Dioxo-vanadium(V), oxo-rhenium(V) and dioxo-uranium(VI) complexes with a tridentate Schiff base ligand

Abstract

The complexation of a julolidine–quinoline based tridentate ligand with three oxo-metal ions, dioxo-vanadium(V), oxo-rhenium(V) and dioxo-uranium(VI), has been investigated with four new complexes being synthesised and structurally characterised. (VO₂L)·2/3H₂O (1) {HL (C₂₂H₂₁N₃O) = ((E)-9-((quinolin-8-ylimino)methyl)-1,2,3,5,6,7-hexahydropyrido[3,2,1-ij]quinolin-8-ol)} has a VO₂L neutral mononuclear structure with a five-fold coordinated vanadium metal centre in a distorted trigonal bipyramidal geometry. (ReOL₂)₂(ReCl₆)·7DMF (2) [DMF = dimethylformamide] exhibits a mixed valent rhenium complex with a (ReOL₂)⁺ cationic unit in a distorted octahedral metal coordination geometry, charge balanced with (ReCl₆)²⁻ anions. [(UO₂)L(H₂O)₂]₂·2(NO₃)·HL·4H₂O (3) and [(UO₂)L(CH₃OH)₂](NO₃)·CH₃OH (4) both have (UO₂L)⁺ cationic mononuclear structures with either coordinated water or methanol molecules in pentagonal bipyramidal coordination geometries for the uranium metal centres. Intra-/intermolecular interactions including hydrogen bonding and π–π interactions are common and have been discussed. In addition, optical absorption and photoluminescence properties have been investigated.

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Introduction

As a strong signal unit, the julolidine functional group is well known for its visible and fluorescence optical properties.¹ As such it has become a popular functional group in the colourimetric studies of metallo-supramolecular systems. Consequently, it has often been observed in chemosensor systems which exhibit selective molecular recognition for metal ions² or anions.³ In addition, julolidine also exhibits fluorescence properties which can be further explored in the application of fluorescence sensors.^4,5 So far, research effort into julolidine systems is mainly focussed on the solution studies of transition and heavy metal ions, the cases of solid state studies are rather limited.⁶

A Schiff base ligand, HL (see below), consisting of 8-hydroxyjulolidine-9-carboxylaldehyde and 8-aminoquinoline has been reported as a Co²⁺ chemosensor.⁷ The presence of CoL₂ in solution has been confirmed by using ESI-MS. In the solid state, complexes of HL with selected 3d transition metal ions are available.⁸ Both mononuclear and dinuclear complexes have been identified but with various coordination arrangements due to the different coordination geometries adopted by different 3d metal ions, from square planar for Cu²⁺ to octahedral for Mn²⁺, Co³⁺ and Ni²⁺.⁸ In addition, two dimeric structures of HL with Eu³⁺ or Gd³⁺ ion bridged by acetate were also reported more recently.⁹

Oxo-metal ions contain highly covalent metal–oxygen multiple bonds and are the building blocks of metal oxides thus have a high impact on the oxide's desirable chemical, magnetic, electronic, non-linear optical and thermal properties.¹⁰ Some oxo-metal complexes with Schiff base ligands have been investigated.^11–13 Compared to normal metal ions, oxo-metal ions are unique as the presence of short multiple bonded oxygen atoms around the metal centre makes it strong affinity to both O- and N-donor ligands. In addition, it is anticipated that the complex formation for oxo-metal ions with HL will be mononuclear dominating due to the limited coordination sites available, especially for dioxo-metal ions where only restricted coordination positions are accessible.

