Element misidentification in X-ray crystallography: A reassessment of the [MCl2(diazadiene)] (M = Cr, Mo, W) series

Article abstract of DOI:10.1021/ic402246d

A series of reports describing the syntheses and structures of [MCl ₂(diazadiene)] (M = Cr, Mo, W) complexes is reassessed in the context of known chemistry of low-valent Group VI metal complexes, crystallographic trends such as M-Cl bond lengths and unit cell volumes, and calculated metal-ligand bond lengths. Crystallographic data and computational results are inconsistent with any of these species being second or third row transition metal complexes. A review of the crystallographic information files accompanying the [MCl₂(diazadiene)] (M = Mo, W) published structures reveals that the metal atoms were inappropriately treated with partial site occupancy factors (0.775 for Mo; 0.4005 and 0.417 for W), the effect of which was to manifest lighter-element behavior and better refinement in accord with the metal atoms' correct identity. A deliberate synthesis and characterization by X-ray diffraction of [ZnCl₂(^Mesdad^Me)] ( ^Mesdad^Me = 1,4-bis(2,4,6-trimethylphenyl)-2,3-dimethyl-1, 4-diaza-1,3-butadiene) are reported. Refinement of this structure with the same combination of second or third row metal and offsetting partial site occupancy is shown to provide final refinement statistics essentially the same as with the correct model employing M = Zn at site occupancy 1.00. Use of the published method for synthesis of [WCl₂(diazadiene)] with ^Mesdad^Me and [WBr₄(MeCN)₂] in lieu of [WCl₄(MeCN)₂] is shown to produce [ZnBr ₂(^Mesdad^Me)], which has also been characterized by X-ray diffraction. It is concluded that the unusual putative 12-electron [MCl₂(diazadiene)] (M = Cr, Mo, W) complexes are in all cases the corresponding [ZnCl₂(diazadiene)] complexes, Zn having been commonly employed as reducing agent in their synthesis.

Full text of DOI:10.1021/ic402246d

Article
pubs.acs.org/IC
Element Misidentification in X‑ray Crystallography: A Reassessment
of the [MCl2(diazadiene)] (M = Cr, Mo, W) Series
Angelique F. Greene,† P. Chandrasekaran,‡ Yong Yan,†,# Joel T. Mague,† and James P. Donahue*,†
†Department of Chemistry, Tulane University, 6400 Freret Street, New Orleans, Louisiana 70118-5698, United States
‡Department of Chemistry and Biochemistry, Lamar University, Beaumont, Texas 77710, United States
S
* Supporting Information
ABSTRACT: A series of reports describing the syntheses and structures of [MCl2(diazadiene)] (M = Cr, Mo, W) complexes is
reassessed in the context of known chemistry of low-valent Group VI metal complexes, crystallographic trends such as M−Cl
bond lengths and unit cell volumes, and calculated metal−ligand bond lengths. Crystallographic data and computational results
are inconsistent with any of these species being second or third row transition metal complexes. A review of the crystallographic
information files accompanying the [MCl2(diazadiene)] (M = Mo, W) published structures reveals that the metal atoms were
inappropriately treated with partial site occupancy factors (0.775 for Mo; 0.4005 and 0.417 for W), the effect of which was to
manifest lighter-element behavior and better refinement in accord with the metal atoms’ correct identity. A deliberate synthesis
and characterization by X-ray diffraction of [ZnCl2(MesdadMe)] (MesdadMe = 1,4-bis(2,4,6-trimethylphenyl)-2,3-dimethyl-1,4-diaza-
1,3-butadiene) are reported. Refinement of this structure with the same combination of second or third row metal and offsetting
partial site occupancy is shown to provide final refinement statistics essentially the same as with the correct model employing M
= Zn at site occupancy 1.00. Use of the published method for synthesis of [WCl2(diazadiene)] with MesdadMe and
[WBr4(MeCN)2] in lieu of [WCl4(MeCN)2] is shown to produce [ZnBr2(MesdadMe)], which has also been characterized by X-
ray diffraction. It is concluded that the unusual putative 12-electron [MCl2(diazadiene)] (M = Cr, Mo, W) complexes are in all
cases the corresponding [ZnCl2(diazadiene)] complexes, Zn having been commonly employed as reducing agent in their
synthesis.
these room-temperature structures, other aspects of the
chemical crystallography within the larger context of many
other structurally identified chromium, molybdenum, and
tungsten complexes, cast considerable doubt upon the
formulation of these compounds as Group VI species and
suggest that they are in all cases the corresponding
[ZnCl2(dad)] complexes. We note that Theopold has ex-
pressed skepticism regarding the identity of the chromium
complexes on the basis that the observed diamagnetism is
inconsistent with a first row d4 ion in a tetrahedral
environment,5 while Konchenko and co-workers have very
recently reported that, in their hands, the synthetic procedure
described for the chromium complexes yields the zinc species.6
In the following,7 we present a series of general observations
about the chemistry of low-valent Group VI metals, particularly
tungsten, that are incompatible with the compounds reported
INTRODUCTION
■
The synthesis, structures, and reactivity of low and middle
valent coordination complexes of tungsten have been of
ongoing interest to our group, in part because of the promise
such compounds have to effect multielectron, small molecule
transformations.1 A series of reports by Bouachir and co-
workers in 20062−4 describing the syntheses and structures of
diazadiene (dad) complexes of the type of [MCl2(dad)]
(Figure 1) drew our attention because the combination of this
low coordination number and formal oxidation state requires
the formulation of these compounds as highly unusual, if not
unprecedented, 12-electron species. We are unaware of any
comparable compounds for any of the Group VI metals.
