Ronald Snaith

Structures of Organonitrogen—Lithium Compounds: Recent Patterns and Perspectives in Organolithium Chemistry

Publisher Summary This chapter discusses the current patterns and perspectives in organolithium chemistry, including the structures of organonitrogenlithium (N-Li) compounds. The chapter explains the compounds with N-Li bonds. These are chiefly lithium imides [iminolithiums (RR’C=NLi) n ] and their complexes with added Lewis bases (L), and lithium amides [amidolithiums (RR’NLi) n ] and their complexes. For the lithium amide species, particularly, only those whose R,R’ groups that do not contain additional functionalities are described—that is, the R,R’ groups remain largely uninvolved with lithium centers. These species are termed “simple” lithium amides. N-Li compounds and, particularly, lithium amides (R 2 NLi) are widely used both in organic and in organometallic syntheses. For the former, these strong bases are employed as proton abstractors to generate new organolithiums. Although synthetic uses have dominated the interest in N-Li compounds, the chapter focuses on the structures. Most of the physical properties of organolithium compounds (the marked exception being conductance) arise, because of the overall size and shape of the units making up these materials and the nature of the peripheries of these units. The basic structural building block of any organolithium is an ion pair, R – L + . The identities and structures of complexed organolithiums are of particular importance.

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.

Towards an understanding of the conjugate addition of organolithium reagents to α,β-unsaturated ketones: the isolation and solid-state structure of a monomeric lithium aluminate with very short agostic Li⋯HC interactions

Abstract Reaction of methylaluminium bis(2,6-di-tert-butyl-4-methylphenoxide) (MAD), 1, with alkyllithium reagents, R′Li (R′=Me, n-Bu or t-Bu), yields the solvent-dependent products lithium bis(2,6-di-tert-butyl-4-methylphenoxide)-THF complex, 2·THF, lithium dimethylbis(2,6-di-tert-butyl-4-methylphenoxide)aluminate, 3, a new type of lithium aluminate in which the lithium centre is stabilised by very short agostic Li⋯H(t-Bu) interactions, and tris(alkyl)aluminium. The observation of these products suggests an explanation for the tendency of α,β-unsaturated ketones to undergo conjugate (rather than 1,2-) addition in the presence of MAD and organolithium reagents.

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.

Azomethine derivatives. Part 20. Crystal and molecular structures of the lithioketimine [{Li(NCBut2)}6] and lithioguanidine [{Li[NC(NMe2)2]}6]; electron-deficient bridging of Li3 triangles by methyleneamino-nitrogen atoms

The lithioketimine Li(NCBut2)(1) and lithioguanidine Li[NC(NMe2)2](2) have remarkably similar hexameric structures [{Li(NCR2)}6](R = But or NMe2) in the crystal phase, based on slightly folded chair-shaped Li6 rings held together by triply-bridging methyleneamino-groups, NCR2, thus providing examples of electron-deficient bridging by the nitrogen atoms of organonitrogen ligands. The mean distance between adjacent metal atoms in the Li6 rings is 2.35(2)A in (1), and 2.445(2)A in (2), and the mean dihedral angles between Li6 chair seats and backs are 85 and 78° respectively. The nitrogen atoms of the bridging methyleneamino-groups are approximately equidistant from the three bridged metal atoms, the mean Li–N distance being 2.06(1)A in (1) and 2.00(1)A in (2). The NC distances of 1.30(1) and 1.244(3)A respectively lie within the range expected for carbon–nitrogen double bonds. Features of these structures are compared with those of related compounds, and some bonding implications are discussed.

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.

THE FIRST SOLID-STATE STRUCTURE OF A MIXED-ANION ROLI/LIOH COMPOUND : (TBUOLI)10.(LIOH)6

Abstract X-Ray analysis of the title compound ( t BuOLi) 10 · (LiOH) 6 ( 1 ) which was obtained by prolonged exposure of a concentrated t BuOLi/hexane solution to moist air, has revealed a mixed-anion aggregate structure comprised of two hexagonal ( t BuO) 5 (OH)Li 6 subaggregates and a crossed LiOH ladder structure.

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 first crystallographic evidence for the structures of ortho-lithiated aromatic tertiary amides

Herein is reported the first crystal structures of the products of tertiary amide directed orthometalation reactions. Thus, reaction of N,N-diisopropylbenzamide with tBuLi in di-Et ether gives the corresponding N,N-diisopropyl-2-lithiobenzamide-diethylether complex, 4. Reaction of N,N-diisopropyl-1-naphthamide in THF with tBuLi gives the corresponding N,N-diisopropyl-2-lithionaphthamide-THF complex, 5. The crystal structures of 4 and 5 are presented. [on SciFinder (R)]