Dr Ian Riddlestone

The use of a sterically demanding pincer ligand to prepare an unusual square planar aluminium complex is reported. Due to the constrained geometry imposed by the ligand scaffold, this four-coordinate aluminium centre remains Lewis acidic and reacts via differing metal-ligand cooperative pathways for activating ketones and CO2. It is also a rare example of a single-component aluminium system for the catalytic reduction of CO2 to a methanol equivalent at room temperature.

[Ni(IMes) 2 ] reacts with chloroboranes via oxidative addition to form rare unsupported Ni-boryls. In contrast, the oxidative addition of hydridoboranes is not observed and products from competing reaction pathways are identified. Computational studies relate these differences to the mechanism of oxidative addition: B–Cl activation proceeds via nucleophilic displacement of Cl − , while B–H activation would entail high energy concerted bond cleavage.

The room temperature reaction of C6F6 or C6F5H with [Ru(IEt2Me2)2(PPh3)2H2] (1; IEt2Me2 = 1,3-diethyl-4,5-dimethylimidazol-2-ylidene) generated a mixture of the trans-hydride fluoride complex [Ru(IEt2Me2)2(PPh3)2HF] (2) and the bis-carbene pentafluorophenyl species [Ru(IEt2Me2)2(PPh3)(C6F5)H] (3). The formation of 3 resulted from C–H activation of C6F5H (formed from C6F6via stoichiometric hydrodefluorination), a process which could be reversed by working under 4 atm H2. Upon heating 1 with C6F5H, the bis-phosphine derivative [Ru(IEt2Me2)(PPh3)2(C6F5)H] (4) was isolated. A more efficient route to 2 involved treatment of 1 with 0.33 eq. of TREAT-HF (Et3N·3HF); excess reagent gave instead the [H2F3]− salt (5) of the known cation [Ru(IEt2Me2)2(PPh3)2H]+. Under catalytic conditions, 1 proved to be an active precursor for hydrodefluorination, converting C6F6 to a mixture of tri, di and monofluorobenzenes (TON = 37) at 363 K with 10 mol% 1 and Et3SiH as the reductant.

Thermally robust expanded ring carbene adducts of AlH3 have been synthesized with a view to probing their ligating abilities via Al–H σ-bond coordination. While κ2 binding to the 14-electron [Mo(CO)4] fragment is readily demonstrated, interaction with [Mo(CO)3] results in μ:κ1,κ1 and μ:κ2,κ2 bridging linkages rather than terminal κ3 binding.

The sequence of fundamental steps implicit in the conversion of a dihydroborane to a metal borylene complex have been elucidated for an [Ir(PMe3)3] system. B–H oxidative addition has been applied for the first time to an aminodihydroborane, H2BNR2, leading to the generation of a rare example of a primary boryl complex, Ln(H)M{B(H)NR2}; subsequent conversion to a borylene dihydride proceeds via a novel B-to-M α-hydride migration. The latter step is unprecedented for group 13 ligand systems, and is remarkable in offering α- substituent migration from a Lewis acidic center as a route to a two-coordinate ligand system.

α versus γ: [CpFe(CO)(PCy3)(BNCMes2)]+, synthesized by halide abstraction, represents the first example of a BN allenylidene analogue, and features an unsaturated MBNC π system. Although DFT calculations show significant LUMO amplitude at the γ (carbon) position, primary reactivity towards nucleophiles occurs at the sterically less hindered α (boron) center.

