Halogen_Bonding

Table 2 (continued)

BAQTOCf	Triphenylphosphine sulfide tris(diiodine)	2.591		2.982			178.11			BA
CEWMOGg	Bis(2-imidazolidinethione) tris(diiodine)	2.580		2.984			177.67			BA
ICUXAGh	2-Thioxo-1,3-dithiole-4-carboxylic acid-S-diiodine hemikis(diiodine)	2.733		2.819			175.72			BA
LOPQEMi	Bis(thiourea-diiodine) diiodine	2.503		3.054			176.04			BA
TPHPSI10j	Bis(S-iodine-triphenylphosphinesulfide) iodine	2.729		2.837			175.24			BA
TURMEYk	Bis(1,3-diisopropylimidazolidine-2,4,5-trithionato)-nickel(II)	2.824		2.815			177.48			BA
	diiodine
TURMICl	Diiodo-bis(1,3-diisopropylimidazolidine-2,4,5-trithionato)-	2.849		2.790			178.78			BA
	nickel(II) bis(1,3-diisopropylimidazolidine-2,4,5-trithionato)-
	nickel(II) diiodine
XOVREFm	Bis(benzimidazole-2-thione-(diiodine)) diiodine dihydrate	2.572		2.989			176.76			BA
XOVROPn	Benzothiazole-2-thione-(diiodine) diiodine	2.588		2.969			177.79		◦	BA
REFCODE	Compound name	+	˚	+	–	˚	+	–		) Mode
		Bd– X (A)		X	· · · X	(A) Bd-X		· · · X	(
LOPQIQ	1,3-Bis(thiourea)triiodonium thiourea-diiodine diiodine triiodide	2.437		3.168			171.59			I
		2.466		3.145			176.08
S· · · I–Br			˚		˚		Bd · · ·		◦
REFCODE	Compound name									Mode
		Bd · · · Xa (A)		Xa–Y (A)				Xa–Y ( )
BIBQUY	1,4,8,11-Tetrathiacyclotetradecane bis(iodobromide)	2.679		2.654			175.52			A
BIBSIO	1,5,9,13-Tetrathiacyclohexadecane tetrakis(iodobromide)	2.617		2.705			177.64			A
		2.687		2.645			177.59
BIBSOU	(1,4,7,10,13,16-Hexathiacyclooctadecane) bis(iodobromide)	2.620		2.694			174.95			A
BIMNUG	4,5,-Bis(methylsulfanyl)-1,3-dithiole-2-thione-iodine monobromide	2.589		2.714			175.62			A
DTHIBR10	1,4-Dithiane bis(iodine monobromide)	2.687		2.646			178.16			A
HAMCEE	4,5-Bis(bromomethyl)-1,3-dithiole-2-thione-bromoiodine	2.597		2.747			178.13			A
		2.564		2.765			176.13

Interhalogens and Dihalogens with Bonding Halogen

81

Table 2 (continued)

IPOGUP	4,5-Bis(2-cyanoethanesulfanyl)-1,3-dithiole-2-bromiodothione	2.605		2.709		177.24				A
	(N,N -Dimethylperhydrodiazepine-2,3-dithione)-(iodobromide-S)	2.607		2.706		177.71
JADYAP		2.574		2.752		175.90				A
LOSWEV	Dimethyl 1,3,-dithiole-2-(bromoiodo)thione-4,5-dicarboxylate	2.605		2.711		178.01				A
	iodomonobromide
NAHQOD	Dibenzylthio iodine bromide	2.616		2.690		174.80				A
QAQTAE	S-(Bromoiodo)-N,N -dimethylbenzimidazole-2(3H)-thione	2.607		2.751		173.28				A
ZZZHUE01	Bromoiodo-triphenylphosphine sulfide	2.665		2.668		175.08				A
ZZZHUE02	Bromoiodo-triphenylphosphine sulfide	2.656		2.683		175.13				A
S· · · I–Cl
ICUWUZ	Dimethyl 2-thioxo-1,3-dithiole-4,5-dicarboxylate-S-iodine chloride	2.591		2.603		176.11				A
LIFXIH	N-Methylbenzothiazole-2(3H)-thione iodine chloride	2.556		2.604		179.89				A
NAHQIX	Dibenzylthio iodine chloride	2.575		2.558		176.14				A
SIBJOC	Chloroiodo-triphenylphosphine sulfide	2.641		2.586		174.86				A
HAMCIIo	4,5-Bis(bromomethyl)-1,3-dithiole-2-thione-chloroiodine	2.534		2.761		176.57				BA
	hemikis(diiodine)
S· · · Br–Br
RORNIV	Dimethylsulfide-dibromide	2.299		2.717		175.05				A
RORNIV01	Dibromo-dimethyl sulfide	2.328		2.705		176.05				A
TEBKIU	1,2,4,5-Tetrakis(ethylthio)benzene bis(dibromine)	2.808		2.408		171.54				B
		3.239				164.45			◦
REFCODE	Compound name	+	˚	+	–	˚	– +	–		) Mode
		Bd– X (A)		X · · · X		(A) B d	X	· · · X (
BEMLIO	5-Bromosulfenyl-1,3,2,4-dithiadiazolium tribromide	2.206		3.204		174.53				I
FIMDOU	Bromodimethyl sulfide bromide tetrabromodiide	2.260		2.969		178.67				I

