The origin of hindered rotation around the Pt-N bond in platinum amides

Article abstract of DOI:10.1021/ic010317f

Several platinum amides of formula trans-[PtCl(NHAr)(PEt₃)₂] (Ar = 3-FC₆H₄, 2; 4-FC₆H₄, 3; 4-ClC₆H₄, 4; 4-IC₆H₄, 5; 4-Cl,3-NO₂-C₆H₃, 6) have been synthesized by reaction of [PtHCl(PEt₃)₂] with aryl azides. All the complexes feature planar arylamido moieties and hindered rotation around the N-aryl and Pt-N bonds have been detected and separately studied. The X-ray crystal structures of complexes 5 and 6 have been determined. Complex 5 crystallizes in the orthorhombic space group Pnma, with a = 23.806(4) A, b = 15.099(2) A, c = 6.7593(10) A α = β = γ = 90°, and Z = 4. Compound 6 shows an N-H·middot;middot;O(NO) hydrogen bond and it crystallizes in the monoclinic space group P2(1)/n, with a; = 12.215(3) A, b = 8.078(2) A, c = 13.052(4) A, α = γ = 90°, β = 90;057(6)°, and Z = 2. Except for Ar = 4-Cl,3-NO₂-C₆H₃, the activation energies obtained for the complexes indicate that both dynamic processes occur simultaneously with a common barrier which originates in the multiple bond character of the N-aryl bond due to a strong π-donor behavior of the N atom in the N-aryl bond. The rotation about the Pt-N bond is unfavorable because of steric congestion with the planar amide, which can be overcome only when the aromatic ring can rotate. For the complex trans-[PtCl{NH(4-Cl,3-NO₂-C₆H₃)} (PEt₃)₂] the barrier to rotation is mostly due to hydrogen bond interaction between the NO₂ ortho substituent and the amide H atom.

Full text of DOI:10.1021/ic010317f

Inorg. Chem. 2001, 40, 4211-4216
4211
The Origin of Hindered Rotation around the Pt-N Bond in Platinum Amides
Ana C. Albe´niz, Virginia Calle, Pablo Espinet,* and Sandra Go´mez
Departamento de Qu´ımica Inorga´nica, Facultad de Ciencias, Universidad de Valladolid,
Prado de la Magdalena s/n, 47005 Valladolid, Spain
ReceiVed March 23, 2001
Several platinum amides of formula trans-[PtCl(NHAr)(PEt3)2] (Ar ) 3-FC6H4, 2; 4-FC6H4, 3; 4-ClC6H4, 4;
4-IC6H4, 5; 4-Cl,3-NO2-C6H3, 6) have been synthesized by reaction of [PtHCl(PEt3)2] with aryl azides. All the
complexes feature planar arylamido moieties and hindered rotation around the N-aryl and Pt-N bonds have been
detected and separately studied. The X-ray crystal structures of complexes 5 and 6 have been determined. Complex
5 crystallizes in the orthorhombic space group Pnma, with a ) 23.806(4) Å, b ) 15.099(2) Å, c ) 6.7593(10)
Å, R ) â ) γ ) 90°, and Z ) 4. Compound 6 shows an N-H‚‚‚O(NO) hydrogen bond and it crystallizes in the
monoclinic space group P2(1)/n, with a ) 12.215(3) Å, b ) 8.078(2) Å, c ) 13.052(4) Å, R ) γ ) 90°, â )
90.057(6)°, and Z ) 2. Except for Ar ) 4-Cl,3-NO2-C6H3, the activation energies obtained for the complexes
indicate that both dynamic processes occur simultaneously with a common barrier which originates in the multiple
bond character of the N-aryl bond due to a strong π-donor behavior of the N atom in the N-aryl bond. The
rotation about the Pt-N bond is unfavorable because of steric congestion with the planar amide, which can be
overcome only when the aromatic ring can rotate. For the complex trans-[PtCl{NH(4-Cl,3-NO2-C6H3)}(PEt3)2]
the barrier to rotation is mostly due to hydrogen bond interaction between the NO2 ortho substituent and the
amide H atom.
Introduction
Chart 1
Amido complexes are unevenly distributed among the transi-
tion metals. In contrast with the numerous and stable early
transition metal compounds bearing an amido group, late
transition metal amido complexes are less well represented. This
is due in part to the stabilization of the M-amido bond by
π-donation of the lone electron pair of the amido N to empty d
orbitals in the early transition metals, which cannot occur in
the late transition metals.1 However, the number of reported
amido complexes of the platinum group metals is growing.2-4
They are involved in the catalytic amination reactions of organic
compounds that have been recently developed,5 and the interest
in their properties and reactions has increased thereof.
Coordination mode C (Chart 1) is also found for group 10
transition metals and most of the monomeric derivatives fall
into this type.2-4a,c-e,g,h,j,k,o As suggested from the discussion
above, generally the substituents on N for complexes of type C
can delocalize electron density (R, R′ ) SiR3, Ph, ...) since it is
unlikely that the metal could itself contribute to this delocal-
The amido group single bonded to a metal has one lone pair
on the N atom that makes it very nucleophilic (mode A in Chart
1). This lone pair is used to bind a second metal atom in bridging
amido groups, which bear a tetrahedral sp3 N atom (mode B in
Chart 1). This type of coordination is found in late transition
metal complexes, especially group 10, and the dimeric com-
plexes formed are usually very stable, the amido bridge being
difficult to cleave.2 Actually, dimerization may be a serious
drawback in the isolation of monomeric amido complexes of
the platinum group metals. Alternatively, this lone pair can get
involved in the formation of π-bond, being donated to empty d
orbitals in the metal (as with the early metals) or being
delocalized into the R substituents, thus producing a N-M or
N-R multiple bond. In this case the N atom becomes sp2
hybridized (mode C in Chart 1).
