Joachim Ballmann

The hydride route to the preparation of dinitrogen complexes

That the inert dinitrogen molecule can act as a ligand to a transition metal complex is one of the key discoveries in inorganic chemistry of the past century. This feature article summarises a body of work up to 2010 that describes a particularly attractive route to dinitrogen complexes that involves the direct reaction of N(2) with metal hydride derivatives. This process is shown to be general across the transition series and, depending on the metal, different levels of activation of the coordinated dinitrogen unit are observed.

A Synthetic Analogue of Rieske-Type [2Fe-2S] Clusters

}] (3) were formed as a byproduct andidentified by X-ray diffraction (Figure S38 in the SupportingInformation). Compound 2 was crystallized for X-ray dif-fraction (Figure 1, top) by diffusion of diethyl ether into aDMF solution. Prominent intracore distances and angles, aswell as bond lengths and angles to the terminal donor atoms,are in agreement with the corresponding values determinedfor the related homoleptic {N

Support-controlled chemoselective olefin–imine addition photocatalyzed by cadmium sulfide on a zinc sulfide carrier

The semiconductor catalyzed photoaddition of cyclopentene or cyclohexene to various novel electron-poor imines of type p-XC(6)H(4)(CN)C[double bond, length as m-dash]N(COPh) (X = H, F, Cl, Br, Me, MeO) was investigated as a function of the nature of the cadmium sulfide photocatalyst. Irradiation (lambda>/= 350 nm) of silica supported cadmium sulfide surprisingly did not afford the expected olefin-imine adducts but an imine hydrocyanation product via an unprecedented dark reaction. However, when silica was replaced by zinc sulfide as the support for cadmium sulfide, the expected homoallylic N-benzoyl-alpha-amino cyanides were isolated in yields of 65-84%. Thus, chemoselectivity is introduced through replacing an insulating by a semiconducting support, a hitherto unknown effect in semiconductor photocatalysis. From the sign of the time resolved photovoltage it is found that the mixed metal sulfide interface CdS/ZnS increases the lifetime of photogenerated electron-hole pairs by about one order of magnitude as compared to the SiO(2)/CdS system. The reaction rate increases with increasing imine sigma-Hammett constants and decreasing stability of intermediate benzyl radicals.

Carbon–Nitrogen Bond Formation by the Reaction of 1,2‐Cumulenes with a Ditantalum Complex Containing Side‐On‐ and End‐On‐Bound Dinitrogen

Synthetic ammonia serves as the key precursor for the synthesis of many nitrogen-containing chemicals and is produced by the Haber–Bosch process on the 100 million ton scale annually. Given the energy-intensive nature of this reaction, there is considerable interest in the search for alternative methods to generate nitrogen-containing molecules directly from N2. [2] One approach towards this goal is the functionalization of coordinated dinitrogen to form new nitrogen–carbon bonds. Typically, reagents such as alkyl halides or triflates can be employed for this purpose. A major drawback of those conversions is the formation of inorganic salts or transition metal halides as byproducts, rendering these reactions atom-inefficient. However, concerted addition reactions that involve the activated dinitrogen unit and substrates with C X double or triple bonds (X = C, N, O, S) could potentially lead to catalytic cycles for the production of nitrogen-containing heterocycles without any waste products. Unfortunately, only a very few stoichiometric cycloadditions at coordinated N2 units have been discovered to date. In all cases, highly activated side-on bridging h:hN2 complexes of Group 4 metals have been employed to achieve the desired nitrogen–carbon bond formations by reaction with arylalkynes, isocyanates, carbon dioxide, or carbon monoxide (via migratory insertion). The question we posed was: Could a Group 5 ditantalum dinitrogen complex with the side-on/end-on coordination mode engage in cycloaddition-type processes? Our initial attempts to try atom-efficient cycloadditiontype chemistry with the previously reported side-on end-on bound dinitrogen in [{(NPN)Ta}2(m-H)2(m-h :h-N2] (1; NPN = PhP(CH2SiMe2NPh)2) [3d] led to a different behavior. For example, with phenyl acetylene and simple alkenes such as propene, we observed displacement of the N2 unit or migratory insertion reactions into the tantalum hydrides, respectively (Scheme 1). In contrast, a variety of E H hydride reagents (EH = R2BH, R2AlH, RSiH3) have been shown to add across the side-on/end-on N2 unit of 1 to generate new functionalized nitrogen fragments and the formation of terminal Ta H bonds. As these E H additions can be considered to be additions across a quasi-Ta N multiple bond, we continued our search for other reagents that could engage in [2+2] chemistry with the side-on/end-on h:h-N2 moiety. Herein, we report our studies on the reaction of 1,2-cumulenes of the type X=C=Y (X, Y= NR, O, S) with the dinitrogen complex 1. Owing to its low cost and its recent successful application in dinitrogen functionalization with Group 4 metals, CO2 was chosen as a substrate for preliminary experiments. Parent dinitrogen complex 1 reacts instantaneously with carbon dioxide, but yields intractable solids that have so far resisted characterization. However, the bis(N-phenylimino) analogue of CO2, N,N’-diphenyl carbodiimide, reacts cleanly with one equivalent of 1 to give 2 as a single product in 70 % isolated yield (Scheme 2). Complex 2 gives rise to two doublets (JP,P = 16.0 Hz) at d = 3.0 and 8.6 ppm in the P{H} NMR spectrum, verifying that it is an unsymmetrical dinuclear species. Along with a fairly crowded aromatic region and a complicated pattern for the methylene protons, eight resonances for the silyl methyl groups and two signals for the bridging hydrides (at d = 11.9 and 12.1 ppm) are observed in the proton NMR spectrum. The latter signals show complex splitting patterns, as both Scheme 1. Reactivity of 1 towards C C double and triple bonds.

