Topological and Electronic Influences on Magnetic Exchange Coupling in Fe(III) Ethynylbenzene Dendritic Building Blocks

Significant variance in the magnitude of reported exchange coupling parameters (both experimental and computed) for paramagnetic transition metal–ethynylbenzene complexes suggests that nuances of the magnetostructural relationship in this class of compounds remain to be understood and controlled, toward maximizing the stability of high-spin ground states. We report the preparation, electrochemical behavior, magnetic properties, and results of computational investigations of a series of iron ethynylbenzene complexes with coordination environments suitable for metallodendrimer assembly: [(dmpe)2FeCl(C2Ph)](OTf) (1), [(dmpe)4Fe2Cl2(μ-p-DEB)](BArF4)2 (2), [(dmpe)6Fe3Cl3(TEB)] (3), [(dmpe)6Fe3Cl3(μ3-TEB)](OTf)3 (4), and [(dmpe)4Fe2Cl2(μ-m-DEB)](BArF4)2 (5) [dmpe = 1,2-bis(dimethylphosphino)ethane; p-H2DEB = 1,4-diethynylbenzene; BArF4 = tetrakis[3,5-bis(trifluoromethyl)phenyl]borate; H3TEB = 1,3,5-triethynylbenzene; m-H2DEB = 1,3-diethynylbenzene]. As expected, the ligand topology drives the antiferromagnetic coupling in 2 (J = −134 cm–1 using the Ĥ = −2JŜ1·Ŝ2 convention) and the ferromagnetic coupling in 4 and 5 (J = +37 cm–1, J′ = +5 cm–1 for 4; J = +11 cm–1 for 5); the coupling is comparable to but deviates significantly from values reported for related Cp*-containing species (Cp* = η5-C5Me5). The origins of these differences are explored computationally: a density functional theory (DFT) approach for treating the coupling of three spin centers as a linear combination of single-determinantal descriptions is developed and described, and the results of these computations can be generalized to other paramagnetic systems. Unrestricted B3LYP hybrid DFT calculations performed on rotamers of 4 and 5 and related complexes, as well as Cp* analogues, provide J values that correlate with the experimental values. We find that geometric considerations dominate the magnetism of the Cp* complexes, while topology and alkynyl ligand electronics combine more subtly to drive the magnetism of the new complexes reported here. These calculations imply that substantial magnetic exchange parameters, with accompanying well-isolated high-spin ground states, are achievable for ethynylbenzene-bridged paramagnetic metallodendrimers.