Studies of hydrogen bonding—XXVI. Proton NMR studies of hydrogen bonding between thiophenol and various phosphoryl compounds. The Hiquchi plot. Solvent effect

Spectroe.h!m!o~ Acta, Vol. 30A, pp. 2121 to 2131. Pergamon Press 1974. Printed in l~orthern Ireland

Studies of hydrogen bonding--YYVL * Proton NMR studies of hydrogen bonding between thiophenol and various phosphoryl compounds. The Hiquchi plot. Solvent effect THOR GR~MST~U and TORBJ~Rlf OLSEN Chemical Institute, University of Bergen, N-5000 Bergen, Norway (Received 27 F e b ~ !

1973)

Abstract--The Kass, AHand AS values for the association of thiophenol with six phosphoryl compounds have been determined by using the Hiquchi plot. Log Ka~ and AH form linear relationships with the change in NMR chemical shift, A~=,and with i.r. frequency shift, A~SH. Also a linear relationship exists between A~e and A~SH. On replacing carbon tetrachloride with cyclohexane as solvent, the Kus and --AH values are increased considerably, whereas the ~= and A~z are strongly lowered. Furthermore, a relationship was found to exist between corresponding energy changes of S--H, C---H and O--H proton donors when hydrogen bonding is established. I ~ PREVIOUS publications [2, 3] it was shown by studying the association of chloroform with phosphoryl compounds (0PC), N.N-disubstituted amides and sulphoxides, t h a t the Hiquchi plot [4] was an excellent method of studying weak hydrogenbonded complexes. Consequently, we found it logical to extend our study to the system thiophenol/OPC, since it is well known t h a t thiophenol forms weak hydrogen bonds [5-9]. However, the main purposes for the present work were (a) to study the temperature dependence of a S - - H proton in a hydrogen bond and thus making a comparison between the systems C6HsSH/0PC , CHCla/OPC and CeHsOH/0PC, and (b) to investigate the effect of solvent interaction on C6HsSH/0PC as compared with CHC13/OPC. EXPERIMENTAL Thiophenol and the liquid phosphoryl compounds w e r e distilled at reduced pressure in an atmosphere of nitrogen. Triphenylphosphine oxide was recrystallized from benzene/petroleum ether. Carbon tetrachloride and eyclohexane were purified chromatographically by using Molecular Sieve Type 4A and basic aluminum oxide. The chemicals were purified just before use and the solutions were made up by weighing. The molarity of each solution was calculated at various temperatures from the change in density with temperature of the solvent. All compounds were * For Part XXV see Ref. [1]. [1] [2] [3] [4] [5] [6] [7] [8] [9]

J. EK~I~-J~and T. G~AlVr~TAD,Spe~'ochi~n. A c ~ ~ 2465 (1972). T. GBAM~TADand 0. Mu~-Dn~.~, Spe~trochim. Act~ 28A, 1405 (1972). T. GRAMST~Uand O. VnrA~r~,S~trochi~n. Acta ~SA, 2131 (1972). M. NAXA_~O,N. N. NAgA~O and T. I-IzGuC~,J. Phys. Ghem. 71, 395 (1967). M. J. CoP~r, C. S. ~A~VF~ and E. Gn~SB~.~G,J. Am. Chem. Soc. 61, 3161 (1939). M. L. Josr~.N, P. DIY~ABOand P. SAu~G~r~, Bull. Soc. Ch/m. France 423 (1957). R. A. SPv-xtRand H. F. B x ' ~ s , J. Phys. Chem. ~ , 425 (1958). R. MATure, E. D. B~.c~.~, R. B. BRADLEYand N. C. LE, J. Phys. Checn. 67, 2190 (1963). R. MAT~,v~, S. H. W~,z~G and N. C. L~., J. Phys. Chem. 68, 2140 (1964). 2121

T. GRAMSTADand T. OLSEN

2122

manipulated in a drying box under dry nitrogen atmosphere. Tables 1 and 2 show the concentration range and the temperature used. All N M R spectra were recorded on a JNM-C-60H spectrometer with temperature regulating accessories. The temperature accuracy was estimated to be ± I°C. Chemical shifts of the S - - H proton in thiophenol were measured with respect to tetramethylsilane (TMS) b y internal locking. B y sweeping with a 100 Hz sweep width three times in each direction, the reported shifts are accurate to at least 0 . 0 0 3 ppm. RESULTS AND DISCUSSION Self-association of thiophenol

