Synthesis, spectroscopic characterization, X-ray structure and DFT calculations of rhenium(III) complex with 1-isoquinolinyl phenyl ketone

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Polyhedron 27 (2008) 1739–1746 www.elsevier.com/locate/poly

Synthesis, spectroscopic characterization, X-ray structure and DFT calculations of rhenium(III) complex with 1-isoquinolinyl phenyl ketone B. Machura a,*, R. Kruszynski b, J. Mrozin´ski c, J. Kusz d b

a Department of Crystallography, Institute of Chemistry, University of Silesia, 9th Szkolna Street, 40-006 Katowice, Poland Department of X-ray Crystallography and Crystal Chemistry, Institute of General and Ecological Chemistry, Lodz University of Technology, 116 Z´eromski Street, 90-924 Ło´dz´, Poland c Faculty of Chemistry, Wroclaw University, F. Joliot-Curie 14 Street, 50-383 Wrocław, Poland d Institute of Physics, University of Silesia, 4th Uniwersytecka Street, 40-006 Katowice, Poland

Received 17 January 2008; accepted 6 February 2008 Available online 18 March 2008

Abstract The [ReCl3(MeCN)(PPh3)] complex reacts with 1-isoquinolinyl phenyl ketone (N–O) to give [ReCl3(N–O)(PPh3)]. The compound has been studied by IR, UV–Vis spectroscopy, magnetic measurements and X-ray crystallography. The magnetic behavior is characteristic of mononuclear octahedral Re(III) complex with d4 low-spin (3T1g ground state) and arises because of the large spin–orbit coupling, which gives diamagnetic ground state. The molecular orbital diagram of [ReCl3(N–O)(PPh3)] has been calculated with the density functional theory (DFT) method, and time-dependent DFT (TD-DFT) calculations have been employed in order to discussion of its spectroscopic properties in more detail. Ó 2008 Elsevier Ltd. All rights reserved. Keywords: Rhenium(III) complexes; 1-Isoquinolinyl phenyl ketone; X-ray structure; DFT calculations; Magnetic measurements

1. Introduction Recent interest in the coordination chemistry of rhenium arises mainly from the introduction of b emitting isotopes 188Re and 186Re in radiotherapy and its similarity with technetium, whose metastable c-emitting isotope 99m Tc plays important role in diagnostic nuclear medicine. The success of the 186Re(Sn)HEDP radiopharmaceutical as a palliative of bone pain has particularly reawakened interest in the coordination chemistry of rhenium in order to

*

Corresponding author. E-mail addresses: [email protected] (B. Machura), rafal.kruszynski@ p.lodz.pl (R. Kruszynski), [email protected] (J. Mrozin´ski). 0277-5387/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2008.02.012

develop new 186/188Re radiopharmaceuticals which could be used for the treatment of cancer [1–6]. Consequently, the inorganic chemistry studies with the naturally occurring non-radioactive rhenium isotopes are very attractive, and design, synthesis and reactivity of novel rhenium complexes has become the aim of several laboratories, including ours. The [ReCl3(MeCN)(PPh3)] complex has been proven to be useful precursor in the synthesis of Re(III) compounds. It easily reacts with mono- and bidentate N-donor ligands to give [ReCl3L3], [ReCl3L2(PPh3)] and [ReCl3(L–L)(PPh3)] compounds [7,8]. Here, we present the synthesis, spectroscopic characterization, crystal and molecular structure of fac-[ReCl3(N–O)(PPh3)] (1) with 1-isoquinolinyl phenyl ketone (N– O). The binding of 1-isoquinolinyl phenyl ketone to the

