Oxidovanadium(IV), oxidomolybdenum(VI) and cobalt(III) complexes of o-phenylenediamine derivatives: oxidative dehydrogenation and photoluminescence

Abstract

Reactions of o-phenylenediamine derivatives (L₃H₂) incorporating a (Ph)(Py)(H)C–N(H)– function with the oxidovanadium(IV) and oxidomolybdenum(VI) ions afford amide complexes of types [V^IVO(L₃²⁻)] (3), [V^IVO(L₃^{t-Bu 2−})] (4) and cis-[Mo^VIO₂(L₃²⁻)] (5) (L₃H₂ = ((E)-2-(((2-((phenyl(pyridin-2-yl)methyl)amino)phenyl)imino)methyl)phenol); L₃^t-BuH = ((E)-2,4-di-tert-butyl-6-(((2-((phenyl(pyridin-2-yl)methyl)amino)phenyl)imino)methyl)phenol)), while the similar reaction of L₃H₂ with the anhydrous CoCl₂ in air results in oxidative dehydrogenation (OD) of the (Ph)(Py)(H)C–N(H)– function, affording a cobalt(III) diimine complex, trans-[Co^III(L₄⁻)Cl₂] (6) (L₄H = 2-((E)-(2-((E)-phenyl(pyridin-2-yl)methyleneamino)phenylimino) methyl)phenol), contradicting the participation of the higher oxidation states of the metal ions in OD reaction of amines. 3–6 are characterized by elemental analyses and mass, IR, ¹H NMR and EPR spectra. The molecular geometries of 4·CH₃OH, 5 and 6 were confirmed by single crystal X-ray structure determinations. The V^IV–O_phenolatocis to the V=O bond and the V^IV=O lengths in 4·CH₃OH are 1.925(2) and 1.612(2) Å. Two cis Mo=O lengths are 1.710(2) Å and 1.720(2) Å in 5. The aliphatic –C–N– lengths in 4·CH₃OH and 5 are 1.448(3) and 1.479(2) Å, while the same is 1.285(4) Å in 6. DFT calculations on 3 and 6 inferred a significant mixing among d_M and NN-ligand backbone favoring a t₂⁶ state of the metal ion for the OD of the amine fragment to have stronger d_M → π_ketimine^* back-bonding. The π_NHPh → π_aldimine^* transition of L₃H₂ is red shifted in 3 and 4 quenching the emissive π_Phenolato → π_aldimine^* transitions, elucidated by the TD DFT calculations on 3 (and 3⁺). The π_NPh → π_aldimine^* transitions are blue shifted in the oxidovanadium(V) analogues, [V^VO(L₃²⁻)]⁺ (3⁺) and [V^VO(L₃^{t-Bu 2−})]⁺ (4⁺), which are fluorescent (3⁺, λ_ex = 331, λ_em = 444 nm; 4⁺, λ_ex = 339, λ_em = 490 nm) recorded by the fluorescence-spectroelectrochemical measurements in CH₂Cl₂. 5 and 6 emit weakly at 466 and 473 nm (5, λ_ex = 336 nm, ϕ = 0.003; 6, λ_ex = 324 nm, ϕ = 0.027).

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Introduction

Oxidation of amine is a significant reaction in biology.¹ In the laboratory, the transition metal promoted oxidative dehydrogenation (OD) reaction of amines has been an area of research since 1960 and the reaction was first reported by Curtis et al.² To date, several OD reactions mediated by transition metal ions with different mechanistic aspects have been reported.³ In many cases, participation of the transition metal ions to the −(2e + 2H⁺) transfer reaction has been proposed. The accepted mechanism is that the metal ion is oxidized first to a higher oxidation state that will either stepwise oxidize the amine by 1e⁻ transfer via a ligand radical intermediate or directly oxidize by 2e⁻ transfer, eliminating protons.³ It has been reported that the OD reactions of [Ru(bpy)₂(ampy)]²⁺ (ampy = 2-(aminomethyl)pyridine),^4a [Ru(tame)₂]²⁺ (tame = 1,1,1-tris(aminomethyl)ethane),^4b [Ru(en)₃]³⁺ (en = ethylenediamine)^4b [Ru(sar)]²⁺ (sar = sarcophagine, 3,6,10,13,16,19-hexaazabicyclo(6,6,6)eicosane),^4c and [Ru(bpy)₂(NC₅H₄CH(CH₃)OH)]²⁺ (NC₅H₄CH(CH₃)OH) = 2-(hydroxymethyl)pyridine)^4d proceed via a ruthenium(IV) intermediate. Similarly, intermediacy of M(IV) ions was proposed in the OD reactions of [Os(bpy)₂(ampy)]²⁺,^4e [Os(en)₃]³⁺ (ref. 4f) and [Fe(sar)]³⁺ (ref. 3a) complexes. A Ni(III) intermediate also was proposed to participate in the OD reaction of a tetraazamacrocyclic complex of nickel(II) ion.^4g However, the proposals of the participation of Fe(IV), Ni(III), Os(IV) and Ru(IV) ions in OD reactions are futile.

The reduction of the metal ion during OD reactions has been established only in the cases of copper(II), iron(III) and ruthenium(III) ions by isolating their reduced analogues. Reduction of copper(II) to copper(I) has been established in OD reactions of [Cu^II₂(H₄L)]⁴⁺ (L = octaazamacrocyclic dinucleating ligand),^5a Cu^II(boradiazaindacene (BODIPY) derivative)^5b and [Cu^II(L)]²⁺ (L = N,N-bis-quinolin-2-ylmethyl-cyclohexane-trans-1,2-diamine)^5c complexes. Reduction of iron(III) to iron(II) in the OD reactions of [Fe^IIIH₂L]³⁺ (H₂L = 1,9-bis(2′-pyridyl)-5-[(ethoxy-2′′-pyridyl)methyl]-2,5,8-triazanonane),^5d tetracyano(1,2-diamine derivative)ferrate(III)^5e complexes and formation of a ruthenium(II) analogue in the OD reaction of [Ru^III(O–N)(bpy)₂]²⁺ (O–N = unsymmetrical bidentate phenolate type ligand, bpy = 2,2′-bipyridine)^5f were authenticated. However, the reports of air and base promoted OD reactions of the amine complexes of nickel(II), cobalt(III) and rare earth metal ions are significant.^5g–i

