'Hemilabile' silyl pincer ligation: Platinum group PSiN complexes and triple C-H activation to form a (PSiC)Ru carbene complex

Article abstract of DOI:10.1039/c2cc16751d

The first example of PSiN mixed-donor silyl pincer ligation is described. Studies involving platinum group metal complexes of [(2-^tBu ₂PC₆H₄)(2-Me₂NC₆H ₄)SiMe]^- (^tBu-PSiN-Me) confirmed that the ligand amino donor is labile. Within the coordination sphere of Ru, ^tBu-PSiN-Me is transformed into a PSiC ligand via multiple C-H bond activation events. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2cc16751d

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COMMUNICATION
‘Hemilabile’ silyl pincer ligation: platinum group PSiN complexes and
triple C–H activation to form a (PSiC)Ru carbene complexw
Adam J. Ruddy,a Samuel J. Mitton,a Robert McDonaldb and Laura Turculet*a
Received 31st October 2011, Accepted 20th November 2011
DOI: 10.1039/c2cc16751d
The first example of PSiN mixed-donor silyl pincer ligation is
described. Studies involving platinum group metal complexes of
[(2-tBu2PC6H4)(2-Me2NC6H4)SiMe]ꢀ (tBu-PSiN-Me) confirmed
that the ligand amino donor is labile. Within the coordination
sphere of Ru, tBu-PSiN-Me is transformed into a PSiC ligand via
multiple C-H bond activation events.
utility of PSiP and NSiN pincer species, we viewed hemilabile
PSiN-ligated complexes as being attractive targets of inquiry.
We report herein on the establishment of PSiN ligation,
including the synthesis of Ru, Rh, Ir, Pd and Pt complexes
derived from the new PSiN silyl pincer precursor
[(2-tBu2PC6H4)(2-Me2NC6H4)SiMe]H ((tBu-PSiN-Me)H). Our
preliminary studies confirm that the amino donor of the
tBu-PSiN-Me ligand is indeed labile. Furthermore, we disclose
herein that within the coordination sphere of Ru, the tBu-PSiN-Me
ligand is transformed into a PSiC ligand by a process involving
multiple C–H bond activation events.
Transition metal pincer complexes supported by LXL-type
(L = neutral donor, X = anionic donor) ligands are of
substantial interest due to the remarkable stoichiometric and
catalytic reactivity exhibited by such complexes.1 While PCP,
NCN, and PNP pincer ligation has been widely explored in
this context, significant effort has also been devoted to the
development of alternative pincer architectures.1,2 In this
regard, there has been increased interest in the chemistry of
pincer complexes supported by PCN or PNN ligands that
contain both hard (N) and soft (P) neutral donors.3 Upon
complexation to electron-rich late metal centers, such ligands
have been shown to exhibit hemilabile4 coordination involving
the L-donors due to the more labile amine donor, which has
been demonstrated to lead to unique reactivity.3
Our synthetic strategy for the preparation of PSiN-type
ligands involved the synthesis of [2-(NMe2)C6H4]SiMeHCl,
which was achieved by the reaction of [2-(NMe2)C6H4]MgBr
with excess MeSiHCl2. The tertiary silane (tBu–PSiN–Me)H (1)
was prepared in 78% yield by the reaction of [2-(NMe2)C6H4]-
SiMeHCl with 2-(tBu2P)C6H4Li.
Group 10 complexes of the type (tBu–PSiN–Me)MX (2, M =
Pd, X = Br; 3, M = Pt, X = Cl) were prepared by the reaction of
1 with either PdBr2 or (COD)PtCl2 (COD = 1,5-cyclooctadiene)
in the presence of Et3N (Scheme 1). The room temperature
1H NMR (benzene-d6) spectra of 2 and 3 each feature a broad
singlet corresponding to the ligand NMe2 protons (2: 3.17 ppm;
3: 3.22 ppm). At low temperature, this resonance decoalesces
(toluene-d8, 218 K: for 2, 3.30 and 3.05 ppm; for 3, 3.29 and
3.01 ppm), which is indicative of inequivalent NMe groups in
square planar complexes of the type k3-(tBu-PSiN-Me)MX.
