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Structural Investigation of Magnesium Complexes Supported by a Thiopyridyl Scorpionate Ligand.

ABSTRACT: Herein, we report the synthesis of a series of heteroleptic magnesium complexes stabilized with the scorpionate ligand tris(2-pyridylthio)methanide (Tptm). The compounds of the general formula [Mg(Tptm)(X)] (1-X; X = Cl, Br, I) were obtained via protonolysis reaction between the proligand and selected Grignard reagents. Attempts to isolate the potassium derivative K(Tptm) lead to decomposition of Tptm and formation of the alkene (C₅H₄N-S)₂C=C(C₅H₄N-S)₂, and this degradation was also modelled using DFT methods. Compound 1-I was treated with K(CH₂Ph), affording the degradation product [Mg(Bptm)₂] (2; Bptm = {CH(S-C₅NH₃)₂}^-). We analyzed and quantified the steric properties of the Tptm ligand using the structural information of the compounds obtained in this study paired with buried volume calculations, also adding the structural data of HTptm and its CF₃-substituted congener (HTptm^CF3). These studies highlight the highly flexible nature of this ligand scaffold and its ability to stabilize various coordination motifs and geometries, which is a highly desirable feature in the design of novel organometallic reagents and catalysts.

SUBMITTER: Stevens MP

PROVIDER: S-EPMC9318590 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description:Six-coordinate MnIII complexes are typically high-spin (S = 2), however, the scorpionate ligand, both in its traditional, hydridotris(pyrazolyl)borate form, Tp- and Tp*- (the latter with 3,5-dimethylpyrazole substituents) and in an aryltris(carbene)borate (i.e., N-heterocyclic carbene, NHC) form, [Ph(MeIm)3B]-, (MeIm = 3-methylimidazole) lead to formation of bis(scorpionate) complexes of MnIII with spin triplet ground states; three of which were investigated herein: [Tp2Mn]SbF6 (1SBF6), [Tp*2Mn]SbF6 (2SBF6), and [{Ph(MeIm)3B}2Mn]CF3SO3 (3CF3SO3). These trigonally symmetric complexes were studied experimentally by magnetic circular dichroism (MCD) spectroscopy (the propensity of 3 to oxidize to MnIV precluded collection of useful MCD data) including variable temperatures and fields (VTVH-MCD) and computationally by ab initio CASSCF/NEVPT2 methods. These combined experimental and theoretical techniques establish the 3A2g electronic ground state for the three complexes, and provide information on the energy of the "conventional" high-spin excited state (5Eg) and other, triplet excited states. These results show the electronic effect of pyrazole ring substituents in comparing 1 and 2. The tunability of the scorpionate ligand, even by perhaps the simplest change (from pyrazole in 1 to 3,5-dimethylpyrazole in 2) is quantitatively manifested through perturbations in ligand-field excited-state energies that impact ground-state zero-field splittings. The comparison with the NHC donor is much more dramatic. In 3, the stronger ?-donor properties of the NHC lead to a quantitatively different electronic structure, so that the lowest lying spin triplet excited state, 3Eg, is much closer in energy to the ground state than in 1 or 2. The zero-field splitting (zfs) parameters of the three complexes were calculated and in the case of 1 and 2 compare closely to experiment (lower by < 10%, < 2 cm-1 in absolute terms); for 3 the large magnitude zfs is reproduced, although there is ambiguity about its sign. The comprehensive picture obtained for these bis(scorpionate) MnIII complexes provides quantitative insight into the role played by the scorpionate ligand in stabilizing unusual electronic structures.

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Structural Investigation of Magnesium Complexes Supported by a Thiopyridyl Scorpionate Ligand.

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