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

Various complexes of the types [Mo­(DAD)­(CO)3L] (L = CO, MeCN, MeOH, THF, DMSO, DMF, Me2CO, EtOAc, THT, ImH, Im1, tBuNC, nBu3P), (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 = N,N′-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 N,N-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.