31P NMR Chemical Shift Tensors: Windows into Ruthenium Phosphinidene Complex Electronic Structures

Article abstract of DOI:10.1021/acs.inorgchem.1c01099

A series of octamethylcalix[4]pyrrole/ruthenium phosphinidene complexes (Na2[1=PR]) can be accessed by phosphinidene transfer from the corresponding RPA (A = C14H10, anthracene) compounds (R = tBu, iPr, OEt, NH2, NMe2, NEt2, NiPr2, NA, dimethylpiperidino). Isolation of the tert-butyl and dimethylamino derivatives allowed comparative studies of their 31P nuclear shielding tensors by magic-angle-spinning solid-state nuclear magnetic resonance spectroscopy. Density functional theory and natural chemical shielding analyses reveal the relationship between the 31P chemical shift tensor and the local ruthenium/phosphorus electronic structure. The general trend observed in the 31P isotropic chemical shifts for the ruthenium phosphinidene complexes was controlled by the degree of deshielding in the δ11 principal tensor component, which can be linked to the σRuP/πRuP? energy gap. A δ22-δ33 crossover effect for R = tBu was also observed, which was caused by different degrees of deshielding associated with polarizations of the σPR and σPR? natural bond orbitals.

Full text of DOI:10.1021/acs.inorgchem.1c01099

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31P NMR Chemical Shift Tensors: Windows into Ruthenium
Phosphinidene Complex Electronic Structures
Wesley J. Transue, Yizhe Dai, Martin-Louis Y. Riu, Gang Wu, and Christopher C. Cummins*
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ABSTRACT: A series of octamethylcalix[4]pyrrole/ruthenium phosphinidene complexes (Na2[1=PR]) can be accessed by
t
i
phosphinidene transfer from the corresponding RPA (A = C14H10, anthracene) compounds (R = Bu, Pr, OEt, NH2, NMe2, NEt2,
NiPr2, NA, dimethylpiperidino). Isolation of the tert-butyl and dimethylamino derivatives allowed comparative studies of their 31P
nuclear shielding tensors by magic-angle-spinning solid-state nuclear magnetic resonance spectroscopy. Density functional theory
and natural chemical shielding analyses reveal the relationship between the 31P chemical shift tensor and the local ruthenium/
phosphorus electronic structure. The general trend observed in the 31P isotropic chemical shifts for the ruthenium phosphinidene
complexes was controlled by the degree of deshielding in the δ11 principal tensor component, which can be linked to the σRuP/πRuP
energy gap. A “δ22−δ33 crossover” effect for R = Bu was also observed, which was caused by different degrees of deshielding
associated with polarizations of the σPR and σPR* natural bond orbitals.
*
t
hemical shift tensors inform us about electronic structure
through both their magnitudes and anisotropies, a fact
C
readily seen through Ramsey’s classic perturbational treatment
of nuclear shielding.1 This link between the solid-state nuclear
magnetic resonance (SSNMR) spectroscopy and electronic
structure has proven especially useful in investigations of
transition metal−ligand multiple bonds,2−5 including our own
studies of PnMo (Pn = 15N, 31P)6,7 centers. Particularly
́
sophisticated have been the studies from the Coperet
laboratory using 13C magic-angle-spinning (MAS) SSNMR
spectroscopy as a central tool in the design and understanding
of d0 olefin metathesis catalysts.8−11 While there have been
studies of the ways 31P chemical shifts may inform the
electronic structure of transition-metal complexes,12,13 the field
remains relatively unexplored. Against this backdrop, we have
sought to expand the understanding of metal−phosphinidene
bonding through 31P MAS SSNMR studies.
Figure 1. Ruthenium(II) complex Na2(DME)6[1], which accepts
phosphinidene fragments from a series of RPA compounds.
