Spectroscopic, structural, and thermodynamic aspects of the stereochemically active lone pair on lead(II): Structure of the lead(II) dota complex

Polyhedron 91 (2015) 120–127

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Spectroscopic, structural, and thermodynamic aspects of the stereochemically active lone pair on lead(II): Structure of the lead(II) dota complex Joseph W. Nugent a, Hee-Seung Lee a, Joseph H. Reibenspies b, Robert D. Hancock a,⇑ a b

Department of Chemistry and Biochemistry, University of North Carolina Wilmington, Wilmington, NC 28403, USA Department of Chemistry, Texas A&M University, College Station, TX 77843, USA

a r t i c l e

i n f o

Article history: Received 11 December 2014 Accepted 23 February 2015 Available online 3 March 2015 Keywords: Lead(II) Lone pair Stereochemically active lone pair Structure Dota

a b s t r a c t Steric, thermodynamic, and spectroscopic consequences of a stereochemically active lone pair (Lp) in Pb(II) complexes are discussed. The structure of Na3[Pb(dota)]NO3
2H2O (dota = 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetate) is reported: the four Pb–N bonds average 2.665 Å, while the four Pb–O bonds to the acetate groups average 2.772 Å. Normally, M–N bond lengths are longer than M–O bond lengths, as reported for the Sr(II) dota complex. This reversal in bond length order is attributed to the Pb–O bonds being closer to a stereochemically active Lp on Pb(II). In line with the latter, a very long Pb


O contact of 3.022 Å is found to a water molecule coordinated over the proposed site of the Lp on Pb in its dota complex. Steric control of whether the Lp is stereochemically active, that produces long Pb–N bonds close to 2.8 Å in length, can be exerted by ligands such as cryptands that prevent the N donors of the ligand from coordinating close together near the site on the Pb(II) opposite the Lp, the ‘antipodal’ site. The intense band that occurs in the electronic spectrum of Pb(II) complexes in the range 210–270 nm, that is attributed to a 6s2 ? 6sp transition, is examined. The band shifts from 209 nm in the Pb2+(aq) ion in aqueous solution to about 260 nm as the number of N donors increases in a series of ligands containing only N and O donors. Within this shift it is found that in Pb(II) complexes with the same number of N donors, there is a shift to shorter wavelength, that appears to correlate with a change in the Lp from stereochemically inactive to stereochemically active. DFT calculations support the idea that the observed band in Pb(II) complexes is due to a transition from the filled 6s orbital to an empty 6p orbital, and suggest that the 6s orbital increasingly becomes hybridized with the 6p orbital as the Lp becomes more stereochemically active. The DFT calculations also support the interpretation that for complexes with the same number of N donors, the 6s2 ? 6sp transition shifts to shorter wavelength as the Lp becomes stereochemically active. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction The steric consequences of the lone pair (Lp) of electrons on Pb(II) has been of enduring interest [1–18], and there has been some doubt expressed even as to the existence of a stereochemically active Lp on Pb(II) [14,15,17]. Of interest here is that a study of the Tl(I) complex of 18-crown-6 (see Fig. 1 for ligand abbreviations) has been used to suggest that there is no 6sp hybridization involved in producing what appears to be a stereochemically active Lp on the Tl(I) [17]. As is discussed below,

⇑ Corresponding author. Tel.: +1 910 962 3025. E-mail address: [email protected] (R.D. Hancock). http://dx.doi.org/10.1016/j.poly.2015.02.033 0277-5387/Ó 2015 Elsevier Ltd. All rights reserved.

this analysis is probably not correct. The stereochemically active Lp has implications for the design of ligands selective for the highly toxic Pb2+ ion, as discussed below [19]. The Pb(II) ion may display structures where the Lp appears to have no steric effects, i.e., be sterically inactive or holodirected, to use the terminology of Glusker et al. [12], or where steric effects of the Lp are shown, and the structure is hemidirected. In the holodirected case all the Pb–L bonds are of intermediate length, and there are no marked differences in Pb–L bond lengths, once corrected for differences in the ionic radii of the donor atoms. In the hemidirected case, the characteristics of the complex are that: (1) There should be a gap in the coordination geometry at the proposed site of the stereochemically active Lp, or very long Pb–L contacts or bonds at that site.

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O

O -

N

N O

O O

O N

N O

O

N

-

H 2N

N

NH2

N

N H

O

cyclam

H

H N

OH

O

N

-

OH N

N H

O

N

HO

N

NH2

dotam

HO

H N

N

H 2N

dota N

H

O

O

N

N

N

O N

H

H

N O

O O

O

-

cyclen

THP-cyclen

-

O

-

O

edta

O O -

N N

O

-

O O

O

-

N

HO

OH OH

nta

O

HO

N O

theen

O O O

NH N

H 2N

en

N O

-

-

-

O

O

-

O

NH2

NH2

egta H N

NH2 H2N

trien N

O O

O

O

O

O

18-crown-6

NH2 H2N

N

N

H N

O

dien

phen

N H

O

O

O

NH2

ntam O

HN

H 2N

O

O

O

O

O

O O

O

O

N

18-ane-N2O4

cryptand-222

Fig. 1. Key to abbreviations for ligands discussed in this paper.

