Steric Influence on the Structure, Magnetic Properties, and Reactivity of Hexa- and Octaisopropylmanganocene

Abstract

The reaction of KCpni (Cpni = (C3H7) n C5H5 - n ; n = 3, 4) with MnCl2 in THF produces the metallocenes (Cp3i)2Mn or (Cp4i)2Mn in good yield. Solid-state magnetic susceptibility measurements indicate that orange-red (Cp3i)2Mn is in high-spin/low-spin equilibrium at room temperature. In the solid-state, it has a centrosymmetric sandwich structure with an average Mn−C bond length (2.130(6) Å) typical of low-spin manganocenes. (Cp3i)2Mn reduces tetracyanoethylene (TCNE) to generate the TCNE radical anion. Despite the presence of additional electron-donating isopropyl groups, pale yellow (Cp4i)2Mn is entirely high-spin from room temperature to 10 K. The crystal structure of (Cp4i)2Mn reveals a bent metallocene structure, with a centroid−Mn−centroid angle of 167.4° and an average Mn−C distance of 2.42(2) Å. This structure is consistent with its high-spin electron configuration; the longer Mn−C bonds provide relief from the steric strain that would be present in a low-spin complex. The reaction of (Cp4i)2Mn with TCNE produces a tricyanovinylation product, C5(i-Pr)4HC(CN)C(CN)2, which was characterized by X-ray diffraction.

Electronic and Steric Control of the Spin-Crossover Behavior in [(CpR)2Mn] Manganocenes

“…Thus, the energy of those orbitals goes down, and the progressive functionalization of the [(Cp n ‑iPr ) 2 Mn] and [(Cp n ‑tBu ) 2 Mn] families moves us from SCO molecules to high-spin systems. Indeed, the [(Cp 1,2,3,4‑iPr ) 2 Mn] system has been characterized as a high-spin system. , Although the computed values agree with the experimentally observed trend in solution, for some systems the solid-state data provide spin-crossover temperatures that move in the opposite direction. Experimentally, the functionalization from [(Cp 1‑tBu ) 2 Mn] to [(Cp 1,3‑tBu ) 2 Mn] leads to a decrease in the spin-crossover temperature when measurements are done in solution, but the opposite trend (an increase of the T 1/2 ) is observed for solid-state data.…”

Structural, Electrochemical, and Magnetic Studies of Bulky Uranium(III) and Uranium(IV) Metallocenes

“…We had simultaneously been interested in studying complexes featuring the (Cp iPr4 ) 2 U moiety (Cp iPr4 = tetra­(isopropyl)­cyclopentadienyl), as the Cp iPr4 ligand is quite sterically encumbering but less bulky than Cp iPr5 , which should enable access to a wide variety of mononuclear complexes without the need for additional donor ligands, while also promoting more facile substitution of equatorial ligands. While metallocene complexes featuring Cp iPr4 have been prepared with transition metals, − s- and p-block metals, − and the lanthanides, − the only reported example with uranium is (Cp iPr4 )­U­(BH 4 ) 3 , which has not been structurally characterized . As an initial demonstration of the utility of the Cp iPr4 ligand in uranium chemistry, we were interested in targeting dihalide series for both uranium­(III) and uranium­(IV).…”

Mn2bis(pentalene): a mixed-spin bimetallic with two extremes of bonding within the same molecule

“…[2][3][4][5] Molecular manganocene displays a partial spin-crossover at low temperatures, and further work substituting the rings with bulky or electron-donating substituents has created a whole series of both partial and complete spin-crossover compounds. [6][7][8][9] Mn(C 5 Me 5 ) 2 is a low-spin metallocene, with shorter Mn-C bonds, lower reactivity and no observed ligand exchange. 10,11 Recent work has also explored the supramolecular chemistry of the [Mn(C 5 H 5 ) 3 ] 2 anion.…”