Transition-Metal Carbonyl Complexes and Electron-Donating Properties of N-Heterocyclic-Carbene–Phosphinidene Adducts

Article abstract of DOI:10.1002/ejic.201600483

Rhodium(I), tungsten(0), and molybdenum(0) carbonyl complexes of the N-heterocyclic-carbene–phosphinidene adducts IPr·PR [1a, R = H; 1b, R = Ph; 1c, R = Mes; IPr = 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene; Mes = 2,4,6-trimethylphenyl] were prepa

Full text of DOI:10.1002/ejic.201600483

DOI: 10.1002/ejic.201600483
Full Paper
Carbene Phosphinidenes
Transition-Metal Carbonyl Complexes and Electron-Donating
Properties of N-Heterocyclic-Carbene–Phosphinidene Adducts
Dirk Bockfeld,[a] Adinarayana Doddi,[a] Peter G. Jones,[a] and Matthias Tamm*[a]
Abstract: Rhodium(I), tungsten(0), and molybdenum(0) carb- Ph; 4c, R = Mes) were isolated from the reactions of 1a–1c
onyl complexes of the N-heterocyclic-carbene–phosphinidene with [(Me3N)W(CO)5], whereas the reactions of 1a and 1b with
adducts IPr·PR [1a, R = H; 1b, R = Ph; 1c, R = Mes; IPr = 1,3- [(thf)Mo(CO)5] (thf = tetrahydrofuran) gave the corresponding
bis(2,6-diisopropylphenyl)imidazolin-2-ylidene; Mes = 2,4,6-tri- molybdenum complexes [(IPr·PR)Mo(CO)5] (5a, R = H; 5b, R =
methylphenyl] were prepared. The reaction of 1b with [Rh- Ph). The molecular structures of the five carbonyl complexes
(μ-Cl)(CO)2]2 afforded the dicarbonyl rhodium(I) complex cis- were established by X-ray diffraction analyses. The IR spectro-
[(IPr·PPh)RhCl(CO)2] (2) as the major product together with the scopic analysis of the CO stretching frequencies of the carbonyl
tetranuclear complex [{μ-(IPr·PPh)}2Rh4(μ-Cl)4(CO)4] (3). The lat- complexes 2, 4, and 5 revealed the strong electron-donating
ter was characterized by X-ray diffraction analysis. The tungsten abilities of the phosphorus(I) ligands.
pentacarbonyl complexes [(IPr·PR)W(CO)5] (4a, R = H; 4b, R =
Introduction
Structures B and C also indicate the possibility that the carb-
ene–phosphinidenes could act as P-donor ligands through two
lone pairs of electrons, as was demonstrated by the isolation of
the bis(borane) adduct [(IMes·PPh)(BH3)2] [IMes = 1,3-bis(2,4,6-
trimethylphenyl)imidazol-2-ylidene] by Arduengo and Cowley.[6]
In agreement with the results reported for related phosphin-
idene systems,[7] this ability was recently confirmed by Dias[8]
and our group,[9] who independently prepared a series of coin-
age metal complexes of the type [(IMes·PPh)(MX)2] and
[(IPr·PPh)(MX)2] [IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-
ylidene], in which the phosphorus atoms coordinate to two
CuCl, AgCl, AuCl, CuBr, or CuOTf (OTf = triflate) moieties. In the
latter study, it was also shown that cationic digold complexes
such as [(IPr·PPh){Au(tht)}2][SbF6]2 (tht = tetrahydrothiophene)
may serve as robust single-source catalysts in cyclo-
isomerization or carbene-transfer reactions, and these were the
first applications of carbene–phosphinidenes as ancillary li-
gands in homogeneous catalysis. Moreover, the isolation of the
monometallic complexes [(IPr·PPh)MCl] (M = Cu, Ag, Au) indi-
cates that the stepwise complexation of the phosphorus atom
in IPr·PPh is possible.[9] This ability was also demonstrated pre-
viously by Lavoie, who reported the ruthenium(II) benzylidene
complex [{IMes·PPh}RuCl2(CHPh)(PPh3)].[10]
In 1997, Arduengo reported the first N-heterocyclic-carbene–
phosphinidene adducts (NHC·PR), which were prepared directly
through the reactions of free carbenes with cyclic phosphorus(I)
compounds such as (PPh)5.[1] Alternatively, an extensive series
of similar phenylphosphinidene adducts were synthesized by
Bertrand and co-workers through the reactions of stable carb-
enes with PPhCl2 and subsequent reduction of the resulting
phosphorus(III) adducts with 2 equiv. of KC8.[2] These com-
pounds can be represented by structures A–C (Scheme 1),[3]
which reveal their nature as inversely polarized phosphaalk-
enes.[4,5] As a mesomeric shift towards the polarized form B can
be expected to afford a more shielded phosphorus atom, the
31P NMR chemical shifts of these adducts were established as
an indicator of the π-accepting properties of the respective
carbene ligands.[2]
Adducts of the parent phosphinidene, in particular IPr·PH,
have recently attracted much interest, and this species was first
generated by Driess and co-workers through silylene-to-carb-
ene PH transfer.[11] Preparative routes were established by
Grützmacher from the imidazolium salt (IPrH)Cl by use of
Na(OCP) or P7(TMS)3 (TMS = trimethylsilyl) as phosphorus-trans-
fer reagents.[12] Together with Gudat, the same group recently
showed that NHC·PH species can also be accessed directly from
imidazolium salts and P4 or Na3P7.[13] Na(OCP) also served as
the phosphorus source for the synthesis of the more bulky de-
Scheme 1. N-Heterocyclic imidazolin-2-ylidene–phosphinidene adducts;
IMes·PPh (R = Ph, R′ = Mes = 2,4,6-trimethylphenyl), IPr·PR (1a, R = H; 1b,
R = Ph; 1c, R = Mes; R′ = Dipp = 2,6-diisopropylphenyl).
[a] Institut für Anorganische und Analytische Chemie, Technische Universität
Braunschweig,
Hagenring 30, 38106 Braunschweig, Germany
E-mail: m.tamm@tu-bs.de
http://www.tu-braunschweig.de/iaac
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/ejic.201600483.