o-Phenylene diamine derivatives are strong chelating agents and furnished several bioactive transition metal complexes.⁶ Thus, the coordination chemistry of o-phenylenediamine derivatives is the subject of investigation here. Recently, we reported the OD reaction of a tetradentate o-phenylenediamine derivative (L₃H₂) (L₃H₂ = (E)-2-(((2-((phenyl(pyridin-2-yl)methyl)amino)phenyl)imino)methyl)phenol). It was disclosed that the reaction of L₃H₂ with tris(triphenylphosphine)ruthenium(II) precursor results in the OD reaction converting L₃H⁻ to L₄⁻, affording the trans-[Ru(L₄⁻)(PPh₃)₂]⁺ (2⁺) cation (L₄H = 2-((E)-(2-((E)-phenyl (pyridinyl)methyleneamino)phenylimino)methyl)phenol.⁷ However, in the presence of an easily reducible iron(III) ion, no OD reaction occurs and the reaction ends up with the formation of an amine complex, cis-[Fe(L₃H⁻)Cl₂] (1), as shown in Scheme 1.

Scheme 1

In this work the role of the metal ions in the OD reaction of the (Ph)(Py)(H)C–N(H)– function of L₃H₂ was further investigated. The question is whether the reaction requires the higher oxidation state of the metal ion to promote the OD reaction acclaimed so far. To explore it, the chemistry of L₃H₂ towards the oxidovanadium(IV) and oxidomolybdenum(VI) ions, which are redox active and participate in electron transfer reaction at lower potential and at neutral pH in several redox-enzymes, has been investigated.^8,9 Further, oxidovanadium(IV) and dioxidomolybdenum(VI) complexes have been reported as effective catalysts for oxo transfer,^10a epoxidation of olefins,^10b hydrosilylation of carbonyls^10c and oxidative bromination^10d reactions. The reactions of oxidovanadium(IV), oxidomolybdenum(VI) and cobaltous ions with L₃H₂ and L₃^t-BuH₂ in air were performed. Surprisingly, the OD reaction does not occur, with oxidizing oxidovanadium(IV) and oxidomolybdenum(VI) ions yielding only the amide products, [V^IVO(L₃²⁻)] (3), [V^IVO(L₃^{t-Bu 2−})] (4) and cis-[Mo^VIO₂(L₃²⁻)] (5) (L₃^t-BuH₂ = (E)-2,4-di-tert-butyl-6-(((2-((phenyl(pyridin-2-yl)methyl)amino)phenyl)imino)methyl) phenol), while the cobaltous ion promotes the OD reaction, affording a cobalt(III) complex of the ketimine derivative, trans-[Co^III(L₄⁻)Cl₂] (6).

The metal ion dependent fluorescence features of the organic chromophore is a significant investigation.¹¹ It is observed that L₃H₂ is fluorescent due to the internal charge transfer (ICT) from the π_phenolato to the π_aldimine^* orbital (λ_ex = 330; λ_em = 470 nm).⁷ Lifetimes measurements and time resolved emission spectra (TRES) have confirmed that the lower energy excited state at 390 nm has a higher non-radiative rate constant (k_nr). It was noted that due to molecular aggregation at higher concentration, the fluid solution fluorescence spectra of L₃H₂ depend on concentration, which has been investigated by ¹H NMR and temperature dependent fluorescence spectra.⁷ An interesting observation is that the molecular aggregation of L₃H₂ depends reversibly on temperature. At higher concentration, in addition to the emission band at 470 nm, L₃H₂ displays a lower energy emission band at 525 nm, which disappears upon dilution. It was recorded that 1 has eighty fold stronger emission than L₃H₂ itself, while the ketimine analogue 2⁺ ion is non-emissive.⁷ The fluid solution fluorescence features of 3, 4, 5 and 6 are also recorded at 298 K. It is found that 3 and 4 in fluid solutions at 298 K are non-emissive while the electrogenerated one-electron oxidized analogues, [V^VO(L₃²⁻)]⁺ (3⁺) and [V^VO(L₃^{t-Bu 2−})]⁺ (4⁺) are emissive. The complex 5 is weakly emissive. The fluorescence of L₃H₂ ligand is completely quenched in presence of the reducing cobalt(II) ion, while 6 is brightly emissive.

In this article, to substantiate the role of the metal ions in the OD reaction of L₃H₂, syntheses, spectra and X-ray structures including the diverse fluorescence spectra and the redox series of 3, 4, 5 and 6 are reported. Density functional theory (DFT) and time dependent (TD) DFT calculations were performed to elucidate the fluorescent as well as the quenched electronic states of the complexes.

Experimental section

Materials and physical measurements

Reagents or analytical grade materials were obtained from commercial suppliers and used without further purification. VO(acac)₂ (acac = acetylacetonate) was prepared by the reported procedure.^10e Spectroscopic grade solvents were used for spectroscopic and electrochemical measurements. After evaporating MeOH solvents of the sample under high vacuum, elemental analyses and spectral measurements were performed. The C, H and N content of the compounds were obtained using a Perkin-Elmer 2400 series II elemental analyzer. Infrared spectra of the samples were measured from 4000 to 400 cm⁻¹ with KBr pellets at room temperature on a Perkin-Elmer Spectrum RX 1 FT-IR spectrophotometer. ¹H NMR spectra in CDCl₃ were obtained on a Bruker DPX-300 MHz spectrometer with tetramethylsilane (TMS) as an internal reference. ESI mass spectra were recorded on a micromass Q-TOF mass spectrometer. Electronic absorption spectra in solutions at 298 K were recorded on a Perkin-Elmer Lambda 750 spectrophotometer in the range of 3000–200 nm. Magnetic susceptibility at 298 K was measured on a Sherwood Magnetic Susceptibility Balance. The electroanalytical instrument, BASi Epsilon-EC, for cyclic voltammetric experiments in CH₂Cl₂ solutions containing 0.2 M tetrabutylammoniumhexafluorophosphate as supporting electrolyte was used. A BASi platinum working electrode, platinum auxiliary electrode and Ag/AgCl reference electrode were used for the measurements. The redox potential data are referenced vs. the ferrocenium/ferrocene, Fc⁺/Fc, couple. In all cases, the experiments were performed with multiple scan rates to analyze the reversibility of the electron transfer waves. BASi Epsilon-EC was used for spectroelectrochemistry measurements. The X-band electron paramagnetic resonance (EPR) spectra were measured on a Magnettech GmbH MiniScope MS400 spectrometer (equipped with temperature controller TC H03), where the microwave frequency was measured with a frequency counter FC400.

