Intern
    Professur für Anorganische Chemie

    Huheey, Keiter, Keiter, Anorganische Chemie - Prinzipien von Struktur und Reaktivität,

    R. Steudel (Hrsg.): deGruyter, Berlin, 2014

    Udo Radius und Maik Finze, Koordinationschemie: Kapitel 1114.


    J. Zhou, M. Kuntze-Fechner, R. Bertermann, U. S. D. Paul, J. H. J. Berthel, A. Friedrich, Z. Du, T. B. Marder, U. Radius, J. Am. Chem. Soc. 2016, 138, 52505253.

    Abstract

    The [Ni(IMes)2]-catalyzed transformation of fluoroarenes into arylboronic acid pinacol esters via CF bond activation and transmetalation with bis-(pinacolato)diboron (B2pin2) is reported. Various partially fluorinated arenes with different degrees of fluorination were converted into their corresponding boronate esters.


    S. Würtemberger-Pietsch, U. Radius, T. B. Marder, Dalton Trans. 2016, 45, 58805895.

    Abstract

    N-Heterocyclic carbenes (NHCs) are widely used ligands and reagents in modern inorganic synthesis as well as in homogeneous catalysis and organocatalysis. However, NHCs are not always innocent bystanders. In the last few years, more and more examples were reported of reactions of NHCs with main-group elements which resulted in modification of the NHC. Many of these reactions lead to ring expansion and the formation of six-membered heterocyclic rings involving insertion of the heteroatom into the C–N bond and migration of hydrides, phenyl groups or boron-containing fragments. Furthermore, a few related NHC rearrangements were observed some decades ago. In this Perspective, we summarise the history of NHC ring expansion reactions from the 1960s till the present.


    N. Arnold, S. Mozo, U. Paul, U. Radius, H. Braunschweig, Organometallics 2015, 34, 57095715.

    Abstract

    A series of iridium dihydroborate complexes [(tBuPOCOP)IrH(κ2-H2BHR)] (tBuPOCOP= κ3-C6H3-1,3-[OPtBu2]2; R = Mes = 2,4,6-Me3C6H2; R = Dur = 2,3,5,6-Me4C6H), [LIrH(κ2-H2BHDur)] (L = tBuPCP= κ3-C6H3-1,3-[CH2PtBu2]2, L = η5-C5Me5), and an osmium dihydroborate compound [OsH(κ2-H2BHDur)(CO)(PiPr3)2] have been prepared by using two different synthetic strategies. The first approach is based on direct borane coordination to the metal center whereas the second is based on a salt-elimination protocol using the lithium salts Li[H3BR] (R = Mes or Dur) and the corresponding metal halides. The compounds have been characterized by multinuclear NMR and IR spectroscopy and X-ray diffraction analysis. The results constitute the first syntheses of κ2-s:s-dihydroborate complexes featuring bulky aryl groups.


    F. Hering, U. Radius, Organometallics 2015, 34, 32363245.

    Abstract

    The widely-held belief that N-heterocyclic carbenes (NHCs) act only as innocent spectator ligands is not always accurate, even in the context of well explored reactions. Ligand exchange in the conversion of [Pt(PPh3)2(η2-C2H4)] 3 to [Pt(iPr2Im)2] 2 depends critically on the particular reaction conditions employed, with slight changes leading to vastly different outcomes. In addition to [Pt(iPr2Im)2] 2, complexes [Pt(iPr2Im)(PPh3)(η2-C2H4)] 5 and trans-[Pt(iPr2Im)2(iPr-Im*)(H)] 6 were isolated and in the case of 6 fully characterized. Complex 5 represents the first mixed-olefin complex in transition metal chemistry containing both an NHC and a phosphine ligand. Chemical degradation of the NHC was shown to yield the new imidazole-2-yl iPr-Im* in 6. Therefore, the synthesis of [Pt(iPr2Im)2] 2 via metallic reduction of the ionic precursor [Pt(iPr2Im)3(Cl)]+Cl- 9 is favorable – a procedure adaptable to analogous palladium compounds. While [Pd(iPr2Im)3(Cl)]+Cl- 8 is the only product obtained from the reaction of iPr2Im and PdCl2, neutral [Pt(iPr2Im)2(Cl)2] 10, formed as a mixture of its two stereoisomers cis-10 and trans-10, is available through precise control of the stoichiometry in the reaction of PtCl2 and exactly two equivalents of iPr2Im.


