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The di-mixed-valence complex [{(eta(5)-C5H5)Fe(eta(5)-C5H4)}4(eta(4)-C4)Co(eta(5)-C5H5)]2+, 1(2+), has been evaluated as a molecular four-dot cell for the quantum cellular automata paradigm for electronic devices. The cations 1(1+) and 1(2+) are prepared in good yield by selective chemical oxidation of 1(0) and are isolated as pure crystalline materials. The solid-state structures of 1(0) and 1(1+) and the midrange- and near-IR spectra of 1(0), 1(1+), 1(2+), and 1(3+) have been determined. Further, the variable-temperature EPR spectra of 1(1+) and 1(2+), magnetic susceptibility of 1(1+) and 1(2+), Mössbauer spectra of 1(0), 1(1+), and 1(2+), NMR spectra of 1(0), and paramagnetic NMR spectra of 1(1+) and 1(2+) have been measured. The X-ray structure determination reveals four ferrocene "dots" arranged in a square by C-C bonds to the corners of a cyclobutadiene linker. The four ferrocene units project from alternating sides of the cyclobutadiene ring and are twisted to minimize steric interactions both with the Co(eta(5)-C5H5) fragment and with each other. In the solid state 1(2+) is a valence-trapped Robin and Day class II compound on the 10(-12) s infrared time scale, the fastest technique used herein, and unambiguous evidence for two Fe(II) and two Fe(III) sites is observed in both the infrared and Mössbauer spectra. Both EPR and magnetic susceptibility measurements show no measurable spin-spin interaction in the solid state. In solution, the NMR spectra show that free rotation around the C-C bonds connecting the ferrocene units to the cyclobutadiene ring becomes increasingly hindered with decreasing temperature, leading to spectra at the lowest temperature that are consistent with the solid-state structure. Localization of the charges in the cations, which is observed in the paramagnetic NMR spectra as a function of temperature, correlates with the fluxional behavior. Hence, the alignment between the pi systems of the central linker and the ferrocene moieties most likely controls the rate of electron exchange between the dots.

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.

Products of the reaction of nido-1,2-(Cp*RuH)2B3H7, 1, and phenylacetylene demonstrate the ways in which cluster metal and main group fragments can combine with an alkyne. Observed at 22 °C are (a)...

Reaction of nido-1,2-(Cp*RuH)2B3H7, 1, and methyl acetylene monocarboxylate under kinetic control generates nido-1,2-(Cp*Ru)2(mu-C[[CO2Me]Me])B3H7 (a pair of geometric isomers, 3 and 5) and nido-1,2-(Cp*Ru)2(1,3-mu-C[[CH2CO2Me]H])B3H7, 4, which display the first examples of exo-cluster mu-alkylidene Ru-B bridges generated by hydrometalation of an alkyne on the cluster framework. Both 3 and 5, but not 4, rearrange into arachno-2,8-mu(C)-5-eta1(O)-Me[CO2Me]C-1,2-(Cp*Ru)2B3H7, 2, in which an unprecedented intramolecular coordination of the carbonyl oxygen atom of the alkyne substituent to a boron framework site opens the ruthenaborane skeleton. Compound 2, in turn, is an intermediate in the formation of the ruthenacarborane nido-1,2-(Cp*Ru)2-3-OH-4-OMe-5-Me-4,5-C2B2H5, 12, in which the carbonyl-oxygen double bond has been cleaved as its oxygen atom inserts into a B-H bond and the carbonyl carbon inserts into the metallaborane framework. In a parallel reaction pathway, nido-1,2-(Cp*Ru)2-5-CO2Me-4,5-C2B2H7, 6, nido-1,2-(Cp*Ru)2-4-B(OH)2-5-CO2Me-4,5-C2B2H6, 16, and nido-1,2-(Cp*Ru)2(mu-H)(mu-BH2)-3-(CH2)2CO2Me-CO2Me-4,5-C2B2H4 (a pair of geometric isomers, 7 and 14, which contain an unusual Ru-B borane bridge) are formed. On heating, 7 rearranges to yield nido-1,2-(Cp*Ru)2-3-(CH2)2CO2Me-4-BH2-5-CO2Me-4,5-C2B2H5, 13, whereas 14 converts to nido-1,2-(Cp*Ru)2-3-(CH2)2CO2Me-4-CO2Me-4,5-C2B2H6, 8. Under thermodynamic control, nido-1,2-(Cp*Ru)2-4,5-B[(CH2)2CO2Me]CO(MeO)[C(CH2)CO2Me]-4,5-C2B2H6, 11, is the major product accompanied by lesser amounts of 6 and 1,2-(Cp*Ru)2-4-OMe-5-Me-4,5-C2B2H6, 10. Compound 11 features a five-membered heterocycle containing a boron atom. The structure of 7, which is an intermediate in the formation of 11, provides the basis for an explanation of this complex condensation of three alkynes. A previously unrecognized role for an exo-cluster bridging borene generated from the metallaborane skeleton by addition of the alkyne is also a feature of this chemistry. Reinsertion or loss of this boron fragment accounts for much of the chemistry observed. NMR experiments reveal labile intermediates, and one has been sufficiently characterized to provide mechanistic insight on the early stages of the alkyne-metallaborane addition reaction. All isolated compounds have been spectroscopically characterized, and most have been structurally characterized in the solid state.

