Synthesis of planar macrocyclic tetradentate phosphine–Mo complexes

Abstract

The synthesis of transition-metal complexes bearing a macrocyclic tetradentate phosphine ligand is relatively undeveloped. Herein, an open-chain tetradentate phosphine ligand with two P-containing rings was synthesized and coordinated to a Mo-oxo template. After deprotonation and cyclization with suitable alkyl chains, a series of planar macrocyclic tetradentate phosphine–Mo complexes was obtained without stereoisomers.

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It is well known that phosphines are excellent ligands which impart many interesting properties to their metal complexes;^1–5 in particular, the macrocyclic effect⁶ of the cyclic phosphine ligand makes the complex more stable.⁷ For example, Mock and coworkers reported the first 16-membered tetradentate macrocyclic phosphine chromium complexes trans-[Cr(N₂)₂(PPh₄NBn₄)] for activation of dinitrogen molecules.^8,9 An extensive development of the chemistry of phosphorus containing macrocyclic compounds in the past years was stimulated by the use of these compounds for the design of new types of supramolecular and coordination systems.¹⁰

Macrocyclic phosphine complexes can be synthesized via two major strategies. One involves the synthesis of macrocyclic phosphine ligands followed by their coordination to a metal center. While in the other strategy, which is normally called template synthesis, components are coordinated to the target metal prior to the final cyclization. Despite the development of the synthesis of macrocyclic phosphine ligands in recent years,^11–13 macrocyclic phosphine complexes of group 6 transition-metals are rare. Helm reported a metal-free synthesis of a [9]-aneP₃R₃ ring and the further coordination of such a ligand to Mo(CO)₃.¹⁴ By placing reactive groups in close proximity to each other, template synthesis has demonstrated superior selectivity and relatively high yields.¹² Norman first employed a Mo(0) template to prepare the macrocyclic triphosphine–molybdenum complex fac-[(CO)₃Mo(PHC₃H₆)₃] via intramolecular hydrophosphination reactions of coordinated primary alkenyl phosphines.¹⁵ This method was followed by Edwards’ group in the template synthesis of a series of 1,5,9-triphosphacyclododecane complexes with both molybdenum and chromium.^16–20 Influenced by the ring size, macrocyclic tridentate phosphine ligands ([9]-aneP₃R₃ to [12]-aneP₃R₃) in the aforementioned complexes exclusively exhibit a facial coordination geometry. Macrocyclic tetradentate phosphine ligands offer the possibility of fully encircling a metal center. The first example of a tetraphosphine macrocyclic complex was reported in 1977 by Rosen using a Ni(II) template.²¹ Unfortunately, no crystallographic characterization was performed, nor was there any information about the stereoisomers in this pioneering work. Recently, Tyler reported a macrocyclic tetradentate phosphine complex with Cu(I) in a tetrahedral geometry.²² To date, most of the currently synthesized macrocyclic tetradentate phosphine complexes have been based on Cu(I), Ni(II), Pd(II) or Pt(II).^22–27 Synthesis of planar macrocyclic tetradentate phosphine complexes with group 6 transition-metals has remained absent since no effective route has been established.

Moreover, compared with oxa-, aza-, and thia-carbocyclic systems, the synthesis of cyclic phosphine ligands especially planar macrocyclic tetradentate phosphine is still difficult.²⁸ This may also be related to the stereochemistry at the phosphorus atoms. Unlike tertiary amines, the inversion barrier of phosphines is sufficiently high (29–36 kcal mol⁻¹), so they do not undergo inversion at room temperature.²⁹ This means that the macrocyclic phosphines with asymmetric phosphine groups (PR₁R₂R₃) are chiral, resulting in multiple possible stereoisomers for each macrocycle (Scheme 1).^12,30,31 For instance, the bisphosphane complex of Pd(II) undergoes reaction with 2 equiv. of acetylacetone in ethanol to form two isomeric cycloadducts which are derived from the (RSSR)-form and the (RSRS)-form of the starting compound, respectively. In the (RSSR)-form derived product, the methyl groups of two adjacent P atoms are oriented above the equatorial plane and the other two methyl groups are oriented below the plane. However, in the (RSRS)-form derived product, the methyl groups of all four P atoms are above the equatorial plane.³² Introduction of a side ring can effectively avoid stereoisomerism (Scheme 1). Herein, we report the synthesis of a series of Mo complexes bearing a planar macrocyclic tetradentate phosphine ligand with two side rings.

