David R. Armstrong

Mixed‐Metal Sodium–Magnesium Macrocyclic Amide Chemistry: A Template Reaction for the Site Selective Dideprotonation of Arene Molecules

A remarkable pair of macrocyclic amides has been synthesized and crystallographically characterized; these consist of a twelve-membered (N6Na4Mg2)2+ cationic ring host with [C6H3(CH3)]2- or (C6H4)2- dianionic guests (see picture) derived from toluene and benzene, respectively.

The first molecular main group metal species containing interstitial hydride

Lithium cages containing hydride: The reaction of tBuLi with Me(2)AlN(2-Pyr)Ph in toluene gave [Li(8)(H){N(2-Pyr)Ph}(6)](+)[Li(Me(2)AltBu(2))(2)](-), whose cation is the first molecular main group metal species to contain interstitial hydride (the cluster core is shown in the picture). Treatment of the reaction mixture with THF gave the neutral hydride Li(7)(H)[N(2-Pyr)Ph](6), which has a capped octahedral (Li(+))(7) cluster core. 2-Pyr=2-pyridyl.

The laddering principle in lithium amide chemistry: the crystal and molecular structure of the pyrrolididolithium adduct [H2C(CH2)3NLi]3·MeN(CH2CH2NMe2)2

The title compound, {[H2[graphic omitted]NLi]3·PMDETA}n, (1)(PMDETA = pentamethyldiethylenetriamine), is shown to be the first example of an organonitrogen–lithium laddered structure, consisting in the solid (n= 2) of two attached (NLi)2 rings, or alternatively four (N–Li) rungs, with two terminal NLi units complexes by PMDETA, so preventing further association; cryoscopic and 7Li n.m.r. spectroscopic studies imply that extension of the ladder framework can occur in arene solutions of (1), and these results, together with those from ab initio m.o. calculations on model systems, suggest that similar compounds of type (RR′NLi·xdonor)n, but of various ladder lengths, should be preparable.

Alkali-metal-mediated zincation (AMMZn) meets N-heterocyclic carbene (NHC) chemistry: Zn–H exchange reactions and structural authentication of a dinuclear Au(I) complex with a NHC anion

Merging two evolving areas in synthesis, namely cooperative bimetallics and N-heterocyclic carbenes (NHCs), this study reports the isolation of the first intermediates of alkali-metal-mediated zincation (AMMZn) of a free NHC and a Zn–NHC complex using sodium zincate [(TMEDA)NaZn(TMP)(tBu)2] (1) as a metallating reagent. The structural authentication of (THF)3Na[:C{[N(2,6-iPr2C6H3)]2CHCZn(tBu2)}] (2) and [Na(THF)6]+[tBu2Zn:C{[N(2,6-iPr2C6H3)]2CHCZn(tBu2)}]− (4), resulting from the reactions of 1 with unsaturated free NHC IPr (IPr = 1,3-bis(2,6-di-isopropylphenylimidazole-2-ylidene) and NHC complex ZntBu2IPr (3) respectively demonstrates that in both cases, this mixed-metal approach can easily facilitate the selective C4 zincation of the unsaturated backbone of the NHC ligand. Furthermore, the generation of anionic NHC fragments enables dual coordination through their normal (C2) and abnormal (C4) positions to the bimetallic system, stabilising the kinetic AMMZn intermediates which normally go undetected and provides new mechanistic insights in to how these mixed-metal reagents operate. In stark contrast to this bimetallic approach when NHC-complex 3 is reacted with a more conventional single-metal base such as tBuLi, the deprotonation of the coordinated carbene is inhibited, favouring instead, co-complexation to give NHC-stabilised [IPr·LiZntBu3] (5). Showing the potential of 2 to act as a transfer agent of its anionic NHC unit to transition metal complexes, this intermediate reacts with two molar equivalents of [ClAu(PPh3)] to afford the novel digold species [ClAu:C{[N(2,6-iPr2C6H3)]2CHCAu(PPh3)}] (6) resulting from an unprecedented double transmetallation reaction which involves the simultaneous exchange of both cationic (Na+) and neutral (ZntBu2) entities on the NHC framework.

