Synthesis, spectroscopic and electrochemical properties of manganese, nickel and iron octakis-(2-diethylaminoethanethiol)-phthalocyanine

The syntheses, spectroscopic and electrochemical properties of manganese (3), nickel (4) and iron (5) phthalocyanine complexes, octa-substituted at the peripheral positions with diethlyaminoethanethiol substituent, are reported. The electrochemistry of these complexes and the corresponding cobalt complex (6) are reported. Complex 3 showed two reversible reduction couples attributed to the Mn^IIIPc⁻²/Mn^IIPc⁻² (E_½ = −0.12 V versus Ag|AgCl) and Mn^IIPc⁻²/Mn^IIPc⁻³ (E_½ = −0.82 V versus Ag|AgCl) species. Two ring-based reduction couples were also observed for complex 4. Two reduction couples, assigned to the Fe^IIPc⁻²/Fe^IPc⁻² (E_½ = −0.35 V versus Ag|AgCl) and Fe^IPc⁻²/Fe^IPc⁻³ (E_½ = −0.96 V versus Ag|AgCl) species, and an oxidation couple, attributed to Fe^IIIPc⁻²/Fe^IIPc⁻² (E_½ = 0.26 V versus Ag|AgCl) species, were observed. For complex 6, two reductions and one oxidation were also observed with the potential range of 1.2 to −1.8 V versus Ag|AgCl Spectroelectrochemical studies were used to confirm some of the assigned processes.     

Synthesis of bis N-heterocyclic carbenes,derivatives and metal complexes

A method for the synthesis and isolation of 1,1′-methylene-bis-(3-aryl-imidazol-2-ylidene) ligands, aryl = 2,6-diisopropyl-phenyl (DiPP), L^DiPP, mesityl (mes), L^mes, is reported, which provides synthetically useful quantities of high purity. Derivatisation of L^DiPP with chalcogenides gave the adducts L^DiPPE₂, E = S, Se, Te. Reaction of L^DiPP with [Pd(tmeda)Me₂], [Pt(μ-SMe₂)Me₂]₂, [Ir(1,5-COD)(μ-Cl)]₂/KPF₆ and [NiBr₂(dme)] gave [Pd(L^DiPP)Me₂] (1), [Pt(L^DiPP)Me₂] (2), [Ir(L^DiPP)(1,5-COD)](PF₆) (3) and [Ni(L^DiPP)Br₂] (4), respectively. The latter was reduced in the presence of CO to [Ni(L^DiPP)(CO)₂] (5). The structures of L^mes, L^DiPPTe₂, and 1–5 are also reported.     

Interconversion of acetyl- and terminal-carbonyl groups on labeled (η5-indenyl)(CO)2Fe13C(O)CH3

The ¹³C-labeled (95–99%) acetyl complex (η⁵-In)(CO)₃Fe¹³C(O)CH₃ (8) (In = indenyl) has been prepared by acylating In(CO)₂Fe⁻ Na⁺ (1) with CH₃ ¹³C(O)Cl. All of the starting 1 must be consumed in this reaction (at −78°C), or 45% of the product results as In(CO)(¹³CO)FeC(O)CH₃ (9). Once isolated, neither 8 nor mixtures of 8 and 9 further redistribute or lose this label after pressurizing under 800 atm CO, or after heating in heptane, THF, or acetonitrile solution. Treating 8 with even trace amounts of 1 or of Cp(CO)₂Fe⁻ Na⁺ (5) rapidly interconverts the acetyl and terminal carbonyls, thus transforming 8 into mixtures of 8 and 9. A mechanism is proposed that involves a labile metalla-β-diketonate In(CO)Fe(Fp-CO)(CH₃¹³CO)⁻ Na⁺.     

