Τίτλος:
Isolation, Characterization, and Computational Studies of the Novel
[Mo-3(mu(3)-Br)(2)(mu-Br)(3)Br-6](2-) Cluster Anion
Περίληψη:
The novel trimolybdenum cluster [Mo-3(mu(3)-Br)(2)(mu-Br)(3)Br-6](2-)
(1, Mo-3(9+), 9 d-electrons) has been isolated from the reaction of
[Mo(CO)(6)] with 1,2-C2H4Br2 in refluxing PhCl. The compound has been
characterized in solution by electrospray ionization mass spectrometry
(ESI-MS), UV-vis spectroscopy, cyclic voltammetry, and in the solid
state by X-ray analysis (counter-cations: (n-Bu)(4)N+ (1), Et4N+,
Et(3)BzN(+)), electron paramagnetic resonance (EPR), magnetic
susceptibility measurements, and infrared spectroscopy. The least
disordered (n-Bu)(4)N+ salt crystallizes in the monoclinic space group
C2/c, a = 20.077(2) angstrom, b = 11.8638(11) angstrom, c = 22.521(2)
angstrom, alpha = 90 deg, beta = 109.348(4) deg, gamma = 90 deg, V =
5061.3(9) angstrom(3), Z = 4 and contains an isosceles triangular metal
arrangement, which is capped by two bromine ligands. Each edge of the
triangle is bridged by bromine ions. The structure is completed by six
terminal bromine ligands. According to the magnetic measurements and the
EPR spectrum the trimetallic core possesses one unpaired electron.
Electrochemical data show that oxidation by one electron of 1 is
reversible, thus proceeding with retention of the trimetallic core,
while the reduction is irreversible. The effective magnetic moment of 1
(mu(eff), 1.55 mu(B), r.t.) is lower than the spin-only value (1.73
mu(B)) for S = 1/2 systems, most likely because of high spin-orbit
coupling of Mo(III) and/or magnetic coupling throughout the lattice. The
ground electronic state of 1 was studied using density functional theory
techniques under the broken symmetry formalism. The ground state is
predicted to exhibit strong antiferromagnetic coupling between the three
molybdenum atoms of the core. Moreover, our calculated data predict two
broken symmetry states that differ only by 0.4 kcal/mol (121 cm(-1)).
The antiferromagnetic character is delocalized over three magnetic
orbitals populated by three electrons. The assignment of the infrared
spectra is also provided.
