The reduction chemistry of cobalt complexes with HPNP (HPNP = HN(CH2CH2P(i)Pr2)2) as a supporting ligand is described. Reaction of [(HPNP)CoCl2] (1) with n-BuLi generated both the deprotonated Co(II) species [(PNP)CoCl] (2) along with the Co(I) complex [(HPNP)CoCl] (3). Products resulting from reduction of 2 with KC8 vary depending upon the atmosphere under which the reduction is performed. Monomeric square planar [(PNP)CoN2] (4) is obtained under dinitrogen, whereas dimeric [(PNP)Co]2 (5) is formed under argon. Over time, 5 activates a C-H bond in the PNP ligand to form the species [Co(H)(μ-PNP)(μ-(i)Pr2PCH2CH2NCHCH2P(i)Pr2)Co] (6). We also observed the oxidative addition of H-Si bond to complex 3 to form [(HPNP)CoCl(H)SiH2Ph] (7). (1)H NMR studies showed that species 7 is in equilibrium with 3 and silane in solution. Complex 3 can be oxidized with AgBPh4 to generate {(HPNP)CoCl}BPh4 (8), a square planar species with a formal electron count of 15 electrons.
A series of Co, Ni and Cu complexes with the ligand HN(CH(2)CH(2)P(i)Pr(2))(2) (HNP(2)) has been isolated and their electrochemical behaviour investigated by cyclic voltammetry. The nickel complexes [(HNP(2))NiOTf]OTf and [(HNP(2))NiNCCH(3)](BF(4))(2) display reversible reductions, as does the related amide derivative (NP(2))NiBr. The related copper(I) and cobalt(II) derivatives were also isolated and characterized. The addition of piperidine to [(HNP(2))NiNCCH(3)](BF(4))(2) led to the formation of the new species [(HNP(2))Ni(N(H)C(CH(3))NC(5)H(10))](BF(4))(2). The nucleophilic addition of piperidine to acetonitrile to produce HN=C(CH(3))NC(5)H(10) was found to be catalyzed by [(HNP(2))NiNCCH(3)](BF(4))(2).
The reactivity of cobalt complexes supported by a PNP pincer ligand towards H2 varies depending on whether the N-donor atom is protonated; the synthesis of [(HPNP)CoCl(H)2] (2), [(PNP)CoH]2 (4), and the trihydride species [(HPNP)CoH3] (7) (HPNP = HN(CH2CH2P(i)Pr2)2) are described.
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