Stabilization of a Two‐Coordinate Mononuclear Cobalt(0) Compound

Compound (Me₂‐cAAC:)₂Co⁰ ( 2 ; Me₂‐cAAC:=cyclic (alkyl) amino carbene; :C(CH₂)(CMe₂)₂N‐2,6‐iPr₂C₆H₃) was synthesized by the reduction of the precursor (Me₂‐cAAC:)₂Co^ICl ( 1 ) with KC₈ in THF. The cyclic voltammogram of 1 exhibited one‐electron reduction, which suggests that synthesis of a bent 2‐metallaallene ( 2 ) from 1 should be possible. Compound 2 contains one cobalt atom in the formal oxidation state zero, which is stabilized by two Me₂‐cAAC: ligands. Bond lengths from X‐ray diffraction are 1.871(2) and 1.877(2) Å with a C‐Co‐C bond angle of 170.12(8)°. The EPR spectrum of 2 exhibited a broad resonance attributed to the unique quasi‐linear structure, which favors near degeneracy and gives rise to very rapid relaxation conditions. The cAAC?Co bond in 2 can be considered as a typical Dewar–Chatt–Duncanson type of bonding, which in turn retains 2.5 electron pairs on the Co atom as nonbonding electrons.     

Synthesis and Characterization of Tris(oxazolinyl)borato Copper(II) and Copper(I) Complexes

The reaction of To^MTl (To^M=tris(4,4-dimethyl-2-oxazolinyl)phenylborate) and CuBr₂ in benzene at 60 °C provides To^MCuBr ( 1 ) as an entry-point into tris(oxazolinyl)phenylborato copper chemistry. To^MCuO^tBu ( 2 ) and To^MCuOAc ( 3 ) are prepared by the reactions of To^MCuBr with KO^tBu and NaOAc, respectively. To^MCuO^tBu is transformed into (To^MCuOH)₂ ( 4 ) through hydrolysis. NMR, FT-IR, and EPR spectroscopies are used to determine the electronic and structural properties of these copper(II) compounds, and the solid-state structures were characterized by X-ray crystallography. Reduction of copper is observed upon treatment of To^MCuO^tBu with phenylsilane in an attempt to synthesize monomeric copper(II) hydride. To^MCu ( 5 ) and To^M₂Cu ( 6 ) were independently synthesized and characterized for comparison.     

A Ferromagnetically Coupled (S=1) Peroxodicopper(II) Complex

Copper enzymes play important roles in the binding and activation of dioxygen in biological systems. Key copper/dioxygen intermediates have been identified and studied in synthetic analogues of the metalloprotein active sites, including the μ‐η²:η²‐peroxodicopper(II) motif relevant to type III dicopper proteins. Herein, we report the synthesis and characterization of a bioinspired dicopper system that forms a stable μ‐η¹:η¹‐peroxo complex whose Cu‐O‐O‐Cu torsion is constrained to around 90° by ligand design. This results in sizeable ferromagnetic coupling between the copper(II) ions, which is detected by magnetic measurements and HF‐EPR spectroscopy. The new dicopper peroxo system is the first with a triplet ground state, and it represents a snapshot of the initial stages of O₂ binding at type III dicopper sites.     

Copper(II) Complexes of a Hexadentate Mixed‐Donor N3S3 Macrobicyclic Cage: Facile Rearrangements and Interconversions

The potentially hexadentate mixed‐donor cage ligand 1‐methyl‐8‐amino‐3,13,16‐trithia‐6,10,19‐triazabicyclo[6.6.6]eicosane (AMME‐N₃S₃sar; sar=sarcophagine) displays variable coordination modes in a complex with copper(II). In the absence of coordinating anions, the ligand adopts a conventional hexadentate N₃S₃ binding mode in the complex [Cu(AMME‐N₃S₃sar)](ClO₄)₂ that is typical of cage ligands. This structure was determined by X‐ray crystallography and solution spectroscopy (EPR and NIR UV/Vis). However, in the presence of bromide ions in DMSO, clean conversion to a five‐coordinate bromido complex [Cu(AMME‐N₃S₃sar)Br]⁺ is observed that features a novel tetradentate (N₂S₂)‐coordinated form of the cage ligand. This copper(II) complex has also been characterized by X‐ray crystallography and solution spectroscopy. The mechanism of the reversible interconversion between the six‐ and five‐coordinated copper(II) complexes has been studied and the reaction has been resolved into two steps; the rate of the first is linearly dependent on bromide ion concentration and the second is bromide independent. Electrochemistry of both [Cu(AMME‐N₃S₃sar)]²⁺ and [Cu(AMME‐N₃S₃sar)Br]⁺ in DMSO shows that upon reduction to the monovalent state, they share a common five‐coordinated form in which the ligand is bound to copper in a tetradentate form exclusively, regardless of whether a six‐ or five‐coordinated copper(II) complex is the precursor.     