The extreme electron withdrawing properties of the perfluoropyridinoxy ligand –OC5F4N were used for the preparation of new (weakly) coordinating borate and aluminate anions of the type [E(OC5F4N)4]– (E = B or Al). These new anions are based on the potent parent Lewis acids E(OC5F4N)3, which possess exceptionally high calculated fluoride ion affinities (FIAs) of 500 and 587 kJ mol–1 for E = B and Al respectively. For aluminum, this extreme Lewis acidity dominates the chemistry and from mixtures of the neutral polymeric Lewis acid [Al(OC5F4N)3]n, the five‐ and six‐coordinate complexes Al(OC5F4N)3(OEt2)2 (1) and [Al(OC5F4N)2(µ‐OC5F4N) (NCMe)2]2 (2) were crystallized upon addition of ether or MeCN. The aluminate salts M[Al(OC5F4N)4] (M = Li or K) were prepared from the reaction between the alcohol 4‐HO–C5F4N and either LiAlH4 or K[AlEt4] respectively. The aluminate anion [Al(OC5F4N)4]– remains Lewis acidic coordinating small donor molecules forming [Al(OC5F4N)4(L)]– (L = THF or NMe3) and even supports formation and structural characterisation of the aluminum dianion containing salt [Na(OEt2)2][Na][Al(OC5F4N)5] (8). The from NaBH4 and 4‐HO–C5F4N accessible borate salt Na[B(OC5F4N)4] shows increased kinetic stability in comparison to the aluminum analogue.

This Review gives a comprehensive overview of the most topical weakly coordinating anions (WCAs) and contains information on WCA design, stability, and applications. As an update to the 2004 review, developments in common classes of WCA are included. Methods for the incorporation of WCAs into a given system are discussed and advice given on how to best choose a method for the introduction of a particular WCA. A series of starting materials for a large number of WCA precursors and references are tabulated as a useful resource when looking for procedures to prepare WCAs. Furthermore, a collection of scales that allow the performance of a WCA, or its underlying Lewis acid, to be judged is collated with some advice on how to use them. The examples chosen to illustrate WCA developments are taken from a broad selection of topics where WCAs play a role. In addition a section focusing on transition metal and catalysis applications as well as supporting electrolytes is also included.

By reaction of AlEt₃ with less than 3 equiv of HORF (RF = C(CF₃)₃) the ethylaluminum sesquialkoxide (Et)₂Al(μ-ORF)₂Al(Et)(ORF) (1a; NMR, XRD) can be obtained. As a univalent electronegative residue, the perfluorinated alkoxy moieties can be seen as pseudohalides. In this respect, 1a represents the closest approximation to the hitherto unknown crystal structure of the alkylaluminum sesquihalide Al₂R₃X₃. By further reaction of 1a with HORF, the Lewis superacid Al(ORF)₃ is formed, which reacts with Me₃SiCl to give Me₃Si–Cl–Al(ORF)₃ (2a; NMR, XRD, IR, Raman). 2a can be used for further reactions as prepared but slowly decomposes at ca. 0 °C to give the known Me₃Si–F–Al(ORF)₃ (2b) and several byproducts. The observed decomposition products, combined with quantum chemical calculations, provide evidence for an even higher silylating potential of 2a over that of 2b.

The mononuclear N‐heterocyclic carbene (NHC) copper alkoxide complexes [(6‐NHC)CuOtBu] (6‐NHC=6‐MesDAC (1), 6‐Mes (2)) have been prepared by addition of the free carbenes to the tetrameric tert‐butoxide precursor [Cu(OtBu)]4, or by protonolysis of [(6‐NHC)CuMes] (6‐NHC=6‐MesDAC (3), 6‐Mes (4)) with tBuOH. In contrast to the relatively stable diaminocarbene complex 2, the diamidocarbene derivative 1 proved susceptible to both thermal and hydrolytic ring‐opening reactions, the latter affording [(6‐MesDAC)Cu(OC(O)CMe2C(O)N(H)Mes)(CNMes)] (6). The intermediacy of [(6‐MesDAC)Cu(OH)] in this reaction was supported by the generation of Cu2O as an additional product. Attempts to generate an isolable copper hydride complex of the type [(6‐MesDAC)CuH] by reaction of 1 with Et3SiH resulted instead in migratory insertion to generate [(6‐MesDAC‐H)Cu(P(p‐tolyl)3)] (9) upon trapping by P(p‐tolyl)3. Migratory insertion was also observed during attempts to prepare [(6‐Mes)CuH], with [(6‐Mes‐H)Cu(6‐Mes)] (10) isolated, following a reaction that was significantly slower than in the 6‐MesDAC case. The longer lifetime of [(6‐Mes)CuH] allowed it to be trapped stoichiometrically by alkyne, and also employed in the catalytic semi‐reduction of alkynes and hydrosilylation of ketones.