A molecular-orbital study of the ground and excited-state properties of metallocenes of the first-row transitionmetal ions. I

Abstract The CNDO-MO formalism has been used to study the ground- and excited-state properties of unsubstituted metallocenes of the first-row transition-metal ions. The multi-electron configuration interaction (MECI) method has been applied to the calculation of both photo-electron and absorption spectra and the agreement with experiment is satisfactory. The electronic properties of the metalloceneseries are described and the variations in the bonding schemes within the series are rationalised.

Structural Basis for Regioisomerization in the Alkali-Metal-Mediated Zincation (AMMZn) of Trifluoromethyl Benzene by Isolation of Kinetic and Thermodynamic Intermediates

Performed with a desire to advance knowledge of the structures and mechanisms governing alkali-metal-mediated zincation, this study monitors the reaction between the TMP-dialkylzincate reagent [(TMEDA)Na(TMP)(tBu)Zn(tBu)] 1 and trifluoromethyl benzene C6H5CF32. A complicated mixture of products is observed at room temperature. X-ray crystallography has identified two of these products as ortho- and meta-regioisomers of heterotrianionic [(TMEDA)Na(TMP)(C6H4-CF3)Zn(tBu)], 3-ortho and 3-meta, respectively. Multinuclear NMR data of the bulk crystalline product confirm the presence of these two regioisomers as well as a third isomer, 3-para, in a respective ratio of 20:11:1, and an additional product 4, which also exhibits ortho-zincation of the aryl substrate. Repeating the reaction at 0 °C gave exclusively 4, which was crystallographically characterized as [{(TMEDA)2Na}+{Zn(C6H4-CF3)(tBu)2}−]. Mimicking the original room-temperature reaction, this kinetic product was subsequently reacted with TMP(H) to afford a complicated mixture of products, including significantly the three regioisomers of 3. Surprisingly, 4 adopts a solvent-separated ion pair arrangement in contrast to the contacted ion variants of 3-ortho and 3-meta. Aided by DFT calculations on model systems, discussion focuses on the different basicities, amido or alkyl, and steps, exhibited in these reactions, and how the structures and bonding within these isolated key metallic intermediates (prior to any electrophilic interception step), specifically the interactions involving the alkali metal, influence the regioselectivity of the Zn−H exchange process.

Pre-inverse-crowns: synthetic, structural and reactivity studies of alkali metal magnesiates primed for inverse crown formation

Two new alkali metal monoalkyl-bisamido magnesiates, the potassium compound [KMg(TMP)2nBu] and its sodium congener [NaMg(TMP)2nBu] have been synthesised in crystalline form (TMP = 2,2,6,6-tetramethylpiperidide). Devoid of solvating ligands and possessing excellent solubility in hydrocarbon solvents, these compounds open up a new gateway for the synthesis of inverse crowns. X-ray crystallography established that [KMg(TMP)2nBu] exists in three polymorphic forms, namely a helical polymer with an infinite KNMgN chain, a hexamer with a 24-atom (KNMgN)6 ring having endo-disposed alkyl substituents, and a tetramer with a 16-atom (KNMgN)4 ring also having endo-disposed alkyl substituents. Proving their validity as pre-inverse-crowns, both magnesiates react with benzene and toluene to generate known inverse crowns in syntheses much improved from the original, supporting the idea that the metallations take place via a template effect. [KMg(TMP)2nBu] reacts with naphthalene to generate the new inverse crown [KMg(TMP)2(2–C10H7)]6, the molecular structure of which shows a 24-atom (KNMgN)6 host ring with six naphthalene guest anions regioselectively magnesiated at the 2-position. An alternative unprecedented 1,4-dimagnesiation of naphthalene was accomplished via [NaMg(TMP)2nBu] and its NaTMP co-complex “[NaMg(TMP)2nBu]·NaTMP”, manifested in [{Na4Mg2(TMP)4(2,2,6-trimethyl-1,2,3,4-tetrahydropyridide)2}(1,4-C10H6)]. Adding to its novelty, this 12-atom (NaNNaNMgN)2 inverse crown structure contains two demethylated TMP ligands as well as four intact ones. Reactivity studies show that the naphthalen-ide and -di-ide inverse crowns can be regioselectively iodinated to 2-iodo and 1,4-diiodonaphthalene respectively.