(2) The Pb–L bonds on the side of the Pb opposite the proposed site of the Lp, referred to here as the antipodal site, are shorter than in the rest of the complex, and may be unusually short for a Pb–L bond of that type. These short bonds usually involve the most covalently binding donor atoms present. (3) The Pb–L bonds tend to become progressively longer as one moves from the antipodal site, to the position of the Lp. The activity of the Lp on Pb(II) is affected by the ionicity or covalence of the Pb–L (L = ligand) bonds formed by the donor atoms bound to the Pb(II) ion. Glusker et al. [12] have carried out wavemechanical calculations on Pb(II) complexes, which agree with the empirical observation that Pb(II) complexes with ionically bound oxygen donor ligands and high coordination numbers are more likely to be holodirected, while hemidirected complexes are more likely to be of low coordination number and involve more covalently bound nitrogen, sulfur, or carbon donor ligands. Of the 78 structures of Pb(II) reported in the CSD [20] that have only O donors, most appear to be best described as holodirected. Even so, eight of the structures show evidence of being hemidirected

in terms of criteria 1–3 above. For example, the structures of a tetranitrato Pb(II) complex [21] (CSD code: RARZUG) and of a bis-phenylalanine complex [22] of Pb(II) (CSD code: QIXHUB01) appear to be hemidirected, in spite of having coordination spheres comprised only of O donors. The clearest cases of a stereochemically active Lp on Pb(II) occur with highly covalent donors such as are afforded by three phenyl groups coordinated to Pb(II) with very short Pb–C bonds, as in the [Pb(C6H5)3] anion where a Li(I) is coordinated to the lone pair with a Pb–Li distance of 2.865 Å [23] or three thiolates bound to the Pb with very short Pb–S bonds and a trigonal pyramidal structure as expected from VSEPR considerations [24] with rather small S–Pb– S angles [25]. Examples of a stereochemically active Lp are observed in [Pb(en)(NO3)2] (CSD: RAQFAQ01) [26], [Pb(phen)(NO3)2] [27] (CSD: RAQGOF) and the [Pb(trien)(NO3)2] (CSD: RAQFIY) complex [28]. In ligands with the (R2NCH2CH2OCH2–)2 moiety, which is present in 18-ane-N2O4, cryptand-222, and egta (Fig. 1), the long diether ‘strap’ connecting the two nitrogen donors together prevents them from both coordinating near the antipodal site. In such

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a case, the Pb(II) becomes holodirected, and has bonds of similar length to the Sr(II) ion, which has an ionic radius [29] of 1.18 Å compared to Pb(II) (1.19 Å). Pb(II) is holodirected when [15] the complex is nearly identical to that of the Sr(II) analogue, which cannot be hemidirected. The holodirected Pb(II) and Sr(II) complexes can form isomorphous crystals [15], as observed for complexes of Pb(II) and Sr(II) with a dicyclohexyl crown ether [30,31]. In holodirected complexes, the Pb(II) has Pb–N bonds very similar in length to those of Sr(II), which average 2.75 ± 0.06 Å for Sr–N bonds to saturated N donors (CSD [20] 30 structures). For Pb(II) complexes that are holodirected, the shortest Pb–N bonds to saturated N donors average 2.84 ± 0.05 Å, similar to Sr–N bonds in Sr(II) complexes. If the Pb(II) complex is hemidirected, the Pb–N bonds near the antipodal site should be closer to 2.40 Å. The holodirected Pb(II) cryptand-222 complex [32] is nearly identical to the very similar Sr(II) complex [33]. Cuenot et al. [34] have suggested that the Pb(II) in the [Pb(dotam)](NO3)2
3.5H2O complex is holodirected, and that a previous study of the very similar [Pb(dotam)](ClO4)2
4.5H2O complex [35] was in error in suggesting that the Pb(II) in the dotam complex was hemidirected. In this paper is reported the structure of the Pb(II) dota complex, which can be compared to the reported structure of the Sr(II) dota complex [36] to apply the most stringent criteria for holodirectedness to complexes of the dota or dotam type. Whether Pb(II) is holodirected or hemidirected can affect the thermodynamics of complex formation [19]. A rule of ligand design [19] is that large metal ions (ionic radius >1.0 Å) show increases in log K1 when groups bearing neutral oxygen donors are added to a ligand. Thus, log K1 for the ligand en with Pb(II) is [37] 5.0, and on adding four hydroxyethyl groups to en to give THEEN, log K1 for Pb(II) rises to 7.5 [37]. However, if the oxygen donors are added as an ethereal strap so as to thrust the two N donors of the ligand apart, as in turning edta into egta, then the Pb(II) appears to become holodirected. The resulting long weak Pb–N bonds lead to a large drop in log K1, but Sr(II) experiences no such large drop in log K1, since it does not go through the hemidirected to holodirected transition. The Pb2+(aq) cation has in aqueous solution a fairly intense peak at 209 nm, which is considered to be a Laporte-allowed intraatomic 6s2 ? 6sp transition [14] involving the lone pair on Pb(II). Little work appears to have been done on this transition: Byrne et al. [38–40] reported that the shifts in the 209 nm peak on formation of carbonato complexes could be used to monitor CO23 concentrations in natural waters; [38] that there was a monotonic shift of the 6s2 ? 6sp transition to longer wavelength in aqueous solution as the Cl concentration was increased to give successively the species Pb2+ (209 nm), PbCl+ (225 nm), PbCl2 (240 nm), PbCl3 (255 nm) [39]; and a further study of the use of the 6s2 ? 6sp transition of Pb(II) to monitor CO23 concentrations in seawater [40]. The variation of the 6s2 ? 6sp transition of Pb(II) as a function of ligand concentration was used to determine log K values for the Pb(II) NTAM complex [41]. Payne et al. [42] and Busenlehner et al. [43] have reported the use of Pb ? S charge transfer bands to monitor binding of Pb2+ into thiolate binding sites in proteins. A more extensive study was undertaken here with a variety of ligands containing differing numbers of saturated nitrogen donors and oxygen donors to determine the effect of these on the 6s2 ? 6sp transition of Pb(II) in aqueous solution. The nature of the shifts in the 6s2 ? 6sp transition was further investigated by DFT calculations. 2. Experimental 2.1. Materials H4dota was obtained from Macrocyclics, and Pb(NO3)2 was obtained from Fisher.

