Bridged 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. 

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 

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. 

The tetrahedrally coordinated boron center in the base stabilized complex 6 shows the expected increase of the metal-boron distances, which thus range between those of cobalt boryl complexes114,115 and cobaltaboranes.116 

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

Already in 1995 we obtained the first bridged borylene complexes 11a, d from the unusual reaction of l,2-dihalodiboranes(4) with K[CpMn(CO)2H] (Equation 3). Although this synthesis was recently extended to further examples lib, c and also to the rhenium analogue lie we aimed at a more general access to this interesting class of compounds. 

The bridging borylene complexes (V, VI) are of some special relevance since they represent the first structurally authentic examples for the stabilization of the borylene ligand -BR in the coordination sphere of a transition metal. Species where the borylene unit adopts a bridging position between two metals represent the most numerous and well-studied class of these compounds. 

Scheme 11 Synthesis of the base-stabilized bridging borylene complex 27...

Scheme 13 Synthesis of a bridging borylene complex with an unsupported borylene ligand and photolytic conversion to the corresponding supported derivative...

Scheme 16 Reaction of the bridging borylene complex 26b with [Pd(PCy3)2]...

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. 

Scheme 18 Synthesis of the heterodinuclear bridging borylene complexes 44a and 44b...

Scheme 20 Synthesis of the semi-bridging borylene complexes 48 and 49...

Synthesis and Reactivity of Semi-bridging Borylene Complexes [79] ... 

Already in 1995 we obtained the first bridged borylene complexes 11a, d fi-om the unusual reaction of l,2-dihalodiboranes(4) with K[CpMn(CO)2H] (Equation 3). 

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