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Introduction

The original proposal was to develop rational syntheses of metallocarbohedrynes (met-cars) such as Ti₈C₁₂ (pictured). This class of compounds had previously been synthesized in plasmas and had been studied only in the gas phase at high temperatures. However, the properties of several metcars are quite stunning. For instance, Ti₈C₁₂ has an ionization potential between those of sodium and potassium and a near zero HOMO-LUMO gap, properties most unusual for molecular substances and suggesting metallic properties and extremely interesting chemistry. This compound is found to coordinate methane, raising the possibility that it might exhibit interesting properties as an alkane functionalization catalyst, while computations suggest that Ti₈C₁₂ should be able to bind sufficient hydrogen molecules per cluster that it might function as a high capacity hydrogen storage system. Synthetic routes to metcars in the condensed phase are clearly desirable and the original proposal outlined several possible rational approaches to this end.  

The Basic Procedures

Last year's annual report outlined our general approach to the problem, described the types of experiments which we had carried out, and need not be repeated in detail. In essence, we consider the C₂ units to be acetylidic (C₂^2-) in nature and therefore the average formal oxidation state of the titanium atoms in e.g. Ti₈C₁₂ is +1.5. Our general synthetic procedures therefore initially involved combining sources of C₂^2- with titanium precursors in the +3 and +4 oxidation states in the presence of reducing agents capable of reducing the Ti(III) and Ti(IV) precursors to the oxidation state(s) desired.

In view of its ready availability, TiCl₄ was the first titanium source to be utilized but it turned out to not be very convenient because of its hydrolytic instability. We therefore turned to crystalline complexes of TiCl₄, which were much more convenient to handle. Titanium(II) and -(III) complexes have also been employed as titanium sources. Reactions of acetylene with titanium sources were carried out in the present of reducing agents lower the oxidation state of the titanium during reactions. We experimented with sodium amalgam, sodium and lithium metals and sodium naphthalenide as reducing agents, and all do effect reduction of Ti(III) and Ti(IV) compounds.

A major source of acetylide dianion was acetylene, obtained in pressurized cylinders in acetone solution and thus requiring purification; we developed a procedure to remove the acetone, and the purity of the acetylene obtained was assured using NMR spectroscopy. Another source utilized was sodium acetylide, NaC₂H, which was reacted directly with titanium sources directly or treated with butyl lithium to produce Na₂C₂. Note that these compounds also deprotonate both acetylene and NaC₂H

As pointed out previously, report, a large number of experiments was carried out in attempts to synthesize Ti₈C₁₂. The low I.P. and near zero HOMO-LUMO gap of Ti₈C₁₂ suggested that this material would be obtained as a black, metallic solid which might well be partially oxidized by traces of water or oxygen to the [Ti₈C₁₂]⁺ cation. We obtained a large number of interesting, insoluble solids which were characterized by DRIFT spectroscopy, XPS and elemental analyses. We also analyzed the resulting solutions for metcar cations using electrospray ionization (ESI) mass spectrometry. Unfortunately, while intriguing observations were made in some cases, we had difficulties with reproducibility. We do not know the reasons for the problems, but we suspect that very low yields may in some cases have rendered the products sensitive to low levels of impurities.

Research during the second year included more stringent drying of the solvents utilized and a series of attempts to duplicate some of the more promising reactions previously carried out. We also extended our previous research with acetylene utilizing Ti(II) precursors with no reducing agent. It was hoped that disproportionation of e.g. soluble TiCl₂ complexes might give us the mix of oxidation states that the formation of Ti₈C₁₂ requires. Black, air-sensitive, metallic looking materials were obtained in THF at room temperature, and these exhibited broad peaks in the DRIFTS spectra at 1300 and 1656 cm^-1. These materials turned yellow in air, and thus are quite different from the metallic substances, reported a year ago from somewhat similar reactions, which may have been various forms of polyacetylene. ESI MS studies of the supernatants from the reactions revealed no peaks suggestive of metcars.

In attempts to prevent the aggregation of metcars clusters to insoluble materials by stabilizing the Ti₈C₁₂ units, some reactions were also carried out under an atmosphere of CO. Previous gas phase MS studies of the [Ti₈C₁₂]⁺ cation had shown that it can coordinate several Lewis bases, and we hoped that the neutral species might be trapped by CO as it formed and be prevented from aggregation to insoluble materials. Unfortunately no carbonyl stretching bands were observed in the IR spectra of the solids or of the supernatants.

As a different approach, one which had not occurred to us previously, we utilized two titanium precursors already in their desired formal oxidation states, TiCl₃(THF)₃ and Ti(bipyridal)₃, in a 1:1 molar ratio, with the acetylide salt Li₂C₂, formed by reacting acetone-free acetylene with two equivalets of butyl lithium. This approach avoided actually using a reducing agent, and a reaction carried out in THF at -20 °C gave a black precipitate which exhibited bands in the DRIFT spectrum which could be attributed to a metcar or to bipyridal (possibly coordinated to a metcar as suggested for CO above). ESI MS experiments on the supernatant showed no interesting peaks but an XPS spectrum suggested the value of pursuing this line of research further. We are continuing to pursue this project with funding from a different source, and we are also utilizing a literature procedure found very recently for generating Na₂C₂. Heating of commercially available NaHC₂ under reduced pressure induces disproportionation to Na₂C₂ and acetylene, and the thus formed Na₂C₂ is being used in parallel with Li₂C₂ in experiments involving TiCl₃(THF)₃ and Ti(bipyridal)₃.