2.1.1. Synthesis of Na3[Pb(dota)](NO3)
2H2O (1) 1 equiv of H4dota (76 mg, 0.15 mmol) was dissolved in 8 mL MeOH/H2O (10:1 v/v) with 4 equiv (60 lL of 10 M) NaOH added in a 20 mL sample vial. A solution of 1 equiv of Pb(NO3)2 (50 mg, 0.15 mmol) in 6 mL MeOH/H2O (10:1 v/v) adjusted to pH 5 with HClO4 was added drop-wise to the DOTA solution while stirring. The resulting solution was refluxed while stirring to reduce volume. The solution was allowed to slowly evaporate for 3 months, but with no deposition of crystals. The small volume remaining was reconstituted in 8 mL MeOH/H2O (3:1 v/v) and the sample vial containing the Pb(II) dota solution placed upright in a glass jar containing a few mL of diethyl ether, which jar was sealed tightly to allow ether diffusion into the sample. Diffusion of diethyl ether resulted in the deposition of transparent plate-like colorless crystals on the walls of the vial over the course of a few days. The solution was filtered off under vacuum and the crystals were air-dried. Elem. Anal. Calc. for C16H28N5Na3O13Pb: C, 24.81; N, 9.04. Found C, 25.13; N, 8.89%.

2.2. Molecular structure determination Data was collected on a three-circle Bruker GADDS instrument with Cu-source and MWPC (multiwire proportional counter) detector. The structure was solved by Patterson synthesis, and refined to convergence [44]. Details of the structure analysis are given in Table 1. The structure of Na3[Pb(dota)](NO3)
2H2O (1) is shown in Fig. 2, and significant bond lengths and angles in the structure of 1 are given in Table 2. Crystal coordinates and details of the structure determination of 1 have been deposited with the CSD [20].

2.3. Density Functional Theory (DFT) calculations All DFT/TDDFT calculations reported in this work were carried out with the ab initio quantum chemistry package GAMESS [45]. Geometry optimization of the Pb(II) nta, edta, dota, and egta complexes was performed within the framework of Kohn–Sham DFT with B3LYP exchange–correlation functional [46,47]. The SV(P) basis set [48,49] was used for the main group elements. For Pb, the effective core potential of Ahlrichs et al. [48,49] with 60 core electrons removed (ECP-60) was employed, along with the SV(P) basis set. The polarizable continuum model (PCM) as implemented in GAMESS was used in all calculations to imitate the solution environment. No explicit water molecule coordinated to Pb(II) is included in the calculations.

Table 1 Crystal data and structure refinement for Na3[Pb(dota)]NO3
2H2O (1). Empirical formula Formula weight T (K) Crystal system Space group a (Å) b (Å) c (Å) a (°) b (°) c (°) V (Å3) Z Radiation Final R indexes [I P 2r(I)] Final R indexes [all data]

C16H28N5Na3O13Pb 774.59 110.0 monoclinic P21/n 9.4598(3) 8.6462(3) 29.6487(10) 90 98.390(2) 90 2399.05(14) 4 Cu Ka (k = 1.54178) R1 = 0.0522, wR2 = 0.1296 R1 = 0.0580, wR2 = 0.1322

J.W. Nugent et al. / Polyhedron 91 (2015) 120–127

123

Fig. 2. Structure of the Na3[Pb(dota)]NO3
2H2O (1) complex, showing the long Pb


O bond or contact to a water coordinated very close to the proposed site of the stereochemically active lone pair on Pb(II). Hydrogen atoms, except for those on the two water molecules, are omitted for clarity. Thermal ellipsoids drawn at 50% probability level. Drawing made with ORTEP [53].