Recently, lVlXRCUS and ~JTJER [10] have shown, b y using a modified curve fitting method of SAU~DE~S and HYdE [11), that the dilution shift data for thiophenol in carbon tetrachloride indicate an apparent monomer-tetramer association (K 4 = 10-4~-8). They believe, however, that the existence of tetramer is illusory and that the anomaly is caused b y medium anisotropy effect because dilution data of thiophenol in chlorobenzene, where the concentration of aromatic is nearly constant, fit a monomer-dimer model. We found it, however, necessary to consider the behaviour of thiophenol at low concentration and at various temperatures. The result of the dilution study in carbon tetrachioride and in cyclohexane is presented in Table 1. As can be seen, the resonance of the thiophenol S ~ H in CC14 is, within the concentration range study, nearly concentration independent, whereas in cyclohexane the resonance peak is shifted a little downfield. This might mean that thiophenol lust as phenol [12, 13] and chloroform [2] has a greater tendency to self-association in C6H12 than in CC14. The chemical shift, 8f, found b y extrapolation to zero concentration of thiophenol, differs also in the two solvents. J u s t as for chloroform [2] this m a y be attributed to stronger interaction via hydrogen bonding and other less specific solvent effects such as van der Waals forces, rather than to Table 1. Temperature and solvent effect on the S---H proton chemical shift of thiophenol. I)ownfield from TMS Solvent CC1a

C6H1~

Temp. (°C)

Cons.range (~.10 s)

Number sol.

~obs. range (ppm)

~1" (ppm)

20 35 50

2.59--79"62 2.54-78.36 2.50-76"92

7 7 7

3.248-3.248 3.240-2.242 3.232-3.233

3.248 3.240 3.232

20 35 50

3-58-86'21 3.51-84.63 3.45-83.09

7 7 7

3-149-3.160 3.147-3.158 3.143-3.153

3.148 3.147 3.143

~t* found by extrapolation to zero concentration of thiophenol. [10] [11] [12] [13]

S. H. MA~C~Sand S. I. MULLER,J. Am. Chem. Soc. 88, 3719 (1966). M. SA~rDZRS and J. B. HYNE, J. Chem. Phys. 29, 253 (1958); 31, 270 (1959). T. GR~MSTXDand E. D. BECEER,J. _Mol.Stunt. 5, 253 (1970). A. J. DALE and T. G~ST~V, Spectrochim. Acta BSA, 639 (1972).

Studies of hydrogen bonding--XXVI

2123

greater self-association tendency of thiophenol in CC14. Furthermore, 81, was found to be more temperature dependent in CC14 than in CeHz2. Thiophenol and chloroform were found to have the same temperature coefficient (0.0005 ppm/°C) in CC14, but differ a little in CeHI~.

Complexing of thiophenol with phosphoryl compounds Since the association constants, Kass, for the association of thiophenol with phosphoryl compounds are rather small, the Hiquchi plot (1) has been used to evaluate, Kas~, and the chemical shifts, ~=, of the S - - H proton in the complex Ca

1

1

~obs, the chemical shift observed relatively to TMS; ~t, the chemical shift of the non-bonded donor; Ca, Ca and C~ are respectively the total concentration of proton donor and accepter, and of the proton donor in complexed form (for more details see Part X X I I I , Ref. [2]). The results are tabulated in Table 2. In Fig. 1 the observed chemical shift, 8ob~, of thiophenol S---H is plotted, at various temperatures, as a function of concentration of the accepter [(CH3)2N]3PO in CC14 and Cell12. The plots are characterized by a more rapid change in 8obs at low base concentration in C8H19 than in CC14, thus indicating greater tendency of C6HsSH to associate with [(CH3)2N]sPO in CeHz2 {;¢~o" 2°° 0"58M--z; --AH, 3.0 kcal/mole). , - - a s s , 3.05; --AH, 3.9) than in CCI4 (K~, Furthermore, l~ig. 2 shows, for the same system in CC14, the final convergent line (4 iterations) of the CJ~ob 8 -- ~1 vs (Ca + Ca -- C®) plot. From these lines, the final K a s s and 8® values were calculated from the limiting slope and the intercept . , ~ ~ ~

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Fig. 1. Chemical shift of thiophenol S - - H proton for the system CsHsSH] [(CH3)=NlaPO in CC14 and Cell12.

