Metal amides are amongst the most important classes of chemical compounds. These molecules have an extensive chemistry, with considerable and versatile applications as synthetic intermediates, reagents, catalysts and sources of valuable materials, including in nanotechnology.

Metal Amide Chemistry is a comprehensive overview of these important compounds, written by leading experts in the field. An introduction to the chemistry of these molecules is followed by in-depth discussions of the amide compounds of:

• alkali metals
• alkaline earth metals
• group 3 and lanthanide metals
• the actinide metals
• the transition metals
• zinc, cadmium and mercury
• group 13 metals
• silicon and the group 14 metals (subvalent compounds)
• group 15 metals

Metal Amide Chemistry is an essential guide to this important class of compounds for researchers and advanced students in academia and industry working in synthetic organometallic, organic and inorganic chemistry, materials chemistry and catalysis.

Michael Lappert is Emeritus Research Professor of Chemistry at the University of Sussex. A graduate of Northern Polytechnic, his B.S.c. was followed by a Ph.D. (with W. Gerard to which in 1960 he added a D.Sc. (University of London). He has been at Sussex since 1964, having previously been at UMIST (1959-1964). He was the recipient of the first chemical Society Award for Main Group Metal chemistry (1970) and then of the chemistry (1976); with the RSC (he was its Dalton Division President, 1989-1991), he has been a Tilden (1972), Nyholm (1994), and Sir Edward Frankland (1998) Medallist and Lecturer. In 2008, he was awarded the GDCh Alfred-Stock Prize. he was elected FRS in 1979 and was award an honorary doctorate from Ludwig-Maximilians-Universitat (Munchen, 1989). With co-workers he has published 2 books, edited 4 others, and more than 780 papers and reviews and a few patents on various aspects of inorganic and organometallic chemistry.

Philip Power received a b.A. from trinity college Dublin in 1974 and a D. Phil, under M. F. Lappert from the University of Sussex in 1977. After postdoctoral studies with R.H. Holm at Stanford University, he joined the faculty at the department of Chemistry at the University of California at Davis in 1981, where he is a Distinguished Professor of chemistry. His main interests lie in the exploratory synthesis of new main-group and transition-metal complexes. A major theme of his work has been the use of sterically crowded ligands to stabilize species with new types of bonding, low coordination numbers, and high reactivity. He has been the recipient of several awards including the A. P. Sloan Foundation Fellow (1985), Alexander von Humboldt Fellowship for Senior U. S. Scientist (1992), Ludwig Mond Medal of the Royal Society of Chemistry (2005), F.A. Cotton Award in Synthetic Inorganic Chemistry of the American Chemical Society (2005) and was elected Fellow of the Royal Society of London (FRS) (2005).

Andrey Protchenko was born in 1961 in Rostov-on-Don, Russia. A graduate of the Moscow State University (Russia) in 1983, he carried out his doctoral work at the Karpov Institute of physical Chemistry (Moscow) and completed his Ph.D. in chemistry (1997) at the Razuvaev Institute of Organometallic Chemistry (Nizhny Novgorod) under the supervision of Prof. M. N. Bochkarev studying naphthalene complexes of the lanthanides. In 1998 he joined the group of Prof. M. F. Lappert at the University of Sussex (England) working on a series of projects (Royal Society, EPSRC and Leverhulme trust grants) involving lanthanide redox chemistry based on cyclopentadienyl, N- and N, N’ -centred ligands.

Alexandra Seeber, nee Pickering, was born in 1979 in Lichfield, UK. After graduating in chemistry at Keele University in 2001 and completing a Ph.D. in Inorganic Chemistry into the design and synthesis of ligands for the controlled self-assembly of tr5ansition metal complexes at the University of Glasgow (Scotland) in 2004, she carried out postdoctoral research into nanoparticle synthesis at the University of California, Davis with Prof. Philip Power under a Department of energy grant for the Design of high surface area materials as hydrogen storage vessels. Alexandra Seeber has worked since 2006 in Research and development in the catalysis department at BASF in Ludwigshafen (Germany).