Metal complexes of anionic 3-borane-1-alkylimidazol-2-ylidene derivatives

Addition of BH₃·thf to 1-alkylimidazoles (alkyl=methyl, butyl) and 1-methylbenzimidazole leads to BH₃ adducts, which are deprotonated by BuLi to yield the organolithium compounds (L)Li⁺(1b–d)⁻. In the solid state (thf)Li⁺1b⁻ is dimeric. The acyl–iron complexes (thf)₃Li⁺(3b,d)⁻ are formed from (thf)Li⁺(1b,d)⁻ and Fe(CO)₅. (L)Li⁺(1a–c)⁻ react with [CpFe(CO)₂X], however, the only complex obtained is [CpFe(CO)₂1a] (5a). The analogous reaction of (L)Li⁺1a⁻ with the pentadienyl complex [(C₇H₁₁)Fe(CO)₂Br] yields the corresponding iron compound 6a. Their compositions follow from spectroscopic data. Treatment of Cp₂TiCl with (L)Li⁺1a⁻ leads to [Cp₂Ti1a] (7a), which could not be oxidized with PbCl₂ to give the corresponding Ti(IV) complex. The compounds [Li(py)₄]⁺9a⁻ and [Li(L)₄]⁺(10b–d)⁻ are obtained when (L)Li⁺1⁻ are reacted with VCl₃ and ScCl₃. The X-ray structure analysis of the vanadium complex reveals a distorted tetrahedron of the anion [V(1a)₄]⁻ with two smaller and four larger CVC angles. The scandium compound [Li(dme)₂⁺10c⁻] has a different structure: the distorted tetrahedron of the anion [Sc(1c)₄]⁻ contains two larger (140.2 and 142.9°) and four smaller CScC angles (93.9–98.7°). This arrangement allows the formation of four bridging BHSc 3c,2e bonds to give an eight-fold coordination. The anion 10c⁻ is formally a 16e complex.     

A new mode of formation of the phosphorus—phosphorus bond: Synthesis and crystal structure determination of 1,5-dichloro-2,4-dimethyl-2,4- diaza-1,5-diphospha-1,5-diphenylpentan-3-one and of 1,4-dimethyl-2,3-diphenyl-1,4,3,2- diazadiphospholidin-5-one-2-oxide and its oxidation with tetrachloroorthobenzoquinone

Reaction of phenyldichlorophosphine, 2b, with N,N′-dimethyl-N,N′-bis(trimethylsilyl)urea, 1, leads to a symmetrical diphosphorylation product, 3b. A mechanism for the formation of 3b, on the basis of ³¹P NMR-data, is proposed. 3b reacts with oxalic acid bis(trimethylsilyl)ester, 5, with intramolecular P—P-coupling and formation of the λ³P–λ⁴P-mixed-valence diphosphorus compound, 1,4-dimethyl-2,3-diphenyl-1,4,3,2- diazadiphospholidin-5-one-2-oxide, 6, which has been oxidized by means of tetrachloroorthobenzoquinone (TOB), 7, to the λ⁴P–λ⁵P-compound, 8. The structures of 3b, 6 and 8 have been elucidated by ¹H- and ³¹P-NMR-spectroscopy; single crystal X-ray diffraction studies of 3b and of 6 have been conducted. The identity of both compounds has been confirmed. In the acyclic diphosphorus compound, 3b, a remarkably short non-bonding PP distance of 280.6 pm has been found while the PP bond length in the cyclic compound 6 is of the usual order of magnitude (221.1 and 221.8 pm, respectively).     

Base-promoted rearrangement of cage α-haloketones—II : 12-oxa-3,5,9,10-tetrachlorohexacyclo[5.4.1.02,6.03,10.05,9.08,11]-dodecane-4-one

A synthesis of 12-oxa-3,5,9,10-tetrachlorohexacyclo[5.4.1.0^2,6.0^3,10.0^5,9.0^8,11]dodecane-4-one (6) from 4,4-dimethoxy-2,3,5,6-tetrachloropentacyclo [[5.4.0.0^2,6.0^3,10.0^5,9]undecane-8,11-dione (1) is described. Reaction of 6 with sodium hydroxide in refluxing benzene, toluene, or tetrahydrofuran affords 11-oxa-3,4,5-exo-6-tetrachloropentacyclo [[6.2.1.0^2,7.0^4,10.0^5,9]undecane-endo-3-carboxylic acid (7a, 80·2% yield). The corresponding reaction of 6 with refluxing aqueous sodium hydroxide solution affords 4,12-dioxa-8,11-dichlorohexacyclo-[5.4.1.0^2,6.0^3,10.0^5,9.0^8,11]dodecane-1-carboxylic acid (8a, 66·5% yield). A mechanism which accounts for the formation of 7a and 8a from 6 is presented.     