    H. Schneider, D. Schmidt, U. Radius, Chem. Commun. 2015, 51, 10138–10141.

    Abstract

    The dehydrogenative coupling of primary and secondary phosphines with the N-heterocyclic carbene iPr2Im (1,3-di-iso-propyl-imidazolin-2-ylidene) is reported. Dehydrogenation of R2PH affords diphosphines R2P–PR2. The reaction of iPr2Im with ArPH2 leads to NHC phosphinidene adducts iPr2Im=PAr and cyclic oligophosphines P4Ar4, P5Ar5 and P6Ar6, depending on the stoichiometry used. The NHC acts in these reactions as phosphine activator and hydrogen acceptor.


    S. Pietsch, U. Paul, I. A. Cade, M. J. Ingleson, Udo Radius, T. B. Marder, Chem. Eur. J. 2015, 21, 9018–9021.

    Abstract

     

    We report the isolation and detailed structural characterization, by solid-state and solution NMR spectroscopy, of the neutral mono- and bis-NHC adducts of bis(catecholato)diboron (B2cat2). The bis-NHC adduct undergoes thermally induced rearrangement, forming a 6-membered -B–C=N–C=C–N-heterocyclic ring via C–N bond cleavage and ring expansion of the NHC, whereas the mono-NHC adduct is stable. Bis(neopentylglycolato)diboron (B2neop2) is much more reactive than B2cat2 giving a ring expanded product at room temperature, demonstrating that ring expansion of NHCs can be a very facile process with significant implications for their use in catalysis.


    H. Schneider, D. Schmidt, U. Radius, Chem. Eur. J. 2015, 21, 27932797.

    Abstract

    The reaction of iPr2Im (iPr2Im = 1,3-di-iso-propyl-imidazolin-2-ylidene) with diphenyldichlorosilane Ph2SiCl2 leads to the adduct (iPr2Im)-SiCl2Ph2 1. Prolonged heating of isolated 1 at 66 °C in THF affords the backbone tethered di(imidazolium) salt [(aHiPr2Im)2SiPh2]2+2Cl- 2 (“a” denotes “abnormal” coordination of the NHC), which can be synthesized in high yields in one step starting from two equivalents iPr2Im with Ph2SiCl2. Imidazolium salt 2 can be deprotonated in THF at room temperature using sodium hydride as a base and catalytic amounts of sodium tert-butoxide to give the stable N-heterocyclic dicarbene (aiPr2Im)2SiPh2 3, in which two NHCs are backbone tethered with a SiPh2 group. This easy-to-synthesize dicarbene 3 can be used as a novel ligand type in transition metal chemistry for the preparation of dinuclear NHC complexes, as exemplified by the synthesis of the homodinuclear copper(I) complex [{a(ClCu-iPr2Im)}2SiPh2] 4.


    P. Hemberger, A. Bodi, J. H. J. Berthel, U. Radius, Chem. Eur. J. 2015, 21, 14341438.

    Abstract

    Ring expansion reactions (RER) of the N-heterocyclic carbene 1,3-dimethylimidazolin-2-ylidene was observed in the gas phase upon VUV photoexcitation. Similarly to RERs reported in the condensed phase for the reaction of NHCs with some main group element hydrides, depopulation of the NHC carbon atom seems to be the crucial initial step. In an ionization-mediated protonation, 1,3-dimethylimidazolin-2-ylidene forms an imidazolium ion, which is the rate limiting step on the pathway to two six-membered ring products, namely methylpyrimidinium and -pyrazinium ions. In order to unravel the reaction path, we have used imaging photoelectron photoion coincidence spectroscopy with vacuum ultraviolet (VUV) synchrotron radiation as well as high-level composite method calculations. Similarities and differences between the mechanism in the gas phase and in the condensed phase will be discussed.


