Trans-Effect in Cobalt Metal Complexes

1
Trans-Influence in Cobalt Metal Complexes
By Angel Peng
2
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
3
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
4
Nature of Ligands

• ?-donor
• (ex. NH3, CH3-, H-)
• ?-donor
• (ex. Cl-, OH-, NR2-)
• ?-acceptor
• (ex. CO, NO, CN-)

Pictures taken from ref 2.
5
Effect of Metal-Pyridine Bond

• ?-donor
• ?-acceptor

• ?-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.
Unmodified pictures taken from ref 3.
6
Synthetic Procedure

• Synthesis of Co(dimethylglyoxime)2(pyridine)X
• XCN, Br, SCN)4
• Step 1 Boil mixture 5 min. Step 2
  Boil another 5 min.
• Step 3 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
Solution of 2 mL of pyridine in 5 mL of THF added.
7
Synthetic Procedure Cont

• Step 4 Aerate for 20 min.
• 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.

Air Pipe
8
Synthetic Procedure Cont

• Synthesis of Co(dimethylglyoxime)2(pyridine)CH35
• Continuing from step 6, Co(dimethylglyoxime)2(pyri
  dine)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
9
Results
-OH
-CN of dimethyl-glyoxime11
-CN
Co-pyridine stretch8 at 514.82 cm-1
Pyridine ring12
IR of Co(dimethylglyoxime)(pyridine)CN
All IR values taken from reference 7, unless
otherwise indicated.
10
Results
-SCN9
Co-pyridine stretch at 512.74 cm-1
IR of Co(dimethylglyoxime)(pyridine)SCN
11
Results
-CH3
Co-pyridine stretch 512.49 cm-1
IR of Co(dimethylglyoxime)(pyridine)CH3
12
Results
Co-Br10
Co-pyridine stretch
IR of Co(dimethylglyoxime)(pyridine)Br
13
Results
-OH
Co-pyridine stretch at 503.86 cm-1
IR of Co(dimethylglyoxime)(pyridine)OH
14
Discussion

• Cobalt-pyridine IR stretch in increasing
  wavenumber according to trans ligand
• 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.

15
Discussion

• Characteristics of OH
• Greatest electron density contribution on cobalt
  atom
• Oxygens lone pair electrons donated into metals
  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

16
Discussion

• Characteristics of SCN
• Sulfur normally would contribute to metal
  electron density as much as oxygen but the -CN
  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.

17
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.

18
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
  trans-cobaloxime type Co(III) complexes prepared
  from Co(II) triflate precursor Synthesis
  structure and properties, Polyhedron 25 (2006),
  3417-3426.

19
Reference

• 9.) Lu G.W., Xia H.R., Zheng W.Q, Vibrational
  Spectrum and Structure of 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)cobaloxime
  s 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 cis-planaramineplatinum(II) complexes of
  the form cis-PtL(NH3)Cl2 where L
  3-hydroxypyridine and 2,3-diaminopyridine, BMC
  Chemical Biology 2006, 63