Dinuclear Nickel(II) Complexes of Phenol-Based End-Off Compartmental Ligands and Their Urea Adducts Relevant to the Urease Active Site

Article abstract of DOI:10.1021/ic971209p

Two phenol-based compartmental ligands of the end-off type, 2,6-bis{N-[2-(dimethylamino)ethyl]iminomethyl}-4-methylphenol (HL¹) and 2-{N[2-(dimethylamino)ethy]iminomethyl}-6-{N-methyl-N-[2-(dimethylamino)ethyl]- aminomethy}-4-bromophenol (HL² ), have been used to form [Ni₂ (L¹)(AcO)(NCS)₂ (MeOH)] (1), [Ni₂ (L¹)(AcO)₂ -(MeOH)]BPh₄ (2), [Ni₂ (L²)(AcO)(NCS)₂ (MeOH)] (3), [Ni₂ (L²)(AcO)₂ ]BPh₄ (4), and [Ni₂ (L²)(NCS)₃ (MeOH)] (5). X-ray crystallographic studies were done for 2-5. 2: triclinic, space group P1?, a = 13.613(3) ?, b = 16.475(4) ?, c = 11.307(4) ?, α = 99.90(2)°, β= 104.16(2)°, γ = 109.01(2)°, V = 2253(1) ?³Z = 2. The complex cation has a dinuclear core triply bridged by the phenolic oxygen of (L¹)^- and two acetate groups in the syn-syn mode. One Ni has a six-coordinate geometry together with a methanol oxygen. 3: triclinic, space group P1?, a = 10.167(1) ?, b = 16.119(2) ?, c = 9.472(3) ?, α = 103.53(2)°, β=100.91(1)°, γ = 85.62(1)°, V = 1481(1) ?³, Z = 2. A pair of Ni ions are bridged by the phenolic oxygen of (L²)^-, an isothiocyanate nitrogen, and an acetate group. The sixth position of one Ni is occupied by a methanol oxygen and that of the other Ni by isothiocyanate nitrogen. 4: monoclinic, space group P2₁/n, a = 15.146(2) ?, b = 9.442(3) ?, c = 30.844(2) ?, β= 93.427(9)°, V = 4402(1) ?³, Z = 4. Two Ni ions are bridged by the phenolic oxygen of (L²)^- and an acetate group. The Ni bound to the iminic pendant arm is nearly planar whereas the Ni bound to the aminic pendant arm is of six-coordination together with a bidentate acetate group. 5: monoclinic, space group C2/c, a = 26.744(5), b = 10.323(4) ?, c = 21.785(6) ?, β= 94.73(2)°, V = 5993(3) ?³, Z = 8. Two Ni ions are bridged by the phenolic oxygen of (L²)^- and an isothiocyanate nitrogen. The Ni bound to the iminic pendant arm is of five-coordination along with an isothiocyanate nitrogen. The Ni bound to the aminic pendant arm has a six-coordinate geometry together with a methanol oxygen and an isothiocyanate nitrogen. 1-5 were examined regarding their ability to bind urea, and 4 and 5 were shown to form [Ni₂(L²)(AcO)2(urea)]BPh₄ (4′) and [Ni₂-(L²)(NCS)₃(urea)] (5′), respectively. 5′ crystallizes in the monoclinic system, space group P2₁/n, with a = 11.709-(5), b = 15.100(4), c = 17.548(5) ?, β= 95.58(3)°, V = 3087(1) ?³, and Z = 4. Its dinuclear core is very similar to that of 5 except that the methanol of 5 is replaced by a urea molecule.

Full text of DOI:10.1021/ic971209p

Inorg. Chem. 1998, 37, 989-996
989
Dinuclear Nickel(II) Complexes of Phenol-Based “End-Off” Compartmental Ligands and
Their Urea Adducts Relevant to the Urease Active Site
Takeichiro Koga, Hideki Furutachi, Takako Nakamura, Nobuo Fukita, Masaaki Ohba,
Kazuhiro Takahashi, and Hisashi Oh kawa*
Department of Chemistry, Faculty of Science, Kyushu University, Hakozaki, Higashiku,
Fukuoka 812, Japan
ReceiVed September 19, 1997
Two phenol-based compartmental ligands of the “end-off” type, 2,6-bis{N-[2-(dimethylamino)ethyl]iminomethyl}-
4-methylphenol (HL1) and 2-{N-[2-(dimethylamino)ethyl]iminomethyl}-6-{N-methyl-N-[2-(dimethylamino)ethyl]-
aminomethyl}-4-bromophenol (HL2), have been used to form [Ni2(L1)(AcO)(NCS)2(MeOH)] (1), [Ni2(L1)(AcO)2-
(MeOH)]BPh4 (2), [Ni2(L2)(AcO)(NCS)2(MeOH)] (3), [Ni2(L2)(AcO)2]BPh4 (4), and [Ni2(L2)(NCS)3(MeOH)]
(5). X-ray crystallographic studies were done for 2-5. 2: triclinic, space group P1h, a ) 13.613(3) Å, b )
16.475(4) Å, c ) 11.307(4) Å, R ) 99.90(2)°, â ) 104.16(2)°, γ ) 109.01(2)°, V ) 2253(1) Å3, Z ) 2. The
complex cation has a dinuclear core triply bridged by the phenolic oxygen of (L1)- and two acetate groups in the
syn-syn mode. One Ni has a six-coordinate geometry together with a methanol oxygen. 3: triclinic, space group
P1h, a ) 10.167(1) Å, b ) 16.119(2) Å, c ) 9.472(3) Å, R ) 103.53(2)°, â)100.91(1)°, γ ) 85.62(1)°, V )
1481(1) Å3, Z ) 2. A pair of Ni ions are bridged by the phenolic oxygen of (L2)-, an isothiocyanate nitrogen,
and an acetate group. The sixth position of one Ni is occupied by a methanol oxygen and that of the other Ni
by isothiocyanate nitrogen. 4: monoclinic, space group P21/n, a ) 15.146(2) Å, b ) 9.442(3) Å, c ) 30.844(2)
Å, â ) 93.427(9)°, V ) 4402(1) Å3, Z ) 4. Two Ni ions are bridged by the phenolic oxygen of (L2)- and an
acetate group. The Ni bound to the iminic pendant arm is nearly planar whereas the Ni bound to the aminic
pendant arm is of six-coordination together with a bidentate acetate group. 5: monoclinic, space group C2/c, a
) 26.744(5), b ) 10.323(4) Å, c ) 21.785(6) Å, â ) 94.73(2)°, V ) 5993(3) Å3, Z ) 8. Two Ni ions are
bridged by the phenolic oxygen of (L2)- and an isothiocyanate nitrogen. The Ni bound to the iminic pendant
arm is of five-coordination along with an isothiocyanate nitrogen. The Ni bound to the aminic pendant arm has
a six-coordinate geometry together with a methanol oxygen and an isothiocyanate nitrogen. 1-5 were examined
regarding their ability to bind urea, and 4 and 5 were shown to form [Ni2(L2)(AcO)2(urea)]BPh4 (4′) and [Ni2-
(L2)(NCS)3(urea)] (5′), respectively. 5′ crystallizes in the monoclinic system, space group P21/n, with a ) 11.709-
(5), b ) 15.100(4), c ) 17.548(5) Å, â ) 95.58(3)°, V ) 3087(1) Å3, and Z ) 4. Its dinuclear core is very
similar to that of 5 except that the methanol of 5 is replaced by a urea molecule.
