Bridging Nitride Research Articles

Progress in the chemistry of molecular actinide-nitride compounds.

The chemistry of actinide-nitrides has witnessed significant advances in the last ten years with a large focus on uranium and a few breakthroughs with thorium. Following the early discovery of the first terminal and bridging nitride complexes, various synthetic routes to uranium nitrides have since been identified, although the range of ligands capable of stabilizing uranium nitrides still remains scarce. In particular, both terminal- and bridging-nitrides possess attractive advantages for potential reactivity, especially in light of the recent development of uranium complexes for dinitrogen reduction and functionalization. The first molecular thorium bridged-nitride complexes have also been recently identified, anticipating the possibility of expanding nitride chemistry not only to low-valent thorium, but also to the transuranic elements.

Amidinate Supporting Ligands Influence Molecularity in Formation of Uranium Nitrides.

Uranium nitride complexes are attractive targets for chemists as molecular models for the bonding, reactivity, and magnetic properties of next-generation nuclear fuels, but these molecules are uncommon and can be difficult to isolate due to their high reactivity. Here, we describe the synthesis of three new multinuclear uranium nitride complexes, [U(BCMA)2]2(μ-N)(μ-κ1:κ1-BCMA) (7), [(U(BIMA)2)2(μ-N)(μ-NiPr)(K2(μ-η3:η3-CH2CHNiPr)]2 (8), and [U(BIMA)2]2(μ-N)(μ-κ1:κ1-BIMA) (9) (BCMA = N,N-bis(cyclohexyl)methylamidinate, BIMA = N,N-bis(iso-propyl)methylamidinate), from U(III) and U(IV) amidinate precursors. By varying the amidinate ligand substituents and azide source, we were able to influence the composition and size of these nitride complexes. 15N isotopic labeling experiments confirmed the bridging nitride moieties in 7-9 were formed via two-electron reduction of azide. The tetra-uranium cluster 8 was isolated in 99% yield via reductive cleavage of the amidinate ligands; this unusual molecule contains nitrogen-based ligands with formal 1-, 2-, and 3- charges. Additionally, chemical oxidation of the U(IV) precursor U(N3)(BCMA)3 yielded the cationic U(V) species [U(N3)(BCMA)3][OTf]. Magnetic susceptibility measurements confirmed a U(IV) oxidation state for the uranium centers in the three nitride-bridged complexes and provided a comparison of magnetic behavior in the structurally related U(III)-U(IV)-U(V) series U(BCMA)3, U(N3)(BCMA)3, and [U(N3)(BCMA)3][OTf]. At 240 K, the magnetic moments in this series decreased with increasing oxidation state, i.e., U(III) > U(IV) > U(V); this trend follows the decreasing number of 5f valence electrons along this series.

Mapping the Reactivity of Dicobalt Bridging Nitrides in Constrained Geometries.

Low-nuclearity nitrides of the late transition metals are rare and reactive molecular species, with little experimental precedent. The first putative examples of dicobalt bridging nitrides, [(nPDI2)Co2(μ-N)(PMe3)2][OTf]3 (n[Co2N]3+; PDI = pyridyldiimine; n = 2 or 3, representing the length of the aliphatic chain linking PDI imino groups), were reported recently and shown to undergo a range of intramolecular reaction pathways, including N-H bond formation, C-H bond insertion, and P═N bond formation at the bridging nitride. The specific mode of reactivity changed with the phase of the reaction and the size of the macrocycle used to support the transient species. The present contribution offers a computational investigation into both the geometric and electronic structures of these nitrides as well as the factors governing their reaction selectivity. The compounds n[Co2N]3+ exhibit μ-N-based lowest unoccupied molecular orbitals (LUMOs) that are consistent with subvalent, electrophilic nitrides. The specific orientations of the LUMOs induce ring-size-dependent stereoelectronic effects, thereby causing the product selectivity observed experimentally. Notably, the nitrides also exhibit a degree of nucleophilicity at μ-N by way of a high-energy, μ-N-based lone pair. This ambiphilic character appears to be a direct result of the constrained environment imposed by the folded-ligand geometries of n[Co2N]3+. When combined with the experimental findings, these data led to the conclusion that the folded-ligand isomers are the reactive species and that the constrained geometry imposed by the macrocyclic ligand plays an important role in controlling the reaction outcome.

