Ligands stannylenes

Tin amidinates display a rich coordination chemistry with the metal in both the di- and tetravalent oxidation states. The first results in this area were mainly obtained with N-silylated benzamidinate ligands. Typical reactions are summarized in Scheme 48. A stannylene containing unsymmetrically substituted amidinate ligands, [o-MeC6H4C(NSiMe3)(NPh)]2Sn, has been prepared accordingly and isolated in the form of colorless crystals in 75% yield. ... 

Ligand transfer from tin(IV) to tin(II) compounds with appropriate ligands such as chlorine (d) has been used infrequently in the synthesis of stannylenes. The only reactions reported so far have been performed with oxygen as substituents at tin (see also Sect. 6.4.3). 

As heavier analogs of carbenes141) stannylenes can be used as ligands in transition-metal chemistry. The stability of carbene complexes is often explained by a synergetic c,7t-effect cr-donation from the lone electron pair of the carbon atom to the metal is compensated by a a-backdonation from filled orbitals of the metal to the empty p-orbital of the carbon atom. This concept cannot be transferred to stannylene complexes. Stannylenes are poor p-a-acceptors no base-stabilized stannylene (SnX2 B, B = electron donor) has ever been found to lose its base when coordinated with a transition metal (M - SnXj B). Up to now, stannylene complexes of transition metals were only synthesized starting from stable monomoleeular stannylenes. Divalent tin compounds are nevertheless efficient cr-donors as may be deduced from the displacement reactions (17)-(20) which open convenient routes to stannylene complexes. 

Another very interesting reaction involving insertion of dicyclopentadienyl-stannylene into metal-hydrogen bond with displacement of its ligands has been described by J. G. Noltes et al.16S). The resulting product was identified by X-ray structural analysis. 

The synthesis of asymmetrically substituted stannylenes is most efficiently achieved by ligand exchange reactions between two stannylenes, SnXj and SnY2 (see Chapter 4). For example, the stannylenes [(Me3Si)2NJClSn and (C5H5)ClSn can be synthesized according to Eqs. (36) and (37) 86,93 94). 

Reactions with Stannylenes SnX2 Displaying Trihapto-Ligand Properties... 

The NMR spectra of the two-coordinate stannylenes in solution show values of Sn ranging from about 1150 (e.g., in ArSnl) to 3750 (in (Ar3Sn)Sn ), with a large anisotropy. The stannylenes behave as Lewis acids, for example, in the three- or four-coordinate complexes (e.g., 78, 79, and 80), which are formed when the molecule carries an intramolecular ligand, and as Lewis bases, particularly in complexing to transition metals (e.g., 81, 82, and 83). The dimerization of stannylenes to give distannenes can be regarded as a result of this amphoteric character (Equation (179)). 

Their structures had been much less explored than those of their heavier congeners such as germylenes and stannylenes. Although dodecamethylsilicocene (19), formally also a silylene, has been known since 1986, compound 19 is stabilized by ri -coordination of pentamethylcyclopentadienyl ligands and is not a congener of a carbene. 

The structures of some stable stannylenes, such as several amino-, " alkoxy-, and arylthio-substituted intermediates, have been revealed by X-ray crystallography. They are monomeric crystals and the tin atom has the coordination number 2. The divalent tin in such compounds is stabilized by the effects of electronegativity of the ligand atoms and by the donation of the lone-pair electrons to the vacant 5p orbital of the tin. Although the first monomeric dialkyl- and diaryl- stannylenes in... 

V. I. Shiryaev, V. P. Kochergin and V. F. Mironov, Stannylenes as Electron-donating Ligands in Complexes, NIITEKhIM, Moscow, 1977 (Russian). 

One further unique diarylstannylene which is worth discussing in more detail is the polycyclic derivative (22)34. This particular molecule possesses three divalent Sn atoms, two of them being coordinated by two amido ligands and the remaining one by two aryl substituents. The Sn atom (SnA) of the bis(aryl)stannylene fragment shows no unusual tight contacts to any other atom — there are close intramolecular contacts to the other Sn atoms, but they are most probably superimposed by the geometric needs of a bicyclic... 

The ability of allyltin halides to extend their coordination sphere allowed the preparation of chiral hypervalent complexes with diamine ligands, which have been efficient in the asymmetric synthesis of homoallylic alcohols with up to 82% ee100. Similarly, a chiral hypervalent allyltin was prepared from a low valent tin (II) catecholate, chiral dialkyl tartrate and ally lie halide101. The allylation of aldehydes and activated ketones proceeded with high enantiomeric excess. Allyltins prepared from Lappert s stannylene and allylic halides were shown to be efficient as well, although the Lewis acid character of the tin atom is much less marked in that case102,103. 

In addition to serving as ligands, organostannylenes can also insert into metal-ligand bonds (cf. Fig. 10). In the reaction shown in Eq. (50) (78), two moles of the divalent tin compound react, one forming a terminal stannylene complex, and the other inserting into the Pt—Cl bond. 

This indicates that the stannylene ligand is strongly held. Additionally, attempts at coordinating other bases to the vacant orbital of the stannylene failed with this and related complexes of Sn[CH(SiMe3)2 (78). That is noteworthy, since analogous dialkylstannylene-chromiumpenta-carbonyl complexes with smaller alkyl groups attached to the tin are unstable in the absence of coordinated base (101). 

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