Transition Metal Chain Complexes Supported by Soft Donor Assembling Ligands.

The chemistry of discrete molecular chains constituted by metals in low oxidation states, displaying metal-metal proximity and stabilized by suitable metal-bridging, assembling ligands comprising at least one soft donor atom is comprehensively reviewed; complexes with a single (hard or soft) bridging atom (e.g., µ-halide, µ-sulfide, or µ-PR2 etc.) as well as "closed" metal arrays (that fall in the realm of cluster chemistry) are excluded. The focus is on transition metal-based systems, with few excursions to cases combining transition and post-transition elements. Most relevant supporting ligands have neutral C, P, O, or S donor (mainly, N-heterocyclic carbene, phosphine, ether, thioether) or anionic donor (mainly phenyl, ylide, silyl, phosphide, thiolate) groups. A supporting-ligand-based classification of the metal chains is introduced, using as the classifying parameter the number of "bites" (i.e., ligand bridges) subtending each intermetallic separation. The ligands are further grouped according to the number of donor atoms interacting with the metal chain (called denticity in the following) and the column of the Periodic Table to which the set of donor atoms belongs (in ascending order). A complementary metal-based compilation of the complexes discussed is also provided in a concise tabular form.

N-Heterocyclic Carbene Complexes of Copper, Nickel, and Cobalt.

The emergence of N-heterocyclic carbenes as ligands across the Periodic Table had an impact on various aspects of the coordination, organometallic, and catalytic chemistry of the 3d metals, including Cu, Ni, and Co, both from the fundamental viewpoint but also in applications, including catalysis, photophysics, bioorganometallic chemistry, materials, etc. In this review, the emergence, development, and state of the art in these three areas are described in detail.

Linear CuI2 Pd0 , CuI Pd02 , and AgI2 Pd0 Metal Chains Supported by Rigid N,N'-Diphosphanyl N-Heterocyclic Carbene Ligands and Metallophilic Interactions.

Selective copper(I) to palladium(0) transmetallation of P-donors from the rigid N,N'-diphosphanyl-imidazol-2-ylidene C3 H2 [NP(tBu)2 ]2 (PCNHC P) was observed when known [Cu3 (µ3 -PCNHC P,κP,κCNHC ,κP)2 ](OTf)3 was reacted with [Pd(PPh3 )4 ]. When 1.2 equivalents of [Pd(PPh3 )4 ] was used, the product [Cu2 Pd(µ3 -PCNHC P,κP,κCNHC ,κP)2 ](OTf)2 (2(OTf)2 ) was obtained, which features a CuI -CuI -Pd0 chain and appears to be the first linear heterotrinuclear complex with d10 -d10 interactions between Pd0 and CuI . When the Cu3 precursor was reacted with 3.0 equivalents of [Pd(PPh3 )4 ], the complex [CuPd2 (µ3 -PCNHC P,κP,κCNHC ,κP)2 ](OTf)2 (3(OTf)2 ) was obtained, which, on the basis of magnetic measurements, DFT calculations, and computed nuclear shieldings, was formulated as containing a Pd0 -CuI -Pd0 chain with an electron hole delocalized over the whole cation, including the metal chain. Similarly, selective transmetallation of the P-donors in [Ag3 (µ3 -PCNHC P,κP,κCNHC ,κP)2 ](OTf)3 from silver to palladium (originating from [Pd(PPh3 )4 ]) gave the linear chain [Ag2 Pd(µ3 -PCNHC P,κP,κCNHC ,κP)2 ](OTf)2 (5(OTf)2 ), which on the basis of NMR spectroscopy comprises an AgI -AgI -Pd0 metal core. However, X-ray diffraction data collected on various samples of 5(OTf)2 were modeled with 50:50 metal disorder at the terminal positions, corresponding to a (AgI /Pd0 )-AgI -(AgI /Pd0 ) formulation. Upon standing in solution, 5(OTf)2 transformed to 6(OTf)2 , the regioisomer of 5(OTf)2 in which the Pd center has migrated to the central position of an AgI -Pd0 -AgI chain. Prolonged standing in CH2 Cl2 or by reaction with [PtCl2 (NCMe)2 ] converts complex 6(OTf)2 to the AgI /PdII complex [Ag2 PdCl2 (µ3 -PCNHC P,κP,κCNHC ,κP)2 ](OTf)2 (7(OTf)2 ). The structural data of 2(OTf)2 , 3(OTf)2 , and 7(OTf)2 establish significant heterometallophilic interactions.