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central ion occurs through the N–O chelating site. This coordination produces a five-membered ring containing an unsaturated a-iminoketo function which can act as a p acceptor towards a bound p electron-rich metal center such as rhenium(III). The a-iminoketo chelation plays an important role in the biosynthesis of the pterins, lumazines, flavins coenzymes [9,10]. Accordingly, complex 1 may be viewed as a simple model for such metalloenzymes. Furthermore, to our knowledge, fac-[ReCl3(N–O)(PPh3)] is the first structurally characterised complex with 1-isoquinolinyl phenyl ketone. The electronic structure of 1 has been determined with the density functional theory (DFT) method, and timedependent DFT (TD-DFT) calculations have been employed in order to discussion of its spectroscopic properties in more detail. Currently, density functional theory (DFT) is commonly used to examine the electronic structure of transition metal complexes. It meets with the requirements of being accurate, easy to use and fast enough to allow the study of relatively large molecules of transition metal complexes [11]. Recent studies have also supported the TD-DFT method to be applicable for open- and closed-shell of 5d-metal complexes giving good assignment of experimental spectra [12,13]. 2. Experimental

2.3. X-ray diffraction studies A dark red crystal of 1 was mounted on a KM-4-CCD automatic diffractometer equipped with a CCD detector, and used for data collection. X-ray intensity data were collected with graphite monochromated Mo Ka radiation ˚ ) at 295(1) K. Details concerning crystal (k = 0.71073 A data and refinement are given in Table 1. Lorentz, polarization and empirical absorption correction using spherical harmonics implemented in SCALE3 ABSPACK scaling algorithm [14] were applied. The structure was solved by the Patterson method and subsequently completed by the difference Fourier recycling. All the non-hydrogen atoms were refined anisotropically using full-matrix, least-squares technique. The hydrogen atoms were treated as ‘‘riding” on their parent carbon atoms and assigned isotropic temperature factors equal 1.2 times the value of equivalent temperature factor of the parent atom. SHELXS97 [15], SHELXL97 [16] and SHELXTL [17] programs were used for all the calculations. Atomic scattering factors were those incorporated in the computer programs. 2.4. Magnetic measurement Magnetization measurements of polycrystalline samples were carried out with a Quantum Design SQUID

2.1. General procedure All the reagents used to the syntheses were commercially available and were used without further purification. The [ReCl3(MeCN)(PPh3)2] complex was prepared according to the literature method [7]. IR spectrum was recorded on a Nicolet Magna 560 spectrophotometer in the spectral range 4000–400 cm1 with the samples in the form of KBr pellets. Electronic spectrum was measured on a spectrophotometer Lab Alliance UV– VIS 8500 in the range 1000–200 nm in acetonitrile solution. Elemental analyses (C, H, N) were performed on a Perkin– Elmer CHN-2400 analyzer. 2.2. Preparation of [ReCl3 (N–O)(PPh3)] (1) [Re(MeCN)Cl3(PPh3)2] (0.5 g, 0.58 mmol) and 1-isoquinolinyl phenyl ketone (0.15 g, 0.64 mmol) in dichloromethane (40 cm3) were refluxed for 4 h. The starting material gradually dissolved and the colour of the reaction solution became brown. The volume was condensed to 10 cm3, diethyl ether (50 cm3) was added and brown microcrystalline solid was filtered. The product was washed with EtOH and cold ether, and dried in vacuo (Yield 80%). The crystals suitable for X-ray investigation were obtained by recrystallization from a mixture dichloromethane/methanol. IR (KBr; m/cm1): mCO = 1677 cm1; mCN and mC@C = 1586(m), 1572(w) and 1539(m) mCN and mC@C. Anal. Calc. for C34H26Cl3NOPRe: C, 51.82; H, 3.33; N, 1.78. Found: C, 52.15; H, 3.24; N, 1.63%.