The EPR spectra of CH₂Cl₂ solutions of the paramagnetic complexes 3 and 4 were recorded at 298 K. The EPR spectrum of the CH₂Cl₂ frozen glass of 3 at 25 K was also recorded. The fluorescence spectra of the complexes were recorded in CH₂Cl₂ at 298 K. The spectral features of 3⁺ and 4⁺ ions were recorded by fluorescence spectro-electrochemical measurements in CH₂Cl₂ solvent at 298 K.

Excitation and emission spectra were recorded using quartz sample tubes in a Perkin Elmer LS 55 luminescence spectrophotometer. The fluorescence quantum yield (ϕ_D) was determined in each case by comparing the corrected emission spectrum of the samples with that of anthracene in MeOH (ϕ_D = 0.20) and CH₂Cl₂ (ϕ_D = 0.30) using the following equation¹² considering the total area under the emission curve.

where Q is the quantum yield of the compounds, F is the integrated fluorescence intensity (area under the emission curve), OD is the optical density, and n is the refractive index of the medium. It is assumed that the reference and the unknown samples are excited at the same wavelength. The subscript R refers to the reference fluorophore (anthracene in this case) of known quantum yield. The standard quantum yield value thus obtained is used for the calculation of quantum yields of the systems under various conditions.

Syntheses

(E)-2-((2-(Phenyl(pyridin-2-yl)methylamino)phenyl imino)methyl)phenol (L₃H₂). The compound was prepared by a reported procedure from a zinc complex, [Zn(L₁)Cl₂] (L₁ = (E)-N¹-(phenyl(pyridin-2-yl)methylene)benzene-1,2-diamine)).⁷

((E)-2,4-Di-tert-butyl-6-((2((phenyl(pyridin-2-yl)methyl) amino)phenyl)imino)methyl)phenol) (L₃^t-BuH₂). The compound was prepared using [Zn(L₁)Cl₂] as a precursor. To a MeOH solution (25 ml) of [Zn(L₁)Cl₂] (410 mg, 1 mmol), sodium borohydride was added in portions with constant stirring until the reddish orange solution turned light yellow. The solution was evaporated under low pressure and the residue was extracted with diethyl ether. After evaporation of the ether, a yellow oily liquid of L₂H was obtained (L₂H = N¹-(phenyl (pyridin-2-yl)methyl)benzene-1,2-diamine).⁷ To L₂H, MeOH (10 ml) followed by 3,5-di-tert-butyl-2-hydroxybenzaldehyde (240 mg, 1 mmol) were added and the resulting solution was refluxed for 30 min and then cooled to 298 K. A yellow crystalline solid of L₃^t-BuH₂ separated out and was filtered and dried in air. Yield: 110 mg (60% with respect to 2-benzoyl pyridine). Mass spectral data (ESI, positive ion, CH₃OH): m/z 492 for [L₃^t-BuH₂]⁺. ¹H NMR (CDCl₃, 300 MHz): δ (ppm) 13.54 (s, 1H), 8.65 (s, 1H), 8.60 (d, 1H), 7.59 (t, 1H), 7.52 (t, 3H), 7.32 (m, 3H), 7.07 (m, 3H), 6.69 (t, 3H), 6.54 (t, 2H) 5.61 (d, 1H), 1.51 (s, 9H), 1.35 (s, 9H). Anal. calcd (%) for C₃₃H₃₇N₃O: C, 80.61; H, 7.59; N, 8.55. Found: C, 80.10; H, 7.37; N, 8.42. IR/cm⁻¹ (KBr): ν 3370 (vs), 2962 (vs), 1595 (vs), 1508 (vs), 1437 (s), 1330 (s), 1250 (s), 998 (s), 754 (s), 581 (m).

[V^IVO(L₃²⁻)] (3). To a MeOH solution (30 ml) of L₃H₂ (380 mg, 1 mmol), VO(acac)₂ (260 mg, 1 mmol) was added and the resulting solution was heated at 327 K for 10–15 min. The solution was cooled at 298 K and filtered. The filtrate was allowed to evaporate slowly in air. After 2–3 days, a dark brown crystalline compound of 3 separated out, which was filtered and dried in air. Yield: 20 mg (40% with respect to vanadium). Mass spectral data (ESI, positive ion, CH₃OH): m/z 445 for [3]⁺. Anal. calcd (%) for C₂₅H₁₉N₃O₂V: C, 67.57; H, 4.31; N, 9.46. Found: C, 65.98; H, 4.15; N, 9.41. IR/cm⁻¹ (KBr): ν 3411 (m), 3055 (m), 1604 (vs), 1528 (s), 1464 (vs), 1381 (vs), 1328 (vs), 1202 (m), 1154 (s), 1030 (m), 960 (vs), 844 (m), 739 (vs), 707 (s), 555 (s), 409 (m).

