Cp*Ir(dab) (dab = 1,4-Bis(2,6-dimethylphenyl)-1,4-diazabutadiene): A Coordinatively Unsaturated Six-π-Electron Metallaheteroaromatic Compound?

Article abstract of DOI:10.1021/ic9514814

1,4-Bis(2,6-dimethylphenyl)-1,4-diaza-1,3-butadiene (dab) forms the structurally characterized iridium(III) complex [Cp*IrCl(dab)](PF₆): C₂₈H₃₅CIF₆IrN₂P, orthorhombic, space group Pnma, a = 16.187(2) ??,b = 15.823(2) ?, c = 11.677(1) ?, V = 2990.8(6) ?3, Z = 4, and R = 0.0588. On reaction with NaBH₃CN this compound does not form an iridium(III) hydride but the coordinatively unsaturated reduced product Cp*Ir(dab): C₂₈H₃₅IrN₂, monoclinic, space group P2₁In, a = 8.484(2) ?, b = 14.535(3) ?, c = 20.956(4) ?, β= 98.88(3)°, V = 7553.2(9) ?³, Z = 4, and R = 0.0586. The inverted relation dcc (=1.334(15) ?) < dCN (=1.379(13) and 1.366(14) ?) in the dab ligand of Cp*Ir(dab) suggests that the reduction has occurred primarily at that ligand to form an ene-1,2-diamido/ iridium(III) moiety or, alternatively, a six-π-electron metallaheteroaromatic system. Ab initio pseudopotential calculations of model complexes [CpIr(HNCHCHNH)]^0/2+ support this description of the bonding.

Full text of DOI:10.1021/ic9514814

3998
Inorg. Chem. 1996, 35, 3998-4002
Cp*Ir(dab) (dab ) 1,4-Bis(2,6-dimethylphenyl)-1,4-diazabutadiene): A Coordinatively
Unsaturated Six-π-Electron Metallaheteroaromatic Compound?
Stefan Greulich, Wolfgang Kaim,* and Andreas F. Stange
Institut fu¨r Anorganische Chemie der Universita¨t, Pfaffenwaldring 55, D-70550 Stuttgart, Germany
Hermann Stoll
Institut fu¨r Theoretische Chemie der Universita¨t, Pfaffenwaldring 55, D-70550 Stuttgart, Germany
Jan Fiedler and Stanislav Za´lisˇ
J. Heyrovsky´ Institute of Physical Chemistry, Academy of Sciences of the Czech Republic,
Dolejsˇkova 3, CZ-18223 Prague, Czech Republic
ReceiVed NoVember 17, 1995X
1,4-Bis(2,6-dimethylphenyl)-1,4-diaza-1,3-butadiene (dab) forms the structurally characterized iridium(III) complex
[Cp*IrCl(dab)](PF6): C28H35ClF6IrN2P, orthorhombic, space group Pnma, a ) 16.187(2) Å, b ) 15.823(2) Å, c
) 11.677(1) Å, V ) 2990.8(6) Å3, Z ) 4, and R ) 0.0588. On reaction with NaBH3CN this compound does not
form an iridium(III) hydride but the coordinatively unsaturated reduced product Cp*Ir(dab): C28H35IrN2, monoclinic,
space group P21/n, a ) 8.484(2) Å, b ) 14.535(3) Å, c ) 20.956(4) Å, â ) 98.88(3)°, V ) 2553.2(9) Å3, Z )
4, and R ) 0.0586. The inverted relation dCC ()1.334(15) Å) < dCN ()1.379(13) and 1.366(14) Å) in the dab
ligand of Cp*Ir(dab) suggests that the reduction has occurred primarily at that ligand to form an ene-1,2-diamido/
iridium(III) moiety or, alternatively, a six-π-electron metallaheteroaromatic system. Ab initio pseudopotential
calculations of model complexes [CpIr(HNCHCHNH)]0/2+ support this description of the bonding.