other cyclen-based ligands such as dotam or THP–cyclen (Fig. 2), without steric crowding problems, the M–N bonds are longer than the M–O bonds. For the Lu(III) complex of dota [50], the Lu–N bonds average 2.614 Å, while the Lu–O bonds average 2.279 Å. In the edta complex of Eu(II) [51], the Eu–N bonds average 2.748 Å, while the Eu–O bonds average 2.597 Å. The fact that for Sr(II), Eu(II), or Ln(III) ions M–N bonds are longer than M–O bonds is to be expected from the fact that the ionic radius of N at 1.32 Å is larger than that for O at 1.26 Å [29]. It is generally observed that M–N bond lengths are longer than M–O bond lengths, in the absence of special features such as a hemidirected structure. This is illustrated by structures of edta complexes reported in the CSD [20]: excluding structures for Pb(II), Bi(III), and Sb(III), which appear to be hemidirected, or Cu(II) which is Jahn–Teller distorted, for larger metal ions 72 edta complexes show M–N bonds averaging some 0.26 ± 0.03 Å longer than the M–O bonds. One finds that for Pb(II) in its dota complex, as noted above, that the Pb–N bonds average 0.107 Å shorter than the Pb–O bonds: in the reported structures of the Pb(II) dotam complex this difference averages 0.130 Å [34] and 0.144 Å [35], and in the THP–cyclen complex it averages 0.117 [52]. All four cyclen-based Pb(II) complexes thus show the important feature that the Pb–N bonds are distinctly shorter than the Pb–O bonds, which long Pb–O bonds are related to the fact that they are closer to the site of the stereochemically active Lp on Pb(II) [35].

3. Results and discussion

3.2. Coordination at the proposed site of the lone pair on the Pb(II) dota complex

The structure of Na3[Pb(dota)](NO3)
2H2O (1) (Fig. 2) has several features of interest, such as bridging waters between two of the sodium cations, and between a sodium cation and the Pb(II). Of importance here are the criteria raised in the introduction that relate to the Pb(II) being hemidirected, namely a gap in the coordination geometry around the Pb(II), or a very long Pb–L (L = ligand) bond at that site, and the variation in Pb–L bond lengths in relation to the proposed position of the Lp, which points are discussed below.

3.1. Pb–N and Pb–O bond lengths in Na3[Pb(dota)](NO3)
2H2O (1) The Pb has the eight donor atoms from dota coordinated to it, with an average of the four Pb–N lengths of 2.665 ± 0.021 Å, and an average of the four Pb–O lengths of 2.772 ± 0.111 Å (Table 2). Applying the test of similarity of the Pb(II) to the Sr(II) complex as a test for holodirectedness, in the Sr(II) complex of dota [36] the Sr–N bonds average 2.732 Å, which is 0.067 Å longer than in the Pb(II) complex, while the Sr–O bonds average 2.548 Å, which is 0.224 Å shorter than in the Pb(II) complex. It should be noted that for all other large metal ions with a preference for coordination numbers of eight or above, such as Sr(II), or Ln(III) ions (Ln = lanthanide), which can therefore coordinate with dota, or

Table 2 Bond lengths and bond angles of interest in the structure of Na3[Pb(dota)]NO3
2H2O (1). Bond lengths (Å) Pb(1)–N(1) 2.687(9) Pb(1)–N(4) 2.676(9) Pb(1)–O(5) 2.827(9) Na(2)–O(12) 2.367(8)

Pb(1)–N(2) Pb(1)–O(1) Pb(1)–O(7) Na(3)–O(12)

Bond angles (°) N(2)–Pb(1)–N(1) N(3)–Pb(1)–N(4) O(7)–Pb(1)–N(1) O(3)–Pb(1)–O(1)

67.9(3) 66.8(3) 64.1(3) 85.6(3)

2.638(9) 2.796(8) 2.609(8) 2.403(8)

Pb(1)–N(3) Pb(1)–O(3) Na(1)–O(13)

N(2)–Pb(1)–N(3) N(4)–Pb(1)–N(1) O(7)–Pb(1)–N(2) O(5)–Pb(1)–O(7)

2.660(9) 2.856(8) 2.346(8)

68.7(3) 68.1(3) 92.5(3) 74.2(3)

An important diagnostic feature for Pb(II) or other nd10(n + 1)s2 configuration ions such as Bi(III), Sb(III), or Tl(I), being hemidirected, is a gap in the coordination geometry, or very long M


L bond lengths or contacts, at or near the proposed site of the Lp. Cuenot et al. [34] stated that there was no gap in the coordination sphere of the [Pb(dotam)]2+ complex where a Lp might be situated. Examination of the structures of the Pb(II) dotam [34,35], THP– cyclen [52], and dota complexes, which have almost identical coordination spheres as far as coordination of the macrocyclic ligand and its pendant O donor groups is concerned, shows that there is clearly a gap in each of the coordination spheres, if any very long Pb


O contacts to waters on the axial site are disregarded. In Fig. 3 is shown a space-filling drawing of the Pb(II) dota complex, with the water molecule contacted at the long Pb


O length of 3.022 Å removed to show that, if the water is disregarded, there is indeed a gap in the coordination sphere of the Pb(II). Interestingly, a structure of a Pb(II) complex that is clearly hemidirected has been reported [54] where there is what might be regarded as an agostic acetonitrile molecule with two of its H atoms making Pb


