Pentanuclear Complexes for a Series of Alkylzinc Carboxylates

Sep 18, 2009 - Neil J. Brown , Andrés García-Trenco , Jonathan Weiner , Edward R. White , Matthew Allinson , Yuxin Chen , Peter P. Wells , Emma K. G...
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Organometallics 2009, 28, 5828–5832 DOI: 10.1021/om900683z

Pentanuclear Complexes for a Series of Alkylzinc Carboxylates Katherine L. Orchard, Andrew J. P. White, Milo S. P. Shaffer,* and Charlotte K. Williams* Department of Chemistry, Imperial College London, Exhibition Road, London SW7 2AZ, U.K. Received August 2, 2009

In contrast to previously reported alkylzinc carboxylates based on aryl carboxylates, the reaction between diethylzinc and a series of zinc bis(alkyl carboxylate)s yields complexes with a ligand stoichiometry of carboxylate to ethyl of 3:2. The analytical data, including single-crystal X-ray diffraction, indicate that, for “ethylzinc acetate”, the product is an unusual pentameric complex of the form [Zn5(OAc)6(Et)4].

Zinc carboxylate interactions are common in biology, where they perform key roles in metabolic processes.1 Zinc carboxylate complexes are also important catalysts for various functional group transformations and polymerizations,2-7 and recently, they have attracted attention as structural units in metal-organic polymers (MOPs) and frameworks (MOFs).8-10 Alkylzinc carboxylates in particular, of the general form [RZn(OOCR0 )], have been studied as catalysts for the copolymerization of CO2 with epoxides,4 as modified Smith-Simmons reagents for cyclopropanation reactions,7 and as efficient precursors for zinc oxocarboxylates (Zn4O(OOCR)6) and zinc sulfidocarboxylates, which can subsequently be used as building units for high-porosity MOFs for chemical storage applications.11 A variety of related alkylzinc complexes containing carboxylate-based ligands have been prepared, most notably alkylzinc carbamato complexes, [RZn(OOCNR0 2)], and alkylzinc carboxylates for which the carboxylate group contains a second coordinating functionality, [RZn(OO*Corresponding authors. E-mail: [email protected]; c.k. [email protected]. (1) Lipscomb, W. N.; Strater, N. Chem. Rev. 1996, 96, 2375–2433. (2) Ree, M.; Bae, J. Y.; Jung, J. H.; Shin, T. J. J. Polym. Sci., Part A: Polym. Chem. 1999, 37, 1863–1876. (3) Luinstra, G. Polym. Rev. 2008, 48, 192–219. (4) Coates, G. W.; Moore, D. R. Angew. Chem., Int. Ed. 2004, 43, 6618–6639. (5) Kuran, W.; Nieslochowski, A. Polym. Bull. (Berlin) 1980, 2, 411– 416. (6) Noltes, J. G.; Verbeek, F.; Overmars, H. G. J.; Boersma, J. J. Organomet. Chem. 1970, 24, 257–262. (7) Lorenz, J. C.; Long, J.; Yang, Z.; Xue, S.; Xie, Y.; Shi, Y. J. Org. Chem. 2004, 69, 327–334. (8) Yaghi, O. M.; O’Keefe, M.; Ockwig, N. W.; Chae, H. K.; Eddaoudi, M.; Kim, J. Nature 2003, 423, 705–714. (9) Kaye, S. S.; Dailly, A.; Yaghi, O. M.; Long, J. R. J. Am. Chem. Soc. 2007, 129, 14176–14177. (10) Huang, L.; Wang, H.; Chen, J.; Wang, Z.; Sun, J.; Zhao, D.; Yan, Y. Microporous Mesoporous Mater. 2003, 58, 105–114. (11) Lewinski, J.; Bury, W.; Dutkiewicz, M.; Maurin, M.; Justyniak, I.; Lipkowski, J. Angew. Chem., Int. Ed. 2008, 47, 573–576. (12) Redshaw, C.; Elsegood, M. R. J. Angew. Chem., Int. Ed. 2007, 46, 7453–7457. (13) Tang, Y.; Kassel, W. S.; Zakharov, L. N.; Rheingold, A. L.; Kemp, R. A. Inorg. Chem. 2005, 44, 359–364. (14) Hursthouse, M. B.; Malik, M. A.; Motevalli, M.; O’Brien, P. J. Chem. Soc., Chem. Commun. 1991, 1690–1691. pubs.acs.org/Organometallics

