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This article describes a detailed comparison of the organometallic chemistry of high-valent nickel and palladium model complexes supported by tris(pyrazolyl)borate and cycloneophyl ligands. The accessibility of the MIII and MIV oxidation states with each metal is investigated through electrochemical and chemical oxidation of the MII precursors. These studies show that the NiII precursor readily undergoes both one- and two-electron oxidations to generate stable NiIII and NiIV products. In contrast, under the conditions examined, the PdII analogue undergoes exclusively two-electron-oxidation reactions to form PdIV. Reactivity studies of isolated NiIV and PdIV complexes show that both participate in C(sp3)-heteroatom coupling reactions and that the reactions at NiIV are approximately 2 orders of magnitude faster than those at PdIV. Experimental and computational mechanistic studies implicate outer-sphere SN2-type pathways for these processes. With most nucleophiles (e.g., phenoxide, acetate, thiophenoxide), the C(sp3)-heteroatom coupling reaction yields a TpMII(σ-aryl) product. However, with azide as the nucleophile, the NiII product of initial C(sp3)-N3 coupling undergoes a subsequent C(sp2)-N insertion reaction. Computations implicate an anionic NiIII-nitrene intermediate in this process and show that the Pd analogue of this species is a much higher energy species. Overall, the combined experimental and computational studies demonstrate remarkable similarities in the chemistry of NiIV and PdIV but an enhanced role for NiIII in enabling reactivity which is distinct from that of palladium. [ABSTRACT FROM AUTHOR]

The copper nanocluster [Cu3(μ3-H)(μ3-BH4)LPh3](BF4), 1a·BF4 (LPh = (PPh2)2NH = dppa), can potentially react with substrates at either the coordinated hydride or borohydride sites. Reaction of 1a·BF4 with CS2 has given rise to [Cu3(μ3-H)(μ2,μ1-S2CH)LPh3](BF4), (2a·BF4), which was structurally characterised using electrospray ionisation (ESI) with high-resolution mass spectrometry (HRMS), X-ray crystallography, NMR, IR and UV-Vis spectroscopy. The copper(i) atoms adopt a planar trinuclear Cu3 geometry coordinated on the bottom face by a μ3-hydride, on the top face by a μ2,μ1-dithioformate and surrounded by three bridging LPh ligands. Reaction of 1a·BF4 with elemental sulfur gives the known cluster [Cu4(LPh-H + 2S)3](BF4), (3·BF4), which was structurally characterised via X-ray crystallography. ESI-MS of 2a·BF4 produces [Cu3(H)(S2CH)LPh3]+ and its gas-phase ion chemistry was examined under multistage mass spectrometry conditions using collision-induced dissociation (CID). The primary product, [Cu3(H)(S2CH)LPh2]+, formed via ligand loss, undergoes further fragmentation via loss of thioformaldehyde to give [Cu3(S)LPh2]+. DFT calculations exploring rearrangement and fragmentation of the model system [Cu3(H)(S2CH)LMe2]+ (LMe = (PMe2)2NH = dmpa) provide a feasible mechanism. Thus, coupling of the coordinated hydride with the dithioformate ligands gives [Cu3(S2CH2)LMe2]+, which then undergoes CH2S extrusion via C–S bond cleavage to give [Cu3(S)LMe2]+. [ABSTRACT FROM AUTHOR]

This report describes a computational study of C(sp3)–OR bond formation from PdIV complexes of general structure PdIV(CH2CMe2-o-C6H4-C,C′)(F)(OR)(bpy-N,N′) (bpy = 2,2′-bipyridine). Dissociation of −OR from the different octahedral PdIV starting materials results in a common square-pyramidal PdIV cation. An SN2-type attack by −OR (−OR = phenoxide, acetate, difluoroacetate, and nitrate) then leads to C(sp3)–OR bond formation. In contrast, when −OR = triflate, concerted C(sp3)–C(sp2) bond-forming reductive elimination takes place, and the calculations indicate this outcome is the result of thermodynamic rather than kinetic control. The energy requirements for the dissociation and SN2 steps with different −OR follow opposing trends. The SN2 transition states exhibit “Pd…C…O” angles in a tight range of 151.5 to 153.0°, resulting from steric interactions between the oxygen atom and the gem-dimethyl group of the ligand. Conformational effects for various OR ligands and isomerisation of the complexes were also examined as components of the solution dynamics in these systems. In all cases, the trends observed computationally agree with those observed experimentally. [ABSTRACT FROM AUTHOR]