Introduction
It is considered that urease employs a nonequivalent pair of
nickel ions to facilitate the concerted binding of the substrate
Urease is a nickel-dependent metalloenzyme that catalyzes
the hydrolysis of urea to ammonia and carbon dioxide,1 in
conjunction with biological functions using urea as a nitrogen
source.2 It was first isolated from jack beans in 19263 and later
at one center and nucleophilic attack of water or hydroxide at
the other center.5,6 It is noted that the water attached to Ni2 is
situated close to Ni1, suggesting a possible formation of a (µ-
hydroxo)(µ-carboxylato)dinickel core under physiological con-
ditions. Such dinuclear cores bridged by one oxygenic ligand
(oxo, hydroxo, or water) and one or two carboxylate groups
are widely known in metallobiosites.7 Some urease models are
provided8-13 and urea adducts of dinickel complexes are
from various origins such as bacteria, fungi, and plants.1
A
recent X-ray crystallographic study on microbial urease from
Klebsiella aerogenes has identified a dinickel active site.4 The
two nickel ions are bridged by a carboxylate group of a
carbamylated lysine residue with a Ni--Ni separation of 3.5 Å.
The geometrical structure around one nickel (Ni1) is pseudo-
tetrahedral with two histidine nitrogens, an oxygen of the
carbamate bridge, and a water molecule; the fourth position is
only partially occupied by a water molecule and considered to
be a likely candidate for binding urea in the catalysis. The
geometry around another nickel (Ni2) is depicted as trigonal
bipyramidal with further coordination of an asparagine oxygen.
(5) Dixon, N. E.; Riddles, P. W.; Gazzola, C.; Blakeley, R. L.; Zerner,
B. Can. J. Biochem. 1980, 58, 1335.
(6) Lippard, S. J. Science 1995, 268, 996.
(7) Fenton D. E.; Ohkawa, H. In PerspectiVes on Bioinorganic Chemistry,
Vol. 2; Hay, R. W., Dilworth, J. R., Nolan, K. B., Eds.; JAI Press:
London, 1993; pp 81-138.
(8) Chaudhuri, P.; Ku¨ppers, H.-J.; Wieghardt, K.; Gehring, S.; Haase, H.;
Nuber, B.; Weiss, J. J. Chem. Soc., Dalton Trans. 1988, 1367.
(9) Buchanan, R. M.; Mashuta, M. S.; Oberhausen, K. J.; Richardson, J.
F.; Li, Q.; Hendrickson, D. N. J. Am. Chem. Soc. 1989, 111, 4497.
(10) Wages, H. E.; Taft, K. L.; Lippard, S. L. Inorg. Chem. 1993, 32, 4985.
(11) Stemmler, A. J.; Kamph, J. W.; Kirk, M. L.; Pecoraro, V. L. J. Am.
Chem. Soc. 1995, 117, 6368.
(1) Hausinger, R. P. Biochemistry of Nickel; Plenum Press: New York,
1993; Chapter 12, pp 23-50.
(2) Mobley, H. L. T.; Hausinger, R. P. Microbiol. ReV. 1989, 53, 85.
(3) Sumner, J. B. J. Biol. Chem. 1926, 69, 435.
(4) Jabri, E.; Carr, M. B.; Hausinger, R. P.; Karplus, P. A. Science 1995,
268, 998.
(12) Volkmer, D.; Ho¨rstmann, A.; Griesar, K.; Haase, W.; Krebs, B. Inorg.
Chem. 1996, 35, 1132.
S0020-1669(97)01209-3 CCC: $15.00 © 1998 American Chemical Society
Published on Web 02/12/1998

990 Inorganic Chemistry, Vol. 37, No. 5, 1998
Chart 1. Chemical Structures of (L1)- and (L2)-
Koga et al.
Preparation of Nickel Complexes. (a) [Ni2(L1)(AcO)(NCS)2-
(MeOH)] (1). A solution of 2,6-diformyl-4-methylphenol (0.164 g,
1.0 mmol) and N,N-dimethylethylenediamine (0.176 g, 2.0 mmol) in
methanol (5 cm3) was refluxed for 10 min. To the resulting yellow
solution of HL1 was added nickel(II) acetate tetrahydrate (0.498 g, 2.0
mmol), and the mixture was refluxed for 30 min. The addition of
sodium thiocyanate (0.162 g, 2.0 mmol) resulted in the precipitation
of green microcrystals. Yield: 0.310 g (49.3%). Anal. Found: C,
42.08; H, 5.53; N, 13.27; Ni, 18.4. Calcd for C22H34N6Ni2O4S2: C,
42.07; H, 5.46; N, 13.38; Ni, 18.7. Selected IR data (ν/cm-1) using
KBr disk: 3000-2800, 2080, 2020, 1660, 1570, 1540, 1460. Molar
conductance: nonelectrolyte in CH2Cl2. Visible spectral data [λ/nm
(ꢀ/M-1cm-1)] in CH2Cl2: 610 (53), 780 (30), 880 (65).
(b) [Ni2(L1)(AcO)2(MeOH)]BPh4 (2). This complex was prepared
as green microcrystals by a method similar to that for 1 except for the
use of sodium tetraphenylborate instead of sodium thiocyanate.
Yield: 80.4%. Anal. Found: C, 62.33; H, 6.35; N, 6.38; Ni, 12.9.
Calcd for C46H57BN4Ni2O6: C, 62.07; H, 6.45; N, 6.29; Ni, 13.4.
Selected IR data (ν/cm-1) using KBr disk: 3520, 3050-2800, 1640,
1600, 1550, 1430, 740, 720. Molar conductance (ΛM/S cm2 mol-1) in
CH2Cl2: 46. Visible spectral data [λ/nm (ꢀ/M-1 cm-1)] in CH2Cl2:
655 (58), 780 (25), 880 (65).
(c) [Ni2(L2)(AcO)(NCS)2(MeOH)] (3). A solution of 5-bromo-3-
{N-[2-(dimethylamino)ethyl]-N-methyl}aminomethylsalicylaldehyde
(0.314 g, 1.0 mmol) and N,N-dimethylethylenediamine (0.088 g, 1.0
mmol) in methanol (5 cm3) was refluxed for 10 min. To the resulting
yellow solution of HL2 was added nickel(II) acetate tetrahydrate (0.498
g, 2.0 mmol), and the mixture was refluxed for 30 min. The addition
of sodium thiocyanate (0.162 g, 2.0 mmol) resulted in the precipitation
of green microcrystals. Yield: 0.320 g (45.2%). Anal. Found: C,
37.34; H, 5.07; N, 11.70; Ni, 16.7. Calcd for C22H35BrN6Ni2O4S2: C,
37.27; H, 4.98; N, 11.85; Ni, 16.6. Selected IR data (ν/cm-1) using
KBr disk: 3520, 3000-2800, 2075, 2025, 1650, 1580, 1550, 1450.
