Weakly-coordinating anions stabilise the unprecedented monovalent and divalent η-benzene nickel cations [(η-C5H5)Ni(ν-C6H6)Ni(ν- C5H5)]2+ and [Ni(ν-C6H6)2]2+

Article abstract of DOI:10.1039/b001190h

Nickelocene in benzene reacts with the Bronsted acid H₂O-B(C₆F₅)₃ to give the salt [(η-C₅H₅)Ni(η-C₆H₆)Ni(η-C₅H₅)][B₃(μ-O)₃(C₆F₅)₅] which is the first example of a triple-decker nickel sandwich with a bridging η-benzene ligand; the borate anion is also unprecedented; treatment of Ni(η-C₅H₅)₂ with Brookhart's acid [H(OEt₂)₂][B(3,5-(CF₃)₂C₆H₃)₄] in benzene gives the paramagnetic bis(η-benzene)nickel derivative {[Ni(η- C₆H₆)₂][B(3,5(CF₃)₂C₆H₃)₄]₂·Ni(η-C₅H₅)₂} in which nickelocene is present as a molecule of crystallisation.

Full text of DOI:10.1039/b001190h

Weakly-coordinating anions stabilise the unprecedented monovalent and
divalent h-benzene nickel cations [(h-C5H5)Ni(h-C6H6)Ni(h-C5H5)]2+ and
[Ni(h-C6H6)2]2+
José L. Priego, Linda H. Doerrer, Leigh H. Rees and Malcolm. L. H. Green*
Inorganic Chemistry Laboratory, South Parks Road, Oxford, UK OX1 3QR. E-mail: malcolm.green@chem.ox.ac.uk
Received (in Cambridge, UK) 9th March 2000, Accepted 15th March 2000
Nickelocene in benzene reacts with the Brønsted acid H2O–
B(C6F5)3 to give the salt [(h-C5H5)Ni(h-C6H6)Ni(h-
C5H5)][B3(m-O)3(C6F5)5] which is the first example of a
triple-decker nickel sandwich with a bridging h-benzene
ligand; the borate anion is also unprecedented; treatment of
Ni(h-C5H5)2 with Brookhart’s acid [H(OEt2)2][B(3,5-
(CF3)2C6H3)4] in benzene gives the paramagnetic bis(h-
magnetic moment of 1 at room temperature is 1.69 mB and
corresponds to one unpaired electron, consistent8 with the
4
2 1
ground state configuration d s p .
Treatment of nickelocene with H2O–B(C6F5)3 in CH2Cl2–
benzene, gave brown crystals of the triple-decker sandwich
compound
[(h-C5H5)Ni(h-C6H6)Ni(h-C5H5)][B3(m-O)3-
(C6F5)5] 2. In contrast, the reaction between nickelocene and the
Brønsted acid HBF4·Et2O gave the cation [Cp3Ni2]+.9 We
envisage the cation [(h-C5H5)Ni(h-C6H6)Ni(h-C5H5)]2+ is
formed by the addition of two intermediate [Ni(h-C5H5)]+
fragments to a benzene molecule. Each nickel centre is formally
benzene)nickel
derivative
{[Ni(h-C6H6)2][B(3,5-
(CF3)2C6H3)4]2·Ni(h-C5H5)2} in which nickelocene is
present as a molecule of crystallisation.
Large and very weakly coordinating anions [BR4]2 [R = 3,5-
(CF3)2C6H3, C6F5] have recently attracted interest owing to
their ability to stabilise highly electrophilic metal cations.1 The
strong Lewis acid B(C6F5)3, is currently of interest as a methyl
abstracting agent and activator of early transition metal Ziegler–
Natta polymerisation systems.2 Recently the mono-aquo adduct
H2O–B(C6F5)3 was reported, and the solution behaviour as a
Brønsted acid was described.3,4
Here, we report reaction between nickelocene and the
Brønsted acids H2O–B(C6F5)3 or [H(OEt2)2][B(3,5-
(CF3)2C6H3)4] (Brookhart’s acid) which give unexpected new
chemistry.
equivalent to the 18-electron centres in [Ni(h-C5H5)(h -
3
allyl)].10 The crystal structure of 2 (Fig. 2) shows the rings are
eclipsed with an average Ni–CCp distance of 2.121 Å, and the
C–C–C angles lie between 106.6(5) and 109.2(4)°; the corre-
sponding Ni–Cbenzene distance is 2.168 Å, with the C–C–C
angles between 118.5(6) and 121.8(5)°. This compound is the
first h-benzene–nickel(II) sandwich compound to be structur-
ally characterised. The only other h-benzene–nickel compound
is the Ni(0) phosphine compound, [Ni(h-C6H6)(But2PCH2-
PBut2)].11 Nickel(II) compounds with one h-toluene or h-
mesitylene ligand have been structurally characterised.12
The counter ion in 2 is a new dianion. The six-membered
B3O3 ring is surrounded by five C6F5 groups, giving one
neutral, three-coordinate boron, B(1), and two anionic tetra-
hedral boron centres, B(2) and B(3). There are only two non-
solvent moieties present in the crystal structure, and to maintain
electrostatic balance we assign a charge of 22 to the anion. The
average B–O bond distance is 1.448 Å and the average B–O–B
angle is 120.9°. The anion likely results from acidic decomposi-
tion and aryl group loss in H2O–B(C6F5)3.
Treatment of nickelocene with Brookhart’s acid
[H(OEt2)2][B(3,5-(CF3)2C6H3)4] in toluene gives green–blue
crystals of the Ni(III
) compound [Ni(h-C5H5)2][B(3,5-
(CF3)2C6H3)4] 1†. The nickelocenium cation has been isolated
previously as the BF4 salt,5 but no crystal structure was
2
reported. The cation [Ni(h-C5Me5)2]+ has been structurally
characterised as C60 and [HTCNQF4]2 salts.6 The crystal
2
structure of 1 has been determined and the ORTEP of [Ni(h-
C5H5)2]+ (Fig. 1) shows eclipsed cyclopentadienyl rings as
observed in [Ni(h-C5H5)2].7 The average Ni–C distance of
2.075 Å is slightly shorter than in [Ni(h-C5H5)2] (2.178 Å). The
Treatment of nickelocene with Brookhart’s acid in benzene
gives red–brown crystals of 3 and the crystal structure of 3
shows the presence of Ni(h-C5H5)2, Ni(h-C6H6)2 and two
Fig. 1 Crystal structure of 1. Selected bond lengths (Å) and angles (°):
Cpcent–Ni(1) 1.70, B(1)–C(1) 1.631(8), B(1)–C(9) 1.636(8); C(23)–C(22)–
C(21) 109.3(8), C(22)–C(21)–C(25) 105.3(7), C(23)–C(24)–C(25)
106.6(7), C(22)–C(23)–C(24) 108.4(7), C(24)–C(25)–C(21) 109.1(7).
Fig. 2 Crystal structure of 2. Selected bond lengths (Å) and angles (°):
Cpcent–Ni(1) 1.744, benzenecent–Ni(1) 1.795, B(1)–O(1) 1.349(4), B(2)–
O(2) 1.438(4), B(3)–O(3) 1.549(4); O(1)–B(1)–O(2) 123.8(3), O(2)–B(2)–
O(3) 108.0(2), O(1)–B(3)–O(3) 107.6(2).
DOI: 10.1039/b001190h
Chem. Commun., 2000, 779–780
This journal is © The Royal Society of Chemistry 2000
779

