Jonathan Bould

Ten-vertex metallaborane chemistry. Aspects of the iridadecaborane closo→isonido→isocloso structural continuum

Reaction of the arachno-[B9H14]– or nido-[B9H12]– anions with [IrCl(PPh3)3] at ca. 298 K gives, in addition to previously reported species, pale violet [7,7,9-(PPh3)3-isonido-7-IrB9H10]2 in yields of ca. 2%. Single crystal X-ray diffraction analysis and multinuclear NMR spectroscopy reveal a ten-vertex {IrB9} cluster of an ‘isonido’ type that is based upon the closo eleven-vertex structure of [B11H11]2– from which a four-connected vertex is removed to generate a four-membered open face. There are two fluxional bridging hydrogen atoms associated with the open face. Thermolysis at ca. 355 K of solutions in 1,1,2,2-tetrachloroethane of yellow [5,7-(PPh3)2-5-H-5-(o-Ph2P[graphic omitted]]4 result in loss of hydrogen and the formation of deep purple [8-Cl-7,9-(PPh3)2-7-(o-Ph2P[graphic omitted]0]5(0–5%) and royal blue [7,9-(PPh3)2-7-(o-Ph2P[graphic omitted]0]6(0–5%). Single-crystal X-ray diffraction shows that compounds 5 and 6 also have isonido cluster structures, although 5 does not have bridging hydrogen atoms on the open face, suggesting that any extra electrons required for it to be isoelectronic with 2 and 6 may originate from the metal vertex. The structure of the crystal of compound 6 examined was found to contain ca. 25 mole % of the isostructural 3-chloro derivative 6a. The ten-vertex clusters 2, 5 and 6 are part of a structural continuum ranging from closo through isonido to isocloso. It is proposed that these isonido compounds represent intermediates in a variety of reactions involving nido to isocloso cluster oxidations and nido to nido rearrangements.

An approach to megalo-boranes. Mixed and multiple cluster fusions involving iridaborane and platinaborane cluster compounds. Crystal structure determinations by conventional and synchrotron methods

Abstract Several new macropolyhedral metallaboranes have been isolated from thermolytic mixed cluster fusion reactions involving metallaboranes and molten B10H14 as solvent. Co-thermolysis of B10H14 with nine-vertex [(CO)(PMe3)2HIrB8H12] (1) engenders 18-vertex [(CO)(PMe3)2IrB17H20] (3), via double cluster fusion; this has the 18-vertex configuration of syn-B18H22, but with a metal atom in the 10-position. From the same reaction, triple cluster fusion engenders 28-vertex [(PMe3)2IrB26H24Ir(CO)(PMe3)2] (4), which structurally is based on an intimate interfusion of closed 10-vertex and 12-vertex subclusters, to generate a tetrahedral tetraboron core that also has a more open commo one-boron linkage to a nido nine-vertex {IrB8} subcluster. Compound 4 exhibits interesting consequences of cluster-crevice formation and introduces the concept of globular megalo-borane structures that have borons-only cores surrounded by boron-hydride sheaths. Examination for incipient megalo-borane globular behaviour in another system, viz. [IrCl(PPh3)3] (7) with anti-B18H22, reveals a four-atom core feature in 19-vertex [(PPh3)HIrB18H18(PPh3)] (6), which has a closo-type {IrB10} 11-vertex subcluster fused to a nido 10-vertex {B10} subcluster to generate a four-atom {IrB3} tetrahedron. Examination for mixed cluster fusion in other systems reveals the generation of [(PMe2Ph)2Pt-anti-B18H20] (8), from the co-thermolysis of [(PMe2Ph)2PtB8H12] (2) and B10H14, and examination for multiple cluster fusion reveals the formation of 30-vertex [(PMe2Ph)2(PMe2C6H4)2Pt2B28H32] (10), 29-vertex [(PMe2Ph)2PtB28H32] (11) and 27-vertex [(PMe2Ph)2PtB26H26(PMe2Ph)] (12) from the same reaction. Structurally, compound 10 is based on a 10-vertex arachno-{6,9-Pt2B8} unit linked, via one B–B two-electron two-centre bond each, to two 10-vertex nido-{B10} units; it also exhibits molecular condensation in the form of two P-phenylene ortho-cycloboronations. Compound 11 is based on the 19-vertex [(PMe2Ph)2Pt-η4-anti-B18H22] configuration with an additional 10-vertex nido-{B10H13} moiety bound to the non-platinated subcluster via one B–B two-electron two-centre bond. Compound 12 is based on two nido 11-vertex {PtB10} units joined by a single commo Pt vertex, with one of these units conjoined to an arachno eight-boron unit via a two-boron common edge and an open bridging {B–H(exo)–Pt–μ-B2} link. Thermolysis of [(PMe2Ph)2PtB8H12] (2) with the pre-formed double-cluster compound anti-B18H22 generates triple-contiguity 27-vertex [(PMe2Ph)PtB26H26(PMe2Ph)] (13) which, structurally, consists of a nido 11-vertex {PtB10} unit that is fused to a second 11-vertex nido {PtB10} unit with a triangular {PtB2} face in common, and also fused to a 10-vertex nido {B10} unit with a {B2} edge in common. The sequence 12→11→10→13→4 represents a progression of increasing intimacy of cluster fusion. Small crystals of compounds 3, 11 and 12 necessitated synchrotron X-radiation for sufficient diffraction intensity.