Metal Amide Chemistry

By Michael Lappert Andrey Protchenko Philip Power Alexandra Seeber

John Wiley & Sons

Copyright © 2009 John Wiley & Sons, Ltd
All right reserved.
ISBN: 978-0-470-72184-1

Chapter One

Introduction

1.1 Scope and Organisation of Subject Matter

Our principal focus is to describe the synthesis, structure, reactions and applications of metal amides and related compounds which are stable at ambient temperatures, and particularly those which are mono-, di- or oligonuclear. Thus, the main thrust is on compounds having one or more [bar.N]](R)R’ ligands attached to a metal M. Such species are now known for all the natural elements (and Np and Pu), except for the lighter rare gases. In the present treatise, M is restricted to an element of Groups 1, 2, 3 and a 4f metal, a lighter 5f metal, and a metal of Groups 4 to 11, 12, 13 (except B), 14 (excluding [M.sup.IV] compounds as reagents), and 15 (excluding P); these are described in Chapters 2 to 10, respectively.

The amido ligand may be [bar.N][H.sub.2], [bar.N]](H)R, and [bar.N]](R)R’, in which R and R’ are the same or different, and each is an alkyl, aryl, or silyl (particularly Si[Me.sub.3]) group; selected N[H.sub.2]-metal compounds feature mainly in Chapters 2, 3, 6, 7 and 8. The amido ligand may be bound to the metal in a terminal or bridging (double or single) fashion; examples of the latter are [[Sn(N[Me.sub.2])(-N[Me.sub.2])].sub.2] and [[Cu(-N[Me.sub.2])].sub.4]. Within our scope for main group and f-elements are bi- and tri-dentate ligands, including not only N,N’– and N,N’,N”-centred species such as 1; 2-[C.sub.6][H.sub.4][([bar.N]C[H.sub.2][Bu.sup.t]).sub.2] and MeC[{Si[(Me).sub.2][bar.N][Bu.sup.t]}.sub.3] but also others with a single amido site such as 1. Tetradentate encapsulating ligands are excluded; their more natural home is to be found for the most part in textbooks of organic and biological chemistry and, more recently, materials chemistry. Amide-free metal complexes containing some N,N’-bidentate ligands which are neutral (e.g. bipy) or monoanionic (e.g. amidinates, guanidinates, or -diketiminates) are also outside our scope.

Imides are usually included for main group elements because bi- and oligonuclear metal imides are ubiquitous and were often developed in parallel with the amides. They generally feature trivalent nitrogen, as in [[As(Cl)( -N[bu.sup.t])].sub.2]. Imido derivatives of the transition metals are not widely covered, as their chemistry is quite extensive and distinct from that of the amides. In general mononuclear metal imides are more frequent in d– and f-block chemistry; examples are [Ti(=N[C.sub.6][H.sub.3][Pr.sup.i.sub.2]-2,6) (N[Me.sub.2])[(NH[Me.sub.2]).sub.3]][B[([C.sub.6][F.sub.5]).sub.4]] or [U{[(N(Si[Me.sub.2][Bu.sup.t])C[H.sub.2]C[H.sub.2]).sub.3]N}(=Nsi[Me.sub.3])], but rare examples are also found in main group metal compounds having bulky ligands, as in [In{[C.sub.6][H.sub.3][([C.sub.6][H.sub.3] [Pr.sup.i.sub.2]-2′,6′).sub.2]-2,6}-{=N[C.sub.6][H.sub.3][([C.sub.6][H.sub.3] [Me.sub.2]-2′,6′).sub.2]-2,6}]. The majority of metal amides and related compounds are neutral, but cationic and anionic complexes have featured; examples are [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]. Compounds considered are mainly homometallic, but several are heterometallic such as the inverse crown complex [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII].

