Multinuclear mesoionic 1,2,3-triazolylidene complexes: design, synthesis, and applications.

The chemistry of multinuclear metal complexes bearing by N-heterocyclic carbene (NHC) ligands, is an area of fast growing interest in modern organometallic chemistry. In particular, complexes supported by mesoionic (MIC) 1,2,3-triazolylidenes are attracting a great deal attention due to their postulated superior donor capacity compared to classical NHC ligands. Despite the readily available synthetic routes to MIC-based ligand platforms featuring several substitution levels, most of the coordination chemistry of triazolylidenes is still dominated by mononuclear complexes. In this short review article, recent progress on the design and synthesis of multinuclear triazolylidene complexes (ranging from di- to tetranuclear species) is discussed. Special emphasis is placed on their structural features, electronic properties and catalytic applications.

NHC-Au(i) complexes bearing trispyrazolyl borate (Tp) ligands: efficient platforms for bimetallic species.

Treatment of NHC-AuCl (NHC = IPr and IMes) complexes with equimolar amounts of KTpR2 (R = Me, H) salts in tetrahydrofuran produces in high yields the heteroleptic complexes 3-6 with the general formula NHC-Au-TpR2. As the TpR ligand in complexes 3-6 features a k1-N type coordination toward the gold(i) center, the subsequent addition of group 10 and 11 metal precursors (NiII, PtII, CuII) results in the isolation of heterobimetallic Au/M complexes supported by TpR platforms. All new metal complexes have been fully characterized by elemental analysis and NMR spectroscopy, and in the case of 3, 4 and 6 by X-ray crystallography.

Hydroxyl-functionalized triazolylidene-based PEPPSI complexes: metallacycle formation effect on the Suzuki coupling reaction.

We report the preparation and full characterization of a series of hydroxyl functionalized 1,2,3-triazolylidene-based PEPPSI complexes 2a-c and their catalytic application in the Suzuki cross coupling reaction of aryl chlorides/amides with boronic acids. Under basic reaction conditions, complexes 2a-c show a notable increase in their catalytic efficiency compared with two ether-wingtip functionalized PEPPSI analogues (3 and 4) and a commercially available NHC-Pd complex (5). The catalytic results suggest that deprotonation of the hydroxyl group in complexes 2a-c plays an important role in the overall process. Deprotonation of the alcohol moiety of complexes 2a-b with sodium tert-butoxide allows for the isolation of metallacycles 6a-b, which are proposed as the active species of cross coupling reactions.

Study of the Coordination Modes of Hybrid NNCp Cyclopentadienyl/Scorpionate Ligands in Ir Compounds.

A new coordination mode for the hybrid scorpionate/cyclopentadienyl ligand bpzcp, [bpzcp = 2,2-bis(3,5-dimethylpyrazol-1-yl)-1,1-diphenylethylcyclopentadienyl] is observed in iridium complexes. The reaction of the lithium precursor, [Li(bpzcp)(THF)], with a range of [IrCl(diene)]2 compounds leads to an unprecedented binding mode of the hybrid scorpionate/cyclopentadienyl ligand as η5-Cp-coordinated and the formation of Ir(I) derivatives [Ir(η5-Cp-bpzcp)(η4-cod)] (1), [Ir(η5-Cp-bpzcp){η4-CH2═C(Me)C(Me)═CH2}] (2), [Ir(η5-Cp-bpzcp)(η2-coe)2] (3), and [Ir(η5-Cp-bpzcp)(η2-CH2═CH2)2] (4). The Ir(I) complex 4 reacts with CO or bromine to afford the compound [Ir(η5-Cp-bpzcp)(CO)2] (5) and the 18e- Ir(III) complex [Ir(κ-N-η5-Cp-bpzcpBr2)Br2] (6), respectively. Reaction of the iridium compounds (2-4) with CuI or [PdCl2(CH3CN)2] yields the heterobimetallic iridium-copper or iridium-palladium complexes [Ir(η5-Cp-bpzcp){η4-CH2═C(Me)C(Me)═CH2}(µ-bpzcp){CuI(κ2-NN-bpzcp)}] (7), [Ir(η5-Cp-bpzcp)(η2-coe)2}(µ-bpzcp){CuI(κ2-NN-bpzcp)}] (8), [Ir(η5-Cp-bpzcp)(η2-CH2═CH2)2}(µ-bpzcp){CuI(κ2-NN-bpzcp)}] (9), [Ir(η5-Cp-bpzcp)(coe)2}(µ-bpzcp){PdCl2(κ2-NN-bpzcp)}] (10), and [Ir(η5-Cp-bpzcp)(η2-CH2═CH2)2(µ-bpzcp){PdCl2(κ2-NN-bpzcp)}] (11). All products were characterized by spectroscopic methods and the X-ray crystal structures of 1, 2, 3, 4, and 6 were also established.