H contacts near the evident site of the Lp. With nitrate counterions, in the structure reported by Cuenot et al. [34], where it was claimed there was no gap at the possible site of the lone pair, there is a nitrate over what appears to be a gap on the Pb(II) dotam complex, with a Pb


O distance of 3.806 Å, which appears to be too long to be regarded as a bond. It should be noted that in the Ln(III) dotam complexes (where there is no Lp) there is a unidentate ligand coordinated on the axial site of the complex, corresponding to the position of the Lp on the Pb(II) dotam complexes [20]. In the Gd(III) dotam complex [55], there is a water molecule on the axial site with a normal Gd–O bond length of 2.384 Å, compared to an average distance of 2.373 Å for the four Gd–O bonds to the amide oxygens of dotam. It cannot be argued that the very long Pb


O contacts over the proposed site of the Lp, or the gaps in the coordination geometry, present in the Pb(II) dotam complexes are due to steric crowding, since the Pb(II) ion is considerably larger than Gd(III) [29]. In the case of the Gd(III) dota complex, there is similarly a water on the

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Fig. 3. Space-filling drawing of the Pb(II) dota complex reported here, looking down the approximate fourfold axis. The diagram shows the gap apparent in the coordination sphere, if the water molecule contacting the Pb(II) with a long Pb


O(13) contact of 3.022 Å at the site of the gap is disregarded. Drawing made with the Mercury program available as part of the CSD suite of programs [20].

axial site (important in MRI applications) with a fairly normal Gd–O bond length of 2.458 Å [56]. 3.3. Variation of Pb–L lengths with distance from the lone pair and antipodal site One can analyze the variation of Pb–L bond lengths with respect to distance from the site of the Lp by bonding a dummy atom representing the Lp to the Pb at the proposed site of the Lp, and then measure the angle each Pb–L bond makes with the Pb–Lp ‘bond’ [57]. A plot of Pb–L length against L–Pb–Lp angle will then show how bond length decreases with increasing distance of the Pb–L bond from the Lp. This is seen for the [Pb(edta)]2 complex [58] in Fig. 4: the Lp of the Pb(II) edta complex is placed on the approximate twofold axis running through the Pb atom, on the side of the Pb opposite to the coordinated N donor atoms. The polynomial curve drawn on the diagram was fitted to these points only. The points for Pb–L length and L–Pb–Lp angle for the dota complex, the THP–cyclen, and the dotam complex with perchlorate counterions [33] have been superimposed on the plot. The diagram shows that the Pb–O and Pb–N bond lengths of the dota and dotam complex are as one would expect from their orientation relative to the proposed position of the Lp. The cyclen moiety of ligands such as dota, dotam, and THP–cyclen provides rigidity that keeps the four N donors some distance from the antipodal site opposite the Lp, where the shortest bonds are expected to form, making these Pb–N bonds somewhat longer. Similarly, the structure of the cyclen-based ligands keeps the O donors further from the site of the Lp, making these Pb–O bonds somewhat longer. This sterically controlled smaller difference between the M–N and M–O bond lengths initially led to the idea that the Pb(II) THP–cyclen complex was holodirected [52], and may also have led Cuenot et al. [34] to this conclusion regarding the Pb(II) dotam complex.

Fig. 4. The relationship between Pb–L bond length and the Lp–Pb–L angle (Lp = lone pair) for some hemidirected Pb(II) complexes: edta [58], dotam [35], THP–cyclen [52] and dota (this work). The Lp is situated at an Lp–Pb–L angle of zero: the larger the Lp–Pb–L angle up to a value of 180°, the further that donor atom is from the Lp, and the shorter is the Pb–L bond, being potentially shortest (2.4 Å) at the antipodal site where the Lp–Pb–L angle = 180°. The curve is fitted only to the points for the edta complex, and the plot is intended to show that the variation in Pb–L length for the dotam and dota complexes is in accord with the idea that they have a stereochemically active Lp.

appear to be two ligand properties that control the wavelength at which the 6s2 ? 6sp transition occurs in aqueous solution. One is that of increasing numbers of N donors in the ligand, which moves the transition to longer wavelength, as is apparent from Fig. 5. This resembles the shifts to longer wavelength that occur on substituting water ligands on the Pb(II) ion with more covalently binding Cl ions [37]. It appears that increasing covalence in the Pb–L bond moves the 6s2 ? 6sp transition to longer wavelengths on substituting more covalently binding N donors for O donors, as seen in Fig. 5. The 18-crown-6 complex 6s2 ? 6sp transition occurs at longer wavelength than that of the Pb2+(aq) cation, which is possibly due to the greater inductive effects of the ethereal oxygens of 18-crown-6 as compared to water as a ligand [19], leading to greater covalence in the Pb–O bonds of the crown ether complex. A second possible effect on the 6s2 ? 6sp transition of Pb(II) appears to be that of whether the Pb(II) is holodirected or hemidirected. This is, of course, a tentative observation, and requires study of a much larger set of complexes than is the case here. In Fig. 6 are

