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

The pincer complexes [NiBr(CNC)]Br (4), [CrBr(CNC)] (5 a) and [CrBrCl(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)Br and [NiBr(PPh)] or from [Cr{N(SiMe)}(THF)] and (CHNCH)Br by aminolysis, respectively. The tetrahedrally distorted square planar (τ≅0.30) geometry and the singlet ground state of Ni in 4 were attributed to steric constraints of the CNC backbone.

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

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

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 (., μ-halide, μ-sulfide, or μ-PR .) as well as "closed" metal arrays (that fall in the realm of cluster chemistry) are excluded.

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.

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

Metallotropism of the M[N(SiMe ) ] metal fragment in the tautomeric system IA ⇌AC involving imidazolium salts/N-heterocyclic carbenes with remote aminide/amine substituents, respectively, is manifested by its C carbophilicity (R=tBu, M=Co, Fe) or NR 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(SiMe ) } ]/ IA ⇌AC system are also substituent-dependent, leading to a rearranged aminide-functionalized aNHC Co 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 Cu Pd , Cu Pd , and Ag Pd 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 C H [NP(tBu) ] (PC P) was observed when known [Cu (μ -PC P,κP,κC ,κP) ](OTf) was reacted with [Pd(PPh ) ]. When 1.2 equivalents of [Pd(PPh ) ] was used, the product [Cu Pd(μ -PC P,κP,κC ,κP) ](OTf) (2(OTf) ) was obtained, which features a Cu -Cu -Pd chain and appears to be the first linear heterotrinuclear complex with d -d interactions between Pd and Cu .

Reactivity of a dearomatised pincer CoBr complex with PNC 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.

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-(R-imidazol-2-ylidene)-6-(RN[double bond, length as m-dash]CH)-pyridine (R = 2,6-diisopropylphenyl (DiPP), R = 2,4,6-trimethylphenyl (Mes), 4B; R = R = DiPP, 4C), have been accessed by a Zn-promoted modular synthesis involving the quaternization of R-imidazole by [Zn(κNκN)(2-(RN[double bond, length as m-dash]CH)-6-bromo-pyridine)Cl], followed by Zn removal and deprotonation of imidazolium pro-ligands. Redox active forms of 4B were implicated in the two complexes obtained by its reaction with FeBr/KC; metrical data analysis pointed to the occurrence of radical anionic and dianionic redox states of 4B.

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

Transamination of [Co{N(SiMe ) } ] with C H (NHSiiPr ) gave the centrosymmetric trinuclear [{Co N(SiMe ) (μ-η-[o-C H (κNSiiPr ) ])} Co ] (1) (Co , Co =terminal, internal Co, respectively), with 3-coordinate Co , and Co "sandwiched" between the o-phenylenes of the two ligands; experimental and computational data support Co 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) ][{Co N(SiMe ) (μ-η-[o-C H (κNSiiPr ) ])} Co ] (4) with the anion isostructural to 1. The centrosymmetric Fe complex [{Fe N(SiMe ) (μ-η-[o-C H (κNSiiPr ) ])} Fe ] (5), analogous to 1, was also obtained.

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

The use of the dialkene divinyltetramethyldisiloxane (dvtms) allows easy access to the reactive 16 valence-electron complexes [Fe(L-L)(dvtms)], (L-L) = dppe (1,2-bis(diphenylphosphino)ethane), (), dppp (1,2-bis(diisopropylphosphino)propane), (), pyNMeP(Pr) (N-(diisopropylphosphino)-N-methylpyridin-2-amine), (), dipe (1,2-bis(diisopropylphosphino)ethane), (), and [Fe(L)(dvtms)], L = PMe, (), by a mild reductive route using AlEt(OEt) as reducing agent. In contrast, by the same methodology, the 18 valence-electron complexes [Fe(L-L)(ethylene)], (L-L) = dppm (1,2-bis(diphenylphosphino)methane), , (L-L) = dppa (1,2-bis(diphenylphosphino)amine) or (L-L)=dppe, , 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 .

Heteroleptic, two-coordinate [M(NHC){N(SiMe)}] (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(SiMe)}] (IPr = 1,3-bis(diisopropylphenyl)-imidazol-2-ylidene), formally M complexes (M = Co, 3; Fe, 4) were obtained by the reduction of [M(IPr)Cl{N(SiMe)}] with KC, or [Co(IPr){N(SiMe)}] with mes*PH, mes* = 2,4,6-tBuCH. The magnetism of 3 and 4 implies Co and Fe 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(SiMe)}] with excess KC in toluene gave the heteronuclear 'inverse-sandwich' Fe-K complex 7, featuring η-toluene sandwiched between one Fe and one K centre.

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.

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.

