Daniel J. Mindiola

A Mononuclear Fe(III) Single Molecule Magnet with a 3/2↔5/2 Spin Crossover

The air stable complex [(PNP)FeCl(2)] (1) (PNP = N[2-P(CHMe(2))(2)-4-methylphenyl](2)(-)), prepared from one-electron oxidation of [(PNP)FeCl] with ClCPh(3), displays an unexpected S = 3/2 to S = 5/2 transition above 80 K as inferred by the dc SQUID magnetic susceptibility measurement. The ac SQUID magnetization data, at zero field and between frequencies 10 and 1042 Hz, clearly reveal complex 1 to have frequency dependence on the out-of-phase signal and thus being a single molecular magnet with a thermally activated barrier of U(eff) = 32-36 cm(-1) (47-52 K). Variable-temperature Mössbauer data also corroborate a significant temperature dependence in δ and ΔE(Q) values for 1, which is in agreement with the system undergoing a change in spin state. Likewise, variable-temperature X-band EPR spectra of 1 reveals the S = 3/2 to be likely the ground state with the S = 5/2 being close in energy. Multiedge XAS absorption spectra suggest the electronic structure of 1 to be highly covalent with an effective iron oxidation state that is more reduced than the typical ferric complexes due to the significant interaction of the phosphine groups in PNP and Cl ligands with iron. A variable-temperature single crystal X-ray diffraction study of 1 collected between 30 and 300 K also reveals elongation of the Fe-P bond lengths and increment in the Cl-Fe-Cl angle as the S = 5/2 state is populated. Theoretical studies show overall similar orbital pictures except for the d(z(2)) orbital, which has the most sensitivity to change in the geometry and bonding, where the quartet ((4)B) and the sextet ((6)A) states are close in energy.

Structural, Spectroscopic, and Theoretical Elucidation of a Redox-Active Pincer-Type Ancillary Applied in Catalysis

Pincer-type ligands are believed to be very robust scaffolds that can support multifarious functionalities as well as highly reactive metal motifs applied in organometallic chemistry, especially in the realm of catalysis. In this paper, we describe the redox and, therefore, noninnocent behavior of a PNP (PNP- = N[2-P(CHMe2)2-4-methylphenyl]2) pincer ancillary bound to nickel. A combination of structural, spectroscopic, and theoretical techniques suggests that this type of framework can house an electron hole when coordinated to Ni(II).

Evidence for the Existence of a Terminal Imidoscandium Compound: Intermolecular CH Activation and Complexation Reactions with the Transient ScNAr Species

Terminal imide ligands in early-transition-metal chemistry have recently undergone a dramatic renaissance, given their potential applications in processes such as group transfer and catalysis. Absent from this extensive list are Group 3 transition-metal imides, an antithesis given the inherent affinity of the highly electropositive metal ions for a hard donor such as nitrogen. To date, complexes of Group 3 transition-metal elements (including the lanthanides) with terminal imido ligands have been neither directly detected nor isolated; their existence during the formation of a narrow list of dinuclear or polynuclear bridging imides has only been speculated. The inability to isolate a terminal imide linkage may be due to the discrepancy in energy between the lanthanide and imide-nitrogen orbitals, rendering this type of bond highly polarized and thus prohibiting the formal M=NR or M NR bond that is prototypical among most early transition metals. As a result, such a mismatch in orbital energies should bestow unprecedented nucleophilic behavior to the imido nitrogen atom when coordinated to an ion such as a lanthanide. Herein, we present credible evidence for the existence of a terminal scandium imido complex by applying a combination of isotopic labeling and reactivity studies of a transient Sc=NR complex, evidenced by the intermolecular C H activation of pyridine and benzene as well as complexation with Al(CH3)3. The fact that we can generate transient, reactive titanium alkylidynes of the type [(PNP)Ti CR] (PNP = N[2-P(CHMe2)2-4-methylphenyl]2, R = Ph, SiMe3, and tBu, among other groups) encouraged the search for an isolobal {(PNP)Sc NR} fragment, owing to the comparable atomic radii between titanium(IV) and scandium(III) when weighed against the other Group 3 congeners. Likewise, the PNP ligand type has been recently demonstrated to be an ideal ancillary support in the preparation of an unprecedented bridging phosphinidene moiety on lutetium(III). For us, assembling the PNP ancillary ligand and Sc to form [(PNP)ScCl2] (1) in 95% yield proved straightforward by treatment of Li(PNP) with [ScCl3(thf)3] in toluene at 70 8C over 48 h. Bright yellow 1 can be readily transmetalated with LiNHAr (Ar = 2,6-iPr2C6H3) to afford [(PNP)Sc(NHAr)Cl] (2) in 76% yield (Scheme 1). To incorporate a

Lewis acid stabilized methylidene and oxoscandium complexes.

