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The far infra-red spectra of 2,2′-dipyridyl and 1,10-phenan-throline complexes of alkyl tin halides
SpectrochimicaActa, 1965, Vol. 21, pp. 1861 to 1868. PergamonPress Ltd. Printed in Northern Ireland
The far i&a-red spectra of 2,2’-dipyridyl and l,lO-phenanthroline complexes of alkyl tin halides R. J. H. CLARK and C. S. WIL~MS William
Ramsay
and Ralph Forster Laboratories, Gower Street, London
University
College,
(Received 11 January 1965) Ah&act-The infrared spectra (650-200 cm-l) of eighteen six-co-ordinate complexes of the alkyl tin halides with 2,2’-dipyridyl and l,lO-phenanthroline have been recorded as nujol mulls, and where possible also in solution. As a result of raising the co-ordination number of the tin from four in the parent alkyl tin halides to six in the complexes, the tin-chlorine stretching frequencies are lowered by some 80-100 cm-r. While the tin-carbon stretching frequencies (024-470 cm-l) are relatively insensitive to the number of alkyl groups in the complexes, the tin-chlorine stretching frequencies (299-215 cm-l) are lowered by successive replacement of chlorine for alkyl groups. This result is similar to that found for the parent alkyl tin halides. The tin-chlorine stretching frequencies are sharp for the parent alkyl tin halides (the infra-red spectrum of trimethyl tin chloride shows an isotopic splitting of 5.6 cm-l), but those for the complexes are broad and insufficiently well resolved to assist in distinguishing between the various possible stereochemical isomers. A new adduct of trimethyltin chloride with dipyridyl is reported. of the increasing interest in alkyl tin halides and their derivatives, we have undertaken a study of the infra-red spectra (650-200 cm-i) of the series of addition compounds formed between alkyl tin halides and the bidentate ligands 2,2’-dipyridyl and l,lO-phenanthroline. The compounds were reported recently by ALLESTON and DAVIES [l]. Metal-halogen (MX) stretching frequencies normally give rise to relatively intense absorption bands in this region of the spectrum, and they are known to provide information on the stereochemistry [2], oxidation state [3] and coordination number of the metal [2, 41. With these compounds it was of interest to discover the effect on the tin-chlorine stretching frequencies of raising the coordination number of the tin from four for the parent alkyl tin halides in non-polar solutions, to six for the complexes. It was also of interest to investigate whether progressive replacement of chloride for alkyl groups led to a monotonic decrease in the remaining MX stretching frequencies, such as has been observed in the parent alkyl tin halides [5], and furthermore whether the number of infra-red-active MX stretching frequencies could provide information on which of the possible stereochemical In the course of the investigation a isomers were formed during the syntheses. new adduct of trimethyl tin chloride with dipyridyl was synthesised.
IN VIEW
[l] D. L. ALLESTON and A. G. DAVIES, J. Chem. Sot. 2050 (1962). [2] R. J. H. CLARK and C. S. WILLIAMS, Chem. I&. (London), 1317 (1964); Inorg. Chem. 4, 350 (1965). [3] R. J. H. CLARK and T. M. DUNN, J. Ghem. Sot. 1198 (1963). [4] R. J. H. CLARK, Symposium in Inorganic Chemistry, Bratislava, Czechoslovakia (1964); Spectrochim. Acta 21, 955 (1965). [5] I. R. BEATTIE and G. P. MCQUILLAN, J. Chem. Sot. 1519 (1963). I
1861
1862
R. J. H. CLARKand C. S. WILLIAMS EXPERIMENTAL
Infra-red spectra The infra-red spectra of the solid compounds were recorded as nujol mulls with a Grubb-Parsons double-beam grating spectrometer, type D.M.2, over the range 200-455 cm-l. The calibration was carried out by reference to the rotational spectrum of water vapour in this region, the spectrometer being used as a single beam instrument [6]. Spectra above 400 cm-l were recorded on a Grubb-Parsons doublebeam grating spectrometer, type GS2A and on a Perkin-Elmer 337 double-beam grating spectrometer, calibrated by reference to the spectra of polystyrene and indene. The frequencies should be accurate to f2 cm-l for sharp bands. The mulls were supported between polythene or potassium bromide plates. Spectra of the most soluble of the adducts were obtained in several solvents using variable path-length polythene containers. Only the butyl and octyl derivatives were sufficiently soluble to be investigated in this way, benzene and dichloromethane being convenient solvents. The general appearance of the solution spectra is closely similar to that of the mull spectra, with the tin-chlorine stretching modes as broad in solution as in the solid state. These modes, however, suffer a solvent shift of up to 18 cm-l in solution compared to their solid state values (e.g. dioctyltin dichloride dipyridyl and dioctyltin dichloride phenanthroline in benzene solutions). The tin-carbon and ligand vibrations are relatively independent of solvent effects. The infra-red spectra of certain of the parent alkyl tin halides were also recorded in solution in the same way. Syntheses and characterisation The adducts were prepared by the methods described previously [l]. Molecular weights were determined with a Mechrolab vapour pressure osmometer using benzil as a standard. Dioctyl tin dichloride phenanthroline is a monomer in dichloromethane (calculated for monomer, 594; found 600, 594, 628 in the concentration range O*Ol-0*05M) [7]. On the bases of the similarities in chemical and physical properties (melting points, tin-chlorine stretching frequencies etc.) between this adduct and all others, it is believed that all the adducts are six-co-ordinate monomers in the solid state, and in solution in non-polar or weakly polar solvents. In more polar solvents such as acetone, however, the adducts give low molecular weights suggesting that some dissociation takes place. In an attempt to form dipyridyl and phenanthroline adducts of trimethyl tin chloride, the components were added to each other in ether solution. On concentration, the phenanthroline solution gave an ill-defined product, but colourless crystals formed slowly from the dipyridyl solution; m.p. 69-70% (uncorrected). Calculated for a 1: 1 adduct C,,HI,N,CISn; C, 43.89; H, 4.78; N, 7.88. Found, C. 43.3; H, 5.1; N, 8.1. In view of the fact that the melting point of the adduct is the same as that of dipyridyl itself, the melting point of a 50 : 50 mixture of the adduct with dipyridyl was determined. There was a 9” depression of the melting point, indicating that the adduct is not a mixed crystal. It breaks down to the starting products in all solvents [6] L. R. BLAINE,E. K. PLYLER~~~W. S. BENEDICT, J. Res. Nat. Bur.Stand., 6&i, 223 (1962). [7] We are indebted to Mr. P. R. PALANfor this measurement.
