Syntheses, crystal structures, and characterization of heteronuclear complexes based on a versatile ligand with both acetylacetonate and bis(2-pyridyl) units

New homodinuclear tris(3-alkylpyrazolyl)borate complexes of CoII and NiII with a tetraacetylethane dianion as a bridging ligand

Polynuclear complexes and coordination polymers of 3d metals have attracted significant interest evoked by a number of their unique properties. One of the most common approaches to the directed synthesis of coordination polymers is the linking of pre-prepared discrete coordination units by polydentate ligands. The formation of polynuclear complexes is usually a spontaneous process and precise prediction of the products of such reactions is virtually impossible in most cases. Tris(pyrazolyl)borates (Tp) act as tripodal `capping' ligands which form stable complexes with 3d metal ions. In such 1:1 compounds, three metal-ion coordination sites are occupied by N atoms from a Tp anion. This limits the number of remaining coordination sites, and thus the number of additional ligands which may coordinate, and opens an attractive approach for the directed design of desirable structures by exploiting ligands with appropriate composition and topology. In the present study, Tp anions with neopentyl [Tp<sup>Np</sup>, tris(3-neopentylpyrazolyl)borate] and cyclohexyl [Tp<sup>Cy</sup>, tris(3-cyclohexylpyrazolyl)borate] substituents were used as `capping' ligands and the dianion of tetraacetylethane (3,4-diacetylhexa-2,4-diene-2,5-diolate, tae^2-) was employed as a bridge. The dinuclear complexes (μ-3,4-diacetylhexa-2,4-diene-2,5-diolato-κ⁴O²,O³:O⁴,O⁵)bis{[tris(3-cyclohexyl-1H-pyrazol-1-yl-κN²)borato]cobalt(II)} acetonitrile disolvate, [Co₂(C₂₇H₄₀BN₆)₂(C₁₀H₁₂O₄)]·2CH₃CN, (I)·2CH₃CN, and (μ-3,4-diacetylhexa-2,4-diene-2,5-diolato-κ⁴O²,O³:O⁴,O⁵)bis{[tris(3-neopentyl-1H-pyrazol-1-yl-κN²)borato]nickel(II)}, [Ni₂(C₂₄H₄₀BN₆)₂(C₁₀H₁₂O₄)], (II), were synthesized by the reaction of the mononuclear complexes Tp^CyCoCl or Tp^NpNiCl with H₂tae (3,4-diacetylhexane-2,5-dione or tetraacetylethane) in the presence of NEt₃ as base. Compounds (I) and (II) were characterized by mass spectrometry, elemental analysis, and X-ray crystallography. They possess similar molecular structures, X-ray diffraction revealing them to be dinuclear in nature and composed of discrete Tp-M units in which two metal ions are linked by a tae^2- dianion. Each metal ion possesses a five-coordinate square-pyramidal environment. The interplanar angles between the acetylacetonate fragments are significantly smaller than the near-90° values commonly observed.

Dithiolene dimetallic molybdenum(V) complexes displaying intraligand charge transfer (ILCT) emission

Bifunctional dithiolene ligands have been coordinated to the Mo(V)(O)(μ-S2)Mo(V)(O) unit to afford [Mo2O2(μ-S)2(BPyDTS2)2](2-) (1(2-)) (BPyDTS2 (2-bis-(2-pyridyl)methylene-1,3-dithiolene) dianions. Reaction of the 1(2-) molybdenum dimer with pentacarbonylchlorothenium(i) affords a tetrametallic complex of formula [Mo2O2(μ-S)2(BPyDTS2)2{Re(CO)3Cl}2](2-) (2(2-)). The monomeric (CH3)2Sn(BPyDTS2) (3) tin complex has also been prepared for comparative purposes. In the structure of (Et4N)2[1], the two metal atoms are in a square pyramidal coordination environment defined by two bridging sulfur atoms, one terminal oxygen atom and the two sulfur atoms of the bifunctional dithiolene ligand. This arrangement leaves two nitrogen atoms on each side which coordinate to two Re atoms in the 2(2-) tetrametallic complex. Compound 3 has a distorted tetrahedral structure defined by two carbon atoms of the methyl groups and two sulfur atoms of the dithiolene ligand. The luminescence properties of all three complexes in acetonitrile have been investigated. Detailed studies supported on quantum mechanical calculations revealed that complex 1(2-) shows photoluminescence in the 600-800 nm region with a maximum wavelength of 628 nm and an emission quantum yield of 0.092, associated with an intraligand charge transfer (ILCT) transition. Coordination of two Re(CO)3Cl fragments to 1(2-) to afford 2(2-) does not affect the emission spectrum and shape although it decreases the quantum yield, approximately by a factor of 4.6. Compound 3 exhibits a similar emission spectrum to those of the complexes 1(2-) and 2(2-) in good agreement with the ILCT assignment. The quantum yield of 3 lies between that of the 1(2-) and 2(2-) complexes.