Cyanide attack at low-spin iron(II)–diimine complexes: the structure of a

cyanide-containing derivative of a leaving ligand

John Burgess and John Fawcett*Department of Chemistry, University of Leicester, Leicester LE1 7RH, UK

Received 9 March 2004; accepted 16 July 2004

Abstract

The isolation and structural characterisation of the product of addition of HCN to the Schiff base derived fromphenyl 2-pyridyl ketone and 3,4-dimethylaniline (Me2bsb) provides evidence in favour of a mechanism involvingnucleophilic attack at the coordinated ligand for reaction of the complex [Fe(Me2bsb)3]

2+ with cyanide.

Introduction

Low-spin iron(II) complexes with diimine ligands, forexample the tris-ligand complexes [Fe(LL)3]

2+ withLL ¼ 1,10-phenanthroline (phen), 2,2¢-bipyridyl (bipy),diazabutadienes (e.g. (1)), Schiff bases of the type (2),or their variously substituted derivatives, are

substitution-inert thanks to the high Crystal Fieldstabilisation of their t2g

6 configuration by these stronglyinteracting ligands. They aquate very slowly [1], butdissociate less slowly at high pH [2] or in the presence ofcyanide [3]. The reactions with hydroxide and cyanideunder most conditions obey the two-term rate law [2, 3]

d½complex�=dt ¼ fk1 þ k2½Nu�g½complex�

with the k2, nucleophile(Nu)-dependent, term dominant.The mechanism of this k2 term has been a matter forlively debate over many years, the controversy centeringon the site of attack by the nucleophile [4, 5]. This couldbe at the central iron atom, but there are many reasonsto believe, or at least consider, that attack at carbonadjacent to one of the donor nitrogens of a diimineligand might provide a lower energy route to ligandremoval [5]. There are several precedents in organicchemistry for such nucleophilic attack on suitablyactivated aromatic systems, as in the generation ofMeisenheimer [6] and Riessert [7] intermediates. How-ever, although there is much circumstantial evidence,

both kinetic and spectroscopic, to support initial attackat diimine ligands coordinated to iron(II) [4, 8], nointermediate containing hydroxide or cyanide bonded tothe coordinated diimine ligand has been unequivocallycharacterised or isolated.We have now obtained evidence from the products of

reaction of a Schiff base complex of this type with

cyanide which provides further support for the hypoth-esis of such reactions proceding by initial attack at theligand, followed by ligand loss and concurrent transferof cyanide (or hydroxide) to the metal. Reaction of thetris-ligand complex of the Schiff base derived fromphenyl 2-pyridyl ketone and 3,4-dimethylaniline(Me2bsb (3) ) [9] gave a mixture of an intensely bluecomplex and a white solid. On the basis of earlier studiesof this type of reaction [10] and indeed of the pioneeringstudies of reactions of [Fe(bipy)3]

2+ and of [Fe(phen)3]

2+ with cyanide [11, 12] we believed thisreaction to occur according to the equation

½FeðMe2bsbÞ3�2þ þ 2CN�

¼ ½FeðMe2bsbÞ2(CN)2� þMe2bsb:

However, an X-ray diffraction structure determinationcarried out on a crystal taken from the white solidproduct revealed the formation of the hydrocyanationproduct of the released ligand (4). This product is thatexpected from the ligand-attack mechanism, althoughthere is also the possibility that it could have beenformed by hydrocyanation of the free Schiff base ligandsubsequent to its dissociation from the central metal ion.* Author for correspondence

Transition Metal Chemistry (2005) 30: 40–43 � Springer 2005

Experimental

Materials

A solution containing the tris–Schiff base complex[Fe(Me2bsb)3]

2+ was prepared in the usual manner[9, 13] by mixing aqueous solutions of iron(II)ammonium sulphate (AnalaR) and EtOH solutionsof phenyl 2-pyridyl ketone (Aldrich) and 3,4-dimethyl-aniline (Aldrich). The mixture was allowed to standfor 24 h (Figure 1). Then to this intensely coloureddark blue–purple solution was added, as in Barbieri’s[11] and Schilt’s [12] preparations of the analogousparent complexes [Fe(bipy)2(CN)2] and [Fe(phen)2-(CN)2], three equivalents of solid KCN. The solutionquickly changed colour to dark blue; some dark bluesolid (the dicyano-complex) settled out of the solutionand was filtered off for characterisation and use insolvatochromic studies [10]. The filtrate was reducedto a small volume to produce a second crop of thedicyano-complex. However the solid obtained con-sisted of a mixture of the required complex and theorganic product. It was a very pale blue crystal of thelatter whose structure was established by X-raydiffraction.

Crystal structure determination

Data for (4) were collected on a Bruker APEX 2000CCD diffractometer at 160 K using graphite monochro-mated Mo-Ka radiation (k = 0.7107 A). The reflectionswere corrected for Lorentz and polarisation effects. Thestructure was solved by direct methods, refined by full-matrix least-squares on F 2 using SHELXTL [14]. Allhydrogen atoms were included in calculated positions(CAH = 0.96 A) with isotropic displacement parame-ters set to 1. 5 Ueq(C) for the methyl groups and 1. 2Ueq(C) for all other hydrogen atoms. All non-hydrogenatoms were refined with anisotropic displacementparameters. Crystal data and structure refinement aredetailed in Table 1. Full crystallographic data may beobtained from the Cambridge Crystallographic DataCentre, where the CIF file (deposition number 236367)has been deposited.

Results and discussion

The structure of the Schiff base–HCN adduct conformsto expectation for such a species, with normal bonddistances and angles for the phenyl and pyridyl rings.

