Boratranes

Metallaboratranes

Metallaboratrane compounds are a class of molecules with a cage-like structure that feature metal-to-ligand MB s–dative bonds, as illustrated below.

While dative bonding is a common feature of transition metals, the metal is normally the electron pair acceptor, rather than the donor. Of particular interest, therefore, is the nature of the bonding in {[B(mim^R)₃]M} complexes and the electronic impact that the borane ligand exerts on a metal center. In this regard, two important factors pertaining to the coordination of any ligand to a metal center are the effects that it has on (i) the electron count and (ii) the dⁿ configuration, both of which play an important role in evaluating the stability and reactivity of a molecule.

The essential feature of a general M–B bonding interaction is illustrated by the simplified molecular orbital diagram shown below.

Thus, the interaction between a filled metal based orbital and the empty orbital on boron results in a filled M–B bonding orbital and an unoccupied M–B antibonding orbital. Since the M–B bonding orbital is occupied by a pair of electrons that were originally on the metal, a metal center that originally possessed a dⁿ configuration becomes d^n–2 upon coordination to boron. The transition from a dⁿ to d^n–2 configuration upon coordination of a BR₃ fragment bears a close analogy to the change resulting from the interaction of a metal center with another Lewis acid, namely H⁺. Thus, it is widely recognized that protonation of a dⁿ metal center results in the formation of a metal–hydride in which the metal center has a d^n–2 configuration.

The reactivity of the MB bond in metallaboratranes has received very little attention. Therefore, we have started to investigate the chemistry of metallaboratranes, with the intention of uncovering new reactivity that is associated with cleavage of the MB bond.

We recently described that the FeB bond of the ferraboratrane [k⁴-B(mim^But)₃]Fe(CO)₂ could be cleaved by a variety of reagents to give B–functionalized tris(mercaptoimidazolyl)borate complexes of the type [YTm^But]FeZ. For example, [k⁴-B(mim^But)₃]Fe(CO)₂ reacts with (i) CHX₃ (X = Cl, Br) to give [XTm^But]FeX, and (ii) I₂ in CHCl₃ to give [ClTm^But]FeI.

In a similar manner, the NiB bond of the nickel boratrane compounds [k⁴-B(mim^But)₃]NiX (X = Cl, OAc, SCN, N₃) may also be cleaved by suitable reagents to afford B–functionalized derivatives, [YTm^But]NiZ.

For example, [k⁴-B(mim^But)₃]NiCl reacts with CHCl₃, CHBr₃, and I₂ to give [ClTm^But]NiCl, [ClTm^But]NiBr, and [ClTm^But]NiI, respectively. Likewise, the azide and isothiocyanate complexes [k⁴-B(mim^But)₃]NiN₃ and [k⁴-B(mim^But)₃]NiNCS react with I₂ to give [N₃Tm^But]NiI and [SCNTm^But]NiI. In each of these examples, the boron is functionalized by the ligand originally attached to nickel. It is, therefore, significant that [k⁴-B(mim^But)₃]NiCl reacts with XeF₂ to yield [FTm^But]NiCl in which the boron is functionalized by the reagent and the chloride ligand remains attached to nickel.

Selected References

“Synthesis and Structural Characterization of [h³–B,S,S–B(mim^R)₃]Ir(CO)(PPh₃)H (R = Bu^t, Ph) and [h⁴–B(mim^But)₃]M(PPh₃)Cl (M = Rh, Ir): Analysis of The Bonding in Metal Borane Compounds.” Victoria K. Landry, Jonathan G. Melnick, Daniela Buccella, Keliang Pang, Joseph C. Ulichny, and Gerard Parkin Inorg. Chem. 2006, 45, 2588-2597.

“Reactivity of the Metal®BX₃ Dative s-Bond: 1,2-Addition Reactions of the Fe®BX₃ Moiety of the Ferraboratrane Complex [k⁴-B(mim^But)₃]Fe(CO)₂.” Joshua S. Figueroa, Jonathan G. Melnick and Gerard Parkin Inorg. Chem. 2006, 45, 7056-7058.

“Palladium Complexes with PdB Dative Bonds: Analysis of the Bonding in the Palladaboratrane Compound [k⁴–B(mim^But)₃]Pd(PMe₃).” Keliang Pang, Stephanie M. Quan and Gerard Parkin Chem. Commun. 2006, 5015-5017.

“Reactivity of the NiB Dative s–Bond in the Nickel Boratrane Compounds [k⁴–B(mim^But)₃]NiX (X = Cl, OAc, NCS, N₃): Synthesis of a Series of B-Functionalized Tris(2-mercapto-1-t-butylimidazolyl)borato Complexes, [YTm^But]NiZ.” Keliang Pang, Joseph M. Tanski, and Gerard Parkin Chem. Commun. 2008, 1008-1010.

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