Miguel A. Esteruelas

Complex OsH₆(PⁱPr₃)₂ releases H₂ at 50 °C. The resulting tetrahydride OsH₄(PⁱPr₃)₂ promotes head-to-head reductive dimerization of phenylacetylenes to give the 1,4-dibranched-butenediyl derivatives OsH₂{η⁴-[C₄H₄R₂]}­(PⁱPr₃)₂ (R = C₆H₅, C₆H₄–CF₃, C₆H₄–NMe₂). DFT calculations suggest that the formation of these compounds proceeds via five-coordinate unsaturated bis­(alkenyl)-osmium­(II)-(Kubas-type dihydrogen) intermediates, which evolve by alkenyl coupling and H–H cleavage of the dihydrogen. The reactions are sensitive to temperature and the amount of alkyne used. At higher temperatures and excess alkyne, the reductive coupling is accompanied by two dehydrogenation reactions, one at the metal center and the other involving an isopropyl substituent of a phosphine. As a result, mixtures of the dihydrides and Os­{η⁴-[C₄H₄R₂]}­(PⁱPr₃)­{η²-C,C;κ¹-P-[(CH₂CMe)­PⁱPr₂]}­(PⁱPr₃) (R = H, CF₃, NMe₂) are formed. Both families react with H₂ to regenerate OsH₆(PⁱPr₃)₂ and release the corresponding 1,4-diarylbutane. According to these reactions, 1,4-diarylbutanes have been obtained in approximately 20% yield, by stirring phenylacetylenes with 5 mol % of OsH₆(PⁱPr₃)₂, in toluene, under 1 atm of H₂.

A bare lead atom is a σ-donor ligand capable of linearly bonding and stabilizing two units of a classical polyhydride complex, with a high-valent metal center. As a proof of concept, we have prepared and characterized the μ₂-tetrylide complex (PⁱPr₃)₂H₄Os═Pb═OsH₄(PⁱPr₃)₂ in the reaction of OsH₆(PⁱPr₃)₂ with Pb{N(SiMe₃)₂}₂. Although the Pb-Os bonds exhibit electrostatic interaction, the main orbital interactions result from two dative σ bonds from the lead atom to the osmium centers. The latter also provide much weaker π-backdonations.

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.

The C–C triple bond of phenylacetylene undergoes the anti-Markovnikov addition of the Rh–H bond of RhH­{κ³-P,O,P-[xant­(PⁱPr₂)₂]} (1; xant­(PⁱPr₂)₂ = 9,9-dimethyl-4,5-bis­(diisopropylphosphino)­xanthene) to give Rh­{(E)–CHCHPh}­{κ³-P,O,P-[xant­(PⁱPr₂)₂]} (2), which reacts with a second alkyne molecule to produce Rh­(CCPh)­{κ³-P,O,P-[xant­(PⁱPr₂)₂]} (3) and styrene before the transformation from 1 to 2 is complete. The metal center of 3 undergoes the oxidative addition of the C­(sp)–H bond of another alkyne molecule to produce RhH­(CCPh)₂{κ³-P,O,P-[xant­(PⁱPr₂)₂]} (4), which also reacts with more phenylacetylene before completing the transformation from 3 to 4. The reaction leads to Rh­{(E)–CHCHPh}­(CCPh)₂{κ³-P,O,P-[xant­(PⁱPr₂)₂]} (5), which reductively eliminates (E)-1,4-diphenyl-1-buten-3-yne to regenerate 3. Complexes 3, 4, and 5 constitute a cycle for head-to-head dimerization of phenylacetylene. Consequently, complex 1 promotes the catalytic homocoupling of terminal alkynes to (E)-enynes, including the dimerization of α-hydroxyacetylenes to (E)-enyne-diols. The rate-determining step of the couplings depends on the nature of the alkyne, being the insertion of the C–C triple bond into the Rh–H bond of a bis­(acetylide)-rhodium­(III)-hydride intermediate for phenylacetylenes and the reductive elimination of the product (E)-enyne-diol for α-hydroxyacetylenes. In support of the latter, complex Rh­{(E)–CHCHC­(OH)­Ph₂}­{CCC­(OH)­Ph₂}₂{κ³-P,O,P-[xant­(PⁱPr₂)₂]} (6) has been isolated and characterized by X-ray diffraction analysis. Complex 1 also effectively promotes the formation of compounds of the type (E)-5-phenyl-2-penten-4-yn-1-ol, by cross-coupling between phenylacetylenes and α-hydroxyacetylenes. These reactions take place through two cycles similar to the cycle that produces the homocouplings, the rate-determining step being the reductive elimination of (E)-enyn-ol for both. The catalytic performance of 1 provides good efficiency in homocoupling and cross-coupling reactions involving progestin-type compounds such as ethisterone.

