1972 923Complexes of Some Metal Bromides with I ,4-DioxanBy John C. Barnes and Miss C. S. Duncan, Department of Chemistry, The University, DundeeThe thermal decomposition reactions of MBr,,Zdx (where M = Mn, Co, or Ni ; and dx = 1.4-dioxan) and MBr,,dx(M = Co, Ni) have been investigated by thermogravimetry, differential enthalpic analyses, and dynamic reflectancespectroscopy. The 1 : 1 complexes decompose in a single step to MBr,. The 1 : 2 complexes also decompose toMBr, but each by a different route. Temperatures and enthalpies of decomposition are reported.Crystalline MBr2,2dx.4H,O are formed by NiBr, and CoBr, but not by MnBr,. The visible spectra suggest thatthe chromophore is trans-M (2Br,4H2O) with the dioxan molecules on lattice sites. Thermal decompositionconverts these compounds into MBr2,2dx.3H,O, MBr,,dx and finally MBr,. The d-d spectrum of CoBr2.2dx,3H,Osuggests the presence of two environments for Co2+, one identical with that in CoBr2,Zdx,4H,O and the otherpossibly tetrahedral.UNLIKE most weak solvating ligands, 1,4-dioxan (dx)forms well characterised hydrated complexes MX,,bdx,-cH20 as well as anhydrous solvates MX,,bd~.l-~ Inrecent papers the present authors reported the spectraand thermal decomposition reactions of MCl,,dx,2H20(where M is Ca, Mn, Fe, Co, and Ni),* and of some copperE.Giesbrecht, W. G. R. de Camargo, G. Vicentini, and M.Perrier, J . Inorg. Nuclear. Chem., 1962, 24, 381.J. C. Barnes and C. S. Duncan, J . Chetn. SOC. ( A ) , 1970,l F. Reiff, 2. anorg. Chem., 1932, 208, 321.a J.A. Weicksel and C . C . Lynch, J . Amer. Chcm. SOC., 1950,72, 2632. 1442924 J.C.S. Daltonc~mplexes.~ Previous workers 6-B have reported MBr,,- compose thermally by reaction (1) but each of the 1 : 22dx for Mn, Co, and Ni and MBr,,dx for Co and Ni. The complexes decomposes by a distinctive route. Thevibrational and electronic spectra of these compoundsMBr,,dx, -+ MBr,, + dx, have been reported.6 There are no previous studies ofthe hydrated complexes. results of d.e.a., t.g., and d.r.s. are given in Tables 1 and 2In this paper the thermal decomposition of both the and in Figure 1.(1)TABLE 1Differential enthalpic analysis of anhydrous compoundsEndothernir A% Atmospheric pressure 1 Torrr AHT,/(product) TllK T I P T2.W kJ mol-1 AWobs.% AwTh. %Reactantil!nT?r,, 2dx 373 435 383 419 180 -j= 20 0.0 46.07aa a 45.2 & 0.7 } 280f20b (MnBr,) 450490 443CoBr,,2dx 383 412 363 388 69.0 5 1-2 22.1 & 0.3 22.28(CoBr.. ,dx)CoBr;,c!x 441 477 76.2 & 3.5 28.9 & 0.3 28.72336 179 f 4 45.4 & 0.4 44.6644 168.8 f 1.2 28-8 f 0.3 28.73NiBr2,2dx 378 436(NiBr,)NiBr,,dx 45 1 515(CuBr,)Experiments were conducted a t atmospheric pressure under nitrogen flowing a t 30 cn13 min-l or under a dynamic vacuum ofTl was the minimum tempcrature a t which decomposition was detected.AWoba combines d.e.a. and(CoBr2){ 430038(CuBr2),,2dx 368 462 2321 Torr, heating a t 8.33 deg min-l on sample of 2-10 mg.T , was the peak temperature of the transition.t.g.measurements.AHF, refers to experiments a t atmospheric pressure.a T, concealed by previous endotherm. CAH (T = 375 to T = 500 K) = 462 f 5 k J mol-l. J. C. Barnes, J . Inorg. NuclearChena., 1969, 31, 95.anhydrous and the hydrated metal brornide-l,4-dioxancomplexes