Sandwich multidecker

The reactions of the Group VIII di- and triborolyl or carboranes sandwiches are directed basically to their extension to multidecker derivatives. 

The di- and triborolyl ligands tend to i -coordination in sandwichforming reactions. There is a clear-cut tendency for stacking processes followed by the formation of multidecker species and often stabilization of the unusual oxidation states of the transition metals. The route to the linked sandwich and multidecker complexes is attractive for materials chemistry. Thia- and azaborolyl organome-tallic chemistry follows the same trends, although in the azaborolyl complexes the i -rather than i -coordination is sometimes realized. Moreover, coordination via the boron atom is known. In the B, N, Si-heterocycles, the heteroring is j " -coordinated. 

The borole ring and various 1,3-diborolyl anions have been extensively employed as ligands to prepare a huge array of transition metal complexes and multidecker sandwich compounds.96 97 Inevitably, the electronic character of the borole is profoundly changed upon complexation, so a study of such complexes can reveal nothing certain about B—C multiple bonding in the isolated ligand. 

Reaction of 137 with [NBri4][SCN] leads to formation of the anionic complex [137-SCN]-, which adopts a multidecker structure with the anion sandwiched between successive molecules of 137 (Figure 13).212 The sulfur atoms of the SCN- ion in [ 137-SCN]- interacts unsymmetrically with the mercury atoms of the neighboring molecules of 137 forming four short Hg-S bonds that range from 3.06(1) to 3.36(1) A and two long bonds of 3.74(1) and 3.87(1) A. While the longer Hg-S bonds approach the limit for the involvement of dative interactions, the shorter ones are comparable to those observed in [163-SCN]. ... 

Boratabenzene-bridged multidecker sandwiches are few in number, all prepared in the Herberich group. Stacking reactions of Cp Ru(HsCsBMe) with electrophilic metal reagents gave complexes 106 and 107. 2 interestingly,... 

Multidecker sandwiches and coordination compounds, formed by unstable organic particles such as radicals, biradicals, carbenes, and carbynes [7,112,113] (see Sec. 2.2.4.1), also belong to MCCC. 

Treatment of 3-borolenes or 2-boraindans with bulky lithium amides yields the dihthiated aromatic borolide dianions, which are applicable as hgand precursors for transition-metal complexes. Many borole complexes including a number of unusual multidecker sandwich complexes and mixed-metal clusters have been described. 4 3,2i2 An unexpected new entry into the synthesis of borole complexes has been recently discovered. Bochmann found that attack of B(C6Fs)3 at a zirconium bound diene leads to a pentafluorophenylborole complex throngh snccessive C-H activation steps. ... 

These systems exhibit considerable affinity for halide anions. X-ray analysis ascertained the formation of an anionic 2 1 chloride adduct of (1) where the chloride is simultaneously bound by four mercury atoms. In the crystal structures of (2) Cl and (2) l2, the anions are located within the macrocycle and complexed cooperatively by the four mercury(II) centers. Several anionic complexes of (3), including bromide, iodide, and thiocyanide salts, have been isolated. The compounds adopt multidecker stmctures with the hexacoordinated anions effectively sandwiched between two successive molecules of (3). The Lewis acidity of perfluoro-ortAo-phenylenemercury (3) has also been substantiated by its ability to form stable adducts with neutral substrates (HMPA, DMSO, ethyl acetate, and acetonitrile) and arenes. The (3) -CeHe adduct exists as extended stacks of nearly parallel, staggered molecules of (3), which sandwich benzene molecules. Similar structures have been reported for the corresponding adducts with biphenyl, naphthalene, pyrene, and triphenylene. 

Figure 3.24. Routes to multidecker sandwich compounds incorporating C2B3 rings.

Figure 10. Multidecker metall-acarborane sandwich complexes. [9] The rotational orientations of the C2B3 rings vary in different complexes, and are arhitrarily depicted here.

Treatment of 3-borolenes or 2-boraindans with bulky lithium amides yields the dilithiated aromatic borolide dianions, which are applicable as ligand precursors for transition-metal complexes. Many borole complexes including a number of unusual multidecker sandwich complexes and mixed-metal clusters have been described. 

Interesting electrical properties are to be expected with the stepwise extension of this TT-system. The preparation of multilayered cyclophanes proved to be laborious [6] nevertheless new synthetic methods in transition metal chemistry of arenes have opened up a promising alternative approach via preparation of multidecker sandwich complexes (structure type D in Fig. 3). First row transition metals like chromium, iron and cobalt [51] form strong coordinative bonds with arenes when their oxidation state is low [48a] whereas second and third row elements like ruthenium, rhodium and iridium are strongly bonded towards arenes in higher oxidation states [48a, 51]. Sandwich complexes of cyclophanes can be divided into two groups ... 

Fig. 21, Reaction paths (a tof) to multidecker sandwich complexes of ruthenium, cobalt, rhodium and iridium with [2.2]paracyclophane (8) as ligand...

Examplified with 8 as ligand, the possible multidecker sandwich complexes are depicted in Fig. 21. Only four sandwich complexes of ruthenium-II are yet known for ansa-phanes [81]. 

Complexation of both benzene rings of 192 or 8 give the multidecker sandwich complexes 193 and 195 respectively [174]. Cyclic voltammetry of 193 clearly demonstrates two distinct 1-electron reduction steps [90]. On the basis of its rigid structure [175] //" -coordination is less easily adopted than in hexamethylbenzene, even though its redox potential is lower [89]. This implies that the phane geometry is widely preserved, and so 194 probably represents the first example of a 20-electron complex of ruthenium (Fig. 49) [90]. 

Multidecker sandwich structures can be classified in the forms shown in Fig. 3.21 (a) and (b) and contrast with the linear polymer structures discussed so far (Fig. 3.21 c). 

Exclusively ir-aromatic metal clusterslike X X=Bq, Mg, Ca, NaX, Na2 3, Y Y= Zn, Cd, Hg, Be, Mg, Ca have also been studied. The NICS(O) value for Ca3 is positive whereas its NICS(l) value is negative. Some of them exhibit bond stretch isomerism . Trigonal dianion aromatic metal clusters " " " like Y F=Be, Mg, Ca containing two TT-electrons can replace the cyclopentadienyl rings to form various metallocene analogues (Fig. 17 and 18) and multidecker sandwich complexes They can stabilize a direct Zn-Zn bond and can trap hydrogen molecules. ... 

In addition to biscyclopentadienyl sandwich-type complexes, triple-decker and multidecker compounds are known.[structures (9.10)]. 

Both 1,2,5-thiadiborole and 2,3-dihydro-l,3-diborole derivatives (9.61) are utilized for the preparation of multidecker sandwich complexes[equations (9.62)-(9.64)]. 

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