Bifunctional activation of amine-boranes by the W/Pd bimetallic analogs of "frustrated Lewis pairs"

Abstract

The reaction between basic [(PCP)Pd(H)] (PCP = 2,6-(CH₂P(t-C₄H₉)₂)₂C₆H₄) and acidic [LWH(CO)₃] (L = Cp (1a), Tp (1b); Cp = η⁵-cyclopentadienyl, Tp = κ³-hydridotris(pyrazolyl)borate) leads to the formation of bimolecular complexes [LW(CO)₂(μ-CO)⋯Pd(PCP)] (4a, 4b), which catalyze amine-borane (Me₂NHBH₃, ^t BuNH₂BH₃) dehydrogenation. The combination of variable-temperature (¹H, ³¹P{¹H}, ¹¹B NMR and IR) spectroscopies and computational (ωB97XD/def2-TZVP) studies reveal the formation of an η¹-borane complex [(PCP)Pd(Me₂NHBH₃)]⁺[LW(CO₃)]^- (5) in the first step, where a BH bond strongly binds palladium and an amine group is hydrogen-bonded to tungsten. The subsequent intracomplex proton transfer is the rate-determining step, followed by an almost barrierless hydride transfer. Bimetallic species 4 are easily regenerated through hydrogen evolution in the reaction between two hydrides.

Scheme 1. Reaction between two metal hydrides, (PCP)PdH and 1.

Fig. 1. IR spectra (top) and the corresponding kinetic curves (bottom) of a mixture of TpWH(CO)₃ (1a, c = 0.003 M) and (PCP)PdH (c = 0.045 M) at 240 K in THF.

Fig. 2. Molecular structures of bimetallic products 4a (top) and 4b (bottom). Atom color code: white, H; gray, C; yellow, B; red, O; blue, N; navy, W; pink, P; and cyan, Pd. Thermal ellipsoids are given at 50% probability level; hydrogen atoms except for the BH group in 4b are omitted for clarity.

Scheme 2. Reaction between the ionic pair 4 and amine-borane.

Fig. 3. ¹¹B{¹H} NMR spectra (128.3 MHz, THF-d₈) of DMAB (5 equiv.) in the presence of 4a (top) and the corresponding changes in molar fractions in time (bottom).

Fig. 4. Kinetic curves obtained from the IR spectra of DMAB (1 equiv. (top) and 5 equiv. (bottom)) dehydrogenation catalyzed by 4b (c = 0.003 M). 298 K, THF.

Fig. 5. Kinetic curves of DMAB (c = 0.06 M) dehydrogenation by reused [CpW(CO)₂(μ-CO)⋯Pd(PCP)] (4a, 5 mol%, c = 0.003 M) in THF at 313 K.

Fig. 6. Dependence of the initial reaction rate on the substrate (DMAB) concentration (c = 0.015 M, 0.03 M, 0.06 M, 0.15 M; THF; 313 K).

Fig. 7. Kinetic curves of DMAB dehydrogenation by [CpW(CO)₂(μ-CO)⋯Pd(PCP)] (4a, c = 0.003 M) in THF at 313 K. (Red) c(DMAB) = 0.015 M, (5 eq.); (blue) c(DMAB) = 0.15 M, (50 eq.).

Scheme 3. Mechanism of R′RNHBH₃ dehydrogenation by bimetallic ion pairs 4.

Fig. 8. Computed (DFT/ωB97XD) energy profile for the DMAB dehydrogenation by [TpW(CO)₂(μ-CO)⋯Pd(PCP)] (4b) in toluene (SMD model).

Fig. 9. DFT/ωB97XD-optimized structure of the complex 5b. Key bond lengths are reported in Å.