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The mono-metal analog of B 5 H 9 , 2-CpCoB 4 H 8 , is used as a benchmark for the interpretation of molecular orbital calculations on 2-CpWH 3 B 4 H 8 and 2-CpTaCl 2 B 4 H 8 , all of which have square pyramidal cluster cores. The latter two molecules are shown to obey the prescription of the cluster electron counting rules for a nido , five atom cluster. On the other hand, although both 2-CpCoB 4 H 8 and 2-CpWH 3 B 4 H 8 possess metals that obey the 18 electron rule, the metal atom in 2-CpTaCl 2 B 4 H 8 must be viewed as a 16 electron center. Further, it is shown that the asymmetric ligand arrangements about the metal centers in 2-CpWH 3 B 4 H 8 and 2-CpTaCl 2 B 4 H 8 are required in order to satisfy the demands of cluster bonding. Finally, the magnitude of the barrier for rotation of the CpML n fragment in the same two compounds relative to the borane core depends on the number of ligands, n , via the magnitude of the splitting of the metal frontier orbitals.

Recent d-block element metallaborane chemistry, in which metal identity is varied with constant ancillary ligand, demonstrates how the rising energy of the d orbitals as one moves to earlier metals gives rise to non spherical cluster shapes that permit low formal cluster electron counts. In essence, the separation of frontier orbitals from “nonbonding” orbitals required by the isolobal analogy breaks down and the resulting mixing generates additional high-lying empty orbitals concurrently with shape change. A very similar mechanism explains recent p-block cluster chemistry albeit with variation in extent of external cluster ligation as the variable and separation of external lone pair orbitals from cluster bonding as the problem. Sensible, novel explanations of the shape/electron count relationships can be discovered for large group 13 clusters by recognizing the perturbation in cluster orbital energies when stabilization by ligand interactions is removed. These observations are pertinent to an understanding of large p-block clusters with internal atoms often referred to as nanoparticles.

Addition of ethynylferrocene to nido-1,2-(Cp∗RuH)2B3H7 (1) at ambient temperature leads to nido-1,2-(Cp∗Ru)2(1,5-μ-C{Fc}Me)B3H7 (2, 3) and closo-4-Fc-1,2-(Cp∗RuH)2-4,6-C2B2H3 (4). Compounds 2 and 3 represent a pair of geometric isomers, nido-species in which the regiochemistry of the alkyne reduction conforms to the Markovnikoff rule. Compound 4 is an octahedral structure in which the inserted alkyne is on an open face of the closo cluster.

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.

The reaction of nido-1,2-(η5-C5Me5Ru)2(μ-H)2B3H7 (1) with the dichloroborane BHCl2·SMe2 results in the formation of three sets of B−Cl inserted metallaborane products: nido-1,2-(η5-C5Me5Ru)2(μ-H)B...

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)...

The reaction of [Cp*IrB 4 H 1 0 ] (1) with [{η 6 -(C 6 H 4 )(CH 3 ) 2 }Mo(CO) 3 ] in tetrahydrofuran (THF) leads to [Cp*Ir(CO) 3 (THF)MoB 4 H 8 ] (2), which was characterized by NMR spectroscopy. Intermediate 2 subsequently decomposes to give the tetracarbonyl analogue [Cp*Ir-(CO) 4 MoB 4 H 8 ] (3). In the presence of free CO, the same reaction constitutes a high-yield synthesis of 3. The treatment of 2 (formed in situ) with different Lewis bases led to the replacement of the THF ligand at the molybdenum center and generation of [Cp*Ir(CO) 3 -(L)MoB 4 H 8 ] (L = PPh 3 (4), NCPh (5), CNBu (6), NH 3 (7)). Reaction of the benzonitrile derivative 5 with PPh 3 =CHCO 2 Me afforded the ylide compound [Cp*Ir(CO) 3 (PPh 3 =CHCO 2 -Me)MoB 4 H 8 ] (8). All compounds have been spectroscopically characterized, and 3 and 5-7 have been structurally characterized in the solid state. This work demonstrates that, similarly to metal clusters, metallaboranes with labile metal ligands can serve as versatile synthons.

