Heteroaryl carbene complexes : synthesis, reactivity and redox behaviour

A series of Fischer mono- and biscarbene complexes of the type [MLn{C=(XR)R'}] was synthesized and characterized. The redox behavior of the complexes was studied by different techniques, including cyclic voltammetry, spectroelectrochemistry, ESR and computational methods. Different transition metals (M) and carbene substituents (XR, R') were employed to compare both the effect of the central metal atom as well as the carbene substituent. Thienyl, furyl and ferrocenyl chromium(0) mono- and biscarbene complexes with ethoxy and amino substituents were electrochemically studied in CH2Cl2. Results were mutually consistent with computational data showing that the carbene double bond of all complexes is reduced pseudo reversibly to an anion radical, -Cr-C•. The Cr centers are oxidized in two successive one electron transfer steps to Cr(II) via the Cr(I) intermediate. For all ferrocenyl carbene complexes the Fe(II) is oxidized after the first oxidation of Cr. It was found that with respect to the aryl substituents the donating effect decreases from Fc>Fu>Th. Stabilization from the XR substituent, where XR = NHR, also resulted in lower redox potentials compared to their OEt analogues. The inclusion of ferrocene in the carbene substituent was done, as its redox activity and increased donating effect are well known. Mono- and biscarbene complexes with ethoxy and amino substituents of both chromium and tungsten were electrochemically studied. Again experimental data were supported by computational studies. Similar to the ethoxy chromium complexes, reduction of the W=C fragment to -W-C• was observed. However oxidation of the Fc group occurred first before the electrochemically irreversible oxidation process for W(0) involving a three electron-mediated process as seen in chronocoulometric analyses. The tungsten oxidation was restricted to a W0/II, consistent with computational studies, by the use of the electrolyte [NnBu4][B(C6F5)4]. The short-lived W(II) species were calculated to be stabilized by agostic CH···W interactions, similar to the chromium analogues. To extend linkers between the metal-carbene termini and investigate metal-metal interaction, biferrocenyl and 2,5-thienylbiferrocenyl tungsten(0) mono-and biscarbene complexes were synthesized and studied by spectroelectrochemistry. A metal-metal charge transfer transition between the tungsten carbonyl increment and the biferrocenyl / 2,5-thienylbiferrocenyl unit was confirmed by infrared spectroelectrochemical studies. The electronic interaction in the corresponding cationic species can be described as weakly coupled class II systems according to Robin and Day. The cymantrenyl moiety, Mn(η5-C5H4)(CO)3, provides an interesting alternative to ferrocene as an organometallic molecular tag, however the instability of the radical cation impairs its use. Improved stability of the cation, monitored by electrochemical measurements, was accomplished by substituting a carbonyl with a ferrocenyl Fischer carbene ligand resulting in Mn(I) oxidation occuring at lower potentials than ferrocenyl oxidation. These uncommon Fischer carbene complexes is the first organometallic multi-tags reported. The mono cationic species, [CpMn(CO)2{=C(OEt)Fc}][PF6], could be isolated and characterized with ESR analysis. Finally, transmetallation from the tungsten(0) Fischer carbene complexes yielded examples of rare acyclic alkoxy- and aminocarbene complexes of gold(I). Single x-ray structures for all complexes could be obtained including the novel ferrocenophane dinuclear biscarbene Au(I) complex. All structures display unsupported aurophilic interactions, while the bridging biscarbene shows a semi-supported Au-Au interaction. In the case of the furanyl/thienyl methoxy monocarbene complexes, extended Au-Au interactions result in oligomeric structures. Although this study is of a fundamental nature, it is imperative for the understanding and design of gold compounds with specific applications.Thesis (PhD)--University of Pretoria, 2013.gm2014Chemistryunrestricte

Synthetic investigation of Mn(I) and Re(I) N-heterocyclic carbene complexes

The study involves synthetic approaches towards the preparation of novel NHC complexes of low valent rhenium and manganese transition metals. Diverse methods of synthesis were studied. The direct approach, in which the ylidene obtained from deprotonation of 1,3-bis(2,4,6- trimethylphenyl)imidazolium chloride was added to the metal substrate, proved to be unsuccessful as isolation of the free carbene should rather be performed in an argon filled glove box under extreme inert conditions. By way of further investigation the ylidene was prepared by in situ methods and then quenched with the metal substrate. Different bases for deprotonation purposes and reaction conditions were explored. All routes employed were investigated and compared using group VII transition metal substrates Re(CO)5Br, Mn(CO)5Br, Re2(CO)10 and Mn2(CO)10. Isolation and purification of these products proved to be very challenging due to the insolubility in some organic solvents with consequent problematic spectroscopic analyses of the complexes. The tendency of the products to undergo various side reactions is observed in all reactions. Specifically, hydrolysis of the imidazolium ligand, followed by vinyl formation, yielded the mesitylformamide compound (3). The results obtained for some of the monometal substrates indicated that the target complexes were formed but could not be isolated. However, the synthesis route employing deprotonation by nBuLi as base and [Mn2(CO)10] as dimetal substrate lead to the isolation of the target dinuclear complex [Mn2(CO)9IMes] (9). Other novel complexes obtained during the course of this study include the biscarbene tetrarhenium complex [Re2(CO)9.C(OEt)C4H2OC(OEt)Re2(CO)9] (12) and various side reaction products. In many cases, metal-metal bond cleavage and carbonyl insertion was observed, as is evident in the complex IMesH[ReO4] (6) and ketene product (13). Structural and theoretical studies were performed to investigate the bond character between the carbene ligand and the metal.Dissertation (MSc)--University of Pretoria, 2010.Chemistryunrestricte

