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Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Dinuclear and Mononuclear Rhenium Coordination Compounds upon Employment of a Schiff-Base Triol Ligand: Structural, Magnetic, and Computational Studies Dimitris A. Kalofolias,† Marek Weselski,‡ Milosz Siczek,‡ Tadeusz Lis,*,‡ Athanassios C. Tsipis,*,§ Vassilis Tangoulis,*,∥ and Constantinos J. Milios*,† †

Department of Chemistry, University of Crete, Voutes 71003, Heraklion, Greece Faculty of Chemistry, University of Wrocław, Joliot-Curie 14, Wrocław 50-383, Poland § Department of Chemistry, University of Ioannina, 45110 Ioannina, Greece ∥ Department of Chemistry, University of Patras, 26504 Patras, Greece ‡

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S Supporting Information *

ABSTRACT: The 1:1 reaction of trans-[ReIIICl3(PPh3)2(MeCN)] with 2-(βnaphthalideneamino)-2-hydroxymethyl-1-propanol, H3L, in toluene gave the dinuclear complex [ReIII2Cl4(HL)(PPh3)]·2C7H8 (1·2C7H8), while the 1:2 reaction led to the formation of complex [ReIVCl2(HL)(PPh3)] (2). In both species, the Schiff-base ligand exists in its doubly deprotonated form, HL2−, forming chelate rings around the metallic centers. In addition, 1·2C7H8 displays a unique triple metal-to-metal bond between the two trivalent rhenium ions separated at a 2.229(1) Å bond distance, while in complex [ReIVCl2(HL)(PPh3)] (2) the two aromatic ligands, HL2− and PPh3, occupy axial positions, with the terminal Cl− ions in the trans position. Investigation of the magnetic properties revealed a Curie paramagnetic behavior (S = 1/2) with a pronounced temperature independent paramagnetism (TIP) for 1·2C7H8 and 2. Both the geometry and the electronic structure of both compounds have been studied by means of density functional theory (DFT) calculations, confirming the triplet and doublet spin ground state of the complexes and furthermore establishing an electron-rich σ2π4δ1δ*1 bond order of 3 for 1·2C7H8. In addition, the absorption spectrum of 1·2C7H8 in CH2Cl2 was simulated by means of DFT calculations and is in excellent agreement with both the crystallographic and theoretical studies. Complex 1·2C7H8 is the first dinuclear rhenium complex with a triple metal− metal bond between trivalent rhenium centers.

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INTRODUCTION

reduced by one electron to deliver Re(II) and Re(III) metal sites. As in the previous case, most dinuclear rhenium complexes containing a triple metal−metal bond form upon chemical modification of the [Re2Cl8]2− and [Re2Br8]2− precursors. The quadruple metal bonded Re26+ units may often undergo (i) substitution reactions, in which ligands are exchanged, while the metal core is retained, (ii) redox reactions, in which the bond order between the rhenium atoms is decreased, and (iii) total breakdown of the metal−metal bond.4 Although there are more than 100 Cambridge Structural Database (CSD)5 records of dinuclear rhenium complexes with bond orders of 3−3.5, only a handful of them originate from a mononuclear rhenium source.6 As we continue our efforts to explore and expand the coordination chemistry of the H3L ligand (Scheme 1), we herein report our initial results upon employment of the 2-(β-

The ground-breaking discovery of the quadruple metal-tometal bond in salts of [M+]2[Re2Cl8]2− and [M+]2[Re2Br8]2− (M+ = various cations) in 1964 was a landmark for the synthesis and study of coordination compounds featuring multiple metal−metal bonds.1 Since then, the versatile {ReIII2X8}2− (X = Cl, Br) core has been extensively used as a scaffold for the preparation of numerous rhenium species,2 either as analogues of the parent compound or as complexes with smaller bond orders within the metallic core, as in the case of [Re2Cl5(DTH)2] (DTH = 2,5-dithiahexane), which is the first rhenium compound reported to feature a triple Re≡Re bond within the {ReIIIReII} metallic core.3 The latter asymmetric complex adopts a staggered conformation and an σ2π4δ2δ* electronic configuration, with the Re−Re bond slightly elongated by 0.04 Å, while the parent molecule [ReIII2Cl8]2− exhibits an σ2π4δ2 electronic configuration and an eclipsed conformation, with the two metals separated at a ∼2.25 Å distance. Furthermore, the initial Re26+ core is now © XXXX American Chemical Society

Received: March 29, 2019

A

DOI: 10.1021/acs.inorgchem.9b00886 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