82

.al et Pennington .T.W

Table 2 (continued)

YESGUY			1,3-Diisopropyl-4,5-dimethylimidazolium-												2.167		3.217		155.59	I
			2-bromodithiocarboxylate tribromide
a B	d	represents the electron donor; X				a	represents the electron acceptor
REFCODE					˚			˚			◦	)				◦	)
			Xd	–Xa	(A)		Xa	–X (A)	Xa –Xs	· · · Xd	(		Xs · · · Xa		–X	(
bCEWMIA			3.003				2.849		92.42				177.13
c HAMCAA			3.270				2.747		81.70				175.53
dVUTPEF			3.403				2.736		80.44				176.13
eZEBQOM			3.550						79.03
			3.453				2.729		88.39				178.44
f BAQTOC			3.373				2.744		93.02				175.35
			3.561				2.710		86.30				178.53
			3.503						82.75				177.27
g CEWMOG			3.582						86.06				171.24
			3.475				2.760		85.12				177.70
hICUXAG			3.441				2.725		80.90				175.50
iLOPQEM			3.407				2.754		84.64				175.20
j TPHPSI10			3.570				2.757		96.30				176.67
k TURMEY			3.213				2.755		93.68				168.98
lTURMIC			3.671				2.786		86.56				176.24
mXOVREF									102.09				175.75
			3.399				2.767		88.74				176.02
n XOVROP			3.423				2.750		102.71				174.04
o HAMCII			3.445						78.46				172.26
			3.257				2.708		84.67				174.57

Interhalogens and Dihalogens with Bonding Halogen

83

Table 3 Geometric parameters for halogen bonded complexes with arsenic and selenium electron donors

	As· · · I–I		˚	a	˚	◦
	REFCODE	Compound name					Mode
			B d · · · Xa (A)		Xa –Y (A)	Bd · · · Xa –Y ( )
	FESKAP	Diiodo-triphenylarsane	2.641		3.004	174.81	A
	FESKAP01	(Triphenylarsine)-diiodine	2.653		3.005	174.32	A
	FESKAP02	(Triphenylarsine)-diiodine	2.615		3.013	180	A
	FESKAP10	(Triphenylarsine)-diiodine	2.641		3.005	174.79	A
	FUTTAP	(Triphenylarsine)-diiodine toluene solvate	2.638		3.002	180	A
	ZODJOR	Diiodo-trimethyl-arsenic	2.271		3.392	180	B
	As· · · I–Br
	CUXCON	(Iodobromo)-triphenyl-arsenic(III)	2.590		2.855	174.78	A
	As· · · Br–Br		˚		˚	◦
	REFCODE	Compound name					Mode
			Ba · · · Xd (A)		Xd–Y (A)	Ba · · · Xd–Y ( )
	TMASBR01	Dibromo-trimethyl-arsenic	2.275		3.363	180	I
	Se· · · I–I		˚		˚	◦
	REFCODE	Compound name					Mode
			Bd · · · Xa (A)		Xa –Y (A)	Bd · · · Xa –Y ( )
	DSEIOD	1,4-Diselenane bis(iodine)	2.830		2.870	178.67	A
	EZOXUM	Bis(tri-t-butylphosphine)-bis(diiodo)-diselenium	2.760		2.914	171.69	A
		(m2-iodo)-bis(tri-t-butylphosphine)-diselenium triiodide
	GIHZIG	Diphenyldiselenide iodine	2.992		2.774	174.21	A
	HECMOR	Di-t-butyliodophosphane selenidediiodine	2.782		2.878	175.84	A
	KUWDUB	5,5-Dimethyl-2-selenoimidazolidine-4-one diiodine	2.699		2.962	170.73	A
	OXSELI	1,4-Oxaselenane iodine	2.755		2.955	174.77	A
	PAQKAT	Triphenylphosphineselenido-diiodine	2.802		2.881	173.67	A
	PAQKEX	(Tris(dimethylamino)phosphine)selenide-diiodide	2.721		2.965	176.78	A
			2.711		2.960	177.35