(4) Recent examples of monomeric Pt-group metal amido complexes: (a)
Moriuchi, T.; Bandoh, S.; Miyaji, Y.; Hirao, T. J. Organomet. Chem.
2000, 599, 135-142. (b) Koo, K.; Hillhouse, G. L. Organometallics
1996, 15, 2669-2671. (c) Kim, Y.-J.; Choi, J.-C.; Osakada, K. J.
Organomet. Chem. 1995, 491, 97-102. (d) Campbell, C. J.; Casti-
n˜eiras, A.; Nolan, K. B. J. Chem. Soc., Chem. Commun. 1995, 1939-
1940. (e) Vanderlende, D. D.; Abboud, K. A.; Boncella, J. M. Inorg.
Chem. 1995, 34, 5319-5326. (f) Rahim, M.; Ahmed, K. J. Organo-
metallics 1994, 13, 1751-1756. (g) Villanueva, L. A.; Abboud, K.
A.; Boncella, J. M. Organometallics 1994, 13, 3921-3931. (h)
Matsunaga, P. T.; Hess, C. R.; Hillhouse, G. L. J. Am. Chem. Soc.
1994, 116, 3665-3666. (i) Rahim, M.; White, C.; Rheingold, A. L.;
Ahmed, K. J. Organometallics 1993, 12, 2401-2403. (j) Schaad, D.
R.; Landis, C. R. Organometallics 1992, 11, 2024-2029. (k) Villan-
ueva, L. A.; Abboud, K. A.; Boncella, J. M. Organometallics 1992,
11, 2963-2965. (l) Glueck, D. S.; Newman Winslow, L. J.; Bergman,
R. G. Organometallics 1991, 10, 1462-1479. (m) Hartwig, J. F.;
Bergman, R. G.; Andersen, R. A. J. Am. Chem. Soc. 1991, 113, 6499-
6508. (n) Joslin, F. L.; Johnson, M. P.; Mague, J. T.; Roundhill, D.
M. Organometallics 1991, 10, 2781-2794. (o) Seligson, A. L.; Cowan,
R. L.; Trogler, W. C. Inorg. Chem. 1991, 30, 3371-3381. (p) Schaad,
D. R.; Landis, C. R. J. Am. Chem. Soc. 1990, 112, 1628-1629. (q)
Casalnuovo, A. L.; Calabrese, J. C.; Milstein, D. J. Am. Chem. Soc.
1988, 110, 6738-6744.
* To whom correspondence should be sent. E-mail: espinet@qi.uva.es.
Fax: 34-983-423013.
(1) Chisholm, M. H.; Rothwell, I. P. In ComprehensiVe Coordination
Chemistry; Wilkinson, G., Gillard, R. D., McCleverty, J. A., Eds.;
Pergamon Press: Oxford, 1987; Vol. 2, p 176.
(2) Fryzuk, M. D.; Montgomery, C. D. Coord. Chem. ReV. 1989, 95, 1-40.
(3) Bryndza, H. E.; Tam, W. Chem. ReV, 1988, 88, 1163-1188.
(5) (a) Hartwig, J. F. Angew. Chem., Int. Ed. 1998, 37, 2046-2067. (b)
Beller, M. Angew. Chem., Int. Ed. Engl. 1995, 34, 1316-1317. (c)
Seligson, A. L.; Trogler, W. C. Organometallics 1993, 12, 744-751.
10.1021/ic010317f CCC: $20.00 © 2001 American Chemical Society
Published on Web 07/17/2001

4212 Inorganic Chemistry, Vol. 40, No. 17, 2001
Albe´niz et al.
ref
Table 1. M-N Distances for Some Group 10 Amido Complexesa
complex
M-N (Å)
N-E (E ) C, Si) (Å)
trans-[Ni(Mes)(NHPh)(PMe3)2]
trans-[PdPh(NHPh)(PEt3)2]
cis-[PtCl(NPh2)(PEt3)2]
1.932(3)
2.116(13)
2.09(2)
2.049(6)
2.043(6)
2.063(2)
2.02(1)
1.354(5)
1.32(2)
4e
4g
9
[PtCl{N(SiMe3)2}(COD)]
2
[PdCl{N(SiMe3)2}(tmeda)]
[PdCl{κ3-N(SiMe2CH2PPh2)2}]
trans-[Pt{o-C6H4(PPh2)(NH)}2]
[Pd{C6H4C(Me)dNNPh}{P(OMe)3}]2
1.700(6), 1.706(6)
1.713(2), 1.711(2)
4c
10
11
12
2.089(5)
1.371(8)
a Carbamide complexes are excluded.
Chart 2
(M ) Mo,Wo)13 and L4Os(H)(NHAr).14 In these cases steric
effects were again deemed responsible for the restricted rotation.