Synthesis,structure, and photoluminescence of ZnII and CdII coordination complexes constructed by structurally related 5,6-substituted pyrazine-2,3-dicarboxylate ligands

Aiming at exploring the effect of substituting groups of three structurally related ligands, 5,6-diethyl-pyrazine-2,3-dicarboxylic acid (H₂L¹), 5,6-diphenyl-pyrazine-2,3-dicarboxylic acid (H₂L²), and dibenzo[f,h]quinoxaline-2,3-dicarboxylic acid (H₂L³), seven new coordination polymers constructed from these three substituted dicarboxylate ligands, {[Zn(L¹)(H₂O)₃]·2H₂O}_∞ (1), {[Cd₂(L^2ʹ)₄(H₂O)]·3H₂O}_∞ (2), [Zn(L²)(CH₃OH)]_∞ (3), {[Zn(L²)(H₂O)₂]·H₂O}_∞ (4), {[Zn(L^2ʹ)]·H₂O}_∞ (5), [Zn₂(L³)(DMF)₄]_∞ (6), [Zn(L³)(2,2ʹ-bipy)(H₂O)]_∞ (7), have been prepared and structurally characterized. 1 is a 1D chain structure in which Zn^II ion is six-coordinated with octahedron geometry. 2 is also a 1D chain structure in which there are two crystallographically independent Cd^II ions in the asymmetric unit and exist transformative L^2ʹ ligands in the resulting complex. 3 and 4 both possess 2D layer network with the same (4, 8²) topology, while the two complexes take different coordination modes during the forming of the compounds. 5 has a 1D chain structure based on the transformative L^2ʹ ligand in which Zn^II ion is five-coordinated with bipyramidal geometry. 6 and 7 both have 1D chain structure constructed from L³ ligand. Thereinto, Zn^II ion in 6 is five-coordinated by three oxygen atoms from two individual L³ ligands and two oxygen atoms from two DMF molecules. While in 7 there are also five coordination sites occupied by two carboxylate oxygen atoms from two L³ ligands. In addition, the compounds are characterized by elemental analysis, IR spectra. The luminescent properties of the compounds are also discussed and exhibit strong fluorescent emissions in the solid state.     

Isomere n-aryltetrazole—IV: 1H und 13c NMR-untersuchungen zur struktur von tetrazoliumkationen

¹H and ¹³C NMR spectra of protonated N-phenyltetrazoles 1⁺ and 2⁺ and of tetrazole 3⁺ have been measured in sulphuric acid. The structures of the cations 1⁺–3⁺ were unequivocally derived from a comparison of the data obtained with spectral parameters (chemical shifts and ¹J(¹³C,H) coupling constants) of the tetrazolium salts 4 and 5 used as model compounds. Protonation occurs always exclusively at ring nitrogen N-4. Changes of the spectral parameters due to protonation were briefly discussed and the protonation site is compared with the results of quantum chemical calculations.     

Synthesis,structural characterisation and reactivity of molybdenum half-sandwich complexes containing keto- and amido-phosphines

