Mechanistic studies on the interaction of [(κ3- P, P, P -NP3)IrH3] [NP3 = N(CH2CH 2PPh2)3] with HBF4 and Fluorinated Alcohols by Combined NMR, IR, and DFT Techniques

Article abstract of DOI:10.1021/ic100313j

The novel iridium(III) hydride [(κ³-P,P,P-NP ₃)IrH₃] [NP₃ = N(CH₂CH ₂PPh₂)₃] was synthesized and characterized by spectroscopic methods and X-ray crystallography. Its reactivity with strong (HBF₄) and medium-strength [the fluorinated alcohols 1,1,1-trifluoroethanol (TFE) and 1,1,1,3,3,3-hexafluoroisopropanol (HFIP)] proton donors was investigated through low-temperature IR and multinuclear NMR spectroscopy. In the case of the weak acid TFE, the only species observed in the 190-298 K temperature range was the dihydrogen-bonded adduct between the hydride and the alcohol, while with the stronger acid HBF₄, the proton transfer was complete, giving rise to a new intermediate [(κ³-P,P,P-NP₃)IrH₄]⁺. With a medium-strength acid like HFIP, two different sets of signals for the intermediate species were observed besides dihydrogen bond formation. In all cases, the final reaction product at ambient temperature was found to be the stable dihydride [(κ⁴-NP₃)IrH₂] ⁺, after slow molecular dihydrogen release. The nature of the short-living species was investigated with the help of density functional theory calculations at the M05-2X//6-31++G(df,pd) level of theory.

Full text of DOI:10.1021/ic100313j

Inorg. Chem. 2010, 49, 4343–4354 4343
DOI: 10.1021/ic100313j
Mechanistic Studies on the Interaction of [(K3-P,P,P-NP3)IrH3]
[NP3 = N(CH2CH2PPh2)3] with HBF4 and Fluorinated Alcohols by Combined NMR,
IR, and DFT Techniques
†
‡
,‡
‡
†
,§
ꢀ
´
Andrea Rossin, Evgenii I. Gutsul, Natalia V. Belkova,* Lina M. Epstein, Luca Gonsalvi, Agustı Lledos,*
Konstantin A. Lyssenko,‡ Maurizio Peruzzini,*,† Elena S. Shubina,*,‡ and Fabrizio Zanobini†
†Instituto di Chimica dei Composti Organometallici (ICCOM-CNR), Via Madonna del Piano 10,
50019 Sesto Fiorentino (Firenze), Italy, ‡A. N. Nesmeyanov Institute of Organoelement Compounds,
Russian Academy of Sciences (INEOS-RAS), Vavilov strasse 28, 119991 Moscow, Russian Federation, and
§
´
ꢁ
Departament de Quımica, Universitat Autonoma de Barcelona, Bellaterra, Barcelona, Spain
Received February 15, 2010
The novel iridium(III) hydride [(κ3-P,P,P-NP3)IrH3] [NP3 = N(CH2CH2PPh2)3] was synthesized and characterized by
spectroscopic methods and X-ray crystallography. Its reactivity with strong (HBF4) and medium-strength [the fluorinated
alcohols 1,1,1-trifluoroethanol (TFE) and 1,1,1,3,3,3-hexafluoroisopropanol (HFIP)] proton donors was investigated
through low-temperature IR and multinuclear NMR spectroscopy. In the case of the weak acid TFE, the only species
observed in the 190-298 K temperature range was the dihydrogen-bonded adduct between the hydride and the alcohol,
while with the stronger acid HBF4, the proton transfer was complete, giving rise to a new intermediate [(κ3-P,P,P-
NP3)IrH4]þ. With a medium-strength acid like HFIP, two different sets of signals for the intermediate species were observed
besides dihydrogen bond formation. In all cases, the final reaction product at ambient temperature was found to be the stable
dihydride [(κ4-NP3)IrH2]þ, after slow molecular dihydrogen release. The nature of the short-living species was investigated
with the help of density functional theory calculations at the M05-2X//6-31þþG(df,pd) level of theory.
Introduction
hydrogen bonds.6 Hydride ligands possessing a partly negative
charge, if present, are unusual proton-accepting sites, forming a
Hydrogen bonding and proton transfer to organometallic
complexes, especially to transition-metal hydrides, as proton
acceptors have attracted considerable attention in the recent
decade.1-3 Transition-metal complexes often bear ligands
potentially capable of acting as proton donors or proton
acceptors in hydrogen bonding. These functionalities are used
to fine-tune the complex properties in catalysis or in molecular
recognition and supramolecular assembly design.4,5 If formed,
the hydrogen bonds with these ligands would be qualitatively
similar to those in traditional organic hydrogen complexes. On
the other hand, core transition metals are able to accept a
so-called dihydrogen bond (DHB), M-H HA.1,4 These two
3 3 3
types of hydrogen bonding, M HA and M-H HA, are
3 3 3
3 3 3
unique for transition-metal (hydride) complexes, and the terms
“nonclassical” or “unconventional” hydrogen bonding have
been coined to address these interactions.2 Thus, the dichotomy
between different sites of proton donor attacks, classical and
nonclassical, could arise for transition-metal complexes.
Another important aspect of a proton donor interaction
with a transition-metal hydride is the structure of the dihy-
drogen-bonded complex formed. Our most recent study of
hydrogen bonding with the hydride complexes [Cp*MH3-
(dppe)] [M = Mo, W; dppe =1,2-bis(diphenylphosphino)-
ethane; Cp* = η5-C5Me5] revealed the delicate balance of the
hydrogen bond through their d lone pairs, forming M HA
3 3 3
*To whom correspondence should be addressed. E-mail: nataliabelk@
ineos.ac.ru (N.V.B.), agusti@klingon.uab.es (A.L.), mperuzzini@iccom.cnr.it
(M.P.), shu@ineos.ac.ru (E.S.S.).
(1) Belkova, N. V.; Shubina, E. S.; Epstein, L. M. Acc. Chem. Res. 2005,
38, 624.
M-H HA and M HA interactions within the same
3 3 3
3 3 3
hydrogen complex (Scheme 1).7,8
The prevalence of either of the interactions apparently
determines the hydride reactivity: M-H HA bonding
3 3 3
(2) Peruzzini, M.; Poli, R. Recent Advances in Hydride Chemistry; Elsevier
SA: Amsterdam, The Netherlands, 2001.
(7) Belkova, N. V.; Besora, M.; Baya, M.; Dub, P. A.; Epstein, L. M.;
ꢀ
(3) Besora, M.; Lledos, A.; Maseras, F. Chem. Soc. Rev. 2009, 38, 957.
ꢀ
Lledos, A.; Poli, R.; Revin, P. O.; Shubina, E. S. Chem.;Eur. J. 2008, 14,
(4) Brammer, L. Dalton Trans. 2003, 3145.
(5) Das, S.; Brudvig, G. W.; Crabtree, R. H. Chem. Commun. 2008, 413.
(6) (a) Shubina, E. S.; Belkova, N. V.; Epstein, L. M. J. Organomet. Chem.
1997, 536-537, 17. (b) Epstein, L. M.; Shubina, E. S. Coord. Chem. Rev. 2002,
231, 165.
9921.
(8) Belkova, N. V.; Revin, P. O.; Besora, M.; Baya, M.; Epstein, L. M.;
ꢀ
Lledos, A.; Poli, R.; Shubina, E. S.; Vorontsov, E. V. Eur. J. Inorg. Chem.
2006, 2192.
r
2010 American Chemical Society
Published on Web 04/08/2010
pubs.acs.org/IC

