A Nontrigonal Tricoordinate Phosphorus Ligand Exhibiting Reversible “Nonspectator” L/X-Switching

Article abstract of DOI:10.1002/anie.201909686

We report here a “nonspectator” behavior for an unsupported L-function σ³-P ligand (i.e. P{N[o-NMe-C₆H₄]₂}, 1a) in complex with the cyclopentadienyliron dicarbonyl cation (Fp⁺). Treatment of 1a?Fp

Full text of DOI:10.1002/anie.201909686

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International Edition: DOI: 10.1002/anie.201909686
German Edition: DOI: 10.1002/ange.201909686
Phosphorous Ligands Very Important Paper
A Nontrigonal Tricoordinate Phosphorus Ligand Exhibiting Reversible
“Nonspectator” L/X-Switching
Gregory T. Cleveland and Alexander T. Radosevich*
Abstract: We report here a “nonspectator” behavior for an
unsupported l-function s3-P ligand (i.e. P{N[o-NMe-C6H4]2},
1a) in complex with the cyclopentadienyliron dicarbonyl
cation (Fp+). Treatment of 1a·Fp+ with [(Me2N)3S][Me3SiF2]
results in fluoride addition to the P-center, giving the isolable
crystalline fluorometallophosphorane 1aF·Fp that allows
À
a crystallographic assessment of the variance in the Fe P
bond as a function of P-coordination number. The nonspecta-
tor reactivity of 1a·Fp+ is rationalized on the basis of electronic
structure arguments and by comparison to trigonal analogue
(Me2N)3P·Fp+ (i.e. 1b·Fp+), which is inert to fluoride addition.
These observations establish a nonspectator L/X-switching in
(s3-P)–M complexes by reversible access to higher-coordinate
phosphorus ligand fragments.
T
ricoordinate phosphorus (s3-P) compounds are archetypal
donor ligands in coordination chemistry.[1–3] Within the
covalent bond classification,[4,5] s3-P compounds are desig-
nated L-function ligands for transition metals (M) and are
overwhelmingly construed as inert, ancillary, spectator
ligands within (s3-P)–M complexes. A rich “nonspectator”
reaction chemistry of metal-bound s3-P compounds, however,
belies this prevailing view. Abstraction of a P-substituent
from (s3-P)–M complexes accesses dicoordinate phosphorus
ligands (Figure 1a; s2-PÀ, phosphide; s2-P+, phosphenium),[6]
and the s2-P+/À/s3-P interconversion has been the focus of
extensive stoichiometric[7–13] and catalytic[14] investigation. By
complement, addition of an exogenous nucleophile to phos-
phorus in an L-function (s3-P)–M complex increases the P-
Figure 1. Nonspectator modes of reactivity for (s3-P)–M complexes.
[20]
À
metallophosphorane by formal insertion to a Ru H bond.
An interpretation of XANES data for B and related
compounds A attributed the propensity of the phosphorus
center to attain higher coordination to the presence of a low-
energy P-based orbital made accessible by the nontrigonal
local environment.[21] The presence of the low-lying P-
coordination number, resulting in a “metallophosphorane” centered orbital in A and related compounds raised the
complex with an X-function (s4-P)–M formula.[15] Literature
concerning the addition of a P-substituent to (s3-P)–M
complexes to give higher-coordinate phosphorus congeners
is comparatively sparse.[16] Verkade has postulated that
fluoride addition to PdII-(bis)phosphines induces PdII!Pd0
reduction via initial addition of FÀ to P.[17] Further, Nakazawa
and Miyoshi have shown the possibility of nucleophilic
substitution of P-substituents in cationic FeII-phosphite com-
plexes, in some cases leading to persistent (s4-P)–M prod-
ucts.[18,19]
prospect of accentuated intermolecular electrophilic reactiv-
ity of such nontrigonal s3-P ligands. We report here the
reversible addition of an exogenous nucleophile to the P-
center of an unsupported (s3-P)–M complex C that demon-
strates a nonspectator behavior of ligands A. With this study,
direct experimental evidence is provided that delineates:
(1) the inherent electronic impact on metal-binding arising
from nontrigonal distortion of s3-P ligands without convolu-
tion from chelate effects, and (2) the direct crystallographic
observation of a nonspectator phosphorus ligand in a higher-
coordination state following exogenous nucleophile addition.