The keto-functionalised N-pyrrolyl phosphine ligand PPh₂NC₄H₃{C(O)CH₃-2} L¹ reacts with [MoCl(CO)₃(η⁵-C₅R₅)] (R=H, Me) to give [MoCl(CO)₂(L¹-κ¹P)(η⁵-C₅R₅)] (R=H 1a; Me 1b). The phosphine ligands PPh₂CH₂C(O)Ph (L²) and PPh₂CH₂C(O)NPh₂ (L³) react with [MoCl(CO)₃(η⁵-C₅R₅)] in an analogous manner to give the compounds [MoCl(CO)₂(L-κ¹P)(η⁵-C₅R₅)] (L=L², R=H 2a, Me 2b; L=L³, R=H 3a, Me 3b). Compounds 1–3 react with AgBF₄ to give [Mo(CO)₂(L-κ²P,O)(η⁵-C₅R₅)]BF₄ (L=L¹, R=H 4a, Me 4b; L=L², R=H 5a, Me 5b; L=L³, R=H 6a, Me 6b) following displacement of chloride. The X-ray crystal structure of 4a revealed a lengthening of both Mo–P and CO bonds on co-ordination of the keto group. The lability of the co-ordinated keto or amido group has been assessed by addition of a range of phosphines to compounds 4–6. Compound 4a reacts with PMe₃, PMe₂Ph and PMePh₂ to give [Mo(CO)₂(L¹-κ¹P)(L)(η⁵-C₅H₅)]BF₄ (L=PMe₃ 7a; PMe₂Ph 7b; PMePh₂ 7c) but does not react with PPh₃, 5a reacts with PMe₂Ph, PMePh₂ and PPh₃ to give [Mo(CO)₂(L²-κ¹P)(L)(η⁵-C₅H₅)]BF₄ (L=PMe₂Ph 8b; PMePh₂ 8c; PPh₃ 8d), and 6a reacts with PMe₃, PMe₂Ph, PMePh₂ and PPh₃ to give [Mo(CO)₂(L³-κ¹P)(L)(η⁵-C₅H₅)]BF₄ (L=PMe₃ 10a; PMe₂Ph 10b; PMePh₂ 10c; PPh₃ 10d). No reaction was observed for the pentamethylcyclopentadienyl compounds 4b–6b with PMe₃, PMe₂Ph, PMePh₂ or PPh₃. These results are consistent with the displacement of the co-ordinated oxygen atom being influenced by the steric properties of the P,O-ligand, with PPh₃ displacing the keto group from L² but not from the bulkier L¹. In the reaction of [Mo(CO)₂(L²-κ²P,O)(η⁵-C₅H₅)]BF₄ (5a) with PMe₃ the phosphine does not displace the keto group, instead it acts as a base, with the only observed molybdenum-containing product being the enolate compound [Mo(CO)₂{PPh₂CHC(O)Ph-κ²P,O}(η⁵-C₅H₅)] 9. Compound 9 can also be formed from the reaction of 2a with BuLi or NEt₃, and a single crystal X-ray analysis has confirmed the enolate structure.     

Titanium complexes containing new dianionic tetradentate [ONNO]-type ligands with benzyl substituents on bridging nitrogen atoms: Syntheses, X-ray structures, and catalytic activities in ring opening polymerization of lactide

Several titanium isopropoxides 1–8 have been prepared by the reaction of Ti(O-i-Pr)₄ with a series of corresponding tetradentate Salan-type [ONNO] ligands with benzyl or methyl substituents on bridging nitrogen atoms. They have been characterized by ¹H NMR, ¹³C{¹H} NMR, and elemental analysis. Solid state structures of compounds 2, 4, 6, and 7 have been determined by X-ray crystallography. X-ray diffraction analysis and ¹H NMR confirmed that these titanium complexes were all monomeric species with a six-coordinated central titanium in their solid and solution structures. Complexes 2, 4, 6, and 8 with benzyl substituents on bridging nitrogens gave PLA with higher molecular weight than compounds 1, 3, 5, and 7 with methyl substituents did.     

Addition reactions of the trichlorocyclopropenylium ion with alkenes: A novel access to cyclopropene and cyclopropenone derivatives

Trichlorocyclopropenylium tetrachloroaluminate 1⁺-AlCl⁻₄ reacts with alkenes in nitromethane at −35°C to give the 1:1 addition products 2⁺-AlCl⁻₄, which can be converted into the 2-chlorocyclopropenones 5 or the ä,β-acetylenic carboxylates 6.     

Alkaloids of NIRAM,natural dye from Polygonum tinctorium,and their anti-inflammatory activities

Phytochemical investigation of EtOH extract of NIRAM, natural dye from Polygonum tinctorium, resulted in the purification of nine alkaloid compounds (1–9) including four new compounds (1–4). Structures of these new compounds were elucidated by 1D and 2D NMR (¹H and ¹³C NMR, ¹H–¹H COSY, ¹H–¹³C HSQC, ¹H–¹³C HMBC), IR, UV, HR-ESI-MS, and ECD spectra. Isolated compounds (1–9) were tested for their inhibitory effects on nitric oxide (NO) production in lipopolysaccharide (LPS)-activated BV-2 cells. Compounds 1–3, 5, and 7 showed potent NO production inhibitory activities, with IC₅₀ values of 3.88–22.87 μM.     