Introduction
state distribution Cp*MII(R-diimine-I) with strong spin-spin
pairing for the most frequently used1,2d,3a,b,4 combination M )
Rh and R-diimine ) 2,2′-bipyridine. In order to drive the
apparent intramolecular metal-to-ligand electron transfer still
further, we became interested in combining the 14-electron
fragment Cp*Ir,5 an extremely strong π donor,2d with the better6a
π-accepting 1,4-diaza-1,3-butadiene R-diimine ligands.6
Considering the coordinative unsaturation of the critical
intermediates Cp*M(R-diimine), we employed the sterically
protecting 1,4-bis(2,6-dimethylphenyl)-1,4-diaza-1,3-butadiene
(dab) as an R-diimine ligand. Previous electrochemical studies
of corresponding (pentamethylcyclopentadienyl)rhodium com-
plexes have indicated the ability of that ligand to slow down
the (chemical) addition step after (electrochemical) electron
transfer.2e 2,6-Dimethyl- or 2,4,6-trimethyl-substituted aryl
Coordinatively unsaturated complexes Cp*M(R-diimine) are
crucial intermediates1-3 in catalyzed hydride transfer reactions1-4
which require a two-electron reduction of [Cp*MCl(R-di-
imine)]+ precursors (M ) Rh, Ir; R-diimine ) 2,2′-bipyridine
or derivatives) prior to protonation. However, the propensity
of the electron-rich intermediates Cp*M(R-diimine) for oxidative
addition of electrophiles has rendered their investigation and
isolation difficult. On the basis of electrochemical and spec-
troscopic data, we have formulated2a,b,f the effective oxidation
X Abstract published in AdVance ACS Abstracts, May 15, 1996.
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S0020-1669(95)01481-9 CCC: $12.00 © 1996 American Chemical Society

A Six-π-Electron Metallaheteroaromatic Compound
Table 1. Crystallographic Data
Inorganic Chemistry, Vol. 35, No. 13, 1996 3999
Chart 1
[Cp*IrCl(dab)](PF6)
chem formula C28H35ClF6IrN2P
Cp*Ir(dab)
C28H35IrN2
591.78
fw
772.20
space group
a (Å)
b (Å)
c (Å)
â(deg)
V (Å3)
Z
Pnma (No. 62)
16.187(2)
15.823(2)
P21/n (No. 14)
8.484(2)
14.535(3)
20.956(4)
98.88(3)
2553.2(9)
4
1.540
5.246
0.710 73
-100
11.677(1)
2990.8(6)
4
1.715
4.665
0.710 73
-100
F
calc (g cm-3
)
µ (mm-1
λ (Å)
)
T (°C)
R indices
R ) 0.0588, Rw ) 0.1454 R ) 0.0586, Rw ) 0.1479
(I > 2σ(I))a,b
2
2
2
a R ) (∑||Fo| - |Fc||)/∑|Fo|. b Rw ) {∑[w(|Fo| - |Fc| )2]/∑[w(Fo )
2]}1/2
.
groups are well-known to protect axial sites such as singly
occupied boron pz7 or platinum dz orbitals.8
2
Both the precursor [Cp*IrCl(dab)](PF6) and the coordinatively
unsaturated Cp*Ir(dab), obtained unexpectedly from the reaction
with cyanoborohydride, could be crystallized and characterized
spectroscopically; their electronic structures were studied with
the help of ab initio pseudopotential9 calculations.
Experimental Section
ence. Spectroelectrochemical measurements were performed using an
optically transparent thin-layer electrode (OTTLE) cell.11
Materials and Procedures. 1,4-Bis(2,6-dimethylphenyl)-1,4-diaza-
1,3-butadiene (dab) and [Cp*IrCl(µ-Cl)]2 were prepared according to
published procedures.6a,10
Crystallography. Data were collected on a Syntex P21 diffracto-
meter. An empirical absorption correction (ψ scans) was applied. The
structures were solved by the Patterson method using the SHELXTL-
PLUS program.12 Refinement was achieved with SHELXL-9313
employing full-matrix least-squares methods.