This contribution deals with the complex formation of HL with three oxo-metal ions, e.g. VO₂⁺, ReO³⁺ and UO₂²⁺ focusing on their structural characterisation and optical properties. Herein we report the synthesis, spectroscopies, crystal structures and optical properties of four new complexes of HL with VO₂⁺, ReO³⁺ and UO₂²⁺ ions: (VO₂L)·2/3H₂O (1) – a neutral mononuclear structure; (ReOL₂)₂(ReCl₆)·7DMF (2) – a mixed valent rhenium complex with a (ReOL₂)⁺ cationic unit in a distorted octahedral geometry, charge balanced with (ReCl₆)²⁻ anions; [(UO₂)L(H₂O)₂]₂·2(NO₃)·HL·4H₂O (3) and [(UO₂)L(CH₃OH)₂](NO₃)·CH₃OH (4) both with (UO₂L)⁺ cationic mononuclear structures in pentagonal bipyramidal coordination geometries for the uranium metal centres.

Experimental section

Materials

All reagents were purchased from commercial sources and used without further purification.

Physical measurements

Scanning electron microscope-electron dispersive spectroscopy (SEM-EDS) was conducted using a Zeiss Ultra Plus SEM (Carl Zeiss NTS GmbH, Oberkochen, Germany) under an accelerating voltage of 20 kV. Electrospray ionisation mass spectroscopy (ESI-MS) analysis was conducted utilising a Waters Xevo QToF/nanoacquity UPLC in positive ion mode. Raman spectra were recorded on a Bruker Senterra using the OPUS software and excitation laser 785 nm in the range of 2000–100 cm⁻¹. Absorption spectra were collected using an Agilent Cary 100 spectrophotometer. Fluorescent emission measurements were conducted on a Shimadzu RF-5301PC instrument with a xenon lamp and excitation wavelength of 360 and 480 nm.

Synthesis

The tridentate ligand, HL, was synthesised using a method reported earlier.^7–9

(VO₂L)·2/3H₂O (1). HL (276 mg, 0.5 mmol) was dissolved in 10 mL of methanol. Vanadyl sulphate tetrahydrate (118 mg, 0.5 mmol) in 5.0 mL of methanol was slowly added to the above ligand solution. 0.05 mL of concentrated triethylamine solution was added and heated at 70 °C under stirring for 1 h. The product of 1 in dark crystalline form was obtained by slow evaporation in 24 h with 43% yield. Elemental analysis for (VO₂L)·2/3H₂O (calc., found): C (60.42, 60.28); H (4.92, 4.97); N (9.61, 9.46). UV/vis (MeOH): λ_max/nm = 487, Raman (solid state, laser = 514 nm): 1568 (s), 1500 (s), 1473 (w), 1375 (vs), 1302 (m), 1239 (m), 1095 (m), 1029 (w), 807 (w), 726 (w), 517 (w), 493 (w), 204 (w) cm⁻¹.

(ReOL₂)₂(ReCl₆)·7DMF (2). Complex 2 was synthesised in a similar way to complex 1 but using ReCl₃ (74 mg, 0.25 mmol) instead. A dark precipitate was separated from the reaction mixture by filtration and was then dissolved in 5.0 mL of DMF. Dark red crystalline product of 2 was obtained via diethyl ether vapour diffusion, with ∼35% yield. Elemental analysis for (ReOL₂)₂(ReCl₆)·4DMF (calc., found): C (48.82, 48.70); H (4.22, 4.35); N (9.11, 9.02). UV/vis (MeOH): λ_max/nm = 360, 450, Raman (solid state, laser = 514 nm): 1625 (s), 1585 (m), 1567 (m), 1520 (vs), 1474 (w), 1424 (s), 1378 (m), 1362 (m), 1335 (w), 1321 (w), 1295 (w), 1262 (w), 1226 (w), 1187 (w), 1093 (w), 1028 (w), 968 (vs), 950 (vs), 830 (w), 812 (w), 630 (w), 585 (w), 520 (w), 445 (w) cm⁻¹.

[(UO₂)L(H₂O)₂]₂·2(NO₃)·HL·4H₂O (3). HL (277 mg, 0.5 mmol) was dissolved in 10.0 mL of ethanol. While stirring, an ethanol solution of uranyl nitrate hexahydrate (0.2514 g, 0.5 mmol) was added at room temperature. The resulting dark solution was left for slow evaporation. Dark coloured crystalline product of 3 was formed on the wall of the glass vial after a few days with very low yield.