Molecular design by cycloaddition reactions—XVIII: Formations of highly strained heterocycles

Reactions of pentacyclo [7.6.1.0^2.80^3.50^10.15]hexadecatetraen-4-yl acetic acids (3a, b), azatricyclo-[5.3.0^3.90^4.6]decaen-5-yl acetic acid (3c), and its alcohol derivatives (12a, b) with bromine gave bromocyclization products (10a, b, c) and (13a, b), respectively.The Curtius reaction of the car?ylic acids (3a, b, c) with diphenylphosphoryl azide gave the corresponding urethanes (19a, b, c) together with compound 20 as a by-product. The cyclization reaction of 23, which was easily prepared from reduction of the urethane (19a) followed by chlorination with N-chlorosuccinimide, in refluxing methanol in the presence of silver nitrate gave mainly the parent amine compound (22) accompanied with a small amount of the cyclized product (24).     

Reactions of half-sandwich rhodium(III) and iridium(III) compounds with pyridinethiolate ligands: Mono-, di-, and tri-nuclear complexes

Neutral trinuclear metallomacrocycles, [Cp^*RhCl(μ-4-PyS)]₃ (3) and [Cp^*IrCl(μ-4-PyS)]₃ (4) [Cp^* = pentamethylcyclopentadienyl, 4-PyS = 4-pyridinethiolate], have been synthesized by self-assembly reactions of [Cp^*RhCl₂]₂ (1) and [Cp^*IrCl₂]₂ (2) with lithium 4-pyridinethiolate, respectively. In situ reaction of complex 3 with three equivalent of lithium 4-pyridinethiolate resulted in [Cp^*Rh(μ-4-PyS)(4-PyS)]₃ (5) containing both skeleton and pendent 4-PyS groups. Chelating coordination of 2-pyridinethiolate broke down the triangular skeleton to give mononuclear metalloligands Cp^*Rh(2-PyS)(4-PyS) (6) and Cp^*Ir(2-PyS)(4-PyS) (7) [2-PyS = 2-pyridinethiolate], which could also be synthesized from Cp^*RhCl(2-PyS) (10) and Cp^*IrCl(2-PyS) (11) with lithium 4-pyridinethiolate. The coordination reactions of 6 with complexes 1 and 2 gave dinuclear complexes [Cp^*Rh(2-PyS)(μ-4-PyS)][Cp^*RhCl₂] (8) and [Cp^*Rh(2-PyS)(μ-4-PyS)][Cp^*IrCl₂] (9), respectively. Molecular structures of 3, 4, 6 and 11 were determined by X-ray crystallographic analysis. All the complexes have been well characterized by elemental analysis, NMR and IR spectra.     

Synthese von Tricarbonyl-η5-cyclohexadienyl-wolframat und Tricarbonyl-η5-cyclohexadienyl-methyl-wolfram

η⁶-Arene-tricarbonyl-tungsten (arene = benzene (1a), toluene (1b), m-xylene (1C), P-xylene (1D), o-xylene (1E), mesitylene (1F)) yield with potassium-tri-sec-butylboranate correspondingly methyl-substituted tricarbonyl-η⁵-cyclohexadienyl-tungstates (2A–2F). Similarly 1A reacts with methyllithium to tricarbonyl-η⁵-anti-6-methylcyclohexadienyl-tungstate (4A). In THF 2A–2F and 4A are converted by methyliodide to tricarbonyl-μ⁵-cyclohexadienyl-tungsten (3A–3F) and tricarbonyl-η⁵-anti-6-methylcyclophexadienyl-methyl-tungsten (5A). The complexes were characterized by C, H elemental analyses and by IR and ¹H-NMR spectroscopy.     

Preparation and reactivity of CpRu(OMe)PCy3 and CpRuHL2 (L = PCyPh2, PCy2H) from [CpRu(OMe)]2 and bulky phosphines

The reaction of [CpRu(OMe)]₂ (1) with PCy₃ yields the 16-electron alkoxo derivative, CpRu(OMe)(PCy₃) (2). 2 reacts with H₂ and HBF₄ to give the known CpRuH₃PCy₃ (3) and [CpRu(C₆H₉PCy₂)]BF₄ (4). The reaction of 1 with one or two equivalents of L yields CpRuHL₂ (L = PCyPh₂ (5), PCy₂H (6)) through a β-elimination process. Upon protonation, 5 and 6 are converted into [CpRuH₂L₂]BF₄ (L = PCyPh₂ (7), PCy₂H (8)).     

