Reactions of dizincocene with sterically demanding bis(iminodi(phenyl) phosphorano)methanes

Article abstract of DOI:10.1039/c0cc02859b

Reactions of Cp*₂Zn₂ with sterically demanding bis(iminodi(phenyl)phosphorano)methanes LH (LH = CH₂(Ph ₂PNR)₂ (R = Ph L¹H, SiMe₃L ²H, 2,6-i-Pr₂C₆

Full text of DOI:10.1039/c0cc02859b

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Reactions of dizincocene with sterically demanding
bis(iminodi(phenyl)phosphorano)methanesw
Stephan Schulz,*a Sebastian Gondzik,a Daniella Schuchmann,a Ulrich Westphal,a
Lukasz Dobrzycki,a Roland Boesea and Sjoerd Harderb
Received 28th July 2010, Accepted 26th August 2010
DOI: 10.1039/c0cc02859b
Reactions of Cp*2Zn2 with sterically demanding bis(iminodi-
(phenyl)phosphorano)methanes LH (LH = CH2(Ph2PQNR)2
(R = Ph L1H, SiMe3 L2H, 2,6-i-Pr2C6H3 (Dipp) L3H) at
ambient temperature occurred with elimination of Cp*H and
to proceed with preservation of the Zn–Zn bond. Protonation
of the Cp* substituent yielded the Zn(I) complex
Mesnacnac2Zn2.7 Recently, [Zn2(dmap)6][Al(OC(CF3)3)4]2
containing the first base-stabilized [Zn2]2+ dication was
synthesized by reaction of Cp*2Zn2 with [H(OEt2)2]-
[Al(OC(CF3)3)4].8 In contrast, the reaction of Dipp–
BIAN2Zn2 with C–H acidic phenylacetylene rather occurred
with H2-elimination and formation of a binuclear acetylene
bridged Zn(II) complex (redox reaction) than with protonation
of the Dipp–BIAN substituent.9
subsequent formation of the homoleptic complex L1 Zn2 1 and
2
the heteroleptic complexes LZnZnCp* (L = L2 2, L3 3, L1 4).
3 is the first structurally characterized heteroleptic organozinc
complex with the zinc atoms in the formal oxidation state +1.
The synthesis of decamethyldizincocene Cp*2Zn2,1 the first
stable molecular compound containing a direct Zn–Zn bond
with the Zn atoms in the formal oxidation state +1,2 by
Carmona et al. in 2004 has very much intensified research
activities on group 2 and 12 metal complexes containing
metal–metal bonds. Since then, a large number of Zn(I)
complexes3 and Mg(I) complexes containing Mg–Mg4 bonds
have been synthesized, most of them by Wurtz-analogous
coupling reaction of the corresponding halide-substituted
compounds RMX. In addition, the nature of the Zn–Zn bond
has been theoretically investigated in detail.5
In order to elucidate the general applicability of protonation
reactions for the synthesis of Zn–Zn bonded complexes,
we studied reactions of Cp*2Zn2 with H-acidic bis-
(iminophosphorano)methanes H2C(P(Ph2)NR)2, which are
easily accessible by the Staudinger reaction. These were
expected to be promising reagents since reactions with metal
alkyls such as LiMe, AlMe3 and ZnMe2 have previously been
shown to proceed with alkane elimination and formation of
the corresponding bis(iminodi(phenyl)phosphorano)methanide
complexes, exhibiting a singly deprotonated, monoanionic
ligand.10 In addition, these ligands are able to bind also as
neutral and dianionic ligands toward a large variety of main
group and transition metals as well as lanthanides.11 Herein,
we report on our results obtained from reactions of Cp*2Zn2
with three bis(iminodi(phenyl)phosphorano)methanes.
The Zn–Zn bonded complexes are typically stabilized by
sterically bulky (often chelating) organic ligands. While these
ligands have been shown in the past to be extremely useful for
the stabilization of metal–metal bonded complexes, they
rather inhibit studies concerning the chemical reactivity of
these complexes due to the effective shielding of the metal
centers. As a consequence, only a handful of reports concerning
the chemical reactivity of such compounds is available.
