Coordination of N,N-Chelated Re(CO)₃Cl Units Across a Mo₂ Quadruple Bond: Synthesis, Characterization, and Photophysical Properties of a Re–Mo₂–Re Triad and Its Component Pieces

2-(2-Pyridyl)-4-methylthiazole carboxylic acid (PMT-H) and rhenium tricarbonyl chloride react to form the red crystalline compound <i>fac-</i>Re­(PMT-H)­(CO)₃Cl, <b>I</b>, which is an analog of the well-known Re­(bpy)­(CO)₃Cl molecule, where bpy is 2,2′-bipyridine. The acids PMT-H (2 equiv) and Re­(PMT-H)­(CO)₃Cl (2 equiv) also react with Mo₂(T^<i>i</i>PB)₄ (T^<i>i</i>PB = 2,4,6-triisopropylbenzoate) in toluene to give the red compound <i>trans-</i>Mo₂(T^<i>i</i>PB)₂(PMT)₂, <b>II</b>, and the royal blue compound <i>trans</i>-Mo₂(T^<i>i</i>PB)₂[(PMT)­Re­(CO)₃Cl]₂, <b>III</b>, respectively. The X-ray and spectroscopic characterization of <b>I</b> confirms its close relationship with Re­(bpy)­(CO)₃Cl, as does the spectroscopic characterization of compounds <b>II</b> and <b>III</b> as analogs of other compounds of the form <i>trans</i>-M₂(TiPB)₂L₂, where L is a π-acceptor ligand. Electronic structure calculations on model compounds <b>II</b>′ and <b>III</b>′, where formate ligands substitute for T^<i>i</i>PB, show that the highest occupied molecular orbital (HOMO) in <b>II</b> is Mo₂δ. When the Re­(CO)₃Cl unit is attached to the PMT ligand to form <b>III</b>, this orbital is stabilized significantly and now becomes associated with a close in energy band of Re d⁶, t_2g type orbitals. Oxidation of <b>III</b> is shown to be Mo₂-based, as evident by EPR spectroscopy, and the lowest-energy electronic absorption corresponds to a Mo₂δ-to-PMT π* transition. The S₁ states in both <b>II</b> and <b>III</b> are metal-to-ligand charge-transfer (MLCT), and the lowest-energy triplet sate, T₁ is ³MoMoδδ*, as evidenced by its steady state emission spectral features. The excited states of compounds <b>I</b> (T₁) and <b>III</b> (S₁ and T₁) have been investigated by time-resolved infrared spectroscopy (TRIR). The spectral features of <b>I</b> parallel those for Re­(bpy)­(CO)₃Cl, with the lowest-energy T₁ state corresponding to Re dπ to PMT-H π* charge transfer, producing higher-energy CO stretching vibrations relative to the ground state. For <b>III</b>, the CO vibrations are shifted to lower energy, consistent with charge being located on the PMT ligand, which enhances Re-to-CO backbonding. In the MoMoδδ* T₁ state, however, the backbonding is reduced to the PMT ligand, and the CO stretches are at slightly higher energy relative to the ground state