1803
Infra-red spectra of 2,2’-dipyridyl and l,lO-phenanthroline complexes
tried, including benzene, cyclohexane, chloroform and acetonitrile, as evidenced by solution infra-red spectra. However a nujol mull spectrum of the compound indicated complex formation, as the SnCl stretching mode is depressed by at least 100 cm-l from its position in trimethyl tin chloride, and the ring vibration of dipyridyl at 402 cm-l is raised to 415 cm-l, a rise typical of co-ordineted dipyridyl. RESULTS AND DISCUSSION
The tin-carbon and tin-chlorine stretching frequencies for the relevant alkyl tin halides are in Table 1 and for the adducts in Table 2. Representative scans for the mono- and di-alkyl tin derivatives are depicted in Fig. 1. For the latter there Table 1. Infra-red-active tin-chloride and tin-carbon stretchingfrequenciesin alkyl tin chlorides* Compound
v(Sn C)
407 * 382 vs., 377 “8, 378 vs., 361 s, 359, 356, 354, 331 “8 337 8
SnCl,
MeSnCl, EtSnCl, n-BuSnCi, Me,SnCl, Et,SnCl, a--Bu@nCl, Oc&lCl, Me,SnCl Et&Cl a. b.
o. d. e. f. g.
560 531 602 606 542 518
Ref.
v(Sn X)
551 w-m 522 Wb 608 VW 524 497 517 518 s 613 w v@ 489 89
368 366 367 356 352 340 345
d c c!
sh ah sh sh sh sh
: d d d I3 0, f
Obtained for oyclohexene solutions unless otherwise stated. Obtained for benzene solution. This investigation. F. K. BUTCHER, W. GERRARD, E. F. MOONEY, R. G. REES, H. A. WILLIS, A. A. ANDERSONand H. A GEBBIE, J. OrganometaZ.Chem. 1, 431 (1964). Ref. [b]. P. TAIMSALUand J. L. WOOD, Spectrochim. Acta 20, 1043 (1964). Obtained for pure liquid.
Table 2. Tin-carbon and tin-halogen stretching frequencies in the 2,2’-Dipyridyl and 1,lO phenanthrolinecomplexes of alkyl tin trihdides and dialkyl tin dihalidesa Compound M&G& dipy MeSnCl, phen EtSnCl, dipy EtSnCl, phen BuSnCl, dipy BuSnCls phen Me,SnCl, dipy Me&Xl, phen Et,SnCl, dipy Et$nCl, phen Bu,SnCl, dipy Bu,SnC!lz phen Bu,SnBra dipy Bu,SnBr, phen Bu,SnI, dipy Bu,SnI, phen Oc,SnCl, dipy Oc.SnCl. Dhen
a@n C) 536 w 529 w 504 w 507 w 592 w 608 w 572 m 578m 529 m 481 525 m 470 b -592 624 vw 594 ~620 sh 587 588 ~615 VW 613 w 584 610 w, br 582 595 b 622 w 591
%J(Sn x1
w w “\v VW w, br w w w, br w, br VW
289 296 292 299 294 285 244 247 234 242 243 242 <200 < 200 (200 < 200 235 235
vs, 267 m 8, asym, 272 vs 8, 283 s, 273 8 s, 270 vs 8, 281 8, 264 m YB 8, br s, 239 sh 8, 215 w 8, 220 8 8, br s, br
B 8, br
a obtained as nujol mulls; ) obscured by ligand absorption Me = methyl, Et = ethyl, Bu = n-butyl, Oc = n-octyl; VW = very weak, w = week, br = broad, m = medium, B = strong, sh = shoulder, asym = asymmetric
R. J. H. CLARK and C. S. WILLIAMS
1864
are three different stereoisomers possible ; with the methyl groups in cis positions (C,, molecular symmetry), with the halogen groups in cis positions (also C,, symmetry), or with both the methyl and the halogen groups in cis positions (no symmetry). 200
I 50
250
300
400
cm-’
I
42
P
34
26
Fig. 1. Far infra-red spectra of A, methyl tin trichloride dipyridyl and B, dimethyl tin dichloride dipyridyl as nujol mulls.
