Calcium carbene complexes with boranophosphorano side-arms: CaC[P(Ph) 2BH3]2

Article abstract of DOI:10.1039/b823224e

The ligand H₂C(PPh₂BH₃)₂ (4-H₂) reacted with one or a half equivalent of (para-tBu-C ₆H₄CH₂)₂Ca·(THF)₄ to form the calcium complexes Ca[HC(PPh

Full text of DOI:10.1039/b823224e

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Calcium carbene complexes with boranophosphorano side-arms:
CaC[P(Ph) BH ]
2
3 2†
a
a
b
a
Lars Orzechowski, Georg Jansen, Martin Lutz and Sjoerd Harder*
Received 5th January 2009, Accepted 9th February 2009
First published as an Advance Article on the web 27th February 2009
DOI: 10.1039/b823224e
The ligand H
para-tBu-C
CaC(PPh BH
4-Ca·(THF)]
complexes Ca[HC(PPh
2
C(PPh
CH
(4-Ca), respectively. The crystal structures of their THF adducts (4-H)
follow the same trends as observed for the corresponding iminophosphorano substituted
NR) and [CaC(PPh NR) . The P–C bonds shorten upon gradual
2
BH
Ca·(THF)
3
)
2
(4-H
2
) reacted with one or a half equivalent of
to form the calcium complexes Ca[HC(PPh
(
6
H
4
2
)
2
4
2
BH
3
)
2
]
2
(4-H)
2
Ca and
2
3
)
2
2
Ca·THF and
[
2
2
2
]
2
2
]
2 2
deprotonation, whereas the P–B and P–N bonds elongate. The geometries of DFT-optimized model
systems and complete molecular structures show similar trends. Also the charge distribution within the
boranophosphorano complexes is similar to that in the iminophosphorano complexes. The high
positive charges on Ca (1.74–1.75) indicate a predominantly ionic ligand–Ca bonding. High negative
charges on the central carbon atom (-1.103 in Ca[HC(PH
compare well to those calculated for analogous iminophosphorano complexes (-1.126
Ca[HC(PH NH) and -1.847 [CaC(PH NH) , respectively). Thus, in both types of complexes
delocalization of electron density over the boranophosphorano or iminophosphorano substituents does
not play a major role. Complex [4-Ca·(THF)] does not react with adamantyl cyanide. Instead the
adduct (4-Ca)
·(THF)·(adamantyl-C∫N) formed (a crystal structure has been determined).
2
BH
3
)
2
]
2
and -1.775 in [CaC(PH
2
BH
3
)
2
]
2
)
2
2
]
2
2
]
2 2
2
2
Introduction
The first calcium carbene complexes (R CCa) are based on
2
ligands with chelating iminophosphorano side-arms (R =
1
,2
–
PPh
2
NSiMe
3
). This ligand system, that has been known for
3
quite some time, is well-known for its formation of unusual
metal carbene complexes. Depending on the size of the imino
4
substituent, the calcium complexes either crystallize as a dimer (1-
1
2
Ca)
2
or monomer (2-Ca·(THF)
2
). As the Ca–C bond in general
5
is predominantly ionic, metal–carbon bonding in such complexes
differs substantially from that in more covalent Schrock or Fischer
carbenes. In its extreme case it could be described as a methandiide
2
-
2+
that carries a double negative charge on carbon: R
2
C
Ca (3).
There has been some debate on the bonding situation in po-
6
lar bis(iminophosporano)methandiide metal complexes. Rather
short P–C and long P–N bond distances suggest extensive
delocalization of p-electron density from the central carbon
to the terminal nitrogen atoms (Scheme 1). However, as the
+
-
dipolar resonance form R
the most dominant for iminophosphoranes, we suggested
a similar strongly charge-separated resonance structure for
3
P –N (R) has been shown to be
7
1
-
+
2-
the bis(iminophosporano)methandiide dianion: (RN -R
2
P ) C .
2
This proposed resonance form has found strong support by
1,8
calculation of the charge distribution in such systems. Short
a
Fachbereich Chemie, Universit a¨ tsstraße 5, 45117, Essen, Germany. E-mail:
sjoerd.harder@uni-due.de; Fax: +49-201-1832621; Tel: +49-201-1833684
b
Bijvoet Center for Biomolecular Research, Department of Crystal and Struc-
tural Chemistry, Utrecht University, 3584, CH Utrecht, The Netherlands
†
Electronic supplementary information (ESI) available: Details for the
theoretical calculations. CCDC reference numbers 714440 (4-H)
Ca·THF,
14438 [4-Ca·(THF)] and 714439 (4-Ca)
·(THF)·(adamantyl-C∫N). For
ESI and crystallographic data in CIF or other electronic format see DOI:
0.1039/b823224e
2
7
2
2
1
Scheme 1
2
958 | Dalton Trans., 2009, 2958–2964
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◦
P–C and long P–N distances should be explained by electrostatic
Table 1 Selected bond distances ( A˚ ) and angles ( ) for (4-H) Ca·THF,
2
2
-
+
2-
-
9
∫
[
4-Ca·(THF)]
2
and (4-Ca)
2
·(THF)·(adamantyl-C N)
C ◊ ◊ ◊ P attraction and C ◊ ◊ ◊ N repulsion, respectively.