4344 Inorganic Chemistry, Vol. 49, No. 9, 2010
Rossin et al.
Scheme 1. Simplified Scheme of the Transition-Metal Hydride-Acid
Interaction
techniques. 1,1,1,3,3,3-Hexafluoroisopropanol (HFIP), 1,1,1-
trifluoroethanol (TFE), HBF4 OMe2 (1:1 solution in OMe2),
3
and HOTf (OTf=trifluoromethanesulfonate, OSO2CF3) were
used as purchased (Aldrich), without further purification.
Deuterated solvents (Aldrich) were degassed by three freeze-
pump-thaw cycles before use. NMR spectra were recorded on
a Bruker AVANCE II 300 spectrometer, equipped with a low-
temperature measurement tool. 1H NMR chemical shifts are
reported in parts per million (ppm) downfield of tetramethyl-
silane and were calibrated against the residual protiated reso-
nance of the deuterated solvent, while 31P{1H} and 31P NMR
were referenced to 85% H3PO4 with the downfield shift taken
prevails for molybdenum, entailing [Cp*Mo(η2-H2)H2-
(dppe)]þ complex formation, whereas M HA bonding wins
3 3 3
in the case of electron richer tungsten, where proton transfer
yields directly the classical tetrahydride product [Cp*WH4-
(dppe)]þ. In the search for other experimental examples for
such interactions with hydrides of the third-row transition
metals, we carried out synthesis and protonation studies of
the iridium hydride [(κ3-P,P,P-NP3)IrH3] [3; NP3 = N(CH2-
CH2PPh2)3] presented herein. The potentially tetradentate
aminotriphosphane tripodal ligand N(CH2CH2PPh2)3 offers
diversity of coordination geometries because it may act as κ4,
κ3-P3, κ3-N,P,P, and κ2-P,P ligands.9-11 The presence of the
amine N donor and of two CH2 spacers separating the P donors
from the bridgehead atom makes NP3 more flexible than other
tripodal polyphosphines, e.g., MeC(CH2PPh2)3, and also in-
creases the overall basicity at the metal center.9,10,12 In addition
to the hydride ligand and the metal, the bridgehead N atom of
NP3 is also a potential protonation site, although examples of
N-protonation are almost unknown in transition-metal com-
plexes with this tripodal ligand.13 A different behavior has been
observed for other aminophosphines. Thus, protonation stu-
dies of ruthenium half-sandwich hydride complexes [CpRRuH-
(P,N)] (CpR = Cp, Cp*) with bidentate P,N-aminophosphane
ligands have shown that when stoichiometric amounts of HBF4
are used, only the hydride is protonated, yielding the dihydro-
gen tautomer [CpRRu(η2-H2)(P,N)]þ at low temperatures and
dihydride tautomer [CpRRu(H)2(P,N)]þ upon warming. In
contrast, reaction with an excess of HBF4 affords the dicationic
dihydride complexes [CpRRu(H)2(P,N-H)]2þ from proton-
ation of both the hydride and the N lone pair.14 The presence of
three hydrides and a third-row transition metal and the
coordinative flexibility of the aminophosphane ligand make 3
a suitable system for exploring its interaction with proton
donors of different strength.
as positive. 11B NMR was referenced to BF3 OEt2. The IR
3
spectra were recorded on an FT Infralum-801 spectrometer.
All measurements were carried out by use of a home-modified
cryostat(Carl Zeiss Jena) inthe 190-290K temperature range.
The cryostat modification allows operation under an inert
atmosphere and transfer of the reagents (premixed either at
low or room temperature) directly into the cell precooled to the
required temperature. The accuracy of the temperature adjust-
ment was (1 K.
Synthesis of [(K4-NP3)IrCl] (1). A total of 335 mg (0.5 mmol)
of [Ir(COD)Cl]2 was dissolved in 35 mL of THF. A total of
653 mg of solid NP3 (1 mmol) was added to this solution under
stirring, and the mixture was stirred at room temperature, until a
crystalline brick-red powder started to precipitate out. The
supernatant was then filtered off, and the solid residue was
washed with fresh THF (2 ꢀ 10 mL) and n-pentane (2 ꢀ 10 mL)
and dried under a nitrogen stream. Yield: 80%. Anal. Calcd for
C42H42ClIrNP3: C, 57.23; H, 4.80; N, 1.59. Found: C, 57.06; H,
4.82; N, 1.52. The compound is insoluble in all solvents; there-
fore, no NMR characterization could be obtained.
Synthesis of [(K4-NP3)Ir(H)Cl]BPh4 (2). A total of 100 mg of 1
(0.1 mmol) was suspended in 20 mL of THF. A total of 50 mg
(0.3 mmol) of pure HOTf was added to this suspension at room
temperature under stirring, and the solid slowly started to dissolve
to give a colorless solution. The mixture was stirred until complete
dissolution of the reagent. At this stage, 200 mg of NaBPh4 (0.6
mmol) dissolved in 15 mL of EtOH was then added to the initial
mixture, and the final solution was concentrated under a nitrogen
stream until a white precipitate of 2 formed. The supernatant was
filtered off, and the solid was washed with fresh EtOH (2 ꢀ 10 mL)
and n-pentane (2 ꢀ 10 mL) and dried under a nitrogen current.
Yield: 78%. IR (Nujol, cm-1): ν(Ir-H) 2100 s. 1H NMR (300.13
MHz, ppm, CDCl3): -10.34 (Ir-H, dt, 2JH-P(trans) = 149.8 Hz;
2JH-P(cis) = 15.2 Hz). 31P{1H} NMR (121.49 MHz, ppm, AM2
spin system, CDCl3): 1.33 (PM, d, 2JP-P = 13.5 Hz), -9.45 (PA, t).
11B NMR (96.29 MHz, ppm, CDCl3): -6.6 (br s). Anal. Calcd for
C66H63BClIrNP3: C, 65.96; H, 5.20; N, 1.16. Found: C, 65.48; H,
5.65; N, 1.06.
Experimental Section
All reactions were performed using the standard Schlenk
procedures under a dry nitrogen or argon atmosphere, unless
specified. The NP3 ligand15 and [Ir(COD)Cl]216 (COD = 1,5-
cyclooctadiene) were prepared according to published proce-
dures. All solvents were purified by standard distillation
Synthesis of [(K3-P,P,P-NP3)IrH3] (3). A total of 400 mg of 2
(0.3 mmol) was dissolved in 40 mL of THF. At room tempera-
ture, 400 mg of solid LiAlH4 (10 mmol) is slowly added to this
solution under vigorous stirring. When the addition was com-
plete, the mixture was gently heated to the boiling point and then
refluxed for 3.5 h. The reaction flask was then cooled at room
temperature, and 11 mL of a H2O/THF mixture (1:10) was
carefully added in small portions in order to hydrolyze the excess
of LiAlH4. The supernatant was then filtered off, and the
resulting clear solution was concentrated under a brisk stream
of nitrogen. Further EtOH addition and concentration under
nitrogen caused an off-white powder of 3 to precipitate from the
solution. The crude compound was collected by filtration and
washed with EtOH (2 ꢀ 5 mL) and n-pentane (10 mL) before
being recrystallized from THF/EtOH (1:1). Yield: 72%. IR
(9) (a) Sacconi, L.; Mani, F. Transition Met. Chem. (N.Y.) 1982, 8, 214.
(b) Sacconi, L.; Bertini, I. J. Am. Chem. Soc. 1968, 90, 5443. (c) Sacconi, L.
Coord. Chem. Rev. 1972, 8, 351. (d) Morassi, R.; Bertini, I.; Sacconi, L. Coord.
Chem. Rev. 1973, 11, 343. (e) Bertolasi, V.; Bianchini, C.; de los Ríos, I.;
Marvelli, L.; Peruzzini, M.; Rossi, R. Inorg. Chim. Acta 2002, 327, 140–146.
(10) Mealli, C.; Ghilardi, C. A.; Orlandini, A. Coord. Chem. Rev. 1992,
120, 361.
ꢀ
(11) Palacios, M. D.; Puerta., M. C.; Valerga, P.; Lledos, A.; Veilly, E.
Inorg. Chem. 2007, 46, 6958.
(12) Bianchini, C.; Meli, A.; Peruzzini, M.; Vizza, F.; Zanobini, F. Coord.
Chem. Rev. 1992, 120, 193.
(13) A unique example of protonation at the N atom in NP3 metal
complexes has been reported. See: Cecconi, F.; Ghilardi, C. A.; Innocenti,
P.; Mealli, C.; Midollini, S.; Orlandini, A. Inorg. Chem. 1984, 23, 922.
1
(CH2Cl2, 290 K, cm-1): ν(Ir-H) 2040 (s). H NMR (300.13
ꢀ
(14) Jimenez-Tenorio, M.; Palacios, M. D.; Puerta, M. C.; Valerga, P.
MHz, ppm, CD2Cl2): -12.24 (Ir-H, m, AA0A00XX0X00 spin
system), 2.11 [CH2 close to a P atom, s(br)], 2.97 [CH2 close to a
N atom, s(br)], 7.19-7.68 (aromatic CH, m). 31P{1H} NMR
Inorg. Chem. 2007, 46, 1001.
(15) Sacconi, L.; Bertini, I. J. Am. Chem. Soc. 1968, 90, 5443.
(16) Herde, J. L.; Lambert, J. C.; Senoff, C. V. Inorg. Synth. 1974, 15, 18.