N-H Bond Formation at a Diiron Bridging Nitride.

Despite their connection to ammonia synthesis, little is known about the ability of iron-bound, bridging nitrides to form N-H bonds. Herein we report a linear diiron bridging nitride complex supported by a redox-active macrocycle. The unique ability of the ligand scaffold to adapt to the geometric preference of the bridging species was found to facilitate the formation of N-H bonds via proton-coupled electron transfer to generate a μ-amide product. The structurally analogous μ-silyl- and μ-borylamide complexes were shown to form from the net insertion of the nitride into the E-H bonds (E=B, Si). Protonation of the parent bridging amide produced ammonia in high yield, and treatment of the nitride with PhSH was found to liberate NH3 in high yield through a reaction that engages the redox-activity of the ligand during PCET.

N−H Bond Formation at a Diiron Bridging Nitride

AbstractDespite their connection to ammonia synthesis, little is known about the ability of iron‐bound, bridging nitrides to form N−H bonds. Herein we report a linear diiron bridging nitride complex supported by a redox‐active macrocycle. The unique ability of the ligand scaffold to adapt to the geometric preference of the bridging species was found to facilitate the formation of N−H bonds via proton‐coupled electron transfer to generate a μ‐amide product. The structurally analogous μ‐silyl‐ and μ‐borylamide complexes were shown to form from the net insertion of the nitride into the E−H bonds (E=B, Si). Protonation of the parent bridging amide produced ammonia in high yield, and treatment of the nitride with PhSH was found to liberate NH3 in high yield through a reaction that engages the redox‐activity of the ligand during PCET.

Isolation of a Bimetallic Cobalt(III) Nitride and Examination of Its Hydrogen Atom Abstraction Chemistry and Reactivity toward H2.

Room temperature photolysis of the bis(azide)cobaltate(II) complex [Na(THF)x][(ketguan)Co(N3)2] (ketguan = [(tBu2CN)C(NDipp)2]-, Dipp = 2,6-diisopropylphenyl) (3a) in THF cleanly forms the binuclear cobalt nitride Na(THF)4{[(ketguan)Co(N3)]2(μ-N)} (1). Compound 1 represents the first example of an isolable, bimetallic cobalt nitride complex, and it has been fully characterized by spectroscopic, magnetic, and computational analyses. Density functional theory supports a CoIII═N═CoIII canonical form with significant π-bonding between the cobalt centers and the nitride atom. Unlike other group 9 bridging nitride complexes, no radical character is detected at the bridging N atom of 1. Indeed, 1 is unreactive toward weak C-H donors and even cocrystallizes with a molecule of cyclohexadiene (CHD) in its crystallographic unit cell to give 1·CHD as a room temperature stable product. Notably, addition of pyridine to 1 or photolyzed solutions of [(ketguan)Co(N3)(py)]2 (4a) leads to destabilization via activation of the nitride unit, resulting in the mixed-valent Co(II)/Co(III) bridged imido species [(ketguan)Co(py)][(ketguan)Co](μ-NH)(μ-N3) (5) formed from intermolecular hydrogen atom abstraction (HAA) of strong C-H bonds (BDE ∼ 100 kcal/mol). Kinetic rate analysis of the formation of 5 in the presence of C6H12 or C6D12 gives a KIE = 2.5 ± 0.1, supportive of a HAA formation pathway. The reactivity of our system was further probed by photolyzing benzene/pyridine solutions of 4a under H2 and D2 atmospheres (150 psi), which leads to the exclusive formation of the bis(imido) complexes [(ketguan)Co(μ-NH)]2 (6) and [(ketguan)Co(μ-ND)]2 (6-D), respectively, as a result of dihydrogen activation. These results provide unique insights into the chemistry and electronic structure of late 3d metal nitrides while providing entryway into C-H activation pathways.

