J. CHEM. SOC. DALTON TRANS. 1987 2089 Thermal Investigation and Stereochemical Studies of Some Cyclic Diamine Complexes of Nickel(fi), Zinc(ii), and Cadmium(ii) in the Solid State Langonjam Kanhai Singh and Samiran Mitra Department of Chemistry, Manipur University, Canchipur, lmphal- 795003, India Nickel(ii), zinc(ii), and cadmium(ii) complexes of piperazine (pipz), N-methylpiperazine (mpipz), and 1,4-diazacycloheptane (dach) with the compositions [NiL,( NCS),], [Ni(dach),] [SCN],, [ZnL(NCS),], [Zn(dach),] [SCN],, [CdL(NCS),], and [Cd(dach)(NCS),] (L = pipz or mpipz) have been synthesised. Attempts to prepare N,N'-dimethylpiperazine complexes failed. Some intermediate complexes were isolated by pyrolysis. Configurational and conformational changes have been studied by elemental analyses, i.r.spectra, magnetic moment measurements, and thermal analysis. All the complexes of pipz and mpipz appear to be octahedral and those of dach to be square planar. Activation energies (Ea), enthalpy (AH) and entropy changes (AS) for the dehydration and decomposition reactions show that the order of stability of the complexes (with respect to EJ follows the trend pipz > mpipz > dach. A linear correlation has been found between E, and AS for the decomposition of the nickel complexes. Acyclic diamines having the N(CH,),N grouping act as chelating agents for transition-metal ions. ' 7 , Work on cyclic diamine complexes is scanty. 1,3 There has been little thermal investig- ation of solid cyclic diamine complexes. The main aim of our work is to synthesize some cyclic diamine (six- or seven- membered ring) complexes of transition and non-transition metals, and study stereochemical changes during thermal decomposition.In addition to six-membered cyclic diamine ligands, we have studied a seven-membered cyclic diamine to see whether the strain in the ligand could be reduced by intro- ducing a methylene group between the amine functions,' but have failed to draw any definite conclusion on this point. Before heating, the pipz, mpipz, and dach ligands in the Ni" complexes and the dach ligand in the complexes of Zn" and Cd" function as bidentate chelating agents (boat and in the remaining pipz and mpipz complexes of Zn" and Cd" the ligands are bridging and bidentate (chair f ~ r m ) . ~ , ~ , ' If these complexes are heated under non- isothermal conditions they decompose via stable intermediates in which the cyclic diamine ligands may function as bridging bidentate ligands (chair form).This kind of conformational change of the ligand (boat form-chair form) has been confirmed by the i.r. spectral data.3*6 Thiocyanate in these complexes functions as a unidentate ligand8 but more usually as a bridging bidentate ligand.8-'2 Parameters like E,, AH, and A S for the dehydration and decomposition reactions of the complexes in the solid state have been calculated. Experimental Materials and Methods.-All metal salts were of A.R. grade and used as received. Metal thiocyanates were freshly prepared by mixing alcoholic solutions of metal salts and potassium thiocyanate and subsequent crystallization from the filtrates obtained.Piperazine obtained from Merck (Germany), N- methylpiperazine, N,N'-dimethylpiperazine, and 1,6diaza- cycloheptane obtained from Fluka (Switzerland) were used as received. Diethyl ether and ethanol were dried by standard procedures. ' Preparation of the Complexes.-[NiL,(NCS),] (L = pipz or mpipz). The ligand (ca. 6 mmol) in dry ethanol (20 cm3) was added with constant stirring to a dry ethanolic solution (35 U Me I H piperazine N - methylpiperazine 1,4 - diazacycloheptane ( pipz) ( mpipz) ( dach ) cm3) containing freshly prepared nickel thiocyanate (ca. 3 mmol). The blue nickel complex was collected by filtration, washed carefully with dry diethyl ether, and dried over fused calcium chloride in a desiccator. Yield ca.70%. The complex [Ni(dach),][SCN], was prepared similarly. [ZnL(NCS),] and [Zn(dach),][SCNl, (L = pipz or mpipz). A clear solution of freshly prepared zinc thiocyanate (ca. 3 mmol) in dry ethanol (35 cm3) was treated with the ligand to give a turbid solution. An excess of the ligand in dry ethanol (20 cm3) was then added till a clear solution was obtained. On addition of an excess of dry diethyl ether a cream precipitate of the zinc complex appeared. It was collected by filtration, washed with dry diethyl ether, and dried over fused calcium chloride in a desiccator. Yield ca. 40-50%. [CdL(NCS),] (L = pipz, mpipz, or dach). Freshly prepared cadmium thiocyanate (3 mmol) in dry ethanol (35 cm3) was treated with the ligand (ca. 3-4 mmol in 20 cm3 of dry ethanol) to give a white precipitate of the cadmium complex which was collected by filtration, washed with the dry ethanol followed by a little dry diethyl ether, and dried over fused calcium chloride in a desiccator.Yield ca. 60%. Nickel, zinc, and cadmium were estimated gravimetrically by standard proced~res,'~ C, H, and N by a Perkin-Elmer 240 C elemental analyser. Elemental analyses are given in Table 1. Thermal investigations (t.g:a. and d.t.a.) was carried out on a Shimadzu DT-30 thermal analyzer under a nitrogen atmos- phere, with a heating rate of 10°C min-' and a-alumina as a standard. Indium metal was used as a calibrant for the evaluation of enthalpy changes. Infrared spectra were recorded with Beckmann IR 20A and Perkin-Elmer 783 spectrometers, in KBr as a medium.The effective magnetic moments were evaluated from magnetic susceptibility measurements with an EG and G PAR 155 vibrating-sample magnetometer at room temperature.2090 J. CHEM. SOC. DALTON TRANS. 