[V^IVO(L₃^{t-Bu 2−})]·CH₃OH (4·CH₃OH). To a MeOH solution (30 ml) of L₃^t-BuH₂ (492 mg, 1 mmol), VO(acac)₂ (260 mg, 1 mmol) was added and the resulting solution was heated at 327 K for 10–15 min. The solution was cooled at 298 K and filtered. The filtrate was allowed to evaporate slowly in air. After 2–3 days, a dark brown crystalline compound of 4·CH₃OH separated out, which was filtered and dried in air. Yield: 23 mg (45% with respect to vanadium). Mass spectral data (ESI, positive ion, CH₃OH): m/z 557 for [4]⁺. Anal. calcd (%) for C₃₃H₃₅N₃O₂V: C, 71.21; H, 6.34; N, 7.55. Found: C, 70.18. H, 6.19; N, 7.41. IR/cm⁻¹ (KBr): ν 3422 (m), 2949 (s), 1597 (vs), 1477 (vs), 1382 (s), 1326 (vs), 1169 (s), 1031 (m), 954 (vs), 760 (s), 734 (vs), 702 (m), 570 (m).

cis-[Mo^VIO₂(L₃²⁻)] (5). To a MeOH solution (30 ml) of L₃H₂ (380 mg, 1 mmol), (NH₄)₂[MoO₄] (175 mg, 1 mmol) was added and the resulting solution was heated at 327 K for 60 min. The orange-yellow solid of 5 separated out, which was filtered, dried in air and collected. The product was further re-crystallized by diffusing n-hexane to the CH₂Cl₂ solution of the crude product at 298 K for single crystal X-ray structure determination. Yield: 120 mg (∼68% with respect to molybdenum). Mass spectral data (ESI, positive ion, CH₃OH): m/z 507.89 for [5]. ¹H NMR (CDCl₃, 300 MHz): δ(ppm) 8.66 (s, 1H), 8.34 (d, 1H), 7.79 (t, 1H), 7.61–7.43 (m, 5H), 7.37–7.35 (m, 5H), 7.18–6.91 (m, 4H), 6.37 (s, 1H), 5.30 (s, 1H). Anal. calcd (%) for C₂₅H₁₉MoN₃O₃: C, 59.41; H, 3.79; N, 8.31. Found: C, 58.75; H, 3.62; N, 8.15. IR/cm⁻¹ (KBr): ν 1614 (vs), 1600 (s), 1547 (s), 1472 (s), 1441 (m), 1384 (m), 1233 (m), 1022 (s), 901 (vs), 915 (vs), 886 (vs), 796 (m), 746 (s), 694 (m), 624 (m).

trans-[Co^III(L₄⁻)Cl₂] (6). To a MeOH solution (30 ml) of L₃H₂ (380 mg, 1 mmol), anhydrous CoCl₂ (136 mg, 1 mmol) was added and the resulting solution was heated at 327 K for 60 min. The solution was cooled to 298 K and filtered. The filtrate was allowed to evaporate slowly in air. After 2–3 days, a dark brown crystalline compound of 6 separated out, which was filtered and dried in air. Yield: 90 mg (∼66% with respect to cobalt). Mass spectral data (ESI, positive ion, CH₃OH): m/z 435 for [6 − 2Cl]⁺. ¹H NMR (CDCl₃, 300 MHz): δ(ppm) 10.04 (s, 1H), 8.73 (s, 1H), 8.53 (s, 3H), 7.79 (m, 3H), 7.89–6.16 (m, 10H). Anal. calcd (%) for C₂₅H₁₈Cl₂CoN₃O: C, 59.31; H, 3.58; N, 8.30. Found: C, 58.95; H, 3.52; N, 8.15. IR/cm⁻¹ (KBr): ν 3422 (s), 1609 (vs), 1528 (s), 1438 (m), 1384 (m), 1350 (m), 1145 (m), 754 (m).

X-Ray crystallographic data collection and refinement of the structures (CCDC 842402 (6), 972492 (4·CH₃OH) and 972493 (5))

Single crystals of 4·CH₃OH, 5 and 6 were picked up with nylon loops and were mounted on a Bruker AXS Enraf-Nonius Kappa CCD diffractometer equipped with a Mo-target rotating-anode X-ray source and a graphite monochromator (Mo-Kα, λ = 0.71073 Å). 4·CH₃OH and 5 were measured at 100 K while 6 was measured at 296 K. Final cell constants were obtained from least squares fits of all measured reflections. The intensity data was corrected for absorptions using intensities of redundant reflections. The structures were readily solved by direct methods and subsequent different Fourier techniques. The crystallographic data of 4·CH₃OH, 5 and 6 are listed in Table 1.

Table 1 X-ray crystallographic data for 4·CH₃OH, 5 and 6

	4·CH3OH	5	6
a R 1 = ∑||Fo| − |Fc||/∑|Fo|. b GOF = {∑[w(Fo^2 − Fc^2)^2]/(n − p)}^1/2. c wR2 = [∑[w(Fo^2 − Fc^2)^2]/∑[w(Fo^2)^2]]^1/2 where w = 1/[σ^2(Fo^2) + (aP)^2 + bP], P = (Fo^2 + 2Fc^2)/3.
Formula	C33H35N3O2V	C25H19MoN3O3	C25H18Cl2CoN3O
FW	572.60	505.37	506.21
Cryst. color	Red	Orange	Green
Cryst. syst.	Triclinic	Triclinic	Monoclinic
Space group	P1̄	P1̄	P21/c
a (Å)	9.340(3)	7.9292(2)	9.3416(5)
b (Å)	11.616(7)	11.4808(6)	12.8676(7)
c (Å)	13.836(4)	11.7269(9)	19.8225(11)
α (°)	76.22(5)	76.524(4)	90.00
β (°)	83.19(5)	83.276(5)	103.333(3)
γ (°)	88.86(5)	81.837(3)	90.00
V (Å^3)	1447.6(11)	1023.71(10)	2318.5(2)
Z	2	2	4
T (K)	100(2)	100(2)	296(2)
2θ	60.00	62.00	48
Calcd (g cm^−3)	1.314	1.640	1.450
Reflns collected	18 275	15 702	9921
Unique reflns	8372	6499	3489
Refection [I > 2σ(I)]	5603	6061	2567
λ (Å)/μ (mm^−1)	0.71073/0.380	0.71073/0.675	0.71073/0.993
F(000)	604	512	1032
R 1 [I > 2σ(I)]/GOF^b	0.0682/1.036	0.0263/1.059	0.0432/1.051
R 1 (all data)	0.1118	0.0293	0.0609
wR2^c [I > 2σ(I)]	0.1469	0.0675	0.1145
No. of param./restr.	378/0	289/0	289/0
Residual density (e Å^−3)	0.852	0.729	0.432

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The Siemens SHELXS-97^13a and SHELXL-97^13b software packages were used for the solution and the refinement. All non-hydrogen atoms were refined anisotropically. Hydrogen atoms were placed at the calculated positions and refined as riding atoms with isotropic displacement parameters.