Here we present the synthesis and structure of
bis(boranophosphorano)methandiide calcium complex for which
a
(
4-H)
2
Ca·THF
Ca–C1
Ca–O1
Ca ◊ ◊ ◊ H
2.754(3) C1–P1
2.361(2) C1–P2
2.45(3) P1–B1
2.815(4) P2–B2
2.908(4) C26–P3
2.816(4) C26–P4
2.840(4) P3–B3
P4–B4
1.762(3) P1–C1–P2
1.758(3) P3–C26–P4
1.932(3) C1–P1–B1
1.919(3) C1–P2–B2
1.707(3) C26–P3–B3
1.713(3) C26–P4–B4
1.927(3) B1 ◊ ◊ ◊ Ca ◊ ◊ ◊ B2
1.924(3) B3 ◊ ◊ ◊ Ca ◊ ◊ ◊ B4
125.4(2)
130.3(2)
113.3(2)
113.5(2)
120.3(2)
120.7(2)
92.7(1)
p-type electron delocalization over the terminal BH substituents
3
a
can be ruled out.
Ca ◊ ◊ ◊ B1
Ca ◊ ◊ ◊ B2
Ca ◊ ◊ ◊ B3
Ca ◊ ◊ ◊ B4
Results and discussion
Although the ligand H
2
C(PPh
quite some time, we conveniently prepared it from H
and H according to a route from Schmidbaur et al. It is
B·SMe
a well-defined ligand system for which a crystal structure also has
2
BH
3
)
2
(4-H
2
) has been known for
78.1(1)
10
2
2 2
C(PPh )
(
4-Ca·THF)
2
11
3
2
Ca1–C1
2.595(3) Ca2–C1
2.541(3) P1–C1–P2
Ca1–C26 2.561(3) Ca2–C26 2.650(3) P3–C26–P4
Ca1–O1 2.379(2) Ca2–O2 2.398(3) Ca1–C1–Ca2
Ca1 ◊ ◊ ◊ H 2.45(4) Ca2 ◊ ◊ ◊ H 2.44(4) Ca1–C26–Ca2
Ca1 ◊ ◊ ◊ B1 2.785(4) Ca2 ◊ ◊ ◊ B2 2.773(4) B1 ◊ ◊ ◊ Ca1 ◊ ◊ ◊ B3 166.5(1)
Ca1 ◊ ◊ ◊ B3 2.725(4) Ca2 ◊ ◊ ◊ B4 2.768(4) B2 ◊ ◊ ◊ Ca2 ◊ ◊ ◊ B4 176.0(1)
128.9(2)
132.6(2)
85.2(1)
83.6(1)
11
been determined. Reaction of (para-tBu-C
with two equivalents of 4-H in benzene gave essentially quantita-
tive formation of (4-H) Ca (Scheme 2). This complex crystallized
in 67% yield as (4-H)
Ca·(THF).
6
H
4
CH
2
)
2
Ca·(THF)
4
a
a
2
2
2
(
4-Ca)
Ca1–C1
Ca1–C26 2.521(4) Ca2–C26 2.538(4) P3–C26–P4
Ca1–O1 2.402(3) Ca2–N1 2.496(4) Ca1–C1–Ca2
2
·(THF)·(adamantyl-C∫N)
2.535(4) Ca2–C1
2.561(4) P1–C1–P2
132.8(3)
130.8(3)
87.0(1)
87.8(1)
a
a
Ca1 ◊ ◊ ◊ H 2.44(4) Ca2 ◊ ◊ ◊ H 2.44(4) Ca1–C26–Ca2
Ca1 ◊ ◊ ◊ B1 2.824(6) Ca2 ◊ ◊ ◊ B2 2.791(6) B1 ◊ ◊ ◊ Ca1 ◊ ◊ ◊ B3 173.0(2)
Ca1 ◊ ◊ ◊ B3 2.822(7) Ca2 ◊ ◊ ◊ B4 2.834(6) B2 ◊ ◊ ◊ Ca2 ◊ ◊ ◊ B4 174.6(2)
a
Average Ca ◊ ◊ ◊ H distance.