Article
Inorganic Chemistry, Vol. 49, No. 9, 2010 4345
Table 1. Crystal Data and Structure Refinement Parameters for 3 and 5
to be applied using multiple measurements of equivalent reflec-
tions with the Bruker program SADABS. The structures were
solved by direct methods and refined by the full-matrix least-
squares technique against F2 in the anisotropic-isotropic ap-
proximation. The hydrides were located from the Fourier
density synthesis and refined within a riding model with fixed
3
5
CCDC
formula
fw
713236
C42H45IrNP3
848.90
713237
C42 50H45BClF4IrNP3
977.17
.
˚
T
100
100
Ir-H distances equal to 1.640 A, according to neutron data
coming from the iridium complex fac-IrH3(PPh2Me)3.17 The
positions of all of the other H atoms of the NP3 ligand were
located geometrically, and their thermal factors were related to
the heavier atoms that they are bound to. Analysis of the Fourier
density synthesis revealed that the BF4- anion in 5 is disordered
by two positions with occupancies of 0.2 and 0.8. The disorder
on the N atom of NP3 in 1 (see the Supporting Information) was
not explicitly treated because no significant improvement of the
R factors was observed. All calculations were performed using
the SHELXTL software.18 The crystallographic data for 1, 3,
and 5 have been deposited with the Cambridge Crystallographic
Data Centre (CCDC nos. 769848, 713236, and 713237). The
coordinates can be obtained, upon request, from the Director,
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, U.K.
cryst syst
space group, Z
monoclinic
P21/c, 4
10.3882(8)
8.3745(6)
41.369(2)
90.523(3)
3598.8(4)
1.567
monoclinic
P21/n, 4
9.8444(10)
27.254(4)
15.109(2)
105.211(3)
3911.6(9)
1.659
˚
a (A)
˚
b (A)
˚
c (A)
β (deg)
3
V (A )
˚
Dcalc (g cm-1
)
μ (cm-1
F(000)
)
38.75
1704
36.56
1948
θ range (deg)
no. of reflns measd
no. of indep reflns (Rint
58
76 022
9568 (0.0495)
8697
436
58
63 497
10 415 (0.0468)
8825
493
)
no. of obsd reflns [I>2σ(I)]
no. of param
final R(Fhkl):
R1
0.0355
0.0666
1.081
1.339/-3.240
0.0363
0.0965
0.996
2.482/-1.896
wR2
GOF
ΔFmax, ΔFmin (e A
Computational Details. All of the calculations were per-
formed with the Gaussian03 software package19 at the density
functional theory (DFT)/M05-2X level.20 In order to have
reasonable computational times, a model system obtained by
replacing the phenyl groups on NP3 with H atoms was used
instead of the real molecule. Thus, all of the model complexes
and intermediates contain the N(CH2CH2PH2)3 ligand, and
they are indicated with the apex “t” on the corresponding
numbers throughout the text. In the basis set employed for the
geometry optimization procedure (BS1), core electrons of the Ir
and P atoms were described using the pseudopotentials of
Hay-Wadt,21 and their valence electrons were expressed
through a LANL2DZ basis set.21 An extra p-type polarization
function for the P atom and an extra f-type function for the Ir
atom were added to the standard set.22 A 6-31þG(d,p) basis set
was used on the hydride ligands, while a 6-31G basis set was
chosen for all of the other atoms. On the optimized structures,
frequency calculations with a more extended basis set [BS2:
same basis set on Ir and P atoms, 6-31þþG(df,pd) on the
hydride ligands, and 6-31þG(d,p) on all of the other atoms]
were performed to calculate zero-point energies, enthalpies,
-3
˚
)
(121.49 MHz, ppm, CD2Cl2): -12.12 (s). Anal. Calcd for
C42H45IrNP3: C, 59.42; H, 5.34; N, 1.65. Found: C, 59.76; H,
5.84; N, 1.55.
Synthesis of [(K4-NP3)IrH2] BF4 (5). To 200 mg (0.2 mmol)
of 3 suspended in 25 mL of THF was added via syringe 40 μL
(0.3 mmol) of HBF4 OMe2, causing slow dissolution of the
3
starting trihydride after gentle warming of the solution with a
water bath at 40 °C. The evolution of gaseous dihydrogen was
also observed during the reaction. Concentration of the result-
ing clear solution under a stream of nitrogen gave a white
crystalline precipitate of 5, which was filtered and washed with
degassed EtOH (2 ꢀ 10 mL) and n-pentane (2 ꢀ 10 mL) before
being dried under nitrogen. Yield: 85%. IR (Nujol, cm-1):
ν(Ir-H) 2071 (s). 1H NMR (300.13 MHz, ppm, CD2Cl2):
2
-9.57 (Ir-H, H trans to P, br d, JH-P(trans) = 142.8 Hz),
-18.50 (Ir-H, H trans to N, br s). 31P{1H} NMR (121.49
MHz, ppm, CD2Cl2): 18.50 (P trans to H, t, 2JP-P = 13.3 Hz),
20.11 (P trans to P, d). 11B NMR (96.29 MHz, ppm, CD2Cl2):
-0.64 (s). Anal. Calcd for C42H44BF4IrNP3: C, 67.91; H, 5.52;
N, 1.20. Found: C, 67.74; H, 5.79; N, 1.14.
(17) Bau, R.; Schwerdtfeger, C. J.; Garlaschelli, L.; Koetzle, T. F.
J. Chem. Soc., Dalton Trans. 1993, 3359.
(18) SHELXTL, version 6.1; Bruker AXS Inc.: Madison, WI, 2005.
(19) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Montgomery, J. A. J.; Vreven, T.; Kudin, K. N.;
Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.;
Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.;
Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa,
J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li,
X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo, C.; Jaramillo, J.;
Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.;
Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.;
Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A.
D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari,
K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.;
Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.;
Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng,
C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.;
Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, revision
E.01; Gaussian, Inc.: Wallingford, CT, 2004.
Variable-Temperature NMR Experiments on Hydride Proton-
ation with HBF4, HFIP, and TFE. A screw-cap NMR tube was
loaded with 50 mg of 3(0.06 mmol) under an inert atmosphere, and
then 1 mL of dry and degassed CD2Cl2 was transferred into the
tube via a cannula, under nitrogen. The suspension obtained was
1
1
first used to record the 31P{1H}, H, 11B, and H{31P} NMR
spectra of the starting material at variable temperatures, by cool-
ing of the sample in 20° steps from ambient conditions (300 K) to
1
190 K. The H{31P} T1 values of 3 were also measured via the
inversion-recovery sequence implemented on the software of
the Bruker DRX spectrometer. A total of 7 μL of HBF4 OMe2
3
(0.06 mmol) was syringed into this suspension kept at 195 K in a
dry ice-acetone bath, and immediate dissolution was observed.
The clear mixture was then transferred back into the NMR
spectrometer (still at 190 K) and warmed stepwise to room
temperature with the same procedure as that above. A new set of
(20) Zhao, Y.; Schultz, N. E.; Truhlar, D. G. J. Chem. Theor. Comput.
2006, 2, 364.
1
multinuclear NMR and H{31P} T1 data were recorded during
(21) (a) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270. (b) Wadt, W.
R.; Hay, P. J. J. Chem. Phys. 1985, 82, 284.
warming and following the reaction course. The reaction of 3 with
TFE or HFIP was carried out and monitored in the same manner.
Crystallographic Studies. All diffraction data were taken
using a Bruker SMART APEX II CCD diffractometer [λ(Mo
KR) =0.710 72 A, ω scans; see Table 1]. The substantial
redundancy in the data allows empirical absorption correction
€
€
(22) (a) Hollwarth, A.; Bohme, M.; Dapprich, S.; Ehlers, A. W.; Gobbi,
€
A.; Jonas, V.; Kohler, K. F.; Stegmann, R.; Veldkamp, A.; Frenking, G.
€
Chem. Phys. Lett. 1993, 208, 237. (b) Ehlers, A. W.; Bohme, M.; Dapprich, S.;
˚
€
€
Gobbi, A.; Hollwarth, A.; Jonas, V.; Kohler, K. F.; Stegmann, R.; Veldkamp, A.;
Frenking, G. Chem. Phys. Lett. 1993, 208, 111.

4346 Inorganic Chemistry, Vol. 49, No. 9, 2010
Rossin et al.
Scheme 2
Figure 1. Molecular structure of 3 (50% probability level). All of the H
atoms, apart the hydride ligands, are omitted for clarity. Selected bond
lengths and angles are given in Table 2.
Protonation of 1 with a strong acid like HOTf in THF
at room temperature, followed by triflate/tetraphenylbo-
rate anion exchange, leads to formation of the iridium-
(III) cationic species 2, in good yield. Compound 2 shows
an octahedral coordination geometry at iridium, with the
NP3 ligand being again tetradentate, as evidenced by the
31P{1H} NMR spectrum, where two different phosphorus
resonances can be detected, with a 2:1 intensity ratio
entropies, and gas-phase Gibbs energies at 298 K more accu-
rately. Finally, a continuum modeling of the reaction medium
was also included in the computational treatment, using BS2 on
all of the BS1-level optimized structures. Bulk solvent effects
(CH2Cl2, ε = 8.93) were expressed through the polarizable
continuum model (PCM-UA0 solvation spheres).23 Individual
solvation cavities were added on the H atoms of the nonclassical
η2-H2 ligand, when present.
1
(AM2 spin system). Turning off the H decoupler in the
Results and Discussion
31P NMR spectrum clearly splits the phosphorus reso-
nance at -9.45 ppm (2JP-H(trans) = 149.8 Hz), then
confirming that protonation takes place with complete
stereoselectivity at the metal as the proton atom is
exclusively delivered to the equatorial position cis to the
N atom (see Scheme 2). In keeping with this stereochemi-
cal evidence, the 1H NMR spectrum shows two tempera-
ture-invariant high-field triplets with similar separation
that collapse to a singlet (δ=-10.34 ppm) after decou-
pling the PA resonance [1H{31PA}sel experiment]. Treat-
ment of 2 with a strong excess of LiAlH4 in THF gives,
after workup, off-white crystals of the neutral trihydride
complex 3 in excellent yield. The iridium(III) trihydrido
complex 3 is air-stable in the solid state and does not
decompose in a THF solution, from which very pale-
yellow crystals suitable for an X-ray diffraction analysis
were grown, after standing overnight in the presence of
EtOH.
A thermal ellipsoid representation of the solid-state
structure of 3, along with atomic numbering schemes, is
˚
depicted in Figure 1, whereas selected bond distances (A)
for 3 and 5 are provided in Table 2.
According to the single-crystal X-ray diffraction ana-
lysis, the Ir atom in 3 adopts a pseudooctahedral coordi-
nation geometry formed by the three P atoms of the
tripodal ligand and three hydrides, without the N atom
of the NP3 ligand entering the metal coordination sphere
Synthesis and Characterization of Compounds 1-3. The
iridium NP3 derivatives described in this paper (see
Scheme 2) were prepared starting from the chloro
compound 1, obtained by the straightforward substitu-
tion of the labile COD ligand in [Ir(COD)Cl]2 by the
tripodal tetradentate ligand NP3. Complex 1 was ob-
tained as brick-red microcrystalline material air stable
in the solid state. 1 is insoluble in both polar and apolar
solvents; it shows a little solubility only in dichloro-
methane, where it decomposes in a short time. Conse-
quently, any spectroscopic characterization in solution
was precluded. As expected from the coordination
abilities of tripodal polyphosphines, 1 exhibits a trigo-
nal bipyramidal structure, as confirmed by indepen-
dent crystallographic results.24 The complex has a C3
symmetry axis coincident with the N-Ir-Cl direction
(Figure S1 and Tables S1-S2 in the Supporting In-
formation); its hexagonal space group (P63) is chiral,
with the optical activity being generated by the “pro-
peller-like” orientation of the three NP3 arms when they
bind the iridium center. An interesting feature is the
strong disorder of the N atom along the N-Ir direction,
even at 150 K, which can be seen as proof of the coordi-
nation “flexibility” of the tripodal phosphine (easy κ4 T
κ3 coordination mode switch).
˚
(23) (a) Miertus, S.; Scrocco, E.; Tomasi, J. J. Chem. Phys. 1981, 55, 117.
(b) Barone, V.; Cossi, M.; Tomasi, J. J. Chem. Phys. 1997, 107, 3210.
(24) Complex 1 shares its geometric properties with the related tripodal
polyphosphine complex [(PP3)IrCl]. See: Bianchini, C.; Peruzzini, M.;
Zanobini, F. J. Organomet. Chem. 1987, 326, C79.
(Figure 1). The Ir(1) N(1) separation in 3 is 3.518(1) A,
with the N(1) atom deviating from the plane of the C(2),
3 3 3
˚
C(4), and C(6) atoms by 0.32 A, pointing toward the
metal. The observed pyramidalization of the N(1) atom is

