Triaminoborane-bridged diphosphine complexes with Ni and Pd: Coordination chemistry, structures, and ligand-centered reactivity

Article abstract of DOI:10.1039/c7dt02144e

The synthesis, coordination chemistry, and reactivity of two diphosphines containing the cyclic triaminoborane 1,8,10,9-triazaboradecalin (TBD) are described. To evaluate the ligand-centered reactivity of ^PhTBDPhos and ^iPrTBDPhos, th

Full text of DOI:10.1039/c7dt02144e

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Triaminoborane-bridged diphosphine complexes
with Ni and Pd: coordination chemistry, structures,
and ligand-centered reactivity†
Cite this: DOI: 10.1039/c7dt02144e
Kyounghoon Lee, Courtney M. Donahue and Scott R. Daly
*
The synthesis, coordination chemistry, and reactivity of two diphosphines containing the cyclic triamino-
borane 1,8,10,9-triazaboradecalin (TBD) are described. To evaluate the ligand-centered reactivity of
PhTBDPhos and iPrTBDPhos, the complexes (PhTBDPhos)MCl2 and (iPrTBDPhos)MCl2, where M = Ni and
Pd, were prepared and characterized by elemental analysis, multinuclear NMR spectroscopy (1H, 13C, 31P,
and 11B), and single-crystal X-ray diffraction (XRD). Despite very low boron Lewis acidity in the TBD back-
bone, (PhTBDPhos)NiCl2 (1) and (PhTBDPhos)PdCl2 (3) react with H2O, alcohols, and hydrated fluoride
reagents in the presence of NEt3 to yield trans H–O or H–F addition across the bridgehead N–B bond. In
contrast, iPrTBDPhos shows no appreciable reactivity when bound to NiCl2 (2) and PdCl2 (4), which is
attributed to the sterically-bulky isopropyl substituents blocking substrate access to boron in the TBD
backbone. The new complexes {[(PhTBDPhos-H2O)Ni]2(μ-OH)2}Cl2 (5), {[(PhTBDPhos-H2O)Pd]2(μ-OH)2}
Cl2 (6), (PhTBDPhos-MeOH)NiCl2 (7), (PhTBDPhos-MeOH)PdCl2 (8), (PhTBDPhos-C3H5OH)PdCl2 (9), and
{[(PhTBDPhos-HF)Ni]2(μ-OH)2}Cl2 (10) were isolated, and all but 6 were structurally characterized by
single-crystal XRD. Multinuclear NMR studies revealed that isolated, crystallographically-authenticated
samples of 5–9 lose ligand-bound water or alcohol with reappearance of starting materials 1 and 3 when
1
dissolved in NMR solvents. Addition of NEt3 attenuated the water and alcohol loss from 5–9 to allow H,
13C, 31P, and 11B NMR data to be collected for all the compounds, confirming the determined structures.
Additional reactivity experiments with NaOMe and fluoride reagents suggested that participation of the
bridgehead nitrogen in the TBD backbone is important for promoting reactivity at boron when
PhTBDPhos is bound to Ni and Pd. The term “cooperative ligand-centered reactivity” (CLR) is proposed to
define chemical reactions that appear to require participation of more than one atom on the ligand, such
as those reported here.
Received 12th June 2017,
Accepted 30th June 2017
DOI: 10.1039/c7dt02144e
rsc.li/dalton
example, are configured so that the boron can form Z-type
interactions with metals, thereby providing access to small
Introduction
The incorporation of trisubstituted boranes in multidentate molecule transformations via cooperative metal–ligand reac-
ligand platforms has allowed new types of cooperative metal– tions (Chart 1). Other ligand designs rely on locating the
ligand transformations, as well as other forms of ligand-cen- borane in the second coordination sphere to serve as a remote
tered reactivity to be realized.1–5 These reactions often exploit Lewis acid binding site. This latter approach has been used in
the vacant p-orbital on boron, which can accept electron diverse applications, including catalysis and optical sensing of
density from Lewis bases, nucleophiles, and electron-rich fluoride and cyanide.8–12
metals. Borane ligand function and reactivity are typically gov-
erned by Lewis acidity and position of boron in the ligand
scaffold. Boratranes and related ambiphilic ligands,6,7 for
The University of Iowa, Department of Chemistry, E331 Chemistry Building,
Iowa City, IA 52242-1294, USA. E-mail: scott-daly@uiowa.edu
†Electronic supplementary information (ESI) available: Molecular structure of 3
and NMR spectra. Tabulated crystallographic details and data in CIF format.
CCDC 1511356–1511359 and 1550914–1550920. For ESI and crystallographic
Chart 1 Structural comparison of triaminoboranes in [κ4-B(mimR)3]M
data in CIF or other electronic format see DOI: 10.1039/c7dt02144e
and TBDPhos complexes reported here.
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We recently began investigating triaminoborane ligands from interacting with the metal, and (2) diphosphines provide
with the goals of accessing new forms of ligand-centered reac- access to coordination chemistry with a wide range of tran-
tivity and overcoming challenges associated with the high oxo- sition metals. As we will show, despite the remote location and
philicity of more Lewis-acidic organoboranes in cooperative low Lewis acidity, the bridgehead nitrogen in the TBD back-
metal–ligand reactions.13,14 Triaminoboranes are poor Lewis bone helps facilitate new ligand-centered reactions at boron
acids: they have dramatically lower Lewis acidity than trialkyl- when coordinated to Ni and Pd.
and triarylboranes,15,16 especially those commonly encoun-
tered in frustrated Lewis pair chemistry (e.g., B(C6F5)3).17–22
The decreased Lewis acidity stems from donation of the nitro-
Results and discussion
gen lone pairs into the vacant p-orbital on boron, which
imparts significant N–B double-bond character.23 The low
TBD was prepared by refluxing 3,3′-diaminodipropylamine
with NaBH4 and I2 in THF for several days, as described pre-
Lewis acidity of triaminoboranes, however, does not necess-
viously (Scheme 1).41 It was then treated with ClPR2 (R = Ph or
arily imply a lack of reactivity when used in ligands: [κ4-B
iPr) in the presence of base to yield the new diphosphines
PhTBDPhos and iPrTBDPhos, as confirmed by multinuclear
NMR spectroscopy and single-crystal X-ray diffraction (Fig. 1).
The 11B NMR resonance of TBD at δ 22.4 in CDCl3 shifts down-
field slightly to δ 26.2 and 25.8 in PhTBDPhos and iPrTBDPhos,
respectively, and new 31P NMR resonances were observed at
δ 47.7 and 61.3 (Table 1). XRD data confirmed sp2 hybridization
at boron in both diphosphines, as indicated by the 360.0°
sum of the three N–B–N angles (Table 2). The bridgehead N–B
bond distances of 1.415(3) and 1.414(3) Å, respectively, are
0.03–0.04 Å shorter than the N–B distances for N atoms bound
to P. For comparison, the N–B bond distances in B(NMe2)3
range from 1.43–1.44 Å.42
(mimR)3]M and related complexes have been shown to form
dative M→B σ bonds (i.e., Z-type bonds) that, in some
instances, facilitate reactivity across the metal and boron
(Chart 1).24–37 We postulated that similar cooperative reactivity
could be achieved across dative N→B π bonds on the ligand if
the triaminoborane was reconfigured to prevent interactions
with the metal.38
To test our hypothesis, we prepared two diphosphines
derived from a cyclic triaminoborane called 1,8,10,9-triaza-
boradecalin (TBD; Scheme 1).39,40 The flexible, chelating triamino
framework in TBD was selected to (1) prevent ligand decompo-
sition and boron loss via dissociative B–N bond cleavage, and
(2) accommodate the anticipated trigonal planar-to-tetrahedral
distortions at boron during reactions (i.e., sp2 → sp3). The
diphosphine scaffold was selected because (1) it prevents TBD
We next established the coordination chemistry of TBD-
bridged diphosphines with NiCl2 and PdCl2. Treatment of
(DME)NiCl2 (DME = 1,2-dimethoxyethane) with PhTBDPhos or
iPrTBDPhos in CH2Cl2 yielded
(
PhTBDPhos)NiCl2 (1) or
(
iPrTBDPhos)NiCl2 (2), respectively. A similar reaction, starting
from (PhCN)2PdCl2, was used to prepare (RTBDPhos)PdCl2,
i
where R = Ph (3) or Pr (4). All four compounds were crystal-
lized in good yield from Et2O/CH2Cl2 (70–79%). The 11B NMR
resonances are similar to those observed for the free ligands
and range from δ 23.9–24.5, whereas the 31P NMR resonances
shift downfield from
δ
47.7
(
PhTBDPhos) and 61.3
(
iPrTBDPhos) upon metalation to 70.1 (1), 147.0 (2), 68.7 (3),
and 97.4 (4). The 31P NMR resonance for (iPrTBDPhos)NiCl2 is
Scheme 1 Synthesis of TBD, PhTBDPhos, and iPrTBDPhos.
very broad compared to (PhTBDPhos)NiCl2 (FWHM = 1200 Hz
Fig. 1 Molecular structures of PhTBDPhos (left) and iPrTBDPhos (right) with thermal ellipsoids drawn at the 35% probability level. Hydrogen atoms
were omitted from the figure.
