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 7-fold rate increase (τSF = 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.

Lanthanide Complexes Supported by a Trizinc Crown Ether as Catalysts for Alternating Copolymerization of Epoxide and CO2 : Telomerization Controlled by Carboxylate Anions.

A new family of heterometallic catalysts based on trimetalated macrocyclic tris(salen) ligands and rare-earth metals was prepared and structurally characterized. The LaZn3 system containing anionic ligands such as acetate plays a critical role in catalyzing the alternating copolymerization of cyclohexene oxide (CHO) and CO2 with a high proportion of carbonate linkages. Among the lanthanide metals, the CeZn3 system exhibits high catalytic activity with a turnover frequency (TOF) of over 370 h-1 . NMR analysis of the complex and end-group analysis of the polymer suggest that the acetate ligands are rapidly exchanged, not only among coordinated acetates, but also between coordinated acetates and added carboxylate anions. These unique properties make this the first example of telomerization for the copolymerization of CHO and CO2 .

Hemilabile N-xylyl-N'-methylperimidine carbene iridium complexes as catalysts for C-H activation and dehydrogenative silylation: dual role of N-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) (1) (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 (2) (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 ((Xy)C(∧)C:)(C(∧)N)IrOAc (2d) ((Xy)C(∧)C: = a cyclometalated N-xylyl-N'-methylperimidine-carbene ligand and C(∧)N = a 2-pyridylphenyl ligand), reacted with 2 equiv of Et3SiH to give an iridium hydride complex, (L(4))(C(∧)N)Ir(H)(SiEt3) (8d) (L(4) = N-CH3, N-3,5-(CH3)2C6H3 perimidine), via demetalation of a N-3,5-xylyl ring of the carbene ligand of 2d. The formation of 8d was confirmed by isolating the corresponding chloro complex (L(4))(C(∧)N)Ir(Cl)(SiEt3) (8d-Cl) by treatment with CCl4. The N-methyl moiety of the carbene ligand coordinated to 8d was cyclometalated in the presence of norbornene at room temperature to afford ((Me)C(∧)C:)(C(∧)N)Ir(SiEt3) (10d) ((Me)C(∧)C: = a cyclometalated N-xylyl-N'-methylperimidine-carbene), while at high temperature 8d reacted with norbornene and Et3SiH to afford the silylated product, 2-(2-triethylsilyl)phenylpyridine (3a) and norbornane. A deuterium labeling experiment using 2d and Et3SiD (excess) revealed the incorporation of deuterium atoms at two ortho-positions of the N-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 N-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 N-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 N-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 = t-Bu, SiMe(3), hexyl, CF(3), p-MeOC(6)H(4), Ph, and p-CF(3)C(6)H(4)). 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 R(M-C/H-C) = 1.32), and follows energy correlations previously established for unsubstituted sp(2) and sp(3) C-H bonds in aliphatic and aromatic hydrocarbons.