84

.al et Pennington .T.W

Table 3 (continued)

	PAQKIB	(Tris(diethylamino)phosphine)selenido-diiodide	2.715	2.985	178.05	A
	PELHUK	6-Propyl-2-(diiodoseleno)uracil	2.781	2.893	176.75	A
	REBNER	Bis(N,N -dimethylimidazolidin-2-yl)-di-selenone	2.683	3.025	175.52	A
		bis(triiodide) N,N-dimethylimidazolidine-2-selone iodine
	RIZMES	Dimethyl-selenium diiodine	2.768	2.915	174.31	A
	RUQPOI	1,3,5-Triselenacyclohexane diiodine	2.734	2.944	179.31	A
	THSEL01	Tetrahydroselenophene iodine	2.765	2.913	179.06	A
	YEYFEN	(N-Methyl-1,3-thiazolidine-2(3H)-selenone)-diiodine	2.725	2.983	173.27	A
	ZOBDOJ	1,1 -Bis(3-methyl-4-imidazolin-2-selenone)methane	2.716	2.996	175.63	A
		bis(diiodine)	2.776	2.912	176.86
	KIGFEL	Bis(2,4,6-triisopropylphenyl) diselenide diiodide	3.483	2.722	169.18	B
	REFCODE	Compound name	˚	˚	◦	Mode
			Bd · · · Xa (A)	Xa–Y (A)	Bd · · · Xa–Y ( )
	RIJQACb	(Tris(dimethylamino)phosphaneselenide)iodine(I) triiodide	2.596	3.215	174.78	AA
	RIJQEGc	(Tris(N-morpholyl)phosphane-selenide)iodine(I) triiodide di-iodine	2.590	3.186	172.01	AA
						BA
	YEYFOXd		2.639	3.071	176.66	AA
			2.639	3.071	176.66	BA
			2.662	3.059	178.84	BA
			2.662	3.059	178.84	BA
			2.720	2.960	171.52	BA
	Se· · · I–Br		˚	˚	◦
	REFCODE	Compound name				Mode
			Bd · · · Xa (A)	Xa–Y (A)	Bd · · · Xa–Y ( )
	NOWLOA	Se,Se-Diphenylselenium bromoiodide	2.808	2.641	177.29	A
	NOWLUG	Se,Se-Dimethylselenium bromoiodide	2.664	2.797	175.82	A
	WIPPAM	Se-(Bromoiodo)-N-methylbenzothiazole-2-selone	2.636	2.813	177.10	A

Interhalogens and Dihalogens with Bonding Halogen

85

Table 3 (continued)																							86
REFCODE			Compound name												˚		˚	Bd · · · Xa –Y (	◦	)	Mode
													Bd · · · Xa (A)				Xa–Y (A)
YEYFUDe			N-Methylbenzothiazole-2(3H)-selenone-iodine dibromoiodide										2.564				3.129	174.31			AA
Se· · · I–Cl
REFCODE			Compound name												˚		˚	Bd · · · Xa –Y (	◦	)	Mode
													Bd · · · Xa (A)				Xa–Y (A)
LIGFIQ			N-Methylbenzothiazole-2(3H)-selenone iodinechloride										2.625				2.690	178.89			A
LIGFIQ01			Se-(Chloroiodo)-N-methylbenzothiazole-2-selone										2.618				2.690	178.75			A
OXSEIC			1-Oxa-4-selenacyclohexane iodine monochloride complex										2.630				2.731	175.76			A
Se· · · Br–Br
IRABEI			5 : 5 -Tribromo-5, 10 -dibromo-5,10-bromo-										2.645				2.358	174.19			A
			bis(5,10-selenanthrene) bis(5,5-dibromoselenanthrene)
a B	d	represents the electron donor; X			a	represents the electron acceptor
REFCODE				˚		˚			◦	)				◦	)
			X -X (A)			X -X (A)	Xa– X · · · X	d	(		X	· · · X	– X	(
			d	a		a	s				s	a
bRIJQAC			3.175			2.778	80.59				173.44
cRIJQEG			3.100			2.786	89.29				179.35												.W
			3.338			2.754	165.87				174.43
dYEYFOX			3.155			2.746	85.46				178.25												.T
																							Pennington
			3.670			2.762	73.95				165.11
			3.341			2.762	117.98				173.28
			3.525			2.764	97.20				176.44
eYEYFUD			3.380			2.764	108.99				176.48												et
			2.803			2.645	89.27				175.37
																							.al