2124

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2125

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Fig. 2. F i n a l plots of O=/~ob. -- ~! vs (O= + O~ -- (7=) for the system OeHsSH /

[(CHa)~JaPO in CCI~at 20, 30 and 50°C. value. The slope value was calculated throughout by the method of least squares by using a computer IBM 360/50. Previously we have shown [2], for the association of chloroform with phosphoryl compounds that --AH and log K w form linear relationships with the change in NMR chemical shift, A~®,and with i.r. frequency shift, A~cv, measured with CDC1s. Also a linear relationship was found to exist between A~= and A~cv. Consistent with these findings we have now found, as shown in Fig. 3, that the Badger-Bauer relationship also is valid for the system CeHsSH/OPC in CC14 and C6HI=. The equations correlating --AH (kcal/mole) and log K~°'(M-z) with A~SH are given by:

--AH(± 0.3) = 0.010A~s. + 1.5 in CCl, - - A H ( ± 0.3) = 0"017A~SD + 1.5 i n Cell1= log K~ °" = 0"0029A~SH -- 0"60 in CC1, log ~ = 0"0047A~s~ -- 0"10 in C,HI= As shown in Fig. 4, there exists also a linear relationship between A~= and --AH and log Kass. The equations are: --AH(±0.3) = 0.41A~® + --AH(±0.3) ~- 0.71A~® ~log ~00" _ 0.14A~® -log K ~°" = 0.19A~ --

1.3 in CC14 1.5 in C6H12 0.73 in CC1, 0.16 in C6HI=

Furthermore, there exists, as shown in Fig. 5, a fairly good linear correlation between A ~ and A~SH in both solvents.

2126

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Studies of hydrogen bonding--XXVI

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Frequencyshift, ~sH, cm-~ Fig. 5. Change in S--H chemical shift, A~z, of thiophenol in hydrogen bonding to various phosphoryl compounds vs i.r. frequency shift, AVsH, on hydrogen bonding. As reported earlier [3], a relationship exists between corresponding energy changes between C - - H and O - - H proton donors, i.e. the log E , , , values for the association of chloroform with various proton acceptors (P~---O, C~---O, S---~O compounds) forms a linear relation with the log Kas Bvalues for the association of phenol with the corresponding proton accoptors. We have now found a siml]ar relationship between S - - H and C - - H (shown in Fig. 6) and between S - - H and O - - H proton donors. The equations correlating the log K u , values in CC14 for phenol and chloroform [2, 3] at 20°C with those of thiophenol with the same acceptors at 20, 35 and 50°C are given by: log I~°~(CeHsSH) ----0.41 log K~(CHC13) -- 0.42 = 0.16 log K~(CeHsOH ) -- 0.77

(2) log K~'(CeHsSH ) = 0.35 log K~(CHC13) -- 0.49 = 0.14 log K~°_~(CeHsOH) - 0.78 (3) log K~'(CeHsSH) = 0.30 log J~°~(CHCI3) -- 0.56 = 0.12 log I~°~(CeH~OH) -- 0.80

(4) I n accordance with the result of TAFT et al. [14] and previous findings [2, 3] we have found t h a t both slope and intercept are decreasing with increasing temperature. [14] N. MUT.T,maand R. C. R z ~ R , J. G~r~. Phys. 4~, 3265 (1985).

2128

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Fig. 6. Correlation of log -Rz0o values for the association of thiophenol with -au various phosphoryl compounds with l o g / ~ values for the association of chloroform with the corresponding proton aceeptors, solvent, CC14 and C6H1~. (1) (CzH50)2P(O)CCla; (2) (C~H50)sPO; (3) (C=H50)aP(O)CHs; (4) (CeHs)sPO: (5) [(CHa)~T]sPO.

Furthermore, the slope and intercept values taken from equations (2), (3) and (4) are, as shown in Fig. 7, directly proportional to lIT. A similar relationship has been shown by T ~ et al. [14] to exist between O--H proton donors. On replacing CC14with C6Hls as solvent, the following equations were obtained between the log K a , values for chloroform at 20° with those of thiophenol with the same accepters at 20, 35 and 50°C.' log K~°*(CeHsSH) = 0-59 log K2~°*(CHCIa) -- 0.22 (5) log K~(CaH5SH ) -- 0.49 log ~ ( C H C l a ) -- 0.26

(6)

log K~(C6HsSH) = 0-39 log K~(CHC13) - 0.30

(7)

Also in Cell12 as solvent, the slope and intercept values are decreasing with increasing temperature, and they are found to be directly proportional to lIT. Furthermore, replacement of CC14 with CeHz2 causes for the systems CHCIa]OPC [2] and CeHsSH/OPC a considerable increase in Kass, --AT/, and in the slope and intercept values [compare equations (2)-(7)].

