Cyanide-bridged trinuclear complexes based on central bis(hexafluoroacetylacetonate)metal units

www.elsevier.nl/locate/ica Inorganica Chimica Acta 309 (2000) 65 – 71

Cyanide-bridged trinuclear complexes based on central bis(hexafluoroacetylacetonate)metal units Z.N. Chen, R. Appelt, H. Vahrenkamp * Institut fu¨r Anorganische und Analytische Chemie der Uni6ersita¨t Freiburg, Albertstraße 21, D-79104 Freiburg, Germany Received 27 May 2000; accepted 4 July 2000

Abstract By treatment of the complexes (hfa)2M (M= Mn, Fe, Co, Ni, Cu, Zn, Cd; hfa = hexafluoroacetylacetonate) with the metallocyanides Ln M%CN (Ln M%=Cp(dppe)Fe and Cp(PPh3)2Ru) 14 trinuclear complexes Ln M%CNM(hfa)2NCM%Ln were obtained. Their IR data and four structure determinations have shown that they contain essentially linear M%CNMNCM% arrays, i.e. have trans-configured M(hfa)2(NC)2 centers. Only complex Cp(PPh3)2RuCNCo(hfa)2NCRu(PPh3)2Cp was also obtained and structurally characterized as the cis isomer. Cyclic voltammetry has yielded no indication that there is electronic communication between the external Ln M units upon oxidation. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Trinuclear complexes; Cyanide bridging; Electrochemistry; Structure

1. Introduction The challenge of finding and using electronic communication along chains of cyanide-linked transition metal atoms [1–4] has motivated numerous preparative and physical investigations on the existence and electronic nature of metal complexes containing [MCN]x arrays. During these investigations it has turned out that it is difficult to prepare chain-like [MCN]x complexes with more than three metal atoms, and to our knowledge no such complex with more than three metal atoms has been subjected to a structure determination. However, various procedures to synthesize di- and trinuclear complexes have been described. Among the preparatively oriented research groups those of Connelly, Denning, Vogler, Ku¨hn and ourselves have made significant contributions to the chemistry of chain-like oligonuclear [MCN]x complexes [5 – 17]. We have started a systematic investigation of trinuclear complexes of the general type M%(m-CN)M(mCN)M%, trying to learn which factors control the extent of electronic communication between the metal centers * Corresponding author. Tel.: + 49-761-203 6120; fax: + 49-761203 6001. E-mail address: [email protected] (H. Vahrenkamp).

[11 –17]. We have varied the orientation of the m-CN ligands (cyanide versus isocyanide), the oxidation states and ligand sets of the metals M and M%, and the geometry at the central M units (linear, square, tetrahedral, octahedral, cis/trans). We found out that a linear array is the best, but not the only, prerequisite for good electronic communication [12,13]. Exceptions to this rule are complexes like Ln MCNZnCl2NCMLn which show communication across a tetrahedral center [15], or like Ln MCNAgNCMLn which show no communication across linearly coordinated ‘naked’ Ag [17]. One important observation was that the amount of electronic interactions depends significantly on the types of ligand on the central metal M [13,14,16]. This paper reports on our attempts to gain insight by another kind of variation, namely that of changing the central metals in the same oxidation state and ligand set along the transition series. For this investigation we chose the metal(II) acetylacetonates because they are easily obtainable for many transition metals. It is known that good donor ligands add to M(acac)2 to form trans-M(acac)2L2 [18,19]. If the ligands L are cyanometal complexes Ln M%CN this would mean that the resulting complexes Ln M%CNM(acac)2NCM%Ln contain linear M%CNMNCM% chains. In order to increase the Lewis acidity of the central M units and

0020-1693/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 0 - 1 6 9 3 ( 0 0 ) 0 0 2 3 0 - 9

