Polyhedron 25 (2006) 1247–1255 www.elsevier.com/locate/poly
Intramolecular hydrogen bonding in complexes containing bicyclic guanidine ligands Sarah H. Oakley a, Delia B. Soria b, Martyn P. Coles
a,*
, Peter B. Hitchcock
a
a
b
Department of Chemistry, University of Sussex, Falmer, Brighton BN1 9QJ, UK CEQUINOR and LANAIS EFO (UNLP–CONICET), Departamento de Quı´mica, Facultad de Ciencias Exactas, Universidad Nacional de La Plata, C.C. 962, 1900 La Plata, R. Argentina Received 8 July 2005; accepted 31 August 2005 Available online 6 October 2005
Abstract A series of complexes with the general formula MX2(hppH)2 [hppH = 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine: 1, M = Mn, X = Cl; 2, M = Fe, X = Cl; 3, M = Ni, X = Cl; 4 M = Zn, X = Br] have been synthesized and structurally characterized. In all cases, the neutral guanidine ligand bonds through the imine nitrogen atom to a tetrahedral metal centre, with additional interaction between the secondary amino NH and the metal bound halide. The intramolecular hydrogen bonding gives two isomeric forms in the solid-state, referred to as D and K, which co-crystallize in the unit cell. An example of intramolecular hydrogen bonding from the neutral hppH to oxygen within the dimeric core lithiated siloxide, [Ph3SiOLi(hppH)(THF)]2 (5), is also reported. 2005 Elsevier Ltd. All rights reserved. Keywords: Guanidine; Hydrogen bond; Manganese; Iron; Nickel; Zinc; Siloxide; Crystal structure
1. Introduction Hydrogen bonding is recognized as an extremely important phenomenon by which molecules may be stabilized through self-organization, with far reaching consequences in diverse areas including, for example, biological systems (e.g., DNA, protein folding), molecular sensing (e.g., host/guest sensing, anion recognition), catalysis (e.g., organocatalysis) and materials science (e.g., templated synthesis, crystal engineering) [1–6]. The amine functionality is a classic example of a group that readily forms hydrogen bonds, in this instance of the type N–H X, where X is an atom or group that contains electrons available for interaction with the acidic proton (e.g., oxygen, halogen). Whilst intermolecular hydrogen bonding has been extensively investigated, the stabilizing influence of such interac-
*
Corresponding author. Tel.: +44 1273 877339; fax: +44 1273 677196. E-mail addresses:
[email protected] (D.B. Soria), m.p.coles@ sussex.ac.uk (M.P. Coles). 0277-5387/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2005.08.038
tions between a ligand and a metal containing fragment have received substantially less attention. We have been involved in the application of neutral, tetrasubstituted guanidine compounds of general formula R2NC{NR 0 }{NHR 0 } as ligands in coordination chemistry [7]. In particular, we have investigated the bicyclic guanidine, 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine (hppH), where the annular framework constrains the amidine moiety into the Eanti configuration, making the NH functionality available for additional interaction with the metal fragment to which it is coordinated. It has been demonstrated that this extremely versatile ligand will bond to a number of main-group and transition metals, supporting a variety of geometries (Fig. 1). For instance, we have shown that copper(I) [8] and silver(I) [9] halides form monomeric trigonal planar complexes (a, Fig. 1) in which the guanidine coordinates as a pseudo-bidentate ligand, interacting with the metal and halide via the Nimine and NHÆ moieties, respectively. This intramolecular hydrogen bonding motif, containing NH Cl interactions, has also been described for tetrahedral iron and cobalt compounds (b) [10] and
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N N
X
H
X
X
H
H
N
M
N
N
N
N
N
N H
H
M
N
N
N
N
Ph Ph
Cu P
N
P
Ph N
X
P
Cu C N
N
N
N H N
N f
N N H
N
Cl
e
N N H
Li
H
Ph Ph
N
N
Cl
N
N
H
Li
Ph
X = Br, I
N
N
N
N Cu
N
d
N
N H
X Cu
N
N Li N N Li N H N N
N
N
N H
Ph
N
c
M = Fe, X = Cl M = Co, X = Cl
H
N
N H N
Cl b
Ph Ph
N
N
Pd
N
a
M = Cu, X = Cl, Br, I M = Ag, X = Cl
Cl H
Cl
N
N
N H
Cu
N Cu
C
N g
N
N
C
N N H
N
N
PPh3
Cu C N
Cu
Ph3P Ph3P
H N h
C N PPh3 i
Fig. 1. Examples of compounds containing the neutral bicyclic guanidine, hppH, as a ligand.
