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Introduction

The Berry Group is investigating the synthesis, characterization, electronic structure, and chemistry of transition metal complexes with polydentate, polyanionic tripodal ligands.  Tris-amidoamine ligands ([N(CH₂CH₂NR)₃]^3–, abbreviated [^RN₃N]^3–, R = alkyl, aryl, pyridyl, or α-imino) constitute a general class of polyanionic ligands that we have investigated to support mononuclear or dinuclear metal centers.  In the first year of this grant, we investigated coordination chemistry of these ligands with iron. We next tackled the challenge of designing a framework to hold two metal atoms together.  We envisioned three ways in which Schrock-type trisamidoamone ligands can be modified to accommodate two metals: (1) Pyridylamido groups could be added as tripodal ‘arms’; (2) amidines could be used as ‘arms’; and (3) urea groups could be used as arms.  Ligand systems (1) and (2) are unknown; urea type ligands (3) have been used by Borovik to study the effects of hydrogen bonding on axial ligand in mononuclear complexes (Acc. Chem. Res. 2005, 38, 765-774), but have never been used for dinuclear complexes.

Results and Discussion

Ligand Synthesis

Ligand (1), tris-2-pyridylaminoethyl amine, was synthesized by palladium-catalyzed Buchwald coupling of three equivalents of 2-bromopyridine with tren.  This resulted in the formation of (1) along with an undesired side product having two pyridine rings bound to one of the tripodal arms.  In order to get around this problem, we modified the reaction conditions by first preparing tris-BOC-protected tren, and treating this protected compound to the Buchwald reaction conditions, yielding tris-BOC-protected-tris-2-pyridylaminoethyl amine.  The BOC protecting groups are then removed by treatment with trifluoroacetic acid.

Ligand (2), with N-phenyl-acetamidine arms, was prepared in a two step reaction starting with N-phenyl acetamide.  This amide was treated with triflic anhydride forming an activated triflate salt.  This activated salt then reacts with tren to form the desired compound in good yield.

Ligand (3) was prepared according to Borovik’s procedure (Acc. Chem. Res. 2005, 38, 765-774).

Complexation

Reactions of these ligands with transition metal starting materials are currently underway.  We envision two main synthetic routes to bimetallic complexes of these ligands.  First, we are employing transition metal salts that contain a pre-formed metal-metal bond to see if we can chelate the metal-metal bonded unit with these ligands, leaving the metal-metal bond intact.  The transition metal salts we are using here are Cr₂(OAc)₄, Mo₂(OAc)₄, and Ru₂(OAc)₄Cl.  Our second approach involves self-assembly of a metal-metal bonded product from monometallic transition metal salts.  For this approach, we are focusing on metals that have been shown to form stable trigonally symmetric compounds such as Cr, Co, and Fe.

Summary.

We have synthesized new expanded Schrock-type triammidoamine ligands to support novel metal-metal bonded compounds.  Metal complexation is underway.  These new compounds will be examined for their potential use in the activation of small molecules such as CO₂.