Solvent effects on charge-transfer spectra of compounds of the type Fe(LL)2(CN)2

spectro&imicaActa,V~l. 26A.pp.1967 tolQ62. PenjMikiPfMni6P0. Pri~tedinNorthtiIrQand

Solvent elbcts on c~e=Wer

spectra

tspeFeU),(cN),

of compounds of the

J. BIJIXIESS Chemistry Department, University of L&ester, Leiceeter (Received 10 Jammy

1970)

Ah&a&--W~velengthelengths of m&mum absorption for charge-tmnsfer spectra of Fe(LL),(CN)*, where LL = bidentate Schiff base, Fe(bipy)(CN),*, and Fe(ophen)e(CN)r+ in a range of solvents are reported. The frequencies correl&e with Reichardt’e solvent parameter ET. The sensitivitiesof these chwge-transfer bands to solvent variation sre compared for these systems and for Fe(LL)&N),, where LL = bipy, ophen, or one of their substituted derivatives, and M(bipy)(GO),, where M = MO or W. WAVEWUMBERS for

maximum absorption correlate well with the solvent parameters Er [l] or 2 [2] for metal to ligand charge-transfer bands in the spectra of 2,2’bipyridyl molybdenum tetracarbonyl, Mo(bipy)(CO,), and its tungsten rtnalogue [3]. Such a correlation has also been observed for bis-(2,2’-bipyridyl)biscyanoiron(II), Fe(bipy),(CN),, and analogous complexes containing l,lO-phemmthroline or substituted derivatives [a]. In the present paper we report similar correlations for the following related complexes :

(1) Fe(W2PhIo~

where LL = Schiff base I or II

A

(3) the iron(II1) derivative [Fe(ophen)2(CN),]+ and for the hexathiocyanatochromium(II1) anion, [Ck(NCS)$-. The solvent dependences of absorption wavenumbers for these compounds and for the recently investigated complex Fe(nioxime)2(py)a]0 [b], are compared. EXPERIME~AL

The Schiff base complexes Fe(LL),(CN), were prepared by a method exactly analogous to Schilt’s preparation of Fe(bipy),(CN), [6], in this case by reaction of an excess of cyanide with the respective complexes [Fe(LL),](ClO,),. These were in turn [I] K.DIMROTH,C. REICHARDT, T. SIEPMANN andF. BOELMANN, J~~tu-9 LiebipAnn.C?mn. 861, 1 (1963); C. REICEARDT, Angew. Chem. Id. Ed. 4, 29 (1965). [2] E. M. KOSOWER, J. Am. C&m. Sot. 80, 3253 (1968). [3] J. BURGESS, J. Organometal. Chem. 19, 218 (1969). [4] J. BURUESS, Spectroohim. Acta, aSA, 1369 (1970). [5] N. SANDERS and P. DAY, J. Ch-em.Sot. A 2303 (1969). [61 A. A. SCHILT.J. Am. Chem. Sot. 83, 3000 (1960). 1

1057

1958

J.

