Download Trans-Effect in Cobalt Metal Complexes

By Angel Peng
Introduction
• Hypothesis: The nature of the X ligand can
be determined by observing the effects of
the metal-ligand bond trans to it.
– General structure of compound used
Co(dimethylglyoxime)(pyridine)X
– X ligands are CN, Br, CH3, SCN, and OH.
2 dimethylglyoxime moieties
in its equatorial plane
stabilizes the unpaired
electron in the cobalt atom6
Background
• Trans influence is a change in the bond
distance between the metal and a ligand
that is trans to the ligand exerting the
effect through the same d-orbital
interaction.1
Ligand exerting
influence
Trans ligand
Nature of Ligands
• -donor
(ex. NH3, CH3-, H-)
• -donor
(ex. Cl-, OH-, NR2-)
• -acceptor
(ex. CO, NO+, CN-)
Pictures taken from ref 2.
Effect of Metal-Pyridine Bond
• -donor
• -acceptor
Unmodified pictures taken from ref 3.
• -donor
Bond length shortens due to
electron density increase in
the metal, building a stronger
bond between Co-pyridine
Metal electron density decreases
causing a weaker elongated bond
between Co-pyridine.
Synthetic Procedure
• Synthesis of Co(dimethylglyoxime)2(pyridine)X:
[X=CN, Br, SCN)4
Step 1: Boil mixture 5 min.
Step 2: Boil another 5 min.
Solution of 5 mmol
of KX or NaX in 5
mL water added
6.25 mmol
dimethylglyoxime
3.3 mmol
cobalt nitrate
hexahydrate
Step 3: Boil another 5 min.
Solution of 2 mL
of pyridine in 5
mL of THF
added.
Synthetic Procedure Con’t
• Step 4: Aerate for 20 min.
Air Pipe
• Step 5: Collect crystals by vacuum filtration
• Step 6: Wash crystals with 5 mL water and ethanol and
10 mL of diethyl ether.
Synthesis of Co(dimethylglyoxime)2(pyridine)OH:
- Replace step 2 with addition of 0.2 mL of ammonia hydroxide instead
of salt solution.
Synthetic Procedure Con’t
Synthesis of Co(dimethylglyoxime)2(pyridine)CH3:5
- Continuing from step 6, Co(dimethylglyoxime)2(pyridine)Br of
0.8 mmol was mixed 5 mL of methanol under argon
atmosphere.
-1.7 mmol of NaBH4 was added, followed
by 1.5 mmol of iodomethane.
- Another 1.7 mmol of NaBH4 was added
into the solution
-Stir for 15 min and then pour into 10 mL
of cold water.
-Collect and wash crystals with 10 mL
ether.
Ar. atmosphere setup
Results
-OH
-C≡N
IR of Co(dimethylglyoxime)(pyridine)CN
All IR values taken from reference 7, unless otherwise indicated.
-C=N of
dimethylglyoxime11
Co-pyridine stretch8
at 514.82 cm-1
Pyridine
ring12
Results
-SCN9
IR of Co(dimethylglyoxime)(pyridine)SCN
Co-pyridine stretch
at 512.74 cm-1
Results
-CH3
IR of Co(dimethylglyoxime)(pyridine)CH3
Co-pyridine stretch
512.49 cm-1
Results
Co-Br10
IR of Co(dimethylglyoxime)(pyridine)Br
Co-pyridine
stretch
Results
-OH
IR of Co(dimethylglyoxime)(pyridine)OH
Co-pyridine stretch
at 503.86 cm-1
Discussion
• Cobalt-pyridine IR stretch in increasing
wavenumber according to trans ligand:
X ligand
OH
Br
CH3
SCN
CN
IR
stretch
(cm-1)
503.86
512.40
512.49
512.74
514.82
• Co-py bond shifting to lower wavenumbers
indicated a shortening of bond length. An
increase in electron density on the metal created
a stronger hold between the Co-pyridine.
Discussion
• Characteristics of OH:
– Greatest electron density contribution on cobalt atom
– Oxygen’s lone pair electrons donated into metal’s empty d-orbital
– -donor
• Characteristics of Br:
– Electron density contribution not as great due to slight electron
withdrawal from inductive effect
– Partial -donor/-acceptor
• Characteristics of CH3:
– -donors
Discussion
• Characteristics of SCN:
– Sulfur normally would contribute to metal electron density as
much as oxygen but the -C≡N counters with its electron affinity
nature.
• Characteristics of CN:
– Electrons from the metal fills the empty -orbital of CN.
– Lowering electron density around cobalt decreases the Co-py
bond.
– Mostly -acceptor and slightly -donor nature.
Conclusion
• Nature of ligands can be comparatively characterized by
observing the effect they have on its metal trans bond.
- Change in metal electron density and trans bond length can be
measured by IR analysis.
• Originally intend to use iron-porphyrin as the general
compound in which the metal complex is stabilized due
electron contribution from the porphyrin core to the
- Metallation of iron into porphyrin was unsuccessful after 4 attempts.
- Extremely low percent yield was achieved after many hours of reflux,
but product was essentially nonexistent.
- Experiment can be improved with better solvent (higher b.p.) and
longer reflux time.
• Experiment can be improved by using more ligands to
attain a broader range of characteristic spectrum.
References
1.) Tadeusz Andruniow, Jadwiga Kuta, Marek Z, Zgierski, Pawel M. Kozlowski,
Molecular orbital analysis of anomalous trans effect in cobalamins,
Chemical Physics Letters 410, (2005), 410-416.
2.) http://www.chemistry.mcmaster.ca/courses/3Q03%2005.pdf
3.) http://cnx.org/content/m15057/latest/ds.GIF
4.) Ndahi Naomi, Kolawole Gabriel, Cobalt(III) complexes of diphenylglyoxime
as possible non-organometallic models of vitamin B12, South African Journal
of Science 101, Sept/Oct. 2005.
5.) Roth Justine P, A Laboratory Manual for Advanced Inorganic Chemistry, The
Johns Hopkins University, Maryland, Fall 2007.
6.) Harwood James H., Shim Anne K., Polymers and copolymers prepared or
modified by using cobalt complexes, US Patent 6602960, 5 Aug, 2003.
7.) http://www.mpcfaculty.net/ron_rinehart/12A/IR/primclue.htm
8.) Gradinaru Julieta, Malinovskii Stanislav,Trinuclear tris-Co(II) and transcobaloxime type Co(III) complexes prepared from Co(II) triflate precursor:
Synthesis structure and properties, Polyhedron 25 (2006), 3417-3426.
Reference
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CdHg(SCN)4 Single Crystals, Physica status solidi (b), Vol 231, Isssue 2,
554-560
10.) Meehan Paul R., Alyea Elmer C., Structural and spectroscopic studies of
cobalt(II) complexes incorporating the bidentate phosphine ligand
Ph2PCH2Si(CH3)2CH2PPh2, Polyhedron, vol. 17, issue 1 (1998), 11-16.
11.) Brown Kenneth, Satyanarayana S. Infrared and 13C and 15N NMR
Studies of Cyano(ligand)cobaloximes: Evidence for Cobalt-to-Cyanide Bonding, Inorganic Chemistry, Vol 31, Issue 8 (1992), 1367.
12.) Abdullah Ahmed, Huq Fazlul, Chowdhury Ashraf, Studies on the synthesis,
characterization, binding with DNA and activities of two cisplanaramineplatinum(II) complexes of the form: cis-PtL(NH3)Cl2 where L =
3-hydroxypyridine and 2,3-diaminopyridine, BMC Chemical Biology 2006,
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