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Spectroscopic and magnetic properties of high spin chromium(II) and copper(II) complexes with bidentate α,α′-diimine ligands: 2,2′- pyridylbenzimidazole; 2,2′-pyridylimidazole and 2,2′-pyridylimidazoline
PolyhedronVol. 10, No. 22, pp. 254-2549, Printed in Great Britain
0277-5387/91 $3.00+.00 0 19% Pergamon Press plc
1991
SPECTROSCOPIC AND MAGNETIC PROPERTIES OF HIGH SPIN CHROMIUM(I1) AND COPPER(H) COMPLEXES WITH BIDENTATE a,a’-DIIMINE LIGANDS : 2,2’PYRIDYLBENZIMIDAZOLE ; 2,2’-PYRIDYLIMIDAZOLE AND 2,2’-PYRIDYLIMIDAZOLINE BALTAZAR
DE CASTRO,*
Departamento
CRISTINA FREIRE, DEOLINDA JOiiO GOMES
DOMINGUES
and
de Quimica, Faculdade de Cigncias do Porto, P-4000 Porto, Portugal (Received 22 February 1991; accepted 11 July 1991)
Abstract-Synthesis and physical properties of chromium(I1) and copper(I1) complexes of general formula [M(L-L’)Cl,] are reported, where L-L’ is one of the bidentate a,a’-diimine ligands : 2,2’-pyridylbenzimidazole ; 2,2’-pyridylimidazole ; 2,2’-pyridylimidazoline. The chromium(I1) complexes are high spin and spectroscopic data suggest a polymeric hexacoordinate distorted chloride bridged structure ; the IR active bands in the region 35& 250 cn- ’ have been assigned to Cr-N and Cr-Cl stretching vibrations. The spectroscopic, EPR and magnetic data for the copper(I1) compounds suggest that they are structurally similar to the chromium(I1) compounds.
The complexes formed by cl,a’-diimine ligands, such as 2,2’-bipyridine (bipy), 1,10-phenanthroline (phen) and related ligands with bivalent transition metals ions have been extensively investigated and several examples of mono-, bis- and tris-diimine complexes have been thoroughly characterized.’ For chromium(II), most of the known complexes with these ligands have two or three diimines bound to the metal,‘T3 and the few examples of monodiimine compounds occur with halides or pseudohalides and possess polymeric hexacoordinate distorted coordination geometries. 3,4 As part of a more general investigation on the coordination chemistry of chromium(I1)’ we report the synthesis and characterizaton of chromium(I1) complexes of the type [Cr(L-L’)Cl,], where L-L’ is one of the following bidentate a&-diimine ligands : 2,2’-pyridylbenzimidazole (pyb) ; 2,2’pyridylimidazole (pyi) and 2,2’-pyridylimidazoline (pyim).
*Author
to whom correspondence
should be addressed.
pyb
xvi
prim
It is also generally observed that most high-spin chromium(I1) complexes are structurally similar to the analogous copper(I1) compounds as revealed by X-ray powder diffraction data,“8 an observation that has been attributed to the occurrence of JahnTeller distortions for both metal ions. This fact prompted us to prepare copper(I1) complexes with the same ligands and stoichiometry, [Cu(L-L’)Cl,], that are more amenable to be studied than the extremely air-sensitive chromium(I1) compounds,
2541
2542
B. DE CASTRO et al.
and to obtain indirect indications on the coordination geometry of the chromium complexes. EXPERIMENTAL Materials
Metallic chromium (electrolytic grade) was from Fluka Balzers and copper chloride dihydrate from Merck; all other reagents were from Aldrich and used without further purification, except for ethylenediamine (Merck) that was freshly distilled prior to use. The solvents (Merck ; pro analysi) were dried over molecular sieves (Merck) and thoroughly degassed before use. Preparation of compounds
(a) Ligands. The ligands 2,2’-pyridylbenzimidazole (pyb), 2,2’-pyridylimidazole (pyi) and 2,2’-pyridylimidazoline (pyim) were prepared and purified by literature methods. 9,‘oThe ligands were characterized by their ‘H NMR spectra and melting points. (Observed and literature values for the melting points-for pyi : 133-135 and 134-135”C;9 for pyb : 218-220 and 216219°C ; lo and for pyim : 95-97 and 9698°C. lo) (b) Dichloro(a’-diimine)chromium (II) complexes. All manipulations involving chromium(I1) compounds were carried out under dry nitrogen using standard Schlenk techniques. Hydrated chromium(I1) chloride was prepared by dissolving chromium metal in dilute hydrochloric acid. ’ ’ All complexes were prepared by treating hot ethanolic solutions of CrC12 with ethanolic solutions of the ligands in a stoichiometric amount. For the complex [Cr(pyi)Cl& 1.2 g of CrCl* - HZ0 in 20 cm3 of hot ethanol were reacted with a solution of 1.0 g of pyi in 20 cmJ of hot ethanol and the resulting solution was then cooled in an ice bath. The microcrystalline solids that formed immediately were collected by filtration, washed with cold diethyl ether and dried by continuous pumping for several hours. Yields of purified complexes ranged from 50 to 70%. These brown compounds are air- and moisture-sensitive, and change colour to green in a few hours when exposed to air, but are stable for several weeks when kept in sealed tubes under dry nitrogen. (c) Dichloro(cr’-diimine)copper(II) complexes. These complexes were prepared similarly by treating hot solutions of CuCl, * 2H20 (1 g) in ethanol (20 cm’) with ethanolic solutions of the ligands in a stoichiometric amount. The resulting bright green complexes, collected and dried as described above for the chromium(H) compounds with a yield of 7&90%, are moisture sensitive.
Physical measurements Analysis. Elemental analysis (C, H, N) were performed at the Microanalytical Laboratory, University of Manchester. Copper and chromium were determined by adding an excess of EDTA (Merck ; Titrisol) and back titrating the excess of EDTA with standard calcium solutions (Merck ; Titrisol) using a Ca2+ selective electrode. Chloride was titrated potentiometrically with silver nitrate using a Clselective electrode. The analytical data are summarized in Table 1. Visible-IR spectra. Visible and NIR spectra (solution and Nujol mulls) of the copper(H) and chromium(I1) complexes were recorded at room temperature on a Cary 17 DX spectrophotometer. IR spectra. IR spectra were recorded in a Philiphs PU- 1800 FTIR spectrophotometer. In the region from 4000 to 300 cm-’ the spectra were obtained using KBr pellets or Nujol mulls between KBr plates. For the region 600-200 cm-’ CsI pellets of Nujol mulls between polyethylene plates were used. Conductivity measurements. Conductivity measurements for solutions of the Cu” complexes were carried out with a Metrohm E-518 conductivity meter at 25°C for 10d3 M solutions in dimethylformamide (dmf) and methanol (MeOH). Magnetic measurements. Magnetic susceptibility measurements on powdered solids in sealed tubes were carried out by the Gouy method, from room to liquid nitrogen temperature. The apparatus, calibrated with Hg[Co(NCS),], was supplied by Newport Instruments. The magnetic moments were corrected for diamagnetism by using Pascal’s constants and for temperature independent paramagnetism, TIP = 0.75 mm3 mol- ’ for copper(H). ” EPR spectra. EPR spectra were obtained with an X-band Varian El09 spectrometer (9 GHz) equipped with a variable-temperature accessory. The spectra were calibrated with diphenylpicrylhydrazyl (dpph ; g = 2.0037) ; the magnetic field was calibrated by use of Mn2+ in MgO. The spectra were recorded at - 140°C using sealed quartz tubes.
RESULTS
AND DISCUSSION
The chromium compounds are soluble or partially soluble in the majority of coordinating solvents (water, dimethyl sulphoxide, NJ’-dimethylformamide, methanol) but their solutions are dark green, except in dmf where the solutions remain brown. The copper complexes are not soluble in most solvents, being partially soluble in methanol, dmf, (CH,)$O and water.
