Anionopentaaminecobalt(III) complexes with polyamine ligands

Inorganica Chimica Acta 288 (1999) 181 – 188

Anionopentaaminecobalt(III) complexes with polyamine ligands Part 30. The base hydrolysis kinetics and structure determination of some [CoCl(N)5]2 + complexes containing pyridine ligands Donald A. House a,*, Jane Browning b, Luc Zipper c, Werner Marty c,1 b

a Department of Chemistry, Uni6ersity of Canterbury, Christchurch, New Zealand Department of Chemistry, Uni6ersity of Victoria, PO Box 3065, Victoria, BC V8W 3V6, Canada c Institute of Chemistry, Uni6ersity of Neuchaˆtel, 2100 Neuchaˆtel, Switzerland

Received 7 October 1998; accepted 1 February 1999

Abstract The base hydrolysis rates (kOH, M − 1 s − 1, I=0.1 M, 25°C) have been measured for cis-[CoBr(en)2(py)]2 + (kOH = 3.33×103), cis-[CoBr(en)2(d5py)]2 + (3.34× 103), cis-[CoCl(en)2(4-NH2-py)]2 + (78.2), cis-[CoCl(en)2(4-N(Me)2-py)]2 + (77.0), cis-[CoCl(en)2(4-CNpy)]2 + (326), trans-[CoCl(tn)2(py)]2 + (3.87×103), pf-[CoCl(dien)(NH3)2]2 + (252) and pf-[CoCl(dien)(py)2]2 + (2.77 ×104) using pH-stat techniques. The crystal structures of pf-[CoCl(dien)(py)2](NO3)(ClO4) and p3f-[CoCl(2,3-tri)(py)2]ZnCl4 have been determined. The kOH values for py complexes are significantly greater than those observed for the analogous methylamine complexes and the rate enhancement is attributed to weak p[3d(Co)] “ p*[pyridine] back donation. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Kinetics and mechanism; Hydrolysis; Cobalt complexes; Amine complexes; Polyamine complexes

1. Introduction The base hydrolysis kinetics of Werner-type transition-metal complexes is a well studied area of investigation [1–4]. A conjugate base mechanism (SN1CB or DCB) is accepted widely [2,3] but the nature of the non-replaced ligands can have a significant effect on the reaction rate [5]. For a reaction of the type (1) kOH

[CoCl(N)5]2 + + OH − “ [Co(OH)(N)5]2 + +Cl −

versus ma-[CoCl(dien)(en)]2 + (kOH = 1.87× 104 M − 1 s − 1 at 25°C) [7]. The stereochemical change outlined results in the orientation of the sec-NH proton changing from folded to planar [5,7]2; (c) an aromatic conjugated heterocyclic amine ligand, e.g. pyridine, is introduced into the coordination sphere. It is an investigation of the factors that contribute to the latter effect that is the subject of this publication.

(1)

acceleratory effects are manifest when: (a) the steric bulk of N5 is increased, i.e. the change from (NH3)5 to (NH2Me)5 results in a 7 ×103 increase in the value of kOH [6]; (b) for an N5 polyamine (or combination of polyamines), the stereochemistry changes from fac to mer, i.e. pf-[CoCl(dien)(en)]2 + (kOH =23.9 M − 1 s − 1)

* Corresponding author. Tel.: +64-3-364 2461; fax: +64-3-364 2110. 1 Deceased 20.9.1986.

2. Experimental The previously characterised complexes, cis-[CoCl(en)2(py)]Cl2 [8], cis-[CoCl(en)2(d5py)](ClO4)2 [9], trans-[CoCl(tn)2(py)]ZnCl4 [10], pf-[CoCl(dien)(MeNH2)2]S2O6 [11a], and pf-[CoCl(dien)(py)2]ZnCl4 [11a] were prepared by literature methods. Pentadeuteropyridine (d5py) was purchased from Fluka AG, Buchs. 2 The stereochemical nomenclature used is that proposed by R.M. Hartshorn and D.A. House, J. Chem. Soc., Dalton Trans. (1998) 2577.

