Conformational analysis of mono- and di-C-substituted derivatives ofcloso-3,3-[η2,3-(2-methylenebicyclo[2.2.1]hepta-2,5-dien-2-yl)]-1-R-2-R1-3,1,2-dicarbollylrhodium (R and R1=h, Me, or PhCH2)-(2-methylenebicyclo[2.2.1]hepta-2,5-dien-2-yl)]-1-R-2-R1-3,1,2-dicarbollylrhodium (R and R1=h, Me, or PhCH2)

Abstract

A model has been developed that allows one to carry out molecular mechanics calculations of metallacarborane complexes with various ligands with the use of the MM3 program package. Based on X-ray structural data on a number of rhodium complexes in which the rhodium atom is coordinated by the ~12.3-norbomadienyl and rlS-dicarbollyl ligands, a parameter set was developed for the compounds of the above-mentioned series. The use of this model made it possible to study the effects of various factors on the structures of theKey words: molecular mechanics, closo-rhodacarboranes, stereochernistry.An important characteristic feature of the nS-dicarbollyl ligand is the absence of symmetry, in its monosubstituted derivatives that contain substituents at C atoms and some B atoms. Therefore, the presence of an additional asymmetrical center in mono-C-substi- The single-crystal X-ray diffraction study of two pairs of the diastereomers, (RS/SR)-and (SS/RR)-closo-3,3-(rl2,3-CTHTCH2)-I-R-3,1,2-RhC2BgHt0 (la,b, R = PhCH2; 3 and 2a,b, R = Me),* made it possible to find an interesting dependence of the solid-state conformation of the norbomadienyl ligand on the stereochemical configuration of the diastereomeric complexes. Thus, in isomers la and 2a, which have RS/SR relative configurations, the norbomadienyl ligand is oriented so that its allylic moiety is projected onto the pentagonal C2B 3 plane of the ~15-dicarbollyl ligand and screens the C(2) and B (7) atoms. In the diastereomers with the opposite configuration (lb and 2b), this group is located approximately above the B(8)--B(4) bond (see below). It is interesting that in the solid state the hydrocarbon ligand in the complex, closo-3,3-(r~2'LC7HTCH2)-3,1,2-RhC2BgHII (3) (three independent molecules), 4 and in its 1,2-dimethyl derivative, closo-3,3-(~2.3-C7HTCH2) -1,2-Me2-3 , 1,2-R_hC2BgH 9 (4), t adopts a conformation similar to that observed for isomers la and 2a in spite of the fact that in compound 4 the distance between the exocyclic C atom of the norbornadienyl ligand and the C atom of the nearest methyl substituent in the ~lS-dicarboUyl Iigand is 3.074 A, Le., it is substantially smaller that the sum of the van der Waals radii of the C atoms (3.4 k).