Toward Factors Affecting the Degree of Zinc Alkyls ... - ACS Publications

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Toward Factors Affecting the Degree of Zinc Alkyls Oxygenation: A Case of Organozinc Guanidinate Complexes Michał K. Leszczyński,† Iwona Justyniak,† and Janusz Lewiński*,†,‡ †

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

‡

S Supporting Information *

ABSTRACT: While extensive research has been carried out on the oxygenation of alkylzinc complexes for decades, this issue still remains unresolved and significant uncertainties concerning the mechanism of these reactions and the composition of the resulting products persist. These reactions are believed to proceed via the initial formation of ROOZn(L) species, but studies on the oxygenation reactions have been substantially impeded by the low stability of the alkylperoxides. This report describes the oxygenation of a tert-butylzinc guanidinate, i.e. a tBuZn(L)-type complex (L = deprotonated 1,4,6-triazabicyclo[3.3.0]oct4-ene), and the isolation and characterization of an unprecedented aggregate based on a combination of a ROOZn(L) moiety and two parent tBuZn(L) molecules, {[tBuZn(L)]2[tBuOOZn(L)]}. Further study revealed that examined tert-butylzinc guanidinate molecules exhibit an ability to entrap other oxygenated species, which was demonstrated by an aggregate containing a tert-butylzinc tert-butylperoxide and two tBuZn(L) molecules, {[tBuZn(L)]2[tBuOZntBu]}. Thus, the reported studies indicate that entrapment of the product of an oxygenation reaction by the parrent alkylzinc species is another important factor controlling the oxygenation of organometallics.

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instead of a well-defined magnesium alkoxide, the tested reaction system afforded an intractable mixture of products. This result along with close examination of magnesium alkylperoxide stability9 demonstrated that the formation of magnesium alkoxides is not a result of the metathesis reaction but rather of direct decomposition of the alkylperoxide species. Our systematic studies on the controlled oxygenation of organozincs7,10 have provided compelling evidence for a new mechanism of O2 insertion into the metal−carbon bond: i.e., the inner-sphere electron transfer mechanism.9,10 The controlled oxygenation of homoleptic R2Zn compounds leads to the selective oxygenation of one R−Zn bond, affording alkylperoxide or alkoxide species.2g,7,10a In turn, the use of alkylzinc RZn(L) complexes (where L = monoanionic multidentate ligand) leads to a vast variety of products: alkylperoxides, alkoxides, peroxides, oxides, hydroxides, or even carboxylates, depending on the character of the supporting organic ligand L.7,10 These transformations are believed to proceed via the initial formation of ROOZn(L) species that in some cases were successfully isolated and structurally characterized. For example, reactions of O2 with alkylzinc complexes stabilized by β-diketiminate,4b,10g aminotroponiminate,11 or bisoxazolinate4 ligands lead to the selective formation of dimeric alkylperoxide zinc compounds, [(N,N)Zn(μOOR)]2 (Scheme 1a), and the analogous reaction of a tertbutylzinc complex incorporating an N,N′-pyrrolylketiminate

rguably the longest debate in organometallic chemistry is related to the mechanism of reactions between metal alkyls and O2, which was originated by Frankland’s pioneering study in 1849.1 Longstanding investigations of the reactivity of alkylzinc complexes toward O2 have been motivated not only by the fundamental pursuance of novel chemical transformations2 but also by their practical applications in organic chemistry, e.g. as initiators of radical reactions3 and catalysts of asymmetric epoxidation of enones,4 as well as in materials science in the controlled preparation of ZnO nanocrystals.5 Strikingly, while the formation of ZnOOR species was proposed by Demuth and Meyer back in 1890 as a product of insertion of an O2 molecule into a Zn−R bond,6 the first alkylzinc peroxides were isolated and structurally characterized over 110 years later.4b,7 For decades the free radical chain mechanism has been commonly accepted as a pathway leading to alkylperoxides in the oxygenation of metal alkyl species. Furthermore, this mechanism is usually supplemented by the postulated metathesis reaction involving an M−OOR moiety with the parent M−R moiety being responsible for the formation of an alkoxide product (eq 1).2e,8

