Coordination chemistry umpolung: A gallane ligand on a phosphine Lewis acceptor

Article abstract of DOI:10.1021/ja012081k

Pentaphenylphosphinophosphonium trifluoromethanesulfonate reacts with amino-iminogall(I)ane to effect quantitative substitution of Ph₃P and formation of the first complex of a gallium Lewis base on a phosphine Lewis acid. Copyright

Full text of DOI:10.1021/ja012081k

Published on Web 12/28/2001
Coordination Chemistry Umpolung: A Gallane Ligand on a Phosphine Lewis
Acceptor
Neil Burford,* Paul J. Ragogna, Katherine N. Robertson, and T. Stanley Cameron
Department of Chemistry, Dalhousie UniVersity, Halifax, NoVa Scotia B3H 4J3, Canada
Ned J. Hardman and Philip P. Power
Department of Chemistry, UniVersity of California at DaVis, One Shields AVenue, DaVis, California 95616
Received August 31, 2001
“Coordinate” bonds result from the interaction between electron-
rich centers (Lewis base or donor) and electron-deficient (Lewis
acid or acceptor) centers. As such, this bonding model is essentially
restricted to the chemistry of transition metals and the elements of
group 13. Reports of transition metal complexes behaving as Lewis
donors (ligands) toward other metals1 and compounds involving
phosphines (bearing lone pairs) behaving as Lewis acceptors toward
amines,2-5 phosphines,6-8 and arenes9 highlight a new dimension
in coordination chemistry. The coordinate nature of the (phosphine)-
PfP(phosphenium) bond in phosphinophosphonium cations 1 has
been demonstrated by phosphine ligand exchange reactions,10 which
we have now exploited to demonstrate the first (gallane)GafP-
(phosphine) coordination.
Figure 1. Molecular structure of 3. Selected bond lengths (Å) and angles
(deg): Ga(1)-P(1) 2.312(3), Ga(1)-O(3) 1.979(6), Ga(1)-N(1) 1.950(8),
Ga(1)-N(2) 1.931(8), P(1)-O(3) 3.242(7), and C(36)-P(1)-C(30) 105.4(5),
C(36)-P(1)-Ga(1) 102.3(3) C(30)-P(1)-Ga(1) 108.4(4), N(1)-Ga-P(1)
132.5(3), N(2)-Ga-P(1) 122.7(3), O(3)-Ga-P(1) 97.9(2), N(2)-Ga-
N(1) 98.3(3), N(2)-Ga-O(3) 102.0(3), N(1)-Ga-O(3) 95.4(3).
(Dipp) substituents, and the conformational features are retained
in solution as the 1H NMR data shows distinct isopropyl and methyl
signals. The gallium center adopts a distorted tetrahedral geometry
and is dislodged from the heterocyclic C3N2 plane, in contrast to
The 31P NMR spectrum of the reaction mixture containing
[Ph3P-PPh2][SO3CF3] (1, R ) Ph) and gall(I)ane11 2 shows two
singlets, assigned to Ph3P (δ ) -5 ppm) and a new complex 3 (δ
the planar free ligand 2.11 Nevertheless, the Ga-N distances
) -57 ppm), which has been isolated and comprehensively
[1.950(8) Å and 1.931(8) Å] are slightly shorter [cf. 2, 2.0528(14)
characterized.12 In the context of the recently reported (gallane)-
Å and 2.0560(13) Å] and the N-Ga-N angle [98.3(3)°] is larger
[cf. 2, 87.56(6)°]. The Ga-O distance [1.979(6) Å] is substantially
shorter than the P-O distance [3.242(7) Å], suggesting a preference
for Ga-O covalency and avoiding coordinative unsaturation and
electron deficiency at gallium.
Gafgroup 13 complexes12,13 the quantitative reaction is best
described as a ligand exchange of Ph3P by 2 at the Ph2P+ Lewis
acceptor site.
The structures of 1, 2, and 3 all represent diversions from
traditional p-block chemistry in terms of the environments of
gallium and phosphorus, and conventional Lewis models need to
evolve to appreciate the unusual bonding. Despite the Lewis acidic
tradition for the chemistry of group 13 elements, the coordinatively
unsaturated and lone pair-bearing nature of the gallium center in
the recently reported gall(I)anes13-15 such as 2,11 and the related
alane,16 identifies them as viable Lewis bases. In this context, the
formation of compound 3 from 1 can be considered in terms of the
introduction of a gallium ligand on a phosphenium cation and
represents17 a “coordination chemistry umpolung” (a converse of
the traditional coordinate bond) in that the metal center (gallium)
behaves as the donor (ligand) and the electron-rich nonmetal center
(phosphorus) behaves as the Lewis acceptor. Tetracoordinate
gallium bound to an electron-rich tricoordinate phosphine is unusual,
The solid-state structure of 3 (Figure 1) confirms the Ga-P
connectivity and reveals an association of one oxygen atom of the
triflate anion with the gallium center. The geometry about phos-
phorus is typical for a phosphine, and the C-P-C angle [105.4(5)°]
is comparable with those observed in the precursor [Ph3P-
PPh2][SO3CF3], [104.25(16)°].10 The phenyl substituents on phos-
phorus are apparently sterically involved with the diisopropylphenyl
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382 VOL. 124, NO. 3, 2002 J. AM. CHEM. SOC.
10.1021/ja012081k CCC: $22.00 © 2002 American Chemical Society

