E-Selective Synthesis and Coordination Chemistry of Pyridine-Phosphaalkenes: Five Ligands Produce Four Distinct Types of Ru(II) Complexes

Article abstract of DOI:10.1021/acs.organomet.9b00425

Pyridine-phosphaalkene (PN) ligands 2a-e were prepared in an E-selective fashion using phospha-Wittig methodology. Treatment of these five ligands, varying only in their 6-substituent with RuCl₂(PPh₃)₃, produced four distinct types of coordination complexes: Pyridine-phosphaalkene-derived 3b,d, cyclized 4e, and six-coordinate 5a and 6c. Prolonged heating of 3b,d in THF resulted in C-H activation of the Mes? group and cyclization to give 4b,d featuring a bidentate pyridine-phospholane ligand bound to the metal center. Complex 5a, also possessing a newly formed phospholane ring, contained a different spatial arrangement of donors to Ru(II) with an agostic Ru-H-C interaction serving as the sixth donor to the transition metal center. Ligands 2b,d,e and Ru(II) complexes 3b, 4b,e and 5a were all characterized by X-ray crystallography. Six-coordinate 6c featured a structure similar to 4b,d,e, but with the CF₃ substituent acting as a weakly bound sixth ligand to the Ru(II) center, as observed by ³¹P{¹H} and ¹⁹F NMR spectroscopy. The calculated structure of 6c established that the closest Ru-a?-a?-F contact was at 2.978 ?.

Full text of DOI:10.1021/acs.organomet.9b00425

Article
pubs.acs.org/Organometallics
Cite This: Organometallics XXXX, XXX, XXX−XXX
E‑Selective Synthesis and Coordination Chemistry of Pyridine-
Phosphaalkenes: Five Ligands Produce Four Distinct Types of Ru(II)
Complexes
Mika L. Nakashige,† Jarin I. P. Loristo,†,∥ Lesley S. Wong,†,∥ Joshua R. Gurr,† Timothy J. O’Donnell,†
Wesley Y. Yoshida,† Arnold L. Rheingold,§ Russell P. Hughes,‡ and Matthew F. Cain*,†
†Department of Chemistry, University of Hawai’i at Manoa, 2545 McCarthy Mall, Honolulu, Hawaii 96822, United States
̅
‡6128 Burke Laboratory, Department of Chemistry, Dartmouth College, Hanover, New Hampshire 03755, United States
§Department of Chemistry, University of California, San Diego, 9500 Gilman Drive, La Jolla, California 92093, United States
S
* Supporting Information
ABSTRACT: Pyridine-phosphaalkene (PN) ligands 2a−e
were prepared in an E-selective fashion using phospha-Wittig
methodology. Treatment of these five ligands, varying only in
their 6-substituent with RuCl2(PPh3)3, produced four distinct
types of coordination complexes: pyridine-phosphaalkene-
derived 3b,d, cyclized 4e, and six-coordinate 5a and 6c.
Prolonged heating of 3b,d in THF resulted in C−H activation
of the Mes* group and cyclization to give 4b,d featuring a
bidentate pyridine-phospholane ligand bound to the metal
center. Complex 5a, also possessing a newly formed
phospholane ring, contained a different spatial arrangement
of donors to Ru(II) with an agostic Ru−H−C interaction
serving as the sixth donor to the transition metal center.
Ligands 2b,d,e and Ru(II) complexes 3b, 4b,e and 5a were all characterized by X-ray crystallography. Six-coordinate 6c featured
a structure similar to 4b,d,e, but with the CF3 substituent acting as a weakly bound sixth ligand to the Ru(II) center, as observed
by 31P{1H} and 19F NMR spectroscopy. The calculated structure of 6c established that the closest Ru- - -F contact was at 2.978
Å.
Scheme 1. Unusual Metal Geometries Supported by
Phosphaalkene-Based PNP Pincers (Top) and
Intramolecular C−H Activation and Cyclization of
Phosphaalkenes Catalyzed by [Pt(PCy3)2] (Bottom)
INTRODUCTION
■
Phosphaalkene-based ligands featuring low-lying π* orbitals
and large P substituents have emerged as a successful class of
pincers for stabilizing low-coordinate and electron-rich metal
centers,1 some in unusual geometries including trigonal-
monopyramidal A2 and T-shaped B (Scheme 1, top).3
The versatility of the PC unit is remarkable, functioning
as a strong π acceptor,4 as an electron reservoir,5 and as a site
for both external nucleophilic attack6 and intramolecular C−H
activation and cyclization.7 These cyclizations generally afford
phospholanes,8 and in the case of phosphaalkene-derived PCP
pincer C,9 a catalytic amount of [Pt(PCy3)2] promotes the
formation of bis(phospholane) D.7 Interestingly, the PNP
analogue E10 was far more resistant to double cyclization,
requiring forcing conditions (80 °C, 11 days) to produce F
(Scheme 1, bottom).11 However, a single cyclization of
bis(phosphaalkene) E occurs readily at metal centers,12
affording square-planar G12 and octahedral H11 with a
modified PNP pincer featuring phospholane, pyridine, and
phosphaalkene donors. Akin to the metal complexes reported
by Milstein that are supported by neutral, tridentate pincers
bearing a central N-heterocyclic donor,13 treatment of G and
H with base results in deprotonation of the benyzlic arm and
subsequent dearomatization of the pyridine functionality and
isolation of I and J (Scheme 2).11,12
Received: June 25, 2019
© XXXX American Chemical Society
A
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formation of PN ligands 2a−e as yellow, crystalline solids in
Scheme 2. Synthesis of Dearomatized Pincer Complexes I
and J
good yields (Scheme 3).22
Scheme 3. E-Selective Synthesis of 2a−e
Their formation was identified by the diagnostic downfield
shift of the PC unit observed by 31P{1H} NMR spectros-
copy, with the 1H NMR spectra confirming that the
Dearomatized I is capable of facilitating the N−H activation
of ammonia and other primary amines,12,14 while a related
analogue of J promotes catalytic N-alkylation of amines with
alcohols.15 In both these cases12,14,15 and in the majority of the
Milstein-type systems,16 the third donor of the pincer acts as a
spectator, leading us to consider if simpler and largely
unexplored bidentate pyridine-phosphaalkene ligands (1) in
combination with a third exogenous ligand would behave in a
similar fashion. Although it was synthesized concurrently with
E in 1992,10 no reports of cyclization of the Mes* group of
parent pyridine-phosphaalkene (PN) 1a (R′ = H) across its
PC double bond have been described (Chart 1).17 In fact,
phosphaalkenes were present in the E conformation (JPH
=
25 Hz).21 Furthermore, the new PN ligands were characterized
by 13C{1H} NMR spectroscopy, mass spectrometry, and
elemental analysis. X-ray crystallography unequivocally estab-
lished the structures of 2b,d,e (Figures 1−3).
