Planar tetracoordinate carbon

Synthesis of Stable Planar Tetracoordinate Carbon Zr/AI Compounds... 

In 1874, varft Hoff [26] and Le Bel [27] independently surmised that tetracoordinate carbon is surrounded by substituents in a tetrahedral geometry. This perception marked the very beginning of modern organic chemistry, which is increasingly being determined by stereochemical argumentation. Some time ago, attempts were made to synthesize stable planar tetracoordinate carbon compounds [28—30]. 

Table 7.1. Compounds 10 containing planar-tetracoordinate carbon.

Figure 7.2. Crystal structure of the bimetallic complex Cp2Zr(ju-r n r 2-Me3SiCCPh)(p-H)AlMe2 10a (X - Cl R = Ph R1 — SiMe3 R2 — Me) the planar-tetracoordinate carbon atom is Cl 5. Adapted by the authors.

R1 = ph R2 = Me) with the double hydrocarbonyl-bridged Cp2Zr(p-C=CPh)(p-CPh—CMeJAlMe 2 complex exhibiting a planar-tetracoordinate carbon atom within the central metallacyclic ring system. Adapted by the authors. 

Bimetallic Boriozirconocene Complexes with Planar Tetracoordinate Carbon... 

Figure 7.9. X-ray crystal structure of the Ga/Zr complex 112. The carbon atom C-2 is planar tetracoordinate. Adapted by the authors.

Crystals of complex 112 suitable for an X-ray structure determination were obtained on cooling a solution in pentane to — 30 °C. The structure determined is shown in Fig. 7.9. The most remarkable structural feature of 112 is that the gallium center is connected to the zirconium through two different c-carboxylic bridges. One of them contains the cyclo-C6Hg system, which is t]1-bonded to gallium and r 2-coordinated to zirconium. It is noteworthy that carbon atom C-2 is planar tetracoordinate. It is connected to four neighboring atoms in the c-plane, specifically to carbon atoms C-l and C-3 and to both metal centers [175]. 

As discussed in the introduction (Section 3.2.1), derivatives of the diboracyclo-propane 1C are non-classical organoboranes having 8 SE, and according to the 2n+2 SE rule may be classified as the simplest doso-carboranes of the series CH(BH) H ( = 2). Compounds IB and 1C have been computed [5] to be 17.5 and 47.6 kcal mol-1 lower in energy, respectively, than the classical diboracyclopropane 1A. They are 2e aromatics and possess planar-tetracoordinate centers in IB this unusual geometry is found at the carbon and one boron atom, in 1C at both boron atoms (Scheme 3.2-2). 

Protonation of 4b leads to the symmetrically substituted 3b (Scheme 3.2-3) and methylation of 4b at temperatures higher than —60 °C gives 3c (Scheme 3.2-5) [19]. In the latter reaction, 6a can be identified as an intermediate at —80 °C by 13C NMR spectroscopy [19]. Its planar-tetracoordinate carbon atom is strongly de-shielded 3 13 C = 144 ppm) as compared with tetrahedrally-coordinated carbon atoms connected to three boron and one silicon center (d 13C = 70-100 ppm). Computations for the model compounds 6A and 6B give 144 and 104 ppm, re-... 

Scheme 3.2-S. Transformations with conservation of 2e aromaticity. 6a is a derivative of IB (Scheme 3.2-2) and has a planar tetracoordinate carbon atom. The dashed lines of the transition state 7 represent two 3c2e bonds (BBB and CBB).

Transition Metai Chemistry of 1,3-Diynes, Poiy-ynes, and Reiated Compounds 195 8. Planar Tetracoordinate Carbon... 

The zirconium-benzyne complex 78 reacts with 2 equiv trimethylalumi-num57 or trimethylgallium58 to give 79 and 80, respectively [Eq. (17)]. As shown by X-ray analysis, these complexes contain a planar-tetracoordinate carbon center (C2) at the bridgehead position. Triethylaluminum and diiso-butylaluminum hydride react similarly.57... 

