Neal P. Mankad

Triggering N 2 uptake via redox-induced expulsion of coordinated NH 3 and N 2 silylation at trigonal bipyramidal iron

The biological reduction of N2 to give NH3 may occur by one of two predominant pathways in which nitrogenous NxHy intermediates, including hydrazine (N2H4), diazene (N2H2), nitride (N3−) and imide (NH2−), may be involved. To test the validity of hypotheses on irons direct role in the stepwise reduction of N2, model systems for iron are needed. Such systems can test the chemical compatibility of iron with various proposed NxHy intermediates and the reactivity patterns of such species. Here we describe a trigonal bipyramidal Si(o-C6H4PR2)3Fe–L scaffold (R = Ph or i-Pr) in which the apical site is occupied by nitrogenous ligands such as N2, N2H4, NH3 and N2R. The system accommodates terminally bound N2 in the three formal oxidation states (iron(0), +1 and +2). N2 uptake is demonstrated by the displacement of its reduction partners NH3 and N2H4, and N2 functionalizaton is illustrated by electrophilic silylation. In nature, iron takes a direct role in converting nitrogen to ammonia through a variety of NxHy intermediates. A series of synthetic iron complexes that mimic these intermediates could lead to an increased understanding of the process and eventually to effective catalysts.

Application of Fundamental Organometallic Chemistry to the Development of a Gold‐Catalyzed Synthesis of Sulfinate Derivatives

The development of a gold(I)-catalyzed sulfination of aryl boronic acids is described. This transformation proceeds through an unprecedented mechanism which exploits the reactivity of gold(I)-heteroatom bonds to form sulfinate anions. Further in situ elaboration of the sulfinate intermediates leads to the corresponding sulfones and sulfonamides, two pharmacophores routinely encountered in drug discovery.

Base Metal Catalysts for Photochemical C–H Borylation That Utilize Metal–Metal Cooperativity

Heterobimetallic Cu-Fe and Zn-Fe complexes catalyze C-H borylation, a transformation that previously required noble metal catalysts. The optimal catalyst, (IPr)Cu-FeCp(CO)2, exhibits efficient activity at 5 mol% loading under photochemical conditions, shows only minimal decrease in activity upon reuse, and is able to catalyze borylation of a variety of arene substrates. Stoichiometric reactivity studies are consistent with a proposed mechanism that exploits metal-metal cooperativity and showcases bimetallic versions of the classical organometallic processes, oxidative addition and reductive elimination.

Two Metals Are Better Than One in the Gold Catalyzed Oxidative Heteroarylation of Alkenes

We present a detailed study of the mechanism for oxidative heteroarylation, based on DFT calculations and experimental observations. We propose binuclear Au(II)-Au(II) complexes to be key intermediates in the mechanism for gold catalyzed oxidative heteroarylation. The reaction is thought to proceed via a gold redox cycle involving initial oxidation of Au(I) to binuclear Au(II)-Au(II) complexes by Selectfluor, followed by heteroauration and reductive elimination. While it is tempting to invoke a transmetalation/reductive elimination mechanism similar to that proposed for other transition metal complexes, experimental and DFT studies suggest that the key C-C bond forming reaction occurs via a bimolecular reductive elimination process (devoid of transmetalation). In addition, the stereochemistry of the elimination step was determined experimentally to proceed with complete retention. Ligand and halide effects played an important role in the development and optimization of the catalyst; our data provides an explanation for the ligand effects observed experimentally, useful for future catalyst development. Cyclic voltammetry data is presented that supports redox synergy of the Au···Au aurophilic interaction. The monometallic reductive elimination from mononuclear Au(III) complexes is also studied from which we can predict a ~15 kcal/mol advantage for bimetallic reductive elimination.

C-C coupling reactivity of an alkylgold(III) fluoride complex with arylboronic acids.

Previously, alkylgold(III) fluorides have been proposed as catalytic intermediates that undergo C-C coupling with reagents such as arylboronic acids in Au(I)/Au(III) cross-coupling reactions. Here is reported the first experimental evidence for this elementary mechanistic step. Complexes of the type (NHC)AuMe (NHC = N-heterocyclic carbene) were oxidized with XeF(2) to yield cis-(NHC)AuMeF(2) products, which were found to be in equilibrium with their fluoride-dissociated, dimeric [(NHC)AuMe(μ-F)](2)[F](2) forms. In one case, a monomeric cis-(NHC)AuMeF(2) complex was favored exclusively in solution, and it was found to react with a variety of ArB(OH)(2) reagents to yield Ar-CH(3) products.

