José G. Andino

Mechanism of Heterolysis of H2 by an Unsaturated d8 Nickel Center: via Tetravalent Nickel?

Collision of H(2) with the unusual nickel complex, (PNP)Ni(+), where PNP = ((t)Bu(2)PCH(2)SiMe(2))(2)N, forms a rare dihydrogen complex of the d(8) configuration which then rearranges to heterolytically cleave the H-H bond. Experimental studies support a short H/H distance in the coordinated diatomic, and DFT calculations show that the transition state for heterolysis, in spite of the fact that this involves an amide nitrogen located trans to the H(2), has the H/H bond fully split, and has all the geometric features of Ni(IV), but this is a local maximum, not a minimum.

Phosphinidene Complexes of Scandium: Powerful PAr Group-Transfer Vehicles to Organic and Inorganic Substrates

The first phosphinidene complexes of scandium are reported in this contribution. When complex (PNP)Sc(CH(3))Br (1) is treated with 1 equiv of LiPH[Trip] (Trip = 2,4,6-(i)Pr(3)C(6)H(2)), the dinuclear scandium phosphinidene complex [(PNP)Sc(mu(2)-P[Trip])](2) (2) is obtained. However, treating 1 with a bulkier primary phosphide produces the mononuclear scandium ate complex [(PNP)Sc(mu(2)-P[DMP])(mu(2)-Br)Li] (3) (DMP = 2,6-Mes(2)C(6)H(3)). The Li cation in 3 can be partially encapsulated with DME to furnish a phosphinidene salt derivative, (PNP)Sc(mu(2)-P[DMP])(mu(2)-Br)Li(DME)] (4). We also demonstrate that complex 3 can readily deliver the phosphinidene ligand to organic substrates such as OCPh(2) and Cl(2)PMes* as well as inorganic fragments such as Cp(2)ZrCl(2), Cp*(2)TiCl(2), and Cp(2)TiCl(2) in the presence of P(CH(3))(3). Complexes 2-4 have been fully characterized, including single crystal X-ray diffraction and DFT studies.

Intermolecular C-H bond activation of benzene and pyridines by a vanadium(III) alkylidene including a stepwise conversion of benzene to a vanadium-benzyne complex†

Breaking of the carbon–hydrogen bond of benzene and pyridine is observed with (PNP)V(CH2tBu)2 (1), and in the case of benzene, the formation of an intermediate benzyne complex (C) is proposed, and indirect proof of its intermediacy is provided by identification of (PNP)VO(η2-C6H4) in combination with DFT calculations.

Mechanism of N/O bond scission of N2O by an unsaturated rhodium transient.

The mechanism of formation of triplet (PNP)RhO and (PNP)Rh(N(2)) (PNP = N(SiMe(2)CH(2)P(t)Bu(2))(2)) from reaction of two molecules of (PNP)Rh with N(2)O has been studied by DFT, evaluating mechanisms which (1) involve free N(2), and (2) which effect N/O bond scission in linearly coordinated (PNP)RhNNO. This work shows the variety of modes of binding N(2)O to this reducing, unsaturated metal fragment and also evaluates why a mechanism avoiding free N(2) is preferred. Comparisons are made to isoelectronic CO(2) in its reaction with (PNP)Rh.

Ligand influence on metal aggregation: a unique bonding mode for pyridylpyrrolides.

The synthesis and characterization of a Cu(I) complex with a cis-bidentate monoanionic nitrogenous ligand, 2-pyridylpyrrolide, L, is reported. This shows binding of one base B = MeCN or CO per copper in a species LCu(B), but this readily releases the volatile ligand under vacuum with aggregation of transient LCu to a mixture of two enantiomers of a chiral trimer: a zwitterion containing inequivalent Cu(I) centers, possible via a new bonding mode of pyridylpyrrolide, and one with nitrogen lone pairs donating to two different metals. Density functional theory calculations show the energetics of both ligand binding and aggregation (including dimer and monomer alternatives), as well as the ability of this ligand to rotate away from planarity to accommodate a bridging structural role. The trimer serves as a synthon for the simple fragment LCu.

New approaches to functionalizing metal-coordinated N2.

