Chromium(III) acetylacetonate is the

coordination compound A coordination complex consists of a central atom or ion, which is usually metallic and is called the ''coordination centre'', and a surrounding array of bound molecules or ions, that are in turn known as ''ligands'' or complexing agents. Man ...

with the

formula In science, a formula is a concise way of expressing information symbolically, as in a mathematical formula or a ''chemical formula''. The informal use of the term ''formula'' in science refers to the general construct of a relationship betwe ...

Cr(C₅H₇O₂)₃, sometimes designated as Cr(acac)₃. This purplish

coordination complex A coordination complex consists of a central atom or ion, which is usually metallic and is called the ''coordination centre'', and a surrounding array of bound molecules or ions, that are in turn known as '' ligands'' or complexing agents. M ...

is used in

NMR spectroscopy Nuclear magnetic resonance spectroscopy, most commonly known as NMR spectroscopy or magnetic resonance spectroscopy (MRS), is a spectroscopic technique to observe local magnetic fields around atomic nuclei. The sample is placed in a magnetic fi ...

as a relaxation agent because of its solubility in

nonpolar In chemistry, polarity is a separation of electric charge leading to a molecule or its chemical groups having an electric dipole moment, with a negatively charged end and a positively charged end. Polar molecules must contain one or more polar ...

organic solvents and its

paramagnetism Paramagnetism is a form of magnetism whereby some materials are weakly attracted by an externally applied magnetic field, and form internal, induced magnetic fields in the direction of the applied magnetic field. In contrast with this behavior ...

Synthesis and structure

The compound is prepared by the reaction of

chromium(III) oxide Chromium(III) oxide (or chromia) is an inorganic compound with the formula . It is one of the principal oxides of chromium and is used as a pigment. In nature, it occurs as the rare mineral eskolaite. Structure and properties has the corundum ...

with acetylacetone (Hacac): :Cr₂O₃ + 6 Hacac → 2 Cr(acac)₃ + 3 H₂O The complex has idealized D₃ symmetry. The Cr-O distances are 1.93 Å. 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. The complex is relatively inert toward substitution but undergoes

bromination In chemistry, halogenation is a chemical reaction that entails the introduction of one or more halogens into a compound. Halide-containing compounds are pervasive, making this type of transformation important, e.g. in the production of polyme ...

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 The nuclear Overhauser effect (NOE) is the transfer of nuclear spin polarization from one population of spin-active nuclei (e.g. 1H, 13C, 15N etc.) to another via cross-relaxation. A phenomenological definition of the NOE in nuclear magnetic res ...

. 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.{{cite book , last1=Berger , first1=Stefan , last2=Braun , first2=Siegmar , date=2004 , title= 200 And More NMR Experiments: A Practical Course , location=Weinheim , publisher=Wiley-VCH , isbn=3-527-31067-3 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

Acetylacetonate complexes Chromium complexes Chromium–oxygen compounds Chromium(III) compounds

Synthesis and structure

Use in NMR

See also

References