Alkylzinc diorganophosphates: synthesis, structural diversity and unique ability to incorporate zincoxane units

Abstract

Equimolar reactions between ZntBu₂ and diphenyl phosphate (dpphe-H) or dimethyl phosphate (dmphe-H) result in the formation of [tBuZn(O₂P(OR′)₂)]-type compounds which crystallize as tetranuclear aggregates, [tBuZn(dpphe)]₄ (1₄) or [tBuZn(dmphe)]₄ (2₄), with the phosphate ligands spanning 4-coordinate Zn centers. The utility of ZnMe₂ instead of ZntBu₂ dramatically changes the reaction outcome and leads to [MeZn(O₂P(OR′)₂)]-type moieties incorporating zincoxane units, i.e., pentanuclear [{MeZn(dpphe)}₃(Me₂Zn₂O)(THF)₂] (3) and nonanuclear [{MeZn(dmphe)}₆(Me₂Zn₃O₂)] (4) aggregates. The resulting compounds were characterized by ¹H and ³¹P NMR spectroscopy and single-crystal X-ray diffraction analysis.

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Since the first zinc-based phosphate molecular sieves related to the structure of zeolitic aluminosilicates were published in 1991,¹ there has been rapid and continuous growth in the area of the synthesis and the application of metal complexes and the corresponding functional materials supported by a vast array of phosphorus oxyacids and their organic derivatives.² Such materials revealing versatile structures from zero-dimensional molecular complexes to three-dimensional frameworks have been used mainly in catalysis, ion-exchange and materials chemistry including rational design of porous materials or gas storage.² Moreover, molecular metal complexes of phosphorus oxyacids have also received considerable interest as potential secondary building blocks of extended structures of various dimensionalities and architectures.^2–4 Within this realm, zinc phosphates occupy a prominent position. However, both the number of structurally well-characterized molecular zinc phosphates as well as the knowledge on their transformation modes from low- to high-dimensional structures remain relatively limited. This obstacle mainly arises from the fact that zinc and other metal phosphates are commonly synthesized by hydrothermal synthetic methods.⁵ In order to isolate molecular complexes, addition of ancillary ligands is usually required and is in line with this strategy, for example, a number of novel inorganic zinc-based molecular complexes incorporating organophosphate ligands were isolated and structurally characterized.⁶ An alternative to this inorganic route may potentially be an organometallic approach. However, the corresponding organozinc compounds still represent a highly unexplored field. Strikingly, only a few organometallic zinc compounds bearing a characteristic structural motif [RZn(O_xPR′_y)] (wherein R and R′ = alkyl or aryl, x = 2–4, y = 1–2) have been crystallographically characterized to date. For example, in the course of extensive investigations on metal phosphonates,^2a,7 Roesky and co-workers described the reaction of tert-butylphosphonic acid with zinc dialkyls in different molar ratios, which resulted in the formation of structurally varied organozinc phosphonates: tetranuclear [{(ZnMe)₄(THF)₂}{tBuPO₃}₂] and hexanuclear [{(ZnEt)₃(Zn(THF))₃}{tBuPO₃}₄{μ₃-OEt}] complexes, and a dodecanuclear aggregate [{Zn₂(THF)₂(ZnEt)₆Zn₄(μ₄-O)}{(tBuPO₃)₈}] with a [Zn₄(μ₄-O)]⁶⁺ core, respectively.⁸ To the best of our knowledge, heretofore only one structurally characterized organozinc phosphate, namely a dimeric iodomethylzinc phosphate [ICH₂Zn(dpphe)]₂ (where, dpphe – diphenyl phosphate monoanion), was reported. This compound was derived from the reaction of dpphe-H with 1 equiv. of ZnEt₂ along with the consecutive addition of 1 equiv. of CH₂I₂ and then was used as an efficient reagent for the cyclopropanation of alkenes.⁹

As part of our ongoing interest in both new molecular organozinc building blocks for inorganic–organic functional materials¹⁰ and well-defined precursors of organic ligand-coated ZnO nanocrystals,¹¹ we turn our attention to molecular complexes incorporating organophosphate ligands. Herein we report the preliminary experiments involving the reactions of ZnR₂ (R = tBu and Me) with organophosphate diesters. To gain insights into the factors determining reaction outcomes and structures of the resulting alkylzinc phosphates, except various substituents at zinc, two different organophosphates were selected as proligands, i.e., diphenyl phosphate (dpphe-H) and dimethyl phosphate (dmphe-H) (Scheme 1a).