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rivative IPr*·PH with 2,6-dibenzhydryl-4-methylphenyl substitu- single crystals suitable for X-ray diffraction analysis, which con-
ents, which was employed by Bertrand to isolate the parent firmed the connectivity and formation of the tetrametallic com-
phosphenium species (IPr*·PH2)+ together with the complexes plex 3 (Scheme 2; see Supporting Information, Figure S1 for an
[(IPr*·PH)(BH3)2], [(IPr*·PH)(AuCl)2], and [(IPr*·PH)Fe(CO)4].[14]
As shown by our group, IPr·PH can also be prepared conve-
niently from the 2,2-difluoroimidazoline IPrF2 (PhenoFluorTM) by
reaction with P(SiMe3)3, followed by desilylation. Notably, the
intermediate IPr·PSiMe3 was used as a starting material for the
synthesis of carbene–phosphinidyne complexes of the type
[(IPr·P)MLn] with MLn = [(η6-cymene)RuCl] or [(η5-C5Me5)-
RhCl].[15] Related transition-metal complexes such as [(η5-
C5Me5)Fe(CO)2{PC(NMe2)2}] were reported by Weber,[5,16] and,
more recently, Grützmacher described the synthesis of the mer-
cury(II) complex [Hg(P·IMesH2)2] from the parent phosphin-
idene adduct IMesH2·PH; the latter was prepared from IMesH2
by carbene insertion into the P–H bond of PH3, followed by
dehydrogenation with ortho-quinone.[17]
ORTEP representation). The formation of the minor product 3
shows that 1b and related carbene–phosphinidene species are
able to bridge two metal atoms; this is accompanied by loss of
two CO ligands and dimerization to form [{μ2-(IPr·PPh)}2Rh4-
(μ-Cl)4(CO)4], in which the four rhodium atoms are connected
by two different types of bridging chlorine atoms.
As indicated above, carbene–phosphinidene adducts can be
regarded as promising ancillary phosphorus ligands in homoge-
neous catalysis;[9] therefore, an assessment of their electronic
and steric properties is required. Transition-metal carbonyl com-
plexes can be used to study ligand donor properties through
the monitoring of their CO stretching frequencies by IR spectro-
scopy. Arguably, the largest data set is available for rhodium(I)
dicarbonyl complexes of the type cis-[(L)RhCl(CO)2][18–20] and,
consequently, we describe herein the preparation and charac-
terization of cis-[(IPr·PPh)RhCl(CO)2]. In addition, a series of
tungsten and molybdenum pentacarbonyl complexes of the
type [(IPr·PR)M(CO)5] (R = H, Ph, Mes) are reported, and their
CO stretching frequencies are compared with those of related
phosphine and carbene complexes.[21] It should be noted that
these complexes can be regarded as carbene adducts of elusive
phosphinidene group VI complexes of the type [(RP)M(CO)5].
According to Mathey, these species can be generated thermally
from 7-phosphanorbornadiene complexes, a process that is ac-
companied by benzene formation, and trapped in situ through
(cyclo)addition reactions or the addition of nucleophiles,[22] in-
Scheme 2. Synthesis of the carbene–phosphinidene rhodium(I) complexes 2
and 3.
The presence of two carbonyl groups in 2 is confirmed by
its 13C NMR spectrum, which shows two doublets of doublets
2
at δ = 184.7 (1JRh,C = 72.0, JP, C = 22.5 Hz) and 183.5 ppm (with
1
2
almost identical, unassignable JRh,C and JP, C coupling con-
stants of 65.7 and 60.9 Hz) that can be assigned to the cis- and
trans-CO ligands, respectively (Figure 1).[25] The signal of the
phosphorus-bound carbene carbon atom appears as a doublet
at δ = 169.3 ppm (1JP, C = 98.2 Hz), which is slightly upfield
and associated with a somewhat larger coupling constant in
comparison with the signal of the free carbene–phosphinidene
cluding a carbene of the diaminocyclopropenylidene-type.[23]
A
1
1b (cf. δ = 172.9 ppm, JP, C = 83 Hz; all spectra were measured
in C6D6).[2] Likewise, the 31P NMR signal of 2 is shifted by ca.
10 ppm to higher field and appears as a doublet at δ =
–28.9 ppm (1JRh,P = 48.4 Hz).
related route was reported recently by Streubel et al. for the
preparation of [{IMe·PR}W(CO)5] [R = CH(SiMe3)2, IMe = 1,3,4,5-
tetramethylimidazol-2-ylidene], which proceeds through the
formation of an anionic phosphinidenoid complex [(RPCl)W-
(CO)5]–.[24]
Results and Discussion
Reaction of IPr·PPh with [Rh(μ-Cl)(CO)2]2
The carbene–phosphinidene adduct IPr·PPh (1b), prepared by
the Bertrand route,[2] was treated with 0.5 equiv. of [Rh(μ-Cl)-
(CO)2]2 in toluene at room temperature to afford a dark red
solution and small amounts of an orange precipitate. The solu-
tion was filtered, the solvent was evaporated, and the solid was
Figure 1. Part of the 13C{1H} NMR spectrum of 2 in C6D6 at 30 °C.
