3.4 Complex Synthesis and Reactivity
3.4.1 Zinc Complexes
Pincer complexes with zinc are only barely known and a complex with a phosphine side arm is not described so far.[411–413] Hence, zinc complexes were synthesized with both ligands, HLtBu and HLiPr, and with triflate and acetate as bridging units between the two metal centers. Zinc(II) complexes are useful for investigating structural properties due to their air stability, redox inertness and diamagnetism (for NMR spectroscopy). When HLtBu was reacted with Zn(OAc)2·2H2O in MeCN, no additional base was needed to deprotonate the pyrazole and potassium hexafluorophosphate (KPF6) was added to introduce a non‐coordinating counterion (Scheme 3.14).
Scheme 3.14: Synthesis of acetate bridged [Zn2(µ-OAc)LtBu](PF6) 48 from zinc acetate and HLtBu.
The reaction gave the symmetric, acetate bridged complex 48 in high yield. The 1H NMR spectrum is depicted in Figure 3.10 showing the symmetry by having three resonances for the pyridine protons between 7.96 ppm and 7.42 ppm, the singulet for the pyrazole 4‐H proton at 7.21 ppm, one doublet for the methylene group at 3.48 ppm and one doublet for the tert‐butyl groups at 1.35 ppm resulting from a coupling to the phosphorus nuclei. The signal of the acetate is superimposed by the MeCN signal. In a 31P NMR spectrum a shift of 19.48 ppm was observed for the phosphorus side arms. Crystals of this compound were not obtained, thus the zinc salt
48
67 3.4 Complex Synthesis and Reactivity was changed to zinc triflate in order to vary the counterion and to change the crystallization properties.
Figure 3.10: 1H NMR spectrum of [Zn2(µ-OAc)LtBu](PF6)2 48 (acetonitrile-d3, 298 K). Residual solvent resonances are marked with an asterisk.
For the synthesis of complexes 49 (ZntBu-OTf) and 50 (ZniPr-OTf) zinc triflate was added to a suspension of ligand HLX and KOtBu in MeCN. The crude products were extracted with DCM to separate the formed potassium triflate from the complexes.
Scheme 3.15: Synthesis of [(Zn(OTf))2(µ-OTf)LtBu] 49 and [(Zn(MeCN))(Zn(OTf))(µ-OTf)LiPr](OTf) 50 by reacting HLtBu and HLiPr with Zn(OTf)2.
The formation of the desired, air stable complexes was shown by NMR spectroscopy. In 31P spectra only single resonances at 22.55 ppm and 6.04 ppm were observed for the tBu and the
iPr ligand, respectively. Both 1H NMR spectra exhibit one doublet for the CH2 groups evidencing symmetric species in solution (Figure 3.11). These signals of HLtBu (3.62 ppm) are slightly more deshielded as in HLiPr (3.50 ppm), which was also visible in the 13C NMR resonances (29.03 ppm vs. 26.39 ppm). ESI-MS spectra showed several species which could be assigned to different oxygenated species. Even under inert conditions, results showing only one species were not detected. It was found that complex 50 was not stable in solution under aerobic conditions for longer time. In NMR measurements a shift of the 31P NMR resonance from 6.04 ppm to 71.56 ppm for the oxidized species was observed. In addition, the signal for the
CH2
pz OAc
tBu
50 49
py *
*
68
3 Two-in-one Pincer Ligands and Their Metal Complexes for Catalysis
CH2 groups is located at 3.66 ppm, a shift of 0.17 ppm to lower field. This instability was not found for the tert-butyl complex, which may indicate that the steric crowd of the substituents on the phosphine play an important role in the corresponding oxygenation reaction.
Figure 3.11: 1H NMR spectra of 49 and 50 (acetonitrile-d3, 298 K). Residual solvent resonances are marked with an asterisk.
Crystals for 49 and 50 were obtained by slow Et2O diffusion into MeCN solutions. Both complexes crystallize in the triclinic space group P1̅ with two molecules in the unit cell.
Figure 3.12: Thermal displacement ellipsoids (shown at 30 % probability) of the molecular structure of [(Zn(OTf))2(µ-OTf)LtBu] 49 in two orientations. Hydrogen atoms and solvent molecules were omitted for clarity.
In 49 both zinc atoms are coordinated by the meridional {PNN} ligand pocket and two oxygen atoms of two triflate ions, giving a distorted square pyramidal geometry around the metal 49 (tBu)
50 (iPr)
CH2
tBu
CH2
py pz
iPr
iPr py
pz
*
*
*
* *
69 3.4 Complex Synthesis and Reactivity centers with triflate in the axial positions (Figure 3.12). According to the method of ADDISON, the geometric parameter τ5 is 0.14 and 0.38 reflecting the found geometry.[414] The zinc atoms are bridged by one of the triflates and separated by dZn-Zn = 4.326 Å. The metal centers are only slightly above and below the pyrazole-pyridine-methylene backbone plane, whereas the phosphorus atoms are located 0.604 Å and -0.447 Å outside of that plane. The Zn-P bond lengths are dZn1-P1 = 2.432 Å and dZn2-P2 = 2.462 Å, which is slightly longer compared to zinc phosphine complexes reported in literature.[415] Weak π-stacking interactions can be found between the pyridine C-3 carbon atoms of the ligand backbones. The minimum distances were found to be dC-C = 3.351 Å (Figure 3.13).