The ability for nontrigonal s3-P ligands to reversibly expand
local coordination number while remaining s-bound in the
primary ligand sphere of a metal complex forecasts emerging
opportunities for functional nonspectator ligands within (s3-
P)–M complexes.[22]
Recently, a k3-chelate containing a nontrigonal s3-P center
(Figure 1, B) was shown to access directly a (s4-P)–M
[*] G. T. Cleveland, A. T. Radosevich
Department of Chemistry, Massachusetts Institute of Technology
77 Massachusetts Avenue, Cambridge, MA 02139 (USA)
E-mail: radosevich@mit.edu
On the basis of precedent from Martin[23] and Nakazawa
and Miyoshi,[18,19] the cyclopentadienyliron dicarbonyl cation
(Fp+) was selected as a coordinatively saturated “ancillary
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under https://doi.org/10.
1002/anie.201909686.
[24]
metal” fragment for study. Iron complexes 1a·Fp+ and
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1b·Fp+ were prepared by ligand exchange of [thf·Fp][PF6][25]
with P{N[o-NMe-C6H4]2} (1a)[26,27] and (Me2N)3P (1b), respec-
tively (Figure 2).
Figure 2. Synthesis of [R3P·Fp][PF6] complexes, where R3P=1a or 1b.
Figure 3. A) Left: Thermal ellipsoid plot for 1a·Fp+ rendered at 50%
À
probability level. Hydrogen atoms, noncoordinating PF6 counterion,
According to IR spectroscopy, the CO stretching frequen-
and a THF solvent molecule are omitted for clarity. Selected metrical
data for 1a·Fp : d(Fe P): 2.1809(4) Š, d(Fe (CO)1): 1.7879(17) Š,
d(Fe (CO)2): 1.7893(16) Š, ](N1-P-N2): 93.42(6)8, ](N1-P-N3): 93.04-
cies of 1a·Fp+ (nasym = 2017 cmÀ1, nsym = 2061 cmÀ1) are higher
+
À
À
in energy than those of 1b·Fp+ (nasym = 2000 cmÀ1, nsym
=
À
2045 cmÀ1). This trend tracks qualitatively with the JSe-P
coupling constants for phosphorus selenides 1a·Se (JSe-P
À
(7)8, ](N2-P-N3): 116.39(7)8. Right: Schematic projection down the P
Fe axis for 1a·Fp+ illustrating dihedral angles f(N-P-Fe-N). B) Left:
Thermal ellipsoid plot for 1b·Fp+ rendered at 50% probability level.
Only one of two molecules in the asymmetriÀc unit is depicted.
=
907 Hz) and 1b·Se (JSe-P = 784 Hz), suggesting to a first
approximation that 1a is a weaker s-donor than 1b (see
Table 1 for collected metrical data). The 57Fe NMR chemical
Hydrogen atoms and a noncoordinating PF6 counterion are omitted
+
À
for clarity. Selected metrical data for 1b·Fp : d(Fe P): 2.2381(5) Š,
d(Fe (CO)1): 1.7739(19) Š, d(Fe (CO)2): 1.7792(18) Š, ](N1-P-N2):
À
shifts (obtained indirectly by 2D Fe P correlation solution
À
À
NMR experiments due to the low receptivity of the 57Fe
101.59(8)8, ](N1-P-N3): 105.03(9)8, ](N2-P-N3): 107.09(9)8. Right:
nucleus[28]
)
for 1a·Fp+ (d = 616 ppm) and 1b·Fp+ (d =
Schematic projection down the P Fe axis for 1b·Fp+ illustrating
À
688 ppm) are consistent with this interpretation, based on
trends established for related cyclopentadienyliron com-
plexes.[29]
dihedral angles f(N-P-Fe-N). See Supporting Information for full
details.