Secondary Bonding Interactions in Biomimetic (2Fe-2S) Clusters

A series of synthetic [2Fe-2S] complexes with terminal thiophenolate ligands and tethered ether or thioether moieties has been prepared and investigated in order to provide models for the potential interaction of additional donor atoms with the Fe atoms in biological [2Fe-2S] clusters. X-ray crystal structures have been determined for six new complexes that feature appended Et (1(C)), OMe (1(O)), or SMe (1(S)) groups, or with a methylene group (2(C) ), an ether-O (2(O)), or an thioether-S (2(S)) linking two aryl groups. The latter two systems provide a constrained chelate arrangement that induces secondary bonding interactions with the ether-O and thioether-S, which is confirmed by density functional theory (DFT) calculations that also reveal significant spin density on those fifth donor atoms. Structural consequences of the secondary bonding interactions are analyzed in detail, and effects on the spectroscopic and electronic properties are probed by UV-vis, Mössbauer, and (1)H NMR spectroscopy, as well by SQUID measurements and cyclic voltammetry. The potential relevance of the findings for biological [2Fe-2S] sites is considered.

Reduction of Carbon Dioxide Promoted by a Dinuclear Tantalum Tetrahydride Complex

The reaction of 1 equiv of carbon dioxide with the dinuclear tetrahydride complex ([NPN]Ta)(2)(μ-H)(4) [where NPN = PhP(CH(2)SiMe(2)NPh)(2)] results in the formation of ([NPN]Ta)(2)(μ-OCH(2)O)(μ-H)(2) via a combination of migratory insertion and reductive elimination. The identity of the ditantalum complex containing a methylene diolate fragment was confirmed by single-crystal X-ray analysis, NMR analysis, and isotopic labeling studies. Density functional theory calculations were performed to provide information on the structure of the initial adduct formed and likely transition states and intermediates for the process.

Synthesis and Reactivity of Cyclometalated Triamidophosphine Complexes of Niobium and Tantalum

The triamidophosphine protioligand 1 reacts with the homoleptic pentakis(dimethylamido) precursors of niobium and tantalum [M(NMe2)5, where M = Nb, Ta] to form cyclometalated complexes of the type [N2PCN-κ(5)-N,N,P,C,N]M(NMe2) (2-M). Apart from the three amido donors, one benzylic position of the ligand backbone is deprotonated over the course of this reaction, resulting in the formation of a new M-C bond. As a consequence, a metallaziridine substructure is formed, and the triamidophosphine moiety thus serves as a tetraanionic pentadentate ligand. The dimethylamido complexes 2-M can be converted into the corresponding triflates [N2PCN-κ(5)-N,N,P,C,N]M(OTf) (3-M) and alkyl complexes [N2PCN-κ(5)-N,N,P,C,N]M(CH2SiMe3) (4-M) by treatment with triethylsilyl triflate (Et3SiO3SCF3) followed by (trimethylsilyl)methyllithium (LiCH2SiMe3). The alkyl complexes exhibit interesting reactivities, including a second cyclometalative backbone activation affording the trimethylphosphine-stabilized complexes [NP(CN)2-κ(6)-N,P,C,N,C,N]M(PMe3) (5-M). In the case of tantalum, the formation of a dinuclear hydrido complex (6) is observed upon hydrogenation of 4-Ta. In the case of niobium, the metallaziridine substructure in 4-Nb is prone to ring opening via protonation with triphenylsilylamine (Ph3SiNH2), resulting in formation of the corresponding imido complex [PN3-κ(4)-P,N,N,N]Nb=NSiPh3 (7).

