Experimental and Computational Studies of Ruthenium Complexes Bearing Z-Acceptor Aluminum-Based Phosphine Pincer Ligands.

Reaction of [Ru(C6H4PPh2)2(Ph2PC6H4AlMe(THF))H] with CO results in clean conversion to the Ru-Al heterobimetallic complex [Ru(AlMePhos)(CO)3] (1), where AlMePhos is the novel P-Al(Me)-P pincer ligand (o-Ph2PC6H4)2AlMe. Under photolytic conditions, 1 reacts with H2 to give [Ru(AlMePhos)(CO)2(µ-H)H] (2) that is characterized by multinuclear NMR and IR spectroscopies. DFT calculations indicate that 2 features one terminal and one bridging hydride that are respectively anti and syn to the AlMe group. Calculations also define a mechanism for H2 addition to 1 and predict facile hydride exchange in 2 that is also observed experimentally. Reaction of 1 with B(C6F5)3 results in Me abstraction to form the ion pair [Ru(AlPhos)(CO)3][MeB(C6F5)3] (4) featuring a cationic [(o-Ph2PC6H4)2Al]+ ligand, [AlPhos]+. The Ru-Al distance in 4 (2.5334(16) Å) is significantly shorter than that in 1 (2.6578(6) Å), consistent with an enhanced Lewis acidity of the [AlPhos]+ ligand. This is corroborated by a blue shift in both the observed and computed νCO stretching frequencies upon Me abstraction. Electronic structure analyses (QTAIM and EDA-ETS) comparing 1, 4, and the previously reported [Ru(ZnPhos)(CO)3] analogue (ZnPhos = (o-Ph2PC6H4)2Zn) indicate that the Lewis acidity of these pincer ligands increases along the series ZnPhos < AlMePhos < [AlPhos]+.

Extreme g-Tensor Anisotropy and Its Insensitivity to Structural Distortions in a Family of Linear Two-Coordinate Ni(I) Bis-N-heterocyclic Carbene Complexes.

We report a new series of homoleptic Ni(I) bis-N-heterocyclic carbene complexes with a range of torsion angles between the two ligands from 68° to 90°. Electron paramagnetic resonance measurements revealed a strongly anisotropic g-tensor in all complexes with a small variation in g∥ ∼ 5.7-5.9 and g⊥ ∼ 0.6. The energy of the first excited state identified by variable-field far-infrared magnetic spectroscopy and SOC-CASSCF/NEVPT2 calculations is in the range 270-650 cm-1. Magnetic relaxation measured by alternating current susceptibility up to 10 K is dominated by Raman and direct processes. Ab initio ligand-field analysis reveals that a torsion angle of <90° causes the splitting between doubly occupied dxz and dyz orbitals, which has little effect on the magnetic properties, while the temperature dependence of the magnetic relaxation appears to have no correlation with the torsion angle.

Zinc-Promoted ZnMe/ZnPh Exchange in Eight-Coordinate [Ru(PPh3 )2 (ZnMe)4 H2 ].

The syntheses, reactivity and electronic structure analyses of [Ru(PPh3 )2 (ZnMe)4 H2 ], 1 a, and [Ru(PPh3 )2 (ZnPh)4 H2 ], 2 b, are reported. 1 a exhibits an 8-coordinate Ru centre with axial phosphines and a symmetrical (2 : 2) arrangement of ZnMe ligands in the equatorial plane. The ZnMe ligands in 1 a undergo facile, sequential exchange with ZnPh2 to give 2 b, which shows a 3 : 1 arrangement of ZnPh ligands. Both 1 a and 2 b exist in equilibrium with their respective 3 : 1 and 2 : 2 isomers. Mechanisms for ZnMe/ZnPh exchange and isomerisation are proposed using DFT calculations. The relationships of these {Ru(ZnR)4 H2 } species to isoelectronic Group 8 transition metal polyhydrides and related Schlenk equilibria in the Negishi reaction are discussed.

[Ni(NHC)2 ] as a Scaffold for Structurally Characterized trans [H-Ni-PR2 ] and trans [R2 P-Ni-PR2 ] Complexes.