Halogen Bonding with Dihalogens and Interhalogens

87

Fig. 2 Structures illustrating bonding modes from Fig. 1. a PAQKIB, mode A; b DOXABR, mode B; c HAMCAA, mode AA; d QARGIZ, mode BA

reduced. This mode is illustrated in Fig. 2b for the dioxane complex of dibromine (DOXABR), in which interactions at either end of the Br2 molecule with oxygen electron donors of two different dioxane molecules result in an extended donor–acceptor chain.

For some strong electron donor molecules the polarization of the X2 molecule may be sufficient that the X atom not complexed to B serves as an electron donor to a second X2 molecule, i.e., the dihalogen is “amphoteric”, acting as a Lewis acid to Lewis base B, and as a Lewis base to the second X2 molecule, acting as a Lewis acid. For a 1 : 1 B · X2 : X2 ratio, an extended adduct (Fig. 1, mode AA) is formed, as illustrated in Fig. 2c for 4,5- bis(bromomethyl)-1,3-dithiole-2-thione-diiodine diiodine (HAMCAA) [58]. This is often referred to as an “extended spoke” structure. If the second X2 acts as Lewis acid acceptor at either end of the molecule, then a bridged amphoteric adduct (Fig. 1, mode BA) is formed, as illustrated for (acridine · I2)2 · I2 (QARGIZ) [31] in Fig. 2d.

2.2

Nitrogen and Oxygen Electron Donors

Second only to sulfur-based systems, nitrogen complexes are relatively well represented in the structural literature with 41 complexes reported. Of these, 25 are with I2 as the electron acceptor, 11 are with the interhalogen ICl, three are with Br2, and two are with IBr. As expected, in every case the halogen bond forms between the nitrogen and the softest halogen atom, i.e., iodine, in all of the complexes except those with dibromine. Most N · I2 complexes, and all N · Br2, N · IBr, and N · ICl complexes are simple adducts, mode A. Exceptions for the diiodine complexes include: bridging mode (B) observed for diazines, such as pyrazine [86], tetramethylpyrazine [86], phenazine, and quinoxaline [87], and for 9-chloroacridine [89] and the 1 : 1 complex of diiodine with hexamethylenetetramine [144]; and amphoteric bridging mode (BA) observed for 2,2 -bipyridine [85], acridine [89], 9-chloroacridine [89], and 2,3,5,6-tetra-2 -pyridylpyrazine [91]. The occurrence of both B and BA complexes with 9-chloroacridine, and of B and A complexes and an

88	W.T. Pennington et al.

ionic salt [145] for tetramethylenetetramine, highlights the obvious difficulty in predicting which mode will be favored for a given donor–acceptor pair.

Oxygen complexes with dihalogens are relatively rare, with only eight complexes known, and none of these involves interhalogens. Diiodine and dibromine complexes are equally represented with four and three known examples of each, respectively, and one dichlorine complex is known. It should be pointed out that this complex, dioxane dichlorine [41], is the only halogen bonded complex of dichlorine that has been structurally characterized. As opposed to those observed for nitrogen, all of the oxygen complexes except one crystallize with bridging dihalogens (mode B). The sole exception is a relatively complex manganese formate salt, in which an I2 molecule interacts as a simple adduct [146].

2.3

Phosphorus and Sulfur Electron Donors

True halogen bonded complexes with phosphorus as the electron donor atom are even more rare than those with oxygen, and all of these involve diiodine as the electron acceptor. Three examples of simple adducts and one with a bridging diiodine were found. All other phosphorus halogen complexes are better described as salts with ion pairs closely associated through X· · ·X interactions. One bromo- (JOMSEJ) and two iodo salts (SORPUK and ZEKMOR) are related to simple adduct (mode A, Eq. 1) complexes, in which the P· · ·X interaction has strengthened to a covalent bond, while the X–X bond has weakened to the point of being an interaction. Likewise, one bromo- (SUHJIO) and five iodo- (LOBKOC, LOBKUI, LOBLAP, FILTEZ, and FILTID) salts with trihalide anions can be considered as coming from extended adducts (mode AA, Eq. 2).

