Lanthanide(III) complexes have been prepared with [L 1 ] -[the tetradentate chelating ligand bis{3-(2-pyridyl)-pyrazolyl}dihydroborate], [L 2 ] -[the tetradentate chelating ligand bis{3-(2-pyrazinyl)pyrazolyl}dihydroborate], [L 3 ] -[the hexadentate chelating ligand bis[3-{6′-(2,2′-bipyridyl)}pyrazol-1-yl]dihydroborate], and [L 4 ] 2-[the 12-dentate compartmental ligand hexakis{3-(2-pyridyl)pyrazol-1-yl}diboran(IV)ate, which has two hexadentate tris-(pyrazolyl)borate-based cavities linked "back-to-back" by a B-B bond]. [Ln(L 1 ) 2 (NO) 3 ] are 10-coordinate with two tetradentate N-donor ligands and one bidentate nitrate. [Ln(L 2 ) 2 (NO) 3 ] have 10-coordinate structures similar to those of the [L 1 ] -complexes except that the coordinated N 1 of the pyrazine rings is not such a good donor as the pyridine rings in the [L 1 ] -complexes, leading to marked lengthening of these Ln-N bonds. [Ln(L 3 )(NO 3 ) 2 ] are also 10-coordinate from one hexadentate chelating ligand which has a pseudoequatorial coordination mode and two pseudoaxial bidentate nitrate ligands; the hexadentate ligand has a shallow helical twist to prevent steric interference between its ends. Finally [{Ln(NO 3 ) 2 (L 4 )] are dinuclear, with each metal center being 10-coordinate from a tripodal hexadentate ligand cavity and two bidentate nitrates. Five complexes were structurally characterized: [Tb(L 2 ) 2 (NO 3 )]‚dmf is monoclinic (P2 1 /c) with a ) 14.881(3) Å, b ) 15.5199(12) Å, c ) 15.845(2) Å, ) 92.387(12)°, and Z ) 4. [Gd(L 2 ) 2 (NO 3 )]‚dmf is monoclinic (P2 1 /c) with a ) 14.926(2) Å, b ) 15.465(2) Å, c ) 15.878(2) Å, ) 92.698(11)°, and Z ) 4. [Eu(L 3 )(NO 3 ) 2 ]‚dmf‚0.5Et 2 O is triclinic (P1 h) with a ) 10.020(3) Å, b ) 13.036(3) Å, c ) 14.740(3) Å, R ) 70.114(14)°, ) 71.55(2)°, γ ) 79.66(2)°, and Z ) 2. [{La(NO 3 )-(dmf) 2 } 2 (L 4 )](NO 3 ) 2 ‚dmf is orthorhombic (Pbca) with a ) 18.813(2) Å, b ) 15.241(2) Å, c ) 27.322(2), and Z ) 4. [{Gd(NO 3 ) 2 } 2 (L 4 )]‚2.4dmf is tetragonal (P4 2 /n) with a ) 16.622(6), c ) 24.19(5) Å, and Z ) 4. Detailed photophysical studies have been performed on the free ligands and their complexes with Gd(III), Eu(III), and Tb(III) in several solvents. The results show a wide range in the emission properties of the complexes which can be rationalized in terms of subtle variations in the steric and electronic properties of the ligands. In particular the dinuclear Tb(III) complex of [L 4 ] 2-has an emission quantum yield of ca. 0.5 in D 2 O and MeOD.
Reaction of 3-(2-pyridyl)pyrazole with 3,3Ј-bis(bromomethyl)biphenyl resulted in the new ligand L 1 which contains two bidentate chelating pyrazolyl/pyridine fragments separated by a meta-biphenyl spacer; this ligand is designed to act only as a bridging ligand, as the two bidentate sites are too far apart to co-ordinate to the same metal ion. The dinuclear copper() complex [Cu 2 (L 1 ) 2 (OAc) 2 ][BF 4 ] 2 is a double helicate in which each copper() centre is in a square pyramidal co-ordination geometry, arising from two bidentate pyrazolyl/pyridine groups (one from each ligand L 1 ) and a monodentate acetate. The structure is stabilised by extensive inter-ligand π-stacking interactions. The complex [Ag 2 (L 1 ) 2 ][ClO 4 ] 2 is also assumed to be a double helicate. In contrast, reaction with Co II afforded the tetranuclear cage complex [Co 4 (L 1 ) 6 ][BF 4 ] 8 , in which each bridging ligand links two metal centres by spanning one edge of the Co 4 tetrahedron. Each metal is therefore in a pseudo-octahedral tris-chelate geometry, with the three bidentate chelating arms each coming from a different ligand L 1 . Again there is substantial inter-ligand π stacking. Unlike other complexes with the same {M 4 L 6 } tetrahedral cage structure, the central cavity is not occupied by a counter ion, showing that although the templating effect of a counter ion can be beneficial in the assembly of such cages it is clearly not essential. 1 H NMR spectroscopy suggests that there is a mixture of species in solution arising from other metal : ligand combinations; 11 B NMR spectroscopy shows that at Ϫ40 ЊC a [BF 4 ] Ϫ anion can become trapped in the cavity of the cage, giving a characteristic high-field resonance in addition to that for the free [BF 4 ] Ϫ anions. Reaction of L 1 with Pd II afforded a mixture of products arising from ligand decomposition, of which [Pd 2 (L 1 )(pypz) 2 ][BF 4 ]-[OH] was structurally characterised. It has a near-planar {Pd 2 (µ-pypz) 2 } 2ϩ core [Hpypz = 3-(2-pyridyl)pyrazole, which has arisen from decomposition of L 1 ] with an additional bridging ligand L 1 co-ordinating in a 'basket-handle' mode, straddling the central core.The course of a self-assembly reaction between a labile metal ion and a multidentate bridging ligand depends principally on the stereoelectronic properties of the metal ion, and the number and disposition of the binding sites of the bridging ligands. 1,2 In some cases the interaction between these is well understood and a considerable degree of control can be exerted over the self-assembly process by judicious choice of components, such that the number, denticity and disposition of the binding sites in the ligand, and the co-ordination number and geometric preferences of the metal ions, can precisely be matched to achieve a remarkable degree of specificity in the self-assembly process. 3 This is exemplified by the recent work of Raymond and co-workers who have developed a symmetry-based argument to rationalise and predict the assemblies of some quite complicated cage struc...
