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Pincer-ligated iridium complexes have been widely developed, and (pincer)Ir(III) complexes, particularly five-coordinate, are central to their chemistry. Such complexes typically bear two formally anionic ligands in addition to the pincer ligand itself. Yet despite the prevalence of halides as anionic ligands in transition metal chemistry there are relatively few examples in which both of these ancillary anionic ligands are halides or even other monodentate low-field anions. We report a study of the fragment (iPrPCP)IrCl2 (iPrPCP = 3-2,6-C6H3(CH2PiPr2)), and adducts thereof. These species are found to be thermodynamically disfavored relative to the corresponding hydridohalides. For example, DFT calculations and experiment indicate that one Ir-Cl bond of (iPrPCP)IrCl2 complexes will undergo reaction with H2 to give the (iPrPCP)IrHCl or an adduct thereof. In the presence of aqueous HCl, (iPrPCP)IrCl2 adds a chloride ion to give an unusual example of an anionic transition metal complex ((iPrPCP)IrCl3–) with a Zundel cation (H5O2+). (iPrPCP)IrCl2 is not stable as a monomer at room temperature but exists in solution as a mixture of clusters which can add various small molecules. DFT calculations indicate that dimerization of (iPrPCP)IrCl2 is more favorable than dimerization of (iPrPCP)IrHCl, in accord with its observed tendency to form clusters.

 

We report a family of cobalt complexes based on bidentate phosphine ligands with two, one, or zero pendent amine groups in the ligand backbone. The pendent amine complexes are active electrocatalysts for the formate oxidation reaction, generating CO2 with near-quantitative faradaic efficiency at moderate overpotentials (0.45 – 0.57 V in acetonitrile). These homogeneous electrocatalysts are the first cobalt example and second first-row transition metal example for formate oxidation. Thermodynamic measurements reveal these complexes are energetically primed for formate oxidation via hydride transfer to the cobalt center, followed by deprotonation of the resulting cobalt-hydride by formate acting as a base. The complex with the strongest cobalt- hydride bond, given by its thermodynamic hydricity, is the fastest electrocatalyst in this series, with an observed rate constant for formate oxidation of 135 ± 8 h−1 at 25 °C. Electrocatalytic turnover is not observed for the complex with no pendent amine groups: decomposition of the complex structure is evident in the presence of high formate concentrations.

 

Iridium dibromide complexes of the phenyldiimine ligand 2,6-bis(1-((2,6-dimethylphenyl)imino)ethyl)phenyl, trans-(XyPhDI)IrBr2L, have been synthesized, and relative Ir-L BDFEs have been experimentally determined for a wide range of corresponding adducts of ligands L. An estimate of the absolute enthalpy of Ir-L binding has been obtained from dynamic NMR measurements. The results of DFT calculations are in very good agreement with the relative and absolute experimental values. Computational studies were extended to the formation of adducts of (XyPhDI)IrH2 and (XyPhDI)Ir(I), as well as other (pincer)Ir(I) fragments, (Phebox)Ir(I) and (PCP)Ir(I), to enable a comparison of electronic and steric effects with these archetypal pincer ligands. Attempts to reduce (XyPhDI)IrBr2(MeCN) to a hydride or an Ir(I) complex yielded a dinuclear CN-bridged complex with a methyl ligand on the cyanide-C-bound Ir center (characterized by scXRD), indicating that C-CN bond cleavage took place at that Ir center. DFT calculations indicate that the C-CN bond cleavage occurs at one Ir center with strong assistance by coordination of the CN nitrogen to the other Ir center.

 

Metal ligand cooperativity (MLC) has revealed a plethora of unusual reactivity in catalysis in the last couple of decades. Since Milstein's report of aromatization-dearomatization of the pincer backbone of pyridine-based-pincer complexes, ruthenium has played a partic ularly important role in the develo pment of M LC. We have recently reported a (H- P3 )Ir complex which is the fastest known catalyst for alkane-transfer dehydrogenation. The active species results from P- to-Ir migration of H in this system. We further explored the possib ility of MLC in an analogous Ru system. Surprisingly, when metalating the same H-P3 ligand with a RuCl2 precursor we only isolated a (Cl-P3 )Ru(H)Cl complex where H had migrated to Ru from P, and Cl to P from Ru ("P- H/M-X exchange"). We have demonstrated that the thermodynamically favored direction of such exchanges depends strongly on the ancillary ligands, with particular driving force for formation of 5-coordinate (pincer)MHCl complexes (M = d6 metal center) . However, for 6- coordinate Ru complexes (H- pincer)MXYL, the electronic nature of L appears to determine if P-H/M-X exchange occurs. Strongly pi-accepting ligands promote P-X/M-H exchange with the reaction observed for L = CO, xylylisonitrile and N O+ , but not for L = N2 , C H3 CN, or PMe3 . While exchange at 5- coordinate (16e- ) Ru centers appears to proceed through initial P-to-Ru migration of X or H, to give a phosphide interme diate, in the case of 6- coordinate (18e- ) Ru centers exchange is believed to proceed through phosphoranyl intermediates. DFT and intrinsic bond orbital anal. has been used to better understand this reactivity. 

Controlling electronic coupling between two or more redox sites is of interest for tuning the electronic properties of molecules and materials. While classic mixed-valence (MV) systems are highly tunable, e.g., via the modular organic bridges connecting the redox sites, metal-bridged MV systems are difficult to control because the electronics of the metal cannot usually be altered independently of redox-active moieties embedded in its ligands. Herein, we overcome this limitation by varying the donor strengths of ancillary ligands in a series of cobalt complexes without directly perturbing the electronics of viologen-like redox sites bridged by the cobalt ions. The cobaltoviologens [1X-Co]n+ feature four 4-X-pyridyl donor groups (X = CO2Me, Cl, H, Me, OMe, NMe2) that provide gradual tuning of the electronics of the bridging CoII centers, while a related complex [2-Co]n+ with NHC donors supports exclusively CoIII states even upon reduction of the viologen ligands. Electrochemistry and IVCT band analysis reveal that the MV states of these complexes have electronic structures ranging from fully localized ([2-Co]4+; Robin-Day Class I) to fully delocalized ([1CO2Me-Co]3+; Class III) descriptions, demonstrating unprecedented control over electronic coupling without changing the identity of the redox sites or bridging metal. Additionally, single-crystal XRD characterization of the homovalent complexes [1H-Co]2+ and [1H-Zn]2+ revealed radical-pairing interactions between the viologen ligands of adjacent complexes, representing a type of through-space electronic coupling commonly observed for organic viologen radicals but never before seen in metalloviologens. The extended solid-state packing of these complexes produces 3D networks of radical π-stacking interactions that impart unexpected mechanical flexibility to these crystals.