Influence of Multisite Metal-Ligand Cooperativity on the Redox Activity of Noninnocent N2S2 Ligands.

Metal-ligand cooperativity (MLC) relies on chemically reactive ligands to assist metals with small-molecule binding and activation, and it has facilitated unprecedented examples of catalysis with metal complexes. Despite growing interest in combining ligand-centered chemical and redox reactions for chemical transformations, there are few studies demonstrating how chemically engaging redox active ligands in MLC affects their electrochemical properties when bound to metals. Here we report stepwise changes in the redox activity of model Ru complexes as zero, one, and two BH3 molecules undergo MLC binding with a triaryl noninnocent N2S2 ligand derived from o-phenylenediamine (L1). A similar series of Ru complexes with a diaryl N2S2 ligand with ethylene substituted in place of phenylene (L2) is also described to evaluate the influence of the o-phenylenediamine subunit on redox activity and MLC. Cyclic voltammetry (CV) studies and density functional theory (DFT) calculations show that MLC attenuates ligand-centered redox activity in both series of complexes, but electron transfer is still achieved when only one of the two redox-active sites on the ligands is chemically engaged. The results demonstrate how incorporating more than one multifunctional reactive site could be an effective strategy for maintaining redox noninnocence in ligands that are also chemically reactive and competent for MLC.

CNS and CNP Iron(II) Mono-Iron Hydrogenase (Hmd) Mimics: Role of Deprotonated Methylene(acyl) and the trans-Acyl Site in H2 Heterolysis.

We report syntheses and H2 activation involving model complexes of mono-iron hydrogenase (Hmd) derived from acyl-containing pincer ligand precursors bearing thioether (CNSPre) or phosphine (CNPPre) donor sets. Both complexes feature pseudo-octahedral iron(II) dicarbonyl units. While the CNS pincer adopts the expected mer-CNS (pincer) geometry, the CNP ligand unexpectedly adopts the fac-CNP coordination geometry. Both complexes exhibit surprisingly acidic methylene C-H bond (reversibly de/protonated by a bulky phenolate), which affords a putative dearomatized pyridinate-bound intermediate. Such base treatment of Fe-CNS also results in deligation of the thioether sulfur donor, generating an open coordination site trans from the acyl unit. In contrast, Fe-CNP maintains a CO ligand trans from the acyl site both in the parent and dearomatized complexes (the -PPh2 donor is cis to acyl). The dearomatized mer-Fe-CNS was competent for H2 activation (5 atm D2(g) plus phenolate as base), which is attributed to both the basic site on the ligand framework and the open coordination site trans to the acyl donor. In contrast, the dearomatized fac-Fe-CNP was not competent for H2 activation, which is ascribed to the blocked coordination site trans from acyl (occupied by CO ligand). These results highlight the importance of both (i) the open coordination site trans to the organometallic acyl donor and (ii) a pendant base in the enzyme active site.

Iron Hydride Detection and Intramolecular Hydride Transfer in a Synthetic Model of Mono-Iron Hydrogenase with a CNS Chelate.

We report the identification and reactivity of an iron hydride species in a synthetic model complex of monoiron hydrogenase. The hydride complex is derived from a phosphine-free CNS chelate that includes a Fe-C(NH)(═O) bond (carbamoyl) as a mimic of the active site iron acyl. The reaction of [((O═)C(HN)N(py)S(Me))Fe(CO)2(Br)] (1) with NaHBEt3 generates the iron hydride intermediate [((O═)C(HN)N(py)S(Me))Fe(H)(CO)2] (2; δFe-H = -5.08 ppm). Above -40 °C, the hydride species extrudes CH3S(-) via intramolecular hydride transfer, which is stoichiometrically trapped in the structurally characterized dimer µ2-(CH3S)2-[((O═)C(HN)N(Ph))Fe(CO)2]2 (3). Alternately, when activated by base ((t)BuOK), 1 undergoes desulfurization to form a cyclometalated species, [((O═)C(NH)NC(Ph))Fe(CO)2] (5); derivatization of 5 with PPh3 affords the structurally characterized species [((O═)C(NH)NC)Fe(CO)(PPh3)2] (6), indicating complex 6 as the common intermediate along each pathway of desulfurization.

Influence of the substituents on the electronic and electrochemical properties of a new square-planar nickel-bis(quinoxaline-6,7-dithiolate) system: synthesis, spectroscopy, electrochemistry, crystallography, and theoretical investigation.