By reaction of the Lewis acid Me3Si–F–Al(ORF)3 with a series of [PF6]− salts, gaseous PF5 and Me3Si–F are liberated and salts of the anion [F–Al(ORF)3]− ([f–al]−; RF = C(CF3)3) can be obtained. By addition of another equivalent of Me3Si–F–Al(ORF)3 to [f–al]−, gaseous Me3Si–F is released and salts of the least coordinating anion [(RFO)3Al–F–Al(ORF)3]− ([al–f–al]−) are formed. Both procedures work for a series of synthetically useful cations including Ag+, [NO]+, [Ph3C]+ and in very clean reactions with 5 g batch sizes giving excellent yields typically exceeding 90%. In addition, the synthesis of Me3Si–F–Al(ORF)3 has been optimized and scaled up to 85 g batches in an one-pot procedure. These anions could previously only be obtained by difficult to control decomposition reactions of [Al(ORF)4]− or by halide abstraction reactions with Me3Si–F–Al(ORF)3, generating relatively large countercations that are unsuited for further use as universal starting materials. Especially [al–f–al]− is of interest for the stabilization of reactive cations, since it is even weaker coordinating than [Al(ORF)4]− and more stable against strong electrophiles. This bridged anion can be seen as an adduct of [f–al]− and Al(ORF)3. Thus, it is similarly Lewis acidic as BF3 and eventually reacts with nucleophiles (Nu) from the reaction environment to yield Nu–Al(ORF)3 and [f–al]−. This prevents working with [al–f–al]− salts in ethereal or other donor solvents. By contrast, the [f–al]− anion is no longer Lewis acidic and may therefore be used for reactions involving stronger nucleophiles than the [al–f–al]− anion can withstand. Subsequently it may be transformed into the [al–f–al]− salt by simple addition of one equivalent of Me3Si–F–Al(ORF)3.

Halide abstraction from the ruthenium N-heterocyclic carbene complex Ru(IPr)2(CO)HCl (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) with NaBAr4F (BAr4F = B{C6H3(3,5-CF3)2}4) gave the salt [Ru(IPr)2(CO)H]BAr4F (2), which was shown through a combined X-ray/neutron structure refinement and quantum theory of atoms in molecules (QTAIM) study to contain a bifurcated Ru···η3-H2C ξ-agostic interaction involving one iPr substituent of the IPr ligand. This system complements the previously reported [Ru(IMes)2(CO)H]+ cation (IMes =1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene), where a non-agostic form is favored. Treatment of 2 with CO, H2, and the amine–boranes H3B·NR2H (R = Me, H) gave [Ru(IPr)2(CO)3H]BAr4F (3), [Ru(IPr)2(CO)(η2-H2)H]BAr4F (4), and [Ru(IPr)2(CO)(κ2-H2BH·NR2H)H]BAr4F (R = Me, 5; R = H, 6), respectively. Heating 5 in the presence of Me3SiCH═CH2 led to alkene hydroboration and formation of the C–H activated product [Ru(IPr)(IPr)′(CO)]BAr4F (7). X-ray characterization of 3 and 5–7 was complemented by DFT calculations, and the mechanism of H2/H exchange in 4 was also elucidated. Treatment of 2 with HBcat resulted in Ru–H abstraction to form the boryl complex [Ru(IPr)2(CO)(Bcat)] BAr4F (8), which proved to be competent in the catalytic hydroboration of 1-hexene. In 8, a combined X-ray/neutron structure refinement and QTAIM analysis suggested the presence of a single Ru···η2-HC ξ-agostic interaction.

Reaction of [Ru(IPr)2(CO)H]BArF4 with ZnEt2 forms the heterobimetallic species [Ru(IPr)2(CO)ZnEt]BArF4 (2), which features an unsupported Ru-Zn bond. 2 reacts with H2 to give [Ru(IPr)2(CO)(η2-H2)(H)2ZnEt]BArF4 (3) and [Ru(IPr)2(CO)(H)2ZnEt]BArF4 (4). DFT calculations indicate that H2 activation at 2 proceeds via oxidative cleavage at Ru with concomitant hydride transfer to Zn. 2 can also activate hydridic E-H bonds (E = B, Si), and computed mechanisms for the facile H/H exchange processes observed in 3 and 4 are presented. © 2016 American Chemical Society.

