Fluorocarbene, fluoroolefin, and fluorocarbyne complexes of Rh

Article abstract of DOI:10.1039/c6sc05391b

The manuscript reports the synthesis, characterization, and analysis of electronic structure in a series of complexes of small perfluorocarbon ligands with the (PNP)Rh fragment (where PNP is a diarylamido/bis(phosphine) pincer ligand). Reactions of (PNP)Rh(TBE) as the source of (PNP)Rh with CHF₃ and C₂HF₅ produced perfluoroalkylidene complexes (PNP)Rh═CF₂ and (PNP)Rh═C(F)(CF₃). (PNP)Rh═CF₂ could also be obtained via the reaction of (PNP)Rh(TBE) with Me₃SiCF₃/CsF, with an admixture of (PNP)Rh(C₂F₄), where TBE = tert-butylethylene. Abstraction of fluoride from these neutral (PNP)RhC_xF_y complexes was successful, although only abstraction from (PNP)Rh═CF₂ allowed unambiguous identification of the Rh product, [(PNP)Rh=CF]⁺. DFT computational studies allowed comparison of relative energies of (PNP)Rh(C₂F₄) and [(PNP)Rh(C₂F₃)]⁺ isomers as well as comparisons between the electronic structure of the ═CF₂, C₂F₄, and =CF⁺ complexes and their hydrocarbon analogues.

Full text of DOI:10.1039/c6sc05391b

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ARTICLE TYPE
Fluorocarbene, Fluoroolefin, and Fluorocarbyne Complexes of Rh†
a
a
Christopher J. Pell¶ , Yanjun Zhu¶ , Rafael Huacuja║a, David E. Herberta‡, Russell P. Hughesb*, and Oleg
V. Ozerova*
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
The manuscript reports the synthesis, characterization, and analysis of electronic structure in a series of
complexes of small perfluorocarbon ligands with the (PNP)Rh fragment (where PNP is
a
diarylamido/bis(phosphine) pincer ligand). Reactions of (PNP)Rh(TBE) as the source of (PNP)Rh with
CHF3 and C2HF5 produced perfluoroalkylidene complexes (PNP)Rh=CF2 and (PNP)Rh=C(F)(CF3).
10 (PNP)Rh=CF2 could also be obtained via the reaction of (PNP)Rh(TBE) with Me3SiCF3/CsF, with an
admixture of (PNP)Rh(C2F4), where TBE = tertꢀbutylethylene. Abstraction of fluoride from these neutral
(PNP)RhCxFy complexes was successful, although only abstraction from (PNP)Rh=CF2 allowed
unambiguous identification of the Rh product, [(PNP)Rh≡CF]+. DFT computational studies allowed
comparison of relative energies of (PNP)Rh(C2F4) and [(PNP)Rh(C2F3)]+ isomers as well as comparisons
15 between the electronic structure of the =CF2, C2F4, and ≡CF+ complexes and their hydrocarbon analogues.
characterized for Ir by Bergman et al. (Figure 1, bottom),18 and
Introduction
one square planar 16ꢀelectron complex was mentioned in passing
50 for Rh by Werner et al.,19 as a component of a reaction mixture.
The “concentration” of metal carbyne complexes in the middle of
the transition metal series can be compared with similar trends for
other metalꢀelement multiple bonds. 20 , 21 In this report, we
describe the synthesis, characterization, and analysis of electronic
55 structure of a rare cationic fluoromethylidyne complex of Rh, as
well as related Rh perfluoroalkylidene complexes.
Organofluorine chemistry’s major impact on the world of
industrial chemistry has inspired many investigations into the
unique properties that are inherent to molecules and materials
20 containing CꢀF bonds. Transition metal complexes containing
perfluorocarbon ligands are an important subset of these studies
since they exhibit distinctive bonding properties1 and can mediate
2
perfluoroalkylꢀcarbon bond forming processes.
Group 9
perfluoroalkylidenes have garnered interest in the past decade
25 after Hughes developed a simple reductive method for making
Ir=CFR complexes (Figure 1, top) from iridiumꢀfluoroalkyl
precursors.3 These complexes have been analyzed in the context
of their potential intermediacy in perfluorolefin metathesis,4 and
more recently the Baker group has shown that analogous cobalt
30 perfluorocarbenes (Figure 1, top)5 are capable of undergoing a
6
[2+1] cycloaddition with :CF2 and [2+2] cycloaddition with
7
C2F4.
Analogous chemistry was also reported for
a
difluorocarbene complex of Ni(0).8 Baker has also shown that
cationic cobalt (III) difluorocarbenes could undergo migratory
35 insertion into perfluoroalkyl ligands, possibly providing
a
blueprint for transition metal catalyzed perfluoroolefin
polymerization.9
The only family of isolable terminal fluoromethylidyne
complexes known to date are the Cp*M(CO)2(CF) compounds
Figure 1. Perfluoroalkylidenes from Hughes and Baker. Isolated
fluorocarbynes by Hughes, the Ir carbyne by Bergman, and matrixꢀ
trapped fluorocarbynes by Andrews.
