Polydentates synthetic

Complexes are similarly formed by polydentate phosphine and arsine ligands synthetic routes involve oxidation of the platinum(II) complex, either with the halogen or with nitric acid ... 

Polydentate ligands do not necessarily need to bind a metal at all possible locations. In some cases, only one site is bound to the metal and the other one or more are left dangling. Such complexes are rare, and it has been necessary to develop special synthetic methods to prepare them. Explain in terms of thermodynamic arguments why polydentate ligands will almost always form chelating complexes rather than leave dangling arms. 

The design of polydentate ligands containing imines has exercised many minds over many years, and imine formation is probably one of the commonest reactions in the synthetic co-ordination chemist s arsenal. Once again, the chelate effect plays an important role in stabilising the co-ordinated products and the majority of imine ligands contain other donor atoms that are also co-ordinated to the metal centre. The above brief discussion of imine formation will have shown that the formation of the imine from amine and carbonyl may be an intra- or intermolecular process. In many cases, the detailed mechanism of the imine formation reaction is not fully understood. In particular, it is not always clear whether the nucleophile is metal-co-ordinated amine or amide. Some intramolecular imine formation reactions at cobalt(m) are known to proceed through amido intermediates. A particularly useful intermediate (5.24) in metal-directed amino acid chemistry is... 

Synthetically useful phosphorane-derived phenyliodonium triflates have been synthesized from the highly electrophilic pyridinium complex 20 <2002TL2359>. Similarly, benziodoxole 21 reacts with trimethylsilyl trifluoro-methanesulfonate (TMSOTf) and pyridine to form a precipitate of complex 22 (Equation 6) <2002TL5735>. The first example of a pentavalent iodine complex with a chelating polydentate nitrogen ligand 24 was obtained from diacetate 23 under similar conditions (Equation 7) <2002TL5735>. 

Finally, just as polydentate amines find wide application in biological and synthetic coordination chemistry, so too do bi- and to a lesser extent tridentate phosphines. In addition to simple kinetic stabilization through... 

Neutral polydentate ligands are widely used for the synthesis of coordination compounds of transition metal ions. -i When different synthetic routes are possible for a given ligand, those which give the highest yields with the least number of steps and minimize the number of possible isomeric or by-products will obviously be preferred. The above criteria may be exemplified by a series of neutral tetradentate ligands of the tripod or umbrella type with a nitrogen atom as the top donor these may be described as 2-substituted triethylamine derivatives (I) ... 

It was obvious to chemists in the early twenties that an extension of Werner s ideas into compounds with polydentate ligands involved the syntheses of suitably constituted, organic substances. Among these were the higher homologs of aliphatic amines related structurally to 1, 2-diaminoethane (en), dH, 2-diaminopropane (pn), and 1, 3-diaminopropane (tn). Efficient synthetic routes to these long-chain bases were at best tedious and frequently very difficult with conventional laboratory equipment. 

Polydentate phosphines continue to attract much interest, especially in the complexes they form, and also in the catalytic applications displayed by some compounds.214-216 To differentiate this class from that of Section 1.12.2.7 on bidentate tertiary phosphines, polyphosphines are compounds possessing three or more phosphorus donor atoms. Most synthetic routes are similar to those outlined before,50,216 so only a selection of polydentate phosphines will be given here including a brief summary, where appropriate, of their syntheses and key reactions. 

The 0-C6H4(SbPh2)2 is also known but little used.75 The synthetic difficulties arising from the weak Sb—C bonds have seriously hindered attempts to make polydentate stibines and in marked contrast to the many P or As examples, only one tritertiarystibine is known. This is MeC(CH2SbPh2)3 made in 15% yield from MeC(CH2Br)3 and Ph2SbNa in liquid ammonia 76... 

As for phosphine macrocycles, arsine macrocycles involve very difficult synthetic chemistry. The reasons for the difficulties are essentially threefold (i) the volatility and well-known toxicity of the arsenic-containing precursors (ii) the instability of coordinated As—II functions, which renders template cyclizations of limited use for arsenic macrocycles (iii) the high energy barrier to inversion at arsenic (167.4kJmol 1),41 which means that high-dilution cyclizations of polydentate arsine macrocycles leads to mixtures of stereoisomers which are not at all trivial to separate. On the other hand, the macrocyclic (tertiary) arsines are less susceptible to oxidation than their phosphine analogs. 

Generally, polydentate phosphines are synthesized by sequential introduction of phosphine units into a precursor [9]. Nevertheless, Whitesides and co-workers proposed a synthetic route by which diphosphine 4 is obtained in situ, as shown in Eq. (1) [10-12],... 

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