Molar conductance: nonelectrolyte in CH2Cl2. Visible spectral data
[λ/nm (ꢀ/M-1 cm-1)] in CH2Cl2: 615 (45), 780 (20), 875 (40).
reported.10 Recently, catalytic ethanolysis of urea by a (µ-
alkoxo)(µ-carboxylato)dinickel(II) complex was was reported.14
To provide further analogues of the urease active site, we
have studied dinuclear nickel complexes derived from two
phenol-based end-off compartmental ligands, 2,6-bis{N-[2-
(dimethylamino)ethyl]iminomethyl}-4-methylphenol (HL1)15 and
2-{N-[2-(dimethylamino)ethyl]iminomethyl}-6-{N-methyl-N-[2-
(dimethylamino)ethyl]aminomethyl}-4-bromophenol (HL2):16
[Ni2(L1)(AcO)(NCS)2(MeOH)] (1), [Ni2(L1)(AcO)2(MeOH)]-
BPh4 (2), [Ni2(L2)(AcO)(NCS)2(MeOH)] (3), [Ni2(L2)(AcO)2]-
BPh4 (4), and [Ni2(L2)(NCS)3(MeOH)] (5). The chemical
structures of HL1 and HL2 are shown in Chart 1. The crystal
structure analyses for 2-5 indicate a phenoxo-bridged dinuclear
core structure in a geometric asymmetry. It is found that 4
accommodates a urea molecule at the four-coordinate nickel to
give [Ni2(L2)(AcO)2(urea)]BPh4 (4′). Further, the methanol
ligand in 5 is substituted by urea to give [Ni2(L2)(NCS)3(urea)]
(5′), whose crystal structure is determined by X-ray crystal-
lography.
(d) [Ni2(L2)(AcO)2]BPh4 (4). This was prepared as a brown powder
by a method similar to that for 3 using sodium tetraphenylborate instead
of sodium thiocyanate. Yield: 56.4%. Anal. Found: C, 57.50; H,
5.80; N, 5.90; Ni, 12.9. Calcd for C45H54BrN4Ni2O5: C, 57.56; H,
5.80; N, 5.97; Ni, 12.5. Selected IR data (ν/cm-1) using KBr disk:
3050-2800, 1630, 1580, 1550, 1450, 740, 710. Molar conductance
(ΛM/S cm2 mol-1) in CH2Cl2:45. Visible spectral data [λ/nm (ꢀ/M-1
cm-1)] in CH2Cl2: ∼500 (sh), 625 (40), 780 (25), 865 (25).
Experimental Section
Measurements. Elemental analyses of C, H, and N were obtained
at the Elemental Analysis Service Center of Kyushu University.
Analyses of Ni were obtained using a Shimadzu AA-660 atomic
absorption/flame emission spectrophotometer. Infrared spectra were
recorded on a JASCO IR-810 spectrometer using KBr disks. Molar
conductances were measured at ∼1 × 10-3 M concentration using a
DKK AOL-10 conductivity meter at 20 °C. Electronic spectra were
recorded on a Shimadzu MPS-2000 spectrometer. Magnetic suscep-
tibilities were determined on a Faraday balance in the temperature range
80-300 K and on a HOXAN HSM-D SQUID susceptometer in the
range 4.2-80 K. Calibrations were done using [Ni(en)3]S2O3.17
(e) [Ni2(L2)(NCS)3(MeOH)] (5). This was obtained as a green
precipitate in a manner similar to that for 3, using nickel(II) thiocyanate
instead of nickel(II) acetate tetrahydrate and sodium thiocyanate.
Yield: 49.3%. Anal. Found: C, 35.64; H, 4.57; N, 13.80; Ni, 16.6.
Calcd for C21H32BrN7Ni2O2S: C, 35.64; H, 4.56; N, 13.85; Ni, 16.6.
Selected IR data (ν/cm-1) using KBr disk: 3000-2800, 2080, 1980,
1650. Molar conductance: nonelectrolyte in CH2Cl2. Visible spectral
data [λ/nm (ꢀ/M-1 cm-1)] in CH2Cl2: 660 (70), 785 (35), 890 (45).
Effective magnetic moments were calculated by the equation µeff
)
2.828(øAT)1/2, where øA is the magnetic susceptibility per nickel
corrected for the diamagnetism of the constituent atoms using Pascal’s
constants.18
Preparation of Urea Adducts. (a) [Ni2(L2)(AcO)2(urea)]BPh4 (4′).
To a solution of 4 (0.47 g, 0.5 mmol) in dichloromethane (5 cm3) was
added powdered urea (excess), and the mixture was stirred for 1 h.
Unreacted urea was removed by filtration, and the resulting green
solution was diffused with hexane to give green microcrystals. Yield:
0.395 g (78%). Anal. Found: C, 55.11; H, 5.87; N, 8.12; Ni, 12.3.
Calcd for C46H58BBrN6Ni2O6: C, 55.30; H, 5.85; N, 8.41; Ni, 11.8.
Selected IR (ν/cm-1) using KBr disk: 3550-3300, 3050-2800, 1660,
1580, 1550, 1450. Molar conductance (ΛM/S cm2 mol-1) in CH2Cl2:
16. Visible spectral data [λ/nm(ꢀ/M-1 cm-1)] in CH2Cl2: 625 (35),
780 (sh), 880 (25).
(b) [Ni2(L2)(NCS)3(urea)] (5′). This was obtained as green micro-
crystals by the reaction of 5 with excess urea in dichloromethane.
Yield: 50%. Anal. Found: C, 34.18; H, 4.38; N, 16.99; Ni, 15.9.
Calcd for C21H32BrN9Ni2O2S3: C, 34.27; H, 4.38; N, 17.13; Ni, 15.9.
Selected IR data (ν/cm-1) using KBr disk: 3600-3200, 3050-2750,
2100, 1990, 1650, 1640, 1610, 1590. Molar conductance (ΛM/S cm2
mol-1) in CH2Cl2: nonelectrolyte. Visible spectral data [λ/nm (ꢀ/M-1
cm-1)] in CH2Cl2: 655 (58), 780 (40), 880 (50).
Material. Unless otherwise stated, all reagents were purchased from
commercial sources and used without further purification. Solvents
were dried by standard methods. 2,6-Diformyl-4-methylphenol19 and
3-{N-methyl-N-(2-dimethylamino)ethyl}aminomethyl-5-bromosalicyl-
aldehyde16 were prepared by literature methods.
(13) Volkmer, D.; Hommerich, B.; Griesar, K.; Haase, W.; Krebs, B. Inorg.
Chem. 1996, 35, 3792.
(14) Yamaguchi, K.; Koshino, S.; Akagi, F.; Suzuki, M.; Uehara, A.;
Suzuki, S. J. Am. Chem. Soc. 1997, 119, 5752.
