John J. Curley

Shining Light on Dinitrogen Cleavage: Structural Features, Redox Chemistry, and Photochemistry of the Key Intermediate Bridging Dinitrogen Complex

The key intermediate in dinitrogen cleavage by Mo(N[t-Bu]Ar)3, 1 (Ar = 3,5-C6H3Me2), has been characterized by a pair of single crystal X-ray structures. For the first time, the X-ray crystal structure of (mu-N2)[Mo(N[t-Bu]Ar)3]2, 2, and the product of homolytic fragmentation of the NN bond, NMo(N[t-Bu]Ar)3, are reported. The structural features of 2 are compared with previously reported EXAFS data. Moreover, contrasts are drawn between theoretical predictions concerning the structural and magnetic properties of 2 and those reported herein. In particular, it is shown that 2 exists as a triplet (S = 1) at 20 degrees C. Further insight into the bonding across the MoNNMo core of the molecule is obtained by the synthesis and structural characterization of the one- and two-electron oxidized congeners, (mu-N2)[Mo(N[t-Bu]Ar)3]2[B(Ar(F))4], 2[B(Ar(F))4] (Ar(F) = 3,5-C6H3(CF3)2) and (mu-N2)[Mo(N[t-Bu]Ar)3]2[B(Ar(F))4]2, 2[B(Ar(F))4]2, respectively. Bonding in these three molecules is discussed in view of X-ray crystallography, Raman spectroscopy, electronic absorption spectroscopy, and density functional theory. Combining X-ray crystallography data with Raman spectroscopy studies allows the NN bond polarization energy and NN internuclear distance to be correlated in three states of charge across the MoNNMo core. For 2[B(Ar(F))4], bonding is symmetric about the mu-N2 ligand and the NN polarization is Raman active; therefore, 2[B(Ar(F))4] meets the criteria of a Robin-Day class III mixed-valent compound. The redox couples that interrelate 2, 2(+), and 2(2+) are studied by cyclic voltammetry and spectroelectrochemistry. Insights into the electronic structure of 2 led to the discovery of a photochemical reaction that forms NMo(N[t-Bu]Ar)3 and Mo(N[t-Bu]Ar)3 through competing NN bond cleavage and N2 extrusion reaction pathways. The primary quantum yield was determined to be Phi(p) = 0.05, and transient absorption experiments show that the photochemical reaction is complete in less than 10 ns.

Thermodynamic, Kinetic, and Computational Study of Heavier Chalcogen (S, Se, and Te) Terminal Multiple Bonds to Molybdenum, Carbon, and Phosphorus

Enthalpies of chalcogen atom transfer to Mo(N[t-Bu]Ar)3, where Ar = 3,5-C6H3Me2, and to IPr (defined as bis-(2,6-isopropylphenyl)imidazol-2-ylidene) have been measured by solution calorimetry leading to bond energy estimates (kcal/mol) for EMo(N[t-Bu]Ar)3 (E = S, 115; Se, 87; Te, 64) and EIPr (E = S, 102; Se, 77; Te, 53). The enthalpy of S-atom transfer to PMo(N[ t-Bu]Ar) 3 generating SPMo(N[t-Bu]Ar)3 has been measured, yielding a value of only 78 kcal/mol. The kinetics of combination of Mo(N[t-Bu]Ar)3 with SMo(N[t-Bu]Ar)3 yielding (mu-S)[Mo(N[t-Bu]Ar)3]2 have been studied, and yield activation parameters Delta H (double dagger) = 4.7 +/- 1 kcal/mol and Delta S (double dagger) = -33 +/- 5 eu. Equilibrium studies for the same reaction yielded thermochemical parameters Delta H degrees = -18.6 +/- 3.2 kcal/mol and Delta S degrees = -56.2 +/- 10.5 eu. The large negative entropy of formation of (mu-S)[Mo(N[t-Bu]Ar)3]2 is interpreted in terms of the crowded molecular structure of this complex as revealed by X-ray crystallography. The crystal structure of Te-atom transfer agent TePCy3 is also reported. Quantum chemical calculations were used to make bond energy predictions as well as to probe terminal chalcogen bonding in terms of an energy partitioning analysis.