[Cp*IrCl(dab)]PF6. Dark green crystals were obtained by slow
diffusion of diethyl ether into a methanol solution at room temperature.
The Cp* ring showed disorder in terms of a “staggered” (55%, Figure
1) or “eclipsed” orientation (45%, designated as A) of the Ir-Cl bond
relative to Cp* methyl groups. All non-hydrogen atoms apart from
those of the disordered Cp* ring were refined anisotropically. Hydrogen
atoms were placed in their ideal positions and were allowed to ride on
the corresponding carbon atoms.
[Cp*Ir(dab)]. Dark orange crystals suitable for X-ray diffraction
were obtained by slow cooling of an ethanol/water mixture (4/1, v/v).
The DIFABS absorption correction14 was applied. All non-hydrogen
atoms were refined anisotropically. Hydrogen atoms were placed in
their ideal positions and were allowed to ride on the corresponding
carbon atoms.
Calculations. Ab initio calculations were performed for (η5-C5H5)-
Ir(HNCHCHNH) using Dunning’s valence double-ú basis with polar-
ization functions for H, C, and N15a and quasirelativistic effective core
pseudopotentials and corresponding basis functions for the iridium
[Cp*IrCl(dab)](PF6). A 202 mg (0.253 mmol) amount of [Cp*IrCl-
(µ-Cl)]2 was reacted with 167 mg (0.632 mmol) of dab for 4 h in
methanol (30 cm3). Precipitation of the green hexafluorophosphate by
addition of 390 mg (1 mmol) of Bu4NPF6 was almost quantitative
(95%). Anal. Calcd for C28H35ClF6IrN2P: C, 43.55; H, 4.57; N, 3.63.
Found: C, 43.51; H, 4.50; N, 3.65. UV/vis (acetone, λmax in nm): 580,
425.
Cp*Ir(dab). A suspension of 77.2 mg (0.10 mmol) of [Cp*IrCl-
(dab)](PF6) in a mixture of 20 cm3 of ethanol and 5 cm3 of water was
reacted with 31.5 mg (0.50 mmol) of sodium cyanoborohydride under
argon. After the mixture was stirred for 5 h at ambient temperature,
the air-sensitive yellow-orange precipitate was filtered off, washed with
water, and dried under vacuum. Yield: 42 mg (71%). Anal. Calcd
for C28H35IrN2: C, 56.83; H, 5.96; N, 4.73. Found: C, 56.61; H, 5.90;
N, 4.72. UV/vis (toluene): λmax ) 431 nm (log ꢀ ) 4.00).
[Cp*Ir(CH3CN)(dab)](PF6)2. About 20 mg (0.061 mmol) of solid
[Cp2Fe](PF6) was added to a solution of 18 mg (0.0304 mmol) of Cp*Ir-
(dab) in 10 cm3 of acetonitrile. After the mixture was stirred for 1 h,
the unreacted ferrocenium hexafluorophosphate was filtered off and
the solvent removed from the reddish brown filtrate. Extraction of
the solid residue with n-hexane to remove ferrocene and brief drying
under vacuum produced 20 mg (71%) of a reddish brown material
which was immediately dissolved in CD3CN for NMR spectroscopy
(Table 2).
Instrumentation. 1H and 13C NMR spectra were recorded on a
Bruker AC 250 spectrometer. UV/vis absorption spectra were recorded
on a Bruins Instruments Omega 10 spectrophotometer. Cyclic volta-
mmetry was carried out in acetonitrile/0.1 M Bu4NPF6 using a three-
electrode configuration (glassy-carbon electrode, Pt counter electrode,
Ag/AgCl reference) and a PAR 273A potentiostat and function
generator. The ferrocene/ferrocenium couple served as internal refer-
(11) Krejcik, M.; Danek, M.; Hartl, F. J. Electroanal. Chem. Interfacial
Electrochem. 1991, 317, 179.