In this paper we have synthesized the new aryl amido
complexes of Pt(II), trans-[PtCl(NHAr)(PEt3)2] (Ar ) C6H4X,
X ) 3-F, 4-F, 4-Cl, 4-I; C6H3XY, X ) 4-Cl, Y ) 2-NO2), which
show a planar structure of type C (Chart 1). We have studied
the rotation about the Pt-N and N-C bond separately. From
the spectroscopic data obtained, along with the crystal structure
determinations for two complexes, useful information about the
cause of restricted rotation around the metal amido bond can
be drawn, proving that this restriction is not associated to the
metal amido bond itself.
ization. Nonetheless, some studies have proposed a π-donor
contribution in the metal amido bond.6
The crystal structures of amido complexes of type C reported
in the literature for Pd and Pt (Table 1) do not show a decrease
of the distances M-N(amido) compared to the M-N(amino)
distances (values of 2.103 and 2.074 Å are found for two Pt(II)
complexes with primary arylamines).7 Only the Ni complex
trans-[Ni(Mes)(NHPh)(PMe3)2] (entry 1, Table 1) shows a
Ni-N(amido) distance slightly shorter than the Ni-N(amino)
distance for coordinated anilines (1.932 vs. 2.06 Å respec-
tively).7,8 On the other hand the M-N distances reported for
those complexes (Table 1) are very close to each other and do
not show significant differences; the same occurs for the N-C
(or N-Si) bond lengths. Thus, the structural data suggest that
the M-amido bond (M ) Ni, Pd, Pt) is essentially a single bond.
In solution, hindered rotation of the amido ligand (about the
M-N or N-R bonds) could be a consequence of an interaction
with a bond order higher than one, but sterics can also be very
important. To our knowledge, only three examples of restricted
rotation about the M-N bond of group 10 amides have been
reported (Chart 2).4e,g,h For complexes D and E (Chart 2), the
hindered rotation was detected in an indirect way, since the ortho
(and meta) positions of the mesityl (D)4e or phenyl (E)4g groups
bound to the metal were found inequivalent, which is consistent
with a rigid amido ligand. The rotation of the amido group could
not be studied since the only process the authors could follow
was the rotation of the aryl-M bond. The different values of
∆Gq determined for complexes D (Chart 2) were attributed to
the different rotation rates of the amido groups, although they
were obtained indirectly by looking at the rotation of the mesityl
group. For complex F, restricted rotation about the N-C(tolyl)
bond and Ni-N bond were analyzed separately. Two different
free energies of activation were found, the N-C rotation (∆Gq
) 48.9 kJ mol-1) being easier than the Ni-N rotation (∆Gq )
67.3 kJ mol-1). The high activation barrier for this last process
was assigned a steric origin.4h Other examples of slow rotating
amides have been found for L3-n(NR2)nMtMn(NR2)L3-n
Experimental Section
General. 1H, 13C, 19F, and 31P NMR spectra were recorded on Bruker
AC-300 and ARX-300 spectrometers. Chemical shifts (in δ units, ppm)
were referenced to TMS for H and 13C, to CFCl3 for 19F, and to H3-
1
PO4 for 31P. The spectral data were recorded at 293 K unless otherwise
noted. IR spectra were recorded using Nujol mulls on a Perkin-Elmer
883 spectrophotometer. C, H, and N elemental analyses were performed
on a Perkin-Elmer 240 microanalyzer. Solvents were dried following
standard procedures and distilled before use. The aromatic azides used
in this work are known compounds that were prepared by the standard
method starting from commercial anilines.15 [PtHCl(PEt3)2] was
prepared as described elsewhere.16
Preparation of the Amido Complexes. Synthesis of trans-[PtCl-
{NH(p-I-C6H4)}(PEt3)2] (5). To a solution of trans-[PtHCl(PEt3)2]
(0.341 g, 0.73 mmol) in freshly distilled THF (15 mL) was added
p-iodophenyl azide (0.179 g, 1.46 mmol). The mixture was stirred for
3 days at room temperature and protected from light. The brownish
solution was then evaporated to dryness, and n-hexane (3 mL) was
added to the residue. The ocher solid was filtered, washed with n-hexane
(3 × 3 mL), and air-dried: 0.447 g (89% yield). Anal. Calcd for C18H35-
ClINP2Pt: C, 31.57; H, 5.13; N, 2.04. Found: C, 31.80; H, 4.85; N,
2.06. 31P{1H} NMR (CDCl3, δ, 121.4 MHz, 293 K), 15.96 (1JPt-P
)
1
2549 Hz); H NMR (CDCl3, δ, 300.13 MHz, 293 K), 7.05 (m, 2H,
C6H4), 6.35 (b, 2H, C6H4), 1.75 (m, 12H, CH2), 1.45 (b,2JPt-H ) 40
Hz, 1H, NH), 1.15 (m, 18H, CH3). 1H NMR (CDCl3, δ, 300.13 MHz,
223 K), 6.12 (dd, J ) 8.5, 2.5, 1H, H2-arom), 6.60 (dd, J ) 8.5, 2.5,
1H, H6-arom)), 6.94 (dd, J ) 8.5, 2, 1H, H3-arom), 7.14 (dd, J ) 8.5,
2, 1H, H5-arom), 1.80 (m, 6H, CH2), 1.65 (m, 6H, CH2), 1.10 (m, 18H,
(9) Eadie, DT.; Pidcock, A.; Stobart, S. R. Inorg. Chim. Acta 1982, 65,
L111-L112.
(10) Fryzuk, M. D.; MacNeil, P. A.; Rettig, S. J.; Secco, A. S.; Trotter, J.