Tin-Carbon
stretching frequencies
The tin-carbon stretching frequencies in all the adducts are located in the cm-l region, and they vary in intensity from weak to medium for the methyl derivatives to very weak for the butyl derivatives. However, for a given Hence the tin-carbon stretching alkyl group, the range is very much narrower. frequencies seem to be relatively insensitive to the coordination number of the tin and to the degree of substitution of alkyl for halide groups. For the mono-alkyl derivatives, only one tin-carbon stretching frequency is expected. This is observed for all such compounds, except for the parent mono-ethyl and mono-butyl tin trichlorides, for which an additional very weak band is observed for benzene solutions at 484 and 527 cm-i respectively. In the latter case it has been suggested [8] that the extra band implies the presence of both trans- and gauche rotational isomers. For the dimethyl tin dichloride adducts, again only one band is observed. Thus a trans arrangement of the methyl groups is implied [5], for although both tin-carbon stretching frequencies are permitted in the infrared the totally symmetric 624-470
[S] R. A. CUMMINS, Au&al.
J. Chem. 16, 985 (1963).
Infrared
spectra of 2,2’-dipyridyl
and I,lO-phenanthroline
complexes
1865
vibration leads to such a small chsnge in dipole moment that the band is unlikely to be seen. However, the other dielkyl tin dihalide derivatives hrtve two tin-carbon stretching frequencies which are of such low intensity that no comment on the stereochemistry of the compounds appears to be justified. Tin-chlorine
stretching frequencies
The tin-chlorine stretching frequencies of given alkyl tin-chlorides sre lowered some 80-100 cm-l on formation of six-coordinate adducts with dipyridyl and This lowering of MX stretching frequencies with increased cophensnthroline. ordirmtion number of the metal is quite striking, and has been observed previously with cobalt-pyridine complexes [2] snd with the addition compounds of titanium tetrachloride [a].
250
300
cm-1
350
400
Fig. 2. Tin-chlorine stretching frequencies for methyl tin chlorides A, in cyclohexane solution, B, in benzene solution, and C, for their adducts with dipyridyl obtained as nujol mulls. (The form of this graph is scarcely altered if degeneracy-weighted averages of the Y(SnC1) modes are plotted, rather than the infra-red active modes.)
However, it is apparent that the tin-chlorine stretching frequencies are ELISO lowered by successive replscement of chloride for alkyl groups (different alkyl groups having much the same effect in this regard). As may be seen from Fig. 2, this conclusion applies both to the parent alkyl tin chlorides as well as to the six-coordinate adducts with dipyridyl and phenantbroline. The probable explanation of this phenomenon is that a decrease in the electronegativity of the groups attached to the tin (i.e. replacement of chloride by alkyl groups) leads to a lowering of the effective nuclear charge on the tin atoms, and to a consequent weakening of the tin-halogen bonds. The effect on the MX stretching frequencies is similar but smaller thsn that observed on lowering the oxidation state of a metal, for which a similar explanation seems likely e.g. the v3(t2) vibration of the tetrahedral anions [3] FeCl,- occurs at 378 cm-l and for FeCld2- at 282 cm-l. It is also of interest to note
1866
R. J. H.
CLARKand C. S. WILLIAMS
that while the tin-carbon bond lengths are approximately constant in the methyl tin halides, the tin-halogen bond length increases very slightly with increasing substitution of alkyl for halide group: e.g. the tin-chlorine bond lengths in the methyl tin chlorides are as follows [9]: SnCl,, 2.30 A; CH,SnCI,, 2.32 A; (CH,),SnCl, 2.34 A; (CH,),SnCl 2.37 A. In the case of the dibutyl tin adducts, the bromides and iodides were also studied. While the tin-carbon stretching frequencies are very similar to those for the chlorides, the tin-bromine and tin-iodine stretching frequencies must all occur below 200 cm-l. This result is to be expected in view of previous experimental findings [IO] which indicate that for octahedral complexes which differ only in that bromide replaces chloride in the first coordination sphere about the metal, the ratio y(MBr): v(MC1) = 0.68 - 0.75. It was hoped that the number of infra-red-active tin-chlorine stretching frequencies together with the number of such tin-carbon stretching frequencies would lead to unambiguous assignments for the possible stereochemical isomers. In general, this has not proved to be possible, because of the large band widths and poor resolution of the tin-chlorine stretching modes, and the low intensity of the tin-carbon stretching modes. The breadths of the tin-chlorine absorption bands in the adducts are in marked contrast to their sharpness in the parent halides. In particular, the SnCl stretching mode of trimethyl tin chloride in cyclohexane at 342 cm-l is very sharp, but has a shoulder some 5.5 cm-l to lower frequencies. This shoulder is interpreted as arising from the Sn-CF7 stretching mode, Cl 37 being the less abundant (24.5%). The chlorine isotopic splitting is thus of closely similar magnitude to that reported previously [lo] for certain metal pentacarbonyl chlorides (6 cm-l). Ligand absorption bands The infra-red spectra of both free and co-ordinated phenanthroline and dipyridyl in the 450-200 cm-l region have been reported previously [ll]. In the case of dipyridyl, the medium-strong band at 401 cm-l in the infra-red spectra of the free ligand has been assigned [12] to vibration 16b, an out-of-plane ring deformation, by analogy with the similar vibration of pyridine [13]. This band is raised on co-ordination to 419 cm-l in the compound SnCl, dipyridyl, and to 412 f 3 cm-l in the adducts R SnCl, dipyridyl and R,SnCI, dipyridyl (R = alkyl group). A further very weak band in the spectrum of dipyridyl at 427 cm-l occurs in practically A corresponding band of phenanthroline the same position in all the complexes. is raised from 410 cm-l in the free ligand to 425 f 1 cm-l in RSnCl, phenanthroline and to 415 + 3 cm-l in R,SnCl, phenanthroline. In the adducts R,SnCl, dipyridyl, a weak band appears at 349 & 3 cm-l which [9] L. PAULING and L. 0. BROCKWAY, J. Am. Chem. Sot. 57, 2684 (1935). H. A. SKINNER and L. E. SUTTON, Trans. Faraday Sot. 40, 164 (1944). [lo] M. A. BENNETT and R. J. H. CLARK, Chem. and Ind. 861(1963); J. Chem.Soc. 5560 (1964). [Ill R. J. H. CLARK,J. Chem.Soc. 1377 (1963). [12] A. I. POPOV, J. C. MARSHALL, F. B. STUTE and W. B. PERSON, J. Am. Chem. Sot. 83,358s (1961). [13] C. H. KLINE and J. TURKEVICH, J. Chem. Phys. 12, 300 (1944). L. CORRSIN, B. J. FAX and R. C. LORD, J. Chem. Phys. 21, 1170 (1953).
