Rational selection of co-catalysts for the deaminative hydrogenation of amides.

The catalytic hydrogenation of amides is an atom economical method to synthesize amines. Previously, it was serendipitously discovered that the combination of a secondary amide co-catalyst with (iPrPNP)Fe(H)(CO) (iPrPNP = N[CH2CH2(PiPr2)]2 -), results in a highly active base metal system for deaminative amide hydrogenation. Here, we use DFT to develop an improved co-catalyst for amide hydrogenation. Initially, we computationally evaluated the ability of a series of co-catalysts to accelerate the turnover-limiting proton transfer during C-N bond cleavage and poison the (iPrPNP)Fe(H)(CO) catalyst through a side reaction. TBD (triazabicyclodecene) was identified as the leading co-catalyst. It was experimentally confirmed that when TBD is combined with (iPrPNP)Fe(H)(CO) a remarkably active system for amide hydrogenation is generated. TBD also enhances the activity of other catalysts for amide hydrogenation and our results provide guidelines for the rational design of future co-catalysts.

Synthesis and characterization of heterobimetallic complexes with direct Cu-M bonds (M = Cr, Mn, Co, Mo, Ru, W) supported by N-heterocyclic carbene ligands: a toolkit for catalytic reaction discovery.

Building upon the precedent of catalytically active (NHC)Cu-FeCp(CO)2 complexes, a series of (NHC)Cu-[M] complexes were synthesized via the addition of Na(+)[M](-) reagents to (NHC)CuCl synthons. The different [M](-) anions used span a range of 7 × 10(7) relative nucleophilicity units, allowing for controlled variation of nucleophile/electrophile pairing in the heterobimetallic species. Direct Cu-M bonds (M = Cr, Mn, Co, Mo, Ru, W) formed readily when the bulky IPr carbene was used as a support. Crystallographic characterization and computational examination of these complexes was conducted. For the smaller IMes carbene, structural isomerism was observed when using the weakest [M](-) nucleophiles, with (IMes)Cu-[M] and {(IMes)2Cu}{Cu[M]2} isomers being observed in equilibrium. Collectively, the series of complexes provides a toolbox for catalytic reaction discovery with precise control of structure-function relationships.

X-ray absorption spectroscopy systematics at the tungsten L-edge.

A series of mononuclear six-coordinate tungsten compounds spanning formal oxidation states from 0 to +VI, largely in a ligand environment of inert chloride and/or phosphine, was interrogated by tungsten L-edge X-ray absorption spectroscopy. The L-edge spectra of this compound set, comprised of [W(0)(PMe3)6], [W(II)Cl2(PMePh2)4], [W(III)Cl2(dppe)2][PF6] (dppe = 1,2-bis(diphenylphosphino)ethane), [W(IV)Cl4(PMePh2)2], [W(V)(NPh)Cl3(PMe3)2], and [W(VI)Cl6], correlate with formal oxidation state and have usefulness as references for the interpretation of the L-edge spectra of tungsten compounds with redox-active ligands and ambiguous electronic structure descriptions. The utility of these spectra arises from the combined correlation of the estimated branching ratio of the L3,2-edges and the L1 rising-edge energy with metal Zeff, thereby permitting an assessment of effective metal oxidation state. An application of these reference spectra is illustrated by their use as backdrop for the L-edge X-ray absorption spectra of [W(IV)(mdt)2(CO)2] and [W(IV)(mdt)2(CN)2](2-) (mdt(2-) = 1,2-dimethylethene-1,2-dithiolate), which shows that both compounds are effectively W(IV) species even though the mdt ligands exist at different redox levels in the two compounds. Use of metal L-edge XAS to assess a compound of uncertain formulation requires: (1) Placement of that data within the context of spectra offered by unambiguous calibrant compounds, preferably with the same coordination number and similar metal ligand distances. Such spectra assist in defining upper and/or lower limits for metal Zeff in the species of interest. (2) Evaluation of that data in conjunction with information from other physical methods, especially ligand K-edge XAS. (3) Increased care in interpretation if strong π-acceptor ligands, particularly CO, or π-donor ligands are present. The electron-withdrawing/donating nature of these ligand types, combined with relatively short metal-ligand distances, exaggerate the difference between formal oxidation state and metal Zeff or, as in the case of [W(IV)(mdt)2(CO)2], exert the subtle effect of modulating the redox level of other ligands in the coordination sphere.

Small molecule activation chemistry of Cu-Fe heterobimetallic complexes toward CS2 and N2O.