As expected, the IR spectrum of 2 shows two strong absorp-
washed with hexane to furnish the desired complex cis- tions at ν = 2047 and 1965 cm–1, which can be assigned to the
˜
[(IPr·PPh)RhCl(CO)2] (2) in analytically pure form in 62 % yield, symmetric and antisymmetric CO stretching vibrations, respec-
as indicated by elemental analysis and NMR spectroscopy (see tively. From the average value ν(CO)av = 2006 cm–1, a Tolman
˜
below). The recrystallization of the remaining orange solid gave electronic parameter (TEP) of 2024 cm–1 can be calculated,
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Figure 2. Rhodium scale showing the average carbonyl stretching [ν(CO)av] frequencies of some selected rhodium complexes of the type cis-[(L)RhCl(CO)2]
˜
(L = carbene, phosphine, etc.).[27,29–37]
which refers to the frequency of the A1 carbonyl mode of com- similar polarization of the exocyclic C–C double bond can be
plexes of the type [(L)Ni(CO)3].[26] In comparison with the values accorded to so-called N-heterocyclic olefins (NHOs), which act
reported for other phosphorus and carbon donor ligands as similarly strong electron donor ligands in comparison with
(Table 1), the low ν(CO) and TEP values of 2 indicate very 1b.[27,32,33]
˜
av
strong Rh–CO π-backbonding and, therefore, a strong electron-
donating ability of the IPr·PPh ligand (1b). In addition, the rela-
tive donor capacities of selected ligands are illustrated in Fig-
Preparation and Characterization of [(IPr·PR)M(CO)5]
ure 2, in which the ν(CO)av values of the respective cis-
˜
[(L)RhCl(CO)2] complexes are ranked (“rhodium scale”).[27]
Group 6 pentacarbonyl complexes have also been used widely
to assess ligand donor properties;[21] therefore, we aimed to
prepare the tungsten and molybdenum complexes
[(IPr·PR)M(CO)5] (R = H, Ph, Mes; M = Mo, W) by employing the
previously reported carbene–phosphinidene adducts 1a (R =
H)[15] and 1b (R = Ph).[2] In addition, the sterically more encum-
bered derivative 1c was synthesized analogously to 1b through
the potassium graphite reduction of the adduct IPr·PMesCl2 ob-
tained from the carbene IPr and dichloromesitylphosphine
(PMesCl2). However, 1c was isolated in only 8 % yield as a yel-
low solid, whereas Layfield and co-workers recently reported
the synthesis of 1c from [(IPr)Fe{N(SiMe3)2}] and MesPH2 in 57 %
yield.[38] In agreement with the reported spectroscopic data,
the 1H NMR spectrum of 1c in C6D6 shows a septet at δ =
3.28 ppm, and two broad signals at δ = 1.45 and 1.08 ppm for
the isopropyl CH and CH3 hydrogen atoms indicate hindered
rotation around the Ccarbene–P bond. The 31P NMR signal of 1c
in C6D6 is found at δ = –51.3 ppm, which is intermediate be-
tween the chemical shifts for 1a (δ = –133.9 ppm) and 1b (δ =
–18.9 ppm).[2]
Table 1. Average CO stretching frequencies and TEPs of a few selected ligands
in complexes of the type [(L)Rh(CO)2Cl][a] and [(L)Ni(CO)3].
Ligand
M
ν(CO)av [cm–1
]
TEP [cm–1
]
Solvent/method
˜
[28]
PPhCl2
Ni
Ni
Ni
Rh
Ni
Ni
2054
2049
2029
2052
2023
2015
2014
2038
2037
2014
2010
2007
2006
2003
2002
2092
2085
2068
2062
2064
2056
2056
2050
2050
2031
2029
2025
2024
2022
2022
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
Nujol
[28]
P(OPh)3
[28]
PPh3
[29]
PPh3
[28]
PMe3
[28]
PCy3
[28]
PtBu3
Ni
IMes[30]
Rh
Rh
Rh
Rh
Rh
Rh
Rh
Rh
IPr[29]
Carbodicarbene[31]
[32]
IPrCH2
[33][b]
IMe4CH2
IPr·PPh (1b)
ATR[c]
ATR[c]
[27][d]
IMe2CH2
ATR[c]
IMe4CHPh[33][b]
ATR[c]
[a] TEP = 0.8001ν(CO)av + 420.0 cm–1 [20]
[b] IMe4 = 1,3,4,5-tetramethylimid-
.
˜
azol-2-ylidene. [c] Attenuated total reflection. [d] IMe2 = 1,3-dimethylimid-
azol-2-ylidene.
Initial attempts to isolate the tungsten pentacarbonyl com-
Accordingly, phosphines and related phosphorus ligands plex [(IPr·PPh)W(CO)5] (4b) involved the photolytically prepared
such as phospholes[34] can be regarded as significantly weaker tetrahydrofuran (THF) adduct [(thf)W(CO)5]. The addition of 1b
donors than the carbene–phosphinidene 1b.[29,35] N-Hetero- afforded 4b in high purity as a precipitate after several days at
cyclic carbenes usually act as stronger donors than phosphines; room temperature but with low and irreproducible yields (ca.
however, the corresponding ν(CO) values,[29,30,36] including 12–34 %). The monitoring of the reaction by 31P NMR spectro-
˜
av
those of unusual and rather exotic NHCs,[18,37] are still markedly scopy revealed that this reaction proceeded slowly even at ele-
higher than that of 1b (Figure 2). A much lower ν(CO) of vated temperature and with the formation of unidentified side
˜
av
2014 cm–1 was reported for a “carbodicarbene”, which displays products. The trimethylamine complex [(Me3N)W(CO)5] proved
a similar arrangement of electrons to that of 1b with two po- to be a better starting material[39] and afforded 4b as a yellow
tentially available lone pairs of electrons on the donor atom. A crystalline solid in 58 % yield from the reaction with 1b in THF
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at 50 °C. Similarly, 4a was obtained in 57 % yield from 1a, 244 Hz for R = Ph)[42] and increase in the order 4a < 4b < 4c
whereas bulky 1c gave 4c in only 18 % yield. The corresponding (Table 2).
molybdenum complexes were prepared from thermally gener-
ated [(thf)Mo(CO)5],[39,40] which provided 5a and 5b as yellow
and pale green solids in 67 and 60 % yield, respectively
(Scheme 3). Complexes 4a and 5a were isolated as THF solvates,
as was also confirmed by X-ray diffraction analysis (vide infra).
Single crystals of 4a·THF, 4b, 4c, 5a·THF, and 5b suitable for
X-ray diffraction analysis were grown by cooling saturated solu-
tions in THF to –30 °C. Even though the steric demand of the
parent phosphinidene is far lower than that of the phenylphos-
phinidene moiety, complexes 4a·THF, 4b, 5a·THF, and 5b crys-
tallize isotypically in the orthorhombic space group Pbca. In
4a·THF and 5a·THF, the solvent molecules occupy, to a good
approximation, the space that is occupied by the phenyl moie-
ties in 4b and 5b. The molecular structures of the tungsten
complexes 4a and 4b are shown in Figures 3 and 4, whereas
the molybdenum complexes 5a and 5b are presented in the
Supporting Information. Complex 4c crystallizes in the mono-
clinic space group P21/n, and the resulting molecular structure
is shown in Figure 5. The unit-cell parameters and selected
bond lengths and angles are listed in Table 3. In all cases, the
formation of pentacarbonyl complexes is confirmed, and they
display slightly distorted octahedral geometries around the
metal atoms. Similarly to those of other monometallic carbene–
phosphinidene complexes,[9,10,24] the phosphorus atoms reside
in rather acute trigonal-pyramidal environments, as indicated
by angle sums of 329.4, 341.8, and 330.0° in 4b, 4c, and 5b,
respectively. The P–M–C4 angles involving the axial carbonyl
group are close to linearity for the parent phosphinidene com-
plexes 4a and 5a [175.53(17) and 175.63(9)°, respectively],
whereas a stronger deviation is observed for the sterically more
Scheme 3. Synthesis of the carbene–phosphinidene tungsten(0) and molybd-
enum(0) complexes 4 and 5.
