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Syntheses and structures of nickel(II) and copper(II) complexes with a 14-membered tetraaza macrocyclic ligand
Inorganica Chimica Acta 286 (1999) 67 – 73
Syntheses and structures of nickel(II) and copper(II) complexes with a 14-membered tetraaza macrocyclic ligand Ju Chang Kim a,*, James C. Fettinger b, Yeong Il Kim a b
a Department of Chemistry, Pukyong National Uni6ersity, Pusan 608 -737, South Korea Department of Chemistry and Biochemistry, Uni6ersity of Maryland, College Park, Maryland, MD 20742, USA
Received 30 June 1998; accepted 23 September 1998
Abstract Nickel(II) and copper(II) complexes of 3,14-dimethyl-2,6,13,17-tetraazatricyclo[16.4.0.07.12]docosane (L) were prepared, isolated and characterized as trans-[Ni(CH3CN)2(L)][PF6]2 (1) and trans-[Cu(CH3CN)2(L)][PF6]2 (2) by UV – Vis, IR and X-ray crystallography. In complex 1 the coordination geometry about the nickel atom is close to octahedral with four equatorial nitrogen atoms from the macrocycle and two axial nitrogen atoms from acetonitrile. The solid-state electronic spectrum of 1 using the diffuse reflectance method shows a characteristic high-spin d8 nickel(II) ion in a distorted octahedral environment. However, the spectrum of 1 dissolved in acetonitrile indicates that the square-planar species is predominant over the octahedral species. The copper ion environment in 2 is similar to that of nickel complex 1, but the great tetragonal distortion in 2 is observed with much longer Cu–N (acetonitrile) distances than Ni–N (acetonitrile) distances in 1. The electronic spectrum of square-planar [Cu(L)][PF6]2 in acetonitrile or the spectrum of 2 dissolved in acetonitrile in the visible region is basically similar to that of the Nujol mull solid-state diffuse reflectance spectrum of 2, indicating that the square-planar [Cu(L)][PF6]2 coordinates to acetonitrile molecules to form 2. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Crystal structures; Nickel complexes; Copper complexes; Azamacrocyclic complexes
1. Introduction Many nickel(II) complexes of tetraaza macrocyclic ligands undergo a square-planar low-spin and octahedral high-spin equilibrium in aqueous solutions according to Eq. (1) [1– 4]. Much lower molar extinction coefficients for the square-planar nickel(II) complexes with saturated macrocyclic ligands have usually been observed in coordinating solvents than those in noncoordinating solvents due to this equilibrium [4,5]: [Ni(macrocycle)]2 + + 2S X trans-[Ni(S)2(macrocycle)]2 +
(1)
(S= donor solvents) The equilibrium has been reported to be susceptible to * Corresponding author. Tel.: + 82-51-620 6382; fax: + 82-51-628 8147; e-mail: [email protected].
temperature, addition of salts, ionic strength, introduction of substituents on the macrocycle, etc. [3,4,6]. Shifting the equilibrium completely to the octahedral side is very hard, affording square-planar nickel(II) complexes in the presence of perchlorate or hexafluorophosphate anions. Mochizuki and Kondo [1] successfully isolated the trans-diaqua nickel(II) cyclam(1,4,8,11-tetraazacycloteradecane, L1) and determined the structure by X-ray crystallography as wll as its single-crystal Vis–absorption spectrum. Similarly, in a coordinating solvent such as acetonitrile, spectrophotometric titration of solutions of the nickel(II) complex in nitromethane with acetonitrile revealed that low-spin square-planar and high-spin octahedral species coexist in solution by the following reactions (Eqs. (2) and (3)) [7]. [Ni(macrocycle)]2 + + CH3CN X [Ni(CH3CN)(macrocycle)]2 + K1
0020-1693/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S 0 0 2 0 - 1 6 9 3 ( 9 8 ) 0 0 3 8 2 - X
(2)
J.C. Kim et al. / Inorganica Chimica Acta 286 (1999) 67–73
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[Ni(CH3CN)(macrocycle)]2 + +CH3CN X [Ni(CH3CN)2(macrocycle)]
2+
K2
b = K1K2
(3)
spectra were obtained on a Varian Cary 1C UV–Vis spectrophotometer or a Milton Roy Spectronic 1201 recording spectrophotometer.
(4)
However, no acetonitrile-coordinated octahedral complex has been isolated and structurally characterized with perchlorate or hexafluorophosphate anions1 [8– 10]. Square-planar copper(II) complexes of tetraaza macrocyclic complexes have been assumed to have a similar coordination environment around the central copper(II) ion as those of nickel(II) complexes [4,11– 13]. Recently, we have isolated and characterized the trans-diaqua tetraaza macrocyclic copper(II) complexes in which two water ligands are coordinated to the central copper(II) ion by the formation of chelate rings with chlorides and secondary amine nitrogens [14,15]. However, the corresponding diacetonitrile-coordinated copper complex with ligand L has not yet been isolated. In the present work, we have prepared and isolated trans-diacetonitrile complexes of nickel(II) and copper(II) as trans-[Ni(CH3CN)2(L)][PF6]2 (1) and trans[Cu(CH3CN)2(L)][PF6]2 (2) for the first time, and displayed their X-ray structures as well as solid-state Vis – absorption spectra.
2.2. Preparation and isolation The free ligand L, [Ni(L)][ClO4]2 and [Cu(L)][PF6]2 were synthesized by previously reported methods [4].
2.2.1. Preparation and isolation of 1 [Ni(L)][PF6]2 was dissolved in a minimum amount of warm acetonitrile and the same amount of water was added, which was allowed to stand in an open beaker at ambient temperature. After several days, diacetonitrilecoordinated complex 1 crystallized out as pale blue blocks while they were wet. On further evaporation of the solution or of the dried solution, the color of the complex rapidly changed from pale blue to yellow which is a characteristic color for the square-planar nickel(II) complex. 2.2.2. Preparation and isolation of 2 Clear, purple plates of complex 2 were prepared from [Cu(L)][PF6]2 by a similar procedure to that for 1. Copper complex 2 also showed extremely unstable nature upon drying, resulting in a color change from clear purple (octahedral form) to bright orange (square-planar form). Crystals of 1 and 2 were picked up while they were wet and stored in sealed vials in mother liquors without drying procedures for subsequent spectroscopic measurements. Caution! The perchlorate salts used in this study are potentially explosive and should be handled in small quantities. 2.3. X-ray crystallography
2. Experimental
2.1. Materials and physical measurements All chemicals obtained from commercial sources were reagent grade and used without further purification. Distilled water was used for all procedures. Infrared spectra of solid samples were recorded on a Perkin-Elmer Paragon 1000 FT-IR spectrophotometer between 4000 and 400 cm − 1 as Nujol mulls on KBr discs. Solid-state electronic spectra were measured by the diffuse reflectance method as Nujol mulls on a Varian Cary 1C UV – Vis spectrophotometer. Solution 1
Oshio reported nickel(II) and copper(II) complexes, [M(cyclam)(CH3CN)2][Ni(dmit)2]2 (M=Cu, Ni; dmit = isotrithionedithiolate) in [8], which is related to the present work. Also, see Barefield’s papers for the acetonitrile-coordinated nickel(II) complexes with tertiary amine macrocycles [9,10].