Published on Web 09/18/2009

CR0 X)] (X=OH, NH2, SH).12-15 The alkylzinc carbamato species, prepared by CO2 insertion into Zn-N bonds, form discrete tetramers of the form [RZn(μ3-OOCNR0 2)]4, which become dimers of the form [RZn(μ2-OOCNR0 2)(py)]2 on the addition of pyridine (py) (R = Me; NR0 2=N(iPr)2, N(iBu)2, and piperidinyl).13,14 A variety of structures have been obtained for [RZn(OOCR0 X)]-type complexes, from tetrameric rings of [RZn(μ2-OOCR0 (μ2-X))] units (R=Et; R0 X= CPh2(NH2)) and more complex structures such as [Zn6R3(μ3-OOCR0 (μ2-X))3L3] (R = Et; R0 X = CPh2(OH) ; L = solvent),12 to extended polymers such as [RZn(μ2-OOCR0 (μ2-X))(py)]n (R=Et; R0 X=(CH2)2SH).15 However, despite having been synthesized and utilized since the 1960s,16 the chemistry of simple17 alkylzinc carboxylates has been relatively unexplored, and very little structural information is known; early studies were restricted to cryoscopic measurements, and to date, only two crystal structures have been reported,11,18 both of which are for complexes with aryl carboxylate groups. Dickie et al. reported ethylzinc 2,6-bis(2,4,6-trimethylphenyl)benzoic acid, in which the bulky carboxylate groups enforce low coordination geometry at the metal centers, giving a dimer of the form [EtZn(μ2-OOCR)]2.18 Lewinski et al. reported ethylzinc benzoate, which was found to be a cyclic hexamer of the form [EtZn(μ3-OOC(C6H5))]6, with each zinc atom coordinated to three carboxylate groups.11 Here we present our findings that, for a series of ethylzinc alkyl carboxylates, an unusual pentameric complex forms in which the ratio of carboxylate to ethyl groups is 3:2. This structure is favored in the solid state and in solution, regardless of the reagent excess and synthetic route. Alkylzinc carboxylates can be prepared via several routes, most commonly by alkane elimination in the reaction of a dialkylzinc with 1 equiv. of the respective acid (eq 1). In our work, however, we have favored a different route, utilizing (15) Boyle, T. J.; Pratt, H. D. I.; Alam, T. M.; Headley, T.; Rodriguez, M. A. Eur. J. Inorg. Chem. 2009, 855–865. (16) Coates, G. E.; Ridley, D. J. Chem. Soc. 1965, 1870–1877. (17) By “simple” we are referring to complexes comprising carboxylate groups that possess a carbon-only backbone and for which there are no additional coordinating groups present. (18) Dickie, D. A.; Jennings, M. C.; Jenkins, H. A.; Clyburne, J. A. Inorg. Chem. 2005, 44, 828–830. r 2009 American Chemical Society

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ligand exchange between the dialkylzinc and the zinc bis(carboxylate) (eq 2); this route minimizes the introduction of small traces of water that are unavoidable with the use of free carboxylic acids. We have found that the primary product formed is the same using either route (1H NMR, see below):

5ZnEt2 þ 6RCOOH f ½Zn5 ðOOCRÞ6 ðEtÞ4  þ 6EtH ð1Þ 2ZnEt2 þ 3ZnðOOCRÞ2 f ½Zn5 ðOOCRÞ6 ðEtÞ4 