(12) Sheldrick, G. M. SHELXTL-PLUS: An Integrated System for Solving,
Refining and Displaying Structures from Diffaction Data; Siemens
Analytical X-Ray Instruments Inc., 1989.
(13) Sheldrick, G. M. SHELXL-93, Program for Crystal Structure Deter-
mination; Universita¨t Go¨ttingen, Go¨ttingen, Germany, 1993.
(14) Walker, N.; Stuart, D. Acta Crystallogr. 1983, A39, 158.
(15) (a) Dunning, T. H.; Hay, P. J. Modern Theoretical Chemistry;
Plenum: New York, 1976; Chapter 1. (b) Frisch, M. J.; Trucks, G.
W.; Schlegel, H. B.; Gill, P. M. W.; Johnson; B. G.; Robb, M. A.;
Cheeseman, J. R.; Keith, T. A.; Petersson, G. A.; Montgomery, J. A.;
Raghavachari, K.; Al-Laham, M. A.; Zakrzewski, V. G.; Ortiz, J. V.;
Foresman, J. B.; Cioslowski, J.; Stefaonov, B. B.; Nanayakkara, A.;
Challacome, M.; Peng, C. Y.; Ayala, P. Y.; Chen, W.; Wong, M. W.;
Andres, J. L.; Replogle, E. S.; Gomperts, R.; Martin, R. L.; Fox, D.
J.; Binkley, J. S.; Defrees, D. J.; Baker, J.; Stewart, J. P.; Head-Gordon,
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4000 Inorganic Chemistry, Vol. 35, No. 13, 1996
Greulich et al.
Figure 2. Molecular structure of Cp*Ir(dab) in the crystal. Selected
bond lengths (Å) and angles (deg): Ir-N1 ) 1.977(9), Ir-N2 ) 1.978-
(9), N1-C19 ) 1.366(14), C19-C20 ) 1.334(15), N2-C20 ) 1.379-
(13), N1-C17 ) 1.419(14), N2-C27 ) 1.432(14), Ir-C1 ) 2.175(12),
Ir-C2 ) 2.199(12), Ir-C3 ) 2.199(12), Ir-C4 ) 2.151(14), Ir-C5
) 2.209(12); N1-Ir-N2 ) 76.9(4), Ir-N1-C19 ) 118.7(7), Ir-N2-
C20 ) 116.0(7), N1-C19-C20 ) 112.2(10), N2-C20-C19 ) 116.1-
(10). Dihedral angles (deg) between rings: IrNCCN/Cp* ) 88.9(4),
IrNCCN/R ) 89.0(4) (R ) 2,6-dimethylphenyl).
Figure 1. Molecular structure of [Cp*IrCl(dab)](PF6) in the crystal
(“staggered” configuration of Cp*). Selected bond lengths (Å) and
angles (deg): Ir-Cl ) 2.354(3), Ir-N1 ) 2.091(7), N1-C15 ) 1.280-
(10), C15-C15a ) 1.482(15), N1-C13 ) 1.450(10), Ir-C1 ) 2.215-
(21), Ir-C2 ) 2.198(14), Ir-C3 ) 2.196(15); N1-Ir-N1a ) 76.0(4),
Ir-N1-C15 ) 115.3(5), N1-C15-C15a ) 115.3(4). Dihedral angles
(deg) between rings: IrNCCN/Cp* ) 74.7(6), IrNCCN/R ) 81.7(3)
(R ) 2,6-dimethylphenyl); angle IrCNNC/IrCl ) 78.2°.