Z.N. Chen et al. / Inorganica Chimica Acta 309 (2000) 65–71

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hence the binding strength of the coordinated Ln M%CN units [20] we chose hexafluoroacetylacetonate (hfa) as the acac ligand. The following seven M(hfa)2 species were thus used as the central building blocks of the trinuclear complexes. M(hfa)2 M = Mn, Fe, Co, Ni, Cu, Zn, Cd The external units to be attached to M(hfa)2 were those that we had used successfully before in trinuclear complexes. The ‘ligands’ a and b form stable and inert linkages to central metals, and they show clean and reversible one-electron oxidations at favorable potentials, thereby facilitating the investigation of electronic communication between them by electrochemical means. Cp(dppe)FeCN a

Cp(PPh3)2RuCN b

2. Results and discussion

2.1. Preparations In general, the preparative chemistry of the new complexes was straightforward. Combination of the hydrated metal hexafluoroacetylacetonates with two equivalents of the metallocyanides a or b in methanol or dichloromethane led to the trinuclear complexes in moderate yields. The products were obtained as crystalline solids. Only one of the trinuclear complexes could not be obtained: Fe(hfa)2 did not bind two Cp(PPh3)2RuCN ‘ligands’ strongly enough to from a stable FeRu2 species. The numbering of the isolated complexes is as follows:

pound as its cis isomer 3b%. Subsequently it was found out that 3b is thermodynamically preferred: heating of 3b% to 140°C led to quantitative conversion to 3b. These observations correspond to previous ones in the chemistry of transition metal acetylacetonates [18 –20]. While it seems to be a rule that M(acac)L2 derivatives are trans-configured, the cobalt(II) complexes show cis– trans isomerism and sometimes appear as equilibrium mixtures of isomers in solution. While the constitutions of the trinuclear complexes were verified by the structure determinations (see below), their identity could easily be checked by their IR data, cf. Table 1. All new compounds, with the exception of 3b%, show one distinctive, though rather broad, band for w(CN). As expected for a cis-configured complex, 3b% shows two closely spaced w(CN) bands of equal intensity. In each case the w(CN) bands appear at higher wavenumbers than those of the free ‘ligands’ a or b. The variation of the corresponding Dw allows for a simple explanation, arguing along the following line: of the three factors affecting w(CN) in cyanide bridged complexes (s-acceptor strength of M at the N-terminus, p-backdonation from M% at the C-terminus, kinematic effect due to the mass of M and M%) the latter two undergo very little variation when varying the central metal from Mn to Zn. Hence it will be the s-acceptor strength of the central metal which determines the value of w(CN). As can be seen from the Dw values there is a rather steady increase when going from Mn to Cu, which has to be interpreted by a corresponding increase in the s-acceptor qualities of the metals. The trend is broken by Zn and Cd, which can be explained by their closed-shell electronic configurations and in case of Cd by the change in kinematic influences. It is also noticeable that the symmetry change from cis-3b% to trans-3b is accompanied by a significant change in Dw. Table 1 Colors and IR data (KBr, cm−1) of the trinuclear complexes

3b%

As expected, the complexes were obtained as transconfigured derivatives of their central M(hfa)2 units, i.e. they contain linear MCNM%NCM arrays. There was one exception to this rule: the product obtained from Co(hfa)2 and Cp(PPh3)2RuCN was a crystalline mixture of a red and a yellow species. Structure determinations (see below) identified the yellow compound to be the expected 3b with trans-configuration and the red com-

Cp(dppe)FeCN(a) 1a 2a 3a 4a 5a 6a 7a Cp(PPh3)2Ru-CN(b) 1b 3b 3b% 4b 5b 6b 7b a

Color

w(CN)

Orange Yellow Black Yellow Green Brown Yellow Yellow Yellow Yellow Yellow Red Green Brown Green Green

2062 2096 2099 2105 2112 2116 2107 2073 2072 2088 2100 2098/2077 2109 2104 2104 2088

As referred to the ‘ligands’ a or b.

Dw

34 37 43 50 54 45 11 16 28 15 37 32 32 16

a

Z.N. Chen et al. / Inorganica Chimica Acta 309 (2000) 65–71

Fig. 1. Molecular structure of 2a.

2.2. Structures

67

Fig. 3. Molecular structure of 3b%.