The methylidene scandium complex (PNP)Sc(mu3-CH2)(mu2-CH3)2[Al(CH3)2]2 (PNP = N[2-P(CHMe2)2-4-methylphenyl]2-) can be prepared from the reaction of (PNP)Sc(CH3)2 and 2 equiv of Al(CH3)3. The Lewis acid stabilized methylidenes candium complex has been crystallographically characterized, and its bonding scheme analyzed by DFT. In addition, we report preliminary reactivity studies of the Sc-CH2 ligand with substrates such as H2NAr and OCPh2. While the former results in an Brønsted acid-base reaction, the latter reagent produces the olefin H2C CPh2 along with the novel oxoscandium complex (PNP)Sc(mu3-O)(mu2-CH3)2[Al(CH3)2]2, quantitatively.

Low-Coordinate and Neutral Nitrido Complexes of Vanadium

Two neutral and four-coordinate vanadium(V)-nitrido complexes have been prepared via the thermolysis of metastable vanadium(III)-azido precursors. All complexes have been fully characterized by multinuclear NMR, FT-IR, isotopic labeling, and, in most instances, single crystal X-ray diffraction. On the basis of activation parameters, N(2) extrusion to form the V[triple bond]N moiety is proposed to occur via an ordered and early transition state having three- or four-triazametallacycle frameworks. In addition, we demonstrate the nitrido ligand to undergo incomplete N-atom transfer to CO and CN{2,6-Me(2)-C(6)H(3)) to form the bent V-N=C=X (X = O, N{2,6-Me(2)-C(6)H(3)}) ligands with concomitant 2e(-) reduction at the vanadium center.

Isocyanate and carbodiimide synthesis by nitrene-group-transfer from a nickel(ii) imido complexElectronic supplementary information (ESI) available: complete experimental, spectroscopic, analytical, and crystallographic details. See http://www.rsc.org/suppdata/cc/b2/b204846a/

The imido complex (dtbpe)Ni(N(2,6-(CHMe2)2C6H3)) reacts with CO and CNCH2Ph with addition at the Ni-N bond to give (dtbpe)Ni(C,N:eta 2-C(O)N(2,6-(CHMe2)2C6H3)) and (dtbpe)Ni(C,N:eta 2-C(NCH2Ph)N(2,6-(CHMe2)2C6H3)); both complexes react further with CO to liberate the isocyanate and carbodiimide ligands with formation of (dtbpe)Ni(CO)2.

A tribute to Frederick Nye Tebbe. Lewis acid stabilized alkylidyne, alkylidene, and imides of 3d early transition metals

Frederick Nye Tebbe distanced himself from the spotlight despite increasing appreciation of his discoveries by the scientific world. Although his research accomplishments are numerous and significant, perhaps his most highly recognized contribution is the reagent that bears his name, Cp2Ti(mu2-CH2)(mu2-Cl)Al(CH3)2: an inspiring molecule to the area of olefin metathesis and methylidene transfer. Masking its potential via Lewis acid stabilization, Tebbe tamed the titanium methylidene moiety, leading many other scientists to exploit the wide-ranging reactivity of such a unit, including olefin metathesis, methylenation, and metallacyclobutane formation, among others. In this perspective, we pay tribute to the life and chemistry of the man behind the masked methylidene and the research progeny spawned by his discovery, focusing on the Lewis acid stabilization of multiply-bonded ligands of the early metals such as Sc and Ti.

Evidence for the Existence of Terminal Scandium Imidos: Mechanistic Studies Involving Imido–Scandium Bond Formation and C–H Activation Reactions

