Inorg. Chem. 2002, 41, 2032−2040
Weakly Coordinating Anions: Crystallographic and NQR Studies of Halogen−Metal Bonding in Silver, Thallium, Sodium, and Potassium Halomethanesulfonates Gary Wulfsberg,* Katherine D. Parks, Richard Rutherford, Debra Jones Jackson, Frank E. Jones, Dana Derrick, and William Ilsley National Center for Applications of NQR Spectroscopy in Inorganic Chemistry, Department of Chemistry, Middle Tennessee State UniVersity, Murfreesboro, Tennessee 37132 Steven H. Strauss, Susie M. Miller, and Oren P. Anderson Department of Chemistry, Colorado State UniVersity, Ft. Collins, Colorado 80523 T. A. Babushkina† and S. I. Gushchin‡ Institute of Biophysics, Ministry of Health, Moscow 123182, Russia E. A. Kravchenko and V. G. Morgunov Institute of General and Inorganic Chemistry, Russian Academy of Sciences, 117907 Moscow, GSP-1, Russia Received July 5, 2001 35Cl, 79,81Br,
and 127I NQR (nuclear quadrupole resonance) spectroscopy in conjunction with X-ray crystallography is potentially one of the best ways of characterizing secondary bonding of metal cations such as Ag+ to halogen donor atoms on the surfaces of very weakly coordinating anions. We have determined the X-ray crystal structure of Ag(O3SCH2Cl) (a ) 13.241(3) Å; b ) 7.544(2) Å; c ) 4.925(2) Å; orthorhombic; space group Pnma; Z ) 4) and compared it with the known structure of Ag(O3SCH2Br) (Charbonnier, F.; Faure, R.; Loiseleur, H. Acta Crystallogr., Sect. B 1978, 34, 3598−3601). The halogen atom in each is apical (three-coordinate), being weakly coordinated to two silver ions. 127I NQR studies on Ag(O3SCH2I) show the expected NQR consequences of three-coordination of iodine: substantially reduced NQR frequencies ν1 and ν2 and a fairly small NQR asymmetry parameter η. The reduction of the halogen NQR frequency of the coordinating halogen atom in Ag(O3SCH2X) becomes more substantial in the series X ) Cl < Br < I, indicating that the coordination to Ag+ strengthens in this series, as expected from hard−soft acid−base principles. The numbers of electrons donated by the organic iodine atom to Ag+ have been estimated; these indicate that the bonding to the cation is weak but not insignificant. We have not found any evidence for the bonding of these organohalogen atoms to another soft-acid metal ion, thallium. A scheme for recycling of thallium halide wastes is included.
* To whom correspondence should be addressed. E-mail: wulfsberg@ mtsu.edu. † Current address: A. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, 117813 Moscow, Russia. ‡ Current address: Department of Chemistry, Perm State University, Perm, Russia.
