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Anal. Chem. 1981, 53, 393-399
Binuclear Complexes of Lanthanide(" and Silver(1) and Their Function as Shift Reagents for Olefins, Aromatics, and Halogenated Compounds T. J. Wenzel and R. E. Sievers" Department of Chemistry and Cooperative Institute Colorado 80309
for Research in Environmental Sciences (CIRES),
Universi@ of Colorado, Boulder,
New binuclear complexes Involving a lanthanide(111) ion and silver(I) and their appllcatlons as nuclear magnetic resonance (NMR) shift reagents for olefins, aromatics, and halogenated compounds are described. The sllver in these complexes binds to the substrate, and shifts are observed in the NMR spectrum because of the paramagnetic lanthanide metal. Cls-trans Isomers of olefins can be distinguished by examlnlng their NMR spectra in the presence of these shift reagents. The NMR spectra of compounds such as 1-hexene, d-llmonene, 2-octyne, I-bromopentane, I-iodohexane, and chlorocyclohexane are simpllfied and considerably more informative In the presence of these shlfl reagents. The structures of the binuclear complexes, as well as the sllver P-dlketonates used In the formatlon of the blnuclear complexes, are discussed.
chelates. Damska and Janowski have recently used silver trifluoroacetate with lanthanide shift reagents for simplification of the spectra of aromatics (15). In an earlier communication we reported novel binuclear complexes containing a lanthanide(II1) ion and silver(1) pdiketonate that are capable of inducing larger shifts in the NMR spectra of aromatics than observed using silver heptafluorobutyrate or silver trifluoroacetate (16). In this paper, the use of these new shift reagents has been extended to olefins and halogenated substrates. In these complexes the substrate bonds to silver(1) and is held near the lanthanide in a binuclear complex. The binuclear complexes are formed in solution from lanthanide P-diketonates and silver P-diketonates. Because the silver is bonded to an olefin or aromatic substrate, the NMR spectrum of the substrate in these complexes exhibits substantial shifts due to the paramagnetic lanthanide metal.
Since 1969, when Hinckley first reported the use of the dipyridine adduct of tris(2,2,6,6-tetramethyl-3,5-heptanedionato)europium(III),Eu(thd)B.py2,as a nuclear magnetic resonance shift reagent (I),the utilization and understanding of lanthanide shift reagents have grown considerably. In 1970, Sanders and William8 (2) observed improved shifts by using the anhydrous E ~ ( t h d compound. )~ In the same year, Whitesides and Lewis (3) described the use of a chiral lanthanide shift reagent, [ (3-tert-butylhydroxymethylene)dcamphorate]europium, for enantiomeric differentiation. The synthesis and utilization of other cliiral lanthanide shift reagents, most notably those of 3-trifluoroacetyl-d-camphor and 3-heptafluorobutyryl-d-camphor, have been described (4-7).In 197'1, Rondeau and Sievers (8)first described the properties of lanthanide chelates of 6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octanedione [H(fod)] as shift reagents. These compounds are stronger Lewis acids than the thd chelates, are considerably more soluble in NMR solvents such as CCll and CHC13,and have been found to induce larger shifts than thd chelates in many substrates. 'The fod complexes are now the most widely used of the various commercially available lanthanide shift reagents. Even stronger Lewis acids, the lanthanide chelates of 1,1,1,2,2,3,3,7,7,7-decafluoro-4,5heptanedione (dfhd) developed by Sievers et al. (9) and 1,1,1,2,2,6,6,7,'7,7-decafluoro-3,5-heptanedione reported by Burgett and Warner ( l o ) ,have been found to induce substantial shifts on rather weak Lewis bases for which the fod and thd chelates are less effective. Over the years a number of reviews have. appeared on the subject (11-13), describing many practical applications as well as theory. These shift reagents are effective on compounds containing basic heteroatoms, primarily oxygen and nitrogen, but do not bond to and shift the spectra of many soft Lewis bases such as olefins, aromatics, phosphines, and halogenated compounds. Evans et al. (14) have previously reported shifts in the spectra of olefins by using silver heptafluorobutyrate with lanthanide
