71 research outputs found
Synthesis, Electronic Structure, and Reactivity of a Planar Four‐Coordinate, Cobalt–Imido Complex
No full textA four-coordinate cobalt-imido complex, ((tBu)mPNP)Co=NMes ((tBu)mPNP=modified PNP pincer ligand) has been synthesized from addition of 2,4,6-trimethylphenylazide (Mes-N-3) to the corresponding dinitrogen complex. The solid-state structure determined by X-ray diffraction established a rare, idealized planar geometry with a Co=N bond distance of 1.716(2) angstrom. Magnetic measurements revealed an S=1 ground state with CAS-SCF calculations supporting radical character on the imide nitrogen. Thermolysis of the cobalt-imido compound induced selective insertion of the imido group into a Co-P bond and yielded a three-coordinate cobalt complex with a distorted T-shaped geometry. Transition state analysis conducted with DFT calculations established the thermodynamic stability of the P-N coupled product and provided insight into the exclusive selectivity.
LIGAND INDUCED DINITROGEN CLEAVAGE AND FUNCTIONALIZATION BY GROUP FOUR METALLOCENES
No full textThe reactivity of the dihafnocene dinitrogen compound towards a variety of E-H (E = Si, B, C) bonds was explored. For E = Si or B, 1,2-addition of the bond across the dinitrogen fragment was observed giving hafnium diazenido hydride complexes which underwent further ligand-induced N-N bond cleavage. For E = C, addition of nitrogen heterocycles with sp2 C-H bonds leads to precoordination of the substrate to one metal center followed by addition across the dinitrogen unit to give hafnium diazenido alkyl complexes.
The 1,2,4-trimethylcyclopentadienyl hafnium platform was investigated in ligand-induced N2 cleavage reactions, providing a unique example of a dihafnium nitride following CO-induced N2 cleavage, whose electronic structure was explored computationally. Additional N-C and C-C bond forming reactions were investigated by addition of excess carbon monoxide, affording hafnium "oxamidide" compounds. The smaller steric profile of the cyclopentadienyl ligand allowed access to unique tetrametallic clusters. This N2 chemistry was extended to isocyanides, providing hafnium carbodiimidyl isocyanide complexes and mixed carbodiimidyl isocyanate complexes which proved competent for additional C-C coupling reactions. The divergent reactivity of isocyanides versus carbon monoxide for ligand-induced N2 cleavage in other systems was also explored.
The chemistry of the CO and N2 derived base-free dihafnium nitride toward a number of small molecule substrates was investigated. Activated alkynes, mono-substituted allenes, and heterocummulenes underwent cycloaddition to forge new N-C bonds, reactivity reminiscent of early metal-imido complexes. Nitriles underwent preferential insertion into the Hf-N bond rather than cycloaddition and could be used to enable cyclization cascades which produced complex small molecule architectures from N2 and CO. N-C bond formation could also be enabled by addition of alkyl triflates and chlorosilanes; these triggered cascade chemistry involving the pendent terminal isocyanate ligand, the mechanism of which was elucidated by deuterium labeling experiments. The base-free nitrido complex was also promiscuous in C-H bond activation chemistry; reaction with terminal alkynes, bulky allenes, dihydrogen and carbonyl containing substrates afforded μ2-NH fragments and the corresponding Hf-X species, consistent with the potent basicity of the nitrido fragment.
The electronic structure of the anionic ansa-zirconocene dinitrogen complex bearing an adamantyl substituted ligand was explored by X-ray crystallography, EPR spectroscopy, isotope labeling experiments, and DFT calculations. The complex is best described as a resonance between Zr(III) and Zr(IV) valence states with rare examples of [N2]1- and [N2]3- ligands. The similar spectroscopic signatures between this complex and the purported monomeric, side-on bound zirconium N2 complex reported by Lappert prompted reinvestigation of this species. Single crystal X-ray diffraction revealed that it was in fact the end-on anionic zirconium dinitrogen complex. The ability of both of these complexes to undergo redox-induced N2 hapticity changes was explored.
The dinitrogen chemistry of titanocene complexes bearing variably disubstituted cyclopentadienyl ligands was explored. Variation of the size and distribution of the Cp substituents allowed the isolation of rare examples of side-on bound titanocene dinitrogen complexes, as well as a unique trimetallic cluster. The electronic structures of these complexes were investigated by magnetic and spectroscopic measurements as well as DFT calculations.