[(UO₂)L(CH₃OH)₂](NO₃)·CH₃OH (4). Complex 4 was synthesised similar to the procedure for 1. Dark coloured crystalline product of 4 was obtained after slow evaporation with 24% yield. Elemental analysis for [(UO₂)L(CH₃OH)₂](NO₃)·CH₃OH (calc., found): C (39.02, 38.91); H (4.06, 3.96); N (7.28, 7.35). UV/vis (MeOH): λ_max/nm = 360, 450, Raman (solid state, laser = 785 nm): 1634 (m), 1577 (vs), 1520 (m), 1479 (m), 1370 (vs), 1334 (w), 1302 (w), 1268 (w), 1212 (w), 1087 (w), 1053 (w), 845 (vs), 807 (w), 793 (w), 743 (w), 440 (w) cm⁻¹. ESI-MS (positive-ion detection, MeOH): m/z = 612.15 calc. 612.20 for {[C₂₂H₂₀N₃O₃U]}¹⁺ (Fig. S1, ESI†).

Single crystal X-ray diffraction

The single crystal X-ray diffraction measurements for complexes 1–4 were carried out on the MX1 beamline at the Australian Synchrotron. Diffraction data were collected using a Si 〈111〉 monochromated synchrotron X-ray radiation (λ = 0.71074) at 100(2) K with BlueIce software¹⁴ and were corrected for Lorentz and polarization effects using the XDS software.¹⁵ An empirical absorption correction was then applied to the data using SADABS.¹⁶ The structures were solved by using SHELXT¹⁷ and the full-matrix least-squares refinements were carried out using a suite of SHELXL program¹⁸ via Olex² interface.¹⁹ All non-hydrogen atoms were located from the electron density maps and refined anisotropically. Hydrogen atoms bound to carbon atoms were added in the ideal positions and refined using a riding model. The data completeness for complexes 1 and 3 was low due to their low symmetry space group as well as suffering radiation damage gradually limiting only one-circle data collection possible. The structure of complex 1 was refined by two-component twin with BASF value of 0.211(2). Complex 2 has large solvent accessible voids and SQUEEZE function of PLATON²⁰ was used to remove the contribution of a disorder DMF molecule. Complex 3 also has solvent accessible void of 145 ų with diffuse electron density ∼ 1.5 e Å⁻³ in it due to the presence of uncounted disordered solvent molecules. In addition, some ripples arise around the heavy metal ions due to ineffective absorption corrections. All potential hydrogen bonds were calculated using PLATON.²⁰

Results and discussion

Synthesis and general characterisation

Complexes 1–2 and 4 were prepared by reactions of metal salts and HL in methanol solutions with addition of triethylamine. Complex 3 was prepared in mixed ethanol and water. In contrast to the preparation of complexes 3 and 4 in which U(VI) ion is the most stable valence state of uranium, the preparation of complexes 1–2 involves partial oxidation of the starting metal ions, from (VO)²⁺ to (VO₂)⁺ for 1, from Re³⁺ to (ReO)³⁺ and (ReCl₆)²⁻ for 2. In addition, complexes with metal to ligand ratios of 1 : 1 (1, 3 and 4) and 1 : 2 (2) have been found. SEM-EDS analysis confirmed the presence of C, N, O and V in 1; C, N, O, Re and Cl in 2; and C, N, O, U in 4 (Fig. S2–S3, ESI†).