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Table 1 11B and 31P NMR resonances for compounds reported herein.
Chemical shifts are reported in δ units relative to BF3·Et2O (11B) and 85%
H3PO4 (
31P)
Compound
11B
31P{1H}
Solvent
TBD
22.4
26.2
25.8
23.9
24.5
24.2
24.5
1.0
1.8
1.9
3.2
2.9
—
CDCl3
CDCl3
CDCl3
CDCl3
CDCl3
CDCl3
CDCl3
CDCl3
CD2Cl2
CDCl3
DMSO-D6
CD2Cl2
CDCl3
PhTBDPhos
47.7
61.3
70.1
147.0
68.7
97.4
70.8
65.3
69.8
64.5
66.1
72.0
iPrTBDPhos
1
2
3
4
5
6
7
8
9
10
Fig. 2 Molecular structure of (PhTBDPhos)NiCl2 (1) with thermal ellip-
soids drawn at the 35% probability level. Hydrogen atoms were omitted
from the figure. The molecular structure of the isomorphous Pd conge-
ner 3 is provided in the ESI (Fig. S1†).
1.6
vs. 130 Hz), suggesting that the Ni adopts a paramagnetic,
high-spin d8 electron configuration in contrast to the low
spin d8 configuration in 1, 3, and 4. The change in electron
configuration and corresponding structural distortion from
square planar (low spin) to tetrahedral (high spin) is common
for Ni(II) phosphine complexes, especially those with bulky
substituents.43
increased from 88.47(2)° in (PhTBDPhos)PdCl2 to 95.32(2)° in
(
from 89.47(2)° to 85.80(2)°. Despite the angular variations, no
significant differences were observed in the Pd–P and Pd–Cl
bond distances. Collectively, the metal–ligand bond distances
in 1–4 are comparable to other NiCl2 and PdCl2 complexes
with phenyl- and isopropyl-substituted diphosphines.46 As
iPrTBDPhos)PdCl2, and the Cl–Pd–Cl angle decreased
Single-crystal XRD studies confirmed the identities of 1–4
(Fig. 2 and 3). As the NMR data suggested, 1, 3, and 4 adopt
low spin, square planar coordination geometries whereas shown in Table 2, only subtle differences were observed in the
(
iPrTBDPhos)NiCl2 (2) is distorted tetrahedral. The chloride
ligands in 2 are rotated 50° out of the plane defined by the Ni
ligand bond distances and angles upon metallation.
Once we established the syntheses of 1–4, we began investi-
and P atoms. As reported for other Ni(II) phosphine complexes, gating their reactivity. We first tested the Lewis acidity of TBD
and PhTBDPhos in C6D6, and 1 in CD2Cl2 using the Gutmann–
we suspected that the tetrahedral distortion was attributed to
the greater steric demands of isopropyl substituents in 2
Beckett method.15,16 No change was observed in the 31P NMR
coupled with the energetically accessible high spin configur- shift of OPEt3 due to B ← OPEt3 interactions, thus confirming
ation of Ni(II).43–45 Evidence of the increased steric profile of
iPrTBDPhos compared to PhTBDPhos is provided by compari-
their very low Lewis acidity.47 We next investigated the hydro-
lysis susceptibility of free and bound PhTBDPhos (Scheme 2). A
son of the Pd bond angles in 3 and 4: the P–Pd–P angle solution of 1 in CDCl3 was layered with excess water in air and
Table 2 Selected bond distances and angles for PhTBDPhos, iPrTBDPhos, 1–5, and 7–10 from single-crystal XRD data (X = Cl or O and Y = O or F)
PhTBDPhos iPrTBDPhos
1
2
3
4
5
7
8
9
10 a
M—P
M–X
P–N
—
—
—
—
—
—
—
—
1.715(1)
1.718(1)
1.422(2)
1.447(2)
1.451(2)
—
2.1463(6) 2.1349(5) 2.2311(5) 2.2421(7) 2.1131(9) 2.1538(8) 2.2227(9) 2.2286(5) 2.1309(6)
2.1556(6) 2.2334(6) 2.2430(6) 2.110(1) 2.146(1) 2.2331(6) 2.2368(4) 2.1357(8)
2.2070(7) 2.2149(5) 2.3550(5) 2.3666(6) 1.882(2) 2.2000(8) 2.367(1) 2.3599(5) 1.892(2)
2.3566(8) 2.3710(6) 1.900(2)
2.2087(6)
1.678(2)
1.672(2)
1.408(3)
1.454(3)
1.454(3)
—
92.91(2)
90.45(2)
87.54(2)
92.91(2)
170.78(2)
177.85(2)
359.9(2)
2.3604(6) 2.3667(7) 1.899(3)
2.207(1)
1.660(3)
1.655(3)
1.650(4)
1.514(3)
1.516(4)
1.450(4)
96.39(3)
89.85(4)
85.88(4)
88.02(3)
1.707(1)
1.687(1)
1.671(1)
1.679(1)
1.415(2)
1.460(2)
1.460(2)
—
88.47(2)
89.47(2)
90.45(2)
1.679(2)
1.681(2)
1.418(3)
1.453(3)
1.458(3)
—
95.32(2)
85.80(2)
89.49(2)
89.44(2)
1.638(3)
1.645(3)
1.645(4)
1.525(5)
1.542(4)
1.427(4)
92.77(3)
78.2(1)
1.649(2)
1.654(2)
1.661(4)
1.523(3)
1.526(4)
1.442(3)
95.35(3)
88.53(3)
86.63(3)
89.71(3)
1.654(1)
1.656(1)
1.658(2)
1.520(2)
1.525(2)
1.452(2)
95.44(2)
88.67(2)
86.89(2)
89.05(2)
1.649(2)
1.650(2)
1.613(3)
1.510(3)
1.513(3)
1.424(3)
89.66(3)
77.06(7)
96.43(5)
96.77(5)
B–N
1.418(3)
1.413(3)
1.459(2)
B–N(P) 1.448(2)
B–Y
—
—
—
—
—
P–M–P
X–M–X
P–M–X
—
—
—
95.94(3)
96.69(3)
95.02(2)
94.17(8)
94.86(8)
143.36(2) 91.07(2)
172.94(2) 174.92(2) 170.85(8) 175.18(4) 174.47(3) 175.29(2) 173.47(6)
175.50(2) 175.00(2) 172.37(8) 175.46(4) 174.50(3) 176.79(2) 173.19(5)
∑NBN 360.0(1)
360.1(1)
360.0(2)
360.0(1)
360.0(1)
322.2(3)
327.5(2)
327.0(2)
328.4(1)
331.5(2)
a Values from one half of the asymmetric dimer. Values for the other half of the dimer are similar.
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Fig. 3 Molecular structures of (iPrTBDPhos)NiCl2 (2; left) and (iPrTBDPhos)PdCl2 (4; right) with thermal ellipsoids drawn at the 35% probability level.
Hydrogen atoms were omitted from the figure.
Ni]2(μ-OH)2}Cl2 (5; Fig. 4). The dimeric structure of 5 contains
two Ni ions bridged by hydroxide ligands, but the most
notable change was observed at the TBD backbone: a water
molecule was added across the bridgehead N–B bond to give
trans B–OH and N–H. In response, the bridgehead N–B bond
elongated from 1.415(3) Å in 1 to 1.645(4) Å in 5, consistent
with transformation of the covalent N–B bond to a dative N→B
bond. For comparison, N→B bond lengths for the archetypal
dative complex Me2NH·BH3 and the more closely related
Me2NH·B(methimazolyl)3 are 1.5965(13) and 1.605(3) Å,
respectively.49,50 The remaining N–B distances in 5 increased
by ca. 0.1 Å, corresponding to the loss of partial N–B double-
bond character and a geometric change from trigonal planar
to tetrahedral boron (i.e., sp2 to sp3). Half the chlorides orig-
inating from 1 remained in the outer coordination sphere of 5,
balancing the unquenched charge on each Ni(II).
Scheme 2 Reactivity studies of (PhTBDPhos)MCl2 (M = Ni or Pd).
monitored by 31P and 11B NMR spectroscopy. Approximately
half of the PhTBDPhos in 1 reacted with H2O over the course of
two days, as indicated by new resonances in the 11B and 31P
NMR spectra at δ 1.0 and 70.8, respectively. The relatively large
upfield shift in the 11B NMR spectrum (22.9 ppm) is known to
be diagnostic for a change from three-coordinate to four-coor-
dinate boron.48 These results are notable when compared to
reaction of H2O with free PhTBDPhos under identical con-
ditions: the ligand decomposes, as is indicated by the dis-
appearance of PhTBDPhos NMR resonances and appearance of
B(OH)3 in the water layer.