Scheme 2
The crystal structure (Fig. 1, Table 1) shows two different
2
+
ligand ◊ ◊ ◊ Ca interactions. For one ligand a tridentate coordi-
2
+
nation mode with two BH
3
◊ ◊ ◊ Ca contacts and a somewhat
2
+
˚
longer C ◊ ◊ ◊ Ca bond of 2.754(3) A is observed. The other ligand
2
+
chelates the metal solely by two BH
3
◊ ◊ ◊ Ca contacts leaving the
Ca
Fig. 1 Crystal structure of (4-H)
of the Ph substituents and only selected hydrogen atoms are shown.
2
Ca·THF. For clarity, only the Cipso atoms
central carbon as a free carbanion. As the complex (1-H)
also crystallized with rather long C ◊ ◊ ◊ Ca bonds (average:
.765(1) A) and alkali metal complexes M[HC(PPh
2
2
+
1
˚
˚
2
2
NSiMe
3
)
2
]
significantly longer (1.760(3) A). Also the P–C–P valence angles
12
(
M = Li, Na, K) generally show no bonding C ◊ ◊ ◊ M contacts,
are in line with the different degrees of hybridization: the P–C–P
◦
these must be of minor importance. The hydrogen atoms attached
to the central carbon atom have been located in the difference-
Fourier and were refined isotropically. The carbon atom bonded to
angle for C26 (130.3(2) ) is significantly larger than that at C1
◦
(125.4(2) ).
All BH
3
hydrogen atoms have been located in the difference-
-substituents
2
+
Ca (C1) is considerably more pyramidalized than non-bonding
C26 which is essentially sp -hybridized (the sum of the valence
angles at C1 and C26 is 342(1) and 358(1) respectively). These
differences in hybridization are also in agreement with the different
P–C bond distances. The P–C bonds for sp -hybridized C26, the
Fourier and were refined isotropically. The BH
coordinate to Ca as bifurcate ligands with H ◊ ◊ ◊ Ca contacts
˚
ranging from 2.28(3) to 2.81(3) A. Although these contacts lie
in a large range, the average value for the H ◊ ◊ ◊ Ca contact
˚ ˚
(2.45(3) A) compares well to that of 2.42(3) A in 5, a calcium
complex in which the BH
contact between carbanion and Ca . It is also very similar to
3
2
2+
2+
◦
◦
2
+
2
2
+
carbon with the highest percentage of s-character, are on average
3
◊ ◊ ◊ Ca interaction represents the only
2+ 13
˚
1
.710(3) A. P–C bonds to the more pyramidilized carbon C1,
2
3
2+
14
˚
for which a hybridization between sp and sp is estimated, are
the average H ◊ ◊ ◊ Ca contact of 2.46(3) A in (BH
4
)
2
Ca·(DME)
2
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2
+
Table 2 General trends observed for the P–C and P–B (or P–N) bond
˚
but significantly longer than the BH ◊ ◊ ◊ Ca contact of 2.25(2) A
distances ( A˚ ) upon stepwise deprotonation of the ligands 4-H
2
(or 1-H )
-
2+
2
in a recently reported complex with a C
8
H
14BH
2
◊ ◊ ◊ Ca bond
and a comparison with calculated values (B3LYP/TZVPP) for the simple
15
2+
˚
(
C
8
H
14BH = 9-BBN). Shorter BH ◊ ◊ ◊ Ca contacts (2.31(4) A)
model systems 7-H
2
2
(or 6-H )
+
-
16
also have been observed in [(Me
3
Si)
3
CpCa ·(THF)
3
][HBEt ].
3
Complex
P–C
P–B
Under slightly more drastic reaction conditions, complex
(
4-H)
2
Ca can be further deprotonated by addition of another
CH
Ca·(THF)
Scheme 2). The product crystallizes from benzene in the form
1
1
4
-H
2
Exp.
1.833(2)
1.847
1.760(3)
1.917(2)
1.930
1.925(4)b
1.926(4)
1.937
1.947(4)
1.960
equivalent of the strong base (para-tBu-C
(
of a dimeric complex: [4-Ca·(THF)]
6
H
4
2
)
2
4
Model 7-H
2
Calc.
Exp.
a
b
a
(4-H) Ca·THF
2
1
.710(3)
2
(Fig. 2, Table 1).
Model (7-H)
2
Ca
Calc.
Exp.
Calc.
1.777
1.724(3)
1.740
(
4-Ca·THF)
2
Model (7-Ca)
2
Complex
P–C
P–N
3
0
1
-H
Model 6-H
1-H) Ca
Model (6-H)
2
Exp.
1.825(1)
1.822
1.739(1)
1.725
1.706(2)
1.705
1.536(2)
1.567
1.592(1)
1.608
1.605(2)
1.631
2
Calc.
Exp.
Calc.
Exp.
Calc.
(
2
2
Ca
(
1-Ca)
2
Model (6-Ca)
2
a
b
Value for ligand with Ca–C contact. Value for ligand without Ca–C
contact.
