Both the Haber-Bosch and biological ammonia syntheses are thought to rely on the cooperation of multiple metals in breaking the strong N≡N triple bond and forming an N-H bond. This has spurred investigations of the reactivity of molecular multimetallic hydrides with dinitrogen. We report here the reaction of a trinuclear titanium polyhydride complex with dinitrogen, which induces dinitrogen cleavage and partial hydrogenation at ambient temperature and pressure. By (1)H and (15)N nuclear magnetic resonance, x-ray crystallographic, and computational studies of some key reaction steps and products, we have determined that the dinitrogen (N2) reduction proceeds sequentially through scission of a N2 molecule bonded to three Ti atoms in a μ-η(1):η(2):η(2)-end-on-side-on fashion to give a μ2-N/μ3-N dinitrido species, followed by intramolecular hydrogen migration from Ti to the μ2-N nitrido unit.

The cleavage of carbon-carbon (C-C) bonds by transition metals is of great interest, especially as this transformation can be used to produce fuels and other industrially important chemicals from natural resources such as petroleum and biomass. Carbon-carbon bonds are quite stable and are consequently unreactive under many reaction conditions. In the industrial naphtha hydrocracking process, the aromatic carbon skeleton of benzene can be transformed to methylcyclopentane and acyclic saturated hydrocarbons through C-C bond cleavage and rearrangement on the surfaces of solid catalysts. However, these chemical transformations usually require high temperatures and are fairly non-selective. Microorganisms can degrade aromatic compounds under ambient conditions, but the mechanistic details are not known and are difficult to mimic. Several transition metal complexes have been reported to cleave C-C bonds in a selective fashion in special circumstances, such as relief of ring strain, formation of an aromatic system, chelation-assisted cyclometallation and β-carbon elimination. However, the cleavage of benzene by a transition metal complex has not been reported. Here we report the C-C bond cleavage and rearrangement of benzene by a trinuclear titanium polyhydride complex. The benzene ring is transformed sequentially to a methylcyclopentenyl and a 2-methylpentenyl species through the cleavage of the aromatic carbon skeleton at the multi-titanium sites. Our results suggest that multinuclear titanium hydrides could serve as a unique platform for the activation of aromatic molecules, and may facilitate the design of new catalysts for the transformation of inactive aromatics.

Complexes to toy with: Ring‐closing metathesis of the bis(phosphane) complexes 1 a–c (n=4–6) followed by hydrogenation gives the “molecular gyroscopes” 2 a–c. The crystal structure of 2 c and the NMR data for the analogous {Fe(CO)2(NO)}+ complex indicate facile rotation of the {Fe(CO)2(L)}m+ moieties within the P(CH2)14P spokes. Shorter bridges as in 2 a lock the rotators.

The hydrogenolysis of the PNP-ligated titanium dialkyl complex {(PNP)Ti(CHSiMe)} (1, PNP = N(CH-2-PPr-4-CH)) with H (1 atm) in the presence of N (1 atm) afforded a binuclear titanium side-on/end-on dinitrogen complex {[(PNP)Ti](μ,η,η-N)(μ-H)} (2) at room temperature, which upon heating at 60 °C with H gave a μ-imido/μ-nitrido/hydrido complex {[(PNP)Ti](μ-NH)(μ-N)H} (3) through the cleavage and partial hydrogenation of the N unit. The mechanistic aspects of the hydrogenation of the N unit in 2 with H have been elucidated by the density functional theory calculations.

Activation and functionalization of N2 : A mixed diimide/dinitride tetranuclear titanium complex formed by activation of dinitrogen served as a unique platform for the synthesis of nitriles. Functional groups such as aromatic C-X (X=Cl, Br, I) bonds, nitro groups, and ammonia-sensitive aldehyde and chloromethyl moieties were compatible with the synthetic method.

The reaction of the tetranuclear rare earth metal polyhydrido complexes {Cp'Ln(mu-H)2}4(THF) (Cp' = C5Me4SiMe3, Ln = Y (1a), Lu (1b)) with carbon monoxide (1 atm) yielded ethylene and the corresponding tetraoxo cubane complexes (Cp'Ln)4(mu3-O)4 (Ln = Y (5a), Lu (5b)). Stepwise formation of some key reaction intermediates, such as oxymethylene complexes (Cp'Ln)4(mu-OCH2)(mu-H)6(THF) (Ln = Y (2a), Lu (2b)), enolate species (Cp'Y)4(OCH=CH2)(mu-O)(mu-H)5(THF) (3), and dioxo complex (Cp'Y)4(mu3-O)2(mu-H)4(THF) (4), was confirmed. The molecular structures of 2a, 4, and 5b were determined by X-ray diffraction studies.

Reactions of trans-MCl2(P((CH2)6(CH=CH2)3)2 (M = a, Pd; b, Pt) and Grubbs' catalyst, followed by hydrogenation (ClRh(PPh3)3), give the title compounds trans-MCl2(P((CH2)14)3P) (2a, 37%; 2b, 43%). These react with LiBr, NaI, and KCN to give the corresponding MBr2, MI2, and M(CN)2 species (58-99%). 13C NMR data show that the MX2 moieties rapidly rotate within the diphosphine cage on the NMR time scale, even at -120 degrees C. The reaction of 2b and KSCN gives separable Pt(SCN)2 and Pt(SCN)(NCS) species (5b, 27%; 6b, 30%), and that with Ph2Zn gives a PtPh2 species (7b, 55%). NMR data for 5b-7b show that MX2 rotation is no longer rapid. Reactions of 2b with excess NaCCH or KCN afford the free dibridgehead diphosphine P((CH2)14)3P (66-83%), presumably as an "in/in" isomer, as addition of PtCl2 regenerates 2b. The crystal structures of 2a and 7b are analyzed with respect to MX2 rotation.

Studies on N2 activation and transformation by transition metal hydride complexes are of particular interest and importance. The synthesis and diverse transformations of a dinitrogen dititanium hydride complex bearing the rigid acridane‐based acriPNP‐pincer ligands {[(acriPNP)Ti]2(μ2‐η1:η2‐N2)(μ2‐H)2} are presented. This complex enabled N2 cleavage and hydrogenation even without additional H2 or other reducing agents. Furthermore, diverse transformations of the N2 unit with a variety of organometallic compounds such as ZnMe2, MgMe2, AlMe3, B(C6F5)3, PinBH, and PhSiH3 have been well established at the rigid acriPNP‐ligated dititanium framework, such as reversible bonding‐mode change between the end‐on and side‐on/end‐on fashions, diborylative N=N bond cleavage, the formal insertion of two dimethylaluminum species into the N=N bond, and the formal insertion of two silylene units into the N=N bond. This work has revealed many unprecedented aspects of dinitrogen reaction chemistry.