Synthesis, Structure, and Solution Dynamics of Neutral Allylnickel Complexes of N-Heterocyclic Carbenes

t-Butyl and Trimethylsilyl Substituents in Nickel Allyl Complexes: Similar but Not the Same

Metal complexes with t-Bu-substituted allyl ligands are relatively rare, especially compared to their conceptually similar trimethylsilyl-substituted analogs. The scarcity partially stems from the few general synthetic entry points for the t-Bu versions. This situation was studied through a modified synthesis for the allyl ligand itself and by forming several mono(allyl)nickel derivatives. After 2,2,6,6-tetramethyl-4-hepten-3-one was converted to the related 5-bromo-2,2,6,6-tetramethylhept-3-ene (A2tBr), a mixture of Ni(COD)2 and A2tBr in the presence of a neutral donor ligand such as MeCN was found to produce the dark red dimeric π-allyl complex [{A2tNiBr}2]. Both NMR and X-ray crystallographic data confirmed that the t-Bu substituents are in a syn, syn-conformation, like that in the previously described [{A'NiBr}2] (A' = 1,3-(TMS)2C3H3) complex. [{A2tNiBr}2] will form adducts with neutral donors such as PPh3 and IMes (IMes = 1,3-dimesitylimidazol-2-ylidene), but the resulting [A2tNi(PPh3)Br] complex is not as stable as its trimethylsilyl analog. The [A2tNi(IMes)Br] complex crystallizes from hexanes as a monomer, with an η3-coordinated [A2t] ligand, and in contrast to the starting arrangement in [{A2tNiBr}2], the t-Bu groups on the A2t ligand are in a syn, anti-relationship. This structure is paralleled in the trimethylsilyl analog [A'Ni(IMes)Br]. DFT calculations were used to compare the structures of t-Bu- and related trimethylsilyl-substituted complexes.

Luminescent Complexes of Platinum, Iridium, and Coinage Metals Containing N-Heterocyclic Carbene Ligands: Design, Structural Diversity, and Photophysical Properties

The employment of N-heterocyclic carbenes (NHCs) to design luminescent metal compounds has been the focus of recent intense investigations because of the strong σ-donor properties, which bring stability to the whole system and tend to push the d-d dark states so high in energy that they are rendered thermally inaccessible, thereby generating highly emissive complexes for useful applications such as organic light-emitting diodes (OLEDs), or featuring chiroptical properties, a field that is still in its infancy. Among the NHC complexes, those containing organic chromophores such as naphthalimide, pyrene, and carbazole exhibit rich emission behavior and thus have attracted extensive interest in the past five years, especially carbene coinage metal complexes with carbazolate ligands. In this review, the design strategies of NHC-based luminescent platinum and iridium complexes with large spin-orbit-coupling (SOC) are described first. Subsequent paragraphs illustrate the recent advances of luminescent coinage metal complexes with nucleophilic- and electrophilic-based carbenes based on silver, gold, and copper metal complexes that have the ability to display rich excited state emissions in particular via thermally activated delayed fluorescence (TADF). The luminescence mechanism and excited state dynamics are also described. We then summarize the advance of NHC-metal complexes in the aforementioned fields in recent years. Finally, we propose the development trend of this fast-growing field of luminescent NHC-metal complexes.

Ni(II) Precatalysts Enable Thioetherification of (Hetero)Aryl Halides and Tosylates and Tandem C-S/C-N Couplings

Ni‐catalyzed C−S cross‐coupling reactions have received less attention compared with other C‐heteroatom couplings. Most reported examples comprise the thioetherification of most reactive aryl iodides with aromatic thiols. The use of C−O electrophiles in this context is almost uncharted. Here, we describe that preformed Ni(II) precatalysts of the type NiCl(allyl)(PMe₂Ar’) (Ar’=terphenyl group) efficiently couple a wide range of (hetero)aryl halides, including challenging aryl chlorides, with a variety of aromatic and aliphatic thiols. Aryl and alkenyl tosylates are also well tolerated, demonstrating, for the first time, to be competent electrophilic partners in Ni‐catalyzed C−S bond formation. The chemoselective functionalization of the C−I bond in the presence of a C−Cl bond allows for designing site‐selective tandem C−S/C−N couplings. The formation of the two C‐heteroatom bonds takes place in a single operation and represents a rare example of dual electrophile/nucleophile chemoselective process.

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.

Reactivity of the diphosphinodithio ligated nickel(0) complex toward alkyl halides and resultant nickel(i) and nickel(ii)-alkyl complexes

Diphosphinodithio ligated complexes of nickel(0), nickel(i) and nickel(ii)-alkyl with a reactivity relevant to the C-C bond formation were described. Stoichiometric reactions of the nickel(0) complex, [(P2S2)Ni] ([1]0, P2S2 = (Ph2PC6H4CH2S)2(C2H4)), with alkyl halides (RX) such as C6H5CH2Br, C2H3CH2Br, C2H5I and (CH3)2CHI were investigated, from which the products were found to be highly dependent on the nature of RX used. Oxidative addition of C2H3CH2Br to [1]0 provides the stable Ni(ii)-alkyl complexes [1-allyl]+. The reaction of [1]0 with C6H5CH2Br proceeds through a radical pathway resulting in the formation of the nickel(i) complex [1]+ and an organic homo-coupled product 1,2-diphenylethane. Oxidative addition of C2H5I or (CH3)2CHI to [1]0 can be achieved but it competes with the halogen atom abstraction reaction as found for C6H5CH2Br. [1]0 was shown to be an active catalyst for the coupling reactions of primary halides and alkyl Grignard reagents.