0020-1693/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S 0 0 2 0 - 1 6 9 3 ( 9 9 ) 0 0 0 8 4 - 5

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2.1. cis-Bromobis(ethylenediamine)(perdeuteropyridine)cobalt(III) bromide (cis-[CoBr(en)2(d5py)]Br2) The corresponding chloro chloride (2 g) [9] was added to a solution of Na2CO3 (1 g) in water (10 ml) and stirred at room temperature for 5 min. An equal volume of 48% HBr was then added (caution, effervescence) and the mixture heated to 80 – 90°C for 5 min. The violet bromo bromide (1.8 g) that deposited from the hot solution was collected by filtration and washed with 2-propanol and then ether. The above procedure was repeated with the crude bromo bromide to ensure complete conversion to the desired product. The sample used in the kinetic runs showed no evidence of chloro bromide impurity as there was no drift in the ‘infinity’ volume of NaOH added. [CoBr(en)2(d5py)]Br2: Calc. FW 502.8 Da. Found 50595 Da.

2.2. cis-Bromobis(ethylenediamine)(pyridine)cobalt(III) bromide (cis-[CoBr(en)2(py)]Br2) This complex was prepared similarly from the chloro chloride. [CoBr(en)2(py)]Br2: Calc. C, 21.69; H, 4.25; N, 14.07%; FW 497.8 Da. Found C, 21.78; H, 4.21; N, 13.99%; FW 495 9 5 Da. The bromo di(nitrate) and bromo di(perchlorate) salts were prepared by metathesis from the bromo bromide. [CoBr(en)2(py)](NO3)2: Calc. FW 452 Da. Found 462910 Da. [CoBr(en)2(py)](ClO4)2: Calc. FW 538 Da. Found 5299 12 Da.

2.3. p3f-Chloro(1,4,8 -triaazaoctane)bis(pyridine) cobalt(III) tetrachlorozincate(II) (p3f-[CoCl(2,3 -tri) (py)2]ZnCl4) This was prepared in a similar manner to the dien analogue [11a] using CoCl3(2,3-tri) [12] as starting material. Single crystals, suitable for X-ray structural determination, were obtained from 100 mg dissolved in 3 ml of 1 M HCl after room temperature evaporation.

2.4. pf-Chloro(diethylenetriamine)bis(pyridine) cobalt(III) nitrate perchlorate (pf-[CoCl(dien)(py)2] (NO3)(ClO4)) This salt was obtained by metathesis from the tetrachlorozincate(II) [11a] in 0.1 M HCl using equal weights of NaNO3 and NaClO4 · H2O. X-ray quality single crystals were obtained from room temperature evaporation of 100 mg dissolved in 0.1 M HCl (3 ml).

2.5. cis-Chlorobis(ethylenediamine)(4 -aminopyridine) cobalt(III) chloride monohydrate (cis-[CoCl(en)2(4 NH2 -py)]Cl2 ·H2O) [11b] This was prepared from trans-[CoCl2(en)2]Cl as for the pyridine analogue and recrystallised from 0.05 M

HCl by addition of lithium chloride. [CoCl(en)2(4-NH2py)]Cl2 · H2O: Calc. FW 397.5 Da. Found 404919 Da. This complex can be converted to the bromo nitrate as for the py analogue. [CoBr(en)2(4-NH2-py)](NO3)2: Calc. C, 22.65; H, 4.65; N, 23.48%. Found: C, 22.75; H, 4.60; N, 23.66%.

2.6. cis-Chlorobis(ethylenediamine)(4 -dimethylaminopyridine)cobalt(III) chloride dihydrate (cis-[CoCl(en)2(4 N(Me)2 -py)]Cl2 · 2H2O) This was prepared as for the 4-aminopyridine analogue. [CoCl(en)2(4-N(Me)2-py]Cl2 · 2H2O: Calc. C, 29.76; H, 5.78; N, 18.94; Cl, 24.01%. Found: C, 29.55; H, 5.97; N, 18.57; Cl, 23.92%.