[ReIVCl2(HL)(PPh3)] (2). Complex 2 was prepared in an manner analogous to 1·2C7H8 using trans-[ReIIICl3(PPh3)2(MeCN)] (257.46 mg, 0.3 mmol) and excess H3L ligand (155.58 mg, 0.6 mmol). Brown crystals suitable for X-ray diffraction were formed after 8 days. Yield (based on Re) ∼25%. Elemental anal. calcd. (found) for C32H30Cl2NO3PRe: C 50.26 (50.73), H 3.95 (3.66), N 1.83 (1.32)%. IR bands (cm−1): 422w, 448w, 459w, 497m, 509m, 522s, 541sh, 561w, 580w, 595w, 603w, 618w, 639sh, 693s, 722sh, 743s, 778w, 819m, 848w, 863w, 987m, 998w, 1008w, 1027sh, 1053w, 1094m, 1142w, 1236sh, 1279sh, 1333m, 1411w, 1436sh, 1511w, 1544m, 1595m, 1615w, 1727w, 3050w, 3524w. FIR bands (cm−1): 136w, 207w, 242m, 298brd, 353s. Physical Methods. Elemental Analyses (C, H, N) was performed by the University of Ioannina microanalysis service. Solid state magnetic measurements were carried out on a Quantum design SQUID MPMS-XL magnetometer equipped with a 5 T DC magnet at the University of Wroclaw. Diamagnetic corrections were applied to the observed paramagnetic susceptibilities using Pascal’s constants. Both IR and far-IR spectra were recorded on a Bruker Vartex 70 FTIR spectrometer as nujol mulls. X-ray Crystallography and Structure Solution. Diffraction data for 1·2C7H8 were collected at 80 K on an Xcalibur PX diffractometer and for 2 on KUMA KM4 diffractometer at 100 K. Both structures were refined by full-matrix least-squares techniques on F2 with SHELXL.11 Full details can be found in the CIF files: CCDC 1899701 and 1899702, for 1 and 2, respectively. Crystal data for 1: C40H38Cl4NO3PRe2, M = 1125.88, a = 9.483(3) Å, b = 12.958(4) Å, c = 15.783(5) Å, α = 96.74(3)°, β = 91.82(3)°, γ = 98.31(3)°, V = 1903.5(11) Å3, T = 80(2) K, space group P1̅, 20779 reflections measured, 10671 independent reflections (Rint = 0.0386). The final R1 values were 0.0367 (I > 2σ(I)). The final wR(F2) values were 0.0641 (I > 2σ(I)). The final R1 values were 0.0571 (all data). The final wR(F2) values were 0.0708 (all data). Crystal data for 2: C33H30Cl2NO3PRe, M = 776.65, a = 19.938(8) Å, b = 8.943(2) Å, c = 17.518(5) Å, β = 108.01(4)° V = 2970.5(17) Å3, T = 100(2) K, space group P21/c, 21120 reflections measured, 5790 independent reflections (Rint = 0.0841). The final R1 values were 0.0475 (I > 2σ(I)). The final wR(F2) values were 0.1069 (I > 2σ(I)). The final R1 values were 0.0707 (all data). The final wR(F2) values were 0.1174 (all data).

Scheme 1. Structure of H3L and Its Coordination Mode in Complexes 1·2C7H8 and 2

naphthalideneamino)-2-hydroxymethyl-1-propanol ligand in rhenium coordination chemistry, since according to our previous experience, this versatile ligand can act as both a bridging and chelate ligand to form high nuclearity 3d and mixed metal 3d-4f complexes.7,8 We herein report the synthesis, characterization, and theoretical studies of two Re clusters: [Re III 2 Cl 4 (HL)(PPh 3 )]·2C 7 H 8 (1·2C 7 H 8 ) and [ReIVCl2(HL)(PPh3)] (2). To the best of our knowledge complex 1·2C7H8 is the first example of a dirhenium complex with two triply bonded ReIII sites.

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EXPERIMENTAL SECTION

All manipulations were performed under nitrogen atmosphere using standard Schlenk techniques. All reagents were purchased from Sigma-Aldrich. Solvents were purified by distillation over CaH2 and under N2, before use. Complex trans-[ReIIICl3(PPh3)2(MeCN)] was synthesized according to the procedure published by Rouschias et al.9 The H3L ligand was prepared according to the modified literature method.10 [ReIII2Cl4(HL)(PPh3)]·2C7H8 (1·2C7H8). H3L (77.79 mg, 0.3 mmol) was transferred to a stirred solution of toluene (10 mL) in a round-bottom flask connected to a Schlenk line and flushed with N2. trans-[ReIIICl3(PPh3)2(MeCN)] (257.46 mg, 0.3 mmol) was subsequently added, and the mixture was intensively stirred and refluxed for 2 h. The solution was then filtered under vacuum and left for slow evaporation under N2 atmosphere at room temperature. Xray quality crystals of 1·2C7H8 formed within 1 week. Yield (based on Re) ∼40%. Data presented below were collected after solvent molecules were removed from the sample under vacuum. Elemental Anal. Calcd (found) for C32H30Cl4NO3PRe2: C 37.61 (37.94), H 2.96 (2.78), N 1.37 (1.50)%. IR bands (cm−1): 424m, 448sh, 510w, 523m, 559m, 580w, 594w, 639sh, 693sh, 706w, 744s, 778m, 820s, 864w, 902w, 951w, 988s, 1010s, 1066m, 1095s, 1142m, 1194w, 1237s, 1280s, 1333m, 1436w, 1512w, 1539s, 1578w, 1595s, 1616s, 3059w, 3526w. FIR bands (cm−1): 137w, 207w, 242m, 299brd, 353brd, 391w.