The amine functionality of the linker on the dinuclear complex [trans-Ru(dppm)(2)(Ctbd1;CFc)(NCCH(2)CH(2)NH(2))][PF(6)] reacts with Si-Cl bonds of a chlorinated, highly B doped Si (111) surface to yield Si-N surface-complex bonds. The surface bound complex is constrained to a near vertical orientation by the chain length of the linker as confirmed by variable angle XPS. Oxidation of the dinuclear complex with ferrocenium ion or electrochemically generates a stable, biased Fe(III)-Ru(II) mixed-valence complex on the surface. Characterization of the array of surface bound complexes with spectroscopic as well as electrochemical techniques confirms the presence of strongly bound, chemically robust, mixed-valence complexes. Capping the flat array of complexes with a minimally perturbing mercury electrode permits the equalization of the Fe and Ru energy wells by an applied electric field. The differential capacitance of oxidized and unoxidized bound complexes is compared as a function of voltage applied between the Hg gate and the Si. The results show that electron exchange between the Fe and Ru sites of the array of dinuclear mixed-valence complexes at energy equalization generates a fluctuating dipole that produces a maximum in the capacitance versus voltage curve for each complex-counterion combination present. Passage through the capacitance maximum corresponds to switching of the molecular quantum cellular automata (QCA) cell array by the electric field from the Fe(III)-Ru(II) configuration to the Fe(II)-Ru(III) configuration, thereby confirming that molecules possess an essential property necessary for their use as elements of a QCA device.

The rhenaborane Cp*ReH3B3H8, which is generated by the reaction of Cp*ReH6 with BH3·THF, has been characterized spectroscopically in solution and by a single-crystal X-ray diffraction study in the solid state as a hydrogen-rich arachno-metallatetraborane.