Table 2 gives a comparison of P–C and P–B bond lengths in
4
-H
of ligand 4-H
lengths. An anomaly is found for (4-H)
2
, (4-H)
2
Ca·THF and [4-Ca·(THF)]
2
. Gradual deprotonation
, results in a stepwise shortening of the P–C bond
2
2
Ca·THF: the ligand with
Fig. 2 Crystal structure of [4-Ca·(THF)]
2
. For clarity, only the Cipso
atoms of the Ph substituents and only selected hydrogen atoms are shown.
Cocrystallized benzene molecules are not shown.
the non-bonding carbon shows a significant shorter P–C bond
than expected on account of a different hybridization (vide supra).
This general trend compares well to that in the corresponding
1
iminophosporano series: 1-H
2
, (1-H)
2
Ca and (1-Ca) (Table 2).
2
The structure of this calcium carbene complex is similar to that
1
The P–B bond lengths show a gradual lengthening upon stepwise
deprotonation of 4-H . The increase in the P–B bond lengths along
the series 4-H , (4-H) Ca·THF and [4-Ca·(THF)] , however, is less
of (1-Ca)
Me
compensated for by coordination of additional THF to each Ca .
2
. Loss of bulkiness of the ligand 1 by substitution of the
2
3
SiN unit for the much smaller BH group in 4 is apparently
3
2
+
2
2
2
pronounced than the observed increase in P–N bond lengths in the
row 1-H , (1-H) Ca and (1-Ca)
Although the rather short P–C bond of 1.724(3) A in [4-Ca·
(THF)] could be explained by resonance (Scheme 3), we propose
that the dipolar resonance form (H B -Ph P ) C is the most
The Ca–C bonds are significantly shorter than that in (4-H)
and vary from 2.541(3) to 2.650(3) A. This relatively large range
2
Ca
˚
2
2
2
.
˚
originates from slight asymmetric bridging of the ligand. The
˚
2
rather short Ca1–C26 bond of 2.561(3) A is nearly coplanar with
-
+
2-
◦
3
2
2
the P3–C26–P4 plane (24.2(1) ) and roughly aligned along C26’s
sp -orbital, whereas the longer Ca2–C26 contact of 2.650(3) A
would correspond to interaction of Ca2 with the spatially more
2
important. It should be noted that this particular dianionic ligand
˚
is isolobal to the carbodiphosphorane (Ph
3
P) C by substitution
2
-
of two Ph substituents with BH
3
groups. This molecule can be
extended p-orbital. All the BH
and were isotropically refined. Also in [4-Ca·(THF)]
substituents coordinate to Ca in a bidentate fashion. Although
3
hydrogen atoms could be located
+
-
described by three resonance forms, Ph
3
P=C=PPh
3
↔ Ph P –C =
3
2
the BH -
3
+
2-
+
2
+
PPh ↔ Ph P –C –P Ph , for which the last dipolar form has
3
3
3
17
2
+
been shown to be the most predominant. This underscores the
˚
the H ◊ ◊ ◊ Ca contacts are in a narrower range (2.35(4)–2.51(4) A)
-
+
2-
˚
importance of the resonance form (H
3
B -Ph
2
P ) C in Scheme 3.
2
than those in (4-H)
well to that of 2.45(3) A in (4-H)
2
Ca, its average value of 2.45(4) A compares
˚
2
Ca.
Scheme 3
We investigated the ligand geometries and charge distribution
in the complexes by DFT calculations. Simple model systems in
which all substituents have been replaced by hydrogen atoms gave
very satisfying results in the analyses of the iminophosphorano
complexes (6-H
2
, (6-H)
2
Ca, 6-Ca and (6-Ca) ; Table 2). Therefore,
2
we analyzed the boranophosphorano complexes by optimization
2
960 | Dalton Trans., 2009, 2958–2964
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of the simple model compounds 7-H
2
, (7-H)
2
Ca, 7-Ca and (7-Ca)
2
Table 3 NPA group charges (i.e. the charge of the heavy atom + that of
the attached hydrogens) for model systems (B3LYP/TZVPP)
with a high-quality basis set (triple-zeta valence plus an extended
set of polarization functions; Fig. 3). All compounds have
been optimized within symmetry restrictions and without taking
THF-solvation into account. Therefore the calcium coordination
spheres do not fully compare to those in the corresponding
crystal structures. However, P–C and P–B bond distances in these
simple model systems follow the same trends as observed in the
comparable crystal structures (Table 2).