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RESULTS AND DISCUSSION Syntheses. Refluxing a mixture of trans-[ReIIICl3(PPh3)2(MeCN)] and H3L in a 1:1 ratio, in toluene, gave a dark brown solution and a small amount of precipitate. The precipitate was removed upon filtration under vacuum, and

Figure 1. Molecular structures of molecules 1 (left) and 2 (right). The distorted octahedral geometry of Re(IV) in complex 2 is presented by dashed lines. Solvent molecules and H atoms are omitted for clarity. Color code: ReIII = light-blue, ReIV = dark blue, Cl = green, O = red, N = blue, P = orange and C = light-gray. B


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Inorganic Chemistry slow evaporation of the filtrate under N2 atmosphere yielded complex [ReIII2Cl4(HL)(PPh3)]·2C7H8 (1·2C7H8), in the form of brown block-shaped X-ray quality crystals. The triol Schiff-base ligand exists in its doubly deprotonated form, HL2−, and as such our next step was to perform the reaction in the presence of base, NEt3, but our attempts proved fruitless, since we were not able to isolate any crystalline material. Furthermore, compound 1·2C7H8 presents a 2:1 metal/ligand product, despite the initially employed 1:1 ratio. Therefore, we increased the amount of the ligand employed and repeated the reaction; the solution formed under reflux conditions remained precipitate-free, until it cooled down to room temperature and a brown solid was formed. The solid was recrystallized in CHCl3/MeCN (1:1 v/v) and gave a few brown block crystals of complex 1·2C7H8 and needle-like crystals of complex [ReIVCl2(HL)(PPh3)] (2), as was established by single crystal X-ray measurements. The remaining filtrate was left upon standing to slowly evaporate, and after 8 days only needle-like crystals of 2 were formed in ∼25% yield. Description of Structures. The molecular structures of complexes [ReIII2Cl4(HL)(PPh3)]·2C7H8 (1·2C7H8) and [ReIVCl2(HL)(PPh3)] (2) are presented in Figure 1, while representative bond lengths and angles are given in Tables S1 and S2. Compound 1·2C7H8 crystallizes in the triclinic P1̅ space group, with disordered toluene solvate molecules. The [ReIII2Cl4(HL)(PPh3)]·2C7H8 (1·2C7H8) cluster consists of two units, [ReIIICl(HL)] (unit 1, containing Re2) and [ReIIICl3PPh3] (unit 2, containing Re1), that are held together via a metallic Re−Re bond, at a distance of 2.229(1) Å, typical for dirhenium species with a bond order of 3.12−14 The doubly deprotonated Schiff-base ligand in unit 1 coordinates in a κ1O:κ1N:κ1O’ mode, forming a five-member and a six-member chelate ring around Re2, while one of its hydroxyl groups remain uncoordinated. Finally, a terminal Cl− ion completes the coordination sphere of Re2. On the other hand, Re1 is found coordinated by three chloro anions and a neutral PPh3 group. The strong trans influence of the PPh3 ligand results in an elongated Re−Cl bond for Cl3 (2.356(2) Å) compared to the rest of the halogen ligands in the structure. Furthermore, complex 1 adopts a near eclipsed rotomeric configuration (Figure 2, left) with torsion angles in the −13.7−0.91° range

allow for a ligand rotation around the axis of the Re−Re bond, and thus the staggered conformation is not favored. In addition, the large value of the Cl2−Re1−Re2−O2A torsion angle may be further rationalized if we consider the coordination environment at the two metallic centers of complex 1·2C7H8; the formation of two chelate rings on Re2 forces the three coordination sites of the doubly deprotonated ligand, HL2−, in a “tight” arrangement around the metal center (∠O1A−Re2−N1A = 84.91(14)° and ∠N1A−Re2−O2A = 80.25(14)°), while the angles between the two hydroxo sites and the Cl3 with the Re ion are ∠O1A−Re2−Cl3 = 88.34(11)° and ∠Cl3−Re2−O2A = 89.27(11)°. For Re1, the ∠L−M−L′ angles (Figure 2, right) are closer to 90° (average deviation by ∼3.25°), as Cl2 and Cl4 ligands are “pushed” further away from the bulky PPh3 group (Tolman cone angle ∼145°).15 For 1·2C7H8, the ∠L−M−L′ and ∠L− M−M angles, as well as the L−M−M−L′ torsion angles, are summarized in Table 1. Table 1. Bond Angles ∠L−M−L′, ∠L−M−M, and Torsion Angles L−M−M−L′ for Molecule 1 bond angles (°) ∠L−M−L′ Cl1−Re2−O1A = 88.34(11) O1A−Re2−N1A = 84.91(14) N1A−Re2−O2A = 80.25(14) O2A−Re2−Cl1 = 89.27(11) Cl2−Re1−Cl3 = 88.10(5) Cl3−Re1−Cl4 = 87.99(5) P11−Re1−Cl2 = 86.83(5) Cl4−Re1−P11 = 84.58(5)

∠L−M−M Cl1−Re2−Re1 = 117.12(4) O1A−Re2−Re1 = 103.55(9) O2A−Re2−Re1 = 102.83(10) N1A−Re2−Re1 = 99.32(11) Cl4−Re1−Re2 = 105.50(4) Cl3−Re1−Re2 = 104.99(4) Cl2−Re1−Re2 = 102.75(4) P11−Re1−Re2 = 100.72(4)

torsion angles (°) L−M−M−L′ Cl2−Re1−Re2−O2A = −13.7(1) P11−Re1−Re2−Cl1 = −6.95(6) Cl3−Re1−Re2−N1A = −4.22(11) Cl4−Re1−Re2−O1A = 0.91(11)