more recently in a square-planar palladium complex (c) [11]. We have shown that the extent of aggregation differs in the mixed phosphine/guanidine copper(I) halides (d) and (e), where the chloride was isolated as the monomer and the heavier congeners formed l,l 0 -dihalobridged dimers [9]. In contrast to complexes of type (a), the bisligand, monovalent metal halide species formed with lithium chloride is dimeric in the solid-state, comprised of chloride bridges and tetrahedral metal centres (f) [9]. Hydrogen bonding between tetrasubstituted guanidines and atoms other than the halides has also been observed, albeit less frequently. The acyclic guanidine, Me2NC{NiPr}{NHiPr}, forms intramolecular NH O bonds within the monomeric zinc complexes Zn(OAr)Me(Me2NC{NiPr}{NHiPr}) (Ar = 2,6-tBu2C6H3) [12] and the guanidinate/boroxide species, Zn(OBmes2)(Me2NC{NiPr}2)(Me2NC{NiPr}{NHiPr}) [13]. Intramolecular hydrogen bonding by hppH to nitrogen is restricted to the mixed guanidine/guanidinate dimer, [Li(hpp)(hppH)]2 (g) [14]. Finally, examples have been structurally characterized in which the coordinated guanidine does not undergo further interaction with the metal fragment. This situation was first noted by Bailey and co-workers with the trisubstituted guanidine, PhN = C{NHPh}2, in the cationic silver species Ag[PhN = C(NHPh)2]2[OTf] [15]. In this
case, however, the preferential formation of intermolecular hydrogen bonds to the triflate anion are observed, via two S@O oxygen atoms. We have shown that the hppH ligand is monodentate in the polymeric copper(I) cyanide complex, [CuCN(hpp)]1 (h), and bimetallic complex, (Ph3P)2(hppH)Cu(l-CN)Cu(PPh3)(CN) (i), where no readily available groups are present to interact with the NH atom. In all cases discussed above, bonding occurs exclusively through donation of the imine lone pair to an orbital of suitable symmetry and energy on the metal in question. In this contribution we report the synthesis and structural analysis of series of divalent tetrahedral metal complexes of general formula MX2(hppH)2 (M = Mn, Fe, Ni, Zn; X = Cl, Br). We also report the solid-state structure of the dimeric lithium siloxide species, [Ph3SiOLi(hppH)(THF)]2, which contains an intramolecular NH O hydrogen bond. 2. Experimental 2.1. General experimental procedures All manipulations were carried out under dry nitrogen using standard Schlenk line and cannula techniques, or in a conventional nitrogen-filled glovebox operating at