The addition of PPh2 H, PPhMeH, PPhH2 , P(para-Tol)H2 , PMesH2 and PH3 to the two-coordinate Ni0 N-heterocyclic carbene species [Ni(NHC)2 ] (NHC=IiPr2 , IMe4 , IEt2 Me2 ) affords a series of mononuclear, terminal phosphido nickel complexes. Structural characterisation of nine of these compounds shows that they have unusual trans [H-Ni-PR2 ] or novel trans [R2 P-Ni-PR2 ] geometries. The bis-phosphido complexes are more accessible when smaller NHCs (IMe4 >IEt2 Me2 >IiPr2 ) and phosphines are employed. P-P activation of the diphosphines R2 P-PR2 (R2 =Ph2 , PhMe) provides an alternative route to some of the [Ni(NHC)2 (PR2 )2 ] complexes. DFT calculations capture these trends with P-H bond activation proceeding from unconventional phosphine adducts in which the H substituent bridges the Ni-P bond. P-P bond activation from [Ni(NHC)2 (Ph2 P-PPh2 )] adducts proceeds with computed barriers below 10 kcal mol-1 . The ability of the [Ni(NHC)2 ] moiety to afford isolable terminal phosphido products reflects the stability of the Ni-NHC bond that prevents ligand dissociation and onward reaction.

Bonding and Reactivity of a Pair of Neutral and Cationic Heterobimetallic RuZn2 Complexes.

A combined experimental and computational study of the structure and reactivity of two [RuZn2Me2] complexes, neutral [Ru(PPh3)(Ph2PC6H4)2(ZnMe)2] (2) and cationic [Ru(PPh3)2(Ph2PC6H4)(ZnMe)2][BArF4] ([BArF4] = [B{3,5-(CF3)2C6H3}4]) (3), is presented. Structural and computational analyses indicate these complexes are best formulated as containing discrete ZnMe ligands in which direct Ru-Zn bonding is complemented by weaker Zn···Zn interactions. The latter are stronger in 2, and both complexes exhibit an additional Zn···Caryl interaction with a cyclometalated phosphine ligand, this being stronger in 3. Both 2 and 3 show diverse reactivity under thermolysis and with Lewis bases (PnBu3, PCy3, and IMes). With 3, all three Lewis bases result in the loss of [ZnMe]+. In contrast, 2 undergoes PPh3 substitution with PnBu3, but with IMes, loss of ZnMe2 occurs to form [Ru(PPh3)(C6H4PPh2)(C6H4PPhC6H4Zn(IMes))H] (7). The reaction of 3 with H2 affords the cationic trihydride complex [Ru(PPh3)2(ZnMe)2(H)3][BArF4] (12). Computational analyses indicate that both 12 and 7 feature bridging hydrides that are biased toward Ru over Zn.

Zn-Promoted C-H Reductive Elimination and H2 Activation via a Dual Unsaturated Heterobimetallic Ru-Zn Intermediate.

Reaction of [Ru(PPh3)3HCl] with LiCH2TMS, MgMe2, and ZnMe2 proceeds with chloride abstraction and alkane elimination to form the bis-cyclometalated derivatives [Ru(PPh3)(C6H4PPh2)2H][M'] where [M'] = [Li(THF)2]+ (1), [MgMe(THF)2]+ (3), and [ZnMe]+ (4), respectively. In the presence of 12-crown-4, the reaction with LiCH2TMS yields [Ru(PPh3)(C6H4PPh2)2H][Li(12-crown-4)2] (2). These four complexes demonstrate increasing interaction between M' and the hydride ligand in the [Ru(PPh3)(C6H4PPh2)2H]- anion following the trend 2 (no interaction) < 1 < 3 < 4 both in the solid-state and solution. Zn species 4 is present as three isomers in solution including square-pyramidal [Ru(PPh3)2(C6H4PPh2)(ZnMe)] (5), that is formed via C-H reductive elimination and features unsaturated Ru and Zn centers and an axial Z-type [ZnMe]+ ligand. A [ZnMe]+ adduct of 5, [Ru(PPh3)2(C6H4PPh2)(ZnMe)2][BArF4] (6) can be trapped and structurally characterized. 4 reacts with H2 at -40 °C to form [Ru(PPh3)3(H)3(ZnMe)], 8-Zn, and contrasts the analogous reactions of 1, 2, and 3 that all require heating to 60 °C. This marked difference in reactivity reflects the ability of Zn to promote a rate-limiting C-H reductive elimination step, and calculations attribute this to a significant stabilization of 5 via Ru → Zn donation. 4 therefore acts as a latent source of 5 and this operational "dual unsaturation" highlights the ability of Zn to promote reductive elimination in these heterobimetallic systems. Calculations also highlight the ability of the heterobimetallic systems to stabilize developing protic character of the transferring hydrogen in the rate-limiting C-H reductive elimination transition states.

