60th Annual Report on Research 2015

The proposed research focuses on the design of a new series of homogeneous bimetallic catalyst systems for conversion of syngas to value added chemicals and fuels such as alkanes, alkenes, and methanol. To date we have explored two bimetallic systems with unique ligand frameworks as shown in Figure 1.

Figure 1. Bimetallic systems developed in this work.

In the first system, we have prepared an unprecedented bisimidazole phosphine ligand, PhP[(CH₂)₂Im^ArMe2]₂, that is capable of coordinating two metal centers in a geometry where one metal is bound between the imidazole C1 atoms and the second is bound between the unsubstituted wingtip nitrogen atoms. The bisimidazole-phosphine ligand PhP[(CH₂)₂Im^ArMe2]₂ was prepared using a three step synthesis from commercially available 5,6-dimethylbenzimidazole using methods derived from the literature. In the final step, vinyl-substituted benzimidazole (2 equiv.) was added to a −35 °C solution of phenylphosphine (1 equiv.) and KO^tBu (0.8 equiv.) in THF (Scheme 1). Monitoring the reaction by ³¹P{¹H} NMR spectroscopy showed conversion to PhP[(CH₂)₂Im^ArMe2]₂, with a characteristic singlet at −35 ppm. We found that metalation of this ligand with [RuCp*(µ³‑Cl)]₄ affords a dimeric complex prior to tautomerization to give the desired monometallic ruthenium biscarbene complex (Scheme 1). Subsequent deprotonation with n-BuLi and salt metathesis with MCl₂ salts in acetonitrile yields the desired bimetallic species in good yield (Scheme 2). Single crystal X-ray diffraction studies confirm formation of the desired coordination geometry (Figure 2).

Scheme 1. Synthesis of the bisimidazole phosphine ligand.

Scheme 2. Dimetallation of the bisimidazole phosphine ligand.

Figure 2. Single crystal X-ray diffraction structure of the Ru-Fe bimetallic complex.

In the second system we have discovered a simple method to prepare a versatile cobalt diimine dioximate metalloligand that allows for synthesis of a wide variety of bi- and tetrametallic complexes. The doubly deprotonated cobalt(III) diimine dioximate cation ligated by two axial 1-methylimidazole groups, [(DO)₂en(Im^Me)₂Co]⁺, is synthesized as the perchlorate salt by addition of excess 1-methyl imidazole and one equivalent of zinc perchlorate to the cobalt(III) diimine monooxime monooximate starting material, (DO)(DOH)enCoBr₂, in CH₃CN (Scheme 3). Addition of various Lewis acids to [(DO)₂en(Im^Me)₂Co]⁺ affords both bimetallic and tetrametallic complexes, the structures of which are confirmed using single crystal X-ray diffraction (Figure 3). All of the new complexes have been verified to electrocatalytically generate hydrogen from mild acid sources, however catalyst instability under electrocatalytic conditions leads to heterogeneous film formation.

Scheme 3. Synthesis of cis-doubly deprotonated diimine dioximate metalloligand.

Figure 3. Single crystal X-ray diffraction structures of mono- bi- and tetrametallic complexes containing the cobalt diimine dioximate metalloligand.

With these two systems in hand we are poised to continue forward towards exploring syngas conversion chemistry. In the biscarbene system it is essential that we first learn how to open a coordination site at the ruthenium center by replacing the Cp* ligand in our synthesis. Towards this end we have begun exploring the metalation chemistry of ruthenium precursors with open or readily exposed coordination sites. Early experiments confirm our ability to access the desired biscarbene complex and verify its reactivity with unsaturated substrates like acetonitrile. In the oximate system we are working to explore the heterogeneous catalysts that result upon reduction of the bi- and tetrametallic complexes. Our hope is that these molecular precursors may give us an unprecedented method to access alloyed metal nanoparticles with high reactivity.