Borylenes formation

The formation of the iron borylene complexes 7 and 8 was observed under all conditions applied, however, the synthesis of the corresponding ruthenium analogue 9 depends on the reaction conditions and stoichiometry. 

More recently, Aldridge further exploited this approach for the synthesis of [p-BMcs [(q S-CsI Is)Fc(CO)2 2] (10) [Eq. (5)] which represents a rare example of a structurally authentic complex with an unsupported borylene ligand B-R (R = alkyl, aryl, silyl). Interestingly, the formation of 10 is not accompanied by CO liberation although rather harsh conditions had to be applied.97... 

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

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. 

The addition of borylenes to cyclohexene was reported to give reactive boranes (46) (76TL4765, 77JA3182), but reinvestigation of this reaction proved the formation of boratricyclic derivative (47 R = Me) (83POL1337). 

The reactivity of the aminoborylene complexes 1 and 2 under thermal conditions leads to the formation of semi-bridged borylene or bis-borylene complexes (vide infra). The photochemical transfer of the borylene ligand is useful for the preparation of borirenes [23], turned out to be an alternative for the synthesis of 1, and was also used for the synthesis of the first half-sandwich borylene complex [(775-C5H5)(OC)3V=B=N(SiMe3)2] (5) [24] (Scheme 4). In the crystal, the borylene ligand adopts the expected linear geometry (V-B-N 177.9(4)°) and the B-N bond (137.8(7) pm) is... 

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]. 

A number of tethered osmium boryl complexes have been developed recently in which one of the boryl substituents features an additional pendant donor which coordinates to the metal centre as a tether . For systems resulting from reactions with bifunctional donors such as 2-aminopyridine two possibilities exist, depending on which donor atom interacts with the boron centre and which with the metal. Thus, tethered boryl complexes result from coordination of the anionic donor at boron, with the neutral donor tether coordinated at osmium the reverse coordination possibility leads to the formation of intramolecular base-stabilized borylene complexes. 

Scheme 1 S.24 Formation of a stable borylene, boronium salt and radical cation [295].

Scheme 11 Reversible dehydrogenation of mesitylborane at Ru(ll) leading to the formation of borylene complex (Cy3P)2(CI)(H)Ru(=BMes) (54). ...

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