Chromium(III) acetylacetonate is the coordination compound with the formula Cr(C<sub>5</sub>H<sub>7</sub>O<sub>2</sub>)<sub>3</sub>, sometimes designated as Cr(acac)<sub>3</sub>. This purplish coordination complex is used in NMR spectroscopy as a relaxation agent because of its solubility in nonpolar organic solvents and its paramagnetism.
The compound is prepared by the reaction of chromium(III) oxide with acetylacetone (Hacac):
The complex has idealized D<sub>3</sub> symmetry. The Cr-O distances are 1.93 ÃÂ . The complex has been resolved into individual enantiomers by separation of its adduct with dibenzoyltartrate.
Like many other Cr(III) compounds, it has a quartet ground state, meaning that it has three unpaired electrons. This situation is consistent with the electronic configuration (t<sub>2g</sub>)<sup>3</sup>(e<sub>g</sub>)<sup>0</sup>. The color of the complex arises from d-d electronic transitions.
The complex is relatively inert toward substitution (hence it is susceptible to optical resolution). It reacts with a variety of electrophiles at the 3-positions of the chelate rings, giving the corresponding bromo-, nitro-, and formyl-substituted derivatives.
Cr(acac)<sub>3</sub> is paramagnetic, a property which is often detrimental for NMR spectroscopy as the spin-lattice relaxation times are very short, leading to excessively broad peaks. However, this can be put to advantage in the right circumstances, particularly quantitative 13C NMR.
The spin-lattice relaxation times for diamagnetic nuclei can be variable. In particular, <sup>13</sup>C 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)<sub>3</sub> to an NMR sample reduces the relaxation time by providing an alternative relaxation pathway - namely through the unpaired electron. 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 <sup>13</sup>C 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.