    F. Hering, J. Nitsch, U. Paul, A. Steffen, F. M. Bickelhaupt, U. Radius, Chem. Sci. 2015, 6, 14261432.

    Abstract

    Synthesis, characterization and investigations on the reactivity of the novel metal basic, yet isolable 14 VE NHC-complexes [M0(iPr2Im)2] (M = Pd 1, Pt 2; iPr2Im = 1,3-di-isopropyl-imidazolin-2-ylidene; VE = valence electron; NHC = N-heterocyclic carbene) is reported and compared to the chemistry of the corresponding nickel complex. Quantum chemical analyses reveal that differences in the reactivity of group 10 NHC complexes are caused by differences in the rigidity and thus activation strain associated with bending the corresponding d10-[M(NHC)2] fragments during reaction. These results should have implications for the understanding of the fundamental steps in catalytic cycles, in which such complex fragments are employed.


    D. Schmidt, T. Zell, T. Schaub, U. Radius, Dalton Trans. 2014, 43, 1081610827.

    Abstract

    The unique reactivity of the nickel(0) complex [Ni2(iPr2Im)4(COD)] (1) (iPr2Im = 1,3-di-isopropyl-imidazolin-2-ylidene) towards hydrosilanes in stoichiometric and catalytic reactions is reported. A series of nickel hydrido silyl complexes cis-[Ni(iPr2Im)2(H)(SiHn-1R4-n)] (n = 1, 2) and nickel bis(silyl) complexes cis-[Ni(iPr2Im)2(SiHn-1R4-n)2] (n = 1, 2, 3) were synthesized by stoichiometric reactions of 1 with hydrosilanes HnSiR4-n, and fully characterized by X-ray diffraction and spectroscopic methods. These hydrido silyl complexes are examples, where the full oxidative addition step is hindered. They have, as a result of remaining Si–H interactions, remarkably short Si–H distances and feature a unique dynamic behavior in solution. Cis-[Ni(iPr2Im)2(H)(SiMePh2)] (cis-5) shows in solution at room temperature a dynamic site exchange of the NHC ligands, H-D exchange with C6D6 to give the deuteride complex cis-[Ni(iPr2Im)2(D)(SiMePh2)] (cis-5-D), and at elevated temperatures an irreversible isomerization to trans-[Ni(iPr2Im)2(D)(SiMePh2)] (trans-5-D). Reactions with sterically less demanding silanes give cis-configured bis(silyl) complexes under liberation of dihydrogen. These complexes display, similar to the hydrido silyl complexes, interestingly short Si–Si distances. Complex 1 reacts with 4 eq. HSi(EtO)3, in contrast to all the other silanes used in this study, to give the trans-configured bis(silyl) complex trans-[Ni(iPr2Im)2Ni(Si(OEt)3)2] (trans-12). The addition of two equivalents Ph2SiH2 to 1 results at elevated temperatures in the formation of the dinuclear complex [{(iPr2Im)Ni-m2-(HSiPh2)}2] (6). This diamagnetic, formal Ni(I) complex exhibits a long Ni–Ni bond in the solid state, as established by X-ray diffraction.

    The capability of the electron rich {Ni(iPr2Im)2} complex fragment to activate Si–H bonds was applied catalytically in the deuteration of Et3Si–H to Et3Si–D employing C6D6 as convenient deuterium source. Furthermore, we show that 1 serves as a catalyst for the acceptorless dehydrogenative coupling of Ph2SiH2 to the corresponding disilane Ph2HSi–SiHPh2 and trisilane Ph2HSi–Si(Ph)2–SiHPh2, and the coupling of PhSiH3 to give a mixture of cyclic and linear polysilanes with high polydispersity (Mw = 1119; Mn = 924 ; Mw/Mn = 1.2). The capability of 1 to catalyze the formal reverse reaction as well is demonstrated by the hydrogenation of disilanes. The hydrogenation of the disilanes Ph2MeSi–SiMePh2 and PhMe2Si–SiMe2Ph to the corresponding hydrosilanes Ph2MeSi–H and PhMe2Si–H, respectively, proceeds effectively in the presence of 1 under very mild conditions (room temperature, 1.8 bar H2 pressure).


