The Importance of Magnetic Coupling Through Atoms with Large Spin Densities—Structure and Magnetic Properties of meso‐Tetrakis‐(4′‐tert‐Butylphenyl)Porphinatomanganese(III) Hexacyanobutadienide, [MnIIITtBuPP]+[C4(CN)6].‐

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Abstract
[MnIIITtBuPP]+[C4(CN)6].‐ · 5PhMe [MnIIITtBuPP = 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 [C4(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 Å (C = NMn). The Mn–N–C angle is 172.3(4)°, and the intrachain Mn — Mn separation is 10.685 Å. Each [C4(CN)6].‐]' unit is bonded to two MnIII atoms through the interior nitrogen atoms in a trans‐μ2‐N‐σ manner with N–Mn bond lengths of 2.353 Å. The ṽCN absorptions are at 2217 (w, br) and 2190 (m) cm−1. Above 50 K the magnetic susceptibility of [MnIIITtBuPP]+[C4(CN)6].‐ can be fitted to the Curie‐Weiss expression, χ∝1(T – θ), with an effective θ of ‐13 K. This is consistent with weak antiferromagnetic coupling, which is in contrast to the effective θ of +67 K for the uniform chain [MnIIIOEP]+[C4(CN)6].‐ [OEP = octaethylporphinato]. Here, the [C4(CN)6].‐'‐ units are bonded to the MnIII centers through endo CN nitrogen atoms in a similar trans‐μ2 manner. Density functional theory MO calculations reveal that the spin density of the CN nitrogen atom bound to [MnIIITtBuPP]+ (0.019 μBÅ−3) is significantly lower than that of the N atom bound to [MnIIIOEP]+ (0.102 μBÅ−3). This is consistent with the reduced spin coupling observed for [MnIIITtBuPP]+[C4(CN)6].‐with respect to [MnIIITtBuPP]+[C4(CN)6].‐, as evidenced by the lower θ value. The different orientations of the [C4(CN)6].‐ units—almost perpendicular (84.72°) for [MnIIITtBuPP]+[C4(CN)6].‐ and substantially tilted (32.1°) for [MnIIIOEP]+ [C4(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.