Magnetic Excitations in Metalloporphyrins by Inelastic Neutron Scattering: Determination of Zero-Field Splittings in Iron, Manganese, and Chromium Complexes

Zero field splitting (ZFS) parameters of several nondeuterated metalloporphyrins [M­(TPP)­Cl] and [Mn­(TPP)] (H₂TPP = tetraphenylporphyrin) have been directly determined by inelastic neutron scattering (INS). The ZFS values are the following: <i>D</i> = 6.33(8) cm^–1 for [Fe­(TPP)­Cl], −2.24(3) cm^–1 for [Mn­(TPP)­Cl], 0.79(2) cm^–1 for [Mn­(TPP)], and |<i>D</i>|= 0.234(12) cm^–1 for [Cr­(TPP)­Cl]. The work shows that compounds with magnetic excitations below ∼30 cm^–1 could be determined using nondeuterated samples

Bismuth-Based, Disposable Sensor for the Detection of Hydrogen Sulfide Gas

A new sensor for the detection of hydrogen sulfide (H₂S) gas has been developed to replace commercial lead­(II) acetate-based test papers. The new sensor is a wet, porous, paper-like substrate coated with Bi­(OH)₃ or its alkaline derivatives at pH 11. In contrast to the neurotoxic lead­(II) acetate, bismuth is used due to its nontoxic properties, as Bi­(III) has been a reagent in medications such as Pepto-Bismol. The reaction between H₂S gas and the current sensor produces a visible color change from white to yellow/brown, and the sensor responds to ≥30 ppb H₂S in a total volume of 1.35 L of gas, a typical volume of human breath. The alkaline, wet coating helps the trapping of acidic H₂S gas and its reaction with Bi­(III) species, forming colored Bi₂S₃. The sensor is suitable for testing human bad breath and is at least 2 orders of magnitude more sensitive than a commercial H₂S test paper based on Pb­(II)­(acetate)₂. The small volume of 1.35-L H₂S is important, as the commercial Pb­(II)­(acetate)₂-based paper requires large volumes of 5 ppm H₂S gas. The new sensor reported here is inexpensive, disposable, safe, and user-friendly. A simple, laboratory setup for generating small volumes of ppb–ppm of H₂S gas is also reported

Preparation of Zirconium Guanidinate Complexes from the Direct Insertion of a Carbodiimine and Aminolysis Using a Guanidine. Comparison of the Reactions

Direct insertion of 1 equiv of CyNCNCy (<b>1</b>; Cy = cyclohexyl) into the Zr–NMe₂ bonds in (Me₂N)₃Zr­[N­(SiMe₃)₂] (<b>2</b>) and (Me₂N)₃Zr­[Si­(SiMe₃)₃] (<b>3</b>) gave exclusively [CyNC­(NMe₂)­NCy]­Zr­(NMe₂)₂[N­(SiMe₃)₂] (<b>5</b>) and [CyNC­(NMe₂)­NCy]­Zr­(NMe₂)₂[Si­(SiMe₃)₃] (<b>6</b>), respectively. The reaction between <b>2</b> and guanidine CyNHC­(NMe₂)NCy (<b>9</b>) gave <b>5</b> and HNMe₂ through the preferred cleavage of a Zr–NMe₂ bond in <b>2</b>. The reaction between <b>3</b> and <b>9</b> led to the preferred cleavage of the Zr–Si­(SiMe₃)₃ bond in <b>3</b>, yielding [CyNC­(NMe₂)­NCy]­Zr­(NMe₂)₃ (<b>7</b>) and HSi­(SiMe₃)₃ and, upon cleavage of another Zr–NMe₂ bond, forming [CyNC­(NMe₂)­NCy]₂Zr­(NMe₂)₂ (<b>8</b>). The aminolysis of Zr­(NMe₂)₄ (<b>4</b>) by <b>9</b> first afforded <b>7</b> and then <b>8</b>. The structures of <b>5</b>, <b>6</b>, and <b>9</b> have been determined by X-ray diffraction