S.H. Oakley et al. / Polyhedron 25 (2006) 1247–1255
2r(I) Data/restraints/parameters Goodness-of-fit on F2 Final R indices [I > 2r(I)] R indices (all data) ˚ 3) Largest difference in peak and hole (e A
1
2
3
4
C14H26Cl2MnN6 404.25 173(2) 0.71073 0.25 · 0.20 · 0.20 orthorhombic Fdd2 (No. 43) 32.0384(6) 16.6254(3) 6.8663(1) 90 3657.34(11) 8 1.47 1.02 3.73–27.48 10 912 2022 (0.040) 1959 2022/1/128 1.073 R1 = 0.026, wR2 = 0.065 R1 = 0.027, wR2 = 0.066 0.91 and 0.19
C14H26Cl2FeN6 405.16 173(2) 0.71073 0.30 · 0.30 · 0.30 orthorhombic Fdd2 (No. 43) 31.7631(7) 16.6091(4) 6.8574(1) 90 3617.67(13) 8 1.49 1.14 4.54–27.48 10 864 1902 (0.037) 1839 1902/1/138 1.048 R1 = 0.020, wR2 = 0.049 R1 = 0.022, wR2 = 0.050 0.17 and 0.18
C14H26Cl2N6Ni 408.02 253(2) 0.71073 0.35 · 0.35 · 0.30 monoclinic P21/n (No. 14) 8.2851(4) 14.0068(7) 15.6742(6) 97.259(3) 1804.38(14) 4 1.50 1.38 3.82–25.06 18 247 3172 (0.041) 2749 3172/0/216 1.062 R1 = 0.035, wR2 = 0.093 R1 = 0.044, wR2 = 0.098 0.56 and 0.29
C14H26Br2N6Zn 503.60 173(2) 0.71073 0.40 · 0.30 · 0.20 monoclinic C2/c (No. 15) 9.2893(2) 13.9906(3) 14.2981(3) 95.197(1) 1850.58(7) 4 1.81 5.65 3.74–27.49 11 000 2105 (0.055) 1892 2105/0/139 0.920 R1 = 0.029, wR2 = 0.066 R1 = 0.035, wR2 = 0.070 0.31 and 0.87
Table 2 ˚ ) and angles () for MnCl2(hppH)2 (1), FeCl2(hppH)2 (2), CoCl2(hppH)2, [10] NiCl2(hppH)2 (3) and ZnBr2(hppH)2 (4) Selected bond lengths (A 1 M–N(1) M–Xa C@Nimine C–NH DCN C–NR2 Namine X H X N(1)–M–N(1 0 )b X-M-X 0 b N(1)–M-Xb N(1)–M-X 0 b N(1 0 )–M-Xb N(1 0 )–M-X 0 b a b
2 2.107(2) 2.3681(5) 1.319(2) 1.348(3) 0.029 1.351(2) 3.28 2.46
108.68(9) 110.67(3) 110.77(5) 107.97(4)
CoCl2(hppH)2 2.0395(14) 2.3190(4) 1.322(2) 1.354(3) 0.032 1.352(2) 3.30 2.52
107.52(9) 111.63(3) 110.87(4) 107.94(5)
2.003(4) 2.2881(13) 1.321(6) 1.335(7) 0.014 1.356(6) 3.23 2.57 107.6(2) 110.00(7) 109.72(12) 109.85(11)
3 1.983(2) 2.2911(8) 1.313(3) 1.354(4) 0.041 1.355(3) 3.24 2.45 103.81(10) 116.08(3) 108.03(7) 111.03(7) 111.12(7) 106.09(7)
4 1.977(2) 2.2749(8) 1.316(4) 1.359(4) 0.043 1.349(3) 3.21 2.47
1.993(5) 2.4321(4) 1.325(6) 1.360(6) 0.035 1.355(3) 3.40 2.59 110.8(3) 112.24(2) 103.92(18) 113.11(16)
1, 2, CoCl2(hppH)2 and 3, X = Cl; 4, X = Br. 3, N(1 0 ) = N(4); X = Cl(1) and X 0 = Cl(2).