Chart 1. Previously Reported PNP Pincer E and PN Ligands
1a−c and Targeted 6-Substituted PN Ligands of Type 2
Figure 1. X-ray crystal structure of 2b. Selected bond lengths (Å) and
angles (deg) are shown in Table 1.
the coordination chemistry of 1a in general has been scarcely
reported with only a few examples, including a poorly
characterized Cu(I) species.10 Other C-functionalized ana-
logues such as 1b (R′ = Ph) can support square-planar Pt(II)
and Pd(II) complexes, with the latter acting as a catalyst for
the Overman−Claisen rearrangement.18 A recent report by
Ozawa also highlighted that 1b could stabilize unusual Ni(I)
species and highly distorted square-planar Ni(II) complexes
and promote unusual P−CAr (CAr = Mes*) bond cleavage
events,19 all of which were hypothesized to originate from the
presence of low-lying π* orbitals on the PC unit. Here, we
report that the PC double bond of pyridine-phosphaalkenes
of type 2, featuring sterically and electronically diverse
substituents at the 6-position of the pyridine ring, is susceptible
to C−H activation of the Mes* group and cyclization to a
phospholane on coordination to Ru(II). Specifically, after
preparing five PN ligands (2a−e) in an E-selective fashion via a
phospha-Wittig methodology,20 we discovered that, remark-
ably, on exposure to RuCl2(PPh3)3 four distinct types of
coordination complexes (3−6) are produced, three of which
feature newly formed phospholane rings.
Figure 2. X-ray crystal structure of 2d. Selected bond lengths (Å) and
angles (deg) are shown in Table 1.
Solid-State Structures of 2. The solid-state structures of
ligands 2a,22 and 2b,d,e all feature PC bond lengths (1.66−
1.69 Å) within the normal range of other bidentate and pincer-
type phosphaalkene-based ligands.4a Although the P−Mes*
bond lengths show little variation (1.85−1.87 Å), they are
marginally longer than most other phosphaalkene−aryl single
bonds,23,24 while their CP−C bond angles are approximately
98°, consistent with a lack of hybridization at P.4a Other bond
RESULTS AND DISCUSSION
■
Synthesis of Ligands of Type 2. Treatment of 6-
substituted pyridine-2-carboxyaldehyde derivatives21 with the
phospha-Wittig20 reagent Mes*PPMe3 resulted in the
B
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Scheme 4. Synthesis of Ru(II) Complexes 3−6
Figure 3. X-ray crystal structure of 2e. Selected bond lengths (Å) and
angles (deg) are shown in Table 1.
solid-state structures of 3b and 4b,e were determined by X-ray
crystallography (Figures 5−7).
lengths and angles of interest are tallied in Table 1. In addition,
the X-ray structures of 2b,d,e (Figures 1−3) revealed that the
E configuration of the PC bond in combination with the
two-carbon linker between the pyridine N atom and the P
center create a promising binding pocket for a metal center.
Ru(II) Complexes. Treatment of 2a−e with RuCl2(PPh3)3
in benzene resulted in the formation of four distinct type of
coordination complexes, pyridine-phosphaalkene-derived 3,
cyclized 4, and six-coordinate 5 and 6, all in reasonable yields
(Scheme 4). The existence of the four different types of
coordination complexes (3−6) was readily identified by NMR
spectroscopy. Red solutions of 3b,d showed that the
phosphaalkene functionality remained intact, maintaining a
significantly downfield shifted 31P NMR signal (δ >290 ppm),
which was coupled to the bound PPh3 unit (3b, δ 36.0; 3d, δ
34.0 ppm, JPP = 36 Hz). On the other hand, a green solution of
cyclized derivative 4e featured an upfield-shifted 31P NMR
resonance (δ 90.3 ppm) reminiscent of other Ru-bound
phospholanes,25 also displaying coupling to PPh3 (δ 48.6 ppm,
Like 4b,d,e, the 31P{1H} NMR spectrum of 5a quickly
established that the PC functionality was no longer present.
However, the two 31P NMR signals detected were much
different from those of cyclized 4b,d,e, resonating at 64.0 and
66.8 ppm (JPP = 39 Hz), suggesting that a different type of
coordination complex was generated. Diastereotopic benzylic
type protons were observed by 1H NMR spectroscopy,
confirming that C−H activation and cyclization had occurred,
but one of the newly formed diastereotopic phospholane
protons was drastically shifted upfield to 0.73 ppm. We
hypothesized that the sterically diminutive C−H group of
ligand 2a created a coordination environment in which the
phospholane, pyridine, and PPh3 donors could all be mutually
cis to one another. This would orient the strongly π donating
chlorides trans to both the phospholane and pyridine,
providing a rationale for the high-field PCH2 signal and the
unusual N-Ar resonance observed at 6.43 ppm. Recrystalliza-
tion of 5a from a concentrated THF solution exposed to
pentane vapors afforded X-ray-quality crystals partially
validating our hypothesis, but also establishing that 5a was
six-coordinate via an agostic interaction from one of the two
remaining t-Bu groups (Figure 8). Evidence of this Ru−H−C
interaction in solution, specifically an unusually upfield shifted
t-Bu proton signal, was not observed.
J
PP = 36 Hz). A comparison of the 31P{1H} NMR spectra of 3b
and 4e is shown in Figure 4. Moreover, the formation of the
phospholane ring in 4e generated a chiral P center, making the
CH2 protons of the new five-membered ring, the benzylic-type
protons linking the pyridine to the phospholane unit, and the
methyl groups on the phospholane diastereotopic, resulting in
1
a series of H NMR signals with complex splitting patterns
Ru(II) complexes featuring agostic interactions are com-
mon,26 normally resulting in five- or six-membered chelates.
For example, Gusev synthesized Ru(II) dichloride K with one
of the P-alkyl groups of the POP pincer utilizing an agostic
interaction (Ru- - -H = 2.23 Å) to establish a five-membered
ring,27 while Shaw showed using azine phosphines in L that
agostic interactions could be forced by placing the H atom in
close proximity to the Ru center, affording six-membered rings
(Figure 9, left).28 Furthermore, Caulton29 and Baratta (Ru- - -
integrating in a 1:1:1:1:3:3 ratio. Interestingly, prolonged
heating of uncyclized 3b,d in THF (>80 °C) resulted in
smooth conversion to cyclized derivatives 4b,d with the
31P{1H} (phospholane P atoms: 4b, δ 91.1 ppm; 4d, δ 89.9
ppm) and 1H NMR spectra closely resembling that of 4e. Both
phosphaalkene-based 3b,d and phospholane-derived 4b,d,e
were further characterized by 13C{1H} NMR spectroscopy,
mass spectrometry, and elemental analysis. Ultimately, the
a
Table 1. Selected Bond Lengths (Å) and Angles (deg) of Ligands 2a22 and 2b,d,e
2a (R = H)22
2b (R = Me)
2d (R = OMe)
2e (R = Ar-m -NO2)
PC
1.663(3)
1.847(3)
99.15(14)
1.39(5)
1.31(5)
119(4)
1.674(2)
1.8607(19)
98.35(9)
1.351(3)
1.337(2)
118.26(18)
1.466(3)
1.504(3)
1.694(4)
1.873(3)
98.27(16)
1.368(4)
1.319(5)
116.4(3)
1.444(5)
1.364(4)
1.669(3)
1.851(3)
98.17(13)
1.348(3)
1.334(3)
118.4(2)
1.461(4)
1.494(4)
P−Mes*
C−PC
N−CAr
N−CAr
C−NC
backbone C−C
CAr−R
1.470(4)
0.950
a
R factor (%): 2a (6.0);22 2b (4.73); 2d (6.71); 2e (4.50).