These lineshape changes are consistent with a dynamic process
involving decomplexation of the amine arm and inversion and
We have recently reported on the synthesis and reactivity of
complexes featuring bis(phosphino)silyl ligation of the type
[k3-(2-R2PC6H4)2SiMe]ꢀ (R-PSiP),5,6 including rare examples
of trigonal pyramidal Ru complexes,5a Ir species that undergo
C–H and N–H bond activation,5b,c and Group 10 complexes that
undergo reversible sp2–sp3 and sp3-sp3 C–Si bond cleavage.5d
Our group, as well as that of Iwasawa, has also demonstrated the
catalytic utility of PSiP ligation, including examples of ketone
transfer hydrogenation,5e dehydrogenative borylation of alkenes,6a
and hydrocarboxylation of alkenes6b and allenes.6c The synthesis
and reactivity of NSiN pincer species has also been investigated.7
However, no previous example of PSiN pincer ligation has been
reported. Given the unique reactivity properties and catalytic
a Department of Chemistry, Dalhousie University, 6274 Coburg Road,
P.O. Box 15 000, Halifax, Nova Scotia, Canada B3H 4R2.
E-mail: laura.turculet@dal.ca; Fax: 1 902 494 1310;
Tel: 1 902 494 6414
b X-Ray Crystallography Laboratory, Department of Chemistry,
University of Alberta, Edmonton, Alberta, Canada T6G 2G2
w Electronic supplementary information (ESI) available: Experimental
details and characterization data. CCDC 785979–851920. For ESI and
crystallographic data in CIF or other electronic format see DOI:
10.1039/c2cc16751d
Scheme 1 Synthesis of Group 9 and 10 tBu-PSiN-Me complexes.
(i) for M = Pd, PdBr2; for M = Pt, (COD)PtCl2 (COD = 1,5-
cyclooctadiene); Et3N; (ii) AgOTf; (iii) PMe3; (iv) M = Pd, BPh3;
(v) 0.5 [(COD)MCl]2; (vi) PMe3.
c
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Fig. 1 ORTEP diagrams for 4ꢁ0.5C6H6, 7, 8, 11, and 13 shown with 50% ellipsoids. Solvate molecules and selected H-atoms have been omitted
for clarity. Bond lengths (A) and angles (1) for 4ꢁ0.5C6H6: Pd-P 2.2731(4), Pd-Si 2.2336(4), Pd-N 2.2345(12), Pd-O1 2.3518(11), P-Pd-N 157.86(4),
Si-Pd-O1 168.92(3); for 7: Pt-P1 2.3188(8), Pt-P2 2.3026(8), Pt-Si 2.3077(9), Pt-Cl 2.4846(8), Cl-Pt-Si 176.86(3), P1-Pt-P2 179.36(3); for 8: Rh-Si
2.2175(4), Rh-P 2.2421(4), Rh-N 2.2349(12), Rh-Cl 2.4365(4), P-Rh-N 163.70(3), Si-Rh-H1 67.4(9), Cl-Rh-Si 132.103(14); for 11: Ru-Si 2.2461(5),
Ru-P 2.3698(4), Ru-C41 1.9101(15), N-C41 1.353(2), Ru-C2 2.2752(16), Ru-C3 2.1737(15), Ru-C4 2.2115(16), P-Ru-C41 97.47(5); for 13: Ru-P
2.3458(4), Ru-Si 2.3322(4), Ru-C41 2.1558(13), N-C41 1.4653(18), P-Ru-Si 83.001(13), P-Ru-C41 88.42(4). Si-Ru-C41 74.16(4).
rotation at N, which renders the NMe groups equivalent at
elevated temperatures (DGz = 12 kcal molꢀ1 for 2; DGz =
13 kcal molꢀ1 for 3). This phenomenon highlights the labile
as was previously observed for the analogous Cy–PSiP ligated
species (for M = Rh, 65.8(12)1; for M = Ir, 68.7(18)1).5b
Treatment of 8 with PMe3 led to the quantitative formation of
k2-(tBu-PSiN-Me)Rh(H)(Cl)(PMe3) (10), in keeping with the
hemilabile character of tBu-PSiN-Me. The solid state structure
of 10 was confirmed using single-crystal X-ray diffraction
analysis (Fig. S2w). No reaction was observed upon treatment
of 10 with BPh3.
t
coordination of the amino donor of the Bu-PSiN-Me ligand.
