Controlling Singlet Fission with Coordination Chemistry-Induced Assembly of Dipyridyl Pyrrole Bipentacenes

Singlet fission has the potential to surpass current efficiency limits in next-generation photovoltaics and to find use in quantum information science. Despite the demonstration of singlet fission in various materials, there is still a great need for fundamental design principles that allow for tuning of photophysical parameters, including the rate of fission and triplet lifetimes. Here, we describe the synthesis and photophysical characterization of a novel bipentacene dipyridyl pyrrole (HDPP-Pent) and its Li- and K-coordinated derivatives. HDPP-Pent undergoes singlet fission at roughly 50% efficiency (τ_(SF) = 730 ps), whereas coordination in the Li complex induces significant structural changes to generate a dimer, resulting in a 7-fold rate increase (τ_(SF) = 100 ps) and more efficient singlet fission with virtually no sacrifice in triplet lifetime. We thus illustrate novel design principles to produce favorable singlet fission properties, wherein through-space control can be achieved via coordination chemistry-induced multipentacene assembly

Controlling Singlet Fission with Coordination Chemistry-Induced Assembly of Dipyridyl Pyrrole Bipentacenes

Singlet fission has the potential to surpass current efficiency limits in next-generation photovoltaics and to find use in quantum information science. Despite the demonstration of singlet fission in various materials, there is still a great need for fundamental design principles that allow for tuning of photophysical parameters, including the rate of fission and triplet lifetimes. Here, we describe the synthesis and photophysical characterization of a novel bipentacene dipyridyl pyrrole (HDPP-Pent) and its Li- and K-coordinated derivatives. HDPP-Pent undergoes singlet fission at roughly 50% efficiency (τ_(SF) = 730 ps), whereas coordination in the Li complex induces significant structural changes to generate a dimer, resulting in a 7-fold rate increase (τ_(SF) = 100 ps) and more efficient singlet fission with virtually no sacrifice in triplet lifetime. We thus illustrate novel design principles to produce favorable singlet fission properties, wherein through-space control can be achieved via coordination chemistry-induced multipentacene assembly

Early Metal Di(pyridyl) Pyrrolide Complexes with Second Coordination Sphere Arene−π Interactions: Ligand Binding and Ethylene Polymerization

Early metal complexes supported by hemilabile, monoanionic di(pyridyl) pyrrolide ligands substituted with mesityl and anthracenyl groups were synthesized to probe the possibility of second coordination sphere arene−π interactions with ligands with potential for allosteric control in coordination chemistry, substrate activation, and olefin polymerization. Yttrium alkyl, indolide, and amide complexes were prepared and structurally characterized; close contacts between the anthracenyl substituents and Y-bound ligands are observed in the solid state. Titanium, zirconium, and hafnium tris(dimethylamido) complexes were synthesized, and their ethylene polymerization activity was tested. In the solid state structure of one of the Ti tris(dimethylamido) complexes, coordination of Ti to only one of the pyridine donors is observed pointing to the hemilabile character of the di(pyridyl) pyrrolide ligands

Olefin Polymerization by Dinuclear Zirconium Catalysts Based on Rigid Teraryl Frameworks: Effects on Tacticity and Copolymerization Behavior

Toward gaining insight into the behavior of bimetallic catalysts for olefin polymerization, a series of structurally related binuclear zirconium catalysts with bisamine bisphenolate and pyridine bisphenolate ligands connected by rigid teraryl units were synthesized. Anthracene-9,10-diyl and 2,3,5,6-tetramethylbenzene-1,4-diyl were employed as linkers. Bulky Si^iPr_3 and SiPh_3 substituents were used in the position ortho to the phenolate oxygen. Pseudo-C_s and C_2 symmetric isomers are observed for the binuclear complexes of bisamine bisphenolate ligands. In general, binuclear catalysts show higher isotacticity compared to the monozirconium analogues, with some differences between isomers. Amine bisphenolate-supported dizirconium complexes were found to be moderately active (up to 1.5 kg mmol_(Zr)^(–1) h^(–1)) for the polymerization of 1-hexene to isotactically enriched poly-1-hexene (up to 45% mmmm) in the presence of stoichiometric trityl or anilinium borate activators. Moderate activity was observed for the production of isotactically enriched polypropylene (up to 2.8 kg mmol_(Zr)^(–1) h^(–1) and up to 25.4% mmmm). The previously proposed model for tacticity control based on distal steric effects from the second metal site is consistent with the observed behavior. Both bisamine bisphenolate and pyridine bisphenolate supported complexes are active for the production of polyethylene in the presence of MAO with activities in the range of 1.1–1.6 kg mmol_(Zr)^(–1) h^(–1) and copolymerize ethylene with α-olefins. Little difference in the level of α-olefin incorporation is observed between mono- and dinuclear catalysts supported with the pyridine bisphenolate catalysts. In contrast, the size of the olefin affects the level of incorporation differently between monometallic and bimetallic catalysts for the bisamine bisphenolate system. The ratio of the incorporation levels with dinuclear vs mononuclear catalysts decreases with increasing comonomer size. This effect is attributed to steric pressure provided by the distal metal center on the larger olefin in dinuclear catalysts

