Han Vinh Huynh

Sulfur-Functionalized N-Heterocyclic Carbene Complexes of Pd(II): Syntheses, Structures and Catalytic Activities

N-heterocyclic carbenes (NHCs) can be easily modified by introducing functional groups at the nitrogen atoms, which leads to versatile coordination chemistry as well as diverse catalytic applications of the resulting complexes. This article summarizes our contributions to the field of NHCs bearing different types of sulfur functions, i.e., thioether, sulfoxide, thiophene, and thiolato. The experimental evidence for the truly hemilabile coordination behavior of a Pd(II) thioether-NHC complex has been reported as well. In addition, complexes bearing rigid CSC-pincer ligands have been synthesized and the reasons for pincer versus pseudo-pincer formation investigated. Incorporation of the electron-rich thiolato function resulted in the isolation of structurally diverse complexes. The catalytic activities of selected complexes have been tested in Suzuki-Miyaura, Mizoroki-Heck and hydroamination reactions.

Pd-carbene catalyzed carbonylation reactions of aryl iodides†

A series of carbene complexes [PdBr(2)((i)Pr(2)-bimy)L] (C2-C13) with different types of co-ligands (L) have been tested for their catalytic activities in the carbonylative annulation of 2-iodophenol with phenylacetylene in DMF to afford the respective flavone 2a. Complex C12 with an N-phenylimidazole co-ligand showed the best activity and also afforded high yields when the substrate scope was extended to other aryl or pyridyl acetylenes. In addition, catalyst C12 was also efficient in the carbonylative annulation of 2-iodoaniline with acid chlorides giving the desirable 2-substituted 4H-3,1-benzoxazin-4-ones (4) in good yields. Additionally, this Pd-NHC complex also proved to be a very efficient catalyst for the hydroxycarbonylation of iodobenzene derivatives at low catalyst loading and under low CO pressure. These results demonstrate the versatility and efficiency of this phosphine-free Pd(II)-NHC complex in different types of carbonylations of aryl iodides under mild conditions.

Electronic Structure Trends in N-Heterocyclic Carbenes (NHCs) with Varying Number of Nitrogen Atoms and NHCTransition-Metal Bond Properties

Carbenes derived from five-membered heterocycles with different numbers of nitrogen atoms ranging from two to four lead formally either to normal N-heterocyclic or mesoionic carbenes with, in some cases, the same skeletal structure. The electronic structures of fourteen of these compounds were examined by means of DFT calculations at the B3LYP/aug-cc-pVTZ level. The examined parameters include the energies of the σ-lone pair at Ccarbene and the π-HOMO of the protonated form, which are correlated to the first and second proton affinities. The singlet-triplet energy gap was used as a measure of the stability of the N-heterocyclic carbene (NHC) towards dimerisation. Natural population analysis provided insight into the variation of the pπ population and the natural charge at Ccarbene with NHC structure. Additionally, the transition metalNHC bond in L-AuCl and L-TiCl4 and the nature of the orbital interactions between the NHC and the transition-metal fragment were analysed in detail by the extended transition state-natural orbitals for chemical valence (ETS-NOCV) approach at the BP86/TZ2P level. Similarities and differences between the NHCgold and the NHCtitanium bond are discussed, and trends in key bonding properties can be traced back to the variation of the electronic parameters of the NHC.

Syntheses and catalytic activities of pseudo-pincer and CSC pincer-type Pd(II) complexes derived from benzannulated N-heterocyclic carbenes

Three new dibenzimidazolium salts bearing thioether (B.2HBr, B.2HNO3) and sulfoxide (C.2HBr) containing bridges have been synthesized as sulfur-functionalized dicarbene precursors. Palladation of B.2HBr and C.2HBr afforded two new pseudo-pincer complexes cis-[PdBr2(B-kappa2C)] (1) and cis-[PdBr2(C-kappa2C)] (2), in which the sulfur-donor remains pendant. Reaction of precursor B.2HNO3 in the presence of 1 equiv of KBr, on the other hand, yields the first CSC-Pd(II) pincer complex [PdBr(B-kappa3CSC)]NO3 (3) bearing two carbene moieties. All three complexes have been fully characterized by multinuclei NMR spectroscopies, ESI mass spectrometry and X-ray diffraction analysis. Their catalytic activities in the Mizoroki-Heck reaction have been evaluated as well.