Unexpected Vulnerability of DPEphos to C-O Activation in the Presence of Nucleophilic Metal Hydrides.

C-O bond activation of DPEphos occurs upon mild heating in the presence of [Ru(NHC)2 (PPh3 )2 H2 ] (NHC=N-heterocyclic carbene) to form phosphinophenolate products. When NHC=IEt2 Me2 , C-O activation is accompanied by C-N activation of an NHC ligand to yield a coordinated N-phosphino-functionalised carbene. DFT calculations define a nucleophilic mechanism in which a hydride ligand attacks the aryl carbon of the DPEphos C-O bond. This is promoted by the strongly donating NHC ligands which render a trans dihydride intermediate featuring highly nucleophilic hydride ligands accessible. C-O bond activation also occurs upon heating cis-[Ru(DPEphos)2 H2 ]. DFT calculations suggest this reaction is promoted by the steric encumbrance associated with two bulky DPEphos ligands. Our observations that facile degradation of the DPEphos ligand via C-O bond activation is possible under relatively mild reaction conditions has potential ramifications for the use of this ligand in high-temperature catalysis.

Impact of the Novel Z-Acceptor Ligand Bis{(ortho-diphenylphosphino)phenyl}zinc (ZnPhos) on the Formation and Reactivity of Low-Coordinate Ru(0) Centers.

The preparation and reactivity with H2 of two Ru complexes of the novel ZnPhos ligand (ZnPhos = Zn(o-C6H4PPh2)2) are described. Ru(ZnPhos)(CO)3 (2) and Ru(ZnPhos)(IMe4)2 (4; IMe4 = 1,3,4,5-tetramethylimidazol-2-ylidene) are formed directly from the reaction of Ru(PPh3)(C6H4PPh2)2(ZnMe)2 (1) or Ru(PPh3)3HCl/LiCH2TMS/ZnMe2 with CO and IMe4, respectively. Structural and electronic structure analyses characterize both 2 and 4 as Ru(0) species in which Ru donates to the Z-type Zn center of the ZnPhos ligand; in 2, Ru adopts an octahedral coordination, while 4 displays square-pyramidal coordination with Zn in the axial position. Under photolytic conditions, 2 loses CO to give Ru(ZnPhos)(CO)2 that then adds H2 over the Ru-Zn bond to form Ru(ZnPhos)(CO)2(µ-H)2 (3). In contrast, 4 reacts directly with H2 to set up an equilibrium with Ru(ZnPhos)(IMe4)2H2 (5), the product of oxidative addition at the Ru center. DFT calculations rationalize these different outcomes in terms of the energies of the square-pyramidal Ru(ZnPhos)L2 intermediates in which Zn sits in a basal site: for L = CO, this is readily accessed and allows H2 to add across the Ru-Zn bond, but for L = IMe4, this species is kinetically inaccessible and reaction can only occur at the Ru center. This difference is related to the strong π-acceptor ability of CO compared to IMe4. Steric effects associated with the larger IMe4 ligands are not significant. Species 4 can be considered as a Ru(0)L4 species that is stabilized by the Ru→Zn interaction. As such, it is a rare example of a stable Ru(0)L4 species devoid of strong π-acceptor ligands.

Reductive Elimination at Carbon under Steric Control.

It has been previously demonstrated that stable singlet electrophilic carbenes can behave as metal surrogates in the activation of strong E-H bonds (E = H, B, N, Si, P), but it was believed that these activations only proceed through an irreversible activation barrier. Herein we show that, as is the case with transition metals, the steric environment can be used to promote reductive elimination at carbon centers.

Well-Defined Heterobimetallic Reactivity at Unsupported Ruthenium-Indium Bonds.

