BOROLE DERIVATIVES .22. IODINE DEGRADATION OF THE TRIPLE-DECKER COMPLEXES (MU-C4H4BR)[RH(C4N4BR)](2) (R = PH, ME) - SYNTHESIS AND STRUCTURAL CHARACTERIZATION OF HETEROCUBANES [RH(MU(3)-I)(C4H4BR)](4) AND OF BIS(BOROLE)RHODIUM COMPLEXES RHI(C4H4BR)(2)

The triple-decker complexes (mu-C4H4Br)[Rh(C4H4BR)](2) (R double bond Ph, Me) (1) react with elemental iodine in toluene or dichloromethane to produce heterocubanes [Rh(mu(3)-I)(C4H4BR)](4) (2) and bis(borole)i odorhodium complexes RhI(C4H4BR)(2) (3). The heterocubanes 2 exchange fragments RhI(C4H4BR) at rates which are fast with respect to the NMR time scale, The bis(borole) compounds 3 exist in an equilibrium of rot ational isomers C-s-3 double bond C-2v-3 with Delta(R)H = (-1.7 +/- 0. 2) kJ mol(-1) and Delta(R)S = (-16 +/- 1)J mol(-1) K-1 for 3b. Activat ion parameters for the forward and reverse isomerization reactions are reported, The isomerization takes place by the mechanism of ring-ring rotation; this is demonstrated for 3b by TOCSY NMR spectra (H-1 NMR, -80 degrees C, mixing time 150 ms). Crystal structure determinations o f 2a, 2b, and 3b are reported. The higher-energy isomer C-s-3b is foun d in the crystal of 3b. One borole ligand of C-s-3b is planar and eta( 5)-bonded with a normal Rh-B1, bond distance [236.7(6) pm]. The second ligand shows an unusual folding of 15.8(9)degrees along the line C2-C 5, a very weak bonding interaction Rh-B2 [252.8(6) pm] approaching eta (4)-coordination, and a close contact I-B2 [295.3(6) pm], Both borole ligands show diene-like C-C bond length patterns with weak Rh-to-borol e back-bonding.