Chlorido-[5,10,15,20-tetra-kis-(quinoline-7-carboxamido)-porphinato]iron(III).

The title compound, [Fe(C84H52N12O4)Cl], crystallizes in space group C2/c. The central FeIII cation (site symmetry 2) is coordinated in a fivefold manner, with four pyrrole N atoms of the porphyrin core in the basal sites and one Cl atom (site symmetry 2) in the apical position, which completes a slightly distorted square-pyramidal environment. The porphyrin macrocycle shows a characteristic ruffled-shape distortion and the iron atom is displaced out of the porphyrin plane by 0.42 Šwith the average Fe-N distance being 2.054 (4) Å; the Fe-Cl bond length is 2.2042 (7) Å. Inter-molecular C-H⋯N and C-H⋯O hydrogen bonds occur in the crystal structure.

(2,5-Di-methyl-imidazole){N,N',N'',N'''-[porphyrin-5,10,15,20-tetra-yltetra-(2,1-phenyl-ene)]tetra-kis(pyridine-3-carboxamide)}manganese(II) chloro-benzene disolvate.

In the title compound, [Mn(C68H44N12O4)(C5H8N2)]·2C6H5Cl, the central MnII ion is coordinated by four pyrrole N atoms of the porphyrin core in the basal sites and one N atom of the 2,5-di-methyl-imidazole ligand in the apical site. Two chloro-benzene solvent mol-ecules are also present in the asymmetric unit. Due to the apical imidazole ligand, the Mn atom is displaced out of the 24-atom porphyrin mean plane by 0.66 Å. The average Mn-Np (p = porphyrin) bond length is 2.143 (8) Å, and the axial Mn-NIm (Im = 2,5-di-methyl-imidazole) bond length is 2.171 (8) Å. The structure displays inter-molecular and intra-molecular N-H⋯O, N-H⋯N, C-H⋯O and C-H⋯N hydrogen bonding. The crystal studied was refined as a two-component inversion twin.

Evidence for Carbene Intermediates in Isocyanide Homologation by Aluminium(I).

The C-C bond formation between C1 molecules plays an important role in chemistry as manifested by the Fischer-Tropsch (FT) process. Serving as models for the FT process, we report here the reactions between a neutral AlI complex (Me NacNac)Al (1, Me NacNac=HC[(CMe)(NDipp)]2 , Dipp=2,6-diisopropylphenyl) and various isocyanides. The step-by-step coupling mechanism was studied in detail by low-temperature NMR monitoring, isotopic labeling, as well as quantum chemical calculations. Three different products were isolated in reaction of 1 with the sterically encumbered 2,6-bis(benzhydryl)-4-Me-phenyl isocyanide (BhpNC). These products substantiate carbene intermediates. The reaction between 1 and adamantyl isocyanide (AdNC) generated a trimerization product, and a corresponding carbene intermediate could be trapped in the form of a molybdenum(0) complex. Tri-, tetra-, and even pentamerization products were isolated with the sterically less congested phenyl and p-methoxyphenyl isocyanides (PhNC and PMPNC) with concurrent construction of quinoline or indole heterocycles. Overall, this study provides evidence for carbene intermediates in FT-type chemistry of aluminium(I) and isocyanides.

A Borane Lewis Acid in the Secondary Coordination Sphere of a Ni(II) Imido Imparts Distinct C-H Activation Selectivity.

Two borane-functionalized bidentate phosphine ligands that vary in tether length have been prepared to examine cooperative metal-substrate interactions. Ni(0) complexes react with aryl azides at low temperatures to form structurally unusual κ2-(N,N)-N3Ar adducts. Warming these adducts affords products of N2 extrusion and in one case, a Ni-imido compound that is capped by the appended borane. Reactions with 1-azidoadamantane (AdN3) provide a distinct outcome, where a proposed nickel imido intermediate activates the sp2 C-H bonds of arenes, even in the presence of benzylic C-H sites. Combined experimental and computational mechanistic studies demonstrate that the unique reactivity is a consequence of Lewis-acid-induced polarization of the Ni-NR bond, potentially providing a synthetic strategy for chemoselective reaction engineering.