2.7. cis-Chlorobis(ethylenediamine)(4 -acetylaminopyridine)cobalt(III) chloride (cis-[CoCl(en)2(4 CH3CONH-py)]Cl2) This was prepared by direct acetylation of the 4NH2-py complex. [CoCl(en)2(4-CH3CONH-py)]Cl2: Calc. FW 421.5 Da. Found FW 4209 5 Da.

2.8. cis-Chlorobis(ethylenediamine)(4 -cyanopyridine) cobalt(III) chloride 2.5 hydrate (cis-[CoCl(en)2(4 CNpy)]Cl2 · 2.5H2O) The 4-cyanopyridine (31.23 g, 0.30 mol) was suspended in 225 ml of 0.01 M HClO4 and the mixture heated to 70°C. The ligand dissolved completely and 64.14 g (0.225 mol) of trans-[CoCl2(en)2]Cl were added. Heating was maintained at 70°C for 1 h and, after cooling for 15 min, the solution was diluted with 300 ml of 50 mM HCl. The product was isolated by chromatographic separation on a 7× 24 cm DOWEX 50 WX2 (200–400 mesh) column in the H + form. After cations from the reaction mixture were absorbed at the top of the column, elution was commenced with 1 M HCl and three bands separated, a small green band containing unreacted trans-[CoCl2(en)2] + , a violet band containing cis-[CoCl2(en)2] + and a substantial red band emerging from the orange at the top of the column. After removal of the green band with 1 M HCl, elution was continued with 1.5 M HCl which removed the violet band and allowed the collection of the red band (the desired product), in fractions, until the column effluent was colourless. Continued elution with 2 M HCl gave first purple cis-[CoCl(en)2(OH2)]2 + and then orange [Co(en)2(OH2)2]3 + isomers. Evaporation of the red band fractions gave a slightly hygroscopic product which was dissolved in the minimum volume of 0.1 M HCl and precipitated with ethanol to give 4.87 g (5% yield) of cis-[CoCl(en)2(4-

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183

Table 1 Kinetic data for the base hydrolysis of some [CoX(N)5]2+ complexes containing pyridine (I =0.1 M NaClO4) Complex

T (°C)

pHa

kOH (mean) (M−1 s−1)

kOH (calc)b (M−1 s−1)

c-[CoCl(en)2(py)]2+ c-[CoBr(en)2(py)]2+ c-[CoBr(en)2(d5py)]2+ t-[CoCl(tn)2(py)]2+ pf-[CoCl(dien)(py)2]2+ pf-[CoCl(dien)(MeNH2)2]2+ c-[CoCl(en)2(4-NH2-py)]2+c

25.0 25.0 25.0 25.0 25.0 25.0 25.0 30.0 35.0 40.0 25.0 30.0 35.0 40.0 17.2 21.2 25.2 29.9 34.4

8.20–8.40(3) 7.40–7.90(9) 7.40(6) 7.40(3) 6.56–6.80(5) 8.48–8.88(4) 9.05–9.45(6) 8.85–9.08(5) 8.07–8.49(8) 8.05–8.45(6) 9.06–9.6(7) 8.55–9.13(9) 8.03–8.49(6) 8.07–8.42(4) 7.5–8.75(6) 7.5–8.75(6) 7.5–8.75(6) 7.5–8.75(6) 7.5–8.75(6)

338 912 3330 970 3340 9 80 3870 9 100 27700 9 200 252 94 78.2 9 5 154 99 309 9 28 568 9 43 75.9 97 157 9 20 295 925 551 917 67 912 181 9 38 380 9 94 808 9 250 1710 9 700

78.2 155 302 574 77.0 152 295 558 75.6 161 337 784 1710

c-[CoCl(en)2(4-N(Me)2-py)]2+c

c-[CoCl(en)2(4-CN-py)]2+

a

Number in parenthesis is the number of individual runs used to determine the mean. Calculated using the activation parameters cited in Table 2. c I=0.1 M NaCl. b

