Role of Phosphine Sterics in Strained Aminophosphine Chelate Formation

Article abstract of DOI:10.1021/acs.inorgchem.8b03514

The preparation of four-membered aminophosphine (PN) chelates from common metal precursors has largely evaded realization because of the ring strain associated with these species. We report a straightforward approach to the synthesis of such PN metallacycles using simple α-PN ligands analogous to the popular class of small-bite-angle diphosphinomethane ligands. It is demonstrated that bulky phosphine substituents are important to the formation of these chelates.

Full text of DOI:10.1021/acs.inorgchem.8b03514

Communication
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Role of Phosphine Sterics in Strained Aminophosphine Chelate
Formation
Eric G. Bowes, D. Dawson Beattie, and Jennifer A. Love*
Department of Chemistry, The University of British Columbia, Vancouver, British Columbia V6T 1Z1, Canada
S
* Supporting Information
Scheme 1. (a) Formation of κ1-P Products, (b) Halide
Abstraction To Provide Cationic κ2-PN Complexes, and (c)
ABSTRACT: The preparation of four-membered amino-
phosphine (PN) chelates from common metal precursors
has largely evaded realization because of the ring strain
associated with these species. We report a straightforward
approach to the synthesis of such PN metallacycles using
simple α-PN ligands analogous to the popular class of
small-bite-angle diphosphinomethane ligands. It is dem-
onstrated that bulky phosphine substituents are important
to the formation of these chelates.
Bulky Phosphines That Permit the Preparation of Strained
Chelates
ncillary ligand design represents a critical strategy in
Asynthetic chemistry. Metal complexes of ligands with labile
donor functionalities have proven invaluable in catalysis. These
“hemilabile” ligands enhance reactivity by facilitating the
generation of low-coordinate species and have the potential
for the decoordinated group to interact with substrates as a
Lewis or Brønsted base.1 Hemilability and associated reactivity
is governed by the ligand dissociation barrier, and mixed-donor
ligands featuring a combination of hard and soft bases enhance
reactivity by providing disparate metal−donor interactions. As
such, ligands with both nitrogen and phosphorous donors (PN),
the largest class of hybrid ligands, have proven to be superior to
bidentate diamine (NN) and diphosphine (PP) ligands in a
variety of catalytic applications.2
bulky substituents were found to lower ring strain through a
Thorpe−Ingold-like effect,8 our key finding was that steric
protection of vacant sites adjacent to P was the determining
factor in chelate formation.
Building on our previous efforts toward hydrocarbon
functionalization, we were interested in platinum(II) complexes
of PN ligands bearing sp3-hybridized N-atom donors capable of
deprotonating platinum(IV) hydride intermediates.3 We
anticipated that strained chelates would afford enhanced
hemilability, increasing reactivity toward small molecules such
as methane. The coordination chemistry of N(sp3)-hybridized
α-PN ligands is well developed and characterized by a distinct
preference for κ1-P coordination (Scheme 1a) that limits their
utility as hemilabile ligands.4 Examples of transition-metal
complexes featuring κ2-PN coordination of these ligands are rare
and are typically formed under forced conditions such as ligand
abstraction to form cationic products (Scheme 1b).5 PN ligands
with N(sp2)-hybridized donor functionalities are well-known to
form four-membered chelates,6 but these species often prefer to
adopt κ1-P or bridging μ-PN coordination modes with late
transition metals.7
We initially attempted to prepare a four-membered PN
chelate via ligation of L1 to [Pt2Me4(μ-SMe2)2], resulting in a
mixture of (SMe2)2PtMe2 and two κ1-P products, 1 and 2
(Scheme 2). Notably, the desired κ2-PN complex 3 was not
formed in appreciable quantities. Similar observations were
made in attempts to prepare 3 from (nbd)PtMe2 (nbd =
norbornadiene) and L1 under a number of conditions.9 The
treatment of [Pt2Me4(μ-SMe2)2] with the more strongly
i
donating Pr-substituted phosphine L2 resulted in a similar
distribution of κ1-P products 4 and 5, but a third Pt-bound
phosphine product 6 was detected in small quantities (8%) by
NMR spectroscopy.10
Given that the desired chelate was observed using an alkyl-
substituted phosphine, we explored a number of other PNs
bearing electron-rich P centers. In contrast to what was observed
with L1/L2, the reaction of L3 (R = tBu) and L4 (R = Ad = 1-
We report the preparation of archetypical (κ2-PN)PtX2
complexes via the ligation of simple metal precursors and
demonstrate that chelate formation is facilitated by increasing
the steric profile of the phosphine group, despite increased
repulsion between P and N substituents (Scheme 1c). While
Received: December 17, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.8b03514
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Communication
putative complex 6, with 195Pt−1H coupling to the N−CH3
groups (20−21 Hz) confirming a N → Pt dative interaction.