1987 Table 1. Analytical data (calculated values in parentheses) for piperazine (L'), N-methylpiperazine (L'), and 1,4-diazacycloheptane (L3) complexes of Ni", Zn", and Cd" Analysis/% Compound (la) [NiL',(NCS),] (2a) [NiL22(NCS)2]-2H,0 (3a) [NiL3 '3 [ SCN] ,-2H,O (4a) [ZnL'(NCS),]*H,O (5a) [ZnL2(NCS),] (7a) [CdL' (NCS),] (8a) [CdL'(NCS),] (9a) [CdL'(NCS),]*H,O (Ib) CNi2L13(NCS)41 (2c) "i2L2,(NCS),I (6a) CZnL321CSCN1, r Colour Blue Bluish Bluish Bluish Yellow White White Cream White White White M 16.9 (16.95) 19.3 (19.35) 14.2 (14.3) 21.4 (21.35) 14.3 (14.3) 22.8 (22.9) 23.2 (23.2) 17.15 (17.15) 35.75 (35.75) 34.25 (34.2) 32.2 (32.45) C 34.5 (34.6) 33.6 (33.6) 35.1 (35.05) 30.6 (30.6) 35.1 (35.05) 25.25 (25.25) 29.8 (29.85) 37.75 (37.75) 22.9 (22.9) 25.6 (25.6) 24.6 (24.25) H 5.80 (5.75) 2.95 (2.95) 6.80 (6.80) 4.35 (4.35) 6.80 (6.80) 4.20 (4.20) 4.25 (4.25) 6.30 (6.30) 3.15 (3.20) 3.65 (3.65) 4.00 (4.05) N Peff. 24.2 (24.25) 3.26 23.0 (23.05) 3.08 20.4 (20.45) 3.08 20.45 (20.45) 19.6 (19.65) 19.95 (19.9) 22.0 (22.05) 17.8 (17.8) 17.0 (1 7.05) 16.05 (16.15) 20.35 (20.4) 2.22 H e a t -L( Piperazinc) 1 M=Zn or Cd (4a)A 5a) (7a),and (8a) Scheme. Results and Discussion [Ni(pipz),(NCS),] (la).-This complex was reported earlier by Mel'nik et al.'' who found that it exists in the dimeric form. On heating, we found that it first loses one molecule of the ligand in the temperature range 200-255 "C.The correspond- ing d.t.a. curve shows one exotherm with a peak at 255 "C. The intermediate product [Ni2L3(NCS)J (1 b) (Scheme) is stable over the range 255-300 "C, but loses ligand in the range 300- 320 "C showing two exothermic (d.t.a.) peaks at 305 and 318 "C and giving Ni(SCN), (Figure). The parameter E, has been evaluated from the t.g.a. curve using Horowitz and Metzger's equation l 6 and the d.t.a. curve by Borchardt and Daniels' equation. '' The values for the conversion of complex (la) into (lb) from the t.g.a. and d.t.a. curves are 183 and 259 kJ mol-' respectively and that for the conversion of (lb) into Ni(SCN), from the t.g.a. curve is 742 kJ mol-'. The latter high value (Table 2) may be due to the polymeric nature6*' 1 v 1 8 * 1 9 of complex (lb) as compared with (la).For the first step, AH is found to be 21 kJ mol-', and AS, evaluated from AH/T, where T, = d.t.a. peak temperature in K,,' is 39 J K-' mol-'. In the blue dimeric complex (la), the ligand functions as a chelating agent in the boat form as shown by the appearance of more i.r. bands between 700 and 1 400 cm-' (Table 3) than for the free ligand which exists in the chair f ~ r m . ~ . ~ . ~ ' Thiocyanate acts as a bridging bidentate ligand as shown by the very strong band of v(CN) at 2 120 cm-'. Complex (lb) has an octahedral structure as indicated by the value of its magnetic moment (Table 1) and characteristic i.r. bands showing that the ligand is both bridging bidentate and chelating (Table 3).The thio- cyanate is also both bridging bidentate and terminal unidentate, as shown by the bands 4 9 8 at 2 140 and 2 080 cm-' for v(CN) and 480 cm-' for G(NCS). The decomposition path and structure of complexes (la) and (lb) are given in the Scheme. [Ni(mpipz),(NCS),].2H2O (2a).-This complex was not reported earlier. It has two molecules of lattice water as con- firmed by i.r. spectral bands at 3 440, 3 260 [v(OH)] and 1 670 cm-' [G(HOH)]. Further the weight loss in the t.g.a. curve of complex (2a) in the range 10@-140"C and the endothermic peak (d.t.a.) at 130 "C (Table 2) correspond to two molecules of lattice water. The complex is expected to be dimeric'2*'8 like complex (la). The anhydrous complex [Ni(mpipz),- (NCS),] (2b) is converted into Ni(SCN), via the formation of [Ni(mpipz)(NCS),] (2c) in two steps in the ranges 140-210 and 210-290 "C respectively.The corresponding d.t.a. curveJ. CHEM. SOC. DALTON TRANS. 1987 209 1 Endo. I Exo . 200 'C * I /-: I \ I \ I I d.t.a. / ---' \,,, \ \ \ \ \ \ 2l;c 160'C \.. \.. \ <..-.. -.. Figure. Thermal decomposition curves of 12.26 mg [Ni(pipz),(NCS),] (-), 12.71 mg [Ni(mpipz),(NCS),].2H20 (- - -), and 9.68 mg [Ni(dach),][SCN],.2H,O (- * - * . -1 Table 2. Thermal parameters of cyclic diamine complexes of Ni", Zn", and Cd" (ligands as in Table 1) Decomposition reaction [NiL',(NCS),] - [NIL' 1.5(NCS)2] [NiL',,5(NCS),] - Ni(SCN), [NiL2,(NCS),]*2H20 - [NiL2,(NCS),] [NiL2,(NCS),] - [NiL2(NCS),] [NiL2(NCS),] - Ni(SCN), [NiL3,][SCN],-2H,0 - [NiL3,][SCN], cis-[NiL',][SCN] , - tvuns-[NiL3,] [SCN] , trans-[NiL3 ,] [SCN] , - Ni(SCN), [ZnL'(NCS),]-H,O - [ZnL'(NCS),] [ZnL'(NCS),] - Zn(SCN), [ZnL2(NCS),] - [ZnLZo.,(NCS),] [ZnL2,,,(NCS),] - Zn(SCN), [ZnL',] [SCN] , - [ZnL3(NCS),] [ZnL3(NCS),] - Zn(SCN), [CdL'(NCS),] - Cd(SCN), [CdLz(NCS),] - [CdL20.5(NCS)2] [Cd Lz o. (NCS) ,] - Cd(SCN) , [Cd L3( NCS),].H ,O --+ [CdL3(NCS) 21 [CdL3(NCS),] - Cd(SCN), * For evaluation of AS, 527 K is considered.Temperature range ("C) 200-255 300-320 100-140 140-210 2 10-290 160-210 210-230 230-490 120-220 2-0 6 0 - 169 1 6 9 4 0 0 40-227 2 2 7 - 4 10 2-20 150-260 26&295 140-245 245-320 Peak temperature ("C) - Endo. Exo. 255 305, 318 130 210 250. 