Self-Isomerized–Cyclometalated Rhodium NHC Complexes as Active Catalysts in the Hydrosilylation of Internal Alkynes

Abstract

A range of unique Rh-based bidentate NHC complexes that are formed in a base-free tandem isomerization/cyclometalation process were synthesized (1–4) from a range of imidazolium salts with an N-alkenyl tether. Cyclometalation occurred with complex 1, leading to an unprecedented complex, which is the first and only example in the literature of a nonaromatic C(sp2)–H activation leading to a C(sp3)–Rh cyclometalated product with a concomitant intramolecular/isomerization process. Dealkylation of the N-alkenyl substituent occurred to form byproducts that showed metal N-coordination (1b and 2b). These byproducts, 1b and 2b, were further reacted with the anion exchange reagent NH4PF6 to form the dimeric complexes 1bd and 2bd. All of the complexes were applied as precatalysts in the hydrosilylation of internal alkynes with excellent performance (conversions of 66–100%) after only 1 h at 80 °C without the use of an additive. Anticancer studies showed that complexes presented half-maximum inhibitory concentrations ranging from 3.71 to 25.85 μM. Depending on the cell line, complex 4 was the most cytotoxic complex, especially in the BT-20 triple-negative breast carcinoma, MCF-12A nontumorigenic mammary gland cell, MDA-MB-231 triple-negative breast carcinoma, and MCF-7/TAMR-1 tamoxifen-resistant subtype of the MCF-7 estrogen- and progesterone-positive luminal breast carcinoma cell lines.