Binding of Amino-Dialkylated Adenines to Rhenium(III) and Rhenium(IV) Centers

Article abstract of DOI:10.1021/ic970763w

Amino-dialkylated adenines R₂AdH (R = Me, Et) react with ReCl₃MeCN)(PPh₃)₂ to form mer,cis-ReCl₃(R₂-AdH)₂(PPh₃) complexes. Attempts to protonate the coordinated adenine with HCl led to the oxidized transReCl₄(R₂AdH)(PPh₃) compounds. In both types of complexes, the purine binds via the N3 site of the six-membered ring and makes an intramolecular N9-H?Cl hydrogen bond with a chloride ligand. A material of composition ReCl₄(R₂AdH)(McCN) was obtained by reacting R₂AdH with cis-ReCl₄(MeCN)₂. It turned out to be a ReCl₄[R₂-AdC(Me)-NH] complex containing a cyclic amidine ligand resulting from the insertion of the acetonitrile C≡N group in the N9-H bond of the N3-coordinated adenine unit. Crystal structures were determined for one compound of each type: ReCl₃(Me₂AdH)₂(PPh₃)·2C ₆H₅CH₃, triclinic, P1, a = 14.181(2) A?, b = 17.591(3) A?, c = 19.015(5) A?, α = 76.66(2)°, β= 86.80(2)°, γ = 87.02(1)°, Z = 4, R = 0.0606; ReCl₄(Me₂Adh)(PPh₃), triclinic, P1?, a = 10.391(4) A?, b= 10.801(5) A?, c= 13.651(6) A?, α = 89.64(4)°, β- 83.49(3)°, γ = 65.14(4)°, Z = 2, R = 0.0432; ReCl₄[Me₂AdC(Me)-NH]·C₆H ₆·CH₂Cl₂, monoclinic, P2₁/c, a - 13.941(3) A?, b = 13.656(3) A?, c = 12.874(2) A?, β= 105.47(2)°, Z = 4, R = 0.0406. Most of the signals in the ¹H NMR spectra of these paramagnetic species could be assigned.

Full text of DOI:10.1021/ic970763w

1242
Inorg. Chem. 1998, 37, 1242-1248
Binding of Amino-Dialkylated Adenines to Rhenium(III) and Rhenium(IV) Centers
Ce´line Pearson and Andre´ L. Beauchamp*
De´partement de Chimie, Universite´ de Montre´al, C.P. 6128, Succ. Centre-ville,
Montre´al, QC, Canada H3C 3J7
ReceiVed June 19, 1997
Amino-dialkylated adenines R2AdH (R ) Me, Et) react with ReCl3(MeCN)(PPh3)2 to form mer,cis-ReCl3(R2-
AdH)2(PPh3) complexes. Attempts to protonate the coordinated adenine with HCl led to the oxidized trans-
ReCl4(R2AdH)(PPh3) compounds. In both types of complexes, the purine binds via the N3 site of the six-membered
ring and makes an intramolecular N9-H‚‚‚Cl hydrogen bond with a chloride ligand. A material of composition
ReCl4(R2AdH)(MeCN) was obtained by reacting R2AdH with cis-ReCl4(MeCN)2. It turned out to be a ReCl4[R2-
AdC(Me)dNH] complex containing a cyclic amidine ligand resulting from the insertion of the acetonitrile CtN
group in the N9-H bond of the N3-coordinated adenine unit. Crystal structures were determined for one compound
of each type: ReCl3(Me2AdH)2(PPh3)‚2C6H5CH3, triclinic, P1h, a ) 14.181(2) Å, b ) 17.591(3) Å, c )
19.015(5) Å, R ) 76.66(2)°, â ) 86.80(2)°, γ ) 87.02(1)°, Z ) 4, R ) 0.0606; ReCl4(Me2AdH)(PPh3), triclinic,
P1h, a ) 10.391(4) Å, b ) 10.801(5) Å, c ) 13.651(6) Å, R ) 89.64(4)°, â ) 83.49(3)°, γ ) 65.14(4)°, Z ) 2,
R ) 0.0432; ReCl4[Me2AdC(Me)dNH]‚C6H6‚CH2Cl2, monoclinic, P21/c, a ) 13.941(3) Å, b ) 13.656(3) Å, c
1
) 12.874(2) Å, â ) 105.47(2)°, Z ) 4, R ) 0.0406. Most of the signals in the H NMR spectra of these
paramagnetic species could be assigned.
Introduction
Chart 1
Purine nucleobases contain various pairs of donors suitably
oriented to bridge adjacent metal atoms.1 For instance, the N3/
N9 region of the deprotonated form has the same topology as
a carboxylate and compounds are known in which it plays a
similar bridging role.2 N6,N6-dialkyladenine (or 6-(dialkylami-
no)purine, R2AdH, Chart 1) attracted our attention as a binuclear
6+
agent to support metal-metal bonded units such as Re2
,
because the alkyl groups hide the reactive N1 and N7 sites and
prevent the formation of extended polymeric materials. Since
our work with CH3Hg+ has shown that these sites remain
accessible for protons,3,4 dinuclear species containing several
of these ligands should exhibit spectroscopic and other physical
properties sensitive to H+ over a broad pH range.
In a previous paper,5 we reported on Re2Cl2(R2Ad)4 com-
pounds whose properties were consistent with a “paddle-wheel”
structure. However, their preparation via substitution on
[Re2Cl8]2- and Re2Cl2(acetate)4 generated mixtures of isomers
corresponding to various relative orientations of the purines
about the metal-metal axis. Considering that these ligands
sometimes promote the formation of dimers from monomeric
species,2 we considered the possibility of preparing pure isomers
by coupling monomeric rhenium complexes containing the
purine specifically coordinated in a monodentate manner. This
goal has not been reached yet, but the monomeric precursors
have been found to contain adenine bonded via the unusual N3
site.
We are describing here the preparation and structure of these
compounds, together with our efforts to interpret the 1H NMR
data of these paramagnetic Re(III) and Re(IV) monomers.
Experimental Section
† Telephone: (514) 343-6446. Fax: (514) 343-7586. E-mail:
beauchmp@ere.umontreal.ca.
