N,C,N-Pincers in Platinum Bimetallic Complexes: Influence of the Pincer and Bridging Ligands on the Metal-Metal Bond and the Photophysical Properties.

Precursors PtCl{κ3-N,C,N-[py-C6HMe2-py]} (1), PtCl{κ3-N,C,N-[py-O-C6H3-O-py]} (2), Pt(OH){κ3-N,C,N-[py-C6HMe2-py]} (3), and Pt(OH){κ3-N,C,N-[py-O-C6H3-O-py]} (4) were used to prepare d8-platinum bimetallic complexes. Precursors 1 and 2 react with AgBF4 and 7-azaindole (Haz) to give [Pt{κ3-N,C,N-[py-C6HMe2-py]}{κ1-N-[Haz]}]BF4 (5) and [Pt{κ3-N,C,N-[py-O-C6H3-O-py]}{κ1-N-[Haz]}]BF4 (6) and 3 and 4 with indolo[2,3-b]indole (H2ii) to generate Pt{κ1-N-[Hii]}{κ3-N,C,N-[py-C6HMe2-py]} (7) and Pt{κ1-N-[Hii]}{κ3-N,C,N-[py-O-C6H3-O-py]} (8). Subsequent addition of 3 and 4 to 5-7 affords bimetallic derivatives [{Pt[κ3-N,C,N-(py-C6HMe2-py)]}2{µ-N,N-[az]}]BF4 (9), [{Pt[κ3-N,C,N-(py-O-C6H3-O-py)]}2{µ-N,N-[az]}]BF4 (10), and {Pt[κ3-N,C,N-(py-C6HMe2-py)]}2{µ-N,N-[ii]} (11). X-ray structures of 9-11 reveal separations between the metals in sequence 9 (3.0515(4) Å) < 10 (3.2689(9) Å) < 11 (3.2949(2) Å). DFT calculations support σ overlap of the dz2 orbitals of platinum atoms, for 9 and 10. Accordingly, their absorption spectra show a MMLCT transition. Complex 9 is a red emitter. The excited state has 3MMLCT characteristics and a Pt-Pt separation of 2.763 Å. Complex 11 is a dual emitter in the red and NIR regions, in solid. Both excited states have a 3LC/LMCT characteristic and platinum-platinum separations of 3.290 and 3.202 Å. Intermediate 5 is a green emitter that achieves quantum yields close to unity, when diluted in PMMA and 1,2-dichloroethane at low concentrations.

Preparation, Aromaticity, and Bromination of Spiro Iridafurans.

Iridium centers of [Ir(µ-Cl)(C8H14)2]2 (1) activate the Cß(sp2)-H bond of benzylideneacetone to give [Ir(µ-Cl){κ2-C,O-[C(Ph)CHC(Me)O]}2]2 (2), which is the starting point for the preparation of the spiro iridafurans IrCl{κ2-C,O-[C(Ph)CHC(Me)O]}2(PiPr3) (3), [Ir{κ2-C,O-[C(Ph)CHC(Me)O]}2(MeCN)2]BF4 (4), [Ir(µ-OH){κ2-C,O-[C(Ph)CHC(Me)O]}2]2 (5), Ir{κ2-C,O-[C(Ph)CHC(Me)O]}2{κ2-C,N-[C6MeH3-py]} (6), and Ir{κ2-C,O-[C(Ph)CHC(Me)O]}2{κ2-O,O-[acac]} (7). The five-membered rings are orthogonally arranged with the oxygen atoms in trans in an octahedral environment of the iridium atom. Spiro iridafurans are aromatic. The degree of aromaticity and the negative charge of the CH-carbon of the rings depend on ligand trans to the carbon directly attached to the metal. Aromaticity has been experimentally confirmed by bromination of iridafurans with N-bromosuccinimide (NBS). Reactions are sensitive to the degree of aromaticity of the ring and the negative charge of the attacked CH-carbon. Iridafurans can be selectively brominated, when different ligands lie trans to metalated carbons. Bromination of 3 occurs in the ring with the metalated carbon trans to chloride, whereas the bromination of 6 takes place in the ring with the metalated carbon trans to pyridyl. The first gives IrCl{κ2-C,O-[C(Ph)CBrC(Me)O]}{κ2-C,O-[C(Ph)CHC(Me)O]}(PiPr3) (8), which reacts with more NBS to form IrCl{κ2-C,O-[C(Ph)CBrC(Me)O]}2(PiPr3) (9). The second yields Ir{κ2-C,O-[C(Ph)CBrC(Me)O]}{κ2-C,O-[C(Ph)CHC(Me)O]}{κ2-C,N-[C6MeH3-py]} (10). The origin of the selectivity is kinetic, with the rate-determining step of the reaction being the NBS attack. The activation energy depends on the negative charge of the attacked atom; a higher negative charge allows for a lower activation energy. Accordingly, complex 7 undergoes bromination in the acetylacetonate ligand, giving Ir{κ2-C,O-[C(Ph)CHC(Me)O]}2{κ2-O,O-[acacBr]} (11).