3.4. The 6s2 ? 6sp transition of Pb(II) The wavelengths at which the 6s2 ? 6sp intra-atomic transition of Pb(II) occur in aqueous solution for a selection of Pb(II) complexes (10 5 M) is shown in Fig. 5. The peaks for a wide range of ligands corresponding to the 6s2 ? 6sp transition appear quite similar in shape, as a simple Gaussian curve, to those seen in Fig. 6 for a few complexes of Pb(II), including the reported [37] spectra of the Pb(II) chloro complexes in aqueous solution. There

Fig. 5. The wavelength at which the 6s2 ? 6sp transition occurs for Pb(II) complexes plus the Pb2+(aq) cation in aqueous solution. All spectra were recorded with Pb(II) = 10 5 M, and ligand concentration and pH adjusted to ensure as near complete formation of the complex as possible.

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Table 3 Absorption wavelengths (k) and oscillator strengths (f) associated with the 6s2 ? 6sp transitions in some Pb(II) complexes as predicted by TDDFT calculations with the PCM solvation.

Fig. 6. The spectra of Pb(II) complexes (Pb(II) = 10 5 M) in aqueous solution showing the 6s2 ? 6sp transition for complexes with two saturated N donors coordinated to the Pb(II). As discussed in the text, it is tentatively suggested that the transition appears broader and occurs at longer wavelength for complexes where the Pb(II) is holodirected than where it is hemidirected.

shown the 6s2 ? 6sp transitions for the edta, egta, 18-ane-N2O4, and cryptand-222 complexes of Pb(II). One notes that the 6s2 ? 6sp transition for the hemidirected edta complex occurs at shorter wavelength (242 nm) than for the egta, 18-ane-N2O4, and cryptand-222 complexes (249–252 nm), which appear to be holodirected. These holodirected complexes all share the structural feature discussed in the introduction of a (NCH2CH2OCH2)2 strap in the ligand which holds the two N donors well apart, and prevents them from simultaneously binding close to the favored antipodal site. The structures of the 18-ane-N2O4 [59] and cryptand-222 [30] complexes have the long Pb–N bonds in the vicinity of 2.8 Å characteristic of holodirected Pb(II), while DFT calculations below support the idea that the egta complex of Pb(II) is holodirected. In contrast, the edta complex has short Pb–N bonds [58] of 2.52 Å, characteristic of a hemidirected complex.

Ligand

k (nm)

f

Composition

Nta

191

0.1296

202 220

0.0702 0.0790

H (H (H H H

Edta

221 234

0.1119 0.1289

H ? L+1 H ? L⁄

83 98

Dota

263

0.1165

H ? L⁄

98

3 ? L+2⁄ 5 ? L+1) 3 ? L+3) 3 ? L+1 3?L

% contribution 17 29 16 80 91

The major contributions to each transition are also reported along with their % contributions. The excitations enclosed by parenthesis involve an orbital with a negligible contribution from Pb(II). The orbitals labeled with an asterisk have 6p contribution aligned along the line connecting Pb and the antipodal site (also labeled in pink in Fig. 7).

are between 170° and 180° (180° = exact planarity, 170° places metal ion some 0.3 Å above the plane of the O donors), this still leads to few examples. By this quite loose criterion, K+ still has only 37% of its 18-crown-6 complexes ‘planar’ (out of 369 structures), Rb+ (59 structures) has 4.5%, and Tl+ (20 structures) has 10%. The very large Cs(I) forms no planar complexes with 18-crown-6 (50 structures). In the structures where the metal ion is not considered by the above criterion to be ‘planar’ in its 18-crown-6 complex, the K+ on average lies 0.60 Å above the plane of the O donors, the Rb+ 0.99 Å, and the Tl+ 0.78 Å. There is nothing in these structures that suggests anything but that in its complexes with 18-crown-6 the Tl+ ion is holodirected and lies mostly above the plane of the O donors because of its size, as suggested by relating these structures to the ionic radii [29] for six-coordination for the relevant cations: K+, 1.38 Å; Tl+, 1.50 Å; and Rb+ 1.52 Å.

3.5. Is Thallium(I) in its 18-crown-6 complexes hemidirected? Mudring and Rieger [17] analyzed the structure of two Tl(I) complexes with 18-crown-6, and on the basis of comparison with the complexes of K(I) and Rb(I) with 18-crown-6 concluded that the Tl(I) 18-crown-6 complexes had what would conventionally be regarded as a stereochemically active lone pair. They then performed DFT calculations on the Tl(I) 18-crown-6 complexes, and concluded that the 6s orbital showed no evidence of hybridizing with a 6p orbital, and that ‘one can state that s–p hybridization on the heavier main-group metals is not responsible for the stereochemical activity of a lone pair’. This analysis is probably not correct for the reason that Tl(I) in its 18-crown-6 complexes does not, as discussed below, actually appear to have a stereochemically active lone pair. It is therefore not surprising that their DFT analysis showed no evidence of sp hybridization, since the essence of holodirected Tl(I) will be the absence of a 6sp hybrid, and the presence of 6s2 orbital occupation. The basis for supposing that the Tl(I) 18-crown-6 complexes were hemidirected was that the Tl(I) in both cases studied [17] resided some 0.65–0.75 Å above the plane of the O donors of the 18-crown-6 ligand. These authors claimed that by contrast K+ and Rb+ showed ‘a strong preference for symmetric complexation by 18-crown-6’, i.e., were coordinated lying in the plane of the O donors. Examination of structures in the CSD [20] shows that this is not true. In very few cases do K+, Rb+, or Tl+ lie exactly in the plane of the O donors of 18-crown-6. Even if one assumes that the metal ion can be regarded as lying in the plane of the O donors where the O–M–O angles diagonally across the 18-crown-6 ligand

