IPr Ligand Research Articles

Reactivity of Strontium Hydride Supported by the Superbulky Hydrotris(pyrazolyl)borate Ligand.

Hydrogenolysis of [(TpAd,iPr)Sr{CH(SiMe3)2}] (1) (TpAd,iPr = hydrotris(3-adamantyl-5-isopropyl-pyrazolyl)borate) in hexane solution under 20 atm of H2 allowed for the isolation of strontium hydride [(TpAd,iPr)Sr(μ-H)]2 (2) in good yield. Complex 2 exhibits the dimeric nature in solid state, featuring two different bond modes between the Sr center and TpAd,iPr ligand. Treatment of complex 2 with PhC(H)═NtBu or PhCH2Bpin (Bpin = pinacolateborane) afforded the strontium amide complex [(TpAd,iPr)Sr{N(CH2Ph)(tBu)}] (4) and hydroborate complex [(TpAd,iPr)Sr{μ-HBpin(CH2Ph)}] (5), respectively. Reactions of complex 2 with 2-picoline, 2-phenylquinoline, or 2-phenylpyridine led to the formation of strontium 2-pyridylmethylene/2-picoline complex [(TpAd,iPr)Sr(2-CH2-Py)(2-picoline)] (6), reductively coupling diphenyl-biquinolide complex [{(TpAd,iPr)Sr}2(2,2'-Ph2-4,4'-dihydro-4,4'-biquinolide)] (7), and diphenyl-bipyridyl radical complex [(TpAd,iPr)Sr(6,6'-Ph2-2,2'-bipyridyl)] (8), separately. All of the complexes have been well characterized, including NMR spectrum and single-crystal X-ray analysis.

Gold(I) N‐heterocyclic carbene (NHC) complexes containing 6‐mercaptopurine derivatives and their in vitro anticancer and anti‐inflammatory effects

A series of eight N‐heterocyclic carbenes (NHC) gold(I) complexes, involving 1,3‐bis(2,6‐diisopropylphenyl)imidazol‐2‐ylidene (iPr) ligand in combination with 6‐mercaptopurine derivatives (HL1–8), has been prepared and thoroughly characterized, including elemental analysis, mass spectrometry, infrared and multinuclear NMR spectroscopy, and single crystal X‐ray analysis. The complexes, showing general composition of [Au (iPr)(Ln)] 1–8, were evaluated for their in vitro cytotoxicity against four human cancer cell lines including A2780 (ovarian) and A2780R (ovarian Cisplatin resistant), PC3 (prostate) and MCF‐7 (breast), and normal human MRC‐5 cells (lung fibroblasts). The complexes revealed significant cytotoxicity, with the best IC50 values ≈ 3.4–6.4 μM against A2780 and reasonable selectivity. Cellular effects of the selected complexes on the A2780 cells were evaluated using various flow cytometry assays. Complexes 1, 3, and 4 showed a strong pro‐apoptotic effect and a strong effect on the loss of mitochondrial membrane potential. These findings indicate that their major mechanism of action is based on the collapse of the mitochondrial metabolism and activation of the intrinsic signaling pathway of apoptosis, consequently resulting in cell death. The complexes 1–8 revealed only negligible effect on the production of inflammatory‐related cytokine (TNF‐α), as well as the activation of nuclear factor kappa‐light‐chain‐enhancer of activated B cells (NF‐κB) or peroxisome proliferator‐activated receptor gamma (PPARγ). Moreover, the shotgun proteomic analysis was performed, and the obtained results suggest that the mechanism of action of complexes 1, 3, and 4 differs somewhat from that of Auranofin.

Divergent Mechanistic Scenarios for the C−sp2 and C−sp3 C−H Insertion of Aurated Aryl Cations

AbstractBy employing the sterically highly demanding steering ligand IPr* at a gold(I) center, it is possible to induce a 6‐endo‐dig cyclization pathway in 1,5‐diynes bearing terminal alkyne moieties. The resulting aurated naphthyl cations are utilized for the intermolecular formation of C−C bonds via C−H insertion of aromatic and aliphatic C−H bonds. The mechanisms of these transformations were studied by quantum chemical calculations.

