In the Laboratory
Experiments in Thermodynamics and Kinetics of Phosphine Substitution in (p-Cymene)RuCl2(PR3)
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Oleg V. Ozerov* and Claudia M. Fafard Department of Chemistry, Brandeis University, Waltham, MA 02454; *
[email protected] Norris W. Hoffman Department of Chemistry, University of South Alabama, Mobile, AL 36688
Studies of the thermodynamics and of the kinetics of chemical reactions are at the heart of chemical research. In the field of organotransition-metal chemistry, proficiency and experience in such studies is considered a requisite skill of the well-trained chemist. Consequently, incorporation into an inorganic laboratory course of experiments that quantitatively probe the thermodynamic and the kinetic parameters of organometallic reactions is strongly warranted. Herein we describe a sequence of experiments that combines thermodynamic and kinetic studies of phosphine-ligand substitution in Ru complexes with their synthesis.1 The experiments are designed to give the student an experience of dealing with a “real-life” research problem, with the sequence designed to be completed in three 4.5-hour lab periods. All the manipulations can be performed in the air without any significant adverse effects owing to formation of phosphine oxide, given the timescale of the experiments.
ment of equilibrium (Scheme II), then the mixtures were cooled to ambient temperature (it is assumed that the equilibrium is effectively “frozen” upon cooling) and analyzed by 31 1 P{ H} NMR. Initially, we also intended to study the reaction of 2a with P(C6H4Me-p)3. However, the overlap of 31P NMR resonances of 2a and 2d made this impractical. Instead, we utilized the reactions in Scheme III. These were accomplished by mixing 1 with two different phosphines and heating the mixture at 80 C for 1 h. For each combination of the PR3 ligands (Schemes II and III), three experiments were performed, each with different phosphine-to-phosphine
Synthesis and Characterization The first period is dedicated to the synthesis of the Ru complex 2a (1). The precursor 1 is commercially available or, more economically, it can be prepared (by instructors ahead of time) via a standard procedure from hydrated RuCl3 and α-terpinene (2). The straightforward preparation of 2a (Scheme I) follows the procedure reported by Nolan et al. (1). Compound 2a is then characterized by 1H, 31P, and 19F NMR in solution. The students were asked to judge its purity from the NMR spectra and assign all the signals in the NMR spectra. To avoid redundancy in having several groups of students obtaining the same spectra, we had students use different NMR solvents [C6D6, (CD3)2CO, and CDCl3] and then compare the results.
Scheme II. Reaction 2a with PPh3 or P(OPh)3.
Thermodynamic Studies The second period was dedicated to the study of the equilibria between various complexes 2 by means of solution 31P NMR. A solution of 2a was treated with PPh or P(OPh) 3 3 and then heated at 80 C for 1 hour to ensure the establish-
Scheme I. Preparation of 2a.
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Scheme III. Reaction of 1 with two different phosphines.
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In the Laboratory Table 1. Equilibrium Data for the Phosphine-Substitution Reactions Forward Reaction
Keq
∆G350o/ (kcal mol−1)
2a → 2b
1.17(3)
0.11(2)
0.1(2)
∆rxnH/ (kcal mol−1)
2a → 2c
0.49(7)
0.5(1)
0.5(2)
2d → 2b
0.30(9)
0.9(3)
0.7(2)
2d → 2c
0.077(32)
1.8(3)
1.1(2)
Note: The enthalpy data is from ref 1.
that the entropy change in the phosphine exchange is insignificant (the entropy change is determined primarily by the change in the translational component, that is, by the change in the number of particles) and thus ∆rxnG ≈ ∆rxnH. This approximation allows the comparison of our equilibria data with the thermochemical data of Nolan et al. The agreement was good (Table 1). The students were asked to analyze the stereoelectronic trends in the affinity of the Ru center not only for the phosphines that were studied by them but also for the entire Nolan series. Their general conclusion was that both the increase in the donicity and decrease in cone angle favor greater P–Ru bond strength (1, 3, 4). The students were introduced to the core concepts of the Tolman steric and electronic parameters (5). The instructor might also require analysis in terms of other organometallic stereoelectronic techniques (6–12). A comparison can also be made with the cymene–Os system (4), in which Os–P bonding is stronger than Ru–P in the Ru analogues. Kinetic Studies
Scheme IV. Reaction 2a with P(OMe)3.