CNpy)] Cl2 · 2.5H2O. Calc.: C, 27.64; H, 5.79; N, 19.34; H2O, 10.36%. Found: C, 27.65; H, 5.74; N, 19.43; H2O, 10.29%. Visible absorption spectra (0.1 M HCl): l (nm, (o (M − 1 cm − 1)); max. 518 (82.4), min. 420 (23.5), max. 364 (97.1), min. 342 (81.2). Kinetic parameters for the Hg2 + -assisted chloride release for this complex (I= 1.0 M, TFA) are kHg (25°C) 2.38 × 10 − 3 M − 1 s − 1, DH c =67.4 9 3 kJ mol − 1, DS c = − 69 9 9 J K − 1 mol − 1.

uptake trace using the expression tkobs = ln[(V −Vo)(V − Vt) − 1] where Vo, V and Vt are the chart reading values at the start, the end and at time, t, for the reaction. From V , and the weight taken, an estimate for the formula weight of the complex salt could be obtained. Values for kOH (M − 1 s − 1) (Table 1) were obtained from the expression kOH = kobs[OH − ] − 1 where [OH − ] was estimated from the set pH using:

2.9. Kinetics and data analysis

− log[OH − ]= pKwc + 0.105−pH

The kinetics of base hydrolysis of the complexes were monitored using a Metrohm pH-stat. The general experimental technique has been outlined [13,14]. A high alkalinity combination electrode, Phillips CA-11, was used and the pH-meter (Model E300B) was calibrated ( 9 0.01 pH) using 0.01 M Na2B4O7 · 10H2O [pH 9.180 (25°C), 9.139 (30.0°C), 9.102 (35.0°C) and 9.068 (40.0°C)]. For each run, a solution (0.1 M, ca. 90 ml) of NaClO4 or NaCl was equilibrated at the appropriate temperature and the set pH adjusted with 1.24 × 10 − 2 M NaOH. A known amount of the powdered solid (20 – 50 mg) or a solution of the solid in ca. 5 ml H2O was added and the base consumption was recorded for 8 – 10 half-lives. During each run the pH fluctuated by ca. 9 0.01 unit from the set mean and the set pH was adjusted to give half-lives of 5 – 10 min. Pseudo-first-order rate constants (kobs) were calculated from 8 to 10 points over the time versus OH −

Values used for pKwc were 13.779, 13.607, 13.458 and 13.309 at 25.0, 30.0, 35.0 and 40.0°C, respectively [15]. Activation parameters (Table 2) were calculated from the variation of kOH with temperature using a non-linear least-squares fit of ln kOH versus 1000/T (K). All kOH data were included in these calculations, not just the means cited in Table 1.

2.10. X-ray crystallography The crystal data, data collection and refinement parameters for the two structures are listed in Table 3. Data were collected with a Siemens SMART CCD area detector, using graphite monochromatized Mo Ka radi˚ ). The structures were solved by ation (l=0.71073 A direct methods using SHELXS [16], and refined on F 2 using all data by full-matrix least-squares procedures with SHELXTL Version 5.10 [17a]. Hydrogen atoms

D.A. House et al. / Inorganica Chimica Acta 288 (1999) 181–188

184

Table 2 Second order rate constants and activation parameters for the base hydrolysis of some [CoX(N)5]2+ complexes at 25°C and I =0.1 M Complex

kOH (M−1 s−1)

DH c (kJ mol−1)

DS c (J K−1 mol−1)

Ref.

[CoCl(NH3)5]2+ [CoCl(MeNH2)5]2+ c-[CoCl(en)2(NH3)]2+ c-[CoCl(en)2(MeNH2)]2+ c-[CoCl(en)2(py)]2+

0.47 3300 8.1 12.7 338 332 337 199 (I= 1.0 M) 335 326 240 279 212 200 78.2 64.0 (I = 1.0 M) 77.0 28.8 508 765 3870 8.1 252 27 700 23.9 1740 154 ca. 100 7.4×106 2.4×107 4.5 69 3330 3340 233