The treatment of [Cp*IrB4H10] (1) and [Cp*IrB3H9] (2) with typical Lewis bases afforded the new substituted iridatetra- and iridatriboranes [Cp*IrB3H7L] (L = PMe3 (3), PMe2Ph (5), PMePh2 (7), PPh3 (10), py (12), NEt3 (14)) and [Cp*IrB2H6L] (L = PMe3 (4), PMe2Ph (6), PMePh2 (8), PPh3 (11), py (13), NEt3 (15)), respectively, plus the adduct BH3L as coproduct. With PPh3, py, and NEt3, the formation of the iridapentaborane [1-Cp*-nido-1-IrB4H8] (9) was also observed. The molecular structure of [Cp*IrB2H6L] was confirmed by X-ray diffraction analysis on compounds 4, 8, and 13. The substituted iridatetraboranes were found to decompose in solution and in the solid state with decomposition pathways that depend on the nature of the L−B bond and the concentration of the free Lewis bases. Two routes involving the apparent disproportionation of iridaborane−base adducts were found to compete with the loss of a {BH} unit. A reaction cycle driven by the Lewis acid/base chemistry of BH3 was constructed from these reactio...

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 reactions of [Cp*MH 2 ] 2 , Cp* = η 5 -C 5 Me 5 ; M = Fe, Ru, with BH 3 .THF have been explored. As with [Cp*RuCl 2 ] 2 , [Cp*RuH 2 ] 2 readily reacts with borane to generate nido-1,2-(Cp*RuH) 2 B 3 H 7 . In contrast to the chloride, intermediates are detectible in the hydride reaction and product selectivity is higher. Benefits of the apparently lower reaction barrier appear in the reaction of [Cp*FeH 2 ] 2 with BH 3 .THF. The formation and isolation of the novel hydrogen-rich ferraborane arachno-1-Cp*FeB 4 H 1 1 from the iron hydride contrasts with the production of pentamethylferrocene from a pentamethylcyclopentadienyl iron halide. This metastable ferraborane has been characterized spectroscopically as well as by reaction with Co 2 (CO) 8 to give a good yield of the more stable derivative nido-1-(Cp*Fe)-2-{Co(CO) 3 }B 4 H 8 by metal fragment addition. The latter compound has been spectroscopically characterized in solution as well as in the solid state by a single-crystal X-ray diffraction study as an example of a mixed metal dimetallahexaborane.

Addition of the internal alkyne, 2-butyne, to nido -1,2-(Cp*RuH) 2 B 3 H 7 ( 1 ) at ambient temperature produces nido -1,2-(Cp*Ru) 2 (μ-H)(μ-BH 2 )-4,5-Me 2 -4,5-C 2 B 2 H 4 ( 2 ), nido- 1,2-(Cp*RuH) 2 -4,5-Me 2 -4,5-C 2 B 2 H 4 ( 3 ), and nido- 1,2-(Cp*RuH) 2 -4-Et-4,5-C 2 B 2 H 5 ( 4 ), in parallel paths. On heating, 2 , which contains a novel exo -polyhedral borane ligand, is converted into closo -1,2-(Cp*RuH) 2 -4,5-Me 2 -4,5-C 2 B 3 H 3 ( 5 ) and nido -1,6-(Cp*Ru) 2 -4,5-Me 2 -4,5-C 2 B 2 H 6 ( 6 ) the latter being a framework isomer of 3 . Heating 2 with 2-butyne generates nido- 1,2-(Cp*RuH) 2 -3-{CMeCMeB(CMeCHMe) 2 }-4,5-Me 2 -4,5-C 2 B 2 H 3 ( 7 ) in which the exo -polyhedral borane is triply hydroborated to generate a boron bound CMeCMeB(CMeCHMe) 2 cluster substituent. Along with 3 , 4 , 5 , 6 , and 7 , the reaction of 1 with 2-butyne at 85 °C gives closo -1,7-(Cp*Ru) 2 -2,3,4,5-Me 4 -6-(CHMeCH 2 Me)-2,3,4,5-C 4 B ( 8 ). Reaction of 1 with the terminal alkyne, phenylacetylene, at ambient temperature permits the isolation of nido -1,2-(Cp*Ru) 2 (μ-H)(μ-CHCH 2 Ph)B 3 H 6 ( 9 ) and nido -1,2-(Cp*Ru) 2 (μ-H)(μ-BH 2 )-3-(CH 2 ) 2 Ph-4-Ph-4,5-C 2 B 2 H 4 ( 11 ). The former contains a RuB edge-bridging alkylidene fragment generated by hydrometallation on the cluster framework whereas the latter contains an exo -polyhedral borane like that of 2 . Thermolysis of 11 results in loss of hydrogen and the formation of closo -1,2-(Cp*RuH) 2 -3-(CH 2 ) 2 Ph-4-Ph-4,5-C 2 B 3 H 3 ( 12 ).

