Synthesis, characterization, and O2 reactivity of iridium(I) complexes supported by guanidinato ligands

Article abstract of DOI:10.1021/ic801867r

Mononuclear [Ir{ArNC(NR₂)NAr}(C₈H₁₂)] complexes (where R = Me or Et; Ar = Ph, 4-MeC₆H₄, or 2,6-Me₂C₆H₃; and C₈H₁₂ = 1,5-cyclooctadiene) were synt

Full text of DOI:10.1021/ic801867r

Inorg. Chem. 2008, 47, 11461-11463
Synthesis, Characterization, and O2 Reactivity of Iridium(I) Complexes
Supported by Guanidinato Ligands
Jan-Uwe Rohde,* Matthew R. Kelley, and Wei-Tsung Lee
Department of Chemistry, The UniVersity of Iowa, Iowa City, Iowa 52242
Received September 30, 2008
Scheme 1. Resonance Structures of the Guanidinate Monoanion
Mononuclear [Ir{ArNC(NR2)NAr}(C8H12)] complexes (where R )
Me or Et; Ar ) Ph, 4-MeC6H4, or 2,6-Me2C6H3; and C8H12 ) 1,5-
cyclooctadiene) were synthesized from the neutral N,N-dialkyl-
N′,N′′-diarylguanidines via deprotonation and transmetalation. As
confirmed by single-crystal structure determinations, the guanidi-
nato(1-) ligands coordinate the low-valent d8 IrI center in an N,N′-
chelating binding mode, and the 13C NMR chemical shifts of the
alkene carbon atoms establish that these ligands function as
stronger donors than related monoanionic, bidentate nitrogen-based
ligands. In the reactions of the complexes with O2, the observed
reactivity trends correlate with the electronic and steric influences
of the substituents of the guanidinato ligands.
Chart 1. Guanidinate, Amidinate, and Triazenide Anions
electron-deficient metal centers can be ascribed to lone-pair
donation from the nitrogen atom of the NR2 group, which
increases the electron density at the two donor atoms
available for metal coordination and, thereby, makes this
class of ligands stronger donors than the closely related
amidinate and triazenide anions10 (Scheme 1 and Chart 1).
Consequently, if coordinated to a lower-valent metal center,
guanidinato ligands confer greater oxidizability on the metal,
as is evident, for example, from the redox potentials of
Monoanionic nitrogen-donor ligands represent an impor-
tant class of supporting ligands and have been widely used
in the coordination chemistry of transition metals to impart
diverse properties and reactivity. Prominent examples include
poly(1-pyrazolyl)borate,1 ꢀ-diiminate,2 aminotroponiminate,3
and amidinate4 scaffolds. Because guanidinates are conceptu-
ally related to the latter three systems, they may be expected
to be similarly well-suited as supports for reactive metal
centers. Complexes of guanidinates are known for many
metals and metalloids, in particular, for early transition metals
and lanthanides in mid- to high-valent oxidation states.5-9
The exceptional ability of guanidinate anions to stabilize
[MoII (µ-L)4] complexes.11,12 Guanidinato complexes of
2
electron-rich transition-metal centers in oxidation states lower
than +II are limited to only a few di- and tetranuclear
complexes, which involve mainly coinage metals.5,13,14
We have targeted IrI complexes of bidentate N,N-dialkyl-
N′,N′′-diarylguanidinate anions, {ArNC(NR2)NAr}-, to ex-
plore the coordination ability of this ligand platform to
transition metals in low oxidation states. Herein, we report
*
To whom correspondence should be addressed. E-mail:
jan-uwe-rohde@uiowa.edu.
(1) (a) Trofimenko, S. Chem. ReV. 1993, 93, 943. (b) Trofimenko, S.
(10) (a) Gantzel, P.; Walsh, P. J. Inorg. Chem. 1998, 37, 3450. (b) Hauber,
S.-O.; Niemeyer, M. Inorg. Chem. 2005, 44, 8644.
Polyhedron 2004, 23, 197.
(2) Bourget-Merle, L.; Lappert, M. F.; Severn, J. R. Chem. ReV. 2002,
102, 3031.
