Bond Dissociation Energies of Carbene-Carbene and Carbene-Main Group Adducts.

A range of carbene structures and their adducts with one another and with a selection of small-molecule electrophiles and nucleophiles were examined at the composite correlated molecular orbital theory G3MP2 level to explore ground-state "carbenic" structures, their stabilities, and reactivities. Differences between carbene general classification as a singlet electrophilic carbene or singlet nucleophilic carbene and their given reactivity are discussed. A key quantity is the carbon-carbon bond dissociation energy for carbene dimers or the carbene-adduct dissociation energy for other species. The carbene dimer bond dissociation energies span a wide range from 10 to 170 kcal/mol. The hydrogenation energies and singlet-triplet splitting were found to correlate best with the carbene's self-dimerization energy, whereas other descriptors do not. The proton and fluoride affinities of the carbenes alone prove inadequate for classifying reactivity among classes of carbenes. The self-dimerization bond dissociation energy, hydrogenation energy, and singlet-triplet splitting of various carbenes, despite sometimes large differences in proton affinity and other indicators of reactivity, provide usable metrics to correlate substantial amounts of thermodynamic and kinetic (reactivity) information regarding these structures.

Lewis Acidity and Basicity: Another Measure of Carbene Reactivity.

The structural and energetic reactivities of various carbenes are evaluated against a standard electrophile (proton) and a standard nucleophile (fluoride). The proton and fluoride affinities of the carbenes studied provide an increased understanding of reactivity modes and mechanisms. General classification of carbenic reactivity as a singlet nucleophilic carbene or a singlet electrophilic carbene is facilitated by this present study, and a need for further classification means along the border between electrophilic and nucleophilic reactivity is considered. The results are based on electronic structure calculations at the composite correlated molecular orbital theory G3MP2 level.

Experimental and Computational Study of the Structure, Steric Properties, and Binding Equilibria of Neopentylphosphine Palladium Complexes.

Steric properties of crystallographically and computationally determined structures of linear palladium(0) and square planar palladium(II) complexes of di(tert-butyl)neopentylphosphine (P(t-Bu)2Np), tert-butyldineopentylphosphine (P(t-Bu)Np2), and trineopentylphosphine (PNp3) have been determined. Structures of linear palladium(0) complexes show that steric demand increases as tert-butyl groups are replaced with neopentyl groups (P(t-Bu)2Np < P(t-Bu)Np2 < PNp3). In square planar palladium(II) complexes, PNp3 gives the smallest steric parameters, whereas P(t-Bu)Np2 has the largest steric demand. The change in the steric demand of PNp3 compared to P(t-Bu)2Np and P(t-Bu)Np2 results from a significant conformational change in PNp3 depending on the coordination number of the metal. The steric properties of these ligands were also probed by measuring the equilibrium constant for coordination of free phosphine to dimeric [(R3P)Pd(µ-Cl)Cl]2 complexes. Binding equilibria follow the same trend as the steric parameters for square planar complexes with PNp3 having the highest binding constant. In contrast to the normal trend, the neopentylphosphines show increased pyramidalization at phosphorus with increasing steric demand. We hypothesize that this unusual dependence reflects the low back side strain of the neopentyl group, which allows the ligand to be more pyramidalized while still exerting a significant front side steric demand.

Synthesis, Structural Characterization, and Coordination Chemistry of (Trineopentylphosphine)palladium(aryl)bromide Dimer Complexes ([(Np3P)Pd(Ar)Br]2).

A series of [(PNp3)Pd(Ar)Br]2 complexes (PNp3 = trineopentylphosphine, Ar = 4-tolyl, 4-tert-butylphenyl, 2-tolyl, 4-methoxy-2-methylphenyl, 2-isopropylphenyl, and 2,6-dimethylphenyl) were synthesized and structurally characterized by X-ray crystallography and density functional theory optimized structures. The trineopentylphosphine ligand is able to accommodate coordination of other sterically demanding ligands through changes in its conformation. These conformational changes can be seen in changes in percent buried volume of the PNp3 ligand. The binding equilibria of the [(PNp3)Pd(Ar)Br]2 complexes with pyridine derivatives were determined experimentally and analyzed computationally. The binding equilibria are sensitive to the steric demand of the pyridine ligand and less sensitive to the steric demand of the aryl ligand on palladium. In contrast to previous studies, the binding equilibria do not correlate with pyridine basicity. The binding equilibria results are relevant to fundamental ligand coordination steps in cross-coupling reactions, such as Buchwald-Hartwig aminations.

Preparation and Characterization of Group†13 Cyanides.

Binary Group 13 cyanides [PPh4 ][Ga(CN)4 ], [PPh4 ]2 [In(CN)5 ], and [PPh4 ]2 [Tl(CN)5 ] were obtained in quantitative yields from the corresponding metal trifluorides MF3 (M=Ga, In, Tl) by fluoride-cyanide exchange reactions with Me3 SiCN in the presence of stoichiometric amounts of [PPh4 ]CN in acetonitrile solution. The 2,2'-bipyridine (bipy) adducts [(bipy)2 Ga(CN)2 ][Ga(CN)4 ], [(bipy)In(CN)3 ], and [(bipy)Tl(CN)3 ] were obtained from the reaction of MF3 with Me3 SiCN and bipy in acetonitrile. While the reaction of the metal trifluorides with Me3 SiCN in acetonitrile resulted in recovery of the starting materials, the reaction of MF3 with Me3 SiCN in pyridine (py) solution resulted in the formation of the pyridine adducts [(py)2 Ga(CN)3 ], [(py)3 In(CN)3 ], and [(py)2 Tl(CN)3 ]. The cyano compounds were characterized by their vibrational spectra and, in most cases, by their X-ray crystal structures.

Preparation and Characterization of Antimony and Arsenic Tricyanide and Their 2,2'-Bipyridine Adducts.

The arsenic(III) and antimony(III) cyanides M(CN)3 (M=As, Sb) have been prepared in quantitative yields from the corresponding trifluorides through fluoride-cyanide exchange with Me3 SiCN in acetonitrile. When the reaction was carried out in the presence of one equivalent of 2,2'-bipyridine, the adducts [M(CN)3 ⋅(2,2'-bipy)] were obtained. The crystal structures of As(CN)3 , [As(CN)3 ⋅(2,2'-bipy)] and [Sb(CN)3 ⋅(2,2'-bipy)] were determined and are surprisingly different. As(CN)3 possesses a polymeric three-dimensional structure, [As(CN)3 ⋅(2,2'-bipy)] exhibits a two-dimensional sheet structure, and [Sb(CN)3 ⋅(2,2'-bipy)] has a chain structure, and none of the structures resembles those found for the corresponding arsenic and antimony triazides.