Phosphine-functionalized NHC Ni(ii) and Ni(0) complexes: synthesis, characterization and catalytic properties

Ni(ii) and Ni(0) complexes bearing chelating diphenylphosphine-functionalized NHC ligands have been prepared and characterized. Their catalytic behavior in various cross-coupling reactions has also been examined.

Boron complexes of aromatic ring fused iminopyrrolyl ligands: synthesis, structure, and luminescence properties

New fluorescent aromatic ring-fused 2-iminopyrrolyl diphenyl boron complexes are emitters in the range blue to orange.

Ruthenium-Catalyzed Asymmetric Hydrohydroxyalkylation of Butadiene: The Role of the Formyl Hydrogen Bond in Stereochemical Control

The catalyst generated in situ from RuH2(CO)(PPh3)3, (S)-SEGPHOS, and a chiral phosphoric acid promotes asymmetric hydrohydroxyalkylation of butadiene and affords enantioenriched α-methyl homoallylic alcohols. The observed diastereo- and enantioselectivities are determined by both the chiral phosphine and chiral phosphate ligands. Density functional theory calculations (M06/SDD-6-311G(d,p)-IEFPCM(acetone)//B3LYP/SDD-6-31G(d)) predict that the product distribution is controlled by the kinetics of carbon-carbon bond formation, and this process occurs via a closed-chair Zimmerman-Traxler-type transition structure (TS). Chiral-phosphate-dependent stereoselectivity arising from this TS is enabled through a hydrogen bond between the phosphoryl oxygen and the aldehyde formyl proton present in TADDOL-derived catalysts. This interaction is absent in the corresponding BINOL-derived systems, and the opposite diastereo- and enantioselectivity is observed. Additional factors influencing the stereochemical control are determined.

Orientation and stereodynamic paths of planar monodentate ligands in square planar nickel N2S complexes

The well-established presence of histidine donors in binding sites of Ni-containing biomolecules prompts the study of orientational preference and stereodynamic nature of flat monodentate ligands (L = imidazoles, pyridine and an N-heterocyclic carbene) bound to planar N(2)SNi moieties. Square planar [N(2)SNiL](n+) complexes are accessed through bridge-splitting reactions of dimeric, thiolate-S bridged [N(2)SNi](2) complexes. The solid state molecular structures of three mononuclear products, and three monothiolate bridged dinickel complexes, reveal that the plane of the added monodentate ligand orients largely orthogonal to the N(2)SNiL square plane. Variable temperature (1)H NMR characterization of dynamic processes and ground state isomer ratios of imidazole complexes in their stopped exchange limiting spectra, readily correlate with density functional theory (DFT)-guided interpretation of Ni-L rotational activation barriers. Full DFT characterization finds Ni-L bond lengthening as well as a tetrahedral twist distortion in the transition state, reaching a maximum in the NHC complex, and relating mainly to the steric hindrance derived both from the ligand and the binding pocket. In the case of the imidazole ligands a minor electronic contribution derives from intramolecular electrostatic interactions (imidazole C-2 C-H(delta+)- - S(delta-) interaction). Computational studies find this donor-acceptor interaction is magnified in O-analogues, predicting coplanar arrangements in the ground state of N(2)ON(imid)Ni complexes.

Half-sandwich NHC-nickel(II) complexes as pre-catalysts for the fast Suzuki coupling of aryl halides: a comparative study

Cationic half-sandwich nickel complexes of general formula [Ni(NHC)(NCMe)(eta(5)-C5R5)](PF6) [NHC = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr) a, 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene (IMes) b; R = H, Me] were prepared from the reaction of their neutral homologues [Ni(NHC)Cl(eta(5)-C5R5)] with 1 equiv. of KPF6 in acetonitrile at room temperature. The new cationic complexes [Ni(IPr)(NCMe)(eta(5)-C5Me5)](PF6) 3a, [Ni(IMes)(NCMe)(eta(5)-C5Me5)](PF6) 3b and [Ni(IMes)(NCMe)(eta(5)-C5H5)](PF6) 4b were obtained in high yield and were fully characterized by 1H and 13C NMR spectroscopy, IR spectroscopy, elemental analyses, and in the case of 3a by a single-crystal X-ray diffraction study. The neutral analogue of 3a, [Ni(IPr)Cl(eta(5)-C5Me5)] 1a was also structurally characterized. Their geometries were compared and no significant structural differences were observed. Nevertheless solution NMR spectroscopy established that the acetonitrile ligand of the cationic species is labile in solution. This results in the absence of any rotational significant barrier about the nickel-carbene carbon bond at ambient temperature in solution in the sterically congested cationic complexes 3a and 3b, in contrast to their neutral analogues 1a and [Ni(IMes)Cl(eta(5)-C5Me5)] 1b. The neutral and the cationic complexes catalyzed the cross-coupling of phenylboronic acid with aryl halides in the absence of co-catalysts or reductants. Surprisingly, the neutral or cationic nature of the complexes proved to have almost no influence on the reaction yields and rates. However, complexes bearing the bulky electron-rich pentamethylcyclopentadienyl ligand were much more active than those bearing the cyclopentadienyl ligand, and TOF of up to 190 h(-1), a high rate for nickel(II) complexes under similar conditions, were observed with these species.

