Net hydrogenation of Pt-NHPh bond is catalyzed by elemental Pt

Article abstract of DOI:10.1021/ja9102309

Synthesis and characterization of the monomeric complex ( ^tbpy)Pt(Me)(NHPh) (^tbpy = 4,4--di- tert -butyl-2,2--dipyridyl) has been accomplished. Mechanistic studies reveal that 1,2-addition of dihydrogen across the Pt-anilido bond to initially produce (^tbpy)Pt(Me)(H) and free aniline is catalyzed by elemental Pt rather than through a pathway that involves direct activation of H₂ by Pt and 1,2-addition across the Pt-NHPh bond.

Full text of DOI:10.1021/ja9102309

Published on Web 03/11/2010
Net Hydrogenation of Pt-NHPh Bond Is Catalyzed by Elemental Pt
†
§
‡
,†
Joanna R. Webb, Aaron W. Pierpont, Colleen Munro-Leighton, T. Brent Gunnoe,*
,
§
‡
Thomas R. Cundari,* and Paul D. Boyle
Department of Chemistry, UniVersity of Virginia, CharlottesVille, Virginia 22904, Department of Chemistry, North
Carolina State UniVersity, Raleigh, North Carolina 27695, and Department of Chemistry, Center for AdVanced
Scientific Computing and Modeling (CASCaM), UniVersity of North Texas, Denton, Texas 76203
Received December 3, 2009; E-mail: tbg7h@virginia.edu
The addition of H
central reaction for several synthetic transformations. For example,
Stryker’s reagent, [(Ph P)CuH] , a catalyst for conjugate addition
reactions, is generated by hydrogenolysis of a Cu-O Bu bond.
Additionally, net H addition across M-O CH bonds completes
catalytic conversion of CO and H to formic acid, H
across Pt-OH bonds has been proposed in a cycle for olefin
2
across M-NHR or M-OR moieties is a
2A). The observation of an induction period led us to consider Pt(s)
as a heterogeneous catalyst.
As an initial probe of a Pt(s) catalyzed process, we used a
2
1
3
6
t
1,2
21
standard Hg test. The addition of Hg to a solution of 2 in C
followed by pressurization with H completely suppresses reactivity
(see Supporting Information). Next, a solution of 2 pressurized with
was allowed to react until visible formation of Pt(s), at which
point conversion of 2 to 3 was observed. The reaction solution was
decanted from the tube, a fresh solution of 2 in C was added to
the original NMR tube and pressurized with H , and the reaction
was followed by H NMR. A plot of [2] versus time reveals no
6 6
D
2
2
2
3
2
2
2
addition
H
2
4
epoxidation, and H addition across M-NHR bonds has been
2
5
implicated in asymmetric hydrogenations. Despite their importance,
reports of well-defined reactions with late(r) transition metal systems
6 6
D
2
5
-8
1
are rare and mechanistic studies are limited.
Recently, C-H
2
2
activation of hydrocarbons via net 1,2-addition of C-H bonds
across metal-heteroatom bonds using late transition metals (with
g6 d-electrons) bearing formally anionic ligands (e.g., -NHR or
induction period (see Supporting Information). Maitlis’ test was
performed by monitoring the reaction until past the induction period.
The reaction tube was then vented, the solution was filtered through
Celite, and the filtrate was placed in a clean NMR tube, pressurized
9
-14
-
OR) has been reported.
Despite interest in these C-H and
H-H bond transformations, questions regarding the mechanism
2
with H , and monitored. Figure 2B shows that a second induction
period is observed after removal of Pt(s). As additional confirmation,
in a separate experiment 10 wt % Pt on activated carbon was added
1
0,15-17
remain.