Organometallics 1982, 1, 918-930.
(11) Cooper, M. K.; Downes, J. M.; Goodwin, H. J.; MacPartlin, M.;
Rosalky, J. M. Inorg. Chim. Acta 1983, 76, L155-L156.
(12) Espinet, P.; Garc´ıa, G.; Herrero, F. J.; Jeannin, Y.; Philoche-Levisalles,
M. Inorg. Chem. 1989, 28, 4207-4211.
(13) (a) Chetcuti, M. J.; Chisholm, M. M.; Folting, K.; Maitko, D. A.;
Huffman, J. C.; Janos, J. Am. Chem. Soc. 1983, 105, 1163-1170. (b)
Chisholm, M. M.; Folting, K.; Huffman, J. C.; Rothwell, I. P.
Organometallics 1982, 1, 251-259.
(6) Poulton, J. T.; Folting, K.; Streib, W. E.; Caulton, K. G. Inorg. Chem.
1992, 31, 3190-3191.
(7) Orpen, A. G.; Brammer, L.; Allen, F. H.; Kennard, O.; Watson, D.
G.; Taylor, R. J. Chem. Soc., Dalton. Trans. 1989, S1-S83.
(8) Even shorter bond lengths ranging from 1.819(8) to 1.889(3) Å have
been reported in three-coordinated Ni complexes. Hope, H.; Olmstead,
M. M.; Murray, B. D.; Power, P. P. J. Am. Chem. Soc. 1985, 107,
712-713.
(14) Flood, T. C.; Lim, J. K.; Deming, M. A.; Keung, W. Organometallics
2000, 19, 1166-1174.
(15) Smith, P. A. S.; Boyer, J. H. Organic Syntheses; Wiley: New York,
1963; Collect. Vol. IV, p 75-77.
(16) Parshall, G. W. Inorg. Synth. 1970, 12, 27-29.

Hindered Rotation around the Pt-N Bond
Inorganic Chemistry, Vol. 40, No. 17, 2001 4213
Table 2. Crystallographic Data for Complexes 5 and 6
5
6
empirical formula
formula weight
temperature
C18 H35 Cl I N P2 Pt
684.85
298(2) K
C18 H34 Cl2 N2 O2 P2 Pt
638.40
300(2) K
wavelength
0.71073 Å
0.71073 Å
crystal system
space group
unit cell dimensions
orthorhombic
Pnma
a ) 23.806(4) Å, R ) 90°
b ) 15.099(2) Å, â ) 90°
c ) 6.7593(10) Å, γ ) 90°
2429.7(6) Å3
monoclinic
P2(1)/n
a ) 12.215(3) Å, R ) 90°
b ) 8.078(2) Å, â ) 90.057(6)°
c ) 13.052(4) Å, γ ) 90°
1287.9(6) Å3
volume
Z
4
2
density (calculated)
absorption coefficient
final R indices [I > 2σ(I)]
R indices (all data)
1.872 Mg/m3
1.646 Mg/m3
7.289 mm-1
R1 ) 0.0158, wR2 ) 0.0410
R1 ) 0.0179, wR2 ) 0.0419
5.794 mm-1
R1 ) 0.0440, wR2 ) 0.1296
R1 ) 0.0523, wR2 ) 0.1368
CH3). 13C{1H} (CDCl3, δ, 75.4 MHz, 293 K),157 (s, C1), 137 (s,
C3+C5), 117 (s, 3JPt-C ) 58 Hz, C2+C6), 68.5 (s, C4), 12.9 (m, AA′XX′
using a VT-100 temperature control unit on Bruker AC300 and
ARX300 spectrometers. The temperature was calibrated by measuring
the difference between the chemical shifts of MeOH signals at each
temperature.17 First-order rate constants for ligand rotation (krot) were
obtained from line shape analysis by matching the observed variable
spin system, P-CH2-CH3, 1JP-C ) 15 Hz), 7.7 (m, AA′XX′ spin system,
2
P-CH2-CH3, JP-C ) 5 Hz). IR, ν(N-H): 3338 cm-1
.
SAFETY NOTE. Aryl azides are potentially explosive. Although
we did not have any problems, great caution should be exercised when
handling these materials.
Complexes 2-4 and 6 were prepared in a similar way. The small
variations on the procedure are specified in each case:
1
temperature 19F NMR (2) or H NMR (3-5, aromatic region) spectra
in CDCl3 with those simulated using the computer programs DNMR6
(2) or the gNMRV3.6.5 (3-5).18 An Eyring plot of ln(krot/T) vs 1/T was
represented. Activation parameters, ∆Hq and ∆Sq, were calculated from
the slope and the intercept respectively of the best fit line drawn by a
least-squares analysis. Uncertainties in the activation parameters were
calculated from the uncertainties of the slope and the intercept of the
best fit line, as reported before.19
2. Molar ratio Pt/azide ) 1:10; reaction time, 3 h; yield, 72%. Anal.
Calcd for C18H35ClFNP2Pt: C, 37.47; H, 6.11; N, 2.43, Found: C,
37.52; H, 5.71; N, 2.51. 31P{1H} NMR (CDCl3, δ, 121.4 MHz, 293
1
K), 15.83 (1JPt-P) 2551 Hz); H NMR (CDCl3, δ, 300.13 MHz, 293
∆Gq values obtained from the phosphine methylene region of the
1H NMR spectra for complexes 3-5 were determined using the Eyring
equation for exchanging sites of equal population at the coalescence
temperature: ∆Gq ) 19.14Tc(9.97 + log(T/δν)). Errors were estimated
assuming the following uncertainties in Tc (( 2 K) and δν (( 2.5 Hz).19
X-ray Crystal Structure Determinations. Crystals were obtained
by slow diffusion of n-hexane in a solution of 5 in acetone or by slow
evaporation of a solution of 6 in a mixture of acetone and n-hexane.
