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Inorg. Chem. 2001, 40, 1926-1935
Tetracyanomanganate(II) and Its Salts of Divalent First-Row Transition Metal Ions Jamie L. Manson,† Wayne E. Buschmann,‡ and Joel S. Miller* Department of Chemistry, University of Utah, 315 South 1400 E. Rm. 2124, Salt Lake City, Utah 84112-0850 ReceiVed September 27, 2000 The first known paramagnetic, tetrahedral cyanide complex, [MnII(CN)4]2-, is formed by the photoinduced decomposition of [MnIV(CN)6]2- in nonaqueous solutions or by thermal decomposition in the solid state. In acetonitrile or dichloromethane, photoexcitation into the ligand-to-metal charge transfer band (λmax ) 25 700 cm-1, ) 3700 cm-1 M-1) causes the homolytic cleavage of cyanide radicals and reduction of MnIV. Free cyanide in dichloromethane leads to the isolation of polycyanide oligomers such as [C12N12]2- and [C4N4]-, which was crystallographically characterized as the PPN+ salt C40H30N5P2: monoclinic space group ) I2/a, a ) 18.6314(2) Å, b ) 9.1926(1) Å, c ) 20.8006(1), β )106.176(2)°, Z ) 4]. In the solid state MnIV-CN bond homolysis is thermally activated above 122 °C, according to differential scanning calorimetry measurements, leading to the reductive elimination of cyanogen. The [MnII(CN)4]2- ion has a dynamic solution behavior, as evidenced by its concentration-dependent electronic and electron paramagnetic spectra, that can be attributed to aggregation of the coordinatively and electronically unsaturated (four-coordinate, 13-electron) metal center. Due to dynamics and lability of [MnII(CN)4]2- in solution, its reaction with divalent first-row transition metal cations leads to the formation of lattice compounds with both tetrahedral and square planar local coordination geometries of the metal ions and multiple structural and cyano-linkage isomers. R-MnII[MnII(CN)4] has an interpenetrating sphaleriteor diamond-like network structure with a unit cell parameter of a ) 6.123 Å (P4h3m space group) while a β-phase of this material has a noninterpenetrating disordered lattice containing tetrahedral [MnII(CN)4]2-. Linkage isomerization or cyanide abstraction during formation results in R-MnII[CoII(CN)4] and MnII[NiII(CN)4] lattice compounds, both containing square planar tetracyanometalate centers. R-MnII[CoII(CN)4] is irreversibly transformed to its β-phase in the solid state by heating to 135 °C, which causes a geometric isomerization of [CoII(CN)4]2from square planar (νCN ) 2114 cm-1, S ) 1/2) to tetrahedral (νCN ) 2158 cm-1, S ) 3/2) as evidenced by infrared and magnetic susceptibility measurements. MnII[NiII(CN)4] is the only phase formed with NiII due to the high thermodynamic stability of square planar [NiII(CN)4]2-.
Introduction Photochemical and thermal decomposition of homoleptic metal cyanide complexes is documented with several examples.1-3 The photochemical processes fall into two general categories, photooxidation-reduction and photosubstitution reactions.1 Photooxidation-reduction reactions are typically initiated by a ligand-to-metal charge transfer (LMCT) transition into an excited state that favors ligand-metal bond homolysis. Photosubstitution (e.g., aquation), in contrast, is typically initiated by ligand-field excited states. Thermolysis of cyanometalates in the solid state can result in loss of cyanide radicals in the form of cyanogen3 accompanied by reduction of the metal center. This type of process has also been reported for Li2MnIVF6 in the solid state as a preparation of Li2MnIIF4.4 Both photochemical5 and thermal decomposition of [MnIV(CN)6]2- lead to the formation of [MnII(CN)4]2- (1),6 the first † Present address: Chemistry and Materials Science Divisions, Argonne National Laboratory, 9700 S. Cass Ave., Argonne, IL 60439. ‡ Present address: Bioscience Division, Los Alamos National Laboratory, Los Alamos, NM 87544. (1) Balzani, V.; Carassiti, V. Photochemistry of Coordination Compounds; Academic Press: New York, 1970. (2) (a) Sharpe, A. G. The Chemistry of Cyano Complexes of the Transition Metals; Academic Press: New York, 1976. (b) Shriver, D. F. Struct. Bonding 1966, 1, 33. (3) (a) Buschmann, W. E.; Ensling, J.; Gu¨tlich, P.; Miller, J. S. Chem.s Eur. J. 1999, 5, 3019. (b) Mohai, V. B. Z. Anorg. Allg. Chem. 1972, 392, 287. (c) Seifer, G. B.; Belova, V. I.; Makarova, Z. A. Russ. J. Inorg. Chem. (Engl.) 1964, 9, 844.
