Halogen_Bonding
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X-ray Structures and Electronic Spectra of π-Halogen Complexes |
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Fig. 9 Solid-state spectra of bromide complexes with TCP (solid lines): 1 Pr4N+[Br–, (TCP)4], 2 (Et4N+)2[(Br–)2,(TCP)3], 3 Bu4N+ [Br–,(TCP)4]. Note: spectra of the corresponding complexes in solution are shown as gray dashed line [23]
Table 3 Solid-state characteristics of halide associates with π-acceptors a
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Molar ratio |
X–· · ·C b |
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˚ |
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[A] |
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Br–/TCP |
2 |
: 3 d |
3.16 |
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1 |
: 4 e |
3.15 |
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I–/TCP |
1 |
: 2 d |
3.52 |
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1 |
: 1 f |
3.49 c |
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Cl–/TCP |
1 |
: 4 f |
3.07 |
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NCS– /TCP |
1 |
: 1 f |
2.951 i |
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Br–/TCNE |
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: 1 d |
3.288 j |
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2 |
3.20 |
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1 |
: 1 e |
3.11 |
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1 |
: 1 f |
3.20 |
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I–/TCB c |
1 |
: 2 g |
3.46 |
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1 |
: 2 h |
3.45 |
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Br–/TCB c |
1 |
: 2 h |
3.34 |
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Br–/o-CA |
1 |
: 1 e |
2.93 |
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a |
From [23] unless noted otherwise |
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b |
Halide–carbon distances with closest contacts. |
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˚ |
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Note that van der Waals radii are (in A) 1.70 (C), 1.52 (O), 1.55 (N), 1.80 (S), 1.80 (Cl), |
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c |
1.85 (Br), 2.1 (I) [20] |
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From [24] |
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d |
Et4N+ salt |
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e |
Pr4N+ salt |
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f |
Bu4N+ salt |
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g |
Na(18-crown-6)+ salt |
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h |
K(18-crown-6)+ salt |
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i |
C· · ·N separation |
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j |
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C· · ·S separation |
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S.V. Rosokha · J.K. Kochi |
tronic spectra of the corresponding of 1 : 1 complexes [X–/TCP] measured in solution (Fig. 9) [23].
However, X-ray analysis of these salts reveals the overall ratio of the acceptor to donor to vary (depending on the counterion and halide) from 4 : 1 to 1 : 1 (Table 3), and they show halide anions in close contact with two to four acceptor moieties, as illustrated in Fig. 10.
Fig. 10 Fragment of crystal structures of tetracyanopyrazine complexes with halides showing coordination of anions to four, three or two acceptor moieties in Bu4N+[Cl–, (TCP)4] (a), Et4N+[(Br–)2, (TCP)3] (b) and Bu4N+ [I–, (TCP)] (c) salts, respectively [23]
In related complexes of bromide and iodide anions with tetracyanobenzene, the halide anions are also surrounded by four acceptor molecules [24]. The coordination of the halides in two of these moieties is similar to that observed in TCP complexes, i.e., the anion is arranged above (or slightly outside) of the ring and forms close contacts with the cyano-bearing carbons. On the other hand, coordination with the third TCB occurs via the unsubstituted carbon, and the halide is positioned far outside the ring in this case The fourth acceptor moiety is hydrogen-bonded to the halide (Fig. 11).
Fig. 11 Fragment of the crystal structures of tetracyanobenzene/bromide complex showing different modes of anion coordination to the acceptor moieties (coordinates from [24])
Notably, the thiocyanide anion as a pseudohalide forms similar complexes with the aromatic π-acceptors. Indeed, in the recently characterized complex
X-ray Structures and Electronic Spectra of π-Halogen Complexes |
153 |
with tetracyanopyrazine, the NCS– is arranged over one side of the acceptor moiety (Fig. 12), and the presence of very short nitrogen and sulfur contacts with the cyano-bearing carbons (Table 3) indicate an especially strong donor/acceptor bonding3.
Fig. 12 Fragment of crystal structure of Bu4N+[NCS–, TCP] salt showing close contact of nitrogen and sulfur atoms with the acceptor moiety (Han et al. private communication)
In anion–π interactions with electron-deficient (neutral) aromatic π-ac- ceptors, the halide lies preferentially over the periphery of the aromatic ring (as illustrated in Fig. 13a) and this is apparently related to the shape of the acceptor LUMO (presented for comparison in Fig. 13b).
