The Reactivity of Co (III) Complexes of a Polydentate NHC Ligand

Ismael, Fouad
Blackman, Allan
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Doctor of Philosophy
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Auckland University of Technology

This thesis presents the outcomes of investigations into the structure and reactivity of Co(III) complexes featuring the (1,3-bis(bis(2-pyridyl)methyl)imidazol-2-ylidene) PY4Im ligand.

Several new complexes, including [(PY4Im)Co(OH)]²⁺, [(PY4Im)Co(OH₂)]³⁺, [(PY4Im)Co (CNCH₃)]³⁺, [(PY4Im)Co(N₃)]²⁺,[(PY4Im)Co(SCN)]²⁺, [(PY4Im)Co(F)]²⁺, Na[(PY4Im-κ⁴N ,N’,N’’,C)Co(N₃)₂]⁺, [Co(PY4Im-κ³N,N’,N’’,C)(O₂CO)]⁺, [(PY4Im)Co(ONO)]²⁺ and [(PY4Im)Co(Py)]³⁺ have been synthesized and characterized using various techniques including ESI-MS, ¹H and ¹³C NMR, IR spectroscopy, and X-ray crystallography.

The X-ray crystallography analysis of (PY4Im)Co(O₂CO) confirmed the presence of both a chelated carbonate ligand and a hypodentate PY4Im ligand within the complex. The complexity observed in the ¹H and ¹³C NMR spectra of this complex was attributed to its lower symmetry compared to complexes featuring monodentate ligands. Interestingly, the ¹³C NMR spectrum of [Co(PY4Im-κ³N,N’,N’’,C)(O₂CO)]ClO₄.2H₂O displayed a peak at 170.22 ppm assigned to the carbene carbon atom, contrary to expectations based on previous complexes of this ligand. However, this finding finds support in prior reports of chemical shifts of carbene carbons in cobalt complexes, shedding light on this unexpected observation.

The dissolution of [(PY4Im)Co(NCMe)]³⁺ in water and subsequent chromatography on Sephadex SP-C25 confirmed the formation of both [(PY4Im)Co(OH)]²⁺ and [(PY4Im)Co(OH₂)]³⁺ ions, a remarkable observation given the typically rapid equilibrium between aqua complexes and their hydroxido conjugate bases. The ability to separate these species on a cation exchange resin is highly unusual and hints at a remarkably slow proton transfer between the two ions. One possible explanation could be steric hindrance, as the aqua/hydroxido oxygen atom is surrounded by four pyridine rings, effectively shielding it from proton access. However, the presence of a water molecule near the hydroxido ligand suggests otherwise, as evidenced by a hydrogen bond with moderate strength. The ¹H NMR spectra of a mixture of these complexes in both D₂O and d₆-DMSO revealed a single species, indicating rapid equilibrium in solution.

The pKa of the aqua complex [(PY4Im)Co(OH₂)]³⁺ was determined using the standard potentiometric titration procedure outlined by Albert and Serjeant. The data indicate a pKa value of 6.54 ± 0.04 for the [(PY4Im)Co(OH₂)]³⁺ ion. This falls within the range typically observed for other Co(III) aqua complexes, demonstrating consistency in the acidity of these species. For instance, it closely resembles the pKa value of 6.5 reported for [Co(NH₃)₅OH₂]³⁺. Additionally, the measured pKa values of other complexes are within 1 pKa unit of this, further supporting the notion of similarity in acidity among Co(III) aqua complexes. This suggests that the presence of the carbene donor atom in the trans position does not significantly alter the acidity of the complex.

The X-ray structures of the carbene complexes consistently demonstrate a shorter Co-C bond compared to other bonds involving the metal ion, indicating a strong coordination between cobalt and carbon atoms. The Co-O bonds in Co-OH complexes are notably shorter than that in [(PY4Im)Co(OH)]²⁺, suggesting a potential trans influence. However, the presence of a strongly hydrogen-bonded water molecule in [(PY4Im)Co(OH)]²⁺ raises questions about its impact on the Co-O bond length. Nonetheless, similar complexes like [Co(terpy)(bipy)OH]²⁺ show minimal effect from hydrogen bonding on the Co-O bond length, indicating its likely negligible influence. Comparing bond lengths across different complexes further elucidates trends, such as longer Co-N azide and Co-N thiocyanate bonds in [(PY4Im)Co(N₃)]²⁺ and [(PY4Im)Co(NCS)]²⁺, respectively, compared to their counterparts. However, the situation with fluorido complexes is less clear-cut, although some evidence suggests longer Co-F bonds in [(PY4Im)Co(F)]²⁺. Interestingly, [(PY4Im)Co(OH₂)]³⁺ presents a shorter Co-O bond than its analogue, contrary to the general trend. Conversely, [(PY4Im)Co(NCMe)]³⁺ exhibits a much longer Co-N acetonitrile bond compared to the tabulated complexes. Finally, except for the aqua complex, the Co-X bonds in PY4Im complexes tend to be longer than those in structurally similar complexes where a pyridyl or tertiary amine N atom lies trans to the monodentate X ligand, suggesting a trans influence in Co(III) complexes of the PY4Im ligand. The DFT calculations show strong agreement with experimental structures, albeit with notable deviations primarily observed in the positioning of monodentate ligands. These discrepancies may arise from the limitations of the calculations, conducted for isolated species in the gas phase, neglecting intermolecular electronic interactions and steric effects arising from crystal packing.

The DFT calculations have shown a strong overall agreement with experimental structures, although there are some notable discrepancies, particularly concerning the positioning of monodentate ligands. These differences likely stem from the inherent limitations of DFT calculations, which typically model isolated species in the gas phase and do not account for intermolecular electronic interactions or steric effects arising from crystal packing. Despite these limitations, the 6-311+G(2df,2p) basis set consistently demonstrates the lowest overlay value in most cases, suggesting that it may more accurately capture the structural features of monodentate complexes and align them better with experimental data compared to other basis sets tested. However, it's crucial to remember the inherent constraints of DFT calculations and the necessity for experimental validation when interpreting these results.

The complexes [Co(bpy)₂BO₂(OH)]·[B₅O₆(OH)₄]·H₃BO₃·H₃O·H₂O and [Co(bipy)₂(O₂CO)] [B₅O₆(OH)₄]·H₃BO₃·2H₂O have been comprehensively characterized using ¹H and ¹³C NMR spectroscopy, ESI mass spectrometry, and X-ray crystallography. Initially identified as [Co(bpy)₂BO₂(OH)]·[B₅O₆(OH)₄]·H₃BO₃·H₃O·H₂O, the complex was believed to contain a Co(II) ion coordinated to chelated hydrogenborate ligands. However, subsequent analysis revealed that it is actually [Co(bipy)₂(O₂CO)][B₅O₆(OH)₄]·H₃BO₃·2H₂O, constituting a Co(III) complex with chelated carbonate ligands. The carbonate ligand originates from atmospheric CO₂, highlighting the necessity of conducting experiments under inert atmospheres when investigating Co(II) amine complexes in aqueous solutions. This discrepancy underscores the importance of thorough characterization techniques to accurately determine the composition and structure of coordination complexes.

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