Fig. 7. Energy levels related to the 6s2 ? 6sp transitions in some Pb(II) complexes, as calculated from TD-DFT calculations. The arrows indicate the electronic excitations reported in Table 3. The states drawn as bold lines have a major contribution from the 6s orbital, whereas the orbitals shown as broken or dotted lines have more 6p contributions than other nearby states. The orbitals shown as broken lines have a 6p orbital aligned along the line connecting Pb and the antipodal site.

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Fig. 8. Plots of frontier orbitals of the Pb(II)(nta) complex involved in 6s2 ? 6sp transitions. These show the effect of 6s/6p hybridization in the Pb(II) ion that leads to contributions from a non-spherical partly sp hybridized orbital in HOMO 3, displaced toward the position of the lone pair, and aligned along the axis connecting the lone pair and the antipodal site. The LUMO, LUMO+1 and LUMO+2 orbitals have significant contributions from the p orbitals of Pb.

3.6. DFT calculations on Pb(II) complexes with saturated N donors In order to gain some physical insights on the nature of the 6s2 ? 6sp transition in the Pb(II) complexes and its relation to the structural parameters, DFT and time-dependent DFT calculations were performed for a group of closely related Pb(II) complexes. The group includes the Pb(II) nta, edta and dota complexes, where the coordinating oxygen atoms are of the same type (acetate) and the number of donor N atoms increases from one to two and then to four. To mimic solution environment, the polarizable continuum model was employed with no explicit water molecule coordinated to the metal ion. The first thing one noticed was that DFT optimized structures do not predict the Pb–L bond lengths that are consistent with the crystallographic structures of the Pb(II) complexes studied here: in particular, the Pb–N bonds of hemidirected complexes were predicted to be too long. Thus, the average Pb–N bond lengths for the Pb(II) edta complex were predicted to be 2.705 Å, as compared to observed values of 2.52 Å [58]. For the Pb(II) dota complex reported here, the observed Pb–N bond lengths average 2.665 Å, while the DFT predicted Pb–N lengths averaged 2.841 Å. In addition, DFT optimized structures did not predict the fact that Pb–N bonds are shorter than Pb–O bonds, which is typically observed in hemidirected complexes of Pb(II). Thus, in the Pb(II) dota complex studied here, the DFT calculations predicted that the four Pb–O bonds close to the proposed site of the Lp averaged 2.627 Å, significantly shorter than the predicted Pb–N bond lengths averaging 2.841 Å: in contrast, the crystallographic structure for the Pb(II) dota complex reported here has Pb–O bonds that average 2.706 Å (Table 2), significantly longer than the Pb–N bonds that average 2.665 Å. The interpretation is that, as discussed above, the unusual case of Pb(II)–O bonds that are longer than Pb(II)–N bonds, as compared to other metal ions lacking a lone pair, is attributed to the O donors being coordinated to the Pb(II) closer to the site of the lone pair than is the case for the N donors. In spite of the inability of the DFT calculations to predict the shortening of the Pb–N bonds in hemidirected complexes of Pb(II) correctly, the trends observed in the energy levels involving the 6s and 6p orbitals of the Pb(II) are quite instructive. In Table 3, we reported the results of time-dependent DFT calculations, including the absorption wavelengths and the oscillator strengths associated with the 6s2 ? 6sp transitions in Pb(II) nta, edta, and dota complexes. These transitions have significantly larger oscillator strengths than other low-lying excitations nearby. The energy level diagram associated with all transitions reported in Table 3 is also shown in Fig. 8. For each system, the most intense excitation involves the 6p orbital that is better aligned along the line connecting Pb(II) and the antipodal site. It is clear from the Table 3 and Fig. 8 that TD-DFT calculations were able to reproduce the general trend we observed in Fig. 6: excitation shifts to longer wavelengths as the number of N donors

