The importance of magnetic coupling through atoms with large spin densities - Structure and magnetic properties of meso-tetrakis-(4'-tert-butylphenyl)porphinatomanganese(III) hexacyanobutadienide, Mn(III)TtBuPP (+) C-4(CN)(6) (.-)

Journal article, 1997

[Mn(III)TtBuPP](+)[C-4(CN)(6)](.-) . [(MnTrBuPP)-Tr-III = meso-tetrakis(4'-tert-butylphenyl)porphinatomanganese(III)] has been prepared and structurally and magnetically characterized. The uniform, linear-chain (1-D) coordination polymer comprises alternating cations and anions. The bond lengths in planar ion [C-4(CN)(6)](.-) are 1.377(10) (CC-CC), 1.418(7) (C-CCC), 1.414 (C-CN), 1.457 (C-CNMn), 1.150 (C=N), and 1.134 Angstrom (C=NMn). The Mn-N-C angle is 172.3 (4)degrees, and the intrachain Mn ... Mn separation is 10.655 Angstrom. Each [C-4(CN)(6)](.-) unit is bonded to two Mn-III atoms through the interior nitrogen atoms in a times-mu(2)-N-sigma manner with N-Mn bond lengths of 2.353 Angstrom. The <(nu)over tilde>(CN) absorptions are at 2217 (w,br) and 2190 (m) cm(-1). Above 50 K the magnetic susceptibility of [Mn(III)TtBuPP](+)[C-4(CN)(6)](.-) can be fitted to the Curie-Weiss expression, chi proportional to 1/(T - theta), with an effective theta of -13 K. This is consistent with weak antiferromagnetic coupling, which is in contrast to the effective theta of + 67 K for the uniform chain [(MnOEP)-O-III](+)[C-4(CN)(6)](.-) [OEP = octaethylporphinato]. Here, the [C-4(CN)(6)](.-) units are bonded to the Mn-III centers through endo CN nitrogen atoms in a similar trans-mu(2) manner. Density functional theory MO calculations reveal that the spin density of the CN nitrogen atom bound to [Mn-III- TtBuPP](+) (0.019 mu(B) Angstrom(-3)) is significantly lower than that of the N atom bound to [(MnOEP)-O-III]+ (0.102 mu(B) Angstrom(-3)). This is consistent with the reduced spin coupling observed for [Mn(III)TtBuPP](+)[C-4(CN)(6)](.-) with respect to [(MnOEP)-O-III](+)[C-4(CN)(6)](.-), as evidenced by the lower theta value. The different orientations of the [C-4(CN)(6)](.-) units-almost perpendicular (84.72 degrees) for [Mn(III)TtBuPP](+)[C-4(CN)(6)](.-) and substantially tilted (32.1 degrees) for [(MnOEP)-O-III](+)[C-4(CN)(6)](.-)-may also contribute to the poorer overlap and weaker spin coupling. Hence, binding between sites with large spin densities is needed to stabilize strong ferromagnetic coupling.