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

Abstract

The magnetic properties of monomeric bis(indenyl)chromium(II) complexes are sensitive to the relative orientation of the ligands (staggered, eclipsed, or gauche), regardless of whether such a conformation is imposed by sterically bulky substituents or is a consequence of crystal-packing forces. The staggered monomethylated compound (2-MeC 9 H 6 ) 2 Cr has previously been found to display a high-spin state in the solid state and solution, whereas the monomeric (1-MeC 9 H 6 ) 2 Cr is eclipsed in the solid state and exhibits spin-crossover behavior over a wide temperature range. The present work examines the synthesis, structures, and magnetic properties of polymethylated indenyl complexes containing two, three, and five substituents on the indenyl ligand. Methyl groups on the benzo portion appear to be critical in supporting a low-spin ground state; the order of the preference to exist as a low-spin complex is approximately (2,4,5,6,7-Me 5 C 9 H 2 ) 2 Cr > (2,4,7-Me 3 C 9 H 4 ) 2 Cr ≈ (4,7-Me 2 C 9 H 5 ) 2 Cr > (1,2,3-Me 3 C 9 H 4 ) 2 Cr > (1-MeC 9 H 6 ) 2 Cr g (2-MeC 9 H 6 ) 2 Cr. The (2,4,7-Me 3 C 9 H 4 ) 2 Cr complex crystallizes with two conformations in the unit cell: one with an eclipsed geometry and the other staggered. When the temperature is raised from 173 to 293 K, the staggered form undergoes approximately 3 times the bond length increase as the eclipsed form. The difference is attributed to the symmetry-driven preference of the staggered configuration for the high-spin state.

Synthesis and Reactivity Studies of Benzo-Substituted Bis(indenyl) Iron and Zirconium Complexes: The Difference a Methyl Group Can Make

“…The steric and electronic effects of benzo ligand substitution on metal reactivity have been examined to some extent. , With group 4 metal catalysts active for olefin polymerization, electron-withdrawing groups tend to decrease catalyst activity and polymer molecular weight, while electron-rich groups have little effect on catalyst or subsequent polymer production. , Structural studies of the effect of permethylation of indenyl , and fluorenyl complexes of both early and late transition metals suggest the groups impart an increase of electron density, as expected. Reports of selective methyl group appendage on indenyl ligands have also appeared involving both Cr and Fe, demonstrating that the spin state and ligand electronics can be modulated considerably on the basis of the position of the substituents. Recently, we have found methyl substitutions of the Cp or benzo rings of bis(indenyl) Co or Zr complexes suggest that steric effects are most pronounced when groups are placed on the five-membered ring, while benzo substitution can attenuate the electron-donating ability of the methyl group .…”

“…37,38 Structural studies of the effect of permethylation of indenyl 39,40 and fluorenyl 41 complexes of both early and late transition metals suggest the groups impart an increase of electron density, as expected. Reports of selective methyl group appendage on indenyl ligands have also appeared involving both Cr 42 and Fe, 43 demonstrating that the spin state and ligand electronics can be modulated considerably on the basis of the position of the substituents. Recently, we have found methyl substitutions of the Cp or benzo rings of bis(indenyl) Co or Zr complexes suggest that steric effects are most pronounced when groups are placed on the five-membered ring, while benzo substitution can attenuate the electrondonating ability of the methyl group.…”

Stabilization of η3-indenyl compounds by sterically demanding N,N-chelating ligands in the molybdenum coordination sphere

“…[25][26][27][28][29] The kinetic "indenyl effect" has been comprehensively scrutinized on various molybdenum compounds by 4 ] is unusually stable, while the indenyl analogue undergoes a spontaneous and very fast ringopening to [(h 5 -Ind)(h 5 -C 5 H 4 CH 2 -h 1 -CH 2 )Mo(CO)] [BF 4 ]. 36,37 Results and discussion Reactions of [{(h 5 -4,7-Me 2 C 9 H 5 )Mo(CO) 2 (m-Cl)} 2 ] (2) with py, bpy, phen and pyma Reaction of [{(h 5 -4,7-Me 2 C 9 H 5 )Mo(CO) 2 (m-Cl)} 2 ] (2) with pyridine (py) gives a monomeric compound with h 5 -bonded indenyl ligand [(h 5 -4,7-Me 2 C 9 H 5 )Mo(CO) 2 (py)Cl] (3), see Scheme 2. It will be shown that a steric congestion of the chelating ligand is critical for the stability of [(h 3 -4,7-Me 2 C 9 H 5 )Mo(CO) 2 ( N,N L)Cl] and could be used for the control of the indenyl ring slippage.…”

“…22 This substitution pattern usually has only negligible effect on the reactivity due to subtle steric and electronic effects of two methyl groups in the C 6ring. 36,37…”

Addition of pnictogen atoms to chromium(ii): synthesis, structure and magnetic properties of a chromium(iv) phosphide and a chromium(iii) arsenide

“…The spin congurations of rst-row transition metal Cp complexes can be very sensitive to the ligand substituents, particularly in the case of manganese(II) 8 and, to a lesser extent, chromium(II). 9 Chromocene itself has a low-spin, S ¼ 1 conguration, 10 but substituting a [Cp] À ligand by [(Me 3 Si) 2 E] À to give [CpCr{m-E(SiMe 3 ) 2 }] 2 (E ¼ P or As) evidently weakens the ligand eld sufficiently to give high-spin S ¼ 2 chromium(II). 6 In an attempt to induce thermal spin crossover in [CpCr{m-E(SiMe 3 ) 2 }] 2 by forcing the ground state of the metal to switch from S ¼ 2 to S ¼ 1 with the aid of a stronger ligand eld, we have targeted the analogous pentamethyl-cyclopentadienyl (Cp*) dimers [(Cp*)Cr{m-E(SiMe 3 ) 2 }] 2 .…”