Electronic transitions in salicylaldimine complexes

/. inorg, nucl Chem., 1975, Vol. 37, pp. 1669-1674. Pergamon Press. Printed in Great Britain

ELECTRONIC TRANSITIONS IN SALICYLALDIMINE COMPLEXES* A. C. BRAITHWAITE, P. E. WRIGHT and T. N. WATERS Chemistry Department, University of Auckland,New Zealand (Received 22 July 1974)

Abstract--Shifts in the absorption maxima of substituted salicylaldiminecomplexes consequent upon changes in solvent, metal ion and substitution pattern are examined and the presence of a hydrogen bonding interaction in chloroform solutions established. Other effects of the solvent and the possibility of hydrolytic reactions are commented upon. The band at ca, 41,000cm ', previously attributed to a o--~ d charge transfer transition in copper complexes, is confirmed as an n ~ 7r* absorption.

INTRODUCTION WE HAVE recently examined the electronic absorption spectra of the salicylaldimine complexes of copper(II)[1] and zinc(II)[2], making a comparison between them for the purpose of assigning transitions. We have now observed band behaviour as the degree of covalency between cation and ligand is varied by substitution or by utilising hydrogen-bonding solvents[3-5]. A further study, not unconnected with a consideration of covalency, concerns the assignment of the band at 41,000 c m ~. Originally ascribed to a ~ -+ d transition it has been more recently labelled as n--, 7r* [2], but its absence from the spectra of uncomplexed ligands suggests, at least superficially, a more complicated genesis.

which was therefore chosen as the working solvent, and suggests that changes in co-ordination geometry are not normally reflected in u.v. band positions to any great extent. The two low energy bands of bis(N-methylsalicylaldiminato)zinc(II) are, however, exceptions and indicate that where there is a profound alteration in geometry--from trigonal bipyramidal to tetrahedral-shifts do occur[7]. A change is not seen in bis(N-rnethylsalicylaldiminato)copper(II) which is 5- or 6-coordinate [8] in crystals but planar in solvents [9] or in N,N'-ethylenebis(salicylaldiminato)copper(II) [10], and probably its zinc(II) analogue[Ill, which also switch to planar coordination[12] in solution from a tetragonalpyramidal stereochemistry in the solid. Bis(Nisopropylsalicylaldiminato)zinc(II)remains tetrahedral in EXPERIMENTAL both states[13,14]. The shifts seen in dioxan and Preparations of most copper and zinc complexes and ligands methanol are thus the result of "solvent effects" and are have been described elsewhere[l, 2,6]; those not detailed were not due to coordination changes, those in methanol being made by similar procedures. The cadmium(II) compounds were generally more pronounced in accord with its increased prepared from pre-formed ligands using dried methanol as solvent polarity[15]. to avoid hydrolysis reactions. They were yellow or dull red in Regardless, therefore, of changes in solvent, in colour, similar to their zinc(II) analogues, and were of low geometry, or in metal, the transition energies remain solubility in most solvents. Only the quadridentate complexes remarkably similar. (The band at ca. 33,000cm-' in were sufficiently resistant to hydrolysis at spectral dilution for satisfactory measurements. Spectroscopic-grade solvents were bidentate trans-planar copper complexes is a ligand-toused after drying over molecular sieves or by more vigorous metal charge transfer transition[l] necessarily absent in methods. Solid-state spectra were obtained by dispersing the zinc(II) compounds.) A major chemical effect which can confuse band sample in Nujol mulls or in pressed KBr discs. Data were recorded on a Cary-14 spectrophotometer. positions is the presence of hydrolysis reactions. Table 2 records the extinction coefficients for absorption bands of DISCUSSION bis(N-methylsalicylaldiminato)zinc(II) as a function of The general correspondence between solution and solid the "wetness" of the solvent. The decline of some bands state spectra (Table 1) is good, especially so for dioxan and the appearance of new ones at positions characteristic of uncomplexed ligands supports the assumption of List of abbreviations. (salimhM, bis(salicylaldiminato)M(II); hydrolysis as does the appearance of metal hydroxide (N-MesalimhM, bis(N-methylsalicylaldiminato)M(II); salenM, precipitates in some instances. Its effect on peak positions N,N'-ethylenebis(salicylaldiminato)M(II); (sal)2M, bis(salicylaldehydato)M(II); (N-i-PrsalimhM, bis(N-isopropyl- must be noted and can be illustrated by Table 2. The band salicylaldiminato)M(II);(N-Me-3,5-Clsalim)zM,bis(N-methyl-3,5- at 45,400 cm -j will be seen to increase in height on the dichlorosalicylaldiminato)M(II)and similarlyfor other substituted initial addition of water but to then shift to higher energies complexes; (salhAl(OMe), Methoxobis(salicylaldehydato)- as the absorption decreases again. Eventually, at aluminium(III); sal-o-phenCd, N,N'-o-phenylenebis 46,700cm-', it is a "free-ligand" transition. Similar (salicylaldiminato)cadmium(II). circumstances occur elsewhere, e.g. with the 38,800 cm ' 1669