The first osmaboranes and a new iridatetraborane

Abstract The reactions of [Os(CO)ClH(PPh 3 ) 3 ] under mild conditions with the anions arachno -[B 3 H 8 ] − and nido -[B 5 H 8 − yield the first air-stable polyhedral osmaborane species arachno -[(HOsB 3 H 8 )(CO)(PPh 3 ) 2 ] (65%) and nido -[(OsB 5 H 9 )(CO)(PPh 3 ) 2 ] (80%) respectively. The 11 B and 1 H NMR properties of these osmaboranes are similar to those of their iridium analogues arachno -[(IrB 5 H 8 )(CO)(PPh 3 ) 2 ]. Mild thermolysis of nido -[(OsB 5 H 9 (CO)(PPh 3 ) 2 ] yields nido -[(OsB 4 H 8 )(CO)(PPh 3 ) 2 ] (40%) for which there is, as yet, no iridium analogue.

Alkene Hydrogenation on an 11-Vertex Rhodathiaborane with Full Cluster Participation

The facile synthesis of the metallaheteroborane [8,8-(PPh 3) 2- nido-8,7-RhSB 9H 10] ( 1) makes possible the systematic study of its reactivity. Addition of pyridine to 1 gives in high yield the 11-vertex nido-hydridorhodathiaborane [8,8,8-(PPh 3) 2H-9-(NC 5H 5)- nido-8,7-RhSB 9H 9] ( 2). 2 reacts with C 2H 4 or CO to form [1,1-(PPh 3)(L)-3-(NC 5H 5)- closo-RhSB 9H 8] [L = C 2H 4 ( 3), CO ( 4)]. In CH 2Cl 2 at reflux temperature 2 undergoes a nido to closo transformation to afford [1,1-(PPh 3) 2-3-(NC 5H 5)- closo-1,2-RhSB 9H 8] ( 5). Reaction of 2 with alkenes leads to hydrogenation and isomerization of the olefins. NMR spectroscopy indicates the presence of a labile phosphine ligand in 2, and DFT calculations have been used to determine which of the two phosphine groups is labile. Rationalization of the hydrogenation mechanism and the part played by the 2 --> 3 nido to closo cluster change during the reaction cycle is suggested. In the proposed mechanism the classical hydrogen transfer from hydride metal complexes to olefins occurs twice: first upon coordination of the alkene to the rhodium centre in 2, and second concomitant with formation of a closo-hydridorhodathiaborane intermediate by migration of a BHB-bridging hydrogen atom to the metal. Reaction of H 2 with 3 or 5 regenerates 2, closing a reaction cycle that under catalytic conditions is capable of hydrogenating alkenes. Single-site versus cluster-bifunctional mechanisms are discussed as possible routes for H 2 activation.

Macropolyhedral boron-containing cluster chemistry. Nineteen-vertex [S2B17H17(SMe2)]. An unusual apical boron atom of cluster connectivity six that introduces a new polyhedral borane building block

Mild thermolysis of [SB8H10(SMe2)] results in the formation of a small amount of macropolyhedral [S2B17H17(SMe2)] of which the stucture is based on the fusion, with two boron atoms in common, of a conventional nido-type eleven-vertex {SB10H9} subcluster with an unprecedented arachno-type ten-vertex {SB9H8(SMe2)} subcluster that exhibits an apical boron atom of cluster connectivity six.

Facile thermally-induced cluster oxidations in metallaborane chemistry: arachno→nido→closo reaction sequences exhibited by iridanonaboranes and iridadecaboranes, and the stabilization of the iridium(V) oxidation state

Iridanonaboranes and iridadecaboranes which have adjacent open-face bridging H atoms and terminal Ir–H atoms readily lose H2 in formal cluster oxidations which involve stable isolable iridium (V) species.