is reported. These reactions have been ex-amined by differential enthalpic analysis (d.e.a.),MnBr2,2dx showed an endothermic structural re-arrangement followed by decomposition into hlnBr, intwo overlapping endotherms. MnBr,,dx was not ob-tained pure at any temperature. CoBr2,2dx gave wellresolved endotherms for the formation of CoBr,,dx and1 I 130 25 20 15kKFIGURE 1 Dynamic reflectance spectra showing decompositionof CoBr,,Bdx a t 1 Tom; A , 300-330K; CoBr2,2dx; B,360 K; C, 370 K, CoBr,,dx; D, 400 K; E 440-500 K, CoBr,.Parts of the intermediate (dotted) curves are omitted forclarity.Isosbestic points were observed a t 19.5 x lo3 cm-lfor both reactions and a t 16.8 x 103 cm-1 in the decompositionof CoBr,,dxthermogravimetry (t .g.) and dynamic reflectance spec-troscopy (d.r.s,) .4,9JO D.e.a. and t.g. measurementshave been made at atmospheric pressure in flowingnitrogen, and under a dynamic vacuum of 1 Torr.D.r.s. measurements were made only a t 1 Torr.Anhydrous CompZexes.-All the 1 : 1 complexes de-IS J. C. Barnes, J . Inorg. Nuclear. Chem., 1969, 31, 96.6 G. W. A. Fowles, D. A. Rice, and R. A. Walton, J . Chem.R. Juhaez and L. F. Yntema, J . Amer. Chem. SOC., 1940,SOG. ( A ) , 1968, 1842.02, 3522.TABLE 2Dynamic reflectance spectral studies on MBr,,dx andStarting Temp.material range/KMnBr2,2dx 370-400420-450CoBr,,dx 330-370CoBr,,dx 423-450NiBr,,2dx 300-333NiBr,,dx 400-460MBr, , 2dxSpectrumObservations of productIncrease in absorption a tDecrease in intensity in dx,26-30 x lo3 cm-lovertones 4700and 5400 cm-lRlnBr,CoBr,,dx See Figure 1See Figure 1 CoBr,Increased absorption a t NiBr,,dxIncreased absorption at NiBr,30 x lo3 cm-l, smallchanges in d-d spectrum26 x lo3 cm-', smallchanges in d-d spectrumAll measurements were made a t 1 Torr.min-lregions were all studied for each compound.observations from all regions were mutually consistent.Heating rate 2 KThe electron transfer (d-d) and vibrational overtoneIn each case thethen CoBr, at atmospheric pressure.The second re-action was observed in t.g. and d.r.s. a t 1 Ton but did8 H. Rheinboldt, A. Luyken, and M. Schittman, J . prukt.9 W. W. Wendlandt, P. M. Franke, and J. P. Smith, Analyt.Chem., 1937, 149, 30.Chem., 1963, 35, 105.10 J. C . Barnes, J . Sci. I~zstv., 1968, 11, 2151972200I? c - cr4 A 150-5. s.-100 s925.... --,not appear in the d.e.a. trace, possibly because of thereduced sensitivity of the instrument a t this pressure.NiBr2,2dx decomposed directly into NiBr, at atmos-pheric pressure but d.r.s. and d.e.a. indicated the form-ation of NiBr,,dx as an intermediate at 1 Torr.The enthalpies of decomposition of the compounds ofCoBr, and NiBr, fall within the range found for thecorresponding chloride^.^ The enthalpy of decomposi-tion of MnBr2,2dx is very high even allowing for theinitial phase change.Hydrated Compowzds.