We discovered that the reaction of 1 with water could be
expedited by adding excess NEt3, which resulted in immediate
and quantitative conversion to the new species observed pre-
Fig. 4 Molecular structure of {[(PhTBDPhos-H2O)Ni]2(μ-OH)2}Cl2 (5)
with thermal ellipsoids drawn at the 35% probability level. The outer-
viously in the 11B NMR spectrum at δ 1.0. Single-crystal XRD
studies on crystals grown by vapor diffusion of pentane into
sphere chloride and hydrogen atoms attached to carbon were omitted
CHCl3 or CDCl3 revealed the complex to be {[(PhTBDPhos-H2O) from the figure.
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The exact role of the NEt3 in the accelerated production of 5 ible B–O bond formation is known to limit the utility of some
is under investigation, but it is evident that formation of OH− trisubstituted borane ligands in catalytic reactions.14
with elimination of [HNEt3]Cl is important to the overall reac-
As shown in eqn (1), half of the chloride ligands from 1
tion. A proposed balanced reaction for the synthesis of 5 is were removed via production of [HNEt3]Cl in the synthesis of
provided in eqn (1).
5. Hence, we postulated that addition of excess [HNEt3]Cl to
solutions of 5 could be used to quantitatively form 1. Indeed,
31P and 11B resonances assigned to 5 disappeared after
addition of excess [HNEt3]Cl and resonances assigned to 1
reappeared (Fig. 6). The reaction, however, is not quantitative,
as indicated by a small unidentified peak in the 31P NMR spec-
2ðPhTBDPhosÞNiCl2 þ 2NEt3 þ 4H2O
ð1Þ
! f½ðPhTBDPhos‐H2OÞNiꢀ ðμ‐OHÞ gCl2 þ 2½HNEt3ꢀCl
2
2
trum at
δ
27.5 attributed to PhTBDPhos decomposition
[HNEt3]Cl was isolated from the reaction mixtures, confirming
its proposed formation. The arrangement of N–H and B–OH
on opposite faces of the TBD backbone suggests that NEt3 may
play a role in shuttling the bridgehead N–H proton trans to the
B–OH in a stepwise process. Also relevant, the bridgehead
nitrogen in 5 remains protonated despite the use of excess
NEt3, suggesting that the bridgehead nitrogen in PhTBDPhos is
more basic.
(vide infra). The new 11B and 31P NMR resonances of δ 23.7
and 68.4, respectively, are slightly different than those
observed for 1 in CDCl3, but are identical to those obtained
when excess [HNEt3]Cl was added to CDCl3 solutions of 1. The
small differences in the NMR spectra of 1 in the presence and
absence of [HNEt3]Cl may be attributed to changes in TBD pro-
tonation in (PhTBDPhos)NiCl2 in solution. However, despite
repeated attempts, no compounds other than 1 and [HNEt3]Cl
were isolated from these mixtures.
1H NMR data collected on crystalline samples of 5 in CDCl3
revealed three new peaks assigned to N–H, B–OH, and Ni–OH
at δ 8.44, 0.25, and −4.48, respectively. The upfield Ni–OH
assignment is similar to the upfield shift reported for [(dippe)
Ni(μ-OH)]2(PF6)2 at δ −1.59.51 The three propylene 1H reson-
ances that form the TBD backbone in 1 at 1.37, 2.80, and 3.05
split into six diastereotopic resonances in 5, as expected due to
the loss of the BN3 mirror plane. In addition to resonances
assigned to 5, the 1H NMR data revealed smaller resonances in
the baseline that increased in intensity over time. We discov-
ered that addition of NEt3 to NMR samples of 5 in CDCl3 atte-
nuated the in-growth of these smaller resonances and
increased the solubility of 5 so that 13C NMR data could be col-
lected overnight. The 13C NMR spectrum revealed resonances
consistent with 5, but also captured the slow in-growth of res-
onances matching those previously observed for 1 (Fig. 5). The
11B NMR spectrum collected after the overnight 13C NMR data
acquisition showed a broad feature at δ 23.9 that corroborated
the reappearance of 1 (Fig. 5; right inset). The resonance at δ
23.9 was not observed in the 11B NMR spectrum collected
immediately before the 13C NMR data collection. These results
suggested that 5 was losing H2O from the TBD backbone when
Fig. 6 11B (left) and 31P (right) NMR spectra collected at specified inter-
vals over 48 h for the reaction of 5 with excess NEt3·HCl in CDCl3. The
asterisks indicates the 31P NMR resonance assigned to decomposition of
removed from wet solvent, and is notable given that irrevers- PhTBDPhos.
Fig. 5 13C and 11B NMR spectra of {[(PhTBDPhos-H2O)Ni]2(μ-OH)2}Cl2 (5) collected in CDCl3. The 11B NMR spectrum was collected at the end of the
overnight 13C data collection. The * symbol indicates resonances assigned to added NEt3, and the # symbol indicates resonances assigned to 1.
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Next we explored the effect of metal identity on PhTBDPhos
reactivity by treating (PhTBDPhos)PdCl2 (3) with water in the
presence of NEt3, as described previously for the synthesis of
5. The reaction yielded mixtures from which the hydroxide-
bridged Pd congener {[(PhTBDPhos-H2O)Pd]2(μ-OH)2}Cl2 (6)
could be obtained (Scheme 2). As with the Ni congener 5, 31P
and 11B NMR spectra collected on the reaction mixtures
revealed only single resonances assigned to 6 at δ 65.3 and 1.8,
respectively. Crystals grown from the reaction mixtures did not
yield XRD data suitable for publication, but were sufficient to
confirm the dimeric structure and trans water addition across
the backbone. The composition of 6 was further corroborated
by a sharp bridging Pd–OH peak in the IR spectrum at
3607 cm−1 and a broad B–OH feature centered at 3340 cm−1
.
As observed with 5, 1H and 13C NMR data collected on isolated
samples of 6 in CD2Cl2 with added NEt3 yielded resonances
consistent with 6, as well as resonances assigned to 3 that
grew in over time. Several small unassigned resonances were
also observed in the baseline and were most apparent in the
1H NMR data (Fig. S33; ESI†). We have so far been unable to
determine if these are attributed to intermediate species
associated with the reverse transformation from 6 to 3 or
Fig. 7 Space-filling models of (PhTBDPhos)NiCl2 (1), (iPrTBDPhos)NiCl2
(2), (PhTBDPhos)PdCl2 (3), and (iPrTBDPhos)PdCl2 (4) generated using
XRD data and the software program Mercury.52
ligand decomposition. However, their concentrations 5 at 1.427(4) Å. As observed in the structure of 5, the bridge-
remained low in all samples analyzed compared to 6 and 3.
head N–B bond distance elongates to 1.650(4) (7), 1.656(3) (8),
Once the reactivity of 1 and 3 with H2O was established, we and 1.660(1) Å (9), indicative of dative N→B bond formation.
explored how changing phosphorus substituents affected reac-
Remarkably, no immediate decomposition of PhTBDPhos
tivity at the TBD backbone. Unexpectedly, we discovered that was observed despite preparing 7–9 in neat alcohols, but we
switching the phosphorus substituents from phenyl in 1 and 3 found that addition of NEt3 was important for attenuating
to isopropyl in 2 and 4 effectively shut down TBD reactivity ligand decomposition over extended reaction times. Stirring 1
with H2O, as well as MeOH and fluoride substrates (vide infra). in MeOH without NEt3 for several hours, for example, yielded
We suspected that the increased sterics afforded by the bulky mixtures that contained 7, but new resonances were also
isopropyl groups were likely responsible for the attenuated observed at δ 19.0 and 29.9 in the 11B and 31P NMR spectra,
reactivity. Indeed, analysis of space-filling diagrams generated respectively. The 11B resonance at δ 19.0 is consistent with the
from the XRD data revealed that the isopropyl substituents in formation of B(OMe)3,53 which suggests that the unidentified
2 and 4 impede substrate access to boron from both sides of 31P NMR resonance at δ 29.9 can be assigned to the remaining
the complex. By comparison, the planar phenyl substituents in phosphorus-containing decomposition product. Attempts to
1 and 3 can rotate to provide substrate access (Fig. 7).
isolate the latter product were unsuccessful.