natometal cations, (porphyrinato)Fe+, and various transition metal organometallic cations which are important industrially as catalysts, and which require “vacant” coordination sites. It is now acknowledged that there is no such thing as a noncoordinating anion; the focus is now on seeking very weakly basic, easily dissociated neutral ligands and weakly coordinating anions1 that act as good leaving groups from latent coordination sites.2 A symposium on weakly coordinating anions was held at the spring 1998 American Chemical Society meeting in Dallas.3
2032 Inorganic Chemistry, Vol. 41, No. 8, 2002
10.1021/ic010715i CCC: $22.00
Introduction Chemists have long sought “noncoordinating” anions to serve as inert counterions in salts of very elusive, very reactive cations such as silylium ions, R3Si+, porphyri-
© 2002 American Chemical Society Published on Web 03/28/2002
Weakly Coordinating Anions
Weakly coordinating anions should be as nonbasic4 as possible: they should have low charge and a large size over which the charge can be dispersed. Examples of this type of anion include methanesulfonate (CH3SO3-), tetraphenylborate ([B(C6H5)4]-), and 1-carba-closo-dodecaborate (CB11H12)-.5 Coordinating ability of the anion is further reduced by substituting the outside surface of the anion with weakly coordinating functional groups containing very electronegative atoms such as halogen, to give anions such as CF3SO3-6 and [B(C6F5)4]-.7 For the most important industrial application requiring weakly coordinating anions, the metallocene process of producing stereoregular polymerization of alkenes,8 the most active catalyst of all is obtained when (CB11H6X6)- (X ) Cl, Br, I) anions are present.9 One might postulate that this is connected with the fact that most weakly coordinating anions now being investigated have fluorinated surfaces, and fluorine is a hard base, more likely to coordinate to the (presumably) hard zirconium(IV) active site in the metallocene catalysts, while the carborane anions have softer surfaces. If this postulate is valid, then the softest carborane anion, the iodinated one, should show the weakest coordination to zirconium and, conversely, the strongest coordination to the (more easily isolated) silver salt. The crystal structures of all six silver salts Ag(CB11H6X6) and Ag(CH3CB11H5X6) have been determined,10 but the varying numbers, types, and bond distances of donors to silver did not present an easily analyzed pattern. Although X-ray crystallography is the most readily available and most commonly used method of detecting weak secondary bonding11 of ligands to metal ions, it leaves several important questions unanswered. Secondary bond distances commonly show wide ranges, even in chemically equivalent bonds in the same complex (e.g., from 2.640 to 2.926 Å in Ag(CB11H6Cl6)). Is there any bonding significance to the differences in contact distances commonly found, or does (1) Strauss, S. H. Chem. ReV. 1993, 93, 927-42. Seppelt, K. Angew. Chem., Intl. Ed. Engl. 1993, 32, 1025-1027. (2) Strauss, S. H. Chemtracts: Inorg. Chem. 1994, 6, 1-13. (3) Dagani, R. Chemical Superweaklings. Chem. Eng. News 1998, 76 (May 4), 49-54. This work was first presented in that symposium (Wulfsberg, G.; Rutherford, R.; Jackson, D.; Jones, F.; Jones, M.; Derrick, D.; Strauss, S.; Terao, H.; Babushkina, T. A.; Gushchin, S. I. Presented at the National Meeting of the American Chemical Society, Dallas, TX, March 1998; Paper INOR 102). (4) Wulfsberg, G. Inorganic Chemistry; University Science Books: Sausalito, CA, 2000; pp 68-73, 116-120. (5) Reed, C. A. Acc. Chem. Res. 1998, 31, 133-139. (6) Lawrance, G. A. Chem. ReV. 1986, 86, 17-33. (7) Lambert, J. B.; Zhang, S.; Stern, C. L.; Huffman, J. C. Science 1993, 260, 1917-1918. (8) Jordan, R. F. AdV. Organomet. Chem. 1991, 32, 325-382. Brintzinger, H. H. Fischer, D.; Mu¨lhaupt, R.; Rieger, B.; Waymouth, R. M. Angew. Chem., Intl. Ed. Engl. 1995, 34, 1143-1170. Deutsch, C. H. Finding Flexibility in Plastics: High Technology Could Add New Life to an Old Product. New York Times, Sept 9, 1997, p C1, C6. (9) (a) Xie, Z.; Bau, R.; Reed, C. A. Angew. Chem., Intl. Ed. Engl. 1994, 33, 2433-2434. (b) Xie, Z.; Manning, J.; Reed, R. W.; Mathur, R.; Boyd, P. D. W.; Benesi, A.; Reed, C. A. J. Am. Chem. Soc. 1996, 118, 2922-2928. (10) (a) Xie, Z.; Wu, B.-M.; Mak, T. C. W.; Manning, J.; Reed, C. A. J. Chem. Soc., Dalton Trans. 1997, 1213-1217. (b) Xie, Z.; Tsang, C.W.; Xue, F.; Mak, T. C. W. J. Organomet. Chem. 1999, 577, 197204. (11) Alcock, N. W. AdV. Inorg. Chem. Radiochem. 1972, 1, 1-58.