The crystal packing of complex 1·2C7H8 (Figure 3) reveals intermolecular O−H···H and C−H···Cl interactions between the noncoordinated hydroxyl group of the HL2− ligand and the toluene solvate molecules. More specifically, complex molecules are hydrogen-bonded via the free hydroxyl groups of the triol ligand [Ο3Α−Η3Α···Ο2Α (2 − x, 1 − y, 1 − z) 2.13 Å; O3A···O2A 2.955(5) Å; ∠O3A-H3A···O2A 168°] to form dimers. Layers of dimers on the ac plane of the unit cell are arranged parallel to each other with respect to the b axis via intermolecular hydrogen bonds between the toluene molecules and the coordinated Cl3 ligands [C3F−H3F Cl3(1 − x, −y, −z)···2.79 Å; C3F···Cl3 3.595(6) Å; ∠C3F−H3F···Cl3 144°; C6F−H6F···Cl3 2.86 Å; C6F···Cl3 3.795(6) Å, ∠C6F−H6F··· Cl3 169°]. The crystal lattice is further supported by T-shaped (∼3.352 Å) and parallel offset (∼3.389 Å) π−π interactions between the solvent molecules and the aromatic rings of the HL2− and PPh3 ligands. Complex [ReIVCl2(HL)(PPh3)] (2) crystallizes in the monoclinic P21/c space group with one crystallographic independent molecule in the asymmetric unit (Figure 1, right). The coordination environment of the metallic center consists of two chloro atoms in trans positions, a tridentate HL2− ligand and a terminal triphenylphosphine group, with the Re(IV) ion located at the center of a distorted octahedron.

Figure 2. (Left) Near eclipsed rotomeric geometry of molecule 1 and (right) schematic presentation of ∠L−M−L′ and ∠L−M−M angles (see text). Color code: Cl− = green, O = red, N = blue, P = orange and C = light-gray.

(Cl2−Re1−Re2−O2A: −13.7(1)°, P11−Re1−Re2−Cl1: −6.95(6)°, Cl3−Re1−Re2−N1A: −4.22(11)°, and Cl4− Re1−Re2−O1A: 0.91(11)o). The deviation from the perfect eclipsed geometry is most probably due to the chelate coordination mode of the Schiff-base ligand and the steric requirements enforced by the two bulky aromatic ligands, HL2− and PPh3. Normally, a severe torsion angle of 13.65° accounts for a possible staggered conformation. The steric crowding introduced by the two aromatic ligands does not C


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Inorganic Chemistry

Figure 3. Dimers of molecules 1·2C7H8 are hydrogen-bonded (red dashed lines) by the noncoordinated hydroxyl groups of HL2− ligands and form layers of dimers on the ac plane of the unit cell through hydrogen bonds from the solvent molecules of the lattice (see text). Color code: ReIII = light-blue, Cl− = green, O = red, N = blue, P = orange and C = black for toluene and light-gray for complex 1·2C7H8.

Figure 4. Hydrogen bonding (red dashed lines) and π−π stacking (green dashed lines) in the crystal lattice of complex 2 (see text).

Figure 5. χMT vs T plots for crystals 1·2C7H8 (left) and 2 (right) under an applied dc field of 1000 G, and M vs H/T plots for both complexes (insets); the solid lines represent fit of the data (see text for details).

smaller than 90° (∠O1A−Re1−N1A = 88.4(2)°; ∠N1A− Re1−O2A = 81.6(2)°), but slightly larger than the corresponding values of molecule 1, and this may be attributed to the higher oxidation state of the metallic center in 2. The alkoxo oxygens at complex 2 are closer to the tetravalent rhenium atom by an average of 0.054 Å compared to 1, while

Again, as in the case of molecule 1, the chelate coordination mode of the triol ligand and the steric effect introduced by the bulky PPh3 group dictate the geometry of complex 2. The doubly deprotonated ligand is found again in a κ1O:κ1N:κ1O′ coordination mode as in 1, with one noncoordinated hydroxyl group, forming two chelate rings. The binding angles are again D


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Inorganic Chemistry

Figure 6. Equilibrium geometries of 1 (S = 1) and 2 (S = 1/2) ground states with selected structural parameters along with the natural atomic charges and the 3D plots of the spin density distribution (isospin surfaces = 0.002 au) calculated by the PBE0/Def2-TZVP(Re) 6-31G(d,p)(E) computational protocol.

uncoordinated hydroxyl oxygen and the aromatic hydrogen at the outer side of the naphthalene ring. Magnetochemistry. The magnetic susceptibility measurements of 1·2C7H8 are shown in Figure 5 (left), revealing a Curie paramagnetic behavior (S = 1/2) with a pronounced temperature independent paramagnetic (TIP) contribution. A fitting of χM = χTIP + C/T leads to a value of χTIP = 6.0 × 10−3 cm3 mol−1 and a Curie constant C = 0.30 cm3 mol−1 K. Such a large TIP value is not unusual for a Re ion because of the large orbital contribution to its magnetic moment.16,17 Furthermore, the magnetization measurements of 1·2C7H8 (Figure 5, left inset) confirmed this paramagnetic behavior since the theoretical Brillouin function of an S = 1/2 system with an effective g = 1.8818 and the experimental magnetization

the Re−N bond is elongated by 0.041 Å. Furthermore, the axial coordination of the PPh3 group “pushes” the angles between the P atom and the ligand sites at the base of the octahedron over 90° (average deviation ∼3.14°). In the crystal packing (Figure 4), HL2− and PPh3 ligands of two neighboring molecules are linked by an extensive offset π−π stacking (∼3.34 Å and ∼3.76 Å for naphthalene and phenyl rings respectively) to form a chain of molecules running along the a axis, while inversion centers that lie in the mid-distance between the interacting rings result in zigzag-shaped chains. The molecular chains are stacked in a parallel corrugated manner along the c axis (due to the 2-fold screw axis at the P21/c symmetry) via C−H···O interactions between the E


