Design and Demonstration of Ferromagnetic Exchange Interactions in Organic Molecules

Solvent dependence of the structure and magnetic ordering of ferrimagnetic manganese(III) meso-tetraphenylporphyrin tetracyanoethenide, [MnTPP]+[TCNE]*-

Tetraphenylporphinatomanganate(III) tetracyanoethenide, [MnTPP][TCNE], is the prototype of a growing family of linear chain (1-D) coordination polymers that magnetically order as ferrimagnets. [MnTPP][TCNE].xS [S = PhMe (x = 2), 1,2-C(6)H(4)Me(2) (x = 1), 1,2-C(6)H(4)Cl(2) (x = 3), 1,2,4-C(6)H(3)Cl(3) (x = 2), and 1,3-C(6)H(4)Cl(2) (x = 2)] have been prepared and structurally and magnetically characterized. All form 1-D chain structures with intrachain Mn.Mn separations ranging from 9.202 to 10.218 A. The 173 K crystal structure of [MnTPP][TCNE].2PhMe has been rerefined, revealing that the [TCNE](*)(-) is 2-fold-disordered and coordinated to Mn(III) by a pair of trans cyano nitrogen atoms to form parallel one-dimensional chains. The two orientations of [TCNE](*)(-) are related by a 180 degrees rotation about the diagonal axis joining the trans nitrogen atoms bound to Mn(III). The major form has an occupancy of 83.3(4)% with a Mn-N(TCNE) distance of 2.328(3) A and a MnNC angle of 146.8(8) degrees. The minor form, with 16.7(4)% occupancy, has a Mn-N(TCNE) distance of 2.176(15) A and a MnNC angle of 152.3(39) degrees. Lattice packing and molecular bonding imply static as opposed to dynamic disorder. The magnetic properties depend on the type and quantity of the solvent present in the structure. Desolvation via heating in n-octane (127 degrees C), n-dodecane (216 degrees C), and/or vacuum thermolysis (175 degrees C) leads to numerous different desolvated materials with differing magnetic properties. At higher temperatures the magnetic susceptibility can be fit by the Curie-Weiss expression, chi varies with (T - theta)(-1), with theta = 44, 52, 72, 55, and 77 K for the toluene, 1,2-xylene, 1,2-dichlorobenzene, 1,3-dichlorobenzene, and 1,2,4-trichlorobenzene solvates, respectively. The T(c)'s were taken as the maximum in 10 Hz chi'(T) and are 7.8, 9.2, 11.3, 10.8, and 8.2 K for the PhMe, 1,2-xylene, 1,2-dichlorobenzene, 1,3-dichlorobenzene, and 1,2,4-trichlorobenzene solvates, respectively. Upon desolvation the T(c)'s increase for the PhMe, 1,2-xylene, 1,2,4-trichlorobenzene solvates and decrease for the 1,2- and 1,3-dichlorobenezene solvates. The compounds show one-dimensional ferrimagnetic exchange behavior at high temperatures with intrachain exchange of J/k(b) = -63, -99, -234, -100, and -200 K for toluene, 1,2-xylene, 1,2-dichlorobenzene, 1,3-dichlorobenzene, and 1,2,4-trichlorobenzene solvates, respectively, as determined from fits to the Seiden expression, which models isolated 1-D interactions among alternating S = 2 classical and S = (1)/(2) quantum spins. For variation in the temperature at which the peak occurs per decade of frequency, phi, (DeltaT(f)/T(f))/Delta(log omega) is 0.167, 0.168, 0.066, 0.171, and 0.024 for toluene, 1,2-xylene, 1,2-dichlorobenzene, 1,3-dichlorobenzene, and 1,2,4-trichlorobenzene, respectively, typical of spin glass behavior. Since [TCNE](*)(-) is only disordered for the PhMe solvate and all five solvates exhibit spin glass behavior, the spin glass behavior cannot be attributed to this disorder.

Roles of Bridging Ligand Topology and Conformation in Controlling Exchange Interactions between Paramagnetic Molybdenum Fragments in Dinuclear and Trinuclear Complexes

The magnetic properties of two series of dinuclear complexes, and one trinuclear complex, have been examined as a function of the bridging pathway between the metal centers. The first series of dinuclear complexes is [{Mo(V)(O)(Tp)Cl}(2)(&mgr;-OO)], where "OO" is [1,4-O(C(6)H(4))(n)O](2)(-) (n = 1, 1; n = 2, 3), [4,4'-O(C(6)H(3)-2-Me)(2)O](2)(-) (4), or [1,3-OC(6)H(4)O](2)(-) (2) [Tp = tris(3,5-dimethylpyrazolyl)hydroborate]. The second series of dinuclear complexes is [{Mo(I)(NO)(Tp)Cl}(2)(&mgr;-NN)], where "NN" is 4,4'-bipyridyl (5), 3,3'-dimethyl-4,4'-bipyridine (6), 3,8-phenanthroline (7), or 2,7-diazapyrene (8). The trinuclear complex is [{Mo(V)(O)(Tp)Cl}(3)(1,3,5-C(6)H(3)O(3))] (9), whose crystal structure was determined [9.5CH(2)Cl(2): C(56)H(81)B(3)Cl(13)Mo(3)N(18)O(6); monoclinic, P2(1)/n; a = 13.443, b = 41.46(2), c = 14.314(6) Å; beta = 93.21(3) degrees; V = 7995(5) Å(3); Z = 4; R(1) = 0.106]. In these complexes, the sign and magnitude of the exchange coupling constant J is clearly related to both the topology and the conformation of the bridging ligand [where J is derived from H = -JS(1)().S(2)() for 1-8 and H = -J(S(1)().S(2)() + S(2)().S(3)() + S(1)().S(3)()) for 9]. The values are as follows: 1, -80 cm(-)(1); 2, +9.8 cm(-)(1); 3, -13.2 cm(-)(1); 4, -2.8 cm(-)(1); 5, -33 cm(-)(1); 6, -3.5 cm(-)(1); 7, -35.6 cm(-)(1); 8, -35.0 cm(-)(1); 9, +14.4 cm(-)(1). In particular the following holds: (1) J is negative (antiferromagnetic exchange) across the para-substituted bridges ligands of 1 and 3-8 but positive (ferromagnetic exchange) across the meta-substituted bridging ligands of 2 and 9. (2) J decreases in magnitude dramatically as the bridging ligand conformation changes from planar to twisted (compare 3 and 4, or 6 and 8). These observations are consistent with a spin-polarization mechanism for the exchange interaction, propagated across the pi-system of the bridging ligand by via overlap of bridging ligand p(pi) orbitals with the d(pi) magnetic orbitals of the metals. The EPR spectrum of 9 is characteristic of a quartet species and shows weak Deltam(s) = 2 and Deltam(s) = 3 transitions at one-half and one-third, respectively, of the field strength of the principal Deltam(s) = 1 component.