Using sterics to promote reactivity in fac-Re(CO)3 complexes of some 'non-innocent' NNN-pincer ligands

Article abstract of DOI:10.1039/c1dt10030k

Two new redox active ligands based on di(2-(3-organopyrazolyl)-p-tolyl) amine have been prepared in order to investigate potential effects of steric bulk on the structures, electronic properties, or reactivity of tricarbonylrhenium(i) complexes. Replacing

Full text of DOI:10.1039/c1dt10030k

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Pincers and other hemilabile ligands
Guest Editors Dr Bert Klein Gebbink and Gerard van Koten
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PERSPECTIVE:
Cleavage of unreactive bonds with pincer metal complexes
Martin Albrecht and Monika M. Lindner
Dalton Trans., 2011, DOI: 10.1039/C1DT10339C
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Mechanistic analysis of trans C–N reductive elimination from a square-planar macrocyclic aryl-
copper(III) complex
Lauren M. Huffman and Shannon S. Stahl
Dalton Trans., 2011, DOI: 10.1039/C1DT10463B
CSC-pincer versus pseudo-pincer complexes of palladium(II): a comparative study on
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Dalton Trans., 2011, DOI: 10.1039/C1DT10269A
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PAPER
Using sterics to promote reactivity in fac-Re(CO)3 complexes of some
‘non-innocent’ NNN-pincer ligands†
Sarath Wanniarachchi,a Brendan J. Liddle,a John Toussaint,a Sergey V. Lindeman,a Brian Bennettb and
James R. Gardinier*a
Received 7th January 2011, Accepted 26th February 2011
DOI: 10.1039/c1dt10030k
Two new redox active ligands based on di(2-(3-organopyrazolyl)-p-tolyl)amine have been prepared in
order to investigate potential effects of steric bulk on the structures, electronic properties, or reactivity
of tricarbonylrhenium(I) complexes. Replacing the hydrogens at the 3-pyrazolyl positions with alkyl
groups causes significant distortion to the ligand framework due to potential interactions between these
groups when bound to a fac-Re(CO)3 moiety. The distortions effectively increase the nucleophilic
character of the central amino nitrogen and ligand-centered reactivity of the metal complexes.
Introduction
Metal complexes of pincer ligands are receiving increased atten-
tion for studies in a wide range of topical areas from catalysis
to bioinorganic and materials chemistry.1 The appeal of these
complexes arises from their generally high stability and the
unusual reactivity that suitably designed ligands can impart on
a metal center. Further interest is educed by emergent reports
documenting non-innocent pincer variants that promote unex-
pected chemistry.2 We recently introduced a new non-innocent
Fig. 1 Electrochemical response of a tricarbonylrhenium(I) NNN-pincer
complex in CH2Cl2 to HBF4.
NNN-pincer ligand based on di(2-(pyrazolyl)-p-tolyl)amine and
its various tricarbonylrhenium(I) complexes (Fig. 1).3 The quasi-
reversible electrochemistry associated with the (metal-bound)
ligand oxidation could be reproducibly turned ‘off’ or ‘on’ by
protonation and deprotonation reactions with Brønsted acids or
bases, respectively. Moreover, the one-electron oxidized product
[Re(CO)3(LH)]+ was demonstrated to contain a ligand-centred rad-
ical by IR and EPR experiments. These results were also suggested
by a theoretical (DFT) study that showed that most of the spin
density was located on the central amido nitrogen, substantial
contributions were found at the ortho- and para- aryl carbons, and
a smaller contribution extended onto a metal d-orbital. During
the course of that work it occurred to us that if the stability of
the ligand cation radical results from hole delocalization over the
entire p-conjugated diarylamine framework, it should be possible
to alter the stability (i.e. increase the reactivity) of this cation
or even its precursors by increasing the aryl-aryl dihedral angle,
effectively disrupting conjugation. Inspection of the structures of
Re(CO)3(LH) and associated derivatives suggested that this goal
could be achieved simply by placing steric bulk at the 3-position
of the pyrazolyls. Herein we fully document the successful, yet
surprising, results of these efforts including the preparation of
two new NNN-pincer ligands (R = Me, iPr, Scheme 1) and the
properties of their various Re(CO)3 complexes.
Results and discussion
Preparation
aDepartment of Chemistry, Marquette University, Milwaukee, WI, USA
53201-1881. E-mail: james.gardinier@marquette.edu; Fax: 414-288-7066;
Tel: 414-288-3533
The syntheses of the ligands and fac-Re(CO)3 complexes follows
methodology similar to that reported for di(2-(pyrazolyl)-p-
tolyl)amine, H(LH) and its complexes.3 The preparative routes to
the complexes are summarized in Scheme 1. For the ligand syn-
theses described in the experimental, the CuI-catalyzed amination
bDepartment of Biophysics, Medical College of Wisconsin, Milwaukee, WI,
USA 53226. E-mail: bbennett@mcw.edu; Fax: 414-456-6512; Tel: 414-456-
4787
† Electronic supplementary information (ESI) available: Experimental
procedures, crystal data, other characterization data and further discus-
sion. CCDC reference numbers 806478–806485 contain the supplementary
5
reactions4 between HN(2-Br-p-tolyl)2 and either 3-methyl- or 3-
isopropyl-pyrazole6 proceeded smoothly to give 60–65% yields
of H(LMe) or H(LiPr) simply by heating neat mixtures for 1 d at
200 ◦C followed by conventional workup. In contrast, low yields of
crystallographic data for H(LMe), 1Me, 1iPr, 2Me, 2iPr, 3Me, 3iPr, and 4iPr
,
respectively. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c1dt10030k
8776 | Dalton Trans., 2011, 40, 8776–8787
This journal is
The Royal Society of Chemistry 2011
©

reaction with hydroxide to give H2O). Of the two routes to 3R,
that starting from 1R is preferred since one less synthetic step
is required (and in our hands it was easier to separate 3R from
NEt4Br than from NEt4(PF6)). In either case, it is noted that the
reaction time is best kept short (15 min) as longer reaction times
give lower yields due to a slow but competing decomposition
reaction that produces increasing amount of ‘free’ ligand H(LR);
the nature of the rhenium-containing decomposition by-product
is unclear. Fortuitously, the separation of 3R and other products is
facilitated by the significantly different solubilities of the desired
and unwanted products in MeOH or in benzene and Et2O.
Solid State Structures
The structures of H(LMe) and the six rhenium complexes 1R,
2R, and 3R (R = Me, iPr) were determined by single crystal X-
ray diffraction; those of H(LH), 1H, 2H, and 3H were reported
previously. Representative structures for 1Me, 2Me, and 3Me are
provided in Fig. 2–4 while other new structures are provided
in the Electronic Supplementary Information (ESI).† Selected
interatomic distances and angles are listed in Table 1. All of
the rhenium complexes retain the fac-Re(CO)3 moiety and all
are chiral (with C1-symmetry) as a result of the various ligand
conformations (vide infra).
Scheme 1 Summary of preparative routes to the various Re(CO)3
complexes of the NNN-pincer ligands used in this work.
H(LH) are obtained when heating neat reaction mixtures because
unsubstituted pyrazole distills out of the reaction mixture and
condenses as a solid onto cooler parts of the reaction apparatus;
here, the addition of minimal xylenes helps to wash pyrazole back
to the heterogeneous reaction mixture. The longer reaction time
required for the preparation of H(LH) (2d, monitored by TLC,
69% isolated yield) is likely limited by the distillation temperature
◦
of xylenes (bp = 151 C). For the 3-organopyrazolyl derivatives,
only the desired isomer of H(LMe) or H(LiPr) as depicted in the
top left of Scheme 1 was obtained (from NMR spectral data and
crystallographic determinations of the free ligand, H(LMe)† and
of all metal complexes with these ligands). Hypothetical di(2-(5-
R-pyrazolyl)-p-tolyl)amine isomers (with both R groups situated
proximal rather than distal to the aryls) or mixed 3,5-isomers
have not been detected. In the IR spectrum (KBr) of each ligand,
the N–H stretching frequency occur as a medium intensity, sharp
bands at rather low energy for 2◦ arylamines (3261 cm-1 for H(LH);
3297 cm-1 for H(LMe); 3296 cm-1 for H(LiPr)) which typically occur
nearer to 3400 cm-1, presumably a result of the intramolecular
hydrogen bonding.7
Fig. 2 Structure of fac-ReBr(CO)3[H(LMe)], 1Me
.
The reactions between the free ligands [of general notation
H(LR)] and Re(CO)5Br in boiling toluene causes elimination of
two equivalents of CO concomitant with the precipitation of
the fac-ReBr(CO)3[H(LR)] complexes (1R) as analytically pure
colorless powders. The ensuing reactions of 1R with TlPF6 in
CH3CN provide {fac-Re(CO)3[H(LR)]}(PF6) (2R). As found in
related diarylamine systems,8 complexation of the ligands to
metal centers causes a progressive red-shift in the N–H stretching
frequency with increasing electron density of the metal center.
For instance, nNH = 3243 cm-1 for 2H and nNH = 3147 cm-1 for 1H.
Finally, the reactions of colorless 1R or 2R in CH3CN with the
Brønsted base (NEt4)(OH) leads immediately to the formation of
the corresponding yellow fac-Re(CO)3(LR) complexes (3R) where
the hydrogen on the diarylamine has been eliminated (after
Fig. 3 Structure of the cation in {fac-Re(CO)3[H(LMe)]}(PF6), 2Me
.
