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Recent studies have focused on the design of model compounds to conceive the effect of diverse metal fragments on the Si-H mojety. In this context the metallo-silanes 1 -5 containing one, two or three Si-bound metal fragments as well as metallodisilanes 6 have been made available by convenient routes.These "special" silanes show extraordinary activation of the SiH-bonds by the transition metal fragment which guarantees smooth "transition metallation" of the silicon to afford inter alia the cluster compounds 7.

 

 

 

 

Interestingly the metallo-silanes can be oxygenated by using dimethyldioxyrane or H2O2/urea in the presence of MeReO3 as an oxygen transfer reagent.
According to this method the metal fragment substituted silanols, silanediols or silanetriols 8, 9 are accessible. An interesting aspect refers to the regiospecifity of the oxygen transfer, which guarantees the formation of unusual ligands at the metal centre like Si(OH)2SiH3 in 10 starting with the metallo-disilanes 6. This product indicates the extraordinary hydride activity of SiH-bonds directly connected to the metal - a consequence of the electron releasing effect of the transition metal. Moreover bis(metallo)silanols 11 can be realized showing for the first time hydroxysilylenes bridging two metal centres. Regiospecifity of the oxygenation process is also valid for the bis-metallated species as is proved by the controlled conversion of Cp(OC)2Fe-Si(H)(OSiMe2H)Fe(CO)2Cp, characterized by two types of SiH bonds, to the bis(metallo)siloxanol 12.

 

All metal fragment substituted silanols show unusual high stability with respect to self condensation offering the possibility to perform controlled condensation reaction with chlorosilanes, alkyls of the group 13 elements and metal halides of Ti, Zr and Hf respectively, as is demonstrated by the metallo-heterosiloxanes 13 and 14. These results characterize metallo-silanols as attractive starting materials for connecting the late metals Fe, Ru, Mo, W with Al, Ga, In or the early metals Ti, Zr, Hf via SiO-units.

The SiH-functionalized metallosiloxanes deriving from 8 or 9 undergo facile oxidative addition of the Si-H bonds under photochemical conditions to yield siloxanediyl bridged dinuclear complexes of the type 15 and 16.

Recent efforts have been focused on metallo-silanols, in which the "transition metal effect" is reduced by introducing a spacer unit. Such metallo-silanols promiss elegant access to metal-substituted oligo- and polysiloxanes via controlled self-condensation, an approach that has been realized for the first time with the metallo-silanols 17 having the metal and the silanol separated by a methylene spacer.


The Cp(OC)2Fe-CH2-substituted silanols 17, showing in the solid state extensive hydrogen bonding (fig. 1 and 2), exhibit enhanced reactivity towards self-condensation compared to the analogous Fe-Si systems. As a consequence, 17 exerts already at room temperature quantitative conversion to the 1,3-bis(ferriomethyl)disiloxane 18.

These studies made avalable a vast number of transition metal fragment substituted phophines, arsines, stibines and even bismutines of the the metals Cr, Mo, W, Fe,and Ru. Half sandwich complexes of the type 1 (R=R´= i-Pr) and 2 offered for the first to determine the influence of transition metals on the phosphorus inversion, resulting in asignifant lowering of the barrier. On the other hand nucleophilicity of the phosphorus is considerably raised and gurantees spontaneous coupling with diverse multiple bonding system to yield diastereoselctively products like 3. In the case of secondary metallo-phosphines (R´= H) an additional insertion process is observed to generate 4.

Functionalized Metallo-phosphines like 5 act as a source of phophinidines as well as the trapping reagent for this short-lived species after transformation to a M=P doubled bond complex via decarbonylation. The P-P coupling product 6 is produced in a stereochemically controlled manner. Further interacction with elctrophiles or chalcogens affords diphosphine ligands which can be removed from the metal under appropriate conditions.

In context with the heavier group 15 elements full transition metalliation of antimony has been realized for the first time as well as the coordination of the tris(ferrio)stibine 1 to give 2. First proof has been also given for Lewis acitity of metallo-dihalostibines by the formstion of 3 after phosphine addition and the existence of the transition metal substituted pentavalent antimony species 4.


Previous studies concerning the coupling reactions of metallo-phosphines with organic multiple bond systems (see Section II) suggest an advantageous modification starting with primary phosphine complexes of the chromium, manganese and iron group metals e. g. 1 ( R' = alkyl, aryl, py) and generating the corresponding phosphanido metal species via deprotonation as a short-lived intermediate. Prominent examples arise from the stepwise diastereoselctive hydrophosphination of electron deficient alkynes resulting in the succesive formation of the secondary vinylphophine complex 2 and the dinuclear iron complexes 3 and 4 bearing novel types of P-H-functionalized bis-phosphinoethane ligands as a bridging unit.

 

 

The introduction of a further functional groups into the secondary phosphine complexes 5 obtained by hydrophosphination involving the C=O- or C=C-bond of quinones or olefins as well as the epoxy unit of cyclohexeneoxide can be achieved by insertion of acrylonitrile or vinyl pyridine, which leads to the regio- and diastereoselctive formation of the tertiary phosphine complexes 6 - 11 . The phosphorus ligand in these complexes offers an additional nitrogen donor site, implying hybrid character appropriate for the coordination of “hard” metals.


 

In general the multifunctionalized chiral phosphines can released from the metal under mild conditions and used in conventional organophosphorus synthesis. Ongoing studies concentrate on the enantioselective generation of the novel phosphines using chiral metal fragments or/ and chiral phosphorus donors.

 

Especially P-H-functionalized phosphenium complexes Cp(OC)2W=P(H)R (1 ), which offer a tremendous reactivity potential, create serious problems concerning isolation due to insufficient shielding of the M=P bond. While the Mes-compound shows controlled dimerisation to the phosphinidene-bridged dinuclear complex 2 , the Ph-derivative undergoes [2+2]-cycloaddition to give 3. The tert -butyl-complex suffers immediate decomposition just below room temperature . Only the supermesityl phosphenium complex Cp(OC)2W=P(H) s -Mes (4) could be isolated and structurally identified.

Despite the notorious instability of most phosphenium complexes, a vast chemistry has been established involving cycloaddition with organic multiple bonded systems like isothiocyanates, dienes and diazoalkanes followed in special cases by P-H-insertion to yield the coupling products 5 – 8 .

 

 

Reactivity of Metal Coordinated Carbonmonoxide towards Phosphorus ylides

This topic investigates routes of C-C-coupling between CO coordinated to a metal and strongly carbanionic phosphorus ylides. Starting with cationic cyclopentadienyl carbonyl complexes opens up controlled interaction involving nucleophilic attack at the CO carbon followed by transylidation to yield metalloacyl-phhosphorusylides like 1. This species develops ambidentate behavior towards carbon electrophiles resulting in the formation of either the phosphonioacyl complex 2 or the phosphoniovinyl complex 3.

 

A special metall centered C-C coupling is initiated, when the silyl-methylene phosphorane iron speciec 4 is deprotonated. Consecutive C-C bond formation and transfer of the phophine to the metal generates the ketenyl complex 5. An analogous process is responsible for the formation of 6 starting from methyl carbonyl complexes of Mo, W and trimethylmethylenephosphorane. The chelating ligand is composed of two CO ligands the alkyl group and the ylidic carbon fragment.

 

Carbonmonoxide cleaveage is achieved in the case of tri(carbonyl)cyclopentadieny)manganese via the carbonmonoxide/ylide reaction to give the anionic acylylid complex 7, which on silylation converts to he siloxyvinyl complex 8. Clean silanol elimination on heating produces the phosphonioalkinyl complex 9.

 

 


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