Structure details and discussion

The crystal data and refinement details for complexes 1–4 are summarised in Table 1. Selected bond lengths and angles are listed in Table 2. The asymmetric unit of 1 contains six distinguished VO₂L monomers, each containing a (VO₂)⁺ ion [V=O bonds range from 1.625(3) to 1.637(3) Å with O=V=O angles from 107.8(2)° to 110.2(2)°] bonded with a L ligand [2.117(3) to 2.131(3) Å for V–(N1, N4, N7, N10, N13 and N16); 2.123(3) to 2.136(3) Å for V–(N2, N5, N8, N11, N14 and N17); 1.886(3) to 1.913(2) Å for V–(O3, O6, O9, O12, O15 and O18)] making distorted trigonal bipyramidal coordination geometries for the five-fold coordinated vanadium metal centres (Fig. 1a). Strong π–π interactions (Fig. 1b and c) between neutral monomeric complexes are present. The unique hydrogen bonding between (VO₂)⁺ and lattice water molecules (Table 3) link the monomers into trimmers in the crystal lattice (Fig. 1d).

Table 1 Crystal data and refinement details for complexes 1–4

a R1 = Σ∥Fo| − |Fc∥/|Fo|.b wR2 = {Σ[w(Fo^2 − Fc^2)^2]/Σ[w(Fo^2)^2]}^1/2.
Complex	1	2	3	4
Formula	C22H21.33N3O3.67V	C109H127N19O13Cl6Re3	C66H76N11O21U2	C25H32N4O9U
Formula weight	437.36	2682.59	1835.43	770.57
Crystal system	Triclinic	Orthorhombic	Triclinic	Monoclinic
Space group	P1̄	Pbcn	P1̄	P21/c
a (Å)	15.390(3)	16.838(3)	15.416(3)	11.447(2)
b (Å)	17.480(4)	20.449(4)	15.959(3)	15.650(3)
c (Å)	22.770(3)	31.770(6)	16.435(3)	15.962(3)
α (°)	100.25(3)	90	105.45(3)	90
β (°)	98.44(3)	90	113.33(3)	110.58(3)
γ (°)	106.11(3)	90	90.57(3)	90
Volume (Å^3)	5665(3)	10 939(4)	3548.2(15)	2677.0(10)
Z/μ (mm^−1)	12/0.560	4/3.528	2/4.639	4/6.123
Min./max. θ [°]	0.929/25.999	1.282/27.929	1.335/27.925	1.884/27.902
dcalcd (g cm^−3)	1.538	1.629	1.718	1.912
GOF	1.034	1.016	1.035	1.134
Final R1^a [I > 2σ(I)]	0.0605	0.0479	0.0510	0.0371
Final wR2^b [I > 2σ(I)]	0.1619	0.1108	0.1153	0.0872

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Table 2 Selected bond lengths and angles for complexes 1–4

1
V1–N1	2.125(3)	V1–N2	2.132(3)	V1=O1	1.626(3)
V1=O2	1.625(3)	V1–O3	1.911(2)	V2–N4	2.119(3)
V2–N5	2.136(3)	V2=O4	1.633(3)	V2=O5	1.637(3)
V2–O6	1.886(3)	V3–N7	2.129(3)	V3–N8	2.125(3)
V3=O7	1.634(3)	V3=O8	1.632(3)	V3–O9	1.913(2)
V4–N10	2.131(3)	V4–N11	2.133(3)	V4=O10	1.627(3)
V4=O11	1.628(2)	V4–O12	1.912(2)	V5–N13	2.117(3)
V5–N14	2.133(3)	V5=O13	1.642(2)	V5=O14	1.629(3)
V5–O15	1.892(3)	V6–N16	2.129(3)	V6–N17	2.123(3)
V6=O16	1.634(3)	V6=O17	1.626(3)	V6–O18	1.913(2)
O=V1=O	110.2(2)°	O=V2=O	109.4(2)°	O=V3=O	107.8(2)°
O=V4=O	109.7(2)°	O=V5=O	110.2(2)°	O=V6=O	107.8(2)°
2
Re1=O1	1.691(4)	Re1–O2	1.990(4)	Re1–O3	1.958(4)
Re1–N1	2.137(5)	Re1–N2	2.029(5)	Re1–N5	2.135(5)
Re2–Cl1	2.3566(16)	Re2–Cl2	2.3597(17)	Re2–Cl3	2.348(2)
3
U1=O1	1.780(5)	U1=O2	1.776(5)	U1–O3	2.208(4)
U1–N1	2.620(6)	U1–N2	2.493(6)	U1–O1W	2.407(6)
U1–O2W	2.437(6)	O=U1=O	177.1(2)°	U2=O4	1.779(5)
U2=O5	1.768(5)	U2–O6	2.194(5)	U2–N4	2.614(6)
U2–N5	2.506(7)	U2–O3W	2.463(7)	U2–O4W	2.429(7)
O=U2=O	176.5(2)°
4
U1=O1	1.784(4)	U1=O2	1.783(4)	U1–O3	2.199(4)
U1–O4	2.428(4)	U1–O5	2.449(4)	U1–N1	2.604(5)
U1–N2	2.486(4)	O=U=O	176.07(17)°

