2,6-Bis[1-phenyl-2-(2,4,6-tri-tert-butylphenyl)-2-phosphaethenyl]pyridine (BPEP-Ph) reacts with [Ru3(CO)12] or [RuCl(η3-allyl)(CO)3] in toluene at elevated temperatures to afford the three types of PNP-pincer complexes 1–3. The reaction of BPEP-Ph with [Ru3(CO)12] under vacuum forms a mixture of [Ru(CO)2(BPEP-Ph)] (1) and a dicarbonylruthenium(0) complex (2); the latter complex has an unsymmetrical PNP-pincer ligand with 1-phenyl-2-phosphaethenyl and phenyl(benzo[b]phospholan-1-yl)methyl groups at the 2,6-positions of the pyridine core, which is formed by intramolecular C–H addition of a t-Bu group to the P═C bond. On the other hand, the reaction of BPEP-Ph with [RuCl(η3-allyl)(CO)3] affords a dicarbonylruthenium(II) chloride (3) with an unsymmetrical PNP-pincer ligand bearing a dearomatized pyridyl group with 1-phenyl-2-phosphaethenyl and α-(benzo[b]phospholan-1-yl)benzylidene substituents at the 2,6-positions. Treatment of 3 with i-PrOK in THF results in selective formation of 2. Complexes 1–3 have been isolated as crystalline compounds and examined in detail by IR and NMR spectroscopy, X-ray diffraction studies, and DFT calculations. Complex 1 exhibits catalytic activity toward N-alkylation of amines (RNH2) with alcohols (R′CH2OH) to give imines (RN═CHR′) as major products.

Dihydrogen activation by a triruthenium μ3-imido complex, (CpRu)3(μ3-NH)(μ-H)3 (1; Cp = η5-C5H5), was theoretically investigated with the DFT method. Each of the three Ru centers can react with dihydrogen via a transition state, (CpRu)3(μ3-NH)(η2-H2)(μ-H)3 (TS1), with a moderate energy barrier. This TS1 corresponds to a transition state for the approach of a dihydrogen molecule to one of the Ru centers. After TS1, the hydrogen–hydrogen bond cleavage occurs without any other energy barrier to produce the pentahydrido intermediate, (CpRu)3(μ3-NH)(μ-H)3(H)2 (INT1), without formation of a dihydrogen complex as an intermediate. In the hydrogen–hydrogen cleavage, the dπ orbitals of three ruthenium centers overlap with the hydrogen–hydrogen σ* orbital to form the charge transfer from the ruthenium to the hydrogen–hydrogen moiety. Natural population analysis indicates that all three ruthenium centers cooperatively participate in the changes in electronic structure during this process. INT1 further undergoes a nitrogen–hydrogen bond formation, which occurs through drastic changes in the binding mode of the amide group and hydrides, to afford a μ-amido complex, (CpRu)3(μ-NH2)(μ-H)4 (2). This process proceeds with a moderate activation barrier. Natural population analysis of this step reveals that one bridging hydride approaches the imido N atom, concomitantly changing from a hydride to a proton-like hydrogen atom with a simultaneous increase in the N atomic population. Fluxional positional changes of hydrides are crucial for the facile N–H bond formation, in which the Ru3 core plays important roles.

Redox reactions of iron complexes bearing a PNP-pincer-type phosphaalkene ligand, 2,6-bis[1-phenyl-2-(2,4,6-tri-tert-butylphenyl)-2-phosphaethenyl]pyridine (BPEP), are reported. The Fe(II) dibromide [FeBr2(BPEP)] (1) is readily reduced by [Cp2Co] to afford the four-coordinate Fe(I) monobromide [FeBr(BPEP)] (2), while 2 reacts with PhCH2Br to reproduce 1. Treatment of 1 with MesMgBr or Mes2Mg(THF)2 (Mes = 2,4,6-Me3C6H2) results in one-electron reduction of 1, followed by transmetalation of the resulting 2 with mesitylmagnesium compounds to give the Fe(I) mesityl complex [FeMes(BPEP)] (3). The single-crystal diffraction study of 3 has revealed a distorted trigonal monopyramidal arrangement around the iron center. SQUID magnetometry has established a low-spin ground state (S = 1/2) of 3. Complex 2 reacts with Me2Mg(THF)2 to afford Fe(0) and Fe(II) complexes (4 and 5, respectively) coordinated with novel multidentate ligand systems containing a phosphonium ylide structure. The formation processes of 4 and 5 via an [FeMe(BPEP)] intermediate are discussed on the basis of their X-ray structures.

CuI complexes bearing BPEP as a PNP-pincer type phosphaalkene ligand undergo effective bonding interactions with SbF6− and PF6− as non-coordinating anions to give [Cu(SbF6)(BPEP)] and [Cu2(BPEP)2(μ-PF6)]+, respectively [BPEP = 2,6-bis(1-phenyl-2-phosphaethenyl)pyridine]. NMR and theoretical studies indicate a reduced anionic charge of the μ-PF6 ligand, which is induced by the strong π-accepting ability of BPEP.

CuI complexes bearing BPEP as a PNP-pincer type phosphaalkene ligand undergo effective bonding interactions with SbF6− and PF6− as non-coordinating anions to give [Cu(SbF6)(BPEP)] and [Cu2(BPEP)2(μ-PF6)]+, respectively [BPEP = 2,6-bis(1-phenyl-2-phosphaethenyl)pyridine]. NMR and theoretical studies indicate a reduced anionic charge of the μ-PF6 ligand, which is induced by the strong π-accepting ability of BPEP.