Protonation Studies of a Tungsten Dinitrogen Complex Supported by a Diphosphine Ligand Containing a Pendant Amine

Treatment of trans-[W­(N2)2(dppe)­(PEtNMePEt)] (dppe = Ph2PCH2CH2PPh2; PEtNMePEt = Et2PCH2N­(Me)­CH2PEt2) with 3 equiv of tetrafluoroboric acid (HBF4·Et2O) at −78 °C generated the seven-coordinate tungsten hydride trans-[W­(N2)2(H)­(dppe)­(PEtNMePEt)]­[BF4]. At higher temperatures, protonation of a pendant amine is also observed, affording trans-[W­(N2)2(H)­(dppe)­(PEtNMe(H)­PEt)]­[BF4]2, with formation of the hydrazido complex [W­(NNH2)­(dppe)­(PEtNMe(H)­PEt)]­[BF4]2 as a minor product. A similar product mixture was obtained using triflic acid (HOTf). The protonated products are thermally sensitive and do not persist at ambient temperature. Upon acid addition to the carbonyl analogue cis-[W­(CO)2(dppe)­(PEtNMePEt)], the seven-coordinate carbonyl hydride complex trans-[W­(CO)2(H)­(dppe)­(PEtNMe(H)­PEt)]­[OTf]2 was generated. A mixed diphosphine complex without the pendant amine in the ligand backbone, trans-[W­(N2)2(dppe)­(depp)] (depp = Et2P­(CH2)3PEt2), was synthesized and treated with HOTf, selectively generating a hydrazido complex, [W­(NNH2)­(OTf)­(dppe)­(depp)]­[OTf]. Computational analysis probed the proton affinity of three sites of protonation in these complexes: the metal, pendant amine, and N2 ligand. Room-temperature reactions with 100 equiv of HOTf produced NH4+ from reduction of the N2 ligand (electrons come from W). The addition of 100 equiv of HOTf to trans-[W­(N2)2(dppe)­(PEtNMePEt)] afforded 0.81 equiv of NH4+, while 0.40 equiv of NH4+ was formed upon treatment of trans-[W­(N2)2(dppe)­(depp)] with HOTf, showing that the complexes containing proton relays produce more products of reduction of N2.