The pertinent spectroscopic data of 4 and 5 are summarized
in Table 2. Unfortunately, the low solubility of the complexes
precluded the observation of the 13C NMR signals of the trans-
CO ligands and, in some cases, the Ccarbene atoms. For 4a and
5a, which contain the parent carbene–phosphinidene IPr·PH
(1a), the proton-coupled 31P NMR spectra exhibit sharp dou-
blets at δ = –163.4 (1JP, H = 215 Hz) and –147.1 ppm (1JP, H
=
211 Hz), which are shifted to higher field with an increased P–
H coupling in comparison with the signals for 1a. This resem-
bles the observation reported for the iron tetracarbonyl com-
1
plex [(IPr*·PH)Fe(CO)4] (vide supra, δ = –96.4 ppm, JP, H
=
208 Hz).[14] Similarly, shifts to higher field are also observed
for the 31P NMR resonances of 4b, 4c, and 5b upon the metal
coordination of 1b and 1c, as was also reported for related
W(CO)5 complexes of inversely polarized phosphaalkenes such
as [{tBuP=C(NMe2)2}W(CO)5] (δ = –25.1, 1JP, W = 154 Hz).[41] Strik-
ingly, the opposite trend is usually found for phosphine com-
plexes of the type [(R3P)W(CO)5], which exhibit shifts of ca.
26 ppm to lower field in comparison with the free PR3 li-
gands.[42] These results indicate that metal complexation of the
carbene–phosphinidene species 1 leads to a stronger polariza-
tion of the carbon–phosphorus double bond, as illustrated by
the canonical form B (Scheme 1). In addition, a significant
spreading of electron density over the W(CO)5 moiety can be
proposed on the basis of theoretical calculations.[24] Notably,
Figure 3. ORTEP diagram of 4a in 4a·THF with thermal displacement parame-
ters drawn at the 50 % probability level. Hydrogen atoms are omitted for
clarity (except H1); pertinent structural data can be found in Table 3.
1
the coupling constants J(31P, 183W) of complexes 4 are smaller
than those established for [(R3P)W(CO)5] complexes (e.g., 1JP, W
=
Table 2. NMR spectroscopic data of 4 and 5 (L = carbene–phosphinidene).
Complex
δ [ppm] (J, Hz)
31P
[(L)M(CO)5] (in [D8]THF)
13C of [(L)M(CO)5] (in [D8]THF)
L (in C6D6)
Ccarbene
CCO
4a
4b
4c
5a
5b
–133.9 (1JP, H = 165)
–18.9
–163.4 (1JP, H = 215, 1JP, W = 97)
–57.7 (1JP, W = 120)
–95.1 (1JP, W = 159)
–147.1 (1JP, H = 211)
–40.2
174.4
–
199.8
200.9
201.6
208.7
–
–51.3
–
–133.9 (1JP, H = 165)
–18.9
176.2 (1JC,P = 83)
–
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congested complexes [170.24(10)° in 4b, 163.39(6)° in 4c, and
170.45(4)° in 5b].
Figure 5. ORTEP diagram of 4c with thermal displacement parameters drawn
at the 50 % probability level. Hydrogen atoms are omitted for clarity; perti-
nent structural data can be found in Table 3.
Figure 4. ORTEP diagram of 4b with thermal displacement parameters drawn
at the 50 % probability level. Hydrogen atoms are omitted for clarity; perti-
nent structural data can be found in Table 3.
The Ccarbene–P bond lengths range from 1.805(2) Å (in 4c) to
In all complexes, the oxygen atom (O5) of one carbonyl li-
1.819(3) Å (in 4b) and are significantly longer than those in the gand points intramolecularly towards one of the 2,6-diiso-
free ligands [1.7510(16) Å for R = H,[12] 1.7658(10) Å for R = propylphenyl (Dipp) aryl groups; the resulting distances be-
Ph,[9] 1.766(2) Å for R = Mes],[38] in agreement with the ex- tween O5 and the centroid of the arene ring range from 3.335
pected decrease of the Ccarbene–P bond order upon metal com- (in 4b) to 3.704 Å (4c), which could be cautiously interpreted
plexation.[9] The metal–phosphorus bond lengths in the pairs as weak M–CO(lone pair)···π(arene) interactions.[45] Some short
4a/4b and 5a/5b are almost identical, whereas the steric de- intermolecular contacts of the type C–H···O, which indicate
mand of the mesityl substituent in 4c affords a slightly elon- weak hydrogen bonds, are observed in the isotypic complexes
gated W–P bond (Table 3). In comparison, phosphine–pentacar- 4a, 4b, 5a, and 5b. Short contacts between the hydrogen atoms
bonyl tungsten and molybdenum complexes such as H3 of the NHC backbone and the axial CO oxygen atoms O4
[(Ph3P)W(CO)5]
[2.545(1)
Å][43]
and
[(Ph3P)Mo(CO)5] form chains of molecules parallel to the b axis, which are further
[2.560(1) Å][44] exhibit markedly shorter M–P distances. In all connected by contacts between the methyl hydrogen atoms of
complexes, the metal–carbon bond to the axial trans-CO group the Dipp moiety and equatorial CO groups (C222–H···O6) to
is shorter than those to the equatorial cis-CO groups, as ex- form planes perpendicular to the c axis (see Table 4). The struc-
pected in the presence of a strong σ-donor/weak π-acceptor tures of 4a and 5a feature additional contacts involving the THF
ligand. This is also reflected by shorter axial M–C4 bonds in molecule; however, as the THF molecules are disordered in both
comparison with those in [(Ph3P)W(CO)5] [2.006(5) Å][43] and structures, such contacts should be interpreted with caution. In
[(Ph3P)Mo(CO)5] [1.995(3) Å].[44]
4c, chains parallel to the a axis are formed by contacts between
Table 3. Cell parameters and selected bond lengths of 4–5.
