J. Phys. Chem. 1995,99, 137-143
137
Copper(I1) and Cobalt(I1) Complexes with Derivatives of Salen and Tetrahydrosalen: An Electron Spin Resonance, Magnetic Susceptibility, and Quantum Chemical Study Marian Valko," R6bert Klement, and Peter Pelikiin Department of Physical Chemistry, Slovak Technical University, Radlinskkho 9, SK-812 37 Bratislava, The Slovak Republic
Roman BolSa and L'ubor Dlhiiii Department of Inorganic Chemistry, Slovak Technical University, Radlinskkho 9, SK-812 37 Bratislava, The Slovak Republic
Arnd Btittcher, Horst Elias, and Lutz Muller Anorganische Chemie HI, Eduard-Zintl-Institut der Technischen Hochschule Darmstadt, 0-64289 Darmstadt, Federal Republic of Germany Received: May 30, 1994; In Final Form: October I , 1994@
The salen complexes ML (M(I1) = Cu and Co) and the corresponding tetrahydrosalen complexes M[I&]L (M(I1) = Cu) were investigated by ESR spectroscopy, by magnetic susceptibility, and by quantum chemical study (L2- and [&]L2- are anions of the following: (H2L' = N,N'-bis(3-tert-butyl-5-methylsalicylidene)2,3-diamino-2,3-dimethylbutane; H2[I&]L1 = N,N-bis(2-hydroxy-3-tert-butyl-5-methylbenzyl)-2,3-diamino2,3-dimethylbutane; H2L2 = N,~-bis(3-tert-butyl-5-chlorosalicylidene)-2,3-di~no-2,3-dimethylbutane; H2[H4]L2 = N,~-bis(2-hydroxy-3-tert-butyl-5-chlorobenzyl)-2,3-diamino-2,3-dimethylbutane). The ESR spectra of Cu(I1) complexes in frozen (100 K) toluene solution exhibit a well-resolved perpendicular part. In addition to hyperfine structure, superhyperfine lines are also seen. The superhyperfine structure in the perpendicular region for both CuL' and CuL2 complexes could be well accounted for by the interaction of two equivalent protons along with the two nitrogen nuclei. The protons here belong to the carbon atoms adjacent to the nitrogen nuclei. In the presence of pyridine (5% v/v) there is a considerable shift in both gll and gL values. The higher gll values compared with those of the parent complexes are consistent with the square pyramidal geometry implying axial (py) coordination. An almost negligible effect of an electronwithdrawing substituent (X5= C1) on spin Hamiltonian parameters was observed. Due to the aggregation of molecules, no resolved spectrum could be obtained from frozen toluene solutions of both CoL' and CoL2 complexes. The addition of an axial pyridine leads to a better resolution of the spectra. In the presence of dioxygen and pyridine (5% v/v) the frozen (100 K) toluene solution of both CoL' and CoL2 exhibits rhombic symmetry with well-resolved hyperfine structures in all three directions. The shape of the spectra and spin Hamiltonian parameters indicate the interaction of the square pyramidal cobalt core with dioxygen. The interaction of the complexes CoL(py) with molecular oxygen leads to a spin-spin pairing process which results in a partial ligand to metal charge transfer and a large spin density on the oxygen moiety. CoL complexes are low-spin d7 systems with peff= 2 . 4 9 , ~for~ CoL'. The copper complexes CuL and Cu[&]L are magnetically normal = 1 . 8 1 , ~for ~ CuL2). The calculated spin densities show that the unpaired electron is localized on the molecular orbital of b2 symmetry which is almost the dv orbital of the central atom. Only negligible spin density appears at the pyridine nitrogen atom, which is in agreement with the ESR measurements. The [CoL'] system exhibits its unpaired electron at the molecular orbital of a1 symmetry which is the net d,z metal orbital. The IND0/2 method yields the description of the dioxygen adduct which matches well with the generally accepted MO model. The QR-INDO/I failed in the prediction of the spin pairing process. It prefers either Co(t). * -02(?4) or Co(4). -02(tt) types of interaction. This may be due to an improper balance of the resonance and exchange contributions to the magnetic coupling.