2.2.3. ZnBr2(hppH)2 (4) As described for 1 (relative occupancies in disordered rings: a 0.74, b 0.26, c 0.78, d 0.22). 2.2.4. [Ph3SiOLi(hppH)(THF)]2 (5) The hydrogen atom on N(2) was refined; other hydrogens were in riding mode. C(3) is disordered. 3. Results and discussion 3.1. MX2(hppH)2 The synthesis of the hppH adducts, MnCl2(hppH)2 (1), FeCl2(hppH)2 (2) and ZnBr2(hppH)2 (4) was achieved in
a straight forward manner by combining the correct ratio of guanidine and metal salt in THF, with the exclusion of air/moisture (Scheme 1). The previously reported synthesis of 2 and the cobalt analogue, CoCl2(hppH)2, employed toluene as solvent and the mixture was refluxed to ensure complete reaction had occurred. Using our modification, each reaction proceeded with gradual dissolution of the metal starting reagent, coincident with precipitation of the product, indicating similar solubility properties. Recrystallization by slowly cooling a saturated solution (1, toluene; 2 and 4, acetonitrile) to room temperature afforded analytically pure products. Attempted synthesis of NiCl2(hppH)2 (3), using the direct combination of reagents analogous to the procedure
S.H. Oakley et al. / Polyhedron 25 (2006) 1247–1255
(v)
N
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(iii)
N
(hpp) 3SiMe
Li N
N SiPh3
N
N
(iv)
+ Ph Ph Ph Si N HN O THF N Li Li N THF O N NH Si Ph Ph Ph 5
(ii) N (i)
N N
N H
N M
N H
N
X X H N
N 1 2 3 4
M M M M
= Mn, X = Cl = Fe, X = Cl = Ni, X = Cl = Zn, X = Br
Scheme 1. (i) 1/2 MX2, THF; (ii) nBuLi; (iii) 1/3 SiCl3Me; (iv) NiCl2(Py)4/H2O; (v) SiPh3Cl/H2O (trace).
described above were unsuccessful. The desired product was, however, repeatedly isolated in moderate yield from the reaction between NiCl2(Py)4 and (hpp)3SiMe [17] (Scheme 1). Clearly, to give the product in this case, protonolysis of the silylated reagent must have occurred, forming the parent hppH reagent during the course of the reaction. We feel that it is unlikely that this was the result of accidental exposure to a moist atmosphere and a more likely explanation is residual water present in the nickel starting reagent (which was synthesized from the hydrate) [16]. NMR analysis of the manganese, iron and nickel complexes gave broad resonances commensurate with paramagnetic species; attempts at determining the magnetic moment using Evans method were frustrated by limited solubility. 1H NMR analysis of the diamagnetic zinc complex 4 suggested a static structure in solution, indicated by the presence of six resonances for the methylene groups within the bicyclic framework. This situation is in contrast to the three coordinate copper and silver complexes, MCl(hppH)2, [8,9] in which a fluxional process occurs in solution equating the two dissimilar rings of the bicyclic framework, but has recently been noted in the palladium adduct, PdCl2(hppH)2 [11]. In the absence of further solution-state measurements for these systems, it is premature to definitively allocate the causes of these differences to purely steric or electronic factors. It does, however, suggest that intramolecular hydrogen bonding is an important feature of solution-state stability of complexes of this general type, which will become central in any proposed catalytic applications of these species in solution. X-ray structural analyses of compounds 1–4 were conducted, primarily in order to acquire information about the nature of any hydrogen bonding involving the guanidine NH atom. The molecular structures are illustrated in
Figs. 2–5; crystal structure and refinement data are given in Table 1 and selected bond lengths and angles in Table 2. To complete the analogous series of MCl2(hppH)2 compounds where M is a metal from groups 7 to 10, the results from this study have been compared with the previously reported cobalt complex, CoCl2(hppH)21 [10]. Despite repeated attempts, the corresponding copper(II) chloride complex was unattainable, precluding extension of this series through to group 12. It is also noted that the presence of bromides in place of the chlorides in 4 will necessarily affect bond lengths and angles within the zinc complex and comparisons with the structure of 4 should therefore be treated accordingly. The manganese, iron and cobalt complexes are isostructural, each crystallizing in the orthorhombic crystal system (space group Fdd2), whilst the nickel and zinc compounds crystallize in the monoclinic form (space groups P21/n and C2/c, respectively). The former molecules each lie on a twofold rotation axis which relates the two halves of the molecule, as does the zinc complex; the two halves of the nickel species are crystallographically distinct. The guanidine rings in compounds 1, 2 and 4 exhibit varying degrees of disorder and were modelled accordingly (see Section 2); again due caution must therefore be exercised when discussing bond distances and angles. In each case a monomeric, distorted tetrahedral complex forms consisting of two imine-bound guanidine ligands about the metal dihalide fragment. The metal nitrogen dis1 The crystal structure of the cobalt complex, CoCl2(hppH)2, has been previously reported by Cotton and co-workers. However, the crystallographic data has not been deposited, as stated in the publication, and is therefore unavailable from the Cambridge Structural Database. We have independently synthesized and characterized the complex by X-ray diffraction studies and have deposited the data with this publication.
