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NMR Studies on the Dynamic Behavior of Zirconaaziridinium Ion Pairs in Solution Luca Rocchigiani, Alceo Macchioni, and Cristiano Zuccaccia* Dipartimento di Chimica, Università degli Studi di Perugia, via Elce di Sotto 8, I-06123, Perugia, Italy S Supporting Information *
ABSTRACT: The fluxional behavior of zirconaaziridinium ion pairs [Cp2Zr(η2-CH2NMePh][X] (X− = MeB(C6F5)3− (1a), B(C6F5)4− (1b)) in solution has been investigated by means of 2D EXSY and line shape analysis NMR methods at different temperatures. It is found that two dynamic processes are operative: (i) the inversion of the absolute configuration of the coordinated nitrogen atom (PI process) and (ii) the dynamic symmetrization of the ion pair occurring through a 180° rotation around Zr−CH2 bond and anion relocation from one side of the cation to the other (BS process). Analysis of the kinetic data indicates that both processes necessitate nitrogen decoordination from the metal that, by the way, is not the rate-determining step; the nature of the borate anion has a negligible effect on the rates of both PI and BS processes.
P
species in olefin polymerization.7,8 By introducing R′ alkyl chains of different lengths, we mimicked the growth of the polymer chain in the proximity of the catalytic center and investigated its effects on the interionic structure and selfaggregation tendency in solution of both inner- and outersphere ion pairs.7 Under proper experimental conditions, the ion pairs reported above undergo controlled single insertion of an α-olefin into the Zr−C bond, allowing clarification of the relative contributions of counterions and solvents to the activation enthalpy (ΔH⧧) and entropy (ΔS⧧) of the olefin insertion reaction into the zirconium−carbon bond.2a,8 As a complement to previous studies, herein we report on the dynamic behavior in solution of the ion pairs [Cp2Zr(η2CH2NMePh)][X] (X− = MeB(C6F5)3− (1a), B(C6F5)4− (1b)) investigated by means of 2D EXSY and line shape analysis of variable-temperature NMR experiments. These studies also represent an extension of previous investigations by Norton and co-workers on neutral ((dimethylamino)alkyl)zirconocene analogues, [Cpx2M(η2-CH(R)NMe2)(Y)] (M = Zr, Hf; Y = Me, Cl, O2CCF3, CpMo(CO)3),9 where only one dynamic process, consisting of the inversion of the configuration of nitrogen, was detected. As will be shown in the following, an additional fluxional motion, involving the dynamic symmetrization of the ion pairs, is operative in 1a,b, likely due to the presence of weakly coordinating borate anions.10
rotonation of an alkyl group of early-transition-metal dialkyl precursors [LnMR 2 ] with ammonium salts [HNR3][X] produces the active olefin polymerization catalysts [LnMR][X], HR, and 1 equiv of the corresponding tertiary amine.1 The subsequent coordination of the amine to the cationic center, which may be crucial in determining the catalytic performance of the system, depends on the nature of the organometallic fragment as well as on the basicity and steric characteristics of the amine.2 When coordination takes place, the resulting adducts can be relatively stable (in a few cases crystals suitable for X-ray analysis were also obtained)2g,3 or may undergo C−H activations involving both aliphatic4 or aromatic5 carbons of the coordinated amine (eq 1). In the case
of relatively unencumbered [Cpx2ZrMe][X] zirconocenium ion pairs and tertiary amines containing at least one methyl group (NMeR′2), amine coordination is followed by the selective activation of the methyl moiety to give zirconaaziridium ion pairs of the type [Cpx2Zr(η2-CH2NR′2)][X] (X− = MeB(C6F5)3−, B(C6F5)4−).2a,6 Over the past few years we have deeply investigated zirconaaziridium ion pairs from both the thermodynamic and kinetic points of view, since they possess some remarkable requisites to be used as good models for the catalytically active © 2012 American Chemical Society
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RESULTS AND DISCUSSION The room-temperature phase-sensitive 1H NOESY NMR spectrum of 1a in benzene-d6 is shown in Figure 1. Cross peaks having the same sign as the diagonal peaks are observed between Cpu and Cpd resonances and between Zr−CHu and Received: February 28, 2012 Published: May 7, 2012 4076
dx.doi.org/10.1021/om300164r | Organometallics 2012, 31, 4076−4079
Organometallics
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“back-skip”, BS). This motion inverts the relative positions of the two Cp rings, leaving unaffected those of the diasterotopic methylene hydrogen atoms. An additional distinction between PI and BS processes is that the latter formally needs an anion relocation from one side of the cation to the other since, from a thermodynamic point of view, the anion is always preferentially located on the side of the nitrogen atom.7,13 According to the data reported by Norton and co-workers, the BS process is not active in the neutral (α-aminoalkyl)zirconocene analogues up to 398 K.9 Assuming that only PI and BS processes are operative in 1a, kapp(Cp) is given by the sum of kPI and kBS, kapp(CH2) is equal to kPI, and an estimation of kBS can be obtained as the difference between kapp(Cp) and kapp(CH2). Activation parameters for both processes were obtained by measuring apparent rate constants at different temperatures by means of line shape analysis14 of 1H NMR spectra recorded in toluene-d8 from 273 to 336 K (Table 1). Eyring analysis15 of the data for 1a
Figure 1. 1H NOESY NMR spectrum of 1a recorded in benzene-d6 at 297 K with a mixing time of 600 ms. Arrows indicate exchange cross peaks. The asterisk denotes the residual solvent resonance.
