B═B and B≡E (E = N and O) Multiple Bonds in the Coordination Sphere of Late Transition Metals

Abstract

Because of their unusual structural and bonding motifs, multiply bonded boron compounds are fundamentally important to chemists, leading to enormous research interest. To access these compounds, researchers have introduced sterically demanding ligands that provide kinetic as well as electronic stability. A conceptually different approach to the synthesis of such compounds involves the use of an electron-rich, coordinatively unsaturated transition metal fragment. To isolate the plethora of borane, boryl, and borylene complexes, chemists have also used the coordination sphere of transition metals to stabilize reactive motifs in these molecules. In this Account, we summarize our results showing that increasingly synthetically challenging targets such as iminoboryl (B≡N), oxoboryl (B≡O), and diborene (B═B) fragments can be stabilized in the coordination sphere of late transition metals. This journey began with the isolation of two new iminoboryl ligands trans-[(Cy3P)2(Br)M(B≡N(SiMe3))] (M = Pd, Pt) attached to palladium and platinum fragments. The synthesis involved oxidative addition of the B-Br bond in (Me3Si)2N═BBr2 to [M(PCy3)2] (M = Pt, Pd) and the subsequent elimination of Me3SiBr at room temperature. Variation of the metal, the metal-bound coligands, and the substituent at the nitrogen atom afforded a series of analogous iminoboryl complexes. Following the same synthetic strategy, we also synthesized the first oxoboryl complex trans-[(Cy3P)2BrPt(BO)]. The labile bromide ligand adjacent to platinum makes the complex a viable candidate for further substitution reactions, which led to a number of new oxoboryl complexes. In addition to allowing us to isolate these fundamental compounds, the synthetic strategy is very convenient and minimizes byproducts. We also discuss the reaction chemistry of these types of compounds. In addition to facilitating the isolation of compounds with B≡E (E = N, O) triple bonds, the platinum fragment can also stabilize a diborene (RB═BR) moiety, a bonding motif that thus far had only been accessible when stabilized by N-heterocyclic carbenes (NHCs). In the new π-diborene [(Et3P)2Pt(B2Dur2)] (Dur = 2,3,5,6-Me4-C6H) complex, the diborene ligand receives electron density from Pt, leading to a strong Pt-B bond and a B═B bond. We attribute this result to the very short B═B bond distance (1.51(2) Å) while coordinated to platinum. Overall, an increasing number of chemists are examining the chemistry of multiply bound boron compounds. The isolation of an oxoboryl complex is of special interest not only from a structural standpoint but also because of its orbital similarities to the ubiquitous CO ligand. Detailed computational studies of the π-diborene complex [(Et3P)2Pt(B2Dur2)] show that the bonding properties of this molecule violate the widely accepted Dewar-Chatt-Duncanson (DCD) bonding model.