The formation of planar-tetracoordinate carbon is an unusnal sitnation that can be obtained, for instance, by treatment of the in im-generated [Cp2Zr =C H3]+ with one eqnivalent of starting bis(propynyl)zirconocene. In homodimetallic system (57), C36 is a planar-tetracoordinate carbon atom. In fact, it is a part of a C=C donble bond (C37-C36 I.3I7(8)A). C36 is also coimected to the acetylene carbon atom C35 by a Csp Csp a-bond (C36-C35 1.401(8) A) and exhibits an unsymmetrical three-center-two-electron interaction with the adjacent Zr atom (C36 Zrl 2.435(6) A C36-Zr2 2.530(5)A). The central core of three atoms around C36 is coplanar. An activation barrier of AGreatr(190K) = 9.5 0.5kcalmor was determined by studying the dynamic feature of (57). 

Most of the known examples of complexes incorporating a planar-tetracoordinate carbon contain alkyl, aryl, or even bulky substituents on the carbon center. However, recently a... 

The group 4 benzynemetallocenes combined with boron, aluminum, and gallium budding blocks were successftdly employed in the stabihzation of planar-tetracoordinate carbon compounds (equation 26). ... 

In boriozirconocene aUcenes, the chemistry of Csp -Zr and Csp -B bonds differ considerably and this feature allows a sequential route to substituted aUcenes. Furthermore, cleavage of the two types of bonds generally occurs with retention of geometry. On the other hand, heating of aUcynezirconocene complexes with 9-BBN, to 80-90 °C, affords a bimetallic complex (95) showing a planar-tetracoordinate carbon and an wo-BBN structure. 

Gabriel Merino was born in Puebla, Mexico, in 1975. He was educated at Universidad de las Americas, Puebla and Cinvestav (where he studied with Prof. Alberto Vela). After a 2-year postdoctoral stay at TU-Dresden, Germany, where he worked with Prof. Gotthard Seifert and Dr. Thomas Heine in several aspects of magnetic response, he joined the Department of Chemistry at the Universidad de Guanajuato. His research interests include design of molecules containing planar tetracoordinate carbon atoms, study of molecular scalar fields, and electron delocalization. He is a member of the Mexican National System of Researchers. 

Crystal structures have been reported for 2,6-dimethoxyphenyllithium, for 2,6-dimethylaminophe-nylliAium and for o-f-butylthiophenyllithium. The crystal structure of the latter compound is characterized diagrammatically as the infinite polymer (121) with relatively planar tetracoordination at the ipso carbon. In THE solution this polymer dissociates into monomers. Planar four-coordinate carbons are dso observed in the 2,6-dimethoxyphenyl anion (122) as a dimeric unit (123) which forms the basic building block of the solid of this anion. In this solid two of these simple dimers (123) combine to form... 

Theoretical design of electronically stabilized molecules containing planar tetracoordinate carbons... 

Terminal alkyne complexes have also been implicated in the generation of planar tetracoordinate carbon (Scheme 34). These compounds are rapidly trapped with main group organometallics, typically alkylaluminums, to afford bimetallic products.113... 

Zirconocene complexes 705 that contain an acetylide ligand bridging between a main group metal (aluminum) and a transition metal (zirconium) are obtained by treatment of dimethyl zirconocene with (alkynyl)dimethylaluminum, (Equation (43)).529 In this reaction, an ( 72-alkyne)zirconocene complex is presumably formed in situ, and it is then trapped by the excess (alkynyl)dimethylaluminum to yield the final product. The molecular structures of the complexes 705 (R = SiMe3, Cy) contain a dimetallabicyclic framework, and one of the bridgehead positions is a planar tetracoordinate carbon center. In these complexes, the -C=CR bridge between zirconium and aluminum can be described as being mainly of /x-(cr-acetylide) character. 

Treatment of the bis(propynyl)zirconocene 759 with B(C6Fs)3 results in a linear G-G coupling of the alkynyl ligands to form the zwitterionic complex 76 0582,583 (Scheme 186). Complex 760 reacts with nitriles R CN to form initially the 1 1 adduct 761 that concurrently equilibrates with 760 and the metallacyclocumulene 762 (and the nitrile-borane adduct) subsequently, irreversible reaction in the presence of excess nitrile yields the methylene-cyclopropene derivative 763.584,585 Calculations have shown that the conversion 760 —> 763 is probably triggered by nitrile addition to the metal with formation of a planar-tetracoordinate carbon intermediate that features coordination of the three-membered carbocycle through one of its G-G cr-bonds. 

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