Further distinctions between 1a·Fp+ and 1b·Fp+ are
manifest in structural analyses based on X-ray diffractometry
data obtained with single-crystalline samples (Figure 3). Most
evidently, compound 1a·Fp+ features a shorter Fe-P bond
length (dFe-P = 2.1809(4) Š) as compared to compound 1b·Fp+
(dFe-P = 2.2381(5) Š). Also, consistent with the aforemen-
1b·Fp+ shows only a span of dihedral angles W(f) = 5.3(3)8
and a maximum dihedral of f(N1-P-Fe-N3) = 121.87(14)8.
These metrics illustrate the enhanced nontrigonal local
geometry about phosphorus for 1a·Fp+ as compared to
1b·Fp+, consistent with the structural distinctions between
the free ligands.[26] For reference, the N2-P-N3 bond angle of
1a·Fp+ (116.40(7)8) is almost unchanged from that of 1a
(115.21(7)8), showing that complexation does not significantly
perturb the phosphorus triamide framework.
À
tioned vibrational data, the average Fe CCO bond length in
1a·Fp+ (dFe-C = 1.7886(17) Š) is slightly longer than in 1b·Fp+
(dFe-C = 1.7766(19) Š). A further feature of note concerns the
dihedral angles f(N-P-Fe-N); by projection down the P-Fe
axis (Figure 3A, right), compound 1a·Fp+ shows a span of
dihedral angles W(f) = 28.11(26)8, with a maximum dihedral
of f(N2-P-Fe-N3) = 137.95(13)8. By contrast, compound
À
In an effort to parse the s- and p-contributions to the Fe
P bonding interactions in 1a·Fp+ and 1b·Fp+, an energy
partitioning into pairwise orbital interactions between s3-P
ligand (1a and 1b, respectively) and Fp+ fragments was
Table 1: Collected spectroscopic, structural, and computational data for compounds 1a, 1b, 1a·Fp+, 1b·Fp+, and 1aF·Fp.
Metric 31P d
1JP-Se 57Fe d
d(Fe1 P1) n(CO)
FIA
EDA-NOCV[d]
À
!
[ppm][a] [Hz] [ppm][b] [Š]
[cmÀ1
]
[kcalmolÀ1 [c]
]
Etot
EPauli Eestat
Esteric Edisp
Eorb
s(P!Fe) p(P Fe)
1a
1b
160.4
122.4
907[e]
784[e]
–
–
–
616
688
1013
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
1a·Fp+ 183.5
1b·Fp+ 141.4
1aF·Fp À3.0
2.1809(4) 2017, 2061 59.3
2.2381(5) 2000, 2045 32.9
2.3047(9) 1952, 2007
À91.9 122.7 À105.0 17.7 À16.1 À93.6 À61.7
À18.1
À13.2
–
–
À99.8 130.0 À118.4 11.6 À18.4 À92.9 À65.8
–
–
–
–
–
–
–
–
–
[a] ppm vs. 85% H3PO4. [b] ppm vs. Fe(CO)5. [c] Computed (M06L/def2-TZVP(CPCM:CH2Cl2)) according to the method in Ref. [32]. [d] EDA-NOCV
computational results represent attractive and repulsive energies (kcalmolÀ1) between the Fp+ fragment and phosphorus ligands at the fragment
geometry of the complex. The direction of donation is defined to be from phosphorus to iron. [e] Values from Ref. [25].