The ruthenium–zinc heterobimetallic complexes, [Ru(IPr)₂(CO)ZnMe][BArF₄] (7), [Ru(IBiox6)₂(CO)(THF)ZnMe][BArF₄] (12) and [Ru(IMes)′(PPh₃)(CO)ZnMe] (15), have been prepared by reaction of ZnMe₂ with the ruthenium N-heterocyclic carbene complexes [Ru(IPr)₂(CO)H][BArF₄] (1), [Ru(IBiox6)₂(CO)(THF)H][BArF₄] (11) and [Ru(IMes)(PPh₃)(CO)HCl] respectively. 7 shows clean reactivity towards H₂, yielding [Ru(IPr)₂(CO)(η²-H₂)(H)₂ZnMe][BArF₄] (8), which undergoes loss of the coordinated dihydrogen ligand upon application of vacuum to form [Ru(IPr)₂(CO)(H)₂ZnMe][BArF₄] (9). In contrast, addition of H₂ to 12 gave only a mixture of products. The tetramethyl IBiox complex [Ru(IBioxMe₄)₂(CO)(THF)H][BArF₄] (14) failed to give any isolable Ru–Zn containing species upon reaction with ZnMe₂. The cyclometallated NHC complex [Ru(IMes)′(PPh₃)(CO)ZnMe] (15) added H₂ across the Ru–Zn bond both in solution and in the solid-state to afford [Ru(IMes)′(PPh₃)(CO)(H)₂ZnMe] (17), with retention of the cyclometallation.

The room temperature reaction of C6F6 or C6F5H with [Ru(IEt2Me2)2(PPh3)2H2] (1; IEt2Me2 = 1,3-diethyl-4,5-dimethylimidazol-2-ylidene) generated a mixture of the trans-hydride fluoride complex [Ru(IEt2Me2)2(PPh3)2HF] (2) and the bis-carbene pentafluorophenyl species [Ru(IEt2Me2)2(PPh3)(C6F5)H] (3). The formation of 3 resulted from C–H activation of C6F5H (formed from C6F6via stoichiometric hydrodefluorination), a process which could be reversed by working under 4 atm H2. Upon heating 1 with C6F5H, the bis-phosphine derivative [Ru(IEt2Me2)(PPh3)2(C6F5)H] (4) was isolated. A more efficient route to 2 involved treatment of 1 with 0.33 eq. of TREAT-HF (Et3N·3HF); excess reagent gave instead the [H2F3]− salt (5) of the known cation [Ru(IEt2Me2)2(PPh3)2H]+. Under catalytic conditions, 1 proved to be an active precursor for hydrodefluorination, converting C6F6 to a mixture of tri, di and monofluorobenzenes (TON = 37) at 363 K with 10 mol% 1 and Et3SiH as the reductant.

The hydride complex [Ru(IPr)2(CO)H][BArF₄], 1, reacts with InMe₃ with loss of CH₄ to form [Ru(IPr)₂(CO)(InMe)(Me)][BArF₄], 4, featuring an unsupported Ru−In bond with unsaturated Ru and In centres. 4 reacts with H₂ to give [Ru(IPr)₂(CO)(η²‐H₂)(InMe)(H)][BArF₄], 5, while CO induces formation of the indyl complex [Ru(IPr)₂(CO)₃(InMe₂)][BArF₄], 7. These observations highlight the ability of Me to shuttle between Ru and In centres and are supported by DFT calculations on the mechanism of formation of 4 and its reactions with H₂ and CO. An analysis of Ru‐In bonding in these species is also presented. Reaction of 1 with GaMe₃ also involves CH₄ loss but, in contrast to its In congener, sees IPr transfer from Ru to Ga to give a gallyl complex featuring an η⁶ interaction of one aryl substituent with Ru.