A Terminal Molybdenum Arsenide Complex Synthesized from Yellow Arsenic

A terminal molybdenum arsenide complex is synthesized in one step from the reactive As(4) molecule. The properties of this complex with its arsenic atom ligand are discussed in relation to the analogous nitride and phosphide complexes.

Relaxation and dissociation following photoexcitation of the (μ-N2)[Mo(N[t-Bu]Ar)3]2 dinitrogen cleavage intermediate.

Frequency resolved pump-probe spectroscopy was performed on isolated (μ-N(2))[Mo(N[t-Bu]Ar)(3)](2) (Ar = 3,5-C(6)H(3)Me(2)), an intermediate formed in the reaction of Mo(N[t-Bu]Ar)(3) to bind and cleave dinitrogen. Evidence is presented for 300 fs internal conversion followed by subpicosecond vibrational cooling on the ground electronic state in competition with bond dissociation. Fast cooling following photoexcitation leads to a relatively low overall dissociation yield of 5%, in quantitative agreement with previous work [Curley, J. J.; Cooke, T. R.; Reece, S. Y.; Mueller, P.; Cummins, C. C. J. Am. Chem. Soc. 2008, 130, 9394]. Coupling of vibrational modes to the excitation and internal conversion results in a nonthermal distribution of energy following conversion, and this provides sufficient bias to allow the nitrogen cleavage reaction to compete with breaking of the Mo-NN bond despite a higher energetic barrier on the ground state.

Synthesis and Reversible Reductive Coupling of Cationic, Dinitrogen-Derived Diazoalkane Complexes

A series of cationic diazoalkane complexes [4-RC(6)H(4)C(H)NNMo(N[t-Bu]Ar)(3)][AlCl(4)], [1-R][AlCl(4)] (R = NMe(2), Me, H, Br, CN; Ar = 3,5-C(6)H(3)Me(2)) has been prepared by treatment of the N(2)-derived diazenido complex Me(3)SiNNMo(N[t-Bu]Ar)(3) with 4-RC(6)H(4)CHO and 2 equiv of AlCl(3). The structures of [1-H][AlCl(4)] and [1-NMe(2)][AlCl(4)] were determined by X-ray crystallography. The C-N and N-N stretching modes were identified by a combined IR and Raman spectroscopy study, and other physical properties are discussed in detail. The electrochemical reduction potential for [1-R][AlCl(4)] was shown to be linear with the Hammett sigma parameter. This reduction process forms the C-C bonded dimer, mu-(4-RC(6)H(4)C(H)NN)(2)[Mo(N[t-Bu]Ar)(3)](2), that was characterized by X-ray crystallography for R = H. Possible mechanisms for the formation of this dimer are presented. Both electrochemical investigations and quantum chemical calculations are used to describe the odd-electron complex 4-RC(6)H(4)C(H)NNMo(N[t-Bu]Ar)(3), 1-R, that is an intermediate in the formation of [1-R](2). The C-C bond in [1-R](2) is redox-noninnocent and is broken upon oxidation. This reaction was used to prepare [1-H][A] (A = PF(6)(-), OTf(-)), and possible uses of this property in charge-storage devices are discussed.