The structures of four of these compounds (Figure 1, 1a, 2c,
3a, 3c), determined by room temperature X-ray crystallog-
raphy, were described. Although neither the refinement
statistics (as-presented R-factors, goodness-of-fit, and magni-
tude of residual peaks and holes) nor the shapes of the thermal
ellipsoids for any of the atoms appear obviously anomalous for
Received: September 3, 2013
Published: December 10, 2013
© 2013 American Chemical Society
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Figure 1. [MCl2(dad)] compounds (dad = diazadiene, M reportedly Cr (1), Mo (2), or W (3)) described by Bouachir and co-workers.2−4 The letter
notation in the compound name refers to a specific diazadiene ligand; for example, 1c and 2c share in common the ligand with Me groups on the
chelate backbone and N-mesityl substituents. Not all combinations of metal (1, 2, or 3) with ligand were reported.
Table 1. Summary of Structural and Crystallographic Data for [MCl2(diazadiene)] Compounds
1a
2c·CH2Cl2
3a
3c·CH2Cl2
4·THF
M−N, Å
2.070(8)
2.074(8)
2.190(3)
2.197(4)
1.283(12)
1.287(11)
1.502(12)
81.2(3)
2.087(5)
2.089(5)
2.204(2)
2.220(2)
1.267(8)
1.258(8)
1.523(9)
78.17(19)
115.96(9)
293(2)
2.063(14)
2.099(13)
2.173(7)
2.178(6)
1.227(19)
1.27(2)
2.038(4)
2.066(4)
2.2139(19)
2.227(2)
1.255(7)
1.264(7)
1.512(7)
78.57(17)
117.44(8)
293(2)
2.0647(12)
2.0699(12)
2.2055(4)
2.2130(4)
c
M−Cl, Å
N−Cchelate, Å
C−Cchelate, Å
N−M−N, deg
Cl−M−Cl, deg
temp, K
1.59(2)
c
81.3(6)
78.89(5)
116.568(16)
91.0(2)
120.65(18)
293(2)
120.5(3)
293(2)
xtl system
space grp
a, Å
orthorhombic
P212121
monoclinic
P21/c
orthorhombic
P212121
monoclinic
P21/c
monoclinic
P21/c
11.825(5)
12.034(4)
16.393(8)
90
12.8712(8)
14.6594(9)
14.4046(9)
90.662(7)
2717.8(3)
5680
11.733(3)
12.003(3)
16.332(5)
90
12.810(3)
14.552(4)
14.349(3)
90.68(3)
2674.6(11)
5249
12.9137(4)
14.2762(4)
14.3954(5)
90.766(1)
2653.68(14)
7164
b, Å
c, Å
β, deg
V, Å3
2332.8(17)
5028
2300.1(11)
3446
indep data
parmtrs refined
c
280
268
215
272
-
a
R1, wR2
0.0735, 0.1921
0.2420, 0.2487
0.972
0.0808, 0.2265
0.1876, 0.2884
0.996
0.0881, 0.2403
0.2698, 0.3313
1.004
0.0697, 0.2110
0.1219, 0.2518
1.021
0.029, 0.068
b
c
R1, wR2
-
GOF
1.02
resid density
0.466, −0.461
1.112, −1.128
0.633, −0.504
1.308, −0.823
0.44, −0.39
a
b
c
Data cut at I < 2σI. All data used. Data unavailable.
and note points in the published work wherein room for doubt
has been admitted. The similarity of metal ligand lengths within
the set of [MCl2(dad)] crystal structures is noted, as is their
conformity with the set of known [ZnCl2(dad)] complexes. A
survey of M−Cl (M = Cr, Mo, W) ligand bond lengths and of
unit cell volumes for isostructural, isomorphous chromium and
tungsten compounds shows consistent and significant differ-
ences between the two elements that are not reflected in the set
of compounds posited as being as [MCl2(dad)] (M = Cr, W)
species. Although we do not believe that diamagnetic
tetrahedral [MCl2(dad)] (M = Cr, Mo, W) species would be
stable and amenable to characterization in the fashion reported,
calculated gas phase structures do reflect bond length
differences between the Cr and W species that should be 
and are not  manifested experimentally. Examination of the
Supporting Information accompanying the structures of 2c·
CH2Cl2, 3a, and 3c·CH2Cl2 shows the metal atoms were
refined with partial occupancies. We report a deliberate
preparation and crystallographic characterization of
ZnCl2(MesdadMe)·CH2Cl2 (MesdadMe = 1,4-bis(2,4,6-trimethyl-
phenyl)-2,3-dimethyl-1,4-diaza-1,3-butadiene) and show the
results of refinement as the identity of M is varied as Cr, Mo,
and W with full and partial site occupancy factors. Finally, we
show that the procedure reported to produce
Mes-
[WCl2(diazadiene)] complexes, when implemented with
dadMe and [WBr4(MeCN)2] in lieu of the corresponding
chloride complex, produces in our hands crystalline
ZnBr2(MesdadMe)·CH2Cl2.
