A photocatalytic noble metal-free system for the generation of hydrogen has been constructed using Eosin Y (1) as a photosensitizer, the complex [Co(dmgH)(2)pyCl](2+) (5, dmgH = dimethylglyoximate, py = pyridine) as a molecular catalyst, and triethanolamine (TEOA) as a sacrificial reducing agent. The system produces H(2) with an initial rate of approximately 100 turnovers per hour upon irradiation with visible light (lambda > 450 nm). Addition of free dmgH(2) greatly increases the durability of the system addition of 12 equiv of dmgH(2) (vs cobalt) to the system produces approximately 900 turnovers of H(2) after 14 h of irradiation. The rate of H(2) evolution is maximum at pH = 7 and decreases sharply at more acidic or basic pH. Spectroscopic study of photolysis solutions suggests that hydrogen production occurs through protonation of a Co(I) species to give a Co(III) hydride, which then reacts further by reduction and protolysis to give Co(II) and molecular hydrogen.

A comprehensive mechanistic study of N2 activation and splitting into terminal nitride ligands upon reduction of the rhenium dichloride complex [ReCl2(PNP)] is presented (PNP– = N(CH2CH2PtBu2)2–). Low-temperature studies using chemical reductants enabled full characterization of the N2-bridged intermediate [{(PNP)ClRe}2(N2)] and kinetic analysis of the N–N bond scission process. Controlled potential electrolysis at room temperature also resulted in formation of the nitride product [Re(N)Cl(PNP)]. This first example of molecular electrochemical N2 splitting into nitride complexes enabled the use of cyclic voltammetry (CV) methods to establish the mechanism of reductive N2 activation to form the N2-bridged intermediate. CV data was acquired under Ar and N2, and with varying chloride concentration, rhenium concentration, and N2 pressure. A series of kinetic models was vetted against the CV data using digital simulations, leading to the assignment of an ECCEC mechanism (where “E” is an electrochemical step and “C” is a chemical step) for N2 activation that proceeds via initial reduction to ReII, N2 binding, chloride dissociation, and further reduction to ReI before formation of the N2-bridged, dinuclear intermediate by comproportionation with the ReIII precursor. Experimental kinetic data for all individual steps could be obtained. The mechanism is supported by density functional theory computations, which provide further insight into the electronic structure requirements for N2 splitting in the tetragonal frameworks enforced by rigid pincer ligands.

The development of a sustainable ammonia synthesis by proton-coupled electroreduction of dinitrogen (N 2 ) requires knowledge of the thermodynamics described by standard reduction potentials. The first collection of N 2 reduction standard potentials in an organic solvent are reported here. The potentials for reduction of N 2 to ammonia (NH 3 ), hydrazine (N 2 H 4 ), and diazene (N 2 H 2 ) in acetonitrile (MeCN) solution are derived using thermochemical cycles. Ammonia is thermodynamically favored, with a 0.43 V difference between NH 3 and N 2 H 4 and a 1.26 V difference between NH 3 and N 2 H 2 . The thermodynamics for reduction of N 2 to the protonated products ammonium (NH 4 + ) and hydrazinium (N 2 H 5 + ) under acidic conditions are also presented. Comparison with the H + /H 2 potential in MeCN reveals a 63 mV thermodynamic preference for N 2 reduction to NH 3 over H 2 production. Combined with knowledge of the kinetics of electrode-catalyzed H 2 evolution, a wide working region is identified to guide future electrocatalytic studies.

A series of Boron-dipyrromethene (Bodipy) dyes were used as photosensitizers for photochemical hydrogen production in conjunction with [Co(III)(dmgH)2pyCl] (where dmgH = dimethylglyoximate, py = pyridine) as the catalyst and triethanolamine (TEOA) as the sacrificial electron donor. The Bodipy dyes are fully characterized by electrochemistry, X-ray crystallography, quantum chemistry calculations, femtosecond transient absorption, and time-resolved fluorescence, as well as in long-term hydrogen production assays. Consistent with other recent reports, only systems containing halogenated chromophores were active for hydrogen production, as the long-lived triplet state is necessary for efficient bimolecular electron transfer. Here, it is shown that the photostability of the system improves with Bodipy dyes containing a mesityl group versus a phenyl group, which is attributed to increased electron donating character of the mesityl substituent. Unlike previous reports, the optimal ratio of chromophore to catalyst is established and shown to be 20:1, at which point this bimolecular dye/catalyst system performs 3-4 times better than similar chemically linked systems. We also show that the hydrogen production drops dramatically with excess catalyst concentration. The maximum turnover number of ∼ 700 (with respect to chromophore) is obtained under the following conditions: 1.0 × 10(-4) M [Co(dmgH)2pyCl], 5.0 × 10(-6) M Bodipy dye with iodine and mesityl substituents, 1:1 v:v (10% aqueous TEOA):MeCN (adjusted to pH 7), and irradiation by light with λ > 410 nm for 30 h. This system, containing discrete chromophore and catalyst, is more active than similar linked Bodipy-Co(dmg)2 dyads recently published, which, in conjunction with our other measurements, suggests that the nominal dyads actually function bimolecularly.

Fe(iv) alkylidenes are produced via protonation of Fe(ii) vinyl chelate complexes.