116 77.7

+140 +72

96

+100

[6,11a] [6] [6] [6]

c-[CoCl(en)2(d5py)]2+ c-[CoCl(en)2(4-CNpy)]2+ c-[CoCl(en)2(4-Etpy)]2+ c-[CoCl(en)2(4-Mepy)]2+ c-[CoCl(en)2(4-CH3Opy)]2+ c-[CoCl(en)2(4-CH3CONH-py)]2+ c-[CoCl(en)2(4-NH2py)]2+ 2+

c-[CoCl(en)2(4-N(Me2)py)] c-[CoCl(en)2(N-Meimid)]2+ c-[CoCl(tn)2(NH3)]2+ c-[CoCl(tn)2(MeNH2)]2+ t-[CoCl(tn)2(py)]2+ pf-[CoCl(dien)(NH3)2]2+ pf-[CoCl(dien)(MeNH2)2]2+ pf-[CoCl(dien)(py)2]2+ pf-[CoCl(dien)(en)]2+ pf-[CoCl(dien)(bipy)]2+ f-[CoCl(tacn)(ampy)]2+ m-[CoCl(bamp)(en)]2+ ffms-[CoCl(bispicdien)]2+ ffms-[CoCl(bispicditn)]2+ [CoBr(NH3)5]2+ c-[CoBr(en)2(NH3)]2+ c-[CoBr(en)2(py)]2+ c-[CoBr(en)2(d5py)]2+ c-[CoBr(en)2(N-Meimid)]2+ a b

a

83.6

132 96 95.8 90.8 96.5

+84

+247918 +122 +107 +115

[8] [9] [42] [9] a

[8] [8] b a

100 9 2

+1299 6

a a

100 93 93.2 84.3 70.4

+1279 10 +95 +90 +38

a

[48] [14] [14] a

[11] a a

83.8 89.9 83.3 57 38 123

+62 +120 +69 +76 +23 +187

[7] [55] [52] [42] [53] [53] [6] [6] a a

85

+84

[48]

This research. D. Van Vinh, University of Neuchatel, Diploma (1983).

were included in calculated positions with isotropic displacement parameters 1.2 times the isotropic equivalent of their carrier atoms. The functions minimised were w(Fo 2 −Fc 2), with w =[s 2(Fo 2) + (aP)2 +bP] − 1, where P= [max(Fo 2) +2Fc 2]/3.

3. Results Tables 1 and 2 present kinetic data for the base hydrolysis of some [CoX(N)5]2 + complexes containing pyridine and where known, their ammonia and methylamine analogues of the same configuration. Inclusive of prior determinations (Table 2), the base hydrolysis rate constant for cis-[CoCl(en)2(py)]2 + has now been determined on three different samples, using different pH-stat instruments and different background electrolytes, with excellent agreement (kOH (25°C)= 3349 3 M − 1 s − 1, I = 0.1 M). The analogous bromo

complex has a base hydrolysis rate 10 times greater (kOH (25°C)= 33309 70 M − 1 s − 1, I= 0.1 M, NaClO4), similar to the Br/Cl rate ratio observed for [CoX(NH3)5]2 + (Table 2). Like the chloro complex [9], replacement of the pyridine by pentadeuteropyridine has no effect on the base hydrolysis rate within experimental error. On increasing the number of pyridine ligands in the [CoCl(N)5]2 + unit, the rate enhancement is even more marked, as pf-[CoCl(dien)(py)2]2 + is 110 times more reactive than pf-[CoCl(dien)(MeNH2)2]2 + , when compared with cis-[CoCl(en)2(py)]2 + which is 26 times more reactive than cis-[CoCl(en)2(MeNH2)]2 + (Table 2). Structures of pf-[CoCl(dien)(py)2](NO3)(ClO4) (1) and p3f-[CoCl(2,3-tri)(py)2]ZnCl4 (2) have also been determined (Tables 3 and 4) (Figs. 1 and 2). Of more interest, perhaps is the effect of substituents on the py ligand and comparable base hydrolysis data are now available for 4-X-py where X=H, CH3, CH3CH2, N(Me)2, NH2, CN, CH3O and NHCOCH3.