Preparation of Zirconium Guanidinate Complexes from the Direct Insertion of a Carbodiimine and Aminolysis Using a Guanidine. Comparison of the Reactions

Direct insertion of 1 equiv of CyNCNCy (<b>1</b>; Cy = cyclohexyl) into the Zr–NMe₂ bonds in (Me₂N)₃Zr­[N­(SiMe₃)₂] (<b>2</b>) and (Me₂N)₃Zr­[Si­(SiMe₃)₃] (<b>3</b>) gave exclusively [CyNC­(NMe₂)­NCy]­Zr­(NMe₂)₂[N­(SiMe₃)₂] (<b>5</b>) and [CyNC­(NMe₂)­NCy]­Zr­(NMe₂)₂[Si­(SiMe₃)₃] (<b>6</b>), respectively. The reaction between <b>2</b> and guanidine CyNHC­(NMe₂)NCy (<b>9</b>) gave <b>5</b> and HNMe₂ through the preferred cleavage of a Zr–NMe₂ bond in <b>2</b>. The reaction between <b>3</b> and <b>9</b> led to the preferred cleavage of the Zr–Si­(SiMe₃)₃ bond in <b>3</b>, yielding [CyNC­(NMe₂)­NCy]­Zr­(NMe₂)₃ (<b>7</b>) and HSi­(SiMe₃)₃ and, upon cleavage of another Zr–NMe₂ bond, forming [CyNC­(NMe₂)­NCy]₂Zr­(NMe₂)₂ (<b>8</b>). The aminolysis of Zr­(NMe₂)₄ (<b>4</b>) by <b>9</b> first afforded <b>7</b> and then <b>8</b>. The structures of <b>5</b>, <b>6</b>, and <b>9</b> have been determined by X-ray diffraction

Formation of the Imide [Ta(NMe₂)₃(μ-NSiMe₃)]₂ through an Unprecedented α-SiMe₃ Abstraction by an Amide Ligand

Ta­(NMe₂)₄[N­(SiMe₃)₂] (<b>1</b>) undergoes the elimination of Me₃Si-NMe₂ (<b>2</b>), converting the −N­(SiMe₃)₂ ligand to the NSiMe₃ ligand, to give the imide “Ta­(NMe₂)₃(NSiMe₃)” (<b>3</b>) observed as its dimer <b>4</b>. CyNCNCy captures <b>3</b> to yield guanidinates Ta­(NMe₂)_3–<i>n</i>(NSiMe₃)­[CyNC­(NMe₂)­NCy]_<i>n</i> [<i>n</i> = 1 (<b>5</b>), 2 (<b>6</b>)]. The kinetic study of α-SiMe₃ abstraction in <b>1</b> gives Δ<i>H</i>^⧧ = 21.3(1.0) kcal/mol and Δ<i>S</i>^⧧ = −17(2) eu

Density Functional Theory Study of the Reaction between d⁰ Tungsten Alkylidyne Complexes and H₂O: Addition versus Hydrolysis

The reactions of early-transition-metal complexes with H₂O have been investigated. An understanding of these elementary steps promotes the design of precursors for the preparation of metal oxide materials or supported heterogeneous catalysts. Density functional theory (DFT) calculations have been conducted to investigate two elementary steps of the reactions between tungsten alkylidyne complexes and H₂O, i.e., the addition of H₂O to the WC bond and ligand hydrolysis. Four tungsten alkylidyne complexes, W­(CSiMe₃)­(CH₂SiMe₃)₃ (<b>A-1</b>), W­(CSiMe₃)­(CH₂^tBu)₃ (<b>B-1</b>), W­(C^tBu)­(CH₂^tBu)₃ (<b>C-1</b>), and W­(C^tBu)­(O^tBu)₃ (<b>D-1</b>), have been compared. The DFT studies provide an energy profile of the two competing pathways. An additional H₂O molecule can serve as a proton shuttle, accelerating the H₂O addition reaction. The effect of atoms at the α and β positions has also been examined. Because the lone-pair electrons of an O atom at the α position can interact with the orbital of the proton, the barrier of the ligand-hydrolysis reaction for <b>D-1</b> is dramatically reduced. Both the electronic and steric effects of the silyl group at the β position lower the barriers of both the H₂O addition and ligand-hydrolysis reactions. These new mechanistic findings may lead to the further development of metal complex precursors