4a·THF
4b
4c
5a·THF
5b
Space group
a [Å]
b [Å]
c [Å]
ꢀ [°]
P–C1 [Å]
P–C31 [Å]
M–P [Å]
M–C4(ax.) [Å]
M–C(eq.)av [Å]
C1–P–C31 [°]
C1–P–M [°]
C31–P–M [°]
M–P–C4 [°]
N1–C1–N2 [°]
N1–C1–P–C31 [°]
N2–C1–P–M [°]
Pbca
Pbca
P21/n
Pbca
Pbca
14.5504(4)
18.9292(4)
26.5711(9)
90
1.818(5)
–
2.6030(13)
1.985(6)
2.049(6)
–
114.73(15)
–
14.9384(6)
18.0388(6)
26.1322(8)
90
10.13568(10)
19.0478(2)
20.1863(2)
99.2915(8)
1.805(2)
1.851(2)
2.6224(5)
1.976(2)
14.5765(4)
18.9211(3)
26.5983(6)
90
1.808(3)
–
2.6249(7)
1.984(3)
2.045(3)
–
115.25(8)
–
14.95011(10)
18.05522(16)
26.1307(2)
90
1.819(3)
1.847(3)
2.6067(7)
1.984(3)
2.051(4)
105.42(14)
115.22(9)
108.75(10)
170.24(10)
104.7(3)
10.7(3)
1.8165(14)
1.8408(14)
2.6158(4)
1.9790(15)
2.0509(16)
105.65(6)
115.47(4)
108.91(5)
170.45(4)
104.92(11)
10.93(15)
76.52(11)
2.049(2)
101.65(9)
133.50(6)
106.69(7)
163.39(6)
104.30(16)
52.36(18)
16.6(2)
175.53(17)
104.8(4)
–
175.63(9)
105.4(2)
–
82.0(4)
76.8(2)
80.7(2)
Eur. J. Inorg. Chem. 2016, 3704–3712
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3708
© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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the NHC backbones and axial CO groups [C3–H3 0.95 Å, H3···O7 4b as well as those of 5a and 5b are very similar; therefore, 1a
2.55 Å, C3···O7 3.380(3) Å, C3–H3···O7 146.0°; the hydrogen at- and 1b have similar donor properties, whereas the higher val-
oms were not refined freely].
ues for 4c indicate a slightly reduced donating ability of the
mesityl derivative 1c. The latter observation could tentatively
be ascribed to a less favorable phosphorus–metal interaction
because of steric overcrowding.
Table 4. Short contacts in 4a, 4b, 5a, and 5b (the hydrogen atoms were not
refined freely).
4a
4b
5a
5b
Table 5. CO stretching frequencies of various [LM(CO)5] complexes.
A1(2) [cm– B1 [cm– E [cm– A1(1) [cm–
C3–H3 [Å]
0.95
2.49
3.323(6)
145.9
0.98
0.95
2.45
3.237(4)
140.5
0.98
2.59
3.526(5)
158.8
0.95
2.50
3.339(3)
146.5
0.98
2.55
3.522(4)
171.7
0.95
2.45
3.2376(18)
140.4
0.98
2.61
3.534(2)
157.6
H3···O4 [Å]
C3···O4 [Å]
O4···H3–C3 [°]
C222–H [Å]
H···O6 [Å]
L
M
Solvent/method
1
1
1
1
]
]
]
]
[47]
P(OPh)3
PPh3
PMe3
PtBu3
IPr[50]
1c
W
W
W
W
W
W
W
W
W
Mo
Mo
Mo
Mo
Mo
2083
2072
2070
2068
2056
2058
2054
2055
2054
2073
2069
2057
2057
2055
–
–
–
1958
1942
1947
1934
1918
1912
1909
1902
1909
–
1965
1939
1937
1929
1880
1869
1868
1861
1860
1946
1936
1882
1866
1864
cyclohexane
cyclohexane
cyclohexane
n-pentane
Nujol
ATR
KBr
ATR
ATR
CH2Cl2
n-pentane
ATR[a]
ATR
ATR
[48]
2.54
3.522(7)
[48]
[49]
C222···O6 [Å]
C222–H···O6 [°] 175.3
1970
1981
1964
–
1961
1963
–
1978
–
1970
1970
[41]
tBuPC(NMe2)2
The IR spectra of 4a–4c, 5a, and 5b exhibit four carbonyl
stretching bands. For [(L)M(CO)5] complexes with idealized C4v
symmetry, three IR-active CO stretching modes are expected:
1a
1b
PPh3
[51]
(1)
[49]
an A1 vibration, which can essentially be assigned to the
PtBu3
1935
1921
1910
1915
IPr
1a
1b
stretching of the axial carbonyl group, and two additional vibra-
tions of A1 and E symmetry, which represent the stretching
(2)
of the equatorial carbonyl groups. If the symmetry of the com-
plexes is lowered by the ligand L, as in the C1-symmetric com-
plexes 4 and 5, a fourth band of weak intensity is usually ob-
served and corresponds to the B1 fundamental.[46] As an exam-
ple, the IR spectrum of the tungsten complex 4b is shown in
Figure 6.
[a] [(IPr)Mo(CO)5] was prepared by the procedure reported for (IPr)W(CO)5.[50]
Conclusions
The IR spectra of the rhodium(I), tungsten(0), and molyb-
denum(0) carbonyl complexes 2, 4, and 5 demonstrate clearly
that the carbene–phosphinidene adducts 1a–1c act as strong
electron-donating phosphorus ligands; substantially lower CO
stretching frequencies are observed in comparison with those
of complexes with other important ancillary ligands such as
phosphines and N-heterocyclic carbenes. In view of the poten-
tial application of carbene–phosphinidene ligands in homoge-
neous catalysis,[9] it can be concluded that these systems are
particularly electron-rich additions to the large family of P-
based ligands, whereas related cationic NHC-based phosphorus
ligands such as phosphenium cations of the type [(NHC)PR2]+
were presented recently as electron-poor and strongly π-ac-
cepting ligands.[52] The NHC skeleton and also the phosphin-
idene moiety in NHC·PR ligands can be varied extensively to
fine-tune the electronic and steric properties to adapt to the
demands of a specific catalytic process.
Figure 6. Excerpt from the infrared spectrum of 4b.