For the 1R series, the ligand is bound to the metal in a chelating
k N-manner via the central amino nitrogen and one pyrazolyl
2
This journal is
The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 8776–8787 | 8777
©

Table 1 Selected bond distances and interatomic angles for 1R, 2R, and 3R (R = H, Me, iPr)
1H
1Me
1iPr
2H
2Me
2iPr
3H
3Me
3iPr
4iPr
˚
Distance (A)
Re–Br
Re–N1
Re–N11
Re–N21
Re–C41
Re–C42
Re–C43
C41–O1
2.6193(8)
2.265(6)
2.182(6)
—
1.899(8)
1.928(8)
1.946(8)
1.165(9)
1.143(10)
1.091(9)
1.913(9)
0.383(8)
2.6312(3)
2.268(2)
2.198(2)
—
1.915(3)
1.922(3)
1.932(3)
1.147(3)
1.147(3)
1.113(3)
2.03(3)
2.6123(3)
2.263(3)
2.198(2)
—
1.932(3)
1.934(3)
1.947(4)
1.132(4)
1.136(4)
1.050(4)
2.13(4)
—
—
—
—
—
—
—
2.257(4)
2.174(3)
2.180(3)
1.916(5)
1.944(5)
1.933(5)
1.149(5)
1.129(5)
1.140(5)
—
2.251(2)
2.175(2)
2.181(2)
1.923(3)
1.927(3)
1.929(3)
1.141(4)
1.148(4)
1.148(4)
—
2.265(2)
2.187(2)
2.205(2)
1.921(3)
1.937(3)
1.925(3)
1.144(4)
1.140(4)
1.141(4)
—
2.163(2)
2.173(2)
2.148(2)
1.914(3)
1.923(3)
1.948(3)
1.158(3)
1.147(3)
1.138(3)
—
2.178(3)
2.191(3)
2.207(3)
1.919(3)
1.921(4)
1.909(3)
1.154(4)
1.155(5)
1.153(4)
—
2.183(2)
2.198(2)
2.219(2)
1.923(2)
1.922(2)
1.915((2)
1.150(3)
1.151(3)
1.152(3)
—
2.306(1)
2.177(1)
2.185(1)
1.905(2)
1.922(2)
1.925(2)
1.153(2)
1.153(2)
1.146(2)
—
C42–O2
C43–O3
N1H1 ◊ ◊ ◊ N21
^N1 ◊ ◊ ◊ (C2Re)a
Angles/torsions (◦)
N1–Re–N11
N1–Re–N21
Fold (N11)b
Fold (N21)c
ReN11–N12C2
ReN21–N22C32
pz(N11)–tol(C1)
pz(N21)–tol(C31)
tol(C1)–tol(C31)
R∠¢s about N1d
0.396(2)
0.385(3)
0.492(5)
0.515(3)
0.519(3)
0.191(2)
0.431(3)
0.426(2)
0.576(2)
77.2(2)
—
138.8(5)
—
-16.1(9)
—
40.8(6)
13.6(6)
76.9(6)
344.4(7)
77.91(8)
—
136.4(2)
—
-12.8(3)
—
39.8(3)
23.7(3)
66.7(3)
343.2(3)
77.61(9)
—
135.1(2)
—
12.0(4)
—
40.7(2)
28.6(2)
67.5(2)
344.2(3)
77.72(13)
83.68(13)
130.6(5)
159.2(4)
14.0(5)
-9.8(6)
43.6(5)
27.2(5)
74.6(4)
334.5(4)
77.03(9)
83.45(8)
127.0(4)
152.6(4)
5.1(3)
76.30(9)
83.57(8)
125.2(2)
153.2(2)
-1.9(3)
15.9(3)
37.3(2)
37.2(2)
69.7(2)
331.7(2)
79.26(8)
81.57(8)
136.2(2)
147.4(2)
-14.3(3)
8.3(3)
45.0(2)
36.3(2)
28.4(2)
355.6(2)
76.59(10)
84.27(11)
127.2(3)
164.1(3)
-1.3(4)
-36.(4)
38.5(2)
37.9(2)
78.8(2)
75.27(7)
85.09(7)
125.5(2)
161.9(2)
6.4(3)
-33.0(3)
35.7(1)
38.6(1)
77.9(1)
339.2(2)
78.25(5)
83.30(5)
121.0(2)
144.2(2)
-11.6(2)
1.5(2)
50.9(1)
28.9(1)
72.9(1)
326.1(2)
0.4(4)
38.5(3)
29.8(3)
71.5(3)
332.0(2)
338.6(3)
pz = mean plane of pyrazolyl ring, tol = mean plane of C6 ring of tolyl group;a Distance of normal vector between N1 and mean plane of atoms Re, C1,
and C31; b fold angle between Re and the centroids (Ct) of N1 and N11 and Ct of C1 and N12; c fold angle between Re and the centroids (Ct) of N1 and
N21 and Ct of C31 and N22; d involving Re, C1, and C31.
the interatomic distances and angles associated with the ‘free’
arm of the ligand. For 1H, there is a relatively short hydrogen
bonding interaction between the amino hydrogen H1 and the free
◦
˚
pyrazolyl nitrogen N21 (N1H1 ◊ ◊ ◊ N21: 1.91 A, 140 ) that brings
the ‘free’ pyrazolyl and tolyl groups closer to coplanarity (dihedral
between mean planes of 14◦) than those rings that are bound to
rhenium (dihedral between mean planes of 41◦). For 1Me and 1iPr
,
the hydrogen bonding interaction becomes progressively longer
(and presumably weaker) and the pz-tolyl dihedral becomes larger
◦
˚
with increasing steric bulk (N1H1 ◊ ◊ ◊ N21: 2.02 A, 141 and pz-
tolyl dihedral 24◦ for 1Me and N1H1 ◊ ◊ ◊ N21: 2.13 A, 153 and
pz-tolyl dihedral 29◦ for 1iPr). A similar observation is made for
the structures of the free lig◦ands [two independent molecules: avg.
◦
˚
Fig. 4 Structure of fac-Re(CO)3(LMe), 3Me
.
◦
H
˚
˚
N1H1 ◊ ◊ ◊ N21: 2.04 A, 132 and pz-tolyl dihedral 30 for H(L );
◦
◦
N1H1 ◊ ◊ ◊ N21: 2.20 A, 129 and pz-tolyl dihedral 43 for H(LMe)].
nitrogen. In each of these cases, the amino hydrogen is oriented
toward the axial bromide rather than the axial carbonyl. For
each, the rhenium-nitrogen bond involving the amino group (Re–
For each ionic derivative 2R, the ligand binds rhenium in a
3
k N- manner giving a fac-ReN3C3 kernel. The average Re–N
distances in 2R are shorter than the corresponding distances in
1R, as expected from the cationic nature of the former. Within the
series of 2R, 3-pyrazolyl substitution results in gradual increase
in Re–Npz distances with increasing steric bulk but, as with 1R,
substitution has little impact on the Re–N1 distances. In 2R, there
are two six-member ReN3C2 chelate rings that can be differentiated
by small differences in Re–Npz bond lengths, chelate bite and fold
angles. As found in Table 1, one chelate ring (containing N11)
has a shorter Re–Npz bond, a smaller chelate bite (N1ReN11
angle) and a greater chelate ring puckering (more acute fold
angle) than the other chelate ring containing N21. The chelate
ring with smaller bite and fold angles in 2R has similar metrical
parameters to those found in 1R. A final small but noteworthy
effect of changing 3-pyrazolyl substituents is found by examining
the local coordination geometry around the amino nitrogen N1.
˚
N1, or Re–NAr, ca. 2.27 A) is longer than that involving the
˚
pyrazolyl (Re–N11, or Re–Npz, ca. 2.19 A). The bond distances
in this series of complexes are typical of other N,N-chelating
ligands containing the fac-Re(CO)3Br moiety such as in the
closely related Re(CO)3Br[H(pzAnMe)] (H(pzAnMe) is 2-pyrazolyl-
˚
˚
4-toluidine; Re–Br = 2.628 A, Re–NAr = 2.219 A, and Re–Npz
9
˚
2.179 A) or those in the NNN-pincer- relative, ReBr(CO)3[bis(1-
methyl-1H-benzoimidazol-2-ylmethyl)amine)] (Re–Br, Re–Navg
=
10
2.23–2.28 A). Within the series 1R, the steric profile of the
3-R-pyrazolyl substituent has the expected but small effect on
Re–Npz bond distances with the unsubstituted derivative having
˚
˚
a shorter bond (2.18 A) than the 3-substituted derivatives (ca.
˚
2.20 A) but there is no significant difference in the Re–N1
˚
(amino nitrogen) bond distances (ca. 2.26 A). Interestingly, the
most striking influence of 3-pyrazolyl substitution occurs with
8778 | Dalton Trans., 2011, 40, 8776–8787
This journal is
The Royal Society of Chemistry 2011
©

The 3-organopyrazolyl groups in 2Me and 2iPr enforce greater
pyramidalization about N1 (relative to the mean plane defined
by C1 C31 and Re) compared to the unsubstituted pyrazolyl
derivative 2H. That is, the sum of angles about N1 (R∠¢s about N1,
not involving N1–H1) and the perpendicular distance between N1
and the mean plane defined by C1 C31 and Re, ^N1 ◊ ◊ ◊ (C2Re),
Table 2 IR and electrochemical data for various Re(CO)3 complexes
Compound nC–O (cm-1)a E1/2 (V vs. Fc/Fc+)a,b
1H
2029, 1921, 1898; avg 1949c irr. Epa = +1.07, +0.67, +0.23
2027, 1920, 1896; avg. 1948c irr. Epa = +1.04, +0.75, +0.25
2027, 1919, 1894; avg. 1947c irr. Epa = +1.04, +0.75, +0.20
2040, 1950, 1930; avg. 1973 irr. Epa = +1.17
2040, 1935, 1919; avg 1965 irr. Epa = +1.27
2038, 1936, 1921; avg 1965 irr. Epa = +1.25
2013, 1901, 1876; avg. 1930 +0.001
1Me
1iPr
2H
◦
◦
Me
are 332 and 0.52 A for 2 and 2iPr but are 334 and 0.49 A for
˚
˚
2Me
2iPr
3H
◦
H
˚
2 ; a planar nitrogen would have ideal values of 360 and 0 A.
In a manner similar to 2R, the ligands in 3R bind rhenium in
a k N-manner giving fac-ReN3C3 kernels. Deprotonation of the
3Me
3iPr
(3H)+
2009, 1903, 1879; avg. 1930 -0.011
3
2008, 1898, 1876; avg. 1927 -0.015
amino hydrogen is accompanied by a significant shortening of
2034, 1927; avg. 1963
2038, 1931; avg. 1967
2038, 1933; avg. 1968
—
—
—
R
(3Me
(3iPr
4Me
)
+
+
˚
the Re–NAr bond in 3 (ca. 2.19 A) relative to the corresponding
distances in 1R (ca. 2.27 A) or 2 (ca. 2.26 A). Within the
R
˚
˚
)
2036, 1930, 1923; avg. 1963 not measured
2033, 1927, 1915; avg. 1958 not measured
series 3R, the Re–NAr bond is longer for derivatives with 3-
4iPr
H
Me
˚
˚
organo substituents (2.163(2) A for 3 , 2.178(3) A for 3 and
a CH2Cl2 solution; b CH2Cl2, 100 mV s-1, TBAH; c major species, see ESI†
for more details.
