Activation Small Molecules At ?A Low-Valent Metal Centers

The chemistry of metal-metal bonds and carbine metal complex with low-coordinate metal ions in the unusual low oxidation states has attracted much attention in recent years.These unusual valence metals are extremely sensitive to water and oxygen and difficult to obtain under normal conditions.These species have attracted great interest not only because of the novel bonding nature of the low-valent,low-coordinate metal centers,but also because they display fascinating reactivity toward a variety of small molecules as well as potential applications in catalysis.The key to the formation of metal-metal bonds is the need for appropriate steric hindrance of the ligand to stabilize.Therefore,we use the redox ligands to synthesize main groups of metal-metal bond compounds,explore the reactivity of these compounds to small molecules,and obtain the catalytic intermediates that are difficult to capture under the conventional conditions.This work can not only expand the types of new metal-metal bonds,but also provide further evidence for the application of main elements in catalysis.This thesis consists of three parts:1.The ?-diimine-stabilized dialumane[L2-(THF)Al?-Al?(THF)L2-]L=dpp-dad=[(2,6-iPr2C6H3)NC(CH3)]2)(1)reacts with 2 equiv of tert-butyl isocyanide(tBuNC),through reductive C-C coupling,to give the aluminum complex[L2Al?(tBuN=C-C=N/Bu)2-Al?L2-](2)with formation of the 1,4-diazabutadiene-2,3-diyl fragment.Reaction of 1 with 3 equiv of tBuNC in the presence of Na metal proceeds via reductive homocoupling of three isocyanide molecules,affording two paramagnetic isomers[Na]+[L2-Al?{(tBuNC)3}3·-Al?L2-]-(3 and 4).More interestingly,dialumane 1 can also promote cyclo-trimerization of the isocyanide,in the presence of Na and excess of tBuNC,to form the dinuclear complex[Na]+[L2-Al?{?-?1:?2-(tBuNC)3}2-Al?(C?N)-L2-]-(5)containing a three-membered carbocycle[C3N3(tBu)3]2-('deltamidinate' dianion).Compounds 3-5 represent first examples of reductive linear and cyclo-trimerization of isocyanides mediated by a metal-metal-bonded complex.On the other hand,the dianionic[tBuN=C-C=NtBu]2-fragment in product 2 can be reduced by Na to the trianion and tetraanion,yielding complexes[(Na(THF)6)]+[(L2-Al?(tBuN(?)C(?)C(?)NtBu)3·-Al?L2-)]-(6)and[Na2]2+[L2-Al?(tBuN-C=C-N/Bu)4-Al?L2-]2-(7)respectively.Products 2-7 have been fully characterized by X-ray diffraction,spectroscopic methods,and DFT computations.2.In the presence of Na metal the related digallane[LGa-GaL](2)was converted to the gallium(?)carbene analogue[LGa:]-(2A),which undergoes a variety of reactions with isocyanate substrates.These include the cycloaddition of ethyl-isocyanate to[LGa:]-affording[Na2(THF)5]{LGa[EtN-C(O)]2GaL}(8),cleavage of the N=C bond and release of 1 equiv of CO to give[Na(THF)2]2[LGa(p-MeC6H4)(N-C(O))2-N(p-MeC6H4)]2(9),cleavage of C=O bond to yield the di-O-bridged digallium compound[Na(THF)3]2[LGa-(?-O)2-GaL](10),and the further addition product[Na2(THF)5][LGa(CyNCO2)]2(11).Complexes 8-11 have been characterized by NMR(1H,13C),IR spectroscopy,elemental analysis and X-ray diffraction.Their electronic structures have been examined by DFT calculations.