Synthesis, Characterization, and CO Elimination of Ferrio-Substituted

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Synthesis, Characterization, and CO Elimination of Ferrio-Substituted Two-Coordinate Germylenes and Stannylenes Hao Lei,† Jing-Dong Guo,‡ James C. Fettinger,† Shigeru Nagase,‡ and Philip P. Power*,† †

Department of Chemistry, University of California, Davis, One Shields Avenue, Davis, California 95616, United States Department of Theoretical and Computational Molecular Science, Institute for Molecular Science, Okazaki, Aichi 444-8585, Japan

‡

S Supporting Information *

ABSTRACT: The reactions of [ArE(Cl)]2 (E = Ge, Sn; Ar = −C 6 H 3 -2,6-(C 6 H 3 -2,6- i Pr 2 ) 2 (Ar Pri 4 ), −C 6 H 3 -2,6-(C 6 H 2 2,4,6-iPr3)2 (ArPri6)) with K[(η 5-C5H5)Fe(CO)2] afforded the deep green ferriogermylenes ArGeFe(η 5-C5H5)(CO)2 (Ar = ArPri4 (1), ArPri6 (2)) and the ferriostannylenes ArSnFe(η 5C5H5)(CO)2 (Ar = ArPri4 (3), ArPri6 (4)), respectively. Complexes 1−4 were characterized by NMR, UV−vis, and IR spectroscopy, as well as by X-ray crystallography. The solid-state structures of 1−4 feature two-coordinate Ge or Sn atoms with bent C−E−Fe (E = Ge, Sn) geometries. Although germylenes 1 and 2 remain intact upon heating or UV irradiation, stannylenes 3 and 4 were shown to eliminate one carbonyl group upon exposure to UV light, affording the brown dimeric species {ArSnFe(η 5-C5H5)(CO)}2 (Ar = ArPri4 (5), ArPri6 (6)), respectively.

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(M = Cr, Mo, W; Ar = ArMe6, ArPri6)7 and metalloplumbylenes (η 5-C5H5)(CO)3M-PbArPri6 (M = Cr, Mo, W).8 In these singly bonded species, the Ge, Sn, Pb atoms are two-coordinate with bent C−E−M (M = Cr, Mo, W; E = Ge, Sn, Pb) geometry. More recently, two Ge(II) compounds containing Ge−Fe σ bonds, (Piso)GeFe(η 5-C5H5)(CO)2 (Piso− = [ArNC(tBu)NAr]−, Ar = −C6H3-2,6-iPr2)9 and LGeFe(η 5-C5H5)(CO)2 (L = CH{CMe(NAr)}2, Ar = −C6H3-2,6-iPr2),10 were synthesized by using bidentate amidinate or diketiminate ligands at germanium. The solid-state structures of these two compounds feature three-coordinate germaniums. A tin analogue of the diketiminato complex LSnFe(η 5-C5H5)(CO)2 (L = CH{CMe(NAr)}2, Ar = −C6H3-2,6-iPr2) was also synthesized and isolated, but a crystallographic structure was not provided. 10 Very recently, Jutzi and co-workers reported the synthesis and characterization of the related metallosilylene (η 5-C5Me5)(CO)2FeSiC5Me5, which contains a Si−Fe σ bond.11 X-ray crystallography showed that the silicon has a bent coordination geometry and is η 3-bound to the cyclopentadienyl ring. Furthermore, DFT calculations on the structure and bonding of the related metallo-ylene complexes [(η 5-C5H5)(Me3P)(H)2M(EPh)] (M = Fe, Ru, Os; E = Si, Ge, Sn, Pb) have been reported.12 Herein, we describe the synthesis and characterization of ferriogermylenes ArGeFe(η 5-C5H5)(CO)2 (Ar = ArPri4 (1), ArPri6 (2)) and ferriostannylenes ArSnFe(η 5-C5H5)(CO)2 (Ar = ArPri4 (3), ArPri6 (4)). In addition, the decarbonylation of 3 and 4 under UV irradiation results in the SnFe compounds {ArSnFe(η 5-C5H5)(CO)}2 (Ar = ArPri4 (5), ArPri6 (6)).