Dinitrogen Complexes Supported by Tris(phosphino)silyl Ligands

The tetradentate tris(phosphino)silyl ligand [SiP(iPr)(3)] ([SiP(iPr)(3)] = [Si(o-C(6)H(4)P(i)Pr(2))(3)](-)) has been prepared, and its complexation with iron, cobalt, nickel, and iridium precursors has been explored. Several coordination complexes have been thoroughly characterized and are described. These include, for example, the divalent trigonal bipyramidal metal chlorides [SiP(iPr)(3)]M-Cl (M = Fe, Co, Ni), as well as the monovalent dinitrogen adducts [SiP(iPr)(3)]M-N(2) (M = Fe, Co, Ir), which are compared with related [SiP(Ph)(3)]M-Cl and [SiP(Ph)(3)]M-N(2) species (M = Fe, Co). Complexes of this type represent the first examples of terminal dinitrogen adducts of monovalent iron, and the ligand architecture allows examination of a unique class of dinitrogen adducts with a trans-disposed silyl donor. Oxidation of the appropriate [SiP(R)(3)]M-N(2) precursors affords the divalent iron triflate [SiP(Ph)(3)]Fe(OTf) and trivalent cobalt triflate {[SiP(iPr)(3)]Co(OTf)}{OTf} complexes, which are of interest for group transfer studies because of the presence of a labile triflate ligand. Comparative electrochemical, structural, and spectroscopic data are provided for these complexes.

Catalytic N−N Coupling of Aryl Azides To Yield Azoarenes via Trigonal Bipyramid Iron−Nitrene Intermediates

The reactivity of the trigonal bipyramidal iron(I) complex [SiP(iPr)(3)]Fe(N(2)) ([SiP(iPr)(3)] = (2-iPr(2)PC(6)H(4))(3)Si(-)) toward organoazides has been examined. 1-Adamantylazide was found to coordinate the iron center to form stable [SiP(iPr)(3)]Fe(eta(1)-N(3)Ad). Aryl azides instead afforded unstable [SiP(iPr)(3)]Fe(N(3)Ar) species that decayed gradually to regenerate [SiP(iPr)(3)]Fe(N(2)) with release of azoarenes (ArN horizontal lineNAr). The conversion of aryl azides to azoarenes can thus be achieved catalytically. Competitive trapping experiments strongly suggest the intermediacy of reactive nitrene complexes of the type [SiP(iPr)(3)]Fe(NAr) that couple bimolecularly in the N-N bond forming step. Evidence for one such intermediate was provided by electron paramagnetic resonance spectroscopy via photolysis of [SiP(iPr)(3)]Fe(N(3)Ar) in a frozen glass. The electronic structures of these putative nitrene intermediates have been examined by DFT methods.

Three-coordinate copper(I) amido and aminyl radical complexes.

A three-coordinate Cu-NR(2) system (R = p-tolyl) supported by the anionic bis(phosphino)borate ligand [Ph(2)B(CH(2)P(t)Bu(2))(2)](-) has been isolated and structurally characterized in both its anionic Cu(I) and neutral (formally) Cu(II) oxidation states. A large rate constant for the self-exchange electron-transfer reaction (k(S) >or= 10(7) M(-1) s(-1)) makes this system a functional model for the type-1 active sites in blue copper proteins. Multiedge X-ray absorption spectroscopy, multifrequency electron paramagnetic resonance, and density functional theory analyses collectively indicate that the oxidized form is best regarded as a Cu(I)-aminyl radical complex rather than a Cu(II)-amido species, with about 70% localization of the unpaired electron on the NR(2) unit. Hydrogen-atom transfer and C-C coupling reactions are presented as examples of chemical reactivity manifested by this unusual electronic structure.

Dinitrogen complexes of sulfur-ligated iron.

We report a unique class of dinitrogen complexes of iron featuring sulfur donors in the ancillary ligand. The ligands utilized are related to the recently studied tris(phosphino)silyl ligands (2-R(2)PC(6)H(4))(3)Si (R = Ph, iPr) but have one or two phosphine arms replaced with thioether donors. Depending on the number of phosphine arms replaced, both mononuclear and dinuclear iron complexes with dinitrogen are accessible. These complexes contribute to a desirable class of model complexes that possess both dinitrogen and sulfur ligands in the immediate iron coordination sphere.

Four-Coordinate, Trigonal Pyramidal Pt(II) and Pd(II) Complexes

We report herein the characterization of electrophilic, trigonal bipyramidal {[SiP(3)(R)]Pt(L)}(+) cations ([SiP(3)(R)] = [(2-R(2)PC(6)H(4))(3)Si]; R = Ph, (i)Pr) that feature weakly coordinated ligands including CH(2)Cl(2), Et(2)O, toluene, and H(2). A cationic toluene adduct that shows a close platinum aryl C-H σ-contact is perhaps most noteworthy in this context. For the isopropyl-substituted ligand, [SiP(3)(iPr)], it has proven possible to exclude the fifth axial donor to afford the rigorously four-coordinate, trigonal pyramidal (TP) complex {[SiP(3)(iPr)]Pt}(+). An isostructural TP palladium complex {[SiP(3)(iPr)]Pd}(+) is also accessible. Prototypical four-coordinate d(8) platinum and palladium complexes are square planar. The TP d(8) cations described herein are hence geometrically distinct.