The complex [( bpy)Pt(Me)(NH
reaction of aniline and ( bpy)Pt(Me)(O CCF
t
2
Ph)][O
2
CCF
3
] (1) is formed upon
t
18
t
2
to a solution of 2 before pressurization with H . The consumption
2
3
)
( bpy ) 4,4′-di-
t
of 2 reached 50% conversion after 5 min and completion in <1 h.
tert-butyl-2,2′-dipyridyl). The deprotonation of 1 produces ( bpy)-
A control experiment with 2 and elemental Pt in the absence of H
2
Pt(Me)(NHPh) (2). For 2, a broad singlet is observed at 3.99 ppm
1
resulted in no reaction.
(
H NMR) due to the amido proton. The methyl ligand resonates
2
1
at 1.85 ppm (singlet with Pt satellites, JPt-H ) 84 Hz) in the H
1
NMR spectrum and at -14.2 ppm (singlet with Pt satellites, JPt-C
)
813 Hz) in the 13C{ H} NMR spectrum. The solid-state structure
1
of 2 (Figure 1) confirms its monomeric nature.
Reactions of a benzene solution of 2 pressurized with H
temperature ultimately produce free bpy ligand, methane, and
2
at room
t
aniline. Careful monitoring of the reaction at 45 psi of H
2
revealed
the formation of ( bpy)Pt(Me)(H) (3) as an intermediate (Scheme
and Figure S4 in the Supporting Information). For complex 3, a
t
1
1
hydride resonance is observed at -14.8 ppm ( JPt-H ) 1575 Hz)
in the H NMR spectrum as well as a new methyl resonance at
1
t
Figure 1. ORTEP of ( bpy)Pt(Me)(NHPh) (2) (50% probability; H atoms
omitted for clarity). Selected bond lengths (Å): Pt1-N1 2.077(2), Pt1-N2
2
2.21 ppm ( JPt-H ) 83 Hz). The mechanism for dihydrogen addition
2
.026(2), Pt1-C1 2.040(3), Pt1-N3 2.005(2), N3-C20 1.357(4).
II
across Pd -OR (R ) H or CH
Kemp, Muller, and Fulmer involves 1,2-addition of H
Pd-OR moiety, via a σ-bond metathesis type transition state, to
3
) bonds proposed by Goldberg,
t
2
across the
Scheme 1. Reaction of ( bpy)Pt(Me)(NHPh) (2) and H
2
II
8
produce ROH and a Pd -H complex. Thus, reaction of H
2
with
2
via a similar mechanism would involve Pt mediated 1,2-addition
t
2 2
of H to give ( bpy)Pt(Me)(NH Ph)(H) with aniline dissociation to
form 3 and subsequent decomposition of 3 to methane, Pt(s), and
t
free bpy. We initially presumed this straightforward pathway, which
is consistent with the general notion of reactions with related
5
,7,8,13,19,20
1
systems.
Monitoring ( H NMR) the reaction of 2 with
H
2
and a plot of [2] versus time reveals an induction period (Figure
The proposed pathway of H addition across the Pt-NHPh bond
2
of 2 contrasts the proposed mechanism of hydrogenolysis of related
Pd(II) hydroxide and methoxide complexes. To probe the source of
these differences, DFT calculations (B3LYP, pseudopotentials) were
†
University of Virginia.
University of North Texas.
North Carolina State University.
8
§
‡
4
520 9 J. AM. CHEM. SOC. 2010, 132, 4520–4521
10.1021/ja9102309  2010 American Chemical Society

C O M M U N I C A T I O N S
t
employed. Reaction of 2 and H
be exothermic by 6 kcal/mol, but with a barrier of 45 kcal/mol. In
contrast, the calculated barrier for H addition across the Pd-OH bond
of (PCP)Pd(OH) {PCP ) 2,6-bis(CH PMe }, to give (PCP)Pd-
H) and water, is 21.0 kcal/mol, despite the similar reaction enthalpy
2
to give 3 and aniline is calculated to
The activation of H
2
by ( bpy)Pt(Me)(NHPh) (2) produces
( bpy)Pt(Me)(H) (3). However, kinetic studies lead to the conclusion
that, rather than direct activation of H across the Pt-NHPh bond,
t
2
2
2
2
)
2
C H
6 3
Pt(s) is catalyzing the hydrogenation of the Pt-NHPh moiety,
though the specific mechanism of this transformation is not known.
