Forays into rhodium macrocyclic chemistry stabilized by a P2N2donor set. Activation of dihydrogen and benzene

Article abstract of DOI:10.1039/d0dt03688a

The reaction of the dilithium diamido-diphosphine macrocycle, Li2[N(SiMe2CH2P(Ph)CH2SiMe2)2N] (Li2[P2N2]) with [Rh(COD)Cl]2 generates the dirhodium macrocyclic compound, [P2N2][Rh(COD)]2 (where COD = η4-1,5-cyclooctadiene), wherein both rhodium-COD units are syn to each other and have square planar geometries. While this dirhodium derivative does react with H2, no clean products could be isolated. Upon reaction of Li2[P2N2] with [Rh(COE)2Cl]2 (where COE is η2-cyclooctene), the dilithium-dihodium derivative ([Rh(COE)][P2N2]Li)2(dioxane) forms, which was characterized by single-crystal X-ray analysis and NMR spectroscopy. The cyclooctene derivative reacts with dihydrogen in benzene to generate the dilithium-dirhodium-dihydride complex ([Rh(H)2][P2N2]Li)2(dioxane); also formed is the dilithium-dirhodium-phenylhydride complex ([Rh(C6H5)H][P2N2]Li)2(dioxane) via oxidative addition of a C-H bond of the solvent. The phenyl-hydride is eventually converted to the dihydride derivative via further reaction with H2. This process is complicated by adventitious H2O, which leads to the isolation of the amine-dihydride, Rh[P2N2H](H)2; drying of the H2 eliminates this side product. Nevertheless, careful addition of H2O to ([Rh(COE)][P2N2]Li)2(dioxane) results in protonation of one of the amido units and the formation of the rhodium-amine cyclooctene derivative, Rh[P2N2H](COE), which upon reaction with H2 generates the aforementioned amine-dihydride, Rh[P2N2H](H)2. The mechanism by which dihydrogen and C-H bonds of benzene are activated likely involves initial dissociation of cyclooctene from the 18-electron centers in ([Rh(COE)][P2N2]Li)2(dioxane), followed by H-H and C-H bond activation. The ability of one of the amido units of the P2N2 macrocycle to be protonated is a potentially useful proton storage mechanism and is of interest in other bond activation processes.

Full text of DOI:10.1039/d0dt03688a

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PAPER
Forays into rhodium macrocyclic chemistry
stabilized by a P N donor set. Activation of
2
2
Cite this: DOI: 10.1039/d0dt03688a
dihydrogen and benzene†
Alyssa Yeo,
Corey A. Sanz
and Michael D. Fryzuk
*
The reaction of the dilithium diamido–diphosphine macrocycle, Li
2
[N(SiMe
2
CH
2
P(Ph)CH
2
2 2
SiMe ) N]
(
Li
2
[P
2
N
2
]) with [Rh(COD)Cl]
4
2
generates the dirhodium macrocyclic compound, [P
2
N
2
][Rh(COD)]
2
(where
COD = η -1,5-cyclooctadiene), wherein both rhodium-COD units are syn to each other and have square
2
planar geometries. While this dirhodium derivative does react with H , no clean products could be iso-
2
lated. Upon reaction of Li
dium derivative ([Rh(COE)][P
lysis and NMR spectroscopy. The cyclooctene derivative reacts with dihydrogen in benzene to generate
the dilithium–dirhodium-dihydride complex ([Rh(H) ][P ]Li) (dioxane); also formed is the dilithium–dir-
hodium–phenylhydride complex ([Rh(C )H][P ]Li) (dioxane) via oxidative addition of a C–H bond of
the solvent. The phenyl-hydride is eventually converted to the dihydride derivative via further reaction
with H . This process is complicated by adventitious H O, which leads to the isolation of the amine-dihy-
dride, Rh[P H](H) ; drying of the H eliminates this side product. Nevertheless, careful addition of H
to ([Rh(COE)][P ]Li) (dioxane) results in protonation of one of the amido units and the formation of the
rhodium–amine cyclooctene derivative, Rh[P H](COE), which upon reaction with H generates the
aforementioned amine-dihydride, Rh[P H](H) . The mechanism by which dihydrogen and C–H bonds
of benzene are activated likely involves initial dissociation of cyclooctene from the 18-electron centers in
[Rh(COE)][P ]Li) (dioxane), followed by H–H and C–H bond activation. The ability of one of the amido
units of the P macrocycle to be protonated is a potentially useful proton storage mechanism and is of
interest in other bond activation processes.
2
[P
2
N
2
] with [Rh(COE)
2 2
Cl] (where COE is η -cyclooctene), the dilithium–diho-
2
N
2
]Li) (dioxane) forms, which was characterized by single-crystal X-ray ana-
2
2
2
N
2
2
6
H
5
2
N
2
2
2
2
2
N
2
2
2
2
O
2
N
2
2
2
N
2
2
2
N
2
2
Received 25th October 2020,
Accepted 2nd December 2020
(
2
N
2
2
DOI: 10.1039/d0dt03688a
2 2
N
rsc.li/dalton
The synthesis of a number of rhodium complexes employing
phosphorus- and/or nitrogen-containing ligand arrangements
The organometallic chemistry of rhodium is dominated by the has previously been reported by our group, in particular,
study of Rh complexes that have relevance to catalytic pro- early work focused on amidodiphosphine complexes of
Introduction
7
1
cesses. A large number of pincer complexes of rhodium have rhodium utilizing the [PNP] ligand framework (where [PNP] =
8
been developed, containing a variety of mixed-donor ligand [N(SiMe CH PPh ) ]). Other related PNP donor sets have
2
2
2 2
2
3,5,6,9
arrays. Several rhodium pincer complexes have proven useful been prepared and their rich chemistry explored.
in catalytic transformations, such as dimerization of terminal thesis of the [PNP] rhodium cyclooctene (COE) complex and
alkynes, alkane transfer-dehydrogenation, metal–ligand subsequent reactivity with H2
The syn-
3
4
8c
are shown in Scheme 1.
5
6
cooperativity, and functionalization of C–H bonds.
