Tetracarbene Ligand Research Articles

A symmetrical tetra N-heterocyclic carbene binuclear silver(I) complex: synthesis, characterization and anticancer study

The synthesis of dinuclear carbene complexes from tetrakis-N-heterocyclic (NHC) and silver(I) ions has been achieved through molecular self-assembly. The reaction between 1,2,4,5-tetra(bromomethyl)benzene with four equivalents of N-substituted alkyl benzimidazole resulted in formation of various tetra-benzimidazolium salts, [H4L]·4Br- (1–5). Further reaction of the respective salts with Ag2O yielded mono tetra-NHC dinuclear silver(I) complexes, [Ag2L1]·2PF6 (6–10). The salts and their silver(I) complexes were characterized by 1H and 13C NMR, FTIR, elemental analysis, molar conductivity, and UV–Visible spectroscopy. Structure elucidation from single crystal X-ray diffraction analyses data revealed that 10 is a dinuclear complex in a tweezer-like structure containing two silver(I) ions that are bridged by one tetra-carbene ligand with the cyclic aromatic backbone of the ligand perpendicular to the benzimidazolium moieties. Anticancer activities of the salts (1–5) and complexes (6–10) were studied, with Etoposide used as the positive control. Subsequently, 1 showed less activity at 24 h while all salts, 1–5, showed better activity at 48 h. The complexes (6–10) indicated moderate and better activity, which is higher compared to 1–5 and Etoposide, both within 24 and 48 h incubation periods.

Cyclic iron tetra N-heterocyclic carbenes: synthesis, properties, reactivity, and catalysis.

Cyclic iron tetracarbenes are an emerging class of macrocyclic iron N-heterocyclic carbene (NHC) complexes. They can be considered as an organometallic compound class inspired by their heme analogs, however, their electronic properties differ, e.g. due to the very strong σ-donation of the four combined NHCs in equatorial coordination. The ligand framework of iron tetracarbenes can be readily modified, allowing fine-tuning of the structural and electronic properties of the complexes. The properties of iron tetracarbene complexes are discussed quantitatively and correlations are established. The electronic nature of the tetracarbene ligand allows the isolation of uncommon iron(III) and iron(IV) species and reveals a unique reactivity. Iron tetracarbenes are successfully applied in C-H activation, CO2 reduction, aziridination and epoxidation catalysis and mechanisms as well as decomposition pathways are described. This review will help researchers evaluate the structural and electronic properties of their complexes and target their catalyst properties through ligand design.

Single-Molecule Junction of a Cationic Rh(III) Polyyne Molecular Wire.

Single-molecule conductance studies on metal-containing inorganic and organometallic molecular wires are relatively less explored compared to those on organic molecular wires. Furthermore, conductance and transmission profiles of the metal-containing wires insensitive to the metal centers often hinder rational design for high performance wires. Here, synthesis and single-molecule conductance measurements of the bis(butadiynyl)rhodium wires with tetracarbene ligands 1H and 1Au are reported as rare examples for Rh(III) diacetylide molecular wires. The rhodium wires derived from the terminal acetylene and gold-functionalized precursors show comparable, high single-molecule conductance ((6-7) × 10-3 G0) as determined by the STM break-junction measurements, suggesting formation of virtually the same covalently linked metal electrode-molecule-metal electrode junctions. The values for the metallapolyynes are larger than those of the organic polyyne wires having the similar molecular lengths. The hybrid DFT-NEGF calculations of the model systems suggest that profiles of transmission spectra are highly sensitive to the presence and species of the metal fragments doped into the polyyne molecular wire because the conductance orbitals of the metallapolyynes molecular junctions carry significant metal fragment characters. Thus, the metallapolyyne junctions turn out to be suitable platforms for rationally designed molecular wires.

Spin-State Variations of Iron(III) Complexes with Tetracarbene Macrocycles.