Article
Inorganic Chemistry, Vol. 49, No. 9, 2010 4347
a
˚
Table 2. - Selected Bond Distances (A) and Angles (deg) for Complexes 3 and 5
complex 3
complex 5
Ir(1)-N(1) = 2.217(4)
Ir(1)-P(1) = 2.3257(11)
Ir(1)-P(2) = 2.3073(11)
Ir(1)-P(3) = 2.2745(11)
N(1)-C(2) = 1.513(6)
N(1)-C(4) = 1.505(5)
N(1)-C(6) = 1.512(5)
N(1)-Ir(1)-P(9) = 85.35(10)
N(1)-Ir(1)-P(10) = 84.00(10)
P(9)-Ir(1)-P(10) = 157.64(4)
N(1)-Ir(1)-P(8) = 86.49(11)
P(9)-Ir(1)-P(8) = 102.69(4)
P(10)-Ir(1)-P(8) = 96.22(4)
Ir(1)-P(1) = 2.3403(9)
Ir(1)-P(2) = 2.3162(9)
Ir(1)-P(3) = 2.3362(9)
N(1)-C(2) = 1.449(5)
N(1)-C(4) = 1.455(5)
N(1)-C(6) = 1.464(5)
P(9)-Ir(1)-P(4) = 99.98(3)
P(9)-Ir(1)-P(10) = 99.93(3)
P(4)-Ir(1)-P(10) = 99.61(3)
a Refer to Figures 1 and 3 for atom numbering.
similar to that observed in the known κ3-P,P,P-NP3
Figure 2. IR spectra (CH2Cl2, νIr-H region) of 3 (0.006 M, a) in the
presence of ca. 1.5 equiv of HBF4 at 190 K (b) and 250 K (c) with the
intermediate spectra at 210 and 230 K. (d) The spectrum of 5 (0.006 M) at
298 K is given for comparison.
˚
complexes (0.29-0.4 A), as are the N-C distances (of
ca. 1.45 A).
25
˚
The Ir(1)-P bonds are merely equal and slightly longer
than those found in the related complex fac-[IrH3-
17
(PPh2Me)3] (2.308 A). There are short (less than the
while a singlet at -12.1 ppm is observed in the 31P{1H}
˚
1
NMR spectrum. The H{31P} NMR spectrum clearly
˚
sum of van der Waals radii, 2.4 A) intramolecular con-
tacts between phenyl CH groups and hydride ligands
reveals that the hydride signal multiplicity is due to P
atoms only, ruling out any magnetic inequivalence of the
three hydrido H atoms because a singlet is obtained when
the 31P coupling is turned off. Complex 3 is stereochemi-
cally rigid on the NMR time scale; neither the 31P NMR
˚
[H(1M) H(32A) = 2.07 A, H(2M) H(8A) = 1.97 A,
˚
3 3 3
3 3 3
˚
and H(3M) H(30A) = 2.14 A]. The crystal packing
3 3 3
analysis revealed also that one of the hydride atoms of 3
is at a short distance to the CH2 group: the H(3M) H(6A)
˚
separation is equal to 2.07 A (with normalization of the
1
3 3 3
spectrum nor the H NMR spectrum shows significant
changes over the temperature window of dichloromethane
except for the slight drift of the signals with temperature
(δIr-H going from -12.2 to -12.4 ppm and δP going from
-12.1 to -12.3 ppm at 298 and 190 K, respectively).
The recorded values of the spin-lattice T1 relaxation time
(300 MHz, CD2Cl2) are typical of a classical polyhydride
falling in the 200-450 ms range,27 with a T1,min value of ca.
200 ms (at 223 K).
C-H bond to the ideal distance), and the corresponding
CH H and IrH H angles are equal to 128 and 175°,
3 3 3
3 3 3
respectively. Another close contact between the two
adjacent molecules via phenyl CH pointing toward the
hydride ligand is also present; CH H(Ir) distance =
3 3 3
˚
2.89 A, angle=169°. On the basis of geometrical para-
meters, we can suggest that these contacts correspond to
weak intermolecular H H hydrogen bonds.
Complex 3 has C3v symmetry, which should give three
3 3 3
Protonation of 3 with Strong Protic Acids: HBF4.
Protonation of 3 in dichloromethane by the strong acid
HBF4 OMe2 was followed by variable-temperature IR
ν
Ir-H vibrations in the IR spectrum.26 Indeed, three νIr-H
bands are observed (at 2022 and 2036 cm-1 with a
shoulder at 2055 cm-1) in the solid-state spectrum. This
3
and NMR spectroscopies. Upon the low-temperature
(190 K) addition of a slight excess of HBF4 OMe2, the
3
splitting is lost in solution, where the broad (Δν1/2
=
νIr-H band of 3 disappears and a new band appears at
2062 cm-1 with a shoulder at 2050 cm-1 (Figure 2). Such a
high-frequency shift (ΔνIr-H = 37 cm-1) is in line with
the formation of a new cationic polyhydrido species [(κ3-
P,P,P-NP3)IrH4]þ (4). No evidence of N-protonation on
the 1H NMR spectra was found, even after the addition of
a large excess of acid (to a final 4:1 acid-to-complex
stoichiometric ratio). This is probably due to both the
“spatial shielding” offered by the dangling NP3 arms and
the particular N lone-pair orientation, pointing inward in
the direction of the metal center and not outward (where
it would be more exposed to electrophilic attacks).10,13
To establish the precise chemical nature of the new com-
plex 4, the reaction was also monitored by 1H and 31P{1H}
NMR spectroscopies at variable temperature. In agreement
with the IR analysis, NMR signals of a new species were
observed upon the addition of less than 1 equiv of HBF4 to a
CD2Cl2 solution of 3 at 190 K, besides those of the starting
56 cm-1 at 290 K) asymmetric νIr-H band is observed at
2040 cm-1 (εIr-H = 570 L mol-1 cm-1) in CH2Cl2. The
position (νIr-H), half-height at full width (Δν1/2), and
extinction coefficient (εIr-H) for this band are tempera-
ture-dependent, being, at 190 K, 2025 cm-1, 54 cm-1, and
700 L mol-1 cm-1, correspondingly. In other words, the
band ascribed to the Ir-H stretching becomes more
intense and shifts to lower frequency upon cooling
(Figure S2 in the Supporting Information) as observed
for other transition-metal hydrides [see the references in
the Introduction section]. At room temperature, hydride
3 exhibits an AA0A00XX00X00 second-order 1H NMR
resonance in the hydride region centered at -12.2 ppm,
(25) Examples of κ3-P,P,P-NP3 complexes characterized through X-ray
diffraction include: (a) Dapporto, P.; Midollini, S.; Sacconi, L. Angew.
Chem., Int. Ed. 1979, 18, 469. (b) Bianchini, C.; Meli, A.; Peruzzini, M.; Vizza,
F.; Bachechi, F. Organometallics 1991, 10, 820. (c) Ghilardi, C. A.; Innocenti, P.;
Midollini, S.; Orlandini, A.; Vacca, A. J. Chem. Soc., Chem. Commun. 1992,
1691. (d) Barbaro, P.; Cecconi, F.; Ghilardi, C. A.; Midollini, S.; Orlandini, A.;
Vacca, A. Inorg. Chem. 1994, 33, 6163.
1
material. In the H NMR spectrum, a new doublet of
(26) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Co-
ordination Compounds, 5th ed.; John Wiley & Sons Inc.: New York, 1997.
(27) Bakhmutov, V. I. Practical NMR Relaxation for Chemists; Wiley-
VCH: New York, 2004.

