Exploring the variable hapticity of the arylamide ligand: Access to σ-amidophenyl and π-cyclohexadienylimine structures

Article abstract of DOI:10.1021/om4005252

A study of the preference for σ vs π coordination of the arylamido ligand to a late transition metal shows that LiNPh₂ reacts with RuHCl(PPh₃)₃ (1) to yield the bent-seat piano-stool complex RuH[(η⁵-C₆H₅)NPh](PPh ₃)₂ (2a) but with RuHCl(CO)(PPh₃)₃ (3) to yield the σ-amide RuH(η¹-NPh₂)(CO) (PPh₃)₂ (4). The stability of the σ-bound NPh ₂ ligand in 4 reflects the π acidity of the CO ligand, which inhibits PPh₃ loss. Carbonylation of 2a at 50 C affords Ru(CO) ₃(PPh₃)₂ (8) and HNPh₂, suggesting sequential π → σ isomerization and reductive elimination. The phenoxide ligand behaves similarly: RuH(η⁵-C₆H ₅O)(PPh₃)₂ (2b) is formed from 1 but RuH(η¹-OPh)(CO)(PPh₃)₃ (5) is formed from 3, and carbonylation of 2b gives 8 and phenol, although more forcing conditions are required (90 C). The crystal structure of 2a is reported.

Full text of DOI:10.1021/om4005252

Communication
pubs.acs.org/Organometallics
Exploring the Variable Hapticity of the Arylamide Ligand: Access to
σ‑Amidophenyl and π‑Cyclohexadienylimine Structures
Benjamin J. Ireland,† Robert McDonald,‡ and Deryn E. Fogg*,†
†Center for Catalysis Research & Innovation and Department of Chemistry, University of Ottawa, Ottawa, Ontario, Canada K1N
6N5
‡X-ray Crystallography Laboratory, Chemistry Department, University of Alberta, Edmonton, Alberta, Canada T6G 2G2
S
* Supporting Information
ABSTRACT: A study of the preference for σ vs π coordination of the
arylamido ligand to a late transition metal shows that LiNPh2 reacts with
RuHCl(PPh3)3 (1) to yield the bent-seat piano-stool complex RuH[(η5-
C6H5)NPh](PPh3)2 (2a) but with RuHCl(CO)(PPh3)3 (3) to yield the σ-
amide RuH(η1-NPh2)(CO)(PPh3)2 (4). The stability of the σ-bound NPh2
ligand in 4 reflects the π acidity of the CO ligand, which inhibits PPh3 loss. Carbonylation of 2a at 50 °C affords
Ru(CO)3(PPh3)2 (8) and HNPh2, suggesting sequential π → σ isomerization and reductive elimination. The phenoxide ligand
behaves similarly: RuH(η5-C6H5O)(PPh3)2 (2b) is formed from 1 but RuH(η1-OPh)(CO)(PPh3)3 (5) is formed from 3, and
carbonylation of 2b gives 8 and phenol, although more forcing conditions are required (90 °C). The crystal structure of 2a is
reported.
xamples of amido complexes of the late transition metals
Ehave expanded greatly in recent years, driven largely by
interest in their catalytic properties.1−3 Arylamido ligands are of
particular interest for their potential steric and electronic
tunability. In the dominant anilido derivatives, the anionic
nitrogen is typically monodentate or bridging (Chart 1a),1−3
but the η5-cyclohexadienylimine structure (Chart 1b) is also
known.4,5 Analogous bent-seat piano-stool complexes are well
documented in Ru−aryloxide chemistry.6−11
we report a study of the RuHX′(PPh3)n system, a well-
developed model that enables examination of these effects
without the difficulties associated with identification of metal
complexes formed during catalysis. We provide evidence that
the arylamide ligand can likewise adapt its hapticity and that it
may do so even more readily than phenoxide. For either ligand
class, however, incorporation of a single π-acid carbonyl ligand
proves sufficient to bias selectivity toward the σ-EArn
coordination mode (E = O, N).
Addition of Li[NPh2] as a solution in THF to a purple
suspension of RuHCl(PPh3)3 (1) in benzene caused a color
change to orange within 15 min at 45 °C. Formation of
RuH[(η5-C6H5)NPh](PPh3)2 (2a) (Scheme 1a) was complete
within 1 h, as judged by 31P{1H} NMR analysis. The sole
species observed in the crude reaction mixture were free PPh3
and 2a (molar ratio 1/1). Pale orange 2a was isolated in 73%
yield after workup. X-ray-quality crystals deposited from
toluene−hexanes at −30 °C.