Table 1 Crystal data and structure refinement for 1 Empirical formula Formula weight Temperature (K) ˚) Wavelength (A Crystal system Space group Unit cell dimensions

˚ 3) V (A Z Dcalc (Mg/m3) Absorption coefficient (mm1) F(0 0 0) Crystal size (mm) h Range for data collection (°) Index ranges

Reflections collected Independent reflections (Rint) Completeness to 2h Maximum and minimum transmission Data/restraints/parameters Goodness-of-fit on F2 Final R indices [I > 2r(I)] R indices (all data) Absolute structure parameter ˚ 3) Largest difference in peak and hole (e A

C34H26Cl3NOPRe 788.08 293(2) 0.71073 orthorhombic P212121 ˚ ) = 13.706(3) a (A ˚ ) = 14.172(3) b (A ˚ = 15.150(3) cA 2942.7(10) 4 1.779 4.486 1544 0.12  0.10  0.07 2.87–25.04 14 6h 6 16 16 6 k 6 16 18 6 l 6 16 18 558 5206 (0.0448) 99.8% 0.586 and 0.260 5206/0/370 0.988 R1 = 0.0269 wR2 = 0.0438 R1 = 0.0395 wR2 = 0.0451 0.004(6) 0.641 and 0.684

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magnetometer (MPMSXL-5-type) at a magnetic field of 0.5 T over the temperature range 1.8–300 K. Magnetization measurements versus magnetic field (0–5 T) were made at 2 K. Corrections are based on subtracting the sample–holder signal and contribution vD estimated from the Pascal constants [18] and equal 411  106 for complex Re(III). The effective magnetic moment was calculated from the equation, leff ¼ 2:83vcorr  T 1=2 B:M. M 2.5. Computational details GAUSSIAN03 program [19] was used in the calculations. The geometry optimizations of 1 in triplet and singlet states were carried out with the DFT method with the use of

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B3LYP functional [20,21]. The energy of the singlet state is of 12.8 kcal higher. All vibrations in the calculated vibrational spectrum of triplet state were real thus the geometry corresponds to true energy minimum. The electronic spectrum of 1 was calculated with the TDDFT method [22]. The calculations were performed by using ECP basis set on the rhenium atom, the standard 6-31 + g** basis for chlorine, phosphorus, and nitrogen, 631g* basis for carbon and 6-31G basis for hydrogen atoms. The Xe core electrons of Re were replaced by an effective core potential and DZ quality Hay and Wadt Los Alamos ECP basis set (LANL2DZ) [23] was used for the valence electrons. Additional d function with exponent a = 0.3811 and f function with exponent a = 2.033 on the rhenium atom were added.

Fig. 1. The molecular structure of 1. Displacement ellipsoids are drawn at 30% probability.

Table 2 ˚ ) and angles (°) for 1 The experimental and optimized bond lengths (A Bond lengths

Experimental

Optimized

Bond angles

Experimental

Optimized

Re(1)–O(1) Re(1)–N(1) Re(1)–P(1) Re(1)–Cl(1) Re(1)–Cl(2) Re(1)–Cl(3)

1.990(3) 2.026(3) 2.5005(14) 2.3352(15) 2.3519(13) 2.3694(15)

1.992 2.074 2.565 2.416 2.402 2.422

O(1)–Re(1)–N(1) O(1)–Re(1)–P(1) N(1)–Re(1)–P(1) O(1)–Re(1)–Cl(1) O(1)–Re(1)–Cl(2) O(1)–Re(1)–Cl(3) N(1)–Re(1)–Cl(1) N(1)–Re(1)–Cl(2) N(1)–Re(1)–Cl(3) Cl(1)–Re(1)–P(1) Cl(2)–Re(1)–P(1) Cl(3)–Re(1)–P(1) Cl(1)–Re(1)–Cl(2) Cl(1)–Re(1)–Cl(3) Cl(2)–Re(1)–Cl(3)

74.75(15) 87.92(9) 92.34(13) 169.07(12) 95.10(12) 91.96(9) 94.63(11) 169.78(11) 90.46(13) 89.94(5) 85.97(6) 177.06(6) 95.44(5) 90.72(6) 91.12(7)

75.20 93.48 96.33 171.60 90.87 89.15 96.40 166.07 87.36 87.24 84.58 175.92 97.53 90.61 92.27

