Isolation, Characterization of an Intermediate in an Oxygen Atom-Transfer Reaction, and the Determination of the Bond Dissociation Energy Victor N. Nemykin, † Julia Laskin,* ,‡ and Partha Basu* ,† Department of Chemistry and Biochemistry, Duquesne UniVersity, Pittsburgh, PennsylVania 15282, and Chemical Sciences DiVision, Pacific Northwest National Laboratory, Richland, Washington 99352 Received February 17, 2004; E-mail: basu@duq.edu; Julia.Laskin@pnl.gov Redox reactions coupled with the formal loss or gain of an oxygen atom are ubiquitous in chemical processes. Such reactions proceed through the reduction of the donor center (XO) and the oxidation of the acceptor (Y) molecule. 1-3 Among many examples of the metal-centered oxygen atom transfer (OAT) reactivity, those involving molybdenum complexes have been widely investigated due to their involvement in mononuclear molybdenum enzymes. 4 The heat of reaction of the overall atom transfer process can be expressed as a difference between the bond dissociation energies (BDEs) of the oxygen-donor (X) and oxygen-acceptor (Y) bond, i.e., ΔH ) D XdO - D YdO . 5 It has recently become apparent that the OAT reactions from many [MoO 2 ] 2+ cores proceed via multiple steps. 6 Here we describe the isolation and characterization of an intermediate of OAT reaction, LMoO(OPMe 3 )Cl (1), (where L ) hydrotris(3,5-dimethyl- 1-pyrazolyl)borate ligand) generated by reacting LMoO 2 Cl with PMe 3 . Surface-induced dissociation (SID) of the complex was studied using a novel Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR MS) specially configured for SID experiments. 7 The Mo-OPMe 3 BDE was determined using RRKM modeling of the time- and energy-dependent SID data. The determination of the BDE allows description of the overall heat of the reaction in a stepwise fashion for the first time. Room-temperature anaerobic reaction of LMoO 2 Cl with PMe 3 in benzene resulted in a rapid change in color from light yellow to dark green. The target compound, 1, has been precipitated as a crystalline solid by adding hexane into the reaction mixture (Supporting Information). The compound is soluble in acetonitrile and benzene but decomposes over a period of time. In the solid state, however, the compound is relatively stable. X-ray quality single crystals (Supporting Information) were grown by slow diffusion of hexane into a solution of benzene. A thermal ellipsoid diagram of the molecular structure is shown in Figure 1. The Mo-N distances are consistent with other reported structures of oxo-Mo(IV) of hydrotrispyrazolyl borate com- plexes. 6,8 Compared to the average of the equatorial Mo-N distances, the Mo-N (trans to the terminal oxo group) has been elongated by 0.23 Å. The ModO distance of 1.727 Å is signifi- cantly shorter than the Mo-O(P) distance (2.139 Å), indicating a reduction in the bond order consistent with a reduced molybdenum center. The phosphorus-to-oxygen distance of 1.492 Å is also consistent with the formation of a P-to-O double bond. The O(t)- Mo-O-P torsion angle of 46° is smaller than the angle reported for L i PrMoO(OPEt 3 )(OPh). 6 Collision-energy resolved SID spectra for 1 are shown in Figure 2. The ions of 1 were produced in a high-transmission electrospray (ESI) source that allowed external accumulation of mass-selected ions prior to their collision with a surface. The ions were then extracted from the source, transferred into the ICR cell using an electrostatic ion guide, and collided with a surface (a self-assembled monolayer of dodecanethiol (HSAM) on Au {111} crystal). 9 Scattered ions were collected and analyzed in the ICR cell. 10 Relative abundance of the precursor ion and its fragments were recorded at different collision energies and reaction times. The only product ion observed at m/z 446 corresponds to cleavage of the Mo-OPMe 3 bond. Survival curves (SCs) displaying the relative abundance of the precursor ion as a function of collision energy are shown in Figure 3 for four different reaction times (1ms, 10ms, 0.1s, and 1s). The curves corresponding to a longer reaction time are slightly shifted to lower collision energies as a result of the decrease in the kinetic shift. SCs were modeled using a previously described modeling approach 11 that utilizes RRKM theory and a † Duquesne University. ‡ Pacific Northwest National Laboratory. Figure 1. Thermal ellipsoid (30% probability) diagram of 1. Selected bond distances and angles: Mo(1)-O(1) 1.727(12), Mo(1)-O(2) 2.139(11), Mo- (1)-N(11) 2.382(14), Mo(1)-N(31) 2.183(15), Mo(1)-N(21) 2.124(16), Mo(1)-Cl(1) 2.422(6), O(1)-Mo(1)-O(2) 96.3(5), O(1)-Mo(1)-N(11) 170.1(6), O(1)-Mo(1)-N(31) 93.6(6), Mo(1)-O(2)-P(1) 133.6(6). Figure 2. FT-ICR SID spectra of 1 as a function of collision energy for reaction delay of 1 s. Published on Web 06/24/2004 8604 9 J. AM. CHEM. SOC. 2004, 126, 8604-8605 10.1021/ja049121f CCC: $27.50 © 2004 American Chemical Society