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undertaken with the energy decomposition analysis–natural
To quantify the relative electrophilicity of P-based
orbitals for chemical valence (EDA-NOCV) method[30] as
implemented in the ADF modeling program[31] at the BP86/
def2-TZVP level of density functional theory (Table 1, see
Supporting Information for full details). Along lines de-
scribed by Michalak,[32] deconvolution of the covalent bond-
ing portion (Eorb) into s- and p-symmetry components for
1a·Fp+ gives donation s(P!Fe) = À61.7 kcalmolÀ1 (65.9% of
acceptor orbitals for 1a·Fp+ vs. 1b·Fp+, solvation-corrected
fluoride ion affinities (FIAs) were computed at the M06L/
def2-TZVP(CPCM:CH2Cl2) level of theory by isodesmic
reaction enthalpies according to Christeꢀs method.[33] The FIA
for 1a·Fp+ is computed to be significantly larger (-DH =
59.3 kcalmolÀ1) than that for 1b·Fp+ (ÀDH = 32.9 kcal
molÀ1). The low absolute values for the FIAs are indicative
a modest overall fluoride affinity,[34] but the difference
D(FIA) = 26.4 kcalmolÀ1 conforms to the interpretation that
P-based electrophilic reactivity should be favored at the
nontrigonal complex 1a·Fp+.
!
Eorb) and back-donation p(P Fe) = À18.1 kcalmolÀ1 (19.3%
of Eorb). An illustration of the electron deformation densities
for the three principal NOCV interactions of 1a·Fp+ is
presented in Figure 4. NOCV deformation density channel
D11 depicts depletion of electron density at P (red) and
accrual of electron density at Fe (blue) as would be expected
for an L-function s-dative interaction. NOCV deformation
density channels D12 and D13 correspond to the backflow of
electron density from an Fe dp orbital into P-based p-
acceptor orbitals with two distinct interaction energies
(DE2orb = À10.7 kcalmolÀ1, DE3orb = À7.35 kcalmolÀ1), consis-
tent with the lifting of pp degeneracy at nontrigonal 1a shown
The reactivity of 1a·Fp+ and 1b·Fp+ toward fluoride
addition was probed experimentally. Treatment of compound
1a·Fp+ with tris(dimethylamino)sulfonium trimethyldifluoro-
silicate (TASF) in acetonitrile resulted in an immediate
change in color from yellow to deep orange (Figure 5a). The
formation of a single new phosphorus-containing species was
evident by 31P NMR spectroscopy, as indicated by the doublet
resonance at d = À3.0 ppm, which displayed large scalar
coupling (J = 971 Hz) consistent with the presence of
by previous XAS evidence.[21] By way of comparison, EDA-
+
À
À
NOCV partitioning of the Fe P bond in 1b·Fp gives
a single fluorine bound to phosphorus via a direct P F
donation s(P!Fe) = À65.8 kcalmolÀ1 (70.8% of Eorb) and
bond. The large upfield shift in 31P NMR chemical shift is
consistent with an increased coordination number at phos-
phorus by fluoride addition, and this inference is confirmed
by observation of the complementary coupling in the lone
19F NMR resonance (d = 27.4 ppm, J = 971 Hz, Figure 5b).
The product was thus assigned to be fluorometallophosphor-
ane 1aF·Fp, in which a fluoride has been added to the
phosphorus of 1a·Fp+ to generate a neutral complex. In
solution, compound 1aF·Fp exhibits time-averaged molecular
Cs-symmetry with a persistent P-Fe bond; 13C NMR spectra
demonstrate an equivalence of the CO ligands (one reso-
!
back-donation p(P Fe) = À13.2 kcalmolÀ1 (14.2% of Eorb).
This analysis therefore quantifies the relatively weaker s-
donating ability of nontrigonal s3-P compound 1a as com-
pared to a compositionally related phosphorous triamide 1b
evident from spectroscopy (see above). Further, a combined
consideration of the spectroscopic, structural, and theoretical
data suggests a relatively stronger p-accepting ability of 1a vs.
1b.