Infra-red spectra of 2,2’-dipyridyl and l,lO-phenanthroline complexes
186’7
is neither present in the infra-red spectra of the corresponding mono-alkyl derivatives nor in that of free dipyridyl. This band, as well as the very weak band occurring in dipyridyl complexes near 427 cm-l has been assigned previously to tin-nitrogen stretching vibrations [ 141. These assignments are considered improbable because (a) the bands do not appear in all our dipyridyl complexes (b) the bands are only slightly below those corresponding to metal-ammonia vibrations [15] in ammonia complexes, an unlikely result from mass considerations (c) metal-phenanthroline vibrations would be expected at similar frequencies, but no such bands occur and (d) metal-pyridine vibrations have been convincingly assigned [2] near 220 cm-i; thus metal-dipyridyl and metal-phenanthroline stretching vibrations might be expected to occur at about or below 200 cm-l. Moreover the band is also observed in the compounds ZnCl, dipyridyl (366 cm-l) and HgBr, dipyridyl(353 cm-l). If the band were a tin-nitrogen stretching mode, it would be expected to be dependent on the oxidation state of the metal, and thus lower for the zinc and mercury complexes than for the tin complex. However, alternative assignments for these bands may be suggested. The lowest ring vibrations of pyridine are 374 cm-l (vibration 16a, a2 symmetry, Raman active), and 404 cm-l (vibration 16b, b, symmetry, infra-red and Raman active). These two vibrations are degenerate in benzene. In dipyridyl, vibrations 16~ of each ring couple to yield modes of symmetry, a, + b, and so also do vibrations 16b of each ring. As all of these are infra-red-active, four such bands should be seen in the infra-red spectrum of free dipyridyl (two are seen, see discussion above). The bands which are apparently too weak to be seen could gain intensity on co-ordination to a metal, and could appear in the 350 cm-l region. Furthermore there are six inter-ring fundamentals in dipyridyl (C,, symmetry) which are non-pyridine like. They are the inter-ring stretch (a,), the inter-ring torsion (a,), the out-of-plane (b,) and in-plane (al) scissoring modes, and the out-ofplane (az) and in-plane (b2) shearing modes. The first mode is probably above 1000 cm-l (cf. biphenyl) [16], the second is only Raman active of unknown frequency, the scissoring modes are probably around 130 cm-l (cf. biphenyl) [16], and only the shearing modes might be expected in the 200-400 cm-l region. Of these two, the b, mode should be infra-red active in the free molecule, but both modes could be active on co-ordination to a metal in which the other ligands are unsymmetrically placed. The in-plane shearing mode could also be coupled with the asymmetric metal-nitrogen stretching mode. The correct assignment of the 349 cm-l band in co-ordinated dipyridyl must await Raman data on the free ligand. Further band of medium strength appears in the infra-red spectra of the dibutyl tin adducts of dipyridyl at 216 cm-l in the chloride, 218 cm-i in the bromide and 228 cm-l in the iodide. It also appears very weakly in the spectra of the other dialkyl tin dipyridyl derivatives. The absorption band at 241 cm-l in free phenanthroline moves to 247 f 1 cm-l in the adducts RSnCl, Phenanthroline and to 239 f 2 cm-1 in the adducts R,SnCl, 1141 T. TANAKA, M. KOMURA, Y. KAWASAKI and R. OEAWABA, J. Organometal. Cl&m. 1, 484 (1964). [15] K. NAKAMOTO, Infrared Spectra of Inorganic and Coordination Compounds, p. 150, John Wiley and Sons, Now York (1963). [16] J. E. KATON and E. R. LIPPINCOTT, Spectrochim. Acta 15, 627 (1959).
1868
R. J. H. CLARK and C. S. WILLIAMS
phenanthroline. A new band not present in the free ligand appears at 277 f 4 cm-l in all the dialkyl tin adducts, and may also be present in the mono-alkyl derivatives; it would then however be obscured by the much stronger absorption due to the tin-chlorine stretching vibrations in this region. No assignments for these bands can be advanced until unambiguous assignments for the low-lying fundamentals of free phenanthroline are available. Acknowledgenzents-The authors thank Dr. D. L. ALLESTON and Mr. P. R. PALAN for the syntheses of some of the adducts, and Dr. A. G. DAVIES for several helpful discussions. One of us (C. S. W. ) thanks the British council for 6nancial assistance.