ð2Þ

Reaction between zinc bis(acetate) and diethylzinc enabled the isolation of “ethylzinc acetate”, 1, as a white powder (81% yield). Crystals were obtained either directly from the reduced reaction mixture (saturated toluene solution) or from a saturated hexane solution at low temperature. In both cases, X-ray crystallographic analysis showed formation of an unusual pentanuclear structure with the molecular formula [Zn5(OAc)6(Et)4]. The complex crystallized with two independent molecules (A and B) in the asymmetric unit; molecule A is shown in Figure 1, while molecule B is shown in Figure S2 (Supporting Information). The only significant difference between the two structures is in the position of the terminal methyl of the Zn(4)-bound ethyl moiety. The complexes contain one central, six-coordinate Zn connected to four four-coordinate Zn centers via bridging acetate groups. Each acetate group bridges three Zn centers in an η1:η2:μ3 coordination mode, analogous to the coordination in Lewinski’s structure.11 The complex is constructed of fused networks of four-, six-, and eight-membered heterocycles. Although pentameric zinc complexes have been observed previously, largely as motifs in extended metal-organic frameworks,19-22 only one other discrete complex has been reported with a similar structure to that of 1. Ullrich et al. found that [Zn5(ONMe2)6(iPr)4] has a similar central, sixcoordinated Zn and four outer ZniPr units, which bind to the six O-N units by either an N,O,O or an N,N,O coordination mode.23 As such, the binding of the hydroxylamine ligands is equivalent to that of the acetate groups in 1. The structure of [Zn5(ONMe2)6(iPr)4] differs from that of 1 in that it is of higher symmetry: in [Zn5(ONMe2)6(iPr)4], the two fourmembered rings are related by a C2 axis, whereas 1 is chiral-at-metal. The Zn-O bond lengths of 1 are comparable to those observed in [Zn5(ONMe2)6(iPr)4], falling within the range 1.997(2)-2.1695(17) A˚ (see Table S1, Supporting Information). Additionally, the O-Zn-O bond angles observed in the four-membered rings are similar, ranging between 75.67(7)° and 79.63(7)° (average 78.1°). Although the structure shown in Figure 1 is chiral-atmetal, the determined space group is centrosymmetric, and there are equal numbers of both enantiomers present in the crystal. (19) Wang, J.; Lin, Z.; Ou, Y.-C.; Yang, N.-L.; Zhang, Y.-H.; Tong, M.-L. Inorg. Chem. 2008, 47, 190–199. (20) Fang, Q.-R.; Zhu, G.-S.; Jin, Z.; Xue, M.; Wei, X.; Wang, D.-J.; Qiu, S.-L. Cryst. Growth Des. 2007, 7, 1035–1037. (21) Li, X.; Cao, R.; Sun, D.; Yuan, D.; Bi, W.; Li, X.; Wang, Y. J. Mol. Struct. 2004, 694, 205–210. (22) Wang, J.-J.; Liu, C.-S.; Hu, T.-L.; Chang, Z.; Li, C.-Y.; Yan, L.F.; Chen, P.-Q.; Bu, X.-H.; Wu, Q.; Zhao, L.-J.; Wang, Z.; Zhang, X.-Z. CrystEngComm 2008, 10, 681–692. (23) Ullrich, M.; Berger, R. J. F.; Lustig, C.; Frohlich, R.; Mitzel, N. W. Eur. J. Inorg. Chem. 2006, 4219–4224.

Figure 1. Molecular structure of one (A) of the two crystallographically independent complexes present in the crystals of 1 showing the five zinc atoms (50% thermal ellipsoids), the six bridging acetates (dark bonds), and the four ethyl units (open bonds).

The complex stoichiometry is strongly supported by 1H NMR spectroscopy. Regardless of preparation method, the 1 H NMR spectrum of 1 consistently showed ratios of the methyl group resonances on the acetate to ethyl ligands of 3:2 (Figure 2a, peaks labeled a and b, respectively). The reaction between equimolar quantities of ZnEt2 and Zn(OAc)2, in situ in d6-benzene, was monitored by NMR spectroscopy (Figure 2b). The spectrum shows the resonances for [Zn5(OAc)6Et4], with an integration ratio for OOCH3:CH3 as before; however, resonances corresponding to the excess ZnEt2 also appear (Figure 2b, labeled d and e). The sum of the integrations for the methyl groups of ZnEt2 and [Zn5(OAc)6Et4] gives a ratio of OOCH3:[total ethyl CH3] of 1:1, consistent with the original equimolar stoichiometry (Figure 2b, peaks labeled d and b, respectively). Inoue et al. have previously suggested that alkylzinc carboxylates can be in equilibrium with the homoleptic complexes (i.e., ZnR2 and Zn(OOCR0 )2).24 However, the presence of such an equilibrium in our system can be ruled out, as the addition of excess ZnEt2 only increased the area of the ZnEt2 resonances and did not shift the intramolecular ratio of OAc:Et from the original 3:2 value (peaks labeled a and b in Figure 2b). Similarly, application of a dynamic vacuum removed the excess diethylzinc to yield a product with a 1H NMR spectrum equivalent to Figure 2a. In other words, the intramolecular OAc:Et ratio remained at 3:2 despite the changing diethylzinc content of the system. When a solution of equimolar quantities of ZnEt2 and Zn(OAc)2 reacted in situ in toluene-d8 was heated, the ethyl peaks coalesced at 80 °C, suggesting that there is dynamic exchange between the ethyl groups of [Zn5(OAc)6(Et)4] and the free diethylzinc (Figure 3). On cooling from 80 to 20 °C, the spectrum becomes identical to that originally obtained at 20 °C. On heating a sample without excess diethylzinc, no (24) Inoue, S.; Kobayashi, M.; Tozuka, T. J. Organomet. Chem. 1974, 81, 17–21.