(15) Sakiyama, H.; Tamaki, H.; Kodera, M.; Matsumoto, N.; Ohkawa, H.
J. Chem. Soc., Dalton Trans. 1993, 591.
(16) Higuchi, C.; Sakiyama, H.; Ohkawa, H.; Isobe, R.; Fenton, D. E. J.
Chem. Soc., Dalton Trans. 1994, 1097.
(17) Curtis, N. F. J. Chem. Soc. 1961, 3147.
(18) Boudreaux, E. A.; Mulay, L. N. Theory and Applications of Molecular
Paramagnetism; Wiley: New York, 1976; pp 491-494.
(19) Denton, D. A.; Suschitzky, H. J. Chem. Soc. 1963, 4741.

Phenol-Based Compartmental Ligands
Inorganic Chemistry, Vol. 37, No. 5, 1998 991
Table 1. Crystallographic Data for 2, 3, 4, 5, and 5′
2
3
4
5
5′
formula
crystal color
fw
crystal size, mm
crystal system
space group
a, Å
C46H57BN4Ni2O6
green
890.19
0.20 × 0.30 × 0.03
triclinic
P1h
13.613(3)
16.475(4)
11.307(4)
99.90(2)
104.16(2)
109.01(2)
2235(1)
2
C22H35BrN5Ni2O4S2
green
708.98
0.25 × 0.15 × 0.30
triclinic
P1h
10.167(1)
16.119(2)
9.472(3)
103.53(2)
100.91(1)
85.62(1)
1481.2(5)
2
C45H54BBrN4Ni2O5
brown
939.06
0.20 × 0.20 × 0.25
monoclinic
P21/n
15.146(2)
9.442(3)
30.844(2)
90
93.427(9)
90
4402(1)
4
C21H32BrN7Ni2O2S3
green
708.01
0.60 × 0.40 × 0.30
monoclinic
C2/c
26.744(5)
10.323(4)
21.785(6)
90
94.73(2)
90
5993(3)
8
C21H32BrN9Ni2O2S3
green
736.03
0.20 × 0.20 × 0.10
monoclinic
P21/c
11.709(5)
15.100(4)
17.548(5)
90
95.58(3)
90
3087(1)
4
b, Å
c, Å
R, deg
â, deg
γ, deg
V, Å3
Z
D
calcd, g cm-3
1.322
1.589
1.417
1.569
1.583
λ(Mo KR), Å
no. of reflns
Ra
0.710 69
7878
0.055
0.710 69
5532
0.050
0.710 69
8597
0.063
0.710 69
5734
0.067
0.710 69
5953
0.041
b,c
Rw
0.047
0.054
0054
0.051
10.1
2
a R ) ∑||Fo| - |Fc||/∑|Fo|. b Rw ) {∑[w(|Fo| - |Fc|)2]/∑[w|Fo| ]}1/2
.
c w ) 1/σ2(Fo).
Crystal Structure Analyses. Single crystals of 2, 3, and 5 were
obtained by crystallization from methanol. Single crystals of 4 and 5′
were grown from a 2-propanol/dichloromethane (1:1 in volume)
mixture. Each crystal of 2, 3, 4, and 5′ was coated with epoxy resin,
and a crystal of 5 was sealed in a glass tube. Intensities and lattice
parameters were obtained on a Rigaku AFC-7R automated four-circle
diffractometer, using graphite-monochromated Mo KR radiation (λ )
0.710 69 Å) and a 12 kW rotating anode generator at 20 ( 1 °C. The
cell parameters were determined by 25 reflections in the 2θ ranges
29.33° < 2θ < 29.96° for 2, 29.49° < 2θ < 29.98° for 3, 27.79° < 2θ
< 29.64° for 4, 29.57° < 2θ < 29.97° for 5, and 6.94° < 2θ < 14.51°
for 5′. For the intensity data collections, the ω-2θ scan mode was
used to a maximum 2θ value of 50.0°. The scan rates were 16° min-1
for 2-5 and 8° min-1 for 5′. The weak reflections (I < 10.0σ(I)) were
rescanned (maximum of four scans), and the counts were accumulated
to ensure good counting statistics. Stationary-background counts were
recorded on each side of the reflection. The ratio of peak counting
time to background counting time was 2:1. The diameter of the
incident-beam collimator was 1.0 mm, the crystal to detector distance
was 235 mm, and the computer-controlled detector aperture was set to
9.0 × 13.0 mm (horizontal × vertical). Three standard reflections were
monitored every 150 reflections. Over the course of data collection,
the standards decreased by 7.22% for 2, increased by 1.35% for 3,
decreased by 3.24% for 4, increased by 0.7% for 5, and decreased by
0.8% for 5′. A linear correction factor was applied to the data to
account for the phenomena. The linear absorption coefficients, µ, for
Mo KR radiation were 8.9 cm-1 for 2, 14.0 cm-1 for 3, 18.1 cm-1 for
4, 28.3 cm-1 for 5, and 27.6 cm-1 for 5′. An empirical absorption
correction based on azimuthal scans of several reflections was applied
which resulted in transmission factors ranging from 0.88 to 1.00 for 2,
from 0.62 to 1.00 for 3, from 0.89 to 1.00 for 4, from 0.77 to 1.00 for
5, and from 0.89 to 1.00 for 5′. The intensity data were corrected for
Lorentz and polarization factors. A correction for secondary extinction
was applied (coefficients: 2.398 20 × 10-7 for 2, 1.900 028 × 10-7
for 3, 3.165 72 × 10-7 for 4, 5.689 42 × 10-7 for 5, and 8.158 92 ×
10-7 for 5′). Crystal data parameters for 2, 3, 4, 5, and 5′ are
summarized in Table 1.
Figure 1. ORTEP view of [Ni2(L1)(AcO)2(MeOH)]BPh4 (2).
and ∆f ′′ were those of Creagh and McAuley.21 The values for the
mass attenuation coefficients were those of Creagh and Hubbell.22 All
calculations were performed using the teXsan crystallographic software
package23 of the Molecular Structure Corp.
Results and Discussion
Crystal Structures of Complexes 2-5. Complex 2. An
ORTEP24 view of [Ni2(L1)(AcO)2(MeOH)]BPh4 (2) is shown
in Figure 1 together with the numbering scheme. Selected bond
distances and angles are given in Table 2.
The complex has a dinuclear core bridged by the phenolic
oxygen of (L1)- and two acetate groups in the syn-syn mode,
providing a (µ-phenoxo)bis(µ-carboxylato)dinickel(II) core in
the Ni--Ni separation of 3.311(1) Å. A methanol molecule
The structures were solved by direct methods and expanded using
Fourier techniques. Refinements were carried out by the full-matrix
least-squares methods, where the function minimized was ∑w(|Fo| -
|Fc|)2 with w ) 1/σ2(Fo). Non-hydrogen atoms were refined anisotro-
pically. Hydrogen atoms were fixed at the calculated positions and
were not refined.
(21) Creagh, D. C.; McAuley, W. J. International Tables for Crystal-
lography, Kluwer: Boston, MA, 1992; pp 219-222.