A combinationof experimental and density functional theory (DFT) investigationssuggests that the Cu-catalyzed fluorination of unsymmetrical diaryliodoniumsalts with general structure [Mes(Ar)I]+in N,N′-dimethylformamide proceeds through aCuI/CuIIIcatalytic cycle. A low concentrationof fluoride relative to combined iodonium reagent plus copper ensuresthat [Mes(Ar)I]+is available as the reactive species foroxidative “Ar+” transfer to a CuIcenter containing one or two fluoride ligands. A series of differentpossible CuIactive catalysts (containing fluoride, triflate,and DMF ligands) have been evaluated computationally, and all showlow-energy pathways to fluorinated products. The oxidation of theseCuIspecies by [Mes(Ar)I]+to form cis-Ar(F)CuIIIintermediates is proposed to be rate-limitingin all cases. Ar–F bond-forming reductive elimination fromCuIIIis computed to be very facile in all of the systemsexamined. The conclusions of the DFT experiments are supported byseveral experimental studies, including tests showing that CuIis formed rapidly under the reaction conditions and thatthe fluoride concentration strongly impacts the reaction yields/selectivities. [ABSTRACT FROM AUTHOR]

A computational analysis of the Pd-catalyzed couplingof 3-methyl-2-phenylpyridine(mppH) with [Ph2I]BF4to form mppPh issupportive of a prior synthetic and kinetic study implicating binuclearpalladium species in a rate-limiting oxidation step. The Pd(OAc)2precatalyst forms the “clamshell” orthopalladatedcomplex [Pd(mpp)(μ-OAc)]2(8) as the active catalyst, which is oxidized by [Ph2I]+in a reaction having the highest energy requirement of allsteps in the catalytic cycle. In the oxidation reaction, involvingformal transfer of Ph+, the electrophilic iodine centerinteracts initially with a bridging acetate oxygen atom of [Pd(mpp)(μ-OAc)]2(8), “Pd–O···IPh2”, which transforms to a transition structure withretention of the O···I interaction and formation ofa “Pd(μ-Ph-η1)I”bridge in a four-membered ring, “Pd···Ph···I(Ph)···O–Pd”,followed by elimination of PhI with formation of a binuclear Pd(III)cation containing a Pd–Pd bond, [Ph(mpp)Pd(μ-OAc)2Pd(mpp)]+(14). Cation 14undergoes mpp···Ph coupling at one Pd centerto form the binuclear Pd(II) cation [(mppPh-N)Pd(μ-OAc)2Pd(mpp)]+(Da). Cation Damay fragment to release mppPh and mononuclear palladiumspecies, followed by orthopalladation at a mononuclear center. However,in an environment of very low acetate concentration and high nitrogen-donorconcentration, it is considered far more likely that Daundergoes ligand exchange with release of mppPh and formation of[(mppH-N)Pd(μ-OAc)2Pd(mpp)]+(I). Computation shows a low-energy pathwayfor orthopalladation at cation Ithat involves nitrogen-donorreagents mppH and mppPh acting as bases to remove a proton as [HN-donor]+. This orthopalladation would complete the cycle and regeneratethe catalyst, [Pd(mpp)(μ-OAc)]2(8). A Hammett plot obtained from a computational analysisof the reaction of [(p-X-C6H4)(Mes)I]BF4(X = H, Me, OMe, F, Cl, COMe,CF3) has a reaction constant (ρ) of 1.8, which compareswell with the experimental result (ρ = 1.7 ± 0.2). Consistentwith this, the analysis reveals the dominant role of the interactionenergy for palladium- and iodine-containing fragments in the transitionstructure. [ABSTRACT FROM AUTHOR]

Abstract: The structural chemistry of dihalogenopalladium(II) and platinum(II) complexes of 2-organochalcogenomethylpyridine ligands is described. Complexes with a methyl group in the 6-position of the pyridyl ring, 6-MepyCH2ER, form dimeric complexes [trans-PdX2(μ-6-MepyCH2SePh-N,Se)]2 (X=Br (1), I (2)) or mononuclear complexes trans-PdI2(6-MepyCH2SR-N)2 (R=Me (5), Ph (6)). Absence of a 6-methyl substituent results in the bidentate configuration observed for PdI2(pyCH2SePh-N,Se) (3) and PdI2(4-MepyCH2SMe-N,S) (4). Related platinum(II) complexes are mononuclear including PtCl2(6-MepyCH2SPh-N,S) (8) as an analogue of trimeric [trans-PdCl2(μ-6-MepyCH2SPh-N,S)]3. Differences between palladium and platinum appear to result from a combination of steric and electronic factors. [Copyright &y& Elsevier]