40 (M = Cr, Mo, W) reported by Hughes and coꢀworkers (Figure 1,
10
bottom).
The Andrews group has reported a number of
Results and Discussion
fluoromethylidyne complexes of the general formula X3M(CF)
(Figure 1, bottom; X = halogen) via trapping laser ablated metal
atoms in argon/halocarbon matrices at ca. 10 K.11 Most of the
45 isolable terminal carbyne complexes are complexes of metals of
groups 6,12,13 7,13,14 and 8.15,16 A few examples are known for
group 5.17 In group 9, one 18ꢀelectron complex has been fully
Synthesis of CF2, C2F4, and CFCF3 complexes.
60
We recently reported reactions of the (PNP)Rh fragment with
aryl carboxylates, including arylꢀoxygen oxidative addition.22 The
(PNP)Rh acylꢀoxygen oxidative addition product of phenyl
trifluoroacetate, (PNP)Rh(C=OCF3)(OPh), could be thermalized
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to produce (PNP)Rh(CO) and (PNP)Rh(CF3)(CO)(OPh) as major
products. In that report, we noted that some other unidentified
products were evident in trace amounts. We continued to be
proceeded via CꢀH oxidative addition of HCF3 to Ir followed by
loss of HF. In a similar vein, we found that (PNP)Rh(TBE)
reacted with HCF3 at 80 °C to provide a mixture of compounds
intrigued by one apparent trace product in particular that was 55 containing (PNP)Rh=CF2 as a major product (>80%) with
5
consistently observed in 2ꢀ5% yield. For it, we observed a
doublet of triplets both in the 31P{1H} NMR and 19F NMR spectra
(coupling constants: JRhꢀP = 146 Hz, JRhꢀF = 49 Hz, JPꢀF = 30
(PNP)Rh(CO) and [(PNP)Rh]2(ꢁꢀN2) as minor products.
Commercial HCF3 contains dinitrogen as an impurity.
1
2
3
Hydrolysis of a difluorocarbene complex to a carbonyl complex
26
Hz). These multiplicities implied a P2RhF2 NMR spin system –
has precedent,
but attempts to purposefully hydrolyze
rather unexpected given the three fluorines in the CF3 group of 60 (PNP)Rh=CF2 proved to be unsuccessful, reminiscent of Baker’s
10 the starting material. We noted that the 19F NMR chemical shift
was itself uncommon (95.6 ppm) and in the range reported for
various difluorocarbene complexes (i.e., M=CF2).5,10a, 23 , 24 The
cobalt fluorocarbene complexes.5 It is possible that hydrolysis of
(PNP)Rh=CF2 only takes place in the presence of HF (a byꢀ
product of (PNP)Rh=CF2 generation). We observed no
intermediates 27 in the reaction of (PNP)Rh(TBE) with HCF3,
65 which may indicate that dissociation of TBE28 is the rateꢀlimiting
step.
2
observed JRhꢀF = 49 Hz was also similar to that of Grushin’s
transꢀ(Ph3P)2(F)Rh=CF2, which possessed a 2JRhꢀF of 33 Hz.23
An analogous reaction of (PNP)Rh(TBE) with C2HF5 was
attempted as a potential means to access (PNP)Rh(C2F4).
However, the major product of this reaction turned out to be a
70 tetrafluoroethylidene complex (PNP)Rh=C(F)(CF3). Dinitrogen
impurity in C2HF5 led to the known29 [(PNP)Rh]2(ꢁꢀN2) as a
major side product, whose content could be reduced by degassing
C2HF5 using the “freezeꢀpumpꢀthaw” technique. (PNP)Rh=CF2
was also observed as a side product composing 13% of the
75 reaction mixture when (PNP)Rh(TBE) was treated with 2 atm of
Scheme 1. Initial observation of (PNP)Rh=CF2.
15
We hypothesized that this minor side product might be
(PNP)Rh=CF2 and attempted an independent synthesis of it based
on the procedure of Grushin et al. that yielded transꢀ
(F)(PPh3)2Rh=CF2.23 Indeed, treatment of (PNP)Rh(TBE) (TBE =
20 tertꢀbutylethylene) with CsF/Me3SiCF3 (Rupert’s reagent)
resulted in complete consumption of (PNP)Rh(TBE) and the
formation of (PNP)Rh=CF2 and (PNP)Rh(C2F4) in about 85:15
ratio (NMR evidence).25 We were able to isolate (PNP)Rh=CF2 in
52% yield and of >98% purity by recrystallization. The 31P{1H}
25 NMR and 19F NMR spectra of (PNP)Rh=CF2 so obtained were
identical to that of the impurity we observed in the reaction in
Scheme 1.
(PNP)Rh(TBE) showed no reaction with Me3SiCF3 alone.
