Jennifer Scott

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.

Multiple Pathways for Dinitrogen Activation during the Reduction of an Fe Bis(iminepyridine) Complex

Reduction of the bis(iminopyridine) FeCl(2) complex {2,6-[2,6-(iPr)(2)PhN=C(CH(3))](2)(C(5)H(3)N)}FeCl(2) using NaH has led to the formation of a surprising variety of structures depending on the amount of reductant. Some of the species reported in this work were isolated from the same reaction mixture, and their structures suggest the presence of multiple pathways for dinitrogen activation. The reaction with 3 equiv of NaH afforded {2-[2,6-(iPr)(2)PhN=C(CH(3))]-6-[2,6-(iPr)(20PhN-C=CH(2)](C(5)H(3)N)}Fe(micro,eta(2)-N(2))Na (THF) (1) containing one N(2) unit terminally bound to Fe and side-on attached to the Na atom. In the process, one of the two imine methyl groups has been deprotonated, transforming the neutral ligand into the corresponding monoanionic version. When 4 equiv were employed, two other dinitrogen complexes {2-[2,6-(iPr)(2)PhN=C(CH(3))]-6-[2,6-(iPr)(2)PhN-C=CH(2)](C(5)H(3)N)}Fe(micro-N2)Na(Et(2)O)(3) (2) and {2,6-[2,6-(iPr)(2)PhN=C(CH(3))](2)(C(5)H(3)N)}Fe(micro-N(2))Na[Na(THF)(2)] (3) were obtained from the same reaction mixture. Complex 2 is chemically equivalent to 1, the different degree of solvation of the alkali cation being the factor apparently responsible for the sigma-bonding mode of ligation of the N(2) unit to Na, versus the pi-bonding mode featured in 1. In complex 3, the ligand remains neutral but a larger extent of reduction has been obtained, as indicated by the presence of two Na atoms in the structure. A further increase in the amount of reductant (12 equiv) afforded a mixture of {2-[2,6-(iPr)(2)PhN=C(CH(3))]-6-[2,6-(iPr)(2)PhN-C=CH(2)](C(5)H(3)N)}Fe-N(2) (4) and [{2,6-[2,6-(iPr)(2)PhN=C(CH(3))](2)(C(5)H(3)N)}Fe-N(2)](2)(micro-Na) [Na(THF)(2)](2) (5) which were isolated by fractional crystallization. Complex 4, also containing a terminally bonded N(2) unit and a deprotonated anionic ligand bearing no Na cations, appears to be the precursor of 1. The apparent contradiction that excess NaH is required for its successful isolation (4 is the least reduced complex of this series) is most likely explained by the formation of the partner product 5, which may tentatively be regarded as the result of aggregation between 1 and 3 (with the ligand system in its neutral form). Finally, reduction carried out in the presence of additional free ligand afforded {2,6-[2,6-(iPr)(2)PhN=C(CH(3))](2)(C(5)H(3)N)}Fe(eta(1)-N(2)){2,6-[2,6-(iPr)(2)PhN=C(CH(3))](20(NC(5)H(2))}[Na(THF)(2)] (6) and {2,6-[2,6-(iPr)(2)PhN=C(CH(3))](2)(C(5)H(3)N)}Fe{2,6-[2,6-(iPr)(2)PhN=C(CH(3))](2)(NC(5)H(2))}Na(THF)(2)) (7). In both species, the Fe metal is bonded to the pyridine ring para position of an additional (L)Na unit. Complex 6 chemically differs from 7 (the major component) only for the presence of an end-on coordinated N(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.

Carbenes and alkylidenes: Spot the difference

Transition-metal carbenes and alkylidenes are sometimes considered similar species with subtly different bonding and reactivities. Investigations into scandium and yttrium carbenes have raised questions about our understanding — and definition — of these widely used compounds.

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.

Oxidative C–C coupling of 2,6-di-tert-butylphenol in aqueous media via catalytically active molybdate surfactants

Long-chain N-alkyliminodiacetatomolybdate catalytic surfactants can be incorporated within a CTAB micellar solution and have shown catalytic activity for the oxidation of 2,6-di-tert-butylphenol to the corresponding diphenoquinone in aqueous media.