Solvent effect As shown earlier [2, 12, 13], the thermodynamic properties of hydrogen bonds are greatly effected by solvent interaction. It was shown [2] by replacing carbon tetraehloride with cyclohexane that there was for the system CHCla[0PC an average increase in --AH of 1-5 kcal/mole whereas the Ka,5 values are increased by a factor of 4.4. It was suggested that the major cause for the fall in the association constant and - - ~ / w a s due to interaction of CHC18 with CC14. Similar comparisons have

2129

Studies of hydrogen bonding~XXVI -0.4 --0"5

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Fig. 7. The 1/T dependence of slope and intercept values taken from the equations (2), (3) and (4) in text. Solvent CC14. been made for the system C6HsSH/0PC in CC14and Cell12. By considering (Table 2) 20 ° 20 ° Km(CeH~2)/Km(CCI4) and AH(CeHzs ) -~ A?/(CCli) we see that the average ratio and difference is 4.7 and 0.6 keal/mole. With reference to earlier study [2] on the system CHCIs/OPC in CeHls and CCI4, we conclude that the major cause for the decrease in K,B, and --AH by replacing CeHls with CC14is due to greater interaction of CeH6SH with CC14 than with CeHls. This view is also supported by the dilution study of thiophenol in the two solvents. Hence the average difference value of 0.6 keal/mole should correspond to the energy of the H-bond in the complex CsHsSH • • • C1--CC13. The H-bond energy in C1sCH • • • C1--CC18 was earlier [2] found to be 1.5 kcal/mole. The tendency of CHC1S is greater than of CeHsSH to associate with phosphoryl compounds in both solvents. For the association of (CsH50)sP(O)CCIs with CHC1s is 2 in both solvents; for and CeHsSH the ratio, K~(CHC13)/Ka~(C6H~SH) 2°° 20" (C2H60)sPO the ratio is 3 (in CC14) and 4 (in Cell12); for [(CHs)sN]aPO it is 5 (in CC1,) and 4.4 (in CeHI~). The ratio is increasing with increasing base strength. With regard to the H-bond energy, however, we have found that the systems CHC13/OPC and CeHsSH/OPC have, with the same phosphoryl compounds, nearly equal --AH in CCI~, e.g. CHCI~/(CsHsO)3PO (--AH, 2.0) should be compared with CeHsSH/(C~H~O)aPO (--AH, 2.2kcal/mole). In CeHz~, however, CHC1a forms stronger H-bonds than does C6HsSH, e.g. CHCla/(C2H50)aPO (--AH, 3-6) compared with CeHsSH/(CsHsO)3PO (--AH, 2.7). This might be explained by stronger complexing of CHCI8 (1.5) than of C6HsSH (0.6 kcal/mole) with CCI4. In CCla there is equilibria between monomer and the hydrogen bonded complexes, ClsCH • • • O ~ P ( (solvated) and C13CH • • • C1--CCla (solvated). Hence the measured H-bond energy corresponds to the difference in energy between CHCla/OPC and CHC13/CC14. Similarly, the measured H-bond energy of the system C6HsSH/OPC corresponds to the dltTerence between CeHsSH]OPC (solvated) and C~H~SH/CCI~ (solvated). CoincidentaUy, the --AH values turn up, in this case, to be equal. In CeHzs there is

