High-Spin and Spin-Crossover Behavior in Monomethylated Bis(indenyl)chromium(II) Complexes

Abstract

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.

Indenyl Complexes of Manganese(II). Conformational Flexibility of the Manganese(II)−(RnC9H7−n) Bond

“…The average Mn−C distance of 2.409(3) Å is comparable to those found in high-spin Cp′ 2 Mn complexes such as Cp 2 Mn (2.380(6) Å) and [(Me 3 Si)C 5 H 4 ] 2 Mn (2.375(6) Å) . The slip parameter for the Mn−C distances (Δ Mn−C = 0.14 Å) is substantially larger than that found in bis(indenyl)iron complexes (i.e., Δ Fe−C < 0.07 Å). ,− It is, however, the same as in η 5 -(1-MeC 9 H 6 ) 2 Cr) and could be considered “distorted η 5 ” . The slippage is not large enough to consider the metal−ring bonding as significantly less than η 5 (as in η 3 -(C 9 H 7 ) 2 Ni, with Δ Ni−C = 0.44 Å).…”

“…It remains to be seen whether bis(indenyl)manganese(II) complexes are suitable for incorporation into charge transfer complexes. The resulting d 4 Mn(III) centers may be able to replicate some of the rich magnetic behavior observed for their Ind′ 2 Cr(II) counterparts. ,, …”

Synthesis and Complexation Behavior of Indenyl and Cyclopentadienyl Ligands Functionalized with a Naphthyridine Unit

“…The dimeric complex (indenyl) 3 Cr 2 Cl A and its derivative (indenyl) 4 Cr 2 B are representatives of a chromium species (Figure ). They are available by reduction of chromium­(III) chloride with sodium indenide, followed by gradual substitution of chloride with an indenyl group . Complexes representing palladium are the dimeric species (μ,η 3 -Ind)­(μ-Cl)­Pd 2 (PR 3 ) 2 (R = Cy, Ph) C , which are accessible via a reversible conproportionation of Ind-Pd­(II) and Pd(0)­(PR 3 ) n species .…”

Tunable Spin-Crossover Behavior in Polymethylated Bis(indenyl)chromium(II) Complexes: The Significance of Benzo-Ring Substitution

“…This is the most straightforward case, as the molecule is minimally substituted, adopts a staggered conformation in the solid state, and is high spin both in solution and in the solid state at all temperatures. 14 (Ind Me-1 ) 2 Cr. Although the molecule exhibits spin-crossover behavior in the solid state, this is certainly the result of its eclipsed configuration, which is dictated by crystal-packing forces.…”

“…Other than the fully methylated complex, 11 the only attempt to prepare a complex with substitution on the benzo portion led to the isolation of the dinuclear (Ind Me-4 ) 3 Cr 2 Cl complex. 14 We report here a study of polymethylated Ind′ 2 Cr complexes that further illuminates the interrelationship among ligand conformation, electron donation, and substitution patterns on the magnetic properties of the compounds. Such knowledge is valuable in the pursuit of tunable magnetism in systems such as charge-transfer salts 15 and in predicting and controlling metal center reactivity.…”