Dehydrogenation of formic acid by Ir–bisMETAMORPhos complexes: experimental and computational insight into the role of a cooperative ligand

The synthesis of Ir-complexes with three bisMETAMORPhos ligands is reported. The activity of these systems towards HCOOH dehydrogenation and the dual role of the ligand during catalysis is discussed, using spectroscopic and computational methods.


Computational details:
Geometry optimizations were carried out with the Turbomole program package S2 , coupled to the PQS Baker optimizer S3 via the BOpt package S4 , at the spin unrestricted ri-DFT level using the BP86 functional S5 , the resolution-of-identity (ri) method S6 , and the def2-TZVP basis set S7 for the geometry optimizations. Energy profiles are shown below and all structure are conveniently added as separate .xyz and .pdb files.
To a solution of 9,9-dimethylxanthene (4.0 g, 19.02 mmol) and TMEDA (4.53 g, 5.85 mL, 39.0 mmol) in diethylether (150 mL) was added a solution of nBuLi (15.3 mL, 2.5 M in hexane, 38.24 mmol) at 0 °C and a deep purple/brown solution was obtained. The reaction mixture was allowed to warm up to room temperature and stirred overnight. [racemic mixture of diastereomers (RR/SS, SR/RS) of the phosphinamine]. The reaction mixture was carefully quenched with a 2 M HCl solution (100 mL) and stirred for 1 hour. The phases were separated and the aqueous phase was extracted with ethylacetate (3×). The organic phases were combined and concentrated. Azeotropic drying with toluene (2×) and stripping with diethylether (3×) yielded a white foam. Purification by column chromatograpy (SiO 2 /H 2 O 8:2, eluens Et 2 O/MeOH 97:3, deposited in CH 2 Cl 2 ) yielded a as a racemic mixture of diastereomers (RR/SS, RS/SR) as a white foam (4.11g, 47% yield).
Compound Lb was obtained pure in its mesomeric form (RS/SR) and this species exists in two tautomeric forms Lb1, Lb2 and Lb3 with a ratio of 1 : 1.8 : 0.2, respectively, according to 1 H and 31 P NMR integrations.

Complex 1a
Commercially available Ir(acac)(COD) (6 mg, 0.015 mmol) was dissolved in CH 2 Cl 2 (1 mL) together with ligand La (12.8 mg, 0.015 mmol). The reaction mixture turned bright orange instantly and was stirred for 15 minutes. Evaporation of solvent and volatiles left an orange solid in near-quantitative yield.

Complex 1c
Commercially available Ir(acac)(COD) (6 mg, 0.015 mmol) was dissolved in CH 2 Cl 2 (1 mL) together with ligand Lc (14.8 mg, 0.015 mmol). The reaction mixture turned bright orange instantly and was stirred for 15 minutes. Evaporation of solvent and volatiles left an orange solid in near-quantitative yield.

Complex 2b
Complex 1b was stirred at room temperature in toluene (1 mL) for 40 hours at 70 °C, during which time a color change from orange to yellow, reaction mixture was concentrated. Complex 2b was formed quantitatively as a diastereomeric mixture.

Complex 2c
Complex 1c was stirred at room temperature in CH 2 Cl 2 (1 mL) for 16 hours at room temperature, during which time a color change from orange to light yellow, reaction mixture was concentrated. Complex 2c was formed quantitatively as a diastereomeric mixture.

Energy profile of structure 7I
Rearrangement of HCOOH in 7I to orient the substrate in the right position for direct hydride-transfer to yield structure 7II was found to be endergonic by 23.5 kcal mol -1 . The transition state (7III) of the direct hydride-transfer toward the dihydride structure 7IV was found to be significantly higher (29.9 kcal mol -1 ) than for the axial structures 5III and 6III. Similar to the transition states previously found (5III and 6III) hydrogen-bonding interactions were also observed in 7III. The release of H 2 has been described above, for complete energy profile of 7I see supporting information. Starting from complex 8I, rearrangement of HCOOH to enable direct hydride-transfer led to an unstable species and no transition state could be identified. Figure S7. Potential energy surfaces (DFT, BP86, def2-TZVP) for dehydrogenation of formic acid by 7I (∆G ᴼ 298K in kcal mol -1 ).