Reactants and Methods. ReCl5 (Aldrich) was kept in the glovebox
under argon and used without further purification. The dialkyladenines
were prepared from 6-chloropurine (Aldrich) and dialkylamines as
described in the literature.6 Dimethyladenine was C8,N9-deuterated
by refluxing in D2O for 2.5 h and evaporating to dryness. ReCl3-
(MeCN)(PPh3)2 was prepared by the method of Rouschias and
(1) Hodgson, D. J. Prog. Inorg. Chem. 1977, 23, 255. Lippert, B. Prog.
Inorg. Chem. 1989, 37, 1. Crawford, C. A.; Day, E. F.; Saharan, V.
P.; Folting, K.; Huffman, J. C.; Dunbar, K. R.; Christou, G. J. Chem.
Soc., Chem. Commun. 1996, 1113. Catalan, K. V.; Mindiola, D. J.;
Ward, D. L.; Dunbar, K. R. Inorg. Chem. 1997, 36, 2458. Day, E. F.;
Crawford, C. A.; Folting, K.; Dunbar, K. R.; Christou, G. J. Am. Chem.
Soc. 1994, 116, 9339. Dunbar, K. R.; Matonic, J. H.; Saharan, V. P.;
Crawford, C. A.; Christou, G. J. Am. Chem. Soc. 1994, 116, 2201.
(2) Beauchamp, A. L.; Terzis, A.; Rivest, R. Inorg. Chem. 1973, 12, 1166
and references therein.
(3) Charland, J.-P.; Beauchamp, A. L. Inorg. Chem. 1986, 25, 4870.
Charland, J.-P.; Phan Viet, M. T.; St.-Jacques, M.; Beauchamp, A. L.
J. Am. Chem. Soc. 1985, 107, 8202.
(4) L. Grenier, L.; Charland, J.-P.; Beauchamp, A. L. Can. J. Chem. 1988,
66, 1663.
1
Wilkinson7 and checked by H NMR.8
The procedure described by the same authors was used to prepare
cis-ReCl4(MeCN)2.7 1H NMR ((CD3)2CO) (δ, ppm): -63.9. The
structure was checked by X-ray diffraction.9
(6) Itaya, T.; Matsumoto, H.; Ogawa, K. Chem. Pharm. Bull. 1980, 28,
1920.
(7) Rouschias, G.; Wilkinson, G. J. Chem. Soc. A 1968, 489.
(8) Pearson, C.; Beauchamp, A. L. Inorg. Chim. Acta 1995, 237, 13.
(5) Lebuis, A.-M.; Beauchamp, A. L. Inorg. Chim. Acta 1994, 216, 131.
S0020-1669(97)00763-5 CCC: $15.00 © 1998 American Chemical Society
Published on Web 02/25/1998

Binding of Adenines to Re(III) and Re(IV) Centers
Inorganic Chemistry, Vol. 37, No. 6, 1998 1243
1H NMR spectra were recorded between 160 and -160 ppm on a
Bruker AMX-300 spectrometer. The residual solvent signal (C6D6, 7.15
ppm; CD2Cl2, 5.32 ppm; (CD3)2CO, 2.04 ppm) was used as internal
reference. Infrared spectra were obtained from CsI pellets with a
Perkin-Elmer 1600 FT-IR instrument. FAB+ mass spectra were
recorded on a VG-Autospec Q apparatus in nitrobenzylic alcohol.
Magnetic moments were determined on an Evans balance (Johnson-
Matthey) calibrated with HgCo(SCN)4 (ø ) 16.44 × 10-6 cgsemu).
Corrections for diamagnetism were calculated from Pascal’s constants.10
Elemental analyses were performed by the Guelph Chemical Labora-
tories (Guelph, Canada).
Table 1. Crystal Data for ReCl3(Me2AdH)2(PPh3)‚2C6H5CH3 (1),
ReCl4(Me2AdH)(PPh3) (2), and
ReCl4[Me2AdC(Me)dNH]‚C6H6‚CH2Cl2 (3)
1
2
3
formula
fw
space group P1h
T, K
a, Å
C46H49Cl3N10PRe C25H24Cl4N5PRe C16H20Cl6N6Re
1065.50
753.47
P1h
215
695.29
P21/c
298
268
14.181(2)
17.591(3)
19.015(5)
76.66(2)
86.80(2)
87.02(1)
4605(2)
4
10.391(4)
10.801(5)
13.651(6)
89.64(4)
83.49(3)
65.14(4)
1380(1)
2
13.941(3)
13.656(3)
12.874(2)
90
105.47(2)
90
2362.1(8)
4
1.955
b, Å
c, Å
R, deg
â, deg
γ, deg
V, Å3
Z
Preparation of ReCl3(R2AdH)2(PPh3). A mixture of ReCl3-
(MeCN)(PPh3)2 (0.20 g, 0.23 mmol) and the dialkyladenine (0.92 mmol)
is heated in toluene (40 mL). The initially insoluble solids have
dissolved to a dark orange solution when the boiling temperature is
reached, at which point heating is immediately stopped. The cooled
solution is filtered to remove unreacted material, and the solvent is
F
calc, g cm-3 1.537
1.814
µ, mm-1
R1a
7.29
0.0606
0.1324
12.72
0.0432
0.1119
16.27
0.0406
0.1040
1
evaporated to ∼ /4 volume. Addition of a small amount of hexane
may be needed to induce precipitation, especially for the diethyladenine
complex. The orange solids are soluble in most common organic
solvents including ether. Yields: ∼65%.
wR2a
a R1 ) ∑||Fo| - |Fc||/∑|Fo|; wR2 ) [∑w(Fo - Fc2)2/∑w(Fo )2]1/2
.
2
2
R ) Me. Anal. Calcd for ReCl3PN10C32H33: C, 43.62; H, 3.77;
N, 15.89; Cl, 12.07. Found: C, 44.05; H, 3.94; N, 15.56; Cl, 12.36.