Recent advances in the field of multicarbene and multimetal carbene complexes of the Fischer-type

This review article covers the development of Fischer carbene complexes since the year 2000, with specific focus on carbene complexes bearing metal-containing fragments as substituents, as well as multicarbene systems. The role of the metal-containing substituents on the character and reactivity of such complexes are discussed. In addition, larger systems containing more than one carbene ligand are also covered (rodlike biscarbenes, chelates, macrosystems, etc.) in terms of the synthesis, reactivity and structural aspects.http://www.elsevier.com/locate/ccrhb201

1,3-Bis(2,4,6-trimethylphenyl)-3H imidazol-1-ium tetraoxidorhenate(VII)

The title compound, (C21H25N2)[ReO4], was formed as the unexpected product in an attempted synthesis of a rhenium(I)–N-heterocyclic carbene (NHC) complex. The compound has crystallographic mirror symmetry with both the cation and the tetrahedral anion located across a mirror plane. The cation and anion are linked by a C—H O hydrogen bond.The University of Pretoria and the National Research Foundationhttp://journals.iucr.org/e/journalhomepage.htmlnf201

N-(2,4,6-Trimethyl­phen­yl)formamide

The title compound, C10H13NO, was obtained as the unexpected, almost exclusive, product in the attempted synthesis of a manganese(I)–N-heterocyclic carbene (NHC) complex. The dihedral angle between the planes of the formamide moiety and the aryl ring is 68.06 (10)°. In the crystal, mol­ecules are linked by N—H⋯O hydrogen bonds, forming infinite chains along the c axis

Monitoring Patient Response to Pembrolizumab With Peripheral Blood Exhaustion Marker Profiles

Exhausted T cells are effector T cells that are silenced due to continuous T cell receptor (TCR) stimulation from persistent antigens. Characteristics of exhaustion include the increased expression of multiple inhibitory receptors such as programme death-1[PD-1], lymphocyte activation gene 3 [LAG-3], T cell Ig and mucin domain [TIM-3], the loss of effector cytokine secretion and altered transcriptional profile. The PD-1/PD-L1 interaction induces functional exhaustion of tumor-reactive cytotoxic T cells and interferes with anti-tumor T cell immunity. T cell exhaustion has been observed in metastatic melanoma patients where the exhaustion of tumor specific T cells suggests that tumor clearance has been impeded and contributed to tumor immune escape. Checkpoint immunotherapies are antibodies designed to block the interaction between the inhibitory receptors expressed on T cells and their respective ligands. Therapies such as anti-PD-1 (Pembrolizumab and Nivolumab) block these inhibitory receptors and are associated with a significant improvement in overall survival and progression free survival. However, only 20–40% of metastatic melanoma patients experience long-term benefit. In a cohort of 16 metastatic melanoma patients receiving pembrolizumab, blood was serially collected before each infusion (mean 8.3; range 1–12 cycles). The presence of inhibitory markers LAG-3, TIM-3, and PD-1 on the surface of T cells was examined and assessed in relation to patient response to identify if inhibitory markers can be used to differentiate responders from non-responders for Pembrolizumab. We confirmed that across a range of cycles (range 1–26) of pembrolizumab, PD-1 expression was significantly higher on CD4+ T cells from non-responders compared to responders and TIM-3 expressed on the surface of CD8+ T cells was significantly higher in non-responders compared to responders. This longitudinal data confirms previous studies that assessed single timepoints. This study provides preliminary evidence that PD-1 and TIM-3 may be predictive of non-responders when assessed over multiple treatment cycles

(Spectro)electrochemical investigations on (ferrocenyl)thiophenes modified by tungsten Fischer carbenes

Please abstract in the article.National Research Foundation, South Africa and Fonds der Chemischen Industrie and Chemifonds Fellowships.http://www.elsevier.com/locate/jorganchem2015-12-31hb201

Electrochemical and computational study of tungsten(0) ferrocene complexes: observation of the mono-oxidized tungsten(0) ferrocenium species and intramolecular electronic interactions