C
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Figure 4. 31P{1H} NMR spectra (202 MHz, CDCl3) of 3b (left) and 4e (right).
Figure 7. X-ray crystal structure of 4e. Selected bond lengths (Å) and
angles (deg) are shown in Table 2.
Figure 5. X-ray crystal structure of 3b with the solvent molecule
omitted for clarity. Selected bond lengths (Å) and angles (deg) are
shown in Table 2.
Figure 8. X-ray crystal structure of 5a. Selected bond lengths (Å) and
angles (deg) are shown in Table 2.
Figure 6. X-ray crystal structure of 4b. Selected bond lengths (Å) and
angles (deg) are shown in Table 2.
right), both indicative of a bona fide agostic interaction (d(M−
H) = 1.8−2.3 Å; M−H−C ≈ 90−140°).32 A second-order
perturbation theory analysis of the Fock matrix using NBO
indicated that the three-center, two-electron Ru−H−C bond
provided 11 kcal/mol of stabilization energy to 5a. In all cases,
these agostic interactions serve to coordinatively saturate the
Ru(II) center, affording 18-electron complexes.
H = 2.185 and 2.198 Å)30 demonstrated that double agostic
interactions31 could be formed by generating coordinatively
unsaturated Ru(II) centers via halide abstraction or through
the use of bulky phosphine ligands. Here, a seven-membered
chelate to the Ru center was formed via a Ru−H contact of
2.069 Å with a Ru−H−C bond angle of 126.67° (Figure 9,
D
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transition metals that a “true” M−F interaction must place the
metal and fluorine atom within 3.0 Å of each other;35 however,
Togni has shown via a combination of NMR spectroscopy and
X-ray crystallography that these interactions can persist above
3.0 Å.36 Unfortunately, neither 6c or M has been crystallo-
graphically characterized; thus, making an assignment of
agostic versus anagostic based on distance is premature.
However, the structure of 6c was calculated at the B3LYP-D3/
LACV3P** level, establishing the closest Ru- - -F contact at
2.978 Å (Figure 11). Although right on the 3 Å threshold,
Figure 9. Structures of K and L (left) with trimmed X-ray crystal
structure of 5a showing the agostic interaction.
The other six-coordinate complex formed was 6c, which is
supported by CF3-substituted ligand 2c. Its NMR spectra
closely mirrored 4b,d,e with the exception that the 31P NMR
signal of the phospholane (δ 85.8 ppm) was not a doublet but
rather an overlapping doublet of quartets with additional
coupling to fluorine (JPP = 40 Hz, JPF = 20 Hz; Figure 10). This
Figure 11. Computed structure of 6c with comparison to 4b (inset).
The labels on the chloride donors were omitted, and the t-Bu groups
were trimmed for clarity.
NBO calculations revealed that no bonding interaction could
be found between the Ru center and the C−F bond or the F
lone pairs, begging the question: is the “interaction” merely a
geometric constraint? Reexamination of the structure of 4b
featuring the sterically similar methyl group suggests that this
may be the case, as the nearest Ru- - -H contact sits at 2.917 Å
(see inset, Figure 11). Furthermore, the broadened 31P NMR
signal of the phospholane in 4b is likely due to a related
dynamic process involving interaction of all three H atoms of
the Me unit with the Ru center on the NMR time scale.
Solid-State Structures of Five-Coordinate Ru(II)
Complexes. Complexes 3b and 4b,e all have distorted five-
coordinate geometries skewed toward square pyramidal with
narrow N−Ru−P bite angles.37 The extremely small bite angle
of the PN ligand in 3b (80.23°) is likely the result of an
abundance of short backbone bonds including the pyridine N−
C bond, an sp2−sp2 C−C bond, and an intact PC double
bond, leading to a “tied-back” five-membered ring; a related
square-planar Pd(Me)(Cl) complex supported by ligand 1c
(see Chart 1 for structure) also featured a N−Ru−P angle of
approximately 80°.22 Cyclization of the Mes* arm across the
PC bond in 4b,e generates a new sp3 benzylic-type carbon
atom and a P−C single bond, providing more flexibility in the
chelate and opening up the bite angle to almost 85°. As the
bite angle increases, the phospholane unit shifts to a more
“apical” position, and the overall geometries of the cyclized
species are significantly closer to square pyramidal than is the
case for uncyclized 3b. A comparison of the bite angle and τ
Figure 10. Enlarged 31P{1H} NMR spectrum (202 MHz, CDCl3) of
6c highlighting the overlapping doublet of quartets for the
phospholane donor.