Chart 1. (a) Selected Ru(σ-NRAr) Derivatives15−19 and (b)
η5-Cyclohexadienylimine Complexes of Iron4,5
Scheme 1. Piano-Stool Products Formed by Reaction of
RuHCl(PPh3)3 with Diphenylamide or Phenoxide10
The potential for reversible isomerization between such σ-
and π-bound structures is of interest in catalysis, as it may affect
the lifetime and productivity of coordinatively unsaturated σ-
NRAr complexes. We earlier suggested that Ru(π-OAr)
structures may be expected for monodentate aryloxides
wherever the ancillary ligands are sufficiently labile to give
access to the required three coordination sites.10 Subsequent
work demonstrated that binaphtholate complexes of ruthenium
can adapt between the extremes of η1:η1 and η3:η3 binding,
depending on the number of other ligands present.12−14 Here
Received: June 7, 2013
© XXXX American Chemical Society
A
dx.doi.org/10.1021/om4005252 | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Communication
Crystallographic analysis of 2a (Figure 1) reveals that the
amide ligand is bound to the metal via an η5-cyclohexadienyl
Scheme 2. σ-Arylamide and σ-Aryloxide Complexes
Accessible from RuHCl(CO)(PPh3)3
analysis of crude 4 revealed a singlet at 41.7 ppm,
accompanying that for equimolar free PPh3. Additional Ru
species present in minor amounts include known24 RuH2(CO)-
(PPh3)3 and unidentified hydride coproducts that give rise to
signals between −3 and −8.5 ppm. Clean 4 was isolated in 76%
yield after washing with hexanes.
Figure 1. Perspective view of 2a. Non-hydrogen atoms are represented
by Gaussian ellipsoids at the 20% probability level. The hydride ligand
is shown with an arbitrarily small thermal ellipsoid. Key metrics: C21−
N, 1.406(6) Å; C11−N, 1.313(6) Å; C21−N−C11, 120.9(4)°.
The hydride signal for 4, a triplet at −17.19 ppm (2JPH = 22
Hz), is ca. 5 ppm upfield of that for 2a. These data imply a
square-pyramidal structure with an apical hydride and two
mutually trans phosphine ligands. σ coordination of the
diphenylamide ligand is confirmed by the downfield location
ring. Loss of aromaticity is evident from the nonplanar structure
of the ring, C11 being bent by 24° out of the plane. Partial
imine character is suggested by the short N−C11 bond distance
of 1.313(6) Å; cf. a value of 1.406(6) Å for the unperturbed
NPh moiety and NC bond lengths of ca. 1.26−1.27 Å for
related aniline-derived ketimines.20−22 The 120.9(4)° angle for
the C11−N−C21 bond is consistent with formulation as an
imine.
1
of all phenyl signals in the H NMR spectrum (6.0−7.7 ppm;
Figure 2).
The σ-bound amide ligand in 4 does not convert into the π
structure 6 over 24 h at 50 °C (C6D6; <3% loss vs internal
standard). We attribute this stability to the presence of the π-
acid CO ligand, which inhibits phosphine loss. Treating piano-
stool complex 2a with CO, however, triggers release of HNPh2
and formation of Ru(CO)3(PPh3)2 (8),25 most plausibly via
reductive elimination of HNPh2 from RuH(σ-NPh2)-
(CO)2(PPh3)2 (7a) (Scheme 3). Formation of 8 reaches 92%
NMR analysis of 2a confirms the distorted piano-stool
1
structure. Diagnostic H NMR markers for the η5-bound ring
are depicted in Figure 2. Five NC6H5 signals are shifted
Scheme 3. Proposed Pathway for Carbonylation of 2a,b To
Form HEPhn and Ru(CO)3(PPh3)2 (8)
Figure 2. Diagnostic 1H NMR locations (C6D6 solvent) for the σ- and
π-bound N(C6H5)2 ligand in 2a and 4.
dramatically upfield from the aromatic region: they appear at ca.
