Heterobimetallic ruthenium–zinc complexes with bulky N-heterocyclic carbenes: syntheses, structures and reactivity

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) (¿2-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 isol- able 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 cyclometallati

Diverse Cooperative Reactivity at a Square Planar Aluminium Complex and Catalytic Reduction of CO 2

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

Stoichiometric and catalytic C-F bond activation by the<i> trans</i>-dihydride complex [Ru(IEt₂Me₂)₂(PPh₃)₂H₂] (IEt₂Me₂ = 1,3-diethyl-4,5-dimethylimidazol-2-ylidene)

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

Contrasting reactivity of B–Cl and B–H bonds at [Ni(IMes)2] to form unsupported Ni-boryls

[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

Use of ring-expanded diamino- and diamidocarbene ligands in copper catalyzed azide-alkyne "click" reactions

The two-coordinate ring-expanded N-heterocyclic carbene copper­(I) complexes [Cu­(RE-NHC)₂]⁺ (RE-NHC = 6-Mes, 7-<i>o</i>-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

Mechanistic study of Ru-NHC-catalyzed hydrodefluorination of fluoropyridines : the influence of the NHC on the regioselectivity of C–F activation and chemoselectivity of C–F versus C–H bond cleavage

We acknowledge financial support from the EPSRC through grant numbers EP/J010677/1 (D.M.) and EP/J009962/1 (I.M.R.).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.Publisher PDFPeer reviewe

A Comparison of the Stability and Reactivity of Diamido- and Diaminocarbene Copper Alkoxide and Hydride Complexes

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

Coordination and activation of E-H Bonds (E=B, Al, Ga) at transition metal centers

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