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Fig. 1 Structure of 1: an ellipsoidal plot (probability 50%) of the mononuclear structure (a), π–π interactions between monomers (b and c) and hydrogen bonds (in yellow) with water molecules leading to the formation of trimers (d) in the crystal lattice.

Table 3 Calculated hydrogen bonds for complexes 1, 3 and 4

D–H⋯A	d(D–H)	d(H⋯A)	d(D⋯A)	∠(DHA)
1
O(1W)–H(1WA)⋯O(4)	0.85	1.98	2.831(4)	175
O(1W)–H(1WB)⋯O(1)	0.85	1.93	2.769(4)	168
O(2W)–H(2WA)⋯O(5)	0.85	1.98	2.809(5)	166
O(2W)–H(2WB)⋯O(7)	0.85	1.92	2.764(6)	172
O(3W)–H(3WA)⋯O(10)	0.85	1.92	2.765(4)	174
O(3W)–H(3WB)⋯O(14)	0.85	2.01	2.852(4)	172
O(4W)–H(4WA)⋯O(13)	0.85	2.01	2.862(5)	176
O(4W)–H(4WB)⋯O(16)	0.85	1.94	2.781(5)	168
3
O(1W)–H(1WA)⋯O(8W)	0.92	1.87	2.70(3)	149
O(1W)–H(1WB)⋯O(3N1)	0.92	1.87	2.775(13)	167
O(2W)–H(2WA)⋯O(9W)	0.92	1.71	2.62(2)	174
O(2W)–H(2WB)⋯O(5W)	0.92	1.76	2.658(12)	164
O(3W)–H(3WA)⋯O(2N1)	0.93	1.86	2.749(11)	159
O(3W)–H(3WB)⋯O(7W)	0.92	1.64	2.56(3)	171
O(4W)–H(4WA)⋯O(1N3)	0.93	1.77	2.62(3)	150
O(4W)–H(4WB)⋯O(2N3)	0.93	2.45	3.32(4)	155
O(4W)–H(4WB)⋯O(1N1)	0.93	1.98	2.854(13)	155
O(7)–H(7A)⋯N(8)	0.82	1.89	2.652(12)	153
O(5W)–H(5WA)⋯O(1N1)	0.85	2.02	2.861(14)	167
O(5W)–H(5WB)⋯O(2N2)	0.85	2.40	3.01(2)	129
O(5W)–H(5WB)⋯O(3N2)	0.85	2.22	2.96(2)	145
O(5W)–H(5WB)⋯O(10W)	0.85	2.33	3.18(3)	175
O(6W)–H(6WA)⋯O(2N1)	0.85	2.45	2.924(15)	116
O(6W)–H(6WA)⋯O(3N1)	0.85	2.40	3.206(17)	160
O(6W)–H(6WB)⋯O(7)	0.85	1.83	2.621(16)	154
4
O(4)–H(4)⋯O(6)	0.92	1.70	2.614(8)	173
O(4)–H(4)⋯O(8)	0.92	2.53	3.098(8)	121
O(5)–H(5)⋯O(9)	0.85	1.78	2.611(7)	166
O(9)–H(9)⋯O(7)	0.82	1.97	2.774(8)	169
O(9)–H(9)⋯O(8)	0.82	2.57	3.209(8)	136