1670

A. C, BRAITHWAITEet al.

Table 1. Solventshifts* (unitsof cm-~) Bis(N-methylsalicylaldiminato)copper(II) Hexane soln. 49500 43100 Solid state 49300 43100 Dioxan soln. 43300 Shift +200 Methanol soln. 44000 Shift +900

40800 37300,36600 40800 37300,36600 41300 36900 +500 0 42000 37000 +1200 +100

Bis(N-methylsalicylaliminato)zinc(II) Solid state 44300 Dioxan soln. 44400 Shift -100 Methanol soln. 45400 Shift +1100

42000 41700 -300 41800 -200

36800 38600,37000 +1000 38600,37000 +1000

28400 27300 -1100 28200 -200

Bis(N-isopropylsalicylaldiminato)zinc(II) Solid state 44400 Dioxan soln. 44300 Shift - 1O0 Methanol soln. 45900 Shift + 1500

41200 41300 + 100 42000 +800

36800 36800 0 37000 +200

26700 27100 +400 27800 + 1100

N,N'-ethylenebis(salicylaldiminato)copper(II) Solid state 49500 44000 40800 Dioxan soln. 40500 Shift -300 Methanol soln. 44100 41700 Shift + 100 +900

36500 36600 +100 36800 +300

27000 27000 0 28100 + 1100

33000 33000 33600 +600 34000 +1000

27300 27300 27800 +500 28200 +900

N,N'-ethylenebis(salicylaldiminato)zinc(II) Solid state Dioxan soln. Shift Methanol soln. Shift

44400 44400 0 44600 +200

42200 37900 40800 37000 -1400 -900 41700 38300,36500 -500 -500

27700 27800 +100 28700 + 1000

*Absences meanthat the transitiondoes not occur or that the transmissionlimitof the solvent has been passed.

band in the same complex, so that hydrolysis products must be kept to low levels, or corrected for, when peak positions are determined. We believe that we have done so within ---150cm-~ and that differences between positions are genuine when greater than 200--300cm-~. The observations of Table 1 are extended and compared in Table 3, which records shifts in the three ~r -~ zr* bands from their positions in the anionic form of the ligands, and in Table 4, which compares the solid state spectra of copper(II) and zinc(II) complexes. Coordination lowers the energy of the 37,000 cm-~ band, raises that of the 26,000 cm-~ band, and has an opposing effect for the highest energy transition. Table 4 confirms the deduction and since these "ligand bands" are of the zr~Tr* type[I,2] coordination must indeed influence the ~energy levels to some extent, most obviously by back-bonding. Hydrogen bonding to donor oxygens as seen in the crystal structures of the chloroform[3], nitrophenol[4], water[16] and methylammonium[5] adducts of a number of copper(II) complexes is also thought to enhance the ligand field effect of the donors by facilitating backbonding. A comparison of absorption energies has thus

