Electronic structure of [Ga 2(tren) 2(CA sq,cat)](BPh 4) 2(BF 4): An EPR, ENDOR, and density functional study

• Published in: Inorganica Chimica Acta 2008-09, Vol.361 (12), p.3539-3547
• Main Authors: Schrauben, Joel N., Guo, Dong, McCracken, John L., McCusker, James K.
• Format: Article
• Language: English
• Subjects: DFT; ENDOR; EPR; Gallium; Semiquinone; Spin exchange
• ISSN: 0020-1693; 1873-3255
• DOI: 10.1016/j.ica.2008.03.021

The electronic structure of a semiquinone-bridged Ga III dimer has been investigated. EPR and ENDOR hyperfine coupling between the organic radical and quadrupolar Ga III nuclei as well as atoms within the bridge itself were observed and accurately simulated. The hyperfine interactions were semi-quantitatively reproduced by DFT calculations to yield additional insights into the spin polarization mechanisms operative within these complexes. The bimetallic [M 1M 2(tren) 2(CA n− )] m+ series, where M = Ga III or Cr III and CA is the chloranilate ligand which can take on diamagnetic (CA cat,cat) 4− or paramagnetic (CA sq,cat) 3− forms, comprises an electronically diverse series of compounds ranging from the closed-shell [Ga 2(tren) 2(CA cat,cat)] 2+ to the S = 5/2 ground state of [Cr 2(tren) 2(CA sq,cat)] 3+. This report deals with the interpretation of the EPR and ENDOR spectra of [Ga 2(tren) 2(CA sq,cat)](BPh 4) 2(BF 4) ( 2) and the related derivative [Ga 2(tren) 2(DHBQ)](BPh 4) 2(BF 4) ( 2a) (where DHBQ is the fully deprotonated trianionic form of 2,5-dihydroxy-1,4-benzoquinone) in an effort to further characterize the electronic structure of this radical species. The X-band (∼9.5 GHz) EPR spectrum of complex 2 acquired in a butyronitrile/propionitrile glass at 4 K reveals a rhombic g-tensor with g xx = 2.0100, g yy = 2.0097, and g zz = 2.0060 with hyperfine interactions due to spin delocalization onto the two Ga nuclei ( a xx = 4.902 G, a yy = 4.124 G, a zz = 3.167 G); the origin of the hyperfine coupling was confirmed by analysis of the room temperature spectra of complexes 2 and 2a. The low-temperature spectrum of complex 2 also indicates the presence of a triplet electronic state characterized by a g-value of 2.009 and axial zero-field splitting of D = 150 G (0.012 cm −1) as determined from measurements carried out at both X- and W-band (∼95 GHz) frequencies. This triplet state is believed to arise due to a weak intermolecular Heisenberg exchange interaction between two aggregating complexes. ENDOR measurements on complex 2a at 20 K allowed for a determination of the magnitude of hyperfine coupling to the protons associated with the radical bridge as well as providing a rare example of an ENDOR signal arising from coupling to a gallium nucleus. Finally, these results were combined with literature data on the free semiquinone form of the bridging ligand in order to assess the extent to which density functional theory can predict unpaired spin density dist