Dual coordination modes of ethylene-linked NP2 ligands in cobalt(II) and nickel(II) iodides

Article abstract of DOI:10.1021/ic201213c

Here we report the syntheses and crystal structures of a series of cobalt(II) and nickel(II) complexes derived from _RNP2 ligands (where R = OMe_Bz, H_Bz, Br_Bz, Ph) bearing ethylene linkers between a single N and two P donors. The Co^II complexes generally adopt a tetrahedral configuration of general formula [(NP2)Co(I) ₂], wherein the two phosphorus donors are bound to the metal center but the central N-donor remains unbound. We have found one case of structural isomerism within a single crystal structure. The Co^II complex derived from _BzNP2 displays dual coordination modes: one in the tetrahedral complex [(_BzNP2)Co(I)₂]; and the other in a square pyramidal variant, [(_BzNP2)Co(I)₂]. In contrast, the Ni^II complexes adopt a square planar geometry in which the P(Et)N(Et)P donors in the ligand backbone are coordinated to the metal center, resulting in cationic species of formula [(_RNP2)Ni(I)]⁺ with iodide as counterion. All Ni^II complexes exhibit sharp ¹H and ³¹P spectra in the diamagnetic region. The Co ^II complexes are high-spin (S = 3/2) in the solid state as determined by SQUID measurements from 4 to 300 K. Solution electron paramagnetic resonance (EPR) experiments reveal a high-spin/low-spin Co^II equilibrium that is dependent on solvent and ligand substituent.

Full text of DOI:10.1021/ic201213c

ARTICLE
pubs.acs.org/IC
Dual Coordination Modes of Ethylene-Linked NP2 Ligands in Cobalt(II)
and Nickel(II) Iodides
Qingchen Dong,†,‡ Michael J. Rose,*,† Wai-Yeung Wong,*,‡ and Harry B. Gray*,†
†Beckman Institute, Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125,
United States
‡Institute of Molecular Functional Materials (Areas of Excellence Scheme, University Grants Committee, Hong Kong) and Department
of Chemistry and Centre for Advanced Luminescence Materials, Hong Kong Baptist University, Waterloo Road, Hong Kong,
P.R. China
S Supporting Information
b
ABSTRACT:
Here we report the syntheses and crystal structures of a series of cobalt(II) and nickel(II) complexes derived from RNP2 ligands
(where R = OMeBz, HBz, BrBz, Ph) bearing ethylene linkers between a single N and two P donors. The CoII complexes generally
adopt a tetrahedral configuration of general formula [(NP2)Co(I)2], wherein the two phosphorus donors are bound to the metal
center but the central N-donor remains unbound. We have found one case of structural isomerism within a single crystal structure.
The CoII complex derived from BzNP2 displays dual coordination modes: one in the tetrahedral complex [(BzNP2)Co(I)2]; and the
other in a square pyramidal variant, [(BzNP2)Co(I)2]. In contrast, the NiII complexes adopt a square planar geometry in which the
P(Et)N(Et)P donors in the ligand backbone are coordinated to the metal center, resulting in cationic species of formula
[(RNP2)Ni(I)]+ with iodide as counterion. All NiII complexes exhibit sharp 1H and 31P spectra in the diamagnetic region. The CoII
complexes are high-spin (S = 3/2) in the solid state as determined by SQUID measurements from 4 to 300 K. Solution electron
paramagnetic resonance (EPR) experiments reveal a high-spin/low-spin CoII equilibrium that is dependent on solvent and ligand
substituent.
’ INTRODUCTION
more efficient ethylene polymerization catalysts.1,8 Other inves-
tigators have used multidentate ligands bearing groups present-
ing “mismatched” donor strengths, as in those containing a wider
variety of hard (N,O) and soft (P,S) donors.9 Inclusion of
nitrogen atoms in predominantly phosphine-based ligands has
led to the development of efficient dihydrogen (H2) evolution
and oxidation catalysts containing both nickel and cobalt.10 In
each case, researchers recognized that the structural and coordi-
nation properties of a chosen ligand to a given metal center
dictate the reactivity and magnetic properties of the resulting
complex.
Complexes containing late transition metals bound to phos-
phine donors have found application in a wide variety of
industrial transformations and synthetic methodologies. Phos-
phorus ligands are of interest because they stabilize low oxidation
states (MII/I) that are active species in many catalytic cycles, and
cobalt phosphines promote ethylene polymerization1 and hydro-
formylation reactions.2ꢀ4 Synthetically, there is ongoing interest
in substituting palladium with more earth abundant metals such
as nickel in CꢀC bond forming reactions.5ꢀ7 Chelating tris-
phosphines bearing different anionic heteroatoms have been
employed to shed light on ligand field effects for both anionic
and neutral phosphines.
In the case of mixed phosphorus- and nitrogen-containing
ligands, the additional functionality of an N-donor can allow the
metal center to access a greater range of oxidation states than
with phosphines alone. For example, cobalt and nickel complexes
derived from pyridine-containing phosphine ligands often are
Three main types of PxNy ligands differ on the basis of
structural flexibility and linker type.11 In the case of the pyr-
idine-based PMP ligand (Scheme 1), the structural rigidity of the
pyridine ring enforces binding of the N-donor to the metal
Received: June 7, 2011
Published: September 13, 2011
r
2011 American Chemical Society
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|

Inorganic Chemistry
ARTICLE
Scheme 1
the solid and solution state properties of the CoII complexes by
X-ray, superconducting quantum interference device (SQUID),
and electron paramagnetic resonance (EPR) methods. The
corresponding NiII complexes are all square planar with NP2
coordination.
’ EXPERIMENTAL SECTION
Reagents and General Procedures. Benzylamine, bromo-
benzylamine, 2-bromoethanol and phosphorus tribromide (PBr3)
were used as received from Sigma-Aldrich. The anyhydrous metal salts
cobalt(II) iodide and nickel(II) iodide also were obtained from Sigma-
Aldrich, while diphenylphosphine (Ph2PH) was from Strem. The PNP
ligand Ph2P(PhN)PPh2 was synthesized according to a published
procedure.17 Solvents (MeCN, THF, Et2O, toluene and hexane) were
obtained from a solvent purification system employing the Grubbs
method,18 while acetone from J.T. Baker was used without further
purification. Deuterated solvent (CDCl3) was from Cambridge Iso-
topes. Reactions with diphenylphosphine, metalations with CoI2 and
NiI2, and crystallizations were performed under dry, inert N2 atmo-
sphere in a glovebox.
center.8,12,13 Coordination of the P- and N-donors in PMP
results in 5-membered ring chelates that are thermodynamically
stable. For example, Rieger and co-workers prepared trigonal
bipyramidal complexes [(PMP)CoII(Cl)2] and [(PMP)FeII(Cl)2]
in which the entire PꢀNꢀP donor set binds to the metal
centers.
In contrast, the ligand PNP contains a methylene linker between
the N- and P-atoms, which disfavors simultaneous coordination
of the three donor atoms. DuBois and co-workers synthesized
several NiII complexes derived from ligands of general formula
R1P-N(R2)-PR1 (R1 = Et; R2 = Me, nBu). The resulting
structures adopt slightly distorted square planar geometries that
display only P-coordination, while the nitrogen remains un-
bound. More recent work utilizing CoII led to the isolation of
[(PNP)CoII(MeCN)3],14,15 which also excludes the N-donor
from the coordination sphere. Further work using methylene-
linked cyclic ligands (P2N2) produced complexes in which
phosphorus atoms are bound to NiII and CoII centers.
