Synthesis and Structural Characterization of a Dimeric Cobalt(I

Communication pubs.acs.org/Organometallics

Synthesis and Structural Characterization of a Dimeric Cobalt(I) Homoleptic Alkyl and an Iron(II) Alkyl Halide Complex Pei Zhao,† Zachary Brown,† James C. Fettinger,† Fernande Grandjean,‡ Gary J. Long,‡ and Philip P. Power*,† †

Department of Chemistry, University of California, Davis, California 95616, United States Department of Chemistry, Missouri University of Science and Technology, University of Missouri, Rolla, Missouri 65409-0010, United States

‡

S Supporting Information *

ABSTRACT: The homoleptic cobalt(I) alkyl [Co{C(SiMe2Ph)3}]2 (1) was prepared by reacting CoCl2 with [Li{C(SiMe2Ph)3}(THF)] in a 1:2 ratio. Attempts to synthesize the corresponding iron(I) species led to the iron(II) salt [Li(THF)4 ][Fe 2(μ-Cl)3 {C(SiMe2Ph)3}2] (2). Both 1 and 2 were characterized by X-ray crystallography, UV−vis spectroscopy, and magnetic measurements. The structure of 1 consists of dimeric units in which each cobalt(I) ion is σ-bonded to the central carbon of the alkyl group −C(SiMe2Ph)3 and π-bonded to one of the phenyl rings of the −C(SiMe2Ph)3 ligand attached to the other cobalt(I) ion in the dimer. The structure of 2 features three chlorides bridging two iron(II) ions. Each iron(II) ion is also σ-bonded to the central carbon of a terminal −C(SiMe2Ph)3 anionic ligand. The magnetic properties of 1 reveal the presence of two independent cobalt(I) ions with S = 1 and a significant zero-field splitting of D = 38.0(2) cm−1. The magnetic properties of 2 reveal extensive antiferromagnetic exchange coupling with J = −149(4) cm−1 and a large second-order Zeeman contribution to its molar magnetic susceptibility. Formation of the alkyl 1 and the halide complex 2 under similar conditions is probably due in part to the fact that Co(II) is more readily reduced than Fe(II). recent example is [AriPr8FeCl]2 (AriPr8 = −C6H-2,6-(C6H2-2,4,6iPr3)2-3,5-iPr2), featuring two iron(II) ions bridged by chlorides.17 Herein we report the synthesis and characterization of the unusual homoleptic alkylcobalt(I) complex [Co{C(SiMe2Ph)3}]2 (1) and alkyliron(II) chloride complex [Li(THF)4][Fe2(μ-Cl)3{C(SiMe2Ph)3}2] (2). The reaction of anhydrous CoCl2 and 2 equiv of [Li{C(SiMe2Ph)3}(THF)]20 in diethyl ether afforded, after workup, green crystals of the cobalt(I) alkyl [Co{C(SiMe2Ph)3}]2 (1) in 20% yield. The objective was to synthesize a homoleptic dialkyl cobalt(II) complex. However, the lithium salt of the [C(SiMe2Ph)3]− anion apparently effected reduction of the cobalt(II) chloride to give the cobalt(I) product [Co{C(SiMe2Ph)3}]2 (1). This result is in contrast to the reaction of CoCl2 with the lithium salt of the chelating ligand, [Li{C(SiMe3)2(SiMe2C5H4N-2)}], which affords the chloro-bridged dimer [{(2-C5H4N)Me2Si}(Me3Si)2CCoCl]2.21 The π-complexation of the phenyl ring apparently plays an important role in the stabilization of 1. This is supported by the corresponding reaction of CoCl2 with [Li{C(SiMe3)3}(THF)2]