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Figure 2. 1H NMR spectra (in C6D6) for (a) “ethylzinc acetate” prepared from a 3:2 mixture of Zn(OAc)2 and ZnEt2, and (b) “ethylzinc acetate” prepared in situ from a 1:1 mixture of ZnEt2 and Zn(OAc)2. In (b), the remaining ZnEt2 peaks are indicated by d (methyl) and e (methylene).

peak broadening or variation in chemical shift of the ethyl resonances was observed, further suggesting that the dynamic behavior is not a true equilibrium but simply an exchange mechanism; for this sample, heating did not alter the 1H NMR, indicating the thermal stability of the complex. The reagent stoichiometry (3:2) can also be derived from the apparent conversion of ZnEt2. For the 1:1 ZnEt2:Zn(OAc)2 experiment, the [Zn5(OAc)6(Et)4] ethyl resonances constitute 64% of the total ethyl resonances. Assuming that 100% of the Zn(OAc)2 is converted, the ratio of OAc:Et is 100:64 or 3.0:1.9. Note that Zn(OAc)2 has negligible solubility in C6D6, so all of the acetate signal must arise from [Zn5(OAc)6(Et)4] acetate groups. In addition, the preparation of 1 in bulk proceeds cleanly from a 3:2 ratio of Zn(OAc)2:ZnEt2. Elemental analysis supports the stoichiometry of the pentanuclear complex (calculated carbon content 30.12, found 30.08 wt %). A gas evolution study indicated 13.7 wt % hydrolyzable ethyl content, which matches the calculated value for the pentanuclear complex (14.6 wt %) to within the accuracy of the testing method. Pulsed gradient spin-echo (PGSE) NMR allows determination of molecular diffusion coefficients, from which hydrodynamic radii can be estimated.25 The diffusion coefficient for 1 was calculated to be 1.2  10-9 m2 s-1, which, using the Stokes-Einstein relation, gave an estimate of the hydrodynamic radius of 4 A˚. The radius estimated from the crystal data is 5 A˚, given that there are four (25) Price, W. S. Concepts Magn. Reson. 1997, 9, 299.

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Figure 3. 1H NMR (C7D8) spectra for solutions of 1:1 ZnEt2 þ Zn(OAc)2 (a) at 293 K, (b) at 353 K. The spectrum shown in (b) has been scaled to allow clearer viewing of the broad, coalesced peaks. The peak at 0.3 ppm is due to silicon grease contamination.

molecules per unit cell and the molecules occupy 69% of the unit cell volume.26 The radius estimates are comparable, especially considering that the ethyl ligands are flexible, suggesting that the pentanuclear complex persists in solution (benzene). In order to investigate whether the observed ligand stoichiometry is common for all alkylzinc alkyl-carboxylates, the hexanoate, dodecanoate, and stearate derivatives were also studied (carbon chain lengths of 6, 12, and 18, respectively). The complex formed by reaction between zinc bis(stearate) and ZnEt2, [Zn5(OOC(CH2)16CH3)6(Et)4], 2, can be isolated in the bulk as a white powder (37% yield). Although the flexibility of the stearate chain prevented successful crystallization, elemental analysis of 2 supports the stoichiometry of the pentanuclear complex (calculated C 64.98 H 10.81, found C 64.90 H 10.87). The infrared (IR) spectra of 1 and 2 both support a bridging arrangement of carboxylate ligands, based on the difference, Δ, between the asymmetric and symmetric stretches (Δ = 165 and 140 cm-1 for 1 and 2, respectively) (see Supporting Information for spectra).27 The IR spectra for both compounds show two resonances in the asymmetric CdO stretching region, and overall, the spectra show many comparable features. (26) Unit cell occupancy estimated using PLATON software (69% occupied space, 4 molecules per unit cell), giving a volume per molecule of 543 A˚3. Radius calculated assuming molecule is spherical. (27) Nakamoto, K. In Infrared and Raman Spectra of Inorganic and Coordination Compounds Part B: Applications on Coordination, Organometallic, and Bioinorganic Chemistry; 1997; pp 59-60.