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Inorganic Chemistry

changes are observed in the excited singlet 1(S = 0) state (Figure S5). Exception is the Re−Re bond length where a striking structural change is observed upon going from the S = 1 to the S = 0 excited state. The estimated distance of 2.197 Å is shorter by 0.055 Å than that calculated for the S = 1 ground state (2.252 Å), while it is shorter by 0.032 Å compared to the respective experimentally determined value. The Re−Cl bond lengths are found in the range of 2.310−2.340 Å, while the Re−O bond lengths are 1.921 and 1.976 Å. The estimated Re−N and Re−P bond lengths are 2.067 and 2.485 Å. In order to analyze in depth the Re−Re multiple bond in the dinuclear Re(III), 1 (S = 1) complex, we employed the natural bond orbital (NBO) population analysis method.27,28 The nature and composition of the Re−Re NBOs for 1 (S = 1) and 1 (S = 0) are compiled in Tables 2 and Table S3 respectively.

curves at various temperatures are superimposable. The magnetic susceptibility measurements of complex 2 (Figure 5, right) suggest again a Curie paramagnetic behavior (S = 1/ 2) with a pronounced TIP contribution with a value of χTIP = 2.7 × 10−3 cm3 mol−1 and a Curie constant C = 0.27 cm3 mol−1 K (g = 1.63).18 The ground state of a six-coordinate Re(IV) with three unpaired electrons, 4A2g, under an intermediate coupling scheme is removed by the action of spin−orbit interaction and the tetragonal crystal field (CF) splitting leading to two Kramers doublets (±1/2, ± 3/2) separated by an energy gap 2D.19 Although in the majority of Re(IV) complexes there is a depopulation of the ground state in the 2−300 K temperature range, this is not our case since the ground state is well isolated from the excited S = 3/2 in the whole temperature range, probably due to a significantly large D value. The magnetization curve is fitted as well with the same model and the same parameters (Figure 5, right inset). Electronic Structure and Bonding Probed by DFT Studies. The electronic structure and bonding features of 1 and 2 were calculated employing DFT computational approaches as implemented in the Gaussian09, version D.01 program suite.20 The geometries of the complexes were fully optimized, without symmetry constraints, employing the 1999 hybrid functional of Perdew, Burke, and Ernzerhof21−23 denoted as PBE0 which mixes the Perdew−Burke-Ernzerhof (PBE) exchange energy and Hartree−Fock exchange energy in a set 3 to 1. Geometry optimization of the complexes was done using the Def2-TZVP basis set24 for Re and the 6-31G(d,p) basis set for all main group elements (E) employing the molecular structures determined by X-ray crystallography as input structures. Hereafter, the computational protocol used in DFT calculations is abbreviated as PBE0/Def2-TZVP(Re) 631G(d,p)(E). To achieve chemical accuracy for the thermodynamics of the complexes in the case of the 2 (S = 1/2) and 2 (S = 3/2) states which exhibit multiconfigurational character, single point energy calculations were performed employing the mPW2-PLYP functional which includes a second-order perturbation correction for nonlocal correlation effects,25 as well as at the ab initio MP2 level of theory.26 The equilibrium geometries of 1 and 2 in their triplet 1 (S = 1) and doublet 2 (S = 1/2) ground states with selected structural parameters along with the natural atomic charges and the 3D plots of the spin density distribution (isospin surfaces = 0.002 au) are given in Figure 6. The equilibrium geometries of the excited singlet 1 (S = 0) and quartet 2 (S = 3/2) states with selected structural parameters and natural atomic charges are given in the Supporting Information (Figure S5). Complex 1 adopts the triplet 1 (S = 1) as the ground state, with the singlet 1(S = 0) excited state located on the potential energy surface (PES) 5.8 and 18.5 kcal/mol higher in energy at the PBE0/Def2-TZVP(Re) 6-31G(d,p)(E) and MP2/Def2TZVP(Re) 6-31G(d,p)(E) level of theory, respectively. The main experimental bond lengths (in parentheses) for the triplet ground state 1 (S = 1) are well-reproduced by the computational approach; e.g., the Re−Re bond distance is computed to be 2.252 Å (2.229(1) Å), and the Re−Cl bond lengths are found in the range 2.316−2.335 Å (2.307(2)− 2.356(2) Å). The Re−O bond lengths in the 1 (S = 1) state are 1.929 and 1.992 Å, while the experiment gives the values of 1.961(3) and 1.983(3) Å. The estimated Re−N and Re−P bond distances are 2.051 and 2.500 Å respectively (exptl. = 2.043(4) and 2.487(2) Å). Overall, only marginal structural

