Recent Advances in the Synthesis and Reactivity of Transition Metal σ-Borane/Borate Complexes

Abstract

Conspectus The coordination of an element–element σ bond to a transition metal (TM) is both a fundamentally intriguing binding mode and of critical importance to metal-mediated bond activation mechanisms and catalysis, particularly the hotly contested field of C–H activation. TM σ complexes of dihydrogen (i.e., H–H) and silanes (H–SiR3) have been extensively studied, the latter being of interest as models for the (generally unstable and unisolable) σ complexes of alkanes (i.e., H–CR3). TM σ complexes of hydroboranes and hydroborates (i.e., H–BR2, H–BR3, (H−)2BR2) are somewhat less well studied but similarly have relevance to catalytic borylation reactions that are of high current interest to organic synthesis. Our two research groups have made significant contributions to elaborating the family of σ-borane/-borate complexes using two distinct approaches: while the Ghosh group generally starts from hydrogen-rich tetracoordinate boron species such as borates, the Braunschweig group starts from hypovalent and/or hypocoordinate boron building blocks. Through these two approaches, a wide range of species containing one or two σ-bound B–H ligands have been prepared, some with additional chelating donor sites. Over the past 2 years, the body of work on σ-borane/-borate complexes from our two research groups has significantly expanded, with a combined nine published articles in 2019–2020 alone. Very recent work from the Braunschweig group has led to the synthesis of the first bis­(σ)-borane complexes of group 6 metals, as well as the synthesis of a series of novel bis­(σ)-borane and bis­(σ)-borate complexes of ruthenium and iridium, the former being useful precursors for pentacoordinate borylene complexes of Ru. Recent work from the Ghosh group has uncovered a remarkable diversity of structures with σ­(B–H)-bound ligands from the combination of borohydrides and nitrogen/chalcogen-containing groups and heterocycles. These reactions, while in some cases producing conventional scorpionate-type chelating products, more frequently undergo fascinating rearrangements with unpredictable outcomes. This Account aims to highlight this recent acceleration of research progress in this area, particularly the distinct but related approaches ofand complexes produced byour two research groups, in addition to relevant works from other groups where appropriate.

Coordination and Hydroboration of Ru(II)‐Borate Complexes: Dihydridoborate vs. Bis(dihydridoborate)

“…Surprisingly, only one dihydridoborate unit participates in the reaction, other one probably provided ancillary support to the metal centre. The C=C and B−C bond distances of 1.386(4) and 1.548(4) Å in 10 b respectively, are analogous to those of η 4 ‐HBCC‐coordinated ruthenium complexes [4b] . The Ru1‐B1 bond distance of 2.353(3) Å is significantly elongated upon coordination with alkene as compared to uncoordinated one (Ru1‐B2 2.143(3) Å).…”

Stereoselective hydrogen atom transfer to acyclic radicals: a switch enabling diastereodivergent borylative radical cascades

“…To investigate the chelation mode of ZnI 2 with Int-III, a variety of possible complexes with different chelation modes were extensively screened. The results revealed that the most favored chelation mode contains a ZnI 2 -chelated six-membered ring, where ZnI 2 coordinates with both the carbonyl oxygen atom and the B-H bond 58,59 (Int-III-B-1 in Fig. 7a).…”

Cooperative B–H bond activation: dual site borane activation by redox active κ2-N,S-chelated complexes

“…As listed in Table 1, the Ru-B distance of 2.172(4) Å for trans-4 is consistent with that of bis(dihyroborate) complex, Ru[(μ-H) 2 BC 8 H 14 ] 2 (PCy 3 ) 29 (2.160(2) Å) and other reported dihyroborate species. [29][30][31][32][33] The Ru-B distances in trans-4 fall in the range of2.103(2)-2.266(8) Å, 15 which are significantly longer as compared to those of σ-borane complexes, for example, [Ru(PCy 3 ) 2 (H) 2 (BH 2 Mes)] 29b (1.938(4) Å) and [Cp*Ru(P i Pr 3 )(BH 2 Mes)]B(C 6 F 5 ) 4 34 (1.921(2) Å). The B1-Ru1-B1 angle of 180.0(2)° shows a perfect symmetrical coordination of borane to metal center, unlike tetrahydroborate species, 2 Ru(η 2 -BH 4 )] 32 (177.4(4)°) and Ru[(μ-H) 2 BC 8 H 14 ] 2 (PCy 3 ) 30 (147.68(8)°).…”

“…As part of our current interest in activation of boranes utilizing cooperative reactivity, 15 we have synthesized a number of molybdenum( ii ) hydroborate species, in which BH 3 is stabilized through Mo–H–B(H 2 )–E molybdacycles (E = S, Se or Te). 16 Further, very recently, we have established cooperative Si–H and B–H bond activations by a κ 2 - N , S -chelated borate complex, trans – mer -1b that led to the formation of six-membered ruthenahetero-cycles through hemilabile ring-opening of Ru–N bonds.…”

“…12,13 On the other hand, for the B-H activation the engagement of MLC with multifunctional reactive sites with redox-active ligands is more useful for accessing key organic transformations. 14 As part of our current interest in activation of boranes utilizing cooperative reactivity, 15 we have synthesized a number of molybdenum(II) hydroborate species, in which BH 3 is stabilized through Mo-H-B(H 2 )-E molybdacycles (E ¼ S, Se or Te). 16 Further, very recently, we have established cooperative Si-H and B-H bond activations by a k 2 -N,S-chelated borate complex, trans-mer-1b that led to the formation of six-membered ruthenahetero-cycles through hemilabile ring-opening of Ru-N bonds.…”