4.0−5.3 ppm vs the region of 6.6−7.7 ppm occupied by the
remaining phenyl protons. A hydride triplet appears at −11.94
2
(t, JPH = 33 Hz). The quaternary imine carbon appears at
in 2 h at 50 °C. The potential intermediacy of 4 was verified by
exposing 4 to CO: this effected complete conversion into 8 and
HNPh2 within 2 h at 50 °C. Room-temperature experiments
indicated that the reductive elimination step was considerably
faster for 4 than for 2a. For 4, evolution of HNPh2 reached 90%
within 20 min vs 65% for 2a at 24 h. Initial π → σ isomerization
of phenylamide thus appears to be the principal barrier to
carbonylation of 2a. (The higher proportion of HNPh2 than 8
(Table S1 in the Supporting Information) also indicates that
reductive elimination occurs prior to formation of 8.) Attempts
152.5 ppm; cf. a value of 175.6 ppm reported for CyNPh in
CDCl3.23 Similar structural and H NMR data were reported
1
for the η5-C6H5O moiety in the known 2b (Scheme 1b).6,7
To assess the impact of a π-acid ligand on the preferred
coordination mode of the anionic donor, we explored the
corresponding reaction of RuHCl(CO)(PPh3)3 (3) with
LiNPh2. A color change from white to red occurred over 15
min at 23 °C in THF, with full conversion to RuH(σ-
NPh2)(CO)(PPh3)2 4 (Scheme 2) after 1 h. 31P{1H} NMR
B
dx.doi.org/10.1021/om4005252 | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Communication
to independently prepare 7a by treating RuHCl(CO)2(PPh3)2
with LiNPh2 yielded ca. 1/1 4 and 8: while the observed CO
disproportionation is unexpected, this finding confirms the
instability of 7a toward reductive elimination.
REFERENCES
■
(1) Lappert, M.; Protchenko, A.; Power, P.; Seeber, A. Metal Amide
Chemistry; Wiley: Chichester, U.K., 2009.
(2) Fulton, J. R.; Holland, A. W.; Fox, D. J.; Bergman, R. G. Acc.
Chem. Res. 2002, 35, 44−56.
The generality of this arylamide behavior is demonstrated by
the accessibility of a σ-aryloxide complex related to 4 via
addition of KOPh to 3. The reaction was complete within 2 h at
room temperature in THF, and clean RuH(σ-OPh)(CO)-
(PPh3)3 (5) was obtained as a white powder in 91% yield
(Scheme 2). Retention of all three phosphine ligands in 5a
function of the reduced bulk of the OPh ligand, relative to the
NPh2 ligand in 4is indicated by the multiplicity and location
of the hydride signal, which appears as a doublet of triplets at
−6.63 ppm (2JHP = 112 and 24 Hz). Other spectroscopic data
are consistent with the proposed structure. Thus, the room-
temperature 31P{1H} NMR spectrum revealed two broad
singlets at 38.1 and 16.3 ppm (ratio 2:1; C6D6), which resolve
into an A2B pattern (2JPP = 17 Hz) at 263 K in C7D8.
As with the arylamide derivatives, aryloxide 5 resisted σ → π
isomerization at 50 °C, but thermolysis of piano-stool complex
2b at 90 °C under CO liberated 8 and phenol (Scheme 3; 93%
after 6 h). No reaction was observed over 24 h at 50 °C,
suggesting a higher barrier to π → σ isomerization of phenoxide
than for diphenylamide.
The foregoing demonstrates that monodentate arylamide
and aryloxide ligands exhibit qualitatively similar tendencies in
terms of the parameters that favor σ binding via the heteroatom
vs π coordination via a dearomatized ring. For either ligand
class, piano-stool structures are favored where three binding
sites are available. When the lability of the PPh3 ligands is
restrictedeven by introduction of a single CO ligandσ-
arylamide or -aryloxide derivatives are formed. In the presence
of additional ligands, however, the piano-stool complexes can
slip to lower-hapticity structures. Exposure to CO is shown to
induce π → σ interconversion and reductive elimination of
diphenylamine or phenol. Of note is the lower barrier of this
transformation for the amido complex, which may indicate that
the arylamide ligand adjusts its hapticity to accommodate
incoming ligands more readily than does aryloxide. This
potential advantage may be offset, for hydride derivatives, by
relatively facile reductive elimination. Whether nonhydride
derivatives also readily eject the arylamide ligand is presently
under study.