4348 Inorganic Chemistry, Vol. 49, No. 9, 2010
Rossin et al.
triplets, centered at -11.1 ppm (2JH-P(trans) = 121.5 Hz,
2JH-P(cis) = 17.5 Hz) comes out in the hydride region, while
a new singlet appears in the 31P{1H} NMR spectrum (δ =
-22.7 ppm). The 1H{31P} NMR signal exhibits a high-field
singlet, thus indicating exclusive coupling with P atoms. The
occurrence of a temperature-invariant singlet resonance in
the 31P NMR spectrum asks for the rapid H-atom exchange
in 4, while the magnetic nonequivalence of the P atoms is
preserved during the scrambling process and, consequently,
mirrored in the 1H NMR spectrum. The T1 values measured
for this new compound are in the 200 ms to 1.5 s range, with
the T1,min value of 200 ms at 243 K. Notably, the tempera-
ture corresponding to T1,min increases upon protonation
from 223 to 243 K, whereas the T1,min value remains the
same. This phenomenon agrees with a decreased correlation
time (τC), related to a slower tumbling motion for the
protonated species, as expected from its larger size.27 Thus,
this new species can be formulated as the cationic tetrahy-
drido complex 4 featuring a κ3-coordination of the tripodal
aminophosphine and a pentagonal-bipyramidal-coordina-
tion polyhedron around the metal. This geometrical assign-
ment is supported by a comparison of the different NMR
behavior of 4 with that of the known classical rhenium
tetrahydride [(κ4-NP3)ReH4]þ, for which a temperature-
dependent A3 f A2M f AMX change of the 31P{1H}
NMR spectrum was observed to decrease with the tempera-
ture and a distorted dodecahedral eight-coordinated geo-
metry was authenticated in the solid state by X-ray crys-
tallography.28
Warming the solution of 4 initiates a subsequent slow
transformation accompanied by dihydrogen evolution
(evident from the appearance of the characteristic singlet
resonance at 4.6 ppm). The transformation of 4 into the
dihydrido complex 5 completes at 293 K when no residual
resonances of either the tetrahydride or the starting material
3 are still present in the NMR spectrum. Complex 5 fea-
tures an AM2 31P{1H} NMR pattern (δA = 18.5 ppm, t,
2JP-P = 13.3 Hz; δM = 20.1 ppm, d) and two distinct hydride
resonances (δH =-9.6 ppm, br d, 2JHP(trans to H) =142.8Hz;
δH = -18.5 ppm, br s). Interestingly, discernible couplings
between the (chemically and magnetically) nonequivalent H
and P atoms could not be observed when HBF4 is used to
protonate 4. A much better spectral resolution was observed
when weaker acids, such as HFIP, are used (vide infra). The
IR spectra confirmed the NMR data: complex 5 generated
in situ by the reaction of 3 with HBF4 features a sharp νIr-H
band at 2071 cm-1 (ε= 480 L mol-1 cm-1), which coincides
with that of the isolated crystalline [(κ4-NP3)IrH2]BF4 salt
isolated as off-white crystals from a bulk preparative reac-
tion carried out in CH2Cl2. The presence of a κ4-coordinated
ligand in 5 is also indirect proof of the aforementioned
absence of N-atom protonation, which could not coordinate
to the iridium center if it had previously reacted with Hþ.
The structure of 5 was unambiguously identified through
single-crystal X-ray analysis carried out on crystalline sam-
ples of the compound prepared via an independent synthesis.
Complex 5 also features the pseudooctahedral coordination
geometry around iridium formed by all four donor atoms of
the tripodal ligand and two cis-disposed hydrides (Figure 3
Figure 3. Molecular structure of the complex cation in 5 (50% prob-
ability level). All of the H atoms, apart from the hydride ligands, are
omitted for clarity. Selected bond lengths and angles are given in Table 2.
and Table 2). The Ir(1)-N(1) bond in 5 is almost equal to
the corresponding value in the [(κ4-NP3)IrH(σ-C8H11)]þ
cation.29 Participation of the N atom in the coordination
leads to some elongation of the N-C bonds from 1.449-
˚
˚
(5)-1.465(5) A in 3 to 1.505(5)-1.513(6) A in 5 as well as to
an increase of the N-atom pyramidalization. Deviation of
the N(1) atom from the plane of the C(2), C(4), and C(6)
˚
˚
atoms becomes as much as 0.53 A (vs 0.32 A in 3). The
Ir(1)-P(2) and Ir(1)-P(3) bonds are slightly shortened to
2.274(1) and 2.307(1) A. Finally, there is an extremely short
˚
intramolecular C-H H-Ir separation between the H
3 3 3
atom of the phenyl group and the hydride ligand, with the
˚
H(3M) H(20A) distance being of 1.90 A and C-H
H
3 3 3
3 3 3
and Ir-H H angles being 128 and 113°, respectively.
3 3 3
Following our well-developed approach,7,8,28,30 we de-
cided to study the reaction mechanism of proton transfer
to the trihydride 3 by using weaker proton donors, i.e.,
fluorinated alcohols of variable strength such as TFE and
HFIP. These alcohols should indeed offer better reaction
kinetics control with respect to stronger acids like HBF4.
Reaction of 3 with TFE. As can be expected, interac-
tion of hydride 3 with the least acidic TFE (pKa=12.5) did
not lead to proton transfer. The addition of 5-20 equiv
(29) Bianchini, C.; Masi, D.; Meli, A.; Peruzzini, M.; Sabat, M.;
Zanobini, F. Organometallics 1986, 5, 2557.
(30) (a) Shubina, E. S.; Belkova, N. V.; Bakhmutova, E. V.; Vorontsov, E.
V.; Bakhmutov, V. I.; Ionidis, A. V.; Bianchini, C.; Marvelli, L.; Peruzzini,
M.; Epstein, L. M. Inorg. Chim. Acta 1998, 280, 302. (b) Bakhmutov, V. I.;
Bianchini, C.; Peruzzini, M.; Vizza, F.; Vorontsov, E. V. Inorg. Chem. 2000, 39,
1655. (c) Bakhmutov, V. I.; Bakhmutova, E. V.; Belkova, N. V.; Bianchini, C.;
Epstein, L. M.; Peruzzini, M.; Shubina, E. S.; Vorontsov, E. V.; Zanobini, F. Can.
J. Chem. 2001, 79, 479. (d) Gutsul, E. I.; Belkova, N. V.; Sverdlov, M. S.; Epstein,
L. M.; Shubina, E. S.; Bakhmutov, V. I.; Gribanova, T. N.; Minyaev, R. M.;
Bianchini, C.; Peruzzini, M.; Zanobini, F. Chem.;Eur. J. 2003, 9, 2219.
(e) Gutsul, E. I.; Belkova, N. V.; Babakhina, G. M.; Epstein, L. M.; Shubina,
E. S.; Bianchini, C.; Peruzzini, M.; Zanobini, F. Russ. Chem. Bull. Int. Ed. 2003,
~
52, 1204. (f) Bolano, S.; Gonsalvi, L.; Barbaro, P.; Albinati, A.; Rizzato, S.;
Gutsul, E.; Belkova, N.; Epstein, L.; Shubina, E.; Peruzzini, M. J. Organomet.
Chem. 2006, 691, 629. (g) Belkova, N. V.; Gribanova, T. N.; Gutsul, E. I.;
Minyaev, R. M.; Bianchini, C.; Peruzzini, M.; Zanobini, F.; Epstein, L. M.;
Shubina, E. S. J. Mol. Struct. 2007, 844/845, 115.
(28) Albinati, A; Bakhmutov, V. I.; Belkova, N. V.; Bianchini, C.; de los
Rios, I.; Epstein, L.; Gutsul, E. I.; Marvelli, L.; Peruzzini, P.; Rossi, R.;
Shubina, E.; Vorontsov, E. V.; Zanobini, F. Eur. J. Inorg. Chem. 2002, 1530.

