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An amphiphilic block copolymer of polyphosphinoborane has been prepared by a mechanism-led strategy of the sequential catalytic dehydropolymerization of precursor monomers, H

The synthesis of new Schrock–Osborn Rh(i) pre-catalysts with ortho-substituted DPEphos ligands, [Rh(DPEphos-R)(NBD)][BArF4] [R = Me, OMe, iPr; ArF = 3,5-(CF3)2C6H3], is described. Along with the previously reported R = H variant, variable temperature 1H NMR spectroscopic and single-crystal X-ray diffraction studies show that these all have axial (C–H)⋯Rh anagostic interactions relative to the d8 pseudo square planar metal centres, that also result in corresponding downfield chemical shifts. Analysis by NBO, QTAIM and NCI methods shows these to be only very weak C–H⋯Rh bonding interactions, the magnitudes of which do not correlate with the observed chemical shifts. Instead, as informed by Scherer's approach, it is the topological positioning of the C–H bond with regard to the metal centre that is important. For [Rh(DPEphos–iPr)(NBD)][BArF4] addition of H2 results in a Rh(iii) iPr–C–H activated product, [Rh(κ3,σ-P,O,P-DPEphos-iPr′)(H)][BArF4]. This undergoes H/D exchange with D2 at the iPr groups, reacts with CO or NBD to return Rh(i) products, and reaction with H3B·NMe3/tert-butylethene results in a dehydrogenative borylation to form a complex that shows both a non-classical B–H⋯Rh 3c-2e agostic bond and a C–H⋯Rh anagostic interaction at the same metal centre., Rh(i) complexes of ortho-substituted DPEphos-R (R = H, Me, OMe, iPr) ligands show anagostic interactions; for R =iPr C–H activation/dehydrogenative borylation forms a product exhibiting both B–H/Rh 3c-2e agostic and C–H/Rh anagostic motifs.

Using solid-state molecular organometallic (SMOM) techniques, in particular solid/gas single-crystal to single-crystal reactivity, a series of σ-alkane complexes of the general formula [Rh(Cy2PCH2CH2PCy2)(ηn:ηm-alkane)][BArF4] have been prepared (alkane = propane, 2-methylbutane, hexane, 3-methylpentane; ArF = 3,5-(CF3)2C6H3). These new complexes have been characterized using single crystal X-ray diffraction, solid-state NMR spectroscopy and DFT computational techniques and present a variety of Rh(I)···H–C binding motifs at the metal coordination site: 1,2-η2:η2 (2-methylbutane), 1,3-η2:η2 (propane), 2,4-η2:η2 (hexane), and 1,4-η1:η2 (3-methylpentane). For the linear alkanes propane and hexane, some additional Rh(I)···H–C interactions with the geminal C–H bonds are also evident. The stability of these complexes with respect to alkane loss in the solid state varies with the identity of the alkane: from propane that decomposes rapidly at 295 K to 2-methylbutane that is stable and instead undergoes an acceptorless dehydrogenation to form a bound alkene complex. In each case the alkane sits in a binding pocket defined by the {Rh(Cy2PCH2CH2PCy2)}+ fragment and the surrounding array of [BArF4]− anions. For the propane complex, a small alkane binding energy, driven in part by a lack of stabilizing short contacts with the surrounding anions, correlates with the fleeting stability of this species. 2-Methylbutane forms more short contacts within the binding pocket, and as a result the complex is considerably more stable. However, the complex of the larger 3-methylpentane ligand shows lower stability. Empirically, there therefore appears to be an optimal fit between the size and shape of the alkane and overall stability. Such observations are related to guest/host interactions in solution supramolecular chemistry and the holistic role of 1°, 2°, and 3° environments in metalloenzymes.

Rhodium-alkene complexes of the pincer ligand κ3-C5H3N-2,6-(OPiPr2)2 (PONOP-iPr) have been prepared and structurally characterized: [Rh(PONOP-iPr)(η2-alkene)][BArF4] [alkene = cyclooctadiene (COD), norbornadiene (NBD), ethene; ArF = 3,5-(CF3)2C6H3]. Only one of these, alkene = COD, undergoes a reaction with H2 (1 bar), to form [Rh(PONOP-iPr)(η2-COE)][BArF4] (COE = cyclooctene), while the others show no significant reactivity. This COE complex does not undergo further hydrogenation. This difference in reactivity between COD and the other alkenes is proposed to be due to intramolecular alkene-assisted reductive elimination in the COD complex, in which the η2-bound diene can engage in bonding with its additional alkene unit. H/D exchange experiments on the ethene complex show that reductive elimination from a reversibly formed alkyl hydride intermediate is likely rate-limiting and with a high barrier. The proposed final product of alkene hydrogenation would be the dihydrogen complex [Rh(PONOP-iPr)(η2-H2)][BArF4], which has been independently synthesized and undergoes exchange with free H2 on the NMR time scale, as well as with D2 to form free HD. When the H2 addition to [Rh(PONOP-iPr)(η2-ethene)][BArF4] is interrogated using pH2 at higher pressure (3 bar), this produces the dihydrogen complex as a transient product, for which enhancements in the 1H NMR signal for the bound H2 ligand, as well as that for free H2, are observed. This is a unique example of the partially negative line-shape effect, with the enhanced signals that are observed for the dihydrogen complex being explained by the exchange processes already noted., η2-Alkene complexes [Rh(PONOP-iPr)(η2-alkene)][BArF4] [alkene = cyclooctadiene (COD), norbornadiene (NBD), ethene] are reported, for which only the COD complex undergoes significant onward reaction with H2 to form a cyclooctene (COE) complex, the reactivity is which is suggested to be due to intramolecular alkene-assisted reductive elimination. The putative product of H2 addition, [Rh(PONOP-iPr)(η2-H2)][BArF4], is made by an alternative route. Remarkably, enhanced signals for this dihydrogen complex are observed when pH2 is added to [Rh(PONOP-iPr)(η2-ethene)][BArF4], a unique example of the partially negative line-shape effect.

Ligand exchange reactions of [Rh(COD){η

A detailed study of H3B·NMeH2 dehydropolymerization using the cationic precatalyst [Rh(DPEphos)(H2BNMe3(CH2)2tBu)][BArF4] identifies the resting state as dimeric [Rh(DPEphos)H2]2 and boronium [H2B(NMeH2)2]+ as the chain-control agent. [Rh(DPEphos)H2]2 can be generated in situ from Rh(DPEphos)(benzyl) and catalyzes polyaminoborane formation (H2BNMeH)n [Mn = 15 000 g mol–1]. Closely related Rh(Xantphos)(benzyl) operates at 0.1 mol % to give a higher molecular weight polymer [Mn = 85 000 g mol–1] on the gram scale with low residual [Rh], 81 ppm. This insight offers a mechanistic template for dehydropolymerization.

The use of solid-state molecular organometallic chemistry (SMOM-chem) to promote the efficient double bond isomerization of 1-butene to 2-butenes under flow-reactor conditions is reported. Single crystalline catalysts based upon the σ-alkane complexes [Rh(R2PCH2CH2PR2)(η2η2-NBA)][BArF4] (R = Cy, tBu; NBA = norbornane; ArF = 3,5-(CF3)2C6H3) are prepared by hydrogenation of a norbornadiene precursor. For the tBu-substituted system this results in the loss of long-range order, which can be re-established by addition of 1-butene to the material to form a mixture of [Rh(tBu2PCH2CH2PtBu2)(cis-2-butene)][BArF4] and [Rh(tBu2PCH2CH2PtBu2)(1-butene)][BArF4], in an order/disorder/order phase change. Deployment under flow-reactor conditions results in very different on-stream stabilities. With R = Cy rapid deactivation (3 h) to the butadiene complex occurs, [Rh(Cy2PCH2CH2PCy2)(butadiene)][BArF4], which can be reactivated by simple addition of H2. While the equivalent butadiene complex does not form with R = tBu at 298 K and on-stream conversion is retained up to 90 h, deactivation is suggested to occur via loss of crystallinity of the SMOM catalyst. Both systems operate under the industrially relevant conditions of an isobutene co-feed. cis:trans selectivites for 2-butene are biased in favor of cis for the tBu system and are more leveled for Cy.

A simple Co(ii)-based amine-borane dehydropolymerisation catalyst system is reported that operates at low loadings, to selectively give (H2BNMeH)n polymer on scale, with catalyst control over Mn, narrow dispersities and low residual metal content.

An easy, direct and room temperature silicon-carbon bond activation is reported. The reaction of [RhCl(coe)2]2 with the silane Si(Me)2(o-C6H4SMe)2 in the presence of an halide extractor provokes a Si-CH3 bond cleavage yielding a cationic silyl-methyl-Rh(iii). In contrast, if the reaction is performed using the Rh(i) bis-alkene dimers, [RhCl(cod)]2 or [RhCl(nbd)]2, the Si-CH3 bond activation does not occur.

The air tolerant precatalyst, [Rh(L)(NBD)]Cl ([1]Cl) [L = κ3-(iPr2PCH2CH2)2NH, NBD = norbornadiene], mediates the selective synthesis of N-methylpolyaminoborane, (H2BNMeH)n, by dehydropolymerization of H3B·NMeH2. Kinetic, speciation, and DFT studies show an induction period in which the active catalyst, Rh(L)H3 (3), forms, which sits as an outer-sphere adduct 3·H3BNMeH2 as the resting state. At the end of catalysis, dormant Rh(L)H2Cl (2) is formed. Reaction of 2 with H3B·NMeH2 returns 3, alongside the proposed formation of boronium [H2B(NMeH2)2]Cl. Aided by isotopic labeling, Eyring analysis, and DFT calculations, a mechanism is proposed in which the cooperative “PNHP” ligand templates dehydrogenation, releasing H2B═NMeH (ΔG‡calc = 19.6 kcal mol–1). H2B═NMeH is proposed to undergo rapid, low barrier, head-to-tail chain propagation for which 3 is the catalyst/initiator. A high molecular weight polymer is formed that is relatively insensitive to catalyst loading (Mn ∼71 000 g mol–1; Đ, of ∼ 1.6). The molecular weight can be controlled using [H2B(NMe2H)2]Cl as a chain transfer agent, Mn = 37 900–78 100 g mol–1. This polymerization is suggested to arise from an ensemble of processes (catalyst speciation, dehydrogenation, propagation, chain transfer) that are geared around the concentration of H3B·NMeH2. TGA and DSC thermal analysis of polymer produced on scale (10 g, 0.01 mol % [1]Cl) show a processing window that allows for melt extrusion of polyaminoborane strands, as well as hot pressing, drop casting, and electrospray deposition. By variation of conditions in the latter, smooth or porous microstructured films or spherical polyaminoboranes beads (∼100 nm) result.

The sequential solid/gas single-crystal to single-crystal reaction of [Rh(Cy2P(CH2)3PCy2)(COD)][BArF4] (COD = cyclooctadiene) with H2or D2was followed in situ by solid-state31P{1H} NMR spectroscopy (SSNMR) and ex situ by solution quenching and GC-MS. This was quantified using a two-step Johnson–Mehl–Avrami–Kologoromov (JMAK) model that revealed an inverse isotope effect for the second addition of H2, that forms a σ-alkane complex [Rh(Cy2P(CH2)3PCy2)(COA)][BArF4]. Using D2, a temporal window is determined in which a structural solution for this σ-alkane complex is possible, which reveals an η2,η2-binding mode to the Rh(I) center, as supported by periodic density functional theory (DFT) calculations. Extensive H/D exchange occurs during the addition of D2, as promoted by the solid-state microenvironment.

In 2007 two of us defined the σ-Complex Assisted Metathesis mechanism (Perutz and Sabo-Etienne, Angew. Chem. Int. Ed. 2007 , 46 , 2578-2592), i.e. the σ-CAM concept. This new approach to reaction mechanisms brought together metathesis reactions involving the formation of a variety of metal-element bonds through partner-interchange of σ-bond complexes. The key concept that defines a σ-CAM process is a single transition state for metathesis that is connected by two intermediates that are σ-bond complexes while the oxidation state of the metal remains constant in precursor, intermediates and product. This mechanism is appropriate in situations where σ-bond complexes have been isolated or computed as well-defined minima. Unlike several other mechanisms, it does not define the nature of the transition state. In this review, we highlight advances in the characterization and dynamic rearrangements of σ-bond complexes, most notably alkane and zincane complexes, but also different geometries of silane and borane complexes. We set out a selection of catalytic and stoichiometric examples of the σ-CAM mechanism that are supported by strong experimental and/or computational evidence. We then draw on these examples to demonstrate that the scope of the σ-CAM mechanism has expanded to classes of reaction not envisaged in 2007 (additional s-bond ligands, agostic complexes, sp 2 -carbon, surfaces). Finally, we provide a critical comparison to alternative mechanisms for metathesis of metal-element bonds.

Reversible encapsulation of CH 2 Cl 2 or Xe in a non–porous solid–state molecular organometallic framework of [Rh(Cy 2 PCH 2 PCy 2 )(NBD)][BAr F 4 ] occurs in single–crystal to single–crystal transformations. These processes are also probed by solid–state NMR spectroscopy, including 129 Xe SSNMR. Non–covalent interactions with the –CF 3 groups, and hydrophobic channels formed, of [BAr F 4 ] – anions are shown to be important, and thus have similarly to the transport of substrates and products to and from the active site in metalloenzymes.

We describe the reactivity of the hypersilyl-functionalized Zintl cluster salt K[Ge9(Hyp)3] towards the nickel reagents Ni(COD)2 and Ni(Cp)2, which gives rise to markedly different complexes. In the case of Ni(COD)2 (COD = 1,5-cyclooctadiene), a dianionic sandwich-like cluster [Ni{Ge9(Hyp)3}2]2− (1) was obtained, in line with a simple ligand substitution reaction of COD by [Ge9(Hyp)3]−. By contrast, when an analogous reaction with Ni(Cp)2 (Cp = cyclopentadienyl) was performed, vertex substitution of the [Ge9(Hyp)3]− precursor was observed, giving rise to the nine-vertex nido-cluster (Cp)Ni[Ge8(Hyp)3] (2). This is the first instance of vertex substitution at a hypersilyl-functionalized Zintl cluster cage. The electrochemical behavior of these compounds was explored and showed reversible redox behaviour for both clusters.

Single‐crystal to single‐crystal solid‐state molecular organometallic (SMOM) techniques are used for the synthesis and structural characterization of the σ‐alkane complex [Rh(tBu2PCH2CH2CH2PtBu2)(η2,η2‐C7H12)][BArF 4] (ArF=3,5‐(CF3)2C6H3), in which the alkane (norbornane) binds through two exo‐C−H⋅⋅⋅Rh interactions. In contrast, the bis‐cyclohexyl phosphine analogue shows endo‐alkane binding. A comparison of the two systems, supported by periodic DFT calculations, NCI plots and Hirshfeld surface analyses, traces this different regioselectivity to subtle changes in the local microenvironment surrounding the alkane ligand. A tertiary periodic structure supporting a secondary microenvironment that controls binding at the metal site has parallels with enzymes. The new σ‐alkane complex is also a catalyst for solid/gas 1‐butene isomerization, and catalyst resting states are identified for this., Importance of microenvironment: By using solid‐state molecular organometallic (SMOM) techniques, the σ‐alkane complex [Rh(tBu2PCH2CH2CH2PtBu2)(η2,η2‐C7H12)][BArF 4], in which the alkane binds through two exo‐C−H⋅⋅⋅Rh interactions, is synthesized and structurally characterized. This is different from the analogous complex with PCy2 groups. Subtle differences in the microenvironment, as encoded by the diene precursor complex, are shown to determine the selectivity of alkane binding.

We report on the synthesis of an alkane-soluble Zintl cluster, [η4-Ge9(Hyp)3]Rh(COD), that can catalytically hydrogenate cyclic alkenes such as 1,5-cyclooctadiene and cis-cyclooctene. This is the first example of a well-defined Zintl-cluster-based homogeneous catalyst.

Vapour-phase surface-initiated cationic polymerisation of ethylvinylether occurs at single-crystals of the σ-alkane complex [Rh(Cy2PCH2CH2PCy2)(NBA)][BArF4]. This new surface interface makes these normally very air sensitive materials tolerant to air, while also allowing for onward single-crystal to single-crystal reactivity at metal sites within the lattice.

A cobalt σ‐alkane complex, [Co(Cy2P(CH2)4PCy2)(norbornane)][BArF 4], was synthesized by a single‐crystal to single‐crystal solid/gas hydrogenation from a norbornadiene precursor, and its structure was determined by X‐ray crystallography. Magnetic data show this complex to be a triplet. Periodic DFT and electronic structure analyses revealed weak C−H→Co σ‐interactions, augmented by dispersive stabilization between the alkane ligand and the anion microenvironment. The calculations are most consistent with a η1:η1‐alkane binding mode., A cobalt σ‐alkane complex was synthesized by a single‐crystal to single‐crystal solid/gas hydrogenation from a norbornadiene precursor, and its structure was determined by X‐ray crystallography. Periodic DFT and electronic structure analyses revealed weak C−H→Co σ‐interactions, augmented by dispersive stabilization between the alkane ligand and the anion microenvironment.

In an attempt to prepare new Fe catalysts for the dehydrocoupling of amine-boranes and to provide mechanistic insight, the paramagnetic FeII dimeric complex [Cp′FeI]2 (1) (Cp′=η5-((1,2,4-tBu)3C5H2)) was used as a precursor to a series of cyclopentadienyl FeII and FeIII mononuclear species. The complexes prepared were [Cp′Fe(η6-Tol)][Cp′FeI2] (2) (Tol=C6H5Me), [Cp′Fe(η6-Tol)][BArF 4] (3) (BArF 4=[B(C6H3(m-CF3)2)4]−), [N(nBu)4][Cp′FeI2] (4), Cp′FeI2 (5), and [Cp′Fe(MeCN)3][BArF 4] (6). The electronic structure of the [Cp′FeI2]− anion in 2 and 4 was investigated by SQUID magnetometry, EPR spectroscopy and ab initio Complete Active Space Self Consistent Field-Spin Orbit (CASSCF-SO) calculations, and the studies revealed a strongly anisotropic S=2 ground state. Complexes 1–6 were investigated as catalysts for the dehydrocoupling of Me2NH⋅BH3 (I) in THF at 20 °C to yield the cyclodiborazane product [Me2N-BH2]2 (IV). Complexes 1–4 and 6 were active dehydrocoupling catalysts towards I (5 mol % loading), however 5 was inactive, and ultra-violet (UV) irradiation was required for the reaction mediated by 3. Complex 6 was found to be the most active precatalyst, reaching 80 % conversion to IV after 19 h at 22 °C. Dehydrocoupling of I by 1–4 proceeded via formation of the aminoborane Me2N=BH2 (II) as the major intermediate, whereas for 6 the linear diborazane Me2NH-BH2-NMe2-BH3 (III) could be detected, together with trace amounts of II. Reactions of 1 and 6 with Me3N⋅BH3 were investigated in an attempt to identify Fe-based intermediates in the catalytic reactions. The σ-complex [Cp′Fe(MeCN)(κ2-H2BH⋅NMe2H][BArF 4] was proposed to initially form in dehydrocoupling reactions involving 6 based on ESI-MS (ESI=Electrospray Ionisation Mass Spectroscopy) and NMR spectroscopic evidence. The latter also suggests that these complexes function as precursors to iron hydrides which may be the true catalytic species.

The rhodium complex [Rh(Ph2PCH2CH2CH2PPh2)(η6-FC6H5)][BArF 4], 2, catalyzes BH/BD exchange between D2 and the boranes H3B⋅NMe3, H3B⋅SMe2 and HBpin, facilitating the expedient isolation of a variety of deuterated analogues in high isotopic purities, and in particular the isotopologues of N-methylamine-borane: R3B⋅NMeR2 1-dx (R=H, D; x=0, 2, 3 or 5). It also acts to catalyze the dehydropolymerization of 1-dx to give deuterated polyaminoboranes. Mechanistic studies suggest a metal-based polymerization involving an unusual hybrid coordination insertion chain-growth/step-growth mechanism.

Crabtree's catalyst was encapsulated inside the pores of the sulfonated MIL‐101(Cr) metal–organic framework (MOF) by cation exchange. This hybrid catalyst is active for the heterogeneous hydrogenation of non‐functionalized alkenes either in solution or in the gas phase. Moreover, encapsulation inside a well‐defined hydrophilic microenvironment enhances catalyst stability and selectivity to hydrogenation over isomerization for substrates bearing ligating functionalities. Accordingly, the encapsulated catalyst significantly outperforms its homogeneous counterpart in the hydrogenation of olefinic alcohols in terms of overall conversion and selectivity, with the chemical microenvironment of the MOF host favouring one out of two competing reaction pathways.

The synthesis of rhodium complexes with weakly binding highly fluorinated benzene ligands is described: 1,2,3‐F3C6H3, 1,2,3,4‐F4C6H2 and 1,2,3,4,5‐F5C6H are shown to bind with cationic [Rh(Cy2P(CH2)xPCy2)]+ fragments (x=1, 2). Their structures and reactivity with alkenes, and use in catalysis for promoting the Tishchenko reaction of a simple aldehyde, are demonstrated. Key to the synthesis of these complexes is the highly concentrated reaction conditions and use of the [Al{OC(CF3)3}4]− anion., Weakly binding highly fluorinated benzene ligands 1,2,3‐F3C6H3, 1,2,3,4‐F4C6H2 and 1,2,3,4,5‐F5C6H are shown to bind with cationic [Rh(Cy2P(CH2)xPCy2)]+ fragments (x=1, 2), and the resulting complexes have been used in synthesis and catalysis.

The σ-amine-borane pincer complex [Rh(PONOP)(η1-H3B·NMe3)][BArF4] [2, PONOP = κ3-NC5H3-2,6-(OPtBu2)2] is prepared by addition of H3B·NMe3 to the dihydrogen precursor [Rh(PONOP)(η2-H2)][BArF4], 1. In a similar way the related H3B·NMe2H complex [Rh(PONOP)(η1-H3B·NMe2H)][BArF4], 3, can be made in situ, but this undergoes dehydrocoupling to reform 1 and give the aminoborane dimer [H2BNMe2]2. NMR studies on this system reveal an intermediate neutral hydride forms, Rh(PONOP)H, 4, that has been prepared independently. 1 is a competent catalyst (2 mol%, ∼30 min) for the dehydrocoupling of H3B·Me2H. Kinetic, mechanistic and computational studies point to the role of NMe2H in both forming the neutral hydride, via deprotonation of a σ-amine-borane complex and formation of aminoborane, and closing the catalytic cycle by reprotonation of the hydride by the thus-formed dimethyl ammonium [NMe2H2]+. Competitive processes involving the generation of boronium [H2B(NMe2H)2]+ are also discussed, but shown to be higher in energy. Off-cycle adducts between [NMe2H2]+ or [H2B(NMe2H)2]+ and amine-boranes are also discussed that act to modify the kinetics of dehydrocoupling.

The non-oxidative catalytic dehydrogenation of light alkanes via C-H activation is a highly endothermic process that generally requires high temperatures and/or a sacrificial hydrogen acceptor to overcome unfavorable thermodynamics. This is complicated by alkanes being such poor ligands, meaning that binding at metal centers prior to C-H activation is disfavored. We demonstrate that by biasing the pre-equilibrium of alkane binding, by using solid-state molecular organometallic chemistry (SMOM-chem), well-defined isobutane and cyclohexane σ-complexes, [Rh(Cy2PCH2CH2PCy2)(η: η-(H3C)CH(CH3)2][BArF4] and [Rh(Cy2PCH2CH2PCy2)(η: η-C6H12)][BArF4] can be prepared by simple hydrogenation in a solid/gas single-crystal to single-crystal transformation of precursor alkene complexes. Solid-gas H/D exchange with D2 occurs at all C-H bonds in both alkane complexes, pointing to a variety of low energy fluxional processes that occur for the bound alkane ligands in the solid-state. These are probed by variable temperature solid-state nuclear magnetic resonance experiments and periodic density functional theory (DFT) calculations. These alkane σ-complexes undergo spontaneous acceptorless dehydrogenation at 298 K to reform the corresponding isobutene and cyclohexadiene complexes, by simple application of vacuum or Ar-flow to remove H2. These processes can be followed temporally, and modeled using classical chemical, or Johnson-Mehl-Avrami-Kologoromov, kinetics. When per-deuteration is coupled with dehydrogenation of cyclohexane to cyclohexadiene, this allows for two successive KIEs to be determined [kH/kD = 3.6(5) and 10.8(6)], showing that the rate-determining steps involve C-H activation. Periodic DFT calculations predict overall barriers of 20.6 and 24.4 kcal/mol for the two dehydrogenation steps, in good agreement with the values determined experimentally. The calculations also identify significant C-H bond elongation in both rate-limiting transition states and suggest that the large kH/kD for the second dehydrogenation results from a pre-equilibrium involving C-H oxidative cleavage and a subsequent rate-limiting β-H transfer step.

H3B·NMe3 σ-complexes of d8 [(L1)Rh][BArF4] and d10 [(L1)Ag][BArF4] (where L1 = 2,6-bis-[1-(2,6-diisopropylphenylimino)ethyl]pyridine) have been prepared and structurally characterised. Analysis of the molecular and electronic structures reveal important but subtle differences in the nature of the bonding in these σ-complexes, which differ only by the identity of the metal centre and the d-electron count. With Rh the amine–borane binds in an η2:η2 fashion, whereas at Ag the unsymmetrical {Ag⋯H3B·NMe3} unit suggests a structure lying between the η2:η2 and η1 extremes.

[Rh(κ2-PP-DPEphos){η2η2-H2B(NMe3)(CH2)2tBu}][BArF4] acts as an effective precatalyst for the dehydropolymerization of H3B·NMeH2 to form N-methylpolyaminoborane (H2BNMeH)n. Control of polymer molecular weight is achieved by variation of precatalyst loading (0.1–1 mol %, an inverse relationship) and use of the chain-modifying agent H2: with Mn ranging between 5 500 and 34 900 g/mol and Đ between 1.5 and 1.8. H2 evolution studies (1,2-F2C6H4 solvent) reveal an induction period that gets longer with higher precatalyst loading and complex kinetics with a noninteger order in [Rh]TOTAL. Speciation studies at 10 mol % indicate the initial formation of the amino–borane bridged dimer, [Rh2(κ2-PP-DPEphos)2(μ-H)(μ-H2BN=HMe)][BArF4], followed by the crystallographically characterized amidodiboryl complex [Rh2(cis-κ2-PP-DPEphos)2(σ,μ-(H2B)2NHMe)][BArF4]. Adding ∼2 equiv of NMeH2 in tetrahydrofuran (THF) solution to the precatalyst removes this induction period, pseudo-first-order kinetics are observed, a half-order relationship to [Rh]TOTAL is revealed with regard to dehydrogenation, and polymer molecular weights are increased (e.g., Mn = 40 000 g/mol). Speciation studies suggest that NMeH2 acts to form the precatalysts [Rh(κ2-DPEphos)(NMeH2)2][BArF4] and [Rh(κ2-DPEphos)(H)2(NMeH2)2][BArF4], which were independently synthesized and shown to follow very similar dehydrogenation kinetics, and produce polymers of molecular weight comparable with [Rh(κ2-PP-DPEphos){η2-H2B(NMe3)(CH2)2tBu}][BArF4], which has been doped with amine. This promoting effect of added amine in situ is shown to be general in other cationic Rh-based systems, and possible mechanistic scenarios are discussed.

Continuous monitoring of catalyzed reactions using infrared spectroscopy (IR) measures the transformation of reactant into product, whereas mass spectrometry delineates the dynamics of the catalytically relevant species present at much lower concentrations, a holistic approach that provides mechanistic insight into the reaction components whose abundance spans 5 orders of magnitude. Probing reactions to this depth reveals entities that include precatalysts, resting states, intermediates, and also catalyst impurities and decomposition products. Simple temporal profiles that arise from this analysis aid discrimination between the different types of species, and a hydroacylation reaction catalyzed by a cationic rhodium complex is studied in detail to provide a test case for the utility of this methodology.

The straightforward synthesis of two cationic iridium norbornadiene species bearing simple bidentate phosphines is reported [Ir(R 2 PCH 2 CH 2 PR 2 )(NBD)][ BAr 4 F ] [NBD = norbornadiene; R = t Bu, Cy; Ar F = 3,5–C 6 H 3 (CF 3 ) 2 ]. The hydrogenation of [Ir( t Bu 2 PCH 2 CH 2 P t Bu 2 )(NBD)][ BAr 4 F ] in the solution phase and in the solid state is described in which saturated (solution) or unsaturated (solid–state) dimeric species with bridging hydrides are formed. The solid–state structures, as determined by single crystal X-ray diffraction, of these dimeric species are also discussed.

The pentane σ-complex [Rh{Cy2 P(CH2 CH2 )PCy2 }(η(2) :η(2) -C5 H12 )][BAr(F) 4 ] is synthesized by a solid/gas single-crystal to single-crystal transformation by addition of H2 to a precursor 1,3-pentadiene complex. Characterization by low temperature single-crystal X-ray diffraction (150 K) and SSNMR spectroscopy (158 K) reveals coordination through two Rh⋅⋅⋅H-C interactions in the 2,4-positions of the linear alkane. Periodic DFT calculations and molecular dynamics on the structure in the solid state provide insight into the experimentally observed Rh⋅⋅⋅H-C interaction, the extended environment in the crystal lattice and a temperature-dependent pentane rearrangement implicated by the SSNMR data.

The dehydropolymerization of amine–boranes, exemplified as H2RB⋅NR′H2, to produce polyaminoboranes (HRBNR′H)n that are inorganic analogues of polyolefins with alternating main‐chain B−N units, is an area with significant potential, stemming from both fundamental (mechanism, catalyst development, main‐group hetero‐cross‐coupling) and technological (new polymeric materials) opportunities. This Concept article outlines recent advances in the field, covering catalyst development and performance, current mechanistic models, and alternative non‐catalytic routes for polymer production. The substrate scope, polymer properties and applications of these exciting materials are also outlined. Challenges and opportunities in the field are suggested, as a way of providing focus for future investigations.

The role that the supporting anion has on the stability, structure, and catalytic performance, in solid-state molecular organometallic systems (SMOM) based upon [Rh- (Cy2PCH2CH2PCy2)(η2 η2 -NBD)][BArX 4], [1-NBD][BArX 4], is reported (X = Cl, F, H; NBD = norbornadiene). The tetra-aryl borate anion is systematically varied at the 3,5-position, ArX= 3,5- X2C6H3, and the stability and structure in the solid-state compared with the previously reported [1-NBD][BArCF34] complex. Single-crystal X-ray crystallography shows that the three complexes have different packing motifs, in which the cation sits on the shared face of two parallelepipeds for [1- NBD][BArCl4], is surrounded by eight anions in a gyrobifastigium arrangement for [1-NBD][BArF 4], or the six anions show an octahedral cage arrangement in [1-NBD][BArH 4], similar to that of [1-NBD][BArCF34]. C−X···X−C contacts, commonly encountered in crystal-engineering, are suggested to be important in determining structure. Addition of H2 in a solid/gas reaction affords the resulting σ-alkane complexes, [Rh(Cy2PCH2CH2PCy2)(η2 η2 -NBA)][BArX 4] [1-NBA][BArX 4] (NBA = norbornane), which can then proceed to lose the alkane and form the zwitterionic, anion-coordinated, complexes. The relative rates at which hydrogenation and then decomposition of σ-alkane complexes proceed are shown to be anion dependent. [BArCF34] − promotes fast hydrogenation and an indefinitely stable σ-alkane complex. With [BArH 4] − hydrogenation is slow and the σ-alkane complex so unstable it is not observed. [BArCl4] − and [BArF 4] − promote intermediate reactivity profiles, and for [BArCl4] −, a single-crystal to single-crystal hydrogenation results in [1-NBA][BArCl4]. The molecular structure derived from X-ray diffraction reveals a σalkane complex in which the NBA fragment is bound through two exo Rh···H−C interactions-different from the endo selective binding observed with [1-NBA][BArCF34]. Periodic DFT calculations demonstrate that this selectivity is driven by the microenvironment dictated by the surrounding anions. [1-NBA][BArX 4] are catalysts for gas/solid 1-butene isomerization (298 K, 1 atm), and their activity can be directly correlated to the stability of the σ-alkane complex compared to the anion-coordinated decomposition products.

The detailed mechanism of the hydroacylation of β-amido-aldehyde, 2,2-dimethyl-3-morpholino-3-oxopropanal, with 1-octyne using [Rh(cis-κ2-P,P-DPEPhos)(acetone)2][BArF4]-based catalysts, is described [ArF = (CF3)2C6H3]. A rich mechanistic landscape of competing and interconnected hydroacylation and cyclotrimerization processes is revealed. An acyl-hydride complex, arising from oxidative addition of aldehyde, is the persistent resting state during hydroacylation, and quaternary substitution at the β-amido-aldehyde strongly disfavors decarbonylation. Initial rate, KIE, and labeling studies suggest that the migratory insertion is turnover-limiting as well as selectivity determining for linear/branched products. When the concentration of free aldehyde approaches zero at the later stages of catalysis alkyne cyclotrimerization becomes competitive, to form trisubstituted hexylarenes. At this point, the remaining acyl-hydride turns over in hydroacylation and the free alkyne is now effectively in excess, and the resting state moves to a metallacyclopentadiene and eventually to a dormant α-pyran-bound catalyst complex. Cyclotrimerization thus only becomes competitive when there is no aldehyde present in solution, and as aldehyde binds so strongly to form acyl-hydride when this happens will directly correlate to catalyst loading: with low loadings allowing for free aldehyde to be present for longer, and thus higher selectivites to be obtained. Reducing the catalyst loading from 20 mol % to 0.5 mol % thus leads to a selectivity increase from 96% to ∼100%. An optimized hydroacylation reaction is described that delivers gram scale of product, at essentially quantitative levels, using no excess of either reagent, at very low catalyst loadings, using minimal solvent, with virtually no workup.

A range of cationic rhodium bisphosphine η6-fluorobenzene (fluorobenzenes = C6H6−nFn, n = 1–3) and related complexes have been synthesized and characterized. These complexes act as useful organometallic precursors for catalysis or further synthetic elaboration. The relative binding affinity of the arene ligands has been investigated using Electrospray Ionisation Mass Spectrometry (ESI–MS) and two different collision-induced dissociation (CID) techniques. The influence of arene fluorination upon arene binding affinity is discussed as well as the comparison of different bis–phosphine ligands with regard to bite angle and phosphine substitution. We show that this simple technique allows fast and easy comparison of the binding affinity of arene ligands to cationic organometallic fragments.

The complexes [Rh(iBu2PCH2CH2PiBu2)L2][BArF4] [L2 = C4H6, (C2H4)2, (CO)2, (NH3)2; ArF = 3,5-C6H3(CF3)2] have been synthesized by solid−gas reactivity via ligand exchange reactions with, in some cases, crystallinity retained through single-crystal to single-crystal transformations. The solid-state structures of these complexes have been determined, but in only one case (L2 = (NH3)2) is the cation ordered sufficiently to enable its structural metrics to be determined by single crystal X-ray diffraction. The onward solid-state reactivity of some of these complexes has been probed. The bis-ammonia complex [Rh(iBu2PCH2CH2PiBu2)(NH3)2][BArF4] undergoes H/D exchange at bound NH3 when exposed to D2. The bis-ethene complex [Rh(iBu2PCH2CH2PiBu2)(C2H4)2][BArF4] undergoes a slow dehydrogenative coupling reaction to produce a material containing a 1:1 mixture of the butadiene complex and a postulated mono-ethene complex. The mechanisms of these processes have been probed by DFT calculations on the isolated Rh cations. All the solid materials were tested as heterogeneous catalysts for the hydrogenation of ethene. Complexes with weakly bound ligands (e.g., L2 = (C2H4)2) are more active catalysts than those with stronger bound ligands (e.g., L = (CO)2). Surface-passivated crystals, formed through partial reaction with CO, allow for active sites to be probed, either on the surface or the interior of the single crystal.

Single-crystal to single-crystal solid/gas reactivity and catalysis starting from the precursor sigma-alkane complex [Rh(Cy2PCH2CH2PCy2)(η2η2-NBA)][BArF4] (NBA = norbornane; ArF = 3,5-(CF3)2C6H3) is reported. By adding ethene, propene and 1-butene to this precursor in solid/gas reactions the resulting alkene complexes [Rh(Cy2PCH2CH2PCy2)(alkene)x][BArF4] are formed. The ethene (x = 2) complex, [Rh(Cy2PCH2CH2PCy2)(ethene)2][BArF4]-Oct, has been characterized in the solid-state (single-crystal X-ray diffraction) and by solution and solid-state NMR spectroscopy. Rapid, low temperature recrystallization using solution methods results in a different crystalline modification, [Rh(Cy2PCH2CH2PCy2)(ethene)2][BArF4]-Hex, that has a hexagonal microporous structure (P6322). The propene complex (x = 1) [Rh(Cy2PCH2CH2PCy2)(propene)][BArF4] is characterized as having a π-bound alkene with a supporting γ-agostic Rh⋯H3C interaction at low temperature by single-crystal X-ray diffraction, variable temperature solution and solid-state NMR spectroscopy, as well as periodic density functional theory (DFT) calculations. A fluxional process occurs in both the solid-state and solution that is proposed to proceed via a tautomeric allyl-hydride. Gas/solid catalytic isomerization of d3-propene, H2C[double bond, length as m-dash]CHCD3, using [Rh(Cy2PCH2CH2PCy2)(η2η2-NBA)][BArF4] scrambles the D-label into all possible positions of the propene, as shown by isotopic perturbation of equilibrium measurements for the agostic interaction. Periodic DFT calculations show a low barrier to H/D exchange (10.9 kcal mol−1, PBE-D3 level), and GIPAW chemical shift calculations guide the assignment of the experimental data. When synthesized using solution routes a bis-propene complex, [Rh(Cy2PCH2CH2PCy2)(propene)2][BArF4], is formed. [Rh(Cy2PCH2CH2PCy2)(butene)][BArF4] (x = 1) is characterized as having 2-butene bound as the cis-isomer and a single Rh⋯H3C agostic interaction. In the solid-state two low-energy fluxional processes are proposed. The first is a simple libration of the 2-butene that exchanges the agostic interaction, and the second is a butene isomerization process that proceeds via an allyl-hydride intermediate with a low computed barrier of 14.5 kcal mol−1. [Rh(Cy2PCH2CH2PCy2)(η2η2-NBA)][BArF4] and the polymorphs of [Rh(Cy2PCH2CH2PCy2)(ethene)2][BArF4] are shown to be effective in solid-state molecular organometallic catalysis (SMOM-Cat) for the isomerization of 1-butene to a mixture of cis- and trans-2-butene at 298 K and 1 atm, and studies suggest that catalysis is likely dominated by surface-active species. [Rh(Cy2PCH2CH2PCy2)(η2η2-NBA)][BArF4] is also shown to catalyze the transfer dehydrogenation of butane to 2-butene at 298 K using ethene as the sacrificial acceptor.

A combined experimental and computational study on the fluxional processes involving the M-H and B-H positions in the sigma amine-borane complexes [M(PR3)2(H)2(η(2)-H3B·NMe3)][BAr(F)4] (M = Rh, Ir; R = Cy for experiment; R = Me, Cy for computation; Ar(F) = 3,5-(CF3)2C6H3) is reported. The processes studied are: B-H bridging/terminal exchange; reaction with exogenous D2 leading to exchange at M-H; and intramolecular M-H/B-H exchange. Experimentally it was found that B-H bridging/terminal exchange is most accessible and slightly favoured for Rh; D2/M-H exchange occurs at qualitatively similar rates for both M = Rh and Ir, while M-H/B-H exchange is the slowest overall, with the Ir congener having a lower barrier than Rh. Evidence for the isotopic perturbation of equilibrium is also reported for the BH/BD isotopologues of [Ir(PCy3)2(H)2(η(2)-H3B·NMe3)][BAr(F)4]. DFT calculations using model complexes (R = Me) qualitatively reproduce the relative rates of the various exchange processes for both M = Rh and Ir, i.e. barriers for B-H bridging/terminal exchange are less than those for M-H/H2 exchange, which in turn are less than those for M-H/B-H exchange. Which metal promotes these processes more effectively depends upon the nature of the rate-limiting transition state, which can change between Rh and Ir. Computational analysis of the full experimental system (R = Cy) reveals similar overall trends in terms of the relative ease of the various exchange processes. However, there are differences in the details, and these are discussed.

The CH2Cl2 complex [Rh((tBu)PONOP)(κ(1)-ClCH2Cl)][BAr(F)4] is reported, that also acts as a useful synthon for other complexes such as N2, CO and H2 adducts; while the analogous PNP complex undergoes C-Cl activation.

Complexes that show C-C···Rh sigma interactions have been synthesized and characterised by NMR spectroscopy and X-ray diffraction: [Rh(PR3)(Binor-S)][BArF4], R = P tBu2Me, PCy2Ph; Ar = C6H 3(CF3)2, Binor-S = 1,2,4,5,6,8-dimetheno-S- indacene. These studies, in concert with previously published work, show that there is a rather narrow window of phosphine steric profile that supports the isolation of these interesting complexes, with the phosphine cone angle lying between 160° and 170°. © 2013 Elsevier B.V. All rights reserved.

We explore the use of stable, pre-formed, oligomeric aminoboranes as precursors for the chemical vapour deposition growth of few-layered hexagonal boron nitride (h-BN) films on Cu foils under atmospheric pressure conditions. Dimeric diborazane H3B·NH2BH2·NH3 (DAB), and trimeric triborazane H3B·(NH2BH2)2·NH3 (TAB), derivatives of ammonia borane, H3B·NH3 (AB), are compared with AB, a commonly used precursor for the CVD growth of h-BN. Both DAB and TAB show similar effectiveness to AB in growing h-BN few layered films. Using DAB as the precursor instead of AB leads to fully continuous h-BN films in a shorter period of time. Analysis of the surface of the h-BN films reveals that DAB and TAB precursors deposit more nanoparticles on the surface of the h-BN films during their CVD growth within the same time period as when using AB. The viability of these two new h-BN precursors (DAB and TAB), opens up a wider range of solid-state sources for growing wide band gap h-BN films using CVD techniques.

At concentrations greater than the critical micellar concentration, cmc, Aerosol-OT, AOT, adsorbs at the air−water and liquid−solid interfaces as a lamellar phase, with long-range lamellar ordering normal to the surface or interfaces. Coincident with the specular Bragg scattering from the ordered structure is pronounced off-specular scattering, characteristic of conformal roughness in the multilayer structure. A detailed analysis of the off-specular scattering, interpreted in terms of a correlation length of the conformity and an amplitude of the fluctuations, is presented. Additional off-specular scattering is observed, and its origin, arising from an effect analagous to that responsible for Newton's interference fringes in optics, is discussed. Analysis of both the specular reflectivity and off-specular scattering is consistent with increasing structural order with increasing temperature.

Reaction of the functionalized phosphine P tBu 2CH 2(C 3H 5) with [Rh(C 8H 14) 2Cl] 2 (C 8H 14 = cis-cyclooctene) resulted in selective C-C bond activation of the cyclopropyl group and formation of a tetrameric rhodacyclobutane, [RhCl(- 3-P tBu 2CH 2CH(CH 2) 2)] 4, which was characterized in the solid state by X-ray crystallography. This complex acts as a latent source of the [Rh(- 3-P tBu 2CH 2CH(CH 2) 2)] + fragment, forming a range of new complexes by salt metathesis with NaCp, [RhCp(- 3-P tBu 2CH 2CH(CH 2) 2)], chloride abstraction in the presence of arenes, [Rh(- 6-arene)(- 3-P tBu 2CH 2CH(CH 2) 2)][BAr F4] [arene = C 6H 5F, C 6H 3Me 3; Ar F = 3,5-C 6H 3(CF 3) 2], or fragmentation by addition of phosphine ligands, trans-[RhCl(PR 3)(- 3-P tBu 2CH 2CH(CH 2) 2)] (R = iBu, Cy). In contrast, reaction with carbon monoxide results in the reductive elimination of the tethered cyclopropane, demonstrating reversible C-C bond activation, and formation of cis-[RhCl(CO) 2(P tBuCH 2(C 3H 5))]. These complexes were characterized in solution by NMR spectroscopy and in the solid state by X-ray diffraction. Chloride abstraction from [RhCl(P iBu 3)(- 3-P tBu 2CH 2CH(CH 2) 2)] resulted in the regioselective isomerization of the tethered cyclopropane to a terminal alkene, viz., the formally 14-electron complex [Rh(P iBu 3)(- 3-P tBu 2CH 2CH 2CH - CH 2)][BAr F4], notable for the presence of a strong agostic interaction with a phosphine iBu substituent, apparent both from the solid-state structure and in solution by 1H NMR spectroscopy. Addition of hydrogen to this complex resulted in hydrogenation of the alkene and formation of [Rh(H) 2(P iBu 3)(P tBu 2nBu)][BAr F4]. These steps overall correspond to selective C-C bond functionalization (hydrogenation) of the tethered cyclopropane and are of direct significance to a related catalytic process [J. Am. Chem. Soc. 2003, 125, 886] and, more generally, to the rhodium-mediated transformation of alkanes. © 2010 American Chemical Society.

Density functional theory has been used to investigate the structures, bonding and properties of a family of hydride rich late transition metal clusters of the type [Rh(6)(PH(3))(6)H(12)](x) (x = 0, +1, +2, +3 or +4), [Rh(6)(PH(3))(6)H(16)](x) (x = +1 or +2) and [Rh(6)(PH(3))(6)H(14)](x) (x = 0, +1 or +2). The positions of the hydrogen atoms around the pseudo-octahedral Rh(6) core in the optimized structures of [Rh(6)(PH(3))(6)H(12)](x) (x = 0, +1, +2, +3 or +4) varied depending on the overall charge on the cluster. The number of semi-bridging hydrides increased (semi-bridging hydrides have two different Rh-H bond distances) as the charge on the cluster increased and simultaneously the number of perfectly bridging hydrides (equidistant between two Rh centers) decreased. This distortion maximized the bonding between the hydrides and the metal centers and resulted in the stabilization of orbitals related to the 2T(2g) set in a perfectly octahedral cluster. In contrast, the optimized structures of the 16-hydride clusters [Rh(6)(PH(3))(6)H(12)](x) (x = +1 or +2) were similar and both clusters contained an interstitial hydride, along with one terminal hydride, ten bridging hydrides and two coordinated H(2) molecules which were bound to two rhodium centers in an eta(2):eta(1)-fashion. All the hydrides were on the outside of the Rh(6) core in the lowest energy structures of the 14-hydride clusters [Rh(6)(PH(3))(6)H(14)] and [Rh(6)(PH(3))(6)H(14)](+), which both contained eleven bridging hydrides, one terminal hydride and one coordinated H(2) molecule. Unfortunately, the precise structure of [Rh(6)(PH(3))(6)H(14)](2+) could not be determined as structures both with and without an interstitial hydride were of similar energy. The reaction energetics for the uptake and release of two molecule of H(2) by a cycle consisting of [Rh(6)(PH(3))(6)H(12)](2+), [Rh(6)(PH(3))(6)H(16)](2+), [Rh(6)(PH(3))(6)H(14)](+), [Rh(6)(PH(3))(6)H(12)](+) and [Rh(6)(PH(3))(6)H(14)](2+) were modelled, and, in general, good agreement was observed between experimental and theoretical results. The electronic reasons for selected steps in the cycle were investigated. The 12-hydride cluster [Rh(6)(PH(3))(6)H(12)](2+) readily picks up two molecules of H(2) to form [Rh(6)(PH(3))(6)H(16)](2+) because it has a small HOMO-LUMO gap (0.50 eV) and a degenerate pair of LUMO orbitals available for the uptake of four electrons (which are provided by two molecules of H(2)). The reverse process, the spontaneous release of a molecule of H(2) from [Rh(6)(PH(3))(6)H(16)](+) to form [Rh(6)(PH(3))(6)H(14)](+) occurs because the energy gap between the anti-bonding SOMO and the next highest energy occupied orbital in [Rh(6)(PH(3))(6)H(16)](+) is 0.9 eV, whereas in [Rh(6)(PH(3))(6)H(14)](+) the energy gap between the anti-bonding SOMO and the next highest energy occupied orbital is only 0.3 eV. At this stage the factors driving the conversion of [Rh(6)(PH(3))(6)H(14)](+) to [Rh(6)(PH(3))(6)H(12)](2+) are still unclear.

Release of the primary amino-borane H2B NtBuH from [Ir(PCy3)2(H)2(η2-H2B NtBuH)] [ BA r 4 F ] , formed from dehydrogenation of [Ir(PCy3)2(H)2(η2-H3B·NtBuH2)][ BA r 4 F ] , results in the eventual formation of the corresponding cyclic borazine [HBNtBu]3. Observations point to the fact that off-metal hydrogen redistribution reactions play a major role in the final product formation, with H3B·NtBuH2 being formed alongside [HBNtBu]3.

Iridium hydridophosphine complexes of general formula [Ir(PR3)2H2(anion)](PR3= PPh3, PMe2Ph; anion =[1-closo-CB(11)H(6)Cl(6)]-, [1-closo-CB(11)H(6)I(6)]-, [BAr(F)4]-) have been prepared by hydrogenation of cyclooctadiene precursor complexes. Solid-state structures of selected examples of these complexes reveal intimate contacts between the carborane anion and cation, with the anion binding through two lower-hemisphere halogen ligands. In CD2Cl2 solution the very weakly coordinating anions [1-closo-CB(11)H(6)Cl(6)]- and [BAr(F)4]- are suggested to favour the formation of solvent complexes such as [Ir(PR3)2H2(solvent)n][anion], while the [1-closo-CB(11)H(6)I(6)]- anion forms a tightly bound complex with the cationic iridium fragment. Calculated DeltaG values for anion reorganisation in d8-toluene reflect this difference in interaction between the anions and cation. With the bulky anion [1-closo-CB(11)Me(5)I(6)]- different complexes are formed: Ir(PPh3)H2(1-closo-HCB(11)Me(5)I(6)) and [(PPh3)3Ir(H2)H2][1-closo-HCB(11)Me(5)I(6)] which have been characterised spectroscopically. Diffusion measurements in CD2Cl2 are also consistent with larger, solvent coordinated, complexes for the more weakly coordinating anions and a tighter interaction between anion and cation for [1-closo-CB(11)H(6)I(6)]-. All the complexes show some ion-paring in solution. Comparison with data previously reported for the [1-closo-CB(11)H(6)Br(6)]- anion shows that this anion--as expected--fits between [1-closo-CB(11)H(6)Cl(6)]- and [1-closo-CB(11)H(6)I(6)]- in terms of coordinating ability. Although not coordinating, the large [1-closo-CB(11)H(6)Cl(6)]- and [BAr(F)4)]- anions do provide some stabilisation towards the metal centre, as decomposition to the hydride bridged dimer [Ir2(PPh3)4H5]+ is retarded. This is in contrast to the [PF6]- salt where decomposition is immediate. As expected, complexes with the smaller phosphine PMe2Ph form tighter interactions with the carborane anions. These observations on the interaction between anion and cation in solution are reflected in benchmark hydrogenation studies that show a significant attenuation in rate of hydrogenation of cyclohexane on using the [1-closo-CB(11)H(6)I(6)]- anion or complexes with the PMe2Ph phosphine. We also comment on the reusability of the catalysts and their tolerance to water and oxygen impurities. Overall the catalyst with the [1-closo-CB(11)H(6)Br(6)]- anion shows the best combination of rate of hydrogenation, reusability and tolerance to impurities.