High Valent Osmium Complexes Incorporating a Tetradentate Tetraanionic Chelating Ligand; Stabilization Modification Resulting from the Formation of Non-Planar Amides

Citation

Keech, John Tyler (1987) High Valent Osmium Complexes Incorporating a Tetradentate Tetraanionic Chelating Ligand; Stabilization Modification Resulting from the Formation of Non-Planar Amides. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/A4N9-5E16. https://resolver.caltech.edu/CaltechTHESIS:02262010-145408928

Abstract

A new tetradentate tetraanionic chelating ligand has been synthesized: 1,2-bis(3,5-dichloro-2-hydroxybenzamido)-4,5-dichlorobenzene, H₄CHBA-DCB. This ligand is one of a class of polyanionic chelating (PAC) ligands synthesized by the Collins group for stabilizing high valent metal centers. H₄CHBA-DCB has an aromatic framework that is chlorinated for added chemical resistance. Reacting equimolar amounts of H₄CHBA-DCB and K₂[Os(OH)₄(O)₂] coordinates the ligand as trans-K₂[Os(η⁴-CHBA-DCB)(O)₂], 3. Compound 3 undergoes reversible protonation at an oxo ligand to produce trans-K[Os(η⁴-CHBA-DCB)(O)(OH)], 4. Compound 4 reacts with solid MgSO₄ and forms the square-pyramidal mono-oxo complex Os(η⁴-CHBA-DCB)(O), 5. Most oxidizing agents, with the exception of bromine, decompose 3 into a black amorphous material. Bromination of 3 produces a compound similar to 4 and is proposed to be trans-K[Os(η⁴-CHBA-DCB)(O)(OBr)], 6. Triphenylphosphine readily reduces Os(VI)3 to a neutral Os(IV) complex, trans-Os(η⁴-CHBA-DCB)(PPh₃)₂, 7. The ancillary phosphine ligands of 7 prove to be substitutionally labile and can be exchanged for other Lewis bases (e.g., py, t-Bupy, bipy, dppe, and t-BuNC) to produce a series of Os(IV) complexes. Using cyclic voltammetry, the formal potentials of the Os(V/IV), Os(IV/III), and Os(III/II) couples are determined in CH₂Cl₂. In liquid SO₂, the formal potentials of the Os(V/IV), Os(2+/+), and Os(3+/2+) couples are measured for three compounds, the highest potential being at +1.70 V vs. Fc⁺/Fc or ca. +2.4 V vs. NHE. A fourth irreversible couple is observed at +2.12 to +2.20 V vs. Fc⁺/Fc. These osmium complexes display temperature-independent paramagnetism.

Os(IV) complexes with bidentate ligands (e.g., bipy and dppe) and Lewis acid ligands (t-BuNC) form cis complexes where the (η⁴-CHBA-DCB)^4- ligand is non-planar, and the auxiliary ligands are cis to one another. Two such complexes have been crystallographically characterized: cis-β-Os(η⁴-CHBA-DCB)(bipy), 11, and cis-α-Os(η⁴-CHBA-DCB)(PPh₃)(t-BuNC), 14. Compound 11 crystallizes with 0.5 equiv EtOH solvate in the triclinic space group P1̅ with a = 10.860(3) Å, b = 12.633(3) Å, c = 12.844(4) Å, α = 117.42(2)°, β = 90.42(3)°, γ = 95.90(3)°, V = 1552.2(7) ų, Z = 2, D_calc = 1.97 g/cm³, and R_f = 0.047 (I > 0; 2888 reflections). Compound 14 crystallizes in the orthorhombic space group P_bca with a = 22.09(2) Å, b = 19.92(2) Å, c = 19.40(2) Å, V = 8537(4) ų, Z = 8, D_calc = 1.69 g/cm³, and R_f = 0.063 (I > 0; 1564 reflections). These structures contain novel non-planar amide groups. The amount of deformation in each amide is quantified by a torsion angle analysis. Two bonding changes cause the amide non-planarity: amide C-N bond rotation and pyramidalization of the amide nitrogen atom. Consistent with the presence of non-planar amides, unusually high amide carbonyl stretching frequencies are observed (1650-1695 cm^-1), which imply that amide delocalization stabilization has been restricted.

The cis-α and trans isomers of [Os(η⁴-CHBA-DCB)(t-Bupy)₂]⁺ are in equilibrium with one another. From the measured equilibrium constant of 1.3 (in favor of the trans) and the formal reduction potentials for each isomer, the equilibrium constants for the interconversion of the neutral (3.4 x 10³), anionic (2.4 x 10¹¹), and dianionic (2.0 x 10¹⁵) species are derived. The trend in equilibrium constants shows that the cis-α ligand set is increasingly favored as the metal is oxidized. A linear free energy relationship (LFER) is established between the cis-α ⇆ trans equilibrium constants of Os(η⁴-CHBA-DCB)(p-X-py)₂ [X = MeO, t-Bu, Et, Me, H, Br, Cl, Ac] and both Fischer σ̅ (ρ = -2.10, r = 0.948) and Hammett σ_p (ρ = -1.60, r = 0.949) substituent parameters. The formal potentials of the Os(V/IV), (IV/III), and Os(III/II) couples of both the cis-α and trans isomers also correlate with σ̅ constants, showing that the electron density at the metal center is perturbed in a predictable manner by the pyridine substituents. These LFERs show that the isomerization equilibria are controlled by electronic demand for stabilization at the metal center with the cis-α ligand set being more electron-donating than the trans set. This difference apparently results from the increased localization of the amide nitrogen lone pair due to restricted amide delocalization in the non-planar amide groups of the cis-α isomers. The localized lone pair probably enhances σ- and π-bonding between the amide nitrogen atoms and the osmium center. The balance between the stabilization derived from the cis-α isomer and the destabilization incurred from restricted amide delocalization may determine which isomer is thermodynamically most stable.

The mechanism for the interconversion of cis-α and trans isomers is probed through kinetic rate measurements and ligand exchange studies. Most of the results suggest that an intramolecular "twist" mechanism, T, requiring no ligand dissociation steps, controls the isomerization processes. The mechanism explains why no cis-β isomers are detected during the isomerization of cis-α and trans isomers. The activation parameters are derived for the cis-α → trans isomerizations of three systems: Os(η⁴-CHBA-DCB)(t-Bupy)₂ (ΔH^≠ = 21.6(19) kcal/mol; ΔS^≠ = -10(6) eu), Os(η⁴-CHBA-DCB)(O =PPh₃)₂ (ΔH^≠ = 21.6(2) kcal/mol; ΔS^≠ = 0.3(6) eu), and [Os(η⁴-CHBA-DCB)(O =PPh₃)₂]⁺ (ΔH^≠ = 23.7(6) kcal/mol; ΔS^≠ = + 17(2) eu). This suggests that the different rates of isomerization result from entropic differences in the transition state, not enthalpic differences. That is, the t-Bupy system has the "tightest" transition state, as evidenced by the most negative ΔS^≠, and consequently the slowest isomerization rate. The rates are observed to be faster in the d³ Os(V) phosphine oxide system than in its d⁴ Os(IV) analog.

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