Science

Polyhedron 26 (2007) 1758–1762 www.elsevier.com/locate/poly

Synthesis and crystal structures of the chromium(II) and chromium(III) hexafluoroacetylacetonate complexes trans-Cr(hfac)2(thf)2 and Cr(hfac)3
Yoshiko Harada, Gregory S. Girolami
Received 9 November 2006; accepted 8 December 2006 Available online 13 December 2006

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School of Chemical Sciences, University of Illinois atUrbana-Champaign, 600 South Mathews Avenue, Urbana, IL 61801, United States

Abstract Treatment of a tetrahydrofuran solution of CrCl2(thf) with Na(hfac), hfac = 1,1,1,5,5,5-hexafluoroacetylacetonate, followed by crystallization from diethyl ether, affords the six-coordinate chromium(II) complex Cr(hfac)2(thf)2. The crystal structures of Cr(hfac)2(thf)2 and the chromium(III) complex Cr(hfac)3 have beendetermined by single-crystal X-ray diffraction. Cr(hfac)2(thf)2 adopts a trans octa˚ hedral geometry, in which the Cr–O(hfac) and Cr–O(thf) distances are 1.936(3) and 2.019(6) A, respectively. Cr(hfac)3 is an octahedral ˚ compound with a Cr–O distance of 1.943(5) A. Structural comparisons with related molecules are given. Ó 2006 Elsevier Ltd. All rights reserved.
Keywords: Chromium; Chromouscomplex; Chromic complex; Hexafluoroacetylacetonate; Diketonate complexes; X-ray crystal structure; Twinned; Twinning

1. Introduction Transition metal b-diketonates are important compounds in inorganic chemistry and in materials science owing to their usefulness as CVD precursors [1,2] and as building blocks for molecule-based magnets [3–5]. Hexafluoroacetylacetonate (hfac) derivatives of the transitionmetals are especially useful because the fluorine substituents lend the complexes increased solubility, volatility, and Lewis acidity [6–9]. For example, Pd(hfac)2 is an excellent precursor for the deposition of palladium metal by CVD [10,11], and Mn(hfac)2 reacts with nitroxide spin radicals to make one-dimensional molecule-based magnets [3]. Owing to their importance, metal b-diketonates havebeen extensively investigated, but to date no structural information has been reported for chromium hexafluoroacetylacetonates. We report here the structures of a new chromium(II) hfac complex, bis(hexafluoroacetylacetonato)bis(tetrahydrofuran)chromium(II), Cr(hfac)2(thf)2, as well as the previously known chromium(III) complex tris(hexafluoroacetylacetonato)chromium(III), Cr(hfac)3.
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2. Resultsand discussion 2.1. Synthesis of Cr(hfac)2(thf)2 Treatment of CrCl2(thf) with sodium 1,1,1,5,5,5-hexafluoroacetylacetonate, Na(hfac), in tetrahydrofuran at room temperature, followed by recrystallization from diethyl ether, affords dark reddish brown crystals of the new high-spin chromium(II) complex Cr(hfac)2(thf)2 (1). The crystals of 1 easily oxidize in air and turn green. CrCl2 ðthfÞ þ 2NaðhfacÞ þthf ! CrðhfacÞ2 ðthfÞ2 þ2NaCl
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Attempts to synthesize Cr(hfac)2(thf)2 from the bis(trimethylsilyl)amido complex Cr[N(SiMe3)2]2(thf)2 and hexafluoroacetylacetone in thf afford instead the air-stable chromium(III) complex Cr(hfac)3 (2) as maroon needles, after crystallization from diethyl ether or pentane. Evidently, hexafluoroacetylacetone is a strong enough acid (oxidant) to convertchromium(II) to chromium(III), probably with formation of H2: Cr½NðSiMe3 Þ2 Š2 ðthfÞ2 þ 3Hhfac ! CrðhfacÞ3 þ2HNðSiMe3 Þ2 þ H2 þ 2thf
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Corresponding author. Tel.: +1 217 333 2729; fax: +1 217 244 3186. E-mail address: girolami@scs.uiuc.edu (G.S. Girolami).

0277-5387/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2006.12.007

Y. Harada, G.S. Girolami / Polyhedron26 (2007) 1758–1762

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Compound 2 has been reported previously by Sievers and Morris [12–14]. It is usually described as a green crystalline material [12,14,15], but is evidently dichroic and the crystals also can appear as maroon-colored. The infrared spectrum of Cr(hfac)3 matches that reported previously [13]. The spectrum of Cr(hfac)2(thf)2 is very similar to that of Cr(hfac)3 except...