Crystals of dimensions 0.05 × 0.1 × 0.42 mm3 (5) or 0.05 × 0.1 ×
0.16 mm3 (6) were mounted on the tip of glass fibers. X-ray
measurements were made using a Bruker SMART CCD area-detector
diffractometer. Reflections were collected, intensities integrated, and
the structure was solved by direct methods procedure.20 Non-hydrogen
atoms were refined anisotropically and hydrogen atoms (except H(1)
for both structures) were constrained to ideal geometries and refined
with fixed isotropic displacement parameters. Relevant crystallographic
data are contained in Table 2.
K), 6.80 (m, 1H), 6.30 (b, 2H), 5.88 (m, 1H), 1.80 (m, 12H, CH2,), 1.5
1
(b, 1H, NH),1.20 (m, 18H, CH3); H NMR (CDCl3, δ, 300.13 MHz,
223 K), 2a/2b: 6.44/5.97 (d, 3JF-H ) 13 Hz, 1H, H2-arom), 5.86/5.88
3
(m, J ) 7.5, 2 Hz, JF-H ) 9.5 Hz, 1H, H4-arom), 6.72/6.89 (q, J )
4
7.5, 7.5 Hz, JF-H ) 7.5 Hz, 1H, H5-arom) 6.07/6.55 (dd, J ) 7.5, 2
Hz, 1H, H6-arom), 1.85 (m, 6H, CH2), 1.65 (m, 6H, CH2), 1.10 (m,
18H, CH3); 19F NMR (CDCl3, δ, 282 MHz, 223 K), -114.5 (m, 2a),
-116.8 (m, 2b). 31P{1H} NMR (CDCl3, δ, 121.4 MHz, 223 K), 17.05
(2a), 16.93 (2b). IR: ν(N-H): 3329 cm-1
.
3. Molar ratio Pt/azide ) 1:10; reaction time, 9 h; yield, 89%. Anal.
Calcd for C18H35ClFNP2Pt: C, 37.47; H, 6.11; N, 2.43; Found: C,
37.63; H, 5.75; N, 2.41. 31P{1H}NMR (CDCl3, δ, 121.4 MHz), 15.76
(1JPt-P ) 2586 Hz); 19F NMR (CDCl3, δ, 282 MHz), -137.14 (m, 1F);
1H NMR (CDCl3, δ, 300.13 MHz, 293 K), 6.62 (t, J ) 8.5 Hz, 2H,
H3, H5-arom), 6.45 (b, 2H, H2, H6-arom), 1.8 (m, 12H, CH2), 1.60 (b,
1H, NH), 1.15 (m, 18H, CH3); 1H NMR (CDCl3, δ, 300.13 MHz, 208
K), 6.70 (b, 2H, H,6 H5-arom), 6.57 (b, 1H, H3-arom), 6.18 (b, 1H,
H2-arom), 1.81 (b, 6H, CH2), 1.63 (b, 6H, CH2) 1.1 (m, 18H, CH3).
IR, ν(N-H): 3321 cm-1
.
Results and Discussion
4. Molar ratio Pt/azide ) 1:2; reaction time, 2 days; crystallized
from Et2O; yield, 86%. Anal. Calcd for C18H35Cl2NP2Pt: C, 36.43; H,
5.94; N, 2.29. Found: C, 36.37; H, 5.57; N, 2.36. 31P{1H} NMR
(CDCl3, δ, 121.4 MHz, 293 K), 15.94 (s, 1JPt-P ) 2558 Hz); 1H NMR
(CDCl3, δ, 300.13 MHz, 293 K), 6.8 (b, 2H), 6.46 (b, 2H), 1.75 (m,
12H, CH2), 1.38 (s,2JPt-H ) 32.7 Hz, 1H, NH), 1.15 (m, 18H, CH3);
1H NMR (CDCl3, δ, 300.13 MHz, 223 K), 6.88 (dd, J ) 8.6, 2.5 Hz,
1H, H3-arom), 6.70 (b, 2H, H2, H4-arom), 6.21 (dd, J ) 8.6, 3 Hz, 1H,
H5-arom), 1.80 (m, 6H, CH2), 1.65 (m, 6H, CH2), 1.45 (s, 1H, NH),
Synthesis and Structure of Complexes. Platinum amido
complexes were synthesized by reaction of the hydrido complex
1 with the corresponding aryl azides, as depicted in eq 1.21
1.10 (m, 18H, CH3). IR, ν(N-H): 3327 cm-1
.