2.3.1. Data collection for 1 and 2 Crystal data, a summary of data collection and details of structure refinement are given in Table 1. Single crystals of 1 and 2 were grown as described in Section 2.2. For 1 and 2, data were collected at 153 K on an Enraf-Nonius CAD4 diffractometer that was controlled by a Micro Vax II computer and control program [16]. The crystals’ final cell parameters and crystal orientation matrix were determined from 25 reflections in the range 13.7 BuB 18.3° for 1 and 12.4 B uB 18.6° for 2; these constants were confirmed with axial photographs. ˚) Data were collected using Mo Ka (l= 0.71073 A radiation with v–2u scans over the range 1.8BuB 27.5° for 1 and 2.2 BuB 27.5° for 2. Four standard reflections for 1 and six standard reflections for 2 in well-dispersed reciprocal space were monitored at 30 min intervals of X-ray exposure. Data were not corrected. For 1, eight c-scan reflections were collected over the range 8.1 B uB 15.8°. For 2, seven c-scan
J.C. Kim et al. / Inorganica Chimica Acta 286 (1999) 67–73
reflections, and Friedel pairs, were collected over the range 7.0BuB 16.3°; the absorption correction was applied with transmission factors ranging from 0.6528 to 0.6840 for 1 and 0.7576 to 0.8126 for 2. For 1, one form of data was collected: indices −h −k9 l, resulting in the measurement of 3844 reflections, 3629 unique [Rint =0.0105]. For 2, two forms of data were collected: indices − h − k 9l and hk 9 l (Friedel), resulting in the measurement of 7867 reflections, 3717 unique [Rint =0.0289].
2.3.2. Structural determination and refinement for 1 and 2 Data were corrected for Lorentz and polarization factors and reduced to F o2 and s(F o2) using the program XCAD4 [17]. Systematic absences clearly determined the centrosymmetric monoclinic space group P21/n (no. 14) for 1 and P21/c (no. 14) for 2. The SHELXTL program package [18] was now implemented to confirm the monoclinic centric space group P21/n for 1 and P21/c for 2, to apply the absorption, and set up the initial files. For 1, the structure was determined by direct methods with the successful location of the Ni atom along with all of the remaining non-
69
hydrogen atoms using the program XS [19]. The structure was refined with XL [20]. All of the non-hydrogen atoms were refined anisotropically. Hydrogen atoms attached to carbon atoms were allowed to refine freely (Uxyz ). The structure of 2 was also determined by direct methods with the successful location of the copper atom along with all of the carbon, nitrogen and oxygen atoms present using the program XS. Since the molecule sits at a special position (Cu 0,0,0 Wyckoff 2a) only half of the molecule required location. The structure was refined with XL. All the non-hydrogen atoms were refined anisotropically. Hydrogen atoms attached to carbon atoms were initially placed in calculated positions and hydrogen atoms attached to nitrogen atoms were directly located from an additional difference-Fourier map. All non-hydrogen atoms were refined anisotropically. It was determined that the unique PF6 group possessed disorder in the F2–F3–F5–F6 plane and this was modeled with the resulting occupancies found to be 0.72:0.28. All non-hydrogen atoms were, once again, refined anisotropically while all of the hydrogen atom positions were allowed to refine freely (Uxyz ).
Table 1 Crystal data and structure refinement for [C24H46N6Ni][PF6]2 (1) and [C24H46N6Cu][PF6]2 (2)
Empirical formula Formula weight Crystal system Space group ˚) a (A ˚) b (A ˚) c (A b (°) ˚ 3) V (A Z Dcalc (g cm−3) Absorption coefficient (mm−1) F(000) Crystal size (mm) Limiting indices
Completeness to u (%) Absorption correction Data/restraints/parameters Goodness-of-fit on F 2 Observed reflections Final R a indices R indices (all data) ˚ −3) Largest difference peak and hole (e A
1
2
C24H46F12N6NiP2 767.32 monoclinic P21/n 9.3110(6) 13.2713(12) 13.4763(10) 107.629(5) 1587.0(2) 2 1.606 0.810 796 0.450×0.425×0.375 −125h50 −175k50 −165l517 100.0 (u= 27.48) empirical 3629/0/297 1.042 3148 (I\2s(I)) R1 = 0.0314, wR2 = 0.0789 R1= 0.0390, wR2 = 0.0835 0.502 and −0.309
C24H46F12N6CuP2 772.15 monoclinic P21/c 9.5512(8) 13.3001(7) 13.5418(10) 110.119(7) 1615.3(2) 2 1.588 0.872 798 0.400×0.275×0.150 05h512, −125h50 05k517, −175k50 −175l517, −175l517 100.0 (u = 27.49) empirical 3717/0/333 1.015 2771 (I\2s(I)) R1 = 0.0346, wR2 = 0.0790 R1 = 0.0575, wR2 = 0.0889 0.630 and −0.412
a R1 = Fo−Fc / Fo and wR2={[w(F o2−F c 2)2]/[w(F o2)2]}1/2; w =1/[s 2(Fo)2+(0.0435P)2+0.8917P] for 1 and w=1/[s 2(Fo)2+ (0.0369P)2+0.9637P] for 2, where P= (F o2+2F c 2)/3.
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70
Fig. 1. ORTEP drawing of trans-[Ni(CH3CN)2(L)][PF6]2 (1) with atom-labeling scheme.