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The asymmetric unit of complex 2 contains a ReO³⁺ ion [1.691(4) Å for Re1=O1] coordinated by two L ligands, one in tridentate [2.137(5) Å for Re1–N1, 2.029(5) Å for Re1–N2 and 1.990(4) Å for Re1–O2] and the other in bidentate [2.135(5) Å for Re1–N5 and 1.958(4) Å for Re1–O3], a half of (ReCl₆)²⁻ anion [2.3566(16) Å for Re2–Cl1, 2.3597(17) Å for Re2–Cl2 and 2.348(2) Å for Re2–Cl3] and three DMF molecules (Fig. 2a). The anion, (ReCl₆)²⁻, has a perfect octahedral geometry and the cationic unit, ReOL₂, has a slightly distorted octahedral polyhedron. The tridentate ligand has a fairly flat conformation whilst the bidentate ligand has the quinoline unit significantly twisted to allow the double bonded oxygen atom occupying one of the octahedral positions leading to the undulating monomers packing along the crystallographic b-axis (Fig. 2b).

Fig. 2 Structure of 2: an ellipsoidal plot (probability 50%) of the mononuclear structure (a) and a crystal packing view along the c-axis (b). DMF molecules are omitted in (a) for clarity.

The asymmetric unit of 3 contains two distinguished UO₂L(H₂O)₂⁺ cationic units, a free HL, two nitrate anions and four water molecules. The cationic structure of 3 is constructed with a uranyl ion [U1=O bond lengths of 1.776(5) and 1.780(5) Å, with O=U1=O angle of 177.1(2)°; U2=O bond lengths of 1.768(5) and 1.779(5) Å, with O=U2=O angle of 176.5(2)°] coordinated by a tridentate L ligand [2.620(6) Å for U1–N1, 2.493(6) Å for U1–N2, 2.208(4) Å for U1–O3; 2.614(6) Å for U2–N4, 2.506(7) Å for U2–N5, 2.194(5) Å for U2–O6] and two water molecules [U1–O1W/O2W bonds from 2.407(6) to 2.437(6) Å; U2–O3W/O4W bonds from 2.463(7) to 2.429(7) Å] in the equatorial plane making a pentagonal bipyramidal geometry for the uranium metal centre (Fig. 3a). The strong π–π interacted cationic units (Fig. 3b) are packed in parallel in the crystal lattice along the a-axis (Fig. 3c), together with nitrate anions for charge balance. The free HL molecules are also packed in parallel between the π–π interacted cationic columns. Hydrogen bonds among nitrate anions and water molecules (Table 3) link the monomeric structure into one dimensional (1D) polymers (Fig. 3d, S4, ESI†) along the crystallographic b-axis.

Fig. 3 Structure of 3: an ellipsoidal plot (probability 50%) of the mononuclear structures (a and b), π–π interactions between monomers (c) and a crystal packing view along the b-axis (d).

The asymmetric unit of 4 contains a uranyl ion, a tridentate L, a nitrate anion and three methanol molecules. The cationic structure of 4 is constructed with a uranyl ion [U=O bond lengths of 1.783(4) and 1.784(4) Å, with O=U=O angle of 176.07(17)°] coordinated by a L ligand [2.604(5) Å for U–N1, 2.486(4) Å for U–N2, 2.199(4) Å for U–O3] and two methanol molecules [U–O bonds from 2.428(4) to 2.449(4) Å] in the equatorial plane making a pentagonal bipyramidal geometry for the uranium centre (Fig. 4a). The isolated cationic units are packed in the crystal lattice (Fig. 4b), together with nitrate anions for charge balance. Both julolidine and quinoline units are bending away from the equatorial plane of the uranyl ion but in opposite directions. Hydrogen bonds (Table 3) among nitrate anions and methanol molecules lead the monomeric cationic structure into 1D polymers (Fig. S5, ESI†).