The reaction of (Cp*Rh)2B2H6 (1) with BH3THF under mild conditions leads to the intermediate (Cp*Rh)2B3H7, which is shown to exist in the two isomeric forms 1,2-(Cp*Rh)2B3H7 (2a) 1,2-(Cp*Rh)2(μ-H)B3H6 (2b) by a solid-state structure of 2a and low-temperature NMR data of the mixture. 1 slowly decomposes at room temperature to yield a number of products including pileo-(Cp*Rh)3B4H4 (4), which has been characterized by a solid-state structure determination. 2 converts on heating to 2,3-(Cp*Rh)2B3H7 (3), of known structure. 1−3 catalyze the cyclotrimerization of both terminal and internal alkynes with differing activities. The rate of catalysis is inhibited by PPh3 and pyridine and competition experiments show evidence for a multistep mechanism with concentration-dependent selectivity. The pathway proposed suggests the borane fragment acts in an ancillary role by providing a cluster environment for the dimetal fragment. Metal polyhedral edge opening on alkyne (or base) addition is proposed as the novel activa...

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.

The reaction of 1,2-{Cp*RuH}2B3H7 (1), Cp* = η5-C5Me5, with Co2(CO)8 in hexane yields two major ruthenaborane products, nido-1,2-{Cp*RuH}2-3-Co(CO)4B3H4 (2) and arachno-{Cp*RuH}(CO)B3H7 (3), as well as two other ruthenaboranes in low yield. In toluene, benzene, and THF one of these minor ruthenaborane products is found with an enhanced yield intermediate between those of 2 and 3. This compound has been isolated and characterized in solution and the solid state as the 8 skeletal electron pair (sep) arachno 1,2-{Cp*Ru}2(CO)2B3H7 (4) and shown to convert into a second isomeric form of similar stability. One isomer constitutes a diruthenium analogue of 8-sep pentaborane(11), whereas the other mimics the geometry of a known metal cluster with 8 sep. The variation in product concentrations in benzene, including [Cp*Ru(CO)2]2, {Cp*Ru(CO)2}Co(CO)4, and HCo(CO)4, as a function of time and Co2(CO)8/1 ratio has been determined by 1H and 11B NMR spectroscopy and interpreted in terms of a reaction pathway featuring co...

The existence of p-block element analogs of transition metal hydrocarbon π-complexes presents opportunities to examine the appropriateness of the Dewar model in describing the metal–main group η 2 -bonding interaction. Geometric structures and molecular orbital descriptions of electronic structures reveal both similarities and differences in the mutual perturbation of the metal and main group atom centers. Literature examples containing boron, silicon and phosphorus are used to illustrate the point in detail and the variety of metallaborane analogs of larger hydrocarbon π-complexes illustrates the scope of possibilities.

Comparison of the structures of two metallaboranes possessing the same borane fragment and ancillary metal ligands but differing transition metal atoms defines the geometric consequences of the addition (or removal) of two valence electrons from a bicapped tetrahedral metallaborane cluster structure. Likewise the effects of the cluster distortion on electronic structure is explored utilizing approximate molecular orbital calculations on hypothetical (CpMn) 2 B 4 H 8 , Cp=η 5 -C 5 H 5 , as it is changed from the shape characteristic of five skeletal electron pair (sep) (Cp*Cr) 2 B 4 H 8 to that of six sep (Cp*Re) 2 B 4 H 8 , Cp=η 5 -C 5 Me 5 . In doing so it is demonstrated that the observed changes in the metal–metal distance (a counter-intuitive increase with smaller sep) and endohydrogen positions (more like BH with smaller sep) are required to electronically accommodate the removal of a pair of electrons from a saturated bicapped tetrahedral cluster.