Fig. 13 a Location of the halide anions above the tetracyanopyrazine (◦) and tetracyanobenzene ( ) π-systems (adapted from [24]) in comparison with b the LUMO shape of the TCP acceptor
3 |
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Han et al. private communication. Note that the S – C distance of 3.29 A in the complex with TCP |
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– |
associates with CBr4 |
[53]. |
is close to separations of 3.16, 3.24 and 3.27 A measured in the NCS |
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S.V. Rosokha · J.K. Kochi |
On the other hand, the over-the-center coordination in a “carousel” copper(II)-triazine complex (Fig. 14) [92] appears to be the result of multiple hydrogen bonding and steric effects.
Fig. 14 Fragment of crystal structure of “carousel” copper(II)-triazine complex showing over-the-center coordination of chloride anion to a triazine (coordinates from [92])
In a similar manner, the diffusion of hexane into dichloromethane solutions containing mixtures of the alkylammonium salts of bromide and the olefinic acceptors o-CA and TCNE result in the formation of brown-red crystals [23]. X-ray analysis reveals the (1 : 1) complex of bromide with o-CA, in which the anion is located over the center of the C – C bond of the accep-
tor moiety (Fig. 15b) and Br |
– |
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· · ·C contacts are shortened by as much as 0.6 A |
relative to the sum of van der Waals radii (Table 3). In bromide complexes with TCNE, the location of the anion relative to the acceptor is variable. In fact, a 2 : 1 complex [(Br–)2,TCNE] is isolated in which both anions reside over the olefinic bond when the tetraethylammonium salt of bromide is used. In comparison, if the tetrapropylor tetrabutylammonium salts of the same anion are employed, the (1 : 1) complexes [Br–,TCNE] are formed in which the bromide donors are shifted toward the cyano substituents (Fig. 15a). In both cases however, the short intermolecular separations that are characteris-
Fig. 15 Molecular structures of bromide complexes with TCNE (a) and o-CA (b) acceptors
X-ray Structures and Electronic Spectra of π-Halogen Complexes |
155 |
tic of π–π bonded CT complexes [26–28] are indicative of strong anion/TCNE interactions (Table 3).
3.4
Donor/Acceptor (Structural) Effects on π-Halogen Interactions
3.4.1
Intermolecular Separations Relevant to Halogen Bonding
Halogen–carbon separations in π-complexes are significantly less than the sum of their van der Waals radii. It is notable, however, that the contrac-
0.4 ˚ relative to equilibrium van der Waals separations, which
tions
of A
are observed in these Br2 complexes, are somewhat less than those measured earlier with various n-type donors. For example, the X· · ·Br distance contraction (relative to the corresponding equilibrium van der Waals sepa-
rations) is 0.55 |
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0.56 |
˚ in the |
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A in the acetone/Br2 complex (O· · ·Br 2.82 A), |
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2– |
A |
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acetonitrile/Br |
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the [Te Cl |
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/Br |
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com- |
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complex (N· · ·Br 2.84 A), |
0.57 A in |
2– |
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10 |
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/Br2 |
complex (Br· · ·Br |
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plex (Cl· · ·Br 3.03 A), and |
0.60 A in the [Se2Br10] |
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3.10 ˚) [41]. A similar tendency is observed in tetrabromomethane com-
A
plexes with π-type (aromatic) vs. n-type donors, with the former showing
contractions of up to 0.3 ˚ relative to the sum of the van der Waals radii,
A
while the contraction in the oxygen, nitrogen, or halide complexes reach
as much as 0.5–0.8 ˚. Most importantly, the shortening of the interatomic
A
distances within various halogen complexes apparently correlates with the donor/acceptor strength of the components. Thus, the average C· · ·Br sepa-
ration of 3.156 ˚ in the toluene/Br2 complex is somewhat shorter than that
A
in the benzene complex (3.18 ˚), as expected from the better donor strength
A
of toluene [63, 64]. In the carbon tetrabromide complexes with π-donors, the separation between the bromine atom and the benzene plane is decreased
3.34 ˚ in the tetrabromomethane complex with the weak p-xylene from A
donor to 3.21 ˚ in the associate with the stronger durene donor, and further
A
to 3.14 ˚ in the complex with OMTP (oxidation potentials for these donors
A
are 2.01, 1.84 and 1.75, respectively [53]. Such data provide clear indications of the increase in the halogen-bonding strength (structurally represented as the interatomic distance) with increasing donor/acceptor strengths that are similar to those observed in halogen bonding with n-type donors [53].
