Closed-Shell Molecules That Ionize More Readily Than Cesium
F. Albert Cotton,1*
Nadine E. Gruhn,2
Jiande Gu,3*
Penglin Huang,1
Dennis L. Lichtenberger,2*
Carlos A. Murillo,1*
Laura O. Van Dorn,2
Chad C. Wilkinson1
We report a class of molecules with extremely low
ionization enthalpies, one member of which has been determined to have
a gas-phase ionization energy (onset, 3.51 electron volts) lower than
that of the cesium atom (which has the lowest gas-phase ionization energy of the elements) or of any other known closed-shell molecule or
neutral transient species reported. The molecules are dimetal complexes
with the general formula M2(hpp)4 (where M is
Cr, Mo, or W, and hpp is the anion of
1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine), structurally characterized in the solid state, spectroscopically characterized in the gas phase, and modeled with theoretical
computations. The low-energy ionization of each molecule corresponds to
the removal of an electron from the delta bonding orbital of the
quadruple metal-metal bond, and a strong interaction of this orbital
with a filled orbital on the hpp ligands largely accounts for the low ionization energies.
1 Department of Chemistry and Laboratory for
Molecular Structure and Bonding, Texas A&M University, Post Office Box
30012, College Station, TX 77842-3012, USA.
2 Department of Chemistry and Laboratory for
Gas-Phase Electron Spectroscopy, The University of Arizona, Tucson, AZ
85721, USA.
3 Drug Design and Discovery Center,
State Key Laboratory of Drug Research, Shanghai Institute of Materia
Medica, Shanghai Institutes for Biological Sciences, Chinese Academy of
Sciences, Shanghai 200031, China.
*
To whom correspondence should be addressed. E-mail:
cotton{at}tamu.edu (F.A.C.); jiandegush{at}go.com (J.G.);
dlichten{at}email.arizona.edu (D.L.L.); murillo{at}tamu.edu (C.A.M.)
