Synthesis and Structure of Alkylzinc 3,5-Diphenylpyrazolates

Communication pubs.acs.org/IC

Synthesis and Structure of Alkylzinc 3,5-Diphenylpyrazolates: Dramatic Influence of Steric and Solvent Effects Szymon Komorski,† Michał K. Leszczyński,‡ Iwona Justyniak,‡ and Janusz Lewiński*,†,‡ †

Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, 00-664 Warsaw, Poland Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland

‡

S Supporting Information *

structurally diverse alkylzinc pyrazolate complexes whose structures were determined by the character of both the Znbonded alkyl substituents and solvents used (Scheme 1). The observed molecular structures provide a new look at the aggregation and stabilization of alkylzinc species.

ABSTRACT: The reaction of R2Zn (R = Et, tBu) with 3,5-diphenylpyrazole results in the formation of three structurally diverse alkylzinc pyrazolates: a novel dinuclear tetrahydrofuran solvate, an unprecedented trimeric structure, and a spiro trinuclear aggregate. Structural analysis of the resulting complexes provides a new look at the aggregation and stabilization of alkylzinc species.

Scheme 1. Syntheses of 1−3

P

yrazole and its derivatives play an important role in coordination chemistry mainly for their rich coordinative flexibility and relatively easy synthesis of a considerable amount of structurally varied derivatives.1 Particularly interesting are 1Hpyrazoles, which upon deprotonation form monoanionic pyrazolate ligands. The two adjacent Lewis basic N-donor atoms provide a bridging platform for metal centers and facilitate the formation of di- and polynuclear complexes, which often selforganize into metallamacrocycles.2 Employing pyrazolate ligands in main-group metal chemistry as bridging scaffolds has led to many advances, such as the first examples of η2-pyrazolate ligand coordination to a group 13 element,3 new coordination modes for acetylide groups,4 stabilization of the bridging alkyl and hydride groups in pyrazolate-bridged dimers,5 and probing of the role of geometric requirements in the mechanism of activation of O2 by four-coordinate aluminum alkyls.6 In the past decade pyrazole-based ligands have attracted a lot of attention in the synthesis of coordination polymers and metal−organic frameworks.7 The driving force behind that is the generally higher thermal and chemical robustness of the pyrazolate-linked materials, compared to their carboxylate-based equivalents.8 We have recently become interested in the chemistry of various low-coordinate alkylzinc complexes stabilized by intramolecular Zn−π interactions.9 For example, using an ethylzinc ohydroxybiphenyl derivative, we came across a new πinteraction-mediated pathway in the oxygenation of zinc alkyls.10 In the course of these investigations, we turned our attention to organozinc pyrazolate complexes. A literature search revealed a relative paucity of structurally well-defined organozinc pyrazolates, including only complexes supported by compartmental multidentate pyrazolate ligands, which promoted the formation of dimeric compounds with four-coordinate Zn centers.11 However, we anticipated that sterically encumbered 3,5diphenylpyrazole (HpzPh2) may be a reliable model proligand with purposefully positioned aromatic rings capable of enforcing low coordination numbers at Zn sites stabilized by Zn−π interactions. Herein, we report the synthesis of three novel © XXXX American Chemical Society

To obtain a tert-butylzinc derivative of HpzPh2, 1 equiv of Bu2Zn was introduced to a slurry of HpzPh2 in toluene at −15 °C. The reaction mixture was left to stir at this temperature for ca. 0.5 h and then for another 0.5 h at ambient temperature. The concentration of the mother solution followed by the addition of hexane and crystallization yielded colorless crystals of the original trimeric aggregate [tBuZn(pzPh2)]3 (1). When Et2Zn was used instead of tBu2Zn in an analogous reaction, a raw product deposited after ca. 0.5 h of stirring at ambient temperature. Dissolution of this solid in hot toluene followed by crystallization at ambient temperature afforded a trinuclear aggregate [Et2Zn3(pzPh2)4] (2); when crystallization was carried at 15 °C, a 2·PhMe solvate was obtained. The same reaction between Et2Zn and HpzPh2 in a tetrahydrofuran (THF) solution resulted in formation of a novel dinuclear cluster [Et2Zn2(pzPh2)2(μt

Received: March 25, 2016

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DOI: 10.1021/acs.inorgchem.6b00745 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

Figure 1. Molecular structures of compounds 1 (a) and 3 (b). The Zn2N4 dinuclear core is highlighted with dark-red bonds, while the H atoms and phenyl groups of the pyrazolate ligands (a) are omitted for clarity.