BTJRUESS

prepared from iron(I1) ammonium aulphate, 3,kdimethylaniline and pyridine 2aldehyde or phenyl2-pyridyl ketone [7]. The Fe(LL),(CN), compounds were characterised by analysis (C, H, N; Fe by acid hydrolysis followed by spectrophotometric determination using l,lO-phenanthroline). The stoichiometry of Fe(LL),(CN), for LL derived from pyridine 2-aldehyde was confirmed by spectroscopic titration of Fe(LL)aa+ against cyanide, in water and in methanol. Spectra run after addition of aliquots of cyanide (less than two equivalents) exhibited an isosbestic point; the spectrum of the solution after addition of exactly two equivalents of cyanide corresponded to that of a solution of Fe(LL),(CN), prepared as described above. Addition of more than two equivalents of cyanide caused no further changes to the spectrum. It was not possible to carry out a similar procedure for LL derived from phenyl %pyridyl ketone since reaction rates were too slow. The compounds K,[Fe(bipy)(CN),] and [Fe(ophen),(CN),](NOs) were prepared by published methods [6] and characterised from published spectra. K,[Cr(NCS),] was also prepared by a standard procedure [S]. Spectra were run using a Unicam SP 8OOArecording spectrophotometer, whose wavelength calibration was periodically checked against standard holmium and didymium glass filters. RESULTSAND DISCUSSION By analogy with the Fe(bipy),(CN), class of compounds [4] we can assume a cis geometry for these Fe(LL),(CN),, LL = Schiff base, complexes. The lowest energy band in the visible region, the band for which results are reported here, can be assigned to charge-transfer from iron to the Schiff base ligand (tzg+ n*). Again by comparison with Fe(bipy),(CN),, W(bipy)(CO),, and related compounds, the solvent dependence of this charge-transfer band can be ascribed to the effect of solvation variation at the cyanide ligands on the appropriate iron and Schiff base ligand energy levels. Wavelengths and frequencies of maximum absorption for the Fe(LL),(CN),, LL = Schiff base, complexes are reported, with the respective solvent ET values, in Table 1. These Schiff base complexes are soluble in a wide range of solvents so the absorption frequency/E, correlation extends over a wide range of ET values for these compounds. This correlation is illustrated, for the complex derived from pyridine 2-aldehyde, in Fig. 1. The correlation plot for the complex derived from phenyl 2-pyridyl ketone looks very similar. The ionic compounds K,[Fe(bipy)(CN)4] and [Fe(ophen),(CN)s](NOs) are soluble only in a limited range of solvents, the hydroxylic solvents of high ET values. Maximum absorption wavelengths and frequencies are reported in Table 2, and the correlations with solvent ET values illustrated in Fig. 2. The omission of published values for WFG4--WW11 in 95o/o ethanol [6] is deliberate ; selective solvation in mixed solvents complicates ET correlations. anion, Charge-transfer frequencies for the hexathiocyanatochromium(II1) [Cr(NCS),]s-, are also known to be solvent sensitive [9]. Results for this complex [7]

P. KRDMHOLZ, Inorg. Chem. 4, 609 (1965); J. BURGESSand R. H. PRINCE, J. Chem. Sot. A 434 (1967). [S] W. 0. PALMER,~3cpetimenkZ~Inorganic Chemistry, p. 391. CambridgeUniversity Press (1969). [9] J. BJERRUM, A. W. ADAMSON and 0. BOSTRUP, A&z. Chewa.Semd. 10,329 (1966).

Solvent effects on charge-transferspectra of compounds of the type Fe(LL),(CN)a

1959

Table 1. Wavelengths (A) and frequencies (3) of maximum absorption for the lowest energy charge-transferbands of b&m- (2-pyridylmethylene)-3,4-dimethylaniline]bis-cyanoiron(II)and of bie-[a-(2-pyridyl)benzylidene-3,4-dimethyl~iline]biscyanoiron(II), Fe(LL)2(CN),, where LL = formula I and II of the text respectively. Solvent ET values from Ref. [l]. Temperature 25’C Fe(LL II),(

Fe&L %(W, m NO.

solvent

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

water bOH* glyool MeOH EtOH PhCH,OH n-PrOH n-BuOH i-PrOH I&NO, MeCN DMSO t-BuOH DMJ? amtone CW% PhCl Et,0 C&l

ET

63.1 59.4 56.3 55.5 51.9 60.8 50.7 50.2 48.6 46.3 46.0 45.0 43.9 43.8 42.2 41.1 37.5 34.6 34.5

n

(mpc)

&I

682 597 613 617 629 632 634 637 640 650 665 669 654 617 683

bv.4

17.18 16.75 16.31 16.21 15.90 15.82 15.71 15.70 15.63 15.38 15.04 14.95 15.29 14.77 14.64

&,

(597) t

16.75

617 619 628 633 634 638 639 651 662 668 652 673 676 658 683 (796) t 691

16.21 16.16 15.92 15.80 lb.77 15.67 15.65 15.36 15.11 14.97 15.34 14.86 14.81 15.20 14.64 14.29 14.47

l 2,2,3,3-t.&mfhmropropssol. t Compound spsringly soluble.

