Steric Effects Dictate the Formation of Terminal Arylborylene Complexes of Ruthenium from Dihydroboranes

B-P Coupling: Metal Stabilized Phosphinoborate Complexes

In an effort to establish B-P coupling reactions without the use of phosphine-borane dehydrocoupling agent, we have developed a new synthetic methodology employing group 8 metal σ-borate complex [{κ3 -H,S,S'-BH2 L2 }Ru{κ3 -H,H,S-BH3 L}] (L=NC5 H4 S), 1. Treatment of 1 with chlorodiphenyl phosphine (PPh2 Cl) yielded 1,5-P,S chelated Ru-dihydridoborate species [PPh2 H{κ3 -H,H,S-BH(OH)L}Ru{κ2 -P,S-(Ph2 P)BH2 L}], 2. The insertion of phosphine moiety (PPh2 ) by the cleavage of 3c-2e σ(Ru… H-B) bonding interaction led to the formation of B-P bond. The κ2 -P,S chelated six-membered ring adopted a boat conformation in complex 2. The heterocycle is made of all different atoms, which is one of the rarest examples of heteroatomic ring systems. Theoretical outcomes demonstrated the electronic insight of B-P coupling and stabilization through transition metal. In order to explore an alternate route of B-P bond formation, we have further explored the reaction of 1 and Ru-bis(dihydridoborate) complex, 5 with secondary phosphine oxide (SPO). Although, thermolysis of 1 with diphenylphosphine oxide yielded analogous σ-borate complex 3, the similar reaction of 5 at room temperature led to the formation of novel phosphinous(III) acid incorporated Ru(σ-borate)(dihydridoborate) complex, 6. In a similar fashion, the reaction of 5 with phosphite ligand generated Ru(σ-borate)(dihydridoborate) complex, 7, which is analogous to 6.

Rhodium-Mediated Dehydrogenation of Hydroboranes and Group 14 Compounds: Base-Stabilized Silylene and Germylene Complexes vs. Transmetalation

Monocarbonyl rhodium complex LRh(CO), 1, which is stabilized by a pyrrole-based bis(phosphinimine) pincer ligand (L=κ3 -NNN'=2,5-[i Pr2 P=N(4-i PrC6 H4 )]2 -N'(C4 H2 )- ), serves as a versatile platform for the dehydrogenation of group 14 substrates. Reaction with primary and secondary silanes and germanes (MesSiH3 , Et2 SiH2 , Ph2 GeH2 , t BuGeH3 ; Mes=mesityl) liberates H2 and yields base-stabilized tetrylene compounds of the form κ2 -L(CO)Rh(ER2 ) (E=Si: R=Mes, H, 2; R=Et, 5; E=Ge: R=Ph, 6; R=t Bu, H, 8). The ":ER2 " fragment in these species bridges between the rhodium center and a phosphinimine donor. Preliminary reactions between pinacol (Pin) and κ2 -L(CO)Rh(ER2 ), E=Si, Ge, indicate that such complexes can serve as silylene and germylene synthons, releasing :ER2 and catalytically generating PinER2 . In contrast, combination of complex 1 and MesGeH3 does not yield the anticipated dehydrogenation product, but rather, transmetalation similar to that observed upon reaction between 1 and 3,5-dimethylphenylborane prevails.

From trihydroborates to bisborylenes: a route to dinuclear bisborylene complexes

A new route for the synthesis of dinuclear bisborylene complexes was described. A series of novel diruthenium bisborylenes were prepared through unprecedented triple B-H oxidative addition of trihydroborates with concomitant hydrogen liberation. Conversion of trihydroborates to bisborylenes involved the formation of tris(σ-B-H) borate as the crucial intermediate stage.

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

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.

Intermetallic transfer of unsymmetrical borylene fragments: isolation of the second early-transition-metal terminal borylene complex and other rare species

Transition metal borylene complexes of the type [(OC)5M[double bond, length as m-dash]BN(SiMe3)(tBu)] (M = Cr, Mo, W) have been synthesised by salt elimination of the corresponding dibromoborane and the dianionic metallates Na2[M(CO)5]. The borylene complexes have been characterised by multinuclear solution-state NMR spectroscopy and solid-state molecular structure determination. The group 6 borylene complexes can be used to effectively transfer the borylene ligand to other transition metal complexes by replacing one or two carbonyl ligands upon irradiation of the reaction mixture with UV light. This borylene transfer reaction led to the formation of new terminal and bridging borylene complexes which cannot be formed by the corresponding salt elimination reactions, including a rare example of a bis(terminal borylene) complex and only the second reported terminal borylene complex of an early transition metal (vanadium).