1H NMR (C6D6) (δ, ppm): 14.3 (ortho PPh3), 12.2 (H2), 10.7, 9.1, 8.9
(CH3), 8.6 (meta PPh3), 8.1 (H2), 8.0 (para PPh3), 5.3, 3.4 (H8), -3.4,
-6.3 (NH). µ: 1.48 µB. FAB+ (exact mass, m/e): calcd, 880.12506;
found, 880.12800.
Chart 2
R ) Et. 1H NMR (C6D6) (δ, ppm): 14.3 (ortho PPh3), 12.3 (H2),
8.6 (meta PPh3), 8.0 (H2 and para PPh3), 6.8, 6.4, 5.9, 5.7 (CH2), 5.3,
3.1 (H8), 2.7, 2.6, 0.7, 0.6 (CH3), -3.1, -7.1 (NH). µ: 1.72 µB. FAB+
(exact mass, m/e): calcd for ReCl3PN10C36H41, 936.18768; found,
936.19120.
Preparation of ReCl4[R2AdC(Me)dNH]. A stoichiometric amount
of cis-ReCl4(MeCN)2 (0.05 g) and the purine (0.02 g) is stirred in
acetonitrile (10 mL) at room temperature for 1 d. The initially green
solution turns orange-brown slowly. The solution is pumped to near
dryness leaving a black crude product. Recrystallization in CH2Cl2-
benzene yields very dark crystals giving an orange solution in CH2Cl2.
The composition of the dimethyladenine compound was determined
by X-ray diffraction work on a specimen from the crystallographically
homogeneous sample. Yields: 45-55%. 1H NMR (CD2Cl2) (δ,
ppm): R ) Me, 136.0 (H2), 48.1, 43.3 ((N)-CH3), 3.5 (H8), 1.3
(amidine CH3); R ) Et, 137.3 (H2), 28.4, 27.5 (CH2), 3.2 (H8), 1.4
(amidine CH3), 0.3 broad (CH2CH3).
X-ray Diffraction Work. Crystallographic data for the three
structures were collected with an Enraf-Nonius CAD-4 diffractometer
using graphite-monochromatized Cu KR radiation. The unit cells were
determined from 25 centered reflections. The Niggli parameters
unambiguously indicated that no higher symmetry was present. Crystal
data are presented in Table 1. The intensity data collected (octants
h,(k,(l for ReCl3(Me2AdH)2(PPh3)‚2C6H5CH3, whole sphere for the
other two compounds) were corrected for the effects of Lorentz,
polarization, and absorption (NRCVAX package,11 Gaussian integra-
tion). Reflections with intensity I g 2σ(I) were retained for structure
determination and refinement. Resolution was done by the direct
methods of SHELXS-86.12 Refinement was carried out on F2 with
the SHELXL-93 package.13 Except when otherwise stated, the non-
hydrogen atoms were refined anisotropically. Hydrogens were fixed
at idealized positions with isotropic temperature factors related to those
of the supporting atom by UH ) 1.5 UC (CH3) or 1.2UC,N (other cases).
The highest residuals in the final ∆F maps were near the metal.
ReCl3(Me2AdH)2(PPh3)‚2C6H5CH3. The compound crystallized
from toluene as thin orange plates. A specimen of dimensions (mm)
0.18 × 0.12 × 0.03 was used, and the data were collected at -5 °C
due to a phase transition occurring a lower temperature. The structure
was solved and refined in the centric P1h space group. The asymmetric
unit consists of two independent complexes and four lattice toluene
molecules, three of which are involved in a 2-fold orientational disorder
of the type depicted in Chart 2. All (whole or fractional) toluene
molecules were constrained to be hexagonal planar with C-C distances
of 1.39 Å. The methyl group was also restricted to occupy this plane
with a C-ring distance of 1.46 Å. Occupancy factors were refined,
but the sum for each pair was normalized to unity. These atoms were
refined isotropically, whereas the refinement for the remaining non-
hydrogen atoms was anisotropic. Hydrogen positions were idealized
at distances of 0.93 Å (C-H), 0.86 Å (N-H), or 0.96 Å (CH3).
ReCl4(Me2AdH)(PPh3). Red-orange plates were obtained from
CH2Cl2. The specimen used had the following dimensions (mm): 0.30
× 0.18 × 0.09. The intensity data were collected at -58 °C. Space
group P1h was used for refinement. The hydrogens were fixed at 0.94
Å (C-H), 0.87 Å (N-H), or 0.97 Å (CH3).
ReCl4[Me2AdC(Me)dNH]‚C6H6‚CH2Cl2. Thin orange plates crys-
tallized in a CH2Cl2-benzene mixture. The crystal used had the
following dimensions (mm): 0.16 × 0.16 × 0.02. The P21/c space
group was unambiguously determined from the monoclinic Laue
symmetry and the systematic absences (h0l, l odd; 0k0, k odd). The
intensity data were collected at room temperature. There was no
evidence of disorder for the lattice CH2Cl2 and benzene molecules.
Hydrogen atoms were fixed at distances of 0.93 Å (C-H), 0.86 Å (N-
H), or 0.96 Å (CH3).
(9) The compound belongs to the orthorhombic space group Pnma with
a ) 10.334(6) Å, b ) 13.482(4) Å, and c ) 9.692(7) Å. A full data
set was collected and the structure was solved, but poor crystal quality
precluded refinement below 0.15. Nevertheless, the connectivity and
cis configuration were unambiguously established.
(10) Drago, R. S. Physical Methods in Chemistry; Saunders: Toronto, 1977.
(11) Gabe, E. J.; Le Page, Y.; Charland, J. P.; Lee, F. L.; White, P. S. J.
Appl. Crystallogr. 1989, 22, 384.
(12) Sheldrick, G. M. SHELXS-86, Program for the Solution of Crystal
Structures; University of Go¨ttingen: Go¨ttingen, Germany, 1985.
(13) Sheldrick, G. M. SHELXL-93, Program for the Refinement of Crystal
Structures; University of Go¨ttingen: Go¨ttingen, Germany, 1993.
