Terminal borylene complexes

An extension of this synthetic approach to dianionic carbonylates and suitable dihaloboranes [Eq. (9)] gave access to the first structurally authentic terminal borylene complexes [(OC)4Fe-B(r 5-C5Me5)] (21)122 and 

By applying the corresponding precursors K2[Fe(CO)4] and Na2[Cr (CO)5] we also obtained the related aminoborylene complexes [(OC)4 Fe=B=N(SiMe3)2] (23)123 and [(OQ5Cr=B=N(SiMe3)2] (24)123 (Fig. 6), the latter of which was subsequently characterized by X-ray structure analysis.124 

The formation of 21-24 implies that the boron center in such terminal borylene complexes requires stabilization by a ligand that is both sterically demanding and electron releasing. Evidently, the r 5-C5Me5 and the N(SiMe3)2 substituents provide sufficient steric shielding and 7i-electron stabilization. In the case of the former, however, an alternative point of view may be applied, namely that the electron deficiency of the boron center is relieved by its incorporation into a non-classical polyhedral skeleton. Thus, 

Silyl and germyl complexes with suitable functionalities were successfully used as precursors for the synthesis of compounds with corresponding transition metal element multiple bonds.137 145 Due to the aforementioned 

however, succeeded in converting the osmium boryl complex [Cl2BOsCl(CO)(PPh3)2] (E) into the terminal borylene complex [Os (=BNHC9H6N)Cl2(CO)(PPh3)2]147 (26) upon reaction with 8-aminoquino-line according to Fig. 7. Compound 26 is noteworthy as it represents the first example of a base stabilized terminal borylene complex of the type [LXM = B(L)-R] (IVa, Fig. 1). 

---

Fig. 6. Terminal borylene complexes from salt elimination reactions.

Fig. 7. Formation and reactivity of base-stabilized terminal borylene complexes of osmium.

The obvious need for an alternative and more general synthetic access to terminal borylene complexes initiated our studies on the potential use of complexes of the type [(OC)5M=B=N(SiMe3)2] (22, M = W 24, M = Cr) as a source for the aminoborylene B=N(SiMe3)2. Recently, we reported on the first photochemically induced intermetal borylene transfer according to Eq. (II).98... 

In solution, the common and most characteristic spectroscopic feature of all terminal borylene complexes with coordinatively unsaturated, i.e., two coordinate boron atoms, are the nB-NMR resonances, which are significantly deshielded with respect to those of corresponding amino- and silylboryl complexes of the type [LxM-B(X)NR2]53 56 and [LXM B(X)SiR3].146... 

The decisive influence of the boron bound ligand R on the metal boron linkage in terminal borylene complexes [LXM = B-R] was already demonstrated to some extent by the aforementioned differences in the nB-NMR shifts. In the solid state, this influence becomes even more evident if one compares the results of the X-ray structure analyses of [(OC)5Cr=B=N(SiMe3)2] (24) and [(OC)5Cr=B-Si(SiMe3)3] (25) (Fig. 9). 

As already indicated, the chemistry of terminal borylene complexes is as yet almost unexplored. In addition to the photochemically induced borylene transfer, which was already discussed in Chapter 3.2, studies of the reactivity of terminal borylene complexes are restricted to two recent reports by Roper.147,148 The base-stabilized borylene complex [Os (=BNHC9H6N)Cl2(CO)(PPh3)2] (26) undergoes a reaction with ethanol to yield the ethoxy(amino)boryl complex [Os B(OEt)NHCgH6N Cl(CO) (PPh3)2] (35) according to Eq. (13) with a 1,2-shift of the quinoline nitrogen atom from the boron to the osmium center. The alcoholysis of 26 indicates that even the boron atom in base-stabilized borylene complexes displays some electrophilic character—a fact already predicted by a theoretical study.117... 

Reaction of [W(CO)s] with Br2BN(SiMe3)2 afforded the terminal borylene complex [ (Me3Si)2NB= W(CO)5], which is octahedral, with a linear W-B-N group and short B-N bonds (1.34 A). 

The missing link in this series of compounds, terminal borylene complexes 17 - 19 (Figure 6) with twofold coordinated boron atoms, were obtained very recently again by... 

Photo chemically induced borylene transfer has already been discussed as a valuable synthetic method for the generation of terminal borylene complexes not accessible through salt elimination reactions (vide supra). Photolysis of [(CO)5W=BN(SiMe3)2] (2) in the presence of [( 5-C5H5)Re(CO)3] yielded the... 

Until very recently, the formation of the semi-bridging borylene compounds 48 and 49 represented the only instance of clean thermal, i.e., non-photolytic, reactivity of the terminal borylene complexes 1 and 2. Interestingly, however, an unprecedented metal-to-metal borylene transfer occurred from [(OC)5M=BN(SiMe3)2] (M = Cr, 1 M = W, 2) to [RhCl(CO)2]2 at room temperature, yielding the tetranuclear bis(borylene) complex [Rh4 /x-BN(SiMe3)2 2Cl4(/x-CO)(CO)4] (50) (Scheme 21) [74]. 

Cationic Terminal Borylene Complexes Interconversion of Amino and Alkoxy Borylenes by an Unprecedented Meerwein-Ponndorf Hydride Transfer [80] ... 

A single bonded cationic terminal borylene complex [81] ... 

Scheme 9 Synthesis of mesityl(halo)boryl complexes via halide addition to a cationic terminal borylene complex...

Metallaborane and borylene complexes are related to some extent because both types possess direct metal-boron bonds however, the nature of these interactions is varied. In metallaborane clusters, the framework is made up by nonclassical, electron-deficient, multicenter two-electron bonds, while the borylene ligand (BR) is related to one or more transition metal centers by classicrJ, electron-precise, two-center two-electron (2c-2e) bonds [280 -283]. Since the discovery of various bridged and terminal borylene complexes by Braunschweig and Aldrige, the chemistry of this subarea of transition metal complexes of boron has received significant attention from both structure/bonding and... 

A novel 3-borane-l-alkylimidazol-2-ylidene complex has been obtained by reaction of a 3-borane-l,4,5-trimethyl-imidazole-2-ylidene anion with GpFe(GO)2Br, A newly developed route to terminal borylene complexes makes use of halide abstraction to generate the first example of a cationic [L MBR] species. The compound so generated, [Gp Fe(GO)2(BMes)] [BAr 4] (Ar = G6H3(GF3)2-3,5), contains the shortest M-B distance yet reported, a feature indicative of a novel Fe=B double bond. 

Access to unsaturated ruthenium complexes has been achieved via phosphine complexation with triphenylborane. The synthesis and structure of the zwit-terionic arene complex (riCph-BPh2H)Ru(PMe3)2(SiMe3) is reported. A base-stabilised terminal borylene complex of osmium has been derived from reaction between a dichloroboryl complex and 8-aminoquinoline. ... 

Braunschweig H, Colling M, Kollaim C, Merz K, Radacki K (2001) [(OC)5Cr=BSi (SiMe3)3] a terminal borylene complex with an electronically unsaturated boron atom. Angew Chem Int Ed 40 4198... 

---