D.A. House et al. / Inorganica Chimica Acta 288 (1999) 181–188 Table 3 Crystal data, data collection and refinement parameters for [CoCl(dien)(py)2](NO3)(ClO4) (1) and [CoCl(2,3-tri)(py)2]ZnCl4 (2)

Formula MR (Da) Crystal system ˚) a (A ˚) b (A ˚) c (A a (°) b (°) g (°) ˚ 3) V (A Space group Z F(000) Dx (g cm−1) Dimensions (mm) m (mm−1) Temperature (K) 2u range (°) Data collected

No. measured Independent reflections Restraints/parameters Good-of-fit on F 2 wR2 (all data) R1 [I\2s(I)]

1

2

C14H20Cl2CoN6O7 514.19 monoclinic 7.7963(6) 25.738(2) 10.5207(10) 90 101.3500(10) 90 2069.8(3) Cc 4 1052 1.650 0.35×0.20×0.11 1.137 161(2) 3.09–26.27 −35h59 −315k520 −125l512 3301 2495 2/271 1.017 0.1742 0.0661

C15H25Cl5CoN5Zn 576.95 triclinic 8.5857(6) 10.5350(7) 13.9134(10) 104.4970(10) 103.2510(10) 104.5270(10) 1120.74(13) P1( 2 584 1.710 0.47×0.20×0.15 2.417 161(2) 3.03–26.37 −65h510 −115k513 −135l517 3624 3624 0/244 1.028 0.0604 0.0269

The introduction of the 4-N(Me)2− and 4-NH2− substituents reduces considerably the acceleratory effect observed with the unsubstituted pyridine. Although synthesised in an analogous fashion to the alkylamine analogues, which are cis [10,14], [CoCl(tn)2(py)]2 + is believed [10] to have the trans-stereochemistry and, as such, is not strictly comparable with the more common cis-isomers. While the synthesis of trans-[CoCl(en)2(py)]2 + has been described, the base hydrolysis rates have not been published [17b] and appropriate comparisons are not yet possible.

185

Table 4 ˚ ), bond angles (°) and torsion angles (°) for Selected bond lengths (A [CoCl(dien)(py)2](NO3)(ClO4) (1) and [CoCl(2,3-tri)(py)2]ZnCl4 (2)a

Co–Cl(1) Co–N(1) (p) Co–N(2) (s) Co–N(3) (p) Co–N(4) (py) Co–N(5) (py) N(1)–Co–N(2) N(2)–Co–N(3) C(41)–N(4)–C(45) C(51)–N(5)–C(55) N(1)–C(1)–C(2)–N(2) N(2)–C(3)–C(4)–N(3) N(2)–C(3)–C(4)–C(5) C(3)–C(4)–C(5)–N(3)

1

2

2.256(2) 1.954(8) 1.959(7) 1.963(7) 1.971(7) 1.974(7)

2.2545(6) 1.960(2) 1.972(2) 1.974(2) 1.978(2) 1.980(2)

84.5(3)b 83.9(3)b 117.1(7) 117.3(8) −43.3(9) (l) −39.8(9) (l)

85.58(8)b 88.94(8)c 117.6(2) 117.4(2) 46.5(2) (d) 61.3(3) −62.0(3)

a See Figs. 1 and 2 for the atom numbering system used. p = primary NH2 group, s =secondary NH group. b Bite angle for the fused five-membered ring. c Bite angle for the fused six-membered ring.

Werner’s research [28]. The interest in pyridine complexes was motivated by the search for arguments against the ‘nitrogen catenation’ structural proposals (Blomstrand and Jørgensen) for Co(III) ammines [29] – the incorporation of pyridine molecules in the proposed chains requiring quite speculative imagination. Therefore, the close similarities observed at that time, between the pyridine complexes and their ammine

4. Discussion

4.1. Co(III) complexes with pyridine ligands Co(III) amine complexes containing pyridine ligands were first prepared by Werner’s students [17c,18 –26]3. These investigations appear to have fallen into oblivion as they have not been quoted in subsequent work [27] and only incompletely in a restrospective survey of 3 The two theses in Refs. [25,26] have been submitted by World War I repatriates after Werner’s retirement and death. Although the work had been carried out under Werner’s personal supervision, they have been refereed by his successor P. Karrer.