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3. Results and discussion The [ReCl3(N–O)(PPh3)] (1) complex was prepared in good yield by ligand exchange reaction starting from [ReCl3(MeCN)(PPh3)] and 1-isoquinolinyl phenyl ketone (N–O): [ReCl3 (MeCN)(PPh3 )] + 1-isoquinolinyl phenyl ketone ! [ReCl3 (N–O)(PPh3 )] + PPh3 It was isolated as brown microcrystalline solid, soluble in common organic solvents. The elemental analysis of the complex is in good agreement with its formulation. A band due to the m(C@O) vibration of 1-isoquinolinyl phenyl ketone appears at 1677 cm1, and characteristic bands of the C@C and C@N stretching modes are observed in the range 1600–1500 cm1 [24].

reflects the symmetry of the chelate arrangement in such ˚ for 1, indicating that the structures, is equal to 0.036 A Re center is situated symmetrically between N(1) and O(1) donor atoms. The chloride ions of 1 are arranged in a facial fashion, and the phosphorous atom of the phosphine ligand occupies the trans position to the Cl(3) ion. Geometrical preferences of the ReCl3 unit in the [ReCl3(L–L)(PPh3)] complexes can be rationalized in terms of the electronic nature of the L–L ligand. Triphenylphosphine is a p-accepting ligand, and trivalent rhenium is prone to

3.1. Crystal structure The molecular structure of 1 is given in Fig. 1, and the selected bond distances and angles are collected in Table 2. The overall structure of 1 can be considered as a distorted octahedral with the largest deviations from the expected 90° bond angles coming from the bite angle of 1-isoquinolinyl phenyl ketone. It equals to 74.75(15)°, and it is similar to that reported for the related [Re(CO)3(6-ATML)Cl]  3C6H6 (6-ATML = 6-acetyl1,3,7-trimethyllumazine) [75.2(1)°] [10]. The bite angles O–M–N in the structures with a-iminoketo chelate ligands bounded to transition metals depend mainly on the size of the metal center, the smallest values (down to 67°) were found for AgI complexes and the largest angles (up to 85°) were observed for compounds with CuII [9]. The central ion of 1 shows slightly larger affinity towards the less basic but strongly p accepting carbonyl oxo atom, and the Re–O(1) is shorter than the Re–N(1) bond (Table 2). The value D = d(M–N)  d(M–O), which

Fig. 3. The variation of the magnetization (M) vs the magnetic field (H) at 2 K for complex 1.

3.00

2.00

1.00

0.00 200.00

Fig. 2. The effective magnetic moment (leff) vs temperature for complex 1.

400.00

600.00

800.00

1000.00

Fig. 4. The experimental (black) and calculated (red) electronic absorption spectrum of 1. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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back-bonding. When the L–L ligand is a strong p-acid, the facial geometry is preferred as it maximizes the back-bonding effect. Assuming idealized octahedral geometry the back-bonding has t2g(Re) ? r*(P–C) and t2g(Re) ? p*(L– L) components. This bonding is maximized when the competition between ligands for identical metal orbitals is minimal – what requires facially disposed p acceptor ligands [25–31]. ˚) It is significant that the Re–N distance in 1 (2.026(3) A is shorter than comparable distances for saturated amine complexes, where metal-to-ligand p-back-bonding is not possible [32]. Similarly, the interatomic distance between the rhenium atom and the oxygen atom of 1-isoquinolinyl phenyl ketone is shorter than an ideal single Re–O bond ˚ ) [33]. length (ca. 2.04 A The Re–X(3) bond length is longer in comparison with the corresponding Re–X(1) and Re–X(2) distances. Fur-

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thermore, the Re–X(2) bond lengths in trans position to the N atom of the bidentate ligand are longer in comparison with the Re–Cl(1) bonds opposite the O atom of the 1-isoquinolinyl phenyl ketone. This is consistent with the presence of t2g(Re) ? r*(P–C) and t2g(Re) ? p*(N–O) back-bonding discussed above. 3.2. Magnetic properties The magnetic properties of complex under the form leff versus temperature (leff – the effective magnetic moment) is shown in Fig. 2. The complex of rhenium(III) shows room temperature magnetic moment 1.91 B.M. (0.456 cm3 mol1 K). In the whole temperature range lowering magnetic moment is observed from 1.91 B.M. at 300 K up to 0.82 B.M. at 1.8 K.