is effectively protected by all three mutually parallel Cp* and
2,6-dimethylphenyl ring systems:
atom.9 Geometry optimizations were performed within Cs constrained
symmetry using the GAUSSIAN 94 program package.15b
Results and Discussion
When the conventionally synthesized2b,3d,g and structurally
characterized [Cp*IrCl(dab)](PF6) (dab ) 1,4-bis(2,6-dimeth-
ylphenyl)-1,4-diaza-1,3-butadiene) was reacted with sodium
cyanotrihydridoborate in ethanol/water (4/1, v/v), the result was
yellow-orange Cp*Ir(dab), which could also be crystallized and
characterized spectroscopically and electrochemically. The
reaction of the chloroiridium(III) precursor with Na[BH3CN]
did thus not result in the familiar2b,3 substitution of Cl by H but
in a reduction of the complex (eq 1). Bearing in mind the
Whereas the R-diimine ligand in a previously reported related
compound described as Cp*Ir(bpy-4,4′-COOH) (bpy-4,4′-
COOH ) 2,2′-bipyridine-4,4′-dicarboxylic acid)3g shows no
significant structural evidence for ligand reduction, as is evident
from the single-bond length of 1.47(1) Å between the pyridine
moieties,3g,17 the C-C, C-N,6,16 and Ir-N18 distances in the
chelate ring in Cp*Ir(dab) suggest strong contributions from
the ene-1,2-diamide structure RN-sCHdCHsNR- of the dab
ligand. This also implies that the reduction producing this
compound has effectively occurred at the R-diimine ligand and
not at the metal site (which remains IrIII). Similarly short CdC
distances and long CsN bond lengths in chelate complexes of
1,4-diaza-1,3-butadienes were previously only known from
carbene19 and silylene20 compounds, from main-group21 or
-
[Cp*IrH(dab)](PF6) 7“H//” [Cp*IrCl(dab)](PF6) “H ”8
-
Cp*IrCl(dab) (1)
noninnocent nature of dab ligands,6,16 the product of this
reduction was studied by experimental methods and by ab initio
pseudopotential calculations.
(18) (a) Zambrano, C. H.; Sharp, P. R.; Barnes, C. L. Organometallics
1995, 14, 3607 and literature cited therein. (b) Ghedini, M.; Longeri,
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1995, 128, 245. (d) Herrmann, W. A.; Runte, O.; Artus, G. J.
Organomet. Chem. 1995, 501, C1.
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Verne, H. P.; Haaland, A.; Wagner, M.; Metzler, N. J. Am. Chem.
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Hitchcock, P. B.; Kennard, C. H. L.; Lamb, R. N.; Raston, C. L. J.
Chem. Soc., Chem. Commun. 1990, 1394. (b) Geoffrey, F.; Cloke, F.
G. N.; Hanson, G. R.; Henderson, M. J.; Hitchcock, P. B.; Raston, C.
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W. J. Chem. Soc., Chem. Commun. 1991, 597. (c) Weidenbruch, M.;
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The results of the crystal structure analyses are summarized
in Table 1, Chart 1, and Figures 1 and 2. [Cp*IrCl(dab)](PF6)
shows the expected3g pseudotetrahedral arrangement, the short
CdN and long CsC and NsIr bonds within the chelate ring
clearly indicating an IrIII/dab0 oxidation state situation (Chart
1).6,16 On the other hand, the neutral compound Cp*Ir(dab)
exhibits a metal center in a pseudo-trigonal-planar environment
with virtually orthogonal Cp* and IrNCCN(dab) chelate rings.
The coordinatively unsaturated metal in that five-membered ring
(16) (a) Richter, B.; Scholz, J.; Neumu¨ller, B.; Weimann, R.; Schumann,
H. Z. Anorg. Allg. Chem. 1995, 621, 365. (b) Chamberlain, L. R.;
Durfee, L. D.; Fanwick, P. E.; Kobriger, L. M.; Latesky, S. L.;
McMullen, A. K.; Steffey, B. D.; Rothwell, I. P.; Folting, K.; Huffman,
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(17) The compound formulated as Cp*Ir(bpy-4,4′-COOH) has a pseudopy-
ramidal configuration at the metal with a tilt angle of about 75°
between the Cp* ring and the heterocyclic plane; two of the carboxylic
oxygen atoms show very large thermal motion.3g Intramolecular
protonation of the unprotected IrI site by the acidic COOH groups
may be considered.