Table 2. Natural Bond Orbital (NBO) Analysis of the Re− Re Bond for the Ground States of Binuclear Complex 1 (S = 1), Calculated at the PBE0/Def2-TZVP(Re) 6-31G(d,p)(E) Level of Theory in a Vacuum composition of NHOs NHO1

NHO2

(Re−Re) NBOs

occupancy

6s%

6p%

5d%

6s%

6p%

5d%

σα πα πα π*α π*α σ*α σβ πβ δα π*β π*β σ*β

0.94964 0.93614 0.89808 0.17976 0.17199 0.16562 0.93717 0.92944 0.89540 0.14444 0.14432 0.15171

19.3

4.8 16.3 4.7

75.7 83.5 92.8

11.2

2.9 3.7 22.5

85.8 95.9 76.7

4.6 11.9 5.1

75.5 87.6 92.7

21.9

2.8 6.3 22.3

75.1 91.7 77.5

2.4

19.7 2.5

The 3D plots of the bonding and antibonding natural bond orbitals (NBOs) along with the molecular orbital energy level diagram for 1 (S = 1) and 1 (S = 0) are shown in Scheme 2 and in the Supporting Information (Scheme S1) respectively. According to the NBO analysis of the Re−Re bond in 1 (S = 1), the estimated effective bond order (EBO), the Mayer bond order (MBO), and the Wiberg bond order (WBO) are 2.296, 2.182, and 2.156 respectively. All these bond order indices having noninteger values are lower29 than the bond order expected from restricted molecular orbital theory, which predicts a σ21π22π2δ1δ*1 electron configuration for 1 (S = 1) (Scheme 2b) that gives a formal bond order (FBO) of 3, characteristic for a triple Re≡Re bond. The occupancy of the σ(Re−Re), π(Re−Re), and δ(Re−Re) ΝΒΟs (Table 2) corresponds also to a σ21π22π2δ1δ*1 electron configuration for 1 (S = 1). The EBO obtained using the natural orbital occupation numbers for the σ(Re−Re), σ*(Re−Re), π(Re− Re), and π*(Re−Re) NBOs shows 0.786 σ contribution and 1.509 π contribution to the ΕΒΟ value. The σ(Re−Re) NBO arises from the overlap of spd hybrids of the interacting Re atoms, while the two π(Re−Re) NBOs arise from the overlap of pd hybrids of the interacting Re atoms (Table 2). For the singlet 1 (S = 0) state the EBO, MBO, and WBO values of 2.892, 2.667, and 2.591 respectively are lower than the bond order expected from molecular orbital theory, which F


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Inorganic Chemistry

Scheme 2. Three-Dimensional Plots of the Bonding and Antibonding Natural Bond Orbitals (NBOs) (a) along with the Molecular Orbital Energy Level Diagram for 1 (S = 1) (b)

predicts a σ21π22π2δ2 electron configuration for 1 (S = 0) (Scheme S1) that gives a formal bond order (FBO) of 4. characteristics for a quadruple Re≡Re bond. The occupancy of the σ(Re−Re), π(Re−Re), and δ(Re−Re) ΝΒΟs (Table S3) corresponds also to a σ21π22π2δ2 electron configuration characteristic for a quadruple Re−Re bond. Τhe σ(Re−Re), σ*(Re−Re), π(Re−Re), π*(Re−Re), δ(Re−Re), and δ*(Re− Re) NBOs show 0.639 σ, 1.464 π, and 0.789 δ contribution to the ΕΒΟ value of 2.892. The σ(Re−Re) NBO arises from the overlap of spd hybrids, the π(Re−Re) NBOs arise from the overlap of pd hybrids, and the δ(Re−Re) NBO arises from the overlap of 5d atomic orbitals of the interacting Re atoms (Table S3). For complex 2, the quartet 2 (S = 3/2) is predicted to be the ground state with the doublet 2 (S = 1/2) state computed to be 3.7 kcal/mol higher in energy at the PBE0/Def2-TZVP(Re) 6-31G(d,p)(E) level of theory in the gas phase. However, at the MP2/Def2-TZVP(Re) 6-31G(d,p)(E) level of theory, the doublet 2(S = 1/2) is predicted to be the ground state with the quartet 2 (S = 3/2) state computed to be 0.9 kcal/mol higher in energy. Noteworthy, the computed structure for the doublet 2 (S = 1/2) state matches the crystal structure of 2 better than the computed structure for the quartet 2 (S = 3/2) state (Figures 6 and S1). In effect, the main experimental bond lengths (in parentheses) for the doublet 2 (S = 1/2) are wellreproduced by the computational approach; e.g., the Re−Cl bond distances are computed to be 2.376 and 2.405 Å (2.367(2), 2.399(2) Å), while the Re−Cl bond distances of the quartet 2 (S = 3/2) computed to be 2.349 and 2.375 Å deviate significantly from the experimental values. The same holds true

