[2.1.1]-(2,6)-pyridinophane(L)-controlled alkane C-H bond cleavage: (L)PtMe2H+ as a precursor to the geometrically tense transient (L)PtMe+

Article abstract of DOI:10.1039/b302055j

(η²-L)PtMe₂ {L = [2.1.1]-(2,6)-pyridinophane} is protonated at Pt to give (η³-L)PtHMe₂⁺, where three pyridine ligands of the macrocycle L bind to Pt in a facial manner. This cation eliminates methane at a rate convenient for trapping of the T-shaped, 14-valence electron (η²-L)PtMe⁺ (based on DFT geometry optimization) by ethane, propane, n-butane, cyclopentane and cyclohexane to give (η³-L)Pt(olefin)H⁺ and CH₄. The crystal structure of (η³-L)Pt(cyclohexene)H⁺ as its BAr₄^F- salt is reported.

Full text of DOI:10.1039/b302055j

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[2.1.1]-(2,6)-Pyridinophane(L)-controlled alkane C–H bond cleavage:
(L)PtMe2H+ as a precursor to the geometrically ‘‘tense’’ transient
(L)PtMe+ y
Andrei N. Vedernikov,* John C. Huffman and Kenneth G. Caulton*
L e t t e r
Department of Chemistry and Molecular Structure Center, Indiana University, Bloomington,
IN 47405, USA
Received (in New Haven, CT, USA) 12th February 2003, Accepted 12th February 2003
First published as an Advance Article on the web 18th March 2003
(g2-L)PtMe2 {L ¼ [2.1.1]-(2,6)-pyridinophane} is protonated at
Pt to give (g3-L)PtHMe2+, where three pyridine ligands of the
macrocycle L bind to Pt in a facial manner. This cation elimi-
nates methane at a rate convenient for trapping of the T-shaped,
14-valence electron (Z2-L)PtMe+ (based on DFT geometry
Scheme 1
optimization) by ethane, propane, n-butane, cyclopentane and
cyclohexane to give (Z3-L)Pt(olefin)H+ and CH4 . The crystal
structure of (Z3-L)Pt(cyclohexene)H+ as its BArF4ꢀ salt is
reported.
ligand 27 shown in Fig. 1(a). This ligand is shown here to allow
unusually fast room temperature CH bond cleavage of alipha-
tic hydrocarbons RH (RH ¼ C2H6 , C3H8 , n-butane, cyclo-
C5H10 , cyclo-C6H12).
Mild transition metal catalyzed hydrocarbon functionalization
is the focus of much research work.1–12 Despite many metal
complexes able to effect alkane CH bond oxidative addition,
producing alkyl hydrido species, the potential of this funda-
mental reaction in elaboration of new catalytic hydrocarbon
transformations remains still unused; few examples of catalysis
involving this step are known.13,14,15 To serve such a catalytic
role, metal alkyl hydrides should be sufficiently transient
(‘‘unstable’’) under catalytic conditions but this makes their
characterization more difficult.16 Particular examples include
platinum(IV) alkyl hydrido species,17 which are supposed to
be responsible for so-called Shilov-type chemistry,18 and
recently for methane transformation into methylhydrosulfate19
and some other reactions involving hydrocarbon CH bond
cleavage.17 Both moderately persistant20,21 and even quite
persistant (‘‘stable’’) platinum(IV) alkyl hydrides22–24 permit
testing the intermediacy of platinum(IV) alkyl hydrides in
alkane functionalization and to reveal some factors controlling
their reactivity.17 Moreover, alkane oxidative addition to
the presumably 14-electron platinum(II) transient species
Tp*PtMe8 has proven this is a feasible process. However, use
of platinum(IV) hydrocarbyl hydrides in alkane activation,
the simplest example of which is hydrocarbyl ligand exchange,
has been demonstrated only recently.25
According to DFT calculations (program package Pri-
roda,28 functional PBE,29 basis set SBK30), the cationic
hydrido dimethyl platinum(IV) complex [PtMe2H(L)]+ is
¼
expected to allow facile (DG
¼ 16 kcal molꢀ1) and thermo-
298
dynamically only slightly unfavorable (DGꢁ
¼ +11 kcal
298
molꢀ1) methane elimination to produce [PtMe(Z2-L)]+, I.
Due to the tridentate ligand constraints, its three nitrogen
donor atoms cannot be arranged in one plane with the Pt–C
bond, thus preventing formation of a stable (i.e., less reactive)
16-electron square planar complex.
The new cationic hydridodimethyl platinum(IV) complex
[PtMe2H(L)]+ can be obtained by protonation of the corre-
sponding [PtMe2(Z2-L)]27,31 with a suitable Brønsted acid
according to the known method.20–23 To preserve the reactivity
of the platinum(II) species I, we have used the anion tetra-
kis[3,5-bis(trifluoromethyl)phenyl]borate, BArF4 ꢀ and a solvent
of low donicity, dichloromethane. This is accomplished by one-
pot protonation of the corresponding dimethylplatinum(II)
Thus, platinum(IV) alkyl hydrides of intermediate thermo-
dynamic and kinetic stability would be of special interest pre-
cisely because they could be convenient sources of the reactive
II
unsaturated fragment LnPt . Recently we have developed an
approach to designing new ligands and complexes that opti-
mize the reactivity of 14-electron d8 metal complexes.26 We