Early Metal Di(pyridyl) Pyrrolide Complexes with Second Coordination Sphere Arene−π Interactions: Ligand Binding and Ethylene Polymerization

Early metal complexes supported by hemilabile, monoanionic di(pyridyl) pyrrolide ligands substituted with mesityl and anthracenyl groups were synthesized to probe the possibility of second coordination sphere arene−π interactions with ligands with potential for allosteric control in coordination chemistry, substrate activation, and olefin polymerization. Yttrium alkyl, indolide, and amide complexes were prepared and structurally characterized; close contacts between the anthracenyl substituents and Y-bound ligands are observed in the solid state. Titanium, zirconium, and hafnium tris(dimethylamido) complexes were synthesized, and their ethylene polymerization activity was tested. In the solid state structure of one of the Ti tris(dimethylamido) complexes, coordination of Ti to only one of the pyridine donors is observed pointing to the hemilabile character of the di(pyridyl) pyrrolide ligands

Controlling Singlet Fission with Coordination Chemistry-Induced Assembly of Dipyridyl Pyrrole Bipentacenes

Singlet fission has the potential to surpass current efficiency limits in next-generation photovoltaics and to find use in quantum information science. Despite the demonstration of singlet fission in various materials, there is still a great need for fundamental design principles that allow for tuning of photophysical parameters, including the rate of fission and triplet lifetimes. Here we describe the synthesis and photophysical characterization of a novel bipentacene dipyridyl pyrrole (HDPP-Pent) and its Li- and K-coordinated derivatives. HDPP-Pent undergoes singlet fission at roughly 50% efficiency (τSF = 730 ps), whereas coordination in the Li complex induces significant structural changes to generate a dimer, resulting in a 5-fold rate increase (τSF = 140 ps) and near fully efficient singlet fission with virtually no sacrifice in triplet lifetime. We thus illustrate novel design principles to produce favorable singlet fission properties, wherein through-space control can be achieved via coordination chemistry-induced multi-pentacene assembly

Hemilabile <i>N</i>‑Xylyl‑<i>N</i>′‑methylperimidine Carbene Iridium Complexes as Catalysts for C–H Activation and Dehydrogenative Silylation: Dual Role of <i>N</i>‑Xylyl Moiety for ortho-C–H Bond Activation and Reductive Bond Cleavage

Direct dehydrogenative silylation of pyridyl and iminyl substrates with triethylsilane was achieved using (L)­Ir­(cod)­(X) (<b>1</b>) (L = a perimidine-based carbene ligand, X = OAc and OCOPh) complexes as catalysts under toluene refluxing conditions in the presence of norbornene as a hydrogen scavenger, and the silylated products were obtained in good yields. The isolated bis­(cyclometalated)iridium complexes, (C^∧C:)­(C^∧N)­IrOAc (<b>2</b>) (C^∧C: = a cyclometalated perimidine-carbene ligand and C^∧N = a cyclometalated pyridyl- and iminyl-ligated aromatic substrate), were key intermediates, where cyclometalated five-membered metallacycles of substrates such as phenylpyridine were selectively formed before yielding mono-ortho-silylation products. The bis­(cyclometalated)­iridium complex (^XyC^∧C:)­(C^∧N)­IrOAc (<b>2d</b>) (^XyC^∧C: = a cyclometalated <i>N</i>-xylyl-<i>N</i>′-methylperimidine-carbene ligand and C^∧N = a 2-pyridylphenyl ligand), reacted with 2 equiv of Et₃SiH to give an iridium hydride complex, (L⁴)­(C^∧N)­Ir­(H)­(SiEt₃) (<b>8d</b>) (L⁴ = <i>N</i>-CH₃, <i>N</i>-3,5-(CH₃)₂C₆H₃ perimidine), via demetalation of a <i>N</i>-3,5-xylyl ring of the carbene ligand of <b>2d</b>. The formation of <b>8d</b> was confirmed by isolating the corresponding chloro complex (L⁴)­(C^∧N)­Ir­(Cl)­(SiEt₃) (<b>8d-Cl</b>) by treatment with CCl₄. The <i>N</i>-methyl moiety of the carbene ligand coordinated to <b>8d</b> was cyclometalated in the presence of norbornene at room temperature to afford (^MeC^∧C:)­(C^∧N)­Ir­(SiEt₃) (1<b>0d</b>) (^MeC^∧C: = a cyclometalated <i>N</i>-xylyl-<i>N</i>′-methylperimidine-carbene), while at high temperature <b>8d</b> reacted with norbornene and Et₃SiH to afford the silylated product, 2-(2-triethylsilyl)­phenylpyridine (<b>3a</b>) and norbornane. A deuterium labeling experiment using <b>2d</b> and Et₃SiD (excess) revealed the incorporation of deuterium atoms at two ortho-positions of the <i>N</i>-xylyl group (>90%) and at the 3-position of 2-pyridylphenyl ligand (ca. 40%) within 3 h at room temperature, indicating that the cyclometalation/demetalation of the <i>N</i>-xylylperimidine carbene and 2-phenylpyridine ligands were reversible processes. Isolation of these cyclometalated iridium complexes under controlled conditions and D-labeling experiments thus revealed a dual function of the <i>N</i>-aryl group bound to the perimidine-carbene ligand, which acted as both a neutral carbene ligand and a monoanionic ortho-metalated aryl-carbene ligand through reversible C–H bond activation and Ir–C bond cleavage of the <i>N</i>-aryl group during the catalytic cycle