Apparatus. Proton NMR spectra were recorded on a Varian EM-390 spectrometer. Carbon-13 spectra were recorded on a JEOL PFT-100 spectrometer with a Nicolet 1080 data system. Infrared analyses were performed by using KBr pellets in a Perkin-Elmer 337 grating infrared spectrophotometer. Elemental analyses were performed by Huffman Laboratories, Inc., Wheatridge, CO. Reagents. All chemicals were used as received without further purification. Silver nitrate was obtained from Fischer Scientific Co., Fair Lawn, NJ. The 6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octanedionewas obtained from PCR Inc., Gainsville, and l,l,l-trifluoroFL; 1,1,1,5,5,5-hexafluoro-2,4-pentanedione 2,4-pentanedione were obtained from Pierce Chemical Co., Rockford, IL. Chloroform-d was obtained from Stohler Isotope Chemicals, Waltham, MA. Tetramethylsilanewas obtained from Alfa Division, Danvers, MA. Synthesis Procedures. The L n ( f ~ dchelates )~ were synthesized and purified according to the procedure of Springer et al. (17) or obtained from Aldrich Chemical Co., Milwaukee, WI, and used after drying over P4Ol0. The H(dfhd) ligand was synthesized according to the procedure of Scribner et al. (18)and L ~ ~ ( d f hcomplexes d)~ synthesized according to Richardson and Sievers (19) or obtained from Aldrich Chemical Co. Preparation of (6,6,7,7,8,8,8-Heptafluoro-2,2-dimethyl-3,5octanedionato)siluer(l),Ag(fod). A solution of 9.6 g (0.0324 mol) of H(fod) in 5 mL of methanol was neutralized with 8.1 mL of 4 M NaOH. This solution was added to a stirred solution of 5.5 g (0.0324 mol) of silver nitrate in 75 mL of distilled water. A white precipitate immediately separated, was collected by suction filtration, and was dried in vacuo over P4Ol0for 24 h. The gray-white product slowly decomposes when exposed to light and was stored in a light-proof container; yield 9.7 g (74%). The dried material decomposesover the range 134-135 "C, changing to a blackened residue. Anal. Calcd for Ag(fod) (AgCloHloOzF7):Ag, 26.76; C, 29.80; H, 2.50; F, 33.00. Calcd for Ag(fod)H20(AgC10H1203F7): Ag, 25.62; C, 28.52; H, 2.87; F, 31.58. Found: Ag, 23.9 xt 1;C, 28.39; H, 2.38; F, 31.44. All attempts at recrystallization led to some decomposition of the material while in solution and the purity could not be improved over that of the precipitate originally collected.
EXPERIMENTAL SECTION
0003-2700/81/0353-0393$01.00/0 0 1981 American Chemlcal Society
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ANALYTICAL CHEMISTRY, VOL. 53, NO. 3, MARCH 1981
Preparation of (1,I ,I -Trifluoro-2,4-pentanedionato)siluer(Z), Ag(tfa). This compound was prepared and dried by the same method as above. It is also light sensitive and is stored in a light-proof container; yield 76%. The white product decomposed over the range 108-133 O C . Anal. Calcd: Ag, 41.34;C, 23.01;H, 1.54;F, 21.84. Found Ag, 40.67 f 1; C,22.47;H,1.52;F, 20.80. Preparation of (1,l,I,5,5,5-Hexafluoro-2,4-pentanedionato)siluer(0, Ag(hfa). When hexafluoroacetylacetonewas subjected to the above procedure, a dark brown solution with no precipitate resulted. On continued addition of 4 M NaOH, a brown precipitate formed, was collected by suction filtration, and dried in vacuo over P4Ol0for 24 h. Above a pH of 9, no more precipitate resulted. The product was stored in a light-proof container; yield 57%. The material did not melt or decompose when heated to 300 O C . Anal. Calcd Ag, 34.25;C, 19.07;H, 0.32;F, 36.20.Found Ag, 36.67 f 1; C, 18.68;H,0.74;F, 32.94. NMR Studies. The required amounts of lanthanide chelate, silver compound, and substrate were added to a solution of 1% tetramethylsilane (Me4Si)in chloroform-dand shaken for 2 min. The mixture was then centrifuged and the supernatant decanted into a NMR tube for analysis. During these procedures, and up until recording the spectrum, the solutions were kept covered with aluminum foil to exclude light. In the figure captions the concentrations shown are those calculated on the basis of what was added but represent only upper limits since some insoluble material was removed in most cases.
