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We prepared two new analogues of ([CH 3 CO((DO)(DOH)pn)L] + ) [(DO)(DOH)pn = N 2 ,N 2 '-propane-1,3-diylbis(2,3-butanedione-2-imine-3-oxime)] B 12 models but with an O-BF 2 -O unit replacing the O-H···O unit as follows: [CH 3 Co((DO)(DOBF 2 )pn)L]PF 6 with L = pyridine (py) and 1,5,6-trimethylbenzimidazole (Me 3 Bzm). Our goal was to compare the properties of these new O-BF 2 -O complexes with the well-established O-H···O analogues. The Co-CH 3 1 H NMR shifts indicate that the BF 2 group makes the Co(III) less electron rich. The X-ray crystal structures determined for the new compounds were compared to the one known structure with L = imidazole (Im). With increasing size of L, in the series Im < py < Me 3 Bzm, the plane of L orients so as to avoid the bulky BF 2 group. This orientation effect becomes apparent in the L 1 H NMR shifts, which are not sensitive to Co(III) electronic properties. Thus, in the O-BF 2 -O versus the O-H···O analogue, the Me 3 Bzm H4 signal shifts 0.41 ppm upfield from the anisotropic effect of the equatorial ligand double bonds. We advance the concept (applicable to a broad series of complexes) that steric interactions between L and the equatorial ligands are alleviated by a combination of Co-N ax bond elongation and opening of the N eq -Co-N ax angles. |
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