Download Tandem Mass Spectrometry of Cu(II) Complexes

Tandem Mass Spectrometry of Cu(II) Complexes: The Effects of Ligand Donor Group on
Dissociation
Amanda L. Chaparro and Richard W. Vachet
Department of Chemistry, University of Massachusetts, Amherst MA 01003
Introduction:
A quadrupole ion trap mass spectrometer equipped with electrospray ionization is used to study the
dissociation patterns of Cu(II) complexes with organic ligands. In order to most effectively use CID to get
coordination structure information from metal complexes, it is important to understand the physical and
chemical features that affect the dissociation patterns of metal complex ions. Some previous studies have
observed the effect of the transition metal on dissociation patterns of metal complexes, but few studies have
investigated the effect of ligand binding groups on the dissociation patterns. The goal of this study is to
evaluate the effect of different ligand functional groups on the dissociation patterns of Cu(II) complexes.
Method:
Copper(II) is complexed with a series of ligands having different combinations of nitrogen-, oxygen-, and
sulfur-containing functional groups attached to a diethylenetriamine (DIEN) framework. These ligands were
synthesized following the method used by Hartman et
N
al.1 and Deroche et al.2 This method involves two
H
DIEN-R1/R2
steps. The first one is the reaction between an amine
NH
HN
and aldehyde, and the second is the reduction of the
resultant Schiff base using H2 over Pd or NaBH4. The
R1
R2
ligands are shown in Figure 1.
H
N
N
N
O
N
R1 and R2 =
pyr
3pyr
-(CH 2) 3-S-CH 3
imi
THF
-(CH 2) 2-NH 2
thioether
amino
Doubly-charged complexes are generated using
electrospray
ionization,
and
collision-induced
dissociation (CID) is used to generate product ions in a
Bruker Esquire quadrupole ion trap.
The CID
spectrum of each complex is obtained and the
appearance of specific product ions is determined to
evaluate the preference of Cu for different functional
groups during the dissociation process.
Figure 1. Structure of the ligands
Results:
Differences in the dissociation patterns are observed depending on the donor groups around the metal.
Figure 2 shows the common products ions and table 1 shows the relative intensities of these ions obtained
upon dissociating Cu(II) complexes with various R1 and R2 groups (see Fig. 1).
In figure 2 each of the following pairs A and B and C
and D are complementary ions. Most of the product
ions are formed via heterolytic cleavages of C-C bonds
with Cu remaining with one of the functional groups. In
most cases a clear preference for one functional group
is observed. The two products ions in which Cu(II) is
reduced to Cu(I) are I and J. These product ions
appear to form via the loss of a hydrogen radical from A
and C respectively with the other electron formally
reducing the metal.
A=
H
N
H
N
R
-
CH 2
B=
R
H
N
CH 2+
H
N
CH 2+
Cu (II)
H
N
C= R
CH 2
D= R
H
N
Cu (II)
F= R
+
G=
RH+
.
H
J= R N CH 2
CH 2
N
I= R N
Cu(I)
For ligands with nitrogen-containing groups (e.g. pyr,
Cu(I)
amino, imi, and 3pyr) Cu prefers to remain with the
Figure 2. Structure of the product ions
functional groups with the following tendency: amino >
imi > pyr > 3pyr. This order of preference reflects a
balance between binding strength and the optimum overlap of the functional group, which is affected by the
group’s flexibility. Even though it does not bind as strongly to the metal as the imi or pyr groups, the amino
group has the greater flexibility, allowing it to more effectively orient its dipole toward the metal. For ligands
containing both nitrogen and sulfur groups, it is observed that Cu prefers to remain coordinated to the sulfur
group upon dissociation. For all complexes with sulfur, the predominant product ions contain Cu(I),
suggesting that the preference for the thioether group is explained by the preference of the soft acid (Cu(I))
for the soft base (thioether). For ligands containing nitrogen and oxygen groups, a strong preference is
observed for remaining with the nitrogen group upon dissociation with the exception of the DIEN-3pyr/THF
ligand, where the metal prefers to remain with THF. This result may be due to differences in the size of the
chelate ring and the flexibility of each R group, which prevent 3pyr from orienting its dipole effectively toward
the metal. In some cases the product ions contain Cu(I), and in other cases the product ions contain Cu(II).
In either case, however, the metal prefers the intermediate bases (N-containing groups) over the harder base
(THF). For ligands with sulfur and oxygen groups, Cu prefers to remain coordinated to the sulfur group. In
general the preference of Cu to remain coordinated to various functional groups upon dissociation is
thioether > amino > imi > pyr > THF > 3pyr. Because some product ions contain Cu(I) and others contain
Cu(II), this ordering presumably reflects a balance between the metal binding affinity of each group with Cu(I)
or Cu(II) and the flexibility of the ligand. The more flexible functional groups (i.e. thioether, amino) are better
able to orient their dipoles to optimally interact with the metal.
Table 1. Types of products ions formed from Cu(DIEN-R1/R2) complexes
Product Ion
Cu(DIEN-Pyr/amino) +2
m/z 150
Cu(DIEN-Pyr/imi) +2
m/z 168.5
Cu(DIEN-imi/ amino) +2
m/z 144.5
Cu(DIEN-3Pyr/ amino) +2
m/z 150
Cu(DIEN-3Pyr/imi) +2
m/z 168.5
Cu(DIEN-Thioether/ amino)+2
m/z 148.5
Cu(DIEN-Thioether/imi) +2
m/z 167
Cu(DIEN-Thioether/Pyr) +2
m/z 172.5
Cu(DIEN-Thioether/3Pyr) +2
m/z 172.5
Cu(DIEN-THF/ amino) +2
m/z 146.5
Cu(DIEN-THF/imi) +2
m/z 165
Cu(DIEN-THF/Pyr) +2
m/z 170.5
Cu(DIEN-THF/3Pyr) +2
m/z 170.5
Cu(DIEN-THF/Thioether)+2
m/z 169
A
R1
R2
R1
R2
R1
R2
R1
R2
R1
R2
R1
R2
R1
R2
R1
R2
R1
R2
R1
R2
R1
R2
R1
R2
R1
R2
R1
R2
B
7%
C
6%
34%
19%
11%
F
100%
50%
16%
36%
100%
78%
D
16%
7%
G
100%
89%
29%
100%
25%
30%
41%
100%
86%
12%
34%
I
27%
79%
40%
58%
57%
96%
J
5%
20%
62%
17%
54%
18%
57%
100%
5%
5%
30%
10%
5%
100%
65%
5%
9%
45%
100%
40%
88%
52%
26%
44%
8%
72%
8%
24%
16%
38%
14%
100%
100%
4%
74%
77%
100%
25%
22%
100%
15%
32%
8%
76%
100%
5%
9%
100%
References:
1. Hartman, J.A.; Vachet, R.W.; Callahan, J.H. Inorg. Chim. Acta. 2000, 297, 79.
2. Deroche, A.; Morgenstern-Badarau, I.; Cesario, M.; Guilhem, J.; Keita, B.; Nadjo, L. Houee-Levin, C.
J. Am. Chem. Soc. 1996, 118, 4567.