Spin-State Alteration from Sterically Enforced Ligand Rotation in Bis(indenyl)chromium(II) Complexes¹

Abstract

The rotational orientation of cyclopentadienyl rings usually has no effect on d-orbital energy levels and splitting in transition metal complexes. With related but less symmetrical carbocyclic ligands, however, the magnetic properties of the associated complexes can be altered by the alignment of the ligands. Examples of this effect are found in substituted organochromium(II) bis(indenyl) complexes. The monosubstituted compounds (1-RC(9)H(6))(2)Cr (R = t-Bu, SiMe(3)) are prepared from the substituted lithium indenides and CrCl(2) in THF; they are high-spin species with four unpaired electrons. Their spin state likely reflects that in the unknown monomeric (C(9)H(7))(2)Cr, which is calculated to have a high-spin (S = 2) ground state in the staggered configuration (180 degrees rotation angle). However, the analogous bis(indenyl) complexes containing t-Bu or SiMe(3) groups in both the 1 and 3 positions on the indenyl ligands ((1,3-R(2)C(9)H(5))(2)Cr) are low-spin compounds with two unpaired electrons. X-ray diffraction results indicate that [1-(t-Bu)C(9)H(6)](2)Cr exists in a staggered conformation, with Cr-C (av) = 2.32(4) A. In contrast, the average Cr-C distances in [1,3-(t-Bu or SiMe(3))(2)C(9)H(5)](2)Cr are 2.22(2) and 2.20(2) A, respectively, and the rings are in a gauche configuration, with rotation angles of 87 degrees. The indenyl conformations are sterically imposed by the bulk of the t-Bu and SiMe(3) substituents. The change from a staggered to a gauche indenyl orientation lowers the symmetry of a (C(9)H(7))(2)M complex and allows greater mixing of metal and ligand orbitals. Calculations indicate that previously nonbonding pi orbitals of the indenyl anion are able to interact with the chromium d orbitals, producing bonding and antibonding combinations. The latter remain unpopulated, and the resulting increase in the HOMO-LUMO gap forces the complexes to adopt a low-spin configuration. The possibility of using sterically imposed ligand rotation as a means of spin-state manipulation makes indenyl compounds a potentially rich source of magnetically adjustable molecules.

Zirconium Bis(indenyl) Sandwich Complexes with an Unprecedented Indenyl Coordination Mode and Their Role in the Reactivity of the Parent Bent‐Metallocenes: A Detailed DFT Mechanistic Study

“…The validity of the theoretical model used may be crucial, since the electronic and steric influence of the substituents on the C b/b' atoms of indenyl, present in the experimental study, may change considerably the relative energies of the relevant species, such as different isomers, transition states or MECP. For example, in a recent work, Hanusa et al [35] found that SiMe 3 or tBu on those same positions of Ind change the preferred conformation of [Cr(Ind) 2 ], favouring a low spin state.…”

Synthetic Explorations Towards Sterically Crowded 1,2,3‐Substituted Bis(indenyl)zirconium(IV) Dichlorides

“…tection of 2-methylhexane (0.3 parts). Deprotonation of 4 with potassium hydride in refluxing THF failed to provide a well-defined, insoluble potassium salt (K [5]). Due to the solubility of these salts, washing with pentane was accompanied by a loss of material.…”

“…A compelling explanation for these solubilities can be derived from a quenching experiment of Li [5] with methyl iodide in THF. GC/MS analysis of the reaction products revealed ten major products, of which one could be identified as 1-tBu-2-TMS-Ind.…”

“…GC/MS analysis of the reaction products revealed ten major products, of which one could be identified as 1-tBu-2-TMS-Ind. Apparently Li [5] equilibrates between its indenyllithium and alkyllithium derivative, i.e. the lithium migrates from the Cp moiety of the indenyl through the iPr group resulting in a covalent lithium carbon bond.…”

Catalysis and Organometallic Chemistry of Monometallic Species

“…116 The bis(indenyl) chromium() complex 13 has a high spin state whereas 14 exhibits a low spin state due to restricted rotation of the indenyl rings allowing the possibility of using restricted rotation as a means of spin state manipulation in these systems. 117 The structural effect due to Jahn-Teller distortion in the high spin chromium() complex [Cr(η 5 -C 5 Me 5 )(Tp)] has also been studied. 118 DFT, NMR spectroscopy, and X-ray crystallography have been used to investigate the bonding in the unusual trigonal prismatic structure of tris(cis-1,3-butadiene)-molybdenum 15 and related complexes with the conclusion that metallocyclopentene resonance structures rather than π-bonded di-olefins offer a more accurate representation of the bonding.…”