Localized π-conjugated structure and EPR investigation of g-C3N4 photocatalyst

The π-conjugated structure of graphitic carbon nitride (g-C3N4) is particularly vital to many photocatalytic reactions. Herein, the hybrid structure of tri-s-triazine unit in g-C3N4 framework is chemically analyzed and expounded according to the hybrid orbital theory. The localized π-conjugated structure of g-C3N4 is also monitored by the electron paramagnetic resonance (EPR) or electron spin resonance (ESR) technique. The experimental results indicate that this π-conjugated structure is attributed to the orbital overlapping of the hybrid carbon and nitride atoms in their 2pz orbits. Unlike graphene with the nonlocalized π-conjugated structure, this orbital overlapping in the whole two-dimensional plane of g-C3N4 is separated by the electrons pairs in 2pz orbits of the bridging nitride atoms, leading to the localized π-conjugated structure. Therefore, the g-C3N4 exhibits the typical features of a semiconductor with band gap and visible-light response. Meanwhile, the EPR or ESR technique can be acted as the ideal tool to indirectly evaluate the yield of photoelectrons by detecting the superoxide radicals (O2−) in g-C3N4-based photocatalytic reactions.

Thorium- and uranium-azide reductions: a transient dithorium-nitride versus isolable diuranium-nitrides.

Molecular uranium-nitrides are now well known, but there are no isolable molecular thorium-nitrides outside of cryogenic matrix isolation experiments. We report that treatment of [M(TrenDMBS)(I)] (M = U, 1; Th, 2; TrenDMBS = {N(CH2CH2NSiMe2Bu t )3}3-) with NaN3 or KN3, respectively, affords very rare examples of actinide molecular square and triangle complexes [{M(TrenDMBS)(μ-N3)} n ] (M = U, n = 4, 3; Th, n = 3, 4). Chemical reduction of 3 produces [{U(TrenDMBS)}2(μ-N)][K(THF)6] (5) and [{U(TrenDMBS)}2(μ-N)] (6), whereas photolysis produces exclusively 6. Complexes 5 and 6 can be reversibly inter-converted by oxidation and reduction, respectively, showing that these UNU cores are robust with no evidence for any C-H bond activations being observed. In contrast, reductions of 4 in arene or ethereal solvents gives [{Th(TrenDMBS)}2(μ-NH)] (7) or [{Th(TrenDMBS)}{Th(N[CH2CH2NSiMe2Bu t ]2CH2CH2NSi[μ-CH2]MeBu t )}(μ-NH)][K(DME)4] (8), respectively, providing evidence unprecedented outside of matrix isolation for a transient dithorium-nitride. This suggests that thorium-nitrides are intrinsically much more reactive than uranium-nitrides, since they consistently activate C-H bonds to form rare examples of Th-N(H)-Th linkages with alkyl by-products. The conversion here of a bridging thorium(iv)-nitride to imido-alkyl combination by 1,2-addition parallels the reactivity of transient terminal uranium(iv)-nitrides, but contrasts to terminal uranium(vi)-nitrides that produce alkyl-amides by 1,1-insertion, suggesting a systematic general pattern of C-H activation chemistry for metal(iv)- vs. metal(vi)-nitrides. Surprisingly, computational studies reveal a σ > π energy ordering for all these bridging nitride bonds, a phenomenon for actinides only observed before in terminal uranium nitrides and uranyl with very short U-N or U-O distances.

The role of bridging ligands in dinitrogen reduction and functionalization by uranium multimetallic complexes.

Cooperativity between metal centres is identified as a crucial step in dinitrogen reduction both for the industrial Haber-Bosch process and for the natural fixation of nitrogen by nitrogenase enzymes, but the mechanism of N2 reduction remains poorly understood. This is in large part because multimetallic complexes that reduce and functionalize dinitrogen in the absence of strong alkali reducing agents are crucial to establish a structure-activity relationship, but remain extremely rare. Recently, we reported a multimetallic nitride-bridged diuranium(III) complex capable of reducing and functionalizing dinitrogen. Here we show that an analogous complex assembled with an oxo instead of a nitride linker also effects the four-electron reduction of dinitrogen, but the reactivity of the resulting oxo-(N2) complex differs significantly from that of the nitride-(N2). Computational studies show a different bonding scheme for the dinitrogen where the bridging nitride does participate in the binding and consequent activation of N2, while the oxide does not.