2130

T. G ~ t ~ r ~ u and T. OLSm~

an equilibrium between monomer and CHC13/OPC (or CeHsSH/OPC ) with little solvent interaction. In this solvent CHCI3 [2] is a much stronger proton donor than CeHsSH, e.g. CHCls/(C2HsO)3PO (K~, 5.60; --AH, 3.6) should be compared with CeHsSH/(C2H50)sPO (Kay, 1.42 ~-1, - - A H , 2-7 kcal/mole). Previously, we have shown [2] that the chemical shift, ~ , of a proton in a complex, C18C--H... O--P~, was relatively independent of the solvent used (CC14 and C6H1~) as compared with the great difference in K~B~ and --AH. The 5~ was shifted only a little more downfield in CsH12 than in CCI4. Similar comparison of the system C~HsSH/OPC (see Table 2) revealed that 8~ is strongly solvent dependent. In the present case, all 8~ values are shifted downfield by replacing C6H1~ with CCl~. From this the conclusion might be drawn that the complex is solvated in such a way that the CC14 (or of a mixture of CC14 and OPC) causes a deshielding effect as compared with Cell12 (or of a mixture of C6H12 and OPC). This is, however, just the opposite effect of what has been found for the system C6HsOH/OPC [12] and by dilution study of phenol [12, 13], i.e. in these systems the ~ values are shifted upfield by replacing C6H1~ with CC14. I t has also been shown [2] that there is for the system CHC18/OPC in CC14 and C6H12, no significant temperature dependence of 8~. From this the conclusion was drawn that the tendency of the solvent to interact with the hydrogen-bonded complex is reduced, due to the weak complexes formed between chloroform and organophosphorus compounds. However, for the systems CsHsOH/OPC [12] and for self-association of phenol [13], ~ is clearly temperature dependent. With few exceptions the ~x values for the system C6HsSH/OPC are lowered on increasing temperature. With reference to the discussion above, we should expect greater temperature dependence of ~x in CC14 than in C6H12. As can be seen in Table 2, however, the reverse is rather the case, e.g. for the system CeHsSH/(C2H60)aPO. ~20" _ ~50o _ 0.0025 in CCI4 and 0.167 ppm in C8H1~. The most likely explanation for this behaviour is that the solvation around the hydrogen-bonded complex is temperature independent due to large excess of CC14 molecules. Another explanation might be that the 8~ values have been corrected insofar as possible for solvent effects during the Higuehi procedure. On the other hand the small change observed in 8~ with temperature may be attributed to excitation of the hydrogen bond stretching vibration mode, i.e. from changes in the effective length of the S ~ H • • • O bond. Our results are with some exceptions within the range 0.0020.008 ppm/°C calculated by M~T.T.~.Rand R~.rr~R [14] for the temperature dependence of the chemical shift, 8~, for a proton in a general O H . • • O bond. Furthermore, there is no obvious correlation between the strength of the hydrogen bond and the temperature dependence of the chemical shift. I t is also of interest to compare the A ~ values of CeH~SH/OPC with those of CHCI~/OPC. The change in chemical shift, A~, are very much greater for the systems CeH~SH/OPC than for CHCls/OPC , i.e. it would be impossible by taking ~B~ or --AH values from different systems to maintain the linear relationship A ~ vs log K ~ or A ~ vs --AH (Fig. 4). Furthermore, the A ~ values for C~H~SH/ OPC are greater in CCI~ than in CeHI~. This is just the opposite of what has been found for the system CHC1s/OPC e.g. CeH~SH/(C~H~O)~PO K ~ , 0.41 (CCl~); 1.42 (CeHI~); --AH, 2.2 (CCI~); 2-7 (CsHI~); A8~, 2.103 (CCI~), 1-837 (C~H~) should be

Studies of hydrogen bonding--XXVI

2131

compared with [2] CHCls/(CsHsO/sPO , K2~, 1.26 (CCI~), 5.60 (C6Hlg); - - ~ / , 2"0 (CCI4), 3.6 (Cell12); A~®,1.155 (CC14), 1.330 (CeHlz). Also should be mentioned that the difference in A ~ for the systems C6H6SH/0PC and CHCla/0PC are greater in CC14than in C6H1~, and that the difference is increasing with increasing base strength, e.g. for the systems CsH6SH/(CzH50)aPO and CHCls/(C~H50)sP0 the difference in A~2°° are 0.948 (CC14) and 0.507 (Cell12), and for the systems CeHsSH/[(CHs)aN]3PO and CHCla/[(CHs)2N]sPO the difference are 2.412 (CCI4) and 1.415 ppm (CeHlz). These figures demonstrate very clearly the different behaviour of a C--H and a S - - H proton donor in the two solvents.

Acknowl~d41eme~ta--Oneof the authors (T. G.) gratefully acknowledges financial support from Norges Alrnenviten~b~peligeForA]rningsr/~d.