Lewis acid-base interactions between platinum(II) diaryl complexes and bis(perfluorophenyl)zinc: Strongly accelerated reductive elimination induced by a Z-type ligand

Article abstract of DOI:10.1039/c6cc02433e

Z-type interactions between bis(perfluorophenyl)zinc and platinum(ii) diaryl complexes supported by 1,10-phenanthroline (phen), 2,2′-bipyridine (bpy), and bis(dimethylphosphino)ethane (dmpe) ligands are reported. In the solid state, the nature of the Pt-Zn interaction depends on the bidentate ligand; the phen-supported complex exhibits an unsupported Pt-Zn bond, while the dmpe derivative features additional bridging aryl interactions. A strongly accelerated rate of reductive elimination is observed for phen- and bpy-supported complexes, while aryl exchange between Pt and Zn is observed for the dmpe complex.

Full text of DOI:10.1039/c6cc02433e

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Lewis acid–base interactions between platinum(II)
diaryl complexes and bis(perfluorophenyl)zinc:
strongly accelerated reductive elimination
induced by a Z-type ligand†
Cite this:DOI: 10.1039/c6cc02433e
Received 21st March 2016,
Accepted 5th May 2016
Allegra L. Liberman-Martin, Daniel S. Levine, Micah S. Ziegler, Robert G. Bergman*
and T. Don Tilley*
DOI: 10.1039/c6cc02433e
www.rsc.org/chemcomm
Z-type interactions between bis(perfluorophenyl)zinc and platinum(II) organozinc reagents to palladium(II) complexes during the Negishi
diaryl complexes supported by 1,10-phenanthroline (phen), 2,20- coupling reaction.6 These heterobimetallic intermediates have also
bipyridine (bpy), and bis(dimethylphosphino)ethane (dmpe) ligands been proposed to lead to cis/trans isomerization of L2PdR1R2
are reported. In the solid state, the nature of the Pt–Zn interaction complexes (R1,R2 = alkyl or aryl) and undesirable secondary
depends on the bidentate ligand; the phen-supported complex transmetalation events that form homocoupling products under
exhibits an unsupported Pt–Zn bond, while the dmpe derivative Negishi cross-coupling conditions.7 Understanding the binding
features additional bridging aryl interactions. A strongly accelerated motifs and reactivity of organozinc complexes with Group 10
rate of reductive elimination is observed for phen- and bpy-supported organometallic complexes could provide insight into the behavior
complexes, while aryl exchange between Pt and Zn is observed for the of these proposed intermediates and inspire improved catalysts
dmpe complex.
and conditions.
We have previously reported the ligand-based binding of
Understanding the interactions of a transition metal with bis(perfluorophenyl)zinc, ZnArF , to a 2,20-bipyrimidyl–platinum(II)
2
its surrounding ligands is a central focus of organometallic complex, which leads to dramatically enhanced rates of reductive
chemistry. Many s-donor ligands are Lewis bases that interact elimination.8 In the current work, we have demonstrated that if
with metal centers by the donation of electron density. In fewer remote binding sites are omitted, ZnArF can directly bind to a
2
cases, Lewis basic transition metals donate electron density platinum(II) center. Addition of ZnArF (Z2-toluene)9 to a solution of
2
to Lewis acids (denoted Z-type ligands) to form retro-dative (phen)PtAr2 (1, Ar = 4-tert-butylphenyl) led to rapid formation of
s-bonding interactions.1 Often, the Lewis acidic component is the heterobimetallic complex (phen)PtAr2(ZnArF2) (2, eqn (1)).
incorporated into a multi-dentate ligand to favor coordination.2 Yellow crystals of 2 were formed by vapor diffusion of pentane
Relatively few examples of unsupported Z-type interactions into a toluene solution at ꢀ35 1C (56% isolated yield). Treat-
exist, particularly those featuring metal–metal bonds, termed ment of (bpy)PtAr2 (3, Ar = 4-tert-butylphenyl) with ZnArF
-
2
metal-only Lewis pairs.3
(Z2-toluene) afforded an analogous (bpy)PtAr2(ZnArF2) adduct
Although Group 13 elements (B, Al, Ga) feature prominently (4) (64% isolated yield).
in reports of Z-type ligands,3 interactions of this type with
Group 12 elements (Zn, Cd, Hg) remain rare.4 The recently
reported [(Cy3P)2Pt–ZnBr2] complex contains the first unsupported
M-Zn interaction,5 and to our knowledge, there are no reported
examples of Z-type bonds featuring organozinc components. How-
ever, these heterobimetallic interactions may be present during
reactions between transition metal complexes and organozinc
reagents. For example, transient Pd–Zn bonds have been invoked
to account for the low energy barrier for transmetalation from
(1)
X-ray crystallographic studies confirmed the presence of an
unsupported platinum–zinc interaction in 2 (Fig. 1), with the
zinc center occupying the apical site of a square pyramidal
platinum complex. The platinum–zinc distance of 2.5526(5) Å in
2 is longer than the distance in [(Cy3P)2Pt–ZnBr2] (2.4040(6) Å)5
Department of Chemistry, University of California, Berkeley, USA.
