Chapter 2 p-Complexes with Heterocyclic Ligands

Norbert Kuhn

Institut für Anorganische Chemie, Eberhard-Karls-Universität Tübingen, Auf der Morgenstelle 18, D-72076 Tübingen, Germany

With contributions from

Al-Taweel, S. M., Al-Karak

Ashe III, A. J., Ann Arbor

Baudry, D., Dijon

Charrier, C., Palaiseau

Jendral, K., Tübingen

King, R. B., Athens, GA

Kuhn, N., Tübingen

Lampe, E.-M., Tübingen

Maigrot, N., Palaiseau

Mathey, F., Palaiseau

Meyer-Zaika, W., Essen

Nief, F., Palaiseau

Ricard, L., Palaiseau

Schmid, G., Essen

Stubenrauch, S., Tübingen

Zarkrzewski, J., Lodz

2.1 Introduction

2.2 Pentamethylpyrrole Complexes

2.3 (?5-Pyrrolyl)tricarbonylinanganese (Azacymantrene)

2.4 (?5-Cyclopentadienyl)(?5-l/H-pyrroi-l-yljiron (Azafcrrocene)

2.5 2,5-Di-tert.-butyl-l-azacyclopentadienylmetal Complexes

2.6 ?5-Phospholyl Complexes of Early Transition Metals

2.6.1 Synthesis of the Phospholyl Precursors

2.6.2 Synthesis of the Phospholyl Complexes

2.7 4,5-Diphenyl-l,3-Diphospholide Anion and (?5-4,5-Diphenyl-l,3-diphospholyl)(?5-cyclopentadienyl)iron

2.8 2,2´,5,5´-Tetrakis(trimethylsilyl)-3,3´,4,4´-tetraethyl-l, l´-distibaferrocene

2.9 Bis(1-tert.-butyl-2-methyl-lH-l,2-azaborolyl)iron and (1-tert.-Butyl-2-methyl-1H-azaborolyl)(dicarbonyl)cobalt

2.1 Introduction

Starting with the pioneering synthesis of (C4H4S)Cr(CO)3 by E. O. Fischer and K. Öfele in 1958,1 the chemistry of p-complexes with heterocyclic ligands represents a continuously developing chapter of organometallic chemistry. 2 Most of the heterocyclic compounds used for coordination act as fivemembered, 6p-electron donor ligands derived in a formal sense from the carbocyclic cyclopentadienide ion (1; for structures, see ? Figure 2.1), and the heavier group 16 analogues (3, 4) of thiophene (2) were coordinated in the following years. 3,4

? Figure 2.1Structures of five-membered heterocyclic species 1 - 15.

Although isoelectronic to the cyclopentadienide ion, the neutral molecule pyrrole (5) forms only labile complexes,5 and no ?5-coordination of the corresponding phosphole ligand is known as yet, presumably as a consequence of the pyramidal geometry at its phosphorus center.6

A marked increase in stability of p-complexes is observed on going from neutral to anionic ligands, as demonstrated by the properties of complexes containing the azacyclopentadienide ligand (6)7 Owing to the more electropositive nature of the heavier group 15 elements, phospholide complexes containing (7) as ligand are amongst the most stable p-complexes in organometallic chemistry,8 and even complexes containing the arsa-, stiba-, and bismacyclopentadienide ligands (8 - 10) are known.9 Complexes containing di-and triphosphacyclopentadienide ligands (11 - 13) have also been reported.10 The coordination of a germacyclopentadienide ligand (14) has been mentioned just recently,11 while complexes of the borolide dianion (15) have been known for more than 20 years.12

The chemistry of the borolide ligands containing additional heteroatoms as members of the five-membered ring system forms a predominant chapter of p-coordination chemistry. This part includes both 4p-electron (16 - 18)13 and ?p-electron donor ligands (19, 20; see ? Figure 2.2).14,15

? Figure 2.2Structures of five-membered heterocyclic species 16 - 20.