DISCUSSION
■
Figure 1 illustrates the family of complexes (1−3) that are the
subject of this reexamination, while Table 1 summarizes the key
structural parameters, unit cell information, and refinement
data associated with the compounds that were characterized by
X-ray crystallography. The reported syntheses proceed from
[CrCl3(thf)3] or [MCl4(solv)2] (M = Mo, solv = thf; M = W,
solv = MeCN) by reduction with elemental zinc in the presence
of the diazadiene ligand. We note that the method of synthesis
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Figure 2. Crystallographically characterized [ZnCl2(dad) complexes.
does not obviate the formation of the [ZnCl2(dad)] complexes
by separating the Zn(II) byproduct prior to the introduction of
the diazadiene ligand and further observe that the adulteration
of minor amounts of impurities into a lightly colored, crystalline
Zn(II) compound can readily present virtually any color, even
black. An elemental analysis (C, H, N) for 1d was reported that
was consistent with its formulation, although the calculated
mass percentages change relatively little if the compound is
assumed to contain Zn rather than Cr. The Mo and W
compounds were characterized only spectroscopically by IR
and 1H and 13C NMR. Physical techniques such as mass
spectrometry or analytical methods such as elemental analysis
that would have been more decisive in corroborating their
identity were not employed.
reported, particularly those obtained when all data are included
(Table 1). Even for diffraction data collected at room
temperature, convergence with wR2 values well below 0.200
would be generally expected.
Shown in the rightmost column of Table 1 are selected
structural and crystallographic data from a recently reported
structural study6 of [ZnCl2(MesdadMe)]·THF, 4·THF, the zinc
compound that is isostructual to compounds 2c and 3c. Overall
residuals are appreciably better for 4 than 2c and 3c, which is
consistent with the latter species being incorrectly formulated.
The Zn−Cl and Zn−N bond lengths shown for 4, which show
lower uncertainties reflecting the higher quality of the data, are
indistinguishable from the M−Cl and M−N bond lengths
reported for 2c and 3c.
A moderate number of [ZnCl2(dad)] compounds have been
deliberately prepared and structurally identified by X-ray
crystallography. Figure 2 illustrates this set of compounds,
while Table 2 summarizes selected structural data. Table 2
reveals a range of 2.173(2)−2.252(7) Å for Zn−Cl bond
lengths, the average value being 2.207 Å, and a range for Zn−N
bond distances of 2.0647(12)−2.147(4) Å, the average being
2.091 Å. The imine NC bond lengths and C−Cchelate bonds
lengths are in all cases typical at ∼1.27 and ∼1.54 Å,
respectively. The metal ligand bond lengths reported for
compounds 1a and 2c (Table 1) occur within the ranges noted
above and near the average values. However, bond lengths for
the compounds formulated as tungsten species are noticeably
more irregular and do not compare simply to the values
collected in Table 2. For example, metal−chloride bond lengths
for 3a (2.173(7), 2.178(6) Å) occur at the low end of the range
of Zn−Cl values, while metal−nitrogen bond lengths for 3c
(2.066(4), 2.038(4) occur at and below the low end of the range
of Zn−N values in Table 2.
A survey of M−Cl bond lengths is presented in Table 3 for
chromium, molybdenum, and tungsten complexes across a
range of formal oxidation states (0−6), coordination numbers,
and geometries. While not an exhaustive listing of Group VI
M−Cl bond lengths, Table 3 is nonetheless extensive and
supports the identification of an unambiguous structural trend.
At parity of ligand environment and redox state, Mo−Cl and
W−Cl bond lengths are typically ∼0.1 Å longer than analogous
Cr−Cl values. While certain specific comparisons in Table 3
admit of smaller differences, we emphasize that Table 3 shows
The chemistry of divalent molybdenum and tungsten is
comprised largely of quadruply bonded species,8,9 the
formation of the metal−metal multiple bond being an avenue
by which the low electron count, noted at the outset as
implausible, is alleviated. Noncarbonyl mononuclear CrII, MoII,
and WII compounds, while known,10−19 are generally six-
coordinate species and still prone to the formation of metal−
metal multiple bonds.20−22 The divalent state, for tungsten at
least, is typically achieved by reduction of WCl4 with strong
reducing agents such as Mg0,17 Na/Hg amalgam,16,18−20,22
23
Na[HBEt3],8 or KC8 in the presence of the desired
supporting ligand. To the best of our knowledge, potentials
for reduction of [WCl4(MeCN)2], the starting material
employed for the synthesis of [WCl2(dad)], have not been
reported, but we view it as unlikely that Zn0 would have
reducing power sufficient to prepare divalent tungsten from a
tetravalent precursor.
Compounds 1a and 3a are isostructural and isomorphous in
orthorhombic space group P212121 (Table 1), while 2c and 3c
are similarly related but occur in monoclinic space group 14. An
immediately apparent anomaly is a set of M−Cl and M−N
bond lengths for 1a and 3a that are the same within the
resolution of the data despite the former being a first row metal
and the latter a third row metal with significantly greater ionic
radius that should manifest itself with significantly longer
metal−ligand bond lengths. The atomic radius of W is ∼9%
larger than that of Cr, and the difference, while variable
depending on redox state, is generally more pronounced for
ionic radii.24 We note also the generally high R-factors that are
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improbable92 that a tungsten complex would display a smaller
unit cell volume than its chromium homologue. The smaller
unit cell volume shown in Table 1 for 3a (2300.1(11) Å3) is
therefore implausible in view of the corresponding volume of
2332.8(17) Å3 given for isostructural 1a.
Table 2. Summary of Crystallographically Identified
[ZnCl2(dad)] Complexes
Cl−Zn−Cl,
cmpd #
Zn−Cl, Å
Zn−N, Å
2.0647(12),
deg
ref
6
4
2.2055(4),
2.2130(4)
116.568(16)
Both hybrid and pure density functionals have been reported
to be generally useful in replicating the structures and
properties of transition metal complexes. Although calculated
metal−ligand bond lengths using the B3LYP functional
employed here are typically larger than experimentally
determined values,93 the “errors” associated with the series of
compounds optimized here are likely systematic and therefore
would not obscure an assessment of bond length differences
between compounds. Starting with the crystallographic
coordinates for the tetrahedral compound reported as
[CrCl2(MesdadMe)], we find that, with M = Cr, Mo, or W and
an S = 0 spin state, convergence toward an energy minimum
corresponding to a tetrahedral structure of C2v symmetry is
quickly achieved. Although we do not believe these gas phase
energy-minimized structures correspond to isolable or observ-
able species in solution or solid phase, it is noteworthy that the
calculated metal−ligand bond lengths for this M = Cr, Mo, W
series (Table 4) reflect a significant ∼0.1 Å difference between
the first row metal and its second and third row congeners that
is comparable to the differences observed experimentally by X-
ray diffraction, as summarized in Table 3. We find the
nonadherence to this general trend by the compounds
identified as [MCl2(dad)] (M = Cr, Mo, W) species to be a
further basis for our contention that these molecules are
misidentified.
To directly assess the effect of variation of the identity of the
metal atom upon the refinement behavior of diffraction data for
the type of zinc compound in question, we have deliberately
prepared, crystallized, and obtained our own X-ray diffraction
data for [ZnCl2(MesdadMe)]·CH2Cl2 (MesdadMe = 1,4-bis(2,4,6-
trimethylphenyl)-2,3-dimethyl-1,4-diaza-1,3-butadiene). It is
this compound that we assert is the true identity of both 2c·
CH2Cl2 and 3c·CH2Cl2 in Table 1. Crystallographic data for
[ZnCl2(MesdadMe)]·CH2Cl2 are collected in Table 5, where the
highly similar unit cell dimensions between [ZnCl2(MesdadMe)]·
CH2Cl2 and 2c·CH2Cl2 and 3c·CH2Cl2 are easily seen. As
gauged by R1 and wR2 values, overall refinement for
[ZnCl2(MesdadMe)]·CH2Cl2 is better than given for 2c·CH2Cl2
and 3c·CH2Cl2, which may be attributed at least in part to
correct identification of the metal atom. A greater amount of
data collected at lower temperature also contributes to these
lower values. The results of refining Zn as Cr, Mo, and W
instead are summarized in Table 6. When M = Cr, final R1 and
wR2 values for data cut at I < 2σI (0.0926, 0.2712) are roughly
comparable to the values presented by Bouachir et al. (0.0735,
0.1921). Unsurprisingly, we find that while prompted to refine
an extinction parameter because the metal atom is now too
light, all non-hydrogen atoms can be refined anisotropically and
produce thermal ellipsoids that are not untoward in appearance.
Notably, if the metal atom is identified as Mo, we find final R1
and wR2 values (0.0730, 0.2308) that are quite similar to those
reported for 2c·CH2Cl2 (0.0808, 0.2265). Again, no obviously
anomalous-looking thermal ellipsoids result for any atoms when
anisotropic refinement is implemented. With M = W, the
refinement behavior dramatically worsens, as expected, with R1
and wR2 ballooning to 0.1893 and 0.5145, respectively (cf.
0.0697, 0.2110 for 3c·CH2Cl2) and nonpositive definite
2.0699(12)
a
5
2.2085(7),
2.2026(8)
2.0873(15)
116.73(3)
6
b
6
2.2095(4)
2.0754(10)
121.47(3)
6
6
7
2.2163(3),
2.2110(3)
2.0754(8),
2.0825(9)
115.254(10)
b
8
2.252(7)
2.064(3)
109.8(2)
25
26
27
28
28
29
30
9
2.219(1), 2.225(1)
2.2173(7)
2.083(3), 2.078(3)
2.076(2)
118.44(4)
122.67(4)
119.61(3)
116.27(8)
114.51(9)
116.80(2)
b
10
11
12
13
14
2.214(8), 2.218(7)
2.194(2), 2.224(2)
2.081(2), 2.085(2)
2.093(5), 2.096(5)
2.204(2), 2.2035(9) 2.079(7), 2.079(1)
2.2013(6),
2.2180(6)
2.085(2), 2.071(2)
a
15
2.1967(15),
2.1979(15)
2.114(2)
117.67(6)
118.03(6)
31
31
32
16
2.1863(15),
2.2138(14)
2.113(3), 2.125(3)
c
17
2.173(2), 2.187(2)
2.188(2),
2.195(2)
2.109(4), 2.121(4),
2.128(5), 2.147(4)
120.26(6)
121.17(6)
a
Compound resides on mirror plane coincident with ZnCl2 plane and
b
bisecting the diazadiene ligand. Compound resides on a crystallo-
graphic 2-fold axis bisecting the Cl−Zn−Cl angle. Two half-molecules
occur in the asymmetric unit.
c
no W−Cl or Mo−Cl bond lengths that are shorter than the
corresponding values in related Cr complexes. This observation
is at stark variance with the data in Table 2, where 3a is seen to
have nominally shorter M−Cl bond lengths (2.173(7), 2.178(6)
Å) than 1a (2.190(3), 2.197(4) Å). That the M−Cl bond
lengths listed for 2c, 3a, and 3c are all significantly shorter by
∼0.1 Å than even the shortest Mo−Cl and W−Cl values
catalogued in Table 3 is a compelling indication that M is a first
row element in all these compounds. This point regarding
metal−ligand bond length differences for first row versus
second and third row transition metals is obviously pertinent to
the M−Nimine bond lengths as well. Although a broad survey of
M−Nimine (M = Cr, Mo, W) bond lengths has not been
undertaken, arbitrarily selected comparisons, such as the M−N
bonds in [M(CO)5(pyridine)] (M = Cr: 2.165(4) Å;89 M = W:
2.258(7), 2.272(6) Å90) and [M(2,2′:6′,2″-terpyridine)2] (M =
Cr: 1.949(1), 2.018(1) Å;91 M = W, 2.050(2), 2.077(2) Å91)
corroborate the expectation.
The occurrence of isostructural, isomorphous species for
transition metals within the same triad is relatively common,
even for the first and third row metals. The generally longer
metal−ligand bond lengths for complexes with the third row
elements versus first row metals, noted above, are typically
reflected in unit cell volumes that are appreciably larger, within
the derived uncertainties, for the third row species over their
first row counterparts. Table S1, Supporting Information
presents a compilation of the unit cell volumes for isostructural,
isomorphous chromium and tungsten complexes taken from a
survey of the Cambridge Structural Database. The entries in
Table S1 are restricted to data sets measured at the same or
similar temperature ( 30 K), as temperature has a direct and
important effect upon the physical dimensions of the unit cell.
Although not exhaustive, Table S1 includes all homologous
Cr−W pairs of which we are aware and shows that it is
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Table 3. Representative Cr−Cl (red), Mo−Cl (green), and W−Cl (blue) Bond Lengths
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Table 3. continued
a
b
c
d
This work. dmpe = 1,2-bis(dimethylphosphino)ethane. depe = 1,2-bis(diethylphosphino)ethane. dppe = 1,2-bis(diphenylphosphino)ethane.
e
f
g
h
dppb = 1,2-bis(diphenylphosphino)benzene. Terminal M−Cl bond length(s). dippe = 1.2-bis(diisopropylphosphino)ethane. HB(pz)3
=
i
j
k
hydridotris(pyrazolyl)borate(1-). HB(3,5-Me2pz)3 = hydridotris(3,5-dimethylpyrazolyl)borate(1-). Data are for the P21/n polymorph. Five
independent structures reported. Cat = 1/2[4,4′-bipyridinediium]2+, [4-methylquinolinium]+, [6-methylquinolinium]+, [N,N′-ethylenediaminium]+,
[5-Cl-2-pyridyl)(2-pyridyl)NH2]+, respectively, for the five given Cr−Cl values.
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Table 4. Calculated Metal−Ligand Bond Lengths, Angles for [MCl2(MesdadMe)] (M = Cr, Mo, W)
[CrCl2(MesdadMe)]
[MoCl2(MesdadMe)]
[WCl2(MesdadMe)]
M−Cl, Å
M−N, Å
2.20
1.86
2.33
1.95
2.30
1.94
Cl−M−Cl, deg
138.10
138.00
128.82
atoms  and only for the metal atoms  that were
substantially less that unity: 0.775 for Mo in 2c·CH2Cl2,
0.417 and 0.4005 for W in 3a and 3c·CH2Cl2, respectively.94
While it is not uncommon to refine parts of a crystal structure
with fractional site occupancies, for example, in a situation
involving compositional disorder or partial occupancy by an
interstitial solvent molecule, the effect of this selective
treatment of the metal atoms is to cause them to behave as
an element with proportionately smaller atomic number and
scattering power. The specific site occupancy factors used are
notably close to the ratios of the atomic numbers of Zn over
Mo (30/42 = 0.714) and Zn over W (30/74 = 0.405), meaning
that the specific lighter element whose appearance is educed
using the foregoing combination of heavier element scattering
factors and offsetting partial site occupancies is zinc. For
emphasis, Table 6 shows that, when the diffraction data
collected for [ZnCl2(MesdadMe)]·CH2Cl2 are refined with metal
as Mo and partial site occupancy of 0.775, or as tungsten with
partial site occupancy of 0.409, final refinement statistics are
obtained that are nearly the same as with the correct model
employing Zn at a site occupancy of 1.00 (cf. Tables 5 and 6).
As a final test of the general procedure reported for the
synthesis of [WCl2(dad)], we have applied it using
[WBr4(MeCN)2] in place of the corresponding chloride and
found it to produce yellow crystalline, diamagnetic
[ZnBr2(MesdadMe)], which we have identified by 1H NMR
spectroscopy, mass spectrometry, and X-ray crystallography
(Table 5, Figure 3). The compound is isostructural and
isomorphous to the chloride homologue, differing only in
having slightly longer metal−halide bond lengths (2.3313(8),
2.3480(7) Å vs 2.2033(6), 2.2140(6) Å) that result in a slightly
larger unit cell volume. The tungsten(IV) precursor, while it
evidently undergoes some degree of reduction, appears to do so
incompletely and to deposit from the crystallization mixture as
an ill-defined brown oily material that is not amenable to
further characterization.
Table 5. Unit Cell and Refinement Data for
[ZnX2(MesdadMe)]·CH2Cl2 (X = Cl, Br)
formula
C23H30Cl4N2Zn
541.71
C23H30Br2Cl2N2Zn
630.58
fw
temperature, K
wavelength, Å
2θ range, deg
crystal system
space group
a, Å
150
100
0.71073
0.71073
4.02−66.28
monoclinic
P21/c
3.98−55.66
monoclinic
P21/c
12.6703(2)
14.4861(2)
14.1761(2)
90
12.625(3)
14.625(3)
14.336(3)
90
b, Å
c, Å
α, deg
β, deg
90.837(1)
90
90.002(30)
90
γ, deg
volume, Å3
2601.65(7)
4
2646.9(9)
4
Z
density, g/cm3
μ, mm−1
1.378
1.582
1.367
4.162
crystal size
color, habit
limiting indices, h
limiting indices, k
limiting indices, l
reflections collected
independent data
restraints
0.12 × 0.15 × 0.21
yellow slab
−16 < h < 19
−22 < k < 22
−21 < l < 21
50166
0.10 × 0.13 × 0.17
yellow block
−16 < h < 16
−19 < k < 19
−18 < l < 18
29645
9919
4742
69
0
parameters refined
285
274
a
GOF
1.118
0.964
bc
,
dc
,
R1, wR2
be
0.0514, 0.1343
0.0682, 0.1442
1.340
0.0465, 0.1177
0.0640, 0.1225
0.926
de
,
,
R1, wR2
largest diff peak, e·Å−3
largest diff hole, e·Å−3
−1.348
−1.204
a
2
GOF = {Σ[w(Fo − Fc2)2]/(n − p)}1/2, where n = number of
b
reflections and p is the total number of parameters refined. R1 = Σ||Fo|
c
d
2
− |Fc||/Σ|Fo|. R indices for data cut off at I > 2σ(I). wR2 = {Σ[w(Fo2
2
2
− Fc2)2]/Σ[w(Fo )2]}1/2; w = 1/[σ2(Fo ) + (xP)2 + yP], where P = (Fo
e
+ 2Fc2)/3. R indices for all data.
SUMMARY AND CONCLUSIONS
■
The primary conclusion emerging from our examination of the
crystallographic characterization of the chromium, molybde-
num, and tungsten [MIICl2(diazadiene)] complexes reported
by Bouachir and co-workers is that the compounds isolated in
their work were Zn species rather than the intended Group 6
complexes, zinc metal having been employed as common
reducing agent in their synthesis from higher-valent precursors.
behavior resulting for nearly every atom of the diazadiene
ligand.