A recurring characteristic amongst the aforementioned and related macrocyclic [P
rhodium derivatives is the use of P and/or N-containing ligands. synthesis of rhodium complexes of [P N ] was not previously
Although numerous transition metal complexes of our [PNP]
1
0
2 2
N ] ligand sets are known, the
2
2
explored.
Macrocyclic complexes of rhodium are known and typically
involve rigid, planar arrays of N-donors as in porphryins
University of British Columbia, Department of Chemistry, Vancouver, BC, Canada.
E-mail: fryzuk@chem.ubc.ca
1
1
and tetraazaannulene derivatives. For example, Rh(TMP)
where TMP = tetramesitylporphyrin) and Rh(TMTAA) (where
†
Electronic supplementary information (ESI) available: Additional experimental
(
details, copies of NMR spectra for all new complexes, X-ray data collection and
TMTAA = dibenzotetranethylazaannulene) display remarkable
refinement procedures. CCDC 2004254, 2004255, 2005434–2005436 and
behavior that includes C–H activation, Rh–H addition to CO,
2024716. For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/d0dt03688a
2
alkenes, and activation of H O.
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shows the formation of this new rhodium species as [P
Rh(COD)] (1).
2 2
N ]
[
2
Single crystals of 1 were isolated and subjected to X-ray
diffraction studies. The solid-state molecular structure of this
dirhodium species is shown in Fig. 1 with selected bond
lengths and angles. Both rhodium atoms are coordinated on
the same side of the [P
2 2
N ] macrocycle. Each Rh(I) center is co-
ordinated to one amido and one phosphine donor on [P N ],
2
2
as well as one COD molecule, generating a distorted square
planar geometry, thus generating essentially identical environ-
ments around each rhodium. The Rh–P and Rh–N bond dis-
tances (2.3082(10), 2.2834(10) Å and 2.163(3), 2.143(3) Å
Scheme 1 Synthesis of the Rh[PNP](COE) complex and its reaction
with H in CH Cl .
2 2 2
2 2 2
respectively) in [P N ][Rh(COD)] (1) are consistent with those
reported for rhodium COD complexes coordinated to P,N-
13
donor ligands. The Rh–COD bond distances differ with respect
to the CvC moiety coordinating in either a cis or trans position
relative to the phosphine. The Rh–COD(trans) bond distances of
In this report, the synthesis and reactivity of macrocyclic
2 2
[P N ] rhodium complexes are investigated with particular inter-
est in exploring the ability of these systems to undergo oxidative
2.108 and 2.101 Å are longer than the Rh–COD(cis) bond dis-
tances of 2.005 and 1.984 Å. In addition, the C–C bond lengths
of the trans CvC moieties (1.373(6) and 1.361(6) Å) are slightly
shorter than the bond lengths of the cis CvC moieties (1.398(6)
and 1.405(5) Å). This difference in bond distances suggests that
the trans disposed CvC moieties are less activated and less
strongly bound to the rhodium centres, as a result of the trans
addition of H . Such studies could set the stage to utilizing
2
these macrocyclic rhodium derivatives in catalytic processes.
Results and discussion
7d
Synthesis and reactivity of [P N ][Rh(COD)]
influence from the phosphines. One further point is the fact
that the P N framework is quite flexible as evident from the con-
2
2
2
Initial exploration into coordinating [P N ] to rhodium began
2 2
2
2
formation of the 12-membered heteroatom ring that supports
the two Rh(COD) units in local square-planar environments.
by treatment of syn-Li [P N ] with half an equivalent of the
2
2 2
readily available Rh(I) starting material, [Rh(COD)Cl]
pating the formation of square-planar, anionic Rh(I)
species such as Li[P N ]Rh, by release of 1,5-cyclooctadiene.
2
, antici-
In an effort to drive the reaction of syn-Li [P N ] with [Rh
a
2
2 2
(
2
COD)Cl] towards formation of a complex with only one
2
2
3
1
1
rhodium coordinated to [P N ] (potentially Li[P N ]Rh via dis-
Surprisingly, upon examination of the P{ H} NMR spectrum
of the reaction mixture, peaks corresponding to unreacted syn-
Li [P N ] were observed along with a doublet at δ 20.0 indicat-
2
2
2 2
placement of COD), a THF solution of syn-Li [P N ] with 0.5
2
2 2
2
equivalents of [Rh(COD)Cl] was heated at 60 °C for 12 h. In
2
2 2
3
1
1
the P{ H} NMR spectrum of the reaction mixture, three new
doublet resonances at δ 13.2, 22.8 and 32.8 (with Rh–P coup-
ling constants of 202, 127, and 108 Hz, respectively) were
ing a new Rh-containing complex was generated. The Rh–P
3
1
1
coupling (143 Hz) observed in the P{ H} NMR spectrum is
1
2
1
indicative of a Rh(I) species. The H NMR spectrum of the
material isolated from the reaction also revealed resonances
suggestive of two coordinated COD molecules (via integration
relative to the silyl-methyl protons). As a result, it was pro-
posed that the new species contains two rhodium COD units
per [P N ] ligand. Treatment of syn-Li [P N ] with one equi-
2
2
2
2 2
valent of dimer [Rh(COD)Cl]2 afforded yellow crystalline
material, giving rise to the same resonances observed in the pre-
vious NMR spectra, absent of unreacted syn-Li [P N ]. Scheme 2
2
2 2
Fig. 1 ORTEP depiction (ellipsoids at 50% probability) of [P
2 2
N ][Rh
(
COD)] (1). Silyl methyl carbons and all hydrogen atoms are omitted for
2
clarity. Selected bond lengths (Å) and angles (°): Rh1–N1, 2.163(3); Rh1–
P1, 2.3082(10); C29–C30 (trans to P1), 1.373(6); C25–C26 (cis to P1),
1
2
1
.398(6); Rh1–COD(trans to P1)cent
.005; Rh2–N2, 2.143(3); Rh2–P2, 2.2834(10); C33–C34 (trans to P2),
.361(6); C37–C38 (cis to P2), 1.405(5); Rh2–COD(trans to P2)cent, 2.101;
, 2.108; Rh1–COD(cis to P1)cent,
Scheme 2 Reaction of syn-Li
][Rh(COD)] (1).