2543
Properties of high spin chromium(I1) and copper(I1) complexes Table 1. Magnetic data and elemental analyses
kfp (BM)
Curie-Weiss Law
298 K
97 K
p(K)
4.b3
4.bb
-1
Cr(pyi)Cl,* 1/2H,O
4.57
4.11
-38
3.71
UZr&>m>C)z *H,D
4,XS
4.33
-34
3.93
Cu(pyb)Cl,
1.79
1.80
2
0.51
Cu(pyi)Cl 2
1.81
1.85
6
0.51
Cu(pyim)Cl,
1.88
1.90
6
0.54
Complex uy_qYbjC~_~
;H_,o
10’ Cb 3.44
Analyses (%) g
C
42.8 (42.9) 34.5 (34.7) 33.b (33.4) 2.07 43.2 (43.7) 2.08 34.5 (34.4) 2.14 34.4 (34.1)
pCalculated from p(e,r= 797.8 (~~7’)“’ and the Cur&Weiss ‘Values in m3 mol- ’ K. ‘Calculated values are given in parentheses.
Compounds of [Cr(L-L)Cl,] Magnetic data. The observed room temperature magnetic moments are somewhat below the spinonly values expected for systems with four unpaired electrons (,u r 4.83 BM) and their values for [Cr(pyi)Cl,] and [Cr(pyim)Cl,] decrease with decreasing temperature. The Curie-Weiss law is obeyed for all complexes down to 97 K, although for [Cr(pyb)Cl,] the value of 0 z 0 (Table 1). The magnetic moments for monomeric, tetrahedral high-spin chromium complexes are assumed to be greater than the spin only value and to show a marked temperature dependence, I3 as expected from a T2 ground state. Our data do not fit this pattern, as the room temperature magnetic moments are smaller than the spin-only value and thus rule out tetrahedral coordination for the chromium complexes reported in this work. The observed magnetic behaviour is similar to those found in other high-spin chromium(I1) complexes of general formula [Cr(L-L’)X,], where L-L’ is a bidentate ligand and X a halide,3*4,‘4for which a polymeric species with a hexacoordinate sphere around the chromium was asserted. Assuming that a similar structure applies to our complexes, the magnetic moment dependence with temperature can be attributed to antiferromagnetic interaction between chromium atoms. A superexchange mechanism involving chlorine bridges between the metal ions can account for partial spin coupling and is consistent with the proposed polymeric structure.
H 3.5 (3.3) 3.0 (2.9) 3.5 (3.8) 2.7 (2.8) 2.5 (2.5) 3.2 (3.2)
N
Cr
Cu
12.4 13s (12.5) (15.5) 15.0 18.9 (15.2) (18.8) 13.9 133.8 (14.6) (18.0) 12.5 19.1 (19.3) (12.8) 15.0 22.5 (22.7) (15.0) 14.9 22.9 (22.6) (14.9)
Cl21% (21.1) 25.5 (25.6) 243 (24.6) 21.3 (21.5) 25.1 (25.4) 25.0 (25.2)
Colour mark brown Brown Bark brown Green Green Green
law, xA-’ a (T--8).
Although for [Cr(pyb)Cl,] the value of 8 z 0, a polymeric structure is also assumed, due to the similarity of the spectroscopic data with those of the other two chromium(I1) complexes (vide infra). Electronic spectra. The band maxima are presented in Table 2 ; the electronic spectra of the complexes in Nujol (Fig. 1A) show two bands in the region 45&600 nm (22,200-16,700 cm-‘) and a weaker one at x 750 mn (13,300 cm- ‘). This behaviour is typical of distorted high-spin six-coordinate chromium(I1) complexes, 3*4,6~8*’ 4*’’ where the low energy (distortion) band is associated with transitions between the components of the ‘E ground state (5B, + ‘A,), and the higher energy (main) bands is assigned to superimposed ‘B’ + (‘Bz, ‘E) transitions. The main bands, although affected by stereochemical distortions of the complex, are mainly determined by the ligand field strength, and their values are similar to those of polymeric hexacoordinate species with monoamine bidentate ligands and chloride. 3,4 The complexes prepared, dissolve readily in water, methanol and other hydroxylic solvents, but the resulting solutions are dark green. The electronic spectra of the complexes in these solutions (Fig. 1B) show new high intensity bands in the region from 1250 to 400 nm (25,000-8000 cm- ‘). These spectra show identical bands to those observed in Nujol mulls and in solution for the lowspin complexes [Cr(L-L’)3]C12, with the same ligands (L-L’) used in our study. ’ This observation implies the existence of low-spin complexes in
2544
B. DE CASTRO et al. Table 2. Solution and Nujol electronic spectra of the Cr(L-L’)Cl, complexes”
Complex Cr(pyb)Cl, * H,O
Absorption bands (cm- ‘) 20,10O(sh) 20,100 21,730 21,500
Cr(pyi)Cl,* 1/2Hz0
22,80O(sh) 22,700 21,970 21,730
Cr(pyim)Cl, - Hz0
17,240 16,660 16,800 20,0OO(sh) 20,100 17,200 16,000 16,000
15,310 15,450 14,490 14,380 15,030
13,330h 13,330b
14,280’ 14,280’
13,330 13,500 13,690 13,330 13,600
Solvent
94306 9520 9250
10,100 10,200 10,300
8620 8330 8360
Nujol dmf (CH&SO Methanol Water
9090 9170 9210
Nujol dmf (CH s)SO Methanol Water
14,20O(sh) 14,000 9250 13,150 13,880 10,lOOb 9090 12,980’ 20,400 14,080h 10,100~ 9090 12,820’ 20,830 13,880’ “The molar extinction coefficients for the solution spectra are not reported as the high reactivity prevents their determination. ‘These absorption bands were obtained by spectral deconvolution. 20,50O(sh) 21,000
18,20O(sh) 18,400 17,100 16,390 16,260
(A)
Nujol dmf (CH,)zSO Methanol Water of these species
hydroxylic solvents and provides spectral evidence for the disproportion of monoamine complexes in solution.
3/n[Cr(L-L’)Cl,], z$ [Cr(L-L’),]C12 + [CrCl,].
Fig. 1. Electronic specta of Cr(L-L’) Cl, complexes. (A) Nujol spectra: (a) Cr(pyi)Cl,; (b) Cr(pyim)Cl,; (c) Cr(pyb)Cl,. (B) Solution spectra of Cr(pyi)Cl, in: (a) dmf; (b) (CH&SO ; (c) H,O and (d) MeOH. The molar extinction coefficients are not reported as the high reactivity of the chromium(I1) species prevents their determination.