3. Results and discussion
3.1. Description of the structure and physicochemical properties of 1 The crystal structure of 1 consists of monomeric cations of the indicated formula and noninteracting hexafluorophosphate anions. The nickel atom is six-coordinate with bonds to the four secondary amine nitrogen atoms of the macrocyclic ligand and to the nitrogen atoms of the acetonitrile molecules, which are coordinated in a trans fashion. Fig. 1 shows the structure of 1 determined by single-crystal X-ray crystallography. A listing of selected interatomic distances and angles is given in Table 2. Although molecular mechanics (MM) investigations, density functional theory (DFT) analysis and NMR studies have suggested that both trans-I and trans-III forms are of similar energies [21 – 24], the ligand skeleton of the present complex adopts the transIII(R,R,S,S) conformation with two chair form six-
Table 2 ˚ ) and angles (°) for [C24H46N6Ni][PF6]2 (1)a Selected bond lengths (A Ni(1)–N(1) Ni(1)–N(11) N(1)–C(10)i N(2)–C(7)
2.0648(15) 2.1684(15) 1.485(2) 1.498(2)
Ni(1)–N(2) N(1)–C(1) N(2)–C(6) N(11)–C(11)
2.0997(14) 1.493(2) 1.490(2) 1.137(2)
N(1)i–Ni(1)–N(1) N(1)–Ni(1)–N(2)i N(1)–Ni(1)–N(11)i N(2)–Ni(1)–N(11) N(11)–Ni(1)–N(11)i N(11)–C(11)–C(12)
180.00(11) 96.01(6) 92.14(6) 93.91(5) 180.00(7) 178.6(2)
N(1)–Ni(1)–N(2) 83.99(6) N(2)–Ni(1)–N(2)i 180.00(13) N(1)–Ni(1)–N(11) 87.86(6) N(2)–Ni(1)–N(11)i 86.09(5) C(11)–N(11)–Ni(1) 173.91(14) C(10)–C(9)–C(7) 118.32(16)
a Symmetry transformations used to generate equivalent atoms: (i) −x, −y, −z.
Fig. 2. Electronic absorption spectra at room temperature: 1 in Nujol mull by the diffuse reflectance method ( — ); [Ni(L)][PF6]2 in BaSO4 by the diffuse reflectance method (…); 1 dissolved in acetonitrile ( – .. – ): [Ni(L)][PF6]2 dissolved in acetonitrile (- - -).
membered and two gauche-five-membered chelate rings. The attached two cyclohexane rings take the stable chair conformations. The central nickel(II) atom lies on an inversion center with four nitrogens of the macrocycle and two axially coordinated acetonitrile molecules, which is a distorted octahedral high-spin form. The nickel–secondary amine nitrogen atom distances vary ˚ with an average disfrom 2.0648(15) to 2.0997(14) A ˚ tance of 2.082 A. The average nickel–nitrogen ˚ is slightly longer than (macrocycle) distance of 2.082 A those found in other trans-nickel complexes of L1 (trans-[NiX2(L1)] (X = Cl, 2.050, 2.066; NO3, 2.050(5), 2.060(6); H2O, 2.072(2), 2.065(3); NCS, 2.069(2), ˚ , [Ni(L1)(CH3CN)2][Ni(dmit)2]2 (dmit= 2.064(2) A ˚ ) [1,8– isotrithionedithiolate), 2.060(6), 2.065(7) A 10,25–27]. However, it is well within the general trend that weaker nickel–nitrogen bonds are involved in the ˚ ) than in the octahedral species (Ni–N=2.07–2.10 A ˚ ) [28]. The square-planar species (Ni–N=1.88–1.91 A ˚, nickel–nitrogen (acetonitrile) distance is 2.1684(15) A which is considerably longer than that found in ˚ ) [8–10]. It is [Ni(L1)(CH3CN)2][Ni(dmit)2]2 (2.137(5) A most reasonable to explain the observed in-plane and axial nickel–nitrogen distances to be a result of a combination of the steric effect of the macrocycle L and the electronically controlled cis effect [28]. The nickel– nitrogen (macrocycle) must be elongated due to the greater antibonding character of the nickel dx 2 − y 2 and dz 2 orbitals that results from the more nearly planar arrangement of the nickel–secondary amine nitrogen atoms when the axial ligand, such as acetonitrile, is present. The trans-acetonitrile ligands are end-bonded to the central nickel ion, and the attached acetonitrile is slightly bent (C11–N11–Ni angle 173.91(14)°) and is tilted 3.91(5)° with respect to the right angle of the
J.C. Kim et al. / Inorganica Chimica Acta 286 (1999) 67–73
macrocyclic plane. As is usual in the saturated 14-membered tetraaza macrocyclic complexes, the N – Ni–N angle of the six-membered chelate rings (96.01(6)°) is larger than that of the five-membered chelate rings (83.99(6)°). However, a substantial amount of strain must be accompanied by the large C7 – C9 – C10 and C7A –C9A–C10A angles (118.32(16)°) of the macrocyclic rings, contributing to longer Ni – N (macrocycle) bond distances as shown above. The infrared spectrum of 1 contains a typical n(CN) stretching band at 2300 cm − 1 and a secondary n(N– H) stretching band 3260 cm − 1 [29,30]. Upon coordination, the n(CN) shifts to higher frequencies (free n(CN) is 2080 cm − 1). The Nujol mull diffuse reflectance absorption spectrum of 1 over the range 300 – 800 nm is composed of three bands at 330, 491 and 670 nm, which is the characteristic spectrum expected for a high-spin d8 nickel(II) ion in a distorted octahedral environment (Fig. 2). These are assigned to the 3B1g 3Eg, 3B1g 3 Eg, 3B1g 3B2g + 3B1g 3A2g transitions, respectively [1,28]. However, the electronic spectrum for 1 dissolved in acetonitrile (lmax =464 nm) is similar to those of the solid-state spectrum of square-planar [Ni(L)][PF6]2 (lmax =468 nm) or to the spectrum of [Ni(L)][PF6]2 dissolved in acetonitrile (lmax =463 nm), indicating that the square-planar species is predominant over octahedral species in acetonitrile. Similar results are obtained from the spectrum run in nitromethane (lmax = 463 nm). Although numerous spectroscopic and magnetic studies have reported that the nickel(II) complexes may exist in an equilibrium between square-planar and the tetragonally distorted octahedral species in coordinating solvents such as acetonitrile, water and N,Ndimethylformamide, etc., structurally determined
Fig. 3. ORTEP drawing of trans-[Cu(CH3CN)2(L)][PF6]2 (2) with atom-labeling scheme. Hydrogen atoms are omitted for clarity.
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Table 3 ˚ ) and angles (°) for [C24H46N6Cu][PF6]2 (2)a Selected bond lengths (A Cu(1)–N(1) Cu(1)–N(11) N(1)–C(10)i N(2)–C(7)
2.0188(18) 2.594(2) 1.483(3) 1.498(3)
Cu(1)–N(2) N(1)–C(1) N(2)–C(6) N(11)–C(11)
N(1)–Cu(1)–N(1)i 189.00(13) N(1)–Cu(1)–N(2) N(1)–Cu(1)–N(2)i 95.34(7) N(2)–Cu(1)–N(2)i N(1)–Cu(1)–N(11)i 92.49(8) N(1)–Cu(l)–N(11) N(2)–Cu(1)–N(11) 97.21(7) N(2)–Cu(1)–N(11)i N(11)i–Cu(1)–N(11) 180.00(11) C(11)–N(11)–Cu(1) N(11)–C(11)–C(12) 179.4(3) C(10)–C(9)–C(7)
2.0481(18) 1.495(3) 1.486(3) 1.132(3) 84.66(7) 180.00(12) 87.51(8) 82.79(7) 166.0(2) 116.4(2)
a Symmetry transformations used to generate equivalent atoms: (i) −x, −y, −z.
octahedral nickel(II) complexes with tetraaza macrocycles are still rare.
