Chapter 1 Amido Ligands in Coordination Chemistry

Thomas Schareina,a Rhett Kempeb

a Institut für Organische Katalyseforschung Rostock, Buchbinderstraße 5 - 6, 18055 Rostock, Germany

b Carl von Ossietzky Universität Oldenburg, P.O. Box 2503, 26111 Oldenburg, Germany

1.1 Introduction

1.2 Amidinates

1.2.1 Ligands

1.2.2 Group 3 Metal and Lanthanide Complexes

1.2.3 Titanium and Zirconium Complexes

1.2.4 Complexes of the Vanadium Triad

1.2.5 Group 6 Metal and Later Transition Metal Complexes

1.3 Aminopyridinates

1.3.1 Ligands

1.3.2 Group 3 Metal and Lanthanide Complexes

1.3.3 Titanium and Zirconium Complexes

1.3.4 Complexes of the Vanadium Triad

1.3.5 Late Transition Metal Complexes

1.3.6 Heterobimetallics

1.1 Introduction

In organometallic chemistry considerable effort is being expended on modifying the reactivity of metal-carbon bonds with coligands. The information thus obtained is being extensively used, for example, in homogeneous catalysis.1 In the choice of such coligands, the cyclopentadienyl (Cp) fragment and phosphanes have hitherto played the decisive role as anionic and neutral donor functions, respectively.2 In addition to these classic possibilities for the control of reactivity of complexes, since the beginning of the 1990's exhaustive efforts have been devoted to the development of novel ligands.3 Besides the Cp ligands (? Fig. 1.1, left), alkoxy (right), and amido ligands4 (center) have proved to be suitable for the stabilization of early, electron-poor transition metals in medium to high oxidation states.

? Figure 1.1 Most important ligands to stabilize early transition metals in medium and high oxidation states.

Of these two alternatives, the amido ligand is especially interesting since it offers the greater potential in ligand and complex design because of the potential for double substitution at the donor atom. The foundations of amido-metal chemistry were laid in the 1960's and 1970's and are associated with names such as Bürger, Wannagat, Bradley, and Lappert. The motivation for these investigations was mainly the exploration of the reactivity of amido-metal bonds in comparison to the metal-carbon bond. However, what was found was rather disappointing. The amido-metal bond is kinetically inert and thermodynamically more stable, and thus synthetically far less interesting than the corresponding metal-carbon bond. Today amido-metal chemistry has come to mean the utilization of the presumed disadvantage of a stable amido-metal bond to produce well-defined reaction centers in transition metal complexes. In this way, the reactivity of the resulting compounds can be specifically tailored to allow applications in areas such as the activation of small, poorly reactive molecules, homogeneous catalysis, or organic synthesis. Insights into the mechanisms of elementary reactions such as C-H activation, a-H elimination, the cleavage of the N/N triple bond, reactions of N2 with H2 within the coordination sphere of transition metals, new routes for the synthesis of macrocycles, polar metal-metal bonds, and complexes with unusual terminal ligands such as phosphorus, arsenic, or carbon are, in addition to interesting transfer or polymerization reactions, just a few selected examples which illustrate what has become possible by the use of amido ligands.5 In this chapter the syntheses of early and late transition metal complexes useful as starting material in amido-metal chemistry are described. Amidinate ligands have been used intensively to synthesize preferably late metal coordination compounds. With the introduction of the silyl-substituted benzamidinates by Dehnicke et al.6 and Roesky el al.7 early transition metal organometallic chemistry became enriched by a new class of ligands. Amidinato ligands have been used to stabilize reactive metal centers in a Cp-analogous fashion. Aminopyridinato ligands are defined as deprotonated 2-aminopyridines. An intensive exploration of this type of ligand was initiated in the middle 1990's in the course of the renaissance of the amido-transition metal chemistry. Aminopyridinato ligands are interesting due to the flexibility of their binding mode and the great variety of substitution patterns that can be easily established. Since the aminopyridinato ligands contain an amido and a pyridine functionality in close proximity, interesting bimetallic coordination compounds can be synthesized.8 The steric bulk of the aminopyridinato ligands is rather small in comparison to cyclopentadienyl ligands and silyl-substituted amidinates. Thus, the chemistry of aminopyridinato ligands differs drastically from that of these two types of ligand (Cp and amidinato ligands) if steric bulk is considered to be very important to stabilize reactive organometallics, as it is known to be in group 3 or lanthanide chemistry. Nevertheless, analogies between cyclopentadienyl and aminopyridinato ligands could be drawn in connection with reactivity studies of complexes of the significantly smaller group 5 metals.9