The molecular structures of 2a, 3b, 3b% and 4b were determined with the purpose of obtaining a horizontal comparison (Fe, Co, Ni) as well as a measure of cis/trans effects. Figs. 1 – 4 show the results, Table 2 lists the data for comparisons. In the figures the phenylgroups of the phosphine ligands were omitted for clarity. The overall similarity of the three trans-complexes is obvious. In all three cases the strict linearity of the central NMN array is caused by crystallographic inversion symmetry. The MN and MO distances correspond to similar radii of Fe(II) and Co(II) and a slightly smaller radius of Ni(II). As usual the CN bond lengths do not respond to the varying electronic situation of the cyanide ligand, and the FeC(cyanide) and RuC(cyanide) bond lengths are within their standard ranges [11 –17]. There can be no doubt that the cyanide ligands are oriented such that their N termini are attached to the central metals, the indicators being the M–NC angles which are typically and significantly

more bent away from the ideal 180° values than the NCM% angles. Although we have described quite a number of trinuclear complexes containing external Cp(dppe)FeCN and Cp(PPh3)2RuCN units, almost none of them contains octahedrally coordinated transition metals at the center. Those that were structurally characterized contain central Fe(III), e.g. {[Cp(PPh3)2RuCN]2Fe(salen)}PF6 [14] or {[Cp(dppe)FeCN]2Fe(phthalocyanine)}PF6 [16]. According to the higher oxidation state of iron their FeN(CN) distances are, on the average, 0.1 A, shorter than that in 2a. The closest approximation to 4b exists in the complex Cp(dppe)FeCNZn(phthalocyanine) [16] whose ZnN(CN) distance of 2.04 A, is close to that in 4b, corresponding to the similar radii of nickel and zinc. The two isomers 3b and 3b% have offered a rare chance to assess the trans effects of the cyanometal ligands. As it turns out, they are considerable. The CoN distance shrinks from 2.12 A, in the trans complex to 2.04 A, in the cis complex, and at the same time

Fig. 2. Molecular structure of 3b.

Fig. 4. Molecular structure of 4b.

Z.N. Chen et al. / Inorganica Chimica Acta 309 (2000) 65–71

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Table 2 Comparison of relevant bond lengths (A, ) and angles (°) 2a

3b

3b%

4b

M%···M···M% Fe···Fe···Fe

Ru···Co···Ru Ru···Co···Ru Ru···Ni···Ru

MO MN NC CM%

2.092(av.) 2.102(2) 1.151(4) 1.872(3)

2.095(av.) 2.116(4) 1.145(5) 2.002(4)

180 168.4(2) 179.3(3)

180 165.1(4) 170.7(4)

NMN MNC NCM%

2.086/2.133 2.037(av.) 1.157(av.) 1.973(av.) 100.6(4) 163.2(av.) 171.2(av.)

2.055(av.) 2.048(3) 1.155(4) 1.995(3) 180 168.4(3) 170.8(3)

the CoO distance increases from 2.09 to 2.13 A, when it is transformed from a trans-OCoO to a transNCoO situation. Thus the cyanide N-termini compete successfully with the hfa O-donors for the s-acceptor abilities of the central metal ion. Even the RuC(CN) distances respond to this, being roughly 0.03 A, shorter in 3b% than in 3b and 4b. This means that the increased s-donation from the CN’s N-terminus in 3b% induces an increased p-backdonation to the C-terminus. As we have observed before [16] this overcompensates the loss of electron population in the CN’s p* orbitals with the result of a weakening of the CN bond strength, which shows up most visibly in the w(CN) values of 3b and 3b%.