C O M M U N I C A T I O N S
(11) Hardman, N. J.; Eichler, B. E.; Power, P. P. J. Chem. Soc., Chem. Commun.
2000, 1991-1992.
but reminiscent of the recently reported base-induced dissociation
of 13/15 oligomers.18,19 More importantly, the isolation and
characterization of the gallaphosphorus triflate 3 highlights the
synthetic utility of the ligand exchange process at phosphorus as a
versatile approach for element-P bond formation. We are currently
assessing the potential diversity of this procedure.
(12) Using established procedures for handling compounds under vacuum,20
a solution of 2 (0.15 g (0.33 mmol) in C6H6 (60 mL) was added dropwise
to a slurry of [Ph3P-PPh2][OTf] (0.20 g, 0.33 mmol) in C6H6 (60 mL)
over 1 h with stirring at room temperature. After 12 h, the solvent was
removed from the colorless solution in vacuo, and the white solid 3 was
washed with C6H6 (10 mL) and recrystallized from CH2Cl2; yield: 69%;
Anal. Calcd C 61.40, H 6.26, N 3.41; Found C 61.27, H 6.46, N 3.42; IR
cm-1 (ranked intensities): 627(11), 727(18), 736(7), 744(20), 806(17),
939(19), 1011(4), 1154(12), 1164(10), 1184(6), 1202(2), 1235(8), 1260(16),
1288(15), 1305(14), 1317(9), 1339(5), 1345(3), 1366(1), 1507(13); NMR
(CDCl3): 31P{1H} (rel. H3PO4) -57 ppm. Crystal data for C42H51F3-
GaN2O3PS: MW ) 821.63 g mol-1, monoclinic, P21, a ) 10.266(1) Å,
b ) 19.810(2) Å, c ) 25.714(2) Å, R ) 90°, â ) 113.41(1)°,γ ) 90°, V
) 2062.2(4) Å3, T ) 293(2)K, Z ) 2, µ(Mo KR) ) 21.9 cm-1, 3679
Acknowledgment. We thank the Natural Sciences and Engi-
neering Research Council of Canada and the Killam Foundation
for funding, and the Atlantic Region Magnetic Resonance Center
and Dalx for the use of instrumentation.
measured reflections, 3465 unique, 479 refined parameters, 2θmax
)
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0.0394, wR2(F2) ) 0.0876; CCDC 167029.
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