member of a new class of paramagnetic, tetrahedral, and coordinatively unsaturated cyanometalates. This result is unusual since percyano complexes of the first-row transition metals are well-known to favor coordinatively saturated octahedral geometries.2,7 Occasionally, however, there are deviations from this trend. Some well-characterized nonoctahedral examples include seven-coordinate [VIII(CN)7]4- 7d and the diamagnetic fourcoordinate square planar-d8 [MII(CN)4]2- (e.g., M ) Ni, Pd, Pt) and tetrahedral-d10 (e.g., M ) ZnII, CdII, HgII) complexes.2 The rare, d7 low-spin, S ) 1/2, square planar [CoII(CN)4]2- attests to the strong ligand-field strength imposed by the cyanide ligand.7b,c [MnII(CN)4]2- elicits an interesting question regarding its ligand-field properties; since it is tetrahedral and consists of a strong ligand field, CN-, is it high- or low-spin? All known homoleptic cyano complexes are low-spin;2,7 however, virtually all known tetrahedral metal complexes are high-spin resulting from the decreased crystal field stabilization with respect to (4) (a) Wandner, K.-H.; Hoppe, R. Z. Anorg. Allg. Chem. 1988, 557, 153. (b) Wandner, K.-H.; Hoppe, R. Z. Anorg. Allg. Chem. 1987, 546, 113. (5) Buschmann, W. E.; Vazquez, C.; McLean, R. S.; Ward, M. D.; Jones, N. C.; Miller, J. S. Chem. Commun. (Cambridge) 1997, 409. (6) Buschmann, W. E.; Arif, A. M.; Miller, J. S. Angew. Chem., Int. Ed. 1998, 37, 781; Angew. Chem. 1998, 110, 813. (7) (a) Dunbar, K. Prog. Inorg. Chem. 1996, 45, 823. (b) Meier, I. K.; Pearlstein, R. M.; Ramprasad, D.; Pez, G. Inorg. Chem. 1997, 36, 1707. (c) Carter, S. J.; Foxman, B. M.; Stuhl, L. S. J. Am. Chem. Soc. 1984, 106, 4265. (d) Towns, R. L. R.; Levenson, R. A. J. Am. Chem. Soc. 1972, 94, 4345.
10.1021/ic0012726 CCC: $20.00 © 2001 American Chemical Society Published on Web 03/09/2001
Tetracyanomanganate(II) and Its Salts octahedral complexes, although a few are low spin.8 From magnetic susceptibility and electron paramagnetic resonance (EPR) studies, it was unambiguously determined that [Mn(CN)4]2- is indeed high-spin (S ) 5/2) with an essentially temperatureindependent moment of 5.99 µB (5.92 µB predicted) and a Curie θ value of 0 K.6 With the discovery of [MnII(CN)4]2-, a synthon to develop novel magnetic 3-D networks akin to the Prussian Blue family of magnetic materials emerged. Prussian Blue, FeIII4[FeII(CN)6]3‚ yH2O (14 < y < 16), is the prototype of a class of 3-D network solids composed of FeII-C-N-FeIII linkages extended in three dimensions.9 Several members of this class of materials magnetically order below an ordering temperature, Tc, which, in several cases, exceeds room temperature.10 Herein, we report on the formation of [Mn(CN)4]2- in solution and the solid state, its solution behavior, and a variety of network solids formed by reacting [Mn(CN)4]2- with first-row transition metal dications. Experimental Section All manipulations were performed under N2 or argon using standard Schlenk techniques or a Vacuum Atmospheres inert atmosphere DriLab. Dichloromethane was dried and distilled under N2 from CaH2. Acetonitrile was dried and