Complex IrH₅(PⁱPr₃)₂ (1) activates two different σ-bonds of 3-phenoxy-1-phenylisoquinoline, 2-(1H-benzimidazol-2-yl)-6-phenylpyridine, 2-(1H-indol-2-yl)-6-phenylpyridine, 2-(2-hydroxyphenyl)-6-phenylpyridine, N-(2-hydroxyphenyl)-N'-phenylimidazolylidene, and 1,3-di(2-pyridyl)-4,6-dimethylbenzene to give IrH{κ³-C,N,C-[C₆H₄-isoqui-O-C₆H₄]}(PⁱPr₃)₂ (2), IrH{κ³-N,N,C-[NBzim-py-C₆H₄]}(PⁱPr₃)₂ (3), IrH{κ³-N,N,C-[Ind-py-C₆H₄]}(PⁱPr₃)₂ (4), IrH{κ³-C,N,O-[C₆H₄-py-C₆H₄O]}(PⁱPr₃)₂ (5), IrH{κ³-C,C,O-[C₆H₄-Im-C₆H₄O]}(PⁱPr₃)₂ (6), and IrH{κ³-N,C,C-[py-C₆HMe₂-C₅H₃N]}(PⁱPr₃)₂ (7), respectively. The activations are sequential, with the second generally being the slowest. Accordingly, dihydride intermediates IrH₂{κ²-C,N-[C₆H₄-isoqui-O-C₆H₅]}(PⁱPr₃)₂ (2d), IrH₂{κ²-N,N-[NBzim-py-C₆H₅]}(PⁱPr₃)₂ (3d), IrH₂{κ²-N,N-[Ind-py-C₆H₅]}(PⁱPr₃)₂ (4d), and IrH₂{κ²-N,C-[py-C₆HMe₂-py]}(PⁱPr₃)₂ (7d) were characterized spectroscopically. Complexes 3 and 5 are green phosphorescent emitters upon photoexcitation, exhibiting good absorption over a wide range of wavelengths, emission quantum yields about 0.70 in solution, long enough lifetimes (10-17 μs), and reversible electrochemical behavior. In agreement with these features, complex 3 promotes the photocatalytic α-amino C(sp³)-H arylation of N,N-dimethylaniline and N-phenylpiperidine with 1,4-dicyanobenzene and 4-cyanopyridine under blue LED light irradiation. The C-C coupling products are isolated in high yields with only 2 mol % of photocatalyst after 24 h.

Deprotonation of the thioamidate group of [OsH{kappa(2)-N,S-[NHC(CH3)S]}(equivalent to CPh)(IPr)((PPr3)-Pr-i)]OTf [1; IPr = 1,3-bis(2,6-diisopropylphenyl)imidazolylidene; OTf = CF3SO3] results in the release of acetonitrile and formation of the terminal sulfide complex OsH(S)(equivalent to CPh)(IPr)((PPr3)-Pr-i) (2), which has been transformed into the hydrosulfide [OsH(SH)(equivalent to CPh)(IPr)((PPr3)-Pr-i)]OTf (3) and the methylsulfide [OsH(SMe)(equivalent to CPh)(IPr)((PPr3)-Pr-i)]OTf (4) through protonation and methylation reactions, respectively. The structure, spectroscopic characteristics, and reactivity of these compounds are compared. Reactions of 3 and 4 with 2-hydroxypyridine and 2-mercaptopyridine afford [OsH{kappa(2)-X,N-[X-py]}(equivalent to CPh)(IPr)((PPr3)-Pr-i)]OTf [X = O (5), S(6)].

Two complementary procedures are presented to prepare cis-pyridyl-iridium(III) emitters of the class [3b+3b+3b'] with two orthometalated ligands of the 2-phenylpyridine type (3b) and a third ligand (3b'). They allowed to obtain four emitters of this class and to compare their properties with those of the trans-pyridyl isomers. The finding starts from IrH₅(PⁱPr₃)₂, which reacts with 2-(p-tolyl)pyridine to give fac-[Ir{κ²-C,N-[C₆MeH₃-py]}₃] with an almost quantitative yield. Stirring the latter in the appropriate amount of a saturated solution of HCl in toluene results in the cis-pyridyl adduct IrCl{κ²-C,N-[C₆MeH₃-py]}₂{κ¹-Cl-[Cl-H-py-C₆MeH₄]} stabilized with p-tolylpyridinium chloride, which can also be transformed into dimer cis-[Ir(μ-OH){κ²-C,N-[C₆MeH₃-py]}₂]₂. Adduct IrCl{κ²-C,N-[C₆MeH₃-py]}₂{κ¹-Cl-[Cl-H-py-C₆MeH₄]} directly generates cis-[Ir{κ²-C,N-[C₆MeH₃-py]}₂{κ²-C,N-[C₆H₄-Isoqui]}] and cis-[Ir{κ²-C,N-[C₆MeH₃-py]}₂{κ²-C,N-[C₆H₄-py]}] by transmetalation from Li[2-(isoquinolin-1-yl)-C₆H₄] and Li[py-2-C₆H₄]. Dimer cis-[Ir(μ-OH){κ²-C,N-[C₆MeH₃-py]}₂]₂ is also a useful starting complex when the precursor molecule of 3b' has a fairly acidic hydrogen atom, suitable for removal by hydroxide groups. Thus, its reactions with 2-picolinic acid and acetylacetone (Hacac) lead to cis-Ir{κ²-C,N-[C₆MeH₃-py]}₂{κ²-O,N-[OC(O)-py]} and cis-Ir{κ²-C,N-[C₆MeH₃-py]}₂{κ²-O,O-[acac]}. The stereochemistry of the emitter does not significantly influence the emission wavelengths. On the contrary, its efficiency is highly dependent on and associated with the stability of the isomer. The more stable isomer shows a higher quantum yield and color purity.