Fig. 1. A view of the cation and anions in pf-[CoCl(dien)(py)2](NO3)(ClO4) (1).

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Fig. 2. A view of the cation and anion in p3f-[CoCl(2,3-tri)(py)2]ZnCl4 (2).

analogues, lent further support to Werner’s coordination theory. We can only speculate why much of this work was never published in the open literature but perhaps Werner was uncertain as to the stereochemistry of [CoCl(en)2(py)]2 + which is produced from the reaction between trans-[CoCl2(en)2] + and pyridine [27,30,31a, 32,33,34a]. This uncertainty persisted for a considerable time [34a,34b]. The resolution (in 1974) of the product into L and D forms established the cis-stereochemistry [31b,32] and this has been substantiated by subsequent X-ray structural determinations [35 – 37]. Nevertheless, stereochemical change is not always observed [38] especially with linear tetraamines [39,40] and [CoCl(tn)2(py)]2 + is believed to have the trans-configuration [10].

4.2. Base hydrolysis rates and the origin of the acceleratory effect The first measurements [41] of the base hydrolysis rate of cis-[CoCl(en)2(py-X)]2 + complexes, gave unusually high values for kOH with respect to the known alkylamine analogues. Subsequent kinetic data [8,9,42,43] showed this rate enhancement is real though relatively modest (ca. 30 times at comparable ionic strengths, for pyridine versus the EtNH2 analogue, (Table 2)). The origin of this acceleratory influence has generated considerable discussion [3]. Alternative mechanisms (pseudo-base), conjugate base stabilisation of the transition state within the DCB mechanism, steric effects and electronic effects have all been proposed as possible contributing influences. The pseudo-base mechanism [3] was expected to lead to a secondary deuterium isotope effect when pentadeuteropyridine complexes are used, but none was found [9]. A similar result is observed here in the base hydrolysis of cis-[CoBr(en)2(py)]2 + and cis-[CoBr(en)2(d5py)]2 + (Table 1).

In terms of electronic effects, we now have data for 4-X-py complexes where X= H, Me, Et, nPr, CH2Ph, NH2, CN, CH3O, NHCOCH3 and N(Me)2. Assuming a constant steric effect, it is difficult to explain the rate reduction for 4-NH2-py and 4-N(Me)2-py in other than an electron donating influence and hence a decrease in the p[3d(Co)]“p*[pyridine] back donation. Indeed, there is quite a reasonable linear correlation between log kOH and the s + [44] values for the substituents. In the absence of back donation we would expect the pyridine ligand to exert a steric effect similar to that of imidazole as the latter has no aromatic conjugation and the C–N–C angles at the Co–N center are similar: 117° [39,45] for pyridine (Table 4) and 106° for imidazole [46]. Both of these are much larger than the 58° C–N–C angle observed for coordinated aziridine [47]. For simplicity, we will use N-methyl imidazole as our model, as this avoids complications due to ligand NH deprotonation [48]. On this basis, the steric contribution to the pyridine rate enhancement would not be much more than a factor of two when compared with MeNH2 (kOH (25°C, N-methyl imidazole)=28.8 M − 1 s − 1: kOH (25°C, MeNH2)= 12.7 M − 1 s − 1). A related back-bonding effect is probably causing the approximately 70-fold increase in kOH between pf-[CoCl(dien)(en)]2 + and pf-[CoCl(dien)(bipy)]2 + (Table 2) and the back bonding is obviously more efficient when the two pyridine ligands are not linked into the bipy arrangement as the rate enhancement increases to approximately 110-fold when pf-[CoCl(dien)(MeNH2)2]2 + and pf-[CoCl(dien)(py)2]2 + are compared. The bonding of pyridine ligands to Co(III) has also been considered on the basis of ligand field spectral parameters. A major s donation and a minor p backbonding contribution was proposed [49]. This is entirely consistent with all structural determinations where there is no significant difference between Co–N(py) and Co– N(aliphatic) bond distances (Table 4).

