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.