FT-ICR studies of polypyrazolyl-1-yl borates of europium(II) adducts

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Abstract

In this work we report the synthesis and characterization of europium(II) hydrotris(pyrazol-1-yl)borate complexes, Eu(HBPz3)2L2 (L=bis(pentamethylene)urea, diphenylphosphineamide, 2-azacyclononanone, 2-azacycloheptanone). The characterization of the complexes was accomplished by Fourier transform ion cyclotron resonance mass spectrometry using electrospray ionization. For all the complexes, the Eu(HBPz3)2L2+ ion was observed. To evaluate the relative bond strength of the ligand L to europium polypyrazolylborate, infrared multiphoton dissociation (IRMPD) experiments were performed using a 40-W continuous-wave CO2 (λ=10.46 μm) laser aligned on-axis with the ion cloud. The parent ion was dissociated to form the daughter ions, Eu(HBPz3)2L+ and Eu(HBPz3)2+. The kinetics for unimolecular dissociation showed the relative bond strengths of the different adducts of the europium complex to be: diphenylphosphineamide>2-azacyclononanone≈2-azacycloheptanone. Dissociation experiments could not be made for the adduct with bis(pentamethylene)urea due to the weak bond.

Introduction

The polypyrazolyl borate ligands (Tp) provide a versatile coordination environment for a wide range of metal centers including lanthanides (III) and (II) [1], [2].

In the course of our studies of lanthanide chemistry with Tp ligands we investigated Eu(HBpz3)2L2 complexes [3], [4] and we verified that the polypyrazolylborate ligands seemed to provide an interesting antenna effect for the luminescence of the europium compounds. Additional ligands necessary to complete the coordination sphere of the europium poly(pyrazolyl)borate compounds seemed also to influence the luminescent properties of the complexes, with the weaker ligands favouring the luminescent efficiency [4].

In this work we report the synthesis of Eu(HBPz3)2L2 (L=diphenylphosphineamide, 2-azacyclononanone, 2-azacycloheptanone) and the evaluation of the relative bond strengths of the different adducts by a gas phase study in a Fourier transform ion cyclotron resonance spectrometer using electrospray ionization.

The ligands used in this study were considered because previous work with Eu(III) complexes have shown that ligands with either carbonyl or phosphoryl groups produce adducts that present satisfactory luminescent properties [5], [6], [7].

Section snippets

Experimental

The syntheses of all the complexes were carried out under nitrogen using glove box and Schlenk-line techniques by the procedure previously described for Eu(HBpz3)2L2 (L=diphenylsulfoxide and bis(pentamethylene)urea) [4].

The MS experiments were carried out on a homebuilt 9.4 T ESI FT-ICR instrument (NHMFL) configured for external ion accumulation [8], [9]. Samples were infused into a 50-μm micro-electrospray capillary at a flow rate of 300 nl min−1 [10], [11]. Ions were accumulated in a linear

Results and discussion

For all the complexes, the electrospray generated Eu(HBPz3)2L2+ ion was observed along with the Eu(HBPz3)2L+ ion. This Eu(HBPz3)2L2+ ion was isolated and fragmented by the use of a CW CO2 laser with the exception of L=bis(pentamethylene)urea which was so weakly bonded that it was not possible to isolate an appreciable quantity to perform the dissociation studies. Typical mass spectra for the dissociation of the isolated Eu(HBPz3)2L2+ metal complexes are shown in Fig. 1 (Eu(HBPz3)2L2+ m/z

Final remarks

In conclusion we showed the relative bond strengths of the different adducts of the europium complex to be: diphenylphosphineamide>2-azacyclononanone≈2-azacycloheptanone>bis(pentamethylene)urea.

Acknowledgements

We acknowledge the support of the Fundação Luso Americana para o Desenvolvimento (A.P.M.), the cooperation programme ICCTI/CNPq-99, the National Science Foundation (CHE-99-09502), Florida State U., and the National High Magnetic Field Laboratory in Tallahassee, FL.

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