Synthesis and structural characterisation of

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DOI: 10.1039/b000000x [DO NOT ALTER/DELETE THIS TEXT]. The synthesis and crystal structures of. [Co4Na2(HL)2(H2L)2(MeOH)4] and [Co5(HL)2(H2L)2] ...
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Synthesis and structural characterisation of polynuclear cobalt complexes with partially-deprotonated Bis-tris Alan Ferguson,a Andrew Parkina and Mark Murrie*a 5

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The synthesis and crystal structures of [Co4Na2(HL)2(H2L)2(MeOH)4] and [Co5(HL)2(H2L)2] are reported as the first examples of polynuclear transition metal complexes assembled using partially-deprotonated Bis-tris {2[bis(2-hydroxyethyl)amino]-2-(hydroxymethyl)propane-1,3-diol, H5L}

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Pro-ligands containing multiple alcohol groups have been utilised with great success in the assembly of high-nuclearity complexes containing the first row transition metal ions. D-sorbitol has been used to synthesise a {Cu16} complex1 whereas tripodal alcohols have been favoured for vanadium2 and manganese complexes.3 Mixed donor-set N-O ligands have also been utilised: for example 2,2’,2”-nitrilotriethanol in the synthesis of iron complexes both as the primary ligand4 and in combination with carboxylate ligands.5 However, examples of polynuclear cobalt complexes synthesised using ligands containing three or more alcohol groups and a nitrogen donor are rare: 2,2’,2”-nitrilotriethanol in combination with a co-ligand, acetate gives a heterometallic complex with a {Co3Cu2} core.6 We are interested in the synthesis and characterisation of polynuclear cobalt complexes due to wide-ranging potential applications as: catalysts,7 electron transfer mediators in dyesensitised solar cells,8 antiviral agents9 and molecular nanomagnets.10 As part of a larger project to synthesise polynuclear cobalt complexes with flexible multidentate ligands, we now extend our search to the pro-ligand 2-[bis(2-hydroxyethyl)amino]2-(hydroxymethyl) propane-1,3-diol (Bis-tris, H5L) (Scheme 1). Bis-tris is more commonly used as a biological buffer, although monomeric complexes have been reported with nickel, copper and cobalt.11 These complexes contain the ligand in it’s fully protonated form (H5L), binding with a {NO4} donor set, leaving one CH2OH arm unbound, with a water ligand completing the octahedral OH

OH

Fig. 1 Structure of complex 1 (Co, pale grey; O, dark grey; N, black; Na, grey; C, rods; H shown only for protonated ligand hydroxyl groups). Selected bond lengths: Co(1)-O(4) 1.928, Co(1)-O(6) 1.951, Co(1)-O(7) 1.928, Co(1)-O(13) 1.883, Co(1)-O(17) 1.891, Co(1)-N(15) 1.892; Co(2)O(4) 1.927, Co(2)-O(6) 1.941, Co(2)-O(7) 1.925, Co(2)-O(11) 1.895, Co(2)-O(12) 1.885, Co(2)-N(8) 1.948 Å [average e.s.d. 0.002 Å]. The asymmetric unit comprises half of the molecular unit shown, with the remainder of the molecule being related by inversion symmetry. Labels for symmetry-related atoms (suffixed a) are given only for metals, and are related to the asymmetric cell by the operation [2-x, -y, 1-z].

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HO

OH

Scheme 1 H5L a

WestCHEM, Department of Chemistry, University of Glasgow, Glasgow, G12 8QQ, UK. Fax: +44 141 330 4888; Tel: +44 141 330 4486; E-mail: [email protected] † Electronic Supplementary Information (ESI) available: [details of any supplementary information available should be included here]. See http://dx.doi.org/10.1039/b000000x/

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environment at the transition metal ion. We have employed reaction conditions to deprotonate the ligand and hence, utilise the alkoxide oxygens as bridging atoms to prepare larger complexes. Addition of Co(NO3)2.6H2O (1 mmol) to a solution of H5L (1 mmol) and NaOMe (3 mmol) in MeOH (20 mL) under aerobic conditions, followed by vapour diffusion of Et2O into the MeOH solution gives crystals of [Co4Na2(HL)2(H2L)2(MeOH)4] 1 in 5% yield over 10 days.† Although the synthesis is low-yielding, the reaction is reproducible, producing a homogeneous product consisting of small single crystals.

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Single crystal X-ray diffraction reveals a {Co2Na2Co2} core bridged by ligand alkoxide groups (Figure 1). Consideration of bond lengths, charge balance and bond valence sum analysis confirms the presence of four Co(III) centres.12 The ligands are present in both the tri- (H2L3-) and tetra-deprotonated (HL4-) forms, displaying different bonding modes. HL4- bonds to Co(1) through N(15): two CH2O- arms bridge Co(1) and Co(2) through O(6) and O(7) while the third arm is unbound and protonated forming a hydrogen bond to an adjacent complex {O(21)…O(11)’=2.67 Å}. One CH2CH2O- arm is monodentate {O(17)} while the second is tridentate, bridging Co(1) and the two Na centres through O(13). HL3- bonds to Co(2) through N(8): in contrast to the tetradeprotonated ligand, two CH2O- arms are monodentate binding Co(2) through O(11) and O(12). The third CH2OH arm is unbound and protonated, as is one CH2CH2OH arm: both form hydrogen [journal], [year], [vol], 00–00 | 1