compounds, was observed. This indicates that a rapid exchange of fod ligands between the two metal centers exists, and the rate of silver exchange between its free and lanthanide coordinated form is fast on the NMR time scale. This is not surprising, as similar results have been observed when mixtures of Eu(fod), and Pr(fod), were studied by NMR (24). Also, silver bonds with either carbon or oxygen are known to be labile (20, 25). The rate of substrate exchange between free and coordinated forms is also rapid on the NMR time scale. In all cases, only one set of resonances was observed for a substrate when it was combined with the silver-lanthanide binuclear complex. Fast exchange has also been observed in previous NMR studies of silver complexes with olefins (26). Because both of these equilibria are rapid on the NMR time scale, the resonances observed for a substrate in the presence of these shift reagents probably are a time-averaged result of three species: the free substrate, the silver complex of the substrate, and the lanthanide-silver complex of the substrate. The mode of bonding between silver and the 0-diketone ligands under study remains uncertain. An X-ray structure of a binuclear silver-nickel compound in which the silver bonds to the methine carbon of a 0-diketone ligand has been reported (27). In earlier studies of silver 0-diketonecomplexes, RESULTS AND DISCUSSION Gibson et al. (23) and Partenheimer and Johnson (22)used Complexes of silver with olefins have been studied and infrared, NMR, and molecular weight data to support their characterized extensively (20))and the bonding is well unconclusion that the ligands form chelates in which the metal derstood (21). Complexes of the type (olefin)Ag(P-diketone) is bonded to the oxygens. Oxygen-bound &diketone ligands usually exhibit two IR bands in the region 1500-1700 cm-l have been synthesized and characterized by Partenheimer and Johnson (22) and Gibson et al. (23). In their studies, the (28). These correspond to the C=O and C=C stretching P-diketone was either 1,1,1,5,5,5-hexafluoro-2,4-pentanedione frequencies and occur at about 1600 and 1525 cm-l, respecor l,l,l-trifluoro-2,4-pentanedione, and a variety of cyclic tively. For carbon-bound metal complexes, the carbonyl band is usually found at a much higher frequency than 1600 cm-l olefins or triphenylphosphine was used. These compounds are soluble in nonpolar solvents. and the C=C stretching frequency does not appear in the The silver 0-diketonates form relatively stable complexes spectrum (23). Also, for oxygen-bonded cases, the C-H with soft Lewis bases such as phosphines and olefins. Silver out-of-plane bend absorption is observed as a sharp band can exhibit coordination numbers of three or four (23)while around 800 cm-', while in the carbon-bonded complex, the the monomeric units of the silver P-diketonates may only have band is not observed. In the infrared spectra for Ag(fod) and Ag(tfa), both the C=C stretching and C-H out-of-plane a coordinationnumber of two or exist as oligomers with higher bending frequencies are observed. The carbonyl stretching coordination numbers. These compounds are also capable of frequencies, 1625 cm-l for Ag(fod) and 1650 cm-' for Ag(tfa), bonding to the lanthanide chelates. The larger shifts in the spectra of olefins and aromatics using binuclear complexes are also indicative of oxygen-bondedligands. The possiblity formed from these silver P-diketonates result in part from their that the silver is a bonded to a carbon-carbon double bond in the chelate ring, however, cannot be eliminated. An inenhanced solubility over those of silver heptafluorobutyrate. teraction of this type has been reported for a platinum 0In addition though, it appears that increased stability of the binuclear complex, an enhanced magnetic environment, or diketone complex (29). Finally, the silver 0-diketonates may be ionic compounds with no specific bonding. An amorphous both lead to the better shifts. oligomerization involving some or all of the above mentioned In order for shifts to be induced in the nuclear magnetic interactions could take place. resonance spectrum of alkene and aromatic substrates by the shift reagents described, two equilibria probably exist (eq 1 A number of structures can be envisioned for the silverand 2). In one, the substrate, S, coordinates to the silver lanthanide binuclear complex. We believe that addition of the silver P-diketonate to the lanthanide tris(0-diketonate) Ag(@-dik) S Ag(@-dik)(S) (1) in solution results in the formation of a lanthanide tetrakis Ln(@-dik)s Ag(@-dik)(S)* Ln(@-dik),Ag(S) (2) chelate anion. The negative charge is balanced by the silver counterion to form an ion pair. The ability of lanthanide(II1) 0-diketonate, Ag(0-dik). The silver 0-diketone-substrate ions to form tetrakis complexes is well-known, and X-ray complex is further associated with a lanthanide chelate, Lncrystal structures for some of these have been reported (30, (0-dik),, to form in situ what is probably a tetrakis complex. 31). The set of experiments that led us to this conclusion is The NMR spectrum of the substrate in these binuclear comdescribed in Table I. plexes exhibits substantial shifts due to the presence of the Several combinations of europium complexes and silver paramagnetic lanthanide metal. Admittedly, the equilibria compounds were studied in an effort to reproduce the shifts taking place may involve many more steps and species, but observed in the NMR spectrum of toluene obtained by using the simplified treatment presented adequately describes the E ~ ( f o dwith ) ~ Ag(fod). The only pair producing appreciable phenomena observed. For the majority of shift reagent shifts involved mixing K[Eu(fod),] with AgBF4. Both of these studies, it is desirable that both of these processes be rapid are soluble in chloroform, and adding the two together proon the NMR time scale. A variety of concentrations of Euduces an insoluble precipitate, presumably KBFI. The ex(f0d)3, P r ( f ~ d )or~ Y , b ( f ~ dwith ) ~ Ag(fod) have been studied change of counterionsprobably results in formation of the ion by NMR and in all cases only one tert-butyl resonance, with pair Ag[Eu(fod)dthat induces shifts similar to those observed a chemical shift value intermediate between those of the pure
+
+
ANALYTICAL CHEMISTRY, VOL. 53,NO. 3, MARCH 1981 CHCl3
Table I. Comparative Shift Data for 0.1 M Toluene in CDCI, with Several Shift Reagent Combinations Hb
309
A
Ho
shift, ppm shift reagent Hb 0.2 M Eu(fodL. 1.00 0.2 M Ag(fd$) 0.10 0.2 M Eu(fod),, 0.2 M AaBJ?, 0.2 M Eu(jodc, 0 0.2 M AgBF,, 0.2 M H(fod) 0.2 M KEu(fod), 0 0.2 hl KEu(fod),, 0.81 0.2 M AeBF, C
H, 0.76
CH3 0.40
0.10
0.05
0
0
0 0.56
0 0.30
Bl
I
Proton NMR spectrum of 0.1 M rn-xylene in CDCI, with (a) no shift reagent, (b) 0.2 M Yb(fod),, 0.2 M Ag(tfa),and (c) 0.1 M
Flgure 2.