Syntheses of the Ligands. N,N-bis(2-hydroxyethyl)-4-methox-
ybenzylamine. Batches of 4-methoxybenzylamine (5.29 g, 38.5 mmol),
2-bromoethanol (10.6 g, 84.7 mmol) and excess triethylamine (20 mL)
were dissolved together in 30 mL of toluene, and the mixture was
allowed to reflux for 5 h. After cooling, the solvent was removed in vacuo,
and tetrahydrofuran (THF) was added to the flask to dissolve the
desired species. The flask was placed at ꢀ20 °C to precipitate the
NEt3 HBr completely. After filtration, the product was collected by
3
Here we report work on complexes of NP2 ligands that retain
the stable 5-membered chelates found in PMP but impart
structural flexibility by using ethylene linkers instead of a rigid
pyridyl moiety (Scheme 2). Hii and co-workers found that
evaporating the solvent. Yield: 6.94 g (80%). 1H NMR in CDCl3 (δ from
TMS, in ppm): δ 7.30ꢀ6.83 (m, 4 H); 3.79 (s, 2 H, ArꢀCH2-N); 3.61
(t, 4 H, N(CH2CH2OH)2); 2.69 (t, 4 H, (NCH2CH2OH)2); 2.49 (m, 2
H, N(CH2CH2OH)2).
t
metalation of NP2 ligands (RNP2; R = Bu, Ph, 4-OMeC6H4)
N,N-bis(2-bromoethyl)-4-methoxybenzylamine. The bis-hydroxy
starting material (4.00 g, 17.8 mmol) from the previous step was
dissolved in 50 mL of CH2Cl2, and the solution was cooled to 0 °C in
an ice bath and brought under N2 atmosphere. To this stirred solution
was added PBr3 (3.40 mL, 35.6 mmol) dropwise via syringe over the
course of 20 min. During the next 2 h, the reaction temperature was
maintained at 0 °C, and a sticky yellow precipitate was observed. The
reaction was allowed to come to room temperature over 2 h. Next, the
reaction was quenched by addition of 10 mL of water at 0 °C, and then
neutralized to pH 7 with a saturated solution of Na2CO3. The organic
layer was separated from the aqueous phase, dried over MgSO4, and the
solvent removed under reduced pressure. The product was purified via
silica gel chromatography and eluted with CH2Cl2:hexanes (1:1) to
afford a colorless oil. Yield: 4.40 g (71%). 1H NMR in CDCl3 (δ from
TMS, in ppm): δ 7.26ꢀ6.82 (m, 4 H, Ar-H); 3.80 (s, 3 H, MeO); 3.65
(s, 2 H, ArꢀCH2-N); 3.32 (t, 4 H, N(CH2CH2Br)2); 2.94 (t, 4 H,
(NCH2CH2Br)2).
with PdII salts afforded N-bound complexes [(RNP2)PdII(Cl)2],
whereas treatment with a Pd0 starting material produced only
P-bound [(RNP2)Pd(dba)] (as determined by 31P shifts).16 Of
interest is whether ligation/deligation of the N-donor is attribu-
table to electronic factors associated with the change in oxidation
state PdIIfPd0, or to a simple geometric preference of square
planar PdII for the N-donor.
Scheme 2
N,N-bis(2-(diphenylphosphino)ethyl)-4-methoxybenzylamine
{MeOBzNP2}. Under N2 atmosphere in a glovebox, diphenylpho-
sphine (0.880 g, 4.70 mmol) was diluted in 15 mL of THF and treated
with 0.580 g (5.18 mmol) of tBuOK. The clear Ph2PH solution turned
to a bright red color upon addition of base, and the mixture was stirred
at room temperature for 30 min. Separately, bis-bromo-4-methoxy-
benzylamine (0.790 g, 2.25 mmol) described above was dissolved in
5 mL of degassed THF. The deprotonated phosphine was brought to
0 °C, and the solution of dibromo-species was added dropwise over
the course of 10 min. After the addition was complete, the solution was
colorless, and the reaction was allowed to come to ambient tempera-
ture over the next 5 h. Next, 5 mL of water were added to quench the
reaction, and the THF was removed in vacuo; 30 mL of CH2Cl2 were
added and the organic layers collected and dried over MgSO4, then the
We have investigated NP2 complexes of two transition metals
in the same oxidation state (NiII and CoII) that typically adopt
different coordination geometries (square planar and tetrahedral,
respectively). Here we report that NP2 ligands with para-
substituted benzylamines and aniline as N-substituent can adopt
dual binding motifs when bound to CoII, designated NP2 and
NP2, where the donor atoms are underlined. We have explored
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Inorganic Chemistry
ARTICLE
solvent was removed under reduced pressure. The product was purified
on silica gel using CH2Cl2 EtOAc/hexanes (1:10), and isolated as a white
solid. Yield: 1.01 g (80%). H NMR in CDCl3 (δ from TMS, in
30 mL of THF was carried out, and the product purified as described
above. Yield: 1.20 g (76%). 1H NMR in CDCl3 (δ from TMS, in ppm): δ
7.35ꢀ7.27 (m, 22 H, Ar-H); 7.09, 7.07 (d, 2 H, Ar-H); 3.48 (s, 2 H,
ArꢀCH2-N); 2.59ꢀ2.51 (m, 4 H, N(CH2CH2PPh2)2); 2.12ꢀ2.09 (m,
4 H, (NCH2CH2PPh2)2). 31P NMR in CDCl3 (δ from H3PO4): δ
ppm ꢀ20.34 (s). 13C NMR in CDCl3 (δ from TMS, in ppm): δ 25.4 (d, 2
C), 49.3 (d, 2 C), 57.4 (s, 1 C), 120.6 (s, 1 C), 128.5 (m, 12 C), 130.5 (s, 2
C), 131.2 (s, 2 C), 132.6 (d, 8 C), 138.3 (m, 5 C).
N,N-bis(2-bromoethyl)phenylamine. The reaction of N-phenyl-
diethanolamine (5.00 g, 27.6 mmol) with 2 equiv of PBr3 (5.40 mL,
55.2 mmol) was performed as described above, and the crude product
was isolated according to the same extraction procedure. The product
was purified via silica gel chromatography and eluted with CH2Cl2:
hexanes (1:2) to afford a colorless oil. Yield: 6.60 g (78%). 1H NMR in
CDCl3 (δ from TMS, in ppm): δ 6.67ꢀ7.30 (m, 5 H, Ar-H); 3.78 (t, 4
H, N(CH2CH2Br)2); 3.46 (t, 4 H, (NCH2CH2Br)2).
1
ppm): δ 7.34ꢀ6.74 (m, 24 H, Ar-H); 3.79 (s, 3 H, MeO); 3.49 (s,
2 H, ArꢀCH2-N); 2.59ꢀ2.51 (m, 4 H, N(CH2CH2PPh2)2); 2.11ꢀ
2.06 (m, 4 H, (NCH2CH2PPh2)2). 31P NMR in CDCl3 (δ from
H3PO4): δ ꢀ20.15 (s). 13C NMR in CDCl3 (δ from TMS, in ppm):
δ 158.5 (s, 1 C), 138.5 (d, 4 C), 132.6 (d, 8 C), 131.1 (s, 1 C), 130.0
(s, 2 c), 128.5 (s, 4 C), 128.3 (d, 8 C), 113.6 (s, 2 C), 57.3 (s, 1 C),
55.3 (s, 1 C), 49.2 (d, 2 C), 25.4 (d, 2 C).