3.2. Description of the structure and physicochemical properties of 2 The crystal structure consists of monomeric cations and noninteracting hexafluorophosphate anions. A perspective drawing of the cation is shown in Fig. 3. The copper atom is six-coordinate as a consequence of interaction of the four secondary amine nitrogen atoms and two of the axial acetonitrile nitrogen atoms. The ligand skeleton in 2 is identical to that of nickel complex 1. Bond distances between copper and secondary ˚ , and amine nitrogen atoms are 2.0188(18)–2.0481(18) A ˚ . This the average of the four distances is 2.033 A value falls within the range of comparable reports for such a geometry ([Cu(L1)(CH3CN)2][Ni(dmit)2]2; ˚ ; [Cu(L)(OH2)2][Cl]2; 2.017(2), 2.028(7), 2.019(7) A ˚ 2.038(2) A; [Cu(TML1)(OH2)2[Cl]2 (TML1=3,5,10,12tetramethyl-1,4,8,11-tetraazacyclotetradecane); 2.051 ˚ ) [8–10,14,15]. The copper–nitrogen (2), 1.996(2) A ˚ . The longer cop(acetonitrile) distances are 2.594(2) A per–axial nitrogen distances are found in this complex in comparison with those determined in related complexes ([Cu(L1)(CH3CN)2][Ni(dmit)2]2; Cu–N= ˚ , [Cu(L1)(CH3CN)2][CF3SO3]2; Cu–N= 2.491(6) A ˚ ) [8–10,31]. The strong tetragonal distortion 2.570(5) A in 2 can be attributed to the well-known Jahn–Teller theorem, which indicates that the structures of copper(II) complexes must be distorted due to the octahedral d9 configuration which is orbitally degenerate and therefore susceptible to distortion [32,33]. In this connection it is interesting to compare the structural parameters around copper ions with the parameters for [Cu(H2O)2(L)][Cl2] and [Cu(OH2)2(TML1)][Cl2] in which they have considerably longer copper–oxygen ˚ than a normal distances of 2.649(2) and 2.666(2) A
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J.C. Kim et al. / Inorganica Chimica Acta 286 (1999) 67–73
copper–oxygen bond. In those cases the long contact is believed to be stabilized by hydrogen bonding chelate rings composed of dihydrate, chloride and secondary nitrogen [14,15]. Although no hydrogen bonding interaction is involved in 2, acetonitrile is able to interact with the central copper ion due to its fairly good s-donor and p-acceptor properties. Thus, acetonitrile molecules, which form more strongly than water molecules, have a preference for coordination to the copper(II) ion in the present acetonitrile/water cosolvents. Selected bond distances and angles for 2 are listed in Table 3. The infrared spectrum for 2 contains a typical n(CN) stretching band at 2257 cm − 1 and a coordinated secondary n(N – H) stretching band at 3243 cm − 1 with a broad strong absorption at about n 840 cm − 1 due to hexafluorophosphate anions [29,30]. The Nujol mull diffuse reflectance absorption spectrum of 2 over the range 300 – 800 nm is composed of two bands at about 270 and 519 nm (Fig. 4). The lower energy d–d transition (519 nm) in the visible region of the spectrum is believed to be a composite of the three possible transitions dxy, dxz, dyz dx 2 − y 2 [34]. The expected lowest energy absorption transition dz 2 dx 2 − y 2, arising from the tetragonal splitting of the formally octahedral eg orbitals, was not observed in the visible region. The higher energy band ( 270 nm) is tentatively assigned to ligand – metal charge transfer transitions associated with the nitrogen (axial acetonitrile or macrocycle) donors. The visible spectrum of square-planar [Cu(L)][PF6]2 in acetonitrile or the spectrum of 2 dissolved in acetonitrile is basically similar to that of the Nujol mull diffuse reflectance spectrum of 2, indicating that the square-planar [Cu(L)][PF6]2 coordinates to acetonitrile molecules to form the tetragonally elongated octahedral copper(II) complex, while the visible band of square-planar [Cu(L)][PF6]2 (lmax = 478
Fig. 4. Electronic absorption spectra at room temperature: 2 in Nujol mull by the diffuse reflectance method (—); [Cu(L)][PF6]2 in BaSO4 by the diffuse reflectance method (- - -); 2 dissolved in acetonitrile (…); [Cu(L)][PF6]2 dissolved in acetonitrile (–. –).
nm) taken in Nujol mull by the diffuse reflectance method is observed at 476 nm ( 33 nm shorter than that in acetonitrile) with an asymmetric skewed shape of its maximum around 531 nm. A similar shape is observed from the spectrum of [Cu(L)][PF6]2 taken in non-donor solvents such as nitromethane. The shoulder around 531 nm in the spectrum of square-planar [Cu(L)][PF6]2 is due to the partially resolved dxy dx 2 − y 2 transition from the dxz, dyz dx 2 − y 2 transitions in the lower symmetry complex, but their separation is not large enough to allow clear resolution of the two components [34].
4. Supplementary material Full crystal data and structure refinement details, atomic coordinates and equivalent isotropic displacement parameters, full interatomic distances and angles, anisotropic displacement parameters, and hydrogen coordinates and isotropic displacement parameters for 1 and 2 are available from the Cambridge Crystallographic Data Center, citing the deposition numbers 102995 for 1 and 102996 for 2.
Acknowledgements Financial support from the Pukyong National University (1997 fund) is gratefully acknowledged. We appreciate a reviewer bringing Ref. [31] to our attention.
