Trans-Influence 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. -donor -acceptor -donor Effect of Metal-Pyridine Bond 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: [X=CN, 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 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

8. 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

9. Results -OH -C=N of dimethyl-glyoxime11 -C≡N 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 IR of Co(dimethylglyoxime)(pyridine)OH Co-pyridine stretch at 503.86 cm-1

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 • 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

16. 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.

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.

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