Starting from their six-coordinate iron(II) precursor complexes [L8R Fe(MeCN)]2+ , a series of iron(III) complexes of the known macrocyclic tetracarbene ligand L8H and its new octamethylated derivative L8Me , both providing four imidazol-2-yliden donors, were synthesized. Several five- and six-coordinate iron(III) complexes with different axial ligands (Cl- , OTf- , MeCN) were structurally characterized by X-ray diffraction and analyzed in detail with respect to their spin state variations, using a bouquet of spectroscopic methods (NMR, UV/Vis, EPR, and 57 Fe Mößbauer). Depending on the axial ligands, either low-spin (S=1/2) or intermediate-spin (S=3/2) states were observed, whereas high-spin (S=5/2) states were inaccessible because of the extremely strong in-plane σ-donor character of the macrocyclic tetracarbene ligands. These findings are reminiscent of the spin state patterns of topologically related ferric porphyrin complexes. The ring conformations and dynamics of the macrocyclic tetracarbene ligands in their iron(II), iron(III) and μ-oxo diiron(III) complexes were also studied.

Correction: Synthesis and structural characterization of metal complexes with macrocyclic tetracarbene ligands

Correction for ‘Synthesis and structural characterization of metal complexes with macrocyclic tetracarbene ligands’ by Fan Fei et al., New J. Chem., 2017, 41, 13442–13453.

A Chromium(II) Tetracarbene Complex Allows Unprecedented Oxidative Group Transfer.

Multiple distinct oxidative group transfer reactions to low valent chromium were examined. Six new chromium complexes were prepared from a highly electronically unsaturated Cr(II) square planar complex that was supported by a macrocyclic tetracarbene ligand. This complex's reactivity with Me3NO and disparate azides was investigated. The reaction with Me3NO generated a highly stable Cr(IV)-oxo complex. Less bulky organic azides such as p-tolyl and n-octyl azides gave rise to metallotetrazenes, while more sterically demanding mesityl and adamantyl azides generated Cr(IV)-imido complexes. The reaction of the square planar Cr(II) complex with TMS-azide yielded the first linearly bridging nitrido chromium species. Reductive group transfer was explored for a Cr(IV)-imido complex, and organic products, such as aziridines, were formed after addition. Cr(IV) imidos and oxos are quite rare, while tetrazenes and bridging nitridos are virtually unknown. This is the most detailed study on oxidative group transfer reactions using chromium based complexes on a single auxiliary ligand to date.

Synthesis and structural characterization of metal complexes with macrocyclic tetracarbene ligands

Fourteen Ag(i), Au(i), Ni(ii), Pd(ii) and Pt(ii) complexes were prepared with macrocyclic tetradentate N-heterocyclic carbene (NHC) ligands.

Magnetic Circular Dichroism Evidence for an Unusual Electronic Structure of a Tetracarbene-Oxoiron(IV) Complex.

In biology, high valent oxo-iron(IV) species have been shown to be pivotal intermediates for functionalization of C-H bonds in the catalytic cycles of a range of O2-activating iron enzymes. This work details an electronic-structure investigation of [FeIV(O)(LNHC)(NCMe)]2+ (LNHC = 3,9,14,20-tetraaza-1,6,12,17-tetraazoniapenta-cyclohexacosane-1(23),4,6(26),10,12(25),15,17(24),21-octaene, complex 1) using helium tagging infrared photodissociation (IRPD), absorption, and magnetic circular dichroism (MCD) spectroscopy, coupled with DFT and highly correlated wave function based multireference calculations. The IRPD spectrum of complex 1 reveals the Fe-O stretching vibration at 832 ± 3 cm-1. By analyzing the Franck-Condon progression, we can determine the same vibration occurring at 616 ± 10 cm-1 in the E(dxy → dxz,yz) excited state. Both values are similar to those measured for [FeIV(O)(TMC)(NCMe)]2+ (TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane). The low-temperature MCD spectra of complex 1 exhibit three pseudo A-term signals around 12 500, 17 000, and 24 300 cm-1. We can unequivocally assign them to the ligand field transitions of dxy → dxz,yz, dxz,yz → dz2, and dxz,yz → dx2-y2, respectively, through direct calculations of MCD spectra and independent determination of the MCD C-term signs from the corresponding electron donating and accepting orbitals. In comparison with the corresponding transitions observed for [FeIV(O) (SR-TPA)(NCMe)]2+ (SR-TPA = tris(3,5-dimethyl-4-methoxypyridyl-2-methy)amine), the excitations within the (FeO)2+ core of complex 1 have similar transition energies, whereas the excitation energy for dxz,yz → dx2-y2 is significantly higher (∼12 000 cm-1 for [FeIV(O)(SR-TPA)(NCMe)]2+). Our results thus substantiate that the tetracarbene ligand (LNHC) of complex 1 does not significantly affect the bonding in the (FeO)2+ unit but strongly destabilizes the dx2-y2 orbital to eventually lift it above dz2. As a consequence, this unusual electron configuration leads to an unprecedentedly larger quintet-triplet energy separation for complex 1, which largely rules out the possibility that the H atom transfer reaction may take place on the quintet surface and hence quenches two-state reactivity. The resulting mechanistic implications are discussed.

Complete Series of {FeNO}(8), {FeNO}(7), and {FeNO}(6) Complexes Stabilized by a Tetracarbene Macrocycle.

Use of a macrocyclic tetracarbene ligand, which is topologically reminiscent of tetrapyrrole macrocycles though electronically distinct, has allowed for the isolation, X-ray crystallographic characterization and comprehensive spectroscopic investigation of a complete set of {FeNO}(x) complexes (x = 6, 7, 8). Electrochemical reduction, or chemical reduction with CoCp2, of the {FeNO}(7) complex 1 leads to the organometallic {FeNO}(8) species 2. Its crystallographic structure determination is the first for a nonheme iron nitroxyl {FeNO}(8) and has allowed to identify structural trends among the series of {FeNO}(x) complexes. Combined experimental data including (57)Fe Mössbauer, IR, UV-vis-NIR, NMR and Kβ X-ray emission spectroscopies in concert with DFT calculations suggest a largely metal centered reduction of 1 to form the low spin (S = 0) {FeNO}(8) species 2. The very strong σ-donor character of the tetracarbene ligand imparts unusual properties and spectroscopic signatures such as low (57)Fe Mössbauer isomer shifts and linear Fe-N-O units with high IR stretching frequencies for the NO ligand. The observed metal-centered reduction leads to distinct reactivity patterns of the {FeNO}(8) species. In contrast to literature reported {FeNO}(8) complexes, 2 does not undergo NO protonation under strictly anaerobic conditions. Only in the presence of both dioxygen and protons is rapid and clean oxidation to the {FeNO}(7) complex 1 observed. While 1 is stable toward dioxygen, its reaction with dioxygen under NO atmosphere forms the {FeNO}(6)(ONO) complex 3 that features an unusual O-nitrito ligand trans to the NO. 3 is a rare example of a nonheme octahedral {FeNO}(6) complex. Its electrochemical or chemical reduction triggers dissociation of the O-nitrito ligand and sequential formation of the {FeNO}(7) and {FeNO}(8) compounds 1 and 2. A consistent electronic structure picture has been derived for these unique organometallic variants of the key bioinorganic {FeNO}(x) functional units.

Exploring Coordination Modes: Late Transition Metal Complexes with a Methylene-bridged Macrocyclic Tetra-NHC Ligand.

A tetranuclear silver(I) N-heterocyclic carbene (NHC) complex bearing a macrocyclic, exclusively methylene-bridged, tetracarbene ligand was synthesized and employed as transmetalation agent for the synthesis of nickel(II), palladium(II), platinum(II), and gold(I) derivatives. The transition metal complexes exhibit different coordination geometries, the coinage metals being bound in a linear fashion forming molecular box-type complexes, whereas the group 10 metals adapt an almost ideal square planar coordination geometry within the ligand's cavity, resulting in saddle-shaped complexes. Both the Ag(I) and the Au(I) complexes show ligand-induced metal-metal contacts, causing photoluminescence in the blue region for the gold complex. Distinct metal-dependent differences of the coordination behavior between the group 10 transition metals were elucidated by low-temperature NMR spectroscopy and DFT calculations.