Complex
CH
x
PH
2
BH
3
Ca
7-H2
-0.465
-1.103
-1.665
-1.775
+0.865
+0.841
+0.800
+0.789
-0.633
-0.727
-0.809
-0.773
—
(
7-H)
-Ca
7-Ca]
2
Ca
+1.750
+1.684
+1.744
7
[
2
Complex
CH
x
PH
2
NH
Ca
6
-H
6-H)
-Ca
6-Ca]
2
-0.533
-1.126
-1.629
-1.847
+1.179
+1.177
+1.098
+1.128
-0.912
-1.070
-1.163
-1.116
—
(
2
Ca
+1.822
+1.760
+1.821
6
[
2
negative charge on the central carbon increases sharply by stepwise
deprotonation of 7-H (dimerization induces a higher negative
charge on the central carbon by polarization), (iii) the PH and
BH group charges are hardly affected by stepwise deprotonation
of the central carbon atom. The slight increase of negative charge
on the BH group must originate from inductive effects rather than
from mesomeric p-type charge delocalization. As the increase of
negative charge on the BH group is less pronounced than that on
the NH group along the iminophosphorane series 6-H
2
2
3
3
3
2
→ (6-Ca)
2
,
mesomeric effects might play a small additional role for charge
delocalization in iminophosphorane complexes. This is in line with
the more pronounced P–N bond lengthening when compared to
P–B bond lengthening along the row 7-H
The high negative charge of -1.775 on the central carbon atom
in (7-Ca) supports the importance of the proposed dipolar reso-
nance form (H
considerably less polarized than the H
2
→ (7-Ca)
2
(Table 2).
2
-
+
2-
3
B -Ph
2
P )
2
C . Interestingly, the H
2
P-BH arms are
3
2
P-NH substituents. This is
likely due to the much higher electronegativity of N compared
to B.
In order to strengthen the conclusions based on accurate calcu-
lations on simple model systems, we also calculated the complete
molecular structures of (4-H)
2
Ca·THF and [4-Ca·(THF)]
2
with
a different functional and a somewhat less extended basis set
18
(
BP86/TZVP). These DFT-optimized structures compare well
to the crystal structures and show essentially the same features (see
ESI† for full details). The calculated structure of (4-H)
Ca·THF
also shows two different ligand ◊ ◊ ◊ Ca interactions: one ligand
2
2
+
2
+
displays a C–Ca contact, whereas the carbon atom in the other
ligand is non-coordinating. The calculated NPA charges of -1.348
and -1.276, respectively, differ only slightly. This suggests that the
2
+
contact to Ca hardly influences the charge on carbon and only
induces slight polarization. The much higher negative charge of
-
1.718 on the central carbon atom in [4-Ca·(THF)]
to that calculated in the simpler model system (-1.775 in 7-Ca)
We screened [4-Ca·(THF)] for its reactivity with unsaturated
substrates. Like (1-Ca) , it does not react with
2
compares well
2
.
2
2
and 2-Ca·(THF)
2
benzophenone. We only found reaction with the more active
substrate cyclohexyl isocyanate. However, we have not been able
to isolate a well-defined product. Although it does not react
with adamantyl cyanide, we could isolate an adduct in good
Fig. 3 DFT optimized structures (B3LYP/TZVPP) for 7-H
2
, (7-H)Ca,
7
-Ca and (7-Ca)
2
. Bond distances to Ca are give in A˚ . See Table 2 and 3
for P–C and P–B distances and NPA group charges, respectively.
Likewise, NPA charges in these model systems show similar
trends as we observed earlier for the iminophosphorano model
systems (Table 3): (i) the charges on Ca are close to 2+, indicating
highly ionic Ca–C and Ca ◊ ◊ ◊ H contacts (>80% ionicity), (ii) the
crystalline yields: (4-Ca)
structure corresponds to that of [4-Ca·(THF)]
2
·(THF)·(adamantyl-C∫N). Its crystal
2
in which one
of the THF molecules has been exchanged for a coordinating
adamantyl cyanide ligand (Fig. 4). As can be seen from Table 1,
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towards unsaturated substrates like ketones and cyanides is
low.
Experimental
General procedures
All experiments were carried out under argon atmosphere using
freshly dried solvents and standard Schlenk line and glove box
1
1
techniques. The ligand [H
reagent (para-tBu-benzyl)
3
B(Ph)
2
P]
2
CH
2
and organocalcium
1
9
2
Ca·(THF)
4
have been prepared ac-
1
13
cording to literature. H and C NMR spectra were recorded on
a Bruker DPX300 spectrometer operating at 300 and 75.5 MHz,
respectively. Crystals were measured on a Siemens Smart diffrac-
tometer with APEXII area detector system or on a Nonius
KappaCCD diffractometer with a rotating anode.
Fig. 4 Crystal structure of (4-Ca)
2
·(THF)·(adamantyl-C∫N). For clar-
ity, only the Cipso atoms of the Ph substituents and only selected
hydrogen atoms are shown. Cocrystallized benzene molecules are not
shown.
