Organometallics 2011, 30, 1231–1235 DOI: 10.1021/om101149z
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Defining Reactivity Differences in Sterically Crowded (η5-C5Me5)3M Complexes Based on Metal Size and Lanthanide vs Actinide Effects Thomas J. Mueller, Gregory W. Nyce, and William J. Evans* Department of Chemistry, University of California Irvine, California 92697-2025, United States Received December 7, 2010
Reactions between (η5-C5Me5)3M and (η5-C5Me5)2M0 (μ-Ph)2BPh2 (M, M0 = La, Ce, Pr, Nd, Sm, and U; M radius < M0 radius) were examined to evaluate the relative steric crowding in the (C5Me5)3M series as a function of metal size and 4f vs 5f electron configuration. The sterically more crowded (C5Me5)3M complexes transfer (C5Me5)- to (C5Me5)2M0 (μ-Ph)2BPh2 to generate less crowded (C5Me5)3M0 products and (C5Me5)2M(μ-Ph)2BPh2 in mixtures with equilibrium constants in the range that allows all four components to be observed by NMR spectroscopy. The (C5Me5)3M to (C5Me5)3M0 ratios depend on the difference in size of the two metals. Hence, (C5Me5)3Sm and (C5Me5)2La(μ-Ph)2BPh2 form a mixture with 99% (C5Me5)3La and 1% (C5Me5)3Sm, but the analogous reaction with (C5Me5)2Nd(μ-Ph)2BPh2 contains 90% (C5Me5)3Nd and 10% (C5Me5)3Sm. In analogous reactions with (C5Me5)2U(μ-Ph)2BPh2 and (C5Me5)3Ln lanthanide complexes, a size dependence is also observed, but the (C5Me5)3Ln complexes are favored over (C5Me5)3U to a greater extent than expected based on size differences.
Introduction The discovery that sterically crowded complexes of formula (η5-C5Me5)3M,1-6 (η5-C5Me5)3ML,7-9 (η5-C5Me5)3 MX,10-12 and (η5-C5Me5)3ML29,13 (L = CO, N2, CNCMe3, NCCMe; X = F, Br, Cl, H, Me) could be synthesized despite a generally accepted value of approximately 142° for the cone angle of (C5Me5)- 14 demonstrated that entire series of complexes were accessible with a 120° (C5Me5)- cone angle. *To whom correspondence should be addressed. Fax: 949-824-2210. E-mail:
[email protected]. (1) Evans, W. J.; Davis, B. L.; Ziller, J. W. Inorg. Chem. 2001, 40, 6341. (2) Evans, W. J.; Perotti, J. M.; Kozimor, S. A.; Champagne, T. M.; Davis, B. L.; Nyce, G. W.; Fujimoto, C. H.; Clark, R. D.; Johnston, M. A.; Ziller, J. W. Organometallics 2005, 24, 3916. (3) Evans, W. J.; Seibel, C. A.; Ziller, J. W. J. Am. Chem. Soc. 1998, 120, 6745. (4) Evans, W. J.; Gonzales, S. L.; Ziller, J. W. J. Am. Chem. Soc. 1991, 113, 7423. (5) Evans, W. J.; Davis, B. L.; Champagne, T. M.; Ziller, J. W. Proc. Natl. Acad. Sci. 2006, 103, 12678. (6) Evans, W. J.; Forrestal, K. J.; Ziller, J. W. Angew. Chem., Int. Ed. Engl. 1997, 36, 774. (7) Evans, W. J.; Kozimor, S. A.; Nyce, G. W.; Ziller, J. W. J. Am. Chem. Soc. 2003, 125, 13831. (8) Evans, W. J.; Kozimor, S. A.; Ziller, J. W. J. Am. Chem. Soc. 2003, 125, 14264. (9) Evans, W. J.; Mueller, T. J.; Ziller, J. W. Chem.;Eur. J. 2010, 16, 964. (10) Evans, W. J.; Nyce, G. W.; Johnston, M. A.; Ziller, J. W. J. Am. Chem. Soc. 2000, 122, 12019. (11) Evans, W. J.; Kozimor, S. A.; Ziller, J. W. Organometallics 2005, 24, 3407. (12) Evans, W. J.; Nyce, G. W.; Ziller, J. W. Organometallics 2001, 20, 5489. (13) Evans, W. J.; Mueller, T. J.; Ziller, J. W. J. Am. Chem. Soc. 2009, 131, 2678. (14) Shannon, R. D. Acta Crystallogr. 1976, A32, 751. r 2011 American Chemical Society