We next set out to determine if the reactivity of PhTBDPhos
Numerous attempts to collect NMR data on crystallographi-
in
1 and 3 could be replicated with other substrates. cally-authenticated samples of 7 in anhydrous CDCl3 yielded,
Dissolving 1 and 3 in neat MeOH with excess NEt3 quantitat- unexpectedly, only 31P and 11B NMR resonances for 1 and the
ively yielded single 11B NMR resonances at δ 1.9 and 3.2, aforementioned 31P NMR peak attributed to PhTBDPhos
respectively. Single-crystal XRD studies performed on crystals decomposition (Fig. 10). Similar issues were encountered for 8
grown via vapor diffusion of Et2O into the reaction mixtures and 9. However, as with our NMR studies of 5 and 6, we dis-
revealed the monomeric complexes (PhTBDPhos-MeOH)NiCl2 covered that multinuclear NMR data consistent with the deter-
(7) and (PhTBDPhos-MeOH)PdCl2 (8) (Fig. 8). As in 5 and 6, 7 mined structures could be collected in deuterated solvents by
and 8 reveal trans O–H addition of MeOH across the bridge- adding several equivalents of NEt3. 11B resonances of 7–9 with
head N–B bond. The reactivity could be extended to other alco- cleaved alcohols were shifted downfield several ppm compared
hols; dissolving 3 in allylic alcohol in the presence of base to those with water (5 and 6), whereas the 31P resonances
yielded (PhTBDPhos-C3H5OH)PdCl2 (9), which was isolated and shifted slightly upfield at δ 69.8 (7), 64.5 (8), and 66.1 (9)
structurally characterized (Fig. 9). Unlike the reactions with (Table 1). The 1H NMR spectrum of 7 revealed the expected
water, both chloride ligands from starting materials 1 and 3 splitting of the PhTBDPhos alkyl and aryl resonances due to
remain bound to the metals in 7–9. The new B–OR bond dis- the loss of the BN3 mirror plane. A new sharp singlet associ-
tances in all three structures were similar at 1.450(4), 1.443(3), ated with B-OMe protons was observed at δ 2.62 and a broad
and 1.453(2) Å, for 7–9, respectively, despite differences in multiplet assigned to the NH proton was located at δ 8.49.
metal identity and alcohol substrate. Collectively, the B–OR Similar diagnostic resonances were assigned in the 1H NMR
distances in 7–9 are slightly longer than the B–OH distance in spectra of 8 and 9.
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Fig. 8 Molecular structures of (PhTBDPhos-MeOH)NiCl2 (7; left) and (PhTBDPhos-MeOH)PdCl2 (8; right) with thermal ellipsoids drawn at the 35%
probability level. All hydrogen atoms except those attached to nitrogen and the methoxy groups were omitted from the figure.
TBD backbone. To test our hypothesis, we treated CH2Cl2 solu-
tions of 1 with an excess of the anhydrous methoxide salt
NaOMe. Despite formation of B-OMe in reactions with MeOH,
NaOMe revealed no evidence of methoxide binding to boron
after 2 days.
We next tested the reactivity of 1 with fluoride, an anion with
a high affinity for Lewis acids. Addition of the anhydrous fluor-
ide source tris(dimethylamino)sulfonium difluorotrimethyl-
silicate (TASF) to 1 yielded an equilibrium mixture of products
containing three- and four-coordinate 11B NMR resonances
after 24 hours (Fig. 11). In stark contrast, we found that addition
of hydrated fluoride sources such as [nBu4N]F·(H2O)n or
addition of TASF in air (i.e. in the presence of ambient humid-
ity) yielded rapid formation of {[(PhTBDPhos-HF)Ni]2(μ-OH)2}Cl2
(10), which was confirmed by multinuclear NMR spectroscopy
(
PhTBDPhos-C3H5OH)PdCl2 (9) with
and single-crystal XRD studies (Fig. 12). As shown in eqn (2),
water was cleaved in the reaction to yield the N–H proton on the
TBD backbone and the bridging hydroxide ligands.
Fig. 9 Molecular structure of
thermal ellipsoids drawn at the 35% probability level. All hydrogen atoms
except those attached to nitrogen were omitted from the figure.
2ðPhTBDPhosÞNiCl2 þ 2½nBu4NꢀFꢁðH2OÞ
ð2Þ
! f½ðPhTBDPhos‐HFÞNiꢀ ðμ‐OHÞ gCl2 þ 2½nBu4NꢀCl
2
2
Fig. 10 31P (top) and 11B (bottom) NMR spectra of crystalline
(
PhTBDPhos-MeOH)NiCl2 (7) dissolved in CDCl3 without NEt3.
Given that the reactions of 1 and 3 with H2O and alcohols
all resulted in protonation of the bridgehead nitrogen atom in
PhTBDPhos, we postulated that nitrogen participation was
Fig. 11 11B NMR spectra of reaction mixtures containing 1 and anhy-
drous TASF (top) and 1 and TASF exposed to ambient humidity in air
important for overcoming the low Lewis acidity of boron in the (bottom). Both spectra were collected in CH2Cl2.
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show no appreciable reactivity. The lack of iPrTBDPhos reac-
tivity is attributed to the sterically bulky isopropyl groups,
which block substrate access to boron in the TBD backbone.
The reactivity studies suggest that protonation of the bridge-
head nitrogen in PhTBDPhos helps to overcome the lack of
measurable Lewis acidity at boron, an observation supported
by methoxide and fluoride binding studies with TASF and
hydrated [nBu4N]F. Furthermore, when H2O and alcohol reac-
tion products 5–9 are dissolved in anhydrous solvent, especially
without addition of NEt3, the compounds revert back to some
proportion of starting materials 1 and 3. As described in a
recent review by Maity and Teets,14 irreversible B–O formation is
a long-standing challenge when using borane ligands in cata-
lytic and stoichiometric reactions. Our results hint at reversible
B–O reactivity, but more work is needed to account for the
small amounts of PhTBDPhos decomposition.
Returning to Chart 1 in the introduction, we noted that tria-
minoboranes participating in Z-type M→B σ bonds can
promote cooperative metal–ligand reactivity. By way of analogy,
our results suggest that dative N→B π bonds in PhTBDPhos can
promote reactions without obvious participation of the metal.
Given that participation of the bridgehead nitrogen in
TBDPhos appears to be important for promoting reactivity at
boron, we propose to define these types of reactions (i.e.,
chemical reactions requiring more than one atom on a ligand)
as “cooperative ligand-centered reactivity” (CLR). Our rational
for defining CLR here is two-fold: (1) to distinguish the reactiv-
ity from cooperative metal–ligand reactions and ligand-cen-
tered reactivity involving a single atom on the ligand (e.g.,
Lewis acid or base binding), and (2) to highlight it as a poten-
tial ligand design principle.
Li and Hall recently proposed a catalytic cycle that proceeds
via concerted transfer of two hydrogen atoms on MeOH to
carbon and nitrogen atoms on a Ru-bound ligand in a
“ligand–ligand bifunctional mechanism”.59 We chose to use
“cooperative” in defining CLR instead of “bifunctional”
because bifunctional has been used historically to imply con-
certed reactivity across metal–ligand bonds in metal–ligand
bifunctional catalysts. Dub and Gordon recently proposed that
the long-entrenched view of concerted reactivity is incorrect in
Noyori’s archetypal catalysts and, in turn, offered an expanded
description of metal–ligand cooperativity (MLC) to more ade-
quately define cooperative ligands:60,61
“Therefore in order for the principle of metal–ligand coop-
erativity to take place, a ligand should act together in a syner-
gistic manner with the metal to facilitate a chemical event, i.e.,
any act of bond cleavage/formation⋯ligands defined in such a
way can be called cooperative”.60
Fig. 12 Molecular structure of {[(PhTBDPhos-HF)Ni]2(μ-OH)2}Cl2 (10)
with thermal ellipsoids drawn at the 35% probability level. The outer-
sphere chloride, hydrogen atoms attached to carbon, phenyl groups,
and co-crystallized CH2Cl2 were omitted from the figure.
The reaction eliminated an equivalent of [nBu4N]Cl, which
was co-crystallized from the reaction mixtures. The structure of
10 revealed trans B–F and N–H bonds and an elongated dative
N→B bond distance of 1.613(3) Å, which is ca. 0.03–0.04 Å
shorter than those in 5 and 7–9 (Table 2). The B–F distances of
1.415(3) and 1.424(3) Å are similar to the B–OH distance in 5
of 1.427(4) Å, but slightly longer than those observed in the
N3B-F core of subporphyrin complexes (1.39–1.41 Å).54–56
Despite the similar B–O and B–F distances, none of our data
suggested that 5 and 10 were the same compound. Attempts to
replace fluoride in the XRD data collected for 10 with oxygen
led to unsatisfactory ellipsoids and a higher R-factor. 19F NMR
spectra collected on crystallized samples of 10 yielded a broad
resonance at δ −164.5 in CDCl3. The 19F NMR shift for 10 is
similar to those in the aforementioned fluorinated subpor-
phyrin complexes, which range between δ −156 and −158.54,55
For added comparison, the 19F NMR resonance for the [nBu4N]
F starting material appears at δ −125.7 in CDCl3.57 The 11B
and 31P NMR resonances for 10 were observed at δ 1.6 and
72.0, respectively, and are slightly shifted compared to those
observed for 5 in CDCl3 at δ 1.0 and 70.8 (Table 1). No 11B–19F
coupling was observed, as is typical for some O→B and N→B
dative complexes containing B–F bonds (e.g., BF3·Et2O and
BF3·MeCN),58 but the 11B resonance for 10 (FWHM = 180 Hz)
was significantly broadened compared to 5 (FWHM = 80 Hz).
The 1H NMR spectrum of 10 corroborated the determined
structure in Fig. 12 and revealed no resonances assigned to B–
OH (Fig. S49; ESI†). Moreover, acquisition of the NMR data for
10 did not require addition of NEt3, nor was any HF loss or
decomposition observed.
Notably, reactions captured in the expanded description
can proceed via concerted or step-wise reaction steps. Hence,
Conclusion
In summary, we have reported two new triaminoborane- we submit that “cooperative”, as used in Dub and Gordon’s
bridged diphosphine ligands, PhTBDPhos and iPrTBDPhos, definition of MLC, provides a less restrictive qualifier to define
and established their coordination chemistry with NiCl2 and ligand-centered reactions involving more than one atom, such
PdCl2. The bridgehead B–N bond in PhTBDPhos selectively as those described here. Future work is aimed at elucidating
reacts with water, alcohols, and fluoride when bound to Ni(II) the mechanism of PhTBDPhos reactivity and exploring the
(1) and Pd(II) (3), whereas the iPrTBDPhos complexes 2 and 4 applications of these and other CLR-type reactions.
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mixture was stirred overnight and the solvent was removed
under vacuum. The solid residue was extracted with Et2O
(3 × 100 mL), filtered, and the filtrate was evaporated to
Experimental
General considerations
Reactions were carried out under an atmosphere of N2 or Ar dryness under vacuum. The resulting white solid was dissolved
using glovebox or standard Schlenk techniques unless stated in CH2Cl2 (15 mL), concentrated, and cooled to −30 °C to yield
otherwise. Glassware used for reactions performed under inert colorless blocks after 1 day. Yield: 7.00 g (96%). Mp: 167 °C.
conditions were dried in an oven at 150 °C for at least 1.5 h Anal. calcd for C30H32BN3P2: C, 71.0; H, 6.36; N, 8.28. Found:
and allowed to cool under vacuum before use. Solvents used C, 70.4; H, 6.12; N, 8.30. 1H NMR (CDCl3, 20 °C): δ 1.37 (quint,
for anhydrous reactions were dried and deoxygenated using a CH2–CH̲2–CH2, 4H), 2.80 (t, NCH2, 4H), 3.05 (m, NCH2, 4H),
Pure Process Technologies Solvent Purification System. NaBH4, 7.30–7.40 (m, Ph, 12H), 7.51–7.55 (m, Ph, 8H). 11B NMR
I2, 3,3′-diaminodipropylamine, ClPPh2, ClPiPr2, nBuLi (2.5 M (CDCl3, 20 °C): δ 26.2 (br s, FWHM = 580 Hz). 13C NMR
in hexanes), (DME)NiCl2, (PhCN)2PdCl2, MeOH, allylic alcohol, (CDCl3, 20 °C): δ 27.3 (s, CH2–C̲H2–CH2), 44.3 (s, NCH2), 48.4
3
[nBu4N]F·(H2O)n, tris(dimethylamino)sulfonium difluorotri- (s, NCH2), 127.7 (vt, Ph, JPC = 2.7 Hz), 127.8 (s, Ph), 132.9 (vt,
1
2
methylsilicate (TASF), NaOMe, and ampules of deuterated sol- Ph, JPC = 11.0 Hz), 139.7 (vt, Ph, JPC = 8.0 Hz). 31P{1H} NMR
vents were used as received from commercial vendors. NEt3 (CDCl3, 20 °C): δ 47.7 (s). MS (FI) [fragment ion, relative abun-
was dried over KOH and distilled before use.
dance]: m/z 108 [PPh, 12], 183 [PPh2, 31], 322 [L − PPh2, 18],
1H, 19F, and 31P NMR data were recorded on a Bruker 430 [L − Ph, 100], 507 [L, 42]. IR (Nujol, cm−1): 1582 w,
AVANCE-300 instrument operating at 300 MHz for 1H, 1501 m, 1348 vw, 1316 vw, 1298 w, 1286 w, 1274 vw, 1205 s,
282.2 MHz for 19F, and 121.4 MHz for 31P. 11B and 13C NMR 1170 m, 1110 s, 1095 w, 1067w, 1022 s, 997 m, 984 vw, 964 m,
data were acquired on a Bruker DRX-400 instrument operating 930 w, 919 w, 878 s, 847 w, 814 s, 772 m, 746 vs, 724 w, 700 vs,
at 128.3 and 75.5 MHz, respectively. Chemical shifts are 656 vw, 648 m, 634 m, 617 vw, 578 vw.
reported in δ units relative to residual solvent peaks (1H and
iPrTBDPhos
13C), 0.05% C6H5CF3 in C6D6 (19F), 85% H3PO4 (31P), and
BF3·Et2O (11B). Microanalysis data (CHN) were collected using To a stirring solution of TBD (2.00 g, 14.4 mmol) in Et2O
an EAI CE-440 Elemental Analyzer at the University of Iowa or (50 mL) cooled to −78 °C was added a solution of 2.5 M BuLi
n
by Midwest Microlab, LLC in Indianapolis, IN. IR spectra were in hexanes (11.5 mL, 28.8 mmol). The white suspension was
collected on a Thermo Scientific Nicolet iS5 using an attenu- stirred for a day. A solution of ClPiPr2 (5.20 g, 28.8 mmol) in
ated total reflection (ATR) accessory, as KBr pellets, or as Nujol Et2O (50 mL) was added and the reaction mixture was stirred
mulls between NaCl plates. Melting points were determined for 5 days. The solvent was removed under vacuum and the
using a REACH MP Device. HR-EI mass spectra were recorded solid residue was extracted with Et2O (3 × 100 mL) and filtered.
on Waters GCT Premier instrument using TOF, and ESI-MS The filtrate was evaporated to dryness under vacuum and the
spectra were collected on Waters Q-TOF premier. Fragment resulting white solid was dissolved in CH2Cl2 (10 mL), concen-
ions (M: molecule, L: ligand) were assigned based on compari- trated, and stored at −30 °C to yield colorless blocks. Yield:
son to calculated natural abundance isotopic distributions.
1.77 g (33%). Mp: 70 °C. Anal. calcd for C18H40BN3P2: C, 58.2;
H, 10.9; N, 11.3. Found: C, 58.0; H, 10.8; N, 11.2. 1H NMR
1,8,10,9-Triazaboradecalin (TBD)
(CDCl3, 20 °C): δ 1.00–1.15 (m, CH
̲
3–CH–CH3
̲
, 24 H), 1.72
–CH3, 4 H),
Prepared with slight modification to a published procedure.41 (quint, –CH2–CH
2̲ –CH2–, 4 H), 1.87 (sept, CH3–CH
̲
To a suspension of NaBH4 (4.30 g, 0.114 mol) and 3,3′- 2.84 (t, NCH2, 4 H), 2.95 (m, NCH2, 4 H). 11B NMR (CDCl3,
diaminodipropylamine (13.1 g, 0.100 mol) in THF (200 mL) 20 °C): δ 25.8 (br s, FWHM = 200 Hz). 13C NMR (CDCl3, 20 °C):
1
was added a solution of I2 (12.7 g, 0.0500 mol) in THF δ 20.2 (vt, CH3–C
(100 mL). The mixture was heated to reflux for 3 days. After
cooling to RT, the solution was filtered. The solvent was (s, CH2–C
̲
̲
2
2
C̲H3, JPC = 8.9 Hz), 26.1 (vt, C̲H3–CH–C̲
̲
removed under vacuum and the residue was distilled under NMR (CDCl3, 20 °C): δ 61.3 (s). MS (FI) [fragment ion, relative
vacuum. The distillate was heated at 150 °C overnight to com- abundance]: m/z 200 [M − 4iPr, 9], 242 [M − 3iPr, 9], 285 [M −
plete the reaction. The resulting solid was dissolved in 2iPr, 29], 328 [M − Pr, 100], 371 [M, 5]. IR (Nujol, cm−1):1500
i
pentane (20 mL), filtered, and stored at −30 °C to yield color- s, 1356 vw, 1347 vw, 1315 s, 1290 s, 1236 vw, 1211 s, 1182 m,
less needles. Yield: 7.68 g (55%). The purity and identity of 1153 w, 1115 s, 1068 m, 1024 s, 966 s, 927 m, 875 s, 817 s,
TBD were confirmed by 1H and 11B NMR spectroscopy. 777 m, 764 m, 749 m, 722 m, 639 s, 598 w.
1H NMR (CDCl3, 20 °C): δ 1.69 (br s, NH, 2H), 1.79 (quint,
(
PhTBDPhos)NiCl2 (1)
CH2–CH2̲ –CH2, 4H), 2.84 (vt, NCH2, 4H), 2.98 (vt, NCH2, 4H).
11B NMR (CDCl3, 20 °C): δ 22.4 (s).
To a solution of (DME)NiCl2 (0.10 g, 0.46 mmol) in CH2Cl2
(10 mL) was added
solution of PhTBDPhos (0.23 g,
0.45 mmol) in CH2Cl2 (10 mL). After 30 min, the color of the
a
PhTBDPhos
A solution of ClPPh2 (6.40 g, 29.0 mmol) in CH2Cl2 (40 mL) reaction mixture changed from yellow to dark orange. The
was slowly added to a solution of TBD (2.00 g, 14.4 mmol) and mixture was evaporated to dryness under vacuum and
NEt3 (2.92 g, 28.9 mmol) in CH2Cl2 (40 mL). The reaction extracted with CH2Cl2 (10 mL). Vapor diffusion with Et2O
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yielded orange needles and dark orange blocks that were deter- 2.81–2.97 (m, NCH2, 8 H), 7.34–7.50 (m, Ph, 12 H), 7.72–7.84
mined to be polymorphs of 1. Yield: 0.23 g (79%). Mp: (m, Ph, 8 H). 11B NMR (CDCl3, 20 °C): δ 24.2 (br s, FWHM =
>250 °C. Anal. calcd for C30H32BCl2N3NiP2·2CH2Cl2: C, 47.6; 440 Hz). 13C NMR (CDCl3, 20 °C): δ 26.2 (s, CH2–C
̲
̲
4H), 2.81 (t, NCH2, 4H), 7.37–7.53 (m, Ph, 12H), 7.85–7.99 δ 68.7 (s). MS (FI) [fragment ion, relative abundance]: 183
(d, Ph, 8H). 11B NMR (CDCl3, 20 °C): δ 23.9 (br s, FWHM = [PPh2, 83], 430 [L − Ph, 100], 507 [L, 14], 535 [M − 2Cl − Ph, 1],
540 Hz). 13C NMR (CDCl3, 20 °C): δ 26.0 (s, CH2–CH2–CH2), 613 [M − 2Cl, 4], 648 [M − Cl, 2]. IR (Nujol, cm−1): 1395 m,
̲
47.3 (s, NCH2), 48.1 (s, NCH2), 128.4 (s, Ph), 130.8 (s, Ph), 1298 m, 1281 m, 1269 w, 1257 w, 1211 s, 1172 s, 1157 w, 1119 w,
132.9 (s, Ph). 31P{1H} NMR (CDCl3, 20 °C): δ 70.1 (br s, FWHM = 1093 s, 1056 w, 1026 s, 1010 w, 997 w, 918 s, 892 s, 834 m,
140 Hz). MS (FI) [fragment ion, relative abundance]: m/z 183 786 m, 742 m, 723 s, 691 s, 638 w, 617 w, 601 w.
[PPh2, 10], 322 [L − PPh2, 9], 353 [L − 2Ph, 1], 379 [M − 2Cl−
PPh2, 3], 415 [M − Cl− − PPh2, 6], 430 [L − Ph, 100], 446
−
(
iPrTBDPhos)PdCl2 (4)
[M − Cl− − 2Ph, 13], 523 [M − Cl− − Ph, 5], 564 [M − 2Cl, 2], To a stirring solution of (PhCN)2PdCl2 (0.30 g, 0.782 mmol) in
600 [M − Cl, 13], 637 [M, 1]. IR (Nujol, cm−1): 1322 w, 1305 w, toluene (10 mL) was added a solution of iPrTBDPhos (0.29 g,
1291 s, 1223 w, 1214 vw, 1206 s, 1179 s, 1170 vw, 1154 vw, 1119 0.781 mmol) in toluene (10 mL). The reaction mixture was
vw, 1098 s, 1071 vw, 1024 s, 1009 w, 998 w, 938 m, 909 s, stirred overnight, filtered, and evaporated to dryness under
891 m, 826 s, 783 s, 749 s, 724 s, 711 m, 693 s, 648 m, 629 m, vacuum. The resulting pale yellow powder was dissolved in
618 w, 595 m, 568 w, 535 w.
CH2Cl2 (5 mL) and filtered. Colorless blocks were obtained by
vapor diffusion with Et2O. Yield: 0.30 g (70%). Mp 142 °C
(dec.). Anal. calcd for C18H40BCl2N3P2Pd: C, 39.4; H, 7.35;
(
iPrTBDPhos)NiCl2 (2)
To a stirring solution of (DME)NiCl2 (0.30 g, 1.37 mmol) in N, 7.66. Found: C, 39.1; H, 7.29; N, 7.52. 1H NMR (CDCl3,
CH2Cl2 (20 mL) was added a solution of iPrTBDPhos (0.51 g, 20 °C): δ 1.32–1.54 (m, CH
1.37 mmol) in CH2Cl2 (10 mL). Stirring the mixture overnight –CH2–CH2–CH2–, 4 H), 2.94 (t, NCH2, 4 H), 3.19–3.37 (m,
yielded a dark blue solution. The solution was concentrated to NCH2, CH3–CH
–CH3, 8 H). 11B NMR (CDCl3, 20 °C): δ 24.5
ca. 10 mL and dark blue blocks were obtained by vapor (br s, FWHM = 320 Hz). 13C NMR (CDCl3, 20 °C): δ 19.6 (s,
diffusion with Et2O. Yield: 0.52 g (76%). Mp: 226 °C. Anal. H3–CH–CH3), 20.4 (s, CH3–CH–CH3), 26.9 (s, CH2–CH2–CH2),
calcd for C18H40BCl2N3NiP2: C, 43.2; H, 8.05; N, 8.39. Found: 29.9 (d, CH3–CH–CH3, JPC = 2.6 Hz), 30.3 (d, CH3–CH–CH3,
C, 42.6; H, 7.92; N, 8.07. 1H NMR (CDCl3, 20 °C): δ 1.45 (d, 1JPC = 2.7 Hz), 45.8 (s, NCH2), 48.9 (s, NCH2). 31P{1H} NMR
CH3–CH–CH3, 12 H), 1.64 (d, CH3–CH–CH3, 12 H), 1.81 (quint, (CDCl3, 20 °C): δ 97.4 (s). MS (FI) [fragment ion, relative abun-
–CH2–CH
2–CH2–, 4 H), 2.92 (t, NCH2, 4 H), 3.16 (sept, CH3– dance]: m/z 305 [M − 2Cl − 4iPr, 100], 348 [M − 2Cl − 3iPr, 64],
CH
–CH3, 4 H), 3.21 (m, NCH2, 4 H). 11B NMR (CDCl3, 20 °C): 391 [M − 2Cl − 2iPr, 50], 434 [M − 2Cl − iPr, 53], 477 [M − 2Cl,
δ 24.5 (br s, FWHM = 320 Hz). 13C NMR (CDCl3, 20 °C): δ 19.4 63], 512 [M − Cl, 66]. IR (Nujol, cm−1): 1508 s, 1354 w, 1324 m,
(s, CH3–CH–CH3), 19.6 (s, CH3–CH–CH3), 27.1 (s, CH3–CH–CH3 1296 m, 1284 vw, 1263 m, 1225 w, 1211 vw, 1202 s, 1173 s,
or CH2–CH2–CH2), 28.3 (s, CH2–CH2–CH2 or CH3–CH
3̲ –CH–CH3̲ , 24 H), 1.83 (quint,
̲
̲
C̲
̲
̲
̲
̲
1
̲
̲
̲
̲
̲
̲
̲
̲
̲
̲
̲
̲
̲
̲
̲
̲ –CH3), 1121 w, 1094 s, 1054 m, 1022 s, 1011 vw, 966 w, 943 m, 913 m,
45.4 (s, NCH2), 48.4 (s, NCH2). 31P{1H} NMR (CDCl3, 20 °C): 897 m, 886 m, 822 s, 781 s, 722 s, 656 s, 635 w, 613 m.
δ 147.7 (br s, FWHM = 1200 Hz). MS (FI) [fragment ion, relative
{[(PhTBDPhos-H2O)Ni]2(μ-OH)2}Cl2 (5)
abundance]: m/z 200 [L − 4iPr, 72], 242 [L − 3iPr, 65], 254
[M − 2Cl − 4iPr, 67], 285 [L − 2iPr, 78], 298 [M − 2Cl − 3iPr, 100], To a solution of 1 (0.20 g, 0.31 mmol) in CHCl3 (10 mL) in air
328 [L − iPr, 84], 342 [M − 2Cl − 2iPr, 65], 384 [M − 2Cl − iPr, 34], was added NEt3 (0.5 mL) and deionized H2O (5 mL). A single
466 [M − Cl, 98], 501 [M, 2]. IR (Nujol, cm−1): 1526 w, 1514 s, agitation of the solution completed the reaction as indicated
1327 m, 1303 w, 1292 w, 1277 m, 1241 m, 1212 s, 1161 s, by a color change from dark to bright orange and subsequent
1101 m, 1079 vw, 1064 vw, 1042 s, 1008 w, 968 m, 938 w, 11B and 31P NMR analysis of the reaction mixture. The CHCl3
900 m, 879 m, 846 w, 819 m, 765 m, 722 vs, 655 m, 623 m.
fraction was separated from the water layer and pentane was
added to precipitate a yellow-orange powder. Yield: 0.10 g
(50%). Single crystals were obtained by layering of the CHCl3
(
PhTBDPhos)PdCl2 (3)
To a stirring solution of (PhCN)2PdCl2 (0.20 g, 0.52 mmol) in solution with hexane. Anal. calcd for C60H70B2Cl2N6Ni2O4P4:
toluene (10 mL) was added a solution of PhTBDPhos (0.26 g, C, 56.6; H, 5.54; N, 6.60. Found: C, 56.1; H, 5.79; N, 6.34.
0.52 mmol) in toluene (10 mL). A dark brown precipitate 1H NMR (CDCl3, 20 °C): δ −4.48 (s, 1H, Ni–OH), 0.25 (s, 1H,
formed and the mixture was stirred overnight. The precipitate B–OH), 1.04 (m, 2H), 1.89 (m, 2H), 2.64 (br s, 2H), 2.73 (m,
was filtered and evaporated to dryness under vacuum. Yield: 2H), 2.94 (m, 2H), 3.26 (m, 2H), 7.11–7.22 (m, 6H, Ph),
0.27 g (75%). Brown needles and blocks were subsequently 7.38–7.51 (m, 10H, Ph), 7.66 (m, 4H, Ph), 8.44 (m, 1H, NH).
obtained by vapor diffusion of Et2O into CH2Cl2 solutions of 3. 11B NMR (CDCl3, 20 °C): δ 1.0 (s). 13C NMR (CDCl3, 20 °C):
Mp: 198 °C (dec). Anal. calcd for C30H32BCl2N3P2Pd·2CH2Cl2: δ 26.4 (s, CH2–C̲H2–CH2), 45.9 (s, NCH2), 49.4 (s, NCH2), 128.9
C, 45.0; H, 4.25; N, 4.92. Found: C, 45.0; H, 4.17; N, 4.91. (m, Ph), 129.0 (vt, Ph, JPC = 5.1 Hz), 130.6 (d, Ph, JPC = 9.2 Hz),
1H NMR (CDCl3, 20 °C): δ 1.56 (quint, –CH2–CH
2̲ –CH2–, 4 H), 131.6 (vt, Ph, JPC = 5.5 Hz), 132.1 (m, Ph), 133.2 (vt, Ph,
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JPC = 5.0 Hz). 31P{1H} NMR (CDCl3, 20 °C): δ 70.8 (s). MS (FI)
[fragment ion, relative abundance]: m/z 353 [TBDPhos − 2Ph,
12], 591 [(M – H2O − 2HCl)2+, 22], 600 [(TBDPhos)Ni(OH)2, 100],
618 [M − (PhTBDPhos-H2O)Ni2+, 70], 676 [M − PhTBDPhos-
H2O, 23], 1237 [M − Cl−, 23]. IR (KBr, cm−1): 3630 s (Ni–OH),
3372 br (B–OH), 3048 w, 3012 m, 2937 w, 2857 m, 1483 m,
1436 s, 1388 m, 1369 m, 1352 w, 1298 m, 1261 m, 1242 m,
1220 w, 1165 m, 1092 vs, 1047 s, 1027 w, 1003 m, 977 w,
912 m, 880 s, 836 w, 801 vw, 778 w, 756 m, 745 m, 710 vw,
697 s, 637 m, 561 m, 527 m, 507 w, 489 m.
(
PhTBDPhos-MeOH)PdCl2 (8)
To a mixture of 3 (0.10 g, 0.14 mmol) and NEt3 (0.5 mL) was
added a 1 : 1 mixture of CH2Cl2 and MeOH (15 mL) in air. The
orange-yellow solution was stirred for 10 min, filtered, and
concentrated to ca. 2 mL. Single crystal were obtained by vapor
diffusion with Et2O. Yield: 0.065 g (62%). Anal. calcd for
C31H36BCl2N3OP2Pd: C, 51.9; H, 5.06; N, 5.86. Found: C, 51.6;
H, 5.34; N, 6.25. 1H NMR (DMSO-d6, 20 °C): δ 1.24 (m, 4H),
2.54–2.81 (m, 9H), 2.91 (m, 2H), 5.85 (s, 1H, NH), 7.50 (m,
12H), 7.94 (m, 8H). 11B NMR (DMSO-d6, 20 °C): δ 3.2 (s).
13C NMR (DMSO-d6, 20 °C): δ 25.3 (s, CH2–C
̲H2–CH2), 45.9 (s,
{[(PhTBDPhos-H2O)Pd]2(μ-OH)2}Cl2 (6)
OCH3), 47.6 (s, NCH2), 48.9 (s, NCH2), 127.6 (m, Ph), 128.5 (m,
Ph), 130.1 (d, Ph, JPC = 20.0 Hz), 130.6 (d, Ph, JPC = 14.7 Hz),
131.2 (s, Ph), 132.6 (m, Ph), 133.1 (m, Ph), 133.6 (m, Ph), 134.2
(s, Ph). 31P{1H} NMR (DMSO-d6, 20 °C): δ 64.5 (s). IR (ATR,
cm−1): 3189 m, 3043 w, 2975 m, 2941 m, 2909 w, 2867 m,
2807 m, 1569 w, 1478 m, 1447 w, 1431 s, 1399 w, 1368 m,
1352 m, 1292 m, 1243 s, 1189 s, 1157 w, 1132 s, 1108 w, 1088
vs, 1037 vs, 1023 w, 999 w, 972 w. 942 m, 912 s, 870 vs, 827 m,
792 s, 755 s, 745 s, 693 vs, 622 m, 610 s, 599 w, 590 w, 579 m.
A mixture of 3 (0.10 g, 0.14 mmol) and NEt3 (0.5 mL) was dis-
solved in CH2Cl2 (10 mL) in air. The pale-yellow solution was
layered with deionized H2O (10 mL) and stirred vigorously for
30 min. The CH2Cl2 layer was separated and filtered. Addition
of pentane (10 mL) precipitated a pale yellow powder. Yield:
0.026 g (26%). Anal. calcd for C60H70B2Cl2N6O4P4Pd2: C, 52.7;
H, 5.16; N, 6.14. Found: C, 52.1; H, 5.10; N, 5.82. 1H NMR
(CD2Cl2, 20 °C): δ −2.89 (s, 1H, Pd–OH), 0.87 (s, 1H, B–OH),
1.10 (m, 2H), 1.91 (m, 2H), 2.65 (m, 2H), 2.90 (m, 4H), 3.47 (m,
2H), 7.20 (m, 6H, Ph), 7.36 (m, 10H, Ph), 7.68 (m, 4H, Ph), 8.58
(br s, 1H, NH). 11B NMR (CD2Cl2, 20 °C): δ 1.8 (s). 13C NMR
(
PhTBDPhos-C3H5OH)PdCl2 (9)
A mixture of 3 (0.10 g, 0.14 mmol) and NEt3 (0.5 mL) was dis-
solved in 15 mL solvent mixture of CH2Cl2 and allyl alcohol
(1 : 1) in air. The green-yellow solution was stirred for 10 min,
filtered, and concentrated to ca. 2 mL. Single crystals were
obtained by vapor diffusion with Et2O. Yield: 0.086 g (80%).
Anal. calcd for C33H38BCl2N3OP2Pd: C, 53.4; H, 5.16; N, 5.66.