Scheme 1 Stereochemistry of macrocyclic tetradentate phosphine ligands.

Starting from compound P1,³³ the quaternary phosphonium diiodide [PP^CprPP]I₂ (P2)^33,34 (PP^CprPP = 4-{[(octahydro[1,2]diphospholo[1,2-a][1,2]diphosphol-4-ium-4-ylmethyl)cyclopropyl]methyl}-octahydro-[1,2]diphospholo-[1,2-a][1,2]diphosphol-4-ium) was synthesized in a good yield by alkylation with 1,1-bis(iodomethyl)cyclopropane (Scheme 2). ³¹P{H} NMR spectroscopy of the phosphonium salt P2 exhibits two doublets of triplets at 79.6 ppm and −50.0 ppm (¹J_PP = 252.0 Hz, ⁴J_PP = 14.3 Hz), respectively, corresponding to the two different types of P atoms. Four P atoms in P2 form an AA′BB′ spin system. Four coupling constants are used to determine the NMR pattern in such a system. With |J_AB| = ¹J_PP = 252.0 Hz and |J_AA′| = ⁴J_PP = 14.3 Hz (coupling between P2 and P3 along the carbon chain), and the other two coupling constants close to zero, P atoms in P2 will give two doublets of triplets. Crystals of compound P2 suitable for X-ray analysis were grown by recrystallization using methanol (Fig. 1).

Scheme 2 Synthesis of compound P3.

Fig. 1 X-ray structure of compound P2. Thermal ellipsoids are drawn at 50% probability. H atoms are omitted for clarity. Selected bond lengths (Å) and angles (°): P1–P2 2.193(2), P3–P4 2.186(2), P3–C12 1.811(7), P4–C7 1.863(7), C14–C17 1.509(9), P3–C9 1.810(6), C9–C8 1.522(9), C9–P3–C12 109.9(3), C9–P3–C15 112.9(3), C12–P3–C15 111.9(4), C9–P3–P4 100.2(2), C12–P3–P4 100.9(3), C15–P3–P4 119.9(2), C13–P2–C6 116.4(3), C13–P2–C3 109.6(3), C6–P2–C3 112.2(3), C13–P2–P1 118.3(2).

Treatment of compound P2 with sodium bis(2-methoxyethoxy)aluminium hydride (Red-Al) broke the P–P bonds, leading to the 1,5-diphosphocane moieties in [HPP^CprPPH] (P3) (HPP^CprPPH = 1-{[(1,5-diphosphocan-1-ylmethyl)cyclopropyl]methyl}-1,5-diphosphocane). ³¹P{H} NMR of P3 shows two doublets at −34.9 ppm and −73.6 ppm. The resonance at −73.6 ppm becomes a broad doublet with ¹J_PH = 199.1 Hz in the proton coupled ³¹P NMR, which clearly proves that P–H bonds are present in P3. Correspondingly, P–H bonds of P3 in ¹H NMR are a doublet at 3.30 (¹J_PH = 196.4 Hz) ppm, which collapses to a singlet in ¹H{P} NMR. When ligand P3 was added to the suspension of Mo(O)Cl₂(PPh₂Me)₃ (Mo0)^35,36 and MeOLi in MeOH at room temperature, the green solid of Mo0 gradually disappeared during the reaction, accompanied by the formation of a yellow solution (Scheme 3). After the anion exchange with NaPF₆, ³¹P{H} NMR of this yellow solution displays two resonances, indicating a typical AA′BB′ spin system. Again, the signal at −36.1 ppm shows a P–H coupling constant of ¹J_PH = 314.8 Hz in the proton coupled ³¹P NMR and the P–H gives a doublet with a coupling constant of ¹J_PH = 316.3 Hz at 5.88 ppm in the ¹H NMR, suggesting the presence of P–H bonds in this complex; thus, a Mo-oxo complex [(HPP^CprPPH)Mo(O)(OMe)][PF₆] (Mo1) with a tetradentate phosphine ligand was proposed. Gratifyingly, yellow crystals of Mo1 were grown by layering n-hexane on top of the dichloromethane solution. An X-ray crystallography study reveals a pseudo octahedral geometry at the Mo center, with the tetradentate phosphine ligand in the equatorial plane and a methoxy group trans to the oxo ligand (Fig. 2, left). The O–Mo–O is linear (O1–Mo1–O2 179.06(9)°). The Mo–O single bond (Mo1–O2 1.9487(19) Å) is longer than the Mo=O double bond (Mo1–O1 1.719(2) Å). The lengths of the Mo–P bonds in Mo1 are similar (2.46–2.50 Å). In contrast, since the planar macrocycle has not been formed, the bite angle of the two secondary phosphine ligands (P1–Mo1–P4 (111.01(3)°)) is larger than other three bite angles.