Density functional theory (DFT) calculations

All the calculations reported in this article were done with the Gaussian 03W¹⁴ program package supported by GaussView 4.1.

The DFT¹⁵ and TD DFT¹⁶ calculations were performed at the level of the Becke three parameter hybrid functional with the non-local correlation functional of Lee–Yang–Parr (B3LYP).¹⁷ The gas-phase geometries of 3 with doublet spin state, 3⁺ and 6 with singlet spin state were optimized using Pulay's Direct Inversion¹⁸ in the iterative Subspace (DIIS), ‘tight’ convergent SCF procedure¹⁹ ignoring symmetry. The optimized coordinates are listed in Tables S8–S11 (ESI†). In all the calculations, a LANL2DZ basis set,²⁰ along with the corresponding effective core potential (ECP) was used for the metal atom. Valence double zeta with polarization and diffuse functional basis set, 6-31++G**²¹ were used for the C, N, O and Cl atoms in all the calculations. For the H atoms, the 6-31G basis set was used.²² The percentage contributions of the metal, chloride and ligand to the frontier orbital of the optimized geometries were calculated using the GaussSum program package.²³ The sixty excitation energies on the optimized geometries of 3, 3⁺ and 6 were calculated by TD DFT²⁴ calculations.

Results and discussion

The coordination complexes of the amide and imine derivatives of L₃H₂ isolated in this work are depicted in Scheme 2. Details of the syntheses of 3–6 are given in the Experimental section. o-Phenylene derivatives are synthesized using the reported procedures.⁷ L₃^t-BuH₂ and 3–6 are characterized by the elemental analyses and IR, mass, EPR and ¹H NMR spectra. The V=O stretching vibrations of 3 and 4 are at 966 and 959 cm⁻¹, while the symmetric and asymmetric stretching vibrations of two cis Mo=O²⁵ resonate at 901 and 915 cm⁻¹. UV/vis absorption spectral data are summarized in Table 2. UV/vis spectra are shown in Fig. S1.† The lower energy absorption bands of 3 and 4 disappear in 3⁺ and 4⁺ ions. Complexes 5 and 6 do not display any lower energy absorption bands.

Scheme 2

Table 2 UV-vis/NIR absorption spectral data of 3, 3⁺, 4, 4⁺, 5 and 6 in CH₂Cl₂ at 298 K

Compound	λ max (ε, 10^4 M^−1 cm^−1) (nm)
3	496 (0.28), 386 (0.44), 327 (0.51)sh, 313 (0.80)sh, 300 (1.24), 266 (1.67)
3 ^+	491 (0.13)sh, 407 (0.33)sh, 376 (0.49), 331 (0.64)sh
4	492 (0.43), 393 (0.63), 333 (1.72)sh, 316 (2.30)sh, 298 (2.78), 264 (3.20)
4 ^+	512 (0.13)sh, 421 (0.55), 357 (0.74)sh
5	450 (0.31), 351 (1.01), 326 (1.71)sh 310 (2.23), 256 (2.67)sh
6	423 (0.24), 352 (0.41)sh, 331 (0.53)sh, 303 (0.66), 250 (1.10), 206 (1.74)

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The paramagnetic 3 and 4 complexes are redox active. The redox series of 3 and 4 were investigated by cyclic voltammetry in CH₂Cl₂ containing 0.2 M tetrabutylammoniumhexafluorophosphate as supporting electrolyte. The cyclic voltammogram of 3 is shown in Fig. S2.† The anodic redox waves of 3 and 4 at 0.35 and 0.33 V are assigned to the VO³⁺/VO²⁺ redox couple.

No electron transfer occurs in the reactions of L₃H₂ with VO(acac)₂ and molybdate ion producing amide complexes 3, 4 and 5. However, the reaction of CoCl₂ with L₃H₂ affords ketimine complex, 6. In the reaction with CoCl₂, both the metal ion and the ligand undergo oxidation. Overall it is a −(3e + 2H⁺) transfer reaction involving an external dioxygen molecule as an oxidizing agent. Scheme 3 illustrates the probable intermediates of this −(3e + 2H⁺) transfer redox reaction, which involves: the coordination of the M(II) ion to the monoanionic L₃H⁻ ligand affording A, deprotonation of the monoanionic L₃H⁻ ligand to the dianionic L₃²⁻ affording B, and the oxidation of the dianionic L₃²⁻ to L₄⁻ by an external oxygen molecule affording C. The intermediate A has been isolated as a iron(III) complex as 1 (Scheme 1).⁷ The intermediate B has been isolated as oxidovanadium(IV) and cis-dioxidomolybdenum(VI) complexes as 3, 4 and 5. In case of ruthenium, C is the final product furnishing the 2⁺ ion (Scheme 1). However, in the case of the cobalt(II) ion, the e_g¹ electron is delocalized over the low-lying π_diimine^* orbital and reacts easily with air, affording the cobalt(III) complex, D.

Scheme 3

The OD reaction with the cobalt(II) ion is informative. The higher valent cobalt(IV) ion will never be achieved in air. The conversion of cobalt(II) to cobalt(III) ion in presence of a chelating ligand is easier, however, cobalt(III) ion is not an oxidizing agent. It completely defies the participation of the higher oxidation state of the metal ion in an OD reaction of the amine. One of the important roles of the metal ions is to de-protonate the NH functionality upon coordination, which is achieved in the cases of oxidovanadium and oxidomolybdenum ions. The oxidation occurs via the external O₂ molecule facilitating the d_M → π_imine* back-bonding, which is favored with a t₂⁶ state, e.g. Ru(II) and Co(III) ions. For effective back-bonding increasing the M–N bond order, it needs a lower oxidation state of the metal ions. Oxidovanadium(IV) and dioxidomolybdenum(VI) ions in 3–5 are, respectively, d¹ and d⁰ ions and lack the ability to back donate, significantly disfavoring the OD of the amide ligand.