From reaction of [(Cp*Ir)2HxCl4−x] (x=1, 0) and LiBH4, arachno-[{Cp*IrH2}B3H7](1) is produced in moderate yield concurrently with [Cp*IrH4]. In contrast, reaction of [(Cp*Ir)2H2Cl2] with LiBH4 results in arachno-[{Cp*IrH}2(μ-H)B2H5] (3) in high yield at room temperature but a mixture of 1 and [{Cp*IrH}2(μ-H)BH4] (2) at 0 °C. BH3⋅THF converts 1 to arachno-[{Cp*IrH}B4H9] (4) and 2 to 3 with 1 as a minor product. Further, reaction of 3 with excess of BH3⋅THF results in formation of nido-[{Cp*Ir}2(μ-H)B4H7] (6) formed by loss of H2 from the intermediate arachno-[{Cp*IrH}2B4H8] (5). Reaction of 1 with [Co2(CO)8] permits the isolation of two metallaboranes, arachno-[{Cp*Ir(CO)}B3H7] (7) and nido-[1-{Cp*Ir}-2, 3-Co2(CO)4(μ-CO)B3H7] (8). Treatment of 4 with [Co2(CO)8] gives only one single mixed-metal metallaborane nido-[1-{Cp*Ir}-2-Co(CO)3B4H7 (9) in high yield. Finally, pyrolysis of 8 results in loss of hydrogen and formation of pileo-[1-{Cp*Ir}-2, 3-Co2(CO)5B3H5] (10) with a BH-capped square-pyramidal structure. With kinetic control rational synthesis of a variety metallaboranes has been achieved by varying the number of chlorides in the monocyclopentadienylmetal halide dimer, reaction temperature, types of monoborane, and metal fragment sources.

An isomer of a sandwich complex, compound 1 has been prepared from nido-[1,2-{Cp*Ru}2B3H9] and structurally characterized. Owing to the connection of the two dimetallahexaborane cages, the three metals form a linear array. The environment of the unique central Ru atom is different from that normally found in metallaboranes. The isolation of 1 suggests the accessibility of variants of multidecker sandwich complexes with linear chains of metal atoms.

In a single step, from [Cp*RuCl2]2 (Cp* = η5-C5Me5) and Li[BH4], nido-1,2-(Cp*Ru)2(μ-H)2B3H7, 1, is produced in high yield. Addition of BH3·THF to 1 results in conversion to nido-1,2-(Cp*Ru)2(μ-H)B...

An analysis of selected sets of metallaboranes in terms of a molecular orbital (MO) model of 11B chemical shift change is used to demonstrate the origin of transition metal effects on boron shifts for: (i) M-B edge protonation; (ii) replacement of direct B-B by M-B interactions; (iii) encapsulation of B in a metal cluster; (iv) change in metal identity; and (v) change in vertex coordination number. Metal effects on both filled and unfilled MO's are important but changes in the latter appear to dominate. Consequently, models based solely on filled orbital properties, e.g., electronic charge, are inadequate. A short review with 56 references.

The ability to synthesize metallaboranes of fixed cluster shape with varying numbers of metal atoms and metal identities highlights the unique ways in which the metals differ from their main-group counterparts in a cluster environment. Thus we have found that electronic unsaturation introduced by the use of early transition metals is expressed in an intriguing and novel manner in a metallaborane cluster.

The controlled pyrolysis of transition metal cluster substituted metal carboxylates, e.g., M4O[(CO)9Co3CCO2]6, where M = Co and Zn, and M'2(CO)9Co3CCOO4, where M' = Co, Mo, and Cu, results in the formation of high surface area, amorphous solids that are active and selective catalysts for the hydrogenation of crotonaldehyde. In contrast to conventional metal catalysts that are selective for the double bond hydrogenation, these new solids exhibit high regioselectivities for the conversion of crotonaldehyde (2-butenal) to crotyl alcohol (2-butenol). Further, the observed selectivities depend on the metal cluster carboxylate structure.

The clusters of clusters M[sub 4]O[(CO)[sub 9]Co[sub 3]CCO[sub 2]][sub 6], M = Zn (I), Co (II), are single-source precursors for porous, high surface area materials, referred to as ZnCo and CoCo, respectively. These two cobalt metal based materials were investigated as catalysts for the hydrogenation of 1,3-butadiene under a variety of reaction conditions. Both ZnCo and CoCo are active catalysts in the 50-300[degrees]C temperature range, however, different activities and selectivities for the formation of products are exhibited. At reaction temperatures below 150[degrees]C these catalysts show no loss of activity at up to 4 days on stream. Comparison of the catalytic properties of CoCo with a cobalt-supported alumina catalyst at similar metal loading shows that the cluster-derived catalyst exhibits both higher activity and different selectivity than the conventional supported cobalt catalyst. The observed hydrogenation activity and selectivity of these new materials relative to conventional catalysts or catalysts prepared from simpler precursors suggest that both the composition and microstructure derived from the complex precursor architecture contribute to the catalytic behavior. At reaction temperatures around 300[degrees]C the activity of ZnCo and CoCo catalysts decrease due to the formation of carbon on the surface of the cluster materials and the partial collapse of themore » porous structure. 22 refs., 9 figs., 2 tabs.« less