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Fig. 4. ORTEP representation of the D isomer of NiCl2(hppH)2 (3) with thermal ellipsoids drawn at the 20% probability level.
Fig. 2. ORTEP representation of the K isomer of MnCl2(hppH)2 (1) with thermal ellipsoids drawn at the 30% probability level (x + 1, y, z).
Fig. 5. ORTEP representation of the D isomer of ZnBr2(hppH)2 (4) with thermal ellipsoids drawn at the 30% probability level ( 0 x, y, z + 1/2).
Fig. 3. ORTEP representation of the D isomer of FeCl2(hppH)2 (2) with thermal ellipsoids drawn at the 30% probability level ( 0 x + 3/2, y + 1/2, z).
tances are, in general, shorter than in related formamidine and benzamidine complexes of the dihalides, [20–22] in agreement with greater electron donation by the guanidine. The angles about the metal in the Mn, Fe and Co compounds are not substantially distorted from the predicted tetrahedral value [ranges: 1, 107.97(4) to 110.77(5); 2, 107.52(9) to 111.63(3); CoCl2(hppH)2, 107.6(2) to 110.00(7)], whilst the range is appreciably greater in the nickel [103.81(10) to 116.08(3)] and zinc [103.92(18) to 113.11(16)] analogues. In the latter two examples, this distortion manifests itself further in different orientations of the neutral donor ligand and halide atoms with respect to
the metal centre (illustrated in Fig. 6 for the CN3 core of the D isomers – vide infra). Thus, the guanidine ligands are aligned almost parallel to the nickel-halide vector in 3, while a pronounced twist between the Br–Zn–Br and N–Zn–N planes is observed in 4. These observations serve to illustrate a degree of flexibility in the geometry generated at the metal upon coordination, dependant on the position of the metal in the periodic table and the nature of the halide atom present in the complex. The carbon–nitrogen distances within the guanidine are consistent with the presence of single and double bonds in which partial delocalization has occurred (DCN values: [23] ˚ ; 2, 0.032 A ˚ ; CoCl2(hppH)2, 0.014 A ˚ ; 3, 0.041 1, 0.029 A ˚ ; 4, 0.035 A ˚ ). The Eanti configuration about and 0.043 A the amidine component of hppH, enforced by the bicyclic framework, encourages hydrogen bonding to the halide atoms (Table 2). In each complex, the amine hydrogen atoms were located and refined indicating interaction with
S.H. Oakley et al. / Polyhedron 25 (2006) 1247–1255
Fig. 6. Core structures of the D isomers of compounds 1, 2, 3 and 4 (viewed along the N–M–N plane with the guanidine ligands facing out and the halides facing into the page).
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the siloxide anion, [Ph3SiO], in the solid-state. The product was analyzed by elemental analysis and X-ray diffraction and since one of the aims of this work is to further the understanding of intramolecular hydrogen bonding involving hppH, we feel that a brief discussion of the molecular structure of 5 is warranted (Fig. 8, Tables 3 and 4). Compound 5 crystallizes as the symmetry related dimer in which the lithium cations and siloxide anions form a rhombohedral Li2O2 core, commonly observed with lithiated alkoxides [24,25], boroxides [25–28] and siloxides [29,30] (provided sufficient bulk is present to prevent further aggregation). Within the central ring, the angles are obtuse at lithium and acute at oxygen [99.24(15) and 80.76(15), respectively], with essentially equal Li–O bond
the chloride and bromide, with H X distances in the ˚. range 2.45–2.59 A The presence of these intramolecular hydrogen-bonds in the solid-state generates chiral molecules which, by analogy with D3-tris(chelate) octahedral complexes, are assigned as the D or K isomer (Fig. 7). All of the compounds crystallise as the racemic mixture; however the way in which these molecules are arranged within the unit cell differs depending to which space group they belong. Thus, in the noncentrosymmetric space group Fdd2 (compounds 1, 2 and the cobalt analogue) molecules with the D and K configuration are related by a d-glide perpendicular to a, while in the centrosymmetric space groups P21/n (3) and C2/c (4) either a centre of inversion and/or a glide plane relates the isomers. 3.2. [Ph3SiOLi(hppH)(THF)]2 During the synthesis of the triphenylsilyl-substituted guanidine, hppSiPh3, using a procedure analogous to that used to generate the trimethylsilyl derivative, [18] a small amount of colourless crystalline material was isolated, identified by X-ray crystallography as the base stabilized siloxide, [Ph3SiOLi(hppH)(THF)]2 (5). We assume that the origin of 5 is due to the reaction between hppLi with a small amount Ph3SiOH, generated from hydrolysis of the silylchloride by adventitious water. The lithiated guanidinate is therefore a sufficiently strong base to deprotonate the silanol, with concomitant formation of the neutral guanidine which, in combination with THF solvent, stabilizes NH N
X M X
∆
X N
N
HN
NH
M
HN N
X
Λ
Fig. 7. Delta (D) and lambda (K) isomers of complexes 1–4, viewed along the N–M–N vector.