N,N-bis(2-hydroxyethyl)benzylamine. A large batch of benzyl bro-
mide (7.14 g, 41.7 mmol) was dissolved in 30 mL of acetone. Separately,
diethanolamine (4.82 g, 45.9 mmol) was dissolved in 5 mL of acetone
and added to a stirred solution of the benzyl bromide, followed by
addition of solid Na2CO3 (4.42 g, 41.7 mmol). The solution was
refluxed for 4 h and cooled to room temperature. Next, acetone was
removed and 20 mL of water were added to dissolve the NaBr, followed
by extraction with 30 mL of CH2Cl2. The organic layer was dried over
MgSO4, and the solvent evaporated to dryness. The resulting oil was
washed with 2 ꢁ 10 mL of hexane, affording the desired product as a
clear oil. Yield: 6.76 g (83%). 1H NMR in CDCl3 (δ from TMS, in ppm):
δ 7.33ꢀ7.29 (m, 5 H, Ar-H); 3.70 (s, 2 H, ArꢀCH2-N); 3.61 (t, 4 H,
N(CH2CH2OH)2); 2.71 (t, 4 H, (NCH2CH2OH)2); 2.44 (m, 2 H,
N(CH2CH2OH)2).
N,N-bis(2-bromoethyl)benzylamine. The reaction of bis-hydroxy-4-
benzylamine (5.65 g, 29.0 mmol) with 2 equiv of PBr3 (5.70 mL, 60.5
mmol) was performed as described above, and the crude product was
isolated according to the same extraction procedure. The product was
purified via silica gel chromatography and eluted with CH2Cl2:
hexanes (1:2) to afford a colorless oil. Yield: 8.70 g (75%). 1H
NMR in CDCl3 (δ from TMS, in ppm): δ 7.34ꢀ7.28 (m, 5 H, Ar-H);
3.73 (s, 2 H, ArꢀCH2-N); 3.34 (t, 4 H, N(CH2CH2Br)2); 2.97 (t,
4 H, (NCH2CH2Br)2).
N,N-bis(2-(diphenylphosphino)ethyl)benzylamine {BzNP2}. The re-
action of the bis-bromoethyl-benzylamine (2.25 g, 5.64 mmol) with 2.10 g
of Ph2PH (11.3 mmol) and 1.41 g of tBuOK (12.4 mmol) in 30 mL of
THF was carried out according to the procedure for MeOBzNP2 as
stated above. The product was purified on silica gel using CH2Cl2 as
solvent, and isolated as a white solid. Yield: 2.30 g (77%). 1H NMR in
CDCl3 (δ from TMS, in ppm): δ 7.33ꢀ7.23 (m, 25 H, Ar-H); 3.56 (s,
2 H, ArꢀCH2-N); 2.62ꢀ2.57 (m, 4 H, N(CH2CH2PPh2)2); 2.13ꢀ2.08
(m, 4 H, (NCH2CH2PPh2)2). 31P NMR in CDCl3 (δ from H3PO4):
δ ppm ꢀ20.34 (s). 13C NMR in CDCl3 (δ from TMS, in ppm): δ 25.4
(d, 2 C), 49.3 (d, 2 C), 58.0 (s, 1 C), 126.9 (s, 1 C), 128.1ꢀ128.9 (m, 18
C), 132.6 (d, 8 C), 138.5 (d, 2 C), 139.1 (s, 1 C).
N,N-bis(2-(diphenylphosphino)ethyl)phenylamine {PhNP2}. The
reaction of the bis-bromoethyl-phenylamine (1.69 g, 5.50 mmol) with
t
2.05 g of Ph2PH (11.0 mmol) and 1.36 g of BuOK (12.1 mmol) in
30 mL of THF was carried out, and the product purified as described
above. Yield: 2.13 g (75%). 1H NMR in CDCl3 (δ from TMS, in ppm):
δ 7.43ꢀ6.33 (m, 25 H, Ar-H); 3.38ꢀ3.30 (m, 4 H, N(CH2CH2PPh2)2);
2.30ꢀ2.25 (m, 4 H, (NCH2CH2PPh2)2). 31P NMR in CDCl3 (δ from
H3PO4): δ ppm ꢀ21.35 (s). 13C NMR in CDCl3 (δ from TMS, in
ppm): δ 25.9 (d, 2 C), 47.7 (d, 2 C), 112.3 (s, 2 C), 116.2 (s, 1 C), 128.6
(m, 12 C), 129.3 (s, 2 C), 132.7 (d, 8 C), 137.9 (d, 4 C), 146.7 (s, 1 C).
Syntheses of the CoII and NiII Complexes. All CoII and NiII
complexes were prepared and recrystallized by procedures similar to
those employed for [(BzNP2)Co(I)2] (Co-2) and [(BzNP2)Ni(I)]I
(Ni-2), as detailed below.
[(BzNP2)Co(I)2] (Co-2). The BzNP2 ligand (0.200 g, 0.380 mmol)
was dissolved in 10 mL of degassed THF. Separately, anhydrous CoI2
(0.118 g, 0.380 mmol) was slurried in 3 mL of THF and added dropwise
to the stirred solution of the NP2 ligand at room temperature; this
immediately generated a dark brown color. The solution was stirred for
12 h, and then filtered to remove a small amount of unreacted CoI2. The
brown solution was layered with 3 volumes of degassed hexane and
after 3ꢀ4 days, orange-green crystals suitable for X-ray diffraction
were obtained. Yield: 312 mg (90%). UV/vis in THF, λ in nm (ε in
Mꢀ1 cmꢀ1): 778 (220), 714 (300), 486 sh (440), 363 (1 900). Anal.
Calcd. for C35H35NP2CoI2: C 49.79, H 4.18, N 1.66; found: C 50.45, H
4.02, N 1.92.
[(MeOBzNP2)Co(I)2] (Co-1). Reaction of the methoxy-appended
MeOBzNP2 ligand (0.20 g, 0.36 mmol) with 0.11 g of CoI2 (0.35 mmol)
was performed as described above; the product crystallized from THF/
hexane after several days. Yield: 277 mg (88%). UV/vis in THF, λ in nm
(ε in Mꢀ1 cmꢀ1): 778 (630), 715 (750), 487 sh (890), 363 (2 330). Anal.
Calcd. for C36H37NOP2CoI2: C 49.45, H 4.27, N 1.60; found: C 48.83, H
4.77, N 1.45.
N,N-bis(2-hydroxyethyl)-4-bromobenzylamine. The reaction of
4-bromobenzylbromide (3.00 g, 12.0 mmol) with 1 equiv of dietha-
nolamine and Na2CO3 was carried out under the same conditions as
1
described above. Yield: 2.73 g (83%). H NMR in CDCl3 (δ from
TMS, in ppm): δ 7.46, 7.43 (d, 2 H, Ar-H); 7.28, 7.21 (d, 2 H, Ar-H);
3.65 (s, 2 H, ArꢀCH2-N); 3.62 (t, 4 H, N(CH2CH2OH)2); 2.69 (t,
4 H, (NCH2CH2OH)2).
[(BrBzNP2)Co(I)2] (Co-3). Reaction of the bromine-appended BrBzNP2
ligand (0.50 g, 0.82 mmol) with 0.26 g of CoI2 (0.82 mmol) was performed
as described above; the product crystallized from THF/hexane after
1ꢀ2 days. Yield: 700 mg (93%). UV/vis in THF, λ in nm (ε in
N,N-bis(2-bromoethyl)-4-bromobenzylamine. The reaction of bis-
hydroxy-4-bromobenzylamine (1.50 g, 5.49 mmol) with 2 equiv of PBr3
(2.07 mL, 11.0 mmol) was performed as described above, and the crude
product was isolated according to the same extraction procedure. The
product was purified via silica gel chromatography and eluted with
CH2Cl2:hexanes (1:2) to afford a colorless oil. Yield: 1.71 g (78%).1H
NMR in CDCl3 (δ from TMS, in ppm): δ 7.46, 7.44 (d, 2 H, Ar-H);
7.25, 7.22 (d, 2 H, Ar-H); 3.68 (s, 2 H, ArꢀCH2-N); 3.34 (t, 4 H,
N(CH2CH2Br)2); 2.95 (t, 4 H, (NCH2CH2Br)2).