Neutral-atom scattering factors were taken from Cromer and Waber.20
Anomalous dispersion effects were included in Fc; the values for ∆f ′
(22) Creagh, D. C.; Hubbell, H. H. International Tables for Crystallography;
Kluwer: Boston, MA, 1992; pp 200-206.
(23) teXsan, Molecular Structure Corp.: Houston, TX, 1985.
(24) Johnson, C. K. ORTEP; Report 3794; Oak Ridge National Labora-
tory: Oak Ridge, TN, 1965.
(20) Cromer, D. T.; Waber, J. T. International Tables for X-ray Crystal-
lography; Kynoch Press: Birmigham, U.K., 1974; Vol. IV.

992 Inorganic Chemistry, Vol. 37, No. 5, 1998
Koga et al.
Table 3. Relevant Bond Distances (Å) and Angles (deg) for 3
Distances
Table 2. Relevant Bond Distances (Å) and Angles (deg) for 2
Distances
Ni1-O1
Ni1-O4
Ni1-N2
Ni2-O3
Ni2-O6
Ni1--Ni2
2.022(3)
1.973(3)
2.138(4)
2.006(3)
2.270(4)
3.311(1)
Ni1-O2
Ni1-N1
Ni2-O1
Ni2-O5
Ni2-N3
1.982(3)
1.983(4)
2.044(3)
2.037(3)
2.002(4)
Ni1-O1
Ni1-O4
Ni1-N2
Ni2-O1
Ni2-N3
Ni2-N5
Ni1--Ni2
2.027(4)
2.180(4)
2.143(5)
2.042(4)
2.113(5)
2.105(6)
2.993(1)
Ni1-O2
Ni1-N1
Ni1-N6
Ni2-O3
Ni2-N4
Ni2-N6
2.036(4)
1.990(5)
2.131(5)
2.034(4)
2.167(5)
2.172(5)
Angles
O1-Ni1-O2
O1-Ni1-N1
O2-Ni1-O4
O2-Ni1-N2
O4-Ni1-N2
O1-Ni2-O3
O1-Ni2-O6
O1-Ni2-N4
O3-Ni2-O6
O3-Ni2-N4
O5-Ni2-N3
O6-Ni2-N3
N3-Ni2-N4
98.8(1)
89.2(2)
103.9(1)
91.5(2)
88.7(2)
97.8(1)
84.8(1)
171.3(2)
83.9(1)
90.9(1)
93.3(2)
86.6(2)
81.6(2)
O1-Ni1-O4
O1-Ni1-N2
O2-Ni1-N1
O4-Ni1-N1
N1-Ni1-N2
O1-Ni2-O5
O1-Ni2-N3
O3-Ni2-O5
O3-Ni2-N3
O5-Ni2-O6
O5-Ni2-N4
O6-Ni2-N4
Ni1-O1-Ni2
96.2(1)
167.1(2)
97.3(2)
157.0(2)
81.8(2)
91.1(1)
89.8(2)
96.8(1)
167.2(2)
175.9(1)
87.8(2)
96.3(2)
109.1(1)
Angles
O1-Ni1-O2
O1-Ni1-N1
O2-Ni1-N6
O2-Ni1-N1
O2-Ni1-N6
O4-Ni1-N2
N1-Ni1-N2
N2-Ni1-N6
O1-Ni2-N3
O1-Ni2-N5
O3-Ni2-N3
O3-Ni2-N5
N3-Ni2-N4
N3-Ni2-N6
N4-Ni2-N6
Ni1-O1-Ni2
91.1(2)
89.7(2)
86.9(2)
93.0(2)
93.0(2)
94.7(2)
83.9(2)
99.3(2)
91.6(1)
85.0(2)
89.9(2)
173.9(2)
85.3(2)
176.2(2)
97.9(2)
94.7(2)
O1-Ni1-O4
O1-Ni1-N2
O2-Ni1-O4
O2-Ni1-N2
O4-Ni1-N1
O4-Ni1-N6
N1-Ni1-N6
O1-Ni2-O3
O1-Ni2-N4
O1-Ni2-N6
O3-Ni2-N4
O3-Ni2-N6
N3-Ni2-N5
N4-Ni2-N5
N5-Ni2-N6
Ni1-N6-Ni2
84.4(2)
173.5(2)
172.2(2)
90.5(2)
93.3(2)
80.4(2)
173.1(2)
90.5(2)
175.8(2)
85.4(2)
92.1(2)
87.7(2)
94.3(2)
92.6(2)
87.8(2)
88.1(2)
(Ni1) is occupied by a methanol oxygen (O4), and the sixth
position of the other Ni (Ni2) is occupied by an isothiocyanate
nitrogen (N5), giving a {6/6} coordination number set. The
methanol molecule on Ni1 and the thiocyanate group on Ni2
are situated cis relative to each other with respect to the mean
molecular plane. The equatorial plane about Ni1 is formed by
N1 and N2 of the iminic pendant arm and O1 and N6 of the
bridges. The equatorial plane about Ni2 is formed by N3 and
N4 of the aminic pendant arm and O1 and N6 of the bridges.
The two equatorial planes sharing O1 and N6 form a good
coplane, and the acetate group bridges the two nickel ions at
their axial sites. The Ni1-to-ligand bond distances fall in the
range 1.990(5)-2.180(4) Å, whereas the Ni2-to-ligand bond
distances fall in the smaller range 2.034(4)-2.172(5) Å. The
Ni1-N1(imine) bond distance (1.990(5) Å) is fairly short
relative to the Ni2-N3(amine) distance (2.113(5) Å).
Complex 4. An ORTEP view of [Ni2(L2)(AcO)2]BPh4 (4)
is shown in Figure 3 together with the numbering scheme.
Selected bond distances and angles are given in Table 4.
Figure 2. ORTEP view of [Ni2(L2)(AcO)(NCS)2(MeOH)] (3).
coordinates to one nickel, forming a geometrically asymmetric
core with a {5/6} coordination number set. The geometry
around the five-coordinate nickel (Ni1) is depicted as a square
pyramid with N1, N2, and O1 of (L1)- and O4 of one acetate
bridge on the basal plane and O2 of another acetate bridge at
the apex. The geometry around the six-coordinate nickel (Ni2)
forms a distorted octahedron. The six-coordinate Ni2 has long
metal-to-ligand bond distances (2.006(3)-2.270(4) Å) relative
to the five-coordinate Ni1 (1.973(3)-2.022(3) Å). In particular,
the Ni2-O6(methanol) bond distance (2.270(4) Å) is elongated.
It must be mentioned that the core of this complex resembles
that of [Mn2(L1)(AcO)2NCS],15 where the sixth position of the
six-coordinate manganese is occupied by the isothiocyanate
nitrogen.
A pair of nickel ions are bridged by the phenolic oxygen of
(L2)- and an acetate group in the syn-syn mode: the Ni1--Ni2
intermetallic separation is 3.254(1) Å. The nickel bound to the
iminic pendant arm (Ni1) has a four-coordinate geometry with
short Ni-N and Ni-O bond distances (1.843(7)-1.963(7) Å).