We describe the synthesis, crystal structures, electronic absorption spectra, and electrochemistry of a series of square-planar nickel-bis(quinoxaline-6,7-dithiolate) complexes with the general formula [Bu(4)N](2)[Ni(X(2)6,7-qdt)(2)], where X = H (1a), Ph (2a), Cl (3), and Me (4). The solution and solid-state electronic absorption spectral behavior and electrochemical properties of these compounds are strongly dependent on the electron donating/accepting nature of the substituent X, attached to the quinoxaline-6,7-dithiolate ring in the system [Bu(4)N](2)[Ni(X(2)6,7-qdt)(2)]. Particularly, the charge transfer (CT) transition bands observed in the visible region are greatly affected by the electronic nature of the substituent. A possible explanation for this influence of the substituents on electronic absorption and electrochemistry is described based on highest occupied molecular orbital (HOMO) to lowest unoccupied molecular orbital (LUMO) gaps, which is further supported by ground-state electronic structure calculations. In addition to this, the observed CT bands in all the complexes are sensitive to the solvent polarity. Interestingly, compounds 1a, 2a, 3, and 4 undergo reversible oxidation at very low oxidation potentials appearing at E(1/2) = +0.12 V, 0.033 V, 0.18 V, and 0.044 V vs Ag/AgCl, respectively, in MeOH solutions, corresponding to the respective couples [Ni(X(2)6,7-qdt)(2)](-)/[Ni(X(2)6,7-qdt)(2)](2-). Compounds 1a, 3, and 4 have been characterized unambiguously by single crystal X-ray structural analysis; compound 2a could not be characterized by single crystal X-ray structure determination because of the poor quality of the concerned crystals. Thus, we have synthesized the tetraphenyl phosphonium salt of the complex anion of 2a, [PPh(4)](2)[Ni(Ph(2)6,7-qdt)(2)]·3DMF (2b) for its structural characterization.

6,7,6',7'-Tetra-phenyl-2,2'-bi[1,3-dithia-5,8-diaza-cyclo-penta-[b]naphthalenyl-idene] chloro-form disolvate.

The title compound, C(42)H(24)N(4)S(4)·2CHCl(3), a symmetrical tetra-thia-fulvalene (TTF) derivative, was prepared by a tri-ethyl-phos-phite-mediated self-coupling reaction of 6,7-diphenyl-1,3-dithia-5,8-diaza-cyclo-penta-[b]napthalen-2-one. The asymmetric unit contains two TTF mol-ecules and four chloro-form solvent mol-ecules. Cl⋯Cl inter-actions [contact distances = 3.263 (1)-3.395 (2) Å] are present between the solvent mol-ecules, resulting in a tape along the bc plane. The crystal packing features weak C-H⋯Cl and C-H⋯N hydrogen bonds, resulting in the formation of a two-dimensional supramolecular network.

Substitution reactions of iron(ii) carbamoyl-thioether complexes related to mono-iron hydrogenase.