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Despite considerable effort, it was not possible to isolate the hexanoate and dodecanoate derivatives; therefore full characterization could not be performed: these complexes are highly solvophilic (toluene, hexane, pentane, tetrahydrofuran), and the solvent could not be fully removed even after extended times under high vacuum. However, the hexanoate and dodecanoate derivatives were prepared and studied in situ in deuterated solvent. For all three derivatives (including 2), the 1H NMR spectra show integration ratios for OOCR:Et of 3:2, based on the methylene peaks (Figure S5, Supporting Information). For each, the apparent conversion from a solution prepared from a 1:1 ratio of Zn(OOCR)2:ZnEt2 gives a 3:2 ratio as found for 1. Additionally, variable-temperature 1H NMR of each derivative demonstrated coalescence with diethylzinc peaks, where excess diethylzinc was present. By analogy to the fully characterized complexes 1 and 2, we believe that the formation of a pentanuclear complex is common to this series of ethylzinc carboxylate compounds: acetate, hexanoate, dodecanoate, stearate. The difference between the structures of our series of alkylzinc carboxylates and the ethylzinc benzoate reported previously remains to be explained. The much greater bulk and rigidity of the benzoate groups must be a critical factor. Preliminary results using zinc bis(formate) (Zn(OOCH)2) and ZnEt2 suggest the formation of a different structure with a ratio of formate:ethyl groups of 8:6, akin to complexes that have been isolated for some ethylzinc alkoxides.28-31 Further work to ascertain the structure is being carried out. In conclusion, a series of ethylzinc carboxylates have been prepared and, in contrast to previous reports of similar compounds based on aryl carboxylates, were found to form stable complexes with ligand ratios of OOCR:Et of 3:2 (where R= Me, (CH2)4CH3, (CH2)10CH3, (CH2)16CH3), as evidenced by 1H NMR spectroscopy and elemental analysis (actetate and stearate derivatives). The integrations of the methyl group resonances on the zinc-bound carboxylate and ethyl groups are in a relative ratio of 3:2, regardless of diethylzinc excess; however the ethyl groups appear to be labile and in exchange with any excess diethylzinc present. X-ray crystal data for the acetate derivative indicates that the 3:2 ratio manifests as a pentanuclear complex, of the form [Zn5(OAc)6Et4], and 1H NMR/PGSE experiments provide good evidence that this structure is maintained in benzene solutions. We propose that the pentanuclear complex is also present for the hexanoate, dodecanoate, and stearate derivatives. It seems likely that the same structure will be formed for all straight-chain carboxylates from acetate to longer pendant chains.

Experimental Section General Considerations. Unless otherwise stated, all reactions were conducted under a nitrogen atmosphere using either standard Schlenk techniques or in a nitrogen-filled glovebox. Solvents were distilled from sodium and stored under nitrogen. (28) Allen, G.; Bruce, J. M.; Farren, D. W.; Hutchinson, F. G. J. Chem. Soc. B 1966, 799–803. (29) Bruce, J. M.; Cutsforth, B. C.; Farren, D. W.; Hutchinson, F. G.; Rabagliati, F. M.; Reed, D. R. J. Chem. Soc. B 1966, 1020–1024. (30) Boyle, T. J.; Bunge, S. D.; Andrews, N. L.; Matzan, L. E.; Sieg, K.; Rodriguez, M. A.; Headley, T. J. Chem. Mater. 2004, 16, 3279–3288. (31) Jana, S.; Berger, R. J. F.; Frohlich, R.; Pape, T.; Mitzel, N. W. Inorg. Chem. 2007, 46, 4293–4297.