We anticipated that the use of bulky phosphine substituents
might be a general strategy for accessing a variety of small-bite-
angle chelates. Precedent for this hypothesis is provided by
observations from Grotjan and co-workers showing that κ2-PN
chelates are formed in the oxidation of (κ1-PN)2Pd complexes
Scheme 2. Reaction of [Pt2Me4(μ-SMe2)2] with L1 (R = Ph)
and L2 (R = iPr) and 1H NMR Spectroscopic Yields
t
(PN = imidazolylphosphine derivatives) featuring Bu-sub-
stituted phosphines but not with smaller iPr substituents.6a We
thus sought to prepare (κ2-PN)PtMe2 complexes featuring an
sp2-hybridized donor in place of the −NMe2 group. Gratifyingly,
t
the use of Bu-substituted (L5) and Ad-substituted (L6)
pyridylphosphines allowed for the straightforward isolation of
N(sp2) derivatives 9 and 10 (Scheme 3b). This is significant
because attempts to prepare analogous Ph-substituted deriva-
tives with commercially available (2-pyridyl)PPh2 have been
unsuccessful.13
ORTEP diagrams of 7, 9, and 10 are shown in Scheme 3c. To
address the structural properties affected by changes in the
chelate size, five-membered PN chelates (L7)PtMe2 (11; L7 =
tBu2PCH2CH2NMe2) and (L8)PtMe2 [12; L8 = (2-pyridyl)-
CH2PtBu2] were prepared by the addition of the appropriate
ligand to [Pt2Me4(μ-SMe2)2]. The X-ray structure of 11 was
obtained and is presented for comparison with four-membered
chelates in Scheme 3c, while the structure of 12 was reported
previously by Goldberg and co-workers.14 The data for the four-
membered chelate complexes confirm their monomeric nature
and show distorted square-planar geometries (τ4 = 0.13−0.15)15
because of the small ligand bite angles of close to 70° [7,
71.4(2)°; 9, 70.0(2)°; 10, 68.90(7)°]. These angles are
significantly smaller than those in related diphosphinomethane
complexes (∼74°).16
Scheme 3. Synthesis of Strained PN Chelates Featuring (a)
N(sp3) and (b) N(sp2) Donors and (c) X-ray Structures of
Four- and Five-Membered PN Chelates
The most notable structural change upon an increase in the
chelate size from four (7/9) to five (11/12) is the ligand bite
angle (P−Pt−N), which increases by ∼14° when an additional
methylene spacer is added to the ligand backbone. The effects of
ring strain in the four-membered rings are evident in the changes
in pyramidalization at the N- and P-atom donors as a function of
the chelate size. The sum of the C−P−C angles about P in 7 is
greater than that in 11 [Δ∑∠C−P−C = +6.7(5)°]. Similar
observations were made for changes in the C−N−C angles in 7
and 11, as well as the C−P−C angles in 9 and 12. This is
rationalized by considering the effect of ∑∠C−E−C (E = P, N)
on the orientation of the E lone pair, which has better alignment
with Pt as ∑∠C−E−C becomes larger. In both the N(sp3) and
N(sp2) systems, the changes in the Pt−N bond lengths were
found to be insignificant.