Expedient Synthesis of Highly Functionalized Abnormal Carbenegold(I) Complexes.

The reaction of 4-substituted imidazol-2-ylidenes with various electrophiles produces a series of 2,4-functionalized imidazolium salts. Subsequent metalation of these precursors using AuCl(SMe2) provides the first examples of highly functionalized abnormal carbenegold(I) complexes. The present protocol introduces a new strategy for the synthesis of metallic abnormal carbenes featuring diverse functional groups.

Bridged N-Heterocyclic/Mesoionic (NHC/MIC) Heterodicarbenes as Ligands for Transition Metal Complexes.

Following a copper catalyzed alkyne azide cycloaddition (CuAAC) and N-alkylation protocols, we report the preparation of a hybrid N-heterocyclic/mesoionic [NHC(H+)-MIC(H+)][2I]2- salt (1) in high yields. The treatment of salt 1 with Cu2O and KI yields a second hybrid NHC/MIC proligand featuring a tetraiodocuprate anion [NHC(H+)-MIC(H+)][Cu2I4]2- (2). Through selective deprotonation and metalation, both salts 1 and 2 can generate either the chelate heterodicarbene complexes (3) with the rare [NHC·(M)·MIC]+[MX2]- general formula (M = PdII, RhI) or NHC-anchored/pendent triazolium species (4) [NHC·(M)-MIC(H+)]. If the triazolium moiety of type 4 complexes is deprotonated with KHMDS in the presence of a second metal center, a series of heterobimetallic complexes of the type [NHC·(M)-MIC·(M')] (5) are achieved. Interestingly, the reaction of salt 2 with KHMDS yields the bimetallic copper heterodicarbene (6) which can be a useful transfer reagent for the preparation of type 3 complexes. A variety of synthetic routes for the preparation of complexes 3-5 and their full characterization in solution and in the solid state will be discussed.

Visible-Light-Promoted AuI to AuIII Oxidation in Triazol-5-ylidene Complexes.

Reaction of triazolium precursors [MIC(CH2 )n - H+ ]I- (n=1-3) with potassium hexamethyldisilazane (KHMDS) and AuCl(SMe2 ) generates the gold(I) complexes of the type MIC(CH2 )n ⋅AuI. Visible light exposure of the latter complexes promotes a spontaneous disproportionation process rendering gold(III) complexes of the type [{MIC(CH2 )n }2 ⋅AuI2 ]+ I- . Both the AuI and AuIII complex series were tested in the catalytic hydrohydrazination of terminal alkynes using hydrazine as nitrogen source.

Activation of aldehydes by exocyclic iridium(i)-η4:π2-diene complexes derived from 1,3-oxazolidin-2-ones.

The Ir(i) complexes [TpMe2Ir(η4-1,4-diene)] 2b and 2c react thermally with a variety of aromatic aldehydes, 3a-e, to generate the metallabicyclic compounds 4e-k and the Fischer-type carbenes 5a-b in moderate yields. These reactions are proposed to take place with the initial formation of η1-aldehyde adducts as key intermediates. The formation of the metallabicyclic compounds 4e-k involves a formal decarboxylation process at the exo-2-oxazolidinone diene and an ortho metallation of the aromatic ring. The generation of the Fischer-type carbenes 5a-b is the result of a series of metal-based rearranged intermediates with no decarboxylation observed. Treatment of the η4-diene complex 2b with a variety of Lewis bases induces a change in the binding mode of the diene ligand from η4:π2 to η2:σ2 to form the Ir(iii) derivatives 6b-d of composition TpMe2Ir-(η4:π2-1,4-diene)(L) (L = CO, MeCN, and C5H5N). A study of reactions of complex 2b with either mono- or poly-deuterated aldehydes was performed to understand the mechanisms of such processes. The results of these studies were used to determine plausible formation mechanisms of the metallabicyclic compounds 4e-4k and Fischer-type carbenes 5a-b compound series. These mechanisms were corroborated by density functional theory (DFT) calculations of the free energy profiles.

Synthesis and characterization of a Bi10O8(OAr)16 oxo-cluster supported by p-tert-butylcalix[5]arene ligands.

Reaction of a non-dried monobenzylated [(t)BuC5(OBn)(OH)4] ligand with excess of Bi[N(SiMe3)2]3 yields a mixture of the dimeric complex 1 [Bi{(t)BuC5(OBn)(OH)}]2 in 12% yield and complex 2 [Bi10O8{(t)BuC5(OBn)(OH)}4] in 48% yield. The highly symmetric complex 2 features a Bi10O8(OAr)16 central core with tetra-, hexa- and heptacoordinated bismuth(iii) centers, and (µ2)-(µ4) bridging oxygen atoms all arranged in a I4 symmetrical pattern. Cluster 2 represents the largest bismuth oxo-cluster supported by calix[n]arene ligands reported to date.