Four-electron Reduction and Functionalization of N₂ by a Uranium(III) Bridging Nitride.

N2 is a cheap and widely available but very unreactive molecule. Notably, the only industrial process for the conversion of dinitrogen is the Haber-Bosch process to form ammonia, but under very harsh conditions (high temperature and pressure). Here, we review our recent research leading to the reduction of dinitrogen by four electrons, under ambient conditions by a uranium(III) bridging nitride, K3 U-N-U, where two uranium(III) cations are linked by a nitride group and a flexible metal-ligand scaffold. We also show that the bound dinitrogen can be further functionalized under mild conditions: the addition of acid, hydrogen and protons or carbon monoxide to the uranium hydrazido complex yields to the cleavage of the N-N single bond and to the formation of new N-H and N-C bonds.

Towards Catalytic Ammonia Oxidation to Dinitrogen: A Synthetic Cycle by Using a Simple Manganese Complex.

Oxidation of the nucleophilic nitride, (salen)Mn≡N (1) with stoichiometric [Ar3 N][X] initiated a nitride coupling reaction to N2 , a major step toward catalytic ammonia oxidation (salen=N,N'-bis(salicylidene)-ethylenediamine dianion; Ar=p-bromophenyl; X=[SbCl6 ]- or [B(C6 F5 )4 ]- ). N2 production was confirmed by mass spectral analysis of the isotopomer, 1-15 N, and the gas quantified. The metal products of oxidation were the reduced MnIII dimers, [(salen)MnCl]2 (2) or [(salen)Mn(OEt2 )]2 [B(C6 F5 )4 ]2 (3) for X=[SbCl6 ]- or [B(C6 F5 )4 ]- , respectively. The mechanism of nitride coupling was probed to distinguish a nitridyl from a nucleophilic/electrophilic coupling sequence. During these studies, a rare mixed-valent MnV /MnIII bridging nitride, [(salen)MnV (μ-N)MnIII (salen)][B(C6 F5 )4 ] (4), was isolated, and its oxidation-state assignment was confirmed by X-ray diffraction (XRD) studies, perpendicular and parallel-mode EPR and UV/Vis/NIR spectroscopies, as well as superconducting quantum interference device (SQUID) magnetometry. We found that 4 could subsequently be oxidized to 3. Furthermore, in view of generating a catalytic system, 2 can be re-oxidized to 1 in the presence of NH3 and NaOCl closing a pseudo-catalytic "synthetic" cycle. Together, the reduction of 1→2 followed by oxidation of 2→1 yield a genuine synthetic cycle for NH3 oxidation, paving the way to the development of a fully catalytic system by using abundant metal catalysis.

Cover Picture: Nucleophilic Reactivity of a Nitride‐Bridged Diuranium(IV) Complex: CO2 and CS2 Functionalization (Angew. Chem. Int. Ed. 12/2016)

The Venus flytrap … … is used to illustrate the importance of a framework built from siloxide ligands and a Cs+ ion in capturing abundant CO2 and toxic CS2 under ambient conditions and in promoting their reactions with a bridging nitride in a diuranium(IV) complex. In their Communication on page 4074 ff., M. Mazzanti et al. show how these processes lead to N−C bond formation and afford cyanate, thiocyanate, and unprecedented dicarbamate products. The Venus flytrap … … is used to illustrate the importance of a framework built from siloxide ligands and a Cs+ ion in capturing abundant CO2 and toxic CS2 under ambient conditions and in promoting their reactions with a bridging nitride in a diuranium(IV) complex. In their Communication on page 4074 ff., M. Mazzanti et al. show how these processes lead to N−C bond formation and afford cyanate, thiocyanate, and unprecedented dicarbamate products. Red Phosphorus Magnetic Materials Enantioselective Cyanosilylation

Nucleophilic Reactivity of a Nitride-Bridged Diuranium(IV) Complex: CO2 and CS2 Functionalization.