The small cone angles were possible due to unusually long M-C(η5-C5Me5) bond distances. In all of the complexes containing three (η5-C5Me5)- ligands, the M-C distances are approximately 0.1 A˚ longer than those previously observed in sterically normal analogues with only two of these large ligands. Associated with these long distances is unusual ligand reactivity. For example, the normally inert (C5Me5)ligand can act as an η1-alkyl ligand2,6,13,15-17 or as a oneelectron reducing agent.2,15,18-20 (C5Me5)- can also be displaced by ligands of lower hapticity in these sterically crowded complexes,13,21 a reaction quite unusual for this mainstay ancillary ligand. Although it is clear that these sterically crowded complexes have higher reactivity than conventional complexes of (C5Me5)-, differentiating the relative reactivity within the sterically crowded series is more difficult. Reactivity studies of the lanthanide series, (η5-C5Me5)3Ln, show increasing (C5Me5)--based reactivity as the metal size decreases.2,18,19 This is reasonable since there is more crowding when the three rings are attached to a smaller metal. However, with some substrates, complexes of several metals exhibit similar reactivity.2 Differentiating the relative reactivity of the lanthanide complexes and the analogous actinide, (15) Evans, W. J.; Forrestal, K. J.; Ziller, J. W. J. Am. Chem. Soc. 1998, 120, 9273. (16) Evans, W. J.; Forrestal, K. J.; Ziller, J. W. J. Am. Chem. Soc. 1995, 117, 12635. (17) Mueller, T. J.; Ziller, J. W.; Evans, W. J. Dalton Trans. 2010, 39, 6767. (18) Evans, W. J. Inorg. Chem. 2007, 46, 3435. (19) Evans, W. J.; Davis, B. L. Chem. Rev. 2002, 102, 2119. (20) Evans, W. J.; Nyce, G. W.; Clark, R. D.; Doedens, R. J.; Ziller, J. W. Angew. Chem., Int. Ed. 1999, 38, 1801. (21) Evans, W. J.; Kozimor, S. A.; Ziller, J. W.; Kaltsoyannis, N. J. Am. Chem. Soc. 2004, 126, 14533. Published on Web 02/22/2011
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(η5-C5Me5)3U, is more difficult since the U3þ complex has a redox-active metal center. In this study the reactivity of the metallocene cations, [(C5Me5)2M]þ, loosely ligated by the tetraphenylborate anion in the complexes, (C5Me5)2M(μ-Ph)2BPh2,3 is used to evaluate the effects of crowding on (C5Me5)3M reactivity as a function of the metal. The test reaction is a (C5Me5)- transfer reaction. Specifically, a series of reactions between (C5Me5)3M and (C5Me5)2M0 (μ-Ph)2BPh2 complexes (where M in this paper has a smaller radius than M0 ) have been examined to determine how readily a sterically crowded complex will transfer a (C5Me5)- ligand to a larger metal, eq 1. The reaction of (C5Me5)2M(μ-Ph)2BPh2 complexes with
Mueller et al. (μ-Ph)2BPh2, (C5Me5)3M, (C5Me5)2M(μ-Ph)2BPh2, and (C5Me5)3M0 were used in each case, accuracy cannot be assumed to be better than (5% since these are NMR experiments and paramagnetic species are involved. Exchange rates were slow enough on the NMR time scale to observe signals of comparable width and position to those observed in isolated samples. Samples were monitored for as long as 100 days until the equilibrium value was invariant over the course of two weeks. The equilibrium mixtures were approached from both directions, i.e., from (C5Me5)3M and (C5Me5)2M0 (μ-Ph)2BPh2 and from (C5Me5)3M0 and (C5Me5)2M(μ-Ph)2BPh2, to ensure that the reactions were fully reversible under the reaction conditions. An example of the reaction procedure follows. In a glovebox, (C5Me5)3Ce (5 mg, 9 μmol) was dissolved in 1 mL of C6D6 and added to (C5Me5)2La(μ-Ph)2BPh2 (7 mg, 9 μmol) in an NMR tube capped with a rubber septum. The sample was removed from the glovebox and frozen in liquid nitrogen. A needle attached to a source of vacuum was inserted into the rubber septum, and the NMR tube was subsequently flame-sealed under vacuum. The reaction was monitored by 1 H NMR spectroscopy.