3.4.2
Molecular Geometries of Donor/Acceptor Moieties
The C(arene)· · ·Br bonding does not markedly perturbed the geometry of the dibromine, which is rather sensitive to coordination/ polarization effects and
the bond readily elongates from 2.284 ˚ in the non-coordinated molecule to
A
2.53 ˚in the [Br3]– anion [41]. Indeed, the Br – Br bond lengths of 2.301(2) ˚
A A
156 S.V. Rosokha · J.K. Kochi
in the benzene complex and an average of 2.302(1) ˚ in the toluene com-
A
plex do not exhibit significant elongation during complex formation, with
the longest Br – Br bond length being 2.307(1) ˚. The shortest contact C· · ·Br
A
3.053(4) ˚ is found in the toluene complex with unsymmetrical coordination
A
of bromine. Interestingly, a similar asymmetric coordination of dibromine is found in the complex with methanol, in which the O· · ·Br distance is shorter
˚ ˚
(2.705 vs. 2.723 A) and the Br – Br bond length is longer (2.324 vs. 2.303 A) than those in the closely related (but symmetric) dioxane complex. However, the precision of the bond-length determination (σCC = 0.6 pm) is insufficient to allow the detection of (small) polarization effects in the arene donor since such changes in C – C bonds are typically less than 0.5 pm [41].
Minimal changes are also observed in the C – Br bond length of coordinated tetrabromomethane. Indeed, the average C – Br bond length in tetrabromomethane for most of the π-bonded complexes in Table 2 is about
± 0.003 ˚, i.e., within the accuracy limit of the free acceptor (measured 1.930 A
at 123 K) of 1.930 ± 0.006 ˚ [5]. The somewhat higher tetrabromomethane
A
average bond length of 1.941 in the complex with durene is still within 3σ of that in the free acceptor to preclude a reliable conclusion to be drawn. Notably however, in complexes with n-type donors, the elongation of the C – Br bond is more pronounced and shows some correlation with donor strength [53]. In a similar way, the small degree of charge transfer occurring in the halide complexes is insufficient to produce notable changes in the molecular geometry of the aromatic and olefinic acceptors.
4
Summary and Conclusions
X-ray structural analyses reveal that the π-bonding of dihalogens, halocarbons and halides to arene donors and acceptors are characterized mostly by over-the-rim coordination in which the dihalogen acceptor generally follows the position of highest electron density on the aromatic donor, and the arrangement of halide donor mostly follows the LUMO shape of the aromatic acceptor.
In the arene complexes of halogen acceptors, the X – X or X – R bonds are directed perpendicular to the aromatic planes, in comparison with nearly 180 deg between these bonds and halogen bonds with n-type donors. The halogen-bond lengths show an apparent correlation with the donor/acceptor strengths: the stronger donor and/or acceptor leading to more significant shortening of the Br· · ·X separation. However, the effects of π-complex formation on the reactant moieties are rather minor, indicating a relatively small degree of donor/acceptor charge transfer.
The structures of the benzene/Br2 and toluene/Br2 complexes at 123 K show over-the-rim coordination with hapticities varying from about 1.5
X-ray Structures and Electronic Spectra of π-Halogen Complexes |
157 |
to 1.9, but X-ray measurements of the benzene/Br2 associate at 230 K are consistent with the symmetrical arrangement of dibromine over the ring center. π-Bonded complexes of tetrabromomethane also show over-the-rim and over-the-center arrangement of the coordinated bromine atom. Furthermore, the halide associates with tetracyanoarenes are characterized by significant scattering of the anion positions over the aromatic ring. In the complexes with tetracyanoethylene, the bromide anion is located over the double bond and also shifted toward the cyano group. Such structural data suggests that various modes of coordination are possible. Such a structural variability on the halogen π-bonding is reminiscent of that observed in the π-complexes of aromatic ion-radicals with their diamagnetic parents [93]. The latter suggests that in long-distance π-bonding, the subtle balance between the attractive interaction of partially occupied frontier orbitals vis á vis the repulsion of filled atomic orbitals led to several shallow, close in energy, local minima involving various mutual donor/acceptor arrangements. As such, we posit that the interactions between halogen acceptors and π-donors (similar to ion-radical π-bonding) can be readily modulated by temperature, electrostatics, crystal packing, solvation, etc., to produce a variety of polymolecular associates within a relatively narrow range of intermolecular separations.
Spectral studies of the intermolecular interaction of dihalogens, halocarbons and halide anions with various organic π-receptors (including the unified Mulliken dependence of their absorption bands) show the direct relationship of the spectral characteristics and formation thermodynamics of the corresponding associates with those of traditional organic donor/acceptors complexes. This indicates the common (charge-transfer) origin of the longdistance bonding of halogen centers. Such a conclusion is of particular interest for the π-interactions of halides, since the formation constants of the halide complexes with neutral π-acceptors, together with their intense absorptions and compression of the intermolecular separations found by X-ray structural analysis, indicate the existence of substantial anion–π interactions. As such, we believe that the relatively strong complex formation together with the distinctive colorations of various anion–π interactions encourage their use in the design of anion-sensing receptors, provided systems with multicentered binding sites are offered for optimal recognition.
Acknowledgements We thank the R.A. Welch Foundation for financial support of this study.
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