Results and Discussion
Synthesis and Reactivity of the ReCl3L2(PPh3) Com-
pounds. The acetonitrile and one phosphine are cleanly
displaced from ReCl3(MeCN)(PPh3)2 by the dialkyladenines in
toluene. In a series of exploratory runs to optimize reaction

1244 Inorganic Chemistry, Vol. 37, No. 6, 1998
Pearson and Beauchamp
Chart 3
already N3-bonded as in the previous compounds. The electron-
withdrawing effect of the metal at N3 increases the acidity of
the N9-H proton, making the N9- lone pair available for
nucleophilic attack on the nitrile carbon, whose nucleophilicity
is simultaneously increased by coordination. Nevertheless, the
alternate mechanism, whereby acetonitrile would be attacked
by the purine before coordination, cannot be ruled out, since
out attempts to isolate an intermediate failed.
These amidine chelates are stable in the solid state and in
chlorinated hydrocarbon solution, as were those obtained by
Rouschias and Wilkinson with anilines.7
Infrared Spectroscopy. At low frequency, a single broad
band is observed at 316 cm-1 for the ν(Re-Cl) vibration in
ReCl4(R2AdH)(PPh3), whereas two vibrations (∼298 and ∼309
cm-1) are found for ReCl3(R2AdH)2(PPh3). The amidine
complexes exhibit a single band at ∼321 cm-1 with a low-
frequency shoulder.
conditions, changes in reactant ratios and heating time showed
no effect on the composition of the product. However,
temperature had to be high enough for ligand substitution to
take place: low-boiling solvents such as benzene and CH2Cl2
were inefficient. The complexes have the mer-cis configuration
(refer to X-ray structure below) also exhibited by pyridine and
imidazole complexes.14,15 In these compounds, the adenines
retain their acidic N9-H proton and coordinate as neutral
ligands via the N3 site of the six-membered ring.
Individual assignment of the numerous bands to either PPh3
or the purine was not attempted. Among the spectral features
of dialkyladenines analyzed previously,5 the band at 1137 cm-1
(Me2AdH) or 1140 cm-1 (Et2AdH), located in the N7-C8-
N9 region, confirms that the N9-H bond is retained, since it is
not affected by coordination in all our compounds: when the
ligand binds via N9, this band loses intensity and a strong band
at ∼1180 cm-1 appears. For the amidine complex, no bands
due to CdN stretching could be identified, since the purine
already possesses strong bands at ∼1600 cm-1, where this
vibration should occur for the amidine group.7
Coordinated adenine should show basic properties via the free
lone pairs at N1 and N7. It has been shown that N7 can be
accessed by H+ despite the presence of interfering NR2 groups
in dialkyladenines.4,5 Thus, protonation was attempted by
adding a few drops (10-fold excess) of concentrated aqueous
HCl to a toluene solution of ReCl3(R2AdH)2(PPh3). Immediate
reaction yielded a red-orange solid, which was not a simple
protonated species. The new complex was first identified as a
1H NMR Spectroscopy. As shown in Figure 1, the spectrum
of ReCl3(Me2AdH)2(PPh3) is complex, consisting of signals for
one PPh3 and two nonequivalent adenines spread between -7
and +15 ppm by the paramagnetic Re(III) center. The PPh3
protons were first identified from the typical pattern of chemical
shifts noted in our earlier work on Re(III) complexes.8 1H-1H
coupling is observed for the meta and para protons at 8.6 and
8.0 ppm, respectively, but not for the ortho proton, whose signal
is broadened by the proximity to the paramagnetic center. The
resonances of the adenine ligands were identified from a
combination of variable-temperature, exchange, and deuteration
experiments. Three signals are found for -N(CH3)2 protons,
at 10.7, 9.1, and 8.9 ppm, respectively. An extra peak at ∼7.2
ppm is masked by C6D6 at room temperature, but its presence
is readily revealed by changing the temperature. Thus, each
-N(CH3)2 group in the nonequivalent adenines gives two
distinct signals due to slow rotation about the C6-N6 bond. In
the free ligand, faster rotation leads to averaging, but ring
protonation or metal coordination induce π-electron transfer
from the amino group into the ring, thereby increasing the
double-bond character in the C6-NR2 unit.18 As the temper-
ature is raised, these methyl signals gradually broaden and get
closer. At the highest temperature studied (365 K), coalescence
was close, but not yet reached at 400 MHz, which places ∆Gq
slightly above 17.8 kcal mol-1. A similar barrier of 16.6 kcal
mol-1 has been found for Re2Cl2(Me2Ad)4.5 The N-H signals
were located (-3.4, -6.3 ppm) by exchange with CD3OD.
Finally, the H8 protons were identified (5.3, 3.4 ppm) by
comparison with the compound prepared from C8-deuterated
dimethyladenine. The diethyladenine complex gives a very
similar spectrum for the ring protons. In this case, the ethyl
groups show CH2 signals between 5.7 and 6.8 ppm and CH3
signals between 0.6 and 2.7 ppm.
1
Re(IV) compound from the well-defined pattern of H NMR
signals of PPh3, which have been found to be differently
downfield-shifted by the paramagnetic Re(III) and Re(IV)
centers.8 This conclusion was confirmed by an X-ray study
(see below) showing the compound to be cis-ReCl4(Me2AdH)-
(PPh3). Thus, the purine trans to PPh3 in ReCl3(Me2AdH)2-
(PPh3) had been replaced by a Cl- ligand probably provided
by HCl. Since the reduction product has not been identified, it
is not sure H+ is the oxidizing agent, but the reaction is certainly
+
promoted by H+, which removes the purine as the Me2AdH2
ion (pka ) 3.9).16 The diethyladenine complex reacts in the
same manner.
Reacting the dialkyladenines with cis-ReCl4(MeCN)2 in
acetonitrile always yielded the same major product, no matter
the heating time or stoichiometric ratio. The 1H NMR spectra
appeared to be consistent with a ReCl4(MeCN)(R2AdH) species,
since they showed two equal peaks for H2 and H8 and three
signals three times as intense initially assigned to -NR2 and
MeCN. The absence of a ν(CtN) vibration at ∼2000 cm-1
provided no useful information, since this band is often
extremely weak and undetected.8 X-ray work on the Me2AdH
product (see below) showed that the two ligands were not
independently coordinated: the N3-bonded adenine was con-
nected via N9 to the quaternary carbon of acetonitrile, thereby
producing a six-membred chelate including an amidine group
(Chart 3). A related reaction has been reported between primary
7
aromatic amines and ReCl4(MeCN)2 or a few other transition
metal complexes.17 This reaction probably proceeds via a cis-
ReCl4(MeCN)(R2AdH) intermediate, in which the purine is
(14) Pearson, C.; Beauchamp, A. L. Can. J. Chem. 1997, 75, 220.