Thermolysis of the nitride-bridged diuranium(IV) complex Cs{(μ-N)[U(OSi(O(t) Bu)3)3]2} (1) showed that the bridging nitride behaves as a strong nucleophile, promoting N-C bond formation by siloxide ligand fragmentation to yield an imido-bridged siloxide/silanediolate diuranium(IV) complex, Cs{(μ-N(t) Bu)(μ-O2 Si(O(t) Bu)2)U2 (OSi(O(t) Bu)3)5}. Complex 1 displayed reactivity towards CS2 and CO2 at room temperature that is unprecedented in f-element chemistry, affording diverse N-functionalized products depending on the reaction stoichiometry. The reaction of 1 with two equivalents of CS2 yielded the thiocyanate/thiocarbonate complex Cs{(μ-NCS)(μ-CS3 )[U(OSi(O(t)Bu)3)3]2} via a putative NCS(-)/S(2-) intermediate. The reaction of 1 with one equivalent of CO2 resulted in deoxygenation and N-C bond formation, yielding the cyanate/oxo complex Cs{(μ-NCO)(μ-O)[U(OSi(O(t) Bu)3 )3]2}. Addition of excess CO2 to 1 led to the unprecedented dicarbamate product Cs{(μ-NC2O4)[U(OSi(O(t) Bu)3)3]2}.

Nucleophilic Reactivity of a Nitride‐Bridged Diuranium(IV) Complex: CO2 and CS2 Functionalization

AbstractThermolysis of the nitride‐bridged diuranium(IV) complex Cs{(μ‐N)[U(OSi(OtBu)3)3]2} (1) showed that the bridging nitride behaves as a strong nucleophile, promoting N−C bond formation by siloxide ligand fragmentation to yield an imido‐bridged siloxide/silanediolate diuranium(IV) complex, Cs{(μ‐NtBu)(μ‐O2Si(OtBu)2)U2(OSi(OtBu)3)5}. Complex 1 displayed reactivity towards CS2 and CO2 at room temperature that is unprecedented in f‐element chemistry, affording diverse N‐functionalized products depending on the reaction stoichiometry. The reaction of 1 with two equivalents of CS2 yielded the thiocyanate/thiocarbonate complex Cs{(μ‐NCS)(μ‐CS3)[U(OSi(OtBu)3)3]2} via a putative NCS−/S2− intermediate. The reaction of 1 with one equivalent of CO2 resulted in deoxygenation and N−C bond formation, yielding the cyanate/oxo complex Cs{(μ‐NCO)(μ‐O)[U(OSi(OtBu)3)3]2}. Addition of excess CO2 to 1 led to the unprecedented dicarbamate product Cs{(μ‐NC2O4)[U(OSi(OtBu)3)3]2}.

Stepwise N–H bond formation from N2-derived iron nitride, imide and amide intermediates to ammonia

Reduction of N2 to ammonia in nature and in electrocatalysis takes place through 1-proton/1-electron steps, motivating efforts to experimentally study the steps during proton/electron transfer to well-characterized N2-derived species with bridging nitrides. We report here the protonation and reduction reactions of an N2-derived iron bis(nitride) complex (Rodriguez et al., Science, 2011, 334, 780). We isolate and definitively characterize triiron imido and amido intermediates that lie along the path to ammonia formation, and Mössbauer spectroscopy shows the oxidation level of iron atoms in these mixed-valence clusters. The first two H atoms add to one of the two nitrides of the bis(nitride) complex, and the proton-coupled electron transfer in the second step can be concerted or stepwise depending on the sources of protons and electrons. The characterization of partially protonated nitrides and their mechanisms of formation are expected to guide efforts to convert N2 to ammonia with mild acids.

Electron‐rich, Nitrido‐bridged Ruthenium Complexes

AbstractTerminal RuIV nitride [RuN{N(CH2CH2PtBu2)2}] (1) and [RuCl{N(CH2CH2PtBu2)2}] (2a) form an equilibrium with bridging nitride [{(tBu2PCH2CH2)2N}Ru(μ‐N)RuCl{N(CH2CH2PtBu2)2}] (3a). While 3a could only be spectroscopically characterized yet not isolated, the reaction of the divinylamido chloro complex [RuCl{N(CHCHPtBu2)2}] (2b) with half an equivalent azide directly gave the analogous bridging nitride [{(tBu2PCHCH)2N}Ru(μ‐N)RuCl{N(CHCHPtBu2)2}] (3b), which was isolated and fully characterized.

