Title: Coordination-Induced Weakening of a C(sp3)–H Bond: Homolytic and Heterolytic Bond Strength of a CH–Ni Agostic Interaction
The scission of a C(sp3)−H bond to form a new metal−alkyl bond is a fundamental step in coordination chemistry and catalysis. However, the extent of C−H bond weakening when this moiety interacts with a transition metal is poorly understood and quantifying this phenomenon could provide insights into designing more efficient C−H functionalization catalysts. We present a nickel complex with a robust adamantyl reporter ligand that enables the measurement of C−H acidity (pKa) and bond dissociation free energy (BDFE) for a C(sp3)−H agostic interaction, showing a decrease in pKa by dozens of orders of magnitude and BDFE decrease of about 30 kcal/mol upon coordination. X-ray crystallographic data is provided for all molecules, including a distorted square planar NiIII metalloradical and “doubly agostic” NiII(κ2-CH2) complex. more »« less
Wong, Anthony; Chakraborty, Arunavo; Bawari, Deependra; Wu, Guang; Dobrovetsky, Roman; Ménard, Gabriel
(, Chemical Communications)
null
(Ed.)
We report the facile activation of aryl E–H (ArEH; E = N, O, S; Ar = Ph or C 6 F 5 ) or ammonia N–H bonds via coordination-induced bond weakening to a redox-active boron center in the complex, (1 − ). Substantial decreases in E–H bond dissociation free energies (BDFEs) are observed upon substrate coordination, enabling subsequent facile proton-coupled electron transfer (PCET). A drop of >50 kcal mol −1 in H 2 N–H BDFE upon coordination was experimentally determined.
Galvin, Conor M.; Marron, Daniel P.; Dressel, Julia M.; Waymouth, Robert M.
(, Inorganic Chemistry)
Coordination of the leucoverdazyl ligand 2,4-diisopropyl-6-(pyridin-2-yl)-1,4-dihydro-1,2,4,5-tetrazin-3(2H)-one VdH to Ru significantly weakens the ligand’s N-H bond. Electrochemical measurements show that the metalated leucoverdazyl Ru(VdH)(acetylacetonate)2 RuVdH has a lower pKa (-5 units), BDFE (-7 kcal/mol), and hydricity (-22 kcal/mol) than the free ligand. DFT calculations suggest that the increased acidity is in part attributable to the stabilization of the conjugate base Vd-. When free, Vd- distorts to avoid an 8πe- antiaromatic state, but it remains planar when bound to Ru. Proton-coupled electron transfer (PCET) behavior is observed for both the free and metalated leucoverdazyls. PCET equilibrium between Vd radical and TEMPOH affords a VdH BDFE that is in good agreement with that obtained from electrochemical methods. RuVd exhibits electrocatalytic PCET donor behavior. Under acidic conditions, it reduces the persistent trityl radical ·CAr3 (Ar = p-tert-butylphenyl) to the corresponding triarylmethane HCAr3 via net 1e-/1H+ transfer from RuVdH.
González, Jonathan Sánchez; Tanko, JM
(, Chemical Thermodynamics and Thermal Analysis)
C-H Bond dissociation energies for a unique selection of tertiary amines that are known substrates or inhibitors of monoamine oxidase have been calculated using density functional theory. These amines are unusual because they are the only tertiary amines that exhibit MAO substrate or inhibitor behavior. The unique structural feature common to these specific compounds is an sp3-hybridized CH2 moiety, which is -both to nitrogen and an C=C or C≡C. The stabilization afforded the resulting radicals by extended delocalization dramatically lowers both the C-H bond strength of the substrate (R-H → R• + H•) and pKa of the corresponding radical cation (RH•+ → R• + H+). This interplay of structure and thermodynamics may provide the driving force for an electron transfer mechanism for MAO catalysis and inhibition.
Bouley, Bailey S.; Tang, Fengzhi; Bae, Dae Young; Mirica, Liviu M.
(, Chemical Science)
Herein we report the direct observation of C–H bond activation at an isolated mononuclear Pd( iii ) center. The oxidation of the Pd( ii ) complex ( Me N4)Pd II (neophyl)Cl (neophyl = –CH 2 C(CH 3 ) 2 Ph; Me N4 = N , N ′-dimethyl-2,11-diaza[3.3](2,6)pyridinophane) using the mild oxidant ferrocenium hexafluorophosphate (FcPF 6 ) yields the stable Pd( iii ) complex [( Me N4)Pd III (neophyl)Cl]PF 6 . Upon the addition of an acetate source, [( Me N4)Pd III (neophyl)Cl]PF 6 undergoes Csp 2 –H bond activation to yield the cyclometalated product [( Me N4)Pd III (cycloneophyl)]PF 6 . This metalacycle can be independently prepared, allowing for a complete characterization of both the starting and final Pd( iii ) complexes. The C–H activation step can be monitored directly by EPR and UV-Vis spectroscopies, and kinetic isotope effect (KIE) studies suggest that either a pre-association step such as an agostic interaction may be rate limiting, or that the C–H activation is partially rate-limiting in conjunction with ligand rearrangement. Density functional theory calculations support that the reaction proceeds through a κ 3 ligand coordination and that the flexible ligand structure is important for this transformation. Overall, this study represents the first example of discrete C–H bond activation occurring at a Pd( iii ) center through a concerted metalation–deprotonation mechanism, akin to that observed for Pd( ii ) and Pd( iv ) centers.