Competition between N,C,N-Pincer and N,N-Chelate Ligands in Platinum(II).

Replacement of the chloride ligand of PtCl{κ3-N,C,N-[py-C6HR2-py]} (R = H (1), Me (2)) and PtCl{κ3-N,C,N-[py-O-C6H3-O-py]} (3) by hydroxido gives Pt(OH){κ3-N,C,N-[py-C6HR2-py]} (R = H (4), Me (5)) and Pt(OH){κ3-N,C,N-[py-O-C6H3-O-py]} (6). These compounds promote deprotonation of 3-(2-pyridyl)pyrazole, 3-(2-pyridyl)-5-methylpyrazole, 3-(2-pyridyl)-5-trifluoromethylpyrazole, and 2-(2-pyridyl)-3,5-bis(trifluoromethyl)pyrrole. The coordination of the anions generates square-planar derivatives, which in solution exist as a unique species or equilibria between isomers. Reactions of 4 and 5 with 3-(2-pyridyl)pyrazole and 3-(2-pyridyl)-5-methylpyrazole provide Pt{κ3-N,C,N-[py-C6HR2-py]}{κ1-N1-[R'pz-py]} (R = H; R' = H (7), Me (8). R = Me; R' = H (9), Me (10)), displaying κ1-N1-pyridylpyrazolate coordination. A 5-trifluoromethyl substituent causes N1-to-N2 slide. Thus, 3-(2-pyridyl)-5-trifluoromethylpyrazole affords equilibria between Pt{κ3-N,C,N-[py-C6HR2-py]}{κ1-N1-[CF3pz-py]} (R = H (11a), Me (12a)) and Pt{κ3-N,C,N-[py-C6HR2-py]}{κ1-N2-[CF3pz-py]} (R = H (11b), Me (12b)). 1,3-Bis(2-pyridyloxy)phenyl allows the chelating coordination of the incoming anions. Deprotonations of 3-(2-pyridyl)pyrazole and its substituted 5-methyl counterpart promoted by 6 lead to equilibria between Pt{κ3-N,C,N-[pyO-C6H3-Opy]}{κ1-N1-[R'pz-py]} (R' = H (13a), Me (14a)) with a κ-N1-pyridylpyrazolate anion, keeping the pincer coordination of the di(pyridyloxy)aryl ligand, and Pt{κ2-N,C-[pyO-C6H3(Opy)]}{κ2-N,N-[R'pz-py]} (R' = H (13c), Me (14c)) with two chelates. Under the same conditions, 3-(2-pyridyl)-5-trifluoromethylpyrazole generates the three possible isomers: Pt{κ3-N,C,N-[pyO-C6H3-Opy]}{κ1-N1-[CF3pz-py]} (15a), Pt{κ3-N,C,N-[pyO-C6H3-Opy]}{κ1-N2-[CF3pz-py]} (15b), and Pt{κ2-N,C-[pyO-C6H3(Opy)]}{κ2-N,N-[CF3pz-py]} (15c). The N1-pyrazolate atom produces a remote stabilizing effect on the chelating form, pyridylpyrazolates being better chelate ligands than pyridylpyrrolates. Accordingly, reactions of 4-6 with 2-(2-pyridyl)-3,5-bis(trifluoromethyl)pyrrole yield Pt{κ3-N,C,N-[py-C6HR2-py]}{κ1-N1-[(CF3)2C4(py)HN]} (R = H (16), Me (17)) or Pt{κ3-N,C,N-[pyO-C6H3-Opy]}{κ1-N1-[(CF3)2C4(py)HN]} (18), displaying κ1-N1-pyrrolate coordination. Complexes 7-10 are efficient green phosphorescent emitters (488-576 nm). In poly(methyl methacrylate) (PMMA) films and in dichloromethane, they experience self-quenching, due to molecular stacking. Aggregation occurs through aromatic π-π interactions, reinforced by weak platinum-platinum interactions.