(CO)3H4 at low low pressure has been studied by using the techniques of X-ray diffraction and Mbsbauer spectroscopy. The resulta unambiguously show that the films are iron rich in terms of the ideal stoichiometry of Fe/B = 3, signifying some loss of boron during film formation. The iron-main group atom phase that crystallizes from the amorphous film is orthorhombic Fe,B1,Cx, with x for the specific film examined lying between 0.3 and 0.4. The cleavage of CO is postulated to account for the overall Fe/B ratio of 3, formation of the mixed boride/carbide phase and the presence of B203 in the film. That is, the boron sequesters the oxygen atom as B203, whereas the carbon atom replaces boron so lost in Fe3B to form the Fe3B1,C,phase. Deposition of films from a closely related precursor, HFe3(CO)laH2, have also been examined.Most of the boron is lost during deposition even at the lowest substrate temperature. Crystallization of these amorphous films requires higher temperatures and yields a-Fe and a phase indistinguishable from orthorhombic Fe3C. Decomposition during sublimation with the production of Fe(CO), accounts for the qualitatively different behavior of HFe3(CO)loBH2.

International audience; 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.

The reactions of alkynes with nido-dimetallaboranes containing monocyclopentadienyl metal fragments are highly dependent on metal identity. Typical metals from groups 6 and 7 do not react under mild conditions, Rh gives exclusively alkyne cyclotrimerization via a cluster-catalyzed reaction, and Ru reacts predominantly via insertion to give new ruthenacarborane clusters. The facile reaction observed for Ru permits the characterization of intermediates exhibiting novel structural features, e.g., external cluster-bridging monoborane or alkylidene fragments. Despite the large differences in reactivity, circumstantial evidence suggests the alkyne adds to and opens the metallaborane to an arachno-structure. The metal present determines the character of the metallaborane, which in turn determines whether further reaction occurs. For Ru, it is the hydrogen-rich character fostered by the metal fragment that leads to the copious chemistry observed.

The reaction of (CpReH(2))(2)B(4)H(4) with monoborane leads to the sequential formation of (CpRe)(2)B(n)()H(n)() (n = 7-10, 1-4). These species adopt closed deltahedra with the same total connectivities as the closo-borane anions [B(n)()H(n)()](2)(-), n = 9-12, but with flattened geometries rather than spherical shapes. These rhenaborane clusters are characterized by high metal coordination numbers, Re-Re cross-cluster distances within the Re-Re single bond range, and formal cluster electron counts three skeletal electron pairs short of that required for a canonical closo-structure of the same nuclearity. An open cluster, (CpReH)(2)B(7)H(9) (5), is isolated that bears the same structural relationship to arachno-B(9)H(15) as 1-4 bear to the closo-borane anions. Chloroborane permits the isolation of (CpReH)(2)B(5)Cl(5) (6), an isoelectronic chloro-analogue of known open (CpWH(2))(2)B(5)H(5) and (CpRe)(2)B(6)H(4)Cl(2) (7), a triple-decker complex containing a planar, six-membered 1,2-B(6)H(4)Cl(2) ring. Both are putative five- and six-boron intermediates in the formation of 1. Electronic structure calculations (extended Hückel and density functional theory) yield geometries in agreement with the structure determinations, large HOMO-LUMO gaps in accord with the high stabilities, and (11)B chemical shifts accurately reflecting the observed shifts. Analyses of the bonding in 1-4 reveal that the CpRe.CpRe interaction generates fragment orbitals that are able to contribute the "missing" three skeletal electron pairs required for skeletal bonding. The necessity of a Re.Re interaction for strong cluster bonding requires a borane fragment shape change to accommodate it, thereby explaining the noncanonical geometries. Application of the debor principle of borane chemistry to the shapes of 1-4 readily rationalizes the observed geometries of 5 and 6. This evidence of the scope of transition metal fragment control of borane geometry suggests the existence of a large class of metallaboranes with structures not found in known borane or metal clusters.

