Interactions of Cationic and Neutral Molybdenum Complexes with -Cyclodextrin Host Molecules Se ´rgio Lima, † Isabel S. Gonc ¸ alves,* ,†,‡ Paulo Ribeiro-Claro, †,§ Martyn Pillinger, † Andre ´ D. Lopes, † Paula Ferreira, † Jose ´ J. C. Teixeira-Dias,* ,† Joa ˜ o Rocha, † and Carlos C. Roma ˜o | Department of Chemistry, University of Aveiro, Campus de Santiago, 3810-193 Aveiro, Portugal, Quı ´mica-Fı ´sica Molecular, Faculdade de Cie ˆ ncias e Tecnologia, Universidade de Coimbra, 3004-353 Coimbra, Portugal, and Instituto de Tecnologia Quı ´mica e Biolo ´ gica, Quinta do Marque ˆ s, EAN, Apt 127, 2781-901 Oeiras, Portugal Received December 27, 2000 The inclusion compounds formed between -cyclodextrin (-CD) and the tetrafluoroborate salts [Cp′Mo(η 4 -C 6 H 8 )(CO) 2 ][BF 4 ] and the neutral derivatives Cp′Mo(η 3 -C 6 H 7 )(CO) 2 [Cp′ ) Cp (η 5 -C 5 H 5 ), Ind (η 5 -C 9 H 7 )] were studied by means of elemental analysis, FTIR spectroscopy, thermogravimetric analysis (TGA), powder X-ray diffraction (XRD), and magic-angle spinning (MAS) NMR ( 13 C, 11 B). Additional information concerning the possible structure of the inclusion compounds was obtained from ab initio calculations using a two-layer approxima- tion. The cationic and neutral η 5 -cyclopentadienyl analogues form stable two-to-one (host- to-guest) channel-type inclusion compounds in a crystalline state. By contrast, the η 5 -indenyl analogues form only weak complexes with -CD and it is evident that the organometallic guests are easily liberated from the host cavities. The ab initio calculations revealed that the steric hindrance arising from the presence of the indenyl ligand is a possible explanation for the experimentally observed lower stability of these compounds. Introduction The past few years have witnessed an increasing effort toward the modification of steric, electronic, and other properties of well-known organometallic first sphere ligands as a means of achieving tuneable reac- tivity control of their complexes. 1 An interesting exten- sion of this concept is the introduction of a second sphere ligand noncovalently attached to the first sphere ligand. This situation arises, for example, when an organome- tallic molecule (the guest) is encapsulated within a supramolecular host. Cyclodextrins (CD) are a group of macrocyclic oligosaccharides that are well-known to form such inclusion complexes. 2 They selectively bind a variety of metalloorganic and organometallic mol- ecules bearing hydrophobic ligands such as cyclopen- tadienyl (Cp) and η 6 -arene groups. 3 Second sphere coordination adducts have been reported with ferrocene and its derivatives, 4 titanocene dihalides, 5 aromatic ruthenium complexes, 6 mixed sandwich complexes such as [(η 5 -C 5 H 5 )Fe(η 6 -C 6 H 6 )][PF 6 ], 7 and half-sandwich com- plexes such as CpFe(CO) 2 X (X ) Cl, Me), 8,9 CpMn- (CO) 3 , 10 (η 6 -C 6 H 6 )Cr(CO) 3 , 11 and Cp′Mo(η 3 -C 3 H 5 )(CO) 2 (Cp′ ) Cp, Ind). 12 Half-sandwich complexes encapsulated within CDs have been shown to exhibit markedly different physical and chemical properties compared to those of the bulk material, for example, in their nonlinear optical proper- ties 13 and ligand substitution/insertion reactions. 8 Also, metallocenes such as Cp 2 MCl 2 (M ) Ti, Mo) are potent organometallic antitumor agents 14 and CD inclusion compounds are interesting for pharmaceutical use. 5 The driving forces for the inclusion complexation of CD with † University of Aveiro. ‡ E-mail: igoncalves@dq.ua.pt. 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