The Importance of Magnetic Coupling Through Atoms with Large Spin Densities—Structure and Magnetic Properties of <i>meso</i>‐Tetrakis‐(4′‐<i>tert</i>‐Butylphenyl)Porphinatomanganese(III) Hexacyanobutadienide, [Mn^IIIT<i>t</i>BuPP]⁺[C₄(CN)₆]^.‐

Abstract

Abstract [Mn III T t BuPP] + [C 4 (CN) 6 ] .‐ · 5PhMe [Mn III T t BuPP = 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 Å (C = NMn). The Mn–N–C angle is 172.3(4)°, and the intrachain Mn — Mn separation is 10.685 Å. Each [C 4 (CN) 6 ] .‐ ]' unit is bonded to two Mn III atoms through the interior nitrogen atoms in a trans ‐μ 2 ‐ N ‐σ manner with N–Mn bond lengths of 2.353 Å. The ṽ CN absorptions are at 2217 (w, br) and 2190 (m) cm −1 . Above 50 K the magnetic susceptibility of [Mn III T t BuPP] + [C 4 (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 [Mn III OEP] + [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 ‐μ 2 manner. Density functional theory MO calculations reveal that the spin density of the CN nitrogen atom bound to [Mn III T t BuPP] + (0.019 μ B Å −3 ) is significantly lower than that of the N atom bound to [Mn III OEP] + (0.102 μ B Å −3 ). This is consistent with the reduced spin coupling observed for [Mn III T t BuPP] + [C 4 (CN) 6 ] .‐ with respect to [Mn III T t BuPP] + [C 4 (CN) 6 ] .‐ , as evidenced by the lower θ value. The different orientations of the [C 4 (CN) 6 ] .‐ units—almost perpendicular (84.72°) for [Mn III T t BuPP] + [C 4 (CN) 6 ] .‐ and substantially tilted (32.1°) for [Mn III OEP] + [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.