285 195 220 247 300,430 150 280 300 160 250, 335 125 170 255 270 280 218, 240 254 28 5 236 245 EJkJ mol-' - t.g.a.d.t.a. 183 259 742 172 144 138 146 183 42 18 75 39 203 99 14 62 146 81 82 296 186 96 49 ASIJ K-' AHlkJ mol-I mol-' 21 39 61, 206 106, 349 112 278 71 147 27,46 52, 82 14 64 147 34 64* 17 30 66 128 shows one exothermic peak at 210 "C for the second step (Figure). In complex (2a) the ligand functions as a chelate and exists in the boat form [as in complex (la)] while the thiocyanate acts as a bridging bidentate ligand as shown by the band* at 2 100 cm-I (Table 3). The blue colour of the complex and its magnetic moment value indicate an octahedral structure (Scheme). The intermediate complexes (2b) and (2c) were unstable.2092 J. CHEM. SOC. DALTON TRANS. 1987 Table 3. 1.r. spectral data (cm-') for cyclic diamine complexes of Nil', Zn", and Cd" (ligands as in Table 1)* Stretching vibrations of skeleton v(C-N) + v(C-C) 1 090vs 1 080 (sh) 1 020 (sh) 1005s 990vs 1 l00m 1 070s 1015vs 995m 1100s 1040m 1025s 1 Ol5vw 990vs 1105s 1040m 1025s 1 015 (sh) 990vs W H , ) + PJNH2) + C H , ) 1330m 1 250w 1 130m 1 320w 1250vw 1300w 1280m 1 205 (sh) 1 190vs 1 180s 1 150s 1 320w 1305vw 1 280m 1 200 (sh) 1 190vs 1 180s 1 150vs PKH2) + 870s 560w 800m 740vw 680w v(CS) v(MN) Com- pound v(NH,) (la) 3420br 3 220s 3 200 (sh) WH2) W N ) 298Ow 2 120vs 2 9% 2 950 (sh) 2 940w W H J 1650br 1640br 1670m 1670br I 520w 1510w 1 620br 164ow 1610w 1590br 1550m 1530w 1 570m WH,) PJCH2) 1455s 1385vs f44Os 1350vw P,(CH,) 940w 900w 875 (sh) 905w 880vs 940w 910w 940 (sh) 91 5m 870s G(NCS) 480w 480w 480w 475w 480m 470w 495vw 470ms 490 (sh) 475m (lb) 3440br 3 24Om 3000w 2 140vs 2 94Ovw 2 080 (sh) 2 920vw 1450m 1390m 1425m 1340vw 835w 560w 780m 530vw 720vw 650vw 6OOW 870vs 510 (sh) 785 (sh) 500 (sh) 780s 710vw 650m (2a) 344Ovs 326Om 2 9 8 0 ~ 2100vs 2 960s 2 820 (sh) 2 81Om 2 74ow 1 460 (sh) 1 380w 1455s 1370m 1450m 144ow 1 420m (2b) 3 440s 3 270vw 3 250vw 2 98Ow 2 130 (sh) 2960s 2lOOvs 2 900vw 2 86Ow 2 81Om 2 780 (sh) 2 740w 2 700vw 296Om 2 17Ow 2 740 (sh) 2 1 lOvs 2 72Om 2 700 (sh) 1450vs 1380w 1440w 1370m 1 420m 790 (sh) 585vw 780ms 520 (sh) 710br 500m 650m (2c) 3440br 3 23Om 1 460 (sh) 1 370vw 1450vs 1 340vw 1430w 1420w 1320vw 1 270w 1210vw 1 200vw 1 180vw 1 150 (sh) 1 310 (sh) 1 300 (sh) 1290vs 1 270 (sh) 1 240m 1235 (sh) 1 220m 1335m 1 265m 1 150vs 1 120s 1105w 1070vw 1050vw 1 030w 1 OOOm 1 134vs 1 128 (sh) 1 115s 1 100 (sh) 1 060vs 1020w 1085s 1025s 1 mow 995w 095vs 070 (sh) 055vs 0 1 ovs 095vs 080vs 070 (sh) 060vw 040vs 980vs 900w 880 (sh) 865vs 980vs 960 (sh) 950 (sh) 940w 920vw 900 (sh) 890ms 880 (sh) 860vw 950w 895m 885 (sh) 840vw 815vw 775m 770 (sh) 730vw 700vw 670vw 650w 620w 845ms 830vw 780vs 755ms 720vw 660vw 645 (sh) 633ms 870vs 830m 670w 580w 550w 520w 600w 550ms 540 (sh) 530w 565vw 550vw 525w (3a) 3460br 3 160vs 2950w 2050vs 2 918w 2 878w 1490vw 1385w 1 460 (sh) 1 375w 1 450 (sh) 1 360vs 1440vs 1 350 (sh) 1 430 (sh) 1 335vw 1420 (sh) (4a) 3450br 3 130m 2990vw 2 l00vs 2 950w 2 940 (sh) 2 860w 2980s 2 100(sp 2940w 2040(sh 2 920 (sh) 2 860w 2 840vw 2960w 2 lOOvs 2 924m 2 920 (sh) 2 m v w 2 860w 1450vs 1390w 1430m 1 380w 1 360w 940w 895w 910w 880 (sh) 875vs 770w 635w 990 (sh) 860w 980vs 840vs 960w 830 (sh) 930vs 770w 910vw 745w 505vw 575vw 470m 550br 460m (Sa) 3450br 3 240vs 1460 (sh) 1 375w 1445s 1430vs 650w 690w 670vw 650w 630vw 610vw 1330w 1 250m 1335vw 1325vw 1 310vw 1290vw 1275 (sh) (6a) 3 500br 3 240vs 3 230 (sh) 1465w 1365m I460m 1345vw 1450w 1 440ms 1430vw 1425vw 1 415vw 590w 490vw 570vw 465m 550vw 455m 540w 530 (sh) 1550s 1 520m 1260vw 1020 (sh) 5 l0vw 1 240w 1210m 1135vs 1 125 (sh) 1330m 1095s 990s 890(sh) 550vw 470ms 1250ms 1070 (sh) 940w 885 (sh) 520vw 455ms 1050s 910w 875s 1 OlOVS 765m 635m (7a) 3480br 3 240s 2980s 2100vs 2 940m 2 060 (sh) 2 900 (sh) 2 86Om 2 840 (sh) I 670 (sh) 1 620 (sh) 1 590 (sh) 1 560br 1445s 1375w 1430ms 1 345vwJ.CHEM. SOC. DALTON TRANS. 1987 Table 3 (conrinued) 2093 Com- pound v(NH,) v(CH,) (8a) 3460br 3000vw 2 950vw 2 930vw 2 870vw 325Om 2980m (9a) 3460br 2940m 3230br 2860w V(CN) W H , ) WH,) 2 llOvs 1690vw 147Om 2 060 (sh) 1 670vw 1 455m 1640vw 1440m 1620br 1415w 2 075vs (sp) 1 550s 1 475 (sh) 1520w 1450m 1 515 (sh) 1420ms 1 490w Stretching vibrations .r(NH,) + of skeleton PJNH2) + 4C-N + P"(CH2) WH2) W-C) PSH2) 1370w 1330vw 1 l00m 995w 1355vw 1280w 1045 (sh) 950s 1180w 104Ow 910(sh) 1145w 1015m 1370w 1300br 1060vw 980 (sh) 1 360w 1290 (sh) 1050br 975 (sh) 1 340 (sh) 1 270 (sh) 1 04Ovw 970 (sh) 1250 (sh) 1020w 965m 1230w 960 (sh) 1145w 910vw 1 130w 900vw 1 115w 1 ll0vw W H * ) + v(CS) v(MN) G(NCS) 890m 530w 470w 840s 460 (sh) 78Om 770w 650w 880vw 550br 475m 855vw 520br 830w 800w 785 (sh) 780w * For complexes containing H,O molecules bands for v(NH,) and 6(NH,) overlapped with v(0H) and 6(HOH), respectively, in some cases.vs = very strong, s = strong, m = medium, br = broad, w = weak, sh = shoulder, and sp = split. However, they could be isolated at 140 and 210 "C respectively by keeping the rate of heating at 1 "C min-' in the respective temperature ranges. In both the complexes the ligand exists in the boat form3--' as shown by the i.r. spectral data (Table 3) and the probable structures are shown in the Scheme. (2b), (2b) + (2c), and (2c) - Ni(SCN), are shown in Table 2. Complex (la) is more stable than (2a); N-alkylation * of the ligand might have been expected to increase stability due to the increased basicity but this is offset by steric [Ni(dach),][SCN],*2H20 (3a).