Characterization
of Cr(acac)3
Abstract
Chromium (III) complexes are commonly used in inorganic chemistry labs because many of them have been studied extensively. The complex tris(acetylacetonato)chromium(III) Figure 1, henceforth referred to as Cr(acac)3, was characterized using infrared spectroscopy and measuring its melting point. The infrared spectrum of the complex is quite different from the spectra for the free ligand.
Figure 1. General structure of Cr(acac)3
Introduction
The complex Cr(acac)3 is well known in the literature and has been studied extensively[1]. Chromium (III) in Cr(acac)3 a d3 metal with an octahedral crystal field splitting is low spin where S = 3/2 indicating that it is strongly paramagnetic. The free ligand is 2,4-pentadione Figure 2 (a), has 2 acidic α protons, of which one can be extracted to created an overall negative charge that is stabilized by resonance[2] Figure 2 (b).
Figure 2. Structure of 2,4-pentadione a) showing the two acidic protons in red, and b) anion stabilized by resonance.
a)
b) 
Experimental
The procedures of Cr(acac)3 synthesis can be found in the literature (the synthesis equation). The chemicals were purchased from XXX companies (use the chemical companies name& Aldrich, Alfa, Acros&). The IR spectra were recorded on a Midac (insert model #) FTIR instrument purged with nitrogen gas. The compound was suspended in nujol pressed between two KBr plates. The melting point was collected (with or without) calibrating the thermometer in a Mel-temp model # from Fisher.
Observation
The
synthesis of Cr(acac)3 was according to the
Schlenk technique. The
operation was under Ar and then we used distilled solvent. Acac (3.0
× 10-4mol)
and Cr (1.0×10-4mol)
were mixed with distilled MeOH in a Schlenk flask and
stirred for 3 hours to get yellow solution. The solvent was removed
under
reduced pressure and the final product was yellow powder
(1.0×10-4mol).
The product was recrystalized by the
slow evaporation method. The yellow compound was re-dissolved in 20ml
dichloromethane in a 50ml beaker. The beaker was covered by aluminum
foil and
left inside the cabinet. The solvent was evaporated slowly for 7 days.
The
yellow needle like crystals formed on the bottom of the beaker.
Discussion
The melting point of the complex was
a sharp range of
208-210, which matches the reported value, indicating that the compound
is
pure.
The infrared spectra of 2,4-pentadione Attachment 1
shows peaks at XXXX cm-1 which is characteristic
of a C=O stretch,
as well as && (peak assignments many websites give peak
assignments
many of you may have used them for Organic chem..) There is a large
noticeable
difference between the free ligand spectra and that for the complex
(Attachment
2). One of the most noticeable is the disappearance of the 1XXX cm-1
peak corresponding to the C=O stretch. Other peaks were shifted as well
(examples &.) these shifts to higher energies are most likely
due increased
rigidity in the structure. There is also a new peak at XXX cm-1
due
to the C=C resonance stretching. Any peaks in the 3000-2000 range are
suspect
due to Nujol absorptions. If the spectra of the complex were run in KBr
and
there were still peaks in this range then assignments could be made
with out
suspicion. The IR spectra provided by Fisher-Acros chemical company
shows IR
absorption in the 2200 cm-1 region.. elaborate...
(include experimental details reaction details etc& in
the future when applicable)
Conclusion
From the IR data, it can be shown that there is a large difference between the free ligand and the complex. The disappearance of the C=O band and growth of a C=C band indicates that there is resonance in the complex. These characteristics could allow industry to monitor the reaction to form Cr(acac)3 by simply observing the C=C or C=O IR frequency as it appears or disappears.