Synthesis of Ir(III) complexes with Tp(Me2) and acac ligands and their reactivity with electrophiles.

The reaction of the bis(ethylene) complex [Tp(Me2)Ir(C2H4)2] () (Tp(Me2) = hydrotris(3,5-dimethylpyrazolyl)borate) with an excess of 2,4-pentanedione (acetylacetone, Hacac) at 70 °C produced a mixture of the Ir(iii) complex [Tp(Me2)Ir(acac)(C2H5)] () as a major product (67% yield) and two other side complexes [Tp(Me2)Ir(acac)(H)] () and [Tp(Me2)Ir(C9H14O2)] () in 20 and 13% yields, respectively. According to the proposed reaction mechanism and DFT calculations, complexes and are generated from an 18e(-) intermediate [Tp(Me2)Ir(C2H4)(acac)(C2H3)] () which undergoes either hydrogen insertion or ß-hydride elimination followed by the subsequent loss of a molecule of ethylene. The lowest yielding complex which features a 2-iridafuran is presumably generated from an unusual thermal coupling between one vinylic and one acac moiety. The availability of the acidic α-proton of the acac ligand was tested by the treatment of complex with Br2 and Cu(NO3)2 rendering the substitution complexes [Tp(3-Br,Me2)Ir(3-Br-acac)Br] () and [Tp(Me2)Ir(3-NO2-acac)(C2H5)] () in good yields. The series of heteroleptic iridium(iii) compounds display air and moisture stability and have been characterized by NMR, IR, and elemental analyses, and, in the case of , and , by single-crystal X-ray diffraction analyses.

Copper(II) complexes supported by click generated mixed NN, NO, and NS 1,2,3-triazole based ligands and their catalytic activity in azide-alkyne cycloaddition.

The preparation and characterization of four new copper(ii) complexes supported by click generated mixed NN, NO, and NS 1,2,3-triazoles are reported. The four complexes display a 1 : 2 copper/ligand ratio and give monomeric units in the solid state. Crystal structures demonstrate that depending on the flexibility of the ligand NX (X = O, N, S) pendant arm, the coordination environment around the metal center can feature square planar or octahedral geometries. All four complexes are catalytically active at room temperature in a copper-catalyzed alkyne-azide cycloaddition (CuAAC) reaction using sodium ascorbate as a reducing agent and water-ethanol as a solvent mixture. Complex 8 supported by the NS ligand displayed the best catalytic performance of the series allowing for the easy and high yielding preparation of a variety of mono-, bis- and tris-1,2,3-triazoles under low catalyst loadings.

Pseudo-four component synthesis of mono- and di-benzylated-1,2,3-triazoles derived from aniline.

The pseudo-four component click synthesis of dibenzylated 1,2,3-triazoles derived from aniline is reported. The cycloaddition of sodium azide to N-(prop-2-ynyl)-benzenamine (I) in the presence of equimolar amounts of p-substituted benzyl derivatives, yields a mixture of mono- and dibenzylated 1,2,3-triazoles. When two equivalents of the benzyl derivative are added to the multicomponent reaction, the selective preparation of the dibenzylated compounds is achieved. The reactivity of the aniline N-H bond in monobenzylated 1,2,3-triazoles was tested by treatment with one equivalent of a p-substituted benzyl chloride at 40 °C, rendering the dibenzylated derivatives quantitatively.

Ynamides: stable ligand equivalents of unstable oxazol-4-ylidenes (novel mesoionic carbenes).

Deprotonation of an oxazolium salt induces a ring opening process leading to the corresponding ynamide. Although the expected mesoionic carbene is not obtained, the acyclic ynamide readily reacts with various transition metals yielding robust mesoionic carbene complexes.

Mesoionic thiazol-5-ylidenes as ligands for transition metal complexes.

The first examples of thiazol-5-ylidene complexes featuring group 9, 10 and 11 metal centers, have been prepared by deprotonation of a series of 2,3,4-triaryl-susbtituted thiazolium salts in the presence of the corresponding transition metal precursor.

Facile preparation of homo- and hetero-dimetallic complexes with a 4-phosphino-substituted NHC ligand. Toward the design of multifunctional catalysts.

A series of bimetallic complexes have been synthesized that are supported by a 4-phosphino-substituted NHC ligand. The use of this stable ligand reduces the number of synthetic steps and allows for a wide range of metal combinations to be introduced into the complexes.

A persistent (amino)(ferrocenyl)carbene().