is due to interaction of the CF3 unit with the 16-electron Ru
center.33 Shaw reported an analogous interaction in M with
strikingly similar NMR spectra (structure shown in inset of
Figure 10);34 like 6c, only the functionalized phosphine donor
trans to F shows JPF coupling (68 Hz). Interestingly, all three F
atoms in 6c and both F atoms in M are interacting with the Ru
center on the NMR time scale, indicating that rotation about
their sp2−sp3/sp2 C−C bonds (shown in magenta in the inset
of Figure 10) is facile and the Ru−F interaction is weak.34
In fact, by analogy to agostic (three-center, two-electron)
versus anagostic (electrostatic) M−H−C bonds,32 the reduced
coupling constant in 6c may reflect an anagostic/electrostatic
Ru−CF3 interaction, while the Ru−F “bond” in M is
significantly more agostic. Distance is often the determining
factor distinguishing agostic M−H−C interactions from
anagostic interactions, with agostic interactions falling between
1.8 and 2.3 Å and anagostic interactions ranging from 2.3 to
2.9 Å.32 Plenio has argued that for second- and third-row
E
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Table 2. Selected Bond Lengths (Å) and Angles (deg) of Ligands 3b, 4b,e, and 5a and Comparison with a Previously
a
Reported22 Pd Complex Supported by Ligand 1c (See Chart 1 for Structure of 1c)
b
[Pd(Me)(Cl)(1c)] 3b (R = Me)
4b (R = Me)
4e (R = Ar-m-NO2)
5a (R = H)
N−Ru−PPC
79.45(10)
80.23(5)
84.43(7)
84.69(5)
84.11(14)
c
(bite angle)
τ38
N/A
0.38
0.19
0.23
N/A
P−CPN‑Chelate
P−Mes*
1.674(4)
1.808(4)
2.1744(14)
N/A
1.671(3)
1.825(2)
2.1498(6)
2.3382(7)
116.48(11)
1.838(3)
1.863(3)
2.1826(9)
2.2986(9)
1.853(2)
1.860(2)
2.2007(6)
2.3072(6)
1.847(7)
1.835(7)
2.2436(17)
2.2447(19)
c
Ru−PPC
Ru−PPPh3
c
C−PC
114.26(19)
110.42(14), 105.29(14),
92.94(14)
111.30(10), 103.23(11),
92.75(10)
105.8(3), 101.9(3),
91.9(3)
Ru−Npy
2.164(4)
2.147(2)
2.145(2)
2.1608(19)
2.103(5)
R factor (%): [Pd(Me)(Cl)(1c)] (5.14);22 3b (3.93); 4b (3.75); 4e (4.12); 5a (5.78). The bond lengths and angles given in this column are for
a
b
c
a Pd complex, not Ru. For complexes 4b,e and 5a, cyclization converted the PC bond into a P−C bond, and therefore, the P atom indicated is
for the newly formed phospholane P.
values38 (τ = 1 = idealized TBP; τ = 0 = idealized square
pyramidal) is shown in Table 2.
bond (which it does), but why is the Ru−P bond length to the
phospholane shorter than that to PPh3? Recent reports by
Glueck41 and Higham42 have compared phosphine and
phosphirane donors, finding that when they are of similar
size, phosphiranes feature shorter M−P bonds (M = Pt, Rh,
Au) because of increased s character in the P lone pair
enforced by the severely acute angles of the three-membered
ring. We expected similar arguments could be made for the
phospholane donor; however, DFT calculations revealed that
Ru−Pphospholane and Ru−PPh3 bonds contained the same
amount of s character (33% s, Table 3). Instead, we speculate
that the shorter Ru−Pphospholane bond is due to the fact that the
trans coordination site is vacant. Without a trans ligand, the Ru
center and phospholane P maximize their orbital overlap,
resulting in a shorter Ru−P bond relative to the PPh3 unit,
which has to compete with the pyridine donor for electron
density.43,44
Solid-State Structure of Six-Coordinate 5a. Ru(II)
complex 5a adopts a distorted-octahedral geometry featuring
cis-oriented chlorides in plane with the PN chelate; the PPh3
ligand is bound in the “apical” position and the agostic Ru−
H−C interaction is located trans to it, residing in the sixth
coordination site. This is in contrast to 3b and 4b,e, in which
the chlorides and PPh3/pyridine donors are mutually trans to
one another with the phosphaalkene-derived P atom occupying
the apical site in the square pyramid (Chart 2). This different
In addition, the X-ray structure determination of the methyl
series, specifically, 2b (see Table 1 for structural data), 3b, and
4b provided valuable insight into the structural changes of the
phosphaalkene functionality upon binding and cyclization. At
first glance, the most striking observation is that the PC
bond length does not change on coordination to the Ru(II)
center (2b vs 3b). Phosphaalkenes are strong π acceptors;4a
thus, the bond is expected to elongate. However, on
complexation to metals, the P center undergoes extensive
rehybridization from largely unhybridized (high s character in
lone pair/high p character in P−C bonds) toward sp2, adding s
character to the P−C bonds and resulting in their contraction;
the two opposing effects cancel each other out, yielding no
overall change in the PC bond length.39 Further evidence to
support this rehybridization is observed in the shortening of
the P−Mes* bond and widening of the C−PC bond angle
to 116.48(11)°; NBO calculations corroborate these findings
(Table 3). Addition of the C−H bond on the Mes* arm across
Table 3. Selected Atom Hybridization of Ligand 2b, Its
Cyclized Counterpart (Unbound to a Metal Center), and
Ru(II) Complexes 3b and 4b
2b
free phospholane
3b
4b
P LP or P−Ru bond
P−CMes* bond
sp0.51
sp5.95
sp3.86
N/A
sp0.80
sp5.30
sp6.15
N/A
sp1.51
sp2.37
sp2.17
sp2.04
sp2.02
sp3.16
sp3.68
sp1.98
Chart 2. Comparison of the Spatial Arrangement of Donor
Atoms to the Ru Center in 3b and 4b,e versus 5a
PC or P−CH2 linker
Ph3P−Ru bond
the PC bond also has profound consequences (3b → 4b,
Table 2). Most obvious is the drastic elongation of the P−C
bond as it is transformed from a three-coordinate, PC
double bond into a four-coordinate, P−C single bond, but the
P−Mes* bond also lengthens. Likewise, these observations can
be rationalized by invoking hybridization arguments. The P
donor in the pyridine-phosphaalkene ligand is ∼sp2 (C−PC
bond angle = 116.48°), while the phospholane functionality
adopts nearly sp3 hybridization. The added p character in the
P−C bonds leads to their elongation, while the P center
simultaneously reorganizes to a more tetrahedral geometry
(average C−P−C bond angle = 102.9°).40 A final more subtle
observation is that the Ru−P bond to the new PN ligands are
all shorter than that to PPh3. Again, the sp2-hybridized P atom
in the pyridine-phosphaalkene should afford the shortest Ru−P
spatial arrangement of ligands at Ru(II) has subtle structural
consequences that are a manifestation of the trans influence.43
For example, the Ru−Npyridine bond (Ru−Navg = 2.151 Å) in 3b
and 4b,e is approximately 0.05 Å longer than in 5a (Ru−N =
2.103(5) Å), and the longest Ru−Cl bond (2.4612(16) Å) by
roughly 0.06 Å (Ru−Cl = 2.4016(6) Å in 4e) is found trans to
the phospholane in 5a; both observations are consistent with
the pronounced trans influence of a PR3 group relative to a
chloride or pyridine donor.44
F
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On the other hand, the most significant structural effect on
going from five-coordinate 3b and 4b,e to six-coordinate 5a is
the drastic pyramidalization of the phospholane center with the
sum of the C−P−C angles totaling less than 300° (Table 2,
above).45 This may be rooted in geometric constraints
involved in the fact that the P atom is incorporated into a
few unusual ring structures, including a fused 5,7-system with a
Ru/P ring junction and a phosphindoline,46 which is formally
annulated to a seven-membered ring through formation of the
Ru−H−C agostic bond. This agostic interaction may also
contribute to the pyramidalization of the phospholane, as the
PN chelate is now “tridentate”47 and bound in a facial
arrangement,48 enforcing bite angles at Ru to approach 90°.
