title: Topological and electronic influences on magnetic exchange coupling in Fe(III) ethynylbenzene dendritic building blocks.
abstract: 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)(2)FeCl(C(2)Ph)](OTf) (1), [(dmpe)(4)Fe(2)Cl(2)(μ-p-DEB)](BAr(F)(4))(2) (2), [(dmpe)(6)Fe(3)Cl(3)(TEB)] (3), [(dmpe)(6)Fe(3)Cl(3)(μ(3)-TEB)](OTf)(3) (4), and [(dmpe)(4)Fe(2)Cl(2)(μ-m-DEB)](BAr(F)(4))(2) (5) [dmpe = 1,2-bis(dimethylphosphino)ethane; p-H(2)DEB = 1,4-diethynylbenzene; BAr(F)(4) = tetrakis[3,5-bis(trifluoromethyl)phenyl]borate; H(3)TEB = 1,3,5-triethynylbenzene; m-H(2)DEB = 1,3-diethynylbenzene]. As expected, the ligand topology drives the antiferromagnetic coupling in 2 (J = -134 cm(-1) using the Ĥ = -2JŜ(1)·Ŝ(2) 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* = η(5)-C(5)Me(5)). 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.
authors: Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523-1872, USA.
authors: Wesley A
authors: WA
authors: Hoffert
authors: Anthony K
authors: AK
authors: Rappé
authors: Matthew P
authors: MP
authors: Shores