M
ꢀ1 cmꢀ1): 778 (790), 718 (910), 486 sh (990), 363 (2 630). Anal.
Calcd. for C35H34NBrP2CoI2: C 45.53, H 3.71, N 1.52; found: C
44.93, H 3.76, N 1.71.
[(PhNP2)Co(I)2] (Co-4). Reaction of PhNP2 ligand (0.15 g, 0.29
mmol) with 0.09 g of CoI2 (0.29 mmol) was performed as described
above; the product crystallized from THF/hexane after 1ꢀ2 days. Yield:
220 mg (90%). UV/vis in THF, λ in nm (ε in Mꢀ1 cmꢀ1): 363 (950).
Anal. Calcd. for C34H33NP2CoI2: C 49.18, H 4.01, N 1.69; found: C
49.28, H 3.82, N 1.49.
N,N-bis(2-(diphenylphosphino)ethyl)-4-bromobenzylamine {BrBzNP2}.
The reaction ofthe bis-bromoethyl-benzylamine (1.03 g, 2.58 mmol) with
0.960 g of Ph2PH (5.15 mmol) and 0.640 g of tBuOK (5.68 mmol) in
[(P(NPh)P)Co(I)2] (Co-PNP). Reaction of the PNP type of ligand
P(NPh)P (0.30 g, 0.61 mmol) with 0.19 g of CoI2 (0.61 mmol) was
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Table 1. Crystal Data and Refinement Parameters for NP2-CoII Complexes
Co-1
Co-2
Co-3
Co-4
empirical formula
FW
C
36H37NOP2CoI2
C35H35NP2CoI2 C4H8O
C35H34NP2BrCoI2
923.21
C34H33NP2CoI2
830.28
3
874.34
916.41
color
green
green
green
dark green
block
habit
needle
blade
plate
size (mm)
T (K)
0.38 ꢁ 0.10 ꢁ 0.07
100(2)
0.20 ꢁ 0.10 ꢁ 0.07
100(2)
0.19 ꢁ 0.14 ꢁ 0.07
100(2)
0.31 ꢁ 0.23 ꢁ 0.11
100(2)
wavelength (Å)
lattice system
space group
a (Å)
0.71073
orthorhombic
P212121
9.2880(4)
13.5281(5)
27.2546(11)
90
0.71073
monoclinic
P21/c
0.71073
0.71073
triclinic
orthorhombic
P212121
P1
9.1301(5)
14.38705(7)
28.7247(14)
90
9.5306(4)
11.7513(5)
15.7593(7)
98.758(2)
93.890(2)
101.072(2)
1703.35(13)
2
11.7095(6)
14.2950(7)
19.3910(11)
90
b (Å)
c (Å)
α (deg)
β (deg)
γ (deg)
V (Å3)
90
94.000(2)
90
90
90
90
3424.5(2)
4
3763.9(3)
4
3237.6(3)
4
Z
dcalc (g/cm3)
1.696
1.617
1.800
1.703
μ (mmꢀ1
)
2.427
2.212
3.609
2.560
GOF on F2
final R indices
[I > 2σ(I)]
R indices
1.548
1.663
1.433
1.268
R1 = 0.0241
wR2 = 0.0376
R1 = 0.0283
wR2 = 0.0381
R1 = 0.0287
wR2 = 0.0411
R1 = 0.0427
wR2 = 0.0424
R1 = 0.0280
wR2 = 0.0381
R1 = 0.0445
wR2 = 0.0393
R1 = 0.0224
wR2 = 0.0316
R1 = 0.0306
wR2 = 0.0326
all data
performed as described above; the product crystallized from THF/
hexane after 3ꢀ4 days. Yield: 400 mg (82%). UV/vis in THF, λ in nm
(ε in Mꢀ1 cmꢀ1): 781 (630), 718 (790), 487 (1 170), 363 sh (3 170).
Anal. Calcd. for C32H29NP2CoI2: C 47.91, H 3.64, N 1.75; found: C
47.58, H 3.79, N 1.77.
337 sh (3 310), 294 sh (6 630). Anal. Calcd. for C36H37NOP2NiI2: C
49.46, H 4.27, N 1.60; found: C 49.05, H 4.15, N 1.48.
[(BrBzNP2)Ni(I)]I (Ni-3). Reaction of the bromine-appended BrBzNP2
ligand (0.20 g, 0.33 mmol) with 0.11 g of NiI2 (0.33 mmol) was
performed as described above; the product crystallized from THF/
hexane after 1ꢀ2 days. Yield: 270 mg (88%). 1H NMR in CDCl3
(δ from TMS, in ppm): δ 8.74, 8.44 (d, 2 H, Ar-H); 7.82ꢀ7.43 (m, 22 H,
Ar-H); 4.41 (s, 2 H, ArꢀCH2-N); 4.04 (m, 2 H, N(CH2CH2PPh2)2);
3.22 (m, 2 H, N(CH2CH2PPh2)2); 2.91ꢀ2.74 (m, 2 H, (NCH2-
CH2PPh2)2); 2.70ꢀ2.17 (m, 2 H, (NCH2CH2PPh2)2). 31P NMR
in CDCl3 (δ from H3PO4): δ ppm 35.46 (s). UV/vis in THF, λ in
nm (ε in Mꢀ1 cmꢀ1): 338 sh (2 660), 294 sh (7 140). Anal. Calcd.
for C35H34NBrP2NiI2: C 45.54, H 3.71, N 1.52; found: C 45.30,
H 3.83, N 1.40.
[(PhNP2)Ni(I)]I (Ni-4). Reaction of the PhNP2 ligand (190 mg,
0.36 mmol) with 120 mg of NiI2 (0.37 mmol) was performed as des-
cribed above; the product crystallized from THF/hexane after several
days. Yield: 260 mg (85%). 1H NMR in CDCl3 (δ from TMS, in ppm):
δ 8.09, 8.07 (d, 2 H, Ar-H); 7.64ꢀ7.43 (m, 23 H, Ar-H); 4.62 (m, 3 H,
N(CH2CH2PPh2)2); 3.46 (m, 3 H, (NCH2CH2PPh2)2); 2.68ꢀ2.64 (m,
2 H, (NCH2CH2PPh2)2). 31P NMR in CDCl3 (δ from H3PO4): δ
ppm 36.06 (s). UV/vis in THF, λ in nm (ε in Mꢀ1 cmꢀ1): 360 (1 610),
294 (3 720). Anal. Calcd. for C34H33NP2NiI2: C 49.20, H 4.01, N 1.69;
found: C 48.66, H 4.12, N 1.52.
[(BzNP2)Ni(I)]I (Ni-2). The BzNP2 ligand (0.15 g, 0.28 mmol) was
dissolved in 5 mL of degassed THF. Separately, anhydrous NiI2 (0.088 g,
0.28 mmol) was dissolved in 5 mL of THF by vigorously stirring and
then added dropwise to the stirred solution of the NP2 ligand at room
temperature; this immediately generated a red-violet color. The
solution was stirred for 12 h, and then filtered to remove the
unreacted NiI2. The red-violet solution was layered with 3 volumes
of degassed hexane and after 3ꢀ4 days, red crystals suitable for X-ray
diffraction were obtained. Yield: 210 mg (89%). 1H NMR in CDCl3
(δ from TMS, in ppm): δ 8.48ꢀ8.46 (m, 2 H, Ar-H); 7.86ꢀ7.42
(m, 23 H, Ar-H); 4.41 (s, 2 H, ArꢀCH2-N); 3.96 (m, 2 H,
N(CH2CH2PPh2)2); 3.26ꢀ3.21 (m, 2 H, N(CH2CH2PPh2)2);
2.98ꢀ2.94 (m, 2 H, N(CH2CH2PPh2)2); 2.71ꢀ2.64 (m, 2 H,
(NCH2CH2PPh2)2). 31P NMR in CDCl3 (δ from H3PO4): δ
ppm 35.65 (s). UV/vis in THF, λ in nm (ε in Mꢀ1 cmꢀ1): 341 sh
(4 230), 294 sh (8 780). Anal. Calcd. for C35H35NP2NiI2: C 49.80, H
4.18, N 1.66; found: C 49.97, H 3.99, N 1.80.