Redox-Controlled Reactivity at Boron: Parallels to Frustrated Lewis/Radical Pair Chemistry.

We report the synthesis of new Lewis-acidic boranes tethered to redox-active vanadium centers, (Ph2N)3V(µ-N)B(C6F5)2 (1a) and (N(CH2CH2N(C6F5))3)V(µ-N)B(C6F5)2 (1b). Redox control of the VIV/V couple resulted in switchable borane versus "hidden" boron radical reactivity, mimicking frustrated Lewis versus frustrated radical pair (FLP/FRP) chemistry, respectively. Whereas heterolytic FLP-type addition reactions were observed with the VV complex (1b) in the presence of a bulky phosphine, homolytic peroxide, or Sn-hydride bond cleavage reactions were observed with the VIV complex, [CoCp2*][(N(CH2CH2N(C6F5))3)V(µ-N)B(C6F5)2] (3b), indicative of boron radical anion character. The extent of radical character was probed by spectroscopic and computational means. Together, these results demonstrate that control of the VIV/V oxidation states allows these compounds to access reactivity observed in both FLP and FRP chemistry.

Neutral and Anionic Monomeric Zirconium Imides Prepared via Selective C=N Bond Cleavage of a Multidentate and Sterically Demanding ß-Diketiminato Ligand.

A sterically encumbering multidentate ß-diketiminato ligand, tBu L2 (tBu L2=[ArNC(tBu)CHC(tBu)NCH2 CH2 N(Me)CH2 CH2 NMe2 ]- , Ar=2,6-iPr2 C6 H3 ), is reported in this study along with its coordination chemistry to zirconium(IV). Using the lithio salt of this ligand, Li(tBu L2) (4), the zirconium(IV) precursor (tBu L2)ZrCl3 (6) could be readily prepared in 85 % yield and structurally characterized. Reduction of 6 with 2 equiv of KC8 resulted in formation of the terminal and mononuclear zirconium imide-chloride [C(tBu)CHC(tBu)NCH2 CH2 N(Me)CH2 CH2 NMe2 ]Zr(=NAr)(Cl) (7) as the result of reductive C=N cleavage of the imino fragment in the multidentate ligand tBu L2 by an elusive ZrII species (tBu L2)ZrCl (A). The azabutadienyl ligand in 7 can be further reduced by 2 e- with KC8 to afford the anionic imide [K(THF)2 ]{[CH(tBu)CHC(tBu)NCH2 CH2 N(Me)CH2 CH2 N(Me)CH2 ]Zr=NAr} (8-2THF) in 42 % isolated yield. Complex 8-2THF results from the oxidative addition of an amine C-H bond followed by migration to the vinylic group of the formal [C(tBu)CHC(tBu)NCH2 CH2 N(Me)CH2 CH2 NMe2 ]- ligand in 7. All halides in 6 can be replaced with azides to afford (tBu L2)Zr(N3 )3 (9) which was structurally characterized, and reduction with two equiv of KC8 also results in C=N bond cleavage of tBu L2 to form [C(tBu)CHC(tBu)NCH2 CH2 N(Me)CH2 CH2 NMe2 ]Zr(=NAr)(N3 ) (10), instead of the expected azide disproportionation to N3- and N2 . Solid-state single crystal structural studies confirm the formation of mononuclear and terminal zirconium imido groups in 7, 8-Et2 O, and 10 with Zr=NAr distances being 1.8776(10), 1.9505(15), and 1.881(3) Å, respectively.

Probing Hydrogen Atom Transfer at a Phosphorus(V) Oxide Bond Using a "Bulky Hydrogen Atom" Surrogate: Analogies to PCET.