Tailor-made poly(vinylidene sulfide)s by Rh(I)–NHC catalyzed regioselective thiol-yne click polymerization

The [Rh(μ-Cl)(IPr)(η2-coe)]2/pyridine system efficiently catalyzes the polyhydrothiolation of a series of dialkynes with dithiols, producing sulfur-rich poly(vinylidene sulfide)s with a typical Mw in the range 20.000–124.000 and vinylidene content of 75–87%. A combination of flexible aliphatic dithiols, including 1,6-hexanedithiol and 2,2′-(ethylendioxy)diethanethiol, and the rigid aromatic dithiol 4,4′-thiobisbenzenethiol, with rigid aromatic dialkynes, 1,3-diethynylbenzene and 1,4-diethynylbenzene, and flexible dialkynes, including propargyl ether and 1,7-octadiyne, have been used to prepare poly(vinylidene sulfide)s. The copolymerization of flexible dithiols with rigid aromatic dialkynes or vice versa results in high molecular weight polymers, Mw up to 259.000, with low polydispersities. However, polyhidrothiolation of flexible dialkynes with flexible dithiols is much less efficient and usually results in the formation of oligomers. The interplay of the IPr and pyridine ligands on the RhCl(IPr)(py)(η2-coe) catalyst, which controls the regioselectivity of the alkyne insertion step towards the branched vinyl sulfide, is key in the preparation of these poly(vinylidene sulfide)s.

Synthesis and Reversible H2 Activation by Coordinatively Unsaturated Rhodium NHC Complexes

AbstractWe report the synthesis of coordinatively unsaturated cationic rhodium complexes bearing the sterically encumbered electron‐rich NHC ligand IPr*OMe. The COD (1,5‐cyclooctadiene) complex [Rh(IPr*OMe)(COD)]BF4 adopts a tilted, pseudo‐square planar coordination geometry, where bonding to the ipso‐carbon of the NHC aryl substituent was observed in the solid state. Hydrogenation of this complex afforded a metastable dihydride complex [Rh(IPr*OMe)(H)2]BF4 with an unusual internal coordination to an arene of the ligand. In the absence of a hydrogen atmosphere, spontaneous reductive elimination of H2 afforded a rhodium complex [Rh(IPr*OMe)]BF4 with a single chelating ligand that stabilizes the highly unsaturated metal by two‐fold π‐face donation as suggested by NMR spectroscopy and computational studies. This unusual complex might serve as a versatile precatalyst for a variety of transformations.

Nickel-catalyzed divergent Mizoroki–Heck reaction of 1,3-dienes

Developing efficient strategies to realize divergent arylation of dienes has been a long-standing synthetic challenge. Herein, a nickel catalyzed divergent Mizoroki–Heck reaction of 1,3-dienes has been demonstrated through the regulation of ligands and additives. In the presence of Mn/NEt3, the Mizoroki–Heck reaction of dienes delivers linear products under Ni(dppe)Cl2 catalysis in high regio- and stereoselectivities. With the help of catalytic amount of organoboron and NaF, the use of bulky ligand IPr diverts the selectivity from linear products to branched products. Highly aryl-substituted compounds can be transformed from dispersive Mizoroki–Heck products programmatically. Preliminary experimental studies are carried out to elucidate the role of additives.

Markovnikov Hydrosilylation of Alkynes with Tertiary Silanes Catalyzed by Dinuclear Cobalt Carbonyl Complexes with NHC Ligation.