Introduction The chemistry of transition metal complexes with the tetradentate Schiff base ligand salen' and, in particular, the dioxygen affinity of Co(salen) and its derivatives has been extensively studied.2 In contrast to the salen complexes, rather little is known about the corresponding tetrahydrosalen complexes. Comparing the ligand properties of salen and its hydrogenated analogue tetrahydrosalen, one expects increased
* Author to whom correspondence should be addressed. @Abstractpublished in Advance ACS Abstracts, November 15, 1994. 0022-365419512099-0137$09.00/0
N-basicity and greater flexibility as a consequence of C=N bond hydrogenation. Tetrahydrosalen should thus coordinate more easily in a folded fashion as well, which is found to be the case.3 We reported very recently4" on the salen complexes ML1 and ML2 (M(II) = Cu, Ni, and Co) as well as the corresponding tetrahydrosalen complexes M[&]L (M(I1) = Cu and Ni) (see Figure 1). Cu[h]L complexes are well-characterized crystalline solids, which, in organic solution, are stable toward oxygen. Complexes Ni[&]L are not stable in aerated organic solution: they activate the 02 molecule and one of the two C-N bonds is dehydrogenated to form a C-N imine bond, corresponding 0 1995 American Chemical Society
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138 J. Phys. Chem., Vol. 99, No. 1, 1995
TABLE 1: Evaluated EPR Spin Hamiltonian Parameters for the Adducts of the Salen Complexes ML (M(II) = Cu and CO) and Tetrahydrosalen Complexes M[b]L (M(I1) = Cup gl
2.042 2.042 2.052 2.049 2.040 2.040 2.049 2.049 2.431 2.432 2.014 2.014
gz
g3 b l )
2.232 2.232 2.078 2.079
2.213 2.213 2.247 2.247 2.191 2.191 2.222 2.222 2.001 2.007 1.989 1.989
A3
32 33 60 60 80 80 46 46 152 153 39 39
(Ail)
590 588 530 530 627 629
550 56 56
50 51
550 283 285 31 31
4 ('47) 32' 33c 40' 40' 4 1c d 41C*d 40' 4oe
AP;'
A: (A;)
a2ref 18
note
0.84 0.85 41'~~ 4lCd 40' 40'
0.83 0.83
Figure 3 Figure 4
Figure 5
45'
44c
Figure 6
a Hyperfine splitting constants are given in MHz. (To convert hyperfine constants to cm-* divide values in table by 3). A, Cu[I&]L1;B, Cu[H4]Lz; C, CuL'; D,CuLz;E, CoL'; F, CoLz. Two equivalent nitrogens. In addition to interaction with two equivalent nitrogen nuclei, interaction of two equivalent protons (the protons belong to the carbon atoms adjacent to the nitrogen nuclei) was observed with A: = 17.5 MHz. One
nitrogen from the pyridine molecule. used: frequency, 666.7 Hz; amplitude, 320 Am-'. The recorded volume susceptibility was corrected for demagnetization. Eventually a correction for addenda (signal of the sample holder filled with the gas) was applied. Electronic spectra were recorded in nujol suspension on a Specord M-40. { X X t-BU
Results and Discussion
Me
Figure 1. Structural formulas of the ligands.