The suitability of [{(eta5-C5H5)Fe(eta5-C5H4)}4(eta4-C4)Co(eta5-C5H5)][PF6]2, [1][PF6]2, for use as a molecular quantum cellular automata (QCA) cell is demonstrated. To this end the structure of 1 in the solid state and the conversion of 1 to mono- and dicationic mixed-valence complexes have been accomplished. The latter compounds have been isolated as pure materials and characterized by IR, EPR, and Mössbauer spectroscopies and single-crystal XRD (monocation only) and magnetic susceptibility measurements. Near-IR spectra demonstrate the mixed valence character of the cations (valence trapped on the IR, EPR and Mössbauer time scales), and the energies of the intervalence charge-transfer bands provide a measure of the hole hopping frequency.

The reaction of the alkyne HC⋮CC(O)OMe with 7 sep 1,2-(Cp*RuH)2B3H7 leads to hydroboration plus hydroruthenation to produce nido-1,3-μ-Me{C(O)OMe}C-1,2-(Cp*Ru)2B3H7, a compound with an exocluster ruthenium−boron μ-alkylidene that exists in two isomeric forms. Both isomers undergo rearrangement with intramolecular chelation of the carbonyl oxygen at a boron site, thereby opening the cluster and generating arachno-2,3,-μ(C)-5-η1(O)-Me{C(O)OMe}C-1,2-(Cp*Ru)2B3H7. Further heating leads to deoxygenation of the carbonyl fragment by a boron center concurrent with insertion of the carbon atom into the metallaborane cage to give nido-1,2-(Cp*RuH)2-3-HOB-4-MeC-5-MeOC-BH3.

The synthesis and characterization of Cp*2Re2B7H11 provides the first example of an open hypoelectronic metallaborane which is shown to possess an arachno geometry based on a total vertex connectivity identical with that of a canonical arachno nine-vertex, twelve-skeletal electron pair cluster.

Sigma bond metathesis of B–H in Cp*2Rh2B3H6Cl with Co–Co in Co2(CO)8 leads directly to the title compound.

Geometric and molecular orbital analyses of the metallaboranes (Cp*Re)2B7H7 and (Cp*W)2B7H9, which display unusual, but identical, core structures and skeletal electron pair counts of n - 2, demonstrate a close connection between these molecular compounds and anionic, hypoelectronic main-group clusters found in the solid state.

Reaction between Cp*TaCl4 (Cp* = η5-C5H5) and BH3·thf at 40 °C yields the pale red electronically unsaturated cluster Cp*TaCl2B4H8 1 which despite having two electrons fewer than the seven pairs required for the observed square pyramidal geometry is stable in air; viewed as a cluster, 1 is the first example of an unsaturated cluster containing a single metal atom; viewed as a chiral 16-electron organometallic complex it is isoelectronic with Cp(C2B9H11)TaCl2 and Cp[(Me3Si)2C2B4H 4]TaCl2.

Mild pyrolysis of (eta5-C5Me5Ru)2B6H12 with Fe2(CO)9 yields the 12 skeletal electron pair (sep) Fe2(CO)6(eta5-C5Me5RuCO)(eta5-C5Me5Ru)B6H10 cluster; the title compound represents a novel class of hybrid multiple cluster in which a Fe2B2 tetrahedron has been fused to a ruthenaborane substrate.