Synthesis and reactivity of a parent phosphathioethynolato-borane and a boraarsaketene

Abstract

We present the synthesis and isolation of the first main-group phosphathioethynolates, [LBH₂(SCP)] (L = SIMes, CAAC^Me; SIMes = 1,3-bis(2,4,6-trimethylphenyl)-4,5-dihydroimidazol-2-ylidene, CAAC^Me = 1-(2,6-diisopropylphenyl)-3,3,5,5-tetramethylpyrrolidin-2-ylidene), in which phosphathioethynolate coordination occurs exclusively through the sulfur atom, giving rise to a phosphaalkyne-type structural motif. This is reflected in the reactivity of the SIMes derivative towards organic azides and [(η²-C₂H₄)₂CoCp] (Cp = C₅H₅), which mirrors the behavior of 1-adamantylphosphaalkyne, yielding triazaphospholes and a mixed cyclopentadienyl-(1,3-diphosphete) sandwich complex, respectively. Deviations from typical phosphaalkyne reactivity are observed in reactions with boron-containing heterocycles such as pentaphenylborole (PPB) and a carboranyl-substituted 9-borafluorene, which yield an unprecedented bicyclic structure and a zwitterionic spiro compound, respectively. Furthermore, we report the synthesis of the first arsaketenylborane, [(SIMes)BH₂(AsCO)], which, although too unstable for isolation, generates an arsinidene in situ, which can be trapped by coordination to a PPB C[double bond, length as m-dash]C double bond.

Fig. 1. Selected examples of PCO⁻-substituted main-group compounds (A and B), as well as the only literature known SCP⁻ complexes (C–D) and the new boron SCP⁻- and AsCO⁻-substituted complexes presented herein. Dipp = 2,6-diisopropylphenyl.

Scheme 1. Synthesis of the carbene-stabilised phosphathioethynolato boranes 2-SIMes and 2-CAAC^Me, and crystallography-derived solid-state structure of 2-SIMes. Atomic displacement ellipsoids set at 50% probability. Ellipsoids of ligand periphery and hydrogen atoms omitted for clarity except for boron-bound hydrides. OTf = trifluoromethylsulfonate = triflate; diox = 1,4-dioxane.

Scheme 2. Phosphaalkyne-like reactivity of 2-SIMes. Ph^CF3 = p-trifluoromethylphenyl. Mes = 2,4,6-trimethylphenyl.

Fig. 2. Crystallography-derived solid-state structure of 3-Ph^CF3. Atomic displacement ellipsoids set at 50% probability. Ellipsoids of ligand periphery and hydrogen atoms omitted for clarity except for boron-bound hydrides.

Fig. 3. Crystallography-derived solid-state structures of (a) 5 and (b) 6. Atomic displacement ellipsoids set at 50% probability. Ellipsoids of ligand periphery and hydrogen atoms omitted for clarity except for B- and P-bound protons.

Scheme 3. Reactivity of 2-SIMes towards PPB and CBBF.

Scheme 4. Proposed mechanism for the formation of 5 from the reaction of 2-SIMes with PPB. Gibbs free energies in parentheses (kcal mol⁻¹) are referenced to 2-SIMes + PPB (no solvent correction) and calculated at the B3LYP/D3(BJ)/def2-SVP level of theory (solvent not included).

Scheme 5. Proposed mechanism for the formation of 6 from the reaction of 2-SIMes with CBBF. Gibbs free energies in parentheses (kcal mol⁻¹) are referenced to 2-SIMes + CBBF and calculated at the B3LYP/D3(BJ)/def2SVP level of theory (solvent not included).

Scheme 6. Synthesis of 7 and its reactivity towards PPB.

Fig. 4. (a) Solid-state structure of 8. Atomic displacement ellipsoids set at 50% probability. Ellipsoids of ligand periphery and hydrogen atoms omitted for clarity except for B-bound protons. (b) Truncated ball-and-stick representation of 8 (Mes and Ph substituents and hydrogen atoms omitted for clarity, except for B-bound protons) with shared numbers of electrons (SNEs) in blue. (c and d) Intrinsic bond orbitals (IBOs) of 8 (stick representations, hydrogen atoms omitted for clarity, except for B-bound protons) involved in As–PPB bonding. Isovalues set at 0.04.