2.183(2) A for 3iPr). As highlighted in Fig. 5, the structure of 3H is
˚
distinct from those of 3Me and 3iPr in that the former approaches
mirror symmetry (disregarding the tolyl-tolyl dihedral and slight
differences in chelate ring distortions that give the complex actual
2
presumably five-coordinate [Re(CO)3(k -HLR)+](Br-)†. All 2R and
C1 symmetry) with a short average Re–Npz distance (2.16 A)
ionized forms of 1R are also involved in dynamic exchange
processes. Full details of the complex NMR data for these
complexes can be found in the ESI.†
˚
and a nearly planar amido nitrogen (R∠¢s about N1 = 356◦). In
contrast, the latter two complexes more closely resemble their
protonated counterparts 2Me and 2iPr each with decidedly C1
The NMR spectra for 3R are simpler than expected based on the
low-symmetry solid state structures owing to rapid processes that
interchange supposedly symmetrically inequivalent halves of the
ligands (or that invert conformations of chelate rings). That is, if
the solid state structures were retained, two sets of resonances for
pyrazolyl and tolyl hydrogens would be expected but only one set is
observed (vide infra). In surprising contrast to 1R, 2R or 4R, the rate
of the exchange process in 3R could not be slowed down enough to
be measured by NMR even when CD2Cl2 or acetone-d6 solutions
are cooled to 193 K. Given that the exchange processes can be
frozen at low temperatures for derivatives with quaternary amino
nitrogens (1R, 2R or 4R, vide infra),† nitrogen inversion facilitates
the exchange processes of 3R.
symmetry. Relative to 3H, 3Me and 3iPr have longer Re–Npz bond
Me
distances (2.20 A for 3 and 2.21 v for 3iPr) and more pyramidal
˚
amido nitrogens (R∠¢s about N1 = 339◦ for each). Presumably,
potential steric interactions involving 3-organopyrazolyl groups
enforce the observed C1 symmetric conformations, and make
hypothetical pseudo-Cs symmetric conformations of either 3Me or
3iPr much higher energy.
In either the solid state or solution, the IR spectrum of each
3R gives a characteristic pattern of three C–O stretching bands
(Table 1) for fac-Re(CO)3 units; the N–H stretching band is
also absent. In accord with expectations based on the increasing
electron density at metal centres (and greater back-bonding), the
CO stretches appear at lower energy relative to 1R and 2R where
average stretching frequencies decrease in the order 2R > 1R > 3R.
For 3R, replacement of 3-pyrazolyl hydrogens for more electron
donating methyl or isopropyl substituents has a surprisingly small
electronic effect, as indicated by the nearly identical average CO
stretching frequencies. It is likely that any potential inductive
electronic effects may be offset by steric interactions that enforce
Fig. 5 Overlay of structures for 3R (R = H, black thin wireframe; R =
Me, red capped stick; R = iPr, green capped stick) referenced to common
NArRe(CO)3 cores.
longer Re–N bonds along the series 3H < 3Me < 3iPr
.
Solution Characterization
The electrochemistry of each 3R is distinct from their counter-
parts 1R or 2R (Table 1) as each 3R in CH2Cl2 shows a quasi-
reversible oxidation near 0 V versus Fc/Fc+ (Fig. 6, ipc/ipa = 1,
but DE = Epa - Epc increases as a function of scan rate); 1R and
2R have irreversible oxidations (ipc/ipa ꢀ 1 and DE ꢁ 59 mV)
at higher potentials. The oxidation potentials of 3Me and 3iPr are
nearly equivalent and are only slightly (10–15 mV) more favourable
than that of 3H. Interestingly, in CH3CN the oxidation becomes
reversible for 3H and 3Me but not for 3iPr.† Spectrophotometric
titrations with organic oxidants indicate that the oxidation is a
one-electron event, as discussed later.
Selected electrochemical and IR spectral data for complexes 1R–3R
(R = H, Me, iPr) are given in Table 2. The current discussion of
solution properties will center on the data for 3R because of their
interesting electronic properties and disparate reactivity patterns
is the focus of this work. The solution characterization data of
analytically pure 1R–3R (R = H, Me, iPr) are less germane to
the central point of the work but are noteworthy since they are
unexpectedly complex, as described previously for R = H.3 That
is, NMR and other solution data show that all 1R are involved in
ionization equilibria to form 2R and another ionic intermediate,
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Fig. 6 Cyclic voltammograms of fac-Re(CO)3(LR) (3R) complexes in
CH2Cl2 each taken with scan rates of 50 (inner), 100, 200, 400, and 800
(outer) mV s-1.
The electronic absorption spectra of 3R complexes in CH2Cl2
are found in Fig. 7. The spectra are qualitatively similar, as
might be expected, but there are subtle differences that distinguish
the R = H from the R = Me, iPr derivatives. Each spectrum
has two bands above about 350 nm that give rise to the yellow
colour of the complexes. For 3H these low energy bands are more
intense (e ~ 8000–10000 M-1cm-1) than those of either 3Me or 3iPr
(e ~ 5000 M-1cm-1). For 3H the lowest energy band (400 nm,
e ~ 8000 M-1cm-1) is less intense than the second lowest energy
band (360 nm, e ~ 10000 M-1cm-1) while the opposite is true
for either 3Me or 3iPr; for the latter deconvolution is necessary
to observe the second lowest energy band. Since these two
bands are absent in 1R and 2R, they are attributed to transitions
between electronic states involving an engaged dp–pp interaction
(between the metal and available lone pair on the central amido
nitrogen of the ligand). Such an assessment was bolstered by
theoretical calculations (TD-DFT, see ESI for full details) where
the lowest energy band enveloped transitions between the HOMO
and various LUMO(+N) (N = 0–4) levels and the second-lowest
energy band involves transitions between the HOMO(-1) and the
various LUMO(+N) (N = 0–4) levels. As illustrated in Fig. 8,
the HOMO is mainly a p-based orbital centralized on the pincer
ligand but extends onto a d-orbital of rhenium. The HOMO(-1)
is qualitatively similar to the HOMO but with greater rhenium
character. For 3H, conjugation across both 2-pyrazolyl-p-tolyl
‘arms’ of the pincer ligand is evident from the atomic orbital
Fig. 8 Comparison between frontier orbitals of 3H (left) and 3iPr (right)
from theoretical calculations (B3LYP/LACVP).
contributions to the HOMO and to a lesser extent the HOMO(-1)
but for 3Me and 3iPr the conjugation appears confined to only
one ‘arm’ of the ligand. The LUMO and LUMO(+1) are mainly
p*-orbitals of the pincer ligand while next three higher-energy
virtual orbitals are those of the tricarbonyl fragment. As such
these two lowest energy bands can be considered to have metal–
ligand-to-ligand charge transfer (MLLCT) character in accord
with conventions used elesewhere.11 The higher energy band found
at 300 nm is likely due to charge transfer transitions involving the
tricarbonylrhenium fragment as found in related systems7 while
the high-intensity bands found below 275 nm are likely p–p*
transitions on the basis of energy and intensity considerations.
Reactivity
Given the availability of a lone pair of electrons on the central
nitrogen in 3R, the potential for these complexes to engage in
nucleophilic substitution (SN2) reactions12 as in Scheme 2 was
evaluated. Initial stoichiometric NMR experiments performed in
C6D6 at room temperature showed either no or trace reaction after
a couple of hours. However, in hot (45 ◦C) acetone and with a 10-
fold excess of MeI, complexes 3R (R = Me, iPr) underwent clean
conversion to give {fac-Re(CO)3[Me(LR)]}(I), 4R, over the course
of about four hours, detected by both NMR and IR (Table 2)
spectroscopy. Complex 3H failed to react with MeI even after days
under similar reaction conditions (of temperature and reagent
concentrations). The NMR spectrum of each 4R shows two sets
of resonances for pyrazolyl and tolyl hydrogens whereas that of
3R shows only one set. Additionally, the solution IR spectrum
(CH2Cl2) of 4R exhibited C–O stetching bands with avg. nco
~
1960 cm-1 which is comparable to that of 2R. Single crystal X-ray
diffraction of 4iPr (Fig. 9) confirmed that the methyl group was
indeed bound to the central nitrogen of the ligand rather than to a
pyrazolyl nitrogen. Also, in contrast to the 1iPr where the bromide
was bound to rhenium, the iodide in 4iPr is a spectator ion and
Fig. 7 Overlay of electronic absorption spectra for 3R in CH2Cl2 (R = H,
black; R = Me, red; R = iPr, green).
3
the ligand binds rhenium in a k N-manner similar to that in 2iPr
.
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Scheme 2 Attempted reactions of 3R with MeI intended to form
{fac-Re(CO)3[Me(LR)]}(I), 4R complexes.
Fig. 10 Portions of the 1H NMR spectra obtained by heating a 1 : 10
mixture of 3Me: MeI, highlighting resonances for 4-pyrazolyl hydrogens of
3Me (red-shaded doublet near dH = 6.4 ppm) and of the product 4Me (two
indigo-shaded doublets near dH = 7.0 and 6.3 ppm).
Fig. 9 Structure of {fac-Re(CO)3[Me(LiPr)]}(I)·2CH2Cl2, 4iPr·2CH2Cl2
with solvate molecules removed for clarity.
The greater steric profile of an methyl versus a hydrogen bound to
nitrogen subtly impacts the cation structure by increasing the bond
distances around rhenium and distorting the ligand framework (by
comparing values in Table 1).13
The resonances for various 3-organopyrazolyl hydrogens for 3R
(R = Me, iPr) and the corresponding 4R products are sufficiently
well separated to allow for a convenient means to monitor the rates
of reaction by using relative integration of signals (Fig. 10). As
illustrated in Fig. 11, the pseudo-first order conditions ([MeI]/[3R]
≥ 10) gave straight-line plots with statistically identical half-lives;
t1/2 of 62 ( 3) min for 3Me and 65 ( 3) min for 3iPr where the
uncertainty arises from the measurements of different types of
resonances within the same experiment. In accord with eqn (1)
and the experimental conditions, the corresponding second-order
rate
Fig. 11 Pseudo-first order plots of ln (mol fraction of 3R) (R = Me, red;
R = iPr, green) versus time from integration of 4-pyrazolyl hydrogen NMR
resonances observed during conversions of 3R to 4R with MeI.