References

1 B. Cornils, W. A. Herrmann, Applied Homogenous Catalysis with Organometallic Compounds, VCH, Weinheim, 1996.

2 Ch. Elschenbroich, A. Salzer, Organometallchemie, Teubner, Stuttgart, 1993.

3 A. Togni, L. M. Venanzi, Angew. Chem. 106, 517 (1994); Angew. Chem. Int. Ed. Engl. 33, 497 (1994).

4 M. F. Lappert, P. P. Power, A. R. Sanger, R. C. Srivastava, Metal and Metalloid Amides, Ellis Norwood Ltd., Chichester, England, 1980.

5 R. Kempe, Angew. Chem. 112, 478, (2000); Angew. Chem., Int. Ed. 39, 468 (2000).

6 D. Fenske, E. Hartmann, K. Dehnicke, Z Naturforsch. 43b, 1611 (1988).

7 H. W. Roesky, B. Meller, M. Noltemeyer, H.-G. Schmidt, U. Scholz, G. M. Sheldrick, Chem. Ber. 121, 1403 (1988).

8 A. Spannenberg, M. Oberthür, H. Noss, A. Tillack, P. Arndt, R. Kempe, Angew. Chem. 110, 2190 (1998); Angew. Chem., Int. Ed. Engl. 37, 2079 (1998).

9 A. Spannenberg, H. Fuhrmann, P. Arndt, W. Baumann, R. Kempe Angew. Chem. 110, 3565 (1998); Angew. Chem., Int. Ed. Engl. 37, 3363 (1998).

1.2 Amidinates

1.2.1 Ligands

N,N'-Bis(trimethylsilyl)benzamidinatolithium, N,N,N'-Tris(trimethylsilyl)benzamidine - [PhC(NSiMe3)2]Li, PhC(NSiMe3)[N(SiMe3)2]

CAUTION:

All manipulations are performed under rigorous exclusion of air and moisture usually under an atmosphere of pure argon or nitrogen using Schlenk techniques.

Procedure for [PhC(NSiMe3)2]Li1

A solution of benzonitrile (44.03 g, 0.427 mol) in diethyl ether (50 mL) is added dropwise to a slurry of (Me3Si)2NLi·Et2O (103.1 g, 0.427 mol) in diethyl ether (400 mL) in a 1-L round-bottomed flask. After 24 h, the formation of the salt was shown to be quantitative by NMR. In smaller batches 1 h is sufficient. This solution can be used as an equivalent of [PhC(NSiMe3)2]Li.

Procedure for PhC(NSiMe3)[N(SiMe3)2]1

The ether is distilled off from the above solution and toluene (350 mL) is added. Chlorotrimethylsilane (46.4 g, 0.427 mol) in toluene (50 mL) is then added and the mixture heated at reflux for 5 h. After cooling, LiCl is separated from the light orange solution by decanting and filtration, and the solvent removed by distillation. High vacuum distillation using an air-cooled condenser gives PhC(NSiMe3)[N(SiMe3)2]. Yield: quantitative.

References

1 R. T. Boeré, R. T. Oakley, R. W. Reed, J. Organomet. Chem. 331, 161 (1987).

N,N'-Bis(trimethylsilyl)benzamidinatosodium · 0.5 Diethyl Etherate - Na[C6H5C(NSiMe3)2] · 0.5 Et2O

CAUTION:

All manipulations are performed under rigorous exclusion of air and moisture usually under an atmosphere of pure argon or nitrogen using Schlenk techniques.

Procedure1

NaN(SiMe3)2 (55.01 g, 0.30 mol) is dissolved in diethyl ether (200 ml) and the solution filtered, if necessary. With vigorous stirring pure benzonitrile (30.94 g, 0.30 mol) is added. After 24 h stirring at room temperature the solvent is removed in vacuum. Yield: 95.1 g (98%); mp 76 °C.

Properties

1H NMR (80 MHz, CD2Cl2): d = 6.90 - 7.44 (m, 5H, Ph), 3.25 (q, 2H, CH3CH2O), 1.00 (t, 3H, CH3CH2O), -0.15 - 0.13 (m, 18H, SiMe3).

References

1 M. Wedler, F. Knösel, M. Noltemeyer, F. T. Edelmann, U. Behrens, J. Organomet. Chem. 388, 21 (1990).

1.2.2 Group 3 Metal and Lanthanide...