Reactivity of a mononuclear iridium(I) species bearing a terminal phosphido fragment embedded in a triphosphorus ligand

Article abstract of DOI:10.1021/ic302301h

The first example of an iridium(I) species bearing a terminal phosphido (PR₂^-) ligand is reported. This stable compound shows well-behaved reactivity toward various electrophiles, owing to its exposed phosphorus lone pair, allowing reversible protonation, selective alkylation, isolation of a phosphidoborane of iridium, and generation of a phosphido-bridged iridium(I)-gold(I) dinuclear species.

Full text of DOI:10.1021/ic302301h

Communication
pubs.acs.org/IC
Reactivity of a Mononuclear Iridium(I) Species Bearing a Terminal
Phosphido Fragment Embedded in a Triphosphorus Ligand
Yann Gloaguen,† Wesley Jacobs,† Bas de Bruin,† Martin Lutz,‡ and Jarl Ivar van der Vlugt*,†
†Homogeneous & Supramolecular Catalysis, van ’t Hoff Institute for Molecular Sciences, University of Amsterdam, Science Park 904,
1098 XH Amsterdam, The Netherlands
‡Department of Crystal and Structural Chemistry, Utrecht University, Padualaan 8, 3584 CH Utrecht, The Netherlands
S
* Supporting Information
formation.2 Despite their importance, structurally well-charac-
ABSTRACT: The first example of an iridium(I) species
bearing a terminal phosphido (PR2 ) ligand is reported.
terized examples of complexes incorporating a phosphidoborane
adduct are mainly limited to alkali and some group 8 (iron and
ruthenium) and group 10 (palladium and platinum) metals.3,4
Most iridium phosphido complexes exist as di- or multinuclear
species, and low-valent iridium complexes featuring a terminal
phosphido are elusive.5 As a result, little is known about the
stability of these species or the residual reactivity of the
phosphide fragment. For example, no phosphidoborane adduct
of iridium has been reported to date. Heterodinuclear analogues
bearing a bridging iridium(I) phosphido unit are also unknown.
Currently, there is ongoing interest in the application of
(monoanionic) “pincer” ligands to tune the properties of late
transition metals, especially in combination with reactive ligand
design and metal−ligand bifunctional bond activation pro-
cesses.6 To the best of our knowledge, bona fide phosphido-
based systems have not yet been considered (Figure 1).7
Given our interest in reactive ligands and their potential
applications,8 we envisaged bis[(2-diphenylphosphino)phenyl]-
phosphine (1H) as a monoanionic scaffold for group 9 metals.
Copper(I) dimers with 1− as the dinucleating entity were
previously reported,9 and coordination of 1− to palladium(II) or
platinum(II) afforded square-planar mononuclear complexes.10
In contrast herewith, we reported combined experimental and
computational data on the group 9 metal rhodium(I), wherein
this ligand coordinated preferentially as the intact PPHP
fragment, withstanding deprotonation.11 We herein describe
the coordination of monoanionic 1− onto iridium(I) and its
selective reactivity with various electrophiles to yield stable, well-
defined species.
−
This stable compound shows well-behaved reactivity
toward various electrophiles, owing to its exposed
phosphorus lone pair, allowing reversible protonation,
selective alkylation, isolation of a phosphidoborane of
iridium, and generation of a phosphido-bridged iridium-
(I)−gold(I) dinuclear species.
he coordination chemistry of monoanionic phosphido
−
T
ligands (PR2 ) to main-group elements or transition metals
is well developed, with preference for the ligand to act as a
bridging moiety spanning two metal centers.1 In the case of
terminal phosphido coordination, there are two extreme bonding
situations of this monoanionic scaffold to a metal center, i.e., only
σ-bond coordination and a single metal−phosphorus bond (2e
donor) or σ−π coordination (4e donor) via additional π-bond
interaction between the phosphorus lone pair and a suitable
metal d orbital, yielding formal double-bond character (Figure
1). The latter results in trigonal-planar geometry around
In pursuit of an iridium(I) species bearing 1− rather than 1H,
which would also lead to the first known terminal iridium(I)
phosphido species, we initially turned to [Ir(μ-Cl)(coe)2}2] and
related dimeric iridium precursors, but these reactions led to
Figure 1. (A) Trigonal pyramidal vs trigonal planar terminal phosphide
coordination, (B) concept of a cooperative PH/P− couple based on 1H,
and (C) metalloligand reactivity of a terminal IrI(PR2) species.
1
multiple products, as indicated by 31P and H NMR spectros-
copy, and attempts to isolate a single species failed. This is
reminiscent of our results with rhodium11 as well as reported
reactions with related PNHP ligands.12 However, in contrast to
other synthetic protocols for such bis(phosphinoaryl)amine
systems,13 deprotonation of ligand 1H or formation of a lithiated
derivative prior to introduction of a suitable (halide-based) metal
precursor did not proceed cleanly. Efforts to enforce
coordination of ligand 1− using, e.g., [{Ir(μ-OH)(coe)2}2] or
phosphorus, whereas the former implies a trigonal-pyramidal
phosphorus atom with its lone pair pointing away from the metal
center, imparting residual Lewis basicity at this phosphorus
donor. As a result, this type of phosphide can be regarded as
coordinatively unsaturated, despite its σ-bonding interaction
with the metal center. Such metal phosphido complexes may
therefore act as (metallo)ligands (Figure 1).
