Elsevier

Inorganica Chimica Acta

Volume 477, 24 May 2018, Pages 284-291
Inorganica Chimica Acta

Research paper
Redox processes in the Cu/(phen)/[B12H12]2−/solv system: Selective preparation of copper(I), copper(II), and heterovalent copper(I/II) compounds

https://doi.org/10.1016/j.ica.2018.03.024Get rights and content

Highlights

  • Cu(I), Cu(II), and Cu(I,II) complexes with [B12H12]2− and phen were isolated.

  • Structure of six copper complexes were characterized by X-ray diffraction.

  • Electronic structure of [CuII(phen)2Cl]2[B12H12] 2CH2I2 was studied by EPR.

Abstract

The complexation of copper and 1,10-phenanthroline (phen) in the presence of the [B12H12]2− anion is studied in organic solvents. The starting reagents contain copper(I) salts. The reactions are carried out in air and accompanied with oxidation of copper(I). Varying solvents and starting reagents, we succeeded in synthesizing selectively copper(I), copper(II), and heterovalent copper(I,II) complexes. Mononuclear, tetranuclear, and polymeric copper(II) complexes as well as a mononuclear copper(I) complex and a mixed cationic mononuclear copper(I)/copper(II) complexes are isolated and characterized by IR-spectroscopy and X-ray diffraction. The electronic structure of mononuclear copper(II) complex [CuII(phen)2Cl]2[B12H12] 2CH2I2 is studied by EPR.

Introduction

The boron cluster anions [BnHn]2− (n = 6, 10, 12) are electron-deficient structures characterized by high delocalization of the electron density over the boron cluster, 3D aromaticity, and relatively low charge [1], [2], [3], [4]. It is known that redox transformations proceed in complexation of metals which have at least two non-zero stable oxidation states when N-donor organic ligands and boron clusters are present (in particular, [B10H10]2− possesses reduction activity) (e.g., iron [5], cobalt [6], and copper [7] complexation). Moreover, it was shown that copper and cobalt complexation reactions can be accompanied with the substitution of terminal hydrogen atoms in the decahydro-closo-decaborate anion to form metal complexes with substituted closo-decaborates [6], [7], [8], [9], [10].

Complexation processes in the [CuI2[B10H10]]/L/solv systems, where L are N-donor ligands (1,10-phenanthroline (phen), 2,2′-bipyridyl (bipy), 2,2′-bipyridylamine (bpa), solv = CH3CN, DMF, and DMSO) were studied systematically [7], [8], [9], [10], [11], [12], [13], [14], [15]. In these studies it was shown that one can obtain copper(I) complexes [CuI2L2[B10H10]] with the inner-sphere position of the boron cluster, copper(II) complexes with the closo-decaborate anion as counterion (for example, [CuIIL2][B10H10], L = phen), and heterovalent copper(I,II) complexes with different structures, namely cationic-anionic copper(I)/copper(II) complex [CuI2[B10H10]3][CuII4(OH4)L4] (L = bipy or bpa) and mixed cationic copper(I)/copper(II) complex [CuIL2][CuIIL3][B10H10]2 (L = phen). It was found that copper complexation can be accompanied with redox processes. When using copper(I) as the starting reagent, we can prepare copper(I) complexes in the presence of [B10H10]2− and L in acetonitrile (at room temperature in air when L = bipy and phen; at an inert atmosphere when L = bpa); copper(I,II) complexes in acetonitrile (on heating in air when L = bipy and phen; at −20 °C when L = bpa); and copper(II) complexes containing OH or CO3 groups when DMSO or DMF were added to acetonitrile reaction solutions (L = bipy or bpa) or in air in acetonitrile for more reactive L = bpa.

When copper(II) was used as the starting reagent, copper(II) complexes can be synthesized in organic ligands (acetonitrile, DMF) and no redox transformation proceeds. It should be noted that redox processes CuI → CuII (because of air oxygen) or CuII → CuI (under the action of [B10H10]2− anion) can be realized at the reagent ratio M: L = 1: 1 or 1: 2, while the threefold excess of bidentate ligands prevents copper(II) cations to be reduced.

It is known that the [B12H12]2− has minor reaction ability among boron cluster anions [BnHn]2− (n = 6–12). As for redox properties, this anion it known to be stable in the presence of oxidizing agents, such as Fe(III) or Ce(IV). The closo-structure of the boron cluster is retained even on boiling in 30% hydrogen peroxide; in this case, the persubstituted [B12(OH)12]2− anion was formed in high yield [16]. Only addition of a reducing agent (Na2SO3 or SO2) to an aqueous solution of CuII and [B12H12]2− salts allowed us to prepare CuI complexes {Cat[CuI[B12H12]]}n [17] while no reaction was observed to proceed between copper(II) salts and the [B12H12]2− anion in water. The high stability of [B12H12]2− compounds determines their application as polyfunctional materials including neutron-protective coatings [18].

Here, we study the copper(I)/copper(II) complexation reactions with phen in the presence of [B12H12]2− anion starting from copper(I) compounds. Taking into account redox processes that accompany copper complexation in air due to the nature of ligand L and solvent, the reaction conditions to prepare Cu(I), Cu(I/II), and Cu(II) compounds with the [B12H12]2− anion are determined.

Section snippets

Results and discussion

In copper complexation reactions we used CuICl, salts of the decahydro-closo-decaborate anion Cs[Ag[B12H12]] or Ph4P[CuI[B12H12]], and phen as starting reagents (Table 1).

When the complexation reaction was carried out using copper(I) salts and phen in DMF in the presence of [B12H12]2−, only copper(II) complexes I and II were formed (Table 1, reaction 1). The scheme of this reaction is shown in Scheme 1.

In this case, copper(I) is oxidized in air because both azaheterocyclic ligand L (phen) and

Conclusions

Thus, in the present work we succeeded in synthesizing selectively copper(I), copper(II), and heterovalent copper(I,II) coordination compounds with 1,10-phenanthroline in the presence of the [B12H12]2− anion starting from copper(I) compounds. Because of redox process CuI/CuII resulted from oxygen air, mono- and polynuclear copper complexes of various composition and structure have been isolated and characterized. In acetonitrile and DMF, mononuclear, tetranuclear and polymeric copper(II)

Synthesis

Triethylammonium dodecahydro-closo-dodecaborate (Et3NH)2[B12H12] was obtained by pyrolysis of a solution of decaborane-14 in triethylamine borane according to the procedure reported [44]. Cesium closo-dodecaborate was obtained by boiling an aqueous solution of (Et3NH)2[B12H12] in the corresponding metal hydroxide until triethylamine is completely removed. Tetraphenylphosphonium closo-dodecaborate was synthesized by the exchange reaction between cesium closo-dodecaborate and the corresponding

Methods

Elemental analysis of dried compounds was carried out on a Carlo Erba Instruments EA1008 automatic CHN analyzer. Boron was determined by electrothermal atomic absorption on a Perkin-Elmer 2100 spectrophotometer with an HGA-700 furnace [45]. Copper was determined on a Perkin-Elmer 303 atomic absorption spectrophotometer in an acetylene-air flame.

Infrared spectra of compounds I–VI were recorded on an Lumex InfraLum FT–02 FT–IR spectrometer (St. Petersburg, Russia) in the range of 400–4000 cm−1

Acknowledgements

This study was supported by the Grant of the President of the Russian Federation NSh-2845.2018.3. EPR studies were performed within project 14-13-01115 of the Russian Scientific Foundation. The authors thank O.N. Belousova (the Kurnakov Institute) for measurement and interpretation of the IR spectra of the synthesized compounds.

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