The dissolution of these complexes in hydroxylic solvents breaks the halogen bridges and the polymeric structure is destroyed. Similar behaviour has been observed for the analogous complexes with ophenanthroline, where no bands from CrCl, are observed in the spectra, since the absorption coefficients of CrCl, are negligible relative to that of the low spin complex. The observation that solution spectra in (CH3)$0 keep some of the features of the Nujol spectra but also exhibit new intense bands, suggests the existence in solution of some of the original species, thus implying only a partial breakdown of the solid structure. Also, as the spectra in dimethylformamide are similar to that obtained in Nujol, it is also conceivable that the solid structure must be retained to a large extent in solution. IR spectra. Information on metal-ligand stretching vibrations for high-spin chromium(I1) complexes is very scarce’6-‘9 and no data for complexes with bidentate a&-diimines and halogens were found in the literature. Larkworthy et al. reported IR data for several high-spin chromium(H) chlorocomplexes with pyridine derivatives and assigned the strong band at ~320-330
cm-’ to the stretching vibration Cr-Cl, and the less intense band at 27@-290 cm- ’ to the stretching
2545
Properties of high spin chromium(I1) and copper(I1) complexes vibration Cr-N. In these complexes the halogens are bridging the chromium atoms and one v(Cr-N) vibration and two IR active v(Cr-Cl) vibrations are expected for these polymeric compounds. The other v(Cr-Cl) band was assumed to occur at ca 160 cm-‘, but in the spectra” of [Cr(4-mepy)z X,], a band at 280 cm-’ observed for the chlorocomplex was shifted to 25 1 cm- ’ when the chlorine was replaced by bromine. It is conceivable that this latter band is also v(Cr-Cl). For octahedral monomeric complexes with the same ligands (L) and axial terminal halogen atoms [CrL,X,], the stretching vibrations Cr-Cl are observed at a much lower frequency (M 150 cm- ‘), but the stretching Cr-N showed little or no variation. This result is observed for metal ions with Jahn-Teller effect, where long axial bond lengths are expected, thus shifting v(Cr-Cl) to lower frequencies.2S22 To assign the IR spectra of our complexes, the spectra were compared with those of analogous compounds with bromide.23 The observed stretching frequencies for the chloro and bromo complexes in the region 40&200 cm-’ are listed in Table 3, and typical IR spectra are shown in Fig. 2A. For [Cr(pyb)C12] and [Cr(pyi)Cl,] the spectra show one strong anion dependent band at x330 cm- ‘, that is shifted by ca 50 and 30 cm- ‘, respectively, when the halogen is replaced by bromide (Table 3). Halogen replacement also shifts the band at 305 cm- ’ of [Cr(pyb)Cl,] to 249 cm- ’ ; and for [Cr(pyi)C12] the band at 295 cm-’ is shifted to 260 cm- ‘, thus allowing an unambiguous assignment of these two bands to v(Cr-Cl). The bands at 322 and 305 cm- ’ for [Cr(pyb)Cl,] and for [Cr(pyi)Cl,], respectively, are anion independent and are assigned to v(Cr-N).
400
360
300
Fig. 2. IR spectra of M(pyi)X, complexes. (A) (1) Wpyi)Cl,; (2) Cr(pyi)Br,. (B) (1) Cu(Pyi)Cl,; (2) Cu(pyi)Br,.
As the complex [Cr(pyim)Br,] could not be prepared, v(Cr-X) was not assigned by halogen substitution, but as v(Cr-N) are not strongly ligand dependent, the bands at 336 cm- ’ and at 305 cm- ’ are assigned to v(Cr-Cl), and the one at 278 cm- ’ is then assigned to v(Cr-N). The similarity of high energy band v(Cr-Cl) in our complexes with those of other chromium(I1)” complexes with polymeric structure also provides evidence for the postulated structure of our complexes, where asymmetric halogen bridges take place with two short and two long bonds,
Table 3. IR spectra of Cr(L-L’)X, and Cu(L-L’)X, complexes IR bands, v (cm- ‘) x = Cl Complex Cr@yb)Xz Cr@yi)X 2
Cr(pyim)X, Cu(wb)Xz
Cu(pyi)Xz Cu(pyim)X,
(M-N)
260 Shi’)
X=Bf
(M-Cl)
(M-N)
(M-Br)
322
332
305
322
283
249
305 278
329 335
295 305
310 h
291 h
260
322 285 258
304 322 304
282 280 277
321 287 264
291 297 289
249 249 247
a From ref. 33. bThe complex could not be prepared.
b
2546
B. DE CASTRO et al.
Compounds of [Cu(L-L’)Cl j
Contrasting with chromium complexes, the known stereochemistries for copper complexes of general formula [Cu(L-L/)X,] or [CuL,X,] are much more diverse, and the choice of the correct one can hardly be accomplished without X-ray data from single crystals. ‘,24 However, combination of spectroscopic and magnetic data, allows for some insight into the stereochemistry of the copper complexes studied. EPR data. Powdered samples of the pure complexes exhibit at low temperature, EPR spectra that are all different (Fig. 3A and Table 4). For [Cu(pyi)C12] the spectrum is of the rhombic type with three g values ; for [Cu(pyb)Cl,] only two g values are obtained (axial type) ; and for [Cu(pyim)Cl,] a pseudo-isotropic spectrum is obtained. No hyperfine splitting with copper or with the ligand nuclei ( 14Nor 3‘v3‘Cl) are observed, probably due to dipole-dipole or exchange interaction. As no signal at half field (AM, = +2) is observed for any spectra, the existence of dimeric structures bridged either by the cr,a’-diimine ligand or by the halogen, can be ruled out. However several structures are still possible, monomeric (pseudo-tetrahedral) and polymeric ; the latter situation may include doubly halogen-bridged hexacoordinate structure, and singly halogen-bridged five-coordinate complexes. Polycrystalline EPR data do not allow for a clear-cut distinction between monomeric (pseudo-tetrahedral) and polymeric geometries. Better resolution was achieved in copper-doped (a,cr-diimine) cadmium dichloride compounds, for which axial type EPR spectra with coupling to copper in the low field region (I = 3/2 ; four bands) are observed (Fig. 3B). For [Cu(pyb)C12] a hyperfine structure is also observed in the high field region. The structures of these (a&-diimine) cadmium dichlorides are not known, but those of [Cd (mpyb)Cl& where mpyb is 2-(4’-methyl-2’-pyridyl)benzimidazole, and of other compounds of general formula [Cd(L-L’)Cl,] with bidentate diimine ligands, are polymeric distorted hexacoordinate with bridging halogens.2’ This coordination geometry must be imposed on the dopant copper complexes and the values of G = (g,, - 2)/(gl - 2) = 4 are indicative of a dxz_yz ground state for these pseudo-octahedral complexes.26 For the doped complexes, the g values are invariant within experimental error, suggesting similar electronic properties. However there is a difference between the A,, values of the [Cu(pyb)Cld and the other two complexes ; this may be interpreted as resulting from stronger stereochemical distortions imposed for the complex with the bulkier ligand. Indeed, gll and
(B)
: : :...:
Fig. 3. EPR spectra of Cu(L-L’)Cl* complexes. (A) Powder spectra : (a) Cu(pyi)Cl, ; (b) Cu(pyim)Cl, ; (c) Cu(pyb)Cl,. (B) (a) Cu/doped Cd(pyi)Cl, ; (b) Cu(pyi)Cl, in dmf.
A,, values seem relatively insensitive to strong axial perturbations, but rather sensitive to in-plane distortions for hexacoordinate species. ’ Frozen solution EPR spectra of the pure complexes in dmf (Fig. 3B) and methanol show two sets of bands that must be due to the existence of at least two different species in solution, indicating the compounds to have undergone some structural changes when in the solvents (see below electronic spectra and conductivity data). All spectra show coupling to copper in the low field region and to the nitrogen atoms in the high field region. ZR spectra. To assign the IR spectra of our complexes, the analogous bromo complexes were also prepared. The observed stretching frequencies for the chloro and bromo complexes in the region
2547
Properties of high spin chromium(I1) and copper(I1) complexes Table 4. EPR data for Cu(L-L’)Cl, complexes Complex Cu@yb)Cl,
Cu(pyi)Cl,
Cu(pyim)Cl,
gla powder Cu/Cd dmf MeOH
2.23 2.29
92a
93
A”
All
Gh
2.07 2.07
2.12 2.14
133
4.1
2.30
2.07
2.15
162
4.1
powder Cu/Cd dmf MeOH
2.20 2.28
2.10 2.07
2.12 2.14
143
4.0
powder Cu/Cd dmf MeOH
2.12 2.28
2.12 2.14
140
4.0
d
2.05
d d
2.07
d d
“In methanol and in the doped cadmium complex, g,, is g,, and g2 is gl, except for Cu(pyim)Cl, whose powder spectrum is pseudo-isotropic. bCalculated as G = (g,, -2)/b, -2).25 ’ Cu/Cd refer to EPR data for copper doped (a&-diimine) cadmium dichloride compounds. dEPR data show that there are more than one complex in solution, see Fig. 3B.
