Binding of Nitriles and Isonitriles to V(III) and Mo(III) Complexes: Ligand vs Metal Controlled Mechanism

Abstract

The synthesis and structures of nitrile complexes of V(N[^tBu]Ar)₃, 2 (Ar = 3,5-Me₂C₆H₃), are described. Thermochemical and kinetic data for their formation were determined by variable temperature Fourier transform infrared (FTIR), calorimetry, and stopped-flow techniques. The extent of back-bonding from metal to coordinated nitrile indicates that electron donation from the metal to the nitrile plays a less prominent role for 2 than for the related complex Mo(N[^tBu]Ar)₃, 1. Kinetic studies reveal similar rate constants for nitrile binding to 2, but the activation parameters depend critically on the nature of R in RCN. Activation enthalpies range from 2.9 to 7.2 kcal·mol^-1, and activation entropies from -9 to -28 cal·mol^-1·K^-1 in an opposing manner. Density functional theory (DFT) calculations provide a plausible explanation supporting the formation of a π-stacking interaction between a pendant arene of the metal anilide of 2 and the arene substituent on the incoming nitrile in favorable cases. Data for ligand binding to 1 do not exhibit this range of activation parameters and are clustered in a small area centered at ΔH^‡ = 5.0 kcal·mol^-1 and ΔS^‡ = -26 cal·mol^-1·K^-1. Computational studies are in agreement with the experimental data and indicate a stronger dependence on electronic factors associated with the change in spin state upon ligand binding to 1.

Scheme 1. Summary of Reactions of Complex 1 with Dinitrogen and Nitriles Ar = 3,5-Me₂C₆H₃; R′ = SiMe₃. Adapted from Ref (11). Copyright 2006 American Chemical Society

Figure 1

Solid-state structure of DFBN-2 (left) and Me₂NCN-2 (right) with thermal ellipsoids at 50% probability. Hydrogen atoms have been omitted for clarity. Selected distances (Å) and angles (degree). For DFBN-2: V1–N1 = 1.9439(15), V1–N2 = 1.9412(16), V1–N3 = 1.9416(16), V1–N4 = 2.0417(16), N4–C41 = 1.151(2), N1–V1–N4 = 99.89(6), N2–V1–N4 = 89.68(6), N3–V1–N4 = 98.71(6), V1–N4–C41 = 161.79(15); for Me₂NCN-2: V1–N1 = 1.9351(15), V1–N2 = 2.038(3), N2–C21 = 1.149(4), N1–V1–N1 = 116.49(3), N1–V1–N2 = 100.94(5), V1–N2–C21 = 180.0(4).

Figure 2

Solid-state structure of [PhCN-2][BAr₄^F] with thermal ellipsoids at 50% probability. Hydrogen atoms, [BAr₄^F]⁻, and interstitial diethyl ether are omitted for clarity. Selected bond lengths (Å) and angles (degree): V1–N4 = 2.0598(15), N4–C41 = 1.145(2), V1–N4–C41 = 172.84(15).

Figure 3

Time-resolved spectra of DFBN (1 mM) binding to 2 (0.3 mM) in toluene solution at −44 °C, acquired over 2 s. Selected traces are shown for clarity. The initially recorded spectrum is shown in black, and the final spectrum is shown in red.

Figure 4

Single-wavelength kinetic traces of aromatic RCN (1 mM) binding to 2 (0.3 mM) in toluene solution at −44 °C (DFBN and PhCN at λ = 687 nm; MesCN at λ = 705 nm).

Figure 5

Second-order rate plot for DFBN binding to 2 at various concentrations (1–10 mM) over a temperature range of −62 to −35 °C with [2]₀ = 0.3 mM.

Figure 6

Optimized structures at the PBE0-D3(BJ)/Def2-SV(P) level of theory⁻ of the two most stable configurations (in terms of ΔG (25 °C)) of 2 with the anilide ligands adopting a three-down (configuration A, left) or a one-down, two-up arrangement (configuration B, right). Hydrogen atoms are omitted for clarity.

Figure 7

DFT-Optimized structures of 2 interacting with a benzene molecule (in orange-brown color) at the PBE0-D3(BJ)/Def2-SV(P) level of theory.⁻ Hydrogen atoms are omitted for clarity.

Scheme 2. Interconversion between the A and B Configurations of Adducts of 2

Scheme 3. Thermochemical Values for the Reaction of the Most Stable Structure from Figure 7 and DFBN Containing a π-Stacking Interaction with a Benzene Molecule to Yield Complex DFBN-2 and the Most Stable Tilted T-Shape Benzene Dimer

Figure 8

Optimized structure of the transition state for binding MeCN to 2 in the A configuration. Hydrogen atoms are omitted for clarity. Selected distances (Å) and angles (degrees): V–N1 = 3.358; N1–C1 = 1.156 Å; V–N1–C1 = 146.8°.

Figure 9

Overlay of optimized configurations at fixed V···N_nitrile distances (from 2 to 3 Å) for nitrile dissociation from AdCN-2 (left) or DFBN-2 (right) showing a linear dissociation of AdCN and a more angular trajectory for DFBN. ^tBu groups, Me substituents in the aryl groups, and hydrogen atoms are omitted for clarity.

Figure 10

Energy (z-axis, kcal·mol^–1), V–N_nitrile distance (x-axis, Å), and V–N–C angle (y-axis, degrees) for dissociation of AdCN (left) and DFBN (right).

Figure 11

Plot of the center point distance between the top three C atoms of the arene of the anilide ligand and the bottom three C atoms of the arene of DFBN as a function of V···N distance.

Figure 12

Computed energies as a function of distance for binding of MeNC (blue squares) and MeCN (red triangles) to 1′ in the quartet (solid lines) and doublet (dashed lines) states.

Figure 13

Plot of −ΔS^‡ (cal·mol^–1·K^–1) vs ΔH^‡ (kcal·mol^–1) for binding of nitriles to 2 (left) and 1 (right). All data are for nitriles, except for AdNC (green diamond).