatography on silica gel. Yield: 0.625 g (1.1 mmol, 50%). 1H NMR (400 MHz, CDCl3): δ 8.72 (s, 2H), 8.60 (dm, J = 4.7 Hz, 2H), 7.82 (dm, J = 7.3 Hz, 2H), 7.26 (ddm, J1 = 7.5 Hz, J2 = 4.9 Hz), 0.38 (s, 12H), 0.076

Article (s, 12H), 0.045 (s, 12H) ppm. 13C NMR (100 MHz, CDCl3): δ 153.9, 150.6, 141.0, 134.7, 123.4, 1.36, 1.23, 0.93 ppm. 29Si NMR (80 MHz, CDCl3): δ -2.4, -19.6, -21.3 ppm. Anal. Calcd for C22H44N2Si6O5: C, 45.16; H, 7.58; N, 4.79. Found: C, 45.19; H, 7.69; N, 4.83. 1,13-Bis(3-pyridyl)tetradecamethylheptasiloxane (4f). 4f was prepared from 1 g (3.3 mmol) of 3b and 0.213 g (1.65 mmol) of dichlorodimethylsilane, according to the synthesis of 4d. The product was purified by chromatography on silica gel. Yield: 0.586 g (0.89 mmol, 54%). 1H NMR (400 MHz, CDCl3): δ 8.72 (s, 12H), 8.60 (dm, J = 4.9 Hz), 7.83 (dm, J = 7.5 Hz), 7.27 (ddm, J1 = 7.5 Hz, J2 = 4.9 Hz), 0.38 (s, 12H), 0.080, (s, 12H), 0.056 (s, 6H), 0.051 (s, 12H) ppm. 13C NMR (100 MHz, CDCl3): δ 153.9, 150.5, 141.0, 134.7, 123.4, 1.37, 1.25, 1.24, 0.94 ppm. 29 Si NMR (80 MHz, CDCl3): δ -2.4, -19.7, -21.33, -21.40 ppm. Anal. Calcd for C24H50N2Si7O6: C, 43.72; H, 7.64; N, 4.25. Found: C, 43.90; H, 7.67; N, 4.37. 1,19-Bis(3-pyridyl)icosamethyldecasiloxane (4g). 4g was prepared from 1 g (3.3 mmol) of 3b and 0.58 g (1.65 mmol) of 1,7dichlorooctamethyltetrasiloxane, according to the synthesis of 4d. Purified by chromatography on silica gel (4:1 hexanes/ethyl acetate). Yield: 0.727 g (0.83 mmol, 50%). 1H NMR (400 MHz, CDCl3): δ 8.72 (s, 12H), 8.60 (dd, J1 = 4.8 Hz, J2 = 1.7 Hz, 2H), 7.83 (dt, J1 = 7.5 Hz, J2 = 1.7 Hz, 2H), 7.26 (ddd, J1 = 7.5 Hz, J2 = 5.0 Hz, J3 = 0.7 Hz), 0.38 (s, 12H), 0.083 (s, 12H), 0.067 (s, 12H), 0.065 (s, 12H), 0.056 (s, 12H) ppm. 13C NMR (100 MHz, CDCl3): δ 153.8, 150.5, 140.9, 134.7, 123.3, 1.35, 1.26, 1.24, 0.94 ppm. 29Si NMR (80 MHz, CDCl3): δ -2.5, -19.6, -21.34, -21.45, -21.49 ppm. HRMS (ESI): calcd for C30H69N2O9Si10 ([M þ H]þ) m/z 881.2690; found 881.2679. Anal. Calcd for C30H68N2O9Si10: C, 40.87; H, 7.77; N, 3.18. Found: C, 41.00; H, 7.95; N, 3.18. Competitive Binding Studies in 1H NMR. General Procedures. Competitive binding experiments were performed using 1 H NMR spectroscopy to quantify the concentrations of bound and unbound species. In the majority of cases spectra were collected in toluene-d8 at 353 K, mostly due to the favorable properties of the NMR spectra at this temperature (good separation between signals from bound and unbound ligand). The NMR probe temperature was calibrated by the neat ethylene glycol method and found to be accurate to within 1 K. A standard solution of 6.4 ( 0.2 mM Pd(OAc)2 in toluene-d8 was prepared using a 10 mL volumetric flask and stored in the dark. Separately, standard 0.20 ( 0.01 N solutions of pyridined5 and the ligands of interest in toluene were prepared gravimetrically. The ligand solutions were prepared on the 0.5 mL scale, while the pyridine-d5 solution was prepared on the 3 mL scale to reduce measurement errors and batch-to-batch variations. The ligand solutions were found to be stable indefinitely, while the Pd(OAc)2 solution darkened noticeably over a period of several months. Displacement Experiments using Pyridine-d5. In a typical displacement experiment, 0.70 mL (4.5 μmol) of Pd(OAc)2 solution was transferred to a thin-walled 5 mm NMR tube, followed by the desired amount of ligand solution (typically 45 μL or 9.0 μmol) to attain the stoichiometric 2:1 ligand/metal ratio. After acquiring a 1H NMR spectrum at 353 K (16 transients,