2
nance at d = 211 ppm) with well-resolved JC-P = 49 Hz and
3JC-F = 5.7 Hz coupling constants. Treatment of 1b·Fp+ to
identical fluorinating conditions (TASF, MeCN, rt) does not
result in fluorination but instead returns starting materials
alongside some decomposition of 1b·Fp+. It is evident that
fluoride addition to a higher coordinate phosphorus ligand is
enabled by the enhanced electrophilicity of 1a·Fp+ as
compared to 1b·Fp+.
The air and moisture sensitive orange 1aF·Fp can be
crystallized by slow evaporation of a saturated CH2Cl2
solution at À358C (Figure 5c). X-ray diffractometry confirms
the structural assignment of 1aF·Fp as a metallophosphorane
resulting from addition of an exogenous fluoride to s3-P
ligand 1a without further substitution. With respect to the Fe
bonding environment, compound 1aF·Fp features an
À
increased Fe P bond length (dFe-P = 2.3047(9) Š) as com-
+
À
pared to 1a·Fp , as well as a shorter average Fe CCO bond
length (dFe-C = 1.764(3) Š) that coincides with a bathochromic
shift of the carbonyl stretching frequencies (nasym = 1952 cmÀ1,
nsym = 2007 cmÀ1). With respect to the P bonding environment,
metrical parameters give a geometry index of t = 0.35,
indicating a geometry closer to that of a square pyramid
than a trigonal bipyramid.[35] The addition of fluoride results
Figure 4. Contours of electron deformation density channels D11, D12,
and D13 describing the bonding between 1a and the Fp+ metal
fragments with corresponding energies and charge estimations
obtained from EDA-NOCV method.
À
in an increase in all of the P N bond lengths by 0.05 Š < DdP-
< 0.09 Š as is common for higher-coordinate main group
N
compounds that compensate for their formal “hypervalent”
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Figure 6. Bonding analysis for 1a·Fp+ and 1aF·Fp. A) Left: NLMO
+
À
representing P Fe bond for 1a·Fp . Right: Contour plot of the Lap-
lacian of the electron-density topology 1a·Fp+ in the plane containing
the Fe, P, and N atoms. Areas of charge depletion are depicted in red
and areas of charge concentration are depicted in blue. Black dots
indicate bond critical points. Metrics represent relevant properties at
the bond critical points (1 in eŠÀ3, 5 1 in eŠÀ5, H/1 in atomic units).
2
B) Left: NLMO representing P-Fe bond for 1aF·Fp. Right: Contour plot
of the Laplacian of the electron-density topology 1aF·Fp in the plane
containing the Fe, P, and F atoms.
0.53; 1aF·Fp: WBI = 0.48), in line with the observed increase
in bond length from crystallography (DdP-Fe =+ 0.12 Š). For
comparison, similar qualitative trends are reported by Gabbai
for addition of fluoride to antimony in Pt–Sb bimetallics.[38]
Here, we invoke a decreased importance of p-backbonding
effects in 1aF·Fp to account for this observation; the P-based
acceptor orbital is saturated by addition of exogenous
fluoride and unavailable for metal bonding.
Figure 5. A) Reversible fluorination of 1a·Fp+ and the resulting fluoro-
metallophosphorane 1aF·Fp. B) Solution 31P NMR spectra in CD3CN:
(top) spectrum of 1a·Fp+; (middle) spectrum of 1aF·Fp from addition
of TASF to 1a·Fp+; (bottom) spectrum of 1a·Fp+ following treatment of
1aF·Fp with AgPF6 and removal of precipitate (AgF). C) Thermal
ellipsoid plot rendered at 50% probability level for 1aF·Fp. Hydrogen
F
À
atoms are removed for clarity. Relevant metrical data for 1a ·Fp: d(Fe
Topological analysis of the computed electron density
within the quantum theory of atoms in molecules (QTAIM)
À
P): 2.3047(9) Š, d(P F): 1.6687(18) Š, ](N1-P-F): 158.11(12)8, ](N2-P-
N3): 134.91(13)8, f(C2-Fe-P-F)=2.638, f(C1-Fe-P-N3)=8.128. See Sup-
framework[39] returns bond paths defined by (3, À1) critical
porting Information for full details.