INTRODUCTION The synthesis and characterization of isolable germylene and stannylene species has attracted considerable interest over the past several decades.1 A large number of such low-valent germanium and tin compounds have been successfully isolated and structurally characterized by using various bulky aryl, alkyl, amido, and related ligands to stabilize them. In addition, they have been shown to have a very rich chemistry and that includes transition-metal-like reactivity with small molecules 2 such as dihydrogen, ammonia, and ethylene under mild conditions.3 In contrast, metallo-substituted germylene and stannylene complexes, which contain direct M−E (E = Ge, Sn) σ bonds, have received less attention. In 1994, Jutzi and coworkers reported the preparation of the first organometallogermylenes Mes*GeFe(CO)2(η 5-C5R5) (R = H, CH3, Mes* = −C6H2-2,4,6-tBu3).4 Both compounds were isolated as airsensitive solids in moderate yields and characterized by NMR and IR spectroscopy. However, no detailed structures were reported. Our group reported the reactions of Na[M(η 5C5H5)(CO)3] (M = Cr, Mo, W) with the m-terphenylstabilized germanium species ArGe(Cl) (Ar = −C6H3-2,6(C6H2-2,4,6-Me3)2 (ArMe6), ArPri6), resulting in either the singly bonded metallogermylenes (η 5-C5H5)(CO)3M-GeAr or the triply bonded metallogermylynes (η 5-C5H5)(CO)2MGeAr.5 The latter species were obtained as a result of CO elimination from the corresponding metallogermylene complexes and represented the first stable compounds with multiple bonds between a transition metal and a heavier group 14 element. Several further examples of transition-metal germanium triply bonded species as well as examples of transition-metal tin and lead multiply bonded species were reported by Filippou and coworkers.6 We were also able to extend our results to the preparation of metallostannylenes (η 5-C5H5)(CO)3M−SnAr © 2011 American Chemical Society

Received: October 4, 2011 Published: November 3, 2011 6316

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overnight. All volatile fractions were removed under reduced pressure, and the residue was extracted with hexanes. The solution was filtered, and the deep green filtrate was concentrated to incipient crystallization. Storage at −18 °C overnight afforded X-ray-quality deep green crystals of 4. Yield: 0.725 g (69%). Mp: 224−225 °C. 1H NMR (300.1 MHz, C6D6, 20 °C): δ 1.18 (d, 3JHH = 6.9 Hz, 24H, oCH(CH3)2), 1.47 (d, 3JHH = 6.9 Hz, 12H, p-CH(CH3)2), 2.74 (sept, 3 JHH = 6.9 Hz, 2H, p-CH(CH3)2), 3.58 (br m, 4H, o-CH(CH3)2), 3.80 (s, 5H, η 5-C5H5), 7.05−7.49 (br m, 7H, aromatic H ). 