-No phase studies have beenreported on the systems MBr,,dx,H,O.The presentwork suggests that the phase diagrams for CoBr, andNiBr, at 298 K are qualitatively similar to those re-ported by Schott and Lynchll for the correspondingchlorides (Figure 2). Only one solid phase, MBr2,2dx,-4H20 was found which contained all three components.This appears in the equilibrium solid over almost all thediagram, corresponding to MCl2,dx,2H,O in the chlorideseries.No mixed solvate was obtained from MnBr, by eitherroute. Addition of dioxan to ethanolic MnBr,,GH,OL ' I IMBr,.LFIGURE 2 Quantitative phase diagram for the system RIBr,-dx-H,O a t 300K; A, MBr,,6H20; B, MBr2,2dx,4H,0; C,MBr,,dx.The shaded area contains two immiscible liquidphases. The less dense is moist dioxan, the denser layer con-tains all the salt in dilute aqueous dioxan. The upper layeris colourless, the lower deep blue for CoBr, and green forNiBr,.precipitated MnBr2,2dx. The explanation must lie inthe unusually high total heat of decomposition ofMnBr2,2dx which shows that the lattice energy of thatTABLE 3Lattice energies of MBr,,Sdx and MBr,,dxAHL(complex) = AH,(MBr,) + AHD(complex)Complex AHL/kJ mol-lMnBr,,Bdx 2898CoBr,,2dx 2796CoBr,,dx 2726NiBr2,2dx 2879NiBr,,dx 2769The values for AH,(MBr,) are taken from K. B. Yatsimirskii,compound must be similar to that of NiBr2,2dx (Table 3).Assuming that the enthalpy of reaction 2 would beZhur.xeovg. Khim., 1958, 3, 2244.MBr,,2dx,4H20, -+ MBr, + 2dx, + 4H20, (2)similar for Mn, Co, and Ni, AH, must lie between 180 and460 kJ mol-I since the Ni and Co compounds exist andMnBr2,2dx,4H,O does not. The corresponding enthalpyfor NiCl2,dx,2H,O is 160 kJ mol-I which suggests avalue about 320 kJ mol-l for NiBr2,2dx,4H,O. TheI -.I..,.... .... ..... 4 FIGURE 3 Decomposition of NiBr2,2dx,4H,0 a t atmosphericpressure; d.e.a. and t.g. datacomplicated thermal decomposition beliaviour de-scribed below prevents AH, from being measured byd.e.a.Thermal Decomposition.-Table 3 summarises thed.e.a. data. At atmospheric pressure MBr,,2dx,4H20,like MCl2,dx,2H,O and many other salt hydrates, ' melt'on heating to give a saturated solution phase.Thisliquid phase dries out a t higher temperatures and thesolid residue decomposes further (Figure 3).Under a dynamic vacuum of 1 TOIT, water can be re-moved from the sample as fast as it is liberated and noliquid phase is formed. CoBr2,2dx,4H,O showed threewell separated endotherms. D.r.s. measurements(Figure 4) show that a reaction occurs between 300and 313 K during which the intensity of the 5100 cm-lcombination band of water (v, + v3) is reduced and anew band appears in the d-d spectrum at 13.8 x lo3cm-l. Both this band and the water vibration vanishby 350 K to leave the spectrum of CoBr, between 383 andI I I I25 20 15 10 5Dynamic reflectance spectra showing decompositionof CoBr,,Sdx,4H,O a t 1 Torr; A , 300 K, CoBr,,2dx,4H20;B, 313 K, CoBr2,2dx,3H,O; C, 353 K, CoBr,,dx; D, 463 K,CoBr,; the intermediate spectra are omitted for claritykKFIGURE 4433 K.The weight losses show that the reactions areclose to stoicheiometric and involve the formation of11 H. Schott and C. C. Lynch, J . Chem. Eng. Data, 1966, 11,216926 J.C.S. DaltonCoBr2,2dx,3H,0 below 313 K and the conversion of thiscompound into CoBr,,dx by 350 K and CoBr, by 450 K.NiBr2,2dx,4H,0 undergoes somewhat similar reactionsto CoBr2,2dx,4H,O a t 1 Torr. Figure 5 shows that twoendotherms and two weight-loss steps are observed butthat these do not correspond in temperature. D.r.s.FIGURE 5 Decomposition of NiBr2,2dx,4H20 a t 1 Torr;d.e.a., t.g., and d.r.s.datameasurements on the 5100 cm-l