Salt metathesis has been exploited in the synthesis of M–Al bonds, stabilized by a variety of chelating N-donor substituents at aluminium and including the first examples of such systems featuring ancillary guanidinato frameworks. Importantly, this synthetic approach can be extended to the synthesis of σ-alane complexes through the use of hydride-containing transition metal nucleophiles. Cp′Mn(CO)2-[H(Cl)Al{(NiPr)2CPh}] synthesized via this route features an alane ligand bound in a more ‘side-on’ fashion than other alane complexes, although DFT calculations imply that the potential energy surface associated with variation in the Mn–H–Al angle is a very soft one.

Treatment of the thioether‐substituted secondary phosphanes R2PH(C6H4‐2‐SR1) [R2=(Me3Si)2CH, R1=Me (1PH), iPr (2PH), Ph (3PH); R2=tBu, R1=Me (4PH); R2=Ph, R1=Me (5PH)] with nBuLi yields the corresponding lithium phosphanides, which were isolated as their THF (1–5Pa) and tmeda (1–5Pb) adducts. Solid‐state structures were obtained for the adducts [R2P(C6H4‐2‐SR1)]Li(L)n [R2=(Me3Si)2CH, R1=nPr, (L)n=tmeda (2Pb); R2=(Me3Si)2CH, R1=Ph, (L)n=tmeda (3Pb); R2=Ph, R1=Me, (L)n=(THF)1.33 (5Pa); R2=Ph, R1=Me, (L)n=([12]crown‐4)2 (5Pc)]. Treatment of 1PH with either PhCH2Na or PhCH2K yields the heavier alkali metal complexes [{(Me3Si)2CH}P(C6H4‐2‐SMe)]M(THF)n [M=Na (1Pd), K (1Pe)]. With the exception of 2Pa and 2Pb, photolysis of these complexes with white light proceeds rapidly to give the thiolate species [R2P(R1)(C6H4‐2‐S)]M(L)n [M=Li, L=THF (1Sa, 3Sa–5Sa); M=Li, L=tmeda (1Sb, 3Sb–5Sb); M=Na, L=THF (1Sd); M=K, L=THF (1Se)] as the sole products. The compounds 3Sa and 4Sa may be desolvated to give the cyclic oligomers [[{(Me3Si)2CH}P(Ph)(C6H4‐2‐S)]Li]6 ((3S)6) and [[tBuP(Me)(C6H4‐2‐S)]Li]8 ((4S)8), respectively. A mechanistic study reveals that the phosphanide–thiolate rearrangement proceeds by intramolecular nucleophilic attack of the phosphanide center at the carbon atom of the substituent at sulfur. For 2Pa/2Pb, competing intramolecular β‐deprotonation of the n‐propyl substituent results in the elimination of propene and the formation of the phosphanide–thiolate dianion [{(Me3Si)2CH}P(C6H4‐2‐S)]2−.

The electrophilic character of free diamidocarbenes (DACs) allows them to activate inert bonds in small molecules, such as NH3 and P4. Herein, we report that metal coordinated DACs also exhibit electrophilic reactivity, undergoing attack by Zn and Cd dialkyl precursors to afford the migratory insertion products [(6‐MesDAC‐R)MR] (M=Zn, Cd; R=Et, Me; Mes=mesityl). These species were formed via the spectroscopically characterised intermediates [(6‐MesDAC)MR2], exhibiting barriers to migratory insertion which increase in the order MR2 = ZnEt2 < ZnMe2 < CdMe2. Compound [(6‐MesDAC‐Me)CdMe] showed limited stability, undergoing deposition of Cd metal, by an apparent β‐H elimination pathway. These results raise doubts about the suitability of diamidocarbenes as ligands in catalytic reactions involving metal species bearing nucleophilic ligands (M‐R, M‐H).

Cationic Ir(III) systems supported by a bis(expanded NHC) framework and featuring both agostic C–H and cis alkyl/hydride ligand sets have been targeted by protonation of the corresponding bis(alkyl) hydride complexes. Remarkably, the steric shielding afforded by the NHC substituents is such that these and related putative 14-electron cations are air and moisture stable. In solution, degenerate fluxional exchange is brought about by reversible σ-bond activation within the agostic alkyl C(sp3)–H bond; a non-dissociative mechanism is implied by the activation parameters ΔH⧧ = 8.8(0.4) kcal mol–1 and ΔS⧧ = −12.2(1.7) eu.