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There are, however, pyridine containing ligands where base hydrolysis acceleration is not observed. Among these are the tridentate bamp [42] and a pyridine capped quinquedentate [50]. In both cases the pyridine component is constrained by pendant arms and the back bonding contribution may be insufficient to overcome the rigidity of such structures. Distortion to the five-coordinate intermediate implicit in the DCB mechanism may be restricted considerably in complexes with these inflexible ligands. It should be noted, however, that when the bamp component is incorporated into a macrocyclic ligand containing ‘flat’ sec-NH proton sites, the sensitivity to base hydrolysis is once again increased [3]. Indeed, a combination of ‘flat’ sec-NH protons and pyridyl groups is usually a potent combination for rate enhancement in base hydrolysis [51]. The base hydrolysis rates of [CoCl(N)5]2 + complexes containing 2-aminomethylpyridine (ampy) have also been studied [52] (Table 2). This ligand is like half of bamp and there is a 10-fold rate increase when [CoCl(tacn)(en)]2 + and [CoCl(tacn)(ampy)]2 + are compared. However, detailed comparisons between bamp and ampy complexes become difficult as there are major stereochemical changes between the systems. Data for the base hydrolysis of [CoCl(tacn)(py)2]2 + would be most interesting.

normally greater than 90° and the observed N–C–C–C torsion angles (Table 4) for this ring are more flattened than those observed in trans-[CoCl(2,2,3-tet)(NH3)]ZnCl4 [39]. This may be a consequence of the facarrangement of the 2,3-tri ligand, as in merCo(NO2)3(2,3-tri), the six-membered ring bite angle is expanded to 92.8° [54] and the corresponding angle in trans-[CoCl(2,2,3-tet)(NH3)]ZnCl4 is 93.3° [39].

4.3. Hg 2 + -assisted equation kinetics

One of us (J.B.) is indebted to the University of Canterbury and especially to Professor W.T. Robinson for provision of facilities and a stimulating environment during her leave of absence from the University of Victoria.

Kinetic data for reaction (2) [CoCl(N)5]2 + + Hg2 + “[Co(N)5(OH2)]3 + + HgCl + (2) have now been determined for a number of complexes where (N5)=cis-(en)2(X-py) [11b,56]. The data for X =4-CN obtained here, fit well on the plot of ln kHg (25°C, 1 M TFA) versus pKa (X-py) reported earlier [11b] and activation parameters are normal for this type of process. The cis-[CoCl(en)2(4-CNpy)]2 + system is the least labile of all the 4-X-py complexes so far investigated.

4.4. X-ray structural determinations As part of this work, and to confirm the previously assigned stereochemistry [11b], we have determined the crystal structures of pf-[CoCl(dien)(py)2](NO3)(ClO4) (1) and [CoCl(2,3-tri)(py)2]ZnCl4 (2). For the latter, there are two potential isomers in the pf- configuration, viz. p3f- and p1f-, as the tridentate ligand is unsymmetrical. The X-ray structure shows the isolated isomer to have the p3f arrangement (Fig. 2). The bond angles and bond lengths within these complexes are similar to other [CoCl(N)5]2 + systems [53]. There is, however, a contraction of the bite angle in the chair six-membered chelate ring in 2 (88.9°) as this is

5. Conclusion We have studied three further examples of the acceleratory role of coordinated pyridine in the base hydrolysis of [CoCl(N)5]2 + complexes. However, the introduction of electron donating substituents on the 4-position of the pyridine results in a rate reduction. Thus, the origin of the rate increase is primarily electronic due to weak p[3d(Co)] “ p*[pyridine] back donation with only minor contributions from specific non-bonded (steric) effects. In situations where back bonding is unfavourable due to the pyridine ligand stereochemistry, no rate enhancement is observed.

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