2
[P
2
N
2
] with [Rh(COD)Cl]
2
to generate
Rh2–COD(cis to P2)cent, 1.984; N1–Rh1–P1, 82.07(9); N2–Rh2–P2,
82.37(9).
[
P
2
N
2
2
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observed, in addition to the doublet at δ 20.0 for the aforemen-
tioned complex [P N ][Rh(COD)] . Attempts to isolate or separ-
2
2
2
ate these species were unsuccessful. Postulating that these new
species could arise from thermal rearrangement of 1, its
thermal stability was also explored. After heating a solution of
3
1
1
{
at 60 °C for 12 h, there was no change observed in the
P
1
1
H} NMR and H NMR spectra, indicating that it is not prone
to thermal rearrangement or decomposition under these con-
ditions. Accordingly, the addition of heat in the treatment of
syn-Li
2 2 2 2
[P N ] with [Rh(COD)Cl] gave rise to new rhodium com-
plexes independently of [P N ][Rh(COD)] .
2
2
2
Reactivity of [P N ][Rh(COD)] with H
2
2
2
2
In the interest of activating H2 and generating a rhodium
hydride species, treatment of the dirhodium-COD derivative
with H
2
was explored. Exposure of a toluene solution of 1 to 4
atm of H resulted in the appearance of new resonances in the
2
3
1
1
P{ H} NMR spectrum of the reaction mixture. However, after
stirring under 4 atm of H at RT for 3 days, the starting dirho-
2
Scheme 3 Synthesis of the dirhodium–dilithium complex 2.
dium complex was still present in a significant amount.
Interestingly, upon heating the reaction mixture at 80 °C, both
in the presence and absence of H
2 2 2
, the remaining [P N ][Rh
(COD)]2 complex was consumed. Unfortunately, the reaction
does not proceed cleanly and a mixture of products is observed
via NMR spectroscopy. The major species present displays a
3
1
1
doublet resonance at δ 33.6 in the P{ H} NMR spectrum,
with a Rh–P coupling of 96 Hz suggestive of a Rh(III) species.
1
In the H NMR spectrum, resonances corresponding to the co-
ordinated COD in 1 have disappeared and new resonances at δ
−14 and −18 have appeared, suggesting the presence of
rhodium hydrides. Attempts to isolate species for further NMR
spectroscopy and X-ray crystallography studies were unsuccess-
ful and thus the nature of the products remains unknown.
Synthesis and reactivity of ([Rh(COE)][P
As outlined above, preliminary oxidative addition experiments
of H with the [P ][Rh(COD)] system proved challenging,
perhaps due to the difficulty in displacing the strongly-co-
ordinated COD unit. In the interest of synthesizing a [P
2 2 2
N ]Li) (dioxane)
2
2
N
2
2
2 2
N ]
rhodium species with a more labile unit than COD, coordi-
nation of [P N ] to rhodium using [Rh(COE) Cl] was investi-
2
2
2
2
gated, the idea being that cyclooctene (COE) would be mono-
dentate as opposed to the bidentate 1,5-cyclooctadiene (COD).
Treatment of syn-Li [P N ] with [Rh(COE) Cl] afforded a yellow
2
2
2
2
2
solid, giving rise to two new doublet of doublets at δ 11.5 and δ
3
1
1
2
3.1 in the P{ H}NMR spectrum. The phosphines are inequi-
valent, coupling to each other (JPP = 384 Hz), and coupling to
rhodium (JRhP = 123 Hz and 114 Hz, suggestive of a Rh(I)
1
31
species). In the H{ P} NMR spectrum, a strong singlet was
observed at δ 3.6, suggestive of the presence of a coordinated
dioxane molecule. X-ray diffraction studies confirmed the struc-
ture of a single complex that matches the NMR spectroscopy
data. Scheme 3 shows the formation of the new dirhodium–
Fig. 2 ORTEP depiction (ellipsoids at 50% probability) of ([Rh
COE)][P ]Li) (dioxane) (2). Silyl methyl carbons and all hydrogen
atoms are omitted for clarity. Selected bond lengths (Å) and angles (°):
(
2
N
2
2
dilithium complex, ([Rh(COE)][P
The solid-state molecular structure of 2 is shown in Fig. 2.
This complex contains two [P N ] rhodium–lithium units N2, 85.94(11); N1–Li1–N2, 116.1(4).
2 2 2
N ]Li) -(dioxane) (2).
Rh1–N1, 2.453(4); Rh1–N2, 2.453(4); Rh1–P1, 2.2996(11); Rh1–P2, 2.344
(8); N1–Li1, 1.951(7); N2–Li1, 1.971(8); P1–Rh1–P2, 166.10(17); N1–Rh1–
2
2
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bridged by a Li-coordinated dioxane molecule. The geometry be in fast exchange to the other amido unit to generate the
1
about each Rh(I) centre is distorted trigonal bipyramidal, with higher symmetry observed by
H
NMR spectroscopy
one COE coordinated to each rhodium; the lithium ion (vide infra). The geometry about the Rh(III) centre is distorted
bridges the two disilylamido units of the macrocycle. The octahedral; the P1–Rh1–P2, N1–Rh1–H1R and N2–Rh2–H2R
2
inequivalent phosphines are generated by the η -COE unit bond angles (172.072(15)°, 177(1)° and 176.1(8)° respectively)
binding mode and lack of rotation on the NMR time scale. demonstrate the distortion from idealized octahedral geometry
Attempts to equilibrate the phosphorus-31 environments by
Reproducibility was initially problematic. While all three
heating in non-aromatic solvents (vide infra) led to some products were evident for these experiments, the relative
broadening but also new products were formed that were not amounts varied considerably. We were able to determine early
pursued.
on that the source of the protonated amido unit in Rh[P
2 2
N H]
(
H)2 (3) resulted from adventitious water likely from the H2
source. Careful drying of the dihydrogen resulted in formation
of only two lithium–rhodium hydride products: the major
product is the dihydride 4 and the minor species formed is the
phenyl-hydride 5 (Scheme 4). Both of these Rh(III) products
were characterized by single crystal X-ray diffraction (Fig. 4 and
Reactivity of ([Rh(COE)][P N ]Li) (dioxane) with H
2
2
2
2
2
Given our earlier success with oxidative addition of H to the
square planar Rh(COE)[PNP] system (see Scheme 1), we exam-
ined the reaction of dihydrogen (1–4 atm) with 2 in benzene.
