CO Release from N,C,S-Pincer Iron(III) Carbonyl Complexes Induced by Visible-to-NIR Light Irradiation: Mechanistic Insight into Effects of Axial Phosphorus Ligands

Light-induced CO release from newly synthesized N,C,S-pincer iron­(III) carbonyl complexes with two phosphorus ligands<i>trans</i>-[Fe­(L-κ³<i>N,C,S</i>)­(CO)­(PR₂R′)₂]­PF₆ ([<b>1</b>]­PF₆, R = Me, R′ = Ph; [<b>2</b>]­PF₆, R = R′ = Me; [<b>3</b>]­PF₆, R = R′ = OEt)were investigated. All the iron­(III) carbonyl complexes were stable in solution and showed light-inducible CO release under ambient conditions. Studies on the wavelength dependence of photoreaction revealed that the phosphite complex [<b>3</b>]­PF₆ exhibited the most extended photosensitivity including all visible and a part of near-IR light (390–800 nm wavelengths). The phosphine complexes [<b>1</b>]­PF₆ and [<b>2</b>]­PF₆ showed sensitivity to only the higher-energy region of visible light (390–450 nm). Quantum-chemical calculations and spectroscopic data suggested that all complexes [<b>1</b>]­PF₆–[<b>3</b>]­PF₆ have dπ–dπ excitation modes to depopulate Fe–C­(carbonyl) bonding and potentially induce the CO release by irradiation of light in the near-IR region, although moderately weakened Fe–C­(carbonyl) bonding due to stronger π-backbonding by the phosphite ligand rendered the excitation effective on the CO release exclusively in [<b>3</b>]­PF₆

Carbon–Sulfur Bond Cleavage Reactions of Quinolyl-Substituted Thiophenes with Iron Carbonyls

Thermal reactions of quinolyl-substituted thiophenes (2-(8′-quinolyl)­thiophene (QT), 2-methyl-5-(8′-quinolyl)­thiophene (MeQT), 2-(8′-quinolyl)-5-(trimethylsilyl)­thiophene (TMSQT)) with [Fe₃(CO)₁₂] gave the corresponding thiolate-bridged diiron complexes [Fe₂(μ-L^R)­(CO)₅] (R = H, Me, SiMe₃), where L^H, L^Me, and L^TMS are dianionic N,C,S-tridentate ligands (SC­(R)­CHCHC­(Q)^2–, Q = 8-quinolyl) formed by the oxidative addition of the C–S bond in QT, MeQT, and TMSQT, respectively. In contrast, the formation of photoreaction products of the quinolyl-substituted thiophenes with [Fe­(CO)₅] was dependent on the R group of the thiophene ring. The photoreaction of QT gave the sulfur-free diiron complex [Fe₂{CHCHCHC­(Q)}­(CO)₅], whereas the photoreactions of MeQT and TMSQT gave the thiolate-bridged triiron complex [Fe₃(μ-L^Me)­(CO)₈] and diiron complex [Fe₂(μ-L^TMS)­(CO)₅], respectively, as the major products. In the triiron complexes [Fe₃(μ-L^R)­(CO)₈], an Fe­(CO)₃ unit is bound to the C­(R)­CHCH moiety in the S-metallacycle of the diiron complexes [Fe₂(μ-L^R)­(CO)₅]. The difference in the photoreaction products is described on the basis of the reactivity of the thiolate complexes [Fe₂(μ-L^R)­(CO)₅] and [Fe₃(μ-L^R)­(CO)₈]. Although the photoreactions of the diiron complexes [Fe₂(μ-L^R)­(CO)₅] with [Fe­(CO)₅] produced the corresponding triiron complexes [Fe₃(μ-L^R)­(CO)₈], desulfurization leading to the formation of [Fe₂{CHCHCHC­(Q)}­(CO)₅] was predominant for R = H, and a fast conversion of the triiron complex to a CO elimination product was observed for R = SiMe₃

Skeletal Modification of Benzothiophene Mediated by Iron Carbonyls: Insertion of Terminal Alkynes with Migration of Amino and Alkoxy Groups

A thiolate-bridged diiron carbonyl complex derived from benzothiophene, [Fe₂(μ-SC₆H₄CHCH)­(CO)₆], reacted with terminal alkynes HCCR (R = SiMe₃, Ph, isobutyl) under photoirradiation conditions to afford diiron complexes with a 2,4-pentadienoyl moiety, [Fe₂{μ-SC₆H₄(CH)₃C­(R)­CO}­(CO)₅], via alkyne and CO insertion. In a similar reaction with <i>N,N</i>-dimethylpropargylamine, a diiron complex with a pentadienyl moiety, [Fe₂{μ-SC₆H₄(CH)₃C­(NMe₂)­CH₂}­(CO)₅], was obtained as an alkyne insertion product without CO insertion. This reaction involves 1,2-migration of a dimethylamino group. The corresponding reactions with alkyl propargyl ethers also produced diiron complexes containing pentadienyl moieties with an alkoxycarbonyl group via alkoxy migration with CO insertion. The migration process via C–N or C–O bond cleavage could be related to the coordination ability of N or O in the propargyl compounds