Article
Inorganic Chemistry, Vol. 49, No. 9, 2010 4349
Reaction of 3 with HFIP. In the presence of 10 equiv of
the more acidic HFIP (pKa = 9.3), only the [IrH] HO-
3 3 3
CH(CF3)2 adduct (3b) could be observed between 190 and
230 K, with the original peaks of 3 being shifted as in the case
of the interaction with TFE [ΔδH = -0.4 ppm; ΔδP = -2.1
ppm, T1,min = 352 ms (190 K)]. At variance with our finding
with in situ protonation using HBF4 (vide supra), at 250 K
two species, 4b0 and 4b00, were detected. They have NMR
parameters very close to those of 4, obtained from the
reaction with HBF4. 4b0 shows a singlet on the 31P{1H}
NMR spectrum at -23.7 ppm and a doublet of triplets
centered at -11.2 ppm on the hydride region of the 1HNMR
2
spectrum (2JH-P(trans) = 119.3 Hz; JH-P(cis) = 17.6 Hz),
while 4b00 appears as a singlet at -23.2 ppm on the 31P{1H}
NMR spectrum and a doublet of triplets (partially over-
lapped with that of 4b0) centered at -11.07 ppm on the
Figure 4. IR spectra (νIr-H region) of 3 (0.01 M) in CH2Cl2 at 200 K in
the presence of 0 equiv (a), 5 equiv (b), 10 equiv (c), 20 equiv (d) of TFE.
2
1H NMR (2JH-P(trans) = 120.6 Hz; JH-P(cis) = 18.0 Hz;
Figure 5). The T1,min values found for 4b0 and 4b00 at 250 K
are 181 and 177 ms, respectively. The possible chemical
nature of both 4b0 and 4b00 are critically discussed with the
help of DFT calculations (vide infra). Further warming leads
to a gradual signal intensity redistribution, indicating com-
plete conversion of 3 into 4b0/4b00. Leaving the sample at
room temperature led to further conversion to the dihydride
derivative [(κ4-NP3)IrH2][OCH(CF3)2] (5b), which became
complete in ca. 18 h. Unlike 5, obtained by HBF4 proton-
ation, the 1H NMR spectrum was well-resolved, and it was
possible to observe all of the couplings (Figure 6). Thus, the
hydride resonances appear as two well-separated doublets of
triplets of doublets centered at -10.6 ppm (H trans to P;
2JH-P(trans) = 116.1 Hz; 2JH-P(cis) = 19.5 Hz; 2JH-H = 5.1
Hz) and -18.4 ppm (H trans to N; 2JH-P(cis1) = 14.4 Hz;
2JH-P(cis2) = 15.0 Hz; 2JH-H = 5.1 Hz), respectively. The
31P{1H} NMR pattern is of an AM2 system similar to that of
5 (Figure S4 in the Supporting Information ): a doublet
centered at 19.5 ppm (δM) and a triplet centered at 18.2 ppm
(δA, 2JP-P = 11.7 Hz) for the two sets of nonequivalent P
atoms. Finally, a 2D 1H-31P HETCOR NMR spectrum
of the reaction mixture taken at room temperature before
completion was also recorded, as additional proof of
the proposed assignment (Figure S5 in the Supporting
Information).
In good agreement with NMR analysis, the IR spectra
of 3 in the presence of 10 equiv of HFIP at 190-230 K in
dichloromethane show only the low-frequency shift and
the intensity increase of the νIr-H band (Figure S6 in the
Supporting Information), evidencing once again DHB
formation. Above 250 K, a new band appears in the IR
spectrum at 2060 cm1 (Figure 7), which increases slowly
with time and/or with temperature. This band is at the
position similar to the one observed in the spectra of 3 in
the presence of 1 equiv of HBF4 (under the conditions of
complete proton transfer) and is assigned to species 4.
Theoretical Study of Hydrogen Bonding to 3. DFT
optimizations of the various structures described above
were carried out, together with frequency calculations,
using the model ligand N(CH2CH2PH2)3, where the three
PPh2 ends of the NP3 ligand have been replaced by
PH2 groups. The functional of choice was Truhlar’s
M05-2X,31 specially designed for the computational
of TFE entails only the intensity increase of the νIr-H
band (Figure 4), which could be assigned to the forma-
tion of dihydrogen-bonded complex [IrH] HOCH2-
3 3 3
CF3 (3a).1,2,6-9,28,30
In order to confirm this hypothesis, the 31P{1H} and 1H
NMR spectra of 3 were measured in CD2Cl2 in the presence
of 6 equiv of TFE. At 190 K, the δΟH value of TFE shifts
considerably in the presence of 3, passing from 2.9 ppm in the
free alcohol to 5.6 ppm. This downfield shift of the acidic
proton signal is in line with the hydrogen-bond formation. A
new hydride signal appears at stronger field (δ=-12.6 ppm;
ΔδH=-0.2 ppm; see Figure S3 in the Supporting Informa-
tion), while the “new” δP is observed at -13.7 ppm instead of
-12.3 ppm. The T1,min value of 143 ms was found at 233 K
for the hydride signal in the presence of TFE, which is 1.4
times lower than that of free hydride. Altogether, these obser-
vations evidence DHB formation.1 No signals belonging to a
different species (such as 4 or 5) were detected on the NMR
spectra in the entire temperature range up to 290 K; thus,
neither proton transfer nor dihydrogen evolution takes place.
As previously reported for other similar systems,30 the
Ir-H HOR distance can be estimated from the T1-min
data. According to eq 1
3 3 3
1
3
2
obs
obs
1=T1,min
¼
½1=T1,min
HORÞꢁ þ ½1=T1,minðIrH Þꢁ
3
ðIrH
3 3 3
3
ð1Þ
value was employed to calculate
for the dihydrogen-bonded hydride li-
obs
the observed T1,min
T1,min
(IrH HOR)
3 3 3
obs
gand in 3a. In turn, this value was used together with
obs
T1,min
of complex 3 to calculate the T1,min(H
(IrH3)
H)
3 3 3
value, which is exclusively due to hydride-proton dipole-
dipole interactions (eq 2).
obs
1=T1,min
¼ 1=T1,minðIrHÞ þ 1=T1,minðH
HÞ
ðIrH HORÞ
3 3 3
3 3 3
ð2Þ
Finally, application of eq 3
rH- H ¼ 5:815½T1;minðH
1=6
=νꢁ
ð3Þ
HÞ
3 3 3
gave an estimation of the H H distance in the dihydrogen-
˚
bonded adduct 3a of 1.66 A, falling in the typical range of
3 3 3
(31) (a) Zhao, Y.; Truhlar, D. G. Theor. Chem. Acc. 2008, 120, 215.
(b) Zhao, Y.; Truhlar, D. G. Acc. Chem. Res. 2008, 41, 157.
DHB lengths.1,27

4350 Inorganic Chemistry, Vol. 49, No. 9, 2010
Rossin et al.
t
˚
3 is 1.59 A, while the calculated ν(Ir-H) stretching
frequencies are 2228 (fully symmetrical ν1Ir-H ), 2197 (ν2),
and 2151 (ν3) cm-1 (for the graphical represen3tation of the
Ir-H normal modes and their intensities, see Figure S7 in
the Supporting Information). The overestimation with
respect to 3 is usual for DFT calculations in the gas phase.
t
˚
The Ir N separation in 3 is 3.611 A, pointing out a good
agreement with the X-ray-determined structure of 3 (experi-
3 3 3
˚
mental value of 3.518 A; see above).
The optimized structure of the DHB adduct between 3t and
TFE (3at; Figure 8a) shows that the proton donor is close to
two hydride ligands, interacting more strongly with one of the
two. The calculated (O)H H(Ir) distances are 1.818 and
˚
3 3 3
2.115 A, and the O-H H(Ir) angles are 144.4 and 127.8°.
3 3
Thus, this adduct can be considered as an example of the
asymmetric bifurcated dihydrogen-bonded complex. A simi-
lar structural motif for DHB species has been theoretically
found as the most stable adduct for the interaction of TFE
with Cp*Mo(dppe)H3.7,8 The proton of TFE is slightly below
the plane defined by two of the three hydrides and the Ir atom
[H-Ir-H-H(O) dihedral angle of -17.0°], leading to a
relatively short separation from the Ir atom [dOH
=
Ir
3 3 3
˚
2.719 A; RO-H-Ir = 158.4°]. This arrangement suggests also
participation of the metal atom in hydrogen bonding.7 The 3at
formation energy is -13.2 kcal mol-1 in the gas phase and
-5.3 kcal mol-1 in dichloromethane. The alcohol νO-H
stretching falls at 3690 cm-1 in 3at, while in free (optimized)
TFE, it appears at 3939 cm-1. The wavenumber decrease in
3at is in accordance with a common O-H bond weakening
due to hydrogen bond formation. The calculated ν(Ir-H)
normal modes have different contributions from different
Ir-H bond vibrations (see Figure S6 in the Supporting
Information), and two out of three appear at lower frequen-
cies [2224 (ν1), 2169 (ν2), 2184 (ν3) cm-1] in comparison to
those of 3t. This illustrates the difficulty in the interpretation of
the spectroscopic changes caused by dihydrogen bonding in
the case of polyhydride compounds.
The dihydrogen-bonded adduct between 3t and HFIP (3bt;
Figure 8b) shows features very similar to those of 3at,
although with a stronger interaction, in agreement with
the stronger acidity of HFIP. Both the (O)-H H distance
3 3 3
with the closer hydride and the (O)H Ir distance have
˚
decreased (1.722 and 2.605 A, respectively), whereas the
3 3 3
(O)-H H distance with the second hydride has increased
3 3 3
˚
(2.164 A). The incoming proton is now placed further below
the plane defined by the two hydrides and the Ir atom [H-
Ir-H-H(O) dihedral angle of -25.9°], pointing toward the
Ir-H bond, with RO-H-H(Ir) and RO-H-Ir angles of 163.2
and 159.2°, respectively. The energetic parameters of the
HFIP adduct (-15.0 kcal mol-1 in the gas phase and -5.4
kcal mol-1 in dichloromethane) reflect such a stronger inter-
action. The calculated ν(Ir-H) vibrations in 3bt appear at
frequencies lower than those in 3at: 2218 (ν1), 2144 (ν2), and
2113 (ν3) cm-1. The νOH stretching falls at 3503 cm-1 (in free
HFIP, νOH = 3907 cm-1), with ΔνΟ-H being much bigger
than that in the case of 3at (-404 vs -249 cm-1) because of
the stronger interaction with a more acidic proton donor.
Using these ΔνO-H values, the DHB formation enthalpy can
be estimated as -4.6 kcal mol-1 for 3at and -6.5 kcal mol-1
for 3bt using the known ΔνO-H/ΔH correlation.6b,9
Figure 5. Variable-temperature 1H NMR spectra (hydride region) of
the 3/HFIP (10 equiv) mixture in CD2Cl2.
treatment of both third-row transition-metal thermo-
chemistry and general noncovalent interactions (e.g.,
hydrogen bonding).32 The optimized Ir-H distance in
€
(32) (a) Buhl, M.; Reimann, C.; Pantazis, D. A.; Bredow, T.; Neese, F.
Proton-Transfer Process. Looking at complex 3, there
are five centers that could be susceptible to interaction with
proton donors. These are nitrogen, iridium, and the three
J. Chem. Theor. Comput. 2008, 4, 1449. (b) Zhao, Y.; Truhlar, D. G. J. Chem.
Theor. Comput. 2008, 4, 1849. (c) Sousa, S. F.; Fernandes, P. A.; Ramos, M. J.
J. Phys Chem. A 2007, 111, 10439.