The anilide-methyl complex (PNP)Sc(NH[DIPP])(CH(3)) (1) [PNP(-) = bis(2-diisopropylphosphino-4-tolyl)amide, DIPP = 2,6-diisopropylphenyl] eliminates methane (k(avg) = 5.13 × 10(-4) M(-1) s(-1) at 50 °C) in the presence of pyridine to generate the transient scandium imido (PNP)Sc═N[DIPP](NC(5)H(5)) (A-py), which rapidly activates the C-H bond of pyridine in 1,2-addition fashion to form the stable pyridyl complex (PNP)Sc(NH[DIPP])(η(2)-NC(5)H(4)) (2). Mechanistic studies suggest the C-H activation process to be second order overall: first order in scandium and first order in substrate (pyridine). Pyridine binding precedes elimination of methane, and α-hydrogen abstraction is overall-rate-determining [the kinetic isotope effect (KIE) for 1-d(1) conversion to 2 was 5.37(6) at 35 °C and 4.9(14) at 50 °C] with activation parameters ΔH(‡) = 17.9(9) kcal/mol and ΔS(‡) = -18(3) cal/(mol K), consistent with an associative-type mechanism. No KIE or exchange with the anilide proton was observed when 1-d(3) was treated with pyridine or thermolyzed at 35 or 50 °C. The post-rate-determining step, C-H bond activation of pyridine, revealed a primary KIE of 1.1(2) at 35 °C for the intermolecular C-H activation reaction in pyridine versus pyridine-d(5). Complex 2 equilibrated back to the imide A-py slowly, as the isotopomer (PNP)Sc(ND[DIPP])(η(2)-NC(5)H(4)) (2-d(1)) converted to (PNP)Sc(NH[DIPP])(η(2)-NC(5)H(3)D) over 9 days at 60 °C. Molecular orbital analysis of A-py suggested that this species possesses a fairly linear scandium imido motif (169.7°) with a very short Sc-N distance of 1.84 Å. Substituted pyridines can also be activated, with the rates of C-H activation depending on both the steric and electronic properties of the substrate.

Room Temperature Dehydrogenation of Ethane to Ethylene

The transient titanium alkylidyne, (PNP)Ti≡C(t)Bu (PNP = N[2-P(i)Pr(2)-4-methylphenyl](2)(-)), activates a C-H bond of ethane at room temperature, and a β-hydrogen of the resulting ethyl ligand is subsequently transferred to the adjacent alkylidene ligand to form an ethylene adduct of titanium. Treatment of the ethylene complex with two-electron oxidants such as organic azides results in extrusion of ethene concomitant with formation of a mononuclear titanium imido complex.

Nacnac …︁ Are You Still There? The Evolution of β‐Diketiminate Complexes of Nickel

alongside a narrow list of popular ancillary supports, given its ability to stabilize or generate unique coordination environments and to support reactive organometallic reagents or catalysts. The nacnac ligand skeleton is analogous to the acac (acetylacetonate) ligand, but the oxygen atoms are exchanged for nitrogen-based moieties such as NR (R = alkyl, silyl, Ar). As a result, the substituent at the nitrogen donor atom can allow for steric protection at the metal center unlike acac could offer. The explosion in popularity of nacnac amongst synthetic chemists is driven, in part, by the monoanionic nature of the b-diketiminate group, the chelating nature but also variable mode of hapticity, the ease in preparation, and the versatility to tune both electronic and steric parameters. Therefore, it is of no surprise that nacnac ligands can stabilize most elements of the periodic table to form main-group, transition-metal, lanthanide, and actinide complexes. The first documented cases of b-diketiminate metal complexes were reported by McGeachin and Parks and Holm in 1968. In their studies, the metal of choice was Ni, as two N-substituted b-diketiminate ligands could be used to sterically tune the Ni ion through a range of geometries from square-planar to tetrahedral. Now, more than 40 years later, Limberg and co-workers demonstrate that a sterically modified version of the b-diketiminate ligand used in 1968 can also stabilize unsaturated {({ArNC(tBu)}2CH)Ni } (Ar = 2,6iPr2C6H3) scaffolds, and by tweaking the conditions to generate the Ni complex, these systems can be made to activate and reduce the bond order of a chemically resistant and biologically relevant molecule such as N2. In their studies, Limberg and co-workers use several synthetic approaches to isolate and characterize the complexes [{({ArNC(tBu)}2CH)Ni}2(m2,h :h-N2)] x (x = 0, 1, 2), therefore allowing them to collect one-electron reduction snapshots of two nickel centers bridged by N2. [4] An outline of these synthetic strategies is described in Scheme 2. Impressively, Limberg and co-workers are able to isolate a neutral complex (4) and odd or evenly charged (complexes 5 and 6) reduction products stemming from a precursor material (1). The authors perform a series of spectroscopic studies to elucidate the connectivity and spin state of these paramagnetic complexes, including single-crystal X-ray diffraction studies and theoretical analysis of their frontier orbitals. In their studies, it was independently found that the monoanion N2 salt 5 could be prepared by comproportionation reactions using the appropriate stoichiometry of 1 and 6 or 4 and 6 (Scheme 2). Surprisingly, few N2 complexes of nickel are known, [5]