2.3. Redox properties Having established that the trinuclear complexes contain linear M%CNMNCM% arrays, it could be expected that they allow electronic communication between the external metal units. To make this visible, MMCT bands would have to be observed in the NIR after one-electron oxidation lifting the complexes into the mixed-valent state with Fe(II)/Fe(III) or Ru(II)/ Ru(III) in the external positions. All complexes were therefore subjected to cyclic voltammetry, expecting a splitting of the redox waves for the external metal units which is indicative of electronic communication between them and which has been observed before in similar trinuclear complexes [12 – 15]. All complexes showed two redox waves, c.f. Table 3. Yet a straightforward interpretation of the data was not possible. The main reason for this was lack of reversibility for all redox steps. Fig. 5 is meant to visualize this. In addition the electrical currents represented by the redox waves did not have integral ratios (1:1 or 2:1) and hence could not be assigned clearly to the central or the external metal units. Thus it remains ambiguous whether the two redox waves represent stepwise oxidation of the two external metals, simultaneous

oxidation of the two external metals followed by the oxidation of the central metal or are dominated by decomposition processes. The situation could not be improved satisfactorily by cooling the solutions or increasing the scan rates. It must be concluded that the trinuclear complexes which are labile as such (c.f. the 3b/3b% isomerization) are too labile after oxidation to be suitable as mixed-valent species. In summary, it must be concluded that the advantage of the M(hfa)2 complexes (existence for a full series of first-row transition metals, suitability to bind two additional donors in trans-positions) is outweighed by their lability and their non-suitability for reversible redox reactions in the context of this study. Therefore other ML4 building blocks featuring a planar environment of M will be tested by us as central units in trinuclear complexes with long-range MMCT. Suitable candidates seem to be M(salen) [14] and M(cyclam) [21] units.

Table 3 Cyclic voltammetry a E1/2(Ox1)

E1/2(Ox2)

Cp(dppe)FeCN 1a 2a 3a 4a 5a 6a 7a

0.48 0.30 0.48 0.44 0.44 0.42 0.34 0.36

0.54 0.89 0.65 0.60 0.67 0.69 0.62

Cp(PPh3)2RuCN 1b 3b 3b% 4b 5b 6b 7b

0.79 0.79 0.78 0.95 0.85 0.82 0.84 0.78

1.00 1.00 1.27 1.06 1.08 1.10 1.01

a CH2Cl2 with TBA.PF6, 100 mV s−1, potentials in volts against Ag/AgCl.

Fig. 5. Cyclic voltammogram of complex 2a (for details see Table 3).

Z.N. Chen et al. / Inorganica Chimica Acta 309 (2000) 65–71 Table 4 Preparative details for the trinuclear complexes Product

1a 2a 3a 4a 5a 6a 7a 1b 3b 3b% 4b 5b 6b 7b a

M(hfa)2·(aq)

Yield

M.p. (°C)

M

mg

mg

%

Mn Fe Co Ni Cu Zn Cd Mn Co Co Ni Cu Zn Cd

50 50 51 51 51 51 56 50 51 51 51 51 51 56

85 60 82 91 68 72 98 88 89 89 95 35 104 100

53 37 51 55 43 46 58 46 55 a 55 a 50 18 54 51

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M(hfa)2·2H2O (M =Mn, Fe, Co, Ni, Zn, Cd) and Cu(hfa)2·H2O were prepared according to the literature procedures [20].

3.1. General synthetic procedure 212 228 234 236 178 185 114 220 246 130 (dec.) 226 150 204 228

Combined yield of 3b and 3b%.

3. Experimental The general experimental methods and measuring techniques were as in Ref. [13]. The hydrated complexes

109 mg (0.20 mmol) of a in 20 ml of methanol or 143 mg (0.20 mmol) of b in 20 ml of dichloromethane were added to a solution of 0.10 mmol of the hydrated M(hfa)2 complex in 10 ml of methanol. After stirring at room temp. for 3 h the solvent was removed in vacuo and the residue picked up in a minimum amount of dichloromethane (ca. 5 ml). This solution was carefully layered with 20 ml of petroleum ether (b.p. 60–70°C) and left standing for one week. The crystalline product which had precipitated was filtered off, washed with a few ml of petroleum ether and dried in vacuo. The experimental details are given in Table 4, the analytical data are given in Table 5. Complexes 3b and 3b% were obtained as a mixture of crystals from which the crystals for the structure determinations and the elemental analyses were picked. Table 4 lists the combined yield. Heating of the mixture to 140°C converted it to pure 3b.