The configuration around the metal can be regarded as a square
plane showing a slight distortion to a tetrahedron: the dihedral
angle between the least-squares plane defined by Ni1, N1, and
N2 and the plane defined by Ni1, O1, and O2 is 15.4°. Ni2
bound to the aminic pendant arm has a six-coordinate geometry
with further coordination of an acetate group in the chelating
mode. Thus, the molecule has a {4/6} coordination number
set. The O4-Ni2-O5 angle in the chelating acetate is small
(59.8(2)°). In contrast, the O3-Ni2-O4, O3-Ni2-N3, and
O5-Ni2-N3 angles are enlarged (98.3(3), 101.2(2), and 100.4-
(3)°, respectively). When we presume the donor atoms O1, N3,
N4, and O4 to form an equatorial plane about Ni2, the bridging
acetate group coordinates to an axial site of Ni2 through O3
and to an equatorial site of Ni1 through O2. Because of this,
the {NiN2O2} entity of the planar Ni1 is not coplanar with the
conjugated part of (L2)-. This is reflected by the nonplanar
Complex 3. An ORTEP view of [Ni2(L2)(AcO)(NCS)2-
(MeOH)] (3) is shown in Figure 2 together with the numbering
scheme. Selected bond distances and angles are given in Table
3.
X-ray crystallography has confirmed that a pair of nickel ions
are bridged by the phenolic oxygen of (L2)-, an isothiocyanate
nitrogen, and an acetate group in the syn-syn mode. The Ni1-
-Ni2 separation is 2.993(1) Å. The sixth position of one Ni

Phenol-Based Compartmental Ligands
Inorganic Chemistry, Vol. 37, No. 5, 1998 993
Figure 4. ORTEP view of [Ni2(L2)(NCS)3(MeOH)] (5).
Figure 3. ORTEP view of [Ni2(L2)(AcO)2]BPh4 (4).
Table 5. Relevant Bond Distances (Å) and Angles (deg) for 5
Table 4. Relevant Bond Distances (Å) and Angles (deg) for 4
Distances
Ni1-O1
Ni1-N2
Ni1-N6
Ni2-O2
Ni2-N4
Ni2-N7
2.013(4)
2.108(7)
2.039(7)
2.159(5)
2.113(6)
2.025(7)
Ni1-N1
Ni1-N5
Ni2-O1
Ni2-N3
Ni2-N6
Ni1--Ni2
1.961(7)
1.991(7)
2.044(5)
2.100(6)
2.170(6)
3.142(2)
Distances
Ni1-O1
Ni1-N1
Ni2-O1
Ni2-O4
Ni2-N3
Ni1--Ni2
1.858(5)
1.843(7)
2.118(5)
2.178(7)
2.058(6)
3.254(1)
Ni1-O2
Ni1-N2
Ni2-O3
Ni2-O5
Ni2-N4
1.862(6)
1.963(7)
2.014(5)
2.097(6)
2.158(7)
Angles
O1-Ni1-N1
O1-Ni1-N5
N1-Ni1-N2
N1-Ni1-N6
N2-Ni1-N6
O1-Ni2-O2
O1-Ni2-N4
O1-Ni2-N7
O2-Ni2-N4
O2-Ni2-N7
N3-Ni2-N6
N4-Ni2-N6
N6-Ni2-N7
89.8(3)
95.6(3)
83.8(3)
152.9(3)
96.7(2)
78.7(2)
96.1(2)
168.4(3)
174.6(2)
91.5(3)
169.3(3)
93.6(3)
94.4(3)
O1-Ni1-N2
O1-Ni1-N6
N1-Ni1-N5
N2-Ni1-N5
N5-Ni1-N6
O1-Ni2-N3
O1-Ni2-N6
O2-Ni2-n3
O2-Ni2-N6
N3-Ni2-N4
N3-Ni2-N7
N4-Ni2-N7
Ni1-O1-Ni2
165.2(3)
82.9(2)
105.0(3)
99.0(4)
101.7(3)
90.4(2)
79.0(2)
92.9(2)
86.6(2)
85.9(3)
96.3(3)
93.9(3)
101.5(2)
Angles
O1-Ni1-O2
O1-Ni1-N2
O2-Ni1-N2
O1-Ni2-O3
O1-Ni2-O5
O1-Ni2-N4
O3-Ni2-O5
O3-Ni2-N4
O4-Ni2-N3
O5-Ni2-N3
N3-Ni2-N4
93.3(2)
176.7(3)
88.1(3)
82.8(2)
91.2(2)
171.0(3)
157.6(3)
90.0(3)
159.9(3)
100.4(3)
85.8(3)
O1-Ni1-N1
O2-Ni1-N1
N1-Ni1-N2
O1-Ni2-O4
O1-Ni2-N3
O3-Ni2-O4
O3-Ni2-N3
O4-Ni2-O5
O4-Ni2-N4
O5-Ni2-N4
Ni1-O1-Ni2
95.1(3)
163.0(3)
84.4(3)
87.3(2)
90.3(2)
98.3(3)
101.2(2)
59.8(2)
99.1(3)
97.5(3)
109.7(3)
configuration about the phenolic oxygen O1; the sum of the
bite angles C1-O1-Ni1, C1-O1-Ni2, and Ni1-O1-Ni is
356.3°.
Complex 5. An ORTEP view of [Ni2(L2)(NCS)3(MeOH)]
(5) is shown in Figure 4. Selected bond distances and angles
are given in Table 5.
O1 and N3 of (L2)-, N6 of the bridging isothiocyanate group,
and N7 of the monodentate isothiocyanate group. The axial
sites are occupied by the terminal nitrogen N4 of the pendant
arm and a methanol oxygen O2. The Ni2-O2 bond (2.159(5)
Å) is relatively long among the Ni2-to-ligand bonds.
Properties. (a) IR Spectra. Complex 1 shows the anti-
symmetric and symmetric vibrations of the acetate group at 1570
and 1460 cm-1, respectively. The small separation between
the two vibrations suggests a bidentate mode for the acetate
group.25 Similarly, a small separation between the antisym-
metric and symmetric vibrations of the acetate group is observed
for 2-4, all of which have at least one bidentate (bridging or
chelating) acetate group as discussed above. Each of 1, 3, and
5 shows two distinct ν(CN) modes for the NCS- group (2080
and 2022 cm-1 for 1, 2075 and 2025 cm-1 for 2, and 2080 and
1980 cm-1 for 5). This is in harmony with the X-ray
crystallographic results indicating that one bridging and one
monodentate thiocyanate group is involved in each crystal. The
higher frequency band can be assigned to the monodentate
X-ray crystallography indicates that the aminic pendant arm
adopts an “out-of-plane” chelating mode in this complex. The
two Ni ions are bridged by the phenolic oxygen of (L2)- and
an isothiocyanate nitrogen; the Ni--Ni separation is 3.142(2)
Å. Ni1 bound to the iminic pendant arm has a five-coordinate
structure with O1, N1 and N2 of (L2)-, N5 of a monodentate
isothiocyanate group, and N6 of the bridging isothiocyanate
group. The geometry about the metal is best depicted as a
square pyramid where the basal plane is formed by O1, N1,
N2, and N6. The equatorial bond distances vary from 1.961-
(7) to 2.108(7) Å. The axial Ni-N5 bond is relatively short
(1.991(7) Å). Ni1 deviates 0.373 Å from the basal least-squares
plane toward N5. Ni2 bound to the aminic pendant arm has a
pseudo octahedral geometry with further coordination of a
methanol molecule. Thus, this complex has a {5/6} coordina-
tion number set. The equatorial plane about Ni2 is formed by
(25) Deacon, G. B.; Phillips, R. J. Coord. Chem. ReV. 1980, 33, 227.