1.2 Developments and Perspectives

In 1980, metal and metalloid amides were already known for 57 elements, but then excluded were amides of the following elements: Sr, Ba, Tb, Dy, Er, Tm, Ru, Os, Rh, Ir and Pd (for several others, the first publications were post-1970: Sc, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Ho, Yb, Lu, Re, In and Tl). Since then, these lacunae have been filled. The variety of metal oxidation states for metal amides has also greatly expanded since 1980. The literature on metal amides has likewise burgeoned. For example, in the 1980 book there were 119 and 317 bibliographic citations for Group 1 and a conglomerate of Group 3 and 4f/5f and d-metal amides, respectively, whereas there are now 263 and 1110 for these two sections, respectively. Our aim has been to provide a comprehensive, but not exhaustive, coverage of the field of metal amides, with emphasis on the post-1980 developments through to the end of 2007. The total number of references for the succeeding Chapters 2 to 10 is in excess of 2500, while for these topics there were 923 citations in 1980.

Tables 1.1 and 1.2, laid out in the form of a Periodic Table, provide (i) a list of the elements, (ii) in brackets after each entry, the metal oxidation states for its amides, and (iii) the number of literature citations for each group of metals. Highlighted in bold are those metals for which amides were reported in the post-1979 period, while in italics are those metal oxidation states which likewise are of more recent date.

The variety of amido ligands now in use is very large. Many are bulky and often are free of -hydrogen atoms. Among the newer monodentate amides are [bar.N][Cy.sub.2] (Cy = cyclohexyl), [bar.N][(SiH[Me.sub.2]).sub.2], [bar.N][(Si[Me.sub.2][Bu.sup.t]).sub.2], [bar.N][(Si[Me.sub.3])[C.sub.6][H.sub.3][Pr.sup.i.sub.2]-2,6], [bar.N][(Si[Me.sub.3]){[C.sub.6][H.sub.3][([C.sub.6][H.sub.2] [Me.sub.3]-2′,4′,6′).sub.2]-2,6}], and [bar.N][Ad[C.sub.6][H.sub.3] [Me.sub.2]-3,5)]. Bi- and tridentate amides include not only those which are solely N-centred such as [bar.N](R)CH=CH[bar.N]R or X[{C[H.sub.2]C[H.sub.2] [bar.N][Bu.sup.t]}.sub.3] (X = Si[Bu.sup.t] or N), but others in which the ligating atoms are not exclusively nitrogen: for illustrations of such ligands see 6-12 and 26-54 in Chapter 4.

Numerous amidometallates (several as cluster compounds) and, more rarely, cationic complexes, are well established. Representative examples of such salts are [[KMg[(N[Pr.sup.i.sub.2]).sub.3]].sub.[infinity]], [Ti[(N[Me.sub.2]).sub.3] [(py).sub.2]][B[Ph.sub.4]], [[Pt(-Cl){SnCl[(N[R.sub.2]).sub.2]} (P[Et.sub.3])].sub.2] (R = Si[Me.sub.3]), [Li[(thf).sub.4]] [Ni[(N[Ph.sub.2]).sub.3]], [U[(N[Et.sub.2]).sub.2][(py).sub.2]] [[B[Ph.sub.4]].sub.2], [As([C.sub.5][Me.sub.5])(N[Me.sub.2])][Al[Cl.sub.4]] and Li[[Sb.sub.3][(-NCy).sub.4][(N[Me.sub.2]).sub.2]].