In initial experiments, after a period of 4–6 h, 2 was consumed
5
) and NMR spectroscopy. Additional experiments showed that
the phenyl-hydride 5 slowly converts to the dihydride 4 under
continued reaction with H . When the reaction is conducted in
C D under H , we still observed the two products in the
3
1
as evidenced by P NMR spectroscopy and varying amounts of
three rhodium–phosphine products were observed; overlap-
ping doublets at δ 32.9 with Rh–P coupling constants of
2
3
1
P
6
6
2
1
1
08 Hz, and a doublet at δ 24.1 with a Rh–P coupling of
10 Hz. Based on the magnitude of the these coupling con-
NMR spectrum, which are the dihydride 4 and the deuterio-
phenyl-deuteride, ([Rh(C )D][P ]Li) (dioxane). In the
6
D
5
2
N
2
2
stants, these are likely Rh(III) species. Upon workup, we were
able to obtain crystals of one of these species; the H NMR
absence of H , dissolution of 2 in benzene and slight heating
produces the phenyl-hydride selectively.
2
1
spectrum of the crystalline material, which corresponds to the
P NMR doublet at δ 32.9, shows a hydride pattern (doublet of
Monitoring the reaction of ([Rh(COE)][P
with dry H2 by H NMR spectroscopy in C D shows free
2
N
2
]Li)
2
(dioxane) (2)
3
1
1
6
6
triplets) at δ −17.5 and a singlet at δ 2.0; these two peaks inte-
grate to 2H and 1H respectively versus the silyl methyl reso-
nances. There is no evidence of coordinated or free cyclooc-
cyclooctene in the early stages, but this is eventually converted
to cyclooctane. This suggests that the dissociation of cyclooc-
tene is the first step in the hydrogenation of each of the
18-electron rhodium centers in 2.
tene and the overall symmetry of this species is C . The
2
v
single-crystal X-ray diffraction study (Fig. 3) revealed its struc-
ture (vide infra) as the Rh(III)–amine dihydride, Rh[P H](H)
3); the bond lengths and angles match other similar Rh
As mentioned above, carbon–hydrogen bond activation of
benzene to generate phenyl-hydride 5 was confirmed by
2
N
2
2
(
systems, but what is evident is that the N–H unit is localized
on one disilylamido unit in the structure, but in solution must
Fig. 3 ORTEP depiction (ellipsoids at 50% probability) of Rh[P
3). Silyl methyl carbons and all hydrogen atoms except for those bound
directly to Rh1 and to N1 are omitted for clarity. Selected bond lengths
Å) and angles (°): Rh1–N1, 2.3305(13); Rh1–N2, 2.2583(13); Rh1–P1,
.2868(4); Rh1–P2, 2.2670(4); Rh1–H1R, 1.57(3); Rh1–H2R, 1.62(2); N1–
2 2 2
N H](H)
(
(
2
H1, 0.74(2); P1–Rh1–P2, 172.072(15); N2–Rh1–H1R, 177(1); N1–Rh1–
H2R, 176.1(8).
2 6 6
Scheme 4 Reaction of 2 with dry H in C H .
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Fig. 5 ORTEP depiction (ellipsoids at 50% probability) of ([Rh(H)
2 2 2
][P N ]
Li) (dioxane) (4). Silyl methyl carbons are omitted for clarity. The hydride
2
atoms could not be located and refined accurately on the density map;
1
however, their presence is confirmed by H NMR spectroscopy. The
compound crystallized with two molecules in the asymmetric unit; only
one molecule is shown but bond metrics for both molecules are listed
below. Selected bond lengths (Å) and angles (°): Rh1–N1, 2.338(5)/2.336
Fig. 4 ORTEP depiction (ellipsoids at 50% probability) of 5. Silyl methyl
carbons and all hydrogen atoms except for those bound directly to Rh
are omitted for clarity. Selected bond lengths (Å) and angles (°): Rh1–N1,
(6); Rh1–N2, 2.300(6)/2.289(5); Rh1–P1, 2.289(2)/2.281(2); Rh1–P2,
2.330(1); Rh1–N2, 2.4144(9); Rh1–P1, 2.2929(4); Rh1–P2, 2.2921(4);
2.275(2)/2.270(2); P1–Rh1–P2, 174.29(6)/−174.99(6); N1–Rh1–N2, 89.7
Rh1–H1, 1.47(2); N1–Li1, 1.997(2); N2–Li1, 2.052(3); P1–Rh1–P2, 173.27
14
(2)/89.0(2).
(
3); N1–Rh1–H1, 89.1(8); N1–Rh1–N2, 88.86(3).
heating 2 in neat C
6
H
6
in the absence of H
2
, which generated bone generate two sets of multiplets. The two lithium–rhodium
good yields of the lithium–rhodium phenyl-hydride species. P N units are equivalent as are the two Rh–H moieties. The
2
2
Other aromatic solvents such as toluene and 1,2-difluoroben- Rh–Ph unit displays slightly upfield multiplets for the aromatic
zene also underwent C–H activation with mild heating; this protons as compared to the P-phenyl protons of the P ligand.
The observation of the mononuclear amine-dihydride 3 in
2 2
N
will be reported separately.