Table 1. Average Metal-Organohalogen Distances for Halocarbon and Halocarborane Complexes of Silver, Silicon, Thallium, and Cesium complex
d(M‚‚‚X)
excess da
CN(Ag)b
ref
Ag(CH3CB11Cl11) Ag(CB11H6Cl6) Ag(CB11H6Br6) Ag(CB11H6I6) iPr Si(CB H Cl ) 3 11 6 6 iPr Si(CB H Br ) 3 11 6 6 iPr Si(CB H I ) 3 11 6 6 Tl(CB11H6Br6)‚2C7H8 Cs(HCB11Br11) Cs(HCB11I6Br5)‚THF Cs(HCB11I6Br5)‚THF [Ag(Cl2CH2)n]+ [Ag(Br2CH2)n]+ [Ag(I2CH2)n]+ Ag(ClCH2SO3) Ag(BrCH2SO3)
2.882 Å 2.780 2.824 2.946 2.323 Å 2.479 Å 2.661 Å 3.469 Å 3.733 Å 3.89 Å (Br) 4.07 Å (I) 2.832 Å 2.865 Å 2.851 Å 2.945 Å 2.971 Å
0.37 Å 0.27 Å 0.14 Å 0.10 Å 0.15 Å 0.16 Å 0.15 Å 0.86 Å 0.24 Å 0.40 Å 0.39 Å 0.32 Å 0.20 Å 0.00 Å 0.43 Å 0.31 Å
6 6 6 5
c d d d e e e f c g g h h h this work i
6 6 4 6 6
a Average distance in excess of M-X single bond distance ) sum of covalent radii except in the case of Cs+, for which the single-bonded metallic radius is used. b Coordination number of Ag+. c Ref 13a. d Ref 10a. e Ref 9b. f Ref 18. g Ref 13b. h From sources cited in Table 1 of ref 31. i Ref 16.
secondary bonding correspond to such a shallow potential well that the differences merely represent variations in what is needed to achieve good packing at the lowest energy in the solid state? The normal criterion for weak secondary bonding is that the metal-donor distance should be greater than the sums of single covalent or ionic radii but less than the sums of van der Waals radii. But van der Waals radii are notoriously hard to determine, because one must first be confident that there is indeed no bonding in the direction in which the contacts are measured. As an illustration of the difficulties, solid halocarbons commonly pack with halogen-halogen distances that are less than the sums of their van der Waals radii. Consequently, one must ask whether the van der Waals radii are incorrect, or differ in different directions around an organohalogen atom, or whether there are hithertounsuspected secondary bonding interactions present.12 In Table 1, we summarize metal-halogen distances found in silver, silicon, thallium, and cesium salts of weakly coordinating anions and related ligands.13 We can attempt to analyze these distances by computing the excess bond distance: the amount that the observed Ag-X distance is in excess of the distance expected for a normal single bond between Ag and X. For a neutral halogen atom in contact with a cation, the types of radii that should be summed to give the expected distance are not entirely clear, but the most sensible results are obtained from the sums of covalent radii, which are 2.51 Å for Ag-Cl, 2.66 Å for Ag-Br, and 2.85 Å for Ag-I. How much longer than the sums of covalent radii must the metal-halogen contact be before we can decide that it no longer represents coordination? Overall, it may be seen that excess bond distances are broadly similar for Ag, Cs, and Si, although cations of these elements would be expected to bond in quite different ways to halogens. (12) Desiraju, G. R.; Parthasarathy, R. J. Am. Chem. Soc. 1989, 111, 87258726. (13) (a) Xie, Z.; Tsang, C.-W.; Sze, E.; Yang, Q.; Chan, D.; Mak, T. C. W. Inorg. Chem. 1998, 37, 6444-6451. (b) Tsang, C.-W.; Yang, Q.; Sze, E.; Mak, T. C. W.; Chan, D.; Xie, Z. Inorg. Chem. 2000, 39, 5851-5858.