Carmona et al. already mentioned in their initial publication
on reactions of Cp*2Zn2, which typically proceeded with
disproportionation and formation of elemental zinc and the
corresponding Zn(II) complexes. This reaction pattern turned
out to be the most prominent pathway for Zn(I) complexes
until we reported on the reaction of Cp*2Zn2 with the
Lewis base 4-dimethylaminopyridine (dmap), yielding
Cp*Zn–Zn(dmap)2Cp*, the first Lewis acid–base adduct of
dizincocene.6 In addition, the reaction with N–H acidic
[{(2,4,6-Me3C6H2)N(Me)C}2CH]H (MesnacnacH) was found
Reactions of Cp*2Zn2 with two equivalents of
CH2(Ph2PQNR)2 (R = Ph L1H, SiMe3 L2H, 2,6-i-Pr2C6H3
(Dipp) L3H) yielded the expected homoleptic complex Zn2L2
only in case of the sterically less demanding Ph-substituted
substituent
L1H,
whereas
heteroleptic
complexes
LZn–ZnCp* (L2 2, L3 3) were formed with the sterically
a University of Duisburg-Essen, Universitatsstr., 5-7, S07 S03 C30,
¨
45117 Essen, Germany. E-mail: stephan.schulz@uni-due.de;
Fax: +201 1833830; Tel: +201 1834635
b University of Groningen, Nijenborgh 4, 9747 AG Groningen,
The Netherlands. E-mail: s.harder@rug.nl; Tel: +31 50 3634322
w Electronic supplementary information (ESI) available: Experimental
details and characterization of 1–4 including single crystal structure
(1, 3) as well as computational details on homo- and heteroleptic
complexes are given. CCDC 785388 (1) and 780366 (3). For ESI and
crystallographic data in CIF or other electronic format see DOI:
10.1039/c0cc02859b
Scheme 1 Synthesis of 1–4.
c
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more demanding bis(iminodi(phenyl)phosphorano)methanes,
respectively (Scheme 1).12
2 and 3 as well as L1Zn–ZnCp* 4 were also obtained
by reactions of equimolar amounts of Cp*2Zn2 and
CH2(Ph2PQNR)2. The formation of H2 (redox reaction) or
elemental zinc (disproportionation reaction) was not observed
in any case. 1–4 are soluble in organic solvents such as toluene
and THF, respectively. 1H and 13C NMR spectra of both
complexes show the resonances due to the organic groups of
the bis(iminodi(phenyl)phosphorano)methane (1) and the Cp*
substituent in a 1 : 1 ratio (2, 3, 4). Signals at 3.28 (2) and
3.71 ppm (3) in the 1H NMR spectra (C6D6) due to the
presence of a methanide unit in the backbone clearly reveal
the monoanionic character of the ligand, whereas no
resonance of the C–H moiety was observed for 1 and 4. A
single resonance of the Cp* substituent (2.35 2, 2.07 3,
1.91 4 ppm) indicates Z5-bonded Cp* groups in solution.
In situ 31P NMR spectroscopy showed the quantitative
conversion of the free ligand L–H into the new complexes
1–4. The 31P NMR spectra of the isolated complexes
each exhibit one sharp singlet, indicating two equivalent
phosphorus atoms. The resonances (22.3 1, 24.4 2, 27.4 3,
21.9 4 ppm) are shifted downfield compared to the free ligands
(L–H). The IR spectra of 1–4 show strong absorptions due to
the PQN moiety between 1250 and 1260 cmꢀ1. Heating of 1–4
in sealed capillaries yielded greyish solids (135 1C 1, 110 1C 2,
150 1C 3, 105 1C 4) due to disproportionation reactions with
subsequent formation of elemental Zn.
Fig. 2 Solid state structure of 3; H atoms are omitted and C atoms
are reduced in size for clarity except for C1.
complex L1ZnMe. The Zn–N bond lengths of 1 (N1–Zn1
2.042(2), N2–Zn1 2.075(2) A) are comparable to those
observed for L1ZnMe (N1–Zn1 2.083(3), N2–Zn1 2.042(3) A),
whereas those of 3 (N1–Zn1 1.989(1), N2–Zn1 1.979(1) A) are
significantly shorter, resulting from the reduced steric demand
of the Cp* substituent. The P–N and P–C bond lengths in 1
(P1–N2 1.615(1), P2–N1 1.615(1), C1–P1 1.725(1), C1–P2
1.732 (1) A) and 3 (P1–N1 1.6277(11), P2–N2 1.6266(11),
C1–P1 1.7018(13), C1–P2 1.7163(13) A) are comparable to
those observed in L1ZnMe (P1–N1 1.585(3), P2–N2 1.600(3),
C1–P1 1.728(4), C1–P2 1.739(4) A).
Single crystals of 1 and 3 suitable for X-ray structure
determinations were obtained from solutions in toluene (1)
and C6D6 (3), respectively (Fig. 1 and 2).
1 and 3 are both monomeric complexes. 1 contains two
threefold-coordinated Zn atoms in an almost ideal trigonal-
planar coordination sphere whereas the Zn atoms in the
heteroleptic complex 3 show different coordination modes.