-This complex was not reported earlier.It is yellow, diamagnetic, and has a planar geometry. A purple complex (expected to be a tris complex ' and existing in solution) was observed when the bluish filtrate obtained from the separation of complex (3a) was treated with an excess of the ligand but it could not be isolated. Complex (3a) has two molecules of lattice water as in (2a). After dehydration it is converted into [Ni(dach),][SCN], (3b). The latter under- goes a transition in the range 21Ck230 "C (Figure) and then is converted into Ni(SCN), in a single step between 230 and 490 "C. As the dehydration peak and the peak for cis - trans transformation merge it is not possible to evaluate AH and A S for each step. In complex (3a) the ligand (dach) functions as a chelate in the boat conformation3.5 and as shown by the i.r. data (Table 3).1.r. evidence suggests that the complex has the cis configur- a t i ~ n . , ~ - ~ ~ The transition at 220 "C in the d.t.a. curve where there is no weight loss in the t.g.a. curve could then correspond to a change to a trans structure.34 The values of E, for the conversions (2a) [Zn(pipz)(NCS),]-H,O (4a) and [Cd(pipz)(NCS),] (7a).- These complexes were reported by Grecu et al.35 Our thermal investigation has confirmed that the lattice water in (4a) (Tables 2 and 3) is lost in the range 120-220 "C. The decompositions of [Zn(pipz)(NCS),] (4b) and [Cd(pipz)(NSC),] (7a) into the corresponding metal thiocyanates take place in single steps as reflected by the t g a . curves in the ranges 24-40 and 20-20 ' C respectively but their d.t.a.curves show multiple peaks (Table 2). In complexes (4a) and (7a) both piperazine (in the chair form) and thiocyanate are bridging bidentate 3,6*7 as indicated by *The same trend is also observed in complexes of Zn" and Cd" (Table 2). their i.r. spectral bands (Table 3). They are probably poly- as shown in the Scheme. merit 3,6,18.19,36,37 [Zn(mpipz)(NCS),] (5a) and [Cd(mpipz)(NCS),] @a).- These white complexes were not reported earlier. Both decompose via the intermediates [ZnL,.,(NCS),] (5b) and [CdL,.,(NCS),] (8b) in the ranges 6Ck169 and 15+260 'C in t.g.a. and their corresponding d.t.a. curves show a single endothermic peak at 160 "C, and two endothermic peaks at 218 and 240 "C and one exothermic peak at 254 "C respectively.Complexes (5b) and (8b) decompose into the corresponding metal thiocyanates in the ranges 169-400 and 26&-295 "C. The structures of complexes (5a) and (8a) are probably similar to those of (4a) and (7a) (Scheme). [Zn(dach),][SCN], (6a) and [Cd(dach)(NCS),]*H,O (9a).-These complexes were not reported earlier. The former is cream while the latter is white with one molecule of lattice water. Complex (6a) after losing one ligand molecule in the range 40-227 "C is converted into [Zn(dach)(NCS),] (6b). Complex (9a) loses its water molecule in a single step (140- 245 "C) and is converted into [Cd(dach)(NCS),] (9b). Complexes (6b) and (9b) decompose into their corresponding metal thiocyanates in a single step in the ranges 2 2 7 4 1 0 and 245-320 "C respectively and the corresponding d.t.a.peak (exothermic) appears at 255 and 245 "C respectively. The i.r. spectral data (Table 3) show that the ligand (dach) in com- plexes (6a) and (9a) acts as a bidentate chelate in the boat form.3-' Further, these complexes may exist in the cis configur- a t i ~ n . ' " ' - ~ ~ Both may be similar in structure to complex (3a). If we consider the activation energy (evaluated from the t.g.a. curves), the order of stability of the complexes follows the trend pipz > mpipz > dach (Table 2). Further, a linear correlation is observed upon plotting E, versus AS for the decomposition reactions of the pipz and mpipz complexes of Ni". A system having a higher entropy change will require less energy, E,, for its thermal decomposition.20 Acknowledgements We thank the Indian Association for the Cultivation of Science, Calcutta for instrumental help and the Goverment of Manipur for financial support (to L.K. S.) under the Faculty Improvement Programme.2094 J. CHEM. SOC. DALTON TRANS. 1987 References 1 W. K. Musker and M. S. Hussain, Inorg. Chem., 1969, 8, 528. 2 G. De P. K. Biswas and N. Ray Chaudhuri, J. Chem. Soc., Dalton 3 P. J. Hendra and D. B. Powell, J. Chem. Soc., 1960, 5105. 4 R. A. Walton, J. Chem. Soc. A, 1967, 1852. 5 G. W. A. Fowles, D. A. Rice, and R. A. Walton, J. Inorg. Nucl. Chem., 1969, 31, 3119. 6 G. W. A. Fowles, D. A. Rice, and R. A. Walton, J. Chem. Soc. A, 1968, 1842. 7 G. W. A. Fowles, D. A. Rice, and R. A. Walton, Spectrochim. Acta, Part A , 1970, 26, 143. 8 K.Nakamoto, ‘Infrared and Raman Spectra of Inorganic and Coordination Compounds,’ 3rd edn., Wiley-Interscience, New York, 1978, pp. 208, 209, 270, and 274. 9 J. L. Cox, W. Marprida, H. Stockton, and J. Howatrou, J. Inorg. Nucl. Chem., 1976, 38, 1217. 10 B. W. Dockum, G. A. Eisman, E. H. Wilten, and W. M. Reiff, Inorg. Chem., 1983, 22, 150. 11 G. De and N. Ray Chaudhuri, Bull. Chem. Soc. Jpn., 1985,58, 715. 12 R. J. H. Clark and C. S. Williams, Spectrochim. Acta, 1966, 22, 1081. 13 A. I. Vogel, ‘A Text Book of Practical Organic Chemistry,’ 4th edn., ELBS and Longman, London, 1980, pp. 269 and 272. 14 A. I. Vogel, ‘A Text Book of Quantitative Inorganic Analysis,’ 3rd edn., ELBS and Longmans, London, 1968, pp. 389, 390, and 480. 