Molybdenum 1,4-Diazabuta-1,3-diene Tricarbonyl Solvento Complexes Revisited: From Solvatochromism to Attractive Ligand–Ligand Interaction

Abstract

Various complexes of the types [Mo­(DAD)­(CO)₃L] (L = CO, MeCN, MeOH, THF, DMSO, DMF, Me₂CO, EtOAc, THT, Im^H, Im¹, <i>t</i>BuNC, <i>n</i>Bu₃P), (ER₄)­[Mo­(DAD)­(CO)₃X] (ER₄ = NEt₄⁺; X^– = Cl^–, Br^–, I^–, NCS^–, CN^– and ER₄⁺ = PPh₄⁺; X^– = N₃^–), and (ER₄)­[{Mo­(DAD)­(CO)₃}₂(μ-X)] (ER₄⁺ = NEt₄⁺; X^– = CN^–, OAc^– and ER₄⁺ = PPh₄⁺; X^– = N₃^–; DAD = <i>N</i>,<i>N</i>′-bis­(2,6-dimethylphenyl)­butane-2,3-diimine) were prepared by ligand exchange from cycloheptatriene molybdenum tricarbonyl. A total of 19 crystal structures were determined, including unprecedented structural characterization of molybdenum(0) coordination by dimethyl sulfoxide (DMSO), methanol, ethyl acetate (EtOAc), acetone, and <i>N</i>,<i>N</i>-dimethylformamide (DMF). Correlation of ¹³C NMR shifts with the complex geometry suggests a direct ligand–ligand interaction between DAD and O-bonded coligands with CO and SO double bonds, such as EtOAc, Me₂CO, DMF, and DMSO. Unexpectedly, the solvatochromic properties of these tricarbonyl complexes [Mo­(DAD)­(CO)₃L] are unfavorable for the determination of Kamlet–Taft parameters of the corresponding solvent L. Contrastingly, the UV/vis absorption of [Mo­(DAD)­(CO)₄] is strongly correlated with the Kamlet–Taft parameter π*, which is shown for 22 solvents, including seven room temperature ionic liquids