Fig. 4 Structure of 4: an ellipsoidal plot (probability 50%) of the mononuclear structure (a) and a crystal packing view along the a-axis (b).

In the preparation of complexes 1–2, oxidation of the starting metal ions has been observed, from (VO)²⁺ to (VO₂)⁺ in 1 and from Re³⁺ to (ReO)³⁺ and (ReCl₆)²⁻ in 2. Such aerial oxidation of low valent transition metal ions in solutions is a common phenomenon in the literature. However, the extent of the oxidation can vary significantly depending on the local solution redox and the presence of ligands to stabilise certain species in a given valence state. Therefore the end products are often controlled by the kinetics of the oxidation reaction, complex formations, solution redox as well as the kinetics on the formation of the final product. There are many tridentate Schiff base complexes with (VO₂)⁺ ion in the literature.¹¹ However, only a few tridentate Schiff base complexes with (ReO)³⁺ exist.¹² In general, the low valence oxo-metal ions are not very easy to be stabilised. However, in this work, (ReO)³⁺ cation is coordinated by two L ligands and crystallised from a DMF solution with diethyl ether vapour diffusion. Although large numbers of Schiff base complexes with (UO₂)²⁺ ion have been reported, tridentate Schiff base complexes are rare.¹³ In addition, the solvent effect has been observed in the preparation of complexes 3 and 4 with two coordinated water molecules when a mixture of water and ethanol is used, and two coordinated methanol molecules instead when methanol is used as the solvent. Note the solvent dependent syntheses of (UO₂)²⁺ complexes incorporating methanol or water molecules in the uranium coordination spheres have been reported previously with different types of organic ligands.²¹

Earlier work⁸ on 3d metal ions has demonstrated that L ligand can adopt flexible conformations, from fairly flat to bending conformations. In this work, fairly flat tridentate conformation was found in 1, 3 and 4 whilst a twisted bidentate conformation was identified in 2. In addition, due to the closely packing of the ligands in the crystal lattices, extensive inter-/intra-molecular interactions including π–π interactions and hydrogen bonds are present in all these complexes.

Optical properties

The absorption spectra of HL and its metal complexes with M : L ratio of 1 : 1 for VOSO₄ and UO₂(NO₃)₂·6H₂O], and 1 : 2 for ReCl₃ in methanol solutions are shown in Fig. 5. Pure HL ligand gives double absorption maxima at ∼437 and ∼460 nm. The complexes have similar absorption maxima in the same range corresponding to LMCT bands²² except for 1 which has the absorption maxima shifted to ∼487 nm. In addition, all complexes have second absorption maxima at ∼360 nm related to MLCT for 1 and 2.²² Based on the absorption maxima of the complexes, both 360 nm and 480 nm excitation wavelengths were chosen to study their fluorescence emissions. Upon excitation at 360 nm (Fig. 6a), HL and all complexes have the similar emission maxima at ∼540 nm with obvious fluorescence enhancement in the orders of 4, 2 whilst 1 has similar level of fluorescence emission as HL. Upon excitation at 480 nm (Fig. 6b), all complexes have the similar emission maxima as HL at ∼570 nm with fluorescence quenched in the orders of 2, 4 and 1. In general, uranyl compounds have typical multi-band adsorption in the range of 300–400 nm and emission in the range of 460–540 nm corresponding to the electronic transitions S₁₁ → S₀₀ and S₁₀ → S_0ν (ν = 0–4) of the uranyl ion.²³ It is likely that the uranyl absorption contributes to the relatively higher absorption at around 360 nm for 4. However, the emission of uranyl ion has not been observed for 4 in this work. It is possible that the complexation of HL ligand may quench the fluorescence emission of the uranyl ion.

Fig. 5 UV-vis absorption spectra of HL, 1, 2 and 4.

Fig. 6 Fluorescence emission spectra of HL, 1, 2 and 4 with excitation wavelengths of 360 nm (left) and 480 nm (right).