3779

recycle delay 4 s), successive aliquots of the 0.2 M pyridine-d5 solution were added by micropipet. After the addition of each aliquot, the sample was thoroughly mixed and equilibrated to 353 K in the NMR probe, and the spectrum retaken to monitor the progress of the displacement. For several of the ligands, the displacement was also performed in the reverse fashion, adding the ligand of interest to a preformed Pd(OAc)2(py-d5)2 complex. These experiments could be analyzed using the same methods and gave similar values for the relative binding constant, indicating that a true equilibrium was reached in these experiments. Successive Dilution Experiments. For the analysis of ringchain equilibria, a more concentrated solution was prepared by dissolving 4.0 mg of Pd(OAc)2 in 0.57 mL of toluene-d8 and adding 180 μL of 0.2 N ligand solution to give 0.75 mL of a 23.8 mM solution at a 2:1 ligand/metal ratio. After analyzing the sample by 1H NMR at 353 K (2 transients, recycle delay 4 s), 0.39 mL, or 50%, of the sample was withdrawn by syringe, and the volume was replaced with pure toluene-d8. After thorough mixing, the sample was analyzed again by 1H NMR and the process was repeated until a final concentration of 1.4 mM was obtained. The number of transients was increased by a factor of 4 after each dilution step, to maintain a constant signal-to-noise ratio throughout the experiment. A combined displacementdilution experiment was also performed for some ligands. In this experiment 0.7 mL of a 6.4 mM Pd(OAc)2 solution was combined with 45 μL of a 0.2 N solution of the ligand of interest, as well as 45 μL of the 0.2 M pyridine-d5 solution. This sample was analyzed and succesively diluted as before, and the effect of dilution on the equilibrium point of the displacement was thereby measured. Variable-Temperature Experiments. Variable-temperature experiments in the range 233-363 K were performed in order to assess the temperature sensitivity of the 1H NMR chemical shifts and peak integrals (i.e., equilibrium concentrations). Two types of samples were analyzed in this way: 0.7 mL of a 6.4 mM solution of Pd(OAc)2 with 45 μL of the ligand solution of interest (2:1 ligand/metal ratio); and the same sample plus 50 μL of 0.2 N pyridine-d5 solution (i.e., a partially displaced ligand sample). The former type of sample is suitable for determining the temperature dependence of the ring interconversion equilibria, while the latter type is suitable for studying the effect of temperature on the displacement equilibria.

Acknowledgment. This work was supported by the Office of Basic Energy Sciences, U.S. Department of Energy, DE-FG02-01ER15184. Spectroscopic characterization was performed at the Integrated Molecular Structure Education and Research Center (IMSERC) at Northwestern University. Supporting Information Available: Derivation of ring-chain equilibrium model, method of model fitting to experimental data, fitting results and evaluation of quality of fit, and complete 1 H NMR spectra of complexes. This material is available free of charge via the Internet at http://pubs.acs.org.

Bis(pyridyl)siloxane Oligomeric Ligands for Palladium(II) Acetate

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