Metal–Metal Redox Exchange to Produce Heterometallic Manganese–Cobalt Oxo Cubanes via a “Dangler” Intermediate

Pendent metals bound to heterocubanes are components of well-known active sites in enzymes that mediate difficult chemical transformations. Investigations into the specific role of these metal ions, sometimes referred to as "danglers", have been hindered by a paucity of rational synthetic routes to appropriate model structures. To generate pendent metal ions bonded to an oxo cubane through a carboxylate bridge, the cubane Co4(μ3-O)4(OAc)4(t-Bupy)4 (OAc = acetate, t-Bupy = 4-tert-butylpyridine) was exposed to various metal acetate complexes. Reaction with Cu(OAc)2 gave the structurally characterized (by X-ray diffraction) dicopper dangler Cu2Co4(μ4-O)2(μ3-O)2(OAc)6(Cl)2(t-Bupy)4. In contrast, the analogous reaction with Mn(OAc)2 produced the MnIV-containing cubane cation [MnCo3(μ3-O)4(OAc)4(t-Bupy)4]+ by way of a metal-metal exchange that gives Co(OAc)2 and [CoIII(μ-OH)(OAc)]n oligomers as byproducts. Additionally, reaction of the formally CoIV cubane complex [Co4(μ3-O)4(OAc)4(t-Bupy)4][PF6] with Mn(OAc)2 gave the corresponding Mn-containing cubane in 80% yield. A mechanistic examination of the related metal-metal exchange reaction between Co4(μ3-O)4(OBz)4(py)4 (OBz = benzoate) and [Mn(acac)2(py)2][PF6] by ultraviolet-visible (UV-vis) spectroscopy provided support for a process involving rate-determining association of the reactants and electron transfer through a μ-oxo bridge in the adduct intermediate. The rates of exchange correlate with the donor strength of the cubane pyridine and benzoate ligand substituents; more electron-donating pyridine ligands accelerate metal-metal exchange, while both electron-donating and -withdrawing benzoate ligands can accelerate exchange. These experiments suggest that the basicity of the cubane oxo ligands promotes metal-metal exchange reactivity. The redox potentials of the Mn and cubane starting materials and isotopic labeling studies suggest an inner-sphere electron-transfer mechanism in a dangler intermediate

Efficient C-H Bond Activations O_2 Cleavage by a Dianionic Cobalt(II) Complex

A dianionic, square planar cobalt(II) complex reacts with O_2 in the presence of acetonitrile to give a cyanomethylcobalt(III) complex formed by C-H bond cleavage. Interestingly, PhIO and -tolylazide react similarly to give the same cyanomethylcobalt(III) complex. Competition studies with various hydrocarbon substrates indicate that the rate of C-H bond cleavage greatly depends on the pK_a of the C-H bond, rather than on the C-H bond dissociation energy. Kinetic isotope experiments reveal a moderate KIE value of ca. 3.5 using either O_2 or PhIO. The possible involvement of a cobalt(IV) oxo species in this chemistry is discussed

Olefin Hydroarylation Catalyzed by (Pyridyl-Indolate)Pt(II) Complexes: Catalytic Efficiencies and Mechanistic Aspects

A series of Pt(II) complexes of the type (N–N)PtPh(SR_2) (N–N = 2,2′-pyridyl-indolate) were prepared, and their performance as catalysts for the hydroarylation of olefins was assessed. Evidence that the catalysis is homogeneous and is Pt-mediated is provided by control experiments with added hindered base (2,6-di-tert-butyl-4-methylpyridine) and Hg(0). Two potential catalytic intermediates, (^tBuPyInd)PtPh(C_2H_4) and (^tBuPyInd)Pt(CH_2CH_2Ph)(C_2H_4), were synthesized, and their catalytic efficacy was explored. Additionally, decomposition and deactivation pathways, including styrene formation via β-hydride elimination and ligand reductive demetalation, were identified

Structural and Chemical Properties of Zwitterionic Iridium Complexes Featuring the Tripodal Phosphine Ligand [PhB(CH_2PPh_2)_3]^-

Several new iridium compounds bearing the PhB(CH_2PPh_2)_3^- (herein abbreviated as [PhBP_3]) ligand have been prepared and characterized, and a comparison of steric, electronic, and chemical properties is made with those of related pentamethylcyclopentadienyl (Cp^*) and hydridotris(3,5-dimethylpyrazolyl)borate (Tp^(Me)_2) complexes. The complexes [PhBP_3]Ir(H)(η_3-C_8H_(13)) (2) and [PhBP_3]Ir(H)(η^3-C_3H_5) (3) were synthesized from the reaction of [Li(TMED)][PhBP_3] (1) with the corresponding [(alkene)_2IrCl]_2 complex. These allyl complexes serve as precursors to the dihalides [PhBP_3]IrX_2 (10, X = I; 12, X = Cl). In addition to these dihalides, the five-coordinate species [PhBP_3]IrMe_2 (16) and [ClB(CH_2PPh_2)_3]IrCl_2 (13) have been isolated. Addition of CO to 2 or 3 gave [PhBP_3]Ir(CO)_2 (7), while reaction of H_2 with 2 yielded {[PhBP_3]IrH_2}_2 (8) in benzene and [PhBP_3]Ir(COE)H_2 (9) in THF (where COE = cyclooctene). Complex 2 reacted with PMePh_2 to give [PhBP_3]Ir(PMePh_2)H_2 (5) and 1,3-cyclooctadiene. The protonation of 5 with [H(OEt_2)]{B[3,5-C_6H_3(CF_3)_2]_4} gave the classical hydride complex {[PhBP_3]Ir(PMePh_2)H_3}{B[3,5-C_6H_3(CF_3)_2]_4} (6). In addition to the formation of allyl complexes 2 and 3, several C−H activation reactions have been observed; addition of P^(Me)_3 to 2 provided the cyclometalated product {PhB[(CH_2PPh_2)_2(CH_2PPhC_6H_4)]}Ir(H)(PMe_3)(4) and COE. Photolysis of 5 gave PhB[(CH_2PPh_2)_2(CH_2PPhC_6H_4)]}Ir(H)(PMePh_2) (A) and [PhBP_3]Ir(H)(PMePhC_6H_4) (B). Complex 9 catalyzes H/D exchange between COE and benzene-d_6. Metathesis reactions of diiodide 10 with LiBHEt_3 gave [Li(THF)_n]{[PhBP_3]Ir(H)_2I} (14a) and [Li(THF)_n]{[PhBP_3]Ir(H)_3} (15). Comparison of the spectroscopic properties of related [PhBP_3]Ir, Cp^*Ir, and Tp^(Me)_2Ir complexes suggests that relative donating abilities follow the trend [PhBP_3] ≥ Cp^* > Tp^(Me)_2, and structural comparisons indicate that [PhBP_3] is the most sterically demanding ligand

