Chromium(III) acetylacetonate is the

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

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 is a chemical compound consisting of a central atom or ion, which is usually metallic and is called the ''coordination centre'', and a surrounding array of chemical bond, bound molecules or ions, that are in turn known as ' ...

is used in

NMR spectroscopy Nuclear magnetic resonance spectroscopy, most commonly known as NMR spectroscopy or magnetic resonance spectroscopy (MRS), is a spectroscopic technique based on re-orientation of atomic nuclei with non-zero nuclear spins in an external magnetic f ...

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, structure, bonding

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 s ...

with

acetylacetone Acetylacetone is an organic compound with the chemical formula . It is classified as a 1,3-diketone. It exists in equilibrium with a tautomer . The mixture is a colorless liquid. These tautomers interconvert so rapidly under most conditions that ...

(Hacac): :Cr₂O₃ + 6 Hacac → 2 Cr(acac)₃ + 3 H₂O The complex has idealized D₃

symmetry Symmetry () in everyday life refers to a sense of harmonious and beautiful proportion and balance. In mathematics, the term has a more precise definition and is usually used to refer to an object that is Invariant (mathematics), invariant und ...

. The Cr-O distances are 1.93 Å. The complex has been resolved into individual

enantiomer In chemistry, an enantiomer (Help:IPA/English, /ɪˈnænti.əmər, ɛ-, -oʊ-/ Help:Pronunciation respelling key, ''ih-NAN-tee-ə-mər''), also known as an optical isomer, antipode, or optical antipode, is one of a pair of molecular entities whi ...

s 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 In atomic physics and quantum chemistry, the electron configuration is the distribution of electrons of an atom or molecule (or other physical structure) in atomic or molecular orbitals. For example, the electron configuration of the neon atom ...

(t_2g)³(e_g)⁰. 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 Optical resolution describes the ability of an imaging system to resolve detail, in the object that is being imaged. An imaging system may have many individual components, including one or more lenses, and/or recording and display components. E ...

). It reacts with a variety of electrophiles at the 3-positions of the chelate rings, giving the corresponding bromo-, nitro-, and formyl-substituted derivatives.

Use in NMR

Cr(acac)₃ 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, 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 spin polarization, nuclear spin polarization from one population of Spin (physics), spin-active nuclei (e.g. 1H, 13C, 15N etc.) to another via Relaxation (NMR), cross-relaxation. A phenomenolog ...

. 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. 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,{{cite journal , last1=Cookson , first1=David J , last2=Smith , first2=Brian E , title=Optimal conditions for obtaining quantitative 13C NMR, data , journal=Journal of Magnetic Resonance , date=May 1984 , volume=57 , issue=3 , pages=355–368 , doi=10.1016/0022-2364(84)90253-1, bibcode=1984JMagR..57..355C 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, structure, bonding

Use in NMR

See also

References