Di- and Mononuclear Iron Complexes of N,C,S-Tridentate Ligands Containing an Aminopyridyl Group: Effect of the Pendant Amine Site on Catalytic Properties for Proton Reduction

A series of diiron complexes of N,C,S-tridentate ligands containing a 6-, 5-, or 4-amino-2-pyridyl group, [{Fe­(μ-L-κ³<i>N,C,S</i>)­(CO)₂}­Fe­(CO)₃] (<b>2</b>, L = <i>o</i>-apyBPT; <b>3</b>, L = <i>m</i>-apyBPT; <b>4</b>, L = <i>p</i>-apyBPT), was synthesized: apyBPT is a doubly deprotonated form of 3′-(amino-2″-pyridyl)-1,1′-biphenyl-2-thiol. Complexes <b>2</b>–<b>4</b> were converted to the mononuclear iron­(II) complexes <i>trans</i>-[Fe­(L-κ³<i>N,C,S</i>)­(CO)­(PMe₂Ph)₂] (<b>6</b>, L = <i>o</i>-apyBPT; <b>7</b>, L = <i>m</i>-apyBPT; <b>8</b>, L = <i>p</i>-apyBPT). In <b>2</b> and <b>6</b>, the <i>o</i>-amino group is close to Fe bound to the aminopyridyl group. Cyclic voltammograms of <b>2</b>–<b>4</b> exhibit two consecutive one-electron reduction events, and catalytic current for proton reduction appears in the presence of acetic acid. The reduction potentials of <b>2</b>–<b>4</b> are similar to each other, while the overpotential for proton reduction with <i>o</i>-amino complex <b>2</b> is ca. 0.2 V lower than those with <b>3</b> and <b>4</b>. In the mononuclear complexes <b>6</b>–<b>8</b>, the redox potentials for the Fe^III/Fe^II couple are dependent on the position of the amino group in the pyridine ring, which is described by electronic and steric effects of the amino group. Such effects on the redox potentials are suppressed in the diiron complexes because the reduction occurs at the diiron core with π-accepting CO ligands, which is supported by DFT calculations. The lower overpotential in <b>2</b> compared with <b>3</b> and <b>4</b> is attributed to the concerted effect of the amino group proximal to the iron center. The amino group probably acts as a proton acceptor and assists the formation of the H–H bond from a hydride on the iron centers and a proton bound to the amino group

Skeletal Modification of Benzothiophene Mediated by Iron Carbonyls: Insertion of Terminal Alkynes with Migration of Amino and Alkoxy Groups

A thiolate-bridged diiron carbonyl complex derived from benzothiophene, [Fe₂(μ-SC₆H₄CHCH)­(CO)₆], reacted with terminal alkynes HCCR (R = SiMe₃, Ph, isobutyl) under photoirradiation conditions to afford diiron complexes with a 2,4-pentadienoyl moiety, [Fe₂{μ-SC₆H₄(CH)₃C­(R)­CO}­(CO)₅], via alkyne and CO insertion. In a similar reaction with <i>N,N</i>-dimethylpropargylamine, a diiron complex with a pentadienyl moiety, [Fe₂{μ-SC₆H₄(CH)₃C­(NMe₂)­CH₂}­(CO)₅], was obtained as an alkyne insertion product without CO insertion. This reaction involves 1,2-migration of a dimethylamino group. The corresponding reactions with alkyl propargyl ethers also produced diiron complexes containing pentadienyl moieties with an alkoxycarbonyl group via alkoxy migration with CO insertion. The migration process via C–N or C–O bond cleavage could be related to the coordination ability of N or O in the propargyl compounds

Diiron Carbonyl Complexes Bearing an N,C,S-Pincer Ligand: Reactivity toward Phosphines, Heterolytic Fe–Fe Cleavage, and Electrocatalytic Proton Reduction