A

lkyl derivatives of cobalt have been known for more than 50 years since the first alkylcobalt compound, [CH3Co(CO)4], was prepared and characterized.1,2 Most such alkyls are stabilized by carbonyl, phosphine, or other coligands.3 The only homoleptic alkylcobalt compound that has been reported is tetrakis(1-norbornyl)cobalt(IV).4 Its crystal structure and magnetic properties were determined by Theopold and coworkers in 1986.5 Three homoleptic, donor-free cobalt aryls are known.6−9 The dimeric cobalt aryl [Co(μ-mesityl)(mesityl)]2 was reported in 1989.6 The use of a bulkier aryl afforded the monomeric compounds Co(ArMe6)2 (ArMe6 = C6H3-2,6-(C6H22,4,6-Me3)2)7,8 and Co(AriPr4)2 (AriPr4 = C6H3-2,6-(C6H3-2,6iPr2)2),8,9 which were obtained by the reaction of CoCl2 with 2 equiv of the lithium aryl. They were found to have twocoordinate structures with nonlinear cobalt coordination. Homoleptic iron alkyls are also scarce,10 and only three tetrakis(1-norbornyl)iron(IV),4,11 Fe[C(SiMe3)3]2,12,13 and the Fe[C(SiMe3)3]2− anion,14 have been structurally characterized. Two iron diaryls, Fe(ArMe6)2 (ArMe6 = C6H3-2,6-(C6H2-2,4,6Me3)2)7,8 and Fe(AriPr4)2 (AriPr4 = C6H3-2,6-(C6H3-2,6iPr2)2),8,15 which have structures similar to those of the aforementioned monomeric cobalt aryl compounds are also known. Donor-free organoiron(II) halides are equally scarce, and only a few have been structurally characterized.16−19 One © 2014 American Chemical Society

Received: February 19, 2014 Published: April 15, 2014 1917

dx.doi.org/10.1021/om500180u | Organometallics 2014, 33, 1917−1920

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proceeded smoothly and pale yellow-green crystals of [Li(THF)4][Fe2(μ-Cl)3{C(SiMe2Ph)3}2] (2) (Figure 2) were

in a 1:2 ratio, which also was investigated by our group but did not afford an isolable species, but only intractable mixtures of solids and no crystalline products. The structure of 1 (Figure 1) was determined by X-ray crystallography. It features a dimeric arrangement in which two

Figure 1. Molecular structure of 1 with thermal ellipsoids presented at the 50% probability level. Hydrogen atoms are not shown for clarity. Cobalt is shown in blue, carbon in gray, and silicon in orange. Selected distances (Å) and angles (deg): Co1···Co2 4.3480(7), Co1−C1 2.037(2), Co2−C26 2.028(2), C1−Co1−CNT1 173.56(8), C26− Co2−CNT2 173.83(8) (CNT = centroid).

cobalt(I) ions are each σ-bonded to the central carbon of one −C(SiMe2Ph)3 ligand and π-bonded to one of the phenyl rings of a −C(SiMe2Ph)3 ligand that is bonded to the other cobalt(I) ion of the dimer. The Co1···Co2 distance of 4.3480(7) Å is much longer than double the single-bond radius (2.46 Å)22 and is also longer than twice the van der Waals radius for cobalt metal (4.00 Å),23 suggesting the existence of little if any Co··· Co bonding. The Co1−C1 and Co2−C26 bond lengths are 2.037(2) and 2.028(2) Å, respectively. These are somewhat shorter than the Co−C distances in the alkylcobalt(II) halides [{(2-C5H4N)Me2Si}(Me3Si)2CCoCl]221 (2.048(4) Å) and [{(Me2N)Me2Si}(Me3Si)2CCoBr]224 (2.065(8) Å) and those in the five-coordinated alkylcobalt(I) complexes [2(diphenylphosphinyl)tolyl-C,P]tris(trimethylphosphine)cobalt (2.080(5) Å)25 and [{8-(diphenylphosphinyl)-1,2,3,4-tetrahydronaphthyl-(C1,P)}tris(trimethylphosphine)cobalt] (2.082(6) Å).26 The overall structure of 1 resembles that of the dimeric terphenylcobalt(I) complex [CoAriPr4]2 (AriPr4 = C6H3-2,6(2,6iPr2C6H3)2),27 in which the cobalt ions (Co−Co = 2.8033(5) Å) are σ-bonded to the ipso carbon of a central aryl ring of a terphenyl ligand with a Co−C bond length of 2.0058(16) Å and π-bonded to a flanking aryl ring of a terphenyl ligand attached to the other cobalt ion. The Co−centroid distances in 1 are 1.6817(10) and 1.6749(10) Å, respectively, which are close to the 1.659(1) Å distance in the monomeric cobalt(I) species (η6-C7H8)CoAriPr8 (AriPr8 = −C6H-2,6-(C6H2-2,4,6iPr3)2-3,5-iPr2)28 but are shorter than the 1.764(2) Å distance in [CoAriPr4]2.27 The C1−Co1−centroid(1) and C2−Co2− centroid(2) angles are 173.56(8) and 173.83(8)°, respectively. These angles are closer to 180° than the C−Co−centroid angles in (η6-C7H8)CoAriPr8 (167.6(2)°) and [CoAriPr4]2 (143.7(3)°). The −SiMe2Ph groups of the ligands in 1 are oriented differently from those in the ligand precursor (PhMe2Si)3CH,29 which has a propeller-type configuration of phenyl rings. In 1, a phenyl ring in each ligand is coordinated to a cobalt(I) ion, causing one of the −SiMe2Ph groups in each ligand to be oriented differently from the other two. The reaction between anhydrous FeCl2 and 2 equiv of [Li{C(SiMe2Ph)3}(THF)] in diethyl ether afforded an intractable mixture of products. However, the same reaction in THF