-
to half-salen complexes N~[Hz]L?~ With H202 instead of 0 2 the conversion Ni[&]L Ni[H2]L is considerably faster and followed by the further dehydrogenation of this half-salen species to the salen complex with two C=N bonds. Of interest is that the reaction of cobalt acetate with Hz[&]Ll and Hz[&]L2 in aerated ethanol is remarkably different. The red products are surprisingly found to be CoL' and CoL2,respectively,instead of the expected tetrahydrosalen complexes. When the reaction of cobalt acetate with H2[&]L1 and H2[&]L2 is carried out strictly under nitrogen, formation of the hydrogenated complexes CO[&]L',~obviously occurs without being accompanied by a substantial color change. Admission of oxygen (air) initiates a strong color change, leading to the CoL' and CoL2 Schiff base complexes. This observation led us to investigate the dioxygen affinity of Co(I1) complexes in greater detail. Experimental Section The preparation of the ligands HzL1, HzL2, Hz[&IL', and H2[&]Lz as well as the synthesis of the complexes was described elsewhere?a All chemicals used were reagent grade. For the ESR measurements, the solvent toluene was dried over Na before use. The ESR spectra were recorded on a Bruker SRC-200 D spectrometer operating at X-band (ca. 9.40 GHz) equipped with a variable temperature unit. Cylindrical quartz sample tubes with 3.5 mm 0.d. (ca. 3.0 mm i.d.) were used for measurements. In g factor evaluations field gradients were corrected using the internal reference standard marker (gM = 2.0052). ESR computer simulations and spin Hamiltonian parameter optimization were done using a computational method and program described el~ewhere.~ The temperature dependence of the magnetic susceptibility of powdered samples was recorded by the AC susceptometer (Lakeshore, Model 7221). The following field parameters were
ESR Spectroscopy of the Copper(@ Complexes. In frozen solution the ESR spectra of the copper(I1) complexes exhibit a rhombic or an axial symmetry with well-resolved hyperfine and superhyperfine structures. Spin Hamiltonian parameters associated with the ESR spectra were determined by assuming a rhombic or axially symmetric spin Hamiltonian:
for S = 1/2, FU(copper nuclear spin) = 3/2, IN (nuclear spin of a nitrogen donor atom) = 1, and P(hydrogen nuclear spin) = 112. The remaining parameters have their usual meaning.6 Spin Hamiltonian parameters obtained by a computer simulation of the ESR spectra are listed in Table 1. As the (MN202) chromophore in the complexes under study has C2, symmetry, we applied the convention introduced elsewhere7 for definition of the coordinate systems: the x-axis bisecting the angle 0-M-0. Consequently the central atom d-orbital contributions broken into individual irreducible representations are al(d,z, dxz-?), a2(dy,), bl(dxz), and b2(dq). It has to be pointed out that this convention is different from that used for octahedral complexes. In order to understand the ESR spectra of (MN202) and eventually {MNi02N") systems a simplified molecular orbital diagram is instructive (Figure 2). The singly occupied molecular orbital (HOMO) of b2 symmetry involves an a-portion of the metal dq orbital and a complementary a'-portion of the appropriate ligand group orbitals (both of equatorial nitrogen and oxygen atoms). Thus the coefficient a2serves as a measure for the spin density localized on the central atom. In order to evaluate the mixing coefficients (a, a', p, p, etc.) the electron transition energies are helpful as they enter relationships such as
x
g,,- g, = -8La2y2/AzE(B2
-
A,)
(2) When the electron transition energies are available (as taken from the absorption spectra) the evaluation of the crucial parameter a is possible.
Copper@) and Cobalt(I1) Complexes
J. Phys. Chem., Vol. 99, No. 1, 1995 139
"
o/cp\
/N
0
. L x
2400
Figure 2. Simplified energy level (MO) scheme for the (CuNi02) and (CuNiOfi"} chromophore of Cz, symmetry.
I
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1
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Magnetic FieldlGauss Figure 3. First-derivative ESR spectrum of a frozen toluene solution at 100 K due to Cu[&]L2 (1.0 x loe3 mol dm-3) containing pyridine (5% v/v). (a) Experimental spectrum and (b) computer simulation using the spin Hamiltonian parameters in Table 1.