Iron and Cobalt Metallotropism in Remote-Substituted NHC Ligands: Metalation to Abnormal NHC Complexes or NHC Ring Opening.

Metallotropism of the M[N(SiMe3 )2 ]2 metal fragment in the tautomeric system IAR ⇌ACR involving imidazolium salts/N-heterocyclic carbenes with remote aminide/amine substituents, respectively, is manifested by its CNHC carbophilicity (R=tBu, M=Co, Fe) or NRaminido nitrogenophilicity (R=Cy, M=Co, Fe; R=Mes, M=Fe) and has been rationalized on the basis of steric and electronic effects. The thermolysis products of the [M{N(SiMe3 )2 }2 ]/ IAR ⇌ACR system are also substituent-dependent, leading to a rearranged aminide-functionalized aNHC Co2 complex of an unprecedented type or to ring-opened metallaketenimines; they are postulated to originate from different metalloisomers. The results are interpreted on the basis of the X-ray diffraction analysis of 11 new compounds.

Linear, Trinuclear Cobalt Complexes with o-Phenylene-bis-Silylamido Ligands.

Transamination of [Co{N(SiMe3 )2 }2 ]2 with C6 H4 (NHSiiPr3 )2 gave the centrosymmetric trinuclear [{Coter N(SiMe3 )2 (µ-η-[o-C6 H4 (κNSiiPr3 )2 ])}2 Coint ] (1) (Coter , Coint =terminal, internal Co, respectively), with 3-coordinate Coter , and Coint "sandwiched" between the o-phenylenes of the two ligands; experimental and computational data support CoII centres and ditopic o-amido-imino-cyclohexen-allyl ligands; magnetic studies reveal intermetallic ferromagnetic interactions and single-molecule magnet (SMM) character. One-electron reduction of 1 yielded the salt [K(18-crown-6)(THF)2 ][{Coter N(SiMe3 )2 (µ-η-[o-C6 H4 (κNSiiPr3 )2 ])}2 Coint ] (4) with the anion isostructural to 1. The centrosymmetric Fe complex [{Feter N(SiMe3 )2 (µ-η-[o-C6 H4 (κNSiiPr3 )2 ])}2 Feint ] (5), analogous to 1, was also obtained.

Homo- and Heterodinuclear Ir and Rh Imine-functionalized Protic NHC Complexes: Synthetic, Structural Studies, and Tautomerization/Metallotropism Insights.

The influence of the potentially chelating imino group of imine-functionalized Ir and Rh imidazole complexes on the formation of functionalized protic N-heterocyclic carbene (pNHC) complexes by tautomerization/metallotropism sequences was investigated. Chloride abstraction in [Ir(cod)Cl{C3 H3N2 (DippN=CMe)-κN3}] (1 a) (cod=1,5-cyclooctadiene, Dipp=2,6-diisopropylphenyl) with TlPF6 gave [Ir(cod){C3 H3N2 (DippN=CMe)-κ(2) (C2,Nimine )}](+) [PF6 ](-) (3 a(+) [PF6 ](-)). Plausible mechanisms for the tautomerization of complex 1 a to 3 a(+) [PF6 ](-) involving C2-H bond activation either in 1 a or in [Ir(cod){C3 H3 N2 (DippN=CMe)-κN3}2](+) [PF6 ](-) (6 a(+) [PF6 ](-)) were postulated. Addition of PR3 to complex 3 a(+) [PF6 ](-) afforded the eighteen-valence-electron complexes [Ir(cod)(PR3){C3 H3N2 (DippN=CMe)-κ(2) (C2,Nimine )}](+) [PF6 ](-) (7 a(+) [PF6 ](-) (R=Ph) and 7 b(+) [PF6 ](-) (R=Me)). In contrast to Ir, chloride abstraction from [Rh(cod)Cl{C3H3N2 (DippN=CMe)-κN3}] (1 b) at room temperature afforded [Rh(cod){C3 H3N2 (DippN=CMe)-κN3}2](+) [PF6 ](-) (6 b(+) [PF6 ](-)) and [Rh(cod){C3 H3N2 (DippN=CMe)-κ(2) (C2,Nimine )}](+) [PF6 ](-) (3 b(+) [PF6 ](-) ) (minor); the reaction yielded exclusively the latter product in toluene at 110 °C. Double metallation of the azole ring (at both the C2 and the N3 atom) was also achieved: [Ir2 (cod)2 Cl{µ-C3H2N2 (DippN=CMe)-κ(2) (C2,Nimine ),κN3}] (10) and the heterodinuclear complex [IrRh(cod)2 Cl{µ-C3H2N2 (DippN=CMe)-κ(2) (C2,Nimine ),κN3}] (12) were fully characterized. The structures of complexes 1 b, 3 b(+) [PF6 ](-) , 6 a(+) [PF6 ](-) , 7 a(+) [PF6 ](-), [Ir(cod){C3HN2 (DippN=CMe)(DippN=CH)(Me)-κ(2) (N3,Nimine )}](+) [PF6 ](-) (9(+) [PF6 ](-)), 10⋅ Et2 O⋅toluene, [Ir2 (CO)4 Cl{µ-C3H2N2 (DippN=CMe)-κ(2) (C2,Nimine ),κN3}] (11), and 12⋅2 THF were determined by X-ray diffraction.