Catalytic conversion of N2 into value-added products is the challenge of attack on a refractory molecule. We develop herein ideas, using product thermodynamics, to gauge which derivatizing reagents hold special promise. Since N2 is an especially stable molecule, it must be paired with an endothermic molecule to make the overall reaction exothermic. A number of unusual nitrogen heterocycle products are targeted in this way, based on derivatization with alkynes, with alkenes, or with allenes. Since the late transition elements (M) have M–heteroatom bonds which are readily subject to hydrogenolysis, aldehydes and ketones are also identified as potential traps. It is proposed that advantages arise from targeting organonitrogen products retaining an N N bond, including double bonds. While fundamentally thermodynamic in character, such advantages may also translate into lowering of barriers in elementary processes, hence to improved rates. The ideas are general in nature, and thus invite testing among newly formed N2 complexes of any metal. A current goal of global importance is the conversion of diatomic nitrogen to value-added organonitrogen compounds. While improved, less-energy intensive hydrogenation of coordinated N2 (“nitrogen fixation”, forming NH3) is an attractive, if challenging goal owing to the importance of agricultural fertilizers, there are broader goals for organonitrogen compounds, including other fertilizers (e.g., urea), but also compounds useful in pharmaceutical and electronic materials applications. The purpose of this Essay is to show that, in the absence of available experimental thermochemical data, it can be useful to go “prospecting”, using density functional theory (DFT) calculations, for organic molecules with which to “derivatize” N2. Our goal is to design new reactions for the conversion of N2 into chemicals based on a thermodynamic concept: high energy reagents serve as fuel to convert the very stable, refractory molecule N2. The challenge of hydrogenation of coordinated, hopefully electron rich N2 to NxHy compounds may be due in part to the fact that diatomic H2 is not very electrophilic. Indeed, earliest efforts in this quest have involved first protons (targeting NH3) and carbon electrophile reagents (alkyl halides, etc.). A solution to this problem is proton-coupled electron transfer (PCET), which is the alternating delivery of electrons and protons, instead of H2. [21–29] This approach however creates the challenge of preventing the protons from combining with the electrons to simply generate H2 and Schrock and Yandulov have nicely solved this problem by delivering the electrons with [Cp*2Cr] (Cp*= h-C5Me5) and using a weak and highly sterically hindered acid: an anilinium cation. From this success, H clearly has a demonstrated ability to react with one metal nitride in the Schrock PCETreduction of N2 to ammonia on molybdenum. However, this reaction is highly species-dependent, since another molybdenum-bound terminal nitride is found to be less Bronsted basic than even a neighboring coordinated tertiary phosphine. It is a truism of catalysis that one must only attempt reactions that are exergonic (DG8< 0), hence thermodynamically favorable to permit high conversion into product. When chosen well, this thermodynamic preference is what makes it possible to convert alkynes into arenes ... even with beach sand! The thermodynamics makes alkyne polymerizations sometimes dangerously explosive whereas olefin polymerizations are harder to accomplish, and less exergonic, since the entropy change is unfavorable. Likewise there are many cyclizations of diynes and enynes, but fewer with dienes. The Huisgen “click reaction” cyclizing alkyne and azide to a triazole is easily accomplished, since it is hugely exergonic. On the negative side, hydrogenation of N2 to ammonia is not very exothermic, so, coupled with an unfavorable DS8, this approach to ammonia production is thermodynamically challenging. The limited quantity of experimental organonitrogen thermodynamic data is a hindrance to establishing what reactions are exergonic; some body of data exists, but less than desired. With the broadening availability of density functional methods, which give chemically useful accuracy even for larger molecules and those outside the experimental database, this situation is improved. We apply this approach herein to an important problem, that of choosing reagents [*] Dr. J. G. Andino, S. Mazumder, Dr. K. Pal, Prof. K. G. Caulton Department of Chemistry, Indiana University 800 East Kirkwood Avenue, Bloomington, IN 47405 (USA) E-mail: caulton@indiana.edu

Synthesis and oxidative reactivity of 2,2'-pyridylpyrrolide complexes of Ni(II).

Synthesis and characterization of divalent nickel complexed by 2-pyridylpyrrolide bidentate ligands are reported, as possible precursors to complexes with redox active ligands. Varied substituents on the pyrrolide, two CF3 (L(2)), two (t)Bu (L(0)), and one of each type (L(1)) are employed and the resulting Ni(L(n))2 complexes show different Lewis acidity toward CO, H2O, tetrahydrofuran (THF), or MeCN, the L(2) case being the most acidic. Density functional theory calculations show that the frontier orbitals of all three Ni(L(n))2 species are localized at the pyrrolide groups of both ligands and Ni(L(n))2(+) can be detected by mass spectrometry and in cyclic voltammograms (CVs). Following cyclic voltammetry studies, which show electroactivity primarily in the oxidative direction, reactions with pyridine N-oxide or Br2 are reported. The former yield simple bis adducts, Ni(L(2))2(pyNO)2 and the latter effects electrophilic aromatic substitution of the one pyrrolide ring hydrogen for both chelates, leaving it brominated.