The reactions of the mixed-metal metallaborane {Cp*Ru}{Cp*Ru(CO)}{Co(CO)2}(μ-CO)B3H6, 1, Cp* = η5-C5Me5, with PPh3 and PMe2Ph as representative phosphine bases and pyridine as a representative nitrogen base have been studied. Phosphines lead to substitution of CO on Co and the formation of {Cp*Ru}2{Co(CO)L}(μ-CO)2B3H4, L = PPh3 (2) and PMe2Ph (3). The latter has been structurally characterized in the solid state and exhibits the fac isomeric form of an octahedral M3B3 cluster. NMR data demonstrate that 3 is fluxional in solution and exists in two isomeric forms: in the major one the phosphine is adjacent to the B3 face, and in the minor one it is adjacent to the M3 face. Reaction with pyridine leads to three products, two of which have been characterized by X-ray structure determinations. Formation of 4, demonstrates borane fragment displacement plus pyridine addition and subsequent orthometalation. Formation of {Cp*Ru}2}{Co(CO)2}(μ-CO)2B2H2B(NC5H5), 5, establishes a parallel pathway involving H substitu...

The reaction of Cp*ReCl4, [Cp*ReCl3]2, or [Cp*ReCl2]2 (Cp* = eta 5-C5Me5) with LiBH4 leads to the formation of 7-skeletal-electron-pair (7-sep) (Cp*ReH2)2(B2H3)2 (1) together with Cp*ReH6. Compound 1 is metastable and eliminates H2 at room temperature to generate 6-sep (Cp*ReH2)2B4H4 (2). The reaction of 2 with BH3.thf produces 7-sep (Cp*Re)2B7H7, a hypoelectronic cluster characterized previously. Heating of 2 with 1 atm of CO leads to 6-sep (Cp*ReCO)(Cp*ReH2)B4H4 (3). Both 2 and 3 have the same bicapped Re2B2 tetrahedral cluster core structure. Monitoring the reaction of 2 with CO at room temperature by NMR reveals the formation of a 7-sep, metastable intermediate, (Cp*ReCO)(Cp*ReH2)(B2H3)2 (4), which converts to 3 on heating. An X-ray structure determination reveals two isomeric forms (4-cis and 4-trans) in the crystallographic asymmetric unit which differ in geometry relative to the disposition of the metal ancillary ligands with respect to the Re-Re bond. The presence of these isomers in solution is corroborated by the solution NMR data and the infrared spectrum. In both isomers, the metallaborane core consists of fused B2Re2 tetrahedra sharing the Re2 fragment. On the basis of similarities in electron count and spectroscopic data, 1 also possesses the same bitetrahedral structure. The reaction of 2 with CO2(CO)8 results in the formal replacement of the four rhenium hydrides with a 4-electron CO2(CO)5 fragment, thereby closing the open face in 2 to produce the 6-sep hypoelectronic cluster (Cp*Re)2CO2(CO)5B4H4 (5). These reaction outcomes are compared and contrasted with those previously observed for 5-sep (Cp*Cr2)2B4H8.

Based on existing connections between borane cages and boron-rich metal borides on the one hand and metal clusters containing boron atoms and metal-rich borides on the other, selected metallaboranes of intermediate stoichiometry are investigated as models for transition metal borides with similar M/B ratios. This study is possible due to the recent development of a general route to metallaboranes for group 5–9 metals that permits the distinct role of the transition metal in metallaborane cluster bonding to be revealed. The comparison shows that it is MM and MB covalent bonding rather than M and B electronic charges that drives change in structure with metal identity.

The reaction of limited quantities of BH3THF with Cp*WCl4, Cp*=η5-C5Me5, leads to the formation of [Cp*WCl2(μ-H)]2 and BH2Cl in good yield. Structural characterization by spectroscopic and crystallographic techniques reveals the presence of two Cp*WCl2 fragments in a transoid arrangement connected by a formal WW double bond (dWW=2.5150(5) A) bridged by two hydrogen atoms. Reaction of [Cp*WCl2(μ-H)]2 with additional BH3THF leads ultimately to Cp*2W2B5H9, thereby providing information on the role of [Cp*WCl2(μ-H)]2 as an intermediate in the formation of Cp*2W2B5H9 from the reaction of Cp*WCl4 with an excess of BH3THF.

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.

Monocyclopentadienyl metal chlorides of transition metals of groups 5−9 react with monoboranes (LiBH4 or BH3THF) to give metallaboranes (and LiCl or BH2Cl). The composition of the metallaborane pro...

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.