Yb(fOd)a, 0.1 M Ag(fOd).
Flgure 1. Proton NMR spectrum of 0.1 M rn-xylene in CDCI3 with (a) no shift reagent, (b) 0.2 M Pr(fo&, 0.2 M Ag(fod), and (c) 0.2 M Pr(fodb, 0.2 M Ag(tfa). In all figures the concentrationsshown are those that would have resulted, based on amounts added, if everything were soluble; see Experimental Section.
with an equimolar mixture of Eu(fod), and Ag(fod). These shifts are not observed when E ~ ( f o dand ) ~ AgBF4 are mixed, and this combination is not expected to form stable ion pairs in solution. A similar result has been obtained by Krasutsky, who saw no shifts when AgBF4 and Eu(fod)3 were added to 3,7-dimethylenebicyclo[3.3.llnonane (32?).The use of K[Eu(fod),] by itself does not result in any shifts, which shows the necessity for the silver ion to bond to toluene. Also, the combination of Eu(fod), with AgBF4 and H(fod) does not produce any shifts. These results all point to the formation of an ion pair, Ag[Eu(fod),], that is the active binuclear species in the alteration of spectra of aromatic compounds. There appear to be a variety of reasons why the use of different silver P-diketonates leads to different lanthanideinduced shifts. One definite advantage of Ag(fod) and Ag(tfa) over Ag(hfa), silver heptafluorobutyrate, or silver trifluoroacetate is the enhanced solubility of the former in chloroform and carbon tetrachloride. Due to the larger distances between the lanthanide and the substrate in these binuclear complexes compared to those where the substrate i s bound directly to the lanthanide, higher concentrations of the shift reagent are needed to force the equilibria 1and 2 to the right, so useful shifts can be observed. The spectra obtained for m-xylene by using Pr(fod), with Ag(fod) and Ag(tfa) are shown in Figure 1. In the spectrum obtained with Ag(fod), complete resolution of the A and B protons is not observed. The shifts obtained by using Ag(tfa) are substantially larger and all three sets of protons are re-
solved. In all studies done with Pr(fod),, larger induced shifts were observed with Ag(tfa) compared to Ag(fod). In Figure 2, the spectra obtained for m-xylene by using Yb(fod), with Ag(tfa) or Ag(fod) are shown. In this case, the reverse of the above is noted, and larger shifts are observed in the spectrum obtained with Ag(fod). In studies in which Yb(fod), was employed, larger shifts were always found with Ag(fod) vs. Ag(tfa). A similar result was observed for Eu(fod),, as the use of Ag(fod) resulted in larger shifts for the substrate than those obtained by using Ag(tfa). The reason for this phenomenon is not known with certainty. One possibility considered was that the different lanthanide silver ion pairs altered the ability of the silver to bond to the substrate due to steric encumbrances. Studies done with La(fod), and Lu(fod),, which are diamagnetic complexes and close to Pr and Yb in ionic radii, respectively, seem to eliminate this possibility. The shifts that result in a substrate upon bonding to these diamagnetic binuclear complexes are due to shifts in electron density upon complexation, since paramagnetism plays no role. The shifts observed in the spectrum of 1-hexene using La(fod)3 with Ag(fod), La(fod), with Ag(tfa), Lu(fod)a with Ag(fod), and Lu(fod), with Ag(tfa) were essentially identical in all cases (-0.5 ppm for the olefinic proton G). This indicates that the stability of the bond between the silver and the substrate is the same throughout this group. For this reason, the most probable cause for the variations in observed shifts with the choice of silver p-diketonate is due to different magnetic properties of the resulting binuclear complexes. The lanthanide-induced shifts in the NMR spectrum of a substrate are dependent on the magnetic environment created in the complex it forms with the shift reagent. Subtle differences in this magnetic environment can lead to substantial changes in the observed shifts. Slight changes in the structure of the ion pairs as the lanthanide ion changes its size may lead to changes in the magnetic environment of the substrate that are rather significant. The shifts were compared for toluene by using Pr(fod), with Ag(fod), Ag(tfa), silver heptafluorobutyrate, and silver trifluoroacetate. A first-order spectrum was obtained for the aromatic protons with both Ag(fod) and Ag(tfa), while the use of the other two silver compounds only resulted in small shifts and incomplete resolution of the aromatic resonances. These shift reagents were tested in seven typical NMR solvents. Induced shifts were measured for 4-methylstyrene by using Eu(fod), and Ag(f0d). In only chloroform and carbon tetrachloride were adequate shifts observed. The shifts in
396
ANALYTICAL CHEMISTRY, VOL. 53, NO. 3, MARCH 1981
Jdb(b) G 1
Flgure 4. Proton NMR spectrum of 0.1 M d-llmonene In CDCI, with (a) rx) shm reagent, (b) 0.1 M Yb(fodh, 0.1 M Ag(fod), (c)0.2 M Yb(fodh, 0.2 M Ag(fod), and (d) 0.4 M Yb(fod),, 0.4 M Ag(fod).