+
À
À
points for P Fe in 1a·Fp (Figure 6a, right), and both P Fe
F
À
character by distribution of electron density toward the
and P F in 1a ·Fp (Figure 6b, right). No bond paths were
substituents.[36] The P-F bond length is quite long (dP-F
=
located for any F···Fe or N···Fe trajectory, conforming to an
h1-formulation of metallophosphorane 1aF·Fp. Qualitatively,
P-based valence shell charge concentrations are evident in the
Laplacian of the electron density for both 1a·Fp+ and 1aF·Fp
1.6687(18) Š), but falls within the range (1.64(11) Š < dP-F
< 1.69(1) Š) observed for the only prior example of a struc-
turally characterized fluorometallophosphorane (i.e. Ir-
(CO)Cl2(PEt3)2(PF4)) from Holloway.[37]
À
along the P Fe bond path, in line with an L- and X-function
Bonding analysis in 1a·Fp+ and 1aF·Fp reveals changes to
ligand classification, respectively. By contrast, the Laplacian
À
À
the nature of the Fe P s-interactions as a function of fluoride
distribution for the P F bond is indicative of a “closed-shell
À
binding. NBO analysis reports a dative covalent P!Fe s-
interaction for 1a·Fp+ described by an NLMO comprising
modest polarization toward the phosphorus (P 56.2%/Fe
38.5%; Figure 6a, left) and involving a P donor NBO with
interaction” and a dominant ionic contribution to the P F
bonding in 1aF·Fp.
À
Consistent with the ionic character of the P F bonding
interaction, treatment of 1aF·Fp with fluoride abstracting
reagents leads to removal of the FÀ ligand and regeneration of
1a·Fp+. Specifically, the addition of 1 equiv of AgPF6 to
a CD3CN solution of 1aF·Fp induces the orange solution to
become yellow with immediate formation of precipitate.
Following filtration, 31P NMR spectroscopy (Figure 5b) con-
sp1.10 hybridization. The NLMO corresponding to the P Fe
À
bonding interaction in 1aF·Fp indicates an increased distribu-
À
tion across Fe P (P 49.8%/Fe 43.9%; Figure 6b, left) with
similar phosphorus parentage (sp1.16). Moving from 1a·Fp+ to
1aF·Fp, the Wiberg bond indices decrease (1a·Fp+: WBI =
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firms full consumption of 1aF·Fp and clean return of
compound 1a·Fp+. Evidently, both the nontrigonal phospho-
[12] S. Oh, S. Kim, D. Lee, J. Gwak, Y. Lee, Inorg. Chem. 2016, 55,
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[13] S. Oh, Y. Lee, Organometallics 2016, 35, 1586 –1592.
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À
rus framework and the P Fe bond are sufficiently robust as to
be retained during the course of the nonspectator L!X!L-
switching cycle.
The data reported herein define the spectroscopic,
structural, and electronic changes that accrue to phosphorus
ligand 1a as it undergoes increase in coordination number
upon exogenous fluoride addition. The conversion from L- to
X- function roles results in little change to the donor capacity
of the phosphorus ligand, but the acceptor capacity is
diminished. Further, the reversible nonspectator behavior of
tricoordinate phosphorus ligand 1a calls to mind recent
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phosphorus-based ligands in catalysis and sensing warrant
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Acknowledgements
Financial support was provided by NIH NIGMS (R21
GM134240). G.T.C. was supported by an NSF Graduate
Research Fellowship. We thank Dr. Clemens Anklin (Bruker)
for assistance with 57Fe NMR studies and Drs. Charlene Tsay
and Peter Müller (MIT) for assistance in crystallographic data
collection.
Conflict of interest
The authors declare no conflict of interest.
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