Electrophilic Activation of Silicon-Hydrogen Bonds in Catalytic Hydrosilations

Hydrosilation reactions represent an important class of chemical transformations and there has been considerable recent interest in expanding the scope of these reactions by developing new catalysts. A major theme to emerge from these investigations is the development of catalysts with electrophilic character that transfer electrophilicity to silicon via Si—H activation. This type of mechanism has been proposed for catalysts ranging from Group 4 transition metals to Group 15 main group species. Additionally, other electrophilic silicon species, such as silylene complexes and η3-H2SiRR' complexes, have been identified as intermediates in hydrosilation reactions. In this Review, different types of catalysts are compared to highlight the range of hydrosilation mechanisms that feature electrophilic silicon centers, and the importance of these catalysts to the development of new hydrosilation reactions is discussed

Efficient C-H Bond Activations O_2 Cleavage by a Dianionic Cobalt(II) Complex

A dianionic, square planar cobalt(II) complex reacts with O_2 in the presence of acetonitrile to give a cyanomethylcobalt(III) complex formed by C-H bond cleavage. Interestingly, PhIO and -tolylazide react similarly to give the same cyanomethylcobalt(III) complex. Competition studies with various hydrocarbon substrates indicate that the rate of C-H bond cleavage greatly depends on the pK_a of the C-H bond, rather than on the C-H bond dissociation energy. Kinetic isotope experiments reveal a moderate KIE value of ca. 3.5 using either O_2 or PhIO. The possible involvement of a cobalt(IV) oxo species in this chemistry is discussed

Binding, Release and Functionalization of Intact Pnictogen Tetrahedra Coordinated to Dicopper Complexes

The bridging MeCN ligand in the dicopper(I) complexes [(DPFN)Cu2(μ,η1 : η1-MeCN)][X]2 (X=weakly coordinating anion, NTf2 (1 a), FAl[OC6F10(C6F5)]3 (1 b), Al[OC(CF3)3]4 (1 c)) was replaced by white phosphorus (P4) or yellow arsenic (As4) to yield [(DPFN)Cu2(μ,η2 : η2-E4)][X]2 (E=P (2 a–c), As (3 a–c)). The molecular structures in the solid state reveal novel coordination modes for E4 tetrahedra bonded to coinage metal ions. Experimental data and quantum chemical computations provide information concerning perturbations to the bonding in coordinated E4 tetrahedra. Reactions with N-heterocyclic carbenes (NHCs) led to replacement of the E4 tetrahedra with release of P4 or As4 and formation of [(DPFN)Cu2(μ,η1 : η1-MeNHC)][X]2 (4 a,b) or to an opening of one E−E bond leading to an unusual E4 butterfly structural motif in [(DPFN)Cu2(μ,η1 : η1-E4DippNHC)][X]2 (E=P (5 a,b), E=As (6)). With a cyclic alkyl amino carbene (EtCAAC), cleavage of two As−As bonds was observed to give two isomers of [(DPFN)Cu2(μ,η2 : η2-As4EtCAAC)][X]2 (7 a,b) with an unusual As4-triangle+1 unit

An efficient one-pot synthesis of carbazole fused benzoquinolines and pyridocarbazoles

Cobalt­(II), in the presence of acetate and nitrate, quantitatively adds to the manganese–cobalt oxido cubane Mn^IVCo^III₃O₄(OAc)₅(py)₃ (<b>1</b>) to furnish the pentametallic dangler complex Mn^IVCo^III₃Co^IIO₄(OAc)₆(NO₃)­(py)₃ (<b>2</b>). Complex <b>2</b> is structurally reminiscent of photosystem II’s oxygen-evolving center, and is a rare example of a transition-metal “dangler” complex. Superconducting quantum interference device magnetometry and density functional theory calculations characterize <b>2</b> as having an <i>S</i> = 0 ground state arising from antiferromagnetic coupling between the Co^II and Mn^IV ions. At higher temperatures, an uncoupled state dominates. The voltammogram of <b>2</b> has four electrochemical events, two more than that of its parent cubane <b>1</b>, suggesting that addition of the dangler increases available redox states. Structural, electrochemical, and magnetic comparisons of complexes <b>1</b> and <b>2</b> allow a better understanding of the dangler’s influence on a cubane