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Scheme 1
=
0.1973. The fluorine atoms of the CF3 groups were found to be
disordered and modelled as occupying staggerred positions (F121–F123
s.o.f. 84%; F151–F153 s.o.f. 87% and F41–F43 s.o.f. 84%).
2·CH2Cl2: C47H18B3Cl2F25Ni2O3, Mw = 1326.37, triclinic, space group
¯
P1, a = 11.873(7), b = 13.526(8), c = 16.304(8) Å, a = 73.658(6), b =
75.969(6), g = 71.607(6)°, U = 2350.0(5) Å3, Z = 2, m(Mo-Ka) = 1.06
mm21. T = 150 K; 8130 independent reflections were collected (Rint
0.03); R = 0.0462 and Rw = 0.0572.
=
=
¯
3: C43H23BF24Ni, Mw
= 1065.13, triclinic, space group P1, a
12.889(5), b = 13.191(6), c = 14.420(4) Å, a = 86.902(6), b = 90.131(6),
g = 61.171(6)°, U = 2143.55(14) Å3, Z = 2, m(Mo-Ka) = 0.590 mm21
.
T = 150 K; 7199 independent reflections were collected (Rint = 0.0000); R
= 0.0797 and Rw = 0.2037. The fluorine atoms of the CF3 groups have
been modelled as disordered over two staggerred positions (F171–F173
s.o.f 49%, F341–F343 s.o.f 72% and F472–F473 s.o.f 74%).
CCDC 182/1576. See http://www.rsc.org/suppdata/cc/b0/b001190h/ for
crystallographic files in .cif format.
Fig. 3 Crystal structure of 3. Selected bond lengths (Å) and angles (°):
benzenecent–Ni(1) 1.713, Cpcent–Ni(2) 1.710, C(51)–C(52)–C(53)
124.2(15), C(56)–C(55)–C(54) 113.8(9), C(52)–C(51)–C(56) 126.2(16),
C(63)–C(62)–C(61) 109.7(7), C(61)–C(65)–C(64) 107.0(7).
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[B(3,5-(CF3)2C6H3)4]2 anions. The room temperature magnetic
moment of 3 is 4.26 mB, corresponding to four unpaired
electrons. These data are consistent with the presence in the
crystal of neutral Ni(h-C5H5)2 and dicationic [Ni(h-C6H6)2]2+;
therefore we formulate this compound as {[Ni(h-
C6H6)2][B(3,5-(CF3)2C6H3)4]2·Ni(h-C5H5)2}, and the nickel-
ocene is present as a molecule of crystallisation (Fig. 3). The
average Ni–C distance in the dication [Ni(h-C6H6)2]2+ is 2.079
Å and the C–C–C angles are between 113.8(9) and 126.2(16)°.
Bis(h-hexamethylbenzene)nickel(II) has been reported,13 but
not structurally characterised.
The new and often surprising reactions are summarised in
Scheme 1.
J. L. P. thanks the Spanish Goverment for financial support.
L. H. D thanks St. John’s College Oxford for a Junior Research
Fellowship.
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343.
Notes and references
† Crystal data: 1: C42H22BF24Ni, Mw = 1052.12, monoclinic, space group
C2/c, a = 15.792(3), b = 12.679(3), c = 21.434(4) Å, b = 91.52(3), U =
4290.1(15) Å3, Z = 4, m(Mo-Ka) = 0.588 mm21. T = 150 K, 2802
independent reflections were collected (Rint = 0.0000); R = 0.0948 and Rw
780
Chem. Commun., 2000, 779–780