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[24] K.R. Adam, I.M. Atkinson, L.F. Lindoy, Inorg. Chem. 36 (1997) 480. [25] B. Bosnich, R. Mason, P.J. Pauling, G.B. Robertson, M.L. Tobe, J. Chem. Soc., Chem. Commun. (1965) 97. [26] V.J. Tho¨m, C.C. Fox, J.C.A. Boeyens, R.D. Hancock, J. Am. Chem. Soc. 106 (1984) 5947. [27] T. Ito, M. Kato, H. Ito, Bull. Chem. Soc. Jpn. 57 (1984) 2641. [28] L.Y. Martin, C.R. Sperati, D.H. Busch, J. Am. Chem. Soc. 99 (1977) 2968. [29] K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compounds, 4th ed., Wiley, New York, 1986, p. 272. [30] M.P. Suh, S.-G. Kang, Inorg. Chem. 27 (1988) 2544. [31] M.J. Scott, R.H. Holm, J. Am. Chem. Soc. 116 (1994) 11357. [32] B.J. Hathaway, Coord. Chem. Rev. 41 (1982) 423. [33] A.W. Addison, E. Sinn, Inorg. Chem. 22 (1983) 1225. [34] T.M. Donlevy, L.R. Gahan, T.W. Hambley, G.R. Hanson, K.L. McMahon, R. Stranger, Inorg. Chem. 33 (1994) 5131.
Syntheses and structures of nickel(II) and copper(II) complexes with a 14-membered tetraaza macrocyclic ligand Ju Chang Kim a,*, James C. Fettinger b, Yeong Il Kim a b
a Department of Chemistry, Pukyong National Uni6ersity, Pusan 608 -737, South Korea Department of Chemistry and Biochemistry, Uni6ersity of Maryland, College Park, Maryland, MD 20742, USA
Received 30 June 1998; accepted 23 September 1998
Abstract Nickel(II) and copper(II) complexes of 3,14-dimethyl-2,6,13,17-tetraazatricyclo[16.4.0.07.12]docosane (L) were prepared, isolated and characterized as trans-[Ni(CH3CN)2(L)][PF6]2 (1) and trans-[Cu(CH3CN)2(L)][PF6]2 (2) by UV – Vis, IR and X-ray crystallography. In complex 1 the coordination geometry about the nickel atom is close to octahedral with four equatorial nitrogen atoms from the macrocycle and two axial nitrogen atoms from acetonitrile. The solid-state electronic spectrum of 1 using the diffuse reflectance method shows a characteristic high-spin d8 nickel(II) ion in a distorted octahedral environment. However, the spectrum of 1 dissolved in acetonitrile indicates that the square-planar species is predominant over the octahedral species. The copper ion environment in 2 is similar to that of nickel complex 1, but the great tetragonal distortion in 2 is observed with much longer Cu–N (acetonitrile) distances than Ni–N (acetonitrile) distances in 1. The electronic spectrum of square-planar [Cu(L)][PF6]2 in acetonitrile or the spectrum of 2 dissolved in acetonitrile in the visible region is basically similar to that of the Nujol mull solid-state diffuse reflectance spectrum of 2, indicating that the square-planar [Cu(L)][PF6]2 coordinates to acetonitrile molecules to form 2. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Crystal structures; Nickel complexes; Copper complexes; Azamacrocyclic complexes
1. Introduction Many nickel(II) complexes of tetraaza macrocyclic ligands undergo a square-planar low-spin and octahedral high-spin equilibrium in aqueous solutions according to Eq. (1) [1– 4]. Much lower molar extinction coefficients for the square-planar nickel(II) complexes with saturated macrocyclic ligands have usually been observed in coordinating solvents than those in noncoordinating solvents due to this equilibrium [4,5]: [Ni(macrocycle)]2 + + 2S X trans-[Ni(S)2(macrocycle)]2 +
(1)
(S= donor solvents) The equilibrium has been reported to be susceptible to * Corresponding author. Tel.: + 82-51-620 6382; fax: + 82-51-628 8147; e-mail: [email protected].
temperature, addition of salts, ionic strength, introduction of substituents on the macrocycle, etc. [3,4,6]. Shifting the equilibrium completely to the octahedral side is very hard, affording square-planar nickel(II) complexes in the presence of perchlorate or hexafluorophosphate anions. Mochizuki and Kondo [1] successfully isolated the trans-diaqua nickel(II) cyclam(1,4,8,11-tetraazacycloteradecane, L1) and determined the structure by X-ray crystallography as wll as its single-crystal Vis–absorption spectrum. Similarly, in a coordinating solvent such as acetonitrile, spectrophotometric titration of solutions of the nickel(II) complex in nitromethane with acetonitrile revealed that low-spin square-planar and high-spin octahedral species coexist in solution by the following reactions (Eqs. (2) and (3)) [7]. [Ni(macrocycle)]2 + + CH3CN X [Ni(CH3CN)(macrocycle)]2 + K1
0020-1693/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S 0 0 2 0 - 1 6 9 3 ( 9 8 ) 0 0 3 8 2 - X
(2)
J.C. Kim et al. / Inorganica Chimica Acta 286 (1999) 67–73
68
[Ni(CH3CN)(macrocycle)]2 + +CH3CN X [Ni(CH3CN)2(macrocycle)]
2+
K2
b = K1K2
(3)
spectra were obtained on a Varian Cary 1C UV–Vis spectrophotometer or a Milton Roy Spectronic 1201 recording spectrophotometer.
(4)
However, no acetonitrile-coordinated octahedral complex has been isolated and structurally characterized with perchlorate or hexafluorophosphate anions1 [8– 10]. Square-planar copper(II) complexes of tetraaza macrocyclic complexes have been assumed to have a similar coordination environment around the central copper(II) ion as those of nickel(II) complexes [4,11– 13]. Recently, we have isolated and characterized the trans-diaqua tetraaza macrocyclic copper(II) complexes in which two water ligands are coordinated to the central copper(II) ion by the formation of chelate rings with chlorides and secondary amine nitrogens [14,15]. However, the corresponding diacetonitrile-coordinated copper complex with ligand L has not yet been isolated. In the present work, we have prepared and isolated trans-diacetonitrile complexes of nickel(II) and copper(II) as trans-[Ni(CH3CN)2(L)][PF6]2 (1) and trans[Cu(CH3CN)2(L)][PF6]2 (2) for the first time, and displayed their X-ray structures as well as solid-state Vis – absorption spectra.
2.2. Preparation and isolation The free ligand L, [Ni(L)][ClO4]2 and [Cu(L)][PF6]2 were synthesized by previously reported methods [4].
