A Thorough Characterization of the Tellurocyanate Anion

Abstract

Tellurocyanate, [TeCN]^-, is the heaviest group 16 congener of the cyanate anion, [OCN]^-. Due to the relative instability of the C─Te bond, tellurocyanate chemistry has seen only scarce attention. Here, we present the facile synthesis and thorough characterization of [K@crypt-222][TeCN]. The anion is essentially linear with interatomic distances C─N = 1.150(6)Å and C─Te = 2.051(4)Å, thus approximating a C≡N triple bond and for C─Te a bond order between 1 and 2. Fully ¹³C and ¹⁵N labeled [Te¹³C¹⁵N]^- allowed for the extraction of chemical shifts and all possible coupling constants (¹³C = 77.8 ppm, ¹⁵N = 285.7 ppm, ¹²⁵Te = -566 ppm, ¹J_13C-15N = 8 Hz, ¹J_13C-125Te = 748 Hz, ²J_15N-125Te = 55 Hz), which were also determined independently by quantum chemical calculations. In the series [ChCN]^- (Ch = O─Te), [TeCN]^- shows the strongest spin-orbit coupling (SOC) induced heavy-atom effect on the light-atom shielding (SO-HALA-effect). In contrast, ¹⁵N shifts are also well described without considering relativistic effects and/or SOC. Negative-ion photoelectron spectroscopy was used to extract the electron affinity (EA = 3.034 eV) and spin-orbit splitting (3807 cm^-1) of [TeCN]^•. These values continue the trends of falling EA and rising SOC in the series [ChCN]^•.

Keywords: Bonding analysis; Cyanates; DFT; Negative ion photoelectron spectroscopy; Nuclear magnetic resonance.

Figure 1

Synthesis of [K@crypt‐222][TeCN] (left). Bulk sample of [K@crypt‐222][TeCN] (right).

Figure 2

Molecular structure of [K@crypt‐222][TeCN] in the single crystal drawn with 50% displacement ellipsoids at 100K. Hydrogen atoms are omitted for clarity. Color code: C white, N blue, O red, K purple, and Te grey.

Figure 3

Images of localized molecular valence orbitals (LMOs) of [ChCN]⁻ for the C─N bond a) and the Ch–C bond b). For both bonds, images are shown for Ch = O and Ch = Te, contours are drawn at +/‐ 0.07 a.u. The diagrams left of the LMO images show WBI, also separated into σ and π contributions, for Ch = O, S, Se, Te.

Figure 4

Chemical shifts of [Te¹³C¹⁵N]⁻ in the ¹³C a), ¹⁵N b), and ¹²⁵Te c) NMR spectra in ppm. ¹³C Shifts are referenced to (CH₃)₄Si, ¹⁵N are referenced to NH₃ and ¹²⁵Te to Me₂Te. ¹²³Te and ¹²⁵Te Satellites are shown enlarged. Couplings and their respective values within [Te¹³C¹⁵N]⁻ are: ¹ J _13C‐125Te = 748 Hz, ¹ J _13C‐123Te = 621 Hz, ² J _15N‐125Te = 55 Hz, and ¹ J _13C‐15N = 8 Hz.

Figure 5

Calculated and measured chemical shifts for ¹³C (w.r.t. TMS, left) and ¹⁵N (w.r.t. NH₃, right) in [ChCN]⁻, Ch = O, S, Se, Te. Asterisks represent measurements, filled circles non‐relativistic calculations, colorless cycles scalar relativistic calculations (i.e., neglecting SOC), rhombi relativistic calculations with SOC. The latter can be decomposed to their contributions from the unperturbed density (filled triangles) and from the magnetic response density (open triangles), which both can be further decomposed into spin‐orbit‐free parts (green) and spin‐orbit parts (red).

Figure 6

T = 20 K NIPE spectra of the [TeCN]^– anion recorded with 370 nm (3.351 eV, a) and 300 nm (4.133 eV, b) photodetachment wavelengths.