Luminescent Tetrahedral Manganese(II) Pentaphluorophenolate Complex as a Highly Sensitive Molecular Thermometer

A mononuclear tetrahedral manganese complex containing all O-donor ligands has been prepared under mild conditions starting from a dialkylcarbamato manganese(II) precursor. Manganese(II) <i>N,N</i>-dibutylcarbamate [Mn(O₂CNBu₂)₂]_<i>n</i>, <b>1</b>, can be conveniently prepared by extraction from a deoxygenated water solution of manganese(II) sulfate using a CO₂-saturated toluene solution of dibutylamine. Access to the <i>N</i>,<i>N</i>-dibenzylcarbamato manganese complex [Mn(O₂CNBz₂)₂]_<i>n</i>, <b>2</b>, occurs through metathesis by reaction with dibenzylamine and carbon dioxide. By reaction of <b>2</b> with pentafluorophenol, an almost quantitative reaction affords [Bz₂NH₂]₂[Mn(OC₆F₅)₄], <b>3</b>, that has been crystallographically characterized through single-crystal X-ray diffraction. Compound <b>3</b> exhibits absorption and emission spectral features characteristic of Mn²⁺ ions in a tetrahedral coordination environment. Upon cooling, the emission intensity was observed to increase by approximately two orders of magnitude. The excited-state lifetimes exhibited significant temperature dependence, ranging from 12.7 ms at 80 K to 10 μs at 290 K. The temperature-dependent trends of both emission intensity and lifetimes showed nearly identical profiles. As a result, compound <b>3</b> functions as a dual-mode highly sensitive luminescent molecular thermometer, with a maximum relative thermal sensitivity (<i>S</i>_r) of 7.4% K^-1 at 220 K and <i>S</i>_r >1 over the temperature range 170-270 K. A distinctive feature of compound <b>3</b> is its capacity to yield equivalent luminescent molecular thermometers (LMT) using either the emission intensity or lifetime, thus enhancing its versatility in thermal sensing applications.