Inspection of the crystallographic data files provided as
Supporting Information for the crystal structures of 2c·CH2Cl2,
3a, and 3c·CH2Cl2 reveals that the models refined against the
diffraction data employed site occupancy factors for the metal
Table 6. Refinement Statistics for [ZnCl2(MesdadMe)] Refined with M = Cr, Mo, or W
[MoCl2(MesdadMe)]
[WCl2(MesdadMe)]
c
c
c
c
[CrCl2(MesdadMe)]
SOF = 1
SOF = 0.775
SOF = 1
SOF = 0.409
a
R1, wR2
0.0926, 0.2712
0.1154, 0.2997
1.056
0.0730, 0.2308
0.0944, 0.2535
1.048
0.0530, 0.1311
0.0747, 0.1469
1.044
0.1893, 0.5145
0.2141, 0.5454
2.399
0.0529, 0.1371
0.0753, 0.1561
1.015
b
R1-wR2
GOF
residual density
3.687, −1.420
1.755, −2.232
1.307, −1.369
15.611, −4.687
1.364, −1.313
a
b
c
Data cut at I < 2σI. All data used. SOF = site occupancy factor.
314
dx.doi.org/10.1021/ic402246d | Inorg. Chem. 2014, 53, 308−317

Inorganic Chemistry
Article
EXPERIMENTAL SECTION
■
[ZnCl2(MesdadMe)]. A 15 mL CH2Cl2 solution of the MesdadMe
ligand (0.531 g, 1.66 mmol), prepared according a literature
procedure,96 was treated with an excess (several grams) of commercial
ZnCl2·Et2O in 25 mL of H2O. This heterogeneous, two-phase mixture
was stirred in air for 36 h, whereupon it was diluted with 50 mL
portions each of CH2Cl2 and H2O. The organic layer was separated,
and the aqueous phase extracted with additional portions of CH2Cl2 (2
× 50 mL). The combined organic portions were dried over anhydrous
Na2SO4, filtered, and reduced to a solid residue under reduced
pressure. The residue was redissolved in a minimal volume of CH2Cl2
(∼15 mL). Slow evaporation of the solution afforded a mixture of
crystalline [ZnCl2(MesdadMe)]·CH2Cl2 and free MesdadMe ligand, as
determined by X-ray crystallography.97 The more soluble free ligand
was separated by extraction with CH2Cl2 (2 × 1 mL), and the
remaining yellow [ZnCl2(MesdadMe)] was recrystallized by diffusion of
n-pentane vapor into a saturated THF solution. Rf (1:1, CH2Cl2/
hexanes): 0.26. 1H NMR (δ, ppm in CDCl3): 6.94 (s, 4H, aromatic C-
H), 2.28 (s, 6H, -NC−CH3), 2.21 (s, 6H, mesityl p-CH3), 1.54 (s,
12H, mesityl o-CH3). MALDI MS: m/e C22H28Cl2N2Zn, 454.756;
Observed: 418.868 (M-Cl).
Figure 3. Thermal ellipsoid plot at the 50% probability level for
[ZnBr2(MesdadMe)]. All H atoms are omitted for clarity.
[ZnBr2(MesdadMe)]. The procedure and crystallization method of
Bouachir et al.4 was followed using [WBr4(MeCN)2]98 in place of
[WCl4(MeCN)2]. After the crystalline compound thus obtained was
identified as [ZnBr2(MesdadMe)], the reaction was repeated on a scale
employing 0.967 g of [WBr4(MeCN)2] (1.60 mmol), 1.54 g of
MesdadMe (4.82 mmol), and 0.525 g of Zn dust (8.03 mmol). After 48 h
of stirring, the reaction mixture was filtered, and the filtrate was
reduced to dryness to afford 0.419 g of crude solid. This solid was
redissolved in a minimal volume of CH2Cl2 and purified on a silica
column eluted with the same solvent. The leading yellowish band was
collected and identified as a mixture of [ZnBr2(MesdadMe)] and free
MesdadMe ligand. The MesdadMe ligand was separated by extracting the
mixture of solids with CH2Cl2 (2 × 1 mL). The resulting yellow solid
residue could be recrystallized by the diffusion of n-pentane vapor into
a saturated THF or CH2Cl2 solution. Rf (1:1, CH2Cl2/hexanes): 0.30.
1H NMR (δ, ppm in CDCl3): 6.94 (s, 4H, aromatic C-H), 2.29 (s, 6H,
-NC−CH3), 2.25 (s, 6H, mesityl p-CH3), 1.54 (s, 12H, mesityl o-
CH3). MALDI MS: m/e C22H28Br2N2Zn, 545.676; observed: 465.077
(M-Br). Anal. Calcd for [ZnBr2(MesdadMe)]·3/4CH2Cl2,
C22.75H29.5Br2Cl1.5N2Zn: C, 44.84; H, 4.88; N, 4.60. Found: C,
44.72; H, 4.88; N, 4.58.
The metal−chloride and metal−nitrogen bond lengths
reported for the compounds identified as molybdenum and
tungsten species are significantly shorter by ∼0.1 Å than typical
bond lengths of these kinds, as shown by a broad survey of
structural data within the Cambridge Structural Database, and
are consistent instead with the metal being a first row element.