NMR spectroscopy was crucial in characterizing both the the initial reactions with H
2
was investigated further. Addition
O to ([Rh(COE)][P ]Li) (dioxane) (2)
phenyl-hydriide 6. For 5, a doublet at δ 32.9 in the P{ H} in THF led to reasonable yields of the rhodium(I) amine-
NMR spectrum with a Rh–P coupling of 107 Hz is observed. In cyclooctene complex Rh[P H](COE) (6) as shown in
the H NMR spectrum, an upfield doublet of triplets resonance Scheme 5. Unlike the lithium–rhodium-cyclooctene complex 2,
lithium–rhodium-dihydride
5
and the lithium–rhodium– of a small amount of H
2
2
N
2
2
3
1
1
2 2
N
1
3
1
1
at δ −16.8, that integrates to 4H (relative to the silyl-methyl the P NMR spectrum displays a broad doublet ( J
=
Rh–P
signals), is consistent with the presence of two equivalent 134 Hz), which indicates free rotation of the COE unit, and
rhodium hydrides. There are two silyl-methyl singlets at δ 0.20 equivalent phosphorus nuclei on the NMR time scale. Cooling
and 0.29, each of which integrates to 24H for the two LiRh a solution of the rhodium(I) amine-cyclooctene complex 3 in
[P
2
N
2
] fragments; there is also a sharp singlet at δ 3.5 that inte-
7 8
C D shows broadening and eventual formation of a similar 8
grates to 8H for the bridging 1,4-dioxane molecule. Overall, line pattern (AMX) to 2 indicative of slow rotation of the COE
the lithium–rhodium-dihydride displays C2v symmetry. The unit, which generates inequivalent phosphines. We attribute
lithium–rhodium–phenyl hydride 6 is less symmetric as there this difference in the COE rotational barriers to the subtle
1
are four silyl-methyl singlets evident in the H NMR spectrum, steric demands of the splayed P-phenyl groups in solution for
and the methylene (PCH Si) protons in the macrocyclic back- these complexes.
2
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These geometries are different than the lithium congeners, 2
and 4, which are trigonal bipyramidal and octahedral, respect-
ively, in the solid state. The rhodium–amido bond lengths are
7
–9
comparable to others reported here and elsewhere. The Rh–
1
4
H bond distances are similar to others reported.
We briefly investigated the ability of the protonated Rh(I)
system, Rh[P H](COE) to activate aromatic C–H bonds akin
to the lithium–rhodium cyclooctene analog 2. Dissolution of
Rh[P H](COE) in C and heating overnight resulted in
2 2
N
2
N
2
6 6
D
loss of the signals due to the starting material and formation
of a number of new products, none of which we could identify
conclusively. It may well be that the putative Rh(III) amine–
phenyl-hydride, Rh[P N H](Ph)H, is inaccessible via this route
2 2
or unstable once formed.
Scheme 5 Protonation of the dilithium–dirhodium-cyclooctene
Conclusions
complex 2 to generate the amine–rhodium species, Rh[P
followed by addition of H to produce the amine dihydride Rh[P
H) 3.
2
N
2
H](COE), 5
2
2 2
N
H]
We report the preparation, structure and reactivity of the
(
2
macrocyclic [P N ] rhodium complexes, [P N ][Rh(COD)] (1)
2
2
2
2
2
and ([Rh(COE)][P
. Rhodium hydride species were observed upon exposure of
both Rh(I) complexes to H2 gas. In the case of [P N ][Rh
2 2 2
N ]Li) (dioxane) (2) and their reactivity with
H
2
As mentioned above, we first observed protonation of the
macrocyclic P rhodium–lithium species in the reaction
with H , presumably due to adventitious H O. Reaction of Rh
2
N
2
2 2
1
(
2
COD)] , hydride signals were detected in the H NMR spec-
2
2
trum along with consumption of the starting dirhodium start-
ing material; however, no products could be isolated or charac-
terized. In contrast, the more labile cyclooctene precursor, 2
[
P N H](COE) with H leads to the identical rhodium–dihy-
2 2 2
dride-amine 3 observed as a minor side product in the reaction
of 2 with H2 due to the presence of H O. The apparent C
symmetry in solution suggests that the N–H unit is shared or
rapidly moves between the two disilyl units as shown in
Scheme 5.
2
2v
undergoes oxidative addition of H
when exposed to dihydrogen in aromatic solvents. The reaction
with H was initially complicated by the presence of H O,
which resulted in the isolation of the dihydride-amine, Rh
P N H](H) (3), which could be prepared separately by proto-
2
and aromatic C–H bonds
2
2
The solid-state single crystal X-ray structures of both 3 and
(Fig. 3 and 6, respectively) show localized N–H units and in
[
6
2
2
2
nation of 2 followed by addition of dihydrogen. In all of the
reactions with dihydrogen, coordinated cyclooctene is hydro-
genated to cyclooctane, which bodes well for the use of these
complexes in catalytic processes.
one case, the H–NSi2 moiety is not coordinated to the
rhodium center of 6; in other words, the former Rh(III) is octa-
hedral and the latter Rh(I) is square planar, in the solid state.
Experimental
General procedures
All experiments used oven- or flame-dried glassware cooled
under vacuum and were performed under an atmosphere of
dry, oxygen-free dinitrogen, using standard Schlenk tech-
niques or glovebox (equipped with
a −40 °C freezer).
Anhydrous hexanes, toluene and THF were purchased from
Aldrich, sparged with dinitrogen, passed through towers con-
taining activated alumina and collected over activated mole-
cular sieves. Pentane was distilled from sodium benzophenone
ketyl and collected over activated molecular sieves.
Hexamethyldisiloxane (HMDSO) was dried over activated mole-
cular sieves and degassed using three freeze–pump–thaw
cycles. Benzene-d and toluene-d were dried with sodium ben-
Fig. 6 ORTEP depiction (ellipsoids at 50% probability) of Rh[P
2 2
N H]
(
COE) (6). Silyl methyl carbons and all hydrogen atoms except the NH
6
8
are omitted for clarity. The H atom attached to N2 was located on the
difference map and refined isotropically. Selected bond lengths (Å) and
angles (°): Rh1–N1, 2.117(2); Rh1–P1, 2.3358(7); Rh1–P2, 2.3095(7); P1–
Rh1–P2, 157.73(3); N1–Rh1–P1, 87.54(6); N1–Rh1–P2, 84.89(6).
zophenone ketyl, trap-to-trap distilled, and freeze–pump–thaw
1
31 29
7
degassed. H, P, Si, Li and 2D NMR spectra were recorded
on either a Bruker Avance 300 or 400 MHz spectrometer at
Dalton Trans.
