Borylene terminal

Borane Boryl Bridging borylene Terminal borylene... 

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... 

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,... 

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

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

Roper, 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). 

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... 

The metal-ligand complex, (CO)4Fe-B(r 5-C5Me5) is reported as a terminal borylene metal complex, i.e., it possesses a B-R fragment coordinated to a single metal center. Formulate this compound as a metal cluster. 

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... 

The reaction of the terminal borylene [(OC)sM=B=N(SiMe3)2] (M = Cr, W) compounds with [ RhCl(CO)2 2] in deuterated benzene for a few minutes led to formation of a tetranuclear rhodium bis-borylene compound as black crystals 18a. The bis-borylene derivative possesses two fused B-Rh-Rh, dirhodium-borirane rings <2006AGE2132>. Compound 18a was characterized by multinuclear NMR and infrared spectroscopy, elemental analysis and single crystal X-ray diffraction. The B NMR (C6D6) spectrum presents a single signal at 74 ppm, a low frequency shift when compared with similar systems, which normally appear between 98 and 120 ppm. 

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... 

The photochemistry of bridging borylene complexes differs significantly from that of their terminal counterparts. The aminoborylene complex 23a,b proved to be entirely unreactive under photolytic conditions, which is in sharp contrast to the terminal aminoborylene complexes 1 and 2. The chloro derivative 34, however, yielded upon irradiation in the presence of a CO donor (i.e. M(CO)6> M = Cr, Mo, W) the dimetalla-mdo-tetraborane [B2Cl2 ( 75-C5R5)Mn(CO)2 2] (43) as shown in Scheme 17 [64]. The reaction provides a direct synthetic link between electron-precise borylene complexes and electron-deficient metallaboranes. 

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] ... 

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