Table 3 The spin-allowed triplet–triplet electronic transitions calculated with the TDDFT method and the assignments of the calculated transitions to the experimental absorption bands for 1 The most important orbital excitations

Character

k (nm)

E (eV)

f

Experimental K (nm) (E [eV]) e

H(b) ? L + 2(b) H(a) ? L(a) H  1(a) ? L(a) H  2(a) ? L(a)

d/p(N–O)/p(Cl) ? d d/p(Cl) ? p*(N–O) d/p(Cl) ? p*(N–O)

862.3 694.7 648.4

1.44 1.78 1.91

0.0001 0.0022 0.0015

777.1 (1.60) 760

H  1(a) ? L(a) H  2(a) ? L(a)

d/p(Cl) ? p*(N–O)

547.7

2.26

0.1501

570.0 (2.18) 3800

H  2(b) ? L(b) H(b) ? L + 3(b) H(b) ? L + 4(b) H  2(b) ? L + 2(b) H  2(b) ? L + 2(b) H  7(a) ? L(a)

p(Cl) ? d d/p(N–O)/p(Cl) ? p*(N–O) d/p(N–O)/p(Cl) ? p*(N–O) p(Cl) ? d p(Cl) ? d p(Cl) ? p*(N–O)

461.8 450.9 414.0 406.1 403.8 402.2

2.68 2.75 3.00 3.05 3.07 3.08

0.0068 0.0369 0.0051 0.0101 0.0051 0.0071

432.4 (2.87) 2520

H  3(b) ? L + 1(b) H(a) ? L + 2(a) H(b) ? L + 5(b) H  7(a) ? L(a) H(b) ? L + 5(b) H  5(b) ? L + 1(b) H  5(b) ? L + 2(b) H  3(b) ? L + 2(b) H  13(a) ? L(a) H  12(a) ? L(a) H  7(b) ? L + 1(b) H  11(b) ? L(b) H  4(b) ? L + 2(b) H  8(b) ? L(b) H  14(b) ? L + 2(b) H  2(a) ? L + 1(a)

p(N–O)/p(Cl) ? d/p* (N–O) d/p(Cl) ? d/p* (PPh3)/p*(N–O) d/p(N–O)/p(Cl) ? d/p*(PPh3) p(Cl) ? p*(N–O) d/p(N–O)/p(Cl) ? d/p*(PPh3) p(Cl)/p(N–O)/p(PPh3) ? d/p*(p*(N–O)) p(Cl)/p(N–O)/p(PPh3) ? d p(N–O)/p(Cl) ? d p(Cl)/p(PPh3) ? p*(N–O) p(N–O)/p(Cl)/p(PPh3) ? p*(N–O) p(N–O)/p(PPh3) ? d/p*(N–O) p(N–O)/p(Cl)/p(PPh3) ? d p(PPh3)/p(Cl) ? d p(Cl)/p(PPh3) ? d p(Cl) ? d d/p(Cl) ? p*(N–O)

387.2 374.5

3.20 3.31

0.0376 0.0193

347.0 (3.57) 5960

372.4

3.33

0.0126

365.2 364.5

3.40 3.40

0.0138 0.0229

332.7 330.2

3.73 3.75

0.0203 0.0135

325.7 322.5 320.3

3.80 3.84 3.87

0.0123 0.0150 0.0165

319.1

3.88

0.0126

H  14(b) ? L(b) H(a) ? L + 3(a) H  14(b) ? L + 1(b) H  7(b) ? L + 2(b) H  9(b) ? L + 2(b) H(b) ? L + 11(b) H  2(a) ? L + 4(a) H  18(b) ? L + (b)

p(Cl) ? d d/p(Cl) ? d/p*(N–O) p(Cl) ? d/p*(N–O) p(N–O)/p(PPh3) ? d p(PPh3)/p(Cl)/p(N–O) ? d d/p(N–O)/p(Cl) ? p* (N–O) d/p(Cl) ? p*(PPh3) p(Cl) ? d