in the complex increases. For the Pb(II) nta complex, the excitations from the state with a major contribution from 6s (HOMO 3) to all three 6p states (LUMO, LUMO+1 and LUMO+2) are allowed with large oscillator strengths. The frontier orbitals involved in these transitions are plotted in Fig. 8. It is evident from the plot of HOMO 3 that electron density is found more on the side of Pb opposite to the coordinated N donor atom. This is possible because the contribution to HOMO 3 from Pb(II) has significant 6p character (15% 6p and 85% 6s), leading to asymmetric distribution of 6s electrons around Pb. The DFT calculations suggest that the degree of mixing of 6p orbital with 6s decreases as the number of N donor increases. For the edta complex, the 6s orbital (HOMO) has 9% 6p character, and in the dota complex it is calculated that the 6s orbital (HOMO) has only 2% 6p character. It can be seen from Fig. 8 that the decrease in 6p contribution pushes the energy of the 6s orbital upward, resulting in the increase of absorption wavelengths. The shift of the 6s2 ? 6sp band to longer wavelengths thus appears to correspond to a drop in the p character of the 6s orbital. In the same vein, increasing the number of N donor will shift the N donors from being coordinated close to the antipodal site opposite the lone pair to being closer to the site of the Lp, leading to a shift to longer wavelengths. This is further confirmed by the DFT/TD-DFT study on more holodirected Pb(II) egta complex. It was found that the excitation wavelength associated with the transition from 6s (HOMO) to the lowest energy 6p (LUMO) is longer in the Pb(II) egta complex (241 nm) than in the Pb(II) edta complex. Given the fact that the Pb(II) egta and Pb(II) edta complexes have the same number of N donors, pushing N donors away from the antipodal site is most likely the cause of absorption being shifted toward the longer wavelengths.

4. Conclusions The Pb(II) dota complex appears to be hemidirected in terms of the criteria that (1) there is a gap in the coordination sphere, or alternatively a very long Pb


O bond or contact of 3.022 Å to a water molecule above the proposed site of the Lp, (2) the Pb–N bonds to the dota nitrogens average 2.665 Å, and are fairly short in terms of the criteria that such bonds may be as short as 2.4 Å in strongly hemidirected complexes of Pb(II), and the shortest Pb–N bonds are close to 2.8 Å in Pb(II) complexes that are holodirected. The Pb–N bonds at about 2.67 Å in the dota, dotam [34,35] and THP–cyclen [52] complexes suggest weaker holodirectedness, sterically controlled by the architecture of these ligands. This steric control prevents the four N donors of dota from all closely approaching the favored antipodal site, (3) the Pb–O bonds in the dota complex average 2.772 Å, which is longer than the Pb–N bonds at 2.665 Å, which effect is not found in dota, dotam, or THP–cyclen complexes of other large metal ions such as Sr(II) or La(III) that do not have a stereochemically active Lp. This satisfies

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criterion 3, which requires that the Pb–L bonds become progressively longer as the bonds move away from the antipodal site and approach the site of the Lp. An important diagnostic structural feature in analyzing the holodirected/hemidirected state of a Pb(II) complex is the length of Pb–N bonds in complexes containing a mixture of mainly N and O donors: if there are any Pb–N bond lengths close to 2.4 Å in length, these must be near the antipodal site, and the complex is hemidirected. If the shortest Pb–N bonds are 2.8 Å or longer, and there are no other short Pb–L lengths, then the complex is holodirected. If there are Pb–N bonds intermediate in length between 2.4 and 2.8 Å, the complex is very likely hemidirected, but more weakly so, which would be the case for the Pb(II) dota, dotam [34,35], and THP–cyclen [52] complexes, with Pb–N bonds close to 2.67 Å in length. In a study of the Tl(I) complexes of 18-crown-6, it was claimed that Tl(I) had a stereochemically active lone pair [17], i.e., was hemidirected. DFT calculations reporting [17] that there is no 6sp hybrid present in these complexes were then used to claim that in hemidirected complexes of Tl(I), as well as Pb(II), 6sp hybridization does not contribute to the structural effects observed for heavy metal ions with an ‘inert’ pair of electrons. However, analysis of the 18-crown-6 complexes of Tl(I) and alkali metal ions suggests strongly that Tl(I) in its 18-crown-6 complexes is holodirected. Therefore, these calculations do not allow one to draw the conclusion, as claimed [17], that 6sp hybridization plays no part in the structures of heavy metal ions with an ‘inert’ pair of electrons. The 6s2 ? 6sp transition in Pb(II) complexes with combined N and O donor sets shows a strong shift to longer wavelength as the set of coordinated donor atoms is comprised of more N donors, being at 209 nm in the Pb2+(aq) ion, and at 258 nm in the Pb(II) dota complex with four N donors. Within this set of shifts in the 6s2 ? 6sp transition, for complexes with N2On donor sets, the band occurs at shorter wavelengths in a complex such as the edta complex, whose structure [58] suggests it is hemidirected, as compared to complexes such as the cryptand-222 and 18-ane-N2O4 complexes [32,59] of Pb(II), whose structures suggest that they are holodirected. DFT calculations have been found not to reproduce well the structures of the Pb(II) complexes studied here with donor sets comprised of N and O donors: however, they can be tentatively viewed as showing that the further the more covalent N donors are from the antipodal site on the Pb(II), the less is the 6p character of the sp hybridized 6s orbital, and the less strongly hemidirected is the complex. The limited work reported here on the 6s2 ? 6sp transition of Pb(II) complexes suggests that this transition could become an important tool in understanding the structure and bonding in complexes of Pb(II).

Acknowledgements The authors thank the University of North Carolina Wilmington for a research assistantship to J.W.N., and for generous support for this study.

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