Metal-catalyzed hydrosilylation of alkynes is an ideal atom-economic method to prepare vinylsilanes that are useful reagents in the organic synthesis and silicone industry. Although great success has been made in the preparation of β-vinylsilanes by metal-catalyzed hydrosilylation reactions of alkynes, reported metal-catalyzed reactions for the synthesis of α-vinylsilanes suffer from narrow substrate scope and/or poor selectivity. Herein, we present selective Markovnikov hydrosilylation reactions of terminal alkynes with tertiary silanes using a dicobalt carbonyl N-heterocyclic carbene (NHC) complex [(IPr)2Co2(CO)6] (IPr = 1,3-di(2,6-diisopropylphenyl)imidazol-2-ylidene) as catalyst. This cobalt catalyst effects the hydrosilylation of both alkyl- and aryl-substituted terminal alkynes with a variety of tertiary silanes with good functional group compatibility, furnishing α-vinylsilanes with high yields and high α/β selectivity. Mechanistic study revealed that the stoichiometric reactions of [(IPr)2Co2(CO)6] with PhC≡CH and HSiEt3 can furnish the dinuclear cobalt alkyne and mononuclear cobalt silyl complexes [(IPr)(CO)2Co(μ-η2:η2-HCCPh)Co(CO)3], [(IPr)(CO)2Co(μ-η2:η2-HCCPh)Co(CO)2(IPr)], and [(IPr)Co(CO)3(SiEt3)], respectively. Both dicobalt bridging alkyne complexes can react with HSiEt3 to yield α-triethylsilyl styrene and effect the catalytic Markovnikov hydrosilylation reaction. However, the mono(NHC) dicobalt complex [(IPr)(CO)2Co(μ-η2:η2-HCCPh)Co(CO)3] exhibits higher catalytic activity over the di(NHC)-dicobalt complexes. The cobalt silyl complex [(IPr)Co(CO)3(SiEt3)] is ineffective in catalyzing the hydrosilylation reaction. Deuterium labeling experiments with PhC≡CD and DSiEt3 indicates the syn-addition nature of the hydrosilylation reaction. The absence of deuterium scrambling in the hydrosilylation products formed from the catalytic reaction of PhC≡CH with a mixture of DSiEt3 and HSi(OEt)3 hints that mononuclear cobalt species are less likely the in-cycle species. These observations, in addition to the evident of nonsymmetric Co2C2-butterfly core in the structure of [(IPr)(CO)2Co(μ-η2:η2-HCCPh)Co(CO)3], point out that mono(IPr)-dicobalt species are the genuine catalysts for the cobalt-catalyzed hydrosilylation reaction and that the high α selectivity of the catalytic system originates from the joint play of the dicobalt carbonyl species to coordinate alkynes in the Co(μ-η2:η2-HCCR')Co mode and the steric demanding nature of IPr ligand.

Synthesis of N-heterocyclic carbene gold(I) complexes.

N-heterocyclic carbene gold(I) chloride and hydroxide complexes are regularly used as synthons to access various oxygen-, nitrogen- or carbon-bound gold complexes. They are also widely employed as efficient catalysts in addition reactions of hydroelements to unsaturated bonds and in several rearrangement and decarboxylation protocols. Here we describe the multigram synthesis of the most common mononuclear N-heterocyclic carbene gold(I) chloride complexes bearing the N,N'-bis-(2,4,6-trimethylphenyl)imidazol-2-ylidene (IMes), N,N'-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr) and N,N'-bis(2,6-bis(diphenylmethyl)-4-methylphenyl)imidazol-2-ylidene (IPr*) ligands. Their synthesis is achieved through the straightforward and practical weak base approach in a total time of 4-5 h. This straightforward methodology is conducted under air and possesses considerable advantages over alternative routes, such as the use of a sustainable reaction solvent, minimal amounts of a mild base and commercially available or easily obtained starting materials. Additionally, we describe the synthesis of the mononuclear gold(I) hydroxide complex bearing the IPr ligand, using the state-of-the-art method requiring 24 h. Finally, the improved synthesis of the dinuclear gold(I) hydroxide complex [{Au(IPr)}2(μ-OH)][BF4] is described (~3 h). All procedures can be performed by researchers with standard training and lead to high yields (76-99%) of microanalytically pure bench-stable materials.

