“… a series of thoughtfully written, specialised and topical accounts of different areas of organometallic research … a good source of ideas … ” Journal of Organometallic Chemistry, Vol 689, Issue 10, 15 May 2004

Perspectives in Organometallic Chemistry

By C. G. Screttas, B. R. Steele

The Royal Society of Chemistry

Copyright © 2003 The Royal Society of Chemistry
All rights reserved.
ISBN: 978-0-85404-876-2

Contents

Group 15 element imido and phosphido cages; Coordination chemistry and synthetic applications E.L Doyle, A.D. Hopkins, G.T. Lawson, M. McPartlin, A.D. Woods and D. S. Wright, 1,
Neutral clusters EnRn of the monovalent elements gallium and indium. Recent results in synthesis and reactivity W. Uhl, 16,
New titanium imido chemistry with polydentate N-donor ligands P. Mountford, 28,
Organometallic complexes with 1,2-dichalcogenolate-o-carboranes Guo-Xin Jin, 47,
Synthesis and reactivities of multinuclear sulfur-bridged metal complexes ranging from dinuclear to hexanuclear cores M. Hidai, 62,
α,ω-Bis[(triphenylphosphine)gold(I)] hydrocarbons K.A. Porter, A. Schier and H. Schmidbaur, 74,
Researches on non-classical organolanthanide chemistry P.B. Hitchcock, A.G. Hulkes, A.V. Khvostov, M.F. Lappert and A.V. Protchenko, 86,
Hyper-structured alkynylruthenium complexes: Effect of dimensional evolution on NLO properties M.G. Humphrey, M.P. Cifuentes, M. Samoc, T. Isoshima and A. Persoons, 100,
Cycloaddition of alkynes mediated by [RuCp(L)]+ (L = CO, NCH, PH3) and RuCpCl complexes – Metallacyclopentatrienes as key intermediates – A DFT study M.J. Calhorda, K. Kirchner and L.F. Veiros, 111,
Selective C-C coupling reactions of Me2N-C [equivalent to] C-NMe2 at iron(0) centers A.C. Filippou, T. Rosenauer and G. Schnakenburg, 120,
Routes to fluorinated organic derivatives by nickel mediated C-F activation of heteroaromatics T. Braun and R.N. Perutz, 136,
Novel η5 – η6 rearrangement of bis(fluorenyl)lanthanide complexes by the addition of AIR3 H. Yasuda, 152,
Results and perspectives of high oxidation state organomolybdenum chemistry in water E. Collange, F. Demirhan, J. Gun, O. Lev, A. Modestov, R. Poli, P. Richard and D. Saurenz, 167,
Modulation of electronic behaviour of metal carbonyl clusters D. Collini, C. Femoni, M.C. Iapalucci, G. Longoni and P. Zanello, 183,
Interionic and intermolecular solution structure of transition metal complexes by NMR A. Macchioni, 196,
Synthetic and mechanistic pathways in platinum(II) chemistry R. Romeo and L. Monsù Scolaro, 208,
New perspectives for olefin complexes: Synthesis and characterisation of stable rhodium(0)and iridium(0) complexes J. Harmer, G. Frison, M. Rudolph, H. Schönberg, S. Deblon, P. Maire, S. Boulmâaz, F. Breher, C. Böhler, H. Rüegger, A. Schweiger and H. Grützmacher, 222,
Substitution and addition reactions catalyzed by transition metal complexes I. P. Beletskaya, 240,
Late transition metal (Co, Rh, Ir)–siloxide complexes – Synthesis, structure and application to catalysis B. Marciniec, I. Kownacki, M. Kubicki, P. Krzyzanowski, E. Walczuk and P. Blazejewska-Chadyniak, 253,
Cheap chiral ligands for asymmetric transition metal catalyzed reactions M.T. Reetz, 265,
Chiral metal complexes in asymmetric catalysis C. Moberg, O. Belda, K. Hallman, R. Stranne, M. Svensson, J.L Vasse, T. Wondimagegn and R. Zalubovskis, 275,
In search of asymmetric propargylic substitution reactions mediated by optically active indenyl-ruthenium(II) allenylidene complexes V. Cadierno, S. Conejero, M.P. Gamasa and J. Gimeno, 285,
Recent developments on hydride iridium triisopropylphosphine complexes: [IrH2(NCCH3)3(PiPr3)]BF4 as hydrogenation catalyst L.A. Oro, E. Sola and J. Navarro, 297,
Pd complex-catalyzed ring-opening polymerisation of 2-aryl-1-methylene-cyclopropanes S. Kim, D. Takeuchi and K. Osakada, 306,
Subject Index, 317,

