No abstract

Bis(cyclopentadienyl) complexes (Cp 2 M) of the divalent first-row transition metals V-Ni have been known and used for over 50 years. For almost as long, an analogous series of compounds has been known with the indenyl ligand (i.e., Ind 2 M), with the conspicuous exception of M = Mn. Bis(indenyl) complexes of manganese(II), Ind 0 2 MnL n , have now been synthesized by halide metathesis from MnCl 2 and an appropriate potassium indenide. Depending on the indenyl ligand substituents and the presence of coordinated bases, a variety of structural motifs and bonding modes of the indenyl ligand are found in the resulting complexes. Single-crystal X-ray structures obtained for [2-(SiMe 3 )C 9 H 6 ] 2 Mn, [1,3-(SiMe 3 ) 2 C 9 H 5 ] 2 Mn, and [1,3-(i-Pr) 2 C 9 H 5 ] 2 Mn confirm that they possess classic η 5 -bound sandwich structures. In contrast, the unsubstituted parent complex recrystallizes from THF as a disolvate with two differently bonded indenyl ligands: i.e., (η 3 -C 9 H 7 )(η 1 -C 9 H 7 )Mn(thf ) 2 . Without the coordinated solvent, density functional theory calculations suggest that the complex would have two slipped η 5 -bound ligands. When methyl groups are present on the benzo portion of the indenyl ligand, specifically in the 4,7-positions, the corresponding manganese complex is isolated as a cyclic octomer, {(4,7-Me 2 C 9 H 5 ) 2 Mn} 8 , containing both bridging and terminal indenyl ligands. In the presence of 1,4-dioxane, however, attempted synthesis of (4,7-Me 2 C 9 H 5 ) 2 Mn results in the isolation of the [K(dioxane) 1.5 ][Mn(4,7-Me 2 C 9 H 5 ) 3 ] salt, in which each manganese atom is surrounded by a paddlewheel of three η 2 -bound 4,7-dimethylindenyl ligands. Cation-π bonding to the potassium and the presence of coordinated dioxane molecules generates a layered structure for the salt. Magnetic susceptibility measurements on the compounds indicate the presence of high-spin Mn(II) centers in all cases. These compounds demonstrate the high degree of conformational flexibility in the Mn(II)-indenyl bond.

Previous work on bis(indenyl)chromium(II) complexes substituted with bulky groups (i-Pr, t-Bu, SiMe 3 ) found that their spin state (S ) 1 or 2) depended on the symmetry of the molecules. Complexes with inversion symmetry (staggered rings) were high-spin; lower symmetry compounds with twisted (gauche) ligands were low spin. The present work explores the effect of methyl group substitution on the indenyl ligand, which leads to complexes possessing either staggered or eclipsed conformations. The monosubstituted compounds [(1 or 2)-MeC 9 H 6 ] 2 Cr are prepared from the substituted alkali metal indenides and CrCl 2 in THF. X-ray diffraction results indicate that (2-MeC 9 H 6 ) 2 Cr exists in a staggered conformation, with Cr-C (av) ) 2.308(7) Å, and is a high-spin species in the solid state and solution. In contrast, the monomeric (1-MeC 9 H 6 ) 2 Cr is eclipsed in the solid state, where it exhibits spin-crossover behavior over a wide temperature range; the average Cr-C distances vary with temperature, from 2.179(9) Å at 105 K to 2.262(10) Å at 298 K. An attempt to form (4-MeC 9 H 6 ) 2 Cr produced the dimeric, thermally unstable complex (η 5 -indenyl′) 2 (µ,η 3 -indenyl′)Cr 2 (µ-Cl) instead. Correlations between the structure and magnetic properties in bis(indenyl)chromium(II) complexes have been made with density functional theory calculations, which indicate that an eclipsed ligand conformation supports a high spin state, but not to the extent that the staggered form does.