E-mail: rbergman@berkeley.edu, tdtilley@berkeley.edu
† Electronic supplementary information (ESI) available. CCDC 1469636 and
1469637. For ESI and crystallographic data in CIF or other electronic format
see DOI: 10.1039/c6cc02433e
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Fig. 1 ORTEP diagram of 2, with p-tBu groups and hydrogen atoms
omitted for clarity and thermal ellipsoids shown at 50%.
Fig. 2 ORTEP diagram of 6, with p-tBu groups and hydrogen atoms
omitted for clarity and thermal ellipsoids shown at 50%.
but is on the shorter end of the range for previously reported
platinum–zinc interactions (B2.34–3.00 Å).10 The zinc-bound platinum square plane.15 The platinum–zinc distance of
perfluorophenyl ligands of 2 are significantly bent away from 2.7368(4) Å in 6 is significantly longer than the corresponding
the platinum center, with a C–Zn–C angle of 134.81 (compared to bond length for 2 (2.5526(5) Å). The 4-tert-butylphenyl groups
ZnArF (172.61)11 or ZnArF (Z2-toluene) (162.41)12).
unsymmetrically bridge the platinum and zinc centers, as
2
2
NMR studies confirmed that the platinum–zinc interaction evidenced by close contacts between zinc and the ipso carbon
persists in benzene-d6 solution at 25 1C. The 1H NMR signal for atoms (2.307(4) and 2.367(4) Å), and the tilt of the aryl rings
the ortho-aryl protons is shifted upfield and the 3J(195Pt–1H) (average Pt–Cipso–Cpara angles of B1631).
coupling constant is decreased upon zinc binding (8.19 ppm,
The platinum–zinc interaction of complex 6 remains intact
JPtH = 69 Hz for 1; 7.67 ppm, JPtH = 50 Hz for 2). Similar trends in benzene-d6 solution at 25 1C. The 1H NMR signal attributable
were previously observed upon copper(I) and silver(I) triflate to the ortho-aryl position is shifted downfield upon zinc coordina-
binding to platinum(II) complexes.13 There is also a small tion, from 7.85 ppm in 5 to 8.03 ppm for 6, and the J(195Pt–1H)
3
change in the 195Pt NMR chemical shift upon ZnArF binding coupling constant decreases from 57 Hz to 42 Hz. The 31P{1H}
2
(d = ꢀ3355 ppm for 1 and ꢀ3167 ppm for 2).14 The zinc- NMR chemical shift is relatively insensitive to zinc binding
coordinated bpy derivative 4 exhibited similar chemical shift (d = 21.2 and 29.4 ppm for 5 and 6, respectively); however, the
and coupling constant perturbations compared to those of 3 J(195Pt–31P) coupling constant increases significantly upon zinc
(see ESI†).
coordination ( JPtP = 1630 Hz for 5 and 2142 Hz for 6). This
To probe the effect of the platinum-bound chelating ancillary increase in 195Pt–31P coupling constant is consistent with a
ligand on zinc coordination, the dmpe analogue was selected weaker trans influence of the aryl ligands resulting from inter-
for comparison. Treatment of a toluene solution of (dmpe)PtAr2 action with zinc.16 The 195Pt chemical shifts of 5 and 6 are
(5, Ar = 4-tert-butylphenyl) with ZnArF2 generated the zinc adduct similar (d = ꢀ4506 and ꢀ4544 ppm, respectively).
(dmpe)Pt(m-Ar)2(ZnArF ) (6, eqn (2)). Colorless crystals of 6 were
DFT calculations were performed to provide insight into the
2
isolated in 68% yield by vapor diffusion of pentane into a toluene bonding interactions in 2 and 6. The complementary occupied–
solution at ꢀ35 1C.
virtual pairs (COVPs) method was selected to visualize charge
transfer interactions between the platinum and zinc fragments
(Fig. 3).17 Each COVP corresponds to a donation of electron
density from an occupied orbital on one fragment to an
acceptor orbital of the other fragment.
Unsurprisingly, the platinum–zinc interaction in 2 is
dominated by donation from the dz orbital of platinum to a
(2)
2
vacant zinc p orbital. There is a secondary backbonding
interaction from a filled Zn d orbital to an orbital with
antibonding platinum–ligand character, which weakens the
platinum–ligand bonds of 2 (see ESI† for details). In contrast,
2
X-ray crystallographic studies revealed a different binding complex 6 features donations from both the platinum dz
mode of zinc in 6 compared with that of 2 (Fig. 2). Rather than orbital and the aryl ligand p-system into vacant zinc p orbitals.
occupying an apical position above the Pt square plane, the zinc Backbonding from a zinc d orbital to the platinum-bound
atom adopts a position near the ipso carbon atoms of the 4-tert- aryl ligands also strengthens the zinc–aryl interactions of 6
butylphenyl ligands and is 0.895 Å above the least-squares (see ESI†).