318.0 315.6 301.0 300.3 294.6 292.0 283.1 282.9

3.90 3.92 4.12 4.13 4.21 4.25 4.38 4.38

0.0105 0.0158 0.0139 0.0168 0.0110 0.0187 0.0101 0.0112

285.6 (4.34) 9980

266.0 (4.66) 15670 231.2 (5.36) 39500 208.0 (5.96) 79100

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This behavior is characteristic of mononuclear Re(III)complexes (3T1g ground state) with d4 low-spin octahedral [34–42] and arises from the large spin–orbit coupling (f = 2500 cm1 [43]), which gives diamagnetic ground state. It seems that in room temperature, in accordance Boltzmann’s distribution, it is populated higher magnetic state, which is depopulated with temperature lowering and decreasing of the magnetic moment is observed. The variation of the magnetization M versus the magnetic field H for complex at 2 K is shown in Fig. 3. The magnetization versus magnetic field curve for complex very slowly increases and indicates value of the magnetization near zero (0.16 B.M) at 5 T. Magnetization of the sample confirms that the ground state is diamagnetic. 3.3. Geometry optimization The optimized geometric parameters of the triplet state are gathered in Table 2. In general, the predicted bond

lengths and angles are in agreement with the values based upon the X-ray crystal structure data. However, some differences may be noticed between the experimental and optimized structures. The experimental Re(1)–Cl(2) is longer than the Re(1)–Cl(1), whereas the other relation is observed for the optimized structure. It may come from the basis sets which are approximated to a certain extent or may indicate the influence of the crystal packing on the values of the experimental bond lengths. The theoretical calculations do not consider the effects of chemical environment. 3.4. Electronic spectrum The experimental and calculated electronic spectra of 1 are presented in Fig. 4. Each calculated transition is represented by a gaussian function y = c ebx2 with the height (c) equal to the oscillator strength and b equal to 0.04 nm2. The electronic absorption spectrum of 1 is similar to those

Fig. 5. Energy and contour plots of the selected occupied and unoccupied molecular orbitals with a spin active in the electronic transitions for complex 1. Positive values of the orbital contour are represented in blue (0.04 au) and negative values – in yellow (0.04 au). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

B. Machura et al. / Polyhedron 27 (2008) 1739–1746

observed for fac-[ReCl3(L–L)(PPh3)] with bipy-like ligands. It displays transitions of moderate intensity in the form of peaks and shoulders in the visible region (400–1000 nm). Table 3 presents the most important spin-allowed triplet–triplet electronic transitions calculated with the TDDFT method assigned to the observed absorption bands of 1. The assignment of the calculated orbital excitations to the experimental bands was based on an overview of the contour plots and relative energy to the orbitals HOMO and LUMO involved in the electronic transitions. The selected HOMO and LUMO orbitals of 1 with a and b spins active in the electronic transitions are presented in Figs. 5 and 6. The investigated complex is of large size, the number of basis functions is equal 686. A hundred electron transitions calculated by the TDDFT method do not comprise all the experimental absorption bands. The UV–Vis spectrum of 1 was calculated only to 280 nm, so the shortest wavelength

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experimental bands can not be assigned to the calculated transitions. Some additional intraligand and interligand transitions are expected to be found at higher energies in the calculation for 1. The TDDFT calculations show that two longest wavelength experimental bands of 1 at 777.1 and 570.0 nm originate in the HOMO2(a) ? LUMO(a), HUMO1(a) ? LUMO(a) and HOMO(a) ? LUMO(a) transitions. The HOMO2, HOMO1 and HOMO orbitals with a-spin can be considered as pReCl orbitals – in all these MOs the rhenium dxz, dyz and dxy orbitals bear antibonding character towards the pp orbitals of the chloride ligands. The lowest unoccupied molecular orbital with a-spin is centered on the p-antibonding orbitals of the N–O ligand. Consequently, the HOMO2(a) ? LUMO(a), HUMO1(a) ? LUMO(a) and HOMO(a) ? LUMO(a) excitations can be seen as mixed Re ? N–O (MLCT) and Cl ? N–O (LLCT) or a delocalized MLLCT (metal– ligand-to-ligand CT) description can be used.