Article
Inorganic Chemistry, Vol. 49, No. 9, 2010 4351
Figure 6. 1H NMR spectrum of 5b (hydride region only, CD2Cl2, 298 K).
Figure 7. IR spectra (νIr-H region) of 3 (0.005 M, dashed line) and of
3 in the presence of 10 equiv of HFIP (solid line). CH2Cl2, 270 K. The
asterisk indicates a band due to HFIP.
hydride ligands. Inspection of the structure leaves only the
last two as suitable candidates for the electrophilic attack
because the N atom is located in such a way that its lone pair
pointing to the Ir atom is hidden by the ethylene bridges of
the NP3 ligand, in contrast to what happens with chelating
aminophosphane ligands such as 1,2-bis[(diisopropylphos-
phino)amino]ethane (dippae) and 1,2-bis[(diisopropylphos-
phino)amino]cyclohexane (dippach).11,14 Such geometry is
typical for the κ3-coordinated NP3 ligand, as evidenced by
the CSD database inspection.25 Accordingly, the experimen-
tal spectroscopic study does not give any evidence of the
proton attack at the nitrogen site. A CCSD search revealed
only one complex of “protonated” NP3, [(κ3-P,P,P-HNP3)-
Figure 8. Optimized structures of (a) 3at and (b) 3bt. Selected bond
˚
lengths are reported (A). Atom color code: orange, Ir; white, H; red, O;
blue, N; gray, C; green, P; light blue, F. H atoms on the N(CH2CH2PH2)3
ligand and on the alcohols are omitted for clarity. The short contacts are
depicted in yellow (dotted lines).
the presence of alcohol are clearly observable for these sys-
tems, in contrast with what was found for [Cp*MH3-
(dppe)] HOR complexes.7,9
3 3 3
The assignment of a well-defined structure to iridium
polyhydrides is a very difficult task, and the structure of
L3IrH4 complexes has been a matter of debate, leading to
the conclusion that these species generally deny any
simple characterization in terms of individual well-de-
fined structures.33-35 The highly fluxional behavior of the
Ni(CO)]BPh4, featuring the inside-oriented N-H Ni
3 3 3
hydrogen bond, but it was obtained by the reductive intra-
molecular hydrogen transfer upon CO addition to [(κ4-NP3)-
NiH]BPh4 rather than via external proton delivery.13
Protonation of 3 with HFIP or HBF4 gives the tetrahy-
drido complex 4, which is, however, not stable at room tem-
perature and loses dihydrogen slowly, yielding 5. TheIRand
NMR data obtained for the interaction of 3 with fluorinated
ꢀ
(33) Gutierrez-Puebla, E.; Monge, A.; Paneque, M.; Poveda, M. L.;
Taboada, S.; Trujillo, M.; Carmona, E. J. Am. Chem. Soc. 1999, 121, 346.
(34) Webster, C. E.; Singleton, D. A.; Szymanski, M. J.; Hall, M. B.;
Zhao, C.; Jia, G.; Lin, Z. J. Am. Chem. Soc. 2001, 123, 9822.
(35) Hebden, T. J.; Goldberg, K. I.; Heinekey, D. M.; Zhang, X.; Emge,
T. J.; Goldman, A. S.; Krogh-Jespersen, K. Inorg. Chem. 2010, 49, 1733.
alcohols are in agreement with the Ir-H HOR hydro-
3 3 3
gen-bond formation preceding the proton transfer. Notably,
despite the presence of three hydride ligands, changes in the
NMR spectra and a decrease of the T1,min relaxation time in

4352 Inorganic Chemistry, Vol. 49, No. 9, 2010
Rossin et al.
is indisputable in this tautomer from the calculated
34
˚
H-H distance of 0.85 A. As in TpIrH4 and (PCP)-
IrH435 [PCP = κ3-1,3-(CH2PtBu2)2-C6H3], the two forms
are almost isoenergetic [ΔE(DIH-TETRA) = -0.4 kcal
mol-1 (gas phase)/-0.5 kcal mol-1 (CH2Cl2), with the
DIH isomer being slightly more stable], precluding any
discrimination between DIH and TETRA. The Ir-H
stretching vibration frequencies calculated for these two
isomers do not allow for discrimination between the two
species either. For TETRA, νIr-H appears at 2363 (ν1),
2252 (ν2), 2247 (ν3), and 2219 (ν4) cm-1, with the latter one
being the most intense. In DIH, vibrations of the Ir(η2-H2)
moiety are rather weak, whereas vibrations of two terminal
Figure 9. Optimized structures of (a) TETRA and (b) DIH. Selected
˚
bond lengths reported (A). Atom color code: see Figure 8. H atoms on the
N(CH2CH2PH2)3 ligand are omitted for clarity.
hydride ligands (νsIr-H = 2254 and νas
= 2225 cm-1
)
Ir-H2
2
fall in the same region as ν2-ν4 of TETRA and are of
comparableintensity to ν4 (see the Supporting Information
for more details).
hydrogen ligands does not help, and the use of the T1 crite-
rion may only lead to ambiguous or even incorrect conclu-
sions.36 From 1H and 2H NMR observations, hydridotris-
(pyrazolyl)boratoiridum tetrahydride (TpIrH4) has been
proposed to be a classical tetrahydride species with a C3v
structure, in which a hydride ligand is capping the IrH3
face.33 Afterward, DFT optimizations and high-level ab
initio calculations of TpIrH4 have found two minimum-
energy structures almost isoenergetic: a Cs edge-bridged
octahedral structure, with the fourth hydride sitting on an
Ir(H)2 edge, and a C1 η2-dihydrogen dihydride structure.34
The presence of both a dihydrogen ligand and two classical
hydrides in [(triphos)IrH4]þ [triphos = MeC(CH2PPh2)3]37
and in cis,trans-[Ir(4-C5NF4)H4(PiPr3)2] (4-C5NF4 =
2,3,5,6-tetrafluoro-4-pyridyl) has been deduced from
NMR T1 measurements and isotopic labeling with deuter-
ium.38 Anyway, these compounds appear as highly fluxional
species because of the fast hydride exchange processes.39
We carried out DFT calculations to determine the struc-
ture of the protonation product 4, bearing in mind that
different species are observed on both 1Hand31P{1H} NMR
spectra depending on the acid used. Two minima were
located, closely related to those previously found for the
TpIrH4 complexes (Figure 9).34 In the first one, the incoming
H atom is placed on a Ir(H)2 edge in a pseudoplane with the
other two hydrides and two P atoms of the NP3 ligand.
The complex can be described as a pentagonal-bipyrami-
dal classical tetrahydrido species [{κ3-P,P,P-N(CH2CH2-
PH2)3}Ir(H)4]þ (TETRA), although the short distance be-
In order to check if other isomers could be stable geome-
tries on the potential energy surface (PES), optimizations
starting from different structures were carried out. The bis-
dihydrogen [{κ3-P,P,P-N(CH2CH2PH2)3}Ir(η2-H2)2]þ led
to TETRA during optimization, while the end-on coordi-
nation of the η1-bound dihydrogen in {[κ3-P,P,P-N-
(CH2CH2PH2)3}Ir(η1-H2)(H)2]þ converted into DIH. The
possibility of a [H3]þ ligand was theoretically and experi-
mentally considered in the 1980s.42 In our case, optimization
of an initial pseudoallylic geometry [{κ3-P,P,P-N(CH2-
CH2PH2)3}Ir(η3-H3)(H)}þ led again to TETRA. Therefore,
none of them represents a stable structure at the computa-
tional level used.
The theoretical data cannot clearly discriminate be-
tween the two almost isoenergetic species DIH and TET-
RA, which are located on an extremely flat energy surface.
Furthermore, the hypothetical alcoholate neutral com-
plex [{κ3-P,P,P-N(CH2CH2PH2)3}Ir(ΟR)(H)2], where
the counterion acts as a ligand on iridium(III), has to be
discarded because its formation is energetically unfavor-
able. In fact, the internal energy variation (calculated in
CH2Cl2) associated with the reaction
3t þ HFIP f
½fK3-P, P, P-NðCH2CH2PH2Þ3gIrðΟRÞðHÞ2g þ H2
ð4Þ
˚
tween the fourth H atom and one hydride (1.417 A) allows
equals to þ12.7 kcal mol-1, while that of the 3t þ HFIP f
3bt reaction is negative: - 5.4 kcal mol-1. The formation
of homoconjugated pair species [{κ3-P,P,P-N(CH2CH2-
also its description as a compressed dihydride.40,41 The
second minimum is an octahedral mixed classical-
nonclassical compound [{κ3-P,P,P-N(CH2CH2PH2)3}Ir-
(η2-H2)(H)2]þ (DIH). The presence of a dihydrogen ligand
PH2)3}IrH4](OR) nROH, where the excess of free alco-
3
hol may form a hydrogen-bonded counterion [R-O
3 3 3
H-OR]- or [R-O (H-OR)n]- instead of the simple
3 3 3
(36) (a) Gusev, D. G.; Kuhlman, R. L.; Renkema, K. B.; Eisenstein, O.;
Caulton, K. G. Inorg. Chem. 1996, 35, 6775. (b) Desroisiers, P. J.; Cai, L.; Lin,
Z.; Richards, R.; Halpern, J. J. Am. Chem. Soc. 1991, 113, 4173. (c) Gusev, D.
G.; H€ubener, R.; Burger, P.; Orama, O.; Berke, H. J. Am. Chem. Soc. 1997, 119,
3716.
alcoholate RO-, has been shown by IR and NMR
methods for CF3COOH or p-nitrophenol interacting with
hydrides,30d,e,43,44 but it is difficult to be proved for HFIP,
which does not have any suitable spectroscopic label.
(37) Bianchini, C.; Moneti, S.; Peruzzini, M.; Vizza, F. Inorg. Chem. 1997,
36, 5818.
(38) Salomon, M. A.; Braun, T.; Krossing, I. Dalton Trans. 2008, 5197.
ꢀ
(39) Maseras, F.; Lledos, A.; Clot, E.; Eisenstein, O. Chem. Rev. 2000,
(42) (a) Burdett, J. K.; Pourian, M. R. Inorg. Chem. 1988, 27, 4445.
(b) Heinekey, D. M.; Payne, N. G.; Schulte, G. K. J. Am. Chem. Soc. 1988, 110,
2303.
100, 601.
ꢀ
(40) (a) Gelabert, R.; Moreno, M.; Lluch, J. M.; Lledos, A.; Pons, V.;
ꢀ
Heinekey, D. M. J. Am. Chem. Soc. 2004, 126, 8813. (b) Gelabert, R.; Moreno,
(43) Belkova, N. V.; Besora, M.; Epstein, L. M.; Lledos, A.; Maseras, F.;
ꢀ
M.; Lluch, J. M.; Lledos, A.; Heinekey, D. M. J. Am. Chem. Soc. 2005, 127,
Shubina, E. S. J. Am. Chem. Soc. 2003, 125, 7715.
(44) Belkova, N. V.; Collange, E.; Dub, P.; Epstein, L. M.; Lemenovskii,
5632.
ꢀ
ꢀ
(41) Heinekey, D. M.; Lledos, A.; Lluch, J. M. Chem. Soc. Rev. 2004,
D. A.; Lledos, A.; Maresca, O.; Maseras, F.; Poli, R.; Revin, P. O.; Shubina,
33, 175.
E. S.; Vorontsov, E. V. Chem.;Eur. J. 2005, 11, 873.

