Sequential Migrations between Boron and Rhodium Centers: A Cooperative Process between Rhodium and a Monosubstituted Borohydride Unit

Abstract

The sodium salt of a monosubstituted borohydride anion containing a 2-mercaptopyridyl unit (mp) is reported herein. This compound was coordinated to a rhodium(I) center providing the complex [Rh{κ-H,H,S-HB(mp)}(NBD)] (1) (where NBD = 2,5-norbornadiene) in which the boron-based ligand is coordinated to the rhodium center via the thione donor and two of the B-H bonds of the BH unit. Reaction of complex 1 with carbon monoxide results in the activation of the complex leading to the product of a formal intramolecular hydroboration reaction, where the NBD unit has, in effect, inserted into one of the B-H bonds. Three complexes were prepared in which the newly formed norbornenyl unit (nbe) is located at the boron center, namely, [Rh{κ-H,H,S-HB(nbe)(mp)}(CO)] (2), [Rh{κ-H,H,S-HB(nbe)(mp)}(CO)(PCy)] (3), and [Rh{κ-H,H,S-HB(nbe)(mp)}(CO)(PPh)] (4). The identities of the three complexes were confirmed by spectroscopic and analytical techniques. Further confirmation was obtained via structural characterization of 3. Studies confirmed that the reactivity occurs at the metal center. A metal-ligand cooperative mechanism, involving initial migration of hydride from boron to metal center, was postulated for the formation of the new complexes based on previous investigations. The newly formed norbornenyl unit then migrates from metal center to boron.

Adding to the Family of Copper Complexes Featuring Borohydride Ligands Based on 2-Mercaptopyridyl Units

“…This original report provided a borohydride-based ligand substituted by two and three of these heterocycles. Last year, we extended this family to include the monosubstitued ligand, [H 3 B(mp)] − (where mp = 2-mercaptopyridyl; Figure 2) [37]. Herein, we report the synthesis and characterization of the first copper complexes containing this new ligand.…”

“…It was this greater flexibility within the ligand structure that opened up the potential for activation at the boron bridgehead and formation of metal-borane (metallaboratrane) complexes [17][18][19][20][24][25][26][27], giving rise to reactivity not observed in the analogous polypyrazolylborate ligands [10][11][12][13][14][15][16]. Over the following twenty years since the first report of hydride migration from the boron center of a scorpionate ligand, a number of research groups have focused on new, more flexible borohydride ligands containing a range of supporting units based on nitrogen [28][29][30][31] and other sulfur heterocycles [32][33][34][35][36][37][38][39][40][41][42][43][44][45][46]. As part of our research, we have focused on providing new derivative ligand systems.…”

“…The first of the more flexible scorpionate ligands was [Tm] − [hydrotris(methylimidazolyl)borate] (Figure 1; middle) [23]. This new ligand had two major Over the following twenty years since the first report of hydride migration from the boron center of a scorpionate ligand, a number of research groups have focused on new, more flexible borohydride ligands containing a range of supporting units based on nitrogen [28][29][30][31] and other sulfur heterocycles [32][33][34][35][36][37][38][39][40][41][42][43][44][45][46]. As part of our research, we have focused on providing new derivative ligand systems.…”

Versatile Modes of Cooperative B−H Bond Activation Reactions in Ruthenium‐Carbene Complexes: Addition, Ring‐Opening and Insertion

“…With successful activation of the B−H bond in CatBH, we addressed the activation of other borane sources. Parent BH 3 Lewis base adducts have only scarcely been applied in cooperative bond activation reactions . Treatment of complex 1 a with an excess of BH 3 ⋅SMe 2 resulted in a completely different reaction outcome.…”

“…Parent BH 3 Lewis base adducts have only scarcely been appliedi nc ooperative bond activation reactions. [13] Treatmento fc omplex 1a with an excess of BH 3 ·SMe 2 resulted in ac ompletely different reactiono utcome. Monitoring of the reaction process by 31 P{ 1 H} NMR spectroscopy revealed the initial formation of a new species characterized by as ignal at d P = À7.20 ppm, which converts, over the course of ac ouple of hours, to a single new product, exhibiting ab road signal at d P = 31.7 ppm.…”