119Sn{1H} NMR (223.6 MHz, C6D6, 22 °C): δ 2915. UV−vis (hexane): λ max (ε) 374 nm (4100 mol−1 L cm−1), 608 nm (1200 mol−1 L cm−1). IR (Nujol, cm−1): ν 1967 (s), 1926 (s). {ArPri4SnFe(η 5-C5H5)(CO)}2 (5). A hexane solution (40 mL) of 3 (0.172 g, 0.25 mmol) was exposed to UV light for ca. 24 h. The resulting dark brown precipitate was separated from the solution by decanting. The solid was washed with ca. 15 mL hexanes twice and dried under vacuum. Yield: 0.108 g (65%). Mp: 274 °C. 1H NMR (599.7 MHz, C7D8, 20 °C): δ 1.06 (d, 3JHH = 6.6 Hz, 12H, CH(CH3)2), 1.14 (d, 3JHH = 6.6 Hz, 12H, CH(CH3)2), 1.32 (d, 3JHH = 6.6 Hz, 12H, CH(CH3)2), 1.34 (d, 3JHH = 6.6 Hz, 12H, CH(CH3)2), 3.26 (sept, 3JHH = 6.6 Hz, 4H, CH(CH3)2), 3.51 (sept, 3JHH = 6.6 Hz, 4H, CH(CH3)2), 3.71 (s, 10H, η 5-C5H5), 6.97−7.32 (m, 18H, aromatic H ). 119Sn{1H} NMR (223.6 MHz, C6D6, 22 °C): δ 1982. UV−vis (hexane): λ max (ε) 464 nm (2000 mol−1 L cm−1, sh). IR (Nujol, cm−1): ν 2009 (s). {ArPri6SnFe(η 5-C5H5)(CO)}2 (6). A hexane solution (40 mL) of 4 (0.150 g, 0.19 mmol) was exposed to UV light for ca. 24 h. The resulting dark brown precipitate was separated from the solution by decanting. The solid was washed with ca. 15 mL diethyl ether twice and dried under vacuum. Brown X-ray-quality crystals of 6·3C7H8 could be obtained by recrystallization from toluene at ca. 7 °C. Yield: 0.077 g (54%). Mp: 233 °C. 1H NMR (300.1 MHz, C7D8, 20 °C): δ 1.11 (d, 3JHH = 6.9 Hz, 12H, p-CH(CH3)2), 1.20 (d, 3JHH = 6.9 Hz, 12H, p-CH(CH3)2), 1.39 (m, 3JHH = 6.9 Hz, 48H, o-CH(CH3)2), 2.95 (sept, 3JHH = 6.9 Hz, 4H, p-CH(CH3)2), 3.34 (sept, 3JHH = 6.9 Hz, 4H, o-CH(CH3)2), 3.55 (sept, 3JHH = 6.9 Hz, 4H, o-CH(CH3)2), 3.74 (s, 10H, η 5-C5H5), 6.97−7.29 (m, 14H, aromatic H ). 119Sn{1H} NMR (223.6 MHz, C6D6, 22 °C): δ 1988. UV−vis (hexane): λ max (ε) 464 nm (2300 mol−1 L cm−1, sh). IR (Nujol, cm−1): ν 2008 (s). X-ray Crystallography. Crystals of 1−4 and 6 were removed from a Schlenk tube under a stream of nitrogen and immediately covered with a thin layer of hydrocarbon oil. A suitable crystal was selected, attached to a glass fiber on a copper pin, and quickly placed in the cold N2 stream on the diffractometer.15 Data for compounds 1− 4 were collected at 90 K on a Bruker SMART Apex II diffractometer with Mo Kα radiation (λ = 0.710 73 Å). Data for compound 6 were collected at 90 K on a Bruker APEX DUO diffractometer with Cu Kα radiation (λ = 1.541 78 Å). Absorption corrections were applied using SADABS.16 The crystal structures were solved by direct methods and refined by full-matrix least-squares procedures in SHELXTL. 17 All non-H atoms were refined anistropically. All H atoms were placed at calculated positions and included in the refinement using a riding model. DFT Calculations on 6. All the calculations were carried out with the density functional theory (DFT) in the Gaussian 03 program. 18 The geometry optimization was performed at the B3PW91 level by using a LANL2DZ effective core potential19 for Fe and Sn atoms (an extra d-type polarization function with exponent 0.183 added for Sn) and 6-31G(d) basis set for all other atoms. The Wiberg bond index 20 was analyzed using the natural bond orbital (NBO 3.1) program. Nucleus-independent chemical shifts (NICS)21 were evaluated using the gauge-independent atomic orbitals (GIAO).22

