Multiple and Highly Selective Alkyne-Isonitrile C-C and C-N Couplings at Group 4 Metallocenes

Reactivity of Rare Earth Metal Alkyl Complexes with Nitriles or Isonitrile: Versatile Ways toward Multiply Functionalized β-Diketiminato, (Iso)indolyl, and Imidazolyl Chelating Rare Earth Metal Complexes

The reactivity of the rare earth metal alkyl complexes LRE(CH2SiMe3)(THF)2 (1RE) [RE = Y (1Y), Yb (1Yb), Lu (1Lu); L = 2,5-[(2-pyrrolyl)CPh2]2(N-methylpyrrole)] with various nitriles and isonitriles has been fully developed. Treatment of the yttrium monoalkyl complex (1Y) with 2 equiv of aromatic nitriles afforded the symmetric trisubstituted β-diketiminato yttrium complexes (2Y(H), 2Y(Me), and 2Y(F)) through successive cyano group insertion into the RE-C bond and 1,3-H shift or the unsymmetric trisubstituted β-diketiminato yttrium complex (3Y) unexpectedly via a 1,3-SiMe3 shift when 4-(trifluoromethyl)benzonitrile was used in this reaction under the same conditions. By treating 1Y with 2 equiv of tolyl acetonitrile, an activation of the sp3 C-H bond occurred to form the corresponding β-aryl keteniminato complexes 4Y(p-tol) and 4Y(m-tol). Remarkably, a heteroleptic cleavage of the CO-CN bond took place in the reaction of 1Y with benzoyl nitrile, affording the unsymmetric trinuclear yttrium complex 5Y bridged by three cyanide groups. Dinuclear ytterbium and lutetium complexes 6Yb and 6Lu containing a functionalized isoindole fragment were synthesized from the reactions of 1 with phthalonitrile by tandem insertion and cyclization. Further studies indicated that the temperature and stoichiometric ratio have a great influence on the reactivity patterns between the reactions of 1RE with benzylisonitrile: two tetrasubstituted β-diketiminato complexes 8 and 9 were obtained at -30 °C, and tetrasubstituted imidazolyl yttrium and lutetium complexes 7 were isolated at elevated temperature, respectively. In addition, the tetrasubstituted β-diketiminato complexes 8 and 9 could be irreversibly converted to the cyclization products 7 by elevating the reaction temperature not only on the NMR scale but also on the preparative scale. Notably, when the phenyl isonitrile instead of benzyl isonitrile was reacted with 1Yb, a 2,3-functionalized indolyl ytterbium complex 10Yb was isolated.

Crystal structures of two novel iron isocyanides from the reaction of 2,6-di-methyl-phenyl isocyanide, CNXyl, with bis-(anthracene)ferrate(-1)

The reaction of the [K(18-crown-6)(thf)2]1+ (thf is tetra-hydro-furan) salt of bis-(anthracene)ferrate(-1), or [Fe(C14H10)2]-, with 2,6-di-methyl-phenyl isocyan-ide (CNX-yl) in thf resulted in the formation of two new iron isocyanide complexes, namely, [(1,2,3,4-η)-anthracene]tris-(2,6-di-methyl-phenyl isocyanide)iron, [Fe(C14H10)(C9H9N)3] or [Fe(1,2,3,4-η-C14H10)(CNX-yl)3], and {5,6-bis-(2,6-di-methyl-anilino)-3-(2,6-di-methyl-phen-yl)-1,2,7-tris-[(2,6-di-methyl-phen-yl)imino]-3-azoniahept-3-ene-1,4,7-triido}tris-(2,6-di-methyl-phenyl isocyanide)iron tetra-hydro-furan disolvate, [Fe(C54H56N6)(C9H9N)3]·2C4H8O or [Fe(C54H56N6)(CNX-yl)3]·2C4H8O, which were characterized by single-crystal X-ray diffraction. The former is likely an inter-mediate along the path to the known homoleptic [Fe(CNX-yl)5], while the latter contains a tridentate ligand that is formed from the 'coupling' of six CNXyl ligands. A third crystal structure from this reaction, (7-methyl-indol-1-ido-κN)(1,4,7,10,13,16-hexa-oxa-cyclo-octa-decane-κ6 O)potassium, [K(C9H8N)(C12H24O6)] or [K(C9H8N)(18-crown-6)], contains a 7-methyl-indol-1-ide anion, in which one CNXyl ligand has shed a proton during its reductive cyclization.