Figure 3.13: Thermal displacement ellipsoids (shown at 30 % probability) of the molecular structure of 49: The packing diagram in the solid state (left) and the two molecules of the unit cell showing weak interactions of the pyrazole-pyridine backbones (right). Hydrogen atoms and solvent molecules were omitted for clarity.
50 was crystallized under inert conditions. The molecular structure is depicted in Figure 3.14, showing a non‐symmetric arrangement with one acetonitrile molecule coordinated to one of the zinc atoms. On the same side of the backbone plane, a triflate ion is bound to the second zinc ion. A second triflate ion is bridging the metal centers, while the third is found as counterion hampering a stacking of the molecules as observed in 49.
Figure 3.14: Thermal displacement ellipsoids (shown at 30 % probability) of the molecular structure of [(Zn(MeCN))(Zn(OTf))(µ-OTf)LiPr](OTf) 50 in two different orientations. Counterions, hydrogen atoms and non-coordinating solvent molecules were omitted for clarity.
70
3 Two-in-one Pincer Ligands and Their Metal Complexes for Catalysis
The metal‐metal distance is essentially the same (dZn‐Zn = 4.342 A ) and also the lengths of the Zn‐P bonds (dZn1‐P1 = 2.431 A and dZn2‐P2 = 2.445 A ) are very similar. The high structurally similarity is also reflected by the τ5 values, which are close to those of the tBu analogue (0.16 and 0.30 in 50; 0.14 and 0.38 in 49).
If 50 was crystallized under aerobic conditions, oxidation of the phosphines was observed.
Although the quality of the crystallographic data was not sufficient to completely refine the molecular structure of 51 (ZniPr,ox-OTf), the connectivity could be determined and is shown in Figure 3.15.
Figure 3.15: Stick-and-ball representation of the molecular structure of [(Zn2(µ-OTf)LtBu,ox)2(µ-OH)2](OTf)2 51.
Triflate counterions, hydrogen atoms and solvent molecules were omitted for clarity.
The phosphorus atoms were oxidized and the zinc ions are coordinated via the oxygen atoms, which resulted in a more clinched arrangement. This is represented by the shorter metal separation (dZn‐Zn = 3.920 A ) and the fact that the triflate ions are bridging via only one oxygen atom. The two subunits of the dimer are connected by hydroxides, producing a short distance between the zinc atoms of the two subunits (dZn‐Zn = 3.398 A ). A summary of selected bond lengths and angles is depicted in Table 3.1. For the non‐oxidized complexes 49 and 50 similar bond length and angles were found, although the coordination environment of one zinc ion is different in the iso‐propyl complex.
71 3.4 Complex Synthesis and Reactivity
Table 3.1: Selected bond lengths, angles and structural parameter τ5 for the deviation from ideal square pyramidal coordination geometry of complexes 49 - 51.
dZn-Zn [Å] dZn-P [Å] dZn-pz N [Å] dZn-py N [Å] α [°] β [°] τ5
a) For the symmetric structure of 51 values for one subunit are depicted for clarity. Values should be considered with caution because of the low quality of the structure determination. b) Metal-metal distance between zinc ions of the two subunits.
In order to investigate the electronic properties of the Two‐in‐one ligand system, electrochemical measurements of the two zinc triflate complexes (ZntBu-OTf and ZniPr-OTf) were performed. Since zinc(II) is an almost redox inert metal ion, reduction or oxidation events in cyclic voltammograms can be assigned to the ligand itself. Cyclic voltammograms of the reduction (left) and oxidation (right) of ZntBu-OTf in MeCN are depicted in Figure 3.16. Two irreversible reductions were observed at low potentials of approx. –1.40 V and –1.65 V (peak potentials at 100 mV/s). In contrast, no distinct oxidation was found. These findings indicate that the ligand system can be reduced, but not oxidized in the measured potential range. Very similar results were also observed for ZniPr-OTf.
Figure 3.16: Cyclic voltammogram (measured in 0.1 M nBu4NPF6 in MeCN) of ZntBu-OTf at different scan rates for the reduction (left) and at 100 mV/s for the oxidation (right).
Reactions of the synthesized zinc(II) complexes with different strong bases in order to deprotonate the ligand side arm were not successful (Scheme 3.16). Upon addition of KOtBu or KH, the triflate complexes 49 and 50 decomposed and no products could be identified by ESI mass spectrometry or NMR spectroscopy.
72
3 Two-in-one Pincer Ligands and Their Metal Complexes for Catalysis
Scheme 3.16: Attempted deprotonation of complex 48 with KOtBu with neutral ligands L in the binding pocket.