EXPERIMENTAL SECTION

General Methods. All manipulations were carried out by using modified Schlenk techniques under an atmosphere of N2 or in a Vacuum Atmospheres HE-43 drybox. All solvents were dried over an alumina column, followed by storage over 3 Å molecular sieves overnight, and degassed three times (freeze−pump−thaw) prior to use. The compounds [ArE(Cl)]2 (E = Ge, Sn; Ar = ArPri4, ArPri6)13 and K[(η 5-C5H5)Fe(CO)2]14 were prepared according to literature procedures. 1H NMR data were obtained on a Varian Mercury 300 spectrometer or a Varian Inova 600 MHz spectrometer at 293.1 K. 119 Sn NMR spectra were recorded on a Varian Inova 600 MHz spectrometer (223.6 MHz) and referenced externally to neat Sn nBu4. Infrared spectra were recorded as Nujol mulls between KBr plates on a Bruker Tensor 27 FTIR spectrometer. Melting points were measured in glass capillaries sealed under N2 by using a Mel-Temp II apparatus and are uncorrected. UV−vis data were recorded on a Hitachi-1200 spectrometer. ArPri4GeFe(η 5-C5H5)(CO)2 (1). Diethyl ether (40 mL) was added to a mixture of [ArPri4Ge(Cl)]2 (0.237 g, 0.23 mmol) and K[(η 5C5H5)Fe(CO)2] (0.101 g, 0.47 mmol) at ca. −78 °C. The reaction mixture was slowly warmed to room temperature and stirred overnight. All volatile fractions were removed under reduced pressure, and the residue was extracted with hexanes. The solution was filtered, and the deep green filtrate was concentrated to incipient crystallization. Storage at −18 °C overnight afforded X-ray-quality deep green crystals of 1. Yield: 0.071 g (23%). Mp: 200−201 °C. 1H NMR (300.1 MHz, C6D6, 20 °C): δ 1.01 (d, 3JHH = 6.9 Hz, 6H, CH(CH3)2), 1.17 (d, 3JHH = 6.9 Hz, 6H, CH(CH3)2), 1.32 (m, 12H, CH(CH3)2), 3.42 (sept, 3JHH = 6.9 Hz, 2H, CH(CH3)2), 3.56 (sept, 3 JHH = 6.9 Hz, 2H, CH(CH3)2), 3.84 (s, 5H, η 5-C5H5), 6.95−7.44 (m, 9H, aromatic H). UV−vis (hexane): λ max (ε) 428 nm (2200 mol−1 L cm−1), 620 nm (490 mol−1 L cm−1). IR (Nujol, cm−1): ν 1977 (s), 1917 (s). ArPri6GeFe(η 5-C5H5)(CO)2 (2). Diethyl ether (40 mL) was added to a mixture of [ArPri6Ge(Cl)]2 (0.828 g, 0.70 mmol) and K[(η 5C5H5)Fe(CO)2] (0.314 g, 1.45 mmol) at ca. −78 °C. The reaction mixture was slowly warmed to room temperature and stirred overnight. All volatile fractions were removed under reduced pressure, and the residue was extracted with hexanes. The solution was filtered, and the deep green filtrate was concentrated to incipient crystallization. Storage at −18 °C overnight afforded X-ray-quality deep green crystals of 2. Yield: 0.543 g (53%). Mp: 226−227 °C. 1H NMR (300.1 MHz, C6D6, 20 °C): δ 1.08 (d, 3JHH = 6.9 Hz, 6H, oCH(CH3)2), 1.14 (d, 3JHH = 6.9 Hz, 12H, o-CH(CH3)2), 1.23 (d, 3JHH = 6.9 Hz, 6H, o-CH(CH3)2), 1.40 (d, 3JHH = 6.9 Hz, 12H, pCH(CH3)2), 2.70 (sept, 3JHH = 6.9 Hz, 2H, p-CH(CH3)2), 3.48 (sept, 3 JHH = 6.9 Hz, 2H, o-CH(CH3)2), 3.61 (sept, 3JHH = 6.9 Hz, 2H, oCH(CH3)2), 3.86 (s, 5H, η 5-C5H5), 6.97−7.46 (m, 7H, aromatic H). UV−vis (hexane): λ max (ε) 430 nm (2100 mol−1 L cm−1), 616 nm (560 mol−1 L cm−1). IR (Nujol, cm−1): ν 1984 (s), 1936 (s). ArPri4SnFe(η 5-C5H5)(CO)2 (3). Diethyl ether (40 mL) was added to a mixture of [ArPri4Sn(Cl)]2 (0.927 g, 0.84 mmol) and K[(η 5C5H5)Fe(CO)2] (0.368 g, 1.70 mmol) at ca. −78 °C. The reaction mixture was slowly warmed to room temperature and stirred overnight. All volatile fractions were removed under reduced pressure, and the residue was extracted with hexanes. The solution was filtered, and the deep green filtrate was concentrated to incipient crystallization. Storage at −18 °C overnight afforded X-ray-quality deep green crystals of 3. Yield: 0.588 g (50%). Mp: 237−238 °C. 1H NMR (300.1 MHz, C6D6, 20 °C): δ 1.39 (d, 3JHH = 6.9 Hz, 24H, CH(CH3)2), 3.53 (br m, 4H, CH(CH3)2), 3.78 (s, 5H, η 5-C5H5), 7.03−7.51 (br m, 9H, aromatic H ). 119Sn{1H} NMR (223.6 MHz, C6D6, 22 °C): δ 2951. UV−vis (hexane): λ max (ε) 382 nm (sh, 12 000 mol−1 L cm−1), 608 nm (1900 mol−1 L cm−1). IR (Nujol, cm−1): ν 1970 (s), 1921 (s). ArPri6SnFe(η 5-C5H5)(CO)2 (4). Diethyl ether (40 mL) was added to a mixture of [ArPri6Sn(Cl)]2 (0.862 g, 0.68 mmol) and K[(η 5C5H5)Fe(CO)2] (0.294 g, 1.36 mmol) at ca. −78 °C. The reaction mixture was slowly warmed to room temperature and stirred