-d[3R]/dt = kobs[3R] = k2[MeI][3R]
(1)
constants were found to be k2 = 5.7 ¥ 10-4 M-1s-1 for 0.033 M 3Me
and 0.331 M MeI and k2 = 8.4 ¥ 10-4 M-1s-1 for 0.021 M 3iPr and
0.212 M MeI. More in-depth kinetic analysis of these and other
related systems is underway.
Fig. 12 Left: Space-filling diagram of 3H; Right: Overlay of structures for
3
iPr (light green) and the cation in 4iPr (violet) referenced to common ReC3
The difference in reactivity between the various 3-
organopyrazolyl derivatives 3R and that of 3H can be attributed
to inter-related structural and electronic factors. It was antici-
pated and found that the replacement of the two (very close)
hydrogen atoms labeled in Fig. 12 with any other group should
(and does) drastically alter the structure and reactivity of the
complexes. Given the typical inert nature of Re-ligand bonds,
the spectroscopic data, and that no N-methyl pyrazolyls was
detected in reactions with MeI, it is expected that the ligands
remain tridentate in acetone solutions of 3H and 3R and that
cores.
pyrazolyl dissociation is unlikely the origin of increased reactivity
of 3R versus 3H. If the ligands are indeed tridentate, the greater
reactivity of 3R versus 3H toward MeI can be rationalized by the
greater steric accessibility of the more pyramidal nitrogen of 3R
to incoming electrophiles than that in 3H. The pyramidal nature
of nitrogen in 3R has two consequences. First, the complexes 3R
are pre-organized in a conformation similar to that found for 4R
(right of Fig. 12); the activation barrier for the conversion of 3H
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to hypothetical 4H should be higher due to requisite structural
reorganization. Moreover, the basicity of the amido nitrogen in
3R is also expected to be greater owing to the greater s-character,
lower degree of conjugation and the slightly higher energy HOMO
versus 3H (Fig. 8).
The discrepancy in properties and reactivity between 3R (R =
Me, iPr) and 3H perpetuates in the one-electron oxidized products
(3R+)(SbCl6 ). Reactions of 3R with the organic cation radical
-
9,10-dimethoxyocta-hydro-1,4:5,8-dimethanoanthracenium hex-
achloroantimonate [(CRET+)(SbCl6 )],14 a modest oxidant (E1/2
,
-
red
0.58 V vs. Fc/Fc+), affords blue-green (3R+)(SbCl6 ), see Fig. 13
and ESI. While (3H+) was found to be stable as a solid and only
very slowly decomposed at 295 K in aerated CH2Cl2 (t1/2 = 3d),
-
(3R(= Me or iPr)+)(SbCl6 ) decomposed much more rapidly in aerated
-
CH2Cl2 (t1/2 = 3.5 h for both); thus, solution measurements must
be made on freshly prepared samples with exclusion of air. At
295 K, the EPR spectrum of each cation radical (3R+) in CH2Cl2
(Fig. 14) displays a well-resolved sextet signal due to the hyperfine
interaction between the electronic spin and the 185/187Re nuclei (I =
5/2). The isotropic signal for (3H+) (giso = 2.017 aRe 49.5 G) is similar
but distinct from the signals for either (3Me+) (gav = 2.016, aRe = 33.4
G, aN = 7.5 G) or (3iPr+) (gav = 2.016, aRe = 33.8 G, aN = 7.5 G).
In each case, the relatively small deviation of g-values from that
for the free electron ge = 2.0023 and the small hyperfine couplings
are consistent with a ligand-centred rather than a metal-centred
radical, with the spin density on rhenium being highest for (3H+).3,15
Theoretical calculations indicate most of the spin density is located
on the ligand (Fig. 15) in accord with other experimental indicators
of a ligand-centred radical such as the occurrence of intense pi-
radical bands (p(L) → SOMO) in the 650–750 nm range of the
electronic absorption spectrum. Also, the average energy of the C–
O stretching bands in the solution (CH2Cl2) IR spectra, nco(avg),
increases by only 33, 37, and 41 cm-1 on traversing between 3R and
3R+ for R = H, Me, and iPr, respectively (Table 2). Such a relatively
small increase in energy is similar to the 38 cm-1 increase for related
PNP pincer complexes [Re(CO)3(PNP)]n+ (n = 0,1) (measured for
KBr pellets) and is consistent with ligand-centered oxidation.2
Rhenium-centred oxidations would be expected to have nco(avg)
increase on the order of 50–100 cm-1.2,3,16
Fig. 14 Comparison of X-Band (9.63 GHz, 295 K) EPR spectra for
(3R+)(SbCl6-) in CH2Cl2 (R = H, black; R = Me, red; R = iPr, green).
Simulated spectra have dashed lines.
Fig. 15 Spin density isosurface for energy minimized (BP86) structural
model of (3Me+) from theoretical calculations (UB3LYP/LACVP).
A final set of poorly understood observations that highlight
the incongruent reactivity patterns of 3R (R = Me, iPr) and 3H
derivatives is that CH2Cl2 solutions of 3Me or 3iPr were light sensitive
but those of 3H were not. Thus, CH2Cl2 solutions of the latter two
compounds should be protected from light and measurements
should be made on freshly prepared solutions. A more extended
account of the unexpected photodecomposition behaviour can be
found in the Electronic Supporting Information.
Conclusions
The purpose of this study was to investigate whether the reactivity
of tricarbonylrhenium(I) complexes of di(2-(3-R-pyrazolyl)-p-
tolyl)amine derivatives would be altered by substitution at the
3-pyrazolyl position; the properties of various Re(CO)3 complexes
of the unsusbtituted ligand H(LR) R = H were communicated
previously. To this end, two new 3-alkylpyrazolyl ligands (R =
Me, iPr) were prepared in good yield by straghtforward CuI-
catalyzed amination reactions. The availability of the three H(LR)
i
ligands (R = H, Me, and Pr) ligands allowed a series of nine
tricarbonylrhenium(I) complexes to be prepared and fully charac-
terized both in solution and the solid state. The most significant
structural and reactivity differences were found across the series
of fac-Re(CO)3(LR) (3R) complexes with deprotonated, formally
uninegative, NNN-ligands. The bond distances in 3R increased
Fig. 13 Spectroelectrochemical titration reaction between 3Me and
(CRET+)(SbCl6-) in CH2Cl2. Inset: Plot of absorbance versus mol ratio
monitoring bands for (CRET+) at 518 nm (grey squares) and for (3Me+) at
377 nm (orange triangles) and 687 nm (red circles).
8782 | Dalton Trans., 2011, 40, 8776–8787
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with increasing steric bulk of the 3-pyrazolyl substituents. For 3H, a
conformation with near Cs symmetry and a planar amido nitrogen
was found in the solid state whereas for 3Me or 3iPr, the ligands were
greatly distorted with substantial pyramidalization of the amido
nitrogen. This conformation is dictated by unfavorable steric
interactions that would occur between 3-pyrazolyl substituents in
a pseudo-Cs symmetric conformation such as in 3H. The solution
spectroscopic data demonstrate that none of the three complexes
retain their static solid state geometries. Based on comparisons
with other complexes, this behavior is attributed to conformational
changes of intact complexes with tridentate ligands. Pyrazolyl
dissociation to give bidentate ligands and perhaps a coordinatively
unsaturated (or weakly-solvated)metal centers cannot be excluded
in either 2R or 3R cases (which show dynamic solution behavior),
but seems unlikely owing the usual kinetically inert nature of
rhenium-ligand bonds, the flexibility of six-membered chelate
rings, combined with the observed reactivity patterns. The relative
reactivities follow the divisive pattern where 3Me and 3iPr are
reactive towards MeI to afford an N-methyl (amino not pyrazolyl)
derivative but 3H does not react with MeI under similar conditions.
Moreover, CH2Cl2 solutions of the former two complexes are
photosensitive but similar solutions of 3H were photo-stable. A
final difference was found for the one-electron oxidized products
(3R+); the room-temperature EPR spectrum of CH2Cl2 solutions
KBr pellets and as CH2Cl2 solutions using a Nicolet Magna-IR
560 spectrometer. Absorption measurements were recorded on an
Agilent 8453 spectrometer. Electrochemical measurements were
collected under nitrogen atmosphere at a scan rate of 100 mV s-1
for samples as 0.1 mM CH2Cl2 solutions with 0.1 M NBu4PF6
as the supporting electrolyte. A three-electrode cell comprised
of an Ag/AgCl electrode, a platinum working electrode, and a
glassy carbon counter electrode was used for the voltammetric
measurements. With this set up, the ferrocene/ferrocenium couple
had an E1/2 value of +0.53 V consistent with the literature value
in this solvent.18 Mass spectrometric measurements recorded in
ESI(+) or ESI(-) mode were obtained on a Micromass Q-TOF
spectrometer whereas those performed by using direct-probe
analyses were made on a VG 70S instrument. For the ESI(+)
experiments formic acid (approximately 0.1% v/v) was added to
the mobile phase (CH3CN). EPR measurements were obtained
using a Bruker ELEXSYS E600 equipped with an ER4116DM
cavity resonating at 9.63 GHz, an Oxford Instruments ITC503
temperature controller and ESR-900 helium-flow cryostat. The
ESR spectra were recorded with 100 kHz field modulation.
Di(2-(3-methylpyrazolyl)-p-tolyl)amine, H(LMe
)
A reaction vessel was charged with a mixture of 3.44 g (9.69 mmol)
di(2-bromo-p-tolyl)amine, 2.78 g (33.9 mmol, 3.5 equiv) 3-
methylpyrazole, 5.35 g (38.7 mmol, 4.0 equiv) K2CO3, and 0.38 mL
(3.87 mmol, 40 mol %) DMED, and was deoxygenated by three
evacuation and nitrogen back-fill cycles. Then, 0.18 g (0.97 mmol,
10 mol %) CuI was added as a solid under nitrogen. The reaction
mixture was heated under nitrogen at 200 ◦C for 15 h. After
cooling to room temperature, 200 mL of H2O was added and
the mixture was extracted with three 100 mL portions of CH2Cl2.