(15) Rouschias, G.; Wilkinson, G. J. Chem. Soc. A 1967, 993.
(16) Albert, A. Synthetic Procedures in Nucleic Acid Chemistry; Zorback,
W. W., Tipson, R. S., Eds., Wiley-Interscience: New York, 1973, Vol.
2, 28.
(17) Michelin, R. A.; Mozzon, M.; Bertani, R. Coord. Chem. ReV. 1996,
(18) Pitner, T. P.; Sternglanz, H.; Bugg, C. E.; Glickson, J. D. J. Am. Chem.
Soc. 1975, 97, 885.
147, 299.

Binding of Adenines to Re(III) and Re(IV) Centers
Inorganic Chemistry, Vol. 37, No. 6, 1998 1245
Figure 1. 1H NMR spectrum of ReCl3(Me2AdH)2(PPh3) in C6D6. Unassigned multiplets (∼3 and ∼1 ppm) are due to traces of ether.
Figure 2. 1H NMR spectrum of crystals of ReCl4[Me2AdC(Me)dNH]‚C6H6‚CH2Cl2 in CD2Cl2.
The remaining complexes discussed here contain the Re(IV)
center. NMR results are scarce for compounds of this d3 system.
To our knowledge, data are available for only two Re(IV)
complexes, namely ReCl4(PMePh2)2 and ReCl4(tri-m-tolylphos-
phine)2.8,19
ReCl4(R2AdH)(PPh3), the reaction product of HCl with
ReCl3(R2AdH)2(PPh3), was identified as a Re(IV) species from
the three sharp PPh3 signals at ∼19 (ortho), 10.7 (para), and
8.5 ppm (meta). These chemical shifts are typical of Re(IV)
and different from those found for related Re(III) compounds.8
Because of poor solubility, only the PPh3 peaks were clearly
defined.
The spectrum of ReCl4[Me2AdC(Me)dNH] shown in Figure
2 contains relatively sharp signals covering a wide range of
chemical shifts. No signal is observed near -64 ppm, where
the starting material cis-ReCl4(MeCN)2 gives a singlet. Signal
intensities and comparisons between the ethyl and methyl
complexes allowed us to propose assignments for all except
the N-H protons. The signal at 135 ppm is believed to originate
from H2, on the basis of its relative intensity and proximity to
the paramagnetic center. A signal of the same intensity at ∼3
ppm is assigned to the remote H8 proton. Two methyl signals
for the -NMe2 group in slow rotation are observed near 40
ppm. These signals are absent from the spectra of the diethyl
analogue, where the CH2 protons appear at ∼30 ppm and those
of the ethyl CH3 groups give a broad signal at 0.3 ppm. The
amidine CH3 protons (1.3 ppm) are observed close to the signal
water for the Me2AdH complex and masked by this signal for
the Et2AdH complex.
Crystal Structure of ReCl3(Me2AdH)2(PPh3)‚2C6H5CH3.
The asymmetric unit contains two molecules of the complex
and four lattice toluenes. The octahedral complex adopts the
mer-cis configuration (Figure 3) found for ReCl3L2(PPh3)
compounds with pyridines and 1-methylimidazole.14 Departure
from octahedral geometry is relatively large and similar in both
molecules, as evidenced from the distances and angles given
in Table 2. The Re-P distances are 2.443(2) Å, and the Re-N
bonds trans to Cl are ∼0.03 Å shorter than those trans to P, as
observed for the above complexes. However, the pattern of
Re-Cl bond lengths is different. In the above complexes, the
mutually trans Re-Cl bonds (2.369 Å) were appreciably shorter
than those trans to a Re-N bond (mean 2.390 Å). Therefore,
our Re-Cl3 distances (2.385(2), 2.403(2) Å), trans to N, are
normal, as are the Re-Cl2 distances (2.362(2), 2.365(2) Å) trans

1246 Inorganic Chemistry, Vol. 37, No. 6, 1998
Pearson and Beauchamp
Table 3. Selected Torsion Angles (deg) in
ReCl3(Me2AdH)2(PPh3)‚2C6H5CH3 (1A,1B) and
ReCl4(Me2AdH)(PPh3) (2)
1A
1B
2
Cl1-Re-P1-C31
Cl1-Re-P1-C41
Cl1-Re-P1-C51
Cl2-Re-P1-C31
Cl2-Re-P1-C41
Cl2-Re-P1-C51
Cl3-Re-P1-C31
Cl3-Re-P1-C41
Cl3-Re-P1-C51
N13-Re-P1-C31
Re-P1-C31-C32
Re-P1-C41-C42
Re-P1-C51-C56
Cl1-Re-N13-C14
Cl2-Re-N13-C14
Cl1-Re-N23-C24
Cl2-Re-N23-C24
Cl3-Re-N23-C24
89.0(3)
-33.4(3)
-154.3(3)
-89.7(3)
147.9(3)
27.0(3)
177.8(3)
55.4(3)
-65.5(3)
-0.3(4)
60.1(7)
15.9(7)
48.9(7)
-39.9(7)
148.4(7)
130.4(7)
-50.8(6)
41.7(6)
93.7(3)
-29.4(3)
-149.8(3)
-87.2(3)
149.7(3)
29.3(3)
81.0(2)
-36.7(2)
-157.4(2)
-97.8(2)
144.5(2)
23.7(2)
-178.1(3)
172.9(2)
55.2(2)
58.8(3)
-61.6(3)
3.3(3)
63.4(7)
-5.6(8)
43.0(7)
-37.6(7)
149.8(7)
128.5(7)
-50.3(6)
40.6(2)
-65.6(2)
-10.3(2)
-50.6(5)
-72.6(6)
-32.4(6)a
32.2(5)
-145.7(5)
Figure 3. ORTEP drawing of ReCl3(Me2AdH)2(PPh3) (molecule A).