Our recent work on phosphinidene transfer from dibenzo-7-
phosphanorbornadiene RPA compounds (A = C14H10 or
anthracene) has positioned us well to explore [RP] unit
delivery to a metal center. We now report facile phosphinidene
transfer from a series of RPA compounds to the ruthenium(II)
calix[4]pyrrole (“porphyrinogen”) complex Na2(DME)6[1]
(Figure 1, DME = 1,2-dimethoxyethane).14,15 The electronic
structure of the resulting RuP bond is strongly influenced by
the substituent, an effect that we have characterized through
31P MAS SSNMR spectroscopy. The chemical shift tensor is
studies began by interrogating the reaction between
Na2(DME)6[1] and tBuPA by NMR spectroscopy in
tetrahydrofuran (THF)-d8. Although originally reported to
have a 1H NMR spectrum consistent with [1]2− being a
closed-shell complex,17 we have found THF-d8 solutions of
Na2(DME)6[1] to exhibit a single paramagnetically shifted
resonance at 1H δ −28.3 (s, 8H) ppm, indicating that [1]2− is
likely of intermediate spin S = 1, as is typical for square-planar
shown to be a direct experimental handle on the σRuP/πRuP
*
energy gap and its orientation on the nature of the
phosphorus−substituent σPR bond. Density functional theory
(DFT) calculations and natural chemical shielding (NCS)
analysis were central in uncovering these connections.
Received: April 12, 2021
Published: June 21, 2021
First investigated by Floriani et al. in 2001,14,15 the
ruthenium(II) platform [1]2− has been reported to support a
variety of ruthenium−main group multiple bonds.14−16 Our
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d6 ruthenium(II) complexes.18−20 Monitoring the reaction
following recrystallization from hot dioxane (62% isolated
yield). The structural parameters from an XRD study (Figure
2b) are similar to those of Na2(dioxane)5[1PtBu] with a
slightly shorter RuP bond length of 2.0829(8) Å and a Ru−
P−N angle of 121.05(11)°.
The ruthenium calix[4]pyrrole platform proved quite
versatile in phosphinidene complex formation. Treatment
with a medley of RPA compounds showed successful
formation of the anticipated [1PR]2− species in situ by 31P
NMR spectroscopy, resonating over a wide (∼300 ppm) range
of chemical shifts (Figure 3). The only exceptions were
21
t
between Na2(DME)6[1] and BuPA over 24 h showed
gradual loss of the 1H δ −28.3 ppm resonance and growth of a
new resonance in the 31P{1H} NMR spectrum at δ 1047 ppm,
characteristic of a bent phosphinidene.22 This change was
accompanied by a dramatic color change from orange-red to
purple.
The reaction was relatively slow even at elevated temper-
atures [k = 8.7(4) × 10−4 M−1 s−1, 80 °C, THF-d8; see the
Supporting Information (SI), section S1.7] but could be
accelerated by the addition of excess tBuPA. Full conversion to
this new species was achieved by heating Na2(DME)6[1] with
t
3 equiv of BuPA (80 °C, 18 h, THF), and the product
crystallized from a hot dioxane solution to yield
Na2(dioxane)5[1PtBu] (33% isolated) as deep-purple
blocks. Characterization by an X-ray diffraction (XRD) study
revealed a Ru−P−C angle of 121.57(3)° and a RuP bond
length of 2.1156(3) Å (Figure 2a), shorter than any
Figure 3. 31P NMR spectra of several phosphinidene complexes [1
PR]2− (Me2Pip = cis-2,6-dimethylpiperidino).
ClPA25,26 and HPA,27 which yielded complicated mixtures of
products, and (Me3Si)2NPA,21 which we presume was too
sterically encumbered to react. Additionally, extended heating
of Na2(DME)6[1] with (tBuP)328 (48 h, 80 °C) did not lead to
the observation of [1PtBu]2− in the 31P NMR spectrum,
indicating that RPA compounds are superior phosphinidene
group transfer reagents for this transformation.
The wide range of chemical shifts demonstrated high
sensitivity of the resonance to the electronic structure of the
RuPR center. Collection of the 31P MAS SSNMR spectra of
Na2(dioxane)5[1PtBu] and Na2(dioxane)4[1PNMe2]
added insight into the nature of the phosphorus chemical
shift tensors (Figure 4a,b). The large spans of the tensors (Ω =
δ11 − δ33 = 1852 and 1740, respectively) are characteristic of
multiply bonded phosphorus centers,7,29 and their highly
negative skews [κ = 3(δ22 − δiso)/Ω = −0.82 and −0.55,
respectively] indicate a single highly deshielded direction, as
expected of a double bond.30−33 Interestingly, the larger
solution 31P chemical shift δsol of Na2(dioxane)5[1PtBu]
than that of Na2(dioxane)4[1PNMe2] was reflected in all
three directions of the chemical shift tensors obtained by
simulation of the 31P MAS SSNMR spectra.