A single‐component ambiphilic system capable of the cooperative activation of protic, hydridic and apolar HX bonds across a Group 13 metal/activated β‐diketiminato (Nacnac) ligand framework is reported. The hydride complex derived from the activation of H2 is shown to be a competent catalyst for the highly selective reduction of CO2 to a methanol derivative. To our knowledge, this process represents the first example of a reduction process of this type catalyzed by a molecular gallium complex.

Systems of the type [(p-cym)Ru(PR3)(H)(H2BNiPr2)]+ (R = Cy, Ph) can be synthesized from (p-cym)Ru(PR3)Cl2 and H2BNiPr2/Na[BArf4] and are best formulated as (hydrido)ruthenium κ1-aminoborane complexes. VT-NMR measurements have been used to probe the σ-bond metathesis process leading to Ru–H/H–B exchange, yielding an activation barrier of ΔG⧧ = 7.5 kcal mol–1 at 161 K. Moreover, in contrast to the case for related non-hydride-containing systems, reactivity toward alkenes constitutes a viable route to a metal borylene complex via sacrificial hydrogenation.

Photolytic ligand displacement and salt metathesis routes have been exploited to give access to κ1 σ-alane complexes featuring Al–H bonds bound to [W(CO)5] and [Cp′Mn(CO)2] fragments, together with a related κ2 complex of [Cr(CO)4]. Spectroscopic, crystallographic, and quantum chemical studies are consistent with the alane ligands acting predominantly as σ-donors, with the resulting binding energies calculated to be marginally greater than those found for related dihydrogen complexes.

The synthetic and reaction chemistries of cationic iminoborylene complexes [LnM[double bond, length as m-dash]B[double bond, length as m-dash]N[double bond, length as m-dash]CR2]+, which feature a unique heterocumulene structure, have been systematically investigated. Precursors of the type CpFe(CO)2B(Cl)NCAr2 (Ar = p-Tol/Mes, 5c/d) have been generated by B-centred substitution chemistry using CpFe(CO)2BCl2 and suitable lithiated ketimines – a reaction which is found to be highly sensitive to the steric bulk at both the metal fragment and the ketimino group. Carbonyl/phosphine exchange (using PCy3 or PPh3), followed by halide abstraction allows for the generation of the cationic iminoborylenes [CpFe(PR3)(CO)(BNCAr2)]+[BArX4]− (R = Cy, Ar = p-Tol/Mes, 12c/d; R = Ph, Ar = Mes, 13d; ArX = 3,5-X2C6H3 where X = Cl, CF3) which have been characterized spectroscopically and by X-ray crystallography. The reactivity of these iminoborylene systems towards a range of nucleophiles and unsaturated substrates has been investigated. The latter includes the first examples of M[double bond, length as m-dash]B metathesis reactivity with a carbodiimide, and results in Fe[double bond, length as m-dash]B cleavage and formation of the isonitrile complexes [CpFe(PCy3)(CO)(CNR)]+[BArCl4]− (R = iPr/Cy, 16/17).

A metal templated synthetic approach gives access to [Cp*Fe(CO)2{B(NMesCMe)2CH}][BArf4], and represents the first example of coordinative trapping of the elusive [B(NRCMe)2CH] fragment.