(11) (a) Bailey, P. J.; Bone, S. F.; Mitchell, L. A.; Parsons, S.; Taylor,
K. J.; Yellowlees, L. J. Inorg. Chem. 1997, 36, 867. (b) Bailey, P. J.;
Bone, S. F.; Mitchell, L. A.; Parsons, S.; Taylor, K. J.; Yellowlees,
L. J. Inorg. Chem. 1997, 36, 5420.
(12) Cotton, F. A.; Murillo, C. A.; Wang, X.; Wilkinson, C. C. Inorg. Chem.
2006, 45, 5493.
(3) Roesky, P. W. Chem. Soc. ReV. 2000, 29, 335.
(4) (a) Barker, J.; Kilner, M. Coord. Chem. ReV. 1994, 133, 219. (b)
Edelmann, F. T. Coord. Chem. ReV. 1994, 137, 403.
(5) Bailey, P. J.; Pace, S. Coord. Chem. ReV. 2001, 214, 91.
(6) (a) Duncan, A. P.; Mullins, S. M.; Arnold, J.; Bergman, R. G.
Organometallics 2001, 20, 1808. (b) Mullins, S. M.; Duncan, A. P.;
Bergman, R. G.; Arnold, J. Inorg. Chem. 2001, 40, 6952.
(7) Bazinet, P.; Wood, D.; Yap, G. P. A.; Richeson, D. S. Inorg. Chem.
2003, 42, 6225.
(8) Soria, D. B.; Grundy, J.; Coles, M. P.; Hitchcock, P. B. J. Organomet.
Chem. 2005, 690, 2278.
(9) (a) Wilder, C. B.; Reitfort, L. L.; Abboud, K. A.; McElwee-White, L.
Inorg. Chem. 2006, 45, 263. (b) Rische, D.; Baunemann, A.; Winter,
M.; Fischer, R. A. Inorg. Chem. 2006, 45, 269.
(13) (a) Irwin, M. D.; Abdou, H. E.; Mohamed, A. A.; Fackler, J. P., Jr
Chem. Commun. 2003, 2882. (b) Mohamed, A. A.; Mayer, A. P.;
Abdou, H. E.; Irwin, M. D.; Perez, L. M.; Fackler, J. P., Jr Inorg.
Chem. 2007, 46, 11165. (c) Coyle, J. P.; Monillas, W. H.; Yap,
G. P. A.; Barry, S. T. Inorg. Chem. 2008, 47, 683. (d) Fandos, R.;
Otero, A.; Rodriguez, A.; Terreros, P. Collect. Czech. Chem. Commun.
2007, 72, 579.
(14) Noteworthy also is the recent report of an MgI2 species supported by
bulky guanidinates: Green, S. P.; Jones, C.; Stasch, A. Science 2007,
318, 1754.
10.1021/ic801867r CCC: $40.75  2008 American Chemical Society
Inorganic Chemistry, Vol. 47, No. 24, 2008 11461
Published on Web 11/14/2008

COMMUNICATION
Scheme 2. Synthesis of [Ir{ArNC(NR2)NAr}(cod)] Complexes 1-4
Figure 1. Molecular structures of 2 (left) and 4 (right), showing the atomic
numbering scheme. Displacement ellipsoids are drawn at the 50% probability
level; hydrogen atoms have been omitted for clarity. Color key: pink ) Ir,
blue ) N, gray ) C.
the synthesis and spectroscopic and structural characterization
of a series of mononuclear [Ir{ArNC(NR2)NAr}(cod)]
complexes and evaluate the donor properties of the guanidi-
nato ligands (cod ) 1,5-cyclooctadiene). Several previously
reported [MI(L)(cod)]z complexes of Rh and Ir, where L is
a di- or tridentate nitrogen- or oxygen-donor ligand, are
known to react with dioxygen and undergo C-O bond
formation under mild conditions.15 In only two cases,
however, the reactions take an atom-economic route that does
not require additional reagents such as acids or sacrificial
reductants.15a,d Also the new complexes presented here were
found to react readily with dioxygen. In an effort to probe
the utility of the guanidinate platform in tuning reactivity,
we have investigated the effect of substituent variation (Ar
and R) on the reactivity of these complexes.