The structure of molecule B is very similar. Ellipsoids are drawn at
the 50% probability level.
a Re-P1-C51-C52.
Table 4. Distances (Å) and Angles (deg) in the Hydrogen Bonds
Table 2. Selected Distances (Å) and Angles (deg) for
ReCl3(Me2AdH)2(PPh3)‚2C6H5CH3 (1A,1B), ReCl4(Me2AdH)(PPh3)
(2), and ReCl4[Me2AdC(Me)dNH]‚C6H6‚CH2Cl2 (3)
N‚‚‚Cl
H‚‚‚Cl
N-H-Cl
ReCl3(Me2AdH)2(PPh3)‚2C6H5CH3
1A
1B
2
3
N19A-H19A‚‚‚Cl1A
N29A-H29A‚‚‚Cl2A
N29A-H29A‚‚‚Cl3A
N19B-H19B‚‚‚Cl1B
N29B-H29B‚‚‚Cl2B
N29B-H29B‚‚‚Cl3B
3.036(8)
3.344(8)
3.119(7)
3.005(8)
3.278(8)
3.123(7)
2.44
2.96
2.52
2.37
2.88
2.51
127
110
128
131
110
129
Re-Cl1
Re-Cl2
Re-Cl3
Re-Cl4
Re-P
Re-N10
Re-N13
Re-N23
2.391(2)
2.362(2)
2.385(2)
2.392(2)
2.365(2)
2.403(2)
2.339(2)
2.323(2)
2.307(2)
2.367(2)
2.545(2)
2.351(2)
2.343(2)
2.327(1)
2.330(2)
2.445(2)
2.441(2)
2.094(4)
2.100(5)
ReCl4(Me2AdH)(PPh3)
2.170(6)
2.193(7)
2.151(6)
2.188(6)
2.143(5)
N19-H19‚‚‚Cl1
N19-H19‚‚‚Cl4
3.129(6)
3.308(6)
2.67
2.76
114
122
ReCl4[Me2AdC(Me)dNH]‚C6H6‚CH2Cl2
N13-Re-N23 85.1(2)
86.2(2)
90.1(4)
85.8(2)
N10-H10‚‚‚Cl2a
a -x, -y, 2 - z.
3.451(5)
2.60
169
N13-Re-Cl1
N23-Re-Cl1
Cl2-Re-Cl1
Cl3-Re-Cl1
N13-Re-Cl2
N23-Re-Cl2
N13-Re-Cl3
N23-Re-Cl3
Cl2-Re-Cl3
N13-Re-Cl4
Cl3-Re-Cl4
Cl2-Re-Cl4
Cl1-Re-Cl4
N13-Re-P
88.5(2)
83.7(2)
91.5(1)
85.9(1)
171.47(7) 172.60(7) 177.55(5) 173.38(5)
88.90(9)
89.6(2)
87.9(2)
175.8(2)
91.3(2)
92.50(8)
87.91(8)
90.6(2)
86.9(2)
176.7(2)
91.0(2)
91.02(8)
91.47(7)
87.4(1)
91.38(5)
88.5(1)
tion of the adenine ligand via N3 allows the N9-H proton to
form an intramolecular N9-H‚‚‚Cl bond (Table 4). Hydrogen
bonding would be more efficient if the Cl1 (or Cl3) acceptor
were in the plane of the purine, but coplanarity would introduce
extremely short C2-H‚‚‚Cl contacts (<2.5 Å) on the opposite
side. N-H‚‚‚Cl hydrogen bonding is retained and steric strain
is relieved by the ligands being rotated ∼38° about the Re-N3
bond. The resulting hydrogen bonds are far from linearity and
not very strong, although within the normal range of N‚‚‚Cl
separation.20 Rotation of the adenine cis to the phosphine about
the Re-N bond brings the five-membered ring on the side of
the phosphine. In both molecules, the P-C31 bond is eclipsed
with the Re-N13 bond (C-P-Re-N < 3°) and phenyl ring
no. 3 is roughly parallel to the plane of the purine (dihedral
angles ∼13°) at a distance of ∼3.2 Å. The Re-P-C31 angle,
on this side, is also smaller than the other Re-P-C angles.
This arrangement minimizes steric interaction and provides some
van der Waals stabilization via π-stacking. The orientation of
ring no. 3 imposes constraints on the remaining phosphine rings.
Ring no. 4, which is above the Re-Cl1 bond (Cl1-Re-P-
C41 ) -31°), is almost parallel to the Re-P bond (Re-P-
C41-C42 ) 15.9° for A, -5.6° for B). As a result, an ortho
proton H42 lies close (∼2.65 Å) to Cl1 and is likely responsible
176.2(1)
93.9(1)
89.79(7)
88.7(1)
93.74(6)
88.78(7)
89.04(7)
91.1(1)
92.45(6)
173.4(1)
92.64(6)
91.48(6)
93.74(6)
94.3(2)
174.1(2)
91.4(2)
172.9(2)
N23-Re-P
Cl1-Re-P
102.15(8) 100.96(7) 83.11(6)
86.28(7)
89.55(7)
Cl2-Re-P
86.39(7)
91.62(7)
99.06(6)
86.83(6)
172.15(5)
Cl3-Re-P
Cl4-Re-P
N10-Re-N13
N10-Re-Cl1
N10-Re-Cl2
N10-Re-Cl3
N10-Re-Cl4
86.1(2)
86.7(1)
89.5(1)
178.1(1)
87.3(1)
to Cl. The anomaly lies in the Re-Cl1 bonds (2.391(2) Å),
which are longer than the expected 2.36 Å value. It is
noteworthy that these Re-Cl1 bonds are also those involved
in large distortions around the metal: the greatest deviations
from 90° are observed for the Cl1-Re-P (102.15(8),
100.96(7)°) and N23-Re-Cl1 angles (83.7(2), 85.8(2)°), and
they likely reflect constraints imposed by the clustering of many
aromatic units around the metal.
(19) Over, D. E.; Critchlow, S. C.; Mayer, J. M. Inorg. Chem. 1992, 31,
4643.