Synthesis of Fully Aliphatic Aziridines with a Macrocyclic Tetracarbene Iron Catalyst

A second-generation aziridination catalyst supported by a borate-based dianionic macrocyclic tetracarbene ligand has been synthesized. The new macrocyclic tetracarbene iron(II) complex catalyzed the aziridination of alkyl azides and aliphatic alkenes showcasing the first fully aliphatic version of this C2 + N1 reaction. High isolated yields were obtained when no functional groups were present on the organic azides and alkenes, while modest yields were achieved when nonprotic functional groups were included. Even multiple functional groups can be added to the azide and alkene fragments to produce the most complex aziridines yet synthesized by this C2 + N1 catalytic reaction. The catalyst generated higher yields for aziridination with aryl azides and alkenes than the previously reported catalyst, [(Me,EtTCPh)Fe(NCCH3)2](PF6)2. The contrast is particularly apparent with functionalized aryl azides where the second-generation catalyst now provides practical yields for synthetic chemistry. Finally, catalytic in...

Fighting Fenton Chemistry: A Highly Active Iron(III) Tetracarbene Complex in Epoxidation Catalysis.

Organometallic Fe complexes with exceptionally high activities in homogeneous epoxidation catalysis are reported. The compounds display Fe(II) and Fe(III) oxidation states and bear a tetracarbene ligand. The more active catalyst exhibits activities up to 183 000 turnovers per hour at room temperature and turnover numbers of up to 4300 at -30 °C. For the Fe(III) complex, a decreased Fenton-type reactivity is observed compared with Fe(II) catalysts reported previously as indicated by a substantially lower H2 O2 decomposition and higher (initial) turnover frequencies. The dependence of the catalyst performance on the catalyst loading, substrate, water addition, and the oxidant is investigated. Under all applied conditions, the advantageous nature of the use of the Fe(III) complex is evident.

Iron Complexes of a Macrocyclic N-Heterocyclic Carbene/Pyridine Hybrid Ligand

The tetradentate ligand system L1 combines two N-heterocyclic carbene (NHC) and two pyridine donor functions in a macrocyclic scaffold. Its iron(II) complex [FeL1(MeCN)2](PF6)2 (1) has been synthesized and fully characterized. The macrocyclic ligand in 1 is puckered and shows a significant barrier for ring inversion (ΔH⧧ = 15.1 kcal mol–1, and ΔS⧧ = −4.7 cal mol–1 K–1). Axial ligands in 1 can be readily substituted to give heteroleptic [FeL1(CO)(MeCN)](PF6)2 (2) or neutral [FeL1(N3)2] (3). The strong ligand field of the NHC/pyridine hybrid ligand imparts low-spin states (S = 0) for all iron(II) complexes 1–3. Mössbauer data reflect the asymmetric electronic situation that results from the strongly covalent Fe–CNHC bonds in the basal plane constituted by the macrocyclic ligand L1. Oxidation of 1 has been monitored by UV–vis spectro-electro chemistry, and the resulting iron(III) complex [FeL1(MeCN)2](PF6)3 (4) has been isolated after chemical oxidation. SQUID and Mössbauer data have shown an S = 1/2 ground state for 4, and X-ray crystallographic analyses of 1 and 4 revealed that structural parameters of the {FeL1} core are basically invariant with respect to changes in the metal ion’s oxidation state. Density functional theory calculations support the experimental findings. The combined structural, spectroscopic, and electrochemical data for 1 with its {C2N2} hybrid ligand L1 allowed for useful comparison with the related iron(II) complex that has a macrocyclic {C4} tetracarbene ligand.

Structural diversity of late transition metal complexes with flexible tetra-NHC ligands.

The synthesis of copper, gold, nickel, palladium, platinum, and iron complexes with open chain tetra-N-heterocyclic carbene (NHC) ligands via transmetalation using silver NHC complexes is presented. The obtained complexes show differing coordination geometries depending on both ligand structure and metal. While the complexes of the coinage metals form di- or tetranuclear structures, the group 10 metal complexes exhibit a distorted square planar coordination geometry at the metal centers. In the case of iron an enhanced flexibility of the ligand - caused by a longer alkyl bridge - leads to octahedral complexes with a sawhorse-type coordination by the tetracarbene ligand and two cis acetonitrile ligands. To the best of our knowledge, this is the first known example of a tetracarbene ligand in sawhorse-type coordination within an octahedral coordination sphere. The remaining cis-labile sites are prone to exchange reactions as shown by addition of trimethylphosphine.