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RESULTS AND DISCUSSION Synthesis and Characterization of 1−4. Treatment of [ArE(Cl)]213 (Ar = ArPri4 (1, 3), ArPri6 (2, 4); E = Ge (1, 2), Sn (3, 4)) with 2 equiv of K[(η 5-C5H5)Fe(CO)2]14 at −78 °C gave, after workup and recrystallization from hexane, dark green

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Scheme 1. Synthetic Routes for 1−4

crystals of the corresponding ferriogermylene or -stannylene ArEFe(η 5-C5H5)(CO)2 in good yield (Scheme 1). The compounds were characterized by 1H and 119Sn NMR, UV−vis, and IR spectroscopy. The 1H NMR spectra of 1−4 displayed resonances of the protons on the cyclopentadienyl rings at ca. 3.8 ppm for stannylenes and 3.9 ppm for germylenes, respectively. This is close to the values observed for the analogous chromium complexes (η 5 -C 5 H 5 )(CO)3CrGeArPri6 (3.87 ppm)5b and (η 5-C5H5)(CO)3CrSnArPri6 (3.71 ppm),7 as well as those for reported ferriogermylenes (3.88 ppm for LGeFe(η 5-C5H5)(CO)2,10 3.90 ppm for (Piso)GeFe(η 5-C5H5)(CO)29) and ferriostannylene (3.83 ppm for LSnFe(η 5-C5H5)(CO)210). In contrast, the η 5C5H5 signals in the corresponding molybdenum and tungsten complexes appear ca. 0.5 ppm downfield,5,7 which is consistent with stronger metal−ligand interactions for the Mo and W compounds in comparison to their Cr and Fe counterparts. 119 Sn NMR spectra of 3 and 4 show a resonance shifted strongly downfield at 2951 and 2915 ppm, respectively. Interestingly, such values are close to those reported for the germanium-substituted stannylene Ar Me6SnGe tBu 3 (2960 ppm)23 and tin-substituted stannylene Ar Pri6SnSnMe2ArPri6 (2856.9 ppm).24 For comparison, the group 6 metal stannylene species we reported earlier show 119Sn signals in the range of 2116−2650 ppm.7 The electronic spectra of 1−4 all consist of two major absorption bands. In the UV−vis spectra, germylenes 1 and 2 show absorptions around 430 and 620 nm, while stannylenes 3 and 4 show a major absorption at 608 nm and a shoulder near 380 nm. The shift to shorter

Figure 1. Thermal ellipsoid (30%) drawing of 1 (H atoms are not shown). Selected bond distances (Å) and angles (deg): Ge(1)−C(1) = 2.011(2), Ge(1)−Fe(1) = 2.3790(4); C(1)−Ge(1)−Fe(1) = 115.27(6).

wavelengths for the tin derivatives is consistent with previous results.5,7 The IR spectra of 1−4 each feature two CO stretching signals. The observed stretching frequencies (1, 1977 and 1917 cm−1; 2, 1984 and 1936 cm−1; 3, 1970 and 1921 cm−1; 4, 1967 and 1926 cm−1) resemble those of related compounds,4,9−11 indicating that different ligands on Ge or Sn have a limited effect on the electron density at the Fe atoms. X-ray Crystal Structures of 1−4. All four compounds were characterized by X-ray crystallography. Compounds 1 and 3 crystallize in the orthorhombic space group P212121, while 2 and 4 crystallize in the orthorhombic space group Pnma (Table 1). In all four structures, the cyclopentadienyl ring and one of the carbonyl groups are disordered over two positions. In the solid-state structures of 1 (Figure 1) and 2 (Figure 2), the germanium atom is two-coordinate and is bonded to one terphenyl ligand and an Fe(η 5-C5H5)(CO)2 group through direct Ge−C and Ge−Fe bonds. The Ge−Fe distances (2.3790(4) Å (1), 2.3747(8) Å (2)) match the sum of the

Table 1. Selected Crystallographic Data and Collection Parameters for 1−4 and 6 1 formula fw color habit space group a, Å b, Å c, Å α, deg β, deg γ, deg V , Å3 Z dcalcd, Mg/m3 θ range, deg μ, mm−1 no. of obsd data, I > 2σ(I) R1 (obsd data) wR2 (all data)

C37H42FeGeO2 647.15 green plate P212121 8.4367(5) 17.7889(11) 21.5854(13) 90 90 90 3239.5(3) 4 1.327 2.48−27.50 1.406 6431 0.0306 0.0687