Origin of Cyclization of Phosphaalkene to Phospho-
lane at Ru. The presence of an agostic interaction also has
mechanistic implications, because substrate binding is the first
step in activation at a transition-metal center.49 With PN
ligands 2a−e, there is a clear electronic component to whether
or not the t-Bu unit of the Mes* group is activated and added
across the PC bond. Electron-rich ligands 2b,d formed 3b,d
(R = Me, OMe), in which the phosphaalkene unit persisted
(unless heated in THF), while the more electron poor ligands
2c,e (R = CF3, Ar-m-NO2) including parent 2a underwent
cyclization affording TM complexes 6c, 4e, and 5a featuring a
modified PN ligand containing a new phospholane donor.
Literature Precedent. Two relevant mechanisms resulting
in the cyclization of the Mes* group across a PC unit to give
phospholanes have been previously reported (Scheme 5). The
Scheme 6. Two Potential Mechanisms of Cyclization of the
P−Mes* Group across Its PC Double Bond in Ligands
2a−e on Exposure to a Ru(II) Center
phospholane ligand, which coordinates to the Ru(II) center in
a bidentate fashion. On the other hand, initial coordination of
the pyridine-phosphaalkene to the Ru(II) center could
promote oxidative addition of the t-Bu group, giving the 7-
coordinate, 18-electron Ru(IV) complex T (Scheme 6, left).52
Proton transfer to the phosphaalkene carbon atom will furnish
zwitterionic U, featuring a cationic phosphorus donor.
Intramolecular nucleophilic attack of the alkyl arm on the
phosphorus atom will quench the charges on both Ru and P
and simultaneously give the phospholane ring. These
mechanistic possibilities will be explored from both a
computational and experimental standpoint in the future.
Scheme 5. Previously Proposed Mechanisms of Cyclization
of the P−Mes* Group across the PC Double Bond
CONCLUSIONS AND OUTLOOK
■
Like their pincer counterparts (C and E), simple, bidentate
pyridine-phosphaalkene (PN) ligands 2a−e are capable of
cyclizing at transition-metal centers. Unexpectedly, four
distinct types of transition metal complexes (3−6) are
produced on exposure of these five ligands (2a−e), varying
only in their 6-substituent to RuCl2(PPh3)3. Complexes 4−6
feature a new phospholane donor tethered to a pyridine ring
via a CH2 spacer; future studies will assess if these acidic,
benzylic-type protons can be deprotonated to generate
dearomatized complexes such as 7 (Scheme 7, shown using
first was a Lewis acid catalyzed approach experimentally
demonstrated by exposing Mes*PCH2 to AlCl3.50 The
+
reaction generated phosphenium (PR2 ) intermediate N,
which C−H activated the t-Bu group of the Mes* arm,
affording P−Me phospholane O after proton transfer from
zwitterionic P.50,51 The second mechanism, based on reactivity
observed with C (vide supra) and supported largely by
computations (B3PW91 functional with 6-31G* basis set for
H, C, and P atoms and LANL2TZ(f) basis set for Pt), involved
coordination of the phosphaalkene to a phosphine-ligated
Pt(0) catalyst followed by stepwise C−H oxidative addition of
the t-Bu group, intramolecular P−C bond formation to give η3
intermediate Q, and C−H reductive elimination of phospho-
lane R.7
We envisioned that either mechanism could be operative
here. For example, if the Ru(II) center acts purely as a Lewis
acid (M+, no redox chemistry), complexation of the nitrogen
atom to Ru(II) opens up a pathway for dearomatization of the
pyridine moiety and generation of reactive phosphenium
intermediate S (Scheme 6, right). Addition of the C−H
bond50,51 across the cationic P center in S constructs a
phospholane ring; subsequent proton transfer and rear-
omatization results in the formation of the new pyridine-
Scheme 7. Future Dearomatization Studies
4 as a representative example). Analogous complexes
supported by pincer ligands have promoted numerous forms
of strong bond activation (C−H,53 N−H,12,54 O−H,55 etc.)
and dehydrogenation-based catalytic processes.16 We will
target energy relevant substrates with the secondary goal of
developing enantioselective catalytic reactions utilizing the
phospholane P as the source of chirality, a monodentate chiral
phosphine in place of PPh3, or a pyridine featuring an
asymmetric R group.
G
DOI: 10.1021/acs.organomet.9b00425
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
vacuum. The crude dark red solid was recrystallized from toluene
layered with pentane at −35 °C (83 mg, 0.102 mmol, 78% yield).
Crystals suitable for X-ray crystallography were collected by slow
diffusion of pentane vapors into a concentrated toluene solution.
Anal. Calcd for C47H59Cl2NOP2Ru [M + THF]: C, 63.58; H, 6.70;
N, 1.58. Found: C, 63.24; H, 6.85; N, 1.46. HRMS: m/z calcd for
C48H51N2O2P2Ru [M]+, 815.1917; found, 815.2510. 31P{1H} NMR
EXPERIMENTAL SECTION
■
General Experimental Details. Unless otherwise specified, all
reactions and manipulations were performed under a nitrogen
atmosphere in a MBraun glovebox or using standard Schlenk
techniques. All glassware was oven-dried overnight (at minimum) at
140 °C prior to use. Anhydrous solvents were purchased directly from
chemical suppliers (Aldrich or Acros), pumped directly into the
glovebox, and stored over oven-activated 4 or 5 Å molecular sieves
(Aldrich). RuCl2(PPh3)3 and Zn dust were purchased from Strem and
used as is; PMe3 was also bought from Strem and stored over
molecular sieves prior to use. The 6-substituted pyridine-2-
carboxyaldehyde derivatives were obtained from Sigma-Aldrich, and
Mes*PCl2 was prepared via a literature protocol.56 NMR spectra were
obtained on Varian spectrometers operating at 300, 400, or 500 MHz;
all spectra are displayed in the Supporting Information. NMR
chemical shifts are reported as ppm relative to tetramethylsilane and
are referenced to the residual proton or 13C signal of the solvent (1H
1
(CDCl3): δ 290.7 (d, J = 36.4, CP), 36.0 (d, J = 36.4, PPh3). H
NMR (CDCl3): δ 7.56 (t, J = 8.0 Hz, 1H, Ar), 7.46−7.39 (m, 8H,
Ar), 7.38−7.32 (m, 3H, Ar), 7.26−7.22 (m, 6H, Ar), 6.92 (d, J = 7.5
Hz, 1H, Ar), 6.54 (d, J = 4.1 Hz, 1H, Ar), 2.85 (s, 3H, Me), 1.33 (s,
9H, t-Bu), 1.23 (s, 18H, t-Bu).13C{1H} NMR (125 MHz, CDCl3): δ
163.3 (Ar), 159.5 (Ar), 155.2 (Ar), 153.4 (Ar), 136.0 (Ar), 134.7 (d, J
= 10.0 Hz, Ar), 133.2 (Ar), 132.9 (Ar), 129.5 (Ar), 127.8 (d, J = 10.0
Hz, Ar), 125.6 (d, J = 26.0 Hz, Ar), 123.8 (d, J = 10.0 Hz, Ar), 119.2
(d, J = 25.0 Hz, CP), 119.0 (Ar), 39.8 (CMe3), 35.2 (CMe3), 33.9
(t-Bu), 31.0 (t-Bu), 24.7 (Me).