Phosphidoboranes (also termed phosphanyl borohydrides)
are key intermediates during the synthesis of chiral phosphine
ligands and the formation of inorganic P−B polymers via
catalytic dehydrocoupling of phosphineboranes and C−P bond
Received: October 23, 2012
Published: January 29, 2013
© 2013 American Chemical Society
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Inorganic Chemistry
Communication
[{Ir(μ-OMe)(coe)2}2] failed to give selective reactions. Gratify-
ingly, the reaction of [{Ir(μ-OMe)(cod)}2]14 with 2 mol equiv of
1H generated a single isolable product in 81% yield (Scheme 1).
atom and, hence, a very different orientation of the backbone
phenyl rings compared to 2.
Given the observed geometric features of 2, we were interested
in probing reversible protonation at the phosphido bridgehead.
The addition of 1 equiv of NH4PF6 to a solution of 2 in CH2Cl2
led to a darkening of the reaction mixture, concomitant with the
facile formation of cationic [Ir(fac-1H)(cod)]PF6 (3). The P−H
Scheme 1. Synthesis and Versatile Reactivity of Complex 2
1
fragment is readily observed in the H NMR spectrum [δ 6.31
(dt, 1JP−H = 408 Hz, 4JP−H = 4.4 Hz)], with further evidence from
the 31P NMR spectrum [δ 45.9 (dt, 1JP−H = 408 Hz, JP−P = 14.3
Hz), 25.5 (d, JP−P = 14.3 Hz)] as well as IR spectroscopy (νP−H
2330 cm−1). The secondary phosphine in species 3 is smoothly
deprotonated by KOtBu,17 which is in stark contrast to our earlier
findings on Rh(PPHP) complexes.10,18
Density functional theory (DFT) analysis of various five-
coordinated Ir(1)(ethene)2 model structures (see the Support-
ing Information, SI) confirmed the observed rare fac
coordination for 1− to be preferred over the hypothetical mer
analogues for iridium(I). The calculated bond order (1.003) for
the Ir−P bond in 2 is indicative of almost exclusive σ-bonding
character with little involvement of the exposed phosphorus lone
pair, in agreement with the calculated highest molecular orbital
(HOMO) of 2 (Figure 1). Thus, complex 2 contains a formal
terminal phosphido pivot that likely exhibits significant
(metallo)phosphine character.
To support this hypothesis, we investigated reactions with
several electrophiles. The addition of an equimolar amount of
BH3SMe2 to a solution of 2 in dichloromethane resulted in a
beige solid after workup. The 31P NMR spectrum revealed two
signals at δ 53.0 (br s, 1P) and 29.3 (d, JP−P = 13 Hz, 2P). A
chemical shift difference Δδ of −13.6 ppm was noted for the
pivotal phosphorus atom relative to 2. A signal at δ −33.5 for the
BH3 group was observed in the corresponding 11B{1H} NMR
spectrum. Fast atom bombardment mass spectrometry (MS)
data (868.23 [M+]) were consistent with the formation of
iridium(I) phosphidoborane species Ir(fac-1·BH3)(cod) (4; see
Figure S25 for the DFT-calculated HOMO). This borane adduct
did not react with DABCO at room temperature. Nucleophilic
metalloligand 2 also reacted smoothly with 1 equiv of AuCl,
resulting in the formation of a beige species with NMR
spectroscopic features similar to those of parent 2 [δ 69.7 (t,
An IR spectroscopic comparison of this moderately air-stable
beige solid with free ligand 1H indicated deprotonation of the
secondary phosphine upon coordination. The corresponding 31P
NMR spectrum showed signals at δ 28.8 (phosphine) and 66.7
(phosphide). No P−P coupling was observed, suggestive of a
reduced σ character of the iridium−phosphide bond. Two signals
for nonequivalent HCCH groups of a coordinated cod
1
fragment were observed in the H NMR spectrum, indicating
different coordination environments. This was confirmed by the
observation of two characteristic signals in the 13C NMR
spectrum (at δ 78.8 and 54.7), leading us to formulate this
complex as Ir(fac-1)(cod) (2), featuring a terminal iridium
phosphido unit embedded within the facially coordinated
monoanionic donor ligand 1−.
Recrystallization of complex 2 from toluene led to single
crystals suitable for X-ray diffraction (Figure 2 and S24 for
Figure 2. Left: ORTEP plot of complex 2. Solvent and hydrogen atoms
are omitted for clarity. Right: calculated highest occupied molecular
orbital (HOMO) for 2.
J
P−P = 14 Hz, 1P), 27.8 (d, JP−P = 14 Hz, 2P)], for which MS data
indicated the formation of complex Ir(fac-1·AuCl)(cod) (5).