We describe a combined experimental and computational study into the scope, regioselectivity, and mechanism of the catalytic hydrodefluorination (HDF) of fluoropyridines, C5F5–xHxN (x = 0–2), at two Ru(NHC)(PPh3)2(CO)H2 catalysts (NHC = IPr, 1, and IMes, 2). The regioselectivity and extent of HDF is significantly dependent on the nature of the NHC: with 1 HDF of C5F5N is favored at the ortho-position and gives 2,3,4,5-C5F4HN as the major product. This reacts on to 3,4,5-C5F3H2N and 2,3,5-C5F3H2N, and the latter can also undergo further HDF to 3,5-C5F2H3N and 2,5-C5F2H3N. para-HDF of C5F5N is also seen and gives 2,3,5,6-C5F4HN as a minor product, which is then inert to further reaction. In contrast, with 2, para-HDF of C5F5N is preferred, and moreover, the 2,3,5,6-C5F4HN regioisomer undergoes C–H bond activation to form the catalytically inactive 16e Ru-fluoropyridyl complex Ru(IMes)(PPh3)(CO)(4-C5F4N)H, 3. Density functional theory calculations rationalize the different regioselectivity of HDF of C5F5N at 1 and 2 in terms of a change in the pathway that is operating with these two catalysts. With 1, a stepwise mechanism is favored in which a N → Ru σ-interaction stabilizes the key C–F bond cleavage along the ortho-HDF pathway. With 2, a concerted pathway favoring para-HDF is more accessible. The calculations show the barriers increase for the subsequent HDF of the lower fluorinated substrates, and they also correctly identify the most reactive C–F bonds. A mechanism for the formation of 3 is also defined, but the competition between C–H bond activation and HDF of 2,3,5,6-C5F4HN at 2 (which favors C–H activation experimentally) is not reproduced. In general, the calculations appear to overestimate the HDF reactivity of 2,3,5,6-C5F4HN at both catalysts 1 and 2.

The two-coordinate ring-expanded N-heterocyclic carbene copper(I) complexes [Cu(RE-NHC)2]+ (RE-NHC = 6-Mes, 7-o-Tol, 7-Mes) have been prepared and shown to be effective catalysts under neat conditions for the 1,3-dipolar cycloaddition of alkynes and azides. In contrast, the cationic diamidocarbene analogue [Cu(6-MesDAC)2]+ and the neutral species [(6-MesDAC)CuCl]2 and [(6-MesDAC)2(CuCl)3] show good activity when the catalysis is performed on water.

Cationic half-sandwich ruthenium complexes featuring κ2-bound aminoborane ligands can readily be accessed from 16-electron precursors via chloride abstraction in the presence of H2BNR2 (R = iPr, Cy). Complexes [Cp*Ru(L)(κ2-H2BNR2)][BArf4] (2a: R = iPr, L = PCy3; 2b: R = iPr, L = PPh3; 2c: R = iPr, L = 1,3-bis-(2,4,6-trimethylphenyl)-imidazol-2-ylidene; 3a: R = Cy, L = PCy3; Arf = C6H3(CF3)2‐3,5) were isolated in yields of ~60 %, and characterised in the solid state by X-ray crystallography (for 2a, 2c, and 3a). Low-field 11B NMR shifts for the coordinated aminoborane fragment, together with short Ru⋯B contacts (of the order of 1.97 Å) imply a relatively tightly bound borane ligand, a finding which is given further credence by the results of density functional theory studies (e.g. bond dissociation energies in the range 24 kcal mol–1; 1 kcal mol–1 = 4.186 kJ mol–1). In terms of reactivity, κ2 systems of this type, while potentially offering a versatile route to asymmetric κ1 systems, in fact undergo borane extrusion even in the presence of a single equivalent of added ligand.

Developments in the coordination chemistry of BH, AlH, and GaH bonds at transition metal centers are reviewed, with particular emphasis on factors influencing electronic/geometric structure and bond activation.

The modes of interaction of donor‐stabilized Group 13 hydrides (E=Al, Ga) were investigated towards 14‐ and 16‐electron transition‐metal fragments. More electron‐rich N‐heterocyclic carbene‐stabilized alanes/gallanes of the type NHC⋅EH3 (E=Al or Ga) exclusively generate κ2 complexes of the type [M(CO)4(κ2‐H3E⋅NHC)] with [M(CO)4(COD)] (M=Cr, Mo), including the first κ2 σ‐gallane complexes. β‐Diketiminato (′nacnac′)‐stabilized systems, {HC(MeCNDipp)2}EH2, show more diverse reactivity towards Group 6 carbonyl reagents. For {HC(MeCNDipp)2}AlH2, both κ1 and κ2 complexes were isolated, while [Cr(CO)4(κ2‐H2Ga{(NDippCMe)2CH})] is the only simple κ2 adduct of the nacnac‐stabilized gallane which can be trapped, albeit as a co‐crystallite with the (dehydrogenated) gallylene system [Cr(CO)5(Ga{(NDippCMe)2CH})]. Reaction of [Co2(CO)8] with {HC(MeCDippN)2}AlH2 generates [(OC)3Co(μ‐H)2Al{(NdippCme)2CH}][Co(CO)4] (12), which while retaining direct AlH interactions, features a hitherto unprecedented degree of bond activation in a σ‐alane complex.