(20) Stout, G. H.; Jensen, L. H. X-Ray Structure Determination, A Practical
Guide; Collier-Macmillan Canada Ltd.: Toronto, 1968; p 303.
Torsion angles in Table 3 show that the two independent
molecules actually have very similar conformations. Coordina-

Binding of Adenines to Re(III) and Re(IV) Centers
Inorganic Chemistry, Vol. 37, No. 6, 1998 1247
Figure 6. ORTEP drawing of ReCl4[Me2AdC(Me)dNH]. Ellipsoids
are drawn at the 50% probability level.
Figure 4. View of the unit cell for ReCl3(Me2AdH)2(PPh3)‚2C6H5-
CH3 down the a axis. The origin is in the upper left corner, and the b
and c axes are in the plane of the figure, oriented to the right and
vertically, respectively.
time, the rotation takes the five-membered ring away from the
phosphine, whose P-C bonds nevertheless adopt the same
orientation as above with respect to the bonds in the ReCl3N
plane (Table 3). Phenyl ring no. 3 is above the adenine unit
(N13-Re-P1-C31 ) -10°), roughly parallel to it (dihedral
angle ) 12°), and also participates in π-stacking contacts.
Lesser crowding near the phosphine allows greater freedom for
the orientation of the other phenyl rings. Ring no. 4 is roughly
parallel to the ReCl3N plane (Re-P1-C41-C42 ) -73°),
which leaves space for Cl1 to move away from the adenine
unit, decreasing the Cl1-Re-P angle to 83.11(6)°. On the other
hand, ring no. 5 is more parallel to the Re-P bond (Re-P1-
C51-C52 ) -32°) and an ortho hydrogen H52 is close to Cl2
(2.53 Å). This is probably correlated with the large Cl2-Re-P
angle of 99.06(6)°.
The Adenine Unit. In these two structures, the distances
and angles in the adenine unit do not differ appreciably from
those found in the free ligand22 and from the typical values
reported by Taylor and Kennard.23 The only small differences
in angles near C6 are related to the steric effect of the -NMe2
group. Coordination to N3 has no detectable effect on the
geometry in the C2-N3-C4 region. Our data confirm the
presence of adenine as the N9-H tautomer, since changes in
the state of protonation of the N atoms are known to introduce
large differences on the C-N-C angles. The absence of
protonation on N1 or N7 further confirms that ReCl4(Me2AdH)-
(PPh3) is a Re(IV) complex as opposed to a possible Re(III)
species ReCl4(Me2AdH2)(PPh3) containing a cationic adenine
ligand.
Least-squares planes (see Supporting Information) show that
the five- and the six-membered rings are individually planar,
but the adenine unit is bent by 2-5° about the common C4-
C5 bond. The -NMe2 groups are roughly coplanar with the
ring, but the CH3 groups are often appreciably displaced from
it by packing forces. Coordination does not take place exactly
along the expected lone pair direction, the C4-N3-Re angle
being ∼6° greater than C2-N3-Re. This effect contributes
to improving N-H‚‚‚Cl hydrogen-bonding efficiency. Also,
the Re atoms is considerably distant from the planes (0.12-
0.40 Å).
Figure 5. ORTEP drawing of cis-ReCl4(Me2AdH)(PPh3). Ellipsoids
are drawn at the 50% probability level.
for the Cl1-Re-P angle being 10° greater than normal. It is
not clear that such a distortion should lengthen the Re-Cl1,
but it is probably not fortuitous that the abnormally long Re-
Cl bond is also the one involved in the largest angular distortion.
A view of the unit cell is provided in Figure 4. Layers
parallel to a along the bc diagonal contain the complex
molecules interacting by normal van der Waals contacts. The
asymmetric unit contains the unusually large number of four
toluene solvent molecules, whose organization is also quite
unusual: they define layers alternating with the layers of the
complex. Most of the toluene molecules are involved in a 2-fold
disorder, in which however they remain in the same plane and
retain π-stacking interactions with their neighbors.
Crystal Structure of ReCl4(Me2AdH)(PPh3). This Re(IV)
complex is the cis isomer (Figure 5). The Re-P bond (2.545-
(2) Å) is much longer than in the above and other Re(III)
compounds,14,21 but similar to those observed for the Re(IV)
compound trans-ReCl4[P(m-tolyl)3]2.8 In contrast, the Re-Cl
and Re-N distances are shorter than in these other Re(III)
complexes. The mutually trans Re-Cl1 (2.339(2) Å) and Re-
Cl2 (2.323(2) Å) distances are similar to those of ReCl4[P(m-
tolyl)3]2,8 whereas the Re-Cl3 bond (2.307(2) Å), trans to N,
is shorter and Re-Cl4 (2.367(2) Å), trans to P, is longer.
Crystal Structure of ReCl4[Me2AdC(Me)dNH]‚C6H6‚
CH2Cl2. This Re(IV) complex is of the ReCl4(L-L′) type and
contains a six-membered chelate ring resulting from the coupling
of the adenine unit with acetonitrile (Figure 6). The distances
and angles in the chelate ring are given in Table 5. The
(21) Davis, M.; Be´langer-Garie´py, F.; Zargarian, D.; Beauchamp, A. L.
Acta Crystallogr. 1997, C53, 428.
(22) Dahl, T. Acta Chem. Scand. 1987, B41, 379.
(23) Taylor, R.; Kennard, O. J. Mol. Struct. 1982, 78, 1.