The molecular structures of 4 and 5 are depicted in Figure 3 (see
Figure S26 for details).
Both complexes show significant deviation from ideal C2
symmetry at the P1 pivot. For 4, this becomes apparent from,
e.g., the torsion angles B1−P1−Ir−C5 [0.08(9)°] and B1−P1−
Ir−C6 [−38.26(9)°]. The BH3 group is moved out-of-plane, with
the torsion angle B1−P1−Ir1−C5−6 being −19.15(8)° as a result
details). The difference Fourier map did not indicate protonation
at P1. The neutral complex features the monoanionic PP−P
scaffold 1−, with the phosphido unit occupying an axial position
in the distorted trigonal-bipyramidal geometry around iridium-
(I). As a result, 1− acts as a facial tridentate ligand rather than the
meridional “pincer”-like orientation observed with palladi-
um.10,11 The acute ∠P1−Ir−P2 [84.781(15)°] and ∠P2−Ir−P3
[107.964(14)°] indicate a high degree of ligand backbone
rigidity, and there is no apparent mirror plane through Ir−P1.
The Ir−P bond lengths are similar, prohibiting a formal
distinction in donor character between the phosphido (P1) and
phosphine (P2 and P3) donors. For d8 metal complexes with a
trigonal-bipyramidal geometry, the strongest σ-donor ligands
generally bind in the axial positions, while π-back-bonding is
typically more pronounced in the equatorial plane.15 Interest-
ingly, Calimano and Tilley reported the structure of Ir(fac-
PNP)(cod) (A), generated via deprotonation of the cationic
analogue [Ir(mer-PNHP)(cod)]Cl.16 In contrast to our findings,
the molecular structure for A featured a trigonal-planar nitrogen
Figure 3. ORTEP plots of 4 (left) and 5 (right). Solvent and hydrogen
atoms are omitted for clarity, except for BH3.
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Inorganic Chemistry
Communication
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Alkylation with MeI also occurs selectively at the P bridgehead to
yield species 6, bearing a triphosphine ligand (see the SI).
Selective oxidation of the phosphide unit was achieved using
excess Me3NO to give phosphido oxide 7, which was
recrystallized from C6H6 (see Figure S27). Selective removal of
the cod ligand in 2 using H2 (50 bar), syngas (55 bar), silane, or
ZrCl4 was not successful. Also, protonated species 3 resisted
hydrogenation of cod with 50 bar of H2 for 5 days. The presence
of the strongly coordinated cod ligand within the five-
coordinated geometry of 2 has so far hampered bifunctional
H−E bond activation.
In conclusion, we have obtained the first mononuclear group 9
complex, containing 1 as the facial ligand, which bears a
previously unknown terminal phosphidoiridium(I) motif. The P
bridgehead in complex 2 shows well-behaved reactivity, i.e., facile
protonation and alkylation as well as Lewis basic coordination to
BH3 and AuCl. We are currently studying bimetallic compounds
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ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental, DFT, and crystallographic data, CIF files, graphics,
and NMR spectra. This material is available free of charge via the
Internet at http://pubs.acs.org.
(9) Mankad, N. P.; Rivard, E.; Harkins, S. B.; Peters, J. C. J. Am. Chem.
Soc. 2005, 127, 16032−16033.
AUTHOR INFORMATION
Corresponding Author
*E-mail: j.i.vandervlugt@uva.nl.
■
(10) (a) Mazzeo, M.; Lamberti, M.; Massa, A.; Scettri, A.; Pellecchia,
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̈
For a related NPN ligand, see: (c) MacInnis, M. C.; McDonald, R.;
Turculet, L. Organometallics 2011, 30, 6408−6415.
Author Contributions
The manuscript was written through contributions of all authors.
(11) Bauer, R. C.; Gloaguen, Y.; Lutz, M.; Reek, J. N. H.; de Bruin, B.;
van der Vlugt, J. I. Dalton Trans. 2011, 40, 8822−8829.
Notes
(12) Friedrich, A.; Ghosh, R.; Kolb, R.; Herdtweck, E.; Schneider, S.
Organometallics 2009, 28, 708−718.
The authors declare no competing financial interest.
(13) Weng, W.; Yang, L.; Foxman, B. M.; Ozerov, O. V.
Organometallics 2004, 23, 4700−4703.
ACKNOWLEDGMENTS
■
This work was financially supported by NWO-ACTS (ASPECT
053.62.029) and ERC Starting Grant EuReCat, Grant Agree-
ment 279097. The COST network CM0802 “PhoSciNet”, Prof.
J. N. H. Reek, and Dr. A. W. Ehlers are thanked for suggestions.
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(18) Deprotonation was not demonstrated for the related fac-PNP
structure A.15 mer-PNP ligands can undergo shuttling between amine
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NOTE ADDED AFTER ASAP PUBLICATION
■
Due to a production error, this paper was published on the Web
on January 29, 2013, with errors in the SI file. The corrected
version was reposted on February 1, 2013.
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