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room temperature unless otherwise noted. The P{ H} NMR 0.28, 0.36 (s, overlapping, 24 H, SiCH
Paper
3
1
1
3
), 0.65, 0.72 (s, overlap-
spectra were referenced to external 85% H PO at 0.0 ppm. ping, 24 H, SiCH ), 0.9–1.1 (m, overlapping signals, approx.
3
4
3
Elemental analyses (EA) were performed using a Thermo Flash 12H, COE CH
2
, Si–CH
2
–P), 1.2, 1.4 (m, overlapping signals,
, Si–CH –P), 1.6, 1.7 (d, 4H each, Si–
2
000 Elemental Analyzer by Mr Derek Smith or Ms. Jenny Lai, approx. 8H each, COE CH
2
2
at the Department of Chemistry, University of British CH –P) 1.8 (m, 4H COE CH ), 2.0 (br, 4H, COE HCvCH), 3.5
2
2
Columbia. Some microanalysis measurements were performed (s, 8H, C
4
H
8
O
2
), 7.0–7.1 (m, approx. 12H, overlapping with
3
1
in open air due to restrictions caused by the COVID-19 pan- residual C
6
D
6
proton signal, C
demic. The compounds synthesized in this paper are highly { H} NMR (C D , 161.9 MHz): δ 11.5 (dd, J
RhP
6
H
5
–P), 7.5 (m, 8H, C
6
H
5
–P).
P
=
1
= 123 Hz, JPP
6
6
7
6 6
air- and moisture-sensitive and as a result some of the experi- 384 Hz), 23.1 (JRhP = 114 Hz, JPP = 384 Hz). Li NMR (C D ,
mental CHN values differ from the calculated values. X-ray 155.5 MHz): δ 1.31 (s). C{ H} NMR (C
1
3
1
6 6
D , 100.6 MHz): δ
data collection and refinement procedures are described in the 5.7–6.0 (overlapping signals, not well resolved, SiCH ), 7.3
3
ESI.† Unless otherwise noted, reagents were obtained from (SiCH
commercial sources and used as received. H (99.99%), CO (COE HCvCH), 67.4 (C
and Regen gas (10% H , 90% N ) were purchased from Praxair (C H –P). Anal. calcd for C H Li N O P Si Rh : C, 51.24; H,
3
), 26.7, 32.3 (Si–CH
2
–P), 26.4, 29.5, 32.6 (COE CH
2
), 43.0
2
4 8 2
H O ), 127.4, 127.6, 131.2, 131.7
2
2
6
5
68 120
2
4
2
4
8
2
and passed through activated molecular sieves prior to use.
][Rh(COD)] (1). Solid syn-Li [P ](1,4-dioxane)
129 mg, 0.203 mmol) and [Rh(COD)Cl]2 (100 mg, 10 mL of toluene was transferred into a thick-walled glass
.203 mmol) were weighed into the same vial and put in the vessel equipped with a Teflon valve and thoroughly degassed
7.59; N, 3.51. Found: C, 50.56; H, 7.70; N, 3.46.
[P
2
N
2
2
2
2
N
2
Rh[P H](H) (3). A solution of 2 (44 mg, 0.028 mmol) in
2
N
2
2
(
0
glovebox freezer for 30 minutes, along with another vial con- via 3 freeze–pump–thaw cycles. The vessel was cooled under
taining 10 mL of toluene. The cold toluene was added to the liquid N and H gas was added. The vessel was sealed and the
2
2
cold solids and the cloudy orange-yellow mixture was left to frozen solution was allowed to warm to RT. The solution was
stir overnight. The solvent was removed under vacuum and the stirred overnight. The H and solvent was removed under
2
dark yellow residue was extracted into 4 mL of toluene. The vacuum, leaving yellow solid. The solid was extracted into
resulting cloudy solution was filtered over Celite. The clear 2 mL hexane and filtered over a glass filter pad within a
orange-yellow filtrate was condensed to a smaller volume pipette, removing a light solid. The clear yellow filtrate was
(1–2 mL) and the concentrated solution was stored in the evaporated to dryness and the remaining yellow residue was
freezer until crystal growth was observed. The solution was triturated with minimal HMDSO, precipitating a yellow solid.
decanted and the crystals were dried under vacuum. The The solution was decanted and the solid was dried under
yellow crystalline solid was weighed (132 mg, 68% yield) and vacuum (26 mg, 74% yield). Crystals suitable for X-ray diffrac-
1
31
stored in the freezer. H{ P} NMR (C
.7, 1.1 (s, 6H each, SiCH ), 1.4–2.0 (m, overlapping signals, at −40 °C. H NMR (C
approx. 18 H, Si–CH –P, COD CH ), 2.2, 2.3, 2.6, (m, 2 H each, JHP = 14 Hz, JHH = 25 Hz), 0.27, 0.29, (s, overlapping, 24H,
6 6
D , 400 MHz): δ 0.3, 0.5, tion were isolated from a concentrated pentane solution stored
1
0
3
6 6
D , 400 MHz): δ −17.5 (dt, 2H, Rh–H,
2
2
COD CH
H, p-C
2
), 3.0, 3.6, 5.3, 5.4 (m, 2H each, COD HCvCH), 7.0 (t, SiCH
3
), 1.4, 1.6 (dt, 4H each, Si–CH
14 Hz), 2.0 (s, 1H, N–H), 7.1 (m, 2H, C
= 143 Hz). C{ H} 4H, overlapping with C D residual proton, C H –P), 8.1 (m,
2
–P, JHP = 4 Hz, JHH
=
3
1
2
6
H
5
–P), 7.1 (t, 4H, m-C
6
H
5
–P), 7.5 (d, 4H, o-C
6
H
5
–P).
P
6 5
H
–P), 7.2 (m, approx.
