Substantial progress has been made in the coordination chemistry of main-group elements with neutral donor ligands, largely ushered in by the development of stable N-heterocyclic carbenes (NHCs). There is growing interest in the synthesis of well-defined coordination compounds containing s-block metals; however, examples of molecular compounds containing “normal” NHCs bound to magnesium remain relatively understudied. We report that NHCs react with magnesium halides, MgX2 (X = Cl, Br, I), to afford (IPr)­MgCl2(THF) (1), [(IPr)­MgCl2]2 (2), (sIPr)2MgCl2 (3), (sIPr)2MgBr2 (4), and (sIPr)2MgI2 (5), where IPr is 1,3-bis­(2,6-di­iso­propyl­phenyl)­imidazole-2-ylidine and sIPr is 1,3-di­isopropyl-4,5-di­methyl­imidizol-2-ylidine. Using the IPr ligand, weak NHC coordination and dynamic interaction with THF are observed in solution. In contrast, the coordination of two sIPr ligands results in higher purity, enhanced stability, and no observation of THF coordination. Dual carbene complexation with commercially available MgnBu2 produced terminal dialkyl (sIPr)2Mg­(nBu)2 (6). The reaction of methylmagnesium bromide with 2 equiv of sIPr afforded the first structurally characterized example of a terminal Grignard reagent which is stabilized by NHCs, (sIPr)2Mg­(Me)Br (7). Notably, compounds 3–7 represent the first members of a new class of compounds where two untethered NHCs are bound to a single Mg center. These experiments suggest that two less sterically demanding NHCs can have superior stabilizing properties compared to one bulky NHC. The structural identity of each compound was confirmed using single-crystal X-ray diffraction studies, and the bonding in these complexes was investigated by density functional theory.

The synthesis, spectroscopic, and electrochemical characterization of oxidatively stable D-π-A compounds of the form (Me2CpC2Fc)2TiCl2 and RCp2Ti(C2Fc)2CuX (where Fc = ferrocenyl) are reported. Oxidative stability enabled by the addition of CuX is evidenced by voltammagrams of the RCp2Ti(C2Fc)2CuX compounds which all display two chemically-reversible 1e- FeIII/II couples, indicative of electronic communication between the Fc- termini. Differential pulse voltammetry (DPV) in CH2Cl2/[n-Bu4N][PF6], demonstrated that the redox potential difference between the two 1e- FeIII/II couples (ΔE1/2) is between 112 mV and 146 mV, being most pronounced with the electron rich Cp*2Ti(C2Fc)2CuBr. The ΔE1/2 values were unaffected by solvent (THF) and displayed only a small dependence on the identity of the counterion, either PF6- or B(C6F5)4-. For each complex with a measurable ΔE1/2 value, spectroelectrochemical experiments were performed in CH2Cl2/[n-Bu4N][PF6] and gave clear evidence of both the one-electron oxidized mixed-valent (MV) state and the two-electron oxidized state, each with distinct spectroscopic signatures. The MV states of these complexes showed absorbance between 820 and 940 nm which were replaced with a higher energy feature following a second oxidation. A very similar absorption band was also observed in the one-electron oxidized state of an analogue with only a single Fc substituent, namely TMSCp2Ti(C2Fc)(C2Ph)CuBr, suggesting this feature is not an FeII/FeIII intravalence charge-transfer (IVCT) band. Despite DFT calculations suggesting a pathway exists for electronic coupling, NIR spectroscopy on the MV states gave no evidence of an FeII/FeIII IVCT. Possible contributions to ΔE1/2 from inductive effects and a superexchange mechanism are discussed.

Charge transfer (CT) transitions are relevant in the fields of solar energy conversion and nonlinear optical materials. Herein, a series of complexes with an alkynyl linkage between an aryl amine donor and a TiIV (titanocene) acceptor is reported. Each complex displays a strong (15000 < ε < 24,000 M–1cm–1), low-energy (520 < λ < 560 nm) absorption ascribed to an amine to TiIV ligand-to-metal CT. This characterization is supported by UV–vis spectroscopy, cyclic voltammetry, and TD-DFT calculations. These complexes are not photostable; therefore, an alternate architecture, wherein the amine donor is appended to the titanocene cyclopentadienyl ligand, has been designed. The molar absorptivity of the amine to TiIV CT in this latter architecture is lower (2100 M–1 cm–1), indicating weaker donor–acceptor coupling. This architecture is indeed much more photostable.