Table 5 Analytical data for the trinuclear complexes Formula (molecular weight) 1a

C74H60F12Fe2MnN2O4P4 (1559.8)

2a

C74H60F12Fe3N2O4P4 a (1560.7+42.5)

3a

C74H60CoF12Fe2N2O4P4 a (1563.8+42.5)

4a

C74H60F12Fe2N2NiO4P4 b (1563.6+85.0)

5a

C74H60CuF12Fe2N2O4P4 (1568.4)

6a

C74H60F12Fe2N2O4P4Zn (1570.3)

7a

C74H60CdF12Fe2N2O4P4 b (1617.3+85.0)

1b

C94H72F12MnN2O4P4Ru2 (1902.6)

3b

C94H72CoF12N2O4P4Ru2 c (1906.6+170.0)

3b%

C94H72CoF12N2O4P4Ru2 (1906.6)

4b

C94H72F12N2NiO4P4Ru2 (1906.3)

5b

C94H72CuF12N2O4P4Ru2 (1911.2)

6b

C94H72F12N2O4P4Ru2Zn (1913.0)

7b

C94H72CdF12N2O4P4Ru2 (1960.0)

a

Solvated with 0.5CH2Cl2. Solvated with 1CH2Cl2. c Solvated with 2CH2Cl2. b

calc. found calc. found calc. found calc. found calc. found calc. found calc. found calc. found calc. found calc. found calc. found calc. found calc. found calc. found

C

H

N

56.93 56.29 55.75 55.95 55.66 55.44 54.59 54.98 56.62 56.20 56.56 56.24 52.87 52.11 59.34 59.84 55.53 56.36 59.22 59.87 59.22 59.26 59.08 58.32 59.02 59.25 57.60 56.47

3.88 4.20 3.83 4.14 3.83 4.04 3.79 3.95 3.86 4.15 3.85 4.32 3.67 3.63 3.82 4.32 3.69 3.72 3.81 4.25 3.81 4.04 3.80 3.55 3.80 4.33 3.71 3.71

1.80 1.64 1.75 1.71 1.74 1.53 1.70 1.53 1.79 1.57 1.78 1.67 1.64 1.85 1.47 1.40 1.35 1.28 1.47 1.35 1.47 1.37 1.47 1.22 1.46 1.21 1.43 1.52

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Z.N. Chen et al. / Inorganica Chimica Acta 309 (2000) 65–71

Table 6 Crystallographic details

Formula Molecular mass Crystal size (mm) Space group Z Unit cell dimensions A (A, ) B (A, ) c (A, ) h (°) i (°) k (°) V (A, 3) Dcalc (g cm−3) v (Mo Ka) (mm−1) hkl Range

Reflections measured Independent reflections Observed reflections (I\2|(I)) Parameters Reflections refined R1 (observed reflections) wR2 (all reflections) Residual electron density Largest difference peak and hole (e A, −3)

2a

3b

3b%

4b

C74H60F12Fe3N4O4P4·C6H14 1646.9 0.4×0.3×0.3 P21/n 2

C94H72CoF12N2O4P4Ru2·2CH2Cl2 2074.7 0.7×0.4×0.2 P1( 2

C94H72CoF12N2O4P4Ru2·C5H12 1976.8 0.6×0.2×0.2 P1( 2

C94H72F12N2NiO4P4Ru2· 4CH2Cl2 2246.1 0.8×0.7×0.6 P1( 1

16.175(3) 14.575(3) 17.537(4) 90 112.39(3) 90 3822.9(1) 1.36 0.72 h: −21–20 k: −19–18 l: −23–23 33 568 9135 6207 484 9135 0.050 0.164 +1.1 −0.8

13.200(3) 17.213(3) 21.547(4) 101.94(3) 101.53(3) 106.14(3) 4425.0(2) 1.50 0.73 h: 0–16 k: −21–20 l: −26–26 18 195 17 396 12 460 1102 17 396 0.045 0.147 +1.0 −0.8

13.142(3) 17.895(4) 21.879(4) 90.82(3) 99.93(3) 105.28(3) 4879.5(2) 1.35 0.61 h: −14–10 k: −19–12 l: −12–24 15 198 11 939 6533 1126 11 939 0.072 0.234 +1.7 −1.0

