Two pathways of proton transfer reaction to (triphos)Cu(η1-BH4) via a dihydrogen bond [triphos = 1,1,1-tris(diphenylphosphinomethyl)ethane]

Abstract

The interaction of the η¹-tetrahydroborate copper(I) complex (triphos)Cu(η¹-BH₄) (1) with proton donors [CF₃CH₂OH (TFE), (CF₃)₂CHOH (HFIP), (CF₃)₃COH (PFTB), PhOH, p-NO₂C₆H₄OH (PNP), p-NO₂C₆H₄NNC₆H₄OH (PNAP), CF₃OH] was a subject of a combined IR spectroscopic and theoretical investigation. Spectral (Δν) and thermodynamic (ΔH) parameters of dihydrogen bond (DHB) formation were determined experimentally. The terminal hydride ligand (characterized by the basicity factor E_j(BH) = 0.87 ± 0.01) is found to be a site of proton transfer which begins with nucleophilic substitution of BH₄⁻ by the alcohol oxygen atom on the copper center (BH pathway). The activation barrier computed for (CF₃)₂CHOH in CH₂Cl₂ – ΔG^‡_{273 K} = 20.6 kcal mol⁻¹ – is in good agreement with the experimental value (ΔG^‡_{270 K} = 20.0 kcal mol⁻¹). An abnormal dependence of the reaction rate on the proton donor strength found experimentally in dichloromethane is explained computationally on the basis of the variation of the structural and energetic details of this process with the proton donor strength. In the second reaction mechanism found (CuH pathway), DHB complexes with the initial ROH coordination to the bridging hydride lead to B–H_br bond cleavage with BH₃ elimination. “Copper assistance” via the Cu⋯O interaction is not involved. This mechanism can be evoked to explain the occurrence of proton transfer in coordinating solvents.