Figure 2. (top) Molecular structure of the anion of 2 with thermal ellipsoids presented at the 50% probability level and (bottom) view along the Fe···Fe axis. Hydrogen atoms are not shown for clarity. Iron is shown in blue, chlorine in green, carbon in gray, and silicon in orange. Selected bond lengths (Å) and angles (deg): Fe1···Fe2 2.8204(15), Fe1−Cl1−Fe2 71.54(5), Fe1−Cl2−Fe2 71.57(5), Fe1− Cl3−Fe2 71.58(5).

formed in 22% yield. Previously, Viefhaus and co-workers12 and LaPointe13 obtained the dialkyl compound Fe{C(SiMe3)3}2 by a similar salt metathesis route. The anion of 2 contains two iron(II) ions which are each σ-bonded to a −C(SiMe2Ph)3 alkyl ligand and linked by three bridging chloride ions. The cation features a Li+ ion solvated by four THF molecules. The stoichiometry of 2 has a resemblance to that obtained from the reaction of CdI2 with 2 equiv of (THF)2.5LiGe(SiMe3)3, which affords the ion pair [Li(THF)4][Cd2(μ-I)3{Ge(SiMe3)3}2].30 In contrast to 1, no reduction of iron(II) was observed. The standard electrode potentials for Co2+/Co and Fe2+/Fe are −0.28 and −0.447 V, respectively,31 which indicates that cobalt(II) is more easily reduced than iron(II). In the [Fe2(μ-Cl)3{C(SiMe2Ph)3}2]− anion of 2, the Fe···Fe separation is 2.8204(15) Å, which greatly exceeds twice the iron single-bond radius (2.46 Å)22 or double the metallic radius in iron metal (2.52 Å).32 The Fe−C bond length of 2.054(4) Å in 2 is almost identical to the 2.0505(14) Å in the dialkyl Fe{C(SiMe3)3}2.13 Each iron(II) ion is in a distortedtetrahedral environment; each C−Fe−Cl angle is 125.79(2)°, and each Cl−Fe−Cl angle is 89.26(3)°. The average Fe−Cl− Fe angles are more acute (71.56(5)°) than the Fe−Cl−Fe angle of 87.50(4)° in the carbene complex [Fe2Cl2(μ-Cl)2(IPr)2] (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene).33 The Fe2(μ-Cl)3 core is similar to those in the diiron cations [LFe(μ-Cl)3FeL]+ (L = 1,4,7-trimethyl-1,4,7-triazacyclononane)34 and [(iPr-trisox)Fe(μ-Cl)3Fe(iPr-trisox)]+ (trisox = 1,1,1-tris(oxazolinyl)ethane),35 which have longer Fe···Fe separations of ca. 3.02(1) and 3.03(1) Å, respectively. The latter are similar to the Fe···Fe distance of 3.086(8) Å36 in 1918