Treatment of 2 and 3 with AgOTf led to the formation of
(tBu-PSiN-Me)M(OTf) (M = Pd, 4; M = Pt, 5) species. The
structure of 4 was confirmed by X-ray diffraction analysis
(Fig. 1) and indicates approximate square planar geometry
at Pd, with trans-disposed phosphino and amino donors. The
Pd-Si distance of 2.2336(4) A is short and falls outside the
range characteristic of Pd–Si bond distances (2.26–2.64 A).8
As well, the Pd–O distance of 2.3518(11) A is longer than the
Pd–O distance in [k3-(2-Me2NC6H4)2N]PdOTf (2.1067(16) A),9
which highlights the strong trans influence of the silyl donor in 4.
Treatment of 2 and 3 with PMe3 led to selective decomplexation
of the amine arm of the PSiN ligand and the formation of
k2-(tBu-PSiN-Me)MX(PMe3) (6, M = Pd, X = Br; 7, M = Pt,
X = Cl; Scheme 1). The 1H NMR spectra of 6 and 7 each
feature a sharp singlet resonance corresponding to the six
equivalent NMe2 protons of the uncoordinated amino ligand
arm. The coordination of PMe3 to the metal center was
confirmed by use of 31P NMR spectroscopy. The solid state
structure of 7 (Fig. 1) indicates approximate square planar
geometry at Pt, with the PMe3 ligand coordinated trans to the
phosphino ligand arm. Treatment of complex 6 with an equivalent
Attempts to prepare (tBu-PSiN-Me)Ru complexes by the
reaction of 1 with RuII starting materials such as (PPh3)3RuCl2
and [(p-cymene)RuCl2]2 in the presence of Et3N were largely
unsuccessful, typically leading to the formation of multiple
intractable products (31P NMR). Conversely, heating 1 with one
equiv. of (COD)Ru(2-methylallyl)2 for 18 h at 85 1C (THF) led to
the quantitative (31P NMR) formation of a single new product
(11). However, the NMR features of isolated 11 are not consistent
with the formation of a (tBu-PSiN-Me)Ru(2-methylallyl) complex.
1
Rather, the H and 13C NMR spectra of 11 indicate surprisingly
the formation of a cyclooctenyl (PSiC)Ru carbene complex
resulting from C–H bond activation of an NMe group in the
tBu-PSiN-Me ligand and of the 1,5-cyclooctadiene, respectively
(Scheme 2). The RuQCH2 unit exhibits diagnostic 13C and
1H NMR resonances (benzene-d6) at 250.8 and 13.11 ppm,
respectively. The 1H NMR spectrum of 11 also revealed the
presence of only one NMe group (2.82 ppm, s, 3H), which is
consistent with the proposed formulation of 11. The structure
of 11 was confirmed by X-ray diffraction analysis (Fig. 1).
10
of BPh3 in benzene led to precipitation of Ph3BPMe3 and
quantitative regeneration of 2 (Scheme 1). This sequence of
reactions involving complexation of PMe3 to 2 to form 6 followed
by reformation of 2 upon reaction with BPh3 further illustrates the
hemilabile character of tBu–PSiN–Me ligation.
t
In an effort to extend the coordination chemistry of Bu–
PSiN–Me ligation to Group 9 metals, complexes of the type
(tBu-PSiN-Me)M(H)Cl (M = Rh, 8; M = Ir, 9) were prepared
by the reaction of 1 with [(COD)MCl]2. The room temperature
1H NMR (benzene-d6) spectra of 8 and 9 each feature two
resonances corresponding to inequivalent NMe groups (for 8:
3.06 and 2.96 ppm; for 9: 3.17 and 2.94 ppm), consistent with
coordination of the amine ligand arm to the metal center. Single-
crystal X-ray diffraction analysis (Fig. 1 and S1w) established that
complexes 8 and 9 both exhibit distorted square based pyramidal
geometry with Si in the apical site. These structures are similar
to those observed for (Cy-PSiP)M(H)Cl complexes, and feature
acute Si–M–H angles (for M = Rh, 67.4(9)1; for M = Ir, 71.0(12)1)
Scheme 2 Synthesis of (PSiC)Ru complexes. (i) (COD)Ru(2-methylallyl)2;
(ii) 1 atm H2, 75 1C, 3 days or 110 1C, 10 days, both in C6H6.
c
1160 Chem. Commun., 2012, 48, 1159–1161
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The geometry at Ru is distorted square-pyramidal with Si in
the apical site. The Ru–C41 distance of 1.9101(15) A is
consistent with related Ru carbene complexes.11a,b The short
N–C41 bond distance of 1.353(2) A (cf. N–C42 = 1.468(2) A)
and planar environment at N are indicative of p-bonding
between nitrogen and C41.
catalytic behavior of PSiN-ligated complexes and this will be
the topic of future reports. We also observed that within the
coordination sphere of Ru, 1 is transformed into a PSiC ligand
in a process requiring overall three C–H bond activation steps,
as well as Si-H bond activation, at a single metal center. This
represents the first reported example of PSiC ligation, which
also promises to provide access to reactive late metal species.