Scheme 3 Synthesis of complex Mo1.

Fig. 2 X-ray structures of complex Mo1 (cation, left) and Mo2 (cation, right). Thermal ellipsoids are drawn at 50% probability. H atoms are omitted for clarity. Selected bond lengths (Å) and angles (°) of Mo1: Mo1–O1 1.719(2), Mo1–O2 1.9487(19), Mo1–P1 2.5203(8), Mo1–P2 2.4863(7), Mo1–P3 2.4609(7), Mo1–P4 2.5086(8), O1–Mo1–O2 179.06(9), O1–Mo1–P1 94.44(7), O1–Mo(1)–P2 93.07(7), O2–Mo1–P1 86.50(6), O2–Mo1–P2 87.16(6), P1–Mo1–P2 77.42(3), P1–Mo1–P3 169.74(3), P1–Mo1–P4 111.01(3), P2–Mo1–P3 93.99(3); Mo2: Mo1–O1 1.712(13), Mo1–O2 1.971(12), Mo1–P1 2.438(6), Mo1–P2 2.437(5), O1–Mo1–O2 178.2(9), O1–Mo1–P1 95.2(6), O2–Mo1–P1 84.0(6), P1–Mo1–P2 97.6(5), P1–Mo1–P3 170.1(7), P1–Mo1–P4 79.5(6).

Unlike the secondary phosphine ligands in Ni(II) or Pd(II),^25,37 which could be deprotonated with a weak base such as triethylamine or K₂CO₃, the ligand in complex Mo1 could only be deprotonated by a very strong base like KHMDS, although the Mo moiety was overall cationic before deprotonation. The deprotonation of complex Mo1 in THF at −60 °C, followed by the addition of 1 equiv. of 1,1-bis(iodomethyl)cyclopropane gave a yellow solid. The P atoms of this solid resonate as only a singlet at 5.0 ppm in the ³¹P{H} NMR, indicating the formation of the desired macrocyclic complex [(PP^CprPP^Cpr)Mo(O)(OMe)][PF₆] (Mo2) (PP^CprPP^Cpr = dispiro-[cyclopropane-1,11′-1,5,9,13-tetraphosphatricyclo-[11.3.3.3^5,9]docosane-3′,1′′-cyclopropane]) (Scheme 4). The formation of the planar tetradentate structures was also confirmed by X-ray diffraction (Fig. 2, right). In the solid state of Mo2, the four Mo–P bonds have almost the same length (2.44 Å), while the bite angles of the diphosphocane moieties (P2–Mo1–P3 81.8(5)° and P1–Mo1–P4 79.5(6)°) are slightly smaller than the other two bite angles (P1–Mo1–P2 97.6(5)° and P3–Mo1–P4 99.4(6)°). ¹H{P} NMR of Mo2 exhibits broad signals of the protons except the protons of the methoxy group. This phenomenon is attributed to not only the fluxional processes of the molecule, but also the coupling of the protons with very close chemical shifts.

