Diazadiene Ligands Research Articles

DFT Provides Insight into the Additive-Free Conversion of Aqueous Methanol to Dihydrogen Catalyzed by [Ru(trop2dad)]: Importance of the (Electronic) Flexibility of the Diazadiene Moiety.

The mechanism for complete dehydrogenation of aqueous methanol to CO2 and three equivalents of H2 catalyzed by [Ru(trop2dad)] was investigated with DFT (trop2dad = 1,4-bis(5H-dibenzo[a,d]cyclohepten-5-yl)-1,4-diazabuta-1,3-diene). To date, this is the only catalyst that promotes the acceptorless dehydrogenation of aqueous methanol in homogeneous phase under mild conditions without the addition of an additive (base, acid, or a secondary catalyst). A detailed understanding of the mechanism of this transformation may therefore be of significant importance for the conversion of liquid organic fuels. Previous computational studies using simplified models of the catalyst suggested entirely ligand-centered reaction pathways with rather high-energy barriers for complete dehydrogenation of aqueous methanol. These are, however, not consistent with the experimental data. In the present paper, we reveal a different reaction mechanism for aqueous methanol dehydrogenation that involves metal–ligand cooperativity involving the diazadiene (dad) ligand and has substantially lower barriers, in good agreement with the experimental data. The dad moiety of the ligand actively participates in the alcohol activation mechanism. In the first step of the reaction, the dad ligand rearranges from a σ- to a π-bound coordination mode. This adjusts the electronic structure of both the metal and the ligand, leading to an enhanced Brønsted basicity of the nitrogen centers and higher Lewis acidity of the ruthenium center. As a result, concerted proton-hydride transfer to/from metal-hydride and N-protonated dad-ligand moieties becomes possible, leading to low-barrier metal–ligand cooperative elementary steps for alcohol activation and H2 elimination.

A homogeneous transition metal complex for clean hydrogen production from methanol–water mixtures

The development of an efficient catalytic process that mimics the enzymatic function of alcohol dehydrogenase is critical for using biomass alcohols for both the production of H2 as a chemical energy carrier and fine chemicals under waste-free conditions. Dehydrogenation of alcohol-water mixtures into their corresponding acids with molecular hydrogen as the sole by-product from the reaction can be catalysed by a ruthenium complex with a chelating bis(olefin) diazadiene ligand. This complex, [K(dme)2][Ru(H)(trop2dad)], stores up to two equivalents of hydrogen intramolecularly, and catalyses the production of H2 from alcohols in the presence of water and a base under homogeneous conditions. The conversion of a MeOH-H2O mixture proceeds selectively to CO2/H2 gas formation under neutral conditions, thereby allowing the use of the entire hydrogen content (12% by weight). Isolation and characterization of the ruthenium complexes from these reactions suggested a mechanistic scenario in which the trop2dad ligand behaves as a chemically 'non-innocent' co-operative ligand.

On the Analysis of the Cr−Cr Multiple Bond in Several Classes of Dichromium Compounds

Since the discovery of a formal quintuple bond in Ar'CrCrAr' (CrCr = 1.835 A) by Power and co-workers in 2005, many efforts have been dedicated to isolating dichromium species featuring quintuple-bond character. In the present study we investigate the electronic configuration of several, recently synthesized dichromium species with ligands using nitrogen to coordinate the metal centers. The bimetallic bond distances of Power's compound and Cr(2)-diazadiene (1) (CrCr = 1.803 A) are compared to those found for Cr(2)(mu-eta(2)-ArNC(R)NAr)(2) (2) (CrCr = 1.746 A; R = H, Ar = 2,6-Et(2)C(6)H(3)), Cr(2)(mu-eta(2)-Ar(Xyl)NC(H)NAr(Xyl))(3) (3) (CrCr = 1.740(reduced)/1.817(neutral) A; Ar(Xyl)= 2,6-C(6)H(3)-(CH(3))(2)), Cr(2)(mu-eta(2)-TippPyNMes)(2) (4) (CrCr = 1.749 A; TippPyNMes = 6-(2,4,6-triisopropylphenyl)pyridin-2-yl (2,4,6-trimethylphenyl)amide), and Cr(2)(mu-eta(2)-DippNC(NMe(2))N-Dipp)(2) (5) (CrCr = 1.729 A, Dipp = 2,6-i-Pr(2)C(6)H(3)). We show that the correlation between the CrCr bond length and the effective bond order (EBO) is strongly affected by the nature of the ligand, as well as by the steric hindrance due to the ligand structure (e.g., the nature of the coordinating nitrogen). A linear correlation between the EBO and CrCr bond distance is established within the same group of ligands. As a result, the CrCr species based on the amidinate, aminopyridinate, and guanidinate ligands have bond patterns similar to the Ar'CrCrAr' compound. Unlike these latter species, the dichromium diazadiene complex is characterized by a different bonding pattern involving Cr-Npi interactions, resulting in a lower bond order associated with the short metal-metal bond distance. In this case the short CrCr distance is most probably the result of the constraints imposed by the diazadiene ligand, implying a Cr(2)N(4) core with a closer CrCr interaction.