2.2.1. Preparation and isolation of 1 [Ni(L)][PF6]2 was dissolved in a minimum amount of warm acetonitrile and the same amount of water was added, which was allowed to stand in an open beaker at ambient temperature. After several days, diacetonitrilecoordinated complex 1 crystallized out as pale blue blocks while they were wet. On further evaporation of the solution or of the dried solution, the color of the complex rapidly changed from pale blue to yellow which is a characteristic color for the square-planar nickel(II) complex. 2.2.2. Preparation and isolation of 2 Clear, purple plates of complex 2 were prepared from [Cu(L)][PF6]2 by a similar procedure to that for 1. Copper complex 2 also showed extremely unstable nature upon drying, resulting in a color change from clear purple (octahedral form) to bright orange (square-planar form). Crystals of 1 and 2 were picked up while they were wet and stored in sealed vials in mother liquors without drying procedures for subsequent spectroscopic measurements. Caution! The perchlorate salts used in this study are potentially explosive and should be handled in small quantities. 2.3. X-ray crystallography
2. Experimental
2.1. Materials and physical measurements All chemicals obtained from commercial sources were reagent grade and used without further purification. Distilled water was used for all procedures. Infrared spectra of solid samples were recorded on a Perkin-Elmer Paragon 1000 FT-IR spectrophotometer between 4000 and 400 cm − 1 as Nujol mulls on KBr discs. Solid-state electronic spectra were measured by the diffuse reflectance method as Nujol mulls on a Varian Cary 1C UV – Vis spectrophotometer. Solution 1
Oshio reported nickel(II) and copper(II) complexes, [M(cyclam)(CH3CN)2][Ni(dmit)2]2 (M=Cu, Ni; dmit = isotrithionedithiolate) in [8], which is related to the present work. Also, see Barefield’s papers for the acetonitrile-coordinated nickel(II) complexes with tertiary amine macrocycles [9,10].
2.3.1. Data collection for 1 and 2 Crystal data, a summary of data collection and details of structure refinement are given in Table 1. Single crystals of 1 and 2 were grown as described in Section 2.2. For 1 and 2, data were collected at 153 K on an Enraf-Nonius CAD4 diffractometer that was controlled by a Micro Vax II computer and control program [16]. The crystals’ final cell parameters and crystal orientation matrix were determined from 25 reflections in the range 13.7 BuB 18.3° for 1 and 12.4 B uB 18.6° for 2; these constants were confirmed with axial photographs. ˚) Data were collected using Mo Ka (l= 0.71073 A radiation with v–2u scans over the range 1.8BuB 27.5° for 1 and 2.2 BuB 27.5° for 2. Four standard reflections for 1 and six standard reflections for 2 in well-dispersed reciprocal space were monitored at 30 min intervals of X-ray exposure. Data were not corrected. For 1, eight c-scan reflections were collected over the range 8.1 B uB 15.8°. For 2, seven c-scan
J.C. Kim et al. / Inorganica Chimica Acta 286 (1999) 67–73
reflections, and Friedel pairs, were collected over the range 7.0BuB 16.3°; the absorption correction was applied with transmission factors ranging from 0.6528 to 0.6840 for 1 and 0.7576 to 0.8126 for 2. For 1, one form of data was collected: indices −h −k9 l, resulting in the measurement of 3844 reflections, 3629 unique [Rint =0.0105]. For 2, two forms of data were collected: indices − h − k 9l and hk 9 l (Friedel), resulting in the measurement of 7867 reflections, 3717 unique [Rint =0.0289].
2.3.2. Structural determination and refinement for 1 and 2 Data were corrected for Lorentz and polarization factors and reduced to F o2 and s(F o2) using the program XCAD4 [17]. Systematic absences clearly determined the centrosymmetric monoclinic space group P21/n (no. 14) for 1 and P21/c (no. 14) for 2. The SHELXTL program package [18] was now implemented to confirm the monoclinic centric space group P21/n for 1 and P21/c for 2, to apply the absorption, and set up the initial files. For 1, the structure was determined by direct methods with the successful location of the Ni atom along with all of the remaining non-
69
hydrogen atoms using the program XS [19]. The structure was refined with XL [20]. All of the non-hydrogen atoms were refined anisotropically. Hydrogen atoms attached to carbon atoms were allowed to refine freely (Uxyz ). The structure of 2 was also determined by direct methods with the successful location of the copper atom along with all of the carbon, nitrogen and oxygen atoms present using the program XS. Since the molecule sits at a special position (Cu 0,0,0 Wyckoff 2a) only half of the molecule required location. The structure was refined with XL. All the non-hydrogen atoms were refined anisotropically. Hydrogen atoms attached to carbon atoms were initially placed in calculated positions and hydrogen atoms attached to nitrogen atoms were directly located from an additional difference-Fourier map. All non-hydrogen atoms were refined anisotropically. It was determined that the unique PF6 group possessed disorder in the F2–F3–F5–F6 plane and this was modeled with the resulting occupancies found to be 0.72:0.28. All non-hydrogen atoms were, once again, refined anisotropically while all of the hydrogen atom positions were allowed to refine freely (Uxyz ).
Table 1 Crystal data and structure refinement for [C24H46N6Ni][PF6]2 (1) and [C24H46N6Cu][PF6]2 (2)
Empirical formula Formula weight Crystal system Space group ˚) a (A ˚) b (A ˚) c (A b (°) ˚ 3) V (A Z Dcalc (g cm−3) Absorption coefficient (mm−1) F(000) Crystal size (mm) Limiting indices
Completeness to u (%) Absorption correction Data/restraints/parameters Goodness-of-fit on F 2 Observed reflections Final R a indices R indices (all data) ˚ −3) Largest difference peak and hole (e A
1
2
C24H46F12N6NiP2 767.32 monoclinic P21/n 9.3110(6) 13.2713(12) 13.4763(10) 107.629(5) 1587.0(2) 2 1.606 0.810 796 0.450×0.425×0.375 −125h50 −175k50 −165l517 100.0 (u= 27.48) empirical 3629/0/297 1.042 3148 (I\2s(I)) R1 = 0.0314, wR2 = 0.0789 R1= 0.0390, wR2 = 0.0835 0.502 and −0.309
C24H46F12N6CuP2 772.15 monoclinic P21/c 9.5512(8) 13.3001(7) 13.5418(10) 110.119(7) 1615.3(2) 2 1.588 0.872 798 0.400×0.275×0.150 05h512, −125h50 05k517, −175k50 −175l517, −175l517 100.0 (u = 27.49) empirical 3717/0/333 1.015 2771 (I\2s(I)) R1 = 0.0346, wR2 = 0.0790 R1 = 0.0575, wR2 = 0.0889 0.630 and −0.412
a R1 = Fo−Fc / Fo and wR2={[w(F o2−F c 2)2]/[w(F o2)2]}1/2; w =1/[s 2(Fo)2+(0.0435P)2+0.8917P] for 1 and w=1/[s 2(Fo)2+ (0.0369P)2+0.9637P] for 2, where P= (F o2+2F c 2)/3.
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70
Fig. 1. ORTEP drawing of trans-[Ni(CH3CN)2(L)][PF6]2 (1) with atom-labeling scheme.