6. Molar ratio Pt/azide ) 1:1; reaction time, 1 h; crystallized from
Et2O; yield, 70%. Anal. Calcd for C36H128N4Cl4O4P4Pt2: C, 33.86; H,
5.33; N, 4.39. Found: C, 34.10; H, 4.95; N, 4.39. 31P{1H} NMR
(CDCl3, δ, 121.4 MHz), 15.15 (s, 1JPt-P ) 2449 Hz); 1H NMR (CDCl3,
δ, 300.13 MHz), 7.98 (d, J ) 3.3 Hz, 1H, H3-arom), 7.65 (s, 2JPt-H
)
Complexes 2-5 could be isolated as yellow or ocher solids
32.6 Hz, NH, 1H), 7.31 (d, J ) 10 Hz, 1H, H5-arom), 7.05 (dd, J )
10, 3.3 Hz, 1H, H6-arom), 1.65 (m, CH2, 6H), 1.82 (m, CH2, 6H), 1.1
(m, CH3, 18H). IR ν(NO2): 1539 cm-1; ν(N-H) around 2900 cm-1
(overlapped with the C-H stretching bands).
and showed a fluxional behavior in solution due to hindered
(17) Van Geet, A. L. Anal. Chem. 1970, 42, 679-6800.
(18) (a) DNMR6, Quantum Chemical Program Exchange (QCPE 633);
Indiana University: Bloomington, IN, 1995. (b) gNMR V 3.6.5;
IvorySoft, Cherwell Scientific Publishing Ltd.: Oxford.
Determination of the Activation Parameters for the Rotation of
the Amido Ligand. Variable temperature NMR spectra were recorded

4214 Inorganic Chemistry, Vol. 40, No. 17, 2001
Albe´niz et al.
Chart 3
rotation of the amido ligand. Accordingly, their NMR spectra
are broad at room temperature. Complex 6 is a red solid, with
an apparently rigid structure in solution (unchanged 1H and 31
NMR spectra) up to 333 K.
P
The low-temperature spectra of complex 2 shows the presence
of two atropisomers, each of them displaying the same features
that will be described for 3-6. Below their coalescence
temperature complexes 3-6 showed in their 31P{1H} NMR
spectra one singlet with platinum satellites, as expected for a
1
single compound with trans sterochemistry. Their H NMR
spectra showed the inequivalence of all the aromatic protons,
and both hydrogens of the phosphine methylene groups were
diastereotopic, as expected from a rigid planar amido ligand
arranged perpendicularly to the platinum coordination plane.
The NH resonances for 2-5 appeared in the range δ 1.45-
1.60, whereas for 6 it was dramatically shifted downfield (δ
7.65). Since the variations in chemical shift for the amido
hydrogen are very modest when the acceptor group in para is
changed from F to I, it seemed unlikely that this huge downfield
shift could be due to the incorporation of a second acceptor
group (NO2) only. It appeared more reasonable that most of
this effect might be due to the involvement of the N-H group
in hydrogen bond with the ortho NO2 substituent. This interac-
tion should be geometrically favorable if the NO2 and the aryl
ring become roughly coplanar (Chart 3). Moreover, the shift of
ν(N-H) from 3338 to 3321 for complexes 2-5 to low wave-
numbers (around 2900 cm-1 for 6, overlapped with the C-H
stretching bands) supported this proposal.22
X-ray crystal structure determinations were carried out for
complexes 5 and 6. Selected bond lengths and angles are given
in Table 3.
Both structures show a planar amido ligand perpendicular to
the platinum coordination plane (Figure 1). The Pt-N distances
are short, and the one found for complex 5 (2.006 Å) is shorter
than any M-N(amido) (M ) Pd, Pt) bond length reported before
(Table 1). The structures feature an sp2 hybridized N atom with
short C-N distances (closer to a CdN double bond distance
(1.30 Å) than to a C-N single bond distance (1.47 Å)),23
supporting that the lone pair of the N atom is heavily engaged
in the formation of a multiple C-N bond. Not surprisingly, the
distance C-N in complex 6 (1.31 Å) is slightly shorter than in
complex 5 (1.346 Å), as expected from the higher acceptor
character and better ability to delocalize electron density of the
aryl ring in 6. The amido hydrogen atom could be located in
the Fourier map in each case. For complex 6 this confirmed
the existence of N-H‚‚‚O(NO2) hydrogen bond (d(N(1)‚‚‚O(2))
) 2.634 Å; d(H‚‚‚O(2)) ) 1.894 Å; O(2)‚‚‚H(1)-N(1) )
130.53°). The distances found are in the range of other
Figure 1. Molecular structures of complexes 5 and 6.
Table 3. Selected Bond Lengths (Å) and Angles for Complexes 5
and 6
5
6
Pt(1)-N(1)
2.006(4)
2.030(15)
2.322(12)
2.289(9)
2.297(6)
1.31(2)
0.97(12)
1.46(2)
Pt(1)-P(1)
2.3053(10)
2.3053(10)
2.3458(11)
1.346(6)
0.69(4)
1.412(5)
Pt(1)-P(2)
Pt(1)-Cl(1)
N(1)-C(1)
N(1)-H(1)
C(1)-C(2)
C(2)-N(2)
1.39(2)
1.75(5)
C(4)-X(4) (X ) I, 5, Cl, 6)
2.117(4)
N(1)-Pt(1)-P(1)
N(1)-Pt(1)-P(2)
N(1)-Pt(1)-Cl(1)
C(1)-N(1)-Pt(1)
N(1)-C(1)-C(6)
N(1)-C(1)-C(2)
O(2)-H(1)-N(1)
90.05(2)
89.9(5)
90.9(5)
176.8(5)
129.2(12)
121.8(15)
126.0(15)
126.0(15)
90.05(2)
179.29(12)
126.9(3)
123.0(4)
121.0(4)
(19) (a) Casado, A. L.; Casares, J. A.; Espinet, P. Organometallics 1997,
16, 5730-5736. (b) Kirkup, L. Experimental Methods; Wiley:
Brisbane, 1994.