Fig. 8. (a) ORTEP representation of [Ph3SiOLi(hppH)(THF)]2 (5) with thermal ellipsoids drawn at the 20% probability level. (b) Core structure of 5, emphasizing the intramolecular NH O hydrogen bond. Only the ipsocarbon atoms of the phenyl groups shown for clarity.
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Table 3 Crystal structure and refinement data for [Ph3SiOLi(hppH)(THF)]2 (5) 5 Formula Formula weight Temperature (K) ˚) Wavelength (A Crystal size (mm) Crystal system Space group ˚) a (A ˚) b (A ˚) c (A b () ˚ 3) V (A Z Dcalc (Mg/m3) Absorption coefficient (mm1) h range for data collection () Reflections collected Independent reflections Reflections with I > 2r(I) Data/restraints/parameters Goodness-of-fit on F2 Final R indices [I > 2r(I)] R indices (all data) ˚ 3) Largest difference in peak and hole (e A
C58H72Li2N6O4Si2ÆC6H6 1065.38 173(2) 0.71073 0.25 · 0.20 · 0.10 monoclinic P21/n (No. 14) 12.9229(2) 15.7013(5) 14.2653(4) 92.945(2) 2890.70(13) 2 1.22 0.12 3.41–26.02 30 176 5659 [Rint = 0.053] 4499 5659/0/361 1.020 R1 = 0.053, wR2 = 0.128 R1 = 0.071, wR2 = 0.140 0.353 and – 0.371
Table 4 ˚ ) and angles () for [Ph3SiOLi(hppH)(THF)]2 (5) Selected bond lengths (A Li–O(1) Li–O(2) Si–O(1) C(1)–N(2) H(2n) O(1) O(1)–Li–O(1 0 ) O(1)–Li–N(1) O(1 0 )–Li–N(1) Li–O(1)–Li 0 Si–O(1)–Li 0
1.969(4) 2.011(4) 1.5926(14) 1.359(3) 2.04 99.24(15) 109.10(17) 119.57(17) 80.76(15) 130.05(12)
Li–O(1 0 ) Li–N(1) C(1)–N(1) C(1)–N(3) DCN O(1)–Li–O(2) O(1 0 )–Li–O(2) N(1)–Li–O(2) Si–O(1)–Li
1.954(3) 2.054(4) 1.302(3) 1.374(3) 0.057 116.17(17) 111.23(17) 102.29(16) 139.25(13)
distances (within 3r) to each oxygen. A similar trend has been reported in the closely related dme adduct, [Ph3SiOLi(dme)]2 [30]. Overall, each lithium atom is distorted tetrahedral, with the coordination sphere being completed by a molecule of THF and an Nimine bound neutral hppH; the three coordinate siloxide oxygen atoms show significant pyramidalization [Rangles = 350.06]. The hppH molecule is oriented approximately parallel to the Li–O vector and therefore effectively bridges this unit, with a resultant intra˚ . This is the molecular hydrogen bond distance of 2.04 A first reported example of an intramolecular NH O bond involving the neutral hppH guanidine. 4. Summary The series of bis-hppH adducts of the first row transition metal dihalides has been extended, showing a common bonding pattern in which intramolecular hydrogen bonding is evident. The compounds each crystallize as the