Zn1 atom adopts a trigonal-planar coordination sphere (sum
of the bond angles 359.8(4)1) whereas Zn2 is almost linearly
coordinated (Zn1–Zn2–Cp*centr. bond angle 175.41). The six-
membered CP2N2Zn metallacycles in 1 and 3 adopt distorted
boat conformations as was observed for the monomeric Zn(II)
The endocyclic bond angles within the metallacycles of 1
(N1–Zn1–N2 98.82(1), C1–P1–N2 104.14(1), C1–P2–N1
106.95(1), Zn1–N2–P1 102.72(1), Zn1–N1–P2 103.87(1),
P1–C1–P2 122.20(1)1) are comparable to those of L1ZnMe
(N1–Zn1–N2 99.4(1), C1–P1–N1 106.8(2), C1–P2–N2
108.9(2), Zn1–N1–P1 97.8(2), Zn1–N2–P2 99.5(2), P1–C1–P2
120.1(2)1), whereas those of
3 (N1–Zn1–N2 107.47(4),
C1–P1–N1 110.43(6), C1–P2–N2 114.56(6), Zn1–N1–P1
127.25(6), Zn1–N2–P2 118.01(6), P1–C1–P2 128.02(8)1) differ
significantly. However, a CCDC structural database search
revealed that these bond angles typically span a wide range
and that the endocyclic angles observed for 1 and 3 are within
the typical range previously described for metal complexes of
bis(iminodi(phenyl)phosphorano)methanides.13 The Zn–Zn
bond (2.3272(2) A) is only slightly elongated compared to
that observed for Cp*2Zn2 (2.305(3) A),1 but shorter than
those observed in 1 (2.3490(1) A) as well as in the homoleptic
b-diketiminato-stabilized Zn(I) complexes Mesnacnac2Zn2
(2.3813(8) A)7 and Dippnacnac2Zn2 (2.3586(7) A),3
respectively. These findings clearly reflect the influence of the
increasing coordination number at the Zn atom due to the use of
an N,N0-chelating ligand. The Zn–C bond length toward the
Z5-coordinated Cp* substituent (Zn2–Cp*centr. 1.944 A) is
significantly shorter compared to those observed in Cp*2Zn2
(2.04 A) and dmap2Zn2Cp*2 (2.033 A), respectively.
Fig. 1 Solid state structure of 1; H atoms are omitted for clarity
except for that on C1.
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DFT calculations (B3LYP/SVP) of homoleptic LZnZnL
(L1 1, L2) and heteroleptic complexes LZnZnCp* (L1 4,
L2 2, L3 3) were performed to investigate the influence of
steric bulk of the substituents on the Zn–Zn bond lengths. The
structural parameters of the calculated structures of 1 and 3
such as the Zn–Zn (2.396 1, 2.381 A 3) bond length and the
distances within the CP2N2Zn ring (Zn–N 2.099, 2.102;
P–N 1.639, 1.646; P–C 1.736, 1.740 A 1; Zn–N 2.048, 2.042;
P–N 1.657, 1.653; P–C 1.722, 1.731 A 3) agree very well with
the experimental values. The Zn–Zn bond lengths in the
heteroleptic complexes LZnZnCp* only slightly increase
with increasing steric bulk of the substituents (L1 2.369 4,
L2 2.376 2, L32.381 A 3), whereas those of the homoleptic
complexes differ significantly (L1 2.396 1, L2 2.438 A 5).14 The
Zn atoms in 3 carry different partial charges as was expected
due to the different coordination sphere. The Zn atom in 2–4
bound to the Cp* substituent is less electropositive (0.55 (2),
0.57 (3, 4)) compared to the Zn atom bound to two electro-
negative N atoms (0.79 (2), 0.76 (3, 4)). Comparable findings
have been previously observed for the dmap-stabilized
dizincocene Cp*(dmap)2Zn–ZnCp*.6
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Cp*2Zn2 is a promising starting reagent for the synthesis of
novel low-valent organozinc complexes by reaction with
organic substituents containing acidic H atoms including a
C–H moiety as shown here, yielding so far unknown base-free,
heteroleptic Zn(I) complexes of the type LZn–ZnCp*.15
S. Schulz is grateful for financial support by the DFG and
L.D. acknowledges the Foundation for Polish Science for the
Kolumb fellowship.
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Notes and references
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12 Experimental details are given in the ESI.w In situ NMR studies
clearly showed the formation of Cp*H during the reaction.
13 See for instance: M. S. Hill and P. B. Hitchcock, Dalton Trans.,
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2 Zn2H2 has been previously trapped in an Ar matrix at 12 K and
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14 The structure of the homoleptic complex L32Zn2 couldn’t be
calculated within these studies due to limited computing time.
15 Very recently, Carmona et al. reported on the crystal structures of
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c
This journal is The Royal Society of Chemistry 2010
Chem. Commun., 2010, 46, 7757–7759 7759