Novel Di- and Trinuclear Palladium Complexes Supported by N,N'-Diphosphanyl NHC Ligands and N,N'-Diphosphanylimidazolium Palladium, Gold, and Mixed-Metal Copper-Gold Complexes.

The reaction of the trinuclear complex [Ag3(µ3-PC(NHC)P,κP,κC(NHC),κP)2](OTf)3 (Ag3; PC(NHC)P = N,N'-bis(di-tert-butylphosphanyl)imidazol-2-ylidene) with [Pd(dba)2] afforded the trinuclear palladium complex [Pd3(µ3-PC(NHC)P,κP,κC(NHC),κP)2](OTf)2 (Pd3) and the dinuclear palladium(I) complex [Pd2(µ2-PC(NHC)P,κP,κC(NHC),κP)2](OTf)2 (Pd2). The assignment of the oxidation state of the metals in the mixed-valence Pd3 chain as Pd(0)-Pd(II)-Pd(0) was based on the reactivity of the complex with 2,6-dimethylphenyl isocyanide and density functional theory calculations. Reaction of PCNHCP with [PdMe2(tmeda)] afforded the palladium(II) complex [PdMe2(PC(NHC)P,κP,κC(NHC))] (Pd-Me2), with PC(NHC)P acting as a bidentate ligand. The reaction of PC(NHC)P with [Pd(dba)2] led to a dinuclear palladium(0) complex [Pd2(µ2-PC(NHC)P,κP,κC(NHC),κP)](dba) (Pd2-dba); attempted replacement of the remaining dba by PC(NHC)P failed. The imidazolium triflate PCHP, precursor to PC(NHC)P, was reacted with [Pd2(dba)3]·CHCl3 to give the (2 + 2) metalla-mesocyclic cationic palladium(0) complex [Pd2(µ2-PCHP,κP,κP)2] (PCHP-Pd2), which resisted further deprotonation of the imidazolium cation. In contrast, PCHP reacted with [AuCl(tht)] to give [Au2Cl2(µ2-PCHP,κP,κP)] (PCHP-Au2), in which one Au-Cl moiety is bound to each P donor. Further reaction of PCHP-Au2 with [Au{N(SiMe3)2}(PPh3)] afforded a mixture of the trinuclear [Au3(µ3-PC(NHC)P,κP,κC(NHC),κP)2](OTf)3 (Au3) and [AuCl(PPh3)], while reaction with [CuMes]5, where Mes = 2,4,6-trimethylphenyl, resulted in a novel, centrosymmetric, heterometallic complex [Au2Mes2(Cu4Cl4)(PCHP,κP,κP)2] (PCHP-AuCu) featuring a new PCHP-AuMes metalloligand bridging a Cu···Cu diagonal of a Cu4Cl4 cubane via the P and AuMes functionalities.

Bonding, Luminescence, Metallophilicity in Linear Au3 and Au2Ag Chains Stabilized by Rigid Diphosphanyl NHC Ligands.