cm-’ are listed in Table 3, and typical IR spectra are shown in Fig. 2B. For [Cu(pyb)Xd and [Cu(pyim)Xd three bands are observed for both halo complexes. One of them (at 322 and 258, respectively for the pyb and pyim complexes) is invariant with halogen substitution and is attributable to v(Cu-N). The other two are anion dependent and have been assigned to v(Cu-X). The complexes [Cu(pyi)XJ show also two well-resolved bands that are anion dependent and assigned to v(Cu-X), and partially superimposed on these bands the anion independent on assigned to v(Cu-N) can also be observed. Absorptions assignable to v(Cu-N) fall in the region 320-250 cm-’ , typical of copper(I1) complexes with aromatic nitrogen ligands, and increase with the aromatic character of the diimine ligand. The v(Cu-Cl) bands lie in the frequency range indicative of terminally bound chlorides in pseudotetrahedral compounds (320-280 cn- 1),27 although for polymeric hexacoordinate copper complexes with two nitrogen donor atoms the values observed for bridging chlorides lie in the same region, well above the expected values for bridging chlorides in other late first transition divalent metal complexes, that appear below 250 cn- ‘. This anomalous behaviour has been attributed to asymmetric M-X-M bonds caused by the Jahn-Teller effect in d9 systems, and as copperhalogen stretching frequencies are highly dependent on bond lengths, values ranging from 400 to 240 400-200
have been observed for distorted polymeric hexacoordinate compounds. ** The spectra of [Cu(bipy)Xd and of [Cu(phen)X,], whose structure is known to be polymeric hexacoordinate, show three sets of strong or medium intensity bands in the region 320-220 cm- ’ that have been assigned to v(Cu-N) and to v(Cu-X) (two bands).** The similarity of our ligands to bipy and phen, and of the IR spectra of our copper complexes with those of bipy and phen, lend some support to propose a polymeric distorted hexacoordinate structure for the complexes reported. Nevertheless, a polymeric tetragonally distorted five-coordinate structure could not be excluded for all the complexes, as two different types of Cu-Cl bonds (one short and one long) are also expected for this geometry.29 Magnetic data. The Cur&Weiss law is obeyed in the temperature range studied for all compounds with very small and positive Weiss constants, with room temperature magnetic moments (Table 1) lying in the range 1.8-1.9 BM, higher than the spin-only value. I3 Both results provide no evidence for significant magnetic interaction between copper atoms, in the temperature range used, despite the postulated polymeric (hexacoordinate) structure. The g values calculated from the magnetic data show reasonable agreement with those of gay obtained by EPR. Electronic spectra. The visible/NIR spectra in Nujol (Table 5) show the usual h-d type tmsymmetrical band, with maxima at ca 12,500-14,500 cm- ’ tailing into the near IR, I4 and with energies cm-’
B. DE CASTRO et al.
2548
Table
5. Nujol,
solution
electronic
spectra and conductivity complexes
data
Electronic spectra, v (cm- ‘) Complex
Nujol
dmf
Cu(pyb)Cl, Cu(pyi)Cl, Cu(pyim)Cl,
12,820 14,700 13,340
12,820 13,510 13,700
(CHj)$O
CHjOH
12,820 13,340 13,340
13,510 13,890 14,090
of Cu(L-L’)Cl,
Conductivities, AM (S cm’ mol- ‘) H,O
dmf
CH30H
13,890 14,090 14,290
15.9 22.2 25.0
73.6 73.9 84.4
The values of AM (S cm* mall ‘) in methanol are : 8&l 15 for 1 : 1 electrolytes and 160220 for 1 : 2 electrolytes, respectively ; in dmf the values for 1 : 1 electrolytes are 60-90, and for 1 : 2 electrolytes 13&l 70. 3o
comparable to that of [Cu(phen)C1J.30 This broad band is likely due to several overlapping transitions that are not well resolved, although a deconvolution into two broad bands seems obvious, and assuming Ddhsymmetry, the unsymmetrical band are assigned to 2B1, -+ (*B2, 2E) transitions. Solution electronic spectra show marked variation in band maxima and spectral width, suggesting that the postulated bridges are partially broken in solution, corroborating the EPR data that more than one species are present in solution. As the possible structures in solution are not known, no attempts to deconvolute the unsymmetrical band were made, since possible overlap of bands from different species are possible. Nevertheless, the band maximum show a red shift for dmf and (CH3)2S0, and a hypsochromic shift for the hydroxylic solvents (water and methanol). The latter shifts are indicative of appreciable chloride substitution by the solvent molecules yielding a stronger ligand field. The red shift can be explained by noting that dmf and (CH3)2SO are less effective in disrupting the halogen bridges, and that the resulting species could then involve weaker axial coordination to the solvent molecules, thus rendering the total ligand field weaker than in the polymeric structures. Conductiuity data. The molar conductivity values (Table 5) for dmf solutions of the compounds (ca lo- 3M) are indicative of non-electrolyte behaviour, but as EPR and electronic spectroscopy data show, the complexes undergo structural changes in solutions through incorporation of solvent molecules in the coordinating sphere of the metal. The conductivity data for methanol solutions are in the lower limit for 1 : 1 electrolyte type behaviour, 3’ and suggests partial release of chloride ions into solution and an extensive disruption of the polymeric structure. As the EPR spectra also
indicate the presence of at least two species in solution, in this solvent it is possible that some chloride ligands are replaced by solvent molecules, also supported by the large changes induced in their electronic spectra. CONCLUDING
REMARKS
The structural characterization of the chromium(I1) compounds posed no ambiguities and hexacoordinate polymeric structures with JahnTeller distortions were proposed based on magnetic and spectroscopic data. For copper a bewildering range of structural types is known for complexes of the general formulation Cu(L-L/)X2 or CuL2X2,24,32,33and it has proved difficult to predict even gross structural features in these complexes in the absence of crystallographic data. In the lack of relationships between structural characteristics and spectroscopic and magnetic properties for these types of copper(I1) complexes, the non-availability of Xray data raises severe difficulties in obtaining structural information for our complexes. Nevertheless, by comparing the data for our complexes with those of other copper(I1) complexes with similar ligands and with known structures, we were able to suggest for the complexes reported a hexacoordinated polymeric skeleton, similar to that observed for the analogous chromium(I1) complexes. work was supported by Instituto National de Investigacgo Cientifica (INIC, Lisboa) through contract no. 89/EXA/3.
Acknowledgement-This
REFERENCES 1. Comprehensive Coordination Chemistry (Edited by G. Wilkinson, R. D. Gillard and J. A. McCleverty),
Properties of high spin chromiu Im(II) and copper(I1) complexes Vol. 2 and 5. Pergamon Press, New York (1987). 2. A. Eamshaw, L. F. Larkworthy, K. C. Patel, K. S. Patel, R. L. Carlin and E. G. Terezakis, J. Chem. Sot. A 1966, 511. 3. A. Eamshaw, L. F. Larkworthy and K. C. Patel, .Z. Chem. Sot. A 1970,184O. 4. A. Eamshaw, L. F. Larkworthy, K. C. Pate1 and B. J. Tucker, J. Chem. Sot., Dalton Trans. 1977,2209. 5. B. Castro and C. Freire, Synth. React. Znorg. Met.Org. Chem. 1990, 20, 1. 6. L. F. Larkworthy and B. J. Tucker, Znorg. Chim. Acta 1987, 134,95. 7. L. F. Larkworthy and M. H. 0. N. Richardson, Znorg. Chim. Acta 1978,28, 251. 8. L. F. Larkworthy and J. M. Tabatabai, Znorg. Chim. Acta 1977,21, 265. 9. B. Chiswell, F. Lions and B. S. Momis, Znorg. Chem. 1964,3, 110. 10. J. L. Walter and H. Freiser, Analyt. Chem. 1954,26, 217. 11. (a) W. A. Knight and E. M. Rich, J. Chem. Sot. (Land.) 1911, 99, 87; (b) H. Lux and G. Illman, Chem. Ber. 1958,91,2143. 12. E. Maarschall, A. Botterman, S. Veta and A. Miedema, Physica 1969, 41,473. 13. F. E. Mabbs and D. J. Machin, Magnetism and Transition Metal Complexes. Chapman, London (1973). 14. A. B. P. Lever, Inorganic Electronic Spectroscopy, 2nd edn. Elsevier, New York (1984). 15. A. Eamshaw, L. F. Larkworthy and K. C. Patel, J. Chem. Sot. A 1968,2276. 16. F. Mani and G. Scapacci, Znorg. Chim. Acta 1976, 16, 163.