The reactivity of such PSiC-ligated Ru complexes with E–H
bonds is currently under investigation.
Remarkably, the formation of 11 requires three C–H bond
activation steps, as well as Si–H bond activation at a single Ru
center. We propose that the formation of the carbene fragment
may occur via C–H bond activation of the Ru-bound NMe2 ligand
arm in a putative k3-(tBu-PSiN-Me)Ru(Z1-2-methylallyl)(COD)
intermediate to form a cyclometalated species, which undergoes
a–H elimination to form the carbene complex (Scheme S1w).
Double C–H bond activation of a Me group by a RuII center to
generate a carbene complex has been reported,11,12 and analogous
mechanisms have been proposed.11,12 Complex 11 is the first
reported example of a PSiC ligated metal species.
We are grateful to NSERC of Canada and Dalhousie
University for their support of this work.
Notes and references
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and C. M. Jensen, Elsevier, Oxford, 2007; (b) M. E. van der Boom
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G. van Koten, Angew. Chem., Int. Ed., 2001, 40, 3750.
2 For representative examples see: (a) W. Leis, H. A. Mayer and
W. C. Kaska, Coord. Chem. Rev., 2008, 252, 1787; (b) Y. Segawa,
M. Yamashita and K. Nozaki, J. Am. Chem. Soc., 2009, 131, 9201;
(c) R. Gerber, O. Blacque and C. M. Frech, Dalton Trans., 2011,
We envisioned that 11 could serve as a source of the
14-electron species 12 upon undergoing an Z3 to Z1 rearrangement
of the cyclooctenyl ligand (Scheme 2). Indeed, the reaction of a
benzene solution of 11 with an atmosphere of H2 resulted in
the formation of the Z6-benzene RuII alkyl complex 13 with
concomitant formation of cyclooctene (Scheme 2). The 1H NMR
spectrum of 13 (benzene-d6) no longer features a downfield-shifted
carbenic resonance; rather, diastereotopic Ru-CH2N protons are
observed as multiplets at 4.58 and 1.92 ppm. The solid state
structure of 13 was confirmed by X-ray diffraction analysis
(Fig. 1). The Ru–C41 distance of 2.156(1) A is consistent with a
Ru–C single bond, while the N–C41 bond length of 1.465(2) A is
indicative of a N–C single bond.
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6 For additional examples of phenylene-bridged PSiP pincers see:
(a) J. Takaya, N. Kirai and N. Iwasawa, J. Am. Chem. Soc., 2011,
133, 12980; (b) J. Takaya, K. Sasano and N. Iwasawa, Org. Lett.,
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L. Konstantinovski and D. Milstein, Inorg. Chem., 2008, 47, 7177.
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27, 2223; (b) P. Sangtrirutnugul, M. Stradiotto and T. D. Tilley,
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We propose that the formation of 13 occurs via the net
reaction of 12 with H2 to form a cyclooctene hydride
Ru species, which liberates cyclooctene in benzene solution
to form a benzene adduct (Scheme 2). Subsequent a–H
migration from Ru to the carbenic carbon leads to the
formation of 13. Interestingly, 13 can also be synthesized by
simply heating a benzene solution of 11 at 110 1C for ten days.
The mechanism for the formation of 13 in the absence of H2 likely
involves b–H elimination in 12 to form a 1,3-cyclooctadiene
hydride Ru species, which subsequently liberates cyclooctadiene
to form a benzene adduct (Scheme 2).
In summary, the synthesis of the first examples of PSiN-
ligated platinum group pincer complexes has been provided.
t
The amino donor of the Bu-PSiN-Me ligand is labile and is
displaced from the metal coordination sphere by a better
donor, such as PMe3, to form k2-(tBu-PSiN-Me) species.
The pincer structure is reformed upon abstraction of PMe3 from
the metal center. This reversible coordination of the amine pincer
arm is anticipated to render PSiN-ligated complexes responsive
to the changing electronic and coordinative requirements at a
metal center that arise during substrate transformations, and
may provide access to new and/or enhanced reactivity. We are
currently exploring the influence of this hemilability on the
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 1159–1161 1161