ESR Spectra of Cu[H4]L1and Cu[H4]L2. The perpendicular region of the ESR spectrum of Cu[I&]L1 in frozen (100 K) toluene solution consists of two resolved hyperfine bands. In addition to hyperfine splitting, each band is further weakly split into five lines arising from two equivalent nitrogen nuclei. The relatively low value of g l can be correlated with the square planarity of N202. An interesting change in the shape of the spectrum is observed when py is added (5% vh), the spectrum showing only one band in the perpendicular region of the spectrum (Figure 3). This band is further split into superhyperfine structure arising again from two equivalent nitrogen nuclei. Similar ESR spectra were observed for Cu[H4]L2 showing an almost negligible effect of an electron-withdrawing substituent (X5= C1) on spin Hamiltonian parameters. As can be seen from Table 1, there is an increase in both gll and g l values and a decrease in All when the pyridine molecule coordinates. The gll values higher than those for the parent complexes are consistent with the square pyramidal geometry implying axial (py) coordination. The covalency of the in-plane odd electron decreases as the electron-donating ligand pyridine is added along the z-axis. The donation of electrons to copper from the axial ligands causes a back-donation to the in-plane
2600
2600
3000
3200
3400
3600
3800
Magnetic FieldlGauss Figure 4. First-derivative ESR spectrum of a frozen toluene solution mol dn-~-~) (a) Experimental spectrum at 100 K due to CuL' (1.0 x and (b) computer simulation using the spin Hamiltonian parameters in Table 1.
ligands. An appreciable coupling to an axial nitrogen atom of pyridine was not observed, which means that the unpaired electron is still localized in the dv orbital and the almost planar structure of the copper(II) core is essentially intact. ESR Spectra of CuL' and CuL2. Compared to their hydrogenated analogues, the ESR spectra of CUL'.~in frozen (100 K) toluene solutions exhibit a much better resolved superhyperfine structure in the perpendicular region (Figure 4). This splitting with about 15 lines cannot be attributed to the interaction with two equivalent nitrogen nucleik alone, but the splittings and the line intensities could be well accounted for by the interaction of two equivalent protons (the protons belong to the carbon atoms adjacent to the nitrogen nuclei) along with the two nitrogen nuclei. This observation described in greater detail will be published elsewhere.4d Similarly, as in the hydrogenated complexes, the addition of an axially coordinated pyridine leads to some loss of the superhyperfine structure. The addition of a pyridine results in a better resolution in the parallel hyperfine structure and in a predominant shift in the gll value. ESR Spectroscopy of CoL' and CoL2 Complexes. The solid state spectra of the cobalt Schiff base complexes CoL' and CoL2 exhibit axial symmetry and are typical for low-spin d7 systems. Even at 100 K, the lines are very broad without any observed hyperfine splitting. Unfortunately, no resolved spectrum could be obtained from frozen toluene solutions of both CoL' and CoL2complexes. Rapid-freeze experiments were tried but did not produce any considerable increase in resolution. Most likely, the CoL molecules aggregate on freezing, which results in a dipolar broadening.* Literature data7 show that the electronic ground state of related planar Co(II) Schiff base systems is 2A2, with the unpaired electron localized in the outof-plane orbital az(dy,). Even a slight axial perturbation caused by an axial ligand (or solvent molecule) is capable of altering the electronic ground state to 2Al with the unpaired electron localized at the al(d,z) orbital. Co(II) Pyridine Adducts. The addition of an axial ligand such as pyridine makes the above mentioned aggregation more difficult, which leads to a better resolution of the spectra. This is illustrated in Figure 5, which shows the spectrum of the CoLl(py) adduct in toluene. The ESR spectra of both CoL1(py) and CoL2(py) adducts are similar, indicating that the perpendicular
Valko et al.