[(MeOBzNP2)Ni(I)]I (Ni-1). Reaction of the methoxy-appended
MeOBzNP2 ligand (83 mg, 0.15 mmol) with 46 mg of NiI2
(0.15 mmol) was performed as described above; the product crystal-
[(P(NPh)P)Ni(I)2] (Ni-PNP). Reaction of the PhP(NPh)PPh ligand
(0.30 g, 0.61 mmol) with 0.19 g of NiI2 (0.61 mmol) was performed
as described above. Yield: 410 mg (84%). 1H NMR in CDCl3 (δ from
TMS, in ppm): δ ppm, 9.25 (br m, 2 H); 7.37 (br d, 9 H); 7.06 (br m,
8 H); 6.82 (br m, 2 H); 6.13 (br m, 4 H); 4.68 (br s, 4 H). 31P NMR in
CDCl3 (δ from H3PO4): δ ppm ꢀ5.11 (s). Anal. Calcd. for
C32H29NP2NiI2: C 47.92, H 3.64, N 1.75; found: C 48.36, H 3.61,
N 1.97.
1
lized from THF/hexane after 3ꢀ4 days. Yield: 110 mg (83%). H
NMR in CDCl3 (δ from TMS, in ppm): δ 8.36, 8.33 (d, 2 H, Ar-H);
7.82ꢀ7.44 (m, 20 H, Ar-H); 6.92, 6.90 (d, 2 H, Ar-H); 4.34 (s, 2 H,
ArꢀCH2-N); 3.90 (m, 2 H, N(CH2CH2PPh2)2); 3.81 (s, 3 H, MeO);
3.17 (m, 2 H, N(CH2CH2PPh2)2); 2.92 (m, 2 H, (NCH2CH2PPh2)2);
2.74ꢀ2.67 (m, 2 H, (NCH2CH2PPh2)2). 31P NMR in CDCl3 (δ from
H3PO4): δ ppm 35.47 (s). UV/vis in THF, λ in nm (ε in Mꢀ1 cmꢀ1):
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Table 2. Selected Bond Distances (Å) and Bond Angles (deg) from Crystal Structures of the Cobalt Complexesa
Co-1
Co-2 (P21/c)
Co-2 (DFT)
Co-20
Co-3
Co-3 (DFT)
Co-4
CoꢀP1
2.3529(3)
2.3572(3)
2.57678(19)
2.54416(17)
3.690
2.3563(4)
2.3566(4)
2.5703(2)
2.5599(2)
3.674
2.339
2.333
2.569
2.583
3.658
114.68
107.12
115.40
2.1945(5)
2.2228(5)
2.6023(3)
2.7480(4)
2.0769(18)
161.57(2)
95.994(16)
95.229(12)
2.3761(3)
2.3990(3)
2.5766(17)
2.5650(17)
3.675
2.310b
2.354b
2.566
2.3511(3)
2.3420(3)
2.58189(16)
2.54249(15)
3.684
CoꢀP2
CoꢀI1
CoꢀI2
2.585
Co---N
3.633
P1ꢀCoꢀP2
PꢀCoꢀI (avg)
I1ꢀCoꢀI2
113.307(11)
107.284(9)
114.400(6)
114.376(14)
106.696(12)
115.503(8)
115.373(10)
106.594(8)
115.006(6)
115.65
106.57
114.96
113.829(9)
106.651(8)
116.386(5)
a Also included for comparison are DFT-optimized (6-31G*b/PW91) bond distances for Co-2 and Co-3. b Denotes a difference of greater than 0.05 Å.
Table 3. Crystal Data and Refinement Parameters for the NP2-NiII Complexes
Ni-1
Ni-2
Ni-3
Ni-4
empirical formula
FW
[C36H37NOP2INi]+Iꢀ
874.12
[C35H35INP2Ni]+Iꢀ CH2Cl2
[C35H34NP2BrINi]+Iꢀ
922.99
[C34H33NP2INi]+Iꢀ C4H8O
3
3
929.02
902.17
color
purple
deep purple
blade
dark purple
block
red
habit
block
block
size (mm)
T (K)
0.28 ꢁ 0.27 ꢁ 0.18
100(2)
0.20 ꢁ 0.18 ꢁ 0.10
100(2)
0.21 ꢁ 0.18 ꢁ 0.15
100(2)
0.29 ꢁ 0.28 ꢁ 0.15
100(2)
wavelength (Å)
lattice system
space group
a (Å)
0.71073
0.71073
0.71073
0.71073
triclinic
monoclinic
P21/n
triclinic
triclinic
P1
P1
P1
11.9939(5)
12.7222(5)
12.9714(9)
105.152(2)
100.967(2)
110.363(2)
1702.42(15)
2
8.9712(4)
24.7251(10)
16.2706(7)
90
11.9678(6)
12.1991(12)
13.2965(7)
103.056(4)
108.995(3)
102.700(4)
1695.0(2)
2
9.7033(3)
12.3287(4)
15.5781(5)
86.4940(10)
87.5220(10)
89.1920(10)
1858.24(10)
2
b (Å)
c (Å)
α (deg)
β (deg)
γ (deg)
V (Å3)
93.804(2)
90
3601.1(3)
4
Z
dcalc (g/cm3)
1.705
1.714
1.808
1.612
μ (mmꢀ1
)
2.507
2.517
3.693
2.299
GOF on F2
final R indices
[I > 2σ(I)]
R indices
2.336
1.675
2.002
2.081
R1 = 0.0282
wR2 = 0.0497
R1 = 0.0400
wR2 = 0.0502
R1 = 0.0299
wR2 = 0.0414
R1 = 0.0459
wR2 = 0.0426
R1 = 0.0329
wR2 = 0.0520
R1 = 0.0475
wR2 = 0.0529
R1 = 0.0282
wR2 = 0.0586
R1 = 0.0348
wR2 = 0.0601
all data
X-ray Crystallography. Crystals were mounted on a glass fiber
using Paratone oil, and then placed on the diffractometer under a stream
of N2 at 100 K. Refinement of F2 against all reflections: the weighted
R-factor (wR) and goodness of fit (S) were based on F2, conventional
R-factors (R) were based on F, with F set to zero for negative F2. The
threshold expression F2 >2σ(F2) was used only for calculating R-factors(gt)
and was not relevant to the choice of reflections for refinement. Diffraction
intensity data were collected on a Bruker Kappa APEX II diffractometer
equipped with a MoKα X-ray source, and data were collected using APEX2
v2009.7ꢀ0; the data reduction program SAINT-plus v7.66A was used.
Details of the data collections and refinements are given in Tables 1 and 3.
Magnetism. Magnetic susceptibilities for Co-1, Co-2, Co-3, and
Co-4 were recorded at 5000 G using a Quantum Designs SQUID
magnetometer controlled by MPMSR2 software. Data points acquired
from 4 to 300 K were corrected for diamagnetic contributions (Pascal’s
constants).19 The output was converted to effective magnetic moment
(μeff) and plotted as BM (μB) versus temperature (K).
Spectroscopy. The 1H, 13C, and 31P NMR spectra were recorded
on a Varian Mercury 300 MHz spectrometer, and chemical shifts were
referenced to TMS (1H, 13C) or H3PO4 (31P). EPR spectra were
recorded on a Bruker EMX Biospin spectrometer at 20 K using a Gunn
diode microwave source. UV/vis spectra were obtained at 298 K using a
Cary 50 spectrophotometer.
Density Functional Theory (DFT) Calculations. Geometry
optimization on the crystal structure coordinates as well as orbital
calculations were performed using the pure functional PW9120 and
the 6-31G* basis set (6-311G* for iodine) in the Firefly software
package.21 Orbitals were visualized using MacMolPlt,22 and spin density
plots were generated using gOpenMol.23
’ RESULTS AND DISCUSSION
Syntheses. We have synthesized a series of NP2 ligands
with a flexible {P-CH2CH2-N-CH2CH2-P} binding unit.
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Scheme 3
Scheme 4
Many reported PNP compounds were synthesized via a Man-
nich-type reaction with the corresponding phosphine, amines,
and CH2O, thereby providing methylene linked ligands. We
opted for a different approach to prepare ethylene-bridged
ligands. Our first targeted compound was the bis-hydroxyethyl
derivative. Reaction of benzylbromide (or 4-bromobenzylbromide)
with diethanolamine in acetone (Na2CO3 as base) afforded the
corresponding bis-hydroxyethyl derivatives in excellent yields.
Alternatively, the ꢀOMe appended bis-hydroxyethyl derivative
was produced by refluxing a mixture of 4-methoxylbenzylamine,
2-bromoethanol, and NEt3 in toluene for 5 h; N-phenyl-
diethanolamine is commercially available. Subsequent activation
of the alcohols with PBr3 generated the brominated derivatives
(71ꢀ78%), which upon disubstitution with diphenylphosphine
(tBuOK as base) afforded the NP2 ligands as air-stable, white
solids in very good yield (75ꢀ80%, Scheme 3).
Crystal Structures of the Cobalt Complexes. The CoII center
in each of the complexes of general formula [(RNP2)Co(I)2] is
coordinated by two phosphino-P donors provided by bidentate
NP2 as well as two iodide ions, adopting a tetrahedral geometry
(Figure 1; selected bond lengths and bond angles are in Table 2.)
Crystal data and refinement parameters for NP2-CoIIcomplexes are
collected in Table 1. The overall structures of these four-coordinate
CoII complexes are pseudotetrahedral with P1ꢀCoꢀP2 (113.307°ꢀ
115.373°), PꢀCoꢀI (avg) (106.594°ꢀ107.284°) and I1ꢀCoꢀI2
(114.400°ꢀ116.386°) bond angles that deviate only slightly from
ideal values. The CoꢀP bond lengths (2.3511(3)ꢀ2.3761(3) Å)
are longer than those observed in complexes bearing a
bidentate P2N2 ligand, such as [(PPh2NPh2)Co(MeCN)3]2+
(2.198(5) Å) or [(PPh2NPh2)2Co(Cl)]+ (2.2133(8) and
2.2368(8) Å).15 They also are longer than those in [(PhBP3)-
Co(I)] (BP3ꢀ = PhB(CH2P(Ph)2)3ꢀ) (2.200(2), 2.206(2), and
2.282(2) Å).24 The CoꢀI bond lengths (avg CoꢀI = 2.565 Å) are
longer than those observed in [(PhBP3)Co(I)] (2.488(1) Å) and
[(PhBPiPr3)Co(I)] (2.540(1) Å), which may be attributable to the
different number of coordinated iodides in each case. The distances
between the CoII center and the unbound N donor lie in the narrow
range 3.674ꢀ3.690 Å.
Metalation of 1 equiv of a given NP2 ligand with anhydrous
CoI2 in THF at room temperature afforded a dark brown solu-
tion (Scheme 4). Crystals of the resulting CoII complexes of
general formula [(RNP2)Co(I)2] were isolated in nearly quanti-
tative yield (81ꢀ92%). The NiII species also were prepared by
the reaction of 1 equiv of the NP2 ligands with anhydrous NiI2
in THF under inert atmosphere. The resulting violet solutions
generated copious amounts of purple solid that could be
recrystallized from THF/hexane. X-ray crystallography (see
below) identified these complexes as the cationic species of
general formula [(RNP2)Ni(I)]+ in very good yield (82ꢀ
The related P2N2 and PNP ligands are methylene linked. In a
P-N-P bonded chelate, these ligands would generate two un-
favorable 4-membered rings among the {Co-P-CH2-N-(Co)}
atoms. The NP2 ligands, with an ethylene linker between the
phosphorus and nitrogen atoms, could form two ideal {Co-P-
CH2-CH2-N-(Co)} 5-membered rings; in most cases (vide infra)
the nitrogen remains unbound, indicating that the tetrahedral
cobalt center prefers phosphorus ligation.
1
90%). The NiII complexes exhibit sharp H and 31P NMR
spectra in the diamagnetic region, consistent with an S = 0
ground state.
10218
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Figure 1. ORTEP diagrams (50% thermal ellipsoids): [(MeOBzNP2)Co(I)2] (Co-1); [(BzNP2)Co(I)2] (Co-2); [(BrBzNP2)Co(I)2] (Co-3);
[(PhNP2)Co(I)2] (Co-4).
Figure 2. Ball and stick diagram of the asymmetric part of the unit cell (Cc crystal structure) containing [(BzNP2)Co(I)2] structural conformers: Co-2,
tetrahedral (left side); Co-20, square pyramidal (right side).
One notable exception is the five-coordinate CoII complex
obtained from the reaction of CoI2 with 2 equiv of BzNP2. In this
case, the complex crystallized in a different space group (Cc), in
contrast to the crystal structure that contains only the tetrahedral
complex described above (P21/c). Figure 2 displays the asym-
metric part of the Cc unit cell, which contains two structural
conformers denoted Co-2 (tetrahedral) and Co-20 (square
pyramidal). In the case of Co-20 (right side), the cobalt center
adopts a square pyramidal geometry because of binding of the
benzylamino-N donor. The Iꢀ occupies the apical position, while
the other Iꢀ, two phosphorus atoms and nitrogen reside in the
basal plane. The CoꢀN bond length (2.0769(18) Å) is
much shorter than {Co- - -N} in the tetrahedral Co complexes
(∼3.7 Å), but slightly longer than the CoꢀNMeCN bond lengths
10219
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Figure 3. ORTEP diagrams (50% thermal ellipsoids): [(MeOBzNP2)Ni(I)]I (Ni-1); [(BzNP2)Ni(I)]I (Ni-2); [(BrBzNP2)Ni(I)]I (Ni-3);
[(PhNP2)Ni(I)]I (Ni-4).
distances are in Table 4. The N-donor in RNP2 is bound to NiII,
Table 4. Selected Bond Distances (Å) and Bond Angles (deg)
even in the case of PhNP2 (Ni-4). In each case, one Iꢀ ligand
occupies the fourth coordination site, while the other Iꢀ is an
outer coordination counterion. Ni-1, Ni-3, and Ni-4 exhibit
P1ꢀNiꢀP2 and NꢀNiꢀI1 angles in the range 164.876(10)ꢀ
178.26(3)°; Ni-2 exhibits a more distorted square planar geo-
metry, with P1ꢀNiꢀP2 and NꢀNiꢀI1 angles of 157.662(13)°
and 166.37(3)°, respectively. The dihedral angle P1ꢀNiꢀI1/
P2ꢀNiꢀN in Ni-2 is 24.95°. The NiꢀP bonds (2.2039(3) and
2.2319(3) Å) in Ni-2 are elongated by comparison to the other
distorted Ni complexes. The NiꢀP bond lengths (Ni-1 = 2.203,
Ni-2 = 2.221, Ni-3 = 2.205, Ni-4 = 2.222 Å) are very close to
those in [(PNP)Ni(dmpm)](BF4)2 (NiꢀPavg ≈ 2.21 Å).25
ORTEP diagrams for Ni-PNP and Co-PNP complexes are
shown in Figure 4 (for crystal data and refinement parameters see
Supporting Information, Table S1). In contrast to Ni-1 to Ni-4,
only P-donors are coordinated in Ni-PNP, as the aniline
N-donor is approximately 3.8 Å away from the metal center.