Flgure 3. Proton NMR spectrum of 0.1 M 1-hexene in CDCI, with (a) no shift reagent, (b) 0.1 M Y b ( f d ) , 0.1 M Ag(fd), and (c) 0.1 M Yb(dfhd),, 0.1 M Ag(hfa).
chloroform were slightly larger and we have found these shifts to be reproducible for at least 4 days after preparation of the sample. In benzene-& acetone-& acetonitriled3, and dimethyl-d6 sulfoxide either small or no shifts were observed. Each of these solvents is capable of bonding effectively to either the silver or lanthanide chelate and disrupting formation of the lanthanide-silver-substrate complex. Addition of Ag(fod) to carbon disulfide resulted in a red-brown color. It appears that a reaction takes place between silver and the solvent, and no shifts were observed for the substrate in carbon disulfude. Studies of Olefins. Several olefins were studied to determine the applicability of these shift reagents for altering the NMR spectra of the substrates. The spectrum of 1-hexene is shown in Figure 3a. In the spectrum of 1-hexene in chloroform without shift reagent present, the resonances for protons B and C overlap extensively. The first attempt to resolve the resonances involved using E ~ ( f o dand ) ~ Ag(fod). Europium(II1) has been used in most shift reagent studies because it results in minimal broadening (33). Europium shift reagents generally induce downfield shifts. In the presence of E ~ ( f o dand ) ~ Ag(fod), the spectrum of 1-hexene exhibited downfield shifts; however, the resonances for the B and C protons could not be resolved. In all studies performed, the europium chelates were generally found to be ineffective, producing only rather small shifts. Since the magnitude of the lanthanide-induced shifts is related to a distance factor, l/?, the increased distance caused by having a silver compound to hold together the olefii and lanthanide significantly reduces the observed shifts. In an attempt to resolve the resonances for protons B and C, shift reagents containing ytterbium(II1) were tried. Ytterbium shift reagents generally induce downfield shifts that are substantially larger than those of europium(II1). In addition, the broadening caused by ytterbium shift reagents is not severe. The spectrum in Figure 3b resulted when equimolar amounts of Y b ( f ~ d ) Ag(fod), ~, and 1-hexene were present. The shifts are significantly better and the resonances for the B and C protons are now resolved. All the resonances are easily identified and assigned; however, the tert-butyl resonance of the fod ligands obscures some of the region in which the A and B resonances lie. This would severely hamper studies done on olefins containing alkyl groups. To eliminate
this problem, we used a lanthanide chelate containing more extensively fluorinated /3-diketone ligands. The lanthanide chelates of the anion of 1,1,1,2,2,3,3,7,7,7-decafluoro-4,6-heptanedione (dfhd) have been used as shift reagents and have been found to induce even larger shifts, especially with weak Lewis bases, than those observed with the fod chelates (9). An equimolar mixture of Yb(dfhd)3,Ag(fod), and 1-hexene gave the spectrum in Figure 3c. The shifts are not as large as those observed with Yb(fod)3/Ag(fod);however, the resonances for protons B and C are resolved. Furthermore, no resonancesfrom the shift reagent interfere with the spectrum of the substrate. The peaks for the methine proton of both ligands, as well as the tert-butyl resonance from Ag(fod) appear upfield of Mg4Si. Another point worth noting in the ytterbium spectrum is the broadening observed in the resonances corresponding to the olefinic protons. The use of ytterbium(1II) leads to greater broadening than use of europium(II1). For this reason, in studies of protons close to the double bond, europium(II1) chelates are recommended. For protons removed from the double bond, ytterbium(II1) is the metal of choice. Shifts were also recorded for 1-hexene by using L a ( f ~ d ) ~ with Ag(fod) and Lu(fod)awith Ag(fod). Since both lanthanum(II1) and lutetium(II1) are diamagnetic, any shifts observed with these reagents should be due to the formation of the complex. The shifts in both cases were small (-0.5 ppm for the olefinic proton G)and the spectral clarifications observed in the presence of the other metal complexes