Interestingly, while a modest increase in the Pt−P bond
length [0.036(3) Å] occurs upon moving from the four-
membered 9 to five-membered 12, the difference in N(sp3)
derivatives 7 and 11 is negligible [0.004(2) Å]. The 195Pt−31P
coupling constants in 7/9 are reduced by 350 Hz in comparison
to those in the five-membered chelates, highlighting reduced
Pt−P interactions despite similar bond lengths. This is also
evident in the reduced trans influence of the phosphine in 7/9,
which show shorter Pt−C bonds trans to the phosphine in
comparison to 11/12 [−0.020(7) and −0.019(13) Å,
respectively].
adamantyl) with [Pt2Me4(μ-SMe2)2] afforded quantitative
conversion to the κ2-PN metallacycles 7/8 (Scheme 3a). The
195Pt−31P coupling constants for 7/8 (JPt−P = 1674 and 1656 Hz,
respectively) were 140−190 Hz smaller in magnitude than those
reported previously for related PR2R′ ligands trans to hydro-
carbonyl donors at PtII.11 Similar observations have been made
Thorpe−Ingold-type effects have been observed in organo-
metallic chemistry and suggested to promote the formation of
C−H agostic complexes17 and cyclometalation of monodentate
phosphines.18 Significant differences in the coordination
previously for four-membered PN and PC chelates.12 The H
1
19
t
i
NMR spectra for 7/8 exhibited features similar to those for
chemistry of Bu- and Pr-substituted diphosphinomethane
B
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Inorganic Chemistry
Communication
and imidazolylphosphine6a ligands have also been attributed to
the size of alkyl substituents at P. Work by Shaw demonstrated
that larger geminal substituents such as Bu are required to
Although significant changes were found between the four-
and five-membered chelates, our computations suggest that the
differences in the strain (as evaluated by ∑ΔBP−GB) and M−L
bond order metrics are small within the series of four-membered
chelates featuring P substituents of varying bulk (Me, Pr, Bu,
etc.). While the observed changes were small, they were in
agreement with a Thorpe−Ingold-like effect, wherein increased
C−P−C angles in bulky phosphines reduce the ring strain by
improving the Pt and P orbital alignment.
t
observe significant effects for P, resulting in the termed “gem-di-
tert-butyl” effect.20 To probe the factors influencing the
differences in coordination preference of the PN ligands under
investigation, a density functional theory (DFT) study on 17
ligands, expanding on the set studied experimentally, was
conducted to determine the effects of P substitution and chelate
size on the structure and bonding of (κ2-PN)PtMe2 com-
plexes.21
A comparison of the DFT-optimized geometries for 7/9 and
11/12 reproduced the experimentally observed structural
differences between four- and five-membered PN chelates
(Tables S2). Natural bond orbital22 and atoms-in-molecules23
analyses revealed weaker Pt−N and Pt−P bonding interactions
in the small rings. Bond orders were lower by 6−11%,
accompanied by less electron density (0.002−0.009 au) at the
bond critical points (bcps) and greater concentrations of
negative charge on P and N. Partial molecular graphs for 7 and
11 are presented in parts a and b of Figure 1, respectively. The
i
t
The N dissociation enthalpy for 7 (16.4 kcal·mol−1) was
calculated to be greater than that for complex 3 (11.8 kcal·
mol−1) or 6 (11.6 kcal·mol−1), indicating greater chelate stability
for the larger phosphines. As anticipated, the energetic penalty
due to steric repulsion between the PR2 and NMe2 groups in the
bound conformation was calculated to be larger for L3/L4 than
for L1/L2.26 Evidently, the reduction in the ring strain provided
by large R groups compensates for the steric repulsion. The
increased stability of chelate rings with large P substituents is in
agreement with experimental trends in the coordination
preferences for the N(sp3) ligands but does not account for
the behavior of the pyridylphosphine derivatives. The enthalpy
for ring opening in 9 (9.2 kcal·mol−1) and 10 (8.0 kcal·mol−1)
reveals lower chelate stability in comparison with that in 3 and 6,
indicating that a factor other than ring strain is influencing our
ability to isolate κ2-PN chelates with the bulky phosphines.
The calculated ΔG values for chelate opening of all κ2-PN
complexes were positive, suggesting that κ2 coordination of L1
and L2 should be feasible in the absence of a competitive donor.