A C,N,S pincer complex has been synthesized for structural modeling of the organometallic active site of mono-[Fe] hydrogenase (HMD). The C,N,S chelate allows for systematic investigation of the substitution reactions of CO and other exogenous X/L-type ligands, as well as examination of the exact roles of the Fe-carbamoyl and {Fe(CO)2}2+ units in stabilizing the low-spin Fe(ii) center. Reaction of the 'apo-ligand' 6-(2-(methylthio)phenyl)pyridin-2-amine (H2NNpySMe) with [Fe(CO)4(Br)2] affords the organometallic complex [(O[double bond, length as m-dash]CNHNpySMe)Fe(CO)2(Br)] (1). Facile substitutions of the halide with L-type ligands such as MeCN, PR3 (R = C6H5, OEt, Me), pyridine and tBuNC afford diamagnetic cations of the type [(O[double bond, length as m-dash]CNHNpySMe)Fe(CO)2(L)]+ (2a-f). Treatment of 1 with Na[S(2,6-Me2C6H3)] affords the neutral complex [(O[double bond, length as m-dash]CNHNpySMe)Fe(CO)2(S(2,6-Me2C6H3))] (2g). Substitution for CO ligand(s) was achieved with trimethylamine-N-oxide (TMAO), and in the presence of PPh3 or pyridine it afforded the six-coordinate monocarbonyl complexes [(O[double bond, length as m-dash]CNHNpySMe)Fe(CO)(Br)(PPh3)] (3a), [(O[double bond, length as m-dash]CNHNpySMe)Fe(CO)(PPh3)2](BArF4) (3b), and [(O[double bond, length as m-dash]CNHNpySMe)Fe(CO)(py)2](BArF4) (3c). Interestingly the stable low-spin Fe(ii), 5-coordinate complex of the formula [(O[double bond, length as m-dash]CNHNpySMe)Fe(CO)2](BArF4) (4) was accessed by treating 1 with TlBArF4 in non-coordinating solvents (DCE, FPh); notably, 4 does not react with H2 in the presence (or absence) of a base. To elucidate the electronic structure differences between the five-coordinate versus six-coordinate complexes, DFT calculations for 4 and 1 were performed. Geometry optimization indicates that 4+ maintains a square-pyramidal geometry, and the Hessian calculation accurately simulates the ν(C[triple bond, length as m-dash]O) in 4+. The electronic structure of 4+ predicts that the HOMO (comprised of Fe|Ncarb) and LUMO (Fe only) orbitals in 4+ are properly oriented to interact with an incoming ligand. However, we postulate that codirectional orientation of the HOMO and LUMO orbitals explains the lack of H2 reactivity with this equatorial CNS donor set, despite many other structural similarities to the endogenous active site. Based on a related work from our lab, we conclude that a facial C,N,S coordination mode is necessary to promote H2 activation and cleavage.

Tuning coordination modes of pyridine/thioether Schiff base (NNS) ligands to mononuclear manganese carbonyls.

We have investigated the coordination modes of NNS Schiff base, thioether ligands to manganese(I) carbonyls. The ligands contain ortho substituted pyridines (H, CH3, OCH3, fluorophenyl) and varying substituents (H, CH3) at the Schiff base linkage. In general, reaction of [Mn(CO)5Br] with a tridentate NNS ligand in CH2Cl2 affords species in which the thioether-S may be bound or unbound to the manganese center, depending on the steric and electronic substitution in the ligand framework; as a result, the complexes exhibit two or three carbonyl ligands, respectively. Aldehyde-derived ligand frames ((R1)N(H)NS) generally afford complexes of type [((R)NNS)Mn(CO)3Br] (1(CO), 2(CO), 3(CO); R = H, OCH3, CH3) that exhibit incomplete ligation of the chelate (S not bound) in X-ray structures. In contrast, use of the iminomethyl ligand (N(Me)NS) affords a complex of formula [(N(Me)NS)Mn(CO)2Br] (4(CO)), in which the mixed N/thioether-S stabilizes the {Mn(CO)2}(+) fragment. In solid state IR spectra, complexes of type [((R)NNS)Mn(CO)3Br] (1(CO) through 3(CO)) afford three ν(CO) in the range ~2060-1865 cm(-1); the dicarbonyl complex [(N(Me)NS)Mn(CO)2Br] (4(CO)) exhibits two carbonyl stretches in the range ~1920-1845 cm(-1). Prolonged storage of the tricarbonyl [((Me)NNS)Mn(CO)3Br] (3(CO)) in presence of trace dioxygen affords the dibromide species [((Me)NNS)Mn(Br)2] (3(Br)), in which the thioether S reliably binds to the Mn(II) center. Complexes 1(CO)-3(CO) exhibit simple, diamagnetic (1)H NMR spectra in CD2Cl2. The S-ligated complex 4(CO) exhibits spectra consistent with a mixture of an S-bound (6-coordinate) and S-unbound (5-coordinate) species as represented by [(N(Me)NS)Mn(CO)2Br] ↔ [(N(Me)NS)Mn(CO)2Br]. Lastly, we obtained crystal structures of the S-bound and S-unbound conformers derived from the same ligand--the fluorophenyl derived (FPh)NNS, namely [((FPh)NNS)Mn(CO)3Br] (5(CO-a)) and [((FPh)NNS)Mn(CO)2Br] (5(CO-b)). This report represents several examples of a thioether-stabilized {Mn(CO)2}(+) fragment, a deviation from the usual 'piano stool' Mn(I) tricarbonyl motif. We highlight that coordination of these NNS ligands to Mn(I) carbonyls occurs on a soft conformational landscape, and that ligand substituents can be rationally employed to favor the desired coordination mode.