Preliminary investigations into the chemistry of cobalt complexes supported by tridentate, meridionally coordinating bis(phosphino)pyridine pincer ligands are additionally described
Unprecedented Reactivity Initiated by Insertion of 2,6-Xylylisonitrile into the W−Alkyl Linkages of Cp*W(NO)(<i>n</i>-alkyl)(η<sup>3</sup>-CH<sub>2</sub>CHCHMe) Complexes
No full textTreatment of the compounds Cp*W(NO)(R)(η3-CH2CHCHMe) (R = n-C5H11, n-C7H15, n-C8H17) with 2,6-xylylisonitrile first produces the expected complexes bearing η2-iminoacyl ligands arising from migratory insertion of the isonitrile into the tungsten−alkyl linkages, processes that result in the allyl ligands undergoing concomitant η3 → η1 haptotropic shifts. These η1-allyl complexes then undergo subsequent intramolecular nucleophilic attacks by their allyl groups on the nitrogen atoms of the η2-iminoacyl ligands to form novel metallacyclic compounds that contain α-aminocarbene ligands
Unprecedented Reactivity Initiated by Insertion of 2,6-Xylylisonitrile into the W−Alkyl Linkages of Cp*W(NO)(<i>n</i>-alkyl)(η<sup>3</sup>-CH<sub>2</sub>CHCHMe) Complexes
No full textTreatment of the compounds Cp*W(NO)(R)(η3-CH2CHCHMe) (R = n-C5H11, n-C7H15, n-C8H17) with 2,6-xylylisonitrile first produces the expected complexes bearing η2-iminoacyl ligands arising from migratory insertion of the isonitrile into the tungsten−alkyl linkages, processes that result in the allyl ligands undergoing concomitant η3 → η1 haptotropic shifts. These η1-allyl complexes then undergo subsequent intramolecular nucleophilic attacks by their allyl groups on the nitrogen atoms of the η2-iminoacyl ligands to form novel metallacyclic compounds that contain α-aminocarbene ligands
Recommended from our members
N-H and N-C Bond Formation with an N-2-Derived Dihafnium mu-Nitrido Complex
No full textExposure of the base-free isocyanato dihafnocene μ-nitrido complex prepared from CO-induced N2 cleavage to a dihydrogen atmosphere resulted in rapid 1,2-addition across the hafnium–nitrogen bond followed by insertion of the terminal isocyanate ligand into the putative hafnium hydride ligand and formed a bridging formamide ligand. Terminal alkynes and sterically hindered allenes underwent preferential addition of a C–H bond across the hafnium nitride fragment and resulted in isolation of the μ-imido acetylide and allenyl dihafnocene complexes, respectively. Reducing the steric profile of the allene enabled N–C rather than N–H bond-forming chemistry arising from cycloaddition of the π system. In the presence of additional allene, the resulting azahafnacyclobutanes underwent exchange, establishing the reversibility of the N–C bond forming reaction. Ketones with enolizable hydrogens, amines, and guanidines underwent rapid deprotonation upon addition to the isocyanato dihafnocene μ-nitrido complex and offer a route to N–H bond formation, as well as allowing isolation of a rare example of a parent amido compound. The preference of the dihafnium nitrido system for N–H over N–C bond formation was explored by treatment with styrene oxide, which afforded exclusively the E2 elimination product rather than the expected 1,2-amino alkoxide complex
Activation of Dinitrogen‐Derived Hafnium Nitrides for Nucleophilic NC Bond Formation with a Terminal Isocyanate
No full text(PNP)Cobalt-Catalyzed Olefination of Diazoalkanes
No full textAddition
of excess diazoalkane to the pincer-supported cobalt(I)
dinitrogen complex (tBumPNP)CoN2 (tBumPNP = modified 2,6-bis[(ditert-butylphosphino)methyl]pyridine) resulted in the catalytic
formation of the homocoupled alkene product with concomitant loss
of N2. Monosubstituted diazoalkanes, trimethylsilyldiazomethane
and tolyldiazomethane, generated the olefin product in quantitative
yield with exclusive (E)-stereoselectivity. Disubstituted
diazoalkanes, diphenyldiazomethane and 9-diazofluorene, also yielded
the olefin as the major product along with minor azine coupling. Investigations
into the nature of the diazoalkane–cobalt interaction by multinuclear
NMR spectroscopy and X-ray diffraction established end-on diazoalkane
cobalt complexes as the resting states. The isolated four-coordinate
cobalt diazoalkane complexes promoted conversion to the corresponding
olefin. The reaction of (tBumPNP)CoN2 with an α-diazo-β-ketoester
resulted in the formation of a five-coordinate Co(I)-diazoalkane complex
with a chelating ester unit that was unreactive for olefination
Synthesis of a Base-Free Hafnium Nitride from N<sub>2</sub> Cleavage: A Versatile Platform for Dinitrogen Functionalization
No full textThe
synthesis and characterization of a metastable, base-free isocyanato
dihafnocene μ-nitrido complex from CO-induced dinitrogen cleavage
is described. The open coordination site at hafnium suggested the
possibility of functionalization of the nitrogen atom by cycloaddition
and insertion chemistry. Addition of the strained, activated alkyne,
cyclooctyne, resulted in N–C bond formation by cycloaddition.
The alkyne product is kinetically unstable engaging the terminal hafnocene
isocyanate and promoting deoxygenation and additional N–C bond
formation resulting in a substituted cyanamide ligand. Group transfer
between hafnium centers was observed upon treatment with Me3SiCl resulting in bridging carbodiimidyl ligands. Amidinato-type
ligands, [NC(R)N]3– were prepared by addition of
either cyclohexyl or isobutyronitrile to the base free dihafnocene
μ-nitrido complex, which also engages in additional N–C
bond formation with the terminal isocyanate to form bridging ureate-type
ligands. Heterocummulenes also proved reactive as exposure of the
nitride complex to CO2 resulted in deoxygenation and N–C
bond formation to form isocyanate ligands. With substituted isocyanates,
cycloaddition to the dihafnocene μ-nitrido was observed forming
ureate ligands, which upon thermolysis isomerize to bridging carbodiimides.
Taken together, these results establish the base free dihafnocene
μ-nitrido as a versatile platform to synthesize organic molecules
from N2 and carbon monoxide
- …
