Facile Synthesis of a Stable Side-on Phosphinyne Complex by Redox Driven Intramolecular Cyclisation

Abstract

Alkyne complexes with vicinal substitution by a Lewis acid and a Lewis base at the coordinated alkyne are prospective frustrated Lewis pairs exhibiting a particular mutual distance and, hence, a specific activation potential. In this contribution, investigations on the generation of a W^II alkyne complex bearing a phosphine as Lewis base and a carbenium group as Lewis acid are presented. Independently on potential substrates added, an intramolecular cyclisation product was always isolated. A subsequent deprotonation step led to an unprecedented side-on λ⁵ -phosphinyne complex, which is interpreted as highly zwitterionic according to visible absorption spectroscopy supported by TD-DFT. Low-temperature ³¹ P NMR and EPR spectroscopic measurements combined with time-dependent IR-spectroscopic monitoring provided insights in the mechanism of the cyclisation reaction. Decomposition of the multicomponent IR spectra by multivariate curve resolution and a kinetic hard-modelling approach allowed the derivation of kinetic parameters. Assignment of the individual IR spectra to potential intermediates was provided by DFT calculations.

Keywords: alkyne complex; cyclisation mechanism; frustrated Lewis pair; non-innocent ligand; phosphinyne complex.

Figure 1

Heteroaryne complexes: thiophyne complex A, phosphinyne complex B and targeted side‐on complexes of phosphine carbenium substituted alkynes C.

Scheme 1

Proposed generation of an intramolecular phosphine carbenium combination at a coordinated alkyne.

Scheme 2

Synthesis of η²‐C,C’ phosphino propargyl complexes with iodide and cyanide as co‐ligands.

Figure 2

Molecular structure of 3 c in the crystal (50 % thermal ellipsoids). With the exception of OH hydrogen atoms have been omitted for clarity. Selected bond lengths (Å) and angles (°):W1−C1 2.055(3), W1−C2 2.059(2), W1−C3A 1.972(3), P1−C1 1.785(3), C1−C2 1.309(4), C2−C16 1.515(4), P1−C16 3.866, C2‐C1‐P1 141.0(2), C1‐C2‐C16 140.3(2).

Scheme 3

Reaction of complexes 3 a–c with one equivalent of a strong acid.

Figure 3

Molecular structure of 4 a in the crystal (50 % thermal ellipsoids). Hydrogen atoms omitted for clarity. Selected bond lengths (Å) and angles (°): 4 a: W1−C1 2.035(4), W1−C2 2.047(4), W1−C3 1.952(4), P1−C1 1.795(4), C1−C2 1.322(6), C2−C16 1.468(5), C16−C21A 1.336(17), C16−C17A 1.510(15), C2‐C1‐P1 129.7(3), C1‐C2‐C16 136.5(4).

Figure 4

Molecular structure of 4 b‐B(C₆F₅)₄ in the crystal (50 % thermal ellipsoids). Hydrogen atoms and anion have been omitted for clarity. Selected bond lengths (Å) and angles (°): 4 b‐B(C₆F₅)₄: W1−C1 2.0240(17), W1−C2 2.0615(17), W1−C3 1.984(2), P1−C1 1.7440(18), P1−C28 1.7864(18), C23−C28 1.405(2), C16−C23 1.537(2), C2−C16 1.502(2), C1−C2 1.332(2), C2‐C16‐C17 117.21(14), C2‐C16‐C23 112.34(14), C17‐C16‐C23 114.06(14).

Figure 5

Molecular structure of complex cation 4 c⁺ in the crystal (50 % thermal ellipsoids); hydrogen atoms and anion (OTf⁻) have been omitted for clarity. Selected bond lengths (Å) and angles (°):W1−C1 2.009(4), W1−C2 2.050(4), W1−C3 1.980(4), P1−C1 1.753(4), P1−C28 1.808(4), C23−C28 1.386(7), C16−C23 1.517(7), C2−C16 1.501(6), C1−C2 1.341(6), C2‐C16‐C17 124.8(4), C2‐C16‐C23 107.0(3), C23‐C16‐C17 103.4(4).

Scheme 4

Generation and main resonance structures of complex 5.

Figure 6

Molecular crystal structure of the neutral complex 5 (50 % thermal ellipsoids). Hydrogen atoms have been omitted for clarity. Selected bond lengths (Å) and angles (°): W1−C1 1.993(10), W1−C2 2.102(9), W1−C3 1.960(8), C1−P1 1.743(10), P1−C28 1.772(10), C23−C28 1.395(15), C16−C23 1.426(15), C2−C16 1.401(13), C1−C2 1.383(13), C2‐C16‐C17 135.4(10), C2‐C16‐C23 116.3(9), C23‐C16‐C17 108.1(9).

Figure 7

UV/Vis spectra in CH₂Cl₂ of 4 c⁺ (red) and compound 5 (black); difference electron density (drop blue, gain white) for the three main visible transitions calculated by TD‐DFT (Figure S44).

Figure 8

Reaction sequence of 3 b (top) and 3 c (bottom) with [H(Et₂O)₂][(B(C₆F₅)₄] monitored by ³¹P NMR spectroscopy. Reaction mixtures were warmed stepwise from −80 °C to room temperature. Note the slight temperature shift of species [H‐3 b]⁺/[H‐3 c]⁺, which is characterized by a doublet in ¹H coupled ³¹P NMR spectra ([H‐3 b]⁺: δ=−4.4 ppm, ¹ J_PH=527 Hz; [H‐3 c]⁺: δ=−2.5 ppm, ¹ J_PH=501 Hz).

Figure 9

Cyclic voltammetry of complex 3 c (black) and 3 d (red) measured in CH₂Cl₂ (referenced against ferrocene/ferrocenium); oxidation process W(II/III) for 3 c at E _1/2=+0.01 V and for 3 d E _1/2=+0.38 V); IR spectro‐electrochemical measurement confirmed the tungsten based oxidation.

Figure 10

Series of measured IR spectra from the reaction monitoring of 3 c with [H(Et₂O)₂][(B(C₆F₅)₄] in the range of carbonyl vibration (top). Decomposition of the mixture spectra into the pure component spectra (centre) and the corresponding concentration profiles (bottom). The pure component spectra are scaled to a maximum height of 1.

Scheme 5

Derived kinetic model that describes the reaction mechanism.

Scheme 6

Reaction sequence of [H‐3 c]⁺ to 4 c⁺. After dehydration biradical ³ IM1⁺ is at equilibrium with its dimer (IM1)₂ ²⁺, as confirmed by EPR spectroscopy. Cyclisation of the monomer ³ IM1⁺ results two isomers, in which only anti ‐IM2⁺ can lead to the product by a suprafacial sigmatropic rearrangement. For syn ‐IM2⁺ the antarafacial rearrangement is impeded (the kinetic dead end).

Figure 11

Mulliken spin density distribution of intermediate ³ IM1 ⁺ (CAM‐B3LYP, def2‐TZVP, cut‐off 0.01).