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 p athway between the metal centers. The first series of dinuclear comple xes is [(Mo-V(O)(Tp)Cl)(2)(mu-00)], where ''00'' is [1,4-O(C6H4)(n)P] (2-) (n = 1, 1; n = 2, 3), [4,4'-O(C6H3-2-Me)(2)P](2-) (4), or [1,3-OC 6H4O](2-) (2) [Tp = tris(3,5-dimethylpyrazolyl)hydroborate]. The seco nd series of dinuclear complexes is [(Mo-1(NO)(Tp)Cl)(2)(mu-NN)], whe re ''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 i s [(Mo-V(O)(Tp)Cl)(3)(1,3,5-C6H3O3)] (9), whose crystal structure was determined [9.5CH(2)Cl(2): C5(6)H(81)B(3)Cl(13)Mo(3)N(18)O(6); monocl inic, P2(1)/n; a = 13.443, b = 41.46(2), c = 14.314(6) Angstrom; beta = 93.21(3)degrees; V = 7995(5) Angstrom(3); Z = 4; R-1 = 0.106]. In th ese complexes, the sign and magnitude of the exchange coupling constan t J is clearly related to both the topology and the conformation of th e bridging ligand [where J is derived from H = -J S-1.S-2 for 1-8 and H = -J(S-1.S-2 + S-2.S-3 + S-1.S3) 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 (antiferrom agnetic 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 dramaticall y 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 ct-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 show s weak Delta m(s) = 2 and Delta m(s) = 3 transitions at one-half and o ne-third, respectively, of the field strength of the principal Delta m (s) = 1 component.