Deprotonation of N,N-diisopropyl-C-ferrocenylaldiminium triflate 2 cleanly leads to the corresponding 1,2-diamino-1,2-diferrocenylethene 3, the dimer of the desired (amino)(ferrocenyl)carbene. Fulvene 6, obtained by addition of the lithium salt of tetramethylcyclopentadiene to methoxyformamidinium methylsulfate 5, reacts with dicarbonylcyclopentadienylbromoiron(II), and with a mixture of FeCl(2) and Cp* lithium salt, affording the corresponding tetramethylferrocenylaldiminium salt 7, and nonamethylferrocenylaldiminium salt 8, respectively. Although the deprotonation of 7 gives a complex mixture of products, the treatment of 8 at -78 °C with sodium hexamethyldisilazide allowed for the isolation of the corresponding (amino)(ferrocenyl)carbene 9 as a yellow powder. However, even in the solid state, it is stable for less than 48 h at -20 °C. In addition to NMR spectroscopy, evidence for the carbene nature of 9 was found by a trapping experiment with sulfur that leads to the corresponding adduct 10, which was characterized by a single crystal X-ray diffraction study.

Synthesis of 4- and 4,5-functionalized imidazol-2-ylidenes from a single 4,5-unsubstituted imidazol-2-ylidene.

Using the nucleophilicity of NHCs and aNHCs, as well as the leaving group ability of the former, the carbon-carbon double bond of imidazol-2-ylidenes can be readily mono- and difunctionalized. These results provide also a new light on the formation of abnormal carbene adducts from classical unsaturated NHCs.

Bismuth aryloxides.

The synthesis and full characterization (mp, NMR, UV/vis, FTIR, and elemental analysis) of 13 bismuth aryloxides are reported. We have prepared bismuth aryloxides with alkyl, aryl, and allylic substituents on the aryl rings. Eleven of these bismuth aryloxides have been characterized with single crystal X-ray diffraction methods. Bismuth-donor interactions (donor = aryl, methoxy) are observed in several cases. Three unexpected bismuth oxo aryloxides (6c, 9c, 11c) were also isolated. Complex C(77)H(102)Bi(4)Br(6)O(8) (6c) results from apparent C-H activation and Bi-C bond formation as a sideproduct in the synthesis of Bi(O-2,6-(i)Pr(2)-4-BrC(6)H(2))(3) (6). Cluster 9c has a Bi(32)O(56) core, and cluster C(90)H(90)Bi(4)Li(2)O(12) (11c) is the second lithium bismuth oxo cluster reported to date.

Facile synthesis of bismuth(III) and antimony(III) complexes supported by silylated calix[5]arenes.

A series of bismuth(III) and antimony(III) complexes supported by silicon-containing calix[5]arene ligands were synthesized and fully characterized by NMR, X-ray, IR, mp, UV/vis, and elemental analysis. Reaction of the para-tert-butylcalix[5]arene [(t)BuC5(H)(5)] disodium salt, Na(2) x (t)BuC5(H)(3), with 1 equiv of R(2)SiCl(2) (R = Me, (i)Pr, Ph, CH=CH(2)) or treatment of the (t)BuC5(H)(5) lower rim monobenzyl ether [(t)BuC5(Bn)(H)(4)] in a 1:1 ratio with Me(2)Si(NMe(2))(2) yields the (t)BuC5(SiRR')(H)(3) (1-5) and (t)BuC5(Bn)(SiMe(2))(H)(2) (6) ligands, respectively. The (1)H NMR spectra of the (t)BuC5(SiRR')(H)(3) (1-5) ligands show three pairs of doublets and three singlets for the (t)Bu peaks, consistent with a C(s) symmetry. In the case of the (t)BuC5(Bn)(SiMe(2))(H)(2) (6) ligand, the presence of the monobenzyl group changes the (1)H NMR patterns to indicate a C(1) symmetry. Treatment of (t)BuC5(SiRR')(H)(3) (1-5) or (t)BuC5(Bn)(SiMe(2))(H)(2) (6) with 1 equiv of M(O(t)Bu)(3) (M = Bi, Sb) or Sb(NMe(2))(2) readily yields metalated products of the type [M{(t)BuC5(SiRR')}] (7-16) and [MX{(t)BuC5(Bn)(SiMe(2))}] (X = O(t)Bu, (NMe(2))(2)) (17-19), respectively. All monometallic complexes [M{(t)BuC5(SiRR')}] (7-19) display excellent solubility in organic solvents including pentane and hexane. The (1)H NMR patterns for complexes 7-16 are consistent with a 1,2- or 1,3-alternate conformation while complexes [MX{(t)BuC5(Bn)(SiMe(2))}] (17-19) display patterns for a C(1) symmetry. All crystals show monomeric structures. Ligand (t)BuC5(SiPh(2))(H)(3) (3) displays a distorted cone conformation while the presence of the monobenzyl ether in (t)BuC5(Bn)(SiMe(2))(H)(2) (6) forces a partial cone conformation. Complexes 7-19 all display a distorted 1,2-alternate conformation with the metal centers displaying coordination numbers of three, four or five. No Si...M interactions were observed.