been made between dioxan solutions as a non-hydrogen bonding standard and those in chloroform (known to hydrogen bond) and methanol. Results for copper and zinc complexes are given in Table 5. Since chloroform and methanol both have dipole moments (1.15 and 1.66 Debye, respectively) higher than that of dioxan (0.45 Debye) they should produce "solvent shifts" in the same direction but only choroform gives, with two exceptions, the red shifts in the ~-~ or* bands expected to result from hydrogen bonding[17]. (Only the two low energy ~"~ ~r* bands can be recorded because of the chloroform transmission limit at about 42,500 cm-~.) Methanol displays the solvent shift in the reverse direction. One of the exceptions to the chloroform red shift is bis(N-isopropylsalicylaldiminato)zinc(II), which has bulky nitrogen substituents likely to prevent appropriate solvent approaches to the donor oxygens. The other exception is bis(salicylaldiminato)copper(II), a planar complex where the effect of hydrogen bonding is apparently inhibited by the presence of positively charged imino hydrogen atoms in the vicinity of the donor oxygens. The red shift is re-established progressively by a tetrahedral geometry (in bis(salicylaldiminato)zinc(II))

1671

Electronic transitions in salicylaldimine complexes Table 2. Hydrolysis data Maxima found at

46000*

42000

38750*

Maxima ratio A Ao

Dilution factor

Absorbance, A

A

I 0.996 0.989 0'985 0.978 0.967 0.956 0.946 0.915 0.897 0.867

1.180 1'175 1'300 1.290 1.340 1.340 1.330 1.300 1.230 1.210 1.140

Ao = 0.98 0'95 0.79 0.80 0.72 0.69 0.67 0.665 0.63 0.61 0.60

1 0.980 0.801 0.816 0.735 0.704 0.684 0"678 0'643 0'622 0'612

36500 cm

A

A

0.46 0.45 0.48 0.48 0.505 0.50 0.495 0.49 0.48 0.47 0.455

Ao = 0.46 0.45 0-375 0.38 0.35 0.35 0.34 0.35 0.35 0.36 0.36

Maxima ratio A A, t 0.977 0.815 0.826 0.761 0.761 0.739 0.761 0.761 0.782 0.782

*Bands shift to higher energy on hydrolysis. Maxima found at

31600

28200

24700cm '

Dilution factor

Absorbance, A

1

0.993 0.987 0.980 0.971 0.952 0.926 0.909 0.885 0.877

A

A-~o

A

Ao = 0.21

1

0.25 0,26 0.27 0,30 0,31 0,33 0.34 0.36 0.36

1.19 1.24 1.29 1.43 1.48 1.59 1.62 1-71 1,71

A

A,-~,

A

A o = 1.30

I

Ao = 0

1.245 1.200 1.165 1.105 1.030 0.935 0.850 0.735 0.660

0.958 0.923 0.896 0.850 0.792 0.720 0.654 0.565 0.508

0 0 0.09 0.11 0.13 0.18 0.21 0.25 0.25

Table 3. Band shifts on complex formation (solution)

Salicylaldimine Anion Zinc(II) complex Copper(II) complex N-methylsalicylaldimine Anion Zinc(II) complex Copper(lI) complex

Peak (cm-')

Shift

Peak (cm ')

Shift

Peak (cm ')

Shift

43,300 45,000 43,300

+ 1,700 0

37,900 36,200 37,000

- 1,700 - 900

26,300 26,600 27,400

+ 300 + 1, 100

43,900 44,400 43,300

+500 -600

38,000 38,600 37,000 36,900

-200 - 1,100

26,400 27,250 27,800

+850 ~-1.400

43,500 43,700 42,900

+200 -600

38,000 37,000 36,600

- 1,400

26,300 27,800 27,000

+ 1,500 +700

42,400 43,100

+700

36.200 37,200

+ 1,000

25,800 26,700

+900

38,200 38,500 36,800

-550

25,800 26,600

+800

37,000 36,200

-800

25,000 26,600

+ 1,600

N,N'-ethylenebis(salicylaldimine)

Anion Zinc(II) complex Copper(lI) complex N-methyl-3,5-dichlorosalicylaldimine Anion Zinc(II) complex N-methyl-5-chlorosalicylaldimine Anion Zinc(II) complex N-methyl-3-methoxysalicylaldimine Anion Zinc(II) complex

42,700 44,200 42,400 42,200

+1,500 -200

1,000

1672

A. C. BRArrHwArrE et al. Table 4. Band shifts on complex formation (solid state)

N-methylsalicylaldimine Anion Zinc(II) complex Copper(II) complex

Band (cm-')