Tetrazole Photoclick Chemistry: Reinvestigating Its Suitability as a Bioorthogonal Reaction and Potential Applications

The bioorthogonality of tetrazole photoclick chemistry has been reassessed. Upon photolysis of a tetrazole, the highly reactive nitrile imine formed undergoes rapid nucleophilic reaction with a variety of nucleophiles present in a biological system, along with the expected cycloaddition with alkenes. The alternative use of the tetrazole photoclick reaction was thus explored: tetrazoles were incorporated into Bodipy and Acedan dyes, providing novel photo-crosslinkers with one- and two-photon fluorescence Turn-ON properties that may be developed into protein-detecting biosensors. Further introduction of these photo-activatable, fluorogenic moieties into staurosporine resulted in the corresponding probes capable of photoinduced, no-wash imaging of endogenous kinase activities in live mammalian cells.

Pyrazole‐Derived Remote Dicarbenes: Versatile Ligands for Di‐ and Tetranuclear Complexes

The spectacular success of N-heterocyclic carbenes (NHCs) as a new class of ligands in organometallic chemistry and their application to catalytic systems has been primarily attributed to their strong electron-donating nature. The extension to yet stronger donors has recently lead to the design of non-conventional NHCs including remote NHCs (rNHCs), cyclic (alkyl)aminocarbenes (CAACs) and “carbodicarbenes” (CDCs), which can also be regarded as zerovalent carbon complexes. In relation to this development, we have recently introduced complexes of pyrazole-based remote and abnormal NHCs, which are isomeric to imidazolin-2-ylidenes and thus were termed pyrazolin-4ylidenes. However, their true nature is currently still under debate, and as depicted in Scheme 1, these ligands can be represented by a zwitterion (Ia), an abnormal rNHC (Ib) or a neutral cyclic bent allene (Ic). Although more detailed investigations are required to reveal their true identity, there is no doubt that they form a new class of strongly donating ligands (L) as shown by an even lower CO stretching frequency in [RhCl(L)(CO)2] compared to those in classical NHCs or phosphine analogues. Owing to the substantial carbenoid ligand character in their metal complexes we will refer to them as abnormal rNHCs in this work. The coordination chemistry of such abnormal rNHCs is poorly understood and only monodentate examples have been reported. Bior multidentate ligands of this type capable of constructing macrocyclic entities through metal-directed self-assembly, an area of great interest, remain unknown. Among such architectures, molecular squares built from four metal-containing corners linked by four spacers have been widely studied. Commonly-used spacers contain nitrogen donor groups, whereas spacers utilizing carbon donors are very rare. Only recently, Bielawski reported linear dior polynuclear complexes incorporating Janus-type NHCs as spacers. Employing these spacers, a Ni rectangle with 4,4’-bipyridine as a second linker was reported by Hahn. Herein, we report on the coordination chemistry of unprecedented remote and abnormal dicarbenes and their use as a new class of organometallic spacers for the formation of diand tetranuclear Pd complexes. The suitable ligand precursors with bridges of different lengths (2 a–c) were obtained through alkylation of 4,4’-diACHTUNGTRENNUNGiododipyrazoles[15] 1 a–c with Me3OBF4 as white solids in moderate to high yields of 86 %, 93 % and 62 %, respectively (Scheme 1). The topology of 2 a–c ensures the exclusive formation of dior polynuclear complexes and prevents chelation of the resulting dicarbenes. Dipyrazolium salt 2 c was reacted with a pre-mixed solution of [Pd2ACHTUNGTRENNUNG(dba)3]/PPh3 (1:4 ratio, dba=dibenzylideneacetone) in CH2Cl2 (Scheme 2) in an attempt to synthesize a dinuclear Pd–dicarbene complex. Owing to the long propylene chain, the two pyrazolium moieties are not likely to interfere with each other to a significant extent, and oxidative addition is expected to proceed in a similar manner as that [a] Dr. Y. Han, L. J. Lee, Dr. H. V. Huynh Department of Chemistry, Faculty of science National University of Singapore 3 Science Drive 3, Singapore 117543 (Singapore) Fax: (+65) 6779-1691 E-mail : chmhhv@nus.edu.sg Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.200902737. Scheme 1. Selected resonance structures of “pyrazolin-4-ylidenes” (Ia–c) and synthesis of di(pyrazolium) salts 2a–c.