The heterofunctional and rigid ligand N,N'-diphosphanyl-imidazol-2-ylidene (PCNHCP; P = P(t-Bu)2), through its phosphorus and two N-heterocyclic carbene (NHC) donors, stabilizes trinuclear chain complexes, with either Au3 or AgAu2 cores, and dinuclear Au2 complexes. The two oppositely situated PCNHCP (L) ligands that "sandwich" the metal chain can support linear and rigid structures, as found in the known tricationic Au(I) complex [Au3(µ3-PCNHCP,κP,κCNHC,κP)2](OTf)3 (OTf = CF3SO3; [Au3L2](OTf)3; Chem. Commun. 2014, 50, 103-105) now also obtained by transmetalation from [Ag3(µ3-PCNHCP,κP,κCNHC,κP)2](OTf)3 ([Ag3L2](OTf)3), or in the mixed-metal tricationic [Au2Ag(µ3-PCNHCP,κP,κCNHC,κP)2](OTf)3 ([Au2AgL2](OTf)3). The latter was obtained stepwise by the addition of AgOTf to the digold(I) complex [Au2(µ2-PCNHCP,κP,κCNHC)2](OTf)2 ([Au2L2](OTf)2). The latter contains two dangling P donors and displays fluxional behavior in solution, and the Au···Au separation of 2.8320(6) Å in the solid state is consistent with metallophilic interactions. In the solvento complex [Au3Cl2(tht)(µ3-PCNHCP,κP,κCNHC,κP)](OTf)·MeCN ([Au3Cl2(tht)L](OTf)·MeCN), which contains only one L and one tht ligand (tht = tetrahydrothiophene), the metal chain is bent (148.94(2)°), and the longer Au···Au separation (2.9710(4) Å) is in line with relaxation of the rigidity due to a more "open" structure. Similar features were observed in [Au3Cl2(SMe2)L](OTf)·2MeCN. A detailed study of the emission properties of [Au3L2](OTf)3, [Au3Cl2(tht)L](OTf)·MeCN, [Au2L2](OTf)2, and [Au2AgL2](OTf)3 was performed by means of steady state and time-resolved photophysical techniques. The complex [Au3L2](OTf)3 displays a bright (photoluminescence quantum yield = 80%) and narrow emission band centered at 446 nm with a relatively small Stokes' shift and long-lived excited-state lifetime on the microsecond timescale, both in solution and in the solid state. In line with the very narrow emission profile centered in the violet-blue region, fabrication of organic light-emitting devices (OLEDs) comprising the [Au3L2](OTf)3 complex demonstrated its usefulness as a deep-blue emitter in solution-processed OLEDs. Electrochemical and Raman spectroscopic studies were also performed on [Au3L2](OTf)3. Experimental results were rationalized by means of Wave-Function Theory (WFT) and Density Functional Theory (DFT). MP2 calculations gave a satisfactory description of the structures of the cationic complexes [Au3L2](3+) and [Au2L2](2+) and pointed to Au···Au interactions having an electrostatic component owing to the dissimilar charge distribution in the chain caused by the heterofunctional ligand. The nature of the emitting states and their geometric distortions relative to the ground states in [Au3L2](3+) and [Au2L2](2+) was studied by DFT, revealing contraction of the Au···Au distances and coordination geometry changes by association of the dangling P donor, respectively.

A Bis(Diphosphanyl N-Heterocyclic Carbene) Gold Complex: A Synthon for Luminescent Rigid AuAg2 Arrays and Au5 and Cu6 Double Arrays.

A mononuclear bis(NHC)/Au(I) (NHC=N-heterocyclic carbene) cationic complex with a rigid bis(phosphane)-functionalized NHC ligand (PC(NHC)P) was used to construct linear Au3 and Ag2 Au arrays, a Au5 cluster with two intersecting crosslike Au3 arrays, and an unprecedented Cu6 complex with two parallel Cu3 arrays. The impact of metallophilic interactions on photoluminescence was studied experimentally.

Aurophilicity-triggered assembly of novel cyclic penta- and hexanuclear gold(I) complexes with rigid anionic NHC-type ligands.