In this contribution, we report the reactivity of polar, unsupported Cu-Fe bonds toward small-molecule heteroallenes. Insertion of CS2 into the polar Cu-Fe bond of (IMes)Cu-FeCp(CO)2 proceeds at mild conditions and results in the simultaneous presence of two unprecedented CS2 binding modes (µ3:η(4) and µ3:η(3)) in the same product. Reactivity between N2O and (NHC)Cu-FeCp(CO)2 complexes also is observed at mild conditions, resulting in migration of the cyclopentadienyl groups from Fe to Cu. Similar reactivity is observed for new (NHC)Cu-FeCp*(CO)2 analogues, whose structural characterization is reported here and reveals two semibridging Cu···CO interactions per molecule. Stoichiometric oxygen atom transfer from N2O to PPh3 was mediated by (IMes)Cu-FeCp(CO)2, indicating the presence of an N2O-activated intermediate that can be intercepted by exogenous reagents.

Ancillary ligand effects upon dithiolene redox noninnocence in tungsten bis(dithiolene) complexes.

An expanded set of compounds of the type [W(S2C2Me2)2L1L2](n) (n = 0: L1 = L2 = CO, 1; L1 = L2 = CN(t)Bu, 2; L1 = CO, L2 = carbene, 3; L1 = CO, L2 = phosphine, 4; L1 = L2 = phosphine, 5. n = 2-: L1 = L2 = CN(-), [6](2-)) has been synthesized and characterized. Despite isoelectronic formulations, the compound set reveals gradations in the dithiolene ligand redox level as revealed by intraligand bond lengths, υ(CCchelate), and rising edge energies in the sulfur K-edge X-ray absorption spectra (XAS). Differences among the terminal series members, 1 and [6](2-), are comparable to differences seen in homoleptic dithiolene complexes related by full electron transfer to/from a dithiolene-based MO. The key feature governing these differences is the favorable energy of the CO π* orbitals, which are suitably positioned to overlap with tungsten d orbitals and exert an oxidizing effect on both metal and dithiolene ligand via π-backbonding. The CN(-) π* orbitals are too high in energy to mix effectively with tungsten and thus leave the filled dithiolene π* orbitals unperturbed. This work shows how, and the degree to which, the redox level of a noninnocent ligand can be modulated by the choice of ancillary ligands(s).

A tungsten-mediated closed cycle of reactivity for the reduction of CO(2) to CO.

The recycling of CO(2) by reduction to CO is an important objective in the context of renewable carbon feedstock chemicals. A tungsten-mediated reduction of CO(2) to CO reported by Mayer and coworkers has been re-examined, and it is shown that a series of four well-defined stoichiometric steps can be executed which form a closed cycle and sum as CO(2) + 2H(+) + 2e(-)→ CO + H(2)O. Energetic parameters of this system are probed by cyclic voltammetry, by calculations of gas-phase reaction enthalpies for each of the four steps, and by calculation of the W[triple bond, length as m-dash]O bond dissociation energy for the tungsten species that results from oxidation addition of CO(2).

Synthesis, structures, and properties of mixed dithiolene-carbonyl and dithiolene-phosphine complexes of tungsten.

A new, high yield synthesis of [Ni(S(2)C(2)Me(2))(2)] (3) is described using 4,5-dimethyl-1,3-dithiol-2-one, Me(2)C(2)S(2)CO (1), as dithiolene ligand precursor. Reaction of (Me(2)C(2)S(2))Sn(n)Bu(2), 2, with WCl(6) produces tris(dithiolene) [W(S(2)C(2)Me(2))(3)] (6) and demonstrates the potential synthetic utility of this compound in metallodithiolene synthesis. The series of compounds [W(S(2)C(2)Me(2))(x)(CO)(6-2x)] (x = 1-3), obtained as a mixture via the reaction of [Ni(S(2)C(2)Me(2))(2)] with [W(MeCN)(3)(CO)(3)], has been characterized structurally. A trigonal prismatic geometry is observed for [W(S(2)C(2)Me(2))(CO)(4)] (4) and confirmed by a DFT geometry optimization to be lower in energy than an octahedron by 5.1 kcal/mol. The tris(dithiolene) compound [W(S(2)C(2)Me(2))(3)] crystallizes in disordered fashion upon a 2-fold axis in C2/c, a different space group than that observed for its molybdenum homologue (P1), which is attributed to a slightly smaller chelate fold angle, alpha, in the former. The reactivity of 4 and [W(S(2)C(2)Me(2))(2)(CO)(2)] (5) toward PMe(3) has been examined. Compound 4 yields only [W(S(2)C(2)Me(2))(CO)(2)(PMe(3))(2)] (7), while 5 produces either [W(S(2)C(2)Me(2))(2)(CO)(PMe(3))] (8) or [W(S(2)C(2)Me(2))(2)(PMe(3))(2)] (9) depending upon reaction conditions. Crystallographic characterization of 5, 8, and 9 reveals a trend toward greater reduction of the dithiolene ligand (i.e., more ene-1,2-dithiolate character) across the series, as manifested by C-C and C-S bond lengths. These structural data indicate a profound effect exerted by the pi-acidic CO ligands upon the apparent state of reduction of the dithiolene ligand in compounds with ostensibly the same oxidation state.