2

3

C43H54FeGeO2 731.30 green plate Pnma 14.6074(13) 23.995(2) 10.8289(10) 90 90 90 3795.6(6) 4 1.280 2.49−25.25 1.208 2759 0.0454 0.1182 6318

C37H42FeO2Sn 693.25 green plate P212121 8.3971(5) 18.0138(10) 21.7207(13) 90 90 90 3285.6(3) 4 1.401 2.45−25.25 1.232 5436 0.0286 0.0670

4 C43H54FeO2Sn 777.40 green plate Pnma 14.5866(8) 24.0905(13) 10.9156(6) 90 90 90 3835.7(4) 4 1.346 2.33−25.25 1.063 3471 0.0913 0.1970

6·3C7H8 C105H132Fe2O2Sn2 1775.18 brown block P1̅ 11.1938(17) 15.366(2) 15.715(2) 114.251(2) 107.508(2) 99.328(2) 2220.0(6) 2 1.328 1.54−27.50 0.926 9386 0.0499 0.1326

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Figure 2. Thermal ellipsoid (30%) drawing of 2 (H atoms are not shown). Selected bond distances (Å) and angles (deg): Ge(1)−C(1) = 2.008(4), Ge(1)−Fe(1) = 2.3747(8); C(1)−Ge(1)−Fe(1) = 113.14(12).

Figure 5. Thermal ellipsoid (30%) drawing of 6 (H atoms and solvent molecules are not shown). Selected bond distances (Å) and angles (deg): Sn(1)−C(1) = 2.196(3), Sn(1)−Fe(1) = 2.4955(11), Sn(1)− Fe(1A) = 2.4780(11), Sn(1)···Sn(1A) = 3.1091(6), Fe(1)−C(37) = 1.747(9); Sn(1)−Fe(1)−Sn(1A) = 77.38(3), Fe(1)−Sn(1)−Fe(1A) = 102.62(3), C(1)−Sn(1)−Fe(1) = 117.82(9), C(1)−Sn(1)−Fe(1A) = 139.41(9).

Figure 3. Thermal ellipsoid (30%) drawing of 3 (H atoms are not shown). Selected bond distances (Å) and angles (deg): Sn(1)−C(1) = 2.208(3), Sn(1)−Fe(1) = 2.5634(5); C(1)−Sn(1)−Fe(1) = 112.65(9).

Figure 6. Drawing of the optimized structure of 6 (Fe in blue; Sn in green; O in red; C in black; H atoms are not shown for clarity). A comparison of selected calculated and experimental bond distances and angles is given in Table 2.

covalent radii of iron and germanium (2.37 Å),25 but they are about 0.1 Å shorter than the Ge−Fe distances in the ferriogermylenes with bidentate ligands (2.4415(11) Å for (Piso)GeFe(η 5-C5H5)(CO)2;9 2.4961(17) Å for LGeFe(η 5C5H5)(CO)210). The C(1)−Ge(1)−Fe(1) angles (115.27(6)° (1), 113.14(12)° (2)) are similar to the C−Ge−M angles in other reported metallo-germylene species, such as (η 5-C5H5)(CO)3MGeArPri6 (117.8(2)° (M = Cr); 114.7(6)° (M = W)),5b

Figure 4. Thermal ellipsoid (30%) drawing of 4 (H atoms are not shown). Selected bond distances (Å) and angles (deg): Sn(1)−C(1) = 2.251(8), Sn(1)−Fe(1) = 2.6040(16); C(1)−Sn(1)−Fe(1) = 106.6(2).