Information).17 The Ir-N and Ir-C distances are in the ranges
of 2.069(2)-2.099(2) and 2.095(3)-2.122(3) Å, respectively,
which agree well with the values of related complexes.17b,c,18,19
Because of the acute bite angles of the chelating guanidinato
ligands (N1-Ir-N2 ) 63.68(9)° for 2 and 63.72(9)° for 4),
the metal centers adopt a distorted square-planar coordination
geometry. The structural analysis also reveals important intrali-
gand parameters that provide insight into the bonding of the
guanidinato ligands. Remarkably, the C-N distances in the
planar CN3 core of each complex are nearly identical,
1.346(4)-1.361(4) Å for 2 and 1.335(4)-1.354(4) Å for 4.
These parameters indicate a high degree of π-electron delocal-
ization over the CN3 core, implicating a significant contribution
of the iminium/diamide resonance structure (Scheme 1, structure
II). In accordance with the sp2 hybridization expected for
efficient lone-pair donation into the CN3 unit, the N3 atoms
adopt a trigonal-planar geometry with bond angle sums of
359.9(5)° (2) and 359.8(4)° (4). In contrast, the N1 and N2
atoms are more pyramidalized, with bond angle sums ranging
from 346.0(3) to 352.5(3)°. Although the C-N3-C and
N1-C1-N2 planes are slightly twisted with respect to each
other [25.6(1)° for 2 and 18.7(3)° for 4], presumably because
of minor steric interactions between the NR2 alkyl and the NAr
aryl groups, the conjugation of N3 and C1 p orbitals is
apparently not disrupted.
The neutral guanidines ArNdC(NR2)NHAr [where R ) Me,
Ar ) Ph; R ) Et, Ar ) Ph;16a R ) Me, Ar ) 4-MeC6H4; and
R ) Me, Ar ) 2,6-Me2C6H3] were synthesized from the
corresponding thioureas, ArHNC(S)NHAr, and the appropriate
amines, HNR2, according to literature methods for related
compounds.16 As shown in Scheme 2, treatment with MeLi
and subsequent transmetalation using [{Ir(cod)}2(µ-Cl)2] under
an inert atmosphere furnished the yellow IrI complexes of the
anionic guanidinates, [Ir{ArNC(NR2)NAr}(cod)], 1-4. Com-
plexes 1-4 are well soluble in nonpolar organic solvents such
as toluene, diethyl ether, and n-pentane but only sparingly
1
soluble in acetonitrile. The H NMR spectra of each of these
complexes are consistent with a guanidinato-κ2N,N′ and an η4-
cod ligand coordinated to a d8 IrI center in a square-planar
environment. On the basis of the molecular ion peaks observed
by electron impact mass spectrometry, 1-4 are identified as
mononuclear complexes (cf. the Supporting Information).
Yellow single crystals of 2 and 4 were grown from
concentrated n-pentane solutions upon standing at -30 °C. The
solid-state molecular structures of both compounds were
determined from crystallographic analyses and confirm the
mononuclear nature of 2 and 4 (Figure 1 and Supporting
The intraligand structural features of 2 and 4 are unusual
because complexes of guanidinato(1-) ligands typically display
more localized bonding with a longer C-N bond to the
noncoordinated nitrogen atom and shorter ones to the metal-
coordinated nitrogen atoms.5,9b,11,13,14,20 Thus, the bonding in
those guanidinate systems is best described by a relatively larger
contribution of the amine/amidinate resonance structures (I and
III). On the other hand, the greater extent of delocalization
(17) For comparison, with amidinato ligands, both mono- and dinuclear
complexes have been obtained: (a) Lahoz, F. J.; Tiripicchio, A.;
Tiripicchio Camellini, M.; Oro, L. A.; Pinillos, M. T. J. Chem. Soc.,
Dalton Trans. 1985, 1487. (b) Cotton, F. A.; Poli, R. Inorg. Chim.
Acta 1986, 122, 243. (c) Kanematsu, N.; Ebihara, M.; Kawamura, T.
Inorg. Chim. Acta 1999, 292, 244.
(18) Albinati, A.; Bovens, M.; Ru¨egger, H.; Venanzi, L. M. Inorg. Chem.