Synthesis of (4-H)
35.9 mg, 57.6 mmol) and H
2
Ca·THF. (para-tBu-benzyl)
2
Ca·(THF)
4
(
2
C(Ph P-BH (46 mg, 111.6 mmol)
2
)
3 2
were dissolved in 0.5 mL of benzene. After 10 min the volume
of the mixture was reduced to a half. Cooling the solution
this hardly affects the metal coordination geometry and the
bond distances within the ligands. Considering that the cen-
tral carbon atom in these carbene calcium complexes carries
a high negative charge, this sluggish reactivity is remarkable.
The low reactivity of these complexes towards unsaturated
substrates could be due to: (i) electrostatic stabilization of the
negatively charged carbon by its two neighbouring phoshonium
substituents and (ii) the poor accessibility of this nucleophilic
after refluxing leads to colorless needles. The needles (35.0 mg,
◦
6
7.2%) were dried under vacuum (50 C, 1 Torr, 5 min). Crystals
suitable for X-ray diffraction were obtained at room temperature
from a mixture of toluene and hexane. Analyses: Anal calcd for
C
54
H
62
B
4
CaOP
4
, (M = 934.31): C, 69.42; H, 6.69%. Found: C,
1
69.08; H, 6.43%. H NMR (300 MHz, C
6
D
6
/THF-d ): d 0.95–
8
3
2.10 (br, 12H, BH
3
), 1.07 (t, JP–H = 10.7 Hz, 2H, CH), 1.44
carbon atom by neighbouring Ph P-substituents. As this com-
2
(
m, 4H, THF), 3.56 (m, 4H, THF), 7.00–7.14 (m, 24H, m-, p-
pound class is exceptionally sensitive towards hydrolysis, we are
currently testing the reactivity of these compounds towards small
molecules.
3
3
13
Ph), 7.91 (dd, JH–H = 6.9 Hz, JP–H = 9.0 Hz, 16 H, o-Ph);
C
1
NMR (75 MHz, C
6
D
6
/THF-d
8
): d 6.4 (t, JP–C = 81.1 Hz, CH),
3
2
5.8 (THF), 68.0 (THF), 127.9 (t, JP–C = 5.0 Hz, m-Ph), 128.8 (s,
2
p-Ph), 132.1 (t, JP–C = 5.0 Hz, o-Ph), 141.5 (m, AXX¢-spin-system,
ipso-Ph); P NMR (202.3 MHz, C /THF-d ): 11.05 (br, PCP);
): -32.0 (br, BH ).
3
1
6
D
6
8
Conclusions
1
1
B NMR (160 MHz, C
Synthesis of [4-Ca·(THF)]
0.554 g, 0.89 mmol) and H C(Ph
6
D
6
/THF-d
8
3
Stepwise deprotonation of the ligand H
with an organocalcium base gave Ca[HC(PPh
and finally the calcium carbene CaC(PPh
2
C(PPh
2
BH
((4-H)
(4-Ca).
3
)
2
(4-H
2
)
2
.
(para-tBu-benzyl) Ca·(THF)
2
4
2
BH
3
)
2
]
2
2
Ca)
(
2
2
P-BH (0.348 g, 0.84 mmol)
)
3 2
2
BH
3 2
)
were dissolved in 10 mL of benzene. The orange solution was
These boranophosphorano substituted calcium compounds crys-
tallize similar to the earlier reported iminophosphorano ana-
◦
stirred for 3 h at 60 C. Removing all solvent resulted in a
yellow residue. Addition of 5 mL of pentane and stirring gave
precipitation of a white solid which was washed three times
logues Ca[HC(PPh
2
NSiMe
3
)
2
]
2
and CaC(PPh
2
NSiMe
3
2
) . As the
iminophosphorano substituents are more bulky than the bo-
ranophosphorano substituents, the latter crystallized as THF
◦
with 6 mL portions of pentane and dried under vacuum (50 C,
1
Torr, 20 min). The resulting white powder (0.43 g, 95.4%) is
adducts (4-H)
2
Ca·THF and [4-Ca·(THF)]
2
. The boranophos-
essentially pure product. Crystals suitable for X-ray diffraction
have been obtained by slowly cooling a solution in benzene–
phorano substituted complexes show the same trends as
the iminophosphorano substituted compounds. Upon grad-
ual deprotonation of these ligands the P–C bonds shorten.
The P–B bond lengths, however, increase somewhat less pro-
nounced than the P–N bond lengths in the iminophosphorano
ligands.