Similar to the other cases of use of CsF/Me3SiCF3, we propose
30 that these reagents generate a CF3 anion equivalent that displaces
TBE and then loses fluoride, resulting in the formal transfer of
CF2 to Rh. Alternatively, CsF/Me3SiCF3 could be generating free
CF2 which then binds to Rh. Using the CsF/Me3SiCF3 protocol,
we could not avoid the formation of (PNP)Rh(C2F4) due to the
35 generation of free C2F4 from the CsF/Me3SiCF3 mixture. C2F4
was observed by 19F NMR spectroscopy in control experiments
where CsF and Me3SiCF3 were mixed in C6D6 and heated at 80
°C. No reaction was observed when (PNP)Rh=CF2 was treated
with another equivalent of CsF/Me3SiCF3. This contrasts the
40 reactivity of Baker’s difluorocarbene cobalt (I) complexes6 which
undergo a [2+1] cycloaddition with free :CF2 to form cobalt
tetrafluoroethylene complexes.
To date, we have not been able to formulate a reasonable
proposal for how (PNP)Rh=CF2 could be formed from
45 (PNP)Rh(COCF3)(OPh) (Scheme 1). The formation of M=CF2
by fluoride migration from MꢀCF3 is well precedented and is
likely the key step in forming (PNP)Rh=CF2; the difficulty is
with conceiving of a plausible fate of the other atoms of the
original phenyl trifluoroacetate molecule.
Scheme 2. Synthesis of rhodium fluorocarbenes and tetrafluoroethylene
complexes.
C2HF5 and heated overnight at 80 °C. (PNP)Rh=C(F)(CF3) could
be isolated in >90% purity with (PNP)Rh(CO) composing the rest
of the mixture. The synthesis of (PNP)Rh(C2F4) was instead
accomplished by thermolysis of (PNP)Rh(TBE) in a solution
80 containing C2F4 which was generated in situ by reducing C2F4Br2
with 1.5 eq. of Zn powder at 50 °C in THF. (PNP)Rh(C2F4) was
isolated in 64% yield as a pure solid.
The presence of multiple NMRꢀactive nuclei provided for
informationꢀrich
NMR
spectra
of
(PNP)Rh=CF2,
85 (PNP)Rh=C(F)(CF3), and (PNP)Rh(C2F4). All three complexes
displayed C2vꢀsymmetric NMR spectra in solution at ambient
temperature. The carbene complexes (PNP)Rh=CF2 and
(PNP)Rh=C(F)(CF3) displayed characteristic 13C NMR
resonances at 206.3 and 225.0 ppm. In the CF2 complex the
90 observation that the two fluorines couple identically to both
phosphorus nuclei, and vice versa, is consistent with rapid
rotation about the Rh=CF2 bond on the NMR timescale at room
temperature. Likewise the observation of identical coupling of
both Pꢀnuclei to all four fluorines in the C2F4 complex is
95 consistent with rapid rotation about the Rhꢀalkene bond axis.
50
Goldman et al. documented formation of (PCP)Ir=CF2 in a
reaction of a (PCP)Ir source with HCF3.24 This reaction
2
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Small energy barriers to these rotations are calculated by DFT 50 spectroscopy and typically broad or no signals were observed by
(see below).
31P{1H} NMR spectroscopy. The reaction mixtures produced
from the reaction of (PNP)Rh(C2F4) or of (PNP)Rh=C(F)(CF3)
with [(Et3Si)2OTf][HCB11Cl11] did regenerate the corresponding
starting material when treated with CsF. This indicates that
The identity of (PNP)Rh=CF2 and (PNP)Rh(C2F4) was
confirmed by Xꢀray diffraction studies on suitable single crystals.
Treating the CF2 or C2F4 ligands as occupying a sole coordination
5
site, the coordination environment about Rh is approximately 55 fluoride abstraction from these two isomeric complexes generates
square planar in both molecules. The CF2 unit in (PNP)Rh=CF2
lies approximately in that plane, while the CꢀC vector of the C2F4
ligand in (PNP)Rh(C2F4) is approximately perpendicular to it.
isomers of [(PNP)Rh(C2F3)]+ that do not interconvert on the
experimental time scale. Although we were not able to identify
these compounds experimentally, DFT computational studies
were used to investigated possible structures of the
10 The CF2 and C2F4 ligands evidently exert similar transꢀinfluence
as the RhꢀN distances in (PNP)Rh=CF2 and (PNP)Rh(C2F4) are 60 [(PNP)Rh(C2F3)]+ isomers (vide infra).
only different by ca. 0.01 Å. In general, the metrics of the
Rh=CF2 unit in (PNP)Rh=CF2 are very similar to the Rh=CF2
unit in transꢀ(Ph3P)2(F)Rh=CF2. The structures of (PNP)Rh=CF2
15 and (PNP)Rh(C2F4) contain some close CꢀF…H contacts (Cꢀ
F···H distances of 2.33ꢀ2.45) (F···C distances of 3.1ꢀ3.3 Å).
