Synthesis and Structure of a Trigonal Monopyramidal Fe(II) Complex and ItsParamagnetic Carbon Monoxide Derivative

Monopyramidal Fe(II) Complex and Its Paramagnetic Carbon Monoxide Derivative Manabendra Ray,† Adina P. Golombek,‡ Michael P. Hendrich,*,‡ Victor G. Young, Jr.,§ and A. S. Borovik*,† Department of Chemistry, Kansas State UniVersity Manhattan, Kansas 66506 Department of Chemistry, Carnegie-Mellon UniVersity Pittsburgh, PennsylVania 15213 Department of Chemistry, UniVersity of Minnesota Minneapolis, Minnesota 55455 ReceiVed January 4, 1996 We report the isolation and structural characterization of an Fe(II) complex containing a trigonal monopyramidal coordination geometry. Trigonal monopyramidal coordination to a metal ion is rare1 and, to our knowledge, has not been observed previously for Fe(II) ions.2 To stabilize this coordination geometry, the tripodal ligand, tris-(N-isopropylcarbamoylmethyl)amine (H31), was synthesized consisting of a tertiary amine and three deprotonated amides with appended isopropyl groups.3 These isopropyl groups are designed to limit access of exogenous ligands to the Fe(II) center.4 This constrained microenvironment can lead to iron adducts with new structural and physical properties, as is illustrated by the formation of a paramagnetic Fe(II)-carbon monoxide complex. H31 was obtained in a one-step synthesis by treating nitrilotriacetic acid with isopropyl amine and triphenyl phosphite in pyridine (Scheme 1).5,6 Isolation of K[Fe1iPr] was accomplished by the following procedure: a solution of H31 (0.200 g, 0.636 mmol) in 10 mL of dry DMF was treated with solid KH (0.077 g, 1.9 mmol) under an Ar atmosphere. After gas evolution ceased, solid Fe(OAc)2 (0.110 g, 0.632 mmol) was added in one portion. The mixture was stirred (1 h) and filtered, and the resulting light yellow solution was concentrated to dryness under reduced pressure to yield a pale yellow powder (0.24 g, 78%). Pale yellow crystals were obtained by diffusing diethyl ether into a DMF solution of K[Fe1iPr].7 Trigonal monopyramidal coordination in [Fe1iPr]is confirmed by an X-ray diffraction study (Figure 1A).8 Trigonal ligation to Fe(II) is provided by the three amidate nitrogen donors of [1iPr]3with an average Fe-Namid distance and NamidFe-Namid angle of 2.020(3) Å and 118.7(1)°. The Fe(II) ion lies 0.23 Å out of the trigonal plane formed by N(2)-N(3)N(4), displaced toward the vacant coordination site. The apical N(1) is positioned perpendicular to the trigonal plane with an average N(1)-Fe-Namid angle of 83.4(1)°. This positioning of N(1) results in [Fe1iPr]having near C3 symmetry where the axis coincided with the Fe-N(1) bond. The Fe-N(1) bond length is 2.098(3) Å, which is similar to those found in other high-spin Fe(II) complexes.9,10 The molecular structure of [Fe1iPr]also reveals that the appended isopropyl groups form a cavity about the Fe(II) that encompasses the vacant axial coordination site. The isopropyl groups have adopted bisected conformations in which one methyl moiety of one group is oriented into the cleft formed by the two methyl groups of a neighboring isopropyl group. These conformations position the methyl substituents of each isopropyl group inside the cavity with the methine hydrogens disposed outside the cavity toward the amide carbonyl groups. This arrangement of appended groups occurs, in part, to minimize the steric interactions between the isopropyl and carbonyl groups of each amide moiety.11,12 The “gearing” of isopropyl groups13 observed in [Fe1iPr]produces a flexible cavity that can accommodate exogenous ligand binding to the Fe(II) center to form five-coordinate complexes. This is demonstrated by the formation of [Fe1iPr(CO)]whose molecular structure is shown in Figure 1B.14 Note that in [Fe1iPr(CO)]the isopropyl groups have rotated, enlarging the size of the cavity, as can be seen by the 17.4° reduction in the average angle between the planes formed by OamidCamid-Namid:Namid-Cmethine-Hmethine. The binding of an exogenous ligand to [Fe1iPr]contrasts with the results found for TMP complexes of [1t-Bu]3-, a ligand similar to [1iPr]3except for the appended tert-butyl groups.1g In these complexes the conformations of t-Bu groups prevent binding of exogenous ligands to the metal. * Authors to whom correspondence should be addressed. † Kansas State University. ‡ Carnegie-Mellon University. § University of Minnesota. (1) Structurally characterized trigonal monopyramidal metal complexes with tripodal ligands: (a) Ni(0), Mealli, C.; Sacconi, L. J. Chem. Soc., Chem. Commun. 