We emphasize that the M−Cl bond lengths in the putative
[WCl2(dad)] compound (3a, Figure 1, Table 1) are
significantly shorter than any W−Cl bond length we have
been able to identify for any complex of tungsten in any
oxidation state. Similarly, unit cell volumes among the set of
[MCl2(dad)] (M = Cr, Mo, W) complexes are at variance with
the clear trend of larger volumes for tungsten complexes versus
their chromium counterparts among isotructural, isomorphous
Group 6 complexes. Rather high R1 and wR values for the final,
refined crystal structures, especially when all data are included,
are further suggestive of structural models that are
fundamentally incorrect. That acceptable thermal ellipsoids
are observed for the published images of 2c·CH2Cl2, 3a, and
3c·CH2Cl2 has been found to be due to the incorrect usage of
partial site occupancy factors that cause Mo and W to appear as
zinc. An effort to prepare [WBr2(MesdadMe)] from
[WBr4(MeCN)2] and Zn0 by the published procedure
produced, in our hands, [ZnBr2(MesdadMe)] instead.
[Et4N][W(CO)5Cl]. This compound was isolated as single crystals
arising from a reaction between [W(CO)4(piperidine)2]99 and cis-2,3-
dithiolato-2-butene(2-)/Et4N+Cl−. Being an unintended product, it
was not characterized further beyond an X-ray crystal structure. The
W−Cl bond length in this anion is included in Table 3 to assist the
comparison between W−Cl and Cr−Cl bond lengths in related
compounds.
[fac-WCl3(PMe3)3]. This compound was identified as an
adventitious byproduct attending the synthesis of [WIVCl4(PMe3)3]
from WCl6 and PMe3 in the presence of Zn0 as reducing agent. A
deliberate synthesis has been reported by Sharp.19 The compound was
characterized structurally by X-ray crystallography but not subjected to
other physical methods. The W−Cl bond lengths in this compound
are included in Table 3 to assist the comparison between W−Cl and
Cr−Cl bond lengths in compounds of the same or similar type.
Collectively taken, the foregoing observations disprove the
synthetic and structural results reported in the series of papers
published by Bouachir and co-workers. The incorrect
formulation of a crystallographically characterized compound
by element misidentification can easily occur when the
elements in question are similar in diffracting power (e.g., S
and Cl)95 and when a heavy reliance upon X-ray diffraction as a
tool of qualitative analysis has been  or must be  made. We
are unaware, however, of a documented instance in which the
elements in question differ as much as they do in the present
case. The error notwithstanding, we submit that the work we
have reviewed and sought to correct presents an instructive case
study for the crystallography student looking for awareness of
the potential pitfalls of highly automated crystal structure
solving and refining. It offers also a reminder to the reviewing
community that such mundane aspects of crystal structures as
bond lengths are important details for scrutiny when the
structures of unusual or implausible species are reported.
CALCULATIONS
■
All calculations were performed using the Gaussian09 package.
Geometry optimizations were calculated at a B3LYP level of
theory using the default pruned fine grids for energies (75 302)
and the default pruned coarse grids (35 110) for Hessians and
gradients for all atoms except tungsten. All structures were
confirmed by ensuring that imaginary frequencies were not
present. The basis set chosen for all main group elements,
except hydrogen, is the 6-31G(d,p). For chromium, molybde-
num, and tungsten, the LANL2DZ with effective core potential
315
dx.doi.org/10.1021/ic402246d | Inorg. Chem. 2014, 53, 308−317

Inorganic Chemistry
Article
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2492−2496.
ASSOCIATED CONTENT
* Supporting Information
■
(17) Gibson, V. C.; Graimann, C. E.; Hare, P. M.; Green, M. L. H.;
Bandy, J. A.; Grebenik, P. D.; Prout, K. J. Chem. Soc., Dalton, Trans.
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S
Spectroscopic data for [ZnCl2 (M e s dadM e )] and
[ZnBr2(MesdadMe)]. Full description of procedures for crystal
growth, diffraction data collection and processing, and structure
solution and refinement; complete crystallographic data for all
new structures in CIF format; thermal ellipsoid plots with
complete atom labeling; table of unit cell volumes for
isostructural, isomorphous Cr and W compounds. This material
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AUTHOR INFORMATION
Corresponding Author
*E-mail: donahue@tulane.edu.
■
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Present Address
#Department of Chemistry, Princeton University, Princeton,
New Jersey 08544.
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H. Chem. Commun. 2006, 2913−2915.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
The Louisiana Board of Regents is thanked for enhancement
Grant LEQSF-(2002-03)-ENH-TR-67 with which Tulane’s X-
ray diffractometer was purchased, and Tulane University is
acknowledged for its ongoing support with operational costs for
the diffraction facility. Support from the National Science
Foundation (Grant CHE-0845829 to J.P.D.) and from the
Louisiana Board of Regents and the IBM Corporation (to
A.F.G) is gratefully acknowledged.
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