Results
KC5Me5 is one of the main methods developed to synthesize (C5Me5)3M complexes, eq 2. In eq 2, the reaction is driven by the formation of the stable lattice of the KBPh4 byproduct
and its precipitation from solution. In eq 1, the (C5Me5)3M complex is used to deliver the (C5Me5)- ligand to (C5Me5)2M0 (μ-Ph)2BPh2. In this case there is less driving force for complete conversion since the (C5Me5)2M(μ-Ph)2BPh2 byproduct is soluble and the (C5Me5)3M0 product is still a sterically crowded molecule. The direction of the equilibrium for reactions in which M and M0 are lanthanides could be predicted on the basis of metal size and consequent steric crowding, but the magnitude of the equilibrium constant could not. The equilibrium for reactions involving lanthanides and uranium was less certain since 4f vs 5f orbital effects could be involved as well as metal size effects.
Experimental Section The syntheses and manipulations described below were conducted under nitrogen with rigorous exclusion of air and water using glovebox and Schlenk techniques. NMR solvents (Cambridge Isotope Laboratories) were dried over sodiumpotassium alloy, degassed, and vacuum transferred prior to use. The (C5Me5)3M (M = La,1 Ce,2 Pr,2 Nd,3 Sm,3 U6) and (C5Me5)2M0 (μ-Ph)2BPh2 (M0 = La,3 Ce,2 Pr,2 Nd,3 Sm,3 U22) complexes were prepared as previously described. 1H NMR spectra were obtained on a Bruker DRX500 MHz spectrometer at 25 °C. Equilibrium constants were determined by 1H NMR spectroscopy (see Supporting Information). Although the intensities of all four unique (C5Me5)- resonances of (C5Me5)2M0 (22) Evans, W. J.; Nyce, G. W.; Forrestal, K. J.; Ziller, J. W. Organometallics 2002, 21, 1050.