Complexes of the new potentially hexadentate ligand bis{3-[6-(2,2Ј-bipyridyl)]pyrazol-1-yl}hydroborate (L Ϫ ), containing two terdentate chelating arms linked by a ᎐BH 2 ᎐ spacer, were prepared and crystallographically characterised with K ϩ , Cu 2ϩ , Gd 3ϩ and Tl ϩ as representatives of the s-, d-, f-and p-block metals respectively. The crystal structure of the K ϩ complex revealed it to be the double-helical dinuclear [K 2 L 2 ], in which each metal ion is six-co-ordinated by a terdentate arm from each of the two ligands; the two ligands are therefore bridging, and folded at the flexible ᎐BH 2 ᎐ spacer group. The complex [Cu 2 L 2 ][BF 4 ] 2 has a similar double-helical dinuclear cation with six-co-ordinate metal centres, but with a greater metal-metal separation because of the greater electrostatic repulsion between two dipositive metal ions compared to [K 2 L 2 ]. The complex [GdL(NO 3 ) 2 ] in contrast is mononuclear with the ligand co-ordinated in a pseudo-equatorial manner, having a shallow helical twist to avoid steric interference between the terminal pyridyl groups. The two pseudo-axial bidentate nitrate ligands complete the ten-fold co-ordination. Formation of a (triple) helical complex between Gd 3ϩ and L Ϫ , known with other bisterdentate compartmental ligands, is thought to be disfavoured in this case because of the electrostatic repulsion between the two ϩ3 metal centres that would occur given the relatively short metal-metal separations imposed by the ligand. In [TlL] the Tl ϩ ion, which is comparable in size and identical in charge to K ϩ , has a preference for lower co-ordination numbers, which is reflected in the fact that not all of the ligand binding sites are co-ordinated and there are three relatively short M᎐N interactions and two long, weak ones.
The trinuclear complexes [{Mo(O)(Tp*)Cl}(μ-1,n-C6H4O2){Mo(O)(Tp*)}(μ-1,n-C6H4O2){Mo(O)(Tp*)Cl}] (1, n = 4; 2, n = 3) have been prepared [Tp* = tris(3,5-dimethylpyrazolyl)hydroborate], in which a chain of three paramagnetic oxo-Mo(V) fragments are linked by two 1,4-[OC6H4O]2- (for 1) or 1,3-[OC6H4O]2- (for 2) bridging ligands. The crystal structure of 1·(CH2Cl2)3.5·(C6H14)0.5 was determined: C63.5H88B3Cl9Mo3N18O7; triclinic, P1̄; a = 12.052(2) Å, b = 18.487(4) Å, c = 21.039(5) Å; α = 68.95(2)°, β = 86.12(2)°, γ = 78.637(13)°; V = 4289(2) Å3; Z = 2. It shows a V-shaped Mo−L−Mo−L−Mo array of three {(Tp*)Mo(O)} fragments with two 1,4-[OC6H4O]2- bridging ligands. The V-shape arises from the cis arrangement of the two bridging ligands at the central metal atom. Electrochemical measurements show the expected Mo(IV)/Mo(V) and Mo(V)/Mo(VI) couples at potentials consistent with significant electrochemical interactions between the terminal and central metal fragments but not between the two terminal metal fragments. Variable-temperature magnetic susceptibility measurements (1.17−225 K) show the occurrence of intramolecular antiferromagnetic (1) and ferromagnetic (2) exchange interactions between adjacent metal atoms with J = −44.0 (1) and +4.5 cm-1 (2) [based on H = −J(S 1 S 2 + S 1 S 3)] leading to S = 1/2 (1) and 3/2 (2) ground states. These results are in accord with the spin-polarization principle, which predicts that [1,4-C6H4O2]2- should be an antiferromagnetic linker whereas [1,3-C6H4O2]2- should be a ferromagnetic linker.
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