The combined organic layers were dried over MgSO4, filtered, and
solvent was removed by rotary evaporation to give an oily residue
that was purified by column chromatography on silica gel. Elution
with 8 : 1 hexanes:ethyl acetate (Rf 0.7) afforded 2.28 g (66%) of
H(LMe) as a white solid. Mp, 83–85 ◦C. Anal. Calcd (obs.) for
C22H23N5: C, 73.92 (73.68); H, 6.49 (6.53); N, 19.59 (19.41). IR
i
for R = Me or Pr gave signals indicative of a more asymmetric
ligand environment than that for R = H. Moreover, solutions of
(3Me+) and (3iPr+) were considerably more prone to decomposition
than (3H+). The incongruent nature of the structures and electronic
spectra of the two classes of complexes combined with results
of DFT calculations for the various 3R and (3R+) cation radicals
indicate that the differences arise from a combination of the lower
degree of conjugation across the ligand backbone and a (surpris-
ing) greater accessibility to a more pyramidal amido nitrogen on
the ligand. Studies are underway to further explore the chemical
and photochemical potential of these and related complexes.
Experimental
1
(KBr) nNH 3297. H NMR: (CD2Cl2) 8.43 (s, 1H, NH), 7.62 (d,
Materials
J = 2 Hz, 2H, H5pz), 7.22 (d, J = 8 Hz, 2H, Ar), 7.09 (s, 2H,
Ar), 7.00 (d, J = 8 Hz, 2H, Ar), 6.19 (d, J = 2 Hz, 2H, H4pz),
2.29 (s, 12H, CH3). 1H NMR: (acetone-d6) 8.86 (s, 1H, NH), 7.85
(d, J = 2 Hz, 2H, H5pz), 7.24 (d, J = 8 Hz, 2H, Ar), 7.19 (s, 2H,
Ar), 7.05 (d, J = 8 Hz, 2H, Ar), 6.24 (d, J = 2, 2H, H4pz), 2.29
(s, 6H, ArCH3), 2.28 (s, 6H, pzCH3). 13C NMR: (CDCl3) 149.9,
134.6, 131.0, 130.7, 130.2, 128.8, 125.8, 118.9, 106.6, 20.7, 13.8.
UV-VIS lmax, nm (e, M-1cm-1), CD2Cl2: 242(37149), 304(26376).
Single crystals suitable for X-ray diffraction were obtained by slow
evaporation of a hexane solution.
Pyrazole, 3-methylpyrazole, CuI, N,N’-dimethylethylenediamine
(DMED), anhydrous K2CO3 powder, and (NEt4)(OH) (1 M
in MeOH) were purchased from commercial sources and used
without further purification while Re(CO)5Br,17 di(2-bromo-p-
tolyl)amine,5 3-isopropylpyrazole6 were prepared by literature
methods. Methyl iodide was distilled under vacuum before use.
Solvents used in the preparations were dried by conventional
methods and were distilled under nitrogen prior to use.
Instrumentation
Di(2-(3-isopropylpyrazolyl)-p-tolyl)amine, H(LiPr
)
Midwest MicroLab, LLC, Indianapolis, Indiana 45250, performed
all elemental analyses. 1H and 13C NMR spectra were recorded on
a Varian 400 MHz spectrometer. Chemical shifts were referenced
to solvent resonances at dH 7.27, dC 77.23 for CDCl3; dH 5.32,
dC 54.00 for CD2Cl2 and dH 2.05, dC 29.92 for acetone-d6.
Melting point determinations were made on samples contained
in glass capillaries using an Electrothermal 9100 apparatus and
are uncorrected. Infrared spectra were recorded on samples as
In a manner similar to that described above, a mixture of 7.17 g
(0.0202 mol) di(2-bromo-p-tolyl)amine, 7.78 g (0.0706 mol, 3.5
equiv) 3-isopropylpyrazole, 11.05 g (0.0800 mol, 4.0 equiv) K2CO3,
and 0.79 mL (0.65 g, 7.4 mmol, 35 mol %) DMED, 0.38 g
(2.0 mmol, 10 mol %) CuI afforded 5.06 g (61%) of H(LiPr) as a light
yellow oil after workup and purification (SiO2, 8 : 1 Hexane: ethyl
acetate Rf 0.6). Anal. Calcd (obs.) for C26H31N5: C, 75.51 (75.61);
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1
H, 7.56 (7.48); N, 16.93 (16.78). IR (KBr) nNH 3296. H NMR:
15 h. The resulting precipitate was isolated by filtration, washed
with two 5 mL portions Et2O and dried under vacuum which
afforded 0.34 g (71%) as a fine white powder. Mp, 264–267 ◦C
dec. Anal. Calcd (obs.) C29H31BrN5O3Re: C, 45.61 (45.40); H, 4.09
(3.96); N, 9.17 (9.14). IR (KBr) nNH 3143; nco 2020, 1910, 1880 cm-1.
1H NMR: (CD2Cl2, 293 K) two species, see text: I, 92% of signal
integration intensity from resolved resonances in the R–CH3, NH,
H5pz and H4pz regions of spectrum; II, 8% of signal: 11.96 (br s,
1H, NH, II), 10.50 (br s, 1H, NH, I), 8.08 (d, J = 2.9 Hz, 1H, H5pz,
I), 8.06 (br m, 1H, H5pz, II), 7.90 (d, J = 2.5 Hz, 1H, H5pz, II), 7.82
(d, J = 2.5 Hz, 1H, H5pz, I), 7.63–7.53 (br s, 6H, Ar, II), 7.29 (m,
3H, Ar, I), 7.18 (m, 3H, Ar, I), 6.65 (d, J = 2.9 Hz, 1H, H4pz, II),
6.64 (d, J = 2.9 Hz, 1H, H4pz, I), 6.37 (br m, 1H, H4pz, II), 6.25
(d, J = 2.5 Hz, 1H, H4pz, I), 3.85 (sept, J = 7.1 Hz, 1H, Me2CH,
I), 2.51 (sept, J = 6.7 Hz, 1H, Me2CH, II), 2.46 (s, 3H, ArCH3,
I), 2.43 (s, 3H, ArCH3, I), 2.38 (br s, 3H, ArCH3, II), 1.33 (d, J =
7 Hz, 3H, iPrCH3, I), 1.29 (d, J = 7 Hz, 3H, iPrCH3, I), 1.23 (br
m, 3H, iPrCH3, II), 1.04 (d, J = 7 Hz, 3H, iPrCH3, I), 0.99 (d, J =
(CD2Cl2) 8.82 (s, 1H, NH), 7.67 (d, J = 2 Hz, 2H, H5pz), 7.18 (d,
J = 2 Hz, 2H, Ar), 7.14 (s, 2H, Ar), 6.98 (dd, J = 8, 2 Hz, 2H, Ar),
6.21 (d, J = 2 Hz, 2H, H4pz), 2.95 (sept, J = 7 Hz, 2H, Me2CH),
2.29 (s, 6H, ArCH3), 1.22 (d, J = 1 Hz, 6H, iPrCH3), 1.20 (d, J =
1 Hz, 6H, iPrCH3). 1H NMR: (acetone-d6) 8.70 (s, 1H, NH), 7.87
(d, J = 2 Hz, 2H, H5pz), 7.23 (s, 2H, Ar), 7.20 (d, J = 8 Hz, 2H, Ar),
7.01 (d, J = 8 Hz, 2H, Ar), 6.28 (d, J = 2 Hz, 2H, H4pz), 2.99 (sept,
J = 7 Hz, 2H, Me2CH), 2.29 (s, 6H, ArCH3), 1.24 (d, J = 1 Hz,
6H, iPrCH3), 1.22 (d, J = 1 Hz, 6H, iPrCH3), 13C NMR: (CDCl3)
160.4, 134.3, 130.9, 130.5, 129.9, 128.5, 125.6, 119.3, 103.7, 27.9,
22.9, 20.7. UV-VIS lmax, nm (e, M-1cm-1), CD2Cl2: 244(36386),
304(22429).
ReBr(CO)3[H(LMe)], (1Me
)
A mixture of 0.172 g (0.423 mmol) Re(CO)5Br and 0.151 g
(0.422 mmol) of H(LMe) in 20 mL of toluene was heated at reflux
15h. The resulting precipitate was isolated by filtration, washed
with two 5 mL portions Et2O and dried under vacuum which
afforded 0.22 g (75%) 1Me as a fine white powder. Mp, 269–271 ◦C
dec. Anal. Calcd (obs.) for C25H23BrN5O3Re: C, 42.44 (42.20); H,
3.28 (3.21); N, 9.90 (9.74). IR (KBr) nNH 3138; nco 2025, 1915,
7 Hz, 3H, iPrCH3, I), 0.88 (br m, 3H, iPrCH3, II). UV-VIS lmax
,
nm (e, M-1cm-1), CD2Cl2: 295 (10,900), 259 (38,200), 230 (51,500).
13C NMR: (CD2Cl2) 168.3, 161.7, 138.6, 137.5, 133.5, 130.6, 130.5,
129.4, 127.3, 125.2, 121.8, 107.2, 105.1, 31.1, 28.4, 24.3, 23.4, 23.1,
22.8, 21.3, 20.9. X-ray quality crystals were grown by layering an
acetone solution with hexane and allowing the solvents to slowly
diffuse over two days.