The reactions between (C5Me5)3M and (C5Me5)2M0 (μPh)2BPh2 complexes according to eq 1 (radius of M < radius of M0 ) approach equilibrium slowly in benzene. Figure 1 shows plots of the formation of (C5Me5)3M0 from (C5Me5)3Sm and (C5Me5)2M0 (μ-Ph)2BPh2 where M0 = La, Ce, Pr, and Nd. In each case, equilibrium is not reached for days. It was found that the greater the difference in size between the metal centers, the faster the reaction reached equilibrium. The reaction between (C5Me5)3Sm and (C5Me5)2La(μ-Ph)2BPh2 (7.4 pm difference in ionic radii)14 reached equilibrium in 5 days at room temperature, whereas the reaction between (C5Me5)3Ce and (C5Me5)2La(μ-Ph)2BPh2 (2.2 pm difference in ionic radii)14 reached equilibrium in approximately 100 days. The initial rates of formation of (C5Me5)3La from (C5Me5)3M and (C5Me5)2La(μ-Ph)2BPh2 are 6.3, 5.0, 2.9, and 2.7 μmol/day for M = Sm, Nd, Pr, and Ce, respectively. The equilibrium constants for these lanthanide reactions were also dependent on the difference in size of the metals. Table 1 shows the percentages of the two tris(pentamethylcyclopentadienyl) products that exist at equilibrium. Figure 2 shows a plot of final percent of (C5Me5)3La as a function of the radius of M (M = Ce, Pr, Nd, Sm). The percentage of (C5Me5)3La at equilibrium increases as the size of the metal in the (C5Me5)3M delivery reagent decreases, but the correlation is not linear. Reactions were also examined between all the other M/M0 combinations of (C5Me5)3M (M = Pr, Nd, and Sm) with (C5Me5)2M0 (μ-Ph)2BPh2 (M0 = Ce, Pr, Nd). As in the lanthanum reactions, the equilibrium again is approached slowly. At equilibrium, the ratio of less crowded (C5Me5)3M0 to more crowded (C5Me5)3M decreases as the difference in the size between M and M0 decreases. For example, La:Nd is 98:2, Ce:Nd is 88:12, and Pr:Nd is 68:32. Since these are NMR experiments and in many cases involve paramagnetic complexes, the accuracy of the measurements should not be considered to be better than (5%. Figure 3 shows the percent of the less crowded (C5Me5)3M0 as a function of the difference in ionic radii between M0 and M. Again a strong correlation is observed, but it is not linear. (C5Me5)3U reacts with (C5Me5)2Ln(μ-Ph)2BPh2 and (C5Me5)2U(μ-Ph)2BPh2 reacts with (C5Me5)3Ln like the lanthanide reactions described above. The redox reactivity
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Figure 1. Percent of (C5Me5)3La produced vs time (days) from the reaction of (C5Me5)2La(μ-Ph)2BPh2 with (a) (C5Me5)3Sm; (b) (C5Me5)3Nd; (c) (C5Me5)3Pr; (d) (C5Me5)3Ce.
Figure 2. Percent of (C5Me5)3La at equilibrium vs ionic radius of M (pm) from the reaction of (C5Me5)2La(μ-Ph)2BPh2 with (C5Me5)3M (M = Ce, Pr, Nd, Sm). 0
Table 1. (C5Me5)3M :(C5Me5)3M Percentages for Lanthanides at Equilibrium in the Reactiona
Figure 3. Percent of (C5Me5)3M0 at equilibrium vs the difference in ionic radius (pm) of M0 - M. Table 2. (C5Me5)3M0 :(C5Me5)3M Percentages for Uranium/ Lanthanide Combinations at Equilibrium in the Reactiona
ðC5 Me5 Þ3 M þ ðC5 Me5 Þ2 M0 ðμ-PhÞ2 BPh2
0
ðC5 Me5 Þ3 M þ ðC5 Me5 Þ2 M ðμ-PhÞ2 BPh2
f ðC5 Me5 Þ3 M0 þ ðC5 Me5 Þ2 Mðμ-PhÞ2 BPh2
0
f ðC5 Me5 Þ3 M þ ðC5 Me5 Þ2 Mðμ-PhÞ2 BPh2 La:Ce La:Pr La:Nd La:Sm
83:17 95:5 98:2 99:1
Ce:Pr Ce:Nd Ce:Sm
66:34 88:12 98:2
Pr:Nd Pr:Sm
68:32 96:4
Nd:Sm
90:10
La:U La:Ce La:Pr La:Nd La:Sm
84:16 83:17 95:5 98:2 99:1
U:Ce U:Pr U:Nd U:Sm
36:64 73:27 76:24 99:1
Six-coordinate ionic radii (pm): La3þ, 103.2; Ce3þ, 101; Pr3þ, 99; Nd3þ, 98.3; Sm3þ, 95.8.14
a Six-coordinate ionic radii (pm): La3þ, 103.2; U3þ, 102.5; Ce3þ, 101; Pr3þ, 99; Nd3þ, 98.3; Sm3þ, 95.8.14
of U3þ does not interfere in these reactions. These reactions also take a long time to reach equilibrium, and the rate of reaction is related to the difference in the ionic radii between the reacting metals. The reactions of (C5Me5)2U(μ-Ph)2BPh2 with the (C5Me5)3M complexes of Pr, Nd, and Sm form the less
crowded (C5Me5)3U in mixtures that contain increasing amounts of (C5Me5)3U as the size of the metal in the (C5Me5)3M delivery reagent decreases: Pr, 73%; Nd, 76%; Sm, 99%. These numbers were compared with those of analogous reactions of (C5Me5)2La(μ-Ph)2BPh2 with the same set of (C5Me5)3M complexes of Pr, Nd, and Sm since
a
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Figure 4. Percent of (C5Me5)3U vs the ratio of ionic radii (U/Ln).
