Synthesis of an Arenide-Masked Scandium Complex Accom-panied by Reductively Induced C-H Activation

Reduction of 3N-supported ScCl(ketguan)(NImDipp) (ScCl) with K(C10H8) generates the naphthalenide-masked species [(18-c-6)K(μ-η6:η4-C10H8)Sc(ketguan)(NImDipp)] (Scnaph) and cyclometallated [K(18-c-6)(Et2O)][Sc{(DippN)[2-iPr-6-(CMe2)C6H3N]C(NCHtBu2)}(NImDipp)(THF)] (ScC-H·Et2O), the latter formed from a rare instance of oxidative addition of a low valent scandium center across an unactivated C(sp3)-H bond. Moreover, ScC-H displays solid-to-solution phase dependent tautomerism within the moiety of the scandium metallacyle. Finally, a safe and convenient method is described for the dehydration of ScCl3·6H2O

Synthesis of a “Super Bulky” Guanidinate Possessing an Expandable Coordination Pocket

Friedel–Crafts alkylation of 4-tert-butylaniline with 2 equiv of benzhydrol affords bulky 2,6-bis(diphenylmethyl)-4-tert-butylaniline (Ar*NH2) in good yield, which can be readily synthesized on a tens of grams scale. The reaction of 6 equiv of Ar*NH2 with triphosgene generates the symmetric urea (Ar*NH)2CO, which, upon dehydration with a P2O5/Al2O3 slurry in pyridine, produces the sterically encumbered carbodiimide (Ar*N)2C as an air-stable white solid. The treatment of (Ar*N)2C with LiN═CtBu2 in tetrahydrofuran cleanly gives the monomeric lithium guanidinate Li[Ar*ketguan], free of coordinating solvent, in 85% yield. Protonation of Li[Ar*ketguan] with lutidinium chloride produces the guanidine Ar*ketguanH (MW = 1112.60 g/mol), which is easily derivatized to give the monomeric alkali metal complexes M[Ar*ketguan] (M = K, Cs) in 94% and 51% yield, respectively. The solid-state molecular structures of M[Ar*ketguan] (M = Li, K, Cs) show formally two-coordinate alkali metal cations encapsulated within a hydrophobic coordination pocket formed by the peripheral diphenylmethyl substituents of the guanidinate. Remarkably, percent buried volume analyses (% VBur) of M[Ar*ketguan] [M = Li (94.8% VBur), K (92.1% VBur), Cs (81.7% VBur)] reveal a coordination cavity that adjusts to individually accommodate the variously sized metal ions despite the highly encumbering nature of the ligand. This demonstrates a flexible ligand framework that is able to stabilize low-coordinate metal centers within a “super bulky” coordination environment

Ferrocenylene- and Carbosiloxane-Bridged Bis(sila[1]ferrocenophanes) E[SiMe₂-X-SiMe<i>FC</i>]₂ {E = (η⁵-C₅H₄)Fe(η⁵-C₅H₄) or O; X = (CH₂)_<i>n</i> (<i>n</i> = 2, 3, 6); CHCH; <i>FC</i> = (η⁵-C₅H₄)₂Fe}

New FC[SiMe2(CH2)nSiMeFC]2 [FC = 1,1′-ferrocenylene; FC = 1,1′-ferrocenophane, (η5-C5H4)2Fe); n = 2 (7), 3 (8), 6 (9)] and O[SiMe2-X-SiMeFC]2 [X = (CH2)2 (11), CHCH (13)] have been synthesized and characterized. The synthesis involved the reactions of FCLi2·tmeda with FC[SiMe2(CH2)nSiMeCl2]2 [n = 2 (4), 3 (5), 6 (6)] and O[SiMe2-X-SiMeCl2]2 [X = (CH2)2 (10), CHCH (12)] at −78 °C. Compounds 4−13 were characterized by elemental analysis, NMR spectroscopy, and for 11, single-crystal X-ray crystallography. The cyclopentadienyl rings in the structure of 11 are tilted with a dihedral angle of 19.8°. The observed red-shift of the lowest energy transition associated with the ferrocenophanyl groups in UV/vis spectra of 7−9, 11, and 13 also confirms the ring strain. Cyclic voltammetric studies exhibited either a broad (7−9) or sharp (11, 13) wave for the reversible redox process. The broadness of the band for the FC-bridged complexes is a composite of the two types of (η5-C5H4)Fe(η5-C5H4) unit, but no significant inter-Fc interaction could be discerned. All the new bis-ferrocenylene materials are readily ring-opened to form polymeric materials

Molecular Capacitors: Accessible 6- and 8-electron Redox Chemistry from Dimeric “Ti(I)” and “Ti(0)” Synthons Support-ed by Imidazolin-2-Iminato Ligands.

Reduction of the diamagnetic Ti(III)/Ti(III) dimer [Cl2Ti(μ-NImDipp)]2 (1) (NImDipp = [1,3-bis(Dipp)imidazolin-2-iminato]-, Dipp = NC6H3-2,6-Pri2) with 4 and 6 equiv of KC8 generates the intramolecularly arene-masked, dinuclear titanium com-pounds [(μ-N-μ-η6-ImDipp)Ti]2 (2) and {[(Et2O)2K](μ-N-μ-η6:η6-ImDipp)Ti}2 (3), respectively, in modest yields. The compounds have been structurally characterized by X-ray crystallographic analysis and inspection of the bond metrics within the η6-coordinated aryl substituent of the bridging imidazolin-2-iminato ligand show perturbation of the aromatic system most consistent with two-electron reduction of the ring. As such, 2 and 3 can be assigned respectively as possessing metal centers in formal Ti(III)/Ti(III) and Ti(II)/Ti(II) oxidation states. Exploration of their redox chemistry reveal the ability to reduce several substrate equivalents. For instance, treatment of 2 with excess C8H8 (COT) forms the novel COT-bridged complex [(ImDippN)(η8-COT)Ti](μ-η2:η3-COT)[Ti(η4-COT)(NImDipp)] (4) that dissociates in THF solutions to give mononuclear (ImDippN)Ti(η8-COT)(THF) (5). Addition of COT to 3 yields heterometallic [(ImDippN)(η4-COT)Ti(μ-η4:η5-COT)K(THF)(μ-η6:η4-COT)Ti(NImDipp)(μ-η4:η4-COT)K(THF)2]n (6). Compounds 2 and 5 are the products of the 4-electron oxidation of 2, while 6 stands as the 8-electron oxidation product of 3. Reduction of organozides was also explored. Low temperature reaction of 2 with 4 equiv of AdN3 gives the terminal and bridged imido complex [(ImDippN)Ti(=NAd)](μ-NAd)2[Ti(NImDipp)(N3Ad)] (7) that undergoes intermolecular C-H activation of toluene at room temperature to afford the amido compound [(ImDippN)Ti(NHAd)](μ-NAd)2[Ti(C6H4Me)(NImDipp)] (8-tol). These complexes are the 6-electron oxidation products of the reaction of 2 with AdN3. Furthermore, treatment of 3 with 4 equiv of AdN3 produces the thermally sta-ble Ti(III)/Ti(III) terminal and bridged imido [K(18-crown-6)(THF)2]{[(ImDippN)Ti(NAd)](μ-NAd)2K[Ti(NImDipp)]} (10). Alto-gether, these reactions firmly establish 2 and 3 as unprecedented Ti(I)/Ti(I) and Ti(0)/Ti(0) synthons with the clear ca-pacity to effect multi-electron reductions ranging from 4 – 8 electrons