Equation 1

Equation 2

With 112 structurally characterized complexes, sulfur-based electrons are by far the most commonly encountered. This is attributed to its importance in antithyroid drugs, as described above. The majority of these complexes

Halogen Bonding with Dihalogens and Interhalogens

89

(87) have diiodine-based electron acceptors, but all of the halogens and interhalogens except for dichlorine are represented. Of the diiodine complexes, 60 are simple adducts, 17 are bridging, four are extended adducts, and nine are bridged extended adducts. One ionic salt is included (LOPQIQ) due to the presence of a simple adduct structure in the asymmetric unit in addition to two iodonium cations closely linked to one I3– and an additional discrete I3– anion. All of the interhalogen IBr complexes are simple adducts, as are all of the ICl complexes, except one (HAMCII) [59] which is a bridged extended adduct, with the chlorine atoms acting as Lewis base donors to a bridging I2 molecule. Two of the three known Br2 complexes are simple adducts, while the third is bridging. In addition, three bromo salts are known, two with S – Br+ cations and Br3– anions (BEMLIO and YESGUY), and one with bis-donor-iodonium cations and an I–· · ·I2· · ·I– anion (FIMDOU).

2.4

Arsenic and Selenium Electron Donors

Few arsenic based complexes have been characterized. Three sets of structural data (FESKAP, FESKAP01, FESKAP02), a set for a second polymorph (FESKAP10), and a set for a toluene solvate (FUTTAP) have been reported for triphenylarsine·I2 , and all are simple adducts, as is the same electron donor complexed with the interhalogen IBr (CUXCON). For trimethylarsine, on the other hand, two electron donors are bridged by a single diiodine (ZODJOR), but with dibromine a simple ionic salt, Me3AsBr+ · · · Br– (TMASBR01), is formed.

Like sulfur, selenium based electron donors are heavily studied due to their potential as antithyroid agents. Of the 28 complexes reported, the majority (20) are diiodine based, and most of these (16) are simple adducts. One diiodine complex is bridged, one contains only extended adducts, one contains only bridged adducts, and one contains both bridged and extended adducts. All of the interhalogen complexes are simple adducts except one with iodine monobromine, which forms an extended adduct.

In addition to halogen bonded complexes or ionic salts, it is also possible for sulfur and selenium electron donors to form complexes in which the electron donor atom inserts into the X2 bond, giving a hypervalent donor atom with a T-shaped geometry. It has been recently reported [147] that for dibromine and selenium, this type of complex is favored over halogen bonded complexes. While no purely halogen bonded complex is reported for dibromine, there is one complex (IRABEI) in which one selenium atom of each of several selenanthrene molecules in the asymmetric unit does insert into a Br2 bond, but for one of the molecules, the other selenium atom forms a halogen bond with a Br2 molecule to form a simple adduct (A).

90	W.T. Pennington et al.

3

Computation Studies

3.1

General Observations

The Lewis dot formalism shows any halogen in a molecule surrounded by three electron lone pairs. An unfortunate consequence of this perspective is that it is natural to assume that these electrons are equivalent and symmetrically distributed (i.e., that the iodine is sp3 hybridized). Even simple quantum mechanical calculations, however, show that this is not the case [148]. Consider the diiodine molecule in the gas phase (Fig. 3). There is a region directly opposite the I–I sigma bond where the nucleus is poorly shielded by the atoms’ electron cloud. Allen described this as “polar flattening”, where the effective atomic radius is shorter at this point than it is perpendicular to the I–I bond [149]. Politzer and coworkers simply call it a “sigma hole” [150, 151]. This area of positive electrostatic potential also coincides with the LUMO of the molecule (Fig. 4).

Fig. 3 The electron density map (0.002 au contour) for diiodine, colored by the electrostatic potential. Red is the most negative potential and blue is the most positive

Fig. 4 The lowest unoccupied molecular orbital for diiodine

The sigma hole is the key to understanding halogen bonding. Based upon the crystallographic and spectral data presented in the literature and the computational studies outlined below, the following generalizations may be made: