Pursuit of an Electron Deficient Titanium Nitride.

The nitride salt [(PN)2Ti≡N{µ2-K(OEt2)}]2 (1) (PN- = (N-(2-PiPr2-4-methylphenyl)-2,4,6-Me3C6H2) can be oxidized with two equiv of I2 or four equiv of ClCPh3 to produce the phosphinimide-halide complexes (NPN')(PN)Ti(X) (X- = I (2), Cl (3); NPN' = N-(2-NPiPr2-4-methylphenyl)-2,4,6-Me3C6H22-), respectively. In the case of 2, H2 was found to be one of the other products; whereas, HCPh3 and Gomberg's dimer were observed upon the formation of 3. Independent studies suggest that the oxidation of 1 could imply the formation of the transient nitridyl species [(PN)2Ti(≡N•)] (A), which can either oxidize the proximal phosphine atom to produce the Ti(III) intermediate [(NPN')(PN)Ti] (B) or, alternatively, engage in H atom abstraction to form the parent imido (PN)2Ti≡NH (4). The latter was independently prepared and was found to photochemically convert to the titanium-hydride, (NPN')(PN)Ti(H) (5). Isotopic labeling studies using (PN)2Ti≡ND (4-d1) as well as reactivity studies of 5 with a hydride abstractor demonstrate the presence of the hydride ligand in 5. An alternative route to putative A was observed via a photochemically promoted incomplete reduction of the azide ligand in (PN)2Ti(N3) (6) to 4. This process was accompanied by some formation of 5. Frozen matrix X-band EPR studies of 6, performed under photolytic conditions, were consistent with species B being formed under these reaction conditions, originating from a low barrier N-insertion into the phosphine group in the putative nitridyl species A. Computational studies were also undertaken to discover the mechanism and plausibility of the divergent pathways (via intermediates A and B) in the formation of 2 and 3, and to characterize the bonding and electronic structure of the elusive nitrogen-centered radical in A.

Selective P4 Activation by a Highly Reduced Cobaltate: Synthesis of Dicobalt Tetraphosphido Complexes.

Although the chemistry of transition metal polyphosphide anions has attracted significant attention, there are few reports of studies in which such species have been synthesized directly from white phosphorus. [K(OEt2 )2 {Co(BIAN)(cod)}] (1, BIAN=1,2-bis(2,6-diisopropylphenylimino)acenaphthene, cod=1,5-cyclooctadiene), which is readily prepared by ligand exchange from [K(thf)x {Co(cod)2 }], reacts with P4 to afford [{K(thf)}2 {(BIAN)Co}2 (µ-η4 :η4 -P4 )] (2 a) in 61 % yield (isolated product). [{K(OEt2 )}2 {(BIAN)Co}2 (µ-η4 :η4 -P4 )] (2 b) and [K([18]crown-6)(MeCN)]2 [{(BIAN)Co}2 (µ-η4 :η4 -P4 )] (2 c) were obtained by recrystallizing 2 a from diethyl ether and acetonitrile (and using [18]crown-6 in case of 2 c). Oxidation of 2 a with [Cp2 Fe]BArF4 (one equivalent) and subsequent recrystallization of the product from different solvents gave [K(OEt2 ){(BIAN)Co}2 (µ-η4 :η4 -P4 )] (3 a) and [K(dme)4 ][{(BIAN)Co}2 (µ-η4 :η4 -P4 )] (3 b; dme=1,2-dimethoxyethane). Neutral [{(BIAN)Co}2 (µ-η4 :η4 -P4 )] (4) was obtained in moderate yield by oxidizing 2 a with two equivalents of [Cp2 Fe]BArF4 . The new complexes were characterized by NMR, EPR (in the case of 3 a), and UV/Vis spectroscopy, and elemental analysis. The molecular structures revealed by X-ray crystallography display planar cyclic or open-chain P44- units sandwiched between {(BIAN)Co} fragments.

Electrocatalytic Azide Oxidation Mediated by a Rh(PNP) Pincer Complex.

One-electron oxidation of the rhodium(I) azido complex [Rh(N3 )(PNP)] (5), bearing the neutral, pyridine-based PNP ligand 2,6-bis(di-tert-butylphosphinomethyl)pyridine, leads to instantaneous and selective formation of the mononuclear rhodium(I) dinitrogen complex [Rh(N2 )(PNP)]+ (9+ ). Interestingly, complex 5 also acts as a catalyst for electrochemical N3- oxidation (Ep ≈-0.23 V vs. Fc+/0 ) in the presence of excess azide. This is of potential relevance for the design of azide-based and direct ammonia fuel cells, expelling only harmless dinitrogen as an exhaust gas.

Electronic Structure and Magnetic Anisotropy of an Unsaturated Cyclopentadienyl Iron(I) Complex with 15†Valence Electrons.

The 15 valence-electron iron(I) complex [CpAr Fe(IiPr2 Me2 )] (1, CpAr =C5 (C6 H4 -4-Et)5 ; IiPr2 Me2 =1,3-diisopropyl-4,5-dimethylimidazolin-2-ylidene) was synthesized in high yield from the FeII precursor [CpAr Fe(µ-Br)]2 . 57 Fe Mössbauer and EPR spectroscopic data, magnetic measurements, and ab initio ligand-field calculations indicate an S= 3/2 ground state with a large negative zero-field splitting. As a consequence, 1 features magnetic anisotropy with an effective spin-reversal barrier of Ueff =64 cm-1 . Moreover, 1 catalyzes the dehydrogenation of N,N-dimethylamine-borane, affording tetramethyl-1,3-diaza-2,4-diboretane under mild conditions.

Porphyrin Cobalt(III) "Nitrene Radical" Reactivity; Hydrogen Atom Transfer from Ortho-YH Substituents to the Nitrene Moiety of Cobalt-Bound Aryl Nitrene Intermediates (Y = O, NH).

In the field of cobalt(II) porphyrin-catalyzed metallo-radical reactions, organic azides have emerged as successful nitrene transfer reagents. In the pursuit of employing ortho-YH substituted (Y = O, NH) aryl azides in Co(II) porphyrin-catalyzed nitrene transfer reactions, unexpected hydrogen atom transfer (HAT) from the OH or NH2 group in the ortho-position to the nitrene moiety of the key radical-intermediate was observed. This leads to formation of reactive ortho-iminoquinonoid (Y = O) and phenylene diimine (Y = NH) species. These intermediates convert to subsequent products in non-catalyzed reactions, as is typical for these free organic compounds. As such, the observed reactions prevent the anticipated cobalt-mediated catalytic radical-type coupling of the nitrene radical intermediates to alkynes or alkenes. Nonetheless, the observed reactions provide valuable insights into the reactivity of transition metal nitrene-radical intermediates, and give access to ortho-iminoquinonoid and phenylene diimine intermediates from ortho-YH substituted aryl azides in a catalytic manner. The latter can be employed as intermediates in one-pot catalytic transformations. From the ortho-hydroxy aryl azide substrates both phenoxizinones and benzoxazines could be synthesized in high yields. From the ortho-amino aryl azide substrates azabenzene compounds were obtained as the main products. Computational studies support these observations, and reveal that HAT from the neighboring OH and NH2 moiety to the nitrene radical moiety has a low energy barrier.

Photolytic N2 splitting: a road to sustainable NH3 production?

The split up: Recent advances in photochemical dinitrogen splitting have been achieved. Demonstration of the reversibility of the N2 splitting and ammonia formation from a nitride has advanced the field of N2 fixation using a synthetic homogeneous system.

An isolated nitridyl radical-bridged {Rh(N·)Rh} complex.

Photochemical activation of [(PNNH)Rh(N3)] (PNNH = 6-di-(tert-butyl)phosphinomethyl-2,2'-bipyridine) complex 2 produced the paramagnetic (S = 1/2), [(PNN)Rh=N(·)-Rh(PNN)] complex 3 (PNN(-) = methylene-deprotonated PNNH), which could be crystallographically characterized. Spectroscopic investigation of 3 indicates a predominant nitridyl radical ((·)N(2-)) character, which was confirmed computationally. Complex 3 reacts selectively with CO, producing two equivalents of [(PNN)Rh(I)(CO)] complex 4, presumably by nitridyl radical N,N-coupling.