60-

1 ET 50-

40I

15

I

16

17 V-+

Fig. 1. Plot of frequencies of maximum absorption (v, kK) against solvent ET values (kcal.mole-l) for bis-[N-(2-pyridylmethylene)-3,4dimethylaniline]biscyanoiron(I1). Solvent numbers correspondwith those in Table 1.

1960

J. BURQESS

Table 2. Wavelengths of maximum absorption (A) for charge-transfer bands of 2,2’-bipyridyltetracyanoiron(II), Fe(bipy)(CN)q]s-, of bis- ( 1, 10-phcnanthroline)biscyanoiron( III), Fe(ophen),(CN)s]+, and of hexathiocyanatochromium(III), [Cr(NCS),]P-. Temperature: 26“C* No.

Solvent

Fe Wpy) (W41a-

1 2 3 4 5 7 9

water PrrOHt glycol MeOH EtOH n-PrOH i-PrOH

484

528

471

567

GO 537 i i i

524 522 520 516 (513): i

4116 416 416 423 i

657 658 660 570

* 50% for Fe(ophen)s(CN),]+. t 2,2,3,3-tetrafluoropropanol. $ compound sparingly soluble. i compound insoluble. r compound reacted with solvent.

are also included in Table 2 ; again the low solubility of K&r(NCS),]~- in many solvents restricts observ&ions to hydroxylic solvents. Maximum absorption frequencies COIT&~~ sdi&dorily with ET for Alcohols, but the point for water lies well away from the frequency/E, correlation line. In order to obtain more precise absorption frequency/E, correlations for the Fe(bipy)ACN)z and W(bipy)(C% series of compounds a few more spectra have been obtained in various solvents. These new results, which complement earlier published data [3, 4, are reported in Tables 3 and 4. For all these compounds, and also for [Fe(nioxime),(py),] of Ref. [6], graphs of maximum absorption frequencies against solvent ET values have been plotted (cf.

60 -

T ET 50-

I

16

I

I

19

20

t

21

V---t

Fig. 2. Plot of frequencies of maximum absorption (v, kK) against solvent ET values (kcal.mole-l) for the bis-( l,lO-phenanthroline)biscyanoiron(III)cation (0) and for the 2,2’-bipyridyl-tetracyanoferratc(I1) anion (0). Solvent numbers correspondwith those in Table 2.

Solvent effects on charge-transferspectra of compounds of the type Fe(LL),(CN),

1961

Table 3. Additional results for wavelengths of maximum absorption (12,rnp) for the lowest energy charge-transfer band of Fe(LL)@N),, where LL = 2,2’-bipyridyl, IJO-phenanthroline, or one of the latter’s substituted derivatives. Temperature: 26% bipy glycol EtOH PhCH,OH n-PrOH n-BuOH t-BuOH DMSO PhNO,

670

ophen

&i-Me ophen 544 659 564 658 571 586

560

4,7-diMe ophen

551

5-Cl ophen 651 565 672 673 674 695

616 623

Figs. 1 and 2). Generally these plots consist of two lines, one for hydroxylic solvents and another for non-hydroxylic solvents. The slopes of all these lines are reported in Table 6. It should be emphasised that the smaller the numerical value of this slope the grester the solvent variation of maximum absorption frequency. Within the group of closely related compounds Fe(LL),(CN),, where LL = bipy, ophen, or substituted ophen, the solvent sensitivity of the absorption frequencies is nearly constant, despite the wide range of substituents from electron-releasing methyl groups to the strongly electron-withdrawing nitro group. This is consistent with the hypothesis above that the solvent variation of the iron to LL charge-transfer band merely reflects solvent effects st the cyanide ligands. Varying the metal from W to MO in M(bipy)(CO), also has a negligible effect on the solvent variation of the M to bipy charge-trsnsfer bands in these compounds. However, large changes in LL do affect solvent sensitivity. Thus for the entire Fe(LL),(CN), series the frequency/ET slope is least, i.e. solvent sensitivity greatest, for the 2,2’-bipyridyl and l,lO-phenanthroline complexes. The solvent sensitivity is less for the Schiff base complex derived from pyridine 2-aldehyde and least for that derived from the bulky phenyl2-pyridyl ketone. In view of the signiscance attributed to solvation effects st the cyanide or carbony1 ligands it is not surprising that maximum absorption frequencies for complexes containing four of these groups, e.g. W(bipy)(CO), or K.JFe(bipy)(CN)J, are more sensitive to solvent variation than those containing only two, e.g. the Fe(LL),(CN), series. The Fe(bipy)(CN),*- complex shows a greater solvent sensitivity than Table 4. Additional results for wavelengths of maximum absorption (2, m/z) for the lowest energy ohargetransfer band of M(bipy)(CO),, where M = MO, W. Temperature: 26% Solve&