Kadassery, Karthika J.; Crawley, Matthew R.; MacMillan, Samantha N.; Lacy, David C.
(, Dalton Transactions)
The known compound K[( PO ) 2 Mn(CO) 2 ] ( PO = 2-((diphenylphosphino)methyl)-4,6-dimethylphenolate) (K[ 1 ]) was protonated to form the new Mn( i ) complex ( HPO )( PO )Mn(CO) 2 ( H 1 ) and was determined to have a p K a approximately equal to tetramethylguanidine (TMG). The reduction potential of K[ 1 ] was determined to be −0.58 V vs. Fc/Fc + in MeCN and allowed for an estimation of an experimental O–H bond dissociation free energy (BDFE O–H ) of 73 kcal mol −1 according to the Bordwell equation. This value is in good agreement with a corrected DFT computed BDFE O–H of 68.0 kcal mol −1 (70.3 kcal mol −1 for intramolecular H-bonded isomer). The coordination of the protonated O-atom in the solid-state H 1 was confirmed using FTIR spectroscopy and X-ray crystallography. The phenol moiety is hemilabile as evident from computation and experimental results. For instance, dissociation of the protonated O-atom in H 1 is endergonic by only a few kcal mol −1 (DFT). Furthermore, [ 1 ] − and other Mn( i ) compounds coordinated to PO and/or HPO do not react with MeCN, but H 1 reacts with MeCN to form H 1 + MeCN . Experimental evidence for the solution-bound O-atoms of H 1 was obtained from 1 H NMR and UV-vis spectroscopy and by comparing the electronic spectra of bona fide 16-e − Mn( i ) complexes such as [{ PNP }Mn(CO) 2 ] ( PNP = − N{CH 2 CH 2 (P i Pr 2 )} 2 ) and [( Me3SiOP )( PO )Mn(CO) 2 ] ( Me3Si 1 ). Compound H 1 is only meta-stable ( t 1/2 0.5–1 day) and decomposes into products consistent with homolytic O–H bond cleavage. For instance, treatment of H 1 with TEMPO resulted in formation of TEMPOH, free ligand, and [Mn II {( PO ) 2 Mn(CO) 2 } 2 ]. Together with the experimental and calculated weakened BDFE O–H , these data provide strong evidence for the coordination and hemilability of the protonated O-atom in H 1 and represents the first example of the phenolic Mn( i )–O linkage and a rare example of a “soft-homolysis” intermediate in the bond-weakening catalysis paradigm.
Lin, Lirong, Spasyuk, Denis M., Lalancette, Roger A., and Prokopchuk, Demyan E. Coordination-Induced Weakening of a C(sp3)–H Bond: Homolytic and Heterolytic Bond Strength of a CH–Ni Agostic Interaction. Retrieved from https://par.nsf.gov/biblio/10473505. Journal of the American Chemical Society 144.28 Web. doi:10.1021/jacs.2c05667.
Lin, Lirong, Spasyuk, Denis M., Lalancette, Roger A., & Prokopchuk, Demyan E. Coordination-Induced Weakening of a C(sp3)–H Bond: Homolytic and Heterolytic Bond Strength of a CH–Ni Agostic Interaction. Journal of the American Chemical Society, 144 (28). Retrieved from https://par.nsf.gov/biblio/10473505. https://doi.org/10.1021/jacs.2c05667
Lin, Lirong, Spasyuk, Denis M., Lalancette, Roger A., and Prokopchuk, Demyan E.
"Coordination-Induced Weakening of a C(sp3)–H Bond: Homolytic and Heterolytic Bond Strength of a CH–Ni Agostic Interaction". Journal of the American Chemical Society 144 (28). Country unknown/Code not available: American Chemical Society. https://doi.org/10.1021/jacs.2c05667.https://par.nsf.gov/biblio/10473505.
@article{osti_10473505,
place = {Country unknown/Code not available},
title = {Coordination-Induced Weakening of a C(sp3)–H Bond: Homolytic and Heterolytic Bond Strength of a CH–Ni Agostic Interaction},
url = {https://par.nsf.gov/biblio/10473505},
DOI = {10.1021/jacs.2c05667},
abstractNote = {The scission of a C(sp3)−H bond to form a new metal−alkyl bond is a fundamental step in coordination chemistry and catalysis. However, the extent of C−H bond weakening when this moiety interacts with a transition metal is poorly understood and quantifying this phenomenon could provide insights into designing more efficient C−H functionalization catalysts. We present a nickel complex with a robust adamantyl reporter ligand that enables the measurement of C−H acidity (pKa) and bond dissociation free energy (BDFE) for a C(sp3)−H agostic interaction, showing a decrease in pKa by dozens of orders of magnitude and BDFE decrease of about 30 kcal/mol upon coordination. X-ray crystallographic data is provided for all molecules, including a distorted square planar NiIII metalloradical and “doubly agostic” NiII(κ2-CH2) complex.},
journal = {Journal of the American Chemical Society},
volume = {144},
number = {28},
publisher = {American Chemical Society},
author = {Lin, Lirong and Spasyuk, Denis M. and Lalancette, Roger A. and Prokopchuk, Demyan E.},
}
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