Metal Complexes with a Hexadentate Macrocyclic Diamine-Tetracarbene Ligand

A hexadentate macrocyclic N-heterocyclic carbene (NHC) ligand precursor (H₄L)­(PF₆)₄ containing four benzimidazolium and two secondary amine groups, has been synthesized and characterized. Coordination chemistry of this new macrocyclic diamine-tetracarbene ligand has been studied by the synthesis of its Ag­(I), Au­(I), Ni­(II), and Pd­(II) complexes. Reactions of (H₄L)­(PF₆)₄ with different equiv of Ag₂O result in Ag­(I) complexes [Ag­(H₂L)]­(PF₆)₃ (<b>1</b>) and [Ag₂(H₂L)]­(PF₆)₄ (<b>2</b>). A mononuclear Au­(I) complex [Au­(H₂L)]­(PF₆)₃ (<b>3</b>) and a trinuclear Au­(I) complex [Au₃(H₂L)­(Cl)₂]­(PF₆) (<b>4</b>) are obtained by transmetalation of <b>1</b> and <b>2</b> with AuCl­(SMe₂), respectively. Reactions of (H₄L)­(PF₆)₄ with Ni­(OAc)₂ and Pd­(OAc)₂ in the presence of NaOAc yield [Ni­(L)]­(PF₆)₂ (<b>5</b>) and [Pd­(L)]­(PF₆)₂ (<b>6</b>), respectively, containing one Ni­(II) and Pd­(II) ion with distorted square-planar geometry. Using more NaOAc results in the formation of unusual dinuclear complexes [Ni₂(L–2H)]­(PF₆)₂ (<b>7</b>) and [Pd₂(L–2H)]­(PF₆)₂ (<b>8</b>) (L–2H = deprotonated ligand after removing two H⁺ ions from two secondary amine groups in L), respectively, featuring a rare M₂N₂ core formed by two bridging amides. <b>7</b> is also formed by the reaction of <b>5</b> with 1.0 equiv of Ni­(OAc)₂·4H₂O in the presence of NaOAc. Transmetalation of <b>2</b> with 2.0 equiv of Ni­(PPh₃)₂Cl₂ gives [Ni₂(L)­(μ-O)]­(PF₆)₂ (<b>9</b>), the first example of a dinuclear Ni­(II) complex with a singly bridging oxo group. <b>9</b> is converted to <b>7</b> in good yield through the treatment with NaOAc

Syntheses and Characterization of Tantalum Alkyl Imides and Amide Imides. DFT Studies of Unusual α‑SiMe₃ Abstraction by an Amide Ligand