The far i&a-red spectra of 2,2’-dipyridyl and l,lO-phenanthroline complexes of alkyl tin halides R. J. H. CLARK and C. S. WIL~MS William
Ramsay
and Ralph Forster Laboratories, Gower Street, London
University
College,
(Received 11 January 1965) Ah&act-The infrared spectra (650-200 cm-l) of eighteen six-co-ordinate complexes of the alkyl tin halides with 2,2’-dipyridyl and l,lO-phenanthroline have been recorded as nujol mulls, and where possible also in solution. As a result of raising the co-ordination number of the tin from four in the parent alkyl tin halides to six in the complexes, the tin-chlorine stretching frequencies are lowered by some 80-100 cm-r. While the tin-carbon stretching frequencies (024-470 cm-l) are relatively insensitive to the number of alkyl groups in the complexes, the tin-chlorine stretching frequencies (299-215 cm-l) are lowered by successive replacement of chlorine for alkyl groups. This result is similar to that found for the parent alkyl tin halides. The tin-chlorine stretching frequencies are sharp for the parent alkyl tin halides (the infra-red spectrum of trimethyl tin chloride shows an isotopic splitting of 5.6 cm-l), but those for the complexes are broad and insufficiently well resolved to assist in distinguishing between the various possible stereochemical isomers. A new adduct of trimethyltin chloride with dipyridyl is reported. of the increasing interest in alkyl tin halides and their derivatives, we have undertaken a study of the infra-red spectra (650-200 cm-i) of the series of addition compounds formed between alkyl tin halides and the bidentate ligands 2,2’-dipyridyl and l,lO-phenanthroline. The compounds were reported recently by ALLESTON and DAVIES [l]. Metal-halogen (MX) stretching frequencies normally give rise to relatively intense absorption bands in this region of the spectrum, and they are known to provide information on the stereochemistry [2], oxidation state [3] and coordination number of the metal [2, 41. With these compounds it was of interest to discover the effect on the tin-chlorine stretching frequencies of raising the coordination number of the tin from four for the parent alkyl tin halides in non-polar solutions, to six for the complexes. It was also of interest to investigate whether progressive replacement of chloride for alkyl groups led to a monotonic decrease in the remaining MX stretching frequencies, such as has been observed in the parent alkyl tin halides [5], and furthermore whether the number of infra-red-active MX stretching frequencies could provide information on which of the possible stereochemical In the course of the investigation a isomers were formed during the syntheses. new adduct of trimethyl tin chloride with dipyridyl was synthesised.
IN VIEW
[l] D. L. ALLESTON and A. G. DAVIES, J. Chem. Sot. 2050 (1962). [2] R. J. H. CLARK and C. S. WILLIAMS, Chem. I&. (London), 1317 (1964); Inorg. Chem. 4, 350 (1965). [3] R. J. H. CLARK and T. M. DUNN, J. Ghem. Sot. 1198 (1963). [4] R. J. H. CLARK, Symposium in Inorganic Chemistry, Bratislava, Czechoslovakia (1964); Spectrochim. Acta 21, 955 (1965). [5] I. R. BEATTIE and G. P. MCQUILLAN, J. Chem. Sot. 1519 (1963). I
1861
1862
R. J. H. CLARKand C. S. WILLIAMS EXPERIMENTAL
Infra-red spectra The infra-red spectra of the solid compounds were recorded as nujol mulls with a Grubb-Parsons double-beam grating spectrometer, type D.M.2, over the range 200-455 cm-l. The calibration was carried out by reference to the rotational spectrum of water vapour in this region, the spectrometer being used as a single beam instrument [6]. Spectra above 400 cm-l were recorded on a Grubb-Parsons doublebeam grating spectrometer, type GS2A and on a Perkin-Elmer 337 double-beam grating spectrometer, calibrated by reference to the spectra of polystyrene and indene. The frequencies should be accurate to f2 cm-l for sharp bands. The mulls were supported between polythene or potassium bromide plates. Spectra of the most soluble of the adducts were obtained in several solvents using variable path-length polythene containers. Only the butyl and octyl derivatives were sufficiently soluble to be investigated in this way, benzene and dichloromethane being convenient solvents. The general appearance of the solution spectra is closely similar to that of the mull spectra, with the tin-chlorine stretching modes as broad in solution as in the solid state. These modes, however, suffer a solvent shift of up to 18 cm-l in solution compared to their solid state values (e.g. dioctyltin dichloride dipyridyl and dioctyltin dichloride phenanthroline in benzene solutions). The tin-carbon and ligand vibrations are relatively independent of solvent effects. The infra-red spectra of certain of the parent alkyl tin halides were also recorded in solution in the same way. Syntheses and characterisation The adducts were prepared by the methods described previously [l]. Molecular weights were determined with a Mechrolab vapour pressure osmometer using benzil as a standard. Dioctyl tin dichloride phenanthroline is a monomer in dichloromethane (calculated for monomer, 594; found 600, 594, 628 in the concentration range O*Ol-0*05M) [7]. On the bases of the similarities in chemical and physical properties (melting points, tin-chlorine stretching frequencies etc.) between this adduct and all others, it is believed that all the adducts are six-co-ordinate monomers in the solid state, and in solution in non-polar or weakly polar solvents. In more polar solvents such as acetone, however, the adducts give low molecular weights suggesting that some dissociation takes place. In an attempt to form dipyridyl and phenanthroline adducts of trimethyl tin chloride, the components were added to each other in ether solution. On concentration, the phenanthroline solution gave an ill-defined product, but colourless crystals formed slowly from the dipyridyl solution; m.p. 69-70% (uncorrected). Calculated for a 1: 1 adduct C,,HI,N,CISn; C, 43.89; H, 4.78; N, 7.88. Found, C. 43.3; H, 5.1; N, 8.1. In view of the fact that the melting point of the adduct is the same as that of dipyridyl itself, the melting point of a 50 : 50 mixture of the adduct with dipyridyl was determined. There was a 9” depression of the melting point, indicating that the adduct is not a mixed crystal. It breaks down to the starting products in all solvents [6] L. R. BLAINE,E. K. PLYLER~~~W. S. BENEDICT, J. Res. Nat. Bur.Stand., 6&i, 223 (1962). [7] We are indebted to Mr. P. R. PALANfor this measurement.