1
13
1
{
H} NMR (C D , 161.9 MHz): δ 20.0 (d, J
6
6
RhP
6
6
6 5
3
1
1
NMR (C
6
D
6
, 100.6 MHz): δ 6.4, 9.5, 9.6, 10.1 (SiCH
3
), 18.8, 4H, C
6
H
5
–P). P{ H} NMR (C
6
D
6
, 161.9 MHz): δ 32.9 (d, JRhP
=
1
3
1
2
4.2, 28.3, 34.1 (COD CH
2
), 30.5, 31.2, (Si–CH –P), 68.2, 71.1, 108 Hz). C{ H} NMR (C
2
6
D
6
, 100.6 MHz): δ 3.6, 5.4 (SiCH
3
),
9
6.4, 100.9 (COD HCvCH), 128.2, 128.3, 129.0, 130.7 (C H – 28.2 (Si–CH –P), 128.3, 129.8, 132.8, 140.9 (C H –P). Anal.
6 5 2 6 5
P). Anal. calcd for C40
H
66
N
2
P
2
Si
4
Rh
2
: C, 50.30; H, 6.97; N, 2.93. calcd for C24
C, 43.04; H, 7.12; N, 3.09.
([Rh(H) ][P N ]Li) (dioxane) (4). A solution of 2 (63 mg,
45 2 2 4 1
H N P Si Rh : C, 45.12; H, 7.10; N, 4.39. Found:
Found: C, 49.41; H, 7.12; N, 2.43.
[Rh(COE)][P N ]Li) (dioxane) (2). A solution of syn-Li [P N ]
(
2
2
2
2
2
2
2
2
2
2
(
0
0
1,4-dioxane) (197 mg, 0.310 mmol) in 5 mL of toluene with 0.040 mmol) in 15 mL of C
.5 mL of dioxane and a solution of [Rh(COE) Cl]
2 2
6
H
6
was prepared in the glovebox in
(111 mg, a thick-walled glass vessel equipped with a Teflon valve. The
.155 mmol) in 5 mL of toluene was prepared and cooled in solution was degassed via one freeze–pump–thaw cycle and
Cl] solution was then the clear yellow solution was allowed to warm to room
(1,4-dioxane) solution temperature. The vacuum line was filled with 1 atm of H gas
the glovebox freezer for 1 h. The [Rh(COE)
added dropwise to the syn-[P ]Li
2
2
2
N
2
2
2
while stirring. After the addition was complete, the mixture and the Teflon valve was opened, allowing H gas to flow into
2
was immediately put under vacuum to remove the solvent. The the flask. The solution was stirred for 4 days and the color
remaining solids were extracted into 2 mL of toluene and fil- slowly faded to generate a nearly colorless solution. At this
tered through a glass filter pad within a pipette, to remove point a 0.5 mL sample was removed from the reaction and
LiCl. The filtrate was evaporated to dryness and the remaining mixed with 0.5 mL C
brown residue was triturated with 1 mL of cold hexane. The signal existed in the hydride region of the H NMR spectrum
dark brown solution was decanted and the solids were washed and one doublet in the P NMR spectrum, both of which
with another 1 mL of cold hexane. The solution was decanted correspond to the intended dihydride product 4. The H NMR
6 6
D for an in situ NMR analysis. Only one
1
3
1
1
and the yellow-brown solid was dried under vacuum and spectrum shows that the solution consists primarily of a
1
31
weighed (199 mg, 81% yield). H{ P} NMR (C D , 400 MHz): δ mixture of 4 and cyclooctane, confirming that the cyclooctene
6
6
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ligand has been fully hydrogenated to cyclooctane. The solvent through Celite, rinsing with 10 mL more hexanes. The volume
was removed under vacuum and the resulting solid was redis- of the hexanes filtrate was reduced to 4 mL and the solution
solved in minimal hexanes (about 5 ml) and allowed to stand was stored in the freezer overnight. Orange crystals suitable for
in the freezer overnight. The resulting white crystals were iso- X-ray crystallography deposited in the vial. A second crop of
lated by pipetting off the supernatant and were then rinsed crystals was obtained by from the supernatant in a similar
with 3 × 1 mL of cold hexanes and dried under vacuum manner. The orange crystals were combined and dried under
1
(
28 mg, 50% yield). Crystals suitable for X-ray diffraction were vacuum (84 mg, 67% yield). H NMR (C
6
D
6
, 400 MHz, T =
isolated from a concentrated hexanes solution stored in the 298 K): δ 0.47 (s, 12H, Si–CH ), 0.54 (s, 12H, Si–CH ), 0.82–1.04
3
3
1
freezer for several days. H NMR (C
6
D
6
, 400 MHz): δ −16.8 (dt, (m, 4H, COE CH
2
), 1.09–1.24 (m, 8H, COE CH
2
2
, Si–CH –P),
4
0
4
H, Rh–H, JHP = 14 Hz, JHRh = 25 Hz), 0.20 (s, 24H, Si–CH
3
), 1.29–1.38 (m, 2H, COE CH
2
), 1.48 (dt, 4H, Si–CH
2
–P, JHP = 14,
.29 (s, 24H, Si–CH ), 1.49, 1.62 (dt, 8H each, Si–CH –P, J
=
JHH = 5 Hz), 2.09 (broad d, 2H, COE, J = 13 Hz), 2.47 (broad,
3
2
HP
Hz, JHH = 14 Hz), 3.52 (s, 8H, C
4
H
8
O
2
), 7.1–7.2 (m, 12H, over- 2H, COE), 5.60 (broad s, 1H, NH), 6.98–7.03 (m, 2H, C –P),
6 5
H
–P). P{ H}
3
1
1
lapping with C
6
D
6
residual proton, C
6
H
5
–P), 8.2 (m, 8H, C
6
H
5
–
7.07–7.10 (m, 4H, C
6
H
5
–P), 7.43–7.47 (m, 4H, C
6
H
5
3
1
1
P). P{ H} NMR (C D , 161.9 MHz): δ 32.9 (d, J
= 107 Hz). NMR (C D , 161.9 MHz, T = 298 K): δ 14.5 (d, J
= 134 Hz).