Molecular geometries

Molecular bond parameters and cis or trans geometries of 3–6 were confirmed by the single crystal X-ray structure determinations of 4·CH₃OH, 5 and 6. 4·CH₃OH crystallizes in the P1̄ space group. The molecular geometry of 4·CH₃OH in the crystals along with the atom labeling scheme is illustrated in Fig. 1. The significant bond parameters are summarized in Table 3. The tetra-dentate L₃^{t-Bu 2−} dianionic ligand spans the sites of the square (with a mean deviation of 0.09 Å) of the distorted square pyramid coordination sphere around the vanadium ion. The vanadium ion is displaced towards the oxido group by 0.65 Å. The oxidovanadium, V(1)–O(40) and the V(1)–O_phenolatoi.e. V(1)–O(1) bond lengths, 1.612(2) and 1.925(2) Å, respectively, correlate well with the presence of the oxidovanadium(IV) ion in 4·CH₃OH.²⁶ The C(8)–N(9) and C(17)–N(16) lengths, 1.303(3) and 1.448(3) Å, respectively, are consistent with the existence of the aldimine, –CH=N– and (Ph)(Py)(H)C–N(H)– functionalities in 4·CH₃OH.⁷

Fig. 1 Molecular geometry of 4·CH₃OH in crystals (50% ellipsoids; CH₃OH and H atoms are omitted for clarity).

Table 3 Selected experimental bond lengths (Å) and angles (°) of 4·CH₃OH and corresponding calculated parameters of 3

	Exp.	Cal.
	4·CH3OH	3
V(1)–O(1)	1.925(2)	1.923
V(1)–N(9)	2.052(2)	2.066
V(1)–N(16)	1.950(2)	1.968
V(1)–N(23)	2.099(3)	2.110
V(1)–O(40)	1.612(2)	1.599
C(8)–N(9)	1.303(3)	1.302
N(16)–C(17)	1.448(3)	1.448
O(1)–V(1)–N(16)	135.08(9)	133.75
N(9)–V(1)–N(23)	146.09(10)	147.79

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5 crystallizes in the P1̄ space group. An ORTEP plot of the molecule and the atom labeling scheme are illustrated in Fig. 2. Significant bond parameters are listed in Table 4. The orientation of the L₃²⁻ ligand in 5 is different from that in 4·CH₃OH. The Mo–N_py (N(23)) bond is perpendicular to the MoO(1)N(9)N(16) plane, making the two oxido groups cis to each other. Two Mo=O bond lengths, 1.7096(11) and 1.7198(11) Å, are similar to those reported in cis-dioxidomolybdenum(VI) complexes.²⁵ The C(8)–N(9) length, 1.294(2) Å, authenticates the aldimine (–CH=N–) functional group, while the C(17)–N(16) length of 1.479(2) correlates well with a C–N single bond.

Fig. 2 Molecular geometry of 5 in crystals (50% ellipsoids; H atoms are omitted for clarity).

Table 4 Selected experimental bond lengths (Å) and angles (°) of 5

Mo–O(1)	1.9692(11)	C(8)–N(9)	1.2934(19)
Mo–N(9)	2.2958(13)	N(16)–C(17)	1.4789(18)
Mo–N(16)	2.0219(12)	O(1)–Mo–N(16)	147.57(5)
Mo–N(23)	2.3637(12)	N(9)–Mo–N(23)	77.79(4)
Mo–O(30)	1.7198(11)	O(40)–Mo–O(30)	107.32(5)
Mo–O(40)	1.7096(11)

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6 crystallizes in the P2₁/c space group. The molecular geometry of 6 in the crystals and the atom labeling scheme is depicted in Fig. 3. Significant bond parameters are summarized in Table 5. The orientation of the tetra-dentate L₄⁻ ligand is different from the dianionic L₃^{t-Bu 2−} and L₃²⁻ ligands present in 4·CH₃OH and 5. In contrast to the non-planner geometries of L₃^{t-Bu 2−} and L₃²⁻, the L₄⁻ in 6 is completely planar, excluding the pendent phenyl groups, and occupies the square plane of the CoN₃OCl₂ octahedron enforcing the two chloride ligands trans to each other.

Fig. 3 Molecular geometry of 6 in crystals (50% ellipsoids; H atoms are omitted for clarity).

Table 5 Selected experimental and calculated bond lengths (Å) and angles (°) of 6

	Exp.	Cal.
Co(1)–O(1)	1.872(2)	1.888
Co(1)–N(1)	1.933(3)	1.9451
Co(1)–N(2)	1.880(3)	1.916
Co(1)–N(3)	1.870(3)	1.896
Co(1)–Cl(1)	2.2333(10)	2.306
Co(1)–Cl(2)	2.2637(10)	2.306
N(2)–C(6)	1.285(4)	1.299
N(3)–C(19)	1.297(4)	1.306
Cl(1)–Co(1)–Cl(2)	177.55(4)	177.33
O(1)–Co(1)–N(2)	176.30(12)	176.85
N(3)–Co(1)–N(1)	170.08(13)	169.75

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The N(3)–C(19) and N(2)–C(6) lengths, 1.285(4) and 1.297(4) Å, are consistent with the existence of the aldimine (–CH=N–) and ketimine ((Ph)(py)C=N–) functional groups in 6.⁷ The bond parameters and the planarity confirm the −(2e + 2H⁺) oxidation of L₃²⁻ to L₄⁻ in 6. The two trans Co^III–Cl lengths are 2.233(2) and 2.364(2) Å.