Due to its similarity to that for 3b, the synthesis of 3d is included in
the Supporting Information.
1
CDCl3, 7.27 ppm; H C6D6, 7.16 ppm; 13C CDCl3, 77.16 ppm; 13C
C6D6, 128.06 ppm). Mass spectroscopic data were collected on an
Agilent 6545 Accurate-Mass Q-TOF LC/MS instrument (NSF CHE-
1532310). Analytical data were obtained from the CENTC Elemental
Analysis Facility at the University of Rochester, funded by NSF CHE-
065-456. All X-ray-quality crystals were analyzed at the Small
Molecular X-ray Crystallography Facility located at the University
of California, San Diego.
Synthesis of Pyridine-Phosphaalkenes of Type 2. Derivative
2a was previously prepared as an E/Z mixture by Geoffroy10 and
selectively (at low temperature) as the E isomer by Bickelhaupt22
using a phospha-Peterson methodology.17 Its full spectroscopic
characterization is reported in ref 22. Using a phospha-Wittig
methodology, in a manner as described below for 2b, parent 2a was
isolated in 83% yield.
Cyclized (Pyridine-Phospholane) Complex 4b. A solution of 3b
(15 mg, 0.019 mmol) in 1 mL of THF was heated at 80 °C for 48 h,
at which time the phosphaalkene resonance was no longer observed
by 31P NMR spectroscopy. The cyclized complex (31P NMR: δ 91.1,
46.7) was filtered through a pad of Celite, and the filtrate was
concentrated under vacuum. The crude solid was recrystallized from a
1/3 solution of THF/pentane at −35 °C to give dark green crystals
(12 mg, 0.015 mmol, 80% yield). Crystals suitable for X-ray
crystallography were collected by slow diffusion of pentane vapors
into a concentrated THF solution.
HRMS: m/z calcd for C45H54ClN2P2Ru [M − (Cl−) + NCMe]+,
821.2494; found, 821.2516. 31P{1H} NMR (202 MHz, CDCl3): δ
1
91.1 (br, CH2P), 48.7 (d, J = 36.4 Hz, PPh3). H NMR (500 MHz,
CDCl3): δ 7.61−7.56 (m, 2H, Ar), 7.40 (t, J = 9.0 Hz, 6H, Ar), 7.31
(t, J = 8.5 Hz, 4H, Ar), 7.21−7.14 (m, 7H, Ar), 6.96 (d, J = 1.5 Hz,
1H, Ar), 4.23 (dd, J = 17.0, 11.0 Hz, 1H, benzylic NAr), 3.99 (dd, J =
17.0, 10.0 Hz, 1H, benzylic NAr), 2.80 (s, 3H, Me), 2.17−1.96 (m,
2H, CH2P), 1.38 (s, 9H, t-Bu), 1.23 (s, 9H, t-Bu), 1.10 (s, 3H,
CMe2), 0.79 (s, 3H, CMe2). 13C{1H} NMR (125 MHz, CDCl3): δ
162.1 (Ar), 160.4 (Ar), 157.3 (d, J = 21.3, Ar), 156.7 (d, J = 7.5 Hz,
Ar), 153.6 (Ar), 136.2 (Ar), 134.6 (d J = 10.0 Hz, Ar), 133.7 (d, J =
42.5 Hz, Ar), 129.3 (Ar), 129.0 (Ar), 128.3 (d, J = 12.5 Hz, Ar), 127.9
(d, J = 8.8 Hz, Ar), 126.8 (d, J = 10.0 Hz, Ar), 123.8 (Ar), 119.2 (d, J
= 10.0 Hz, Ar), 117.9 (d, J = 10.0 Hz, Ar), 50.9 (d, J = 22.5 Hz,
benzylic), 47.6 (d, J = 43.8 Hz, PCH2CMe2), 43.2 (CMe2), 37.9
(CMe3), 35.4 (t-Bu), 35.0 (CMe3), 34.1 (t-Bu), 31.3 (t-Bu), 28.6 (d, J
= 10.0 Hz, CMe2), 25.0 (Me).
E-Selective Synthesis of 2b using a Phospha-Wittig
Methodology. Mes*PCl2 (1.00 g, 2.88 mmol) and Zn dust (940
mg, 14.4 mmol, 5 equiv) were combined in a vial, and 4 mL of THF
was added. Treatment of the suspension with a room-temperature
solution of PMe3 (560 mg, 7.36 mmol, 2.6 equiv) in THF resulted in
a yellow reaction mixture. The reaction mixture was stirred at room
temperature for 1 h and filtered through a Celite plug directly into a
solution of 6-methylpyridine-2-carboxaldehyde (262 mg, 2.16 mmol,
0.75 equiv) in 3 mL of THF to afford a homogeneous yellow-orange
solution. After it was stirred for 10 min, the reaction mixture was
concentrated under vacuum. The yellow residue was extracted with 5
mL of pentane and filtered through a Celite plug. The filtrate was
concentrated under vacuum and recrystallized from 5 mL of a 50/50
solution of THF and acetonitrile at −35 °C to give a yellow solid (700
mg, 1.84 mmol, 85% yield).
Anal. Calcd for C25H36NP: C, 78.70; H, 9.51; N, 3.67. Found: C,
78.42; H, 9.54; N, 3.59. HRMS: m/z calcd for C25H36NP [M]+,
381.2585; found, 381.2591. 31P{1H} NMR (CDCl3): δ 280.4. 1H
NMR (CDCl3): δ 8.10 (d, J = 25.0 Hz, 1H, PCH), 7.51 (m, 2H,
Ar), 7.43 (s, 2H, Ar), 6.97 (br m, 1H, Ar), 2.53 (s, 3H, Me), 1.52 (s,
18H, t-Bu), 1.35 (s, 9H, t-Bu). 13C{1H} NMR (CDCl3): δ 175.6 (d, J
= 33.8, PC), 158.2 (Ar), 157.5 (d, J = 15.0 Hz, Ar), 153.9 (Ar),
149.7 (Ar), 148.5 (Ar), 139.0 (d, J = 52.5 Hz, Ar), 136.4 (Ar), 123.7
(Ar), 121.6 (Ar), 119.4 (Ar), 117.0 (d, J = 20.0 Hz, Ar), 38.2 (CMe3),
34.0 (t-Bu), 33.9 (t-Bu), 31.8 (CMe3), 31.6 (CMe3), 31.4 (t-Bu), 30.9
(Me).