water band show thatwater is lost in three stages below 400 K. All threespin allowed d-d bands shift to the red between 300 andwith a splitting of 18 cm-l. Both compounds show abroad absorption between 700 and 400 cm-l with amaximum at 550 cm-l, superimposed on which are sharptransitions at 630, 620, 603, 515, and 450 cm-l. Apartfrom the 603 cm-l peak which is the ring bending modefound at 610 cm-l in pure dioxan, the envelope and thesubsidiary peaks represent the rocking and waggingmotions of co-ordinated water. A weak absorption at360 cm-l is probably the M-OH, stretching mode. Thering bending modes of dioxan (285 and 275 cm-l) appearat 300 and 275 cm-l in these complexes.The complexesare transparent towards the low frequency limit of thespectrometer (238 cm-l), where MCl2,dx,2H,O absorbstrongly. This is consistent with energies expected forthe metal-halide stretching frequencies, ca. 240 and200 cm-l for C1 and Br respectively. CoBr2,2dx shows(Co-Br) at 294 and 305 cm-l.The d-d spectra of MBr2,2dx,4H,O are identical withthose of MBr2,6H,O in energy and relative intensity.Table 5 shows that none of the eight observed d-dtransitions of CoBr2,6H,0 lies more than 0.2 x lo3 cm-lfrom the band position in CoBr2,2dx,4H,O. Both com-pounds show similar mauve to orange dichroism.TABLE 4Differential enthalpic analysis of MBr2,2dx,4H20EndothermfA IAtmospheric pressure 1 Torrf ----7 .ATI T2 AWob8 ./a TI T2 Al.T/',b, % AWm.% Product310 344 18.2 f 1.0a 32.0 f 1.0c o < 300 308 4.5 f 0.5 385 CoBr2,2dx,3H20331 350 34.5 f 0.5 34-29 CoBr,,dx433 47 7 62.5 & 1-5 420 463 53.0 f 0.6 53.11 CoBr,Ni b 38-1 f 3.0313 330 6.0 f 1.0 3.9 NiBr2,2dx,3H,0G 355 44.8 f 0.8 43.75 NiBr, + NiBr,,dx45 1 515 52.5 & 0.5 a 52.7 & 0.6 63.16 (NiBr,)358-413 I<, region of fusion with unreproducible endotherm; reproducible sharp exotherm at 383 K. 325-425 K, regionT, obscured by previous endotherm. d WeightAW,,,,of fusion with unreproducible endotherm; reproducible sharp exotherm a t 364 K.loss continued t o 450 K without detectable endotherm.includes measurements by d.e.a. and t.g., at 1 Torr.Experimental condition as in Table 1.AWT~. is the theoretical weight loss from MBr,,2dx,4H20 to the product shown.370 K and decrease in intensity between 410 and 450 K.These observations suggest that the first endothermrepresents the formation of NiBr,,2dx,3H20 which is notthermally stable and decomposes in the overlappingsecond endotherm, the products of which are an equi-molar mixture of NiBr,,clx and NiBr,. The NiBr,,dxthen decomposes below 470 K, without showing theendotherm reported in Table 1. This may again repre-sent the reduced sensitivity of the apparatus when work-ing at 1 Torr.Spectra.-The i.r. spectra of MBr,,2dx,4H20 are simi-lar to those of MCl,,dx,BH,O. Above 900 cm-l thespectra are very closely those of dioxan and water.In CoBr2,2dx,4H,O the dioxan ring stretching modeat 874 cm-l is split by less than 8 cm-l whereas in thenickel compound the components are clearly resolved,Analysis of the spectrum of NiBr2,2dx,4H,0 givesDq = 710 and B' = 973 cm-l.TABLE 5Electronic spectra to 40 k K (band maxima in lo3 cm-1)CoBr2,2dx,4H20CoBr2,2dx,3H20NiBr2,2dx,4H20NiBr,,2dx,3H20CoBr,,6H20NiBr2,6H20 7.5, 13.0 (20), 