Recent computational studies suggest that the phosphate support in the commercial vanadium phosphate oxide (VPO) catalyst may play a critical role in initiating butane C-H bond activation through a mechanism termed reduction-coupled oxo activation (ROA) similar to proton-coupled electron transfer (PCET); however, no experimental evidence exists to support this mechanism. Herein, we present molecular model compounds, (Ph2N)3V═N-P(O)Ar2 (Ar = C6F5 (2a), Ph (2b)), which are reactive to both weak H atom donors and a Me3Si• (a "bulky hydrogen atom" surrogate) donor, 1,4-bis(trimethylsilyl)pyrazine. While the former reaction led to product decomposition, the latter resulted in the isolation of the reduced, silylated complexes (Ph2N)3V-N═P(OSiMe3)Ar2 (3a/b). Detailed analyses of possible reaction pathways, involving the isolation and full characterization of potential stepwise square-scheme intermediates, as well as the determination of minimum experimentally and computationally derived thermochemical values, are described. We find that stepwise electron transfer (ET) + silylium transfer (ST) or concerted EST mechanisms are most likely. This study provides the first experimental evidence supporting a ROA mechanism and may inform future studies in homogeneous or heterogeneous C-H activation chemistry, as well as open up a possible new avenue for main group/transition metal cooperative redox reactivity.

Scandium Terminal Imido Chemistry.

Research into transition metal complexes bearing multiply bonded main-group ligands has developed into a thriving and fruitful field over the past half century. These complexes, featuring terminal M═E/M≡E (M = transition metal; E = main-group element) multiple bonds, exhibit unique structural properties as well as rich reactivity, which render them attractive targets for inorganic/organometallic chemists as well as indispensable tools for organic/catalytic chemists. This fact has been highlighted by their widespread applications in organic synthesis, for example, as olefin metathesis catalysts. In the ongoing renaissance of transition metal-ligand multiple-bonding chemistry, there have been reports of M═E/M≡E interactions for the majority of the metallic elements of the periodic table, even some actinide metals. In stark contrast, the largest subgroup of the periodic table, rare-earth metals (Ln = Sc, Y, and lanthanides), have been excluded from this upsurge. Indeed, the synthesis of terminal Ln═E/Ln≡E multiple-bonding species lagged behind that of the transition metal and actinide congeners for decades. Although these species had been pursued since the discovery of a rare-earth metal bridging imide in 1991, such a terminal (nonpincer/bridging hapticities) Ln═E/Ln≡E bond species was not obtained until 2010. The scarcity is mainly attributed to the energy mismatch between the frontier orbitals of the metal and the ligand atoms. This renders the putative terminal Ln═E/Ln≡E bonds extremely reactive, thus resulting in the formation of aggregates and/or reaction with the ligand/environment, quenching the multiple-bond character. In 2010, the stalemate was broken by the isolation and structural characterization of the first rare-earth metal terminal imide-a scandium terminal imide-by our group. The double-bond character of the Sc═N bond was unequivocally confirmed by single-crystal X-ray diffraction. Theoretical investigations revealed the presence of two p-d π bonds between the scandium ion and the nitrogen atom of the imido ligand and showed that the dianionic [NR]2- imido ligand acts as a 2σ,4π electron donor. Subsequent studies of the scandium terminal imides revealed highly versatile and intriguing reactivity of the Sc═N bond. This included cycloaddition toward various unsaturated bonds, C-H/Si-H/B-H bond activations and catalytic hydrosilylation, dehydrofluorination of fluoro-substituted benzenes/alkanes, CO2 and H2 activations, activation of elemental selenium, coordination with other transition metal halides, etc. Since our initial success in 2010, and with contributions from us and across the community, this young, vibrant research field has rapidly flourished into one of the most active frontiers of rare-earth metal chemistry. The prospect of extending Ln═N chemistry to other rare-earth metals and/or different metal oxidation states, as well as exploiting their stoichiometric and catalytic reactivities, continues to attract research effort. Herein we present an account of our investigations into scandium terminal imido chemistry as a timely summary, in the hope that our studies will be of interest to this readership.

NHC-CAAC Heterodimers with Three Stable Oxidation States.

