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# Dinitrogen Complexation and Extent of NN Activation within the Group 6 “End-On-Bridged” Dinuclear Complexes, {(η^{5}-C_{5}Me_{5})M[N(i-Pr)C(Me)N(i-Pr)]}_{2}(μ-η^{1}:η^{1}-N_{2}) (M = Mo and W)

journal contribution

posted on 2010-09-08, 00:00 authored by Philip P. Fontaine, Brendan L. Yonke, Peter Y. Zavalij, Lawrence R. SitaChemical reduction of Cp*M[N(i-Pr)C(Me)N(i-Pr)]Cl

_{3}(Cp* = η^{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}(μ-η^{1}:η^{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}(μ-η^{1}:η^{1}-N_{2}) (**7**). Single-crystal X-ray analyses of**7**and**8**confirmed the dinuclear end-on-bridged μ-η^{1}:η^{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) Å 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 μ-η^{1}:η^{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}(μ-η^{1}:η^{1}-N_{2}) (**10**) for which a*d*(NN) value of 1.270(2) Å 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 °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}(μ-η^{1}:η^{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) Å that is consistent with two Mo(IV,d^{2}) centers connected by a bridging diazenido, [μ-N_{2}]^{2−}, moiety. Moreover,*meso*-**11**was found to undergo clean dehydrogenation in solution at 50 °C to provide**6**via a first-order process. Chemical oxidation of**8**with an excess of PbCl_{2}in toluene solution at 25 °C provided a 1:1 mixture of*rac-*and*meso*-{Cp*W(Cl)[N(i-Pr)C(Me)N(i-Pr)]}_{2}(μ-η^{1}:η^{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) Å, and for*meso*-**12**,*d*(NN) = 1.192(3) Å]. 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 ν_{CN}or ν_{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 π-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 μ-N_{2}coordination and the extent of NN 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}(μ-N_{2}) compounds where M = Ti, Zr, Hf, Ta, Mo, and W.