Reaction of (CO)9Co3(μ3-CCOOH) with metal trifluoroacetates leads to the isolation of Cr2{μ-OOC-CCo3(CO)9}2{μ-OOCCF3}2(THF)2, 1, and Sm2{μ-OOCCCo3(CO)9}2{μ-OOCCF3}4{(CO)9Co3CCOOH}2(THF)2·2THF, 2, which are cluster substituted analogs of known organic carboxylates. Reaction of Pt(II) acetate, prepared in situ, with (CO)9Co3(μ3-CCOOH) leads to PtCo{μ-OOCCCo3(CO)9}3(μ-OOCCH3){(CO)9Co3CCOOH}, 3, which contains an unusual Pt(II)Co(II) dimeric core. Reaction of Zn(OH)2 with (CO)9Co3(μ3-CCOOH) in THF leads to Zn4(μ4-O){μ-OOC-CCo3(CO)9}6 but in 2-MeTHF the reaction leads to ZnCo{μ-OOCCCo3(CO)9}3{Co(CO)4}{2-(CH3)C4H7O}, 4. This compound features the presence of a [Co(CO)4]− moiety coordinated to a Co(II) center. Reaction of Cr(II) acetate with (CO)9Co3(μ3-CCOOH) in noncoordinating solvents results in CoCr2(μ3-O){μ-OOCCCo3(CO)9}4(μ-OOCCH3)2{(CO)9Co3CCOOH}2(H2O)·C7H8, 5a, and Cr3(μ3-O){μ-OOCCCo3(CO)9}4(μ-OOCCH3)2{(CO)9Co3CCOOH}(CH3OOH)2·C7H8, 5b, which are cluster ligand analogs of known organic carboxylate oxo metal trimers. Finally, deprotonation of (CO)9Co3(μ3-CCOOH) with a bulky base inhibits the base degradation of the tricobalt cluster sufficiently such that [C10H6(N(CH3)2)2H][Co2{μ-OOCCCo3(CO)9}3{OOCCCo3(CO)9}2], 6, can be isolated. All compounds are structurally characterized by X-ray crystallography and selected spectroscopic techniques.

Hydrogen loss from the 7 skeletal electron pair (sep) nido 2-Cp*H3WB4H8, 1 (Cp* = η-C5Me5), the metal analogue of pentaborane (9), has been examined as a potential source of unsaturated, reactive s...

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.

Reaction between Cp*WCl4 or [Cp*WCl2]2 (Cp* = η5-C5Me5) and BH3·thf affords the new tungstaborane (Cp*WCl)2B2H6, 2, which reacts with further BH3·thf to give (Cp*W)2B5H9, 1. The structure of 1 has been determined by a single-crystal X-ray diffraction study and is best described as a bicapped trigonal bipyramidal, the same as previously found for Cr and Mo analogues. The first step of this reaction is reduction of Cp*WCl4 to [Cp*WCl2]2 by BH3·thf, paralleling that of the Cp*MoCl4 system. In contrast with its lighter congener, reaction between LiBH4 and Cp*WCl4 or [Cp*WCl2]2 gives different metallaboranes depending on the oxidation state of the starting material and rate of addition of borane. With Cp*WCl4 and fast addition of LiBH4 the monometallic, crystallographically characterized, metallaborane nido-2-(Cp*WH3)B4H8, 3, is formed in good yield along with the coproduct (Cp*WH3)2B2H6, 4. We have observed the time-dependent behavior of the formation of 3, allowing the elucidation of intermediates on the rea...

The characterization and catalytic properties of a new molecular cluster precursor, Ti 4 O 4 [OCH(CH 3 ) 2 ] 4 {(CO) 9 Co 3 CCO 2 } 4 , denoted Ti 4 Co 12 , were studied during the vapor phase hydrogenation of 2-butenal to form the thermodynamically less favored product, 2-butenol. Pyrolysis of this precursor yields three active catalytic forms, designated LT-, HT1-, and HT2-. This catalyst shows enhanced thermal stability (up to 400°C) as compared to M 2 {(CO) 9 Co 3 CCO 2 } 4 (where M=Co, Cu, or Mo) and M 4 O{(CO) 9 Co 3 CCO 2 } 6 (where M=Zn or Co), which are stable only to 300°C. It has been found that the HT1-Ti 4 Co 12 structure provides significantly better selectivity to 2-butenol than the HT2-Ti 4 Co 12 structure. In-situ DRIFTS measurements during the decomposition of the precursor indicate the presence of COO − groups during the formation of the selective HT1-Ti 4 Co 12 form. These groups disappear at higher temperatures when the HT2-Ti 4 Co 12 catalyst is formed. We speculate that these ions are related to the high selectivity of these materials.

