The Synthesis of a Corrole Analogue of Aquacobalamin (Vitamin B_12a) and Its Ligand Substitution Reactions

Abstract

The synthesis of a Co(III) corrole, [10-(2-[[4-(1H-imidazol-1-ylmethyl)benzoyl]amino]phenyl)-5,15-diphenylcorrolato]cobalt(III), DPTC-Co, bearing a tail motif terminating in an imidazole ligand that coordinates Co(III), is described. The corrole therefore places Co(III) in a similar environment to that in aquacobalamin (vitamin B12a, H2OCbl(+)) but with a different equatorial ligand. In coordinating solvents, DPTC-Co is a mixture of five- and six-coordinate species, with a solvent molecule occupying the axial coordination site trans to the proximal imidazole ligand. In an 80:20 MeOH/H2O solution, allowed to age for about 1 h, the predominant species is the six-coordinate aqua species [H2O-DPTC-Co]. It is monomeric at least up to concentrations of 60 μM. The coordinated H2O has a pKa = 9.76(6). Under the same conditions H2OCbl(+) has a pKa = 7.40(2). Equilibrium constants for the substitution of coordinated H2O by exogenous ligands are reported as log K values for neutral N-, P-, and S-donor ligands, and CN(-), NO2(-), N3(-), SCN(-), I(-), and Cys in 80:20 MeOH/H2O solution at low ionic strength. The log K values for [H2O-DPTC-Co] correlate reasonably well with those for H2OCbl(+); therefore, Co(III) displays a similar behavior toward these ligands irrespective of whether the equatorial ligand is a corrole or a corrin. Pyridine is an exception; it is poorly coordinated by H2OCbl(+) because of the sterically hindered coordination site of the corrin. With few exceptions, [H2O-DPTC-Co] has a higher affinity for neutral ligands than H2OCbl(+), but the converse is true for anionic ligands. Density functional theory (DFT) models (BP86/TZVP) show that the Co-ligand bonds tend to be longer in corrin than in corrole complexes, explaining the higher affinity of the latter for neutral ligands. It is argued that the residual charge at the metal center (+2 in corrin, 0 in corrole) increases the affinity of H2OCbl(+) for anionic ligands through an electrostatic attraction. The topological properties of the electron density in the DFT-modeled compounds are used to explore the nature of the bonding between the metal and the ligands.

Probing the nature of the Co(III) ion in cobalamins: The reactions of aquacobalamin (vitamin B12a), aqua-10-chlorocobalamin and aqua-10-bromocobalamin with anionic and neutral ligands

“…Briefly, we have demonstrated that there is indeed electronic communication between the axial and the equatorial ligands in the cobalt corrins [5] and that the C-10-Cl bond length in [X-(10-Cl)Cbl] depends on the nature of the axial ligand X [1]; that substituting the C-10 H by NO deactivates the metal ion [2]; that the ligand substitution reactions of [H 2 O-(10-Cl)Cbl] + are slower than those of vitamin B 12a , [H 2 OCbl] + , itself [4]; that interrupting the conjugation of the corrin makes the metal ion harder [7,9]; and that the difference in charge on the macrocyle (À3 in corroles, À2 in porphyrins and À1 in corrins), which modifies the net charge at the metal centre, plays an important role in controlling and modifying the chemistry of the metal ion [10,12].…”

“…We have been exploring the nature of Co(III) in the cobalt corrins (derivatives of vitamin B 12 ) in an effort to elucidate the effect of the corrin ligand on the chemistry of the metal ion [1][2][3][4][5][6][7][8][9][10][11][12]. We have suggested that an important factor in modifying the properties of normally inert Co(III) and conferring on it the unusual lability found in the cobalt corrins is the delocalisation of electron density from the corrin equatorial ligand onto Co(III), making the metal softer and imparting on it some measure of labile Co(II) character.…”

“…Firstly, we have perturbed the electronic structure of the corrin by substituting the H atom at C-10 (see Fig. 1 for numbering) by either Cl (which is p electron-donating towards the corrin) [1,4] or NO (which is p electron-withdrawing) [2]; secondly, we have disrupted the p conjugated system by oxidising the C5-C6 double bond [6,7,9]; thirdly, we have prepared an aquacobalamin (vitamin B 12a , H 2 OCbl + ) analogue containing a porphyrin [10]; and more recently we have prepared a B 12a analogue containing a corrole [12]. In each case we have compared the physical properties and the thermodynamics (and in some cases the kinetics) of the substitution of the axial H 2 O by a series of neutral and anionic ligands to determine the effect that perturbing the electronic structure of the corrin, or exchanging a corrin for a porphyrin or a corrole, has on the chemistry of Co(III).…”

Elucidation of Factors That Govern the 2e–/2H+ vs 4e–/4H+ Selectivity of Water Oxidation by a Cobalt Corrole

“…The erstwhile focus of the water oxidation community has been accelerating the rate and efficiency of the water oxidation process using cheap first-row transition-metal-based materials and complexes. − Several Co-based complexes/composites have been recently used as catalysts for water splitting in this regard. − Macrocyclic corrole, a porphyrin-like moiety, has recently emerged as a unique ligand to carry out several catalytic transformations. It has the capability to support both high and low formal metal oxidation states. − This nature of the corrole ligand encouraged exploration of its potential for electrocatalysts for supporting the oxygen evolution reaction (OER), hydrogen evolution reaction (HER), oxygen reduction reaction (ORR), etc. ,− Recently, a system for molecular OER was reported containing a hangman β-octafluoro corrole cobalt complex with a high-valent Co­(IV) corrole cation radical as the precatalyst . β-Halogen substitution on the corrole ring in a metallocorrole complex has a dominant effect in controlling the redox potential of the metal and their subsequent reactivity. , Furthermore, the β-positions of the corrole macrocycle are prone to oxidation, leading to dearomatization .…”

Study of Structure−Properties Relationship for Lanthanide Tetrapyrrolic Macrocycles Modified with Aminopolycarboxylate Substituents

“…Many derivatives of porphyrins and corroles are the basis of numerous natural compounds such as heme, vitamin B12, etc. [1] Corolles consist of completely unsaturated ring, and their main difference from porphyrins is the absence of one methine bridge between the pyrroles. Therefore, the number of carbon atoms in the aromatic chain changes and this macrocycle becomes tribasic ( Figure 1).…”