The products of the reaction between N,N'-diphosphanylimidazol-2-ylidene (P-C-P) and gold(I) precursors depend on the nature of the anions associated with the latter. In contrast to the reported reaction with [Au(tht)2(OTf)], the use of [AuCl(tht)] led to the new hexanuclear complex 1, which features a Au6(µ3-P-C,κC,κN,κP)3 skeleton. The reaction of lithium imidazolide (P-C-Li) and [AuCl(tht)] also afforded 1, together with an unusual salt of the general formula [Au5Cl(µ3-P-C-κP,κC,κN)3]2[AuCl2]2 (2), which contains [Au5(µ3-P-C-κP,κC,κN)](+) subunits. In the solid state, one of these Au5 cations is associated with an [AuCl2](-) anion, while two other cations interact through their unique dicoordinated N-Au-N center with a [AuCl2](-) anion, with the charge of the resulting monocation being compensated for by another [AuCl2](-) anion to give a Au12 salt. Remarkably, the latter displays seven different bonding types at Au(I): C-Au-C, N-Au-N, P-Au-P, Cl-Au-Cl, C-Au-N, P-Au-Cl, and Au···Au.

Relative Lability and Chemoselective Transmetallation of NHC in Hybrid Phosphine-NHC Ligands: Access to Heterometallic Complexes.

The relative lability and transmetallation aptitude of trialkylphosphine and NHC donors, integrated in semi-rigid hybrid ligands attached to [Ag4Br4] pseudo-cubanes, lies in favor of the NHC and is used to selectively access unprecedented NHC complexes with heterobimetallic cores, such as Ag-Cu (4(Cy)) and Ag-Ir (5(tBu)). These can be viewed as an arrested state before the full transmetallation of both donors, which gives the homodinuclear Cu (3(Cy)) and Ir (6(Cy)) complexes. The observed NHC transmetallation aptitude and reactivity urges caution in the common notion that views the NHC as a universal spectator.

New Pyridine Dicarbene Pincer Ligands with Ring Expanded NHCs and their Nickel and Chromium Complexes.

The pincer complexes [NiIIBr(CNC)]Br (4), [CrIIIBr3(CNC)] (5 a) and [CrIIIBr2.3Cl0.7(CNC)] (5 b), where CNC=3,3'-(pyridine-2,6-diyl)bis(1-mesityl-3,4,5,6-tetrahydropyrimidin-2-ylidene), were obtained from the novel ligand CNC, generated in situ from the precursor (CHNCH)Br2 and [NiIIBr2(PPh3)2] or from [CrII{N(SiMe3)2}2(THF)2] and (CHNCH)Br2 by aminolysis, respectively. The tetrahedrally distorted square planar (τ4≅0.30) geometry and the singlet ground state of Ni in 4 were attributed to steric constraints of the CNC backbone. Computational methods highlighted the dependence of the coordination geometry and the singlet-triplet energy difference on the size of the N-substituent of the tetrahydropyrimidine wingtips and contrasted it to the situation in 5-membered imidazolin-2-ylidene pincer analogues. The octahedral CrIII metal center in 5 a and 5 b is presumably formed after one electron oxidation from CH2Cl2. 4/MAO and 5 a/MAO were catalysts of moderate activity for the oligomerization and polymerization of ethylene, respectively. The analogous (CH^N^CH)Br2 precursor, where (CH^N^CH)=3,3'-(pyridine-2,6-diylbis(methylene))bis(1-mesityl-3,4,5,6-tetrahydropyrimidin-1-ium), was also prepared, however its coordination chemistry was not studied due to the inherent instability of the resulting free C^N^C ligand.

Gold complexes with remote-substituted amino N-heterocyclic carbenes.

The 4-RN-1,3-Ar2-imidazolium salt, R = iPr, tBu, Ar = Mes, Dipp, Mes = mesityl, Dipp = 2,6-bis-diisopropyl-phenyl was metalated by AuI at the C2-, C5- and 4-RN positions depending on the reactants and conditions employed; a rare direct rearrangement of a AuI aminide to an abnormal imidazol-5-ylidene AuI complex was also observed and based on a DFT study it may involve TfO- facilitated H+ transfer.

Structural and reactivity studies of "pincer" pyridine dicarbene complexes of Fe0: experimental and computational comparison of the phosphine and NHC donors.

