45243-AC3
Polymorphs and Polymorphic Transformations that Alter Chemical Bonding
During the past year we have examined several types of metal complexes where we expected that polymorphs might exist and have studied the inter-conversion between these polymorphs. We have also examined several cases where variations in crystallization conditions lead to significantly altered luminescence of crystals that contain a single cation that can display different modes of self-association and different luminescence. We are looking for cases where significant changes in chemical interactions between molecules in the different polymorphs or other types of closely related crystals produce marked spectral differences between them. The ability to transform one polymorph or crystalline form into another with different spectroscopic properties suggests that these novel materials may be useful as sensors of volatile organic compounds, temperature, or mechanical stress. Toward the goal of developing these compounds into sensors we will determine conditions under which the transformations can be made reversible.
Four polymorphs of IrI(CO)2(OC(CH3)CHC(CH3)N(p-tol)) have been characterized by single crystal X-ray crystallography. While all contain the same molecular unit with no significant structural variations within the molecules, all show different degrees of metallophilic interactions between the planar molecules. Three of these (the amber, pale yellow and orange forms) are stable at room temperature, while the fourth, the L. T. orange form, is only obtained by cooling the orange polymorph. At 90 K, the amber, pale yellow, and L. T. orange polymorphs show intense luminescence. The variations in the luminescence among the polymorphs are considered in the context of the structural differences between them and the nature of the metallophilic interactions between the iridium centers. These results demonstrate how subtle variations in molecular organization can affect the physical properties of planar d8 transition metal compounds, which are an important class of lumiphores. The observations of externally induced transformations between these different phases suggest that these materials may have sensing applications.
Depending upon the crystallization conditions, [Au{C(NHMe)2}2](AsF6) forms colorless crystals that display a blue or green luminescence. The difference involves the type of solvate molecule that is incorporated into the crystal and the structure of the chains of cations that are formed upon crystallization. The crystallographically determined structures of blue-glowing [Au{C(NHMe)2}2](AsF6) 0.5(benzene), blue-glowing [Au{C(NHMe)2}2](AsF6) 0.5(acetone), green-glowing [Au{C(NHMe)2}2](AsF6) 0.5(chlorobenzene), and blue-glowing, solvate-free [Au{C(NHMe)2}2](EF6), E = P, As, Sb are reported. All pack with the cations forming extended columns, which may be linear or bent, but all show significant aurophilic interactions. The blue-glowing crystals have ordered stacks of cations with some variation in structural arrangement, while the green-glowing crystals have disorder in their stacking pattern. While there is extensive hydrogen bonding between the cations and anions in all structures, in the solvated crystals, the solvate molecules occupy channels but make no hydrogen-bonded contacts. The emission spectra of these new salts taken at 298 and 77 K have been reported.
Crystallographic and luminescence studies on salts of the two-coordinate carbene cation, [Au{C(NHCH3)(NHCH2CH2OH)}2]+, demonstrate the ability of the cation to exist in three different states of aggregation. In colorless, non-luminescent [Au{C(NHCH3)(NHCH2CH2OH)}2]Cl the cation crystallizes as a monomer with the nearest gold(I) center 6.7890(11) away. Colorless, luminescent [Au{C(NHCH3)(NHCH2CH2OH)}2]AsF6 forms dimers with an AuAu separation of 3.1288(4) . These dimers form weakly associated extended chains of cations with additional AuAu separations of 3.6625(5) . [Au{C(NHCH3)(NHCH2CH2OH)}2]PF6 is isostructural. Yellow, luminescent [Au{C(NHCH3)(NHCH2CH2OH)}2]3(AsF6)2Cl.0.5(H2O)2 and [Au{C(NHCH3)-(NHCH2CH2OH)}2]3(PF6)2Cl.0.5(H2O)2 form trimers that further aggregate into extended chains with rather short AuAu separations of 3.1301(14) , 3.1569(14) and 3.1415(14) . Absorption, emission and excitation spectra are reported for these salts. The excitation and emission results from the interactions between the gold centers and involves transitions between the filled dz2 band and the empty pz bands with the z-axis pointing along the chain of cations. These results clearly substantiate the hypothesis that luminescence from gold(I) carbene complexes results from the AuAu interactions between cations that are otherwise non-luminescent.