1803
Infra-red spectra of 2,2’-dipyridyl and l,lO-phenanthroline complexes
tried, including benzene, cyclohexane, chloroform and acetonitrile, as evidenced by solution infra-red spectra. However a nujol mull spectrum of the compound indicated complex formation, as the SnCl stretching mode is depressed by at least 100 cm-l from its position in trimethyl tin chloride, and the ring vibration of dipyridyl at 402 cm-l is raised to 415 cm-l, a rise typical of co-ordineted dipyridyl. RESULTS AND DISCUSSION
The tin-carbon and tin-chlorine stretching frequencies for the relevant alkyl tin halides are in Table 1 and for the adducts in Table 2. Representative scans for the mono- and di-alkyl tin derivatives are depicted in Fig. 1. For the latter there Table 1. Infra-red-active tin-chloride and tin-carbon stretchingfrequenciesin alkyl tin chlorides* Compound
v(Sn C)
407 * 382 vs., 377 “8, 378 vs., 361 s, 359, 356, 354, 331 “8 337 8
SnCl,
MeSnCl, EtSnCl, n-BuSnCi, Me,SnCl, Et,SnCl, a--Bu@nCl, Oc&lCl, Me,SnCl Et&Cl a. b.
o. d. e. f. g.
560 531 602 606 542 518
Ref.
v(Sn X)
551 w-m 522 Wb 608 VW 524 497 517 518 s 613 w v@ 489 89
368 366 367 356 352 340 345
d c c!
sh ah sh sh sh sh
: d d d I3 0, f
Obtained for oyclohexene solutions unless otherwise stated. Obtained for benzene solution. This investigation. F. K. BUTCHER, W. GERRARD, E. F. MOONEY, R. G. REES, H. A. WILLIS, A. A. ANDERSONand H. A GEBBIE, J. OrganometaZ.Chem. 1, 431 (1964). Ref. [b]. P. TAIMSALUand J. L. WOOD, Spectrochim. Acta 20, 1043 (1964). Obtained for pure liquid.
Table 2. Tin-carbon and tin-halogen stretching frequencies in the 2,2’-Dipyridyl and 1,lO phenanthrolinecomplexes of alkyl tin trihdides and dialkyl tin dihalidesa Compound M&G& dipy MeSnCl, phen EtSnCl, dipy EtSnCl, phen BuSnCl, dipy BuSnCls phen Me,SnCl, dipy Me&Xl, phen Et,SnCl, dipy Et$nCl, phen Bu,SnCl, dipy Bu,SnC!lz phen Bu,SnBra dipy Bu,SnBr, phen Bu,SnI, dipy Bu,SnI, phen Oc,SnCl, dipy Oc.SnCl. Dhen
a@n C) 536 w 529 w 504 w 507 w 592 w 608 w 572 m 578m 529 m 481 525 m 470 b -592 624 vw 594 ~620 sh 587 588 ~615 VW 613 w 584 610 w, br 582 595 b 622 w 591
%J(Sn x1
w w “\v VW w, br w w w, br w, br VW
289 296 292 299 294 285 244 247 234 242 243 242 <200 < 200 (200 < 200 235 235
vs, 267 m 8, asym, 272 vs 8, 283 s, 273 8 s, 270 vs 8, 281 8, 264 m YB 8, br s, 239 sh 8, 215 w 8, 220 8 8, br s, br
B 8, br
a obtained as nujol mulls; ) obscured by ligand absorption Me = methyl, Et = ethyl, Bu = n-butyl, Oc = n-octyl; VW = very weak, w = week, br = broad, m = medium, B = strong, sh = shoulder, asym = asymmetric
R. J. H. CLARK and C. S. WILLIAMS
1864
are three different stereoisomers possible ; with the methyl groups in cis positions (C,, molecular symmetry), with the halogen groups in cis positions (also C,, symmetry), or with both the methyl and the halogen groups in cis positions (no symmetry). 200
I 50
250
300
400
cm-’
I
42
P
34
26
Fig. 1. Far infra-red spectra of A, methyl tin trichloride dipyridyl and B, dimethyl tin dichloride dipyridyl as nujol mulls.
