Brendan L. Yonke

Dinitrogen Complexation and Extent of N≡N Activation within the Group 6 “End-On-Bridged” Dinuclear Complexes, {(η5-C5Me5)M[N(i-Pr)C(Me)N(i-Pr)]}2(μ-η1:η1-N2) (M = Mo and W)

Chemical reduction of Cp*M[N(i-Pr)C(Me)N(i-Pr)]Cl(3) (Cp* = eta(5)-C(5)Me(5)) (1, M = Mo) and (2, M = W) using 0.5% NaHg in THF provided excellent yields of the diamagnetic dinuclear end-on-bridged dinitrogen complexes {Cp*M[N(i-Pr)C(Me)N(i-Pr)]}(2)(mu-eta(1):eta(1)-N(2)) (6, M = Mo) and (8, M = W), respectively. Chemical reduction of Cp*Mo[N(i-Pr)C(NMe(2))N(i-Pr)]Cl(2) (4) with 3 equiv of KC(8) in THF similarly yielded diamagnetic {Cp*Mo[N(i-Pr)C(NMe(2))N(i-Pr)]}(2)(mu-eta(1):eta(1)-N(2)) (7). Single-crystal X-ray analyses of 7 and 8 confirmed the dinuclear end-on-bridged mu-eta(1):eta(1)-N(2) coordination mode and the solid-state molecular structures of these compounds provided d(NN) values of 1.267(2) and 1.277(8) A for 7 and 8, respectively. Based on a comparison of (15)N NMR spectra for (15)N(2) (99%)-labeled 6 and (15)N(2) (99%)-labeled 8, as well as similarities in chemical reactivity, a dinuclear mu-eta(1):eta(1)-N(2) structure for 6 is further proposed. For comparison with a first-row metal derivative, chemical reduction of Cp*Ti[N(i-Pr)C(Me)N(i-Pr)]Cl(2) (9) with KC(8) in THF was conducted to provide {Cp*Ti[N(i-Pr)C(Me)N(i-Pr)]}(2)(mu-eta(1):eta(1)-N(2)) (10) for which a d(NN) value of 1.270(2) A was obtained through X-ray crystallography. Compounds 6-8 were all found to be thermally robust in toluene solution up to temperatures of at least 100 degrees C, and 6 and 8 were determined to be inert toward the addition of H(2) or H(3)SiPh under a variety of conditions. Single-crystal X-ray analysis of meso-{Cp*Mo(H)[N(i-Pr)C(Me)N(i-Pr)]}(2)(mu-eta(1):eta(1)-N(2)) (meso-11), which was serendipitously isolated as a product of attempted alkylation of Cp*Mo[N(i-Pr)C(Me)N(i-Pr)]Cl(2) (3) with 2 equiv of n-butyllithium, revealed a smaller d(NN) value of 1.189(4) A that is consistent with two Mo(IV,d(2)) centers connected by a bridging diazenido, [mu-N(2)](2-), moiety. Moreover, meso-11 was found to undergo clean dehydrogenation in solution at 50 degrees C to provide 6 via a first-order process. Chemical oxidation of 8 with an excess of PbCl(2) in toluene solution at 25 degrees C provided a 1:1 mixture of rac- and meso-{Cp*W(Cl)[N(i-Pr)C(Me)N(i-Pr)]}(2)(mu-eta(1):eta(1)-N(2)) (12); both isomers of which provided solid-state structures through X-ray analyses that are consistent with an electronic configuration comprised of two W(IV,d(2)) centers linked through a bridging [N(2)](2-) group [cf. for rac-12, d(NN) = 1.206(9) A, and for meso-12, d(NN) = 1.192(3) A]. Finally, treatment of 6 and 8 with either 4 equiv of CNAr (Ar = 3,5-Me(2)C(6)H(3)) or an excess of CO in toluene provided excellent yields of Cp*M[N(i-Pr)C(Me)N(i-Pr)](CNAr)(2) (13, M = Mo and 14, M = W) and Cp*M[N(i-Pr)C(Me)N(i-Pr)](CO)(2) (15, M = Mo and 16, M = W), respectively. Single-crystal X-ray analyses of 13-16, along with observation of reduced IR vibrational nu(CN) or nu(CO) bond-stretching frequencies, provide strong support for the electron-rich character of the Cp*M[N(i-Pr)C(Me)N(i-Pr)] fragment that can engage in a high degree of back-donation with moderate to strong pi-acceptors, such as N(2), CNR, and CO. The collective results of this work are analyzed in terms of the possible steric and electronic factors that contribute to preferred mode of mu-N(2) coordination and the extent of N[triple bond]N activation, including complete N-N bond scission, within the now completed experimentally-derived ligand-centered isostructural series of {Cp*M[N(i-Pr)C(Me)N(i-Pr)]}(2)(mu-N(2)) compounds where M = Ti, Zr, Hf, Ta, Mo, and W.