While they are probably unavoidable in these molecules, these
distances are short enough to be considered weak F…H
interactions.30 CꢀF···H interactions have been observed in pincerꢀ
20 ligated zirconium complexes bearing a trifluoromethyl as a
pendant group, which have also exhibited throughꢀspace HꢀF
31
coupling visible in their 1H NMR spectra.
However
(PNP)Rh(C2F4) and (PNP)Rh=CF2 showed no throughꢀspace 19Fꢀ
1H coupling to the isopropyl arms.
25 Synthesis of cationic fluoromethylidyne.
Figure 2. ORTEPs of (PNP)Rh=CF2 (left) and (PNP)Rh(C2F4) (right).
The ellipsoids are set at the 50% probability level, and hydrogen atoms
are omitted for clarity. Selected bond distances (Å) and angles (°) for
(PNP)Rh=CF2: Rh1ꢀC1, 1.821(4); Rh1ꢀN1, 2.043(3); C1ꢀF1, 1.335(4);
C1ꢀF2, 1.348(5); N1ꢀRhꢀC1, 171.39(15); F2ꢀC1ꢀF1, 100.8(3); Rh1ꢀC1ꢀ
F2, 130.1(3); Rh1ꢀC1ꢀF1, 128.6. (PNP)Rh(C2F4): Rh1ꢀC14, 2.006(3);
Rh1ꢀN1, 2.054(3); C14ꢀF1, 1.378(3); C14ꢀF2, 1.361(3); C14ꢀC14’,
1.354(7); C14ꢀRhꢀC14’, 39.4(2); C14ꢀRhꢀN1, 160.28(10).
With compounds (PNP)Rh=CF2, (PNP)Rh(=CFCF3), and
(PNP)Rh(C2F4) in hand, we contemplated whether one of the
fluorides could be removed to yield cationic CxFy complexes.
Hughes et al. previously demonstrated protonꢀinduced loss of
30 fluoride from αꢀpositions of Ir perfluoroalkyls,3a and Baker
recently demonstrated a Lewisꢀacid abstraction of a fluoride from
Nꢀheterocyclic fluoroalkenes to yield polyfluoroalkenyl
32
imidazolium salts.
There is significant precedent for
electrophilic abstraction of an anionic heteroatom substituent
On the other hand, reaction of (PNP)Rh=CF2 with [Et3SiꢀHꢀ
SiEt3][HCB11Cl11] cleanly and reproducibly generated a new Rh
65 complex that displayed a P2RhF NMR spin system. The key
NMR spectroscopic features of this compound were the unusual
33
35 from lateꢀmetal carbene complexes by a Lewis acid.18,
Trialkylsilylium cations, in the form of their salts with
halogenated carborane anions, are powerful Lewis acids with
high affinity for fluoride.34 We and others have exploited them in
catalytic CꢀF activation reactions35 and thus a [R3Si]+ reagent
40 appeared perfect for fluoride abstraction.
1
19F NMR chemical shift (66.2 ppm), the very high JCꢀF coupling
constant of 470 Hz,38 and the rather substantial 2JRhꢀF = 136 Hz.
These spectroscopic data are similar to those exhibited by
70 Cp*(CO)2Mo≡CF, whose 19F NMR spectrum contained a singlet
1
at 78.15 ppm, with a large JCꢀF coupling constant of 556 Hz
evident by 13C NMR spectroscopy.10a Hughes’s other
Cp(CO)2M≡CF (M = Cr, W) complexes also exhibited 19F NMR
chemical shifts in this region with high JCꢀF coupling constants.10b
75 Due to limited solubility in nonꢀinteractive solvents and the
extensive coupling inherent to the fluoromethylidyne 13C NMR
resonance in [(PNP)Rh≡CF]+, it was not observed by 13C{31P},
13C{1H}, nor 13C{19F} NMR spectroscopy.
Scheme 3. Synthesis of [(PNP)Rh ≡ CF][CHB11Cl11] via fluoride
abstraction from (PNP)Rh=CF2.
Xꢀray quality crystals of [(PNP)Rh≡CF][CHB11Cl11] were
80 studied using Xꢀray diffraction to yield
a structure fully
supportive of a fluorocarbyne formulation (Figure 3). The
structural and NMR spectroscopic features of [(PNP)Rh≡CF]+ are
best reviewed in comparison with Cl3Rh≡CF and a few other
relevant compounds. Andrews et al. observed IR spectroscopic
85 evidence for Cl3Rh≡CF in reactions of laserꢀablated rhodium
atoms with CFCl3. A DFT calculation of this product predicted a
Reactions
of
(PNP)Rh=CF2,
(PNP)Rh(C2F4),
and
or
36
45 (PNP)Rh=C(F)(CF3) with [Et3SiꢀHꢀSiEt3][HCB11Cl11]
[(Et3Si)2OTf][HCB11Cl11]37 all generated the Et3SiF byꢀproduct,
indicating that fluoride abstraction took place in all three cases.