Table 1. kPI and kBS Values (s−1) as a Function of Temperature (T, K) from Full Line Shape Analysisa and Corresponding Activation Parameters ΔG⧧298 (kcal mol−1), ΔH⧧ (kcal mol−1), and ΔS⧧ (cal mol−1 K−1) for 1a,b in Toluene-d8
Zr−CHd resonances,11 indicating the presence of dynamic processes that exchange the relative positions of both Cp rings and methylene hydrogen atoms with respect to the plane defined by the azazirconacycle (Figure 1).7 The apparent rate constants of chemical exchanges between Cp (kapp(Cp)) and Zr−CH2 (kapp(CH2)) resonances were initially estimated with the Perrin method, by recording a series of 1H EXSY NMR spectra while varying the mixing time (τm).12 For 1a, kapp(Cp) and kapp(CH2) are equal to 5.5 ± 0.1 and 2.9 ± 0.2 s−1 at 297 K, respectively. A second series of 1H EXSY NMR spectra recorded at 337.5 K confirmed that the two apparent rate constants are significantly different (kapp(Cp)337.5 = 94.4 ± 0.4 s−1, kapp(CH2)337.5 = 58.6 ± 0.3 s−1), indicating that at least two fluxional motions are operative in 1a. In close analogy with the behavior of the neutral (α-aminoalkyl)zirconocenes studied by Norton and co-workers,9 one of these processes consists of the inversion of the absolute configuration of the nitrogen atom (we will abbreviate this process as “pyramidal inversion”, PI). As depicted on the right side of Scheme 1, this motion exchanges both CH2 and Cp resonances, necessarily with the same rate. The second process (Scheme 1, left) involves a 180° rotation of the entire fragment CH2NMePh around the Zr−CH2 bond and is similar to the “site epimerization” process typical of cationic group IV olefin polymerization catalysts (we will abbreviate this process as
1ab T
kPI
1bc kBS
291.2 297.4 303.2 312.7 321.1 329 335.6 ΔH⧧
1.1 1.7 3.5 7.2 14.9 30.0 50.2 16.3 ± 0.4
1.9 3.3 5.6 10.3 17.4 30.6 13.3 ± 0.4
ΔS⧧
−2 ± 1
−12 ± 2
ΔG⧧298e
17.0 ± 0.7
17.0 ± 1
kPI 1.6 (2.2)d 2.9 (4.0) 5.9 (6.9) 10.6 (12.6) 18.0 (23.6) 28.0 (42.2) 41.7 (65.0) 13.4 ± 0.7 (14.0 ± 0.3) −11 ± 2 (−9 ± 1) 16.7 ± 1.3 (16.6 ± 0.6)
kBS 0.7 (0.6) 1.0 (1.0) 1.8 (1.6) 3.7 (2.8) 6.1 (6.8) 14.4 (11.0) 25.7 (19.8) 15.0 ± 0.7 (14.5 ± 0.5) −8 ± 2 (−10 ± 2) 17.3 ± 1.4 (17.3 ± 1.1)
a1
H NMR spectral parameters for the frozen exchange regime were obtained at 272.8 K. b[1a] = 20 mM. c[1b] = 1 mM. dValues in parentheses refer to [1b] = 3 mM. eCalculated from ΔH⧧ and ΔS⧧ values.