A feature of the post-1980 literature on mononuclear metal amides has been the discovery of numerous low-coordinate metal complexes. Significant examples among neutral compounds are the crystalline unicoordinate Group 13 metal and the dicoordinate [Si.sup.II] and Group 15 metal amides [Ga{N(Si[Me.sub.3])[C.sub.6][H.sub.3] [Mes.sub.2]-2,6}], [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] and [As{N(H)Mes*}(=NMes*)] (Mes = [C.sub.6][H.sub.2][Me.sub.3]-2,4,6; Mes* = [C.sub.6][H.sub.2][Bu.sup.t.sub.3]-2,4,6). The highest metal coordination number recorded for a homo- or heteroleptic metal amide containing only monodentate ligands is six, in [M[(N[Me.sub.2]).sub.6]] (M = Mo or W), or seven, in [U[(N[Et.sub.2]).sub.2][(py).sub.5]][[B[Ph.sub.4]].sub.2], respectively. The highest metal oxidation state of +6 is found not only in the Mo and W hexa(dimethylamides) but also for U, originally in trans-[U[(O).sub.2]- [{N[(Si[Me.sub.3]).sub.2]}.sub.2][(thf).sub.2]]; U[(N[Me.sub.2]).sub.6] was found to be thermally labile.

Bi-, tri- and oligonuclear compounds have ring, fused ring, or cluster core geometries, which feature [bar.N](R)R’ as bridging ligands. For oligonuclear aggregates, the fused rings are often arranged as ladders or stacks, descriptions which were originally coined in the context of such alkali metal amides. The addition of a neutral ligand often fragments such an aggregate, a simple example being the conversion of [[Li{-N[(Si[Me.sub.3]).sub.2]}].sub.3] into 3[Li-{N[(Si[Me.sub.3]).sub.2]}(tmeda)] (tmeda = [Me.sub.2]NC[H.sub.2]C[H.sub.2]N[Me.sub.2]).

A wider range of amido-containing complexes is of mixed metals, while others have imido bridges. Illustrations are provided by [[NaMg[(N[Pr.sup.i.sub.2]).sub.2](O[Bu.sup.n])].sub.2] (see Chapter 3, Figure 3.5) and [[Li.sub.3][(thf).sub.3][Sb.sub.6][(NCy).sub.6][(N).sub.3]Li(N=NH)] (see 29 in Chapter 10). In the latter compound, each of the bridging cyclohexylimido or nitrido ligands has a coordination number [greater than or equal to] 3 and hence may be regarded as a bis- or tris-(metallo)amide, respectively; the diradicaloid compound [[SnCl([]-Nsi[Me.sub.3])].sub.2] is a simpler compound having bridging imido ligands in which the nitrogen atoms are three-coordinate.

Prior to 1980, 11 methods of synthesis of metal (M) amides were described and continue to be used. The salt elimination procedure [[L.sub.n]MX + M'{N(R)R’}] is the most general, usually with X = Hal and M’ = an alkali metal; whereas lithium is often the metal of choice, employing M’ = Na or K has the advantage of easier separation of the alkali metal halide from the target metal amide. More recently used leaving groups are tosylate, triflate, aryloxide, or a cyclopentadienyl (e.g. in [Al.sup.I] or [Pb.sup.II] chemistry). Newer sources of an amido ligand are BrN[(Si[Me.sub.3]).sub.2] and a N,N’-dihydrocarbyl-1,4-diazabuta-1,3-diene.

Molecular structure elucidation, principally by single crystal X-ray diffraction, has become almost routine and is now available for the majority of the metal amides presently discussed. In the 1980 book, however, such data were provided for just 112 compounds, 54 of which were for d– and f-block metals and 41 for the Group 13 metal amides. The contrast with the developing situation is illustrated by reference to Group 1 metal amides: from four X-ray data sets in 1980 there were more than 200 by the end of 2007.