However, reactions of (PNP)Rh(C2F4) and (PNP)Rh=C(F)(CF3)
resulted in mixtures of several products as seen by 19F NMR
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RhꢀC bond length of 1.740 Å and a RhꢀCꢀF bond angle of
143.4°.11d This compares with our observed RhꢀC bond length of
newly synthesized family of fluorocarbon complexes
(PNP)Rh(C2F4), (PNP)Rh=CF2, and [(PNP)Rh≡CF)]+ prompted a
1.702(7) Å and a RhꢀCꢀF bond angle of 173.46°. Although both 30 computational comparison with their (hypothetical) hydrocarbon
[(PNP)Rh≡CF]+ and Cl3Rh≡CF are fourꢀcoordinate, they contain
different numbers of valence electrons: from a hypothetical point
of view of a [CF]+ ligand, it is attached to a d8 Rh center in
[(PNP)Rh≡CF]+, but to a d7 [Cl3Rh]ꢀ fragment in Cl3Rh≡CF. The
analogues (PNP)Rh(C2H4), (PNP)Rh=CH2 and [(PNP)Rh≡CH]+
in order to assess the effects of fluorination on the metalꢀcarbon
bonding, and, for the carbene and carbyne complexes, to probe
the nature and extent of the multiple bonding between the metal
5
geometry of the RhCF unit in [(PNP)Rh≡CF]+ is similar to 35 and carbon. In addition, since the CF+ ligand is isoelectronic with
Bergman’s iridium carbyne complex (Scheme 1), which
the wellꢀknown ligands NO+ and CO, it was of interest to
establish trends in metal ligand and intraligand bonding between
[(PNP)Rh≡CF]+, [(PNP)Rh(NO)]+, and (PNP)Rh(CO). Full
molecule DFT studies were performed using the M06
10 possesses an IrꢀC bond length of 1.734(6) Å and an IrꢀCꢀC bond
angle of 175.7(4).18 The RhꢀC distance in [(PNP)Rh≡CF]+ is ca.
0.12 Å shorter than that in (PNP)Rh=CF2, consistent with the
increase in the RhꢀC bond order. The RhꢀC bond distance in 40 functional49,50 and the tripleꢀζ LACV3P**++ basis set, which
[(PNP)Rh≡CF]+ is also ca. 0.07 Å shorter than that in Werner’s
15 transꢀ(PiPr3)2ClRh=C=C(Me)(H) square planar vinylidene
complex.39
uses extended core potentials 51 ꢀ 54 on heavy atoms and a 6ꢀ
311G**++ basis 55 ꢀ 58 for other atoms, as implemented in the
Jaguar 59 , 60 suite of programs. Full details are available as
Supplementary Information.
45
Selected bond lengths and computed WBI values for the
calculated complexes are provided in Table 1, with metric
comparisons to the available crystallographic structures reported
here. The DFT calculated metrics are in good agreement with
crystallographic numbers, giving confidence in the DFT metrics
50 for the unknown complexes. One exception appears in
(PNP)Rh(C2F4) in which the CꢀC distance for the coordinated
alkene (1.354 Å) is only slightly longer than that in C2F4 itself
(1.318 Å)61 and is much shorter than all other transition metal
complexes of this perfluoroalkene in the Cambridge Structure
55 Database.62 In contrast, the DFT calculated value (1.416 Å) is in
good agreement with other crystallographically determined
values62 and is much more sensible with respect to calculated
WBI values (see below).
All the complexes examined here can be formally viewed as
60 square planar d8 compounds, i.e., as complexes of a d8, threeꢀ
coordinate fragment (PNP)Rh with neutral or cationic ligands.
The NBO perspective of bonding interactions in such
compounds42,46 requires 4ꢀelectron/3ꢀcenter bonds between the
pair of trans ligands such that the alkene, carbene, and carbyne
65 ligands of interest are always involved in a shared bonding
interaction with the N of the PNP pincer. Clearly contributions to
this shared interaction may be weighted differently in each case,
and the WBI values should reflect this.
Figure 3. ORTEP of [(PNP)Rh≡CF][HCB11Cl11]. The ellipsoids are set
at the 50% probability level, and hydrogen atoms are omitted for clarity.
Selected bond distances (Å) and angles (°): Rh1ꢀC1, 1.702(7); Rh1ꢀN1,
2.019(4); C1ꢀF1, 1.257(8); C1ꢀRh1ꢀN1, 174.1(3); F1ꢀC1ꢀRh1, 173.4(7).
Computational Studies and Discussion
DFT Structural Studies
Modern Density Functional Theory (DFT) is a powerful tool
20 with which to examine electronic structures and bonding trends in
40 , 41
organometallic compounds.
In addition, application of
Natural Bond Orbital (NBO)42 ꢀ 47 methods allows insight into
some of the subtleties of metalꢀligand bonding.42,44,46 The NBO
analysis also generates Wiberg Bond Indices (WBI),48 determined
25 within the natural atomic orbital basis, providing one means of
estimating bond orders between atoms. Trends in WBI values are
also useful in tracking variations in bond multiplicities. The
Comparison of C2H4 and C2F4 ligands.