Synthesis and structures of the dinuclear tin(II) complexes [R = Ph, X(Z) = CPh; R = SiMe3, X(Z) = PPh2] and of related compounds

Tin(II) compounds containing the ligands [CH(C₆H₃Me₂-2,5)C(Bu^t)NSiMe₃]⁻ (≡ L¹), [CH(Ph)C(Ph)NSiMe₃]⁻ (≡L²), [CH(SiMe₃)P(Ph)₂NSiMe₃]⁻ (≡ L³),

Full-size image (3K)

(≡ L⁴), [C(Ph)C(Ph)NSiMe₃]²⁻ (≡ L⁵), and [C(SiMe₃)P(Ph)₂NSiMe₃]²⁻ (≡ L⁶) are reported: the transient SnBr(L¹) (1) and SnBr(L²) (2), Sn(L¹)₂ (3) [P.B. Hitchcock, J. Hu, M.F. Lappert, M. Layh, J.R. Severn, J. Chem. Soc., Chem. Commun. (1997) 1189], the labile Sn(L²)₂ (4), [Sn(L⁵)]₂ (5), SnCl(L³) (6), Sn(L³)₂ (7), [Sn(L⁶)]₂ (8), Sn(L⁴)₂ (9) and Pb(L⁴)₂ (10). They were prepared from (i) SnBr₂ and K(L¹) (1, 3) or K(L²) (2, 4, 5); (ii) SnCl₂ and Li(L³) (6–9); or (iii) PbCl₂ and Li(L⁴) (10). Each of 1, 3 and 5–10 has been characterised by multinuclear NMR spectra; 3, 5, 6, 8, 9 and 10 by EI-mass spectra, but only 3, 5, 8, 9 and 10 were isolated pure and furnished X-ray quality crystals. Of greatest novelty are the title binuclear fused tricyclic ladder-like compounds 5 and 8. Quantum chemical calculations, on alternative pathways to 5 from 2 and to 8 from 7, are reported.     

Organoborierung von alkinylstannanen: XV. Synthese von 1,2-dihydro-1,2,5-disilaborepinen

The reaction between 1,2-diethynyl-tetramethyldisilane (1) and two equivalents of diethylaminotrimethylstannane (2) leads to 1,2-bis(trimethylstannylethynyl)-tetramethyldisilane (3). The new alkyne derivative 3 reacts, already at room temperature, with trialkylboranes, R₃B (5) (R = Me, Et), quantitatively to give 1,1,2,2-tetramethyl-3,7-bis(trimethylstannyl)-4,5,6-trialkyl-1,2-dihydro-1,2,5-disilaborepines (6). The reaction is much slower with R = Prⁱ which allows detection of intermediates by NMR spectroscopy. All products are characterized by ¹H, ¹¹B, ¹³C, ²⁹Si and ¹¹⁹Sn NMR data.     

Tributyltin hydride-mediated free radical cyclization of allene-tethered oxime ethers,oxime esters and hydrazones. Influence of the substituents borne by the C=N double bond on the course of the reaction

A set of allenic derivatives 1 bearing a C=N bond in β position and diversely substituted in R³ and Z was prepared. The behaviour of compounds 1 towards the Bu₃Sn^• radical was then studied. Depending on the nature of R³ and Z, products 2, 3, and 4 were obtained in diverse ratios. When the steric hindrance on the C-atom of the C=N bond was low (R³ = H), the product ‘normally’ obtained was 2. When the steric hindrance of R³ was higher, the formation of 3 and 4 became competitive. In this case, the ratio between 2, 3, and 4 can be explained on the basis of the polarity of R³ and Z. The higher the electroattractive effect of R³ and Z, the higher the quantity of 2 and 4 in comparison with 3.     

Darstellung und eigenschaften von und reaktionen mit metallhaltigen heterocyclen: LXXXII. Cyclocotrimerisierung der P=icr;S-Gruppe mit dem sterisch anspruchsvollen t-Butylpropinoat

The P=icr;S functional group in (η²-PCy₂

S)Mn(CO)₄ (1) undergoes cyclocotrimerization with t-butyl propynoate to give the isomeric compounds 3a und 3b in a 1 : 1 ratio. Oxidation of 3a and 3b with (NH₄)₂[Ce(NO₃)₆] gives thiophene 4. Compounds 3a and 3b were characterized by use of MS spectrometry and IR, ³¹P{¹H}, ¹H, and ¹³C{¹H} NMR spectroscopy.