15 G. S. Mel’nik, M. V. Artemenko, E. S.Sereda, and P. A. Suprumenko, Wkr. Khim. Zh. (Russ. Ed.), 1981, 47, 590. 16 H. H. Horowitz and G. Metzger, Anal. Chem., 1963, 35, 1464. 17 H. J. Borchardt and F. Daniels, J. Am. Chem. SOC., 1957, 79, 41. 18 C. Postmus, J. R. Ferraro, A. Quattrochi, K. Shobatake, and Trans., 1984, 259 1. K. Nakamoto, Inorg. Chem., 1969, 8, 1851. 19 M. Goldstein and W. D. Unsworth, Inorg. Chim. Acta, 1970, 4, 342. 20 R. Roy, M. Chaudhury, S. K. Mondal, and K. Nag, J. Chem. Soc., 21 F. G. Mann and H. R. Watson, J. Chem. Soc., 1958, 2772. 22 F. Basolo and R. K. Murmann, J. Am. Chem. Soc., 1952, 74, 5343. 23 F. Basolo and R. K. Murmann, J. Am. Chem. SOC., 1954, 76, 211. 24 H. Irving and J. M. M. Griffiths, J. Chem. Soc., 1954, 213. 25 J. E. Huheey, ‘Inorganic Chemistry,’ 3rd edn., Harper International, 26 W.K. Musker and M. S. Hussain, Inorg. Chem., 1966, 5, 1416. 27 M. E. Baldwin, J. Chem. Soc., 1960, 4369. 28 J. A. McLean, A. F. Schreiner, and A. F. Laethem, J. Inorg. Nucl. 29 J. M. Rigg and E. Sherwin, J. Inorg. Nucl. Chem., 1965, 27, 653. 30 M. N. Hughesand W. R. McWhinnie, J. Inorg. Nucl. Chem., 1966,28, 31 S. Kida, Bull. Chem. Soc. Jpn., 1966, 39, 2415. 32 E. B. Kipp and R. A. Haines, Can. J. Chem., 1969, 47, 1073. 33 D. A. Buckingham and D. Jones, Inorg. Chem., 1965,4, 1387. 34 F. Basolo and R. G. Pearson, ‘Mechanism of Inorganic Reactions,’ 35 1. Grecu, E. Curea, and M. Pitis, Acad. Repub. Pop. Rom., Fil. Chj, 36 R. D. Willet and R. E. Rundle, J. Chem. Phys., 1964, 40, 338. 37 J. Macicek, V. K. Trunov, and R. I. Machkhosvili, Zh. Neorg.Khim., Dalton Trans., 1984, 1681. New York, 1983, pp. 298 and 299. Chem., 1964, 26, 1245. 1659. 2nd edn., Wiley, New York, 1967, p. 424. Stud. Cercet. Chim., 1962, 13, 35. 1981, 26, 1690. Received 30th June 1986; Paper 611317 J. CHEM. SOC. DALTON TRANS. 1987 2089 Thermal Investigation and Stereochemical Studies of Some Cyclic Diamine Complexes of Nickel(fi), Zinc(ii), and Cadmium(ii) in the Solid State Langonjam Kanhai Singh and Samiran Mitra Department of Chemistry, Manipur University, Canchipur, lmphal- 795003, India Nickel(ii), zinc(ii), and cadmium(ii) complexes of piperazine (pipz), N-methylpiperazine (mpipz), and 1,4-diazacycloheptane (dach) with the compositions [NiL,( NCS),], [Ni(dach),] [SCN],, [ZnL(NCS),], [Zn(dach),] [SCN],, [CdL(NCS),], and [Cd(dach)(NCS),] (L = pipz or mpipz) have been synthesised.Attempts to prepare N,N'-dimethylpiperazine complexes failed. Some intermediate complexes were isolated by pyrolysis. Configurational and conformational changes have been studied by elemental analyses, i.r. spectra, magnetic moment measurements, and thermal analysis. All the complexes of pipz and mpipz appear to be octahedral and those of dach to be square planar. Activation energies (Ea), enthalpy (AH) and entropy changes (AS) for the dehydration and decomposition reactions show that the order of stability of the complexes (with respect to EJ follows the trend pipz > mpipz > dach. A linear correlation has been found between E, and AS for the decomposition of the nickel complexes. Acyclic diamines having the N(CH,),N grouping act as chelating agents for transition-metal ions.' 7 , Work on cyclic diamine complexes is scanty. 1,3 There has been little thermal investig- ation of solid cyclic diamine complexes. The main aim of our work is to synthesize some cyclic diamine (six- or seven- membered ring) complexes of transition and non-transition metals, and study stereochemical changes during thermal decomposition. In addition to six-membered cyclic diamine ligands, we have studied a seven-membered cyclic diamine to see whether the strain in the ligand could be reduced by intro- ducing a methylene group between the amine functions,' but have failed to draw any definite conclusion on this point. Before heating, the pipz, mpipz, and dach ligands in the Ni" complexes and the dach ligand in the complexes of Zn" and Cd" function as bidentate chelating agents (boat and in the remaining pipz and mpipz complexes of Zn" and Cd" the ligands are bridging and bidentate (chair f ~ r m ) .~ , ~ , ' If these complexes are heated under non- isothermal conditions they decompose via stable intermediates in which the cyclic diamine ligands may function as bridging bidentate ligands (chair form). This kind of conformational change of the ligand (boat form-chair form) has been confirmed by the i.r. spectral data.3*6 Thiocyanate in these complexes functions as a unidentate ligand8 but more usually as a bridging bidentate ligand.8-'2 Parameters like E,, AH, and A S for the dehydration and decomposition reactions of the complexes in the solid state have been calculated.Experimental Materials and Methods.-All metal salts were of A.R. grade and used as received. Metal thiocyanates were freshly prepared by mixing alcoholic solutions of metal salts and potassium thiocyanate and subsequent crystallization from the filtrates obtained. Piperazine obtained from Merck (Germany), N- methylpiperazine, N,N'-dimethylpiperazine, and 1,6diaza- cycloheptane obtained from Fluka (Switzerland) were used as received. Diethyl ether and ethanol were dried by standard procedures. ' Preparation of the Complexes.