A multi-iron system capable of rapid N2 formation and N2 cleavage.

The six-electron oxidation of two nitrides to N2 is a key step of ammonia synthesis and decomposition reactions on surfaces. In molecular complexes, nitride coupling has been observed with terminal nitrides, but not with bridging nitride complexes that more closely resemble catalytically important surface species. Further, nitride coupling has not been reported in systems where the nitrides are derived from N2. Here, we show that a molecular diiron(II) diiron(III) bis(nitride) complex reacts with Lewis bases, leading to the rapid six-electron oxidation of two bridging nitrides to form N2. Surprisingly, these mild reagents generate high yields of iron(I) products from the iron(II/III) starting material. This is the first molecular system that both breaks and forms the triple bond of N2 at room temperature. These results highlight the ability of multi-iron species to decrease the energy barriers associated with the activation of strong bonds.

Molybdenum(VI) Complexes of a 2,2′-Biphenyl-bridged Bis(amidophenoxide): Competition between Metal–Ligand and Metal–Amidophenoxide π Bonding

The 2,2'-biphenyl-bridged bis(2-aminophenol) ligand 4,4'-di-tert-butyl-N,N'-bis(3,5-di-tert-butyl-2-hydroxyphenyl)-2,2'-diaminobiphenyl ((t)BuClipH(4)) reacts with MoO(2)(acac)(2) to form ((t)BuClipH(2))MoO(2), where the diarylamines remain protonated and bind trans to the terminal oxo groups. This complex readily loses water on treatment with pyridine or 3,5-lutidine to form mono-oxo complexes ((t)BuClip)MoO(L), which exhibit predominantly a cis-β geometry with an aryloxide trans to the oxo group. Exchange of the pyridine ligands is rapid and takes place by a dissociative mechanism, which occurs with retention of stereochemistry at molybdenum. Oxo-free alkoxide complexes ((t)BuClip)Mo(OR)(2) are formed from ((t)BuClipH(2))MoO(2) and ROH. Treatment of NMo(O(t)Bu)(3) with (t)BuClipH(4) results in complete deprotonation of the bis(aminophenol) and formation of a dimolybdenum complex ((t)BuClip)Mo(μ-N)(μ-NH(2))Mo((t)BuClip) containing both a bridging nitride (Mo-N = 1.848 Å, Mo-N-Mo = 109.49°) and a bridging amide group. The strong π bonding of this bis(amidophenoxide) ligand allows the molybdenum center to interconvert readily among species forming three, two, one, or zero π bonds from multiply bonded ligands.

Heterobimetallic Nitride Complexes from Terminal Chromium(V) Nitride Complexes: Hyperfine Coupling Increases with Distance

