Chromium(III) acetylacetonate

Chromium(III) acetylacetonate^[1]
	; Solid chromium(III) acetylacetonate
Names
	IUPAC name Tris(acetylacetonato)chromium(III)
	Other names Tris(2,4-pentanediono)chromium(III), Cr(acac)3, Cr(pd)3
Identifiers
CAS Number	21679-31-2 ;
3D model (JSmol)	Interactive image;
ChEBI	CHEBI:33035 ;
ChemSpider	2006256 ;
EC Number	244-526-0;
PubChem CID	24884255;
	InChI InChI=1S/3C5H7O2.Cr/c31-4(6)3-5(2)7;/h33H,1-2H3;/q3-1;+3 Key: GMJCSPGGZSWVKI-UHFFFAOYSA-N ; InChI=1/3C5H7O2.Cr/c31-4(6)3-5(2)7;/h33H,1-2H3;/q3-1;+3 Key: GMJCSPGGZSWVKI-UHFFFAOYAF;
	SMILES [Cr+3].O=C([CH-]C(=O)C)C.O=C([CH-]C(=O)C)C.O=C([CH-]C(=O)C)C;
Properties
Chemical formula	Cr(C5H7O2)3
Molar mass	349.32
Appearance	deep maroon
Density	1.34 g/cm3
Melting point	210 °C (410 °F; 483 K)
Boiling point	340 °C (644 °F; 613 K) (sublimes near 110°C)
Solubility in non-polar organic solvents	soluble
Hazards
	GHS labelling:
Pictograms
Signal word	Warning
Hazard statements	H315, H319, H335
Precautionary statements	P261, P264, P271, P280, P302+P352, P304+P340, P305+P351+P338, P312, P321, P332+P313, P337+P313, P362, P403+P233, P405, P501
	Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). verify (what is ?) Infobox references

Chromium(III) acetylacetonate is the coordination compound with the formula Cr(C₅H₇O₂)₃, sometimes designated as Cr(acac)₃. This purplish coordination complex is used in NMR spectroscopy as a relaxation agent because of its solubility in nonpolar organic solvents and its paramagnetism.

Synthesis and structure

The compound is prepared by the reaction of chromium(III) oxide with acetylacetone (Hacac):^[3]

Cr₂O₃ + 6 Hacac → 2 Cr(acac)₃ + 3 H₂O

The complex has idealized D₃ symmetry. The Cr-O distances are 1.93 Å.^[4] The complex has been resolved into individual enantiomers by separation of its adduct with dibenzoyltartrate.^[5]

Like many other Cr(III) compounds, it has a quartet ground state.

The complex is relatively inert toward substitution but undergoes bromination at the 3-positions of the chelate rings.

Use in NMR

Often paramagnetism in compounds is detrimental for NMR spectroscopy, as the spin-lattice relaxation times are very short which leads to excessively broad peaks. However, this can be put to advantage in the right circumstances.

The spin-lattice relaxation times for diamagnetic nuclei can be variable. In particular, ¹³C quaternary carbons suffer from low signal intensity due to long relaxation times and lack of enhancement from the Nuclear Overhauser effect. To circumvent the first issue, the addition of a small quantity (on the order of 0.1 mM) of Cr(acac)₃ to an NMR sample reduces the relaxation time by providing an alternative relaxation pathway - namely through the unpaired electron.^[6] By reducing the relaxation time, more scans can be acquired in a given amount of time, resulting in higher signal intensity. This is particularly advantageous for quantitative ¹³C NMR, which requires that all signals have fully relaxed between pulses. By reducing the relaxation time, the delay between pulses can be reduced without affecting the relative integrations of peaks.

References

^ Chromium acetylacetonate Archived 2015-04-16 at the Wayback Machine at American Elements
^ Semyannikov, P.P.; Igumenov, I.K.; Trubin, S.V.; Chusova, T.P.; Semenova, Z.I. (February 2005). "Thermodynamics of chromium acetylacetonate sublimation". Thermochimica Acta. 432 (1): 91–98. doi:10.1016/j.tca.2005.02.034.
^ Fernelius, W. Conard; Blanch, Julian E. (2007). "Chromium(III) Acetylacetonate". Inorganic Syntheses. Vol. 5. pp. 130–131. doi:10.1002/9780470132364.ch35. ISBN 9780470132364.
^ Morosin, B. (1965). "The crystal structure of trisacetylacetonatochromium(III)". Acta Crystallographica. 19: 131–137. doi:10.1107/S0365110X65002876.
^ Drake, A. F.; Gould, J. M.; Mason, S. F.; Rosini, C.; Woodley, F. J. (1983). "The optical resolution of tris(pentane-2,4-dionato)metal(III) complexes". Polyhedron. 2 (6): 537–538. doi:10.1016/S0277-5387(00)87108-9.{{cite journal}}: CS1 maint: uses authors parameter (link)
^ Berger, Stefan; Braun, Siegmar (2004). 200 And More NMR Experiments: A Practical Course. Weinheim: Wiley-VCH. ISBN 3-527-31067-3.

[americanelements-1] Chromium acetylacetonate Archived 2015-04-16 at the Wayback Machine at American Elements

[2] Semyannikov, P.P.; Igumenov, I.K.; Trubin, S.V.; Chusova, T.P.; Semenova, Z.I. (February 2005). "Thermodynamics of chromium acetylacetonate sublimation". Thermochimica Acta. 432 (1): 91–98. doi:10.1016/j.tca.2005.02.034.

[3] Fernelius, W. Conard; Blanch, Julian E. (2007). "Chromium(III) Acetylacetonate". Inorganic Syntheses. Vol. 5. pp. 130–131. doi:10.1002/9780470132364.ch35. ISBN 9780470132364.

[4] Morosin, B. (1965). "The crystal structure of trisacetylacetonatochromium(III)". Acta Crystallographica. 19: 131–137. doi:10.1107/S0365110X65002876.

[5] Drake, A. F.; Gould, J. M.; Mason, S. F.; Rosini, C.; Woodley, F. J. (1983). "The optical resolution of tris(pentane-2,4-dionato)metal(III) complexes". Polyhedron. 2 (6): 537–538. doi:10.1016/S0277-5387(00)87108-9.{{cite journal}}: CS1 maint: uses authors parameter (link)

[6] Berger, Stefan; Braun, Siegmar (2004). 200 And More NMR Experiments: A Practical Course. Weinheim: Wiley-VCH. ISBN 3-527-31067-3.

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Chromium(III) acetylacetonate

Contents

Synthesis and structure

Use in NMR

See also

References

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