11.441(2) 12.503(3) 16.894(3) 78.09(3) 82.54(3) 83.67(3) 2335.9(8) 1.60 0.89 h: −14–0 k: −15–15 l: −20–20 9706 9220 8030 592 9220 0.042 0.115 +1.3 −1.2

3.2. Structure determinations The crystals were taken directly from the isolated products. Diffraction data were taken by the …/2q technique on a Nonius CAD4 diffractometer using graphite-filtered Mo Ka radiation. They were treated without an absorption correction. The structures were solved with direct methods and refined anisotropically with the SHELX program suite [22]. Hydrogen atoms were included with fixed distances and isotropic temperature factors 1.2 times those of their attached atoms. Parameters were refined against F 2. The R values are defined as R1 = S Fo −Fc /SFo and wR2 ={S[w(F 2o − F 2c )2]/S[w(F 2o)2]}1/2. Drawings were produced with SCHAKAL [23]. Table 6 lists the crystallographic data.

and 144309 for compound 4b. Copies of this information may be obtained free of charge from The Director, CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK (fax: + 44-1223-336033; e-mail: [email protected]. ac.uk or www: http://www.ccdc.cam.ac.uk).

Acknowledgements This work was supported by a postdoctoral stipend to Z.N.C. by the A. v. Humboldt Foundation. We are indebted to Marie-Louise Flay and Alexander Tro¨sch for helping to complete the project.

References 4. Supplementary material Crystallographic data for the structural analysis have been deposited with the Cambridge Crystallographic Data Centre, CCDC No. 144306 for compound 2a, 144307 for compound 3b%, 144308 for compound 3b

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Z.N. Chen et al. / Inorganica Chimica Acta 309 (2000) 65–71 [6] N.G. Connelly, G.R. Lewis, M.T. Moreno, A.G. Orpen, J. Chem. Soc., Dalton Trans. (1998) 1905. [7] N.G. Connelly, D.M. Hicks, G.R. Lewis, M.T. Moreno, A.G. Orpen, J. Chem. Soc., Dalton Trans. (1998) 1913. [8] W.M. Laidlaw, R.G. Denning, Inorg. Chim. Acta 248 (1996) 51. [9] A. Vogler, H. Kunkely, Inorg. Chim. Acta 254 (1997) 195. [10] F.E. Ku¨hn, I.S. Goncalves, A.D. Lopes, J.P. Lopes, C.C. Romao, W. Wachter, J. Mink, L. Hajba, A.J. Parola, F. Pina, J. Sotomayor, Eur. J. Inorg. Chem. (1999) 295. [11] A. Geiß, M. Keller, H. Vahrenkamp, J. Organomet. Chem. 541 (1997) 441. [12] N. Zhu, H. Vahrenkamp, J. Organomet. Chem. 573 (1999) 67. [13] G.N. Richardson, U. Brand, H. Vahrenkamp, Inorg. Chem. 38 (1999) 3070.

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[14] A. Geiß, H. Vahrenkamp, Eur. J. Inorg. Chem. (1999) 1793. [15] G.N. Richardson, H. Vahrenkamp, J. Organomet. Chem. 593– 594 (2000) 44. [16] A. Geiß, H. Vahrenkamp, Inorg. Chem. 39 (2000) 4029. [17] V. Comte, Z.N. Chen, M.L. Flay, H. Vahrenkamp, J. Organomet. Chem., in press. [18] R.C. Mehrotra, Metal b-Diketonates and Allied Derivatives, Academic Press, New York, 1978. [19] J.P. Fackler, Prog. Inorg. Chem. 7 (1966) 361. [20] K.C. Joshi, V.N. Pathak, Coord. Chem. Rev. 22 (1977) 37. [21] R. Appelt, H. Vahrenkamp, unpublished results. [22] G.M. Sheldrick, SHELXS-86 and SHELXL-93, Universita¨t Go¨ttingen, 1986 and 1993. [23] E. Keller, SCHAKAL for Windows, Universita¨t Freiburg, 1999.