twice distilled under N2 from CaH2. Diethyl ether (Et2O) and tetrahydrofuran (THF) were dried and distilled under N2 from sodium benzophenone ketyl radical. [MII(NCMe)6][TFPB]2 (M ) V, Cr, Mn, Fe, Co, Ni; TFPB ) B[C6H3(CF3)2]4) salts,11 K3[MnIII(CN)6],12a and [PPN]3[MnIII(CN)6]12b [PPN+ ) (Ph3P)2N+] were prepared as previously described. [Et4N][PF6] was obtained by mixing a 50 mL aqueous solution of [Et4N]Cl (Aldrich) (3.01 g, 18.2 mmol) with a 50 mL aqueous solution of K[PF6] (Alfa) (3.34 g, 18.2 mmol). The white precipitate was filtered off, washed with water several times, air-dried, recrystallized from a minimum amount of hot MeOH, and dried in vacuo. [Fe(C5H5)2]{B[C6H3(CF3)2]4}. A 50 mL Et2O solution containing p-benzoquinone (1.29 mmol, 0.140 g) and HCl (2.6 mmol, 2.6 mL as a 1.0 M Et2O solution) was added via cannula to an Et2O solution (40 mL) containing K{B[C6H3(CF3)2]4} (2.355 mmol, 2.124 g) and Fe(C5H5)2 (2.59 mmol, 0.482 g). The reaction mixture immediately turned blue in color. The product precipitated from solution by addition of hexanes, was isolated by vacuum filtration, and then was recrystallized from Et2O/hexanes and dried in vacuo. Yield: 1.985 g of deep blue (8) Huheey, J. E.; Keiter, E. A.; Keiter, R. L. Inorganic Chemistry, 4th ed.; Harper Collins, 1993; p 403. Byrne, E. K.; Theopold, K. H. J. Am. Chem. Soc. 1989, 111, 3887. Arnold, J.; Wilkinson, G.; Hussaun, B.; Hursthouse, M. B. J. Chem. Soc., Chem. Commun. 1988, 1349. Stavropoulos, P.; Savage, P. D.; Tooze, R. P.; Wilkinson, G. J. Chem. Soc., Dalton Trans. 1987, 557. (9) Ludi, A.; Gu¨del, H. U. Struct. Bonding 1973, 14, 1. (10) (a) Ferlay, S.; Mallah, T.; Ouahe`s, R.; Veillet, P.; Verdaguer, M. Inorg. Chem. 1999, 38, 229. Holmes, S. D.; Girolami, G. J. Am. Chem. Soc. 1999, 121, 5593. Hatlevik, Ø.; Buschmann, W. E.; Zhang, J.; Manson, J. L.; Miller, J. S. AdV. Mater. 1999, 11, 914. Dujardin, E.; Ferlay, S.; Phan, X.; Desplanches, C.; Moulin, C. C. D.; Sainctavit, P.; Baudelet, F.; Dartyge, E.; Veillet, P.; Verdaguer, M. J. Am. Chem. Soc. 1998, 120, 11347. Ferlay, S.; Mallah, T.; Ouahe`s, R.; Veillet, P.; Verdaguer, M. Inorg. Chem. 1999, 38, 229. Verdaguer, M.; Bleuzen, A.; Train, C. Garde, R.; Debiani, F. F.; Desplanches, C. Philos. Trans. R. Soc. London (A) 1999, 357, 3159. (b) E.g.: Buschmann, W. E.; Paulson, S. C.; Wynn, C. M.; Girtu, M.; Epstein, A. J.; White, H. S.; Miller, J. S. AdV. Mater. 1997, 9, 645; Chem. Mater. 1998, 10, 1386. Ferlay, S.; Mallah, T.; Ouahes, R.; Veillet, P.; Verdaguer, M. Nature 1995, 378, 701. Entley, W. R.; Treadway, C. R.; Girolami, G. S. Mol. Cryst. Liq. Cryst. 1995, 237, 153. Mallah, T.; Thie´baut, S.; Verdaguer, M.; Veillet, P. Science 1993, 262, 1554. Greibler, W. D.; Babel, D. Z. Naturforsch. 1982 87b, 832. (11) Buschmann, W. E.; Miller, J. S. Chem.sEur. J. 1998, 4, 1731. (12) (a) Brauer, G. Handbook of PreparatiVe Inorganic Chemistry, 2nd ed.; Academic Press: New York, 1965; Vol. 2, p 1473. (b) Buschmann, W. E.; Liable-Sands, L.; Rheingold, A. L.; Miller, J. S. Inorg. Chim. Acta 1999, 284, 175. (c) Alexander, J. J.; Gray, H. B. J. Am. Chem. Soc. 1968, 90, 4260.