◦
hexane to 7 C. Analyses: Anal calcd for C58
1
H
68
B
4
Ca
2
O
2
1
P
4
, (M =
044.49): C, 66.70; H, 6.56%. Found: C, 66.19; H, 6.51%. H NMR
300 MHz, C /THF-d ): d 0.95–2.10 (br, 6H, BH ), 1.47 (m,
H, THF), 3.56 (m, 4H, THF), 6.90–7.05 (br, 12H, m-, p-Ph),
.65–7.85 (br, 8 H, o-Ph); C NMR (75 MHz, C
5.8 (THF), 67.9 (THF), 127.4 (br, m-Ph), 127.7 (s, p-Ph), 133.0
(
6
D
6
8
3
4
7
2
1
3
6
D
6
/THF-d
8
): d
Also the trends in the charge distributions are similar, calcula-
tions show that gradual deprotonation results in a drastic increase
of negative charge on the central carbon atom. Only very little
electron density is transferred to the phosphorano, borano or
(
br, o-Ph); the PCP and ipso-Ph carbon atoms are NMR silent.
3
1
11
P NMR (202.3 MHz, C
160 MHz, C /THF-d
6
D
6
/THF-d
8
): 1.65 (br, PCP); B NMR
).
(
6
D
6
8
): -28.3 (br, BH
3
imino groups. These results underscore that both CaC(PPh
2
BH
3
)
2
and CaC(PPh NSiMe should be seen as complexes in which
2
3
)
2
Synthesis of (4-Ca)
cyanide (7.0 mg, 43.4 mmol) was added to a supension of [4-Ca·
(THF)] (23 mg, 22.0 mmol) in 0.5 mL of benzene. Heating this
2
·(THF)·(adamantyl-C∫N). Adamantyl
the negative charge is mainly located on the central carbon
atom. In both types of complexes, mesomeric p-type electron
delocalization does not play a major role. Despite a high con-
centration of negative charge on the central carbon, reactivity
2
suspension to reflux temperature dissolved most of the solids.
After separation of the remaining solids the solution was slowly
2
962 | Dalton Trans., 2009, 2958–2964
This journal is © The Royal Society of Chemistry 2009

View Article Online
◦
23
cooled to 7 C. Colorless crystals precipitated (22.0 mg, 88.0%).
Analyses: Anal calcd for C65 Ca NOP
, (M = 1133.63): C,
8.87; H, 6.67%. Found: C, 68.48; H, 6.81%. H NMR (300 MHz,
/THF-d ): d 1.05–2.20 (br, 6H, BH ), 1.27 (br, adamantyl,
H), 1.42 (m, 4H, THF), 1.50 (br, adamantyl, 3H), 1.66 (d,
B3LYP functional. The TZVPP Gaussian type function basis set
used in these calculations consists of a triple-zeta valence quality
H
75
B
4
2
4
1
24
25
6
C
set and the polarization functions of Dunning’s cc-pVTZ set for
all atoms except for Ca, where a 2p1d polarization function set
26
6
D
6
8
3
6
has been used. This level of theory has also been employed to verify
that the optimized structures were minima by frequency analyses
making use of analytical second derivatives, and to determine
atomic charges by Natural Population Analysis (NPA).
The full molecules and complexes (4-H
)Ca·THF and [4-Ca·
THF] have been optimized without symmetry constraints using
their respective crystal structures as starting geometries. Here the
BP86 exchange–correlation functional was employed.
the geometry was pre-optimized using a SV(P) split-valence plus
polarization basis set, followed by optimization using a TZVP
3
J
2
H–H = 2.5 Hz, adamantyl, 6H), 3.58 (m, 4H, THF), 6.93–7.03 (br,
4H, m-, p-Ph), 7.60–7.80 (br, 16H, o-Ph); C NMR (75 MHz,
/THF-d ): d 25.8 (THF), 27.3 (s, adamantyl-CH), 30.3 (s,
adamantyl-C), 35.7 (s, adamantyl-CH ), 39.9 (s, adamantyl-CH ),
7.7 (THF), 124.9 (s, CN), 127.4 (t, JP–C = 4.5 Hz, m-Ph), 128.2
1
3
27
C
6
D
6
8
2
2
2
3
6
(
2
2
br, p-Ph), 133.0 (t, JP–C = 4.3 Hz, o-Ph). 142.0–142.8 (m, AXX¢-
3
1
23a–d,g
spin system, ipso-Ph); the PCP carbon atom is NMR silent.
P
First
1
1
NMR (202.3 MHz, C
160 MHz, C /THF-d
6
D
6
/THF-d
): -28.0 (br, BH
8
): 1.99 (br, PCP); B NMR
).
28
(
6
D
6
8
3
24
basis set. The valence orbital part of this basis set is essentially
the same as that of the TZVPP basis set described above with,
however, a smaller set of polarization functions (1p instead of
Crystal structure determination
Structures have been solved and refined using the programs
2
p1d for H and Ca, 1d instead of 2d1f for the other atoms) and
20
SHELXS-97 and SHELXL-97, respectively. All geometry cal-
a slightly different contraction scheme for the P atoms (6111 for
the p functions in TZVP, 51111 for TZVPP). The computation of
geometries and NPA charges was carried out in the framework
of the “resolution-of-the-identity” (RI) approximation using the
21
culations and graphics have been performed with PLATON.