Systematic reductive oligomerization of isocyanides with a vanadium(ii) complex

Using a V(ii) complex as a reducing reagent, we demonstrate controlled reduction of isocyanides, resulting in decyanation of alkyl isocyanides to form a V(iii) cyanide complex or oligomerization of aryl isocyanides to form trimers and a tetramer. The pathways leading to the trimers involve a divanadium ynediamido intermediate, which further reacts with the third isocyanide molecule to selectively produce a tri(imino)deltate or an indolenine complex, by altering the temperature and stoichiometry.

A Mononuclear and High-Spin Tetrahedral TiII Complex

A high-spin, mononuclear TiII complex, [(TptBu,Me)TiCl] [TptBu,Me- = hydridotris(3-tert-butyl-5-methylpyrazol-1-yl)borate], confined to a tetrahedral ligand-field environment, has been prepared by reduction of the precursor [(TptBu,Me)TiCl2] with KC8. Complex [(TptBu,Me)TiCl] has a 3A2 ground state (assuming C3v symmetry based on structural studies), established via a combination of high-frequency and -field electron paramagnetic resonance (HFEPR) spectroscopy, solution and solid-state magnetic studies, Ti K-edge X-ray absorption spectroscopy (XAS), and both density functional theory and ab initio (complete-active-space self-consistent-field, CASSCF) calculations. The formally and physically defined TiII complex readily binds tetrahydrofuran (THF) to form the paramagnetic adduct [(TptBu,Me)TiCl(THF)], which is impervious to N2 binding. However, in the absence of THF, the TiII complex captures N2 to produce the diamagnetic complex [(TptBu,Me)TiCl]2(η1,η1;μ2-N2), with a linear Ti═N═N═Ti topology, established by single-crystal X-ray diffraction. The N2 complex was characterized using XAS as well as IR and Raman spectroscopies, thus establishing this complex to possess two TiIII centers covalently bridged by an N22- unit. A π acid such as CNAd (Ad = 1-adamantyl) coordinates to [(TptBu,Me)TiCl] without inducing spin pairing of the d electrons, thereby forming a unique high-spin and five-coordinate TiII complex, namely, [(TptBu,Me)TiCl(CNAd)]. The reducing power of the coordinatively unsaturated TiII-containing [(ΤptBu,Me)TiCl] species, quantified by electrochemistry, provides access to a family of mononuclear TiIV complexes of the type [(TptBu,Me)Ti═E(Cl)] (with E2- = NSiMe3, N2CPh2, O, and NH) by virtue of atom- or group-transfer reactions using various small molecules such as N3SiMe3, N2CPh2, N2O, and the bicyclic amine 2,3:5,6-dibenzo-7-azabicyclo[2.2.1]hepta-2,5-diene.

Carbon Monoxide Coupling Reactions: A New Concept for the Formation of Hexahydroxybenzene

For linear and cyclic coupling reactions of CO, among other products, the formation of the hexapotassium salt of hexahydroxybenzene is of particular interesting. The interaction of metallic potassium and CO offers, via the assumed K[OC≡CO]K as the result of several carbon monoxide coupling reactions, the formation of C6 (OK)6 among other products. To date, only speculations exist about the reaction pathway for these products, which were first described by Liebig in 1834. A novel concept is suggested here, which consists of the single steps (i) reductive coupling of CO, (ii) formation of dihetero-metallacyclopentynes (cis-2,5-diheterobutatriene as formal ethylenedione O=C=C=O complexes), (iii) formation of its dinuclear 1-metalla-2,5-dioxo-cyclopentyne complexes by external coordination of the triple bond, (iv) insertion of CO into the M-C bond of the formed metallacyclopropene, and (v) the reductive elimination of C6 (OK)6 . The novel aspect of this concept is the formation of dihetero-metallacyclopentynes (in analogy to the well characterized all-C-metallacyclopentynes), which have not been considered in the mechanism of reductive CO coupling reactions. It is expected that the presence of transition-metal impurities would promote the reaction.