Found: C, 53.1; H, 5.40; N, 5.87. 1H NMR (CD2Cl2, 20 °C):
δ 1.29 (m, 4H), 2.74 (m, 2H), 3.04 (m, 6H), 3.38 (m, 2H),
4.90–5.09 (m, 2H), 5.66–5.80 (m, 1H), 7.42–7.59 (m, 13H),
7.89–8.04 (m, 8H). 11B NMR (CD2Cl2, 20 °C): δ 2.9 (s). 13C NMR
(CD2Cl2, 20 °C): δ 26.7 (s, CH2–C̲H2–CH2), 46.4 (s, NCH2), 49.6
(s, NCH2), 129.0 (m, Ph), 131.0 (s, Ph), 131.2 (s, Ph), 131.6 (s,
Ph), 132.2 (m, Ph), 133.2 (m, Ph), 133.5 (m, Ph). 31P{1H} NMR
(CD2Cl2, 20 °C): δ 65.3 (s). IR (ATR, cm−1): 3607 m (Pd–OH),
3340 br (B–OH), 3049 m, 2940 m, 2856 m, 1586 w, 1481 w,
1456 w, 1434 s, 1373 m, 1297 m, 1260 w, 1240 m, 1148 w,
1121 w, 1097 s, 1044 s, 1025 w, 997 w, 921 w, 884 sm, 833 w,
744 s, 711 w, 693 vs, 634 s, 619 w, 590 w, 583 w, 574 w, 567 w,
554 m.
(
PhTBDPhos-MeOH)NiCl2 (7)
(CD2Cl2, 20 °C): δ 26.8 (s, CH2–C
(s, NCH2), 60.9 (s, BOCH2), 112.8 (s, –CHvC
131.0 (s, Ph), 131.3 (s, Ph), 131.6 (s, Ph), 133.3 (m, Ph), 134.3
(m, Ph), 134.9 (s, Ph), 138.6 (s, –C
HvCH2). 31P{1H} NMR
̲
̲
To a mixture of 1 (0.20 g, 0.31 mmol) and NEt3 (0.5 mL) was
added MeOH (10 mL) in air. The mixture was stirred for
20 min until everything dissolved. 11B and 31P NMR analysis of
the resulting light orange solution revealed that the reaction
was complete. Vapor diffusion of Et2O into the solution
yielded orange blocks after 3 days. Yield: 0.10 g (49%). Anal.
calcd for C31H36BCl2N3NiOP2·CH3OH: C, 54.8; H, 5.75; N, 5.99.
Found: C, 54.8; H, 5.64; N, 6.32. 1H NMR (CDCl3, 20 °C): δ 0.97
(d, 2H), 1.85 (m, 2H), 2.59 (br s, 2H), 2.62 (s, 3H, MeO), 2.71
(m, 2H), 2.87 (m, 2H), 3.02 (m, 2H), 7.03 (m, 4H), 7.11 (m, 2H),
7.42 (m, 2H), 7.51 (m, 4H), 7.67 (m, 4H), 7.92 (m, 4H), 8.49 (m,
̲
(CD2Cl2, 20 °C): δ 66.1 (s). IR (ATR, cm−1): 3203 m, 3054 w,
2980 m, 2918 w, 2865 w, 2828 m, 1568 w, 1478 m, 1447 w,
1431 s, 1398 w, 1369 m, 1348 m, 1293 m, 1244 m, 1190 m,
1165 m, 1128 s, 1108 w, 1084 vs, 1037 vs, 1024 w, 998 w, 986 w,
949 m, 917 s, 875 vs, 828 m, 786 s, 748 s, 740 w, 694 vs, 623 w,
605 s, 585 w, 577 w, 572 w.
{[(PhTBDPhos-HF)Ni]2(μ-OH)2}Cl2 (10)
1H, NH). 11B NMR (CDCl3, 20 °C): δ 1.9 (s). 13C NMR (CDCl3, To a solution of 1 (0.10 g, 0.16 mmol) in CH2Cl2 (10 mL) was
20 °C): δ 26.2 (s, CH2–C
H2–CH2), 46.4 (s, OCH3), 47.9 (s, added a solution of 1 M [nBu4N]F·(H2O)n in THF (0.16 mL,
̲
NCH2), 49.3 (s, NCH2), 128.7 (m, Ph), 130.2 (s, Ph), 130.8 (m, 0.16 mmol). The solution immediately transformed from dark
Ph), 132.6 (m, Ph), 133.3 (m, Ph). 31P{1H} NMR (CDCl3, 20 °C): orange to orange-yellow. After stirring for 1 h, the reaction was
δ 69.8 (s). MS (FI) [fragment ion, relative abundance]: m/z 78 evaporated to dryness, dissolved in CH2Cl2 (5 mL), and fil-
[Ph, 100], 600 [M − Cl− − CH3OH, 3]. IR (KBr, cm−1): 3199 m, tered. Vapor diffusion with Et2O yielded orange blocks with
3056 w, 2943 m, 2866 m, 2808 m, 1479 m, 1459 vw, 1448 w, small amounts of co-crystallized [nBu4N]Cl. The crystals were
1432 s, 1400 w, 1369 m, 1351 m, 1292 m, 1244 m, 1191 m, dissolved in 3 mL CH2Cl2 and recrystallized with Et2O to yield
1132 m, 1099 vs, 1039 vs, 999 w, 978 w, 941 w, 912 m, 870 s, deep orange blocks free of [nBu4N]Cl. Yield: 0.10 g (88%). Anal.
828 w, 794 m, 756 m, 746 m, 695 vs, 622 w, 609 s, 564 w, calcd for C60H68B2Cl2F2N6Ni2O2P4·2CH2Cl2: C, 51.5; H, 5.01; N,
1
551 w, 541 w, 530 w, 502 m, 483 w.
5.80. Found: C, 51.7; H, 4.88; N, 5.89. H NMR (CDCl3, 20 °C):
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δ −4.51 (s, 1H, Ni–OH), 1.06 (m, 2H), 1.85 (m, 2H), 2.40 (br, s, alcohol (9) were mounted on a MiTeGen micromount with
2H), 2.67 (m, 2H), 3.02 (m, 2H), 3.15 (m, 2H), 7.14–7.27 (m, ParatoneN oil. The data were collected as described pre-
6H, Ph), 7.27–7.45 (m, 10H, Ph), 7.68 (m, 4H, Ph), 8.79 (m, 1H, viously.62 The structures were solved with Direct Methods
NH). 11B NMR (CDCl3, 20 °C): δ 1.6 (s). 13C NMR (CDCl3, (SHELXT or SHELXS) and least squares refinement (SHELXL)
20 °C): δ 26.1 (s, CH2–C̲
H2–CH2), 45.3 (s, NCH2), 49.7 (s, confirmed the location of the non-hydrogen atoms.63 All
NCH2), 129.0 (m, Ph), 130.7 (s, Ph), 130.8 (s, Ph), 131.5 (vt, Ph, hydrogen atom positions were idealized and were allowed to
JPC = 5.5 Hz), 133.3 (vt, Ph, JPC = 5.0 Hz). 19F NMR (CDCl3, ride on the attached carbon, nitrogen or oxygen atoms.
20 °C): δ −164.5 (br s, FWHM = 130 Hz).31P{1H} NMR (CDCl3, Anisotropic temperature factor for all non-hydrogen atoms
20 °C): δ 72.0 (s). IR (ATR, cm−1): 3627 m (Ni–OH), 3049 w, were included at the last refinement. Structure solution and
3003 w, 2956 m, 2884 vw, 2805 m, 2681 vw, 2644 w, 2598 vw, refinement were performed with Olex2.64 Publication figures
2570 w, 2536 w, 1586 w, 1481 vw, 1472 w, 1459 vw, 1431 s, were generated with SHELXP.63 The data collection and refine-
1397 m, 1389 w, 1372 w, 1355 w, 1341 w, 1301 m, 1269 m, ment details are provided in Tables S1 and S2 (ESI†).
1245 m, 1226 w, 1199 m, 1151 s, 1124 vs, 1092 vs, 1049 vs,
1028 w, 994 m, 953 m, 908 s, 884 s, 854 vw, 839 m, 810 s,
795 vw, 755 w, 746 s, 693 vs, 634 s, 614 w.
Acknowledgements
This work was supported by start-up funds and an Internal
Funding Initiative (IFI) grant from the University of Iowa. We
would like to thank Dale Swenson for collecting the single-
crystal XRD data and Matthew Blake for preparing several
batches of TBD.
Reaction of 1 with TASF
To a stirring solution of 1 (0.10 g, 0.16 mmol) in CH2Cl2
(8 mL) was added a solution of dry TASF (0.045 g, 0.16 mmol)
in CH2Cl2 (2 mL). The reaction mixture transformed from dark
orange to orange-yellow over 1 day. Analysis by 11B NMR
spectroscopy revealed a mixture of products (δ 23.8 and 1.6;
Fig. 11).
The reaction was repeated with TASF that was exposed to
humidity in ambient air. 11B NMR data collected revealed that
the reaction converged to a single species at δ 1.6 (Fig. 11).
Subsequent NMR and IR analysis of the isolated compound
confirmed the formation of 10.
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