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The thermolysis of (bpy)PtAr2 (3) in the absence of additives at
150 1C has previously been reported to form tert-butylbenzene
and a platinum–bipyridine polymer.19 These products are
generated by a roll-over cyclometalation reaction, which proceeds
by dissociation and rotation of one pyridyl group, followed by C–H
activation at the remote C3 pyridyl position.20 In the current work,
addition of ZnArF2 switches the reaction selectivity to elimination
of biaryl, rather than intramolecular metalation of a pyridyl group.
In contrast to the reductive elimination observed from phen-
and bpy-containing complexes 2 and 4, for (dmpe)Pt(m-Ar)2-
(ZnArF2) (6), aryl exchange occurs between platinum and zinc to
produce exclusively (dmpe)Pt(C6F5)2 (7) and Zn(4-tBu-Ph)2 at
60 1C (eqn (4)). There is no evidence of an interaction between
(dmpe)Pt(C6F5)2 and either ZnArF2 or Zn(4-tBu-Ph)2 by 1H or 19
F
NMR spectroscopy. DFT calculations are consistent with this
result, and EDA data suggest that electrostatic repulsion between
zinc and the electron-deficient aryl groups of 7, along with a
reduction of the polarizability of 7, disfavors zinc binding.
Fig. 3 Selected COVPs for 2 and 6. Donor orbitals are represented by
solid intense colors, and complementary acceptor orbitals have mesh
t
isosurfaces. Hydrogen atoms and Bu are omitted for clarity.
(4)
To investigate the different geometric preferences of 2 and 6,
energy decomposition analysis (EDA) calculations were performed
to compare the apical and aryl-bridged isomers of both complexes.
The apical isomer of the phen-supported complex 2 is favored by
We propose that coordination of ZnArF favors reductive
2
7.7 kcal molꢀ1, whereas the aryl-bridged isomer of 6 is preferred to elimination from 2 and 4 by withdrawing electron density
a lesser extent (1.1 kcal molꢀ1). These binding differences can be from platinum,21,22 weakening the platinum–aryl bonds, and
rationalized based on the different donor properties of nitrogen- increasing steric congestion at the metal center. In the case of
and phosphorus-based ligands. The phen-supported complex 2 the bpy complex 4, zinc binding also prevents roll-over cyclo-
has a greater partial charge on platinum (relative to the dmpe metalation by blocking the intramolecular C–H activation path-
complex 6), which favors apical coordination due to greater way that requires a vacant coordination site.22 For the dmpe
electrostatic stabilization from the close Pt–Zn contact. In contrast, complex 6, there are strong interactions between the zinc center
the platinum complex is more polarizable when ligated by dmpe and the platinum-bound aryl groups. If this interaction persists,
versus phen, making geometric distortion to adopt the aryl-bridged or even becomes stronger, in the transition state, it could help
isomer more favorable for 6 than 2.
favor aryl exchange over reductive elimination.
To assess the effect of zinc binding on reactivity, a solution of 1 with
In conclusion, dative, heterobimetallic interactions are observed
a 10-fold excess of ZnArF2 was heated to 60 1C in benzene-d6, resulting between platinum(II) diaryl complexes and ZnArF in both solution
2
in quantitative biaryl reductive elimination within 15 minutes, along and the solid state. The zinc binding motif is highly sensitive to the
with formation of Pt(0) and (phen)ZnArF2 byproducts (eqn (3)).18 In the nature of the platinum-bound bidentate ligand. The (phen)PtAr2-
absence of added ZnArF , biaryl formation did not proceed from (ZnArF ) complex 2 features an unsupported Pt–Zn bond, whereas
2
2
(phen)PtAr2 (1) over 48 hours at 200 1C, demonstrating that the zinc the dmpe analogue 6 features additional interactions between the
substituent dramatically accelerates reductive elimination.
platinum-bound aryl ligands and zinc. For the dmpe complex, aryl
transmetalation between Pt and Zn is exclusively observed, which is
reminiscent of the secondary transmetalation pathways that have
been proposed for Negishi catalysts.6 In contrast, the biaryl for-
mation observed from the phen and bpy derivatives 2 and 4 provides
a rare example of reductive elimination promoted by a dative,
heterobimetallic bond. This result raises the possibility that
analogous Pd–Zn interactions may facilitate the reductive elimina-
tion step, as well as transmetalation events, of Negishi cross-
coupling reactions. Future work is needed to determine whether
(3)
Quantitative reductive elimination was also observed upon heat- reductive elimination induced by an organozinc Z-type ligand
ing (bpy)PtAr2(ZnArF ) (4) for 15 minutes at 60 1C in benzene-d6. interaction is a viable mechanism under catalytic conditions.
2
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This work was supported by the Director, Office of Science,
Office of Basic Energy Sciences of the US Department of Energy
under contract no. DE-AC02-05CH11231. We also acknowledge
the US National Institutes of Health for funding of the CheXray
X-ray crystallographic facility (College of Chemistry, University
of California, Berkeley) under grant number S10-RR027172 and
the UC Berkeley College of Chemistry NMR facility under grant
SRR023679A. D. S. L. and M. S. Z. would like to acknowledge
support of NSF Graduate Research Fellowships.
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