Removing the Sting from the Tail:  Reversible Protonation of Scorpionate Ligands in Cobalt(II) Tris(carbene)borate Complexes

Abstract

Low-temperature deprotonation of the phenylborane dications, PhB(RIm)3OTf2 (R = tBu, Mes), followed by in situ reaction with CoCl2(thf)1.5, results in the formation of the four-coordinate complexes, kappa3-PhB(RIm)3CoCl, in which the metal is supported by tripodal N-heterocyclic carbene-based ligands. The chloride complexes are exceptionally sensitive to acid and can be reversibly protonated to form the zwitterions kappa2-{PhB(RIm)2(RIm.H)}CoCl2. This unexpected reactivity is attributed to the highly basic nature of the tris(carbene)borate ligands. Reaction of the chloride complexes with methylating reagents results in products that depend on the N-heterocyclic carbene substituent. For R = tBu, the four-coordinate high-spin complex, kappa3-PhB(tBuIm)3CoMe, is formed, while for R = Mes, reduction to a multitude of complexes occurs.

Mechanistic investigations of the asymmetric hydrosilylation of ketimines with trichlorosilane reveals a dual activation model and an organocatalyst with enhanced efficiency

“…1-Methylimidazole 1 is commercially available, whereas 1-tert-butylimidazole 2 was prepared by condensing a mixture of glyoxal, aqueous formaldehyde, aqueous ammonia and tertbutylamine. 20 These were converted to the dimethylamides 3 and 4 by deprotonation with n-BuLi and quenching with dimethylcarbamoyl chloride (Scheme 1), and evaluated in a benchmark HSiCl 3 reduction of the N-PMP acetophenone ketimine 5 under a standard set of reaction conditions by removing aliquots and analyzing the conversion by 1 H NMR spectroscopy ( Fig. 1).…”

“…20 A 250 mL three-necked flask containing distilled water (50 mL) was equipped with two dropping funnels and a condenser. One dropping funnel was filled with a mixture of 40% aqueous glyoxal (11.5 mL, 0.1 mol) and 37% aqueous formaldehyde (8.10 g, 0.1 mol), the other with t-butylamine (10.6 mL, 0.1 mol) and 35% aqueous ammonia (4.86 g, 0.1 mol).…”

Preparations of a series of coinage metal complexes with pyridine-based bis(N-heterocyclic carbene) ligands including transmetalation to palladium complexes

“…The starting materials, 1-tert-butylimidazole [16] Elemental analyses (C, H, N) were performed on a Perkin Elmer 2400II elemental analyzer.…”

“…2.105 Å). The distortion of the CNHC-M-CNHC bond angles (169.3(5) º (1), 163.03 (16) º (2), and 172.57 (18) º (3)) away from linearity occurs, probably as a result of the metal-metal interactions as shown below.…”

Theoretical investigation of the scope of sequential ligand tuning using a bifunctional scorpionate tris(1,2,4-triazolyl)borate-based architecture

“…More recently, the chemistry of complexes of these ligands and their bulkier second generation tris(imidazolylidene)borate complexes have been explored by several groups including that of Smith. [13][14][15][16][17][18] Just as scorpionate ligands moved beyond Tp to greater structural diversity, N-heterocyclic carbenes quickly moved beyond imidazole based frameworks. A commonly encountered analogue to imidazol-2-3 ylidenes are 1,2,4-triazol-5-ylidenes.…”