However, to our knowledge, no sufficient experimental evidence has been presented to support this claim. In turn, very recently the metathesis reaction was directly tested using a model reaction involving alkyl and alkylperoxide magnesium complexes supported by a β-diketiminate ligand.9 Strikingly, © XXXX American Chemical Society

Received: April 15, 2017

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DOI: 10.1021/acs.organomet.7b00287 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

Scheme 1. Observed Structural Motifs of Zinc Alkylperoxides Formed in the Controlled Oxygenation of RZn(N,N) and RZn(O,N) Complexes

ligand afforded the novel trinuclear tert-butylperoxide [tBuOOZn(N,N′))]2[Zn(N,N′)2]10f (Scheme 1b). A spectacular result was obtained in the oxygenation of the EtZn(O,N)] complex (where O,N = deprotonated 1-aziridine-ethanol), which led selectively to a stable [EtOOZn(O,N)]2[EtZn(O,N)]2 adduct (Scheme 1d).7 The formation of this stable equimolar adduct between the resulting alkylperoxide and the parent organozinc molecules in a 1:1 molar ratio is another instance demonstrating that the metathesis reaction between Zn−OOR and Zn−R moieties is unfavorable. In this communication we advance the importance of stabilization of the oxygenated product by the parent alkylzinc species as one of the factors affecting the oxygenation of organozinc compounds. However, ligands exert oxygenation selectivity such that the selection of appropriate ligands for particular transformations is rather adventitious at the current stage of studies. So far, N,Nbidentate ligands have appeared to be a very graceful group of supporting ligands in oxygenation reactions of alkylzinc complexes.4b,10a,b,e−g Following this direction and our ongoing efforts to gain a more intimate view of the role of supporting ligands in the reactions of zinc alkyls with O2, we turned our attention to highly versatile guanidine-type ligands, which stabilize various alkylzinc cluster compounds.11,12 Herein we report on the oxygenation of the alkylzinc guanidinate [tBuZn(tbo)] (1) (where tbo = deprotonated 1,4,6triazabicyclo[3.3.0]oct-4-ene), which yielded the unprecedented aggregate [RZn(L)]2[ROOZn(L)] based on a combination of a ROOZn(L) moiety and two substrate RZn(L) molecules (Scheme 1c). The equimolar reaction of tBu2Zn and tbo-H afforded quantitatively a [tBuZn(tbo)] (1) complex. Despite many attempts, we were not able to obtain 1 in a form of single crystals of sufficient quality for X-ray structure determination. Nevertheless, NMR data of 1 are fully consistent with the anticipated formula (Figures S1 and S2 in the Supporting Information). 1H NMR and DOSY NMR analysis revealed that 1 forms an equilibrium system between a monomeric form (major) and a dimeric form 12 (minor) in toluene (Table S1 in the Supporting Information). Next, the oxygenation study was conducted by exposure of a toluene solution of 1 to dry O2 for 30 min at −20 °C, which resulted in the formation of the alkylzinc−alkylperoxide aggregate {[tBuZn(tbo)]2[tBuOOZn(tbo)]} (2) (Figure 1a). Compound 2 was characterized spectroscopically (Figures S3 and S4 in the Supporting Information), and its molecular structure was determined by X-ray diffraction analysis (Figure 1b and Figure S7 in the Supporting Information). Crystals of 2 belong to the monoclinic space group P21/n. The molecular structure of 2 is an aggregate consisting of one tert-butylperoxide/guanidinate

Figure 1. (a) Synthesis of 2. (b) Molecular structure of 2 with thermal ellipsoids set at 35% probability. Color scheme: Zn, light blue; O, red; N, blue; C, gray. Hydrogen atoms have been omitted for clarity. Selected bond lengths (Å) and angles (deg): Zn1−O1 2.245(3), Zn2− O2 2.060(2), Zn3−O2 1.995(3), O1−O2 1.457(3); Zn1−O1−O2 112.31(19), O1−O2−Zn2 112.80(17), O1−O2−Zn3 107.41(16), Zn2−O2−Zn3 109.83(11).

