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The borylative cyclisation of 1,2-dialkynyl benzenes with BCl3 leads to dibenzopentalenes (via intramolecular SEAr) or benzofulvenes (via chloride addition) depending on substituents, with stabilised vinyl cation intermediates (e.g. with a p-MeO-C6H4-group) favouring the latter. The use of borocations leads to more selective dibenzopentalene formation, while other diyne frameworks undergo intramolecular SEAr selectively even with p-MeO groups.

Useful α-boryl esters can be synthesized in one step from α,β-unsaturated esters using just a simple to access NHC-BH 3 (NHC = N-heterocyclic carbene) and catalytic I 2 . The scope of this reductive α-borylation methodology is excellent and includes a range of alkyl, aryl substituted and cyclic and acyclic α,β-unsaturated esters. Mechanistic studies involving reductive borylation of a cyclic α,β-unsaturated ester with NHC-BD 3 /I 2 indicated that concerted hydroboration of the alkene moiety in the α,β-unsaturated ester proceeds instead of a stepwise process involving initial 1,4-hydroboration; this is in contrast to the recently reported reductive α-silylation. The BH 2 (NHC) unit can be transformed into electrophilic BX 2 (NHC) moieties (X = halide) and the ester moiety can be reduced to the alcohol with the borane unit remaining intact to form β-boryl alcohols. The use of a chiral auxiliary, 8-phenylmenthyl ester, also enables effective stereo-control of the newly formed C-B bond. Combined two step ester reduction/borane oxidation forms diols, including excellent e.e. (97%) for the formation of S -3-phenylpropane-1,2-diol. This work represents a simple transition metal free route to form bench stable α-boryl esters from inexpensive starting materials.

An intramolecular 1,2-boryl-anion migration from boron to carbon has been achieved by selective activation of the π system in [(vinyl)B 2 Pin 2 )] - using "soft" BR 3 electrophiles (BR 3 =BPh 3 or 9-aryl-BBN). The soft character is key to ensure 1,2-migration proceeds instead of oxygen coordination/B-O activation. The BR 3 -induced 1,2-boryl-anion migration represents a triple borylation of a vinyl Grignard reagent using only B 2 Pin 2 and BR 3 and forms differentially protected 1,1,2-triborylated alkanes. Notably, by increasing the steric bulk at the β position of the vinyl Grignard reagent used to activate B 2 Pin 2 , 1,2-boryl-anion migration can be suppressed in favor of intermolecular {BPin} - transfer to BPh 3 , thus enabling simple access to unsymmetrical sp 2 -sp 3 diboranes., (© 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

B(sp 2 )-B(sp 3 ) diborane species based on bis(catecholato)diboron and N-heterocyclic carbenes (NHCs) underwent catechol/bromide exchange selectively at the sp 3 -hybridized boron atom. The reduction of the resulting 1,1-dibromodiborane adducts led to reductive coupling and isolation of doubly NHC-stabilized 1,2-diboryldiborenes. These compounds are the first examples of molecules exhibiting π-electron delocalization over an all-boron chain., (© 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Benzothiadiazole (BT) directed C-H borylation using BCl 3 , followed by B-Cl hydrolysis and Suzuki-Miyaura cross-coupling enables facile access to twisted donor-acceptor compounds. A subsequent second C-H borylation step provides, on arylation of boron, access to borylated highly twisted D-A compounds with a reduced bandgap, or on B-Cl hydrolysis/cross-coupling to twisted D-A-D compounds. Photophysical studies revealed that in this series there is long lifetime emission only when the donor is triphenylamine. Computational studies indicated that the key factor in observing the donor dependent long lifetime emission is the energy gap between the S 1 /T 2 excited states, which are predominantly intramolecular charge-transfer states, and the T 1 excited state, which is predominantly a local excited state on the BT acceptor moiety., (© 2018 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.)