Mo(bipy)(CO),

W(bipy)(CO),

Pr,OH* glycol PhCH,OH

437 461 463

445 460 476

* 2,2,3,3-tetrsfluoropropanol.

1962

J. BURUESS

Table 5. Slopes of plots of frequencies of maximum absorption for lowest energy chargetransfer bands against solvent ET values; slopes in kcal.mole-1 Compound*

Hydroxylic solvents

Fe(LL II)B(CN),

Non-hydroxylic solvents

13.4

16.3

K#J4NCS),l

-12

-

Fe(nioxime)z(py), Fe -+ py CT

-11

-11

10.3

Fe(LL H&N), Fe(bipy)ACN), Fe(ophen),(CN), Fe(&Me-ophen),(CN), Fe(B-Cl-ophen),(CN), Fe(S-NO,-ophen),(CN), Fe(4,7-diMe-ophen),(CN)2

8.6 8.0 8.1 8.6 (8) 7.7

Mo(bipy)(W, W(bipy)tW,

6.6 6.1

_

(CN),l Fexl’(ophen),(CN),l(NOII)

3.8

%Fe(bipy)

Fe(nioxime),(CN), Fe ---cnioxime CT

9.2 -

8.4

7.7 4.6 4.8 -

-12 -400

-400

* LL I E SchiB base from pyridine 2-aldehyde; LL II 3 Sohiff base from phenyl2-pyridyl ketonwf. formulae I and II of text.

W(bipy)(CO)4, but whether this difference should be ascribed to the difFerence in charge or to specific csrbonyl-cyanide ligand differences cannot be decided from the currently available evidence. It is interesting to contrast the very low solvent sensitivity of the iron + nioxime charge-transfer band in Fe(nioxime),(py), with the reasonable sensitivity of the iron + Schiff base, iron + bipy, or iron + ophen bands in Fe(LL),(CN),. Presumably the difference can be ascribed to much lower solvent interaction with pyridine ligands than with cyanide. That there is some salvation of the pyridine ligands is apparent from the variation of the iron + pyridine charge-transfer frequency with solvent in Fe(nioxime),(py),. Two unsolved problems remain. The first is the negative sign of the frequency/ET slope for the iron(II1) complex [Fe(ophen),(CN),]+. One would obviously expect different behsviour for analogous iron(II1) and iron(I1) complexes, particularly when one is uncharged ; and negative slopes for frequency/E, graphs sre not unknown [lo]. It is, unfortunately, very difficult to estimate the variation in, and hence salvation of, locsl dipoles [lo], here Fen-C-N and Fe*n-C%N. The second anomaly is the non-inclusion of water in the absorption frequency/ET plot for [Cr(NCS)JS-. It may be that alcohols can only interact with the sulphur atoms, whereas the smaller water molecules might interact with nitrogen or chromium as well, thus having a different effect on solvation energies and thus on the chromium + thiocyanate charge-transfer energy. A&nowledge?nent--Iam gratefulto the Royal Society for the award of a Grant-m-aid for the purchase of the spectrophotometerused in these investigations. [lo] P. H. EYSLIE and R. FOSTER,Rec. !#!%a~.Chin-a.84,266

(1966).