Reaction of TaCl₂(NSiMe₃)­[N­(SiMe₃)₂] (<b>1</b>) with alkylating reagents form the alkyl amide imide complexes TaR₂(NSiMe₃)­[N­(SiMe₃)₂] (R = Me (<b>2</b>), CH₂Ph (<b>3</b>)) and mixed amide imide compounds Ta­(NR′₂)₂(NSiMe₃)­[N­(SiMe₃)₂] (R′ = Me (<b>4</b>), Et (<b>5</b>)). The reaction of <b>2</b> and 0.5 equiv of O₂ leads to preferential oxygen insertion into one Ta–Me bond, yielding the alkoxy-bridged alkyl dimer Ta₂(μ-OMe)₂Me₂(NSiMe₃)₂[N­(SiMe₃)₂]₂ (<b>6</b>) as cis and trans isomers. Crystallization of the <i><b>cis</b></i><b>-6</b> and <i><b>trans</b></i><b>-6</b> mixture gave only crystals of <i><b>trans</b></i><b>-6</b>. When the crystals of <i><b>trans</b></i><b>-6</b> were dissolved in benzene-<i>d</i>₆, conversion of <i><b>trans</b></i><b>-6</b> to <i><b>cis</b></i><b>-6</b> occurred until the <i><b>trans</b></i><b>-6</b> ⇌ <i><b>cis</b></i><b>-6</b> equilibrium was reached with <i>K</i>_eq = 0.79(0.02) at 25.0(0.1) °C. Kinetic studies of the exchange gave the rate constants <i>k</i> = 0.018(0.002) min^–1 for the <i><b>trans</b></i><b>-6</b> → <i><b>cis</b></i><b>-6</b> conversion and <i>k</i>′ = 0.022(0.002) min^–1 for the reverse <i><b>cis</b></i><b>-6</b> → <i><b>trans</b></i><b>-6</b> conversion at 25.0(0.1) °C. Complex <b>6</b> reacts with additional O₂, forming the dialkoxy dimer Ta₂(μ-OMe)₂(OMe)₂(NSiMe₃)₂[N­(SiMe₃)₂]₂ (<b>7</b>) as cis and trans isomers. Solid-state structures of <b>3</b> and <i><b>trans</b></i><b>-6</b> have been determined by X-ray diffraction analyses. The mixed amide imide compounds Ta­(NR′₂)₂(NSiMe₃)­[N­(SiMe₃)₂] (R′ = Me (<b>4</b>), Et (<b>5</b>)) have also been prepared by salt metathesis reactions employing TaCl₃[N­(SiMe₃)₂]₂ (<b>8</b>). The pathway from <b>8</b> to <b>4</b> and <b>5</b> eliminates Me₃Si–NR′₂ (R′ = Me, Et), converting the amide N­(SiMe₃)₂ ligand to the imide NSiMe₃ ligand. Such intramolecular imidation is rare. The mechanism of this process has been computationally probed, and α-elimination involving the mixed amide species TaCl₂(NMe₂)­[N­(SiMe₃)₂]₂ (<b>9</b>) is discussed. Diffusion-ordered spectroscopy (DOSY) studies of <b>1</b>–<b>6</b> and <b>8</b> show that only the alkoxy-bridged <i><b>cis-</b></i><b>6</b> and <i><b>trans</b></i><b>-6</b> are dimers in benzene-<i>d</i>₆ solution at 25 °C

Magnetic Transitions in Iron Porphyrin Halides by Inelastic Neutron Scattering and Ab Initio Studies of Zero-Field Splittings