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17. M. M. Khamar, L. F. Larkworthy and M. H. 0. N. Richardson, Znorg. Chim. Acta 1978,28,245. !S. L. F. Larkworthy and M. H. 0. N. Richardson, Znorg. Chim. Acta 1980,40,217. 19. F. Mani and R. Morassi, Znorg. Chim. Acta 1979, 36,63. 20. R. J. H. Clark and C. S. Williams, Znorg. Chem. 1965, 4, 350. 21. D. M. Adams and P. J. Lock, J. Chem. Sot. A 1967, 620. 22. R. H. Nuttall, Talanta 1968, 15, 157. 23. B. Castro, C. Freire and D. Domingues, unpublished results. It must be mentioned that the compound [Cr(pyim)Brd could not be prepared. 24. M. Melnik, Coord. Chem. Rev. 1982,47, 249. 25. M. Masssacesi, M. Biddau, G. Devoto, E. Barni and P. Savarino, Znorg. Chim. Acta 1984,82, 27. 26. B. J. Hathaway and D. E. Billing, Coord. Chem. Rev. 1970,5, 143. 27. K. Nakamoto, Infrared and Raman Spectra of Znorganic and Coordination Cornpour&, 4th edn. Wiley, New York (1986). 28. R. E. Wilde and T. K. K. Srinivasan, J. Znorg. Nucl. Chem. 1973,35,1017. 29. R. A. Bream, E. D. Ester and D. J. Hodgson, Znorg. Chem. 1975,14, 1672. 30. N. Ray, S. Tyagi and B. Hathaway, J. Chem. Sot., Dalton Trans. 1982, 143. 31. W. J. Geary, Coord. Chem. Rev. 1971,7,81. 32. W. E.Marsh, T. L. Bowman, W. E. Hatfield and D. J. Hodgson, Znorg. Chim. Acta 1982,59, 19. 33. W. E. Marsh, D. E. Eggleston, W. E. Hatfield and D. J. Hodgson, Znorg. Chim. Acta 1983,70, 137.
0277-5387/91 $3.00+.00 0 19% Pergamon Press plc
1991
SPECTROSCOPIC AND MAGNETIC PROPERTIES OF HIGH SPIN CHROMIUM(I1) AND COPPER(H) COMPLEXES WITH BIDENTATE a,a’-DIIMINE LIGANDS : 2,2’PYRIDYLBENZIMIDAZOLE ; 2,2’-PYRIDYLIMIDAZOLE AND 2,2’-PYRIDYLIMIDAZOLINE BALTAZAR
DE CASTRO,*
Departamento
CRISTINA FREIRE, DEOLINDA JOiiO GOMES
DOMINGUES
and
de Quimica, Faculdade de Cigncias do Porto, P-4000 Porto, Portugal (Received 22 February 1991; accepted 11 July 1991)
Abstract-Synthesis and physical properties of chromium(I1) and copper(I1) complexes of general formula [M(L-L’)Cl,] are reported, where L-L’ is one of the bidentate a,a’-diimine ligands : 2,2’-pyridylbenzimidazole ; 2,2’-pyridylimidazole ; 2,2’-pyridylimidazoline. The chromium(I1) complexes are high spin and spectroscopic data suggest a polymeric hexacoordinate distorted chloride bridged structure ; the IR active bands in the region 35& 250 cn- ’ have been assigned to Cr-N and Cr-Cl stretching vibrations. The spectroscopic, EPR and magnetic data for the copper(I1) compounds suggest that they are structurally similar to the chromium(I1) compounds.
The complexes formed by cl,a’-diimine ligands, such as 2,2’-bipyridine (bipy), 1,10-phenanthroline (phen) and related ligands with bivalent transition metals ions have been extensively investigated and several examples of mono-, bis- and tris-diimine complexes have been thoroughly characterized.’ For chromium(II), most of the known complexes with these ligands have two or three diimines bound to the metal,‘T3 and the few examples of monodiimine compounds occur with halides or pseudohalides and possess polymeric hexacoordinate distorted coordination geometries. 3,4 As part of a more general investigation on the coordination chemistry of chromium(I1)’ we report the synthesis and characterizaton of chromium(I1) complexes of the type [Cr(L-L’)Cl,], where L-L’ is one of the following bidentate a&-diimine ligands : 2,2’-pyridylbenzimidazole (pyb) ; 2,2’pyridylimidazole (pyi) and 2,2’-pyridylimidazoline (pyim).
*Author
to whom correspondence
should be addressed.
pyb
xvi
prim
It is also generally observed that most high-spin chromium(I1) complexes are structurally similar to the analogous copper(I1) compounds as revealed by X-ray powder diffraction data,“8 an observation that has been attributed to the occurrence of JahnTeller distortions for both metal ions. This fact prompted us to prepare copper(I1) complexes with the same ligands and stoichiometry, [Cu(L-L’)Cl,], that are more amenable to be studied than the extremely air-sensitive chromium(I1) compounds,
2541
2542
B. DE CASTRO et al.
and to obtain indirect indications on the coordination geometry of the chromium complexes. EXPERIMENTAL Materials
Metallic chromium (electrolytic grade) was from Fluka Balzers and copper chloride dihydrate from Merck; all other reagents were from Aldrich and used without further purification, except for ethylenediamine (Merck) that was freshly distilled prior to use. The solvents (Merck ; pro analysi) were dried over molecular sieves (Merck) and thoroughly degassed before use. Preparation of compounds
(a) Ligands. The ligands 2,2’-pyridylbenzimidazole (pyb), 2,2’-pyridylimidazole (pyi) and 2,2’-pyridylimidazoline (pyim) were prepared and purified by literature methods. 9,‘oThe ligands were characterized by their ‘H NMR spectra and melting points. (Observed and literature values for the melting points-for pyi : 133-135 and 134-135”C;9 for pyb : 218-220 and 216219°C ; lo and for pyim : 95-97 and 9698°C. lo) (b) Dichloro(a’-diimine)chromium (II) complexes. All manipulations involving chromium(I1) compounds were carried out under dry nitrogen using standard Schlenk techniques. Hydrated chromium(I1) chloride was prepared by dissolving chromium metal in dilute hydrochloric acid. ’ ’ All complexes were prepared by treating hot ethanolic solutions of CrC12 with ethanolic solutions of the ligands in a stoichiometric amount. For the complex [Cr(pyi)Cl& 1.2 g of CrCl* - HZ0 in 20 cm3 of hot ethanol were reacted with a solution of 1.0 g of pyi in 20 cmJ of hot ethanol and the resulting solution was then cooled in an ice bath. The microcrystalline solids that formed immediately were collected by filtration, washed with cold diethyl ether and dried by continuous pumping for several hours. Yields of purified complexes ranged from 50 to 70%. These brown compounds are air- and moisture-sensitive, and change colour to green in a few hours when exposed to air, but are stable for several weeks when kept in sealed tubes under dry nitrogen. (c) Dichloro(cr’-diimine)copper(II) complexes. These complexes were prepared similarly by treating hot solutions of CuCl, * 2H20 (1 g) in ethanol (20 cm’) with ethanolic solutions of the ligands in a stoichiometric amount. The resulting bright green complexes, collected and dried as described above for the chromium(H) compounds with a yield of 7&90%, are moisture sensitive.
