Cameron et al.
Table 6. Selected bond distances (Å)
617
erties in organic solvents (37), with dissolution in dichloro-
methane avoiding the pyrophoric nature of the complex dis-
solved in pyridine. This complex has been used for many
years as a selective oxidant for organic alcohols, although
many procedures and stoichiometric ratios have been pub-
lished, especially for the selective oxidation of primary alco-
hols to aldehydes (37).
The isolation of the dichromate salts 2–5 offered the op-
portunity to explore their chemistry and to supplement the
limited literature on the structural features of heterocyclic
chromate salts. A picture of compound 2 (50% probability
thermal ellipsoids) is given in Fig. 2 while selected geomet-
rical parameters are given in Table 6.
and bond angles (°) for the non-
hydrogen atoms of [C7H7N2]2Cr2O7 (2).
Bond distances (Å)
Cr1—O1
Cr1—O2
Cr1—O3
Cr1—O4
N1—C1
N1—C2
N2—C1
N2—C7
1.586(3)
1.627(3)
1.794(2)
1.599(3)
1.328(5)
1.391(4)
1.324(5)
1.396(4)
1.391(5)
C2—C7
In compounds 3 and 4, the chromate anions are disordered
about 1, with oxygen atom O(2) occupying two positions and
the two CrO3 groups being staggered with respect to each
other. The geometry of chromate anions, associated with
heterocyclic cations, of general form [R+]2[CrnO3n+1]2–, con-
tinues to attract attention (12, 16, 38, 39). A smooth correla-
tion of terminal and bridging Cr—O distances with
increasing degree of polymerization has been noted (16).
The lengths of the bridging Cr—O bonds in 2 are 1.794(2)
and 1.795(2) Å, but are 1.78(1) and 1.80(1) Å in 5 for a
mean of 1.79 Å; the terminal bonds of both 2 and 5 are clus-
tered around a mean value of 1.60 Å. These may be com-
pared with the literature values of 1.780(7) and 1.615(18) Å,
respectively (16). The nonbridging bond angles are close to
tetrahedral at 105.98(13)–112.0(2)° in 2 and 106.7(7)–
112.3(8)° in 5, whereas the bridging bond angles are
129.25(14), 130.4(3), 134.0(4), and 123.2(6)° for the four
salts, respectively. Again, these are comparable to literature
values, namely, 127.0(4)° (12), 122.7(1)° (13), 124.7(8)°
(14), and 135.3(5)° (15). These numbers show that there is
considerable flexibility in the bonding about the bridging
oxygen atom. The ability of dichromate anions to build an
acentric polar framework has been studied by Pecaut and
Masse (13) in connection with attempts to design efficient
organic–inorganic nonlinear optical crystals. Compounds 2–
5 have nonpolar space groups, with symmetrical molecular
arrangements. The dimensions of the dichromate anions in 3
and 4 are similar to those of 2 and 5 but the structures of 3
and 4 are disordered.
Hydrogen bonds, of the type N—H···O, play a crucial role
in determining the molecular packing in these systems. In-
terestingly, the hydrogen bonding networks in the four struc-
tures are different, as outlined in Table 7. These values are in
general agreement with published values for such hydrogen
bonds (40–42). In 2, each dichromate anion is linked to five
benzimidazolinium cations through three strong hydrogen
bonds and a pair of bifurcated hydrogen bonds (Fig. 2). Five
of the seven oxygen atoms are involved in the hydrogen
bonding, but oxygen atoms O(1) (and by symmetry, O(5))
are not. These two oxygen atoms, therefore, have larger ther-
mal parameters than the other four terminal oxygen atoms.
These facts are mirrored in the chromium—oxygen bond
lengths (Table 6). The molecular structure thus takes on the
form of an infinite molecular chain along a, which is joined
together by the fifth hydrogen bond (N(1)—H(9)···O(4)) in a
molecular layer in plane (010). In contrast to this, structure 3
has two hydrogen bonds which create a molecular sheet in
plane (201), while in 4 there is a bimolecular chain along
Bond angles (°)
O1-Cr1-O2
O1-Cr1-O3
O1-Cr1-O4
O2-Cr1-O3
O2-Cr1-O4
O3-Cr1-O4
Cr1-O3-Cr2
O3-Cr2-O5
O3-Cr2-O6
O3-Cr2-O7
O5-Cr2-O6
O5-Cr2-O7
O6-Cr2-O7
N1-C1-N2
110.60(17)
109.04(16)
112.0(2)
108.99(13)
109.50(15)
106.61(14)
129.25(14)
109.38(16)
105.98(13)
108.86(13)
110.97(17)
110.90(17)
110.60(15)
110.2(3)
(1.578(2) Å) bonds in the chromate ester (CrO2(OCPh3)2) (29).
The mean of the two chromium—nitrogen bonds at 2.150 Å
can be compared with typical lengths of 2.158–2.178 Å for the
nitrogen—chromium(VI) bond (30). More specifically, com-
parison can be made with the lengths of the chromium—nitro-
gen bonds in oxodiperoxopyridinechromium(VI) ([CrO5·C5-
H5N]) at 2.047(13) Å (31, 32) and in the complex with chro-
myl chloride of a secondary amine chlorodioxo-(2,2,6,6,-tetra-
methylpiperidine)chromium(VI) ([CrO2Cl(NC9H18)]) at 1.81(1)
Å (33). The latter compound is not actually a complex, but has
a formal N—Cr bond. It is also instructive to make compari-
sons with some other Lewis acid–base complexes involving
pyridine. Three examples, with the N—X bond distance in
brackets, are pyridine–boron trichloride (1.592(3) Å) and
pyridine–boron trifluoride (1.604(5) Å) (34), examples of
strongly bound complexes to a small electrophilic atom, with
the third example, pyridine 1-sulphonate (1.829 Å) (35), being
a strongly bound larger electrophile.
Another noteworthy feature of 1 is that the planes of the
two pyridine rings are twisted at an angle of ca. 60° with re-
spect to each other, as would be expected if electron-pair re-
pulsions are to be minimized. These data are overall in
accord with the bonding between the pyridine rings and the
chromic anhydride being a classic example of a Lewis acid–
base complex. A trigonal pyramidal structure for 1 was orig-
inally proposed by Sisler et al. (36). The IR spectrum of 1 is
not especially informative, although the strong band at
926 cm–1 likely results from Cr—O stretching vibrations
(31). Confirmation that the pyridine–chromic anhydride
complex is monomeric is consistent with its solubility prop-
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