Key Intermediate Species Reveal the Copper(II)‐Exchange Pathway in Biorelevant ATCUN/NTS Complexes

The amino‐terminal copper and nickel/N‐terminal site (ATCUN/NTS) present in proteins and bioactive peptides exhibits high affinity towards Cu^II ions and have been implicated in human copper physiology. Little is known, however, about the rate and exact mechanism of formation of such complexes. We used the stopped‐flow and microsecond freeze‐hyperquenching (MHQ) techniques supported by steady‐state spectroscopic and electrochemical data to demonstrate the formation of partially coordinated intermediate Cu^II complexes formed by glycyl–glycyl–histidine (GGH) peptide, the simplest ATCUN/NTS model. One of these novel intermediates, characterized by two‐nitrogen coordination, t_1/2≈100 ms at pH 6.0 and the ability to maintain the Cu^II/Cu^I redox pair is the best candidate for the long‐sought reactive species in extracellular copper transport.     

Elucidating the Structural Chemistry of a Hysteretic Iron(II) Spin-Crossover Compound From its Copper(II) and Zinc(II) Congeners

Annealing [FeL₂][BF₄]₂ ⋅ 2 H₂O (L=2,6-bis-[5-methyl-1H-pyrazol-3-yl]pyridine) affords an anhydrous material, which undergoes a spin transition at T_1/2=205 K with a 65 K thermal hysteresis loop. This occurs through a sequence of phase changes, which were monitored by powder diffraction in an earlier study. [CuL₂][BF₄]₂ ⋅ 2 H₂O and [ZnL₂][BF₄]₂ ⋅ 2 H₂O are not perfectly isostructural but, unlike the iron compound, they undergo single-crystal-to-single-crystal dehydration upon annealing. All the annealed compounds initially adopt the same tetragonal phase but undergo a phase change near room temperature upon re-cooling. The low-temperature phase of [CuL₂][BF₄]₂ involves ordering of its Jahn–Teller distortion, to a monoclinic lattice with three unique cation sites. The zinc compound adopts a different, triclinic low-temperature phase with significant twisting of its coordination sphere, which unexpectedly becomes more pronounced as the crystal is cooled. Synchrotron powder diffraction data confirm that the structural changes in the anhydrous zinc complex are reproduced in the high-spin iron compound, before the onset of spin-crossover. This will contribute to the wide hysteresis in the spin transition of the iron complex. EPR spectra of copper-doped [Fe_0.97Cu_0.03L₂][BF₄]₂ imply its low-spin phase contains two distinct cation environments in a 2:1 ratio.     

The Instability of Ni{N(SiMe3)2}2: A Fifty Year Old Transition Metal Silylamide Mystery

The characterization of the unstable Ni^II bis(silylamide) Ni{N(SiMe₃)₂}₂ ( 1 ), its THF complex Ni{N(SiMe₃)₂}₂(THF) ( 2 ), and the stable bis(pyridine) derivative trans‐Ni{N(SiMe₃)₂}₂(py)₂ ( 3 ), is described. Both 1 and 2 decompose at ca. 25 °C to a tetrameric Ni^I species, [Ni{N(SiMe₃)₂}]₄ ( 4 ), also obtainable from LiN(SiMe₃)₂ and NiCl₂(DME). Experimental and computational data indicate that the instability of 1 is likely due to ease of reduction of Ni^II to Ni^I and the stabilization of 4 through dispersion forces.     

Two‐Coordinate Copper(I)/NHC Complexes: Dual Emission Properties and Ultralong Room‐Temperature Phosphorescence

As a kind of photoluminescent material, Cu^I complexes have many advantages such as adjustable emission, variable structures, and low cost, attracting attention in many fields. In this work, two novel two‐coordinate Cu^I‐N‐heterocyclic carbene complexes were synthesized, and they exhibit unique dual emission properties, fluorescence and phosphorescence. The crystal structure, packing mode, and photophysical properties under different conditions were systematically studied, proving the emissive mechanism to be the locally excited state of the carbazole group. Based on this mechanism, ultralong room‐temperature phosphorescence (RTP) with a lifetime of 140 ms is achieved by selective deuteration of the carbazole group. These results deepen the understanding of the luminescence mechanism and design strategy for two‐coordinate Cu^I complexes, and prove their potential in applications as ultralong RTP materials.     