Tantalum−Benzylidene Complexes Supported by C5Me5 and Diazadiene Ligands: Synthesis, Kinetic Analysis of the Formation, and Reactive Studies

Reaction of half-metallocene dichloro complexes Cp*TaCl2(η4-supine-Ar-DAD) (1a: Ar = p-MeO; 1b: Ar = p-Me; 1c: Ar = p-F; 1d: Ar = p-Cl) (Cp* = pentamethylcyclopentadienyl; Ar-DAD = 1,4-di(aryl)-1,4...

Multiple Bonding in the Chromium Dimer Supported by Two Diazadiene Ligands

Analysis of the topology and delocalization of the electron density with the quantum theory of atoms in molecules (QTAIM) shows that the Cr(2) dimer when supported by two diazadiene ligands has a degree of bonding of 3.6--slightly less than that of a formal quadruple bond. The metal-ligand complex is held together by relatively weak dative bonds. Out-of-plane lone-pair-like charge concentrations on the nitrogen atoms are oriented so as to maximally avoid areas of charge concentration in the distorted inner-valence shell of the chromium atoms.

The Shortest Metal−Metal Bond Yet: Molecular and Electronic Structure of a Dinuclear Chromium Diazadiene Complex

Reduction of a diazadiene chromium halide complex, [(HLiPr)Cr(μ-Cl)]2 (1, HLiPr = bis(2,6-diisopropylphenyl)diazadiene), with KC8 gave a diamagnetic, bimetallic complex, (HLiPr)2Cr2 (2). Complex 2 has been structurally characterized by X-ray crystallography and consists of a Cr2 unit spanned by two diazadiene ligands. The very short Cr−Cr distance (1.8028(9) A) and low formal oxidation state of the Cr atoms suggest that the Cr−Cr bond order may be greater than 4. Spin-restricted and spin-unrestricted DFT calculations on a model complex both converge on a singlet spin state and accurately reproduce the metric parameters of 2. The calculations show high-order metal−metal bonding along with extensive delocalization over the diazadiene ligands. Natural resonance theory analysis shows population of resonance structures that contain quintuple bonds, and natural bond order analysis gives a bond order of 4.28 for the Cr−Cr bond.

Preparation and structural characterization of tetracoordinated tungsten(II) diazadiene complexes

Reduction in situ of WCl 4(MeCN) 2 with zinc in the presence of diazadiene ligands 1b– f is reported and produces a series of diamagnetic complexes (DAD)WCl 2 2b– f. All complexes were characterized by 1H, 13C NMR and IR spectroscopic data. The crystal structures of complexes 2b and 2e were solved by X-ray diffraction methods.

