The existence of the homoleptic Fe complex published by S. Samanta114 and the dinickel complexes discussed in sections 1.3 and 1.4 offered interesting perspectives for comparative studies of their reactivity. The challenge of synthesizing a dinuclear complex which combined two different metals is another attractive project that was realised and will be discussed in the following paragraph.
Figure 1.34. UV-vis (VT) spectra of complex 2in CH2Cl2
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VIII was dissolved in methylene chloride and one equivalent of NiDME(Br)2 was added. The suspension turned red in several minutes. After one hour of stirring, one equivalent of Fe(OTf)2(CH3CN)2 dissolved in CH3CN was added, affording a brown solution. Subsequently, triethylamine was added causing the formation of a yellow precipitate corresponding to (Et3NH)(OTf). It was filtered off, and the remaining solution was set to vapor diffusion with diethyl ether, affording single crystals suitable for X-ray diffraction after three days. Complex 3 was particularly sensitive toward oxygen as the colour of a solution of this complex turned from brown to orange under air exposure.
The synthesis of this complex was a delicate exercise which required a detailed inspection of experimental parameters: a) The solubility of VIII in CH2Cl2 was good while NiDME(Br)2 was insoluble. The amount of Ni2+ in solution being limited and diluted, it favoured the incorporation of a single Ni2+ ion rather than formation of the dinickel species. b) The proton of the pyrazole, which is hydrogen bonded to the pyridine moiety, blocks one coordination site of the ligand. The prototropy is lowered by the low polarity of CH2Cl2, thus the N−H proton is localized and operates as a protecting group, so that the binding of a second Ni2+ is disfavoured prior to addition of a base. Thus, it appeared that dichloromethane was the ideal solvent for the binding of a single Ni2+ ion by the ligand VIII. Once the single binding of Ni2+ ion was achieved, another metal could be added in the presence of a base in order to obtain the heterobimetallic complex.
Figure 1.35 Molecular structure (thermal displacement ellipsoids shown at 50 % probability) of the cation of complex 3. Hydrogen atoms and anions have been omitted for clarity. Left: top view of the molecular structure. Right: front view of the molecular structure
Bond Lengths around Ni(1) / Å Bond Lengths around Fe(1) / Å
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The molecular structure shown in Figure 1.35 revealed a nickel ion with a square planar geometry and an iron ion with a square pyramidal geometry. Contrary to the homoleptic dinickel complex 2, the two metals in [LH2Ni(µ-Br)FeBr]OTf shared a bridging bromide. The distance between the equatorial bromide and the iron was about 2.753 Å which was 0.283 Å longer than the apical bromide. Such distances suggested that Br(1) was weakly coordinated to Fe. Comparison of Ni-Br bond lengths between complex 2 (2.287 Å) and complex 3 (2.328 Å) gave a hint about the flexibility of the bridging bromide which varied from 0.041 Å. A Mössbauer spectrum of complex 3 in the solid state was recorded at 80 K (Figure 1.36) and displayed a single doublet with a large isomer shift (Fe = 0.90 mms−1) and large quadrupole splitting (EQ = 3.79 mms−1), which suggested the presence of a high spin Fe2+ (S = 2) ion. SQUID data of complex 3 in solution were complicated to interpret and decent fitting was not possible, possibly due to the presence of dynamic processes. An analysis of complex 3 in solution was carried out by NMR spectroscopy at variable temperatures.
The 31P NMR spectra did not show any signals. The absence of 31P NMR signals was rationalized by the paramagnetic nature of the metal center. However, 1H NMR spectra displayed paramagnetically shifted signals which followed the Curie law (Figure 1.38). The chemical shifts of 1H NMR signals were proportional to 1/T. However, at 298 K, the 1H NMR signals became too broad for reasonable analysis. The assignment and the interpretation of 1H NMR resonances were complicated as 2D NMR spectra did not show any correlation peaks. The presence of more than six NMR signals indicated chemically and magnetically inequivalent halves of the complex.
Two larges 1H NMR signals located at 12.7 ppm and 3.1 ppm probably corresponded to the resonances of the two inequivalent tBu groups of the complex (Figure 1.37). Complex 3 had a limited solubility as it was insoluble in dichloromethane, acetone and THF.
Figure 1.36. Mössbauer spectrum of complex 3 in solid state at 80 K: Fe = 0.90 mms−1 EQ = 3.79 mms−1.
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Finally, complex 3 was analysed by UV-vis at different temperatures (cf experimental section) and ESI mass spectrometry (Figure 1.38). Peaks in the region of 809.2 m/z were consistent with the cation LH2Ni(µ-Br)FeBr+.
Figure 1.37. 1H NMR spectrum of complex 3 in CD3CNat different temperatures (500 MHz).
Figure 1.38. ESI mass spectrum of complex 3 in CH3CN. The inset shows the experimental and simulated isotropic distribution pattern for LH2Ni(µ-Br)FeBr+.
36 1.6 Conclusion
In conclusion, ligand VIII was employed for the synthesis of homobimetallic dinickel(II) complexes as well as heterobimetallic nickel(II)/iron(II) complexes in which both metal ions are hosted in pincer-type PNN compartments of the pyrazolate-based binucleating ligand scaffold.
Complex 1 was green and paramagnetic while complex 2 was red and diamagnetic. As it was demonstrated, Ni2+ might adopt different geometries and spin states depending on the coligand.
It was an important prerequisite for the next chapters. Both complexes showed coalescence phenomena in CH2Cl2 at low temperature. Finally, the synthetic challenge to make a complex of ligand VIII chelating two different metal ions has been achieved within complex 3. It represented an interesting system for the study of the synergy between a nickel and an iron ion for substrate transformations.