As above, the adenine ligand forms an intramolecular
hydrogen bond with Cl1 (Table 4) and it is rotated by ∼32°
about Re-N13 with respect to the Re-Cl1 bond (Table 3). This

1248 Inorganic Chemistry, Vol. 37, No. 6, 1998
Pearson and Beauchamp
Table 5. Distances (Å) and Angles (deg) in the Chelate Ring of
ReCl4[Me2AdC(Me)dNH]‚C6H6‚CH2Cl2
where N9 accepts a hydrogen bond from the amine ligand. The
only example for the N3-M/N9-H pattern is [(cod)Rh(Me2-
AdH)Cl],28 in which an intramolecular N9-H‚‚‚Cl hydrogen
bond is formed. This has also been observed for purine itself
and two other C6-substituted derivatives, and in all cases the
coordination sphere includes a ligand able to accept a hydrogen
bond.29 Thus, the pattern actually adopted seems to depend on
a delicate balance between intrinsic basicity of the donor atom
and interligand interactions. Our compounds ReCl3(Me2AdH)2-
(PPh3) and ReCl4(Me2AdH)(PPh3) belong to the latest class:
like the Rh complex, they contain N3-bonded ligands forming
intramolecular N9-H‚‚‚Cl hydrogen bonds.30
N13-C14
N19-C10
C10-C11
1.359(7)
1.392(7)
1.480(7)
C14-N19
N10-C10
1.371(7)
1.284(7)
Re-N13-C14
C14-N19-C10
N10-C10-C11
Re-N10-C10
122.2(4)
129.2(5)
123.4(5)
132.4(4)
N13-C14-N19
N19-C10-N10
N19-C10-C11
127.3(5)
118.6(5)
118.0(5)
octahedron shows a significant, but relatively small, distortion
keeping the cis angles, including the one in the chelate ring, in
the 86-94° range (Table 2). The Re-N10 distance to the
amidine nitrogen (2.094(4) Å) is longer than expected for a Re-
(IV)-nitrile bond, since a Re(III)-nitrile distance of 2.068(5)
Acknowledgment. We thank M. Simard and F. Be´langer-
Garie´py for their assistance in solving the crystal structures.
The financial support of the Natural Sciences and Engineering
Research Council of Canada and the Fonds FCAR du Ministe`re
de l’EÄ ducation du Que´bec is also acknowledged.
21
Å has been observed in ReCl3(MeCN)(PPh3)2 and the bond
to a Re(IV) center should be slightly shorter. The Re-N13
distance (2.100(5) Å) is shorter than those of ReCl4(Me2AdH)-
(PPh3) (2.143(5) Å) and ReCl4(bipy) (2.133(5) Å).24 The
mutually trans Re-Cl distances (2.343(2), 2.351(2) Å) are
longer than those trans to N donors (2.327(1), 2.330(2) Å). A
similar pattern is noted for the Re-Cl bonds of ReCl4(bipy),
which possesses the same configuration of donor atoms, but
the distances are systematically ∼0.02 Å shorter in the latter
case.
Supporting Information Available: Tables of complete crystal
data, final coordinates, temperature factors, distances and angles, torsion
angles, and distances from the least-squares planes for the crystal-
lographic study (41 pages). Ordering information is given on any
masthead page.
IC970763W
(25) Beck, W. M.; Calabrese, J. C.; Kottmair, N. D. Inorg. Chem. 1979,
18, 176. Sakaguchi, H.; Anzai, H.; Furuhata, K.; Ogura, H.; Iitaka,
Y.; Fujita, T.; Sakaguchi, T. Chem. Pharm. Bull. 1978, 26, 2465.
Kistenmacher, T. J. Acta Crystallogr. 1974, B30, 1610. Prizant, L.;
Olivier, M.; Rivest, R.; Beauchamp, A. L. Can. J. Chem. 1981, 59,
1311. Charland, J. P.; Beauchamp, A. L. Croat. Chem. Acta 1984,
57, 679. Rosopulos, Y.; Nagel, U.; Beck, W. Chem. Ber. 1985, 118,
931. Tiekink, E. R. T.; Kurucsev, T.; Hoskins, B. F. J. Crystallogr.
Spectrosc. Res. 1989, 19, 823.
(26) De Meester, P.; Skapski, A. C. J. Chem. Soc., Dalton Trans. 1973,
424.
(27) Marzotto, A.; Clemente, D. A.; Ciccarese, A.; Valle, G. J. Crystallogr.
Spectrosc. Res. 1993, 23, 119.
(28) Sheldrick, W. S.; Gu¨nther, B. J. Organomet. Chem. 1989, 375, 233.
(29) Sheldrick, W. S. Acta Crystallogr. 1981, B37, 945. Sletten, E.; Sletten,
J.; Froystein, N. A. Acta Chem. Scand. 1988, A42, 413. Chifotides,
H. T.; Dunbar, K. R.; Matonic, J. H.; Katsaros, N. Inorg. Chem. 1992,
31, 4628. Dalby, C.; Bleasdale, C.; Clegg, W.; Elsegood, M. R. J.;
Golding, B. T.; Griffin, R. J. Angew. Chem., Int. Ed. Engl. 1993, 32,
1696.
(30) Note added in proof: N3-coordination has been reported very recently
(Meiser, C.; Song, B.; Freisinger, E.; Peilert, M.; Sigel, H.; Lippert,
B. Chem. Eur. J. 1997, 3, 388.) for the square-planar complex [Pd-
(dien)(N6,N6,N9-trimethyladenine)]2+. N3 is the only site accessible
in this trimethylated ligand and steric hindrance is avoided by ligand
rotation about the Pd-N3 bond to bring the 9-methyl group above
the square plane.
The purine unit shows a small departure from planarity, as
usual: slight puckering of the six-membered ring (maximum
deviation of 0.020(4) Å for N13), almost perfectly planar five-
membered ring, and slight bend of 1.4(2)° about the common
C4-C5 bond. In the chelate ring, small displacements (maxi-
mum of 0.037(4) Å for N19) alternatively above and below the
mean plane through the N13-C14-N19-C10-N10 part are
noted, whereas the ring as a whole adopts an envelope
arrangement, the Re atom lying 0.443(7) Å from this plane.
Binding Mode of Dialkyladenines. Complexation with
monoanionic adenine consistently takes place at the deprotonated
N9 site.25 Neutral adenine (LH) shows greater variability, and
three complexation/protonation patterns have been found. In
(LH2)[Co(LH)2(H2O)4], the ligands adopts the N9-M/N7-H
pattern.26 Coordination to N9 inevitably requires that N3 be
close to other ligands in the sphere, and in this case it acts as
a hydrogen-bond acceptor from an aqua ligand. The N3-M/
N7-H pattern is found for [NiCl(LH){N(CH2CH2NH2)3}]Cl,27
(24) Herrmann, W. A.; Thiel, W. R.; Herdtweck, E. Chem. Ber. 1990, 123,
271.