1997, 36, 5991.
(19) Bernskoetter, W. H.; Lobkovsky, E.; Chirik, P. J. Organometallics
2005, 24, 6250.
(15) (a) Day, V. W.; Klemperer, W. G.; Lockledge, S. P.; Main, D. J. J. Am.
Chem. Soc. 1990, 112, 2031. (b) de Bruin, B.; Brands, J. A.; Donners,
J. J. J. M.; Donners, M. P. J.; de Gelder, R.; Smits, J. M. M.; Gal,
A. W.; Spek, A. L. Chem.sEur. J. 1999, 5, 2921. (c) Flood, T. C.;
Iimura, M.; Perotti, J. M.; Rheingold, A. L.; Concolino, T. E. Chem.
Commun. 2000, 1681. (d) Tejel, C.; Ciriano, M. A.; Sola, E.; del Rio,
M. P.; Rios-Moreno, G.; Lahoz, F. J.; Oro, L. A. Angew. Chem., Int.
Ed. 2005, 44, 3267. (e) de Bruin, B.; Budzelaar, P. H. M.; Gal, A. W.
Angew. Chem., Int. Ed. 2004, 43, 4142.
(16) (a) Srinivasan, N.; Ramadas, K. Tetrahedron Lett. 2001, 42, 343. (b)
Zepik, H. H.; Benner, S. A. J. Org. Chem. 1999, 64, 8080.
(20) Robinson, S. D.; Sahajpal, A.; Steed, J. Inorg. Chim. Acta 2000, 303,
265.
11462 Inorganic Chemistry, Vol. 47, No. 24, 2008

COMMUNICATION
observed for 2 and 4 is precedent in complexes of TiIV,21 ZrIV,7
and NbV.8 It has been argued that significant involvement of
the lone pair of the NR2 nitrogen atom is due to the ligands’
coordination to an electron-deficient metal center. In this context,
the observations made here for low-valent d8 IrI complexes may
seem surprising. Presumably, the π-acceptor alkene ligand
enhances the electron-releasing character of the guanidinato
ligand and thus facilitates participation of the NR2 lone pair in
CN3 π bonding.
To assess the donor strength of the guanidinato ligands in
1-4, we turned to 13C NMR spectroscopy to gauge the extent
of Ir-to-cod π back-bonding. Complexes 1-4 exhibit 13C
NMR resonance signals for the alkene carbon atoms in the
range of 58.8-61.0 ppm. These are upfield-shifted compared
to the corresponding signals in IrI(cod) complexes of
dicoordinating tris(pyrazolyl)borato and ꢀ-diketiminato ligands,
which appear in the range of 63.2-66.7 ppm.18,19 Lower
δ(13CCdC) values reflect increased π back-donation into the
alkene π* orbitals caused by increased electron density at
the metal center. Therefore, the dialkyldiarylguanidinates
may be considered stronger donors than κ2-binding tris(pyra-
zolyl)borates and ꢀ-diketiminates. They are still weaker
donors than tris(pyrazolyl)borate-κ3 and cyclopentadienyl
ligands (54.1 and 45.5-53.1 ppm).18,22
In solution, complexes 1-4 react readily with dioxygen, as
indicated by the disappearance of their characteristic absorption
features in the UV-vis region (Figure 2). For example, at 0
°C, the half-life of 1 in a toluene solution saturated with O2 is
ca. 20 min. Mono- and dioxygenated products were identified
by electrospray ionization mass spectrometry. The masses and
isotope distribution patterns of the peaks at m/z ) 554 and 570
are consistent with {1 + O - H}+ and {1 + 2O - H}+,
respectively, and the reaction of 1 with isotope-enriched 18O2
shows that the oxygen atoms are derived from O2 (m/z ) 556
and 574).23 The m/z value of the monooxygenated product
agrees with the formulation as an oxocyclooctenyl complex.