(20) The data analysis and the drawing were performed with the following
programs: SMART V5.051, 1998; SAINT V6.02, 1999. Sheldrick, G.
M. SHELXTL V5.1; Bruker AXS, Inc.: Madison, WI, 1998.
(21) Beck, W.; Bauder, M. Chem. Ber. 1970, 103, 583-589.
(22) Gilli, P.; Bertolasi, V.; Ferretti, V.; Gilli, G. J. Am. Chem. Soc. 2000,
122, 10405-10417.
intramolecular N-H‚‚‚O hydrogen bonded structures reported
in the literature for coordinated ligands24 and also for â-enami-
nones with comparable H-bonded six-membered stuctures.22 The
H-bond is bent but this is probably imposed by the geometry
of the molecule.
(23) Emsley, J. The Elements; Clarendon Press: Oxford, 1991.

Hindered Rotation around the Pt-N Bond
Inorganic Chemistry, Vol. 40, No. 17, 2001 4215
Chart 4
Table 4. Activation Parameters for the Rotation of the Amido
Ligand about the N-aryl Bond in Complexes 2-5a
complex
∆Sq(Jmol-1K-1
)
∆Hq(kJ mol-1
)
∆G298q(kJ mol-1
)
2
3
4
5
-13 ( 4
-32 ( 5
49.1 ( 1.2
33.3 ( 1.1
32.3 ( 1.4
35.7 ( 0.5
53.0 ( 1.7
42.8 ( 1.5
53 ( 1.8
-68 ( 6
-69.2 ( 1.9
56.3 ( 0.6
a Data obtained by line shape analysis of 1H (3-5) or 19F (2) NMR
signals.
Fluxional Behavior of the Complexes. The substitution of
the aryl ring in the meta or ortho positions, as in complexes 2
and 6, can give rise to atropisomers by restricted rotation around
the N-aryl bond. In fact, the NMR spectra of complex 2 (CDCl3)
in the slow exchange limit (223 K) revealed a mixture of two
isomers in a ratio 2a/2b ) 1:0.7 (Chart 4). Two signals were
observed in the 31P{1H} (δ 17.05, s, JPt-P ) 2504 Hz; and 16.93,
s, JPt-P ) 2520 Hz) and 19F (δ -114.46, m; -116.77, m)
1
spectra. The H spectrum showed clearly two sets of aromatic
signals (two rotamers) and two groups of methylene protons
from the PEt3 ligands each showing that the two H atoms on
each methylene group are diastereotopic. When the temperature
was raised all the methylenic hydrogen atoms became iso-
cronous. This requires fast rotation about both the N-aryl and
the Pt-N bonds. Using the 19F NMR data, line shape analysis
was carried out which gave the rate of exchange at different
temperatures and, by using an Eyring plot, the activation
parameters collected in Table 4. Although 2 does not allow to
study the rotation about the N-aryl bond independently, these
activation parameters are similar to those found for the rotation
about the N-aryl bond in 2-5, which were assigned as discussed
below.
The analysis of the NMR spectra of the complexes with para
substituted amido ligands (3-5) at variable temperature in
CDCl3 turned out to be very informative. Restricted rotation
does not produce atropisomers in this case, but different changes
in the spin systems for the 1H NMR spectra of these derivatives
are expected depending on the motion involved. Fast rotation
about the Pt-N bond should produce equivalence of the
methylenic H atoms only. Fast rotation around the N-aryl bond
should bring about equivalence of the arylic hydrogen atoms
only. Thus, both movements can be studied independently by
looking at the corresponding signals separately.
1
Figure 2. Real (left) and simulated (right) H NMR spectra of the
aromatic region at variable temperature for complex 5.
(Figure 2) was carried out and an Eyring plot led to the
activation parameters for the rotation about the N-aryl bond of
the ligand (Table 4).
The free energies of activation found are also similar to the
data reported for restricted rotation about the C-N bond in
aromatic amines (cf. ∆Gq
4-ClC6H4NMe2 and ∆Gq
) 44.3 kJ mol-1 for
rot(201.5 K)
) 46 kJ mol-1 for 4),25 and
rot(201.5 K)
in the Ni complex [Ni(bipy)(Et)(NEtTol)] (N-C(tolyl) ∆Gq
rot
) 48.9 kJ mol-1).4h
On the other hand, the two protons of the methylene groups
of the phosphines become equivalent only when the platinum
coordination plane becomes a symmetry plane, which requires
rotation of the amido group about the N-Pt bond (Figure 3).
