Borylenes

Braunschweig, Holger, Borylenes as Ligands to Transition Metals. 

Cobaltocene is partially oxidized and in part undergoes insertion of a borylene group, RB. The borinato ligands derive from the unknown borabenzene ( 6.5.3.4). Some porphyrinatoindium and thallium complexes ( 6.5.2.2) can also be synthesized via oxidative addition reactions (TPP)InCl is added oxidatively to Co2(CO)g and Mn2(CO) to give (TPP)In—Co(CO)4 and (TPP)ln — Mn(CO)j, respectively, and (oep)InCl is added to CojfCOg to yield (oep)ln— 0(00)4. 

All potentials versus the saturated calomel electrode (SCE). Borylene shift per ring. c From Ref. 96. 

At present, boron-containing compounds and their metal complexes are of considerable interest. Amongst them are found the coordination compounds 545-547 of transition metals containing boryle and borylene ligands [942] ... 

The synthesis of borylenes is as yet restricted to the aforementioned high and low temperature techniques, respectively. Related hypovalent compounds of other p-block elements, e.g., Al,17 C,18 22 and Si,23 28 however,... 

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... 

Fig. 3. Substitution reactions at a metal coordinated borylene ligand.

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... 

---