are due principally to the paramagnetism of the lanthanide ions. In previous work by Evans et al. (14),the compound d-limonene was studied. This molecule has two double bonds that are capable of bonding to silver. Since the stability of a bond between silver and an olefin is known to be very dependent on steric factors (34,35),in a polyfunctional substrate, bonding at the site of least steric encumbrance should be preferred. Evans concluded that bonding took place at the olefii attached to the 4-position of the ring, and our shift data support an analogous conclusion. The series of spectra obtained when increasing concentrationsof Yb(fod)3and Ag(fod) were present is shown in Figure 4. The large number of individual resonances observed for the ring protons in the presence of the shift reagent suggests distinctive environments for axial and equatorial protons. On the basis of shift and decoupling data, most of the resonances can be assigned. At higher shift reagent concentrations, it appears that the double bond in the ring also complexes with the silver. Evidence for this can be seen in the shifts for protons A and F. These
ANALYTICAL CHEMISTRY, VOL. 53, NO. 3, MARCH 1981 CIS -
TRANS
C B A CH3;C=C/CH2CH2CH3
C B A ,CH2Cti2CH3
D
"E
1
"E
2
I
\
\
1
I"
387
TMS
TMS
C*3
1
0
9
E
7
6
5
4
3
2
1
oppm
Flgure 5. Proton NMR spectrum of 0.1 M cls- and hansQhexene in CDCi, with (a) no shift reagent, (b) 0.1 M Yb(fod),, 0.1 M Ag(fod), and (c) 0.2 M Yb(fod),, 0.2 M Ag(fod).
protons actually exhibit larger shift changes at the higher concentrations of shift reagent than expected if a simple linear relationship existed. For 4-vinyl-1-cyclohexene (I) (a polyfunctional compound
Hd&/Hb
(1) similar to d-limonene except that protons replace the two methyl groups) shift data indicate that the silver bonds to both olefin groups. The shifts observed with 0.2 M Eu(fod),, 0.2 M Ag(fod),and 0.1 M 4-vinyl-1-cyclohexene in CDC13are Ha (1.58 ppm), Hb(1.65), H,(1.94), Hd (1.94), and I& (2.13). Since all of these shifts are quite similar in magnitude, complexation a t both sites is implied. This was further substantiated by analysis of the 13C NMR spectrum. In previous studies by Parker and Roberts (36)and Beverwijk and van Dongen (33, upfield shifts were observed for the resonances of carbons upon binding to silver. This phenomenon was explained as resulting from decreases in the x-orbital energy that produce small changes in the excitation energy. The size of the shifts depends on the degree of substitution on the unsaturated carbons, and in some instances downfield or no shifts were observed. The 13C NMR spectrum of a sample to which was added 0.2 M Eu(fod),, 0.2 M Ag(fod), and 0.1 M 4-vinyl-lcyclohexene exhibited upfield shifts for three of the four unsaturated carbons. The carbon containing proton E did not shift. All the other carbons in the compound exhibited downfield shifts. The fact that the unsaturated carbons shift upfield suggests that some degree of silver complexation takes place at both olefin sites. Resonances arising from cis-trans isomers can also be differentially shifted and resolved by using these shift reagents. Due to the steric influence on silver-olefin bond strengths, the cis isomer of a cis-trans pair forms a stronger complex with silver (38). The spectrum of a mixture of cis-trans 2-hexene is shown in Figure 5a. No differentiation of the cis-trans isomers is observed in the spectrum, and the amount of each isomer cannot be obtained. In Figure 5b, the spectrum obtained starting with equimolar amounts of Yb(fod)3,Ag(fod), and 2-hexene is shown. The two isomers exhibit different shifts for many of the same protons. Decoupling experiments were done, and these verified that there are two equivalent sets of resonances that are not coupled to each other. The sets of resonances can be identified by spiking the mixture with one of the pure isomers. In the spectrum in Figure 5b, the trans form predominates, and, as expected if steric encumbrance makes the complex less stable, a smaller
F E D C B A CH3 C=C C H ~ C H ~ C H ~ C H ; ! C H J
-
7 6 5 4 3 2 I OPPm Flgurr 8. Proton NMR spectrum of 0.1 M P-octyne in CDCI, wlth (a) no shift reagent, (b) 0.2 M Ywfod),, 0.2 M Ag(fod), and (c) 0.2 M Yb(dfhd),, 0.2 M Ag(fod).