Vibration modes

The vibration modes of the complexes have been examined using Raman spectroscopy and assigned with consultation of ref. 24. The Raman spectra of 1, 2 and 4 (Fig. 7) show some typical features related to the ligand HL e.g.: ν(C=N) + ν(C=C) at ∼1625–1567 cm⁻¹; ν(C–C) + ν(CN) at ∼1476–1424 cm⁻¹; ν(C–N) + δCH₂ at ∼1378–1362 cm⁻¹; ν(C–C) + ν(C–N) + δCH₂ at ∼1335–1321 cm⁻¹; ν(C–N) + δ(CH), in-plane bending at ∼1295 cm⁻¹; ν(C–N) + δ(CH), in-plane bending including ν(C–N) as of N(CH₃)₂ at ∼1226 cm⁻¹; δ(CH), in-plane bending +δ(CO) at ∼1187 cm⁻¹; δ(CH), out-of-plane bending + δ(CCC) + δ(CCN) at 830–812 cm⁻¹. In addition, a strong Raman band at 845 cm⁻¹ for 4 due to the symmetric stretching vibrations of uranyl ion, ν_s(UO₂)²⁺, has been observed with the calculated U=O bond length of 1.767 Å,²⁵ slightly shorter than the value [1.783(4) Å] determined by the single crystal X-ray diffraction study. The strong double bands at 968 and 950 cm⁻¹ for 2 are due to Re=O stretching vibrations.²⁶ In general, a single Re=O stretching vibration should be observed. It is likely that the two coordinated L ligands in different conformations lead to the splitting of this Re=O band in the case of 2. Furthermore, the interactions between ring C=C and C=N stretching vibrations result in two strong-to-medium intensity Raman bands at 1625 and 1520 cm⁻¹ for 2 and 4 with split bands observed on the low-frequency side at 1585–1567 cm⁻¹.

Fig. 7 Raman spectra of HL (a), 1 (b), 2 (c) and 4 (d) in 1800–200 cm⁻¹ region.

Conclusions

The complexation of a tridentate Schiff base ligand, HL, with three oxo-metal ions, (VO₂)⁺, (ReO)³⁺ and (UO₂)²⁺, has been investigated with four new complexes being synthesised and structurally characterised. The reaction of VOSO₄ with HL in methanol solution affords the formation of (VO₂L)·2/3H₂O (1), a charge neutral mononuclear structure with VO₂⁺ ions each coordinated by a L with slightly distorted trigonal bipyramidal geometries for the vanadium metal centres. (ReOL₂)₂(ReCl₆)·7DMF (2) was isolated from a DMF solution via diethyl ether vapour diffusion. It is a mixed valent oxo-metal complex with a 1 : 2 mononuclear cationic unit in a distorted octahedral geometry, charge balanced by [ReCl₆]²⁻ anions. [(UO₂)L(H₂O)₂]₂·2(NO₃)·HL·4H₂O (3) was isolated from the reaction in mixed water and ethanol whilst [(UO₂)L(CH₃OH)₂(NO₃)]·CH₃OH (4) was isolated from the reaction in methanol. Both 3 and 4 have 1 : 1 (UO₂ : L) mononuclear cationic units with either coordinated water or methanol molecules leading to pentagonal bipyramidal coordination geometries for the uranium metal centres, charged balanced with nitrate anions. It is interesting to observe that all complexes 1–2 were formed by oxidation of the starting metal ions. In addition, complex 2 is a mixed valent oxo-metal complex highlighting the complexity of the redox environment in the chosen oxo-metal system under the reaction condition.

Acknowledgements

The authors would like to thank Dr F. Li for helpful discussion. DJF acknowledges the Western Sydney University Postgraduate Research Award and CSIRO for a top-up scholarship; NDS thanks the AINSE Honours Scholarship Program. The crystallographic data collections for complexes 1–4 were undertaken on the MX1 beamline at the Australian Synchrotron, Victoria, Australia.

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Footnote

† Electronic supplementary information (ESI) available: SEM-EDS and ESI-MS (1–4). CCDC 1479864–1479867. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6ra12872f

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