-[NiL,(NCS),] (L = pipz or mpipz). The ligand (ca. 6 mmol) in dry ethanol (20 cm3) was added with constant stirring to a dry ethanolic solution (35 U Me I H piperazine N - methylpiperazine 1,4 - diazacycloheptane ( pipz) ( mpipz) ( dach ) cm3) containing freshly prepared nickel thiocyanate (ca.3 mmol). The blue nickel complex was collected by filtration, washed carefully with dry diethyl ether, and dried over fused calcium chloride in a desiccator. Yield ca. 70%. The complex [Ni(dach),][SCN], was prepared similarly. [ZnL(NCS),] and [Zn(dach),][SCNl, (L = pipz or mpipz). A clear solution of freshly prepared zinc thiocyanate (ca. 3 mmol) in dry ethanol (35 cm3) was treated with the ligand to give a turbid solution. An excess of the ligand in dry ethanol (20 cm3) was then added till a clear solution was obtained. On addition of an excess of dry diethyl ether a cream precipitate of the zinc complex appeared. It was collected by filtration, washed with dry diethyl ether, and dried over fused calcium chloride in a desiccator.Yield ca. 40-50%. [CdL(NCS),] (L = pipz, mpipz, or dach). Freshly prepared cadmium thiocyanate (3 mmol) in dry ethanol (35 cm3) was treated with the ligand (ca. 3-4 mmol in 20 cm3 of dry ethanol) to give a white precipitate of the cadmium complex which was collected by filtration, washed with the dry ethanol followed by a little dry diethyl ether, and dried over fused calcium chloride in a desiccator. Yield ca. 60%. Nickel, zinc, and cadmium were estimated gravimetrically by standard proced~res,'~ C, H, and N by a Perkin-Elmer 240 C elemental analyser. Elemental analyses are given in Table 1. Thermal investigations (t.g:a. and d.t.a.) was carried out on a Shimadzu DT-30 thermal analyzer under a nitrogen atmos- phere, with a heating rate of 10°C min-' and a-alumina as a standard.Indium metal was used as a calibrant for the evaluation of enthalpy changes. Infrared spectra were recorded with Beckmann IR 20A and Perkin-Elmer 783 spectrometers, in KBr as a medium. The effective magnetic moments were evaluated from magnetic susceptibility measurements with an EG and G PAR 155 vibrating-sample magnetometer at room temperature.2090 J. CHEM. SOC. DALTON TRANS. 1987 Table 1. Analytical data (calculated values in parentheses) for piperazine (L'), N-methylpiperazine (L'), and 1,4-diazacycloheptane (L3) complexes of Ni", Zn", and Cd" Analysis/% Compound (la) [NiL',(NCS),] (2a) [NiL22(NCS)2]-2H,0 (3a) [NiL3 '3 [ SCN] ,-2H,O (4a) [ZnL'(NCS),]*H,O (5a) [ZnL2(NCS),] (7a) [CdL' (NCS),] (8a) [CdL'(NCS),] (9a) [CdL'(NCS),]*H,O (Ib) CNi2L13(NCS)41 (2c) "i2L2,(NCS),I (6a) CZnL321CSCN1, r Colour Blue Bluish Bluish Bluish Yellow White White Cream White White White M 16.9 (16.95) 19.3 (19.35) 14.2 (14.3) 21.4 (21.35) 14.3 (14.3) 22.8 (22.9) 23.2 (23.2) 17.15 (17.15) 35.75 (35.75) 34.25 (34.2) 32.2 (32.45) C 34.5 (34.6) 33.6 (33.6) 35.1 (35.05) 30.6 (30.6) 35.1 (35.05) 25.25 (25.25) 29.8 (29.85) 37.75 (37.75) 22.9 (22.9) 25.6 (25.6) 24.6 (24.25) H 5.80 (5.75) 2.95 (2.95) 6.80 (6.80) 4.35 (4.35) 6.80 (6.80) 4.20 (4.20) 4.25 (4.25) 6.30 (6.30) 3.15 (3.20) 3.65 (3.65) 4.00 (4.05) N Peff.24.2 (24.25) 3.26 23.0 (23.05) 3.08 20.4 (20.45) 3.08 20.45 (20.45) 19.6 (19.65) 19.95 (19.9) 22.0 (22.05) 17.8 (17.8) 17.0 (1 7.05) 16.05 (16.15) 20.35 (20.4) 2.22 H e a t -L( Piperazinc) 1 M=Zn or Cd (4a)A 5a) (7a),and (8a) Scheme.Results and Discussion [Ni(pipz),(NCS),] (la).-This complex was reported earlier by Mel'nik et al. '' who found that it exists in the dimeric form. On heating, we found that it first loses one molecule of the ligand in the temperature range 200-255 "C. The correspond- ing d.t.a. curve shows one exotherm with a peak at 255 "C. The intermediate product [Ni2L3(NCS)J (1 b) (Scheme) is stable over the range 255-300 "C, but loses ligand in the range 300- 320 "C showing two exothermic (d.t.a.) peaks at 305 and 318 "C and giving Ni(SCN), (Figure). The parameter E, has been evaluated from the t.g.a. curve using Horowitz and Metzger's equation l 6 and the d.t.a. curve by Borchardt and Daniels' equation.'' The values for the conversion of complex (la) into (lb) from the t.g.a. and d.t.a. curves are 183 and 259 kJ mol-' respectively and that for the conversion of (lb) into Ni(SCN), from the t.g.a. curve is 742 kJ mol-'. The latter high value (Table 2) may be due to the polymeric nature6*' 1 v 1 8 * 1 9 of complex (lb) as compared with (la). For the first step, AH is found to be 21 kJ mol-', and AS, evaluated from AH/T, where T, = d.t.a. peak temperature in K,,' is 39 J K-' mol-'. In the blue dimeric complex (la), the ligand functions as a chelating agent in the boat form as shown by the appearance of more i.r. bands between 700 and 1 400 cm-' (Table 3) than for the free ligand which exists in the chair f ~ r m . ~ . ~ . ~ ' Thiocyanate acts as a bridging bidentate ligand as shown by the very strong band of v(CN) at 2 120 cm-'.Complex (lb) has an octahedral structure as indicated by the value of its magnetic moment (Table 1) and characteristic i.r. bands showing that the ligand is both bridging bidentate and chelating (Table 3). The thio- cyanate is also both bridging bidentate and terminal unidentate, as shown by the bands 4 9 8 at 2 140 and 2 080 cm-' for v(CN) and 480 cm-' for G(NCS). The decomposition path and structure of complexes (la) and (lb) are given in the Scheme. [Ni(mpipz),(NCS),].2H2O (2a).