    B. Zarzycki, F. M. Bickelhaupt, U. Radius, Dalton Trans. 2013, 42, 74687481.

    Abstract

    A full theoretical mechanistic investigation on the symmetrical degradation of P4 at the active complex fragments [(η5-C5H5)Co(L)] (L = CO, iPr2Im; iPr2Im = 1,3-di-iso-propylimidazolin-2-ylidene), which results in the formation of the complex [{(η5-C5H5)Co}2(µ, η2:2-P2)2] 9, is presented. The overall reaction mechanism is a complex, multistep process. Rate-determining steps of the reaction sequence are two consecutive dissociations of the co-ligands L, which induce the decisive structural rearrangements of the P4 unit. The choice of the co-ligand L (= CO, iPr2Im) influences the kinetic barrier as well as the energy balance of the overall reaction path significantly. The calculations further reveal a strong influence of the entropic effect on the overall reaction. As a consequence, the energy balance of the overall formation of 9 starting from [(η5-C5H5)Co(CO)] precursors is almost thermoneutral and has to overcome high kinetic barriers, whereas the reaction starting from [(η5-C5H5)Co(iPr2Im)] precursors with L = NHC is exothermic, features lower transition barriers with stabilized intermediates. From the direct comparison of both reaction coordinates it seems that the entropic effect of the co-ligands is even stronger than their electronic influence, as for both investigated systems the reactions' energy profiles are almost identical up to intermediate [{(η5-C5H5)Co(L)}2(μ,η2:2-P4)] 5 (L = CO, iPr2Im). After the formation of 5, the first CO dissociation step renders the reaction endothermic for L = CO, whereas in the case of NHC dissociation the reaction progresses exothermically. Energy decomposition analysis and fragment analysis provide a picture of the bonding mechanisms between the metal complex fragments and P4 in case of the most significant intermediates and the final product.


    C. Hauf, J. E. Barquera-Lozada, P. Meixner, G. Eickerling, S. Altmannshofer, D. Stalke, T. Zell, D. Schmidt, U. Radius, W. Scherer, Z. Anorg. Allg. Chem. 2013, 639, 19962004.

    Abstract

    In general, it is assumed that the reaction between silanes and late transition metal fragments yields silyl hydride species as oxidative addition products. However, the silane complex Ni(iPr2Im)2(SiHMePh2) (iPr2Im = 1,3-diisopropylimidazolin-2-ylidene) (3a), might represent one of the rare systems where a stable η2-(Si–H)Ni intermediate of the oxidative addition process has been isolated. Indeed, 3a is characterized by an acute Si–Ni–H angle of 62.0(2)°, a rather short Si–H bond length of 1.992(6) Å and displaysa silicon-hydride cross peak in Si-H-HMQC 2D-NMR studies. We therefore selected the latter system for a combined experimental and theoretical charge density study to explore the electronic prerequisites which hinder the full completion of the oxidative addition step in transition metal silane complexes and cause the presence of remanent Si–H interactions in these species.


    B. Zarzycki, T. Zell, D. Schmidt, U. Radius, Eur. J. Inorg. Chem. 2013, 20512058.

    Abstract

    The reaction of [Ni2(iPr2Im)4(COD)] 1 with white phosphorus leads to dinuclear [{Ni(iPr2Im)2}2(µ,η2:2-P2)] 2 in excellent yield. This reaction represents the first example of a conversion of white phosphorus to a complex of the type [{L2Ni}2(µ,η2:2-P2)] and the first example for the formation of a complex of the type [{L2M}2(µ,η2:2-P2)] for a group 10 metal stabilized by two simple, non-chelating 2 electron donor ligands via the reaction of a suitable precursor with P4 in general. The X-ray molecular structure of 2 reveals a bent Ni2P2 core with a Ni-P-P-Ni dihedral angle in solid state of 102.95°. According to DFT calculations of the symmetrized model systems planar-D2h- and bent-C2v-[{Ni(iPr2Im)2}2(µ,η2:2-P2)], this deviation of the Ni2P2 core from planarity is caused by a second order Jahn Teller distortion. Calculations on the related platinum compound [{Pt(iPr2Im)2}2(µ,η2:2-P2)] confirm this type of bent structure for the higher congener with an even higher barrier to planarization as calculated for the nickel case. Energy decomposition analysis and fragment molecular orbital analysis further illustrate the bonding mechanisms in these complexes.


    D. Schmidt, J. H. J. Berthel, S. Pietsch, U. Radius, Angew. Chem. 2012, 124, 9011–9015, Angew. Chem. Int. Ed. 2012, 51, 8881–8885. (Highlight: Nachr. Chem. 2012, 60, 1071)

    Abstract

    We report here the direct insertion of silylene moieties into the C-N bond of N-heterocyclic carbenes with ring expansion and formation of diaza-silinanes. These reactions are feasible using primary, secondary and tertiary silanes Ph4-nSiHn and a variety of NHCs, i.e. saturated and unsaturated NHCs with N-alkyl and N-aryl substituents. This ring expansion should be of general interest for the different areas of NHC-based chemistry of main group elements and transition metals, as well as for catalysis, especially if silanes are used in reactions with these species at higher temperature.