The hydride complex [Ru(IPr)2 (CO)H][BArF4 ], 1, reacts with InMe3 with loss of CH4 to form [Ru(IPr)2 (CO)(InMe)(Me)][BArF4 ], 4, featuring an unsupported Ru-In bond with unsaturated Ru and In centres. 4 reacts with H2 to give [Ru(IPr)2 (CO)(η2 -H2 )(InMe)(H)][BArF4 ], 5, while CO induces formation of the indyl complex [Ru(IPr)2 (CO)3 (InMe2 )][BArF4 ], 7. These observations highlight the ability of Me to shuttle between Ru and In centres and are supported by DFT calculations on the mechanism of formation of 4 and its reactions with H2 and CO. An analysis of Ru-In bonding in these species is also presented. Reaction of 1 with GaMe3 also involves CH4 loss but, in contrast to its In congener, sees IPr transfer from Ru to Ga to give a gallyl complex featuring an η6 interaction of one aryl substituent with Ru.

C-F Bond Activation of P(C6F5)3 by Ruthenium Dihydride Complexes: Isolation and Reactivity of the "Missing" Ru(PPh3)3H(halide) Complex, Ru(PPh3)3HF.

The major product of the reaction between Ru(IMe4)2(PPh3)2H2 (1; IMe4 = 1,3,4,5-tetramethylimidazol-2-ylidene) and P(C6F5)3 (PCF) is the five-coordinate complex Ru(IMe4)2(PF2{C6F5})(C6F5)H (2), which is formed via a complex series of C-F/P-C bond cleavage and P-F bond formation steps. In contrast, hydrodefluorination of all six ortho C-F bonds in PCF occurs with Ru(PPh3)4H2 to afford Ru(PPh3)3HF (3). NaBArF4 abstracted the fluoride ligand in 3 to give [Ru({η6-C6H5}PPh2)(PPh3)2H][BArF4], while B2pin2 reacted with 3 in C6D6 to yield a mixture of [Ru({η6-C6D6)(PPh3)2H]+ and Ru(PPh3)4H2. The treatment of 3 with HBpin (5 equiv) and HSiR3 (R = Et, Ph; 2 equiv) afforded Ru(PPh3)3(σ-HBpin)H2 and Ru(PPh3)3(SiR3)3H3, respectively. No stable substitution products were generated when 3 was reacted with Me3SiX (X = CF3, C6F5).

Room Temperature Regioselective Catalytic Hydrodefluorination of Fluoroarenes with trans-[Ru(NHC)4 H2 ] through a Concerted Nucleophilic Ru-H Attack Pathway.

The efficient and highly selective room temperature hydrodefluorination (HDF) of fluoroarenes by the trans-[Ru(IMe4 )4 H2 ] catalyst, 3, is reported. Mechanistic studies show 3 acts directly in catalysis without any ligand dissociation and DFT calculations indicate a concerted nucleophilic attack mechanism. The calculations fully account for the observed selectivities which corroborate earlier predictions regarding the selectivity of HDF.

Activation of H2 over the Ru-Zn Bond in the Transition Metal-Lewis Acid Heterobimetallic Species [Ru(IPr)2(CO)ZnEt](.).

Reaction of [Ru(IPr)2(CO)H]BAr(F)4 with ZnEt2 forms the heterobimetallic species [Ru(IPr)2(CO)ZnEt]BAr(F)4 (2), which features an unsupported Ru-Zn bond. 2 reacts with H2 to give [Ru(IPr)2(CO)(η(2)-H2)(H)2ZnEt]BAr(F)4 (3) and [Ru(IPr)2(CO)(H)2ZnEt]BAr(F)4 (4). DFT calculations indicate that H2 activation at 2 proceeds via oxidative cleavage at Ru with concomitant hydride transfer to Zn. 2 can also activate hydridic E-H bonds (E = B, Si), and computed mechanisms for the facile H/H exchange processes observed in 3 and 4 are presented.

Influence of Ring-Expanded N-Heterocyclic Carbenes on the Structures of Half-Sandwich Ni(I) Complexes: An X-ray, Electron Paramagnetic Resonance (EPR), and Electron Nuclear Double Resonance (ENDOR) Study.

