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

Herein are reported the first π 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) π complexes of boron-boron double bonds, the Cun-π-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

Synthesis and characterization of a mercury-containing metalloborylene

The reaction of phenylmercuric chloride with an anionic dimanganaborylene [Cp2(CO)4Mn2B]Na led to the formation and isolation of a trimetalloborylene featuring at previously unreported bond between mercury and a single boron atom. Examination by 199Hg NMR displayed a small 11B-199Hg scalar coupling (J = 103 Hz), confirming the electronic interaction of the two atoms. The use of ETS-NOCV analysis indicated the nature of bonding to be σ-donation from a B-Mn π-orbital to Hg, in conjunction with weak Hgd→π* back-donation

Understanding, Modulating, and Leveraging Transannular M → Z Interactions

Density functional theory calculations have been performed on metallatranes featuring a group 13 elements at the bridgehead position to understand the factors that influence the nature of the M···Z (M = Fe, Co, Ni; Z = Al, Ga, In) interaction present in these complexes and the resultant reactivity at the metal center. The strength of the M···Z interaction increases with the increase in the size and polarizability of the bridgehead group 13 elements. The calculated reaction free energies (ΔG° values) for binding of different Lewis bases to the metallatranes are found to be significantly more exergonic for the larger In(III) ions. Quantum theory of atoms in molecules calculations reveal the covalent nature of the M···Z interactions, while the EDA-NOCV analysis indicates the strong binding ability of these metallatranes not only to different σ-donor and π-acceptor ligands but also to relatively inert species, such as N2

Theoretical Study on the Effect of Annelation and Carbonylation on the Electronic and Ligand Properties of <i>N</i>‑Heterocyclic Silylenes and Germylenes: Carbene Comparisons begin To Break Down

Quantum chemical calculations have been carried out to investigate the effect of annelation and carbonylation on the electronic and ligand properties of <i>N</i>-heterocyclic silylenes and germylenes. The thermodynamic stability of these ligands has been found to increase with annelation, while the reverse is true for carbonylation. This is in sharp contrast to N-heterocyclic carbenes (NHCs) where annelation leads to a decrease in their thermodynamic stabilities. Compared to nonannelated derivatives, annelated and carbonylated ones are found to be weaker σ donors but better π acceptors. The effect of carbonylation is more pronounced than annelation toward increasing the π acidity of these ligands. Carbonylation at the α-position with respect to the N atom attached to the Si/Ge center has been found to be the most effective way of enhancing the π acidity of these ligands. The computed natural charges reveal that electrophilicity increases upon both annelation and carbonylation. The calculated values of ³¹P NMR chemical shifts of corresponding phosphinidene adducts of these ligands have been found to correlate well with the π acidity of these Si/Ge centers

Tuning the Electronic and Ligand Properties of Remote Carbenes: A Theoretical Study

The effect of annulation and carbonylation on the electronic and ligating properties of remote N-heterocyclic carbenes (rNHCs) has been studied quantum-chemically. The thermodynamic stability of these complexes has been assessed on the basis of their hydrogenation and stabilization energies, while HOMO–LUMO gaps are used to measure the kinetic stabilities. Annulated/carbonylated rNHCs are found to be weaker σ donors but better π acceptors compared with the parent rNHCs. The reactivity of these rNHCs has been studied by evaluating their nucleophilicity and electrophilicity indices. The nucleophilicity values are in good agreement with the σ basicities of all of the rNHCs. The ³¹P NMR chemical shifts of the corresponding rNHC–phosphinidene adducts have been calculated and found to correlate well with the π acidities of these rNHCs

Ligand Properties of Boron-Substituted Five‑, Six‑, and Seven-Membered Heterocyclic Carbenes: A Theoretical Study

The electronic properties of boron-substituted five-, six-, and seven-membered heterocyclic carbenes have been studied using quantum chemical methods. The stability of carbenes has been examined from the values of their respective singlet–triplet and HOMO–LUMO gaps. Both the singlet–triplet and the HOMO–LUMO gaps indicate higher stability for six- and seven-membered P-heterocyclic carbenes (PHCs) containing boron atoms at the α position with respect to phosphorus atoms. While PHCs are better π acceptors, the π acidities of NHCs can be tuned by substituting a boron atom in the α position with respect to nitrogen. This is revealed by the energies of a π-symmetric unoccupied orbital centered at the central carbon atom. Reactivity of these carbenes has been discussed in terms of nucleophilicity and electrophilicity index. The calculated relative redox potential values and ¹³C NMR parameters are found to correlate well with the π acidities of the respective carbenes

Nature of Intramolecular Metal–Metal Interactions in Supported Group 4–Group 9 and Group 6–Group 9 Heterobimetallic Complexes: A Combined Density Functional Theory and Topological Study

Quantum chemical calculations have been carried out on a series of supported group 4–group 9 and group 6–group 9 heterobimetallic complexes designated by the general formulas [Cp₂M­(μ-E)₂M′(H)­(CO)­L] and [(CO)₄M­(μ-E)₂M′(H)­(CO)­L] where E = SH, SeH or PH₂ and L = PH₃, CO, NHC, or <i>a</i>NHC. An analysis of the optimized geometries of these molecules indicates the presence of an M···M′ interaction. The nature of this interaction is investigated by using Bader’s quantum theory of atoms in molecules (QTAIM), electron localization function (ELF), and source function (SF). The results of QTAIM analysis suggest a polar covalent interaction between the two disparate metal centers in these heterobimetallic complexes. ELF analysis identifies a bonding basin between the two metal centers, while SF analysis reveals that the metal–metal bonding is moderately delocalized. The strength of the M···M′ interaction is found to be stronger in group 4–group 9 heterobimetallic complexes compared to group 6–group 9 ones

Probing the Potential of Hitherto Unexplored Base-Stabilized Borylenes in Dinitrogen Binding

Computational investigations were carried out to probe the potential of several dicoordinate, singly base-stabilized borylenes of the form [L→BR] (L=neutral Lewis base) in dinitrogen binding. The calculated reaction free energies and activation barriers associated with the formation of mono- and diborylene-N2 adducts suggest the presence of thermally surmountable kinetic barriers towards their possible isolation. Our results show that the exergonicity of dinitrogen activation and fixation is linearly dependent on the natural charge at the boron center, which can be tuned to design novel boron-based compounds with potential applications to small-molecule activation. EDA-NOCV analysis reveals strong binding of dinitrogen to these base-stabilized borylenes

Spectroscopic Distinction of Different Carbon Bases: An Insight from Theory

Spectroscopic differentiation based on the ¹³C NMR chemical shift of the parent and protonated derivatives of carbon­(II) and carbon(0) bases has been proposed. The ¹³C chemical shift of the central carbon atom of carbenes in their parent and protonated forms will experience more downfield shift, whereas the central carbon atom of carbones will experience a lesser downfield shift; such shifts for compounds that possess “hidden” carbon(0) characteristics will lie between these two extremes. The ¹³C chemical shifts of the protonated derivatives are solely dependent on the out-of-plane p_π occupancies of the central carbon atom. This difference arises due to their unique difference in bonding and may provide an easier distinction between these two classes of compounds