Abstract: Ligands containing the 2-organochalcogenomethylpyridine motif with substituents in the 4- or 6-position of the pyridyl ring, R4,R6-pyCH2ER1 [R4 =R6 =H, ER1 =SMe (1), SeMe (2), SPh (6), SePh (7); R4 =Me, R6 =H, ER1 =SMe (3), SPh (8), SePh (9); R4 =H, R6 =Me, ER1 =SMe (4), SPh (10), SePh (11); R4 =H, R6 =Ph, ER1 =SMe (5), SPh (12), SePh (13)] are obtained on the reaction of R4,R6-pyMe with LiBu n followed by R1EER1. On reaction with PdCl2(NCMe)2, the ligands with a 6-phenyl substituent form cyclopalladated species PdCl{6-(o-C6H4)pyCH2ER1-C,N,E} (5a, 12a, 13a) with the structure of 13a (ER1 =SePh) confirmed by X-ray crystallography; other ligands form complexes of stoichiometry PdCl2(R4,R6-pyCH2ER1). Complexes with R6 =H are monomeric with N,E-bidentate configurations, confirmed by structural analysis for 3a (R4 =Me, ER1 =SMe), 7a (R4 =H, ER1 =SePh) and 9a (R4 =Me, ER1 =SePh). Two of the 6-methyl substituted complexes examined by X-ray crystallography are oligomeric with trans-PdCl2(N,E) motifs and bridging ligands, trimeric [PdCl2(μ-6-MepyCH2SPh-N,S)]3 (10a) and dimeric [PdCl2(μ-6-MepyCH2SePh-N,Se)] 2 (11a). This behaviour is attributed to avoidance of the Me···Cl interaction that would occur in the cis-bidentate configuration if the pyridyl plane had the same orientation with respect to the coordination plane as observed for 3a, 7a and 9a [dihedral angles 8.0(2)–16.8(2)°]. When examined as precatalysts for the Mizoroki–Heck reaction of n-butyl acrylate with aryl halides in N,N-dimethylacetamide at 120°C, the complexes exhibit the anticipated trends in yield (ArI>ArBr>ArCl, higher yield for electron withdrawing substituents in 4-RC6H4Br and 4-RC6H4Cl). The most active precatalysts are PdCl2(R4-pyCH2SMe-N,S) (R=H (1a), Me (3a)); complexes of the selenium containing ligands exhibit very low activity. For closely related ligands, the changes SMe to SPh, 6-H to 6-Me, and 6-H to 6-Ph lead to lower activity, consistent with involvement of both the pyridyl and chalcogen donors in reactions involving aryl bromides. The precatalyst PdCl2(pyCH2SMe-N,S) (1a) exhibits higher activity for the reaction of aryl chlorides in Bu n 4NCl at 120°C as a solvent under non-aqueous ionic liquid (NAIL) conditions. [Copyright &y& Elsevier]

The first of the title compounds, [Fe(C9H10N4O)3]Cl2·5.SH2O, is the first structurally characterized homoleptic tris[bis(imidazol-2-yl) ketone]-metal complex to be structurally defined. In the second of the title compounds, [FeCl2(C9H10N4O)2], a pair of monodentate donors supplants one of the bidentate ligands. In the third complex, [Fe2Cl4(C9H10N4O)2]·2CH3OH, a centrosymmetric binuclear compound with a pair of bridging chloride ligands, the introduction of a third chloro ligand results in five-coordination about each of the metal atoms; the environments of the latter are square pyramidal, with a chloro ligand at the apex and the two bidentate arrays about the base of the pyramid. [ABSTRACT FROM AUTHOR]

The first of the title compounds, [Fe(C12H15N6P)2][FeCl4]·4CH4O, is the first homoleptic bis(ligand)iron(II) species reported for this phosphorus bridgehead ligand; interestingly, the complex is isomorphous with its tris(1-methyl-1Himidazol-2-yl)methanol counterpart, the second of the title compounds, [Fe(C13H16N6O)2][FeCl4]2C1. In both compounds, the Fe atom of the cation lies on an inversion centre. The structure determinations presented here allow an interesting comparison of iron(II) and iron(III) species in similar coordination environments. [ABSTRACT FROM AUTHOR]