In comparison with their five-membered counterparts, only few types of six-membered heterocyclic ligands have been reported as p-ligands. In contrast to its numerous s-complexes, pyridine (21) forms only labile complexes in the p-coordination mode,16 while for the related complexes containing the phosphorus and arsenic ligands (22, 23) a marked increase in stability is observed.17 Surprisingly stable complexes are formed with the anionic borinate ligand (24) which is isoelectronic with the benzene system.18 Only few six-membered ligands containing two or more heteroatoms (25 - 27) have been reported (? Figure 2.3).19-21

In general, heterocyclic ligands form less stable p-complexes than their carbocyclic analogues, presumably as a consequence of the change of symmetry and an increase in ionization energy with electronegative heteroelements.22 Stabilization of the complexes is achieved by use of partially or fully alkylated derivatives.23 In fact, only a minority of the ligands mentioned above has been used as the parent compounds which is, in part, also a consequence of synthetic aspects. In addition, the presence of lone electron pairs at the heteroatom enables these complexes to act as bases. Decomposition reactions starting with a s-rearrangement may be avoided by the introduction of sterically demanding substituents in the a-positions of the ligand24 or by blocking the lone electron pair with electrophiles.25

? Figure 2.3Structures of six-membered heterocyclic species 21 - 27.

Owing to their low p-basicity, the "common" ligands such as pyrrole and pyridine form labile ligand-to-metal bonds, while more basic ligands are difficult to prepare. In every case, the preparation and handling of p-complexes containing heterocyclic ligands needs experience and a high level of instrumentation. This is the reason for the relatively small number of complexes introduced in this chapter.

References

1 E. O. Fischer, K. Öfele, Chem. Ber. 91, 3395 (1958).

2 G. Wilkinson, F. G. A. Stone, E. W. Abel (eds.) Comprehensive Organometallic Chemistry, Pergamon Press, Oxford, 1982.

3 K. Öfele, Chem. Ber. 99, 1732 (1966).

4 K. Öfele, E. Dotzauer, J. Organomet. Chem. 42, C87 (1972).

5 K. Öfele, E. Dotzauer, J. Organomet. Chem. 30, 211 (1971).

6 F. Mathey, New. J. Chem. 11, 585 (1987).

7 (a) R. B. King, A. Bisnette, Inorg. Chem. 3, 796 (1964).
(b) J. K. K. Joshi, P. L. Pauson, A. R. Qazi, W. H. Stubbs, J. Organomet. Chem. 1, 471 (1964).

8 F. Mathey, A. Mitschier, R. Weiss, J. Am. Chem. Soc. 99, 3357 (1977).

9 (a) E. W. Abel, I. W. Nowell, A. G. J. Modinos, C. Towers, J. Chem. Soc., Chem. Commun. 258 (1973).
(b) A. J. Ashe, III, T. R. Diephouse, J. Organomet. Chem. 202, C95 (1980).
(c) A. J. Ashe, III, J. W. Kampf, S. M. Al-Taweel, J. Am. Chem. Soc. 114, 372 (1992).

10 (a) R. Bartsch, P. B. Hitchcock, J. F. Nixon, J. Chem. Soc., Chem Commun. 1146 (1987).
(b) L. Weber, R. Kirchhoff, R. Boese, H.-G. Stammer, J. Chem. Soc., Chem. Commun. 1293 (1991).

11 W. P. Freeman, T. Don Tilley, A. L. Reingold, R. L. Ostrander, Angew. Chem. 105, 1841 (1993); Angew. Chem., Int. Ed. Engl. 32, 1744 (1993).

12 G. E. Herberich, J. Hengesbach, U. Koelie, W. Oschmann, Angew. Chem. 89, 43 (1977); Angew. Chem., Int. Ed. Engl. 16, 42 (1977).

13 (a) W. Siebert, M. Bochmann, Angew. Chem. 89, 483 (1977); Angew. Chem., Int. Ed. Engl. 16, 468 (1977).
(b)W. Siebert, G. Augustin, R. Full, C. Krüger, Y.-H. Tsai, Angew. Chem. 87, 286 (1975); Angew. Chem., Int. Ed. Engl. 14, 262(1975).
(c)W. Siebert, Advan. Organomet. Chem. 18, 301 (1980).

14 J. Schulze, R. Boese, G. Schmid, Chem. Ber. 113, 2348 (1980).

15 L. Weber, G. Schmid, Angew. Chem. 86, 519 (1974); Angew. Chem., Int. Ed. Engl. 13, 467 (1974).

16 H. G. Biedermann, K....