70 The bonding between alkenes and transition metal fragments is
well understood,63 but a comparison between C2H4 and C2F4
coordinated to identical metalꢀligand fragments is rare. A classic
64
intramolecular example involves CpRh(C2H4)(C2F4)
but
4
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Table 1. Calculated and Crystallographic Bond lengths (Å)a and Wiberg Bond Indicesb (WBI).
Compound
RhꢀPave
2.332
0.462
2.378
2.3309(11)
0.436
2.338
0.476
2.331
2.302(12)
0.462
2.378
0.440
2.384
2.337(16)
0.432
2.384
0.453
2.336
0.462
RhꢀN
2.098
0.378
2.100
2.054(3)
0.352
2.216
0.225
2.156
2.043(3)
0.297
2.075
0.332
2.061
2.019(4)
0.341
2.054
0.330
2.117
0.291
2.019
0.445
RhꢀCave
2.165
0.453
2.048
2.006(3)
0.594
1.850
1.250
1.864
1.821(4)
1.168
1.728
1.714
1.740
1.702(7)
1.587
1.734
1.770
1.855
1.041
CꢀC
1.394
1.474
CꢀXave (ligand)
1.088
(PNP)Rh(C2H4)
0.930
1.346
1.369(3)
0.858
1.104
0.955
1.325
1.341(5)
0.911
1.110
0.942
1.247
1.257(8)
1.065
1.499
0.960
1.152
2.031
1.416
1.354(7)c
1.186
(PNP)Rh(C2F4)
(PNP)Rh=CH2
(PNP)Rh=CF2
[(PNP)Rh≡CH]+
[(PNP)Rh≡CF]+
ꢀ
ꢀ
ꢀ
ꢀ
[(PNP)Rh≡CꢀCF3]+
(PNP)Rh(CO)
ꢀ
ꢀ
ꢀ
2.393
0.439
1.778 (RhꢀN)
1.151
1.141 (NO)
1.917
[(PNP)Rh(NO)]+
a DFT calculated (M06/LACV3P**++) values are in plain text; X ray crystallographic values are in bold. b WBI values are in italics
This crystallographic value is questionable. See discussion in the text.
c
Figure 5. NLMOs for the bonding interactions between the CF2 ligand
and the (PNP)Rh fragment. For clarity the PiPr2 groups have been
replaced by PMe2 groups and the aryl part of the pincer truncated to Pꢀ
CH=CHꢀN linkers.
5
the hydrocarbon and fluorocarbon alkenes are necessarily bound
to different fragments in this molecule. Figure 4 illustrates the
key Natural Localized Molecular Orbitals (NLMOs)42,45,46 arising
from NBO calculations of interactions between C2H4 and C2F4
and the truncated65 (PNP)Rh fragment. The bonding orbitals (σ
Figure 4. NLMOs for the bonding interactions between C2H4 (column 1)
and C2F4 (column 2) and the (PNP)Rh fragment. For clarity the PiPr2
groups have been replaced by PMe2 groups and the aryl part of the pincer
truncated to PꢀCH=CHꢀN linkers.
10 and π) are essentially localized on Rh and the alkene ligand,
while the corresponding antibonding NLMOs show significant
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“tailing” involving the σ and π orbitals on the transꢀN of the
pincer ligand. This “tailing” is indicative of delocalization of
these N electrons into the corresponding σ* and π* components
corresponds to a 45 degree rotation about the Rhꢀalkene bond
axis, and the second, lying 8.7 kcal/mol above the ground state is
the conformation in which the fluoroalkene lies in the
of the Rhꢀalkene interaction; it is significantly greater for the σ* 30 coordination plane. These barriers are low enough in energy that
5
component and corresponds to the 3ꢀcenter/4ꢀelectron bonding
expected between transꢀligands in a d8 Rh(I) complex. The WBI
values indicate significantly greater reduction in CꢀC bonding and
increase in RhꢀC bonding in coordinated C2F4 than in C2H4. This
rotation should be fast on the NMR timescale, consistent with the
observed NMR data. The barriers contrast with those for the
corresponding C2H4 analogue, for which the inꢀplane
conformation is a minimum, lying only 0.4 kcal/mol above the
is consistent with the shorter RhꢀC distances and with the idea of 35 ground state, and the 45 degree conformation is a transition state
10 a more metallacyclopropane structure and stronger RhꢀC bonding
for the fluorinated alkene complex. Not surprisingly, stronger
bonding to the fluorinated alkene results in weaker bonding to the
trans ligand, with correspondingly lower WBI values for the Rhꢀ
N bonds (Table 1). In the π*ꢀperp NMLO for the C2F4 there is
15 also evidence for delocalization from Fꢀlone pairs (see figure 4).