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[(THF)3Fe(μ-Cl)3Fe(THF)3]+. The −SiMe2Ph groups of the ligands in 2, in contrast to those in 1, have a propeller-like arrangement similar to that in (PhMe2Si)3CH.29 The iron(II) ions and the two iron−carbon bonds are collinear, and the two ligands in the structure are in an eclipsed conformation. Thus, the silyl groups eclipse each other when viewed along the iron− iron axis, as shown in the lower portion of Figure 2. The magnetic properties of 1 were measured, after zero-field cooling, between 2 and 300 K in a 0.01 T applied magnetic field. The resulting temperature dependence of χMT is shown in Figure 3. The temperature dependence of 1/χM is linear

Figure 4. χMT of 2 obtained at 0.01 T between 2 and 300 K (black points) and fit between 4 and 300 K (black line) with the Heisenberg isotropic exchange coupling Hamiltonian for S = 2 and g = 2 with J = −149(4) cm−1 (red line), Nα = 0.00714(7) emu/mol (blue line), and ca. 4 wt % of a high-spin iron(III) impurity with S = 5/2 and g = 2 (green line).

however, is the virtually linear increase in χMT observed between ca. 75 and 300 K, an increase that results from a substantial second-order Zeeman contribution, Nα, to the molar magnetic susceptibility. Although larger than expected, the Nα value of 0.00714(7) or 0.00394(4) emu/mol of iron(II) is not unprecedented37,38 and indicates that the second-order Zeeman perturbation has mixed one or more higher energy magnetic states into the ground state of 2, a mixing that can be quite extensive in the small applied magnetic field of 0.01 T. The magnetic properties of 2 also indicate the presence of a small amount of an unknown impurity, a presence that is often observed in strongly antiferromagnetically coupled magnetic complexes. If the impurity is a high-spin iron(III) complex with a molecular weight similar to that of 2, then the impurity amounts to ca. 4 wt % of the sample. In view of the earlier structural work (see above) and magnetic studies,34,35 which have reported small ferromagnetic exchange interactions with J values of 5.8 cm−1 in triply bridged chloride iron(II) complexes, it is rather surprising to find that 2 exhibits strong antiferromagnetic exchange with J = −149(4) cm−1. This difference is, no doubt, a consequence of the Fe···Fe bonding interactions in 2, in which the Fe···Fe distance is substantially shorter at 2.8204(15) Å and the average Fe−Cl−Fe angle is much smaller at 71.56(5)° in comparison to those reported earlier34,35 (see above). Thus, it seems that both direct Fe···Fe magnetic exchange and the three Fe−Cl−Fe superexchange pathways greatly favor antiferromagnetic exchange in 2. In conclusion, the alkylcobalt(I) complex [Co{C(SiMe2Ph)3}]2 (1) and the alkyliron(II) chloride complex [Li(THF)4][Fe2(μ-Cl)3{C(SiMe2Ph)3}2] (2) have been synthesized and structurally and magnetically characterized. Compound 1 is the first example of a homoleptic alkylcobalt(I) complex and has an unusual dimeric structure. In the ionic structure of 2, the two iron(II) ions are each in a distortedtetrahedral environment and are bridged by three chloride ions; the three bridging chloride ions, each with a rather small Fe− Cl−Fe bond angle, yield a short Fe···Fe distance, which in turn

Figure 3. Temperature dependence of χMT of 1 obtained, after zerofield cooling, in a 0.01 T magnetic field and a fit between 2 and 300 K (black line) obtained for S = 1 with D = 38.0(2) cm−1, g = 1.802(2), and Nα = 0.00027(3) emu/mol of dimer.

