Transition Metal/Lanthanide-Nitrogen Double Bonds Co-stabilized in a Carbon Cage

Abstract

Metal-nitrogen double bonds have been commonly reported for conventional metal complexes, but the coexistence of both transition metal-nitrogen and lanthanide-nitrogen double bonds bridged by nitrogen within one compound has never been reported. Herein, by encapsulating a ternary transition metal-lanthanide heteronuclear dimetallic nitride into a C₈₄ fullerene cage, transition metal-nitrogen and lanthanide-nitrogen double bonds are costabilized simultaneously within the as-formed clusterfullerene TiCeN@C₁(12)-C₈₄, which is a representative heteronuclear dimetallic nitride clusterfullerene. Its molecular structure was unambiguously determined by single-crystal X-ray diffraction, revealing a slightly bent μ₂-bridged nitride cluster with short Ti-N (1.761 Å) and Ce-N (2.109 Å) bond lengths, which are comparable to the corresponding Ti=N and Ce=N double bonds of reported metal complexes and consistent with the theoretically predicted values, confirming their coexistence within TiCeN@C₁(12)-C₈₄. Density functional theory (DFT) calculations unveil three-center two-electron (3c-2e) bonds delocalized over the entire TiCeN cluster, which are responsible for costabilization of Ti=N and Ce=N double bonds. An electronic configuration of Ti⁴⁺Ce³⁺N^3-@C₈₄ ^4- is proposed featuring an intramolecular four-electron transfer, drastically different from the analogous actinide dimetallic nitride clusterfullerene (U₂)⁹⁺N^3-@C₈₀ ^6- and trimetallic nitride clusterfullerene (Sc₂)⁶⁺Ti³⁺N^3-@C₈₀ ^6-, indicating the peculiarity of 4-fold negatively charged fullerene cage in stabilizing the heteronuclear dimetallic nitride cluster.

Scheme 1. Schematic Illustration of Difference on the Bonding Nature and Electronic Structures of Trimetallic Nitride Clusterfullerene (NCF) Commonly Reported Previously and the First Heteronuclear Dimetallic Nitride Clusterfullerene Discovered in This Work

Figure 1

(a) Drawing of the crystallographically determined structure of TiCeN@C₁(12)-C₈₄·2DPC. Only the major Ti/Ce sites and the major carbon cage are shown. Solvent molecules and H atoms have been omitted for clarity. (b) Molecular structure of TiCeN@C₁(12)-C₈₄. (c) View of the major TiCeN cluster with respect to the closest cage portion within TiCeN@C₁(12)-C₈₄. Gray: C; blue: N; green: Ce; cyan: Ti.

Figure 2

Cluster-based LMOs of TiCeN@C₁(12)-C₈₄ showing (a) a Ce–N dative bond arising from the N lone pair electron; (b) a delocalized 3c-2e σ bond (c); (d) two delocalized 3c-2e π bonds; (e) a singly occupied Ce 4f orbital; and (f) the spin density distribution localized on Ce.

Figure 3

(a) UV–vis–NIR absorption spectrum of TiCeN@C₁(12)-C₈₄ dissolved in toluene along with the photograph shown in the inset. (b) Cyclic voltammogram (top) and differential pulse voltammogram (bottom) of TiCeN@C₁(12)-C₈₄ in o-DCB solution in different scanning regions showing the correlation of each reduction/oxidation step. Scan rate: 100 mV/s, TBAPF₆ as supporting electrolyte. The half-wave potentials (E_1/2) of each redox step are marked with a solid dotted line to aid comparison. Ferrocene (Fc) was added as the internal standard of which the oxidation peak is labeled by the asterisk.