Carbon-to-Metal Hydrogen Atom Transfer:  Direct Observation Using Time-Resolved Infrared Spectroscopy

Abstract

We report the direct spectroscopic observation of hydrogen atom transfer reactions from carbon to metals, in which homolytic cleavage of a C-H bond is accomplished at a single metal center. Laser flash photolysis (355 nm) of a solution of [Cp(CO)2Os]2 leads to homolysis of the Os-Os bond and formation of the osmium-centered radical, Cp(CO)2Os*, as observed by time-resolved infrared (TRIR) spectroscopy. DFT computations on Cp(CO)2Os* support this assignment. Continuous photolysis (lambda > 300 nm) of [Cp(CO)2Os]2 in the presence of excess 1,4-cyclohexadiene produces the osmium hydride Cp(CO)2OsH. The kinetics of this carbon-to-metal hydrogen atom transfer were examined by TRIR spectroscopy. The second-order rate constant for hydrogen atom transfer from 1,4-cyclohexadiene to Cp(CO)2Os* in hexane at 23 degrees C is kH = (2.1 +/- 0.2) x 106 M-1 s-1. The pKa of Cp(CO)2OsH was determined as 32.7 in CH3CN, and use of a thermochemical cycle provided an estimated lower limit of 82 kcal/mol for the Os-H bond dissociation energy, indicating that it is an exceptionally strong M-H bond. Photolysis of [Tp(CO)2Os]2 (Tp = hydridotris(pyrazolyl)borate) results in carbon-to-metal hydrogen atom transfers from even stronger C-H bonds (THF or toluene) and produces Tp(CO)2OsH.

Kinetic and Mechanistic Studies of Carbon-to-Metal Hydrogen Atom Transfer Involving Os-Centered Radicals: Evidence for Tunneling

“…We previously reported our initial studies showing the direct observation of carbon-to-metal HAT using time-resolved infrared (TRIR) spectroscopy. 20 Photochemical homolysis of the Os−Os bond of [Cp(CO) 2 Os] 2 (Cp = η 5 -C 5 H 5 ) generated the osmium-centered radical Cp-(CO) 2 Os • , which abstracts a hydrogen atom from the C−H bond of 1,4-cyclohexadiene, producing the osmium hydride Cp(CO) 2 OsH.…”

“…The synthetic procedures and characterization of [Cp(CO) 2 Os] 2 and [(η 5 -i Pr 4 C 5 H)(CO) 2 Os] 2 have been described previously. 20,24 Continuous Photolysis and KIE Measurements. Continuous photolysis was carried out in NMR tubes sealed with J.…”

Catalysis Involving the H• Transfer Reactions of First‐Row Transition Metals

“…Although free N -H and O -H bonds have high bond dissociation enthalpies, they are weak- In an investigation related to the total synthesis of ( + ) -3 α -hydroxyreynosin, Cuerva and Oltra discovered that the products of a Ti(III) -mediated epoxideopening/cyclization cascade varied with the presence or absence of water. In dry THF they obtained the product originally desired, with an exocyclic ∆ [3,14] double bond, but the addition of water resulted in the formation of the reductively cyclized product in Scheme 1.4 . Apparently H • is transferred from the titanium -water complex to the cyclized radical [35 -37] .…”

“…Bond dissociation energies of most M -H bonds lie between 60 and 65 kcal mol − 1 [4] . A few are much stronger: the Os -H bond of Cp(CO) 2 OsH has a BDE ≥ 82 kcal mol − 1 [14] . On the other hand V -H bonds are particularly weak.…”

Peptides-assisted charge transfers in proteins: relay mechanism and its controllability

“…A survey of charge transfer characteristics reveals that transports of charges or radicals could be divided into two main categories: hole transfer (HOT) and electron transfer (ET). In general, charge transfer process accompanies proton transfer (PT) simultaneously, which may be mainly classified as a hydrogen atom transfer (HAT) [11][12][13][14][15][16][17][18][19][20][21] or as a more complicated protoncoupled ET (PCET) [17][18][19][20][21][22][23][24][25][26][27][28][29][30] process, with the latter further subdivided into several distinct types according to different reaction characteristics [17][18][19][20][21][22].…”