994 Inorganic Chemistry, Vol. 37, No. 5, 1998
Koga et al.
isothiocyanate group and the lower frequency band to the
bridging isothiocyanate group.26 The corresponding ν(CS)
modes are obscured by ligand vibrations. 2 and 4 both have
an intense vibration at 710 cm-1 that is characteristic of the
tetraphenylborate ion. All the complexes show the ν(CdN)
mode of the azomethine group at ∼1650 cm-1
.
(b) Visible Spectra. Complexes 1, 3, and 5 are nonelectro-
lytes in dichloromethane, while 2 and 4 have a molar conduc-
tance typical of 1:1 electrolytes in this solvent (46 and 45 S
cm2 mol-1, respectively).27 Each complex shows a close
resemblance between absorption (in dichloromethane) and
reflectance spectra. Numerical data from the absorption spectra
are given in the Experimental Section.
Complex 1 exhibits two distinct absorption bands at 610 and
880 nm and a weaker band near 780 nm. A similar spectrum
is observed for 3 that has a core with a {6/6} coordination
number set. If we assume Oh symmetry for the two nickel ions
of 1 and 3, the two bands at ∼610 and ∼880 nm can be assigned
3
3
3
to the spin-allowed d-d transitions T2g r A2g and T1g
r
3A2g, respectively.28,29 The weak band near 780 nm can be
assigned to the spin-forbidden transition 1Eg r 3A2g. Complexes
2 and 5, having a core with a {5/6} coordination number set,
exhibit two distinct absorption bands at ∼660 and ∼880 nm
along with a weaker band at ∼780 nm. The spectrum of 4 is
characterized by a discernible shoulder near 500 nm that is
typical of a planar nickel(II) ion.29 Because of this absorption,
4 assumes a brown color, whereas the other complexes are
green.
(c) Magnetism. The room-temperature magnetic moments
of 1-3 (per one Ni) fall in the range 3.10-3.15 µB. The
magnetic moments gradually decrease with decreasing temper-
ature, suggesting the presence of an antiferromagnetic interaction
between a pair of nickel(II) ions. The øA vs T and µeff vs T
curves for 1 are given in Figure 5.
Figure 5. øA vs T (2) and µeff vs T (O) curves for 1 and 5.
interaction. Magnetic simulations based on the expression
containing the correction term Θ for the secondary effect
Magnetic analyses were carried out by using the magnetic
susceptibility expression based on the isotropic Heisenberg
model (H ) -2JS1S2, S1 ) S2 ) 1):
øA ) [Ng2â2/3k(T - Θ)][5 + exp(-4J/kT)]/
[5 + 3 exp(-4J/kT) + exp(-6J/kT)] + NR (2)
øA ) (Ng2â2/3kT)[5 + exp(-4J/kT)]/
[5 + 3 exp(-4J/kT) + exp(-6J/kT)] + NR (1)
gave a good fitting as indicated in Figure 5, using the magnetic
parameters of g ) 2.17, J ) +10 cm-1, Θ ) -1.5 K, and NR
) 200 × 10-6 cm3 mol-1. Magnetostructural correlations have
been extensively studied for dinuclear copper(II) complexes,
and the significance of the Cu-X-Cu angle in a dibridged
system and the coplanarity about the bridging oxygen in
determining the sign of the exchange integral are well
documented.31-33 A similar magnetostructural study has been
conducted for dinuclear nickel(II) complexes.34 For the present
dinuclear nickel(II) complexes, however, we cannot identify any
dominant factor contributing to the positive (for 5) or negative
(for 1-3) exchange integral from inspection of their crystal
structures.
where N is the Avogadro number, â is the Bohr magneton, k is
the Boltzman constant, g is the Zeeman splitting constant, T is
the absolute temperature, J is the exchange integral, and NR is
the temperature-independent paramagnetism. The cryomagnetic
property of 1 can be simulated by this expression as indicated
by the solid curve, using the magnetic parameters g ) 2.21, J
) -0.8 cm-1, and NR ) 200 × 10-6 cm3 mol-1. Similarly,
negative exchange integrals are obtained for 2 (-1.0 cm-1) and
3 (-1.7 cm-1).
The magnetic moment of 5 is 3.25 µB at room temperature
and gradually increases with decreasing temperature up to a
maximum of 3.56 µB near 20 K (Figure 5). Evidently a
ferromagnetic interaction operates between the pair of nickel
ions. With further decreasing temperature, the magnetic mo-
ment tends to drop probably due to a secondary effect such as
zero-field splitting of SNi ) 1 or interdimer antiferromagnetic
The magnetic moment of 4 is 3.31 µB per molecule at room
temperature, and the moment is practically independent of
temperature down to 10 K. This, together with X-ray crystal-
lographic and visible spectral data, shows that 4 is evidently a
(30) Bosnich, B. J. Am. Chem. Soc. 1968, 90, 627.
(31) Melnik, M. Coord. Chem. ReV. 1982, 42, 259.
(32) Cairns, C. J.; Busch, D. H. Coord. Chem. ReV. 1986, 69, 1.
(33) Handa, M.; Koga, N.; Kida, S. Bull. Chem. Soc. Jpn. 1988, 61, 3853.
(34) Nanda, K. K.; Das, R.; Thompson, L. K.; Venkastsubramanian, K.;
Paul, P.; Nag, K. Inorg. Chem. 1994, 33, 1188.
(26) Nakamoto, K. Infrared Spectra of Inorganic and Coordination
Compounds; Wiley Interscience: New York, 1970; p 187.
(27) Geary, W. J. Coord. Chem. ReV. 1971, 7, 81.
(28) Sacconi, L. Coord. Chem. ReV. 1966, 1, 192.
(29) Yamada, S. Coord. Chem. ReV. 1966, 1, 415.

Phenol-Based Compartmental Ligands
Inorganic Chemistry, Vol. 37, No. 5, 1998 995
Figure 6. Visible spectra of 4 and 4′ in dichloromethane.
Figure 7. ORTEP view of [Ni2(L2)(NCS)3(urea)] (5′).
mixed-spin complex having one low-spin (S ) 0) and one high-
spin (S ) 1) nickel(II) ion.