Insights into the Regioselectivity of Hydroheteroarylation of Allylbenzene with Pyridine Catalyzed by Ni/AlMe3 with N-Heterocyclic Carbene: The Concerted Hydrogen Transfer Mechanism.

The hydroheteroarylation of allylbenzene with pyridine as catalyzed by Ni/AlMe3 and a N-heterocyclic carbene ligand has recently been established. Density functional calculations revealed that the common stepwise pathway, which involves the C-H oxidative addition of pyridine-AlMe3 before the migratory insertion of allylbenzene, is unlikely as the migratory insertion needs to overcome a prohibitively high energy barrier. In contrast, the ligand-to-ligand hydrogen transfer pathway is more favorable in which the hydrogen is transferred directly from the para-position of pyridine-AlMe3 to C2 of allylbenzene. Our distortion-interaction analysis and natural bond orbital analysis indicate that the interaction energy is strongly correlated with the extent of the charge transfer from the alkene (hydrogen acceptor) to the pyridine-AlMe3 (hydrogen donor), which dictates the selectivity of the H-transfer to the C2 position of allylbenzene. Then, the subsequent C-C reductive elimination of the regioselective linear product is facilitated by the steric hindrance of the IPr ligand. Understanding these key factors affecting the product regioselectivity is important to the development of catalysts for hydroheteroarylation of alkenes.

Ligand‐Facilitated Reductive Coupling of Benzyl Chlorides with Aryl Chlorides Catalyzed by Well‐Defined Heteroleptic Ni (II)‐NHC Complexes

Novel heteroleptic Ni (II) complexes bearing a highly hindered yet flexible IPr* ligand, Ni (IPr*)(PPh3)Br2 (1) and Ni (IPr*)(PCy3)Br2 (2) (IPr* = 1,3‐bis(2,6‐bis (diphenylmethyl)‐4‐methylphenyl)imidazol‐2‐ylidene), were easily prepared in 78% and 89% yield, respectively. Both were characterized by elemental analysis and NMR spectroscopy, and 1 was subjected to X‐ray crystallography. Compared with 2 and its analogue bearing a less sterically demanding IPr ligand (IPr = 1,3‐bis(2,6‐diisopropylphenyl)imidazol‐2‐ylidene), complex 1 exhibited superior catalytic activity in the magnesium‐mediated reductive coupling of benzyl chlorides with aryl chlorides, featuring outstanding tolerance of both coupling partners with steric demand. This study discloses a ligand‐facilitated reductive coupling of benzyl chlorides with aryl chlorides, which provides a new and practical synthetic tool for the synthesis of diarylmethanes.

Modular Chiral N-Heterocyclic Carbene Ligands for the Nickel-Catalyzed Enantioselective C-H Functionalization of Heterocycles.

N-Heterocyclic carbenes (NHCs) are the ligands of choice in a large variety of transformations entailing different transition metals. However, the number and variety of chiral NHCs suitable as stereo-controlling ligands in asymmetric catalysis remains limited. Herein we highlight the introduction of a modular NHC ligand family, consisting of a chiral version of the widely used IPr ligand. These chiral NHC ligands were applied in the nickel-catalyzed enantioselective C-H functionalization of N-heterocycles. Nickel-NHC catalysis unlocked the stereoselective C-H annulation of 2- and 4-pyridones, delivering fused bicyclic compounds found in many biologically active compounds. Applying a bulky, yet flexible ligand scaffold enabled the highly enantioselective C-H functionalization of pyridones under mild conditions. The introduction of a bulky chiral SIPr analogue enabled the nickel-catalyzed enantioselective C-H functionalization of indoles, yielding valuable tetrahydropyridoindoles. Additionally, pyrrolopyridines, pyrrolopyrimidines and pyrroles were efficiently functionalized, delivering chiral annulated azoles.

Gold(I)-Catalyzed Cascade Cyclization of Anilines with Diynes: Controllable Formation of Eight-Membered Ring-Fused Indoles and Propellane-Type Indolines

Heterocycle-fused indoles or indolines are distributed widely in a variety of natural products, bioactive agents, and pharmaceuticals. Herein, we describe the development of gold-catalyzed cascade reactions of anilines with diynes to form eight-membered ring-fused indoles and propellane-type indolines, both of which proceed through an intramolecular 5-endo-dig hydroamination followed by an 8-endo-dig cycloisomerization. Controllable formation of eight-membered ring-fused indoles and propellane-type indolines was achieved through selection of the ligands and/or solvents. Protic solvents such as alcohols or IPr ligand favored the formation of eight-membered ring-fused indoles, whereas the use of Buchwald's type ligands and/or nonpolar solvents gave propellane-type indoline predominantly. This reaction provides rapid access to two types of fused nitrogen heterocycles from simple aniline derivatives.

Two Carbenes versus One in Magnesium Chemistry: Synthesis of Terminal Dihalide, Dialkyl, and Grignard Reagents

Substantial progress has been made in the coordination chemistry of main-group elements with neutral donor ligands, largely ushered in by the development of stable N-heterocyclic carbenes (NHCs). There is growing interest in the synthesis of well-defined coordination compounds containing s-block metals; however, examples of molecular compounds containing “normal” NHCs bound to magnesium remain relatively understudied. We report that NHCs react with magnesium halides, MgX2 (X = Cl, Br, I), to afford (IPr)MgCl2(THF) (1), [(IPr)MgCl2]2 (2), (sIPr)2MgCl2 (3), (sIPr)2MgBr2 (4), and (sIPr)2MgI2 (5), where IPr is 1,3-bis(2,6-diisopropylphenyl)imidazole-2-ylidine and sIPr is 1,3-diisopropyl-4,5-dimethylimidizol-2-ylidine. Using the IPr ligand, weak NHC coordination and dynamic interaction with THF are observed in solution. In contrast, the coordination of two sIPr ligands results in higher purity, enhanced stability, and no observation of THF coordination. Dual carbene complexation with commercially available Mgn...

Barium tetraalkylaluminate complexes supported by the super-bulky hydrotris(pyrazolyl)borate ligand.

A heteroleptic barium aminobenzyl complex [(TpAd,iPr)Ba(o-CH2C6H4-NMe2)] (1) was obtained in excellent yield from a simple one-pot reaction. Treatment of [(TpAd,iPr)Ba(o-CH2C6H4-NMe2)] (1) with two equivalents of AlR3 (R = Me, Et) led to the formation of barium tetraalkylaluminate complexes [(TpAd,iPr)Ba(AlR4)]n (R = Me, n = 2, 2; R = Et, n = 1, 3) as dimers or monomers in the solid state. The TpAd,iPr ligand-free peralkylated barium complex [Ba(AlEt4)2]n was isolated by the addition of ten equivalents of AlEt3 under the same conditions. The donor-induced aluminate cleavage is not applicable when donor solvents are added to complexes 2 and 3. In the solution of complexes 2 and 3, the alkylaluminate moieties and TpAd,iPr ligands show a rapid fluxional behavior in [D8]toluene solution over the temperature range of -70 to 25 °C, without any significant decoalescence of the corresponding proton signals.

Influence of N-Heterocyclic Carbene Steric Bulk on Selectivity in Nickel Catalyzed C–H Bond Silylation, Germylation, and Stannylation

A series of Ni(0) compounds supported by electronically similar N-heterocyclic carbene (NHC) ancillary ligands with a range of %Vbur were used as catalysts for aryl C–H bond silylation, germylation, and stannylation. The NHC steric bulk strongly influenced the selectivity of C–H functionalization to give new carbon–heteroatom bonds versus alkene hydroarylation, despite little structural change in the resting state of the catalysts. Studies were performed by reacting C6F5H and H2C═CHER3 (ER3 = SnBu3, GePh3, SiMe3) using catalytic amounts of Ni(COD)2 and NHC ligands IPr, IMes, IBn, and iPr2Im. Catalytic C–H stannylation to give C6F5SnBu3 was facile with all ligands. The catalytic C–H germylation reaction was more difficult than stannylation but was demonstrated using H2C═CHGePh3 to give C6F5GePh3 for all but the largest NHC. The bulkiest NHC, IPr, gave a 96:4 ratio of the hydroarylation product C6F5CH2CH2GePh3 versus C6F5GePh3. The C–H silylation reactions required the highest temperatures and gave selective silylation product C6F5SiMe3 only for the smallest IBn and iPr2Im NHC ligands. Using the larger IMes carbene resulted in a 66:34 mixture of silylation and hydroarylation products, and the largest NHC, IPr, gave exclusive conversion to the hydroarylation product, C6F5CH2CH2SiMe3. DFT calculations are provided that give insight into the mechanism and key reaction steps, such as the relative difficulty of the critical β-Sn, Ge, and Si elimination steps.

A monomeric manganese(II) catecholato complex: Synthesis, crystal structure, and reactivity toward molecular oxygen

A monomeric manganese(II) catecholato complex with hydrotris(3-tert-butyl-5-isopropyl-1-pyrazolyl)borate (TptBu,iPr) was synthesized and characterized. X-ray analysis of the compound shows a tetrahedral manganese coordinated to one oxygen atom of the catecholato ligand and three nitrogen atoms of the TptBu,iPr ligand. The MnO distance (1.952(5) Å) is longer than the corresponding FeO distance (1.874(7) Å) in the analogous iron(II) TptBu,iPr complex due to the weaker Lewis acidity of manganese(II). Upon reacting of the manganese(II) complex with molecular oxygen, a manganese(II) semiquinonato complex was obtained, thus serving as an important model for catechol dioxygenase. This differs from the reaction of the iron(II) analog, which generates an iron(III) catecholato complex.

Theoretical Insight into Ligand- and Counterion-Controlled Regiodivergent Reactivity in Synthesis of Borylated Furans: 1,2-H vs 1,2-B Migration

Divergent catalytic reactions provide quick access to diverse molecular scaffolds through controlled reaction pathways by tuning the catalytic conditions, especially changing auxiliary ligands and/or counterions. This study presents a computational study of Au(I)-catalyzed regiodivergent cycloisomerizations of boron-containing alkynyl epoxides toward C2- and C3-borylated furans. The calculations clarified the mechanistic details of the reaction and the regiodivergence induced by two gold catalysts. The proposed catalytic cycle involves two major stages: ring expansion of alkynyl epoxide accompanied by B(MIDA) group migration to form a m-boron furyl heterocyclic intermediate and a formal 1,2-H or a 1,2-B migration to form a C3- or C2-borylated furan. The former is identified as the bottleneck of the reaction, which proceeds via σ activation of the oxirane moiety rather than π activation of the alkynyl moiety proposed in experimental work, and the latter controls the regiodivergence. Calculations show that the counterion and ligand in the gold catalyst play a less important role in the ring expansion, but they are crucial for the divergent formation of borylated furans: an OTf– counterion with an (ArO)3P ligand favors the 1,2-H migration, leading to the formation of C3-borylated furan, whereas an SbF6– anion with an IPr ligand promotes the 1,2-B migration, supporting the predominant formation of C2-borylated furan. The theoretical results rationalize the observed regioselectivity and provide key insights into the mechanism of the migratory cycloisomerization of boron-containing alkynyl epoxides.

Buttressing Effect as a Key Design Principle towards Highly Efficient Palladium/N-Heterocyclic Carbene Buchwald-Hartwig Amination Catalysts.