CCDC reference numbers 714440 (4-H)
Ca·THF, 714438
4-Ca·(THF)] and 714439 (4-Ca)
·(THF)·(adamantyl-C∫N). For
crystallographic data in CIF or other electronic format see DOI:
2
[
2
2
29
appropriate TZVP auxiliary basis set.
1
0.1039/b823224e
Atom coordinates, energies and NPA charges of all optimized
structures are included in the Supplementary Information, which
also contains the calculated frequencies of the model compounds.
◦
(
4-H)
2
Ca·THF. Measurement at -90 C (Mo Ka), formula
˚
C
54
H
62
B
4
CaOP
4
, weight 934.24, monoclinic, a = 13.4423(11) A,
◦
˚
˚
b = 13.8789(11) A, c = 28.380(2) A, b = 101.081(5) , V =
3
-3
˚
5
196.0(7) A , space group P2
1
/c, Z = 4, r
c
= 1.194 g cm , m(Mo
Acknowledgements
-
1
Ka) = 0.281 mm , 83169 measured reflections, 8364 independent
Prof. Dr. R. Boese and D. Bl a¨ ser (Universit a¨ t Duisburg-Essen) are
thanked for collection of X-ray diffraction data.
reflections (Rint = 0.107), 5953 reflections observed with I > 2s(I),
◦
q
max = 24.3 , R = 0.0449, wR
2
= 0.11361, GOF = 1.02, 633
-
3
˚
parameter, min./max. residual electron density -0.29/+0.34 e A .
References
◦
[
4-Ca·(THF)]
2
.
Measurement at -123 C (Mo Ka), formula
C
58
H
68
B
4
Ca
2
O
˚
2
P
4
·(C
6
H
6
)
2.5, weight 1239.68, monoclinic, a =
1 L. Orzechowski, G. Jansen and S. Harder, J. Am. Chem. Soc., 2006,
128, 14676.
˚
˚
2
1
r
0.6437(1) A, b = 11.2943(1) A, c = 33.4613(3) A, b =
2
3
L. Orzechowski and S. Harder, Organometallics, 2007, 26, 2144.
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3
˚
17.6520(3) , V = 6910.62(9) A , space group P2 /c, Z = 4,
1
-
3
-1
c
= 1.192 g cm , m(Mo Ka) = 0.301 mm , 76064 measured
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◦
reflections observed with I > 2s(I), qmax = 25.6 , R = 0.0578,
4
5
wR
2
= 0.1860, GOF = 1.09, 785 parameter, min./max. residual
-3
˚
electron density -0.39/+1.11 e A .
2
The Ca–C bond in Me Ca is calculated to be 89% ionic (RHF/
◦
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4-Ca)
2
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H
75
B
˚
4
Ca
2
NOP
4
·(C
6
H
6
)
˚
3
, weight 1367.87,
˚
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7
8
G. R. Giesbrecht and J. C. Gordon, J. Chem. Soc., Dalton Trans., 2004,
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2
387.
◦
3
˚
b = 112.493(6) , V = 7655.4(14) A , space group P2 /c, Z = 4,
1
N. Kocher, D. Leusser, A. Murso and D. Stalke, Chem.–Eur. J., 2004,
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3
-1
r
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= 1.187 g cm , m(Mo Ka) = 0.278 mm , 173398 measured
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◦
reflections observed with I > 2s(I), qmax = 23.9 , R = 0.0634,
wR
2
= 0.1611, GOF = 1.02, 904 parameter, min./max. residual
-3
˚
electron density -0.39/+0.38 e A .
9
Although short P–C bonds in the Wittig reagent have always been
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just as well be explained by electrostatic bond shortening in the ylide
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Can. J. Chem,., 1975, 53, 3040.
10 K. C. Nainan and G. E. Ryschkewitsch, Inorg. Chem., 1969, 8, 2671.
3
P=CH
2
, they could
DFT calculations
+
-
3 2
P -CH : M.-H. Whangbo, S. Wolfe and F. Bernardi,
All geometries were fully optimized within the designated sym-
metry constraints (using tight criteria) at the density functional
theory level. The Turbomole V 5.10 program package has been
employed throughout. Geometries for the model systems 7-H
7-H) Ca, 7-Ca and [7-Ca] have been optimized employing the
1
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(
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This journal is © The Royal Society of Chemistry 2009
Dalton Trans., 2009, 2958–2964 | 2963

View Article Online
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8 This BP86 functional method and TZVP basis set have been chosen
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. Note that in ref. 1 the footnote a in Table 1
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2
964 | Dalton Trans., 2009, 2958–2964
This journal is © The Royal Society of Chemistry 2009