A Tripodal Benzylene-Linked Trisamidophosphine Ligand Scaffold: Synthesis and Coordination Chemistry with Group(IV) Metals

A new tripodal trisamidophosphine ligand (1) based on the trisbenzylphosphine backbone has been synthesized in three steps starting from NaPH2 and phthaloyl-protected 2-aminobenzyl bromide. At elevated temperatures, 1 reacts directly with M(NMe2)4 (M = Zr, Hf) to afford the dimethylamido complexes [PN3]M(NMe2) (M = Zr, Hf) (2), which are easily converted into the corresponding triflates [PN3]MOTf (M = Zr, Hf) (3) via reaction with triethylsilyl trifluoromethanesulfonate. The related titanium chloro complex [PN3]TiCl (4-Ti) is obtained from 1 and Bn3TiCl via protonolysis. Triple deprotonation of 1 with n-butyllithium affords the tris-lithium salt Li3[PN3] (1-Li), which serves as a common starting material for the preparation of all the group(IV) chlorides [PN3]MCl (M = Ti, Zr, Hf) (4). Upon treatment of 4-Ti with Bn2Mg(thf)2, formation of a benzyltitanium species is observed, which is converted cleanly into a ligand-CH-activated species (5-Ti).

Synthesis of NPN-Coordinated Tantalum Alkyl Complexes and Their Hydrogenolysis: Isolation of a Terminal Tantalum Hydride Incorporating a Doubly Cyclometalated NPN Scaffold

The closely related benzylene-linked diaminophosphines PhP(CH2C6H4-o-NHPh)2 (AH2) and PhP(C6H4-o-CH2NHXyl)2 (BH2 with Xyl = 3,5-Me2C6H3) were employed for the synthesis of tantalum(V) alkyls, which were then studied with respect to hydrogenolysis. In the case of AH2, the tantalum trimethyl complex [Ta(A)Me3] (1) and the tantalum hydrocarbyl complex [Ta(A)(CH2SiMe3)(η2-EtC≡CEt)] (2) were prepared from the ligands dilithium salt (A)Li2(diox). Upon hydrogenolysis of 1 and 2, the formation of methane and SiMe4, respectively, was observed, but well-defined tantalum hydrides could not be detected. In the case of BH2, the cyclometalated species [Ta(B*)(NMe2)2] (3 with B* = κ4-N,P,N,C-(PhP(C6H4-o-CH2NXyl)(C6H4-o-CHNXyl))3-) was isolated and converted to the corresponding diiodo species [Ta(B*)I2] (4). Treatment of 4 with LiCH2SiMe3 resulted in the isolation of the corresponding dialkyl complex [Ta(B*)(CH2SiMe3)2] (5), which was converted to the doubly cyclometalated monoalkyl complexes [Ta(B**)(CH2SiMe3)(PMe3)] (6 with B** = κ5-C,N,P,N,C-(PhP(C6H4-o-CHNXyl)2)4-) and [Ta(B**)(CH2SiMe3)(dmpe)] (7) via reaction with PMe3 and dmpe, respectively. In contrast to 5 and 6, 7 was found to react cleanly with dihydrogen to afford the corresponding terminal tantalum(V) hydride [Ta(B**)(H)(dmpe)] (8). Upon reaction of 7 with D2, the deuteride [Ta(d2-B**)(D)(dmpe)] (9) was obtained and found to contain deuterium atoms in the methine positions of both tantalaaziridine subunits. The partially deuterated derivatives [Ta(B**)(D)(dmpe)] (10) and [Ta(d2-B**)(H)(dmpe)] (11) were generated via reaction of 8 and 9 with PhSiD3 and PhSiH3, respectively. Prior to the addition of gaseous D2 or H2, no H/D scrambling was observed in 10 or 11, indicating that the exchange of the methine positions proceeds via addition of D2 or H2 across the tantalaaziridine Ta-C bonds.