Scheme 4 Synthesis of complexes Mo2, Mo3 and Mo4.

Similarly, after the deprotonation of Mo1, the addition of 1 equiv. of 3,3-bis(iodomethyl)oxetane or α,α′-dibromo-o-xylene produced [(PP^CprPP^Oxt)Mo(O)(OMe)][PF₆] (Mo3) (PP^CprPP^Oxt = dispiro-[oxetane-3,11′-1,5,9,13-tetraphosphatricyclo-[11.3.3.3^5,9]-docosane-3′,1′′-cyclopropane]) or [(PP^CprPP^Xy)Mo(O)(OMe)][PF₆] (Mo4) (PP^CprPP^Xy = spiro[1,5,9,18-tetraphosphatetracyclo[16.3.3.3^5,9.0^11,16]heptacosa-11(12),13,15-triene-3,1′-cyclopropane]) (Scheme 4), respectively. In these cases, P atoms in each molecule become two inequivalent groups, forming AA′BB′ spin systems as observed in the ³¹P{H} NMR spectra. Note that in Mo3 and Mo4, the chemical shift differences of inequivalent P atoms are much smaller than the J_PP coupling, complicated patterns are seen and the coupling constants cannot be simply read from the spectra. Intriguingly, protons of the methoxy group in Mo4 shift to the high field in the ¹H NMR spectra (δ = 2.16 ppm), suggesting the presence of a strong shielding effect. Yellow crystals of Mo3 (Fig. 3, left) and Mo4 (Fig. 3, right) were obtained by vapor diffusion of diethyl ether into a dichloromethane or N,N-dimethylformamide solution. X-ray diffraction analysis reveals that the lengths of the Mo–P bonds and the bite angles in Mo3 are similar to those in Mo2. However in Mo4, owing to the different lengths of the chains of the macrocycle, the bite angle of P1–Mo1–P4 (103.944(15)°) is significantly larger than that of P2–Mo1–P3 (96.715(15)°). Additionally, the C atom of the methoxy group in Mo4 is located 3.84 Å above the face of the benzene ring, which is an NMR shielding region. This rationalizes the unusually low chemical shift of its protons.

Fig. 3 X-ray structures of complex Mo3 (cation, left) and Mo4 (cation, right). Thermal ellipsoids are drawn at 50% probability. H atoms are omitted for clarity. Selected bond lengths (Å) and angles (°) of Mo3: Mo1–O1 1.764(4), Mo1–O3 1.925(4), Mo1–P1 2.4433(13), Mo1–P2 2.4293(15), O1–Mo1–O3 179.5(2), O1–Mo1–P1 92.96(14), O1–Mo1–P2 91.87(14), O3–Mo1–P1 87.32(13), O3–Mo1–P2 87.74(14), P1–Mo1–P2 81.20(6), P1–Mo1–P3 175.67(7), P1–Mo1–P4 98.73(6), P2–Mo1–P3 98.83(5); Mo4: Mo1–O1 1.7142(13), Mo1–O2 1.9696(12), Mo1–P1 2.4616(5), Mo1–P2 2.4627(5), O1–Mo1–O2 177.80(6), O1–Mo1–P1 91.69(5), O1–Mo1–P2 95.01(5), O2–Mo1–P1 87.00(4), O2–Mo1–P2 86.48(4), P1–Mo1–P2 79.101(15), P1–Mo1–P3 172.039(16), P1–Mo1–P4 103.944(15), P2–Mo1–P3 96.715(15).

In conclusion, a new class of planar macrocyclic tetradentate phosphine–Mo complexes was synthesized for the first time though templating. Thanks to the two side rings in the structure, these complexes did not generate stereoisomers during the synthesis. Further studies on the reactivity of these complexes are in progress in our lab.

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors are grateful for the financial support from the National Natural Science Foundation of China (No. 22271037 and 21702024).

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Footnote

† Electronic supplementary information (ESI) available: Experimental procedure, NMR spectra and X-ray diffraction data. CCDC 2433616–2433620 for P2, Mo1, Mo2, Mo3, and Mo4. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d5dt00984g

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