Reductive linear- and cyclo-trimerization of isocyanides using an Al–Al-bonded compound

Dialumane can promote cyclo-trimerization of isocyanides in the presence of Na, affording the product [Na][LAl{μ-η¹:η²-(tBuNC)₃}Al(CN)L] with a unique aromatic tri(tert-butylimino)deltate dianion [C₃N₃(tBu)₃]²⁻.

Modern applications of low-valent early transition metals in synthesis and catalysis

Low-valent early transition metals are often intrinsically highly reactive as a result of their strong propensity toward oxidation to more stable high-valent states. Harnessing these highly reducing complexes for productive reactivity is potentially powerful for C-C bond construction, organic reductions, small-molecule activation and many other reactions that offer orthogonal chemoselectivity and/or regioselectivity patterns to processes promoted by late transition metals. Recent years have seen many exciting new applications of low-valent metals through building new catalytic and/or multicomponent reaction manifolds out of classical reactivity patterns. In this Review, we survey new methods that employ early transition metals and invoke low-valent precursors or intermediates in order to identify common themes and strategies in synthesis and catalysis.

A general benzylic C–H activation and C–C coupling reaction of zirconocenes mediated by C–N bond cleavage in tert-butylisocyanide – unusual formation of iminoacyl complexes

Reactions of the zirconocene alkyne complex [rac-(ebthi)Zr(η²-Me₃SiC₂SiMe₃)] (rac-(ebthi) = rac-1,2-ethylene-1,1′-bis(η⁵-tetrahydroindenyl)) with tert-butylisocyanide and methylbenzenes were investigated.

Insertion of Isonitriles into the M-C Bonds of Group 4 Dialkyl Complexes

The 2,3-dimethylbutadiene complexes of Group 4 metals with constrained geometry (cg) ligands have been prepared and found to adopt a supine orientation with σ²,π bonding. Treatment of cgTi(2,3-dimethylbutadiene) (1-Ti) with ^tBuNC leads to the formation of a titana-aziridine (3) with a coordinated cyclopentenimine that arises from the formal [4+1] addition of the diene to the isonitrile. In contrast, the reactions of cgZr(2,3-dimethylbutadiene) (1-Zr) or cgHf(2,3-dimethylbutadiene) (1-Hf) with 2 equiv of ^tBuNC or XyNC proceeded in a more sophisticated manner to yield unsymmetrical 2,5-diazametallacyclopentane derivatives (4, 6-Zr, and 6-Hf) or symmetrical 2,5-diazametallacyclopentene complexes (7-Zr and 7-Hf). The unsymmetrical products contain coordinated cyclopropanes; the strength of the interaction is measured by the reduction in the ¹ J_CC of the C-C bond that is coordinated. A detailed mechanistic analysis has been possible with the related cgM(Me)₂ (M = Ti and Hf) complexes. The first insertion is too fast to monitor, but allows complete conversion to an alkyl iminoacyl intermediate. The second isonitrile (RNC) may react with that intermediate by either of two different mechanisms, reductive elimination and coordination/insertion. In the first mechanism (Ti), rate-determining C-C coupling gives a titana-aziridine, followed by fast coordination of the isonitrile. In the second mechanism (Hf), coordination is the slow step; insertion to form a bis(iminoacyl) Hf complex is rapid.