A crude 1H NMR spectrum of a reaction of the acetate complex 48 with KOtBu showed some indications that a deprotonation took place (Figure 3.17), however further investigations were not possible, since the signals of the formed species vanished after one day completely and the resonances of the free ligand appeared. The signal for the bridged acetate disappeared and the integral of the former CH2 group was lower and shifted to higher field which is in agreement with a higher electron density at this position. In contrast, the signals for the pyridine protons were only shifted less than 0.25 ppm to higher field, so that a dearomatization was not very likely. But this dearomatization might be crucial for the stabilization of the deprotonated complex. Unfortunately, a 13C NMR spectrum exhibited too low intensity of the resonances to safely assign them and support the assumption of deprotonation without a dearomatization.
The 31P NMR signal shifted from 6.04 ppm to 32.11 ppm upon addition of base.
Figure 3.17: 1H NMR spectra of a reaction mixture of zinc acetate complex 48 and KOtBu with prosposed assignment of the resonances (acetonitrile-d3, 298 K). Residual solvent resonances are marked with an asterisk.
It was tried to follow the deprotonation reaction by UV‐vis spectroscopy but the observations were ambiguous. The spectrum of ZntBu-OTf in MeCN exhibited two intensive π‐π* absorption bands in the UV region (λmax,1 = 259 nm (ε1 = 38 800 M‐1 cm‐1) and λmax,2 = 306 nm (ε2 = 38 100 M‐1 cm‐1)). Upon addition of KOtBu, a shift of the absorption maxima was observed (259 and 306 nm to 272 and 340 nm), indicating a change in the electronic structure of the ligand, since these bands were assigned to ligand related π‐π* transitions (Figure 3.18, left).
This effect was expected for a deprotonation and dearomatization of the ligand. Unfortunately, stable products in a reaction on a larger scale could not be isolated, although the formed species in the cuvettes seemed to be quite stable. In methylene chloride as solvent and using LiOtBu as
48
py and pz CH
tBu
*
*
73 3.4 Complex Synthesis and Reactivity a base, a different behavior was found. Here, the absorption maxima shifted only slightly, which might be a consequence of a ligand exchange with tert‐butoxide anions (Figure 3.18, right).
Figure 3.18: UV-vis spectra of reactions of [(Zn(OTf))2(µ-OTf)LtBu] 49 with KOtBu in MeCN at 0 °C (left) and with LiOtBu in CH2Cl2 at ambient temperature (right).
The behavior of ZntBu-OTf upon addition of base was found to be ambiguous and highly dependent on the reaction conditions. In fact, a deprotonation and a concomitant dearomatization could not be proved.
Zinc alkyl or alkoxy complexes are known as active initiators for the polymerization of rac‐lactide and other cyclic esters.[416,417] Thus, different ways to synthesize zinc alkyl complexes were tried, but both the use of diethylzinc in combination with ligand HLtBu and the reaction of 49 with methyl magnesium bromide did not lead to succesful isolation of the desired zinc alkyl complexes. With the zinc alkyl precursor, a mixture of various species was formed as indicated by 31P NMR spectroscopy. The reaction with a Grignard reagent (Scheme 3.17) seemed to be cleaner, the 31P NMR spectrum of the crude reaction products revealed only one resonance in at 71.01 ppm (49: 22.55 ppm).
Scheme 3.17: Possible reaction pathway for the formation of ligand oxidized zinc bromide complex 53 via the alkyl zinc intermediate 52 upon reaction of 49 with methyl magnesium bromide.
However, the signals of the methylene groups could not be reliably assigned. In addition the signals of the methyl groups were not found in the expected regions (0 ‐ –1 ppm).[416,418] The spectrum is shown in Figure 3.19, exhibiting three signals for the pyridine protons, revealing a symmetric compound. Further purification steps led to fast decomposition of the product.
49
52 53
74
3 Two-in-one Pincer Ligands and Their Metal Complexes for Catalysis
Figure 3.19: 1H NMR spectrum of a crude product of a reaction of 49 and MeMgBr (acetonitrile-d3, 298 K). The inset is showing the CH2 resonance, which is superimposed by the Et2O signal.
Crystallisization attempts were conducted in the presences of a slight excess of methyl magnesium bromide and colorless crystals of complex 53 (ZntBu,ox-Br) were obtained. 53 crystallized in the monoclinic space group P121/c1 with eight molecules in the unit cell. The molecular structure of 53 featured an oxidized ligand with three bromides as coligands presumably due to air in the crystallization flask (Figure 3.20). The metal metal separation (dZn‐Zn = 3.971 A ) was found to be shorter than for 49 due to the smaller brigding unit (bromide vs. triflate). The geometry around the zinc ions can be described as an intermediate between square pyramidal and trigonal bipyramidal with τ5 values of 0.53 (Zn1) and 0.48 (Zn2).
Figure 3.20: Thermal displacement ellipsoids (shown at 30 % probability) of the molecular structure of [(ZnBr)2(µ-Br)LtBu, ox] 53 in two different orientations. Hydrogen atoms and solvent molecules were omitted for clarity.