A Six-π-Electron Metallaheteroaromatic Compound
Inorganic Chemistry, Vol. 35, No. 13, 1996 4001
Table 2. 1H and 13C NMR Data (δ, ppm) for Iridium/dab Complexesa
b
Cp*Ir(dab)
[Cp*Ir(dab)(CH3CN)](PF6)2
[Cp*IrCl(dab)](PF6)
CD3CN
CD3OD
1H Spectra
1.27(s)
C6D6
CD3CN
CH3(Cp*)
CH3(R)d
CH(R)d
1.13(s)
1.21(s)
2.21(s)
7.01(dd)g
7.14(d)h
6.96(s)
1.18(s)
2.07(s)/2.41(s)c
7.28(ps)e
2.08(s)
2.12(s)/2.27(s)c
7.38(ps)e
H4: 6.97(dd)f
H3,5: 7.12(d)h
6.81(s)
CH(imine)
CH3(CH3CN)
9.03(s)
9.12(s)
1.96(s)
13C Spectra
CH3(Cp*)
CH3(R)d
8.40
19.10/20.05c
96.10
nd
8.31
17.02
82.67
124.90
129.00
132.24
nd
CCH3(Cp*)
CH(R)d
129.50i
129.80
130.80
131.16
131.35
132.07
174.30
CN(R)d
CH(imine)
132.60
155.50
b
c
adab ) 1,4-bis(2,6-dimethylphenyl)-1,4-diaza-1,3-butadiene. From oxidation of Cp*Ir(dab) with ferrocenium hexafluorophosphate. Inequivalent
2- and 6-methyl groups due to restricted rotation. R ) 2,6-dimethylphenyl. ps: pseudo-singlet (accidental degeneracy) fDifferent coupling constants:
d
e
3J(H3,H4) ) 6.9 Hz, 3J(H4,H5) ) 8.0 Hz. Different coupling constants: 3J(H3,H4) ) 6.5 Hz, 3J(H4,H5) ) 8.3 Hz. Average coupling constant 3J )
g
h
i
7.4 Hz. Inequivalent 2/6- and 3/5-positions due to restricted rotation.
lanthanoid-element complexes.22 In the transition-metal se-
ries,6,16,22 only the structure of (η2-BH4)2Zr(tBuNCHCHNtBu)
comes close to that of the organometallic system Cp*Ir(dab)
with values of d(CC) ) 1.361(5) Å and d(CN) ) 1.385(4) and
1.387(4) Å.23
Assuming the iridium(III) formulation, the relative stability
of a coordinatively unsaturated 5d6 metal center despite the very
strong ligand-field preference for close-to-octahedral coordina-
tion is not completely without precedent. As in Cp*Ir(dab),
the coordinatively unsaturated organotungsten(0) complex di-
anion [(cat)W(CO)3]2- (cat ) catecholate) is stabilized by the
extremely strong σ- and π-donor effect of a dianionic unsatur-
ated chelate ligand.24 Similarly, coordinatively unsaturated
iridium(III) compounds with 16-electron configurations are
stabilized by simultaneously σ- and π-donating ligands;5b,c
however, Cp*Ir(dab) is an 18-electron species.
Spectroscopic data are in agreement with the proposed
oxidation state distribution in Cp*IrIII(dab-II). An intense (log
ꢀ ) 4.0) and relatively2 high energy absorption at 431 nm points
to strong orbital mixing between metal dπ and dab π* MOs,
creating a π f π* instead of a charge transfer transition.25
Cyclic voltammetry and coulometry reveal that a reversible two-
Figure 3. 1H NMR spectrum (low-field section) of Cp*Ir(dab) in C6D6,
illustrating the nonsymmetry of the CH(aryl) protons (X denotes the
resonance of C6HxD6-x, which obscures one part of the signal from the
protons H3,5
.
electron oxidation occurs at -0.37 V vs Cp2Fe+/0 26
Spectro-
.
as a ligand (Table 2). A comparison of 1H and 13C NMR data
between [Cp*Ir(dab)Cl](PF6) and Cp*Ir(dab) reveals particularly
large effects for the imine-CH centers, the sizable upfield shifts
∆δH > 2 ppm and ∆δC ≈ 20 ppm confirming the predominant
dab-centered reduction (Table 2). In complex ions [Cp*M-
(dab)X]n+ the arylmethyl groups in the 2,6-dimethylphenyl
substituents are nonequivalent due to restricted rotation.2e
Remarkably, the H4 protons of that substituent also show a clear
splitting in Cp*Ir(dab) (Figure 3), suggesting a deviation from
the symmetric situation found in the solid state; weak axial
interactions with one solvent molecule may be responsible for
this phenomenon.
electrochemistry in acetonitrile showed a bleaching of the intense
431 nm band and the emergence of low-intensity shoulders at
540, 420, and 345 nm which are typical for the complexes
[Cp*M(R-diimine)X]n+ 2a,b
Separate chemical oxidation of
.
Cp*Ir(dab) with ferrocenium hexafluorophosphate produced
similar spectra, and the 1H NMR spectrum suggests the
formation of [Cp*Ir(dab)(CH3CN)]2+ with nonlabile acetonitrile
(22) Go¨rls, H.; Neumu¨ller, B.; Scholz, A.; Scholz, J. Angew. Chem. 1995,
107, 732; Angew. Chem., Int. Ed. Engl. 1995, 34, 673.
(23) Herrmann, W. A.; Denk, M.; Scherer, W.; Klingan, F.-R. J. Orga-
nomet. Chem. 1993, 444, C21.
Ab initio pseudopotential9 calculations were performed for
the neutral and dicationic complexes, employing N-hydrogen
substituents instead of the 2,6-dimethylphenyl groups at the 1,4-
diaza-1,3-butadiene ligand and Cp instead of Cp* (Chart 1).
Considering these changes of substituents, the calculated
structures are in good agreement with the experimental data
for Cp*Ir(dab) and [Cp*Ir(dab)Cl]+. Not unexpectedly, the
(24) (a) Darensbourg, D. J.; Klausmeyer, K. K.; Reibenspies, J. H. Inorg.
Chem. 1995, 34, 4676. (b) For a report on related Cp*Rh(cat) species
see: Espinet, P.; Bailey, P. M.; Maitlis, P. M. J. Chem. Soc., Dalton
Trans. 1979, 1542.
(25) tom Dieck, H.; Haarich, M. J. Organomet. Chem. 1985, 291, 71.
(26) The peak potential difference of 48 mV and the coulometric peak
height of 1.6 charge equiv are also compatible with two very close
lying one-electron processes. The oxidized species exhibits no EPR
signal down to 4 K.

4002 Inorganic Chemistry, Vol. 35, No. 13, 1996
Greulich et al.
Scheme 1
tion B (Scheme 1) involves the kind of six-π-electron “metal-
laaromaticity” that has been reported previously for iridaben-
zene27 and iridathiabenzene compounds.28 Similar, albeit not
identical, situations have been reported before in the 3d series
between CpCo and diazabutadienes25 and, on the basis of
structural results, between CpCo29 or CpNi30 fragments and the
extremely strong π-accepting 1,4-diorganotetraaza-1,3-butadiene
chelate ligands. Cationic complexes between tetraazabutadienes
(“tetrazenes”) and Ir(CO)(PPh3)2 also bear some resemblance
to the species reported here;31 the crystallographically deter-
mined N-N bond length distribution and the Ir-N distances
reported for one derivative also supported the formulation B or
C (cf. Scheme 1).
Figure 4. Ab initio/pseudopotential-calculated contour map of the
redox orbital (HOMO) of CpIr(HNCHCHNH), 0.5 Å above the
IrNCCN plane.