    T. Zell, P. Fischer, D. Schmidt, U. Radius, Organometallics 2012, 31, 5065–5073.

    Abstract

    Complex [Ni2(iPr2Im)4(COD)] 1 (iPr2Im = 1,3-di-iso-propylimidazolin-2-ylidene) is a very efficient catalyst for the Suzuki-Miyaura cross coupling reaction of 4-bromotoluene with phenylboronic acid and also mediates the Ullmann-type homo cross coupling reaction of bromo benzene with a moderate efficiency. Stoichiometric reactions of complex 1 with aryl bromides (ArBr) at room temperature lead to mixtures of aryl bromo complexes of the type trans-[Ni(iPr2Im)2(Br)(Ar)] and the bis(bromo) complex trans-[Ni(iPr2Im)2(Br)2] 2. The complexes trans-[Ni(iPr2Im)2(Br)(Ar)] (for Ar = Ph 3, 4-MeC6H4 4, 4-Me(O)CC6H4 5, 4-MeOC6H4 6, 4-MeSC6H4 7, 4-Me2NC6H4 8, C5NH4 9) can be selectively synthesized by working at low temperatures and using high dilution of the starting materials. A major deactivation pathway for trans-[Ni(iPr2Im)2(Br)(Ar)] was identified in the presence of aryl bromides. This deactivation process includes (i) the formation of trans-[Ni(iPr2Im)2(Br)2] from trans-[Ni(iPr2Im)2(Br)(Ar)] and ArBr and (ii) the formation of an imidazolium salt of the type 2[iPr2Im-Ar]+[NiBr4]2- from trans-[Ni(iPr2Im)2(Br)2] 2 and ArBr. The reactions of complex 2 with a series of aryl halides at higher temperatures leads to the decomposition of the bis(carbene) nickel moiety with formation of the the imidazolium salts 2[iPr2Im-Ar]+[NiBr2X2]- (for X = I, Ar = Ph 10 and X = Br, Ar = Ph 11, 4-MeC6H4 12, 4-FC6H4 13, 4-OSi(CH3)3-C6H4 14) in high yields.


    S. Dürr, D. Ertler, U. Radius, Inorg. Chem. 2012, 51, 39043909.

    Abstract

    Degradation of white phosphorus (P4) in the coordination sphere of transition metals is commonly divided into two major pathways depending on the Px ligands obtained. Consecutive metal-assisted P-P bond cleavage of four bonds of the P4 tetrahedron leads to complexes featuring two P2 ligands (symmetric cleavage) or one P3 and one P1 ligand (asymmetric cleavage). A systematic investigation of the degradation of white phosphorus P4 to coordinated µ,η2:2 bridging diphosphorus ligands in the coordination sphere of cobalt is presented herein as well as the isolation of each of the decisive intermediates on the reaction pathway. The olefin complex [Cp*Co(iPr2Im)(η2-C2H4)] 1 (Cp* = η5-C5Me5, iPr2Im = 1,3-di-iso-propylimidazolin-2-ylidene) reacts with P4 to give [Cp*Co(iPr2Im)(η2-P4)] 2, the insertion product of [Cp*Co(iPr2Im)] into one of the P-P bonds. Addition of a further equivalent of the CoI complex [Cp*Co(iPr2Im)(η2-C2H4)] 1 induces cleavage of a second P-P bond to yield the dinuclear complex [{Cp*Co(iPr2Im)}2(μ,η2:2-P4)] 3, in which a kinked cyclo-P44- ligand bridges two cobalt atoms. Consecutive dissociation of the N-heterocyclic carbene with concomitant rearrangement of the cyclo-P4 ligand and P-P dissociation leads to the complexes [Cp*Co(µ,η4:2-P4)Co(iPr2Im)Cp*] 4, featuring a P4 chain, and [{Cp*Co(μ,η2:2-P2)}2] 5, in which two isolated P22- ligands bridge two [Cp*Co] fragments. Each of these reactions is quantitative if performed on an NMR scale, and each compound can be isolated in high yields and large quantities.