Potassium graphite reduction of the half-sandwich Ni(II) ring-expanded diamino/diamidocarbene complexes CpNi(RE-NHC)Br gave the Ni(I) derivatives CpNi(RE-NHC) (where RE-NHC = 6-Mes (1), 7-Mes (2), 6-MesDAC (3)) in yields of 40%-50%. The electronic structures of paramagnetic 1-3 were investigated by CW X-/Q-band electron paramagnetic resonance (EPR) and Q-band 1H electron nuclear double resonance (ENDOR) spectroscopy. While small variations in the g-values were observed between the diaminocarbene complexes 1 and 2, pronounced changes in the g-values were detected between the almost isostructural species (1) and diamidocarbene species (3). These results highlight the sensitivity of the EPR g-tensor to changes in the electronic structure of the Ni(I) centers generated by incorporation of heteroatom substituents onto the backbone ring positions. Variable-temperature EPR analysis also revealed the presence of a second Ni(I) site in 3. The experimental g-values for these two Ni(I) sites detected by EPR in frozen solutions of 3 are consistent with resolution on the EPR time scale of the disordered components evident in the X-ray crystallographically determined structure and the corresponding density functional theory (DFT)-calculated g-tensor. Q-band 1H ENDOR measurements revealed a small amount of unpaired electron spin density on the Cp rings, consistent with the calculated SOMO of complexes 1-3. The magnitude of the 1H A values for 3 were also notably larger, compared to 1 and 2, again highlighting the influence of the diamidocarbene on the electronic properties of 3.

Experimental and Computational Studies of the Copper Borate Complexes [(NHC)Cu(HBEt3 )] and [(NHC)Cu(HB(C6 F5 )3 )].

The synthesis of the Cu-borate complexes [(6Mes)Cu(HBR3 )] featuring the unusual [HBEt3 ]- (5) and [HB(C6 F5 )3 ]- (6) ligands is described. Experimental and computational studies show both compounds feature a direct Cu-H interaction, but that while 5 is two-coordinate, 6 displays an additional, stabilizing Cu-Cipso (C6 F5 ) interaction.

A Comparison of the Stability and Reactivity of Diamido- and Diaminocarbene Copper Alkoxide and Hydride Complexes.

The mononuclear N-heterocyclic carbene (NHC) copper alkoxide complexes [(6-NHC)CuOtBu] (6-NHC = 6-MesDAC (1), 6-Mes (2)) have been prepared by addition of the free carbenes to the tetrameric tert-butoxide precursor [Cu(OtBu)]4, or by protonolysis of [(6-NHC)CuMes] (6-NHC = 6-MesDAC (3), 6-Mes (4)) with tBuOH. In contrast to the relatively stable diaminocarbene complex 2, the diamidocarbene derivative 1 proved susceptible to both thermal and hydrolytic ring-opening reactions, the latter affording [(6-MesDAC)Cu(OC(O)CMe2C(O)N(H)Mes)(CNMes)] (6). The intermediacy of [(6-MesDAC)Cu(OH)] in this reaction was supported by the generation of Cu2O as an additional product. Attempts to generate an isolable copper hydride complex of the type [(6-MesDAC)CuH] by reaction of 1 with Et3SiH resulted instead in migratory insertion to generate [(6-MesDAC-H)Cu(P(p-tolyl)3)] (9) upon trapping by P(p-tolyl)3. Migratory insertion was also observed during attempts to prepare [(6-Mes)CuH], with [(6-Mes-H)Cu(6-Mes)] (10) isolated, following a reaction that was significantly slower than in the 6-MesDAC case. The longer lifetime of [(6-Mes)CuH] allowed it to be trapped stoichiometrically by alkyne, and also employed in the catalytic semi-reduction of alkynes and hydrosilylation of ketones.

Unexpected migratory insertion reactions of M(alkyl)2 (M=Zn, Cd) and diamidocarbenes.

The electrophilic character of free diamidocarbenes (DACs) allows them to activate inert bonds in small molecules, such as NH3 and P4 . Herein, we report that metal coordinated DACs also exhibit electrophilic reactivity, undergoing attack by Zn and Cd dialkyl precursors to afford the migratory insertion products [(6-MesDAC-R)MR] (M=Zn, Cd; R=Et, Me; Mes=mesityl). These species were formed via the spectroscopically characterised intermediates [(6-MesDAC)MR2 ], exhibiting barriers to migratory insertion which increase in the order MR2 = ZnEt2 < ZnMe2 < CdMe2 . Compound [(6-MesDAC-Me)CdMe] showed limited stability, undergoing deposition of Cd metal, by an apparent ß-H elimination pathway. These results raise doubts about the suitability of diamidocarbenes as ligands in catalytic reactions involving metal species bearing nucleophilic ligands (M-R, M-H).