Interpretation of these tensors was assisted by DFT and the
NCS34 analytical subroutine of the natural bond orbital
(NBO) program (see the SI, section S3).35 Nuclear shielding
calculations yielded the tensors shown in Figure 4c−e upon
variation of the substituent from tert-butyl to dimethylamino
and methoxy. The axis of strongest deshielding (δ11) uniformly
resides in the Ru−P−R plane and is approximately
perpendicular to the Ru−P bond, a consequence of the strong
paramagnetic deshielding induced by nucleus−orbit coupling
between the σRuP and πRuP* NBOs. This can be visualized
Figure 2. X-ray crystallographic structures (see the SI, section S2) of
(a) Na2(dioxane)5[1PtBu] and (b) Na2(dioxane)4[1PNMe2]
with sodium, dioxane, and hydrogen centers omitted for clarity. (a)
Selected interatomic distances (Å) and angles (deg): average Ru−N
2.0591(8), Ru−P 2.1156(3), P−C 1.8981(11); Ru−P−C 121.57(3).
(b) Selected interatomic distances (Å) and angles (deg): average Ru−
N 2.053(3), Ru−P 2.0829(8), P−N 1.666(3); Ru−P−N 121.05(11).
ruthenium−phosphinidene bond currently catalogued in the
Cambridge Structural Database.23 It is also slightly shorter
than the sum of the two double-bond covalent radii (2.16 Å).24
Similar studies combining Me2NPA25 and Na2(DME)6[1]
provided analogous formation of dark purple [1PNMe2]2−
(18 h, 22 °C) by 31P NMR spectroscopy (δ 791 ppm). The
product could be isolated as a Na2(dioxane)4[1PNMe2] salt
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Figure 4. (a and b) Experimental (blue) and simulated (red) 31P MAS SSNMR (16.4 T, 85% aqueous H3PO4, δ 0 ppm) spectra of (a)
Na2(dioxane)5[1PtBu] and (b) Na2(dioxane)5[1PNMe2]. Simulations are deliberately offset downfield for easy visual inspection. Minor
impurities are marked with an asterisk (*). (c−e) Plots of the anisotropic components of the calculated chemical shift tensors for (c)
Na (OMe ) [1PtBu], (d) Na (OMe ) [1PNMe ], and (e) Na (OMe ) [1POMe] as r(δ − 1/ Tr δ)r for r = (sin θ cos ϕ, sin θ sin ϕ, cos
⃗
⃗
2
2
5
2
2
5
2
2
2
5
3
θ). Principal axes are illustrated with arrows; purple lobes indicate a downfield shift relative to δiso, and green lobes indicate upfield shift. Sodium
and hydrogen atoms and molecules of solvation are omitted for clarity. Note that the directions of δ22/δ33 are inverted in part c versus parts d and e.
as a 90° rotation of the σ bond about the δ11 principal axis
providing significant orbital overlap with the π* orbital. Such
behavior mirrors that of the 13C NMR chemical shifts for d0
olefin metathesis catalysts.8 The δ11 contributions from the σ/
π* interaction were quantified by NCS analysis to be 1987
ppm (tBu), 1545 ppm (NMe2), and 1740 ppm (OMe), giving
a trend that matches the solution δiso values.
Ramsey’s theory of nuclear shielding can be used for a
qualitative explanation of this phenomenon. Ramsey’s
perturbative treatment relates paramagnetic deshielding to
the sum-over-states (eq 1):1
leading to strong deshielding.36 The δ11 shift is thus controlled
by the energy of the π* orbital, which is, in turn, controlled by
the nature of the phosphinidene substituent. Stronger donors
should destabilize the πRuP* orbital, so the δ11 shift is a direct
experimental readout of the π donicity. Indeed, the tert-butyl
derivative has the most downfield shift of the three and the
strong π-donor dimethylamino derivative the most upfield
shift, and the weaker π-donor methoxy derivative is
intermediate.