Article
Inorganic Chemistry, Vol. 49, No. 9, 2010 4353
Figure 10. Optimized structures of (a) 3t (HFIP)2 and (b) the proton-transfer product DIH [AHA]. Selected bond lengths reported (A). Atom color
˚
3 3 3
code: see Figure 8. H atoms on the N(CH2CH2PH2)3 ligand are omitted for clarity.
3
Scheme 3
The involvement of two alcohol molecules in the proton-
transfer process has been suggested for the [Cp*RuH3-
(PCy3)]/HFIP system45 and confirmed by kinetic studies
for [Cp*FeH(dppe)] protonation by different alcohols.44,46
Binding of additional alcohol molecules would alter the
[{κ3-P,P,P-N(CH2CH2PH2)3}IrH4](OR) ion-pair stability
(weakening the IrH OR interaction, whose presence is
3 3 3
clearly indicated by the dependence of the hydride signal
position on the HFIP concentration) but not the cation
nature/structure, which agrees with both the similarity of
the 1H NMR chemical shift and the multiplicity of 4b0/4b00.
Surprisingly, the optimization of nonclassical and classical
cation structures in the presence of a [(CF3)2CHO
3 3 3
HOCH(CF3)2]- anion, DIH [AHA] and TETRA [AHA],
gave in both cases the dihydrido dihydrogen cation bonded
with the homoconjugated HFIP anion (DIH [AHA];
3
3
3
Figure 10b). Thus, in the presence of the counteranion, only
the dihydrogen dihydride isomer would be stable. The similar
effect of the counteranion favoring the nonclassical structure
was obtained very recently for the [Cp*Mo(CO)(PMe3)2-
(H2)]BF4 system in a parallel study by some of us.47
The energy barrier for the occurrence of the proton-
-
(HFIP)2 system (Figure 10a) and analyzing its internal
energy variation (evaluated in CH2Cl2; single-point calcula-
tion with the BS2 basis set on the BS1-optimized geometries)
with the dO-H reaction coordinate, as performed in similar
transfer reaction was also evaluated, starting from the 3t
From all of these results and in keeping with the
negligible energy differences between the computed spe-
cies, a rationale for the reaction mechanism accounting
for protonation of 3 may be the following: it starts with
dihydrogen-bonded adduct formation, followed by pro-
tonation of one Ir-H bond to give the tetrahydride 4. In
the case of HBF4 protonation, when only one set of
resonances appears in the low-temperature spectrum, it
is impossible to discriminate between the classical tetra-
hydride and the nonclassical dihydrido dihydrogen com-
plex cation. However, on the basis of T1 arguments (vide
supra), the lack of hydrogen-bonding network capabili-
3 3 3
literature cases.7,44 In the starting geometry, d(O-H)opt
=
˚
˚
0.994 A; a maximum was found for dO-H = 2.7 A, 19.5 kcal
mol-1 above the reagents. The final proton-transfer product
is ΔE=þ16.0 kcal mol-1. The values so obtained are much
lower than those in the gas phase (energetic barrier=27.3
kcal mol-1; thermodynamic ΔE = þ21.1 kcal mol-1), as
expected, mirroring the trend found in the literature.7,44
-
ties of the BF4 anion, and literature data,37 it is con-
ceivable that the rapidly exchanging tetrahydride tauto-
mer is preferred, although no final decision can be taken.
A similar degree of uncertainty also stands in the case of
HFIP protonation (Scheme 3), where the situation is
further complicated by the appearance of two similar sets
€
(45) Grundemann, S.; Ulrich, S.; Limbach, H.-H.; Golubev, N.; Denisov,
G. S.; Epstein, L. M.; Sabo-Etienne, S.; Chaudret, B. Inorg. Chem. 1999, 38,
2550.
(46) Belkova, N. V.; Revin, P. O.; Epstein, L. M.; Vorontsov, E. V.;
Bakhmutov, V. I.; Shubina, E. S.; Collange, E.; Poli, R. J. Am. Chem. Soc.
2003, 125, 11106.
(47) Dub, P. A.; Belkova, N. V.; Filippov, O. A.; Daran, J.-C.; Epstein, L.
M.; Lledos, A.; Shubina, E. S.; Poli, R. Chem.;Eur. J. 2010, 16, 189.
1
of resonances on the H NMR spectrum, which slowly
transforms into the κ4-NP3 dihydride 5. From careful
ꢀ

4354 Inorganic Chemistry, Vol. 49, No. 9, 2010
Rossin et al.
examination of the whole body of collected experimental
and theoretical information, we are inclined to suppose
that, after formation of an initial DHB adduct [corres-
DFT results for the “naked” cations, but in the presence of
the [(CF3)2CHO HOCH(CF3)2]- counteranion, only the
3 3 3
dihydrogen/dihydride isomer seems to be stable. The final
loss of molecular dihydrogen at room temperature gives 5. In
the reaction of 3 with the medium-strength acid HFIP, two
distinct intermediates, 4b0 and 4b00, are observed on the low-
ponding to the DFT minimum 3t (HFIP)2], the con-
3 3 3
formational flexibility of the NP3 ligand could allow for
an easy coordination mode “switch” from κ3-P,P,P-NP3
to κ3-P,P,N-NP3, as confirmed by additional theoretical
evidence coming from a PES scan along the dIr-N reaction
coordinate. In fact, starting from either the TETRA or
1
temperature H NMR spectra, having very similar T1,min
values, δH, and related coupling patterns (doublet of triplets).
An unambiguous assignment of their identity is impossible
on the basis of the collected data;this is not surprising, in view
of the extensive literature on related iridium tetrahydrides,
where it is always hard to discriminate between different
tautomers.33-35 In the present case, the high flexibility of the
NP3 backbone, with the κ3-P,P,N-coordination isomer
slightly more stable than the κ3-P,P,P one, together with
the easy switching between the κ3 and κ4 hapticities, further
complicates the situation, vanishing every attempt of a
definite and clear description.
DIH [AHA] geometry and shortening the Ir-N distance,
3
which entails concomitant N-coordination, a simulta-
neous -PH2 detachment is observed, maintaining the
octahedral coordination geometry on the iridium center
and leaving one “dangling” NP3 arm (Figure S8 in the
Supporting Information). The energy of this tautomer is
1.6 kcal mol-1 below(!) that of DIH [AHA] in dichloro-
3
methane, and the small energy difference favors the exis-
tence of an equilibrium between the two isomers. No
stable geometry corresponding to either eight-coordi-
nated [(κ4-NP3)IrH4]þ or seven-coordinated [(κ4-NP3)Ir-
(η2-H2)(H)2]þ was found, despite the fact similar species
were characterized in the case of rhenium.28 As soon as
the temperature is raised from 250 to 294 K, the κ4-NP3
mode is prevalent, and subsequent loss of molecular
dihydrogen from the intermediate(s) generates 5 as the
only stable product observed at ambient temperature
after 18 h.48
Acknowledgment. The work was supported by the
RFBR (Grant 08-03-00464) and CNR-RAS bilateral
agreement. The motu proprio FIRENZE HYDROLAB
project by Ente Cassa di Risparmio di Firenze (http://
www.iccom.cnr.it/hydrolab/) and the PIRODE project
by MATTM (Rome) are kindly acknowledged for fund-
ing of this research activity also through a postdoctoral
grant to A.R. Thanks are also expressed to the GDRE
project “Homogeneous Catalysis for Sustainable Develop-
ment”. A.L. thanks the Spanish MICINN (Projects
CTQ2008-06866-C02-01 and ORFEO Consolider Inge-
nio 2010 CSD2007-00006). The use of the computational
Conclusions
The novel trihydride 3 has been synthesized and character-
ized through IR/multinuclear NMR spectrometry and X-ray
diffraction. Treatment with strong (HBF4) and medium-
strength (TFE and HFIP) proton donors showed that
DHB formation precedes hydride protonation to yield the
species 4, whose nature has been critically examined with the
help of DFT calculations. Discrimination between a classical
and nonclassical hydride cannot be made on the basis of the
ꢀ
facilities of the Centre de Supercomputacio de Catalunya
(CESCA) is greatly appreciated.
Supporting Information Available: Figures S1-S8, crystal-
lographic tables for 1, and Cartesian coordinates and absolute
energies (in the gas phase and in dichloromethane) of the
optimized geometries of 3t, 5t, 3at, 3bt, TETRA, DIH, 3t
-
3 3 3
(HFIP)2, DIH [AHA], and (K3-P,P,N-NP3)-DIH[AHA]. This
3
(48) In 5t, the two hydride ligands appear to be totally independent:
material is available free of charge via the Internet at http://
pubs.acs.org.
ν1Ir-1H = 2188 cm-1, ν2Ir-1H = 2368 cm-1
.