The results of the fifth blind test of crystal structure prediction, which show important success with more challenging large and flexible molecules, are presented and discussed.

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.

We have prepared a series of dinuclear complexes [{MoV(O)(Tp*)Cl}(μ-OO)], where “OO” represents one of the series of diphenolate bridging ligands [1,4-O(C6H4) n O]2- (n = 1−4; complexes 1−4, respectively), [O(C6H3-2-Me)2O]2- (complex 5), or [1,3-OC6H4O]2- (complex 6) [Tp* = tris(3,5-dimethylpyrazolyl)hydroborate]. The complexes therefore contain two paramagnetic (d1), redox-active metal centers. For complexes 1−4 the metal−metal separation increases due to the increasing number of phenyl spacers in the bridge. Complex 5 is similar to 2 with the exception that for steric reasons there is a larger dihedral twist between the halves of the complex. Complex 6 is similar to 1, but with a meta substitution pattern at the bridging ligand instead of para. Complexes 1 and 6 were characterized by X-ray crystallography. For 1·2Et2O:  C44H68B2Cl2Mo2N12O6; orthorhombic, Pnnm; a = 24.566(11), b = 8.085(4), c = 14.172(4) Å; Z = 2. For 6·3CH2Cl2:  C39H54B2Cl8Mo2N12O4; monoclinic, P21/n; a = 15.984(3), b = 16.934(3), c = 20.931(3) Å, β = 105.33(1)°; Z = 4. Due to the cocrystallization of the two diastereoisomers of the complexes (the Mo centers are chiral), disorder is present in both cases between the oxo and chloride ligands which could be completely resolved for 6 but not for 1. Electrochemical measurements showed that each Mo(V) center undergoes a one-electron oxidation [to Mo(VI)] and a one-electron reduction [to Mo(IV)]. Electrochemical interactions across the bridge result in very large splittings between the two oxidations (e.g. 990 mV for 1) and smaller but still significant splittings between the reductions (e.g. 250 mV for 1). The much stronger interaction between the oxidation processes is due to the fact that they are partly delocalized onto the bridging ligand, whereas the reduction processes are almost completely metal-localized. EPR spectra of the complexes showed the presence of magnetic exchange between the paramagnetic centers, with several features characteristic of magnetic exchange:  the hyperfine pattern, line broadening, and the presence of a half-field (Δm s = 2) transition. The magnitudes of the electrochemical interactions and of the magnetic interactions could be related to the structures (length, substitution pattern, dihedral twist) of the bridging ligands.

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