(Supporting Information) afforded ΔG⧧298 = 17.0 ± 0.7 kcal mol−1 (ΔH⧧ = 16.3 ± 0.4 kcal mol−1, ΔS⧧ = −2 ± 1 cal mol−1
Scheme 1. Qualitative Energy Profile for PI (Right) and BS (Left) Processes
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dx.doi.org/10.1021/om300164r | Organometallics 2012, 31, 4076−4079
Organometallics
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K−1) and ΔG⧧298 = 17.0 ± 1.0 kcal mol−1 (ΔH⧧ = 13.3 ± 0.4 kcal mol−1, ΔS⧧ = −12 ± 2 cal mol−1 K−1) for the PI and BS processes, respectively. The analysis of the kinetic data follows the same line of reasoning proposed by Norton and co-workers:9 both processes can be described in terms of the elementary steps shown in Scheme 1. One of these steps is common to both exchange motions, and it consists of the decoordination of the nitrogen atom from the zirconium center to give the corresponding ion pair featuring the η1-coordination mode of the CH2NMePh fragment. This step is necessary to make nitrogen pyramidal inversion possible, since inversion of quaternary nitrogen stereocenters requires a much higher energy.16 Nitrogen decoordination must also occur during the BS process, since the alternative mechanism, involving the 180° rotation around the (CN+)−Zr single bond in the resonance structure where the iminium double bond is π-coordinated to the formally ZrII center, can be excluded for the following reasons. First, the contribution of the (CN+)−ZrII resonance structure should be negligible due to the high π basicity of Zr(II),17 and second, the same patterns of chemical exchanges are observed in related [Cpx2Zr{CH2C(Me)(R)CH2NR′R″-κN,C}][X] ion pairs featuring saturated azazirconapentacycles.18 Because the rates of PI and BS are different, the nitrogen decoordination step cannot be rate determining. In addition, it is reasonable to assume that the entropy variation associated with nitrogen decoordination is very small and that the inverse reaction (nitrogen recoordination to the metal center) would require a negligible activation energy. Consequently, the free energy and the activation free energy should show little dependence on the temperature so that ΔG⧧(Zr−N interaction) ≈ ΔG(Zr−N interaction) ≈ ΔH⧧(Zr−N interaction) ≈ ΔH(Zr−N interaction). The relative contribution of the Zr−N interaction to the overall activation barriers cannot be extracted directly from the experimental data, but its value can be estimated by evaluating the energetics of the second step. For the PI process, the second step is the pyramidal inversion of nitrogen and rotation around the CH2−N single bond. Both of these elementary motions are necessary to give the corresponding syn conformer in which the nitrogen lone pair is in the correct geometrical position to recoordinate at the zirconium center (Scheme 1). Electronic and steric arguments point to a very fast nitrogen inversion. From an electronic point of view, the conjugation of the nitrogen lone pair with the π system of the phenyl ring should decrease the barrier for inversion down to about 2 kcal mol−1 (the value reported for nitrogen inversion in aniline).19 This inversion barrier should be even lower due the presence of the bulky CH2−Zr(Cp2) moiety that destabilizes the pyramidal ground state with respect to the planar transition state required for inversion.20 Therefore, the free energy associated with the rotation around the CH2−N bond should be higher than that of nitrogen pyramidal inversion. Forsyth and Johnson measured ΔG⧧191 = 9.5 ± 0.2 kcal mol−1 for the rotation around the (tBu)CH2−N single bond in N,N-diethyl-2,2-dimethylpropanamine.21 Considering the higher steric bulk of Zr(Cp2) and Ph moieties, we can reasonably set the lower limit of ΔG⧧298(C−N rotation) to 10 kcal mol−1 and, consequently, we can estimate the upper limit of ΔG(Zr−N interaction) to about 7 kcal mol−1 as the difference between the experimentally measured ΔG⧧298(PI) (17.0 kcal mol−1) and ΔG⧧298(C−N rotation). The estimated Zr−N interaction energy in 1a is consistent with that reported for the neutral (α-aminoalkyl)zirconocene analogues (8 kcal mol−1).9
Having an estimation of the upper limit for ΔG(Zr−N interaction), it is possible to quantitatively analyze the energetics of the profile of the BS process. In this case, the additional step is the 180° rotation around the Zr−CH2 single bond and the formal anion relocation from one side of the Cpcentr−Zr−Cpcentr plane to the other (Scheme 1, left). The dynamic motion associated with this step corresponds to the “ion pair symmetrization” process operative in catalytically active metallocenium ion pairs.22 The lower limit of the corresponding activation free energy (ΔG⧧298(IPS)), computed for 1a as the difference between ΔG⧧298(BS) and the upper limit of ΔG298(Zr−N interaction), is about 10 kcal mol−1. Typically, ΔG⧧298(IPS) is about 17−18 kcal mol−1 for ion pairs of the type [L2ZrR][MeB(C6F5)3] (where L = Cp ligand with small steric hindrance and R = Me) in aromatic nonpolar solvents.23 However, substitution of the small methyl ligand with larger groups causes a sensible decrease of the barrier: ΔG⧧298(IPS) becomes 14, 13.5, and