Shift

Band (cm-')

Shift

Band (cm -~)

Shift

43900 44300 43100

+400 -800

38000 36800 37300, 36600

-1200 -1000

26400 28400 27300

+2000 +900

43500 44400 44000

+900 +500

38000 37900 36500

-100 -1500

26300 27700 27000

+1400 +700

N,N'-ethylenebis(salicylaldimine) Anion Zinc(II) complex Copper(If) complex

Table 5. Hydrogen bonding shifts§ (incm -~) Bands at

43000

Complex salen Cu salen Zn (salim)2Cu (salim):Zn (N-Mesalim)2Cu (N-Mesalim)2Zn (N-iPrsalim)2Zn (N-Me-5-Clsalim)2Zn (N-Me-3,5-Clsalim)2Zn (N-Me-3-OMesalim)2Zn

CHCI3 * * * * * * * * * *

MeOH +1200 +900 +600 +500 +700 +1000 +1600 +200 -200 +900

41000 CHCI3 MeOH +500 +1200 +500 +900 * ~ 0 +1400 +400 +700 * +100 -300 +700 * +400 * ~: * ~:

37000

33000

CHCI3 MeOH CHC13 MeOH -500 +200 t t +800 +400 t t 0 +500 +200 +200 -200 + 1300 t t -100 +100 -300 +400 -500 0 + t 0 +200 t t -800 +100 t t - 1200 0 t t -500 0 t t

~:Poor resolution in methanol solution. *Bands absent because of solvent transmission limit. tTransition does not occur. §Shifts relative to dioxan solution.

Table 6. The 41000 cm ' band Medium Complex (salim)2Zn (salim)2Cu (N-Mesalim)zZn (N-Mesalim)2Cu (N-Me-5-Clsalim)2Zn (N-Me-3°OMesalim)2Zn (N-iPrsalim)2Zn salen Zn salen Cu salen Mg salen Cd (sal)2Cu (sal)2Al(OMe) (sal)2Mg (3-OMesal)2Ca 3-OMesalenCd 3,5-ClsalenCd Sal-o-phenCd

Solid state

Dioxan

Methanol

Chloroform

* * 42000 40800 41900 42900 41200 42200 40800 * * 39700 42000 * 42000 * * *

40300 42000 41700 41300 41300 42200 41300 40800 40500 40800 t 41200 41800 42200 t t t t

41700 42200 41800 42000 41700 43300 42000 41700 41700 41700 41000 * t 42000 t 43500 42600 42600

40300 42000 * 41700 41300 * 41000 41300 41000 * t 41300 t t t t t f

*Bands are not included if they could not be resolved or were beyond the solvent limit. tSpectra not obtainable due to low solubility or hydrolysis.