CHAPTER 1

GROUP 15 ELEMENT IMIDO AND PHOSPHIDO CAGES; COORDINATION CHEMISTRY AND SYNTHETIC APPLICATIONS

Emma L. Doyle, Alexander D. Hopkins, Gavin T. Lawson, Mary McPartlin, Anthony A. Woods, Dominic S. Wright

1 INTRODUCTION

This review details recent developments in the synthesis and coordination chemistry primarily of Group 15 imido and phosphido cages containing a variety of anionic arrangements. The review will concentrate on the applications of these Group 15 anionic ligands in organometallic chemistry, and essentially follows the theme of the lecture given at the XXth International Conference on Organometallic Chemistry (Corfu, 2002). Further aspects of this work have been published in separate review articles.

2 MIXED (OR STEP-WISE) METALLATION

In contrast to the alkali metal organometallics (such as the ubiquitous “BuLi), the organometallics of the later p block elements (E = Group 14, Sn, Pb; Group 15, As-Bi) are significantly less polar. As a consequence previous synthetic strategies to imido (RN2-) complexes of the later main group metals had been mainly limited to procedures involving condensation with Group 15 halides (eqn. 1), desilylation with SiR3 reagents (eqn. 2), or (in rare cases) reactions of alkali metal RN2- reagents with p block element salts (eqn. 3). In view of this background it is perhaps not surprising that until fairly recently very few imido complexes of the later p-block metals had been structurally characterised.

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The aim at the beginning of our studies in this area was to develop a range of p-block metal reagents which were strong enough bases to effect smooth deprotonation of primary amines, allowing direct access to complexes containing the RN2- dianion. We showed in preliminary studies that Sb(NMe2)3, which is readily prepared in high yield via the reaction of LiNMe2 with SbCl3 (eqn. 4), will doubly-deprotonate a broad range of primary amines even at low temperature (eqn. 5). Similar dimers of the type [Me2NSb(µ-NR)]2 are isolated from these high-yielding reactions.

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (4)

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (5)

The key point as regards further developments in this field to p-block metal anion arrangements is that the extremely basic nature of reagents like Sb(Nme2)3 contrasts with the lower basicity generally observed for alkali metal organometallics (R’M). In the absence of conjugation within the organic group of the primary amine only single deprotonation will follow (giving RNHM). Thus, there was the possibility that step-wise deprotonation of the primary amine, first with R’M then with Sb(NMe2)3, would lead to mixed alkali metal/p-block element imido complexes. This scenario is shown in Scheme 1.

In practice the strategy of step-wise (or mixed-) metallation of primary amines (and, indeed, of primary phosphines) employing Group 15 bases of the type E(NMe2)3 works very well. Alkali metal cage compounds containing the trianions [E(NR)33] (type I), the dianions [E2(NR)42-] (type II), and the monoanions [Me2NE(µ-NR)}2E]- (type III), can be readily obtained (Figure 1). A significant point in regard to the selection of a particular anion type is that the precise anion unit obtained by these reactions (the syntheses of which are discussed in more detail later) depends on the synthetic route used. In this sense, the reaction products are kinetically controlled and a particular desired ligand grouping can be targeted by the choice of reaction conditions.