The trend of M–N_ketimine and M–N_amine bond lengths in 1–6 complexes is noteworthy. All three types of bond lengths, M–N_amine, M–N_amide and M–N_ketimine, with 3d and 4d metal ions have successfully been determined. The experimental bond lengths are listed in Table 6. It is observed that the M–N_ketimine lengths are significantly shorter than the M–N_amine and M–N_amide lengths. In 2⁺, the Ru^II–N_ketimine length, 1.976(6) Å, is intermediate between the reported Ru^II–N_amine and Ru^II=N_imide lengths. The average Ru^II–N_amine and Ru^II–N_iminoquinone distances in o-phenylenediamine complexes are 2.132 and 2.080 Å.²⁷ The reported average Ru^II=N_imide length is 1.753 Å.²⁸ It claims that the bond order of the Ru^II–N_ketimine in 2⁺ ion is higher than one. A similar trend has been recorded in the case of Co(III) complex 6 also. The Co^III–N_imine distance, 1.880(3) Å, is shorter than Fe^III–N_amine and V^IV–N_amide distances (Table 6). The observed Co^III–N_aldimine length in 6 is 1.870(3) Å. In a o-phenylenediamine complex, Co^III–N_amine length is 1.982(8)–2.016(3) Å,²⁹ while the Co^III=N_imide length in a cobalt(III) aryl imido complex is 1.675 Å.³⁰ In 6, the Co^III–N_ketimine length being intermediate between the Co^III–N single and double bonds corresponds to a bond order higher than one. The features are explained by the mixing of the d_M–π* orbitals (vide infra) that stabilizes the lower oxidation states of the metal ions and increases the M–N_ketimine bond order.

Table 6 Significant experimental M–N_imine, M–N_amide and M–N_amine bond lengths (Å)

Bond type	Length	Complexes
Fe^III–Namine	2.192(2)	1
Ru^II–Nketimine	1.976(6)	2 ^+
V^IV–Namide	1.950(2)	4
Mo^VI–Namide	2.022(2)	5
Co^III–Nketimine	1.880(3)	6

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EPR spectra, fluorescence and fluorescence-spectroelectrochemistry

The EPR spectra with simulation are shown in the panels (a–c) of Fig. S3.† The spectra with the hyperfine coupling from ⁵¹V nuclei corroborate with s = 1/2 spin state and (3, g_iso = 1.9806, A = 86.9 × 10⁻⁴ cm⁻¹; 4, g_iso = 1.9778, A = 86.7 × 10⁻⁴ cm⁻¹) and are consistent with the presence of the oxidovanadium(IV) ion in 3 and 4. The g values of the axial spectrum (panel (b) of Fig. S3†) of the CH₂Cl₂ frozen glass of 3 at 25 K are: g_‖ = 1.9590, A_‖ = 156.4 × 10⁻⁴ cm⁻¹; g_┴ = 1.9828, A_┴ = 103.7 × 10⁻⁴ cm⁻¹. Analysis of the EPR spectra of 3⁺ and 4⁺ ions confirms that the oxidation is metal centered, concluding that 3⁺ and 4⁺ cations are the oxidovanadium(V) complexes of types [V^VO(L₃²⁻)]⁺ (3⁺) and [V^VO(L₃^{t-Bu 2−})]⁺ (4⁺).

3 and 4 are non-emissive while the oxidized analogues 3⁺ and 4⁺ ions are emissive at 298 K. In CH₂Cl₂, 5 and 6 are also fluorescent. The fluorescence data are listed in Table 7 and the relevant spectra are shown in Fig. S4.†

Table 7 Fluorescence spectral parameters of the complexes in CH₂Cl₂ at 298 K

Compound	λ ex/λem (nm)/ϕ^a
a ϕ = quantum yield.
3 ^+	331/444
4 ^+	339/490
5	336/466/0.003
6	324/473/0.027

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In this regard, it is to be noted that the free L₃H₂ ligand is fluorescent (λ_ex = 330; λ_em = 470 nm) due to the internal charge transfer from the π_phenolato → π_aldimine* orbital.⁷3 and 4 absorb strongly at comparatively longer wavelengths (493 and 497 nm) due to π_NPh → π_aldimine^* transition and the complexes are non-emissive. However similar to 1, in 3⁺ and 4⁺ ions these lower energy bands are absent and the cations are fluorescent.

It is to be noted that upon oxidation the lower energy absorption bands gradually disappear while fluorescence intensity at λ_em = 444 and 490 nm, respectively, for 3⁺ and 4⁺ cations gradually increases as depicted in the panels (b) and (d) of Fig. 4. The spectral features of 3⁺ and 4⁺ cations are illustrated in Fig. 4. The lower energy absorption bands of 5 and 6 at 450 and 423 nm are weaker and both the complexes are weakly fluorescent as illustrated in Fig. S4† and Table 7.

Fig. 4 Spectroelectrochemical measurements of the conversion of 3 → 3⁺ [(a) UV-vis/NIR absorption and (b) fluorescence spectra] and 4 → 4⁺ [(c) UV-vis/NIR absorption and (d) fluorescence spectra] in CH₂Cl₂ at 298 K.

The origins of the UV-vis/NIR absorptions of 3–6 were elucidated by the time dependent (TD) density functional theory (DFT) calculations on 3, 3⁺ and 6. The gas phase geometry of 3 was optimized at the B3LYP/DFT level with the doublet spin state while those of 3⁺ and 6 were optimized with the singlet spin state. Calculated bond parameters are listed in Tables 3, 5 and S1.† The calculated bond parameters are similar to those obtained from the single crystal X-ray diffraction studies of 4·CH₃OH and 6 (Tables 3 and 5). The optimized geometries of 3, 3⁺ and 6 are shown in Fig. S2.† Excitation energies were calculated by the TD DFT calculations on the optimized geometries. The excitation energies with the oscillator strengths are listed in Table S2.† Fragmentations of the ligand used for the calculations are shown in Fig. S5.† It is reported that L₃H₂ is emissive due to π_Phenolato → π_aldimine^* transition at λ_ex = 330 nm. 3 and 4 with lower energy absorption bands at λ_max = 490 and 500 nm are non-emissive. The TD DFT calculation on 3 has authenticated that the lower energy absorption band of 3 at λ_max = 493.57 nm with f = 0.13 is due to π_NPh → π_aldimine^* transitions. The π_phenolato → π_aldimine^* transition of 3 appears at 316.72 nm. However, the non-emissive lower energy absorption band at λ_max = 490 nm gradually disappears upon oxidation of 3 to 3⁺ (panel (a) of Fig. 4) and 3⁺ becomes emissive. The calculated excitation band of 3⁺ at 357.8 nm with f = 0.12 is due to π_phenolato → π_aldimine^* transition. Similarly, the calculated π_phenolato → π_aldimne^* emissive excitation wavelength of 6 is 318.06 nm (f = 0.45). The emissive and non-emissive transitions of 3–6 including the L₃H₂ ligand are illustrated in Scheme S1.†