Reversible dehydrogenation of a primary aryl borane

The consecutive activation of B-H bonds in mesitylborane (H2BMes; Mes = 2,4,6-(CH3)3C6H2) by a 16-electron rhodium(i) monocarbonyl complex, (iPrNNN)Rh(CO) (1-CO; iPrNNN = 2,5-[iPr2P[double bond, length as m-dash]N(4-iPrC6H4)]2N(C4H2)-) is described. Dehydrogenative extrusion of the {BMes} fragment led to the isolation of (iPrNNN)(CO)RhBMes (1-BMes). Addition of H2 gas to 1-BMes regenerated 1-CO and H2BMes, highlighting the ability of 1-CO to facilitate interconversion of {BMes} with dihydrogen. Reactivity studies revealed that 1-BMes promotes formal group transfer and that {BAr} fragments accessed by dehydrogenation are reactive entities.

Reactions of [(dmpe)2MnH(C2H4)]: synthesis and characterization of manganese(i) borohydride and hydride complexes

Reactions of trans-[(dmpe)₂MnH(C₂H₄)] (1) with BH₃(NMe₃), 9-BBN, and HBMes₂ yielded the manganese(i) borohydride complexes [(dmpe)₂Mn(μ-H)₂BR₂] (3: R = H, 4: R₂ = C₈H₁₄, 5: R = Mes). The reaction of 1 with BH₃(NMe₃) proceeds via ethylene substitution. By contrast, a detuerium labelling study indicates that the reaction of 1 with HBMes₂ involves initial isomerization of 1 to an unobserved 5-coordinate ethyl intermediate, [(dmpe)₂MnEt], which reacts with the hydroborane to afford EtBR₂ and [(dmpe)₂MnH], followed by reaction with a second equivalent of hydroborane to generate 5 (an analogous pathway is likely followed for other base-free hydroboranes such as 9-BBN). Identification of 3-5 as κ²-borohydride complexes, as opposed to boryl dihydride or hydroborane hydride isomers, is supported by ¹¹B NMR spectroscopy, X-ray diffraction, and Atoms in Molecules calculations. Two byproducts were observed in the syntheses of 3-5: [{(dmpe)₂MnH}₂(μ-dmpe)] (6) and [(dmpe)₂MnH(κ¹-dmpe)] (7). These complexes were independently prepared by exposure of 1 to free dmpe under an atmosphere of Ar or H₂, and the generality of this synthetic route was demonstrated by the reaction of 1 with PMe₃ (under H₂) to form [(dmpe)₂MnH(PMe₃)] (8). Complexes 6-8 can exist as isomers with either a trans or a cis relationship between the hydride and κ¹-coordinated phosphine ligands on manganese. trans to cis isomerization of 6-8 is photochemically induced, whereas the reverse reaction occurs under thermal conditions. X-ray crystal structures were obtained for 3-5, trans,trans-6, cis,cis-6, trans-7, and trans-8.

Toward Transition-Metal-Templated Construction of Arylated B4 Chains by Dihydroborane Dehydrocoupling

The reactivity of a diruthenium tetrahydride complex towards three selected dihydroboranes was investigated. The use of [DurBH₂ ] (Dur=2,3,5,6-Me₄ C₆ H) and [(Me₃ Si)₂ NBH₂ ] led to the formation of bridging borylene complexes of the form [(Cp*RuH)₂ BR] (Cp*=C₅ Me₅ ; 1 a: R=Dur; 1 b: R=N(SiMe₃ )₂ ) through oxidative addition of the B-H bonds with concomitant hydrogen liberation. Employing the more electron-deficient dihydroborane [3,5-(CF₃ )₂ -C₆ H₃ BH₂ ] led to the formation of an anionic complex bearing a tetraarylated chain of four boron atoms, namely Li(THF)₄ [(Cp*Ru)₂ B₄ H₅ (3,5-(CF₃ )₂ C₆ H₃ )₄ ] (4), through an unusual, incomplete threefold dehydrocoupling process. A comparative theoretical investigation of the bonding in a simplified model of 4 and the analogous complex nido-[1,2(Cp*Ru)₂ (μ-H)B₄ H₉ ] (I) indicates that there appear to be no classical σ-bonds between the boron atoms in complex I, whereas in the case of 4 the B₄ chain better resembles a network of three B-B σ bonds, the central bond being significantly weaker than the other two.

Steric Effects Dictate the Formation of Terminal Arylborylene Complexes of Ruthenium from Dihydroboranes

The steric and electronic properties of aryl substituents in monoaryl borohydrides (Li[ArBH₃ ]) and dihydroboranes were systematically varied and their reactions with [Ru(PCy₃ )₂ HCl(H₂ )] (Cy: cyclohexyl) were studied, resulting in bis(σ)-borane or terminal borylene complexes of ruthenium. These variations allowed for the investigation of the factors involved in the activation of dihydroboranes in the synthesis of terminal borylene complexes. The complexes were studied by multinuclear NMR spectroscopy, mass spectrometry, X-ray diffraction analysis, and density functional theory (DFT) calculations. The experimental and computational results suggest that the ortho-substitution of the aryl groups is necessary for the formation of terminal borylene complexes.