Reactions of ytterbium(II) bis(indenyl) complex (C9H7)2Yb(thf)2 with 2,2’-bipyridine and 1,4-bis(2,6-diisopropylphenyl)-1,4-diazabuta-1,3-diene. Structures and properties of (C9H7)2Yb(bipy) and (C9H7)2Yb(2,6-Pri 2C6H3NCHCHNC6H3Pri 2-2,6) complexes

The reaction of the ytterbium(II) bis(indenyl) complex (C9H7)2Yb(thf)2 (1) with 2,2’-bipyridine afforded the diamagnetic (C9H7)2Yb(bipy) compound (2), whose structure was established by X-ray diffraction analysis. Under similar conditions, the reaction of complex 1 with 1,4-bis(2,6-diisopropylphenyl)-1,4-diazabuta-1,3-diene (DAD) led to oxidation of ytterbium giving rise to the paramagnetic (C9H7)2Yb(DAD) complex (3). Magnetic measurements, X-ray diffraction study, and 1H NMR spectroscopy in benzene confirmed the trivalent state of the ytterbium atom and the radical-anionic nature of the diazadiene ligand in complex 3. In the complex 3—solvent system, the oxidation state of metal depends on the coordination ability of the solvent. In benzene, complex 3 exists as (C9H7)2YbIII(DAD·-), whereas (C9H7)2YbII(thf)2 and DAD0 are present in THF.

Controlled radical polymerization of alkyl acrylates and styrene using a half-sandwich molybdenum(III) complex containing diazadiene ligands

The half-sandwich molybdenum(III) complex CpMoCl 2( i Pr 2-dad) ( i Pr 2-dad= i Pr–NCH–CHN– i Pr) proved to be an effective metal catalyst for the controlled radical polymerization of methyl acrylate, butyl acrylate, and styrene. In conjunction with an alkyl iodide [R–I: CH 3CH(COOEt)I] as an initiator and in the presence or absence of Al(O– i-Pr) 3 as a co-catalyst, the molybdenum-based system gave polymers with narrow molecular weight distributions. The in situ addition of styrene to a macroinitiator of poly(methylacrylate) afforded an AB-type block copolymer.

Half-sandwich molybdenum(III) compounds containing diazadiene ligands and their use in the controlled radical polymerization of styrene

The reaction of CpMoCl 2 with diazadiene ligands RNCHCHNR (R 2dad) affords the corresponding paramagnetic complexes CpMoCl 2(R 2dad) (R=Ph, 1; p-Tol, 2; C 6H 3Pr 2 i -2,6, 3; and Pr i , 4). All compounds have been characterized by EPR spectroscopy and have been investigated by cyclic voltammetry. They display one-electron oxidation and reduction processes, these being reversible or irreversible depending on the nature of R. The irreversibility of the reduction wave is due to a chemical follow-up process which consists of chloride loss from the reduced product. This phenomenon is suppressed in the presence of excess chloride in solution. An X-ray structure of 3 verifies the mononuclear nature of the compound and the chelating mode of the dad ligand, the MoN 2C 2 ring being essentially planar. The NC and CC bond distance pattern suggests the important contribution of an enediamido Mo(V) limiting form. In the presence of 1-bromoethylbenzene, complexes 1– 4 catalyze the controlled/‘living’ radical polymerization of styrene. Complex 3 also leads to a controlled/‘living’ radical polymerization of styrene in the presence of AIBN (α,α-azoisobutyronitrile) as a radical generator. Therefore, this is the second example of a compound which is capable of controlling the styrene radical polymerization under both ATRP and SFRP conditions.

Synthesis of new (σ 2- N, N′-diazadiene)(η 2-alkene)platinum(0) compounds

Novel, thermally stable, dark-red to orange Pt 0(σ 2- N, N′-diazadiene)(η 2-alkene) compounds have been synthesized in good yields from Pt 0(COD) 2 or Pt 0(NBE) 3, by stepwise substitution of the respective dienes or alkenes by an electron-poor alkene (dimethyl fumarate, maleic anhydride or fumaronitrile), followed by the appropriate diazadiene ligand in dry diethyl ether at 20 °C (diazadiene=various N, N′-disubstitued-1,4-diaza-1,3-dienes). The complex Pt(DBA) 2 is less suited as a precursor for the synthesis of Pt 0(σ 2- N, N′-diazadiene)(η 2-alkene) compounds. These zerovalent Pt(diazadiene)(η 2-alkene) compounds constitute a useful category of starting materials for synthetic organoplatinum chemistry and catalysis.