Catalytic Degenerate and Nondegenerate Oxygen Atom Transfers Employing N2O and CO2 and a MII/MIV Cycle Mediated by Group 6 MIV Terminal Oxo Complexes

The development of transition-metal-catalyzed transformations that employ molecular oxygen (O2), carbon dioxide (CO2), and nitrous oxide (N2O) as inexpensive and chemically benign “green” oxidants for the industrial-scale production of specialty and commodity chemicals is of significant scientific, commercial, and environmental interest. To achieve this goal, a fine thermodynamic balance must be established for reaction pathways involving oxygen atom transfer (OAT) to, and from, a given metal center in a fashion that favors productive substrate oxidation. Herein, we report a new class of low-valent Group 6 metal complex that, in the case of molybdenum, can mediate the direct oxidation of tert-butyl isocyanide, tBuNC, to the corresponding isocyanate, tBuNCO, through nondegenerate OAT utilizing N2O as a chemical oxidant according to: RN C + N2O!RN=C=O + N2. [5] We further detail the ability of this same class of metal complex to serve as a photocatalyst for the reversible degenerate OAT between CO and CO2 in the case of molybdenum and tungsten. For both nondegenerate and degenerate OAT processes, which proceed at near ambient conditions, key intermediates have been isolated and structurally characterized, including midvalent terminal oxo metal complexes, the first unequivocal examples of h-(OCNR) metal complexes that are supported by a k-O,C bonding motif, and finally, a rare example of a h-CO2 tungsten complex that can engage in elimination of either CO or CO2. Collectively, these results serve to establish catalytically competent OAT cycles that are based on a Group 6 metal M/M formal oxidation state couple. By way of contrast, all biological molybdenumand tungsten-dependent oxotransferase enzymes investigated to date appear to favor thermal OAT mechanisms based on a M/M couple. We have previously reported that the Group 6 dinuclear “end-on-bridged” dinitrogen complexes, [{Cp*M[N(iPr)C(Me)N(iPr)]}2(m-h :h-N2)] (Cp* = h -C5Me5), for M = Mo (1) and W (2), can function as convenient M synthons for [Cp*M{N(iPr)C(Me)N(iPr)}(L)2], where L = CO for M = Mo (3) and W (4), and L = CN(2,6-Me2C6H3) for M = Mo (5) and W (6), according to Scheme 1. In keeping with known literature precedent, this M synthon analogy for 1 and 2 was extended further in the present work through demonstration that the corresponding terminal M oxo compounds, [Cp*M(O){N(iPr)C(Me)N(iPr)}] for M = Mo (7) and W (8), could be isolated in modest to excellent yields through oxidative OAT with N2O under near ambient conditions (25 8C, 10 psi) (see Scheme 1). As the results in Scheme 1 further reveal, it was determined that 7 and 8 could also be obtained from 1 and 2, respectively, through facile OAT employing CO2 (10 psi) at room temperature. Mayer and coworkers have previously reported a similar W!W oxygen atom abstraction of CO2 by [WCl2(PMePh2)4] that yielded the tungsten oxo, carbonyl complex, [W(O)(CO)Cl2(PMePh2)2]. Compounds 7 and 8 are diamagnetic, crystalline solids for which spectroscopic and elemental analyses are fully consistent with the structures depicted. Single-crystal X-ray analyses provided the solid-state molecular structures of 7 and 8 which displayed a high degree of isostructural similarity between the two compounds, and as such, only that of 8 is presented in Figure 1. In keeping with expected periodic trends, however, the second-row molybdenum complex 7 exhibits a slightly shorter molybdenum–oxygen bond of 1.7033(19) as compared with the corresponding tungsten– oxygen bond of 1.7234(17) for 8. While these bonds are significantly longer than the tungsten–oxygen bond of 1.684 determined for [W(O)Cl4] [13] and of 1.689(6) for [W(O)(CO)Cl2(PMePh2)2], [10] both 7 and 8 possess metal– oxygen bonds that are shorter than the corresponding values reported for the bis(h-cyclopentadienyl) (also known as “metallocene”) derivatives, [(MeCp)2M(O)] (MeCp = h C5H4Me) [cf., Mo O 1.721(2) and W O 1.744(5) , respectively] for which a formal metal–oxygen bond order Scheme 1. Synthesis of M and M complexes from a common precursor for M = Mo and W.

Carbon Monoxide-Induced N–N Bond Cleavage of Nitrous Oxide That Is Competitive with Oxygen Atom Transfer to Carbon Monoxide As Mediated by a Mo(II)/Mo(IV) Catalytic Cycle

In the presence of CO, facile N-N bond cleavage of N(2)O occurs at the formal Mo(II) center within coordinatively unsaturated mononuclear species derived from Cp*Mo[N((i)Pr)C(Me)N((i)Pr)](CO)(2) (Cp* = η(5)-C(5)Me(5)) (1) and {Cp*Mo[N((i)Pr)C(Me)N((i)Pr)]}(2)(μ-η(1):η(1)-N(2)) (9) under photolytic and dark conditions, respectively, to produce the nitrosyl, isocyanate complex Cp*Mo[N((i)Pr)C(Me)N((i)Pr)](κ-N-NO)(κ-N-NCO) (7). Competitive N-O bond cleavage of N(2)O proceeds under the same conditions to yield the Mo(IV) terminal metal oxo complex Cp*Mo[N((i)Pr)C(Me)N((i)Pr)](O) (3), which can be recycled to produce more 7 through oxygen-atom-transfer oxidation of CO to produce CO(2).