Attempts to access κ2-PN chelates using (cod)PtMe2 as an
alternative PtII precursor were unsuccessful.26 The treatment of
complexes 2 and 5 with [Ph3C][BF4] to generate a vacant
coordination site via methyl abstraction did, however, result in
chelate formation (Scheme 4a). Cationic complexes 13 and 14
were isolable, demonstrating that L1 and L2 can indeed adopt κ2-
PN coordination modes when alternative ligation is unfavor-
able.27
The treatment of 7 with excess free ligand L3 did not yield κ1-P
product 15, suggesting that the metal cannot accommodate two
Scheme 4. Reactivity Studies
Figure 1. Partial molecular graphs and calculated electron density
topologies ρ(r) for (a) 7 and (b) 11 and topologies of ∇2ρ(r) (negative
contours, red; positive contours, black) for (c) 7 and (d) 11. Dots: red,
(3, −1) bcps; green, (3, +1) rcps.
paths of maximal e− density connecting the Pt,P and Pt,N atoms
(bond paths) in 7 are visibly distorted; strained molecules
typically have bond paths that exceed the geometric bond length
(recall the “banana bonds” of cyclopropane).24 The sum of the
differences between the bond path and geometric bond lengths,
∑ΔBP−GB, for the atoms forming the chelate ring in 7 (0.018 Å)
was significantly larger than that in 11 (0.004 Å), with the
greatest deviation exhibited by the Pt−P bond (0.011 Å).
Analysis of the Laplacian of the electron density, ∇2ρ(r),
provides information about regions of local concentration and
depletion of the electron density and can be used to identify the
locations of nonbonding electron pairs.23,25 The P lone pair (red
contours) in strained 7 (Figure 1c) clearly suffers from poor
alignment with Pt relative to that in 11 (Figure 1d), resulting in
reduced stabilization despite similar M−L bond lengths.
C
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Inorganic Chemistry
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actor ligands as versatile platforms for small molecule activation and
catalysis. RSC Adv. 2013, 3, 11432−49.
large PR3 substituents in a cis configuration (Scheme 4b). This is
further supported by the degree of distortion in 5, with a P−Pt−
P angle of 106.82(2)° (Scheme 2). Tellingly, 7 was found to
(2) (a) Carroll, M. P.; Guiry, P. J. P,N ligands in asymmetric catalysis.
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t
react with the less hindered phosphine Bu2PH to form 16
(43%) along with L3 (23%) and cis-(tBu2PH)2PtMe2 (23%;
Scheme 4c). The steric bulk of the PN ligands were quantified by
means of buried volume (Vbur) calculations,28 showing that
bulky ligands L3−L8 occupy between 30 and 32% of the Pt
coordination sphere. Vbur values for L1 and L2 were found to be
slighter smaller (27−28%), with a less spherical distribution of
bulk, which may limit steric repulsion with cis ligands. While we
have not evaluated the influence of hindered rotation in the
ligand backbone,20 our data are consistent with a mechanism in
which chelate formation is promoted by steric protection of the
site cis to P.
We have demonstrated that the preparation of four-
membered chelates of N(sp3)-hybridized PN ligands using
simple metal precursors is possible when employing sterically
encumbered phosphines. The behavior of these complexes
toward small-molecule activation and the effects of the chelate
size on the reactivity are currently being investigated.
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ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.8b03514.
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Schutte, R. P.; Rettig, S. J.; James, B. R. Coordination chemistry of the
2-pyridyldiphosphine ligands, (py)2P(CH(CH2)3CH)P(py)2 and
(py)2P(CH2)2P(py)2 (py = 2-pyridyl), with platinum(II) and
ruthenium(II). Ruthenium-catalyzed hydrogenation of imines. Inorg.
Full computational and experimental details (PDF)
Accession Codes
CCDC 1872321−1872325 contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/data_request/cif, or by email-
ing data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road, Cam-
bridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Author
*E-mail: jenlove@chem.ubc.ca.
ORCID
Eric G. Bowes: 0000-0003-3605-1814
D. Dawson Beattie: 0000-0002-7909-2416
Jennifer A. Love: 0000-0003-2373-1036
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank The University of British Columbia, NSERC
(Discovery and Research Tools and Instrumentation grants),
Compute Canada Calcul Canada, and Canada Foundation for
Innovation for supporting this research. E.G.B. and D.D.B. are
grateful to the Government of Canada and NSERC for Vanier
CGS and PGS-D scholarships, respectively. Marcus W. Drover
and Yiming Ren are thanked for providing assistance with X-ray
crystallography and ligand synthesis.
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E
DOI: 10.1021/acs.inorgchem.8b03514
Inorg. Chem. XXXX, XXX, XXX−XXX