27000 CHCI3 MeOH 0 +1100 -900 +900 0 +900 -100 +800 -200 +400 -200 +1000 0 +700 -300 +700 -700 +400 -200 +500

Electronic transitions in salicylaldiminecomplexes and by substitution on the nitrogen (in bis(N-methylsalicylaldiminato)copper(II)). In general therefore, a red shift occurs with chloroform, the shift being in both the 26,000 cm -l and 37,000 cm -~ bands in accord with the view that it is the ~r* levels near the oxygen atoms which are particularly stabilised by hydrogen bonding[17]. It is clear, nevertheless, that such bonding does not lead to major changes in transition energies and to evidence of greatly increased conjugation between metal and ligands. The conclusions of the solid state studies are therefore only supported to the extent of verifying the presence of chloroformcomplex bonding. The assignment of the band at c a. 41,000 cm -~ poses a number of difficulties. It has been characterised as a ~r ~ d charge transfer transition following the study of copper(II) complexes[l], but its presence in corresponding zinc compounds makes this assertion untenable and its reassignment as n ~Tr* was tentatively suggested[2]. However the band is absent from spectra of free ligands although present at a remarkably constant position (40,500-42,200cm ') in a range of complexes which includes not only those of copper(II), zinc(II) and cadmium(II) but also those of the d ° metal ions aluminium(III), magnesium(II) and calcium(II)[2]. All these results are brought together in Table 6. Unless extensive orbital mixing occurs to vitiate the assumption that essential ligand and metal character is retained by the constituents of a complex, we conclude that this constancy in energy precludes charge transfer as the absorption process. Thus the band is intra-ligand and either becomes "allowed" on complex formation or shifts from beneath an obscuring transition. Two observations are pertinent. First, we find no trace of the band in oaminobenzylideneiminecomplexes where all donor atoms are nitrogens; secondly, the band is absent from acetylacetone and acetylacetoneimine complexes[18]. The absorption is therefore closely associated with a phenolic oxygen atom when it is the donor of salicylaldimine ligands. One further observation which seems to be important is the blue shift of the band, relative to dioxan, seen in chloroform solutions. It contrasts with the red shifts in the 37,000 cm ~and 26,000 cm ' bands associated earlier with hydrogen bonding and may be interpreted either as a "solvent shift" of the type found with methanol or as evidence for the presence of an n ~ ' * transition in which the base is acting as a hydrogen bond acceptor[17]. We distinguish between these two possibilities by noting the differences in blue shifts with chloroform and methanol as solvents. All the complexes listed in Table 7 show the 41,000 cm ' band moving to higher energies in alcohol solution but with chloroform that of bis(Nisopropylsalicylaldiminato)zinc(II) does not. This is just the complex referred to earlier as being sterically unable to hydrogen bond and it therefore marks the shifts in chloroform as being those expected of an n ~ r r * transition associated with an atom subjected to specific hydrogen interactions and not to general "solvent effects".

1673

Table 7. Solvent effects* on the 41000cm-' band

(salimhZn (N-Mesalim)2Zn salen Zn (N-i-PrsalimhZn (N-Me-5-Clsalim)2Zn (salim)2Cu (N-Mesalim)2Cu salen Cu (N-t-Busalim)2Cu?

CHCI~

MeOH

+500 -300 0 0 +400 +500 +900

+ 1400 + 100 +900 +700 +400 +200 +7iX) + 1200 +700

*Shifts relative to dioxan solution unless otherwise indicated (in cm -J}.

+Shifts relative to hexane solution (in cm %. -Poor resolution at solvent transmission limit.

We thus conclude that the absorption is of the n --, ~r* type arising from the modification of energy levels on the donor oxygens under the influence of a positive centre. This charge stabilises the oxygen "lone pairs" on complex formation and removes their degeneracy so that an n ~ ~'* band, which we assume to be hidden under the large (and often split[l]) 7r-*Tr* absorption at c a . 37,000 cm -~, shifts to higher energies at c a . 41,000 cm ' on co-ordination. Covalent bonding between metal and ligand is not, therefore, directly involved in the energy of the transition, the only interactions likely to shift the peak being hydrogen bonding to the lone pair, as referred to above, or those which involve the ~-* level. Table 8 indicates that shifts do indeed follow substitution on the aromatic ring, but not changes in molecular geometry or substitution on the donor nitrogen atoms; all are observations which support the assignment. The full assignment of bands in the complexes of the various metal ions which have been studied is now; 44,000cm -~ (~r~Tr*), 41,000cm -~ (n ~ r * ) , 37,000cm ~ (~r ~ rr* often appearing as a doublet), 34,000 cm-' (o- -o d but only appearing in bis-bidentate copper complexes), 27,000 cm 1 (Tr ~ ~'*) and 25,000cm-" ( d ~ 7r*).

Table 8. Substituent effects on 41000cm ' band

(salim)2Zn (salim)2Cu (N-MesalimhZn (N-Mesalim)2Cu (N-i-PrsalimhZn (N-t-Busalim)zCu salenZn salenCu (N-Me-5-Clsalim)2Zn (N-Me-3,5-Clsalim)zZn (N-Me-3-OMesalim)2Zn *Poor resolution.

Dioxan

Methanol

40300 * 41700 41300 41300 * 40800 40500 41300 40600 42200

41700 42200 41800 42000 42000 41500 41700 41700 41700 * 43300

1674

A.C. BRAITHWAITEet al. REFERENCES

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