Metallaborane reactivity. A stoichiometric mechanism for the insertion of two alkynes into an iridaborane framework via a disposable molybdenum chaperone

Building on earlier work that showed the formation of [1-Cp*-2,2,2-(CO)3-2-THF-nido-1,2-IrMoB(4)H(8)], 2, from the reaction of [1-Cp*-arachno-1-IrB(4)H(10)], 1, with (arene)Mo(CO)3, the stoichiometric mechanism for the generation of [1-Cp*-5,6,7,8-(R)4-nido-1,5,6,7,8-IrC(4)B(3)H(3)], 8, from the reaction of 2 with RCCR, R = Me, Ph, has been identified. For R = Me, the major product in solution is [1-Cp*-5,6,7,8-(CH3)4-closo-1,5,6,7,8-IrC(4)B(3)H(3)Mo(CO)3], 7, which is in equilibrium with 8. The equilibrium 8 + Mo(THF)3(CO)3 <==> 7 + 3THF is characterized by DeltaH = 8 kcal/mol and DeltaS = 34 cal/mol K. Density functional theory calculations carried out on 7 indicate that the Mo(CO)3 moiety is weakly bound to the cluster mainly through Mo-C rather than Mo-B interactions. Under alkyne deficient conditions, the product [1-Cp*-2,2,2-(CO)3(mu-CO)-3,4-(CH3)2-closo-1,2,3,4-IrMoC(2)B(3)H(3)], 6, can be isolated. Solid-state structures of 1 and 2 have been reported previously, and those of 6, 7, and 8, R = Me, Ph, are reported here. The evolution of products with time was monitored by 1H and 11B NMR and showed the formation and decay of two additional species which have been identified as the structural isomers [1-Cp*-7,7,7-(CO)3-7-THF-2,3-(CH3)2-nido-1,7,2,3-IrMoC(2)B(3)H(5)], 4, and [5-Cp*-7,7,7-(CO)3-7-THF-2,3-(CH3)2-nido-5,7,2,3-IrMoC(2)B(3)H(5)], 5, with the metals nonadjacent in 4 and adjacent in 5. Circumstantial evidence suggests that 4 is the precursor to 5 and 5 is the precursor to both 6 and 7. Cluster 2 also is a catalyst or catalyst precursor for the isomerization of olefins, namely, hex-1-ene to cis-hex-2-ene and tetramethyl allene to 2,4-dimethylpenta-1,3-diene. These novel results also establish that [1-Cp*-2,2,2-(CO)3-2-(alkyne)-nido-1,2-IrMoB(4)H(8)], 3, forms from 2 and constitutes a logical precursor to 4. The entire process, 1 + 2alkyne = 8 + BH3 + 2H2, which is promoted by (arene)Mo(CO)3, constitutes an explicit example of a transition-metal-facilitated process analogous to metal-facilitated organic transformations observed in organometallic chemistry.     

Protonation of cluster molecules: Bridging hydrogen sites in tetrahedral,octahedral and capped square pyramidal clusters: Be4H8, Be4H8

Main group structural models for tetrahedral and octahedral transition metal clusters containing bridging hydrogens are developed using the MNDO quantu     

On the validity of the isolobal principle: Pentaborane(9) and its ferraborane derivatives

We have completed Fenske-Hall LCAO-MO-SCF calculations on B₅H₉, 1-Fe(CO)₃B₄H₈, 2-Fe(CO)₃B₄H₈, and 1,2-[Fe(CO)₃]₂B₃H₇. Comparison of orbital contour diagrams of the a₁ and e cluster MO's for B₅H₉ and 1-Fe(CO)₃B₄H₈ demonstrates the validity of the isolobal principle. In addition it is found that the apical and basal BH units of B₅H₉ have practically identical Mulliken overlap populations for framework cluster-type interactions. Further, in all the ferraboranes the 1-Fe(CO)₃ (apical) units have a larger cluster-type Mulliken overlap population than do the 2-Fe(CO)₃ (basal) units. The Fe(CO)₃ units have less electronic charge than do the BH units. The cluster-type Mulliken overlap population for an Fe(CO)₃ group is much less than that of the isolobal BH unit, but this may result from an artifact of the Mulliken overlap population analysis.     

UV photoelectron spectra of group IV alkyl hydrides

We have had occasion to measure the photoelectron spectra of some heavy group IV alkyls and alkyl hydrides. Although the ionization potentials serve primarily as a body of information which is useful in investigating trends in reactivity for processes involving electron transfer [1], they also provide a pleasing example of the simplicity with which the language of molecular orbital theory can be used to explain trends in the energies and nature of the radical cation states of a series of chemically related molecules [2].     

Dependence of field switched ordered arrays of dinuclear mixed-valence complexes on the distance between the redox centers and the size of the counterions

trans-[(H(2)NCH(2)CH(2)C triple bond N)(dppe)(2)Ru(C triple bond C)(6)Ru(dppe)(2)(N triple bond CCH(2)CH(2)NH(2))][PF(6)](2), 2[PF(6)](2), a derivative of trans-[Cl(dppe)(2)Ru(C triple bond C)(6)Ru(dppe)(2)Cl] functionalized for binding to a silicon substrate, has been prepared and characterized spectroscopically, electrochemically, and with a solid state, single-crystal structure determination. Covalent binding via reaction of one amine group to a boron-doped, smooth Si-Cl substrate is verified by XPS measurements and surface electrochemistry. Vertical orientation is demonstrated by film thickness measurements. Synthesis of the 2[PF(6)](3) mixed-valence complex on the surface is established by electrochemical techniques. Measurement of the ac capacitance of the film at 1 MHz as a function of voltage across the film with a pulse-counter pulse technique demonstrates controlled electric field generation of the two stable mixed-valence forms differing in the spatial location of one electron, that is, switching. As compared to [trans-Ru(dppm)(2)(C triple bond CFc)(NCCH(2)CH(2)NH(2))][PF(6)][Cl], 1[PF(6)][Cl], the magnitude of the capacitance signal per complex observed on switching is shown to increase with increasing distance between the metal centers. Additional experiments on 1[X][Cl] show that the potential for switching 1[X][Cl] increases in the order [X](-) = [SO(3)CF(3)](-) < [PF(6)](-) < [Cl](-). A simple electrostatic model suggests that the smaller is the counterion, the greater is the perturbation of the metal sites and the larger is the barrier for switching.