    S. Dürr, B. Zarzycki, D. Ertler, I. Ivanović-Burmazović, U. Radius, Organometallics 2012, 31, 17301742.

    Abstract

    The complexes [(η5-C5R5)Co(iPr2Im)(η 2-C2H4)] (R = H 1; Me 2) were synthesized in good yields via reaction of one equivalent of the N-heterocyclic carbene (NHC) iPr2Im (R2Im = 1,3-di-alkyl-imidazolin-2-ylidene) and the bis(ethylene) complexes [(η5-C5R5)Co(η2-C2H4)2]. These complexes serve as convenient starting materials for chemistry using the [(η5-C5R5)Co(iPr2Im)] complex fragment. The reaction with carbon monoxide leads to the carbonyl complexes [(η5-C5R5)Co(iPr2Im)(CO)] (R = H 3; Me 4) in good to excellent yields. The carbonyl complexes 3 and 4 are very air sensitive and react readily with oxygen in the solid state and in solution. Whereas the cyclopentadienyl substituted complex [(η5-C5H5)Co(iPr2Im)(CO)] 3 decomposes upon reaction with O2 to untractable products, [(η5-C5Me5)Co(iPr2Im)(CO)] 4 yields the structurally characterized cobalt(III) carbonato complex [(η5-C5Me5)Co(iPr2Im)(η2-CO3)] 5. This reaction represents the first example for O2 oxidation of metal bound carbonyl for a 3d transition metal complex. The oxidation is too fast to be monitored by NMR spectroscopy and application of low-temperature time-resolved UV/Vis spectroscopy combined with stopped-flow techniques led to the detection of a possible intermediate. Based on these experiments and computational investigations using density functional theory (DFT) the peroxo acyl complex [(η5-C5Me5)Co(iPr2Im)(k2-C,O-C{O}OO)] B is assumed to be the key intermediate detected. The DFT calculations further reveal that this reaction is strongly exothermic with two kinetic barriers, one for the exothermic addition of O2 to the carbonyl complexes 4 to give the peroxo acyl complex [(η5-C5Me5)Co(iPr2Im)(k2-C,O-C{O}OO)] B, the other for the rearrangement of B to give the carbonato complex [(η5-C5Me5)Co(iPr2Im)(η2-CO3)] 5. The key step for the rearrangement is the formation of CO2 in the coordination sphere of cobalt and the attack of metal bound oxygen at the carbon atom of CO2.


    P. Fischer, K. Götz, A. Eichhorn, U. Radius, Organometallics 2012, 31, 13741383 (Special Issue: Fluorine in Organometallic Chemistry).

    Abstract

    The hydrodefluorination reaction of perfluorinated arenes using [Ni2(iPr2Im)4(COD)] 1 (iPr2Im = 1,3-bis(isopropyl)imidazolin-2-ylidene) as a catalyst as well as stoichiometric transformations to elucidate the decisive steps for this reaction are reported. The reaction of hexafluorobenzene with five equivalents of triphenylsilane in the presence of 5 mol% 1 affords 1,2,4,5-tetrafluorobenzene after 48 hours at 60 °C and 1,4-difluorobenzene after 96 hours at 80 °C; the reaction of perfluorotoluene and five equivalents Et3SiH for 4 days at 80 °C results in the selective formation of 1-(CF3)-2,3,5,6-C6F4H. Stoichiometric transformations of the complexes cis-[Ni(iPr2Im)2(H)(SiPh3)] and cis-[Ni(iPr2Im)2(H)(SiMePh2)] with hexafluorobenzene at room temperature lead to the formation of trans-[Ni(iPr2Im)2(F)(C6F5)] 2 and trans-[Ni(iPr2Im)2(H)(C6F5)] 4 with elimination of the corresponding silane or fluorosilane. The reactions of the C–F activation products trans-[Ni(iPr2Im)2(F)(C6F5)] 2 and trans-[Ni(iPr2Im)2(F)(4-(CF3)C6F4)] 3 with PhSiH3 and Ph2SiH2 afford the hydride complexes trans-[Ni(iPr2Im)2(H)(C6F5)] 4 and trans-[Ni(iPr2Im)2(H)(4-(CF3)C6F4)] 5, which convert into the compounds trans-[Ni(iPr2Im)2(F)(2,3,5,6-C6F4H)] 7, trans-[Ni(iPr2Im)2(F)(3-(CF3)-2,4,5-C6F3H)] 9a and trans-[Ni(iPr2Im)2(F)(2-(CF3)-3,4,6-C6F3H)] 9b, respectively. In the case of the rearrangement of trans-[Ni(iPr2Im)2(H)(4-(CF3)C6F4)] 5 an intermediate [Ni(iPr2Im)22-C,C-(CF3)C6F4H)] 8 was detected. Reaction of 8 with perfluorotoluene gave the C–F activation product trans-[Ni(iPr2Im)2(F)(4-(CF3)C6F4)] 3. All these experimental findings point to a mechanism for the HDF by [Ni(iPr2Im)2] via the “fluoride route” involving C–F activation of the polyfluoroarene, H/F exchange of the resulting nickel fluoride, reductive elimination of the polyfluoroaryl nickel hydride to an intermediate with η2-C,C-coordinated arene ligand, subsequent ligand exchange with the higher fluorinated polyfluoroarene and renewed C–F activation of the polyfluoroarene.