for the computed Re−O bond lengths for 2 (S = 1/2) and 2 (S = 3/2) states which are 1.898, 1.926, and 1.963, 2.000 Å, respectively, while the experiment gives the values of 1.895(5) and 1.941(5) Å. Similarly the estimated Re−N and Re−P bond distances for 2 (S = 1/2) and 2 (S = 3/2) states are 2.066, 2.067, and 2.456, 2.526 Å respectively (exptl. = 2.087(6) and 2.446(2) Å). It can be concluded that the doublet 2 (S = 1/2) should be the ground state in the solid state in line with the magnetochemistry findings. TD-DFT Simulation of the Absorption Spectrum of 1. The absorption spectrum of 1 calculated by TD-DFT approaches30−32 at the PBE0/Def2-TZVP(Re) 6-31G(d,p)(E)/PCM level of theory in CH2Cl2 is depicted in Figure 7a. The simulated absorption spectrum of 1 reproduces very well the experimentally obtained absorption spectrum in CH2Cl2 solution. Accordingly, the simulated absorption spectrum exhibits two very intense high energy bands at 331 and 378 nm in excellent agreement with those found experimentally at 325 and 383 nm. These absorption bands might arise from the δ* → π* and δ* → σ* excitations (Scheme 2) since the vertical δ* → π* and δ* → σ* excitations are calculated to be 392, 318, and 310 nm, respectively. Three extremely weak bands of lower energy absorb at 530, 601, and 853 nm in line with the experimental bands absorbing at 511, 602, and 865 nm. The highest energy intense bands absorbing at 331 and 378 nm arise from a multitude of electronic transitions. In contrast, the weak bands absorbing at 530, 601, and 853 nm arise from a single electronic transition. The electronic transitions comprising the bands in the absorption spectrum of 1 in CH2Cl2 solution arise from heavily mixed linear G


Article

Inorganic Chemistry

Figure 7. Simulated absorption spectrum of 1 in CH2Cl2 solution (a); 3D plots of MOs involved in the most intense electronic transitions (b) and 3D plots of “hole” and “particle” pair NTOs involved in the most intense electronic transitions (c).

MO analysis, though a charge transfer from the HL ligand to the metal centers (LMCT) could also be observed based on the locations of the “holes” and “particles” after the excitation. Overall, the band at 331 nm exhibits a mixed MC/LC/LMCT character based on either the MO or the NTO analysis (practically arise from the δ* → π* and δ* → σ* electronic transitions). Next, both NTO pairs related to the most intense electronic transition at 375 nm (Figure 7c) are spatially confined indicating again a mixed MC/LC character. The same holds true for the electronic transition absorbing at 383 nm that could be assigned as having a mixed MC/LC character (specifically are due to δ* → π* electronic transitions). Generally, it should be stressed that the MC character of both the main absorption bands arising from the δ* → π* and δ* → σ* excitations (Scheme 2) is related to the Re≡Re triple bond of 1 (S = 1).

combinations of many excitations between occupied and unoccupied MOs. Selected principal triplet− triplet electronic transitions are given in the Supporting Information (Table S4). Figure 7b shows the excitations between the MOs giving rise to the most intense electronic transition among those comprising the bands peaking at 331 and 378 nm. On the basis of the 3D plots of the occupied and unoccupied MOs (Figure 7b), the most intense electronic transitions could be assigned as mixed metal centered and ligand centered MC/ LC transitions. In order to simplify the assignment of the electronic transitions, we performed natural transition orbital (NTO) calculations33 that provide a simpler interpretation of the electronic transitions, based on only a few NTO pairs instead of using a high number of MOs involved in these transitions. The dominant excitation NTO pairs accounting for nearly or over 90% of the electronic transitions are shown in Figure 7c and Figure S7, Supporting Information. According to the NTO’s pictures, the most intense electronic transition at 331 nm could be assigned as mixed MC/LC in line with the H


Article

Inorganic Chemistry Table 3. Previously Characterized Complexes from the Use of H3L Ligand ref 7a

7b

8a

8b

8c

formula

ligand form

[MnIII6LnIII2O2(OH)2(H2O)2(HL)4(L)2(NO3)6] (Ln = Gd,Tb,Dy,Er) [MnIII6YIII2O2(OH)2(H2O)2(HL)6(NO3)6] [MnIII6LnIII6(OH)7(H2O)3(O2CPh)11(L)3(HL)4(NO3)] (Ln = Gd, Dy)

HL L3−

HL2−

HL2− L3−

[DyIII6ZnII4O2(L)2(HL)2(OAc)8(CH3O)4(H2O)2]·4MeOH

CONCLUSIONS In conclusion, in this work we have reported the syntheses, structures, magnetic properties, and electronic studies of two new rhenium complexes, upon employment of the 2-(βnaphthalideneamino)-2-hydroxymethyl-1-propanol, H3L, ligand in Re chemistry. Despite the fact that previous employment of this ligand in 3d-4f chemistry led to the synthesis of relatively large clusters with nuclearities ranging from 8 to 12 (Table 3), in 5d chemistry this trend does not seem to be valid, as so far we were only able to isolate much smaller complexes. However, employment of the H3L ligand in Re chemistry has led to the isolation of the first dinuclear rhenium complex with a triple metal−metal bond between trivalent rhenium centers, complex 1. These findings suggest that the employment of the H3L ligand in the “exotic” 5d chemistry can indeed lead to very interesting and unprecedented compounds.