Coordination-mode control of bound nitrile radical complex reactivity: Intercepting end-on nitrile-mo(III) radicals at low temperature

Variable temperature equilibrium studies were used to derive thermodynamic data for formation of eta(1) nitrile complexes with Mo(N[(t)Bu]Ar)(3), 1. (1-AdamantylCN = AdCN: DeltaH(degrees) = -6 +/- 2 kcal mol(-1), DeltaS(degrees) = -20 +/- 7 cal mol(-1) K(-1). C(6)H(5)CN = PhCN: DeltaH(degrees) = -14.5 +/- 1.5 kcal mol(-1), DeltaS(degrees) = -40 +/- 5 cal mol(-1) K(-1). 2,4,6-(H(3)C)(3)C(6)H(2)CN = MesCN: DeltaH(degrees) = -15.4 +/- 1.5 kcal mol(-1), DeltaS(degrees) = -52 +/- 5 cal mol(-1) K(-1).) Solution calorimetric studies show that the enthalpy of formation of 1-[eta(2)-NCNMe(2)] is more exothermic (DeltaH(degrees) = -22.0 +/- 1.0 kcal mol(-1)). Rate and activation parameters for eta(1) binding of nitriles were measured by stopped flow kinetic studies (AdCN: DeltaH(on)(++) = 5 +/- 1 kcal mol(-1), DeltaS(on)(++) = -28 +/- 5 cal mol(-1) K(-1); PhCN: DeltaH(on)(++) = 5.2 +/- 0.2 kcal mol(-1), DeltaS(on)(++) = -24 +/- 1 cal mol(-1) K(-1); MesCN: DeltaH(on)(++) = 5.0 +/- 0.3 kcal mol(-1), DeltaS(on)(++) = -26 +/- 1 cal mol(-1) K(-1)). Binding of Me(2)NCN was observed to proceed by reversible formation of an intermediate complex 1-[eta(1)-NCNMe(2)] which subsequently forms 1-[eta(2)-NCNMe(2)]: DeltaH(++)(k1) = 6.4 +/- 0.4 kcal mol(-1), DeltaS(++)(k1) = -18 +/- 2 cal mol(-1) K(-1), and DeltaH(++)(k2) = 11.1 +/- 0.2 kcal mol(-1), DeltaS(++)(k2) = -7.5 +/- 0.8 cal mol(-1) K(-1). The oxidative addition of PhSSPh to 1-[eta(1)-NCPh] is a rapid second-order process with activation parameters: DeltaH(++) = 6.7 +/- 0.6 kcal mol(-1), DeltaS(++) = -27 +/- 4 cal mol(-1) K(-1). The oxidative addition of PhSSPh to 1-[eta(2)-NCNMe(2)] also followed a second-order rate law but was much slower: DeltaH(++) = 12.2 +/- 1.5 kcal mol(-1) and DeltaS(++) = -25.4 +/- 5.0 cal mol(-1) K(-1). The crystal structure of 1-[eta(1)-NC(SPh)NMe(2)] is reported. Trapping of in situ generated 1-[eta(1)-NCNMe(2)] by PhSSPh was successful at low temperatures (-80 to -40 degrees C) as studied by stopped flow methods. If 1-[eta(1)-NCNMe(2)] is not intercepted before isomerization to 1-[eta(2)-NCNMe(2)] no oxidative addition occurs at low temperatures. The structures of key intermediates have been studied by density functional theory, confirming partial radical character of the carbon atom in eta(1)-bound nitriles. A complete reaction profile for reversible ligand binding, eta(1) to eta(2) isomerization, and oxidative addition of PhSSPh has been assembled and gives a clear picture of ligand reactivity as a function of hapticity in this system.

Observation of the ÃA″1 state of isocyanogen

The AA″1 state of isocyanogen, CNCN, is observed using photofragment fluorescence excitation spectroscopy in a room temperature cell and in a molecular beam. The spectra are highly congested, but progressions that correspond to the Franck-Condon active C–N–C bending vibration in the excited state are evident. Linewidth measurements indicate that the excited state lifetime is <10ps. These measurements are consistent with previous ab initio calculations, which predicted a bent excited state with a short lifetime due to predissociation. Although we do not believe that we have observed the origin band of the electronic transition, we place an upper limit of 42523cm−1 on the energy of the excited state zero point level.