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bonds to an adjacent complex {O(23)…O(17)’=2.66 Å and O(19)…O(22)’=2.69 Å}. The second CH2CH2O- group is tridentate bridging Co(1), Co(2) and Na(3) through O(4). Two MeOH molecules complete the coordination sphere of each Na centre, which are in a distorted square-based pyramidal environment. Each MeOH is hydrogen bonded to the Bis-tris ligands {O(18)…O(17) = 2.65 Å and O(22)…O(11) = 2.54 Å}. We are aware of only one other complex displaying a similar metal topology, with an alkoxobridged {Fe2Na2Fe2} core.13 Increasing the amount of base produces a different reaction product. Addition of CoCl2.6H2O (1 mmol) to a solution of H5L (1 mmol) and NaOMe (4 mmol) in MeOH (20 mL) under aerobic conditions, followed by vapour diffusion of Et2O reproducibly yields crystals of [Co5(HL)2(H2L)2] 2 in 6% yield over 8 days.† Single crystal X-ray diffraction reveals a mixed-valence pentanuclear complex displaying an unusual structure where four distorted octahedral Co(III) centres surround a distorted tetrahedral Co(II) centre.12 The ligand bonds to Co(1) through N(1) (see Figure 2): one CH2CH2O- arm is monodentate {O(3)} while the second is bidentate, bridging Co(1) to the tetrahedral divalent Co(2) centre through O(4). One CH2O- arm is bidentate bridging Co(1) and Co(1)’ through O(1); one is monodentate, {O(2)} and one unbound and protonated, forming a hydrogen bond to an adjacent complex {O(10)…O(3)’ = 2.61 Å}. The four ligands are crystallographically identical, but from charge balance considerations the complex is best formulated with an equal number of HL4- and H2L3- ligands, [Co5(HL)2(H2L)2]. Consequently, it is necessary to model each ligand as having a 3.5- charge, which is achieved by a halfoccupancy H2 atom along the very short O(2)…O(2)’ hydrogen bond (2.41 Å). Such ‘sharing’ of a proton between oxygen atoms on adjacent ligands has been observed in a tetranuclear nickel(II) complex between the ligands 1,3-diaminopropan-2-ol and 1,3diaminopropan-2-olate (O…O’ = 2.38 Å).14

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Pentanuclear cobalt complexes are rare,15 and none display the structural motif displayed by 2. It is interesting to note that both 1 and 2 comprise two Co(III) dimeric units bridged by an intermediate ‘divalent centre’, either {Na2} or Co(II), despite the ligands exhibiting different binding modes between the two complexes. An extension to anaerobic conditions to obtain new cobalt(II) Bis-tris complexes for magnetic studies is underway. In conclusion, there is great potential for partially-deprotonated Bistris in the assembly of both homo- and hetero-metallic polynuclear complexes and a large parameter space, which we are beginning to explore.

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Financial support from EPSRC is gratefully acknowledged. We would like to thank Dr. E. K. Brechin and Dr. C. J. Milios (University of Edinburgh) for helpful discussions.

Notes and references

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† Dried samples analyse as the desolvated form. Analysis (%) calc. (found) for C36H78N4Co4Na2O24 (1) C, 35.07 (35.10); H 6.38 (6.47); N 4.54 (4.48). For C32H62N4Co5O20 (2) C, 34.38 (34.22); H 5.59 (5.62); N 5.01 (4.98). Crystal data for 1·6CH4O: monoclinic, P21/n, a = 13.1331(2), b = 12.2948(2), c = 18.9290(3) Å, β = 94.0590(10), U = 3048.78(8) Å3, M = 1424.99, Z = 2 , µ(Mo-Kα) = 1.172, T = 100 K; refinement used 334 parameters and gave R1 = 0.0453 for 5432 data with Fo > 4 σ(F), wR2 = 0.1235 for 8878 unique data (2θ ≤ 60°). For 2·2CH4O: tetragonal, P-421c, a = 13.2070(7), c = 12.5890(8) Å, U = 2195.8(2) Å3, M = 1181.61, Z = 2, µ(Mo-Kα) = 1.938, T = 100 K; refinement used 157 parameters and gave R1 = 0.0475 for 1873 data with Fo > 4 σ(F), wR2 = 0.1127 for 2538 unique data (2θ ≤ 55°) and 6 restraints. The refined Flack parameter of 0.50(3) indicates that the absolute configuration is unknown. CCDC reference numbers XXX/XXXX.

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Fig. 2 Structure of complex 2 (colour scheme as in Fig. 1). H atoms are shown only for protonated ligand hydroxyl groups, H2 is constrained by symmetry to be half-occupancy; only one position is shown. Selected bond lengths: Co(1)-O(1) 1.917, Co(1)-O’(1) 1.934, Co(1)-O(2) 1.927, Co(1)-O(3) 1.871, Co(1)-O(4) 1.879, Co(1)-N(1) 1.901; Co(2)-O(4) 1.973 Å [average e.s.d. 0.003 Å]. The asymmetric unit comprises a quarter of the molecular unit shown, with the remainder of the molecule being related by crystallographic -4 symmetry. Labels for symmetryrelated atoms are given only for metals, and are related to Co1 as follows; Co1a [y,1-x,-z]; Co1b and O2’ [1-x,1-y,z]; Co1c [1-y,x,-z] .

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