-This complex was not reported earlier. It has two molecules of lattice water as con- firmed by i.r. spectral bands at 3 440, 3 260 [v(OH)] and 1 670 cm-' [G(HOH)]. Further the weight loss in the t.g.a. curve of complex (2a) in the range 10@-140"C and the endothermic peak (d.t.a.) at 130 "C (Table 2) correspond to two molecules of lattice water. The complex is expected to be dimeric'2*'8 like complex (la).The anhydrous complex [Ni(mpipz),- (NCS),] (2b) is converted into Ni(SCN), via the formation of [Ni(mpipz)(NCS),] (2c) in two steps in the ranges 140-210 and 210-290 "C respectively. The corresponding d.t.a. curveJ. CHEM. SOC. DALTON TRANS. 1987 209 1 Endo. I Exo . 200 'C * I /-: I \ I \ I I d.t.a. / ---' \,,, \ \ \ \ \ \ 2l;c 160'C \.. \.. \ <..-.. -.. Figure. Thermal decomposition curves of 12.26 mg [Ni(pipz),(NCS),] (-), 12.71 mg [Ni(mpipz),(NCS),].2H20 (- - -), and 9.68 mg [Ni(dach),][SCN],.2H,O (- * - * . -1 Table 2. Thermal parameters of cyclic diamine complexes of Ni", Zn", and Cd" (ligands as in Table 1) Decomposition reaction [NiL',(NCS),] - [NIL' 1.5(NCS)2] [NiL',,5(NCS),] - Ni(SCN), [NiL2,(NCS),]*2H20 - [NiL2,(NCS),] [NiL2,(NCS),] - [NiL2(NCS),] [NiL2(NCS),] - Ni(SCN), [NiL3,][SCN],-2H,0 - [NiL3,][SCN], cis-[NiL',][SCN] , - tvuns-[NiL3,] [SCN] , trans-[NiL3 ,] [SCN] , - Ni(SCN), [ZnL'(NCS),]-H,O - [ZnL'(NCS),] [ZnL'(NCS),] - Zn(SCN), [ZnL2(NCS),] - [ZnLZo.,(NCS),] [ZnL2,,,(NCS),] - Zn(SCN), [ZnL',] [SCN] , - [ZnL3(NCS),] [ZnL3(NCS),] - Zn(SCN), [CdL'(NCS),] - Cd(SCN), [CdLz(NCS),] - [CdL20.5(NCS)2] [Cd Lz o.(NCS) ,] - Cd(SCN) , [Cd L3( NCS),].H ,O --+ [CdL3(NCS) 21 [CdL3(NCS),] - Cd(SCN), * For evaluation of AS, 527 K is considered. Temperature range ("C) 200-255 300-320 100-140 140-210 2 10-290 160-210 210-230 230-490 120-220 2-0 6 0 - 169 1 6 9 4 0 0 40-227 2 2 7 - 4 10 2-20 150-260 26&295 140-245 245-320 Peak temperature ("C) - Endo.Exo. 255 305, 318 130 210 250. 285 195 220 247 300,430 150 280 300 160 250, 335 125 170 255 270 280 218, 240 254 28 5 236 245 EJkJ mol-' - t.g.a. d.t.a. 183 259 742 172 144 138 146 183 42 18 75 39 203 99 14 62 146 81 82 296 186 96 49 ASIJ K-' AHlkJ mol-I mol-' 21 39 61, 206 106, 349 112 278 71 147 27,46 52, 82 14 64 147 34 64* 17 30 66 128 shows one exothermic peak at 210 "C for the second step (Figure). In complex (2a) the ligand functions as a chelate and exists in the boat form [as in complex (la)] while the thiocyanate acts as a bridging bidentate ligand as shown by the band* at 2 100 cm-I (Table 3). The blue colour of the complex and its magnetic moment value indicate an octahedral structure (Scheme). The intermediate complexes (2b) and (2c) were unstable.2092 J.CHEM. SOC. DALTON TRANS. 1987 Table 3. 1.r. spectral data (cm-') for cyclic diamine complexes of Nil', Zn", and Cd" (ligands as in Table 1)* Stretching vibrations of skeleton v(C-N) + v(C-C) 1 090vs 1 080 (sh) 1 020 (sh) 1005s 990vs 1 l00m 1 070s 1015vs 995m 1100s 1040m 1025s 1 Ol5vw 990vs 1105s 1040m 1025s 1 015 (sh) 990vs W H , ) + PJNH2) + C H , ) 1330m 1 250w 1 130m 1 320w 1250vw 1300w 1280m 1 205 (sh) 1 190vs 1 180s 1 150s 1 320w 1305vw 1 280m 1 200 (sh) 1 190vs 1 180s 1 150vs PKH2) + 870s 560w 800m 740vw 680w v(CS) v(MN) Com- pound v(NH,) (la) 3420br 3 220s 3 200 (sh) WH2) W N ) 298Ow 2 120vs 2 9% 2 950 (sh) 2 940w W H J 1650br 1640br 1670m 1670br I 520w 1510w 1 620br 164ow 1610w 1590br 1550m 1530w 1 570m WH,) PJCH2) 1455s 1385vs f44Os 1350vw P,(CH,) 940w 900w 875 (sh) 905w 880vs 940w 910w 940 (sh) 91 5m 870s G(NCS) 480w 480w 480w 475w 480m 470w 495vw 470ms 490 (sh) 475m (lb) 3440br 3 24Om 3000w 2 140vs 2 94Ovw 2 080 (sh) 2 920vw 1450m 1390m 1425m 1340vw 835w 560w 780m 530vw 720vw 650vw 6OOW 870vs 510 (sh) 785 (sh) 500 (sh) 780s 710vw 650m (2a) 344Ovs 326Om 2 9 8 0 ~ 2100vs 2 960s 2 820 (sh) 2 81Om 2 74ow 1 460 (sh) 1 380w 1455s 1370m 1450m 144ow 1 420m (2b) 3 440s 3 270vw 3 250vw 2 98Ow 2 130 (sh) 2960s 2lOOvs 2 900vw 2 86Ow 2 81Om 2 780 (sh) 2 740w 2 700vw 296Om 2 17Ow 2 740 (sh) 2 1 lOvs 2 72Om 2 700 (sh) 1450vs 1380w 1440w 1370m 1 420m 790 (sh) 585vw 780ms 520 (sh) 710br 500m 650m (2c) 3440br 3 23Om 1 460 (sh) 1 370vw 1450vs 1 340vw 1430w 1420w 1320vw 1 270w 1210vw 1 200vw 1 180vw 1 150 (sh) 1 310 (sh) 1 300 (sh) 1290vs 1 270 (sh) 1 240m 1235 (sh) 1 220m 1335m 1 265m 1 150vs 1 120s 1105w 1070vw 1050vw 1 030w 1 OOOm 1 134vs 1 128 (sh) 1 115s 1 100 (sh) 1 060vs 1020w 1085s 1025s 1 mow 995w 095vs 070 (sh) 055vs 0 1 ovs 095vs 080vs 070 (sh) 060vw 040vs 980vs 900w 880 (sh) 865vs 980vs 960 (sh) 950 (sh) 940w 920vw 900 (sh) 890ms 880 (sh) 860vw 950w 895m 885 (sh) 840vw 815vw 775m 770 (sh) 730vw 700vw 670vw 650w 620w 845ms 830vw 780vs 755ms 720vw 660vw 645 (sh) 633ms 870vs 830m 670w 580w 550w 520w 600w 550ms 540 (sh) 530w 565vw 550vw 525w (3a) 3460br 3 160vs 2950w 2050vs 2 918w 2 878w 1490vw 1385w 1 460 (sh) 1 375w 1 450 (sh) 1 360vs 1440vs 1 350 (sh) 1 430 (sh) 1 335vw 1420 (sh) (4a) 3450br 3 130m 2990vw 2 l00vs 2 950w 2 940 (sh) 2 860w 2980s 2 100(sp 2940w 2040(sh 2 920 (sh) 2 860w 2 840vw 2960w 2 lOOvs 2 924m 2 920 (sh) 2 m v w 2 860w 1450vs 1390w 1430m 1 380w 1 360w 940w 895w 910w 880 (sh) 875vs 770w 635w 990 (sh) 860w 980vs 840vs 960w 830 (sh) 930vs 770w 910vw 745w 505vw 575vw 470m 550br 460m (Sa) 3450br 3 240vs 1460 (sh) 1 375w 1445s 1430vs 650w 690w 670vw 650w 630vw 610vw 1330w 1 250m 1335vw 1325vw 1 310vw 1290vw 1275 (sh) (6a) 3 500br 3 240vs 3 230 (sh) 1465w 1365m I460m 1345vw 1450w 1 440ms 1430vw 1425vw 1 415vw 590w 490vw 570vw 465m 550vw 455m 540w 530 (sh) 1550s 1 520m 1260vw 1020 (sh) 5 l0vw 1 240w 1210m 1135vs 1 125 (sh) 1330m 1095s 990s 890(sh) 550vw 470ms 1250ms 1070 (sh) 940w 885 (sh) 520vw 455ms 1050s 910w 875s 1 OlOVS 765m 635m (7a) 3480br 3 240s 2980s 2100vs 2 940m 2 060 (sh) 2 900 (sh) 2 86Om 2 840 (sh) I 670 (sh) 1 620 (sh) 1 590 (sh) 1 560br 1445s 1375w 1430ms 1 345vwJ.CHEM. SOC. DALTON TRANS. 