6
6
PRh
6
6
PRh
1
3
1
31
1
C{ H} NMR (C
6
D
6
, 100.6 MHz) 4.9 (s, SiCH
–P, J = 3 Hz), 67.2 (s, C
3
), 6.1 (t, SiCH
3
,
P{ H} NMR (toluene-d
8
, 161.9 MHz, T = 188 K): δ 24.4 (dd,
J = 3 Hz), 30.4 (t, Si–CH
2
4
H
8
O
2
), 129.8 (s,
JRhP = 135 Hz, JPP = 399 Hz), 4.2 (dd, JRhP = 138 Hz, JPP = 398
1
3
1
C H –P), 133.0 (t, C H –P, J = 8 Hz), 140.9 (td, C H –P, J = 20 Hz). C{ H} NMR (C D , 100.6 MHz, T = 298 K, ±/x refers to
6
5
6
5
6
5
CP
6 6
Hz, JCRh = 3 Hz). Anal. calcd for C52
H
96
N
4
O
2
P
4
Si
8
Rh
2
Li
2
: C, up/down/missing in DEPT-135 spectrum): δ 4.7 (t, (+), SiCH
3
,
4
5.34; H, 7.02; N, 4.07. Found: C, 41.56; H, 6.91; N, 3.18.
J = 2 Hz), 5.8 (s, (+), SiCH ), 23.0 (s, (−), CH ), 26.5 (s, (−),
3
2
(
[Rh(C H )H][P N ]Li) (dioxane) (4). A solution of 2 (169 mg, CH ), 31.5 (s, (−), CH ), 32.0 (t, (−), CH , J = 4 Hz), 59.8 (broad,
6
5
2
2
2
2
2
2
0
.106 mmol) in 20 mL of C
6
H
6
was prepared in the glovebox in (+), COE HCvCH), 127.9 (t, (+), C
6
H
5
–P, J = 4 Hz), 128.3 (s, (+),
a thick-walled glass vessel equipped with a Teflon valve. After
C
6
H
5
−P), 130.6 (t, (+), C H –P, J = 6 Hz), 141.4 (t, (x), C H –P,
6 5 6 5
stirring for 5 days, NMR analysis revealed that the reaction was J = 12 Hz). Anal. calcd for C H N P RhSi : C, 51.45; H, 7.69;
3
2
57
2
2
4
only 50% complete. The sealed flask was then brought out of N, 3.75. Found: C, 49.19; H, 7.53; N, 3.18.
the glovebox and immersed in an oil bath set at 90 °C for Rh[P H](H) – synthesis from Rh[P
2
N
2
2
2
N
2
H](COE). A solution
2
hours. The flask was then allowed to cool and brought back of Rh[P N H](COE) 6 (10 mg, 0.013 mmol) in 1 mL of C D
2 2 6 6
into the glovebox. The solution was transferred to a vial and was prepared in the glovebox in a thick-walled glass vessel
the solvent volume was reduced to about 1 mL under vacuum. equipped with a Teflon valve. The solution was degassed via
4
mL of hexanes was then added to the flask and it was one freeze–pump–thaw cycle and then the clear yellow solution
allowed to stand in the freezer overnight. The resulting white was allowed to warm to room temperature. The vacuum line
crystals were isolated by pipetting off the supernatant and was filled with 1 atm of H gas and the Teflon valve was
2
were then rinsed with 2 mL of cold hexanes and dried under opened, allowing H gas to flow into the flask. The solution
2
vacuum (113 mg, 70% yield). Crystals suitable for X-ray diffrac- was stirred for 3 days and was then transferred to a J Young
1
31
1
tion were isolated from a concentrated C
6 6
H /hexanes solution NMR tube. Both H and P{ H} NMR spectra confirm that the
1
stored in the freezer for several days. H NMR (C D , solution consists of a mixture of Rh[P N H](H) 3 and cyclooc-
00 MHz): δ −16.98 (dt, 2H, Rh–H, JHP = 12 Hz, JHRh = 30 Hz), tane (singlet at 1.50 ppm in the H NMR spectrum).
.08 (s, 12H, Si–CH ), 0.23 (s, 12H, Si–CH ), 0.39 (s, 12H, Si–
6
6
2
2
2
1
4
0
3
3
CH ), 0.53 (s, 12H, Si–CH ), 1.25–1.35 ppm (m, 8H, Si–CH –P),
3
3
2
1
3
.52, 1.83 (dt, 4H each, Si–CH
4 8 2 6 5
.37 (s, 8H, C H O ), 6.7–6.85 (m, 6H, C H ), 7.0–7.1 (m, 12H,
2
–P, JHP = 5 Hz, JHH = 14 Hz), Conflicts of interest
3
1
1
There are no conflicts to declare.
C H ), 7.5–7.6 (m, 12H, C H ).
1
1
P{ H} NMR (C D ,
6
5
6
5
6
6
1
3
1
61.9 MHz): δ 24.1 (d, JPRh = 110 Hz). C{ H} NMR (C
00.6 MHz): δ 4.7 (s, SiCH ), 5.4 (m, SiCH ), 5.9 (m, SiCH
6
D
6
,
3
3
3
), 5.4
Acknowledgements
(s, SiCH ), 26.4 (m, Si–CH –P), 26.7 (m, Si–CH –P), 67.3 (s,
3
2
2
C
4
H
8
O
2
), 120.7 (s, C
6
H
5
), 126.1 (s, C
6
H
5
6 5
), 129.2 (s, C H ), 132.0
Funding was provided by NSERC of Canada.
(t, C
6 5 6 5
H , J = 7 Hz), 142.9 (m, C H ). Anal. calcd for
C H N O P Si Rh Li : C, 50.25; H, 6.85; N, 3.66. Found: C,
6
4
104
4
2
4
8
2
2
4
1
6.80; H, 6.17; N, 3.11.
Rh[P H](COE). (A solution of 2 (134 mg, 0.0841 mmol) in
5 mL of THF was prepared in the glovebox and then trans-
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