presence of a three-center Cu–B–C ipso interaction on the basis of Kohn-Sham
orbitals obtained by density functional theory (DFT) calculations. Natural bond
orbital (NBO) analysis showed that among the various donor-acceptor interactions
involving this triangle, a Cu!B donating interaction of similar magnitude as the
η
1 (B) interactions found in related complexes is present [24]. Additionally, natural
population analysis confirms a net charge transfer from Cu to B, reinforcing the
description of the ligand as an acceptor ligand.
Moving to the second and third row transition metal centers, L1 coordinates to
Au
I in an initially unexpected yet most pincerlike manner [61]. A square planar
geometry around the tetracoordinated Au
I is observed, featuring trans-diphosphine
coordination of L1 and a chloride co-ligand trans to the η
1 (B)-coordinated borane
(Table 1). A strong Au!B interaction is observed as evident from a short bond
distance (2.309(8) Å) and strong pyramidalization of the borane center (ΣB α ¼ 341
).
Frontier orbital analysis shows a B–Au–Cl three-center interaction; however, the
charge depletion at gold and charge increase at boron are not large enough to be
considered a 2e
À oxidation of the gold center to Au
III . In addition,
197 Au Mössbauer
spectroscopy supported the classification of L1Au
I Cl as a 16 VE Au
I complex.
The distinct η
1 (B) and arene-supported η
2 (B,C) coordination modes can be
considered two extremes of L1 coordination to d
10 transition metal centers. This
becomes apparent upon the evaluation of the coordination of L1 to Pd
0
[59]. Depending on the donor strength and steric requirements of the phosphine
tethers, the Pd complexes of L1 adopt a strongly distorted square planar geometry
(P
1
¼ PPh 2 , Table 1) with at most a weak Pd–C ipso interaction (Pd–C ¼ 2.463(3) Å)
or a T-shape geometry (P
3
¼ PCy 2 , Cy ¼ cyclohexyl, no co-ligand) [62]. Both
complexes feature a strong Pd!B interaction as evident from short Pd–B bond
distances (Pd–B ¼ 2.194(3) Å for P
1
¼ PPh 2 and Pd–B ¼ 2.243(2) Å for P
3
¼ PCy 2 )
and a significant pyramidalization of the boron atom (ΣB α ¼ 346
for P
1
¼ PPh 2 and
ΣB α ¼ 341
for P
3
¼ PCy 2 ).
In contrast with other d
10 analogues, the silver(I) complex L1Ag
I (I) exhibits a
very weak TM!B interaction (Table 1) [60]. Rather than κ
3 (P,B,P), a trigonal
planar κ
2 (P,P) coordination geometry was proposed based on the sum of angles of
356.5
in the P 2 AgI plane. Competing B–F bond formation prevents the coordination of L1 to AgF, showing that halide abstraction can hamper coordination of
σ-acceptor ligands.
This study of L1 coordination to d
10 transition metals nicely illustrates how
acceptor ligands can give rise to structures that challenge our understanding of the
bonding and geometry of transition metal complexes. More generally, the series of
complexes shown in Table 1 exposes the coordination flexibility of the L1 platform,
demonstrating most importantly that the TM–B interaction is not enforced by the
pincer architecture but rather a possibility among several accessible geometries.
Inagaki and co-workers hypothesized that the electron-depleting nature of L1
would amplify the intrinsic alkynophilicity of a gold cationic center which, in
catalysis, can be utilized for a more effective activation of alkynes towards nucleophilic attack. For this purpose, cationic Au
I complexes of L1 were synthesized
(Scheme 2) [63]. The synthesis of an L1Au
+ fragment from L1Au
I
Cl by direct
Metal-Ligand Cooperation at Phosphine-Based Acceptor Pincer Ligands
31
orbitals obtained by density functional theory (DFT) calculations. Natural bond
orbital (NBO) analysis showed that among the various donor-acceptor interactions
involving this triangle, a Cu!B donating interaction of similar magnitude as the
η
1 (B) interactions found in related complexes is present [24]. Additionally, natural
population analysis confirms a net charge transfer from Cu to B, reinforcing the
description of the ligand as an acceptor ligand.
Moving to the second and third row transition metal centers, L1 coordinates to
Au
I in an initially unexpected yet most pincerlike manner [61]. A square planar
geometry around the tetracoordinated Au
I is observed, featuring trans-diphosphine
coordination of L1 and a chloride co-ligand trans to the η
1 (B)-coordinated borane
(Table 1). A strong Au!B interaction is observed as evident from a short bond
distance (2.309(8) Å) and strong pyramidalization of the borane center (ΣB α ¼ 341
).
Frontier orbital analysis shows a B–Au–Cl three-center interaction; however, the
charge depletion at gold and charge increase at boron are not large enough to be
considered a 2e
À oxidation of the gold center to Au
III . In addition,
197 Au Mössbauer
spectroscopy supported the classification of L1Au
I Cl as a 16 VE Au
I complex.
The distinct η
1 (B) and arene-supported η
2 (B,C) coordination modes can be
considered two extremes of L1 coordination to d
10 transition metal centers. This
becomes apparent upon the evaluation of the coordination of L1 to Pd
0
[59]. Depending on the donor strength and steric requirements of the phosphine
tethers, the Pd complexes of L1 adopt a strongly distorted square planar geometry
(P
1
¼ PPh 2 , Table 1) with at most a weak Pd–C ipso interaction (Pd–C ¼ 2.463(3) Å)
or a T-shape geometry (P
3
¼ PCy 2 , Cy ¼ cyclohexyl, no co-ligand) [62]. Both
complexes feature a strong Pd!B interaction as evident from short Pd–B bond
distances (Pd–B ¼ 2.194(3) Å for P
1
¼ PPh 2 and Pd–B ¼ 2.243(2) Å for P
3
¼ PCy 2 )
and a significant pyramidalization of the boron atom (ΣB α ¼ 346
for P
1
¼ PPh 2 and
ΣB α ¼ 341
for P
3
¼ PCy 2 ).
In contrast with other d
10 analogues, the silver(I) complex L1Ag
I (I) exhibits a
very weak TM!B interaction (Table 1) [60]. Rather than κ
3 (P,B,P), a trigonal
planar κ
2 (P,P) coordination geometry was proposed based on the sum of angles of
356.5
in the P 2 AgI plane. Competing B–F bond formation prevents the coordination of L1 to AgF, showing that halide abstraction can hamper coordination of
σ-acceptor ligands.
This study of L1 coordination to d
10 transition metals nicely illustrates how
acceptor ligands can give rise to structures that challenge our understanding of the
bonding and geometry of transition metal complexes. More generally, the series of
complexes shown in Table 1 exposes the coordination flexibility of the L1 platform,
demonstrating most importantly that the TM–B interaction is not enforced by the
pincer architecture but rather a possibility among several accessible geometries.
Inagaki and co-workers hypothesized that the electron-depleting nature of L1
would amplify the intrinsic alkynophilicity of a gold cationic center which, in
catalysis, can be utilized for a more effective activation of alkynes towards nucleophilic attack. For this purpose, cationic Au
I complexes of L1 were synthesized
(Scheme 2) [63]. The synthesis of an L1Au
+ fragment from L1Au
I
Cl by direct
Metal-Ligand Cooperation at Phosphine-Based Acceptor Pincer Ligands
31