The title complex, [Pd{(py)3COH}2](NO3)2, where (py)3COH is tris(pyridin-2-yl)methanol (C16H13N3O), studied at 150 and 298 K, has the potentially tridentate ligands present as bidentate ligands, with the Pd atom at a centre of symmetry. At 150 K, the Pd-N distances are 2.022 (3) and 2.026 (3) Å. The mean planes of the coordinated pyridine groups form dihedral angles of 42.1 (1) and 45.3 (1)° (at 150 K) with the coordination plane; the uncoordinated rings situated either side of the coordination plane form dihedral angles of 42.2 (1)° with it. The nitrate ions are regular and interact weakly with the hydroxyl group. [ABSTRACT FROM AUTHOR]

In the title compound, [Pd(C6H5)Cl(C3H4N2)[P(C6H5)3]], the phenyl and Cl ligands lie mutually trans. The compound is the first structurally characterized complex with four monodentate Cl, P, N and (non-carbenoid) C ligands in a squareplanar four-coordinate palladium(II) environment. The pyrazole ligand is coplanar with the latter array. The pyrazole NH group forms a bifurcated hydrogen bond to Cl, with an intraand intermolecular component. [ABSTRACT FROM AUTHOR]

In the quasi-trigonal-bipyramidal environment of the fivecoordinate Fen atom in the title compound, [FeCl2(C14H18N6O)(CH4O)], the methanol and one of the N-atom donors of the potentially tridentate ligand are disposed axially: FeN(axial) is 2.149 (2) Å, Fe-N(equatorial) is 2.108 (2) Å and N-Fe-O is 174.14 (7)°. [ABSTRACT FROM AUTHOR]

Gas-phase studies utilizing ion-molecule reactions, supported by computational chemistry, demonstrate that the reaction of the enolate complexes [(CH2CO2--C,O)M(CH3)]- (M = Ni (5a), Pd (5b)) with allyl acetate proceed via oxidative addition to give MIV species [(CH2CO2--C,O)M(CH3)(η¹-CH2--CH-CH2)(O2CCH3--O,O')]- (6) that reductively eliminate 1-butene, to form [(CH2CO2--C,O)M(O2CCH3--O,O')]- (4). The mechanism contrasts with the MII-mediated pathway for the analogous reaction of [(phen)M(CH3)]+ (1a,b) (phen = 1,10-phenanthroline). The different pathways demonstrate the marked effect of electron-rich metal centers in enabling higher oxidation state pathways. Due to the presence of two alkyl groups, the metal-occupied d orbitals (particularly dz²) in 5 are considerably destabilized, resulting in more facile oxidative addition; the electron transfer from dz² to the C-C π* orbital is the key interaction leading to oxidative addition of allyl acetate to MII. Upon collision-induced dissociation, 4 undergoes decarboxylation to form 5. These results provide support for the current exploration of roles for NiIV and PdIV in organic synthesis. [ABSTRACT FROM AUTHOR]

The mechanism for the oxidation of 3'-dGMP by [PtCI4(dach)] (dach = diaminocyclohexane) in the presence of [PtCI2(dach)] has been investigated using density functional theory. We find that the initial complexation, Le., the formation of [PtCl3(dach)(3'-dGMP)], is greatly assisted by the reaction of the encounter pair [PtCl2(dach)--3'-dGMP] with [PtCl4>(dach)], leading to migration of an axial chlorine ligand from platLnum(lV) to platinum(H). A dinuclear platinum (ll)/platinum(rV) intermediate could not be found, but the reaction is predicted to pass through a platinum ( i l l ) / platinum (III) transition structure. A cyclization process, Le., C8--O bond formation, from [PtCl3(dach)(3'-dGMP)] occurs through an intriguing phosphate--water-assisted deprotonation reaction, analogous to the opposite of a proton shuttle mechanism. Followed by this, the guanine moiety is oxidized via dissociation of the Ptiv--CLax bond, and the cyclic ether product is finally formed after deprotonation. We have provided rationalizations, including molecular orbital explanations, for the key steps in the process. Our results help to explain the effect of [PtCl4(dach)] on the complexation step and the effect of a strong hydroxide base on the cyclization reaction. The overall reaction cycle is intricate and involves autocatalysis by a platinum(ll) species. [ABSTRACT FROM AUTHOR]