To our knowledge, this is the first report of a heterogeneous catalyst
for activation of covalent bonds toward addition across an M-X
(X ) NHR or OR) bond and a rare example of net H addition to
2
M-L bonds by a heterogeneous catalyst.
(
8
(
-4 kcal/mol). At the current level of theory, the calculated H
activation barrier and reaction enthalpy for (PCP)Pd(OMe) (24 and
3 kcal/mol, respectively) are similar to those of the hydroxy analogue.
2
-
2
3,24
2
Figure 3. Calculated transition state for activation of H by complex 2.
Most hydrogen atoms omitted for clarity.
Acknowledgment. T.B.G. acknowledges the NSF (CHE-
848693) for funding. T.R.C. thanks the NSF for support (CHE-
701247) and facilities (CHE-0741936). A.W.P. thanks the UNT
0
0
Toulouse Graduate School for a Dissertation Fellowship.
Supporting Information Available: Characterization data, experi-
mental and computational details including kinetic studies and plots,
and X-ray crystallographic data files for 2. This material is available
free of charge via the Internet at http://pubs.acs.org.
2
Figure 2. Plots monitoring [2] versus time: (A) 200 psi of H at room
temperature, (B) reaction solution filtered to remove Pt(s) after initial
induction period (plot started after 14 400 s reaction time).
References
(
1) Deutsch, C.; Krause, N.; Lipshutz, B. H. Chem. ReV. 2008, 108, 2916.
2) Goeden, G. V.; Caulton, K. G. J. Am. Chem. Soc. 1981, 103, 7354.
(
Figure 3 displays the calculated transition state for H-H
activation by 2. For the amido lone pair (conjugated with the phenyl
substituent) to orient to receive a H atom from H , it must be aligned
2
(3) Jessop, P. G.; Joo, F.; Tai, C. C. Coord. Chem. ReV. 2004, 248, 2425.
(
(
4) Wenzel, T. T. Stud. Surf. Sci. Catal. 1991, 66, 545.
5) Sandoval, C. A.; Ohkuma, T.; Muniz, K.; Noyori, R. J. Am. Chem. Soc.
2
003, 125, 13490.
(
(
(
6) Abdur-Rashid, K.; Faatz, M.; Lough, A. J.; Morris, R. H. J. Am. Chem.
perpendicular to the Pt square plane. This orientation would place
Soc. 2001, 123, 7473.
the phenyl substituent in close proximity to either the Me or the
7) Fryzuk, M. D.; Montgomery, C. D.; Rettig, S. J. Organometallics 1991,
10, 467.
t
2
bpy ligand, which provides a steric inhibition to H activation. In
8) Fulmer, G. R.; Muller, R. P.; Kemp, R. A.; Goldberg, K. I. J. Am. Chem.
Soc. 2009, 131, 1346.
the calculated transition state, the phenyl substituent may rotate
perpendicular to the amide plane, breaking Cipso-N conjugation.
Note the longer bond length of 1.46 Å for the Cipso-N bond in the
TS (Figure 3) versus 1.39 Å (calcd., 1.36 Å/experimental) in 2.
(9) Feng, Y.; Lail, M.; Barakat, K. A.; Cundari, T. R.; Gunnoe, T. B.; Petersen,
J. L. J. Am. Chem. Soc. 2005, 127, 14174.
(
(
10) Feng, Y.; Lail, M.; Foley, N. A.; Gunnoe, T. B.; Barakat, K. A.; Cundari,
T. R.; Petersen, J. L. J. Am. Chem. Soc. 2006, 128, 7982.
11) Tenn, W. J., III; Young, K. J. H.; Bhalla, G.; Oxgaard, J.; Goddard, W. A.,
III; Periana, R. A. J. Am. Chem. Soc. 2005, 127, 14172.