Chart 2
Employing the established propensity of iridium to engage
in efficient dπ/pπ delocalization with light-element π systems27
and the extreme π-donor capacity of Cp*Ir,2b,c we have thus
obtained the 1,4-diaza-1,3-butadiene/transition-metal compound
Cp*Ir(dab), which comes close to the extreme resonance
formulation C in Scheme 1. As in the case of carbene19 and
silylene20 compounds the ene-1,2-diamido chelate ligand system
has shown its capacity to stabilize coordinatively unsaturated
centers. Although Cp*Ir(dab) contains formally a “basic metal”
according to Werner’s concept,32 it remains to be studied
whether the unusual structure described here allows for the
associated kind of reactivity.
replacement of the two π-accepting N-aryl substituents in the
dab ligand and of the five electron-donating methyl groups in
the cyclopentadienide ligand by H appears to equilibrate the
C-N and C-C distances in CpIr(HNCHCHNH); in the actual
structure of Cp*Ir(dab) with a more electron-rich metal and
stronger π-accepting dab ligand the ene-1,2-diamide situation
with a shorter C-C distance and longer C-N bonds is favored.
For the oxidized species, the coordination of negatively charged
chloride to the Lewis acidic metal center in [Cp*Ir(dab)Cl]+
lowers the Ir-N distance relative to the calculated value for
[CpIr(HNCHCHNH)]2+ and attenuates the differences of the
intra-diazabutadiene distances (dCN < dCC).
The highest occupied MO of electron-rich Cp*Ir-
(NHCHCHNH) is shown in Figure 4. It is composed from the
lowest unoccupied π MO of 1,4-diaza-1,3-butadiene6,25 and the
distorted dxy orbital of the metal. A side-view representation
illustrates this kind of overlap in Chart 2.
While the oxidation state formulation IrIII/dab-II (Scheme 1;
C) is thus supported by the calculation, the alternative descrip-
Acknowledgment. This work has been supported by grants
from the Deutsche Forschungsgemeinschaft (DFG), the Volk-
swagenstiftung, the Fonds der Chemischen Industrie, and an
Exchange Program between DFG and the Academy of Sciences
of the Czech Republic.
Supporting Information Available: Tables giving details of X-ray
crystal data collection, anisotropic temperature factors, hydrogen atom
coordinates, and bond lengths and angles and additional views and
stereoviews of [Cp*IrCl(dab)](PF6) and Cp*Ir(dab) (16 pages). Order-
ing information is given on any current masthead page.
IC9514814
(28) Chen, J.; Young, V. G., Jr.; Angelici, R. J. J. Am. Chem. Soc. 1995,
117, 6362.
(29) (a) Maroney, M. J.; Trogler, W. C. J. Am. Chem. Soc. 1984, 106,
4144. (b) Gross, M. E.; Trogler, W. C.; Ibers, J. A. Organometallics
1982, 1, 732. (c) Gross, M. E.; Trogler, W. C. J. Organomet. Chem.
1981, 209, 407.
(30) Overbosch, P.; van Koten, G.; Spek, A. L.; Roelofsen, G.; Duisenberg,
A. J. M. Inorg. Chem. 1982, 21, 3908.
(31) Einstein, F. W. B.; Sutton, D. Inorg. Chem. 1972, 11, 2827.
(32) Werner, H. Angew. Chem. 1983, 95, 932; Angew. Chem., Int. Ed. Engl.
1983, 22, 927.
(27) (a) Bleeke, J. R. Acc. Chem. Res. 1991, 24, 271. (b) Bleeke, J. R.;
Bass, L. A.; Xie, Y.-F.; Chiang, M. Y. J. Am. Chem. Soc. 1992, 114,
4213. (c) Bleeke, J. R.; Behm, R.; Xie, Y.-F.; Clayton, T. W., Jr.;
Robinson, K. D. J. Am. Chem. Soc. 1994, 116, 4093. (d) See also:
Yang, J.; Jones, W. M.; Dixon, J. K.; Allison, N. T. J. Am. Chem.
Soc. 1995, 117, 9776.