Hydroboration Reactivity of Niobium Bis(N-heterocyclic carbene)borate Complexes

Abstract

The syntheses of high-valent niobium imido complexes [HB(Im)]Nb(N Bu)Cl (2) and [HB(Im)]Nb(N Bu)Me (3) bearing a bis(NHC)borate (NHC = N-heterocyclic carbene) supporting ligand are described. The reaction of the dimethyl complex (3) with excess CO generates an equivalent of acetone, which inserts into a B-H bond of the bis(NHC)borate ligand to form a boryl isopropoxide/niobium(III) dicarbonyl complex (4). This mode of hydroboration reactivity also occurs readily upon the treatment of either 2 or 3 with ketones, aldehydes, and isocyanates. Modification of the bis(carbene) ligand of 3 via the hydroboration of benzophenone produces the dimethylniobium complex [(OCHPh)B(Im)]Nb(N Bu)Me (12), which undergoes intramolecular η-arene coordination upon hydrogenation.

Hydroboration reactions using transition metal borane and borate complexes: an overview

“…Bergman, Arnold and co-workers demonstrated the hydroboration reactions of bis(NHC)borate complexes of niobium [X 2 ( t BuN)Nb{(NHC) 2 BH 2 }], 1a-1b [X = Cl (1a) or Me (1b); NHC = C 3 H 2 N 2 (Mes)] with different carbonyl group-containing organic substrates and showed the use of these borate complexes (Scheme 2). 11 The room temperature (rt) reaction of benzophenone with 1a exclusively generated the monohydroborated complex [Cl 2 ( t BuN)Nb(OCHPh 2 ){(NHC) 2 BH}], 2a by the addition of one of the B-H bonds across the CvO bond of benzophenone. The 11 B{ 1 H} chemical shift which appeared at δ −0.04 ppm has been shifted downfield when compared to the starting material, 1a (δ −9.65 ppm) due to the presence of an alkyloxy group attached to boron.…”

“…In a similar way, the reaction of paraformaldehyde with 1a-1b generated the mono-and di-hydroborated niobium complexes [X 2 ( t BuN)Nb(OMe){(NHC) 2 BR}], 2c-2d (2c: R = H, X = Cl; 2d: R = OMe, X = Me). 11 The thermolysis of 1a with an excess of benzaldehyde underwent double hydroboration of the carbonyl groups and yielded a double alkyloxylated borate complex [Cl 2 ( t BuN)Nb (OCH 2 Ph){(NHC) 2 BOCH 2 Ph}], 2e. The 11 B{ 1 H}-NMR signal for this complex appeared slightly at the deshielded region (δ 3.45 ppm) when compared to 2c (δ 2.35 ppm).…”

“…The 11 B-NMR spectrum revealed a broad signal at δ 34.7 ppm (hhw of 1550 Hz) that shifted downfield as when compared to the starting material 21 (δ 0.95 ppm). The downfield 11 B chemical shift is indirect evidence for the presence of an alkenylborane unit in 22. Further, the 31 P{ 1 H}-NMR spectrum of 22 showed two signals at δ 47.8 (dd, 1 J RhP = 176 Hz, 2 J PP = 33 Hz) and 31.9 ppm (dd, 1 J RhP = 134 Hz, 2 J PP = 33 Hz).…”

Recent progress on NHC-stabilized early transition metal (group 3–7) complexes: Synthesis and applications

“…Bis-NHC ligands featuring a boryl moiety have been used for the synthesis of high-valent Nb(V) imido complexes. 77 In-situ deprotonation of the imidazolium salt with LiN i Pr2 and subsequent metalation with Nb(N t Bu)Cl3Py2 afforded the bis-(NHC)-Nb(V) imido dichloride complex 99 (Scheme 29). Reaction with MeMgCl leads to the formation of the alkyl derivative 100.…”

Synthesis, Structural Characterization, and Theoretical Studies of Silver(I) Complexes of Dihydrobis(2‐mercapto‐benzothiazolyl) Borate

“…Di or trihydro‐borate ligands supported by mercapto‐benzothiazolyl or other heterocycles typically yield borate and boratrane complexes respectively when these ligands are reacted with group 6–8 and group 9 metal precursors. [10b], [17c], [19d, f], Group 11 metals with dihydrobis(borate) ligands and phosphines as co‐ligands mostly resulted in B–H ··· M interactions , . Here phosphines are utilized as co‐ligands for the stability of the complexes and presence of these co‐ligands around the copper/silver ions lead to distorted tetrahedral or distorted trigonal planar arrangement.…”