moiety, [tBuOOZn(tbo)], and two parent alkylzinc/guanidinate molecules. The tert-butylperoxide unit adopts a μ3,κ2O,κO coordination mode, which is rare for zinc alkylperoxides. To date this type of coordination mode has only been observed in the hexanuclear carboxylate cluster [Zn4(μ3-OOtBu)3(μ4-O)(O2CEt)3]2 obtained by the oxygenation of [tBuZn(O2CEt)].3c The Zn2−O2, Zn3−O2, and O1−O2 bonds within the tBuOOZn moiety in 2 are similar in length to those of other zinc complexes with bridging alkylperoxide groups.7,10a,b However, the Zn1−O1 distance (2.245(3) Å) in 2 is significantly shorter in comparison to the corresponding bonds in the carboxylate cluster [Zn4(μ3-OOtBu)3(μ4-O)(O2CEt)3]2 (distances ranging from 2.416 to 2.455 Å),3c which indicates a stronger interaction between these atoms in compound 2. The molecular structure of 2 represents an unprecedented aggregate involving two parent RZn(L)-type molecules and one oxygenated ROOZn(L) molecule. To our B

DOI: 10.1021/acs.organomet.7b00287 Organometallics XXXX, XXX, XXX−XXX

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Organometallics Accession Codes

knowledge, this type of entrapment mode of metal alkylperoxides has never been observed in any reaction system involving metal alkyls. During the course of our investigations of the oxygenation of 1 we also obtained a novel alkyl(alkoxide)zinc aggregate {[tBuZn(tbo)]2[tBuOZntBu]} (3) (Figure 2 and Figure S8 in

CCDC 1550288−1550289 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail for J.L: [email protected]. ORCID

Janusz Lewiński: 0000-0002-3407-0395 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the financial support of the Ministry of Science and Higher Education of Poland, under project no. 0152/DIA/2012/41 within the Diamond Grant program. The authors also wish to acknowledge the National Science Centre, Grant Maestro DEC-2012/04/A/ST5/00595, for financial support.

Figure 2. Molecular structure of 3 with thermal ellipsoids set at 35% probability. Color scheme: Zn, light blue; O, red; N, blue; C, gray. Hydrogen atoms have been omitted for clarity. Selected bond lengths (Å) and angles (deg): Zn1−O1 2.028(5), Zn3−O1 2.179(3), Zn2− O1 2.174(5); Zn1−O1−Zn3 106.05(15), Zn2−O1−Zn3 88.18(12), Zn2−O1−Zn1 101.93(14).

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the Supporting Information). This trinuclear aggregate was obtained by a serendipitous oxygenation of a toluene solution of 1 at room temperature and was characterized spectroscopically (Figures S5 and S6 in the Supporting Information) upon isolation in the form of colorless single crystals at −20 °C obtained from the concentrated solution. Compound 3 crystallizes in the monoclinic Cc space group. The molecular structure is depicted in Figure 2 and can be seen as an aggregate formed by a [tBuOZntBu] moiety entrapped by the two parent [tBuZn(tbo)] molecules. Two four-coordinate and one threecoordinate alkylzinc species are joined by two monoanionic tbo ligands exhibiting μ2- and μ3-bridging coordination modes. The alkoxide moiety in 3 exhibits a μ3-bridging mode with Zn−O distances of 2.029(4), 2.175(4), and 2.182(4) Å, which are in the typical range for zinc tert-butoxide complexes.13,14 In conclusion, we have successfully synthesized and characterized novel zinc alkylperoxide and alkoxide complexes incorporating guanidinate ligands. The resulting compounds, along with our previous report on the alkylperoxide−alkylzinc complex stabilized by the aminoalcoholate ligand,6b illuminate the usually overlooked tendency of the parent organometallic species and their oxygenated products to form transient or stable aggregates of the type [ROOZn(L)]x[RZn(L)]y.15 These results pave the way to a more in-depth understanding of metal alkyls oxygenation processes.

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REFERENCES

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.7b00287. Experimental procedures, characterization data, and NMR spectra (for 1−3), and crystallographic data (for 2 and 3) (PDF) C

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DOI: 10.1021/acs.organomet.7b00287 Organometallics XXXX, XXX, XXX−XXX