Fig. 1. The structure of the Schiff base–hydrogen cyanide adduct.

41

Bond angles at the carbon to which the cyanide has beenadded [108.99(13), 109.43(13), 109.84(13), and112.07(13)�] correspond closely to the tetrahedral geo-

metry expected for this now-sp3 carbon. The bonddistance between this carbon and the protonated nitro-gen is 1.447(2) A. This distance may be compared with1.28 A for the analogous bond in the tris-ligand iron(II)complex derived from 2-acetylpyridine and methylamine[15], with 1.35, 1.35–1.36, 1.33–1.36, 1.35–1.37, 1.35–

1.36, and 1.35–1.37 A in 2,2¢-bipyridyl [16], 1,10-phe-nanthroline [17], 2,2’-bipyridinium (bipyH+) [18, 19],1,10-phenanthrolium (phenH+) [19 , 20], 2,2’-bis-pyrid-inium (bipyH2

2+) [21] and doubly-protonated 1,10-phenanthroline (phenH2

2+) [22] respectively, and withthe sum of the covalent radii for singly bonded carbonand nitrogen, 1.47 A.The structure determination shows that at least some

of the ligand displaced from the tris-ligand complex bycyanide is hydrocyanated, producing (4) , in the courseof the reaction to form the dicyano-complex [Fe(Me2bsb)2(CN)2]. The formation of (4) can mosteasily be explained by initial rate-determining attackof cyanide at the ketimine-carbon of the coordinatedSchiff base ligand, followed by attack of a secondcyanide which can cause the breaking of a now-weakened iron–nitrogen bond [4]. The second iron–nitrogen bond to this modified ligand will breakquickly, releasing the conjugate anion of (4), whichcan rapidly acquire a proton to give (4) itself, leavinga transient five-coordinate intermediate [Fe(Me2bsb)2(CN)] which can rapidly scavenge another cyanidefrom solution to form the [Fe(Me2bsb)2(CN)2] product.This mechanism is outlined, in the form proposedoriginally for reaction at [Fe(phen)3]

2+ [4], in Scheme1. The alternative mechanism involves rate-determiningSN2 attack of cyanide at the iron to give aseven-coordinate transition state, then an unstablesix-coordinate intermediate [Fe(Me2bsb)2 (monoMe2-bsb)(CN)]+ containing one monodentate Schiff baseligand (monoMe2bsb), and then, by easy and fastreaction with a second cyanide, [Fe(Me2bsb)2(CN)2]and free Me2bsb as products. The compound (4)would then be formed by direct hydrocyanation of thereleased Me2bsb (3).

It is the expected difficulty of carrying out this directhydrocyanation reaction quite rapidly under mild con-ditions which causes us to favour the ligand-attackroute. The direct addition of HCN to double bonds inorganic compounds is a slow and difficult process [23],for which a transition metal complex or compound

Scheme 1.

Table 1. Crystal data and structure refinement for (4)

Empirical formula C21H19N3

Formula weight 313.39

Temperature (K) 160(2)

Wavelength (A) 0.71073

Crystal system Orthorhombic

Space group Pna2(1)

Unit cell dimensions

a (A) 8.3226(14)

b (A) 32.054(5)

c (A) 6.1515(10)

a� 90�b� 90�c� 90�

Volume (A3) 1641.1(5)

Z 4

Density (calculated) (Mg m)3) 1.268

Absorption coefficient (mm)1) 0.076

F(0 0 0) 664

Crystal size (mm)3) 0.34 · 0.27 · 0.14

h range for data collection (�) 2.53–26.00

Index ranges )10 £ h £ 10

)39 £ k £ 39

)7 £ l £ 7

Reflections collected 12225

Independent reflections 3202 [R(int) = 0.0536]

Completeness to h = 26.00� 99.9%

Absorption correction None

Refinement method Full-matrix least-squares on F2

Data/restraints/parameters 3202/1/223

Goodness-of-fit on F2

1.026

Final R indices [I > 2r(I)] R1 = 0.0362, wR2 = 0.0945

R indices (all data) R1 = 0.0386, wR2 = 0.0957

Absolute structure parameter 0(2)

Largest diff. peak and hole (e A)3) 0.272 and )0.193

42

(admittedly generally of nickel rather than iron) isgenerally sought as a catalyst [24]. This contrasts withthe well-known ready addition of cyanide to >C@O[25]. Cyanide addition to >C@N– might be expected tobe of intermediate difficulty, a presumption supportedby the classical method of carrying out cyanide additionacross C@N bonds, in which reaction with bisulphitepreceded the addition of cyanide [23 ]. It is relevant tomention here that treatment of [Fe(pmma)3]

2+, wherepmma is the Schiff base (2) with R1 ¼ H,R2 ¼ C6H4Me)4, EDTA in aqueous methanol givesthe free Schiff base, but that in aqueous acetone theadduct (5), analogous to our cyanide adduct (4), isobtained. However the free Schiff base pmma does notreact with acetone – it has to be activated by coordi-nation to the Fe2+ for addition of acetone to take place[26].

To conclude, we prefer the mechanism of cyanideattack at iron(II)–diimine complexes of this type whichinvolves initial attack at an Fe2+-activated site on thecoordinated ligand, though it must be emphasised thatthe chemistry reported here provides evidence which isstrongly supportive of this mechanism but by no meansprovides unequivocal proof of its operation. This insightinto the mechanism of nucleophilic substitution intotris–Schiff base iron(II) complexes complements recentinsights, resulting from the isolation and characterisa-tion of a key intermediate, into the mechanism offormation of this type of complex [27].

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