Provided that rotation about the N-aryl bond were not required,
the free energies of activation obtained looking at this region
in the spectra should correspond to ∆Gq for the rotation about
the Pt-N bond. Table 5 collects the thermodynamic data
obtained for complexes 3-5. There is a perfect coincidence
(within the experimental error) between the ∆Gq values for
rotation about the N-aryl bond and those for rotation about the
Pt-N bond. This suggests that the rotation around the Pt-N
bond does require simultaneous rotation around the N-aryl bond,
which is the highest barrier and becomes rate determining for
Figure 2 shows the change in the aromatic region of the
spectrum for complex 5 as the temperature increases. Rotation
about the N-aryl bond is the only process required to bring about
the coalescence of both ortho protons on one side and both meta
protons on another, and this movement certainly does not require
any other modification in the molecule (such as rotation around
the Pt-N bond). Line shape analysis at different temperatures
(24) Some recent examples: (a) Moriuchi, T.; Watanabe, T.; Ikeda, I.;
Ogawa, A.; Hirao, T. Eur. J. Inorg. Chem. 2001, 277-287. (b)
Sakagami-Yoshida, N.; Teramoto, M.; Hioki, A.; Fuyuhiro, A.; Kaizari,
S. Inorg. Chem. 2000, 39, 5717-5724. (c) Cini, R.; Grabner, S.;
Bukovec, N.; Cerasino, L.; Natile, G. Eur. J. Inorg. Chem. 2000,
1601-1607. (d) Castarnelas, R.; Esteruelas, M. A.; On˜ate, E. Orga-
nometallics 2000, 19, 5454-5463.
(25) Oki, M. Applications of Dynamic NMR Spectroscopy to Organic
Chemistry; Methods in Stereochemical Analysis 4; VCH Publishers:
New York, 1985; pp 86-87.

4216 Inorganic Chemistry, Vol. 40, No. 17, 2001
Albe´niz et al.
1
Figure 3. Variable temperature H NMR spectra of complex 5 in the methylene region.
Table 5. Free Energies of Activation for the Rotation about the
N-aryl and Pt-N Bonds for complexes 3-5
complex Tcoal (K) ∆Gqrot(Pt-N)(kJ/mol)a ∆Gqrot(C-N)(kJ/mol)b
3
4
5
223
248
258
44.58 ( 0.43o
50.64 ( 0.44
53.07 ( 0.45
40.4 ( 1.6
49 ( 2
53.6 ( 1.7
a Obtained by analysis of the phosphine methylene region of the
1H NMR spectra. b Obtained by analysis of the aromatic region of the
1H NMR spectra.
the whole process (see Conclusions). This is in contrast with
the results reported for [Ni(bipy)(Et)(NEtTol)] where the free
energies of activation found for rotation about the N-C (48.9
kJ mol-1) and Ni-N (67.3 kJ mol-1) are very different and
indicate that both processes are independent.4h Since the Et
substituent on the amido ligand of the Ni complex increases
the steric hindrance in that case, rotation about the N-C(Tolyl)
bond is not sufficient and the Ni-N barrier for rotation is higher
than the energy needed for rotation about the Pt-N bond of
complexes 2-5.
Other alternative mechanisms for rotation about the N-aryl
and Pt-N bonds were considered and discarded. The dynamic
processes are intramolecular and the rates do not increase with
the concentration of the complex, which rules out any bimo-
lecular mechanism. The polarity of the solvent does not
influence the process either, and no change in rate was observed
when complex 5 was dissolved in a CDCl3 solution of (NBu4)-
PF6, which discards the dissociation of the amido ligand to form
an ionic pair as an alternative mechanism. The involvement in
the rotation in an amino ligand (formed by the presence of traces
of acid) or an imino complex (by deprotonation of the amido
ligand) was excluded since the addition of CaH2 to a solution
of 5 in CDCl3 does not affect the rotation rate.
Finally, no fluxional behavior was observed for complex 6
up to 333 K. Only one atropisomer with diastereotopic meth-
ylenic hydrogen atoms was observed in solution, suggesting that
the hydrogen bonding observed in the solid state, which is
preserved in solution, precludes the necessary rotation about
the N-aryl bond.
Figure 4. Ball and stick and space filling models of the partial rotation
about the Pt-N bond for (a) a rigid amido moiety and (b) an amido
ligand after rotation about the C-N bond.
about the N-aryl bond occurs and the hybridization of N changes
in the transition state from sp2 to sp3 and the phenyl ring can,
by N-aryl rotation, relieve the steric hindrance that a the planar
arrangement was imposing (Figure 4b). There is no need to
invoke any multiple character of the Pt-N bond.
The rigidity of complex 6 indicates a much higher barrier
for the rotation around the N-aryl bond. Two effects should
contribute to make this barrier higher than for any other
compound in this study: (i) the aryl ring in 6 bears two electron
acceptor groups and should produce the N-aryl bond with the
highest multiplicity; (ii) the hydrogen bond must be broken.
Considering the more modest range of variations in ∆Gq
rot
produced upon changes of the withdrawing group in 2-5, it
seems that most of the dramatic increase in ∆Gq observed
rot
Conclusions
for 6 must be attributed to the hydrogen bond.
The barrier observed for complexes 2-5 corresponds to that
of the rotation about the N-aryl bond, which is electronic in
origin and reflects the multiplicity of the bond. In effect, the
rotational barrier can be associated to the accessibility of a
transition state similar to A in Chart 1, with a lone electron
pair on a sp3 N atom.
The rotation about the Pt-N bond is probably a low energy
process in the absence of steric considerations, but a planar
amido group with a sp2 N atom should clash with the phosphines
(Figure 4a). This steric problem can be overcome when rotation
Acknowledgment. We thank the Direccio´n General de
Ensen˜anza Superior (Ministerio de Educacio´n y Cultura,
PB96-0363) and the Junta de Castilla y Leo´n (VA80/99 and
VA17/00B)f or financial support.
Supporting Information Available: Crystallographic data, in CIF
format. This material is available free of charge via the Internet at
http://pubs.acs.org.
IC010317F