New talent: Americas

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Methane activation and exchange by titanium-carbon multiple bonds

We demonstrate that a titanium-carbon multiple bond, specifically an alkylidyne ligand in the transient complex, (PNP)Ti≡C^(t)Bu (A) (PNP^− = N[2-P(CHMe_2)_(2)-4-methylphenyl]_2), can cleanly activate methane at room temperature with moderately elevated pressures to form (PNP)Ti=CHtBu(CH_3). Isotopic labeling and theoretical studies suggest that the alkylidene and methyl hydrogens exchange, either via tautomerization invoking a methylidene complex, (PNP)Ti=CH_(2)(CH_(2)^(t)Bu), or by forming the methane adduct (PNP)Ti≡C^(t)Bu(CH_4). The thermal, fluxional and chemical behavior of (PNP)Ti=CH^(t)Bu(CH_3) is also presented in this study

Radical Scission of Symmetrical 1,4-Dicarbonyl Compounds: C−C Bond Cleavage with Titanium(IV) Enolate Formation and Related Reactions

Reaction of Ti(NRAr^1)_3 (1, R = C(CH_3)_3, Ar^1 = 3,5-C_6H_3Me_2) with 0.5 equiv of symmetrical 1,4-diketones (ArCOCH_2)_2 (Ar = p-Tol or p-MeOC_6H_4) in hydrocarbon solvents at ≤25 °C resulted in carbon−carbon bond cleavage with clean formation of titanium-bound enolates, 1-OC(CH_2)Ar. Treatment of Ti(NRAr^1)_3 with esters or amides of succinic acid, under the same mild conditions, smoothly produced titanium(IV) compounds containing the corresponding amide or ester enolate moiety. The amide enolate condenses with benzaldehyde in an aldolic fashion. Differences in the observed reactivity of amido-enolate vs ketone-derived enolate toward aldol condensation were interpreted with the help of computational methods. Upon reaction with Ti(NRAr^1)_3, para-substituted acetophenones yielded equal amounts of enolate and alkoxide products. Under similar experimental conditions, acetophenone itself produced quantitatively a species whose proposed structure incorporates characteristics reminiscent of a Gomberg dimer. This intermediate decomposes cleanly to the expected enolate and alkoxide mixture upon heating. Ti(NRAr^1)_3 reductively complexes substrates such as N-methyl phthalimide. Treatment of Ti(NRAr^1)_3 with 0.5 equiv of o-bromophenyl allyl ether resulted in bromine atom abstraction followed by cyclization of the intermediate aryl radical to generate a titanium-bound 3-methylenedihydrobenzofuran product

Radical Scission of Symmetrical 1,4-Dicarbonyl Compounds: C−C Bond Cleavage with Titanium(IV) Enolate Formation and Related Reactions

Reaction of Ti(NRAr^1)_3 (1, R = C(CH_3)_3, Ar^1 = 3,5-C_6H_3Me_2) with 0.5 equiv of symmetrical 1,4-diketones (ArCOCH_2)_2 (Ar = p-Tol or p-MeOC_6H_4) in hydrocarbon solvents at ≤25 °C resulted in carbon−carbon bond cleavage with clean formation of titanium-bound enolates, 1-OC(CH_2)Ar. Treatment of Ti(NRAr^1)_3 with esters or amides of succinic acid, under the same mild conditions, smoothly produced titanium(IV) compounds containing the corresponding amide or ester enolate moiety. The amide enolate condenses with benzaldehyde in an aldolic fashion. Differences in the observed reactivity of amido-enolate vs ketone-derived enolate toward aldol condensation were interpreted with the help of computational methods. Upon reaction with Ti(NRAr^1)_3, para-substituted acetophenones yielded equal amounts of enolate and alkoxide products. Under similar experimental conditions, acetophenone itself produced quantitatively a species whose proposed structure incorporates characteristics reminiscent of a Gomberg dimer. This intermediate decomposes cleanly to the expected enolate and alkoxide mixture upon heating. Ti(NRAr^1)_3 reductively complexes substrates such as N-methyl phthalimide. Treatment of Ti(NRAr^1)_3 with 0.5 equiv of o-bromophenyl allyl ether resulted in bromine atom abstraction followed by cyclization of the intermediate aryl radical to generate a titanium-bound 3-methylenedihydrobenzofuran product