The closely related thermally induced cyclization of 3d to 4d can
be found in the Supporting Information.
Cyclization of 2e to 4e. RuCl2(PPh3)3 (197 mg, 0.205 mmol) was
loaded into a vial and treated with a solution of 2e (100 mg, 0.205
mmol) in 2 mL of benzene at room temperature. The dark reaction
mixture was transferred to a J. Young tube and monitored by 31P{1H}
NMR spectroscopy. After it was heated at 80 °C for 72 h, the reaction
mixture turned dark green, and the 31P{1H} NMR spectrum showed
two doublets at 90.3 and 48.6 ppm. The mixture was subsequently
filtered through Celite, and the filtrate was concentrated under
vacuum. The dark green residue was recrystallized from a 5 mL
solution of THF layered with pentane at −35 °C (185 mg, 0.201
mmol, 98% yield). Crystals suitable for X-ray crystallography were
collected by slow diffusion of pentane vapors into a concentrated
THF solution.
Ligands 2c−e were prepared in an analogous fashion to 2b using a
phospha-Wittig methodology. Full experimental details are included
in the Supporting Information.
Anal. Calcd for C48H52Cl2N2O2P2Ru was consistently low (three
attempts) in carbon, for example: C, 62.47; H, 5.68; N, 3.04. Found:
C, 61.83; H, 5.69; N, 2.69. HRMS: m/z calcd for
C48H52Cl2N2O2P2Ru [M]+, 922.1925; found, 922.1906. 31P{1H}
NMR (CDCl3): δ 90.3 (d, J = 36.4 Hz, P-Mes*), 48.6 (d, J = 36.4
Hz, PPh3). 1H NMR (CDCl3): δ 8.57 (s, 1H, Ar), 8.11 (d, J = 7.0 Hz,
1H, Ar), 8.03 (d, J = 9.5 Hz, 1H, Ar), 7.87 (t, J = 8.0 Hz, 1H, Ar),
7.58−7.54 (m, 3H, Ar), 7.27 (m, J = 7.0 Hz, 9H, Ar), 7.08 (t, J = 6.5
Hz, 6H, Ar), 6.95 (s, 1H, Mes*), 6.89 (t, J = 8.0 Hz, 1H, Ar), 4.25
Syntheses of Ru(II) Complexes 3−6. Pyridine-Phosphaalkene
Complex 3b. RuCl2(PPh3)3 (126 mg, 0.131 mmol) was loaded into a
vial and treated with a solution of 2b (50 mg, 0.131 mmol) in 1 mL of
benzene at room temperature. The dark reaction mixture was
transferred to a J. Young tube and monitored by 31P{1H} NMR
spectroscopy. After it was heated at 60 °C for 120 h, the solution
turned dark red, and the 31P{1H} NMR spectrum showed two
doublets at 290.7 and 36.0 ppm. The mixture was subsequently
filtered through Celite, and the filtrate was concentrated under
H
DOI: 10.1021/acs.organomet.9b00425
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
(overlapping m, J = 16.5, 10 Hz, 2H, PCH2N), 2.34 (dd, J = 16.5, 5.0
Hz, 1H, PCH2), 1.90−1.81 (m, 1H, PCH2), 1.35 (s, 9H, t-Bu), 1.15
(s, 9H, t-Bu), 1.12 (s, 3H, C(Me)2), 0.78 (s, 3H. C(Me)2). 13C{1H}
NMR (125 MHz, CDCl3): δ 164.1 (Ar), 160.8 (Ar), 157.5 (d, J =
21.3 Hz, Ar), 156.6 (d, J = 7.5 Hz, Ar), 153.7 (Ar), 148.8 (Ar), 142.6
(Ar), 136.8 (Ar), 134.9 (d, J = 8.8 Hz, Ar), 133.8 (d, J = 42.5 Hz, Ar),
131.9 (Ar), 131.4 (Ar), 129.2 (Ar), 127.6 (d, J = 10.0 Hz, Ar), 126.8
(d, J = 10.0 Hz, Ar), 126.4 (Ar), 123.9 (Ar), 123.8 (Ar), 123.2 (Ar),
121.6 (d, J = 10.0 Hz, Ar), 118.1 (d, J = 10.0 Hz, Ar), 50.9 (d, J = 22.5
Hz, benzylic), 45.9 (d, J = 42.5 Hz, Phospholane PCH2), 42.7
(CMe2), 37.7 (CMe3), 35.3 (CMe2), 35.0 (CMe3), 34.0 (t-Bu), 31.0
(t-Bu), 28.8 (d, J = 10 Hz, CMe2).
35.5 (CMe2), 35.0 (CMe3), 34.1 (t-Bu), 31.1 (t-Bu), 28.7 (d, J = 10.6
Hz, CMe2).
DFT Calculations. Full details on the computational methods are
provided in the Supporting Information.
X-ray Crystallography. Details for the seven crystal structures are
given in the Supporting Information. All data were collected at 100 K
(except for 2e, which were collected at 200 K due to an apparent
destructive phase change at lower temperatures) on Bruker
diffractometers using Mo Kα radiation. For 4e, 3b, and 4b, the
program SQUEEZE was used to render highly disordered solvent
molecules. All specimens of 2d and 5a were found to be rotationally
twinned. The structure of 2e contained a disordered t-Bu group.
Six-Coordinate 5a (C−H Agostic Complex). RuCl2(PPh3)3 (55
mg, 0.149 mmol, 1.0 equiv) was loaded into a vial and treated with a
solution of 2a (144 mg, 0.149 mmol, 1.0 equiv) in 1 mL of benzene at
room temperature. The dark reaction mixture was transferred to a J.
Young tube and monitored by 31P{1H} NMR spectroscopy. After it
was heated at 80 °C for 72 h, the solution turned dark red, and the
31P{1H} NMR spectrum showed two doublets at 66.8 and 64.0 ppm.
The mixture was subsequently filtered through a Celite plug using
Et2O, and the filtrate was concentrated under vacuum. The crude
product was recrystallized from a concentrated solution of benzene at
25 °C to give a red solid (50 mg, 0.062 mmol, 42% yield). Crystals
suitable for X-ray crystallography were collected by slow diffusion of
pentane vapors into a concentrated THF solution.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.9b00425.
■
S
Complete experimental details supplemented with NMR
spectra and specifics on DFT computational method-
ology (PDF)
Optimized Cartesian coordinates (XYZ)
Accession Codes
Anal. Calcd for C42H49Cl2NP2Ru: C, 62.92; H, 6.16; N, 1.75.