The targeted high-yield syntheses of a number of new clusters by the controlled addition of BH, Fe(CO)3, or Co(CO)3 fragments to dinuclear group 6 metallaboranes are reported. The reaction of the chromaborane, (Cp*Cr)2B4H8 (1), Cp* = η5-C5Me5, with BHCl2·SMe2 results in a cluster expansion reaction giving the green-brown diamagnetic species (Cp*Cr)2B5H9 (2) in 43% yield. The six skeletal electron pair (sep) cluster has a structure based on a trigonal bipyramidal Cr2B3 unit with adjacent Cr2B faces capped by BH3 fragments. Reaction of 1 with the dinuclear metal carbonyls Fe2(CO)9 and Co2(CO)8 leads to the isolation of the mixed-metal metallaboranes (Cp*Cr)2B4H8Fe(CO)3 (3) and (Cp*Cr)2B4H7Co(CO)3 (4) in yields of 76 and 87%, respectively. 3 and 4 can also be viewed as six-sep bicapped trigonal bipyramidal clusters with the metal carbonyl occupying one of the capping positions. Alternatively, they can be viewed as complexes between M(CO)3 transition metal fragments (M = Fe, Co) and the four-electron (Cp*Cr)2...

Fenske–Hall molecular orbital calculations have been carried out on Cp 2 Cr 2 B 4 H 8 ( 1′ ), Cp 2 Cr 2 (CO) 2 B 4 H 6 ( 2′ ), Cp 2 Cr 2 B 4 H 8 Fe(CO) 3 ( 3′ ), and Cp 2 Cr 2 B 4 H 6 (S 2 CH 2 ) ( 4′ ) (Cp= η 5 -C 5 H 5 ) with structures based on the known compounds with η 5 -C 5 Me 5 ligands. It is demonstrated that the dinuclear Cp 2 Cr 2 fragment is capable of providing an additional low energy (filled) or high energy (unfilled) orbital to the cluster bonding network with only small distortions of the Cr 2 B 4 cluster core geometry. By this mechanism, the cluster core geometry changes required by Wade's rules are decoupled from cluster electron count. Thus, both electron poor and electron rich clusters, as defined by the Wade prescription, are stable and exhibit the same qualitative cluster geometry. The electronic origin of this behavior resides in the close energy match of the metal fragment `t 2g ' orbitals with those of the borane fragment combined with the perturbation of differing ligands associated with the cluster.

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 cluster acid CpM‘(CO)2Co2(CO)6(μ3-CCOOH) (M‘ = Mo, W; Cp = η5-C5H5), 1, has been synthesized in good yield from the protected (CO)9Co3(μ3-CC(O)OR) cluster by metal fragment exchange followed by...

The protonolysis of group 4 M(IV)(OR)4, M = Ti, Zr, organic alkyl- or aryloxides with the cluster acid (CO)9Co3(μ3-CCO2H) leads to a series of novel tricobalt cluster M(IV) alkoxy- or aryloxycarboxylates, most of which contain bridging oxo ligands. The compounds, which are isolated in good yield as crystalline substances and characterized spectroscopically as well as crystallographically, are Zr2(μ-OCH(CH3)2)2(OCH(CH3)2)2{μ-(CO)9Co3(μ3-CCO2)}2{(CO)9Co3(μ3-CCO2)}2, 1; Ti4(μ3-O)4(OR)4{μ-(CO)9Co3(μ3-CCO2)}4, 2 (R = CH(CH3)2, 2a; (CH2)3CH3, 2b; C6H5,2c; 2,6-(CH3)2-C6H3, 2d); Ti6(μ3-O)4(μ-OCH2CH3)4(OCH2CH3)8{μ-(CO)9Co3(μ3-CCO2)}4, 3; and [Ti6(μ3-O)2(μ-O)2(μ-OCH(CH3)2)2(OCH(CH3)2)2(1,2-O2C6H4)4{μ-(CO)9Co3(μ3-CCO2)}4], 4. Further, the mixed-metal derivatives [Ti2Co2{(OCH2)3CCH3}2(OCH(CH3)2)2{μ-(CO)9Co3(μ3-CCO2)}4], 5, and [{Co3Ti(OCH3)6(HOCH3)(THF){μ-(CO)9Co3(μ3-CCO2)}3}2(O2CCO2)]·2HOCH3·2THF,6, in which the Co(II) ions are derived from in situ tricobalt cluster degradation, are characterized in similar fashion....