between 50 and 300 K, and a Curie−Weiss law fit yields a Weiss temperature of −5.9 K, a Curie constant of 1.725 emu K/mol, a corresponding effective magnetic moment, μeff, of 2.63 μB/mol of 3d8 cobalt(I), and, for S = 1, a g value of 1.86. The decrease in χMT for 1 at lower temperatures is indicative of the presence of either cobalt(I) zero-field splitting, weak antiferromagnetic exchange coupling between the two cobalt(I) cations, or perhaps a combination of both. A fit of χMT between 2 and 300 K assuming only zero-field splitting is shown in Figure 3 and, for S = 1, yields D = 38.0(2) cm−1, g = 1.802(2), and Nα = 0.00027(3) emu/mol. An alternative, statistically somewhat poorer, fit of χMT that assumes only antiferromagnetic exchange coupling and uses the / = −2JS1·S2 isotropic Heisenberg exchange coupling Hamiltonian with S1 = S2 = 1 is shown in Figure S2 (Supporting Information). At this point it seems that zero-field splitting is most likely predominant in 1 but also that a combination of some zero-field splitting and some weak antiferromagnetic exchange coupling cannot be eliminated. The magnetic properties of 2 were measured from 2 to 300 K and the observed χMT obtained between 4 and 300 K has been fit with the Heisenberg isotropic exchange coupling Hamiltonian given above to yield the parameters given in the caption of Figure 4. The observed strong antiferromagnetic exchange coupling is somewhat unexpected for the iron(II) dimeric structure of 2 with three chloride bridges. More unexpected, 1919

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(24) Al-Juaid, S. S.; Eaborn, C.; El-Hamruni, S. M.; Hitchcock, P. B.; Smith, J. D.; Sözerli Can, S. E. J. Organomet. Chem. 2002, 649, 121− 127. (25) Klein, H.-F.; Beck, R.; Flörke, U.; Haupt, H.-J. Eur. J. Inorg. Chem. 2003, 2003, 853−862. (26) Klein, H.-F.; Beck, R.; Flörke, U.; Haupt, H.-J. Eur. J. Inorg. Chem. 2003, 2003, 1380−1387. (27) Nguyen, T.; Merrill, W. A.; Ni, C.; Lei, H.; Fettinger, J. C.; Ellis, B. D.; Long, G. J.; Brynda, M.; Power, P. P. Angew. Chem., Int. Ed. 2008, 47, 9115−9117. (28) Lei, H.; Ellis, B. D.; Ni, C.; Grandjean, F.; Long, G. J.; Power, P. P. Inorg. Chem. 2008, 47, 10205−10207. (29) Eaborn, C.; Hitchcock, P. B.; Lickiss, P. D. J. Organomet. Chem. 1984, 269, 235−238. (30) Mallela, S. P.; Schwan, F.; Geanangel, R. A. Inorg. Chem. 1996, 35, 745−748. (31) Electrochemical Series. In CRC Handbook of Chemistry and Physics, 94th ed.; Haynes, W. M., Ed.; CRC Press/Taylor and Francis: Boca Raton, FL, 2014; internet version. (32) Wells, A. F. Structural Inorganic Chemistry, 5th ed.; Oxford University Press: New York, 1984. (33) Przyojski, J. A.; Arman, H. D.; Tonzetich, Z. J. Organometallics 2012, 31, 3264−3271. (34) Bossek, U.; Nühlen, D.; Bill, E.; Glaser, T.; Krebs, C.; Weyhermüller, T.; Wieghardt, K.; Lengen, M.; Trautwein, A. X. Inorg. Chem. 1997, 36, 2834−2843. (35) Gade, L. H.; Marconi, G.; Dro, C.; Ward, B. D.; Poyatos, M.; Bellemin-Laponnaz, S.; Wadepohl, H.; Sorace, L.; Poneti, G. Chem. Eur. J. 2007, 13, 3058−3075. (36) Janas, Z.; Sobota, P.; Lis, T. J. Chem. Soc., Dalton Trans. 1991, 2429−2434. (37) Rohmer, M.-M.; Liu, I. P.-C.; Lin, J.-C.; Chiu, M.-J.; Lee, C.-H.; Lee, G.-H.; Bénard, M.; López, X.; Peng, S.-M. Angew. Chem., Int. Ed. 2007, 46, 3533−3536. (38) Van den Heuvel, W.; Chibotaru, L. F. Inorg. Chem. 2009, 48, 7557−7563.

favors antiferromagnetic exchange coupling between the iron(II) ions.

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ASSOCIATED CONTENT

* Supporting Information S

Text, a table, additional figures, and CIF files giving experimental details, magnetic study details for 1 and 2, and crystallographic data for 1 and 2. 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 for P.P.P.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to the U.S. National Science Foundation (CHE-1263760) for financial support. We thank Dr. Peter Klavins for help with SQUID measurements.

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REFERENCES

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