Figure 1. First-order rate law for the simple dissociative substitution mechanism.
ratio. The students were then asked to calculate the equilibrium constant (as the average of three determinations) for the exchanges in Schemes II and III and convert it to the Gibbs free energy of reaction. As can be extrapolated from the kinetic studies (vide infra), the phosphine substitution for these phosphines is slow at ambient temperature. Thus cooling effectively freezes the equilibrium distribution that exists at 80 C (ca. 350 K). The results are given in Table 1. Nolan et al. reported the experimentally determined enthalpies of reaction of 1 with various PR3 ligands (1).1 This allows the calculation of the predicted enthalpies of reaction in Schemes II and III (Table 1). We make the assumption 490
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In the third period, we undertook the kinetic study of the phosphine substitution (Scheme IV). This particular reaction was selected because (i) it can be easily followed by 19 F NMR; (ii) the rate is appropriate for the variable-temperature NMR (VT-NMR) study in a convenient temperature range; (iii) the reaction is irreversible and (as we find out) is strictly dissociative; (iv) the reagents are air-stable, inexpensive, and readily soluble in CDCl3; and (v) the Nolan data predicts the enthalpy of the reaction as ca. 5 kcal兾mol, a value meaning that the forward reaction, is for practical purposes, irreversible. The general rate law for the dissociative substitution reduces to a simple first-order rate law when k2[L2] >> k-1[L1] (Figure 1). The negative slope of the plot of ln[2a] versus time is the rate constant. The instructors performed an auxiliary study by monitoring the reaction (Scheme IV) with three different P(OMe)3 concentrations. In all three cases, simple first-order decay of 2a was observed and the apparent rate constants were identical within experimental error for the reactions with 10-, 20-, or 30-fold excess of P(OMe)3. This is consistent with the simple first-order dissociative mechanism where the dissociation of P(C6H4F-p)3 is the ratelimiting, irreversible step under these conditions. The students determined the rate constants of the reaction in Scheme IV at six different temperatures. The determinations were performed using a VT-NMR probe. Reaction progress was monitored by 19F NMR; the decrease in concentration of 2a (vs PhF as an internal integration standard) over time was recorded. The class of 12 students was split into two sections, three pairs in each. Each pair was assigned a temperature between 23.6 and 53.6 C. We were able to perform three determinations per afternoon (4–5 h), including the time needed for slow warming and cooling of probe. The rate constants are collected in Table 2. The Eyring plot, ln(k兾T) versus 1兾T, permitted the extraction of the activation parameters: ∆H ‡ = 26.1(16) kcal兾mol and ∆S‡ = 9(5) cal兾(mol K). This plot (Figure 2) is based on six rate-constant determinations by the students and three by the instructors. The results obtained by the students
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In the Laboratory Table 2. First-Order Rate-Constant Data for the Disappearance of 2a at Different Temperatures Temp/K
Temp/°C
k/(105 s1)
296.8(10)
23.6(10)
3.8.(6)
297.9(10)
24.7(10)
5.0.(3)
303.5(10)
30.3(10)
10.0.(8)
309.3(10)
36.1(10)
25.(1)
315.2(10)
42.0(10)
55.(1)
315.5(10)
42.3(10)
50.(1)
321.3(10)
48.1(10)
125.(2)
326.2(10)
53.0(10)
246.(8)
326.8(10)
53.6(10)
246.(6)
Figure 2. Eyring plot for the dissociative substitution of phosphine ligand in 2a.
and those obtained by the instructors differed by a statistically insignificant margin. The error in the determination of the activation parameters was calculated according to Girolami et al. (14). The positive entropy of activation reinforced the operation of a dissociative mechanism for the phosphine substitution. An associative mechanism would be expected to lead to a decidedly negative entropy of activation. The activation enthalpy of a purely dissociative process (late transition state) can be roughly approximated as the bond dissociation energy (BDE) for the bond between the metal and the departing ligand. There exists some discussion in the literature regarding this approximation (15–18). It is important to stress that the two values are not identical, but it is reasonable to expect that this assumption is fairly accurate. If this assumption is taken for granted, then it is possible to estimate the strength of the bridging Ru–Cl bonding in 1. Nolan et al. provide the enthalpy of the reaction of 1 with 2 equiv P(C6H4F)3 (36.5(3) kcal兾mol) (1), while in this work we obtain the value approximating the enthalpy of the reaction of monomeric (p-cymene)RuCl2 with P(C6H4F)3 [52(3) kcal兾mol for 2 mol Ru]. The difference yields the enthalpy of the dimerization of ( p-Cymene)RuCl2, 16(3) kcal兾mol. Half of this (absolute) value formally represents the BDE of the Ru–µ-Cl bond = 8(2) kcal兾mol. The error margins here do not take into account the error of the assumption we took earlier concerning the tentative equivalence of the ∆H‡ we obtained with the Ru–P BDE. However, 16 kcal兾mol for the dimerization of ( p-cymene)RuCl2 in the first estimation appears reasonable. A much lower favorability of dimerization is unlikely because the favorable enthalpy must be large enough to overcome the unfavorable entropy of dimerization.
materials in closed NMR tubes should be done with utmost caution. In our experience, heating screw-capped NMR tubes (Wilmad) containing CDCl3 solutions as described throughout up to 90 C has not led to tube breakage. We use silicon oil baths for heating. The NMR tube is submerged approximately 20–30% of its height into the hot oil. This should be done in a fume hood from which flammable materials have been removed. The screw-capped NMR tubes were closed when heated inside the NMR spectrometer. The temperatures (