1987 Table 3 (conrinued) 2093 Com- pound v(NH,) v(CH,) (8a) 3460br 3000vw 2 950vw 2 930vw 2 870vw 325Om 2980m (9a) 3460br 2940m 3230br 2860w V(CN) W H , ) WH,) 2 llOvs 1690vw 147Om 2 060 (sh) 1 670vw 1 455m 1640vw 1440m 1620br 1415w 2 075vs (sp) 1 550s 1 475 (sh) 1520w 1450m 1 515 (sh) 1420ms 1 490w Stretching vibrations .r(NH,) + of skeleton PJNH2) + 4C-N + P"(CH2) WH2) W-C) PSH2) 1370w 1330vw 1 l00m 995w 1355vw 1280w 1045 (sh) 950s 1180w 104Ow 910(sh) 1145w 1015m 1370w 1300br 1060vw 980 (sh) 1 360w 1290 (sh) 1050br 975 (sh) 1 340 (sh) 1 270 (sh) 1 04Ovw 970 (sh) 1250 (sh) 1020w 965m 1230w 960 (sh) 1145w 910vw 1 130w 900vw 1 115w 1 ll0vw W H * ) + v(CS) v(MN) G(NCS) 890m 530w 470w 840s 460 (sh) 78Om 770w 650w 880vw 550br 475m 855vw 520br 830w 800w 785 (sh) 780w * For complexes containing H,O molecules bands for v(NH,) and 6(NH,) overlapped with v(0H) and 6(HOH), respectively, in some cases.vs = very strong, s = strong, m = medium, br = broad, w = weak, sh = shoulder, and sp = split. However, they could be isolated at 140 and 210 "C respectively by keeping the rate of heating at 1 "C min-' in the respective temperature ranges. In both the complexes the ligand exists in the boat form3--' as shown by the i.r. spectral data (Table 3) and the probable structures are shown in the Scheme. (2b), (2b) + (2c), and (2c) - Ni(SCN), are shown in Table 2.Complex (la) is more stable than (2a); N-alkylation * of the ligand might have been expected to increase stability due to the increased basicity but this is offset by steric [Ni(dach),][SCN],*2H20 (3a).-This complex was not reported earlier. It is yellow, diamagnetic, and has a planar geometry. A purple complex (expected to be a tris complex ' and existing in solution) was observed when the bluish filtrate obtained from the separation of complex (3a) was treated with an excess of the ligand but it could not be isolated. Complex (3a) has two molecules of lattice water as in (2a). After dehydration it is converted into [Ni(dach),][SCN], (3b). The latter under- goes a transition in the range 21Ck230 "C (Figure) and then is converted into Ni(SCN), in a single step between 230 and 490 "C.As the dehydration peak and the peak for cis - trans transformation merge it is not possible to evaluate AH and A S for each step. In complex (3a) the ligand (dach) functions as a chelate in the boat conformation3.5 and as shown by the i.r. data (Table 3). 1.r. evidence suggests that the complex has the cis configur- a t i ~ n . , ~ - ~ ~ The transition at 220 "C in the d.t.a. curve where there is no weight loss in the t.g.a. curve could then correspond to a change to a trans structure.34 The values of E, for the conversions (2a) [Zn(pipz)(NCS),]-H,O (4a) and [Cd(pipz)(NCS),] (7a).- These complexes were reported by Grecu et al.35 Our thermal investigation has confirmed that the lattice water in (4a) (Tables 2 and 3) is lost in the range 120-220 "C.The decompositions of [Zn(pipz)(NCS),] (4b) and [Cd(pipz)(NSC),] (7a) into the corresponding metal thiocyanates take place in single steps as reflected by the t g a . curves in the ranges 24-40 and 20-20 ' C respectively but their d.t.a. curves show multiple peaks (Table 2). In complexes (4a) and (7a) both piperazine (in the chair form) and thiocyanate are bridging bidentate 3,6*7 as indicated by *The same trend is also observed in complexes of Zn" and Cd" (Table 2). their i.r. spectral bands (Table 3). They are probably poly- as shown in the Scheme. merit 3,6,18.19,36,37 [Zn(mpipz)(NCS),] (5a) and [Cd(mpipz)(NCS),] @a).- These white complexes were not reported earlier. Both decompose via the intermediates [ZnL,.,(NCS),] (5b) and [CdL,.,(NCS),] (8b) in the ranges 6Ck169 and 15+260 'C in t.g.a.and their corresponding d.t.a. curves show a single endothermic peak at 160 "C, and two endothermic peaks at 218 and 240 "C and one exothermic peak at 254 "C respectively. Complexes (5b) and (8b) decompose into the corresponding metal thiocyanates in the ranges 169-400 and 26&-295 "C. The structures of complexes (5a) and (8a) are probably similar to those of (4a) and (7a) (Scheme). [Zn(dach),][SCN], (6a) and [Cd(dach)(NCS),]*H,O (9a).-These complexes were not reported earlier. The former is cream while the latter is white with one molecule of lattice water. Complex (6a) after losing one ligand molecule in the range 40-227 "C is converted into [Zn(dach)(NCS),] (6b). Complex (9a) loses its water molecule in a single step (140- 245 "C) and is converted into [Cd(dach)(NCS),] (9b).Complexes (6b) and (9b) decompose into their corresponding metal thiocyanates in a single step in the ranges 2 2 7 4 1 0 and 245-320 "C respectively and the corresponding d.t.a. peak (exothermic) appears at 255 and 245 "C respectively. The i.r. spectral data (Table 3) show that the ligand (dach) in com- plexes (6a) and (9a) acts as a bidentate chelate in the boat form.3-' Further, these complexes may exist in the cis configur- a t i ~ n . ' " ' - ~ ~ Both may be similar in structure to complex (3a). 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