Scheme 1 (a) Selected proligands: diphenyl phosphate and dimethyl phosphate, and (b) syntheses of alkylzinc diorganophosphates.

All the reactions between the selected phosphate proligands and ZnR₂ (R = tBu, Me) were carried out according to Scheme 1b. An equimolar reaction of ZntBu₂ with dpphe-H or dmphe-H in toluene at −78 °C lead to the formation of [tBuZn(O₂P(OR′)₂)]-type compounds which crystallize as tetranuclear aggregates [tBuZn(dpphe)]₄ (1₄), and [tBuZn(dmphe)]₄ (2₄) (for details, see the ESI†). In both cases colourless rectangular crystals suitable for X-ray diffraction analyses were almost quantitatively isolated from the postreaction mixtures. Conversely, the utility of ZnMe₂ instead of ZntBu₂ dramatically changes the reaction outcome and afforded two methylzinc(oxo)phosphate complexes, which exhibited solvent-dependent aggregation degrees. The reaction between ZnMe₂ and 1 equiv. of dpphe-H in a toluene solution leads to an insoluble product. Dissolution of the resulting solid in THF followed by crystallization at low temperature afforded a novel organozinc oxo cluster supported by the diphenyl phosphate ligands with the formula [{MeZn(dpphe)}₃(Me₂Zn₂O)(THF)₂] (3). However, in a similar reaction system involving dmphe-H, a nonanuclear cluster [{MeZn(dmphe)}₆(Me₂Zn₃O₂)] (4) was isolated in high yield. The observed formation of zinc oxo species is an intriguing issue. We note that the reported reactions were performed using the standard Schlenk technique with the same batches of solvents. Because of the observed relatively high reaction yields, the presence of the oxo-zinc species cannot be attributed to adventitious moisture or traces of dioxygen. In our opinion, the origin of zincoxane units may likely be associated with a condensation of organophosphate molecules mediated by ZnMe₂ and the subsequent formation of an organopyrophosphate moiety related to the processes observed in biochemistry (cf. ATP biosynthesis); the presence of tetraphenyl pyrophosphate and products of its decomposition were confirmed by EI-MS in the postreaction mixtures (for details, see the ESI†).

The resulting compounds 1–4 were fully characterized using spectroscopic techniques and single crystal X-ray diffraction studies. The IR spectra are devoid of any characteristic absorption in the region 2700–2500 cm⁻¹ and ∼3600 cm⁻¹, indicating a complete reaction of all P–O–H groups with ZnR₂. Moreover, for 1–4 the characteristic vibrational P–O–Zn stretches appear as bands of strong intensities at 1054 cm⁻¹, 1032 cm⁻¹, 1097 cm⁻¹ and 1025 cm⁻¹, respectively. All ¹H NMR spectra are consistent with the structures found in the solid state. The ¹H NMR spectra of 1–4 contain well-resolved signals characteristic for the alkyl groups bonded to zinc centres and the sets of signals for the corresponding phosphate ligands (Fig. S1a–S4a†). The ³¹P NMR spectra of 1, 2 and 3 show a single resonance at −16.79 ppm, −1.35 ppm and −9.67 ppm, respectively, (Fig. S1b–S3b†). However, the ³¹P NMR of 4 comprises two signals at 1.37 ppm and 5.91 ppm which is consistent with the presence of two nonequivalent dmphe ligands in the solid state structure (Fig. S4b†).

Compounds 1₄ and 2₄ crystallize in the triclinic P1̄ space group as tetrameric structures with four-coordinate zinc centres. The molecular structures of 1 and 2 are shown in Fig. 1 and S6 (see the ESI†), respectively. In both cases the association of four [tBuZn(O₂P(OR′)₂)] moieties results in a quasi-cube-shaped central core, which is made up of two 8-membered heterocycles Zn₂P₂O₄ joined by two 4-membered Zn₂(μ₂-O)₂ rings. Each of these eight-membered rings adopts a pseudo-boat conformation. The periphery of the central Zn₄P₄O₈ polyhedron is surrounded by hydrophobic tert-butyl and –P(OR′)₂ groups (where R′ = –Ph (1₄), –CH₃ (2₄)), which is consistent with the high solubility of 1₄ and 2₄ in common organic solvents. The bridging phosphate groups between three zinc centres display a μ₃–μ₂:η¹ coordination mode. The Zn–C and Zn–O bonds fall within a narrow range of 1.982–1.988 Å (for 1₄), 1.987–1.994 Å (for 2₄) and 1.972–2.151 Å (1₄), 1.977–1.216 Å (2₄), respectively.