There are two Rh→alkene backbonding options involving π*ꢀ
perp or π*ꢀinplane. Clearly the ground state conformation of the
C2F4 complex utilizes the former, but the latter is available for an
inꢀplane C2F4 conformation, leading to a low barrier for C2F4
20 rotation. Similar arguments for facile rotation of C2F4 ligands in
Ru(II) complexes have been put forth elsewhere. 66 The free
energy profile for C2F4 rotation was calculated using a truncated65
version of the PNP ligand (identical to that shown in Figure 4),
and is unusual. Relative to the perpendicular conformation
25 observed in the ground state there two transition states were
located. The first, lying 7.1 kcal/mol above the ground state
lying 2.5 kcal/mol above the ground state.
Comparison of CH2 and CF2 ligands.
Figure 5 presents the corresponding NLMOs for the CF2
complex with a truncated65 PNP ligand. Those for the CH2
40 analogue are very similar, except for the fluorine delocalizations
into the π∗ꢀNLMO, and are not illustrated here but can be found
in the ESI (Figure S17). The σ and π NLMOs are consistent with
a formal double bond between Rh and the CF2 (or CH2) ligands,
with the pꢀorbital on C and dꢀorbital on Rh providing the πꢀ
45 component. These NLMOs look essentially identical to those in
Figure 4, except for a more significant delocalization of the
fluorine lone pairs in the π*ꢀperp NLMO. But now there is
competition between the rhodium dꢀorbital and the fluorine lone
pair pꢀorbitals for πꢀbonding with the carbene carbon, as
50 expected; in (PNP)Rh=CH2 only the metal can provide this πꢀ
bonding. Consequently πꢀbonding with fluorines diminishes πꢀ
bonding with Rh and, relative to the CH2 complex, the RhꢀC WBI
decreases significantly and the RhꢀC distance increases; notably
the CꢀF WBI is greater than in the previously discussed C2F4
55 complex. In contrast to the alkene ligands discussed above there
is overall weaker metalꢀcarbon bonding to CF2 than CH2 and the
corresponding trans RhꢀN WBI value is larger for (PNP)Rh=CF2.
Facile CF2 ligand rotation is expected due to the availability of
the π*ꢀperp and π*ꢀinplane interactions. Calculations on the
60 truncated65 ligand analogue of the CF2 complex reveal the same
trend in conformational energetics observed for the C2F4 complex
(vide supra). The inꢀplane CF2 conformation, with the π*ꢀperp
interaction, is the ground state, with two transition states at 45
degree and 90 degree (perpendicular to the coordination plane)
65 lying 2.0 and 4.8 kcal/mol higher in energy, respectively. The
barriers are consistent with the experimental observation of fast
rotation on the NMR timescale. In contrast, the rotation of the
corresponding CH2 ligand is even more facile, with the
perpendicular, 45 degree and inꢀplane conformations lying at
70 essentially equal energies.
Comparison of CH+, CF+ and CCF3+ ligands.
Figure
6
presents the corresponding NLMOs for
[(PNP)Rh≡CF]+. As with the carbene complexes, those for
[(PNP)Rh≡CH]+ are similar except for enhanced “tailing” in the
75 antibonding NLMOs for the CF+ complex. In contrast to the
alkene and carbene complexes (vide supra) there is now a second
fully engaged πꢀcomponent for the RhꢀC bond involving the inꢀ
plane dꢀorbital and a second pꢀorbital on the CF+ (or CH+) ligand.
In the antibonding NLMOs we see the expected σꢀdonation from
80 the transꢀN in σ*, a small πꢀdonation from transꢀN in π*(perp),
and a small donation from the RhꢀP bonds in π*(inꢀplane). But
once again the largest delocalizations in the π* NLMOs comes
from the F lone pairs, interactions that cannot occur in
Figure 6. NLMOs for the bonding interactions between the CF+ ligand
and the (PNP)Rh fragment. For clarity the PiPr2 groups have been
replaced by PMe2 groups and the aryl part of the pincer truncated to Pꢀ
CH=CHꢀN linkers.
6
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[(PNP)Rh≡CH]+.
Consequently
the
RhꢀC
WBI
for 50 lies perpendicular to the (PNP)Rh plane, in contrast to the CF2
analogue described above, presumably due to steric interactions
between the CF3 and the cis PR2 groups.
[(PNP)Rh≡CH]+ is substantially larger than that for the CF+
analogue, consistent with the shorter RhꢀC distance in the former;
as before, a smaller RhꢀC WBI in [(PNP)Rh≡CF]+ leads to a
larger WBI for the trans RhꢀN bond. In the CCF3 analogue, in
Potential products arising from fluoride abstraction from these
isomeric complexes were also subjected to DFT evaluation.