Comparison of calculated H
HPh) (M ) Pd or Pt) and (PCP)Pd(OMe), (bpy)Pt(Me)(NHMe),
bpy)M(Me)(OMe), and (bpy)Pt(Me)(NHPh*) {Ph* denotes a Ph
2
-scission barriers for (bpy)M(Me)(N-
(12) Kloek, S. M.; Heinekey, D. M.; Goldberg, K. I. Angew. Chem., Int. Ed.
2
007, 46, 4736.
(
(
13) Conner, D.; Jayaprakash, K. N.; Cundari, T. R.; Gunnoe, T. B. Organo-
in the MM region of a QM/MM calculation} suggests that the
remarkable difference in barriers between (bpy)Pt(Me)(NHPh) and
metallics 2004, 23, 2724.
(
(
14) Bercaw, J. E.; Hazari, N.; Labinger, J. A. Organometallics 2009, 28, 5489.
15) Oxgaard, J.; Tenn, W. J., III; Nielson, R. J.; Periana, R. A.; Goddard, W. A.,
III. Organometallics 2007, 26, 1565.
(PCP)Pd(OMe) is due in equal parts to replacement of the metal
(Pt f Pd), activating ligand (NHMe f OMe), and supporting ligand
(bpy/Me f PCP). The predicted enhancement upon substituting
(
(
(
16) Boutadla, Y.; Davies, D. L.; Macgregor, S. A.; Poblador-Bahamonde, A. I.
J. Chem. Soc., Dalton Trans. 2009, 5820.
17) Cundari, T. R.; Grimes, T. V.; Gunnoe, T. B. J. Am. Chem. Soc. 2007,
1
29, 13172.
Pd for Pt might be explained by the anticipated increase in
electrophilicity of Pd (vs Pt), which would facilitate H activation.
The long Pt-H bond distances in the calculated transition state for
(Figure 3) are consistent with weak Pt/H activation as one source
18) Hill, G. S.; Rendina, L. M.; Puddephatt, R. J. J. Chem. Soc., Dalton Trans.
2
1
996, 1809.
(19) Abdur-Rashid, K.; Clapham, S. E.; Hadzovic, A.; Harvey, J. N.; Lough,
A. J.; Morris, R. H. J. Am. Chem. Soc. 2002, 124, 15104.
2
2
(
20) Yang, J. Y.; Bullock, R. M.; Shaw, W. J.; Twamley, B.; Fraze, K.; DuBois,
M. R.; DuBois, D. L. J. Am. Chem. Soc. 2009, 131, 5935.
21) Widegren, J. A.; Finke, R. G. J. Mol. Catal. A: Chem. 2003, 198, 317.
22) Hamlin, J. E.; Hirai, K.; Millan, A.; Maitlis, P. M. J. Mol. Catal. 1980, 7,
543.
of a substantial activation barrier. These influences underscore the
subtle balance required to access systems that can activate H-H
(
(
and C-H bonds. In the transition states for H
2
activation by
(
23) Miller, T. M.; Izumi, A. N.; Shih, Y. S.; Whitesides, G. M. J. Am. Chem.
(
bpy)M(Me)(NHPh) the calculated Pt-H bond distances are 2.48
Soc. 1988, 110, 3146.
and 2.34 Å, while the Pd-H distances for the TS of the complex
(24) Begum, R. A.; Chanda, N.; Ramakrishna, T. V. V.; Sharp, P. R. J. Am.
Chem. Soc. 2005, 127, 13494.
8
reported by Goldberg are indeed shorter, 2.12 and 2.14 Å, despite
the near identical covalent radii of Pd and Pt.
JA9102309
J. AM. CHEM. SOC. 9 VOL. 132, NO. 13, 2010 4521