Thomas P. Fehlner

Fine‐Tuning of Metallaborane Geometries: Chemistry of Iridaboranes Derived from the Reaction of [(Cp*Ir)2HxCl4−x] (x=0‐2; Cp*=η5‐C5Me5) with LiBH4

From reaction of [(Cp*Ir)2HxCl(4-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(mu-H)B2H5] (3) in high yield at room temperature but a mixture of 1 and [[Cp*IrH]2(mu-H)BH4] (2) at 0 degrees C. BH3 x THF converts 1 to arachno-[(Cp*IrHB4H9] (4) and 2 to 3 with 1 as a minor product. Further, reaction of 3 with excess of BH3 x THF results in formation of nido-[[Cp*Ir]2-(mu-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(mu-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.

Comparison of the geometric and molecular orbital structures of (Cp*Cr)2B4H8 and (Cp*Re)2B4H8, Cp*=η5-C5Me5. Structural consequences of delocalized electronic unsaturation in a metallaborane cluster

Abstract 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.

Expansion of iridaborane clusters by addition of monoborane. Novel metallaboranes and mechanistic detail.

This work reports the results of a thermally driven cluster expansion of arachno-1-{eta5-C5Me5IrH2}B3H7, 1, with BH3.THF. In addition to the previously reported product, arachno-1-{eta5-C5Me5IrH}B4H9, 2, formed at lower temperatures, reaction at 100 degrees C permits the isolation of four new iridaboranes. Two products, nido-1-(eta5-C5Me5Ir)B5H9, 3, and nido-3-(eta5-C5Me5Ir)B9H13, 4, contain a single Ir atom and five and nine framework boron atoms, respectively. One, nido-3,4-(eta5-C5Me5Ir)2B8H12, 5, contains two Ir atoms and eight framework boron atoms. Their structures are predicted by the electron counting rules to be a nido-iridahexaborane, 3, nido-iridadecaborane, 4, and nido-diiridadecaborane, 5. The accuracy of these predictions in each case is established experimentally by spectroscopic characterization in solution and structure determinations in the solid state. A less stable metallaborane has been identified and the available spectroscopic and crystallographic information are consistent with the formulation nido-3,4-(eta5-C5Me5Ir)2B8H13(eta-BH2), 6, i.e., a species containing an exopolyhedral bridging BH group. These new observations, along with earlier ones on ruthenaborane cluster systems, are used to fully define a general mechanism for a cluster expansion reaction, i.e., addition of borane to form an exopolyhedral adduct followed by cage insertion.

Metalloboranes: Their Relationships to Metal-Hydrocarbon Complexes and Clusters

Publisher Summary The hydridic character of boranes reflects the polarity of the B–H bond, whereas the Lewis acidity of boranes reflects the fact that boron possesses fewer valence electrons than valence orbitals. Metalloboranes have been classified both according to ligand behavior attributed to the borane via the requirements of the metal fragment or cluster behavior attributed to the entire metalloborane. A significant number of the compounds resulted from serendipitous routes, but it must be kept in mind that these “unusual” methods contain the seeds of future syntheses. This chapter compares and contrasts the metal–boron interaction in metalloboranes with the metal–carbon interaction in organometallic compounds containing hydrocarbon ligands. The key that unlocked the field of metallocarborane chemistry was the theoretical and practical recognition that a five-atom open face of a nido icosahedral carborane could bind a transition-metal fragment. An analogy between boranes and metal clusters has been drawn that allows information on the former to be used to provide a first-order picture of the behavior of the latter. Hydrocarbon metallo derivatives and the analogous metalloboranes can be considered as clusters based on skeletal electron counting rules. There are metalloboranes that can justifiably be described as metalligand complexes, and there are metalloboranes that can only usefully be described as clusters. The usefulness of metalloboranes in synthetic procedures has not been established, but there are indications that these compounds may well eventually be valued intermediates.

Scanning tunneling microscopy and spectroscopy investigations of QCA molecules.

Quantum-dot cellular automata (QCA), a computation paradigm based on the Coulomb interactions between neighboring cells. The key idea is to represent binary information, not by the state of a current switch (transistor), but rather by the configuration of charge in a bistable cell. In its molecular realization, the QCA cell can be a single molecule. QCA is ideally suited for molecular implementation since it exploits the molecules ability to contain charge, and does not rely on any current flow between the molecules. We have examined using an UHV-STM some of the QCA molecules like silicon phthalocyanines and Fe-Ru complexes on Au (111) and Si (111) surfaces, which are suitable candidates for the molecular QCA approach.

Synthesis of metal-rich metallaborane clusters. Evidence for a mechanism involving fragment condensation

Abstract A mechanistic hypothesis of metallaborane cluster build-up by the condensation of metal and boron containing fragments, which is supported by circumstantial evidence from previous work, suggests improved routes to the synthesis of ferraboranes. This work describes two new approaches consisting of the examination of two precursors with properties consistent with such a hypothesis. The first precursor, a neutral mononuclear dimethyl sulfide substituted iron tetracarbonyl, Fe(CO) 4 SMe 2 , is a new compound and the high yield synthesis and structural characterization of it are reported. This compound provides a better route to the ferraborane Fe 2 (CO) 6 B 2 H 6 than those presently known and is an isolatable, alternate source of the Fe(CO) 4 fragment. The second precursor, Fe(CO) 3 (cco) 2 , where cco is η 2 - cis -cyclooctene, is a known, ready source of the Fe(CO) 3 fragment. The reaction of Fe(CO) 3 (cco) 2 with BH 3 sources at low temperatures results in good yields of known ferraboranes and a product distribution that depends primarily on the ratio of boron to iron in the reactants. Both of these results support a mechanism for metallaborane cluster formation involving rapid metal carbonyl fragment condensation as a principal mechanistic component.

The syntheses and X-ray crystal structures of novel transition metal cluster arrays containing 2–5 coordinated [(CO)9Co3(μ3-CCOO)]− ligands in a variety of geometries

Abstract 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.

The Metallic Face of Boron

Publisher Summary This chapter discusses the metallic face of boron. The properties of the element are reviewed and an idea of the relationship of boron to the other elements in terms of metallic character is presented. The chapter presents structural evidence delineating the similarities and differences of borane and transition metal species. Pertinent examples from metallaborane chemistry demonstrate real bridges between the two distinct areas. Both discrete and solid state systems are discussed in the chapter and the transition metal borides constitute bridges in the latter case. Despite real differences, because of the differing electronic structures of the atoms, there are a considerable number of similarities between the structural elements of the discrete complexes and the repeating units in the solid state. This theme receives further emphasis in a discussion of discrete transition metal borides that constitute links between molecular species and solid state structures with extended bonding networks. The chapter presents a brief discussion on how such discrete metallaboranes provide a logical preparative approach to solid state materials containing transition metals and boron.

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

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

A system to demonstrate the bistability in molecules for application in a molecular QCA cell

We present a system to test the bistability of individual molecules for application in a QCA cell. The system presented consists of two polysilicon gate electrodes which sit adjacent to two highly-doped windows of silicon to which molecules can be bound. To enable the detection of switching activity the two highly doped regions are electrically connected to the island of a single-electron transistor (SET) which serves as an electrometer. Using this design we facilitate a differential measurement approach where a single molecular switching event should be seen in the conductance of the SET.