A second interesting difference between the tensors is
readily observed in Figure 4c−e, which shows that δ22 and δ33
are reversed in the tert-butyl derivative.37,38 NCS calculations
clearly reveal that the orientation of the tensor is dictated by a
balance of influences from the σRuP and σPR NBOs (Table S2).
The σPR NBO contributes to deshielding along the Ru−P bond
through coupling to the πRuP* antibonding NBO, and the σRuP
NBO contributes perpendicularly to the Ru−P−R plane
through coupling to the σRuP* antibonding NBO. Both
influences seem to reflect the polarization of the phospho-
rus−substituent interaction (Table 1). As the electronegativity
of the substituent increases, the phosphorus character of the
σPR NBO decreases and that of σPR* correspondingly increases.
The 31P deshielding influence of the NBO parallels its
Re[⟨0|Lu|k⟩⟨k|Lα,vrα
−3|0⟩]
para
δ
∝
∑
uv
E0 − Ek
(1)
k>0
Here, α is the nucleus of interest (the phosphorus-31 center),
u/v indexes the Cartesian directions (xyz), |0⟩ is the ground
state, |k⟩ indexes all excited states, EX is the energy of state X,
Re indicates the real part of a complex number, Lu is the u
component of the angular momentum operator, rα is the
distance from nucleus α, and Lα,v is the v component of the
angular momentum operator relative to nucleus α. The low-
lying πRuP* orbital provides a small E0 − Ek denominator,
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Authors
Table 1. Percent Phosphorus Character (%) of NBOs
Wesley J. Transue − Department of Chemistry, Massachusetts
Institute of Technology, Cambridge, Massachusetts 02139,
United States; orcid.org/0000-0001-7445-5663
Yizhe Dai − Department of Chemistry, Queen’s University,
Kingston, Ontario K7L3N6, Canada
Martin-Louis Y. Riu − Department of Chemistry,
Massachusetts Institute of Technology, Cambridge,
Massachusetts 02139, United States; orcid.org/0000-
0002-0900-3545
substituent
NBO
tBu
NMe2
OMe
σRuP
56
44
67
33
37
63
47
53
72
28
24
76
51
49
68
32
19
81
σRuP
πRuP
πRuP
σPR
*
*
*
σRuP
Gang Wu − Department of Chemistry, Queen’s University,
Kingston, Ontario K7L3N6, Canada; orcid.org/0000-
0002-0936-9432
phosphorus character, meaning the percent phosphorus
character should correlate with the NBO influence. Indeed,
this is seen for both the σPR contribution along the Ru−P bond
(tBu, 1166 ppm, σPR 37% P; NMe2, 497 ppm, σPR 24% P;
OMe, 405 ppm, σPR 19% P) and the σRuP contribution
perpendicular to the Ru−P−R plane (tBu, 434 ppm, σPR* 63%
P; NMe2, 559 ppm, σPR* 76% P; OMe, 693 ppm, σPR* 81% P).
Thus, the orientation of the δ22 and δ33 axes is an experimental
reflection of the polarization of the P−R bond.
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.1c01099
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This material is based on research supported by the National
Science Foundation under Grant CHE-1955612. G.W. thanks
the NSERC of Canada for funding.
Influences on the 31P chemical shift tensor are undoubtedly
multifaceted; however, the wide array of derivatives available
from Na2(DME)6[1] has allowed us to map out several of the
strongest factors. In this way, 31P MAS SSNMR spectroscopy
makes experimentally accessible the local electronic structure
of the RuPR moiety. Leverage of analogous information in
d0 olefin metathesis catalysts has been a major influence in the
design of early-transition-metal alkylidene catalysts,8−11 hinting
that 31P MAS SSNMR spectroscopy may be a boon to similar
advances in phosphinidene chemistry. Such analogies with
metathesis and cyclopropanation catalysts were an early motive
in the preparation of [1PR]2− complexes because we have
recently begun to investigate catalytic “phosphiranation”
reactions.39 We plan to use SSNMR to guide the search for
further phosphinidene transfer catalysts.
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AUTHOR INFORMATION
■
Corresponding Author
Christopher C. Cummins − Department of Chemistry,
Massachusetts Institute of Technology, Cambridge,
Massachusetts 02139, United States; orcid.org/0000-
0003-2568-3269; Email: ccummins@mit.edu
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