An example of an oxocyclooctenyl complex with a different
ancillary ligand was previously reported to form in the reaction
of an IrI complex with H2O2 via an intermediate iridaoxetane.24
In contrast, other IrI(cod) and RhI(cod) complexes that react
with O2 afford metallaoxetanes that rearrange to hydroxycy-
clooctenediyl complexes.15
Figure 2. Reaction of 2 mM 1 (orange solid line) in toluene with O2 at 0
°C as monitored by electronic absorption spectroscopy (path length, 0.5
cm). Inset: Time courses of the reactions of 1 (black solid line; λ, 417 nm),
2 (red dashed line; λ, 418 nm), 3 (red solid line; λ, 410 nm), and 4 (blue
dashed line; λ, 423 nm) with O2.
the order 3 < 2 < 1 < 4 (Figure 2, inset; t1/2 ) 400 s, 1100 s,
1300 s, and 1 h). Thus, both methyl substitution in the para
position of the benzene rings (3) and extension of the alkyl
substituents of the NR2 group (2) have an accelerating effect
on the O2 reaction, whereas methyl groups in the ortho
position of the benzene rings (4) have a decelerating effect.
The higher reactivity of 2 and 3 compared to that of 1 can
be explained by increased electron donation from the
guanidinato ligand to the IrI center and consequent destabi-
lization of the low oxidation state. In the case of 4, the steric
influences of the o-methyl groups appear to exceed their
electronic influences and may be responsible for the slower
rate. Clearly, the IrI centers in 1-3 are somewhat more
exposed than that in 4, and the methyl groups of the latter
may limit access to the metal center (Figure S1 in the
Supporting Information). It also may be possible that the
differences in sterics lead to different reaction pathways.
In summary, we have prepared and characterized mono-
nuclear complexes of a low-valent d-block metal center
coordinated by bidentate guanidinato(1-) ligands and thus
expanded the scope of this ligand platform in transition-metal
chemistry. Spectroscopic evidence suggests that the N,N-dialkyl-
N′,N′′-diarylguanidinates employed here function as stronger
donors than other monoanionic, nitrogen-based scaffolds, such
as tris(pyrazolyl)borates-κ2 and ꢀ-diketiminates, when bound
to the IrI(cod) unit. While complexes 1-4 differ only in the
methyl substitution pattern, the trends observed for the half-
lives of their reactions with O2 illustrate the potential that the
guanidinato(1-) platform has to offer for facile modulation of
the reactivity of transition-metal complexes.
Interestingly, while complexes 2-4 reacted with O2
similarly to 1, the rates of the reactions varied with the
substituents on the guanidinato ligand. Under the same
conditions, the half-lives of the IrI complexes increase in
Acknowledgment is made to the University of Iowa and
the donors of the American Chemical Society Petroleum
Research Fund (46587-G3) for support of this research. We
thank Dr. Dale C. Swenson for the collection of X-ray
diffraction data and Dr. Srikanta Patra for assistance.
(21) (a) Bailey, P. J.; Grant, K. J.; Mitchell, L. A.; Pace, S.; Parkin, A.;
Parsons, S. J. Chem. Soc., Dalton Trans. 2000, 1887. (b) Coles, M. P.;
Hitchcock, P. B. Organometallics 2003, 22, 5201.
(22) Sowa, J. R., Jr.; Angelici, R. J. J. Am. Chem. Soc. 1991, 113, 2537.
(23) The species associated with these peaks, however, have eluded
characterization by NMR and IR spectroscopy, and despite consider-
able efforts, we have not been able to grow single crystals of the
products. The lack of conclusive NMR and IR spectroscopic evidence
indicates that the species identified in the mass spectrum are not the
only products and that the reaction may be complicated. Also a
Supporting Information Available: Experimental procedures,
characterization data of all compounds, details of the crystal
structure determinations (Figure S1 and Tables S1-S4), and CIF
files of 2 and 4. This material is available free of charge via the
Internet at http://pubs.acs.org.
complex of
a monoalkene was synthesized, [Ir{PhNC(NMe2)-
NPh}(coe)2] (5), but in this case, the alkene dissociates upon reaction
with O2 (coe ) cis-cyclooctene).
(24) Sciarone, T.; Hoogboom, J.; Schlebos, P. P. J.; Budzelaar, P. H. M.;
de Gelder, R.; Smits, J. M. M.; Gal, A. W. Eur. J. Inorg. Chem. 2002,
457.
IC801867R
Inorganic Chemistry, Vol. 47, No. 24, 2008 11463