    U. Radius, F. M. Bickelhaupt, Coord. Chem. Rev. 2009, 253, 678686.

    Abstract

    In this contribution, we give a brief overview of studies on the bonding mechanism between transition metals (TM) and N-heterocyclic carbenes (NHC) and report on a systematic bond analysis of the bonding of 1,3-diorganyl-imidazolin-2-ylidenes (R2Im) in a series of nickel, palladium and platinum complexes D2h- and D2d-M(H2Im)2 to exemplify the dependence of the TM-NHC bonding on the group 10 transition metal. Furthermore complexes with seemingly different complex fragment group electronegativities, i.e. [Ni(R2Im)3], [Ni(R2Im)2], [Ni(R2Im)(CO)], [Ni(R2Im)(CO)2], and [Ni(R2Im)(CO)3] have been analyzed, a series that provides theoretical evidence that the bonding mechanism of 1,3-diorganyl-imidazolin-2-ylidene ligands to metal-complex fragments strongly depends on the nature of the ligand environment. Our results confirm the currently accepted idea that NHCs are not pure η-donors. In the series of complexes examined here η-contribution is at least 10% and up to 40%, depending on the transition metal complex fragment bonded to the carbene. The dependence of the bonding mechanism on the R substituent in R2Im also investigated.


    T. Schaub, P. Fischer, A. Steffen, T. Braun, U. Radius, J. Am. Chem. Soc. 2008, 130, 93049317.

    Abstract

    The reaction of [Ni2(iPr2Im)4(COD)] 1a or [Ni(iPr2Im)2(η2-C2H4)] 1b with different fluorinated arenes is reported. These reactions occur with a high chemo- and regioselectivity. In the case of polyfluorinated aromatics of the type C6F5X such as hexafluorobenzene (X = F) octafluorotoluene (X = CF3), trimethyl(pentafluorophenyl)silane (X = SiMe3), or decafluorobiphenyl (X = C6F5) the C-F activation regioselectively takes place at the C-F bond in the para position to the X group to afford the complexes trans-[Ni(iPr2Im)2(F)(C6F5)] 2, trans-[Ni(iPr2Im)2(F)(4-(CF3)C6F4)] 3, trans-[Ni(iPr2Im)2(F)(4-(C6F5)C6F4)] 4, and trans-[Ni(iPr2Im)2(F)(4-(SiMe3)C6F4)] 5. Complex 5 was structurally characterized by X-ray diffraction. The reaction of 1a with partially fluorinated aromatic substrates C6HxFy leads to the products of a C-F activation trans-[Ni(iPr2Im)2(F)(2-C6FH4)] 7, trans-[Ni(iPr2Im)2(F)(3,5-C6F2H3)] 8, trans-[Ni(iPr2Im)2(F)(2,3-C6F2H3)] 9a and trans-[Ni(iPr2Im)2(F)(2,6-C6F2H3)] 9b, trans-[Ni(iPr2Im)2(F)(2,5-C6F2H3)] 10, and trans-[Ni(iPr2Im)2(F)(2,3,5,6- C6F4H)] 11. The reaction of 1a with octafluoronaphthalene yields exclusively trans-[Ni(iPr2Im)2(F)(1,3,4,5,6,7,8-C10F7)] 6a, the product of an insertion into the C-F bond in the 2-position, whereas for the reaction of 1b with octafluoronaphthalene the two isomers trans-[Ni(iPr2Im)2(F)(1,3,4,5,6,7,8-C10F7)] 6a and trans-[Ni(iPr2Im)2(F)(2,3,4,5,6,7,8-C10F7)] 6b in a ratio of 11:1 are formed. The reaction of 1a or of 1b with pentafluoropyridine at low temperatures affords trans-[Ni(iPr2Im)2(F)(4-C5NF4)] 12a as the solely product, whereas the reaction of 1b performed at room temperature leads to the generation of trans-[Ni(iPr2Im)2(F)(4-C5NF4)] 12a and trans-[Ni(iPr2Im)2(F)(2-C5NF4)] 12b in a ratio of