A Stable Organic π‐Radical of a Zinc(II)–Copper(I)–Zinc(II) Complex of Decaphyrin

A Zn^II‐Cu^I‐Zn^II heterotrimetal complex of decaphyrin was synthesized by stepwise metalations: metalation of a [46]decaphyrin with Zn^II ions to produce a 46π decaphyrin bis(Zn^II) complex and its subsequent metalation with Cu^II ion. In the second metalation step, it has been shown that Cu^II ion is reduced to a Cu^I ion in the complex and a dianionic bis(Zn^II) containing [46]decaphyrin ligand is oxidized to the corresponding monoanionic [45]decaphyrin ligand, indicating a non‐innocent nature of the decaphyrin ligand. Despite the radical nature, the heterotrimetal complex is fairly stable under ambient conditions and exhibits almost no intermolecular magnetic interaction, owing to extensive delocalization of an unpaired electron in the large π‐conjugated circuit of decaphyrin moiety.     

Tetrakis(1‐adamantylcarboxylato)dikupfer(II) Cu2(1‐Ad)4 – Synthese,Struktur und X‐/Q‐Band‐EPR‐Untersuchungen

Tetrakis(1‐adamantylcarboxylato)dicopper(II) Cu₂(1‐Ad)₄ – Synthesis, Structure and X‐/Q‐band EPR Investigations The synthesis and the crystal structure of tetrakis(1‐adamantylcarboxylato)dicopper(II) are reported. [Cu₂(1‐Ad)₄·2DMF] ( 1 , 1‐Ad = adamantylcarboxylate) crystallizes in the space group (Z = 2) with two crystallographically distinguishable complexes in the unit cell. The averaged Cu‐Cu distance of 260.5 pm is smaller than that found for Cu₂(ac)₄·2H₂O. The combination of temperature‐dependent X‐ and Q‐band powder EPR investigations in the temperature range 6 ≤ T ≤ 295 K show the presence of an antiferromagnetically coupled Cu‐Cu dimer and allow a precise determination of the spin‐Hamiltonian parameter. A comparison of those with that derived for Cu₂(ac)₄·2H₂O indicate a higher symmetry within the Cu₂O₈ central unit of [Cu₂(1‐Ad)₄·2DMF].     

Cis‐trans Isomerism in Copper(II) Complexes with N‐acyl Thiourea Ligands

The N‐acyl thiourea complexes bis[N,N‐diethyl‐N′‐(p‐nitrobenzoyl)‐thioureato]copper(II) ( 1a,1b ) and bis(N,N‐diphenyl‐N′‐benzoylthioureato)copper(II) ( 2a,2b ) crystallize in each case in two modifications. X‐ray structural analysis shows that 1a and 1b are cis‐trans isomers. This is very unusual for N‐acyl thioureato complexes because with exception of one platinum(II) complex up to now only cis complexes have been found. In contrast X‐ray structural analysis of both forms 2a and 2b of the other complex shows no cis‐trans pair. Both modifications are cis complexes. In solution both isomers of the copper(II) complexes are observable by EPR spectroscopy.     

A Two‐Coordinate Cobalt(II) Imido Complex with NHC Ligation: Synthesis,Structure, and Reactivity

The synthesis, structural characterization, and reactivity of the first two‐coordinate cobalt complex featuring a metal–element multiple bond [(IPr)Co(NDmp)] ( 4 ; IPr=1,3‐bis(2′,6′‐diisopropylphenyl)imidazole‐2‐ylidene; Dmp=2,6‐dimesitylphenyl) is reported. Complex 4 was prepared from the reaction of [(IPr)Co(η²‐vtms)₂] (vtms=vinyltrimethylsilane) with DmpN₃. An X‐ray diffraction study revealed its linear C? Co? N core and a short Co? N distance (1.691(6) Å). Spectroscopic characterization and calculation studies indicated the high‐spin nature of 4 and the multiple‐bond character of the Co? N bond. Complex 4 effected group‐transfer reactions to CO and ethylene to form isocyanide and imine, respectively. It also facilitated E? H (E=C, Si) σ‐bond activation of terminal alkyne and hydrosilanes to produce the corresponding cobalt(II) alkynyl and cobalt(II) hydride complexes as 1,2‐addition products.