Tris boryl Complexes

Tris(boryl) complexes have been shown to have a central role in the iridium-catalyzed borylation of arenes [136,137]. The syntheses of the half-sandwich iridium(III) systems ( 6-toluene)Ir(Bcat)3, 9.34, and rf-mesitylene)Ir(Bcat)3, 9.35, were reported by Marder and co-workers in 1993, by exploiting the reaction of ( 5-Ind)Ir(cod) with excess HBcat using the arene as a solvent. The organic products of the reactions were found not only to include species resulting from the hydroboration or hydrogenation of the cod or indenyl ligands, but also compounds derived from borylation 

---

The range of group 9 mono-, bis- and tris(boryl) complexes discussed below provides a useful structural basis on which to discuss the trans influence... 

Scheme 22 Interplay between formally Rh(III) and Rh(V) boryl complexes formation of bis- and tris(boryl) complexes 9.17 and 9.18 via B-H oxidative addition, and their subsequent reactions with phosphine via B-H reductive elimination...

Fig. 32 Crystallographically characterized iridium tris(boryl) complexes 9.34-9.37...

Nguyen P, Blom HP, Westcott SA, Taylor NJ, Marder TB. Synthesis and structures of the first transition-metal tris(boryl) complexes (T -arene)Ir(B02C6H4)3. J Am Chem Soc. 1993 115 9329-9330. 

Tris(dioxime) complexes that are capped with a boryl group BR at only one end of the molecule are also known, e.g., with technetium(III) [227, 228] and rhenium(III) [229]. 

The direct borylation of arenes was catalyzed by iridium complexes [61-63]. Iridium complex generated from [lrCl(cod)]2 and 2,2 -bipyridine (bpy) showed the high catalytic activity of the reaction of bis (pinaco la to) diboron (B2Pin2) 138 with benzene 139 to afford phenylborane 140 (Equation 10.36) [61]. Various arenes and heteroarenes are allowed to react with B2Pin2 and pinacolborane (HBpin) in the presence of [lrCl(cod)]2/bipyridne or [lr(OMe)(cod)]2/bipyridine to produce corresponding aryl- and heteroarylboron compounds [62]. The reaction is considered to proceed via the formation of a tris(boryl)iridium(lll) species and its oxidative addition to an aromahc C—H bond. 

The mechanism proposed for aromatic C-H borylation of aromatic compounds 1 by B2pin2 3 catalyzed by the Ir-bpy complex is depicted in Scheme 3 [6-9]. A tris(boryl)Ir (III) species [5, 6, 11] 6 generated by reaction of an Ir(I) complex 5 with 3 is chemically and kinetically suitable to be an intermediate in the catalytic process. Oxidative addition of 1 to 6 yields an Ir(V) species 7 that reductively eliminates an aromatic boron compound 4 to give a bis(boryl)Ir(III) hydride complex 8. Oxidative addition of 3 to 8 can be followed by reductive elimination of HBpin 2 from 9 to regenerate 6. 2 also participates in the catalytic cycle via a sequence of oxidative addition to 8 and reductive elimination of H2 from an 18-electron Ir(V) intermediate 10. Borylation of 1 by 2 may occur after consumption of 3, because the catalytic reaction is a two-step process - fast borylation by 3 then slow borylation by 2 [6],... 

The structure of a further iridium bis(boryl) complex, the cationic species [(tbbpy)(cod)Ir(Bpin)2]+[OTf] (9.33), together with that of a related neutral tris(boryl) system (vide infra), has been reported as part of a study to elucidate the mechanism and intermediates of arene borylation chemistry [136]. 9.33 features an approximately octahedral coordination geometry with mutually cis pairs of bipyridyl, diene and Bpin ligands. The two Ir-B bond lengths [2.06(2) and 2.12(1) A] are effectively identical within the standard 3a limit. 

Intensive studies also showed that many transition metal complexes are able to catalyze aromatic C-H borylation of various arenes (Scheme 7), e.g., Cp Ir(H)(Bpin)(PMe3) [64,65], Cp Rh(Ti4-C6Me6) [65,66], ( 75-Ind)Ir(COD) [67], (776-mesitylene)Ir(Bpin)3 [67], [IrX(COD)]2/bpy (X = Cl, OH, OMe, OPh) [68-70]. A very recent study by Marder and his coworkers showed that [Ir(OMe)(COD)]2 can also catalyze borylation of C-H bonds in N-containing heterocycles [71]. For the Ir-catalyzed borylation reactions, it is now believed that tris(boryl)iridium(III) complexes [67,69], 40c, [72] are likely the reactive intermediates and a mechanism involving an Ir(III)-Ir(V) catalytic cycle is operative [67,69]. A recent theoretical study [73] provided further support for this hypothesis. A mechanism, shown in Scheme 8, was proposed. Interestingly, there are no cr-borane complexes involved in the Ir-catalyzed reactions. The very electropositive boryl and hydride ligands may play important roles in stabilizing the iridium(V) intermediates. 

In the bis-boryl complexes 7, 8, and 9 [49-52], the cis arrangements of the two boryl ligands prevent the very strong trans influence ligands from being trans to each other. In the tris-boiyl complexes 10,11, and 12 [53-55], the facial arrangements of the three boryl ligands do the same. 

Theoretical calculations, using the DFT method, examined the iridium-catalyzed borylation of benzene with diboron source B2Cg2 (eg — ethyleneglycolato, —OCH2CH2O—). This study found that oxidative addition of the B—B bond occurred at a lower energy than the corresponding C—H bond activation step. A tris(boryl)iridium(III) complex was also concluded to be catalyticaUy active, and an unusual seven-coordinate iridium(V) species is beheved to be involved as a key intermediate in the catalytic cycle.A tris(boryl)iridium(III) complex has been previously prepared and structurally characterized by Marder from the reaction between [Ir(T] -indenyl) (r " -COD)] (COD = cyclooctadiene) and excess of catecholborane. ... 

Borylated Tris(dioxime)metal Complexes and Related Compounds 4.1.3.1... 

---