Tin-Carbon
stretching frequencies
The tin-carbon stretching frequencies in all the adducts are located in the cm-l region, and they vary in intensity from weak to medium for the methyl derivatives to very weak for the butyl derivatives. However, for a given Hence the tin-carbon stretching alkyl group, the range is very much narrower. frequencies seem to be relatively insensitive to the coordination number of the tin and to the degree of substitution of alkyl for halide groups. For the mono-alkyl derivatives, only one tin-carbon stretching frequency is expected. This is observed for all such compounds, except for the parent mono-ethyl and mono-butyl tin trichlorides, for which an additional very weak band is observed for benzene solutions at 484 and 527 cm-i respectively. In the latter case it has been suggested [8] that the extra band implies the presence of both trans- and gauche rotational isomers. For the dimethyl tin dichloride adducts, again only one band is observed. Thus a trans arrangement of the methyl groups is implied [5], for although both tin-carbon stretching frequencies are permitted in the infrared the totally symmetric 624-470
[S] R. A. CUMMINS, Au&al.
J. Chem. 16, 985 (1963).
Infrared
spectra of 2,2’-dipyridyl
and I,lO-phenanthroline
complexes
1865
vibration leads to such a small chsnge in dipole moment that the band is unlikely to be seen. However, the other dielkyl tin dihalide derivatives hrtve two tin-carbon stretching frequencies which are of such low intensity that no comment on the stereochemistry of the compounds appears to be justified. Tin-chlorine
stretching frequencies
The tin-chlorine stretching frequencies of given alkyl tin-chlorides sre lowered some 80-100 cm-l on formation of six-coordinate adducts with dipyridyl and This lowering of MX stretching frequencies with increased cophensnthroline. ordirmtion number of the metal is quite striking, and has been observed previously with cobalt-pyridine complexes [2] snd with the addition compounds of titanium tetrachloride [a].
250
300
cm-1
350
400
Fig. 2. Tin-chlorine stretching frequencies for methyl tin chlorides A, in cyclohexane solution, B, in benzene solution, and C, for their adducts with dipyridyl obtained as nujol mulls. (The form of this graph is scarcely altered if degeneracy-weighted averages of the Y(SnC1) modes are plotted, rather than the infra-red active modes.)
However, it is apparent that the tin-chlorine stretching frequencies are ELISO lowered by successive replscement of chloride for alkyl groups (different alkyl groups having much the same effect in this regard). As may be seen from Fig. 2, this conclusion applies both to the parent alkyl tin chlorides as well as to the six-coordinate adducts with dipyridyl and phenantbroline. The probable explanation of this phenomenon is that a decrease in the electronegativity of the groups attached to the tin (i.e. replacement of chloride by alkyl groups) leads to a lowering of the effective nuclear charge on the tin atoms, and to a consequent weakening of the tin-halogen bonds. The effect on the MX stretching frequencies is similar but smaller thsn that observed on lowering the oxidation state of a metal, for which a similar explanation seems likely e.g. the v3(t2) vibration of the tetrahedral anions [3] FeCl,- occurs at 378 cm-l and for FeCld2- at 282 cm-l. It is also of interest to note
1866
R. J. H.
CLARKand C. S. WILLIAMS
that while the tin-carbon bond lengths are approximately constant in the methyl tin halides, the tin-halogen bond length increases very slightly with increasing substitution of alkyl for halide group: e.g. the tin-chlorine bond lengths in the methyl tin chlorides are as follows [9]: SnCl,, 2.30 A; CH,SnCI,, 2.32 A; (CH,),SnCl, 2.34 A; (CH,),SnCl 2.37 A. In the case of the dibutyl tin adducts, the bromides and iodides were also studied. While the tin-carbon stretching frequencies are very similar to those for the chlorides, the tin-bromine and tin-iodine stretching frequencies must all occur below 200 cm-l. This result is to be expected in view of previous experimental findings [IO] which indicate that for octahedral complexes which differ only in that bromide replaces chloride in the first coordination sphere about the metal, the ratio y(MBr): v(MC1) = 0.68 - 0.75. It was hoped that the number of infra-red-active tin-chlorine stretching frequencies together with the number of such tin-carbon stretching frequencies would lead to unambiguous assignments for the possible stereochemical isomers. In general, this has not proved to be possible, because of the large band widths and poor resolution of the tin-chlorine stretching modes, and the low intensity of the tin-carbon stretching modes. The breadths of the tin-chlorine absorption bands in the adducts are in marked contrast to their sharpness in the parent halides. In particular, the SnCl stretching mode of trimethyl tin chloride in cyclohexane at 342 cm-l is very sharp, but has a shoulder some 5.5 cm-l to lower frequencies. This shoulder is interpreted as arising from the Sn-CF7 stretching mode, Cl 37 being the less abundant (24.5%). The chlorine isotopic splitting is thus of closely similar magnitude to that reported previously [lo] for certain metal pentacarbonyl chlorides (6 cm-l). Ligand absorption bands The infra-red spectra of both free and co-ordinated phenanthroline and dipyridyl in the 450-200 cm-l region have been reported previously [ll]. In the case of dipyridyl, the medium-strong band at 401 cm-l in the infra-red spectra of the free ligand has been assigned [12] to vibration 16b, an out-of-plane ring deformation, by analogy with the similar vibration of pyridine [13]. This band is raised on co-ordination to 419 cm-l in the compound SnCl, dipyridyl, and to 412 f 3 cm-l in the adducts R SnCl, dipyridyl and R,SnCI, dipyridyl (R = alkyl group). A further very weak band in the spectrum of dipyridyl at 427 cm-l occurs in practically A corresponding band of phenanthroline the same position in all the complexes. is raised from 410 cm-l in the free ligand to 425 f 1 cm-l in RSnCl, phenanthroline and to 415 + 3 cm-l in R,SnCl, phenanthroline. In the adducts R,SnCl, dipyridyl, a weak band appears at 349 & 3 cm-l which [9] L. PAULING and L. 0. BROCKWAY, J. Am. Chem. Sot. 57, 2684 (1935). H. A. SKINNER and L. E. SUTTON, Trans. Faraday Sot. 40, 164 (1944). [lo] M. A. BENNETT and R. J. H. CLARK, Chem. and Ind. 861(1963); J. Chem.Soc. 5560 (1964). [Ill R. J. H. CLARK,J. Chem.Soc. 1377 (1963). [12] A. I. POPOV, J. C. MARSHALL, F. B. STUTE and W. B. PERSON, J. Am. Chem. Sot. 83,358s (1961). [13] C. H. KLINE and J. TURKEVICH, J. Chem. Phys. 12, 300 (1944). L. CORRSIN, B. J. FAX and R. C. LORD, J. Chem. Phys. 21, 1170 (1953).