Neutral, formally Fe(IV) alkylidene species are sought as plausible olefin metathesis catalysts, and the synthesis of several is described herein. The complexes are prepared via nucleophilic attack (Nu = MeLi, PhCH2K, 2-picolyllithium, Me2PCH2Li, MePhPCH2Li, Ph2PCH2Li) at the imine of cationic [mer-{κ-C,N,C-(C6H4-yl)-2-CH=N(2-C6H4-C(iPr)=)}Fe(PMe3)3][B(3,5-CF3-C6H3)4]. In contrast, MeMgCl and mesityllithium displaced and deprotonated bound PMe3, respectively. Structural details are provided for mer-{κ-C,N,C-(C6H4-yl)-2-CH(Bn)N(2-C6H4-C(iPr))}Fe{trans-(PMe3)2}N2 and {κ-C,N,C,P-(C6H4-yl)-2-CH(CH2PMe2)N(2-C6H4-C(iPr)=)}Fe(PMe3)2.

The conversion of metal nitride complexes to ammonia may be essential to dinitrogen fixation. We report a new reduction pathway that utilizes ligating acids and metal-ligand cooperation to effect this conversion without external reductants. Weak acids such as 4-methoxybenzoic acid and 2-pyridone react with nitride complex [(H-PNP)RuN] (H-PNP = HN(CHCHPBu)) to generate octahedral ammine complexes that are κ-chelated by the conjugate base. Experimental and computational mechanistic studies reveal the important role of Lewis basic sites proximal to the acidic proton in facilitating protonation of the nitride. The subsequent reduction to ammonia is enabled by intramolecular 2H/2e proton-coupled electron transfer from the saturated pincer ligand backbone.

Ene-amines Z-3-(2-pyridyl)-1-aza(2,6-i Pr 2 -Ph)propene, (pynac)H, and 2-(2-pyridyl)-1-aza(2,6-R,R′-Ph)propene, (pyEA-ArRR′)H, were synthesized by condensation procedures; corresponding lithium or potassium ene-amides were prepared via standard deprotonation protocols. Addition of 2 equiv of (pynac)H to {(Me 3 Si) 2 N} 2 Fe(THF) or 2 Li(pynac) to FeBr 2 (THF) 2 afforded (pynac) 2 Fe (1), while treatment of CrCl 2 (THF) 2 , MnCl 2 , FeBr 2 (THF) 2 , and CoCl 2 py 4 with 2 equiv of (pyEAAr i Pr 2 )K afforded pseudotetrahedral (pyEA-Ar i Pr 2 ) 2 M (2-M, M = Cr, Mn, Fe) and (pyEA-Ar i Pr 2 ) 2 Co-py (2-Co-py). Diamagnetic (κ-C,N-pyEA-Ar i Pr 2 ) 3 Co (3) was prepared in low yield (∼7%) from CoCl 2 , and its Co−C(sp 3 ) linkages are unusually low in field strength. Reactivity studies yielded little clean reactivity, but thermolysis of 2-Co-py afforded the bis-indolamide derivative {κ-N,N-N(C 6 H 3 (2-i Pr)CMe 2 C(Me)(2-py)} 2 Co (5-Co), and related thermolyses of 2-M (M = Cr, Mn, Fe), conducted on NMR tube scales, generated related 5-M (M = Cr, Mn, Fe) at roughly the same rates. This observation prompted thermolyses of (pyEA-ArRR′)Li, which rearrange to their corresponding indolamides in >90% yields. Rate studies, accompanied by KIE and EIE observations, revealed that an initial hydrogen transfer is reversible and is likely to correspond to an anionic rearrangement, whereas C−C bond formation is ratedetermining, as suggested by accompanying calculations. X-ray structure determinations of 1, 2-Fe, 2-Co-py, 3, and 5-Co were conducted. ■ INTRODUCTIONTransition metal compounds containing pyridine-imine (PI) 1−6 and pyridine-diimine (PDI) 7−19 moieties often exhibit redox noninnocent (RNI) 20−29 behavior due to multiple accessible charged states of the ligands. This capacity is most evident in first-row transition metals, where the ionic character of the metal−ligand bond limits charge distribution via covalency, and in early transition metals 29−32 that have limited redox capability. Ligands designed as PI variants consisting of 2-azaallyls, their precursors, or related chelates have led to intriguing carbon− carbon and C−X bond-forming reactions and afford examples of RNI, as illustrated in Figure 1.Compounds containing 1,3-di-2-pyridyl-2-azaallyl (smif) 33 tridentate ligands exhibit reversible and irreversible C−C bondforming reactions depending on steric factors (A), 34 while the generation of transient azaallyls within a tetradentate chelate have afforded [{Me 2 C(CHNCH-2-py) 2 }M] 2 (M = Cr, Co, Ni) dimers wherein three new carbon−carbon bonds have been formed around unique metal−metal bonds (B). 35 Incorporating PI precursors into a nacnac framework 36−39 permitted the isolation of carbon radical character, leading to C−C bond formation (C), 40 but in related tetradentate ligands, electrostatic stabilization of a 14e − π-system afforded very stable Fe(II) complexes (D). 41,42 Finally, a tetradentate di-PI ligand revealed five redox states (E) that were quite stable, while the metal formally remained Ni(II). 6 The s...