Zero-field splitting (ZFS) parameters of nondeuterated metalloporphyrins [Fe­(TPP)­X] (X = F, Br, I; H₂TPP = tetraphenylporphyrin) have been directly determined by inelastic neutron scattering (INS). The ZFS values are <i>D</i> = 4.49(9) cm^–1 for tetragonal polycrystalline [Fe­(TPP)­F], and <i>D</i> = 8.8(2) cm^–1, <i>E</i> = 0.1(2) cm^–1 and <i>D</i> = 13.4(6) cm^–1, <i>E</i> = 0.3(6) cm^–1 for monoclinic polycrystalline [Fe­(TPP)­Br] and [Fe­(TPP)­I], respectively. Along with our recent report of the ZFS value of <i>D</i> = 6.33(8) cm^–1 for tetragonal polycrystalline [Fe­(TPP)­Cl], these data provide a rare, complete determination of ZFS parameters in a metalloporphyrin halide series. The electronic structure of [Fe­(TPP)­X] (X = F, Cl, Br, I) has been studied by multireference ab initio methods: the complete active space self-consistent field (CASSCF) and the N-electron valence perturbation theory (NEVPT2) with the aim of exploring the origin of the large and positive zero-field splitting <i>D</i> of the ⁶A₁ ground state. <i>D</i> was calculated from wave functions of the electronic multiplets spanned by the d⁵ configuration of Fe­(III) along with spin–orbit coupling accounted for by quasi degenerate perturbation theory. Results reproduce trends of <i>D</i> from inelastic neutron scattering data increasing in the order from F, Cl, Br, to I. A mapping of energy eigenvalues and eigenfunctions of the <i>S</i> = 3/2 excited states on ligand field theory was used to characterize the σ- and π-antibonding effects decreasing from F to I. This is in agreement with similar results deduced from ab initio calculations on CrX₆^3– complexes and also with the spectrochemical series showing a decrease of the ligand field in the same directions. A correlation is found between the increase of <i>D</i> and decrease of the π- and σ-antibonding energies <i>e</i>_λ^X (λ = σ, π) in the series from X = F to I. Analysis of this correlation using second-order perturbation theory expressions in terms of angular overlap parameters rationalizes the experimentally deduced trend. <i>D</i> parameters from CASSCF and NEVPT2 results have been calibrated against those from the INS data, yielding a predictive power of these approaches. Methods to improve the quantitative agreement between ab initio calculated and experimental <i>D</i> and spectroscopic transitions for high-spin Fe­(III) complexes are proposed

Syntheses of Group 5 Amide Amidinates and Their Reactions with Water: Different Reactivities of Nb(V) and Ta(V) Complexes

Chemistries of Nb(V) and Ta(V) compounds are essentially identical as a result of lanthanide contraction. Hydrolysis of M(NMe2)5 (M = Nb, Ta), for example, yields [M(μ3-O)(NMe2)3]4 (M = Nb, 1; Ta, 2) reported earlier. The similar reactivities of Nb(V) and Ta(V) compounds make it challenging, for example, to separate the two metals from their minerals. We have found that the reactions of H2O with amide amidinates M(NMe2)4[MeC(NiPr)2] (M = Nb, 3; Ta, 4) show that the niobium and tantalum analogues take different principal paths. For the Nb(V) complex 3, the amidinate and one amide ligand are liberated upon treatment with water, yielding [Nb(μ3-O)(NMe2)3]4 (1). For the Ta(V) complex 4, the amide ligands are released in the reaction with H2O, leaving the amidinate ligand intact. [Ta(μ3-O)(NMe2)3]4 (2), the analogue of 1, was not observed as a product in the reaction of 4 with H2O. To our knowledge, this is the first example of the formation of two different complexes that maintain the (V) oxidation state in both metals. The new complexes M(NMe2)4[MeC(NiPr)2] (M = Nb, 3; Ta, 4) have been prepared by the aminolysis of M(NMe2)5 (M = Nb, Ta) with iPrN(H)C(Me)=NiPr (5). The hydrolysis of 3 and 4 has been investigated by DFT electronic structure calculations. The first step in each hydrolysis reaction involves the formation of a hydrogen-bonded complex that facilitates a proton transfer to the amidinate ligand in 3 and protonation of an axial dimethylamide ligand in 4. Both proton transfers furnish an intermediate metal-hydroxide species. The atomic charges in 3 and 4 have been computed by Natural Population Analysis (NPA), and these data are discussed relative to which of the ancillary ligands is protonated initially in the hydrolysis sequence. Ligand exchanges in 3 and 4 as well as the exchange in iPrN(H)C(Me)=NiPr (5) were probed by EXSY NMR spectroscopy, giving rate constants of the exchanges: 0.430(13) s–1 (3), 0.033(6) s–1 (4), and 2.23(7) s–1 (5), showing that the rate of the Nb complex Nb(NMe2)4[MeC(NiPr)2] (3) is 13 times faster than that of its Ta analogue 4