1-Boraphenalenes have been synthesized by reaction of BBr 3 with 1-(aryl-ethynyl)naphthalenes, 1-ethynylnaphthalene, and 1-(pent-1-yn-1-yl)naphthalene and they can be selectively functionalized at boron or carbon to form bench-stable products. All of these 1-boraphenalenes have LUMOs localized on the planar C 12 B core that are closely comparable in character to isoelectronic phenalenyl cations. In contrast to the comparable LUMOs, the aromatic stabilization of the C 5 B ring in 1-boraphenalenes is dramatically lower than the C 6 rings in phenalenyl cations. This is due to the occupied orbitals of π symmetry being less delocalised in the 1-boraphenalenes., (© 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The relative (to BEt 3 ) hydride ion affinity (HIA) of a series of acridine borenium salts has been calculated, with some HIAs found to be similar to that for [Ph 3 C] + . The HIA at the acridine C9 position is controlled by both acridine and the boron substituents, the latter presumably affecting the strength of the B=N bond in the acridane-BY 2 products from hydride transfer. Through a range of hydride abstraction benchmarking reactions against organic hydride donors the experimental HIA of [F 5 acr-BCat] + (cat = catechol, F 5 acr = 1,2,3,4,7-pentafluoroacridine) has been confirmed to be extremely high and closely comparable to that of [Ph 3 C] + . The high HIA of [F 5 acr-BCat] + enables H 2 and alkene activation in a FLP with 2,6-di- tert -butylpyridine. Finally, the HIA of pyridine and quinoline borenium cations has been determined, with the HIA at boron in [PinB(amine)] + (pin = pinacol, amine = pyridine or quinoline) found to be relatively low. This enabled the hydroboration of pyridine and quinoline by HBPin to be achieved through the addition of 5-10 mol % of bench-stable cationic carbon Lewis acids such as 2-phenyl- N , N -dimethylimidazolium salts., Competing Interests: The authors declare no competing financial interest.

The Suzuki-Miyaura (SM) reaction is one of the most important methods for C-C bond formation in chemical synthesis. In this communication, we show for the first time that the low toxicity, inexpensive element zinc is able to catalyze SM reactions. The cross-coupling of benzyl bromides with aryl borates is catalyzed by ZnBr 2 , in a process that is free from added ligand, and is compatible with a range of functionalized benzyl bromides and arylboronic acid pinacol esters. Initial mechanistic investigations indicate that the selective in situ formation of triaryl zincates is crucial to promote selective cross-coupling reactivity, which is facilitated by employing an arylborate of optimal nucleophilicity., (© 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Post-polymerization modification of the donor-acceptor polymer, poly(9,9-dioctylfluorene-alt-benzothiadiazole), PF8-BT, by electrophilic C-H borylation is a simple method to introduce controllable quantities of near-infrared (near-IR) emitting chromophore units into the backbone of a conjugated polymer. The highly stable borylated unit possesses a significantly lower LUMO energy than the pristine polymer resulting in a reduction in the band gap of the polymer by up to 0.63 eV and a red shift in emission of more than 150 nm. Extensively borylated polymers absorb strongly in the deep red/near-IR and are highly emissive in the near-IR region of the spectrum in solution and solid state. Photoluminescence quantum yield (PLQY) values are extremely high in the solid state for materials with emission maxima ≥ 700 nm with PLQY values of 44% at 700 nm and 11% at 757 nm for PF8-BT with different borylation levels. This high brightness enables efficient solution processed near-IR emitting OLEDs to be fabricated and highly emissive borylated polymer loaded conjugated polymer nanoparticles (CPNPs) to be prepared. The latter are bright, photostable, low toxicity bioimaging agents that in phantom mouse studies show higher signal to background ratios for emission at 820 nm than the ubiquitous near-IR emissive bioimaging agent indocyanine green. This methodology represents a general approach for the post-polymerization functionalization of donor-acceptor polymers to reduce the band gap as confirmed by the C-H borylation of poly((9,9-dioctylfluorene)-2,7-diyl-alt-[4,7-bis(3-hexylthien-5-yl)-2,1,3-benzothiadiazole]-2c,2cc-diyl) (PF8TBT) resulting in a red shift in emission of >150 nm, thereby shifting the emission maximum to 810 nm.