3. Results and discussion
3.1. Description of the structure and physicochemical properties of 1 The crystal structure of 1 consists of monomeric cations of the indicated formula and noninteracting hexafluorophosphate anions. The nickel atom is six-coordinate with bonds to the four secondary amine nitrogen atoms of the macrocyclic ligand and to the nitrogen atoms of the acetonitrile molecules, which are coordinated in a trans fashion. Fig. 1 shows the structure of 1 determined by single-crystal X-ray crystallography. A listing of selected interatomic distances and angles is given in Table 2. Although molecular mechanics (MM) investigations, density functional theory (DFT) analysis and NMR studies have suggested that both trans-I and trans-III forms are of similar energies [21 – 24], the ligand skeleton of the present complex adopts the transIII(R,R,S,S) conformation with two chair form six-
Table 2 ˚ ) and angles (°) for [C24H46N6Ni][PF6]2 (1)a Selected bond lengths (A Ni(1)–N(1) Ni(1)–N(11) N(1)–C(10)i N(2)–C(7)
2.0648(15) 2.1684(15) 1.485(2) 1.498(2)
Ni(1)–N(2) N(1)–C(1) N(2)–C(6) N(11)–C(11)
2.0997(14) 1.493(2) 1.490(2) 1.137(2)
N(1)i–Ni(1)–N(1) N(1)–Ni(1)–N(2)i N(1)–Ni(1)–N(11)i N(2)–Ni(1)–N(11) N(11)–Ni(1)–N(11)i N(11)–C(11)–C(12)
180.00(11) 96.01(6) 92.14(6) 93.91(5) 180.00(7) 178.6(2)
N(1)–Ni(1)–N(2) 83.99(6) N(2)–Ni(1)–N(2)i 180.00(13) N(1)–Ni(1)–N(11) 87.86(6) N(2)–Ni(1)–N(11)i 86.09(5) C(11)–N(11)–Ni(1) 173.91(14) C(10)–C(9)–C(7) 118.32(16)
a Symmetry transformations used to generate equivalent atoms: (i) −x, −y, −z.
Fig. 2. Electronic absorption spectra at room temperature: 1 in Nujol mull by the diffuse reflectance method ( — ); [Ni(L)][PF6]2 in BaSO4 by the diffuse reflectance method (…); 1 dissolved in acetonitrile ( – .. – ): [Ni(L)][PF6]2 dissolved in acetonitrile (- - -).
membered and two gauche-five-membered chelate rings. The attached two cyclohexane rings take the stable chair conformations. The central nickel(II) atom lies on an inversion center with four nitrogens of the macrocycle and two axially coordinated acetonitrile molecules, which is a distorted octahedral high-spin form. The nickel–secondary amine nitrogen atom distances vary ˚ with an average disfrom 2.0648(15) to 2.0997(14) A ˚ tance of 2.082 A. The average nickel–nitrogen ˚ is slightly longer than (macrocycle) distance of 2.082 A those found in other trans-nickel complexes of L1 (trans-[NiX2(L1)] (X = Cl, 2.050, 2.066; NO3, 2.050(5), 2.060(6); H2O, 2.072(2), 2.065(3); NCS, 2.069(2), ˚ , [Ni(L1)(CH3CN)2][Ni(dmit)2]2 (dmit= 2.064(2) A ˚ ) [1,8– isotrithionedithiolate), 2.060(6), 2.065(7) A 10,25–27]. However, it is well within the general trend that weaker nickel–nitrogen bonds are involved in the ˚ ) than in the octahedral species (Ni–N=2.07–2.10 A ˚ ) [28]. The square-planar species (Ni–N=1.88–1.91 A ˚, nickel–nitrogen (acetonitrile) distance is 2.1684(15) A which is considerably longer than that found in ˚ ) [8–10]. It is [Ni(L1)(CH3CN)2][Ni(dmit)2]2 (2.137(5) A most reasonable to explain the observed in-plane and axial nickel–nitrogen distances to be a result of a combination of the steric effect of the macrocycle L and the electronically controlled cis effect [28]. The nickel– nitrogen (macrocycle) must be elongated due to the greater antibonding character of the nickel dx 2 − y 2 and dz 2 orbitals that results from the more nearly planar arrangement of the nickel–secondary amine nitrogen atoms when the axial ligand, such as acetonitrile, is present. The trans-acetonitrile ligands are end-bonded to the central nickel ion, and the attached acetonitrile is slightly bent (C11–N11–Ni angle 173.91(14)°) and is tilted 3.91(5)° with respect to the right angle of the
J.C. Kim et al. / Inorganica Chimica Acta 286 (1999) 67–73
macrocyclic plane. As is usual in the saturated 14-membered tetraaza macrocyclic complexes, the N – Ni–N angle of the six-membered chelate rings (96.01(6)°) is larger than that of the five-membered chelate rings (83.99(6)°). However, a substantial amount of strain must be accompanied by the large C7 – C9 – C10 and C7A –C9A–C10A angles (118.32(16)°) of the macrocyclic rings, contributing to longer Ni – N (macrocycle) bond distances as shown above. The infrared spectrum of 1 contains a typical n(CN) stretching band at 2300 cm − 1 and a secondary n(N– H) stretching band 3260 cm − 1 [29,30]. Upon coordination, the n(CN) shifts to higher frequencies (free n(CN) is 2080 cm − 1). The Nujol mull diffuse reflectance absorption spectrum of 1 over the range 300 – 800 nm is composed of three bands at 330, 491 and 670 nm, which is the characteristic spectrum expected for a high-spin d8 nickel(II) ion in a distorted octahedral environment (Fig. 2). These are assigned to the 3B1g 3Eg, 3B1g 3 Eg, 3B1g 3B2g + 3B1g 3A2g transitions, respectively [1,28]. However, the electronic spectrum for 1 dissolved in acetonitrile (lmax =464 nm) is similar to those of the solid-state spectrum of square-planar [Ni(L)][PF6]2 (lmax =468 nm) or to the spectrum of [Ni(L)][PF6]2 dissolved in acetonitrile (lmax =463 nm), indicating that the square-planar species is predominant over octahedral species in acetonitrile. Similar results are obtained from the spectrum run in nitromethane (lmax = 463 nm). Although numerous spectroscopic and magnetic studies have reported that the nickel(II) complexes may exist in an equilibrium between square-planar and the tetragonally distorted octahedral species in coordinating solvents such as acetonitrile, water and N,Ndimethylformamide, etc., structurally determined
Fig. 3. ORTEP drawing of trans-[Cu(CH3CN)2(L)][PF6]2 (2) with atom-labeling scheme. Hydrogen atoms are omitted for clarity.