Experimental Section
The IR CO stretching frequencies of 4 and 5 together with
those of representative phosphine, NHC, and phosphaalkene Materials and Instruments: All manipulations were performed un-
der a strictly dry argon atmosphere by using standard Schlenk-line
techniques and dry argon-filled gloveboxes. The solvents were
dried with an MBraun solvent-purification system. The starting ma-
terials, carbene–phosphinidene adducts 1a[15] and 1b,[2] and the
metal precursor [(Me3N)W(CO)5][39] were prepared according to pre-
viously published procedures. Dichloromesitylphosphine was pre-
pared by a modified literature procedure (see Supporting Informa-
tion for details). The 1H, 13C, and 31P NMR spectra were recorded
with Bruker DPX 200 (200 MHz) and Bruker AV 300 (300 MHz) spec-
trometers. The chemical shifts are given in ppm relative to residual
pentacarbonyl tungsten and molybdenum complexes are sum-
marized in Table 5. In agreement with the results obtained for
the rhodium(I) complex 2 (vide supra), the stronger electron-
donating properties of the carbene–phosphinidenes 1a–1c in
comparison with those of phosphine and NHC ligands is con-
firmed by the observation of consistently lower CO stretching
frequencies. Furthermore, these values agree well with those
reported for complexes containing related polarized phos-
phaalkenes, for example, [{tBuP=C(NMe2)2}W(CO)5].[41] Finally, it
should be noted that the CO stretching frequencies of 4a and solvent peaks {δ = 7.16 (C6D6), 3.58 ([D8]THF) ppm}.[53] Coupling
Eur. J. Inorg. Chem. 2016, 3704–3712
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3709 © 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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constants (J) are reported in Hz, and splitting patterns are indicated
as s (singlet), d (doublet), t (triplet), m (multiplet), sept (septet), and
br (broad). All spectra were recorded at room temperature. The IR
spectra were recorded with a Bruker VERTEX 70 FTIR spectrometer
equipped with a Pike Technologies MIRacle attenuated total reflec-
tance (ATR) unit with neat samples unless otherwise noted. Elemen-
tal analyses were performed with a Vario Micro Cube system.
rhodium complex 3. The solvent was removed from the filtrate, and
the remaining solid was washed quickly with cold n-hexane (2 ×
1 mL) and dried under vacuum to afford 2 as an orange solid, yield
0.087 g (62 % based on 1b). 1H NMR (C6D6, 300 MHz): δ = 7.93–
7.87 (m, 2 H, m- or o-CPhH), 7.11–7.06 (m, 2 H, p-CDippH), 6.96–6.95
(m, 4 H, m-CDippH), 6.74–6.62 (m, 3 H, m- or o-CPhH and p-CPhH),
3
6.44 (s, 2 H, NCH), 3.29 (sept, JH,H = 6.9 Hz, 4 H, CHMe2), 1.53 [d,
3JH,H = 6.9 Hz, 12 H, CH(CH3)2], 0.93 [d, 3JH,H = 6.9 Hz, 12 H, CH(CH3)2]
X-ray Structure Determinations: The crystallographic data are
listed in Tables S1 and S2. Suitable single crystals were mounted on
glass fibers in perfluorinated inert oil. The intensity measurements
were performed at 100 K with an Oxford Diffraction Nova A diffrac-
tometer with mirror-focused Cu-Kα radiation. The diffractometer
software CrysAlisPRO was employed.[54] Absorption corrections
were based on multiscans. The structures were refined anisotropi-
cally on F2 by using the SHELXL-97 program.[55] Hydrogen atoms
were included by using a riding model or rigid methyl groups. Ex-
ceptions and special features: The PH hydrogen atoms H1 in 4a and
5a were found and refined freely. In these solvates, the THF mol-
ecules were refined as disordered over two positions with the less-
occupied position isotropic. For 3, residual electron density was in-
terpreted as dichloromethane but was overlaid by other peaks that
could not be interpreted. Therefore, the SQUEEZE program[56] was
used to remove mathematically the effects of the solvent. For the
calculations of the formula masses and associated parameters,
idealized compositions were assumed. Compound 5a·THF showed
thermochromic behavior. At room temperature, the crystalline ma-
terial was green-yellow, and it changed gradually to pink at –173 °C
(see Figure S31 for pictures). The cell determination at room tem-
perature gave approximately the same cell parameters as those de-
termined at low temperature. The color change is reversible several
times without the destruction of the single crystal.
1
2
ppm. 13C NMR (C6D6, 75 MHz): δ = 184.7 (dd, JC,Rh = 72.0, JC,P
=
1
2
22.5 Hz, cis P–Rh–CO), 183.5 (dd, JC,Rh = 65.7, JC,P = 60.9 Hz, trans
1
P–Rh–CO), 169.3 (d, JC,P = 98.2 Hz, CNHCP), 146.4 (CAr), 139.0 (d,
1JC,P = 22.4 Hz, PCPh), 133.5 (CAr), 131.5 (CAr), 127.1 (d, JC,P = 9 Hz,
3
3
5
C=C–N), 125.0 (CAr), 124.5 (d, JP, C = 3.7 Hz, p-CPh), 29.4 (d, JC,P
=
6
3.3 Hz, CHMe2), 26.6 [CH(CH3)2], 23.1 [d, JC,P = 1.8 Hz, CH(CH3)2]
ppm. 31P NMR (C6D6, 121 MHz): δ = –28.7 (d, JP, R h = 48.4 Hz) ppm.
1
C
35H41ClN2O2PRh (691.04): calcd. C 60.83, H 5.98, N 4.05; found C
61.19, H 6.22, N 4.08. IR: ν(CO) = 2047 (s), 1965 (s) cm–1
.
˜
[(IPr·PH)W(CO)5]·THF (4a·THF): Compound 1a (0.048 g,
0.115 mmol) and [(Me3N)W(CO)5] (0.044 g, 0.115 mmol) were dis-
solved in THF (10 mL). The reaction mixture was stirred for 16 h at
50 °C. All volatile components were removed under vacuum, and
the residue was washed with n-hexane (3 mL). The remaining solid
was dissolved in THF, and the solution was filtered through a pad
of aluminum oxide. After the removal of the solvent under vacuum,
4a·THF was isolated as a yellow solid, yield 0.054 g (57 % based on
1a). 1H NMR ([D8]THF, 200 MHz): δ = 7.60–7.34 (m, 8 H, NCH, p-
3
CDippH, m-CDippH), 2.79 (sept, JH,H = 6.7 Hz, 4 H, CHMe2), 2.59 (d,
3
1JH,P = 215.2 Hz, 1 H, PH), 1.44 [d, JH,H = 6.7 Hz, 12 H, CH(CH3)2],
3
1.16 [d, JH,H = 6.8 Hz, 12 H, CH(CH3)2] ppm. 13C{1H} NMR ([D8]THF,
50 MHz): δ = 199.8 (CO), 174.4 (CNHCP), 146.3 (o-CDipp), 133.7
3
(NCDipp), 131.5 (p-CDipp), 125.5 (m-CDipp), 125.3 (d, JC,P = 3.2 Hz,
C=C-N), 67.8 (THF), 29.3 (CHMe2), 26.0 (THF), 25.3 [CH(CH3)2], 22.7
CCDC 1440396 (for 1c), 1440397 (for 4a), 1440398 (for 4b), 1440399
(for 4c), 1440400 (for 5a), 1440401 (for 5b), and 1440402 (for 3)
contain the supplementary crystallographic data for this paper.