Activation of N-heterocyclic carbene boranes (NHC⋅BH 3 ) by I 2 enables the metal-free catalytic C-H borylation of heteroarenes with formation of H 2 as the by-product in a process that uses only bench stable precursors. The borylation of indoles using NHC⋅BH 3 /I 2 produces C2-borylated indoles exclusively in contrast to other catalytic electrophilic C-H borylation methods. Mechanistic studies indicate that this is due to C3 to C2 boron migration facilitated by the absence of exogenous Brønsted bases. Thus this C-H borylation methodology proceeds under sufficiently Brønsted acidic conditions to enable the thermodynamic C2-borylated indole isomer to be formed instead of the C3 borylated-isomer. This demonstrates that electrophilic C-H borylation can be used to access a wider range of borylated regioisomers than reported to date., (© 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Lower Lewis acidity boranes demonstrate greater tolerance to combinations of water/strong Brønsted bases than B(C 6 F 5 ) 3 , this enables Si-H bond activation by a frustrated Lewis pair (FLP) mechanism to proceed in the presence of H 2 O/alkylamines. Specifically, BPh 3 has improved water tolerance in the presence of alkylamines as the Brønsted acidic adduct H 2 O-BPh 3 does not undergo irreversible deprotonation with aliphatic amines in contrast to H 2 O-B(C 6 F 5 ) 3 . Therefore BPh 3 is a catalyst for the reductive amination of aldehydes and ketones with alkylamines using silanes as reductants. A range of amines inaccessible using B(C 6 F 5 ) 3 as catalyst, were accessible by reductive amination catalysed by BPh 3 via an operationally simple methodology requiring no purification of BPh 3 or reagents/solvent. BPh 3 has a complementary reductive amination scope to B(C 6 F 5 ) 3 with the former not an effective catalyst for the reductive amination of arylamines, while the latter is not an effective catalyst for the reductive amination of alkylamines. This disparity is due to the different pK a values of the water-borane adducts and the greater susceptibility of BPh 3 species towards protodeboronation. An understanding of the deactivation processes occurring using B(C 6 F 5 ) 3 and BPh 3 as reductive amination catalysts led to the identification of a third triarylborane, B(3,5-Cl 2 C 6 H 3 ) 3 , that has a broader substrate scope being able to catalyse the reductive amination of both aryl and alkyl amines with carbonyls., (© 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

BCl 3 -induced borylative cyclization of aryl-alkynes possessing ortho-EMe (E=S, O) groups represents a simple, metal-free method for the formation of C3-borylated benzothiophenes and benzofurans. The dichloro(heteroaryl)borane primary products can be protected to form synthetically ubiquitous pinacol boronate esters or used in situ in Suzuki-Miyaura cross couplings to generate 2,3-disubstituted heteroarenes from simple alkyne precursors in one pot. In a number of cases alkyne trans-haloboration occurs alongside, or instead of, borylative cyclization and the factors controlling the reaction outcome are determined., (© 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Whilst hydrogen is a potentially clean fuel for energy storage and utilisation technologies, its conversion to electricity comes at a high energetic cost. This demands the use of rare and expensive precious metal electrocatalysts. Electrochemical-frustrated Lewis pairs offer a metal-free, CO tolerant pathway to the electrocatalysis of hydrogen oxidation. They function by combining the hydrogen-activating ability of frustrated Lewis pairs (FLPs) with electrochemical oxidation of the resultant hydride. Here we present an electrochemical-FLP approach that utilises two different Lewis acids - a carbon-based N -methylacridinium cation that possesses excellent electrochemical attributes, and a borane that exhibits fast hydrogen cleavage kinetics and functions as a "hydride shuttle". This synergistic interaction provides a system that is electrocatalytic with respect to the carbon-based Lewis acid, decreases the required potential for hydrogen oxidation by 1 V, and can be recycled multiple times.

New aryl- and heteroarylboronate esters chelated by dipropanolamine are synthesised directly from boronic acids. The corresponding anionic borates are readily accessible by deprotonation and demonstrate an increase in hydrocarbyl nucleophilicity in comparison to other common borates. The new borates proved competent for magnesium or zinc additive-free, direct boron-to-iron hydrocarbyl transmetallations with well-defined iron(II) (pre)catalysts. The application of the new borate reagents in representative Csp(2)-Csp(3) cross-coupling led to almost exclusive homocoupling unless coupling is performed in the presence of a zinc additive.

BCl3 is an inexpensive electrophile which induces the borylative cyclization of a wide range of substituted alkynes to regioselectively form polycycles containing synthetically versatile C(sp(2) )boronate esters. It proceeds rapidly, with good yields and is compatible with a range of functional groups and substitution patterns. Intermolecular 1,2-carboboration of alkynes is also achieved using BCl3 to generate trisubstituted vinyl boronate esters., (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The direct C(sp(2) )-C(sp(3) ) cross-coupling of diaryl zinc reagents with benzylic, primary, secondary, and tertiary alkyl halides proceeded in the absence of coordinating ethereal solvents at ambient temperature without the addition of a catalyst. The C(sp(2) )-C(sp(3) ) cross-coupling showed excellent functional-group tolerance, and products were isolated in high yields, generally without the requirement for purification by chromatography. This process represents an expedient, operationally simple method for the construction of new C(sp(2) )-C(sp(3) ) bonds., (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

N-methylacridinium salts are Lewis acids with high hydride ion affinity but low oxophilicity. The cation forms a Lewis adduct with 4-(N,N-dimethylamino)pyridine but a frustrated Lewis pair (FLP) with the weaker base 2,6-lutidine which activates H2, even in the presence of H2O. Anion effects dominate reactivity, with both solubility and rate of H2 cleavage showing marked anion dependency. With the optimal anion, a N-methylacridinium salt catalyzes the reductive transfer hydrogenation and hydrosilylation of aldimines through amine-boranes and silanes, respectively. Furthermore, the same salt is active for the catalytic dehydrosilylation of alcohols (primary, secondary, tertiary, and ArOH) by silanes with no observable over-reduction to the alkanes., (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The reaction of 8-(trimethylsiloxy)quinoline (QOTMS) with BCl3 and (aryl)BCl2 forms QOBCl2 and QOBCl(aryl). The subsequent addition of stoichiometric AlCl3 follows one of two paths, dependent on the steric demands of the QO ligand and the electrophilicity of the resulting borenium cation. The phenyl- and 5-hexylthienylborenium cations, QOBPh(+) and QOBTh(+), are formed, whereas QOBCl(+) is not. Instead, AlCl3 preferentially binds with QOBCl2 at oxygen, forming QOBCl2 ⋅AlCl3, rather than abstracting chloride. A modest increase in the steric demands around oxygen, by installing a methyl group at the 7-position of the quinolato ligand, switches the reactivity with AlCl3 back to chloride abstraction, allowing formation of  Me 2QOBCl(+). All the prepared borenium cations are highly chlorophilic and exhibit significant interaction with AlCl4(-) resulting in an equilibrium concentration of Lewis acidic "AlCl3" species. The presence of "AlCl3(") species limits the alkyne substrates compatible with these borenium systems, with reaction of [QOBPh][AlCl4 ] with 1-pentyne exclusively yielding the cyclotrimerised product, 1,3,5-tripropylbenzene. In contrast, QOBPh(+) and QOBTh(+) systems effect the syn-1,2-carboboration of 3-hexyne. DFT calculations at the M06-2X/6-311G(d,p)/PCM(DCM) level confirm that the higher migratory aptitude of Ph versus Me leads to a lower barrier to 1,2-carboboration relative to 1,1-carboboration., (© 2014 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.)

A range of frustrated Lewis pairs (FLPs) containing borenium cations have been synthesised. The catechol (Cat)-ligated borenium cation [CatB(PtBu(3))](+) has a lower hydride-ion affinity (HIA) than B(C(6)F(5))(3). This resulted in H(2) activation being energetically unfavourable in a FLP with the strong base PtBu(3). However, ligand disproportionation of CatBH(PtBu(3)) at 100 °C enabled trapping of H(2) activation products. DFT calculations at the M06-2X/6-311G(d,p)/PCM (CH(2)Cl(2)) level revealed that replacing catechol with chlorides significantly increases the chloride-ion affinity (CIA) and HIA. Dichloro-borenium cations, [Cl(2)B(amine)](+), were calculated to have considerably greater HIA than B(C(6)F(5))(3). Control reactions confirmed that the HIA calculations can be used to successfully predict hydride-transfer reactivity between borenium cations and neutral boranes. The borenium cations [Y(Cl)B(2,6-lutidine)](+) (Y = Cl or Ph) form FLPs with P(mesityl)(3) that undergo slow deprotonation of an ortho-methyl of lutidine at 20 °C to form the four-membered boracycles [(CH(2){NC(5)H(3)Me})B(Cl)Y] and [HPMes(3)](+). When equimolar [Y(Cl)B(2,6-lutidine)](+)/P(mesityl)(3) was heated under H(2) (4 atm), heterolytic cleavage of dihydrogen was competitive with boracycle formation., (Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Electrophilic direct borylation is facilitated, and arene substrate scope enhanced, by using electrophiles derived from inexpensive reagents; specifically an amine, BCl(3) and AlCl(3).

Postsynthetic metal-organic framework (MOF) derivatization introduces accessible secondary amine functionalities that react with nitric oxide (NO) to form N-diazenium diolates. This is in contrast to the parent MOF that binds NO essentially irreversibly at open metal coordination sites.

Reaction of PNPCo, where PNP is (tBu2PCH2SiMe2)2N-, with the persistent radical galvinoxyl, G, gives PNPCoIIG, a nonplanar S = 3/2 species. Reaction with PhCH2Cl or with 0.5 mol I2 gives PNPCoX (X = Cl or I, respectively), but additional I2, seeking CoIII, gives instead oxidation at phosphorus: (tBu2P(I)CH2SiMe2NSiMe2CH2PtBu2)CoI2. Hydrogen-atom transfer reagents fail to give PNPCoH, but H2 gives instead PNPCo(H)2, a result rationalized thermodynamically based on DFT calculations. Multiple equiv of PhSiH3 give a product of Co(V), where N/SiPh and P/Si bonds have formed. N2CH(SiMe3) gives a 1:1 adduct of PNPCo, whose metric parameters suggest partial oxidation above CoI; N2CHPh gives a 1:1 adduct but with very different spectroscopic features. PhN3 reacts fast, via several intermediates detected below 0 degrees C, to finally release N2 and form a CoI product where one phosphorus has been oxidized, PN(P=NPh)Co. Whereas PNPCo(N3) resists loss of N2 on heating, one electron oxidation gives a rapid loss of N2, and the remaining nitride nitrogen is quickly incorporated into the chelate ligand, giving [tBu2PCH2SiMe2NSiMe2NP(tBu2)=CH2Co]. O2 or PhI=O generally gives products where one or both phosphorus centers are converted to its oxide, bonded to cobalt.

Reduction of (PNP)MCl [PNP = ((t)Bu(2)PCH(2)SiMe(2))(2)N] with Mg gives three-coordinate, T-shaped (PNP)M for M = Fe(S = 3/2) and Ni. Their reactivity was tested toward CO; Ni binds one CO, but only reversibly (i.e., CO is completely lost in vacuum), and has a CO stretching frequency showing effective back-donation by NiI. The structure of (PNP)Ni(CO) is intermediate between planar and tetrahedral, in contrast to the planar d8 analogue, (PNP)Co(CO). This structural reorganization on carbonylation changes the singly occupied molecular orbital from having negligible phosphorus character [no P hyperfine structure in the electron paramagnetic resonance (EPR) spectrum of (PNP)Ni] to having enough P character to have a triplet structure in the EPR spectrum of the CO. The presence of one fewer electron in (PNP)Fe (vs the Co analogue) leads to binding of two CO, and (PNP)Fe(CO)(2) is characterized as a spin doublet with square-pyramidal structure. Density functional theory calculations strengthen the understanding of the structural and spectroscopic changes along this dn series (n = 7-9).

The compounds (PNP)CoX, where PNP is (tBu2PCH2SiMe2)2N- and X is Cl, I, N3, OAr, OSO2CF3 and N(H)Ar, are reported. Some of these show magnetic susceptibility, color, and 1H NMR evidence of being in equilibrium between a blue, tetrahedral S=3/2 state and a red, planar S=1/2 state; the equilibrium populations are influenced by subtle solvent effects (e.g., benzene and cyclohexane are different), as well as by temperature. Attempted oxidation to Co(III) with O2 occurs instead at phosphorus, giving [P(O)NP(O)]CoX species. The single O-atom transfer reagent PhI=O likewise oxidizes P. Even I2 oxidizes P to give the pendant phosphonium species (tBu2P(I)CH2SiMe2NSiMe2CH2PtBu2)CoI2 with a tetrahedral S=3/2 cobalt; the solid-state structure shows intermolecular PI...ICo interactions. Attempted alkyl metathesis of PNPCoX inevitably results in reduction, forming PNPCo, which is a spin triplet with planar T-shaped coordination geometry with no agostic interaction. Triplet PNPCo binds N2(weakly) and CO (whose low CO stretching frequency indicates strong PNP-->Co donor power), but not ethene or MeCCMe.

A new class of transition metal cluster is described, [Rh(6)(PR(3))(6)H(12)][BAr(F)(4)](2) (R = (i)Pr (1a), Cy (2a); BAr(F)(4) = [B{C(6)H(3)(CF(3))(2)}(4)](-)). These clusters are unique in that they have structures exactly like those of early transition metal clusters with edge-bridging pi-donor ligands rather than the structures expected for late transition metal clusters with pi-acceptor ligands. The solid-state structures of 1a and 2a have been determined, and the 12 hydride ligands bridge each Rh-Rh edge of a regular octahedron. Pulsed gradient spin-echo NMR experiments show that the clusters remain intact in solution, having calculated hydrodynamic radii of 9.5(3) A for 1a and 10.7(2) A for 2a, and the formulation of 1a and 2a was unambiguously confirmed by ESI mass spectrometry. Both 1a and 2a take up two molecules of H(2) to afford the cluster species [Rh(6)(P(i)Pr(3))(6)H(16)][BAr(F)(4)](2) (1b) and [Rh(6)(PCy(3))(6)H(16)][BAr(F)(4)](2) (2b), respectively, as characterized by NMR spectroscopy, ESI-MS, and, for 2b, X-ray crystallography using the [1-H-CB(11)Me(11)](-) salt. The hydride ligands were not located by X-ray crystallography, but (1)H NMR spectroscopy showed a 15:1 ratio of hydride ligands, suggesting an interstitial hydride ligand. Addition of H(2) is reversible: placing 1b and 2b under vacuum regenerates 1a and 2a. DFT calculations on [Rh(6)(PH(3))(6)H(x)()](2+) (x = 12, 16) support the structural assignments and also show a molecular orbital structure that has 20 orbitals involved with cluster bonding. Cluster formation has been monitored by (31)P{(1)H} and (1)H NMR spectroscopy, and mechanisms involving heterolytic H(2) cleavage and elimination of [HP(i)Pr(3)](+) or the formation of trimetallic intermediates are discussed.

The four-coordinate compound [(tBu2PCH2SiMe2)2N]RuCH3 undergoes rapid double H-C(sp3) activation at -78 degrees C to generate a "hydrido-carbene" complex. DFT calculations suggest that the origin of the low barrier to methane elimination is an alpha-agostic interaction in the low-lying singlet state of the highly unsaturated (PNP)RuMe. The hydrido-carbene complex can be viewed as a "masked" resting state of the four-coordinate cyclometalated alkyl complex, [(tBu2PCH2SiMe2)N(Me2SiCH2P(tBu)(C(CH3)2CH2)]Ru, where hydride migration from metal to carbon occurs before any subsequent reactivity.

Addition of H2 (4 atm at 298 K) to [Rh(nbd)(PR3)2][BAr(F)4] [R = Cy, iPr] affords Rh(III) dihydride/dihydrogen complexes. For R = Cy, complex 1a results, which has been shown by low-temperature NMR experiments to be the bis-dihydrogen/bis-hydride complex [Rh(H)2(eta2-H2)2(PCy3)2][BAr(F)4]. An X-ray diffraction study on 1a confirmed the {Rh(PCy3)2} core structure, but due to a poor data set, the hydrogen ligands were not located. DFT calculations at the B3LYP/DZVP level support the formulation as a Rh(III) dihydride/dihydrogen complex with cis hydride ligands. For R = iPr, the equivalent species, [Rh(H)2(eta2-H2)2(P iPr3)2][BAr(F)4] 2a, is formed, along with another complex that was spectroscopically identified as the mono-dihydrogen, bis-hydride solvent complex [Rh(H)2(eta2-H2)(CD2Cl2)(P iPr3)2][BAr(F)4] 2b. The analogous complex with PCy3 ligands, [Rh(H)2(eta2-H2)(CD2Cl2)(PCy3)2][BAr(F)4] 1b, can be observed by reducing the H2 pressure to 2 atm (at 298 K). Under vacuum, the dihydrogen ligands are lost in these complexes to form the spectroscopically characterized species, tentatively identified as the bis hydrides [Rh(H)2(L)2(PR3)2][BAr(F)4] (1c R = Cy; 2c R = iPr; L = CD2Cl2 or agostic interaction). Exposure of 1c or 2c to a H2 atmosphere regenerates the dihydrogen/bis-hydride complexes, while adding acetonitrile affords the bis-hydride MeCN adduct complexes [Rh(H)2(NCMe)2(PR3)2][BAr(F)4]. The dihydrogen complexes lose [HPR3][BAr(F)4] at or just above ambient temperature, suggested to be by heterolytic splitting of coordinated H2, to ultimately afford the dicationic cluster compounds of the type [Rh6(PR3)6(mu-H)12][BAr(F)4]2 in moderate yield.

The synthesis of the silver(I) salt of the highly methylated carborane anion [closo-1-H-CB(11)Me(11)](-) is described, Ag[closo-1-H-CB(11)Me(11)] 1, which in the solid state shows close intermolecular Ag...H(3)C contacts. Addition of various monodentate phosphines to 1 results in the formation of the complexes (R(3)P)Ag[closo-1-H-CB(11)Me(11)] [R = Ph, 2; cyclohexyl (C(6)H(11)), 3; (3,5-Me(2)-C(6)H(3)), 4]. All these complexes show close intermolecular Ag.H(3)C contacts in the solid state that are considerably shorter than the sum of the van der Waals radius of methyl (2.00 A) and the ionic radius of silver(I) (1.29 A). For 2 and 3 there are other close intermolecular Ag...H(3)C contacts in the solid state, arising from proximate carborane anions in the crystal lattice. Addition of methyl groups to the periphery of the phosphine ligand (complex 4) switches off the majority of these interactions, leaving essentially a single cage interacting with the cationic silver-phosphine fragment through three CH(3) groups. In solution (CD(2)Cl(2)) Ag...H(3)C contacts remain, as evidenced by both the downfield chemical shift change and the significant line-broadening observed for the cage methyl signals. These studies also show that the metal fragment is fluxional over the surface of the cage. The Ag...H(3)C interactions in solution may be switched off by addition of a stronger Lewis base than [closo-1-H-CB(11)Me(11)](-). Thus, addition of [NBu(4)][closo-1-H-CB(11)H(5)Br(6)] to 2 affords (Ph(3)P)Ag[closo-1-H-CB(11)H(5)Br(6)], while adding Et(2)O or PPh(3) affords the well-separated ion-pairs [(Ph(3)P)(L)Ag][closo-1-H-CB(11)Me(11)] (L = OEt(2) 5, PPh(3) 6,) both of which have been crystallographically characterized. DFT calculations on 2 (at the B3LYP/DZVP level) show small energy differences between the possible coordination isomers of this compound, with the favored geometry being one in which the [(Ph(3)P)Ag](+) fragment interacts with three of the [BCH(3)] vertices on the lower surface of the cage, similar to the experimentally observed structure of 4.

[Cp2ZrMe(12-mu-Me-1-closo-CB11HMe10), isolated by methide abstraction from Cp2ZrMe2 using [CPh3][closo-HCB11Me11] displays an intermolecular Zr...H3C interaction in both the solid state and solution.