Bond-strengthening p backdonation in a transition-metal p-diborene complex

Transition-metal catalysis is founded on the principle that electron donation from a metal to a ligand is accepted by an antibonding orbital of the ligand, thereby weakening one of the bonds in the ligand. Without this, the initial step of bond activation in many catalytic processes would simply not occur. This concept is enshrined in the well-accepted Dewar–Chatt–Duncanson model of transition-metal bonding. We present herein experimental and computational evidence for the first true violation of the Dewar–Chatt–Duncanson bonding model, found in a p-diborene complex in which an electron-rich group 10 metal donates electrons into an empty bonding p orbital on the ligand, and thereby strengthens the bond. The complex is also the first transition-metal complex to contain a bound diborene, a species not isolated before, either in its free form or bound to a metal

Unprecedented luminescence behavior of coinage metal p-diborene complexes

A number of unprecedented photophysical phenomena are observed in the study of luminescent p-diborene complexes of Cu and Ag, including unusually high fluorescence quantum yields in solution for complexes of these metals (up to unity). This indicates that very little or no intersystem crossing between S1 and Tn occurs in the complexes, despite the strong spin-orbit coupling of the metal atoms. The substitution of carbon for boron thus yields luminescent isolobal analogues of otherwise non-emissive olefin complexes of Cu and Ag

Unsupported boron–carbon s-coordination to platinum as an isolable snapshot of s-bond activation

s-Complexes of transition metals—key intermediates in metal-mediated bond activation and homogeneous catalysis—have traditionally been isolable only when chelating or when one of the participating atoms is hydrogen. Here, by treating the Lewis-basic transition metal complex [Pt(PEt3)4] with an electron-poor borirene, we isolate a complex with an unsupported borirene ligand bound, not through the unsaturated C=C bond, but exclusively via a B–C single bond. Using NMR spectroscopy, X-ray crystallography and density functional theory calculations, we show, herein, that coordination of the borirene ligand is based on electron donation from the B–C s bond to the metal, aided by a strong Pt-to-B dative interaction. The complex is the first isolable non-agostic s-complex featuring two p-block elements and has broad implications as a model for the metal-mediated activation of strong p-block-p-block s-bonds

Thienyl-substituted diboranes(4): electronic stabilization of radicals versus increased reactivity towards bond activation

Low-valent main-group chemistry involves a balancing act between steric and electronic stabilization of the electron-rich low-oxidation-state main-group centers and their desired reactivity. Herein we show that the combination of sterically-shielding mesityl and rotationally flexible 2-thienyl groups, the latter having the potential to be either electronically stabilizing or activating, at a diborane(4) provides both radical-anion stabilization and unusual bond activation and rearrangement reactions. The addition of a Lewis base to a 1,2-dimesityl-1,2-dithienyldiborane(4) (1) results in direct and unprecedented C?H borylation of one thienyl substituent with cleavage of the B?B bond. The facile one-electron reduction of 1 yields a stable diboron radical anion through delocalization of its unpaired electron over the entire planar 1,2-dithienyldiboron framework, as evidenced by EPR spectroscopy and DFT calculations. The two-electron reduction of 1 with magnesium-anthracene under more forcing conditions results in B?B-bond cleavage and replacement of one thienyl sulfur atom by a mesitylboron moiety, leading to the formation of a magnesium complex of an ?5-diborafulvene dianion. Salt metathesis of the latter with [(?6-p-cymene)RuCl2] affords a mixed ruthenium sandwich complex of an ?5-borylborole dianion. Calculations highlight the structural and electronic changes in the boron-substituted heterocyclic C4B dianion upon switching coordination from magnesium (diborafulvene dianion) to ruthenium (borylborole dianion)

Strongly phosphorescent transition metal p complexes of boron-boron triple bonds

Herein are reported the first p complexes of compounds with boron-boron triple bonds to transition metals, in this case CuI. Three different compounds were isolated that differ in the number of copper atoms bound to the BB unit. Metallation of the B-B triple bonds causes significant lengthening of the B-B and B-CNHC bonds, as well as large upfield shifts of the 11B NMR signals, suggesting greater orbital interactions between the boron and transition metal atoms than those observed with recently published diboryne / alkali metal cation complexes. In contrast to previously-reported fluorescent copper(I) p complexes of boron-boron double bonds, the Cun-p-diboryne compounds (n = 2, 3) show intense phosphorescence in the red to near-IR region from their triplet excited states, according to their microsecond lifetimes, with quantum yields of up to 58%. The bonding situation, as well as the unusual photophysical properties, has been further corroborated by DFT studies

Exclusive p encapsulation of light alkali metal cations by a neutral molecule

Cation-p interactions are one of the most important classes of non-covalent bonding, and are seen throughout biology, chemistry and materials science. However, in almost every documented case, these interactions play only a supporting role to much stronger covalent or dative bonds, making examples of exclusive cation-p bonding exceedingly rare. In this work, a neutral diboryne molecule is found to encapsulate the light alkali metal cations Li+ and Na+ in the absence of a net charge, covalent bonds, or lone-pair donor groups. The resulting encapsulation complexes are to our knowledge the first structurally authenticated species in which a neutral molecule binds the light alkali metals exclusively through cation-p interactions