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Unless otherwise stated, solvents were degassed prior to use by performing three freeze-pump-thaw cycles. Deuterated solvents were dried by placing over calcium hydride, performing three free-thaw cycles under vacuum, refluxing for at least 48 h, distilling under vacuum, and storing under nitrogen. Diethylzinc was purchased from Aldrich, vacuum distilled, and stored in an ampule, under nitrogen, at -38 °C. Zinc bis(dodecanoate) and zinc bis(hexanoate) were prepared according to the method described by Berkesi et al., from reaction with the respective acid with Zn(OH)2 in boiling octane.32 Zinc bis(stearate) was prepared by the method described by Ekwunife et al., from reaction of in situ prepared potassium stearate with zinc chloride in an ethanol solution.33 All zinc bis(carboxylates) were found to be of sufficient purity by elemental analysis. Unless otherwise stated, all other reagents were purchased from commercial suppliers and used as received. Infrared (IR) spectroscopy was carried out using a Perkin-Elmer Spectrum 100 Fourier transform IR spectrometer, using dried Nujol (sodium). Elemental analysis was carried out using a Carlo Erba CE1108 elemental analyzer, and samples were manipulated under inert atmosphere (helium glovebag); analysis was performed by S. Boyer at London Metropolitan University, North Campus, Holloway Road, London, N7. In general, NMR spectra were collected on a Bruker AV-400 instrument. The 1H PGSE (DOSY) experiments were performed on a Bruker AV-500 spectrometer, equipped with a z-gradient bbo/5 mm tunable probe and a BSMS GAB 10 A gradient amplifier providing a maximum gradient output of 5.35 G/cmA. All experiments were measured using the stebpgp1s pulse program (TopSpin 2.1.3 software) at a constant temperature of 300 K and a gas flow of 400 L per hour. The spectra were collected at a frequency of 500.13 MHz with a spectral width of 4000 Hz (centered on 4 ppm) and 32768 data points. A relaxation delay of 10 s was employed along with a diffusion time (Δ) of 70 ms. Bipolar gradient pulses (δ/2) of 2.2 ms and homospoil gradient pulses of 1.1 ms were used. The gradient strength of the homospoil pulse was -17.13%. A total of 32 experiments were collected with the bipolar gradient strength, initially at 2% (first experiment), linearly increased to 95% (32nd experiment). All gradient pulses were sine shaped, and after each application a recovery delay of 200 μs was used. The spectra were processed using an exponential function with a line broadening of 2 Hz. Further processing was achieved using the Bruker dosy software or DOSYm software (NMRtec). Hydrolyzable ethyl content was determined by a liquid displacement method described in the Supporting Information. Synthesis of 1, [Zn5(OOCCH3)6(Et)4]. Diethylzinc (0.496 g, 4.02 mmol) was added to a suspension of anhydrous zinc bis(acetate) (1.107 g, 6.03 mmol) in toluene (5 mL) and stirred for 6 h to give a hazy solution, which became clear on heating. Volatiles were removed in vacuo to yield a fine, white powder (1.296 g, 1.63 mmol, 81%). 1H NMR (C6D6, 400 MHz): δ 1.89 (s, 18 H, OOCCH3), 1.56 (t, 12 H, J = 8.0 Hz, CH2CH3), 0.61 (q, 8 H, J = 8.0 Hz, CH2CH3) ppm. 13C{1H} NMR (C6D6, 400 MHz): δ 128.1 (-CdO), 24.2 (H3CCOO-), 12.3 (CH3CH2-), -0.8 (CH3CH2-) ppm. IR (Nujol mull): 1572 (br, ν(CdO)asymm.), 1545 (br), 1407 (ν(CdO)symm.), 1350, 1226 (w), 1176 (w) (δ(Zn-Et)), 1027, 990, 953, 921, 694, 617 (Zn-Et rock), 520 (w) (ν(Zn-Et)) cm-1. Anal. Calcd for C20H38O12Zn5: C 30.12, H 4.80. Found: C 30.08, H 4.72. Hydrolyzable ethyl: calcd 14.5%, found 13.7%. Synthesis of 2, [Zn5(OOC(CH2)16CH3)6(Et)4]. Diethylzinc (0.054 g, 0.437 mmol) was added to a suspension of zinc bis(stearate) (0.380 g, 0.601 mmol) in toluene (10 mL) and stirred for 19 h. The resulting suspension was heated gently to (32) Berkesi, O.; Dreveni, I.; Andor, J. A. Inorg. Chim. Acta 1992, 195, 169–173. (33) Ekwunife, M. E.; Nwachukwu, M. U.; Rinehart, F. P.; Sime, S. J. J. Chem. Soc., Faraday Trans. 1 1975, 71 (7), 1432–1446.

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give a clear solution with a few insoluble particulates. The solution was filtered and the solvent reduced to yield a swollen, solvated white solid. The product was resuspended in cold pentane and all volatiles were removed in vacuo to yield a white solid (0.160 g, 37%). 1H NMR (C6D6, 400 MHz): δ 2.54 (t, 12 H, J = 8.0 Hz, OOCCH2(CH2)15CH3), 1.72 (m, 24 H, CH3CH2 þ OOCCH2CH2(CH2)14CH3), 1.35 (m, 168 H, OOCCH2CH2(CH2)14CH3), 0.93 (t, 18 H, J = 6.8 Hz, OOC(CH2)16CH3), 0.80 (q, 8 H, J = 8.0 Hz, CH3CH2) ppm. 13C{1H} NMR (C6D6, 400 MHz): δ 183.5 (CdO), 38.4, 32.4, 30.3 - 29.65 (12 C, carboxylate chain), 26.4, 23.2, 14.4, 12.9 (CH3CH2), -0.2 (CH3CH2) ppm. IR (Nujol mull): 1566 (ν(CdO)asymm), 1540, 1426 (ν(CdO)symm), 1410 (w), 1399 (w), 1337 (w), 1268 (w), 1169 (w) (δ(Zn-Et)), 668, 614 (Zn-Et rock), 530 (w) (ν(Zn-Et)) cm-1. Anal. Calcd for C116H230O12Zn5: C 64.98, H 10.81. Found: C 64.90, H 10.87. Preparation of Ethylzinc Carboxylates in situ for NMR Analysis. In general, a stock solution of diethylzinc (0.093 g, 0.753 mmol) was diluted to 5 mL total volume with C6D6 (0.15 M solution), and portions of this stock solution were added to the respective zinc bis(carboxylate), equilibrated for 16 h, and transferred to NMR tubes for analysis. The ethylzinc stearate and ethylzinc dodecanoate samples required gentle warming, after the incubation period, to aid full dissolution. The relative quantities for the 3:2 ratio samples are given in Table S1, Supporting Information. [Zn5(OOC(CH2)4CH3)6(Et)4]. 1H NMR: δ 2.46 (t, 12 H, J = 8.0 Hz, OOCCH2(CH2)3CH3), 1.64 (m, 24 H, OOCCH2CH2(CH2)8CH3 þ CH2CH3), 1.21 (m, 24 H, OOCCH2CH2(CH2)2CH3),

Orchard et al. 0.83 (t, 18 H, J = 8 Hz, OOC(CH2)4CH3), 0.75 (q, 8 H, J = 8.0 Hz, CH2CH3) ppm. [Zn5(OOC(CH2)10CH3)6(Et)4]. 1H NMR: δ 2.50 (t, 12 H, J = 8.0 Hz, OOCCH2(CH2)9CH3), 1.66 (m, 24 H, OOCCH2CH2(CH2)2CH3 þ CH2CH3), 1.28 (m, 96 H, OOCCH2CH2(CH2)8CH3), 0.93 (t, 18 H, J = 8 Hz, OOC(CH2)10CH3), 0.75 (q, 8 H, J = 8.0 Hz, CH2CH3) ppm. X-ray Crystallography. Crystal data for 1: C20H38O12Zn5, M=797.35, triclinic, P1 (no. 2), a=10.9551(2) A˚, b=12.9319(2) A˚, c = 22.0224(4) A˚, R = 90.0423(14)°, β = 95.0744(14)°, γ = 91.7413(15)°, V = 3106.24(9) A˚3, Z = 4 (2 independent complexes), Dc =1.705 g cm-3, μ(Cu KR) = 4.754 mm-1, T = 173 K, colorless platy needles, Oxford Diffraction Xcalibur PX Ultra diffractometer; 24 305 independent measured reflections (Rint = 0; see Supporting Information), F2 refinement, R1(obs) = 0.0407, wR2(all) = 0.1180, 19 838 independent observed absorption-corrected reflections [|Fo| > 4σ(|Fo|), 2θmax =145°], 680 parameters. CCDC 740016.

Acknowledgment. This work was funded by the Engineering and Physical Sciences Research Council. We thank Mr. P. Haycock and Mr. R. N. Sheppard for the PGSE and VT experiments and helpful discussions. Supporting Information Available: Further discussion, experimental data, and X-ray crystallographic data in pdf format and CIF files are available free of charge via the Internet at http:// pubs.acs.org.