140 J. Phys. Chem., Vol. 99, No. 1, 1995
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2400- 2600
2800 3000
3200 3400 3600 3800 4000 4200
Magnetic FieldlGauss Figure 5. First-derivative ESR spectrum of a frozen toluene solution at 100 K due to COLI(1.0 x mol dm-3) containing pyridine (5% v/v) under an argon atmosphere. (a) Experimental spectrum and (b) computer simulation using the spin Hamiltonian parameters in Table 1.
3000
3100
.
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3200
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3300
.
1
3400
.
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.
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Magnetic FieldlGauss
Figure 6. First-derivative ESR spectrum of a frozen toluene solution at 100 K due to the dioxygen adduct of COLI(1.0 x lo-' mol dm-3) containing pyridine (5% v/v) exposed to an atmosphere of oxygen. (a) Experimental spectrum and (b) computer simulation using the spin Hamiltonian parameters in Table 1.
*
region is split considerably by a rhombic distortion (gl f g2 g3). Seven of eight 59C0 hyperfine lines (Fo= 7/2) for the high field (parallel region) are observable, and except for two of them, all of them are very clearly split into equal intensity triplets by the single nitrogen nucleus (P = 1) of one coordinated pyridine molecule. The superhyperfine splittings with the nitrogen atoms of the chelating ligand in the other two directions of the magnetic field are not resolved. One would expect that the Co-N(py) is of importance for the observed value of the superhyperfine splitting. It is interesting to note that a decrease of the Co-N distance gives rise to an increased transfer from pyridine to cobalt and, consequently, to an increased spin transfer from the cobalt to the nitrogen atom.g Both processes take place through a a-type interaction as it follows from the simple representation of the singly occupied molecular orbital in Co(I1) Schiff base complexes:
When p' increases with decreasing Co-N, distance (covalency), the spin density e(") $s p'2 increases as well. The spin density in these systems cannot be related to the strenth of the donor properties of the axial ligand but only to transfer and polarization mechanisms. Interaction with Dioxygen. In the presence of dioxygen and pyridine, the frozen (100 K) toluene solutions of both CoL' and CoL2 exhibit rhombic symmetry with well-resolved hyperfine structures in all three directions (see Figure 6). The shape of the spectra and spin Hamiltonian parameters indicate unambiguously the interaction of the square-planar cobalt core with oxygen.1° Excluding the term describing nitrogen superhyperfine splitting, the spectra can be analyzed by using the spin Hamiltonian given by eq 1. Earlier ESR investigations of Co(I1). - 0 2 adducts led to several inconsistencies in a simplified ESR interpretation.' The unusually low cobalt hyperfine splitting (see Table 1) was interpreted in terms of an almost complete transfer of an unpaired electron from Co(I1) to 0 2 , leading to a formal Co(1II). G2- description of the adduct. The interpretation of the cobalt (ICo = 7/2) hyperfine structure is complicated mainly by the ambiguity that it may arise from either a direct or indirect
'0Y O," \
Co" (d')
02
Figure 7. Molecular orbital model for the dioxygen CoL-pyridine adduct. mechanism. MO description of the oxygen adduct (Figure 7) would involve one of the ng*orbitals of 0 2 strongly u-interacting with the d$ orbital of the cobalt atom to form a doubly occupied u MO. The electron on the second JC*orbital of the 0 2 moiety remains unpaired.'" Thus the interaction of the pentacoordinate complexes CoLl(py) and CoL2(py) with molecular oxygen leads to a spin-spin pairing process which results in a partial ligand to metal charge transfer and a large spin density on the oxygen moiety. The localization of a negative charge on the terminal oxygen atom would support the tendency to form dimers. This can be explained by an intemal excitation to produce a true superoxide species, which may then very quickly dimerize. This was c o n f i i e d by kinetic studies which support the interpretation that an adduct with activated oxygen is formed.12 AC Magnetic Susceptibility. Within the range 77-300 K the volume susceptibility X of the complexes CuLz and CoL'
Copper(I1) and Cobalt(I1) Complexes
J. Phys. Chem., Vol. 99, No. 1, 1995 141 , , o 040
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