Fig. 6. Energy and contour plots of the selected occupied and unoccupied molecular orbitals with b spin active in the electronic transitions for complex 1. Positive values of the orbital contour are represented in blue (0.04 au) and negative values – in yellow (0.04 au). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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The absorption band at 432.4 nm originates mainly from the HOMO(b) ? LUMO+3(b) transition of d/p (N–O)/p(Cl) ? p*(N–O) character. The highest occupied molecular orbital with b-spin is delocalized among rhenium ion, chloride and N–O ligand, and the LUMO-3 orbital with b-spin is centered on the 1-isoquinolinyl phenyl ketone. To the absorption band at 432.4 nm have been also assigned Ligand–Metal Charge Transfer excitations calculated in 460–400 nm region. Similarly, the absorption at 347.0 nm is attributed to Ligand–Metal Charge Transfer and Ligand–Ligand Charge Transfer excitations. 4. Supplementary data CCDC 674873 contains the supplementary crystallographic data for C26H25Cl5N4PRe. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/ conts/retrieving.html, or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail: deposit@ ccdc.cam.ac.uk. Acknowledgement The GAUSSIAN03 calculations were carried out in the Wrocław Centre for Networking and Supercomputing, WCSS, Wrocław, Poland, under calculational Grant No. 51/96. References [1] D.S. Urch, M.J. Welch, Annu. Rep. Prog. Chem., Sect. A 101 (2005) 585. [2] P. Blower, Dalton Trans. (2006) 1705. [3] F. Tisato, M. Porchia, C. Bolzati, F. Refosco, A. Vittadini, Coord. Chem. Rev. 250 (2006) 2034. [4] Jonathan R. Dilworth, Suzanne J. Parrott, Chem. Soc. Rev. 27 (1998) 43. [5] R.C. Elder, J. Yuan, B. Helmer, D. Pipes, K. Deutsch, E. Deutsch, Inorg. Chem. 36 (1997) 3055. [6] U. Abram, R. Alberto, J. Braz. Chem. Soc. 17 (2006) 1486. [7] G. Rouschias, G. Wilkinson, J. Chem. Soc. A (1967) 993. [8] P. Ghosh, A. Pramanik, N. Bag, A. Chakravorty, J. Chem. Soc., Dalton Trans. (1992) 1883. [9] W. Kaim, B. Schwederski, O. Heilmann, F.M. Hornung, Coord. Chem. Rev. 182 (1999) 323. [10] S.B. Jime´nez-Pulido, M. Sieger, A. Kno¨dler, O. Heilmann, M. Wanner, B. Schwederski, J. Fiedler, M.N. Moreno-Carretero, W. Kaim, Inorg. Chim. Acta 325 (2001) 65. [11] H. Chermette, Coord. Chem. Rev. 178–180 (1998) 699. [12] M.C. Aragoni, M. Arca, T. Cassano, C. Denotti, F.A. Devillanova, F. Isaia, V. Lippolis, D. Natali, L. Niti, M. Sampietro, R. Tommasi, G. Verani, Inorg. Chem. Commun. 5 (2002) 869. [13] P. Romaniello, F. Lelj, Chem. Phys. Lett. 372 (2003) 51. [14] CRYSALIS RED, Oxford Diffraction Ltd., Version 1.171.29.2. [15] G.M. Sheldrick, Acta Crystallogr. A64 (2008) 112.

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