Functionalized nido-C4B2, closo-C2B5 and -C2B10 Carboranes, and Reactivity Studies on Electron-Poor 2,3-Dihydro-1,3-diborolyl Complexes of Ruthenium

The dissertation describes work on organoborane/carborane compounds (Chapters 3.1, 3.2, 3.3, 3.5), on the reactivity of 2,3-dihydro-1,3-diborolyl complexes of ruthenium (3.4), and on aminoborane derivatives (3.6). In Chapter 3.1, a one-pot synthesis of 1,6-diiodo-2,3,4,5-tetracarba-nido-hexaborane(6) derivatives (nido-(RC)4(BI)2, R = Et, Me, Ph, 5a-c) is reported, involving disubstituted alkynes and BI3 (in 1:1 ratio) and NaK2.8 at low temperature. Whereas the reaction of 3-hexyne, BI3 (2 equiv.) and NaK2.8 at r.t. affords a mixture of 5a, nido-(EtC)4(BI)4 (4) and closo-(EtC)2(BI)5 (6). A possible mechanism of the formation of 5a is proposed and studied. The reactivity of 5a towards various nucleophiles has been investigated. In most cases, the substitution occurs regiospecifically at the basal boron atom. Replacement of the ‘inert’ apical iodine is realized by a Pd(0)-catalyzed Negishi-type cross-coupling, as demonstrated by the synthesis of apically alkynyl-substituted nido-(RC)4(BC2Ph)(BC2R) (R = SiMe3, Ph, 5q,r). A series of linked clusters with different types of linkages (via B–C bond, a C6H4C6H4 unit, a C2(CH2)4C2 unit, and an oxygen atom) are obtained. In Chapter 3.2, treatment of the nido-2,3-Et2C2B4H42- dianion with the reagents BX3 (X = Br, I) and PhC2Bcat lead to apically functionalized closo-1-R-2,3-Et2C2B5H4 derivatives (19a-c). A more efficient pathway is developed by the Pd-catalyzed cross-coupling reactions of 19b with R’C2ZnCl to give closo-1-C2R’-2,3-Et2C2B5H4 (R’ = SiMe3, Me, tBu, 19d-f). The reac- tions of the carboranyl-acetylenes with Co2(CO)8 and CpCo(C2H4)2, respectively, afford dicobaltatetrahedrane compounds 21c,d and CpCo(cyclobutadiene) complex 23. In Chapter 3.3, a series of C-boryl-o-carborane derivatives are reported by reacting dilithio-o-carborane and the corresponding aminochloroboranes, respectively. In Chapter 3.5, the unprecedented transformation of 1,3-diiodo-1,3-diborole 37d into the known 2,3,4,5,6-pentacarba-nido- hexaborane(6) cation (63+) is observed. A possible mechanism for its formation is proposed. In Chapter 3.4, the long-sought crystal structure of the (pentamethylcyclopentadienyl)(2,3-dihydro-1,3-diborolyl)-ruthenium derivative 35b is described. It finally confirms the folding along the B…B vector (40.7o) which is similar to the iron analog (41.3o). DFT calculations (by Dr. I. Hyla-Kryspin) on the electronic structures of the model sandwiches Ru (35) and Fe (36) indicate that the folding of the 1,3-diborolyl ligands is of electronic origin, and the parent compounds 35 and 36 with a folded 1,3-diborolyl ligand are more stable in energy (24.5 and 24.9 kcal/mol for Ru and Fe structures, respectively) than 35’ and 36’ with a planar ligand. The reactivity of 35 is studied with respect to insertion of terminal alkynes into its C3B2 heterocycle which results in novel 18 VE ruthenocene analogs Cp*Ru(C5B2) (38) with seven-membered 4-borataborepines as ligands. Additionally, boratabenzene complexes 39 and triarylbenzene species (in some cases) are identified as byproducts, the formation of 39 is a result of elimination of one boranediyl moiety [:BR2] from the intermediate 4-borataborepine complexes. The 4-borataborepine functions as 6e ligand and exhibits a reduced folding along the B…B vector (folding angle = 12.5o in 38a, 26o in 38b, and 15.2o in 38c) compared with 40.7o in 35b. The boratabenzene complexes 39f,g are formed as the main products in the reactions of 35a and disubstituted alkynes (3-hexyne and di-p-tolylacetylene, respectively). The comparative study on the reactivity of the [(1,3-diborole)RhCl]2 dimer 48 with PhCH2C2H is carried out, in which the cyclotrimerization occurs to give (1,3-diborolyl)Rh(arene) 49. The complex reaction of 35a with allylchloride in hexane affords a mixture of products, one is the novel complex 58 [Cp*Ru(C3B2Me5)(RuCp*)n] (n > = 2). Its identity and formation mechanism are not yet clear, however, the results from MS studies and two independent but only partially solved X-ray diffraction analyses indicate that it has a stack of ruthenium atoms with bridging Cp* and 1,3-diborolyl ligands. The stacks are along a threefold axis which would be consistent with either extensive disorder or a tripling of the crystals. In Chapter 3.6, the reactivity of (Me2N)2B4[B(NMe2)2]2 (70) with a rhombohedral B4 unit is studied, and some pyrrolidinoborane derivatives are prepared for new cyclo or polyhedral boranes.