3 STRUCTURE AND ORGANOMETALLIC REACTIONS OF IMIDO COMPLEXES

The synthesis of trianionic frameworks of type I is the most obvious reaction sequence, involving the 3:1 reaction of a primary amido alkali metal precursor (RNHM) with Sb(NMe2)3 (eqn. 6a). In the case of the As(III) analogues, a modified reaction sequence is required owing to the lower basicity of As(NMe2)3, involving reaction of the amine with As(NMe2)3 followed by deprotonation with nBuLi (eqn. 6b). Several structurally characterised examples containing Li have been prepared. All of these have similar structures in which two [E(NR)3]3- anions are associated by six Li+ cations, e.g., [{Sb(NCy)3}2Li6.2Me2NH] (1) (Figure 2). In most cases the central N6Li6 ‘stack’ arrangement within these complexes is retained even in the presence of extensive Lewis base solvation. The [E(NR)3]3- anions (E= Sb, As) are valence isoelectronic with the Group 16 dianions [E(NR)3].

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (6a)

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (6b)

Bearing out the view that the [Sb(NR)3]3- trianions are the robust chemical constituents of alkali metal cages of this type, these anions are transferred intact in reactions with a range of main group and transition metal precursors. For example, the reaction of [Cp2Pb.TMEDA] (2) with 1 gives the heterometallic cage [{Sb(NCy)3}2Pb3] (3) (eqn. 7) (Figure 3), in which the three PbII centres replace the six Li+ cations at the centre of the cage structure. The predominant bonding within the Sb2N6Pb3 core of 3 is undoubtedly between the N and Sb and Pb centres. However, it is of value to note here, in relation to the later discussion of the behaviour of phosphide analogues, that the Pb3Sb2 metal core of the complex would (if an isolated fragment) conform to Wade’s rules (an n+1, closo polyhedron).

Dianions can be accessed via the 1:2 stoichiometric reactions of dimers of the type [Me2NE(µ-NR)]2 (E= As, Sb, Bi) with RNHM (eqn. 8). The structures of [{E2(NCy)4}2Li4] (E = Sb, As) (4) (Figure 4) consist of two [E2(NCy4)]2- dianions that are associated by four Li+ cations (adopting a tetrahedral Li4 arrangement at the centre of the cage). This arrangement can be described as arising from the association of two E2N4Li2 cubane units, a view that is supported by the dissociation of the complexes into these units in arene solutions. Unlike the alkali metal complexes of [Sb(NR)3]3- trianions, Lewis base solvation of the Li+ cations results in dissociation of the cubane constituents. This is seen in the formation of the discrete cubane [Bi2(NtBu)4Li2.2thf] (5) from the reaction of Bi(NMe2)3 with tBuNHLi in thf as the solvent.

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The reactivity patterns of alkali metal cages containing [E2(NR4)]2- dianions mirror that of the trianion counterparts, the dianions being transferred intact to other metal ions. This is illustrated here by the organometallic example of the reactions of the complexes 4 with Cp2Mn leading to the cubanes [E2(NtBu)(MnCp)2] (5) (Figure 5). Investigation of the magnetic behaviour of these complexes reveals that they are predominantly high-spin, with the magnetic properties being subtly dependent on the Group 15 element (E). The origin of this dependence stems from the geometric constraints within the [E2(NCy)4]2–ligands which effect the Mn … Mn separation and therefore the communication between the two MnII centres.

The ability for ligands of this type to influence the architecture of coordinated metal cores is illustrated most dramatically by the comparison of the structures of the two Cu complexes [{Sb2(NtBu)4}2Cu] (6) and [{As2(NtBu)4}2Cu4] (7) (Figure 6). The more compact AS2N2 ring of the [As2 (NCy)4]2- dianion leads to a different ligand coordination mode than in the Sb analogue, in which unfavourably close Cu … Cu contacts are avoided. Consequently, 7 has an unusual Cu4 butterfly arrangement at its centre whereas 6 has a square-planar Cu4 core.