1895 cm-1. H NMR: (CD2Cl2, 303 K) three species, see text: I,
1
88% of signal integration intensity from resolved resonances in
the R–CH3, NH, H5pz and H4pz regions of spectrum; II, 10% of
signal; III 2% of signal): 12.10 (br s, 1H, NH, III), 11.84 (br s, 1H,
NH, II), 10.50 (br s, 1H, NH, I), 8.06 (d, J = 2 Hz, 1H, H5pz, I),
8.03 (d, J = 2 Hz, 1H, H5pz, II), 7.89 (d, J = 2 Hz, 1H, H5pz, II),
7.79 (d, J = 2 Hz, 1H, H5pz, I), 7.58 (d, J = 8 Hz, 1H, Ar, I), 7.33–
7.24 (m, 4H, Ar, I/II/III), 7.21–7.02 (br m, 4H, Ar I/II/III), 6.60
(d, J = 2 Hz, 1H, H4pz, II), 6.59 (d, J = 2 Hz, 1H, H4pz, I), 6.31
(d, J = 2 Hz, 1H, H4pz, II), 6.20 (d, J = 2 Hz, 1H, H4pz, I), 2.74
(s, 3H, pzCH3, I), 2.47 (s, 3H, ArCH3, I), 2.44 (s, 3H, ArCH3, I),
2.41–2.28 (br m, 9H, pz- and ArCH3, II/III), 2.21 (s, 3H, pzCH3,
II), 1.98 (s, 3H, pzCH3, I). 1H NMR: (CD2Cl2, 213 K) 11.70 (br s,
1H, NH, III), 11.48 (br s, 1H, NH, II), 10.29 (br s, 1H, NH, I),
8.17 (d, J = 8 Hz, 1H, Ar, II), 8.13 (br s, 1H, H5pz, III), 8.08 (br s,
1H, H5pz, I), 7.99 (br s, 1H, H5pz, II), 7.95 (br s, 1H, H5pz, III),
7.86 (br s, 1H, H5pz, II), 7.80 (d, J = 8 Hz, 1H, Ar, I), 7.74 (br s,
1H, H5pz, I), 7.53 (br s, 1H, Ar, II), 7.41–7.01(br m, see text, Ar,
I/II/III), 6.91 (s, 1H, Ar, II), 6.83 (d, J = 8 Hz, 1H, Ar, II), 6.61 (d,
J = 2 Hz, 1H, H4pz, II), 6.58 (d, J = 2 Hz, 1H, H4pz, I), 6.38 (br s,
1H, H4pz, III), 6.31 (d, J = 2 Hz, 1H, H4pz, II), 6.18 (br s, 1H,
H4pz, III), 6.16 (br s, 1H, H4pz, I), 6.07 (d, J = 2 Hz, 1H, H4pz, II),
5.87 (1H, H4pz, III), 2. 78 (s, 3H, pzCH3, II), 2.73 (s, 3H, pzCH3,
III), 2.68 (s, 3H, pzCH3, I), 2.62 (s, 3H, pzCH3, III), 2.45 (s, 3H,
ArCH3, II), 2.44 (s, 3H, ArCH3, I), 2.39 (s, 3H, ArCH3, I), 2.33 (s,
3H, ArCH3, III), 2.24 (s, 3H, ArCH3, III), 2.18 (s, 3H, ArCH3, II),
2.02 (s, 3H, pzCH3, II), 1.83 (s, 3H, pzCH3, I). UV-VIS lmax, nm
(e, M-1cm-1), CD2Cl2: 231(50963), 261(34522), 289(11818). X-ray
quality crystals of 1Me·acetone were grown by layering an acetone
solution with hexane and allowing the solvents to slowly diffuse
over two days.
{Re(CO)3[H(LMe)]}(PF6), (2Me
)
A mixture of 0.075 g (0.11 mmol) of 1Me and 0.04 g (0.11 mmol)
of TlPF6 in 10 mL dry CH3CN was heated at reflux overnight.
After cooling to room temperature TlBr was separated by filtration
through Celite, and solvent was removed by rotary evaporation.
The residue was washed with two 5 mL portions Et2O and was
dried under vacuum to give 0.060 g (75%) of 2Me as a colorless to
pale yellow powder.
Mp, 243–246 ◦C dec. Anal. Calcd (obs.) for
C26H25Cl2F6N5O3PRe (2Me·CD2Cl2): C, 36.41 (36.25); H,
2.94 (2.77); N, 8.17 (8.27). IR (KBr) nNH 3253; nco 2030, 1940,
1
1920 cm-1. H NMR: (CD2Cl2, 233 K) 7.98 (d, J = 3 Hz, 1H,
H5pz), 7.54 (s, 1H, NH), 7.53 (d, J = 3 Hz, 1H, H5pz), 7.48 (d, J =
8 Hz, 1H, Ar), 7.41 (d, J = 8 Hz, 1H, Ar), 7.30 (s, 1H, Ar), 6.94
(s, 1H, Ar), 6.89 (d, J = 9 Hz, 1H, Ar), 6.69 (d, J = 9 Hz, 1H, Ar),
6.66 (d, J = 3 Hz, 1H, H4pz), 6.10 (d, J = 3 Hz, 1H, H4pz), 2.80 (s,
3H, pzCH3), 2.49 (s, 3H, ArCH3), 2.23 (s, 3H, ArCH3, ArCH3),
2.04 (s, 3H, pzCH3). 13C NMR: (CD2Cl2, 295 K) no signals were
observed even after prolonged acquisition times. UV-VIS lmax, nm
(e, M-1cm-1), CD2Cl2: 230(34154), 250 (28087), 294 (8854). X-ray
quality crystals were grown by layering an acetone solution with
hexane and allowing the solvents to slowly diffuse over two days.
{Re(CO)3[H(LiPr)]}(PF6), (2iPr
)
A mixture of 0.205 g (0.27 mmol) of 1iPr and 0.084 g (0.27 mmol)
TlPF6 in 20 mL dry THF was heated at reflux overnight. After
cooling to room temperature, TlBr was separated by filtration
through Celite and solvent was removed from the filtrate by rotary
evaporation. The residue was washed with two 5 mL portions
Et2O and was dried under vacuum to give 0.198 g (84%) of 2iPr
as a white powder. Mp, 278–280 ◦C dec. Anal. Calcd (obs.) for
ReBr(CO)3[H(LiPr)] (1iPr
)
A mixture of 0.256 g (0.630 mmol) Re(CO)5Br and 0.260 g
(0.727 mmol) of H(LiPr) in 20 mL of toluene was heated at reflux for
8784 | Dalton Trans., 2011, 40, 8776–8787
This journal is
The Royal Society of Chemistry 2011
©

1
C29H31F6N5O3PRe: C, 42.03 (42.26); H, 3.77 (4.02); N, 8.45 (8.12)
IR (KBr) nNH 3236; nco 2035, 1940, 1911 cm-1. 1H NMR: (CD2Cl2,
233 K) 8.24 (s, 1H, NH), 7.97 (d, J = 3 Hz, 1H, H5pz), 7.56 (d,
J = 3 Hz, 1H, H5pz), 7.55 (d, J = 8 Hz, 1H, Ar), 7.40 (d, J =
8 Hz, 1H, Ar), 7.29 (s, 1H, Ar), 6.90 (s, 1H, Ar), 6.87 (d, J =
8.5 Hz, 1H, Ar), 6.73 (d, J = 3 Hz, 1H, H4pz), 6.70 (d, J = 8.5 Hz,
1H, Ar), 6.15 (d, J = 3 Hz, 1H, H4pz), 3.84 (sept, J = 7 Hz, 1H,
Me2CH), 2.92 (sept, J = 7 Hz, 1H, Me2CH), 2.49 (s, 3H, ArCH3),
2.23 (s, 3H, ArCH3), 1.49 (d, J = 7 Hz, 3H, iPrCH3), 1.36 (d, J =
7 Hz, 3H, iPrCH3), 1.16 (d, J = 7 Hz, 3H, iPrCH3), 0.62 (d, J =
7 Hz, 3H, iPrCH3). 13C NMR: (CD2Cl2, 295 K) no signals were
observed even after prolonged acquisition times. UV-VIS lmax, nm
(e, M-1cm-1), CD2Cl2: 229(31536), 249(24891), 294(5514). X-ray
quality crystals were grown by layering an acetone solution with
hexane and allowing the solvents to slowly diffuse over two days.
2H, H4pz), 2.52 (s, 6H, pzCH3), 2.27 (s, 6H, ArCH3). H NMR:
(acetone-d6) 8.26 (d, J = 3 Hz, 2H, H5pz), 7.10 (d, J = 2 Hz,
2H, Ar), 6.92 (part of AB d, J = 8, 2 Hz, 2H, Ar), 6.61 (part
of AB d, J = 8 Hz, 2H, Ar), 6.44 (d, J = 3 Hz, 2H, H4pz), 2.53
(s, 6H, pzCH3), 2.23 (s, 6H, ArCH3). 13C NMR: (CD2Cl2) 198.2,
196.7, 155.5, 149.8, 132.2, 131.5, 129.6, 128.2, 124.2, 122.1, 108.3,
20.6, 17.1. UV-VIS lmax, nm (e, M-1cm-1), CD2Cl2: 234(44756),
247(41711), 298(14615), 392(3687). X-ray quality crystals were
grown by layering an acetone solution with hexane and allowing
the solvents to slowly diffuse over two days.
Method B. A 0.32 mL aliquot of 0.509 M (NEt4)(OH)
(0.16 mmol) in MeOH was added to a solution of 0.124 g
(0.160 mmol) 2Me in 10 mL of CH3CN immediately giving a yellow
solution. The mixture was stirred for 15 min then solvent was
removed by rotary evaporation. The yellow residue was extracted
with three 5 mL portions of benzene and solvent was removed
by vacuum distillation to leave a mixture of benzene-soluble
3Me contaminated with H(LMe) (NMR). The contaminant was
removed by washing with minimal Et2O (2 mL), to leave 0.030 g
(30%) of 3Me with characterization data identical to above. Selective
precipitation of 3Me using MeOH as in Method A, did not lead to
improved yield.
Re(CO)3(LMe), (3Me
)
Method A. To a solution of 0.201 g (0.28 mmol) 1Me in 20 mL
of CH3CN was added 2.75 mL (0.283 mmol) (NEt4)(OH) solution
in MeOH immediately giving a yellow solution. The mixture was
stirred for 30 min then solvent was removed by rotary evaporation.
The yellow residue was washed with two 5 mL portions MeOH
and was dried under vacuum to leave 0.150 g (88%) of 3Me as
a yellow powder. Mp, 250–254 ◦C dec. Anal. Calcd (obs.) for
C25H22N5O3Re: C, 47.91 (48.01); H, 3.54 (3.58); N, 11.18 (11.23).
Re(CO)3(LiPr), (3iPr
)
In a manner similar to method A of 3Me, 0.091 mmol (NEt4)(OH)
(1.3 mL of 0.07 M solution in MeOH) and 0.073 g (0.095◦mmol)
gave 0.040 g (61%) of 3iPr as a yellow powder. Mp, 240–243 C dec.
