Spin-State Alteration from Sterically Enforced Ligand Rotation in Bis(indenyl)chromium(II) Complexes
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- 作者：Erik D. Brady ; Jason S. Overby ; M. Brett Meredith ; Adam B. Mussman ; Michael A. Cohn ; Timothy P. Hanusa ; Gordon T. Yee ; and Maren Pink
- 刊名：Journal of the American Chemical Society
- 出版年：2002
- 出版时间：August 14, 2002
- 年：2002
- 卷：124
- 期：32
- 页码：9556 - 9566
- 全文大小：343K
- 年卷期：v.124,no.32(August 14, 2002)
- ISSN：1520-5126
文摘
The rotational orientation of cyclopentadienyl rings usually has no effect on d-orbital energylevels 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 theligands. Examples of this effect are found in substituted organochromium(II) bis(indenyl) complexes. Themonosubstituted compounds (1-RC9H6)2Cr (R = t-Bu, SiMe3) are prepared from the substituted lithiumindenides and CrCl2 in THF; they are high-spin species with four unpaired electrons. Their spin state likelyreflects that in the unknown monomeric (C9H7)2Cr, which is calculated to have a high-spin (S = 2) groundstate in the staggered configuration (180
rotation angle). However, the analogous bis(indenyl) complexescontaining t-Bu or SiMe3 groups in both the 1 and 3 positions on the indenyl ligands ((1,3-R2C9H5)2Cr) arelow-spin compounds with two unpaired electrons. X-ray diffraction results indicate that [1-(t-Bu)C9H6]2Crexists in a staggered conformation, with Cr-C (av) = 2.32(4) Å. In contrast, the average Cr-C distancesin [1,3-(t-Bu or SiMe3)2C9H5]2Cr are 2.22(2) and 2.20(2) Å, respectively, and the rings are in a gaucheconfiguration, with rotation angles of 87. The indenyl conformations are sterically imposed by the bulk ofthe t-Bu and SiMe3 substituents. The change from a staggered to a gauche indenyl orientation lowers thesymmetry of a (C9H7)2M complex and allows greater mixing of metal and ligand orbitals. Calculations indicatethat previously nonbonding 
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 increasein the HOMO-LUMO gap forces the complexes to adopt a low-spin configuration. The possibility of usingsterically imposed ligand rotation as a means of spin-state manipulation makes indenyl compounds apotentially rich source of magnetically adjustable molecules.
rotation angle). However, the analogous bis(indenyl) complexescontaining t-Bu or SiMe3 groups in both the 1 and 3 positions on the indenyl ligands ((1,3-R2C9H5)2Cr) arelow-spin compounds with two unpaired electrons. X-ray diffraction results indicate that [1-(t-Bu)C9H6]2Crexists in a staggered conformation, with Cr-C (av) = 2.32(4) Å. In contrast, the average Cr-C distancesin [1,3-(t-Bu or SiMe3)2C9H5]2Cr are 2.22(2) and 2.20(2) Å, respectively, and the rings are in a gaucheconfiguration, with rotation angles of 87. The indenyl conformations are sterically imposed by the bulk ofthe t-Bu and SiMe3 substituents. The change from a staggered to a gauche indenyl orientation lowers thesymmetry of a (C9H7)2M complex and allows greater mixing of metal and ligand orbitals. Calculations indicatethat previously nonbonding 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 increasein the HOMO-LUMO gap forces the complexes to adopt a low-spin configuration. The possibility of usingsterically imposed ligand rotation as a means of spin-state manipulation makes indenyl compounds apotentially rich source of magnetically adjustable molecules.