Donald Barr

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.

X-ray crystallographic studies and comparative reactivity studies of a sodium diisopropylamide (NDA) complex and related hindered amides

Two related sodium amide complexes derived from secondary amines with bulky organic substituents have been synthesised and crystallographically characterised. Both [(Pr-i)(2)NNa(TMEDA)](2) and [Cy(Pr-i)NNa(TMEDA)](2) adopt dimeric crystal structures with a central, planar (nitrogen-metal)(2) azametallocycle, a now familiar feature in both lithium amide and sodium amide chemistry. TMEDA ligands chelate in their usual bidentate manner making the Na+ cations four-coordinate with a distorted tetrahedral geometry. In the latter complex, the amido substituents are disposed in a trans conformation with respect to the (NNa)(2) ring plane. The deprotonating ability of the former complex has been tested against that of the parent amide [(Pr-i)(2)NNa](infinity) and the lithium congener [(Pr-i)(2)NLi](infinity) (LDA) in a series of simple organic reactions: selective enolate formation from 2-octanone and 2-methylcyclohexanone; synthesis of diphenylacetic acid via diphenylmethane. In general, the performance of the sodium reagents compares favourably with that of the lithium reagent.

The ring-stacking principle in organolithium chemistry: its development through the isolation and crystal structures of hexameric iminolithium clusters (RR'C=NLi)6 (R'=Ph, R=But or Me2N; R=R'=Me2N or But)

Synthetic and X-ray crystallographic studies on two new iminolithium compounds, [But(Ph)CNLi]6(1) and [Me2N(Ph)CNLi]6(2), have revealed, in conjunction with previously reported work on [(Me2N)2CNLi]6(3) and (But2CNLi)6(4), a family of hexameric iminolithium species. All four compounds have remarkably similar solid-state structures based on slightly folded chair-shaped Li6 cores, the six smaller (isosceles) Li3 triangular faces of which are bridged by three-electron imino ligands through electron-deficient bonds. The precise determination of the structures of (1) and (2) has revealed features which were not apparent in the earlier limited and less fully refined structural studies on (3) and (4). Recognition of such features and their detailed analysis leads to a ring-stacking principle which envisages the formation of these hexameric structures from two slightly puckered trimeric rings, (RR′CNLi)3, brought together in a staggered arrangement. Extension of this principle allows the rationalisation of many other structures in lithium chemistry and facilitates structural predictions.

Crystal structures of (Ph2CNLi·NC5H5)4 and [CILi·OP(NMe2)3]4; discrete tetrameric pseudo-cubane clusters with bridging of Li3 triangles by nitrogen and by chlorine atoms

X-Ray crystallographic studies of the title compounds (Ph2CNLi·NC5H5)4, (1), and [CILi·OP(NMe2)3]4, (2) show them to have pseudo cubane tetrameric structures in which triply-bonding methyleneamino -nitrogen atoms [(1)] or chlorine atoms [(2)] hold together the tetrahedral Li4 frameworks.

The stabilisation of water as a ligand in lithiated organic compounds: intra- and inter-molecular hydrogen bonding in the structures of the aquo complexes [Li·HMPA·H2O]2 and [Li·HMPA]2·H2O

Abstract The feasibility of using water as a ligand within lithiated organics has been confirmed through the syntheses and crystal structures of the dimeric 2-mercaptobenzoxazoyllithium complexes [ C 6 H 4 O(C⎓S)⎓N Li·HMPA·H2O]2 (1) and [ C 6 H 4 O(C⎓S)⎓N Li·HMPA]2·H2O (2) which exhibit intramolecular hydrogen bonding between H2O and the Sδ− centre of the organic anion and also intermolecular hydrogen bonding, which serves to sew these dimers into extended arrays: ab initio MO calculations on model aquo species have clarified the effects of H2O complexation and hydrogen bonding, in particular, by demonstrating that incorporated H2O molecules are activated, e.g. towards further metallation.

Bonding implications of interatomic distances and ligand orientations in the iminolithium hexamers [LiNC(Ph)But]6 and [LiNC(Ph)NMe2]6: a stacked-ring approach to these and related oligomeric organolithium systems

The ligand orientations and Li–N distances in the title compounds show that their µ-3-imino units NC(Ph)R (R = But or NMe2) function as 3-electron ligands, forming one 2-centre LiN bond and one 3-centre Li2N bond to isosceles triangles of bridged metal atoms, prompting treatment of each hexamer [LiNC(Ph)R]6 as a pair of stacked cyclic trimers [LiNC(Ph)R]3; extension of this ring-stacking principle allows many other structures to be rationalised in lithium chemistry and facilitates structural predictions.

Trimeric dibenzylamidolithium and its dimeric diethyl ether and hexamethylphosphoramide complexes: structural and theoretical studies of reactive organonitrogen–lithium oligomers

The coloured, Crystalline title compounds have properties likely to make them synthetically useful reagents while their isolation and structural characterisation, with the aid of X-ray crystallagraphy and MO calculations, allows predictive rules to be forward for the probable structures and reactivities of other organonitrogen-lithium species.

A unique lithium halide complex, [(LiBr)2·3HMPA·toluene]: synthesis from reaction of solid NH4Br with BunLi in HMPA (hexamethylphosphoramide)/toluene media, crystal structure showing two Li–Br units linked by three µ2-HMPA ligands, and detection of 7Li ⋯31P coupling in solutions of the complex

Reaction of solid NH4Br with BunLi and HMPA (hexamethylphosphoramide)(1 : 1 : 2 ratio) in toluene produces (LiBr)2·3HMPA·toluene, (1), shown by X-ray crystallography to contain (LiBr)2·3HMPA molecules whose two terminal Li–Br units are linked by three µ2-HMPA ligands and with toluene molecules lying between these units in the lattice; in arene solutions, the (LiBr)2·3HMPA units remain intact, giving rise in 7Li n.m.r. spectra to 1 : 3 : 3 : 1 quartets caused by through-bond (OP) coupling of each 7Li nucleus to three 31P(HMPA) centres.

Three model structural types in lithium amide chemistry: the crystal and molecular structures of dimeric [Ph(Me)NLi·TMEDA]2(TMEDA = Me2NCH2CH2NMe2) and monomeric Ph(naphthyl)NLi·PMDETA [PMDETA = MeN(CH2CH2NMe2)2], and of [Ph(naphthyl)NLi·TMEDA]2, a ‘loose dimer’

The crystal structures of three amidolithium complexes are reported, those of [Ph(Me)NLi·TMEDA]2(TMEDA = Me2NCH2CH2NMe2), (1), and Ph(naphthyl)NLi·PMDETA [PMDETA = MeN(CH2CH2NMe2)2], (2), whose Li atoms are all four-co-ordinate, confirming predicted structural types, while a new structural option is found for [Ph(naphthyl)NLi·TMEDA]n, (3), whose monomeric (n= 1) moolecules, each containing a three-co-ordinate Li atom, are prompted to associate looselyin pairs (n= 2)via intermolecular Li ⋯ phenyl interactions.

Experimental and molecular orbital calculational study of the stereoselective Horner—Wittig reaction with phosphine oxides: Control of stereoselectivity by lithium

Abstract Ab initio calculations have been used to determine the possible structure of lithium derivatives of phosphine oxides in THF: extension of these calculations to the docking of a carbonyl compound onto such a lithium derivative suggests an explanation for the anti selectivity of the Horner—Wittig reaction.