Fig. 1 Molecular structure of 1₄; hydrogen atoms and methyl groups of the tert-butyl substituents are omitted for clarity.

The methylzinc(oxo)phosphate 3 crystallizes in the triclinic space group as a pentanuclear complex. The molecular structure of 3 contains a Zn₄O₇P₃ core stabilized by the three phosphate ligands (Fig. 2). Each dpphe ligand bridges three zinc centers and adopts a μ₃–μ₂:η¹ coordination mode. A striking feature of this structure is the presence of a zincoxane unit. Hence, the revealed very unusual oxo-zinc structure of 3 can be formally described as a zincoxane (Me₂Zn₂O) moiety entrapped by three [MeZn(dpphe)] molecules. As a result, the molecular structure of 3 contains a total of two eight-membered Zn₂P₂O₄ rings and three four-membered Zn₂O₂ rings. Moreover, one MeZn subunit of the zincoxane is anchored in the central core and the second, terminal MeZn subunit is stabilized by two η¹-THF molecules. All the zinc centres are tetra-coordinate and possess a near tetrahedral geometry. Due to a poor quality of crystals of 3 for single-crystal X-ray diffraction, it was not possible to obtain a fully satisfactory refinement of the molecular structure, and this precludes also a detailed discussion of the geometrical parameters.

Fig. 2 Molecular structure of 3; hydrogen atoms are omitted for clarity.

Compound 4 crystallizes in the centrosymmetric monoclinic space group and its molecular structure is shown in Fig. 3. The oxo-zinc structure of 4 can be formally described as two trinuclear [MeZn(dpphe)]₃ subunits with an entrapped trinuclear zincoxane moiety, (Me₂Zn₃O₂). It results in the formation of the Zn₉P₆O₁₃ cage, which is made up of a combination of two eight-membered Zn₂P₂O₄, four six-membered Zn₂P₁O₃ and eight four-membered Zn₂O₂ rings. Depending on the coordination modes, the six dmphe groups can be classified into two different types and the phosphate monoanionic ligands are in two distinctive μ₄–μ₂:μ₂ and μ₃–μ₂:η¹ coordination modes. The Zn₄(μ₄-O) bond distances in 4 range from 1.834 to 2.006 Å, and the Zn–O_(phosphate) distances range from 1.968 to 2.196 Å. These values are similar to the average value of those found in the reported zinc oxophosphate tetranuclear cluster [Zn₄(μ₄-O){O₂P(OtBu)₂}]₆ (2.00 and 1.93 Å, respectively).¹² Moreover, there are no formal P–O and P=O bonds in 1–2 and 4, and the average P–O bond lengths: 1.51 Å (for 1₄), 1.50 (for 2₄), 1.50 Å (for 4) are much shorter than a formal P–O bond (1.59–1.60 Å) and considerably longer than a formal P=O bond (1.45–1.46 Å).¹³ It is essential to note that compounds 3 and 4 bear a striking resemblance to each other. Particularly, both methylzinc(oxo)phosphates contain a pentanuclear structural motif with a zincoxane unit incorporated into methylzinc diorganophosphate moieties.

Fig. 3 (a) Molecular structure of 4 and (b) its asymmetric unit; hydrogen atoms are omitted for clarity.

Conclusions

In conclusion, we prepared and characterized novel and structurally diverse organozinc complexes stabilized by organophosphate diesters. While the reaction involving bulky ZntBu₂ results in the formation of [tBuZn(O₂P(OR′)₂)]-type organozinc compounds, the utility of ZnMe₂ dramatically changes the reaction outcome and leads to methylzinc organophosphate complexes incorporating zincoxane units. The resulting structures reproduce the salient bonding features of the phosphate zincoxanes which likely illuminate the structural changes in molecules and intermediate compounds that may exist only briefly during a transformation process of organozinc phosphates into ZnO nanocrystals. Further studies on the chemistry of organozinc complexes supported by a vast array of phosphorus oxyacids and their organic derivatives as well as their transformation into extended inorganic–organic porous materials and ZnO-based nanomaterials are currently in progress.

Acknowledgements

The authors acknowledge the Foundation for Polish Science Team Program co-financed by the EU “European Regional Development Fund” TEAM/2011-7/8 and the National Science Centre, Grant Maestro DEC-2012/04/A/ST5/00595 for financial support.

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available: All experimental details and characterization data. CCDC 1499962–1499965. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6dt03324e

‡ These authors contributed equally to the manuscript.

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