The backbone substitution of the standard 1,3-bis(2,6-diisopropylphenyl)-2H-imidazol-2-ylidene (IPr) ligand by dimethylamino groups was previously shown to induce a dramatic improvement in the catalytic efficiency of the corresponding Pd-PEPPSI (pyridine-enhanced pre-catalyst preparation, stabilization, and initiation) pre-catalysts in N-arylation reactions. Herein, a thorough structure/activity study towards rationalizing this beneficial effect has been described. In addition to the previously reported IPrNMe2 and IPr(NMe2)2 ligands, the new IPrNiPr2 and IPr(NMe2,Cl) ligands, which bear one bulkier diisopropylamino group and a combination of dimethylamino and chloro substituents, respectively, have been designed and analyzed in the study. The influence of the backbone substitution was found to be steric in origin and is related to the well-known buttressing effect encountered in arene chemistry. The usefulness and versatility of this approach was demonstrated through the development of a highly efficient catalytic system for the challenging arylation of bulky α,α,α-trisubstituted primary amines. The optimized system based on the [PdCl(η3 -cinnamyl)(IPr(NMe2)2 )] or [PdCl(η3 -cinnamyl)(IPrNiPr2 )] pre-catalysts operates under unprecedented mild conditions (catalyst loadings: 0.5-2 mol %, reaction temperatures: 40-60 °C) with a wide substrate scope.

Solvent-Reversible Addition of Alkyne C–H Bonds to Water-Soluble NHC Platinum(0) Complexes

Oxidative addition of CH3I and alkynes to the platinum(0) complex [Pt(AE)NHC] (1), in which the NHC ligand is a sulfonated version of the IPr ligand (1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) and AE is diallyl ether, is reported. Reaction with the haloalkane affords the iodomethyl complex [PtI(Me)NHC] (2), whereas the addition of alkynes via C(sp)–H bond activation leads to the corresponding new water-soluble hydride (NHC)PtII complexes 3 and 4. Complexes 3 and 4 were found to be moderately stable in water, displaying an interesting behavior in this solvent. Thus, in the presence of the corresponding olefin, the reaction can be reversible under mild conditions, thus allowing the formation of diolefinic (NHC)Pt0 complexes 1, 5, and 6, with elimination of the alkyne.

Reactions of Platinum Carbonyl Chini Clusters with Ag(NHC)Cl Complexes: Formation of Acid-Base Lewis Adducts and Heteroleptic Clusters.

The reactions of anionic platinum carbonyl Chini clusters [Pt3n(CO)6n]2- [n = 2 (1), 3 (2), 4 (3)] with Ag(IPr)Cl [IPr = C3N2H2(C6H3iPr2)2] afford the neutral acid-base Lewis adducts [Pt9(CO)18(AgIPr)2] (4) and [Pt6(CO)12(AgIPr)2] (5). These are thermally transformed into the homometallic heteroleptic neutral cluster [Pt3(CO)4(IPr)2] (6). Alternatively, 6 can be obtained from the reactions of 1-3 with an excess of the free IPr carbene ligand. The formation of 6 is sometimes accompanied by trace amounts of [Pt4(CO)4(IPr)3] (7). The reaction of 6 with free IPr affords the closely related [Pt3(CO)3(IPr)3] (8) heteroleptic cluster by substitution of the unique terminal CO ligand with a third IPr ligand. The reactions of 1-3 with Ag(IMes)Cl [IMes = C3N2H2(C6H2Me3)2] proceed differently from those involving Ag(IPr)Cl. Indeed, the only product isolated after workup is the bimetallic tetranuclear cluster [Pt3(CO)3(IMes)3(AgCl)] (9). 9 slowly reacts under a CO atmosphere, resulting in the pentanuclear [Pt5(CO)7(IMes)3] (10) complex. All of the new clusters 4-10 have been spectroscopically characterized and their molecular structures determined by X-ray crystallography. 4 and 5 retain the original trigonal-prismatic structures of the parent anionic Chini clusters, which are capped by two [Ag(IPr)]+ moieties. Conversely, 6-9 are based on a Pt3 triangular core decorated by CO and N-heterocyclic carbene ligands as well as Pt(CO) (in the case of 7) and AgCl (9) moieties. 10 displays an edge-bridged tetrahedral geometry.