The new tripodal ligand tris-[3-(2'-pyridyl)pyrazol-l -yl]hydroborate (L-), comprising three N,N-bidentate chelating arms linked by the apical boron atom, has been synthesized; the crystal structure of [EuL(MeOH),F][PF6I reveals the nine-coordinate metal lying within the hexadentate ligand cavity.
The templating effect of the tetrafluoroborate ion leads to assembly of four Co ions and six bridging ligands around this anion to give a tetrahedral complex with a bridging ligand along each edge and the anion trapped in the central cavity (shown below). Surprisingly under identical conditions but with Ni a simpler dinuclear complex forms.
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.
The new compounds N-phenyl-4,4‘-bipyridinium (PhQ+), N-(4-acetylphenyl)-4,4‘-bipyridinium (4-AcPhQ+), and N-(2,4-dinitrophenyl)-4,4‘-bipyridinium (2,4-DNPhQ+), together with the known ligand N-methyl-4,4‘-bipyridinium (MeQ+), have been used to prepare a series of Ru(II) complex salts trans-[Ru(NH3)4(LD)(LA)](PF6)3 [LD = NH3 and LA = MeQ+ (1), PhQ+ (2), 4-AcPhQ+ (3), or 2,4-DNPhQ+ (4); LD = 4-(dimethylamino)pyridine (dmap) and LA = PhQ+ (7) or 4-AcPhQ+ (11); LD = 1-methylimidazole (mim) and LA = PhQ+ (8) or 4-AcPhQ+ (12); LD = 4-(dimethylamino)benzonitrile (dmabn) and LA = PhQ+ (9) or 4-AcPhQ+ (13); LD = phenothiazine (PTZ) and LA = PhQ+ (10) or 4-AcPhQ+ (14)]. These complexes display intense, visible metal-to-ligand charge-transfer (MLCT) absorptions, due to dπ(RuII) → π*(LA) excitations. The MLCT energy decreases as the acceptor strength of LA increases, in the order MeQ+ < PhQ+ < 4-AcPhQ+ < 2,4-DNPhQ+, and/or as the donor strength of LD increases, in the order PTZ < dmabn < NH3 < mim < dmap. X-ray crystal structure determinations have been carried out for [PhQ+]Cl·2H2O and trans-[Ru(NH3)4(PhQ+)(PTZ)](PF6)3·Et2O (10·Et2O). [PhQ+]Cl·2H2O, chemical formula C16H17ClN2O2, crystallizes in the triclinic system, space group P1̄, with a = 7.675(2) Å, b = 9.895(2) Å, c = 10.175(2) Å, α = 96.003(1)°, β = 104.74(2)°, γ = 90.398(1)°, and Z = 2. 10·Et2O, chemical formula C32H44F18N7OP3RuS, crystallizes in the triclinic system, space group P1̄, with a = 10.310(3) Å, b = 10.698(2) Å, c = 20.986(4) Å, α = 95.09(2)°, β = 91.49(2)°, γ = 105.53(2)°, and Z = 2. The dihedral angles between the two pyridyl rings of the 4,4‘-bipyridinium unit are 19.8° in [PhQ+]Cl·2H2O and 2.6° in 10·Et2O. Molecular first hyperpolarizabilities β of the complex salts, obtained from hyper-Rayleigh scattering measurements at 1064 nm, are in the range (698−1214) × 10-30 esu. Static hyperpolarizabilities β0 derived by using the two-level model are very large, with trans-[Ru(NH3)4(4-AcPhQ+)(dmap)](PF6)3 (11) having the largest at 410 × 10-30 esu. When the MLCT absorption is sufficiently far from the second harmonic at 532 nm, both β and β0 increase as the absorption energy decreases.
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