Table 6. Relevant Bond Distances (Å) and Angles (deg) for 5′
Distances
Urea Adducts. Urea adducts of dinuclear nickel complexes
are of current interest because of relevance to the urease active
site.10 In this study, we have found that 4 and 5 form the urea
adducts [Ni2(L2)(AcO)2(urea)]BPh4 (4′) and [Ni2(L2)(NCS)3-
(urea)] (5′), respectively. Urea binding through its oxygen in
these urea adducts is inferred from new IR bands at 3550-
3300 and 1660 cm-1 which are assigned to the ν(N-H) and
ν(CdO) modes, respectively. In Figure 6 is shown the visible
spectrum of 4′ together with the spectrum of the precursor
complex 4. The discernible shoulder near 500 nm of 4, typical
of planar NiII, disappears in the spectrum of 4′. This fact means
that the urea binding occurs at the four-coordinate Ni center.
The room-temperature magnetic moment of 4′ is 3.29 µB per
one nickel (4.65 µB per molecule), clearly indicating that both
nickel ions are paramagnetic. The urea adduct resembles 4 in
cryomagnetic properties; the µeff increases with decreasing
temperature up to a maximum value of 3.69 µB at 8.1 K and
then decreases below this temperature. Magnetic simulations
based on eq 2 gave the best-fit parameters g ) 2.25, J ) +4.0
Ni1-O1
Ni1-N2
Ni1-N6
Ni2-O2
Ni2-N4
Ni2-N7
1.992(4)
2.107(5)
2.077(5)
2.125(5)
2.158(6)
2.030(6)
Ni1-N1
Ni1-N5
Ni2-O1
Ni2-N3
Ni2-N6
Ni1--Ni2
1.981(5)
1.975(6)
2.058(4)
2.094(5)
2.197(5)
3.155(2)
Angles
O1-Ni1-N1
O1-Ni1-N5
N1-Ni1-N2
N1-Ni1-N6
N2-Ni1-N6
O1-Ni2-O2
O1-Ni2-N4
O1-Ni2-N7
O2-Ni2-N4
O2-Ni2-N7
N3-Ni2-N6
N4-Ni2-N6
N6-Ni2-N7
90.3(2)
98.2(2)
83.4(2)
148.5(2)
95.8(2)
87.0(2)
93.1(2)
172.8(2)
176.3(2)
90.5(2)
169.3(2)
95.9(2)
94.1(2)
O1-Ni1-N2
O1-Ni1-N6
N1-Ni1-N5
N2-Ni1-N5
N5-Ni1-N6
O1-Ni2-N3
O1-Ni2-N6
O2-Ni2-N3
O2-Ni2-N6
N3-Ni2-N4
N3-Ni2-N7
N4-Ni2-N7
Ni1-O1-Ni2
166.6(2)
83.5(2)
99.2(2)
94.4(2)
112.2(2)
90.3(2)
79.0(2)
91.4(2)
87.7(2)
84.9(2)
96.5(2)
89.8(2)
102.3(2)
compared with the Ni-O(methanol) bond of 5 (2.159(5) Å).
The Ni1--Ni2 separation in 5′ is 3.155(2) Å.
cm-1, Θ ) -0.7 K, and NR ) 200 × 10-6 cm3 mol-1
.
The spectral resemblance between 5′ and 5 (see Experimental
Section) suggests a similarity in their dinuclear cores. This was
proved by X-ray crystallography. An ORTEP view of 5′ is
shown in Figure 7 together with the numbering scheme.
Relevant bond distances and angles are given in Table 6.
The µeff vs T behavior of 5′ is essentially similar to those of
5 and 4′, indicating a ferromagnetic spin exchange within a
molecule. Magnetic simulations by means of eq 2 gave the
following best-fit parameters: g ) 2.12, J ) +10.0 cm-1, Θ
) -0.7 K, and NR ) 200 × 10-6 cm3 mol-1. Magnetic studies
for urease have provided conflicting results for magnetic
exchange interactions.35-37 The present magnetic studies for
the dinuclear nickel complexes (1-5) and the urea adducts (4′
and 5′) suggest that the magnetic interaction in dinuclear nickel
is complicatedly affected by various factors to give a positive
or negative exchange integral by a slight structural change.
It must be emphasized that urea adduct formation does not
occur for 1-3. 3 has a core with a {6/6} coordination number
set, and 1 probably has a similar core. The coordinative
The urea adduct 5′ retains the dinuclear core of 5 bridged by
the phenolic oxygen O1 of (L2)- and the isothiocyanate nitrogen
N6. The aminic pendant arm of (L2)- adopts a similar out-of-
plane chelating mode toward N2. One essential difference
between 5′ and 5 is seen in the sixth exogenous ligand of Ni2;
this is urea in 5′ while it is methanol in 5. The five-coordinate
Ni1 at the iminic site shows little change on urea binding; the
average Ni1-to-ligand bond distance of 5′ (2.026 Å) is compa-
rable to that of 5 (2.022 Å). Ni2 at the aminic site shows a
slight change in its six-coordinate environment on urea binding.
The average of the Ni2-to-ligand bond distances in 5′ is 2.110
Å, which is short relative to that of 5 (2.168 Å). The Ni2-
O2(urea) bond distance is 2.125(5) Å, which is common for a
Ni-O(urea) distance.10 The Ni-O(urea) bond distance is short
(35) Clark, P. A.; Wilcox, D. E. Inorg. Chem. 1989, 28, 1326.
(36) Finnegan, M. G.; Kowal, A. T.; Werth, M. T.; Clark, P. A.; Wilcox,
D. E.; Johnson, M. K. J. Am. Chem. Soc. 1991, 113, 4030.
(37) Day, E. P.; Peterson, J.; Sendova, M. S.; Todd, M. J.; Hausinger, E.
P. Inorg. Chem. 1993, 32, 634.

996 Inorganic Chemistry, Vol. 37, No. 5, 1998
Koga et al.
saturation of two nickel centers explains why these two
complexes cannot bind urea. Complex 2 has a core with a {5/
6} coordination number set but cannot bind urea. In relating
to this, it must be pointed out that dinuclear Mn(II) complexes
of (L1)- and (L2)- prefer similar asymmetric cores with {5/6}
coordination number sets.15,37 It is plausible that when one metal
is six-coordinate, the other metal cannot take a six-coordinate
geometry owing to intracore strain within (L1)- and (L2)-. That
is, the core of with a {5/6} coordination number set is practically
saturated for weakly donative ligands such as urea. Complex
4 has an unsaturated core with a {4/6} coordination number
set to allow urea binding at the four-coordinate Ni center. In
the case of 5, with a core with a {5/6} coordination number
set, the methanol ligand is more or less released and the resulting
[Ni2(L2)(NCS)3] with a {5/5} coordination number set binds
urea instead of the methanol.
Acknowledgment. This work was supported by a Grant-
in-Aid for Scientific Research (No. 09440231) from the Ministry
of Education, Science, and Culture of Japan. Thanks are also
expressed to the British Council for support.
Supporting Information Available: X-ray crystallographic files,
in CIF format, for 2, 3, 4, 5, and 5′ are available on the Internet only.
Access information is given on any current masthead page.
IC971209P