Self-assembled Transition Metal Coordination Frameworks of ...

Chapter 5 _ H _ g g A
temperature, concentration etc. It is seen that most of the complexes fragmented
extensively and is more for carbohydrazones derived Mn(II) complexes. The
[Mn(HL)]*" fragment was readily observed in all cases, with the presence of
characteristic dinuclear and trinuclear fragments of assigned square structures. The
electronic spectra of all complexes are consistent with Mn(Il) centers. The variable
temperature magnetic studies are in agreement with antiferromagnetic couplings
among Mn(II) centers, with the possible tetranuclear Mn(lI) complexes. The X-band
EPR spectra in frozen DMF solution show many features for 19 and 20, is in
accordance with the presence of more than one metal centers. The features of EPR
spectra of other complexes 21 and 22 are close to indicate mononuclear octahedral
Mn(II), attributed to fragmentation in DMF solution. This is attributed to the
concentration of taken solution; for example a very closely related complex [20] in
acetonitrile remains as tetramer only above 25 uM. An ESI MS study show peaks
corresponding to tetrameric species only at higher concentrations. Also, the same
complex shows different EPR characteristics on varying the concentration; a g = 2
resonance with nearly 2200 G in width at 77 K in acetonitrile/dichloromethane and
EPR silent at 5 K. So for a highly coupled Mn(II) cluster a resonance at g = 2 with
high width is expected; however the fragmentation in DMF is clear in the spectra of
complexes 19, 21 and 22 under the investigation conditions. The EPR spectrum
recorded for complex 23 in DMF at 77 K lacks any signal. However, it is not possible
to get a complete characterization between mononuclear and multinuclear Mn(H)
complexes by X-band EPR spectra alone. The MALDI mass spectrum of 23 also
didn’t show any characteristic peaks attributed to lack of enough concentration or due
to complete fragmentation in solution. Molar conductivity measurement in l0'3 M
DMF solution of complex 19 shows a 2:1 electrolyte (121 Q"cm2mol") [27] and
complex 20 exhibits nonelectrolytic nature (9 Q"cm2mol"). For Mn(H)
carbohydrazone complexes, the conductance values obtained were 68, 62 and 61 Q‘
200