Found: C, 63.31; H, 6.12; N, 1.37. HRMS: m/z calcd for
C42H49ClNP2Ru+ [M − (Cl−)]+, 766.2067; found, 766.2077.
31P{1H} NMR (202 MHz, CDCl3): δ 66.8 (d, J = 39.3 Hz, PPh3),
CCDC 1936294−1936299 and 1937111 contain the supple-
mentary crystallographic data for this paper. These data can be
obtained free of charge via www.ccdc.cam.ac.uk/data_request/
cif, or by emailing data_request@ccdc.cam.ac.uk, or by
contacting The Cambridge Crystallographic Data Centre, 12
Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
1
64.0 (d, J = 39.3 Hz, CH2P). H NMR (500 MHz, CDCl3): δ 9.31
(dd, J = 6.1, 1.8 Hz, 1H, Ar), 7.76−7.82 (m, 5H, Ar), 7.42 (dd, J =
4.8, 1.8 Hz, 1H, Ar), 7.27−7.20 (m, 5H, Ar), 7.19−7.13 (m, 6H, Ar),
7.10 (d, J = 1.8 Hz, 1H, Mes*), 7.04 (d, J = 7.1 Hz, 1H, Ar), 6.43 (t, J
= 7.1 Hz, 1H, Ar), 3.69 (dd, J = 18.4, 11.4 Hz, 1H, CH2Ar), 3.44 (dd,
J = 18.4, 8.5 Hz, 1H, CH2Ar), 2.22 (dd, J = 15.3, 7.8 Hz, 1H, PCH2),
1.30 (s, 9H, t-Bu), 1.26 (s, 3H, Me), 1.25 (s, 3H, Me), 0.86 (d, J = 1.6
Hz, 9H, t-Bu), 0.73 (dd, J = 15.2, 2.2 Hz, 1H, PCH2). 13C{1H} NMR
(125 MHz, CDCl3): δ 164.7 (Ar), 157.0 (Ar), 156.2 (d, J = 14.2 Hz,
Ar), 154.2 (Ar), 152.2 (d, J = 10.6 Hz, Ar), 134.8 (d, J = 22.3 Hz, Ar),
134.6 (Ar), 133.9 (d, J = 9.5 Hz, Ar), 129.0 (Ar), 127.6 (d, J = 10.1
Hz, Ar), 124.6 (d, J = 8.2 Hz, Ar), 121.9 (Ar), 121.7 (d, J = 11.4 Hz,
Ar), 118.8 (d, J = 8.2 Hz, Ar), 46.8 (d, J = 20.0 Hz, CH2Ar), 43.6 (d, J
= 5.5 Hz, CMe2), 39.4 (d, J = 31.3 Hz, PCH2), 35.0 (CMe3), 30.4
(CMe3), 32.8 (t-Bu), 31.7 (d, J = 9.6 Hz, CMe2), 31.2 (t-Bu), 30.5 (d,
J = 5.3 Hz, CMe2).
AUTHOR INFORMATION
Corresponding Author
*E-mail for M.F.C.: mfcain@hawaii.edu.
ORCID
■
Matthew F. Cain: 0000-0002-0673-5839
Author Contributions
∥J.I.P.L and L.S.W. contributed equally.
Notes
The authors declare no competing financial interest.
Six-Coordinate 6c (“C−F Agostic” Complex). RuCl2(PPh3)3 (128
mg, 0.130 mmol) was loaded into a vial and treated with a solution of
2c (60 mg, 0.130 mmol) in 2 mL of benzene at room temperature.
The dark reaction mixture was transferred to a J. Young tube and
monitored by 31P{1H} NMR spectroscopy. After it was heated at 60
°C for 240 h, the solution turned dark green, and the 31P{1H} NMR
spectrum showed two doublets at 85.8 and 50.2 ppm. The mixture
was subsequently filtered through Celite, and the filtrate was
concentrated under vacuum. The crude residue was recrystallized
from THF layered with Et2O at −35 °C to give green crystals (35 mg,
0.039 mmol, 30% yield).
ACKNOWLEDGMENTS
M.F.C. thanks the University of Hawai’i at Manoa (UHM) for
■
̅
generous start-up funds and laboratory space. M.F.C. also
thanks the National Science Foundation (NSF) for a CAREER
Award (NSF CHE-1847711). M.L.N. thanks the National
Institute of Health for a fellowship funded by Grant
U54CA143727. Mass spectroscopic data was obtained at
UHM on an Agilent 6545 Accurate-Mass QTOF-LCMS (NSF
CHE-1532310). R.P.H. thanks Dartmouth College for access
to computational resources. Elemental analyses were con-
ducted by William Brennessel (University of Rochester).
31P NMR (CDCl3): δ 85.8 (overlapping dq, J = 42.8, 21.5 Hz,
CH2P) 50.2 (d, J = 42.8 Hz, PPh3). 1H NMR (CDCl3): δ 7.89 (t, J =
7.8 Hz, 1H, Ar), 7.72 (d, J = 7.8 Hz, 1H, Ar), 7.68 (d, J = 7.8 Hz, 1H,
Ar), 7.60 (dd, J = 4.6, 2.0 Hz, 1H, Ar), 7.40 (t, J = 7.8 Hz, 4H, Ar),
7.30 (m, 2H, Ar), 7.17 (m, 7H, Ar), 6.97 (d, J = 2.0 Hz, 1H, Ar), 4.33
(dd, J = 16.7, 10.7 Hz, 1H, CH2Ar), 4.18 (dd, J = 16.7, 9.8 Hz, 1H,
CH2Ar), 2.27 (dd, J = 16.4, 5.1 Hz, 1H, CH2P), 1.99 (dd, J = 16.4,
10.7 Hz, 1H, CH2P), 1.38 (s, 9H, t-Bu), 1.22 (s, 9H, t-Bu), 1.10 (s,
3H, Me), 0.79 (s, 3H, Me). 19F NMR (CDCl3): δ −63.4 (d, J = 21.5
Hz). 13C NMR (125 MHz, CDCl3): δ 163.9 (Ar), 157.8 (Ar), 156.7
(Ar), 153.8 (Ar), 136.9 (Ar), 134.7 (d, J = 10.0 Hz, Ar), 133.5 (Ar),
133.3 (d, J = 42.8 Hz, Ar), 127.8 (d, J = 9.6 Hz, Ar), 126.8 (Ar), 124.7
(Ar), 122.2 (Ar), 118.1 (Ar), 118.0 (Ar), 52.1 (d, J = 21.5 Hz,
PCH2Ar), 46.5 (d, J = 42.8 Hz, PCH2), 43.0 (CMe2), 38.0 (CMe3),
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