Inner-Sphere versus Outer-Sphere Coordination of BF₄^– in a NHC-Gold(I) Complex

Abstract

ABSTRACT:The role of counterions in chemistry mediated by gold complexes stretches much further than merely providing charge balance to cationic gold species. Interplay between their basicities and coordination strengths influences interactions with both the gold center and substrates in catalysis. Actual monogold(I) active species are generally believed to be mono-coordinated species, formed from the abstraction or the decoordination of a second ligand from precursor complexes, but only little experimental evidence exists to underpin the existence of these transient species. The formation of a bench-stable neutral IPr Cl -gold(I) tetrafluoroborate complex is herein reported. Experimental studies by X-ray diffraction analysis, NMR spectroscopy and theoretical studies by DFT calculations were conducted to determine the composition, structure, and behavior of this complex. The absence of an auxiliary ligand resulted in inner-sphere coordination of the counterion in the solid state. In solution, an equilibrium between two conformations was found with the counterion occupying inner-sphere and outer-sphere positions, respectively. Stoichiometric and catalytic reactivity studies with the tetrafluoroborate complex have been conducted. These confirmed the lability of the inner-sphere coordinating counterion that gives the IPr Cl -gold(I) fragment similar behavior to related systems.

Cycloisomerization of Olefins in Water

“…Initial studies of feasibility and rate under dilute conditions indicated that although the reactions appeared to be slow at a 1 m m concentration (1 equiv 1 , 0.3 equiv catalyst), the rate increased in acetone/water mixtures (Figure ) . Our group previously reported a Co/Ni bimetallic hydroarylation of olefins, where Co(Sal t ‐Bu, t ‐Bu )BF 4 appeared to be superior to Co(Sal t ‐Bu, t ‐Bu )Cl . We found that at high dilution Co(Sal t ‐Bu, t ‐Bu )BF 4 catalyzes cycloisomerization significantly faster than the initially reported Co(Sal t ‐Bu, t ‐Bu )Cl.…”

Optimization of Catalysts and Conditions in Gold(I) Catalysis—Counterion and Additive Effects

“…Also, computational modeling further revealed the interconversion between inner-sphere and outersphere coordinating counterions and the favorable coordination of solvent molecules in the gold complex with an empty coordination site, where the BF 4 − anion resided in the outer sphere. 86 Pulsed-field gradient (PFG) NMR and its implementation DOSY (diffusion-ordered NMR spectroscopy) are useful techniques for the investigation of interactions between substrates and to characterize reactive intermediates. Together with external calibration curves (ECC), Dumez and coworkers 87 could obtain substantial interaction evidence between cationic gold(I) and their counterion or the substrate.…”

Expedient Syntheses of Neutral and Cationic Au(I)–NHC Complexes

“…33 is indicative of a less electrondeficient gold center in the former, as expected from coordination of the second ligand (NCCH3). 14,34 Net electron transfer from the coordinated acetonitrile to the gold center was also apparent from downfield shifts (caused by lower shielding) of the 1 H and 13 C resonances relative to noncoordinated acetonitrile: C-( 13 C) = 116.9, 35 CH3-( 13 C) = 1.9 and CH3-( 1 H) = 2.10. 36 Changes to the electronic configuration of the acetonitrile fragment in 5 were next probed by comparing the C≡N stretching frequencies from infrared spectra to those in noncoordinated acetonitrile.…”