Inorganic Chemistry, Vol. 41, No. 8, 2002
2033
Wulfsberg et al. Hence, determining whether a long metal-donor atom contact actually signifies coordinate covalent (secondary) bonding requires confirmation by another method. Most spectroscopic methods cannot reliably detect the subtle bonding changes that follow the weak coordination of a halogenated anion or other ligand to a metal ion. We propose that, for ligands or anions in which surface donor atoms are chlorine, bromine, or iodine, the method of choice to supplement X-ray crystallography for characterizing weak bonding may be halogen (35Cl, 79,81Br, 127I) nuclear quadrupole resonance (NQR) spectroscopy. A limitation of NQR spectroscopy is the requirement for gram-scale samples of polycrystalline solid materials; such quantities are not affordable or even available for Ag(CB11H6X6).14 Hence, to begin the study of the metal-ion coordinating characteristics of weakly coordinating halogenated anions, we have substituted a much more affordable set of anions, the halomethanesulfonate anions, ICH2SO3(1-), BrCH2SO3- (2-), and ClCH2SO3- (3-). Fluorinated anions of this type are suggested for use as noncoordinating anions in the “Notice to Authors” of this journal;15 results on related weakly coordinating anions will be reported elsewhere. Although the oxygen ends of these anions are the main sites of coordination, the crystal structure of Ag(2)16 shows that it has Ag-Br contacts very similar to those found in Ag(CB11H6Br6). In this study, we have determined the X-ray structure of Ag(3) and find not only that it is isomorphous and isostructural with Ag(2) but also that it shows very similar Ag-Cl contacts to those found in Ag(CB11H6Cl6). Although twinning problems prevented us from determining the structure of Ag(1), we shall show that the NQR evidence suggests that it also shows similar characteristics to silver chloro- and bromomethanesulfonate. Study of the NQR spectra of salts of the halomethanesulfonate anions should show what happens when an anion generally expected to be weakly coordinating to typical cations such as Na+ and K+ does coordinate to a soft cation such as Ag+ via halogen atom(s). The thallium(I) ion is another classical +1-charged softacid metal ion which has been found by X-ray crystallography17 to coordinate to organohalogen atoms of a weakly coordinating ligand and to halogen atoms in the weakly coordinating anion (CB11H6Br6)-.18 This study includes an NQR investigation of the coordinating ability of thallium(I) ion in its halomethanesulfonate salts. Experimental Section All reactions involving the photosensitive silver and thallium halomethanesulfonates were carried out with red-light illumination. Elemental analyses were obtained from Galbraith Laboratories, Knoxville, TN, and are shown in Table 2. It should be noted that (14) Reed, C. A. Personal communication. (15) Notice to Authors. Inorg. Chem. 2001, 39 (1), 14A. (16) Charbonnier, F.; Faure, R.; Loiseleur, H. Acta Crystallogr., Sect. B 1978, 34, 3598-3601. (17) Hurlburt, P. K.; Anderson, O. P.; Strauss, S. H. Can. J. Chem. 1992, 70, 726-731. (18) Mathur, R. S.; Drovetskaya, T.; Reed, C. A. Acta Crystallogr., Sect. C 1997, 53, 881-883.
2034 Inorganic Chemistry, Vol. 41, No. 8, 2002
Table 2. Elemental Analyses of Halocarbon Derivativesa %C
%H
%hal
compound
calcd
found
calcd
found
calcd
found
AgBrCH2SO3 KBrCH2SO3 TlBrCH2SO3 KClCH2SO3 AgClCH2SO3 TlClCH2SO3 AgICH2SO3 TlICH2SO3‚1/12C12H24O6 KICH2SO3
4.26 5.64 3.17 7.12 5.06 3.60 3.63 5.36 4.62
4.24 5.63 2.40 7.05 5.26 3.65 2.73 5.23 4.66
0.72 0.95 0.53 1.20 0.85 0.60 0.61 1.12 0.78
0.67 0.93