Dehydrogenative coupling of 4-substituted pyridines mediated by a zirconium(ii) synthon: reaction pathways and dead ends

The mechanism of the reductive homocoupling of pyridine derivatives mediated by the Zr^II synthon [(PNP)Zr(η⁶-toluene)Cl] (1) has been investigated.

The mechanism of the reductive homocoupling of pyridine derivatives mediated by the Zr^II synthon [(PNP)Zr(η⁶-toluene)Cl] (1) has been investigated. Selective transformation into three different types of product complexes has been observed, depending on the N-heterocyclic substrate employed: the bipyridyl complexes 3-R (R = Me, Et, ^tBu, Bn, Ph, CHCHPh), which are the homocoupling products, the η²-((4-dimethylamino)pyridyl) complex 4 as well as the bis(isoquinolinyl) complex 5. By deuterium labelling experiments the participation of the ligand backbone in the pyridine coupling reaction via potential cyclometallation steps was ruled out. Based on DFT modelling of the possible reaction sequences a reaction mechanism for the coupling sequence could be identified. The latter is initiated by a reductive syn C–C coupling rather than based on an initial C–H activation of the pyridine substrate.

Benzylene-linked [PNP] scaffolds and their cyclometalated zirconium and hafnium complexes

The benzylene-linked [PNP] scaffolds HN(CH₂-o-C₆H₄PPh₂)₂ ([A]H) and HN(C₆H₄-o-CH₂PPh₂)₂ ([B]H) have been used for the synthesis of zirconium and hafnium complexes. For both ligands, the dimethylamides [A]M(NMe₂)₃ ([A]1-M) and [B]M(NMe₂)₃ ([B]1-M) were prepared and converted to the iodides [A]MI₃ ([A]2-M) and [B]MI₃ ([B]2-M) (M = Zr, Hf). Starting from these iodides, the corresponding benzyl derivatives [A]MBn₃ ([A]3-M) and [B]MBn₃ ([B]3-M) (M = Zr, Hf) were obtained via reaction with Bn₂Mg(OEt₂)₂. For zirconium, the benzylic ligand positions in [A]3-Zr and [B]3-Zr were found to cyclometalate readily, which led to the corresponding κ⁴-[PCNP]ZrBn₂ complexes [A]4-Zr and [B]4-Zr. As these complexes failed to hydrogenate cleanly, cyclometalated derivatives with only one alkyl substituent were targeted and the mixed benzyl chlorides κ⁴-[PCNP]MBnCl ([B]5-M, M = Zr, Hf) were obtained in the case of ligand [B]. Upon hydrogenation of [B]5-Zr, the η⁶-tolyl complex [B]Zr(η⁶-C₇H₈)Cl ([B]6-Zr) was generated cleanly, but the corresponding hafnium complex [B]5-Hf was found to decompose unselectively in the presence of H₂. Using a closely related carbazole-based [PNP] ligand, Gade and co-workers have shown recently that zirconium η⁶-arene complexes similar to [B]6-Zr may serve as zirconium(ii) synthons, namely when reacted with 2,6-Dipp-NC (L) or pyridine (py). Both these substrates were shown to react cleanly with [B]6-Zr, which led to the formation of the bis-isocyanide complex [B]ZrCl(L)₂ ([B]7-Zr) and the 2,2'-bipyridine derivative [B]ZrCl(bipy) ([B]8-Zr), respectively. Upon reaction of [B]Zr(η⁶-C₇H₈)Cl ([B]6-Zr) with NaBEt₃H, the cyclometalated derivative κ⁴-[PCNP]Zr(η⁶-C₇H₈) ([B]9-Zr) was isolated. In an attempt to synthesise terminal hydrides, complexes [A]MI₃ ([A]2-M) were treated with KBEt₃H, which led to the isolation of the cyclometalated hydrido complexes κ⁴-[PCNP]M(H)(κ³-Et₃BH) ([A]10-M; M = Zr, Hf) featuring a κ³-bound triethyl borohydride moiety.