Density Functional Study On The Structures And Thermochemical Properties Of Binuclear Manganese Acetylene Carbonyl Derivatives

Transition metal carbonyl chemistry is a major research field in modern organometallic chemistry.The binuclear manganese acetylene carbonyl compounds(Et₂NC=CMe)Mn₂(CO)_n(n=7,8)have been synthesised experimentally,the phenomenon of hydrogen migrations from the methyl group attaching to the alkyne carbon atom has been identified by the X-ray crystal diffraction methods in the isomers of the(Et₂NC=CMe)Mn₂(CO)_n structure.The effects of the ligands connecting to the alkyne carbon atom on the structures,thermochemistry,and the manganese-manganese bonding characters have been investigated systematically in the(RC=CR')Mn₂(CO)_n(R and R'=Meor Me₂N;n=10,9,8,7,6)complexes using the M06-L/DZP method.The main studies are as follows.(1)In the compound(alkyne)Mn₂(CO)_n(n=10,9,8,7,6)with the ligand Me₂NC=CNMe₂,no H migration was observed.For the(Me₂N)₂C₂Mn₂(CO)₁₀ system,the electronic configuration of the manganese atom is saturated,so that the Mn atom is mostly attached to the alkyne carbon atom in a single coordination mode and no metallic bonds were predicted in this system;for the(Me₂N)₂C₂Mn₂(CO)₉system,the nitrogen atoms can provide electrons to manganese atoms.for the eight carbonylsystem(Me₂N)₂C₂Mn₂(CO)₈,the lowest energy structure is similar to the experimental structure without any hydrogen migration;for the(Me₂N)₂C₂Mn₂(CO)₇ system,a variety of coordination modes were predicted to compensate electrons deficiency of the metal atoms;in the hexacarbonyl system(Me₂N)₂C₂Mn₂(CO)₆,the lowest energy structure was predicted to undergo a complete cleavage of the central C=C triple bonds.(2)When one or two diamethylamino ligands Me₂N was(or were)substituted by the methyl group(s)to form the MeC=CMeor Me₂NC=CMeligand,hydrogen migrations from the methyl group to the alkyne carbon atoms were predicted in three different ways:the bridging propadiene ligand RCH=C=CH₂ can be formed when the H atom on one methyl group is transferred to a distant alkyne carbon;the bridging RCH=CH-CH ligand can be obtained when two hydrogens on the methyl groups are transferred to each alkyne carbon atom;if one hydrogen atom on the methyl group is transfered to the adjacent alkyne carbon atom,the bridging vinyl ligand CH₂-CH=C(R)can be formed.(3)Conversion of the analogue structure(MeC₂NMe₂)Mn₂(CO)₈,which is the experimental synthesized and XDR determined(MeC₂NEt₂)Mn₂(CO)₈ structure,to the structure Mn₂(CO)₇[m-?⁴-C₃H₃NMe₂]with hydrogen migrations from the methyl group to the central alkyne carbon atoms may involve eight transition states.The activation energies for the first and the second hydrogen migration are calculated to be?20 kcal/mol and?42kcal/mol,respectively.This thesis is presented in the following four chapters.Chapter 1 is an introduction,Chapter 2 is the introduction of the theoretical method,and Chapter 3 is the analysis of the calculations.Chapter 4 concludes this thesis.