Anal. Calcd (obs.) for C29H30N5O3Re: C, 51.01 (51.24); H, 4.43
1
IR (KBr) nco 2020, 1905, 1885 cm-1. H NMR: (CD2Cl2, 293 K)
7.81 (d, J = 2 Hz, 2H, H5pz),6.92 (s, 2H, Ar–H), 6.90 (part of
AB d, 2H, Ar), 6.63 (part of AB d, 2H, Ar), 6.28 (d, J = 2 Hz,
Table 3 Crystal and structure refinement data for H(LMe), 1Me·acetone, 2Me, and 3Me
H(LMe
)
1Me·acetone
2Me
3Me
Empirical Formula
Formula Weight
T/K
C22H23N5
357.45
100(2)
0.43 ¥ 0.18 ¥ 0.12
Orthorhombic
Pbcn
14.6980(3)
7.7505(2)
33.6382(8)
90
C28H29BrN5O4Re
765.67
100(2)
C25H23F6N5O3PRe
772.65
100(2)
C25H22N5O3Re
626.68
100(2)
0.24 ¥ 0.14 ¥ 0.06
Monoclinic
C2/c
30.8054(4)
11.4882(2)
13.4222(2)
90
94.7730(10)
90
4733.63(12)
8
1.759
1.54178
Crystal Size/mm
Crystal System
Space Group
0.24 ¥ 0.13 ¥ 0.05
0.26 ¥ 0.21 ¥ 0.10
Triclinic
Triclinic
¯
¯
P1
P1
˚
a/A
10.8094(2)
10.9596(2)
13.4496(2)
84.7710(10)
85.0860(10)
60.9190(10)
1385.10(4)
2
8.63640(10)
9.66670(10)
16.7540(2)
78.9030(10)
82.4590(10)
78.8010(10)
1340.01(3)
2
˚
b/A
˚
c/A
a (◦)
b (◦)
g (◦)
90
90
3
˚
V/A
3831.96(16)
8
1.239
1.54178
0.597
multi-scan
1520
2.63 to 67.97
32005
3444
0.0306
0.7835/0.9318
3444/0/253
1.020
0.0382 (0.0432)
0.0988 (0.1024)
0.237/-0.158
Z
dc/g cm-3
1.836
1.54178
10.630
numerical
748
3.30 to 67.89
11557
4733
1.915
1.54178
10.165
numerical
752
2.70 to 67.70
11073
4556
˚
l(Cu-Ka)/A
m/mm-1
10.350
numerical
2448
2.88 to 68.02
19490
4198
Abs. Correction
F(000)
q range (◦)
Reflections collected
Reflections unique
Rint
0.0150
0.0168
0.0204
T
min/Tmax
0.1847/0.6185
4046/0/363
1.001
0.0172 (0.0173)
0.0446 (0.0447)
0.613/-0.548
0.1775/0.4297
4556/0/381
1.007
0.0188 (0.0194)
0.0474 (0.0478)
0.714/-0.646
0.1902/0.5756
4198/0/311
0.993
0.0245 (0.0285)
0.0679 (0.0702)
0.815/-0.565
Data/restraints/parameters
GOF(F2)
R indices[I > 2s(I)]a (all data)
wR indices (all data)b
Residuals/e A-3
2
1/2
a R1 = RꢂFo| - |Fcꢂ/R|Fo|. b wR2 = {R[w(Fo - Fc2)2]/R[w(Fo2)2]}
.
This journal is
The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 8776–8787 | 8785
©

Table 4 Crystal and structure refinement data for 1iPr, 2iPr, 3iPr and 4iPr·2CH2Cl2
1iPr
2iPr
3iPr
4iPr·2CH2Cl2
Empirical Formula
Formula Weight
T/K
C29H31BrN5O3Re
763.70
100(2)
0.15 ¥ 0.14 ¥ 0.10
Orthorhombic
P212121
13.1679(2)
13.4486(2)
16.2017(2)
90
C29H31F6N5O3PRe
828.76
100(2)
0.23 ¥ 0.10 ¥ 0.06
Monoclinic
P21/c
9.49510(10)
14.9707(2)
22.1068(3)
90
100.1190(10)
90
3093.56(7)
4
1.779
1.54178
C29H30N5O3Re
682.78
100(2)
0.28 ¥ 0.16 ¥ 0.06
Monoclinic
P21/c
15.7528(2)
13.73050(10)
13.76030(10)
90
114.662(1)
90
2704.79(4)
4
1.677
1.54178
C32H37Cl4IN5O3Re
994.57
100(2)
Crystal Size
0.33 ¥ 0.07 ¥ 0.04
Crystal System
Triclinic
¯
Space Group
P1
˚
a/A
10.9417(3)
12.1830(2)
15.7817(3)
73.5426(18)
85.7981(19)
65.475(2)
1833.16(7)
2
˚
b/A
˚
c/A
a (◦)
b (◦)
g (◦)
90
90
3
˚
V/A
2869.16(7)
4
1.768
1.54178
10.234
numerical
1496
4.27 to 68.17
23851
5079
0.0216
0.3090/0.4276
5079/0/362
0.992
0.0168 (0.0170)
0.0428 (0.0429)
0.642/-0.458
Z
dc/g cm-3
1.802
0.7107
4.487
numerical
968
3.30 to 32.72
59279
12747
˚
l(Cu or Mo-Ka), A
m/mm-1
8.854
numerical
1632
3.58 to 67.61
25065
5429
9.110
numerical
1352
3.09 to 68.20
22830
4810
Abs. Correction
F(000)
q range (◦)
Reflections collected
Reflections unique
Rint
0.0223
0.0173
0.0330
T
min/Tmax
0.2353/0.6187
5429/0/416
0.992
0.0219 (0.0252)
0.0541 (0.0554)
0.682/-0.628
0.1847/0.6109
4810/0/350
0.999
0.0178 (0.0182)
0.0462 (0.0465)
0.578/-0.606
0.349/0.866
12747/0/422
1.040
0.0191 (0.0241)
0.0456 (0.0463)
1.006/-0.896
Data/restraints/parameters
GOF(F2)
R indices[I > 2s(I)]a (all data)
wR indices (all data)b
Residuals/e A-3
a R1 = RꢂFo| - |Fcꢂ/R|Fo|. b wR2 = {R[w(Fo - Fc2)2]/R[w(Fo2)2]}
.
2
1/2
(4.54); N, 10.26 (10.22). IR (KBr): nco 2010, 1900, 1875 cm-1. H
NMR (CD2Cl2, 293 K) 7.80 (d, J = 3 Hz, 2H, H5pz), 6.89 (s, 2H,
Ar), 6.88 (part of AB d, 2H, Ar), 6.60 (part of AB d, 2H, Ar), 6.33
(d, J = 3 Hz, 2H, H4pz), 3.57 (sept, J = 7 Hz, 2H, Me2CH), 2.26
(s, 6H, ArCH3), 1.34 (d, J = 7 Hz, 6H, iPrCH3), 1.02 (d, J = 7 Hz,
6H, iPrCH3). 1H NMR: (acetone-d6) 8.18 (d, J = 3 Hz, 2H, H5pz),
7.07 (s, 2H, Ar), 6.89 (part of AB d, 2H, Ar), 6.56 (part of AB d,
2H, Ar), 6.55 (d, J = 2 Hz, 2H, H4pz), 3.62 (sept, J = 7 Hz, 1H,
Me2CH), 2.22 (s, 6H, ArCH3), 1.37 (d, J = 7 Hz, 6H, iPrCH3), 1.08
(d, J = 7 Hz, 6H, iPrCH3). 13C NMR: (CD2Cl2) 197.2, 194.5, 166.0,
150.0, 133.0, 131.5,129.5, 128.5, 124.4, 122.0, 104.6, 30.6, 23.6,
23.3, 20.6. UV-VIS lmax, nm (e, M-1cm-1), CD2Cl2: 234(49162),
247(46362), 305(12127), 391(3667). X-ray quality crystals were
grown by layering an acetone solution with hexane and allowing
the solvents to slowly diffuse over two days.
dissolving the residue in CH2Cl2, layering with n-hexane, and
allowing the solvents to slowly diffuse 15 h.
4
1
Me Mp, 265–270 ◦C dec. IR (CH2Cl2) nco 2036, 1930, 1923 cm-1.
1H NMR: (acetone-d6, 293 K) 8.78 (d, J = 2.8 Hz, 1H, H5pz), 8.06
(d, J = 2.8 Hz, 1H, H5pz), 8.00 (part of AB d, Japp = 8.4 Hz, 1H,
Ar), 7.72 (s, 1H, Ar), 7.66 (part of AB, d, Japp = 8.4, 2.1, 1 Hz, 1H,
Ar), 7.57 (s, 1H, Ar), 7.04 (part of AB d, Japp = 8.4 Hz, 1H, Ar),
7.01 (d, J = 2.8 Hz, 1H, H4pz), 6.82 (part of AB d, Japp = 8.4 Hz,
1H, Ar), 6.33 (d, J = 2.8 Hz, 1H, H4pz), 3.76 (s, 3H, NCH3), 2.92
(s, 3H, CH3), 2.53 (s, 3H, CH3), 2.28 (s, 3H, CH3), 2.06 (s, 3H,
CH3). UV-VIS lmax, nm (e, M-1cm-1), CD2Cl2: 242 (52,000), 289sh
(13,000), 368 (1,300).
4
iPr. Mp, 260–265 ◦C dec. IR (CH2Cl2) nco 2033, 1927, 1915 cm-1.
