Spin-Flow Vibrational Spectroscopy of Molecules with Flexible Spin Density: Electrochemistry, ESR, Cluster and Spin Dynamics, and Bonding in TiSc₂N@C₈₀

Abstract

The recently isolated TiSc(2)N@C(80) was used to study the spin state of a Ti(3+) ion in a mixed metal nitride cluster in a fullerene cage. The electronic state of the new clusterfullerene is characterized starting with the redox behavior of this structure. It differs markedly from that of homometallic nitride clusterfullerenes in giving reversible one-electron transfers even on the cathodic scale. Both oxidation and reduction of TiSc(2)N@C(80) occur at the endohedral cluster changing the valence state of Ti from Ti(II) in anion to Ti(IV) in cation. The unpaired electron in TiSc(2)N@C(80) is largely fixed at the Ti ion as shown by low temperature ESR measurements. Isotropic g-factor 1.9454 points to the significant spin-orbit coupling with an unquenched orbital momentum of the 3d electron localized on Ti. Measurements with the frozen solution also point to the strong anisotropy of the g-tensor. DFT computations show that the cluster can adopt several nearly isoenergetic configurations. DFT-based Born-Oppenheimer molecular dynamics (BOMD) simulations reveal that, unlike in Sc(3)N@C(80), the cluster dynamics in TiSc(2)N@C(80) cannot be described as a 3D rotation. The cluster rotates around the Ti-N axis, while the Ti atom oscillates in one position around the pentagon/hexagon edge. Evolution of the spin populations along the BOMD trajectory has shown that the spin distribution in the cluster is very flexible, and both an intracluster and cluster-cage spin flows take place. Fourier transformation of the time dependencies of the spin populations results in the spin-flow vibrational spectra, which reveal the major spin-flow channels. It is shown that the cluster-cage spin flow is selectively coupled to one vibrational mode, thus, pointing to the utility of the clusterfullerene for the molecular spin transport. Spin-flow vibrational spectroscopy is thus shown to be a useful method for characterization of the spin dynamics in radicals with flexible spin density distribution.

Endohedral fullerene with μ3-carbido ligand and titanium-carbon double bond stabilized inside a carbon cage

“…Deeper insight into the molecular and electronic structure of TiLu 2 C@I h -C 80 was obtained by DFT computations and topological analysis of the electronic density using Quantum Theory of Atoms in Molecules (QTAIM) 49 . In agreement with the fast rotation of the cluster that was revealed by 13 C NMR spectroscopy, computations showed the existence of several orientational conformers of the TiLu 2 C cluster inside the I h -C 80 cage in the energy range of 0-10 kJ mol À 1 (see Supplementary Fig. 3 and Supplementary Table 2).…”

“…3 and Supplementary Table 2). A similar situation is also found for group III nitride clusterfullerenes (for example, Lu 2 ScN@I h -C 80 ) [10][11][12]41 in contrast to TiM 2 N@I h -C 80 (M ¼ Sc, Y, Lu), which show more enhanced dependence of the relative energy on the cluster position [13][14][15] . The DFT predicted Ti ¼ C bond length in TiLu 2 C@I h -C 80 is 1.836 Å, B0.04 Å shorter than experimental value.…”

“…The electrochemical behaviour of TiLu 2 C@I h -C 80 at the first reduction step is similar to that of other Ti-based EMFs, such as diamagnetic Ti 2 S@C 78 , paramagnetic TiSc 2 N@C 80 and TiY 2 N@C 80 , which exhibit reversible reductions at À 0.92, À 0.94 and À 1.11 V, respectively, and all EMFs are characterized by the reduction localized at the Ti atom 13,15,19 . Note also that the Ti(III)/Ti(IV) oxidation potential of Tialkylidene (Nacnac)Ti(CH 2 t Bu) 2 in THF reported by Mindiola…”

“…2a for its mass-spectrum) enabled the elucidation of its molecular structure. Two lines in a 3:1 intensity ratio in the 13 Unfortunately, even after accumulation of the spectrum over two weeks, the signal of the central carbon atom could not be observed. The molecular structure of TiLu 2 C@I h -C 80 was obtained by single-crystal X-ray diffraction as shown in Fig.…”

“…The nitride cluster works as a template to enforce a trivalent state of an endohedral Ti atom, which can be then oxidized or reduced to the Ti IV and Ti II states 13 . In addition to the Ti-NCFs, the only known Ti-EMFs are Ti@C 28 (seen only in the gas phase so far) 16 , the carbides Ti 2 C 2 @C 78 and Ti 2 C 2 @C 82 (refs 17,18), and a sulphide Ti 2 S@C 78 (ref.…”