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Tris(Butadiene) Compounds versus Butadiene Oligomerization in Second-Row Transition Metal Chemistry: Effects of Increased Ligand Fields.

ABSTRACT: The geometries, energetics, and preferred spin states of the second-row transition metal tris(butadiene) complexes (C₄H₆)₃M (M = Zr-Pd) and their isomers, including the experimentally known very stable molybdenum derivative (C₄H₆)₃Mo, have been examined by density functional theory. Such low-energy structures are found to have low-spin singlet and doublet spin states in contrast to the corresponding derivatives of the first-row transition metals. The three butadiene ligands in the lowest-energy (C₄H₆)₃M structures of the late second-row transition metals couple to form a C₁₂H₁₈ ligand that binds to the central metal atom as a hexahapto ligand for M = Pd but as an octahapto ligand for M = Rh and Ru. However, the lowest-energy (C₄H₆)₃M structures of the early transition metals have three separate tetrahapto butadiene ligands for M = Zr, Nb, and Mo or two tetrahapto butadiene ligands and one dihapto butadiene ligand for M = Tc. The low energy of the experimentally known singlet (C₄H₆)₃Mo structure contrasts with the very high energy of its experimentally unknown singlet chromium (C₄H₆)₃Cr analog relative to quintet (C₁₂H₁₈)Cr isomers with an open-chain C₁₂H₁₈ ligand.

SUBMITTER: Zhao Y

PROVIDER: S-EPMC8068848 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description:Meso tetraarylporphyrinato gold(iii) cations bearing different substituents at the aryl substituents (COOMe, COOH, NO2, NH2, NHAc, H, O n Bu, CF3) were prepared and characterised. Their reversible one-electron reductions were studied by (spectro)electrochemical means as well as by selective chemical one-electron reduction using cobaltocene. The preferred location of the spin density, namely gold centred or porphyrin centred, was probed by electron paramagnetic resonance spectroscopy (g values, 197Au hyperfine coupling) as well as by density functional theory calculations (spin densities). In all cases studied experimentally and theoretically, the gold(ii) valence isomer (5d9 electron configuration) is preferred over the porphyrin π radical anion. In the hexafluorophosphate salt of the nitro derivative a further nitro π radical anion valence isomeric species is significantly populated. In the presence of chloride ions this nitro π radical anion/AuII valence isomeric equilibrium evolves towards the porphyrin π radical anion. The electronic structures of the nitro π radical and the AuII σ radical valence isomers (5d x2-y2 orbital) could be calculated by DFT methods. The electron transfer pathway between the nitro π radical anion and the AuII valence isomer is well described by the location of the hexfluorophosphate counterion, the Au-N distances (corresponding to the totally symmetric stretching vibration), the symmetric stretching mode of the NO2 substituent and a meso-nitrophenyl rotation. The specific geometric and electronic properties of the favoured gold(ii) σ radical valence isomer, namely counterion dislocation and σ symmetry of the redox orbital, might stabilise charge-shifted states [(gold(ii) porphyrin)-donor˙+] by retarding the back electron transfer to give the ground state (gold(iii) porphyrin)-donor. This will guide the design of (photo-induced) electron transfer pathways with tetraarylporphyrinato gold(iii) complexes as electron acceptors.

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Tris(Butadiene) Compounds versus Butadiene Oligomerization in Second-Row Transition Metal Chemistry: Effects of Increased Ligand Fields.

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