Synthesis, Structure, and Photophysical Properties of Platinum(II) (N,C,N') Pincer Complexes Derived from Purine Nucleobases†.

The synthesis of a series of Pt{κ3-N,C,N'-[L]}X (X = Cl, RC≡C) pincer complexes derived from purine and purine nucleosides is reported. In these complexes, the 6-phenylpurine skeleton provides the N,C-cyclometalated fragment, whereas an amine, imine, or pyridine substituent of the phenyl ring supplies the additional N'-coordination point to the pincer complex. The purine N,C-fragment has two coordination positions with the metal (N1 and N7), but the formation of the platinum complexes is totally regioselective. Coordination through the N7 position leads to the thermodynamically favored [6.5]-Pt{κ3-N7,C,N'-[L]}X complexes. However, the coordination through the N1 position is preferred by the amino derivatives, leading to the isomeric kinetic [5.5]-Pt{κ3-N1,C,N'-[L]}X complexes. Extension of the reported methodology to complexes having both pincer and acetylide ligands derived from nucleosides allows the preparation of novel heteroleptic bis-nucleoside compounds that could be regarded as organometallic models of Pt-induced interstrand cross-link. Complexes having amine or pyridine arms are green phosphorescence emitters upon photoexcitation at low concentrations in CH2Cl2 solution and in poly(methyl methacrylate) (PMMA) films. They undergo self-quenching at high concentrations due to molecular aggregation. The presence of intermolecular π-π stacking and weak Pt···Pt interactions was also observed in the solid state by X-ray diffraction analysis.

Simple ZnEt2 as a catalyst in carbodiimide hydroalkynylation: structural and mechanistic studies.

Expanding the possibilities of the use of simple and available ZnEt2 as a catalyst, the hydroalkynylation of carbodiimides with a variety of alkynes to obtain unsaturated substituted amidines is described in this work. Different stoichiometric studies allow proposing that amidinate complexes are intermediates in this catalytic process, produced by easy activation of the C-H bond of the alkyne and formation of alkynyl derivatives followed by a carbodiimide insertion step. Kinetics studies allowed the generation of a rate law for the hydroalkynylation of N,N'-diisopropylcarbodiimide with phenylacetylene which is second order in [carbodiimide], first order in [catalyst] and zero order in [alkyne], with a negligible PhC[triple bond, length as m-dash]CH/PhC[triple bond, length as m-dash]CD isotopic effect, consistent with a rate-determining state involving carbodiimide insertion. The hydroalkynylation reaction has been coupled with isocyanate (and isothiocyanate) insertion and intramolecular hydroamination to obtain imidazolidin-2-ones (or thione). The structures of different plausible intermediates have been determined by X-ray diffraction studies.

Tris(pentafluorophenyl)borane as an efficient catalyst in the guanylation reaction of amines.

Tris(pentafluorophenyl)borane, [B(C6F5)3], has been used as an efficient catalyst in the guanylation reaction of amines with carbodiimide under mild conditions. A combined approach involving NMR spectroscopy and DFT calculations was employed to gain a better insight into the mechanistic features of this process. The results allowed us to propose a new Lewis acid-assisted Brønsted acidic pathway for the guanylation reaction. The process starts with the interaction of tris(pentafluorphenyl)borane and the amine to form the corresponding adduct, [(C6F5)3B-NRH2] , followed by a straightforward proton transfer to one of the nitrogen atoms of the carbodiimide, (i)PrN[double bond, length as m-dash]C[double bond, length as m-dash]N(i)Pr, to produce, in two consequent steps, a guanidine-borane adduct, [(C6F5)3B-NRC(N(i)PrH)2] . The rupture of this adduct liberates the guanidine product RNC(N(i)PrH)2 and interaction with additional amine restarts the catalytic cycle. DFT studies have been carried out in order to study the thermodynamic characteristics of the proposed pathway. Significant borane adducts with amines and guanidines have been isolated and characterized by multinuclear NMR in order to study the N-B interaction and to propose the existence of possible Frustrated Lewis Pairs. Additionally, the molecular structures of significant components of the catalytic cycle, namely 4-tert-butylaniline-[B(C6F5)3] adduct and both free and [B(C6F5)3]-bonded 1-(phenyl)-2,3-diisopropylguanidine, and respectively, have been established by X-ray diffraction.