The reaction of the cluster-substituted carboxylic acid, (CO) 9 Co 3 CCOOH, with M(CH 3 CO 2 ) 2 permits the isolation of M 2 {(CO) 9 Co 3 CCO 2 } 4 (THF) 2 , M = Mn ( 1 ), Fe ( 2 ), Co ( 3 ) respectively. Single crystals could not be obtained directly from any of these compounds; however, the slow redox degradation of Hg{(CO) 9 Co 3 CCO 2 } 2 in THF leads to crystalline [Co 2 ((CO) 9 Co 3 CCO 2 ) 4 (THF) 2 ], which has been characterized by single crystal X-ray structure analysis. Both compounds 1 and 2 react with methanol and the products [M 2 III Co 2 II (MeO) 6 (MeOH) 2 {(CO) 9 Co 3 CCO 2 } 4 ] · 2MeOH, M = Mn ( 4 ), Fe ( 5 ) respectively, have been isolated as crystalline solids. The formulation of these new mixed-metal cubane clusters is based on analytical data, single crystal X-ray structure analysis, and the Mossbauer spectrum of 5 . The inner Co II centers are derived from sacrificial decomposition of the tricobalt clusters during precipitation of the product. For both 4 and 5 the magnetic susceptibility data show high room temperature magnetic moments which decrease with decreasing temperature consistent with weak antiferromagnetic coupling between the core metal centers and with S = 0 ground states. The formation of these unusual carboxylate coordinated mixed-metal alkoxide cubes derives from the properties of a transition metal cluster as a substituent as well as its tendency to engage in redox chemistry with other metal species.

The reaction of the cluster substituted carboxylic acid [(CO)9Co3(μ3-CCO2H)] with Cd (CH3CO2)2 · H2O in the presence of tetraethylene glycol dimethyl ether(TGM) afforded the binuclear complex [ Cd 2 ∗( CO ) 9 Co 3 (μ 3 - CCO 2 )∗ 4 ( TGM )] . The solid state structure shows that the cluster carboxylate ligands act as chelating and asymmetrical bridging ligands, respectively, and the TGM ligand exhibits a conformation not previously observed. The coordination number of each cadmium atom is seven, resulting in a coordination geometry consisting of a distorted trigonal capped square-based polyhedron.

The reaction of closo-[2,3,4-(η5-C5Me5)3(μ-H)2Co3B2H2], 1, with metal chlorides results in the formation of either closo-1-Cl-[2,3,4-(η5-C5Me5)3(μ-H)2Co3B2H], 2, or closo-1,5-Cl2-[2,3,4-(η5-C5Me5)3...

Complex cobalt–carbonyl ligand clusters of clusters are used as well defined molecular precursors for self-supported single metal or bimetallic catalysts. These precursors incorporate two metals: an outer layer formed from the complex cobalt cluster carbonyl ligands [(CO)9Co3CCO2] and an inner core of the metal carboxylate cores. Two types of precursor structures are used: one with two metal atoms in the core (designated as M2Co12, where M = Co, Cu, and Mo) and the other with four metal atoms in the core bonded to a centering oxygen atom (designated as M4Co18, where M = Co and Zn). The catalysts are preparedin situby partial thermolysis (LT catalysts) or complete thermolysis (HT catalysts) of the cluster of clusters precursors. The catalysts derived from Co4Co18, Zn4Co18, Co2Co12, Mo2Co12, and Cu2Co12precursors are used in the selective hydrogenation reaction of 2-butenal. The desired product of the reaction is the thermodynamically less favored unsaturated alcohol (2-butenol). The highest 2-butenol selectivity observed was 100% over HT-Co2Co12catalyst at 373 K. The highest 2-butenol yield observed in our experiments was ≈28% using HT-Co4Co18at 423 K. The activity and selectivity behavior of HT-Co4Co18was stable over at least 50 h of operation. The factors affecting 2-butenol selectivity and yield during 2-butenal hydrogenation using these catalysts are discussed.