Cobalt(III) Carbonate and Bicarbonate Chelate Complexes of Tripodal Tetraamine Ligands Containing Pyridyl Donors:  The Steric Basis for the Stability of Chelated Bicarbonate Complexes

Abstract

The synthesis and characterization (X-ray crystallography, UV/vis spectroscopy, electrochemistry, ESI-MS, and (1)H, (13)C, and (59)Co NMR) of the complexes [Co(L)(O(2)CO)]ClO(4)xH(2)O (L = tpa (tpa = tris(2-pyridylmethyl)amine) (x = 1), pmea (pmea = bis((2-pyridyl)methyl)-2-((2-pyridyl)ethyl)amine) (x = 0), pmap (pmap = bis(2-(2-pyridyl)ethyl)(2-pyridylmethyl)amine) (x = 0), tepa (tepa = tris(2-(2-pyridyl)ethyl)amine) (x = 0)) which contain tripodal tetradentate pyridyl ligands and chelated carbonate ligands are reported. The complexes display different colors in both the solid state and solution, which can be rationalized in terms of the different ligand fields exerted by the tripodal ligands. Electrochemical data show that [Co(tepa)(O(2)CO)](+) is the easiest of the four complexes to reduce, and the variation in E(red.) values across the series of complexes can also be explained in terms of the different ligand fields exerted by the tripodal ligands, as can the (59)Co NMR data which show a chemical shift range of over 2000 ppm for the four complexes. [Co(pmea)(O(2)CO)](+) is fluxional in aqueous solution, and VT NMR spectroscopy ((1)H and (13)C) in DMF-d(7) (DMF = dimethylformamide) over the temperature range -25.0 to 75.0 degrees C are consistent with inversion of the unique six-membered chelate ring. This process shows a substantial activation barrier (DeltaG(#) = 58 kJ mol(-1)). The crystal structures of [Co(tpa)(O(2)CO)]ClO(4)xH(2)O, [Co(pmea)(O(2)CO)]ClO(4).3H(2)O, [Co(pmap)(O(2)CO)]ClO(4), and [Co(tepa)(O(2)CO)]ClO(4) are reported, and the complexes containing the asymmetric tripodal ligands pmea and pmap both crystallize as the 6-isomer. The carbonate complexes all show remarkable stability in 6 M HCl solution, with [Co(pmap)(O(2)CO)](+) showing essentially no change in its UV/vis spectrum over 4 h in this medium. The chelated bicarbonate complexes [Co(pmea)(O(2)COH)]ZnCl(4), [Co(pmap)(O(2)COH)][Co(pmap)(O(2)CO)](ClO(4))(3), [Co(pmap)(O(2)COH)]ZnCl(4)xH(2)O, and [Co(pmap(O(2)COH)]ZnBr(4)x2H(2)O can be isolated from acidic aqueous solution, and the crystal structure of [Co(pmap)(O(2)COH)]ZnCl(4)x3H(2)O is reported. The stability of the carbonate complexes in acid is explained by analysis of the crystallographic data for these, and other slow to hydrolyze chelated carbonate complexes, which show that the endo (coordinated) oxygen atoms are significantly hindered by atoms on the ancillary ligands, in contrast to complexes such as [Co(L)(O(2)CO)](+) (L = (NH(3))(4), (en)(2), tren, and nta), which undergo rapid acid hydrolysis and which show no such steric hindrance.

Spin Crossover in a Family of Iron(II) Complexes with Hexadentate ligands: Ligand Strain as a Factor Determining the Transition Temperature

“…Below this temperature, the c M T value descends slowly to reach 1.10 cm 3 mol À1 K at 100 K, and then it remains fairly constant with decreasing temperature until 45 K. Further cooling to 15 K leads to the c M T value equal to 0.80 cm 3 mol À1 K. This weak paramagnetism can be related to a residual amount of the HS fraction A C H T U N G T R E N N U N G (%20 %), probably due to the slow kinetics of HS!LS relaxation. [19] In the warming mode the same magnetic behavior as in the cooling one was observed. Because of potentially explosive properties of the perchlorate salts, the complexes 2 c and 3 c were heated to 330 K. For compound 2 c, the variation of c M T in the range of 85-330 K conforms to a partial (55 %) spin conversion with T 1/2 equal to 320 K. The compound 3 c shows a weak increasing tendency for the c M T value up to 330 K, so the presence of spin crossover at higher temperatures can be expected.…”

“…[19][20][21] Although, this feature concerns s-bonding, some effects from p-bonding also can not be excluded. The tilt of the pyridine rings, probably a result of packing effects, modifies the p-acceptor properties of the aromatic-nitrogen atoms.…”

“…For example, significant chelate-ring-size effects have been found in the copper-dioxygen chemistry, [14,15] catechol dioxygenase reactivity of ironA C H T U N G T R E N N U N G (III) compounds, [16] electrochemical properties of manganeseA C H T U N G T R E N N U N G (III) and (IV) complexes, [17] spectroscopic properties of nickel(II) [18] and cobaltA C H T U N G T R E N N U N G (III) compounds. [19][20][21] Some attempts to consider the effect of chelate cycles size on the spin state of iron(II) have been done in the literature. [22][23][24] However, to our knowledge, there is no example in the literature of a series of similar compounds with successive changes in the size of chelate cycles, all of them manifesting a spin-crossover behavior.…”

A Structural Model for HPO42− Binding to Co in a Water Oxidation Catalyst

“…Despite the similar pK a values, 1 and 2 exhibit very different reactivity towards chelate ring opening, and this observation provides a possible explanation for the unusual stability of the chelated HPO 4 2À ligand in 1. We have previously shown that the structurally similar complexes [LCoA C H T U N G T R E N N U N G (O 2 CO)] + (L = tpa, Metpa, Me 2 tpa, Me 3 tpa, pmea, pmap, tepa) containing a chelated carbonate ligand [34][35][36] exhibit extraordinary stability toward acid hydrolysis in acidic aqueous solution owing to the fact that the tripodal ancillary ligands provide sufficient steric hindrance at the coordinated endo O atoms to inhibit protonation at these sites. This renders CoÀO bond cleavage and subsequent chelate ring-opening more difficult than in complexes such as [Co(en) 2 O 2 CO] + , which undergo rapid evolution of CO 2 in acidic aqueous solution.…”

“…We are currently investigating the reactivity of 1 and the synthesis of related HPO 4 2À -containing complexes. [35] , the cation is fluxional at room temperature); 31 (2); Co1-O1, 1.916(2); Co1-N21, 1.922(2); Co1-O2, 1.928(2); Co1-N1, 1.943(2); Co1-N11, 1.945(2); P1-O3, 1.497(2); P1-O4, 1.553(2); P1-O1, 1.554(2); P1-O2, 1.555(2); O1-Co1-O2, 76.11 (8); O1-Co1-N1, 90.34(9); O2-Co1-N11, 94.71(8); N1-Co1-N11, 98.83(10); O3-P1-O4 110.27 (12); O1-P1-O2, 99.32 (10).…”