The NiꢀP bonds in Ni-PNP (NiꢀP1 = 2.1833(3), NiꢀP2 =
2.1760(3) Å) are shorter than the average NiꢀP length in Ni-
1ꢀNi-4 (NiꢀP1 = 2.2126(3), NiꢀP2 = 2.2114(3) Å), most
likely because of the “pinched” geometry in the methylene-linked
PNP. As a result, the NiꢀI1 bond lengths in Ni-PNP
(2.53658(15) Å) are longer than the average NiꢀI1 distance
in Ni-1ꢀNi-4 (2.4806(6) Å), likely attributable to a stronger
trans effect of the more closely bonded P-donors. We suggest that
from Crystal Structures of NiII Complexes
Ni-1
Ni-2
Ni-3
Ni-4
Ni-PNP
NiꢀP1
NiꢀP2
NiꢀI1
NiꢀI2
NiꢀN
2.2139(2) 2.2039(3) 2.2062(4) 2.2264(4) 2.1833(3)
2.1916(2) 2.2319(3) 2.2039(4) 2.2181(4) 2.1760(3)
2.4739(2) 2.4744(16) 2.4871(3) 2.4870(2) 2.53658(15)
2.53083(14)
2.0051(7) 1.9864(8) 1.9887(11) 1.9911(12) 3.771
P1ꢀNiꢀP2 164.876(10) 157.662(13) 175.916(15) 173.670(16) 93.381(10)
P1ꢀNiꢀI1 94.251(7) 92.936(8) 91.784(11) 94.559(11) 176.719(9)
P1ꢀNiꢀI2
174.479(9)
NꢀNiꢀI1 177.22(2) 166.37(3) 174.92(3) 178.26(3)
in [Co(PPh2NPh2)(CH3CN)3](BF4)2 (1.952, 2.078, 1.943 Å)15
or [Co(PtBu2NBz2)(CH3CN)3](BF4)2 (1.961, 1.955, 2.059 Å).14
The CoꢀP distances are both shortened (2.1945(5) and
2.2228(5) Å) once the N-donor is bound. On the other hand,
the CoꢀI bond lengths are elongated (2.6023(3) and 2.7480(4)
Å). The structure of the Cc tetrahedral Co-2 is nearly identical
with that in the P21/c crystal.
Crystal Structures of the Nickel Complexes. Table 3 shows
crystal data and refinement parameters for the NP2-NiII com-
plexes. ORTEP diagrams for the four square planar NP2-NiII
R
complexes are shown in Figure 3; selected bond angles and bond
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Figure 4. ORTEP diagrams (50% thermal ellipsoids): [(P(NPh)P)Co(I)2] (Co-PNP); [(P(NPh)P)Ni(I)2] (Ni-PNP).
Figure 5. DFT (6-31G*/PW91) spin density plot for the S = 3/2
ground state of the geometry optimized Co-2 structure.
Figure 6. Plots of μeff (BM) versus T: [(OMeBzNP2)Co(I)2], Co-1,
(]); [(BrBzNP2)Co(I)2], Co-3, (O); [(PhNP2)Co(I)2], Co-4, (0).
Inset: [(BzNP2)Co(I)2], Co-2, (Δ).
the nitrogen atom of the NP2 ligand acts as a “claw” to pull NiII
back, thus forming two 5-membered rings when the nitrogen is
bound to the metal center. The corresponding CoII structure is
unremarkable, as Co-PNP exhibits a similar P-only coordination
mode as in Ni-PNP and Co-1ꢀCo-4. Thus, the dual coordina-
tion modes of ethylene-linked RNP2 ligands are not observed in
P-only complexes containing PNP.
DFT Calculations. We performed DFT calculations on Co-2
and Co-3 as model compounds. Geometry optimization at the
6-31G*/PW91 (S = 3/2) level afforded bond distances and bond
angles very similar to those found in the crystal structure, with no
differences in bond lengths >0.03 Å (Table 2). DFT calculations
on Co-3 gave similar results, although the CoꢀP bonds in the
calculation (2.310, 2.354 Å) were significantly shorter than those
in the crystal (2.3761(3), 2.3990(3) Å); the CoꢀI bonds,
however, were in good agreement (diff <0.02 Å). In the DFT
calculation, the S = 3/2 ground state of Co-2 is approximately
25 kcal/mol more stable than the S = 1/2 state, as observed in some
tetrahedral CoII complexes bearing anionic phosphine ligands.24
The spin density of Co-2 (shown in Figure 5) is consistent with that
expected for a nearly ideal Td system, a result in accord with SQUID
data (vide infra). It was necessary to establish unambiguously the S=
3/2 ground state in the solid state by SQUID and DFT calculations
in light of unexpected EPR results (vide infra).
range 4 to 300 K using SQUID magnetometry (Figure 6).
Complexes Co-3 and Co-4 exhibit little variation in μeff in the
range 25 to 300 K. At low temperature (4 K), the magnetic
moments are 4.03 μB and 4.07 μB, respectively, in reasonable
agreement with the spin-only value (3.87 μB) for an S = 3/2
system. Such an S = 3/2 ground state is typically observed for
tetrahedral CoII complexes.26ꢀ29
The magnetic data for Co-2 are more difficult to interpret: μeff
decreases slowly from 300 (4.61) to 110 K (4.24 μB), then
increases from 110 to 20 K. Nonetheless, the 4 K magnetic
moment for Co-2 of 4.04 μB is nearly identical with those for Co-
3 and Co-4. The data for Co-2 were obtained using crystals from
the 1:1 ratio (BzNP2:Co) reaction (the P21/c tetrahedral-only
structure). It is possible that deviation from ideal CurieꢀWeiss
behavior is due to trace quantities of Cc crystals (obtained from
the 2:1 ratio reaction of BzNP2:Co) that contain structural
conformers Co-2/Co-20 (vide supra). We conclude that the
RNP2-CoII complexes are high-spin (S = 3/2) in the solid state,
as expected for tetrahedral and pseudotetrahedral structures.30
EPR. We recorded EPR spectra for Co-1 and Co-3 in the solid
state at 4 K (Supporting Information, Figure S1). In each case,
the spectrum is dominated by high-spin CoII signals near g ≈ 6 in
accord with the SQUID data. Peters and co-workers also
reported consistent SQUID and solid state EPR results for both
Magnetic Susceptibilities. We determined magnetic suscept-
ibilities for Co-1, Co-2, Co-3, and Co-4 over the temperature
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Figure 8. EPR spectra of [(BzNP2)Co(I)2] (Co-2) and [(PhNP2)Co(I)2]
(Co-4) in frozen MeCN/tol glass at 20 K.
Figure 7. EPR spectra for CoII complexes in frozen THF at 20 K
(from top to bottom): [(PhNP2)Co(I)2] (Co-4); [(BrBzNP2)Co(I)2]
(Co-3); [(BzNP2)Co(I)2] (Co-2); [(MeOBzNP2)Co(I)2] (Co-1).
Microwave frequency, 9.453 GHz; field modulation, 20 G; microwave
power, 2 mW; frequency modulation, 100 kHz.