Molecular orbital analyses

The constituents of the frontier molecular orbitals of 3 and 6 were investigated by DFT calculations using B3LYP functional. Gas phase geometries of 3 and 6 were optimized, respectively, with doublet and singlet spin states. The constituents of the frontier orbitals are analyzed and the data are summarized in Table S3.† The calculations authenticated a significant mixing among the d orbitals and the benzoyl pyridine fragment of the L₄⁻ ligand in 6. Analyses have shown that the d_xz (HOMO−12) and d_yz (HOMO−11) orbitals of the t₂ set of 6 exhibit strong interactions with the benzoyl pyridine fragment of the tetra dentate diimine ligand. Similar types of mixing among the d orbitals and the L₄⁻ ligand have been attributed in the case of 3 also. However, the d orbitals of the t₂ set of the OV(IV) ion interact equally with the phenolato and the benzoyl pyridine fragments. The mixing of the d orbitals results in diverse effects in 3 and 6. In the case of 6, the d⁶ ion promotes the oxidation of amine to ketimine for π delocalization while the d¹ ion stabilizes the hard amide binding in 3. Similarly, the amide binding is stabilized by the hard acid, d⁰, molybdenum(VI) ion. The result is reversed with the soft d⁶ ruthenium(II) ion, which converts amine to ketimine for π-delocalization. The OD of the amine to ketimine parallels the chemistry of the conversion of NO → NO⁺ reducing metal ions in some cases for effective back-bonding with the lower oxidation states of the metal ions. The results correlate well with the reported conversions of copper(II) to copper(I), d¹⁰ ion, iron(III) to iron(II) t₂⁶ ion and ruthenium(III) to ruthenium(II), t₂⁶ ion oxidizing amines to imines.⁴

Conclusion

The role of the oxidation states of the metal ions in oxidative dehydrogenation (OD) reaction of the (Ph)(Py)(H)C–N(H)– functionality of an o-phenylenediamine derivative (L₃H₂) has been investigated (L₃H₂ = (E)-2-(((2-((phenyl(pyridin-2-yl)methyl)amino)phenyl)imino)methyl)phenol). Recently, we reported that the reaction of L₃H₂ with anhydrous FeCl₃ affords the amine complex cis-[Fe^III(L₃H⁻)Cl₂] (1) while the same reaction with [Ru^II(PPh₃)₃Cl₂] results in an OD reaction affording a ketimine complex, trans-[Ru^II(L₄⁻)(PPh₃)]⁺ (2⁺), in good yield (L₄H = 2-((E)-(2-((E)-phenyl(pyridin-2-yl)methyleneamino) phenylimino)methyl)phenol)). To summarize the effect of the higher oxidation states of the metal ions to the OD reaction of L₃H₂, similar reactions of L₃H₂ with oxidovanadium(IV) and oxidomolybdenum(VI) ions were performed. In each case the reaction produces amide complexes of type [V^IVO(L₃²⁻)] (3), [V^IVO(L₃^{t-Bu 2−})] (4) and cis-[Mo^VIO₂(L₃²⁻)] (5). However, the reaction of anhydrous CoCl₂ with L₃H₂ promotes the OD reaction in air yielding a ketimine complex of type trans-[Co^III(L₄⁻)Cl₂] (6). The study infers that the OD reaction of L₃H₂ is not successful with hard metal ions with higher oxidation states, such as Fe^III, V^IVO and Mo^VIO₄ ions, while the OD reaction occurs with softer, lower valency Ru(II) and Co(II) ions with the filled t₂⁶ set that enhances the d_M → p_π^* back bonding. The work does not justify the previous reports that claim that the metal ion promoted OD reaction of an amine requires the higher oxidation state as an intermediate for oxidation of amines. The work rather concludes that the coordinated amine is de-protonated to an amide that undergoes oxidation to ketimine by an external oxygen molecule to have stronger d_M → p_π^* back bonding with the lower oxidation states of the metal ions.

Fluorescence features of L₃H₂ and 3–6 are noteworthy. L₃H₂ is weakly fluorescent (λ_ex = 330; λ_em = 470 nm) due to a non-emissive lower energy absorption band at 390 nm. 3 and 4 exhibit absorption bands at 493 and 497 nm and are non-emissive, while upon oxidation the lower energy absorption band disappears and [V^VO(L₃²⁻)]⁺ (3⁺) and [V^VO(L₃^{t-Bu 2−})]⁺ (4⁺) cations are fluorescent, recorded by fluorescence-spectroelectrochemical measurements in CH₂Cl₂ at 298 K. 5 and 6 display weaker absorption bands at 445 and 423 nm and are weakly fluorescent (5, λ_ex = 336 nm, λ_em = 466 nm; 6, λ_ex = 324 nm, λ_em = 473 nm). Moreover, in addition to the oxidation state dependent fluorescence features of 3–4, and oxidovanadium(IV) and dioxidomolybdenum(VI) compounds being effective catalysts for several organic transformations, complexes 3–5 appeared to be significant in coordination chemistry.

Acknowledgements

Financial support received from DST (SR/S1/IC/0026/2012) and CSIR (01/2699/12-EMR-II) New Delhi, India is gratefully acknowledged. SB (CSIR no. 8/531(0006)/2012-EMR-I) and MKB (CSIR no. 8/531(0007)/2012-EMR-I) are thankful to CSIR, New Delhi, India, for fellowships.

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Footnote

† Electronic supplementary information (ESI) available: UV-vis/NIR absorption spectra (Fig. S1), cyclic voltammogram of 3 (Fig. S2), X-band EPR spectra of 3 and 4 (Fig. S3), fluorescence spectra of 5 and 6 (Fig. S4), photoactive molecular orbitals (Scheme S1), schematic diagram of the ligand fragmentation considered in MO analyses (Fig. S5), calculated bond lengths of 3, 3⁺ and 6 (Table S1), TD DFT calculations (Table S2), population analyses of selected molecular orbitals of 6, 3, 3⁺ (Table S3) and optimized coordinates (Table S4–S6). CCDC 842402, 972492 and 972493. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c3qi00103b

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