Physical measurements Analysis. Elemental analysis (C, H, N) were performed at the Microanalytical Laboratory, University of Manchester. Copper and chromium were determined by adding an excess of EDTA (Merck ; Titrisol) and back titrating the excess of EDTA with standard calcium solutions (Merck ; Titrisol) using a Ca2+ selective electrode. Chloride was titrated potentiometrically with silver nitrate using a Clselective electrode. The analytical data are summarized in Table 1. Visible-IR spectra. Visible and NIR spectra (solution and Nujol mulls) of the copper(H) and chromium(I1) complexes were recorded at room temperature on a Cary 17 DX spectrophotometer. IR spectra. IR spectra were recorded in a Philiphs PU- 1800 FTIR spectrophotometer. In the region from 4000 to 300 cm-’ the spectra were obtained using KBr pellets or Nujol mulls between KBr plates. For the region 600-200 cm-’ CsI pellets of Nujol mulls between polyethylene plates were used. Conductivity measurements. Conductivity measurements for solutions of the Cu” complexes were carried out with a Metrohm E-518 conductivity meter at 25°C for 10d3 M solutions in dimethylformamide (dmf) and methanol (MeOH). Magnetic measurements. Magnetic susceptibility measurements on powdered solids in sealed tubes were carried out by the Gouy method, from room to liquid nitrogen temperature. The apparatus, calibrated with Hg[Co(NCS),], was supplied by Newport Instruments. The magnetic moments were corrected for diamagnetism by using Pascal’s constants and for temperature independent paramagnetism, TIP = 0.75 mm3 mol- ’ for copper(H). ” EPR spectra. EPR spectra were obtained with an X-band Varian El09 spectrometer (9 GHz) equipped with a variable-temperature accessory. The spectra were calibrated with diphenylpicrylhydrazyl (dpph ; g = 2.0037) ; the magnetic field was calibrated by use of Mn2+ in MgO. The spectra were recorded at - 140°C using sealed quartz tubes.
RESULTS
AND DISCUSSION
The chromium compounds are soluble or partially soluble in the majority of coordinating solvents (water, dimethyl sulphoxide, NJ’-dimethylformamide, methanol) but their solutions are dark green, except in dmf where the solutions remain brown. The copper complexes are not soluble in most solvents, being partially soluble in methanol, dmf, (CH,)$O and water.
2543
Properties of high spin chromium(I1) and copper(I1) complexes Table 1. Magnetic data and elemental analyses
kfp (BM)
Curie-Weiss Law
298 K
97 K
p(K)
4.b3
4.bb
-1
Cr(pyi)Cl,* 1/2H,O
4.57
4.11
-38
3.71
UZr&>m>C)z *H,D
4,XS
4.33
-34
3.93
Cu(pyb)Cl,
1.79
1.80
2
0.51
Cu(pyi)Cl 2
1.81
1.85
6
0.51
Cu(pyim)Cl,
1.88
1.90
6
0.54
Complex uy_qYbjC~_~
;H_,o
10’ Cb 3.44
Analyses (%) g
C
42.8 (42.9) 34.5 (34.7) 33.b (33.4) 2.07 43.2 (43.7) 2.08 34.5 (34.4) 2.14 34.4 (34.1)
pCalculated from p(e,r= 797.8 (~~7’)“’ and the Cur&Weiss ‘Values in m3 mol- ’ K. ‘Calculated values are given in parentheses.
Compounds of [Cr(L-L)Cl,] Magnetic data. The observed room temperature magnetic moments are somewhat below the spinonly values expected for systems with four unpaired electrons (,u r 4.83 BM) and their values for [Cr(pyi)Cl,] and [Cr(pyim)Cl,] decrease with decreasing temperature. The Curie-Weiss law is obeyed for all complexes down to 97 K, although for [Cr(pyb)Cl,] the value of 0 z 0 (Table 1). The magnetic moments for monomeric, tetrahedral high-spin chromium complexes are assumed to be greater than the spin only value and to show a marked temperature dependence, I3 as expected from a T2 ground state. Our data do not fit this pattern, as the room temperature magnetic moments are smaller than the spin-only value and thus rule out tetrahedral coordination for the chromium complexes reported in this work. The observed magnetic behaviour is similar to those found in other high-spin chromium(I1) complexes of general formula [Cr(L-L’)X,], where L-L’ is a bidentate ligand and X a halide,3*4,‘4for which a polymeric species with a hexacoordinate sphere around the chromium was asserted. Assuming that a similar structure applies to our complexes, the magnetic moment dependence with temperature can be attributed to antiferromagnetic interaction between chromium atoms. A superexchange mechanism involving chlorine bridges between the metal ions can account for partial spin coupling and is consistent with the proposed polymeric structure.
H 3.5 (3.3) 3.0 (2.9) 3.5 (3.8) 2.7 (2.8) 2.5 (2.5) 3.2 (3.2)
N
Cr
Cu
12.4 13s (12.5) (15.5) 15.0 18.9 (15.2) (18.8) 13.9 133.8 (14.6) (18.0) 12.5 19.1 (19.3) (12.8) 15.0 22.5 (22.7) (15.0) 14.9 22.9 (22.6) (14.9)
Cl21% (21.1) 25.5 (25.6) 243 (24.6) 21.3 (21.5) 25.1 (25.4) 25.0 (25.2)
Colour mark brown Brown Bark brown Green Green Green
law, xA-’ a (T--8).
Although for [Cr(pyb)Cl,] the value of 8 z 0, a polymeric structure is also assumed, due to the similarity of the spectroscopic data with those of the other two chromium(I1) complexes (vide infra). Electronic spectra. The band maxima are presented in Table 2 ; the electronic spectra of the complexes in Nujol (Fig. 1A) show two bands in the region 45&600 nm (22,200-16,700 cm-‘) and a weaker one at x 750 mn (13,300 cm- ‘). This behaviour is typical of distorted high-spin six-coordinate chromium(I1) complexes, 3*4,6~8*’ 4*’’ where the low energy (distortion) band is associated with transitions between the components of the ‘E ground state (5B, + ‘A,), and the higher energy (main) bands is assigned to superimposed ‘B’ + (‘Bz, ‘E) transitions. The main bands, although affected by stereochemical distortions of the complex, are mainly determined by the ligand field strength, and their values are similar to those of polymeric hexacoordinate species with monoamine bidentate ligands and chloride. 3,4 The complexes prepared, dissolve readily in water, methanol and other hydroxylic solvents, but the resulting solutions are dark green. The electronic spectra of the complexes in these solutions (Fig. 1B) show new high intensity bands in the region from 1250 to 400 nm (25,000-8000 cm- ‘). These spectra show identical bands to those observed in Nujol mulls and in solution for the lowspin complexes [Cr(L-L’)3]C12, with the same ligands (L-L’) used in our study. ’ This observation implies the existence of low-spin complexes in
2544
B. DE CASTRO et al. Table 2. Solution and Nujol electronic spectra of the Cr(L-L’)Cl, complexes”
Complex Cr(pyb)Cl, * H,O
Absorption bands (cm- ‘) 20,10O(sh) 20,100 21,730 21,500
Cr(pyi)Cl,* 1/2Hz0
22,80O(sh) 22,700 21,970 21,730
Cr(pyim)Cl, - Hz0
17,240 16,660 16,800 20,0OO(sh) 20,100 17,200 16,000 16,000
15,310 15,450 14,490 14,380 15,030
13,330h 13,330b
14,280’ 14,280’
13,330 13,500 13,690 13,330 13,600
Solvent
94306 9520 9250
10,100 10,200 10,300
8620 8330 8360
Nujol dmf (CH&SO Methanol Water
9090 9170 9210
Nujol dmf (CH s)SO Methanol Water
14,20O(sh) 14,000 9250 13,150 13,880 10,lOOb 9090 12,980’ 20,400 14,080h 10,100~ 9090 12,820’ 20,830 13,880’ “The molar extinction coefficients for the solution spectra are not reported as the high reactivity prevents their determination. ‘These absorption bands were obtained by spectral deconvolution. 20,50O(sh) 21,000
18,20O(sh) 18,400 17,100 16,390 16,260
(A)
Nujol dmf (CH,)zSO Methanol Water of these species
hydroxylic solvents and provides spectral evidence for the disproportion of monoamine complexes in solution.
3/n[Cr(L-L’)Cl,], z$ [Cr(L-L’),]C12 + [CrCl,].