Reduction of [Fe(C-N-C)(Br)(2)] or [Fe(C-N(Me)-C)(I)(2)] with Na/Hg under N(2) gave the Fe(0) complexes [Fe(C-N-C)(N(2))(2)] and [Fe(C-N(Me)-C)(N(2))(2)] for which C-N-C = 2,6-bis(arylimidazol-2-ylidene)-pyridine, C-N(Me)-C = 2,6-bis(arylimidazol-2-ylidene)-3,5-dimethyl-pyridine and aryl = 2,6-iPr(2)C(6)H(3). Substitution of the coordinated N(2) by CO or CN(2,6-xyl), xyl = 2,6-dimethylphenyl, took place readily to afford [Fe(C-N-C)(L)(2)] and [Fe(C-N(Me)-C)(L)(2)], L = CO or CN(2,6-xyl). The electronic characteristics of the N-heterocyclic carbene and phosphine donors in the complexes [Fe(C-N-C)(CO)(2)], [Fe(C-N(Me)-C)(CO)(2)] and [Fe(P-N-P)(CO)(2)], P-N-P = 2,6-bis(di-tert-butylphosphinomethyl)-pyridine, have been evaluated by spectroscopic, structural and computational methods. The unexpected reduced Fe(0) to CO backbonding in [Fe(C-N-C)(CO)(2)] and [Fe(C-N(Me)-C)(CO)(2)] compared to [Fe(P-N-P)(CO)(2)] is accounted for by backbonding from the Fe-N(pyridine) bond in the imidazol-2-ylidene complexes. Reduction of [Fe(C-N-C)(Br)(2)] under Ar gave mixtures from which a complex with a metalated pyridine and a dangling imidazolium group was isolated. Photolysis of [Fe(C-N-C)(N(2))(2)] in the presence of benzaldehyde phenylimine gave an ortho-metalated benzaldehyde phenylimine. The Fe(II) complex [Fe(C-N-C)(CO)(Br)(2)] was prepared by the reaction of [Fe(C-N-C)(Br)(2)] with CO or reduction of CO(2) with [Fe(C-N-C)(N(2))(2)].

Reactivity of a dearomatised pincer CoIIBr complex with PNCNHC donors: alkylation and Si-H bond activation via metal-ligand cooperation.

The double aminolysis reaction of [Co{N(SiMe3)2}2] by the salt 1-(6-((dicyclohexylphosphaneyl)methyl)pyridin-2-yl)-3-(2,6-diisopropylphenyl)-1H-imidazol-3-ium bromide, which contains one phosphane, one pyridine and one imidazolium groups, of formula [o-Cy2PCH2(C5H3N)(o-C3H3N2DiPP)]Br and abbreviated as (CyPNpyrCim)Br, was previously shown to afford the Co(ii) complex [Co(CyP*NaCNHC)Br] (1) containing a dearomatised picolyl moiety in the tridentate, anionic donor ligand CyP*NaCNHC (Na = anionic amido, P* = vinylic P donor). We now report that formation of 1 is preceded by an intermediate tentatively assignable to the 5-coordinate Co(ii) complex [Co(CyPNpyrCNHC){N(SiMe3)2}Br] (2). The reaction of 1 with LiCH2SiMe3 afforded the dark purple, paramagnetic [Co(CyP*NaCNHC)CH2SiMe3] (3) with a low spin d7 CoII; the electronic configurations of 1 and 3 were corroborated by EPR spectroscopy. Addition of excess (≥4 equiv.) H2SiPh2 to a solution of 1 gave the diamagnetic [Co(CyP(SiHPh2)NCNHC)Br] (4) following Si-H activation and silylation of the ligand backbone at the α-CHP. Reduction of CoII to CoI by silanes is uncommon.

Non-symmetrical, potentially redox non-innocent imino NHC pyridine 'pincers' via a zinc ion template-assisted synthesis.

New non-symmetrical, redox-active imino NHC pyridine pincer ligands, 2-(R1-imidazol-2-ylidene)-6-(R2N[double bond, length as m-dash]CH)-pyridine (R1 = 2,6-diisopropylphenyl (DiPP), R2 = 2,4,6-trimethylphenyl (Mes), 4B; R1 = R2 = DiPP, 4C), have been accessed by a ZnII-promoted modular synthesis involving the quaternization of R1-imidazole by [Zn(κNimineκNpyridine)(2-(R2N[double bond, length as m-dash]CH)-6-bromo-pyridine)Cl2], followed by ZnII removal and deprotonation of imidazolium pro-ligands. Redox active forms of 4B were implicated in the two complexes obtained by its reaction with FeBr2/KC8; metrical data analysis pointed to the occurrence of radical anionic and dianionic redox states of 4B.

A Straightforward Access to Stable, 16 Valence-electron Phosphine-Stabilized Fe0 Olefin Complexes and their Reactivity.