the 102.5 pm Shannon radius of U3þ (available only for six coordination) is closest to that of La3þ, 103.2 pm (Ce3þ is 101 pm).14 The comparable numbers with lanthanum, namely, Pr, 95%; Nd, 98%; Sm, 99%, show that transfer of (C5Me5)to La3þ is favored over transfer to U3þ, Table 2. Consistent with this, the reaction of (C5Me5)2La(μ-Ph)2BPh2 with (C5Me5)3U forms an equilibrium mixture that contains more of the lanthanide complex, 84% (C5Me5)3La and only 16% (C5Me5)3U, even though these metals are similar in size. In the reaction between (C5Me5)3Ce and (C5Me5)2U(μPh)2BPh2, the equilibrium actually favors the production of the more crowded (C5Me5)3Ce (64%) over (C5Me5)3U (36%) even though U3þ is larger than Ce3þ, Figure 4. The compositions of the equilibrium mixtures were also measured when approached from the opposite direction. In these cases, a less crowded (C5Me5)3M0 transfers its (C5Me5)ligand to (C5Me5)2M(μ-Ph)2BPh2 to make a more crowded (C5Me5)3M product. These reactions are also slow, and they approach the same equilibrium constants. The temperature range available to study these equilibria was limited due to the peculiar solubility properties of the cations and the reactivity of both components with polar solvents. Although the (C5Me5)2M(μ-Ph)2BPh2 complexes have solubility in benzene sufficient for this study, they have very low solubility in toluene. This unusual feature was first found with (C5Me5)2Sm(μ-Ph)2BPh2.3 Studies in THF, pyridine, and MeCN are not possible since they solvate the cations to form [(C5Me5)2MLx][BPh4] complexes that react differently with pentamethylcyclopentadienyl reagents,19,23 and these solvents react with (C5Me5)3M complexes.2,13 Heating (C5Me5)2M(μ-Ph)2BPh2/(C5Me5)3M samples to 75 °C in benzene showed no detectable change in equilibrium constants within the limits of the NMR data. This is consistent with an exchange reaction in which the differences in products and reactants are very small.
Discussion Given the high reactivity observed in the sterically crowded (C5Me5)3M complexes such that the normally inert (C5Me5)group engages in reduction,2,15,18,19,24 insertion,2,7,9,13,16 and (23) Evans, W. J.; Ulibarri, T. A.; Chamberlain, L. R.; Ziller, J. W.; Alvarez, D. Organometallics 1990, 9, 2124. (24) Evans, W. J.; Nyce, G. W.; Clark, R. D.; Doedens, R. J.; Ziller, J. W. Angew. Chem., Int. Ed. 1999, 38, 1801.
Mueller et al.