The series [(CO)5W=C(XR)Fc], 1 (XR = OEt) and 3 (XR = NHBu) as well as [(CO)5W=C(XR)-Fc'-(XR)C=W(CO)5], 2 (XR = OEt) and 4 (XR = NHBu) of mono- and biscarbene tungsten(0) complexes with Fc = FeII(C5H5)(C5H4) for monosubstituted derivatives and Fc¢ = FeII(C5H4)2 for disubstituted derivatives were synthesized and characterized spectroscopically. The oxidized ferrocenium complex [1+]•PF6 was also synthesized and characterized. Electrochemical and computational studies were mutually consistent in confirming the sequence of redox events for the carbene derivatives 1 - 4 as first a carbene double bond reduction to a radical anion, -W-C•, at peak cathodic potentials smaller than -2 V, then a ferrocenyl group oxidation in the range 0.206 < Eo' < 0.540 V and finally an electrochemically irreversible three-electron W(0) oxidation at Epa > 0.540 V vs. FcH/FcH+ in CH2Cl2 / [(nBu4)N][PF6]. This contrasts the sequence of oxidation events in ferrocenylcarbene complexes of chromium where Cr(0) is first oxidised in a one electron transfer process, then the ferrocenyl group, and finally formation of a Cr(II) species. The unpaired electron of the reductively formed radical anion is mainly located on the carbene carbon atom. Electronic interactions between two carbene double bonds (for biscarbenes 2 and 4) as well as between two W centers (for 4) were evident. Differences in redox potentials between the “a” and “b” components of the threeelectron W oxidation of 4 in CH2Cl2 or CH3CN / [(nBu4)N][PF6] are DEo' = Epa W(0) oxd 1b – Epa W(0) oxd 1a = ca. 51 and 337 mV respectively. Tungsten oxidation was restricted to a W0/II couple in CH2Cl2 / [(nBu4)N][B(C6F5)4]. From the computational results, the short-lived W(II) species were observed to be stabilized by agostic CH···W interactions.National Research Foundation, South Africa, (DIB, Grant number 76226; JCS, Grant number 81829), and by the Spanish MICINN and CAM (IF, Grants CTQ2010-20714-CO2-01/BQU, Consolider-Ingenio 2010, CSD2007-00006, S2009/PPQ-1634).http://pubs.acs.org/journal/orgnd7hb201

Metal–metal interaction in Fischer carbene complexes : a study of ferrocenyl and biferrocenyl tungsten alkylidene complexes

A series of ferrocenyl (Fc = ferrocenyl; fc = ferrocen-1,1'-diyl) and biferrocenyl (Bfc = 1',1''-biferrocenyl; bfc = 1',1''-biferrocen-1,1'''-diyl) mono- and biscarbene tungsten(0) complexes of the type [(CO)5W=C(OMe)R] (1, R = Fc; 3, R = Bfc) and [(CO)5W=C(OMe)-R'-(OMe)C=W(CO)5] (2, R' = fc; 4, R' = bfc) were synthesized according to the classical synthetic methodology by reacting W(CO)6 with LiR (R = Fc, fc, bfc), followed by a subsequent alkylation using methyl trifluoromethanesulfonate. Electrochemical investigations were carried out on these complexes to get a closer insight into the electronic properties of 1 - 4. The ferrocenyl and biferrocenyl moieties in 1 – 4 show reversible one electron redox events. It was further found that the Fischer carbene unit is reducible in an electrochemical one electron transfer process. For the tungsten carbonyl moieties, irreversible oxidation processes were found. In addition, charge transfer studies were performed on 1 - 4 by the use of in situ UV-Vis-NIR and infrared spectroelectrochemical techniques. During the UV-Vis-NIR investigations typical low energy transitions for the mixed-valent biferrocenyl unit were found. A further observed high energy NIR absorption is attributed to a metal-metal charge transfer transition between the tungsten carbonyl fragment and the ferrocenyl/biferrocenyl group in the corresponding oxidized states, which can be described as class II systems according to Robin and Day. This assignment was verified by infrared spectroelectrochemical studies. The electrochemical investigations are supported by DFT calculations. The structural properties of 1 - 4 in the solid state were investigated by single-crystal Xray diffraction studies showing no substituent effects on bond lengths and angles. The biferrocenyl derivatives exhibit synconformation of the ferrocenyl and carbene building blocks.D.I.B. and B.v.d.W. acknowledge the National Research Foundation, South Africa for financial support (Grant number 76226). We are grateful to the Fonds der Chemischen Industrie for financial support. J.M.S. and M.K. thank the FCI for Chemiefonds Fellowships.http://pubs.acs.org/journal/inocajhb201

Fischer-type gold(I) carbene complexes stabilized by aurophilic interactions

The synthesis and structure of rare acyclic alkoxy- and aminocarbene complexes of gold(I) are reported, including a novel ferrocenophane dinuclear biscarbene complex. X-Ray diffraction analyses and DFT calculations reveal that these complexes are stabilized by genuine aurophilic interactions.National Research Foundation,South Africa (D.I.B., Grant number 76226), and the Spanish MICINN and CAM (I.F., Grants CTQ2010-20714-CO2-01/BQU, Consolider-Ingenio 2010, CSD2007-00006,S2009/PPQ-1634).http://www.rsc.org/dalton2015-02-28hb201