The thiolate-bridged diiron carbonyl complex [{Fe­(μ-PyBPT-κ³<i>N,C,S</i>)­(CO)₂}­Fe­(CO)₃] (<b>1</b>) consists of two units, Fe­(PyBPT)­(CO)₂ and Fe­(CO)₃, where the N,C,S-pincer ligand PyBPT is a doubly deprotonated form of 3′-(2″-pyridyl)-1,1′-biphenyl-2-thiol. The two Fe complex units are connected through a thiolate S atom, π coordination, and an Fe–Fe bond. Diiron complex <b>1</b> reacted with 1 equiv of dimethylphenylphosphine to form the CO substitution product [{Fe­(μ-PyBPT-κ³<i>N,C,S</i>)­(CO)₂}­Fe­(CO)₂(PMe₂Ph)] (<b>3</b>) via the phosphine adduct [{Fe­(μ-PyBPT-κ³<i>N,C,S</i>)­(CO)₂}­Fe­(CO)₃(PMe₂Ph)] (<b>2</b>), which has a polarized Fe–Fe bond. A further reaction of <b>3</b> with PMe₂Ph produced the N,C,S-pincer iron­(II) complex <i>trans</i>-[Fe­(PyBPT-κ³<i>N,C,S</i>)­(CO)­(PMe₂Ph)₂] (<b>4</b>) and the iron(0) complex <i>trans</i>-[Fe­(CO)₃(PMe₂Ph)₂]. 1,2-Bis­(diphenylphosphino)­benzene (dppbz) abstracted the Fe­(CO)₃ unit from <b>1</b> to give the dimeric diiron­(II,II) complex [{Fe­(μ-PyBPT-κ³<i>N,C,S</i>)­(CO)₂}₂] (<b>7</b>) and the iron(0) complex [Fe­(CO)₃(dppbz)]. The asymmetric bridging ligand PyBPT and coordination of the phosphines induce polarization of the Fe–Fe bond, which leads to the formation of the iron­(II) and iron(0) complexes via heterolytic Fe–Fe cleavage. Electrochemical properties of <b>3</b> and <b>4</b> were investigated by cyclic voltammetry. Complex <b>3</b> showed two one-electron-reduction processes, the potentials of which are ca. 0.4 V more negative than those of <b>1</b>. Electrocatalytic proton reduction by <b>3</b> was investigated, and the efficiency was comparable to that of <b>1</b>

Diiron Carbonyl Complexes Bearing an N,C,S-Pincer Ligand: Reactivity toward Phosphines, Heterolytic Fe–Fe Cleavage, and Electrocatalytic Proton Reduction

The thiolate-bridged diiron carbonyl complex [{Fe­(μ-PyBPT-κ³<i>N,C,S</i>)­(CO)₂}­Fe­(CO)₃] (<b>1</b>) consists of two units, Fe­(PyBPT)­(CO)₂ and Fe­(CO)₃, where the N,C,S-pincer ligand PyBPT is a doubly deprotonated form of 3′-(2″-pyridyl)-1,1′-biphenyl-2-thiol. The two Fe complex units are connected through a thiolate S atom, π coordination, and an Fe–Fe bond. Diiron complex <b>1</b> reacted with 1 equiv of dimethylphenylphosphine to form the CO substitution product [{Fe­(μ-PyBPT-κ³<i>N,C,S</i>)­(CO)₂}­Fe­(CO)₂(PMe₂Ph)] (<b>3</b>) via the phosphine adduct [{Fe­(μ-PyBPT-κ³<i>N,C,S</i>)­(CO)₂}­Fe­(CO)₃(PMe₂Ph)] (<b>2</b>), which has a polarized Fe–Fe bond. A further reaction of <b>3</b> with PMe₂Ph produced the N,C,S-pincer iron­(II) complex <i>trans</i>-[Fe­(PyBPT-κ³<i>N,C,S</i>)­(CO)­(PMe₂Ph)₂] (<b>4</b>) and the iron(0) complex <i>trans</i>-[Fe­(CO)₃(PMe₂Ph)₂]. 1,2-Bis­(diphenylphosphino)­benzene (dppbz) abstracted the Fe­(CO)₃ unit from <b>1</b> to give the dimeric diiron­(II,II) complex [{Fe­(μ-PyBPT-κ³<i>N,C,S</i>)­(CO)₂}₂] (<b>7</b>) and the iron(0) complex [Fe­(CO)₃(dppbz)]. The asymmetric bridging ligand PyBPT and coordination of the phosphines induce polarization of the Fe–Fe bond, which leads to the formation of the iron­(II) and iron(0) complexes via heterolytic Fe–Fe cleavage. Electrochemical properties of <b>3</b> and <b>4</b> were investigated by cyclic voltammetry. Complex <b>3</b> showed two one-electron-reduction processes, the potentials of which are ca. 0.4 V more negative than those of <b>1</b>. Electrocatalytic proton reduction by <b>3</b> was investigated, and the efficiency was comparable to that of <b>1</b>

Carbon- and Sulfur-Bridged Diiron Carbonyl Complexes Containing N,C,S-Tridentate Ligands Derived from Functionalized Dibenzothiophenes: Mimics of the [FeFe]-Hydrogenase Active Site