Fig. 1. Electronic specta of Cr(L-L’) Cl, complexes. (A) Nujol spectra: (a) Cr(pyi)Cl,; (b) Cr(pyim)Cl,; (c) Cr(pyb)Cl,. (B) Solution spectra of Cr(pyi)Cl, in: (a) dmf; (b) (CH&SO ; (c) H,O and (d) MeOH. The molar extinction coefficients are not reported as the high reactivity of the chromium(I1) species prevents their determination.
The dissolution of these complexes in hydroxylic solvents breaks the halogen bridges and the polymeric structure is destroyed. Similar behaviour has been observed for the analogous complexes with ophenanthroline, where no bands from CrCl, are observed in the spectra, since the absorption coefficients of CrCl, are negligible relative to that of the low spin complex. The observation that solution spectra in (CH3)$0 keep some of the features of the Nujol spectra but also exhibit new intense bands, suggests the existence in solution of some of the original species, thus implying only a partial breakdown of the solid structure. Also, as the spectra in dimethylformamide are similar to that obtained in Nujol, it is also conceivable that the solid structure must be retained to a large extent in solution. IR spectra. Information on metal-ligand stretching vibrations for high-spin chromium(I1) complexes is very scarce’6-‘9 and no data for complexes with bidentate a&-diimines and halogens were found in the literature. Larkworthy et al. reported IR data for several high-spin chromium(H) chlorocomplexes with pyridine derivatives and assigned the strong band at ~320-330
cm-’ to the stretching vibration Cr-Cl, and the less intense band at 27@-290 cm- ’ to the stretching
2545
Properties of high spin chromium(I1) and copper(I1) complexes vibration Cr-N. In these complexes the halogens are bridging the chromium atoms and one v(Cr-N) vibration and two IR active v(Cr-Cl) vibrations are expected for these polymeric compounds. The other v(Cr-Cl) band was assumed to occur at ca 160 cm-‘, but in the spectra” of [Cr(4-mepy)z X,], a band at 280 cm-’ observed for the chlorocomplex was shifted to 25 1 cm- ’ when the chlorine was replaced by bromine. It is conceivable that this latter band is also v(Cr-Cl). For octahedral monomeric complexes with the same ligands (L) and axial terminal halogen atoms [CrL,X,], the stretching vibrations Cr-Cl are observed at a much lower frequency (M 150 cm- ‘), but the stretching Cr-N showed little or no variation. This result is observed for metal ions with Jahn-Teller effect, where long axial bond lengths are expected, thus shifting v(Cr-Cl) to lower frequencies.2S22 To assign the IR spectra of our complexes, the spectra were compared with those of analogous compounds with bromide.23 The observed stretching frequencies for the chloro and bromo complexes in the region 40&200 cm-’ are listed in Table 3, and typical IR spectra are shown in Fig. 2A. For [Cr(pyb)C12] and [Cr(pyi)Cl,] the spectra show one strong anion dependent band at x330 cm- ‘, that is shifted by ca 50 and 30 cm- ‘, respectively, when the halogen is replaced by bromide (Table 3). Halogen replacement also shifts the band at 305 cm- ’ of [Cr(pyb)Cl,] to 249 cm- ’ ; and for [Cr(pyi)C12] the band at 295 cm-’ is shifted to 260 cm- ‘, thus allowing an unambiguous assignment of these two bands to v(Cr-Cl). The bands at 322 and 305 cm- ’ for [Cr(pyb)Cl,] and for [Cr(pyi)Cl,], respectively, are anion independent and are assigned to v(Cr-N).
400
360
300
Fig. 2. IR spectra of M(pyi)X, complexes. (A) (1) Wpyi)Cl,; (2) Cr(pyi)Br,. (B) (1) Cu(Pyi)Cl,; (2) Cu(pyi)Br,.
As the complex [Cr(pyim)Br,] could not be prepared, v(Cr-X) was not assigned by halogen substitution, but as v(Cr-N) are not strongly ligand dependent, the bands at 336 cm- ’ and at 305 cm- ’ are assigned to v(Cr-Cl), and the one at 278 cm- ’ is then assigned to v(Cr-N). The similarity of high energy band v(Cr-Cl) in our complexes with those of other chromium(I1)” complexes with polymeric structure also provides evidence for the postulated structure of our complexes, where asymmetric halogen bridges take place with two short and two long bonds,
Table 3. IR spectra of Cr(L-L’)X, and Cu(L-L’)X, complexes IR bands, v (cm- ‘) x = Cl Complex Cr@yb)Xz Cr@yi)X 2
Cr(pyim)X, Cu(wb)Xz
Cu(pyi)Xz Cu(pyim)X,
(M-N)
260 Shi’)
X=Bf
(M-Cl)
(M-N)
(M-Br)
322
332
305
322
283
249
305 278
329 335
295 305
310 h
291 h
260
322 285 258
304 322 304
282 280 277
321 287 264
291 297 289
249 249 247
a From ref. 33. bThe complex could not be prepared.
b
2546
B. DE CASTRO et al.
Compounds of [Cu(L-L’)Cl j
Contrasting with chromium complexes, the known stereochemistries for copper complexes of general formula [Cu(L-L/)X,] or [CuL,X,] are much more diverse, and the choice of the correct one can hardly be accomplished without X-ray data from single crystals. ‘,24 However, combination of spectroscopic and magnetic data, allows for some insight into the stereochemistry of the copper complexes studied. EPR data. Powdered samples of the pure complexes exhibit at low temperature, EPR spectra that are all different (Fig. 3A and Table 4). For [Cu(pyi)C12] the spectrum is of the rhombic type with three g values ; for [Cu(pyb)Cl,] only two g values are obtained (axial type) ; and for [Cu(pyim)Cl,] a pseudo-isotropic spectrum is obtained. No hyperfine splitting with copper or with the ligand nuclei ( 14Nor 3‘v3‘Cl) are observed, probably due to dipole-dipole or exchange interaction. As no signal at half field (AM, = +2) is observed for any spectra, the existence of dimeric structures bridged either by the cr,a’-diimine ligand or by the halogen, can be ruled out. However several structures are still possible, monomeric (pseudo-tetrahedral) and polymeric ; the latter situation may include doubly halogen-bridged hexacoordinate structure, and singly halogen-bridged five-coordinate complexes. Polycrystalline EPR data do not allow for a clear-cut distinction between monomeric (pseudo-tetrahedral) and polymeric geometries. Better resolution was achieved in copper-doped (a,cr-diimine) cadmium dichloride compounds, for which axial type EPR spectra with coupling to copper in the low field region (I = 3/2 ; four bands) are observed (Fig. 3B). For [Cu(pyb)C12] a hyperfine structure is also observed in the high field region. The structures of these (a&-diimine) cadmium dichlorides are not known, but those of [Cd (mpyb)Cl& where mpyb is 2-(4’-methyl-2’-pyridyl)benzimidazole, and of other compounds of general formula [Cd(L-L’)Cl,] with bidentate diimine ligands, are polymeric distorted hexacoordinate with bridging halogens.2’ This coordination geometry must be imposed on the dopant copper complexes and the values of G = (g,, - 2)/(gl - 2) = 4 are indicative of a dxz_yz ground state for these pseudo-octahedral complexes.26 For the doped complexes, the g values are invariant within experimental error, suggesting similar electronic properties. However there is a difference between the A,, values of the [Cu(pyb)Cld and the other two complexes ; this may be interpreted as resulting from stronger stereochemical distortions imposed for the complex with the bulkier ligand. Indeed, gll and
(B)
: : :...:
Fig. 3. EPR spectra of Cu(L-L’)Cl* complexes. (A) Powder spectra : (a) Cu(pyi)Cl, ; (b) Cu(pyim)Cl, ; (c) Cu(pyb)Cl,. (B) (a) Cu/doped Cd(pyi)Cl, ; (b) Cu(pyi)Cl, in dmf.