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 unusual 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 reveals complex 1 to undergo a frequency dependent of the out-of-phase signal and thus be 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. Multi-edge 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 30-300 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 is 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

Redox-Catalyzed Binding of Dinitrogen by Molybdenum N-tert-Hydrocarbylanilide Complexes: Implications for Dinitrogen Functionalization and Reductive Cleavage

The splitting of dinitrogen (1 atm, THF, 25 °C) by Mo(N[R]Ar)_3 (R = C(CD_3)_2CH_3, Ar = 3,5-C_6H_3Me_2) giving 2 equiv of nitride N⋮Mo(N[R]Ar)3 is found to be accelerated in the presence of sodium amalgam. Careful control of the Mo(N[R]Ar)_3 concentration led to the isolation and characterization of the anionic dinitrogen complex, [(THF)xNa][(N_2)Mo(N[R]Ar)_3], where x is from 0 to 3. Via electrochemical experiments and synthetic studies, [(THF)xNa][(N2)Mo(N[R]Ar)_3] is found to be a key intermediate in the acceleration of N_2 splitting by Mo(N[R]Ar)_3 in the presence of sodium amalgam. Accordingly, in the presence of an electron acceptor, [(THF)xNa][(N_2)Mo(N[R]Ar)_3] reacts with Mo(N[R]Ar)_3 to give the neutral N2-bridged complex (μ-N_2){Mo(N[R]Ar)_3}_2, which in turn splits to 2 equiv of nitride N⋮Mo(N[R]Ar)3. It is seen that the function of sodium amalgam in this system is as a redox catalyst, accelerating the conversion of Mo(N[R]Ar)_3 to (μ-N2){Mo(N[R]Ar)3}2, a dinuclear dinitrogen complex that does not lose N_2 readily. Electrochemical or chemical outer-sphere oxidation of [(THF)xNa][(N2)Mo(N[R]Ar)_3] leads to rapid N_2 evolution with regeneration of Mo(N[R]Ar)_3, presumably via the neutral mononuclear dinitrogen complex (N2)Mo(N[R]Ar)_3. In situ generated [(THF)xNa][(N_2)Mo(N[R]Ar)_3] was efficiently trapped by ClSiMe3 to give (Me3SiNN)Mo(N[R]Ar)_3. This complex underwent reaction with methyl triflate to give the dimethyl hydrazido cationic species, [(Me_2NN)Mo(N[R]Ar)_3][OTf]. The synthesis of the monomethyl complex (MeNN)Mo(N[R]Ar)_3 also was achieved. Experiments designed to trap the neutral mononuclear dinitrogen complex (N_2)Mo(N[R]Ar)_3 gave rise to efficient syntheses of heterodinuclear dinitrogen complexes including (Ph[tBu]N)3Ti(μ-N_2)Mo(N[R]Ar)_3, which also was synthesized in its ^(15)N_2-labeled form. Synthesis and characterization data for the new N-adamantyl-substituted three-coordinate molybdenum(III) complex Mo(N[Ad]Ar)_3 (Ad = 1-adamantyl, Ar = 3,5-C_6H_3Me_2) are presented. The complex is found to react with dinitrogen (1 atm, THF, 25 °C) in the presence of sodium amalgam to give the dinitrogen anion complex [(THF)xNa][(N_2)Mo(N[Ad]Ar)_3]; the synthesis does not require careful regulation of the Mo(N[Ad]Ar)_3 concentration. Indeed, under no conditions has Mo(N[Ad]Ar)_3 been observed to split dinitrogen or to give rise to a dinuclear μ-N_2 complex; this striking contrast with the reactivity of Mo(N[R]Ar)_3 (R = C(CD_3)_2CH_3) is attributed to the enhanced steric protection at Mo afforded by the 1-adamantyl substituents