Photochemical reactions of [Fe­(CO)₅] with dibenzothiophene (DBT) derivatives bearing a N-donor group produced a series of C,S-bridged diiron carbonyl complexes [{Fe­(μ-L′-κ³<i>N</i>,<i>C</i>,<i>S</i>)­(CO)₂}­Fe­(CO)₃], as previously reported for 4-(2′-pyridyl)­dibenzothiophene (L¹), where L′ represents the N,C,S-tridentate ligands L¹′–L⁵′, formed by C–S bond cleavage of L¹–L⁵, respectively. The DBT derivatives used in this study have Schiff base or oxazoline moieties at the 4-position: L² = PhCH₂NCH-DBT, L³ = 2-MeOC₆H₄CH₂NCH-DBT, L⁴ = (<i>S</i>)-PhC­(Me)­HNCH-DBT, L⁵ = (<i>R</i>)-4-isopropyl-2-oxazolinyl-DBT. The diiron complexes were characterized by NMR, absorption, and circular dichroism spectroscopy, and the dinuclear structures bridged by thiolate S and aryl C atoms were established by X-ray crystallography. The diiron complex [{Fe­(μ-L′-κ³<i>N</i>,<i>C</i>,<i>S</i>)­(CO)₂}­Fe­(CO)₃] consists of two units, Fe­(L′)­(CO)₂ and Fe­(CO)₃: the latter unit is located on a thiolate-containing metallacycle in the former one. The chiral Schiff base ligand precursor L⁴ gave a 55:45 mixture of two diastereomers for [{Fe­(μ-L⁴′-κ³<i>N</i>,<i>C</i>,<i>S</i>)­(CO)₂}­Fe­(CO)₃], while chiral L⁵ with an (<i>R</i>)-4-isopropyl-2-oxazolinyl group afforded [{Fe­(μ-L⁵′-κ³<i>N</i>,<i>C</i>,<i>S</i>)­(CO)₂}­Fe­(CO)₃] in a 9:1 diastereomeric ratio. The diiron carbonyl complexes of the N,C,S-tridentate ligands (L¹′L⁵′) showed two reversible redox couples for [Fe₂(μ-L′)­(CO)₅]^0/– and [Fe₂(μ-L′)­(CO)₅]^−/2–. The two-electron-reduced forms undergo protonation and act as electrocatalysts for proton reduction of acetic acid in acetonitrile

Fine-Tuning the Energy Barrier for Metal-Mediated Dinitrogen NN Bond Cleavage

Experimental data support a mechanism for NN bond cleavage within a series of group 5 bimetallic dinitrogen complexes of general formula, {Cp*M­[N­(^<i>i</i>Pr)­C­(R)­N­(^<i>i</i>Pr)]}₂(μ-N₂) (Cp* = η⁵-C₅Me₅) (M = Nb, Ta), that proceeds in solution through an intramolecular “end-on-bridged” (μ-η¹:η¹-N₂) to “side-on-bridged” (μ-η²:η²-N₂) isomerization process to quantitatively provide the corresponding bimetallic bis­(μ-nitrido) complexes, {Cp*M­[N­(^<i>i</i>Pr)­C­(R)­N­(^<i>i</i>Pr)]­(μ-N)}₂. It is further demonstrated that subtle changes in the steric and electronic features of the distal R-substituent, where R = Me, Ph and NMe₂, can serve to modulate the magnitude of the free energy barrier height for NN bond cleavage as assessed by kinetic studies and experimentally derived activation parameters. The origin of the contrasting kinetic stability of the first-row congener, {Cp*V­[N­(^<i>i</i>Pr)­C­(Me)­N­(^<i>i</i>Pr)]}₂(μ-η¹:η¹-N₂) toward NN bond cleavage is rationalized in terms of a ground-state electronic structure that favors a significantly less-reduced μ-N₂ fragment

Anion-Controlled Assembly of Four Manganese Ions: Structural, Magnetic, and Electrochemical Properties of Tetramanganese Complexes Stabilized by Xanthene-Bridged Schiff Base Ligands

The reaction of manganese­(II) acetate with a xanthene-bridged bis­[3-(salicylideneamino)-1-propanol] ligand, H₄L, afforded the tetramanganese­(II,II,III,III) complex [Mn₄(L)₂(μ-OAc)₂], which has an incomplete double-cubane structure. The corresponding reaction using manganese­(II) chloride in the presence of a base gave the tetramanganese­(III,III,III,III) complex [Mn₄(L)₂Cl₃(μ₄-Cl)­(OH₂)], in which four Mn ions are bridged by a Cl^– ion. A pair of L ligands has a propensity to incorporate four Mn ions, the arrangement and oxidation states of which are dependent on the coexistent anions