report here our observations of hydrocarbon CH bond oxida-
tive addition to a cationic, presumably 14-electron transient
platinum(II) complex resulting (I, Scheme 1) from methane
elimination from a dimethyl hydrido platinum(IV) species
[PtMe2H(L)]+ supported with the [2.1.1]-(2,6)-pyridinophane
Fig. 1 (a) Ligand L. (b) ORTEP diagram of [PtH(cyclohexene)(Z3-
L)]+ ion in its BArF4 ꢀ salt. Hydrogen atoms are omitted. The hydride
ligand presumably located trans to N2 was not located. Selected bond
lengths (A) and angles ( ): Pt–C24, 2.098(3); Pt–C25, 2.077(3); C24–
C25, 1.443(5); Pt–N2, 2.198(3); Pt–N14, 2.221(3); Pt–N21, 2.221(3);
N2-Pt–N14, 84.9(1); N2–Pt–N21, 90.1(1); N14–Pt–N21, 82.1(1); N2–
Pt–C24, 93.3(1); N2–Pt–C25, 91.5(1).
ꢁ
˚
y Electronic supplementary information (ESI) available: complete
experimental and characterization data. See http://www.rsc.org/supp-
data/nj/b3/b302055j/.
DOI: 10.1039/b302055j
New J. Chem., 2003, 27, 665–667
665
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complexes with triflic acid [eqn. (1), > 95% yield] and anion
exchange with solid NaBArF4 in dichloromethane at ꢀ60 ꢁC
([eqn. (2), > 98% yield].
firmed by electrospray ionization MS (M+ ¼ 551.2 for the
195Pt isotopomer). The structure of the cyclohexene complex,
established by X-ray crystallographyz [Fig. 1(b)], shows a
five-coordinated platinum with the pyridinophane ligand
coordinated unsymmetrically (unsym) relative to the Pt–H
fragment and the cyclohexene ring cis to the hydride. Note-
worthy in the structure are the three nearly identical Pt–N
distances.
½PtMe2ðZ2-LÞꢂ þ HOTf ! ½PtMe2HðZ3-LÞꢂOTf
½PtMe2HðZ3-LÞꢂOTf þ NaBArF4
ð1Þ
! ½PtMe2HðZ3-LÞꢂBArF4 þ NaOTf ð2Þ
The mild conversion effected by (Z2-L)PtMe+ of alkane to
coordinated olefin and hydride offers new possibilities for sub-
sequent functionalization of an alkane compared to previous
conversions to coordinated alkyl and hydride. This is a special
advantage of slightly ‘‘oversized’’ ligands like [2.1.1]-(2,6)-pyr-
idinophane, which form constrained octahedral d6 complexes
and therefore can more easily change from Z3 to Z2 than does
Tp or does the corresponding ring slippage occur for C5R5 .
A new alkane CH bond cleavage reagent has been designed
and synthesized that possesses an unusually high [for Pt(II)/
Pt(IV)] room temperature reactivity in hydrocarbon oxidative
addition/reductive elimination. The latter properties might be
used to design new catalytic alkane-to-olefin conversions.
The [PtMe2H(L)]X complexes in both cases (X ¼ OTf,
BArF4 ꢀ
)
exhibit almost identical 1H NMR resonances
(ꢀ20.65, JPtH ¼ 1354 Hz in CD2Cl2 , X ¼ BArF4 ꢀ) and possess
mirror symmetry, showing one signal of equivalent methyl
groups at 1.27 ppm (JPtH ¼ 70 Hz), one set of signals of two
equivalent pyridine rings and one set of AX resonances for
protons of the methylene bridges. The solid tetraarylborate salt
decomposes completely during one week at RT.
Both Pt(IV) complexes synthesized undergo methane
reductive elimination at room temperature.32 Saturated hydro-
carbons, both acyclic and cyclic, introduced into dichloro-
methane solutions of the [PtMe2H(L)]BArF4 complex prepared
according to eqns (1) and (2), react at room temperature with
one and the same rate constant [k295
¼ (1.55 ꢃ 0.03) ꢄ
K
10ꢀ4sꢀ1, hence identical rate-determining steps] to produce
C–H bond activation products in high yield after 8 h.
Acknowledgements
Ethane (2 M) reacts, with methane elimination, to produce
(95 ꢃ 2% vs. BArF4 ꢀ signals) [PtH(Z2-C2H4)(L)]X with a
hydride ligand chemical shift of ꢀ27.49 (JPtH ¼ 1093 Hz) at
13C NMR spectroscopy [see Electronic supplementary infor-
mation (ESI) for this and other adducts].
This work was supported by the U.S. Department of Energy.
We thank Dr. D.N. Laikov for the use of his software.
ANV is on leave from the Chemical Faculty, Kazan State Uni-
versity, Kazan, Russia. This work has been made possible in
part due to support from the Russian Foundation for Basic
Research (grant #01.03.32692).
1
ꢀ40 ꢁC, which has been completely characterized by H and
Higher alkanes, propane (3 M), n-butane (3 M), cyclopen-
tane and cyclohexane (all the liquids are in 1:1 volume ratio)
react similarly (ꢅ95% yield) to produce platinum hydrido
Z2-olefin complexes. This behavior resembles closely the prop-
erties of [Cp*IrMe(PMe3)(ClCH2Cl)]+, with the difference that
the latter is more sensitive to the steric accessibility of the
alkane CH bond.33 In the case of propane, two diastereomers
of [PtH(Z2-C3H6)(L)]X in almost 1:1 ratio have been observed
with Pt–H resonances at ꢀ27.59 (JPtH ¼ 1158 Hz) and ꢀ27.50
(JPtH ¼ 1141 Hz) at ꢀ30 ꢁC. n-Butane gives rise to five iso-
meric products (1:0.7:0.1:0.1:0.2 ratio) with hydride ligand d
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