Among other physicochemical techniques which now feature prominently are those based on vibrational, electronic, NMR (including [sup.2]H, [sup.13]C, [sup.6]Li, [sup.7]Li, [sup.9]Be, [sup.27]Al, [sup.29]Si, [sup.113]Cd, [sup.119]Sn and [sup.171]Yb) and EPR spectroscopy and magnetic measurements. Computational studies are increasingly significant, particularly for subvalent metal-nitrogen compounds. The nitrogen environment is almost invariably close to trigonal, or distorted trigonal, planar for homo- and heteroleptic metal amides having terminal [bar.N](R)R’ ligands, respectively. This fact, as well as the generally relatively short M-N bond lengths (compared with the sum of the covalent radii of M and N) has often been interpreted in terms of the [bar.N](R)R’ ligand being both a [sigma]- and a [pi]-donor. However, as has increasingly become evident, other factors should also be considered, as discussed, for example, in the context of transition metal amides and bis (amino)silylenes in Chapter 6 (Section 6.2.3.1) and Chapter 9 (Section 9.2.3), respectively.

Several metal amides are much employed as reagents, ligand transfer reagents or precursors for more complex molecules. Alkali metal, and mainly lithium, bulky amides, including LiN[(Si[Me[sup.3]).sub.2], LiN[Pr.sup.i.sub.2] and LiN[Cy.sub.2], are important proton-abstractors for organic synthesis, as are recently the synergic couple MN(R)R’/M'[R.sub.n] (M’ = a Group 2, 12 or 13 metal). Magnesium, aluminium, tri(methyl)tin and, particularly, alkali metal amides are valuable ligand transfer reagents, being the most useful precursors to amides of other metals. The ready cleavage of the highly polar M-N(R)R’ bond, especially by protic reagents, make several metal amides important as (i) synthons for a wide range of that metals compounds, and (ii) materials. Examples of (i) are Sn[(N[Me.sub.2]).sub.2] or Sb[(N[Me.sub.2]).sub.3] as the source of unusual Sn-N and Sn-P complexes such as the cubane [[{Sn(-N[Bu.sup.t])}.sub.4]] and [[Li.sub.2][Sn.sub.2][(PR).sub.4][(tmeda).sub.2]] (53 in Chapter 9), or related Sb compounds, such as [[Li.sub.4][{[Sb.sub.2][(NCy).sub.4]}.sub.2]] (30 in Chapter 10). Illustration of (ii) is provided by Tables 4.1 and 4.2 of Chapter 4, showing [Ce[{N[(Si[Me.sub.3]).sub.2]}.sub.3]] as a starting material for a heteroleptic -diketiminato compound and [Y[{N[(SiH[Me.sub.2]).sub.2]}.sub.3][(thf).sub.2]] as a substrate for the formation of yttrium-centred catalysts, usually immobilised, for various organic transformations. A valuable feature for (i) and (ii) is that the amine co-product, NH[Me.sub.2] or NH[(SiH[Me.sub.2]).sub.2], is volatile. Metal amide-catalysed hydroamination/cyclisation of N-unprotected alkenes to form heterocyclic nitrogen compounds is exemplified by the role ofan optically active [Sm.sup.III] amide as an enantioselective catalyst for such a process (e.g. see Equations (4.8) and (4.12) in Chapter 4). Various d– and 4f-block metal amides have been shown to be catalysts for some important processes, including olefin di-, oligo- or polymerisation, olefin hydroamination and hydrosilylation, copolymerisation of an oxirane and carbon dioxide, and ring-opening polymerisation of lactides or [epsilon]-caprolactam. Certain d– and 4f-block metal amides are unusual in their ability to activate [N.sub.2] or [N.sub.2]O, as in the conversion of [Mo-[{N(Ad)R}.sub.3]] into [Mo(X)[{N(Ad)R}.sub.3]], respectively (R = [C.sub.6][H.sub.3][Me.sub.2]-3,5 and X = N or NO), or in the reactions of [Zr(F)[Cl.sub.2]] with K[C.sub.8] and [N.sub.2] (F is a macrocyclic P,P’,N,N’-centred bis-(amido) ligand) (see Schemes 6.3, 6.4 and 6.7 in Chapter 6).

(Continues…)

Excerpted from Metal Amide Chemistryby Michael Lappert Andrey Protchenko Philip Power Alexandra Seeber Copyright © 2009 by John Wiley & Sons, Ltd. Excerpted by permission.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.