Pentacoordinated Si compounds

Arshadi and co-workers [43] investigated sixty different R3SiX/S and R2HSiX/S systems where solvents S of both relatively weak donicity (methylenechloride, sulfolane, acetonitrile, etc.) and relatively strong donicity (dimethylsulfoxide (DMSO), N-methylimidazole (NMI), etc.) were considered. Specific solvation and dissociation of I SiX (or R2HSiX) was assumed to take place in three steps involving equilibria between tetra- or pentacoordinated Si compounds I - IV as shown in Scheme 6. 

Oxidative addition to a related silylene (Scheme 15) afforded a hexacoordinated silicon complex with two Si—I bonds (72), the first crystallographically evidenced hexacoordinated iodosilicon complex so far [203]. Furthermore, addition of chalcogens E (S, Se, Te) afforded pentacoordinated Si compounds with Si=E bonds (73) [204],... 

Finally, a pentacoordinated Si compound 202 with a thiolate type ligand as a 1,2-bidentate chelator (thus forming a thiasilirane, a three-membered Si,C,S heterocycle) is highly noteworthy [358]. In this compound the S atom occupies... 

Generally, the pentacoordinate silicon compounds described in this chapter are sensitive to water and very easily undergo hydrolytic Si-O cleavage reactions in solution. This has been used for the synthesis of the octa(silases-quioxane) 46, which was obtained in 90% yield by treatment of compound 35 with water in boiling acetonitrile (Scheme 8).34... 

A series of binuclear pentacoordinated silicon complexes 85 of diketopiperazine have been synthesized and substituent (or leaving group) effects on the Si-O coordination have been studied for five analogues with X=F, Cl, OTf, Br, and I [226]. Variable-temperature NMR spectroscopy (supported by X-ray crystallography) shows, for the first time in binuclear pentacoordinated silicon complexes, a complex equilibrium with both nonionic (0-Si) and ionic (Si-X) dissociation of the axial bonds in the silicon-centered trigonal bipyramids. The two dissociation pathways are consistent with a model for nucleophilic substitution at the silicon atom in a binuclear pentacoordinated silicon compound (Scheme 21) [226]. 

Scheme 24), the hydride migration from silicon to an adjacent unsaturated imino carbon atom leads to a pentacoordinated silicon complex 89 as final product [170]. For the intermediate 88 a dynamic equilibrium between two conformers 88a and 88b with pentacoordinated Si atom was observed by NMR spectroscopy. For related compounds with hexacoordinated Si atom within a (0,N>2SiMe(H) coordination sphere, the authors observed reversible neutral dissociation of the N-Si dative bond, i.e., an equilibrium between hexa- and pentacoordinated hydrido complexes of silicon [235]. 

Last but not least, a carbene adduct of SiBr4 (175) with pentacoordinated Si atom (with carbene ligand in equatorial position and Si-Br bond lengths of 2.24 and 2.38/2.41 A for equatorial and axial sites, respectively) [108] and a dinuclear pentacoordinated bromosiUcon compound (176) with a central Si2N2 four-membered cycle and rather short axial Si—Br bonds (ca. 2.28 A) [340] have been reported. 

This variety of hypercoordinated bromosilicon compounds is in sharp contrast to hypercoordinated iodosilicon compounds, which are represented by far less crystallographicaUy evidenced examples. In addition to some pentacoordinated monoiodosilicon compounds with tridentate chelators (compound 177, Si—I bond lengths ranging between 2.74 and 2.82 A) [95, 188, 190], the first crystallographi-cally characterized hexacoordinated iodosilicon compound (with two Si—I bonds, 2.64 and 2.66 A) [203] has been reported very recently (compound 72, Scheme 15). 

Genuine hypercoordination has been encountered with isolated and with adjacent hypercoordinated Si atoms within oligosilanes [48, 109, 134, 161, 162, 198-200, 241, 348, 364—368]. Schemes 48 and 49 show such compounds with isolated and with adjacent pentacoordinated Si atoms, respectively, and their Si—Si bond lengths (the value for the longest bond distance within the Si coordination sphere is given in the Schemes). Without exception, the Si—Si bonds are located in equatorial positions within the distorted trigonal-bipyramidal coordination spheres. 

Scheme 48 Compounds with pentacoordinated Si atoms within an oligosilicon skeleton. Si—Si bond lengths in A are given below the formula the longest bond in the Si coordination sphere in A is given in parentheses...

Today it is widely accepted that fivefold coordinated silicon plays a key role in the reaction mechanisms of the nucleophilic substitution having a trigonal bipyramidal transition state species which ressemble these transition states can be isolated in some special cases. The structural features fit well to kinetic data and possibly explain the significantly higher reactivity (proved by experimental data) of Si-pentacoordinated compounds compared to their tetracoordinated analoga. 

These percentages were determined using 29Si and 13C NMR spectroscopies, the highest rates of Si-O bond formation being observed with X = OTf, whereas chlorosilanes rather led to pentacoordinated compounds. 

As demonstrated by single-crystal X-ray diffraction, the /-coordination polyhedra of 85-87 are distorted trigonal bipyramids, with each of the axial positions occupied by the oxygen atoms. This is shown for compound 86 in Fig. 11. In all cases, the crystals are formed from pairs of (A)- and (A)-enantiomers. Selected geometric parameters for 85-87 are listed in Table XIII. As can be seen from the Si-O [1.8004(10)-1.829(6) A], Si-N [1.741(7)-1.764(6) A], and Si-C distances [1.867(8)-1.915(2) A], the A/02N2C frameworks of 85-87 are built up by five normal covalent bonds and do not involve a bonding system in the sense of the 4+1 coordination usually observed for pentacoordinate silicon species with Si-N bonds. 

Plumbene-NHC complex 36 is generated by the reaction of an NHC with a bis(aryl)-lead(II) compound. The NHC-silylene adduct 37 also features a long C-Si bond with significant Si polarity. Pentacoordinated silicon(lV) and... 

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