Isomerization DOI: 10.1002/anie.200804199 Synthesis and Redox-Induced Structural Isomerization of the Pentagonal Bipyramidal Complexes [W(CN) 5 (CO) 2 ] 3À and [W(CN) 5 (CO) 2 ] 2À ** Hemamala I. Karunadasa and Jeffrey R. Long* There has been a surge of interest in the use of cyanometalate complexes for the synthesis of magnetic molecules [1] and solids. [2] The variation of the metal electron configuration and coordination geometry enables adjustment of magnetic properties, such as the spin ground state, magnetic anisotropy, and the strength of the exchange coupling. Second- and third- row transition-metal ions are of particular interest, owing to a large magnetic anisotropy stemming from spin-orbit coupling, as well as to their diffuse valence d orbitals, which can result in strong magnetic exchange. [3] Owing to poorly understood reactivity patterns and the preference for low-spin electron configurations, however, there are relatively few examples of simple, paramagnetic building units containing these metals. While S = 1/2 complexes, such as [Nb(CN) 8 ] 4À and [M(CN) 8 ] 3À (M = Mo, W) have been successfully incorporated into high- spin clusters, [1c, h, 4] the symmetric ligand arrangement and lack of orbital angular momentum have typically led to only a small overall magnetic anisotropy. In contrast, use of the pentagonal bipyramidal species [Mo(CN) 7 ] 4À with a 2 E 1 ’’ ground state has yielded highly anisotropic magnetic solids, [5] and is predicted to give rise to single-molecule magnets with large relaxation barriers. [6] Analogous complexes of 5d metal ions should possess still greater anisotropy. This prediction is indeed borne out in the EPR spectrum of [Re(CN) 7 ] 3À ,a species which, unfortunately, is highly unstable to reduc- tion. [7, 8] Although the diamagnetic complex [W(CN) 7 ] 5À has been claimed on the basis of spectroscopic data, [9] attempts to reproduce these results led only to mixtures containing [W(CN) 8 ] 4À . Herein, we demonstrate the use of a mixed cyanide/carbonyl ligand set [10] to stabilize two new pentagonal bipyramidal complexes: [W(CN) 5 (CO) 2 ] 3À and [W(CN) 5 (CO) 2 ] 2À . The complex [WI 2 (CO) 3 (MeCN) 2 ] [11] was employed as a convenient, labile source of tungsten(II). Reaction with five equivalents of (Bu 4 N)CN in toluene readily afforded (Bu 4 N) 3 [W(CN) 5 (CO) 2 ](1). Compound 1 is soluble in polar organic solvents, such as chloroform, dichloromethane, and acetonitrile. A cyclic voltammogram of 1 in dichloromethane revealed successive oxidation waves at À0.91, À0.25, and À0.04 V versus [Cp 2 Fe] 0/1+ , (Cp = C 5 H 5 À ) with the first wave exhibiting partial reversibility (see Figure S3 in the Support- ing Information). Accordingly, reaction of 1 with slightly more than one equivalent of [Cp 2 Fe](PF 6 ) afforded the one- electron oxidized product (Bu 4 N) 2 [W(CN) 5 (CO) 2 ](2). X-ray analysis of a single crystal of 1 revealed two molecules of [W(CN) 5 (CO) 2 ] 3À in the asymmetric unit, each adopting a slightly distorted pentagonal bipyramidal geome- try (see Figure 1, left). [12] In both molecules, the two carbonyl groups were tentatively assigned to the axial sites on the basis of comparisons of thermal ellipsoids, W À C and W···O/N distances, and refinement residual factors. The mean W À C ax distance was found to be approximately 0.2  shorter than the mean WÀC eq distance, consistent with the stronger p-acceptor ligand occupying the axial sites. Note that this is a significant axial contraction when compared with [Mo(CN) 7 ] 4À [13] and [Re(CN) 7 ] 3À [7] which show no differences between the axial and equatorial bonds within experimental error. For each molecule, the distortion away from a perfect pentagonal bipyramidal coordination of the W II center occurs amongst the cyanide ligands, one of which (the rightmost in Figure 1) is raised above the mean equatorial plane by approximately 108, and one of which (the foremost) is pushed below the plane by Figure 1. Structures of the pentagonal bipyramidal complexes in 1 (left) and 2 (right). Orange W, gray C, blue N, and red O. In 2, the complex resides on a twofold rotation axis of the crystal, resulting in disorder of the axial carbonyl and cyanide ligands. Selected mean interatomic distances [] and angles [8] for two crystallographically independent molecules in 1: W–C(O) 1.99(2), W–C(N) 2.17(3), C eq -W- C eq 73(1), C ax -W-C ax 177.5(4), W-C-O 177(1), W-C-N 177(2). Selected interatomic distances [] and angles [8] for 2 : W–C eq (O) 2.083(7), mean W–C eq (N) 2.1825(5), W–C ax 2.030(4), mean C eq -W-C eq 72(3), mean C ax -W-C ax 173.9(2), W-C eq -O 180.0, mean W-C eq -N 178.05(5), W- C ax -(O/N) 177.4(3). [*] H. I. Karunadasa, Prof. J. R. Long Department of Chemistry, University of California, Berkeley Berkeley, CA 94720-1460 (USA) Fax: (+ 1) 510-643-3546 E-mail: jrlong@berkeley.edu [**] This research was funded by NSF Grant No. CHE-0617063. We thank Tyco Electronics for providing H.I.K. with a predoctoral fellowship, Dr. F.J. Hollander, Dr. K. Durkin, and Dr. Y. Pushkar for assistance, and Prof. K. N. Raymond for helpful discussions. Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/anie.200804199. Communications 738  2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Angew. Chem. Int. Ed. 2009, 48, 738 –741