These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre.
1
[CH(CH3)2] ppm. 31P NMR ([D8]THF, 81 MHz): δ = –163.4 (d, JP, H
=
1
215.2, JP, W = 97.2 Hz) ppm. C36H45N2O6PW (816.58): calcd. C 52.95,
H 5.55, N 3.43; found C 53.52, H 5.64, N 3.63. IR: ν(CO) = 2055 (m),
˜
1961 (w), 1902 (s), 1861 (s) cm–1
.
[(IPr·PPh)W(CO)5] (4b). Procedure A: Carbene–phosphinidene ad-
duct 1b (0.100 g, 0.201 mmol) and [(Me3N)W(CO)5] (0.077 g,
0.201 mmol) were dissolved in THF (20 mL) at room temperature.
The reaction mixture was stirred for 16 h at 50 °C. All volatile com-
ponents were removed under vacuum, and unreacted [(Me3N)W-
(CO)5] was extracted with n-hexane (5 mL). The remaining solid was
then dissolved in THF, and the solution was filtered through a pad
of aluminum oxide. The solvent was removed under vacuum to give
the air-stable complex 4b as a yellow solid, yield 0.095 g (58 %
based on 1b).
IPr·PMes (1c): MesPCl2 (0.61 g, 2.74 mmol) was added slowly to a
stirred suspension of IPr (1.066 g, 2.74 mmol) in n-hexane (150 mL).
Immediately, a white precipitate formed. The mixture was then
stirred at room temperature overnight. The white solid was col-
lected by filtration, washed with n-hexane (3 × 20 mL), and dried
under vacuum. Potassium graphite (0.742 g, 5.48 mmol) was added
to the solid followed by THF (100 mL). The mixture was then stirred
at room temperature overnight and filtered through a pad of Celite;
all volatiles were removed under vacuum. The residue was washed
with n-pentane and recrystallized from toluene at –78 °C to afford
1c as a yellow solid, yield 0.118 g (8 % based on IPr). 1H NMR
([D8]THF, 300 MHz): δ = 7.46–6.72 (br m, 6 H, m-, p-CDippH), 6.54 (s,
Procedure B: W(CO)6 (0.352 g, 1.000 mmol) was suspended in THF
(60 mL), and the suspension was degassed with three freeze–
pump–thaw cycles. The suspension was irradiated with UV light for
4 h. To this solution, 1b (0.497 g, 1.000 mmol) dissolved in THF
(10 mL) was added. The reaction mixture was stirred at room tem-
perature overnight. The solution was concentrated to 10 mL and
stored at –35 °C for 5 d to afford 4b as a crystalline solid. Further
concentration and cooling of the mother liquor yielded small
amounts of 4b, yield 0.279 g (34 % based on 1b). 1H NMR ([D8]THF,
3
2 H, m-CMesH), 6.23 (s, 2 H, NCH), 3.28 (sept, JH,H = 6.8 Hz, 4 H,
CHMe2), 2.56 [s, 6 H, o-CMes(CH3)], 2.06 [s, 3 H, p-CMes(CH3)], 1.46 [br,
12 H, CH(CH3)2], 1.08 [br, 12 H, CH(CH3)2] ppm. 31P{1H} NMR ([D8]THF,
121 MHz): δ = –51.3 (s) ppm. C36H47N2P (538.76): calcd. C 80.26, H
8.79, N 5.20; found C 80.50, H 8.79, N 5.16.
[(IPr·PPh){RhCl(CO)2}] (2): [Rh(μ-Cl)(CO)2]2 (0.040 g, 0.102 mmol) in
toluene (2 mL) was added slowly over 2 min to a stirred solution of
1b (0.100 g, 0.201 mmol) in toluene (3 mL). After the addition of a
few drops, the solution immediately became dark red, and a small
amount of an orange precipitate formed. The mixture was stirred
for 24 h and filtered. The solid was washed with n-hexane and
dried under vacuum. The NMR spectroscopic data of this precipitate
could not be recorded, but crystals were grown by slow evaporation
of a CH2Cl2 solution at room temperature and shown to be the
3
300 MHz): δ = 7.50 (s, 2 H, NCH), 7.44 (t, JH,H = 7.7 Hz, 2 H, p-
3
CDippH), 7.27 (d, JH,H = 7.7 Hz, 4 H, m-CDippH), 7.13–7.06 (m, 2 H,
m-CPhH), 6.88–6.81 (m, 1 H, p-CPhH), 6.75–6.68 (m, 2 H, o-CPhH), 2.95
3
3
(sept, JH,H = 6.8 Hz, 4 H, CHMe2), 1.35 [d, JH,H = 6.8 Hz, 12 H,
CH(CH3)2], 1.12 [d, 3JH,H = 6.8 Hz, 12 H, CH(CH3)2] ppm. 13C{1H} NMR
([D8]THF, 75 MHz): δ = 200.9 (CO), 146.3 (o-CDipp), 138.9 (d, JP, C
1
=
22.5 Hz, PCPh), 135.2 (NCDipp), 131.6 (p-CDipp), 127.7 (p-CPh), 127.2 (d,
Eur. J. Inorg. Chem. 2016, 3704–3712
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3710 © 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
2JP, C = 8.7 Hz, o-CPh), 126.7 (d, 2JC,P = 3.0 Hz, m-CPh), 125.5 (m-CDipp), CH(CH3)2] ppm. A 13C NMR spectrum could not be recorded be-
3
125.4 (d, JC,P = 5.0 Hz, C=C-N), 29.9 (CHMe2), 26.0 [CH(CH3)2], 22.7 cause of the low solubility of 5b. 31P{1H} NMR ([D8]THF, 81 MHz):
[CH(CH3)2] ppm. 31P{1H} NMR ([D8]THF, 121 MHz): δ = 57.7 (s, 1JP, W
=
δ = –40.2 (s) ppm. C38H41N2O5PMo (732.69): calcd. C 62.29, H 5.64,
120.5 Hz) ppm. C38H41N2O5PW (820.57): calcd. C 55.62, H 5.04, N
N 3.82; found C 62.22, H 5.69, N 3.77. IR: ν(CO) = 2055 (m), 1970 (w),
˜
3.41; found C 55.89, H 4.83, N 2.97. IR: ν(CO) = 2054 (m), 1963 (w),
1915 (s), 1864 (s) cm–1
.