C-H bond activation,5 it was expected that the transfer of a ligand from the more crowded (C5Me5)3M complexes to a larger metal, M0 , would be thermodynamically favored. However, it was not certain if this reaction would be kinetically viable due to the steric crowding in the system. Indeed, the reactions are quite slow, requiring well over 24 h to reach equilibrium. This is consistent with the crowded nature of the species involved. Although the equilibrium would be expected to favor sterically less crowded (C5Me5)3M0 over sterically more crowded (C5Me5)3M, the magnitude could not be readily predicted and the effect of metal size on the relative stabilities of the (C5Me5)2M(μ-Ph)2BPh2 and (C5Me5)2M0 (μ-Ph)2BPh2 coproducts was less certain. In these tetraphenylborate complexes, the smaller metals would be more effective Lewis acids, but the larger metal would provide more room for agostic interaction with the phenyl rings of the (BPh4)anion. The data in Table 1 show that the equilibria do not exclusively favor the formation of less crowded (C5Me5)3M0 and (C5Me5)2M(μ-Ph)2BPh2 complexes. Mixtures of four complexes are observed in each case, and the equilibria can be approached from the opposite direction, eq 3, in which a sterically less crowded (C5Me5)3M0 generates a sterically more
crowded (C5Me5)3M complex. This may seem unreasonable at first, but it should be remembered that the reactions are not taking sterically crowded products to sterically normal products as in the reduction, insertion, and C-H activation reactions mentioned above. Instead, there are sterically crowded species on both sides in these exchange reactions, and the equilibrium is a balance between them as well as a balance between the stability of the tetraphenylborate complexes. The results with uranium indicate that similarly sized (C5Me5)3Ln complexes are favored over (C5Me5)3U. Hence, in the case of La and U, (C5Me5)3La is preferred 84:16, and in the Ce/U reaction, the more crowded (C5Me5)3Ce is present in larger concentration than (C5Me5)3U. Unusual results observed between uranium and the lanthanides are normally attributed to higher covalency with U 3þ due to greater 5f orbital participation compared to the contracted nature of the 4f orbitals. In this case, the preference for (C 5 Me 5)3Ln over (C 5Me 5 )3 U could be rationalized by saying the covalent interactions favor the stability of (C 5 Me 5)2U(μ-Ph)2BPh2 over (C5 Me 5 )2 Ln(μ-Ph)2 BPh2 or that (C5 Me 5 )3 U is destabilized compared to (C5 Me 5)3 Ln because the 5f orbitals are overloaded with electron density from the three (C 5Me 5)- rings. Alternatively, since the lanthanides are more electropositive according to Pauling’s scale, 1.10 La to 1.17 Sm vs 1.38 U, the lanthanides may preferentially form more electrostatically favored (C 5 Me 5)3M complexes. Each analysis may be too simple since the stabilities of both (C 5Me 5)3M and
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(C 5Me 5 )2M(μ-Ph)2 BPh2 components are involved, and these may not have the same dependence on metal size and f orbital participation.
Conclusion (C5Me5)3M lanthanide complexes transfer (C5Me5)- ligands to (C5Me5)2M0 (μ-Ph)2BPh2 complexes regardless of the relative size of the metals to form equilibrium mixtures that contain more of the less crowded (C5Me5)3M0 than (C5Me5)3M. The product ratios as well as the rate of the reactions depend on the relative sizes of the metals. Similar
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exchange reactions occur between uranium and lanthanide complexes, but the data indicate that (C5Me5)3Ln lanthanide complexes are favored over (C5Me5)3U. For similarly sized La and U, more (C5Me5)3La than (C5Me5)3U is present, and with the U/Ce combination, the more crowded (C5Me5)3Ce is predominant.
Acknowledgment. We thank the National Science Foundation for support of this research. Supporting Information Available: Representative 1H NMR spectra and equilibrium data are given. This material is available free of charge via the Internet at http://pubs.acs.org.