71
Table 3 ˚ ) and angles (°) for [C24H46N6Cu][PF6]2 (2)a Selected bond lengths (A Cu(1)–N(1) Cu(1)–N(11) N(1)–C(10)i N(2)–C(7)
2.0188(18) 2.594(2) 1.483(3) 1.498(3)
Cu(1)–N(2) N(1)–C(1) N(2)–C(6) N(11)–C(11)
N(1)–Cu(1)–N(1)i 189.00(13) N(1)–Cu(1)–N(2) N(1)–Cu(1)–N(2)i 95.34(7) N(2)–Cu(1)–N(2)i N(1)–Cu(1)–N(11)i 92.49(8) N(1)–Cu(l)–N(11) N(2)–Cu(1)–N(11) 97.21(7) N(2)–Cu(1)–N(11)i N(11)i–Cu(1)–N(11) 180.00(11) C(11)–N(11)–Cu(1) N(11)–C(11)–C(12) 179.4(3) C(10)–C(9)–C(7)
2.0481(18) 1.495(3) 1.486(3) 1.132(3) 84.66(7) 180.00(12) 87.51(8) 82.79(7) 166.0(2) 116.4(2)
a Symmetry transformations used to generate equivalent atoms: (i) −x, −y, −z.
octahedral nickel(II) complexes with tetraaza macrocycles are still rare.
3.2. Description of the structure and physicochemical properties of 2 The crystal structure consists of monomeric cations and noninteracting hexafluorophosphate anions. A perspective drawing of the cation is shown in Fig. 3. The copper atom is six-coordinate as a consequence of interaction of the four secondary amine nitrogen atoms and two of the axial acetonitrile nitrogen atoms. The ligand skeleton in 2 is identical to that of nickel complex 1. Bond distances between copper and secondary ˚ , and amine nitrogen atoms are 2.0188(18)–2.0481(18) A ˚ . This the average of the four distances is 2.033 A value falls within the range of comparable reports for such a geometry ([Cu(L1)(CH3CN)2][Ni(dmit)2]2; ˚ ; [Cu(L)(OH2)2][Cl]2; 2.017(2), 2.028(7), 2.019(7) A ˚ 2.038(2) A; [Cu(TML1)(OH2)2[Cl]2 (TML1=3,5,10,12tetramethyl-1,4,8,11-tetraazacyclotetradecane); 2.051 ˚ ) [8–10,14,15]. The copper–nitrogen (2), 1.996(2) A ˚ . The longer cop(acetonitrile) distances are 2.594(2) A per–axial nitrogen distances are found in this complex in comparison with those determined in related complexes ([Cu(L1)(CH3CN)2][Ni(dmit)2]2; Cu–N= ˚ , [Cu(L1)(CH3CN)2][CF3SO3]2; Cu–N= 2.491(6) A ˚ ) [8–10,31]. The strong tetragonal distortion 2.570(5) A in 2 can be attributed to the well-known Jahn–Teller theorem, which indicates that the structures of copper(II) complexes must be distorted due to the octahedral d9 configuration which is orbitally degenerate and therefore susceptible to distortion [32,33]. In this connection it is interesting to compare the structural parameters around copper ions with the parameters for [Cu(H2O)2(L)][Cl2] and [Cu(OH2)2(TML1)][Cl2] in which they have considerably longer copper–oxygen ˚ than a normal distances of 2.649(2) and 2.666(2) A
72
J.C. Kim et al. / Inorganica Chimica Acta 286 (1999) 67–73
copper–oxygen bond. In those cases the long contact is believed to be stabilized by hydrogen bonding chelate rings composed of dihydrate, chloride and secondary nitrogen [14,15]. Although no hydrogen bonding interaction is involved in 2, acetonitrile is able to interact with the central copper ion due to its fairly good s-donor and p-acceptor properties. Thus, acetonitrile molecules, which form more strongly than water molecules, have a preference for coordination to the copper(II) ion in the present acetonitrile/water cosolvents. Selected bond distances and angles for 2 are listed in Table 3. The infrared spectrum for 2 contains a typical n(CN) stretching band at 2257 cm − 1 and a coordinated secondary n(N – H) stretching band at 3243 cm − 1 with a broad strong absorption at about n 840 cm − 1 due to hexafluorophosphate anions [29,30]. The Nujol mull diffuse reflectance absorption spectrum of 2 over the range 300 – 800 nm is composed of two bands at about 270 and 519 nm (Fig. 4). The lower energy d–d transition (519 nm) in the visible region of the spectrum is believed to be a composite of the three possible transitions dxy, dxz, dyz dx 2 − y 2 [34]. The expected lowest energy absorption transition dz 2 dx 2 − y 2, arising from the tetragonal splitting of the formally octahedral eg orbitals, was not observed in the visible region. The higher energy band ( 270 nm) is tentatively assigned to ligand – metal charge transfer transitions associated with the nitrogen (axial acetonitrile or macrocycle) donors. The visible spectrum of square-planar [Cu(L)][PF6]2 in acetonitrile or the spectrum of 2 dissolved in acetonitrile is basically similar to that of the Nujol mull diffuse reflectance spectrum of 2, indicating that the square-planar [Cu(L)][PF6]2 coordinates to acetonitrile molecules to form the tetragonally elongated octahedral copper(II) complex, while the visible band of square-planar [Cu(L)][PF6]2 (lmax = 478
Fig. 4. Electronic absorption spectra at room temperature: 2 in Nujol mull by the diffuse reflectance method (—); [Cu(L)][PF6]2 in BaSO4 by the diffuse reflectance method (- - -); 2 dissolved in acetonitrile (…); [Cu(L)][PF6]2 dissolved in acetonitrile (–. –).
nm) taken in Nujol mull by the diffuse reflectance method is observed at 476 nm ( 33 nm shorter than that in acetonitrile) with an asymmetric skewed shape of its maximum around 531 nm. A similar shape is observed from the spectrum of [Cu(L)][PF6]2 taken in non-donor solvents such as nitromethane. The shoulder around 531 nm in the spectrum of square-planar [Cu(L)][PF6]2 is due to the partially resolved dxy dx 2 − y 2 transition from the dxz, dyz dx 2 − y 2 transitions in the lower symmetry complex, but their separation is not large enough to allow clear resolution of the two components [34].
4. Supplementary material Full crystal data and structure refinement details, atomic coordinates and equivalent isotropic displacement parameters, full interatomic distances and angles, anisotropic displacement parameters, and hydrogen coordinates and isotropic displacement parameters for 1 and 2 are available from the Cambridge Crystallographic Data Center, citing the deposition numbers 102995 for 1 and 102996 for 2.
Acknowledgements Financial support from the Pukyong National University (1997 fund) is gratefully acknowledged. We appreciate a reviewer bringing Ref. [31] to our attention.
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