˜
1909 (s), 1860 (s) cm–1
.
Acknowledgments
This work was supported by the Deutsche Forschungsgemein-
schaft (DFG) through grant TA 189/16-1.
[(IPr·PMes)W(CO)5] (4c): Carbene–phosphinidene adduct 1c
(0.054 g, 0.100 mmol) and [(Me3N)W(CO)5] (0.038 g, 0.100 mmol)
were dissolved in THF (10 mL). The reaction mixture was stirred for
16 h at 50 °C. All volatile components were removed under vacuum,
and the residue was washed with n-hexane (3 mL). The remaining
solid was dissolved in THF, and the solution was filtered through a
pad of aluminum oxide. The removal of the solvent gave 4c as a
yellow solid, yield 0.016 g (18 % based on 1c). 1H NMR ([D8]THF,
200 MHz): δ = 7.54–7.22 (br m, 8 H, NCH, m, p-CDippH), 6.59 (s, 2 H,
Keywords: P ligands · Carbonyl ligands · Rhodium ·
Tungsten · IR spectroscopy
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m-CMesH), 2.76 (sept, JH,H = 6.8 Hz, 4 H, CHMe2), 2.25 [s, 6 H, o-
3
C
Mes(CH3)], 2.10 [s, 3 H, p-CMes(CH3)], 1.34 [d, JH,H = 6.8 Hz, 12 H,
CH(CH3)2], 1.09 [d, 3JH,H = 6.8 Hz, 12 H, CH(CH3)2] ppm. 13C{1H} NMR
([D8]THF, 75 MHz): δ = 201.6 (CO), 149.5 (o-CDipp), 145.5 (d, JC,P
1
=
12.8 Hz, PCMes), 142.9 (NCDipp), 139.5 (p-CMes), 138.2 (o-CMes), 134.0
3
(m-CMes), 132.1 (d, JC,P = 5.3 Hz, C=C–N), 128.8 (p-CDipp), 128.5 (m-
C
Dipp), 127.9 (p-CDipp), 32.5 (o-CH3,Mes), 29.1 (CHMe2), 28.6
[CH(CH3)2], 25.4 [CH(CH3)2], 23.6 (p-CH3,Mes) ppm. 31P{1H} NMR
1
([D8]THF, 81 MHz):
δ
=
–95.1 (s, JP, W
=
159.5 Hz) ppm.
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55.74, H 5.75, N 3.65. IR: ν(CO) = 2058 (m), 1963 (w), 1911 (s), 1870
˜
(s) cm–1
.
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[(IPr·PH)Mo(CO)5]·THF (5a·THF): Mo(CO)6 (0.055 g, 0.208 mmol)
was suspended in THF (20 mL), and the suspension was degassed
with three freeze–pump–thaw cycles. The suspension was heated
under reflux for 6 h. After the addition of 1a (0.087 g, 0.208 mmol)
in THF (10 mL) to the hot solution, the mixture was stirred at room
temperature overnight. All volatiles were then removed under vac-
uum. The remaining solid was dissolved in THF, and the solution
was filtered through a pad of aluminum oxide. The solvent was
removed to afford 5a·THF as a pale green solid, yield 0.102 g (67 %
based on 1a). 1H NMR ([D8]THF, 200 MHz): δ = 7.59–7.35 (m, 8 H,
3
NCH, p-CDippH, m-CDippH), 2.80 (sept, JH,H = 6.8 Hz, 4 H, CHMe2),
1
3
2.18 (d, JH,P = 211.6 Hz, 1 H, PH), 1.44 [d, JH,H = 6.8 Hz, 12 H,
CH(CH3)2], 1.16 [d, 3JH,H = 6.8 Hz, 12 H, CH(CH3)2] ppm. 13C{1H} NMR
([D8]THF, 75 MHz): δ = 208.7 (CO), 176.2 (d, JC,P = 83.1 Hz, CNHCP),
1
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146.7 (o-CDipp), 134.2 (NCDipp), 131.8 (p-CDipp), 125.8 (m-CDipp), 125.4
(d, 3JP, C = 2.9 Hz, C=C–N), 68.2 (coordinated THF), 29.7 (CHMe2), 26.4
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1
([D8]THF, 81 MHz):
δ
=
147.1 (d, JP, H
=
211.6 Hz) ppm.
C36H45N2O6PMo (728.70): calcd. C 59.34, H 6.22, N 3.84; found C
60.37, H 6.35, N 4.03. IR: ν(CO) = 2057 (m), 1970 (w), 1910 (s), 1866
˜
(s) cm–1
.
[(IPr·PPh)Mo(CO)5] (5b): Mo(CO)6 (0.055 g, 0.210 mmol) was sus-
pended in THF (25 mL), and the suspension was degassed with
three freeze–pump–thaw cycles. The suspension was heated under
reflux for 8 h. To this hot solution, 1b (0.104 g, 0.210 mmol) in THF
(10 mL) was added, and the mixture was stirred at room tempera-
ture for 16 h. All volatile components were removed under vacuum.
The remaining solid was dissolved in THF, and the solution was
filtered through a pad of aluminum oxide. The removal of the sol-
vent gave the air-stable complex 5b as a yellow solid, yield 0.092 g
(60 % based on 1b). 1H NMR ([D8]THF, 200 MHz): δ = 7.45 (s, 2 H,
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NCH), 7.43 (t, JH,H = 7.7 Hz, 2 H, p-CDippH), 7.26 (d, JH,H = 7.7 Hz, 4
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Eur. J. Inorg. Chem. 2016, 3704–3712
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Received: April 26, 2016
Published Online: July 4, 2016
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www.eurjic.org
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© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim