A key step in cytochrome P450 catalysis includes the spin-state crossing from low spin to high spin upon substrate binding and subsequent reduction of the heme. Clearly, a weak perturbation in P450 enzymes triggers a spin-state crossing. However, the origin of the process whereby enzymes reorganize their active site through external perturbations, such as hydrogen bonding, is still poorly understood. We have thus studied the impact of hydrogen-bonding interactions on the electronic structure of a five-coordinate iron(III) octaethyltetraarylporphyrin chloride. The spin state of the metal was found to switch reversibly between high (S=5/2) and intermediate spin (S=3/2) with hydrogen bonding. Our study highlights the possible effects and importance of hydrogen-bonding interactions in heme proteins. This is the first example of a synthetic iron(III) complex that can reversibly change its spin state between a high and an intermediate state through weak external perturbations.

Addition of CO to a low-valent nickel species has been explored with a newly designed PNP pincer ligand (PNP = 4,5-bis(diisopropylphosphino)-2,7,9,9-tetramethyl-9H-acridin-10-ide). This is a crucial step in understanding biological CO conversion to CO found in carbon monoxide dehydrogenase (CODH). A four-coordinate nickel(0) state was reliably accessed in the presence of a CO ligand, which can be prepared from a stepwise reduction of a cationic {(PNP)Ni(II)-CO} species. All three Ni(II), Ni(I), and Ni(0) monocarbonyl species were cleanly isolated and spectroscopically characterized. Addition of electrons to the nickel(II) species significantly alters its geometry from square planar toward tetrahedral because of the filling of the d orbital. Accordingly, the CO ligand position changes from equatorial to axial, ∠N-Ni-C of 176.2(2)° to 129.1(4)°, allowing opening of a CO binding site. Upon addition of CO to a nickel(0)-CO species, a nickel(II) carboxylate species with a Ni(η-CO-κC) moiety was formed and isolated (75%). This reaction occurs with the concomitant expulsion of CO(g). This is a unique result markedly different from our previous report involving the flexible analogous PNP ligand, which revealed the formation of multiple products including a tetrameric cluster from the reaction with CO. Finally, the carbon dioxide conversion to CO at a single nickel center is modeled by the successful isolation of all relevant intermediates, such as Ni-CO, Ni-COOH, and Ni-CO.

We report the synthesis, structure, and spectroscopic characterization of 1,2-bis[μ-hydroxo iron(III) 5-(2,3,7,8,12,13,17,18-octaethylporphyrinyl)]ethane with PF6(–) and SbF6(–) counteranions. The two iron centers are nonequivalent with admixed intermediate spin state (S = 3/2 with a minor contribution of S = 5/2) on each metal both in the solid and in solution. The molecules are compared with previously known μ-hydroxo complexes with other counterions, such as I3(–), BF4(–), and ClO4(–), which demonstrates that the nature of the counterion can affect the spin-state ordering dramatically. To understand how the spin-state ordering is affected by external perturbations, we also have done a comprehensive computational study. The calculations show that subtle environmental perturbations affect the spin-state ordering and relative energies and are likely to be the root cause of the variation in spin-state ordering observed experimentally.

The reaction of vanadium(III) trichloride with tert-butylphosphonic acid (t-BuPO3H2) in the presence of 1,10-phenanthroline/2,2'-bipyridine as an ancillary ligand in acetonitrile at room temperature afforded two dinuclear dicationic vanadium(IV) complexes [(VO)2(phen)2{t-BuPO2(OH)}2(OH2)2]·2Cl [1] and [(VO)2(bipy)2{t-BuPO2(OH)}2(OH2)2]·2Cl [2]. On the other hand, when the reaction was carried out in methanol, the dinuclear vanadium(V) complex [(VO)2(bipy)2(μ2-O)2(t-BuPO3)2]·2CH3OH·0.5CH2Cl2 [3] was isolated. While 1 and 2 contain two six-membered V2P2O4 rings, 3 contains a unique four-membered V2O2 ring. Replacement of tert-butylphosphonic acid by tritylphosphonic acid (Ph3CPO3H2) under the same reaction conditions in methanol leads to the formation of dicationic trinuclear vanadium(IV) complexes [(VO)3(phen)3(Ph3CPO3)2(OH2)3]·CHCl3·2(OH)·2MeOH·1.5H2O [4] and [(VO)3(bipy)3(Ph3CPO3)2(CH3OH)3]·2(OH)·4CH3OH·5H2O [5]. In these complexes, the triangular V(IV) platform is held together by two bicapping tripodal phosphonate ligands. Replacement of the chelating 2,2'-bipyridine ligand with 3,5-dimethyl-1H-pyrazole, under the same reaction conditions, afforded a tetranuclear vanadium(V) complex [{(VO)4(Ph3CPO3Me)4(μ-O)4}Cl]{3,5-Me2PzH2}·3C7H8·H2O·CH3OH [6]. Remarkably 6 possesses a unique bowl-shaped structure encapsulating a chloride anion.