The complexes Ag(L)(n)[WCA] (L=P4S3, P4Se3, As4S3, and As4S4; [WCA]=[Al(ORF)(4)](-) and [F{Al(ORF)(3)}(2)](-); R-F=C(CF3)(3); WCA=weakly coordinating anion) were tested for their performance as ligand-transfer reagents to transfer the poorly soluble nortricyclane cages P4S3, P4Se3, and As4S3 as well as realgar As4S4 to different transition-metal fragments. As4S4 and As4S3 with the poorest solubility did not yield complexes. However, the more soluble silver-coordinated P4S3 and P4Se3 cages were transferred to the electron-poor Fp(+) moiety ([CpFe(CO)(2)](+)). Thus, reaction of the silver salt in the presence of the ligand with Fp-Br yielded [Fp-P4S3][Al(ORF)(4)] (1 a), [Fp-P4S3][F(Al(ORF)(3))(2)] (1 b), and [Fp-P4Se3][Al(ORF)(4)] (2). Reactions with P4S3 also yielded [FpPPh(3)-P4S3][Al(ORF)(4)] (3), a complex with the more electron-rich monophosphine-substituted Fp(+) analogue [FpPPh(3)](+) ([CpFe(PPh3)(CO)](+)). All complex salts were characterized by single-crystal XRD, NMR, Raman, and IR spectroscopy. Interestingly, they show characteristic blueshifts of the vibrational modes of the cage, as well as structural contractions of the cages upon coordination to the Fp/FpPPh(3) moieties, which oppose the typically observed cage expansions that lead to redshifts in the spectra. Structure, bonding, and thermodynamics were investigated by DFT calculations, which support the observed cage contractions. Its reason is assigned to sigma and pi donation from the slightly P-P and P-E antibonding P4E3-cage HOMO (e symmetry) to the metal acceptor fragment.

Polymers with tailored architectures and degradability were prepared through thiocarbonyl addition ring-opening (TARO) atom-transfer radical polymerization (ATRP) using dibenzo[c,e]oxepin-5(7H)-thione (DOT), Cu(I)Br, and tris[2-(dimethylamino)ethyl]amine (Me6TREN) as the thionolactone, catalyst, and ligand, respectively, in combination with a selection of acrylic comonomers. Although copolymers with selectively degradable backbone thioesters and low dispersities (1.10 ≤ D̵ ≤ 1.26) were achieved using DMSO, acetonitrile, or toluene as the solvent, the Cu(I)-catalyzed dethionation of DOT to its (oxo)lactone analogue limited the achievable copolymer DOT content. Using anhydrous polymerization conditions minimized the side reaction and provided degradable copolymers with a higher (≤32 mol %) thioester content. Water-soluble molecular brushes were prepared by grafting poly(ethylene glycol) methyl ether acrylate–DOT copolymers from a pre-made multi-ATRP initiator. Due to copolymerization kinetics, the thioesters were installed close to the junctions and enabled the fast (

Upon coordinating P4 to electron poor cyclopentadienyl-iron cations, the average P-P bond distancesshrink and the respective P4 breathing mode in theRaman spectra (600 cm-1, P4, free) is blueshifted by>40 cm-1 in [CpFe(CO)(L)(h1-P4)]+ cations (L=CO orPPh3). Analysis suggests that this corresponds to an umpolungof the bonding from more phosphidic in theknown, electron-rich systems to more phosphonium-likein the reported electron-poor versions. This may opennew functionalization pathways for white phosphorus P4.