Borylene complexes

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. 

Protic reagents such as primary amines, alcohols, and water led to the corresponding substituted borylene complexes [n-BX (C5H4Me) Mn(CO)2 2] (13, X = NHtBu 14, X = NHPh 15, X = OMe 16, X = OEt 17, X = Oz Pr 18, X = OH) in high yields of up to 94% (Fig. 3).100 102 It should be noted that due to the kinetic lability of the metal-boron bond such reactions at the boron center with retention of the M-B linkage are very rare for both boryl- and borylene complexes and were observed subsequently by Roper only in the case of one particular boryl complex.103... 

Abbreviations a, from monoboranes b, from diboranes(4) c, by borylene transfer d, from boryl complexes e, from borylene complexes x, no X-ray data available y, no "B-NMR data available. 

Fig. 4. Structure of selected bridged borylene complexes in the crystal.

Recently, the borylene complex [p-B(NMe2) (r 5-C5H5)Mn(CO)2 2] (1) was subject to detailed computational studies and the theoretically predicted and the experimentally derived structural parameters were found to be in very good agreement (Table I).117 Density functional theoretical studies have concluded that borylenes BX can be viable ligands in the design of transition metal complexes, which are thermodynamically stable with... 

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 borylene complex 31 turned out to be highly reactive and readily regenerates its precursor G upon reaction with [PPh4]Br. 

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... 

It should be noted that in 1973, Timms had already recognized borylene complexes to be a potentially useful source of borylenes in cases where gaseous species (i.e., haloborylenes being obtained via high temperature synthesis) cannot be used.3 Future investigations may likely show that borylene complexes have a synthetic applicability as a unique source for B-R species in condensed phase far beyond the intermetal transfer reactions discussed here. 

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 complex 30 with Ag[SbF6] in the presence of MeCN and ethanol yielded the cationic borylene complex [Os =BOEt(NMeCsH4N) MeCN(CO)(PPh3)2][SbF6] (38) with abstraction of the osmium bound... 

A detailed comparison of the spectroscopic and structural parameters of the base stabilized borylene complexes 30, 36, 37, and 38 with those of tethered boryl complexes such as F established their close structural relationship and led the authors to the statement that a formal distinction between those species is somewhat artificial. Their reactivity, however,... 

Many experimental and theoretical studies have now contributed to the understanding of the classical metal-boron linkage in borylene complexes. Future work could probably envisage possible applications of those species, especially if one considers their close relationship to carbonyl or carbene complexes and their potential in stoichiometric or catalytic reactions. 

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