1H NMR: (acetone-d6, 293 K) 8.79 (d, J = 3.0 Hz, 1H, H5pz), 8.12
(d, J = 3.0 Hz, 1H, H5pz), 8.00 (part of AB d, Japp = 8.4 Hz, 1H,
Ar), 7.71 (s, 1H, Ar), 7.66 (part of AB, d, Japp = 8.4, 2.1, 1 Hz,
1H, Ar), 7.53 (s, 1H, Ar), 7.17 (d, J = 2.8 Hz, 1H, H4pz), 7.05
General procedure for NMR-scale reactions between 3R and MeI
(part of AB d, Japp = 8.4 Hz, 1H, Ar), 6.82 (part of AB d, Japp
=
8.4 Hz, 1H, Ar), 6.49 (d, J = 2.8 Hz, 1H, H4pz), 3.93 (sept, J =
7 Hz, 1H, Me2CH), 3.75 (s, 3H, NCH3), 2.91 (sept, J = 7 Hz, 1H,
Me2CH), 2.54 (s, 3H, ArCH3), 2.28 (s, 3H, ArCH3), 1.57 (d, J =
7 Hz, 3H, iPrCH3), 1.49 (d, J = 7 Hz, 3H, iPrCH3), 1.22 (d, J =
Solutions were prepared in NMR tubes by dissolving 7–9 mg
3R in 0.35 mL of acetone-d6. A ten-fold excess MeI (7–9 mL,
as appropriate) was injected into the solution, the NMR tube
was immediately sealed and inserted into the pre-heated 45 ◦C
NMR cavity for measurements where time of insertion served as
the reference point (t = 0 min). NMR spectra were acquired at
10 min intervals for the first 30 min, then at 30 min intervals
thereafter.
Colorless crystals of {Re(CO)3[Me(LiPr)]}(I), (4iPr) suitable for
single-crystal X-ray diffraction were obtained by removing volatile
components from the completed reactions by vacuum distillation,
7 Hz, 3H, iPrCH3), 0.81 (d, J = 7 Hz, 3H, iPrCH3). UV-VIS lmax
,
nm (e, M-1cm-1), CD2Cl2: 242 (50,000), 293sh (9,000), 367 (400).
Crystallographic Structure Determinations
X-ray intensity data from a colorless prism of H(LMe), colorless
block of each ReBr(CO)3[H(LMe)]·acetone (1Me·acetone),
8786 | Dalton Trans., 2011, 40, 8776–8787
This journal is
The Royal Society of Chemistry 2011
©

ReBr(CO)3[H(LiPr)] (1iPr), {Re(CO)3[H(LMe)]}(PF6), (2Me),
3 S. Wanniarachchi, B. J. Liddle, J. Toussaint, S. V. Lindeman, B. Bennett;
and J. R. Gardinier, Dalton Trans., 2010, 39, 3167.
4 B. J. Liddle, R. M. Silva, T. J. Morin, F. P. Macedo, R. Shukla, S. V.
Lindeman and J. R. Gardinier, J. Org. Chem., 2007, 72, 5637; M.
Taillefer, N. Xia and A. Ouali, Angew. Chem., Int. Ed., 2007, 46, 934 and
references; J. C. Antilla, J. M. Baskin, T. E. Barder and S. L. Buchwald,
J. Org. Chem., 2004, 69, 5578.
5 L. Fan, B. M. Foxman and O. V. Ozerov, Organometallics, 2004, 23,
326.
6 S. Trofimenko, J. C. Calabrese, P. J. Domaille and J. S. Thompson, Inorg.
Chem., 1989, 28, 1091.
7 K. Chadwick, R. J. Davey, R. G. Pritchard, C. A. Hunter and D.
Musumeci, Cryst. Growth Des., 2009, 9, 1990; V. Zanker, H. H.
Mantsch; and E. Erhardt, Anales de Quimica, 1968, 64, 659; M. A.
Salimov and V. M. Tatevskii, Dokl. Akad. Nauk. SSSR, 1957, 112, 890.
8 R. C. Sharma and R. K. Parashar, J. Inorg. Biochem., 1987, 29, 225;
A. V. Savitskii and T. M. Kosareva, Dokl. Akad. Nauk SSSR, 1971,
197, 359; J. S. Decker and H. Frye, Z. Naturforsch. Teil B, 1966, 21,
527.
and{Re(CO)3[H(LiPr)]}(PF6), (2iPr), of
a yellow block of
Re(CO)3(LMe), (3Me), and of a pale yellow block of Re(CO)3(LiPr),
(3iPr), were measured at 100(2) K with a Bruker AXS 3-circle
diffractometer equipped with a SMART219 CCD detector
using Cu(Ka) radiation. X-ray intensity data from a colorless
needle of {Re(CO)3[Me(LiPr)]}(I)·2CH2Cl2, (4iPr·2CH2Cl2) were
measured at 100(2) K with an Oxford Diffraction Ltd. Supernova
diffractometer equipped with a 135 mm Atlas CCD detector
using Cu(Ka) (or Mo(Ka) for 4iPr·2CH2Cl2) radiation. Raw
data frame integration and Lp corrections were performed
with SAINT+.20 Final unit cell parameters were determined
by least-squares refinement of 5019, and 9735 reflections from
the data sets of 1Me·acetone, and 1iPr, respectively, of 8854, and
7055 reflections from the data sets of 2Me and 2iPr respectively,
and of 8940, and 8003 reflections from the data sets of 3Me
and 3iPr, respectively, and of 38198 reflections from the data set
of 4iPr with I > 2s(I) for each. Analysis of the data showed
negligible crystal decay during collection in each case. Direct
methods structure solutions, difference Fourier calculations and
full-matrix least-squares refinements against F2 were performed
with SHELXTL.21 Numerical absorption corrections based on
the real shape of the crystals for the compounds were applied
with SADABS.20 All non-hydrogen atoms were refined with
anisotropic displacement parameters. Hydrogen atoms were
placed in geometrically idealized positions and included as riding
atoms, except where noted below. The X-ray crystallographic
parameters and further details of data collection and structure
refinements are presented in Tables 3 and 4.
9 B. J. Liddle, S. Wanniarachchi, S. V. Lindeman and J. R. Gardinier,
J. Organomet. Chem., 2010, 1, 695.
ˆ
10 S. Alves, A. Paulo, J. D. G. Correia, A. Domingos and I. Santos,
J. Chem. Soc., Dalton Trans., 2002, 4714.
11 B. Machura, R. Kruszynski, M. Jaworska, P. Lodowski, R. Penczek
and J. Kusz, Polyhedron, 2008, 27, 1767.
12 A. J. Parker, J. Chem. Soc., 1961, 1328.
iPr
˚
˚
13 The average Re–N bond distance of 2.223 A in 4 is similar to 2.219 A
found in 2iPr where the Re–NAr bond (2.306 A) is longer and the two
˚
˚
Re–Npz (2.177, 2.185 A) are shorter for the former than in the latter.
It is also noted that the amino nitrogen is more pyramidal for 4iPr
than 2iPr as evident from the smaller sum of angles around nitrogen
not involving the methyl (326◦ versus 333◦) and larger perpendicular
distance of nitrogen from the mean plane of rhenium and aryl carbon
˚
˚
atoms to which nitrogen is bound (0.576 A versus 0.519 A). The small
but increased ligand distortion in 4iPr versus 2iPr is also found from the
tolyl-tolyl and average pz-tolyl dihedral angles (73◦ versus 70◦ and 40◦
versus 37◦, respectively).
14 R. Rathore, C. L. Burns, M. I. Deselinescu, S. E. Denmark and T. Bui,
Org. Synth., 2005, 82, 1.
15 The difference in EPR signal line shapes between (3H+) and (3Me+) or
(3iPr+) can be attributed to a number of factors. First, the rhenium
hyperfine coupling is smaller than the overall line width in the latter two
cases causing alteration of the hyperfine pattern. The relative intensities
of the four middle lines (line 4 > 3 > 5 > 2) in the latter two spectra
may also arise due to genuine asymmetry of the complexes to give
higher order effects. The possibility for multiple conformers (where each
has a slightly different g-value) and any dynamic equilibria involving
these species may also contribute. Incomplete motional averaging seems
unlikely given the small size of the molecule.
16 D. Chong, D. R. Laws, A. Nafady, P. J. Costa, A. L. Rheingold, M. J.
Calhorda and W. E. Geiger, J. Am. Chem. Soc., 2008, 130, 2692; F. A.
Cotton, K. R. Dunbar, L. R. Falvello and R. A. Walton, Inorg. Chem.,
1985, 24, 4180.
17 S. P. Schmidt, W. C. Trogler and F. Basolo, Inorg. Synth., 1990, 28, 160.
18 I. Noviandri, K. N. Brown, D. S. Fleming, P. T. Gulyas, P. A. Lay, A. F.
Masters and L. Phillips, J. Phys. Chem. B, 1999, 103, 6713.
19 SMART APEX2 Version 2.0-2 Bruker Analytical X-ray Systems, Inc.,
Madison, Wisconsin, USA, 2005.
Acknowledgements
JRG thanks Marquette University and the NSF (CHE-0848515)
for financial support. Support for ESR spectral studies (Grant
NIH-RR001980) is also gratefully acknowledged.
Notes and references
1 O. V. Ozerov, The Chemistry of Pincer Compounds, D. Morales-Morales
and C. M. Jensen, Eds; Elsevier: New York, Chapter 13, 2007.
2 A. T. Radosevich, J. G. Melnick, S. A. Stoian, D. Bacciu, C.-H. Chen,
B. M. Foxman, O. V. Ozerov and D. G. Nocera, Inorg. Chem., 2009,
48, 9214; J. I. von der Vlugt and J. N. H. Reek, Angew. Chem., Int. Ed.,
2009, 48, 8832 and references; A. I. Nguyen, K. J. Blackmore, S. M.
Carter, R. A. Zarkesh and A. F. Heyduk, J. Am. Chem. Soc., 2009, 131,
3307 and references; D. Adhikari, S. Mossin, F. Basuli, J. C. Huffman,
R. K. Szilagyi, K. Mayer and D. J. Mindiola, J. Am. Chem. Soc., 2008,
130, 3676; S. B. Harkins and J. C. Peters, Inorg. Chem., 2006, 45, 4316;
S. B. Harkins and J. C. Peters, J. Am. Chem. Soc., 2005, 127, 2030; S. B.
Harkins and J. C. Peters, J. Am. Chem. Soc., 2004, 126, 2885.
20 SAINT+ Version 7.23a and SADABS Version 2.05 TWINABS Bruker
Analytical X-ray Systems, Inc., Madison, Wisconsin, USA, 2007.
21 G. M. Sheldrick SHELXTL, Version 6.1Bruker Analytical X-ray
Systems, Inc., Madison, Wisconsin, USA, 2000.
This journal is
The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 8776–8787 | 8787
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