2.432(7) and 2.587(7) Å].14a The Zn···Zn distance is 3.382(1) Å and is longer than that in 1. The Zn−N distances are 2.009(2) and 2.026(2) Å, and the Zn−C bond is 1.956(3) Å. The ethylzinc derivative of HpzPh2 prepared in the absence of donor solvents crystallizes in two forms (molecular crystal 2 and solvate 2·PhMe), depending on the crystallization conditions. The crystal form of 2 obtained at ambient temperature is densely packed in the crystal lattice (Figure S3), while the toluene solvate 2·PhMe possesses ca. 8.7 × 4.5 Å2 sized channels filled with guest solvent molecules along the 2-fold axis (Figure S5). The molecular structures of both compounds are almost identical and show only slight differences in the bond distances and angles resulting from the emergence of CH−π interactions with toluene molecules in the crystal lattice. The unexpected stoichiometry of this trinuclear compound may be attributed to the presence of ligand redistribution processes, leading to Schlenk equilibrium, which is hindered in the cases of 1 and 3 because of bulky tBu ligands and THF, respectively.6,15 The molecular structure of 2 is an almost linear spiro trinuclear cluster (Figure 2). The central

THF)] (3). New complexes were characterized using 1H and DOSY NMR spectroscopy and X-ray crystallography. The molecular structures of 1 and 3 are presented in Figure 1. It is essential to note that the structures of 1 and 3 bear a striking resemblance to each other. Specifically, both complexes comprise a dinuclear [RZn(μ-pzPh2)]2 structural motif with the Zn centers bridged by another unit, i.e., the monomeric [RZn(pzPh2)] unit or THF molecule (see Figure 1). The tert-butylzinc derivative 1 crystallizes in the P1 space group as a hexane solvate. The very unusual type of trimeric12 structure of 1 can be formally described as a dimeric [tBuZn(pzPh2)]2 aggregate assembled with the monomeric [tBuZn(pzPh2)] unit (Figure 1a). The core of the dimeric subunit [tBuZn(pzPh2)]2 is comprised of a Zn2N4 ring in a boat conformation. Stabilization of the dimeric subunit is achieved by the entrapment of a tBuZn(pzPh2) subunit through the cooperative anchoring of Zn3 and N5 atoms to N3, N4 and Zn1, Zn2 centers of the dimeric subunit, respectively. Consequently, the pyrazolate ligands are in three distinctive coordination modes: the classic η1:η1-μ2 as well as the previously unreported for zinc pyrazolates η1:η2-μ3 and η2:η2-μ3 modes. Metal centers Zn1 and Zn2 are bridged by two pyrazolate ligands with Zn−N distances between 1.976 and 2.087 Å, being in the expected range, while the 3.271(5) Å Zn···Zn distance is somewhat short (relative to other zinc pyrazolates). The adduct character of this compound is further confirmed by other bond distances: Zn3−N6 is significantly shorter (1.875 Å), while the Zn3−N3 and Zn3−N4 distances are elongated [2.179(2) Å and 2.506(2) Å, respectively]. The distances for Zn1−N5 and Zn2− N5 are also elongated: 2.309(2) and 2.242(2) Å, respectively. Notably, the observed aggregation mode for 1 is entirely different from the commonly anticipated ring-stacking and ring-laddering principle for the related complexes.13 The crystal structure of 3 belongs to the monoclinic C2/c space group. Compound 3 has a novel dinuclear μ-THF solvate structural motif. The two four-coordinate Zn atoms are bridged by two η1:η1-μ2 pyrazolate ligands, forming a Zn2N4 ring in a boat conformation (Figure 1b). The boat conformation is stabilized by anchoring a μ-THF molecule between the Zn atoms. The bridging Zn−OTHF bonds [2.345(2) Å] are similar to those previously reported in [(MeZn)2(tBuPO3)(μ-THF)]2 [2.308(3) and 2.360(3) Å]14b and significantly shorter than those in [Zn4O(Ant)6(μ-THF)(THF)] [Ant = 9-anthracenecarboxylate;

Figure 2. Molecular structure of 2. H atoms are omitted for clarity.

Zn atom adopts tetrahedral geometry and is bridged in a η1:η1-μ2 fashion by four pzPh2 ligands, with bond lengths between 2.003(2) and 2.016(3) Å. The terminal Zn atoms have trigonalplanar geometry, with two pzPh2 ligands and one η1 ethyl group. Zn1−N1 and Zn1−N3 bond lengths are 1.995(3) and 2.008(3) Å, respectively, and Zn1−C1 is 1.967(3) Å. The latter coordinatively unsaturated Zn centers are unsymmetrically B

DOI: 10.1021/acs.inorgchem.6b00745 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

(d) Viciano-Chumillas, M.; Tanase, S.; de Jongh, L. J.; Reedijk, J. Eur. J. Inorg. Chem. 2010, 3403−3418. (e) Hitzbleck, J.; O’Brien, A. Y.; Forsyth, C. M.; Deacon, G. B.; Ruhlandt-Senge, K. Chem. - Eur. J. 2004, 10, 3315−3323. (3) (a) Zheng, W.; Roesky, H. W.; Noltemeyer, M. Organometallics 2001, 20, 1033−1035. (b) Deacon, G. B.; Delbridge, E. E.; Forsyth, C. M.; Junk, P. C.; Skelton, B. W.; White, A. H. Aust. J. Chem. 1999, 52, 733−739. (4) Zheng, W.; Stasch, A.; Prust, J.; Roesky, H. W.; Cimpoesu, F.; Noltemeyer, M.; Schmidt, H. Angew. Chem., Int. Ed. 2001, 40, 3461− 3464. (5) (a) Yu, Z.; Heeg, M. J.; Winter, C. H. Chem. Commun. 2001, 353− 354. (b) Yu, Z.; Wittbrodt, J. M.; Heeg, M. J.; Schlegel, H. B.; Winter, C. H. J. Am. Chem. Soc. 2000, 122, 9338−9339. (c) Yu, Z.; Wittbrodt, J. M.; Xia, A.; Heeg, M. J.; Schlegel, H. B.; Winter, C. H. Organometallics 2001, 20, 4301−4303. (6) Lewinski, J.; Zachara, J.; Gos, P.; Grabska, E.; Kopec, T.; Madura, I.; Marciniak, W.; Prowotorow, I. Chem. - Eur. J. 2000, 6, 3215−3227. (7) (a) Zhang, J.-P.; Zhang, Y.; Lin, J.; Chen, X. Chem. Rev. 2012, 112, 1001−1033. (b) Pettinari, C.; Tăbăcaru, A.; Galli, S. Coord. Chem. Rev. 2016, 307, 1−31. (8) Miras, H. N.; Zhao, H.; Herchel, R.; Rinaldi, C.; Pérez, S.; Raptis, R. G. Eur. J. Inorg. Chem. 2008, 4745−4755. (9) (a) Zelga, K.; Leszczyński, M.; Justyniak, I.; Kornowicz, A.; Cabaj, M.; Wheatley, A. E. H.; Lewiński, J. Dalton Trans. 2012, 41, 5934−5938. (b) Lewiński, J.; Marciniak, W.; Lipkowski, J.; Justyniak, I. J. Am. Chem. ́ Soc. 2003, 125, 12698−12699. (c) Lewiński, J.; Sliwiń ski, W.; Dranka, M.; Justyniak, I.; Lipkowski, J. Angew. Chem., Int. Ed. 2006, 45, 4826− 4829. (d) Wróbel, Z.; Justyniak, I.; Dranka, I.; Lewiński, J. Dalton Trans. 2016, 45, 7240−7243. (10) (a) Mąkolski, L.; Zelga, K.; Petrus, R.; Kubicki, D.; Zarzycki, P.; Sobota, P.; Lewiński, J. Chem. - Eur. J. 2014, 20, 14790−14799. (b) Sobota, P.; Petrus, R.; Zelga, K.; Mąkolski, Ł.; Kubicki, D.; Lewiński, J. Chem. Commun. 2013, 49, 10477−10479. (11) (a) Kloubert, T.; Müller, C.; Krieck, S.; Schlotthauer, T.; Görls, H.; Westerhausen, M. Eur. J. Inorg. Chem. 2012, 5991−6001. (b) Schowtka, B.; Görls, H.; Westerhausen, M. Z. Anorg. Allg. Chem. 2014, 640, 907−915. (c) Kloubert, T.; Görls, H.; Westerhausen, M. Z. Anorg. Allg. Chem. 2010, 636, 2405−2408. (d) Kloubert, T.; Görls, H.; Westerhausen, M. Z. Naturforsch., B: J. Chem. Sci. 2012, 67, 519−531. (12) Linear or cyclic motifs are common for [M3pz3−4] cores. The geometry of the [M2pz2] metallocore is usually controlled by steric factors as shown (e.g., see ref 6). (13) Mulvey, R. E. Chem. Soc. Rev. 1991, 20, 167−209. (14) (a) Bury, W.; Justyniak, I.; Prochowicz, D.; Wróbel, Z.; Lewiński, J. Chem. Commun. 2012, 48, 7362−7364. (b) Anantharaman, G.; Chandrasekhar, V.; Walawalkar, M. G.; Roesky, H. W.; Vidovic, D.; Magull, J.; Noltemeyer, M. Dalton Trans. 2004, 1271−1275. (15) Hao, H. J.; Bhandari, S.; Ding, Y. Q.; Roesky, H. W.; Magull, J.; Schmidt, H. G.; Noltemeyer, M.; Cui, C. M. Eur. J. Inorg. Chem. 2002, 2002, 1060−1065. (16) Alvarez, S. Dalton Trans. 2013, 42, 8617−8636. (17) This interaction can be supported by cooperative interactions between the π density of Ph substituents and the heterocyclic metallocore Zn2(μ-pzPh2)2, as indicated by one of reviewers. (18) Li, D.; Kagan, G.; Hopson, R.; Williard, P. G. J. Am. Chem. Soc. 2009, 131, 5627−5634.

shielded by the phenyl rings of the distal bridging pyrazolate ligands, with the shortest intramolecular Zn−C distance of 3.119(3) Å (Zn1−C16); the sum of rvdW for C and Zn is 4.16 Å,16 indicating the presence of Zn−π attractive interactions.17 The spiro trinuclear structure of 2 with the Zn−π interaction motif is highly relevant to the recently reported organozinc aryloxide [(EtZn)2Zn(OAr)4] (OAr = deprotonated o-hydroxybiphenyl), which appeared as a very valuable model compound for investigations of the π-interaction-assisted activation of O2 by zinc alkyls.10 The crystal structures often do not reflect the reality occurring in solution. Therefore, to gain a further understanding of the real constitution of 1−3 in solution, we studied their diffusion properties using DOSY NMR spectroscopy.18 The corresponding 1H NMR (toluene-d8) spectra of 1 revealed two sets of signals, indicating the presence of two species, which DOSY measurements identified as a trimer (major) and a dimer (minor). The 1HNMR spectra of 2 and 3 were consistent with their corresponding crystal structures (for details regarding DOSY and molecular weight calculations, see the Supporting Information). In conclusion, we have prepared and characterized three new structurally diverse alkylzinc derivatives of HpzPh2 with various coordination modes of pyrazolate ligands, including two coordination modes previously unobserved for zinc pyrazolates, i.e., η1:η2-μ3 and η2:η2-μ3. The solid-state structural diversity of these compounds is derived from the presence or absence of a donor solvent and/or a bulky tert-butyl group at Zn centers. Notably, the entrapment of monomeric [RZn(pzPh2)] species by the pyrazolate-bridged dimeric [RZn(μ-pzPh2)2ZnR)] metallocore is an alternative aggregation mode to the commonly observed ring-stacking and ring-laddering principle.13 Further studies on the reactivity of alkylzinc pyrazolates are in progress.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b00745. Experimental procedures and characterization data of compounds 1−3(PDF) X-ray crystallographic data in CIF format for 1−3 (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge financial support by the National Science Centre (Grant Maestro DEC-2012/04/A/ST5/00595) and the Ministry of Science and Higher Education of Poland (research funds in 2012−2016 under Project DI2011015241 within the “Diamond Grant” program). We thank Dr. Piotr Bernatowicz for his help with DOSY NMR measurements.

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REFERENCES

(1) Fustero, S.; Sánchez-Roselló, M.; Barrio, P.; Simón-Fuentes, A. Chem. Rev. 2011, 111, 6984−7034. (2) (a) La Monica, G.; Ardizzoia, G. A. Prog. Inorg. Chem. 1997, 46, 151. (b) Halcrow, M. A. Dalton Trans. 2009, 2059−2073. (c) Klingele, J.; Dechert, S.; Meyer, F. Coord. Chem. Rev. 2009, 253, 2698−2741. C

DOI: 10.1021/acs.inorgchem.6b00745 Inorg. Chem. XXXX, XXX, XXX−XXX