HL2− L3−

coord mode μ-κ2O:κ1N:κ1O′ μ4-κ2O:κ2O’:κ1O″ μ-κ2O:κ1N:κ1O′ μ4-κ3O:κ2O’:κ1N:κ1O″ μ3-κ2O:κ2O’:κ1N:κ1O″ μ3-κ2O:κ1O’:κ1N:κ1O″ μ3-κ3O:κ1O’:κ1N:κ1O″

μ-κ2O:κ1N:κ1O″ μ5-κ3O:κ3O’:κ1N:κ1O″ μ4-κ3O:κ2O’:κ1N:κ1O″ μ3-κ3O:κ1O’:κ1N:κ1O″ μ4-κ3O:κ3O’:κ1N:κ1O″

Vassilis Tangoulis: 0000-0002-2039-2182 Constantinos J. Milios: 0000-0002-1970-6295 Notes

The authors declare no competing financial interest.

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REFERENCES

(1) Cotton, F. A.; Curtis, N. F.; Harris, C. B.; Johnson, B. F. G.; Lippard, S. G.; Mague, J. T.; Robinson, W. R.; Wood, J. S. Mononuclear and Polynuclear Chemistry of Rhenium (III): Its Pronounced Homophilicity. Science 1964, 145, 1305−1307. (2) Cotton, F. A.; Murillo, C. A.; Walton, R. A. Multiple Bonds Between Metal Atoms; Springer: New York, 2005. (3) (a) Bennett, M. J.; Cotton, F. A.; Walton, R. A. A Rhenium-toRhenium Triple Bond. J. Am. Chem. Soc. 1966, 88, 3866−3867. (b) Cotton, F. A.; Oldham, C.; Walton, R. A. Some reactions of the octahalodirhenate(III) ions. III. The stability of the rhenium-rhenium bond toward oxygen and sulfur donors. Inorg. Chem. 1967, 6, 214− 223. (c) Bennett, M. J.; Cotton, F. A.; Walton, R. A. A structural and magnetic study of pentachloro-bis(1,5-dithiahexane)dirhenium. Proc. R. Soc. 1968, A303, 175−192. (4) Poineau, F.; Sattelberger, A. P.; Lu, E.; Liddle, S. T. Molecular Metal-Metal Bonds: Compounds, Synthesis, Properties, 1st ed.; Liddle, S. T., Ed.; Wiley-VCH: Germany, 2015; Chapter 7, p 202. (5) Groom, C. R.; Bruno, I. J.; Lightfoot, M. P.; Ward, S. C. The Cambridge Structural Database. Acta Crystallogr., Sect. B: Struct. Sci., Cryst. Eng. Mater. 2016, B72, 171−179. (6) (a) Sugimoto, H.; Kamei, M.; Umakoshi, K.; Sasaki, Y.; Suzuki, M. Preparation, Structure, and Properties of Bis(μ-oxo)dirhenium(III,IV) and -dirhenium(IV) Complexes of Tris(2-pyridylmethyl)amine and Its (6-Methyl-2-pyridyl)methyl Derivatives. Inorg. Chem. 1996, 35, 7082−7088. (b) Takahira, T.; Umakoshi, K.; Sasaki, Y. Diμ-oxo-bis{[tris(2-pyridylmethyl)amine-N,N’,N’’,N’’’]rhenium(IV)} tetrakis(hexafluorophosphate) diacetone tetrahydrate. Acta Crystallogr., Sect. C: Cryst. Struct. Commun. 1994, C50, 1870−1872. (c) Douthwaite, R. E.; Wolczanski, P. T.; Merschrod, E. [(silox)2ReO]2 (silox = tBu3SiO) contains a Re≡Re bond and terminal oxo ligands. Chem. Commun. 1998, 0, 2591−2592. (d) Mukiza, J.; Gerber, T. I. A.; Hosten, E.; Taherkhani, F.; Nahali, M. A (μ-O)(μ-Br)ReIV2 metal−metal triple bond complex with a bridging tridentate ligand: Synthesis, structure and DFT study. Inorg. Chem. Commun. 2014, 49, 5−7. (e) Mukiza, J.; Hosten, E. C.; Gerber, T. I. A. Rhenium(III), (IV) and (V) complexes with 6hydroxypicolinic acid. Polyhedron 2016, 110, 106−113. (7) (a) Tziotzi, T. G.; Kalofolias, D. A.; Tzimopoulos, D. I.; Siczek, M.; Lis, T.; Inglis, R.; Milios, C. J. A family of [MnIII6LnIII2] rod-like clusters. Dalton Trans. 2015, 44, 6082−6088. (b) Tziotzi, T. G.; Tzimopoulos, D. I.; Lis, T.; Inglis, R.; Milios, C. J. Dodecanuclear

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.9b00886. IR/FIR spectra, as well as pXRD diagrams for compounds 1 and 2, and theoretical calculation details (PDF) Accession Codes

CCDC 1899701−1899702 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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2−

[NiII6GdIII3(OH)6(HL)6(NO3)3] [NiII6DyIII3(OH)6(HL)6(NO3)3] [NiII6ErIII3(OH)6(HL)6(NO3)3] [CuII7LnIII2(L)4(HL)2(OAc)4] (Ln = Gd, Tb, Dy) [CuII7YIII2(L)4(HL)2(OAc)4]·2MeCN

■

■

HL2−

AUTHOR INFORMATION

Corresponding Authors

*(C.J.M.) E-mail: [email protected]. *(V.T.) E-mail: [email protected]. *(A.C.T.) E-mail: [email protected]. *(T.L.) E-mail: [email protected]. ORCID

Dimitris A. Kalofolias: 0000-0002-8636-7238 Athanassios C. Tsipis: 0000-0002-0425-2235 I


Article

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