set of shifts is observed than for the cis isomer. In Figure 5c, the spectrum obtained with twice as much shift reagent as olefin (on a molar basis) is shown. In this spectrum, the cis-trans differentiation is much more pronounced. In fact, all the protons exhibit separate resonanm for the cis and trans forms, although something quite unexpected has occurred. The larger set of resonances, corresponding to the trans isomer, are now shifted farther than those of the cis isomer. Apparently, at this concentration ratio there is no significant competition between the two isomers for the silver, and the trans isomer exists in magnetic environment producing larger shifts. The same results were observed for a mixture of cis- and trans-2-octene. Once again, no separation of the resonancea for the cis and trans isomers was found in the unshifted spectrum. Addition of Yb(fod)3 and Ag(fod) at the same concentration as 2-octene led to a spectrum in which the cis form had shifted the farthest, presumably because it forms the more stable complex of the two. When twice the concentration of shift reagent to olefin was used,the trans isomer shifted farther. This concentration-dependent phenomenon of the relative shifta appears to constitute a general trend. For cis-trans 2-octene, not all of the protons were resolved, but enough were to allow one to quantitate the relative amounts of both isomers and also perform decoupling experiments to show that there are two independent seta of resonances. Alkynes are one other class of compounds with which these reagents should form complexes and alter NMR spectra. The spectrum of 2-octyne is shown in Figure 6a. Three sets of methylene protons overlap in this spectrum. The spectrum obtained by using Yb(fod), with Ag(fod) is shown in Figure 6b. The resonances are shifted, but the resonance due to the tert-butyl group on the fod ligands overlaps with the resonances of 2-octyne, making interpretation impossible. The use of Yb(dfhd)s with Ag(fod) eliminates this problem. The highly fluorinated dfhd ligands have no resonances that interfere over the region of the spectrum of interest. The resulting spectrum is shown in Figure 6c. The resonances for each set of protons become resolved, and there are no shift reagent resonances that interfere with spectral interpretation. Studies of Halogenated Compounds. Most alkyl halides are not expected to form stable complexes with lanthanide shift reagents. San Filippo et al. (39)have reported shifts in the spectra of n-cetyl fluoride using Yb(fod), or Eu(dfhd), but
398
ANALYTICAL CHEMISTRY, VOL. 53, NO. 3, MARCH 1981
Table 11. Comparative Shift Data for the Halobutanes A
B
C
D
CH,CH,CH,CH,X shift,a ppm X
HA
HB
a
0.08
0.12 0.35
Br I
HC 0.18
HD 0.26 0.68 1.57
0.21 0.35 0.47 0.64 1.00 a All samples were 0.1 M Yb(fod),, 0.1 M Ag(fod), and 0.2 M halobutane in CDCI,.
(d Flgure 8. Proton NMR spectrum of 0.1 M chbocyclohexane in CDCI, with (a) no shift reagent and (b) 0.3 M Dy(fod),, 0.3 M Ag(fod).
A
bs
(bl
1 0
-2
NMR spectra. The resonances are all upfield of MelSi and the protons on the CH2containing the bromide appear farthest upfield, while the methyl protons remain farthest downfield. A large peak from the shift reagent is apparent in this spectrum. The use of D ~ ( d f h d )a~ chelate , containing a more highly fluorinated ligand, with Ag(fod) avoids interferences from the chelate resonances, as shown in Figure 7c. The broadening caused by Dy(II1) eliminates the fine structure, although in a set of decoupling experiments, a sharpening of the coupled peaks was apparent on irradiation of a resonance. Large shifts were also obtained in the spectrum of 1-iodohexane. In the unshifted spectrum, the resonances for three sets of protons overlap. With 0.2 M D ~ ( f o d )0.2 ~ , M Ag(fod), and 0.1 M 1-iodohexane, a spectrum in which the resonances for each proton are widely separated was obtained. In the shifted spectrum, the protons in the CH2adjacent to the iodide are farthest upfield (-23 ppm) and the methyl group appears farthest downfield (-3 ppm). To demonstrate the effectiveness of these shift reagents on chlorine-containing compounds, we have studied chlorocyclohexane. The unshifted spectrum (Figure 8a) is quite complex and the resonance for only one proton can be identified. Since the chloro moiety forms only a weak bond to silver, higher shift reagent concentrations are needed to obtain useful shifts. The use of 0.3 M D y ( f ~ dand ) ~ 0.3 M Ag(fod) with 0.1 M chlorocyclohexaneresulted in the spectrum shown in Figure 8b. Certain of the axial and equatorial protons on the same carbon exhibit different resonances. In chlorocyclohexane, steric factors would favor a conformation in which the chloro group occupies an equatorial position. The axial and equatorial protons cy to the chloro group are not distinguished, and a single resonance with relative area 4 results. Two resonances, each with relative area 2, result for the four /3 protons. The two axial protons (C) are closer to the chloro group and are assigned to the resonance that shifts the farthest, leaving the two equatorial protons (D)assigned to the other resonance. The axial and equatorial protons at the y position also exhibit separate resonances. Once again, the axial proton (E) is closer to the chlorine and is assigned to the resonance of relative area 1 that shifts the farthest. Unfortunately, unlike the other assignments, these could not be confirmed by spin-decoupling due to the absence of fine structure in the spectrum. Chloro-, bromo-, and iodobenzene were also studied with these shift reagents. Only small shifts were observed for chloro- and bromobenzene using Yb(fod)a with Ag(fod). For iodobenzene, the shifts were larger, and, based on the relative shifts for the protons, it appears that the silver bonds preferentially at the iodine rather than at the aromatic -C=Cacross the ring from the iodo group. In a previous study of the equilibrium constants for the complex formation of halobenzenes with silver nitrate, much higher values were observed for the iodo derivatives than for the chloro or bromo
,?J;L
A B C D E CH3CH2CH2 CH2 CH2&
fod
-4
-6
-8
-Ib
-12
-14
-16
-6pprn
Figure 7. Proton NMR spectrum of 0.1 M 1-bromopentane in CDCI, with (a)no shift reagent, (b) 0.2 M Dy(fod),, 0.2 M Ag(fod), and (c) 0.2 M Dy(dfhd),, 0.2 M Ag(fOd).