Scheme 5. Coordination Modes of NP2 CoII Complexes in
Solutions
{[PhBP3]Co(μ-Br)}2 and {[PhBP3]Co(μ-Br)}2.24 The 20 K
EPR spectra of the four CoII complexes in THF are shown in
Figure 7: Co-4 exhibits a primary feature at g = 6.2 (Figure 7, top
spectrum) typical for distorted tetrahedral S = 3/2 systems.
There is a systematic increase in the intensities of g ≈ 2
features with increasing N-donor strength (Ph-N < BrBz-N < Bz-
N < OMeBz-N in Co-4 < Co-3 < Co-2 < Co-1), and a con-
comitant decrease in the broad high-spin system near g ≈ 6. As a
result, the EPR spectrum of [(OMeBzNP2)Co(I)2] (Co-1) at the
opposite end of the donor strength trend is dominated by a
rhombic, low-spin signal with primary features at g = 2.52 and
2.19. Such a spectrum is consistent with low-spin (S = 1/2) CoII
with vastly different axial ligands (e.g., anionic vs neutral; or
neutral vs unoccupied site). The lineshapes of the low-spin
signals in Co-1, Co-2, and Co-3 closely follow those of a related
square pyramidal CoII phosphine complex[(PP3)Co(MeCN)]2+
(g = 2.11).31 Similar features were observed for low-spin
complexes [Co(PPh2NPh2)2(CH3CN)](BF4)2 (g = 2.11) and
[Co(dppe)2(CH3CN)](BF4)2 (g = 2.20) that also are square
pyramidal.15,30,31 Thus, it is likely that [(OMeBzNP2)Co(I)2]
(Co-1) has adopted a square pyramidal coordination geometry
in THF.
We suggest that the two conformers exist in solution whose
relative populations are influenced by the ligand substituent.
Correspondingly, the EPR spectra in Figure 7 clearly show that as
the N-substituent becomes more strongly donating, the EPR
signal attributable to the square pyramidal conformer is domi-
nant. The ꢀOMe appended ligand strongly enforces the five-
coordinate geometry of [(OMeBzNP2)Co(I)2] (Co-10). In con-
trast, the [(PhNP2)Co(I)2] (Co-4) EPR spectrum is consistent
only with a tetrahedral geometry, while those of Co-3 and Co-2
can be interpreted in terms of mixtures of conformers. It is
apparent that subtle differences in ligand substitutent can tune
the coordination modes of NP2-type ligands.
spectrum) in MeCN/tol (3:1, 20 K) that is dominated by low-
spin features (g = 2.42, 2.22) that are consistent with a five-
coordinate geometry. The complex [(BzNP2)Co(I)2] (Co-2)
in frozen MeCN/tol also exhibits exclusively low-spin features
(g = 2.43, 2.22; Figure 8, bottom spectrum). In both cases, it is
likely that conversion to the square pyramidal (or octahedral)
conformer is facilitated by coordination of MeCN. For compar-
ison, [(PP3)Co(MeCN)]2+, [Co(PPh2NPh2)2(CH3CN)]2+, and
[Co(dppe)2(CH3CN)]2+ each exhibit coordinated MeCN in a
square pyramidal geometry. It is well-known that MII centers in
octahedral or square pyramidal geometries have a strong preference
for back-bonding ligands such as MeCN, CNꢀ, and py because of
strong {dπ(M)|π*(L)} overlap. This is in contrast to the relatively
low affinity for tetrahedral MII centers for back-bonding ligands.
In our case, coordination of the N-donor of the NP2 backbone
is likely driven by the high stability of the two 5-membered P-N-P
chelate rings that form the equatorial plane. Once a square pyra-
midal unit is established, MeCN could facilitate the formation
of octahedral geometry bearing the NP2 and Iꢀ donors in the
Frozen MeCN/tol Glass. We observed further dynamic beha-
vior of the NP2 ligands in EPR experiments performed in MeCN.
The complex derived from the weakest N-donor, [(PhNP2)-
Co(I)2] (Co-4), displays an EPR spectrum (Figure 8, top
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different high-spin/low-spin ratios that correlate with the donor
strength of the ligand.
In summary, we have synthesized a series of NP2 type of
ligands and corresponding CoII and NiII complexes. From the
crystal structures of these metal complexes, we have found that
NP2 ligands exhibit dual coordination modes. The dual coordi-
nation modes are due to the geometric preference of the metal
center: CoII (tetrahedral) and NiII (square planar), best exhibited
by the unusual crystal Cc structure containing Co-2/Co-20 with
two conformers in a single unit cell. We intend to exploit this facile
N-donor ligation/deligation property in designs of MII complexes
for reactions involving activation of organic substrates.
’ ASSOCIATED CONTENT
S
Supporting Information. Crystal structure data for all
b
complexes (CIF files), and solid state EPR spectra of Co-1 and
Co-3 (Figure S1). This material is available free of charge via the
Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: mrose@caltech.edu.
’ ACKNOWLEDGMENT
This work was supported by the NSF CCI Solar Fuels
Research Program (CHE-0802907). M.J.R. also received support
from an NSF ACC-F postdoctoral fellowship (NSF CHE-
1042009). W.Y.W. and Q.C.D. thank the Hong Kong Research
Grants Council (HKBU202508), the University Grants Com-
mittee of HKSAR, China (Project No. [AoE/P-03/08]) and
Hong Kong Baptist University for support. We thank Drs. Larry
Henling and Michael Day for assistance in solving crystal
structures, and Dr. Angelo Di Bilio for assistance in recording
EPR spectra. The Bruker KAPPA APEXII X-ray diffractometer
was purchased via an NSF CRIF:MU award to the California
Institute of Technology (CHE-0639094).
Figure 9. (Top) UV/vis spectra of Co-3 measured in THF at 25
and ꢀ80 °C. (Bottom) UV/vis spectra of Co-3 in MeCN at 25, 0, and
ꢀ40 °C.
equatorial plane, while MeCN and the second Iꢀ occupy axial
positions (see Scheme 5). An octahedral formulation is consis-
tent the finding that all NP2 cobalt complexes exhibit low-spin
EPR signals in MeCN. Overall, the EPR experiments illustrate
the role of solvent (MeCN versus THF) in modulating the dual
coordination modes of NP2 ligands.
UV/vis Absorption Spectra. We also investigated the spin
states of the cobalt complexes by UV/vis spectrophotometry in
THF and MeCN. In THF solution at 25 °C, all of the complexes
exhibit the following: Co-3 exhibits peaks at 780 and 720 nm
(790 and 910 Mꢀ1 cmꢀ1, respectively), which we assign high-
spin CoII LF transitions (Figure 9, top). The same solution at
ꢀ80 °C exhibits slightly sharper features at 785 and 725 nm of
comparable intensity, suggesting that even at this low tempera-
ture Co-3 remains high-spin in THF. In contrast, the spectrum of
Co-3 in MeCN (25 °C) exhibits a peak near 800 nm that is much
less intense (∼250 Mꢀ1 cmꢀ1). Furthermore, as the MeCN
solution of Co-3 is cooled first to 0 °C and then ꢀ40 °C, the
feature near 800 nm disappears (Figure 9, bottom), consistent
with the finding that only a low-spin CoII signal is observed in
EPR experiments in MeCN at low temperatures (20 to 80 K).
We conclude that there is a high/low-spin equilibrium (as
evident in the EPR spectra) in MeCN solution, and that the
low-spin state is strongly favored at lower temperatures (below
0 °C). In contrast, THF solutions of Co-3 must be cooled to
much lower temperatures (less than ꢀ80 °C) to observe even
partial populations of the low-spin conformer (EPR experiments).
We found similar behavior for Co-1, Co-2, and Co-4 with slightly
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