Hemilabile <i>N</i>‑Xylyl‑<i>N</i>′‑methylperimidine Carbene Iridium Complexes as Catalysts for C–H Activation and Dehydrogenative Silylation: Dual Role of <i>N</i>‑Xylyl Moiety for ortho-C–H Bond Activation and Reductive Bond Cleavage

Direct dehydrogenative silylation of pyridyl and iminyl substrates with triethylsilane was achieved using (L)­Ir­(cod)­(X) (<b>1</b>) (L = a perimidine-based carbene ligand, X = OAc and OCOPh) complexes as catalysts under toluene refluxing conditions in the presence of norbornene as a hydrogen scavenger, and the silylated products were obtained in good yields. The isolated bis­(cyclometalated)iridium complexes, (C^∧C:)­(C^∧N)­IrOAc (<b>2</b>) (C^∧C: = a cyclometalated perimidine-carbene ligand and C^∧N = a cyclometalated pyridyl- and iminyl-ligated aromatic substrate), were key intermediates, where cyclometalated five-membered metallacycles of substrates such as phenylpyridine were selectively formed before yielding mono-ortho-silylation products. The bis­(cyclometalated)­iridium complex (^XyC^∧C:)­(C^∧N)­IrOAc (<b>2d</b>) (^XyC^∧C: = a cyclometalated <i>N</i>-xylyl-<i>N</i>′-methylperimidine-carbene ligand and C^∧N = a 2-pyridylphenyl ligand), reacted with 2 equiv of Et₃SiH to give an iridium hydride complex, (L⁴)­(C^∧N)­Ir­(H)­(SiEt₃) (<b>8d</b>) (L⁴ = <i>N</i>-CH₃, <i>N</i>-3,5-(CH₃)₂C₆H₃ perimidine), via demetalation of a <i>N</i>-3,5-xylyl ring of the carbene ligand of <b>2d</b>. The formation of <b>8d</b> was confirmed by isolating the corresponding chloro complex (L⁴)­(C^∧N)­Ir­(Cl)­(SiEt₃) (<b>8d-Cl</b>) by treatment with CCl₄. The <i>N</i>-methyl moiety of the carbene ligand coordinated to <b>8d</b> was cyclometalated in the presence of norbornene at room temperature to afford (^MeC^∧C:)­(C^∧N)­Ir­(SiEt₃) (1<b>0d</b>) (^MeC^∧C: = a cyclometalated <i>N</i>-xylyl-<i>N</i>′-methylperimidine-carbene), while at high temperature <b>8d</b> reacted with norbornene and Et₃SiH to afford the silylated product, 2-(2-triethylsilyl)­phenylpyridine (<b>3a</b>) and norbornane. A deuterium labeling experiment using <b>2d</b> and Et₃SiD (excess) revealed the incorporation of deuterium atoms at two ortho-positions of the <i>N</i>-xylyl group (>90%) and at the 3-position of 2-pyridylphenyl ligand (ca. 40%) within 3 h at room temperature, indicating that the cyclometalation/demetalation of the <i>N</i>-xylylperimidine carbene and 2-phenylpyridine ligands were reversible processes. Isolation of these cyclometalated iridium complexes under controlled conditions and D-labeling experiments thus revealed a dual function of the <i>N</i>-aryl group bound to the perimidine-carbene ligand, which acted as both a neutral carbene ligand and a monoanionic ortho-metalated aryl-carbene ligand through reversible C–H bond activation and Ir–C bond cleavage of the <i>N</i>-aryl group during the catalytic cycle

C–H Activation of Terminal Alkynes by Tris-(3,5-dimethylpyrazolyl)boraterhodiumneopentylisocyanide: New Metal–Carbon Bond Strengths

C–H bond activation of terminal alkynes by [Tp′Rh­(CNneopentyl)] (Tp′ = hydridotris-(3,5-dimethylpyrazolyl)­borate) resulted in the formation of terminal C–H bond activation products Tp′Rh­(CNneopentyl)­(CCR)­H (R = <i>t</i>-Bu, SiMe₃, hexyl, CF₃, <i>p</i>-MeOC₆H₄, Ph, and <i>p</i>-CF₃C₆H₄). A combination of kinetic selectivity determined in competition reactions and activation energy for reductive elimination has allowed for the calculation of relative Rh–C_alkynyl bond strengths. The bond strengths of Rh–C_alkynyl products are noticeably higher than those of Rh–C_aryl and Rh–C_alkyl analogues. The relationship between M–C and C–H bond strengths showed a linear correlation (slope <i>R</i>_M–C/H–C = 1.32), and follows energy correlations previously established for unsubstituted sp² and sp³ C–H bonds in aliphatic and aromatic hydrocarbons