Molecular Structures of THF‐Solvated Alkali‐Metal 2,2,6,6‐Tetramethylpiperidides Finally Revealed: X‐ray Crystallographic, DFT, and NMR (including DOSY) Spectroscopic Studies

The often studied THF solvates of the utility alkali-metal amides lithium and sodium 2,2,6,6-tetramethylpiperidide are shown to exist in the solid state as asymmetric cyclic dimers containing a central M(2)N(2) ring and one molecule of donor per metal to give a distorted trigonal planar metal coordination. DFT studies support these structures and confirm the asymmetry in the ring. In C(6)D(12) solution, the lithium amide displays a concentration-dependent equilibrium between a solvated and unsolvated species which have been shown by diffusion-ordered NMR spectroscopy (DOSY) to be a dimer and larger oligomer, respectively. A third species, a solvated monomer, is also present in very low concentration, as proven by spiking the NMR sample with THF. In contrast, the sodium amide displays a far simpler C(6)D(12) solution chemistry, consistent with the solid-state dimeric arrangement but with labile THF ligands.

All-solid-state ultrafast lasers facilitate multiphoton excitation fluorescence imaging

Improvements in ultrafast laser technology have enabled a new excitation mode for optical sectioning fluorescence microscopy: multiphoton excitation fluorescence imaging. The primary advantages of this technique over laser scanning confocal imaging derive from the localized excitation volume; additional advantages accrue from the longer wavelength of the excitation source. Recent advances in all-solid-state, ultrafast (subpicosecond) laser technology should allow the technique to gain widespread use as a commercial instrument. In this paper, we review: optical sectioning fluorescence microscopy, multiphoton excitation fluorescence laser scanning microscopy, developments in laser physics which have enabled all-solid-state lasers to be used as excitation sources for multiphoton excitation fluorescence imaging, and provide current data for all-solid-state ultrafast lasers. A direct comparison between confocal (488 nm) imaging and two-photon excitation (1047 nm) imaging of a mouse brain slice stained with the lipophilic dye FM4-64 shows two-photon imaging can provide usable images more than twice as deep as confocal imaging. Multi-mode images (both two- and three-photon excitation) are presented for fixed and living cells as examples of multiphoton excitation fluorescence imaging applied to developmental biology. Also, a comparison of the axial resolution of our system is presented for confocal imaging (488 nm) and two-photon imaging (1047 nm) with and without a confocal pinhole aperture.

X-ray crystallographic and solution studies of the pentamethyldiethylenetriamine and tetramethylethylenediamine adducts of lithium diphenylphosphide

X-ray crystallographic studies on the lithium diphenylphosphide adducts (Me2NCH2CH2)2NMe2 · LiPPh2 ( 1) and Me2NCH2CH2NMe2 · LiPPh2 (2) are reported. 1 is monomeric, with a terminal PPh2 unit containing a pyramidally coordinated phosphorus atom attached to the four-coordinate metal atom by a Li-P bond of length 2.567(6) A. 2 crystallizes as dimers, (Me2NCH2CH2NMe2 · LiPPh2)2, with bridging PPh2 units containing (distorted) tetrahedrally coordinated phosphorus atoms: their planar (LiP)2 rings are roughly square-shaped (mean PLiP angle 91°, mean LiP distance 2.61 A). Discussion of features of these structures is facilitated by ab initio MO calculations on the model systems LiPH2 and (LiPH2)2. Cryoscopic molecular mass measurements and high-field 7Li/31P NMR spectroscopic studies on solutions of 1 and 2 indicate that both solid-state structures are retained in arene solution, though some dissociation of2 into monomers is apparent.