Overcoming NHCs neutrality: installing tetracarbenes on group 13 and 14 metals.

The first tetracarbene complexes of group 13 and 14 metals have been synthesized by employing dianionic macrocyclic tetracarbene ligands. The tin, indium, and aluminium tetracarbene complexes are structurally analogous to their porphyrin or salen analogues. The aluminium complex is the first example of multiple NHCs bound to this metal centre.

Synthesis of polynuclear Ag(i) and Au(i) complexes from macrocyclic tetraimidazolium salts

The synthesis of the macrocyclic tetraimidazolium salt (H4-1)(Br)4 and the anion exchange to (H4-1)(PF6)4 have been explored. Deprotonation of (H4-1)(PF6)4 with Ag2O yields the mono- and dinuclear silver(I) complexes [Ag(H2-1)](PF6)3 and [Ag2(1)](PF6)2. Transmetallation of the dicarbene and tetracarbene ligands to gold(I) yields the mono- and dinuclear gold(I) complexes [Au(H2-1)](PF6)3 and [Au2(1)](PF6)2. The monosilver complex [Ag(H2-1)](PF6)3, featuring two imidazolium moieties, has been deprotonated and used for the synthesis of the tetranuclear silver(I) octacarbene complex [{Ag(1)}2Ag2](PF6)4.

A Tetracarbene–Oxoiron(IV) Complex

Bioinspired organometallic chemistry: An oxoiron(IV) unit has been trapped within a macrocyclic tetracarbene ligand, merging a key bioinorganic intermediate with a popular organometallic scaffold (see picture). The stability of the new complex has allowed its characterization by a variety of methods, which show a strong σ-donating tetracarbene coordination leading to an S=1 ground state and unusual properties of the oxoiron(IV) species. Detailed facts of importance to specialist readers are published as ”Supporting Information”. Such documents are peer-reviewed, but not copy-edited or typeset. They are made available as submitted by the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

ChemInform Abstract: Synthesis and Coordination Chemistry of Macrocyclic Ligands Featuring NHC Donor Groups

AbstractReview: 104 refs.

Synthesis of Aziridines from Alkenes and Aryl Azides with a Reusable Macrocyclic Tetracarbene Iron Catalyst

A new iron aziridination catalyst supported by a macrocyclic tetracarbene ligand has been synthesized. The catalyst, [((Me,Et)TC(Ph))Fe(NCCH(3))(2)](PF(6))(2), was synthesized from the tetraimidazolium precursor ((Me,Et)TC(Ph))(I)(4) and characterized by NMR spectroscopy, electrospray ionization mass spectrometry, and single-crystal X-ray diffraction. This iron complex catalyzes the aziridination of electron-donating aryl azides and a wide variety of substituted aliphatic alkenes, including tetrasubstituted ones, in a "C(2) + N(1)" addition reaction. Finally, the catalyst can be recovered and reused up to three additional times without significant reduction in yield.

Synthesis and coordination chemistry of macrocyclic ligands featuring NHC donor groups

Poly-NHC (NHC = N-heterocyclic carbene) ligands emerged almost immediately after the first stable NHCs had been described. Macrocyclic ligands, featuring NHC donor groups and their metal complexes, however, remained rare until recently. This perspective highlights modern developments in the fields of synthesis and coordination chemistry of macrocyclic poly-NHC ligands. These include the synthesis of tetracarbene ligands which were obtained from complexes of β-functionalized isocyanides followed by cyclization of the coordinated iscocyanide ligands to NH,NH-functionalized NHCs and the subsequent metal template controlled bridging alkylation of the NH,NH-NHCs to yield the macrocycle. The template synthesis of ligands featuring a mixed NHC/phosphine donor set like [11]ane-P(2)C(NHC) and [16]ane-P(2)C(NHC)(2) by linkage of NH,NH-NHCs to different phosphines is also presented. Finally, methods for the preparation of cyclic polyazolium salts, their deprotonation and metalation and the different modes of coordination of such macrocyclic poly-NHC ligands are discussed.