did not see any shifts in the chloride, bromide, or iodide derivatives. In our studies we have learned that halogenated compounds such as 1-chlorobutane form weak complexes with binuclear shift reagents containing silver(1) and lanthanide(III) ions and shifts are observed in the NMR spectra. The shifts obtained for 1-chloro-, 1-bromo-, and 1-iodobutane with 0.1 M Y b ( f ~ d )0.1 ~ , M Ag(fod), and 0.2 M of the halogen are shown in Table 11. The iodide derivative shifts the farthest since silver(1) is a soft Lewis acid and bonds more strongly to softer bases. In general, the shifts observed for halogenated compounds using these binuclear complexes are too small to be of practical utility, so one must employ dysprosium(III), a lanthanide ion noted for its large shifting ability, to obtain useful spectral alterations. Unfortunately, dysprosium(II1) chelates also broaden the resonances more than the other trivalent lanthanide shift reagents. The increased distances to the dysprosium in the binuclear complexes vs. those in conventional mononuclear shift reagents keep the broadening within tolerable limits. In the halogenated compounds we have studied, the resonances were easily identified in the shifted spectra, although the fine structure that results from coupling was not apparent in the broadened peaks. The NMR spectrum of 1-bromopentane, shown in Figure 7a, has two sets of protons, B and C, the resonances of which overlap. The spectrum obtained for 1-bromopentane using D y ( f ~ dwith ) ~ Ag(fod) is shown in Figure 7b. The shifts are quite large and the resonances are well-resolved. One has to keep in mind that since the shifts are upfield, the spectrum is now the reverse of what is normally observed in the proton
ANALYTICAL CHEMISTRY, VOL. 53,
derivatives (40). This was explained by assuming that coordination of silver is to the iodine rather than the ring. In hopes of obtaining useful shifts for the chloro and bromo derivatives, we examined Tm(fodI3with Ag(fod). The three types of aromatic protons on chlorobenzene were resolved, and, from the relative shifts, it appears that silver bonds to the aromatic ring rather than the chlorine. The same result was observed for bromobenzene. These shift reagents are suitable for studying halogenated aromatic compounds; however, for chloro and bromo derivatives the complexes formed with silver are much weaker. For this reason, to obtain useful shifts in the NMR spectrum, one needs powerful lanthanide shift reagents such as those of thulium(II1). One serious drawback we have encountered that will limit the utility of these shift reagents with halogenated compounds is that certain halogen-containing compounds react with the silver P-diketonate. Cyclohexyl bromide, 2-bromooctane, 2-iodobutane, and 2-chloro-2-methylpropaneare examples of compounds that react with the silver. A precipitate, presumably the silver halide, forms in these cases. It appears that these shift reagents are suitable for studying all the primary halides, as well as secondary chlorides. Secondary and tertiary bromides and iodides and tertiary chlorides react with the shift reagent and are not suitable for study. Also, because small shifts are induced in the resonance of chloroform by the presence of silver-containing shift reagents, the use of CHC13 as an internal reference is not recommended.
ACKNOWLEDGMENT The encouragement of William Harris of the National Science Foundation is gratefully acknowledged. We are greatly indebted to Ted B e t h and Jean Sadlowski for their technical assistance. The assistance of Martin Ashley in obtaining 13C NMR spectra is deeply appreciated. LITERATURE CITED Hinckiey, C. C. J. Am. Chem. SOC. 1060, 97. 5100. Sanders, J. K. M.; Williams, D. H. J. Chem. Soc., Chem. Commun. 1070, 422. WhResMes, G. M.; Lewis, D. W. J. Am. Chem. Soc. 1070, 92, 0979. Goering, H. L.; Elkenberry, J. N.; Koermer, 0.S. J. Am. Chem. Soc. 1071. 93. 5913. Fraser, R. R.; Petit, M. A.; Saunders, J. K. J. Chem. Soc., Chem. Commun. 1071, 1450.
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RECEIVED for review August 14,1980. Accepted November 17, 1980. The support of the National Science Foundation through Grant CHE 79-13022 is greatly appreciated.