A single-molecular pathway from heterometallic MM′ (M = BaII, MnII; M′ = CrIII) oxalato complexes to intermetallic composite oxides

Thermolysis of {(n-C4H9)4N[MnIICrIII(C2O4)3]}n, 1 at 500 °C for 10 h gives spinel Mn1.5Cr1.5O4 which was characterized by powder XRD, SEM, FTIR and elemental analysis. The thermal conversion occurs by an internal redox process at ca. 400 °C in one step (TGA). It offers an alternative molecular source for an intermetallic oxide. Thermolysis of {[BaII6(H2O)17][CrIII(C2O4)3]4}·7H2O, 2 under similar conditions gives a mixture of BaIICrVIO4 and BaIICO3. These results suggested that the oxalate ligand in a heterometallic complex may be a convenient source for oxides in intermetallic composites and that, under suitable conditions, both metals in the heterometallic complexes can be transferred to the intermetallic oxides. It suggested that composite metal oxides can be generated from hetero- and inter-metallic oxalato complexes at relatively low temperatures, which could serve as a convenient route for the preparation of technologically important composites.

Dinuclear Complexes with Bis(benzenedithiolate) Ligands

As a part of a broader study directed towards helical coordination compounds with benzenedithiolate donors, we have synthesized the bis(benzenedithiol) ligands 1,2-bis(2,3-dimercaptobenzamido)ethane (H(4)-1) and 1,2-bis(2,3-dimercaptophenyl)ethane (H(4)-2). Both ligands form dinuclear complexes with Ni(II), Ni(III) and, after air-oxidation, Co(III) ions under equilibrium conditions. Complexes (NEt(4))(4)[Ni(II)(2)(1)(2)] (11 b), (NEt(4))(2)[Ni(III)(2)(1)(2)] (13), and Na(4)[Ni(II)(2)(2)(2)] (14) were characterized by X-ray diffraction. In all complexes, two square-planar [Ni(S(2)C(6)H(3)R)(2)] units are linked in a double-stranded fashion by the carbon backbone and they assume a coplanar arrangement in a stair-like manner. Cyclic voltammetric investigations show a strong dependence of the redox potential on the type of the ligand. The substitution of 1(4-) for 2(4-) on nickel (-785 mV for 11 b versus -1130 mV for 14, relative to ferrocene) affects the redox potential to a similar degree as the substitution of nickel for cobalt (-1160 mV for [Co(2)(1)(2)](2-)/[Co(2)(1)(2)](4-), relative to ferrocene). The redox waves display a markedly less reversible behavior for complexes with the shorter bridged ligand 2(4-) compared to those of 1(4-).

Pincer-type di(1,2,4-triazolin-5-ylidene)Pd(II) complexes and their catalytic activities towards Cu- and amine-free Sonogashira reaction

Pd(II) complexes [PdBr(A-κ(3)CNC)]Br (1) and [PdBr(B-κ(3)CNC)]Br (2) with new CNC pincer-type ligands derived from 1,2,4-triazolin-5-ylidenes have been synthesized and characterized by multinuclei NMR spectroscopies, ESI mass spectrometry and X-ray diffraction analysis. The more soluble complex 1 proved to be an efficient pre-catalyst in copper- and amine-free Sonogashira reaction with high turnover numbers. The potential for recycling of the catalyst was also demonstrated.

Benzimidazolin-2-ylidene N-heterocyclic carbene complexes of ruthenium as a simple catalyst for the N-alkylation of amines using alcohols and diols

Simple air and moisture stable ruthenium complexes 1–3 and 3a were synthesized from readily available benzannulated N-heterocyclic carbene ligands (bimy = benzimidazolin-2-ylidene). These complexes were found to be efficient catalysts for the alkylation of amines using alcohols as alkylating agents. Catalysts 1, 2 and 3a gave excellent yields of up to 99% for the alkylation of various amines using benzylic and aliphatic alcohols at 130 °C for 18 h under solventless conditions. Catalyst 3a bearing both phosphine and carbene ligands gave excellent yields of up to 98% for the synthesis of heterocyclic amines by double alkylation of primary amines using linear diols. The practical utility of these catalysts was demonstrated for the synthesis of pharmaceutically important amines in a more environmentally benign way under solventless conditions.