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The stannylenes 3 (Figure 3) and 4 (Figure 4) show structural features broadly similar to those of their germanium congeners. Each tin is two-coordinate and is bonded to the ipso carbon of the central ring of the terphenyl ligand as well as to the Fe(η 5-C5H5)(CO)2 group through a direct Sn−Fe bond. The Sn−Fe distances (2.5634(5) Å (3), 2.6040(16) Å (4)) are similar to the 2.56 Å predicted from the sum of the covalent radii25 of tin and carbon and to the Sn−C distances reported for Sn(IV)−Fe single bonds. For example, the Sn−Fe distances in [(η 5 -C 5 H 5 )Fe(CO) 2 ] 2 SnPh 2 28 and [(η 5 -C 5 H 5 )Fe(CO)2]2SntBu229 are 2.579(1) and 2.6601(8) Å, respectively. However, they are longer than the Sn−Fe distances in stannylene−iron complexes where a donor−acceptor Sn−Fe bond is formed between the lone pair on Sn and the empty d orbital on Fe, such as (η 6 -PhMe)(η 2 -C 2 H 4 )FeSnR 2 (2.4362(10) Å, R = 2-tert-butyl-4,5,6-trimethylphenyl), 30 [(OC)(NO)2FeSnRR′] (2.484(1) Å, R = 2,4,6-tBu3-C6H2, R′ = CH2C(CH3)2-3,5-t Bu2-C6H3),31 [Fe(CO)4{Sn(OAr)2}] (2.408(1) Å, Ar = 2,6-tBu2-4-CH3-C6H2),32 and (η 6-PhMe)Fe(SnR2)2 (2.432(1) and 2.434(1) Å, R = 2-tert-butyl-4,5,6trimethylphenyl).33 Since 3 and 4 represent the first examples of structurally characterized ferriostannylenes, there are no examples available for comparison of their structural parameters: i.e., the Sn−Fe distance. The C(1)−Sn(1)−Fe(1) angles (112.65(9)° (3), 106.6(2)° (4)) are similar to the C−Sn−M angles in reported group 6 metallo-stannylene species, such as (η 5-C5H5)(CO)3MSnArMe6 (111.0(4)° (M = Cr); 106.7(10)° (M = Mo); 110.8(2)° (M = W)),7 (η 5-C5H5)(CO)3MSnAr ArPri6 (110.12(19)° (M = Cr); 110.14(10)° (M = Mo); 109.9(2)° (M = W)),7 (η 5-1,3-tBu2-C5H3)(CO)3MoSnArPri6 (112.10(8)°), 7 and the group 14 element derivatives Ar Me 6 SnGe t Bu 3 (112.7(2)°) 23 and Ar Pri 6 SnSnPh 2 Ar Pri 6 (108.48(6)°).34 Reactivity of 1−4 under UV Irradiation. Our group has shown that the metallo-germylene (η 5-C5H5)(CO)3MoGeArMe6 was a precursor in the preparation of the compound (η 5-C5H5)(CO)2MoGeArMe6, the first stable species that contains a metal−germanium triple bond.5a Thus one objective of this work was to see if compounds 1−4 could be converted into the corresponding triply bonded germylyne or stannylyne complexes. Triple bonds between heavier group 14 elements and transition metals are rare. Filippou and coworkers have demonstrated that a number of series of compounds containing triple bonds between group 6 metals (Mo and W) and heavier group 14 elements (Si, Ge, Sn, and Pb) could be isolated and fully characterized.6 Complexes featuring triple bonds between a heavier main-group element and a first-row transition metal (i.e., Fe) are less common. 35 Difficulty in their formation was also supported by Jutzi′s report that the ferriogermylenes (η 5-C5H5)(CO)2FeGeCH(SiMe3)2, (η 5-C5H5)(CO)2FeGeMes*, and (η 5-C5Me5)(CO)2FeGeMes* could not be transformed into stable iron−germanium triplebond systems.11 No reaction was observed when solutions of germylenes 1 and 2 were heated at 80 °C for 3 days or irradiated under UV light overnight. Although stannylenes 3 and 4 also displayed no reactivity upon heating, green solutions of 3 or 4 turned reddish brown after several hours of UV irradiation. The brown solids 5 and 6 could be isolated due to their lower solubility in comparison to the precursors 3 and 4. The solid-state structure of 6 was established by X-ray crystallography (Figure 5 and Table 1). Instead of the expected monomeric stannylyne structure, the data for 6 showed that it contained a Sn2Fe2 core

Table 2. Calculated and Experimental Bond Distances (Å) and Angles (deg) for 6 exptl Sn(1)−Fe(1) Sn(1)−Fe(1A) Fe(1)···Fe(1A) Sn(1)···Sn(1A) Fe(1)−C(37) Sn(1)−C(1) Fe(1)−Sn(1)−Fe(1A) Sn(1)−Fe(1)−Sn(1A) Fe(1)−Sn(1)−Fe(1A)−Sn(1A)

2.4955(11) 2.4780(11) 3.882(1) 3.1091(6) 1.747(9) 2.196(3) 102.62(3) 77.38(3) 0

calcd 2.503 2.506 3.889 3.157 1.726 2.239 101.9 78.1 0

Figure 7. Graphic representation of the calculated frontier orbitals for 6 (LUMO, top; HOMO, middle; HOMO-1, bottom; contour value 0.05 au).