A,, values seem relatively insensitive to strong axial perturbations, but rather sensitive to in-plane distortions for hexacoordinate species. ’ Frozen solution EPR spectra of the pure complexes in dmf (Fig. 3B) and methanol show two sets of bands that must be due to the existence of at least two different species in solution, indicating the compounds to have undergone some structural changes when in the solvents (see below electronic spectra and conductivity data). All spectra show coupling to copper in the low field region and to the nitrogen atoms in the high field region. ZR spectra. To assign the IR spectra of our complexes, the analogous bromo complexes were also prepared. The observed stretching frequencies for the chloro and bromo complexes in the region
2547
Properties of high spin chromium(I1) and copper(I1) complexes Table 4. EPR data for Cu(L-L’)Cl, complexes Complex Cu@yb)Cl,
Cu(pyi)Cl,
Cu(pyim)Cl,
gla powder Cu/Cd dmf MeOH
2.23 2.29
92a
93
A”
All
Gh
2.07 2.07
2.12 2.14
133
4.1
2.30
2.07
2.15
162
4.1
powder Cu/Cd dmf MeOH
2.20 2.28
2.10 2.07
2.12 2.14
143
4.0
powder Cu/Cd dmf MeOH
2.12 2.28
2.12 2.14
140
4.0
d
2.05
d d
2.07
d d
“In methanol and in the doped cadmium complex, g,, is g,, and g2 is gl, except for Cu(pyim)Cl, whose powder spectrum is pseudo-isotropic. bCalculated as G = (g,, -2)/b, -2).25 ’ Cu/Cd refer to EPR data for copper doped (a&-diimine) cadmium dichloride compounds. dEPR data show that there are more than one complex in solution, see Fig. 3B.
cm-’ are listed in Table 3, and typical IR spectra are shown in Fig. 2B. For [Cu(pyb)Xd and [Cu(pyim)Xd three bands are observed for both halo complexes. One of them (at 322 and 258, respectively for the pyb and pyim complexes) is invariant with halogen substitution and is attributable to v(Cu-N). The other two are anion dependent and have been assigned to v(Cu-X). The complexes [Cu(pyi)XJ show also two well-resolved bands that are anion dependent and assigned to v(Cu-X), and partially superimposed on these bands the anion independent on assigned to v(Cu-N) can also be observed. Absorptions assignable to v(Cu-N) fall in the region 320-250 cm-’ , typical of copper(I1) complexes with aromatic nitrogen ligands, and increase with the aromatic character of the diimine ligand. The v(Cu-Cl) bands lie in the frequency range indicative of terminally bound chlorides in pseudotetrahedral compounds (320-280 cn- 1),27 although for polymeric hexacoordinate copper complexes with two nitrogen donor atoms the values observed for bridging chlorides lie in the same region, well above the expected values for bridging chlorides in other late first transition divalent metal complexes, that appear below 250 cn- ‘. This anomalous behaviour has been attributed to asymmetric M-X-M bonds caused by the Jahn-Teller effect in d9 systems, and as copperhalogen stretching frequencies are highly dependent on bond lengths, values ranging from 400 to 240 400-200
have been observed for distorted polymeric hexacoordinate compounds. ** The spectra of [Cu(bipy)Xd and of [Cu(phen)X,], whose structure is known to be polymeric hexacoordinate, show three sets of strong or medium intensity bands in the region 320-220 cm- ’ that have been assigned to v(Cu-N) and to v(Cu-X) (two bands).** The similarity of our ligands to bipy and phen, and of the IR spectra of our copper complexes with those of bipy and phen, lend some support to propose a polymeric distorted hexacoordinate structure for the complexes reported. Nevertheless, a polymeric tetragonally distorted five-coordinate structure could not be excluded for all the complexes, as two different types of Cu-Cl bonds (one short and one long) are also expected for this geometry.29 Magnetic data. The Cur&Weiss law is obeyed in the temperature range studied for all compounds with very small and positive Weiss constants, with room temperature magnetic moments (Table 1) lying in the range 1.8-1.9 BM, higher than the spin-only value. I3 Both results provide no evidence for significant magnetic interaction between copper atoms, in the temperature range used, despite the postulated polymeric (hexacoordinate) structure. The g values calculated from the magnetic data show reasonable agreement with those of gay obtained by EPR. Electronic spectra. The visible/NIR spectra in Nujol (Table 5) show the usual h-d type tmsymmetrical band, with maxima at ca 12,500-14,500 cm- ’ tailing into the near IR, I4 and with energies cm-’
B. DE CASTRO et al.
2548
Table
5. Nujol,
solution
electronic
spectra and conductivity complexes
data
Electronic spectra, v (cm- ‘) Complex
Nujol
dmf
Cu(pyb)Cl, Cu(pyi)Cl, Cu(pyim)Cl,
12,820 14,700 13,340
12,820 13,510 13,700
(CHj)$O
CHjOH
12,820 13,340 13,340
13,510 13,890 14,090
of Cu(L-L’)Cl,
Conductivities, AM (S cm’ mol- ‘) H,O
dmf
CH30H
13,890 14,090 14,290
15.9 22.2 25.0
73.6 73.9 84.4
The values of AM (S cm* mall ‘) in methanol are : 8&l 15 for 1 : 1 electrolytes and 160220 for 1 : 2 electrolytes, respectively ; in dmf the values for 1 : 1 electrolytes are 60-90, and for 1 : 2 electrolytes 13&l 70. 3o
comparable to that of [Cu(phen)C1J.30 This broad band is likely due to several overlapping transitions that are not well resolved, although a deconvolution into two broad bands seems obvious, and assuming Ddhsymmetry, the unsymmetrical band are assigned to 2B1, -+ (*B2, 2E) transitions. Solution electronic spectra show marked variation in band maxima and spectral width, suggesting that the postulated bridges are partially broken in solution, corroborating the EPR data that more than one species are present in solution. As the possible structures in solution are not known, no attempts to deconvolute the unsymmetrical band were made, since possible overlap of bands from different species are possible. Nevertheless, the band maximum show a red shift for dmf and (CH3)2S0, and a hypsochromic shift for the hydroxylic solvents (water and methanol). The latter shifts are indicative of appreciable chloride substitution by the solvent molecules yielding a stronger ligand field. The red shift can be explained by noting that dmf and (CH3)2SO are less effective in disrupting the halogen bridges, and that the resulting species could then involve weaker axial coordination to the solvent molecules, thus rendering the total ligand field weaker than in the polymeric structures. Conductiuity data. The molar conductivity values (Table 5) for dmf solutions of the compounds (ca lo- 3M) are indicative of non-electrolyte behaviour, but as EPR and electronic spectroscopy data show, the complexes undergo structural changes in solutions through incorporation of solvent molecules in the coordinating sphere of the metal. The conductivity data for methanol solutions are in the lower limit for 1 : 1 electrolyte type behaviour, 3’ and suggests partial release of chloride ions into solution and an extensive disruption of the polymeric structure. As the EPR spectra also
indicate the presence of at least two species in solution, in this solvent it is possible that some chloride ligands are replaced by solvent molecules, also supported by the large changes induced in their electronic spectra. CONCLUDING
REMARKS
The structural characterization of the chromium(I1) compounds posed no ambiguities and hexacoordinate polymeric structures with JahnTeller distortions were proposed based on magnetic and spectroscopic data. For copper a bewildering range of structural types is known for complexes of the general formulation Cu(L-L/)X2 or CuL2X2,24,32,33and it has proved difficult to predict even gross structural features in these complexes in the absence of crystallographic data. In the lack of relationships between structural characteristics and spectroscopic and magnetic properties for these types of copper(I1) complexes, the non-availability of Xray data raises severe difficulties in obtaining structural information for our complexes. Nevertheless, by comparing the data for our complexes with those of other copper(I1) complexes with similar ligands and with known structures, we were able to suggest for the complexes reported a hexacoordinated polymeric skeleton, similar to that observed for the analogous chromium(I1) complexes. work was supported by Instituto National de Investigacgo Cientifica (INIC, Lisboa) through contract no. 89/EXA/3.
Acknowledgement-This
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