Mono- and bimetallic zwitterionic chromium(0) and tungsten(0) allenyls.

A series of stable chiral (racemic), formally neutral, zwitterionic mono- and bimetallic M(CO)5[C(OEt)═C═CR(NHC)] (M = Cr, W) σ-allenyls are ready available by the addition of N-heterocyclic carbenes (NHCs) to Cr(0) and W(0) alkynyl Fischer carbene complexes. Different classes of NHCs, (e.g., 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene, 1,3-bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene, and their six- and seven-membered analogues and 1,3-bis(dimethyl)imidazol-2-ylidene) were employed as nucleophiles in these C-C bond-forming reactions yielding the novel complexes in essentially quantitative yields. A systematic experimental and computational study of the electronic properties of the Cr- and W-allenyls shows that their UV-vis spectra are directly influenced by the structure of the heterocyclic moiety derived from the NHC (ring size, substituents on the N atoms) and by the nature of the metal fragment (Cr/W). The electron-releasing nature of these complexes allows them to participate in electron-transfer reactions in the ground state, leading to a type of charged α,ß-unsaturated Fischer carbenes that incorporate an NHC fragment in their structure.

Copper diamidocarbene complexes: characterization of monomeric to tetrameric species.

Treatment of CuCl with 1 equiv of the in situ prepared N-mesityl-substituted diamidocarbene 6-MesDAC produced a mixture of the dimeric and trimeric copper complexes [(6-MesDAC)CuCl]2 (1) and [(6-MesDAC)2(CuCl)3] (2). Combining CuCl with isolated, free 6-MesDAC in 1:1 and 3:2 ratios gave just 1 and 2, respectively, while increasing the ratio to >5:1 allowed the isolation of small amounts of the tetrameric copper complex [(6-MesDAC)2(CuCl)4] (3). Efforts to bring about metathesis reactions of 1 with MO(t)Bu (M = Li, Na, K) proved successful only for M = Li to afford the spectroscopically characterized ate product [(6-MesDAC)CuCl·LiO(t)Bu·2THF] (5). Attempts to crystallize this species instead gave a 1:1 mixture of 1 and the monomer [(6-MesDAC)CuCl] (6). The X-ray structures of 1-3 and 1 + 6, along with the cation [Cu(6-MesDAC)2](+) (4), have been determined.

Stereoelectronic effects in C-H bond oxidation reactions of Ni(I) N-heterocyclic carbene complexes.

Activation of O2 by the three-coordinate Ni(I) ring-expanded N-heterocyclic carbene complexes Ni(RE-NHC)(PPh3)Br (RE-NHC = 6-Mes, 1; 7-Mes, 2) produced the structurally characterized dimeric Ni(II) complexes Ni(6-Mes)(Br)(µ-OH)(µ-O-6-Mes')NiBr (3) and Ni(7-Mes)(Br)(µ-OH)(µ-O-7-Mes')NiBr (4) containing oxidized ortho-mesityl groups from one of the carbene ligands. NMR and mass spectrometry provided evidence for further oxidation in solution to afford bis-µ-aryloxy compounds; the 6-Mes derivative was isolated, and its structure was verified. Low-temperature UV-visible spectroscopy showed that the reaction between 1 and O2 was too fast even at ca. -80 °C to yield any observable intermediates and also supported the formation of more than one oxidation product. Addition of O2 to Ni(I) precursors containing a less electron-donating diamidocarbene (6-MesDAC, 7) or less bulky 6- or 7-membered ring diaminocarbene ligands (6- or 7-o-Tol; 8 and 9) proceeded quite differently, affording phosphine and carbene oxidation products (Ni(O═PPh3)2Br2 and (6-MesDAC)═O) and the mononuclear Ni(II) dibromide complexes (Ni(6-o-Tol)(PPh3)Br2 (10) and (Ni(7-o-Tol)(PPh3)Br2 (11)) respectively. Electrochemical measurements on the five Ni(I) precursors show significantly higher redox potentials for 1 and 2, the complexes that undergo oxygen atom transfer from O2.