(3) Ikariya, T. Bull. Chem. Soc. Jpn. 2011, 84, 1−16.
(4) Helling, J. F.; Hendrickson, W. A. J. Organomet. Chem. 1977, 141,
99−105.
(5) Helling, J. F.; Hendrickson, W. A. J. Organomet. Chem. 1979, 168,
87−95.
(6) Cole-Hamilton, D. J.; Young, R. J.; Wilkinson, G. J. Chem. Soc.,
Dalton Trans. 1976, 1995−2001.
(7) Yamamoto, T.; Miyashita, S.; Naito, Y.; Komiya, S.; Ito, T.;
Yamamoto, A. Organometallics 1982, 1, 808−812.
(8) Chaudret, B.; He, X.; Huang, Y. J. Chem. Soc., Chem. Commun.
1989, 1844−1846.
(9) Christ, M. L.; Sabo-Etienne, S.; Chung, G.; Chaudret, B. Inorg.
Chem. 1994, 33, 5316−19.
(10) Snelgrove, J. L.; Conrad, J. C.; Yap, G. P. A.; Fogg, D. E. Inorg.
Chim. Acta 2003, 345, 268−278.
(11) Abdur-Rashid, K.; Fedorkiw, T.; Lough, A. J.; Morris, R. H.
Organometallics 2004, 23, 86−94.
(12) Blacquiere, J. M.; Higman, C. S.; McDonald, R.; Fogg, D. E. J.
Am. Chem. Soc. 2011, 133, 14054−14062.
(13) Blacquiere, J. M.; McDonald, R.; Fogg, D. E. Angew. Chem., Int.
Ed. 2010, 49, 3807−3810.
(14) Panichakul, D.; Su, Y.; Li, Y.; Deng, W.; Zhao, J.; Li, X.
Organometallics 2008, 27, 6390−6392.
(15) Bryndza, H. E.; Fong, L. K.; Paciello, R. A.; Tam, W.; Bercaw, J.
E. J. Am. Chem. Soc. 1987, 109, 1444−1456.
(16) Zhang, J. B.; Gunnoe, T. B.; Boyle, P. D. Organometallics 2004,
23, 3094−3097.
(17) Khaskin, E.; Iron, M. A.; Shimon, L. J. W.; Zhang, J.; Milstein, D.
J. Am. Chem. Soc. 2010, 132, 8542−8543.
(18) Fryzuk, M. D.; Petrella, M. J.; Coffin, R. C.; Patrick, B. O. C. R.
Chim. 2002, 5, 451−460.
(19) Blake, R. E., Jr.; Heyn, R. H.; Tilley, T. D. Polyhedron 1992, 11,
709−710.
(20) Keim, W.; Killat, S.; Nobile, C. F.; Suranna, G. P.; Englert, U.;
Wang, R.; Mecking, S.; Schroder, D. L. J. Organomet. Chem. 2002, 662,
̈
150−171.
(21) Schleis, T.; Heinemann, J.; Spaniol, T. P.; Mulhaupt, R.; Okuda,
J. Inorg. Chem. Commun. 1998, 1, 431−434.
(22) Markowicz, S. W.; Figlus, M.; Lejkowski, M.; Karolak-
̈
Wojciechowska, J.; Dzierzawska-Majewska, A.; Verpoort, F. Tetrahe-
̇
dron: Asymmetry 2006, 17, 434−448.
́ ́
(23) Barluenga, J.; Jimenez-Aquino, A.; Aznar, F.; Valdes, C. J. Am.
Chem. Soc. 2009, 131, 4031−4041.
(24) Hallman, P. S.; McGarvey, B. R.; Wilkinson, G. J. Chem. Soc. A
1968, 3143−50.
ASSOCIATED CONTENT
(25) Dell’Amico, D. B.; Calderazzo, F.; Labella, L.; Marchetti, F. J.
Organomet. Chem. 2000, 596, 144−151.
■
S
* Supporting Information
Text, figures, tables, and a CIF files giving experimental and
crystallographic details. This material is available free of charge
via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail for D.E.F.: dfogg@uottawa.ca.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was funded by the NSERC of Canada. The NSERC
is thanked for a CGS-D award to B.J.I.
C
dx.doi.org/10.1021/om4005252 | Organometallics XXXX, XXX, XXX−XXX