23.57.4, 11.0, 14.0, 17.8, 19.4, 20.4, 21.7, 29.7, 34.55.5 (7.2) ( l l - o ) , 13.8 (14.8) (15-3), 17.8, 19.5,7.1, 13.0 (20.0), 23.3 (36) (40)6.8, 11.8 (14), 20.2 (22.5) (28), 3120.4, 21.5, 34.27.6, 11.0, 14.0, 18-0, 19.5, 20.4, 21.9, 29.9In MBr2,6H20 the inner co-ordination sphere con-tains trans (2Br,4H20). The spectrum of the iso-structural CoCI,,GH,O was only assigned by invokingsymmetry-lowering by significant interaction of Co21972with the C1- ion in the second co-ordination sphere.12This suggests that the structure of CoBr2,2dx,4H20 willprove to be very closely related to that of CoBr2,6H20with octahedral complexes CoBr2(OH2), and the dioxanmolecule occupying lattice sites similar to those occupiedby the remaining 2H20 in the hexahydrate.The d-d spectrum of CoBr2,2dx,3H,0 shows unusualfeatures (Figure 4).All the bands of CoBr2,2dx,4H,0occur essentially unchanged in energy and intensity.In addition there is the strong band at. 13.8 x lo3 cm-lwith two subsidiary peaks to higher energy and a bandat 5.5 x lo3 cm-l. Once established, the spectrum ofthe trihydrate is not time dependent at 313 K showingthat the phase is not simply a mixture of CoBr2,2dx,-4H20 with another species.The most likely explanationis that two non-equivalent Co2+ ions exist in the tri-hydrate, one of which is identical with that in the tetra-hydrate. With so many of the bands identical withthose of the tetrahydrate it seems unlikely that theadditional bands can be assigned to distortion of thechromophore. The spectrum is quite unlike that ofCoBr,,2H20. It is possible that three Co ions out of fourretain the environment of CoBr2,Zdx,4H20 and the re-mainder are converted into a tetrahedral species.Since NiBr2,2dx,3H20 is fugitive, its d-d spectrum isless well characterised. The spectrum shows no evidencel2 J. Ferguson, J . Chern. Phys., 1960, 32, 533.p’927of new bands, the general red shift associated with theweakened crystal field leads to the values Dq = 680 andB’ = 920 cm-l.EXPERIMENTALThe anhydrous compounds were prepared by the methodsdescribed previously.6 MBr,, 2dx,4H20 were prepared byadding dioxan to solutions of MBr,,6H20 in ethanol at roomtemperature. Crystals up to 3 mm of edge appeared as thesolvent evaporated (Found: C, 21.0; H, 5.0; Br, 34.2.Calc. for C,H,,Br&OO,: c, 20.6; H, 5.15; Br, 34.25.Found: c , 20.4; H, 4-5; Br, 34-51. Cak. for C,H,,Br,NiO,:C, 20.6; H, 5.15; Br, 34.25%).Spectra and d.r.s. and d.e.a. measurements at atmos-pheric pressure were made as described previo~sly.~~~-1~A specially constructed furnace housing was used with thePerkin-Elmer DSC 1B for d.e.a. measurements at 1 Torr.The instrument was calibrated at the m.p. of indium andtin but the temperatures reported are less precise than thosea t atmospheric pressure. No enthalpy data are reported a t1 Torr. The Perkin-Elmer TGS 1 thermobalance was usedin a modified form. The difficulties in correlating the d.r.s.measurements with those from d.e.a. and t.g. have been dis-cussed.* The heating rates used in this work were 5-33 I<min-1 for d.e.a. and t.g. and ca. 2 K min-1 for d.r.s.We thank Dr. T. J. R. Weakley for useful discussions and[1/2255 Received, 29th November, 19711the S.R.C. for the provision of apparatus