1973, 886. (b) Co(I), Sacconi, L.; Orlandini, A.; Midollini, S. Inorg. Chem. 1974, 13, 2850. (c) Cu(I), Karlin, K. D.; Dahlstrom, P. L.; Stanford, M. L.; Zubieta, J. J. Chem. Soc., Chem. Commun. 1979, 465. Karlin, K. D.; Dahlstrom, P. L.; Hyde, J. R.; Zubieta, J. J. Chem. Soc., Chem. Commun. 1980, 906. Chuang, C.; Lim, K.; Chen, K.; Zubieta, J.; Canary, J. W. Inorg. Chem. 1995, 34, 2563. (d) Ag(I), de Mendoza, J.; Mesa, E.; Robdrigues-Ubis, J.-C.; Vázques, P.; Vögtle, F.; Windscheif, P.M.; Rissanen, K.; Lehn, J.-M.; Lilienbaum, D.; Ziessel, R. Angew. Chem., Int. Ed. Engl. 1991, 30, 1331. (e) V(III), Cummins, C. C.; Lee, J.; Schrock, R. R.; Davis, W. D. Angew. Chem., Int. Ed. Engl. 1992, 31, 1501. (f) Al(III), Pinkas, J.; Wang, T.; Jacobson, R. A.; Verkade, J. G. Inorg. Chem. 1994, 33, 4202. (g) Ni(II), Ray M.; Yap, G. P. A.; Rheingold, A. L.; Borovik, A. S. J. Chem. Soc., Chem. Commun. 1995, 1777. (2) A nonstructurally characterized trigonal monopyramidal Fe(III) has been reported:1e Cummins, C. C.; Schrock, R. R. Inorg. Chem. 1994, 33, 395. (3) For the use of N-amidate ligands in coordination chemistry, see: Collins, T. J. Acc. Chem. Res. 1994, 27, 279. (4) Examples of metal complexes with C3 symmetric ligands having cavity motifs:1a,b,e-g (a) Plass, W.; Verkade, J. G. J. Am. Chem. Soc. 1992, 114, 2275. (b) Cummins, C. C.; Schrock, R. R.; Davis, W. D. Angew. Chem., Int. Ed. Engl. 1993, 32, 757. (c) Shih, K.-Y.; Schrock, R. R.; Kempe, R. J. Am. Chem. Soc. 1994, 116, 8804. (d) Cummins, C. C.; Schrock, R. R.; Davis, W. D. Inorg. Chem. 1994, 33, 1448. (e) Trofimenko, S. Chem. ReV. 1993, 93, 943 and references therein. (f) Memmler, H.; Walsh, K.; Gade, L.; Lauher, J. W. Inorg. Chem. 1995, 34, 4062 and references therein. (g) Kitajima, N.; Tolman, W. B. In Progress in Inorganic Chemistry; Karlin, K. D., Ed.; Wiley: New York, 1995; p 419. (5) Barnes, D. J.; Chapman, R. L.; Vagg, R. S.; Walton, E. C. J. Chem. Eng. Data 1978, 23, 394. (6) Yield: 60%; FTIR (Nujol, cm-1), ν(NH) 3316 (s); ν(CO) 1676 (sh), 1658 (s), 1631 (s); 1H NMR (CDCl3, 400 MHz, TMS) δ 7.04 (d, 3 H, NH), 4.08 (m, 3 H, (CH3)2CH), 3.24 (s, 6 H, -CH2), 1.18 (d, 18 H, (CH3)2CH); mp 146-147 °C (uncorrected); MH+ m/e 315 (CI, isobutane). (7) Anal. Calcd (Found) for K[Fe1iPr]‚DMF, C18H34FeKN5O4: C, 45.09 (45.42); H, 7.15 (7.00); N, 14.61 (14.53). IR (Nujol, cm-1), ν(CO) 1679 (DMF, s), 1593 (amide, s); λmax (DMF) 343 (2500 M-1 cm-1); μeff ) 5.38μB (solid, 298 K), μeff ) 4.98μB [(CD3)2SO, 298 K]. (8) K[Fe1iPr] crystallized with one DMF solvate per molecule in the triclinic space group P1h with the following cell constants: a ) 8.6150(7) Å, b ) 10.3589(8) Å, and c ) 14.2889(11) Å; R ) 69.800(1)°, â ) 83.897(1)°, and γ ) 84.599(1)°; V ) 1187.7(2) Å3, Z ) 2. Of 4918 reflections collected (1.52 e θ e 24.12°, 173(2) K), 3499 were unique data (F2 > 2.0σ(F2)); R1 ) 0.0574 and wR2 ) 0.1462 with a GOF(F2) ) 1.101. (9) Lubben, M.; Meetsma, A.; Wilkinson, E. C.; Feringa, B.; Que, L., Jr. Angew. Chem., Int. Ed. Engl. 1995, 34, 1512. (10) The molecular structure of a four-coordinate Fe(II) complex with a tripodal ligand has been reported recently; the iron center in this complex has a distorted tetrahedral coordination geometry: Govindaswamy, N.; Quarless, D. A., Jr.; Koch, S. A. J. Am. Chem. Soc. 1995, 117, 8468. (11) Similar conformations of isopropyl groups have been observed in the molecular structures of [Zn1iPr]and [Ni1iPr]-, unpublished results. (12) There are no agostic interactions present in [Fe1iPr]-. (13) “Gearing” has been used to describe the arrangement of geminal or vicinal isopropyl groups attached to a planar framework. In these systems, the isopropyl groups are oriented such that the methine hydrogen of one group is positioned into the space formed by the two methyl groups of an adjacent isopropyl group: Siegel, J.; Gutiérrez, A.; Schweizer, W. B.; Ermer, O.; Mislow, K. J. Am. Chem. Soc. 1986, 108, 1569. Scheme 1 6084 J. Am. Chem. Soc. 1996, 118, 6084-6085