5
which F lone pair participation with the carbyne carbon is 55 Abstraction of fluoride from (PNP)Rh(CFCF3) could occur from
removed,
correspondingly smaller RhꢀN WBI.
a
larger RhꢀC WBI is calculated, with
a
the αꢀposition to yield a carbyne complex (PNP)Rh(CCF3)+,
analogous to the characterized CF+ complex described above, or
from the βꢀposition to afford the corresponding isomeric η1ꢀ
perfluorovinyl cation (PNP)Rh(CF=CF2)+. These are found to
60 have almost identical free energies, with the carbyne complex
lying only 0.4 kcal/mol higher than its perfluorovinyl isomer. An
η2ꢀperfluorovinyl isomer, the potential initial product of fluoride
abstraction from the (PNP)Rh(C2F4) was found to lie 12.1
kcal/mol above its η1ꢀperfluorovinyl analogue. Structures of all
65 these compounds and their relative energies are provided in
Figure 8.
Degree of RhꢀC multiple bonding.
10
The RhꢀC bonding in these carbene and carbyne complexes is
inextricably linked to interactions with the trans N since there are
necessarily shared bonding components between trans ligands,
with additional complexities introduced by the fluorine
substituents on carbon. So we cannot expect the Rh=CX2
15 interaction to be a true double bond, or that in the cationic
Rh≡CX to be a triple bond, even though we may draw resonance
structures that reflect these prejudices. However, the WBI values
for both Rh=CX2 bonds (CH2 1.250; CF2 1.168) are significantly
larger than unity, though not close to two, while those for the
20 Rh≡CX cations are significantly larger still (CH 1.714; CF 1.587;
CCF3 1.770), though not close to the bond order of three. Clearly
there is significant multiple bonding between Rh and these
unsaturated carbon ligands with higher bond orders to these
ligands being reflected in lower bond orders to the trans N.
25
Rh
X
Y
Rh
X
Y
Rh
X
Y
Rh
N
O
Rh
C
F
Rh
C
O
A
Figure 7. Resonance forms for the πꢀsystem in a linear RhꢀXꢀY ligand
array, with WBI values for the bonds in RhꢀCꢀO, RhꢀNꢀO, and RhꢀCꢀF
complexes. All three complexes are isoelectronic and no formal charges
are shown.
Figure 8. (Top) Calculated structures and relative energies for (PNP)Rh
C2F4 isomers. (Bottom) Calculated structures and relative energies for
proposed structures resulting from fluoride abstraction from
(PNP)Rh=C(F)(CF3).
30
Comparison of CF+, NO+, and CO ligands.
The NLMOs for the CO and NO+ complexes are similar to
those of the CF+ compounds discussed previously and are not
shown here. Considering this series of isoelectronic complexes as
Conclusions
In summary, we have shown that (PNP)Rh perfluorocarbene
complexes can be synthesized by treating a Rh(I) precursor with
70 Rupert’s reagent or a fluoroalkane containing a CꢀH bond. Using
35 involving a linear RhꢀXꢀY array the three resonance forms (A, B,
C) for the contiguous πꢀsystem are shown in Figure 7, along with
the WBI values for the appropriate bonds in the RhꢀCꢀO, RhꢀNꢀO
and RhꢀCꢀF complexes. The WBI values are consistent with
progressively increased contributions of resonance forms C>B>A
40 on changing the ligand from CO to NO+ to CF+, as expected from
their relative πꢀacceptor abilities. Similar conclusions were
reached for the fragments M(CO)2(XY) [M = Cr, Mo, W; XY =
CO, NO+, CF+) in a previous study.10b
silylium reagents,
a
fluoride can be abstracted from
(PNP)Rh=CF2 to form a cationic fluoromethylidyne. Thus the
(PNP)Rh system conveniently allows synthesis and comparison
of perfluoroolefin, perfluorocarbene, and perfluorocarbyne
75 complexes. Using DFT calculations we were able to compare the
Natural Localized Molecular Orbitals of these fluoroorganic
complexes to their hypothetical hydrocarbon analogues. We
established that the fluorine atoms on the carbene and carbyne
ligands participate in π donation to the acceptor orbitals on
80 carbon to compete with back donation from the metal. This
resulted in a longer RhꢀC bond in the fluorinated complexes
compared to their hydrocarbon analogues. However, C2F4 was
calculated to form a shorter RhꢀC bond than the C2H4 complex.
Calculated Wiberg Bond Indices also showed that although the
85 unsaturated fluorocarbon ligands have bond orders greater than
Relative energies of isomeric fluorocarbon ligands.
45
It was of interest to compare the relative energies of the
(PNP)Rh(C2F4) complex with its carbene isomer
(PNP)Rh(CFCF3). At the DFT/M06/LACV3P**++ level the free
energy of the carbene isomer is found to be 2.4 kcal/mol uphill
from its alkene analogue. Interestingly the carbene CFCF3 ligand
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one to rhodium, the nitrogen trans to these ligands interacts with
their antibonding orbital and decreases the bond order to less than
a true double and triple bond.
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We are grateful for the support of this research by the Welch
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║Present address: The Dow Chemical Company, Freeport, Texas 77541,
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which the NLMO Figures were derived and for calculations of
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