Terminal nitride complexes of rhenium, osmium and molybdenum can form complexes with either alkylating agents, Lewis acidic metal halides, or low-valent, coordinatively unsaturated metal complexes. The few reactions of this type with a first-row transition-metal complex are limited to vanadium. Recently, the nitride chemistry of the chromium(V) cation has been significantly expanded by introduction of a preparative route which is based on nitrogen transfer from [Mn(N)(salen)] (salen=N,N’-bis(salicylidene)ethylenediamine) to the chromium(V) cation. With a range of chromium nitride complexes at hand we have investigated their reactivity and found that nucleophilicity is a general property which can be observed during formation of imide complexes with, for example, the trityl cation, tris(pentafluorophenyl)boron, and methyl triflate. In addition we report that terminal chromium(V) nitride complexes coordinate through the nitride ligand to low-valent complexes of the platinum metals. These compounds are possible precursors to bimetallic nitride phases which are gaining in importance as heterogeneous catalysts in, for example, the Haber–Bosch process. Solutions of terminal chromium nitride complexes in noncoordinating solvents treated with electrophiles such as B(C6F5)3 or C(C6H5)3 + quickly yield intensely colored orangered or green solutions. The reactions proceed cleanly as shown by EPR spectra which display a signal from a single S= =2 spin species. Similar reactivity was observed in reactions with either [Rh(cod)Cl]2 or cis-[PtCl2(dmso)2] (cod= 1,5cyclooctadiene, dmso= dimethyl sulfoxide). Structures of some of these systems, characterized by single-crystal X-ray diffraction, are shown in Scheme 1. Experimental and crystallographic details such as ORTEP drawings andmetric parameters of complexes 1–5 (Scheme 1) are available in the Supporting Information (Tables S1 and S1a). Inspection of the structures reveals a number of general aspects: there is a strong propensity for the chromium center to increase its coordination number from five to six upon coordination of the nitride ligand. This propensity is expected and a consequence of the trans influence of either an imide or a bridging nitride ligand which is significantly lower than that of a terminal nitride ligand. Accompanying this, the displacement of Cr out of the plane spanned by the equatorial ligators is diminished from about 0.5 to about 0.2 . The Cr N bond length is elongated from 1.55 in the terminal nitride complexes to approximately 1.60–1.62 in the functionalized systems. Comparison of structure 1 with that of [Cr(N)(salen)] reveals that the metal–salen ligand bonds are significantly shorter when the nitride ligand is functionalized, as expected when two ligands compete for electron donation. However, for the systems derived from [Cr(N)(dbm)2] the situation is less clear (dbm= dibenzoylmethanolate). In complex 2 all the Cr–dbm bonds are longer than in the parent terminal nitride complex, while they are shorter or similar within the limits of uncertainty in complex 5. The B N and C N bonds in 1, 4, and 5 are unexceptional but the N Rh and N Pt bond lengths in 2 and 3 are at about 1.970 and 1.906 , respectively, and very short; the first value belongs to the top 5% of the shortest Rh N bonds and the second belongs to the top 1% of the shortest Pt N bonds. Table 1 compares the Pt N bond of 3 with Pt N bonds of other cis[PtCl2(dmso)L] structures. Scheme 1. Schematic representation of the chromium(V) imide and chromium(V) bridging-nitride complexes.

Inner-sphere two-electron reduction leads to cleavage and functionalization of coordinated dinitrogen

Activation of molecular nitrogen by transition metal complexes is an area of current interest as investigations using the inert N2 molecule to produce higher-value organonitrogen compounds intensify. In an attempt to extend the addition of hydride reagents E-H (where E = BR2, AlR2, and SiR3) to the dinitrogen complex ([NPN]Ta)2(mu-H)2(mu-eta1:eta2-N2) [1; where NPN = (PhNSiMe2CH2)2PPh], the reaction with zirconocene chlorohydride, [Cp2Zr(Cl)H]x, was examined. The crystalline product formed in 35% yield was determined to be ([NP(N)N]Ta)(mu-H)2(mu-N)(Ta[NPN])(ZrCp2) (2) in which the coordinated N2 has been cleaved to form a phosphinimide bridging between Ta and Zr and a triply bridging nitride. The mechanism of this reaction was examined to determine the fate of the chloride and hydride ligands attached to Zr in the starting zirconocene reagent. Using the zirconocene dihydride dimer ([Cp2ZrH2]2), a higher yield of 2 was obtained (76%), and H2 was also observed by 1H NMR spectroscopy. To probe the origin of the eliminated H2, the dideuterated dinitrogen complex ([NPN]Ta)2(mu-D)2(mu-eta1:eta2-N2) (d2-1) was allowed to react with ([Cp2ZrH2]2), which resulted in the formation of ([NP(N)N]Ta)(mu-D)2(mu-N)(Ta[NPN])(ZrCp2), (d2-2), with no evidence of hydrogen for deuterium scrambling between the starting zirconocene dihydride and the ditantalum dinitrogen complex. Studies into the use of preformed Zr(II) and Ti(II) reagents were also performed. The proposed mechanism involves initial adduct formation that facilitates inner-sphere electron transfer to cleave the N-N bond to form a species with bridging nitrides, one of which is transformed by nucleophilic attack of a phosphine donor to generate the observed phosphinimide.