Infra-red spectra of 2,2’-dipyridyl and l,lO-phenanthroline complexes
186’7
is neither present in the infra-red spectra of the corresponding mono-alkyl derivatives nor in that of free dipyridyl. This band, as well as the very weak band occurring in dipyridyl complexes near 427 cm-l has been assigned previously to tin-nitrogen stretching vibrations [ 141. These assignments are considered improbable because (a) the bands do not appear in all our dipyridyl complexes (b) the bands are only slightly below those corresponding to metal-ammonia vibrations [15] in ammonia complexes, an unlikely result from mass considerations (c) metal-phenanthroline vibrations would be expected at similar frequencies, but no such bands occur and (d) metal-pyridine vibrations have been convincingly assigned [2] near 220 cm-i; thus metal-dipyridyl and metal-phenanthroline stretching vibrations might be expected to occur at about or below 200 cm-l. Moreover the band is also observed in the compounds ZnCl, dipyridyl (366 cm-l) and HgBr, dipyridyl(353 cm-l). If the band were a tin-nitrogen stretching mode, it would be expected to be dependent on the oxidation state of the metal, and thus lower for the zinc and mercury complexes than for the tin complex. However, alternative assignments for these bands may be suggested. The lowest ring vibrations of pyridine are 374 cm-l (vibration 16a, a2 symmetry, Raman active), and 404 cm-l (vibration 16b, b, symmetry, infra-red and Raman active). These two vibrations are degenerate in benzene. In dipyridyl, vibrations 16~ of each ring couple to yield modes of symmetry, a, + b, and so also do vibrations 16b of each ring. As all of these are infra-red-active, four such bands should be seen in the infra-red spectrum of free dipyridyl (two are seen, see discussion above). The bands which are apparently too weak to be seen could gain intensity on co-ordination to a metal, and could appear in the 350 cm-l region. Furthermore there are six inter-ring fundamentals in dipyridyl (C,, symmetry) which are non-pyridine like. They are the inter-ring stretch (a,), the inter-ring torsion (a,), the out-of-plane (b,) and in-plane (al) scissoring modes, and the out-ofplane (az) and in-plane (b2) shearing modes. The first mode is probably above 1000 cm-l (cf. biphenyl) [16], the second is only Raman active of unknown frequency, the scissoring modes are probably around 130 cm-l (cf. biphenyl) [16], and only the shearing modes might be expected in the 200-400 cm-l region. Of these two, the b, mode should be infra-red active in the free molecule, but both modes could be active on co-ordination to a metal in which the other ligands are unsymmetrically placed. The in-plane shearing mode could also be coupled with the asymmetric metal-nitrogen stretching mode. The correct assignment of the 349 cm-l band in co-ordinated dipyridyl must await Raman data on the free ligand. Further band of medium strength appears in the infra-red spectra of the dibutyl tin adducts of dipyridyl at 216 cm-l in the chloride, 218 cm-i in the bromide and 228 cm-l in the iodide. It also appears very weakly in the spectra of the other dialkyl tin dipyridyl derivatives. The absorption band at 241 cm-l in free phenanthroline moves to 247 f 1 cm-l in the adducts RSnCl, Phenanthroline and to 239 f 2 cm-1 in the adducts R,SnCl, 1141 T. TANAKA, M. KOMURA, Y. KAWASAKI and R. OEAWABA, J. Organometal. Cl&m. 1, 484 (1964). [15] K. NAKAMOTO, Infrared Spectra of Inorganic and Coordination Compounds, p. 150, John Wiley and Sons, Now York (1963). [16] J. E. KATON and E. R. LIPPINCOTT, Spectrochim. Acta 15, 627 (1959).
1868
R. J. H. CLARK and C. S. WILLIAMS
phenanthroline. A new band not present in the free ligand appears at 277 f 4 cm-l in all the dialkyl tin adducts, and may also be present in the mono-alkyl derivatives; it would then however be obscured by the much stronger absorption due to the tin-chlorine stretching vibrations in this region. No assignments for these bands can be advanced until unambiguous assignments for the low-lying fundamentals of free phenanthroline are available. Acknowledgenzents-The authors thank Dr. D. L. ALLESTON and Mr. P. R. PALAN for the syntheses of some of the adducts, and Dr. A. G. DAVIES for several helpful discussions. One of us (C. S. W. ) thanks the British council for 6nancial assistance.