The use of the dialkene divinyltetramethyldisiloxane (dvtms) allows easy access to the reactive 16 valence-electron complexes [Fe0(L-L)(dvtms)], (L-L) = dppe (1,2-bis(diphenylphosphino)ethane), (1), dppp (1,2-bis(diisopropylphosphino)propane), (2), pyNMeP(iPr)2 (N-(diisopropylphosphino)-N-methylpyridin-2-amine), (4), dipe (1,2-bis(diisopropylphosphino)ethane), (5), and [Fe0(L)2(dvtms)], L = PMe3, (3), by a mild reductive route using AlEt2(OEt) as reducing agent. In contrast, by the same methodology, the 18 valence-electron complexes [Fe0(L-L)2(ethylene)], (L-L) = dppm (1,2-bis(diphenylphosphino)methane), 6, (L-L) = dppa (1,2-bis(diphenylphosphino)amine) 7 or (L-L)=dppe, 8, were obtained, which do not contain dvtms. In addition, a combined DFT and solid-state paramagnetic NMR methodology is introduced for the structure determination of 5. A comparative study of the reactivity of 1,2,4-6 and 8 with 3-hexyne highlights emerging mechanistic implications for C-C coupling reactions using these complexes as catalysts.

Heteroleptic, two-coordinate [M(NHC){N(SiMe3)2}] (M = Co, Fe) complexes: synthesis, reactivity and magnetism rationalized by an unexpected metal oxidation state.

The linear, two-coordinate and isostructural heteroleptic [M(IPr){N(SiMe3)2}] (IPr = 1,3-bis(diisopropylphenyl)-imidazol-2-ylidene), formally MI complexes (M = Co, 3; Fe, 4) were obtained by the reduction of [M(IPr)Cl{N(SiMe3)2}] with KC8, or [Co(IPr){N(SiMe3)2}2] with mes*PH2, mes* = 2,4,6-tBu3C6H2. The magnetism of 3 and 4 implies CoII and FeII centres coupled to one ligand-delocalized electron, in line with XPS and XANES data; the ac susceptibility of 4 detected a pronounced frequency dependence due to slow magnetization relaxation. Reduction of [Fe(IPr)Cl{N(SiMe3)2}] with excess KC8 in toluene gave the heteronuclear 'inverse-sandwich' Fe-K complex 7, featuring η6-toluene sandwiched between one Fe0 and one K+ centre.

Coinage metal complexes with bridging hybrid phosphine-NHC ligands: synthesis of di- and tetra-nuclear complexes.

A series of P-NHC-type hybrid ligands containing both PR2 and N-heterocyclic carbene (NHC) donors on meta-bis-substituted phenylene backbones, L(Cy), L(tBu) and L(Ph) (R = Cy, tBu, Ph, respectively), was accessed through a modular synthesis from a common precursor, and their coordination chemistry with coinage metals was explored and compared. Metallation of L(Ph)·n(HBr) (n = 1, 2) with Ag2O gave the pseudo-cubane [Ag4Br4(L(Ph))2], isostructural to [Ag4Br4(L(R))2] (R = Cy, tBu) (T. Simler, P. Braunstein and A. A. Danopoulos, Angew. Chem., Int. Ed., 2015, 54, 13691), whereas metallation of ·HBF4 (R = Ph, tBu) led to the dinuclear complexes [Ag2(L(R))2](BF4)2 which, in the solid state, feature heteroleptic Ag centres and a 'head-to-tail' (HT) arrangement of the bridging ligands. In solution, interconversion with the homoleptic 'head-to-head' (HH) isomers is facilitated by ligand fluxionality. 'Head-to-tail' [Cu2Br2(L(R))2] (R = Cy, tBu) dinuclear complexes were obtained from L(R)·HBr and [Cu5(Mes)5], Mes = 2,4,6-trimethylphenyl, which also feature bridging ligands and heteroleptic Cu centres. Although the various ligands L(R)l ed to structurally analogous complexes for R = Cy, tBu and Ph, the rates of dynamic processes occurring in solution are dependent on R, with faster rates for R = Ph. Transmetallation of both NHC and P donor groups from [Ag4Br4(L(tBu))2] to AuI by reaction with [AuCl(THT)] (THT = tetrahydrothiophene) led to L(tBu) transfer and to the dinuclear complex [Au2Cl2L(tBu)] with one L(tBu) ligand bridging the two Au centres. Except for the silver pseudo-cubanes, all other complexes do not exhibit metallophilic interactions.