Monoanions of type III can be prepared by the reaction of the salt Sb(NHR)4Li with Sb(NMe2)3 (1:2 molar equivalents), the result is a bicyclic, spiro arrangement in which the central Sb(III) atom has a 10e, pseudo-trigonal bipyramidal geometry with the two terminal Sb(III) centres being 8e, pyramidal (eqn. 9). The normal coordination mode observed in these species is illustrated in the structure of [{Me2NSb(NCy)2}2Sb]Li (8) (Figure 7), in which the alkali metal ion is coordinated by two bridging NCy groups and by the terminal Me2N groups of the monoanion ligand. The reactivity of the parent [{Me2NSb(NCy)2}2Sb]- anion is of some interest. Reaction with primary amines leads to replacement of the Me2N groups by RNH groups, with retention of the spiro structure of the original monoanion. In the structure of [{CyNHSb(NCy)2}2Sb]K.toluene (9) (Figure 8) the low coordination number of the K+ ion is made up for by agostic interactions with the Me groups of two toluene ligands.

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However, the reaction of the [{Me2NSb(NCy)2}2Sb]- monoanion with tBuOH leads to rearrangement of the spiro structure into a nido isomer. The structure of the product [{Sb(µ-NCy)}3(µ3-NCy)(OtBu)2]K.toluene (10) is illustrated in Figure 9. One of the major reasons for this rearrangement is the greater Lewis acidity of the Sb(III) centres in 10 compared to those in 9, leading to an overall desire to increase the coordination numbers of the Sb centres.

4 PHOSPHIDE ANALOGUES

As can be concluded from the previous section, the chemically robust nature of Group 15 imido systems gives them some potentially broad applications in various aspects of coordination chemistry. There is, however, a striking difference between these systems and their phosphide analogues. Such phosphide cages decompose into Zintl compounds containing E73- anions at relatively low temperatures, via an apparent step-wise mechanism involving heterocyclic intermediates of the type [(RP)nE]- (Scheme 2). The extent of this decomposition process and whether or not it can be limited to the intermediate heterocycles depends on a number of factors, which include,

• the organic group (R) present within the phosphide groups (RP2-); aromatic groups accelerate the formation of Zintl compounds, whereas aliphatic groups result in the stabilisation of the [(RP)nE]- heterocycles.

• the presence of Me2NH in the reaction, which encourages formation of the Zintl compound.

• the alkali metal present in the Group 15 cage; as Group 1 is descended the decomposition of the cage is encouraged.

• the Group 15 element present; as the group is descended (from As to Bi) formation of the Zintl phase becomes more favourable.

Underlying the thermolability of the phosphide cages is the strength of single P–P bonds, which are the strongest homoatomic bond energies of all the Group 15 elements. The latter provides the thermodynamic driving force for the conversion of the Group 15/alkali metal cages to (effectively) an alloy phase (the Zintl phase). The ultimate formation of cylophosphazanes, [RP]n together with the Zintl compound suggests that the [(RP)nE]- intermediates play the role of metal atom deliverers.

Although the phosphide cage [{Sb(PCy)3}2Li6.6Me2NH] (11) (Figure 10a) can be obtained from the reaction of Sb(NMe2)3 with CyPHLi (eqn. 10), the complex is unstable above ca. 0°C. At 30-40°C 11 undergoes thermal decomposition into the Zintl compound [Sb7Li3.6Me2NH] (12) (Figure 10b). Interestingly, if 11 is held under vacuum and the Me2NH solvation removed then the decomposition process no longer takes place (if Me2NH is bubbled through a solution of the complex rapid decomposition to 12 ensues).

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (10)

The decomposition of 11 can be compared to that of the related complex [{Sb(PtBu)}2Li6.6thf] (13) in which the final product isolated is the bicyclic, distibane [(tBuP3Sb]2 (14) (Figure 11), together with [tBuP]4. This reaction can be monitored by 31NMR spectroscopy and occurs via the heterocyclic anion [(tBuP)3Sb]-. The significance of the different reactivity of 13 to that of 11 is the suggestion that coupling of the heterocyclic [(RP)nSb]- anions via an oxidative process is probably the key metal-metal bond forming process in the ultimate formation of the Sb73- anion. The fact that the decomposition of 13 stops at 14 provides some circumstantial evidence for the importance of the presence of Me2NH bonded to the alkali metal within the cage precursor and intermediates (as implied by the activation and deactivation of 11, mentioned previously).

(Continues…)Excerpted from Perspectives in Organometallic Chemistry by C. G. Screttas, B. R. Steele. Copyright © 2003 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
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