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) (.-)
Artikel i vetenskaplig tidskrift, 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.

polymers

porphyrin

exchange

polarized neutron-diffraction

molecular magnets

metalloporphyrins

charge-transfer complexes

behavior

tetracyanoethenide

electron transfer

spin density

ferromagnetic

oxidation

chain

magnetic properties

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Författare

K. Sugiura

A. M. Arif

D. K. Rittenberg

J. Schweizer

Lars Öhrström

Institutionen för oorganisk kemi

A. J. Epstein

J. S. Miller

Chemistry-a European Journal

Vol. 3 138-142

Ämneskategorier

Kemi