Transition metal (Rh and Fe) complexes and main-group (Se and B) adducts with N,N'-diphosphanyl NHC ligands: a study of stereoelectronic properties.

Attempts to evaluate experimentally the donor characteristics of the N,N'-bis(di-tert-butylphosphanyl)-imidazole-2-ylidene (PC(NHC)P) hybrid ligand are described. Thus, reactions of PC(NHC)P with [Rh(μ-Cl)(COD)]2 and [Rh(μ-Cl)(CO)2]2 led to the formation of the mononuclear and dinuclear complexes, [Rh(PC(NHC)P,κP,κC(NHC))2]Cl (PC(NHC)P-RhCl) and [Rh(CO)(PC(NHC),κP,κC(NHC),κN)]2 (PC(NHC)-RhCO), respectively, the latter resulting after in situ cleavage of one (t-Bu)2P-Nimid bond of PC(NHC)P. With ligands acting as a P,C-chelate, a straightforward evaluation of the Tolman electronic parameter (TEP) of the C(NHC) donor is problematical; the viability of dangling P- and bound C(NHC)-donors (i.

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.

Direct synthesis of doubly deprotonated, dearomatised lutidine PNP Cr and Zr pincer complexes based on isolated K and Li ligand transfer reagents.

Double deprotonation of 2,6-bis-(di-tert-butylphosphinomethyl)-pyridine ((tBu)PN(tBu)P), with KCH2C6H5 afforded K2((tBu)P*Na(tBu)P*), Na = anionic amido N, (tBu)P* = di-tert-butyl vinylic P donor. The analogous [Li2((tBu)P*Na(tBu)P*)]2 (1) was reacted with [CrCl2(THF)2] and [ZrCl4(THT)2] to give the helical [Cr{Cr((tBu)P*Na(tBu)P*)Cl}2] (2) and [Zr((tBu)P*Na(tBu)P*)Cl2] (3), respectively. DFT calculations support dearomatisation of P*NaP* and its high donor ability.

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.

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.

Cobalt PNC(NHC) 'pincers': ligand dearomatisation, formation of dinuclear and N2 complexes and promotion of C-H activation.

Studies of ligand dearomatisation in cobalt phosphino-picoline N-heterocyclic carbene (PNC(NHC))-type pincers have revealed the formation of dinuclear cobalt complexes by either side-arm metalation or C-H activation at the NHC backbone; in the latter case, the product complex, featuring an 'anionic dicarbene', originated from a dearomatised Co(I)-N2 complex.

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.

Dinuclear iridium and rhodium complexes with bridging arylimidazolide-N(3),C(2) ligands: synthetic, structural, reactivity, electrochemical and spectroscopic studies.

Deprotonation of 1-arylimidazoles (aryl = mesityl (Mes), 2,6-diisopropylphenyl (Dipp)), with n-butyl lithium afforded the corresponding derivatives (1-aryl-1H-imidazol-2-yl)lithium (1a, Ar = Mes; 1b, Ar = Dipp) in good yield. Reaction of 1a with 0.5 equiv.

N-Heterocyclic carbene-phosphino-picolines as precursors of anionic 'pincer' ligands with dearomatised pyridine backbones; transmetallation from potassium to chromium.

Deprotonation of the α-picolinyl-CH2 in the hybrid ligands R2P–NCNHC (–N = substituted 2-picoline, R = Cy, t-Bu; CNHC = N-heterocyclic carbene) (L(R)) with KN(SiMe3)2 in ether gave [KL(Cy)–H(ether)]2 featuring a dearomatised picoline backbone. Assisted by 18-crown-6, K+ dissociation afforded the Z- and E-isomers of [L(R)–H](−). Transmetallation of L(R)–H from KL(R)–H led to [CrCl(L(t-Bu)–H)].

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.

Imine-functionalised protic NHC complexes of Ir: direct formation by C-H activation.

N-Arylimine-functionalised protic NHC (pNHC) Ir(i) and Ir(iii) complexes are obtained directly from neutral or cationic Ir(i) imidazole complexes using excess [Ir(cod)(μ-Cl)]2 or TlPF6, respectively. N-Arylimine-functionalised imidazolium salts lead to imidazole or pNHC complexes by competing N-H or C-H bond activation depending on the type of imidazolium counterion.

Unprecedented directed lateral lithiations of tertiary carbons on NHC platforms.

Unexpected and unprecedented directed remote lateral lithiation at one CH(CH3)2 of the 3-(2,6-di-isopropylphenyl) wingtip took place upon the reaction of functionalised N-heterocyclic carbene-type molecules with excess of LiCH2SiMe3, leading to dilithiated dianionic 4-amido-N-heterocyclic carbenes. DFT calculations show that the nature of the isolated species are under thermodynamic control.