Half-Metallocene Tantalum Complexes Bearing Methyl Methacrylate (MMA) and 1,4-Diaza-1,3-diene Ligands as MMA Polymerization Catalysts.

Activation with AlMe3 at low temperature converts half-metallocene complexes of tantalum with methyl methacrylate (MMA) and diazadiene ligands such as [Cp*Ta(η2 -Cy-DAD)(η4 -MMA)] (structure shown) into effective catalysts for the polymerization of MMA. Polymerization is complete after 10 min at -20°C and gives a poly(methyl methacrylate) of narrow polydispersity. Cp*=η5 -C5 H5 , Cy-DAD=1,4-dicyclohexyl-1,4-diaza-1,3-butadiene.

Ni(II) and Pd(II) complexes of camphor-derived diazadiene ligands: steric bulk tuning and ethylene polymerization

Nickel(II) and palladium(II) centers were coordinated to a series of chiral 1,4-diazadiene ligands. The ligand backbones are camphor derivatives and the imine nitrogens are attached to independently varied 2- and 2,6-substituted aryl groups. Upon activation with methyl-aluminoxane, the dibromo nickel complexes polymerize ethylene and 1-hexene. The polymerization properties are dependent on the steric features of the aryl substituents on the imine nitrogens.

Cyclopentadienyl Complexes of Yttrium Aminotroponiminates: An Example for an Improved Alternative Ligand System

Mixed aminotroponiminate ([ATI]−) cyclopentadienyl complexes were prepared using two different synthetic routes. Reaction of [[(iPr)2ATI]YCl2(THF)2]2 (1) with KCp* (Cp* = C5Me5) or Li[C5H3(SiMe3)2] affords [(iPr)2ATI]YCp2* (2a) and [(iPr)2ATI]Y[C5H3(SiMe3)2]2 (2b), respectively. Moreover, [(iPr)2ATI]YCp2* has also been obtained from the reaction of Cp2*Y(μ-Cl)2Li(OEt2)2 (3) and [(iPr)2ATI]K. The single crystal X-ray structure of [(iPr)2ATI]YCp2* shows that the steric demand of the [(iPr)2ATI]− ligand is somewhat similar to that of the well-known cyclopentadienyl group and diazadiene ligand. [(iPr)2ATI]Y[C5H3(SiMe3)2]2 shows a dynamic behavior in solution (1H NMR), which is essentially caused by rotation of the cyclopentadienyl group rather than by migration of the [(iPr)2ATI]− ligand.

Cyclopentadienyl Complexes of Yttrium Aminotroponiminates: An Example for an Improved Alternative Ligand System

Mixed aminotroponiminate ([ATI]−) cyclopentadienyl complexes were prepared using two different synthetic routes. Reaction of [[(iPr)2ATI]YCl2(THF)2]2 (1) with KCp* (Cp* = C5Me5) or Li[C5H3(SiMe3)2] affords [(iPr)2ATI]YCp2* (2a) and [(iPr)2ATI]Y[C5H3(SiMe3)2]2 (2b), respectively. Moreover, [(iPr)2ATI]YCp2* has also been obtained from the reaction of Cp2*Y(μ-Cl)2Li(OEt2)2 (3) and [(iPr)2ATI]K. The single crystal X-ray structure of [(iPr)2ATI]YCp2* shows that the steric demand of the [(iPr)2ATI]− ligand is somewhat similar to that of the well-known cyclopentadienyl group and diazadiene ligand. [(iPr)2ATI]Y[C5H3(SiMe3)2]2 shows a dynamic behavior in solution (1H NMR), which is essentially caused by rotation of the cyclopentadienyl group rather than by migration of the [(iPr)2ATI]− ligand.