ArMe6 GeGetBu 3 (114.87(13)°), 23 Ar Pri4GeGe(CN tBu)Ar Pri 4 (102.77(15)°),26 and K{GeArMe6}3 (111.3(3)°).27 6320

dx.doi.org/10.1021/om200912x | Organometallics 2011, 30, 6316−6322

Organometallics

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formed by bridging of the iron atoms by the two SnAr fragments as a result of CO elimination from each iron. Thus, compounds 5 and 6 are in effect {ArSnFe(η 5-C5H5)(CO)}2 (Ar = ArPri4 (5), ArPri6 (6)) dimers of the desired monomer. The isolation of such dimeric species rather than stannylyne monomers indicates that the steric crowding around the Sn− Fe unit and the Sn−Fe multiple bonding are insufficient to prohibit intermolecular association through Sn−Fe donor− acceptor interactions. The solid-state structure of 6 feature two slightly different sets of Sn−Fe bond lengths (Sn(1)−Fe(1) = 2.4955(11) Å, Sn(1)−Fe(1A) = 2.4780(11) Å). Both bond distances are more than 0.1 Å shorter than the Sn−Fe bond length in 4 (2.6040(16) Å), suggesting a stronger metal−metal interaction. Nevertheless, the Sn−Fe distances are longer than the range of Sn−Fe distances for reported donor−acceptor Sn−Fe bonds (2.41−2.48 Å) in stannylene complexes.30−33 The long Sn(1)···Sn(1A) separation (3.1091(6) Å) and the planar coordination geometry around Sn (sum of interligand angles equals to 359.85°) suggest little or no Sn−Sn bonding interactions. The planarity of the Sn 2Fe2 core is also noteworthy. Several compounds have been reported to contain Sn2Fe2 core structures; however, these compounds all involve four-coordinate Sn(IV) atoms exclusively.36 Furthermore, both complexes were characterized by 119Sn NMR spectroscopy. Compounds 5 and 6 show 119Sn resonances at 1982 and 1988 ppm, respectively. Such values are ca. 1000 ppm upfield of their stannylene precursors 3 and 4, which is consistent with the increase in the coordination number at the Sn atoms from 2 to 3. DFT Calculations on 6. To obtain further insight into the electronic structure of 6, DFT calculations were performed at the B3PW91 level. The optimized structure of 6 (Figure 6) is in excellent agreement with the crystallographic data. As illustrated in Table 2, the two sets of Fe−Sn distances were calculated to be 2.503 and 2.506 Å, respectively. Furthermore, the planarity of the Sn2Fe2 core structure was well reproduced by calculations. The calculated natural charges for Sn (+1.78) and Fe (−1.70) are consistent with the presence of four highly polarized Sn−Fe bonds. The calculated Wiberg bond indexes for the Sn−Fe bonds are quite similar (0.7257, 0.7282, 0.7258, 0.7281), clearly indicating comparable bond strengths for all four Sn−Fe bonds. In addition, the nucleus-independent chemical shift (NICS) was calculated for 6, and the resulting positive value (NICS(0) = 16.3, NICS (1) = 6.5) suggests there is no ring current over the Sn2Fe2 ring. The calculated frontier orbitals (Figure 7) showed the bonding in the Sn2Fe2 core to be symmetric: that is to say, that the Sn−Fe bonding molecular orbitals are delocalized over the Sn2Fe2 unit. The LUMO consists mainly of an empty p orbital combination on the Sn atoms, with some d character from Fe. The HOMO and HOMO-1 are shown as σ-bonding Sn−Fe orbitals.

ASSOCIATED CONTENT S Supporting Information * Figures giving the 1H NMR spectra of 1−6 and 119Sn{1H} NMR spectra of 3−6, CIF files giving crystallographic data for 1−4 and 6, and text giving the full citation for ref 18. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected].

ACKNOWLEDGMENTS We thank the National Science Foundation (Grant No. CHE0948417) for support of this work. REFERENCES

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CONCLUSIONS In summary, ferriogermylene and -stannylene derivatives 1−4 were prepared by a salt elimination route. All four complexes were characterized by NMR, UV−vis, and IR spectroscopy. These complexes represent the first examples of structurally characterized ferrio-substituted two-coordinate germylenes and stannylenes. Compounds 3 and 4 can undergo CO elimination reactions under UV irradiation and afford the unusual tin−iron dimeric species 5 and 6, respectively. This result suggests that more sterically hindered ligand sets should be used in order to isolate monomeric Sn−Fe triply bonded complexes. 6321

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