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

Trans ligand

Ligand exerting influence

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

-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.

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.

Step 3: Boil another 5 min.

6.25 mmol dimethylglyoxime

3.3 mmol cobalt nitrate hexahydrate

Solution of 5 mmol of KX or NaX in 5 mL water added

Solution of 2 mL of pyridine in 5 mL of THF added.

Synthetic Procedure Con’t• 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

Synthetic Procedure Con’tSynthesis 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

-C≡N

Pyridine ring12

-OH

-C=N of dimethyl-glyoxime11

Co-pyridine stretch8 at 514.82 cm-1

IR of Co(dimethylglyoxime)(pyridine)CNAll IR values taken from reference 7, unless otherwise indicated.

Results

-SCN9

Co-pyridine stretch at 512.74 cm-1

IR of Co(dimethylglyoxime)(pyridine)SCN

Results

-CH3

Co-pyridine stretch 512.49 cm-1 IR of Co(dimethylglyoxime)(pyridine)CH3

Results

Co-Br10

Co-pyridine stretch IR of Co(dimethylglyoxime)(pyridine)Br

Results

-OH

Co-pyridine stretch at 503.86 cm-1

IR of Co(dimethylglyoxime)(pyridine)OH

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.

X ligand OH Br CH3 SCN CN

IR stretch(cm-1)

503.86 512.40 512.49 512.74 514.82

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.

References1.) 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.pdf3.) http://cnx.org/content/m15057/latest/ds.GIF4.) 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.htm8.) 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.

Reference9.) 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)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 cis-planaramineplatinum(II) complexes of the form: cis-PtL(NH3)Cl2 where L = 3-hydroxypyridine and 2,3-diaminopyridine, BMC Chemical Biology 2006, 6:3