approximately 1:2. The detection of intermediates as well as kinetic studies give some insight into the mechanistic details for the activation of an aromatic carbon-fluorine bond at the {Ni(iPr2Im)2} complex fragment. The intermediates of the reaction of 1b with hexafluorobenzene and octafluoronaphthalene, [Ni(iPr2Im)22-C6F6)] 13 and [Ni(iPr2Im)22-C10F8)] 14, have been detected in solution. They convert into the C-F bond activation products. Complex 14 was structurally characterized by X-ray diffraction. The rates for the loss of 14 at different temperatures for the C-F activation of the coordinated naphthalene are first order and the estimated activation enthalpy ΔH¹ for this process was determined to be ΔH¹ = 116 ± 8 kJ mol-1 (ΔS¹ = 37 ± 25 J K-1 mol-1). Furthermore, DFT calculations on the reaction of 1a with hexafluorobenzene, octafluoronaphthalene, octafluorotoluene, 1,2,4-trifluorobenzene, and 1,2,3-trifluorobenzene are presented.


    T. Schaub, M. Backes, U. Radius, J. Am. Chem. Soc. 2006, 128, 1596415965. (Highlighted: Chem. Eng. News 2006, 84(59), 31; Nachr. Chem. 2007, 55, 119)

    Abstract

    We report here the first example of a catalytically active system for Suzuki-Miyaura type cross-coupling reactions of perfluorinated arenes such as octafluoro toluene and perfluoro biphenyl with aryl boronic acids. This catalysis combines C-F bond activation of these substrates efficiently with C-C coupling reactions. Although C-F activation appears to be quite general in this system, it was necessary to optimize each reaction separately.


    T. Schaub, U. Radius, Chem. Eur. J. 2005, 11, 50245030.

    Abstract

    The NHC stabilized complex [Ni2(iPr2Im)4(COD)] 1 was isolated in good yield from the reaction of [Ni(COD)2] with 1,3-di(isopropyl)imidazole-2-ylidene (iPr2Im). Compound 1 is a source of the [Ni(iPr2Im)2] complex fragment in stoichiometric and catalytic transformations. The reactions of 1 with ethylene and CO under atmospheric pressure or with equimolar amounts of diphenyl acetylene lead to the compounds [Ni(iPr2Im)22-C2H4)] 2, [Ni(iPr2Im)22-C2Ph2)] 3, and [Ni(iPr2Im)2(CO)2] 4 in good yields. In all cases the [Ni(iPr2Im)2] complex was readily transferred without decomposition or fragmentation. In the infrared spectrum of carbonyl complex 4, the CO stretching frequencies were observed at 1847 cm-1 and 1921 cm-1, significantly shifted to lower wavenumbers compared to other nickel(0) carbonyl complexes of the type [NiL2(CO)2]. Complex 1 activates the C-F bond of hexafluorobenzene very efficiently to give [Ni(iPr2Im)2(F)(C6F5)] 5. Furthermore, [Ni2(iPr2Im)4(COD)] 1 is also an excellent catalyst for the catalytic insertion of diphenyl acetylene into the 2,2’ bond of biphenylene. The reaction of 1 with equimolar amounts of biphenylene at low temperature leads to the insertion product into the strained 2,2’ bond, [Ni(iPr2Im)2(2,2’-biphenyl)] 6. The reaction of diphenyl acetylene and biphenylene at 80 °C using 2 mol% 1 as a catalyst yields diphenylphenanthrene quantitatively and is completed within one hour.

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