The synthesis of N-heterocyclic carbene (NHC)-cyclic (alkyl)(amino) carbene (CAAC) heterodimers is presented. As the free carbenes do not react together in solution, the synthetic approach involves the addition of a free NHC to a cyclic iminium salt, which results in the formation of the protonated heterodimer. Subsequent deprotonation leads to the isolation of the corresponding mixed Wanzlick dimers. One- and two-electron oxidations of these triazaolefins result in the formation of stable cationic radicals and bis(cations), respectively, which have been isolated and fully characterized. Cyclic voltammetry, UV/Vis spectroscopy, spin density, and DFT calculations suggest that these heterodimers feature complementary electronic properties to tetrathiafulvalenes (TTFs).

Synthesis of Hemilabile Cyclic (Alkyl)(amino)carbenes (CAACs) and Applications in Organometallic Chemistry.

A versatile methodology, involving readily available starting materials, allows for the synthesis of stable hemilabile bidentate cyclic (alkyl)(amino)carbenes (CAACs) featuring alkene, ether, amine, imine, and phosphine functionalities. The stability of the free carbenes has been exploited for the synthesis of copper(I) and gold(I) complexes. It is shown that the pendant imine moiety stabilizes the gold(III) oxidation state and enables the C-C bond oxidative addition of biphenylene to the corresponding cationic gold(I) complex. The latter and the corresponding copper(I) complex show high catalytic activity for the hydroarylation of α-methylstyrene with N,N-dimethylaniline, and the copper(I) complex promotes the anti-Markovnikov hydrohydrazination of phenyl acetylene with high selectivity.

Air-Stable (CAAC)CuCl and (CAAC)CuBH4 Complexes as Catalysts for the Hydrolytic Dehydrogenation of BH3NH3.

The first stable copper borohydride complex [(CAAC)CuBH4] [CAAC = cyclic(alkyl)(amino)carbene] bearing a single monodentate ligand was prepared by addition of NaBH4 or BH3NH3 to the corresponding [(CAAC)CuCl] complex. Both complexes are air-stable and promote the catalytic hydrolytic dehydrogenation of ammonia borane. The amount of hydrogen released reaches 2.8 H2/BH3 NH3 with a turnover frequency of 8400 mol H2 molcat(-1) h(-1) at 25 °C. In a fifteen-cycle experiment, the catalyst was reused without any loss of efficiency.

Lewis acid triggered reactivity of a Lewis base stabilized scandium-terminal imido complex: C-H bond activation, cycloaddition, and dehydrofluorination.

A stable scandium-terminal imido complex is activated by borane to form an unsaturated terminal imido complex by removing the coordinated Lewis base, 4-(dimethylamino)pyridine, from the metal center. The ensuing terminal imido intermediate can exist as a THF adduct and/or undergo cycloaddition reaction with an internal alkyne, C-H activation of a terminal alkene, and dehydrofluorination of fluoro-substituted benzenes or alkanes at room temperature. DFT investigations further highlight the ease of C-H activation for terminal alkene and fluoroarene. They also shed light on the mechanistic aspects of these two reactions.

Chameleon behavior of a newly synthesized scandium nitrilimine derivative.

The synthesis, structural characterization, and reactivity of the first example of a scandium-substituted nitrilimine are presented. This unique complex exhibits high thermal stability but shows a rich reactivity toward a variety of unsaturated substrates, including aldehyde, ketone, nitrile, and allene derivatives. The versatility of the complex was further highlighted by density functional theory mechanistic studies.

Reactivity of scandium terminal imido complexes towards metal halides.

Reactions of scandium terminal imido complexes with CuI and [M(COD)Cl](2) (M = Rh, Ir) show two interesting reaction patterns, and the formed heterobimetallic complexes have intriguing structural features and show promising catalytic properties.

Scandium terminal imido complex induced C-H bond selenation and formation of an Sc-Se bond.

The reaction between scandium terminal imido complexes and elemental selenium showed an unprecedented C-H bond selenation and the formation of an Sc-Se bond.