Synthese und Kristallstrukturen von Diazadien-Komplexen der Erdalkalimetalle / Syntheses and Crystal Structures of Diazadiene Complexes of the Alkaline Earth Metals

(DME)2] 1 (R = C6H4-4-CH3; DME = dimethoxyethane) was prepared by reaction of calcium with NR=CPh-CPh=NR in DME solution. The compound forms orange, moisture sensitive crystals, which were characterized by an X-ray structure determination [space group orthorhombic, P212121, Z = 4, 4634 observed unique reflections, R = 0.046; lattice dimensions at –70 °C: a = 1340.2(3), b = 1528.1(3), c = 1609.1(3) pm]. The calcium atom is coordinated by the four oxygen atoms of two chelating DME molecules and two nitrogen atoms of the diazadiene ligand, bonded in its enediamide form. [Ba2(DME)3(NPh–CPh=CPh–NPh)2 · DME] 3 was obtained from barium metal and NPh=CPh-CPh=NPh in DME solution as red crystals [space group monoclinic, P2l/c, Z = 4, 4000 observed unique reflections, R = 0.166; lattice dimensions at -70 °C: a = 1704.5(3), b = 1786.1(4), c = 2177.4(4) pm, β = 105.98(3)°]. The two barium atoms are bridged by two differently bonded diazadiene ligands (μ2-Ν,Ν′;μ-Ν, σ-Ν′). Additionally, one of the barium atoms is coordinated to two DME molecules and the other one to only one of the ether molecules. A further DME molecule is a constituent of the crystal lattice.

Synthesis of bis(cyclopentadienyl)diazadiene complexes of ytterbium. An X-ray structural study of the Cp2Yb(?-?2:Ti2-But-N=CH-CH=N-But)Li(DME) complex

Ytterbium complexes, Cp2Yb(μ-η2:η2-DAB)Li(DME) (1) and CpYb(DAB)K(THF)2 (2), containing a bridging diazadiene ligand were prepared by the reaction of CpYbCl2(THF)3 with (DAB)Li2 (1∶1) and (DAB)−K+ (1∶2) (DAB = But-N=CH-CH=N-But). The structure of complex1 was established by X-ray diffraction analysis. The complex is binuclear: the Yb atom of the Cp2Yb fragment and the Li atom, which is bonded as well with the chelating DME molecule, are linked by the DAB ligand.

Electronic structure and photoelectron spectroscopy of Al(Me3CNCHCHNCMe3)2 and Ga(Me3CNCHCHNCMe3)2

The He I and He II spectra of M(Me3CNCHCHNCMe3)2(M = Al or Ga) have been recorded. A complex band structure arising from ionization of the π3 levels of the diazadiene ligands can be understood by reference to the structural distortion from idealized D2d symmetry observed in the ground state. Other higher-lying bands are assigned to π2 and n+ and n– ionizations. The electronic structure of the neutral molecules is discussed and the differences in bond lengths between the rings is attributed to a Jahn–Teller distortion that traps two electrons in the π3 orbital of one ring and one in the π3 orbital of the other.

Mononuclear Bis(diazadiene)ruthenium(0) Complexes and the Coupling of Butadiene to a Diazadiene System

Complexes (dad)2RuCl2(1) (dad = diazadiene, RN = CR'– CR'=NR) are reduced to (dad)2Ru (2) which react with L = CO or ethylene to give (dad)2RuL (3,5). Reduction of1b( R' = H) with magnesium–butadiene or reaction of2bwith butadiene leads to a new type of C–C coupling of butadiene to one of the diazadiene ligands (6b). Addition of CO induces a rearrangement to7bby a hydride shift.

Diazadienes as control ligands in homogeneous catalysis: Part XV. Diazadiene-iron(0) catalyzed synthesis of tetracyclononenes from norbornadiene and internal alkynes

Iron complexes (DAD)FeCl 2 with diazadiene ligands RNCR'-CR'NR (DAD) activated with butadiene magnesium or other main group organometallics catalyze the homo-Diels-Alder reaction of norbornadiene with internal alkynes to give 7,8-disubstituted tetracyclo[4.3.0.0 4,9.0 3,5]-7-nonenes. Terminal alkynes are polymerized by this iron catalyst, but they can be introduced in the reaction in the form of their silylated derivatives. Thus the reaction is successfully performed with 3-hexyne, 1-phenyl-2-trimethylsilylacetylene and 1-trimethylsilyl-1-hexyne. The products are characterized by NMR methods.