Inorganic Chemistry, Vol. 40, No. 8, 2001 1927 needles (80% yield based on K{B[C6H3(CF3)2]4}). IR (Nujol): 3118, 1608, 1355, 1279, 1164, 1141, 1122, 1098, 950, 929, 892, 853, 838, 744, 719, 712, 683, 670 cm-1. Mp: 120 °C dec. Anal. Calcd for C42H22BF24Fe: C, 48.09; H, 2.11. Found: C, 48.01; H, 2.20. [PPN]2[MnIV(CN)6]. The previous method5 was improved by adding a 1:6 MeCN/Et2O solution (70 mL) containing [Fe(C5H5)2]{B[C6H3(CF3)2]4} (0.7763 mmol, 0.8143 g) to a MeCN solution (35 mL) containing [PPN]3[MnIII(CN)6] (0.7763 mmol, 1.418 g) while excluding light (solutions are stable under red light). The blue color of the [Fe(C5H5)2]+ disappeared immediately upon mixing. Addition of another 20 mL of Et2O with stirring initiates crystallization from solution after ∼1 min. The product was allowed to settle for 30 min, isolated by vacuum filtration, and recrystallized from MeCN/Et2O. Small, yellow platelet crystals were isolated in 90% yield. IR νCN (Nujol, MeCN): 2132 cm-1. Raman νCN (solid): 2135 cm-1. Mp: 142 °C dec. Anal. Calcd for C78H60N8P4Mn: C, 72.73; H, 4.70; N, 8.40. Found: C, 72.49; H, 4.51; N, 8.58. [Et4N]2[MnIV(CN)6]. A CH2Cl2 solution (5 mL) containing [Et4N]PF6 (0.220 mmol, 0.0606 g) was added to a CH2Cl2 solution (8 mL) containing [PPN]2[MnIV(CN)6] (0.1060 mmol, 0.1366 g) while light was excluded. Et2O (1 mL) was added, and the reaction mixture became cloudy with a precipitate. The solid was isolated by vacuum filtration and recrystallized from CH2Cl2/MeCN/Et2O. Yield: 50 mg of yellow microcrystalline solid (50% yield). IR νCN (Nujol): 2132 cm-1. Mp: 123 °C dec. Anal. Calcd for C23.25Cl0.50H40.50MnN8 (includes 0.25 equiv of CH2Cl2): C, 54.23; H, 8.28; N, 22.47. Found: C, 54.20; H, 8.74; N, 22.71. Photochemical Preparation of [PPN]2[MnII(CN)4]. A solution of [PPN]2[Mn(CN)6] (1.546 g, 0.8470 mmol) in 10 mL of MeCN/ CH2Cl2, 1:1, was exposed to ambient fluorescent light until the color had changed from yellow to red. The solution was then layered with Et2O to crystallize out 0.314 g of burgundy-red prisms (30% yield). IR νCN: 2205 cm-1 (Nujol), 2202 cm-1 (MeCN). Raman νCN (solid): 2109 cm-1. Dec: 206 °C (DSC, TGA). Anal. Calcd for C76H60MnN6P4: C, 73.84; H, 4.82; N, 6.80. Found: C, 73.62; H, 4.97; N, 6.75. Thermochemical Preparation of [PPN]2[MnII(CN)4]. A 50.0 mg sample of yellow [PPN]2[Mn(CN)6] was heated in vacuo at 140 °C for 14 h. The remaining solid was pale red and 4.0% (2.0 mg) less in mass than prior to heating. Recrystallization from MeCN/CH2Cl2/Et2O gave burgundy-red crystals in 90% yield. IR νCN: 2205 cm-1 (Nujol), 2202 cm-1 (MeCN). Dec: 206 °C (DSC, TGA). [Et4N]2[MnII(CN)4]. A solution of [Et4N][PF6] (0.0606 g, 0.220 mmol) in 5 mL of 10:1 CH2Cl2/MeCN was added to a red solution of [PPN]2[Mn(CN)4] (0.1236 g, 0.1000 mmol) in 5 mL of CH2Cl2, and a fine precipitate began to form. The volume was approximately doubled with Et2O to precipitate the product. The solid was isolated by vacuum filtration and recrystallized from MeCN/CH2Cl2/Et2O. Yield: 270 mg of dark burgundy-red twinned needles (64% yield). IR νCN: 2209 cm-1 (Nujol), 2201 cm-1 (MeCN). Mp: 137 °C (DSC). Dec: 248 °C (DSC, TGA). Anal. Calcd for C20H40MnN6: C, 57.26; H, 9.61; N, 20.03. Found: C, 57.29; H, 9.53; N, 19.99. [PPN]2[C12N12]. A solution of [PPN]2[Mn(CN)6] (100 mg, 0.077 mmol) in 3 mL of CH2Cl2 was exposed to ambient fluorescent light until the color had changed from yellow to red. This was slowly diluted with Et2O by vapor diffusion while sitting on an optical bench for 1 month. Small pale-yellow crystals were isolated by vacuum filtration in