Ten-vertex metallaborane chemistry: facile, thermally induced, nido→isocloso cluster-closure oxidation reactions in iridadecaborane clusters

Four nido-iridadecaborane cluster compounds [6-H-6,6-(PR3)2-nido-6-IrB9H13][R = Ph, (1); Me, (5)][sym-6-H-6-(PPh3)-6-(PPh2-ortho-[graphic omitted]](2), and [5-H-5-(PPh3)-5-(PPh2-ortho-[graphic omitted]](3) lose hydrogen on heating in dichloroethane solution at ca. 80 °C (bath) to give, in each case, an ‘isocloso’ iridadecaborane cluster compound. These isocloso species have been characterised by multielement n.m.r. studies and, in the case of [1-H-1-(PPh3)-1-(PPh2-ortho-[graphic omitted]](4), by a single-crystal X-ray crystallographic study. Crystals of (4) are monoclinic, space group P21/c with a= 1366.0(3), b= 1906(4), c= 1429.1(3) pm, β= 103.71(2)°, and Z= 4. The ‘isocloso’ structures and a possible mechanism for the nido→isocloso cluster closure are briefly discussed.

Polyhedral metallaheteroborane chemistry. Synthesis, spectroscopy, structure and dynamics of eleven-vertex {RhNB9} and {PtCB9} metallaheteroboranes.

Reaction between [RhCl(PPh(3))(3)] and the [nido-6-NB(9)H(11)](-) anion in CH(2)Cl(2) yields orange eleven-vertex [8,8-(PPh(3))(2)-nido-8,7-RhNB(9)H(11)]. Reaction of the [nido-6-CB(9)H(12)](-) anion with [cis-PtCl(2)(PMe(2)Ph)(2)] in methanol affords yellow eleven-vertex [9-(OMe)-8,8-(PMe(2)Ph)(2)-nido-8,7-PtCB(9)H(10)], which is also formed from the reaction of MeOH with [8,8-(PPh(3))(2)-nido-8,7-PtCB(9)H(10)]. Both compounds have been characterised by single-crystal X-ray diffraction analysis and examined by NMR spectroscopy and have structures based on eleven-vertex nido-type geometries, with the metal centre and the heteroatoms in the adjacent (8)- and (7)-positions on the pentagonal open face. The metal-to-heteroborane bonding sphere of is fluxional, with a DeltaG(double dagger) value of 48.4 kJ mol(-1). DFT calculations on the model compounds [8,8-(PH(3))(2)-nido-8,7-RhNB(9)H(11)] and [8,8-(PH(3))(2)-nido-8,7-RhSB(9)H(10)] have been carried out to define the fluxional process and the intermediates involved.

Alkyne-promoted H2 loss in a metallaborane: nido-to-closo cluster transformation and sp C-H bond oxidative addition.

A sensitive cluster: The labile rhodathiaborane [(PPh(3))(2)(H)-nido-RhSB(9)H(9)(NC(5)H(5))] combines the redox and coordinative flexibility of the {(PPh(3))(2)(H)Rh} fragment with the capability of the 11-vertex rhodathiaborane cluster to undergo oxidative nido-to-closo transformations induced by coordination of alkynes to the metal centre, which leads to hydrogenation of the triple bond, dehydrogenation of the cluster and oxidative addition of sp C-H bonds.

Decaborane thiols as building blocks for self-assembled monolayers on metal surfaces.

Three nido-decaborane thiol cluster compounds, [1-(HS)-nido-B(10)H(13)] 1, [2-(HS)-nido-B(10)H(13)] 2, and [1,2-(HS)(2)-nido-B(10)H(12)] 3 have been characterized using NMR spectroscopy, single-crystal X-ray diffraction analysis, and quantum-chemical calculations. In the solid state, 1, 2, and 3 feature weak intermolecular hydrogen bonding between the sulfur atom and the relatively positive bridging hydrogen atoms on the open face of an adjacent cluster. Density functional theory (DFT) calculations show that the value of the interaction energy is approximately proportional to the number of hydrogen atoms involved in the interaction and that these values are consistent with a related bridging-hydrogen atom interaction calculated for a B(18)H(22)·C(6)H(6) solvate. Self-assembled monolayers (SAMs) of 1, 2, and 3 on gold and silver surfaces have been prepared and characterized using X-ray photoelectron spectroscopy. The variations in the measured sulfur binding energies, as thiolates on the surface, correlate with the (CC2) calculated atomic charge for the relevant boron vertices and for the associated sulfur substituents for the parent B(10)H(13)(SH) compounds. The calculated charges also correlate with the measured and DFT-calculated thiol (1)H chemical shifts. Wetting-angle measurements indicate that the hydrophilic open face of the cluster is directed upward from the substrate surface, allowing the bridging hydrogen atoms to exhibit a similar reactivity to that of the bulk compound. Thus, [PtMe(2)(PMe(2)Ph)(2)] reacts with the exposed and acidic B-H-B bridging hydrogen atoms of a SAM of 1 on a gold substrate, affording the addition of the metal moiety to the cluster. The XPS-derived stoichiometry is very similar to that for a SAM produced directly from the adsorption of [1-(HS)-7,7-(PMe(2)Ph)(2)-nido-7-PtB(10)H(11)] 4. The use of reactive boron hydride SAMs as templates on which further chemistry may be carried out is unprecedented, and the principle may be extended to other binary boron hydride clusters.