Allylansa-Lanthanidocenes: Single-Component, Single-Site Catalysts for Controlled Syndiospecific Styrene and Styrene–Ethylene (Co)Polymerization

A series of new neutral allyl Group 3 metal complexes bearing ansa-bridged fluorenyl/cyclopentadienyl ligands [[Flu-EMe(2)-(3-R-Cp)]Ln(eta(3)-C(3)H(5))(THF)] (E=C, R=H, Ln=Y (2), La (3), Nd (4), Sm (5); R=tBu, Ln=Y (8), Nd (9); E=Si, R=H, Ln=Y (12), Nd (13)) were synthesized in good yields via salt metathesis protocols. The complexes were characterized by elemental analysis, NMR spectroscopy for diamagnetic complexes, and single-crystal X-ray diffraction studies for 2, 4, 9 and 12. Some of the allyl ansa-lanthanidocenes, especially 4, are effective single-component catalysts for the polymerization of styrene, giving pure syndiotactic polystyrenes (rrrr > 99 %) with low to high molecular weights (M(n)=6000-135,000 g mol(-1)) and narrow polydispersities (M(w)/M(n)=1.2-2.6). The catalyst systems are remarkably stable, capable of polymerizing styrene up to 120 degrees C with high activities, while maintaining high syndiotacticity via chain-end control as established by a Bernoullian analysis. Highly effective copolymerization of styrene with ethylene was achieved using neodymium complex 4 (activity up to 2530 kg PS-PE mol(-1) h(-1)) to give true copolymers void of homopolymers with M(n)=9000-152,000 g mol(-1) and narrow polydispersities (M(w)/M(n)=1.2-2.5). The nature of the resultant P(S-co-E) copolymers was ascertained by NMR, size-exclusion chromatography/refractive index/UV, temperature rising elusion fractionation, and differential scanning calorimetry. It is shown that, regardless the amount of ethylene incorporated (1-50 mol %), P(S-co-E) copolymers have a microstructure predominantly made of long highly syndiotactic PS sequences separated by single or few ethylene units. Co-monomers feed and polymerization temperature can be used straightforwardly to manipulate with the physical and mechanical characteristics of the P(S-co-E) copolymers (molecular weights and distributions, co-monomer content, microstructure, T(m), T(g), T(c)).

Structural variation, dynamics, and catalytic application of palladium(ii) complexes of di-N-heterocyclic carbene–amine ligands

A series of palladium(II) complexes incorporating di-NHC-amine ligands has been prepared and their structural, dynamic and catalytic behaviour investigated. The complexes [trans-(kappa(2)-(tBu)CN(Bn)C(tBu))PdCl(2)] (12) and [trans-(kappa(2)-(Mes)CN(H)C(Mes))PdCl(2)] (13) do not exhibit interaction between the amine nitrogen and palladium atom respectively. NMR spectroscopy between -40 and 25 degrees C shows that the di-NHC-amine ligand is flexible expressing C(s) symmetry and for 13 rotation of the mesityl groups is prevented. In the related C(1) complex [(kappa(3)-(tBu)CN(H)C(tBu))PdCl][Cl] (14) coordination of NHC moieties and amine nitrogen atom is observed between -40 and 25 degrees C. Reaction between 12-14 and two equivalents of AgBF(4) in acetonitrile gives the analogous complexes [trans-(kappa(2)-(tBu)CN(Bn)C(tBu))Pd(MeCN)(2)][BF(4)](2) (15), [trans-(kappa(2)-(Mes)CN(H)C(Mes))Pd(MeCN)(2)][BF(4)](2) (16) and [(kappa(3)-(tBu)CN(H)C(tBu))Pd(MeCN)][BF(4)](2) (17) indicating that ligand structure determines amine coordination. The single crystal X-ray structures of 12, 17 and two ligand imidazolium salt precursors (tBu)C(H)N(Bn)C(H)(tBu)][Cl](2) (2) and [(tBu)C(H)N(H)C(H)(tBu)][BPh(4)](2) (4) have been determined. Complexes 12-14 and 15-17 have been shown to be active precatalysts for Heck and hydroamination reactions respectively.