racemic mixture of D and K isomers within the crystal, and it is noted that the long range order of how these isomers are arranged with respect to each other differs depending on the crystal system. The molecular structure of the lithiated siloxide [Ph3SiOLi(hppH)(THF)]2 reveals a dimer in the solid-state, in which the NH hydrogen bonds to the siloxide oxygen atom. 5. Supplementary material Crystallographic data for the structural analyses have been deposited with the Cambridge Crystallographic Data Centre, CCDC nos. 1 (276689), 2 (231326) CoCl2(hppH)2 (276693), 3 (276690), 4 (276691) and 5 (276692). Copies of this information may be obtained free of charge from: The Director, CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (Fax: +44 1223 336033; email: deposit@ccdc. cam.ac.uk or www: http://www.ccdc.cam.ac.uk). Acknowledgements We thank the University of Sussex for financial support and wish to acknowledge the use of the EPSRCs Chemical Database Service at Daresbury. References [1] For reviews on hydrogen-bonding and its importance in a number of key areas of chemistry, M. Meot-Ner, Chem. Rev. 105 (2005) 213. [2] L. Kovbasyuk, R. Kra¨mer, Chem. Rev. 104 (2004) 3161. [3] P.R. Schreiner, Chem. Soc. Rev. 32 (2003) 289. [4] D. Braga, L. Maini, M. Polito, E. Tagliavini, F. Grepioni, Coord. Chem. Rev. 246 (2003) 53. [5] C.R. Bondy, S.J. Loeb, Coord. Chem. Rev. 240 (2003) 77. [6] D. Braga, F. Grepioni, G.R. Desiraju, Chem. Rev. 98 (1998) 1375. [7] P.J. Bailey, S. Pace, Coord. Chem. Rev. 214 (2001) 91. [8] S.H. Oakley, M.P. Coles, P.B. Hitchcock, Inorg. Chem. 42 (2003) 3154. [9] S.H. Oakley, D.B. Soria, M.P. Coles, P.B. Hitchcock, Dalton Trans. (2004) 537. [10] F.A. Cotton, C.A. Murillo, D.J. Timmons, Polyhedron 18 (1999) 423. [11] S.H. Oakley, M.P. Coles, P.B. Hitchcock, Inorg. Chem. 43 (2004) 7564. [12] M.P. Coles, P.B. Hitchcock, Eur. J. Inorg. Chem. (2004) 2662. [13] S.J. Birch, S.R. Boss, S.C. Cole, M.P. Coles, R. Haigh, P.B. Hitchcock, A.E.H. Wheatley, Dalton Trans. (2004) 3568. [14] M.P. Coles, P.B. Hitchcock, Chem. Commun. (2005) 3165. [15] P.J. Bailey, K.J. Grant, S. Pace, S. Parsons, L.J. Stewart, J. Chem. Soc., Dalton Trans. (1997) 4263. [16] D.A. Rowley, R.S. Drago, Inorg. Chem. 6 (1967) 1092. [17] S.H. Oakley, M.P. Coles, P.B. Hitchcock, Dalton Trans. (2004) 1113. [18] D. Kummer, S.H.A. Halim, W. Kuhs, G. Mattern, J. Organomet. Chem. 446 (1993) 51. [19] G.M. Sheldrick, SHELXL-97, Program for the Refinement of Crystal Structures, Go¨ttingen, 1997.. [20] F.A. Cotton, L.M. Daniels, D.J. Maloney, J.H. Matonic, C.A. Murillo, Polyhedron 13 (1994) 815. [21] F.A. Cotton, L.M. Daniels, C.A. Murillo, Inorg. Chim. Acta 224 (1994) 5. [22] D.I. Arnold, F.A. Cotton, D.J. Maloney, J.H. Matonic, C.A. Murillo, Polyhedron 16 (1997) 133.
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